g

UNITED STATES DEPARTMENT OF THE INTERlOR 
U.S. Department of the Interior 

Mission Statement 

As the Nation's principal conservation agency, the Department of 

the Interior has responsibility for most of our nationally-owned 

public lands and natural resources. This includes fostering 

sound use of our land and water resources; protecting our fish, 

wildlife, and biological diversity; preserving the environmental 

and cultural values of our national parks and historical places; and 

providing for the enjoyment of life through outdoor recreation. 

The Department assesses our energy and mineral resources and 

works to ensure that their development is in the best interests of 

all our people by encouraging stewardship and citizen participation in their care. The Department also has a major responsibility 

for American Indian reservation communities and for people who 

live in island territories under U.S. administration. 
Information Circular 9377 

Bleederless Ventilation Systems as 

a Spontaneous Combustion Control 

Measure in U.S. Coal Mines 

By A. C. Smith, W. P. Diamond, T. P. Mucho, 

and J. A. Organiscak 

UNITED STATES DEPARTMENT OF THE INTERIOR 

Bruce Babbitt, Secretary 

BUREAU OF MINES 
Library of Congress Cataloging in Publication Data: 

Bleederless ventilation systems as a spontaneous combustion control measure in 

U.S. coal mines / by A.C. Smith ... [et al.]. 

p. cm. - (Information circular; 9377) 

Includes bibliopphical references @. 35). 

Supt. of Docs. no.: I 28.279377. 

1. Mine ventilation-Equipment and supplies. 2. Combustion, Spontaneous. 3. 

Coal mines and miningdfety measures. I. Smith, AIex C. 11. Series: Information 

circular (United States. Bureau of Mines); 9377. 

TN295.U4 [TN303] [622 a--dc20] [622' .42] 93-39740 CIP 
CONTENTS 

Abstract ........................................................................... Introduction ........................................................................ Ventilation and spontaneous combustion ................................................... Bleederless ventilation designs ........................................................... "U"system ....................................................................... Backreturn"UM ................................................................... "Ysystem ....................................................................... U.S.systems ...................................................................... Bleederless ventilation design considerations for U.S. coal mines ................................. Spontaneous combustion risks using bleederless ventilation system .............................. Heatingbehindseals ............................................................. Heating behind face supports ....................................................... Sources of air leakages .............................................................. Seais and ventilation cuntruls ........................................................... Methane control aspects of bleederless ventilation design for longwall spontaneous combustion control ..... Historical and current methane-drainage practices .......................................... Methane-drainage systems ......................................................... Methane drainage in advance of mining ............................................... Methane-drainage considerations for use of bleederless ventilation systems in the United States ........ Ground control aspects of bleederless design for spontaneous combustion control ..................... Initialplanningandevaluation ........................................................ General ground control considerations .................................................. Numberofopenings .............................................................. Coalbarriers ................................................................... Longpillars .................................................................... Number of entries ............................................................... Floorheave ....... : ............................................................ Ribsloughage .................................................................. Entrystability .................................................................. Sealdesi gn ..................................................................... Thegroundcontrolplan ............................................................. Stiffdesi gn ..................................................................... Yieldingdesign ................................................................. Combination design .............................................................. Monitoring ......................................................................... Spontaneous combustion detection ..................................................... Methanedrainage ................................................................. Seals ........................................................................... Additional considerations ............................................................ Selection criteria for bleeder-bleederless ventilation system ..................................... Conclusions ........................................................................ References ......................................................................... 

ILLUSTRATIONS 

1 . "U" ventilation system on retreat longwall face ........................................... 2 . Methane distribution in gob using "U" ventilation system .................................... 3 . "Back return" system with two predeveloped tailgate entries ................................. 4 . "Back return" system with predeveloped entries and pack wall support .......................... 5 . "Y" ventilation system ............................................................. 6 . Two-entry modified "U" ventilation system used in United States .............................. 
ILLUSTRATIONS-Continued 

Page 

Critical air velocity zone in gob . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pressure balance chamber to minimize seal leakage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . "U" ventilation system on advancing longwall face . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cross-measure methane-drainage holes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Methane-drainage system used with "back return" ventilation system . . . . . . . . . . . . . . . . . . . . . . . . . . . Australian "modified U" ventilation system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Russian "modified U" ventilation system with "auxiliary diagonal air raises" . . . . . . . . . . . . . . . . . . . . . . Shallow-angle methane-drainage holes drilled in advance of entry development . . . . . . . . . . . . . . . . . . . Cross-measure-type methane-drainage holes drilled from drainage entries into surrounding coalbeds . . . Methane drainage behind seals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Horizontal and cross-measure methane-drainage holes used on "back return" ventilation system . . . . . . . Complete longwall gob gas vent hole system commonly used in United States . . . . . . . . . . . . . . . . . . . . Plan view of short horizontal holes used for draining gas from longwall panels . . . . . . . . . . . . . . . . . . . . Gpen hole cased hole coiiip!i~ons f-- *.-- .A,-. --'I.#.-- A--: em-.-. ....-. 11 ' . . ul VCA~;L$ uAbLuaLIb-ulmagb ~~11 iii 2d.vr&?22 of I?iil?iilg . . . . . . . Evolution of detector gas in laboratory tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 

"C degree Celsius mm millimeter 

cm centimeter Pm micrometer 

h hour mm Hg millimeter of mercury 

kg kilogram m3/min cubic meter per minute 

kPa kilopascal Pa pascal 

m meter Pet percent 

mg/m3 milligram per cubic meter PPm part per million 
BLEEDERLESS VENTILATION SYSTEMS AS A SPONTANEOUS 

COMBUSTION CONTROL MEASURE IN U.S. COAL MINES 

By A. C. smith,' W. P. ~iamond,~ T. P. ~ucho,~ and J. A. 0rganiscak3 

ABSTRACT 

The U.S. Bureau of Mines conducted a worldwide literature review of bleederless ventilation practices 

to evaluate their use as a spontaneous combustion control measure in U.S. coal mines. Factors that 

must be taken into account in the design and use of these systems include seal construction, the use of 

ventilation control devices, the use of methane-drainage systems in gassy mines, and the ground control 

plan. Monitoring for the detection of spontaneous combustion and the control of methane when 

methane-drainage techniques are employed is critical to the successful use of a bleederless ventilation 

system. This report describes the types of ventilation systems used throughout the world and the 

spontaneous combustion risks associated with these systems. Methane-drainage systems used in 

conjunction with bleederless ventilation systems are discussed. Ground control considerations such as 

pillar design, entry stability, and seal usage are reviewed. Monitoring systems for spontaneous 

combustion detection, methane-drainage control, and behind seals are examined. Finally, methods for 

evaluating the spontaneous combustion hazard of a mining operation to determine when the use of a 

bleederless ventilation system is warranted are reviewed. 

'~esearch chemist. 

2~eologist. 

3~ining engineer. 

Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA. 
INTRODUCTION 

The primary function of mine ventilation is to dilute, 

render harmless, and carry away dangerous accumulations 

of gas and dust from the working environment. For 

underground U.S. coal mines, the minimum ventilation 

requirements were initially defined under the Federal Coal 

Mine Health and Safety Act of 1969 (Q4 with current 

regulations published in Title =ode of Federal 

Regulations (2). The regulations stipulate that "all active 

workings shall be ventilated by a current of air containing 

not less than 19.5 volume per centum of oxygen, not more 

than 0.5 volume per centum of carbon dioxide and no 

harmful quantities of other noxious or poisonous gases; 

and the volume and velocity of the current of air shall be 

sufficient to dilute, render harmless and to carry away 

flammable, explosive, noxious, and harmful gases, and 

dust, and smoke and explosive fumes." Methane concentration is not allowed to exceed 1 pct by volume at the 

working area, and a worker's respirable dust exposure 

(particles less than 10 pm in size) cannot exceed an 

average of 2 mg/m3 for an 8-h shift. 

Regulations also stipulate that "bleeder entries, bleeder 

systems, or equivalent means should be used in all active 

pillaring areas to ventilate the mined areas from which the 

pillars have been wholly or partially extracted, so as to 

control the methane content in such areas." A ventilation 

pressure differential is required between the active working area and the bleeder system to ensure gob gas drainage through the bleeder entries. The concentration of 

methane in a bleeder split of air immediately before the 

air in the split joins another split of air, or in a return air 

course, cannot exceed 2 pct. Therefore, underground coal 

mines in the United States are designed with bleeder 

entries to ventilate active gob areas while mining is in 

progress. 

With the passage of the most recent Federal ventilation 

regulations, bleederless ventilation systems may now be 

proposed as a spontaneous combustion control method. 

Regulation 75.371 states, "the mine ventilation plan shall 

contain the information described below .... In mines with a 

demonstrated history of spontaneous combustion: a description of the measures that will be used to detect 

methane, carbon monoxide, and oxygen concentration ... If 

a bleeder system will not be used, the plan shall contain 

the methods that will be used to control spontaneous 

combustion, accumulations of methane-air mixtures, and 

other gases, dusts, and fumes in the worked out area" (2). 

With this development, it is necessary to have a 

thorough understanding of current and past experience of 

4~talic numbers in parentheses refer to items in the list of references 

bleederless ventilation systems both overseas and in the 

United States. To accomplish this, the U.S. Bureau of 

Mines (USBM) conducted a worldwide literature review 

of bleederless ventilation practices to evaluate their use as 

a spontaneous combustion control measure in U.S. coal 

mines. The impact of bleederless ventilation practices 

on other mine design parameters, such as ground control, methane control, ventilation, and monitoring, is 

considered. 

In the United States between 1978 and 1988, approximately 15 pct of underground coal mine fues were caused 

by the spontaneous combustion of coal (3). Spontaneous 

combustion fires usually occur in worked-out or gob areas 

and are not easily detected. These fires present a serious 

safety hazard to mine perso~el and are difficult to 

extinguish, often requiring sealing large sections of the 

mine or the entire mine for long periods, resulting in 

severe economic losses. A spontaneous combustion fire 

was discovered in June 1986 in a Colorado mine (4-5). 

The entire mine was sealed and abandoned. Another 

heating occurred at a Colorado mine in a longwall gob 

that required sealing and inert gas injection to suppress 

the heating (6). Several self-heating events due to the 

oxidation of pyrite have also been associated with floor 

heave in an Alabama mine (7). The number of spontaneous combustion fues is expected to increase with the 

increased use of low rank coals, deeper mines, and the 

growth in longwall mining. 

The self-heating of coal occurs when the heat that is 

produced by low-temperature oxidation is not adequately 

dissipated, resulting in a net temperature increase in the 

coal mass. Under conditions that favor a high heating 

rate, a fire ensues. Many factors can contribute to the 

self-heating process, including coal properties, geologic 

factors, and mining conditions and practices. 

Singh grouped these factors into intrinsic and extrinsic 

categories (8). Intrinsic factors are those that cannot be 

controlled by the mine operator, such as coal properties 

and geologic and mining conditions, while extrinsic factors 

are those that can be controlled, such as mining practices. 

Generally, the most important factor is the coal's reactivity, or propensity to oxidize. 

The oxidation of coal requires fuel, the coal, and an 

oxidizer, usually air. Thus, by limiting the amount of 

oxidation, and therefore the amount of heat generated, the 

spontaneous combustion risk can be reduced. To accomplish this, either the coal or the air must be removed. The 

amount of coal in a retreat longwall gob can be somewhat 

at the end of this report. 
controlled by the mining practices, such as pillar design 

and amount of coal left in the roof and floor. The amount 

of airflow through the gob can be reduced by using a 

bleederless ventilation system. This forms the basis of the 

bleederless ventilation design as a spontaneous combustion 

control measure. 

This report will discuss the bleeder less ventilation 

procedures used and the mining methods employed in conjunction with these procedures both in the United States 

and throughout the world. It will discuss bleederless 

ventilation design with respect to current U.S. mining 

practices and regulations, and the effect of a bleederless 

ventilation system design on spontaneous combustion, 

ground and methane control, and ventilation. Monitoring 

will be discussed in terms of early detection of spontaneous combustion and methane control. Finally, selection 

criteria for determining when to use bleederless ventilation 

systems will be examined. 

This work was done in support of a USBM program to 

improve health and safety in the mining industry. 

VENTILATION AND SPONTANEOUS COMBUSTION 

In coalbeds that are reactive to oxidation, critical lowvelocity airflow over the reactive coal increases the risk of 

heathgs, Criiid --flow is de$ded 3s hsi~fficient, 

airflow to remove the heat due to oxidation, but enough 

airflow to maintain the oxidation process (9). Humphreys 

reports that greater than 14 pct oxygen is needed to 

initiate a spontaneous combustion heating and oxygen 

levels of greater than 2 pct are needed to sustain a 

heating, while Liney states that heatings will occur with 

oxygen levels as low as 6 pct (10-11). Either more or less 

airflow than the critical amount can inhibit the self-heating 

of coal. Critical airflows generally exist in gobs, behind 

longwall supports, through crushed pillars, in intake-return 

air crossings, and across stoppings. 

Controls for the spontaneous combustion of coal focus 

on either minimizing the amount of coal exposed to reactive environmental conditions or curtailing the conditions. Mine planning and production practices are of 

utmost importance in controlling spontaneous combustion. 

One of the most common methods of spontaneous cornbustion control used around the world is minimizing 

airflow to prior and active mine workings to reduce the 

available oxygen for spontaneous combustion. Balancing 

the requirements of spontaneous combustion control, 

ground and methane control, and the coal production plan 

can be very demanding because of the interdependence of 

each factor on the others. Therefore, a systems approach 

must be undertaken in mine planning to control the environmental conditions affecting spontaneous combustion 

(reduce fuel (coal) exposure to critical airflow conditions), 

to continuously assess these environmental conditions 

(monitoring), and to formulate alternative action plans for 

the existing or progressing circumstances (safety training, 

firefighting, and sealing). 

Coal-producing countries around the world have used 

methods to limit the amount of airflow or oxygen to areas 

prone to spontaneous combustion. Great Britain (11-13), 

Germany (14-IS), Poland (Id), France (17), Australia (10, 

la), India (19), Russia (20-21), and China (22) all have 

e.qerience in restricting the airflow into working gob areas 

in coalbeds prone to spontaneous combustion. In the 

United States, two mines that have had spontaneous combustion occurrences have been allowed to use a bleederless 

ventilation system (6, 23). 

