Numerical Simulation of the Release of Alkali Metal during the Combustion of Biomass-fired Boiler

Abstract

In this paper, a CFD-based release model is presented to study the release process of alkali metal and predict the deposition in the furnace. In the release model, potassium chloride (KCl) is chosen as a representative alkali. The results showed that: the concentration of KCl, mainly releases in devolatilization and carbon combustion zone of the boiler grate, and the largest concentration appears in the carbon combustion zone. Meanwhile, the concentration of KCl is relatively high in superheater area. According to the simulation results, it is predicted that the right side of the three superheater regions prone to deposition and severe deposition appears on the lower turning-place of the superheater. The simulation results are in good agreement with the measured results in Heishan power plant. Introduction Biomass grate firing as one of the main technologies is currently widely used in biomass combustion for heat and power production. But grate-firing of some biomass fuels with a high alkali metal and Cl content (e.g., straw) may suffer from severe deposition and corrosion problems [1]. Therefore, to understand and master the process of biomass burning alkali metal release and transformation behavior is of big significance. In the combustion process, the alkali metal is easy to combine with inorganic elements generating a variety of complex compounds, so it is difficult to direct study the releasing and transportation of the alkali metal during the combustion [2]. Numerical simulation of biomass combustion represents today a very useful approach, and it is much faster and cheaper than an experimental campaign [3]. In this paper, the CFD software FLUENT is used to simulate the biomass combustion process and the release of alkali metal, KCl is chosen as a representative alkali metal[4], an alkali metal release model is established to study the release behavior of alkali metal during the combustion.