¦ : ? /
CataJog of CHP
Technologies
U.S. Environmental Protection Agency
Combined Heat and Power Partnership
-------
Introduction to CHP Technologies
Introduction
Fueled by electric industry deregulation, environmental concerns, unease over energy security,
and a host of other factors, interest in combined heat and power (CHP) technologies has been
growing among energy customers, regulators, legislators, and developers. CHP is a specific
form of distributed generation (DG), which refers to the strategic placement of electric power
generating units at or near customer facilities to supply on-site energy needs. CHP enhances
the advantages of DG by the simultaneous production of useful thermal and power output,
thereby increasing the overall efficiency.
CHP offers energy and environmental benefits over electric-only and thermal-only systems in
both central and distributed power generation applications. CHP systems have the potential for
a wide range of applications and the higher efficiencies result in lower emissions than separate
heat and power generation system. The advantages of CHP broadly include the following:
• The simultaneous production of useful thermal and electrical energy in CHP systems
lead to increased fuel efficiency.
¦ CHP units can be strategically located at the point of energy use. Such onsite
generation avoids the transmission and distribution losses associated with electricity
purchased via the grid from central stations.
¦ CHP is versatile and can be coupled with existing and planned technologies for many
different applications in the industrial, commercial, and residential sectors.
EPA offers this catalog of CHP technologies as an on-line educational resource for the
regulatory, policy, permitting, and other communities. EPA recognizes that some energy
projects will not be suitable for CHP; however, EPA hopes that this catalog will assist readers in
identifying opportunities for CHP in applications where thermal-only or electric-only generation
are currently being considered.
The remainder of this introductory summary is divided into sections. The first section provides a
brief overview of how CHP systems work and the key concepts of efficiency and power-to-heat
ratios. The second section summarizes the cost and performance characteristics of five CHP
technologies in use and under development.
Overview of Combined Heat and Power
What is Combined Heat and Power?
CHP is the sequential or simultaneous generation of multiple forms of useful energy (usually
mechanical and thermal) in a single, integrated system. CHP systems consist of a number of
individual components - prime mover (heat engine), generator, heat recovery, and electrical
interconnection - configured into an integrated whole. The type of equipment that drives the
overall system (i.e., the prime mover) typically identifies the CHP system. Prime movers for
CHP systems include reciprocating engines, combustion or gas turbines, steam turbines,
microturbines, and fuel cells. These prime movers are capable of burning a variety of fuels,
including natural gas, coal, oil, and alternative fuels to produce shaft power or mechanical
energy. Although mechanical energy from the prime mover is most often used to drive a
generator to produce electricity, it can also be used to drive rotating equipment such as
Introduction to CHP Catalog of Technologies
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compressors, pumps, and fans. Thermal energy from the system can be used in direct process
applications or indirectly to produce steam, hot water, hot air for drying, or chilled water for
process cooling.
Figure 1 shows the efficiency advantage of CHP compared with conventional central station
power generation and on-site boilers. When considering both thermal and electrical processes
together, CHP typically requires only 3A the primary energy separate heat and power systems
require. This reduced primary fuel consumption is key to the environmental benefits of CHP,
since burning the same fuel more efficiently means fewer emissions for the same level of
output.
Figure 1: CHP versus Separate Heat and Power (SHP) Production
Existing SHP
Conventional
Generation (49%
overall efficiency)
68 units
(Losses)
/N
Combined heat and power
produces electricity and thermal
energy from a single fuel
Combined Heat and Power
(75% overall efficiency)
GRD
>30V
/ units v
Electricity
Electricity
BOILER
units
11 units
(Losses)
25 units
(Losses)
Note: Assumes national averages for grid electricity and incorporates electric transmission losses.
Source: Tina Kaarsberg and Joseph Roop, "Combined Heat and Power: How Much Carbon and Energy Can
It Save for Manufacturers?"
Expressing CHP Efficiency
Many of the benefits of CHP stem from the relatively high efficiency of CHP systems compared
to other systems. Because CHP systems simultaneously produce electricity and useful thermal
energy, CHP efficiency is measured and expressed in a number of different ways.1 Table I
summarizes the key elements of efficiency as applied to CHP systems.
1 Measures of efficiency are denoted either as lower heating value (LHV) or higher heating value
(HHV). HHV includes the heat of condensation of the water vapor in the products. Unless
otherwise noted, all efficiency measures in this section are reported on an HHV basis.
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Table 1: Measuring the Efficiency of CHP Systems
System
Component
Efficiency Measure
Description
Separate heat and
power (SHP)
Thermal Efficiency
(Boiler)
Net Useful Thermal Output
EFF -
Energy Input
Net useful thermal output for the fuel
consumed
Electric-only generation
Power Output
.Tp —
Energy Input
Electricity Purchased From Central Stations
via Transmission Grid
Overall Efficiency of
separate heat and power
(SHP)
EFF - P + Q
SHP P/EFFPowe, + Q/EFFThennal
Sum of net power (P) and useful thermal
energy output (Q) divided by the sum of fuel
consumed to produce each.
Combined heat and
power (CHP)
Total CHP System
Efficiency
EFFTo„, = (P + Q)/F
Sum of the net power and net useful thermal
output divided by the total fuel (F)
consumed.
FERC Efficiency
Standard
_ (P + Q/2)
L FERC t—i
F
Developed for the Public Utilities Regulatory
Act of 1978, the FERC methodology
attempts to recognize the quality of electrical
output relative to thermal output.
Effective Electrical
Efficiency (or Fuel
Utilization Efficiency,
FUE):
FUE = P
F - Q/EFFThermal
Ratio of net power output to net fuel
consumption, where net fuel consumption
excludes the portion of fuel used for
producing useful heat output. Fuel used to
produce useful heat is calculated assuming
typical boiler efficiency, usually 80%.
Percent Fuel Savings
S = 1 F
P/EFFp + Q/EFFq
Fuel savings compares the fuel used by the
CHP system to a separate heat and power
system. Positive values represent fuel
savings while negative values indicate that
the CHP system is using more fuel than
SHP.
Key:
P = Net power output from CHP system
Q = Net useful thermal energy from CHP system
F = Total fuel input to CHP system
EFFp = Efficiency of displaced electric generation
EFFq = Efficiency of displaced thermal generation
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As illustrated in Table I the efficiency of electricity generation in power-only systems is
determined by the relationship between net electrical output and the amount of fuel used for the
power generation. Heat rate, the term often used to express efficiency in such power
generation systems, is represented in terms of Btus of fuel consumed per kWh of electricity
generated. However, CHP plants produce useable heat as well as electricity. In CHP systems,
the total CHP efficiency seeks to capture the energy content of both electricity and usable
steam and is the net electrical output plus the net useful thermal output of the CHP system
divided by the fuel consumed in the production of electricity and steam. While total CHP
efficiency provides a measure for capturing the energy content of electricity and steam
produced it does not adequately reflect the fact that electricity and steam have different
qualities. The quality and value of electrical output is higher relative to heat output and is
evidenced by the fact that electricity can be transmitted over long distances and can be
converted to other forms of energy. To account for these differences in quality, the Public
Utilities Regulatory Policies Act of 1978 (PURPA) discounts half of the thermal energy in its
calculation of the efficiency standard (EffFERc)- The EFFferc is represented as the ratio of net
electric output plus half of the net thermal output to the total fuel used in the CHP system.
Opinions vary as to whether the standard was arbitrarily set, but the FERC methodology does
recognize the value of different forms of energy. The following equation calculates the FERC
efficiency value for CHP applications.
P + ^
EFFferc = ^
F
Where: P = Net power output from CHP system
F = Total fuel input to CHP system
Q = Net thermal energy from CHP system
Another definition of CHP efficiency is effective electrical efficiency, also known as fuel
utilization effectiveness (FUE). This measure expresses CHP efficiency as the ratio of net
electrical output to net fuel consumption, where net fuel consumption excludes the portion of
fuel that goes to producing useful heat output. The fuel used to produce useful heat is
calculated assuming typical boiler efficiency, generally 80%. The effective electrical efficiency
measure for CHP captures the value of both the electrical and thermal outputs of CHP plants.
The following equation calculates FEU.
FUE =
r_Q/
EFF,
Where: EffQ = Efficiency of displaced thermal generation
FEU captures the value of both the electrical and thermal outputs of CHP plants and it
specifically measures the efficiency of generating power through the incremental fuel
consumption of the CHP system.
EPA considers fuel savings as the appropriate term to use when discussing CHP benefits
relative to separate heat and power (SHP) operations. Fuel savings compares the fuel used by
the CHP system to a separate heat and power system (i.e. boiler and electric-only generation).
The following equation determines percent fuel savings (S).
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Where:
Effp = Efficiency of displaced electric generation
EffQ = Efficiency of displaced thermal-only facility
In the fuel saving equation given above, the numerator in the bracket term denotes the fuel used
in the production of electricity and steam in a CHP system. The denominator describes the sum
of the fuel used in the production of electricity (P/EffP) and thermal energy (Q/EffQ) in separate
heat-and-power operations. Positive values represent fuel savings while negative values
indicate that the CHP system in question is using more fuel than separate heat and power
generation.
Another important concept related to CHP efficiency is the power-to-heat ratio. The power-to-
heat ratio indicates the proportion of power (electrical or mechanical energy) to heat energy
(steam or hot water) produced in the CHP system. Because the efficiencies of power
generation and steam generation are likely to be considerably different, the power-to-heat ratio
has an important bearing on how the total CHP system efficiency of the CHP system might
compare to a separate power-and-heat system. Figure 2 illustrates this point. The illustrative
curves display how the overall efficiency might change under alternate power-to-heat ratios for a
separate power-and-heat system and a CHP system (for illustrative purposes, the CHP system
is assumed to use 5% less fuel than its separate heat-and-power counterpart for the same level
of electrical and thermal output).
Figure 2: Equivalent Separate Heat and Power Efficiency
Assumes 40% efficient electric and 80% efficient thermal generation
80%
75%
- - CHP System Efficiency Using 5% Less Fuel
Separate Heat and Power Overall Efficiency
> 70%
^ 65%
5 60%
^ 50%
40%
0.1 0.9
10% Electric Output
90% Thermal Output
1.7 2.4
Power-to-Heat Ratio
3.2 4.0
80% Electric Output
20% Thermal Output
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Overview of CHP Technologies
This catalog is comprised of five chapters that characterize each of the different CHP
technologies (gas turbine, reciprocating engines, steam turbines, microturbines, and fuel cells)
in detail. Many of these technologies are commonly used today, some are in the early stages of
commercialization, and others are expected to be available in a few years. The chapters supply
information on the applications of the technology, detailed descriptions of its functionality and
design characteristics, performance characteristics, emissions, and emissions control options.
The following sections provide snapshots of the five technologies, and a comparison of key cost
and performance characteristics across the range of technologies that highlights the
distinctiveness of each. Tables II and III provide a summary of the key cost and performance
characteristics of the CHP technologies discussed in the catalog.
Table II: Summary of CHP Technologies
CHP system
Advantages
Disadvantages
Available
sizes
Gas turbine
High reliability.
Low emissions.
High grade heat available.
No cooling required.
Require high pressure gas or in-
house gas compressor.
Poor efficiency at low loading.
Output falls as ambient
temperature rises.
500 kWto
40 MW
Microturbine
Small number of moving parts.
Compact size and light weight.
Low emissions.
No cooling required.
High costs.
Relatively low mechanical
efficiency.
Limited to lower temperature
cogeneration applications.
30 kW to 350
kW
Spark ignition
(SI)
reciprocating
engine
High power efficiency with part-
load operational flexibility.
Fast start-up.
Relatively low investment cost.
Can be used in island mode
and have good load following
capability.
Can be overhauled on site with
normal operators.
Operate on low-pressure gas.
High maintenance costs.
Limited to lower temperature
cogeneration applications.
Relatively high air emissions.
Must be cooled even if recovered
heat is not used.
High levels of low frequency noise.
<5 MW
Diesel/compre
ssion ignition
(CI)
reciprocating
engine
High speed
(1,200 RPM)
<4MW
Low speed
(60-275
RPM)
<65MW
Steam turbine
High overall efficiency.
Any type of fuel may be used.
Ability to meet more than one
site heat grade requirement.
Long working life and high
reliability.
Power to heat ratio can be
varied.
Slow start up.
Low power to heat ratio.
50 kW to 250
MW
Fuel Cells
Low emissions and low noise.
High efficiency over load range.
Modular design.
High costs.
Low durability and power density.
Fuels requiring processing unless
pure hydrogen is used.
200 kWto
250 kW
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Table III: Summary Table of Typical Cost and Performance Characteristics by CHP Technology Type*
Technology
Steam turbine1
Diesel engine
Nat. gas engine
Gas turbine
Microturbine
Fuel cell
Power efficiency (HHV)
15-38%
27-45%
22-40%
22-36%
18-27%
30-63%
Overall efficiency (HHV)
80%
70-80%
70-80%
70-75%
65-75%
65-80%
Effective electrical efficiency
75%
70-80%
70-80%
50-70%
50-70%
60-80%
Typical capacity (MWe)
0.2-800
0.03-5
0.05-5
1-500
0.03-0.35
0.01-2
Typical power to heat ratio
0.1-0.3
0.5-1
0.5-1
0.5-2
0.4-0.7
1-2
Part-load
ok
good
ok
poor
ok
good
CHP Installed costs ($/kWe)
300-900
900-1,500
900-1,500
800-1,800
1,300-2,500
2,700-5,300
O&M costs ($/kWhe)
<0.004
0.005-0.015
0.007-0.02
0.003-0.0096
0.01 (projected)
0.005-0.04
Availability
near 100%
90-95%
92-97%
90-98%
90-98%
>95%
Hours to overhauls
>50,000
25,000-30,000
24,000-60,000
30,000-50,000
5,000-40,000
10,000-40,000
Start-up time
1 hr-1 day
10 sec
10 sec
10 min -1 hr
60 sec
3 hrs - 2 days
Fuel pressure (psi)
n/a
<5
1-45
120-500
(compressor)
40-100
(compresor)
0.5-45
Fuels
all
diesel, residual oil
natural gas,
biogas, propane,
landfill gas
natural gas,
biogas, propane,
oil
natural gas,
biogas, propane,
oil
hydrogen, natural
gas, propane,
methanol
Noise
high
high
high
moderate
moderate
low
Uses for thermal output
LP-HP steam
hot water, LP
steam
hot water, LP
steam
heat, hot water,
LP-HP steam
heat, hot water,
LP steam
hot water, LP-HP
steam
Power Density (kW/m2)
>100
35-50
35-50
20-500
5-70
5-20
NOx2, Ib/MMbtu
0.03-0.3
1-1,83
0.18
0.05
0.03
0.004
1 b/M Wh j otalOutput
0.13-1.3
4.3-8.24
0.8
0.25
0.15
0.02
* Data are illustrative values for 'typically' available systems; All $ are in 2000$
1 For steam turbine, not entire boiler package
2 New low emitting units without end of pipe controls
3 Present on road diesel requirements are approximately 1 Ib/MMBtu, but most backup diesel generators emit at 1.8 Ib/MMBtu
4 New on road diesel rule would bring emissions rate to approximately 0.3 lb/MWhTotaioutput
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Technology
The first chapter of the catalog focuses on gas turbines as a CHP technology. Gas turbines are
typically available in sizes ranging from 500 kW to 250 MW and can operate on a variety of fuels
such as natural gas, synthetic gas, landfill gas, and fuel oils. Most gas turbines typically operate
on gaseous fuel with liquid fuel as a back up. Gas turbines can be used in a variety of
configurations including (1) simple cycle operation with a single gas turbine producing power
only, (2) combined heat and power (CHP) operation with a single gas turbine coupled and a
heat recovery exchanger and (3) combined cycle operation in which high pressure steam is
generated from recovered exhaust heat and used to produce additional power using a steam
turbine. Some combined cycles systems extract steam at an intermediate pressure for use and
are combined cycle CHP systems. Many industrial and institutional facilities have successfully
used gas turbines in CHP mode to generate power and thermal energy on-site. Gas turbines
are well suited for CHP because their high-temperature exhaust can be used to generate
process steam at conditions as high as 1,200 pounds per square inch gauge (psig) and 900
degree Fahrenheit (°F). Much of the gas turbine-based CHP capacity currently existing in the
United States consists of large combined-cycle CHP systems that maximize power production
for sale to the grid. Simple-cycle CHP applications are common in smaller installations, typically
less than 40 MW.
The second chapter of the catalog focuses on microturbines, which are small electricity
generators that can burn a wide variety of fuels including natural gas, sour gases (high sulfur,
low Btu content), and liquid fuels such as gasoline, kerosene, and diesel fuel/distillate heating
oil. Microturbines use the fuel to create high-speed rotation that turns an electrical generator to
produce electricity. In CHP operation, a heat exchanger referred to as the exhaust gas heat
exchanger, transfers thermal energy from the microturbine exhaust to a hot water system.
Exhaust heat can be used for a number of different applications including potable water heating,
absorption chillers and desiccant dehumidification equipment, space heating, process heating,
and other building uses. Microturbines entered field-testing in 1997 and the first units began
commercial service in 2000. Available and models under development typically range in sizes
from 30 kW to 350 kW.
The third chapter in the catalog describes the various types of reciprocating engines used in
CHP applications. Spark ignition (SI) and compression ignition (CI) are the most common
types of reciprocating engines used in CHP-related projects. SI engines use spark plugs with a
high-intensity spark of timed duration to ignite a compressed fuel-air mixture within the cylinder.
SI engines are available in sizes up to 5 MW. Natural gas is the preferred fuel in electric
generation and CHP applications of SI; however, propane, gasoline and landfill gas can also be
used. Diesel engines, also called CI engines, are among the most efficient simple-cycle power
generation options in the market. These engines operate on diesel fuel or heavy oil. Dual fuel
engines, which are diesel compression ignition engines predominantly fueled by natural gas
with a small amount of diesel pilot fuel, are also used. Higher speed diesel engines (1,200 rpm)
are available up to 4 MW in size, while lower speed diesel engines (60 - 275 rpm) can be as
large as 65 MW. Reciprocating engines start quickly, follow load well, have good part-load
efficiencies, and generally have high reliabilities. In many instances, multiple reciprocating
engine units can be used to enhance plant capacity and availability. Reciprocating engines are
well suited for applications that require hot water or low-pressure steam.
The fourth chapter of the catalog is dedicated to steam turbines that generate electricity from
the heat (steam) produced in a boiler. The energy produced in the boiler is transferred to the
turbine through high-pressure steam that in turn powers the turbine and generator. This
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separation of functions enables steam turbines to operate with a variety of fuels including
natural gas, solid waste, coal, wood, wood waste, and agricultural by-products. The capacity of
commercially available steam turbine typically ranges between 50 kW to over 250 MW.
Although steam turbines are competitively priced compared to other prime movers, the costs of
a complete boiler/steam turbine CHP system is relatively high on a per kW basis. This is
because steam turbines are typically sized with low power to heat (P/H) ratios, and have high
capital costs associated with the fuel and steam handling systems and the custom nature of
most installations. Thus the ideal applications of steam turbine-based CHP systems include
medium- and large-scale industrial or institutional facilities with high thermal loads and where
solid or waste fuels are readily available for boiler use.
Chapter five in the catalog deals with an emerging technology that has the potential to serve
power and thermal needs cleanly and efficiently. Fuel cells use an electrochemical or battery-
like process to convert the chemical energy of hydrogen into water and electricity. In CHP
applications, heat is generally recovered in the form of hot water or low-pressure steam (<30
psig) and the quality of heat is dependent on the type of fuel cell and its operating temperature.
Fuel cells use hydrogen, which can be obtained from natural gas, coal gas, methanol, and other
hydrocarbon fuels. There are currently five types of fuel cells under development. These
include (1) phosphoric acid (PAFC), (2) proton exchange membrane (PEMFC), (3) molten
carbonate (MCFC), (4) solid oxide (SOFC), and (5) alkaline (AFC). Currently, there are only two
commercially available fuel cells, a 200 kW PAFC unit and a 250 kW MCFC unit. Due to the
high installed cost of fuel cell systems, the most prominent DG applications of fuel cell systems
are CHP-related.
Installed cost2
The total plant cost or installed cost for most CHP technologies consists of the total equipment
cost plus installation labor and materials, engineering, project management, and financial
carrying costs during the construction period. The cost of the basic technology package plus
the costs for added systems needed for the particular application comprise the total equipment
cost.
Total installed costs for gas turbines, microturbines, reciprocating engines, and steam turbines
are comparable. The total installed cost for typical gas turbines ranges from $785/kW to
$1,780/kW while total installed costs for typical microturbines in grid-interconnected CHP
applications may range anywhere from $1,339/kW to $2,516/kW. Commercially available
natural gas spark-ignited engine gensets have total installed costs of $920/kW to $1,515/kW,
and steam turbines have total installed costs ranging from $349/kW to $918/kW. Fuel cells are
currently the most expensive among the five CHP technologies with total installed costs ranging
between $4,500/kW (for a 200 PAFC unit) to $5,000/kW (for a 250 MCFC unit).
O&M Cost
Non-fuel operation and maintenance (O&M) costs typically include routine inspections,
scheduled overhauls, preventive maintenance, and operating labor. O&M costs are comparable
for gas turbines, gas engine gensets, steam turbines and fuel cells, and only a fraction higher
for microturbines. Total O&M costs range from $4.2/MWh to $9.6/MWh for typical gas turbines,
from $9.3/MWh to $18.4/MWh for commercially available gas engine gensets and are typically
2 All $ are 2000$.
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less than $4/MWh for steam turbines. Based on manufacturers offer service contracts for
specialized maintenance, the O&M costs for microturbines appear to be around $10/MWh. For
fuel cells O&M costs range approximately between $29/MW and $43/MW.
Start-up time
Start-up times for the five CHP technologies described in this catalog can vary significantly
depending on the technology and fuel used. Gas turbines have relatively short start up time,
though heat recovery considerations may constraint start up times. Microturbines require
several minutes for start-up but requires a power storage unit (typically a battery UPS) for start-
up if the microturbine system is operating independent of the grid. Reciprocating engines have
fast start-up capability, which allows for timely resumption of the system following a
maintenance procedure. In peaking or emergency power applications, reciprocating engines
can most quickly supply electricity on demand. Steam turbines, on the other hand, require long
warm-up periods in order to obtain reliable service and prevent excessive thermal expansion,
stress and wear. Fuel cells also have relatively long start-up times (especially for MCFC and
SOFC). The longer start-up times for steam turbines and fuel cells make it more applicable to
baseload needs.
Availability
Availability indicates the amount of time a unit can be used for electricity and/or steam
production. Availability generally depends on the operational conditions of the unit. Frequent
starts and stops of gas turbines can increase the likelihood of mechanical failure, though steady
operation with clean fuels can permit gas turbines to operate for about a year without a
shutdown. The estimated availability for gas turbines operating on clean gaseous fuels such as
natural gas is over 95 percent.
Given the limited number of microturbines currently in commercial use it is difficult to draw
conclusions on the reliability and availability of these units. At the same time, the basic design
and limited number of moving parts in microturbines suggests that the technology will have
good availability. Manufacturers of microturbines have targeted availabilities between 98 and
99 percent. Natural gas engine availabilities generally vary with engine type, speed, and fuel
quality. Typically demonstrated availabilities for natural gas engine gensets in CHP applications
is approximately 95 percent. Steam turbines have high availability rates ~ usually greater than
99 percent with longer than one year between shutdowns for maintenance and inspections.
However, for purposes of CHP application it should be noted that this high availability rate is
only applicable to the steam turbine itself and not to the boiler or HRSG that is supplying the
steam. Some demonstrated and commercially available fuel cells have achieved greater than
90 percent availability.
Thermal output
The ability to produce useful thermal energy from exhaust gases is the primary advantage of
CHP technologies. Gas turbines produce a high quality (high temperature) thermal output
suitable for most CHP applications. High-pressure steam can be generated or the exhaust can
be used directly for process heating and drying. Microturbines produce exhaust output at
temperatures in the 400°F - 600°F range, suitable for supplying a variety of building thermal
needs. Reciprocating engines can produce hot water and low-pressure steam. Steam turbines
are capable of operating over a broad range of steam pressures. They are custom designed to
deliver the thermal requirements of CHP applications through use of backpressure or extraction
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steam at the appropriately needed pressure and temperature. Waste heat from fuel cells can
be used primarily for domestic hot water and space heating applications.
Efficiency
Total CHP efficiency is a composite measure of the CHP fuel conversion capability and is
expressed as the ratio of net output to fuel consumed. As explained earlier, for any technology
the total CHP efficiency will vary depending on size and power-to-heat ratio. Combustion
turbines achieve higher efficiencies at greater size and with higher power-to-heat ratios. The
total CHP efficiency for gas turbines between 1 MW and 40 MW range from 70 percent to 75
percent for power-to-heat ratio between 0.5 to 1.0 respectively. Unlike gas turbines,
microturbines typically achieve 65 percent to 75 percent total CHP efficiency for a range of
power-to-heat ratios. Commercially available natural gas spark engines ranging between 100
kW to 5 MW are likely to have total CHP efficiency in the 75 percent to 80 percent. The total
CHP efficiency of such engines will decrease with unit-size, and also with higher power-to-heat
ratios. Although performance of steam turbines may differ substantially based on the fuel used,
they are likely to achieve near 80 percent total CHP efficiency across a range of sizes and
power-to-heat ratios. Fuel cell technologies may achieve total CHP efficiency in the 65 percent
to 75 percent range.
Emissions
In addition to cost savings, CHP technologies offer significantly lower emissions rates compared
to separate heat and power systems. The primary pollutants from gas turbines are oxides of
nitrogen (NOx), carbon monoxide (CO), and volatile organic compounds (VOCs) (unburned,
non-methane hydrocarbons). Other pollutants such as oxides of sulfur (SOx) and particulate
matter (PM) are primarily dependent on the fuel used. Similarly emissions of carbon dioxide are
also dependent on the fuel used. Many gas turbines burning gaseous fuels (mainly natural gas)
feature lean premixed burners (also called dry low-NOx burners) that produce NOx emissions
ranging between 0.3 Ibs/MWh to 2.5 Ibs/MWh with no post combustion emissions control.
Typically commercially available gas turbines have CO emissions rates ranging between 0.4
Ibs/MWh - 0.9 Ibs/MWh. Selective catalytic reduction (SCR) or catalytic combustion can further
help to reduce NOx emissions by 80 percent to 90 percent from the gas turbine exhaust and
carbon-monoxide oxidation catalysts can help to reduce CO by approximately 90 percent.
Many gas turbines sited in locales with stringent emission regulations use SCR after-treatment
to achieve extremely low NOx emissions.
Microturbines have the potential for low emissions. All microturbines operating on gaseous
fuels feature lean premixed (dry low NOx, or DLN) combustor technology. The primary
pollutants from microturbines include NOx, CO, and unburned hydrocarbons. They also produce
a negligible amount of S02. Microturbines are designed to achieve low emissions at full load
and emissions are often higher when operating at part load. Typical NOx emissions for
microturbine systems range between 0.5 Ibs/MWh and 0.8 Ibs/MWh. Additional NOx emissions
removal from catalytic combustion is microturbines is unlikely to be pursued in the near term
because of the dry low NOx technology and the low turbine inlet temperature. CO emissions
rates for microturbines typically range between 0.3 Ibs/MWh and 1.5 Ibs/MWh.
Exhaust emissions are the primary environmental concern with reciprocating engines. The
primary pollutants from reciprocating engines are NOx, CO, and VOCs. Other pollutants such
as SOx and PM are primarily dependent on the fuel used. The sulfur content of the fuel
determines emissions of sulfur compounds, primarily S02. NOx emissions from reciprocating
engines typically range between 1.5 Ibs/MWh to 44 Ibs/MWh without any exhaust treatment.
Introduction to CHP Catalog of Technologies
Page 11 of 14
-------
Use of an oxidation catalyst or a three way conversion process (non-selective catalytic
reductions) could help to lower the emissions of NOx, CO and VOCs by 80 percent to 90
percent. Lean burn engines also achieve lower emissions rates than rich burn engines.
Emissions from steam turbines depend on the fuel used in the boiler or other steam sources,
boiler furnace combustion section design, operation, and exhaust cleanup systems. Boiler
emissions include NOx, SOx, PM, and CO. The emissions rates in steam turbine depend largely
on the type of fuel used in the boiler. Typical boiler emissions rates for NOx with any post-
combustion treatment range between 0.2 Ibs/MWh and 1.24 Ibs/MMBtu for coal, 0.22
Ibs/MMBtu to 0.49 Ibs/MMBtu for wood, 0.15 Ibs/MMBtu to 0.37 Ibs/MMBtu for fuel oil, and
0.03lbs/MMBtu - 0.28 Ibs/MMBtu for natural gas. Uncontrolled CO emissions rates range
between 0.02 Ibs/MMBtu to 0.7 Ibs/MMBtu for coal, approximately 0.06 Ibs/MMBtu for wood,
0.03 Ibs/MMBtu for fuel oil and 0.08 Ibs/MMBtu for natural gas. A variety of commercially
available combustion and post-combustion NOx reduction techniques exist with selective
catalytic reductions achieving reductions as high as 90 percent.
S02 emissions from steam turbine depend largely on the sulfur content of the fuel used in the
combustion process. S02 composes about 95% of the emitted sulfur and the remaining 5
percent are emitted as sulfur tri-oxide (S03). Flue gas desulphurization (FGD) is the most
commonly used post-combustion S02 removal technology and is applicable to a broad range of
different uses. FGD can provide up to 95 percent S02 removal.
Fuel cell systems have low emissions profiles because the primary power generation process
does not involve combustion. The fuel processing subsystem is the only significant source of
emissions as it converts fuel into hydrogen and low energy hydrogen exhaust stream. The
hydrogen exhaust stream is combusted in the fuel processor to provide heat, achieving
emissions signatures of less than 0.07 Ibs/MWh of CO, less than 0.06 Ibs/MWh of NOx and
negligible SOx without any after-treatment for emissions. Fuel cells are not expected to require
any emissions control devices to meet current and projected regulations.
While not considered a pollutant in the ordinary sense of directly affecting health, C02 emissions
do result from the use the fossil fuel based CHP technologies. The amount of C02 emitted in
any of the CHP technologies discussed above depends on the fuel carbon content and the
system efficiency. The fuel carbon content of natural gas is 34 lbs carbon/MMBtu; oil is 48 lbs
of carbon/MMBtu and ash-free coal is 66 lbs of carbon/MMBtu.
Introduction to CHP Catalog of Technologies
Page 12 of 14
-------
Appendix 1: Fuel Savings Equations
Absolute Fuel Savings:
Fchp =Fshp*(1-S) and ESHP =ECHP *(1-S)
Fuel Savings = FSHP - FCHP = - FCHP
Where FChp = CHP fuel use
FShp = SHP fuel use
S = % fuel savings compared to SHP
Echp = CHP efficiency
EShp = SHP efficiency
= Fc
1
1
= Fchp
1-S
1 1-S
1-S 1-S
"l-l+s"
= Fchp
1 - s
Fuel Savings = Fc
1-S
= FSHP-Fshp*(1-S) = Fshp*S
Percentage Fuel Savings:
Equivalent separate heat and power (SHP) efficiency
SHP Output P + Q
Eff
SHP
SHP Fuel Input P/ , Q/
/ Effp /Eff Q
divide numerator and denominator by (P+Q)
Eff = i
SHP %P %Q
Effp EffQ
Where P =
power output
Q =
useful thermal output
Effp
= power generation efficiency
EffQ
= thermal generation efficiency
Where %P = P/(P+Q)
%Q = Q/(P+Q)
CHP efficiency
P + Q Eff s
Eff
lii CHP
" CHP
(1-S)
Substitute in equation for EFFShp and isolate S
P + Q
p/ +0/
P + Q /EFFp/EFFQ
F " (1-S)
Introduction to CHP Catalog of Technologies
Page 13 of 14
-------
(1-S)<
P + Q
P + Q
0/
P /^'Q
P/ + ,
/EFR + /EFF
Divide out (P+Q) and multiply by F
F
1-S =
f
p + Q
A
Effp EffQ j
Percent fuel savings calculated from power and thermal output, CHP fuel input, and efficiency of
displaced separate heat and power.
Eff,, Eff
Calculation of percentage power or percent thermal output from power to heat ratio:
Power to Heat Ratio = X = j/Q = °^/o/0q
P + Q = 1
P = X*Q
P = X * (l - P)
P = X-X*P
P + X*P = X
P * (l + X) = X
X
P =
q4
Q =
i-Q
x
Q * X = 1 - Q
Q * (X +1) = 1
1
1 + X
Q =
x + i
Introduction to CHP Catalog of Technologies
Page 14 of 14
-------
Technology Characterization
Gas Turbines
Prepared for:
Environmental Protection Agency
Climate Protection Partnership
Division
Washington, DC
Prepared by:
Energy Nexus Group
1401 Wilson Blvd, Suite 1101
Arlington, Virginia 22209
February 2002
-------
DISCLAIMER:
The information included in these technology overviews is for information purposes only and is
gathered from published industry sources. Information about costs, maintenance, operations, or
any other performance criteria is by no means representative of agency policies, definitions, or
determinations for regulatory or compliance purposes.
Technology Characterization
ii
Gas Turbines
-------
TABLE OF CONTENTS
Introduction and Summary 2
Applications 3
Technology Description 4
Basic Process and Components 4
Modes of Operation 4
Types of Gas Turbines 5
Design Characteristics 6
Performance Characteristics 7
Electrical Efficiency 7
Fuel Supply Pressure 8
Part Load Performance 9
Effects of Ambient Conditions on Performance 10
Heat Recovery 11
Performance and Efficiency Enhancements 13
Capital Cost 15
Maintenance 17
Fuels 18
Availability 19
Emissions 19
Emissions Control Options 21
Gas Turbine Emissions Characteristics 24
Technology Characterization iii Gas Turbines
-------
Technology Characterization - Gas Turbines
Introduction and Summary
Engineering advancement pioneered the gas turbine in the early 1900s, and turbines began to be
used for stationary electric power generation in the late 1930s. Turbines revolutionized airplane
propulsion in the 1940s, and in the 1990s through today (early 2000s) are currently the economic
and environmentally preferred choice for new power generation plants in the United States.
Gas turbines can be used in a variety of configurations: (1) simple cycle operation which is a
single gas turbine producing power only, (2) combined heat and power (CHP) operation which is
a simple cycle gas turbine with a heat recovery heat exchanger which recovers the heat in the
turbine exhaust and converts it to useful thermal energy usually in the form of steam or hot
water, ands (3) combined cycle operation in which high pressure steam is generated from
recovered exhaust heat and used to create additional power using a steam turbine. Some
combined cycles extract steam at an intermediate pressure for use in industrial processes and are
combined cycle CHP systems.
Gas turbines are available in sizes ranging from 500 kilowatts (kW) to 250 megawatts (MW).
The most efficient commercial technology for central station power-only generation is the gas
turbine-steam turbine combined-cycle plant, with efficiencies approaching 60% (LHV).1
Simple-cycle gas turbines for power-only generation are available with efficiencies approaching
40% (LHV). Gas turbines have long been used by utilities for peaking capacity. However, with
changes in the power industry and advancements in the technology, the gas turbine is now being
increasingly used for base-load power.
Gas turbines produce high-quality exhaust heat that can be used in CHP configurations to reach
overall system efficiencies (electricity and useful thermal energy) of 70 to 80%. By the early
1980s, the efficiency and reliability of smaller gas turbines (1 to 40 MW) had progressed
sufficiently to be an attractive choice for industrial and large institutional users for CHP
applications.
Gas turbines are one of the cleanest means of generating electricity, with emissions of oxides of
nitrogen (NOx) from some large turbines in the single-digit parts per million (ppm) range, either
with catalytic exhaust cleanup or lean pre-mixed combustion. Because of their relatively high
efficiency and reliance on natural gas as the primary fuel, gas turbines emit substantially less
1 Lower Heating Value. Most of the efficiencies quoted in this report are based on higher heating value (HHV),
which includes the heat of condensation of the water vapor in the combustion products. In engineering and
scientific literature the lower heating value (LHV - which does not include the heat of condensation of the water
vapor in the combustion products) is often used. The HHV is greater than the LHV by approximately 10% with
natural gas as the fuel (i.e., 50% LHV is equivalent to 45% HHV). HHV efficiencies are about 8% greater for oil
(liquid petroleum products) and 5% for coal.
Technology Characterization
2
Gas Turbines
-------
carbon dioxide (CO2) per kilowatt-hour (kWh) generated than any other fossil technology in
general commercial use.2
Applications
The oil and gas industry commonly use gas turbines to drive pumps and compressors, process
industries use them to drive compressors and other large mechanical equipment, and many
industrial and institutional facilities use turbines to generate electricity for use on-site. When
used to generate power on-site, gas turbines are often used in combined heat and power mode
where energy in the turbine exhaust provides thermal energy to the facility.
There were an estimated 40,000 MW of gas turbine based CHP capacity operating in the United
States in 2000 located at over 575 industrial and institutional facilities.3 Much of this capacity is
concentrated in large combined-cycle CHP systems that maximize power production for sale to
the grid. However, a significant number of simple-cycle gas turbine based CHP systems are in
operation at a variety of applications as shown in Figure 1. Simple-cycle CHP applications are
most prevalent in smaller installations, typically less than 40 MW.
