new approach to fuel cells turns wastewater into clean water and electricity
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24-01-2010, 10:53 PM
new approach to fuel cells turns wastewater into clean water and electricity.DOC (Size: 107.5 KB / Downloads: 292)
Necessity is the mother of invention. Energy crisis is the cry of
The historical and the present-day civilization are closely interwoven
with energy and in future, our existence will be more dependent upon
energy. The conventional sources of energy, the single most important
pre-requisite for power generation, are depleting fast. The world is
heading towards a global energy crisis mostly due to running out of
these energy sources; decreasing the dependency on fossil fuels is
recommended. Yet, the sources of energy are infinite. The greatest task
today is to exploit the non-conventional energy resources for power
Starting with brief history of Fuel Cells,
this paper presents an innovative emission-offset project and implimentation that utilizes
anaerobic digester gas-powered fuel cells to produce electricity. ADG
is primarily a mixture of gasses which are the byproducts of anaerobic
decomposition at waste water treatment plants. At many waste water
treatment plants ADG is being utilized inefficiently, or not at all. If
ADG is released uncombusted, it significantly contributes to the green
house effect. This designation necessitates installation of control and
monitoring technologies, which can be very costly. Fuel Cells provide
the most effective solution to these problems. They efficiently
generate premium quality electricity and much needed thermal energy,
while consuming ADG and emitting negligible amount of regulated
The paper also presents the main applications of the Fuel Cells. The
concept of using fuel cells powered by hydrogen gas provides the ideal
solution to pollution in our world. Fuel cells have been developed that
can be used for virtually any application requiring electrical power or
mechanical energy. They are now classified into three basic categories:
portable devices, transportation applications, and stationary power.
Fuel cell may be defined as an electro chemical device for the
continuous conversion of the free energy change in a chemical reaction
to electrical energy.
Fuel cell systems offers clean and efficient energy production and are
currently under intensive development by several manufacturers for both
stationary and mobile applications. Fuel cells found its first
application during the NASA Apollo moon landing program in the late
1960s. It was a logical choice as an energy source. It had no moving
parts, was compact in size, and was founded with an unlimited budget.
Powered simply by hydrogen gas, the fuel cell produced dc power, pure
water as its exhaust, and heat. The dc supply charged the spacecraft
batteries. The water was used by the crew and the heat generated by the
fuel cell was rejected to the void of space. At that time a brief study
was conducted as to how fuel cells could be used to power homes in the
same manner as spacecrafts. The study was very short lived but the
dream of powering automobiles and houses using fuel cells was born in
Fuel cells, until recently a curiosity largely confined to the space
program, are emerging as a valuable clean and efficient generator of
electricity. A number of companies are developing fuel cells for use in
stationary applications. Most of the current applications for fuel
cells utilize natural gas as a fuel. In certain states, such as New
York and Connecticut, fuel cells operating on natural are recognized by
the states as a renewable energy source. Recently, however, fuel cells,
mostly phosphoric acid, have been shown to operate well on renewable
biogas fuels, such as anaerobic digester gas (ADG) produced at
wastewater treatment plants as well as landfill gas (LFG) and gas
produced at beer breweries.
Wastewater Treatment Facilities
Anaerobic decomposition involves microorganisms that derive energy from
metabolizing organic materials to decompose organic waste at WWTFs. In
the absence of oxygen the byproducts of their metabolism are carbon
dioxide (CO2) and methane (CH4) plus trace quantities of other
components, such as hydrogen sulfide (H2S) and organic halides (mostly
chlorides). ADG is primarily a mixture of these gasses (60% methane and
40% carbon dioxide). A simplified diagram of the Wastewater Treatment
process is shown in Figure.
ADG is generally collected and either used as fuel in boilers to keep
anaerobic digesters war, flared off, or, in some cases, used in
internal combustion engines to produce electricity. At many WWTFs, ADG
is being utilized in efficiently, or not at all. For example, many
facilities are located in temperate climates in which the requirement
for heat in summer is minimal.
If ADG is released uncombusted, it significantly contributes to the
greenhouse effect. This occurs principally through emission of methane,
which traps at least 10 times as much heat as carbon dioxide. For this
reason and for odor control, excess ADG is typically flared (burned) in
flame towers, a process that eliminates methane emission. However,
flaring is only a partial solution, since ADG combustion generates
photo reactive ozone precursors, such as nitrogen oxides and volatile
organic components. This designation necessitates installation of
control and monitoring technologies, which can be very costly. Fuel
cells provide the most effective solution to these problems. They
efficiently generate premium quality electricity and much needed
thermal energy, while consuming ADG and emitting negligible amount of
regulated pollutions. In addition, they permit significant reductions
in carbon dioxide emissions compared to flaring. As a result, WWTFs are
primary candidates for clean distributed generation and for win-win
partnerships between the WWTF operators and utilities.
