Numerical Simulation Of The Release And Deposition Of Alkali Metal During The Combustion Of Biomass-fired Boiler

As a renewable and environmentally friendly energy source, biomass and its utilization are gaining an increasingly important role worldwide. Grate-firing is one of the main competing technologies in biomass combustion for heat and power production, because it can fire a wide range of fuels of varying moisture content, and requires less fuel preparation and handling. Biomass-fired grate boiler technology has been widely used in China, however, due to the higher alkali metal content of biomass, there is a more serious deposition and slagging problem during biomass combustion process, which has become a living major obstacle to the use of biomass energy. Therefore, to understand and master the process of biomass burning alkali metal release and transformation behavior is of big significance.In this paper, the CFD software FLUENT is used to simulate the biomass combustion process, the release and deposition of alkali metal. Firstly, assuming that biomass on the grate undergoing drying, devolatilization, char combustion and char burnout in turn. The conversion of biomass in the bed is expressed by C++code, which is coupled to FLUENT by UDF, a3D full-scale combustion model is established to gain the temperature field, concentration field and combustible gas reaction rate. The simulation results are in good qualitative agreement with measurements at the plants, so the model is reasonable and reliable. Secondly, based on the combustion field and the experimental study of alkali metal, potassium chloride is chosen as an representative alkali, the release model is established and potassium chloride concentration field is gained. Finally, the transport, adhesion and deposit model are established to get the deposition on the heating surface. And the simulation results showed that:deposition occurs throughout the furnace, but the deposition of the third super-heater is more serious.