安全工程专业中英文文献翻译-煤炭自燃.doc

Spontaneous combustion of coalCoal undergoes slow oxidation on exposure to air at ambient temperatures, with the evolution of heat, gases and moisture, the heat generated, if not dissipated, gives rise to an increase in the temperature of the coal. As the temperature of the coal rises, the rate of oxidation increases. If this is allowed to proceed unchecked it can eventually result in the ignition of the coal. This oxidation process is known as spontaneous combustion or spontaneous heating or self-heating. Self-heating, therefore, occurs when the rate of heat generation exceeds the rate of oxidation.During recent years there has been a renewed interest in the spontaneous combustion of coal in all coal mining countries particularly because of the use of caving methods and the thicker seams being mined. Large-scale bulk storage and bulk transport of coal have also become more important with the increase in coal trade.Evaluation of the potential of coal for spontaneous combustion Several methods have been used to evaluate the potential of coal for spontaneous combustion but none is clearly superior. The most common methods used are described blow.Oxygen absorptionIn this method, a coal sample is placed in a container and oxygen or air is added to it. The amount of oxygen absorbed by the coal is estimated from the analysis of the gaseous reaction products. The temperature increase per unit of oxygen consumed indicates potential of coal for spontaneous combustion.Heating rate/crossing-point temperature In this method, a coal sample is placed in a bath and heated at a constant rate. Initially, the temperature of the coal lags behind the temperature of the bath but as coal begins to self-heat, the temperature of the coal first coincides with and then exceeds the temperature of the bath. The crossing-point temperature is known as the relative ignition temperature. Usually, the crossing point temperature is used as a measure of the potential of coal for spontaneous combustion although the index based on the ratio of heating rate to crossing-point temperature is more suitable because the spontaneous combustion potential of coal not only depends on the ignition temperature but also on the rate of heat generation.Adiabatic calorimetry In this method, a coal sample is placed in an insulated bath, and the whole system is heated to a pre-selected temperature. Oxygen or air is then added to it and oxidation of the coal raises its temperature. Since no heat is lost to the surroundings, the change in the temperature of the coal in a given time, the time needed to reach a pre-selected temperature, or the amount of heat generated per unit time indicates the potential of coal for spontaneous combustion.Isothermal calorimetryIn this method, a coal sample is placed in a large bath held at a constant temperature. Heat generated in the coal sample due to spontaneous combustion is measured by thermocouples and dissipated in the relatively large heat sink. The amount of heat generated per unit time gives an indication of the potential of coal for spontaneous combustion.Factors contributing to spontaneous combustion Coal characteristics Some coals are more prone to spontaneous combustion than others. The rate of oxidation of coal depends upon many factors, including rank, presence of pyrite, particle size, moisture content, temperature, extent of previous oxidation of coal and the composition of the ambient air.It is generally accepted that as the rank of coal decreases, the risk of spontaneous combustion increases.The presence of pyrite increases the potential of coal for spontaneous combustion, particularly when the pyrite concentration exceeds 2 % and when it is very finely distributed. Pyrite accelerates spontaneous combustion by swelling and causing disintegration of the coal mass, thereby increasing the surface area available for oxidation.The smaller the coal particle, the greater the exposed surface area and the greater the tendency toward spontaneous combustion. Friable coals which produce a considerable amount of fines when mined are more vulnerable to spontaneous combustion.The changes in moisture content of the coal affect the potential of coal for spontaneous combustion. It has been found that the rate of oxidation increases with an increase in moisture content. Also, wetting is an exothermic process and drying is an endothermic process.Airflow rate For spontaneous combustion to develop, the rate of heat generation should be more than the rate of heat dissipation. At very high airflow rates almost unlimited oxygen for the oxidation of coal is available but dissipation of the heat generated by oxidation is very efficient. A low flow rate restricts the amount of oxygen available , but does not allow the heat generated to be dissipated. A critical flow rate is one that provides sufficient oxygen for widespread oxidation but does not dissipate the heat generated.Geological factorsThe presence of faults in coal seams often contributes to the development of heating in coal mines by allowing air and water to migrate into the coal seams. Zones of weakness which usually develop in the area around the faults also aid in the development of heating.The temperatures of the strata increase with depth. Therefore, the oxidation rate will increase with depth, making deeper seams more vulnerable to spontaneous combustion. On the other hand, the higher rank of coal found in these seams decreases the chances of heating.Thick coal seams are often considered to have more potential for spontaneous combustion because the working of these seams is invariably accompanied by high losses of coal in the goaf areas. The low thermal conductivity of coal compared with that of shale or sandstone is also a contributory factor.When a coal seam under a shallow overburden is mined, the goaf areas become connected to the surface by cracks and