Chapter 112

IMPROVED PLANNING OF HIGH - PERFORMANCE FACES CONCERNING

FACE LAYOUT, CLIMATE AND GAS EMISSION

R. Wesely Department of Mine Ventilation and Air	A. Friederich R. Sdunowski
Conditioning, Deutsche Steinkohle AG, Herne, Germany	Department of Mine Ventilation and Air Conditioning, Deutsche Steinkohle AG, Herne, Germany

ABSTRACT

The suitable face layout is decisive for a high-performance face with increased gas emissions and high climatic load. Gas emission from the adjacent strata and heat influx from the goaf into the face have to be avoided. Therefore so-called windows are installed into the roadside pack. Concerning mine climate and gas emission extensive measuring campaigns were enforced to improve the planning-security. It was shown that the engine output of a face conveyor participates in the total power consumption of the electric equipment to an amount of approximate 70%. In workings with extended face lengths and low rock temperatures the transformed electric output turns consequently into a climate-decisive parameter. When calculating the transformed power of the winning equipment the face length has to be considered for a plough but not for a shearer. Climatic improvements by means of breaks of production are of secondary importance. Previous planning methods assumed a fixed share of the desorbable gas content which flows to the longwall air flow. So former pre-calculations resulted in a proportional increase in the gas emission with an increased face output. As a result of the investigations it is proved that the gas emission proceeds under-proportionally with an increased face output.

KEYWORDS

Face layout, fugitive air flow, drop of pressure, electrical installed capacities, cooling capacity, heat absorption, mine-climate, gas content, gas emission, desorption

INTRODUCTION

Due to the particular economic terms and conditions in German hard coal mining and so, owing to the extraordinary oppression to decrease the expenditure, the mines depend on reducing the quantity of workings and simultaneous increasing the face output. On principle for this reason controlling methane emissions are made more difficult. More­over the coal has to be extracted from continual increasing depth. Linked with that fact rising installed capacities of the engines and the increase in rock temperatures generally creates a raised heat load in the underground working areas. This means a special challenge to the mine ventilation, not only to keep the achieved high level referring to occupational safety and health, but also to improve it, if possible. Last but not least by adapting the techniques of planning to the requirements of high-performance faces, the additional difficulties could be managed satisfactorily. Only by means of reliable mine-ventilation-planning possibly occurring mistakes can be got around beforehand, and uncontrollable increase of expenses can be avoided.

FACE-LAYOUT

In order to run a safe working with respect to ventilation, mine-climate and gas emission attention is not only to be turned to the conditions in the active workings, but also to the goaf and any other abandoned pit room. Planning high-performance faces, the suitable face layout essentially depends on the question if looking at the avoidance of the spontaneous combustion-danger or if on the other hand the climate- and gas emission-control is to be assessed with higher priority.

Retreat Working concerning Mine Ventilation

Talking about retreat working concerning mine ventilation means that the drop of pressure, related to the goaf, is directed towards the upcast face-to-roadway transition (Figure 1). This causes the concentration of both gas emission from the adjacent strata and heat supply from the goaf to that particular point. Moreover there are only restricted possibilities for managing gas emission from the adjacent strata by means of degasification from the overlying beds and the lying side when using u-ventilation with retreat working. Even if the roadside pack is comparatively tight the refreshing air flow which is supplied to the longwall air flow may be polluted by gas or heat when using negative y- or h- ventilation-system. On the other hand retreat working guarantees a merely limited ventilation of the goaf and due to that fact air loss by fugitive air flow can only occur in a limited extent. That is why retreat working is advantageous with regard to avoiding spontaneous combustion.

Figure 1. Panel - layout: retreat working concerning mine ventilation

Advance Working concerning Mine Ventilation

Advance working concerning mine ventilation means that the drop of pressure, related to the goaf, is directed towards the upcast starting line of the panel (Figure 2).

