Numerical modeling of co-firing a light density biomass, oat (Avena sativa) hulls, and chunk coal in fluidized bed boiler

Light weight biomass has been considered as a potential biomass fuel to be used in industrial and power-producing furnaces and boilers. This paper presents a comprehensive numerical study of co-firing chunk coal and a kind of light weight biomass, oat hulls (Avena sativa), in a stoker boiler. In this comprehensive model, a one dimensional steady state coal bed model already verified through experimental measurements was employed to describe the combustion process on the bed and an oat hulls devolatilization sub model derived from experiments was added to predict the devolatilization process of oat hulls. Through the study of oat hulls co-firing with coal combustion it was found that light biomass, similar to pulverized coal, burns in suspension (following the air flow). The particle trajectory shows that the devolatilization of oat hulls occurs shortly after injection into the boiler. The flow pattern indicates the importance of air system tuning in optimizing the design. For larger oat hull particles, they tend to reach the back wall of the furnace which can result in slagging problems. Also, the peak temperature inside the furnace inversely varies with the co-firing percentage of oat hulls. This numerical study supports the thermal and economic potential of using unprocessed light weight biomass on the existing combustion facility.