Figure 1. Existing Simple Cycle Gas Turbine CHP — 9,854 MW at 359 sites
Refin
1,576
Paper
911 MW
Other
1,594 MW
Food
Processing
605
Universities
561 MW
Oil Recovery
2,478 MW
Chemicals
2,131 MW
Source: PA Consulting; Energy Nexus Group
Gas turbines are ideally suited for CHP applications because their high-temperature exhaust can
be used to generate process steam at conditions as high as 1,200 pounds per square inch gauge
(psig) and 900 degree Fahrenheit (°F) or used directly in industrial processes for heating or
drying. A typical industrial CHP application for gas turbines is a chemicals plant with a 25 MW
2 Fuel cells, which produce electricity from hydrogen and oxygen emit only water vapor. There are emissions
associated with producing the hydrogen supply depending on its source. However, most fuel cell technologies are
still being developed, with only one type (phosphoric acid fuel cell) commercially available in limited production.
3 PA Consulting Independent Power Database; Energy Nexus Group.
Technology Characterization
3
Gas Turbines
-------
simple cycle gas turbine supplying base-load power to the plant with an unfired heat recovery
steam generator (HRSG) on the exhaust. Approximately 29 MW thermal (MWth) of steam is
produced for process use within the plant.
A typical commercial/institutional CHP application for gas turbines is a college or university
campus with a 5 MW simple-cycle gas turbine. Approximately 8 MWth of 150 to 400 psig
steam (or hot water) is produced in an unfired heat recovery steam generator and sent into a
central thermal loop for campus space heating during winter months or to single-effect
absorption chillers to provide cooling during the summer.
While the recovery of thermal energy provides compelling economics for gas turbine CHP,
smaller gas turbines supply prime power in certain applications. Large industrial facilities install
simple-cycle gas turbines without heat recovery to provide peaking power in capacity
constrained areas, and utilities often place gas turbines in the 5 to 40 MW size range at
substations to provide incremental capacity and grid support. A number of turbine
manufacturers and packagers offer mobile turbine generator units in this size range that can be
used in one location during a period of peak demand and then trucked to another location for the
following season.
Technology Description
Basic Process and Components
Gas turbine systems operate on the thermodynamic cycle known as the Brayton cycle. In a
Brayton cycle, atmospheric air is compressed, heated, and then expanded, with the excess of
power produced by the expander (also called the turbine) over that consumed by the compressor
used for power generation. The power produced by an expansion turbine and consumed by a
compressor is proportional to the absolute temperature of the gas passing through the device.
Consequently, it is advantageous to operate the expansion turbine at the highest practical
temperature consistent with economic materials and internal blade cooling technology and to
operate the compressor with inlet air flow at as low a temperature as possible. As technology
advances permit higher turbine inlet temperature, the optimum pressure ratio also increases.
Higher temperature and pressure ratios result in higher efficiency and specific power. Thus, the
general trend in gas turbine advancement has been towards a combination of higher temperatures
and pressures. While such advancements increase the manufacturing cost of the machine, the
higher value, in terms of greater power output and higher efficiency, provides net economic
benefits. The industrial gas turbine is a balance between performance and cost that results in the
most economic machine for both the user and manufacturer.
Modes of Operation
There are several variations of the Brayton cycle in use today. Fuel consumption may be
decreased by preheating the compressed air with heat from the turbine exhaust using a
recuperator or regenerator; the compressor work may be reduced and net power increased by
using intercooling or precooling; and the exhaust may be used to raise steam in a boiler and to
generate additional power in a combined cycle. Figure 2 shows the primary components of a
simple cycle gas turbine.
Technology Characterization
4
Gas Turbines
-------
Figure 2. Components of a Simple-Cycle Gas Turbine
Air
Fuel
i
Gas Producer
~ Power Turbine
Combustor
\r
^Electricity
Compressor
Generator
Exhaust
Gas turbine exhaust is quite hot, up to 800 to 900°F for smaller industrial turbines and up to
1,100°F for some new, large central station utility machines and aeroderivative turbines. Such
high exhaust temperatures permit direct use of the exhaust. With the addition of a heat recovery
steam generator, the exhaust heat can produce steam or hot water. A portion or all of the steam
generated by the HRSG may be used to generate additional electricity through a steam turbine in
a combined cycle configuration.
A gas turbine based system is operating in combined heat and power mode when the waste heat
generated by the turbine is applied in an end-use. For example, a simple-cycle gas turbine using
the exhaust in a direct heating process is a CHP system, while a system that features all of the
turbine exhaust feeding a HRSG and all of the steam output going to produce electricity in a
combined-cycle steam turbine is not.
Types of Gas Turbines
Aeroderivative gas turbines for stationary power are adapted from their jet and turboshaft aircraft
engine counterparts. While these turbines are lightweight and thermally efficient, they are
usually more expensive than products designed and built exclusively for stationary applications.
The largest aeroderivative generation turbines available are 40 to 50 MW in capacity. Many
aeroderivative gas turbines for stationary use operate with compression ratios in the range of
30:1, requiring a high-pressure external fuel gas compressor. With advanced system
developments, larger aeroderivative turbines (>40 MW) are approaching 45% simple-cycle
efficiencies (LHV).
Industrial or frame gas turbines are exclusively for stationary power generation and are available
in the 1 to 250 MW capacity range. They are generally less expensive, more rugged, can operate
longer between overhauls, and are more suited for continuous base-load operation with longer
inspection and maintenance intervals than aeroderivative turbines. However, they are less
efficient and much heavier. Industrial gas turbines generally have more modest compression
ratios (up to 16:1) and often do not require an external fuel gas compressor. Larger industrial gas
Technology Characterization
5
Gas Turbines
-------
turbines (>100 MW) are approaching simple-cycle efficiencies of approximately 40% (LHV)
and combined-cycle efficiencies of 60% (LHV).
Industry uses gas turbines between 500 kW to 40 MW for on-site power generation and as
mechanical drivers. Small gas turbines also drive compressors on long distance natural gas
pipelines. In the petroleum industry turbines drive gas compressors to maintain well pressures
and enable refineries and petrochemical plants to operate at elevated pressures. In the steel
industry turbines drive air compressors used for blast furnaces. In process industries such as
chemicals, refining and paper, and in large commercial and institutional applications turbines are
used in combined heat and power mode generating both electricity and steam for use on-site.
Design Characteristics
Thermal output:
Fuel flexibility:
Reliability and life:
Gas turbines produce a high quality (high temperature) thermal
output suitable for most combined heat and power applications.
High-pressure steam can be generated or the exhaust can be used
directly for process drying and heating.
Gas turbines operate on natural gas, synthetic gas, landfill gas, and
fuel oils. Plants typically operate on gaseous fuel with a stored
liquid fuel for backup to obtain the less expensive interruptible rate
for natural gas.
Modern gas turbines have proven to be reliable power generators
given proper maintenance. Time to overhaul is typically 25,000 to
50,000 hours.
Size range:
Emissions:
Gas turbines are available in sizes from 500 kW to 250 MW.
Many gas turbines burning gaseous fuels (mainly natural gas)
feature lean premixed burners (also called dry low-NOx
combustors) that produce NOx emissions below 25 ppm, with
laboratory data down to 9 ppm, and simultaneous low CO
emissions in the 10 to 50 ppm range.4 Selective catalytic reduction
(SCR) or catalytic combustion further reduces NOx emissions.
Many gas turbines sited in locales with stringent emission
regulations use SCR after-treatment to achieve single-digit (below
9 ppm) NOx emissions.
4 Gas turbines have high oxygen content in their exhaust because they burn fuel with high excess air to limit
combustion temperatures to levels that the turbine blades, combustion chamber and transition section can handle
without compromising system life. Consequently, emissions from gas turbines are evaluated at a reference
condition of 15% oxygen. For comparison, boilers use 3% oxygen as the reference condition for emissions, because
they can minimize excess air and thus waste less heat in their stack exhaust. Note that due to the different amount of
diluent gases in the combustion products, the mass of NOx measured as 9 ppm @ 15% oxygen is approximately 27
ppm @ 3% oxygen, the condition used for boiler NOx regulations.
Technology Characterization
6
Gas Turbines
-------
Part-load operation: Because gas turbines reduce power output by reducing combustion
temperature, efficiency at part load can be substantially below that
of full-power efficiency.
Performance Characteristics
Electrical Efficiency
The thermal efficiency of the Brayton cycle is a function of pressure ratio, ambient air
temperature, turbine inlet air temperature, the efficiency of the compressor and turbine elements,
turbine blade cooling requirements, and any performance enhancements (i.e., recuperation,
intercooling, inlet air cooling, reheat, steam injection, or combined cycle). All of these
parameters, along with gas turbine internal mechanical design features, have been improving
with time. Therefore newer machines are usually more efficient than older ones of the same size
and general type. The performance of a gas turbine is also appreciably influenced by the purpose
for which it is intended. Emergency power units generally have lower efficiency and lower
capital cost, while turbines intended for prime power, compressor stations and similar
applications with high annual capacity factors having higher efficiency and higher capital costs.
Emergency power units are permitted for a maximum number of hours per year and allowed to
have considerably higher emissions than turbines permitted for continuous duty.
Table 1 summarizes performance characteristics for typical commercially available gas turbine
CHP systems over the 1 to 40 MW size range. Heat rates shown are from manufacturers'
specifications and industry publications. Available thermal energy (steam output) was calculated
from published turbine data on turbine exhaust temperatures and flows. CHP steam estimates
are based on an unfired HRSG with an outlet exhaust temperature of 280°F producing dry,
saturated steam at 150 psig. Total efficiency is defined as the sum of the net electricity generated
plus steam produced for plant thermal needs divided by total fuel input to the system. Higher
steam pressures can be obtained but at slightly lower total efficiencies. Additional steam can be
generated and total efficiency further increased with duct firing in the HRSG (see heat recovery
section). To estimate fuel savings effective electrical efficiency is a more useful value than
overall efficiency. Effective electric efficiency is calculated assuming the useful-thermal output
from the CHP system would otherwsie be generated by an 80% efficient boiler. The theoretical
boiler fuel is subtracted from the total fuel input and the remaining fuel input used to calculate
the effective electric efficiency which can then be compared to traditional electric generation.
The data in the table show that electrical efficiency increases as combustion turbines become
larger. As electrical efficiency increases, the absolute quantity of thermal energy available to
produce steam decreases per unit of power output, and the ratio of power to heat for the CHP
system increases. A changing ratio of power to heat impacts project economics and may affect
the decisions that customers make in terms of CHP acceptance, sizing, and the desirability of
selling power.
Technology Characterization
1
Gas Turbines
-------
Table 1. Gas Turbine CHP- Typical Performance Parameters*
Cost & Performance Characlerislics
System
1
System
Syslem
Syslem
4
Syslem
5
Electricity Capacity (kW)
1,000
5,000
10,000
25,000
40,000
Total Installed Cost (2000 $/kW)6
$1,780
$1,010
$970
$860
$785
Electric Heat Rate (Btu/kWh), HHV7
15,580
12,590
11,765
9,945
9,220
Electrical Efficiency (%), HHV
21.9%
27.1%
29.0%
34.3%
37.0%
Fuel Input (MMBtu/hr)
15.6
62.9
117.7
248.6
368.8
Required Fuel Gas Pressure (psig)
95
160
250
340
435
CHP Characteristics
Exhaust Flow (1,000 lb/hr)
44
162
316
571
954
GT Exhaust Temperature (Fahrenheit)
950
950
915
950
854
HRSG Exhaust Temperature (Fahrenheit)
280
280
280
280
280
Steam Output (MMBtu/hr)
7.1
26.6
49.6
95.6
136.8
Steam Output (1,000 lbs/hr)
6.7
25.0
46.6
89.8
128.5
Steam Output (kW equivalent)
2,080
7,800
14,540
28,020
40,100
Total CHP Efficiency (%), HHVS
68%
69%
71%
73%
74%
Power/Heat Ratio9
0.48
0.64
0.69
0.89
1.0
Net Heat Rate (Btu/kWh)10
6,673
5,947
5,562
5,164
4,944
Effective Electrical Efficiency (%), HHV11
51%
57%
61%
66%
69%
* For typical systems commercially available in 2001
Source: Energy Nexus Group5
Fuel Supply Pressure
Gas turbines need minimum gas pressure of about 100 psig for the smallest turbines with
substantially higher pressures for larger turbines and aeroderivative machines. Depending on the
supply pressure of the gas being delivered to the site the cost and power consumption of the fuel
gas compressor can be a significant consideration. Table 2 shows the power required to
compress natural gas from supply pressures typical of commercial and industrial service to the
pressures required by typical industrial gas turbines. Required supply pressures generally
increase with gas turbine size.
5 Characteristics for "typical" commercially available gas turbine generator system. Data based on: Solar Turbines
Saturn 20-1 MW; Solar Turbines Taurus 60-5 MW; Solar Turbines Mars 100 - 10 MW; GE LM2500+ - 25
MW; GE LM6000PD - 40 MW.
6 Installed costs based on CHP system producing 150 psig saturated steam with an unfired heat recovery steam
generator.
7 All turbine and engine manufacturers quote heat rates in terms of the lower heating value (LHV) of the fuel. On
the other hand, the usable energy content of fuels is typically measured on a higher heating value basis (HHV). In
addition, electric utilities measure power plant heat rates in terms of HHV. For natural gas, the average heat content
of natural gas is 1,030 Btu/scf on an HHV basis and 930 Btu/scf on an LHV basis - or about a 10% difference.
8 Total Efficiency = (net electric generated + net steam produced for thermal needs)/total system fuel input
9 Power/Steam Ratio = CHP electrical power output (Btu)/ useful steam output (Btu)
10 Net Heat Rate = (total fuel input to the CHP system - the fuel that would be normally used to generate the same
amount of thermal output as the CHP system output assuming an efficiency of 80%)/CHP electric output (kW).
11 Effective Electrical Efficiency = (CHP electric power output)/(Total fuel into CHP system - total heat
recovered/0.8); Equivalent to 3,412 Btu/kWh/Net Heat Rate.
Technology Characterization
8
Gas Turbines
-------
Table 2. Power Requirements For Natural Gas Compression
12
System
1
System
2
System
3
System
4
System
5
Turbine Electric Capacity (kW)
1,000
5,000
10,000
25,000
40,000
Turbine Pressure Ratio
6.5
10.9
17.1
23.1
29.6
Required Compression Power (kW)
50 psig gas supply pressure
17
125
310
650
1,310
150 psig gas supply pressure
NA
26
120
300
675
250 psig gas supply pressure
NA
NA
40
150
380
Source: Energy Nexus Group
Part-Load Performance
When less than full power is required from a gas turbine, the output is reduced by lowering the
turbine inlet temperature. In addition to reducing power, this change in operating conditions also
reduces efficiency. Figure 3 shows a typical part-load derate curve. Emissions are generally
increased at part load conditions, especially at half load and below.
Figure 3. Part Load Power Performance
Part Load
Performance
35-,
30.
?fS-
i5
5*
?0.
c
-------
Effects of Ambient Conditions on Performance
The ambient conditions under which a gas turbine operates have a noticeable effect on both the
power output and efficiency. At elevated inlet air temperatures, both the power and efficiency
decrease. The power decreases due to the decreased air flow mass rate (the density of air
declines as temperature increases) and the efficiency decreases because the compressor requires
more power to compress air of higher temperature. Conversely, the power and efficiency
increase when the inlet air temperature is reduced. Figure 4 shows the variation in power and
efficiency for a gas turbine as a function of ambient temperature compared to the reference
International Organization for Standards (ISO) condition of sea level and 59°F. At inlet air
temperatures of near 100°F, power output can drop to as low as 90% of ISO-rated power for
typical gas turbines. At cooler temperatures of about 40 to 50°F, power can increase to as high
as 105% of ISO-rated power.
Figure 4. Ambient Temperature Effects on Performance
Derate at Altitude
110
100
T3
(5
O
_l
D
LL.
4—
o
+¦»
c
-------
Figure 5. Altitude Effects on Performance
Impact of Ambient
Temperature
120
100
c
v
u
V
Q.
0
10
20
30
40
50
60
70
80
90
100
110
Ambient Temperature (°F)
—•— Power (% ISO Rated Output) M Efficiency (%)
Source: Energy Nexus Group
Heat Recovery
The economics of gas turbines in process applications often depend on effective use of the
thermal energy contained in the exhaust gas, which generally represents 60 to 70% of the inlet
fuel energy. The most common use of this energy is for steam generation in unfired or
supplementary fired heat recovery steam generators. However, the gas turbine exhaust gases can
also be used as a source of direct process energy, for unfired or fired process fluid heaters, or as
preheated combustion air for power boilers. Figure 6 shows a typical gas turbine/HRSG
configuration. An unfired HRSG is the simplest steam CHP configuration and can generate
steam at conditions ranging from 150 psig to approximately 1,200 psig.
Figure 6. Heat Recovery from a Gas Turbine System
Gas Turbine
Electricity
i k Med/High Pressure Steam to Process
(Simple Cycle with Heat Recovery)
Feed water
HRSG
Electricity
Steam Turbine
(Combined Cycle)
Low Pressure Steam to Process or Condenser
Technology Characterization
11
Gas Turbines
-------
CHP System Efficiency
Overall or total efficiency of a CHP system is a function of the amount of energy recovered from
the turbine exhaust. The two most important factors influencing the amount of energy available
for steam generation are gas turbine exhaust temperature and HRSG stack temperature.
Turbine firing temperature and turbine pressure ratio combined determine gas turbine exhaust
temperature. Typically aeroderivative gas turbines have higher firing temperatures than do
industrial gas turbines, but when the higher pressure ratio of aeroderative gas turbines is
recognized, the turbine discharge temperatures of the two turbine types remain somewhat close,
typically in the range of 850 to 950°F. For the same HRSG exit temperature, higher turbine
exhaust temperature (higher HRSG gas inlet temperature) results in greater available thermal
energy and increased HRSG output.
Similarly, the lower the HRSG stack temperature, the greater the amount of energy recovered
and the higher the total-system efficiency. HRSG stack temperature is a function of steam
conditions and fuel type. Saturated steam temperatures increase with increasing steam pressure.
Because of pinch point considerations within the HRSG, higher steam pressures result in higher
HRSG exhaust stack temperatures, less utilization of available thermal energy, and a reduction in
total CHP system efficiency. In general, minimum stack temperatures of about 300°F are
recommended for sulfur bearing fuels. Figure 7 illustrates the increase in overall system
efficiency as the exhaust temperature decreases through effective heat recovery. Generally,
unfired HRSGs can be designed to economically recover approximately 95% the available
energy in the turbine exhaust (the energy released in going from turbine exhaust temperature to
HRSG exhaust temperature).
Figure 7. Effect of Stack Temperature on Total CHP Efficiency*
90
80
70
60
50
40
30
20
100
200
300
400
500
600
700
800
900 1000
HRSG Exhaust Temperature (°F)
* Based on an LM6000 with unfired HRSG
Technology Characterization 12
Gas Turbines
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Overall CHP efficiency generally remains high under part-load conditions. The decrease in
electric efficiency from the gas turbine under part-load conditions results in a relative increase in
heat available for recovery under these conditions. This can be a significant operating advantage
for applications in which the economics are driven by high steam demand.
Supplementary Firing
Since the gas turbine combustion process consumes little of the available oxygen in the turbine
air flow, the oxygen content in the gas turbine exhaust permits supplementary fuel firing ahead
of the HRSG to increase steam production relative to an unfired unit. Supplementary firing can
raise the exhaust gas temperature entering the HRSG up to 1,800°F and increase the amount of
steam produced by the unit by a factor of two. Moreover, since the turbine exhaust gas is
essentially preheated combustion air, the fuel consumed in supplementary firing is less than that
required for a stand-alone boiler providing the same increment in steam generation. The HHV
efficiency of incremental steam production from supplementary firing above that of an unfired
HRSG is often 85% or more when firing natural gas.
Supplementary firing also increases system flexibility. Unfired HRSGs are typically convective
heat exchangers that respond solely to exhaust conditions of the gas turbine and do not easily
allow for steam flow control. Supplementary firing capability provides the ability to control
steam production, within the capability of the burner system, independent of the normal gas
turbine operating mode. Low NOx duct burners with guaranteed emissions levels as low as 0.08
lb NOx/MMBtu can be specified to minimize the NOx contribution of supplemental firing.
Performance and Efficiency Enhancements
Recuperators
Several technologies that increase the output power and/or the efficiency of gas turbines have
been developed and put into limited commercial service. Fuel use can be reduced (and hence
efficiency improved) by use of a heat exchanger called a recuperator that uses the hot turbine
exhaust to preheat the compressed air entering the combustor. Depending on gas turbine
operating parameters, such a heat exchanger can add up to ten percentage points in machine
efficiency (thereby raising efficiency from 30 to 40%). However, since there is increased
pressure drop in both the compressed air and turbine exhaust sides of the recuperator, power
output is typically reduced by 10 to 15%.
Recuperators are expensive, and their cost can normally only be justified when the gas turbine
operates for a large number of full-power hours per year and the cost of fuel is relatively high.
As an example, pipeline compressor station gas turbines frequently operate with high annual
capacity factors, and some pipeline gas turbines have utilized recuperators since the 1960s.
Recuperators also lower the temperature of the gas turbine exhaust, reducing the turbine's
effectiveness in CHP applications. Because recuperators are subject to large temperature
differences, they are subject to significant thermal stresses. Cyclic operation in particular can
fatigue joints, causing the recuperator to develop leaks and lose power and effectiveness. Design
and manufacturing advancements have mitigated some of the cost and durability issues, and
commercial recuperators have been introduced on microturbines and on a 4.2 MW industrial gas
turbine (through a project supported by the U.S. Department of Energy).
Technology Characterization
13
Gas Turbines
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Intercoolers
Intercoolers are used to increase gas turbine power by dividing the compressor into two sections
and cooling the compressed air exiting the first section before it enters the second compressor
section. Intercoolers reduce the power consumption in the second section of the compressor,
thereby adding to the net power delivered by the combination of the turbine and compressor.
Intercoolers have been used for decades on industrial air compressors and are used on some
reciprocating engine turbochargers. Intercoolers generally are used where additional capacity is
particularly valuable. Gas turbine efficiency does not change significantly with the use of
intercooling. While intercoolers increase net output, the reduced power consumption of the
second section of the compressor results in lower temperature for the compressed air entering the
combustor and, consequently, incremental fuel is required.
Inlet Air Cooling
As shown in Figure 4, the decreased power and efficiency of gas turbines at high ambient
temperatures means that gas turbine performance is at its lowest at the times power is often in
greatest demand and most valued. The figure also shows that cooling the air entering the turbine
by 40 to 50°F on a hot day can increase power output by 15 to 20%. The decreased power and
efficiency resulting from high ambient air temperatures can be mitigated by any of several
approaches to inlet-air cooling, including refrigeration, evaporative cooling, and thermal-energy
storage using off-peak cooling.
With refrigeration cooling, either a compression driven or thermally activated (absorption
chiller) refrigeration cycle cools the inlet air through a heat exchanger. The heat exchanger in
the inlet air stream causes an additional pressure drop in the air entering the compressor, thereby
slightly lowering cycle power and efficiency. However, as the inlet air is now substantially
cooler than the ambient air there is a significant net gain in power and efficiency. Electric motor
compression refrigeration requires a substantial parasitic power loss. Thermally activated
absorption cooling can utilize waste heat from the gas turbine, reducing the direct parasitic loss.
However, the complexity and cost of this approach pose potential drawbacks in many
applications.
Evaporative cooling, which is widely used due to its low capital cost, uses a spray of water
directly into the inlet air stream. Evaporation of the water reduces the temperature of the air.
Since cooling is limited to the wet bulb air temperature, evaporative cooling is most effective
when the wet bulb temperature is appreciably below the dry bulb (ordinary) temperature.
Evaporative cooling can consume large quantities of water, making it difficult to operate in arid
climates. A few large gas turbines have evaporative cooling, and it is expected to be used more
frequently on smaller machines in the future.
The use of thermal-energy storage systems, typically ice, chilled water, or low-temperature
fluids, to cool inlet air can eliminate most parasitic losses from the augmented power capacity.
Thermal energy storage is a viable option if on-peak power pricing only occurs a few hours a
day. In that case, the shorter time of energy storage discharge and longer time for daily charging
allow for a smaller and less expensive thermal-energy storage system.
Technology Characterization
14
Gas Turbines
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Capital Cost
A gas turbine CHP plant is a complex process with many interrelated subsystems. The basic
package consists of the gas turbine, gearbox, electric generator, inlet and exhaust ducting, inlet
air filtration, lubrication and cooling systems, standard starting system, and exhaust silencing.
The basic package cost does not include extra systems such as the fuel-gas compressor, heat-
recovery system, water-treatment system, or emissions-control systems such as selective
catalytic reduction (SCR) or continuous emission monitoring systems (CEMS). Not all of these
systems are required at every site. The cost of the basic turbine package plus the costs for added
systems needed for the particular application comprise the total equipment cost. The total plant
cost consists of total equipment cost plus installation labor and materials (including site work),
engineering, project management (including licensing, insurance, commissioning, and startup),
and financial carrying costs during the 6-18 month construction period.
Table 3 details estimated capital costs (equipment and installation costs) for the five typical gas
turbine CHP systems. These are "typical" budgetary price levels; it should be noted that
installed costs can vary significantly depending on the scope of the plant equipment,
geographical area, competitive market conditions, special site requirements, emissions control
requirements, prevailing labor rates, whether the system is a new or retrofit application, and
whether or not the site is a green field or is located at an established industrial site with existing
roads, water, fuel, electric, etc. The cost estimates presented in this section are based on systems
that include DLE emissions control, unfired heat recovery steam generators (HRSG), fuel gas
compression, water treatment for the boiler feed water, and basic utility interconnection for
parallel power generation. There is no SCR system, no supplementary firing or duct burners, no
building construction, and minimal site preparation and support.
The table shows that there are definite economies of scale for larger turbine power systems.
Turbine packages themselves decline only slightly between the range of 5 to 40 MW, but
ancillary equipment such as the HRSG, gas compression, water treatment, and electrical
equipment are much lower in cost per unit of electrical output as the systems become larger.
Technology Characterization
15
Gas Turbines
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13
Table 3. Estimated Capital Costs for Typical Gas Turbine-Based CHP Systems ($000s)
Cost Component
System I
System 2
System 3
System 4
System 5
Nominal Turbine Capacity (MW)
Equipment (Thousands of $)
Turbine Genset
Heat Recovery Steam Generators
Water Treatment System
Electrical Equipment
Other Equipment
Total Equipment
Materials
Labor yota| process Capital
Project/Construction Management
Engineering
Project Contingency
Project Financing
Total Plant Cost
$675
$250
$30
$150
$145
$1,250
$144
$348^
$1,742*
$315
$125
$63
$87$
$129
$2,146
879
$1,800
$450
$100
$375
$3,040
$346
$4,265L
$304
$153$|l
$215
$575
,752
$5,253
10
$4,000
$590
$150
$625
$5,940
$689(
$8,381
$594(
$260"
$419
$1,150
$3,715
$10,272
25
$11,500
$1,020
$200
$990
$14,860
$1,490
<
$20,065
$1,875
$1,486
$537$|4,723
$1,005
$24,576
40
$15,800
$1,655
$225
$1,500
$21,055
$2,054
$27,832
$2,105
$672
$1,392
$34,049
Actual Turbine Capacity (kW)
Total Plant Cost per net kW ($)
1,210
$1,781'
5,200
10,600$i 483 28,600
$316
$1,010
618
2,048
$969
$859
43,400
$785
13 Combustion turbine costs are based on published specifications and package prices. The total installed cost estimation is based in part on the use of a
proprietary cost and performance model - SOAPP-CT.25 - (for state-of-the-art power plant, combustion turbine). The model output was adjusted based on
Energy Nexus Group engineering judgment and experience and input from vendors and packagers. Actual costs can vary widely and are affected by site
requirements and conditions, regional price variations, and environmental and other local permitting requirements.
Technology Characterization
16
Gas Turbines
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Maintenance
Non-fuel operation and maintenance (O&M) costs presented in Table 4 are based on gas turbine
manufacturer estimates for service contracts, which consist of routine inspections and scheduled
overhauls of the turbine generator set. Routine maintenance practices include on-line running
maintenance, predictive maintenance, plotting trends, performance testing, fuel consumption,
heat rate, vibration analysis, and preventive maintenance procedures. The O&M costs presented
in Table 4 include operating labor (distinguished between unmanned and 24 hour manned
facilities) and total maintenance costs, including routine inspections and procedures and major
overhauls.
Daily maintenance includes visual inspection by site personnel of filters and general site
conditions. Routine inspections are required every 4,000 hours to insure that the turbine is free
of excessive vibration due to worn bearings, rotors, and damaged blade tips. Inspections
generally include on-site hot gas path horoscope inspections and non-destructive component
testing using dye penetrant and magnetic particle techniques to ensure the integrity of
components. The combustion path is inspected for fuel nozzle cleanliness and wear, along with
the integrity of other hot gas path components.
A gas turbine overhaul is needed every 25,000 to 50,000 hours depending on service and is
typically a complete inspection and rebuild of components to restore the gas turbine to nearly
original or current (upgraded) performance standards. A typical overhaul consists of
dimensional inspections, product upgrades and testing of the turbine and compressor, rotor
removal, inspection of thrust and journal bearings, blade inspection and clearances and setting
packing seals.
Gas turbine maintenance costs can vary significantly depending on the quality and diligence of
the preventative maintenance program and operating conditions. Although gas turbines can be
cycled, cycling every hour triples maintenance costs versus a turbine that operates for intervals
of 1,000 hours or more. In addition, operating the turbine over the rated capacity for significant
periods of time will dramatically increase the number of hot path inspections and overhauls. Gas
turbines that operate for extended periods on liquid fuels will experience higher than average
overhaul intervals.
Technology Characterization
17
Gas Turbines
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Table 4. Gas Turbine Non-Fuel O&M Costs (Year 2000)
O&M Costs11
System
1
System
System
System
' 4
System
5
Llcclncil) Capacil), k\\
l,uuu
55UUU
1 u,uuu
25,UUU
4U,UUU
Variable (service contract), $/kWh
0.0045
0.0045
0.0045
0.0040
0.0035
Variable (consumables), $/kWh
0.0001
0.0001
0.0001
0.0001
0.0001
Fixed, $/kW-yr
40
10
7.5
6
5
Fixed, $/kWh @ 8,000 hrs/yr
0.0050
0.0013
0.0009
0.0008
0.0006
Total O&M Costs, $/l
-------
Gas turbines operate with combustors at pressure levels from 75 to 350 psig. While the pipeline
pressure of natural gas is always above these levels, the pressure is normally let down during city
gate metering and subsequent flow through the distribution piping system and customer
metering. For example, local distribution gas pressures usually range from 30 to 130 psig in
feeder lines and from 1 to 60 psig in final distribution lines. Depending on where the gas turbine
is located on the gas distribution system, a fuel gas booster compressor may be required to
ensure that fuel pressure is adequate for the gas turbine flow control and combustion systems.
The cost of such booster compressors adds to the installation capital cost - fuel gas compressor
costs can add from $20 to $150/kW to a CHP system's total cost, representing 2% of the total
cost for a large system up to 10% of the total installed cost for a small gas turbine installation.15
Redundant booster compressors ensure reliable operation because without adequate fuel pressure
the gas turbine does not operate.
Availability
Many operational conditions affect the propensity to fail in a gas turbine. Frequent starts and
stops incur damage from thermal cycling, which accelerates mechanical failure. Use of liquid
fuels, especially heavy fuels and fuels with impurities (alkalis, sulfur, and ash), radiate heat to
the combustor walls significantly more intensely than occurs with, clean, gaseous fuels, thereby
overheating the combustor and transition piece walls. On the other hand, steady operation on
clean fuels can permit gas turbines to operate for a year without need for shutdown. Estimated
availability of gas turbines operating on clean gaseous fuels, like natural gas, is in excess of 95%.
Emissions
Gas turbines are among the cleanest fossil-fueled power generation equipment commercially
available. Gas turbine emission control technologies are continuing to evolve, with older
technologies gradually phasing out as new technologies are developed and commercialized.
The primary pollutants from gas turbines are oxides of nitrogen (NOx), carbon monoxide (CO),
and volatile organic compounds (VOCs). Other pollutants such as oxides of sulfur (SOx) and
particulate matter (PM) are primarily dependent on the fuel used. The sulfur content of the fuel
determines emissions of sulfur compounds, primarily SO2. Gas turbines operating on desulfized
natural gas or distillate oil emit relatively insignificant levels of SOx. In general, SOx emissions
are greater when heavy oils are fired in the turbine. SOx control is thus a fuel purchasing issue
rather than a gas turbine technology issue. Particulate matter is a marginally significant pollutant
for gas turbines using liquid fuels. Ash and metallic additives in the fuel may contribute to PM
in the exhaust.
It is important to note that the gas turbine operating load has a significant effect on the emissions
levels of the primary pollutants of NOx, CO, and VOCs. Gas turbines typically operate at high
loads. Consequently, gas turbines are designed to achieve maximum efficiency and optimum
combustion conditions at high loads. Controlling all pollutants simultaneously at all load
conditions is difficult. At higher loads, higher NOx emissions occur due to peak flame
15 American Gas Association, Distributed Generation and the Natural Gas Infrastructure, 1999
Technology Characterization
19
Gas Turbines
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temperatures. At lower loads, lower thermal efficiencies and more incomplete combustion
occurs resulting in higher emissions of CO and VOCs.
The pollutant referred to as NOx is a mixture of mostly NO and NO2 in variable composition. In
emissions measurement it is reported as parts per million by volume in which both species count
equally. NOx is formed by three mechanisms: thermal NOx, prompt NOx, and fuel-bound NOx.
The predominant NOx formation mechanism associated with gas turbines is thermal NOx.
Thermal NOx is the fixation of atmospheric oxygen and nitrogen, which occurs at high
combustion temperatures. Flame temperature and residence time are the primary variables that
affect thermal NOx levels. The rate of thermal NOx formation increases rapidly with flame
temperature. Prompt NOx forms from early reactions of nitrogen modules in the combustion air
and hydrocarbon radicals from the fuel. It forms within the flame and typically is about 1 ppm at
15% O2, and is usually much smaller than the thermal NOx formation. Fuel-bound NOx forms
when the fuel contains nitrogen as part of the hydrocarbon structure. Natural gas has negligible
chemically bound fuel nitrogen.
The control of peak flame temperature, through diluent (water or steam) injection or by
maintaining homogenous fuel-to-air ratios that keep local flame temperature below the
stoichiometric adiabatic temperature, have been the traditional methods of limiting NOx
formation. In older diffusion flame combustion systems, fuel/air mixing and combustion
occurred simultaneously. This resulted in local fuel/air mixture chemical concentrations that
produced high local flame temperatures. These high temperature "hot spots" are where most of
the NOx emissions originate. Many new gas turbines feature lean pre-mixed combustion
systems. These systems, sometimes referred to as dry low NOx (DLN) or dry low emissions
(DLE), operate in a tightly controlled lean (lower fuel-to-air ratio) premixed mode that maintains
modest peak flame temperatures.
CO and VOCs both result from incomplete combustion. CO emissions result when there is
insufficient residence time at high temperature. In gas turbines, the failure to achieve CO
burnout may result from the quenching effects of dilution and combustor wall cooling air. CO
emissions are also heavily dependent on the operating load of the turbine. For example, a gas
turbine operating under low loads will tend to have incomplete combustion, which will increase
the formation of CO. CO is usually regulated to levels below 50 ppm for both health and safety
reasons. Achieving such low levels of CO had not been a problem until manufacturers achieved
low levels of NOx, because the techniques used to engineer DLN combustors had a secondary
effect of increasing CO emissions.
VOCs can encompass a wide range of compounds, some of which are hazardous air pollutants.
These compounds discharge into the atmosphere when some portion of the fuel remains
unburned or just partially burned. Some organics are unreacted trace constituents of the fuel,
while others may be pyrolysis products of the heavier hydrocarbons in the gas.
While not considered a regulated pollutant in the ordinary sense of directly affecting public
health, emissions of carbon dioxide (CO2) are of concern due to its contribution to global
warming. Atmospheric warming occurs because solar radiation readily penetrates to the surface
of the planet but infrared (thermal) radiation from the surface is absorbed by the CO2 (and other
Technology Characterization
20
Gas Turbines
-------
polyatomic gases such as methane, unburned hydrocarbons, refrigerants, water vapor, and
volatile chemicals) in the atmosphere, with resultant increase in temperature of the atmosphere.
The amount of CO2 emitted is a function of both fuel carbon content and system efficiency. The
fuel carbon content of natural gas is 34 lbs carbon/MMBtu; oil is 48 lbs carbon/MMBtu; and
(ash-free) coal is 66 lbs carbon/MMBtu.
Emissions Control Options
NOx control has been the primary focus of emission control research and development in recent
years. The following provides a description of the most prominent emission control approaches:
Diluent Injection
The first technique used to reduce NOx emissions was injection of water or steam into the high
temperature zones of the flame. Water and steam are strong diluents and can quench hot spots in
the flame reducing NOx. However, positioning of the injection is not precise and some NOx is
still created. Depending on uncontrolled NOx levels, water or steam injection reduces NOx by
60% or more. Water or steam injection enables gas turbines to operate with NOx levels as low as
25 ppm (@ 15% O2) on natural gas. NOx is only reduced to 42 to 75 ppm when firing with
liquid distillate fuel. Both water and steam increase the mass flow through the turbine and create
a small amount of additional power. Use of exhaust heat to raise the steam temperature also
increases overall efficiency slightly. The water used needs to be demineralized thoroughly in
order to avoid forming deposits and corrosion in the turbine expansion section. This adds cost
and complexity to the operation of the turbine. Diluent injection increases CO emissions
appreciably as it lowers the temperature in the burnout zone as well as well as in the NOx
formation zone.
Lean Premixed Combustion
As discussed earlier, thermal NOx formation is a function of both flame temperature and
residence time. The focus of combustion improvements of the past decade was to lower flame
hot spot temperature using lean fuel/air mixtures. Lean combustion decreases the fuel/air ratio in
the zones where NOx production occurs so that peak flame temperature is less than the
stoichiometric adiabatic flame temperature, therefore suppressing thermal NOx formation.
