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In biomass combustion, ash deposit formation is a common problem and has been studied by a number of researchers [Miles et al., 1995; Baxter and DeSollar, 1995; Nordin et al., 1995; Bryers, 1996; Miles et al., 1996; Moilanen et al., 1996]. There are also some observations about operational problems in fluidised-bed gasification processes, caused by ash. In pressurised steam-oxygen gasification of peat, ash deposits have been formed in the upper part of the gasifier and in the cyclones [Moilanen, 1993]. Furthermore, straw ash has been found to cause both bed sintering and deposit formation in air-blown gasification [Kurkela et al., 1996], These problems were difficult to overcome in strawalone gasification. In fact, the gasification temperature had to be reduced to below 800–850°C, which resulted in poor carbon conversion and high tar concentrations. On the other hand, co-gasification of coal and straw (up to 50wt% straw) could be carried out without any signs of ash problems in spite of high operation temperatures of the order of 950–980°C [Kurkela et al., 1996]. One variable of significance that was observed to prevent the detrimental behaviour of ash in the gasification process was carbon conversion and measures to achieve this [Kurkela et al., 1996; Skrifvars et al., 1995]. The completeness of fuel carbon conversion is dependent on the reactivity of residual char and the operating conditions. If the reactivity is high, ash is formed rapidly and, consequently, deposits are also formed rapidly. The gasification reactivity of the biomasses has been observed to vary within wide limits [Moilanen and Kurkela, 1995; Moilanen and Saviharju, 1997]. Ash and its composition can be regarded essential in this respect and they vary significantly in different biomasses [Wilen et al., 1996]. The ash components, mainly alkaline metal, contribute catalytically to the rate of gasification, which may increase or decrease as a function of conversion depending of the behaviour of catalytically active substances. However, it is rather unknown, in detail, how these substances react during the gasification of biomass chars. |
