Etude de la combustion de gaz de synthèse issus d'un processus de ...

Chapter 2
- PEFC / PEM - Polymer Electrolyte Fuel Cell / Proton Exchange Membrane
- PAFC - Phosphoric Acid Fuel Cell
- MCFC - Molten Carbonate Fuel Cell
- SOFC - Solid Oxid Fuel Cell
SOFC fuel cell is the most suitable for integration with a gasifier (Klein, 2002). The
kinetics of this cell is fast, and the CO in the syngas is a directly useable fuel. This type
of fuel cell operates at a high temperature, and over 900°C the syngas can be reformed
within the cell.
2.2.2 Fuels
tel-00623090, version 1 - 13 Sep 2011
Several synthetic fuels can be produced from syngas via the Fischer-Tropsch (FT)
process. There are several commercial FT plants in South Africa producing gasoline
and diesel from coal and natural gas.
The FT synthesis involves the catalytic reaction of H 2 and CO to form hydrocarbons
chains of various lengths (CH 4 , C 2 H 6 , C 3 H 8 , etc.). The FT synthesis reaction can be
written in the general form (Ciferno and Marano, 2002):
( n 2 + m) H2 + mCO = CmHn
+ mH2O
(2.1)
where m is the average chain length of the hydrocarbons formed, and n equals 2m+2
when only paraffins are formed, and 2m when only olefins are formed.
Iron catalyst has water gas shift activity, which allows the use of syngas with low H 2 /CO
ratios. Syngas with H 2 /CO ratios of 0.5 to 0.7 is recommended for iron catalysts. The
water - gas shift reaction adjusts the ratio to match the requirements for hydrocarbon
and produce CO 2 as the largest by-product.
On the other hand, cobalt catalysts do not have water-gas shift reaction activity, and
H 2 /CO ratio required is then (2m + 2)/m. Water is the primary by-product of FT
synthesis over a cobalt catalyst.
As shown in Table 2.3, the composition of syngas required for fuel gas application is
different from that required for the synthetic fuel or chemical synthesis. A high H 2 , low
CO 2 and low CH 4 content is essential for fuel or chemical production. In opposite, a
high H 2 is not necessary for power production, as long as a high enough heating value
is supplied by the CH 4 .
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