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Safety and health in mines research advisory board

Annual Review 1999


Methane Emission Prediction

IMC have completed their investigations within this ECSC supported project and delivered their final report. The principal conclusions of the work relate to emission and fracture heights above longwall districts. The emission height is the height of the zone above the extracted level from which gas enters into the working district. The fracture height is some height above the extraction level, above which there is no significant fracture. The principal conclusions of the project were:

  1. Data from a wide range of sources supported the hypothesis that there was a similarity between the emission height (H) and the fracture height, and their dependence upon the depth of extraction (d), the face length (l) and the face height (h). Initial data supported the hypothesis that H } d.l.h, although support for the dependence upon face height was limited and was excluded from further investigations.
  2. Transverse two-dimensional models created using FLAC (Fast Lagrangian Analysis of Continua) produced a fracture pattern in the roof, roughly trapezoidal in shape, enclosing the failed goaf region. The sides of the trapezoid were formed by linear fracture zones tilted in over the goaf and topped by a horizontal fracture. A similar zone existed in the floor, but no horizontal fracture zones were created. The ratio of the height of fracture in the roof to that in the floor is variable, but it has a value of 3.4±1.0 which is close to the 3:1 ratio used in gas emission calculations
  3. The height of fracture showed a systematic increase with face depth and face length although the relationship was not always linear and did not pass through the origin. However, the height of fracture showed a linear relationship with the product of the face length and the face depth (d.l). The gradient of the regression was 0.00138 with an intercept of 12. The gradient of the regression between emission height and dl is 0.00084 with an intercept of 6. The depth of fracture in the floor was found to be non-linear with dl. The height of fracture is generally about 64% greater than the emission height.
  4. One explanation for the difference in height between fractures and emission, for greater depth of working, could be the effect of the emission zone being narrower than assumed, due to the trapezoidal nature of the fracture zone. However, most evidence suggests that emission zones do not usually extend beyond those commonly assumed.
  5. The transverse FLAC models provide strong support for the use of rock mechanical models in providing information on the likely size of emission zones.
  6. Longitudinal models produced standard trapezoidal fracture zones at short gate lengths. However at long face lengths, fractures extended to the surface. The major fractures occurred at intervals of about 60 m, with little fracture between them. The break interval of 60 m is much greater than found in actual mines and would result in major weightings and collapses of the waste. The model was therefore found less effective when using longitudinal models.
  7. In an attempt to resolve difficulties with the longitudinal models and obtain useful strain information, models were constructed which used bridging strata to reduce the length of the fractures and softened strata to encourage a reduction in the degree of major fracture. The use of bridging strata produced little change in the fracture pattern of the longitudinal model. Softened strata only reduced the degree of fracture between the major fractures which occurred at a similar interval. Strain in the longitudinal models was limited to the fracture zones; the strata between showing little or no strain, indicating that the modelled strata was subsiding in blocks.

Computational Fluid Dynamics (CFD) Modellin of Gas Flows in Coal Mine Goafs

A new ECSC project began in 1999 with aim of using CFD modelling to improve understanding and control of methane emissions from the goaf and longwall faces and spontaneous heatings in the goaf by optimisation of inert gas injection systems. The project is being carried out by HSL, as project leaders, in collaboration with the University of Nottingham, DMT (Germany), AITEMIN (Spain) and Deutsche Steinkohle (Germany).

A literature survey has been undertaken by HSL and was being evaluated by the participants at the year end. Tower Colliery have agreed to collaborate in the project and will be used as a test case for the study of methane flows. Following a visit by HSL to Tower, an initial CFD model has been set-up to simulate flow and methane dispersion at a back-return from an operating face. It is intended to extend the model to include flows through waste and strata. Arrangements are currently being made with the University of Nottingham to use FLAC for geotechnical modelling to enable this extension to proceed. The project is due for completion at the end of 2001.

Alternative Ventilation layouts for Mine Drivages

This ECSC supported project by RJB Mining (UK) Ltd in association with the University of Nottingham and DMT of Germany is now in its third year. A world wide literature study has been undertaken by the University but this has not revealed anything fundamental. Practical trials have been conducted in the surface gallery at Welbeck Colliery. A JCM5 machine was sited and various configurations of ventilation systems installed. Airflow and firedamp distribution patterns were mapped and the information used to construct a computer model of a typical heading. Computational fluid dynamic modelling have been used by DMT to predict flow patterns. The computer model is being refined to accurately reflect the actual measurements obtained in the gallery.

