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• A mass balance-based multi-step model was used to study the reaction kinetics of direct chemical looping process. • The mass balance formulated steps required fewer parameters and thus demanded less computational resource. • The activation energy changes were diversified by adding oxygen carrier. • Oxygen carrier altered the yields and distributions of intermediates and products. The investigation into reaction kinetics provides fundamental insights for reactor design in biomass thermochemical conversion. This work studied the kinetics of biomass chemical looping process based on a multi-step model. Cellulose, xylan, and lignin were employed as biomass representatives. The individual step in the multi-step scheme was formulated with a mass-balance equation form, requiring fewer parameters and thus less computational cost. In addition, the decomposition characteristics of different biomass samples in the presence of oxygen carrier (Fe 2 O 3) were analyzed, and the effects of Fe 2 O 3 on the kinetic parameters of reaction steps were investigated. The results revealed that volatiles pyrolyzed from cellulose and xylan were almost not involved with Fe 2 O 3 reduction at high temperatures (∼1000 K) while playing a major role in terms of lignin (∼1050 K). Moreover, adding OC did not always positively affect the activation energy; in particular, it decreased the activation energy of xylan yet elevated the activation energies of cellulose (at low-temperature zone) and lignin to some extent. [ABSTRACT FROM AUTHOR]

[Display omitted] • The effects of HTC on coconut shell fuel properties were quantitatively studied. • The composition of hydrochar is similar in condition of 200 °C–0 h and 180 °C–3 h. • The migration behavior of different forms potassium was studied after HTC process. • Kinetic models were adopted to predict products yield during HTC. Hydrothermal carbonization is a promising renewable technology to produce high quality solid biofuel with characters of sterilization, micronization, and homogeneous. In this work, coconut shell was used as feedstock to investigate the products distribution, reaction kinetics and the migration of potassium during HTC process. Experiments were carried out in 180–240 °C with residence time of 0–5 h. The results showed that the content of fixed carbon increased from 20.76 wt% to 49.80 wt%, and HHV increased from 20.75 MJ/kg to 31.77 MJ/kg at 240 °C with the holding time of 3 h. The ratio of H/C dropped from 1.78 to 0.86 and O/C dropped from 0.68 to 0.16 which all indicated the improvement of fuel performance. The effects of increasing temperature and prolonging residence time on the products were in the same direction but different in severity. The properties of the products changed little after the residence time was more than 1 h. In addition, chemical fractionation method was used to investigate migration behavior of potassium. The best removal temperature of potassium was 200 °C which decreased potassium concentration from 2.7 mg/g to 0.6 mg/g specifically. Due to the increase of specific surface and porosity caused by high temperature, the content of potassium rose slightly at 220 and 240 °C. A kinetic model was adopted to predict the products distribution and yield of solid phase and gas phase. The activation energies (Ea) of different reaction paths were calculated and the results showed that Ea of gas formation is greater than that of solid. Migration behavior of potassium and modeling of HTC process is of great significance for design, optimization of industrial HTC reactor and selecting the best time for discharging and feeding to achieve better performance. [ABSTRACT FROM AUTHOR]

[Display omitted] • Isothermal prediction based on model-free method was established. • Kinetic parameters form model-free and model-fitting method were compared. • Gasification mechanism was revealed from carbon and pore structure evolution. • The fractal dimension D 1 was related with amorphous structure. • Mesopore SSA increased by hundred times whereas active sites changed insignificantly. Biomass gasification integrated with leaching pretreatment can be conducive to the sustainable energy system. In this work, the biochar gasification was studied from structure to reactivity with the consideration of leaching pretreatment. The biochar was prepared from raw and leached corn straw (CS) and corncob (CC) in a fixed bed reactor. The gasification was performed in thermogravimetric analyzer with the emphasis on kinetic parameters comparison and reactivity prediction. The carbon and pore structure were investigated to reveal gasification mechanism. The prediction procedure based on model-free method was proposed. From the results, compared with CS, leaching pretreatment reduced gasification reactivity of CC considerably according to reactivity index and reactivity prediction. The random pore model showed the best fitting performance. Furthermore, the leaching pretreatment reduced inorganics (mainly K) in CS and CC, and removed water-soluble organics (pectin) in CC. The active sites and surface roughness in leached biochar decreased accordingly. The CO 2 gasification can activate initial biochar after pyrolysis. Specifically, the specific surface area of mesopore increased by hundred times and more active sites were also generated. However, as the gasification proceeding, the active sites and the fractional dimensions changed insignificantly for both raw and leached biochar. The leaching pretreatment mainly promoted the micropore development during gasification. The pore structure evolution was consistent with the carbon structure, which appeared to be correlated with the kinetic analysis. The research gives new insight into structure-performance correlation of biomass gasification and provides the implication for sustainable energy system based on gasification technology. [ABSTRACT FROM AUTHOR]