Figure 2. Panel - layout: advance working concerning mine ventilation

Provided that the roadside packs are erected comparatively tight of hardening pack building materials, the influx of the gas emission from the adjacent seams and the heat from the goaf spreads evenly along the return air course. Under economic points of view a high-performance face with increased gas emissions and high climatic load is to be run only by means of advance working with a positive y- or h-ventilation-system. With these kinds of face layout the longwall air flow is freshened up by an additional unpolluted air flow. Moreover favourable starting-points for gas drilling and degasification out of the drift behind the longwall face exist. In case of increased gas emission from the working seam the methane contents gradually increase towards the face bottom end. As well in case of high installed capacities of the engines in the face the climatic load get worse towards the face bottom end. Because the roadside packs necessarily have to be tight, by all means an additional influx of the gas emission from the adjacent seams and the heat from the goaf to the working room have to be avoided. For that purpose a measured fugitive air flow behind the shield support, parallel to the longwall air flow is exactly directed towards the return air course. There the fugitive air flow is mixed with the longwall air flow and with a supplementary unpolluted air flow. For this so-called windows have to be installed into the roadside pack. These are measured openings corresponding to the expected fugitive air flow (Figure 3). Another advantage is that the occurrence of gas accumulations near the longwall face can be prevented if the goaf is well-aimed diluted. With regard to possible spontaneous combustion-danger the windows in the roadside pack have to be closed at most in a distance of 30 - 50 m behind the longwall face. Right behind the face new windows have to be erected. Since a couple of years one aim of Deutsche Steinkohle AG (DSK) for high-performance faces is, to realise consistently a so-called ideal face layout, provided that this is required because of gas emission and air conditioning. On one hand an ideal face layout consists of a positive y-ventilation. The double used tail gate is abandoned right behind the face. Furthermore there is a parallel flow of the haulage and the air flow in the face. Finally the haulage is situated in that part of the main gate, which has not come to contact with the winning till then.

Figure 3. Separation of longwall air flow and fugitive air flow by means of windowing

ADAPTATION OF AN ALREADY EXISTING COMPUTER PROGRAM FOR PRE-CALCULATING MINE CLIMATE IN THE CONDITIONS OF HIGH-PERFORMANCE FACES

In order to secure and to improve the quality of planning it is more and more important to evaluate faithfully the climatic conditions in high-per­form­ance faces. Especially referring to the design of required air-conditioning installations, pre-calculation of mine climate is essential. High-per­form­ance faces were made possible by further technical development for instance of winning equipment, face conveyors or face support. This up to date technology requires conditions, which cannot be satisfied by a calculation program developed about 30 years ago by Voß (1969; 70; 71; 73). Beside the geometrical fundamentals for instance like seam thickness, face length, face width and depth also empirically determined quantities have to be entered to the equations for the calculation. In the first place the calculation of the effective heating capacity concerning the electrical equipment and heat-technological parameters like the `equivalent thermal conductivity of the surrounding strata (λ<sub>eq</sub>)' and the `non-di­men­sional effective humidity-parameter (η_eff)' have to be mentioned here.

Measuring Campaigns on Mine Workings

To determine these empirically discovered algorithms for this calculation extensive measuring campaigns were enforced on several mines of Deutsche Steinkohle AG. In order to collect a wide range of the above-mentioned influencing factors and to find out the extents of validity, the investigations were not only carried out in standard workings but even in such workings, which can be regarded as a borderline case. In total six different workings were investigated which were ventilated by negative as well as by positive y-, h- and u-ventilation systems. Three workings were equipped with a shearer and a plough. The range of the electrical installed capacities in the faces varied between 1,300 kW and 3,850 kW. The effective electrical capacities were determined for face lengths between 300 m and 430 m and for face outputs of raw coal between 850 t/d and 18,500 t/d. There were rock temperatures in the investigated workings between 38.7°C and 55.1°C. The following quantities were measured at these investigations: dry-bulb temperature, relative humidity of the air, air quantity and the effective electrical capacities of the winning equipment, of the armoured face conveyor, of the crusher and of the roadway conveyor. Besides, the effective cooling capacity in the longwall face was determined, if existing (Figure 4). The measured quantities were recorded continuously over a period of several days. The effective electrical capacities were transmitted directly to a personal computer on the surface. The revision of the algorithm for determining the effective electrical capacities in the longwall face was caused by balancing the following measured quantities: firstly the heat absorption of the air flow in the face, secondly the effective real electric power and thirdly the heat sinks.

