Safety and environmental reasons for the use of Ni-, Co-, Cu-, Mn- and Fe-based oxygen carriers in CLC/CLOU applications: An overview

Chemical looping combustion (CLC) is presently considered as a cutting-edge combustion technology that enables capturing CO2 without major energy penalty for its separation. The technology involves the use of solid metal oxides, so-called oxygen carriers (OCs), which are alternately subjected to the redox reactions. In the last years, most investigations on the CLC process have been focused on the development of synthetic oxygen carriers, which are much more efficient and have greater resistance compared to natural materials. Synthetic OCs can be based on Ni-, Co-, Cu-, Mn- and Fe- oxides, the use of which however may cause various kinds of threats. The paper provides an overview of Ni, Co, Cu, Mn, and Fe-based OCs taking into account safety operation of CLC reactors as well as health and environmental issues. Therefore, the purpose of this work is to study the impact of different solid oxygen carriers on human beings, animals and natural environment. It has been revealed among others that OCs based on Ni- and Co- oxides may cause various environmental and health problems, while Cu-oxide might be hazardous especially for aquatic wildlife. Moreover, it was shown that Ni-based OCs have tendency to create Ni-S compounds, such as Ni3S2 or NiSO4, in reactions with combustible sulphur contained in the fuel. Toxicity investigations prove, for instance, that rats exposed to 15 mg/6h/day of NiSO4 die in 10 days after the test. Otherwise, Mn- and Fe- oxides are generally safe in use.