United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-95/034 March 1995
4>EPA Project Summary
Demonstration of Fuel Cells to
Recover Energy from an
Anaerobic Digester Gas—
Phase I. Conceptual Design,
Preliminary Cost, and Evaluation
Study
J.C. Trocciola and H.C. Healy
This document summarizes Phase I
of a study to demonstrate the recovery
of energy from waste methane pro-
duced by anaerobic digestion of waste
water treatment sludge. The U.S. Envi-
ronmental Protection Agency (EPA) is
interested in the fuel cell for this appli-
cation because it is potentially one of
the cleanest energy technologies avail-
able. This program is focused on using
a commercial phosphoric acid fuel cell
power plant because of its inherently
high fuel efficiency, low emissions char-
acteristics, and high state of develop-
ment. The environmental impact of
widespread use of this concept would
be a significant reduction in global
warming and acid rain air emissions.
Phase I is a conceptual design, pre-
liminary cost, and evaluation study. The
conceptual design of the fuel cell en-
ergy system is described and its eco-
nomic and environmental feasibility is
projected. Technology evaluations
aimed at improving the phosphoric acid
power plant operation on Anaerobic
Digester Gas (ADG) are described and
the two optional programs for complet-
ing the project are described. In Option
I, the technical issues of ADG contami-
nant removal and improved, fuel cell
power plant performance on low-Btu
fuel are addressed. In Option II, a one-
year field performance evaluation of the
energy recovery concept is planned.
The demonstration will document the
environmental and economic feasibil-
ity of the fuel cell energy recovery con-
cept.
This Project Summary was developed
by EPA's Air and Energy Engineering
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Introduction
International Fuel Cells Corporation
(IFC) is conducting a three-phase pro-
gram to determine if a fuel cell, which
utilizes the methane (CH4) from a waste-
water treatment (WWT) plant, is economi-
cally feasible and environmentally
beneficial in commercial operation. This
summary includes Phase I results of the
program
CH4 has been identified as a gas that
may contribute to global warming. Recent
information indicates that it is second only
to global carbon dioxide (CO2) in its con-
tribution to radiative forcing. Worldwide,
many sources of CH4 emitted into the
atmosphere include landfills, wastewater/
sewage treatment plants, coal mines, and
livestock waste. In the U.S., CH4 produced
in treatment plants is usually flared and
sometimes utilized for in-plant uses, al-
though plants that employ lagoon digest-
ers frequently vent their gas. If the CH4
emitted at facilities were converted to elec-
tricity, rather than being flared or used
thermally, the amount of electricity gener-
ated at central electric utility plants could
be reduced, thereby lowering emissions
of CO2, another global warming gas.
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Fuel Cell Benefits
The CH4 from WWT can be used ther-
mally or can be converted to electricity
using other technologies. However, con-
version using a fuel cell offers several
advantages:
The fuel cell emits very few pollut-
ants compared to other natural-gas-
fueled equipment (see Figure 1).
It produces electricity at 40% effi-
ciency and, with recovery of waste
heat, thermal efficiencies up to 85%
are possible.
Fuel cell power plants can be eco-
nomical in small ratings (200 kW).
As a consequence, they can be
added incrementally to accommo-
date increases in waste treatment
plant capacity while maintaining ef-
ficiency and emissions benefits.
Utilizing IFC's computer model, a per-
formance comparison has been made be-
tween the estimated performance
characteristics of a fuel cell operating on
natural gas and one operating on anaero-
bic digester gas (ADG). The estimate given
in Table 1 indicates that the performance
of the ADG fuel cell will be excellent and
similar to the natural gas model.
Using the total potential market for WWT
plants, an assessment was made of the
reduction of pollutants and global warm-
ing gases that would result from the use
of fuel cells. This is shown in Table 2.
This reduction in pollutants results from
the generation of electricity using a fuel
cell at the WWT plant site, thereby reduc-
ing the amount of electricity and associ-
ated pollutants generated at an electric
utility central station site. The bases for
these emission reductions are discussed
in the full report.
