| Description |
Chemical looping combustion (CLC) is a carbon capture technology capable of combusting solid fuels while intrinsically separating the greenhouse gas CO2. Compared to gasification or oxy-fuel combustion, CLC eliminates the energy-intensive step of air separation along with the associated cost, making it an attractive green energy production technology. In the variant of CLC known as chemical looping with oxygen uncoupling (CLOU), the oxygen carrier releases O2 in the fuel reactor, allowing for a gas-solid reaction to occur. Research to date has predominantly focused on the development and characterization of oxygen carriers. The largest scale of CLC with solid fuels in operation is 1 MWth while CLOU has only been successfully operated at a scale of 10 kWth. The aim of this work is to understand the degree of solid fuel conversion in CLC systems utilizing oxygen carriers with and without oxygen uncoupling. The competition for oxygen by volatiles and char has been determined in this work for CLOU and non-CLOU carriers. CLOU oxygen carriers convert coal more efficiently than non-CLOU oxygen carriers because of the more favorable conversion pathway in a CLOU environment. However, coal conversion in a CLOU reactor is not as simple as conversion of volatiles and char with gaseous O2. Instead, there are several competing pathways for conversion of coal utilizing a CLOU oxygen carrier because volatiles can react with O2 and CuO while char can be combusted by O2 or gasified into products that can react similarly to volatiles. A novel method for determining the different pathways of coal conversion in a CLOU environment was developed and a complementary model for predicting volatiles and char conversion pathways was iv developed in order to assist in future CLOU reactor design and operation. It was found the conversion pathways are influenced heavily by temperature and fuel particle size. The majority of volatile conversion was by reaction with gas-phase O2 and increased with temperature. Volatiles released from small coal particles form large bubbles that limit contact with O2 and CuO so there are more unconverted volatiles that exit the reactor. Char conversion increased at higher temperatures primarily because of the increase in the char combustion pathway because there is more O2 available in the reactor at higher temperatures. Small char particles have a large surface area-to-mass ratio and diffusion limitations are limited so conversion by both heterogeneous reactions (combustion and gasification) are efficient. However, small char particles have a much higher tendency to elutriate the system than larger char particles so there is a trade-off between using small and large sized coal particles in a CLOU system. |