DMT have also undertaken a climatic study in T16s Maingate at Maltby Colliery. This demonstrated the heat loading in the airstream and the information is being included in the computer model. It will be possible to move items of equipment in the model and evaluate changes to the heat distribution in the heading.

The project research team has made a series of underground visits to a number of collieries to conduct ventilation surveys and climatic studies. The purpose of these studies is to investigate suitable underground survey methods to be employed during a series of underground trials at Welbeck Colliery.

Further development of CFD modelling including the simulation of box cut operations and the clearance of methane gas from the face of the drivage. The project team commenced the development of an auxiliary ventilation simulation tool. The development of this model will eventually assist Environment Engineers in the effective design and operation of auxiliary ventilation systems.

With all the information in the model, different ventilation layouts will be modelled and the effects evaluated. If a suitable system can be identified, this will be mocked up on the surface to test the validity of the computer model.

Other Ventilation Studies

IMC have continued their ECSC-supported studies on several fronts. Detailed CFD layouts for the three dimensional studies of booster and jet fans were developed. The setup was a 100 m long rectangular tunnel (3.5 m high x 5 m wide) with pressure conditions at inlet and outlet. Three scenarios were developed, each with the fan centreline 1 m down from the roof and 1 m in from the side wall:

Steady state solutions were found for all scenarios using the CFD software and velocity and pressure fields were available for analysis. By incorporating porous jumps in the roadway upstream and downstream of the fan, the effects of increased roadway resistance were investigated and the onset of recirculation for the free standing booster and jet fan scenarios quantified.

For the study of ventilation problems when cutting high and wide (for example at junction developments), two CFD scenarios were developed for analysis:

The standard drivage section was 3.5 m high x 5 m wide x 100 m long, opening up to 4 m high x 7 m wide for the last 10 m for the high and wide scenario. The forcing and exhaust airflows were modelled as velocity inlets giving 6.5 m3/s from the forcing duct and 5 m3/s through the exhausting system. Steady state solutions were found for both scenarios, using the standard k-` turbulence model. Work is continuing to refine the models and to investigate the application of physical curtains and air curtains on the drivage machine in order to alter the airflow pattern.

Considering the application of methane utilisation apparatus to a fan evasée, a full three dimensional model of a main fan/evasée system was developed, based on dimensional and fan performance data from a particular installation. The mine resistance was modelled by incorporating a porous region upstream of the fan and tailoring the permeability and inertial resistance values so that the fan was at its correct operating point (350 m3/s at 4 kPa). Solutions were found for the standard evasée, with no methane utilisation apparatus, and the pressure and velocity fields calculated. Work continues to incorporate packed bed models in the evasée to represent methane utilisation apparatus. The data from the previous two dimensional studies will be used to define a set of models.

Mine ventilation network analysis software was employed to study the application of controlled recirculation in a simple mine layout. The study was to investigate the possibility of reducing the main fan duty at low or zero production times (thus making significant power savings), whilst maintaining the air velocities on the production faces by the use of a certain amount of recirculation. A number of main fan-booster combinations were studied and the power savings quantified.

Related to the above work on recirculation, the same network software was used to investigate the replacement of main fan capacity by a distributed fan system. A number of scenarios, based around a simple three-district network, were analysed and the network flows and power consumptions quantified.

The issue of mine climate is a subject of further study. A number of mines operate at depth, with relatively high geothermal gradients and extended workings. Problems of heat stress are also evidenced at shallower depths due to high production levels and large heat fluxes from plant and equipment. An initial scheme definition for pumped satellite boreholes has been researched, particularly for those mines that are relatively deep and laterally extended. In concept, satellite boreholes would be drilled and cased to intersect with the workings. Cooled and dried air would be delivered to the boreholes by surface compressors. This air, since it is delivered to the workings cold and dry, has a far greater capacity to absorb heat than ambient air sucked down a ventilation shaft. Hence relatively modest delivery rates could effect a noticeable improvement in prevailing climatic conditions in the workings. An assessment has been made of the likely impacts and target improvement sought from a practical scheme.