Geologic conditions of European and Asian coalfields 

are notably different than those of both the United States 

and Australia. The European and Asian mining operations commonly extract multiple dipping coalbeds that are 

deeply covered (24). When multiple coalbeds are mined, 

ground and methane control problems become more complex because of the stress and multiple sources. Thus, less 

entry development with extensive secondary support practices is commonly used to control ground stresses, and 

complex in-mine methane-drainage practices are often 

used to control methane emissions. 

Underground coal production in Europe and Asia is 

usually conducted with full extraction techniques or 

longwall mining. Both advancing and retreating longwall 

mining systems have been utilized in these countries. 

Advancing longwalls are more beneficial for methane 

removal from the gob area, since a negative pressure 

differential is created on the gob (25). For spontaneouscombustion-prone coalbeds, retreat longwall mining systems are more beneficial because airflow into the gob can 

be more easily controlled. If a large amount of methane 

is emitted in the gob on the retreat system, in-mine 

methane drainage of the gob is commonly practiced so the 

face ventilation system does not become overloaded with 

explosive levels of methane gas (26). In-mine drainage is 

generally preferred because of the depth of cover, dip of 

the coalbeds, multiple-bed mining practices, and surface 

population density. The entry that contains the methanedrainage system must be well supported so the system 

remains functional during the retreat of the panel. Gas 
pg gp

such as limiting the number of entries to reduce ground 

stresses versus creating more entries to improve ventilation. Since performance of various technologies is mine 

dependent and cannot always have universal application 

throughout the industry, mine planning should include 

alternatives to the original design. 

"U" SYSTEM 

The most common and effective method to limit airflow 

to the gob in a longwall panel is the "U" ventilation system. 

In this system, the air is brought up the headgate entry, 

across the face, and down the tailgate entry. A schematic 

of the "U" system is shown in figure 1. 

European and Australian mines typically operate this 

system using single-entry development of the longwall 

panel. The principal advantage of this type of development is that the system does not require many seals to 

limit air leakage (18). The use of multiple-entry development requires seals between entry crosscuts, thereby 

increasing the risk of air leakage to the parallel entry or 

mined-out panel. These seals isolate the gob from the 

gy

Return Intake 

Figure 1.-'U" ventilation system on retreat longwall face (10). 
Flgure 2.4ethane dlstrlbution In gob uslng "U" ventilation Figure l.-'Ebck return" system with two predeveloped tailgate 

system (16). entries (10). 
Flguro 4.-"Back re4urn" system with predeveloped enMes and 

pack wall support (modified from Bacharach (12)). 

Intake Intakes 

U.S. SYSTEMS 

Two mines in the United States currently use a bleederless ventilation system (6, 23). Both use a modified-"Utype ventilation scheme with multiple predeveloped entries. 

Seals are constructed in entries and crosscuts to isolate the 

gob. Air is directed up the headgate entries, across the 

face, and back along the panel return. A limited amount 

of intake air is coursed through the headgate entry 

Figure 5.-'Y' ventilation system (10). 

adjacent to the sealed gob to maintain access for seal 

examinations, monitoring, gob pressure balancing and 

entry maintenance. An example of a modified "U" system 

with two-entry development is shown in figure 6. Like the 

"Y" system, the major disadvantage of this system is a 

greater reliance on seal integrity to isolate the gob. 
 Seals  Stoppings 

Seals sampled X Overcasts 

for gases 

Figure 6.-Two-entry modified "U" ventilation system on retreat longweli face used in United States. 

BLEEDERLESS VENTILATION DESIGN CONSIDERATIONS FOR U.S. COAL MINES 

Underground coal mine design and practices in the 

United States are notably different than those of many 

other countries. One of the principal differences is the 

number of entries commonly developed. Most coalbeds 

currently mined in the United States are shallow (less than 

600 m) and relatively flat with multiple-entry development 

within the coalbed. Three-entry longwall gate road 

developments are very common, with some gassy mines 

using four-entry developments for additional ventilation. 

One or more of the entries on the headgate side of the 

longwall become the tailgate entries on the next longwall 

panel, connecting adjacent gobs, which are linked to a 

blceder system. 

In Europe and Asia one-entry longwall gate roads are 

common, with some two-entry developments on the tailgate side of the longwall. Barrier pillars are left between 

panels, isolating individual gob areas. These one-entry or 

sometimes two-entry longwall development systems undergo ground convergence (mainly floor heave) inby the mining face, reducing the air pathways or increasing air resistance into gob areas on the retreat longwalls. Also, the 

lower number of entries reduces the number of seals to 

construct to isolate the longwall gob. 

Multiple-entry developments in the United States reduce mine resistance to airflow, so that higher quantities 

of air can be more easily circulated throughout these 
mines and are available for bleeder ventilation systems. 

Higher quantities of air at the mining face are required in 

the United States to meet stricter health and safety standards and higher coal production levels. For example, the 

U.S. regulations limit the 8-h personal airborne dust exposure level of an employee to 2.0 mg/m3 and require the 

mining face to be deenergized when the methane concentration exceeds 1.0 pct (2). In Great Britain the dust 

standards are 7.0 mg/m3 measured at a point in the section return 70 m from the face and 5.0 mg/m3 in entry 

roads upon development, with a methane standard of 

1.25 pct before the mining face is deenergized (30). Also, 

the overall efficiency of the U.S. underground coal mining 

industry is notably higher than that of many other countries, so more ventilation is required to remove the dust 

and methane produced at a higher mining rate. Finally, 

the 5.S. reguiaiions wiih respect to beit eiiiiy t'eiiiZaiioii 

to the face and escapeways require three entries for longwall development, although some petitions for modification 

have been granted for two-entry development. 

SPONTANEOUS COMBUSTION RISKS USING 

BLEEDERLESS VENTILATION SYSTEM 

Although a bleederless ventilation system is designed as 

a spontaneous combustion control measure, certain design 

parameters provide potential spontaneous combustion 

risks. The two areas with the greatest risk of heatings are 

around seals and behind the face supports. If high levels 

of methane exist in the gob (higher than the explosive 

range), there is the additional risk that methane concentrations within the explosive range will be located behind 

a leaking seal or behind the supports. If a heating takes 

place in these areas, dangerous methane accumulations 

may be exposed to a possible ignition source. Therefore, 

when sealing high-methane-emission gobs for spontaneous 

combustion control, the risks from air leakage are not only 

a heating but possibly an explosion hazard. Several reported cases of explosions initiated by self-heatings in the gobs 

of gassy coalbeds are found in reference 16. 

Heating Behind Seals 

In the United States the use of multiple-development 

entries for longwall panels leads to the extensive use of 

seals in a bleederless ventilation design to isolate the gob. 

Each seal provides the potential for a self-heating event, 

through air leakage around or through the seal. Also, the 

larger number of entries increases the number of coal pillars (additional spontaneous ignition and fuel sources) that 

can be exposed to critical airflows when a bleederless 

ventilation system is being implemented. 

Heatings around seals can occur in the floor behind the 

seal or in the pillars and roof adjacent to the seals because 

of air leakage. Therefore, seal design must take into account all mining conditions and consequences that can lead 

to air leakage. These conditions include high pressure 

differentials across seals; weak floor or roof strata; fractured, weathered, or permeable coal pillars; and ground 

control problems and pillar stability. Seal construction 

must utilize appropriate materials and construction methods to meet or exceed the design specifications. Inspection and maintenance of seals is critical for continued 

optimum performance. 