Figure 4. Equipment of an investigated working

Results of the Investigations

At the previously used calculation program the electric power, which is transformed into heating capacity, was assumed only to be dependent on the face output and the installed capacities. From the investigations followed that the engine output of an armoured face conveyor had a share of approximately 70% of that power, which was consumed by the entire electric equipment. Due to this result, more than yet the calculations have to take into consideration the armoured face conveyor and for this reason the face length. In addition the period of extraction and the period of operation, that means the period of extraction plus the period of idling, influence the transformation of power. Measurements at unloaded armoured face conveyors showed that the power consumption amounted merely to 40-50% below the values of loaded conveyors. When calculating the transformed power of the winning equipment it is essential for a plough to consider the face length due to the dead load of the plough chain. However, the face length has not to be considered when calculating the transformed power of a shearer. The recorded measured transformed power of a conveyor and a plough in the face is displayed in the following diagrams (Figures 5 and 6). Supplementary the diagrams show the total heat absorption of the mine air determined by the air quantity and by the respective state of the mine air at the face entrance and the face exit. The diagrams show a coherent process of 72 hours in all with phases of high intensive face output (Figure 5) and a period of operational rest (Figure 6). From the course of the curves it is obvious that in high-performance faces with comparatively low rock temperatures the transformed electric output turns into a decisive parameter concerning the mine-climate. In that investigated working with high face output it could be established that, due to the lower moisture load, after stoppage of winning the total climatic load decreased distinctly. After resumption of winning the state of maximum climatic load came back again within a few hours. Therefore at high-performance faces climatic improvements by means of periods of operational rests or breaks of production at the weekends are of secondary importance. At first, when trying to reproduce the measured temperature-courses along the investigated faces, it was realised that it was not possible to describe and reproduce the conditions precisely enough. The reason for this was the assumption of a constant fugitive air flow related to the whole longwall face. In face sectors with an influx of fugitive air flows spontaneous changes of the effective temperatures about several degrees can occur. The same way the consideration of the face layout, the question if a positive- or negative- type ventilation system is used, and the effect of possibly installed windows require variable calculation methods. Calculating the longwall face section by section turned out to be the solution of that problem. If desired, the modified version of the program makes it possible to subdivide the longwall face in sections. Each section of an air course along the face can be prescribed in any way. Such as this the effect on the climatic conditions because of the influx of fugitive air flows to the face or flowing off from the face can be simulated (Figure 7). Another emphasis of the investigations led to the conclusion to verify the heat-technological parameters like the `equivalent thermal conductivity of the surrounding strata (λ<sub>eq</sub>)' and the `non-di­men­sional effective humidity-parameter (η_eff)', which are calculated automatically by the program. While pre-calculating the mine climate in the face it has to be distinguished between the λ_eq for the supported working room and the λ_eq for the goaf. By means of reproducing a model of the precisely measured climatic states it was possible to receive a method of calculation which is adapted in a practice-oriented manner and which corresponds to the state of the art.

