Transformation and Release to the Gas Phase of Cl, K, and S during Combustion of Annual Biomass

The transformation of inorganic constituents in annual biomass was experimentally investigated at grate-combustion conditions. A laboratory fixed-bed reactor was applied to obtain quantitative information of the release of Cl, K, and S to the gas phase from six distinctively different annual biomass fuels. Samples of 4.0 g of biomass were combusted at well-controlled conditions at temperatures from 500 to 1150 °C. The elemental release was quantified by analysis of the residual ash and a mass balance on the system. The experimental results revealed that potassium was released to the gas phase in significant amounts at combustion above 700 °C. The potassium release increased with the applied combustion temperature for all biomass fuels; however, the quantity released was largely determined by the ash composition. At 1150 °C, between 50 and 90% of the total potassium was released to the gas phase. The biomass fuels with an appreciable content of silicate showed the lower release of potassium. Between 25 and 70% of the fuel chlorine was released below 500 °C; the residual chlorine was released by evaporation of KCl, mainly between 700 and 800 °C. Above 800 °C, the fuel chlorine was completely released to the gas phase for all of the samples. Between 30 and 55% of the fuel sulfur was released at 500 °C. The samples rich in K and Ca, but low in Si, displayed only a minor increase in the sulfur release as the combustion temperature was further increased. On the contrary, the sulfur release increased abruptly above 700−800 °C for the Si-rich samples. On the basis of the release quantification, the overall transformations of the ash-forming elements are discussed at grate-combustion conditions.