Frictional ignitions in underground 


bituminous coal operations 1983 to 2005
 

R.B. KRog and S.J. Schatzel 

R.B. Krog, member SMe, and S.J. Schatzel, are mining 

engineer and research geologist, respectively, with the Pittsburgh 

Research laboratory, national Institute for occupational Safety 

and health (nIoSh), Pittsburgh, Pa. 

Abstract 

A statistical study of available data regarding frictional 

ignitions in the United States from 1983 to 2005 was conducted by the National Institute for Occupational Safety 

and Health (NIOSH) to determine and define conditions 

controlling their occurrence (MSHA, 2006a;MSHA,2006b; 

and MSHA, 2006c). 

 Frictional ignitions are defined as the ignition of a 

flammable mixture of methane and air that is initiated by 

frictional heating. Ignitions created through the addition of 

energy from open flames and exposed electrical circuits are 

not included in this analysis. Energy released in a roof fall 

that creates a spark and ignites a methane mixture would be 

considered a frictional ignition. Frictional ignitions represent the majority of all ignitions in underground coal mines. 

Over the study period of 1983 to 2005, a total of 1,993 ignitions were reported in underground bituminous coal mines 

in the United States,of which 1,589 were frictional ignitions. 

Reportable ignitions to the U.S. Mine Safety and Health 

Administration (MSHA) are any unintentional occurrences 

of flame underground regardless of duration. Continuous 

miners represented the largest source for frictional ignitions 

with 1,090 (68.6% of all frictional ignitions). Longwall 

mining operations represented the highest concentration 

of frictional ignitions during this study period with 1,365 

frictional ignitions (85.9% of all frictional ignitions). 

 From 1983 to 1991,underground productivity and the number of frictional ignitions increased. However, productivity 

continued to increase from 1991 to 2000 while total frictional 

ignitions fell, thereby disproving this connection between 

productivity and frequency of frictional ignitions. Ninety 

percent of all frictional ignitions occurred in underground 

coal mines that liberated more than 393 L/s (1.2 MMcfd 

million cubic feet per day) of methane through the main 

ventilation system. Although methane production is not the 

determining factor for the occurrence of frictional ignitions, 

it is a good indicator of the anticipated frequency. 

Introduction 

A typical frictional ignition is 

defined as a continuous miner or 

shearer bit striking a sandstone layer 

in the coal seam or the surrounding 

strata, creating a frictional hot spot 

or spark that ignites a flammable 

concentration of methane. 

Three conditions need to be present for frictional ignitions to occur: 

Oxygen, methane or other hydrocarbon (fuel), and a heat source. The concentration of 

methane at the active face is controlled by many factors 

including local geology, coalbed characteristics (porosity, 

permeability, cleat system and methane content), mining 

rate, methane drainage and ventilation controls. The creation of a spark or heating of the cutting bits or the rock 

face to a significant temperature usually occur when they 

strike a hard quartz-rich or sulfur-bearing rock within or 

surrounding the coal seam. 

A statistical analysis and review of all frictional ignitions determined possible controlling factors in frictional 

ignitions. Various factors were considered and included 

time of day, month, year, state, basin, coalbed, coal thickness, methane emissions, equipment type, underground 

location, tonnage and hours worked, productivity and underground mining method. For this paper, only a subset 

of the important factors listed above will be discussed. 

One possible controlling factor that was not investigated was face ventilation (blowing, 

exhausting or combination) because 

the authors were not able to determine the ventilation systems used by 

all mines during the study period. 

The main results are: frictional 

ignitions were reported in 155 mines 

and 13 states. The 17 mines with the 

most frictional ignitions were all 

longwall mining operations and had 

1,130 frictional ignitions (307 from 

the shearer) that represented 71.1% of the total. 

In 1983, mines with frictional ignitions represented 

6.9% of operating underground mines and 36.6% of total 

underground coal production. The percentage of operating underground mines with frictional ignitions and their 

total production has increased steadily and in 2005 represented 9.8% of active mines and 46.2% of underground 

coal production. 

