A particle based model for the combustion of municipal waste in grate firing systems

摘要

For the purpose of design and optimization of utility boilers numerical simulations have become a valuable tool to complement or even to replace experimental investigations. However, only strongly simplified numerical models exist for the simulation of the firing equipment for lumpy solids fuels like biomass or municipal waste. In a waste incineration plant the fuel is transported mechanically through the furnace by means of a grate. The current paper concentrates on a forward acting grate with movable bars. The waste layer on the grate is passed by cross flowing preheated primary combustion air. Radiative heat transfer from the furnace initiates the successive processes of drying, devolatilization and char combustion within the fuel bed. The task of the moving grate bars is on the one hand to transport the fuel across the grate but also to generate mixing of fuel particles bed. The objective of the work presented here is to demonstrate the application of a novel 2-dimensional particle-mechanics based, numerical tool, which allows the simulation of a moving and reacting fuel bed in grate firing systems tightly coupled with the 3-dimensional simulation of the furnace atop. Compared to existing continuum models used for the same purpose, the use of an extended Discrete Element Method can reflect the important effect of the grate bars on the movement and the mixing of the bed material. Municipal waste, the most demanding fuel in this context, consists of a broad range of objects with very complex, actually unknown shapes. In addition municipal waste shows overall sticky and cohesive behavior. A new model to approximate the cohesive behavior of the waste and a procedure to derive consistent thermochemical properties for representative waste particles are introduced.