An analysis of ilmenite particles used as bed material for combustion of biomass in a CFB boiler

Abstract

Combustion of biomass in a fluidized bed boiler with silica sand as bed material is related to problems such as agglomeration of bed material and corrosion of the heat transfer surfaces, where potassium plays a major role in both issues. In the autumn of 2011 the Chalmers 12 MWth CFB boiler was run with 40% of the natural ore ilmenite and 60 % silica sand as bed material. The purpose of this thesis has been to study the possibilities of ilmenite to be used as bed material, considering physical and chemical changes which the material undergoes. Except for an investigation on how the ash layers are formed, the ilmenite was leached and investigated as a potassium capturer. A segregation of iron to the surfaces and an enrichment of titanium in the particle core of ilmenite takes place when it is used as bed material in a CFB boiler fired with biomass. The ashes form a double layer on the particle, consisting of Ca(Ti0.7Fe0.3)O2.85, as well as a diffusion of potassium into the particle core forming KTi8O16. The potassium and calcium can be leached out 32 and 7 ppm respectively from the used ilmenite. The diffusion of potassium into the core of ilmenite would be of great value for the chemistry in the boiler if it reduces both agglomeration and corrosion issues. Theoretically potassium could be captured at 5.7 wt % in ilmenite. Since the potassium is only leachable at a small extent, the possibilities to leach out potassium for further capture through regeneration of ilmenite are restricted. However, further studies are needed to appreciate the industrial implementation of ilmenite as a potassium capturer.