An Analysis of the Mass-and-Energy Balance Relationship Between a Direct-fired System and Air Preheater

To solve the problems of an increase in cold-air share in a milling system and a rise in boiler exhaust gas temperature which are caused by a change in coal types for coal-fired boilers at a thermal power plant, the air quantity-temperature characteristic curves of the milling system and air preheater have been identified based on a thermodynamic calculation of the milling system and the three-chamber rotary air preheater. This has been realized with the positive-pressure direct-fired system and three chamber rotary air preheater of a 330 MW coal fired boiler unit at a North China large thermal power plant serving as an object of study. On the basis of the air mass balance and heat quantity balance of the milling system and air preheater, their operational points can be determined. The analysis and calculation results are in good agreement with the on-site operation conditions. Proceeding from the calculation results, an analysis and evaluation of the measures to reduce the exhaust gas temperature and cold air share has also been conducted. A reduction of flue gas temperature at the air preheater inlet can effectively lower the exhaust gas temperature but a reduction of the share of primary air sweeping across the air preheater rotor has little effect in reducing the exhaust gas temperature.