Study On Avoiding Low Temperature Corrosion In Rotary Air Preheaters With High Temperature Flue Gas

Low temperature corrosion in rotary air preheaters of large scale fossil-fuel-fired powerplants is detrimental to the safety of operating power plants, and moreover, it makes thedecrease of flue gas temperature of boilers difficult. To prevent low temperature flowpassages in operating steam boilers from corrosion, steam-air heater and hot-air recyclesystem are usually put into operation. Thus, the flue gas temperature is increased, resultingin lower power plant efficiency.To foresee the low temperature corrosion accurately, the temperature distribution of a rotaryair preheater in a300MW unit was theoretically computed first. After that protection to lowtemperature heat transfer surface with highly individualized method was carried out,bringing in less decrease to the boiler efficiency. In the light of structural features of therotary air preheater, the heat transfer process was simplified to some extent and physical andmathematical models were built accordingly. The models were solved numerically with thefinite difference method, and then the temperature distribution was achieved by means ofiterative computation. The numerical calculation results agreed well with the variation lawobtained via analysis of the temperature distribution in the rotary air preheater. Thecalculated temperature distribution of flue gas, combustion air and rotating matrix underdifferent work conditions, collectively reflected the influence of varying heat transferintensity in rotating direction. The general calculation result was in accordance with theparameters got from thermal calculation. By numerical calculation, the existance of an areathat is more sensitive to low temperature corrosion than other parts of the matrix wasconfirmed, indicating it is more exact to predict low temperature corrosion by the matrixtemperature of the more sensitive area than by the cold-end average temperature.With the numerical calculation above and the mechanisms and occurrence of lowtemperature corrosion taken into account, the temperature distribution of a chosen rotary airpreheater under different work conditions was calculated. During the calculation, thesteam-air heater, hot-air recycle system and a plan in which a part of high temperature fluegas short-circuits some of the heat transfer surface were applied to the investigated rotary airpreheater, separately. Then the performance of different plans were compared to ensure theequipment safety and improve the unit efficiency. The calculation results indicate that the plan in which high temperature flue gas short-circuits is less economical on restrainingcorrosion of low temperature surface than the steam-air heater and hot-air recycle system.But the plan can be used to adjust the steam temperature at the flue gas side when the powerplant is subjected to a large heat loss of attempering water.