Lean premixed combustion (DLN/DLE) pre-mixes the gaseous fuel and compressed air so that
there are no local zones of high temperatures, or "hot spots," where high levels of NOx would
form. Lean premixed combustion requires specially designed mixing chambers and mixture inlet
zones to avoid flashback of the flame. Optimized application of DLN combustion requires an
integrated approach to combustor and turbine design. The DLN combustor becomes an intrinsic
part of the turbine design, and specific combustor designs must be developed for each turbine
application. While NOx levels as low as 9 ppm have been achieved with lean premixed
combustion, few DLN equipped turbines have reached the level of practical operation at this
emissions level necessary for commercialization - the capability of maintaining 9 ppm across a
wide operating range from full power to minimum load. One problem is that pilot flames, which
are small diffusion flames and a source of NOx, are usually used for continuous internal ignition
and stability in DLN combustors and make it difficult to maintain full net NOx reduction over the
complete turndown range.
Technology Characterization
21
Gas Turbines
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Noise can also be an issue in lean premixed combustors as acoustic waves form due to
combustion instabilities when the premixed fuel and air ignite. This noise also manifests itself as
pressure waves, which can damage combustor walls and accelerate the need for combustor
replacement, thereby adding to maintenance costs and lowering unit availability.
All leading gas turbine manufacturers feature DLN combustors in at least parts of their product
lines. Turbine manufacturers generally guarantee NOx emissions of 15 to 42 ppm using this
technology. NOx emissions when firing distillate oil are typically guaranteed at 42 ppm with
DLN and/or combined with water injection. A few models (primarily those larger than 40 MW)
have combustors capable of 9 ppm (natural gas fired) over the range of expected operation.
The development of market-ready DLN equipped turbine models is an expensive undertaking
because of the operational difficulties in maintaining reliable gas turbine operation over a broad
power range. Therefore, the timing of applying DLN to multiple turbine product lines is a
function of market priorities and resource constraints. Gas turbine manufacturers initially
develop DLN combustors for the gas turbine models for which they expect the greatest market
opportunity. As time goes on and experience is gained, the technology is extended to additional
gas turbine models.
Selective Catalytic Reduction
The primary post-combustion NOx control method in use today is selective catalytic reduction
(SCR). Ammonia is injected into the flue gas and reacts with NOx in the presence of a catalyst to
produce N2 and H20. The SCR system is located in the exhaust path, typically within the HRSG
where the temperature of the exhaust gas matches the operating temperature of the catalyst. The
operating temperature of conventional SCR systems ranges from 400 to 800°F. The cost of
conventional SCR has dropped significantly over time — catalyst innovations have been a
principal driver, resulting in a 20% reduction in catalyst volume and cost with no change in
performance.
Low temperature SCR, operating in the 300 to 400°F temperature range, was commercialized in
1995 and is currently in operation on approximately twenty gas turbines. Low temperature SCR
is ideal for retrofit applications where it can be located downstream of the HRSG, avoiding the
potentially expensive retrofit of the HRSG to locate the catalyst within a hotter zone of the
HRSG.
High temperature SCR installations, operating in the 800 to 1,100°F temperature range, have
increased significantly in recent years. The high operating temperature permits the placement of
the catalyst directly downstream of the turbine exhaust flange. High temperature SCR is also
used on peaking capacity and base-loaded simple-cycle gas turbines where there is no HRSG.
SCR reduces between 80 to 90% of the NOx in the gas turbine exhaust, depending on the degree
to which the chemical conditions in the exhaust are uniform. When used in series with
water/steam injection or DLN combustion, SCR can result in low single digit NOx levels (2 to 5
ppm).
Technology Characterization
22
Gas Turbines
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SCR systems are expensive and significantly impact the economic feasibility of smaller gas
turbine projects. For a 5 MW project electric generation costs increase approximately half a cent
per kWh.16 SCR requires on-site storage of ammonia, a hazardous chemical. In addition,
ammonia can "slip" through the process unreacted, contributing to environmental health
concerns.17
Carbon Monoxide Oxidation Catalysts
Oxidation catalysts control CO in gas turbine exhaust. Some SCR installations incorporate CO
oxidation modules along with NOx reduction catalysts for simultaneous control of CO and NOx.
The CO catalyst promotes the oxidation of CO and hydrocarbon compounds to carbon dioxide
(CO2) and water (H20) as the exhaust stream passes the through the catalyst bed. The oxidation
process takes place spontaneously so no reactants are required. The catalyst is usually made of
precious metal such as platinum, palladium, or rhodium. Other formations, such as metal oxides
for emission streams containing chlorinated compounds, are also used. CO catalysts also reduce
VOCs and organic hazardous air pollutants (HAPs). CO catalysts on gas turbines result in
approximately 90% reduction of CO and 85 to 90% control of formaldehyde (similar reductions
can be expected on other HAPs).
Catalytic Combustion
In catalytic combustion, fuels oxidize at lean conditions in the presence of a catalyst. Catalytic
combustion is a flameless process, allowing fuel oxidation to occur at temperatures below
1,700°F, where NOx formation is low. The catalyst is applied to combustor surfaces, which
cause the fuel air mixture to react with the oxygen and release its initial thermal energy. The
combustion reaction in the lean premixed gas then goes to completion at design temperature.
Data from ongoing long term testing indicates that catalytic combustion exhibits low vibration
and acoustic noise, only one-tenth to one-hundredth the levels measured in the same turbine
equipped with DLN combustors.
Gas turbine catalytic combustion technology is being pursued by developers of combustion
systems and gas turbines and by government agencies, most notably the U.S. Department of
Energy and the California Energy Commission. Past efforts at developing catalytic combustors
for gas turbines achieved low, single-digit NOx ppm levels, but failed to produce combustion
systems with suitable operating durability. This was typically due to cycling damage and to the
brittle nature of the materials used for catalysts and catalyst support systems. Catalytic
combustor developers and gas turbine manufacturers are testing durable catalytic and "partial
catalytic" systems that are overcoming the problems of past designs. Catalytic combustors
capable of achieving NOx levels below 3 ppm are in full-scale demonstration and are entering
16 Cost Analysis of NOx Control Alternatives for Stationary Gas Turbines, ONSITE SYCOM Energy Corporation,
November, 1999.
17 The SCR reaction, with stoichiometric (for NOx reduction) ammonia or other reagent should eliminate all NOx.
However because of imperfect mixing in the combustor the NOx is not uniformly distributed across the turbine
exhaust. Additionally the ammonia, or other reagent, also is not injected in a precisely uniform manner. These two
non-uniformities in chemical composition cause either excess ammonia to be used, and to consequently "slip" out
the exhaust, or for incomplete reaction of the NOx in the turbine exhaust.
Technology Characterization
23
Gas Turbines
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18
early commercial introduction. Similarly to DLN combustion, optimized catalytic combustion
requires an integrated approach to combustor and turbine design. Catalytic combustors must be
tailored to the specific operating characteristics and physical layout of each turbine design.
Catalytic Absorption Systems
SCONOx™, patented by Goaline Environmental Technologies (currently EmerChem), is a post-
combustion alternative to SCR that reduces NOx emissions to less than 2.5 ppm and almost
100% removal of CO. SCONOx™ combines catalytic conversion of CO and NOx with an
absorption/regeneration process that eliminates the ammonia reagent found in SCR technology.
It is based on a unique integration of catalytic oxidation and absorption technology. CO and NO
catalytically oxidize to CO2 and NO2. The NO2 molecules are subsequently absorbed on the
treated surface of the SCONOx™ catalyst. The system does not require the use of ammonia,
eliminating the potential for ammonia slip associated with SCR. The SCONOx™ system is
generally located within the HRSG and under special circumstances may be located downstream
of the HRSG. The system operates between 300-700°F. U.S. EPA Region 9 identified
SCONOx™ as "Lowest Achievable Emission Rate (LAER)" technology for gas turbine NOx
control in 1998.
The SCONOx™ technology is still in the early stages of market introduction. Issues that may
impact application of the technology include relatively high capital cost, large reactor size
compared to SCR, system complexity, high utilities cost and demand (steam, natural gas,
compressed air and electricity are required), and a gradual rise in NO emissions over time that
requires a 1 to 2 day shutdown every 6 to 12 months (depending on fuel quality and operation) to
remove and regenerate the absorption modules ex-situ.19
Gas Turbine Emissions Characteristics
Table 5 shows typical emissions for each of the five typical turbine systems for the base year
(2000). Typical emissions presented are based on gas turbine exhaust with no exhaust treatment
and reflect what manufacturers will guarantee. Notable outliers for specific installations or
engine models are identified. Due to the uniqueness of the combustion system of each gas
turbine model, clear distinctions need to be made when discussing emissions technology and the
corresponding emissions levels. Those distinctions are technology that is commercially
available, technology that is technically proven but not yet commercial, and technology that is
technically feasible but neither technically proven nor commercially available. This is
particularly true for pollution prevention and combustion technologies as opposed to exhaust
treatment control alternatives. The later two distinctions do not fall under the category of
commercially available and consequently are noted as footnotes in Table 5 rather than the
representative emissions level.
Add-on control options for NOx and CO can further reduce emissions of each by 80 to 90%. For
many distributed generation gas turbine installations, exhaust treatment options have for the most
part been avoided or not implemented due to the unfavorable capital and operating costs impacts.
18 For example, Kawasaki offers a version of their MIA 13X, 1.4 MW gas turbine with a catalytic combustor with
less than 3 ppm NOx guaranteed.
19 Resource Catalysts, Inc.
Technology Characterization
24
Gas Turbines
-------
Table 5. Gas Turbine Emissions Characteristics Without Heat Recovery or Exhaust Control
Options*
1 Emissions ( luimck-nslics
S\ slcm
1
S\ slcm
S\ slcm
S\ slcm
' 4
S\ slcm
5
Electricity Capacity (kW)
1,000
5,000
10,000
25,000
40,000
Electrical Efficiency (HHV)
22%
27%
29%
34%
37%
NOx, ppm
4220
2521'
25
25
2522
NOx, lb/MWh23
2.43
1.16
1.08
0.92
0.31
CO, ppmv24
20
20
20
20
20
CO, lb/MWh17
0.71
0.56
0.53
0.45
0.85
C02, lb/MWh
1,887
1,510
1,411
1,193
1,106
Carbon, lb/MWh
515
412
385
326
302
* For typical systems commercially available in 2001. Emissions estimates for untreated turbine exhaust
conditions (15% 02i no SCR or other exhaust clean up). Estimates based on typical manufacturers' guarantees
using commercially available dry low NOx combustion technology.
2042 ppm represents the representative guaranteed state-of -the-art for 1 MW gas turbine systems. It has just been
announced that Kawasaki is offering their Ml A-13A equipped with a Xonon catalytic combustion system provided
by Catalytica Energy Systems with a guarantee of less than 3 ppm NOx. Many of the models in this size range do
not have DLN options and still utilize diffusion flame combustion systems.
21 Solar Turbines has permitted a 5 MW turbine guaranteed for 15 ppm NOx. This specific gas turbine installation is
equipped with a developmental ceramic combustor liner that is not standard on the commercial product line.
22 9 ppm is offered on industrial frame machines (e.g., GE 6B, Alstom GT10) which have lower firing temperatures
and pressure ratios (resulting in lower efficiencies), and longer residence time.
23 Conversion from volumetric emission rate (ppm at 15% 02) to output based rate (lbs/MWh) for both NOx and CO
based on conversion multipliers provided by Catalytica Energy Systems
(http://www.catalyticaenergy.com/xonon/emissionsJ'actors.html).
24 CO catalytic oxidation modules on gas turbines result in approximately 90% reduction of CO. Recent permits
have included the utilization of CO catalysts to achieve less than 5 ppm CO.
Technology Characterization
25
Gas Turbines
-------
Technology Characterization
Microturbines
Prepared for:
Environmental Protection Agency
Climate Protection Partnership Division
Washington, DC
Prepared by:
Energy Nexus Group
1401 Wilson Blvd, Suite 1101
Arlington, Virginia 22209
March 2002
-------
Disclaimer:
The information included in these technology overviews is for information purposes only and is
gathered from published industry sources. Information about costs, maintenance, operations, or
any other performance criteria is by no means representative of agency policies, definitions, or
determinations for regulatory or compliance purposes.
Technology Characterization
Microturbines
-------
TABLE OF CONTENTS
Introduction and Summary 1
Applications 1
Technology Description 2
Basic Processes 2
Basic Components 2
CHP Operation 5
Performance Characteristics 6
Part-Load Performance 11
Effects of Ambient Conditions on Performance 12
Heat Recovery 14
Performance and Efficiency Enhancements 15
Capital Cost 17
Maintenance 20
Fuels 21
Availability 21
Emissions 22
Microturbine Emissions Characteristics 23
Technology Characterization
Microturbines
-------
Technology Characterization - Microturbines
Introduction and Summary
Microturbines are small electricity generators that burn gaseous and liquid fuels to create high-
speed rotation that turns an electrical generator. Today's micro turbine technology is the result of
development work in small stationary and automotive gas turbines, auxiliary power equipment,
and turbochargers, much of which was pursued by the automotive industry beginning in the
1950s. Microturbines entered field testing around 1997 and began initial commercial service in
2000.
The size range for microturbines available and in development is from 30 to 350 kilowatts (kW),
while conventional gas turbine sizes range from 500 kW to 250 megawatts (MW).
Microturbines run at high speeds and, like larger gas turbines, can be used in power-only
generation or in combined heat and power (CHP) systems. They are able to operate on a variety
of fuels, including natural gas, sour gases (high sulfur, low Btu content), and liquid fuels such as
gasoline, kerosene, and diesel fuel/distillate heating oil. In resource recovery applications, they
burn waste gases that would otherwise be flared or released directly into the atmosphere.
Applications
Microturbines are ideally suited for distributed generation applications due to their flexibility in
connection methods, ability to be stacked in parallel to serve larger loads, ability to provide
stable and reliable power, and low emissions. Types of applications include:
o Peak shaving and base load power (grid parallel)
o Combined heat and power
o Stand-alone power
o Backup/standby power
o Ride-through connection
o Primary power with grid as backup
o Microgrid
o Resource recovery
Target customers include financial services, data processing, telecommunications, restaurant,
lodging, retail, office building, and other commercial sectors. Microturbines are currently
operating in resource recovery operations at oil and gas production fields, wellheads, coal mines,
and landfill operations, where byproduct gases serve as essentially free fuel. Reliable unattended
operation is important since these locations may be remote from the grid, and even when served
by the grid, may experience costly downtime when electric service is lost due to weather, fire, or
animals.
Technology Characterization
1
Microturbines
-------
In CHP applications, the waste heat from the microturbine is used to produce hot water, to heat
building space, to drive absorption cooling or desiccant dehumidification equipment, and to
supply other thermal energy needs in a building or industrial process.
Technology Description
Basic Processes
Microturbines are small gas turbines, most of which feature an internal heat exchanger called a
recuperator. In a microturbine, a radial flow (centrifugal) compressor compresses the inlet air
that is then preheated in the recuperator using heat from the turbine exhaust. Next, the heated air
from the recuperator mixes with fuel in the combustor and hot combustion gas expands through
the expansion and power turbines. The expansion turbine turns the compressor and, in single-
shaft models, turns the generator as well. Two-shaft models use the compressor drive turbine's
exhaust to power a second turbine that drives the generator. Finally, the recuperator uses the
exhaust of the power turbine to preheat the air from the compressor.
Single-shaft models generally operate at speeds over 60,000 revolutions per minute (rpm) and
generate electrical power of high frequency, and of variable frequency (alternating current -AC).
This power is rectified to direct current (DC) and then inverted to 60 hertz (Hz) for U.S.
commercial use. In the two-shaft version, the power turbine connects via a gearbox to a
generator that produces power at 60 Hz. Some manufacturers offer units producing 50 Hz for
use in countries where 50 Hz is standard, such as in Europe and parts of Asia.
Thermodynamic Cycle
Microturbines operate on the same thermodynamic cycle, known as the Brayton cycle, as larger
gas turbines. In this cycle, atmospheric air is compressed, heated, and then expanded, with the
excess power produced by the expander (also called the turbine) over that consumed by the
compressor used for power generation. The power produced by an expansion turbine and
consumed by a compressor is proportional to the absolute temperature of the gas passing through
those devices. Consequently, it is advantageous to operate the expansion turbine at the highest
practical temperature consistent with economic materials and to operate the compressor with
inlet airflow at as low a temperature as possible. As technology advances permit higher turbine
inlet temperature, the optimum pressure ratio also increases. Higher temperature and pressure
ratios result in higher efficiency and specific power. Thus, the general trend in gas turbine
advancement has been towards a combination of higher temperatures and pressures. However,
microturbine inlet temperatures are generally limited to 1,800°F or below to enable the use of
relatively inexpensive materials for the turbine wheel, and to maintain pressure ratios at a
comparatively low 3.5 to 4.0.
Basic Components
Turbo-Compressor Package
The basic components of a microturbine are the compressor, turbine generator, and recuperator
(see Figure 1). The heart of the microturbine is the compressor-turbine package, which is
commonly mounted on a single shaft along with the electric generator. Two bearings support the
Technology Characterization
2
Microturbines
-------
single shaft. The single moving part of the one-shaft design has the potential for reducing
maintenance needs and enhancing overall reliability. There are also two-shaft versions, in which
the turbine on the first shaft directly drives the compressor while a power turbine on the second
shaft drives a gearbox and conventional electrical generator producing 60 Hz power. The two-
shaft design features more moving parts but does not require complicated power electronics to
convert high frequency AC power output to 60 Hz.
Figure 1. Microturbine-Based CHP System (Single-Shaft Design)
To Stack
60 Hz AC
Electricity
m
Exhaust
Heat
Recovery
Exhaust
Recup-
erator
Inverter
Rectifier
High Frequency
Generator
Compressor
6-
Combustor ¦ Fuel
Turbine
Exhaust
Air
Moderate to large-size gas turbines use multi-stage axial flow turbines and compressors, in
which the gas flows along the axis of the shaft and is compressed and expanded in multiple
stages. However, microturbine turbomachinery is based on single-stage radial flow compressors
and turbines. Radial flow turbomachinery handles the small volumetric flows of air and
combustion products with reasonably high component efficiency.1 Large-size axial flow turbines
and compressors are typically more efficient than radial flow components. However, in the size
range of microturbines — 0.5 to 5 lbs/second of air/gas flow - radial flow components offer
minimum surface and end wall losses and provide the highest efficiency.
In microturbines, the turbocompressor shaft generally turns at high rotational speed, about
96,000 rpm in the case of a 30 kW machine and about 80,000 rpm in a 75 kW machine. One 45
kW model on the market turns at 116,000 rpm. There is no single rotational speed-power size
rule, as the specific turbine and compressor design characteristics strongly influence the physical
size of components and consequently rotational speed. For a specific aerodynamic design, as the
power rating decreases, the shaft speed increases, hence the high shaft speed of the small
microturbines.
The radial flow turbine-driven compressor is quite similar in terms of design and volumetric flow
to automobile, truck, and other small reciprocating engine turbochargers. Superchargers and
turbochargers have been used for almost 80 years to increase the power of reciprocating engines
1 With axial flow turbomachinery, blade height would be too small to be practical.
Technology Characterization
3
Microturbines
-------
by compressing the inlet air to the engine. Today's world market for small automobile and truck
turbochargers is around two million units per year. Small gas turbines, of the size and power
rating of microturbines, serve as auxiliary power systems on airplanes. Cabin cooling (air
conditioning) systems of airplanes use this same size and design family of compressors and
turbines. The decades of experience with these applications provide the basis for the engineering
and manufacturing technology of microturbine components.
Generator
The microturbine produces electrical power either via a high-speed generator turning on the
single turbo-compressor shaft or with a separate power turbine driving a gearbox and
conventional 3,600 rpm generator. The high-speed generator of the single-shaft design employs
a permanent magnet (typically Samarium-Cobalt) alternator, and requires that the high frequency
AC output (about 1,600 Hz for a 30 kW machine) be converted to 60 Hz for general use. This
power conditioning involves rectifying the high frequency AC to DC, and then inverting the DC
to 60 Hz AC. Power conversion comes with an efficiency penalty (approximately five percent).
To start-up a single shaft design, the generator acts as a motor turning the turbo-compressor shaft
until sufficient rpm is reached to start the combustor. Full start-up requires several minutes. If
the system is operating independent of the grid (black starting), a power storage unit (typically a
battery UPS) is used to power the generator for start-up.
Recuperators
Recuperators are heat exchangers that use the hot turbine exhaust gas (typically around 1,200°F)
to preheat the compressed air (typically around 300°F) going into the combustor, thereby
reducing the fuel needed to heat the compressed air to turbine inlet temperature. Depending on
microturbine operating parameters, recuperators can more than double machine efficiency.
However, since there is increased pressure drop in both the compressed air and turbine exhaust
sides of the recuperator, power output typically declines 10 to 15% from that attainable without
the recuperator. Recuperators also lower the temperature of the microturbine exhaust, reducing
the microturbine's effectiveness in CHP applications.
Bearings
Microturbines operate on either oil-lubricated or air bearings, which support the shaft(s). Oil-
lubricated bearings are mechanical bearings and come in three main forms - high-speed metal
roller, floating sleeve, and ceramic surface. The latter typically offer the most attractive benefits
in terms of life, operating temperature, and lubricant flow. While they are a well-established
technology, they require an oil pump, oil filtering system, and liquid cooling that add to
microturbine cost and maintenance. In addition, the exhaust from machines featuring oil-
lubricated bearings may not be useable for direct space heating in cogeneration configurations
due to the potential for contamination. Since the oil never comes in direct contact with hot
combustion products, as is the case in small reciprocating engines, it is believed that the
reliability of such a lubrication system is more typical of ship propulsion diesel systems (which
have separate bearings and cylinder lubrication systems) and automotive transmissions than
cylinder lubrication in automotive engines.
Technology Characterization
4
Microturbines
-------
Air bearings have been in service on airplane cabin cooling systems for many years. They allow
the turbine to spin on a thin layer of air, so friction is low and rpm is high. No oil or oil pump is
needed. Air bearings offer simplicity of operation without the cost, reliability concerns,
maintenance requirements, or power drain of an oil supply and filtering system. Concern does
exist for the reliability of air bearings under numerous and repeated starts due to metal on metal
friction during startup, shutdown, and load changes. Reliability depends largely on individual
manufacturers' quality control methodology more than on design engineering, and will only be
proven after significant experience with substantial numbers of units with long numbers of
operating hours and on/off cycles.
Power Electronics
As discussed, single-shaft microturbines feature digital power controllers to convert the high
frequency AC power produced by the generator into usable electricity. The high frequency AC
is rectified to DC, inverted back to 60 or 50 Hz AC, and then filtered to reduce harmonic
distortion. This is a critical component in the single-shaft microturbine design and represents
significant design challenges, specifically in matching turbine output to the required load. To
allow for transients and voltage spikes, power electronics designs are generally able to handle
seven times the nominal voltage. Most microturbine power electronics are generating three-
phase electricity.
Electronic components also direct all of the operating and startup functions. Microturbines are
generally equipped with controls that allow the unit to be operated in parallel or independent of
the grid, and internally incorporate many of the grid and system protection features required for
interconnect. The controls also allow for remote monitoring and operation.
CHP Operation
In CHP operation, a second heat exchanger, the exhaust gas heat exchanger, transfers the
remaining energy from the microturbine exhaust to a hot water system. Exhaust heat can be used
for a number of different applications, including potable water heating, driving absorption
cooling and desiccant dehumidification equipment, space heating, process heating, and other
building or site uses. Some microturbine-based CHP applications do not use recuperators. With
these microturbines, the temperature of the exhaust is higher and thus more heat is available for
recovery. Figure 1 illustrates a microturbine-based CHP system.
Design Characteristics
Thermal output: Microturbines produce thermal output at temperatures in the 400 to
600°F range, suitable for supplying a variety of building thermal
needs.
Fuel flexibility: Microturbines can operate using a number of different fuels: natural
gas, sour gases (high sulfur, low Btu content), and liquid fuels such as
gasoline, kerosene, and diesel fuel/heating oil.
Technology Characterization
5
Microturbines
-------
Reliability and life:
Design life is estimated to be in the 40,000 to 80,000 hour range.
While units have demonstrated reliability, they have not been in
commercial service long enough to provide definitive life data.
Size range:
Microturbines available and under development are sized from 30 to
350 kW.
Emissions:
Low inlet temperatures and high fuel-to-air ratios result in NOx
emissions of less than 10 parts per million (ppm) when running on
natural gas.
Modularity:
Units may be connected in parallel to serve larger loads and provide
power reliability.
Part-load operation: Because microturbines reduce power output by reducing mass flow
and combustion temperature, efficiency at part load can be below that
of full-power efficiency.
Dimensions:
About 12 cubic feet.
Performance Characteristics
Microturbines are more complex than conventional simple-cycle gas turbines, as the addition of
the recuperator both reduces fuel consumption (thereby substantially increasing efficiency) and
introduces additional internal pressure losses that moderately lower efficiency and power. As the
recuperator has four connections - to the compressor discharge, the expansion turbine discharge,
the combustor inlet, and the system exhaust — it becomes a challenge to the microturbine product
designer to make all of the connections in a manner that minimizes pressure loss, keeps
manufacturing cost low, and entails the least compromise of system reliability. Each
manufacturer's models have evolved in unique ways.
The addition of a recuperator opens numerous design parameters to performance-cost tradeoffs.
In addition to selecting the pressure ratio for high efficiency and best business opportunity (high
power for low price), the recuperator has two performance parameters, effectiveness and
pressure drop, that also have to be selected for the combination of efficiency and cost that creates
the best business conditions. Higher effectiveness recuperation requires greater recuperator
surface area, which both increases cost and incurs additional pressure drop. Such increased
internal pressure drop reduces net power production and consequently increases microturbine
cost per kW.
Microturbine performance, in terms of both efficiency and specific power,2 is highly sensitive to
small variations in component performance and internal losses. This is because the high
efficiency recuperated cycle processes a much larger amount of air and combustion products
flow per kW of net powered delivered than is the case for high-pressure ratio simple-cycle
machines. When the net output is the small difference between two large numbers (the
2 Specific power is power produced by the machine per unit of mass flow through the machine.
Technology Characterization
6
Microturbines
-------
compressor and expansion turbine work per unit of mass flow), small losses in component
efficiency, internal pressure losses and recuperator effectiveness have large impacts on net
efficiency and net power per unit of mass flow.
For these reasons, it is advisable to focus on trends and comparisons in considering performance,
while relying on manufacturers' guarantees for precise values.
Electrical Efficiency
Figure 2 shows a recuperated microturbine electrical efficiency as a function of microturbine
compressor ratio, for a range of turbine firing temperatures from 1,550 to 1,750°F, corresponding
to conservative to optimistic turbine material life behavior. The reported efficiency is the gross
generator output (without parasitic or conversion losses considered). Often this is at high
frequency, so the output must be rectified and inverted to provide 60 Hz AC power. The
efficiency loss in such frequency conversion (about 5%, which would lower efficiency from 30%
to 28.5%) is not included in these charts. Figure 2 shows that a broad optimum of performance
exists in the pressure ratio range from 3 to 4.
Figure 3 shows microturbine specific power for the same range of firing temperatures and
pressure ratios. Higher pressure ratios result in greater specific power. However, practical
considerations limit compressor and turbine component tip speed due to centrifugal forces and
allowable stresses in economic materials, resulting in compressor pressure ratio limits of 3.5 to 5
in microturbines currently entering the market.
Technology Characterization
1
Microturbines
-------
Figure 2. Microturbine Efficiency as a Function of Compressor Pressure Ratio and Turbine
Firing Temperature*
Microturbine Electrical Efficiency
32
30
>
28
> 26
o
c
a>
o 24
LU
22
20
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
Compressor Pressure Ratio
Efficiency at 1,550 deg F
Efficiency at 1,650 deg F Efficiency at 1,750 deg F
Source: Energy Nexus Group
* Most of the efficiencies quoted in this report are based on higher heating value (HHV), which includes the heat of
condensation of the water vapor in the combustion products. In engineering and scientific literature the lower
heating value (LHV) is often used, which does not include the heat of condensation of the water vapor in the
combustion products). Fuel is sold on a HHV basis. The HHV is greater than the LHV by approximately 10% with
natural gas as the fuel (i.e., 50% LHV is equivalent to 45% HHV). HHV efficiencies are about 8% greater for oil
(liquid petroleum products) and 5% greater for coal.
Technology Characterization
8
Microturbines
-------
Figure 3. Microturbine Specific Power as a Function of Compressor Pressure Ratio and
Turbine Firing Temperature
Microturbine Specific Power
80 i
o- 30
o
o 20
v
Q.
10
0 -I
1 1.5 2 2.5 3 3.5 4 4.5 5
Compressor Pressure Ratio
Spec Power at 1,550 deg F
— - — - Spec Power at 1,650 deg F
Spec Power at 1,750 deg F
Source: Energy Nexus Group
Table 1 summarizes performance characteristics for typical microturbine CHP systems. The
range of 30 to 350 kW represents what is currently or soon to be commercially available. Heat
rates and efficiencies shown were taken from manufacturers' specifications and industry
publications. Electrical efficiencies are net of parasitic and conversion losses. Available thermal
energy is calculated based on manufacturer specifications on turbine exhaust flows and
temperatures. CHP thermal recovery estimates are based on producing hot water for process or
space heating applications. Total CHP efficiency is the sum of the net electricity generated plus
hot water produced for building thermal needs divided by total fuel input to the system.
Effective electrical efficiency is a more useful value than overall efficiency to measure fuel
savings. Effective electric efficiency assumes that a water heater would otherwise generate the
useful thermal output from the CHP system at an 80% thermal efficiency. The theoretical water
heating fuel use is subtracted from the total fuel input to calculate the effective electric efficiency
of the CHP system.
The data in the table show that electrical efficiency increases as the microturbine becomes larger.
As electrical efficiency increases, the absolute quantity of thermal energy available decreases per
unit of power output, and the ratio of power to heat for the CHP system increases. A changing
ratio of power to heat impacts project economics and may affect the decisions that customers
make in terms of CHP acceptance, sizing, and other characteristics.
Technology Characterization
9
Microturbines
-------
Table 1. Microturbine CHP - Typical Performance Parameters*
( oM & 1 VifoniKiikv ClKii'MCk'i isiic.s'
Sysk'm 1
Sysk'm 2
Sysk'm
Sysk'm 4
Nominal Electricity Capacity (kW)
30 kW
70 kW
100 kW
350 kW
Package Cost (2000 $/kW)4
$1,000
$950
$800
$750
Total Installed Cost (2000 $/kW)5
$2,516
$2,031
$1,561
$1,339
Electric Heat Rate (Btu/kWh), HHV6
14,581
13,540
12,637
11,766
Electrical Efficiency (%), HHV'
23.4%
25.2%
27.0%
29.0%
Fuel Input (MMBtu/hr)
0.437
0.948
1.264
4.118
Required Fuel Gas Pressure (psig)
55
55
75
135
CI II' C"haiacU'i i.Niics
Exhaust Flow (lbs/sec)
0.72
1.40
1.74
5.00
GT Exhaust Temp (degrees F)
500
435
500
600
Heat Exchanger Exhaust Temp (degrees F)
150
130
131
140
Heat Output (MMBtu/hr)
0.218
0.369
0.555
1.987
Heat Output (kW equivalent)
64
108
163
582
Total CHP Efficiency (%), HHV8
73%
64%
71%
77%
Power/Heat Ratio9
0.47
0.65
0.62
0.60
Net Heat Rate (Btu/kWh)10
5,509
6,952
5,703
4,668
Effective Electrical Efficiency (%), HHV11
62%
49%
60%
73%
* For typical systems commercially available in 2001 (30-, 70- and 100 kW units) or soon to be available
(350 kW model is under development). 30-, 100- and 350 kW systems represented are single-shaft models.
70 kW system represented is a double-shaft model.
Source: Energy Nexus Group.
3 Characteristics presented are representative of "typical" commercially available or soon to be available
microturbine systems. Table data are based on: Capstone Model 330 - 30 kW; IR Energy Systems 70LM - 70 kW
(two-shaft); Turbec T100 - 100 kW; DTE model currently under development - 350 kW.
4 Equipment cost only. The cost for all units except for the 30 kW unit includes integral heat recovery water heater.
All units include a fuel gas booster compressor.
5 Installed costs based on CHP system producing hot water from exhaust heat recovery. The 70 kW and 100 kW
systems are offered with integral hot water recovery built into the equipment. The 30 kW units are currently built as
electric (only) generators and the heat recovery water heater is a separate unit. Other units entering the market are
expected to feature built in heat recovery water heaters.
6A11 turbine and engine manufacturers quote heat rates in terms of the lower heating value (LHV) of the fuel. On the
other hand, the usable energy content of fuels is typically measured on a higher heating value (HHV) basis. In
addition, electric utilities measure power plant heat rates in terms of HHV. For natural gas, the average heat content
of natural gas is 1,030 Btu/scf on an HHV basis and 930 Btu/scf on an LHV basis - or about a 10% difference.
7 Electrical efficiencies are net of parasitic and conversion losses. Fuel gas compressor needs based on 1 psi inlet
supply.
8 Total Efficiency = (net electric generated + net heat produced for thermal needs)/total system fuel input
9 Power/Heat Ratio = CHP electrical power output (Btu)/ useful heat output (Btu)
10 Net Heat Rate = (total fuel input to the CHP system - the fuel that would be normally used to generate the same
amount of thermal output as the CHP system output assuming an efficiency of 80%)/CHP electric output (kW).
11 Effective Electrical Efficiency = (CHP electric power output)/(Total fuel into CHP system - total heat
recovered/0.8).
Technology Characterization
10
Microturbines
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Each microturbine manufacturer represented in Table 1 uses a different recuperator, and each
has made individual tradeoffs between cost and performance. Performance involves the extent to
which the recuperator effectiveness increases cycle efficiency, the extent to which the
recuperator pressure drop decreases cycle power, and the choice of what cycle pressure ratio to
use. Consequently, microturbines of different makes will have different CHP efficiencies and
different net heat rates chargeable to power.
As shown, microturbines typically require 50 to 80 psig fuel supply pressure. Because
microturbines are built with pressure ratios between 3 and 4 to maximize efficiency with a
recuperator at modest turbine inlet temperature, the required supply pressure for microturbines is
much less than for industrial-size gas turbines with pressure ratios of 7 to 35. Local distribution
gas pressures usually range from 30 to 130 psig in feeder lines and from 1 to 50 psig in final
distribution lines. Most U.S. businesses that would use a 30, 70, or 100 kW microturbine receive
gas at about 0.5 to 1.0 psig. Additionally, most building codes prohibit piping higher-pressure
natural gas within the structure. Thus, microturbines in most commercial locations require a fuel
gas booster compressor to ensure that fuel pressure is adequate for the gas turbine flow control
and combustion systems.
Most microturbine manufacturers offer the equipment package with the fuel gas booster
included. It is included in all of the representative systems shown in Table 1. This packaging
facilitates the purchase and installation of a microturbine, as the burden of obtaining and
installing the booster compressor is no longer placed on the customer. Also, it might result in
higher reliability of the booster through standardized design and volume manufacture.
Booster compressors can add from $50 to $100 per kW to a microturbine CHP system's total
cost. As well as adding to capital cost, booster compressors lower net power and efficiency so
operating cost is slightly higher. Typically, the fuel gas booster requires about 5% of the
microturbine output. For example, a single 60 kW unit requires 2.6 kW for the booster, while a
booster serving a system of three 30 kW units would require 4.4 kW. Such power loss results in
a penalty on efficiency of about 1.5 percentage points. For installations where the unit is located
outdoors, the customer can save on cost and operating expense by having the gas utility deliver
gas at an adequate pressure and obtaining a system without a fuel gas booster compressor.
Part-Load Performance
When less than full power is required from a microturbine, the output is reduced by a
combination of mass flow reduction (achieved by decreasing the compressor speed) and turbine
inlet temperature reduction. It takes approximately 15 seconds for a microturbine to go from no-
load to full-load conditions. Rapid off-loading (removal of load) will tend to cause the stored
energy in the microturbine to overspeed and damage the turbine. In addition to reducing power,
this change in operating conditions also reduces efficiency. Figure 4 shows a sample part-load
derate curve for a microturbine.
Technology Characterization
11
Microturbines
-------
Figure 4. Microturbine Part Load Power Performance
Part Load Performance
30 kW Microturbine
120 1
100
>»
o
c
0)
o
»~—
»~—
LU
40
20
10
20
30
40
50
60
70
80
90
100
Percent Load (%)
Afofe: unit represented is a single-shaft, high-speed alternator system.
Source: Energy Nexus Group.
Effects of Ambient Conditions on Performance
The ambient conditions under which a microturbine operates have a noticeable effect on both the
power output and efficiency. At elevated inlet air temperatures, both the power and efficiency
decrease. The power decreases due to the decreased airflow mass rate (since the density of air
declines as temperature increases), and the efficiency decreases because the compressor requires
more power to compress air of higher temperature. Conversely, the power and efficiency
increase with reduced inlet air temperature. Figure 5 shows the variation in power and
efficiency for a microturbine as a function of ambient temperature compared to the reference
International Organization for Standards (ISO) condition of sea level and 59°F. The density of
air decreases at altitudes above sea level. Consequently, power output decreases. Figure 6
illustrates the altitude derate.
Technology Characterization
12
Microturbines
-------
Figure 5. Ambient Temperature Effects on Microturbine Performance
Impact of Ambient Temperature
30 kW Microturbine
100
90
80
70
60
<
c
a>
o
50
v
40
30
20
0
10 20 30 40 50 60 70 80 90 100 110 120 130
Ambient Temperature (°F)
Power (% ISO Rated Output)
Efficiency (%), HHV
Impact of Ambient Temperature
70 kW Microturbine
Ambient Temperature (°F)
Power (% ISO Rated Output)
Efficiency (%), HHV
Source: Energy Nexus Group.
