Oxygen carrier aided combustion with copper smelter slag as bed material in a semi-commercial wood-fired circulating fluidized bed

Fluidized bed combustion (FBC) is a well-established technology in Sweden for the conversion of biomass and waste-derived fuels. Recent research has shown that the process can be upgraded by using oxygen-carrying bed material, for example in the form of iron oxide-containing minerals and slags. The concept, called Oxygen Carrier Aided Combustion (OCAC), has demonstrated enhanced oxygen distribution and thermal efficiency in commercial boilers, using the iron-titanium ore ilmenite as oxygen carrier. This study demonstrates the first large-scale utilization of a commercial copper slag product (Järnsand) as oxygen carrier in a 12 MW_th circulating fluidized bed boiler. By exchanging between 21 and 100 % of the silica sand bed with Järnsand it was possible to successfully reduce the air-to-fuel ratio from 1.2 down to 1.08 with CO emissions remaining low, at concentrations of around 20 mg/nm³ at 6 % O₂. In contrast, an air-to-fuel ratio of 1.08 with silica sand bed gave a 5-min average CO concentration of 800 mg/nm³ at 6 % O₂. The NO emissions were also reduced by about 30 % with the introduction of Järnsand. For 3.5 days of the campaign, the boiler was operated with a 100 % Järnsand bed without any bed material regeneration. K was absorbed by Järnsand, and a K concentration of 3.5 wt.-% was reached. No agglomeration or increasing CO emissions were observed. K-saturation was not reached, and a longer lifetime than the studied 3.5 days is suggested. In conclusion, similar performance as ilmenite is suggested for Järnsand as an oxygen carrier in OCAC operation.