Page 155 - Synthetic Fuels Handbook

FUELS FROM COAL 141

Over short distances coal is generally transported by conveyor or truck, whereas trains,
barges, ships, or pipelines are used for long distances. Preventative measures are taken at
every stage during transport and storage to reduce potential environment impacts. Dust can
be controlled by using water sprays, compacting the coal, and enclosing the stockpiles. Sealed
systems, either pneumatic or covered conveyors, can be used to move the coal from the stock-
piles to the combustion plant. Run-off of contaminated water is limited by appropriate design
of coal storage facilities. All water is carefully treated before reuse or disposal.

5.3.4 Coal Preparation
As-mined coal (run-of-mine coal) contains a mixture of different size fractions, sometimes
together with unwanted impurities such as rock and dirt. Thus, another sequence of events
is necessary to make the coal of a consistent quality and salable. Such events are called coal
cleaning. Effective preparation of coal prior to combustion improves the homogeneity of
coal supplied, reduces transport costs, improves the utilization efficiency, produces less ash
for disposal at the power plant, and may reduce the emissions of sulfur oxides.
Coal cleaning (coal preparation, coal beneficiation) is the stage in coal production
when the run-of-mine coal is processed into a range of clean, graded, and uniform coal
products suitable for the commercial market. In some cases, the run-of-mine coal is of such
quality that it meets the user specification without the need for beneficiation, in which case
the coal would merely be crushed and screened to deliver the specified product.
A number of physical separation technologies are used in the washing and beneficiation
of coals. After the raw run-of-mine coal is crushed, it is separated into various size fractions
for optimum treatment. Larger material (10–150 mm lumps) is usually treated using dense
medium separation—the coal is separated from other impurities by being floated across a
tank containing a liquid of suitable specific gravity, usually a suspension of finely ground
magnetite. The coal, being lighter, floats and is separated off, while heavier rocks and other
impurities sink and are removed as waste. Any magnetite mixed with the coal is separated
using water sprays, and is then recovered, using magnetic drums, and recycled.
The smaller size fractions are treated in a variety of ways—usually based on gravity dif-
ferentials. In the froth flotation method, coal particles are removed in a froth produced by
blowing air into a water bath containing chemical reagents. The bubbles attract the coal but
not the waste and are skimmed off to recover the coal fines. After treatment, the various size
fractions are screened and dewatered or dried, and then recombined before going through
final sampling and quality-control procedures.
Blending also enables selective purchasing of different grades of coal. More expensive,
higher quality supplies can be carefully mixed with lower quality coals to produce an aver-
age blend suited to the plant needs, at optimum cost.

5.4 USES

The use of coal in various parts of the world dates to the Bronze Age, 2000 to 1000 B.C.
The Chinese began to use coal for heating and smelting in the Warring States Period
(475–221 B.C.). They are credited with organizing production and consumption to the
extent that by the year 1000 A.D. this activity could be called an industry. China remained
the world’s largest producer and consumer of coal until the eighteenth century.
Outcrop coal was used in Britain during the Bronze Age (2000–3000 B.C.), where it
has been detected as forming part of the composition of funeral pyres (Britannica, 2004).
It was also commonly used in the early period of the Roman occupation. Evidence of trade