A New Ventilation Method for a Thin Seam Room and Pillar Coal Mine Alan Bilton 1 & Bill Tonks 2

Transcription

1 A New Ventilation Method for a Thin Seam Room and Pillar Coal Mine Alan Bilton 1 & Bill Tonks 2 1 Mine Manager, New Crofton Co-op Colliery, c/o Blast Log Ltd, 30 Grovehall Parade, Leeds. LS11 7AE Tel: albilton@newcroftoncoopcolliery.co.uk 2 Director, Bill Tonks Ventilation Services Ltd, Edwinstowe House, High Street, Edwinstowe, Notts. NG21 9PR Tel: billtonks@gmail.com Presented at the Midland Institute of Mining Engineers 10 th Annual Safety Seminar, 10 th April 2015, Royal Victoria Hotel, Sheffield. Introduction The New Crofton Colliery project proposes to open a new drift mine - operating as a worker s co-operative - that will produce 200,000 tons of coal per year and employ 50 people for 20 years. The New Crofton site is located 6 miles South East of Wakefield and 1 mile South of Nostell Priory (Figure 1). Figure 1 Location map of mine site Surface Arrangements The proposed mine surface (Figure 2) is a disused open-cast coal disposal site (Figure 3) which still has a working rail connection. The reserves are in the Sharlston s Top, (Muck) Low, and Yard seams in an area of coal that was originally proposed to be worked as an open cast coal site. Figure 2 Red line shows mine boundary including access road Figure 3 Previous use of mine site as coal disposal point 1

2 The proposed layout of the surface is as shown in Figure 4. Figure 4 Proposed surface layout A radial stacker will be used as the main coal stocking distribution device. It can drop coal onto the usual radial area, or swing round to feed the first vertical bunker. The vertical bunkers (Nos. 6 and 7 on the plan) for power station coal are situated above the main drift belt, and will discharge onto a feed-out belt mounted above the drift belt. When the first bunker (No 6) is full the stacker can feed a bunker-to-bunker belt to fill the second bunker (No 7) (Figure 5). Figure 5 Section of upcast drift and bunkers Geological Information The major faults in the area can be seen on the geological map in Figure 6, which also shows the coal-take area in the red dashed line. Figure 6 Geological map showing coal-take area and faults 2

3 The seam sections of interest to New Crofton are in the Sharlston group as follows (Figure 7): The Top seam is approximately 1.2m thick in the close vicinity of the drifts and is 50m from the surface. The Low seam is 20m below the Top seam and is approximately 1.2m thick, it has a 0.6m rock band above it and sitting on the rock band is the Muck seam approximately 1.2m thick. The Muck seam is multi-banded coal and rock about half coal and half rock. The Yard seam is approximately 0.9m thick and 20m below the Low seam. Figure 7 Section data from boreholes There are a multitude of boreholes in the New Crofton mining area that confirm these seams continuity (Figure 8). Figure 8 Boreholes and section-line across the mine site The reserves are split into 2 main areas by an East-West fault of approximately 40m throw on the line of the main drifts. This allows a convenient level access from the lowest seam (Yard) in the North area to the highest seam (Top) in the South area (Figure 9). 3

4 Figure 9 Cross-section across north of mine Consultation with the Surrounding Population Preparations for the planning application started with consultations with the locals. New Crofton did not want to force their way into the area instead explaining what they wanted to do - and how the area would benefit from the mine. Figure 10 Working layout for the north area (Sharlston Top seam) 4 From these meetings the main concerns raised were that there was to be: no subsidence; no mine dirt tip; and employment for as many locals as possible. These criteria were all accommodated in the design. In addition, a share of the profit from the mine was proposed to be used to benefit the local area, and create new co-operative ventures that should continue to flourish well after the mine has ceased production. In-seam room and pillar mining was chosen in favour of long wall extraction which would have caused subsidence. Room and pillar mining is also considered to minimise dirt make. In addition, the shallow depths allow small pillars, which helps maximize coal recovery. Underground dirt disposal was designed in favour of a spoil heap. As many locals as possible were proposed to be employed Figure 11 Working layout for the south area (Sharlston Top seam) (tempered by the initial need for qualified miners to get the mine started). The basic working layout for the North and South areas is shown in Figure 10 and Figure 11.

