Page 165 - Synthetic Fuels Handbook

FUELS FROM COAL 151

structure breaks down. Volatile matter, including hydrogen, water, methane, and higher-
boiling tar is produced. This leaves a char consisting of almost pure carbon intermingled
with inert, incombustible ash. Among the important attributes of a particular coal are the
proportions of volatiles, ash, and sulfur; how chemically reactive it is; and whether it tends
to swell or stick together, or cake when burned. Different coals may exhibit very different
physical and chemical behavior.
Using very general equations, when coal is heated in the presence of oxygen and/or water,
carbon monoxide (CO) and carbon dioxide (CO ) are also formed as seven reactions compete:
2
[C] coal + O → CO 2
2
2[C] coal + O → 2CO
2
[C] + CO → 2CO
coal 2
[C] coal + 2H → CH 4
2
[C] coal + H O → CO + H 2
2
CO + H O → CO + H
2 2 2
CO + 3H → CH + H O
2
4
2
The rates of these different reactions depend on the temperature and pressure, the char-
acteristics and physical form of the coal, the amounts of oxygen and steam available, and
the configuration of the reacting materials. All the reactions (except the coal-carbon-dioxide
reaction) are exothermic and release energy as heat.
However, if the process is to produce fuel gas, the coal-oxygen reaction to produce car-
bon dioxide must not dominate because all the energy in the coal will be released as heat
and there will be none left in the gas. The gases needed are carbon monoxide, hydrogen, and
methane. The reaction conditions must, therefore, be carefully controlled by controlling
the amount of oxygen in the combustion chamber so that other reactions can occur. Hence,
coal gasifiers are designed to favor the reactions that produce energy-rich gases, such as
the coal-oxygen reaction to form carbon monoxide reactions and the coal stream reaction
to produce synthesis gas (a mixture of carbon monoxide and hydrogen).
Sulfur in the coal will also react with oxygen or hydrogen to release energy and to form
either sulfur oxides or hydrogen sulfide. Nitrogen present in the air of the combustion
chamber may also form nitrogen oxides.

Reactions. Coal gasification involves the thermal decomposition of coal and the reaction of the
coal carbon and other pyrolysis products with oxygen, water, and fuel gases such as methane.
The presence of oxygen, hydrogen, water vapor, carbon oxides, and other compounds in
the reaction atmosphere during pyrolysis may either support or inhibit numerous reactions
with coal and with the products evolved. The distribution of weight and chemical compo-
sition of the products are also influenced by the prevailing conditions (i.e., temperature,
heating rate, pressure, residence time, and any other relevant parameters) and, not less
importantly, by the nature of the feedstock (Wang and Mark, 1992).
If air is used as the means of combustion, the product gas will have a heat content of
3
3
150 to 300 Btu/ft (5.6–11.2 MJ/m ) (depending on process design characteristics) and will
contain undesirable constituents such as carbon dioxide, hydrogen sulfide, and nitrogen.
The use of pure oxygen, although expensive, results in a product gas having a heat content
3
3
of 300 to 400 Btu/ft (11.2–14.9 MJ/m ) with carbon dioxide and hydrogen sulfide as
by-products [both of which can be removed from low or medium heat-content (low- or
medium-Btu) gas by any of several available processes].