Written by Zili Zhang, Imperial College who is working on Call 1 Project: Chemical looping for low-cost oxygen production and other applications, PI: Paul Fennell
A fossil-fuel power station converts part of thermal energy of combustion of fossil fuel to mechanical energy, and finally to electrical power (electricity). Power stations using solid fossil fuels such as coal are widely deployed around the world, and they are responsible for a large fraction of carbon dioxide emission worldwide. The direct combustion of the carbon based fossil fuel with air (N2+O2) results in a mixture of N2 and CO2 exhaust from the power station. Capture of the carbon dioxide from those power stations often requires energy intensive process of separation of CO2 from N2 in the exhaust gas. Chemical looping (combustion) is an advanced indirect combustion process which can offer the potential for integrated power generation and carbon capture in the fossil-fuel power station with a much lower energy penalty. Principally, the kind of technologies employ a metal oxide that is shuttled back and forth between a fuel and air reactor, picking up oxygen in the air reactor and ‘carrying’ it to the fuel reactor such that direct contact between air and fuel is avoided to preventing the additional energy-intensive process of separation. The metal oxide carriers (so called oxygen carriers, OC) could directly react (burn) with the solid fuel (chemical looping combustion, CLC), and some of them (e.g. CuO, CoO, MnO) can release gas-phase O2, which can then be used in the fuel reactor to burn a solid fuel (coal or biomass) readily (chemical looping with oxygen uncoupling, CLOU). Most of CLC and CLOU process is achieved in a fluidised bed reactor in which the solid fuel and the oxygen carrier are intimately mixed, and suspended under a high velocity of gas. The intimate solid-solid and solid-gas contact offer advantages in terms of rate of fuel conversion and heat integration, but also creates major problems e.g. contamination of the oxygen carrier by impurities in solid fuel, and leak of unreacted fuel from the fuel reactor to the air reactor with the oxygen carrier means that some CO2 is produced in the air reactor, and is mixed with N2 again.
Three institutions, Imperial College London, Cambridge University and Cranfield University involved in the project have significant history of working together in the field of chemical looping, and wish to push forward the field significantly with this project. A series of cutting edge research with a number of different themes are conducted with a common aim of assessing the viability of chemical looping technologies for CCS, together with a determination of which forms of chemical looping (including CLOU) are most likely to be genuinely useful in the context of power generation. The result of the project will lead to the first pilot scale demonstration of chemical looping technology, and will see advances in some new reactor and process design for applying chemical looping for power generation in a large scale.
Various forms of chemical looping are possible with different degrees of integration between the oxygen release and the thermal energy production cycles, e.g. from using the chemical looping to directly react with (burn) gaseous fuels, or to produce oxygen either then combusted with fuel in-situ (e.g. the CLOU process) or ex-situ for oxygen production. The three institutions are investigating routes to achieve the optimal degree of integration of a hybrid form of chemical looping that is more thermally efficient, and avoid the need for intimate contact between the oxygen carrier and the solid fuel. The system as a whole is less a typical chemical looping combustion system, and more a thermal oxygen production system. Ultimately, with good heat integration, there is a negligible efficiency penalty for carbon capture for the system. Imperial College has modelled and designed from first principles a novel counter-flow thermal oxygen reactor for replacing the burner in conventional coal fire power plant. Cambridge University now focuses on setting up detailed chemical looping reactor models containing particle oxidation and reduction kinetics, and linking to power plant flow sheet model. The 50kWth chemical looping rig in Cranfield University is now fully operational and re-configured for CLOU process studies in conjunction with test work on the 3D cold model. The first large-scale UK test of CLOU technology using CuO-Al2O3 composite oxygen carriers will be run using the facility. The procedure of production of large quantity of the oxygen carriers for pilot-scale operation is currently developed in Imperial College London.
Watch this space for more updates!