HISTORICAL RESEARCH REPORT Research Report TM/89/

Transcription

1 HISTORICAL RESEARCH REPORT Research Report TM/89/ A study of the formation and control of dust at the ends of highly mechanised longwall faces Bradley A, Aitken M, Garland RP, Nicholl AGMcK, Weston P

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3 HISTORICAL RESEARCH REPORT Research Report TM/89/ A study of the formation and control of dust at the ends of highly mechanised longwall faces Bradley A, Aitken M, Garland RP, Nicholl AGMcK, Weston P This document is a facsimile of an original copy of the report, which has been scanned as an image, with searchable text. Because the quality of this scanned image is determined by the clarity of the original text pages, there may be variations in the overall appearance of pages within the report. The scanning of this and the other historical reports in the Research Reports series was funded by a grant from the Wellcome Trust. The IOM s research reports are freely available for download as PDF files from our web site: Copyright 2006 Institute of Occupational Medicine. INSTITUTE OF OCCUPATIONAL MEDICINE No part of this publication may be reproduced, stored Research Avenue North, Riccarton, Edinburgh, EH14 4AP or transmitted in any form or by any means without Tel: +44 (0) Fax: +44 (0) written permission from the IOM publications@iomhq.org.uk

4 ii Research Report TM/89/03

5 Report No. TM/89/03 UDC A STUDY OF THE FORMATION AND CONTROL OF DUST AT THE ENDS OF HIGHLY MECHANISED LONGWALL FACES A Bradley RJ Aitken RP Garland AGMcK Nicholl P Weston August 1989 Price: Price: (UK) (Overseas)

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7 Report No. TM/89/03 CEC CONTRACT /022/08 INSTITUTE OF OCCUPATIONAL MEDICINE A STUDY OF THE FORMATION AND CONTROL OF DUST AT THE ENDS OF HIGHLY MECHANISED LONGWALL FACES by A Bradley, RJ Aitken, RP Garland, AGMcK Nicholl, P Weston FINAL REPORT ON CEC CONTRACT /022/08 Duration of project: January 1986 to January 1989 Institute of Occupational Medicine 8 Roxburgh Place EDINBURGH EH8 9SU Tel: Telex: G FAX: August 1989

8 This report is one of a series of Technical Memoranda (TM) published by the Institute of Occupational Medicine. Current and earlier lists of these reports, and of other publications, are available from the Librarian/Information Officer at the address overleaf. For further information about the Institute's facilities for research, service/consultancy and teaching please contact the Librarian/Information Officer in the first instance.

9 CONTENTS Page No SUMMARY 1. INTRODUCTION 3 2. DATA FROM PAST RESEARCH AND SELECTION OF SITES 5 3. METHODS 7 4. RESULTS AND DISCUSSION Advanced Headings at Intake Ends of Faces Advanced Headings at Return Ends of Faces Study of Dust Pluming Effect Using the IOM 15 Model Coal Face 4.4 Observations on the Use of Coanda Tube Air Curtains 4.5 In-line Rippings at Intake Ends of Faces In-line Rippings at Return Ends of Faces Retreat Face Ends CONCLUSIONS AND RECOMMENDATIONS Advanced Headings In-line Rippings Retreat Faces General Conclusions 24 ACKNOWLEDGEMENTS 27 REFERENCES 29 TABLES FIGURES

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11 1 INSTITUTE OF OCCUPATIONAL MEDICINE Final Report on CEC Contract /022/08 A STUDY OF THE FORMATION AND CONTROL OF DUST AT THE ENDS OF HIGHLY MECHANISED LONGWALL FACES by A Bradley, RJ Aitken, RP Garland, AGMcK Nicholl, P Weston SUMMARY With the advance in technology and advent of more powerful and efficient coal-getting machines, recent developments within the mining industry have been concentrated on new techniques of roadway drivage and construction. A variety of new machines and face end configurations have been tried in order to match the greater advance rates being achieved by modern coal-getting machines. This in turn has created a number of problems for dust control engineers. This study set out to evaluate airborne dust conditions of a number of face end configurations, including in-line rippings (where the roadway is advanced in line with the coalface) and mechanised advanced headings using different types of machines and auxiliary ventilation. Because of the stated policy of the British Coal Corporation to change, wherever possible, to retreat mining, two retreat face ends were also included in the study. Early in the trial it was observed that elevated dust concentrations measured in an advanced heading on the return side of the face could be attributed to the dust from the coal-getting machine "pluming" along the face track and being drawn preferentially into the heading. A study was set up on the Institute of Occupational Medicine's V 10 scale model coalface to study the plume profiles. These studies showed that the plume concentration falls approximately exponentially from the coalface to the travelling track, and that the concentrations close to the coalface can be more than 100 times those in the travelling track. The plume also remains essentially intact as it turns into the advanced heading, which indicates where free standing filtration units may best be sited to capture the plume. Underground trials in advanced headings demonstrated the efficiency of air curtains on heading machines, and showed that heading teams are almost always exposed to airborne dust travelling up the heading from behind. They also demonstrated that for headings at the intake ends of faces, providing methane concentrations can be controlled, the preferential method of dust control is by partial recirculation ventilation through a filtration unit.

