Mine ventilation and air-conditioning

The effect of the geothermal gradient means that strata temperatures increase by about 3 degrees for every hundred metres in depth. This has a major impact on climate conditions in underground coal mines. The atmosphere below ground is also affected by the geographic situation of the colliery, the working depth, the rock temperature, the humidity level, the output of the production face, the type of machinery in operation, the roof control measures, the airflow volume and the method of ventilation being used. Face cooler Other sources of heat and heat sinks, such as pipelines, have an additional influence on mine climate. The German coal industry's decision to introduce mechanised coal winning brought about a dramatic change in the conditions prevailing below  ground. When the ECSC Treaty was drawn up in 1952 climate issues were generally of little or no significance for those responsible for operating Europe's coal industries. However, greater face concentration combined with an increase in winning depth was to generate increasing quantities of thermal energy, which in turn placed an additional burden on the underground climate. The workplace temperature limits that were imposed by a number of European countries reflected the efforts that were under way to introduce an acceptable ergonomic workplace threshold for underground personnel. As well as undertaking mine climate research, the first of the ECSC-funded projects also sought to acquire basic information into the occurrence and release of mine gas. This work was to run until the early 1970s, with some specific projects continuing until well after that date. Coal and the surrounding strata - though the latter are much less affected in this respect - contain certain quantities of mine gas that have been created as a result of the conversion of organic matter. This gas usually comprises 90 to 95% methane (CH4) and 2 to 4% carbon dioxide (CO2). As methane is explosive when mixed with air in certain proportions (4.4% by volume for the lower explosion limit and 16.5% by volume for the upper explosion limit) the problem of gas emission became a key part of the European Community's mine-safety R&D programme. On the basis of the fundamental research results, and especially the information relating to the nature of the coal itself, viable methods were developed for assessing the potential risk of an underground gas explosion. This included measuring the gas content and gas pressure, determining the desorption characteristics as a way of assessing the risk of gas outburst and measuring the gas permeability of the coal. Investigations were also carried out with a view to improving planning reliability and devising a better system for controlling gas emissions. Much of this work focused on studies that were aimed at identifying the gas-emission zones in and around the working face and the associated development of methods for calculating emission levels and optimising gas drainage techniques. The main parameters affecting the absorbed and free gas component of the coal, and the quantity of gas that is liable to be released from the solid, are: gas composition, temperature and moisture levels, ash content and the coal's pore ratio and structure. The systematic investigation of all these factors revealed that the phenomenon of gas emission was specific to the nature of the deposits. A series of part-overlapping and part-parallel investigations was therefore undertaken in the different coalfields of the coal-producing countries. Two of the key objectives here were to establish the gas content of the deposits and to pre-calculate the gas make as a basis for improving existing gas-control techniques and developing new technology. Germany's decision to amend its Law on Energy Supply now provides an opportunity for mine-gas to be used in markets beyond that of colliery own consumption. Much greater emphasis is therefore being focused on developing methods for the extraction of methane before, during and after the coal winning phase. Gas content Information on gas content and gas release is used for pre-calculating the gas make (cubic metres of methane per tonne of extracted coal). Determining gas content (cubic metres of methane per tonne or cubic centimetres of methane per gram of in-site coal) was therefore fundamental to the gas-emission research work. Because of the coalfield-specific conditions, almost all the countries of the Community have developed their own independent processes for determining gas content and calculating gas make. However, these methods all differ very little in terms of the results obtained. Gas content map In Germany, for example, direct measurement of gas content (laboratory or desorption measuring equipment) was introduced as the standard method. This means that collieries are able to carry out their own independent gas-content analyses. The parameters required for indirect measurement (sorption method and gas pressure in the coal measures) were also investigated, but this process proved to be difficult to carry out. Reliable methods have still not been developed for measuring gas pressure in the working seam, although a viable laboratory technique does exist for determining the sorption isotherms. Nevertheless, the indirect method can be used for strata that are bored through at an oblique angle to the stratification because the borehole that has to be drilled in the surrounding rock can be effectively sealed. From the early 1980s investigations were conducted in a number of coalfield areas in order to establish the