In addition to providing environmental
benefits, the fuel cell can also provide
economic benefits to the owner of a WWT
plant. In evaluating these benefits, sev-
eral application credits were identified that
may be applicable to facilities that install
on-site electrical generation equipment;
utilizing fuel cell power plants tends to
increase the value of these credits, in-
cluding
Biomass Energy Credits—The En-
ergy Policy Act of 1992 authorizes
financial incentives of 1.50/kWh for
power generated from biomass.
Emission Credits—These credits
could result if the fuel cell at the
WWT facility displaced electricity
that was otherwise generated us-
ing coal. The reduction in coal plant
generation results in lowering the
quantity of NOX and SOX emitted.
This reduction in pollutants was val-
ued at $1.10/kg. This value for SO2
is consistent with guidelines estab-
lished by the EPA for computing
cost effectiveness of New Source
Performance Standards. No guide-
line for NOX has been established.
Backup Power Avoidance Credits—
WWT facilities typically utilize grid
electricity plus backup diesels for
critical loads. By using multiple
200-kW fuel cell modules to pro-
2000
1250
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in
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100
90
80
70
60
50
40
30
20
10
0
- NOV
••••Federal New Source
Performance
Standard for NOVJ
Notes:
1 From Staff Recommendations for
Generic Power Plant Emissions Factors,
California Energy Commission. August
1989.
2 Source: ONSI Corporation.
3 EPA: 40 CFR CRI (7/1/87 Edition).
4 South Coast Air Quality Management
District Rule 11102.
Existing
Equipment
Boilers1
New
Boiler1
Internal Commercial 200-kW
Combustion Phosphoric Acid
Engine4 Fuel Cell2
Figure 1. Power plant emissions comparison (natural gas).
vide the facilities' power, it is esti-
mated that 50% of the backup die-
sels can be eliminated, resulting in
a savings of $500/kW of installed
fuel cell power plant capacity.
Distributed Power Credit—Fuel
cell power plants have been iden-
tified by the Electric Power Re-
search Institute and various
utilities as a dispersed power gen-
eration technology that could miti-
gate the need to install, replace,
or extend utility transmission and
distribution power systems. It is
estimated that the elimination of
this need would save the utility
approximately $500/kW of in-
stalled fuel cell capacity.
These credits may be grouped into vari-
ous economic scenarios ranging from uti-
lizing many of these credits (optimistic
application) to utilizing few of the credits
(pessimistic application). Table 3 summa-
rizes the fuel cell economics for three
scenarios using a cost for grid electricity
of 50/kWhr, which is the U.S. average
cost to large users. The details of each
scenario are discussed in the full report.
The data show that, for an "entry level"
cost of the power plant of $3000/kW, the
fuel cell is economic for the "moderate"
and "optimistic" assumptions. For the ma-
ture fuel cell cost of $1500/kW, the fuel
cell is economical for all the scenarios
considered.
Fuel Cell Operation on ADG
A number of WWT plants have been
surveyed to determine the composition of
their gas streams. The results of the gas
analysis are shown in Table 4. The data
indicate the ADG contains 55 to 65 vol %
CH4, and 30 to 40 vol % CO2. The gas
also contains hydrogen sulfide (H2S) at
the parts-per-million level.
The gas analysis for the various plants
is based typically on a one-time analysis.
The planned fuel cell demonstration por-
tion of this program at the demonstration
site will provide for periodic measurements
of the impurity levels in order to assess
their variabilities with time.
The commercial phosphoric acid fuel
cell (PAFC) power plant has been de-
signed to operate on natural gas, which is
essentially CH4. Since the CH4 from an
anaerobic digester is diluted with CO2, a
greater volume of gas must be ducted
through the power plant to supply enough
CH4 to produce 200 kW of power. These
higher flow rates result in higher pressure
drops through the power plant. A steam-
driven ejector pumps the fuel gas to the
pressure required to overcome system
pressure drops in the fuel cell power plant.
