Co-combustion of biomass fuels with coal in a fluidised bed combustor
Co-combustion of biomass with coal has been investigated in a 0.15 m diameter and 2.3 m high fluidised bed combustor under various fluidisation and operating conditions. Biomass materials investigated were chicken waste, rice husk, palm kernel shells and fibres, refuse derived fuel and wood wastes. These were selected because they are produced in large quantities particularly in the Far East. The carbon combustion efficiency was profoundly influenced by the operating and fluidising parameters in the decreased following order: fuel properties (particle size and density), coal mass fraction, fluidising velocity, excess air and bed temperature. The smaller particle size and lower particle density of the fuels (i.e. coal/chicken waste, coal/rice husk and coal/wood powder), the higher carbon combustion efficiency obtained in the range of 86-90%, 83-88%, 87-92%, respectively. The carbon combustion efficiency increases in the range of 3% to 20% as the coal fraction increased from 0% to 70%, under various fluidisation and operating conditions. Also, the carbon combustion efficiency increases with increasing excess air from 30- 50% in the range of 5 - 12 % at 50% coal mass fraction in the biomass mixture. However, further increased of excess air to 70% will reduced the carbon combustion efficiency. Relatively, increasing fluidising velocity contributed to a greater particle elutriation rate than the carbon to CO conversion rate and hence increased the unburned carbon. Furthermore, the bed temperature had insignificant influence of carbon combustion efficiency among the biomass fuels. Depending upon excess air ranges, fluctuations of CO emissions between 200 - 1500 ppm were observed when coal added to almost all biomass mixtures. In ash analyses, the percentages of unburned carbon were found to have increased in the range 3 to 30% of the ash content with the increases of coal fraction in the coal! biomass mixture. Furthermore, no fouling, ash deposition and bed agglomeration was observed during the combustion runs for all tests due to lower operating bed temperature applied. Lastly, a simple model was developed to predict the amount of combustion in the freeboard. This study demonstrated the capability of co-firing biomass with coal and also demonstrated the capability to be burnt efficiently in existing coal-fired boilers with minimum modification.