Influence of stoking on the combustion of beech wood particles of different shape in an agitated bed

Mass loss of beech wood particles during combustion was measured in a batch reactor which allows stoking of the fuel bed. The test rig allows air staging, where primary air is flowing through the fuel bed and secondary air is added in the freeboard above the bed. The bed is ignited by radiation from electrically heated walls. The shape of the biomass particles (spheres, cylinders and cubes) and the primary to secondary air mass flow ratio were varied. Influence of stoking has been assessed by determining a mixing index for the top particle layer. Results show the general influence of stoking on the mass loss rate of the bulk in different combustion regimes. Stoking delays volatile ignition above the bed but accelerates fuel bed mass loss during combustion. Increased radiative heat flux from the flame into the bed when the bed is stoked was identified as the main reason for accelerated mass loss. Particle shape influences bulk mixing which is reflected in the combustion behavior. In particular, ignition of the volatiles is delayed by increased bulk mixing.