Noise Studies

Vibration testing and modal analysis of a shearer cutting drum were completed by IMC as the preliminary phase in evaluating whether modifications to the drum could result in lower radiated noise.

The next stage of the work used this model to examine, firstly, the response of the drum to cutting forces and, secondly, to investigate ways of reducing this response by means of structural modifications, and hence to reduce the radiated noise during cutting.

Detailed noise and vibration measurements were carried out on a Gopher airleg roofbolter. It was found that body radiated noise was a significant contributor to the overall noise when the machine was free running, but less so when the machine was drilling. Additionally, hand-arm vibration measurements for the operator were made.

Two extensive noise measurement surveys in underground drivages were completed. The first drivage was circular in section with steel rings, concrete sprayed walls and precast concrete sections to form the floor. The drivage machine was an Rh35 loading out on to a short conveyor system and then into tubs for transport outbye. Ventilation was provided by an overlap system, with the forcing duct mounted below the exhaust duct, giving 6.3 m3/s force and 5.2 m3/s exhaust quantities. Noise measurements were made on the heading machine, the conveyor system, the forcing and exhaust ducts, the dust extractor, pneumatic jigger pick equipment and at distances travelling outbye of the work area.

The second site was a conventional drivage in stone. Support was by standard rings with steel mesh, with bolting between the rings for added support. The drivage machine was a Dosco Mk3, loading out into a shuttle car. Ventilation was provided by an exhaust system. At the time of the visit the drivage length was only 50 metres. Noise measurements were made on the heading machine (idling, conveyor only, cutting low, cutting high), a number of roofbolting cycles, shuttle car, exhaust fan and railed manriding system.

Environmental Impact of Opencast Mining

IMC started work on a new ECSC-supported project at the beginning of the year, intended to study various aspects of the environmental problems of dust and noise associated with opencast mining. The project is in response to the need to improve standards of environmental management in all phases of the mining cycle so increasingly stringent planning consents can be met in conjunction with operating with the best available technologies and practice.

Minewater Recovery and Gas Emission at Former Mine Sites

IMC, with funding from the Coal Authority, has continued monitoring of water recovery and gas emissions in a number of coalfields across the country. Minewater data has been used to construct a minewater inflow model in coalmines (MIMIC). The model considers the different ways in which water enters the mine workings under different hydrological conditions. The model divides the coalfield into four separate hydrological zones. Zone 1 includes all shallow workings within the band of increased permeability related to surface unloading and mining induced movement. Zone 2 includes intermediate workings defined as those 45 m below the surface or base of superficial deposits and Zone 3 covers the modern deep workings. The final zone is similar to Zone 3 but covers workings which sub crop into a major aquifer or body of water. The inputs required include rainfall data, permeability and thickness of strata (divided between superficial, surface coal measures, undisturbed coal measures, mining zone and aquifers), flow data from shafts and known water makes, other geologically significant features such as faults and, mining details. The model has been tested using measurements made at a number of sites and good agreements obtained. The model may also be used to predict water flows into active mines where no prior knowledge of inflows is assumed. At one site the model showed reasonable agreement with historic data from the mine. Further work is necessary to test the model and once confirmation is obtained it will be used to predict minewater recovery in areas where no monitoring exists.

Geophysical techniques for non-intrusive mapping mine waters have been examined. The techniques are aimed at detecting the enhanced conductivity produced by the presence of highly saline waters in abandoned workings. Various methods for investigating the relationship between electrical conductivity and depth have been examined and instrumentation suitable for locating conductive layers at depths of up to 500 m found. Suitable sites, where mining has produced a continuous vertical zone of enhanced permeability devoid of perched water tables, have been identified.

The examination of surface gas emissions has confirmed that rising mine waters may be the dominant controlling factor at many locations. At one site, where gas is transmitted several kilometres through permeable sandstone beds the presence or absence of gas at the sandstone outcrop is dependent upon the rate of water inflow to the abandoned workings but independent of water level. The predominance of carbon dioxide in the initial gas subsequently followed by the appearance of methane indicates that mine waters are influencing shallow and deep workings. Testing at another location indicated that significant volumes of flammable gas are present in gas saturated sandstones although concentrations of less than 20% suggest that extracting from this source is unlikely to be commercially viable.

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Updated 2009-05-22