Understanding self-ignition of coal - Sino German Coal fire project

Self-heating and spontaneous combustion page 11
From the low temperature formation of coal-oxygen complexes and the outbreak of
spontaneous combustion, the coal heats at an increasing rate. This heat is dependent on
enough air being available and enough heat being retained to sustain the reaction. At
temperatures above 80°C the coal-oxygen complex breaks down and releases greater
amounts of heat with the reaction:
coal-oxygen complex → CO + CO2 + H2O + heat /1/
The decomposition of the chemisorption intermediates mainly generates CO while the
liberation of CO2 comes from the decomposition of the carboxyl groups. Three
simultaneous reactions proceed in the thermal decomposition process. The carbonyl
groups are supposed to be generated via a dehydroxylation process. The
decarboxylation results in gaseous products, including CO2, CO and H2O, and the
decarbonylation process liberates the same species. At higher temperatures of about
150°C decomposition of carboxyl and carbonyl groups occurs, leading to the production
of carbon dioxide and carbon monoxide along two independent reaction pathways
(Figure 3.4) /18/.
Coal C
O
Coal C
O
OH
R
Coal + CO
Coal C
O
O Coal + CO 2
R
Coal C
Figure 3.4: Decomposition of carboxyl and carbonyl groups /18/
In addition to the coal carbon processes, the reaction involved in the oxidation of coal
hydrogen to form water are also exothermic. This is of particular importance for lowrank
coals that have a high inherent hydrogen content /10/.
3.2 Factors contributing to spontaneous combustion
Aside from the natural affinity of coal to self-heating, a number of factors are
significant when determining the risk of spontaneous combustion and are presented in
this section.
O
OR