The utility of coal molecular models

There are a large number (> 125) of molecular representations for coals that span the rank range over seven decades. However, their utility has mostly been in representing chemical structural features, rather than in probing physical structure or exploring the structure-behavior relationship. This paper examines the utility of coal models and reviews the existing and emerging opportunities for coal models to contribute to coals effective utilization via demystification of the structure-behavior relationship. Coal models have been used to explore the coalification pathway, including contraction with water removal. Physical evaluations have probed the density of models as a check on their accuracy. Pore size distribution and sorption have been explored in simple pores and more recent work with carbon dioxide, water and methane sorption within the porous structure of large-scale (< 20,000 atoms) model. Pair distribution frequency and small angle X-ray scattering simulations have also been compared with experimental observations and offer an additional check on the constitution of the model structure. Simulated HRTEM and simulated (calculated) NMR spectra also exist. Models have been disassembled in efforts to represent the pyrolysis process, char formation, and char reactivity (including the role of ion-exchangable ions). Similar to the pyrolysis models, direct liquefaction has been explored with a pyrolysis style approach. Coal-solvent swelling, and coal-solvent solubility have also been explored. While considerable progress has accompanied improvements in computational power and software advances, it is the generation of the model that is the most significant barrier to the meaningful utility of these models. The ability to generate large-scale models (incorporation of molecular weight diversity and structural diversity) with new automation approaches, coupled with new dynamic force-fields that can simulate reactive events in complicated materials like coals, offers a new hope for the utility of coal or char molecular models to probe our understanding and aid in the scientific method rather than our current informed trial and error approach.     

Numerical modelling of the carrier gas phase in a laboratory-scale coal classifier model

The pneumatic transport of fine ideally combustible coal dust to the burner furnace is an important process in coal fired power plants. The strongly swirling air phase responsible for the particle separation and transport in a coal pulverising mill was characterised experimentally and numerically. Measurements of the swirl velocity component were taken in a scaled laboratory model of the device and compared to CFD model. In particular, an evaluation of the turbulence models used to describe the flow was performed. The modified isotropic k-epsilon turbulence models (RNG k-ε and Realizable k-ε) were compared to the anisotropic Reynolds stress model (RSM) and their ability to predict the bulk flow structure present in the classifier was assessed. The experiments showed that the swirling flow structure, responsible for coarse-fine particle classification, has several flow regimes which are governed by the areas it is bounded by. The numerical model predictions generally corroborate the results. However, a distinction in performance between the three models can be made based on accuracy, solution generation time and numerical stability. The RSM model predicted both the trends and magnitude the most accurately when compared to the isotropic models. However, the Realizable k-epsilon model, with its relatively low solution generation time, shows potential when using CFD as a classifier design optimization tool. The investigation has given some insight on single phase classifier flow and suggests a design improvement based on the results.     

Suppression of floatability of pyrite in coal processing by carrier microencapsulation

Carrier microencapsulation, CME, is a technique to form a thin layer of metal oxide or hydroxide on pyrite surface using a water soluble organic carrier combined with metal ions. The present study investigated the effect of CME using a tris-catecholato complex of Si⁴⁺, Si(cat)₃²⁻ on pyrite-coal separation by dynamic bubble pick-up experiments and Hallimond tube flotation experiments using coal, pyrite, and a coal-pyrite mixture. The mineral samples were treated in 0-5 mol m^− 3 Si(cat)₃²⁻ solutions at pH 4-9 at treatment times of 1-24 h. Dynamic bubble pick-up experiments showed that CME treatment converted the pyrite surface from hydrophobic to hydrophilic but did not affect coal's hydrophobic surface. The results of the Hallimond tube flotation experiments of a coal-pyrite mixture at pH 7-9 in the presence of kerosene as a collector showed that pyrite floatability was selectively suppressed after 1 h CME treatment with 0.5 mol^− 3 Si(cat)₃²⁻ while both coal and pyrite were floated without the treatment. This indicates that CME treatment is effective in suppressing pyrite floatability in coal-pyrite flotation.     

Thermal decomposition of coal fly ash by concentrated sulfuric acid and alumina extraction process based on it

The industrial applications of acidic leaching processes for alumina extraction from coal fly ash (CFA) have been limited due to its excessive usages of acid and fluoride. This limitation can be lifted by calcining the CFA with the concentrated sulfuric acid at high temperature. When CFA was mixed with the concentrated sulfuric acid of same amount and calcined at 300 °C, most of its aluminum components were transformed into aluminum sulfate. Excessive sulfuric acid was recycled by collecting sulfur trioxide produced during heating process. Morphological and X-ray diffraction evidence indicated that mullite inside CFA was thermally decomposed. Alumina and mullite were transformed into aluminum sulfate. A practical process of alumina extraction from CFA was developed based on this observation. The adoption of calcination process significantly reduced the sulfuric acid usage, shortened the process cycle, and eliminated the use of fluoride. The alumina extraction efficiency was improved up to 85%.     

原料粒度对煤矸石烧结砖结构和性能的影响

将煤矸石粉碎至三种不同的粒度,经过陈腐、压制成型、干燥、烧成后制得煤矸石烧结砖。利用X射线衍射仪、扫描电子显微镜研究烧结砖的物相组成和显微结构,并对其抗压强度和吸水率等性能进行了研究。结果显示,煤矸石原料的粒度和烧结保温时间对烧结砖的吸水率和抗压强度有较大影响。利用粒度小于0.15mm的煤矸石原料制备的烧结砖综合性能较好,1100℃烧结3h时,吸水率为17.63%,抗压强度达到22.47MPa,符合普通烧结砖的有关国家标准要求。     

Transferred arc plasma processed mullite from coal ash and bauxite

Coal ash disposal is one of the main problems in thermal power plant unit. Currently, a number of waste management systems are being tried for effective disposal of coal ash. In this paper, coal ash from thermal power plant unit was successfully utilized for synthesis of mullite ceramic by using transferred arc plasma processing (TAP) technique. For this purpose, bauxite was added with coal ash by 0, 25, 50 and75 weight ratio. The compositions were thoroughly mixed by ball milling and were melted in the transferred arc plasma torch at 10 kW input power level for 3 min. Argon was used as plasma forming gas. The phase and microstructure formation of the melted samples were investigated by XRD and SEM images. The results show that the additions of bauxite greatly influenced the phase formation of mullite during the processes.     

