Page 211 - Synthetic Fuels Handbook

CHAPTER 7

FUELS FROM SYNTHESIS GAS

The hydrogen-to-carbon ratio of carbonaceous feedstocks such as heavy oil, tar sands, oil
shale, coal, and biomass to liquid fuels is lower than the hydrogen to carbon ratio of conven-
tional petroleum. In the process, the hydrogen/carbon atomic ratio must be adjusted to that of
transportation fuels. One of the methods by which this can be achieved is through the appli-
cation of the Fischer-Tropsch process to the conversion of synthesis gas to hydrocarbons.
Synthesis gas (syngas) is the name given to a gas mixture that contains varying amounts
of carbon monoxide and hydrogen generated by the gasification of a carbon-containing fuel
to a gaseous product with a heating value (Speight, 2007a and references cited therein).
Examples include steam reforming of natural gas or liquid hydrocarbons to produce hydro-
gen, the gasification of coal, and in some types of waste-to-energy gasification facilities.
The name comes from their use as intermediates in creating synthetic natural gas (SNG)
and for producing ammonia or methanol. Synthesis gas is also used as an intermediate in
producing synthetic petroleum for use as a fuel or lubricant via Fischer-Tropsch synthesis
(Storch, 1945).
In the context of this book, synthesis gas, which can also be generated from biomass
(Chap. 8), is not the same as biogas. Biogas is a clean and renewable form of energy gener-
ated from biomass that could very well substitute for conventional sources of energy. The gas
is generally composed of methane (55–65 percent), carbon dioxide (35–45 percent), nitrogen
(0–3 percent), hydrogen (0–1 percent), and hydrogen sulfide (0–1 percent) (Anunputtikul
and Rodtong, 2004; Coelho et al., 2006).
Gasification to produce synthesis gas can proceed from just about any organic mate-
rial, including biomass and plastic waste. The resulting syngas burns cleanly into water
vapor and carbon dioxide. Alternatively, syngas may be converted efficiently to methane
via the Sabatier reaction, or to a diesel-like synthetic fuel via the Fischer-Tropsch process.
Inorganic components of the feedstock, such as metals and minerals, are trapped in an inert
and environmentally safe form as char, which may have use as a fertilizer.
Regardless of the final fuel form, gasification itself and subsequent processing neither
emits nor traps greenhouse gases such as carbon dioxide. Combustion of syngas or derived
fuels does of course emit carbon dioxide.
Synthesis gas consists primarily of carbon monoxide, carbon dioxide, and hydrogen,
and has less than half the energy density of natural gas. Synthesis gas is combustible and
often used as a fuel source or as an intermediate for the production of other chemicals.
Synthesis gas for use as a fuel is most often produced by gasification of coal or municipal
waste mainly by the following paths:
C + O → CO 2
2
CO + C → 2CO
2
C + H O → CO + H
2 2

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