Numerical study on effects of coal properties on spontaneous heating in longwall gob areas

A computational fluid dynamics (CFD) study was conducted to model effects of coal properties on the potential for spontaneous heating in longwall gob (mined-out) areas. A two longwall panel district using a bleeder ventilation system was simulated. The permeability and porosity profiles for the longwall gob were generated from a geotechnical model and were used as inputs for the three-dimensional CFD modeling. The spontaneous heating is modeled as the low-temperature oxidation of coal in the gob using kinetic data obtained from previous laboratory-scale spontaneous combustion studies. Heat generated from coal oxidation is dissipated by convection and conduction, while oxygen and oxidation products are transported by convection and diffusion. Unsteady state simulations were conducted for three different US coals and simulation results were compared with some available test results. The effects of coal surface area and heat of reaction on the spontaneous heating process were also examined.     

A model for the spontaneous heating of coal

As part of an investigation into the spontaneous heating of coal piles, a one-dimensional model has been developed to describe the spontaneous heating process at relatively low temperatures (< ≈ 100 °C). The ultimate unsteady-state model takes into account depletion of oxygen and production of heat by chemisorption of oxygen in the coal, transport of oxygen by diffusion and convection and transport of heat by conduction, convection and evaporation/condensation of coal moisture. It consists of four differential equations, for conservation of oxygen mass, of coal moisture and of heat and rate of reaction of oxygen with coal. Calculations using data from laboratory and field experiments give results that describe the process of spontaneous heating semiquantitatively. The most important parameters in the process of spontaneous heating, particularly for the time between stacking and spontaneous ignition, are the porosity of the pile (degree of compaction), the initial temperature of the coal and the evaporation and condensation of coal moisture. The influence of other parameters (e.g. reactivity of the coal, thermal conductivity) is much less pronounced.     

Interpretation of biomass gasification yields regarding temperature intervals under nitrogen–steam atmosphere

Gasification of some agricultural waste biomass samples (sunflower shell, pine cone, cotton refuse, and olive refuse) and colza seed was performed using a thermogravimetric analyzer at temperatures up to 1273 K with a constant heating rate of 20 K/min under a dynamic nitrogen–steam atmosphere. Derivative thermogravimetric analysis profiles of the samples were derived from the non-isothermal thermogravimetric analysis data. Gasification yields of the biomass samples at temperature intervals of 473–553 K, 553–653 K, 653–773 K, 773–973 K, and 973–1173 K were investigated considering the successive stages of “evolution of carbon oxides”, “start of hydrocarbon evolution”, “evolution of hydrocarbons”, “dissociation”, and “evolution of hydrogen”, respectively. Although, there were some interactions between these stages, some evident relations were observed between the gasification yields in a given stage and the chemical properties of the parent biomass materials.     

The role of desorption of moisture from coal in its spontaneous heating

As a part of investigations on the influence of humidity on the initial stage of the spontaneous heating of coal, the effect of desorption of moisture from the coal was studied. Laboratory experiments with different coals were carried out under conditions where desorption of water from the coals by air was certain to take place. Thermal changes were measured by a specially designed calorimeter in isothermal conditions, mostly at 30 °C. The results showed that in each case there occurred a cooling of the coal, indicating that the rate of heat loss from a coal due to this process was greater than the rate of heat release due to oxidation of the coal. The data suggest that moisture desorption acts as an inhibitor to the spontaneous heating of coal. For a particular coal, the rate of heat loss increases with the increase in the equilibrium humidity deficiency of the air. The effects of rank, particle size and weathering on the process are also discussed.     

Chemical process opportunities for vibrated powders 3. Provisional design for an experimental sidewards flying bed

At a scale suitable for laboratory experimentation, a provisional mechanical design illustrates a concept for a “sidewards flying bed” (SFB), a layer of particulate matter held upon the inner wall of a cylinder that rotates at varying speed. Acting upon the layer are centrifugal and tangential forces whose patterns of variation create time intervals during which the latter is the greater. During such intervals, the layer is postulated to respond by “flying sidewards.” In a laboratory SFB set-up, experiments can elucidate a new area of non-steady-state soil mechanics, with outcomes bearing upon a full-scale SFB's chemical processing potential. An important question is, what will be the delay in the SFB layer's taking flight after the moment tangential force just begins to exceed centrifugal? For the SFB design to be successful, this delay should be significantly shorter than a time interval during which the tangential force dominates.If experiments were to show this not to be so, a “what-then?” might be to install several stationary rakes near the cylinder's inner wall, parallel to its axis. Loosening the particulate layer as the cylinder turns, these should advance the timing of the layer's flight.Achieving adequate flight in an experimental SFB could point to a design at a commercial scale capable of promoting rapid local mixing — more rapid than that afforded by conventional vibrated or fluid beds. Such should become a strong candidate for applications requiring rapid heating of hydrocarbonaceous matter followed by quick separation of vapor product and carbonaceous residue (e.g., flash pyrolysis of coal or oil-shale). The SFB's competencies should include ability to treat moieties of especially large size (e.g., for producing a smokeless solid fuel from coal, for heat-treating a metal part or a polymer, for flash-cooking foodstuffs without oil, for drying large particulates); ability to treat highly heterogeneous matter (e.g., for burning or gasifying comminuted biomass or solid wastes); and ability to promote excellent heat transfer between SFB and stationary surfaces embedded therein (e.g., in an air-cooled SFB steam-condenser).     