The PC25C phosphoric acid fuel cell was modified to operate on ADG.
This involved modifications to the cell stack assembly, reformer,
thermal management system, piping valves, controls, etc. ADG differs
from pipeline natural gas in the following ways:
Â¢ADG contains trace quantity of sulfur compounds, typically in the form
of hydrogen sulfide and organic compounds, which contain chlorine. Both
of these species can react with the catalysts in the reformer system,
resulting in deactivation of the catalysts.
Â¢ADG typically contains 60% methane, while natural gas contains in
excess of 95%. This lower methane content of ADG results in a higher
volumetric flow of gas, which can increase system pressure drops.
These differences require modification of the PC25C, originally
designed to operate on natural gas only. These modifications were
Â¢Mechanical components, such as piping and valves, in the reactive gas
supply system were modified/ enlarged to accommodate the larger volume
flow rates resulting from the use of diluted methane fuel. This
modification helped reduce system pressure drops.
Â¢An external gas compressor skid was added to raise the inlet pressure
of the ADG to compensate in part for the increased pressure drops of
the diluted fuel.
Â¢An external gas processing unit (GPU) was added to remove the hydrogen
sulfide contained in the ADG stream. This GPU consists of a specially
treated charcoal, which converts the hydrogen sulfide into elemental
sulfur and water. The sulfur is absorbed on the charcoal, which is then
removed on a periodic basis; the water evaporates into the ADG stream;
and the purified gas is fed to the fuel cell.
Â¢A halide absorber was added internally to the PC25C to remove these
compounds (mostly chlorides).
Â¢Fuel-to-air ratios over the entire operating range were adjusted
within the wider-than-usual boundaries to compensate for broader-than-
anticipated methane concentration variations in ADG.
Â¢Additional drains were installed in the facility fuel line to remove
large amounts of entrained water periodically blocking ADG supply to
Â¢A blower was installed for lower-than-anticipated ADG pressure.
Fuel cells at Wastewater Treatment Plants
Wastewater treatment plants that utilize the anaerobic digestion
process produce a gas mixture of about 60% methane (CH4) and 40% carbon
dioxide (CO2), plus ppm levels of hydrogen sulfide and, in some cases,
organic halides (mostly chlorides). This gas mixture, called ADG, may
be utilized in a fuel cell to produce power and heat. However, the
sulfur and halide compounds must be removed to prevent deactivation of
certain key components ion the fuel cell. An ADG fuel cell system is
shown in the figure.
Figure: ADG fuel cell system schematics.
The ADG-powered fuel cells are constructed in three modules: a gas
processing unit (GPU), a power module, and a cooling module. The GPU
module is the unique new component specially developed for this
The GPU accepts ADG directly from the anaerobic digesters and delivers
a pretreated gas to the modified power module. The GPU, developed by
UTC Fuel Cells in cooperation with the United States Environmental
Protection Agency, consists of a demister to remove any entrained water
and two beds of specially treated charcoal, which convert the hydrogen
sulfide (H2S) into elemental sulfur and water by reacting with air,
which is fed separately to the unit. The unit utilizes nonregenerable
potassium hydroxide-impregnated redundant activated carbon beds to
remove hydrogen sulfides from ADG. The unit is sized to process ADG
flows of up to 4,800 scf/hr. The two carbon beds are capable of
operating for about six months with ADG containing 200 ppm of H2S. Each
bed contains approximately 1,200 lbs. of carbon. The GPU contains
sampling ports so that the H2S content may be monitored to determine
when the beds need to be changed. The sulfur is absorbed on the
charcoal, which is then removed on a periodic basis; the water
evaporates into the ADG stream; and the sulfur-free gas is fed to the
fuel cell. The unit is designed such that the charcoal in one bed may
be removed and replaced with fresh charcoal while the second bed is
used to continue to purify the ADG. The chemical reaction that takes
place in the bed is
H2S (gas) + O2 (gas) = H2O (gas) + S (solid sulfur).
After the ADG exists the GPU, it consists of methane, carbon dioxide,
and very low levels of organic halides and water. The methane can be
used as a fuel in the power plant; the carbon dioxide merely acts as an
inert gas in the system and, therefore, need not be removed inside the
fuel cell prior to reaching those components that they can affect. To
achieve this removal, a halide adsorption bed is added to the fuel
processing stream inside the fuel cell power plant, where it is
incorporated into the reactant supply system. Prior to entering this
bed, the organic halide compounds are converted, inside the power
plant, into inorganic halide compounds. These compounds are absorbed
onto the halide bed.
A standard PC25C power module reactant supply system is sized for
natural gas with a nominal heating value of 980 to 1,200 BTU/scf (HHV).