The three coalbeds with the highest number of frictional ignitions (Blue Creek, Pittsburgh No 8, and Pocahontas No 3) had 1,049 ignitions that represented 66% 

of all frictional ignitions. The three states with the highest number of frictional ignitions were Alabama (710), 

Virginia (247) and Pennsylvania (242) that represented 

75.5% of the total. 

Frictional ignitions peaked in 1991 with a high of 114. 

In 2005, only 34 frictional ignitions were reported. If only 

frictional ignitions that occur at the working face are con-
sidered, there have been no fatalities since 1983. Since 

this time, there were 33 ignitions that caused 53 workers 

to lose a total of 2,912 working days. When including 

ground falls, then there have been four frictional ignitions 

with one incident causing two fatalities and injuring eight 

workers. 

Total ignition vs. friction ignitions 

A total of 1,993 ignitions of methane in underground 

bituminous coal mines were reported to the U.S. Mine 

Safety and Health Administration (MSHA) from 1983 to 

2005, with frictional ignitions being the majority. During 

the period of the study, 155 mines in 13 states reported 

a total of 1,589 frictional ignitions (79.7% of total) averaging 69 per year. Total ignitions peaked in 1991 with 

a total of 137, and this corresponded to a peak of 114 

frictional ignitions during the same year (Fig. 1). The 

general trend for the occurrence of frictional ignitions 

has been declining since 1991, with a record low of 34 

frictional ignitions in 2005. 

Figure 1 

Frictional ignitions as a percentage of total ignitions. 

Comparison by mining machine type (continuous

miner, shearer, roof bolter, cutting machine) 

Continuous mining machines represented the largest 

percentage of frictional ignitions followed by longwall 

shearers, roof bolters and cutting machines (Table 1). 

Continuous miners accounted for 68.6% (1,090) of all 

frictional ignitions, longwall shearers 24.1% (383), roof 

bolters 3.1% (50), cutting machines 1.2% (19) and unknown equipment causes 1.3% (20). The four ground 

fall frictional ignitions were given their own classification 

because their occurrences led to destructive mine fires. 

The remaining 1.4% (23) of “other equipment” frictional 

ignitions represent a range of equipment including coal 

loaders, feeder-breakers, longwall components other than 

the shearer (e.g. shields), conveyors, hand tools drills, locomotives and mancars. Machinery directly accounted 

for 98.5% of all frictional ignitions (1,565) with the 20 

unknown equipment not counted because no specific 

equipment was recorded. 

Comparison by state 

During the period of the study, 13 states reported a 

total of 1,589 frictional ignitions (Table 2). Alabama led 

the nation with 710 frictional ignitions (44.7%), followed 

by Virginia with 247 frictional ignitions (15.5%) and 

Pennsylvania with 242 frictional ignitions (15.2%). West 

Virginia with 132 ignitions (8.3%) and Utah with 113 

(7.1%) were fourth and fifth, respectively. The remaining 

states in decreasing order were Kentucky (62), Ohio (22), 

Colorado (22), Illinois (19), Indiana (11), New Mexico 

(5), Oklahoma (2) and Tennessee (2). The reduction in 

the average number of frictional ignitions after 1999 is 

mainly the result of a few ignition-prone Alabama mines 

changing cutting sequence or shutting down. 

Comparison by basin 

The United States was divided into sections using the 

following primary basins: Northern Appalachian, Central Appalachian, Warrior, Illinois, Western Interior and 

Rocky Mountains (Fig. 2). 

Figure 2 

Coal basins in the United States. 

The Rocky Mountain basin is 

a collection of smaller basins including Uinta, San Juan, 

Wasatch Plateau, Emery, Book Cliffs, plus a few more. 