5 Mining Equipment Mining equipment will consist of: a low height continuous miner (Figure 12); 3-off battery powered coal scoops (Figure 13); a feeder breaker (Figure 14); a twin boom roof bolter (Figure 15); and 1.05m wide belt conveyors. Figure 12 Low height continuous miner Figure 13 Battery powered coal-scoops The usual lowest working height is planned to be 1.2m, but a very low-profile continuous miner (CM) has been specified for New Crofton, which is able to mine the Yard seam within the 0.9m section then pull back and rip down 0.3m of roof to give a working height of 1.2m wherever required. The CM is proposed to have on-board dust extraction and scrubbing ideal for working in conjunction with forced air auxiliary ventilation (this essentially creates a shortened version of the force / exhaust overlap arrangement more commonly seen in longwall drivages). The bolting machinery proposed is to be fitted with vacuum dust capture ideal for dust control as drilling dust will not be disturbed by forcing ventilation. The loading machinery proposed is battery scoop. Battery chargers will be situated inbye, ventilated through X slits, with charging gases being vented to the return. The number of conveyor belts used underground has been minimised. They are planned to be: a drift belt; a trunk belt (East and West bound in the Northern area & South bound in the Southern area); a district belt; and where needed a small mop-up belt to get to any outlying coal areas. Figure 14 Feeder-breaker Figure 15 Twin-boom roof bolter 5

6 Mining Method In the Top and Yard seams, it is proposed to take extra height in the belt road and supply road using the continuous miner, the dirt being stowed behind the working area by scoops - between the intake and return roadways - to create air control seals. The preferred method of support is by mechanised roof bolting. As the standard bolt length is 1.8m and the section worked is only 1.2m, coupled or flexible bolts will be required and trials have already successfully matched the characteristics of a solid 1.8m bolt. The proximity between the Muck seam and the Low seam creates a problem in that the Muck seam is within the bolt length of the Low seam and the strata may not be sufficiently competent. If it is not, then an alternative is to mine the muck seam first and support its roof, then after a suitable distance has been mined (around 3 pillars in advance) dint out the Low seam coal (this was common practice in a recent room and pillar mine in the North East). Once again the dirt would be used to create air control seals. Battery powered scoops were chosen because they are versatile. As well as being used for coaling, they can also pack the dirt into the old roadways to form air seals and even clean-up roadways when required, whereas shuttle cars would only be able to drop dirt onto the floor. The supply transport system from the surface will also use a battery scoop charged and loaded on the surface with sufficient range to deliver its load as required - anywhere in the mine. Because drift gradients can quickly drain the battery, to enhance range, a rope assist via a constant pull winch will be used to cancel-out the gradient effect. The scoop driver will control the rope by driving towards or away from the winch while on the gradients and then drop the rope when it is no longer needed. The three scoops will all be interchangeable and will be rotated for coaling or supply duty, thus allowing each of their servicing schedules to be carried out on the surface. Coal Preparation The Top, Low and Yard seams will be clean-mined. This is achievable with continuous miners which are ideal for small mines - as most do not have coal washing facilities. The Muck seam would require washing, to produce a power station product. As there is no spoil heap planned, this dirt must be returned underground for disposal. Returning the dirt to the mined voids removes the need for waste disposal permit. The dirt would be crushed to a suitable size and then transported hydraulically by gravity to the required old workings. Water drain-off would be cleaned by weirs and pumped back to surface via the pit bottom sumps for re-use, so water losses from this cycle should be minimised. Ventilation Design The ventilation of the mine has required some changes to the normal style used in the UK. The ventilation at New Crofton will by force fan from surface and will be homotropal i.e. with belts in the returns. This mine infrastructure is designed to minimise sources of ignition and fuel in the intake airways, allow simple mine degassing and remove the need for belt airlocks. Because of the shallow depths, the virgin strata temperature is only likely to be around 12 C so heat will not be a problem. At shallow depth, coal seams also have a lower gas contents and in addition - room and pillar mining minimises gas emission from any overlying and underlying seams - so methane is unlikely to be a problem - neither from production in the homotropal returns - nor during barometric pressure falls in the mine as a whole. 6

