Release of K, Cl, and S during combustion and co-combustion with wood of high-chlorine biomass in bench and pilot scale fuel beds

Studies of the release of critical ash-forming elements from combustion of biomass are typically conducted with small sample masses under well controlled conditions. In biomass combustion on a grate, secondary recapture and release reactions in the fuel-bed may affect the overall release and partitioning of these elements. Earlier work by the authors on the release of K, Cl, and S from a high-chlorine biomass (corn stover) in a lab-scale setup is, in the present work, supplemented with novel results from a bench-scale fixed bed reactor and a 100 kW moving grate pilot facility. The results from the bench-scale reactor indicate that S and K release are not significantly affected by secondary reactions, while Cl is partly recaptured by secondary reactions in the char. A linear increase in K-release was observed from 50% at 906 °C to almost 80 wt.% at 1234 °C when firing only corn stover. A similar release profile was observed for Cl, from 65% to nearly 100%. Complete release of S was achieved at 1234 °C with a linear increase from 70% at 906 °C. Co-combustion of corn stover with low-Cl wood chips served to increase the bed temperature, resulting in complete and close to complete release of Cl and S, respectively. An increase in the relative K-release was observed when increasing the wood chip fraction from 40% to 100% (energy basis). Pilot scale flue gas results indicate that the share of Cl released as HCl decreases towards 0% as the share of wood chips is increased towards 100%. Hence, co-combustion of corn stover with wood chips is expected to decrease the absolute release of KCl due to the lower feedstock quantity of Cl, however, increase the relative release of Cl as KCl.