United States Air and Energy Engineering

Environmental Protection Research Laboratory

Agency Research Triangle Park, NC 27711

Research and Development EPA/600/SR-95/034 March 1995

Demonstration of Fuel Cells to

Recover Energy from an

Anaerobic Digester Gas—

Phase I. Conceptual Design,

Preliminary Cost, and Evaluation

Study

Project Summary

This document summarizes Phase I

of a study to demonstrate the recovery

of energy from waste methane produced by anaerobic digestion of waste

water treatment sludge. The U.S. Environmental Protection Agency (EPA) is

interested in the fuel cell for this application because it is potentially one of

the cleanest energy technologies available. This program is focused on using

a commercial phosphoric acid fuel cell

power plant because of its inherently

high fuel efficiency, low emissions characteristics, and high state of development. 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, preliminary cost, and evaluation study. The

conceptual design of the fuel cell energy system is described and its economic 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 completing the project are described. In Option

I, the technical issues of ADG contaminant removal and improved, fuel cell

power plant performance on low-Btu

fuel are addressed. In Option II, a oneyear field performance evaluation of the

energy recovery concept is planned.

The demonstration will document the

environmental and economic feasibility of the fuel cell energy recovery concept.

J.C. Trocciola and H.C. Healy

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 documented in a separate report of the same

title (see Project Report ordering information at back).

Introduction

International Fuel Cells Corporation

(IFC) is conducting a three-phase program to determine if a fuel cell, which

utilizes the methane (CH4) from a wastewater treatment (WWT) plant, is economically 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 contribution 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, although plants that employ lagoon digesters frequently vent their gas. If the CH4

emitted at facilities were converted to electricity, rather than being flared or used

thermally, the amount of electricity generated at central electric utility plants could

be reduced, thereby lowering emissions

of CO2, another global warming gas.
2

Fuel Cell Benefits

The CH4 from WWT can be used thermally or can be converted to electricity

using other technologies. However, conversion using a fuel cell offers several

advantages:

• The fuel cell emits very few pollutants compared to other natural-gasfueled equipment (see Figure 1).

• It produces electricity at 40% efficiency and, with recovery of waste

heat, thermal efficiencies up to 85%

are possible.

• Fuel cell power plants can be economical in small ratings (200 kW).

As a consequence, they can be

added incrementally to accommodate increases in waste treatment

plant capacity while maintaining efficiency and emissions benefits.

Utilizing IFC’s computer model, a performance comparison has been made between the estimated performance

characteristics of a fuel cell operating on

natural gas and one operating on anaerobic 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 warming 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 reducing the amount of electricity and associated 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, several 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, including

• Biomass Energy Credits—The Energy Policy Act of 1992 authorizes

financial incentives of 1.5¢/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 using coal. The reduction in coal plant

generation results in lowering the

quantity of NOx and SOx emitted.

This reduction in pollutants was valued at $1.10/kg. This value for SO2

is consistent with guidelines established by the EPA for computing

cost effectiveness of New Source

Performance Standards. No guideline 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 provide the facilities’ power, it is estimated that 50% of the backup diesels 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 identified by the Electric Power Research Institute and various

utilities as a dispersed power generation technology that could mitigate 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 installed fuel cell capacity.

These credits may be grouped into various economic scenarios ranging from utilizing many of these credits (optimistic

application) to utilizing few of the credits

(pessimistic application). Table 3 summarizes the fuel cell economics for three

scenarios using a cost for grid electricity

of 5¢/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 mature 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 portion 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 designed 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 steamdriven ejector pumps the fuel gas to the

pressure required to overcome system

Figure 1. Power plant emissions comparison (natural gas). pressure drops in the fuel cell power plant.

0

10

20

30

40

50

60

70

80

90

100

2000

250

Emissions, ppmv (15% O2 dry)

NOx Federal New Source

Performance

Standard for NOx

3

CO

Non-methane

hydrocarbons

Existing

Equipment

Boilers1

New

Boiler1 Internal

Combustion

Engine4

Commercial 200-kW

Phosphoric Acid

Fuel Cell2

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.
3

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 produced by the stack is adequate to pump

enough ADG to produce 200 kW.

A gas cleanup system has been designed to remove the H2S which, if fed to

the fuel cell, would degrade catalysts in

the power plant. The design of this removal system is based on the use of a

commercially available carbon-based material. 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 Claus reaction:

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 demonstration is the Back River WWT facility in

Baltimore, Maryland.

The Back River plant is owned and operated 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 building, 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 principles 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 required 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 commercial fuel cell for the ADG application.

