Page 36 - Synthetic Fuels Handbook

24 CHAPTER ONE

carbon dioxide from the atmosphere. While other biofuel technologies such as biogas and
biodiesel are also carbon neutral, gasification (a) uses a wider variety of feedstocks, (b) can
be used to produce a wider variety of products, and (c) is an efficient method of extracting
energy from biomass.
Therefore, it is not surprising that biomass gasification is one of the most technically
and economically convincing energy possibilities for the production of alternate fuels.

1.4.3 The Fischer-Tropsch Process

The Fischer-Tropsch process is a catalyzed chemical reaction in which carbon monoxide
and hydrogen are converted into liquid hydrocarbons of various forms. Typical catalysts
used are based on iron and cobalt. The principal purpose of this process is to produce a
synthetic petroleum substitute for use as synthetic lubrication oil or as synthetic fuel. The
process is currently used in South Africa to produce liquid fuels from syngas (produced
from coal by gasification).
Chemically, the Fischer-Tropsch process is represented as the conversion of carbon
monoxide and hydrogen to hydrocarbons and water:
nCO + (2n+1)H → C H (2n+2) + nH O
2
2
n
The initial reactants in the above reaction (i.e., CO and H ) can be produced by other reac-
2
tions such as the partial combustion of a hydrocarbon:
1
C H (2n+2) + / 2 nO → (n+1)H + nCO
2
n
2
Or by the gasification of coal or biomass:
C + H O → H + CO
2 2
The energy needed for this endothermic reaction of coal or biomass and steam is usually pro-
vided by (exothermic) combustion with air or oxygen. This leads to the following reaction:
2C + O → CO
2
The mixture of carbon monoxide and hydrogen is called syngas. The resulting hydrocarbon
products are refined to produce the desired synthetic fuel.
The carbon dioxide and carbon monoxide is generated by partial oxidation of coal and
wood-based fuels. The utility of the process is primarily in its role in producing fluid hydro-
carbons from a solid feedstock, such as coal or solid carbon-containing wastes of various
types. Nonoxidative pyrolysis of the solid material produces syngas which can be used
directly as a fuel without being taken through Fischer-Tropsch transformations. If a liquid
fuel, lubricant, or wax is required, the Fischer-Tropsch process can be applied successfully
in the manufacture.
Part of the issue with the Fischer-Tropsch process is that it produces a mixture of
hydrocarbons—many of which are not useful as fuel. However, the use of molecularly-
specific catalysts to convert these undesirable hydrocarbons into specific liquid fuels
is established. These catalysts work by rearranging the carbon atoms, transforming
six-carbon atom hydrocarbons, for example, into two- and ten-carbon atom hydrocarbons.
The liquid fuels produced in this way have several potential advantages not the least
of which is the absence of the odiferous, particle-producing aromatics. The liquid fuels
formed by the Fischer-Tropsch process burn much cleaner and are environmentally more
acceptable.