Characterization of unburned carbon present in coarse gasification ash

Depending on the mode of operation and quality of the feed material, discrete unburned carbon particles are evident in coarse gasification ash emanating from a commercial gasifier. Black, partially reacted carbon particles in the size range −13 + 4 mm were randomly hand picked from grab sample following a gasifier shutdown. These particles were classified into three major categories namely: unburned carbon, carbonaceous shale, and “shrinking core” particles. The unburned carbon particles were further macroscopically subdivided into remnant “coal” particles, solid carbon, layered carbon, and porous carbon. The unburned carbon particles were characterized using a petrographic analysis, reflectance analysis, chemical analysis, TGA mass loss curves, pore volume and surface area, and mineral characterization. The carbon particles were compared to the feed coal. The petrographic-based characterization technique as developed for the characterization of coarse unburned carbon particles indicated that remnant coal, devolatilised coal, highly porous isotropic carbon, dense anisotropic carbon, and variations in-between occurred in the coarse gasification ash sample.     

Modelling pyrolysis with dynamic heating

In literature, the reaction kinetic of pyrolysis process is often determined and modelled under constant heating rates. In reality, the heating rate of an industrial pyrolysis process is difficult or often not necessary to be kept constant. The variation of heating rate at different reaction stages, termed “dynamic heating”, governs the pyrolysis performance such as production rate, energy consumption, product quality, etc. In this work, pyrolysis progress with dynamic heating is being studied. The rate and reaction heat of tyre pyrolysis at different heating rates are obtained experimentally. A transient model considering the effect of dynamic heating was then developed and compared with the conventional static heating model. Results show that a higher heating rate favours the production of volatiles and shifts the overall pyrolysis heat flow to more endothermic. The significance of the dynamic heating model was observed for processes with large feed size and/or with high heating rate.     

近距离采空区下无煤柱连续开采技术的研究与应用

基于近距离采空区下无煤柱连续开采过程中顶板垮落、初次来压及周期来压对工作面巷道的影响,分析回采上覆煤层后底板岩层移动变化规律,研究近距离采空区下巷道集约化布置沿空留巷方式,总结近距离采空区下沿空留巷围岩矿压变化规律,合理选择沿空留巷巷道支护形式,探索并实践近距离采空区下无煤柱连续开采技术,解放了近距离采空区下采区区段及其他保护煤柱资源,本质上做到了采区的集约集中化高效开采及资源高效利用,具有推广廊用价倌．     

Fractionation of a coal tar pitch by ultra-filtration, and characterization by size exclusion chromatography, UV-fluorescence and laser desorption-mass spectroscopy

Ultra-filtration (UF) provides a new way of generating narrow bands of sample in the fractionation of coal and petroleum-derived liquids. It allows larger quantities of high-mass fractions to be recovered, making more detailed investigations possible, through the use of techniques requiring larger amounts of sample. In this work, UF-separated fractions have been used to study molecular mass distributions of a coal tar pitch, used as laboratory standard. The pitch was fractionated by solvent solubility into three fractions. These were further fractionated by ultra-filtration, using membranes specific to protein molecular sizes classed as “1 kDa”, “5 kDa”, “10 kDa” and “100 kDa”. Planar chromatography was used as a tertiary fractionation method, to sub-divide the UF-fractions. The various fractions were examined by size exclusion chromatography (SEC), UV-fluorescence spectroscopy and laser desorption-mass spectrometry (LD-MS). There was good agreement between mass estimates based on SEC and LD-MS of the smaller UF-fractions, with evidence for the presence of material with molecular masses ranging between 800 and 10,000 u. Examining the largest UF fractions of the pitch-pyridine-insoluble sample also gave clear evidence for material with molecular masses above 10,000 u. Taken together, however, the LD-MS data showed progressively diminishing differences, as the sizes of the UF membranes, and the likely molecular masses of the sample fractions, increased. One likely explanation is incomplete sampling during the laser desorption procedure. The evidence suggests that the upper mass limit detectable for these and similar samples by LD-MS has been reached. Despite these reservations, LD-MS appears as the best method to date, for investigating the mass ranges of samples derived from coal tar pitch and heavy petroleum fractions.     