relationship between the gas content and the depth/distance to the overburden measures. This generally involved using data from surface exploratory drillings to produce gas content maps. Gas-content analyses, which were carried out using the direct process and were based on surface and underground drill-core samples, showed that the gas content of the deposits tended to increase until it reached a maximum limit. This was an extremely important finding as far as the evaluation of new working panels was concerned. Gas-emission predictions The anticipated gas make is one of the basic parameters used for mine planning purposes. The pre-calculation processes are normally based on gas emission measurements, e.g. gas make monitoring. The release of gas into the coal face or mine roadways is the result of rock-mechanical de-stressing processes, which can be described using empirical gas-emission models. The gas-emission projections depend to a large extent on what is known about the formation of the gas-emission cavity around the face or drivage, the rate of gas discharge into this cavity and the gas content of the working seam and the adjacent seams. EU Member States have investigated these relationships and have now developed a standardised projection process. The gas-emission calculation is programmed for computer processing in the same way as the mine ventilation planning data and climate projections. Since 1985 two new methods have been proposed for calculating gas-emission rates. The first involves carrying out short-term gas-emission projections using continuous data recording and processing. The second comprises the use of "expert systems" that are designed to help assess irregular or sudden gas emission occurrences in those areas where emission calculations have previously proved unsuccessful. While these investigations were under way work also began on the first research projects to have as an objective the development of a procedure for the precise calculation and effective control of gas emissions in roadway drivages with auxiliary ventilation. In this connection it is worth noting that the German coal industry carried out trials with the French calculation method for roadway drivages. However, it was found that because of the different operating conditions this system required some adaptation before it could be used effectively. Controlling underground gas emissions The first projects dealing with gas-emission control concentrated mainly on coal faces and sought in particular to investigate how operational factors affected gas emission levels. Gas drainage was considered to be the primary option here. Research into gas-emission control looked at a number of different methods with a view to developing them to full industrial status. These included rich-gas (CH4 < 22%) and lean-gas (CH4 > 3%) drainage, floor-gas drainage, preliminary gas drainage from the coal seam prior to coal extraction and drainage from gas collector roadways above or below the goaf. Apart from lean-gas drainage, all the abovementioned processes are now still in widespread use. These methods were subsequently developed and refined through a series of follow-up projects. They have now been adapted very effectively to suit a range of underground conditions and their application is based on the latest technical equipment (e.g. computer-controlled gas drainage stations above ground with utilisation of the extracted gas). The rich-gas extraction process, which was already a recognized technique when the ECSC research work began, is now used by every coal industry in the world. However, it first had to be adapted effectively to suit the different mining conditions prevailing in each country, which meant modifying not only the gas drainage equipment but also the layout of drainage circuit. Gas drainage rates and airflow levels are monitored by means of the latest measurement technology, which has also been either developed or refined by ECSC-funded research. In the 1970s the first successful trials were also carried out into the extraction of mine-gas from abandoned collieries, the objective being to protect operating collieries from gas inflow. While at the beginning of the ECSC research programme all gas drainage measures were aimed solely at protecting mine workings and active collieries from the risk of explosion, other important aspects have now come to the fore over the years - including the utilisation of mine gas as an energy commodity, both for economic reasons as well as on environmental grounds. Gas drainage trials based on surface boreholes drilled into the roof of the coal seam indicated that there was no detectable relief for the coal winning operation, at least as far as West-European mining conditions were concerned. However, tests involving long-reach boreholes (> 100 metres), which were drilled in order to collect the gas from the upper roof area of the gas drainage zone, did prove promising on a number of coal faces. The idea of gas drainage via long boreholes drilled parallel to the seam in the roof or floor beds of the gas-drainage zone is now being looked at again. Tests were also carried out in which the coal seam was split by hydro-fracturing from the surface in order to increase the strata's potential for the preliminary drainage and commercial-scale utilisation of the gas in the seam. Unfortunately these trials failed to achieve sustainable success. During the 1980s much greater attention was focused on the problem of controlling gas emissions on "high- performance coal faces"; this included gas extraction at the T-junction on