CONTROL AND FUEL FLEXIBILITY OF CIRCULATING FLUIDISED BED

ABSTRACT This paper examines how load control and fuel flexibility may be achieved in circulating fluidised bed combustors. The paper looks specifically at the Studsvik/Babcock & Wilcox unit which achieves control in a unique way using a combination of an uncooled particle storage zone and a controlled return of circulating solids to the combustor. The circulation rates are controlled using L-valves. Using this technique it is typically possible to achieve turndown ratios of 4:1 without the need to use increased excess air or flue gas recirculation at any point in the turndown range. It is however necessary to accept a drop in the furnace exit gas temperature at low load because circulation rates at low load do not permit operation of an isothermal combustor. As long as these processes are understood it is possible to maintain excellent superheat control in superheated steam boilers. Finally, the combustor is presented along with typical density profiles and heat balances which illustrate how the turndown and fuel flexibility are achieved in practice.