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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]

Pyrolysis and catalytic pyrolysis behaviors of two kinds of coals with different ranks were determined by thermogravimetric analyzer–mass spectrometry (TG–MS) and a high-frequency furnace. A quantitative method of on-line gas analysis was proposed using quadrupole mass spectrometry. The effects of pyrolysis temperature (650–800 °C) and Na 2 CO 3 loading amount (0–15 wt.%) on the gaseous compounds evolution were investigated. The results show that loading Na 2 CO 3 on coal was favorable for gas production in pyrolysis process. The yields of light gas species in the high-frequency furnace were lower than those in TG–MS. By comparing the pyrolysis yields of gas and solid in the high-frequency furnace and TG–MS, it was concluded that heating methods had great effects on the reaction mechanism and product distribution of pyrolysis and catalytic pyrolysis of coals. Moreover, the evolution processes of gaseous compounds were related to the presence of different functional groups in the solid samples. [ABSTRACT FROM AUTHOR]

Catalytic pyrolysis and gasification characteristics of coal using Na 2 CO 3 as a catalyst were studied in a high-frequency furnace and thermogravimetric analyzers (TGA), respectively. Interactions of Na 2 CO 3 with carbon at both pyrolysis and gasification phases were observed by in-situ heating stage microscope. The effects of pyrolysis temperature (650–800 °C) and catalyst loading amount (0–15 wt.%) on the associated pyrolysis and gasification processes were investigated. The results show that, H 2 and CO are the main pyrolysis gas products and the yields of these two gas increase with the increasing of Na 2 CO 3 in the range of 0–15 wt.%. The in-situ heating stage experiments indicated that one main gasification reaction form was observed for Shenfu char samples. Most of SF char particles reacted with CO 2 in the form of shrinking core mode without cracking, and certain particles presented fragmentation phenomenon in the reaction process. There is an optimal loading amount of Na 2 CO 3 for the best catalytic gasification effect of Shenfu bituminous char, while Na 2 CO 3 does not reach the saturation amount for the catalytic gasification of Zunyi anthracite coal. The activation energy of catalytic Shenfu coal gasification with 10 wt.% Na 2 CO 3 is 31.5 kJ mol − 1 less than that of non-catalytic coal gasification, which indicates that Na 2 CO 3 can improve the kinetics of coal gasification with CO 2 . [ABSTRACT FROM AUTHOR]

• Carbon structure in thermochemical conversion of coal is characterized in-situ. • Transition mechanism between amorphous carbon and aromatic carbon is clarified. • Oxygen-containing functional groups are active sites for low temperature oxidation reactions. Thermal conversion is pivotal for clean and efficient coal utilization. Evolution of solid structure profoundly affects the gasification mechanism. However, in-situ study on coal structure evolution during gasification is limited. In this study, in-situ Raman spectroscopy was used to investigate the carbon structure evolution during thermal conversion. Results revealed two stages in coal pyrolysis: polycondensation and aromatic ring fusion. During polycondensation, numerous side chain groups break, while aromatic compounds condensed and rings fused in the aromatic ring fusion stage, increasing molecular size. In char gasification, larger particles required more time to achieve stable shrinkage, thus delaying coal char graphitization process. Low temperature oxidation led to decreased order due to oxygen-containing structure formation, evolving into a graphite-like structure as disordered carbon diminished. In addition, oxygen-containing groups acted as active oxidation sites. The insights into real-time structure evolution enhance understanding of the particle behavior during thermal conversion of coal. [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]

This study focuses on characteristics of structure evolution during pyrolysis and subsequent char gasification of lignite and bituminous coal. Coal pyrolysis was conducted on induction-heating reactor. As pyrolysis temperature increased, the graphitization degree of char increased, and the amount of C-O and -CH in char decreased, resulting in poorer gasification reactivity of char. Char gasification was conducted by thermogravimetric analyzer (TGA), high-temperature stage microscope (HTSM) and in-situ Raman system to investigate gasification reactivity, in-situ evolution of morphology structure and in-situ evolution of carbon structure, respectively. As gasification proceeded, the projected area of char particle decreased rapidly first and then kept consistent, and degree of graphitization of char gradually increased. In-situ Raman parameters indicated difference in increasing rate of graphitization between lignite char and bituminous char decreased with increasing temperature. Higher temperature deactivated the alkali metals, leading to attenuation of their inhibitory effect on graphitization. Gasification reactivity index R 0.9 was in positively correlated with in-situ I D1 /I G value, and corresponding correlation coefficient (R2) for TGA and HTSM were 0.8108 and 0.9829, respectively. The higher R2 for HTSM was due to the weaker diffusion effect. In-situ Raman parameter was more suitable for predicting gasification reactivity of single particle char. [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]

