Alkali and alkaline earth metals catalytic steam gasification of ashless lignin: Influence of the catalyst type and loading amount

This work studies the effects of alkali and alkaline earth metallic species (AAEMs) (Na2CO3, K2CO3, and CaCO3), Na+ impregnation amount (2, 5, and 7 mmol) and sodium salts (NaHCO3, Na2SO4, and NaAlO2) on the gasification characteristics and product distribution of G-type lignin in a fixed-bed system for continuous gasification. The results showed that six catalysts all could improve lignin gasification performance by increasing total gas yields, syngas yields, H2/CO ratios, and carbon conversion rates. Na2CO3 had the strongest catalytic ability, followed by NaHCO3 and K2CO3, while CaCO3, Na2SO4 and NaAlO2 had poor catalytic abilities. Gaseous AAEMs, especially NaOH and KOH, could enhance tar cracking, volatiles reforming, coke gasification, the water-gas shift reaction, and methane steam reforming. The high melting points of CaO and NaAlO2 greatly reduced the catalytic capacities of CaCO3 and NaAlO2. AAEMs could combine with the carbon matrices to form carboxylate structures or phenol/aldehyde structures which might volatilize to catalyze homogeneous reactions. Moderately increasing Na+ impregnation amount to 5 mmol was beneficial to lignin gasification, but excessive impregnation reduced the gasification parameters. In addition, all six catalysts had effects on the tar composition, especially K2CO3 and NaAlO2, which could effectively catalyze tar cracking and reduce heavy compounds.