The evolution of catalytically active calcium catalyst during steam gasification of lignite char.

Coal or coal char/pyrocarbon gasification is a viable way to produce cheaper hydrogen. The gasification of three different lignites and their pretreated samples showed that either the intrinsic calcium in lignite or the extrinsic calcium added by ion exchange had high catalytic activity. It was proven that most of the calcium in the lignite was organically associated, which was mainly bound to carboxyl groups and highly dispersed at an atomic level. Furthermore, Yuxi lignite was demineralized to investigate the catalytic characteristics of calcium and potassium catalysts on steam gasification. The changes of gasification rate, gas compositions and oxygen chemisorption capacity with carbon conversion hinted that the catalytic mechanism of calcium was different from that of potassium. X-ray absorption near edge structure (XANES) analysis revealed that carboxyl-bound calcium was the active form of the catalyst, and the deactivation of the calcium catalyst in the gasification process was due to the transformation of carboxyl-bound calcium to other chemical forms. The organically bound calcium also led to the porous char structures at the early gasification stage, which improved the catalytic activity. Image 1 • Insightful characterization of lignite char gasification catalyzed by calcium/potassium. • Quantitative analysis of organically bound active calcium in char. • Detailed explanations of the active sites and deactivation of the calcium catalyst. • Proposal of an active site/intermediate mechanism for calcium catalysis. [ABSTRACT FROM AUTHOR]