• Structure of pyrolytic coal–biomass blends were not additive function of composition. • Co-pyrolysis enhanced the transformation of ion-exchanged AAEMs in residue char. • Synergy of isothermal co-gasification changed from inhibition to promotion. • Maximum reaction rate of non-isothermal co-gasification was increased. This work investigated the structural evolution of rapid co-pyrolysis char and its influence on co-gasification characteristics. The chars were prepared in a high frequency furnace at 900 °C with the heating rate of 300 °C/min, using Xinjiang lignite and cotton straw as raw materials. Blends of coal and biomass were prepared in proportions of 25:75, 50:50 and 75:25 wt.%. Compared to the individual pyrolysis, co-pyrolysis enhanced the transformation of ion-exchanged alkali and alkaline-earth metals (AAEMs). Moreover, the surface area of co-pyrolysis chars were enhanced, and the carbon structure of biomass char in mixtures became more ordered. The gasification reactivity of co-pyrolysis char was evaluated by thermogravimetric analyzer (TGA), including isothermal (800, 850 and 900 °C) and non-isothermal (5 °C/min) gasification using CO 2 as agent. For isothermal gasification, the synergistic effect of reactivity changed from inhibition to promotion with the increasing conversion. The synergy index changed obviously at 800 °C. For non-isothermal gasification, the maximum reaction rate was increased. The blends with 75% biomass had the minimum activation energy. The co-gasification reactivity varied with the temperatures, biomass blending ratios and conversions, which were well related to the char structural evolution caused by rapid co-pyrolysis. [ABSTRACT FROM AUTHOR]

Physicochemical evolution (i.e. pore structure variation, carbon structure change and active AAEM transformation) during rice straw (RS) and Shenfu bituminous coal (SF) co-pyrolysis was quantitatively determined in this work. Moreover, the corresponding char gasification was conducted using a thermogravimetric analyzer (TGA) and relative reactivity was proposed to quantify the co-pyrolysis impact on co-gasification reactivity. The results showed that the development of pore structure in co-pyrolyzed chars was first inhibited and then enhanced with the decrease of SF proportion. The promotion effect of co-pyrolysis on order degree of co-pyrolyzed chars gradually weakened with increasing RS proportion. Co-pyrolysis mainly enhanced active K transformation in co-pyrolyzed chars and the promotion effect was alleviated with increasing RS proportion. The inhibition effect of co-pyrolysis on co-gasification reactivity weakened with increasing RS proportion and gasification temperature, which was mainly attributed to the combination of carbon structure evolution and active AAEM transformation in co-pyrolysis. [ABSTRACT FROM AUTHOR]

In this study, the pyrolysis of low-ash coal (SF) and high-ash coal (NM) was investigated using a thermogravimetric analyzer. The semi-pyrolyzed char was collected to analyze its structural characteristics, and different model-fitting approaches were adopted for non-isothermal kinetics. Partial demineralized coal was also prepared to clarify the mineral–organics–gas correlation. The results showed that coal pyrolysis was a two-step sequence in which the aliphatic chain decomposed first (<600 °C), and then the aromatic ring condensed (>600 °C) with the formation of H 2. Moreover, the amorphous structure in coal char was generated before the graphitization. HF leaching can promote the generation of H 2 during the secondary pyrolysis of high ash coal. Inner minerals can promote graphitization under high-temperature pyrolysis. The n th-order model showed the best fitting performance, and the activation energy (E a) increased with temperature based on piecewise analysis. Moreover, the E a distribution became more concentrated after demineralization, especially for high-ash coal. Finally, the coal pyrolysis mechanism was summarized based on the solid structural evolution with consideration of the gas release property and inner minerals effect. [Display omitted] • The pyrolysis mechanism based on minerals–organics–gas correlation was summarized. • The VM/FC ratio of NM-HF coal decreased whereas H 2 generation increased. • Amorphous carbon was generated first and was then converted into graphitic carbon. • Inert minerals can promote coal char graphitization at high temperatures. • A piecewise combined model was proposed, and the E a variation was obtained. [ABSTRACT FROM AUTHOR]

In this study, the pyrolysis of low-ash coal (SF) and high-ash coal (NM) was investigated using a thermogravimetric analyzer. The semi-pyrolyzed char was collected to analyze its structural characteristics, and different model-fitting approaches were adopted for non-isothermal kinetics. Partial demineralized coal was also prepared to clarify the mineral–organics–gas correlation. The results showed that coal pyrolysis was a two-step sequence in which the aliphatic chain decomposed first (<600 °C), and then the aromatic ring condensed (>600 °C) with the formation of H 2. Moreover, the amorphous structure in coal char was generated before the graphitization. HF leaching can promote the generation of H 2 during the secondary pyrolysis of high ash coal. Inner minerals can promote graphitization under high-temperature pyrolysis. The n th-order model showed the best fitting performance, and the activation energy (E a) increased with temperature based on piecewise analysis. Moreover, the E a distribution became more concentrated after demineralization, especially for high-ash coal. Finally, the coal pyrolysis mechanism was summarized based on the solid structural evolution with consideration of the gas release property and inner minerals effect. [Display omitted] • The pyrolysis mechanism based on minerals–organics–gas correlation was summarized. • The VM/FC ratio of NM-HF coal decreased whereas H 2 generation increased. • Amorphous carbon was generated first and was then converted into graphitic carbon. • Inert minerals can promote coal char graphitization at high temperatures. • A piecewise combined model was proposed, and the E a variation was obtained. [ABSTRACT FROM AUTHOR]