Variation of the pore structure of coal chars during gasification

The variation of the pore structure of several coal chars during gasification in air and carbon dioxide was studied by argon adsorption at 87 K and CO2 adsorption at 273 K. It is found that the surface area and volume of the small pores (10 Å for air gasification is constant over a wide range of conversion (>20%), while for CO2 gasification similar results are obtained using the total surface area. However, in the early stages of gasification (

On the microscopic mechanism of carbon gasification: A theoretical study

In this paper we report the results of ab initio calculations on the energetics and kinetics of oxygen-driven carbon gasification reactions using a small model cluster, with full characterisation of the stationary points on the reaction paths. We show that previously unconsidered pathways present significantly reduced barriers to reaction and must be considered as alternative viable paths. At least two electronic spin states of the model cluster must be considered for a complete description. (C) 2004 Elsevier Ltd. All rights reserved.

Kinetics of reduction of FeO from slag by graphite and coal chars

The rates of reduction of FeO from iron-saturated FeO-CaO-Al2O3-SiO2 slags by graphite, coke, bituminous coal and anthracitic coal chars at temperatures in the range 1 673-1873 K have been measured using a sessile drop technique. The extents of reaction were determined using EPMA analysis of quenched samples, and on line gas analysis using a quadrupole mass spectrometer. The reaction rates have been shown to be dependent critically on carbon type. For the reaction geometry used in this investigation the reduction rates of graphite and coke are observed to be faster than with coal chars. This unexpected finding is shown to be associated with differences in the dominant chemical and mass transfer mechanisms occurring at the reaction interface. High reaction rates are observed to occur with the formation of liquid Fe-C alloy product and the associated gasification of carbon from the alloy. The rates of reduction by coal chars are determined principally by the chemical reaction at the carbon/gas interface and slag phase mass transfer.

Observations of the reduction of FeO from slag by graphite, coke and coal char

The reduction of FeO from iron-saturated FeO-CaO-Al2O3-SiO2 slags by graphite, coke and coal char at 1 673 K has been investigated using a sessile drop technique. Metallographic analysis of samples quenched from the reaction temperature, and in situ observations of the reaction interface, reveal significant differences in the slag/carbon contact, and in the morphologies of the product iron and its composition; these differences were found to depend on the carbon type used in the reduction. In particular it has been shown that, in the case of graphite and coke, liquid Fe-C droplets were rapidly formed at the slag/C interface. Reactions of the slag with coal chars, in contrast, result predominantly in the formation of solid iron. These observations indicate that the reaction pathways, and hence reaction kinetics, are dependent on carbon type.

The kinetics of NO and N2O reduction over coal chars in fluidised-bed combustion

This paper presents a comprehensive and critical review of the mechanisms and kinetics of NO and N2O reduction reaction with coal chars under fluidised-bed combustion conditions (FBC). The heterogeneous reactions of NO and N2O with char/carbon surface have been well recognised as the most important processes in reducing both NOx and N2O in situ FBC. Compared to NO-carbon reactions in FBC, the reactions of N2O with chars have been relatively less understood and studied. Beginning with the overall reaction schemes for both NO and N2O reduction, the paper extensively discusses the reaction mechanisms including the effects of active surface sites. Generally, NO- and N2O-carbon reactions follow a series of step reactions. However, questions remain concerning the role of adsorbed phases of NO and N2O, and the behaviour of different surface sites. Important kinetics factors such as the rate expressions, kinetics parameters as well as the effects of surface area and pore structure are discussed in detail. The main factors influencing the reduction of NO and N2O in FBC conditions are the chemical and physical properties of chars, and the operating parameters of FBC such as temperature, presence of CO, O-2 and pressure. It is shown that under similar conditions, N2O is more readily reduced on the char surface than NO. Temperature was found to be a very important parameter in both NO and N2O reduction. It is generally agreed that both NO- and N2O-carbon reactions follow first-order reaction kinetics with respect to the NO and N2O concentrations. The kinetic parameters for NO and N2O reduction largely depend on the pore structure of chars. The correlation between the char surface area and the reactivities of NO/N2O-char reactions is considered to be of great importance to the determination of the reaction kinetics. The rate of NO reduction by chars is strongly enhanced by the presence of CO and O-2, but these species may not have significant effects on the rate of N2O reduction. However, the presence of these gases in FBC presents difficulties in the study of kinetics since CO cannot be easily eliminated from the carbon surface. In N2O reduction reactions, ash in chars is found to have significant catalytic effects, which must be accounted for in the kinetic models and data evaluation. (C) 1997 Elsevier Science Ltd.

Thermogravimetric analysis of carbon deposition over Ni/gamma-Al2O3 catalysts in carbon dioxide reforming of methane

Carbon formation on Ni/gamma-Al2O3 catalysts and its kinetics during methane reforming with carbon dioxide was studied in the temperature range of 500-700 degrees C using a thermogravimetric analysis technique. The activation energies of methane cracking, carbon gasification in CO2, as well as carbon deposition in CO2-CH4 reforming were obtained. The results show that the activation energy for carbon gasification is larger than that of carbon formation in methane cracking and that the activation energy of coking in CO2-CH4 reforming is also larger than that of methane decomposition to carbon. The dependencies of coking rate on partial pressures of CH4 and CO2 indicate that methane decomposition is the main route for carbon deposition. A mechanism and kinetic model for carbon deposition is proposed.

Carbon structure and porosity of carbonaceous adsorbents in relation to their adsorption properties

A number of carbonaceous adsorbents were prepared by carbonisation at 600 degrees C following acidic oxidation under various conditions. Effects of the chemical nature of the precursor, such as the ratio of aromatic to aliphatic carbons and oxygen content, on the chemical and structural characteristics of the resultant chars were investigated using C-13 NMR and Raman spectroscopy, respectively. The C-13 NMR spectral parameters of the coal samples show that as the severity of oxidation conditions increased, the ratio of aromatic to aliphatic carbons increased. Furthermore, it was also found that the amount of disorganised carbon affects both the pore structure and the adsorption properties of carbonaceous adsorbents. It is demonstrated that higher amount of the disorganised carbon indicates smaller micropore size. (C) 1999 Elsevier Science Ltd. All rights reserved.