Prospects for Coal Briquettes as a Substitute Fuel for Wood and ...

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is coal carbonization, converting it to a "'soft coke" by pyrolysls.
This is simllar to the conversion of wood to charcoal. During the
heating of the coal without its combustion, vol atile content is reduced,
and gases and tars containing some of the other harmful constituents are
glven off. The result is a product high in fixed carbon, which can be
burned with 1 ittle or no smoke and missions no worse than the present
practice of burning coal and firewood. Carbonization can be performed
before or after briquetting -- or wen without briquetting, if irregular
"soft coke" lumps and loss of the coal fines are acceptable. [See
Appendix A,] Because the soft coke is vlrtually smokeless9 it w i l l be
referred to often as ?"smokeless coal" in the remainder of this document.
One of the important aspects of the coal carbonization process is that
many of the potentially harmful constituents of coal are removed. As
mentioned, most of the volatiles are driven off during pyrolysis fnto
the off-gases, tars, or liquors. A certain level of volatiles is left
in the soft coke (approximately 1% to 20%) to promote its easy ignition
C61, but this level is deemed safe and does not allow the fuel to smoke
significantly. Many of the potentially carcinogenic and other organic
constituents, such as benzo(a1pyrene (BaP), to1 uener benzene, etc. , cane
out in the tar.
The sulfur content is also reduced as the sulfur is converted to
hydrogen sulfide gas. Actually, several steps can be taken for sulfur
removal to make the final smokeless coal lump or briquette rather clean
with respect to sulfur oxide emissions. Washing the coal with water
prior to pyrolysis is a simple, inexpensive, and effective step. More
sulfur is removed during the carbonization process. Finally, if the
coal is a higher sulfur coal, lime or limestone can be added to the
final briquette so that any remaining sulfur combines with it and
remains in the ash upon combustion. The cost of 1 ime, if needed, Is
also very lowp representing only 1% of the price of a soft coke
briquette in a model plant scenario constructed by Fabuss and Tatm.C63
Particulates are considerably reduced by the process of carbonization.
Only minor fly ash results from combustion of the carbonized fuel.
Also, while the temperature of carbonization is not high enough to
remove many of the trace elements by volati~ation~ neither is the
temperature of combustion of the smoke1 ess coal. Thus, without
particulates to which to adsorb, the trace elements remain in the ash
when the smokeless coal briquettes are burned. This keeps the level of
human exposure to trace elements at an insignificant level.
One remaining health effect shoul d be mentioned. The production of soft
coke can expose workers to dangerous conditions unless strict control s
are imposed. Coal dust in coking plants is often very high and
difficult to suppress. The potential carcinogens in the coal tarsr such
as BaP, suggest careful hand1 ing of these by-products. In developing
countries, attention to worker safeTy in carbonization plants Is not
a1 ways given due attention. [I61
It should be noted that Carbonization is only needed for coals below the
rank of anthracite, that is, for bituminous, subbituminous, and 1 ignite
coals. Anthracite in its natural state is sufficiently low in volatile