Structure and combustion reactivity of inertinite-rich and vitrinite rich South African coal chars: quantification of the structural factors contributing to reactivity differences

Open Access
Author:
Louw, Enette
Graduate Program:
Energy and Mineral Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 10, 2012
Committee Members:
  • Jonathan P Mathews, Dissertation Advisor
  • Jonathan P Mathews, Committee Chair
  • Adrianus C Van Duin, Committee Member
  • Sarma V Pisupati, Committee Member
  • Semih Eser, Committee Member
Keywords:
  • Coal
  • char
  • reactivity
  • structure
  • molecular modeling
  • South Africa
Abstract:
ABSTRACT South Africa has large reserves of Permian-aged, high-ash yielding bituminous coals. Although these coals are of great economic importance, their behavior has not been well studied. This study compares the devolatilization and subsequent combustion behavior of an inertinite-rich (87.7% dmmf) and a vitrinite-rich (91.8% dmmf) South African coal, wet-screened to a narrow particle size distribution of 200 x 400 mesh. Pyrolysis chars were generated under rapid-heating conditions (104−105 °C/s) in a drop-tube reactor to closely resemble chars generated in pulverized combustion conditions. The inertinite-rich coal took 400ms to devolatilize in the drop-tube, compared to only 240ms for the vitrinite-rich sample. The combustion reactivities of the chars were correlated to a range of chemical, physical, and optical characteristics including the maceral differences and high ash yields. To evaluate the combustion reactivity, isothermal and non-isothermal thermogravimetrical analyses (TGA) were utilized. The vitrinite-rich char had on average 20% higher reaction rates than the inertinite-rich char under the various combustion conditions. To verify these results, temperature programmed oxidation was used and confirmed the higher reactivity of the vitrinite-rich char, where the vitrinite-rich char reached a higher maximum carbon dioxide signal at 590ºC compared to 650ºC for the inertinite-rich char. The char samples were de-ashed with HCl and HF acid which resulted in an increase in combustion reactivity. The maximum reaction rate of the high-ash (36%) inertinite-rich char increased with 80% after de-ashing. This suggested that the ash acted as a barrier, and the removal of ash most likely increased the access to reactive surface area. While the vitrinite-rich char was slightly more reactive than the inertinite-rich char the similar char reactivities once de-ashed are consistent with the emerging recognition that these interitite-chars are far more reactive than N. American inertinite chars The chemical and physical structures of the chars were characterized through a range of different analytical techniques to quantify the factors contributing to reactivity differences. The morphologies of the chars were characterized with SEM and optical microscopy, while quantitative information on the ordered nature of chars was obtained through XRD on de-ashed chars. The inertinite-rich coal experienced limited fluidity during heat-treatment, resulting in slower devolatilization, limited growth in crystallite height (11.8 to 12.6Å), only rounding of particle edges, and over 40% of mixed-dense type chars. The vitrinite-char showed more significant structural transformations; producing mostly (80%) extensively swollen crassisphere, tenuisphere, and network-type chars, and XRD showed a large increase in crystallite height (4.3 to 11.7Å). Nitrogen adsorption and small-angle X-ray scattering (SAXS) were utilized to compare the nitrogen surface areas and pore size distributions of the two chars. Both chars were mostly mesoporous but the inertinite-rich char had double the average pore size, which also resulted in a larger nitrogen surface area since nitrogen can only access surface areas in larger micropores. The BET surface area was 3.9 and 2.7m2/g (dry basis) for the inertinite- and vitrinite-rich chars respectively. SAXS data showed that the vitrinite-rich char had 60% higher frequencies of pores in the micropore range. Helium porosimetry indicated that the inertinite-rich coal and resultant char had higher densities than the vitrinite coal and char; 1.6 and 2.0 g/cm3, compared to 1.3 and 1.9g/cm3(dry basis). To further explore structural influences on reactivity HRTEM analysis of the chars was performed. It allows the direct observation of the distribution of structural features (layer size and stacking) of the chars. A new semi-automated approach for HRTEM image analysis and a new technique for quantifying distributions were developed in Photoshop and Matlab respectively, which enabled the quantitative characterization of the crystalline char structure. Distributions of layer size, interlayer spacing, stacking number, and layer orientation were obtained. By developing this computerized approach for image processing and statistical analysis, it is now possible to analyze more micrographs (larger sample size) in a shorter period of time with less user bias. Therefore, the usefulness of HRTEM to analyze carbonaceous structures is greatly improved. The shrinking core and random pore models were fitted to the reactivity data. These mathematical models, used for predicting the effect of process variables on the reaction rates of chars and carbons in various environments, are not able to predict behavior at conditions higher than 550⁰C. Consequently, there is a need for improved reactivity modeling approaches which would enable the incorporation of the multiple competing structural factors. Current molecular modeling tools and approaches do not allow testing of our understanding of char structure-behavior relationship since most of the above-mentioned structural features cannot be incorporated. Here, a new tool for the construction of molecular models was developed which enables the construction of large scale atomistic representations based on the distribution of structural features such as stacking. This novel approach was able to generate structures which capture some of the structural differences seen in experimental data between the inertinite-rich and vitrinite-rich coal chars. To explore reactivity differences between these two char models, a rules-based simulation approach was developed utilizing Material Studio and Matlab. Oxygen atoms were diffused through the char structure, and if an oxygen atom was within a critical distance of an edge carbon atom, it was assumed that they will react and consequently the carbon atom was deleted. These steps were repeated 15 and 18 times for the vitirinite-rich and inertinite-rich char structures, respectively. After these steps, 80% of the initial char structure was ‘combusted’. Although a simplified approach, the models correctly captured the relative reactivity differences for ash-free models, where the vitrinite-rich char combusted slightly faster than the inertinite-rich char. Given the similarties in the structure the presence of larger micropores (small mesopores) were likely the cause of the greater reactivity at zone I conditions. Increased automation and further enhancements to this approach would enable us to explore structure-reactivity relationship relationships rapidly while adjusting contributing parameters.