The Study Of Boiling Rate’s Effect On The Safety Of Mediumtemperature And Medium Pressure Biomass Boiler’s Economizer

Biomass combustion for power generation has been promoted in China in recent years. But the development of biomass boiler is restricted to the problem of over-temperature of economizer tubes.The reliability of boiler heating surface depends on temperature operating condition of tubes. When the metal tubes’temperature rises, its permanent intensity falls, and the tubes will burst if the temperature reach a certain value. Working in medium temperature and medium pressure boiler raises security problems which is dangerous for back-end heating surfaces. Therefore, research on temperature of tubes and security problems in boiling economizer is significant.Firstly, this paper analyses the heat transfer character of the economizer tube and constructs a temperature model, considering the asymmetrical of gas temperature and mass flow distribution. The model can calculate tube wall temperature. Furthermore the program has given basis in the inspection. Research methods focus on the thermal deviation of mass flow and heat transfer. The program discuss the effect of gas temperature、the maximum heat-flow density and working medium temperature. Results indicate that when the working medium is in liquid state or in phase changing progress, the tube temperature is close to working medium temperature. When the working medium is at superheat state, the tube is easy to overheat. In the normal running state, the thermal deviation decides the state of outlet working medium. Based on the several definitions of boiling rate, the actual boiling rate is calculated at variety of conditions. This paper summed up the relationship between the wall temperature and boiling rate, according to this relationship, the definition of first limit boiling rate is that the working medium state of the calculation point is just reach the saturated steam. When the tube temperature of the calculation point is just reach the limit metal temperature, this state is defined as second limit boiling rate. In the design progress of biomass boilers, economizer boiling rate should be controlled below the first limit boiling rate. Based on the wall temperature and the boiling rate, the wall temperature calculation program is compiled, which is to predict wall temperature.In order to verify the correctness of program results or not, a series of experiments for tube wall temperature is conducted in 35 tons medium pressure and medium temperature boiler. Through compare analyzing experimental data and compute data, the results show that the program results have small errors, and the program can well predicted the tube wall temperature. The program provides a useful reference for biomass boiler design.