Reducing the number of entries is beneficial for bleederless ventilation systems in terms of potential sources of 

air leakage, but also increases the mine ventilation network 

resistance. A higher ventilation resistance increases the 

pi-zssure diffCrer;tids throu&o.;), the =.line if =ir?izg fZCeS 

are being ventilated with the same air quantity. Higher 

pressure differentials increase leakage through ventilation 

controls such as stoppings. Work conducted by the USBM 

has shown that for a newly constructed conventional block 

stopping (dry-stacked with mortar applied to one side) 

leakage was 4 m3/min per 2.5 mm of water gauge pressure 

differential, and the leakage more than doubled after 

1 year of use with slight roof convergence (31). A newly 

constructed universal stopping (keyed into ribs with a 

poured footer) leaked 1.4 m3/min per 2.5 mm of water 

gauge pressure differential, and the leakage more than 

doubled after 1 year of use with slight roof convergence. 

It was shown that a stopping maintenance program could 

restore stopping integrity to its original effectiveness. 

Thus, improving the sealing of ventilation controls such as 

stoppings, overcasts, and seals in bleederless ventilation 

systems can reduce the potential for spontaneous combustion in these areas. 

An example of leakage around seals was seen at a 

room-and-pillar mine in the Western United States. Here, 

seals were used to separate the intake and return airways, 

and there was a high differential pressure across the seals. 

In this case, pillars located within a 150-m burn zone near 

the portal of the mine were extremely weathered, and 

because of the high differential across the pillars, air was 

drawn through the pillars, causing them to heat. This is 

an instance where the seals were more effective in preventing air leakage than the coal itself. The problem was 

alleviated by reducing the differential pressure across the 

pillars. Although this example is not in the context of a 

bleederless ventilation system, the problem of air leakage 

and self-heating due to high pressure differentials is clearly 

evident. 

Another mine in the Western United States that uses a 

bleederless ventilation scheme at its longwall operation still 
experienced heatings (23). This mine has low methane 

emissions and the longwalls use two-entry development. 

Seals are constructed in the entry crosscuts to isolate the 

gob as the longwall retreats. Monitoring behind these 

seals is conducted to measure oxygen depletion in the gob, 

which usually decays to below 5 pct. The oxygen content 

behind several seals only dropped to 15 to 20 pct because 

of leakage, and a heating developed. Because of the 

suspected air leakage, seal modifications were made on the 

next panel. Because the success of the seals depends 

strongly on ground control and mining conditions, seal 

design and construction need to be evaluated on a mineby-mine basis. 

Heating Behind Face Supports 

"-A"" C;~PP AL it I" ;c ;mnnccihl~ I- in practical terms to prevent face r-"----- 

ventilation air from entering the gob behind the supports, 

an oxygen gradient develops. Just behind the supports, 

where the oxygen concentration is the highest, the air 

velocity is high, so that any heat generated by self-heating 

is carried away. Deeper in the gob, the air velocity is too 

low to provide sufficient oxygen to support spontaneous 

combustion. However, there exists a critical air velocity 

zone in the gob where the airflow and oxygen concentration are conducive to self-heating. This is illustrated in 

figure 7. 

The incidence of self-heating depends on how long the 

coal is subjected to this critical air velocity. If the time is 

long enough for the heat to build up to an imminent selfheating event, referred to by the British as the "incubation 

period," a fire may result. Therefore, the rate of face 

advance is extremely important in preventing self-heating 

behind the supports. A similar critical area exists with 

respect to explosive methane concentrations behind the 

supports in gassy coalbeds. This makes the prevention of 

self-heating even more critical in this situation, since the 

body of methane could be ignited. 

SOURCES OF AIR LEAKAGES 

High pressure differentials across the gob and across 

seals provide the impetus for leakage. In the U.S. mines 

using the bleederless ventilation system, this is dealt with 

by attempting to pressure balance across the gob. However, because of pressure gradients in the gob, it may also 

to weather fronts can induce air into the gob, 

causing spontaneous combustion. These fronts can also 

cause outgassing to occur, producing high concentrations 

of methane and/or carbon monoxide in the face area. 

Communication between adjoining gobs, the mine and 

the surface, and between mines in multiple-bed mining 

operations by way of overburden fracturing can also 

provide sources of air for spontaneous combustion in the 

gob. This factor also makes it difficult to characterize gob 

gases and maintain pressure balancing, and should be 

considered in the mine design and planning stages, especially for multiple coalbeds and shallow overburden 

conditions, to minimize these communication paths. 

Return Intake 

be necessary to measure and adjust pressure differentials 

across seals. In addition, barometric pressure changes due Figure 7.--Critical alr velocity zone in gob (66). 
SEALS AND VENTIL ATlON CONTROLS 

The principle behind the bleederless ventilation design 

for the control of spontaneous combustion is to reduce the 

amount of air in the gob, thus limiting coal oxidation and 

the subsequent generation of heat. Bleederless ventilation 

systems are designed to minimize the potential for air 

leakage into the gob. Therefore, of critical importance in 

a bleederless ventilation design are the proper design and 

construction of seals to minimize leakage and maintain 

seal integrity, and ventilation planning to reduce pressure 

differentials across these seals. 

Longwall system developments in Europe and Asia, 

with fewer entries, allow improved control in isolating 

active and inactive gob areas. A smaller number of entries 

reduces the number of seals that need to be constructed 

and maintained, making airfiow isoiation of mined-out 

areas more manageable. European and Asian mines 

usually drive mine entries in rock to gain access to the 

steeply dipping coalbeds; therefore, seals are constructed 

in the mined-out areas in the bedrock, reducing the risk of 

heating at the seal site. When the seals have to be 

constructed in the coalbed, they are usually constructed 

within a substantially sized coal pillar to reduce the 

crushing of surrounding coal and seal (12). In Australia, 

where mine entry development occurs within the coalbed, 

coal barrier pillars (or fire barriers) are designed into the 

mine plan so that mined-out areas can be more easily 

sealed or isolated from airflow to the active mining areas 

(10). Also, in Australia where pillar extraction is practiced, mining methods or sequences of pillar extraction are 

designed to minimize the gob's exposure to airflow. One 

of the key elements practiced in both longwall mining and 

room-and-pillar operations is that production panels are 

designed for quick advance rates, limiting the duration of 

critical airflows in active mining areas. 

Seal construction is important, so various types of seals 

are built to meet the mine's particular requirements. Seal 

construction can vary from a stacked-block stopping to 

explosion-proof seals. Bacharach defines an explosionproof seal as a seal composed of noncombustible material 

to fill the length of the entry equal to one-half the sum of 

the height and width of the entry (12). The USBM defines an explosion-proof seal as one that is able to 

withstand a static load pressure of 137 kPa, provided that 

the area to be sealed contains sufficient incombustible 

material to abate the explosion hazard in that area and 

that adequate incombustible material is maintained in the 

adjoining open passageways (32). 

Seal location and design are crucial in their ability to 

limit air exchange. Seals should be generally located in 

areas where entry closure or seal crushing is minimal so 

airflow leakage can be minimized. Quality construction 

techniques can reduce leakage, such as keying the seal wall 

into the floor and ribs and using some squeezable material 

in the seal construction to accommodate some roof and 

floor convergence. When one or more panels are being 

sealed, building lightweight seals in the same entry outby 

the main seal is recommended to create a void space or 

buffer zone between the sealed area and the mine (12). 