The technology improvements considered 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.

Table 3. Fuel Cell Economics for ADG

Applications

Economic Fuel Cell Cost

Assumptions ($/kW)

______________________________________________________________________________________________________________________________________________________________________________________________________________________________

1500 3000

Optimistic (Scenario “A”) LC* LC

Moderate (Scenario “B”) LC LC

Pessimistic (Scenario “C”) LC EC**

______________________________________________________________________________________________________________________________________________________________________________________________________________________________

* Cost of Electricity From Fuel Cell Lower Than

Cost of Grid Electricity (@5¢/kWhr)

** Cost of Electricity From Fuel Cell Equal to Cost

of Grid Electricity (@5¢/kWhr)

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 carbonbased 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 demonstrated 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 Claus 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 concentration increases above the specified

value due to exhausting the capacity of

the bed, the bed will be replaced.

Table 1. Estimated Performance Comparison for Nominal 200-Kw Output

______________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Natural Gas ADG

Power Plant Power Plant

Fuel Natural Gas ADG

Electrical Efficiency (LHV), % 40 38

Heat Rate (HHV), kg.cal/kWhr 2,395 2,495

Available Heat, kg.cal/hr 190,000 200,000

Ambient Temperature for Fuel Water Recovery, °C 35 35

Start-up Fuel Natural Gas ADG

______________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Table 2. Reduction in Pollutants Through Use of Fuel Cells

Global Warming Gases Acid Rain and Health Related Gases

CO2 , Mg/yr NOx, Mg/yr SO2 , Mg/yr CO, Mg/yr

4.59x 106 15,181 22,983 1269

______________________________________________________________________________________________________________________________________________________________________________________________________________________________________
4

Pretreatment bed(s)

(H2S removal)

Activated

carbon

bed

Fuel cell

power plant

Coalescing Blower

filter

Digester

Fixed

control

valve

Air addition

Figure 2. Gas cleanup unit schematic.

Table 4. Typical Digester Gas Compositions (Dry Basis)

Philadelphia Orange

NYC Water County

Baltimore Nassau County DEP Dept. Calif.

Back Bay Cedar 26th

River Park Creek Ward

_________________________________________________________________________________________________________________________________________________________________

Heating Value N/M 670 N/M 636 N/M N/M

HHV, Btu/SCF

Methane, vol % 60.9 66.0 57.2 62.0 62.0 65.6

Carbon Dioxide, vol % 37.8 32.6 38.9 36.1 34.0 33.4

Nitrogen, vol % 1.0 0.92 3.82 0.97 N/M 1.0

Oxygen, vol % 0.3 (est.) 0.45 N/M 0.20 N/M 0.03

Hydrogen Sulfide, ppmv 6.0 80 170** 100 <500** 81

Halides, ppmv <1.0 ND* N/M <1 N/M <4

NMOCs, vol % <0.0005 ND* 0.01** ND* N/M <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 associated with the use of a gas produced

by such a plant in a fuel cell power plant

designed for natural gas have been identified and straightforward technical solutions 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 recommended 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 demonstrating high operating efficiency and low emissions on ADG.
5

Table 5. Technology Areas Recommended for Further Assessment

Potential Power Plant Benefit

Low Increased Waste Water Further

Technology Reduced Increased Emis- Quan/ Treatment Plant Activities

Area Cost Efficiency sions Qual Heat Impact/Issue Results Warranted

____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Fuel ✓ ✓✓ Operation of • Low emissions Yes

Processor reformer on dilute maintained by

burner gas. increasing

flame temp.

Ejector/Fuel ✓ Operation of • Advanced No

Control ejector on dilute ejector shows

fuel gas. no benefits

compared to

existing ejector.

Water ✓ Shell and tube • Cost savings No

Recovery condenser offset by effipresently used. ciency loss

Look to replace

with lower cost

contact cooler.

Controls ✓ ✓ Advanced • Several areas Yes

controls could look promising

reduce power and warrant

plant cost. Use further effort

of O2 sensors in and monitoring.

exhaust could

provide more

efficient reformer

operation on ADG.

Heat ✓ Maximizing • System No

Recovery waste heat changes,

quantity/quality identified to

could provide increase thermal

for better quality/quantity,

integration with do not require

waste water technology

plant development.

____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
6

United States

Environmental Protection Agency

Center for Environmental Research Information

Cincinnati, OH 45268

Official Business

Penalty for Private Use

$300

EPA/600/SR-95/034

BULK RATE

POSTAGE & FEES PAID

EPA

PERMIT No. G-35

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, 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