Experimental Substantiation of the Sulfur Oxide Concentration Reduction Mechanism in Coal and Biomass Particle Mixture Pyrolysis Products

The results of studies carried out by many researchers show that, in combusting mixtures of dispersed coal with biomass—even with a relatively small fraction of the latter—under conditions corresponding to the combustion process in the furnaces of steam- and hot-water boilers, the concentration of sulfur oxides in the combustion products of such mixtures (flue gases) decreases. This effect results in an essentially lower detrimental impact of coal-fired power plants on the environment. However, the mechanism through which sulfur oxides are sequestered with such combustion has not hitherto been unequivocally clarified. A few hypotheses have been formulated, according to one of which, sulfur acid vapors appear as a consequence of interaction between sulfur oxides and water vapor, followed by interaction of acid vapor with the metal oxides contained in the coal mineral part. It has been found from the results of experiments with weighted samples of wood–coal mixtures (under the conditions of densely “packed” particles) that the fraction of calcium and aluminum sulfates per unit coal mass in the ash of such mixtures after the thermal decomposition of both the components becomes completed is essentially (by a factor of 1.5–2.0) higher in comparison with that in the ash of homogeneous coal. However, the distances between individual coal and biomass particles are significantly larger in the furnace space than they are in a dense weighted sample (pour) of such mixtures. Therefore, for substantiating the sulfur oxide sequestration mechanism determined in the experiments with wood–coal mixture pours, it is necessary to confirm that the calcium and aluminum sulfates contained in the ash of wood–coal mixtures are produced under conditions in which coal and biomass particles are located at distances from each other commensurable with the size of these particles. Such experiments were carried out with mixtures of jet coal and pine wood particles at ambient medium temperatures from 400 to 800°С. It has been found that the concentrations of calcium and aluminum sulfates in the ash of mixtures after completion of pyrolysis in a closed space remain almost unchanged with increasing the distance between the fuel particles from its minimal value (with a dense contact in the mixture pour) to the value corresponding to the particle characteristic size. The deviations do not exceed ±15%, which is insignificantly larger that the uncertainty of the experiments, in comparison with the ash produced after completion of the pyrolysis of dense pours of the same mixtures. An analysis of the results obtained from the accomplished experiments gives grounds to draw the following conclusion: the mechanism governing the sequestration of sulfur oxides produced during joint pyrolysis of coal and wood biomass particles in mixture pours corresponds to the sequestration conditions of these oxides during the pyrolysis of particles between which the distances are quite typical for the furnaces of steam- and hot-water boilers.