7 Coal sampled from Boreholes drilled within this region during the NCB regime show that the seams typically have low gas contents between 1.4 m 3 and 2.7 m 3 of CH 4 per tonne. Even using the high figure of 2.7 m 3 per tonne it is calculated that a New Crofton heading would typically release only 14 litres/second of CH4 during cut-down resulting in less than 0.3% CH 4 vol/vol when ventilated by 5 m 3 /s. Table 1 Local boreholes analysed for gas content by the NCB BH261 Brierley Road Location: E N Seam (Sh = Sharlston) Depth below surface Total CH 4 m 3 /tonne Total C 2 H 6 m 3 /tonne Ash content % by weight Ash free CH 4 m 3 /t Ash free C 2 H 6 m 3 /t Sh Top No name Sh Yard BH121 Two Gates Location: E N Seam (Sh= Sharlston) Depth below surface Total CH 4 m 3 /tonne Total C 2 H 6 m 3 /tonne Ash content % by weight Ash free CH 4 m 3 /t Ash free C 2 H 6 m 3 /t Sh Yard BH359 Cliff Lane Location: E N Seam (Sh = Sharlston) Depth below surface Total CH 4 m 3 /tonne Total C 2 H 6 m 3 /tonne Ash content % by weight Ash free CH 4 m 3 /t Ash free C 2 H 6 m 3 /t Sh Top Sh Low Sh Yard The room and pillar mining method proposed at New Crofton is conventional throughout the world and has been re-introduced into the UK in recent times in the North East, Midlands and latterly into South Wales where it is currently being worked with modern continuous miner and mechanised bolting machinery similar to that proposed at New Crofton. The concept of ventilation of headings in the UK has always relied upon auxiliary fans and ducting. Although the series ventilation using brattice lines prevalent in USA s room and pillar mines (Figure 16) is not illegal, its use has largely been precluded in the UK following lengthy discussion with HMI. This is because the high panel resistance they create produces a reduction in continuity and control of ventilation during machine movement. There is a general leakiness and there is an increased risk regarding series accumulations of dust and CH4 passed from heading to heading. Conversely, fan and ducting systems deliver a continuous known level of clean feed air to each working place, and when using regulation discs, can deliver an accurate jet flow of regulated airflow in each of those headings that are stood down i.e. never leaving headings unventilated (Figure 17). 7

8 Figure 16 Brattice layout in the USA Figure 17 Example of force fan ventilation (UK) New Crofton districts will each have 7 headings. The perimeter force duct ventilation design of headings within each district is novel and takes into account the UK ducted air philosophy, whilst still accommodating machine movement in very low seam heights. To allow the positive ventilation of headings by fan and duct, arrangements in the production districts had to be rethought. With a worked height of just 1.2m and a machine height of 1.0m, it was impossible to run machines under any suitable size of air ducting and it was considered that even letter-box style air crossings would quickly become damaged. So after consideration of the place changing method of working, it was noted that only two rooms were actually working at any one time: the miner room; and the bolter room. All the others were stood down waiting to be cut then bolted. The main ventilation flows were only required in these two rooms and the others only needed to be kept sweet until they were re-visited. The innovative concept at new Crofton was to use two auxiliary fans each fan running its ducting around the outer walls of the district - before running along the last cross cut to feed the 2 working rooms. 8