The Warrior basin had the most frictional ignitions with 

710 (44.7%), followed by the Northern Appalachian basin 

with 346 (21.8%) and the Central Appalachian basin with 

321 (20.2%), as shown in Table 3. The Rocky Mountain 

basin had 140 (8.8%), followed by the Illinois basin with 

70 (4.4%) and the Western Interior basin with 2 (0.1%). 

Figure 3 

The six coalbeds with the highest number of frictional 

ignitions. 

The basin approach was selected because it gives a 
better view of the frictional ignitions than a comparison 

on a state-wide basis since two states are located in two 

different basins. Eastern Kentucky is in the Central Appalachian basin and Western Kentucky is in the Illinois 

basin while West Virginia is in both the Northern and 

Central Appalachian basins. 

Table 1 

Breakdown of frictional ignitions by machine types.

 Continuous Cutting Longwall roof Unknown Other Ground
 

Year miner machines shearer bolter equipment equipment fall Total
 

1983 41 5 10 2 1 59
 

1984 40 2 15 6 1 1 65
 

1985 64 1 9 2 1 77
 

1986 45 2 16 2 2 2 69
 

1987 43 6 30 2 2 2 85
 

1988 49 19 3 1 3 75
 

1989 56 18 1 1 76
 

1990 57 3 26 1 2 89
 

1991 100 11 2 1 114
 

1992 75 24 1 2 1 103
 

1993 67 3 3 1 74
 

1994 47 25 1 2 75
 

1995 46 8 2 56
 

1996 43 20 4 1 68
 

1997 50 25 2 1 1 79
 

1998 33 18 1 2 1 55
 

1999 46 22 3 1 3 75
 

2000 31 11 1 1 44
 

2001 36 11 1 1 49
 

2002 45 10 4 1 60
 

2003 30 21 4 2 2 59
 

2004 29 18 2 49
 

2005 17 13 4 34
 

Total 1090 19 383 50 20 23 4 1589
 

Percent 68.6 1.2 24.1 3.1 1.3 1.4 0.3 100
 

Comparisons by coalbed 

A total of 155 mines in 53 coalbeds had a total of 

1,589 frictional ignitions. The six coalbeds with the highest number of frictional ignitions accounted for 1,261 

(79.4%) of the total in the United States (Fig. 3). These 

included the Blue Creek coal seam (Warrior basin) with 

623 ignitions from five mines, the Pittsburgh coal seam 

(Northern Appalachian basin) with 262 ignitions from 

22 mines, the Pocahontas No. 3 coal seam (Central Appalachian basin) with 164 ignitions from 11 mines, the 

Mary Lee coal seam (Warrior basin) with 83 ignitions 

from three mines, the Jawbone coal seam (Central Appalachian basin) with 67 ignitions from four mines and the 

Springfield coal seam (Illinois basin) with 62 ignitions 

from 13 mines. 

The cumulative total shows that the six coalbeds with 

the highest number of frictional ignitions account for almost 80% of the total (Fig. 4).

Figure 4 

Cumulative frictional ignitions by coalbeds in decreasing order. The Blue Creek coal seam 

dominated frictional ignition numbers; as more ignitions 

occurred in this seam than all other coalbeds combined, 

with the exception of the top three. In other words, the 

fourth to 53rd coalbeds have a total of 540 frictional ignitions while the Blue Creek coalbed has 623 ignitions. 

Comparisons by coalbed production 

The number of frictional ignitions in a coalbed is a 

function of the total coal production from the coalbed. 

For the time period of this study, 262 and 164 frictional 

ignitions occurred in the Pittsburgh and in the Pocahontas No. 3 seams, respectively. To infer that the Pittsburgh 

seam is more prone to frictional ignitions would be incorrect. A total of 1,350 Mt (1,488 million st) of coal was 

extracted from Pittsburgh seam mines and 260 Mt (286 

million st) of coal from Pocahontas No. 3 mines. The ra­
tio of ignitions to tonnage suggests that the Pocahontas 

No 3 is three times more likely to have a frictional ignition than the Pittsburgh seam on a per-ton-of-coal basis. 

