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.
The combustion of coal can result in trace elements, such as mercury, being released from power stations with potentially harmful effects for both human health and the environment. Research is ongoing to develop cost-effective and efficient control technologies for mercury removal from coal-fired power plants, the largest source of anthropogenic mercury emissions. A number of activated carbon sorbents have been demonstrated to be effective for mercury retention in coal combustion power plants. However, more economic alternatives need to be developed. Raw biomass gasification chars could serve as low-cost sorbents for capturing mercury since they are sub-products generated during a thermal conversion process. The aim of this study was to evaluate different biomass gasification chars as mercury sorbents in a simulated coal combustion flue gas. The results were compared with those obtained using a commercial activated carbon. Chars from a mixture of paper and plastic waste showed the highest retention capacity. It was found that not only a high carbon content and a well developed microporosity but also a high chlorine content and a high aluminium content improved the mercury retention capacity of biomass gasification chars. No relationship could be inferred between the surface oxygen functional groups and mercury retention in the char samples evaluated.
The overall objective of the work described in this paper was to determine the behavior of wood ash under entrained-flow gasification conditions. Experimental work in atmospheric and pressurized entrained-flow gasification simulators, combined with thermodynamic equilibrium calculations, has shown that wood ash is not prone to form a molten slag at typical operating conditions of (pressurized, dry-feed, oxygen-blown) entrained-flow gasifiers, in spite of the presence of a relatively high amount of low-melting alkaline elements. This appears mostly due to the formation of mainly high-temperature-melting compounds (e.g., CaO) and only a small fraction of Ca silicates, which are characterized by a lower melting temperature. Phosphor and silicon may contribute to creating a higher melt amount, whereas low-melting alkali metal compounds are mostly partitioned into the vapor phase. Experiments, as well as modeling work performed for three types of wood, have shown consistent results. Addition of a fluxing agent is a promising option to improve the slagging behavior of wood-based systems by reducing the melting point of the slag. Moreover, thermodynamic calculations have shown that slag recycle may represent a feasible option in order to obtain sufficient slag coverage of the refractory wall despite the low ash content of woody fuels (typically 1 order of magnitude lower than in coal). In the present work, the determination of slag viscosity, a parameter critical for continuous operation of a slagging gasifier, has been addressed as well. The results of modeling work, showing the inapplicability of predictive formulas developed in the past for coal slags to wood-based slags, underline that further work is required to allow for a quantitative assessment of the slag viscosity as a function of slag composition and temperature.
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.
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