Molecular dynamic simulation of spontaneous combustion and pyrolysis of brown coal using ReaxFF
应用ReaxFF进行褐煤自燃与热解的分子动态模拟
Fuel, Volume 136, 15 November 2014, Pages 326-333
Abstract: Brown coal is a soft sedimentary organic rock which is complex in nature and is the main source of energy production. In this work, we have studied the combustion and pyrolysis of brown coal using reactive molecular dynamic (MD) simulation. To make the large scale (above 1000 of atoms) reactive system practical, ReaxFF MD system was used which is 100 times faster than the methods of quantum mechanics (QM). To examine the pyrolysis/combustion process and initiation mechanism of brown coal, a fuel lean (ϕ = 2), fuel rich (ϕ = 0.5) and stoichiometric (ϕ = 1) conditions were used in this work. The temperature used was high as per experimental reported condition so as to enable chemical reaction within a computationally affordable time. It was observed that the combustion of brown coal was initialized by thermal degradation subsequently forming small fragments. As the brown coal molecule oxidizes or thermally decomposes, hydrogen is abstracted and reacts with oxygen to form large amount of H2O molecules. Furthermore, the combustion of coal was also studied in the same conditions namely fuel rich, fuel lean and stoichiometric. Potential energy gradually decreases at high temperature while it was the reverse in pyrolysis. It was found that the effects of densities are lesser as compared to temperature. Some important intermediate like formaldehyde (HCHO) generated during the simulation reaction agreed well with the experimental data reported in literature.
An electro-chemical method for determining the susceptibility of Indian coals to spontaneous heating
确定印度煤自发热倾向性的电化方法
International Journal of Coal Geology, Volumes 128–129, 1 August 2014, Pages 68-80
Abstract: Spontaneous combustion is an inherent problem to the coal mining industry. The paper describes an electro-chemical method called wet oxidation potential technique for determining the susceptibility of coal to spontaneous combustion. Altogether 78 coal samples collected from thirteen mining companies spreading over most of the Indian Coalfields have been used for this experimental investigation and 936 experiments have been carried out by varying different experimental conditions to standardize wet oxidation potential method for wider application. The results of wet oxidation potential (WOP) method have been correlated with the intrinsic properties of coal by carrying out proximate, ultimate and petrographic analyses of the coal samples. Experimental results are first analyzed with principal components analysis and then by multiple regression analysis. Results obtained from this method have been compared with one of the widely used method, i.e. crossing point temperature (CPT) in India. The experimental investigation clearly indicates that WOP method with 0.2 N KMnO4 in 1 N KOH solution at 45 °C (susceptibility index PD12) should be used for finding out the susceptibility of coal to spontaneous combustion. PD12 results are well corroborated with CPT results.
CFD simulations of the effect of wind on the spontaneous heating of coal stockpiles
风对煤堆自发热影响的CFD模拟
Fuel, Volume 118, 15 February 2014, Pages 107-112
Abstract: A commercial CFD software program, fluent, was used to study the effect of wind on the spontaneous heating process of a coal stockpile. A two-domain model was developed to simultaneously solve the governing equations of an open porous medium (coal stockpile domain) situated in a homogeneous atmosphere (wind flow domain). Simulations with air blowing from a fixed direction as well as real fluctuations of the airflow both in velocity and direction were performed.
Numerical calculations confirmed the promoting role of wind on the dynamics of the development of spontaneous heating. Under the conditions of the simulations, three possible shifts of the hot spot in the stockpile were distinguished when coal undergoes the self-heating process:
(1)Shift of the hot spot to the pile surface when spontaneous heating of coal is in progress.
(2)Movement of the hot spot inwards the stockpile as the wind speed increases.
(3)Transfer of the hot spot from the upper part of the stockpile to the lower part when the self-heating process progresses. Such movement was found for wind speeds ⩽3 m s−1 and clearly is mainly connected with the effect of buoyancy.
The properties of Çan Basin coals (Çanakkale—Turkey): Spontaneous combustion and combustion by-products
Çan盆地煤炭(Çanakkale—土耳其)特性：自燃及其副产品
International Journal of Coal Geology, Volume 138, 15 January 2015, Pages 1-15
Abstract: The goals of this study were to investigate the susceptibility of Çan Basin (Çanakkale—Turkey) coals to spontaneous combustion and to determine the composition of the gas produced from the coal during combustion. Coal properties were determined using burned and partly burned coal samples; gas samples were analyzed for their composition. The mineralogical variations of burning coals were also investigated. Our results indicated that the pyrite content of Çan Basin coals is a significant factor for promoting combustion in addition to rank and moisture. X-ray diffraction (XRD) and scanning electron microscope (SEM) analyses indicated that the coal samples contained pyrite, quartz, cristobalite, tridymite, kaolinite, amorphous matter, and gypsum. Fumarolic minerals (sulfur blooming and ammonium chloride) forming on the surface of coal seams were monitored. Elements including beryllium, fluorine, scandium, vanadium, cobalt, nickel, copper, zinc, arsenic, selenium, zirconium, molybdenum, tungsten, mercury, tantalum, lead, and uranium were found to be higher in Çan coal samples than the world average. The concentration of arsenic (max. 3319.7 μg/g) was relatively high and is the major hazardous element in the region.