Under stable barometric pressures the zone between this 

pair of seals will tend to fdl with methane, nitrogen, and 

carbon dioxide from the sealed area. Provided the zones 

are made large enough, whatever breathing exists around 

the inner seal(s) will not be associated with an oxygen-rich 

atmosphere. This minimizes the risk of a heating around 

the inner seai. 

Barometric pressure effects on seal leakage are amplified with larger mined-out areas, so a series of seals with 

a buffer zone also reduces atmosphere exchanges with the 

sealed area because the outer seal adds extra resistance 

to the pressure change. An improved means to reduce atmosphere exchanges in sealed areas of the mine is to construct pressure chambers at the seals or to pressure 

balance the sealed areas to limit atmosphere exchanges 

with the mine (12, 17, 33). This is achieved by constructing several seals with a void space in the same entry 

(as described above), generally on the low-ventilationpressure side of the sealed area. The void space is then 

pressurized by connecting it with a duct to a high-pressure 

ventilation network in the mine. This is shown in figure 8. 

An exhaust regulator on the outer seal is then adjusted to 

equalize the atmospheric pressure behind the inner seal 

with the void space pressure. Several pipes are built into 

these seals for gas and pressure sampling behind the seals. 

The U.S. mines with bleederless ventilation systems use 

wood squeeze seals and pressure balancing techniques (6, 

23). At one mine, isolation stoppings are constructed in 

the crosscuts of the headgate entry prior to the longwall 

face passing. The stoppings consist of a wooden pack wall, 

a 3- to 5-m slurried sandfdl, and a 1.2-m-thick wood crib 

block stopping. Even with this method, heatings behind 

the seals continue to be a persistent problem because of 

leakage through and around the seals. At this mine, the 

leakage is primarily due to the weak roof conditions, which 

allow leakage through the roof. At another mine, timber 

squeeze seals are built in advance of the face. As the 

longwall passes the seal, ground convergence "squeezes" 

the timbers, isolating the gob. No self-heatings have been 

reported at this mine using this type of seal in a bleederless system. 
30.5-cm steel tube 

- 3.7 to 5.5 mSIDE VlEW 

damper 

Gauge 'b A, B Balance pipes 

C Sample pipe 

FRONT VlEW 

Figure 8.--Pressure balance chamber to minimize seal leakage (12). 

Ventilation control devices such as stoppings, man 

doors, regulators, and overcasts create air pressure 

differentials within the coalbed. Fractures and cleat in the 

coal around these control devices are conduits for critical 

airflows through the coal, creating the potential for a 

heating. Proper location, improved design, and quality 

construction of these devices can decrease the amount of 

critical air exposure of coal and/or reduce pressure 

differentials around these devices. The coalbed airway 

surfaces around these ventilation control devices can be 

sealed or coated and the coal itself injected with an 

inorganic sealant to restrict airflow through the fissures. 

Coal pillars in mine airways should be properly designed 

to reduce crushing or sloughage, using pillar bolting if 

necessary. Pillars that crack can be surface coated and/or 

injected with an inorganic sealant. 

The mine plan should be designed to minimize ventilation control devices, and the sealed mine areas should be 

located on the low-pressure side of the ventilation network. Also, pressure differentials can be reduced through 

better location of the control devices, or through alternative pressure reduction techniques. For example, 
regulators, in some cases, can be located outby overcasts should be keyed into the floor, roof, and coal pillars. 

or air crossovers. Auxiliary fans can be used with stop- Finally, a compromise between the number of air entries, 

pings to reduce the air pressure differential between air entry dimensions, and exposed coal surfaces in the ventilacrossover entries. Double doors can be constructed to tion network design should be considered while minimizing 

create an airlock. If possible, all these control devices the mine's ventilation pressure characteristics (12). 

METHANE CONTROL ASPECTS OF BLEEDERLESS VENTILATION DESIGN 

FOR LONGWALL SPONTANEOUS COMBUSTION CONTROL 

Spontaneous heatings can be a serious mine safety 

problem because in addition to the obvious fire hazard, 

they can potentially ignite methane accumulations with 

disastrous consequences (16). The preponderance of literature that addresses the problem of spontaneous comhustion in the presence of methane during longwall mining 

recommends or describes using retreat mining and preventing the penetration of ventilation air into the gob 

(10, 13, 15-16, 18, 25, 34-35). Humphreys points out that 

"wherever ventilation air is permitted to pass along the 

edge of a goaf or is forced to pass through the goaf, there 

will be areas in which there is sufficient oxygen to support 

oxidation, and insufficient air movement to carry away the 

heat generated." He further states that "it is much easier 

to ensure no air reaches these places, than to ensure that 

all the potentially hazardous areas are adequately 

ventilated" (10). 

In mines where methane drainage has been used to 

supplement bleeder entries, excluding ventilation air from 

the gob will make drainage the primary methane control 

measure. Methane drainage may also be required with 

bleederless ventilation systems in mines where ventilation 

by bleeder entries alone could have controlled the 

methane accumulations. Humphreys states that "when 

dealing with a spontaneous heating, the source of ignition 

is ever-present and therefore emphasis must be placed on 

preventing accumulation of explosive mixtures." He 

further states that the key element of a methane-drainage 

system is to establish "an extinctive atmosphere in the goaf, 

that is neither potentially explosive nor aids the oxidation 

of coal" (10). Relatively low levels of methane emissions 

that would otherwise have been swept out of the gob could 

eventually build up to the point where they could inundate 

the face as a result of falling barometric pressure, or large 

roof falls. 

The effectiveness of methane drainage is important 

beyond the obvious explosion risk when spontaneous 

combustion is also a concern. One spontaneous combustion control strategy is to mine at rates sufficiently fast so 

that any ventilation of the gob that does occur is sustained 

for a time interval less than the coal's spontaneous combustion incubation period. Thus mining delays due to high 

methane levels not effectively controlled by ventilation and 

methane drainage can potentially expose ventilated gob 

areas to spontaneous combustion conditions past the 

incubation period. This is a particular concern when the 

"Y" or "back return" bleederless ventilation systems are 

used, since they are specifically designed to ventilate the 

return corner of the longwall face. 

HISTORICAL AND CURRENT 

METHANE-DRAINAGE PRACTICES 

Ventilation is the primary methane control measure in 

coal mines throughout the world. However, when ventilation alone cannot control methane levels underground, 

supplementary methane drainage must be used. Methanedrainage practices are generally different in the United 

States than in other countries. Methane drainage for 

longwalls in the United States is primarily accomplished by 

the drilling of several vertical boreholes into the gob from 

the surface. These vent holes are generally equipped with 

surface vacuum pumps to draw gases from the gob. In 

most applications, the pumps are set to draw the maximum volume of gas possible. Minimal monitoring or control of the gas production is attempted. Secondarily, 

horizontal methane-drainage holes are used underground 

to drain gas in advance of mining from the coalbed being 

mined. 

The primary methane-drainage technique used on longwalls outside the United States is cross-measure boreholes. 