Technology Characterization
13
Microturbines
-------
Figure 6. Altitude Effects on Microturbine Performance
Derate at Altitude
LL
70
0
1000
2000
3000
4000
5000
6000
Altitude (ft)
Source: Energy Nexus Group
Heat Recovery
Effective use of the thermal energy contained in the exhaust gas improves microturbine system
economics. Exhaust heat can be recovered and used in a variety of ways, including water
heating, space heating, and driving thermally activated equipment such as an absorption chiller
or a desiccant dehumidifier.
Microturbine CHP system efficiency is a function of exhaust heat temperature. Recuperator
effectiveness strongly influences the microturbine exhaust temperature. Consequently, the
various microturbine CHP systems have substantially different CHP efficiency and net heat rate
chargeable to power. These variations in CHP efficiency and net heat rate are mostly due to the
mechanical design and manufacturing cost of the recuperators and their resulting impact on
system cost, rather than being due to differences in system size.
Technology Characterization
14
Microturbines
-------
Performance and Efficiency Enhancements
Recuperators
Most microturbines include built in recuperators. The inclusion of a high effectiveness (90%)12
recuperator essentially doubles the efficiency of a microturbine with a pressure ratio of 3.2, from
about 14% to about 29% depending on component details. Without a recuperator, such a
machine would be suitable only for emergency, backup, or possibly peaking power operation.
With the addition of the recuperator, a microturbine can be suitable for intermediate duty or
price-sensitive baseload service.
While recuperators previously in use on industrial gas turbines developed leaks attributable to
the consequences of differential thermal expansion accompanying thermal transients,
microturbine recuperators have proven quite durable in testing to date. This durability has
resulted from using higher strength alloys and higher quality welding along with engineering
design to avoid the internal differential expansion that causes internal stresses and leakage. Such
practical improvements result in recuperators being of appreciable cost, which detracts from the
economic attractiveness of the microturbine. The cost of a recuperator becomes easier to justify
as the number of full-power operational hours per year increases.
Incorporation of a recuperator into the microturbine results in pressure losses in the recuperator
itself and in the ducting that connects it to other components. Typically, these pressure losses
result in 10 to 15% less power being produced by the microturbine, and a corresponding loss of a
few points in efficiency. The pressure loss parameter in gas turbines that is the measure of lost
power is 8p/p. As 8p/p increases, the net pressure ratio available for power generation decreases,
and hence the power capability of the expansion process diminishes as well. Figure 7 illustrates
the relationship between recuperator effectiveness and microturbine efficiency.
12 Effectiveness is the technical term in the heat exchanger industry for the ratio of the actual heat transferred to the
maximum achievable
Technology Characterization
Microturbines
15
-------
Figure 7. Microturbine Efficiency as a Function of Recuperator Effectiveness
Recuperator Impact on Efficiency
35
30
X 25
X
20
>.
o
c
0)
b
LU
0
20
40
60
80
100
Recuperator Effectiveness (%)
Source: Energy Nexus Group.
Firing Temperature
Large turbines (25 to 2,000 lbs/second of mass flow) are usually equipped with internal cooling
capability to permit operation with firing temperatures well above those of the metallurgical limit
of the best gas turbine alloys. Indeed, progress to higher and higher gas turbine efficiency, via
higher firing temperatures, has occurred more through the development and advancement of
blade and vane internal cooling technology than through the improvement of the high
temperature capabilities of gas turbine alloys.
Unfortunately for microturbine development, the nature of the three dimensional shape of radial
inflow turbines has not yet lent itself to the development of a manufacturing method that can
produce internal cooling. Consequently, microturbines are limited to firing temperatures within
the capabilities of gas turbine alloys. An ongoing program at the U. S. Department of Energy
(DOE) Office of Energy Efficiency seeks to apply the technology of ceramic radial inflow
turbines (previously advanced for the purpose of developing automotive gas turbines) to
microturbines, to increase their efficiency to 36% (HHV). The design and materials technology
from the previous efforts are applicable, since the automotive gas turbines were in the same size
range, and of the same general geometry, as those used in microturbines.
Technology Characterization
16
Microturbines
-------
Inlet Air Cooling
As shown in Figure 5, the decreased power and efficiency of microturbines at high ambient
temperatures means that microturbine performance is at its lowest at the times power is often in
greatest demand and most valued. The use of inlet air cooling can mitigate the decreased power
and efficiency resulting from high ambient air temperatures. While inlet air cooling is not a
feature on today's microturbines, cooling techniques now entering the market on large gas
turbines can be expected to work their way to progressively smaller equipment sizes, and, at
some future date, be used with microturbines.
Evaporative cooling, a relatively low capital cost technique, is the most likely to be applied to
microturbines. It uses a very fine spray of water directly into the inlet air stream. Evaporation of
the water reduces the temperature of the air. Since cooling is limited to the wet bulb air
temperature, evaporative cooling is most effective when the wet bulb temperature is appreciably
below the dry bulb (ordinary) temperature. In most locales with high daytime dry bulb
temperatures, the wet bulb temperature is often 20°F lower. This affords an opportunity for
substantial evaporative cooling. However, evaporative cooling can consume large quantities of
water, making it difficult to operate in arid climates.
Refrigeration cooling in microturbines is also technically feasible. In refrigeration cooling, a
compression-driven or thermally activated (absorption) refrigeration cycle cools the inlet air
through a heat exchanger. The heat exchanger in the inlet air stream causes an additional
pressure drop in the air entering the compressor, thereby slightly lowering cycle power and
efficiency. However, as the inlet air is now substantially cooler than the ambient air, there is a
significant net gain in power and efficiency. Electric motor compression refrigeration requires a
substantial parasitic power loss. Thermally activated absorption cooling can use waste heat from
the microturbine, reducing the direct parasitic loss. The relative complexity and cost of these
approaches, in comparison with evaporative cooling, render them less likely.
Finally, it is also technically feasible to use thermal energy storage systems, typically ice, chilled
water, or low-temperature fluids, to cool inlet air. These systems eliminate most parasitic losses
from the augmented power capacity. Thermal energy storage is a viable option if on-peak power
pricing only occurs a few hours a day. In that case, the shorter time of energy storage discharge
and longer time for daily charging allow for a smaller and less expensive thermal energy storage
system.
Capital Cost
This section provides typical study estimates for the installed cost of microturbine systems. Two
configurations are presented: power-only and CHP producing hot water for use on-site.
Equipment-only and installed costs are estimated for the four typical microturbine systems.
These are "typical" budgetary price levels. It should also be noted that installed costs can vary
significantly depending on the scope of the plant equipment, geographical area, competitive
market conditions, special site requirements, emissions control requirements, prevailing labor
rates, and whether the system is a new or retrofit application.
Table 2 provides cost estimates for combined heat and power applications, assuming that the
CHP system produces hot water. The basic microturbine package consists of the turbogenerator
Technology Characterization
17
Microturbines
-------
package and power electronics. All of the commercial and near-commercial units offer basic
interconnection and paralleling functionality as part of the package cost. All but one of the
systems offers an integrated heat exchanger heat recovery system for CHP within the package.
All current manufacturers also indicate the package price includes that the gas booster
compressor. It should be noted that the package prices cited in the table represent manufacturer
quotes or estimates. However, only two of the products have any market history, while the other
two are planning to enter the market this year. The manufacturer quotes may not reflect actual
cost plus profit today, but may instead represent a forward pricing strategy in which early units
are sold at a loss to develop the market. The information provided for each sample system is as
follows:
• 30 kW - Single unit $l,000/kW, including fuel gas compressor, DC-to-AC inverter, all
electronic interconnection hardware, but without the heat recovery heat exchanger. Prices
are lower for volume purchases, which are favored. (Capstone: nearly 2,000 units shipped to
dealer network.)
• 70 kW - Price of $79,900 includes commissioning and the first year of maintenance (at
$0.01/kWh and 4,000 hours/year, equivalent to $40/kW). Built-in heat recovery heat
exchanger included in price. Generator is a standard 3,600-rpm AC unit; therefore, there is
no need for an inverter. Electrical interconnection and fuel gas booster compressor included.
For this comparison, prepaid maintenance and commissioning costs have been backed out
from the package price.
• 100 kW - A price of $800/kW is offered to distributors for equipment including heat
recovery heat exchanger (built-in), fuel gas booster, DC-to-AC inverter and all
interconnection hardware.
• 350 kW - Price target of $910/kW for all equipment, including heat recovery heat exchanger,
inverter, fuel gas booster and interconnection, installed. For this comparison, the total
package was separated into a package price plus labor.
There is little additional equipment that is required for these integrated systems. A heat recovery
system has been added where needed, and additional controls and remote monitoring equipment
have been added. The total plant cost consists of total equipment cost plus installation labor and
materials (including site work), engineering, project management (including licensing, insurance,
commissioning and startup), and financial carrying costs during the 6- to 18-month construction
period.
The basic equipment costs represent material on the loading dock, ready to ship. The cost to a
customer for installing a microturbine-based CHP system includes a number of other factors that
increase the total costs by 70 to 80%. Labor/materials represent the labor cost for the civil,
mechanical, and electrical work and materials such as ductwork, piping, and wiring. Total
process capital is the equipment costs plus installation labor and materials.
A number of other costs are incurred on top of total process capital. These costs are often
referred to as soft costs because they vary widely by installation, by development channel and by
approach to project management. Engineering costs are required to design the system and
integrate it functionally with the application's electrical and mechanical systems. In this
characterization, environmental permitting fees are included here. Project and construction
Technology Characterization
18
Microturbines
-------
management also includes general contractor markup and bonding and performance guarantees.
Contingency is assumed to be 3% of the total equipment cost in all cases. Up-front, financing
costs are also included.
Table 2. Estimated Capital Cost for Microturbine Generators in Grid-Interconnected
Combined Heat and Power Application
Cost Component
System 1
System 2
System 3
System 4
Nominal Capacity (kW)
30
70
100
350
Costs ($/kW)
Equipment
Microturbine
$1,000
$1,030
$800
$750
Gas Booster Compressor
incl.
incl.
incl.
Incl.
Heat Recovery
$225
incl.
incl.
Incl.
Controls/Monitoring
$179
$143
$120
$57
Total Equipment
$1,403
$1,173
$920
$807
Labor/Materials
$429
$286
$200
$160
Total Process Capital
$1,832
$1,459
$1,120
$967
Project and Construction
$418
$336
$260
$226
Management
Engineering and Fees
$154
$146
$112
$86
Project Contingency
$72
$58
$45
$38
Project Financing (interest
$40
$32
$25
$21
during construction)
Total Plant Cost ($/kW)
$2,516
$2,031
$1,561
$1,339
Source: Energy Nexus Group
Since heat recovery is not required for systems that are power-only, the capital costs are lower.
For the units that integrate this equipment into the basic package, the savings will be modest,
about a $50/kW reduction in the basic package price. In addition, installation labor and materials
costs are reduced because there is no need to connect with the application's thermal system or to
connect the heat recovery equipment in the case where it is a separate unit. Power-only systems
require less engineering time, as integration is required only with the application's electrical
system. Project management and construction fees also tend to be lower because it is a more
competitive business than CHP. Table 3 shows the power-only cost estimates.
Technology Characterization
19
Microturbines
-------
Table 3. Estimated Capital Cost for Microturbine Generators in Grid-Interconnected
Power-Only Application
Cost Component
System 1
System 2
System 3
System 4
Nominal Capacity (kW)
30
70
100
350
Costs ($/kW)
Equipment
Microturbine
$1,000
$980
$750
$700
Gas Booster Compressor
$0
$0
$0
$0
Heat Recovery
$0
$0
$0
$0
Controls/Monitoring
$179
$143
$120
$57
Total Equipment
$1,179
$1,123
$870
$757
Labor/Materials
$300
$200
$140
$112
Total Process Capital
$1,479
$1,323
$1,010
$869
Project and Construction
$266
$245
$188
$206
Management
Engineering and Fees
$130
$85
$64
$44
Project Contingency
$56
$50
$38
$34
Project Financing (interest
$31
$27
$21
$18
during construction)
Total Plant Cost ($/kW)
$1,962
$1,729
$1,320
$1,171
Source: Energy Nexus Group
As an emerging product, the capital costs shown in the preceding two tables represent the cost
for the early market entry product, though not the cost of the first units into the market. All of
the microturbine developer/manufacturers have cost reduction plans and performance enhancing
developments for the mature market product.
Maintenance
Microturbines are still on a learning curve in terms of maintenance, as initial commercial units
have seen only two to three years of service so far. With relatively few operating hours logged
as a group, the units in the field have not yet yielded enough data to allow much definition in the
area of maintenance.
Most manufacturers offer service contracts for specialized maintenance priced at about
$0.01/kWh. This includes periodic inspections of the combustor (and associated hot section
parts) and the oil bearing in addition to regular air and oil filter replacements. There have been
microturbines operating in environments with extremely dusty air that have required frequent air
filter changes due to the dust in the air.
Technology Characterization
20
Microturbines
-------
A gas microturbine overhaul is needed every 20,000 to 40,000 hours depending on manufacturer,
design, and service. A typical overhaul consists of replacing the main shaft with the compressor
and turbine attached, and inspecting and if necessary replacing the combustor. At the time of the
overhaul, other components are examined to determine if wear has occurred, with replacements
made as required. Microturbines are usually operated with at least one on-off cycle per day.
There is concern about the effects of this type of operation on component durability.
There is no known difference in maintenance for operation on fuels other than natural gas.
However, experience with liquid fuels in industrial gas turbines suggests that liquid fueled
combustors require more frequent inspections and maintenance than natural gas fueled
combustors.
Fuels
Microturbines have been designed to use natural gas as their primary fuel. However, they are
able to operate on a variety of fuels, including:
• Liquefied petroleum gas (LPG) - propane and butane mixtures
• Sour gas - unprocessed natural gas as it comes directly from the gas well
• Biogas - any of the combustible gases produced from biological degradation of organic
wastes, such as landfill gas, sewage digester gas, and animal waste digester gas
• Industrial waste gases - flare gases and process off-gases from refineries, chemical plants
and steel mill
• Manufactured gases - typically low- and medium-Btu gas produced as products of
gasification or pyrolysis processes
Contaminants are a concern with some waste fuels, specifically acid gas components (H2S,
halogen acids, HCN; ammonia; salts and metal-containing compounds; organic halogen-, sulfur-,
nitrogen-, and silicon-containing compounds); and oils. In combustion, halogen and sulfur
compounds form halogen acids, SO2, some SO3 and possibly H2SO4 emissions. The acids can
also corrode downstream equipment. A substantial fraction of any fuel nitrogen oxidizes into
NOx in combustion. Solid particulates must be kept to low concentrations to prevent corrosion
and erosion of components. Various fuel scrubbing, droplet separation, and filtration steps will
be required if any fuel contaminant levels exceed manufacturer specifications. Landfill gas in
particular often contains chlorine compounds, sulfur compounds, organic acids, and silicon
compounds which dictate pretreatment.
Availability
With the small number of units in commercial service, information is not yet sufficient to draw
conclusions about reliability and availability of microturbines. The basic design and low number
of moving parts hold the potential for systems of high availability; manufacturers have targeted
availabilities of 98 to 99%. The use of multiple units or backup units at a site can further
increase the availability of the overall facility.
Technology Characterization
21
Microturbines
-------
Emissions
Microturbines have the potential for extremely low emissions. All microturbines operating on
gaseous fuels feature lean premixed (dry low NOx, or DLN) combustor technology, which was
developed relatively recently in the history of gas turbines and is not universally featured on
larger gas turbines.
The primary pollutants from microturbines are oxides of nitrogen (NOx), carbon monoxide (CO),
and unburned hydrocarbons. They also produce a negligible amount of sulfur dioxide (SO2).
Microturbines are designed to achieve the objective of low emissions at full load; emissions are
often higher when operating at part load.
The pollutant referred to as NOx is a mixture of mostly NO and NO2 in variable composition. In
emissions measurement it is reported as parts per million by volume in which both species count
equally. NOx forms by three mechanisms: thermal NOx, prompt NOx, and fuel-bound NOx. The
predominant NOx formation mechanism associated with gas turbines is thermal NOx. Thermal
NOx is the fixation of atmospheric oxygen and nitrogen, which occurs at high combustion
temperatures. Flame temperature and residence time are the primary variables that affect thermal
NOx levels. The rate of thermal NOx formation increases rapidly with flame temperature.
Prompt NOx forms from early reactions of nitrogen modules in the combustion air and
hydrocarbon radicals from the fuel. It forms within the flame and typically is about 1 ppm at
15% O2, and is usually much smaller than the thermal NOx formation. Fuel-bound NOx forms
when the fuel contains nitrogen as part of the hydrocarbon structure. Natural gas has negligible
chemically bound fuel nitrogen.
Incomplete combustion results in both CO and unburned hydrocarbons. CO emissions result
when there is insufficient residence time at high temperature. In gas turbines, the failure to
achieve CO burnout may result from combustor wall cooling air. CO emissions are also heavily
dependent on operating load. For example, a unit operating under low loads will tend to have
incomplete combustion, which will increase the formation of CO. CO is usually regulated to
levels below 50 ppm for both health and safety reasons. Achieving such low levels of CO had
not been a problem until manufacturers achieved low levels of NOx, because the techniques used
to engineer DLN combustors had a secondary effect of increasing CO emissions.
While not considered a regulated pollutant in the ordinary sense of directly affecting public
health, emissions of carbon dioxide (CO2) are of concern due to its contribution to global
warming. Atmospheric warming occurs because solar radiation readily penetrates to the surface
of the planet but infrared (thermal) radiation from the surface is absorbed by the CO2 (and other
polyatomic gases such as methane, unburned hydrocarbons, refrigerants, water vapor, and
volatile chemicals) in the atmosphere, with resultant increase in temperature of the atmosphere.
The amount of CO2 emitted is a function of both fuel carbon content and system efficiency. The
fuel carbon content of natural gas is 34 lbs carbon/MMBtu; oil is 48 lbs carbon/MMBtu; and
(ash-free) coal is 66 lbs carbon/MMBtu.
Technology Characterization
22
Microturbines
-------
Lean Premixed Combustion
Thermal N0X formation is a function of both the local temperatures within the flame and
residence time. In older technology combustors used in industrial gas turbines, fuel and air were
separately injected into the flame zone. Such separate injection resulted in high local
temperatures where the fuel and air zones intersected. The focus of combustion improvements
of the past decade was to lower flame local hot spot temperature using lean fuel/air mixtures
whereby zones of high local temperatures were not created. Lean combustion decreases the
fuel/air ratio in the zones where NOx production occurs so that peak flame temperature is less
than the stoichiometric adiabatic flame temperature, therefore suppressing thermal NOx
formation.
All microturbines feature lean pre-mixed combustion systems, also referred to as dry low NOx or
dry low emissions (DLE). Lean premixed combustion pre-mixes the gaseous fuel and
compressed air so that there are no local zones of high temperatures, or "hot spots," where high
levels of NOx would form. DLN requires specially designed mixing chambers and mixture inlet
zones to avoid flashback of the flame. Optimized application of DLN combustion requires an
integrated approach to combustor and turbine design. The DLN combustor is an intrinsic part of
the turbine design, and specific combustor designs are developed for each turbine application.
Full power NOx emissions below 9 ppmv @ 15% O2 have been achieved with lean premixed
combustion in microturbines.
Catalytic Combustion
In catalytic combustion, fuels oxidize at lean conditions in the presence of a catalyst. Catalytic
combustion is a flameless process, allowing fuel oxidation to occur at temperatures below
1,700°F, where NOx formation is low. The catalyst is applied to combustor surfaces, which
cause the fuel/air mixture to react on the catalyst surface and release its initial thermal energy.
The combustion reaction in the remaining volume of the lean premixed gas then goes to
completion at design temperature. Data from ongoing long term testing indicates that catalytic
combustion exhibits low vibration and acoustic noise, only one-tenth to one-hundredth the levels
measured in the same turbine equipped with DLN combustors.
Combustion system and gas turbine developers, along with the U.S. DOE, the California Energy
Commission, and other government agencies, are pursuing gas turbine catalytic combustion
technology. Past efforts at developing catalytic combustors for gas turbines achieved low,
single-digit NOx ppm levels, but failed to produce combustion systems with suitable operating
durability. This was typically due to cycling damage and to the brittle nature of the materials
used for catalysts and catalyst support systems. Catalytic combustor developers and gas turbine
manufacturers are testing durable catalytic and "partial catalytic" systems that are overcoming
the problems of past designs. Catalytic combustors capable of achieving NOx levels below 3
ppm are in full-scale demonstration and are entering early commercial introduction.13 As with
DLN combustion, optimized catalytic combustion requires an integrated approach to combustor
13 For example, Kawasaki offers a version of their MIA 13X, 1.4 MW gas turbine with a catalytic combustor with
less than 3 ppm NOx guaranteed.
Technology Characterization
23
Microturbines
-------
and turbine design. Catalytic combustors must be tailored to the specific operating
characteristics and physical layout of each turbine design.
Catalytic combustion may be applied to microturbines as well as industrial and utility turbines.
Because of the low emissions from DLN combustors, combined with the low turbine inlet
temperatures at which microturbines currently operate, it is not expected that catalytic
combustion for microturbines will be pursued in the near term.
Microturbine Emissions Characteristics
Table 4 presents typical emissions for microturbine systems. The data shown reflect
manufacturers' guaranteed levels.
Table 4. Microturbine Emissions Characteristics
1-missions Cluirack'iisiii.'s:
S\ sleni 1
S\ sleni 2
S\ sleni *
S\sk'in 4
Nominal Electricity Capacity (kW)
30
70
100
350
Electrical Efficiency, HHV
23%
25%
27%
29%
NOx, ppmv
9
9
15
9
NOx, lb/MWh14
0.54
0.50
0.80
0.53
CO, ppmv
40
9
15
25
CO, lb/MWh
1.46
0.30
0.49
0.72
THC, ppmv
<9
<9
<10
<10
THC, lb/MWh
<0.19
<0.17
<0.19
<0.19
C02, (lb/MWh)
1,928
1,774
1,706
1,529
Carbon, (lb/MWh)
526
484
465
417
Note: Estimates are based on manufacturers' guarantees for typical systems commercially available in 2001
(30-, 70- and 100 kW models). The emissions figures for the 350 kW system under development are
manufacturer goals.
14 Conversion from volumetric emission rate (ppmv at 15% 02) to output based rate (lbs/MWh) for both NOx and
CO based on conversion multipliers provided by Capstone Turbine Corporation and corrected for differences in
efficiency.
Technology Characterization
Microturbines
24
-------
Technology Characterization
Reciprocating Engines
Prepared for:
Environmental Protection Agency
Climate Protection Partnership
Division
Washington, DC
Prepared by:
Energy Nexus Group
1401 Wilson Blvd, Suite 1101
Arlington, Virginia 22209
February 2002
-------
Disclaimer:
The information included in these technology overviews is for information purposes only and is
gathered from published industry sources. Information about costs, maintenance, operations, or
any other performance criteria is by no means representative of agency policies, definitions, or
determinations for regulatory or compliance purposes.
Technology Characterization
i
Reciprocating Engines
-------
TABLE OF CONTENTS
Introduction and Summary 1
Applications 2
Technology Description 4
Basic Engine Processes 4
Types of Reciprocating Engines 5
Design Characteristics 10
Performance Characteristics 10
Electrical Efficiency 10
Part Load Performance 13
Source: Caterpillar, Energy Nexus Group 13
Effects of Ambient Conditions on Performance 13
Heat Recovery 14
Performance and Efficiency Enhancements 15
Capital Cost 16
Maintenance 17
Fuels 19
Availability 22
Emissions 22
Nitrogen Oxides (NOx) 23
Carbon Monoxide (CO) 24
Unburned Hydrocarbons 24
Carbon Dioxide (CO2) 24
Emissions Control Options 25
Gas Engine Emissions Characteristics 28
Technology Characterization ii Reciprocating Engines
-------
Technology Characterization - Reciprocating Engines
Introduction and Summary
Reciprocating internal combustion engines are a widespread and well-known technology. North
American production exceeds 35 million units per year for automobiles, trucks, construction and
mining equipment, marine propulsion, lawn care, and a diverse set of power generation
applications. A variety of stationary engine products are available for a range of power
generation market applications and duty cycles including standby and emergency power, peaking
service, intermediate and baseload power, and combined heat and power (CHP). Reciprocating
engines are available for power generation applications in sizes ranging from a few kilowatts to
over 5 MW.
There are two basic types of reciprocating engines - spark ignition (SI) and compression ignition
(CI). Spark ignition engines for power generation use natural gas as the preferred fuel, although
they can be set up to run on propane, gasoline, or landfill gas. Compression ignition engines
(often called diesel engines) operate on diesel fuel or heavy oil, or they can be set up to run in a
dual-fuel configuration that burns primarily natural gas with a small amount of diesel pilot fuel.
Diesel engines have historically been the most popular type of reciprocating engine for both
small and large power generation applications. However, in the United States and other
industrialized nations, diesel engines are increasingly restricted to emergency standby or limited
duty-cycle service because of air emission concerns. Consequently, the natural gas-fueled SI
engine is now the engine of choice for the higher-duty-cycle stationary power market (over 500
hr/yr) and is the primary focus of this report.
Current generation natural gas engines offer low first cost, fast start-up, proven reliability when
properly maintained, excellent load-following characteristics, and significant heat recovery
potential. Electric efficiencies of natural gas engines range from 28% LHV for small
stoichiometric engines (<100 kW) to over 40% LHV for large lean burn engines (> 3 MW)1.
Waste heat recovered from the hot engine exhaust and from the engine cooling systems produces
either hot water or low pressure steam for CHP applications. Overall CHP system efficiencies
(electricity and useful thermal energy) of 70 to 80% are routinely achieved with natural gas
engine systems.
Reciprocating engine technology has improved dramatically over the past three decades, driven
by economic and environmental pressures for power density improvements (more output per unit
of engine displacement), increased fuel efficiency and reduced emissions. Computer systems
have greatly advanced reciprocating engine design and control, accelerating advanced engine
1 Lower Heating Value. Most of the efficiencies quoted in this report are based on higher heating value (HHV),
which includes the heat of condensation of the water vapor in the combustion products. In engineering and
scientific literature the lower heating value (LHV - which does not include the heat of condensation of the water
vapor in the combustion products) is often used. The HHV is greater than the LHV by approximately 10% with
natural gas as the fuel (i.e., 50% LHV is equivalent to 45% HHV). HHV efficiencies are about 8% greater for oil
(liquid petroleum products) and 5% for coal.
Technology Characterization
1
Reciprocating Engines
-------
designs and making possible more precise control and diagnostic monitoring of the engine
process. Stationary engine manufacturers and worldwide engine R&D firms continue to drive
advanced engine technology, including accelerating the diffusion of technology and concepts
from the automotive market to the stationary market.
The emissions signature of natural gas SI engines in particular has improved significantly in the
last decade through better design and control of the combustion process and through the use of
exhaust catalysts. Advanced lean burn natural gas engines are available that produce NOx levels
as low as 50 ppmv @ 15% O2 (dry basis).
Applications
Reciprocating engines are well suited to a variety of distributed generation applications.
Industrial, commercial, and institutional facilities in the U.S. and Europe for power generation
and CHP. Reciprocating engines start quickly, follow load well, have good part-load
efficiencies, and generally have high reliabilities. In many cases, multiple reciprocating engine
units further increase overall plant capacity and availability. Reciprocating engines have higher
electrical efficiencies than gas turbines of comparable size, and thus lower fuel-related operating
costs. In addition, the first costs of reciprocating engine gensets are generally lower than gas
turbine gensets up to 3-5 MW in size. Reciprocating engine maintenance costs are generally
higher than comparable gas turbines, but the maintenance can often be handled by in-house staff
or provided by local service organizations.
Potential distributed generation applications for reciprocating engines include standby, peak
shaving, grid support, and CHP applications in which hot water, low-pressure steam, or waste-
heat-fired absorption chillers are required. Reciprocating engines are also used extensively as
direct mechanical drives in applications such as water pumping, air and gas compression and
chilling/refrigeration.
Combined Heat and Power
While the use of reciprocating engines is expected to grow in various distributed generation
applications, the most prevalent on-site generation application for natural gas SI engines has
traditionally been CHP, and this trend is likely to continue. The economics of natural gas
engines in on-site generation applications is enhanced by effective use of the thermal energy
contained in the exhaust gas and cooling systems, which generally represents 60 to 70% of the
inlet fuel energy.
There are four sources of usable waste heat from a reciprocating engine: exhaust gas, engine
jacket cooling water, lube oil cooling water, and turbocharger cooling. Recovered heat is
generally in the form of hot water or low pressure steam (<30 psig). The high temperature
exhaust can generate medium pressure steam (up to about 150 psig), but the hot exhaust gas
contains only about one half of the available thermal energy from a reciprocating engine. Some
industrial CHP applications use the engine exhaust gas directly for process drying. Generally,
the hot water and low pressure steam produced by reciprocating engine CHP systems is
appropriate for low temperature process needs, space heating, potable water heating, and to drive
absorption chillers providing cold water, air conditioning, or refrigeration.
Technology Characterization
2
Reciprocating Engines
-------
There were an estimated 1,055 engine-based CHP systems operating in the United States in 2000
representing over 800 MW of electric capacity. Facility capacities range from 30 kW to 30
MW, with many larger facilities comprised of multiple units. Figure 1 shows the variety of
applications using reciprocating engine CHP. Spark ignited engines fueled by natural gas or
other gaseous fuels represent 84% of the installed reciprocating engine CHP capacity.
Figure 1. Existing Reciprocating Engine CHP - 801 MW at 1,055 sites
Other
Industrial
155 MW
Univers ities
100 MW
Hospitals
95 MW
Source: PA Consulting, Energy Nexus Group
Thermal loads most amenable to engine-driven CHP systems in commercial/institutional
buildings are space heating and hot water requirements. The simplest thermal load to supply is
hot water. Figure 1 shows the primary applications for CHP in the commercial/institutional
sectors are those building types with relatively high and coincident electric and hot water
demand such as colleges and universities, hospitals, nursing homes, and lodging. If space
heating needs are incorporated, office buildings, certain warehousing, and mercantile/service
applications can be economic applications for CHP. Technology development efforts targeted at
heat activated cooling/refrigeration and thermally regenerated desiccants expand the application
of engine-driven CHP by increasing the thermal energy loads in certain building types. Use of
CHP thermal output for absorption cooling and/or desiccant dehumidification could increase the
size and improve the economics of CHP systems in existing CHP markets such as schools,
lodging, nursing homes, and hospitals. Use of these advanced technologies in applications such
as restaurants, supermarkets, and refrigerated warehouses provides a base thermal load that
opens these applications to CHP.
2 PA Consulting Independent Power Database, Energy Nexus Group
Chemicals
Processing
36 MW
Office
Buildings
57 MW
Food
Processing
79 MW
Water
Treatment
92 MW
Other
Commercial
186 MW
Technology Characterization
3
Reciprocating Engines
-------
A typical commercial application for reciprocating engine CHP is a hospital or health care
facility with a 1 MW CHP system comprised of multiple 200 to 300 kW natural gas engine
gensets. The system design satisfies the baseload electric needs of the facility. Approximately
1.6 MW thermal (MWth) of hot water is recovered from engine exhaust and engine cooling
systems to provide space heating and domestic hot water to the facility, and to drive absorption
chillers for space conditioning during summer months. Overall efficiency of this type of CHP
system can exceed 70%.
Figure 1 shows industry uses engine-driven CHP in a variety of applications where hot water or
low-pressure steam is required. A typical industrial application for engine CHP would be a food
processing plant with a 2 MW natural gas engine-driven CHP system comprised of multiple 500
to 800 kW engine gensets. The system provides baseload power to the facility and
approximately 2.2 MWj, low-pressure steam for process heating and washdown. Overall
efficiency for a CHP system of this type approaches 75%.
Technology Description
Basic Engine Processes
There are two primary reciprocating engine designs relevant to stationary power generation
applications - the spark ignition Otto-cycle engine and the compression ignition Diesel-cycle
engine. The essential mechanical components of the Otto-cycle and Diesel-cycle are the same.
Both use a cylindrical combustion chamber in which a close fitting piston travels the length of
the cylinder. The piston connects to a crankshaft that transforms the linear motion of the piston
into the rotary motion of the crankshaft. Most engines have multiple cylinders that power a
single crankshaft.
The primary difference between the Otto and Diesel cycles is the method of igniting the fuel.
Spark ignition engines (Otto-cycle) use a spark plug to ignite a pre-mixed air fuel mixture
introduced into the cylinder. Compression ignition engines (Diesel-cycle) compress the air
introduced into the cylinder to a high pressure, raising its temperature to the auto-ignition
temperature of the fuel that is injected at high pressure.
Engines are further categorized by crankshaft speed (rpm), operating cycle (2- or 4-stroke), and
whether turbocharging is used. Reciprocating engines are also categorized by their original
design purpose - automotive, truck, industrial, locomotive, and marine. Hundreds of small-scale
stationary power, CHP, irrigation, and chiller applications use automotive engine models. These
are generally low-priced engines due to large production volumes. However, unless
conservatively rated, these engines have limited durability. Truck engines have the cost benefit
of production volume and a reasonably long life (e.g., one million miles). A number of truck
engines are available as stationary engines. Engines intended for industrial use are designed for
durability and for a wide range of mechanical drive and electric power applications. Their sizes
range from 20 kW up to 6 MW, including industrialized truck engines in the 200 to 600 kW
range and industrially applied marine and locomotive engines above 1 MW.
Both the spark ignition and the diesel 4-stroke engines most relevant to stationary power
generation applications complete a power cycle in four strokes of the piston within the cylinder:
Technology Characterization
4
Reciprocating Engines
-------
1. Intake stroke - introduction of air (diesel) or air-fuel mixture (spark ignition) into the
cylinder.
2. Compression stroke - compression of air or an air-fuel mixture within the cylinder. In diesel
engines, the fuel is injected at or near the end of the compression stroke (top dead center or
TDC), and ignited by the elevated temperature of the compressed air in the cylinder. In spark
ignition engines, the compressed air-fuel mixture is ignited by an ignition source at or near
TDC.
3. Power stroke - acceleration of the piston by the expansion of the hot, high pressure
combustion gases, and
4. Exhaust stroke - expulsion of combustion products from the cylinder through the exhaust
port.
Types of Reciprocating Engines
Natural Gas Spark Ignition Engines - Spark ignition engines use spark plugs, with a high-
intensity spark of timed duration, to ignite a compressed fuel-air mixture within the cylinder.
Natural gas is the predominant spark ignition engine fuel used in electric generation and CHP
applications. Other gaseous and volatile liquid fuels, ranging from landfill gas to propane to
gasoline, can be used with the proper fuel system, engine compression ratio and tuning.
American manufacturers began to develop large natural gas engines for the burgeoning gas
transmission industry after World War II. Smaller engines were developed (or converted from
diesel blocks) for gas gathering and other stationary applications as the natural gas infrastructure
developed. Natural gas engines for power generation applications are primarily 4-stroke engines
available in sizes up to about 5 MW.
Depending on the engine size, one of two ignition techniques ignites the natural gas:
• Open chamber - the spark plug tip is exposed in the combustion chamber of the cylinder,
directly igniting the compressed fuel-air mixture. Open chamber ignition is applicable to any
engine operating near the stoichiometric air/fuel ratio up to moderately lean mixtures.3
• Precombustion chamber - a staged combustion process where the spark plug is housed in a
small chamber mounted on the cylinder head. This cylinder charges with a rich mixture of
fuel and air, which upon ignition shoots into the main combustion chamber in the cylinder as
a high energy torch. This technique provides sufficient ignition energy to light off lean fuel-
air mixtures used in large bore engines.4
3 Stoichiometric ratio is the chemically correct ratio of fuel to air for complete combustion, i.e., there is no unused
fuel or oxygen after combustion.
4 Lean mixture is a mixture of fuel and air in which an excess of air is supplied in relation to the amount needed for
complete combustion; similarly, a rich mixture is a mixture of fuel and air in which an excess of fuel is supplied in
relation to the amount needed for complete combustion.
Technology Characterization
5
Reciprocating Engines
-------
The simplest natural gas engines operate with natural aspiration of air and fuel into the cylinder
(via a carburetor or other mixer) by the suction of the intake stroke. High performance natural
gas engines are turbocharged to force more air into the cylinders. Natural gas spark ignition
engines operate at modest compression ratios (compared with diesel engines) in the range of 9:1
to 12:1 depending on engine design and turbocharging. Modest compression is required to
prevent auto-ignition of the fuel and engine knock, which can cause serious engine damage.5
Using high energy ignition technology, lean fuel-air mixtures can be burned in natural gas
engines, lowering peak temperatures within the cylinders and resulting in reduced NOx
emissions. The lean burn approach in reciprocating engines is analogous to dry low-NOx
combustors in gas turbines. All major natural gas engine manufacturers offer lean burn, low
emission models and are engaged in R&D to further improve their performance.
Natural gas spark ignition engine efficiencies are typically lower than diesel engines because of
their lower compression ratios. However, large, high performance lean burn engine efficiencies
approach those of diesel engines of the same size. Natural gas engine efficiencies range from
about 28% (LHV) for small engines (<50 kW) to 42% (LHV) for the largest high performance,
lean burn engines. Lean burn engines tuned for maximum efficiency may produce twice the NOx
emissions as the same engine tuned for minimum NOx. Tuning for low NOx typically results in a
sacrifice of 1 to 1.5 percentage points in electric generating efficiency from the highest level
achievable.
