Generation of heat and power from biogas for stationary applications: boilers, gas engines and turbines, combined heat and power (CHP) plants and fuel cells - The Biogas: Science, Production and Applications - page 413

17.3 Fuel cell types and chemical reactions.

Steinhause, 2008; US DOE, 2011). Tables 17.4 and 17.5 list the different
types of available fuel cells and their advantages and disadvantages. Each
fuel cell consists of a negative anode and a positive cathode, which are
separated by a solid or liquid electrolyte (Fig. 17.3). Input fuel (e.g. H 2 )
passes over the anode where it catalytically splits into ions (H + )and
electrons (e ). At the same time, oxygen (usually from air) passes over the
cathode. The ions pass through the electrolyte (electrically non-conductive),
while the electrons flow through the external electric circuit from the anode
to the cathode. A catalyst, such as platinum, is often used to speed up the
reactions at the electrodes. With some fuel cells, the electrolyte is also
permeable to oxygen molecules. In this case, the reaction occurs on the
anode side. Taking into account all losses, the voltage per single cell is 0.6-
0.9 V (Deublein and Steinhause, 2008). The desired voltage can be reached
by single cells arranged in series, a so-called stack. In a stack, the voltages of
the single cells are added. Depending on the type of fuel cell, the biogas has
to be purified, especially by removing CO and H 2 S, before feeding the fuel
cell, especially when working in a temperature range up to 200

C (Deublein
and Steinhause, 2008). Methane from the biogas must be reformed to H 2
and fed into the stack. Unused H 2 , that does not pass through the electrolyte
is used for the generation of heat through after-burning. At the moment,
only a small number of fuel cell plants, mostly pilot plants, are in operation
for the generation of electricity from biogas. Fuel cells are considered as
small-scale power plants of the future for the production of power and heat
with efficiencies exceeding 60% and low emissions (US DOE, 2011).

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17.3.7 Generation of electricity in a gas turbine

Biogas can also be used as fuel in the generation of electricity by gas turbines
(see Chambers and Potter, 2002; Deublein and Steinhause, 2008). A gas
turbine mainly consists of a compressor, combustion chamber and turbine.