Using affordable materials from metallurgical industries in Oxygen Carrier Aided Combustion and Chemical-Looping Combustion

Licentiate thesis, 2022

Oxygen carriers are solid oxides of transition metals that can be used to convert fuel in the absence of gaseous oxygen in a process called Chemical-Looping Combustion (CLC). The use of oxygen carriers makes it possible to produce undiluted CO2 without expensive gas separation. High-concentration CO2 is a requirement in carbon capture and storage (CCS), which is considered a promising path for climate change mitigation. Oxygen carriers can also be used as bed material in fluidized bed conversion, and in that case provides enhanced oxygen distribution in the furnace. The concept is called Oxygen Carrier Aided Combustion (OCAC), and has been implemented in several existing biomass-, or municipal solid waste-fired boilers with ilmenite as oxygen carrier.

This thesis examines utilizing oxygen carriers for the conversion of biomass and waste-derived fuels. Such fuels contain reactive ash species, which cause operational problems in the boiler such as corrosion, bed agglomeration, and slagging. Historically, much of the CLC-research has been on synthetic oxygen carriers with high reactivity. However, since the lifetime is expected to become quite low due to contamination of ash, they are likely not economically viable with these low-grade fuels. Here, the focus instead is on low-cost materials. The situation in Sweden is quite unique, since affordable metal oxide particles are produced in large quantities in our metallurgical industries. Several products and by-products from these industries are potential oxygen carriers. This thesis summarizes and discusses the large-scale utilization of oxygen carriers for OCAC in Sweden. Ilmenite is the most studied oxygen carrier for this purpose, but some other low-cost materials have also been tested in semi-industrial scale. The general findings are that implementing OCAC in already existing fluidized bed boilers is possible and enables a decrease in air-to-fuel ratio.

Since biomass and waste fuels have complex and reactive ash compositions, they react with, and affect the lifetime of oxygen carriers. This thesis, therefore, also discusses ash interactions with some low-cost materials. Potassium is considered the most problematic ash element. Potassium is reactive with bed material and causes deposit formation and corrosion on the boiler, among other problems. Fixed bed interaction experiments have been conducted in this work with different potassium salts and oxygen carriers to study the changes in the materials with respect to composition and reactivity. Two by-products from steelmaking (LD-slag and iron mill scale) were studied in fixed bed interactions experiments. Also, as part of the work, a new lab-scale method which allows for improved experiments in a fluidized bed reactor has been developed. The method was used to study the effect of the accumulation of potassium on reactivity and fluidization of oxygen carriers. The study was conducted with ilmenite as oxygen carrier and K2CO3 as ash model compound. The addition of K2CO3 caused defluidization in ilmenite and diffusion of K into the particles.