NIOSH Research for Controlling Respirable Dust and 

Methane Gas on Continuous Miner Faces

by

Gerrit V.R. Goodman1

, Charles D. Taylor2, Jay F. Colinet3, and Edward D. Thimons4

1

Mining Engineer, 2Industrial Hygienist, 3Team Leader, 4Branch Chief

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Dust and Toxic Substances Control Branch

Pittsburgh, PA 15236 USA

(412)-386-4455 (office)

(412)-386-4917 (fax)

April 10, 2000
ABSTRACT

The importance of controlling respirable dust and methane gas levels in underground

coal mining cannot be underestimated. While respirable dust can significantly affect the

occupational health of underground coal miners, methane gas accumulations pose significant

safety concerns for these same workers. Water sprays and machine mounted dust scrubbers

offer effective control of respirable dust exposures and methane gas accumulations. Water

must not only be applied carefully to avoid dust rollback to the machine operator but must create

sufficient turbulence to remove dead zones that could contain high concentrations of methane

gas. While the flooded-bed dust scrubber has been generally responsible for decreased worker

exposures to respirable dusts, this device has proved effective in controlling methane levels at

the face. This paper reviews practical applications of water sprays and dust scrubbers to

control respirable dust and methane gas on continuous miner faces. 

NIOSH RESEARCH TO CONTROL RESPIRABLE DUST EXPOSURES

The United States Federal Coal Mine Health and Safety Act of 1969 limits personal

exposure to respirable dust to 2,0 mg/m3

. This limit is measured gravimetrically as an 8-hour

time weighted average concentration of the respirable coal dust1

 . If the respirable dust sample

contains more than five percent silica by weight, the respirable dust standard is reduced

according to the formula 10 ÷ (percent silica). The 2 mg/m3

 standard with a silica percentage of

five percent corresponds to a personal exposure limit for silica of 100 μg/m3

. Because the

continuous miner operator is on or near the continuous mining machine, this person is

frequently exposed to the greatest levels of respirable dust. In the years 1988 through 1991,

nearly forty percent of the continuous miner operator samples analyzed contained more than

five percent respirable silica dust2

. Approximately eleven percent of the continuous miner

operator samples exceeded the Federal dust standard of 2,0 mg/m3

. Thirty percent of these

samples exceeded 100 μg/m3

 of silica.

Water sprays remain the most widely used technology for limiting exposures to

respirable dust. Water sprays control dust exposure by suppressing airborne dust, inducing

airflow to drive dust clouds away from personnel, and preventing generation of airborne dust3

.

Much research by NIOSH and its predecessor, the U.S. Bureau of Mines, dealt with balancing

these attributes to provide effective control of respirable coal and silica dusts.

The earliest water sprays on a continuous miner were used for bit lubrication, bit cooling,

and dust control. Although these sprays controlled respirable dust exposure to a limited extent,

they created large quantities of dust rollback.

To control rollback, sprays were situated atop and beneath the cutting drum (figure 1).

The sprays operated at a pressure of approximately 690 kPa and a flow rate of 3,6 liters/min per

spray. Two large orifice, deluge-type, sprays were mounted on the left and right underside of

the boom and directed to spray into the cutting bits. These sprays operated at a low pressure of

roughly 48 kPa and a higher flow rate of 18,9 lpm per spray4

. Dust rollback decreased because

the spray droplets moved only a short distance before impacting on the cutting bits. The short

distance also increased coal surface wetting capabilities while minimizing turbulence. In-mine

evaluations of these boom sprays showed that miner operator dust exposures were reduced

forty percent compared to the factory-issued spray system.
Figure 1. Sprays to control dust rollback

Half the respirable silica generated in underground coal mining has a size between 1,0

and 3,5 microns5

. The small sizes of these particles make their capture difficult with larger

water droplets produced by conventional water sprays. Higher-pressure sprays generate

smaller water droplet sizes and increased spray velocities required to capture these smallersized particles.

High-pressure sprays swept underboom dust behind the return curtain when installed at

the rear corner of the shovel on the side opposite the curtain (figure 2). Extensive underground

testing showed that the shovel sprays reduced coal dust exposures by sixty percent at the miner

operator’s location while virtually eliminating exposures to respirable silica dust4

. These sprays

also were placed on top of the cutting boom and oriented into the cutting head to reduce dust

levels nearly sixty percent with the most dramatic dust reductions occurring in the size range

below 2,9 microns. In fact, the reduction more than doubled for 1,8 micron particles with the

addition of high-pressure sprays. 

