Leaching of Potassium from Rock Ilmenite Used As a Bed Material in Fluidized Bed Boiler Performed Under Acidic and Alkaline Conditions

Ewa Krymarys, Angelica Gyllén, Pavleta Knutsson, Fredrik Lind, Magnus Rydén

Oxygen Carrier Aided Combustion (OCAC) is a new concept for fluidized bed combustion of biomass and waste fuels, where the semi-inert particles typically used as bed material are replaced with chemically active oxygen carrying particles. The oxygen carrier is capable of absorbing oxygen from oxygen-rich zones and release it in zones that are oxygen-depleted, thereby evening out the oxygen fugacity in space and time. The concept has shown promising results and has been implemented on industrially relevant scales from 12 – 75 MW_th. Oxygen carriers particles should have high reactivity with fuel and air, be resistant to attrition, have low tendency to agglomerate, be environmentally- benign and have low cost. The most studied oxygen carrier material for OCAC so far has been ilmenite, which is a common mineral ore consisting mainly of titanium and iron oxides. Apart from being a good oxygen carrier ilmenite has been shown to act also as an ash absorber. Previous studies with ilmenite as oxygen carrier in OCAC have shown that there is distinctive elements patterns for particles that has been applied in the combustion process, where iron migrates to the surfaces, titanium enrichment occurs in the core and a calcium-rich double layer covers the formed iron layer. Potassium has been observed to diffuse into the core forming KTi₈O₁₆.

Perpetual absorption of ash on bed material is not possible. Bed agglomeration arises when ash layers are formed around the bed material particles. Potassium compounds are of low melting point, thus especially prone to evaporate at an early stage during the combustion. Evaporation enhances formation of sticky potassium silicates on the surfaces of bed material particles, which pick up on other ash components such as calcium. In both cases whether potassium stays as a layer or migrates to the core, there is an urge to discover the possible ways to leach the potassium out – to avoid possible agglomeration and regain the capability of potassium-capturing.

The present work involves Norwegian rock ilmenite which has been used as bed material during combustion of wood chips in the Chalmers campus 12-MW_th biomass-fired Circulating Fluidized Bed (CFB) boiler. The aim was to investigate leaching of potassium from ilmenite samples extracted directly from the boiler during operation. If certain amounts of potassium could be removed from the ilmenite particles under moderate process conditions this would allow for recirculating bed material back to the boiler, significantly reducing the cost and environmental footprint of OCAC. Leached potassium would also represent an additional by-product stream for the process.

Samples of bed material were extracted at different time intervals (after 1 day and 3-4 days of operation) directly from the boiler furnace. Consequently, samples were leached both under acidic (pH=2,5) and alkaline (pH=12) conditions by means of acetic and phosphoric acids both at room (21°C) and elevated temperatures (70, 80°C). The results have been interpreted in terms of optimal conditions for leaching alkali, changes in morphology and elemental content of the bed material and influence on its reactivity for further reuse. Based on the obtained results from a flame atomic absorption spectroscopy (AAS), scanning electron microscopy (SEM) analysis combined with electron dispersive X-ray (EDX) the possible implementation of leaching as an industrially applied step to the OCAC can be discussed.