retreat faces, gas drainage in strongly-inclined and steep deposits, gas drainage on working faces and the study of fluctuating gas-emission rates in and around residual pillars. While the gas drainage measures proved to be fairly successful, tests aimed at controlling emission levels at the T-junction on retreat faces made little headway. The "window method" employed by the German coal industry, whether using windows or pipes and both with and without lean-gas extraction, failed to solve the problem. The "back return system" and "bleeder road system" developed to viability in the UK proved to be unsuitable for direct introduction into other coal industries. Because of the operational advantages of retreat mining over the advance method it was recognized that this area was in need of further research. For the sake of completeness we must also mention the research project entitled "Prevention and elimination of increased CH4 concentrations beneath the coal-face conveyor", which involved the use of slotted line-pans and air blower/suction units to draw air into and out of the bottom AFC strand. Neither system proved successful. The need for further research became a matter of irrelevance when the coal industry introduced totally-enclosed AFC floor pans. During the 1990s much greater use was made of models and computer simulation as a cost-effective alternative to large-scale field trials. Computer-assisted simulation is now used to investigate the build-up of localized gas accumulations and to assess the possibility of draining gas through the rock mass and goaf area. Other important fields of research include the drainage of gas from closed collieries and the commercial utilisation of mine-gas. Measurement technology Initial ECSC research work in the field of measurement technology focused on two objectives. The first was the development of equipment for measuring carbon-monoxide and oxygen content and the development of self-rescuer systems, and the second involved extensive research into the occurrence and release of methane gas imprisoned in the coal and surrounding strata. Here great emphasis was laid on the use of computer-assisted monitoring systems, whereby data on mine-gas levels, ventilation flow, pressure levels, temperatures etc. were transmitted to a central station for processing by computer (in the form of daily, weekly or even monthly logs). With a view to examining the particular conditions in one of the ECSC member countries funding was provided between 1988 and 1990 to develop a special type of cap-lamp which featured a gas monitoring unit with alarm function built into the feed cable between the battery and the head-piece. This system was designed for measuring methane, carbon monoxide and oxygen. In the early 1990s research was also carried out with a view to developing a thermographic measurement and analysis system for locating concealed mine fires. However, this work was not taken any further because it was thought that the system lacked the resolution needed for underground application. A few years later greater success was achieved in the development of gas measuring equipment for the early detection of mine-fires. This involved the use of prototype versions of the gas-chromatograph system already approved for use below ground. Unfortunately no manufacturer could be found for this equipment because of the small production run required for the mining industry. Promising results have also been obtained in recent years in the development of processes for the automatic calibration of airflow and gas measuring devices fitted with electrochemical cells. Mine ventilation The first development projects of the 1973 - 1976 period with the word "ventilation" in the title focused more on measurement technology (data collection and processing) than on mine ventilation proper. It was not until a few years later than research began to concentrate on mine ventilation in the general sense. Auxiliary ventilation One of the first projects investigated the operation of closed-circuit ventilation systems with auxiliary-fan control and the impact of this type of system on ventilation stability, pollutant concentration levels and climate changes in the underground workings. The problems associated with closed-circuit ventilation were again studied and re-assessed some years later as part of an investigation into combined auxiliary-ventilation installations. The industry was very keen to develop combined auxiliary-ventilation equipment and also to improve system reliability and availability. As a result of this interest an additional project was undertaken to investigate how the shut-down and start-up of auxiliary-ventilation equipment affected the stability of the main ventilation system. The impact of an underground fire on mine ventilation stability was first investigated as part of a research project in the early 1980s. Some ten years later this scenario was calculated using computer simulation and on this basis a better system was then developed for assessing airflow disruption as a result of a mine fire. In the early 1980s studies were also conducted to measure aerodynamic loss in shafts, air ducts and fan drifts due to poor geometrical design and the presence of fixtures, fittings and shaft conveyances. This scenario was also reconstructed in a series of model tests and the aerodynamic improvements realized as a result of the true-scale modelling work were subsequently introduced by the industry with great success. In the mid-1980s it was found that under certain conditions real cost savings could be made by regulating