Using affordable iron oxide-based materials as oxygen carriers for biomass conversion

Doctoral thesis, 2024

Fluidized bed combustion is a mature technology that is well-established in the Swedish energy system. It is an especially suitable technology for conversion of biomass- and waste-derived fuels, which have high shares of volatiles and contain reactive ash species. Typically, silica sand is used as bed material, and it is inert with respect to the fuel conversion. However, recent developments have shown that fluidized bed combustion can be upgraded by utilizing oxygen carriers – bed materials that participate actively in the oxygen transport in the boiler. This concept is called Oxygen Carrier Aided Combustion (OCAC) and makes it possible to operate existing boilers at a reduced air-to-fuel ratio, increased efficiency, higher load, and potentially improved economic performance and reduced environmental impact.

OCAC makes use of oxygen carriers in the form of solid particles of transition metal-oxides. The material needs to be chemically active and have suitable fluidization properties, as well as sustained particle integrity at high temperatures and mechanical stress. The conversion of solid biomass- and waste-derived fuels is especially challenging because they are associated with corrosion, bed agglomeration, and slagging due to a high content of reactive ash species such as potassium. Fluidized bed combustion of such fuels necessitates a high rate of bed material replacement, so the material must be cheap and abundant. As a future step, oxygen carriers could be applied in a process known as Chemical-Looping Combustion (CLC), to produce undiluted CO2 suitable for storage and utilization, but CLC has yet to be demonstrated on a commercial scale. OCAC could act as an intermediate step between conventional combustion and CLC, and potentially accelerate the implementation of large-scale carbon capture and storage necessary for meeting climate targets.

This thesis explores the possibilities for utilizing oxygen carrier materials that are already available in the Swedish metallurgical industries. Copper smelter slag was identified as a particularly suitable material due to its large production quantities, limited utilization and the low need of material processing required. Its properties were investigated, and its ability to absorb potassium without severe agglomeration was established by means of lab-scale interaction experiments with ash model compounds. Semi-commercial scale operation of a fluidized-bed boiler with copper smelter slag revealed suitable oxygen-carrying and fluidization properties, and good general performance.

Apart from these promising findings, a better understanding of potassium interactions with iron-based oxygen carriers in general was obtained by lab-scale experiments. Iron-oxides (iron mill scale and magnetite fines), iron-titanium-oxide (ilmenite), and two industrial slags (copper smelter slag and LD-slag) and their interaction with ash model compounds under different conditions were studied. The lab-scale studies included the development of a new method that allows for the addition of ash model compounds in connection to redox experiments at fluidized bed conditions. By comparing the results from different methods (fixed bed, fluidized bed, and large-scale boiler operation), it was concluded that the small-scale experiments are useful for an initial understanding of ash interaction. This is therefore a useful tool for estimating the risks of, for example, agglomeration before applying a new bed material in a full-scale boiler.