 

Figure 2. Shovel-mounted sprays control underboom dust

Past work showed that flooded-bed scrubbers effectively controlled occupational dust

exposures when used with either blowing or exhausting face ventilation schemes. As of

1997, approximately sixty percent of the operating continuous mining machines used a

flooded-bed dust scrubber to control occupational dust levels. The U.S. Mine Safety and

Health Administration requires air quantities measured at the curtain mouth to be equal to or
slightly greater than the rated capacity of the dust scrubber6

. Excessive airflow at the mouth

of a blowing curtain is thought to overpower the scrubber and blow dust by the scrubber

inlets. However, excessive airflow at the mouth of an exhausting curtain does not appear to

significantly affect dust capture7

. 

Increased emphasis on controlling occupational dust exposures led to increases in face

airflow and scrubber airflow. Although physical constraints limit scrubber performance,

scrubber airflow was increased by reducing the density of the scrubber filter. However, this

reduction suggested lower dust collection efficiencies and thus increased occupational

exposures to respirable coal and silica dusts. Recent work investigated silica collection

efficiencies of filters containing 10, 15, 20, and 30 layers of stainless steel meshing8

. The 20-

layer scrubber filter is the most common filter in use in the U.S. mining industry. Also

investigated was a filter panel containing a non-woven synthetic fiber and one containing an

array of nylon brushes. Multiple scrubber duct velocities were tested.

 

0

20

40

60

80

100

20-layer 15-layer Brush 10-layer Synthetic 30-layer

Filter panel

Collection efficiency, %

11.1 m/s

17.8 m/s

Figure 3. Silica collection efficiencies for various filters and scrubber velocities

Over the range of velocities tested, respirable silica collection efficiencies ranged from

69 percent to 84 percent for the standard 20-layer scrubber filter. Efficiencies for the denser

30-layer and synthetic fiber filters varied from 91 to 95 percent. The 10-layer filter panel had

the lowest collection efficiencies ranging from 58 percent to 76 percent. All filter panels

showed improved silica collection at higher scrubber velocities.

Dust scrubbers are most effective when dust is contained beneath the cutting boom,

allowing this cloud to be drawn into the scrubber inlets. However, movement of the

continuous mining machine at the face can sometimes cause dust to roll out from under the

boom. Supplemental water sprays were placed along the sides of the continuous miner to

improve suppression and confinement of the dust cloud beneath the cutting boom. Testing

showed that these added sprays not only improved dust suppression but also redistributed

the dust cloud beneath the cutting boom. This led to increased amounts of dust entering the

scrubber inlets that reduced dust concentrations around the machine. This work showed that

the supplemental sprays were most effective at smaller ventilation curtain setback distances9

. 

NIOSH RESEARCH TO CONTROL METHANE GAS LEVELS

Federal regulations require that methane levels in the face area be maintained below

1 percent. Methane readings obtained with a hand held methane monitor are usually taken

as close to the face as practical although they cannot be taken closer than 0,3 m from the
roof, face, ribs, and floor (30 CFR§75.342)10. Machine-mounted methane monitors are used

to continuously monitor methane levels on the mining machine. Whenever methane readings

are 1 percent or higher, mining must stop until concentrations are reduced below this level.

Methane gas at the mining face is controlled by providing sufficient intake air to the

face to maintain methane levels below 1 percent. Although adequate air quantity may be

available in the heading, a fraction of this intake air never reaches the face. Generally, the

quantity of air reaching the face decreases as tubing or curtain setback distance increases. 

Although water sprays and scrubbers are designed primarily for dust control, research has

shown that they also can be used to reduce methane levels by increasing the amount of

intake air that reaches the face. They have been shown to be effective with either blowing or

exhausting face ventilation schemes.

Exhaust face ventilation systems, rather than blowing systems, are generally

preferred for dust control because the dust is removed from the face area before it passes

over the mining machine operator’s location. However, for given setback distances and

intake air quantities, exhaust ventilation generally supplies less airflow to the face than

blowing ventilation. This can lead to higher face methane concentrations. 