or reducing the airflow without compromising safety standards. Several development projects also examined the problem of mine ventilation in the context of controlling gas emissions on the coal face. By switching from retreat-based ventilation to advance ventilation, and by introducing new ventilation systems (such as Y, H and W ventilation), effective solutions were found to operational problems. Air conditioning When the ECSC Treaty was drawn up in 1952 climate issues were generally of little or no significance for those responsible for operating Europe's coal industries. However, greater face concentration combined with an increase in winning depth was to generate increasing quantities of thermal energy, which in turn placed an additional burden on the underground climate. The workplace temperature limits that were imposed by a number of European countries reflected the efforts that were under way to introduce an acceptable ergonomic workplace threshold for underground personnel. The workplace restrictions that had to be introduced for hot and difficult working conditions were a sign of the increasing economic burden affecting the faces concerned, for if suitable measures were not put in place coal extraction would be restricted and in some extreme cases would even become impossible. Because of this situation various measures had to be developed for difficult environments so that workplace conditions could be made more acceptable and production targets met. Several ECSC-funded research projects were therefore launched in order to investigate the thermodynamic processes involved and on the basis of these findings a number of climate projection programs were developed. The research activities also included an investigation and assessment of the components and equipment needed to create an underground air-conditioning system. Thermodynamic investigations As well as the geographic location of colliery itself, the main parameters affecting underground climate are: working depth, strata temperature, air humidity, output per face, degree of mechanisation, method of roof control, mass airflow and ventilation system. The underground environment is also influenced by other heat sources and heat sinks, including pipe runs. Since 1958 the German coal industry has seen many changes in these important factors. Whereas in the early 1960s the total power rating of mechanized coal faces was less than 300 kW, power outputs of 2,000 kW are now commonplace. Some longwall faces have even had an installed power capacity inbye of as much as 3,800 kW. In recent years roadway drivages have also seen a significant increase in the amount of electrical power available. Diagram showing climate developments below ground Initial investigations in this field of study sought to examine the heat-transfer processes taking place in the strata and the way in which thermal energy was transmitted from the workings into the mine air. Increasing face concentration has meant that heat transfer from the mineral and from electrical plant now has a much greater impact on the mine environment, with the result that these heat sources are now the subject of much closer scrutiny. Special attention has also be directed at the fact that much of this heat is not transferred in a dry state. In fact most of it is used for the evaporation of free moisture and is therefore responsible for increasing air humidity. In-situ measurement data were increasingly required to complement the theoretical research work, which was in turn backed up by laboratory measurements of the thermal conductivity of Carboniferous strata and coal. As a result, parameters were developed that could effectively reproduce actual conditions below ground - in spite of the complex nature of the underground heat-transfer relationships. As well as the physical characteristics of the strata, equivalent heat conductivity was also introduced in order to take account of additional thermodynamic effects. This parameter incorporates factors such as the amount of heat released by the extracted coal, the number of winning shifts and the method of roof control used (caving or stowing). The quantity of thermal energy generated by the electrical equipment was initially included as part of the equivalent heat conductivity factor. However, as electrical heat loss became increasingly important as far as the underground environment was concerned, this factor soon had to be treated as a separate consideration. A humidity characteristic was therefore developed that could take sufficient account of the effect of moist heat transfer. This parameter is designed to represent that proportion of the total surface area that is moisture covered. On longwall faces this figure will depend on the quantity of coal extracted and the method of roof control. In the case of mine roadways an important factor is whether the road is being used for coal conveying or not. Because of the large number of factors affecting heat transfer from the workings to the mine air, it was found that no firm characteristic values could be formulated. An extensive programme of measurements was carried out and the findings led to the development of characteristic fields. The measurements focused on establishing climate values as a function of the coal output and the general operational situation. Increasing attention was paid to measuring the actual electrical power consumption of the plant and equipment, which in some cases differed significantly from the rated power output. Different types of face layout were investigated. System variations included the