12 In-line rippings were found to pose few dust control problems, regardless of machine or strata. It was noted, however, that fly-cutting by the main coal-getting machine led to particularly high dust concentrations, and this action should be reduced whenever possible. As expected, due to the lack of major roadway construction operations, retreat faces posed few face end dust control problems. An interesting innovation observed was the use of a very small shot-fired stable which drastically reduced the necessity for fly-cutting. Finally, it appears that British Coal has the equipment and technical ability to control dust at the ends of faces of any configuration. Further investigation is required, however, into the problems of dust pluming, and further development is required of methods for dust capture at points of high dust concentration caused by the pluming effect.

13 3 1. INTRODUCTION In two earlier research studies jointly funded by the CEC and British Coal, (Bradley et al 1976, Bradley el al 1979) the mechanics of dust formation were studied, the former into factors relating to the operation of the main coal-getting machine and the latter as a result of dust production from impact ripping machines, roof supports and shotfiring. With the advance in technology and the advent of more powerful and efficient coal-getting machines, recent developments have been concentrated on new techniques of roadway drivage and construction. A variety of new machines and face-end configurations have been tried in order to match the greater advance rate of coalfaces achieved with the new technology, which in turn has created a number of problems for dust control engineers. This study set out to evaluate the airborne dust conditions of a number of face-end configurations at both intake and return ends of mechanised longwall coalfaces. The sites investigated included in-line rippings and mechanised advanced headings with different types of machines and different auxiliary ventilation layouts. Because of British Coal policy to introduce retreat mining whenever possible, a retreat longwall face was also studied. The results from the individual face ends are given as a series of appendices to this report. Data from past projects have been reassessed to allow comparisons to be made between past and current methods of face end working. Early in the present study, the phenomenon of airborne dust streaming or "pluming" along the conveyor track of the coalface and into the return road from the coal-getting machine was observed. This effect occurs when the dust produced at the shearer drum stays airborne in the stream of air close to the coalface. This forms a "plume" which expands only slowly as it passes down the coalface. This effect has been noted before (Bauer, (1986). In these studies, however, the dust plume was contained in the conveyor track by the physical separation of the airflows in the conveyor and travelling tracks using shielding deflectors between the chock legs. In discussions with British Coal Headquarters Technical Department, it was decided that important lessons in dust control could be learned by studying the conditions of formation of this plume, which could in turn lead to the development of methods of containment and capture of the dust it contained. It was decided that the best way to study the plume in detail would be on a small-scale model face at IOM which was already in use. This study was carried out, and the model results are compared in Section 4 of this report with observations on actual coal faces.

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15 5 2. DATA FROM PAST RESEARCH AND SELECTION OF SITES In an earlier project (Bradley et al, 1979) jointly funded by CEC/British Coal Corporation, the Institute reported the results of investigations into the production of airborne dust by ripping operations. At that time the majority of face ends in the UK were worked by conventional rippings, with only a small percentage of in-line rippings and advanced headings used. The main findings of the study were that a rotary ripping machine at a ripping could produce approximately 17 per cent of the dust reaching the 70 metre site; conventional ripping was less dusty, producing about 8 per cent; and impact ripping machines produced the least dust (about 3 per cent). Shotfiring during conventional ripping accounted for approximately another 5 per cent of the dust reaching the statutory site from each ripping lip. Two in-line rippings using ranging drum shearers were investigated in the course of that study. On average, each of these machines produced 34 per cent of the dust measured at the 70 metre statutory site. The report (Bradley et al, 1979) concluded that the method of ripping selected depended on a number of factors, including rate of advance and strength of strata. For weaker strata, impact machines were found to be less dusty than alternative methods. The use of ranging drum shearers to cut roadway profiles was always found to be a dusty operation. Contributing factors included low pick penetration, low haulage speed and the fact that the drum is never fully buried. As the quest for greater face advance rates has proceeded, new techniques for roadway advancement have been tried. In 1987 the distribution of roadway advancement methods was as shown in Table 1. It was also noted in the previous study (Bradley et al, 1979) that about 8 per cent of rippings still used shotfiring as a means of breaking down the ripping. In 1987 British Coal also reaffirmed their policy to increase, wherever possible, the use of retreat faces. It was felt, therefore, that this study should include advanced headings, in-line rippings and retreat faces. It was not thought necessary to study conventional rippings, either hand or mechanised, as this had been done in the earlier study. It was also considered that equal attention should be paid to intake and return end operations for each type of face end. The final distribution of heading types studied was as shown in Table 2. It became apparent early in the project that dust was relatively easily controlled in intake advanced headings, but was much more difficult to control in return advanced headings. Resources were therefore concentrated on the return ends during the later stages of the study.

