Numerical Investigation Of Bed Combustion Process In Biomass Grate-fired Boilers

Bio mass receives increasing attention all around the world due to its advantage of reproducibility, cleanness and widespread distribution. In China, coal-fired travelling grate boilers have disadvantage of severe pollution, but these problems can be mitigated by using biomass as fuel or introducing new biomass grate-fired boilers. Though bed combustion is the most complex process in biomass grate-fired boilers, numerical investigation on it is relatively few. As a result, this paper takes biomass bed as research object, and uses numerical methods to research on the combustion process.Firstly, this paper develops a one-dimensional unsteady combustion model for the complex combustion process in bed. The model is based on the thermal non-equilibrium porous medium method and considers the decrease of bed height and the influence of non-isothermal particles. What’s more, the model contains the physical and chemical reactions in combustion process and is numerically solved by Fluent combined with UDF programming. The predictions of residual mass percentage and volume fraction of O2, CO and CO2 above the bed are compared with experimental data, and show good agreement. The results also conform to basic characteristics of grate firing, and can verify the model.Secondly, this paper discusses the impacts of fuel parameters such as particle size, bulk density and moisture content in bed combustion process. The results show that the decrease of particle size can obviously enhance the average burning rate of bed, reduce temperature difference between gas-solid two-phase, and help increase volatile release rate and char burning rate in the earlier stage of combustion quickly, and shorten ignition time on bed surface. When bulk density increases, the average burning rate firstly increases and then decreases, volatile release rate reduces and char burning rate raises in the earlier stage, and the ignition time on bed surface is delayed. The increase of moisture content significantly decreases the average burning rate, volatile release rate and char burning rate, and delays ignition time on bed surface.Finally, the variation law of correlated parameters on bed combustion is studied under different primary air and bed heights in this paper. The results indicate that primary air flow has great influence on bed combustion. With the increase of primary air flow, volatile release rate decreases, char burning rate firstly increases and then decreases rapidly, residual mass of bed decreases and then increases, the maximum temperature of bed increases quickly and then changes smoothly. The temperature of primary air has little impact on bed combustion. In the middle stage, volatile release rate raises, but char burning rate and gas temperature above the bed decrease with increasing the temperature of primary air. Moreover, higher temperature of primary air can reduce residual mass of bed, decrease and then increase maximum bed temperature. The bed height has little influence on volatile release.rate, char burning rate and gas temperature above the bed in the early and middle stage. The increase of bed height can increase residual mass of bed and raise maximum bed temperature.