United States
Environmental Protection
Agency
Research and Development
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
EPA/600/S7-91/008 Jan. 1992
Project Summary
Waste Combustion System
Analysis
J. Newhall, G. Taylor, and B. Folsom
This report presents the results of a
study of biomass combustion alterna-
tives. The objective was to evaluate the
thermal performance and costs of avail-
able and developing biomass systems.
The characteristics of available biomass
fuels were reviewed and the perfor-
mance parameters of alternate power
generating systems were evaluated us-
ing a thermodynamic model. The re-
sults were compared with available
information on commercially available
equipment. Capital and operating costs
were also estimated. The selection of
an optimum biomass combustion sys-
tem depends on the available fuel and
the specific application. A case study of
an ethanol plant was conducted to illus-
trate the key considerations.
This Project Summary was devel-
oped by EPA's Air and Energy Engi-
neering Research Laboratory, Research
Triangle Park, NC, to announce key find-
Ings of the research project that Is fully
documented In a separate report of the
same title (see Project Report ordering
Information at back).
Introduction
Biomass is available in large quantities
and offers several benefits compared to
conventional fossil fuels including:
Reduced CO2 emissions (which miti-
gates global warming)
Reduced pollutant emissions, particu-
larly S02
Reduced dependence on fossil fuels
In addition, some forms of biomass are
available as waste materials at low or no
cost compared to fossil fuels. This offers
the potential to achieve the benefits listed
above at a net cost savings.
This report presents the results of a
study of biomass combustion alternatives.
The objective was to evaluate the thermal
performance and costs of available and
developing biomass systems. The charac-
teristics of available biomass fuels were
reviewed and the performance parameters
of alternate power operating systems were
evaluated using a thermodynamic model.
The results were compared with available
information on commercially available
equipment. Capital and operating costs
were also estimated. The selection of an
optimum biomass combustion system de-
pends on the available fuel and the spe-
cific application. A case study of an ethanol
plant was conducted to illustrate the key
considerations.
Biomass Fuels
Biomass fuels include waste materials
from agricultural operations, urban and in-
dustrial wastes, and materials grown spe-
cifically for their fuel value. These fuels
consist of three major components: dry
cellulose, ash, and moisture. Dry cellulose,
the combustible portion of the material, is
essentially the same for all types of bio-
mass. It can be represented by the chemi-
cal formula CH, ^O,, M with a higher heating
value of 8,555 Btu/l'b* (standard deviation
of 4.4% for 50 samples).
The water and ash contents of bio-
mass can vary greatly, and these param-
eters can impact the design and operation
* 1 Btu/lb = 2.324 kJ/kg
-------
of the combustion system. High moisture
content fuels may require auxiliary fuel for
flame stabilization. The mineral matter in
the biomass can cause ash deposition prob-
lems and corrosion of turbine blades in
gas turbine applications.
The pollution emission potential for bio-
mass fuels varies. In general, biomass
contains low sulfur levels so that the SO2
emission potential is tow compared to most
fossil fuels. The average SO2 emission
potential for 50 fuels was 0.55 lb/108 Btu.*
The NO, emission potential varies sub-
stantially. While the tow heating value of
btomass minimizes thermal NO,, some bio-
mass fuels contain substantial amounts of
bound nitrogen. A substantial fraction of
that bound nitrogen can be converted to
NO,, particularly if the fuel is gasified and
fired in a gas turbine combustor without
heat removal (which is the preferred ap-
proach based on thermodynamics). The
highest fuel nitrogen content for the fuels
evaluated in this study was 14.34 lb/108
Btu. If conversion of this nitrogen to NO,
was limited to 10%, NO, emissions would
still be 1.4 lb/108 Btu.
Biomass Combustion Systems
The two major methods of using bio-
mass for power production are direct firing
in a boiler to produce steam for a Rankine
cycle and using it in a Brayton cycle (gas
turbine) w'rth or Without heat recovery from
the turbine exhaust. The gas turbine con-
figuration has many alternate arrangements
such as integrated gasification and addi-
tion of a steam bottoming cycle. The ther-
modynamic performance of several cycles
was evaluated over a range of design pa-
rameters and with alternate biomass fuel
characteristics. The performance achiev-
able by currently available technology,
along with typical costs, is summarized in
Table 1.