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17 7 3. METHODS In order to study the formation of dust from the variety of sources in different face end configurations it was decided to use well established techniques (Bradley et al, 1979). The basic method adopted for measuring mean respirable dust concentrations was to place MRE113A Gravimetric Dust Samplers (Dunmore et al, 1964) on the intake and return side of all major dust sources identified, and at the statutory control sampling point 70 metres along the return roadway. Continuous recordings of respirable dust concentrations were made by siting Simslin Mk 2 continuous recording dust samplers alongside the MRE 113As. The quartz contents of the respirable dust collected using the MRE samplers were measured using the direct on-filter method of infra-red spectrophotometry (Dodgson and Whittaker, 1973). By using the Simslin Mk 2 and MRE113As in this manner, and by carefully timing and recording the various machine operations, it was possible to calculate the contribution made by individual dust sources to the overall respirable dust concentration. From these notes and by comparing the patterns of peaks on the two Simslin charts, observations on dust formation could be made. Details of the Simslin Mk2, its memory unit (Sirnstor) and the use of an integrating chart recorder to decode the Simslin estimates were given in the Final Report on ECSC projects /8/017 and 032 (Mining Research and Development Establishment, 1982). A number of repeat shifts were necessary at each site to allow for day-to-day variations in production. In order to study the pluming of dust along the conveyor track of the coalface which was observed to be preferentially drawn into the advanced heading, a V 10 -scale model coalface was used. The model had been built in its original form for a previous CEC-sponsored Vincent and Mark, 1983). For this study, the system was set up to model as closely as possible the dimensions of a return advanced heading included in this study. In order to set up the model, a special investigation of the ventilation aspects of the actual coalface was carried out.

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19 9 4. RESULTS AND DISCUSSION 4.1 Advanced Headings at Intake Ends of Faces One of the most widely used methods of ventilating a heading at the intake end of a longwall face is by exhaust ventilation through a filtration system followed by partial recirculation of the cleaned air. From the point of view purely of dust control, this type of system is ideal for a number of reasons:- - it takes the dust raised by the heading machine away from the heading team - it draws mainly clean intake air with a small proportion of cleaned air into the heading and over the heading team There are three situations where the partial recirculation system may not be sufficient to provide clean air to the face; these are - - when the intake air is so highly polluted with airborne dust that diluting it with cleaned air will not bring the levels down below the permitted concentrations - when the face conveyor transfer and stage loader create high dust concentrations - when back-up of airborne dust occurs from the heading machine cutting head The intake air to the heading selected for the investigation (coded FE1) had a high average respirable dust concentration of about 3.0 mgm~ 3 and a respirable quartz concentration of 0.19 mgm- 3 (Table 3). As a result of these high intake concentrations, the air approaching the Dosco driver's position (Site 5) had a mean respirable dust concentration of 2.63 mgm- 3 and a mean respirable quartz concentration of 0.15 mgm~ 3. The slight reduction in both concentrations was due to the efficiency of the filtration of recirculation air from the heading mixed with the input air into the main gate. The air entering the exhaust duct (Site 7) had a mean respirable dust concentration of 19.4 mgm" 3 and a respirable quartz concentration of 4.11 mgm~ 3 ; the respective concentrations at the exit from the Dustomac filter were 2.4 mgm" 3 and 0.19 mgm~ 3. This showed the Dustomac to be about 88 per cent efficient when dealing with respirable dust and about 95 per cent efficient when dealing with respirable quartz. As can be seen from comparing the concentrations of dust and quartz at Site 4 with Sites 5 and 8, the face conveyor transfer added no measurable amount of dust to the mine atmosphere.

20 10 The respirable dust concentration at the Dosco driver's position (Site 6) was, at 4.2 mgm- 3, higher than expected. A proportionately high respirable quartz concentration was also noted. Examination of traces from a Simslin sited at the Dosco driver's position showed peak respirable dust concentrations to coincide with times when the machine was cutting the face, which identified this dust as being back-up from the cutting head. A typical Simslin trace with cutting periods highlighted is given as Figure 1 at the end of this report. The conclusions from this exercise were that partial recirculation and filtration successfully removed most of the dust produced in the heading, and resulted in a slightly lower airborne dust concentration entering the heading than had originally entered.the District as intake pollution. Calculations based on data from other exercises on the effects of applying more efficient filtration showed that the very small overall improvement gained would probably not be cost effective. None of the men working in the heading appeared to have been exposed to more than the statutory respirable dust limit of 5.0 mgm- 3 ; there was, however, some cause for concern over the mean respirable quartz concentration of 0.57 mgm- 3 measured at the Dosco driver's position. It was therefore recommended that an air curtain be fitted to the Dosco to reduce dust back-up from the cutting head which appeared to be the source of the elevated concentrations. The effectiveness of the Coanda tube air curtain system fitted is discussed later in this report (Section 4.4). It was further recommended that attention should be paid to dust sources outside the District in order to reduce intake pollution. 4.2 Advanced Headings at Return Ends of Faces Advanced headings at the return ends of longwall faces are usually ventilated by exhaust systems, and three such headings were examined during the project. The main results from each investigation are discussed below in sections The return advanced heading of the first of these coalfaces (designated FE2) was selected as it had a problem due to airborne dust being produced from a band of hard sandstone directly overlying the extracted section. The roadway was formed by a short (circa 20 m) heading worked by an Anderson Strathclyde RH22 roadheader. The cutting head of the roadheader was equipped with high pressure water jets as an aid to cutting the sandstone, and to reduce the risk of frictional ignition. The machine was fitted with a prototype Coanda tube air curtain system to form an air curtain between the heading face and the machine driver, thus preventing dust generated at the face from reaching the driver.