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Table 1. Estimated Performance Comparison for Nominal 200-Kw Output
Fuel
Electrical Efficiency (LHV), %
Heat Rate (HHV), kg.cal/kWhr
Available Heat, kg.cal/hr
Ambient Temperature for Fuel Water Recovery, °C
Start-up Fuel
Natural Gas
Power Plant
Natural Gas
40
2,395
190,000
35
Natural Gas
ADC
Power Plant
ADG
38
2,495
200,000
35
ADG
Table 2. Reduction in Pollutants Through Use of Fuel Cells
Global Warming Gases Acid Rain and Health Related Gases
CO2 Mg/yr
4.59x 106
NOX, Mg/yr
15,181
SO2 Mg/yr
22,983
CO, Mg/yr
1269
Tables. Fuel Cell Economics for ADG
Applications
Economic
Assumptions
Optimistic (Scenario "A ")
Moderate (Scenario "B")
Pessimistic (Scenario "C")
Fuel Cell Cost
($/kW)
1500
LC*
LC
LC
3000
LC
LC
EC**
* Cost of Electricity From Fuel Cell Lower Than
Cost of Grid Electricity (@50/kWhr)
** Cost of Electricity From Fuel Cell Equal to Cost
of Grid Electricity (@5f!/kWhr)
The fluid that provides the energy to pump
the fuel gas is steam-generated by the
fuel cell stack. In this program, testing of
the ejector presently used in the fuel cell
power plant confirmed that the steam pro-
duced by the stack is adequate to pump
enough ADG to produce 200 kW.
A gas cleanup system has been de-
signed to remove the H2S which, if fed to
the fuel cell, would degrade catalysts in
the power plant. The design of this re-
moval system is based on the use of a
commercially available carbon-based ma-
terial. The material has been tested at
the laboratory level under this program
and has been found to be very effective in
removing H2S. The material is believed to
absorb sulfur by the Glaus reaction:
H2S + 1/2 O2-> H2O + S
In order to promote this reaction, low
concentrations of oxygen are required in
the gas stream. Testing of the carbon-
based material on simulated ADG has
shown that 0.3 vol % oxygen, consistent
with the level at the Back River facility, is
sufficient for high adsorbent capacity. Up
to 50 wt % sulfur capacity was demon-
strated in the laboratory testing.
Since this testing was performed in the
laboratory on simulated ADG, a test at a
WWT facility to verify the suitability of the
gas cleanup approach is recommended.
A schematic of the gas cleanup system
for a fuel cell power plant is shown in
Figure 2. In the design, provision is made
for addition of air to the gas stream to
provide additional oxygen, if required, to
promote the Glaus reaction.
This system is designed to accept a
gas of variable inlet H2S concentration. If
the H2S concentration is higher than the
nominal level for that plant and the air
concentration in the gas stream is lower
than required, more air will be added. In
addition, the exit concentration of H2 from
the system will be measured: if its con-
centration increases above the specified
value due to exhausting the capacity of
the bed, the bed will be replaced.
Site Recommendation for Fuel
Cell Demonstration
Based on the favorable environmental
and economic benefits of fuel cells at WWT
plants and identification of a suitable gas
cleanup system, a demonstration of the
technology at a plant would be beneficial.
The plant recommended for this demon-
stration is the Back River WWT facility in
Baltimore, Maryland.
The Back River plant is owned and op-
erated by the city of Baltimore. It is a
secondary treatment facility occupying a
466 acre (1.9 x 106 m2) wooded site in the
eastern part of Baltimore County at the
head of Back River. The collection system
discharging to the Back River plant serves
an area of 140 mi2 (362 x 106 m2) with an
estimated population of 1.3 million. The
plant treats approximately 90% of the
wastewater generated from Baltimore City
and Baltimore County.
Several possible siting options for the
fuel cell have been identified at the facility's
new egg-shaped digesters. Two of the
sites are near the thermal generation build-
ing, which would facilitate heat recovery.
Back River strongly favors heat recovery
for economic reasons, and these are the
preferred sites for the demonstration. While
the H2S content of the ADG produced by
the Baltimore plant is lower than the other
facilities surveyed, Table 4, the basic prin-
ciples of the gas cleanup system will be
verified by testing at the facility. The exit
sulfur concentration from the gas cleanup
system is critical in determining fuel cell
life. Inlet concentration determines the re-
quired intervals between bed replacements
and consequently operating/maintenance
costs. Economic analyses were based on
high inlet concentrations of sulfur to the
gas cleanup system.
