Steel converter slag as an oxygen carrier

Doktorsavhandling, 2023

Thermal conversion of fuels can be used to produce heat and power in addition to chemicals. In order to be aligned with climate targets, it is necessary that such systems do not emit carbon dioxide to the atmosphere. Carbon capture and storage (CCS) can be used together with fuel conversion systems to prevent CO2 from entering the atmosphere. If CCS is used together with biomass-based fuels, it is possible to achieve a net-flow of carbon dioxide out of the atmosphere, so called negative emissions.
Chemical looping technologies for combustion (CLC) and gasification (CLG) are technologies which can be used for heat, power and chemical production with no or low penalties for carbon capture. In any chemical looping applications, a functional oxygen carrier is essential. The oxygen carrier is normally a metal oxide based material that can transport oxygen from one reactor to another. However, when fuel is introduced into the system, ash can react with the oxygen carrier and decrease its operational lifespan, especially reactive ash from biomass and low-grade fuels. Therefore, there is growing interest in low-cost oxygen carriers that can contribute to making the process economically feasible. Low-cost oxygen carriers can be obtained from ores or as byproducts of the steel industry. Of particular interest is steel converter slag, which is also known as Linz-Donawitz (LD) slag. LD slag is generated in significant amounts, contains sufficient amount of iron oxide (that can act as an oxygen carrier) and available at a low cost.
This work presents a comprehensive overview of the chemistry and behavior of LD slag when it is implemented as an oxygen carrier in chemical-looping applications. The material has been investigated in laboratory reactors, in addition to pilot and semi-industrial units, and LD slags interactions with different fuel components, ash, alkali salts, sulfur and tars have been investigated.
It is concluded from this work that LD slag can be viable as material for both CLC and CLG processes with biomass. In contrast to other bed materials, such as silica sand or the commonly investigated iron-based oxygen carrier ilmenite, the slag has limited reactivity with reactive alkali components. This results in more alkali being available in the gas phase, which is beneficial for tar cracking and for the gasification rate of the solid char. The high content of calcium in the LD slag is also favorable in terms of gasification and ash interactions. Calcium oxide catalyzes both the water-gas shift reaction and is catalytic towards tar cracking. A high level of calcium also increases the melting points of both the K-Ca-P and K-Ca-Si matrixes. However, the structural integrity of the material is lower compared to, for example, ilmenite, resulting in more fines being generated during the process. Overall, LD slag is a potential oxygen carrier that is suitable for chemical-looping processes that utilize low-grade fuels.