Water spray systems designed for use with exhaust face ventilation schemes can

maintain and improve face airflow at longer setback distances11. The nozzles in the spray

system are oriented to move air from the intake to return side of the entry. Each spray nozzle

acts as a small fan and the system is referred to as a spray fan system. Tests showed that

increasing water pressure and flow rate increased the air moved by the sprays. Although

typical flow pressure for the spray fan system was at least 689 KPa, high water pressure

often created turbulence that led to dust roll back. This can result in higher dust exposures

for the mining machine operator12.

The orientation and location of spray nozzles on the mining machine are also

important factors in determining face airflow. Nozzles on the mining machine boom should

be directed toward the return side to maintain airflow across the face. Nozzles on the intake

side of the machine should be oriented to maintain intake airflow to the face. The best

design for a particular mining face is the one that provides the greatest airflow quantity with

consistent flow patterns (intake to return) at the face. Smoke or discharge from a fire

extinguisher can identify face airflow patterns to compare effects of sprays on face airflow. 

Test results show that, when compared to a 3-m primary ventilation brattice, the spray fan

system consistently provided better face ventilation up to and including a 13-m brattice

setback13. Use of the spray fan system allowed increases in curtain setback distances while

maintaining the effectiveness of the exhausting face ventilation system. 

When used with a blowing face ventilation scheme, early studies indicated that water

spray systems had little effect on face methane levels14. Spray systems usually were

directed toward the face and were not intended, as with the spray fan system, to direct air

from the intake to return side of the face. 

Using a dust scrubber with exhausting face ventilation reduced face methane levels

and these reductions increased as scrubber flow increased15. The mouth of the exhaust

curtain must be positioned outby the exhaust of the scrubber. With the curtain mouth inby the

scrubber exhaust, air recirculation to the face increases and, more importantly, intake air

reaching the face is reduced. This can lead to increases in face methane levels.

For a given ventilation setback distance, blowing face ventilation systems typically

provide more airflow to the mining face than exhausting systems. Thus, blowing systems are
more effective controllers of face methane concentrations. Scrubbers are used with most

blowing systems to reduce the amount of dust that passes over the mining machine

operator’s work location. Dust control and ventilation plans approved by the U.S. Mine

Safety and Health Administration typically permit blowing curtain ventilation flows to exceed

the rated capacity of the dust scrubber by no more than 0,50 m3

/sec. This additional face

airflow helps control face methane levels.

Methane levels at the face were lower with blowing schemes than exhausting

schemes due to the higher intake air velocities created by the blowing system. Greater

reductions in face methane levels were also seen at higher scrubber flows. With blowing

curtain ventilation, analysis of face airflow patterns indicated that the scrubber maintained

intake airflow on the curtain side of the entry (figure 4). 

Figure 4. Face airflow patterns with and without scrubber

Test results showed that the scrubber sustained intake airflow reaching the face as

curtain setback distance increased from 7,5 to 15 meters16. Not surprisingly, intake air

velocities measured at the end of the blowing curtain increased with the scrubber operating. 

These increases were greater at the higher scrubber flows and lower intake flows. Face

methane levels decreased with increasing scrubber flow, even when scrubber flow was

greater than intake flow (figure 5)17.
0.00

0.05

0.10

0.15

0.20

0.25

2.8 4.7 6.6

Intake flow, m3

/s

Methane, %

2.8 cu. m/s scrubber

4.7 cu. m/s scrubber

6.6 cu. m/s scrubber

Figure 5. Face methane levels with varying intake and scrubber flows

SUMMARY

Water sprays remain the most widely used method for controlling respirable dust

exposures. NIOSH research has evaluated sprays to reduce dust rollback from the cutting

head, high-pressure sprays to control exposures to respirable silica dust, and sprays to

improve performance of a flooded-bed dust scrubber. The dust capture properties of various

wetted screens used in the dust scrubber have been evaluated.

Face methane concentrations at the cutting face have been reduced using water

sprays and flooded-bed scrubbers. NIOSH research has shown that water sprays can be

used to maintain and improve face airflow at longer setback distances with exhaust face

ventilation systems. This work has also shown that dust scrubbers increase intake airflow

with either blowing or exhausting face ventilation systems. Higher face ventilation flows are

needed to control face methane concentrations.