[Display omitted] • Combining wet torrefaction (WT) and pyrolysis for bioresource utilization. • Global reactivity was inhibited whereas local reactivity could be promoted by WT. • WT changed the structure and reactivity of softwood biochar significantly. • WT temperature had limited effect on AAEMs removal efficiency in biochar. • Relationship between X - T - t was determined by model-free method. This study investigated the effect of wet torrefaction (WT) on biochar property derived from softwood (PIW) and hardwood (POW) pyrolysis. The biochar gasification was performed in a thermogravimetric analyzer, followed by kinetic modelling. The biochar structure was analyzed, including inherent metals, pore structure and carbon structure. The results showed that WT affected reactivity and structure of PIW-derived biochar more than that of POW-derived biochar. The critical WT temperature determined the reactivity changes together with kinetic parameters. The WT affected the inorganic and organic component in biochar simultaneously. As results of WT, more than 70 and 50% ion-exchanged AAEMs in PIW-derived biochar and POW-derived biochar were removed. Nevertheless, the organic structure had essential effect on the gasification reactivity. More pronounced structure evolution of PIW-derived biochar was observed, including lower mesopore surface area, more graphite structure and small rings. This research provides new insights into the structure–reactivity correlation for biochar utilization. [ABSTRACT FROM AUTHOR]

A series of thermogravimetric experiments were conducted to study the gasification kinetics of raw and demineralized coal char. The gasification mechanism was reveled through the kinetic analysis and the structure evolution. The systematic analyses showed that the reactivity of demineralized coal char was more sensitive to the heating rate β. The random pore model was more suitable for both raw and demineralized coal chars, where the pre-exponential factor (A) played the essential role in fitting performance. Moreover, the parameters of kinetic compensation effect were found to have a good linear relationship with ln β , and the variations of kinetic triplet (A , E a and f (X)) with the conversion level could be further explored accordingly. The reactivity predictions employing the integral and differential approaches were compared under the isothermal and non-isothermal conditions. The reactivity of raw coal char was more easily affected by diffusion, and the conversion-effectiveness factors increased with the conversion level at high temperatures. Finally, the local gasification mechanism was analyzed by piecewise comparing different single-step global models. The raw coal char featured the closed-pore reopening according to the distribution pore structure. The ash can act as the nuclei for gasification and affect pore growth and coalescence. [Display omitted] • KCE parameters had a good linear relationship with ln β. • Predications for non-isothermal and isothermal gasification were performed. • The local reaction pathway was analyzed by the piecewise SGMs. • The distribution structure parameter based on TGA was proposed. • The ash content affected the pore growth and coalescence during gasification. [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]

• Effect of HTC on softwood and hardwood was compared. • Model-free kinetic prediction was performed. • Cellulose structure was activated firstly and then decomposed with HTC temperature. • Pyrolysis mechanism changed from diffusion model to reaction order model. • Pore structure changed more significantly for pinewood after HTC. Hydrothermal carbonization (HTC) pretreatment is an effective method to upgrade the low quality biomass properties. In this work, the pinewood (PIW) and poplar wood (POW) were selected to study the effect of HTC (180–240 °C) on woody biomass structure and reactivity. The results showed that the slight and moderate HTC can make the cellulose structure more ordered, while the severe HTC leaded to the decomposition of cellulose structure. The mesoporous specific surface area and pore volume increased with the HTC temperature, and the pore structure of PIW was more developed than that of POW. The slight HTC had little effect on cellulose structure and the diffusion effect played the dominant role. The severe HTC increased the relative content of lignin and the pyrolysis followed the high order reaction model. The model-free method was used to determine the relationship between T - β - X , and the specific temperature T HTC / T raw was calculated. The T HTC / T raw value of PIW and POW located in the range of 0.938–1.340 and 0.924–1.422, indicating the HTC pretreatment had great effect on POW pyrolysis reactivity. [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]