21 11 Table 4 gives the mean respirable dust and quartz concentrations averaged over a number of shifts. The respirable dust concentration entering the heading from the coalface was 7.4 mgm~ 3 with a quartz concentration of 0.81 mgm~ 3. At the roadheader driver's position the concentrations had increased to 9.9 mgm~ 3 and 2.15 mgm~ 3 respectively. Simultaneous Simslin traces (e.g. Figure 2) taken at the intake to the heading and at the roadheader driver's position showed that the respirable dust concentration at the driver's position during the roadheader cutting period far exceeded the respirable dust concentration entering the heading; this therefore demonstrated that the increased concentration at the driver's position was due to dust back-up. Visual examination of the site showed that the dust back-up was increased by the high pressure water jets, which induced excessive air turbulence around the heading face. Dust then defeated the tube air curtain system by passing underneath the air curtain where the tubes did not reach low enough on either side of the machine. It was also noted that the positioning of the roadheader affected the amount of dust defeating the air curtain. As the machine moved away from a central position in the roadway, the width of the zone of influence of the side tubes was insufficient to continue to prevent dust back-up. Figures 3 and 4 illustrate the effects of angular change. Positions could be reached where the zone of influence from the tubes ceased to be in contact with the roadway profile, and dust back-up past the air curtain occurred. Modifications were in progress to alleviate these problems and other recommendations included: - to extend the Coanda side tubes downwards, together with endcaps, as had been done on other machines. - to increase airflow through the tube system to extend the zone of influence. - to seek Inspectorate approval to introduce a free-standing fan between the face line and the rear of the heading machine, in order to reduce the concentrations of airborne dust produced in the face from reaching the roadheader driver's position. - to reduce further the very high quartz exposures around the driver's position by use of adequate respiratory protection. The mean respirable dust concentration at the heading intake (Table 4) was 7.4 mgnrr 3, compared with 3.5 mgm~ 3 measured at the 70 metre statutory site in the return road. As only one fifth of the air arriving at the 70 metre site came via the auxiliary ventilation from the heading, and had been cleaned in the process, the difference in concentration implied that more airborne dust was drawn into the heading than was drawn down the return roadway. Visual observations showed that dust produced by the face machine tended to plume along the conveyor track and to be drawn preferentially into the advanced heading. Further investigations

22 of this phenomenon are discussed in later sections of this report The advanced heading designated FE3 was located at the return end of a longwall face worked by an Eickhoff in-web shearer. The heading was driven about 200 metres in advance of the face by a Dosco LH1300 roadheader fitted with water sprays on the cutting head and an air curtain of 100 mm diameter Coanda tubes. Air was exhausted from the heading by a BC70 fan fitted with a cage and wet sock sited in the return roadway outbye the 70 metre control site. A free standing BC40 fan fitted with a cage and sock was sited approximately 5 metres inbye the face line underneath the heading conveyor. The respirable dust concentration at a site 5 metres from the return end of the face was 5.8 mgm~ 3, and respirable quartz was 0.23 mgirr 3. In the return roadway the air split, with some of the air passing up the return heading being cleaned by the free-standing filter unit placed under the heading conveyor 5 metres up the heading. The subsequent small reduction in respirable dust concentration can be seen from samples taken 20 metres inbye the face and 70 metres inbye the face, although no reduction in quartz concentration is indicated. At the Dosco driver's position the mean respirable concentration was 5.0 mgm~ 3, about 25 per cent more than at the 70 metre site; the quartz concentration almost doubled, indicating once again that some back-up of dust from the heading face was occurring. This effect, although small, was confirmed by Simslin traces from the driver's position. Comparison of mean respirable dust concentrations from the site 70 m up the heading, 4.1 mgm- 3, and from the site 70 m down the return road, 1.5 mgm- 3, again showed that dusty air from the face was being drawn preferentially into the heading. This was shown even though the air entering the heading had been partially cleaned. Mean respirable concentrations measured at the entry to the heading exhaust fan duct, 27.3 mgm- 3, and at the exit from the wet sock air cleaner on this duct, 2.9 mgm- 3, indicated the air cleaner to be about 90 per cent efficient for dust of respirable mass. The free-standing filter 5 m up the heading, although of similar design, appeared to have relatively little effect. This was observed to be due to poor siting of the unit, which allowed the dust to plume past it on the far side of the roadway. It was suggested that the filter should be resiled and that some engineering research work might be carried out to design a more efficient captor hood than the present circular duct orifice The third return advanced heading selected (designated FE4) was about eighty metres long, ventilated by an exhaust system and worked by a Dosco LH1300 fitted with an air curtain constructed of 100 mm Coanda tubes. The heading exhaust

