Energy Production and Pollution Prevention at Sewage Treatment Plants Using Fuel Cell Power Plants

ENERGY PRODUCTION AND POLLUTION PREVENTION AT SEWAGE
TREATMENT PLANTS USING FUEL CELL POWER PLANTS

R.J. Spiegel

National Risk Management Research Laboratory
U.S. Environmental Protection Agency (EPA)

Research Triangle Park. NC 27711 U.S.A.

Tel: 919-541-7542. Fax: 919-541-7885
E-Mail: spicgel.ronald@epa.gov

J.L. Preston
ONSI Corp.

South Windsor, CT 06704 U.S.A.

ABSTRACT

Anaerobic digester gas (ADG) is produced at waste water treatment plants during the process of treating
sewage anaerobically to reduce solids. The major constituents are methane (57-66%). carbon dioxide
(33-39%), nitrogen (1-4%), and a small amount of oxygen (<0,5%). Minor constituents include sulfur-
bearing compounds (principally hydrogen sulfide), trace amounts of halogen compounds (chlorides), and
nonmethane organic compounds. Currently, most of this gas is either vented to the atmosphere or
combusted in flares. In either case, the release of these gases (methane or carbon dioxide) can potentially
affect global climate by trapping radiation from the sun and enhancing atmospheric heating. Additionally,
the vented gas contains hazardous air pollutants (e.g., hydrogen sulfide and chlorides). It is desirable to
develop technologies that cannot only produce energy from ADG, but also reduce air emissions. In this
regard, fuel cells are an emerging technology to produce electricity and clean heat. The electricity produced
can either be used by the waste water plant or sold to the electrical grid, while the heat is used to aid the
anaerobic process. EPA, in conjunction with ONSI, has recently utilized a200-kW phosphoric acid fuel
cell power plant to operate on ADG at a sewage treatment plant in Yonkers, NY. After a year of
operation, some 20 tons of ADG have been turned into over 1.2 million kWh of electrical energy. A
summary of test results on the cleanup system used to remove the fuel cell contaminants (chiefly sulfur and
halogen compounds) and fuel cell operational data including air emissions are described,

INTRODUCTION

There are over 400 municipal waste water treatment (WWT) facilities in the U.S. that use anaerobic
digesters to treat sewage to reduce solids [1]. Anaerobic digestion is a natural biological process that
occurs in the absence of oxygen whereby microbes consume organic matter to produce methane, water,
and carbon dioxide as byproducts. Digesters are usually large enclosed concrete containers that are heated
to enhance the rate of digestion of the influent consisting of sewage/sludge. Typically about 40% of the
sol ids are converted to ADG containing about 60% methane, which rises to the top of the digester where
it can be collected. ADG represents a form of renewable energy if utilized. Some WWT plants have
placed emphasis on the use of ADG for heat recovery (boilers), engine generators (produce electricity),

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Spiegel page 2 of 4

or natural gas production (treated and sold to a utility as natural gas).

Because of siting problems, noise, and air emissions (nitrogen oxides and carbon dioxide) associated with
engines, it is desirable to develop superior technologies that cannot only produce energy from ADG, but
also reduce air emissions and ultimately lower capital and operating costs. In this regard, fuel cells are an
emerging technology that may improve the outlook for clean, efficient, and economical use of ADG to
produce electricity and heat. Recently, the U.S. EPA embarked on a project to establish the conceptual
design of a fuel cell energy recovery system for application at anaerobic digester WWT plants, and to
establish the basis for the site-specific demonstration hardware to validate the conceptual design [2].
Figure i is a simplified diagram of the conceptual design consisting of digesters, a gas pretreatment module,
and a fuel cell power plant. After satisfactory assessment of the concept, the world's first application of
a fuel cell using ADG at a WWT plant in Yonkers, NY was conducted. The 200-kW fuel cell (PC 25)
is manufactured by ONSICorp.. a subsidiary of International Fuel Cells Corp., South Windsor, CT. This
paper describes some initial test results.

GAS

ANAEROBIC PRETREATMENT

DIGESTERS MODULE

Figure 1. Fuel Cell/ADG Energy Recovery Concept.
ADG PRETREATMENT MODULE

Based on a survey of available ADG analyses data, the contaminants of chief concern are sulfur and
halogen compounds [2], These contaminants can poison the fuel processor reforming catalyst in the fuel
cell. The data indicate that sulfur levels (hydrogen sulfide) range from 6 up to 200 ppmv. Halogen
compounds range from 0 to around 4 ppmv. The implication of these data is the requirement to focus the
ADG pretreatment system design on removal of sulfur compounds. Halides can be removed in the fuel
cell power plant fuel processor with a halogen guard bed on a site-specific basis. Other potential
contaminants that need to be eliminated are solids, liquid water and condensate, and bacteria which may
be present in the gas.

