Reactor Design of CanmetENERGY's Pilot-Scale Pressurized Chemical Looping Conversion

Canada is taking a leading role in conducting innovative research in carbon capture, utilization, and storage (CCUS) using a cost effective and efficient technique. Chemical looping combustion (CLC) is a newly emerging CCUS technology for fossil fuel conversion with the inherent advantage of economic carbon dioxide separation. In CLC, the oxygen for combustion is provided by a metal oxide-based oxygen carrier, which cycles between an air reactor and a fuel reactor. The oxygen carrier supplies the oxygen for combustion in the fuel reactor and then cycles into the air reactor where it is re-oxidized. The two reactors ensure that the reduction and oxidation reactions remain separate. Therefore, CLC fuel conversion is performed in the absence of air thereby producing a stream of flue gas concentrated in CO₂. CanmetENERGY aims to design and build a 600 kW_th pressurized-CLC (PCLC) pilot-plant to combust natural gas using ilmenite ore as the oxygen carrier. The pilot-plant includes feedstock supply systems (natural gas, air, oxygen carrier, loop seal gas, and water), two fluidized bed reactors (fuel reactor and air reactor), effluent processing, heat exchangers, and utility systems. The core of the pilot-plant is a dual fluidized bed system.

The reactors operate as gas-solid fluidized beds. To promote gas-solid mixing, and therefore increase the rates of reaction, these reactor sections should operate in the turbulent fluidized bed regime. CanmetENERGY has considered a configuration using a riser to transfer the oxygen carrier from the air reactor to the fuel reactor. This paper presents an analysis of hydrodynamics and heat transfer related to this configuration.

The air reactor riser must operate in the fast-fluidized regime to ensure the particles are carried over the top of the bed. The fluidization regime of the beds depends on the properties of the gas, the properties of the solid particles, and the inner diameter of the bed (i.e., the superficial gas velocity). The diameter was optimized to maintain the desired fluidization regime under the widest range of thermal input and pressure. Based on the results of the study, it was determined that the inner diameters of the air reactor bottom, air reactor riser, and fuel reactor should be between 203-305 mm, 51-76 mm and 102-127 mm, respectively.

The oxidation reactions are highly exothermic, whereas the reactions in the fuel reactor are net endothermic. Sensible heat from the hot solid particles entering the fuel reactor are used to drive the endothermic reactions. Therefore, it is necessary to minimize heat loss from each reactor in order to maintain the high temperature of the solids; this is particularly important for small pilot-plants such as the one under design here. A separate case study was completed to determine the optimal outer diameter of each reactor pressure vessel section based on the thickness of insulation required to limit heat loss to acceptable levels. From this study, it was determined that outer diameters of 508 mm, 203 mm, and 406 mm for the air reactor bottom, the air reactor riser and the fuel reactor, respectively, are sufficient to provide enough resistance to heat transfer without compromising cost effectiveness. From the results of this study, the reactor heights can be determined based on the residence time required in each reactor section to meet target fuel and oxygen carrier conversions.