type of support installation, the method of roof control, face length, coal output and the design of the winning equipment. Many different measurements were also taken in development roads, including different types of shotfired and mechanized drivages. As the underlying conditions of the workplaces tended to change from year to year, new measurements were constantly needed in order to fit the changing circumstances. The measurements became ever more detailed in nature as measurement technology developed. While the first measured data were obtained manually, and involved considerable labour costs, recording equipment was gradually introduced in the course of time. Modern computer technology and data transmission systems eventually meant that the relevant data could be stored in the instrument itself for subsequent processing by computer or transferred directly to surface processing systems by means of remote transmission circuitry. This technology was also developed with the support of ECSC-funded research projects. Mine climate projections The first climate projections were undertaken without computer support. This initially meant calculating dry heat transfer only, as the relationships involved in moist heat transfer were at the time not sufficiently understood because of a lack of relevant characteristic data. An increasing understanding of the processes associated with heat transfer below ground, combined with advances in computer capacity, eventually led to the development of powerful climate projection programs. A number of EU countries made great efforts to devise effective methods for pre-calculating environmental conditions in mines and projection programs were developed for coal faces as well as for development drivages. The calculations were based on theoretical model concepts and also on data obtained by on-site measurement. These calculations were then compared with real measurement values as the programs were being developed. It was found that a good correlation could be obtained between the calculated values and the actual climate conditions. Continuous adaptation to changes in the relevant parameters has proved to be an on-going process. Reliable climate planning for working faces and roadway drivages depends on having access to highly-developed and viable climate projection programs. Using tools of this kind it becomes possible to calculate dry-bulb, wet-bulb and effective temperatures as well as the relative humidity, water-vapour content and heat content of the air in different sections of the mine. Variables have included parameters such as roadway and face length, strata temperature, production levels, ventilation layout, roadway cross-section and seam thickness. Air cooling and air conditioning Since the 1950s heat influx due to deeper workings, rock temperatures and face concentration has increased to such an extent that many coal faces can still only be operated when adequate cooling capacity has been installed. While in the 1960s attempts were still being made to deal with mine-climate problems by ventilation measures alone, by 1970 many collieries - especially in Germany - were resorting increasingly to the mechanized cooling of faces and drivages. At one stage (in 1994) the Germany coal industry had more than 250 MW of underground cooling capacity in place. The rapid growth in the refrigeration needs of colliery operators also meant that better air-conditioning equipment and more efficient conditioning systems were urgently needed. ECSC-funded research projects again played a part here in helping with the design and calculation of a number of innovative underground air-conditioning systems. An exhaustive programme of laboratory and in-situ measurements provided valuable data for the development and refinement of various air-conditioning components and operating systems. Effective air conditioning depends on the successful synchronisation of three essential factors - refrigeration, cooling-agent transport and low-temperature transfer. During the development and introduction of air cooling equipment it became apparent that the air-conditioning system had to be designed to suit the individual conditions prevailing at the colliery and at the working faces. If the refrigeration requirement is expected to be low, or if cooling is only needed at a few localized workplaces and for a limited period of time, then a decentralized air-conditioning system is the most appropriate solution. This means that local areas are cooled by means of face chiller units. However, decentralized chilled-water machines can also be deployed to supply several air coolers, such as on a longwall face for example. Arrangements of this type save on the high investment costs that would be required for the design and construction of a centralized cooling installation. One of the disadvantages of this set-up, however, is that the equipment has to be sited in the harsh environment of the workplace, which often results in increased expenditure on maintenance and repair. When there is a large refrigeration requirement in remote areas of the mine air-cooling systems with centralized refrigeration plant are to be preferred to the decentralized system. In this case the expensive machinery and equipment is positioned at a fixed and well-protected location. The chilling medium is delivered to the individual cooler units through cold-water pipes. As the higher investment outlay required for this system is usually offset in the medium term by the relatively low operating costs, cooling installations of this type can operate at very low cost over