Deposit Formation in the Convective Path of a Danish 80MWth CFB-Boiler Co-Firing Straw and Coal for Power Generation

Abstract

The Danish government has committed the national power companies to reduce the emissions from Danish power stations by 20% based on the level of 1988 before the year 2005. In order to meet this goal Elsam (the Jutland, Funen Energy Consortium) has proposed a biomass program, for combustion of neutral straw, according to which three boiler concepts will be investigated: combustion of 100% straw in a separate stoker fired boiler build as an addition to an already existing PC-boiler, co-combustion of coal and straw in a utility type boiler [Hansen et al., 1996; Andersen et al., 1997, Frandsen et al., 1997] and co-combustion of straw, wood chips and coal in a circulating fluidized bed boiler. During the combustion process, the inorganic constituents of the fuel, such as Fe, Si. Al, Mg, Ca, K, and Na, are transformed into ash. The ash may be roughly divided into two ash fractions; a residual ash (mainly containing transformed minerals with a particle size ) and sub-micron ash or aerosols (mainly including vaporized minerals with particle smaller than 1 μ m), [Benson et al., 1993]. In PC boilers approximately 1 wt% of a coal ash will vaporize [Flagan and Friedlander, 1978]. In CFB-boilers the combustion temperatures are some 300–600°C lower than in a PC-boiler. Therefore the evaporation of inorganic minerals, ie. the aerosol formation, may be reduced compared to PC-combustion. However, during straw combustion as much as 30–75 wt% of the inorganic constituents in the ash will vaporize [Livingston, 1991, Baxter, 1993; Olanders and Steenari, 1995] mainly due to high concentrations of simple salts such as KC1 in the straw. Combustion of bituminous and sub-bituminous coal in CFB-boilers with in-situ sulfur capture by limestone, generally pose no real problems with respect to superheater deposit formation. A typical bulk chemical analysis of an upstream superheater deposit