Table 4 shows all coalbeds that experienced a frictional 

ignition along with their coal production and frictional 

ignition rate. The Spring Canyon A and Castle Gate D 

seams have abnormally high frictional ignition rates due 

to the presence of unsaturated carbon compounds in the 

surrounding strata. 

Table 2 

Frictional ignitions by state per year. 

Year AL CO iL in KY nM OH OK PA Tn UT VA WV Total 

1983 23 1 2 11 13 5 4 59
 

1984 30 4 5 8 2 5 2 9 65
 

1985 55 3 10 1 5 3 77
 

1986 25 2 1 13 4 15 9 69 


1987 15 2 3 5 8 25 10 17 85
 

1988 20 2 1 3 13 12 19 5 75
 

1989 23 3 18 16 13 3 76
 

1990 30 2 2 16 10 4 16 6 89
 

1991 62 2 3 1 2 8 3 29 4 114
 

1992 49 3 8 2 36 5 103
 

1993 52 2 7 2 9 2 74
 

1994 43 6 4 1 4 2 14 1 75
 

1995 29 1 1 5 1 17 2 56
 

1996 48 8 12 68
 

1997 38 2 6 1 11 1 17 3 79
 

1998 38 1 1 5 5 2 3 55
 

1999 44 2 1 1 6 13 3 4 1 75
 

2000 10 1 5 15 8 1 4 44
 

2001 15 2 2 2 13 5 10 49
 

2002 23 2 2 13 3 17 60
 

2003 16 2 5 2 2 12 4 7 9 59
 

2004 7 2 4 1 22 1 3 9 49
 

2005 12 1 4 7 1 3 6 34
 

Total 710 22 19 11 62 5 22 2 242 2 113 247 132 1589 

Percent 44.7 1.4 1.2 0.7 3.9 0.3 1.4 0.1 15.2 0.1 7.1 15.5 8.3 100 

Comparisons by mining method 

Figure 5 shows that those operations using the longwall method had a greater occurrence of frictional ignitions (1,365 ignitions) than those operations using only 

the room-and-pillar method (224). Although shearers 

produced 383 ignitions on longwall mining operations, 

continuous miners produced 906 ignitions, most of these 

during longwall gateroad development. Individual longwall operations have more production than room-andpillar individual operation but only since 2000 has more 

coal been produced by longwalls (Fig. 5). 

Figure 5

Longwall and room-and-pillar frictional ignitions per 

year. 

When individual coalbeds were investigated, there 

was evidence that longwall mining produced more frictional ignitions on a per-ton basis than room-and-pillar 

mining (Table 5). There were 13 coalbeds with frictional 

ignitions in both longwall and room-and-pillar mining 

with 10 Mt (11 million st) of production by both methods. 

Nine of the coalbeds had higher frictional ignition rates 

for longwall mining operations than room-and-pillar. The 

Jawbone frictional ignition rate for longwall mining arose 

from a single operation, so this high rate was viewed with 

caution. Also, the Pocahontas No. 3 coalbed had many 

room-and-pillar operations that produced more than 41 

Mt (45 million st) of coal with only one frictional ignition. Three western coalbeds had higher frictional ignition rates from room-and-pillar operations than longwall 

mining operations, although these coalbeds had only a 

limited number of ignitions from a few mines, so the results were not conclusive. The majority of frictional ignitions from the western Somerset (B) and D Seam (CO) 

occurred during roof bolting. If roof bolting controlled 

the frictional ignition rate and not the mining of the coal 

seam, then longwall mining would have had a lower rate 
because fewer roof bolts were required to extract the 

same amount of coal. 

The mining method is inconclusive as a major factor in controlling frictional ignitions because individual 

coalbeds are predisposed to a preferred mining method 

by geology. Only the Springfield No. 5 coal seam has 

large production by both mining methods and has similar 

frictional ignitions rates. 

Table 3 

Frictional ignitions by coal basin per year. 

