Numerical Investigation on the Improvement of Carbon Conversion in a Dual Circulating Fluidized Bed Reactor for Chemical Looping Combustion of Coal

A dual circulating fluidized bed reactor for chemical looping combustion of coal has the advantage of flexible regulation on oxygen carrier circulation, but there is always part of unconverted coal char escaping from the fuel reactor (FR), reducing the carbon capture efficiency of the whole unit. In this work, a numerical investigation is conducted using the computational particle fluid dynamics (CPFD) method to examine how to improve the carbon conversion in a 50 kW(th) coal-fueled dual circulating fluidized bed reactor for chemical looping combustion. The improvement strategies generally fall into two categories. The first category is based on the ideas of extending the residence time of char and strengthening the mixing between the char and oxygen carrier (OC), including enhancing the FR height, increasing the number of coal feeding points, and leading combustible gas generated in the gasification carbon stripper (GCS) to the FR The second category is to physically separate char particles from the binary material stream in a carbon striper (CS) based on the difference of terminal velocity between the char and OC particles and then recycle the stripped char to the FR again. With respect to the 50 kWth reactor, in which a two-chamber GCS is coupled within the loop seal, the full-scale CPFD simulation reveals that, when the GCS is off, the carbon capture efficiency (solely contributed by the FR) is quite low (42.3%), even if some optimizations are adopted for the FR (two oppositely collocated coal feeding points, 56.4%; doubling the FR height, 58.7%). On the other side, when the GCS is on, the carbon capture efficiency reaches over 90% significantly. To further improve the carbon conversion in the reactor, a new non-mechanical valve integrated with a four-chamber CS/GCS is designed numerically in this paper, where three-dimensional CFPD simulation is conducted to optimize the key parameters, such as freeboard height, baffle height, flow rate of OC, and superficial velocity. The integrated and compact device can function as the loop seal and carbon stripper simultaneously, and it can be operated as not only the CS mode (char is physically separated from OC) but also the GCS mode (char is gasified, and then combustible products are recycled back to the FR; i.e., char is chemically separated from OC) by regulating the operational velocity and fluidization agent. The carbon capture efficiencies of GCS and CS modes are 97.8 and 98.9%, respectively. The CS mode is more economical in terms of the operational cost, because a large amount of steam is not required herein. Finally, by means of parallel comparisons on these strategies mentioned above in performance and cost, the optimal strategy to improve carbon conversion in the reactor is proposed successfully.