Long-term effects of ilmenite on a micro-scale bubbling fluidized bed combined heat and power pilot plant for oxygen carrier aided combustion of wood

Abstract

Small-scale solutions for the application of solid biomass fuels for the generation of heat and power with fluidized beds are hardly available. The main issues comprise agglomeration and slagging, incomplete burnout of combustibles and low efficiencies. During so-called oxygen carrier aided combustion, an oxygen carrier replaces the conventional bed material and balances the local and temporal oxygen supply in the fluidized bed. This results in a more homogeneous combustion and increased heat release in the fluidized bed area. Especially for small-scale bubbling fluidized bed solutions, these effects require an adapted plant setup. However, applications of oxygen carriers in small-scale bubbling fluidized bed boilers are barely reported. Process parameters differ significantly from large-scale circulating fluidized beds regarding resident times, particle volume fractions and size distributions. Hence, this research examines the enhancing effect of 50 wt% ilmenite in a 45 kWth bubbling fluidized bed fired combined heat and power plant. Long-term measurements in a field test environment promise a potential decrease of the carbon monoxide emissions of up to 75 %. Lower fluidized bed temperatures and apparently poor combustion conditions even enhance the positive effect of ilmenite. Moreover, the porous ilmenite particles interact with ash compounds such as potassium, calcium and magnesium. The migration of potassium into the inner particle mitigates the formation of low-melting layers around the particle. Calcium and magnesium form titanium oxide phases in the outer particle layer, below an iron enriched particle shell. Thereby, ilmenite prevents the formation of sticky particle layers, which addresses agglomeration and slagging issues. The particle shell is subject of ongoing attrition leading to a constant entrainment of active material from the combustion chamber. Therefore, a continuous addition of fresh material is essential to sustain the beneficial impact of the oxygen carrier. The optimal operation period for the oxygen carrier additive is supposed to cover 1–2 weeks.