Mechanisms of Ultra-low NOx and High Efficiency De-SOx in CO_2 Recycled Coal Combustion

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CO_2 recycled coal combustion is promising because of its easy CO_2 recovery, extremely low NOx emission and high in-furnace desulfurization efficiency. The feasibility of simultaneous CO_2/NOx/SOx reduction in CO_2 recycled coal combustion was studied in order to develop a novel environmentally favorable coal combustion technology.It was identified through experiments that the amount of NOx exhausted from the system is reduced to less than about one fourth of that exhausted with conventional air combustion. The contributions to the total NO decrease from coal combustion with air caused by the use of recycled CO_2 are as follows : 1. The effect of an increase of CO_2 concentration in the furnace is not significant below 10%. 2. The effect of interaction between fuel-N and recycled NO is 10-50 %. 3. The effect of a reduction of recycled NO in the furnace is dominant and amounts to 50-80%. Theoretical simulation also confirmed the validity of this result. The characteristics and mechanism of desulfurization were clarified through experiment and modeling. The desulfurization efficiency in CO_2 recycled coal combustion is about three times as high as that with coal combustion in air and does not suffer so much from CaSO_4 decomposition. The high desulfurization efficiency is mainly attributed to : 1. high concentration of SO_2 inside furnace owing to the recirculation of gases ; 2. decrease in CaSO_4 decomposition resulted from high SO_2 concentration. CO_2 recycled coal combustion was identified to be a promising system with which simultaneous reduction of CO_2/NOx/SOx can be realized.