(177b) Back to the Future: Chemical Looping Beyond Combustion
The clean and efficient use of the world’s energy and materials resources is arguably one of the prime challenges of our time. At present, the world’s energy needs are met predominantly through the combustion of fossil fuels, with no significant change predicted for at least the next ~30 years. In combination with the continuous increase in global energy demand, fossil fuel based energy production is hence expected to result in a strong increase in anthropogenic CO2 emissions, motivating major efforts to develop more efficient energy systems and economically viable CO2 capture and sequestration technologies. At the same time, the vast expansion of recoverable gas reserves through new developments in drilling technology is creating a renaissance in the development of natural gas based processes which could supplement or even replace oil-based processes in the near future.
In this context, Chemical Looping Combustion (CLC) is emerging as a particularly promising novel combustion technology which offers an elegant and efficient route towards clean combustion of fossil fuels. In CLC, the combustion of a fuel is broken down into two, spatially separated steps: The oxidation of an oxygen carrier (typically a metal) with air, and the subsequent reduction of this carrier via reaction with a fuel. CLC is thus a flame-less, low-NOx combustion process, which produces a pure mixture of CO2 and H2O as combustion gases, from which highly concentrated, high-pressure (i.e. sequestration-ready) CO2-streams can be produced via condensation of the water.
While initial interest in chemical looping was almost exclusively focused on combustion, the underlying reaction engineering principle – periodic processes in circulating fluidized beds – is in fact neither new nor limited to combustion, but rather represents a flexible reaction engineering principle: For example, by controlling the degree of carrier oxidation, the fuel oxidation can be controlled such that incomplete, i.e. partial oxidation to synthesis gas (CO + H2) results. Similarly, replacing air with steam or CO2 as oxidizers yields the chemical looping analogue to steam and dry reforming. However, in contrast to conventional reforming processes, chemical looping reforming results in the first case in the direct formation of high purity (i.e. fuel-cell ready) hydrogen streams without the need for further CO removal, and, in the second case, in an efficient process for CO2 activation via reduction to CO. Finally, we have recently demonstrated that appropriate selection of oxygen carrier materials even allows using the chemical looping process scheme for simultaneous, integrated desulfurization of the effluent stream, resulting in highly efficient conversion of S-contaminated fuel streams.
Drawing upon examples from our own research, I will discuss the potential of chemical looping processes for methane conversion, an area which is likely to shape reaction engineering developments for the next few decades due to the vast expansion of natural gas reserves. In particular, I will discuss the design of engineered carrier materials for chemical looping processes as an example of the role of materials chemistry as an enabler for emerging reaction engineering concepts. Overall, I will argue that “chemical looping” represents a widely applicable reaction engineering approach towards the development of strongly intensified processes, which in particular offers exciting possibilities in the development of novel processes for C1 chemistry.