Figure 5. Recorded transformed power of a conveyor and a plough in a longwall face; period of high intensive face output

Figure 6. Recorded transformed power of a conveyor and a plough in a longwall face; period of operational rest

Figure 7. March of heat along a longwall face shown by a Mollier-diagram for humid air

IMPROVED PLANNING RELIABILITY
IN THE PREDETERMINATION
OF GAS EMISSIONS FOR HIGH-PERFORMANCE LONGWALL OPERATIONS

For the mining of hard coal, both legal regulations and operating needs require that a forecast has to be made of the volumes of methane gas expected to occur. As the face output and the length of the working faces increased, the question arose as to the quality and applicability of the methods used up to now for predetermining the gas emission volumes. Therefore, in order to improve planning reliability in the predetermination of gas flows, a research and development project was arranged. By carrying out sophisticated measurement and observation programs in appropriate mine workings, the aim of the project was to obtain information on the future development of methane flows in high-performance mines and in mines with greater face lengths. A distinction is thereby made according to where the methane gas originates, namely on the one hand from the working seam itself, and on the other from adjacent strata in the roof and floor.

Gas Emission from the adjacent Strata

Theoretical basis: For calculating gas emission rates from adjacent strata in the roof, calculation methods that took account of the gas emission of the surrounding rock were used at DSK. For calculation of the gas volume from the roof, methods as put forward by the authors Schulz (1959), Flügge (1971; 72) and Winter (1958; 71; 76) were used in the past. In the case of Schulz and Flügge, these are geometrical methods which, depending on the face length, calculate the gas emission rates of the individual strata at a vertical distance from the working seam. The Winter-method is not dependent on face length, but uses an exponential equation. For calculating the gas emission from the floor by the Koppe-method (1975; 76), a quadratic equation is used which does not include face length as a factor. A comparison of the various authors shows that methods dependent on face length produced forecasts for gas flow that were up to 50% higher than those not dependent on face length. So calculating the gas emission for greater face lengths the impression arose that such bigger face lengths might have a negative effect on the potential face output. In the Winter-method, the gas emission remains constant, despite increasing face lengths. The previously named authors developed their methods from studies carried out in the 1970s and 1980s, with an average face length of around 250 m and a daily disposable output of approx. 2,500 t. For purposes of medium and long-term planning reliability, a need was seen for reviewing whether, in the light of ever greater face lengths and increasing output performance, the methods for the predetermination of gas emissions were still applicable. To this end, long-term observation was undertaken in several mines. As well as observation of the volumes of methane extracted, the data recorded by the underground air-measuring equipment was in each case evaluated over several months in respect of methane volumes.

Results of Investigations into Gas Emission from Adjacent Strata

After determining the flows of methane from the adjacent strata, the results were compared with the predetermination calculations previously carried out using the various methods. The greatest correspondence was found with the Winter-method for the roof strata and the Koppe-method for the floor strata. However, the accuracy of predetermination is also assisted by a relatively good, that is to say widespread exploration of the deposit by a large number of core drillings in the roof and floor. As a result, the determination of the gas content in the adjacent floor and roof seams contributing to the mine gas flow. Possibly occurring differences between predetermined and actual values are, therefore, not due to the methods but to the less complete knowledge of the geological conditions surrounding the actual working seam.

Rock Mechanics and Prospects for better Gas Emission Models: The above-mentioned methods for predetermining gas emissions represent a mathematical description of empirically determined mine gas flows studied in the coal mines. They only describe the symptoms, but do not give detailed consideration to the actual physical processes of gas emission in the fracture area of face operations. While they do take account of the parameters of distance, thickness and gas content of the surrounding strata and seams, they do not deal with the actual fracture behaviour of the space above a working face. So still today it is not possible to make any scientifically proved statement about routes for gas flow from the rock strata in the area of fracture which allow gas to flow into the active workings. In combination with the gas pressures, released by the coal extraction activities, it results in the actual measure for the level of gas emission from adjacent strata. The fracture behaviour of faces of different length was studied by means of computer simulation techniques. To show the possible differences in fracture behaviour over working faces of different lengths, model calculations were performed. Results and conclusions of this investigations are presented in the course of this congress by Dr. Brandt from DMT (Deutsche Montan Technologie, Essen, Germany). However it may be, it seems to make little sense to go on developing the methods for predetermining gas emission without also taking rock mechanics into account. Until a combined geomechanical and gas emission model has been developed up to operational readiness, the method described above remains valid.

