Solids Flow Pattern in Circulating Fluidized-bed Boilers

Doktorsavhandling, 2005

The background of this dissertation is the knowledge of circulating fluidized bed (CFB) technology, which has reached a level enabling units of several hundred megawatts of electricity to be constructed and employed. Yet an improved understanding of the technology is needed in order to further develop and optimize large-scale combustion in CFB boilers, especially since the operating conditions and the geometries of large boilers differ from those typical for laboratory units, in which most of the research hitherto has been performed. The suitability of combusting or co-combusting heterogeneous fuels, such as biomass and waste, in CFB boilers also justifies further research on the technology. Based on measurements in a 235 MWe boiler, this work compares boilers and laboratory units in terms of solids flow, solids distribution and solids wall-layer thickness. The work also studies the origins of pressure fluctuations in large boilers and investigates their influence on the combustion process. The gas composition and the solids flow pattern in a 30 MW boiler loop seal are examined as well. The flow pattern in the 235 MWe boiler was examined and a compilation of literature data show that there are considerable differences in the flow pattern between boilers and laboratory units. Literature correlations for prediction of the solids distribution and the thickness of the solids wall-layer, derived from best fit to data from laboratory units, therefore give poor accuracy when applied to boiler data. Methods to better estimate those parameters are derived in this work. In addition, the pressure fluctuations investigated here were found to originate mainly from two sources: the bubble flow through the dense bed and the irregularities in the fuel feed. Occurrences of high fuel flow give pressure peaks due to a high release of volatiles, causing concurrent dips in the air feed to the furnace. The coincidence of a high release of volatiles and a low air feed results in a reducing environment in virtually the entire furnace, which may hinder burnout and cause undesirable emission of hydrocarbons. Measurements in the 30 MW boiler loop seal show that gas concentration and solids flow depend on load, i.e. on the external solids flow, and on height above the bottom of the loop seal. It was also found that some flue gas enters the loop seal from the cyclone and that combustion occurs in the loop seal.