23 13 fan was fitted with a two-stage Bretby wet filter unit. The heading hit a major fault during the exercise, curtailing some of the planned measurements, and the face closed prematurely. On the first sampling shift (Table 6) it was noted that, although the coalface machine did not run well, a plume formed along the conveyor track on the coalface. MRE113A gravimetric samplers suspended side by side, one in the conveyor track (3.1 mgm~ 3 ) and one in the travelling track (2.7 mgm~ 3 ), indicated a minor pluming effect. The effect was more clearly seen from samples taken 20 metres inbye the face in the advanced heading (4.3 mgm~ 3 ) and 20 metres outbye the face (1.3 mgirr 3 ). These values indicated some of the difficulty of trying to estimate the concentration of dust, which varied across the width of the face, from a measurement at a single site. Small differences in siting can lead to large differences in measured levels. In order to assess the path of the plume into the heading, MRE113As were placed on the coalface in the conveyor track and the travelling track and side by side at three positions across the advanced heading 20 metres inbye the faceline. The results are summarised in Table 7. It can be seen that the locally high dust concentrations in the plume had almost completely dissipated by the time it had travelled 20 metres or so up the advanced heading; in other words, it was almost fully mixed by this point. The fact that the concentrations were found to be higher in the return heading than on the return end of the face could be due partly to the siting of the MRE113As underestimating the respirable dust concentration at the densest part of the plume, i.e. close to the coalface, and partly to the activity of the shearer in cutting out the conveyor head position at the face end. By calculation from Simslin traces, about 25 per cent of the respirable dust measured 20 metres inbye the face was produced by the face machine when cutting the conveyor head and fly-cutting at the return end of the face Further Investigations to Examine the Pluming Effect From all three return advanced headings it was apparent that the major part of the airborne dust produced by the coalface machine was being carried along in the airstream close to the coal face; it was then drawn preferentially into the heading by the exhaust ventilation system causing potential exposure of the heading team to the dust. The phenomenon appeared to be a major factor in the potential exposure of the heading team, and it was therefore decided, in conjunction with British Coal Headquarters Technical Department, to carry out some measurements to quantify the effects of the plume. The aim of this was to study the plume concentration profile in order to give an indication of the ways in which the pluming effect could be utilised to control the dust entering the heading. The investigation was carried out in the following manner: a) measurements were made in order to plot the ventilation movement around a typical longwall face with advanced heading in order to aid scaling of the model (see c).

24 b) a Simslin study was carried out of the mechanics of the plume formation 14 c) a coalface simulation small-scale model was set up in the IOM laboratories based on the longwall face/advanced heading studies in (a). This was used to investigate the behaviour of the pluming effect in greater detail. The general conclusions from these three studies are given in i) and ii) below and Section 4.3 respectively. i) Ventilation Survey of a Return End Advanced Heading with Exhaust Ventilation The study was set up as an aid to scaling and to judge whether the model face could produce realistic simulations of the air movement in the face end and heading. It was shown that: a) On the coalface, the highest ventilation velocity was typically at a point about 1 metre from the coal in the conveyor track with a mean velocity of about 2.0 ms' 1 b) Velocity profiles showed several dead spaces in the return road. c) On the coalface studied, 80 per cent of the air leaving the coalface was drawn into the advanced heading. d) The mean air velocity in the heading was 0.5 ms' 1. ii) Assessment of the Differences in Respirable Dust Concentrations Measured in the Conveyor Track and in the Travelling Track at the Return End of a Longwall Face. Respirable dust concentrations were measured by Simslin instruments mounted side-by-side, one in the centre of the conveyor track and one in the centre of the travelling track. The results showed clearly that, as previously noted, a plume of dust travelled along the conveyor track and that respirable dust concentrations were always higher there than in the travelling track (Figure 5). Surprisingly, there was no indication of total dispersion of the plume even when the coal getting machine was operating at the extreme intake end of the coalface. This is an important finding, indicating that the increased dust concentration is preferentially drawn into the heading throughout the machine operating time and not just for the short period of time as the machine approaches the return end. The ratio of conveyor track dust concentration to travelling track concentration was calculated for each period of 7.5 minutes as the machine completed a full cut from intake to return. Until the machine was within 75 metres of the return road the ratio remained essentially stable at about 1.7:1. From this point the ratio rose steadily until the final 15 metres of cutting, when it rose sharply to a ratio of greater than 10:1.

