Improved Fuel Conversion in Chemical Looping Combustion using Novel Volatiles Distributors and Effective Manganese-based Oxygen Carriers

Doktorsavhandling, 2024

Chemical looping combustion (CLC) is a promising carbon capture technology, as CO2 generated during combustion is inherently obtained in a concentrated form. As the technology moves from pilot-scale to larger-scale operations, factors such as fuel conversion and the development of robust and economically feasible oxygen carriers become increasingly critical. In this thesis, two ways to improve the fuel conversion in the fuel reactor are studied: i) use of a volatile distributor to achieve improved gas-solids contact and ii) use of effective and cheap oxygen carrier materials.

Poor contact between volatiles from solid fuels such as biomass, and oxygen carriers is one reason for insufficient gas conversion in the fuel reactor of CLC systems based on interconnected fluidized beds. Hence, the concept of a volatiles distributor (VD) is proposed for achieving a uniform cross-sectional distribution of volatiles, providing better gas-solids contact and improving the gas conversion in the fuel reactor. In this work, it is clearly demonstrated that the use of the VD results in a more uniform cross-sectional distribution of volatiles under different fluidization regimes. Adjusted designs of the VD were found to improve the uniformity of the horizontal distribution of the simulated volatiles. CFD simulations additionally reveal the improved gas-solids mixing, highlighting the potential of the VD for addressing incomplete conversion of high-volatile fuels in fluidized bed systems. The effectiveness and design flexibility of the VD, together with the most suitable drag model identified in the CFD simulations, are instructive for the large-scale application of the VD.

In chemical looping with oxygen uncoupling (CLOU), the conversion of volatiles and char in the fuel reactor can be notably enhanced compared to regular CLC. This justifies studies of CLOU oxygen carriers with sufficient reactivity, mechanical stability, and reasonable production costs. This thesis covers the investigations of four natural manganese ores and one manufactured calcium manganite as oxygen carriers in a 300 W circulating fluidized-bed system together with detailed characterizations of the fresh and spent oxygen carrier samples. CLC operations with non-calcined manganese ores proceed as smoothly as with calcined ones, indicating that an energy-intensive pre-calcination step for manganese-based oxygen carriers in large-scale applications is not necessary. The content of Fe in manganese ores plays a crucial role in their O2 release. A sufficiently high content of Ca in manganese ores facilitates the formation of perovskite phase, which enhances both the O2 release and reactivity. The manufactured calcium manganite demonstrates higher oxygen release capacity and CH4/syngas conversion compared to the four natural ores, offering another effective way of increasing fuel conversion in the fuel reactor.

In conclusion, the fuel conversion in CLC could be improved using the novel volatiles distributor, as demonstrated in both cold-flow model fluidized bed and CFD simulations, and effective manganese-based oxygen carriers characterized by sufficient mechanical stability and economic feasibility.