9 This way, the centre of the district was left totally free from ducting, allowing free movement of scoops and continuous miner. The waiting rooms would be ventilated with T-offs from the main ducting run regulated by air jetting discs installed at the face end. The working rooms ventilation system was designed to mimic the place changing of the machines i.e. each time the machines moved - the ducting was to be moved too, but always hugging the side and front walls to leave clear passage routes for machinery (Figure 18). Figure 18 New Crofton perimeter ventilation ducts Dust control will achieved by the CM on-board scrubber, by dry exhausters on the bolter and because the mine is homotropal, dust from the belts will be carried away from the working area in the returns, so intake air will be as clean as possible and also offer the safest possible exit route from the mine. The system felt right but would it actually work? Brief History of Ventilation Engineering In the past, ventilation has been considered by many to be a black art. Actually however, it can be designed, calculated and predicted very accurately - using a process otherwise known as Engineering. The fathers of the engineering approach to ventilation engineering in the UK were: John Buddle, ( ) An English engineer who made safer furnaces, invented parallel splits and worked with Davy testing flame safety lamps after the Felling mine explosion; John Job Atkinson ( ) an English mines Inspector, who wrote his work of genius On the Theory of Ventilation in upon which all ventilation calculations are based today - and who s work showed that with ventilation surveys, longhand calculations could be used to predict and design ventilation in mines perfectly accurately; Hardy Cross ( ), an American who developed an iterative method for solving fluid flows in networks (did he know how good computers would eventually become at iteration?); Professor Frederick Hinsley ( ) and Malcolm J McPherson ( ) who both advanced and modified this work to create computer programs for solving airflows in networks at Nottingham University. 9

10 Ventilation Modelling Ventilation modelling quickly became significant in the UK coal mining industry with the introduction of commercially available computers. Early models were created by the NCB s technical department at Bretby firstly run on mainframes with punch cards - then latterly run on smaller department based Tektronix machines by the mid 80 s. By the late 80s, the first desktop PC s had become available and the NCB s numerical Vent3 program was written to work on a twin floppy drive IBM PC; then a pen-plotter version Vent4 to work on an IBM AT or PS2; and finally an Autocad Vent5 version to work on Compaq 386 machines just before the demise of British Coal in Later variants were the XEagle MinCAD version MinEnet program written by Mike Clayton, David Darns and Bill Tonks in the mid 90 s and an American commercial package written by Malcolm J McPherson s company MVS called VnetPC available in the early 2000 s. All of these models were numeric and latterly strived to give a 2D plan presentation with the universally recognised intake and return colours and the facility to superimpose numerical levels of air quantity, pressure and roadway resistance as desired. A revelation however was the introduction of Ventsim (Figure 19) in around 2010, written by Craig Stewart of Chasm Consulting on a gaming platform. This allowed dynamic 3D models to be created and for the first time, you could now see air flowing around a mine in real time in exact scale roadways from any aspect up down and all around, whilst still retaining numerical superimposition facilities where required. In addition, this powerful software also incorporates thermodynamic algorithms, pollutant and fire toolbars (by Dr Rick Brake), together with auxiliary fan and main fan charting graphics. To avoid inputting errors, the import facility can pull-in Autocad or DXF files straight from the Surveyors desk resulting in a perfect mimic of the mine. Figure 19 A screen grab of some VentSim features New Crofton Ventilation Parameters The Sharlston Top, Sharlston Mid and Sharlston Yard working heights in general are 1.2m, 1.2m (plus 1.2m a Muck inferior seam separated by 0.6m dirt) and 0.9m thick respectively. The drifts and South extension roads are to be extracted at an effective 5.2m wide and 2m high square section (assumed supported by steel) and the coal will be extracted at 6.0m wide by 1.2m high (assume bolted) except for the far right hand intake supply roadway which will be 1.8m high 10

