高速煤粉燃烧器内燃烧特性数值模拟及结构优化

高速煤粉燃烧器火焰喷射速度高达60~200 m/s,炉膛内火焰较长,对流换热比例提高,使得炉膛内温度分布均匀,没有传统低速煤粉燃烧器火焰短,炉膛内局部过热和结焦等缺点。笔者以14 MW高速煤粉燃烧器为研究对象,采用数值模拟的方法,研究旋流强度、二次风温度等关键参数对燃烧器内煤粉燃烧的影响,针对燃烧器内煤粉燃烧特点进行结构优化设计。对旋流强度研究结果表明,当旋流强度S=2.2、2.8、3.2及3.7时,燃烧器内回流区形状变化不大,从一次风喷口开始到旋流叶片位置结束,回流区环绕一次风管;最大回流量在一次风喷口附近,距离一次风喷口越远,回流量越小;旋流强度对一次风喷口附近最大回流量影响不大,喷口附近最大回流量均在0.45 kg/s左右,当距喷口超过一定距离(L/H<0.35)时,旋流强度对回流量的影响开始变得明显,表现为旋流强度越大,回流区末端回流量越大,回流区末端回流量最大为0.30 kg/s,最小为0.17 kg/s。研究燃烧器喷口处燃烧状态表明,喷口处火焰旋流强度为0.10~0.28,与入口旋流强度正相关,火焰喷射速度150 m/s,为中等旋流强度的高速旋流火焰;喷口中心区可燃性组分富集,缺氧,燃料和氧气分层分布。当旋流强度提高,喷口中心区可燃性组分浓度降低,CO浓度从11%降低到10%,H2浓度从1.65%降低到1.40%,焦炭浓度从0. 14%降低到0. 11%,喷口边缘O2浓度从13%降低到10%。旋流强度S=3.2和S=3.7时可燃组分和氧气浓度分布变化较小,说明旋流强度提高对燃烧的影响减弱。考察0、100和200℃下二次风温度对燃烧的影响,结果表明,当二次风温度提高,煤粉在燃烧器内的反应时间有所降低,从0.15 s降低到0.11 s,但燃烧器内的煤粉碳转化率提高20%,达到65%。对燃烧器结构进行优化,加入中心风,对比中心风直流和旋流与不加中心风3种状态,结果表明,加入旋流中心风和直流中心风后喷口中心区半径r≤75 mm范围内可燃组分浓度降低,采用直流时由于气流刚性较强,喷口中心区氧气浓度升高,采用旋流中心风对中心区氧浓度影响弱,对可燃组分浓度降低效果优于直流中心风。

Numerical simulation of burning characteristics and structural optimization design of the high speed combustor of pulverized coal

The flame velocity is about 60-200 m/s at the nozzle of the high velocity coal combustor.The flame is long and the convective heat exchange ratio between the flame and membrane wall is increased in the furnace,which comes into being a uniform temperature field without local over-heating and slagging by the traditional low velocity pulverized coal burner.Taking 14 MW high-speed pulverized coal burner as the research object,the key factors liked swirl intensity and secondary air temperature on the combustion of pulverized coal in the burner were studied and the structural optimization design was carried out in this article according to the combustion characteristics of pulverized coal in the burner by way of numerical simulation.The study about swirl intensity shows that the shape of recirculation zone changes little when the swirl intensity increases from 2.2 to 3.7.The recirculation zone starts from the jet of primary duct to the swirl vane,and the recirculation zone surrounds the primary duct.The maximum return flow is near the primary air nozzle,and the farther away from the primary air nozzle is,the smaller the return flow is.The swirl intensity has little effect on the maximum recirculation flow ratio which is around0.45 kg/s and the effect of swirl strength on the return flow becomes larger when the distance from the jet is more than somewhere L/H<0.35.It appears that the recirculation flow ratio increases as the swirl intensity gets stronger and the ratio is from 0.17 kg/s to 0.30 kg/s at the end of the recirculation flow zone.The results shows that flame is 150 m/s at the nozzle of the combustor with moderate swirl intensity which is proportional to the swirl intensity of the secondary air at the inlet of the combustor.The swirl intensity at the nozzle is 0.10-0.28.The fuel and oxygen is hierarchical distribution which means the nozzle center is fuel-rich and oxygen-lean.The study shows that the concentration of combustible component at the middle of the nozzle decreases when the swirl intensity increases. Concentration of CO、H2 and coke is deducing from 11% to 10%,1.65% to 1.40% and 0.14% to 0.11%,respectively.The oxygen concentration at the nozzle edge is deducing from 13% to 10%.At the same time,the study shows that it has little difference on the oxygen and combustible gas distribution when the swirl intensity is increased from 3.2 to 3.7,indicating that the effect of swirl strength on combustion is weakened.In order to study the effect on combustion of secondary air temperature,the three different temperatures such as 0,100 and 200 ℃ were set and studied.The result shows that when the temperature of secondary air increases,the carbon conversion in the combustor is increased by 20% to 65%while the reaction time is deduced from 0.15 s to 0.11 s.Central air is arranged when it comes to structural optimal design of the combustor.The swirl condition of the central air and 3 kinds of conditions were studied.The results show that the combustible gas at the middle of the nozzle is dropped within a radius less than 75 mm when central air is added in the combustor.The oxygen concentration is increased when the central air is direct flow as the jet flow is rigid and can get to the middle of the nozzle.The swirl central air has little effect on the oxygen distribution at the nozzle but the effect of reducing the concentration of combustible components is better than that of the DC center wind.