Transformation and Release of Potassium, Chlorine, and Sulfur from Wheat Straw under Conditions Relevant to Dual Fluidized Bed Gasification

The release and transformation of potassium (K), chlorine (Cl), and sulfur (S) from biomass during thermochemical conversion processes may lead to problems, such as the corrosion and fouling of heat transfer surfaces, agglomeration of bed material, and the poisoning of catalysts used in the downstream processes of gasifiers. To predict and to mitigate effectively these problems, information regarding the quantity and mechanism of the release of these elements under relevant operating conditions is required. In the present work, the release of K, Cl, and S from wheat straw under conditions relevant to dual fluidized bed gasification were quantified in a laboratory-scale bubbling fluidized bed reactor. During the pyrolysis step, the bed temperature ranged from 550 to 900 degrees C, while the residence time for the fuel in the reactor was fixed at 3 mm. The char samples obtained from the pyrolysis step were partially combusted at the same temperature at which they were produced for an additional 3 mm. The fractions of the elements released from the fuel were quantified by chemical analysis of the char/residual ash obtained in each experiment and a mass balance across the system. Overall, 75%-62% of the Cl, 59%-67% of the S, and 14%-31% of the K in the virgin wheat straw were released during pyrolysis conducted within the investigated temperature range. The char obtained from the pyrolysis process contained significantly higher amounts of K, Cl, and S than the virgin fuel. Furthermore, the ash content of the char was about 5-fold higher than that of the virgin fuel. This suggests that at combustion-relevant temperatures, complete combustion of the char is more likely to result in severe ash-related problems than combustion of the virgin fuel. Partial combustion of the char resulted in additional release of K, Cl, and S. In addition to the experimental results, the transformation and release of the elements during this process are discussed with the aid of chemical thermodynamic equilibrium modeling and leaching.