Study on a two-stage combustion method of woody biomass

Abstract

The purpose of this study is to establish a two-stage combustion method, which has advantages of a wide range of combustion temperature and high controllability compared to the conventional methods, from the standpoint of promoting the use of woody biomass as solid fuel and the effective utilization of unused wood materials. In this method, we use a vertical grate-type combustor, into which chipped woody biomass is loaded as a batch. The wood chips are burned at a low air ratio of 0.3 to 0.5 from the top to the bottom of a fuel layer, which is opposite to the direction of supplied air flow. The two-stage combustion method can be achieved through the way described above. Under the new method, wood chip combustion can be divided into two stages, namely, pyrolytic combustion process (generation of carbides) and surface combustion process (combustion of carbides). In this paper, the combustion of wood chips with different moisture contents (11, 20, 35%) was investigated, and the effects of the amount of air and moisture contents on the burning characteristics were discussed. The result clarified the mechanisms of combustion under the new method. In addition, we confirmed that combustion temperature and burning rate could be controlled by simply changing the amount of air. The temperature could be changed in the range of about 800 to 1300 K in the pyrolytic combustion process and about 800 to 1500 K in the surface combustion process.