Page 177 - Synthetic Fuels Handbook

FUELS FROM COAL 163

In direct coal liquefaction, coal is pulverized and mixed with oil and hydrogen in
a pressurized environment. This process converts the coal into a synthetic crude oil
that can then be refined into a variety of fuel products. The direct coal liquefaction
technology has been demonstrated in the United States and is now being commercially
deployed in China and other countries.
The obvious drawback relates to environment issues. The process of converting coal
into liquid and using it for transportation releases nearly twice as much carbon dioxide
as burning diesel made from crude oil does. In a world conscious of climate change,
that excess carbon is a major issue. One way round this problem might be to take the
carbon dioxide and bury it underground. Another would be to replace fossil-fuel feed-
stock with biomass (Chap. 8).
Expense is another issue. The Fischer-Tropsch process has always been relatively expen-
sive. However, the need may have to justify the expense! The first major use of Fischer
Tropsch technology was during World War II, when Germany produced about 90 percent of
the diesel and aviation fuel. South Africa began liquefying coal in response to apartheid-era
sanctions, and in part as a result of its investment back then, continues to derive about
30 percent of its fuel from liquefied coal.

5.7 SOLID FUELS

5.7.1 Coke
In the current context, the term solid fuel refers to various types of solid material that is used
as fuel to produce energy and provide heating, usually released through combustion. Coal
itself is not included in this definition.
The most common coal-based solid fuel, coke, is a solid carbonaceous residue derived
from low-ash, low-sulfur bituminous coal from which the volatile constituents are driven
off by baking in an oven without oxygen at temperatures as high as 1000°C (1832°F) so
that the fixed carbon and residual ash are fused together.
Coke is used as a fuel and as a reducing agent in smelting iron ore in a blast furnace.
Coke from coal is grey, hard, and porous and has a heating value of 24.8 million Btu/ton
(29.6 MJ/kg). By-products of this conversion of coal to coke include coal tar, ammonia,
light oils, and coal-gas. On the other hand, petroleum coke is the solid residue obtained in
oil refining but at lower temperature, which resembles coke but contains too many impuri-
ties to be useful in metallurgic applications.
Next to combustion, carbonization for the production of coke represents one of the most
popular, and oldest, uses of coal (Armstrong, 1929; Forbes, 1950).
The thermal decomposition of coal on a commercial scale is often more commonly
referred to as carbonization and is more usually achieved by the use of temperatures
up to 1500°C (2732°F). The degradation of the coal is quite severe at these tempera-
tures and produces (in addition to the desired coke) substantial amounts of gaseous
products.
The original process of heating coal (in rounded heaps; the hearth process) remained the
principal method of coke production for many centuries, although an improved oven in the
form of a beehive shape was developed in the Newcastle area of England in about 1759.
The method of coke production was initially the same as for the production of charcoal, that
is, stockpiling coal in round heaps (known as milers), igniting the piles, and then covering
the sides with a clay-type soil. The smoke generated by the partial combustion of coal tars
and gases was soon a major problem near residential areas, and the coking process itself
could not be controlled because of climatic elements: rain, wind, and ice. It was this new