Page 105 - Synthetic Fuels Handbook

92 CHAPTER THREE

The heat value of gases is generally determined at constant pressure in a flow calorim-
eter in which the heat released by the combustion of a definite quantity of gas is absorbed
by a measured quantity of water or air. A continuous recording calorimeter is available for
measuring heat values of natural gases (ASTM D1826).
The lower and upper limits of flammability of organic compounds indicate the percent-
age of combustible gas in air below and above which flame will not propagate. When
flame is initiated in mixtures having compositions within these limits, it will propagate
and therefore the mixtures are flammable. Knowledge of flammable limits and their use
in establishing safe practices in handling gaseous fuels is important, for example, when
purging equipment used in gas service, in controlling factory or mine atmospheres, or in
handling liquefied gases.
Synthesis Gas. On the other hand, synthesis gas (syngas) is the name given to a gas mix-
ture that is generated by the gasification of a carbon-containing fuel (e.g., petroleum coke,
qv) to a gaseous product that contains varying amounts of carbon monoxide and hydrogen.
The name synthesis gas originates from their use as intermediates in creating synthetic
natural gas (SNG) and for producing ammonia and/or methanol. Syngas is also used as an
intermediate in producing synthetic fuels via the Fischer-Tropsch reaction.
In the strictest sense, synthesis gas consists primarily of carbon monoxide and hydrogen,
although carbon dioxide and nitrogen may also be present. The chemistry of syntehsis gas
production is relatively simple but the reactions are often much more complex than indi-
cated by simple chemical equations:

[C] + O → CO
petroleum coke 2 2
[C] petroleum coke + CO + C → 2CO
2
[C] petroleum coke + H O → CO + H 2
2
Synthesis gas is combustible and often used as a fuel source or as an intermediate for the
production of other chemicals. 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
Synthesis gas is also manufactured from waste and form coal but these feedtocks and
processes are not discussed in the current context.

3.5.2 Liquid Fuels

Gasoline. Gasoline, also called gas (United States and Canada), or petrol (Great Britain)
or benzine (Europe) is mixture of hydrocarbons that usually boil below 180°C (356°F) or,
at most, below 200°C (392°F).
Gasoline is manufactured to meet specifications and regulations and not to achieve a
specific distribution of hydrocarbons by class and size. However, chemical composition
often defines properties. For example, volatility is defined by the individual hydrocarbon
constituents and the lowest boiling constituent(s) defines the volatility as determined by
certain test methods.
Automotive gasoline typically contains about almost 200 (if not several hundred)
hydrocarbon compounds. The relative concentrations of the compounds vary considerably
depending on the source of crude oil, refinery process, and product specifications. Typical