Page 178 - Synthetic Fuels Handbook

164 CHAPTER FIVE

modification of the charring-coking kiln which laid the foundation for the modern coke
blast furnace: the beehive process.
Carbonization is essentially a process for the production of a carbonaceous residue by
thermal decomposition (with simultaneous removal of distillate) of organic substances.

[C] organic carbon → [C] coke/char/carbon + liquids + gases

The process is a complex sequence of events which can be described in terms of
several important physicochemical changes, such as the tendency of the coal to soften
and flow when heated (plastic properties) or the relationship to carbon-type in the coal.
In fact, some coals become quite fluid at temperatures on the order of 400 to 500°C
(752–932°F) and there is a considerable variation in the degree of maximum plasticity,
the temperature of maximum plasticity, as well as the plasticity temperature range for
various types of coal (Kirov and Stephens, 1967; Mochida et al., 1982; Royce et al.,
1991). The yields of tar and low molecular weight liquids are to some extent variable
but are greatly dependent on the process parameters, especially temperature, as well
as the type of coal (Cannon et al., 1944; Davis, 1945; Poutsma, 1987; Ladner, 1988;
Wanzl, 1988).

5.7.2 Charcoal
Charcoal is the blackish residue consisting of impure carbon obtained by removing water
and other volatile constituents from animal and vegetation substances such as wood (Chap. 10)
and, on occasion, from coal.
Charcoal is usually produced by heating wood, sugar, bone char, or other substances in the
absence of oxygen. The soft, brittle, lightweight, black, porous material resembles coal and is
85 to 98 percent carbon with the remainder consisting of volatile chemicals and ash.
The first part of the word is of obscure origin, but the first use of the term “coal” in English
was as a reference to charcoal. In this compound term, the prefix “chare-” meant “turn,” with
the literal meaning being “to turn to coal.” The independent use of “char,” meaning to scorch,
to reduce to carbon, is comparatively recent and must be a back-formation from the earlier
charcoal. It may be a use of the word charren or churn, meaning to turn, that is, wood changed
or turned to coal, or it may be from the French charbon. A person who manufactured charcoal
was formerly known as a collier (also as a wood collier). The word “collier” was also used for
those who mined or dealt in coal, and for the ships that transported it.

5.8 ENVIRONMENTAL ASPECTS OF COAL
UTILIZATION

During the combustion or conversion of coal, many compounds can be produced, some of
which are carcinogenic. Coal also contains nitrogen, oxygen, and sulfur as well as volatile
constituents and the minerals that remain as ash when the coal is burned. Thus some of the
products of coal combustion can have detrimental effects on the environment. For example,
coal combustion produces carbon dioxide (CO ) and it has been postulated that the amount
2
of carbon dioxide in the earth’s atmosphere could increase to such an extent that changes
in the earth’s climate will occur. Also, sulfur and nitrogen in the coal form oxides [sulfur
dioxide (SO ), sulfur trioxide (SO ), nitric oxide (NO), and nitrogen dioxide (NO )] during
2 3 2
combustion that can contribute to the formation of acid rain.