25 15 Figure 6 shows the shift mean respirable dust concentrations measured at points 70 metres inbye the return end of the face in the advanced heading and 70 metres outbye the face in the return roadway. The concentration measured in the advanced heading was more than double that measured in the return road. Figure 6 also gives the respirable dust concentrations on the faceline taken on the same shift by MRE113A samplers. Two MREllSAs were mounted side by side with the Simslins in the centre of the conveyor track and the centre of the travelling track at the return end of the coalface. The ratio of conveyor track to travelling track concentrations was 1.6:1, similar to that shown by the Simslins when averaged over the full shift. However, as mentioned above, there is some doubt about the ability of a static instrument to give a true measurement of the linear concentrations in the plume. 4.3 Study of dust pluming effect using the */ lo Scale IOM model coalface In previous sections of this report it has been noted that uneven mixing of air on the coalface results in a plume of dust forming over the conveyor track and being preferentially drawn into the advanced heading by the exhaust ventilation system. It has also been demonstrated that, whilst the pluming effect had long been recognised by mining engineers, the present underground dust sampling instruments were incapable of fully investigating the nature and magnitude of the problem. It was therefore concluded, in co-operation with British Coal Headquarters Technical Department, that the most efficient and cost-effective way of investigating the problem further was by use of a small-scale model coalface. The use of scale models to study the properties of aerodynamic systems is not new. Scale models have been used in the past in relation to the coal mining environment, including recent studies at the Institute of Occupational Medicine in relation to both dust and methane (Vincent et al, 1983 and Aitken et al, 1989). These authors showed that the use of scale models had considerable potential provided the appropriate scaling laws were identified and applied. Factors considered in this experiment were main and auxiliary ventilation rates, the position of the shearer and the dimensions and layout of the face. The main ventilation airflow along the model face was provided by a variable speed 2 Kw centrifugal fan enabling face mean air velocities to be varied in the range 0-8 ms" 1. The auxiliary ventilation was capable of providing a mean air velocity of 0.25 ms" 1 in the model advanced heading. The system was set up to model as closely as possible the geometry of a typical coalface, similar to that described in Section This had a face height of 1.2 m and a conveyor track of width 1.7 m. The equivalent model dimensions were m and 0.19 m respectively, giving a model scale of about V 9 for that particular arrangement. Smoke was used as a tracer to simulate the distribution of airborne respirable dust. A diagram of the model setup is given as Figure 7.

26 16 Two main types of measurement were made; direct measurement of air velocities (both for ventilation quantity assessment and to consider structure or flow patterns) and measurement of smoke concentration (including concentration profiles). Initially some qualitative observations were made, followed by quantitative measurements Qualitative observations The model was run at a number of nominal ventilation rates, and a high concentration of smoke was introduced in the region of the shearer drum. High intensity 'slit' illumination was used so that flows could be observed as 2-dimensional 'slices', and these allowed detailed observations to be made of the smoke patterns associated with the mixing processes within the model. On the face downwind of the shearer the smoke concentration in the conveyor track was clearly much higher than in the travelling track. Inceasing the face air velocity did not appear to enhance or reduce the pluming. Moving the smoke source closer to the face end resulted in the concentration imbalance becoming more pronounced. As the tracer exited from the face, most of it appeared to travel across the roadway to the opposite wall, where it turned upwards following the line of the wall to the roof. At this point it was seen to split either to enter the heading or to pass down the return roadway. In the heading itself, violent mixing patterns were observed in the first metre, with a very marked 'hole 1 in the tracer smoke on the face side of the heading, Further into the heading, at around 2 metres (the equivalent of about 20 metres full scale), the tracer was more evenly distributed and the air movement largely unidirectional, A similar pattern was observed in the return roadway. All the qualitative observations were broadly similar to those observed at underground locations Quantitative Results Velocity profiles across the face at 0.5 m from the face end are shown in Figure 8, where the measured velocities are shown as a function of distance from the coalface at three different positions between the detector and the shearer. Also shown are the measurements taken on the face described in Section at a distance of 5 metres from the face ends and with the shearer at the intake end of the face. The profiles for both model and actual faces are similar, with distance to the shearer having little effect. All exhibit the same general form, with velocities in the machine track being the higher and travelling track velocities lower. On the model the lowest velocities corresponded to the position of the front leg of the roof support at about 0.15 m from the coal face. On the actual face measured, the roof support leg was slightly further from the face than this.

27 17 In Figure 9 the ratio between machine track and travelling rack concentrations is plotted as a function of distance between the shearer and the measurement position. Two groups of measurements were made some months apart, and whilst some differences can be seen, the two sets of results are broadly similar, indicating the reproducibility of the technique. The hand-drawn dashed line in Figure 9 shows a sharp increase in ratio as the shearer approaches the face end with a ratio of 85:1 measured for a shearer distance of 0.85 m (equivalent to 7.5 m full scale). An observation made on an actual face (section 4.2.4, ii) suggested that the ratio in the final 15 metres of cutting peaked at a level in excess of 10:1. The effect of varying main airflow rate was investigated for different shearer positions. It was found that the rate of airflow had very little effect on the relative concentrations of dust on the face line and in the travelling track for any position of the shearer (Figure 10). For two shearer positions (1.7 m and 2.6 m from the detector) a series of measurements was taken at increments of m between m and 0.3 m from the wall (Figure 11). Error bars indicate the reproducibility of the averaged profiles, means of 2 profiles at distance 1.7 m and of 5 profiles at distance 2.6 m. These results show that there are not two distinct concentration zones based on the machine track and travelling track, but that there is a continuous, approximately exponential concentration gradient across the whole coal face cross-section. The gradient is steeper when the shearer is closest to the face end. This emphasises the observations made earlier of the inability of static dust samplers on an actual face to measure the concentrations in the plume or the ratios of concentrations across the plume. The concentration ratios measured here of 20:1 for the shearer at 1.7 m and 6:1 for the shearer at 2.6 m are consistent with the single measurement reported earlier. An attempt was made to consider the concentration build-up in the heading due to the plume. For this study, one detector was placed 2 m inside the heading from the face and one was sited 1.5 m down the return roadway. In earlier observations the tracer had been seen to be well mixed at these points, so that samples from the roadway mid points were expected to be reasonably representative of the concentration across the roadway at these positions. Figure 12 shows a plot of the ratio of the measured heading and roadway concentrations against the ratio of the heading to return roadway ventilation quantities. Several points may be noted. Firstly, the concentration ratio was always higher than the ventilation quantity ratio, indicating that more pollutant was drawn into the heading than would be expected on the basis of ventilation quantities alone. The results also indicate that the concentration ratio increased steadily with the ventilation quantity ratio, but was independent of the actual individual values of auxiliary ventilation and return road ventilation quantity.

