The relevance of thermal annealing to the evolution of coal char gasification reactivity

The CO2 gasification of torrefied wood samples was examined by thermogravimetry (TGA) at linear, modulated and constant reaction rate (CRR) temperature programs. The untreated raw materials and chars prepared at 750°C were also included in the study. The gasification temperature range separated sufficiently from the pyrolysis in the experiments for its separate analysis. Characteristic gasification reactivity differences were observed between the samples that were due to the differences of the char-forming reactions at different conditions. Various groups of experiments were evaluated together by the method of least squares, under various hypotheses on the dependence of the reaction rate on the reacted fractions. The differences between the samples were described by different pre-exponential factors while the rest of the kinetic parameters were kept identical during an evaluation. The effect of thermal annealing on the gasification kinetics was also expressed by the values of the pre-exponential factors. When the experiments of the samples prepared from birch were evaluated together, self-accelerating kinetics was obtained with E=225kJ/mol. The experiments belonging to the samples prepared from spruce resulted in n-order kinetics with E=223kJ/mol.

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“…The n-order evaluation gave n=0.34 and n=0. 35 for samples B750 and S---. Each evaluation was carried out by n-order kinetics and by eq 9.…”

Section: Results Of the Kinetic Evaluationmentioning

confidence: 99%

CO₂ Gasification of Torrefied Wood: A Kinetic Study

2014

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“…Figure 1 shows results from the HRTEM technique, which were used to derive the char models used here [45]. It was postulated that during CO2 gasification, the higher reactive sites such as Cz and Czi, from the 3x3, will preferentially lose H. Anealling during pyrolysis however, may give rise to active sites of lower reactivity as the 3x3 grows to 5x5 sizes, because at high temperatures, and chemically-controlled gasification process, char undergoes thermal annealing in competition with gasification [106][107][108]. This causes the growth of crystallite diameter, La, by the coalescence of crystallites along the a-axis (e.g.…”

Section: Dft Reactivity Modelling and The Fukui Functionmentioning

confidence: 99%

“…Suuberg also found that the inherent reactivity of the chars was attributed to the changes in the active site concentrations, which were often thermally induced [110]. Hence, under high temperature regimes and chemically-controlled gasification process, the char actually undergoes a process of thermal annealing (pyrolysis) in competition with gasification [106][107][108]. With the increase of carbon conversion or when the higher reactive sites in the 3x3 model are removed/annealed, the reactivity, and the number of reactive carbon sites reach similar levels and the difference in char reactivities becomes smaller [106,110].…”

Section: Dft Reactivity Modelling and The Fukui Functionmentioning

confidence: 99%

Density functional theory molecular modelling and experimental particle kinetics for CO2–char gasification

Roberts

Everson

Domazetis

et al. 2015

Carbon

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Experimental measurements and DFT atomistic modelling were conducted to elucidate the mechanisms for gasification chemistry of char with CO2 gas. The molecular models used were based on experimental representations of coal chars derived from the vitrinite-and inertinite-rich South African coals at 1000 . The HRTEM and XRD techniques were used to construct parallelogram-shaped PAH stacks of highest frequency in the vitrinite-rich (7x7) and intertinite-rich (11x11) char structures. Computations were executed to get the nucleophillic Fukui functions, at DFT-DNP level, to elucidate the nature and proportions of carbon active sites and quantify their reactivity. The DFT-DNP-computed reaction pathways and transition states, to obtain the energy of reaction and activation energies for the gasification reactions of CO2 with active carbon sites were examined. These results were compared with TGA experimental results at 900-980 . The mean nucleophillic Fukui function of the H-terminated char models and active sites located at similar edge positions

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“…This is related with the condensation degree of the samples: the higher the La value, the higher the condensation degree. This effect is reflected by a decrease in the ratio of edge (more reactive) to basal carbon atoms due to the increase in the diameter of the crystallite [12].…”

Section: Resultsmentioning

confidence: 99%

04/01177 Comparison between the reactivity of coal and synthetic coal models

2004

Fuel and Energy Abstracts

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A mixture of carbon compounds was pyrolysed under an inert atmosphere at different temperatures in a fixed bed reactor. The resultant chars were characterised in terms of texture and thermal behaviour. Textural characterisation of the chars was carried out by N 2 and CO 2 adsorption isotherms at -196 ºC and 0 ºC, respectively. Char isothermal reactivity in air at 500 ºC, and in CO 2 at 1000 ºC, was performed in a thermogravimetric analyser (TGA). Temperature-programmed combustion tests under 20% oxygen in argon were also performed in the TGA linked to a mass spectrometer (TGA/MS). The results showed that char textural properties do not always relate well to their reactivity.Not only do physical properties (e.g. surface area, porosity) but also chemical properties

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The relevance of thermal annealing to the evolution of coal char gasification reactivity

Cited by 109 publications

References 18 publications

CO₂ Gasification of Torrefied Wood: A Kinetic Study

CO₂ Gasification of Torrefied Wood: A Kinetic Study

Density functional theory molecular modelling and experimental particle kinetics for CO2–char gasification

04/01177 Comparison between the reactivity of coal and synthetic coal models

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The relevance of thermal annealing to the evolution of coal char gasification reactivity

Cited by 109 publications

References 18 publications

CO2 Gasification of Torrefied Wood: A Kinetic Study

CO2 Gasification of Torrefied Wood: A Kinetic Study

Density functional theory molecular modelling and experimental particle kinetics for CO2–char gasification

04/01177 Comparison between the reactivity of coal and synthetic coal models

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CO₂ Gasification of Torrefied Wood: A Kinetic Study

CO₂ Gasification of Torrefied Wood: A Kinetic Study