Evaluation of the combustion behaviour and ash characteristics of biomass waste derived fuels, pine and coal in a drop tube furnace

Currently there is an increasing interest on a new generation of low cost biomass fuels derived from human activities like wood and forestry residues, crop residues and refuse derived fuels (RDF) produced from municipal or industrial solid waste. The main objective of this study is to evaluate the combustion behaviour and ash characteristics of a number of these renewable fuels, namely rice husk, straw, coffee husk and RDF derived from municipal waste. For comparisons purposes, the study also includes a bituminous coal and pine branches. The study was carried out in a drop tube furnace (DTF), where data was obtained for gas temperatures, particle burnout, and carbon, hydrogen and nitrogen release along the reactor for the six solid fuels. The analysis of the experimental data included the use of both a one dimensional (1D) model and the Fluent. The results reveal that biomass fuels yield particle burnout values comparable to those of coal due to a number of factors that have opposite effects, mainly their higher volatile matter, which increases the burnout, and their higher moisture content and larger particle size, which decreases it. According to the 1D model, the residence time of the particles in the DTF is strongly affected by the particle size, but the number of size fractions considered has little influence on the calculated burnout. Moreover, the use of the Sauter mean or mass median diameters is not appropriated to represent the biomass particle sizes in model calculations. Predicted burnout with the 1D model is similar to those predicted by the Fluent. The ash composition of the fuels revealed that RDF might have some corrosion and slagging potential, while the use of rice husk and straw could induce slagging through the formation of low melting temperature ashes.