Many natural gas spark ignition engines are derived from diesel engines, i.e., they use the same
block, crankshaft, main bearings, camshaft, and connecting rods as the diesel engine. However,
natural gas spark ignition engines operate at lower brake mean effective pressure (BMEP) and
peak pressure levels to prevent knock.6 Due to the derating effects from lower BMEP, the spark
ignition versions of diesel engines often produce only 60 to 80% of the power output of the
parent diesel. Manufacturers often enlarge cylinder bore about 5 to 10% to increase the power,
but this is only partial compensation for the derated output. Consequently, the $/kW capital
costs of natural gas spark ignition engines are generally higher than the diesel engines from
which they were derived. However, by operating at lower cylinder pressure and bearing loads as
well as in the cleaner combustion environment of natural gas, spark ignition engines generally
offer the benefits of extended component life compared to their diesel parents.
Diesel Engines - Compression ignition diesel are among the most efficient simple-cycle power
generation options on the market. Efficiency levels increase with engine size and range from
about 30% (HHV) for small high-speed diesels up to 42 to 48% (HHV) for the large bore, slow
speed engines. By 2006, it is expected that efficiencies will improve to a maximum of 52%
(HHV). High-speed diesel engines (1,200 rpm) are available up to about 4 MW in size. Low
speed diesels (60 to 275 rpm) are available as large as 65 MW.
5 Knock is produced by explosive auto-ignition of a portion of the fuel in the cylinder due to compression and
heating of the gas mixture ahead of the flame front. The term knock and detonation are often used interchangeably.
6 Brake mean effective pressure (BMEP) can be regarded as the "average" cylinder pressure on the piston during the
power stroke and is a measure of the effectiveness of engine power output or mechanical efficiency.
Technology Characterization
6
Reciprocating Engines
-------
Diesel engines typically require compression ratios of 12:1 to 17:1 to heat the cylinder air to a
temperature at which the injected fuel will ignite. The quality of fuel injection significantly
affects diesel engine operating characteristics, fuel efficiency, and emissions. Fine atomization
and good fuel dispersion by the injectors are essential for rapid ignition, ideal combustion and
emissions control. Manufacturers are increasingly moving toward electronically controlled,
high-pressure injection systems that provide more precise calibration of fuel delivery and
accurate injection timing.
Depending on the engine and fuel quality, diesel engines produce 5 to 20 times the NOx (on a
ppmv basis) of a lean burn natural gas engine. Emergency generators and marine engines often
emit over 20 lb NOx/MWh and present on road engines emit less than 13 lb NOx/MWh. New
diesel engines using low sulfur diesel will achieve rates of approximately 0.65 lb NOx/MWh.
Diesel engines also produce assorted heavy hydrocarbons and particulate emissions. However,
diesel engines produce significantly less CO than lean burn gas engines. The NOx emissions
from diesels burning heavy oil are typically 25 to 30% higher than diesels using distillate oil.
Common NOx control techniques include delayed fuel injection, exhaust gas recirculation, water
injection, fuel-water emulsification, inlet air cooling, intake air humidification, and compression
ratio and/or turbocharger modifications. In addition, an increasing number of larger diesel
engines are equipped with selective catalytic reduction and oxidation catalyst systems for post-
combustion emissions reduction.
High-speed diesel engines generally require high quality fuel oil with good combustion
properties. No. 1 and No. 2 distillate oil comprise the standard diesel fuels. Low sulfur distillate
is preferred to minimize SO2 emissions. High-speed diesels are not suited to burning oil heavier
than distillate. Heavy fuel oil requires more time for combustion and the combination of high
speed and contaminants in lower quality heavy oils cause excessive wear in high-speed diesel
engines. Many medium and low speed diesels designs burn heavier oils including low grade
residual oils or Bunker C oils.
Dual Fuel Engines - Dual fuel engines are diesel compression ignition engines predominantly
fueled by natural gas with a small percentage of diesel oil as the pilot fuel. The pilot fuel auto-
ignites and initiates combustion in the main air-fuel mixture. Pilot fuel percentages can range
from 1 to 15% of total fuel input. Dual fuel operation is a combination of Diesel and Otto cycle
operation, with reduction in the percentage of pilot fuel used it approaches the Diesel cycle more
closely. Most dual fuel engines can be switched back and forth on the fly between dual fuel and
100%) diesel operation. In general, because of lower diesel oil usage, NOx, smoke, and
particulate emissions are lower for dual fuel engines than for straight diesel operation—
particularly at full load. Particulate emissions reduce in line with the percentage reduction in
diesel oil consumption while the level of NOx reduction depends on combustion characteristics
(see Emissions section). However, CO and unburned hydrocarbon emissions are often higher,
partly because of incomplete combustion.
There are three basic types of dual fuel engines:
Conventional low pressure gas injection engines typically require about 5 to 10%> pilot fuel and
may be derated to about 80 to 95% of the rated diesel capacity to avoid detonation. The
Technology Characterization
7
Reciprocating Engines
-------
turndown ratio of the diesel fuel injection system sets the minimum pilot fuel requirement.
Conventional diesel injectors cannot reliably turn down to less than 5 to 6% of the full-load
injection rate. Natural gas input is controlled at each cylinder by injecting gas before the air
intake valves open. NOx emissions of conventional dual fuel engines are generally in the 5 to 8
gm/kWh range (compared to lean burn natural gas engines with NOx emissions in the 0.7 to 2.5
gm/kWh range).
High pressure gas injection engines attempt to reduce derating by injecting natural gas at high
pressures (3,600 to 5,100 psig) directly into the main combustion chamber as the pilot fuel is
injected. However, the parasitic power for gas compression can be as high as 4 to 7% of the
rated power output - partly offsetting the benefit of reduced derating. This technology has not
proved particularly popular because of this issue and the additional equipment costs required for
gas injection. Pilot fuel consumption is typically 3 to 8% and NOx emissions are generally in the
5 to 8 gm/kWh range.
Micropilot prechamber engines are similar to spark ignition prechamber engines in that the pilot
fuel injected into a prechamber provides a high-energy torch that ignites the lean, compressed
fuel air mixture in the cylinder. Leaner mixtures than spark ignition engines are achievable since
the energy provided by the diesel-fueled micropilot chamber is higher than that obtained with a
spark ignition prechamber. Micropilot dual fuel engines with 1% pilot fuel can operate at or
close to the diesel engine's compression ratio and BMEP, so little, if any, derating occurs. In
this case the high power density and low $/kW cost advantage of the original diesel engine are
retained and engine efficiency at 75 to 100% load is close to that of the 100% diesel engine.
NOxand other emissions are comparable to those of lean burn spark ignition prechamber engines
(NOx emissions in the 0.7 to 2.5 gm/kWh range). These engines must be equipped with
conventional diesel fuel injectors in order to operate on 100% diesel.
Several independent developers and engine manufacturers are testing and commercializing dual
fuel retrofit kits for converting existing diesel engines to dual fuel operation. The level of
sophistication of these kits varies widely and some require major engine modifications.
Derating, efficiencies, and emissions also vary widely and have yet to be fully tested or certified.
However, dual fuel conversions are unlikely to be as low in emissions as dedicated natural gas
engines. In addition, manufacturers may not honor warrantees on an engine retrofitted by an
independent third party.
Engine Speed Classifications - Reciprocating engines are classified as high-, medium-, or low-
speed. Table 1 presents the standard speed ranges in each class and the types and sizes of
engines available. Engine driven electric generators typically must run at fixed (or synchronous)
speeds to maintain a constant 50 or 60 Hertz (Hz) output, setting the engine speed needed within
the classifications (i.e., a 60 Hz generator driven by a high speed engine would require engine
speeds of 1200, 1800, or 3600 rpm versus a 50 Hz generator which requires engine speeds of
1000, 1500, or 3000 rpm)
Technology Characterization
8
Reciprocating Engines
-------
Table 1. Reciprocating Engine Types by Speed (Available MWRatings)
Speed
Classification
l-nuine
Speed,
rpm
Stoic Rich
IJiirii. Spark
lunilion
l.ean Burn.
Spark lunilion
Dual I'nel
Diesel
High Speed
1,000-3,600
0.01-1.5 MW
0.15 -3.0 MW
1.0-3.5 MW8
0.01-3.5 MW
Medium Speed
275-1,000
None
1.0 -6.0 MW
1.0 - 25 MW
0.5 - 35 MW
Low Speed
58-275
None
None
2.0 - 65 MW
2-65 MW
Source: SFA Pacific, Inc.
Engine power output is proportional to engine speed, affording high-speed engines the highest
output per unit of displacement (cylinder size) and the highest power density. Consequently,
high-speed engines generally have the lowest $/kW production costs of the three types. The cost
benefits of high-speed engines must be weighed against other factors. Smaller high-speed
engines tend to have lower efficiencies than large bore, lower speed engines due in part to the
higher surface area to volume ratio for small cylinders resulting in slightly higher heat losses. In
addition, higher speed engines tend to have higher wear rates, resulting in shorter periods
between minor and major overhauls. These factors are often less important than capital costs for
limited duty cycle applications.
Medium speed stationary power engines are largely derived from marine and locomotive
engines. Medium-speed engines are higher in cost, but generally higher in efficiency than high-
speed engines. Because of their massive physical size and speed-related power reduction, low-
speed engines are increasingly being displaced by medium and high speed engines as the primary
choice for stationary power applications.
Load Service Ratings - Reciprocating engine manufacturers typically assign three power ratings
to engines depending on the intended load service:
• Standby - continuous full or cycling load for a relatively short duration (usually less than
100 hours) - maximum power output rating
• Prime - continuous operation for an unlimited time (except for normal maintenance
shutdowns), but with regular variations in load - 80 to 85% of the standby rating
• Baseload - continuous full-load operation for an unlimited time (except for normal
maintenance shutdowns) - 70 to 75% of the standby rating.
7 Stoichiometric or rich burn combustion is required for the use of 3 -way catalytic converters for emissions control.
8 Micropilot, prechamber dual fuel engines
Technology Characterization
9
Reciprocating Engines
-------
Design Characteristics
The features that have made reciprocating engines a leading prime mover for CHP and other
distributed generation applications include:
Size range:
Reciprocating engines are available in sizes from 10 kW to over 5
MW.
Thermal output:
Reciprocating engines can produce hot water and low-pressure
steam.
Fast start-up:
The fast start-up capability of reciprocating engines allows timely
resumption of the system following a maintenance procedure. In
peaking or emergency power applications, reciprocating engines
can quickly supply electricity on demand.
Black-start capability:
Availability:
Part-load operation:
Reliability and life:
Emissions:
In the event of an electric utility outage, starting reciprocating
engines requires minimal auxiliary power. Generally only
batteries are required.
Reciprocating engines have typically demonstrated availability in
excess of 95% in stationary power generation applications.
The high part-load efficiency of reciprocating engines ensures
economical operation in electric load following applications
Reciprocating engines have proven to be reliable power generators
given proper maintenance.
Diesel engines have relatively high emissions levels of NOx and
particulates. However, natural gas spark ignition engines have
improved emissions profiles and are easier to site.
Performance Characteristics
Electrical Efficiency
Table 2 summarizes performance characteristics for typical commercially available natural gas
spark ignition engine CHP systems over a 100 kW to 5 MW size range. This size range covers
the majority of the market applications for engine-driven CHP. Heat rates and efficiencies
shown were taken from manufacturers' specifications and industry publications. Available
thermal energy was calculated from published engine data on engine exhaust temperatures and
engine jacket and lube system coolant flows. CHP thermal recovery estimates are based on
producing hot water. As shown in the table, 50 to 60% of the waste heat from engine systems is
recovered from jacket cooling water and lube oil cooling systems at a temperature too low to
produce steam. This feature is generally less critical in commercial/institutional applications
where it is more common to have hot water thermal loads. Steam can be produced from the
Technology Characterization
10
Reciprocating Engines
-------
exhaust heat if required (maximum pressure of 150 psig), but if no hot water is needed, the
amount of heat recovered from the engine is reduced and total CHP system efficiency drops
accordingly.
The data in the table show that electrical efficiency increases as engine size becomes larger. As
electrical efficiency increases, the absolute quantity of thermal energy available to produce
useful thermal energy decreases per unit of power output, and the ratio of power to heat for the
CHP system generally increases. A changing ratio of power to heat impacts project economics
and may affect the decisions that customers make in terms of CHP acceptance, sizing, and the
desirability of selling power.
Technology Characterization
11
Reciprocating Engines
-------
Table 2. Gas Engine CHP - Typical Performance Parameters *
Cost & Performance Characteristics '
Svslcni
1
System
System
Svslcni
" 4
System
5
Baseload Electric Capacity (kW)
100
300
800
3,000
5,000
Total Installed Cost (2001 $/kW),n
$1,515
$1,200
$1000
$920
$920
Electric Heat Rate (Btu/kWh), IIIIV
11,147
10,967
10,246
9,492
8,758
Electrical Efficiency (%), HHV
30.6%
31.1%
33.3%
36.0%
39.0%
Engine Speed (rpm)
1,800
1,800
1,200
900
720
Fuel Input (MMBtu/hr)
1.11
3.29
8.20
28.48
43.79
Required Fuel Gas Pressure (psig)
<3
<3
<3
43
65
CHP Characteristics
Exhaust Flow (1000 lb/hr)
1.0
3.3
10.9
48.4
67.1
Exhaust Temperature (Fahrenheit)
1,060
1,067
869
688
698
Heat Recovered from Exhaust
(MMBtu/hr)
0.20
0.82
2.12
5.54
7.16
Heat Recovered from Cooling Jacket
(MMBtu/hr)
0.37
0.69
1.09
4.37
6.28
Heat Recovered from Lube System
(MMBtu/hr)
0
0
0.29
1.22
1.94
Total Heat Recovered (MMBtu/hr)
0.57
1.51
3.50
11.12
15.38
Total Heat Recovered (kW)
167
443
1,025
3,259
4,508
Form of Recovered Heat
Hot H20
Hot H20
Hot H20
Hot H20
Hot H20
Total Efficiency (%)12
81%
77%
76%
75%
74%
Power/Heat Ratio13
0.60
0.68
0.78
0.92
1.11
Net Heat Rate (Btus/kWh)14
4,063
4,687
4,774
4,857
4,914
Effective Electrical Efficiency15
0.84
0.73
0.71
0.70
0.69
* For typical systems commercially available in 2001
Source: Energy Nexus Group9
9 Characteristics for "typical" commercially available natural gas engine gensets. Data based on: MAN 150 kW -
100 kW; Cummins GSK19G - 300 kW; Caterpillar G3516 LE - 800 kW; Caterpillar G3616 LE - 3 MW; Wartsila
5238 LN - 5 MW; Energy use and exhaust flows normalized to nominal system sizes.
10 Installed costs based on CHP system producing hot water from exhaust heat recovery (250 °F exhaust from heat
recovery heat exchanger), and jacket and lube system cooling
11 All engine manufacturers quote heat rates in terms of the lower heating value (LHV) of the fuel. However the
purchase price of fuels on an energy basis is typically measured on a higher heating value basis (HHV). For natural
gas, the average heat content of natural gas is 1,030 Btu/kWh on an HHV basis and 930 Btu/kWh on an LHV basis -
or about a 10% difference.
12 Total CHP Efficiency = (net electric generated + net thermal energy recovered)/total engine fuel input
13 Power/Heat Ratio = (CHP electric power output (Btus))/useful thermal output (Btus)
14 Net Heat Rate = (Total fuel input to the CHP system - the fuel that would be normally used to generate the same
amount of thermal output as the CHP system thermal output assuming an efficiency of 80%)/CHP electric output
(kW).
15 Effective Electrical Efficiency = (CHP electric power output)/(Total fuel into CHP system - total heat
recovered/0.8); Equivalent to 3,412 Btu/kWh/Net Heat Rate
Technology Characterization
12
Reciprocating Engines
-------
Part-Load Performance
In power generation and CHP applications, reciprocating engines generally drive synchronous
generators at constant speed to produce steady alternating current (AC) power. At reduced loads,
the heat rate of spark ignition engines increases and efficiency decreases. Figure 1 shows the
part-load efficiency curve for a typical lean burn natural gas engine. The efficiency at 50% load
is approximately 8 to 10% less than full-load efficiency. As the load decreases further, the curve
becomes somewhat steeper. While gas engines compare favorably to gas turbines, which
typically experience efficiency decreases of 15 to 25% at half-load conditions, multiple engines
may be preferable to a single large unit to avoid efficiency penalties where significant load
reductions are expected on a regular basis. Diesel engines exhibit even more favorable part-load
characteristics than spark ignition engines. The efficiency curve for diesel engines is
comparatively flat between 50 and 100% load.
Figure 1. Part-Load Efficiency Performance
40%
38%
> 36%
30%
28%
20%
30%
40%
50%
60%
70%
80%
90%
100% 110%
Percent Load (%)
Source: Caterpillar, Energy Nexus Group
Effects of Ambient Conditions on Performance
Reciprocating engines are generally rated at ISO conditions of 77°F and 0.987 atmospheres (1
bar) pressure. Like gas turbines, reciprocating engine performance - both output and efficiency
- degrades as ambient temperature or site elevation increases. While the effect on gas turbines
can be significant, it is less so on engines. Reciprocating engine efficiency and power are
reduced by approximately 4% per 1,000 feet of altitude above 1,000 feet, and about 1% for every
10°F above 77°F.
Technology Characterization
13
Reciprocating Engines
-------
Heat Recovery
The economics of engines in on-site power generation applications often depend on effective use
of the thermal energy contained in the exhaust gas and cooling systems, which generally
represents 60 to 70% of the inlet fuel energy. Most of the waste heat is available in the engine
exhaust and jacket coolant, with smaller amounts recoverable from the lube oil cooler and the
turbocharger's intercooler and aftercooler (if so equipped). The most common use of this heat is
to generate hot water or low-pressure steam for process use or for space heating, process needs,
domestic hot water or absorption cooling. However, the engine exhaust gases can also be used
as a source of direct energy for drying or other direct heat processes.
Heat in the engine jacket coolant accounts for up to 30% of the energy input and is capable of
producing 200 to 210°F hot water. Some engines, such as those with high pressure or ebullient
cooling systems, can operate with water jacket temperatures up to 265°F. Engine exhaust heat
represents from 30 to 50% of the available waste heat. Exhaust temperatures of 850 to 1200°F
are typical. By recovering heat in the cooling systems and exhaust, approximately 70 to 80% of
the energy of the fuel is effectively utilized to produce both power and useful thermal energy..
Closed-loop cooling systems - Figure 2 shows the most common method of recovering engine
heat, closed-loop cooling system. These systems cool the engine by forced circulation of a
coolant through engine passages and an external heat exchanger. An excess heat exchanger
transfers engine heat to a cooling tower or radiator when there is excess heat generated. Closed-
loop water cooling systems can operate at coolant temperatures from 190 to 250°F. Depending
on the engine and CHP system's requirements, the lube oil cooling and turbocharger aftercooling
may be either separate or part of the jacket cooling system.
Figure 2. Closed-Loop Heat Recovery System
Customer Heat
Exchanger
Exhaust
Engine
G ear
Box
A/
Excess Heat
Exchanger
0 il Cooler
Jacket W ater
H eat
R ecovery
Ebullient Cooling Systems - Ebullient cooling systems cool the engine by natural circulation of a
boiling coolant through the engine. This type of cooling system is typically used in conjunction
with exhaust heat recovery for production of low-pressure steam. Cooling water is introduced at
the bottom of the engine where the transferred heat begins to boil the coolant generating two-
Technology Characterization
14
Reciprocating Engines
-------
phase flow. The formation of bubbles lowers the density of the coolant, causing a natural
circulation to the top of the engine.
The coolant at the engine outlet is maintained at saturated steam conditions and is usually limited
to 250°F and a maximum of 15 psig. Inlet cooling water is also near saturation conditions and is
generally 2 to 3°F below the outlet temperature. The uniform temperature throughout the
coolant circuit extends engine life and contributes to improved combustion efficiencies.
Exhaust Heat Recovery - Recovered exhaust heat generates hot water to about 230°F or low-
pressure steam (up to 150 psig). To prevent the corrosive effects of condensation in exhaust
piping exhaust gas temperatures are generally kept above temperature thresholds, recovering
only a portion of the exhaust heat. For this reason, most heat recovery units use a 250 to 350°F
exhaust outlet temperature.
Exhaust heat recovery can be independent of the engine cooling system or coupled with it. For
example, hot water from the engine cooling can be used as feedwater or feedwater preheat to the
exhaust recovery unit. In a typical district heating system, jacket cooling, lube oil cooling,
single stage aftercooling and exhaust gas heat recovery are all integrated for steam production.
Performance and Efficiency Enhancements
BMEP and Engine Speed
Engine speed and the BMEP during the power stroke relate to engine power. BMEP is an
"average" cylinder pressure on the piston during the power stroke, and is a measure of the
effectiveness of engine power output or mechanical efficiency. Engine manufacturers often
include BMEP values in their product specifications. Typical BMEP values are as high as 230
psig for large natural gas engines and 350 psig for diesel engines. Corresponding peak
combustion pressures are about 1,750 psig and 2,600 psig respectively. High BMEP levels
increase power output, improve efficiency, and result in lower specific costs ($/kW).
BMEP can be increased by raising combustion cylinder air pressure through increased
turbocharging, improved aftercooling, and reduced pressure losses through improved air passage
design. These factors all increase air charge density and raise peak combustion pressures,
translating into higher BMEP levels. However, higher BMEP increases thermal and pneumatic
stresses within the engine, and proper design and testing is required to ensure continued engine
durability and reliability.
Turbocharging
Essentially all modern engines above 300 kW are turbocharged to achieve higher power
densities. A turbocharger is a turbine-driven intake air compressor. The hot, high velocity
exhaust gases leaving the engine cylinders power the turbine. Large engines typically are
equipped with two turbochargers. On a carbureted engine, turbocharging forces more air and
fuel into the cylinders, increasing engine output. On a fuel-injected engine, the mass of fuel
injected increases in proportion to the increased air input. Cylinder pressure and temperature
normally increase as a result of turbocharging, increasing the tendency for detonation for both
Technology Characterization
15
Reciprocating Engines
-------
spark ignition and dual fuel engines and requiring a careful balance between compression ratio
and turbocharger boost level. Turbochargers normally boost inlet air pressure on a 3:1 to 4:1
ratio. A wide range of turbocharger designs and models are used. Heat exchangers (called
aftercoolers or intercoolers) cool the discharge air from the turbocharger to keep the temperature
of the air to the engine under a specified limit.
Capital Cost
This section provides typical study estimates for the installed cost of natural gas spark-ignited,
reciprocating engine-driven generators. Two configurations are presented: power-only and
CHP. Capital costs (equipment and installation) are estimated for the five typical engine genset
systems ranging from 100 kW to 5 MW for each configuration. These are "typical" budgetary
price levels; it should also be noted that installed costs vary significantly depending on the scope
of the plant equipment, geographical area, competitive market conditions, special site
requirements, emissions control requirements, prevailing labor rates, and whether the system is a
new or retrofit application.
In general, engine gensets do not show typical economies of scale when costing industrial
equipment of different sizes. Smaller genset packages are typically less costly on a unit cost
basis ($/kW) than larger gensets. Smaller engines typically run at a higher RPM than larger
engines and often are adapted from higher volume production runs from other markets such as
automotive or truck engines. These two factors combine to make the engine package costs lower
than the larger, slow-speed engines.
The basic genset package consists of the engine connected directly to a generator without a
gearbox. In countries where 60 Hz power is required, the genset operates at multiples of 60 -
typically 1,800 rpm for smaller engines and 900 or 720 rpm for the large engines. In areas where
50 Hz power is used such as Europe and Japan, the engines run at speeds that are multiples of 50
- typically 1,500 rpm for the small engines. The smaller engines are skid mounted with a basic
control system, fuel system, radiator, fan, and starting system. Some smaller packages come
with an enclosure, integrated heat recovery system, and basic electric paralleling equipment. The
cost of the basic engine genset package plus the costs for added systems needed for the particular
application comprise the total equipment cost. The total plant cost consists of total equipment
cost plus installation labor and materials (including site work), engineering, project management
(including licensing, insurance, commissioning and startup), and financial carrying costs during
the 6 to 18 month construction period.
Table 3 provides cost estimates for combined heat and power applications. The CHP system is
assumed to produce hot water, although the multi-megawatt size engines are capable of
producing low-pressure steam. The heat recovery equipment consists of the exhaust silencer that
extracts heat from the exhaust system, process heat exchanger for extracting heat from the engine
jacket coolant, circulation pump, control system, and piping. These cost estimates include
interconnection and paralleling. The package costs reflect a generic representation of popular
engines in each size category. The engines all have low emission, lean-burn technology with the
exception of the 100 kW system, which is a rich burn engine that would require a three-way
catalyst in most urban installations. The interconnect/electrical costs reflect the costs of
paralleling a synchronous generator, though many 100 kW packages available today use
Technology Characterization
16
Reciprocating Engines
-------
induction generators that are simpler and less costly to parallel.16 Labor/materials represent the
labor cost for the civil, mechanical, and electrical work and materials such as ductwork, piping,
and wiring. Project and construction management also includes general contractor markup and
bonding and performance guarantees. Contingency is assumed to be 3% of the total equipment
cost in all cases.
Table 3. Estimated Capital Cost for Typical Gas Engine Generators in Grid
Interconnected, Combined Heat and Power Application (SAW)
Cost Component
Svslcm
Svslcm
Svslcm
Svslcm
Svslcm
1
4
5
Nominal Capacity (kW)
100
300
800
3,000
5,000
Costs ($/kW)
Equipment
Gen Set Package
$260
$230
$269
$400
$450
Heat Recovery
$205
$179
$89
$65
$40
Interconnect/Electrical
$260
$90
$40
$22
$12
Total Equipment
$725
$499
$398
$487
$502
Labor/Materials
$359
$400
$379
$216
$200
Total Process Capital
$1,084
$899
$777
$703
$702
Project and Construction
$235
$158
$121
$95
$95
Management
Engineering and Fees
$129
$81
$45
$41
$41
Project Contingency
$43
$34
$28
$25
$25
Project Financing (interest
$24
$25
$31
$55
$55
during construction)
Total Plant Cost ($/kW)
$1,515
$1,197
$1,002
$919
$919
Source: Energy Nexus Group
Maintenance
Maintenance costs vary with type, speed, size and numbers of cylinders of an engine and
typically include:
• Maintenance labor.
• Engine parts and material such as oil filters, air filters, spark plugs, gaskets, valves,
piston rings, electronic components, etc. and consumables such as oil.
• Minor and major overhauls.
16 Reciprocating Engines for Stationary Power Generation: Technology, Products, Players, and Business Issues,
GRI, Chicago, IL and EPRIGEN, Palo Alto, CA: 1999. GRI-99/0271, EPRITR-113894.
Technology Characterization
17
Reciprocating Engines
-------
Maintenance can be either done by in-house personnel or contracted out to manufacturers,
distributors, or dealers under service contracts. Full maintenance contracts (covering all
recommended service) generally cost between 0.7 to 2.0 cents/kWh depending on engine size,
speed, and service. Many service contracts now include remote monitoring of engine
performance and condition and allow for predictive maintenance. Service contract rates
typically are all-inclusive, including the travel time of technicians on service calls.
Recommended service is comprised of routine short interval inspections/adjustments and
periodic replacement of engine oil and filter, coolant and spark plugs (typically 500 to 2,000
hours). An oil analysis is part of most preventative maintenance programs to monitor engine
wear. A top-end overhaul, generally recommended between 8,000 and 30,000 hours of operation
(see Table 4), entails a cylinder head and turbocharger rebuild. A major overhaul after 30,000 to
72,000 hours of operation involves piston/liner replacement, crankshaft inspection, bearings, and
seals (Table 4)
Table 4. Representative Overhaul Intervals for Natural Gas Engines in Baseload Service
Time liclwccn ()\ eihauls (Thousand Operating 1 loin s)
Engine Speed
720 rpm
900 rpm
1,200 rpm
1,500 rpm
1,800 rpm
Minor Overhaul
>30
15-36
24-36
10-20
8 - 15
Major Overhaul
>60
40-72
48-60
30-50
30-36
Source: SFA Pacific, Inc.
Table 5 presents maintenance costs based on engine manufacturer estimates for service contracts
consisting of routine inspections and scheduled overhauls of the engine generator set. Costs are
based on 8,000 annual operating hours expressed in terms of annual electricity generation.
Technology Characterization
18
Reciprocating Engines
-------
Table 5. Typical Natural Gas Engine Maintenance Costs *
Maintenance Costs'
System
1
System
System
System
4
System
5
100
0.017
0.00015
10
0.00125
300
0.012
0.00015
5
0.00063
800
0.009
0.00015
4
0.0005
3,000
0.009
0.00015
1.5
0.00019
5,000
0.009
0.00015
1.1
0.00014
Electricity Capacity, kW
Variable (service contract), $/kWh
Variable (consumables), $/kWh
Fixed. $/kW-vr
Fixed. S/kWh a, 8000 hrs/yr
Total O&M Costs. $/kWh
0.0184
0.0128
0.0097
0.0093
0.0093
* Typical maintenance costs for gas engine gensets 2001
Source: Energy Nexus Group
Fuels
Spark ignition engines operate on a variety of alternative gaseous fuels including:
• Liquefied petroleum gas (LPG) - propane and butane mixtures.
• Sour gas - unprocessed natural gas as it comes directly from the gas well
• Biogas - any of the combustible gases produced from biological degradation of organic
wastes, such as landfill gas, sewage digester gas, and animal waste digester gas
• Industrial waste gases - flare gases and process off-gases from refineries, chemical plants
and steel mill
• Manufactured gases - typically low- and medium-Btu gas produced as products of
gasification or pyrolysis processes.
Factors that impact the operation of a spark ignition engine with alternative gaseous fuels
include:
• Volumetric heating value - Since engine fuel is delivered on a volume basis, fuel volume
into the engine increases as heating value decreases, requiring engine derating on fuels
with very low Btu content. Derating is more pronounced with naturally aspirated
engines, and depending on air requirements turbocharging partially or totally
compensates.
17 Maintenance costs presented in Table 5 are based on 8,000 operating hours expressed in terms of annual
electricity generation. Fixed costs are based on an interpolation of manufacturers' estimates. The variable
component of the O&M cost represents the inspections and overhaul procedures that are normally conducted by the
prime mover original equipment manufacturer through a service agreement usually based on run hours.
Technology Characterization
19
Reciprocating Engines
-------
• Autoignition characteristics and detonation tendency
• Contaminants that may impact engine component life or engine maintenance, or result in
air pollutant emissions that require additional control measures.
• Hydrogen-containing fuels may require special measures (generally if hydrogen content
by volume is greater than 5%) because of hydrogen's unique flammability and explosion
characteristics.
Table 6 presents representative constituents of some of the alternative gaseous fuels compared to
natural gas. Industrial waste and manufactured gases are not included in the table because their
compositions vary widely depending on their source. They typically contain significant levels of
H2 and/or CO. Other common constituents are CO2, water vapor, one or more light
hydrocarbons, and H2S or SO2.
Table 6. Major Constituents of Gaseous Fuels
Natural
( MS
I.PCi
Digester
( MS
1 .andllll
( MS
Methane. C11,. ("0)
80 y7
u
35 05
40 00
Ethane, C2H6, (%)
3-15
0-2
0
0
Propane, C3H8, (%)
0-3
75-97
0
0
Butane.C4H|i). (%)
Higher CXHX, (%)
O
1
O
0-2
0
0
0- 0.2
0 -2018
0
0
C02, (%)
0-1.8
0
30-40
O
1
O
\l"
N2, (%)
0-14
0
1 -2
0 - 13
II . (%)
LHV, (Btu/scf)
0-0.1
830- 1,075
0
2.500
0
300-666
0
350 - 550
Source: SFA Pacific, Inc.; North American Combustion Handbook.
Contaminants are a concern with many waste fuels, specifically acid gas components (H2S,
halogen acids, HCN; ammonia; salts and metal-containing compounds; organic halogen-, sulfur-,
nitrogen-, and silicon-containing compounds); and oils. In combustion, halogen and sulfur
compounds form halogen acids, SO2, some SO3 and possibly H2SO4 emissions. The acids can
also corrode downstream equipment. A substantial fraction of any fuel nitrogen oxidizes into
NOx in combustion. To prevent corrosion and erosion of components solid particulates are kept
to low concentrations. Various fuel scrubbing, droplet separation and filtration steps will be
required if any fuel contaminant levels exceed manufacturer specifications. Landfill gas in
particular often contains chlorine compounds, sulfur compounds, organic acids and silicon
compounds, which dictate pretreatment.
18 High levels of heavier hydrocarbons are found in LPG derived from refinery processing
Technology Characterization
20
Reciprocating Engines
-------
Once treated and acceptable for use in the engine, emissions performance profiles on alternative
fuels are similar to natural gas engine performance. Specifically, the low emissions ratings of
lean burn engines can usually be maintained on alternative fuels.
LPG
LPG is composed primarily of propane and/or butane. Propane used in natural gas engines,
requires retarding of ignition timing and other appropriate adjustments. LPG often serves as a
back-up fuel where there is a possibility of interruption in the natural gas supply. LPG is
delivered as a vapor to the engine. LPG's use is limited in high-compression engines because of
its relatively low octane number. In general, LPG for engines contains 95% propane by volume
with an HHV of 2,500 Btu/scf, and with the remaining 5% lighter than butane. Off-spec LPG
may require cooling to condense out larger volumes of butane or heavier hydrocarbons.
High butane content LPG is recommended only for low compression, naturally aspirated
engines. Significantly retarded timing avoids detonation.
Field Gas
Field gas often contains more than 5% by volume of heavy ends (butane and heavier), as well as
water, salts and H2S and usually requires some scrubbing before use in natural gas engines.
Cooling may be required to reduce the concentrations of butane and heavier components. Field
gas usually contains some propane and normally is used in low compression engines (both
naturally aspirated and turbocharged). Retarded ignition timing eliminates detonation.
Biogas
Biogases (landfill gas and digester gas) are predominantly mixtures of methane and CO2 with
HHV in the range of 300 to 700 Btu/scf. Landfill gas also contains a variety of contaminants as
discussed earlier. Biogases are produced essentially at atmospheric pressure so must be
compressed for delivery to the engine. After compression, cooling and scrubbing or filtration are
required to remove compressor oil, condensate, and any particulates entrained in the original gas.
Scrubbing with a caustic solution may be required if acid gases are present. Because of the
additional requirements for raw gas treatment, biogas powered engine facilities are more costly
to build and operate than natural gas-based systems.
Industrial Waste Gases
Industrial waste gases that are common reciprocating engine fuels include refinery gases and
process off-gases. Refinery gases typically contain components such as H2, CO, light
hydrocarbons, H2S, and ammonia, as well as CO2 and N2. Process off-gases include a variety of
compositions. Generally, waste gases are medium- to low-Btu content. Medium-Btu gases
generally to not require significant engine derating; low Btu gases usually require derating.
Technology Characterization
21
Reciprocating Engines
-------
Depending on their origin and contaminants, industrial gases sometimes require pretreatment
comparable to that applied to raw landfill gas. Particulates (e.g., catalyst dust), oils, condensable
gases, water, C4+ hydrocarbons and acid gases may all need to be removed. Process offgases are
usually available at pressures of several atmospheres or higher, which are generally satisfactory
for delivery to an on-site or nearby reciprocating engine facility.
Availability
Reciprocating engines are maintenance intensive, but they can provide high levels of availability,
even in high load factor applications. While natural gas engine availabilities vary with engine
type, speed, and fuel quality, Table 7 illustrates typical availability numbers based on a survey of
natural gas engine gensets in CHP applications.
Table 7. Availabilities and Outage Rates for Natural Gas Engines
(ias I jiuincs
SO KDMkW
(ias 1'HUNK'S
Soi) k\\"
Availability Factor (%)
94.5
91.2
Forced Outage Rate (%)
4.7
6.1
Scheduled Outage Rate (%)
2.0
3.5
Source: GRI (Liss, 1999)
The use of multiple units or back-up units at a site can further increase the availability of the
overall facility. Some engine manufacturers offer engine exchange programs or other
maintenance options that increase the ability to promptly deliver and install replacement units on
short notice, typically increasing facility availabilities to greater than 95%.
Emissions
Exhaust emissions are the primary environmental concern with reciprocating engines. The
primary pollutants are oxides of nitrogen (NOx), carbon monoxide (CO), and volatile organic
compounds (VOCs - unburned, non-methane hydrocarbons). Other pollutants such as oxides of
sulfur (SOx) and particulate matter (PM) are primarily dependent on the fuel used. The sulfur
content of the fuel determines emissions of sulfur compounds, primarily SO2. Engines operating
on natural gas or desulfurized distillate oil emit insignificant levels of SOx. In general, SOx
emissions are an issue only in large, slow speed diesels firing heavy oils. Particulate matter
(PM) can be an important pollutant for engines using liquid fuels. Ash and metallic additives in
the fuel contribute to PM in the exhaust.
Technology Characterization
22
Reciprocating Engines
-------
Nitrogen Oxides (N0X)
N0X emissions are usually the primary concern with natural gas engines and are a mixture of
(mostly) NO and NO2 in variable composition. In measurement, NOx is reported as parts per
million by volume in which both species count equally (e.g., ppmv at 15% O2, dry). Other
common units for reporting NOx in reciprocating engines are gm/hp-hr and gm/kWh, or as an
output rate such as lbs/hr. Among engine options, lean burn natural gas engines produce the
lowest NOx emissions and diesel engines produce the highest (Table 8).
