Impact of finite-rate kinetics on carbon conversion in a high-pressure, single-stage entrained flow gasifier with coal-CO_2 slurry feed

摘要

Coal-CO_2 slurry feed has been suggested as an attractive alternative to coal-water slurry feed for single-stage, entrained-flow gasifiers. Previous work demonstrated the system-level advantages of gasification-based plants equipped with CO_2 capture and CO_2 slurry feed, under the assumption that carbon conversion remains unchanged. However, gasification in carbon dioxide has been observed to be slower than that in steam. In view of this, the impact of CO_2 slurry feeding on gasification kinetics and ultimately on carbon conversion and oxygen consumption in a pressurized, single-stage entrained-flow gasifier processing bituminous coal is studied here using a 1-D reduced order model. Results show that the CO_2 gasification reaction plays a dominant role in char conversion when the feeding system is CO_2 slurry, increasing the CO content in the products by up to a factor of two. CO inhibition of the gasification reaction and a higher degree of internal mass transport limitations lead to an up to 60% slower gasification rate, when compared to a system based on coal-water slurry. Accordingly, a gasifier with CO_2 slurry feed has 15% less oxygen consumption but a 7%-point lower carbon conversion for a given reactor outlet temperature. The gasifier outlet temperature must be raised by 90 K in order to achieve the same conversion as in a water slurry-fed reactor; the peak reactor temperature increases by 220 K as a result. Net oxygen savings of 8% are estimated for a system with a CO_2 slurry-fed gasifier relative to one with water slurry and the same level of conversion.