Solids Distribution and Gas Concentrations in the Furnace of a CFB Boiler during Co-Combustion of Bituminous Coal with Municipal Sewage Sludge

Rapport, 2006

In this work, bituminous coal was co-fired with municipal sewage sludge in a 12 MWth circulating fluidized bed (CFB) research boiler. The fraction of sewage sludge(both wet and dry) in the feed was varied in the range 0-67 % (on dry mass). The impact of co-combustion on ash leaving the boiler, bed material in the furnace and concentrations of O2, CO2, CO and total hydrocarbons throughout the furnace was studied. In order to evaluate the influence of the wide size distribution of the coal on the results, the coal was pre-sieved into a fine and a coarse size fraction and separately burned. The higher ash content in the sewage sludge resulted in increasing fly ash flows and lower combustible content in the fly ash with increasing fraction of sludge in the fuel mix. The bottom ash flow and combustible content were more sensitive to the feed coal size distribution than to the sludge supply. Compared to coal combustion, co-combustion of coal with sewage sludge was found to lead to a higher concentration of fine solids in the furnace, caused by attrition of sludge ash particles. Furthermore, as the density of sewage sludge ash is lower than that of coal ash, coarser sludge ash particles than coal ash particles could be entrained from the bottom bed. These two phenomena contributed to wider solids size distributions throughout the furnace in the case of co-combustion with sewage sludge. The effect of the entering coal size distribution on the size distribution of the inert ashes was negligible compared to the impact of the sludge fraction in the feed. The size distributions of the combustible solids in the bed material, on the other hand, were dependent on variations in feed size distribution of the coal rather than on sludge supply. The introduction of sewage sludge in the fuel mix resulted in greater measured concentrations of CO and total hydrocarbons in the furnace, leading to an increased combustion of volatiles above the bottom bed and higher temperatures in the top of the furnace.