Table 8. Representative NOx Emissions from Reciprocating Engines
(w/o add on controls)
1 jluilK'S
I'ud
NO
(ppm\ )
NO
(urn k\Vh)
Diesel Engines (high speed &
medium speed)19
Distillate
450- 1,350
7-18
Diesel Engines (high speed &
medium speed)20
Heavy Oil
900- 1,800
12-20
Lean Burn, Spark Ignition
Engine21
Natural Gas
45 - 150
0.7-2.5
Source: SFA Pacific, Inc., Energy Nexus Group
Three mechanism form NOx: thermal NOx, prompt NOx, and fuel-bound NOx. The predominant
NOx formation mechanism associated with reciprocating engines is thermal NOx. Thermal NOx
is the fixation of atmospheric oxygen and nitrogen, which occurs at high combustion
temperatures. Flame temperature and residence time are the primary variables that affect thermal
NOx levels. The rate of thermal NOx formation increases rapidly with flame temperature. Early
reactions of nitrogen modules in the combustion air and hydrocarbon radicals from the fuel form
prompt NOx. It forms within the flame and typically is approximately 1 ppm at 15% O2, and is
usually much smaller than the thermal NOx formation. Fuel-bound NOx forms when the fuel
contains nitrogen as part of the hydrocarbon structure. Natural gas has negligible chemically
bound fuel nitrogen. Fuel-bound NOx can be at significant levels with liquid fuels.
The control of peak flame temperature through lean burn conditions has been the primary
combustion approach to limiting NOx formation in gas engines. Diesel engines produce higher
combustion temperatures and more NOx than lean burn gas engines, even though the overall
diesel engine air/fuel ratio may be lean. There are three reasons for this: (1) heterogeneous near-
stoichiometric combustion; (2) the higher adiabatic flame temperature of distillate fuel; and (3)
fuel-bound nitrogen. The diesel fuel is atomized as it is injected and dispersed in the combustion
chamber. Combustion largely occurs at near-stoichiometric conditions at the air-droplet and air-
fuel vapor interfaces, resulting in maximum temperatures and higher NOx. In contrast, lean-
19 Efficiency range: 37 to 44% LHV
20 Efficiency range: 42 to 48% LHV
21 Efficiency range: 35 to 42% LHV
Technology Characterization
23
Reciprocating Engines
-------
premixed homogeneous combustion used in lean burn gas engines results in lower combustion
temperatures and lower NOx production.
For any engine there are generally trade-offs between low NOx emissions and high efficiency.
There are also trade-offs between low NOx emissions and emissions of the products of
incomplete combustion (CO and unburned hydrocarbons). There are three main approaches to
these trade-offs that come into play depending on regulations and economics. One approach is to
control for lowest NOx accepting a fuel efficiency penalty and possibly higher CO and
hydrocarbon emissions. A second option is finding an optimal balance between emissions and
efficiency. A third option is to design for highest efficiency and use post-combustion exhaust
treatment.
Carbon Monoxide (CO)
CO and VOCs both result from incomplete combustion. CO emissions result when there is
inadequate oxygen or insufficient residence time at high temperature. Cooling at the combustion
chamber walls and reaction quenching in the exhaust process also contribute to incomplete
combustion and increased CO emissions. Excessively lean conditions can lead to incomplete
and unstable combustion and high CO levels.
Unburned Hydrocarbons
Volatile hydrocarbons also called volatile organic compounds (VOCs) can encompass a wide
range of compounds, some of which are hazardous air pollutants. These compounds discharge
into the atmosphere when some portion of the fuel remains unburned or just partially burned.
Some organics are carried over as unreacted trace constituents of the fuel, while others may be
pyrolysis products of the heavier hydrocarbons in the gas. Volatile hydrocarbon emissions from
reciprocating engines are normally reported as non-methane hydrocarbons (NMHCs). Methane
is not a significant precursor to ozone creation and smog formation and is not currently
regulated.
Carbon Dioxide (CO2)
While not considered a pollutant in the ordinary sense of directly affecting health, emissions of
carbon dioxide (CO2) are of concern due to its contribution to global warming. Atmospheric
warming occurs since solar radiation readily penetrates to the surface of the planet but infrared
(thermal) radiation from the surface is absorbed by the CO2 (and other polyatomic gases such as
methane, unburned hydrocarbons, refrigerants, water vapor, and volatile chemicals) in the
atmosphere, with resultant increase in temperature of the atmosphere. The amount of CO2
emitted is a function of both fuel carbon content and system efficiency. The fuel carbon content
of natural gas is 34 lbs carbon/MMBtu; oil is 48 lbs carbon/MMBtu; and (ash-free) coal is 66 lbs
carbon/MMBtu.
Technology Characterization
24
Reciprocating Engines
-------
Emissions Control Options
N0X control has been the primary focus of emission control research and development in natural
gas engines. The following provides a description of the most prominent emission control
approaches.
Combustion Process Emissions Control
Control of combustion temperature has been the principal focus of combustion process control in
gas engines. Combustion control requires tradeoffs - high temperatures favor complete burn up
of the fuel and low residual hydrocarbons and CO, but promote NOx formation. Lean
combustion dilutes the combustion process and reduces combustion temperatures and NOx
formation, and allows a higher compression ratio or peak firing pressures resulting in higher
efficiency. However, if the mixture is too lean, misfiring and incomplete combustion occur,
increasing CO and VOC emissions.
Lean burn engine technology developed during the 1980s as a direct response to the need for
cleaner burning gas engines. As discussed earlier, thermal NOx formation is a function of both
flame temperature and residence time. The focus of lean burn developments was to lower
combustion temperature in the cylinder using lean fuel/air mixtures. Lean combustion decreases
the fuel/air ratio in the zones where NOx is produced so that peak flame temperature is less than
the stoichiometric adiabatic flame temperature, therefore suppressing thermal NOx formation.
Most lean burn engines use turbocharging to supply excess air to the engine and produce the
homogeneous lean fuel-air mixtures. Lean burn engines generally use 50 to 100% excess air
(above stoichiometric). The typical emissions rate for lean burn natural gas engines is between
0.5 to 2.0 gm/bhph.
As discussed above, an added performance advantage of lean burn operation is higher output and
higher efficiency. Optimized lean burn operation requires sophisticated engine controls to
ensure that combustion remains stable and NOx reduction maximized while minimizing
emissions of CO and VOCs. Table 9 shows data for a large lean burn natural gas engine that
illustrates the tradeoffs between NOx emissions control and efficiency. At the lowest achievable
NOx levels (45 to 50 ppmv), almost 1.5 percentage points are lost on full rated efficiency.
Technology Characterization
25
Reciprocating Engines
-------
Table 9. N0X Emissions versus Efficiency Tradeoffs22
Limine ("ha i iicleri sli cs
Low \()..
1 liuli LlYidency
CapaciL\ ( \1\\)
?"?
s ">
Speed (rpm)
720
720
Efficiency, LHV (%)
40.7
42.0
Emissions
\() (gm kW'li_)
(ppmv @15% 02)
0.7
46
1.4
92
CO (gm/kWh)
(ppmv @15% 02)
3.2
361
2.0
227
NMHC (gm/kWh)
(ppmv @15% 02)
0.9
61
0.6
39
Combustion temperature can also be controlled to some extent in reciprocating engines by one or
more of the following techniques:
• Delaying combustion by retarding ignition or fuel injection.
• Diluting the fuel-air mixture with exhaust gas recirculation (EGR), which replaces some
of the air and contains water vapor that has a relatively high heat capacity and absorbs
some of the heat of combustion.
• Introducing liquid water by direct injection or via fuel oil emulsification - evaporation of
the water cools the fuel-air mixture charge.
• Reducing the inlet air temperature with a heat exchanger after the turbocharger or via
inlet air humidification.
• Modifying valve timing, compression ratio, turbocharging, and the combustion chamber
configuration.
Water injection and EGR reduce diesel engine NOx emissions 30 to 60% from uncontrolled
levels. The incorporation of water injection and other techniques to lean burn gas engines is the
focus of ongoing R&D efforts with several engine manufacturers and is being pursued as part of
the Department of Energy's Advanced Reciprocating Engine Systems (ARES) program. One of
the goals of the program is to develop a 45% efficient (HHV) medium sized natural gas engine
operating at 0.3 lb NOx/MWh (0.1 gm NOx/bhph)
22 Based on engine manufacturer's data - Wartsila 18V34SG Prechamber Lean Burn Gas Engine.
Technology Characterization
26
Reciprocating Engines
-------
Post-Combustion Emissions Control
There are several types of catalytic exhaust gas treatment processes that are applicable to various
types of reciprocating engines.
Three - Way Catalyst
The catalytic three-way conversion process (TWC) is the basic automotive catalytic converter
process that reduces concentrations of all three major criteria pollutants - NOx, CO and VOCs.
The TWC is also called non-selective catalytic reduction (NSCR). NOx and CO reductions are
generally greater than 90%, and VOCs are reduced approximately 80% in a properly controlled
TWC system. Because the conversions of NOx to N2 and CO and hydrocarbons to CO2 and H2O
will not take place in an atmosphere with excess oxygen (exhaust gas must contain less than
0.5% O2), TWCs are only effective with stoichiometric or rich-burning engines. Typical
"engine out" NOx emission rates for a rich burn engine are 10 to 15 gm/bhph. NOx emissions
with TWC control are as low as 0.15 gm/bhph.
Stoichiometric and rich burn engines have significantly lower efficiency than lean burn engines
(higher carbon emissions) and only certain sizes (<1.5 MW) and high speeds are available. The
TWC system also increases maintenance costs by as much as 25%. TWCs use noble metal
catalysts that are vulnerable to poisoning and masking, limiting their use to engines operated
with clean fuels - e.g., natural gas and unleaded gasoline. In addition, the engines must use
lubricants that do not generate catalyst poisoning compounds and have low concentrations of
heavy and base metal additives. Unburned fuel, unburned lube oil, and particulate matter can
also foul the catalyst. TWC technology is not applicable to lean burn gas engines or diesels.
Selective Catalytic Reduction (SCR)
This technology selectively reduces NOx to N2 in the presence of a reducing agent. NOx
reductions of 80 to 90% are achievable with SCR. Higher reductions are possible with the use of
more catalyst or more reducing agent, or both. The two agents used commercially are ammonia
(NH3 in anhydrous liquid form or aqueous solution) and aqueous urea. Urea decomposes in the
hot exhaust gas and SCR reactor, releasing ammonia. Approximately 0.9 to 1.0 moles of
ammonia is required per mole of NOx at the SCR reactor inlet in order to achieve an 80 to 90%
NOx reduction.
SCR systems add a significant cost burden to the installation cost and maintenance cost of an
engine system, and can severely impact the economic feasibility of smaller engine projects. SCR
requires on-site storage of ammonia, a hazardous chemical. In addition, ammonia can "slip"
through the process unreacted, contributing to environmental health concerns.
Oxidation Catalysts
Oxidation catalysts generally are precious metal compounds that promote oxidation of CO and
hydrocarbons to CO2 and H20 in the presence of excess O2. CO and NMHC conversion levels
of 98 to 99% are achievable. Methane conversion may approach 60 to 70%. Oxidation catalysts
are now widely used with all types of engines, including diesel engines. They are being used
Technology Characterization
27
Reciprocating Engines
-------
increasingly with lean burn gas engines to reduce their relatively high CO and hydrocarbon
emissions.
Lean -NOx Catalysts
Lean-NOx catalysts utilize a hydrocarbon reductant (usually the engine fuel) injected upstream of
the catalyst to reduce NOx. While still under development, it appears that NOx reduction of 80%
and both CO and NMHC emissions reductions of 60% may be possible. Long-term testing,
however, has raised issues about sustained performance of the catalysts. Current lean-NOx
catalysts are prone to poisoning by both lube oil and fuel sulfur. Both precious metal and base
metal catalysts are highly intolerant of sulfur. Fuel use can be significant with this technology -
the high NOx output of diesel engines would require approximately 3% of the engine fuel
consumption for the catalyst system.
Gas Engine Emissions Characteristics
Table 10 shows typical emissions for each of the five gas engine systems. The emissions
presented assume no exhaust treatment. System 1, 100 kW engine, is a high speed, rich burn
engine. Use of a TWC system would reduce NOx emissions to 0.15 gm/bhph, CO emissions to
0.6 gm/bhph, and VOC emissions to 0.15 gm/bhph. Systems 2 through 5 are all lean burn
engines optimized for low emissions. Use of an oxidation catalyst could reduce CO and VOC
emissions from these engines by 98 to 99%.
With current commercial technology, highest efficiency and lowest NOx are not achieved
simultaneously. Therefore many manufacturers of lean burn gas engines offer different versions
of an engine - a low NOx version and a high efficiency version - based on different tuning of the
engine controls and ignition timing. Achieving highest efficiency operation results in conditions
that generally produce twice the NOx as low NOx versions (e.g., 1.0 gm/bhph versus 0.5
gm/bhph). Achieving the lowest NOx typically entails sacrifice of 1 to 2 points in efficiency
(e.g., 38%) versus 36%>). In addition, CO and VOC emissions are higher in engines optimized for
minimum NOx.
Technology Characterization
28
Reciprocating Engines
-------
Table 10. Gas Engine Emissions Characteristics Without Exhaust Control Options *
Emissions ("haiiiclcrislics
System
1
System
System
j
System
' 4
System
5
Electricity Capacity (kW)
100
300
800
1000
5000
Electrical Efficiency (HHV)
30.6%
31.1%
33.3%
36.0%
39.0%
Engine Combustion
Rich
Lean
Lean
Lean
Lean
NOx (gm/bhph)
NOx. (ppmv @15% 0 )
15.0
l.ioo
2.0
150
l.o
80
0.5
44
0.5
46
NOx, (lb/MWh)
44.3
5.91
2.95
1.48
1.48
CO, (gm/bhph)
12.0
1.8
2.6
2.8
2.2
CO, (lb/MWh)
35.4
5.31
7.68
8.27
6.50
VOC, (gm/bhph)
0.7
0.2
1.0
1.4
0.4
VOC, (lb/MWh)
2.07
0.59
2.95
4.13
1.18
C02, (lb/MWh)
1,338
1,316
1,166
1,139
1,051
Carbon, (lb/MWh)
365
359
318
311
287
* For typical systems commercially available in 2001. Emissions estimates for untreated engine exhaust conditions
(15% 02i no TWC, SCR, or other exhaust clean up). Estimates based on typical manufacturers' specifications.
Source: Energy Nexus Group23
23 Characteristics for "typical" commercially available natural gas engine gensets. Data based on: MAN 150 kW -
100 kW; Cummins GSK19G - 300 kW; Caterpillar G3516 LE - 800 kW; Caterpiller G3616 LE - 1 MW; Wartsila
5238 LN - 3 MW
Technology Characterization
29
Reciprocating Engines
-------
Technology Characterization
Steam Turbines
Prepared for:
Environmental Protection Agency
Climate Protection Partnership Division
Washington, DC
Prepared by:
Energy Nexus Group
1401 Wilson Blvd, Suite 1101
Arlington, Virginia 22209
March 2002
-------
Disclaimer:
The information included in these technology overviews is for information purposes only and is
gathered from published industry sources. Information about costs, maintenance, operations, or
any other performance criteria is by no means representative of agency policies, definitions, or
determinations for regulatory or compliance purposes.
Technology Characterization
Steam Turbines
-------
TABLE OF CONTENTS
Introduction and Summary 1
Applications 1
Industrial and CHP Applications 2
Combined Cycle Power Plants 4
District Heatins Systems 4
Technology Description 4
Basic Process and Components 4
Types of Steam Turbines 6
Design Characteristics 9
Performance Characteristics 10
Electrical Efficiency 10
Process Steam and Performance Tradeoffs 12
CHP System Efficiency 12
Performance and Efficiency Enhancements 12
Capital Cost 13
Maintenance 15
Fuels 16
Availability 16
Emissions 16
Nitrogen Oxides (NOr> 16
Sulfur Compounds (SOr) 16
Particulate Matter (PM) 16
Carbon Monoxide (CO) 17
Carbon Dioxide (CO7) 17
Typical Emissions 17
Boiler Emissions Control Options - NOr 18
Boiler Emissions Control Options - SO, 20
Technology Characterization
Steam Turbines
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Technology Characterization - Steam Turbines
Introduction and Summary
Steam turbines are one of the most versatile and oldest prime mover technologies still in general
production. Power generation using steam turbines has been in use for about 100 years, when
they replaced reciprocating steam engines due to their higher efficiencies and lower costs.
Conventional steam turbine power plants generate most of the electricity produced in the United
States. The capacity of steam turbines can range from 50 kW to several hundred MWs for large
utility power plants. Steam turbines are widely used for combined heat and power (CHP)
applications.
Unlike gas turbine and reciprocating engine CHP systems where heat is a byproduct of power
generation, steam turbines normally generate electricity as a byproduct of heat (steam)
generation. A steam turbine is captive to a separate heat source and does not directly convert
fuel to electric energy. The energy is transferred from the boiler to the turbine through high-
pressure steam that in turn powers the turbine and generator. This separation of functions
enables steam turbines to operate with an enormous variety of fuels, from natural gas to solid
waste, including all types of coal, wood, wood waste, and agricultural byproducts (sugar cane
bagasse, fruit pits, and rice hulls). In CHP applications, steam at lower pressure is extracted
from the steam turbine and used directly or is converted to other forms of thermal energy.
Steam turbines offer a wide array of designs and complexity to match the desired application
and/or performance specifications. Steam turbines for utility service may have several pressure
casings and elaborate design features, all designed to maximize the efficiency of the power plant.
For industrial applications, steam turbines are generally of simpler single casing design and less
complicated for reliability and cost reasons. CHP can be adapted to both utility and industrial
steam turbine designs.
Applications
While steam turbines themselves are competitively priced compared to other prime movers, the
costs of complete boiler/steam turbine CHP systems are relatively high on a per kW of capacity
basis. This is because of their low power to heat (P/H) ratio, the costs of the boiler, fuel
handling, overall steam systems, and the custom nature of most installations. Thus, steam
turbines are well suited to medium- and large-scale industrial and institutional applications where
inexpensive fuels, such as coal, biomass, various solid wastes and byproducts (e.g., wood chips,
etc.), refinery residual oil, and refinery off gases are available. Because of the relatively high
cost of the system, high annual capacity factors are required to enable a reasonable recovery of
invested capital.
However, a retrofit application of steam turbines into existing boiler/steam systems is often an
economic option. A turbine-generator set requires the boiler must be able to support a small
increase in demand. In addition, to continue to satisfy thermal demands, the distribution system
Technology Characterization
1
Steam Turbines
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must be able to accommodate the increased flow rate of lower-Btu steam. In such situations, the
decision involves only the added capital cost of the steam turbine, its generator, controls, and
electrical interconnection, with the balance of plant already in place. Similarly, many facilities
faced with replacement or upgrades of existing boilers and steam systems often consider the
addition of steam turbines, especially if steam requirements are relatively large compared to
power needs within the facility.
In general, steam turbine applications are driven by balancing lower cost fuel or avoided disposal
costs for the waste fuel, with the high capital cost and (hopefully high) annual capacity factor for
the steam plant and the combined energy plant-process plant application. For these reasons,
steam turbines are not normally direct competitors of gas turbines and reciprocating engines.
Industrial and CHP Applications
The primary locations of steam turbine based CHP systems is industrial processes where solid or
waste fuels are readily available for boiler use. In CHP applications, steam extracted from the
steam turbine directly feeds into a process or is converted to another form of thermal energy.
The turbine may drive an electric generator or equipment such as boiler feedwater pumps,
process pumps, air compressors, and refrigeration chillers. Turbines as industrial drivers are
usually a single casing machine, either single stage or multistage, condensing or non-condensing
depending on steam conditions and the value of the steam. Steam turbines operate at a single
speed when driving an electric generator and operate over a speed range when driving a
refrigeration compressor. For non-condensing applications, steam exhausted from the turbine is
at a pressure and temperature sufficient for the CHP heating application.
There were an estimated 19,062 MW of boiler/steam turbine CHP capacity operating in the
United States in 2000 located at over 580 industrial and institutional facilities. Figure 1 shows
the largest amount of capacity is in the chemicals, primary metals, and paper industries. Pulp
and paper mills are often an ideal industrial/CHP application for steam turbines. Such facilities
operate continuously, have a high demand for steam, and have on-site fuel supply at low, or even
negative costs (waste that would have to be otherwise disposed of).
Figure 2 illustrates existing steam turbine CHP capacity by boiler fuel type. While coals fuels
much of the installed boiler/steam turbine system base, large amounts of capacity are fueled by
wood, waste, and a variety of other fuels.
Technology Characterization
2
Steam Turbines
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Figure 1. Existing Boiler/Steam Turbine CHP by Industry
19,062 MW at 582 Sites
Paper
34%
Comm/lnst
7%
Other
Industrial
19%
Primary
Metals
11%
Food
7%
Chemicals
22%
Figure 2. Existing Boiler/Steam Turbine CHP by Boiler Fuel Type
19,062 MW at 582 Sites
Wood
13%
Other
12%
Waste
19%
Natural Gas
7%
Source: Energy Nexus Group/HaglerBailly.
Technology Characterization
3
Steam Turbines
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Combined Cycle Power Plants
The trend in power plant design is the combined cycle, which incorporates a steam turbine in a
bottoming cycle with a gas turbine. Steam generated in the heat recovery steam generator
(HRSG) of the gas turbine is used to drive a steam turbine to yield additional electricity and
improve cycle efficiency. Combined cycle CHP applications use an extraction-condensing type
of steam turbine.
District Heating Systems
There are many cities and college campuses that have steam district heating systems where
adding a steam turbine between the boiler and the distribution system may be an attractive
application. Often the boiler is capable of producing moderate-pressure steam but the
distribution system needs only low-pressure steam. In these cases, the steam turbine generates
electricity using the higher-pressure steam, and discharges low-pressure steam into the
distribution system.
Technology Description
Basic Process and Components
The thermodynamic cycle for the steam turbine is the Rankine cycle. The cycle is the basis for
conventional power generating stations and consists of a heat source (boiler) that converts water
to high-pressure steam. In the steam cycle, water is first pumped to medium to high pressure. It
is then heated to the boiling temperature corresponding to the pressure, boiled (heated from
liquid to vapor), and then most frequently superheated (heated to a temperature above that of
boiling). A multistage turbine expands the pressurized steam to lower pressure and the steam is
then exhausted either to a condenser at vacuum conditions or into an intermediate temperature
steam distribution system that delivers the steam to the industrial or commercial application.
The condensate from the condenser or from the steam utilization system returns to the feedwater
pump for continuation of the cycle. Figure 3 shows the primary components of a boiler/steam
turbine system.
Technology Characterization
4
Steam Turbines
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Figure 3. Components of a Boiler/Steam Turbine System
Steam
Fuel
Boiler
Turbine
Power out
Pump
WW
Process or
Condenser
I
Heat out
The steam turbine itself consists of a stationary set of blades (called nozzles) and a moving set of
adjacent blades (called buckets or rotor blades) installed within a casing. The two sets of blades
work together such that the steam turns the shaft of the turbine and the connected load. The
stationary nozzles accelerate the steam to high velocity by expanding it to lower pressure. A
rotating bladed disc changes the direction of the steam flow, thereby creating a force on the
blades that, because of the wheeled geometry, manifests itself as torque on the shaft on which the
bladed wheel is mounted. The combination of torque and speed is the output power of the
turbine.
The internal flow passages of a steam turbine are similar to those of the expansion section of a
gas turbine (indeed, gas turbine engineering came directly from steam turbine design around 100
years ago). The main differences are the different gas density, molecular weight, isentropic
expansion coefficient, and to a lesser extent viscosity of the two fluids.
Compared to reciprocating steam engines of comparable size, steam turbines rotate at much
higher rotational speeds, which contributes to their lower cost per unit of power developed. The
absence of inlet and exhaust valves that somewhat throttle (reduce pressure without generating
power) and other design features enable steam turbines to be more efficient than reciprocating
steam engines. In some steam turbine designs, the blade row accomplishes part of the decrease
in pressure and acceleration. These distinctions are known as impulse and reaction turbine
designs, respectively. The competitive merits of these designs are the subject of business
competition as both designs have sold successfully for well over 75 years.
The connection between the steam supply and the power generation is the steam, and return
feedwater, lines. There are numerous options in the steam supply, pressure, temperature and
extent, if any, for reheating partially expanded steam. Steam systems vary from low-pressure
Technology Characterization
5
Steam Turbines
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lines used primarily for space heating and food preparation, to medium pressure used in
industrial processes and cogeneration, to high-pressure use in utility power generation.
Generally, as the system gets larger the economics favor higher pressures and temperatures with
their associated heavier walled boiler tubes and more expensive alloys.
In general, utility applications involve raising steam for the exclusive purpose of power
generation. Such systems also use a water-cooled condenser to exhaust the steam from the
turbine at the lowest practical pressure. Some utility turbines have dual use, power generation
and steam delivery at higher pressure into district heating systems or to neighboring industrial
plants at pressure, and consequently do not have condensers. These plants are actually large
cogeneration/CHP plants.
Boilers
Steam turbines differ from reciprocating engines and gas turbines in that the fuel is burned in a
piece of equipment, the boiler, which is separate from the power generation equipment, the
steam turbogenerator. As mentioned previously, this separation of functions enables steam
turbines to operate with an enormous variety of fuels.
For sizes up to (approximately) 40 MW, horizontal industrial boilers are built. This enables rail
car shipping, with considerable cost savings and improved quality as the cost and quality of
factory labor is usually both lower in cost and greater in quality than field labor. Large shop-
assembled boilers are typically capable of firing only gas or distillate oil, as there is inadequate
residence time for complete combustion of most solid and residual fuels in such designs. Large,
field-erected industrial boilers firing solid and residual fuels bear a resemblance to utility boilers
except for the actual solid fuel injection. Large boilers usually burn pulverized coal, however
intermediate and small boilers burning coal or solid fuel employ various types of solids feeders.
Types of Steam Turbines
The primary type of turbine used for central power generation is the condensing turbine. These
power-only utility turbines exhaust directly to condensers that maintain vacuum conditions at the
discharge of the turbine. An array of tubes, cooled by river, lake, or cooling tower water,
condenses the steam into (liquid) water.1 The cooling water condenses the steam turbine exhaust
steam in the condenser creating the condenser vacuum. As a small amount of air leaks into the
system when it is below atmospheric pressure, a relatively small compressor removes non-
condensable gases from the condenser. Non-condensable gases include both air and a small
amount of the corrosion byproduct of the water-iron reaction, hydrogen.
The condensing turbine processes result in maximum power and electrical generation efficiency
from the steam supply and boiler fuel. The power output of condensing turbines is sensitive to
ambient conditions.2
1 At 80°F, the vapor pressure of water is 0.51 psia, at 100°F it is 0.95 psia, at 120°F it is 1.69 psia and at 140°F
Fahrenheit it is 2.89 psia
2 From a reference condition of condensation at 100 degree Fahrenheit, 6.5% less power is obtained from the inlet
steam when the temperature at which the steam is condensed is increased (because of higher temperature ambient
conditions) to 115°F. Similarly the power output is increased by 9.5% when the condensing temperature is reduced
Technology Characterization
6
Steam Turbines
-------
CHP applications use two types of steam turbines: non-condensing and extraction.
Non-Condensing (Back-pressure) Turbine
Figure 4 shows the non-condensing turbine (also referred to as a back-pressure turbine) exhausts
its entire flow of steam to the industrial process or facility steam mains at conditions close to the
process heat requirements.
Figure 4. Non-Condensing (Back-Pressure) Steam Turbine
High pressure steam
Power Out
Low pressure steam
To process
Usually, the steam sent into the mains is not much above saturation temperature/ The term
"back-pressure" refers to turbines that exhaust steam at atmospheric pressures and above. The
specific CHP application establishes the discharge pressure. 50, 150, and 250 psig are the most
typical pressure levels for steam distribution systems. District heating systems most often use
the lower pressures, and industrial processes use the higher pressures. Industrial processes often
include further expansion for mechanical drives, using small steam turbines for driving heavy
equipment that runs continuously for long periods. Power generation capability reduces
significantly when steam is used at appreciable pressure rather than being expanded to vacuum in
a condenser. Discharging steam into a steam distribution system at 150 psig can sacrifice
slightly more than half the power that could be generated when the inlet steam conditions are 750
psig and 800°F, typical of small steam turbine systems.
Extraction Turbine
The extraction turbine has opening(s) in its casing for extraction of a portion of the steam at
some intermediate pressure before condensing the remaining steam. Figure 5 illustrates the
to 80°F. This illustrates the influence of steam turbine discharge pressure on power output and, consequently, net
heat rate (and efficiency.)
3 At 50 psig (65 psia) the condensation temperature is 298°F, at 150 psig (165 psia) the condensation temperature is
366°F, and at 250 psig (265 psia) it is 406°F.
Technology Characterization
1
Steam Turbines
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extracted steam may be used for process purposes in a CHP facility or for feedwater heating as is
the case in most utility power plants.
Figure 5. Extraction Steam Turbine
High pressure steam
Power Out
Medium/low
pressure steam
To process ^
~
Condenser
The steam extraction pressure may or may not be automatically regulated. Regulated extraction
permits more steam to flow through the turbine to generate additional electricity during periods
of low thermal demand by the CHP system. In utility type steam turbines, there may be several
extraction points, each at a different pressure corresponding to a different temperature. The
facility's specific needs for steam and power over time determine the extent to which steam in an
extraction turbine is extracted for use in the process.
In large, often complex, industrial plants, additional steam may be admitted (flows into the
casing and increases the flow in the steam path) to the steam turbine. Often this happens when
using multiple boilers at different pressure, because of their historical existence. These steam
turbines are referred to as admission turbines. At steam extraction and admission locations there
are usually steam flow control valves that add to the steam and control system cost.
Numerous mechanical design features increase efficiency, provide for operation over a range of
conditions, simplify manufacture and repair, and achieve other practical purposes. The long
history of steam turbine use has resulted in a large inventory of steam turbine stage designs. For
example, the division of steam acceleration and change in direction of flow varies between
competing turbine manufacturers under the identification of impulse and reaction designs.
Manufacturers tailor clients' design requests by varying the flow area in the stages and the extent
to which steam is extracted (removed from the flow path between stages) to accommodate the
specification of the client.
When the steam expands through a high-pressure ratio, as in utility and large industrial steam
systems, the steam can begin to condense in the turbine when the temperature of the steam drops
below the saturation temperature at that pressure. If water drops form in the turbine, blade
erosion occurs from the drops impact on the blades. At this point in the expansion the steam is
Technology Characterization
8
Steam Turbines
-------
sometimes returned to the boiler and reheated to high temperature and then returned to the
turbine for further (safe) expansion. In a few large, high pressure, utility steam systems install
double reheat systems.
With these choices the designer of the steam supply system and the steam turbine have the
challenge of creating a system design which delivers the (seasonally varying) power and steam
which presents the most favorable business opportunity to the plant owners.
Between the power (only) output of a condensing steam turbine and the power and steam
combination of a back-pressure steam turbine essentially any ratio of power to heat output can be
supplied. Back-pressure steam turbines can be obtained with a variety of back pressures, further
increasing the variability of the power-to-heat ratio.
Design Characteristics
Custom design:
Thermal output:
Fuel flexibility:
Reliability and life:
Size range:
Emissions:
Steam turbines are designed to match CHP design pressure and
temperature requirements and to maximize electric efficiency
while providing the desired thermal output.
Steam turbines are capable of operating over a broad range of
steam pressures. Utility steam turbines operate with inlet steam
pressures up to 3,500 psig and exhaust vacuum conditions as low
as one inch of Hg (absolute). Steam turbines are custom designed
to deliver the thermal requirements of the CHP applications
through use of backpressure or extraction steam at appropriate
pressures and temperatures.
Steam turbines offer a wide range of fuel flexibility using a variety
of fuel sources in the associated boiler or other heat source,
including coal, oil, natural gas, wood and waste products.
Steam turbine life is extremely long. When properly operated and
maintained (including proper control of boiler water chemistry),
steam turbines are extremely reliable, only requiring overhauls
every several years. They require controlled thermal transients to
minimize differential expansion of the parts as the massive casing
slowly heats up.
Steam turbines are available in sizes from under 100 kW to over
250 MW. In the multi-megawatt size range, industrial and utility
steam turbine designations merge, with the same turbine (high-
pressure section) able to serve both industrial and small utility
applications.
Emissions are dependent upon the fuel used by the boiler or other
steam source, boiler furnace combustion section design and
operation, and built-in and add-on boiler exhaust cleanup systems.
Technology Characterization
9
Steam Turbines
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Performance Characteristics
Electrical Efficiency
The electrical generating efficiency of standard steam turbine power plants varies from a high of
37% HHV4 for large, electric utility plants designed for the highest practical annual capacity
factor, to under 10% HHV for small, simple plants which make electricity as a byproduct of
delivering steam to processes or district heating systems.
Steam turbine thermodynamic efficiency (isentropic efficiency) refers to the ratio of power
actually generated from the turbine to what would be generated by a perfect turbine with no
internal losses using steam at the same inlet conditions and discharging to the same downstream
pressure (actual enthalpy drop divided by the isentropic enthalpy drop). Turbine thermodynamic
efficiency is not to be confused with electrical generating efficiency, which is the ratio of net
power generated to total fuel input to the cycle. Steam turbine thermodynamic efficiency
measures how efficiently the turbine extracts power from the steam itself. Multistage (moderate
to high-pressure ratio) steam turbines have thermodynamic efficiencies that vary from 65% for
small (under 1,000 kW) units to over 90% for large industrial and utility sized units. Small,
single stage steam turbines can have efficiencies as low as 50%. When a steam turbine exhausts
to a CHP application, the turbine efficiency is not as critical as in a power only condensing
mode. The majority of the energy not extracted by the steam turbine satisfies the thermal load.
Power only applications waste the exhaust turbine steam energy in condensers.
Table 1 summarizes performance characteristics for typical commercially available steam
turbines and for typical boiler/steam CHP systems in the 500 kW to 15 MW size range.
4
All turbine and engine manufacturers quote heat rates in terms of the lower heating value (LHV) of the fuel.
However, the usable energy content of fuels is typically measured on a higher heating value basis (HHV). In
addition, electric utilities measure power plant heat rates in terms of HHV. For natural gas, the average heat content
of natural gas is 1,030 Btu/scf on an HHV basis and 930 Btu/scf on an LHV basis - or about a 10% difference.
Technology Characterization
10
Steam Turbines
-------
Table 1. Boiler/Steam Turbine CHP System Cost and Performance Characteristics *
( osl & Performance ( luiraclenslics
S\ slem 1
S\ slem 2
S\ slem 3
Sleam Turbine Parameters
Nominal Electricity Capacity (kW)
500
3,000
15,000
Turbine Type
Back Pressure
Back Pressure
Back Pressure
Equipment Cost ($/l
-------
Operating Characteristics
Steam turbines, especially smaller units, leak steam around blade rows and out the end seals.
When an end is at a low pressure, as is the case with condensing steam turbines, air can also leak
into the system. The leakages cause less power to be produced than expected, and the makeup
water has to be treated to avoid boiler and turbine material problems. Air that has leaked in
needs to be removed, which is usually done by a compressor removing non-condensable gases
from the condenser.
Because of the high pressures used in steam turbines, the casing is quite thick, and consequently
steam turbines exhibit large thermal inertia. Steam turbines must be warmed up and cooled
down slowly to minimize the differential expansion between the rotating blades and the
stationary parts. Large steam turbines can take over ten hours to warm up. While smaller units
have more rapid startup times, steam turbines differ appreciably from reciprocating engines,
which start up rapidly, and from gas turbines, which can start up in a moderate amount of time
and load follow with reasonable rapidity. Steam turbine applications usually operate
continuously for extended periods, although the steam fed to the unit and the power delivered
may vary (slowly) during such periods of continuous operation.
Process Steam and Performance Tradeoffs
The amount and quality of recovered heat is a function of the entering steam conditions and the
design of the steam turbine. Exhaust steam from the turbine is used directly in a process or is
converted to other forms of thermal energy, including hot or chilled water. Steam discharged or
extracted from a steam turbine can be used in a single- or double effect absorption chiller. The
steam turbine can also be used as a mechanical drive for a centrifugal chiller.
CHP System Efficiency
Steam turbine CHP systems generally have low power to heat ratios, typically in the 0.05 to 0.2
range. This is because electricity is a byproduct of heat generation, with the system optimized
for steam production. Hence, while steam turbine CHP system electrical efficiency15 may seem
low, it is because the primary objective is to produce large amounts of steam. The effective
electrical efficiency16 of steam turbine systems, however, is generally high, because almost all
the energy difference between the high-pressure boiler output and the lower pressure turbine
output is converted to electricity. This means that total CHP system efficiencies17 are generally
high and approach the boiler efficiency level. Steam boiler efficiencies range from 70 to 85 %
HHV depending on boiler type and age, fuel, duty cycle, application, and steam conditions.
Performance and Efficiency Enhancements
In industrial steam turbine systems, business conditions determine the requirements and relative
values of electric power and steam. Plant system engineers then decide the extent of efficiency
15 Net power output / total fuel input into the system.
16 (Steam turbine electric power output)/(Total fuel into boiler - (steam to process/boiler efficiency)).
17 Net power and steam generated divided by total fuel input.
Technology Characterization
12
Steam Turbines
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enhancing options to incorporate in terms of their incremental effects on performance and plant
cost, and select appropriate steam turbine inlet and exhaust conditions. Often the steam turbine
is going into a system that already exists and is being modified, so that a number of steam system
design parameters are already determined by previous decisions, which exist as system hardware
characteristics.
As the stack temperature of the boiler exhaust combustion products still contain some heat,
tradeoffs occur regarding the extent of investment in heat reclamation equipment for the sake of
efficiency improvement. Often the stack exhaust temperature is set at a level where further heat
recovery would result in condensation of corrosive chemical species in the stack, with
consequential deleterious effects on stack life and safety.