Methane desorption from a coal-bed

We study the desorption of methane from a coal-bed. A model taking into account both methane diffusion in coal - blocks and its filtration through the system of open pores and cracks is developed. Methane pressure in the coal-bed is found for an arbitrary instant of time. Dependency of the rate of methane release upon the block size, open and closed porosity, viscosity, solubility, bed pressure and temperature is established. We derive the effective coefficient of diffusion of methane in blocks containing closed pores filled with gaseous methane. It is shown that at a hindered diffusion methane is distinctly divided into the “quick” and the “slow” one.     

Biodesulphurized subbituminous coal by different fungi and bacteria studied by reductive pyrolysis. Part 1: Initial coal

One of the perspective methods for clean solid fuels production is biodesulphurization. In order to increase the effect of this approach it is necessary to apply the advantages of more informative analytical techniques. Atmospheric pressure temperature programming reduction (AP-TPR) coupled with different detection systems gave us ground to attain more satisfactory explanation of the effects of biodesulphurization on the treated solid products.Subbituminous high sulphur coal from “Pirin” basin (Bulgaria) was selected as a high sulphur containing sample. Different types of microorganisms were chosen and maximal desulphurization of 26% was registered. Biodesulphurization treatments were performed with three types of fungi: “Trametes Versicolor” - ATCC No. 200801, “Phanerochaeta Chrysosporium” - ME446, Pleurotus Sajor-Caju and one Mixed Culture of bacteria - ATCC No. 39327. A high degree of inorganic sulphur removal (79%) with Mixed Culture of bacteria and consecutive reduction by ∼13% for organic sulphur (S_org) decrease with “Phanerochaeta Chrysosporium” and “Trametes Versicolor” were achieved.To follow the S_org changes a set of different detection systems i.e. AP-TPR coupled “on-line” with mass spectrometry (AP-TPR/MS), on-line with potentiometry (AP-TPR/pot) and by the “off-line” AP-TPR/GC/MS analysis was used. The need of applying different atmospheres in pyrolysis experiments was proved and their effects were discussed. In order to reach more precise total sulphur balance, oxygen bomb combustion followed by ion chromatography was used.     

Effect of chain disentanglement on melt crystallization behavior of isotactic polypropylene

Isotactic polypropylene (iPP) with “disentangled” chains was generated through crystallization of iPP from its mineral oil solution. TGA test assured complete removal of mineral oil from iPP precipitates. Time sweep rheological measurements showed the modulus build-up with time indicating the formation of “disentangled” chains in iPP after the sample disentanglement treatment. The “disentangled” chains could preserve for a certain time before completely re-entangled during melting. Crystallization kinetics of iPP with “disentangled” chains was studied by using polarized optical microscope. The growth rate of spherulites in “disentangled” iPP was faster than that in the entangled one.     

AN ADIABATIC METHOD FOR STUDYING SPONTANEOUS HEATING OF COAL1

An adiabatic calorimeter is described, by means of which the spontaneous heating characteristics of dry pulverized coal can be studied, so as to obtain a direct measure of the rate of spontaneous heating at different temperatures, uninfluenced by various heat interchange factors. Specimen time-temperature curves of different coals are presented, which show approximately the same rate of heating over corresponding temperature ranges. From these curves, it is concluded that the main characteristic of spontaneous heating is that the rate of heating is an increasing function of the temperature. An empirical formula is developed, for which the constants, K, a and b, are determined for a Pittsburgh coal; θ= temperature, and T= time. From this formula, it is concluded: (1) no heating will take place below 26.27°C; (2) the time required for Pittsburgh coal to heat from this temperature to ignition is 144 hours, both the coal and the oxygen used being moisture free.     

火区综合治理技术研究与应用

改扩建矿井在建设期间,经常要用到矿井部分老巷道,这些老巷道顶帮普遍存在高冒区和破碎煤体松动裂隙区,容易引发火灾。针对这一问题,通过对金地煤业13号煤层火区治理的实践,获得了高冒区和破碎煤体松动裂隙区煤体的氧化、自燃及发生和发展规律,积累了大量火区治理技术和火区管理方面的经验,对其他矿区的火灾治理具有重要的借鉴意义。     

地下采空区顶板安全厚度的确定

利用强度折减技术和二分法原理,以塑性区的贯通作为顶板破坏的标准,当采空区顶板的安全系数等于1.2时,利用FLAC软件计算得出各种跨度空区在不同岩层中的最小安全顶板厚度.     