REFERENCES

1. National Institute for Occupational Safety and Health. Occupational Exposure to

Respirable Coal Mine Dust, Criteria for a Recommended Standard. U.S. Dept. of

Health and Human Services, Centers for Disease Control and Prevention, 1995, 336

pp.

2. Ainsworth, S.M., Gero, A.J., Parobeck, P.S., and Tomb, T.F. Quartz Exposure Levels

in the Underground and Surface Coal Mining Industry. Am.Ind. Hyg. Assoc. Journal,

vol. 56, no. 10, 1995, pp.1002-1007.

3. Goodman, G.V.R. and Jankowski, R.A. Optimal Spray System Designs for Continuous

Miner Dust Control. Proceedings, 9th Intl. Conf. on Advances in Prevention of Occ.

Resp. Diseases, Chiyotani, K., Hosoda, Y., and Aizawa, Y. (eds.), Kyoto, Japan, 1997,

pp 1081-1087.
4. Foster-Miller, Inc. Development of Optimal Water Spray Systems for Dust Control in

Underground Mines. Contract Report No. H0199070, U.S. Bureau of Mines, 1986, 146

pp.

5. Jayaraman, N.I. and Jankowski, R.A. Atomization of Water Sprays for Quartz Dust

Control. Appl. Ind. Hyg., vol. 3, no. 12, 1988, pp. 327-331.

6. Schultz, M.J. and Fields, K.G. Dust Control Considerations for Deep Cut Mining

Sections. Preprint 99-163, Soc. of Mining Engineers, Ann. Mtg., Denver, CO, 1999, 4

pp.

7. Colinet, J.F. and Jankowski, R.A. Dust Control Considerations for Deep-Cut Mining

When Utilizing Exhaust Ventilation and a Scrubber. Report of Investigations 9615, U.S.

Bureau of Mines, 1996, 12 pp.

8. Colinet, J.F. and Jankowski, R.A. Silica Collection Concerns When Using Flooded-Bed

Scrubbers. Preprint Number 98-131, Soc. of Mining Engineers, Annual Mtg., Orlando,

FL, 1998, 7 pp.

9. Goodman, G.V.R. Using Water Sprays to Improve Performance of a Flooded-Bed Dust

Scrubber. Appl. Occup. Envir. Hyg., to be published in vol. 15, 2000. pp. 550-560.

10. Mineral Resources. Code of Federal Regulations, Title 30, Parts 70 and 75, U.S.

Government Printing Office.

11. Foster-Miller, Inc. Improved Diffuser and Sprayfan Systems for Ventilation of Coal

Mine Working Faces. Contract Report No. JO113010, U.S. Bureau of Mines, 1985,

292 pp.

12. Goodman, G.V.R. and Listak, J.M. Variation in Dust Levels with Continuous Miner

Position. Mining Eng., Vol. 51, No. 2, 1999, pp. 53-58.

13. Volkwein, J.C., Ruggieri, S.K., McGlothlin, C., and Kissell, F.N. Exhaust Ventilation of

Deep Cuts using a Continuous-Mining Machine. Report of Investigations 8992, U.S.

Bureau of Mines, 1985, 16 pp.

14. Gillies, A.D.S. Studies in Improvements to Coal Face Ventilation with Mining Machine

Mounted Dust Scrubber Systems. Preprint No. 82-24, Soc. of Mining Engineers,

Annual Mtg., Dallas, Texas, 1982, 13 pp.

15. Taylor, C.D., Rider, J., and Thimons, E.D. Changes in Face Methane Concentrations

Using High Capacity Scrubbers with Exhausting and Blowing Ventilation. Preprint 96-

167, Soc. of Mining Engineers, Annual Mtg., Phoenix, Arizona, 1996, 6 pp.

16. Ingersoll Rand Research, Inc. Feasibility of a Machine Mounted Scrubber System for

Ventilating Coal Mine Working Faces. Contract Report No. JO199080, U.S. Bureau of

Mines, 1984.

17. Taylor, C.D., Rider, J., and Thimons, E.D. Impact of Unbalanced Intake and Scrubber

Flows on Methane Concentrations. Proceedings, 6th International Mine Ventilation

Congress, Ramani, R.V. (ed.). Pittsburgh, PA, Soc. for Mining, Metal., and Exploration,

1997, pp. 169-172.