These holes serve a similar function as the vertical gob gas 

vent holes used in the United States. The cross-measure 

holes, however, are drilled underground from the entries 

adjacent to the panel. The holes are usually drilled at an 

angle into the roof, over the longwall panel to intercept 

the "destressed" zone by the retreating face. Methane 

drainage of gob areas through pipes installed in seals is 

also practiced. Methane-drainage holes are also drilled in 

advance of mining to other horizons (usually other minable 
Great Britain for many years before coal mining began in 

the United States, and longwall mining was used in British 

mines before it was introduced in the United States. Historically, longwall mining in Great Britain has been by 

advancing methods into the solid coal, which requires the 

construction of pack walls and the use of extensive roof 

support and floor grading to maintain open entries for 

ventilation, transport of the mined coal, and movement of 

equipment and personnel. An advancing longwall, shown 

in figure 9, presented an ideal situation for methane 

control since the pressure differential between the intake 

and return sides tended to direct gas accumulations in the 

gob away from the face and into the return. However, as 

gassier coalbeds were mined, it became necessary to supplement the ventilation system with methane-drainage 

holes to control methane at the face. Cross-measure 

holes, shown in figure 10, were the most common drainage 

system employed. These holes are drilled from the return 

side over (or under) the pack wall or cribs into the 

destressed zone over (or under) the extracted panel. The 

holes are drilled from the return side to take advantage of 

the pressure differential between the intake and return. 

Figure 9.-"U" ventliatlon system on advancing longwall face 

(modified from Humpreys (10)). 

\ ' L~L 30~\\n A-1 

-\ -+ -1 \ 11 KEY 

1 i + - -3 ' Coalbed 

+ \ + I- ---'- , a Mined-out ore0 

+ & crols-measure '$<\ 35 lrn Height abme ,A rnethandra~nage 

- I,. holes , \ , rnlned coalbed 

Figure 10.--Section view of cross-measure methane-drainage 

holes (modifled from McKensey (18)). 
measure methane-drainage holes can be drilled. The gas 

produced by these holes is collected in an underground 

pipeline and transported to the surface where it is generally utilized. 

The back return airway is generally kept relatively short, 

requiring that seals be periodically constructed to isolate 

the gob. In the first experiments with this system, the 

cross-measure holes into the roof and floor were not left 

connected to the pipeline after seals were installed outby 

their location (37). Apparently the holes were allowed to 

bleed under natural flow into the sealed chambers. Openended methane-drainage pipes were left behind the seals 

to provide drainage of these chambers, shown in figure 11. 

Even with this drainage system in place, the gas pressure behind the seals eventually became sufficient to leak 

gas at high rates into the returns. Subsequently, the crossmeasure holes were left connected to the underground 

pipeline after the seals were constructed, and this successfully controlled the methane leakage problem. No 

access was possible to these cross-measure holes behind 

the seals, and the only control of their gas flow was by 

adjustments to the amount of suction applied to individual 

pipelines. The holes behind the seals could only be shut 

in by closing valves on the pipelines outby the last seal. 

KEY 

C ross-meaure holes 

lnroof --- In floor 

Figure 11.--Methanedrainage systm used with "back return" 

ventllatlon system (modlfled from Highon and Cunliffe (31)). 

Such methane control measurer would most likely not 

be acceptable in U.S. coal miner. When underground 

methane-drainage systems have beta utilized in the United 

States, many ventilation plans ha\-: required that they be 

installed in accessible entries whert repairs could be made 

in the event of leaks (38). The L5BM has developed a 

fail-safe protection system to autoxatically shut in underground pipelines and the methane-rrainage holes connected to them in the event of methaw leaks (39). 

Highton recommends that two xethane-drainage pipelines be maintained in the sealed rnrn airway (13). This 

is to make it more likely that one ?ipeline would survive 

and continue to produce gas from he gob in the event of 

a roof fall. Each pipeline would, zowever, be connected 
to different sets of cross-measure holes, and the consequences of losing one set of drainage holes would vary 

depending on their location. Highton reports that even 

when one pipeline has been damaged and shut in, the remaining methane-drainage holes have captured a portion 

of the gas previously drained by the holes connected to the 

damaged system (40). A decrease in the percent methane 

from a pipeline is generally the first indication of a leak. 

Under most circumstances, it is likely that drainage holes 

from only a limited number of sealed chambers in a back 

return ventilation system would be kept on production as 

the longwall retreats. Eventually the pipeline connected to 

holes farther back in the gob would be cut off, and a new 

pipeline connected to holes closer to the face would be 

started. In this way, the recommended two pipelines 

would be preferentially draining the gob closest to the 

longwall face. It has aiso been a practice to ieave a 

pipeline active to the holes near the start-up end of the 

panel, especially when it is updip of the retreating face. 

Another version of the modified "U" ventilation system 

has been used in gassy coalbeds subject to spontaneous 

combustion in Australia. It is quite similar in design and 

function to the British back return system. The primary 

difference is that the Australian system uses two entries 

on the tailgate side driven on development, shown in figure 12, instead of leaving a line of small pillars and 

constructing a pack wall to create two entries as the 

British do. ow ever, as the longwall retreats, two entries 

are maintained with the British system, whereas one entry 

is sacrificed in the Australian system. The Australian 

system would probably be less labor and time intensive 

than the British back return system, making it more 

attractive to U.S. coal companies. The Australian modified "U" system also provides for two intake airways, which 

would be advantageous in gassy U.S. coalbeds. 

Methane drainage with the Australian modified "U" 

ventilation system is similar to the back return system used 

by the British. A system of cross-measure holes is drilled 

into the roof from the return airway. However, fewer 

holes are used (one per pillar versus every 9 m), and they 

are drilled in advance of the face instead of after. Also, 

no floor methane-drainage holes were reported. These 

differences may be due to site-specific methane conditions, 

and not to any fundamental difference in the ventilation 

system. The cross-measure holes are connected to an underground pipeline hung from the ceiling, and the gas is 

transported to the surface with the aid of vacuum pumps. 

In addition to the cross-measure holes, an open-ended 

pipeline is also installed through the seals to a point near 

the gob at the highest point in the return airway. This 

pipeline is also connected to the vacuum pumps on the 

surface. 

Figure 12.--Australian "modified U" ventilation system (18). 

The basic British back return ventilation system with 

modifications to include two intake entries and "auxiliary 

diagonal air raises" (fig. 13) is used by the Russians to 

control methane emissions on longwall panels susceptible 

to spontaneous combustion (21). This ventilation plan 

diverts gas emitted from broken coal on the belt line 

through the diagonal, thus bypassing the face. The 

methane hazard is reduced along with the air velocity at 

the face, which aids the basic back return system in 

spontaneous combustion control. As in the other countries 

discussed previously, methane drainage by cross-measure 

holes (both above and below the extracted panel) is 

commonly practiced. However, Smorchkov also mentions 

the use of "ventilation holes drilled from the surface to the 

drainage gate," and "gas drainage of contiguous seams" 

using boreholes from the surface without giving specifics 

as to the conf~guration of these holes (21). 

Spontaneous combustion and concurrent methane 

drainage is also a considerable problem in Poland (16, 29). 

The ventilation systems developed by the Polish are most 
Figure 13.--Russian "modifled U" ventilation system with 'auxiliary diagonal air raises' (modified from Smorchkov (21)). 

remarkable for their great number of variations, including several variations of the basic "U" and back return 

systems. Methane control systems are also quite varied, 

utilizing several variations of the cross-measure concept. 

Methane drainage in advance of the face is more commonly practiced than in other countries, primarily from 

underground, but in some instances from the surface. 

Krzystolik states, in reference to surface drainage in 

advance of mining, that in no case did it "avoid the 

necessity of methane drainage from the excavation after 

beginning the exploitation process" (41). 