fissures. Air and water from the surface can gain access to the coal and increase the potential for spontaneous combustion. Similarly, when multi-seams in close proximity are worked, the cracks and fissures developed in the intervening strata increase the potential for spontaneous combustion of the surrounding unmined seams, particularly the undermined seams.Mining practiceSome of the most common places where spontaneous heatings occur are goaf areas and unconsolidated wastes, pack wall a high proportion of coal, the edges of goaves where high strata pressure causes crushing, roof falls and floor heaves, crushed pillars, regulators doors and air crossings and constrictions in the roadways.Coal left in goaf areas is very liable to spontaneous combustion as the air movement there is very sluggish, and any heat generated as g result of oxidation will not be removed. In coal mines, coal is left in the roof and/or floor to support the weak adjoining strata or bands of inferior quality coal which are left unmined. However on long standing, roof falls and floor heaves occur causing large-scale crushing of the left coal and creating conditions susceptible for heating.Pillars that have been standing for a long time are prone to heating, particularly when they are liable to crushing.Regulators, doors and air crossings are points of high air leakage, the air moving through the fractures in the solid coal around them. The greater the pressure difference across them, the greater the leakage. Constrictions of mine roadways also cause leakage of air. Changes in ventilation, either intentional or accidental, may cause excessive air leakages or may suddenly bring moist air into contact with dry coal.Goaf areas, where a large amount of coal is left and particularly where a bleeder ventilation system is used to clear gas from the gofa, present optimal conditions for spontaneous heating.Incubation period The term incubation period generally implies the time required for the oxidation of coal, in suitable circumstances, to cause a rise in temperature to its ignition point. It depends on the characteristics of the coal, the air leakage and the heat accumulation in the environment. For low-rank coals, the time period generally varies between 3 and 6 months, but with high-rank coals the period varies between 9 and 18 months. The incubation period can be extended by reducing fissuration and/or air leakage. Under adverse conditions, the period can be less than 2 weeks, especially with low-rank coals.Prevention of spontaneous combustion Prevention of spontaneous combustion is based on two factors: (1) elimination of coal from the area and (2) control of ventilation so as to exclude oxygen entirely from the area, or to supply a sufficient flow of air to dissipate the heat efficiently as it is generated and before a critical temperature is reached. The methods adopted depend upon the local situation.Mining layout When designing mining layouts for seams liable to spontaneous heating it is essential that the general layout of the mine is simple and that each area can be quickly and effectively sealed off. The relative positions of the various districts in the seam and surrounding seams must also be taken into account. It is essential to follow descending order of extraction when mining multiple seams.The panel system is an appropriate one for mining seams liable to spontaneous combustion. This system facilitates effective sealing with a few stopping. The size and configuration of the panels depend upon the method of mining, the seam contours and other geological considerations. If necessary, the panels must be of a size which would permit complete extraction within the incubation period. The size of panel barriers needs to be sufficient for stability.When working seams by the bord and pillar method, the size of the pillars must be sufficient to avoid excessive crushing. This also applies to coal pillars left at the start of longwall faces.When working a seam by a longwall, the retreating method is preferable as it eliminates leakage currents through the goaf area.On completion of production from a panel, reclamation of material should be completed without delay and the panel adequately sealed as quickly as possible.Air leakageAs far as is practicable, the formation of leakage paths should be minimised by providing adequate support, e.g. adequately sized pillars and good gateside packs. If this is not sufficient to prevent air leakage, leakage paths should be sealed off by sealant coating or injection.Fractures extending to the surface offer a source of air leakage into sealed areas. Artificial sealing from the surface, usually by sand, can prevent such leakage.Doors, regulators and stoppings should be properly sited. Unnecessary stopping and starting of main and booster fans should be avoided. When a panel has ceased production and is to be stopped off, the ventilation pressure difference should be balanced across the old panel. Balancing the ventilation pressure is not a substitute but a complementary requirement for effective stoppings.Inhibitors In storage areas and surface stock piles, certain chemical agents can be applied to the coal surface which can hinder the penetration of oxygen into the coal by sealing the surface pores and thereby stopping initiation of auto-oxidation of coal at ambient temperatures. Surface stock piles can also be sealed off by consolidation and bitumen. Stock piles can be so designed as to reduce air movement through them.Detection of spontaneous combustionThe development of heating underground is accompanied by the progressive appearance of:(1) haze formed when air heated by an incipient fire meets colder air; (2) sweating or condensation on the roof and exposed surfaces due to the moisture formed by combustion;(3) goaf stink or fire stink with a characteristic smell, variously described as musty, oily, petrolic, aromatic