Industried-size Measurements And Numerical Simulations Of The Temperature Distribution Within Medium Speed Mills

The boiler is one of the most important equipment in electric power plant. Heat loss due to exhaust gas is the largest heat loss of boiler which about6～7%of all that greatly affects the boiler thermal efficiency. Reduce the exhaust gas temperature within a reasonable scope to decrease the waste heat loss is beneficial to improve not only the utilization efficiency of coal but also the economic benefit. Improving the medium speed mill outlet temperature to increase the heat exchange between gas and the primary air in air pre-heater is an effective method for reducing the exhaust gas temperature. The safety of the mixing process of gas-solid is the key factor to set the outlet temperature.The dynamic heat transfer characteristics of gas-solid and the temperature fields in the mill in HP863and HP1003medium speed mill were studied with numerical simulation and performance tests. The temperature fields in the mill were analyzed by numerical simulation first, and then the critical region of heat transfer as well as other related parameters like velocity of the air rings and the temperature of the coal were obtained. The practical temperature fields in the critical region were measured by measuring device which was designed accord to the actual structure of the mill on a running mill.The comprehensive results show that the intense heat transfer concentrated in a rather small region that about30mm after the mixing of hot air and pulverized coal in the mill. In this region the gas-solid mixed temperature decreased more than100℃in a very short time, but the temperature fields is rather uneven distribution. The mixed temperature reduced to mill outlet temperature within200mm after the mixing of hot air and pulverized coal and the heat transfer process almost reach the end. What’s more the temperature fields is well distribute now. After the corner of the mill the gas-solid mixed temperature keep steady in the outlet temperature. And the temperature of the primary air varied with different water content of the coal, but the temperature fields of the mill has no difference. At the same time, the temperature of the pulverized coal in the mill is still low enough to ensure the safety of the mill when improving the medium speed mill outlet temperature in suitable scope.