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Abstract 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.

Abstract Combustion tests were undertaken in a vertical pilot-scale furnace (1.2 MWt) at the IHI test facility in Aioi, Japan, to compare the performance of an air fired swirl burner retrofitted to oxy fired pf coal combustion with the oxy fired feed conditions established to match the furnace heat transfer for the air fired case. A turn down test at a reduced load was also conducted to study the impact on flame stability and furnace performance. Experimental results include gas temperature measurements using pyrometry to infer the ignition location of the flames, flue gas composition analysis, and residence time and carbon burnout. Theoretical computational fluid dynamics (CFD) modelling studies using the Fluent 6.2 code were made to infer mechanisms for flame ignition changes. Previous research has identified that differences in the gas compositions of air and oxy systems increase particle ignition times and reduce flame propagation velocity in laminar systems. The current study also suggests changes in jet aerodynamics, due to burner primary and secondary velocity differences (and hence the momentum flux ratio of the flows) also influence flame shape and type. For the oxy fuel retrofit considered, the higher momentum flux of the primary stream of the oxy-fuel burner causes the predicted ignition to be delayed and occur further distant from the burner nozzle, with the difference being accentuated at low load. However, the study was limited to experimental flames being all Type-0 (low swirl with no internal recirculation), and therefore future work consider higher swirl flames (with internal recirculation) more common in industry.

Deposits formation on heat transfer surfaces is one of the main problems associated to biomass co-combustion. It reduces plant efficiency and availability and increases maintenance costs. It is obvious that an increasing amount of low-temperature melting components in fuel ash accelerates and aggravates this process. Research is done to evaluate the validity of thermal analysis methods to characterise fusion of biomass and waste ashes. Laboratory ashes from a set of biomass and waste fuels are leached in successive steps. The original and the leached ashes are analysed by Thermo-Mechanical Analysis (TMA). Traces obtained from TMA show to be promising ash fingerprints to classify deposition tendencies. Additionally Simultaneous Thermal Analysis (STA) is performed on selected samples. Furthermore, improved chemical equilibrium calculations are proposed to predict the proportion of melted species resulting from combustion of biomass fuels. The model takes into account the reactivity of the inorganic matter in the fuel as issued from ash leaching.