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• Effects of pyrolysis temperature, residence time and atmosphere were investigated. • Release of AAEMs was associated with char structure evolution in rapid pyrolysis. • CO 2 promoted Na release in early stage and Ca and Mg release in late stage. The release law of alkali and alkaline earth metals (AAEMs) during coal pyrolysis is crucial for controlling scaling, corrosion and slagging in the utilization of high AAEMs coal. To approximate entrained-flow bed gasification process, a free-falling reactor was used for coal pyrolysis to study the release characteristics of AAEMs in nascent char. Effects of pyrolysis temperature, residence time and atmosphere were investigated. The cumulative release of Na, Ca and Mg at 1100 °C was 68.0%, 47.5% and 69.8%, respectively. As pyrolysis temperature increased from 900 °C to 1100 °C, the release of Na, Ca and Mg increased by 7.0 %, 3.5% and 5.3%, respectively. The cumulative release of AAEMs increased with the pyrolysis residence time, but the release rate decreased significantly in middle and late stages. The promoted thermal decomposition of functional groups by CO 2 resulted in a lower char yield. CO 2 promoted Na release in early stage and Ca and Mg release in late stage. Furthermore, release of AAEMs was associated with char structure evolution in rapid pyrolysis. The presence of CO 2 , high temperature and prolonged residence time led to enhanced particle fragmentation, increased surface cracks, and intensified thermal decomposition of functional groups, resulting in the migration of AAEMs in carbon matrix to char surface and the release of AAEMs organically bound to carbon matrix. [ABSTRACT FROM AUTHOR]

• In-situ Raman spectroscopy was innovatively used to characterize carbon structure. • AAEM promoted continuous breaking/reforming of bonds in polycondensation. • Inherent AAEM promoted the miniaturization of carbon crystal structure. Inherent minerals, especially alkali and alkaline earth metals (AAEM), are reactive constituents that interact with organic matter during pyrolysis and have significant effects on thermal characteristics and carbon structure evolution of char. The pyrolysis of raw and leached coal samples was conducted by using thermogravimetric analyzer and in-situ Raman system to explore the effects of content and chemical form of AAEM on pyrolysis characteristics and carbon structure evolution of single char particle. The water-soluble and ion-exchangeable AAEM promoted continuous breaking/reforming of bonds between alkyl substitutional groups and aromatic ring systems, resulting in the delay of volatiles release and orderly evolution of carbon structure during pyrolysis. The effects of water-soluble AAEM was more severe, because the divalent alkaline earth metal bonds to carbon was much more stable than the monovalent alkali metal for the inherent AAEM participated in the polycondensation reaction. Besides, the ion-exchangeable AAEM inhibited the generation of cross-linked structures more effectively than water-soluble AAEM, because ion-exchangeable AAEM existed in the form of AAEM-carboxylates and delayed the thermal decomposition. Moreover, the inherent AAEM promoted the miniaturization of carbon crystal structure in the carbon matrix, forming char with disordered carbon structure. [ABSTRACT FROM AUTHOR]

• 300 °C torrefaction reduced gasification reactivity mainly in midterm and late stage. • Low-temperature torrefaction had slight effect on char gasification reactivity. • Gasification kinetics were analyzed based on the gas-solid reaction methods. • Pathways of char structure evolution changed after 800 °C pyrolysis. • Gasification reactivity decreased significantly for chars pyrolyzed after 800 °C. In this study, the effect of torrefaction on the gasification reactivity of chars from raw and torrefied biomass was investigated. Three torrefaction temperatures and four pyrolysis temperatures were taken into consideration. It was found that the severe torrefaction (300 °C) would reduce the char gasification reactivity by at least 19% according to the normalized gasification rate. Moreover, the reduction of gasification reactivity appeared after the midterm stage. The gasification reaction were further analyzed by nucleation/growth model and model-free method. The activation energy increased by ~80 kJ/mol with conversion, indicating an enhancement of the reaction resistance. Furthermore, 800 °C pyrolysis was found to be a turning point, beyond which the gasification reactivity reduced significantly. These reactivity changes were implied by the bio-char structure evolution and active alkali and alkaline earth metals (AAEMs) contents variations. The research results provide insights into the effect of torrefaction on biomass gasification. [ABSTRACT FROM AUTHOR]