28 4.4 Observations on the use of Coanda Tube Air Curtains 18 The heading machines in all the return advanced headings visited were fitted with Coanda tube air curtains. The aim of these is to help to redirect the dust produced by the heading machine back to the exhaust duct, and hence to stop airborne dust back-up from the heading face from reaching the machine driver. Measurements were made in order to assess the effectiveness of the air curtains in these headings, and the results are given in Table 8. Respirable dust concentrations were measured by MRE113A in three static positions: near the heading face by the exhaust duct entry, at the machine driver's position and in the advanced heading behind the driver (in order to measure the incoming concentration). Short peaks in the respirable dust concentrations were found at the driver's position during the cutting phase, showing that back-up dust did occasionally penetrate the air curtain. However, from the difference between shift average respirable dust concentrations measured at the driver's position and behind the driver, the amount of dust in these peaks was shown to be a very small proportion of the overall dust produced by the cutting head. The ratio of the estimated amount of back-up dust reaching the driver to the respirable dust concentration measured near the duct entry gave an estimate of the efficiency of the overall ventilation system comprising the exhaust ventilation and the air curtain. From Table 8 it can be seen that the systems appeared to be between 66 and 99 per cent efficient. The lowest estimated efficiencies were for the site mentioned above, where investigations pointed to mechanical deficiencies and the use of water jets to aid cutting (rather than just for dust suppression). Shift average respirable dust concentrations measured close to the cutting zone and exhaust ventilation duct entry reached as high as 48.5 mgm~ 3. Even though the air curtain system on this occasion appeared to be about 93 per cent efficient, the estimated shift mean respirable dust concentration reaching the driver from the other side of the air curtain was 3.5 mgm~'. This observation shows that in addition to a well-maintained air curtain, efficient water sprays are also required at the cutting head for dust suppression purposes. The efficiency of the exhaust ventilation may also need to be examined. 4.5 In-line Rippings at Intake Ends of Coalfaces Two in-line rippings at the intake end of longwall faces were investigated; one employed a Ranging Drum Shearer and the other was worked by a Webster Ripper Ranging Drum Shearer - Intake In-line Ripping On the ripping investigated here (designated FES), the mean respirable dust concentration reaching the ripping as intake contamination was 5.0 mgm~ 3 with a respirable quartz concentration of 0.35 mgm~ 3 (Table 9). The mean concentrations measured 20 metres up the face were 6.1 mgm~ 3 and 0.32 mgm~ 3 respectively indicating that the ripping operations were only a minor dust source.

29 19 The high intake contamination was identified by the investigation and by local Dust Control Engineers as being due to badly set conveyor water sprays, disconnected sprays on the intake gate lump breaker and a malfunctioning free-standing filter unit. All these points received attention, and on a series of return visits the shift mean respirable dust concentration at the intake roadway site was found to have been reduced by 24% to 3.8 mgm~ 3. This was a smaller reduction that expected, but one which indicated the problems of attempting to control airborne dust by water sprays unless they are directed at the dust source. Generally the ripping operations did not appear to be a major source of dust, but records from Simslins sited 20 metres up the face and 10 metres outbye the face in the intake road to showed that on one shift 71 per cent of the respirable dust measured was due to intake contamination, 9 per cent was due to the ranging drum shearer cutting the roadway profile and 20 per cent was due to fly-cutting at the intake end of the face. Past investigation (Bradley et al, 1979) concluded that the use of ranging drum shearers always posed dust control problems due to low pick penetration, low haulage speed and the fact that the drum was not fully buried in the mineral being cut. The present observations appear to contradict the 1979 findings, but disucssions with Mining Engineers indicated that improvements in machine design have increased traverse speeds and pick penetration, and that nowadays much more care is taken over the selection of machine type for different types of strata. The investigators did, however, comment on the amount of dust produced by fly-cutting, but they were advised that fly-cutting was an integral part of the face operations and could not be eliminated. Whilst it was stated that fly-cutting could not be eliminated, it should still be reduced as much as practicable because it not only creates dust but is counter-productive. A similar observation was made most emphatically in the previous report (Bradley et al, 1974) Webster Ripper - Intake In-line Ripping The selected face end (designated FE6) was worked by a Webster Ripping machine operating on an in-line ripping. Reference to Table 10 shows that intake pollution was low at an average of 1.8 mgm~ 3 respirable dust. The mean respirable dust concentration at the Webster operator's position (mean 1.9 mgm~ 3 ) indicated that the ventilation stream was effectively scouring the ripping face and that virtually no back-up of dust was occurring from the Webster cutting head. Simslin traces, an example of which is given in Figure 13, taken 20 metres up the coal face indicated that the Webster was capable of producing very short peak respirable dust concentrations of up to 50 mgm~ 3. However, the overall contribution to the shift mean concentration was very low. The traces also showed that the main coal getting machine cutting at the face end produced lower respirable dust levels than the Webster Ripper.

