Page 214 - Synthetic Fuels Handbook

200 CHAPTER SEVEN

The combustion process occurs as the volatile products and some of the char reacts
with oxygen to form carbon dioxide and carbon monoxide, which provides heat for the
subsequent gasification reactions:
3C + 2O → CO + 2CO
2
2
Gasification is the decomposition of hydrocarbons into a syngas by carefully control-
ling the amount of oxygen present, for example, the conversion of coal into town gas. The
gasification process occurs as the char reacts with carbon dioxide and steam to produce
carbon monoxide and hydrogen:
C + H O → H + CO
2
2
In addition, the gas phase water gas shift reaction reaches equilibrium very fast at the
temperatures in a gasifier. This removes carbon dioxide from the reactor and provides water
for the gasification reaction:

CO + H O ↔ CO + H
2 2 2
In essence, a limited amount of oxygen or air is introduced into the reactor to allow
some of the organic material to be burned to produce carbon monoxide and energy, which
drives a second reaction that converts further organic material to hydrogen and additional
carbon monoxide.
Coal gasification chemistry is reasonably simple and straightforward and, hence, is
reasonably efficient. For many years, such processes were used to manufacture illumi-
nating gas (coal gas) for gas lighting, before electric lighting became widely available.
The simplest method, and the first used, was to heat coal in a retort in the absence of air,
partially converting coal to gas with a residue of coke; the Scottish engineer William
Murdock used this technique in pioneering the commercial gasification of coal in 1792.
Murdock licensed his process to the Gas Light and Coke Company in 1813, and in 1816
the Baltimore Gas Company, the first coal gasification company in the United States,
was established. The process of heating coal to produce coke and gas is still used in
the metallurgical industry.
Currently, hydrogen is produced from coal by gasification and the subsequent process-
ing of the resulting synthesis gas. In its simplest form, coal gasification works by first react-
ing coal with oxygen and steam under high pressures and temperatures to form a synthesis
gas consisting primarily of carbon monoxide and hydrogen. This synthesis gas is cleaned
of virtually all of its impurities and shifted to produce additional hydrogen. The clean gas
is sent to a separation system to recover hydrogen.
The most complete conversion of coal or coke to gas that is feasible was achieved by
reacting coal continuously in a vertical retort with air and steam. The gas obtained in this
3
manner, called producer gas, has low heat content per unit volume of gas (100–150 Btu/ft ).
The development of a cyclic steam-air process in 1873 made possible the production of a
3
gas of higher thermal content (300–350 Btu/ft ), composed chiefly of carbon monoxide
and hydrogen and known as water gas. By adding oil to the reactor, the thermal content
3
of gas was increased to 500 to 550 Btu/ft ; this became the standard for gas distributed to
residences and industry. Since 1940, processes have been developed to produce continu-
ously a gas equivalent to water gas; this involves the use of steam and essentially pure
oxygen as a reactant. A more recently developed process reacts coal with pure oxygen and
steam at an elevated pressure (450 psi) to produce a gas that may be converted to synthetic
natural gas.