Assessing slagging and fouling during biomass combustion: A thermodynamic approach allowing for alkali/ash reactions

The ash behaviour during combustion of a wheat straw sample was assessed using chemical fractionation and thermodynamic equilibrium calculations. Chemical fractionation has been used to distinguish the inorganics into reactive and non-reactive fractions. The reactive part of fuel comprises mainly of alkalis (K and Na), chlorine, sulphur and a part of alkaline earth metals (Ca, Mg), while the non-reactive fraction is dominated by silicon, aluminium and iron. The reactive fraction of inorganics is expected to reach equilibrium during combustion, while the non-reactive fraction simply passes through the combustion process unaffected.Literature on interaction of silica with sodium has indicated that, due to the high temperatures involved, a part of non-reactive fraction may react. The use of chemical fractionation alone does not take these reactions into account. In order to take these reactions into account a second model has been developed based on chemical fractionation and inclusion of a fraction of non-reactive fraction into the reactive fraction. This has been modelled to predict the interaction of reactive layer of larger ash particles with the combustion gases. Two models were developed, one based on only chemical fractionation and the second one based on chemical fractionation as well as secondary reactions involving ash (mainly silica).The results suggest that the amount of potassium sequestered in the condensed phase increased with the increase in the availability of non-reactive fraction (mainly silica) in the high-temperature section (1600-1300 °C) and hence less potassium is available for condensation in the fouling sections, in the case of K: Cl ratio greater than unity. It is observed that the ratio of chlorine to alkalis determines the importance of alkali/ash reactions at the high temperatures and thereby slagging and fouling propensities involving potassium.