30 In-line Rippings at Return Ends of Coalfaces Two in-line rippings at the return ends of faces were examined. worked by a Ranging Drum Shearer and the other by a Webster Ripper. One was Ranging Drum Shearer - Return In-line Ripping (Face End FE7) Measurement by MRE113As placed on the face 10 m from the return end and in the return road 10 m back from the ripping indicated that the operations by the ranging drum shearer on the last ten metres of face produced about one third of all the dust reaching the return roadway (Table 11). The quartz concentration increased in similar proportion to the respirable dust concentration, indicating that the major proportion of the dust reaching the second instrument originated from a similar source to that reaching the coal face instrument, i.e., the ranging drum shearer cutting the coal seam. It did not appear that much of the dust had been raised by cutting the roadway profile. Examination of traces taken by Simslins mounted side-by-side with the MREs showed that the time taken for the ranging drum shearer to cut the roadway profile was very small and that often the production of dust by this operation could not be separated from dust peaks produced on the inbye side of the instruments. It was concluded that the amount of dust produced by the profile cutting operation was very small. However, a number of peaks were produced by the shearer as it cut the tailgate end of the face in a series of flycuts. Over a number of shifts observed the fly-cutting operations averaged 85 minutes (Table 12) and produced about 40 per cent of all the dust measured at the site 10 metres down the return roadway Webster Ripper - Return In-line Ripping (Face End FES) Measurements made by MRE113A 10 metres from the return end of the face (mean respirable concentration 4.9 mgm~ 3 ) and at the Webster driver's position (mean respirable concentration 7.9 mgm~ 3 ) indicated that approximately 40 per cent of the dust reaching the driver's position during each shift was produced by the main coal-getting machine during fly-cutting and by the Webster Ripper (Table 13). Examination of the Simslin charts indicated that almost all this dust was produced by fly-cutting. 4.7 Retreat Face Ends It is the policy of the British Coal Corporation to increase, wherever possible, the number of retreat faces. This method of mining has advantages from the points of view both of mining technology and of production. The number of face end operations is reduced by retreat mining due to the use of preformed roadways, but it was felt necessary to include at least one intake and one return face end in the

31 study because of the rapid increase in this type of mining Retreat Face End - Intake (FE9) The only major operation at the intake end of the face (Face End FE9) was the forming of a small stable hole by shotfiring. This operation was designed to avoid fly-cutting. The mean respirable dust concentration measured over two shifts in the intake roadway 20m outbye the coal face was 0.5 mgm~ 3. This low level of intake contamination was due to factors such as the intake roadway being wet, and also because the district was ventilated homotropally. On the same two shifts, the mean respirable dust concentration at a point 15m down the face was 1.0 mgm~ 3. This indicated that the machine operations and shotfirings in the first 15m of the face contributed only an additional 0.5 mgm~ 3 to the average level. These observations were confirmed by the Simslin charts, which showed that the highest of the shotfiring peaks was 5.0 mgm~ 3 and that these peaks were of very short duration. This may be compared with a shift mean respirable dust concentration at the 70m point of 2.4 mgm~ 3, and with the results from the return face end, FE Retreat Face End - Return (FE10) This face was also ventilated homotropally. The return end of the face (main gate) was worked by the main coal-getting machine, which involved a certain amount of fly-cutting in order to get the machine into the full web for the return run along the face. Respirable dust concentrations at the 70m site were generally low, as on Face FE9, with a highest shift mean concentration of 3.0 mgm~ 3. A Simslin sited at a position 10m from the face in the return road showed that peaks of between 10 and 35 mgm~ 3 were being produced during the fly-cutting operations.

32 22

33 23 5. CONCLUSIONS AND RECOMMENDATIONS S.I Advanced Headings Well constructed and maintained air curtains fitted to heading machines are a very efficient method of reducing dust back-up from the heading face. However, airborne dust concentrations of almost 50 mgm~ 3 have been measured near the heading face itself. Care should therefore always be taken to fit an efficient spray system to the cutting head in order to reduce the concentration of airborne dust at the heading face In both intake and return end advanced headings with exhaust or partial recirculation systems, the heading team is exposed to airborne dust travelling up the heading from behind At intake ends of faces, if methane concentrations can be controlled, a partial recirculation system with a good air filtration system is to be recommended for dust control At return ends of faces the heading team is often exposed to considerable levels of airborne dust due to the pluming of dust from the coal-getting machine along the conveyor track and the fact that these higher dust levels are being drawn preferentially into the heading. Some important work was conducted on the lom's model face to characterise the plume. It is now known that dust concentration in the plume reduces approximately exponentially across the profile, and that the dust concentration close to the coalface can be a hundred times greater than that in the travelling track. It has also been shown that the plume turns into the advanced heading essentially intact. Further work is required on the scientific design and proper siting of captor hoods for attachment of free-standing filters in the return end roadways.