Development of Clean Utilization Technologies of Low-grade Coals Combined with Biomass

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In order to utilize low-grade coals efficiently and cleanly, technologies of co-combustion or co-gasification of coal with biomass will contribute to enhancing utilization of low-grade coals. In this study, three kinds of experiments such as a biobriquette combustion, using an electrically heated batch combustor, co-combustion of coal with biomass, using an electrically heated drop tube furnace, and fundamental gasification of coal or biomass, using a molten salt gasifier, were carried out. For the biobriquette combustion, the ignition lag and burning rate of the biobriquette becomes shorter and faster than that of the coal briquette, respectively, since the biomass in the biobriquette plays a role for a trigger of ignition. Slaked lime can capture in situ SO_2 produced from the briquette. In the co-combustion experiments, the biomass can enhance ignition and burnout of the low-grade coal during the co-combustion. But, trend of NO and N_2O emissions during the co-combustion is similar with that during the coal combustion. The chemical kinetic simulation relating to NOx can also explain the trend of NOx behaviors obtained experimentally. However, heterogeneous reaction scheme seems to affect the trend of N_2O. In the biomass combustion, fine particulates with the particle size of less than 2μm are produced as ash particles. Burning the biomass with the coal, however, the fine particulates from the biomass are captured by the ash from the coal during the combustion. The molten salt coal gasifier seems to have such functions as in-furnace desulfurization, dust collection and catalytic action of carbon. It may be possible for the gasifier to produce a clean synthesis gas without a gas cleaning-up system even at relatively low temperature.