The modifications required to operate on ADG with nominal heating
values of 500 to 700 BTU/scf consists primarily of resizing inlet fuel
valves and piping to reduce pressure drop and increase fuel flow
capacity. Power module controller settings are tuned to maintain the
appropriate level of process fuel, steam, and burner air when running
on ADG. Additionally, ADG software modifications are implemented, and a
separate natural-gas piping to the reformer start-up burner is
provided. This separate piping will supply natural gas during start-up
of the fuel cell.
The purified ADG from the GPU flows to the fuel processor, which
consists of a metal vessel containing catalyst. In this vessel the
methane in the ADG reacts with steam produced by the fuel cell stack to
produce a stream consisting mostly of hydrogen and carbon dioxide; the
CO2 contained in the ADG does not react but passes through as an inert
diluent. The hydrogen production reaction is
CH4 (gas) + 2H2 (gas) = 4H2 (gas) + CO2 (gas).
The hydrogen is fed to the fuel cell stack where it reacts
electrochemically with air to produce power, water vapor, and heat. A
portion of the product water vapor is condensed into a liquid,
vaporized by cooling the fuel cell stack, and then used in the fuel
processor to react with the methane. Any hydrogen not utilized in the
fuel cell stack (<5%) is combusted to provide the heat required by the
fuel processor. The product water and carbon dioxide are exhausted to
the ambient air. Any fuel cell heat not used to boil water for the fuel
processor is available for use in the WWT process.
The GPU includes the gas analysis unit consisting of a sample pump,
regulator, and HÃ‚Â¬2S detector cell. The H2S sensor detects any hydrogen
sulfide in the gas entering the fuel cell, and it provides a signal to
the fuel cell controller to initiate an alarm or a safe shutdown before
damage can occur.
The fuel cell stack dc is converted to 480 Vac using a static inverter.
Applications of Fuel Cells
The concept of using fuel cells powered by hydrogen gas provides the
ideal solution to pollution in our world. Fuel cells have been
developed that can be used for virtually any application requiring
electrical power or mechanical energy. They are now classified into
three basic categories: portable devices, transportation applications,
and stationary power.
For portable applications such as laptop computers, cameras, and cell
phones, direct methanol fuel cells (DMFCs) shoe the most promise as
future replacements for batteries. Potentially, they can store over ten
times as much energy as a lithium battery, which would translate to
longer operating times. Instead of being recharged from plug-in ac
adapters, these units would get their charge from small disposable
cartridges of fuel plugged into the device offering total independence
from a wall plug for energy. For the DMFCs they biggest hurdles are
size and weight versus batteries and the amount of heat radiated. These
devices are low efficiency; thus, more heat is produced in the energy
conversion process. Technology advancements to address these issues are
underway and fuel cells may emerge as the energy source of choice for
many portable power applications.
One of the largest efforts in fuel cell development involves the proton
exchange membrane (PEM) type fuel cells for use in automobiles. Many
demonstration programs have been accomplished and real promise exists
for significant levels of automobile production using PEM fuel cells in
hybrid vehicles. A key challenge in any fuel cell program is volume of
units built. Economy of scale will drastically lower the cost, say
proponents, and lower cost will drive demand.
Stationary power application of fuel cells represents the biggest
opportunity for hydrogen to truly impact the worldâ„¢s environment. At
its ultimate stage of deployment, hydrogen fed fuel cells could produce
all of the energy needs of an average residence and eliminate the need
for many of the worldâ„¢s fossil-fuel power plants. Currently, molten
carbonate fuel cell (MCFC) systems appear to be the leading contender
for this application. They have been built and demonstrated at 200 kW
levels and are powering dozens of demonstration sites ranging from post
offices to personal homes. Many of these sites use existing natural gas
lines as the energy source. Natural gas is reformed onsite to produce
the hydrogen to power the fuel cell. This reforming process does
consume electricity and is a key issue in designing cost-effective
distributed energy devices that can serve as onsite commercial and
residential power plants.
Today a fuel-cell based power system is a very expensive method of
producing electricity when the initial costs are amortized into the
electricity rate. Plus, the average operating reliability and life of
the fuel cell is still an area of concern. All of these issues are
being addressed by fuel cell developers and will continue to improve
with time. One area of significant advancement is lower-cost power
electronics that convert the dc output of the fuel cell to useable ac
power. These converters make it easy to adapt the fuel cell power
system to any country regardless of voltage or frequency allowing the
production of truly universal power systems.
Â¢From IEEE editions
Â¢William H. Hayt, Jr.: Engineering Electromagnetics, McGraw-Hill Book
Â¢J.B. Gupta: Electric Power Systems, S.K.Kataria & sons
Â¢S.O.Pillai: Solid state physics, New Age International Publications.
Â¢M.V. Deshpande: Elements of Power System Design, Wheeler Publishing
Â¢C.L. Wadhwa: Generation and Utilization of Electrical Energy, New
Age International Publications.
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05-07-2010, 10:15 PM
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