11 and the central return belt road which will also be 1.8m high. The belt will be 42 wide on heavy duty structure. For dust control, the minimum air velocity required to prevent dust backing up is 0.3 m/s. Because New Crofton could extract at up to 3m height (in the muck/mid seam), this meant that a maximum of 5 m 3 /s would be required in both the continuous miner and bolter headings. This would however generally be far in excess of requirements where (the majority of) cutting was only to be at 1.2m height or less. Airflow requirements were calculated to be minimum18 m 3 /s to feed the auxiliary fans and another 6 m 3 /s to prevent recirculation, so a minimum of 24 m 3 /s was required to be delivered to the inbye end of the working district. The water pumping sump area to the North was planned to be ventilated by return air in parallel with the main return so no additional air would be consumed whereas the water pumping sump area to the South had to be ventilated by fresh air that was bled back onto the return. A nominal 10 m 3 /s was assigned for this. New Crofton Modelling New Crofton was designed in 3D using Vulcan software. Once drawings had been imported from this directly into Ventsim, it then took just a couple of days to create a series of accurate and fully working models of Crofton that showed various ventilation-critical stages likely to be encountered throughout the mine s life. The objective was to fix sufficient quantity at the surface fan to be able to deliver over 24m 3 /s into the working place of the furthermost district at different stages in time. The models calculated the pressure required to do this so fan duties and sizes could be determined. The modelling process also allowed other critical aspects and sensitivities to be tested. An Atkinson friction factor of kg/m 3 was used for the roadways throughout the model, whilst a friction factor of kg/m 3 was used for the flexible ducting blowing into the headings. Within the models, pit bottom doors and cross cuts local to the inbye end of the districts were assigned a resistance of 50 gauls (a typical resistance of a reasonable set of doors) and as per above, the roadways dimensions, obstructions and layout were input accordingly and their resistances were calculated automatically in the program. The auxiliary fan outlet and perimeter duct were assigned 0.9 m diameter and the manifold face road and heading feed ducts were assigned 0.6 m diameter. Figure 20 Crofton auxiliary ventilation 11 The auxiliary ventilation was shown to be easily achieved using two off 37 kw centrifugal fans (ex NCB design). This was based on any two working headings having 5 m 3 /s delivered and the remaining stood headings being ventilated by a nominal 0.5 m 3 /s through jetting blank plates - ideal for sweeping faces of stood headings (Figure 20).

12 The ventilation models were constructed to illustrate two stages Stage 1 up to 6 years then Stage 2 for remaining life-of-mine. The six year point was selected to broadly coincide with the mine venturing into the mid and lower end of the South take. The first model showed year 2 as the concept for all of Stage 1, where the Sharlston Top seam has been mined by taking districts in a clockwise sequence and has reached the furthest extent to the West (Figure 21). Figure 21 Crofton northern extremities The second model showed the furthest extent of the mine in the South in the lowest seam as the concept for Stage 2 (Figure 22). Figure 22 Crofton southern extremities In Stage 1, the logic was that once the extents were reached, then shortening back would only require less airflow from that point on. The logic also led to the idea that whatever surface fan air this stage required to achieve 24 m 3 /s inbye, then this airflow would also be capable of ventilating to similar extents in the Mid and Yard seams below in the West take too. Extending the logic further, it also pointed to the mine being able to venture into the South workings to a similar extent to that in the North with the same airflow. 12

