Chemiluminescence measurements in a pulverized fuel furnace of pure torrefied biomasses in comparison to solid fossil fuels

摘要

Torrefaction is a thermo-chemical process also known as "low-temperatur pyrolysis" modifying fuel properties of biomass to enhance its combustion in a pulverized fuel burner. Due to changes in water content, elemental composition and cell structure, improved grindabilty and an increased lower heating value (LHV) are achieved. In this study chemiluminescence of OH~*- and CH~*-radicals was measured in flames of torrefied beech wood and a torrefied mixture of different kinds of wood. The torrefaction of beech wood was carried out in a bench scale screw conveyor reactor at a temperature of 280 °C and a residence time of 30 min. Both, i) decrease in mass to calculate the degree of torrefaction and ii) chemical composition of the solid residue were measured afterwards. Torrefied biomass samples were then grinded and particle sizes have been evaluated. The wood mixture has been produced in an industrial scale rotary drum reactor at 295 °C and a residence time of 45 min resulting in a stronger torrefied fuel. Thus, differences in flame structures could be investigated for different torrefaction conditions. In order to investigate differences in the combustion of biomass and fossil coal, chemiluminescence measurements were also conducted using lignite and hard coal as fuel. Again, particle sizes and chemical composition were measured. The combustion experiments were carried out in a small-scale pulverized coal burner with a thermal power of up to 100 kW. Because the burner is vertically adjustable, measurements can be taken at different positions in the pulverized coal flame. The images taken at each position were averaged over time in order to minimize the influence of fluctuations in the flame. The observed flames show certain differences to each other. In general: relative to fossil coal flames, biomass flames tend to be shorter and show different flame shapes as well.