Trace element speciation with oxygen carriers

Research Project, 2018 – 2020

Chemical-looping combustion (CLC) is a form of unmixed combustion for burning hydrocarbon fuels. It is based on a transfer of oxygen from air to fuel using oxygen carriers. The inherent partitioning of the fuel oxidation in two steps - in two reactors - can have benefits for limiting emissions, since these will be concentrated to one reactor. The technique is applicable to carbon capture, but likely also for limiting emissions and effects of impurities. CLC can be easily implemented in fluidized beds, and Chalmers is world-leading in the research. Oxygen carrier aided combustion (OCAC) utilizes the same underlying reactions as CLC, but in one reactor, the combustion chamber. This technology is used commercially in several fluidized bed boilers.

The use of dedicated crops and waste-derived fuels will likely increase in the future. Such fuels, including agricultural and municipal solid waste, could contain significant fractions of potentially toxic elements including Cu, Pb, and Zn. The use of CLC and OCAC with such fuels presents both opportunities and challenges.

In this work, a range of complementary investigations has been carried out to improve understanding of oxygen carrier–trace element interactions. The thermodynamics of oxygen carrier–trace element interactions have been critically evaluated, and oxygen carrier samples from CLC/OCAC pilots have been analyzed in detail with respect to metal speciation. Together, these results provide a significantly improved and practically relevant understanding of the chemistry in such systems, demonstrating that oxygen carriers can actively contribute to controlling potentially toxic elements and reducing operational challenges. The results show that oxygen carriers can strongly influence trace element chemistry as key metals can be effectively retained through interactions with the oxygen carrier material, while others are preferentially directed to specific process streams. These insights provide new strategies for both emission reduction and material handling.