The OXYCOAL-AC research project aims at the development of the main components for an integratedpower plant process based on burning pulverized fuel in a mixture of recirculated flue gas (RFG) andoxygen. The oxygen is produced by a ceramic membrane-based air separation. The paper presents thecurrent status of the project, focusing on the main subprojects namely stability of oxy-flames, design ofoxycoal swirl burners, cleaning of the recycled hot flue gas and heat transfer in a utility scale oxy-firedfurnace.The development, experimental tests and scale up of a pilot scale oxy-fired swirl burner, able to operate inoxy-firing as well as in air firing atmospheres are shown. Measures for oxy-flame stabilization as a functionof flue gas recycling ratio (O_2 content) are derived and the design of a swirl burner able to stable operationin both air and oxy-firing has been developed. With this status reached, OXYCOAL-AC is the first pilottest-plant where coal can be burnt in a stable flame in a CO_2 atmosphere with an oxygen content betweenas low as 18 % up to above 30 % for both dry and wet recycle.The hot flue gas leaving the boiler in the OXYCOAL-AC process (containing fly ash, sulphur dioxide andalkali metal compounds) must be cleaned before reaching the membrane module. In order to reduce theefficiency losses, the RFG is cleaned at temperature levels defined by the membrane operation. For thispurpose, a hot gas cleaning vessel is connected through insulated hot gas piping with the furnaceexhaust. Pilot tests were conducted using different filter candles. First experiments have shown that attemperatures above approximately 500°C, the softening of the fly ash impedes back pulse filter cleaningthus causing major problems. As the ash’s alkali content has a high impact on the softening temperatureof the ash, an addition of alkali getter materials (alumina-silicate) to the coal led to a reduction ofmeasured Na~+ and K~+ concentrations in the gas phase. Thus the adhesiveness and the stickiness of thefly ash at high temperatures (around 800°C) can be reduced. Based on this it is expected that animprovement in the hot gas de-dusting process can be achieved, however, further experiments provingthis are needed.Further, numerical simulations of utility scale (1200 MW_(th)) boiler oxy-firing bituminous coal, based on nongreyimplementation of Exponential Wide Band Model, were performed with respect to retrofit.Comparisons are made between air firing and oxy- firing under boundary conditions varying with respectto oxygen and water content of the oxidizer. Predictions have shown that a significant increase of theoxygen concentration in the O_2/RFG mixture (approx. 27% for wet recycle and 30% for dry recycle) isnecessary for compensation of the higher molar heat capacity of CO_2 and thus for obtaining the similarflame temperatures as those in air-firing. However, due to the changed optical density of the flue gas, thiswill lead to 16 % increase of the incident radiation to the furnace walls in case of wet recycle and 5% fordry recycle. The results have shown that similar heat transfer in the combustion chamber to those in airfiringcan be achieved with an oxygen enrichment of recycled flue gas to around 24 vol.-% for wet and toaround 29 vol.-% for dry recycle respectively.
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