Central northern rocky Western 

Appalachian illinois Appalachian Mountain Warrior interior 

Year Basin Basin Basin Basin Basin Basin Total 

1983 7 1 14 14 23 59 

1984 10 2 14 9 30 65 

1985 8 2 11 1 55 77 

1986 22 2 16 4 25 69 

1987 17 7 19 27 15 85 

1988 25 3 13 14 20 75 

1989 19 18 16 23 76 

1990 22 18 12 4 33 89 

1991 32 5 12 3 62 114 

1992 41 3 8 2 49 103 

1993 10 8 4 52 74 

1994 18 6 6 2 43 75 

1995 18 7 2 29 56 

1996 12 8 48 68 

1997 19 6 13 3 38 79 

1998 5 7 5 38 55 

1999 4 2 20 5 44 75 

2000 4 21 9 10 44 

2001 6 2 24 15 2 49 

2002 4 2 29 2 23 60 

2003 9 3 23 8 16 59 

2004 3 4 31 4 7 49 

2005 6 2 12 2 12 34 

Total 321 70 346 140 710 2 1589 

Percent 20.2 4.4 21.8 8.8 44.7 0.1 100 

Comparisons by production 

Total underground coal production in the United 

States ranged from a low of 265 Mt (292 million st) (1983) 

to a high of 380 Mt (419 million st) (1990) and was 334 

Mt (368 million st) in 2005 (Fig. 6).

Figure 6 

Number of underground mines and total production. 

The number of active underground coal mines has dramatically decreased 

from 1,950 operating in 1984 to only 557 in 2003. This 

steady downward trend was due to the consolidation 

and competition within the coal industry. The low total 

production in 1993 relative to other years was due to a 

coal workers’ strike. 

Comparions by productivity 

As productivity increased from 1983 to 1991, so did 

frictional ignitions, which suggested a link between increasing productivity and the increasing number of frictional ignitions (Schatzel, 1995). However, productivity 

continued to increase from 1991 to 2000, whereas total 

frictional ignitions fell during this time. The initial hypothesis that frictional ignition increases between 1983 

(59) and 1991 (116) were the result of increasing productivity was not supported by Fig. 7 for the data after 1991. 

Production recorded by MSHA is in clean tons not raw, 

so clean tonnage has to be used for comparison. Productivity increased consistently from 1.74 tonnes/worker-hr 

in 1983 to 4.85 tonnes/worker-hr in 2000. Productivity 

from 2001 to 2005 declined from 4.65 tonnes/worker-hr to 

4.15 tonnes/worker-hr. Productivity in 2005 (4.15 tonnes/ 

worker-hr) closely matched the productivity in 1996 (4.13 

tonnes/worker-hr). Increases in productivity were not 

indicators of increased frictional ignitions. 
Table 4 

Frictional ignitions by coal basin per year. 

