分解炉空气分级燃烧及NOx排放特性研究

随着我国经济的飞速发展,作为重要基础材料的水泥产品需求量极大且趋于稳定。水泥生产过程中的NOx排放与燃煤火电厂和汽车尾气产生的NOx排放已成为空气污染的主要来源,而分解炉是降低水泥生产工艺中NOx排放的有效设备。笔者在引入高温烟气的模拟分解炉内进行空气分级燃烧试验,研究配风位置、配风比例以及石灰石/煤比例对分解炉内燃烧和NOx排放特性的影响规律。试验稳定过程中,高温烟气发生装置的给煤量和配风量保持不变。此时,高温烟气发生装置的时间平均温度为911℃,其产生的高温烟气温度稳定在750℃左右,高温烟气中NOx主要以NO和N2O的形式存在,其浓度分别为261.49×10^-6和12.96×10^-6。该股高温烟气将模拟实际回转窑产生的烟气进入分解炉内。在分解炉的上部区域(距离顶部0~2 000 mm区域)的温度为800~1 000℃,与实际分解炉运行温度一致,排放烟气中NOx主要以NO和N2O形式存在。随着中间配风位置的下移,煤粉燃烧放热区域下移,而顶部区域的石灰石吸热量变化较小,则原有热量平衡被打破且原有吸热量高于现有放热量,导致顶部区域内燃烧温度降低。此时,还原气氛中煤粉燃烧和石灰石分解反应时间均变长,导致NOx的还原反应更加充分。但石灰石分解产生的氧化钙(CaO)作为中间产物会促进NO的生成反应,其反应时间增加也促进了NO的生成;另一方面,石灰石作为催化剂参与焦炭和挥发分还原NO的反应过程,分解炉顶部区域的温度下降使得该还原反应变弱。综上,NO的最终排放浓度是以上反应的综合结果。随着配风位置的下移,该变化对NO的生成作用更加明显,故NO的排放浓度逐渐升高。当一级风量与二级风量的配风比例降低时,分解炉上部区域的煤粉燃烧份额减少和石灰石分解量降低,而分解炉下部区域的煤粉燃烧份额增加和未分解的石灰石份额增加,但石灰石的吸热增加量高于燃烧增加份额的放热量,因此分解炉内整体温度均降低。分解炉内NO浓度是由石灰石催化的氧化过程和还原过程综合决定的。一级风量变小时,尾部CO浓度随之增加,烟气中NO浓度呈现降低的趋势。当石灰石/煤比例增加时,分解炉内沿程温度逐渐下降。随着石灰石给粉量增加,分解炉内石灰石受热分解产生的CaO浓度增加,CaO催化NO还原反应更剧烈,从而NO浓度逐渐降低。而石灰石给粉量增加和分解炉温度降低的过程导致尾部的CO浓度升高。

Experimental study on air-staging combustion and NO_x emission characteristics in cement precalciner

With the rapid development of China’s economy,as an important basic material,the demand for cement products is huge and tends to be stable. The NOx emission from cement production process and coal-fired power plants and automobile exhaust has become the main source of air pollution,and decomposition furnace is an effective equipment to reduce NOx emission in cement production process.The air-staging combustion experiments were carried out in a simulated precalciner with high-temperature flue gas,and the effects of air distribution position,air distribution ratio and limestone/coal ratio on combustion and NOxemission characteristics in cement precalciner were investigated. During the stable test,the coal feeding and air distribution of the high-temperature flue gas generator remain unchanged.At this time,the time-average temperature of the high-temperature flue gas generator is 911 ℃,and the temperature of the high-temperature flue gas produced is stable at approximately 750 ℃ . The NOx in the high-temperature flue gas mainly exists in the form of NO and N2O,and the concentration is 261.49×10^-6 and 12.96×10^-6,respectively. The high-temperature flue gas will simulate the actual flue gas from the rotary kiln into the decomposition furnace. The temperature in the upper part of the calciner( 0-2 000 mm from the top) is from800 to 1 000 ℃,which is consistent with the actual operation temperature of the calciner. NOxin the exhaust gas mainly exists in the form of NO and N2O. As the position of the middle air distribution moves down,the exothermic region of pulverized coal combustion moves down,while the heat absorbed by limestone in the top region changes a little,so the original heat balance is broken and the original heat absorbed is higher than the exothermic quantity,causing the combustion temperature in the top zone to drop. At this time,the reaction time of coal combustion and limestone decomposition becomes longer in reducing atmosphere,which leads to more sufficient reduction of NOx.However,calcium oxide( Ca O) produced by limestone decomposition,as an intermediate product,can promote the formation of NO,and its reaction time also promotes the formation of NO. On the other hand,limestone is used as a catalyst in the reduction of NO by char and volatiles,the reduction of NO is weakened due to the decrease in temperature in the top zone of the decomposition furnace. To sum up,the final emission concentration of NO is the combined result of the above reactions. As the position of air distribution moves down,the effect of those changes on NO production becomes more obvious,so the concentration of NO emission increases gradually. When the ratio of the first air flow rate to the second air flow rate is reduced,the coal combustion rate and limestone decomposition rate in the upper part of the calciner decrease,and the proportion of pulverized coal combustion and the proportion of undecomposed limestone in the lower part of the calciner increases,but the heat absorption increase of limestone is higher than the heat release of combustion increase,so the whole temperature of calciner decreases. The concentration of NO in the calciner is determined by the oxidation and reduction processes catalyzed by limestone. The concentration of CO in the tail increases and the concentration of NO in the flue gas decreases with the decrease of the first stage air flow rate. When the ratio of limestone to coal increases,the temperature in the calciner decreases gradually. With the increase of the amount of limestone powder,the CaO concentration produced by thermal decomposition of limestone in the calciner increases,and the NO reduction reaction is more intense,and the NO concentration decreases gradually. The increase of limestone powder and the decrease of calciner temperature lead to the increase of CO concentration in the tail.