Comparing the structural development of sand and rock ilmenite during long-term exposure in a biomass fired 12 mWthCFB-boiler

Paper in proceeding, 2017

Oxygen Carrier Aided Combustion (OCAC) is a novel combustion concept and a spinoff from Chemical-Looping Combustion (CLC). The purpose of the concept is to increase the overall efficiency in conventional circulating fluidized bed (CFB) boilers by replacing the commonly used silica-sand bed material with an oxygen carrier (OC). The conceptual idea is to utilize the fluid dynamics in a CFB and the inherent oxygen transport supported by the OC to increase the oxygen distribution within the furnace in time and space. This is achieved as the OC can buffer oxygen in oxygen rich regions and release oxygen in oxygen poor regions of the furnace, resulting in less emissions of harmful and unburnt species as well as operation at lower air-to-fuel ratios. The OCAC concept has been successfully proven in laboratory and pilot plants and further demonstrated in full scale operation (75 MW) during more than 10'000 hours. However, as far as known to the authors no studies have been reported on the evolution in mechanical stability of an OC during continuous operation in a scale larger than 100 kW. This work aims to make a first evaluation of how ilmenite particles used as OC are affected with regard to mechanical resistance during long-term exposure to combustion conditions in Chalmers semi-industrial scaled (12 MW) CFB-boiler. The mechanical stability of two different types of ilmenite with similar composition, a sand ilmenite and a rock ilmenite, are evaluated in experiments conducted in the Chalmers boiler. Samples of the fresh materials and samples collected during operation in the Chalmers boiler are investigated with regard to their morphology, size distribution but also to attrition in a laboratory test rig. The study shows that the two materials differ in how the mechanical degradation occurs with exposure time. Cavities are formed inside the sand ilmenite particles which are held together by an ash layer before they are shattered into numerous pieces, whereas the rock ilmenite develops distinct cracks that cause splitting of the particles.