Frictional 

Frictional ignitions per 

Coalbed name Basin ignitions Tonnes coal million tonnes 

spring Canyon a (sub. No 3) rMB 33 889,844 37.09 

Castle Gate D rMB 29 4,361,491 6.65 

Blue Creek WrB 623 185,416,792 3.36 

Mary Lee WrB 83 79,885,318 1.04 

Davis ILB 5 5,202,603 0.96 

Jawbone CaB 67 70,093,801 0.96 

Coal Basin B rMB 6 8,688,575 0.69 

Lower Hartshorne WIN 2 2,909,555 0.69 

Pocahontas No 3 CaB 164 260,460,520 0.63 

Gholson rMB 2 3,461,460 0.58 

stearns No 1-1/2 CaB 1 2,005,679 0.50 

elswick CaB 1 2,074,224 0.48 

Upper O’Connor (Castle Gate B) rMB 15 35,421,423 0.42 

Lower Freeport NaB 34 84,319,214 0.40 

Lower Iaeger (Tiller) CaB 4 11,601,016 0.34 

Beckley (Pocahontas No 12) CaB 14 40,623,537 0.34 

Pocahontas No 4 CaB 10 29,831,611 0.34 

apache rMB 2 6,404,879 0.31 

Lower sunnyside rMB 6 21,052,968 0.28
 

rock Canyon rMB 7 25,731,494 0.27 

sewell “B” (Greasy Creek) CaB 2 7,385,694 0.27 

Wattis rMB 7 26,542,319 0.26 

raven (Jewell) CaB 6 23,868,463 0.25
 

Cedar (Dorchester) CaB 11 44,803,077 0.25 

Fruitland No 8 rMB 5 21,977,790 0.23 

Upper Kittanning NaB 12 52,801,491 0.23 

Pittsburgh NaB 262 1,350,221,677 0.19 

Pocahontas No 5 CaB 1 5,163,664 0.19 

Clarion NaB 17 88,887,943 0.19 

Brookville NaB 1 5,443,716 0.18 

Fire Creek (Pocahontas No 11) CaB 4 32,370,140 0.12 

Cameo rMB 1 8,207,194 0.12 

somerset (B) rMB 10 91,117,871 0.11 

springfield (No 5) ILB 62 621,606,355 0.10 

Hiawatha rMB 7 88,593,866 0.08 

Blind Canyon rMB 6 80,187,479 0.07 

Upper Freeport (Davis) NaB 15 233,750,259 0.06 

Hazard No 7 CaB 1 16,690,115 0.06 

splash Dam CaB 3 54,428,415 0.06 

Cedar Grove (U. elkhorn No 3) CaB 15 287,670,223 0.05 

Lower O’Connor (Castle Gate a) rMB 3 59,108,466 0.05 

Pratt rMB 2 45,299,428 0.04 

redstone CaB 1 26,456,358 0.04 

elkhorn Leader CaB 1 26,468,360 0.04 

D seam (CO) rMB 3 80,872,314 0.04 

Lower Kittanning NaB 5 186,282,689 0.03 

No 2 Gas (Imboden, L. elkhorn) CaB 7 371,493,244 0.02 

Low splint (Williamson) CaB 2 127,966,667 0.02 

sewell CaB 1 65,753,750 0.02 

eagle CaB 2 165,854,492 0.01 

alma CaB 2 175,706,990 0.01 

Herrin (No 6) ILB 3 565,824,120 0.005 

Hazard No 4 (Fire Clay) CaB 1 312,230,047 0.003 

CaB = Central appalachian basin WIN = Western Interior basin 

ILB = Illinois basin WrB = Warrior basin 

NaB = Northern appalachian basin 

rMB = rocky Mountain basin 

Frictional ignition rates 

Frictional ignition rates 

increased when compared to 

hours worked, but frictional 

ignitions per million tons 

produced decreased (Fig. 8). 

This means that while total 

frictional ignitions per ton 

of coal decreased over the 

timeframe of the study, the 

productivity per working 

hour increased at a greater 

rate, so frictional ignitions on 

a per-hour-worked basis increased. The 2005 frictional 

ignition rates for both hours 

worked and tons produced 

were below the normal averages because of the low 

frictional ignition total of 34 

in 2005. The 2005 frictional 

ignition rate per two million 

hours worked (0.85) was less 

than half of 2003 (1.73). 

Comparisons by mining 


by mining machine and


mine type
 

Mine machinery and 

its use underground differ between longwall and 

room-and-pillar operations. 