Steam Reheat
Higher pressures and steam reheat increase power generation efficiency in large industrial (and
utility) systems. The higher the pressure ratio (the ratio of the steam inlet pressure to the steam
exit pressure) across the steam turbine, and the higher the steam inlet temperature, the more
power it will produce per unit of mass flow. To avoid condensation the inlet steam temperature
is increased to the economic practical limit of materials. This limit is now generally in the range
of 800 to 900°F for small industrial steam turbines.
When the economically practical limit of temperature is reached, the expanding steam can reach
a condition of temperature and pressure where condensation to (liquid) water begins. Small
amounts of water droplets can be tolerated in the last stage of a steam turbine provided that the
droplets are not too large or numerous. At pressures higher than that point the steam is returned
to the boiler and reheated in temperature and then returned to the expansion steam turbine for
further expansion. When returned to the next stage of the turbine, the steam expands without
condensation.
Combustion Air Preheating
In large industrial systems, air preheaters recover heat from the boiler exhaust gas stream, and
use it to preheat the combustion air, thereby reducing fuel consumption. Boiler combustion air
preheaters are large versions of the heat wheels used for the same purpose on industrial furnaces.
Capital Cost
A steam turbine-based CHP plant is a complex process with many interrelated subsystems that
must be usually be custom designed. A typical breakdown of installed costs for a steam turbine
CHP plant is 25% - boiler, 25% - fuel handling, storage and preparation system, 20% - stack gas
cleanup and pollution controls, 15% steam turbine generator, and 20% - field construction and
plant engineering. Boiler costs are highly competitive. Typically, the only area in which
significant cost reductions can be made when designing a system is in fuel
handling/storage/preparation.
In a steam turbine cogeneration plant, especially one burning solid fuel such as biomass, the
turbine accounts for a much smaller portion of total system installed costs than is the case with
internal combustion engines and industrial gas turbines. Often the solid fuel-handling equipment
alone costs as much as 90% of the cost of the steam turbine. The pollution control and
Technology Characterization
13
Steam Turbines
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electrostatic precipitator cost can reach 80% of the steam turbine cost. A typical coal/wood fired
boiler costs more than the steam turbine.18 The cost of complete solid fuel cogeneration plants
varies with many factors, with fuels handling, pollution control equipment and boiler cost all
being major cost items. Because of both the size of such plants and the diverse sources of the
components, solid fuel cogeneration plants invariably involve extensive system engineering and
field labor during construction. Typical complete plant costs run well over $l,000/kW, with
little generalization except that for the same fuel and configuration, costs per kW of capacity
generally increase as size decreases.
Steam turbine costs exhibit a modest extent of irregularity, as steam turbines are made in sizes
with finite steps between the sizes. The cost of the turbine is generally the same for the upper
and lower limit of the steam flowing through it, so step-like behavior is sometimes seen in steam
turbine prices. Since they come in specific size increments, a steam turbine that is used at the
upper end of its range of power capability costs lest per kW generated than one that is used at the
lower end of its capability.
Often steam turbines are sold to fit into an existing plant. In some of these applications, the
specifications, mass flow, pressure, temperature and backpressure or extraction conditions are
not conditions for which large competition exists. These somewhat unique machines are more
expensive per kilowatt than are machines for which greater competition exists, for three reasons:
1) a greater amount of custom engineering and manufacturing setup may be required; 2) there is
less potential for sales of duplicate or similar units; and 3) there are fewer competitive bidders.
The truly competitive products are the "off-the-rack" type machines, while "custom" machines
are naturally more expensive.
Steam turbine prices vary greatly with the extent of competition and related manufacturing
volumes for units of desired size, inlet and exit steam conditions, rotational speed and
standardization of construction. Quoted prices are usually for an assembled steam turbine-
electrical generator package. The electrical generator can account for 20% to 40% of the
assembly. As the steam turbine/electrical generator package is heavy, due in large part to the
heavy walled construction of the high-pressure turbine casing, it requires careful mounting on an
appropriate pedestal. The installation and connection to the boiler through high pressure-high
temperature steam pipes require engineering and installation expertise. As the high-pressure
steam pipes typically vary in temperature by 750°F between cold standby/repair status and full
power status, care must be taken in installing a means to accommodate the differential expansion
accompanying startup and shutdown. Should the turbine have variable extraction, the cost of the
extraction valve and control system adds to the installation.
Small sized steam turbines, below about 2 MW, have a relatively small market, as complete plant
cost becomes high enough so that the business venture has much less attractiveness. In these
small sizes there is less competition and lower manufacturing volume, so that component costs
are not as competitive, the economies of scale in both size and manufacturing volumes disfavor
18 Spiewak and Weiss, loc. Cit., pages 82 and 95. These figures are for a 32.3 MW multi-fuel fired, 1,250 psig, 900
°F, 50 psig backpressure steam turbine used in an industrial cogeneration plant
Technology Characterization
14
Steam Turbines
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such small sizes, and the fraction of total cost due to system engineering and field construction
are high.19
Boiler combustion produces the steam for a steam turbine and the temperature of the steam is
limited by furnace heat transfer design, manufacturing consideration, and boiler tube bundle
design. Higher heat fluxes in the boiler enable more compact boilers, with less boiler tube
material to be built; however, higher heat fluxes also result in higher boiler tube temperature and
the need for the use of a higher grade (adequate strength at higher temperature) boiler tube
material. Such engineering economic tradeoffs between temperature (with consequential
increases in efficiency) and cost appear throughout the steam plant.
Because of the temperature limitation on boiler tubes, which are exposed to the high temperature
and heat flux in the furnace, steam turbine material selection is easier. An often-overlooked
component in the steam power system is the steam (safety) stop valve, which is immediately
ahead of the steam turbine and is designed to be able to experience the full temperature and
pressure of the steam supply. This safety valve is necessary because if the generator electric load
were lost (an occasional occurrence), the turbine would rapidly overspeed and destroy itself.
Other accidents are possible, supporting the need for the turbine stop valve, which adds
significant cost to the system
Maintenance
Steam turbines are rugged units, with operational life often exceeding 50 years. Maintenance is
simple, comprised mainly of making sure that all fluids (steam flowing through the turbine and
the oil for the bearing) are always clean and at the proper temperature. The oil lubrication
system must be clean and at the correct operating temperature and level to maintain proper
performance. Other items include inspecting auxiliaries such as lubricating-oil pumps, coolers
and oil strainers and checking safety devices such as the operation of overspeed trips.
In order to obtain reliable service, steam turbines require long warmup periods so that there are
minimal thermal expansion stresses and wear concerns. Steam turbine maintenance costs are
quite low, typically less than $0,004 per kWh. Boilers and any associated solid fuel processing
and handling equipment that is part of the boiler/steam turbine plant require their own types of
maintenance.
One maintenance issue with steam turbines is solids carry over from the boiler that deposit on
turbine nozzles and other internal parts and degrades turbine efficiency and power output. Some
of these are water soluble but others are not. Three methods are employed to remove such
deposits: 1) manual removal; 2) cracking off deposits by shutting the turbine off and allowing it
to cool; and 3) for water soluble deposits, water washing while the turbine is running.
19 Data on steam generator costs shows cost increasing with decreasing size, with a 5.25 MW, 900 psig, 850°F, 125
psig backpressure steam turbine/generator costing $285/kW (installed). In that installation the boiler alone,
excluding fuel handling and pollution control equipment, cost 150% of the cost of the steam turbine.
Technology Characterization
15
Steam Turbines
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Fuels
Industrial boilers operate on a variety of fuels, including wood, coal, natural gas, oils (including
residual oil), municipal solid waste, and sludges. The fuel handling, storage, and preparation
equipment needed for solid fuels add considerably to the cost of an installation. Thus, such fuels
are used only when a high annual capacity factor is expected of the facility, or when the solid
material has to be disposed of to avoid an environmental or space occupancy problem.
Availability
Steam turbines generally have 99% plus availability with longer than one year between
shutdowns for maintenance and inspections. This high level of availability applies only to the
steam turbine, not the boiler or HRSG that is supplying the steam.
Emissions
Emissions associated with a steam turbine are dependent on the source of the steam. Steam
turbines can be used with a boiler firing any one or a combination of a large variety of fuel
sources, or they can be used with a gas turbine in a combined cycle configuration. Boiler
emissions vary depending on fuel type and environmental conditions. Boilers emissions include
nitrogen oxide (NOx), sulfur oxides (SOx), particulate matter (PM), carbon monoxide (CO), and
carbon dioxide (CO2).
Nitrogen Oxides (NOx>
The pollutant referred to as NOx is a mixture of (mostly) nitric oxide (NO) and nitrogen dioxide
(N02) in variable composition. NOx forms by three mechanisms: thermal NOx, prompt NOx, and
fuel-bound NOx. In industrial boilers, thermal and fuel-bound are the predominant NOx
formation mechanisms. Thermal NOx, formed when nitrogen and oxygen in the combustion air
combine in the flame, comprises the majority of NOx formed during the combustion of gases and
light oils. Fuel-bound NOx is associated with oil fuels and forms when nitrogen in the fuel and
oxygen in the combustion air react.
The most significant factors influencing the level of NOx emissions from a boiler are the flame
temperature and the amount of nitrogen in the fuel. Other factors include excess air level and
combustion air temperature.
Sulfur Compounds (SOx)
Emissions of sulfur relate directly to the sulfur content of the fuel, and are not dependent on
boiler size or burner design. Sulfur dioxide (S02) composes about 95% of the emitted sulfur and
with the remaining 5% emitted as sulfur trioxide (SO3). SOx are pollutants because they react
with water vapor and form sulfuric acid mist, which is extremely corrosive and damaging in its
air-, water- and soil-borne forms. Boiler fuels containing sulfur are primarily coal, oil, and some
types of waste.
Particulate Matter (PM)
PM emissions are largely dependent on the grade of boiler fuel, and consist of many different
compounds, including nitrates, sulfates, carbons, oxides and other uncombusted fuel elements.
PM levels from natural gas are significantly lower than those of oils, and distillate oils much
Technology Characterization
16
Steam Turbines
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lower than residual oils. For industrial and commercial boilers, the most effective method of PM
control is use of higher-grade fuel, and ensuring proper burner setup, adjustment and
maintenance.
Carbon Monoxide (CO)
CO forms during combustion when carbon in the fuel oxidizes incompletely, ending up as CO
instead of C02. Older boilers generally have higher levels of CO than new equipment because
older burner designs do not have CO controls. Poor burner design or firing conditions are
responsible for high levels of CO boiler emissions. Proper burner maintenance or equipment
upgrades, or using an oxygen control package, can control CO emissions successfully.
Carbon Dioxide (CO2)
While not considered a regulated pollutant in the ordinary sense of directly affecting public
health, emissions of carbon dioxide are of concern due to its contribution to global warming.
Atmospheric warming occurs because solar radiation readily penetrates to the surface of the
planet but infrared (thermal) radiation from the surface is absorbed by the C02 (and other
polyatomic gases such as water vapor, methane, unburned hydrocarbons, refrigerants and volatile
chemicals) in the atmosphere, with resultant increase in temperature of the atmosphere. The
amount of CO2 emitted is a function of both fuel carbon content and system efficiency. The fuel
carbon content of natural gas is 34 lbs carbon/MMBtu; oil is 48 lbs carbon/MMBtu; and (ash-
free) coal is 66 lbs carbon/MMBtu.
Typical Emissions
Table 2 below illustrates typical emissions of NOx, PM and CO for boilers by size of steam
turbine system and by fuel type.
Table 2. Typical Boiler Emissions Ranges
Boiler Fuel
System 1
500 kW
Systems 2 and 3
3 MW/ 15 MW
NOx
CO
PM
NOx
CO
PM
Coal
(lbs/MMBtu)
N/A
N/A
N/A
0.20-1.24
0.02-0.7
Wood
(lbs/MMBtu)
0.22-0.49
0.6
0.33-0.56
0.22-0.49
0.06
0.33-0.56
Fuel Oil
(lbs/MMBtu)
0.15-0.37
0.03
0.01-0.08
0.07-0.31
0.03
0.01-0.08
Natural Gas
(lbs/MMBtu)
0.03-0.1
0.08
"
0.1-0.28
0.08
"
Note: all emissions values are without post-combustion treatment.
Source: EPA, Compilation of Air Pollutant Emission Factors, AP-42, Fifth Edition, Volume I:
Stationary Point and Area Sources
Technology Characterization
17
Steam Turbines
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Boiler Emissions Control Options - N0X
N0X control has been the primary focus of emission control research and development in boilers.
The following provides a description of the most prominent emission control approaches.
Combustion Process Emissions Control
Combustion control techniques are less costly than post-combustion control methods and are
often used on industrial boilers for NOx control. Control of combustion temperature has been the
principal focus of combustion process control in boilers. Combustion control requires tradeoffs
- high temperatures favor complete burn up of the fuel and low residual hydrocarbons and CO,
but promote NOx formation. Lean combustion dilutes the combustion process and reduces
combustion temperatures and NOx formation, and allows a higher compression ratio or peak
firing pressures resulting in higher efficiency. However, if the mixture is too lean, misfiring and
incomplete combustion occurs, increasing CO and VOC emissions.
Flue Gas Recirculation (FGR)
FGR is the most effective technique for reducing NOx emissions from industrial boilers with
inputs below 100 MMBtu/hr. With FGR, a portion of the relatively cool boiler exhaust gases re-
enter the combustion process, reducing the flame temperature and associated thermal NOx
formation. It is the most popular and effective NOx reduction method for firetube and watertube
boilers, and many applications can rely solely on FGR to meet environmental standards.
External FGR employs a fan to recirculate the flue gases into the flame, with external piping
carrying the gases from the stack to the burner. A valve responding to boiler input controls the
recirculation rate. Induced FGR relies on the combustion air fan for flue gas recirculation. A
portion of the gases travel via ductwork or internally to the air fan, where they are premixed with
combustion air and introduced into the flame through the burner. Induced FGR in newer designs
uses an integral design that is relatively uncomplicated and reliable. The physical limit to NOx
reduction via FGR is 80% in natural gas-fired boilers and 25% for standard fuel oils.
Low Excess Air Firing (LAE)
Excess air ensures complete combustion. However, excess air levels greater than 45% can result
in increased NOx formation, because the excess nitrogen and oxygen in the combustion air
entering the flame combine to form thermal NOx. Firing with low excess air means limiting the
amount of excess air that enters the combustion process, thus limiting the amount of extra
nitrogen and oxygen entering the flame. Burner design modification accomplishes this and
optimization uses oxygen trim controls. LAE typically results in overall NOx reductions of 5 to
10%) when firing with natural gas, and is suitable for most boilers.
Low Nitrogen Fuel Oil
NOx formed by fuel-bound nitrogen can account for 20 to 50% of total NOx levels in oil-fired
boiler emissions. The use of low nitrogen fuels in boilers firing distillate oils is one method of
reducing NOx emissions. Such fuels can contain up to 20 times less fuel-bound nitrogen than
Technology Characterization
18
Steam Turbines
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standard No. 2 oil. NOx reductions of up to 70% over NOx emissions from standard No. 2 oils
have been achieved in firetube boilers utilizing flue gas recirculation.
Burner Modifications
Modifying the design of standard burners to create a larger flame achieves lower flame
temperatures and results in lower thermal NOx formation. While most boiler types and sizes can
accommodate burner modifications, it is most effective for boilers firing natural gas and distillate
fuel oils, with little effectiveness in heavy oil-fired boilers. Also, burner modifications must be
complemented with other NOx reduction methods, such as flue gas recirculation, to comply with
the more stringent environmental regulations. Achieving low NOx levels (30 ppm) through
burner modification alone can adversely impact boiler operating parameters such as turndown,
capacity, CO levels, and efficiency.
Water/Steam Injection
Injecting water or steam into the flame reduces flame temperature, lowering thermal NOx
formation and overall NOx emissions. However, under normal operating conditions, water/steam
injection can lower boiler efficiency by 3 to 10%. Also, there is a practical limit to the amount
that can be injected without causing condensation-related problems. This method is often
employed in conjunction with other NOx control techniques such as burner modifications or flue
gas recirculation. When used with natural gas-fired boilers, water/steam injection can result in
NOx reduction of up to 80%, with lower reductions achievable in oil-fired boilers.
Post-Combustion Emissions Control
There are several types of exhaust gas treatment processes that are applicable to industrial
boilers.
Selective Non-Catalytic Reduction (SNCR)
In boiler SNCR, a NOx reducing agent such as ammonia or urea is injected into the boiler
exhaust gases at a temperature in the 1,400 to 1,600°F range. The agent breaks down the NOx in
the exhaust gases into water and atmospheric nitrogen (N2). While SNCR can reduce boiler NOx
emissions by up to 70%, it is difficult to apply to industrial boilers that modulate or cycle
frequently because to perform properly, the agent must be introduced at a specific flue gas
temperature. The location of the exhaust gases at the necessary temperature is constantly
changing in a cycling boiler.
Selective Catalytic Reduction (SCR)
This technology involves the injection of the reducing agent into the boiler exhaust gas in the
presence of a catalyst. The catalyst allows the reducing agent to operate at lower exhaust
temperatures than SNCR, in the 500 to 1,200°F depending on the type of catalyst. NOx
reductions of up to 90% are achievable with SCR. The two agents used commercially are
ammonia (NH3 in anhydrous liquid form or aqueous solution) and aqueous urea. Urea
decomposes in the hot exhaust gas and SCR reactor, releasing ammonia. Approximately 0.9 to
1.0 moles of ammonia is required per mole of NOx at the SCR reactor inlet in order to achieve an
80 to 90% NOx reduction.
Technology Characterization
19
Steam Turbines
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SCR is however costly to use and can only occasionally be justified on boilers with inputs of less
than 100 MMBtu/hr. SCR requires on-site storage of ammonia, a hazardous chemical. In
addition, ammonia can "slip" through the process unreacted, contributing to environmental
health concerns.
Boiler Emissions Control Options - SOx
The traditional method for controlling SOx emissions is dispersion via a tall stack to limit ground
level emissions. The more stringent SOx emissions requirements in force today demand the use
of reduction methods as well. These include use of low sulfur fuel, desulfurizing fuel, and flue
gas desulfurization (FGD). Desulfurization of fuel primarily applies to coal, and, like FGD, is
principally used for utility boiler emissions control. Use of low sulfur fuels is the most cost
effective SOx control method for industrial boilers, as it does not require installation and
maintenance of special equipment.
FGD systems are of two types: non-regenerable and regenerable. The most common, non-
regenerable, results in a waste product that requires proper disposal. Regenerable FGD converts
the waste product into a product that is saleable, such as sulfur or sulfuric acid. FGD reduces
SOx emissions by up to 95%.
Technology Characterization
20
Steam Turbines
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Technology Characterization
Fuel Cells
Prepared for:
Environmental Protection Agency
Climate Protection Partnership Division
Washington, DC
Prepared by:
Energy Nexus Group
1401 Wilson Blvd, Suite 1101
Arlington, Virginia 22209
April 2002
-------
Disclaimer:
The information included in these technology overviews is for information purposes only and is
gathered from published industry sources. Information about costs, maintenance, operations, or
any other performance criteria is by no means representative of agency policies, definitions, or
determinations for regulatory or compliance purposes.
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TABLE OF CONTENTS
Introduction and Summary 1
Applications 2
Combined Heat and Power 3
Premium Power 4
Remote Power 4
Standby Power 4
Peak Shaving 5
Grid Support 4
Technology Description 5
Basic Processes and Components 5
Design Characteristics 11
Performance Characteristics 13
Electrical Efficiency 15
Part Load Performance 15
Effects of Ambient Conditions on Performance 16
Heat Recovery 16
Performance and Efficiency Enhancements 17
Capital Cost 17
Maintenance 19
Fuels 20
Availability 21
Emissions 22
Fuel Cell Emissions Characteristics 23
Technology Characterization ii Fuel Cells
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Technology Characterization - Fuel Cell Systems
Introduction and Summary
Fuel cell systems, currently in the early stages of development, are an entirely different approach
to the production of electricity than traditional prime mover technologies. Fuel cells are similar
to batteries in that both produce a direct current (DC) through an electrochemical process without
direct combustion of a fuel source. However, whereas a battery delivers power from a finite
amount of stored energy, fuel cells can operate indefinitely provided the availability of a
continuous fuel source. Two electrodes (a cathode and anode) pass charged ions in an electrolyte
to generate electricity and heat. A catalyst enhances the process.
Fuel cells offer the potential for clean, quiet, and efficient power generation. As with most new
technologies, fuel cell systems face a number of formidable market entry issues resulting from
product immaturity, over-engineered system complexities, and unproven product durability and
reliability. These translate into high capital cost, lack of support infrastructure, and technical risk
for early adopters. However, the many advantages of fuel cells suggest that they could well
become the prime mover of choice for certain applications and products in the future.
The inventor of fuel cell technology is Sir William Grove, who demonstrated a hydrogen fuel
cell in London in the 1830s. Grove's technology remained without a practical application for
100 years. Fuel cells returned to the laboratory in the 1950s when the United States space
program required the development of new power systems. Today, the topic of fuel cells
encompasses a broad range of different technologies, technical issues, and market dynamics that
make for a complex but potentially promising outlook. Significant amounts of public and private
investment are being applied to the development of fuel cell products for both stationary and
transportation applications.
There are five types of fuel cells under development. These are: 1) phosphoric acid (PAFC), 2)
proton exchange membrane (PEMFC), 3) molten carbonate (MCFC), 4) solid oxide (SOFC), and
5) alkaline (AFC). The electrolyte and operating temperatures distinguish each type. Operating
temperatures range from near ambient to 1,800°F, and electrical generating efficiencies range
from 30 to over 50% HHV.1 As a result, they can have different performance characteristics,
advantages and limitations, and therefore will be suited to distributed generation applications in a
variety of approaches.
The different fuel cell types share certain important characteristics. First, fuel cells are not
Carnot cycle (thermal energy based) engines. Instead, they use an electrochemical or battery-
like process to convert the chemical energy of hydrogen into water and electricity and can
achieve high electrical efficiencies. The second shared feature is that they use hydrogen as their
^ost of the efficiencies quoted in this report are based on higher heating value (HHV), which includes the heat of
condensation of the water vapor in the products. In engineering and scientific literature the lower heating value
(LHV - which does not include the heat of condensation of the water vapor in the products) is often used. The HHV
is greater than the LHV by approximately 10% with natural gas as the fuel (i.e., 50% LHV versus 45% HHV).
Technology Characterization
1
Fuel Cells
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fuel, which is typically derived from a hydrocarbon fuel such as natural gas. Third, each fuel cell
system is composed of three primary subsystems: 1) the fuel cell stack that generates direct
current electricity; 2) the fuel processor that converts the natural gas into a hydrogen-rich feed
stream; and 3) the power conditioner that processes the electric energy into alternating current or
regulated direct current. Finally, all types of fuel cells have low emissions profiles. This is
because the only combustion processes are the reforming of natural gas or other fuels to produce
hydrogen and the burning of a low energy hydrogen exhaust stream that is used to provide heat
to the fuel processor.
Today, there are only two commercially available fuel cells, a 200 kW PAFC unit2 and a 250 kW
MCFC unit.3 With over 200 units sold, the PAFC fleet has achieved over 5 million operating
hours in a variety of distributed generation applications. These range from a New York City
police station to a major postal facility in Alaska and a credit card processing system in
Nebraska. Located in over 15 countries, this initial commercial fuel cell product has successfully
introduced the capabilities and features of fuel cells into the distributed generation marketplace.
The MCFC fleet is more limited, with half a dozen commercial units, but several more in the
development stages. While nearly two dozen companies are currently field testing a variety of
alternative fuel cell systems for market entry, the availability of a wide array of off-the-shelf,
fully warranted fuel cell systems designed for broad customer classes is still several years away.
Applications
Fuel cell systems are envisioned to serve a variety of distributed generation applications and
markets. Since all fuel cells are in an early stage of development, there is limited experience to
validate those applications considered most competitive for fuel cells. This early stage of
development and commercial use causes fuel cells to be high in capital cost and to have a higher
project risk due to unproven durability and reliability. These two characteristics will force
introductory fuel cell products into specific markets and applications that are most tolerant of
risk due to other market or operational drivers.
In DG markets, the primary characteristic driving early market acceptance is the ability of fuel
cell systems to provide reliable premium power. The primary interest drivers have been their
ability to achieve high efficiencies over a broad load profile and low emission signatures without
additional controls.
Potential DG applications for fuel cell systems include combined heat and power (CHP),
premium power, remote power, grid support, and a variety of specialty applications. Figure 1
illustrates two actual sites with fuel cell systems functioning in DG applications.
2 Sold by UTC Fuel Cells as the PC25, www.utcfuelcells.com.
3 Sold by FuelCell Energy as the DFC® 300, www.fce.com.
Technology Characterization
2
Fuel Cells
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Figure 1. FuelCell Energy Molten Carbonate Fuel Cells in Distributed Generation
Applications
Source: www.fuel cells, org/pics.
Combined Heat and Power
Due to the high installed cost of fuel cell systems, the most prevalent DG application envisioned
by product development leaders is CHP. CHP applications are on-site power generation in
combination with the recovery and use of by-product heat. Continuous baseload operation and
the effective use of the thermal energy contained in the exhaust gas and cooling subsystems
enhance the economics of on-site generation applications.
Heat is generally recovered in the form of hot water or low-pressure steam (<30 psig), but the
quality of heat is dependent on the type of fuel cell and its operating temperature. The one
exception to this is the PEM fuel cell, which operates at temperatures below 200°F, and therefore
has only low quality heat. Generally, the heat recovered from fuel cell CHP systems is
appropriate for low temperature process needs, space heating, and potable water heating. In the
case of SOFC and MCFC technologies, medium pressure steam (up to about 150 psig) can be
generated from the fuel cell's high temperature exhaust gas, but the primary use of this hot
exhaust gas is in recuperative heat exchange with the inlet process gases.
The simplest thermal load to supply is hot water. Primary applications for CHP in the
commercial/institutional sectors are those building types with relatively high and coincident
electric and hot water/space heating demand such as colleges and universities, hospitals and
nursing homes, and lodging. Technology developments in heat activated cooling/refrigeration
and thermally regenerated desiccants will enhance fuel cell CHP applications by increasing the
thermal energy loads in certain building types. Use of these advanced technologies in
applications such as restaurants, supermarkets, and refrigerated warehouses provides a base-
thermal load that opens these applications to CHP.
Technology Characterization
3
Fuel Cells
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Premium Power
Consumers who require higher levels of reliability or power quality, and are willing to pay for it,
often find some form of DG to be advantageous. These consumers are typically less concerned
about the initial prices of power generating equipment than other types of consumers. Premium
power systems generally supply base load demand. As a result, and in contrast to back-up
generators, emissions and efficiency become more significant decision criteria.
Fuel cell systems offer a number of intrinsic features that make them suitable for the premium
power market. These market-driving features include low emissions/vibration/noise, high
availability, good power quality, and compatibility with zoning restrictions. As emissions
become more relevant to a business's bottom line in the form of zoning issues and emissions
credits, the fuel cell becomes a more appealing type of DG.
Some types of fuel cell systems have already demonstrated high availability and reliability. As
fuel cells further mature in the market, they are expected to achieve the high reliability associated
with fewer moving parts.
While the fuel cell requires significant power conditioning equipment in the form of direct
current to alternating current conversion, power from fuel cell systems is clean, exhibiting none
of the signal disturbances observed from grid sources.
Finally, zoning issues for fuel cell systems are quite possibly the least problematic of all DG
systems. Fuel cell systems can be designed for both indoor and outdoor installation, and in close
proximity to sensitive environments, people, or animals.
Remote Power
In locations where power from the local grid is unavailable or extremely expensive to install, DG
is a competitive option. As with premium power, remote power applications are generally base
load operations. Consequently, emissions and efficiency become more significant criteria in
much of the remote power DG market. Coupled with their other potential advantages, fuel cell
systems can provide competitive energy into certain segments of the remote power DG market.
Where fuel delivery is problematic, the high efficiency of fuel cell systems can also be a
significant advantage.
Grid Support
One of the first applications that drew the attention of electric utilities to fuel cell technologies
was grid support. Numerous examples of utility-owned and operated distributed generating
systems exist in the U.S. and abroad. The primary application in the U.S. has been the use of
relatively large diesel or natural gas engines for peaking or intermediate load service at
municipal utilities and electric cooperatives. These units provide incremental peaking capacity
and grid support for utilities at substations. Such installations can defer the need for T&D
system expansion, can provide temporary peaking capacity within constrained areas, or be used
for system power factor correction and voltage support, thereby reducing costs for both
Technology Characterization
4
Fuel Cells
-------
customers and the utility system. The unique feature of fuel cell systems is the use of power
conditioning inverters to transform direct current electricity into alternating current. These
power conditioners can be operated almost independent of the fuel cell to correct power factors
and harmonic characteristics in support of the grid.
Standby Power
Fire and safety codes require standby power systems for hospitals, elevator loads, and water
pumping. Standby is an economic choice for customers with high outage costs such as those in
the telecommunications, retail, gaming, banking, and certain process industries. The standby
engine-driven generator set is typically the simplest distributed generation system, providing
power only when the primary source is out of service or falters in its voltage or frequency. This
application requires low capital cost, minimal installation costs, rapid black start capability,
onsite fuel storage, and grid-isolated operation. In standby power applications, efficiency,
emissions, and variable maintenance costs are usually not major factors in technology selection.
Based on this definition of standby power, fuel cells do not appear to have much application.
Fuel cell systems are characteristically high in capital cost and do not have rapid black start
capability.
Peak Shaving
In certain areas of the country, customers and utilities are using on-site power generation to
reduce the need for costly peak-load power. Peak shaving is also applicable to customers with
poor load factor and/or high demand charges. Typically, peak shaving does not involve heat
recovery, but heat recovery may be warranted where the peak period is more than 2,000
hours/year. Since low equipment cost and high reliability are the primary requirements,
equipment such as reciprocating engines are ideal for many peak-shaving applications.
Emissions may be an issue if operating hours are high. Combining peak shaving and another
function, such as standby power, enhances the economics. High capital cost and relatively long
start-up times (particularly for MCFC and SOFC) will most likely prevent the widespread use of
fuel cells in peak shaving applications.
Technology Description
Fuel cells produce direct current electricity through an electrochemical process, much like a
standard battery. Unlike a standard battery, a fuel supply continuously replenishes the fuel cell.
The reactants, most typically hydrogen and oxygen gas, are fed into the fuel cell reactor, and
power is generated as long as these reactants are supplied. The hydrogen (H2) is typically
generated from a hydrocarbon fuel such as natural gas or LPG, and the oxygen (O2) is from
ambient air.
Basic Processes and Components
Fuel cell systems designed for DG applications are primarily natural gas or LPG fueled systems.
Each fuel cell system consists of three primary subsystems: 1) the fuel cell stack that generates
direct current electricity; 2) the fuel processor that converts the natural gas into a hydrogen rich
Technology Characterization
5
Fuel Cells
-------
feed stream; and 3) the power conditioner that processes the electric energy into alternating
current or regulated direct current.
Figure 2 illustrates the electrochemical process in a typical single cell, acid-type fuel cell. A
fuel cell consists of a cathode (positively charged electrode), an anode (negatively charged
electrode), an electrolyte and an external load. The anode provides an interface between the fuel
and the electrolyte, catalyzes the fuel reaction, and provides a path through which free electrons
conduct to the load via the external circuit. The cathode provides an interface between the
oxygen and the electrolyte, catalyzes the oxygen reaction, and provides a path through which
free electrons conduct from the load to the oxygen electrode via the external circuit. The
electrolyte, an ionic conductive (non-electrically conductive) medium, acts as the separator
between hydrogen and oxygen to prevent mixing and the resultant direct combustion. It
completes the electrical circuit of transporting ions between the electrodes.
Figure 2. Fuel Cell Electrochemical Process
oooo
Anode
H+ H+ H+ H+
Electrolyte
Cat]
Source: Energy Nexus Group.
The hydrogen and oxygen are fed to the anode and cathode, respectively. The hydrogen and
oxygen gases do not directly mix and combustion does not occur. Instead, the hydrogen oxidizes
one molecule at a time, in the presence of a catalyst. Because the reaction is controlled at the
molecular level, there is no opportunity for the formation of NOx and other pollutants.
At the anode the hydrogen gas is electrochemically dissociated (in the presence of a catalyst) into
hydrogen ions (H+) and free electrons (e ).
Anode Reaction: 2H2 -> 4H+ + 4e~
Technology Characterization
6
Fuel Cells
-------
The electrons flow out of the anode through an external electrical circuit. The hydrogen ions
flow into the electrolyte layer and eventually to the cathode, driven by both concentration and
potential forces. At the cathode the oxygen gas is electrochemically combined (in the presence
of a catalyst) with the hydrogen ions and free electrons to generate water.
Cathode Reaction: O2 + 4H+ + 4e~ -> 2H20
The overall reaction in a fuel cell is as follows:
Net Fuel Cell Reaction: 2H2 + 02 -> 2H20 (vapor) + Energy
The amount of energy released is equal to the difference between the Gibbs free energy of the
product and the Gibbs free energy of the reactants.
When generating power, electrons flow through the external circuit, ions flow through the
electrolyte layer and chemicals flow into and out of the electrodes. Each process has natural
resistances, and overcoming these reduces the operational cell voltage below the theoretical
potential. There are also irreversibilities4 that affect actual open circuit potentials. Therefore,
some of the chemical potential energy converts into heat. The electrical power generated by the
fuel cell is the product of the current measured in amps and the operational voltage. Based on
the application and economics, a typical operating fuel cell will have an operating voltage of
between 0.55 volts and 0.80 volts. The ratio of the operating voltage and the theoretical
maximum of 1.48 volts represents a simplified estimate of the stack electrical efficiency on a
higher heating value (HHV ) basis.
As explained, resistance heat is also generated along with the power. Since the electric power is
the product of the operating voltage and the current, the quantity of heat that must be removed
from the fuel cell is the product of the current and the difference between the theoretical potential
and the operating voltage. In most cases, the water produced by the fuel cell reactions exits the
fuel cell as vapor, and therefore, the 1.23-volt LHV theoretical potential is used to estimate
sensible heat generated by the fuel cell electrochemical process.
The overall electrical efficiency of the cell is the ratio of the power generated and the heating
value of the hydrogen consumed. The maximum thermodynamic efficiency of a hydrogen fuel
cell is the ratio of the Gibbs free energy and the heating value of the hydrogen. The Gibbs free
energy decreases with increasing temperatures, because the product water produced at the
elevated temperature of the fuel cell includes the sensible heat of that temperature, and this
energy cannot be converted into electricity without the addition of a thermal energy conversion
cycle (such as a steam turbine). Therefore, the maximum efficiency of a pure fuel cell system
decreases with increasing temperature. Figure 3 illustrates this characteristic in comparison to
4 Irreversibilities are changes in the potential energy of the chemical that are not reversible through the
electrochemical process. Typically, some of the potential energy is converted into heat even at open circuit
conditions when current is not flowing. A simple example is the resistance to ionic flow through the electrolyte
while the fuel cell is operating. This potential energy "loss" is really a conversion to heat energy, which cannot be
reconverted into chemical energy directly within the fuel cell.
5 Most of the efficiencies quoted in this report are based on higher heating value (HHV), which includes the heat of
condensation of the water vapor in the products.
Technology Characterization
1
Fuel Cells
-------
the Carnot cycle efficiency limits through a condenser at 50 and 100°C.6 This characteristic has
led system developers to investigate hybrid fuel cell-turbine combined cycle systems to achieve
system electrical efficiencies in excess of 70% HHV.
Figure 3. Effect of Operating Temperature on Fuel Cell Efficiency
100%-
90%-
>; 80%-
H2 Fuel Cell
Carnot at 21 OF Condensor
¦ - Carnot at 120F Condensor
0 H2 Stack Efficiency
o
LU
O 60%-
E
E 40%-
30%-
E
D
E
20%-
10%-
O NG Fuel Cell Systems
0%-
0
200 400 600 800 1000 1200 1400 1600 1800 2000
Operational Temperature, F
Source: Siemens/Westinghouse Electric Corp.
Fuel Cell Stacks
Practical fuel cell systems require voltages higher than 0.55 to 0.80. Combining several cells in
electrical series into a fuel cell stack achieves this. Typically, there are several hundred cells in a
single cell stack. Increasing the active area of individual cells manages current flow. Typically,
cell area can range from 100 cm2 to over 1 m2 depending on the type of fuel cell and application
power requirements.
Fuel Processors
In distributed generation applications, the most viable fuel cell technologies use natural gas as
the system's fuel source. To operate on natural gas or other fuels, fuel cells require a fuel
processor or reformer, a device that converts the fuel into the hydrogen-rich gas stream. While
adding fuel flexibility to the system, the reformer also adds significant cost and complexity.
There are three primary types of reformers: steam reformers, autothermal reformers, and partial
oxidation reformers. The fundamental differences are the source of oxygen used to combine
with the carbon within the fuel to release the hydrogen gases and the thermal balance of the
chemical process. Steam reformers use steam, while partial oxidation units use oxygen gas, and
autothermal reformers use both steam and oxygen
6 Larminie, James and Andrew Dicks, Fuel Cell Systems Explained. John Wiley & Sons, Ltd., West Sussex,
England, 2000.
Technology Characterization 8 Fuel Cells
-------
Steam reforming is extremely endothermic and requires a substantial amount of heat input.
Autothermal reformers typically operate at or near the thermal neutral point, and therefore, do
not generate or consume thermal energy. Partial oxidation units combust a portion of the fuel
(i.e. partially oxidize it), releasing heat in the process. When integrated into a fuel cell system
that allows the use of anode-off gas, a typical natural gas reformer can achieve conversion
efficiencies in the 75 to 90% LHV range, with 83 to 85% being an expected level of
performance. These efficiencies are defined as the LHV of hydrogen generated divided by the
LHV of the natural gas consumed by the reformer.
