Thermolysis of brown coal in the presence of alkali metal hydroxides

Abstract

The thermolysis (≤500°C) of brown coal impregnated with alkali metal hydroxides MOH was studied. Alkalis cause the cleavage of a three-dimensional coal framework into fragments similar to humic acids. The initial rate of the process increases with the diameter of a cation. For the brown coal–KOH compound, an increase in the temperature from 110 to 400°C increased the rate of fragmentation by a factor of ~200 to cause the almost complete (≥90 wt %) decomposition of coal. The promotion of condensational reactions that cause a decrease in the yield of low-molecular-weight volatile organic products is a competing process. In the presence of MOH, the buildup of radicals is changed: a local maximum (at 200–250°C) is formed in the temperature dependence of the concentration of unpaired electrons; the temperature of this maximum and the values of [e –] depend on the nature of MOH. In the series of alkalis from LiOH to CsOH, the EPR signal broadened from 0.63 to 0.84 mT due to the formation of semiquinone radical anions and the interaction of an unpaired electron with a cation. A reaction scheme was proposed for the processes of the thermolysis (to 400°C) of brown coal with MOH; it includes the cleavage of a coal framework by the simultaneous heterolysis and homolysis of C–O and C–C bonds with the subsequent formation of a secondary lattice due to polycondensation and radical polymerization.