空气冷却式煤粉燃烧室数值模拟研究

双锥煤粉燃烧室在小容量工业锅炉中广泛采用水冷却方式,但随着市场对高容量锅炉需求的增加,双锥燃烧室体积增大、数量增多,如仍采用水冷却的方式将导致安装困难、水系统复杂等问题,亟需开发新的冷却方式。空气冷却形式具有结构简单、预热后的空气可以增加煤粉的着火稳定性等优点,需要考察其首次应用于双锥煤粉燃烧室中的效果。为了确定空气冷却式燃烧室燃烧和壁面冷却情况,采用数值模拟技术对14 MW工业锅炉燃烧室和炉膛进行三维建模,得到50%和100%两种负荷下不同内外二次风配风比例下燃烧室内部燃烧情况、金属壁面温度、出口火焰形状和炉膛充满度。结果表明:控制总空气过量系数不变,随着内二次风比例的逐渐增加,燃烧室内的平均温度逐渐降低;50%负荷下金属壁面温度随二次风比例的增加逐渐降低,100%负荷下金属壁面温度先降低后升高,这是内二次风助燃燃烧和外二次风的冷却共同作用的结果。随着内二次风比例的增加,金属壁面的高温区域逐渐后移,集中于后锥出口区域;在50%负荷下内二次风量占总空气量比例为0.4时,金属壁面具有最高温度930 K,100%负荷下内二次风量占总空气量比例为0.2时,壁面金属最高温度835K,2个最高温度均出现在后锥收缩段,据最高温度推荐壁面材料选取0Cr18Ni9,2种负荷下最高温度出现时燃烧室内的内二次风配风量为2 600 Nm3/h,应尽量使内二次风远离此配风量;50%负荷下燃烧室平均温度、金属壁面平均温度及最高温度均高于100%负荷,是空气冷却结构需要重点考察的工况。随着内二次风比例的逐渐增加,火焰长度先增加后减小,当内二次风过小时,出口气速较小,外二次风具有向中心的速度分量,火焰主要集中在炉膛前部。随着内二次风比例的增加,出口速度增大,火焰变长变细。但随着比例的继续增加,外二次风的轴向速度变小,出口火焰的旋流强度完全由二次风决定,出口旋流强度的增大导致了火焰的变短变粗,在2种负荷下,火焰长度较长时,内二次风比例为0.4~0.5。内外二次风比例为0.5∶0.5时,燃烧室内燃烧情况和壁面温度均匀稳定,火焰在炉膛内的充满度最好,是2个考察负荷下均较适合的运行参数。

Numerical simulation on pulverized coal combustion chamber with air cooling

Possessing the advantages of burning stability and low burn-out rate,the double-cone combustion chamber with independent space is suitable for industrial boilers which start-stop frequently.with the increase of market demand for high-capacity boilers,the combustion chamber volume and number increase,and the adoption of water cooling method will lead to problems such as difficult installation and complex water system.Therefore,it is urgent to develop new cooling methods.Air cooling technology has the advantages of simple structure and combustion stability. Whether this technology can be used in the combustion chamber with independent space needs to be explored.In order to determine the combustion and wall cooling of the air-cooled combustion chamber,the three-dimensional modeling of the combustion chamber and furnace of 14 MW industrial boiler was carried out by using numerical simulation technology. The internal combustion,metal wall temperature,exit flame shape and furnace fullness were obtained under the different ratio of internal and external secondary air distribution under 50% and 100% load.The results show that: at the conditions of constant excess air coefficient,the temperature of the inner region decreases gradually with the increasing of inner secondary air ratio,the average temperature of metal wall also shows a decreasing trendwith the increase of the ratio of internal secondary air at 50% load,while first decreased and then increased at100% load,which is the result of the combined effect of combustion supporting by internal secondary air and cooling by external secondary air;With the increase of the proportion of the internal secondary air,the high temperature area of the metal wall gradually moves backward and focuses on the exit area of the back cone;when the inner secondary air ratio is 0.4,the temperature of metal wall is maximum( 930 K)at 50% load,and when the inner secondary air ratio is 0.2,the temperature of metal wall is maximum( 835 K) at 100% load.The two temperatures all appear in the behind cone,at this time,the air volume of secondary air distribution is 2 600 Nm3/h,which should be avoided.According to the maximum temperature,the wall material is selected as 0 Cr18 Ni9;The average temperature of the combustion chamber,the average temperature and maximum temperature of the metal wall under 50% load are all higher than that under 100% load,which is the working condition that air cooling structure needs to focus on.As the inner secondary air ratio increases,the length of flame increases at first and then decreases.When the internal secondary air is too small,the outlet gas velocity is small,and the external secondary air has a velocity component toward the center,and the flame is mainly concentrated in the front of the furnace.With the increase of the ratio of internal secondary air,the exit velocity increases and the flame becomes longer and thinner.However,as the proportion continues to increase,the axial speed of the external secondary air becomes smaller,and the swirl strength of the exit flame is completely determined by the secondary air.The increase of the swirl strength of the exit leads to the flame becoming shorter and thicker.Under two loads,the flame is longer when the ratio of internal secondary air is 0.4-0.5.The ratio internal and external secondary air is 0.5 ∶ 0.5,the flame’s fullness in furnace is the best,the combustion situation and wall temperature in the combustion chamber are also uniform and stable,and the fullness of the flame in the furnace is the best,which is the mostsuitable working parameter for both loads.