Advanced Technology Studies
IFC has ongoing activities to improve
the operating characteristics and lower the
cost of their natural gas fueled PAFC.
Under this program, a number of advanced
technology options were investigated to
determine their potential benefit to a com-
mercial fuel cell for the ADG application.
The technology improvements consid-
ered were related to the fuel processor,
the fuel ejector, water recovery, controls,
and heat recovery.
The results of these investigations
identified several areas of technology
improvements beneficial to fuel cells in
ADG applications that are considered
worthy of further activities. These are
listed in Table 5.
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Table 4. Typical Digester Gas Compositions (Dry Basis)
Baltimore
Back
River
Nassau County
Philadelphia Orange
NYC Water County
DEP Pert. Calif.
Bay
Park
Cedar
Creek
26th
Ward
Heating Value
HHV, Btu/SCF
Methane, vol %
Carbon Dioxide, vol %
Nitrogen, vol %
Oxygen, vol %
Hydrogen Sulfide, ppmv
Halides, ppmv
NMOCs, vol %
N/M
60.9
37.8
1.0
670
66.0
32.6
0.92
N/M
636
N/M
0.3 (est.) 0.45
6.0 80
<1.0 ND*
<0.0005 ND*
57.2 62.0 62.0
38.9 36.1 34.0
3.82 0.97 N/M
N/M 0.20 N/M
170** 100 <500**
N/M <1 N/M
0.01** ND* N/M
N/M
65.6
33.4
1.0
0.03
81
<4
<0.001
N/M—Not measured
* Not detected (level of detection not specified)
** Value set from equipment specifications, not from analyses
Conclusions
This study has confirmed that fuel cell
power plants have many benefits to the
operator of a WWT plant. The issues as-
sociated with the use of a gas produced
by such a plant in a fuel cell power plant
designed for natural gas have been iden-
tified and straightforward technical solu-
tions to these issues have been defined.
One of these issues is associated with
removal of the H2S contained in the ADG.
A test of a cleanup system to remove this
impurity has been designed, and it is rec-
ommended that this system be tested. A
site for this cleanup system test and the
subsequent demonstration fuel cell has
been selected. This site, in Baltimore,
Maryland, at the Back River WWT facility,
provides the opportunity for demonstrat-
ing high operating efficiency and low emis-
sions on ADG.
Air addition
Fuel cell
power plant
Coalescing
filter
Blower
Pretreatment bed(s)
(H2S removal)
Digester
Figure 2. Gas cleanup unit schematic.
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Table 5. Technology Areas Recommended for Further Assessment
Potential Power Plant Benefit
Low
Technology Reduced Increased Emis-
Area Cost Efficiency sions
Fuel / / /
Processor
Ejector/Fuel /
Control
Water /
Recovery
Controls / /
Heat
Recovery
Increased Waste Water
Quan/ Treatment Plant
Qual Heat Impact/Issue
Operation of
reformer on dilute
burner gas.
Operation of
ejector on dilute
fuel gas.
Shell and tube
condenser
presently used.
Look to replace
with lower cost
contact cooler.
Advanced
controls could
reduce power
plant cost. Use
of O2 sensors in
exhaust could
provide more
efficient reformer
operation on ADG.
/ Maximizing
waste heat
quantity/quality
could provide
for better
integration with
waste water
plant
Results
• Low emissions
maintained by
increasing
flame temp.
• Advanced
ejector shows
no benefits
compared to
existing ejector.
• Cost savings
offset by effi-
ciency loss
• Several areas
look promising
and warrant
further effort
and monitoring.
• System
changes,
identified to
increase thermal
quality/quantity,
do not require
technology
development.
Further
Activities
Warranted
Yes
No
No
Yes
No
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J.C. Trocciola and H.C. Healy are with International Fuel Cells Corp., South
Windsor, CT 06074.
Susan A. Thorneloe is the EPA Project Officer (see below).
The complete report, entitled "Demonstration of Fuel Cells to Recover Energy from
an Anaerobic Digester Gas—Phase I. Conceptual Design, Preliminary Cost, and
Evaluation Study,"(Order No. PB95-187381, Cost:$19.50, subjecttochange)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
Penalty for Private Use
$300
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
EPA/600/SR-95/034
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