The ADG pretreatment system is depicted in Figure 2. The absorbent material selected for hydrogen

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Spiegel page 3 of 4

sulfide removal is a potassium-hydroxide-impregnated activated carbon absorbent which is commercially
available. This material had been previously tested at a landfill [3]. The nonregenenible desulfurizerbed
operates at ambient temperature and pressure and converts hydrogen sulfide to elemental sulfur and water
via the Claus reaction. Elemental sulfur produced by the reaction is adsorbed by the carbon. An air
injection system is required to control oxygen levels in the range of 0.3 - 0.5%. A blower conveys the
ADG through the pretreatment module and delivers it to the fuel cell at the required pressure. In addition,
a coalescing filter upstream of the blower precludes the possibility of solids/ liquid canyover, and bacteria
from entering the pretreatment module and fuel cell.

The up-front desulfurizerbed design provides flexibility for ADG applications. For low levels of hydrogen
sulfide, the bed can be nominal in size and operated as a single unit. For higher levels of hydrogen sulfide,
the activated carbon absorbent bed could be incoiporated as parallel units, allowing continuous operation
of the pretreatment system on one bed while the other is being replaced with fresh absorbent. Although
the carbon can be disposed of as solid waste, it can be also regenerated off-site, eliminating waste disposal
considerations. Operating costs associated with the activated carbon are low. Based on hydrogen sulfide
loadings of35 - 50% by weight, lOOppmv hydrogen sulfide in the gas stream, and material costs of about
$4.96/kg, operating costs are less than 0.1 cent/kWh.

Air Addition

^ Control
A~" Valve

CLEAN EXHAUST

COj-f- HjO
+ Nj + °a

200 KW

UTILITY
GRACE

POWER MODULE

CEIL

INVERTER

-REMOVES
03

-REMOVES
RESIDUAL
SULFW,
MAUDES
-CONVERTS
FUEL TO
HYDflOGEN

fVWVH f'f

AIR AMD
HYDR-
OGEN TO
DC POWER
AMD

THERMAL
ENERGY

-coHwms

DC TO AC
- PROVIDES
SAFE
ELECTB-
ICAL
tffTER-
CONNECT

COOL-
ING

REJECTS

WASTS

HEAT

AJR

Fuel Cell Power Plant

Coalescing
Filter

Condenser

Digester*

Waste Water Treatment Plant

8«d 1

Bed 2

Figure 2. Process Schematic Of ADG Pretreatment System

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^piegei page 4 or 4

Hal ides in ADG can be removed in the fuel cell power plant fuel processor with a halide guard bed on a
site-specific basis. This internal design is feasible because of the low levels of halides in the ADG stream.
A vessel with a metal oxide supported on alumina was incorporated into the fuel processor of the power
plant (not shown in Figure 2) to remove the halide contaminants.

TEST RESULTS

ADG samples were taken at the inlet to the fuel cell to ascertain the effectiveness of the pretreatment
system in removing fuel cell catalyst poisons such as sulfur and halide compounds. Samples were collected
in glass-lined stainless steel canisters, and the contents were analyzed for halide compounds by gas
chromatography/mass spectrometry, and for sulfur compounds using gas chromatography/flame
photometric procedures. Initial results have demonstrated that the pretreatment system is extremely
effective in removing hydrogen sulfide, with output levels non-detectable to the detection limit of 10 ppbv.
The measured halide input levels of around 1.6 pprnv are lowered to levels below 0.03 ppmv by the halide
guard in the fuel cell.

Air pollutant emissions from the fuel cell exhaust were monitored according to EPA methods. The
emissions for carbon dioxide, sulfur dioxide, nitrogen oxide, methane, and nonmethane organic compounds
were <0.46, <0.92,0.44, 1.87, and <0.46 ppmv, respectively.

CONCLUSIONS

The fuel cell power plant has successfully operated on ADG for several thousand hours. Depending on
the heating value of the fuel, the power output levels have been as high as 175 kVV with most of the
operation occurring at the 140-kW level due to low methane content (around 50%) in the ADG. The
pretreatment system for cleaning ADG was successfully designed, installed, tested, and validated.
Emissions in the fuel cell exhaust gas stream are far below what would be produced by flaring the ADG,
and are comparable with a conventional fuel cell using natural gas.

