Investigation of Solids Distribution and External Solids Flux in a Circulating Fluidized Bed Boiler

Licentiate thesis, 2006

This work concentrates on two issues regarding combustion in circulating fluidized bed (CFB) boilers. Firstly, the impact of co-combustion of coal with sewage sludge on ash flows, bed material and gas concentrations in a CFB boiler is studied. Secondly, a methodology to determine the solids recirculation rate in a CFB boiler is developed and evaluated. Tests have been performed in a 12 MWth CFB boiler. Co-combustion of bituminous coal with municipal sewage sludge is conveniently carried out in a CFB boiler. Nevertheless, introducing sewage sludge into the fuel mix leads to altered conditions in the boiler. The major differences compared to coal combustion observed in this study are increased fly ash flows, wider bed material size distributions and higher temperatures in the top of the furnace caused by increased volatiles combustion above the bottom bed. The solids recirculation rate is an important operating parameter in a CFB boiler, but difficult to determine, as no standard procedure is available to measure it directly. A method is developed to measure the instantaneous or time-averaged solids recirculation rate, based on heat and mass balances over the return loop. The conditions for its use are that the solids in the circulation loop pass through a well-fluidized device such as a fluidized seal pot with a heat exchanger or an external heat exchanger. Continuous measurement of temperatures and flow rates of gas and water, and of temperatures in gas-solids mixtures at different locations in the return system is required. The method has been experimentally evaluated and proven suitable for external solids flux measurement.