Steady State Fixed Bed Combustion of Wood Pellets, A Simple Theoretical Model

Abstract

The combustion of biomass comes from a sequence of several distinct phases which begin when its ignition occurs, followed by drying, pyrolysis, gasification and combustion of carbonaceous residue. For small solid fuel boilers, as is the case of pellet boilers for domestic heating, biomass particles are continuously burned in fixed bed boilers or heaters. In these appliances the furnace is a kind of metallic basket into which fuel particles are thrown and burn in a fixed bed. The combustion air is introduced through orifices drilled in this metallic basket although some lateral air entrances are also found. The gaseous flow is a piston flow, the particles fall under gravity on the upper bed surface and as they burn they slowly move downwards until their size is small enough to fall down through the air entrance orifices or are dragged by the upcoming combustion gas flow. So the gaseous current has an up flow movement while the solid particles move downwards. It is assumed that the bed is at uniform temperature and all the average properties of the gaseous flow are constant with the exception of the oxygen concentration that reduces as the gas flow rises through the bed. This paper presents a simple mathematical model that allows that the steady state burning time of a solid particle, as well as the amount of energy released, can be calculated and, later on, compared to experimental data.