The Role of Alkali Sulfates and Chlorides in Post Cyclone Deposits from Circulating Fluidized Bed Boilers Firing Biomass and Coal

Abstract

High amounts chlorine and sulphur in a fuel is generally connected with ash related operational problems in the boiler in which the fuel is fired. The problems occur as fireside deposits in different locations of the fluegas channel or as corrosion problems. Sulphur and chlorine together with alkali and earth alkali metals are known to strongly affect the thermal behaviour of the ash. First melting temperatures as low as 515 °C may be found if unsuitable amounts of alkali, sulphur and chlorine is present in the ash. A vast experience on the matter exists from coal firing [Sarofim and Helble 1994; Bryers 1992; Harb and Smith, 1990] as well as from firing different types of waste sludges [Backman et al., 1987] Backman et al., 1996; Salmenoja et al., 1996]. Forrest derived fuels such as wood, bark or forest residue (branches and tops) contain usually low amounts of sulphur and chlorine. The low potential for sulphur dioxide emissions from combustion of these kind of fuels as well as the indication of a fairly well behaving ash in most kind of combustion systems, are generally considered as two important advantages for the fuels. Sometimes these general indications are applied also on any type of biomass. This may, however, lead to serious errors since other biomasses such as straw or annually grown energy crops may contain significant amounts of both chlorine and sulphur [Nordin 1993]. In Fig. 1 the amount of chlorine and sulphur is shown for a number of different fuels, including coal, peat, wheat straw and forest derived fuel. Fluidized bed combustion is regarded as a very flexible combustion system with a capacity to burn a wide range of fuels. From the ash behaviour point of view the low combustion temperature of some 800–900 °C is favourable compared to conventional pulverised systems since operational problems due to a low melting ash are assumed to be avoided. The FBC technique has, however, limits. Common knowledge from conventional