Research On The Key Technology Of Extended Heating Surface Of Large-Scale Circulating Fluidized Bed Boiler

Circulating Fluidized Bed (CFB) combustion is one of the best technologies for the production of clean energy from coal with the advantages of wide fuel flexibility, high efficient desulfurization, low NOx emission, broad turn-down ratio of boiler load, comprehensive ash utilization, etc. CFB boilers have been widely applied around the world, and one unit with supercritical parameters is in development. With the scaling-up of CFB boilers, there are a series of problems affecting the development and application of large-scale CFB boilers. With the scaling-up of CFB boilers, it is necessary to install additional heating surface to insure adequate heat absorption, such as external heating exchanger, platen heating surface. The operation and heat transfer characteristics of external heating exchanger and platen heating surface and thermal matching characteristics have not been well mastered, leading to some problems in the CFB boilers that have been put into operation.Therefore, this article mainly focuses on the extended heating surface of300MW CFB boilers. Due to technical protection and difficult measurement, at present there are few reports about related tests on300MW CFB boilers. In addition, there is no public calculation method which can be applied to engineering porjects. Some research work about the external heat exchanger with cooling shell is carried out on the MW multi-fuel combustion test-bed in the laboratory. The main contents and results are as follows:(1) According to the test data from three units of300MW CFB boilers in the field site, a heat transfer model of platen heating surface from flue gas to tube wall is established, including a radiation heat transfer model and a semi-empirical formula of convection heat transfer, which fully takes the effects of structure and dimension of platen heating surface, solids suspension concentration,furnace temperature, steam temperature, emissivity of tube wall and fluidization velocity on the heat transfer coefficients into consideration. The results indicate that fluidization velocity, furnace temperature and emissivity of tube wall are the leading factors affecting heat transfer. This heat transfer model can reasonably reflect the effects of these factors on the heat transfer of the platen heating surface of CFB boilers.(2) At different boiler loads and the different arrangement types, the thermal-hydraulic characteristics of platen heating surface of a300MWe CFB boiler are investigated. Combining heat transfer model and the results of thermal-hydraulic calculation, the tubes wall temperature characteristics of platen heating surface can be obtained. Meanwhile, the wall temperature of deviation tube of platen heating surface is calculated and analyzed by finite element method. The results show that Z type arrangement header of medium platen superheater I is better than U type arrangement in300MW CFB boilers.(3) Through the tests conducted on300MW CFB boilers, the operation and heat transfer characteristics of external heating exchanger are researched, and the heat transfer coefficients of the heat external exchange at different loads are obtained. And heat transfer model is established which is fully verified by test data. The new arrangement of external heat exchanger with cooling shell is researched, including the operation and heat transfer characteristics.(4) Thermal matching at the flue gas side and steam side in large-scale CFB boilers is studied. The effects of different heating surface arrangement and the changes of steam parameters on the heat distribution of heating surface are investigated.