Although continuous mining machines account for the 

majority of all frictional ignitions in longwall and roomand-pillar operations (Fig. 9), 

their methane environments 

differ. Coal production from 

continuous miners in roomand-pillar sections generally 

exceeds that for continuous 

miners in longwall gateroad 

sections. A continuous miner working a seven-entry 

section in a room-and-pillar 

mine generates more coal 

on a per-shift basis than a 

continuous miner developing a three-entry longwall 

gateroad because of a more 

efficient cutting cycle. A 

continuous miner developing gateroads at a longwall 

mining operation is typically 

operating up to 3.2 km (2 

miles) in virgin coal seams 

and developing quickly and 

deeply into the coalbed 

methane reservoir. Methane 

bleeds into gateroads from 

the virgin coal on both ribs 

and face while a seven-entry 

room-and-pillar section will 
have a barrier pillar as one of 

the ribs that most likely has already been degassed of methane. The lateral development 

rate of a five-to-seven-entry 

room-and-pillar section or 

main/submain section will be 

less than the rate of a threeto-four entry gateroad section and will have more time 

to degas. Methane generation 

of up to 328 L/s (1 MMcfd) 

in headgate development is 

not uncommon. Continuous 

miners account for 82% of all 

frictional ignitions in roomand-pillar operation and 66% 

in longwall mining operations 

(Fig. 9). 

Table 5 

Breakdown of frictional ignitions by machine types. illar frictional ignitions Longwall frictional illar nes coal Longwall million

illar

frictional ignitions

tonnes

Longwall frictional

r million

 roomand-pillar ratio of

frictional ignitions

tonnes 

Coalbed 

oom-and-p

rignitions

oom-and-p

rmillion tontonnes coal

oom-and-p

rper million 

ignitions petonnes coal

Longwall toper million 

Jawbone 9 58 58 12 0.15 4.95 32.14 

Pocahontas No 3 1 163 41 220 0.02 0.74 30.07 

No. 2 Gas (Imboden) 2 5 297 74 0.01 0.07 10.00 

Pittsburgh 2 260 64 1286 0.03 0.20 6.47 

Upper Freeport 3 12 142 92 0.02 0.13 6.18 

Lower Kittanning 2 3 136 50 0.01 0.06 4.11

Mary Lee 10 73 25 54 0.40 1.35 3.40

Clarion 1 16 11 78 0.09 0.20 2.16 

springfield (No 5) 41 21 445 176 0.09 0.12 1.29 

Cedar Grove (U. elk.) 14 1 269 19 0.05 0.05 1.03 

somerset (B) 2 8 11 80 0.19 0.10 0.54 

D seam CO 1 2 10 71 0.10 0.03 0.28 

rock Canyon 6 1 10 15 0.58 0.07 0.11 

Figure 7

Productivity and frictional ignitions.
 

Comparisons by total 

mine emissions 

MSHA collects quarterly gas emissions from each 

mine’s main ventilation system. This methane emission 

data was collected sporadically by the former U.S. Bureau 

of Mines (USBM) for the following three years: 1985, 1988 and 1990. The U.S. Environmental Protection Agency (EPA) started collecting 

this information from 1993 onwards and maintains an 

up-to-date spreadsheet for each year (Bergamo, 2006). 

This yields a total of 953 frictional ignitions, where corresponding methane emission data is available. Only 

the main ventilation systems are measured (main exhaust, bleeder fans). Methane drainage quantities from 

pre-drainage, gob vent boreholes (GVB), cross measure holes and in-seam holes are not included in the 

total methane emissions of the mine, although the existence of such drainage systems is noted by the EPA. 

For each frictional ignition, arranging the total 

methane emissions in ascending order, for better viewing, yields a fairly linear trend (Fig. 10). This graph 

shows that half of all frictional ignitions occur with 

methane emission rates of 3048 L/s (9.3 MMcfd) or 

greater. It should be noted that 90% of all frictional
 

ignitions occurred at mines with methane emissions of 

at least 393 L/s (1.2 MMcfd).
 

While there is a correlation between the number of 

frictional ignitions and the emission rate, no correlation 

is apparent when normalizing methane emissions by 

production for mines operating in the same coalbed. 

There is a correlation among operations in the same 

coalbed where mines having higher emission rates have 

higher yearly frictional ignition rates. However, this 

arises because larger mines in the same coalbed have 

more production and a greater likelihood of frictional 

ignitions. Once the mine emissions were normalized 

for production, there was no correlation. 

