With
the aim at understanding the self-ignition mechanism of low-rank coals,
low-temperature oxidation behaviors of three coals were investigated
by measuring the weight change, gas formation rate, heat generation
rate, and change of functional groups during the oxidation at the
heating rate of 5 K/min in a helium stream containing 22% oxygen.
Detailed examination of the initial stage of oxidation below 200 °C
that will be closely related to the self-ignition clarified the following:
the reactions occurring are the intake of oxygen into aliphatic carbons
as peroxides and the decomposition of the peroxides forming carboxyls
and H2O. For two low-rank coals tested, the former process
was rapid enough and only the latter decomposition reaction of the
peroxides to form carboxyl groups was observed, which resulted in
the monotonous weight decrease and the H2O production.
For a high-rank coal tested, the former process solely occurred below
140 °C and the former process was faster than the latter process
below 250 °C, resulting in the monotonous weight increase. The
amount of oxygen involved in these processes was found to be only
1.7–4.6 mol of O/100 mol of C of coal. The amount of heat generated
during these processes was large enough to raise the coal from 180
to 320 °C.
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