Sulfur Uptake during Oxygen-Carrier-Aided Combustion with Ilmenite

The feeding of sulfur during conventional biomass combustion can reduce ash-related corrosion attacks on heat-transferring surfaces. In this work, oxygen-carrier-aided combustion with ilmenite as the bed material and the feeding of elementary sulfur as an additive is addressed. More precisely, the sulfur uptake by the bottom bed material, i.e., the inventory of the oxygen carrier and its accumulation of the main biomass ash components calcium and potassium, is considered. The experiments, including time-resolved sampling of bed material, were carried out during continuous operation in conditions that can be considered to mimic those in a full-scale boiler. The aim with the investigation was to compare the sulfur uptake by the bed material based on two distinct conditions: (1) sulfur was introduced to the furnace in the absence of an ash layer on the particles (i.e., at the start of an experiment) and (2) sulfur was introduced with an already existing ash layer on the particle (i.e., after long time operation). Convectional ash characteristic methods where used to study the sulfur uptake, and the investigation showed that sulfur was mainly bound to the potassium and calcium phases on/in the particles. Differences in the elemental distribution of sulfur were observed depending upon the emergence of an ash layer coat on the bed material particles. Sulfur was located predominantly between the formed ash layer and the core when it was added from the start of the process and was located mainly in the particle core when an ash layer coated the particle before its addition.