Carbon Capture and Reduced Irreversibility Combustion Using Chemical Looping

Power generation traditionally depends on combustion to ‘release’ the energy contained in fuels. Combustion is, however, an irreversible process and typically accounts for a quarter to a third of the lost work generation in power producing systems. The source of this irreversibility is the large departure from chemical equilibrium that occurs during the combustion of hydrocarbons. Chemical looping combustion (CLC) is a technology initially proposed as a means to reduce the lost work generation in combustion equipment. However, renewed interest has been shown in the technology since it also facilitates carbon capture. CLC works by replacing conventional “oxy-fuel” combustion with a two-step process. In the first, a suitable oxygen carrier (typically a metal) is oxidised using air. This results in an oxygen depleted air stream and a stream of metal oxide. The latter is then reduced in the second reaction step using a hydrocarbon fuel. The products of this second step are a stream of reduced metal, which is returned to the oxidation reaction, and a stream of CO₂ and H₂O that can be separated easily. The thermodynamic benefits of CLC stem from the fact that the oxygen carrier is recirculated and can thus be chosen with a reasonable degree of freedom. This enables the chemistry to be optimised to reduce the lost work generation in the two reactors – the reactions can then be operated much closer to chemical equilibrium. It is widely accepted in the literature that a key issue in CLC is identifying the most effective oxygen carrier. However, most previous work appears to consider systems in which a solid phase metallic oxygen carrier is recirculated between two fluidised bed reactors. In the current paper, we explore the possibility of using liquid or gas phase reactions in the two reaction steps since it is hypothesised that these might be compatible with a wider range of fuels including coal. The paper, however, starts by reviewing the existing literature on CLC and the basic thermodynamics of a conceptual CLC power plant. The thermodynamic analysis is extended to include a general method for calculating the lost work generation in a given chemical reactor. Finally, this method is applied to the oxidation reaction of a proposed CLC reaction scheme.