Page 287 - Dust Explosions in the Process Industries

256 Dust Explosions in the Process Industries

4.1.4
CARBON AND COAL

Research on the explosibility of coal dust ,,as a long tradition. According to Essenhigh
(1961), the possible role of coal dust in coal mine explosions was suggested as early as
in 1630 by Edward Lloyd, when commenting on information received from Anthony
Thomas concerningan explosion in England in about 1580.The role of coal dust in such
explosions was certainly clear to Faraday and Lye11 (1845), discussing the disastrous
explosion in the Haswell collieriesthe year before. More systematic investigationsinto
the ignitability and explosibility of coal dusts started at the end of the 19th and the
beginning of the 20th centuries.
However,the combustion of coal dust particles is not only related to the explosionprob-
lem, the increasing use of pulverized coal in burners for energy production has become
an important area of research and development, and much information on the combus-
tion of coal particles directly applicable to the coal dust explosion problem has been gen-
erated in that context.Furthermore, this use of pulverized coal in industry as well as the
public sector has caused coal dust explosions to become a potential hazard, not only in
mines but also in power generating plants utilizing powdered coal.
Coal normally containsboth solid carbon and combustible volatiles. In addition, there
is usually some ash and some moisture. The simplest system to study is the combustion
of pure carbon or char. Nusselt (1924) proposed that the oxidation of pure carbon was
essentially a direct conversion of solid carbon to COz at the particle surface. However,
later investigationsdisclosed a more complex picture even for oxidation of pure carbon,
as illustrated in Figure 4.2.

Figure 4.2 Compositionof laminar gas layer during
combustion of solid carbon according to the theory
of Held (1 96 1) for surface temperatures >1400 K.
R II Nitrogen is not considered,S = carbon surface; R =
rc LAMINAR DC TURBULENT reaction zone (From Craaf, 1965).

In zone I, the concentration of 0, is 0; whereas in Zone 11, the CO concentration is
0. At the carbon surface, S, COzreacts with the solid carbon according to the endother-
mic scheme CO, + C +2CO. The required heat is supplied from the oxidation zone,
R, where the temperature is at maximum and the exothermic reaction CO + 'h0, +
CO, takes place. Using the theory of Held (1961), Graaf (1965) found that the tem-
perature in the oxidation zone R was about 2500 K for a coal surface temperature of
1800 K.
For low carbon surface temperaturesof 4400 K, a significant concentration of 0, may
exist right at the surface; and at very low surface temperatures of <800 K, direct oxidation