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Fluidized bed gasification processes are generally considered a good choice for biomass and waste because of its fuel flexibility. Furthermore, it is a relatively low‐temperature highly efficient process operating at 700–900°C compared with, for example, coal‐based entrained flow processes that mostly operate at 1400–1600°C. Indirect fluidized bed gasification is becoming increasingly popular for some applications due to the possibility of producing a N2‐free gas without the need for an air separation unit, as well as complete conversion of the fuel. ECN has developed MILENA indirect gasification, in which gasification and combustion are physically separated, but both reactors are placed in the same vessel. This article aims to compare the performance of olivine as bed material in direct‐ and indirect (MILENA) gasification. With this purpose, oxidation/reduction cycles have been simulated in a direct gasifier to determine the effect of olivine preoxidation in terms of tar destruction and oxygen transport. Results show that olivine preoxidation mainly enhances the capacity of oxygen transport of the bed material, which adds up to the catalytic effect of iron in olivine towards tar destruction. Oxygen transport capacity of olivine has been quantified as ER = 0.25 at the maximum initial CO2 peak, and it has been estimated that 20–25% wt of iron in olivine is able to transfer oxygen during the first 10 min operation. On the other hand, it has been found that MILENA operating conditions are equivalent to the point of initial maximum peak of CO2 in the devolatilization stage. This means that oxygen transport capacity of olivine is kept at its maximum due to the continuous combustion/gasification cycles, and olivine is kept activated by cyclical migration of iron into the surface and subsequent fast reduction. The effect of oxygen transport on the overall heat balance of the MILENA gasifier is equivalent to oxidation of additional fuel in the combustor. © 2014 American Institute of Chemical Engineers Environ Prog, 33: 711–720, 2014

The use of domestic high-ash coal reserves contributes to the security of energy supply, and therefore high-ash coal is expected to remain as a key energy source in several countries (e.g., India, Turkey) for at least the next 30-40 years. However, the use of high-ash coals for energy production (currently performed mainly via combustion processes) poses a number of technical and economic challenges, e.g., low efficiency and environmental issues. Gasification is an attractive option, since it allows a more efficient, more environmentally friendly conversion of the coal. In particular, integrated gasification combined cycle (IGCC) offers high efficiency, reduced emissions and potential for the implementation of CO 2 capture. With the aim of optimising the design and operation of high-ash coal fluidised-bed gasification processes, this paper studies the effect of temperature and partial pressure on the conversion and reactivity of coke from an Indian high-ash coal under CO 2 and steam gasification conditions using thermogravimetric analysis. Moreover, additional steam gasification tests have been carried out in order to determine the conversion rate under realistic fluidised-bed gasification conditions (e.g., coal ash as bed material, coal particle size, heating rate, bed hydrodynamics, gasification atmosphere), thus taking into account the effect of mass and heat transfer phenomena. Results of isothermal TGA tests have shown that coke reactivity increases at higher temperatures and/or partial pressures of gasifying agent. The experimental data have been fitted to two conversion models (shrinking core and volumetric). The determination of kinetic parameters (reaction order b, pre-exponential factor A and activation energy E a) has been carried out at three conversion levels: X = 0.2, X = 0.5, and X = 0.8. In the case of CO 2 gasification, the reaction order b ranges between 0.2 and 0.8, although at temperatures of 850-900°C, the reaction order has a value around 0.6. In the case of steam gasification, the reaction order ranges between 0 and 1.1, and increases with reaction temperature. Fluidised-bed steam gasification tests have shown that approximately 23-27% of the carbon contained in the coal (~40-45% of the overall coal, considering also hydrogen and oxygen released in the gas) is quickly converted during the devolatilisation stage. Between 12 and 22% of the carbon contained in the remaining coke is converted to gas within the first 25 min of steam gasification. Finally, 55-80% of the carbon in the coke remaining after steam gasification is converted during the first 25 min of oxidation with air. Conversion of the high-ash coal is favoured at higher steam partial pressures and/or higher gasification temperatures. The conversion rate under fluidisedbed conditions is significantly lower than that obtained in TGA tests at similar temperature and steam partial pressure values. Differences in coal particle size, heating rate during devolatilisation, inhibition issues, and other fluid-dynamic effects influence the re...

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