Methane drainage in many Polish mines is an integral 

part of the drivage of development entries. Shallow-angle 

holes are drilled from specially constructed driig 

"caves" to drain gas from the strata in front of and adjacent to the advancing entries as shown in figure 14. 

Because of the steep dip of the strata in many of the 

Polish mining districts, cross-measure-type holes are also 

drilled from drainage entries driven in rock above or 

below mined coalbeds (fig. 15). Because of the prevalence 

of multiple-coalbed mining, similar holes are also drilled 

from existing entries in other coalbeds in preference to 

driving entries in rock. The more traditional crossmeasure holes are also used during the mining of longwall 

panels. These holes are drilled both above and below the 

panel, in advance of the face and into the destressed zone 

immediately before the longwall face. Additionally, when 

the mined coalbed is a major contributor to the gas emission problem, horizontal holes are drilled into the unmined 

panel to drain gas. This is one of the few instances of a 

foreign country's using a gas-drainage technique commonly 

used in the United States. 

Drainage entries and the remaining entries around 

completed panels are generally sealed, especially when 

spontaneous combustion is likely. These seals, shown in 

figure 16, are equipped with open-ended methane-drainage 

pipelines and gas sampling ports to monitor the gas composition behind the seals. KrzystoIik also mentions a 
Figure 15.--Section vlew of cross-measure-type methane-drainage holes drilled from drainage entries into surrounding coallpds (modified from Krzystolik (42)). 
steep dips, longwalls are oriented with the face parallel to 

the strike and mining progresses downdip (18, 40). This 

enhances methane control by the ventilation system in that 

the buoyancy effect of methane tends to keep it in the gob 

updip of the face (26). Methane drainage of sealed chambers created by the back return ventilation system is also 

enhanced by placing the intake of the open-ended drainage 

pipeline at the highest elevation in the individual chambers. Thorp observed that retreat mining downdip is 

less conducive to spontaneous combustion (25). A disadvantage of mining downdip is that if water is present, it 

will continually accumulate at the working face. 

While retreating downdip can be advantageous, it is not 

without potential problems. Highton reports that because 

of the buoyancy effects of the methane in the gob, it was 

generally found that when retreating downdip, the oxygen 

content is generally higher behind the seals in a back 

return ventilation system (13). 

"Overdrainage" of the gob will tend to move the interface between methane-rich and oxygen-rich air farther back into the gob, creating conditions ideal for 

spontaneous combustion over a more extensive area. 

Swift describes a longwall panel where "overdrainage" from 

estimated incubation period. However, Highton also states 

that the "length of the back return is more critical on faces 

retreating to the rise (updip), and this length should be 

kept as long as possible in order to maintain access to a 

maximum number of boreholes on the methane drainage 

system" (13). 

Longwalls are also oriented with their faces parallel or 

subparallel to the dip. In this orientation, the gate roads 

are then parallel to the strike, and successive panels are 

placed downdip of the previous panel. The intake airway 

is on the downdip margin of the panel, and the return is 

on the updip margin. This is also an advantageous arrangement for cross-measure methane drainage when the 

back return ventilation system is used for spontaneous 

combustion. The buoyancy effect of the methane helps to 

move the methane updip to the cross-measure methanedrainage holes in the return airway on the updip margin of 

the panel. 

Another ventilation system used for spontaneous combustion control in the presence of methane is the "Y" 

system (fig. 5). This system has been used in Australia on 

longwalls retreating downdip (18). As with the back return ventilation system, the "Y" system is designed to 
wall 

Intake 

KEY 

- - - Cross - measure methanedrainage hole 

. . . . . . . . . . Methane-drainage hole in solid coal 

Figure 17.--Horizontal and cross-measure methane-drainage holes used on "back return" ventilation system (26). 

ventilate the gob in a limited area at the return corner as 

well as the tailgate. The advantage of this system is that 

three intake airways are used, which allows for greater 

volumes of air to reach the face and return comer. The 

back return system with a pack wall, as utilized by the 

British, uses only a single intake airway. Another advantage of the "Y system is that the return airway adjacent to the gob is always accessible, permitting access to 

the methane-drainage system. The disadvantage from a 

methane control standpoint is that as the panel retreats 

and the gob area increases, the volume of air decreases 

because of a longer return airflow path with greater resistance. With greater volumes of gas being diluted in less 

airflow, the methane concentration in the returns increases, placing more reliance on the methane-drainage 

system for methane control. The "Y system is probably 

best suited for mines operating with low to moderate 

methane emission problems. 

Methane drainage is achieved in the "Y system by 

drawing gas from the gob through an open-ended pipeline 

installed through the sealed crosscut at the highest 

elevation in the gob. In addition, cross-measure holes are 

drilled over the gob from the return airway that is maintained for ventilation. These cross-measure holes are then 

connected to a separate pipeline to transport the gas out 

of the mine. There are several notable differences in 

methane-drainage practices relative to the cross-measure 

holes described by McKensey for the "Y system versus the 

back return system utilized by the British (18). With the 

Australian "Y" system the cross-measure holes were drilled 

in advance of the longwall face, and the holes were farther 

apart (120 m versus 9 m). Also, the British utilized drainage holes into the floor beneath the panel. 

In the United States, the primary methane-drainage 

technique employed on longwalls is vertical gob gas holes 

drilled from the surface in advance of the longwall, as 

shown in figure 18 (42-44). This technique is generally 

preferred because of the shallow depth of U.S. mines that 

results in relatively low drilling costs. Also, because of 

subsidence-related problems, longwall mining is generally 

practiced in areas with sparse development on the surface, 

which makes it easier to acquire drilling locations. Drilling 

on the surface is also more desirable than in the more 

restrictive underground environment, where logistics are 
Figure 18.Schematlc section view of complete longwall gob 

gas vent hole system commonly used in United States. 

more cumbersome and more stringent safety regulations 

must be observed. 

The vertical gob gas vent holes function very much like 

the cross-measure holes most commonly used on longwalls 

outside the United States. The major diierence (other 

than drilling location) is that the vertical gob gas vent 

holes are usually drilled near the centerline of the panel, 

whereas the cross-measure holes are driied from entries 

along the panel margin. Also, fewer vertical gob gas vent 

holes (two to four per panel) are generally required on a 

panel, whereas cross-measure holes are spaced as close as 

every 10 m. 

These differences in methane-drainage technologies potentially have some signif~cance relative to their use in 

mines that have spontaneous combustion potential. Humphrey~, Muller, and others note the importance of not 

"overdraining," since this can pull mine air into the gob 

and potentially initiate spontaneous combustion (10, 45). 

seveaadvatages eatve to teuse wtbeedeess 

longwall ventilation systems in the presence of a potential 

for spontaneous combustion. First, since the gob gas vent 

holes are generally drilled near the centerline of the panel, 

they are as far from the mine airways as possible. Second, 

there are generally only two to four vertical gob gas vent 

holes drilled per panel, so there are fewer sites that could 

potentially leak air into the gob because of incomplete 

cementing of the annulus between the hole and the casing. 

Third, there is greater length of cemented casing; therefore, it is less likely that a leak will occur between the 

surface and the gob. If leaks were discovered, there is a 

greater chance that they could be corrected, since access 

to the hole is always possible on the surface, as opposed 

to the underground cross-measure methane-drainage holes. 