or tarry;(4) smoke in airways; and(5) fire.In the past, reliance has been placed on these indications for the detection of spontaneous combustion, although it has never been satisfactory for the reason that the spontaneous combustion must have reached an advanced stage, thus seriously limiting the time available for control, reclamation of equipment and sealing off.Modern methods of early detection of spontaneous combustion are based on changes in air composition. The oxidation leading to the spontaneous combustion of coal consumes oxygen from the air and produces carbon dioxide and carbon monoxide. Carbon dioxide is produced in much greater quantities than carbon monoxide but its presence cannot be used as an indication of the onset of spontaneous combustion because of the high base levels in fresh air (3000ppm) which make small changes undetectable. On the other hand, there is no carbon monoxide in fresh air and virtually none in a panel intake so that a change in level of a few parts per million can mean a severalfold increase.Exhausts from diesel engines and blasting fumes are two common sources of carbon monoxide underground but their effects can be distinguished from a gradual increase or trend due to spontaneous combustion because they are basically intermittent in nature.In panels where ventilation conditions are steady, even a small change in the concentration of carbon monoxide in the return airway may be sufficient to detect a spontaneous heating condition. Fluctuations in ventilation affect the concentration of carbon monoxide by dilution but an allowance for this can be made by calculating either the carbon monoxide/oxygen deficiency ratio or the actual production of carbon monoxide.Carbon monoxide/oxygen deficiency ratio(Graham s ratio)The calculation of this ratio depends on the constant ratio of oxygen to nitrogen in fresh air. The formula for the calculation is:where ,andare the percentages of the gases present at any given time in a sample of air coming from the suspected area in a mine.Every mine and every panel has its own typical value or norm for the make of carbon monoxide and for the carbon monoxide/oxygen deficiency ratio depending on the oxidation of the coal and the conditions in which it is mined. Any analysis showing a higher value than the norm determined should be followed by resampling. Confirmation of continuous increase warrants immediate investigation underground.Typical values of the carbon monoxide/oxygen deficiency ratio for underground coal mines are given below:0.4 or less normal value0.5 necessity for a thorough check-up 1.0 heating is almost certain2.0 heating is serious, with or without the presence of active fire 3.0 active fire surely existsContinuous monitoring of carbon monoxide in mine airAutomatic monitoring for carbon monoxide is done in mines susceptible to heating. Automatic monitoring also permits the determination of carbon monoxide trends and absolute values using microprocessors without the need to relate them to oxygen deficiency.Continuous monitoring of carbon monoxide at a number of strategic points in the mine can give timely warning of the onset of spontaneous combustion and permit delineation of areas in a mine. Computerised data collection systems with graphic displays and a continuous graphical record permit easy recognition of the changes in background levels and enable exhausts from diesel equipment or other sources to be distinguished.Two types of analysers are available available for continuous monitoring of carbon monoxide in the air: (1) the infra-red analyzer and (2) the electrochemical analyzer. Only the infra-red analyzer is available in a form approved for use in underground coal mines.There are two systems used in monitoring. In one system, the analysers are installed at various points underground and they either record the percentage of carbon monoxide on site or telemeter the results to some convenient point underground or on the surface. In the other system, lengths of tube are installed from the sampling points to the surface and the samples drawn through these tubes are analysed sequentially. This system is known as the tube bundle system.The main advantage of installing on-site analysers underground lies in the immediate availability of results. But analysers are dedicated instruments and can monitor only carbon monoxide. The advantage of the tube bundle system is that is provides a sample for analysis on the surface which can be analysed for all gases. The limitation of this system is the delay between the air entering the tube at the sampling point and its subsequent analysis on the surface. For detecting spontaneous combustion, a delay of one or possibly two hours in getting the results of the samples is not a serious matter because spontaneous combustion has a relatively long incubation period.Generally, for large installations involving many sampling points, the tube bundle system is much less expensive than a system in which each point has a separate analyzer. The costs of pneumatic tubing are normally comparable with the wiring costs for analysers installed underground; however, the tube bundle system requires only one analyzer, whereas the other system requires an analyzer at each point underground. This reduces the cost of the tube bundle system substantially. Moreover, maintenance costs for a single analyzer and pumping station are lower than for a system containing many individual analysers, each of which must be periodically checked, cleaned, or adjusted for sensitivity. (However, when the system is to be used for monitoring ventilation during a sealing-off operation, on-site analysters are far superior due to the instant availability of results.)Control of spontaneous combustion The method adopted for dealing with spontaneous combustion once it has occurred must depend upon the position and intensity of the hea