Some fuel cells can function as internally steam reforming fuel cells. Since the reformer is an
endothermic catalytic converter and the fuel cell is an exothermic catalytic oxidizer, the two
combine into one with mutual thermal benefits. More complex than a pure hydrogen fuel cell,
these types of fuel cells are more difficult to design and operate. While combining two catalytic
processes is difficult to arrange and control, these internally reforming fuel cells are expected to
account for a significant market share as fuel cell based DG becomes more common.
Power Conditioning Subsystem
The fuel cell generates direct current electricity, which requires conditioning before serving a
DG application. Depending on the cell area and number of cells, this direct current electricity is
approximately 200 to 400 volts per stack. If the system is large enough, stacks can operate in
series to double or triple individual stack voltages. Since the voltage of each individual cell
decreases with increasing load or power, the output is considered an unregulated voltage source.
The power conditioning subsystem boosts the output voltage to provide a regulated higher
voltage input source to an electronic inverter. The inverter then uses a pulse width modulation
technique at high frequencies to generate simulated alternating current output. The inverter
controls the frequency of the output, which can be adjusted to enhance power factor
characteristics. Because the inverter generates alternating current within itself, the output power
is generally clean and reliable. This characteristic is important to sensitive electronic equipment
in premium power applications. The efficiency of the power conditioning process is typically 92
to 96%, and is dependent on system capacity and input voltage-current characteristic.
Types of Fuel Cells
There are five basic types of fuel cell under consideration for DG applications. The fuel cell's
electrolyte or ion conduction material defines the basic type. Two of these fuel cell types,
polymer electrolyte membrane (PEM) and phosphoric acid fuel cell (PAFC), have acidic
electrolytes and rely on the transport of H+ ions. Two others, alkaline fuel cell (AFC) and
carbonate fuel cell (MCFC), have basic electrolytes that rely on the transport of OH" and CO32"
ions, respectively. The fifth type, solid oxide fuel cell (SOFC), is based on a solid-state ceramic
electrolyte in which oxygen ions (02~) are the conductive transport ion.
Each fuel cell type operates at optimum temperature, which is a balance between the ionic
conductivity and component stability. These temperatures differ significantly among the five
basic types, ranging from near ambient to as high as 1800°F. The proton conducting fuel cell
Technology Characterization
9
Fuel Cells
-------
type generates water at the cathode and the anion conducting fuel cell type generates water at the
anode. Table 1 below presents fundamental characteristics for each fuel cell type.
Table 1. Characteristics of Major Fuel Cell Types
PEMFC
AFC
PAFC
MCFC
SOFC
Type of Electrolyte
H+ ions (with
anions bound
in polymer
membrane)
OH" ions
(typically
aqueous KOH
solution)
H+ ions
(H3PO4
solutions)
C032" ions
(typically,
molten
LiKaC03
eutectics)
O2" ions
(Stabilized
ceramic matrix
with free oxide
ions)
Typical construction
Plastic, metal,
or carbon
Plastic, metal
Carbon,
porous
ceramics
High temp
metals, porous
ceramic
Ceramic, high
temp metals
Internal reforming
No
No
No
Yes, Good
Temp Match
Yes, Good
Temp Match
Oxidant
Air to 02
Purified Air to
o2
Air to
Enriched Air
Air
Air
Operational
Temperature
150- 180°F
(65-85°C)
190-500°F
(90-260°C)
370-410°F
(190-210°C)
1,200-1,300°F
(650-700°C)
1,350-1,850°F
(750-1,000°C)
DG System Level
Efficiency, % HHV
25 to 35%
32 to 40%
35 to 45%
40 to 50%
45 to 55%
Primary
Contaminate
Sensitivities
CO, Sulfur,
and NH3
CO, C02, and
Sulfur
CO< 1%,
Sulfur
Sulfur
Sulfur
Source: Energy Nexus Group
PEMFC (Proton Exchange Membrane Fuel Cell or Polymer Electrolyte Membrane)
NASA developed this type of fuel cell in the 1960s for the first manned spacecraft. The PEMFC
uses a solid polymer electrolyte and operates at low temperatures (about 200°F). Over the past
ten years, the PEMFC has received significant media coverage due to the large auto industry
investment in the technology. Due to their modularity and potential for simple manufacturing,
reformer/PEMFC systems for residential DG applications have attracted considerable
development capital. PEMFC's have high power density and can vary their output quickly to
meet demand. This type of fuel cell is highly sensitive to CO poisoning.
AFC (Alkaline Fuel Cell)
F.T. Bacon in Cambridge, England first demonstrated AFC as a viable power unit during the
1940s and 1950s. NASA later developed and used this fuel cell on the Apollo spacecraft and on
the space shuttles. AFC technology uses alkaline potassium hydroxide as the electrolyte. The
primary advantages of AFC technology are improved performance (electrical efficiencies above
60% HHV), use of non-precious metal electrodes, and the fact that no unusual materials are
needed. The primary disadvantage is the tendency to absorb carbon dioxide, converting the
alkaline electrolyte to an aqueous carbonate electrolyte that is less conductive. The
attractiveness of AFC has declined substantially with the interest and improvements in PEMFC
technology.
PAFC (Phosphoric Acid Fuel Cell)
Technology Characterization
10
Fuel Cells
-------
PAFC uses phosphoric acid as the electrolyte and is generally considered the most established
fuel cell technology. The first PAFC DG system was designed and demonstrated in the early
1970s. PAFCs are capable of fuel-to-electricity efficiencies of 36% HHV or greater. A 200 kW
PAFC has been commercially available since the early 1990s. Over 200 of these commercial
units were manufactured, delivered, and are operating in the U.S., Europe, and Japan. The
current 200 kW product is reported to have a stack lifetime of over 40,000 hours, units with
nearly eight years of operation, and commercially based reliabilities in the 90 to 95% range. The
major market barrier has been the initial installed cost that has not yet fallen below the $4,500 to
$5,500/kW range.
MCFC (Molten Carbonate Fuel Cell)
The MCFC uses an alkali metal carbonate (Li, Na, K) as the electrolyte and has a developmental
history that dates back to the early part of the twentieth century. Due to its operating temperature
range of 1,100 to 1,400°F, the MCFC holds promise in both CHP and DG applications. This
type of fuel cell can be internally reformed, can operate at high efficiencies (50% HHV), and is
relatively tolerant of fuel impurities. Government/industry R&D programs during the 1980s and
1990s resulted in several individual pre-prototype system demonstrations. FuelCell Energy is
presently the only company with a commercial molten carbonate fuel cell. The primary
technical issue with MCFC technologies is the degradation of cell components due to the
corrosive nature of the electrolyte/operating temperature combination.
SOFC (Solid Oxide Fuel Cell)
The SOFC uses solid, nonporous metals oxide electrolytes and is generally considered less
mature in its development than the MCFC and PAFC technologies. Several SOFC units up to
100 kW in size and based on a concentric tubular design have been built and tested.7 In addition,
there are many companies developing planar SOFC designs, which offer higher power densities
and lower costs than the tubular design, but these have yet to achieve the reliability of the tubular
design. Despite relative immaturity, the SOFC has several advantages (high efficiency, stability
and reliability, and high internal temperatures) that have attracted development support. The
SOFC has projected service electric efficiencies of 45 to 60% and higher, for larger hybrid,
combined cycle plants. Efficiencies for smaller SOFC DG units are expected to be in the 50%
range.
Stability and reliability of the SOFC are due to an all-solid-state ceramic construction. Test units
have operated in excess of 10 years with acceptable performance. The high internal
temperatures of the SOFC are both an asset and a liability. As an asset, high temperatures make
internal reforming possible. As a liability, these high temperatures add to materials and
mechanical design difficulties, which reduces stack life and increases cost. While SOFC
research has been ongoing for 30 years, costs of these stacks are still comparatively high.
Design Characteristics
The features that have the potential to make fuel cell systems a leading prime mover for CHP
and other distributed generation applications include:
7 By Siemens/Westinghouse Electric Corp.
Technology Characterization
11
Fuel Cells
-------
Size range:
Fuel cell systems are constructed from individual cells that
generate 100 W to 2 kW per cell. This allows systems to have
extreme flexibility in capacity. Systems under development for
DG application range in sizes from 5 kW to 2 MW. Multiple
systems can operate in parallel at a single site to provide
incremental capacity.
Thermal output:
Fuel cells can achieve overall efficiencies in the 65 to 85% range.
Waste heat can be used primarily for domestic hot water
applications and space heating.
Availability:
The commercially available 200 kW PC25 system fleet (200-plus
units) has demonstrated greater than 90% availability during over
four million operating hours. As fuel cell systems mature, their
reliability should improve.
Part-load operation:
Fuel cell stack efficiency improves at lower loads, which results in
a system electric efficiency that is relatively steady down to one-
third to one-quarter of rated capacity. This provides systems with
excellent load following characteristics.
Cycling:
While part-load efficiencies of fuel cells are generally high, MCFC
and SOFC fuel cells require long heat-up and cool-down periods,
restricting their ability to operate in many cyclic applications.
High quality power:
Electrical output is computer grade power, meeting critical power
requirements without interruption. This minimizes lost
productivity, lost revenues, product loss, or opportunity cost.
Reliability and life:
Since only auxiliary components have moving parts, the reliability
of fuel cells is expected to be high. A few of the initial PC25
systems have achieved operational lives of 70,000 hours.
Emissions:
The only combustion within a fuel cell system is the low energy
content hydrogen stream exhausted from the stack. This stream is
combusted within the reformer and can achieve emissions
signatures of < 2 ppmv CO, <1 ppmv NOx and negligible SOx (on
15% O2, dry basis).
Efficiency:
Different types of fuel cells have varied efficiencies. Depending
on the type and design of fuel cells, electric efficiency ranges from
30% to over 50% HHV.
Quiet Operation:
Conversational level (60dBA
installation.
30 ft.), acceptable for indoor
Technology Characterization
12
Fuel Cells
-------
Siting and Size:
Indoor or outdoor installation.
Fuel Use: The primary fuel source for the fuel cell is hydrogen, which can be
obtained from natural gas, coal gas, methanol, and other fuels
containing hydrocarbons.
Performance Characteristics
Fuel cell performance is a function of the type of fuel cell and its capacity. Since the fuel cell
system is a series of chemical, electrochemical, and electronic subsystems, the optimization of
electric efficiency and performance characteristics can be a challenging engineering task. The
electric efficiency calculation example provided in the next section illustrates this.
Table 2 summarizes performance characteristics for representative commercially available and
developmental natural gas fuel cell CHP systems over the 10 kW to 2 MW size range. This size
range covers the majority of the market applications currently envisioned for fuel cell CHP and
represents the most likely units to be commercially introduced within the next five years. Of the
systems included in Table 2, the only commercially available at this time is the PAFC product,
first introduced in 1992. The other systems are in various phases of prototype demonstration.
Cost and performance estimates for these systems are based on initial market entry targets. The
capital cost estimate for the PAFC system represents published cost from the manufacturer for
lots of three or more units. Since the other systems are just emerging from their demonstration
phases, pricing and costing information are subjective and estimates should be considered within
the plus or minus thirty percent range.
Technology Characterization
13
Fuel Cells
-------
Table 2. Fuel Cell CHP - Typical Performance Parameters
Cost and Performance Characteristics8
System 1
System 2
System 3
System 4
System 5
System 6
Fuel Cell Type
PAFC
PEM
PEM
MCFC
MCFC
SOFC
Nominal Electricity Capacity (kW)
200
10
200
250
2,000
100
Commercial Status 20029
Com'l
Demo
Demo
Demo
Demo
Demo
Operating Temperature (°F)
400
150
150
1200
1200
1750
Package Cost (2002 $/kW)10
3,850
4,700
2,950
4,350
2,400
2,850
Total Installed Cost (2002 $/kW)11
4,500
5,500
3,600
5,000
2,800
3,500
O&M Costs ($/kW)12
0.029
0.033
0.023
0.043
0.033
0.023
Electric Heat Rate (Btu/kWh)13
9,480
11,370
9,750
7,930
7,420
7,580
Electrical Efficiency (% HHV)14
36%
30%
35%
43%
46%
45%
Fuel Input (MMBtu/hr)
1.90
0.10
2.00
2.00
14.80
0.80
CHP Characteristics
Heat Avail. >160°F (MMBtu/hr)
0.37
0.00
0.00
0.22
1.89
0.10
Heat Avail. <160°F (MMBtu/hr)
0.37
0.04
0.72
0.22
1.67
0.09
Heat Output (MMBtu/hr)
0.74
0.04
0.72
0.44
3.56
0.19
Heat Output (kW equivalent)
217
13
211
128
1043
56
Total CHP Efficiency (%), HHV15
75%
68%
72%
65%
70%
70%
Power/Heat Ratio16
0.92
0.77
0.95
1.95
1.92
1.79
Net Heat Rate (Btu/kWh)17
4,860
6,370
5,250
5,730
5,200
5,210
Effective Electrical Eff (%), HHV
70.3%
53.6%
65.0%
59.5%
65.7%
65.6%
Source: Energy Nexus Group.
8 Data are representative typical values for developmental systems based on available information from fuel cell
system developers. Only the PAFC data are representative of a commercial product available in 2002. Developers
include but are not limited to UTC Fuel Cells, Toshiba, Ballard Power, Plug Power, Fuel Cell Energy, Siemens-
Westinghouse, H-Power, Hydrogenics, Honeywell, Fuji, IHI, Global Thermal, Mitsubishi Heavy Industries, and
Ztek.
9 Com'l = Commercially Available; Demo = Multiple non-commercial demonstrations completed or underway in
field sites with potential customers; Lab = Characteristics observed in laboratory validation testing of complete
systems; Exp = Only experimental prototypes have been tested.
10 Packaged Cost includes estimates of typical costs for a CHP compatible system with grid interconnection
functionality built into power conditioning subsystem.
11 Total Installed Cost include estimates for packaged cost plus electrical isolation equipment, hot water CHP
interconnections, site labor and preparation, construction management, engineering, contingency, and interest during
construction. See Table 3.
12 O&M costs are estimated based on service contract nominal rate, consumables, fixed costs, and sinking fund for
stack replacement at end of life. See Table 4.
13 All equipment manufacturers quote heat rates in terms of the lower heating value (LHV) of the fuel. On the other
hand, the usable energy content of fuels is typically measured on a higher heating value (HHV) basis. In addition,
electric utilities measure power plant heat rates in terms of HHV. For natural gas, the average heat content of
natural gas is 1,030 Btu/scf on an HHV basis and 930 Btu/scf on an LHV basis - or about a 10% difference.
14 Electrical efficiencies are net of parasitic and conversion losses.
15 Total Efficiency = (net electric generated + net heat produced for thermal needs)/total system fuel input
16 Power/Heat Ratio = CHP electrical power output (Btu)/ useful heat output (Btu)
17 Effective Electrical Efficiency = (CHP electric power output)/(Total fuel into CHP system - total heat
recovered/0.8). Equivalent to 3,412 Btu/kWh/Net Heat Rate and Net Heat Rate = 3412/Effective Elec Eff.
Technology Characterization
14
Fuel Cells
-------
Heat rates and efficiencies shown were taken from manufacturers' specifications, industry
publications, or are based on the best available data for developing technologies. Available
thermal energy was calculated from estimated overall efficiency for these systems. CHP thermal
recovery estimates are based on producing low quality heat for domestic hot water process or
space heating needs. This feature is generally acceptable for commercial/institutional
applications where it is more common to have hot water thermal loads.
The data in the table show that electrical efficiency increases as the operating temperature and
size of the fuel cell increases. As electrical efficiency increases, the absolute quantity of thermal
energy available to produce useful thermal energy decreases per unit of power output, and the
ratio of power to heat for the CHP system generally increases. A changing ratio of power to heat
impacts project economics and may affect the decisions that customers make in terms of CHP
acceptance, sizing, and the desirability of selling power.
Electrical Efficiency
As with all generation technologies, the electrical efficiency is the ratio of the power generated
and the heating value of the fuel consumed. Because the fuel cell system has several subsystems
in series, the electrical efficiency of the DG unit is the multiple of the efficiencies of the
individual section. The concept of stack electric efficiency was introduced earlier. The
following calculates the electric efficiency of a fuel cell system:
ElecEff = (FPS Eff * H2 Utilization * Stack Eff * PC Eff) * (HHV/LHV ratio of the fuel)
Where: FPS Eff = Fuel Processing Subsystem Efficiency, LLV
= (LHV of H2 Generated/LHV of Fuel Consumed)
H2 Utilization = '/< of H2 actually consumed in the stack
Stack Eff = (Operating Voltage/Energy Potential -1.23 volts)
PC Eff = AC power delivered/(DC power generated)
(auxiliary loads are assumed DC loads here)
For example: PAFC = (84%FPS)*(83% util)*(0.75V/1.25V)*(95%PC)*(0.9HHV/LHV)
= 36% electric efficiency HHV
As the operating temperature range of the fuel cell system increases, the electric efficiency of the
systems tends to increase. Although the maximum thermodynamic efficiency decreases as
shown in Figure 2, improvements in reformer subsystem integration and increases in reactant
activity balance out to provide the system level increase. Advanced high temperature MCFC and
SOFC systems are projected to achieve simple cycle efficiencies in the range of 50 to 55% HHV,
while hybrid combined fuel cell-heat engine systems are calculated to achieve efficiencies above
60% in DG applications.
Part-Load Performance
In power generation and CHP applications, fuel cell systems follow either the electric or thermal
load of the applications to maximize DG energy economics. Figure 4 shows the part-load
efficiency curve for a market entry PAFC fuel cell in comparison to a typical lean burn natural
Technology Characterization
15
Fuel Cells
-------
gas engine. The efficiency at 50% load is within 2% of its full load efficiency characteristic. As
the load decreases further, the curve becomes somewhat steeper, as inefficiencies in air blowers
and the fuel processor begin to override the stack efficiency improvement.
38%
36%
>
X 34%
vP
cr-
£? 32%
C
0)
I 30%
.o
I 28%
LU
26%
24%
0% 20% 40% 60% 80% 100% 120%
Part Load, % of Rated Power, %
Source: Gas Research Institute, Caterpillar, and Energy Nexus Group.
Effects of Ambient Conditions on Performance
Fuel cells are rated at ISO conditions of 77°F and 0.987 atmospheres (1 bar) pressure. Fuel cell
system performance - both output and efficiency - can degrade as ambient temperature or site
elevation increases. Ancillary equipment performance, primarily the air handling blowers or
compressors, accounts for the degradation of performance. Performance degradations will be
greater for pressurized systems operating with turbo-chargers or small air compressors as their
primary air supply components.
Heat Recovery
The economics of fuel cells in on-site power generation applications depend less on effective use
of the thermal energy recovered than is the case with lower efficiency prime movers, but thermal
load displacements improves operating economics as in any CHP application. Generally, the
stack and reformer subsystems contain 25% of the inlet fuel energy in the form of higher quality
thermal energy. The exhaust gases (includes the latent heat of the product water generated in the
fuel cell) contain another 25% of the recoverable energy. The most common use of this heat is
to generate hot water or low-pressure steam for process use or for space heating, process needs,
or domestic hot water.
Figure 4. Comparison oj Part Load Efficiency Derate
PAFC Rated at
200kW
/ /' Typical Lean Burn Engine
in 0.5 to 3 MW Range
Technology Characterization
16
Fuel Cells
-------
Heat can generally be recovered in the form of hot water or low-pressure steam (< 30 psig), but
the quality of heat is dependent on the type of fuel cell and its operating temperature. The one
exception to this is the PEM fuel cell, which operates at temperatures below 100°C, and
therefore has only low quality heat.
As an example, there are four primary potential sources of usable waste heat from a fuel cell
system: exhaust gas including water condensation, stack cooling, anode-off gas combustion, and
reformer heat. The PAFC system achieves 36% electric efficiency and 72% overall CHP
efficiency, which means that it has a 36% thermal efficiency or power to heat ratio of one. Of
the available heat, 25 to 45% is recovered from the stack-cooling loop that operates at
approximately 400°F and can deliver low- to medium-pressure steam. The exhaust gas-cooling
loop provides the balance of the heat and serves two functions. The first is condensation of
product water, thus rendering the system water self-sufficient, and the second is the recovery of
by-product heat. Since its primary function is water recovery, the balance of the heat available
from the PAFC fuel cell is recoverable with 120°F return and 300°F supply temperatures. This
tends to limit the application of this heat to domestic hot water applications. Maximum system
efficiency occurs when all of the available anode-off gas heat and internal reformer heat is used
internally.
In the case of SOFC and MCFC fuel cells, medium-pressure steam (up to about 150 psig) can be
generated from the fuel cell's high temperature exhaust gas, but the primary use of these hot
exhaust gas is in recuperative heat exchange with the inlet process gases. Like engine and
turbine systems, the fuel cell exhaust gas can be used directly for process drying.
Performance and Efficiency Enhancements
Air is fed to the cathode side of the fuel cell stack to provide the oxygen needed for the power
generation process. Typically, 50 to 100% more air passes through the cathode than the fuel cell
reactions require. The fuel cell operated at near-ambient pressure, or at elevated pressures to
enhance stack performance. Increasing the pressure, and therefore the partial pressure of the
reactants, increases stack performance by reducing the electrode over potentials associated with
moving the reactants into the electrodes where the catalytic reaction occurs. It also improves the
performance of the catalyst. Assuming optimistic compressor characteristics, these
1 R
improvements appear to optimize at approximately three atmospheres pressure. More realistic
assumptions often result in optimizations at ambient pressure where air movement requires the
least energy. Because of these characteristics, developers focus on both pressurized and ambient
pressure systems.
Capital Cost
This section provides estimates for the installed cost of fuel cell systems designed for DG
applications. Only CHP configurations are presented because the majority of developments
appear to integrate heat recovery capability primarily to support product water condensation and
self-sufficiency. Capital costs (equipment and installation) are estimated in Table 3 for the six
typical fuel cell systems presented in Table 1 in the 10 kW to 2 MW size range. Estimates are
18 Ibid., p. 90.
Technology Characterization
17
Fuel Cells
-------
"typical" budgetary price levels. Installed costs can vary significantly depending on the scope of
the plant equipment, geographical area, competitive market conditions, special site requirements,
prevailing labor rates, and whether the system is a new or retrofit application.
Based on commercially available information and internal analysis, each of five major
component groups with approximately twenty major components was used to define the
individual fuel cell systems and to develop total package cost estimates. Cost and pricing
information was estimated in constant 2002 dollars and totaled across major component groups
to achieve the estimated packaged cost of each system. This process allowed for uniform
estimates for similar components of similar capacity such as the power electronics, and
adjustments for components such as the cell stack and reformer subsystems due to the
technology differences and product requirements
Following the above approach, each fuel cell system was broken down into the following five
major component groups or subsystems:
• Stack subsystem - consisting of the fuel cell stacks, feed gas manifolds, and power take-
offs.
• Fuel processing subsystem - consisting of fuel management controls, reformer, steam
generators, shift reactors, sulfur absorbent beds, and ancillary components.
• Power and electronic subsystem - consisting of a solid-state boost regulator, DC to AC
inverters, grid interconnect switching, load management and distribution hardware, and
inverter controller and overall supervisory controller.
• Thermal management subsystem - consisting of the stack cooling system, heat recovery
and condensing heat exchangers.
• Ancillary subsystems - consisting of the process air supply blowers, water treatment
system, safety controls and monitoring, cabinet ventilation fans, and other miscellaneous
components.
From a cost and complexity standpoint, each individual system and system developer has a
different perspective on the details of these subsystems. The stack subsystem represents from 25
to 40% of equipment cost, the fuel processing subsystem from 25 to 30%, the power and
electronics subsystem from 10 to 20%, the thermal management subsystem from 10 to 20%, and
ancillary subsystems from 5 to 15%. One of the major issues with fuel cell systems is process
complexities and the cost of equipment to maintain expanded features and characteristics.
The cost of the basic fuel cell package plus the costs for added systems needed for the particular
application comprise the total equipment cost. The total plant cost consists of total equipment
cost plus installation labor and materials (including site work), engineering, project management
(including licensing, insurance, commissioning and startup), and financial carrying costs during
construction. The installation costs of fuel cell systems are relatively consistent with engine-
based equipment. The range of $400 to $800/kW used in Table 2 reflects this similarity but
include slight increases due to issues that will arise because of the uniqueness of the equipment.
No additional costs were applied for emission controls technologies or permitting delays.
Technology Characterization
18
Fuel Cells
-------
Table 3. Estimated Capital Cost for Typical Fuel Cell Systems in
Grid Interconnected CHP Applications ($/kW)*
Installed Cost Components
System 1
System 2
System 3
System 4
System 5
System 6
Nominal Capacity (kW)
200
10
200
250
2000
100
Fuel Cell Type
PAFC
PEM
PEM
MCFC
MCFC
SOFC
Equipment Costs (2002 $/kW)
Packaged Cost
3,850
4,700
2,950
4,350
2,400
2,850
Grid Isolation Breakers19
100
250
100
100
15
120
20
Materials and Labor
272
100
272
280
230
250
Total Process Capital
4,222
5,050
3,370
4,780
2,645
2,270
Other Site Costs (2002 $/kW)21
Proj. & Const. Management
124
280
124
112
80
168
Engineering and Fees
52
90
52
60
25
72
Contingencies
94
80
94
90
20
30
Interest during Construction
8
0
8
8
30
10
Total Installed Cost
(2002 $/kW)
4,500
5,500
3,600
5,000
2,800
3,500
* Estimated capital costs for current technology fuel cell systems in the 2003/04 timeframe.
Source: Energy Nexus Group.
Maintenance
Maintenance costs for fuel cell systems vary with type of fuel cell, size, and maturity of the
equipment. Some of the typical costs that need to be included are:
• Maintenance labor.
• Ancillary replacement parts and material such as air and fuel filters, reformer igniter or
spark plug, water treatment beds, flange gaskets, valves, electronic components, etc., and
consumables such as sulfur adsorbent bed catalysts and nitrogen for shutdown purging.
• Major overhauls include shift catalyst replacement (3 to 5 years), reformer catalyst
replacement (5 years), and stack replacement (4 to 8 years).
In-house personnel can perform basic maintenance, or it can be contracted out to manufacturers,
distributors, or dealers under service contracts. Details of full maintenance contracts (covering
all recommended service) and costing are not generally available, but are estimated at 0.7 to 2.0
cents/kWh excluding the stack replacement cost sinking fund. Maintenance for initial
19 Only grid isolation breakers included because functionality of grid interconnection and isolation has been included
into the power conditioning subsystem with package cost. For example, $100/kW was included for System 1 and
System 3 at 200 kW capacity.
20 Materials and labor estimates were based on typical engine CHP systems for units between 100 and 2,000 kW.
The 10 kW system was estimated based on appliance type residential installations.
21 Other site-related costs were estimated based on the extrapolated data from typical engine CHP systems for units
between 100 and 2,000 kW. The interest during construction for the 10 kW system was set at zero because of the
residential appliance-like nature of equipment.
Technology Characterization
19
Fuel Cells
-------
commercial fuel cells has included remote monitoring of system performance and conditions and
an allowance for predictive maintenance. Routine short interval inspections/adjustments and
periodic replacement of filters (projected at intervals of 2,000 to 4,000 hours) comprise the
recommended service.
Table 4 presents maintenance costs based on service contracts consisting of routine inspections
and scheduled overhauls of the fuel cell system and are prorated based on comparable engine
generator service contracts. Stack life and replacement costs are based on developers' estimates
for initial units. Overall, maintenance costs are based on 8,000 annual operating hours expressed
in terms of annual electricity generation.
Table 4. Estimated Operating and Maintenance Costs
Of Typical CHP Fuel Cell Systems*
O&M Cosl A Mill \ sis <2002 $)"
SjsK'in
1
SjsK'in
2
SjsK'in
' 3
SjsK'in
' 4
SjsK'in
5
SjsK'in
' 6
Nominal Capacity (kW)
200
10
200
250
2,000
100
Fuel Cell Type
PAFC
PEM
PEM
MCFC
MCFC
SOFC
Variable Service Contract ($/kWh)
0.0087
0.0121
0.0087
0.0072
0.0054
0.0102
Variable Consumables ($/kWh)
0.0002
0.0002
0.0002
0.0002
0.0002
0.0002
Fixed ($/kW-yr)
6.5
18.0
6.5
5.0
2.1
10.0
Fixed($/kWh @ 8,000 hrs/yr)
0.0008
0.0023
0.0008
0.0006
0.0003
0.0013
Stack Fund ($/kWh)23
0.0193
0.0188
0.0132
0.0350
0.0275
0.0125
Stack Life (yrs)
5
4
4
4
4
8
Recovery Factor (%)
30%
50%
35%
30%
20%
20%
Net O&M cost ($/kWh)
0.029
0.033
0.023
0.043
0.033
0.023
* Estimated costs for current technology fuel cell systems in the 2003/04 timeframe
Source: Energy Nexus Group.
Fuels
Fuel cell systems operate on a variety of alternative gaseous fuels since the primary fuel source is
hydrogen produced from hydrocarbon fuels. These including the following:
22 Maintenance costs presented in Table 4 are based on 8,000 operating hours expressed in terms of annual
electricity generation. Fixed costs are based on an interpolation of engine manufacturers' estimates and applied to
fuel cell system. The variable component of the O&M cost represents the inspections and minor procedures that are
normally conducted by the original equipment manufacturer through a service agreement, and have been estimated
based on 60% of reciprocating engine service contracts. Major overhaul procedures primarily representing stack
replacements have been handled as a separate item.
23 Stack replacement costs have been estimated = (stack original cost*(l -recovery factor))/(stack life*8000hrs/yr).
Stack life was estimated based on type of fuel cell. Recovery factor was based on catalyst recovery, metal scrap
value and non-repeat hardware value at end of life. All estimates are considered first cut projections and have an
uncertainty of +/- one year and +/- 15%. The small PEM recovery factor was increased due to its higher non-repeat
component cost.
Technology Characterization
20
Fuel Cells
-------
• Liquefied petroleum gas (LPG) - propane and butane mixtures.
• Sour gas - unprocessed natural gas as it comes directly from the gas well.
• Biogas - any of the combustible gases produced from biological degradation of organic
wastes, such as landfill gas, sewage digester gas, and animal waste digester gas.
• Industrial waste gases - flare gases and process off-gases from refineries, chemical plants
and steel mill.
• Manufactured gases - typically low- and medium-Btu gas produced as products of
gasification or pyrolysis processes.
Factors that impact the operation of a fuel cell system with alternative gaseous fuels include:
• Volumetric heating value - Since the fuel cell's fuel processing system initially reforms
the fuel, the lower energy content fuels simply result in a less concentrated hydrogen-rich
gas stream feeding the anode. This will cause some loss in stack performance, which can
affect the stack efficiency, stack capacity, or both. Increased pressure drops through
various flow passages can also decrease the fine balance developed in fully integrated
systems.
• Contaminants are the major concern when operating on alternative gaseous fuels. If any
additional sulfur and other components (e.g., chlorides) can be removed prior to entering
the fuel processing catalyst, there should be no performance or life impact. If not, the
compounds can cause decreased fuel processor catalyst life and potentially impact stack
life.
Availability
Although fuel cell systems are perceived as low maintenance devices, their technical immaturity
and market entry status cause concern in DG applications. Close attention has been given to the
availability of the initial fleet of over 200 commercial PAFC fuel cell units. In a recent 12-
month period, the fleet of units in North America achieved 89% availability, with 94% during
the last 30 days of the period. In premium power applications, 100% customer power
94
availability, and 96.3% fleet availability has been reported during the same period. This
performance is a preliminary indicator that fuel cells can provide high levels of availability, even
in high-load factor applications.
The use of multiple units at a site can further increase the availability of the overall facility.
Analysis conducted during the fuel cell field demonstration programs of the 1980s indicated that
three to five units sized to 120% of application load, operating in parallel, could provide 99.99%-
plus availability under typical commercial building load profile characteristics.
24 According to manufacturer United Technology Corporation (www.UTCFuelCells.com, 3/28/02).
Technology Characterization
21
Fuel Cells
-------
Emissions
Fuel cell systems produce few emissions since the primary power generation process does not
involve combustion. In fact, the fuel processing subsystem is the only significant source of
emissions. The anode-off gas that typically consists of 8 to 15% hydrogen is combusted in a
catalytic or surface burner element to provide heat to the reforming process. The temperature of
this lean combustion can be maintained at less than 1,800°F, which also prevents the formation
of oxides of nitrogen (NOx) but is sufficiently high to ensure oxidation of carbon monoxide (CO)
and volatile organic compounds (VOCs - unburned, non-methane hydrocarbons). Typically, an
absorbed bed before the fuel processor removes and eliminates other pollutants such as oxides of
sulfur (SOx).
Nitrogen Oxides (NOx)
Three mechanisms form NOx: thermal NOx, prompt NOx, and fuel-bound NOx. Thermal NOx is
the fixation of atmospheric oxygen and nitrogen, which occurs at high combustion temperatures.
Flame temperature and residence time are the primary variables that affect thermal NOx levels.
The rate of thermal NOx formation increases rapidly with flame temperature. Early reactions of
nitrogen modules in the combustion air and hydrocarbon radicals from the fuel form prompt
NOx. It forms within the flame and typically is on the order of 1 ppm at 15% O2, and is usually
much smaller than the thermal NOx formation. Fuel-bound NOx forms when the fuel contains
nitrogen as part of the hydrocarbon structure. Natural gas has negligible chemically bound fuel
nitrogen. Fuel-bound NOx can be at significant levels with liquid fuels.
Carbon Monoxide (CO)
CO and VOCs both result from incomplete combustion. CO emissions result when there is
inadequate oxygen or insufficient residence time at high temperature. Cooling at the combustion
chamber walls and reaction quenching in the exhaust process also contribute to incomplete
combustion and increased CO emissions. Excessively lean conditions can lead to incomplete
and unstable combustion and high CO levels.
Unburned Hydrocarbons
Volatile hydrocarbons, also called volatile organic compounds (VOCs), can encompass a wide
range of compounds, some of which are hazardous air pollutants. When some portion of the fuel
remains unburned or just partially burned these compounds discharge into the atmosphere. Some
organics are carried over as unreacted trace constituents of the fuel, while others may be
pyrolysis products of the heavier hydrocarbons in the gas. Volatile hydrocarbon emissions from
reciprocating engines are normally reported as non-methane hydrocarbons (NMHCs). Methane
is not a significant precursor to ozone creation and smog formation and is not currently
regulated. Methane is a green house gas and may come under future regulations.
Carbon Dioxide (CO2)
While not considered a pollutant in the ordinary sense of directly affecting health, emissions of
carbon dioxide (CO2) are of concern due to its contribution to global warming. Atmospheric
warming occurs since solar radiation readily penetrates to the surface of the planet but infrared
(thermal) radiation from the surface is absorbed by the CO2 (and other polyatomic gases such as
methane, unburned hydrocarbons, refrigerants, and volatile chemicals) in the atmosphere, with
Technology Characterization
22
Fuel Cells
-------
resultant increase in temperature of the atmosphere. The amount of CO2 emitted is a function of
both fuel carbon content and system efficiency. The fuel carbon content of natural gas is 34 lbs
carbon/MMBtu; oil is 48 lbs carbon/MMBtu; and (ash-free) coal is 66 lbs carbon/MMBtu.
Fuel Cell Emissions Characteristics
Table 6 illustrates the emission characteristics of fuel cell system. Fuel cell systems do not
require any emissions control devices to meet current and projected regulations.
Table 6. Estimated Fuel Cell Emission Characteristics without Additional Controls*
Emissions Analysis25
System 1
System 2
System 3
System 4
System 5
System 6
Electricity Capacity (kW)
200
10
200
250
2000
100
Electrical Efficiency (HHV)
36%
30%
35%
43%
46%
45%
Fuel Cell Type
PAFC
PEM
PEM
MCFC
MCFC
SOFC
Emissions26
NOx (ppmv @15% 02)
1.0
1.8
1.8
2.0
2.0
2.0
NOx (lb/MWh)
0.03
0.06
0.06
0.06
0.05
0.05
CO (ppmv @ 15% 02)
2.0
2.8
2.8
2.0
2.0
2.0
CO (lb/MWh)
0.05
0.07
0.07
0.04
0.04
0.04
VOC (ppmv @15% 02)
0.7
0.4
0.4
0.5
1.0
1.0
VOC (lb/MWh)
0.01
0.01
0.01
0.01
0.01
0.01
C02 (lb/MWh)
1,135
1,360
1,170
950
890
910
Carbon (lb/MWh)
310
370
315
260
240
245
* Electric only, for typical systems under development in 2002. Estimates are based on fuel cell system
developers' goals and prototype characteristics. All estimates are for emissions without after-treatment
and are adjusted to 15% 02.
Source: Energy Nexus Group.
25 Emissions estimates are based on best available data from manufacturers and customer data. Emission expressed
in lb/MWh are for electric only performance and do not credit emissions for CHP operations. Typically CHP
emissions are calculated by Emissions = (lb emissions/(MWh of Elec generated + (MWh of Heat Recovered/80%
Boiler eff)*(ratio of Boiler Regulations/Electric Regulations both in lb/MWh equivalent))) and then compared to the
Electric Only Regulations.
26 Conversion from volumetric emission rate (ppmv at 15% 02) to output based rate (lbs/MWh) for NOx, CO, and
VOC are based on the following conversion multipliers: (0.01418 lb/MWh per ppm NOx) times (System Elec
Efficiency LHV); (0.00977 lb/MWh per ppm of CO) times (System Elec Efficiency, LHV); and (0.00593 lb/MWh
per ppm of VOC) times (System Elec Efficiency, LHV) respectively.
Technology Characterization
23
Fuel Cells
------- |