Hydrothermal carbonization (HTC) can improve biomass quality in both physical and chemical aspects for energy application. This study aims to investigate the characteristics and reactivities of rape straw (RS) hydrochars. Hydrochars were prepared at 160–240 °C with residence time of 15–120 min. Mass yield, energy yield, microstructure, functional group and migration of alkali and alkaline earth metals (AAEMs) were studied to evaluate the influence of different conditions on properties of hydrochar. The results showed that O/C and H/C ratio decreased, while the higher heating value (HHV) increased with increasing temperature and residence time. The effect of increasing temperature on hydrochar properties was more significant than residence time. The structure was changed, and hydrochar possessed a more stable form after the aromatization reaction. For the gasification reactivity of hydrochar, decomposition rate curves showed that the peak of pyrolysis and gasification moved to a higher temperature region with the increasing of HTC temperature because of the developed aromatic structures in hydrochar. The pyrolysis activation energy decreased from raw RS 71.68 to 41.03 kJ/mol in 240 °C, while gasification activation energy increased from 80.42 to 251.30 kJ/mol. Moreover, it was found that HTC can reduce the content of AAEMs efficiently and the best removal condition is 200 °C. Ca content dropped to a minimum value at 200 °C and then increased at higher temperature which may be caused by well-developed pore structure in hydrochars. This study provides basic data for comprehensive utilization of rape straw and migration mechanism of AAEMs in HTC process. [Display omitted] • The effects of HTC on RS structure and reactivity were quantitatively studied. • Temperature effect was more obvious than residence time during HTC. • Effects of HTC on kinetics of pyrolysis and gasification were studied separately. • Energy yield reached max at 180 °C, while AAEMs reached minimum at 200 °C. • The migration behavior of total and active AAEMs is studied. [ABSTRACT FROM AUTHOR]

Producing clean energy from waste biomass via pyrolysis is critical in reducing dependence on fossil fuels and alleviating their environmental impacts. Inherent potassium in low energy density coconut fibers reduces bio-oil yield and compromises bio-oil quality and the integrity of reactors within which pyrolysis occurs. The effect of consecutive hydrothermal treatment (180°C-220 °C) and water washing on coconut fiber demineralization, pyrolysis behavior, and bio-oil composition was investigated. Chemical fractionation method classified the most occurring inorganic (potassium) as ~76% water-soluble, ~19% ion-exchangeable, and ~5% acid-soluble species. Demineralization results show that combined pretreatment at ≤200 °C leached out all water-soluble and exchangeable potassium. However, at 220 °C, electrostatic attraction and increased tortuosity imposed by the high density of oxygenated functional groups and a well-developed porous structure hamper demineralization. Nevertheless, a maximum potassium removal efficiency of 95% could be achieved. Although pretreatment marginally affects apparent activation energy, the reaction mechanism of hydrochars became insensitive to the heating rate between 10 and 50 °C min−1. Due to pretreatment, the selectivity of thermally stable phenolic and anhydrosugar derivatives increased at the expense of the light oxygenates of low thermal stability because of suppressing fragmentation reactions and enriching polymers with a high degree of polymerization. Light oxygenates decreased from 27.45 area % to 24.02, 16.33, and 13.32 area % for hydrochars produced at 180 °C, 200 °C, and 220 °C, respectively. Therefore, this pretreatment approach can produce inorganic-free, thermally stable, and chemically stable bio-oils. • Hydrothermal pretreatment and washing leaches out all water-soluble and ion-exchangeable K. • Demineralization rate is hampered by reactive oxygenated functional groups and hydrochar porosity. • Pyrolysis mechanism becomes less sensitive to heating rate after hydrothermal transformation. • Selectivity of aromatics and anhydrosugars increased with carbonization intensity. [ABSTRACT FROM AUTHOR]

In this paper, the catalytic effect of different components in biomass ash on petroleum coke (PC) gasification was studied. Cotton straw ash with high content of AAEMs and low content of silica, was prepared and pretreated by washing to obtain the soluble (ash leachate) and insoluble (washed ash) components. These ash-based catalysts were added to PC via physical mixing or impregnation. The catalytic gasification performance and kinetics were investigated using a thermal-gravimetric analyzer (TGA), and the carbon structure evolution was determined by Raman spectroscopy. The results showed that all of these ash-based catalysts, even for the washed ash, had catalytic effect on PC gasification. The soluble component, mainly containing K, could speed up the gasification rate especially at the late stage. The catalytic activity of the soluble component was sensitive to temperature and conversion, while that of the insoluble component was steady. Moreover, the modified random pore model and nucleation and growth method were applied to analyze the gasification kinetics. In addition, the washed ash could inhibit the graphitization during gasification, whereas the ash leachate and original ash could enhance the generation of amorphous carbon. Possible mechanisms of catalytic gasification were proposed to explain these phenomena. • Different components of cotton straw ash were used as catalysts for PC gasification. • Ash leachate could accelerate the gasification rate especially at late stage. • Parameters of catalytic gasification kinetics were analyzed. • Pathways of structural evolution changed with presence of different catalysts. • Possible catalytic gasification mechanisms were proposed. [ABSTRACT FROM AUTHOR]