OXYCOAL-AC: Towards the Realisation of a Coal-Fired Power Plant Process with Membrane-based Oxygen Supply - a Survey of Accumulated Experience

摘要

The OXYCOAL-AC research project aims at the development of the main components for an integrated power plant process based on burning pulverized fuel in a mixture of recirculated flue gas (RFG) and oxygen. The oxygen is produced by a ceramic membrane-based air separation. The paper presents the current status of the project, focusing on the main subprojects namely stability of oxy-flames, design of oxycoal swirl burners, cleaning of the recycled hot flue gas and heat transfer in a utility scale oxy-fired furnace. The development, experimental tests and scale up of a pilot scale oxy-fired swirl burner, able to operate in oxy-firing as well as in air firing atmospheres are shown. Measures for oxy-flame stabilization as a function of flue gas recycling ratio (O_2 content) are derived and the design of a swirl burner able to stable operation in both air and oxy-firing has been developed. With this status reached, OXYCOAL-AC is the first pilot test-plant where coal can be burnt in a stable flame in a CO_2 atmosphere with an oxygen content between as 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 and alkali metal compounds) must be cleaned before reaching the membrane module. In order to reduce the efficiency losses, the RFG is cleaned at temperature levels defined by the membrane operation. For this purpose, a hot gas cleaning vessel is connected through insulated hot gas piping with the furnace exhaust. Pilot tests were conducted using different filter candles. First experiments have shown that at temperatures above approximately 500°C, the softening of the fly ash impedes back pulse filter cleaning thus causing major problems. As the ash’s alkali content has a high impact on the softening temperature of the ash, an addition of alkali getter materials (alumina-silicate) to the coal led to a reduction of measured Na~+ and K~+ concentrations in the gas phase. Thus the adhesiveness and the stickiness of the fly ash at high temperatures (around 800°C) can be reduced. Based on this it is expected that an improvement in the hot gas de-dusting process can be achieved, however, further experiments proving this are needed. Further, numerical simulations of utility scale (1200 MW_(th)) boiler oxy-firing bituminous coal, based on nongrey implementation 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 respect to oxygen and water content of the oxidizer. Predictions have shown that a significant increase of the oxygen concentration in the O_2/RFG mixture (approx. 27% for wet recycle and 30% for dry recycle) is necessary for compensation of the higher molar heat capacity of CO_2 and thus for obtaining the similar flame temperatures as those in air-firing. However, due to the changed optical density of the flue gas, this will lead to 16 % increase of the incident radiation to the furnace walls in case of wet recycle and 5% for dry recycle. The results have shown that similar heat transfer in the combustion chamber to those in airfiring can be achieved with an oxygen enrichment of recycled flue gas to around 24 vol.-% for wet and to around 29 vol.-% for dry recycle respectively.