Vol 51, No 3 (2017)

The dependence of the water wettability of coal surface on the structure and properties of coals of different stages of diagenesis and metamorphism was studied based on the evaluation of the angles of contact. It was found that the caking coals of Zh and K ranks exhibited maximum hydrophobic properties. Hydrophilic properties were most pronounced in brown coals and low-rank black coals with the highest concentrations of hydroxyl and carboxyl groups and a developed porous structure.

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.

The results of an experimental study and mathematical simulation of the ignition of coal–water fuel (CWF) particles, the main thermophysical characteristics of which (thermal conductivity (λ), heat capacity (C), and density (ρ)) depend on temperature, are reported. Based on the results of the numerical study, the influence of changes in the thermophysical properties upon the heating of the main bed of fuel on the conditions and characteristics of its ignition was analyzed. The ignition delay times (t_i) of CWF particles were determined under the typical furnace conditions of boiler aggregates. As a result of the mathematical simulation of the process of CWF ignition, it was established that the temperature dependence of thermophysical characteristics can exert a considerable effect on the characteristics and conditions of ignition. In this case, it was found that the ignition of coal–water drops is possible under the conditions of their incomplete dehydration. A good agreement of the theoretical ignition delay times of the CWF particles and the experimental values of t_i was established.

The thermodynamic analysis of the composition of the combustion products of 15 types of coals was carried out with consideration for the formation of potassium and sodium aluminosilicates and solid and liquid slag removal. Based on the results of the analysis, the approximating temperature dependences of the concentrations of condensed components (potassium and sodium sulfates) were obtained for the cases of two-phase and single-phase equilibriums; conclusions on the comparative influence of solid and liquid slag removal on the probability of the formation of submicron particles on the combustion of coals were made. The found dependences will make it possible to perform a numerical simulation of the bulk condensation of potassium and sodium sulfate vapors upon the cooling of coal combustion products in a process flow.

The possibility of controlling the ash content of fuel based on a carbon/oxygen ratio taking into account variable carbon content out of the organic matter was demonstrated. An algorithm was proposed for the determination of the ash content with the use of a pulsed neutron method with the detection of gamma rays from inelastic neutron scattering on the nuclei of carbon, oxygen, and calcium with an instrumental correction for the variable chemical composition of the mineral matter of coals. The procedure was recommended for the operational quality control of coarse dispersed fuel whose chemical composition varies over wide limits.