1

Figure 2 Red line shows mine boundary including access

road

Figure 1 Location map of mine site

A New Ventilation Method for a Thin Seam Room and Pillar Coal Mine

Alan Bilton1 & Bill Tonks

2

1 Mine Manager, New Crofton Co-op Colliery, c/o Blast Log Ltd, 30 Grovehall Parade, Leeds. LS11 7AE

Tel: 0113 385 9870 Email: albilton@newcroftoncoopcolliery.co.uk 2Director, Bill Tonks Ventilation Services Ltd, Edwinstowe House, High Street, Edwinstowe, Notts. NG21 9PR

Tel: 01623 821548 Email: billtonks@gmail.com

Presented at the Midland Institute of Mining Engineers’ 10th

Annual Safety Seminar, 10th

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).

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 3 Previous use of mine site as coal disposal point

2

The proposed layout of the

surface is as shown in Figure

4.

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).

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 4 Proposed surface layout

Figure 5 Section of upcast drift and bunkers

Figure 6 Geological map showing coal-take area and faults

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.

There are a multitude of boreholes in the New Crofton mining area that confirm these seams’

continuity (Figure 8).

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).

Figure 7 Section data from boreholes

Figure 8 Boreholes and section-line across the mine site

4

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.

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

(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.

Figure 9 Cross-section across north of mine

Figure 10 Working layout for the north area (Sharlston Top seam)

Figure 11 Working layout for the south area (Sharlston Top seam)

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.

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 12 Low height continuous miner

Figure 13 Battery powered coal-scoops

Figure 14 Feeder-breaker

Figure 15 Twin-boom roof bolter

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.

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 m3 and 2.7 m

3 of CH4 per tonne.

Even using the high figure of 2.7 m3 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% CH4 vol/vol

when ventilated by 5 m3/s.

Table 1 Local boreholes analysed for gas content by the NCB

BH261 Brierley Road Location: E440017 N412018

Seam

(Sh =

Sharlston)

Depth

below

surface

Total CH4

m3/tonne

Total C2H6

m3/tonne

Ash

content %

by weight

Ash free

CH4 m3/t

Ash free

C2H6 m3/t

Sh Top 120.1 1.86 0.063 3 1.92 0.065

No name 155.0 1.16 0.058 14.8 1.36 0.068

Sh Yard 177.5 2.28 0.145 4.7 2.40 0.152

BH121 Two Gates Location: E441375 N410454

Seam

(Sh=

Sharlston)

Depth

below

surface

Total CH4

m3/tonne

Total C2H6

m3/tonne

Ash

content %

by weight

Ash free

CH4 m3/t

Ash free

C2H6 m3/t

Sh Yard 310.6 2.64 0.178 3.4 2.73 0.184

BH359 Cliff Lane Location: E440366 N410814

Seam

(Sh =

Sharlston)

Depth

below

surface

Total CH4

m3/tonne

Total C2H6

m3/tonne

Ash

content %

by weight

Ash free

CH4 m3/t

Ash free

C2H6 m3/t

Sh Top 210.7 1.40 0.005 8 1.52 0.006

Sh Low 246.0 1.38 0.059 2.2 1.41 0.06

Sh Yard 271.9 1.93 0.124 2.5 1.98 0.127

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).

8

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.

Figure 16 Brattice layout in the USA

Figure 17 Example of force fan ventilation (UK)

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).

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, (1773 – 1843) 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 (1821-1870) an English mines Inspector, who wrote his work of genius

“On the Theory of Ventilation” in 1854 - 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 (1885-1959), 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 (1900-1988) and Malcolm J McPherson (1937-2008) who both

advanced and modified this work to create computer programs for solving airflows in

networks at Nottingham University.

Figure 18 New Crofton perimeter ventilation ducts

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 1994.

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.

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

Figure 19 A screen grab of some VentSim features

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 m3/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 m3/s to feed the auxiliary fans and another

6 m3/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 m3/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 24m3/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 0.01580 kg/m3 was used for the roadways throughout the model,

whilst a friction factor of 0.0029 kg/m3 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.

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 m3/s delivered and the

remaining stood

headings being

ventilated by a nominal

0.5 m3/s through jetting

blank plates - ideal for

sweeping faces of stood

headings (Figure 20).

Figure 20 Crofton auxiliary ventilation

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).

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).

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 m3/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.

Figure 21 Crofton northern extremities

Figure 22 Crofton southern extremities

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 m3/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 m3/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).

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 m3/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 23 Crofton surface fan: Stage 1

Figure 24 Crofton surface fan: Stage 2

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).

In addition, an arbitrary 1 m3/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 m3/s was tried in sensitivity tests

and found to be achievable. Examination of the 0.5 m3/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 25 Crofton cross-cut sealing

Figure 26 Crofton auxiliary ventilation headings (velocity when regulated to 0.5 m3/s)

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).

Figure 27 Crofton surface fan options

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

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 78% duty

A 112 kW motor would be enough

to provide the 80 m3/s @ 844 Pa

required during Stage 1.