REFERENCES

1. U.S. Environmental Protection Agency. 1992 Needs Survey, EPA/DF/MT-94/084,NTIS PB94-
501343, Office of Water, Washington, DC, 1992.

2. Trocciola, J.C. and Healy, H.C., Demonstration of Fuel Cells to Recover Energy from Anaerobic
Digester Gas. Phase I, Conceptual Design, Preliminary Cost, and Evaluation Study, EPA-600/R-95-
034, NTIS PB95-187381, Air and Energy Engineering Research Laboratory, Research Triangle Park,
NC, March 1995.

3. Trocciola, J.C. and Preston, J.L., Demonstration of Fuel Cells to Recover Energy from Landfill Gas.
Phase II, Pretreatment System Performance Measurement, EPA-600/R-95-155, NTIS PB96-

103601, Air and Energy Engineering Research Laboratory, Research Triangle Park, NC, October
1995.

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TECHNICAL REPORT DATA

N RM RL- RT P- P- 43 9 (Please mad Instructions on the reverse before compl PB20Q0-102943

1. REPORT NO. 2.

EPA 600/A-99/097

llllllllllllllllll

4. TITLE AND SUBTITLE

Energy Production and Pollution Prevention at Sewage
Treatment Plants Using Fuel Cell Power Plants

5. REPORT DATE

6. PERFORMING ORGANIZATION CODE

7. AUTHORS

R. J. Spiegel (EPA) and J. L. Preston (ONSI)

8. PERFORMING ORGANIZATION REPORT NO.

9, PERFORMING ORGANIZATION NAME AND ADDRESS'

ONSI Corporation

South Windsor, Connecticut 06704

10. PROGRAM ELEMENT NO.

11. CONTRACT/GRANT NO.

68-02-0186

12 SPONSORING AGENCY NAME AND ADDRESS

U, S. Environmental Protection Agency

Air Pollution Prevention and Control Division

Research Triangle Park, North Carolina 27711

13, TYPE OF REPORT AND PERIOD COVERED

Published paper; 7/98-6/99

14. SPONSORING AGENCY CODE

EPA/600/13

is. supplementary notes appcd project officer is Ronald J. Spiegel, Mail Drop 63, 919/541-
7542. For presentation at Third International Fuel Cell Conference, Nagoya, Japan,
11/30-12/3/99.

16.abstractT"he PaPer discusses energy production and pollution prevention at sewage
treatment plants using fuel cell power plants. Anaerobic digester gas (ADG) is pro-
duced at waste water treatment plants during the anaerobic treatment of sewage to re-'
duce solids. The major constituents are methane (57-66%), carbon dioxide (33-39%),
nitrogen (1-4%), and oxygen (<0.5%). Minor constituents include sulfur-bearing com-
pounds (principally hydrogen sulfide), trace amounts of halogen compounds (chlorides),
and nonmethane organic compounds. Currently, most of this gas is either vented to
the atmosphere or combusted in flares. In either case, the release of these gases
(methane or carbon dioxide) can potentially affect global climate by trapping radiation
from the sun and enhancing atmospheric heating. Additionally, the vented gas contains
hazardous air pollutants (e.g., hydrogen sulfide and chlorides). It is desirable to de-
velop technologies that cannot only produce energy from ADG, but also reduce air
emissions. In this regard, fuel cells are an emerging technology to produce electri-
city and clean heat. The electricity produced can either be used by the waste water
plant or sold to the electrical grid, while the heat is used to aid the anaerobic pro-
cess. EPA, in conjunction with ONSI, recently utilized a 200-kW phosphoric acid fuel
cell power plant to operate on ADG at a sewage treatment plant in Yonkers, NY.

17. KEY WORDS AND DOCUMENT ANALYSIS

a DESCRIPTORS

b. IDENTIFIERS/OPEN ENDED TERMS

c. COSATI Field/Group

Pollution Phosphoric Acids
Fuel Cells

Electric Power Plants

Sewage Treatment
Energy

Anaerobic Processes

Pollution Prevention
Stationary Sources

13 B 07B
10B

14G
06 C

18, distribution statement

19, SECURITY CLASS (This Report)

21. NO. OF PAGES

20. SECURITY CLASS (This Pagej

22. PRICE

EPA Form 2220-1 {Rev. 4-77 } PREVIOUS EDITION IS OBSOLETE forms/admin/techiptfrm 7/8/99 pad

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