6. Coal Staged Conversion Polygeneration Technology Combining with Pyrolysis and Combustion Processes

Coal is the main primary energy in the world. However, most coal is combusted to generate electric and heat, which does not make effective of valuable hydrogen-rich volatile matter. A coal staged conversion process,which coupled circulating fluidized bed (CFB) combustion and coal pyrolysis,is a more efficient and clean technology for coal conversion and the principle is: part of the high temperature circulating material from CFB combustor as solid heat carrier is sent to the pyrolyzer to provide heat for coal pyrolysis and the coal incurs pyrolysis, producing tar and gas. The pyrolysis char is returned to CFB for further combustion to generate electricity and provide heat. Thus, poly-generation of gas, tar, heat and power can be realized in this system. According to the bed type of the pyrolyzer, the coal staged conversion technology is mainly divided into three types: the technology based on fluidized bed pyrolysis, that based on moving bed pyrolysis and that based on downer bed pyrolysis, and a lot of research has been done on this. Take the technology by Zhejiang University for example, industrial application and system analysis are discussed in detail. 12 MW and 40 t/h coal staged conversion process is established and operated. The test results showed that this system can realize poly-generation of heat, electricity, gas and tar. When the pyrolyzer does not run, the circulating fluidized bed can still be normally operated. This shows that the coal staged conversion technology can be applied to industrial scale poly-generation plant. Thermodynamic and economic of poly-generation system coupling 2 × 300 MW circulating fluidized bed and 2 fluidized bed pyrolyzer is analyzed. It shows that the coal staged conversion process system has higher energy and exergy efficiency and more profitable than single traditional 2 × 300 MW CFB plant. This justified the coal staged conversion process in a large scale is an efficient and economic technology.