Evaluation Of Coal Char Gasification Kinetics And Pore Development In High Pressure Steam And Carbon Dioxide

Author : Vijayaragavan Krishnamoorthy
Publisher :
ISBN 13 :
Total Pages : pages
Book Rating : 4.:/5 (15 download)

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Download or read book Structural Characteristics and CO2 Reactivity of Partially Gasified Pittsburgh No.8 Coal Chars Generated in a High-pressure, High-temperature Flow Reactor written by Vijayaragavan Krishnamoorthy and published by . This book was released on 2018 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Integrated gasification combined cycle (IGCC) is an advanced power generation technology based on gasification of coal or solid fuels. Despite many commercial operations, the knowledge of char gasification rates at high pressures and temperatures, crucial to the design and troubleshooting of the gasifiers, are relatively unknown. While many kinetic studies have been performed at atmospheric pressure and low heating rates, there are few studies that examined the reactivity of chars generated at high temperatures and elevated pressuresGasification rate of chars in entrained-flow gasifiers is dependent on both intrinsic reactivity and the gas diffusion rate of reactants into pores. Therefore, the knowledge of intrinsic reaction rate and the structural features of the char are necessary for developing a kinetic model. The aim of the thesis is to determine the intrinsic reactivity and the structural features of the chars generated at elevated pressures and temperatures pertinent to conditions of the entrained-flow gasifiers. A series of interrelated studies were conducted to characterize the gasification behavior of a widely used Pittsburgh No,8 coal. To generate chars under conditions similar to that of the gasifier, a 20 kW high-pressure, high-temperature flow reactor (HPHTFR) was designed to operate up to 1650C and 30 bar. The chars obtained at various temperatures, pressures, and pyrolysis atmospheres were characterized for physical and chemical structure using surface area analyzer, XRD, Raman, and morphological analysis. The CO2 kinetics on chars were obtained using a high pressure thermogravimetric analyzer (HPTGA). The structural properties and intrinsic kinetics of chars widely reported in the literature were generated in inert atmospheres. However, the pyrolysis of feedstock occurs in the presence of reaction gas. This difference can affect char structural properties and intrinsic reactivity. To determine the role of pyrolysis atmosphere, chars were generated in three different atomspheres-CO2/N2, Ar and N2- at 1100C and 6.2 bar. The chars generated in the CO2/N2 atmosphere showed higher conversion compared to that of chars generated in N2 and Ar atmospheres. The increased conversion in the CO2/N2 atmosphere was attributed to increased gasification of tar/soot. While the volatile yield showed some difference, char properties such as surface area, swelling ratio, defects to graphitic band ratio and crystallite sizes showed no difference. The kinetic parameters of the chars were obtained using the nth order model. The activation energy was found to be independent of pyrolysis atmospheres. The order of reaction was found to be significantly affected by the pyrolysis atmosphere. The order of reaction followed the trend: CO2/N2> N2 Ar. The order of the reaction was found to correlate with surface area evolution.Gasification of coal can be impacted by the organic and inorganic compositional heterogeneity, which further impact the char morphology, and the intrinsic reactivity. To account for the compositional heterogeneity, chars were generated from various size fractions (-106+75, -150+106, -212+150, -420+212 m at 1300C and 11.3 bar) and density fractions (1.3 g/cc, 1.3-1.6 g/cc, 1.6g/cc of -106+75 m at 1300C and 11.3 bar). Chars were also generated over a range of temperatures (1100, 1300, and 1400C at 11.3 bar for the -150+106 m fraction), pressures (3.4, 6.2, 11.3, 15.5, and 21.7 bar at 1300C for -150+106 m fraction) to study the effect of temperature and pressure on char structures and reactivity. Chars were characterized for morphology, pore structure (i.e. surface area and pore volume), reflectance, and reactivity using oil immersion microscopy, N2 adsorption technique, reflectance microscopy, and thermogravimetric analyzer, respectively. The swelling ratio, pore volume, and surface area increased up to certain pressure while these parameters decreased with particle size and density fraction. The intrinsic reactivity of chars increased with inorganic matter and feed particle size, while it decreased with increase in char generation temperature. The intrinsic gasification rate is an important parameter for designing a kinetic model. Chars were obtained by partially gasifying Pittsburgh No.8 coal in CO2 atmosphere at 1300C and over a range of pressures (3.4, 6.2, 11.3, 15.5, and 21.7 bar) in the HPHTFR. The intrinsic reaction rate of those chars with CO2 was obtained at the char generation pressure using the HPTGA. The kinetic parameters were obtained using the nth order model. The intrinsic reaction rate, and activation energy were found to be independent of the char generation pressure. The order of reaction was obtained by varying CO2 partial pressures. The order of reaction decreased with increase in char generation pressure. The comparison of initial char with the char obtained at ~20% conversion in the HPTGA for surface area and pore volume showed that the reaction primarily occurs in microporous regions. The order of reaction also closely followed the surface area during gasification in the HPTGA.Through this research, a comprehensive assessment of the entrained-flow gasification behavior of Pittsburgh No.8 coal has been performed using proven experimental techniques under conditions of industrial interest. The structural features and kinetics were obtained. The generated data provide optimum, and trends that can be used as direct inputs to kinetic modelling and gasifier design applications.