long periods of time where there is a fairly high refrigeration requirement. Investigations have already confirmed the economic advantages of large-capacity centralized cooling installations. The ECSC provided funding for research projects whose aim was to improve the cost efficiency of existing refrigeration systems. This work was based on the principle that the total refrigeration capacity installed at the workplace was not required over the entire operating life of the cooling equipment. Significant energy savings can be made, especially during scheduled shutdowns, when only the immediate refrigeration demand is covered. Calculations have shown that the capital outlay required for installing a control system is soon paid off in a short period of time. Mine climate measurements carried out on a number of production faces have shown that in many cases the cooling output can be substantially reduced during non-cutting periods without any risk of negative repercussions as far as the face climate is concerned when coal winning is resumed. Water cooling equipment Another important facet of ECSC-funded research was the examination of new component technology in the field of underground air-conditioning with a view to driving further development. A focal point of this work was the investigation of mine cooling systems. The research began by testing the underground performance of commercially-available heat exchangers based on tried and tested technology. The finned-tube coolers were found to lack the mechanical stability needed for the harsh conditions encountered below ground and tended to become heavily contaminated because of the high humidity levels and dust concentrations present in the mine air. Water sprays proved ineffective as a long-term decontamination method, so the decision was taken to develop a new range of air coolers for underground application. The new vaned-tube coolers that were subsequently produced proved to be better suited for coal-mining use. Further investigations then showed that plain-tube cooler units yielded the best performance in the long term. This development work was driven by performance trials as well as by dust tests under laboratory conditions. The mining suppliers currently offer various designs of mine cooling equipment. All these coolers, and also the evaporators fitted to air cooling systems, feature smooth-tube units. Another interesting development has been the mist cooling system, which was occasionally used at one colliery to provide most of the air pre-cooling capacity. In this system the chilled water is sprayed directly into the mine air. For a given volumetric airflow and water throughput the direct- contact system is capable of transferring a larger cooling capacity than conventional mine coolers. However, as the infrastructural requirements for mist-type coolers are comparatively expensive, only a few collieries have opted to install this type of system. Further development work on face coolers has focused on minimising dust sensitivity and improving cooler performance, while at the same time reducing the size of the units. The latest face coolers are cylindrical in shape and are available in three sizes to suit different seam thicknesses. Heat exchangers in action The chilled-water pipework is an important part of any air cooling installation. Extracting the maximum thermodynamic benefit from the cold water is only possible when this medium is able to arrive at the cooler at the lowest possible temperature. Intensive efforts have therefore been directed at improving the insulation around the chilled-water pipes. These studies have focused on the effect of the insulating material, the insulation thickness, the method of flange insulation and the effectiveness of a water-vapour barrier. Insulation is now used on almost all cold-water feed pipes. Detailed bench tests and underground trials have been carried out on air-cooling and water-cooling machines. As with the air coolers, this research focused initially on the performance characteristics of the machines under different operating conditions. The EU-wide ban on hydrochlorofluorocarbons and on the use of HCFC22 in new plant (in Germany this ban took effect on 01.01.2000) has generated enormous interest in the thermodynamic characteristics of substitute refrigerants. In this respect it became especially important to establish which refrigerants would be suitable for use in underground cooling installations. The special safety regulations applying in the mining industry precluded the use of tried and tested refrigerants such as NH3. Because of the large number of installations in service, an attempt also had to be made to identify agents that could be employed in older types of plant with minimal refrigerant loss. Replacing all the existing installations would have entailed enormous expenditure and such a measure could not be justified in economic terms. Against this background a number of different products were investigated in order to establish their suitability as replacement refrigerants. An extensive programme of test-bench measurements showed that R407C could be used very effectively in installations that had previously been run on R22, as these machines could be converted at justifiable expense and without any significant loss in performance. The first air-cooling installation running on R407C was successfully monitored in an instrumented trial that was carried out as part of an ECSC-funded project. Source: 50 years of ECSC Coal Research European Community, 2002

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