Through experiments on desulfurization, CaSO 4 decomposition, and system approach using theoretical analysis, the in-furnace desulfurization in an O 2 /CO 2 combustion system with partial CO 2 removal from recycled gas was investigated. The results revealed that the SO 2 concentration increased with the CO 2 removal ratio and could be much higher than conventional combustion in air. This high SO 2 concentration came from the enrichment effect of gas recirculation, at a high gas recirculation ratio of 0.989. The system desulfurization efficiency also increased with the CO 2 removal ratio. Under the conditions investigated, the system efficiency of in-furnace desulfurization could be as high as 88.5%. The system desulfurization efficiency for the new scheme could be 6−10 times higher than conventional combustion in air. With this new scheme, easy CO 2 recovery and efficient in-furnace desulfurization could be realized simultaneously.
A new scheme of oxyfuel combustion combined with partial removal of CO 2 from recycled gas and MILD combustion was proposed. Through experiments and theoretical analysis including the Monte Carlo method, the characteristics of in-furnace desulfurization in this new scheme was investigated. It was found that as the initial O 2 concentration decreased, the gas recirculation ratio, SO 2 concentration, and global efficiency of in-furnace desulfurization increased. On the other hand, the gas recirculation ratio, SO 2 concentration, and global efficiency of in-furnace desulfurization increased as the CO 2 removal ratio increased. The practical residence time of SO 2 in oxyfuel-MILD coal combustion increased to about five to thirteen times as long as that of conventional pulverized coal combustion. The contributions of oxyfuel combustion, partial removal of CO 2 from recycled gas, and MILD combustion, to the high desulfurization efficiency, were almost the same importance. At CO 2 removal ratio = 11% and initial O 2 concentration = 16%, the gas recirculation ratio became as high as 99.12%, i.e., only 0.88% of the flue gas was exhausted to the atmosphere. The corresponding system desulfurization efficiency was as high as 95.8%. It was demonstrated that this new scheme can realize extremely high in-furnace desulfurization efficiency in addition to easy CO 2 recovery.
O 2 /CO 2 combustion is a promising technology to facilitate carbon capture in pulverized coal-fired power plants. The reduction of recycled NO, influence of CO 2 concentration on the conversion ratio of fuel N to NO, interaction between recycled NO and fuel N, and other factors were experimentally investigated, and correlations were quantitatively obtained. The conversion ratio from fuel N to NO increased with an increasing CO 2 concentration in the absence of coal but decreased in the presence of coal. The reduction ratio of NO in the recycled gas depended upon the oxygen/fuel stoichiometric ratio, λ, and increased with the NO concentration in the recycled gas when λ = 0.7 and 1.0 but decreased when λ = 1.2. The conversion ratio from fuel N to NO decreased with an increasing NO concentration in the recycled gas, because of the interaction between fuel N and recycled NO. The global conversion ratio from fuel N to exhausted NO in O 2 /CO 2 combustion was derived from the experimental results and analysis of the system. The relative importance of different factors in the low conversion ratio from fuel N to exhausted NO in O 2 /CO 2 combustion has been quantified and found to depend upon λ. Under all conditions investigated, the reduction of recycled NO was the major factor (over 70%) contributing to the low NO emission in O 2 /CO 2 combustion.
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