| The global warming caused by the greenhouse gas emission has become a serious problem that must be solved. The coal-fired power plant is the largest CO2 emission sources. Among various CO2 capture technologies, the O2/CO2 circulating fluidized bed (CFB) combustion technology is considered as an economic and feasible method due to its unique advantages such as low NOx emission and large-scale CO2 capture and utilization with low cost. However, when the flue gas from the O2/CO2 CFB combustion is purified and compressed, mercury in the flue gas will react with aluminum to form the mercury amalgam, which leads to the corrosion of the CO2 compressor and reduces the safety of the O2/CO2 system. Thus, mercury removal in the O2/CO2 CFB combustion technology is of great significance.This paper is devoted to investigating the mercury speciation mechanism with O2/CO2 coal combustion in a fluidized bed via experimental research, theoretical analysis and thermodynamic equilibrium calculation. Efforts are made to simulate and conduct effects of flue gas components on the mercury speciation mechanism, effects of temperature, oxygen concentration and excess oxygen coefficients on the mercury speciation mechanism in a small bubbling fluidized bed, relationship of flue gas components and mercury in a small circulating fluidized bed, mercury emission and speciation in a pilot-scale recycled flue gas circulating fluidized bed, and thermodynamic prediction model of the mercury speciation under the oxygen-enriched atmosphere.A vertical tube furnace is designed and established in this study. Combined with the commercial software FactSage, the experimental study and theoretical analysis are conducted to explore the effects of NO and SO2 on the mercury oxidation by Cl2 under the air and oxygen-enriched atmosphere. Results show that CO2, NO and SO2 inhibit the mercury oxidation. O2 promotes the mercury oxidation by Cl and ClO radicals while SO2 inhibits the mercury oxidation by consuming O radical. When NO and SO2 coexist under the oxygen-enriched atmosphere, the effects of SO2 on the mercury oxidation depends on the NO concentration. Under the air atmosphere, Cl element exists in the form of Cl2, Cl and ClO radical, the concentration of which varies with temperature. CO2 may affect the mercury oxidation by consuming Cl radical to form CCl3, CCl4, COCl and COCl2 radical.A 6 kWth coal-fired bubbling fluidized bed (BFB) is designed and retrofitted in this study. Combined with the commercial software FactSage, effects of temperature, oxygen content, excess oxygen coefficients and different coal types on the mercury speciation mechanism under the oxygen-enriched atmosphere are studied. Results show that under the oxygen-enriched atmosphere, Hg0 concentration increases with the increase of temperature, and Hg0 and Hg2+ concentration increases with the increase of oxygen content and excess oxygen coefficients. Besides, Hg2+(g) distribution of Xuzhou bituminite is less than that under the air atmosphere while Hg2+(g) distribution of Huaibei bituminite shows an adverse result, indicating effects of coal types on the mercury speciation due to different sulfur contents.A 6 kWth coal-fired circulating fluidized bed (CFB) is designed and retrofitted to explore the relationship between the flue gas components and the mercury speciation. Besides, the effects of coal type on the mercury speciation are also conducted. Results show that, for the anthracite, the growth rate of Hg2+ under the air combustion atmosphere is greater than that under the 21%02/79%C02 atmosphere, and the growth rate of Hg2+ under the oxygen-enriched atmosphere increases with increase of oxygen concentration. For anthracite under the oxygen-enriched atmosphere, the mercury speciation mechanism is analyzed by Langmuir-Hinshelwood, Eley-Rideal and Mars-Maessen heterogeneous models. Under both the air and oxygen-enriched atmosphere, Hg2+ concentration of bituminite is lower than that of anthracite. Under the oxygen-enriched atmosphere, Hg0 and Hg2+ proportion in the flue gas of bituminite and anthracite is almost the same. Under the oxygen-enriched atmosphere with different oxygen contents, Hg0 and Hg2+ proportion of bituminite is lower than that of anthracite, but the growth rate of Hg0 and Hg2+ shows little difference.Based on the 50 kWth circulating fluidized bed (CFB) combustion system with warm flue gas recirculation, effects of coal types on the mercury speciation under the oxygen-enriched atmosphere by thermodynamic analysis are analyzed. Results show that, Hg0(g) concentration under the oxygen-enriched atmosphere is higher than that under the air atmosphere; the growth rate of Hg2+ under the oxygen-enriched atmosphere is higher than that under the air atmosphere; NO, HCl and SO2 promote the mercury oxidation; the high H2O under the oxygen-enriched atmosphere is beneficial for the mercury oxidation. Simulation results show that, HgCl2 concentration under the oxygen-enriched atmosphere is higher than that under the air atmosphere at 700-1400℃; HgS concentration reaches the highest under the air atmosphere at 1400℃; H2O inhibits the formation of HgCl2 and HgS, the concentration of which reduces with the increase of H2O; when HCl concentration is higher than 600ppm, HCl promotes the formation of HgCl2. |
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