Cesium as a tracer for alkali processes in a circulating fluidized bed reactor
Artikel i vetenskaplig tidskrift, 2006
Addition of cesium salt has been used to study the removal rate, transport, and fate of alkali components during combustion of a mixture of wood chips and pellets in a 12-MW circulating fluidized bed (CFB) reactor. The alkali concentration in the flue gas was monitored with a recently developed aerosol mass spectrometer to provide on-line measurements of alkali-containing submicron particles. Cs2CO3 was fed with the fuel during a 5.5-h period, and the Cs concentration in the flue gas and in ash samples was followed for 70 h. Cesium addition resulted in a 1000-fold increase in Cs concentrations, followed by a slow decay of the concentrations during the following days. Cs was concluded to bind strongly to the fluidized bed material and to be removed with the bed ash or by slow release to the gas phase. The apparent rate coefficient for Cs removal from the bed was approximately 0.03 h(-1). After 70 h, 33 +/- 10% of the added cesium had been removed with the bottom ash, 7 +/- 5% with secondary cyclone ash, and 45 +/- 10% with the bag-house filter ash. About 15% of the Cs remained in the facility at this time, either bound to the fluidized bed material or as deposits on surfaces. Cesium compounds released from the bed participate in the formation of new particles that are efficiently collected by the bag-house filters. A minor Cs fraction also condenses on fly ash particles that are removed by the secondary cyclone. The study shows that the results of cesium addition can be followed in detail under typical operation conditions, and the potential of the tracer method for studies of transformations and fate of alkali compounds in commercial scale boilers is discussed.