Page 32 - Synthetic Fuels Handbook

20 CHAPTER ONE

removing unwanted constituents such as ash, and thereby to produce a fuel which is cheaper to
transport and handle.
Although most of the emphasis for the production of synthetic fuels is on synthetic fuels
from coal, oil shale, and tar sands, biomass can also be converted to synthetic fuels and the
fermentation of grain to produce alcohol is a well known example. However, in many coun-
tries, grain is an expensive product which is generally thought to be more useful for its food
value. Wood is an abundant and accessible source of bioenergy but it is not known whether its
use to produce synthetic fuels is economic. The procedures for the gasification of cellulose-
containing materials have much in common with the conversion of coal to gas. Most of the
conversion principles to be discussed are, however, applicable to the spectrum of carbonaceous
or cellulosic materials which occur naturally, are grown, or are waste.
Synthetic fuel or synfuel is any liquid fuel obtained from any of the aforementioned fuel
sources (i.e., tar sand, coal, oil shale, natural gas, natural gas hydrates, and biomass) as
well as biologic alcohol and through the agency of the Fischer-Tropsch synthesis. For the
purposes of this text, the term synthetic fuel also includes liquid fuels derived from crops,
wood, waste plastics, and landfill materials. In a similar manner, synthetic gaseous fuel
(syngas) and synthetic solid fuel can also (but less often) refer to gaseous fuels and solid
fuels produced from the same sources.
In fact, 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 con-
taining fuel to a gaseous product with a heating value. Examples include steam reforming
of natural gas or liquid hydrocarbons to produce hydrogen, 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. Syngas is also used as an intermediate in producing synthetic petroleum for use
as a fuel or lubricant via Fischer-Tropsch synthesis and previously the Mobil methanol to
gasoline process.
Syngas consists primarily of carbon monoxide, carbon dioxide, and hydrogen, and has
less than half the energy density of natural gas. It is combustible and often used as a fuel
source or as an intermediate for the production of other chemicals. Syngas for use as a fuel
is most often produced by gasification of coal or municipal waste:
C + O → CO
2 2
CO + C → 2CO
2
C + H O → CO + H 2
2
When used as an intermediate in the large-scale, industrial synthesis of hydrogen and
ammonia, it is also produced from natural gas (via the steam reforming reaction):
CH + H O → CO + 3H 2
2
4
The syngas produced in large waste-to-energy gasification facilities is used as fuel to gener-
ate electricity.
Coal gasification processes (Chap. 5) are reasonably efficient and were used for many
years to manufacture illuminating gas (coal gas) for gas lighting, before electric lighting
became widely available.
When syngas contains a significant amount of nitrogen, the nitrogen must be removed.
Cryogenic processing has great difficulty in recovering pure carbon monoxide when rela-
tively large volumes of nitrogen are present, as carbon monoxide and nitrogen have
very similar boiling points (i.e., −191.5°C and −195.79°C, respectively). Instead there is
technology that selectively removes carbon monoxide by complexation/decomplexation