The efficiency and mechanism of humidity in alleviating CO2 inhibition on H2S adsorption to straw biochars

Abstract

Adsorption tests were conducted to clarify the impact of carbon dioxide (CO2) on hydrogen sulfide (H2S) removal to straw biochars and the role of humidity in affecting the CO2 impact. When straw biochars were dry, CO2 proved significantly detrimental to their H2S adsorption, regardless of the CO2 contents. In contrast, when adjusting the humidity of the biochars to 20%, the presence of CO2 promoted the conversion of H2S into sulfites, though still inhibited the generation of elemental sulfur, through which the original negative CO2 impact was nearly compensated. The presence of low to medium contents of CO2 even slightly improved the adsorption capacity and extended the breakthrough time. This phenomenon was ascribed to a favorable carbon surface environment for H2S dissociation and oxidation, created from the reactions of the major minerals (Ca and Mg) with CO2 and water. To sum up, the CO2 inhibition on the H2S adsorption to biochars was ascribed to the competition of CO2 for the active sites; however, humidity induced a series of CO2-participated chemical reactions between H2S and the minerals, sufficiently alleviating the CO2 inhibition. The results also highlight the necessity to recognize the most critical biochar properties or an efficient balance of crucial biochar properties in achieving an optimal H2S removal when CO2 is present in the syngas.