Pyrolysis And Combustion Of Low No_x Emission Characteristics Of The Chain Furnace And The Furnace Structure Parameters Of Numerical Simulation

In China, the existing problems of industrial boilers which are about more than 500 thousand units and mainly adopt the grate-fired manner are general low combustion efficiency and high emission of pollutant. Among them, an important source of atmospheric pollution caused by nitrogen oxides (NO_X) emission accounts for about 15%～20% of amounting to NO_X emission in China. Therefore, it is very important in view of the environment protection of China to develop new combustion technology of industrial boiler for low NO_X emission.Pyrolysis-combustion chain grate boiler invented by the Institute of Process Engineering, Chinese Academy of Sciences, is a new-style stoker boiler. In comparison with traditional chain grate boiler, pyrolysis-combustion chain grate boiler combined with fuel reburning can be effectively suppressed the generation of NO_X. The pyrolysis-combustion chain grate boiler is composed of two sections including front section of pyrolysis and partial gasification, and back section of semicoke combustion. The flammable gas produced by coal pyrolysis and partial gasification in front section can be imported into the back furnace, which form fuel reburning which can reduce NO_X emission from semicoke combustion on the chain grate.In this paper, fluid dynamics software FLUENT was used to simulate NO_X emission characteristics of pyrolysis-combustion chain grate boiler in order to provide theoretical guidance of design. Meanwhile, the effects of parameters of furnace structure on NO_X emission were investigated. During the simulation process, back combustion furnace is selected as the calculating physical model. Methane is adopted as reburning fuel, while the combustion of air-ethylene mixture added with N element which formed NO in combustion process is employed to simulate the combustion of semicoke on the grate. On the other hand, the mathematical models about furnace combustion , NO formation and reduction, are established. Numerical simulation results show that NO concentration of furnace outlet is 178 ppm in the traditional combustion, whereas on the same combustion conditions, NO emission of furnace outlet can be decreased to 152 ppm with reduction efficiency being 14.6%. Forming a partial reduction zone in the furnace by pyrolysis gas reburnig is the major contribution for the reduction of NO_X. This shows that pyrolysis-combustion technology has a certain effect on the NO_X reduction compared with the traditional combustion. For the pyrolysis-combustion chain grate boiler, the calculation results demonstrate that improving NO reduction efficiency by decreasing excess air ratio, increasing reburn fuel fraction and decreasing air volume in the air plenum underneath forepart of the traveling grate. Varying the configuration of boiler furnace clarified that NO reduction efficiency declined with increasing the front arch length, decreasing the front arch angle and extending the width of the fore-and-aft arch. Nonetheless, changing the back arch angle has little effect on the NO reduction efficiency.