Oxidation reaction of coal at relatively high temperatures were reviewed from the view point of reaction chemistry.
In the gas phase reaction, coal can be oxidized by mostly dehydrogenating mechanism being quite different from the liquid phase reaction due to free radical mechanism, because coal contains many kinds of easily oxidizable structures as well as many oxidation inhibiting structures.
In the liquid phase reaction, coal is oxidized and dissolved into solution as the high molecular weight compounds so called humic acids, which were further oxidized rather slowly to the lower molecular weight aromatic polycarboxylic acids.
The oxygen-oxidation of bituminous coal in alkaline medium results in a good yield of water-soluble aromatic polycarboxylic acids composed of mostly benzene polycarboxylic acids as the functions of alkali concentration and coalification grade of coal.
Nitric acid oxidation of bituminous coal at high temperatures or with concentrated nitric acid also results in a good yield of acid-soluble aromatic carboxylic acids.