Biomass fuel characteristics affect these
cycles differently. For boiler combustion
systems, fuel moisture is a thermodynamic
detriment. The latent heat of the fuel mois-
ture is not recovered, resulting in a direct
reduction in boiler efficiency. However, in
gas turbine systems, the fuel moisture can
actually increase thermodynamic efficiency
since the water is effectively processed
through a Rankine cycle. This benefit is
only achieved if the moisture is vaporized
in the combustion system at pressure with-
out heat removal. Direct firing pulverized
biomass in a gas turbine combustion burner
is one way to achieve this. Another way is
to gasify the biomass under pressure, re-
move particulates and alkali in a hot cleanup
system and then fire the clean gas in the
gas turbine combustor. At present, this
approach to improving performance is lim-
ited by two factors: (1) the moisture con-
tent of the biomass must be less than
about 20% for satisfactory gasifier opera-
tion and to produce a combustible gas with
sufficient heating value for gas turbine com-
bustor operation, and (2) a high tempera-
ture cleanup system has not been
developed.
The STIG cycle involves direct injec-
tion of steam generated in a heat recovery
steam generator. This improves efficiency
in a manner analogous to the moisture
content of the fuel. Taken to the limit of
maximum steam injection, the STIG cycle
offers substantial efficiency improvement.
However, current aero-derivative gas tur-
bines cannot handle the large turbine mass
flows which result. This limits the STIG
cycle to moderate steam injection rates.
Ethanol Plant Case Study
Ethanol production uses sugarcane as
a feedstock and produces bagasse as a
waste material. Substantial electrical power
and heat are required to operate the plant.
A conventional approach would involve
electrical power from a utility (generated
by firing a fossil fuel) and heat generated
on site by fossil fuel combustion in a boiler.
The potential for combustion of the bag-
asse to supplant the electrical power and/
or the heat requirements was evaluated. In
the lost optimized case, net CO2 reduc-
tions of 50,000 Ib/hr* were achieved. In
addition, each case evaluated exhibited a
simple pay back of less than 2.5 years.
Recommendations
The gas-turbine-based systems offer
the greatest potential for efficient use* of
biomass. Among these, systems which fire
the biomass under pressure are attractive
since the fuel moisture content is a benefit.
Development of the following should fully
exploit this concept:
1. A gasifier capable of processing high
moisture biomass
2. A combustor capable of firing low-
heating-value gas without auxiliary
fuel
The thermodynamic performance of the
STIG cycle can be improved by injecting
greater amounts of water. However, the
water injection rate is limited by the gas
turbine design. It may be possible to modify
the gas turbine design to handle increased
water injection.
NO, control is a key issue, particularly
for biomass fuels with high nitrogen con-
tent. Combustion modification has the po-
tential to reduce No, emissions without
large cost or performance penalties. It may
apply to gas-turbine-based systems with
integrated gas'rfiers.
* 1 lb/101 Btu - 435 ng/J
* 1 Ib/hr = 0.0454 kg/hr
-------
Table 1.
Performance of Current Technology
System
Heat Rate,
BtuAWhr'
Cost,
$/kW
Simple Cycle Gas Turbine
Pressurized Gasification
Hot-Gas Cleanup
Simple Cycle Gas Turbine
Pressurized Gasification
Cold-Gas Cleanup
Combined Cycle
Atmospheric Gasification
Cold-Gas Cleanup
Combined Cycle
Pressurized Gasification
Cold-Gas Cleanup
STIG "Cycle
Pressurized Gasification
Cold-Gas Cleanup
STIG Cycle
Atmospheric Gasification
Cold-Gas Cleanup
Combined Cycle + Process Steam
Pressurized Gasification
Hot-Gas Cleanup-
Combined Cycle + Process Steam
Pressurized Gasification
Cold-Gas Cleanup
Stoker Boiler
30 MW
10,092
11,910
11,200
8,944
9,667
12,350
10,100
11,918
11,046
1,415
1,320
1,962
1,552
1,457
1,246
2,237
2,086
2,200
• 1 Btu/kWhr = 1054 J/k Whr
* STIG = steam injected gas turbine cycle
&U.S. GOVERNMENT PRINTING OFFICE: 1992 - 648-080/40149
-------
J. Newhall, G. Taylor, andB. Folsom are with Energy and Environmental Research
Corp., Irvine, CA 92718.
David A. Klrchgessner Is the EPA Project Officer, (see below).
The complete report, entitled "Waste Combustion System Analysis," (Order No. PB92-
125418/AS; Cost: $26.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati, OH 45268
Official Business
Penatly for Private Use $300
EPA/600/S7-91/008
------- |