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a b s t r a c tCombustion in an O 2 /CO 2 mixture (oxyfuel) has been recognized as a promising technology for CO 2 cap ture as it produces a high CO 2 concentration flue gas. Furthermore, biofuels in general contribute to CO 2 reduction in comparison with fossil fuels as they are considered CO 2 neutral. Ash formation and deposi tion (surface fouling) behavior of coal/biomass blends under O 2 /CO 2 combustion conditions is still not extensively studied. Aim of this work is the comparative study of ash formation and deposition of selected coal/biomass blends under oxyfuel and air conditions in a lab scale pulverized coal combustor (drop tube). The fuels used were Russian and South African coals and their blends with Shea meal (cocoa). A horizontal deposition probe, equipped with thermocouples and heat transfer sensors for on line data acquisition, was placed at a fixed distance from the burner in order to simulate the ash deposition on heat transfer surfaces (e.g. water or steam tubes). Furthermore, a cascade impactor (staged filter) was used to obtain size distributed ash samples including the submicron range at the reactor exit. The deposition ratio and propensity measured for the various experimental conditions were higher in all oxyfuel cases. The SEM/EDS and ICP analyses of the deposit and cascade impactor ash samples indicate K interactions with the alumina silicates and to a smaller extend with Cl, which was all released in the gas phase, in both the oxyfuel and air combustion samples. Sulfur was depleted in both the air or oxyfuel ash deposits. S and K enrichment was detected in the fine ash stages, slightly increased under air combustion conditions. Chemical equilibrium calculations were carried out to facilitate the interpretation of the measured data; the results indicate that temperature dependence and fuels/blends ash composition are the major factors affecting gaseous compounds and ash composition rather than the combustion environment, which seems to affect the fine ash (submicron) ash composition, and the ash deposition mechanisms.

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a b s t r a c tThis paper presents a comparative study on ash deposition of two selected coals, Russian coal and lignite, under oxyfuel (O 2 /CO 2 ) and air combustion conditions. The comparison is based on experimental results and subsequent evaluation of the data and observed trends. Deposited as well as remaining filter ash (fine ash) samples were subjected to XRD and ICP analyses in order to study the chemical composition and mineral transformations undergone in the ash under the combustion conditions. The experimental results show higher deposition propensities under oxyfuel conditions; the possible reasons for this are investigated by analyzing the parameters affecting the ash deposition phenomena. Particle size seems to be larger for the Russian coal oxy fired ash, leading to increased impaction on the deposition surfaces. The chemical and mineralogical compositions do not seem to differ significantly between air and oxyfuel conditions.The differences in the physical properties of the flue gas between air combustion and oxyfuel combus tion, e.g. density, viscosity, molar heat capacity, lead to changes in the flow field (velocities, particle tra jectory and temperature) that together with the ash particle size shift seem to play a role in the observed ash deposition phenomena.