Physical, chemical and mineralogical characterisation of hydraulically disposed fine coal ash from SASOL Synfuels

Coal serves as the primary energy source in most parts of the world. It is a fact that coal combustion yields enormous quantities of fly ash some of which are either hydraulically placed or dry dumped. The current study attempts to provide a comprehensive characterisation of a disused alkaline fine coal ash dam (FCAD) towards assessing environmental impact, rehabilitation and utilisation potential. Fine coal ash refers to a combination of approximately 83% power station fly ash and 17% gasification and bottom ash fines (particles <250 μm) at SASOL Synfuels. The hydration products found in Weathered Fine Coal Ash (WFCA) using X-ray Diffraction (XRD) and Differential Scanning Calorimetry (DSC) are analcime, calcite, C-S-H gel, ettringite, hydrated gehlenite (Strätlingite), magnetite, periclase, pyrrhotite and sillimanite. High resolution Scanning Electron Microscope (SEM) results provide additional proof that hydration products are present in WFCA. No indication of appreciable leaching was given by X-ray Fluorescence (XRF) results except calcium and silicon. Thus evidence exists that pollutants from saline brines are immobilised in WFCA and an insight of reaction kinetics was obtained. High content of amorphous phase and lack of alteration in some geotechnical properties suggest that WFCA can be reutilised with lime addition to increase alkalinity and activate pozzolanic reactions.     

Effect of longwall face advance rate on spontaneous heating process in the gob area - CFD modelling

A commercial CFD software programme, FLUENT, was used to study the oxidation process of coal in the mined-out longwall (gob) area. A three-dimensional, single-phase model with a continuously advancing longwall face has been developed. For the model, the gob longwall area was designed on the basis of the actual longwall panel operating in the Ostrava-Karviná Coal Mines (OKD, Czech Republic). The behaviour of the coal to oxygen was modelled using the results arising mainly from the former laboratory-scale experiments with Czech bituminous coals. Basically, the technique of pulse flow calorimetry and measurements at a continuous airflow reactor were applied during the laboratory investigations. In the contribution, the main focus was to understand the effect of the longwall face advancing speed on the oxidation heat production as well as evolution of the gases in the gob area. Simultaneously, the effect of coal crushing in the mined-out area on the spontaneous heating process was examined.Numerical simulations confirmed the existence of a “favourable” zone for the onset and development of the spontaneous heating process in the gob area. The location and the maximal temperature reached in the “favourable” zone were found to be significantly affected by the advancing rate of the coalface. The slower the advancing rate is, the higher the maximal temperature and smaller the depth of the “favourable” zone in the gob area are. When the rate drops to a certain “critical” value, spontaneous heating turns to flammable combustion of the coal. The value of the “critical” advancing rate was confirmed to increase if the grain size of the coal left in the gob decreases. Numerical examinations of carbon monoxide concentrations then proved that small incidents of spontaneous heating could occur in the gob area that need not be detected in the airflow of the longwall tail gate.     

Selective adsorption of organic sulfur in coal extract by using metal-loaded carbon

The objective of this study was to develop an efficient cleaning process for hydrocarbon resources. We investigated the removal of organic sulfur from coal extract by using metal-loaded carbons. Tetralin and tetralin/phenol mixture were used for thermal extraction of Illinois No. 6 coal at 375 °C with a holding time of 1 h, and we examined the effects of temperature on product yield and sulfur-removal behavior using metal-loaded carbons. A decrease in the n-hexane insoluble fraction was observed at a temperature range from 200 °C to 250 °C in the extracts of tetralin and tetralin/phenol mixture. The organic sulfur content in the n-hexane insoluble fraction drastically decreased with increasing temperature. The organic sulfur in the n-hexane insoluble fraction was reduced above 200 °C, while the sulfur content reached 30-35% at 375 °C for both tetralin and tetralin/phenol mixture extracts. We investigated the effect of holding time on the extent of the coal extract desulfurization using Ni-loaded carbon. The sulfur content in the coal extract decreased as the holding time increased. The rate of coal extract desulfurization using tetralin/phenol was slower than that obtained using tetralin. The coal extract obtained using the tetralin/phenol mixture contained a heavier polar fraction, which may have impeded the adsorption. All organic sulfur in the coal extract could be successfully removed Ni-loaded carbon at a treatment temperature of 350 °C and a holding time of 8 h. No appreciable changes were observed in the content of carbon, hydrogen, or nitrogen at any temperature or holding time in the ultimate analysis of the n-hexane insoluble fraction treated with Ni-loaded carbon. The results demonstrated that Ni-loaded carbon was much effective in removing organic sulfurs from the coal extract.     

正交设计法优化配煤

采用正交设计法进行配煤试验,研究各类煤的配比与焦炭强度的相关性。用正交设计法可以大大减少试验次数。根据试验得出的M40、M10、大于40mm和大于25mm粒度的数据,进行统计分析,选择出最佳配煤比为：弱黏结煤0,瘦煤25％,肥煤18％,焦粉3％,焦煤54％。