13 Note: At any distances less than these extents, the Stage 1 fan could be turned down on the VSD - matching the duty exactly and only to the airflow required to deliver 24 m 3 /s inbye - thus saving power. Results It was shown that for Stage 1, the mine could be ventilated adequately using a fan (or fans) having a duty of 80 m 3 /s at 844 Pa total pressure, for 2m diameter fan. This meant an airpower of 67 kw was required. This meant that for a fan of typically 75% efficiency, an electrical load of 90 kw would be created. This meant that to prevent overloading a 112 kw motor would be required (Figure 23). Figure 23 Crofton surface fan: Stage 1 In Stage 2, the logic was that a fan capable of maximum duty for the remaining life of the mine was required. Any lesser duties (during periods of working towards this extent and working away from this extent in each seam) could likewise be achieved by turning the fan duty down on the VSD, saving power during the approach/ retreat from this stage. It was shown that for Stage 2, the mine could be ventilated adequately using a fan (or fans) having a duty of 105 m 3 /s at 2356 Pa total pressure. This meant an airpower of 247 kw was required. This meant that for a fan of typically 75% efficiency, an electrical load of 330 kw would be created. This meant that a standard motor of 375 kw would be required to prevent overloading (Figure 24). Figure 24 Crofton surface fan: Stage 2 13

14 Critical Findings One of the critical aspects of room and pillar work is the conservation of airflow and faced with a profusion of poorly sealed cross cuts, air can quickly leak away. With this in mind, the room and pillar roads to the West and the drift extension roads to the South were originally tested in the models at 50 gauls, but had to be assigned a tighter resistance of 200 gauls to preserve airflow to the districts. A proven technique to achieve this is by foam injection - which is quick, cheap and effective (Figure 25). Figure 25 Crofton cross-cut sealing In addition, an arbitrary 1 m 3 /s was initially assigned to stood headings to keep them sweet, but this was shown to be too excessive for a 37 kw auxiliary fan to deliver when at full stretch (i.e. feeding one working and five stood headings). A stood heading flow of 0.5 m 3 /s was tried in sensitivity tests and found to be achievable. Examination of the 0.5 m 3 /s figure showed it still to be suitable - providing that the air was jetted into the face of the heading to prevent layering (Figure 26). Figure 26 Crofton auxiliary ventilation headings (velocity when regulated to 0.5 m 3 /s) 14

15 Conclusion New Crofton has been carefully designed to exploit the reserve as thoroughly as possible whilst still fulfilling the wishes of the general public in the immediate area. The extensive knowledge of the strata and topography of the area and existence of essential local services has given New Crofton a unique advantage in planning and logistics. The Ventsim 3D modelling process has enabled Engineers to visualize ventilation at New Crofton in an unprecedented manner and has shown that all of their proposals can be achieved. Specifically, it has shown how the auxiliary fans would perform in the headings; how air would be distributed around the districts, pumping lodges and drifts; has highlighted a cross-slit leakage problem in trunk airways; and has allowed the Engineers to specify duties for the main surface fan for life of mine. It has been shown that two standard 37 kw auxiliary fans per district can deliver the quantities of air required in the headings. To support these fans with the correct levels of airflow, sufficient surface fan airflows have been demanded and the pressure required to achieve that flow have been illustrated. Using these flows and pressures it has been determined that a surface fan up to year 6 would require around 112 kw motor size and up to year 20 would require around 375 kw motor size (both assuming 75% fan efficiency). The surface fan is proposed to be powered using a variable speed drive (VSD) that can deliver a full range of duties between the initial and final stages. Using VSD s it is envisaged that these fan(s) would rarely be used at full speed, the concept being that the large size simply ensures there is sufficient power available whenever the far-extents of the mine are reached in each seam. Additionally (or as an alternative to VSD), depending upon the fan design, it may be possible to upgrade from Stage 1 to Stage 2 simply by adding extra fans in series as future demand dictates (Figure 27). 2 No. red line fans (100%) in series Covers all situations, including Stage 2. If a single fan was used, then a 374 kw motor would be needed. For 2 fans in series, 2 No. 225 kw motors will be adequate. 1 fan at 100% duty This speed would need more than 112 kw motor (e.g. 225 kw), but could then run beyond Stage 1 duties 1 fan at 78% duty A 112 kw motor would be enough to provide the Pa required during Stage 1. Figure 27 Crofton surface fan options 15