Modelling Combustion Pollutants from Coal-Sawdust Blends in a Drop Tube Furnace

This paper investigates the emissions released when Hwange coal and Pinus sawdust are combusted in a drop tube furnace using a cocombustion model. This study is motivated by the need to reduce emissions from coal fired industrial boilers whilst tackling waste from sawdust. The submodels that were utilized to develop the cocombustion model included the discrete phase model, eddy dissipation concept reaction model, single kinetic devolatilization model, multiple surface heterogenous char reaction model. The refined Jones and Lindstedt 4 step reaction mechanism was employed to model the homogenous volatile combustion mechanisms whilst the P1 model and weighted sum of grey gases was employed to model the radiation properties during combustion. Increasing the blending ratio of Pinus sawdust in coal from 0% to 30% by mass produced a decrease in NOx emissions from 257 to 187 ppm when the furnace temperature is 1673 K. Sox emissions reduced by 22% as the blending by sawdust increased to 20%. An important deduction that the cocombustion model was able to make apparent was how the Fuel NOx produced during the combustion process forms an integral part of the final NOx as compared to Thermal NOx when blending is being considered.