Gases emitted from coal-fire vents in Çan coalfields were found to consist of a complex mixture of hydrocarbons, greenhouse gases, and toxic concentrations of carbon monoxide (CO), hydrogen sulfide (H2S), and benzene. Hydrocarbon concentrations ranged from 77 to 92%, and the dominant hydrocarbon gas was methane. Ethane (0.3 to 2.1%) and propane (0.2 to 1.4%) were also detected. Hazardous compounds such as 5-methyl-3-propyl-1,2-oxazole (C7H11NO), ethanediimidic acid, 1,2-dihydrazide (C2H8N6), and 2,3-dihydrofuran (C4H6O); high concentrations of nitrogen (N2) (max. 6.8%) and carbon dioxide (CO2) (max. 18.2%); and low concentrations of carbon monoxide were also determined. Greenhouse gases (CO2 and methane (CH4)) from burning coal beds may contribute to climate change and alter ecosystems. Gas components including furan, H2S, CO, carbon disulfide (CS2), benzene etc., can be hazardous to human health, even in trace amounts. As a result, the uncontrolled release of pollutants from burning coal beds presents potential environmental and human health hazards.
Petrographic characterization of coals as a tool to detect spontaneous combustion potential
煤岩相特征化在煤自燃势探测中应用
Fuel, Volume 125, 1 June 2014, Pages 173-182
Abstract: Textural features of 25 worldwide coals were studied after slow oxidation processing (0.5 °C min−1 from 20 to 250 °C in air) using oil immersion microscopy and image analysis techniques. The characterization of samples, before and after oxidation, showed important changes in vitrinite reflectance with high reactive coals, which also related to their intrinsic self-oxidation potential. The morphology of the coal particles was also altered after the oxidation, to produce at least six different morphotypes. Particles with ‘homogeneous change of reflectance’ and particles with ‘oxidation rims’ were predominant in the samples studied, which related to boundary reactive conditions (kinetic and diffusion control of the reaction respectively). These textural characteristics indicate how particles interacted with oxygen at low temperatures, which could be used to predict the most probable pathway during the early stages of oxidation which could then lead to a spontaneous combustion event. The magnitude of the reflectance change and the morphological characteristics of samples studied were also related to the reactivity properties, providing an additional source of information to identify coals prone to spontaneous combustion.
Investigation of the spontaneous combustion susceptibility of coal using the pulse flow calorimetric method: 25 years of experience
脉冲流量热法在煤自燃倾向性研究中的应用：25年之经验
Fuel, Volume 125, 1 June 2014, Pages 101-105
Abstract: The paper summarises 25 years of experience with pulse flow calorimetry (PFC) as a method to assess the potential of coals to spontaneously heat up using the values of oxidation heat q30 (W kg−1). During the period, about 300 coals of various rank and with natural moisture content were investigated, the maximum propensity to oxidation being found in subbituminous coals with moisture content of about 20%. The highest value of oxidation heat q30 was found to be 10.5 W kg−1. Oxidatively altered bituminous coals are presented as coals of evidently increased ability to oxidize, and the reasons for the increased oxyreactivity are reported. Limiting values of q30 heat are then given to rank the coals according to the categories of self-heating risk. Finally, a comparison between oxidation heat q30 and the values of the initial rate of heating (IRH) of adiabatic oxidation tests is presented.
Calorimetric and heat transfer studies of the spontaneous combustion of two low carbon fuels
两种低碳燃料自燃的量热法及传热研究
Journal of Loss Prevention in the Process Industries, Volume 32, November 2014, Pages 44-51
Abstract: The self-heating and heat transfer characteristics of creosote-treated wooden railway tie dust and roofing asphalt shingle particles, which are considered as low carbon fuels, are investigated. Self-heating was measured with isothermal calorimetry and thermal conductivity was measured using a guarded hot plate. The self-heating of petroleum coke and poplar wood pellets were also measured as reference materials. Self-heating models and heat transfer parameters were then defined assuming constant moisture content. The self-heating of petroleum coke was found to be similar to coal, while both poplar pellets and railway tie dust were found to be more reactive compared to oven test results of similar materials. As both poplar pellets and railway ties both had significant water content, the higher reactivity was attributed to the effect of moisture. Furthermore, critical conditions for spontaneous combustion were evaluated with the Frank–Kamenetskii parameter, assuming an ambient temperature of 40 °C. It was found that cubic volumes of railway tie dust and asphalt particles, 1.6 m and 58 m to a side, respectively, would spontaneously combust; however, compaction effects associated with asphalt softening may invalidate the assumptions of the Frank–Kamenetskii spontaneous combustion theory.
Intrinsic reaction kinetics of coal char combustion by direct measurement of ignition temperature
通过点火温度直接测量进行煤焦燃烧本征反应动力学研究
Applied Thermal Engineering, Volume 63, Issue 2, 22 February 2014, Pages 565-576
Abstract: A wire heating reactor that can use a synchronized experimental method was developed to obtain the intrinsic kinetics of large coal char particles ranging in size from 0.4 to 1 mm. This synchronization system consists of three parts: a thermocouple wire for both heating and direct measurement of the particle temperature, a photodetector sensor for determining ignition/burnout points by measuring the intensity of luminous emission from burning particles, and a high-speed camera–long-distance microscope for observing and recording the movement of luminous zone directly. Coal char ignition was found to begin at a spot on the particle's external surface and then moved across the entire particle. Moreover, the ignition point determined according to the minimum of dT/dt is a spot point and not a full growth point. The ignition temperature of the spot point rises as the particle diameter increases. A spot ignition model, which describes the ignition in terms of the internal conduction and external/internal oxygen diffusion, was then developed to evaluate the intrinsic kinetics and predict the ignition temperature of the coal char. Internal conduction was found to be important in large coal char particles because its effect becomes greater than that of oxygen diffusion as the particle diameter increases. In addition, the intrinsic kinetics of coal char obtained from the spot ignition model for two types of coal does not differ significantly from the results of previous investigators.
