Biomass Combustion and Its Utilization to the Distributed Power Generation

A wood chip combustion behavior in a turntable type moving bed combustor is numerically analyzed in order to understand the fundamental combustion behavior in the combustor. An experiment is also carried out to compare the experimental data with numerical results and estimate the performance of the numerical analysis. Wood is used as building materials in most countries, and wooden house is the most popular in Japan. In some countries such as Japan, the period of durability of wooden house is for several decades because of high humidity and warm weather. A great amount of wooden waste is disposed and buried every year. From a viewpoint of effective use of energy, these kind of wooden wastes should be converted to valuable source of energy by efficient combustion operation. It is however difficult to operate the wooden waste conversion system because this waste includes other material such as plastics, plaster board, stone and soil. These kinds of intermingled material cannot be separated easily from wooden waste. In this study, we develop the turntable type moving bed combustor which effectively discharges the intermingled material from the combustor. We also develop the numerical model for the analysis of the combustor. The turbulent gas flow in the combustor is described by k-ε two-equation model and a momentum exchange between gas and moving bed of wood chip is considered. A solid phase is assumed to be a Newtonian fluid. Gas and solid phase temperature are calculated with considering convective and radiative heat transfers. Devolatilization reaction of wood is calculated by a first order chemical reaction model. Chemical reactions of gas and solid surface are also calculated with considering both chemical and gas film diffusion rates. Governing equations above-mentioned are simultaneously solved by control volume method. The geometry of the combustor is 470 mm in diameter and 1,500 mm in height. The combustion air is introduced tangentially from side wall. Wood chip is fed by screw feeder from side wall. Wood chip feed rate is 50 kg/hr, initial temperature of wood chip is 293 K and air ratio is 1.2. Numerical results are fairly in good agreement with experimental data. High temperature and low oxygen gas which contains unburned CO near centerline region of the combustor is observed in both experimental and numerical results. The mixing promotion of this fuel containing gas and oxygen rich gas near sidewall region is a problem of the development of efficient energy conversion system. This combustor would be one of the heat sources for a steam-driven electric power plant utilizing wooden waste as the source of fuel in local area.