Conclusions 

Frictional ignitions in underground coal mines are a 

concern to the safety of workers because of the potential for fires and explosions that may result in injuries 

and fatalities. While most frictional ignitions are small 

and short-lived, the possibility exists that a frictional 

ignition could be the precursor to a more violent fire 

or secondary explosion. The coalbed mined and the 

overlying and underlying strata are the major controlling factors for the occurrence of frictional ignitions in 

the United States. The Blue Creek coalbed located in 

Alabama had the highest reported frictional ignitions 

total as well as the highest frictional ignition rate per 

million tons of coal mined, except for two minor coalbeds in the Rocky Mountains. 

There were 1,589 frictional ignitions between 1983 

and 2005. Reported frictional ignitions have decreased 

from a high of 114 in 1991 to a record low of 34 in 2005, 

which was primarily the result of a few ignition-prone 
Alabama mines changing cutting sequence or ceasing 

production. While the general trend for the occurrences of frictional ignitions has been declining since 

1991, the record low of 34 frictional ignitions in 2005 

was most likely a statistical anomaly. Continuous miners represent 68.6% of all frictional ignitions (1,090) 

and longwall mining operations represent 85.9% of all 

frictional ignitions (1,365), while machinery directly 

accounts for at least 98.5% of all frictional ignitions 

(1,565). 

Figure 8 

Frictional ignition rate per 2 million hours worked and 

per million tonnes coal. 

Figure 9 

Continuous miner frictional ignitions totals and percentage of total by mining method. 

Figure 10 

Annual methane emissions from coal mines for each 

frictional ignition in ascending order (data not available 

for all years). 

Productivity is not a controlling factor in frictional 

ignition rates. The number of frictional ignitions did increase from 1983 to 1991 along with productivity. Frictional ignition rates have decreased since 1991, while 

productivity has continued to rise until 2000. 

When individual coalbeds are investigated, there 

is evidence that longwall mining operations produce 

more frictional ignitions than room-and-pillar mining on a per-ton basis. Operations using the longwall method had a greater occurrence of frictional 

ignitions than the room-and-pillar method. Although 

383 ignitions were reported on shearers at longwall 

operations, 906 ignitions were reported on continuous mining machines, mostly during gateroad development. Continuous miners developing gateroads 

for longwalls are deeply piercing into virgin methane 

reservoirs at a higher advance rate than continuous 

miners at room-and-pillar operations. Consequently, 

methane emission rates can be higher during gateroad 

development. 

While total frictional ignitions on a per-ton-coal basis have decreased during the timeframe of the study, 

the rise in productivity per working hour has increased 

at a greater rate so that frictional ignitions on a perhour-worked basis have increased. There is a correlation between methane emissions and the number 

of frictional ignitions. However, methane emissions 

normalized by production for mines operating in the 

same coalbed showed no correlation with the number 

of frictional ignitions. Therefore, the coalbed mined, 

and not the mine’s methane emissions, is the determining factor for the occurrence of frictional ignitions. 

References 

Bergamo CA [2006]. Methane emission from US coal mines. Prepared 

for EPA 1990-2004. Personal communication. 

Kissell FN, McCulloch CM, Elder CH [1973]. Report of Investigations 

7767: The Direct Method of Determining Methane content of 

Coalbeds for Ventilation Design. U.S. Bureau of Mines, Washington, DC. 

MSHA [2006a]. AE1983-2005 Annual Employment production yearly 

totals by MSHA mine ID number. 

MSHA [2006b]. AI1983-2005 Accident Injury yearly totals by MSHA 

mine ID number. 

MSHA [2006c]. Data Retrieval System Website http://www.msha.gov/ 

drs/drshome.htm 

Schatzel SJ [1995]. A Characterization of Frictional Ignitions in Underground Coal Mines. Proceedings of the 7th US Mine Ventilation 

Symposium, Lexington, KY. June 5-7.