Gas Emission from the working Seam

Theoretical Basis: For calculating the amount of gas occurring from the working seam, it has been customary in the past to assume a fixed proportion of the desorbable gas content (in m³ / t) as entering the airflow. This portion was estimated in each case and amounted to between 50% and 80% of the desorbable gas content of the working seam. However, in long-term observation of the underground methane detection meters in the mines and in measuring campaigns at the face, this assumed level was found to be much too high. To resolve this problem it was decided to make use of previous research work done by DMT.

Results of Investigations into Gas Emissions from the working Face

In the late 1980s and the early 1990s, a statistical equation was developed by DMT on the basis of studies by Janas and Stamer (1987), Noack and Janas (1988), and Noack and Opahle (1993). This method of calculating the rate of gas emission is based on measurement of the residual gas content in the run-of-mine coal and that in the face during and after extraction. An operative review of this equation through practical application for predetermination purposes and verification through follow-up calculation and measurement of the methane flows and gas contents in working mines had not previously been carried out.

r_a =1.07^. ( m_A/m₁ )^.29.9^. ( v_A/v₁ )^-0.4665 + 6.4 [%] (1)

r_a average rate of gas emission from the working seam [%]

m_A total worked seam thickness [m]

m₁ reference value for the seam thickness (1m) [m]

v_A face advance per working day [m/d]

v₁ reference value for the face advance (1m / d) [m/d]

The degree of gas emission from the working seam is described with a potential function which, taking account of the total seam thickness worked (note: not the coal thickness) and the face advance, is designed to incorporate a geomechanical and a dynamic, that means a time-dependent component for gas emission from the working seam itself. However, in the course of the research project, it was found out that a correction seemed appropriate because of the differences in desorption behaviour between the different working seams. This was done by an addend which describes so to speak the propensity of the different seams to release gas with the help of the volatile parts in the coal. The higher the proportion of volatile matter, the faster the gas is emitted.

r_a=1.07^.(m_A/m₁)^.29.9^.(v_A/v₁)^-0.4665 +6.4+0.5411 ^.F-12.2 [%] (2)

F volatile matter (dry and mineral matter free) [%]

With this method, now developed up to operating readiness, for predetermining the gas emission from the working seam, it is possible to present a significantly improved gas emission forecast, especially in cases where the share of gas from adjacent strata is only small and it is therefore the emission from the working seam which determines the achievable face output quantity. A specially chosen example (Figure 8) compares the former and new methods. This clearly shows that as the speed of face advance increases, and hence the daily coal output, the mine gas flow from the working face captured in the airflow rises under-proportionately as compared to previous assumptions. To put this more simply: the faster the face advances, the less time is available to the coal to emit the gas it contains at the working itself, with the result that the limited airflow volumes available at this point have a lower content of gas. However, to join up high-performance working faces to long faces, it is also necessary to look in detail at how gas emissions from the working seam develop as face lengths increase. Thus, Figure 9 shows, in the case of specially chosen input parameters, how the mine gas flows from the working seam develop with increasing face length. For the iso-lines with constant daily coal output quantities, ever increasing gas flows occur as the face length increases. With the new method of calculation, this can be explained by the resulting decrease in the speed of attacking the face. For the sake of completeness, Figure 9 also shows the result using the old calculation method for an assumed medium face output (dotted line). This again underlines the previous overestimation of the volume of gas emission entering the airflow from the working seam.

Figure 8. Comparison of the old and new method for predetermining the gas emission from the working seam

Figure 9. Change in gas emission from the working seam with increasing face length

Outlook for future Production Planning

For the future operative planning of high-performance faces, the new knowledge gained means that because of the merely limited volumes of airflow available it is necessary to think about adjusting, that is to say optimising the face length on the basis of the gas emission and mine ventilation. With a view to further increasing performance and as production technology develops further, consideration should be given early on in the planning phase, when determining the structure of the face operations, to the aspect of gas emissions, in order to bring the production and ventilation technology into co-ordination and so achieve optimum output performance through optimised face lengths.

