The influence of particle size on the pore development of coal chars during gasification

Abstract

In recent times, there is a strong drive towards the reduction and dependency of coal for the production of energy globally, and specifically in Europe and Japan. However, coal utilisation, includes gasification technology, will remain an important energy source in countries like China, India and South Africa for the coming decades. Environmental impact of coal utilisation is seen as the greatest driving force to reduce coal dependency and consumption, with optimisation of coal conversion technologies aimed at increasing technology efficiency and thereby minimising the environmental impact of the operation. The fundamental knowledge of the heterogeneous coal conversion processes is hereby pivotal in the retrofitting and design of clean coal technologies. One aspect in the basic description of coal reaction processes is the change in the solid structure of coal as the carbonaceous material reacts. The changing solid structure is known to influence the carbon reactivity and also impacts internal mass transfer limitations, mostly studied in detail for powdered coal chars. Studies have predominantly focused on single reaction gases, where pore structure development is studied by both gas adsorption and mercury porosimetry techniques. It is hereby useful to combine results from different measuring techniques in order to evaluate a wider pore size range. However, the influence of time-temperature histories on the char behaviour and the dissimilarities between various techniques complicate the application of combining these methods. Small angle scattering techniques have been used to probe the pore structure of coal and char, with the main advantage of small angle scattering measurements probing a wide pore size range (from Ångström to millimetres) with a single experiment, mitigating the difficulties encountered when combining multiple measuring techniques required to evaluate a similar pore size range. A comprehensive study was therefore proposed to evaluate the pore development arising from CO2 and steam gasification using SAXS measurements, focusing on evaluating the effect of reagent gas, particle size and temperature. A typical inertinite rich South African coal with low ash values from the Witbank coalfield was selected, with the parent coal sample being a single source coal (Witbank seam 4) that was density separated at < 1500 kg/m3, with particle size > 30 mm. A low ash coal was selected in order to reduce the influence of mineral matter on experimental results. A detailed coal sampling procedure was developed to prepare representative samples for parent coal characterisation, smaller particle samples and 20 mm particles from the 300 kg bulk sample. Parent coal characterisation results showed typical values for Witbank seam 4 washed coal, specifically resulting in a low ash coal with high calorific value and petrographic classification as an inertinite-rich, medium rank C bituminous coal. A detailed char preparation procedure was developed to reduce the influence of charring parameters on experimental results. Char preparation was conducted at 1000°C and further processed using a detailed sampling procedure which resulted in a three particle size classification (75 μm, 2 and 20 mm). The resulting chars were characterised and generally, it is concluded that the chemical composition of the three particle sizes does not differ significantly. However, pore structure parameters obtained for different sized chars using low pressure CO2 gas adsorption measurements show particle size dependent pore structure with an increase in micropore volume and surface area as char particle size increases. Partially gasified chars were prepared in a thermogravimetric analyser for the three sized samples using steam and CO2. The lowest gasification temperature (800°C for steam and 850°C for CO2) was experimentally proven to be in the chemical reaction controlled regime for the smallest particle size. The highest gasification temperatures were chosen as 950 and 1000°C for steam and CO2 gasification, to illustrate the effect of mass transfer limitations. The partially converted samples were scanned at the Australian Synchrotron in Melbourne using two detector positions, which resulted in a Q-range probed from 1.698•10-3 to 0.8333 Å- 1 and an assumed pore diameter between 0.25 and 147 nm. A novel ratio analysis technique was developed to study the pore development of individually sized pores from SAXS experimental data without requiring non-fractal pore modelling. Three separate ratios were specified from the SAXS data to compare results of different scans: scaled intensity (SI), intensity conversion ratio (ICR) and scaled conversion intensity ratio (SCIR). The scaled intensity allows direct comparison of changes in pore size distributions between samples. The ICR was calculated by dividing the scattering intensity of the gasified char at each Q by that of the unreacted char. The SCIR was calculated by dividing the scaled intensity of the gasified char with the scaled intensity of the char (at each Q). The proposed ratio analysis technique was used to qualitatively compare the relative extent to which the number of pores at different sizes grow, allowing multiple samples comparison, without requiring non-fractal modelling to determine pore volume and/or surface area.. Chars were partially converted to prepare samples with specific conversions of 10, 25, 35 and 50%, with a precise time required for each specified conversion. The influence of reagent gas on the pore development was evaluated using partially gasified 75 μm chars. Comparing the trends observed using large lumped pore ranges (IUPAC) from SAXS results compares well to literature, with broadening of meso- and macropores from onset of CO2 gasification being the only exception, likely to be maceral related. Further, steam shows greater development of pore structure for all conversions of 75 μm chars, with increased pore volume and surface area, compared to CO2. The ratio analysis results showed that, for both CO2 and steam, size dependent pore growth rate. In particular, the intermediate pore sizes (between 1 and 40 nm) showed increased pore growth when compared to other pore sizes. Compared to CO2, steam gasification resulted in an increased pore growth of pore sizes between 1 and 40 nm. The ratio analysis technique further resulted in classification of critical cross over pore size, where the critical cross over pore size indicates at what size the pore generation rate is observed to be below the largest pore size. Comparison of the critical cross over pore size results for steam and CO2 gasification showed that a smaller cross over pore size (0.6 nm for steam, compared to 1 nm for CO2) and a smaller critical cross over value is observed for steam, which may be a direct consequence of the smaller kinetic diameter of water molecules. It was also observed that particle size influenced the pore development of CO2 gasified chars over the entire pore scale studied, specifically the development of micro- and macropores, which prevailed over the intermediate sized pores. For steam gasification, particle size only influenced the growth rate in the macropore range. For both the steam and the CO2 gasified chars, larger particle sizes resulted in a decrease in growth rate for < 0.6 nm pores as well as an increase in critical cross over pore size. A novel application to evaluate the radial pore development of a 20 mm particle was developed, with the sampling spanning from the surface to the interior of the particle. The surface sample showed the greatest pore development over the entire pore range evaluated, followed by interior and centre. For both CO2 and steam chars, the radial changes in growth rate for individual pore sizes confirmed intra-particle mass transfer limitations for 20 mm particles. The results obtained during this study further demonstrates the advantages of using small angle scattering techniques over other techniques, due to increased pore size range and pore size resolution probed using a single measurement. Further, this study developed a novel ratio analysis technique to elucidate pore structure development of gasified char, with results showing different sized pores grow at different rates. The techniques developed here gave greater insight into dependence of reagent gas, with steam and CO2 studied here, on pore development of gasified chars. On a pore scale, the results for micropore surface area development suggest that reported trends (increased surface area development for steam compared to CO2) are mainly due to differences in pore growth of pores smaller than 7 nm and could be a direct implication of the smaller kinetic diameter of water, compared to CO2. The detailed evaluation also highlights the additional complexities that needs to be addressed in advanced dynamic single particle reaction modelling