A thermally responsive, rigid, and reversible adhesive

In this paper we present the development of a unique self-adhesive material that, unlike conventional adhesives, maintains a high degree of rigidity at the “adhesive” state while possessing the ability to easily de-bond upon heating. Consequently, the material is both a rigid and a reversible adhesive. The material is an initially miscible blend of poly(?-caprolactone) (PCL) and diglycidyl ether of bisphenol-A/diaminodiphenylsulfone (DGEBA/DDS) epoxy, processed to a unique morphology via polymerization-induced phase separation (PIPS). The fully cured material features a biphasic, “bricks-and-mortar” morphology in which epoxy forms highly interconnected spheres (“bricks”) that interpenetrate with a continuous PCL matrix (“mortar”). When heated to melt the PCL phase (60 °C < T < 200 °C), the epoxy bricks remain rigid due to the high epoxy T_g (>200 °C) while PCL liquefies to become a melt adhesive. Moreover, the PCL liquid undergoes microscopic dilational flow to wet the sample surfaces due to its high volumetric expansion in excess to epoxy bricks expansion, a phenomenon we term “differential expansive bleeding” (DEB). Remarkably, the samples remain rigid at this state and their surfaces become covered by a thin layer of PCL now able to wet, and subsequently bond through cooling, to a variety of substrates. We observe high bonding strengths, which we attribute to a combination of good wetting and subsequent formation of a thin layer of crystalline PCL with high cohesive strength upon cooling. This adhesive layer can be melted again by heating (T > T_m) to easily de-bond and subsequent rebonding capacity was demonstrated, indicating repeated availability of PCL melt adhesive to the surface by the DEB mechanism.     

Granular-bed filtration assisted by filter-cake formation 5. Treating a liquid in a panel bed with pulseback renewal of liquid-entry granular-bed faces

A “panel bed” of granular solid, held in place by louvers, can serve to filter “dirt” from a liquid (the filtration assisted by filter-cake formation) or to place the liquid in countercurrent contact with the solid. A sharp “reverse” pulse of liquid, at sufficient intensity and not too long duration, produces a body movement of the solid capable of causing it to spill in a substantially uniform amount from each liquid-entry face of the granular solid. The paper reports experiments elucidating the physical behavior of the granular solid during “pulseback” and casting light, as well, upon the behavior of a “sand” during puffback of a panel bed gas filter. In both puffback and pulseback, the quantity of spilled solid is a function of “active time” (time during which a reverse pressure gradient across the panel bed exceeds a value just capable of producing a spill). Puffback quickly achieves a maximum attainable spill rate (a function of louver geometry); accordingly, spill quantity is linear in active time. In pulseback, however, spill rate increases gradually, achieving at last a maximum attainable spill rate (whereupon spill quantity becomes linear in active time).     

Structural and mechanical properties of advanced polymer gels with rigid side-chains using coarse-grained molecular dynamics

Computational modeling was utilized to design complex polymer networks and gels which display enhanced and tunable mechanical properties. Our approach focuses on overcoming traditional design limitations often encountered in the formulation of simple, single polymer networks. Here, we use a coarse-grained model to study an end-linked flexible polymer network diluted with branched polymer solvent chains, where the latter chains are composed of rigid side-chains or “spikes” attached to a flexible backbone. In order to reduce the entropy penalty of the flexible polymer chains these rigid “spikes” will aggregate into clusters, but the extent of aggregation was shown to depend on the size and distribution of the rigid side-chains. When the “spikes” are short, we observe a lower degree of aggregation, while long “spikes” will aggregate to form an additional secondary network. As a result, the tensile relaxation modulus of the latter system is considerably greater than the modulus of conventional gels and is approximately constant, forming an equilibrium zone for a broad range of time. In this system, the attached long “spikes” create a continuous phase that contributes to a simultaneous increase in tensile stress, relaxation modulus and fracture resistance. Elastic properties and deformation mechanisms of these branched polymers were also studied under tensile deformation at various strain rates. Through this study we show that the architecture of this branched polymer can be optimized and thus the elastic properties of these advanced polymer networks can be tuned for specific applications.     

Controlling the excess heat from oxy-combustion of coal by blending with biomass

Two different biomass species such as sunflower seed shell and hazelnut shell were blended with Soma-Denis lignite to determine the effects of co-combustion on the thermal reactivity and the burnout of the lignite sample. For this purpose, Thermogravimetric Analysis and Differential Scanning Calorimetry techniques were applied from ambient to 900 °C with a heating rate of 40 °C/min under dry air and pure oxygen conditions. It was found that the thermal reactivities of the biomass materials and the lignite are highly different from each other under each oxidizing medium. On the other hand, the presence of biomass in the burning medium led to important influences not only on the burnout levels but also on the heat flows. The heat flow from the burning of lignite increased fivefold when the oxidizing medium was altered from dry air to pure oxygen. But, in case of co-combustion under oxygen, the excess heat arising from combustion of lignite could be reduced and this may be helpful to control the temperature of the combustion chamber. Based on this, co-combustion of coal/biomass blends under oxygen may be suggested as an alternative method to the “Carbon Dioxide Recycle Method” encountered in the oxyfuel combustion systems.