FINAL REMARKS

Finally, it should be mentioned that these research projects were carried out under the leadership of the Central Mine Ventilation Department of Deutsche Steinkohle AG with the active help of the mine ventilation engineers of DSK Ruhr and the scientific support of Deutsche Montan Technologie GmbH.

REFERENCES

Flügge, G., 1971, “Die Anwendung der Trogtheorie auf den Raum der Zusatzausgasung”, Glückauf-Forschungshefte, Vol. 32, pp. 122-129

Flügge, G., 1972 “Beispiele verstärkter Zusatzausgasung und Möglichkeiten ihrer Bekämpfung”, Glückauf, Vol. 108 , pp. 337-341

Janas, H. F., 1987, Stamer, R., “Beeinflussung der Grundausgasung”, Glückauf-Forschungshefte, Vol. 48, No. 4, pp. 189-195

Koppe, U., 1975, “Der Ausgasungsgrad von Begleitflözen im Liegenden der flachen Lagerung”, Glückauf-Forschungshefte, Vol. 36, pp. 138-144

Koppe, U., 1976, “Vorausberechnung der Ausgasung von Abbaubetrieben”, Glückauf, Vol. 112, pp. 1154-1156

Noack, K., 1970, “Untersuchungen über Form und Größe des Ausgasungsraumes um Abbaubetriebe in flacher oder mäßig geneigter Lagerung des Ruhrkarbons”, Glückauf-Forschungshefte, Vol. 31, pp. 121-132

Noack, K., Janas, H. F., 1988, “Ein verbessertes Ver-fahren für die Vorausberechnung der Grungausgasung”, Glückauf-Forschungshefte, Vol. 49, No. 2, pp. 76-82

Noack, K., Opahle, M., 1993, “Vorausberechnung der Ausgasung für die Planung moderner Hochleistungs-betriebe”, Glückauf, Vol. 129, No. 4, pp. 276-282

Schulz, P., 1959, “Le dégagement de grisou du charbon causé par l' exploitation”, Rev. Univ. Min., Vol. 102, pp. 41-58

Sdunowski, R., 2000, “Improved Planning Reliability in the Predetermination of Gas Emissions for High-Performance Longwall Operations”, Proceedings, 2^nd International Symposium “High-Performance Longwall Operations” , Aachen University of Technology, June 13^th - 14^th, pp. 313-324

Voß, J., 1969, “Ein neues Verfahren zur Klima-vorausberechnung in Steinkohlenbergwerken”, Glückauf-Forschungshefte, Vol. 30, pp. 321-331

Voß, J., 1970, “Die Bestimmung wärmetechnischer Kenngrößen in Abbaustrecken und Streben”, Glückauf, Vol. 106, No. 5, pp. 215-220

Voß, J., 1971, “Klimavorausberechnung für Abbaube-triebe”, Glückauf, Vol. 107, pp. 412-418

Voß, J., 1973, “Kenngrößen für die Klimavoraus-berechnung”, Glückauf, Vol. 109, No. 13, pp. 678-681

Wesely, R., 1998, “Neuere Entwicklungen auf dem Gebiet der Grubenbewetterung”, VDF Führungskraft, Nov./Dec., pp. 35-42

Winter, K., 1958, “Derzeitiger Stand der Vorausberechnung der Ausgasung beim Abbau von Steinkohlenflözen”, Bergfreiheit, Vol. 23, pp. 439-454

Winter, K., 1971, “Die Anwendung statistischer Verfahren als Grundlage für die Vorausberechnung der Ausgasung”, Glückauf-Forschungshefte, Vol. 32, pp. 220-228

Winter, K., 1976, “Reichweite der Ausgasung im Ein-wirkungsbereich des Abbaus”, Glückauf-Forschungshefte, Vol. 37, pp. 22-27

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IMPROVED PLANNING OF HIGH - PERFORMANCE FACES CONCERNING

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