Even with the apparent positive attributes of the vertical 

gob gas vent holes, it is still possible to "overdrain" and 

pull mine air into the gob. It is essential that the amount 

of suction applied to the gob gas vent hole be controllable 

so that the infiltration of mine air into the gob is minimized. Also, monitoring the gas compositions in the vent 

holes is necessary to detect overdrainage. 

On longwalls in the United States that use bleeder 

entries, there is generally little, if any, control over the 
suction applied to a gob gas vent hole. The intent of most 

mine operators is to produce the highest volume of gas 

possible with the highest capacity blower available at the 

mine. This policy is relatively harmless early on in the 

panel extraction, since most gob gas vent holes reach their 

highest production volume in the first week or two after 

being intercepted by mining. The methane concentration 

generally increases into the high 90-pct range in this same 

early timeframe. However, if the large blowers are left in 

place for an extended time, the methane concentration 

gradually decreases as mine air is drawn through the gob, 

probably partially because of the active bleeder system and 

the suction applied to the vertical gob gas vent hole itself. 

The methane concentration commonly stabilizes for some 

extended period of time in the 50- to 80-pct range. Commonly when the methane concentration drops below 25 pct 

the blowers aPy =ff 2, 2 s?EPyv jlpfPyaUti9n, Lqd the 

holes are allowed to free flow (50). In many cases, the 

only control over the gob gas production is by moving the 

large blower installed at the beginning of the panel to a 

newer hole and replacing it with one of lower capacity. 

This movement is done to keep the capital outlay low by 

having the fewest number of large blowers possible. 

Implicit with control of the suction on gob gas vent 

holes to prevent of mine air into the gob is 

monitoring of the gas stream. Practices in monitoring gas 

flow and composition are quite variable in the United 

States. In general, only those mines producing gob gas 

commercially, where a high methane concentration (greater than 95 pct) is desirable, have comprehensive monitoring (and suction control) capabilities. A mine operating in 

the Mary Lee and Blue Creek Coalbeds of Alabama has 

been producing gas commercially from vertical gob vent 

holes for many years (43). The monitoring system at this 

mine continuously measures the oxygen concentration with 

an automatic oxygen analyzer at each gob gas vent hole. 

The data are relayed to a central monitoring station at the 

mine office. When the oxygen concentration increases to 

0.3 pct, an alarm is sounded, and field crews are dispatched to check the overall system at the hole. The 

system is adjusted, primarily by decreasing the suction 

applied to the hole. This in turn eventually decreases the 

infitration of mine air into the gob, and the oxygen content of the produced gas. In addition, this mine uses a 

bleeder-type ventilation system that effectively excludes 

oxygen from the old gobs, which aids spontaneous combustion control as well as the commercial production of pipeline quality gob gas (51). 

A gas chromatograph lab at the mine is used to routinely check the oxygen concentration in gas samples from 

the gob gas vent holes to calibrate the oxygen analyzers. 

Gas samples from various locations are also analyzed in 

the on-site lab for carbon monoxide concentrations as part 

of the mine's spontaneous combustion detection program. 

It has recently been reported (52) that a mine operating 

in the Pocahontas No. 3 Coalbed, Virginia, has begun to 

produce gas commercially from gob vent holes. Monitoring and suction control to maintain a high methane concentration in the produced gas are also part of this system. 

During informal discussions with USBM personnel, it was 

reported that in addition to remotely monitoring the oxygen concentration in the produced gas, the suction on individual holes could be controlled by changing the voltage 

to the blower. The flow could also be remotely controlled 

by adjusting valves in the surface production facility to 

increase or decrease the back-pressure on the system. 

Such a monitoring and control system would be required 

on any facility draining gas from gobs using bleederless 

.,Pm .VlltLI(ILIVLI ;I~+:~~ i),i)~ems PIIP~ fsr sp~ntmeous comhstion control. 

In addition to monitoring oxygen and/or methane, carbon 

monoxide should be monitored in the methane-drainage 

system for early warning of spontaneous combustion. 

The USBM has developed a monitoring and data collection and transmission system for vertical gob gas vent 

holes. This system monitors methane concentration in the 

produced gas, along with atmospheric pressure, ambient 

temperature, and gas temperature, to correct the measured 

flow rate to standard temperature and pressure. Data 

from the system can be accessed remotely via phone lines. 

While this system was not designed to control the flow of 

gas from the hole, it was set up to shut down the blower 

if the methane concentration was below 25 pct continuously for 10 min. If the blower shutdown is due to low methane percent (or for any other reason such as a power failure), an alarm is triggered in the mine office. 

One way to minimize mine air intiitration, thereby controlling spontaneous combustion in the gob, is by "pressure 

balancing" the gob with the mine ventilation system (1011, 

21, 53). Pressure balancing is a rather complex procedure 

requiring comprehensive control of the ventilation system 

in response to many factors, including changing differential 

pressures, ventilation changes, gob gas composition, and 

the effect of mining rate on the volume of gob created and 

methane released. One of the biggest problems is the effect of methane drainage on the gob when pressure balancing is being practiced. While methane drainage of 

the gob can help achieve the desired pressure balance, it 

is another variable to be considered. Uncontrolled methane drainage could easily overwhelm a traditional pressure 

balancing system. 

Because of the many variables, the Coal Mining Research Center of Japan has developed an autocontrolled 

gas-drainage system in connection with pressure balancing 

of sealed gobs for spontaneous combustion control (53). 
The methane-drainage technique practiced is open-ended 

pipes installed through seals. Pressure balancing is 

achieved through the control of gas extraction from the 

gob by controlling valves on the gas-drainage pipelines 

underground. The system is multicomponent, consisting of 

monitoring, data storage, judgment, control, and alarm. 

Extensive use is made of minicomputers and expert systems to evaluate and respond to the many variables involved in the control of atmosphere behind the seals. In 

field trials of the system for 2 years, it was able to maintain the desired differential pressure range of 0.5 to 

2.0 mm Hg and a nearly constant methane concentration 

of 98 pct, both advantageous conditions for spontaneous 

combustion control. While this system was designed for 

control of underground methane-dr ainage pipelines, there 

is no reason why it could not be adapted to control surface 

gob gas vent holes and underground monitorkg of sealed 

areas. 

Methane Drainage in Advance of Mining 

Control of methane emission or spontaneous combustion problems individually is reasonably straightforward. 

However, when both problems must be dealt with simultaneously, control becomes more difficult and complicated. 

One approach to address this situation is to remove one of 

the problems in advance of mining. Methane is probably 

the problem most amenable to control prior to mining. 

Methane drainage in advance of mining is especially important in mines having high gas emission where ventilation pressure is being minimized for spontaneous 

combustion control. Liney reports that in Great Britain 

"it is becoming extremely difficult to adhere to previously 

accepted limits on ventilating pressure-for example, 50 Pa 

across the wastew because of "production limiting levels of 

methane" (11). 

Methane drainage in advance of mining is less frequently practiced than drainage during and after mining, both in 

the United States and elsewhere. This is primarily because of the reluctance to commit the capital necessary to 

drain gas from coal reserves several years prior to mining. 

Many companies would rather commit capital when a 

problem has reached the point of interfering with production. In view of the complexity and severity of the combined problems of methane and spontaneous combustion 

control, methane drainage in advance of mining may be 

the key to safe and productive longwall mining under these 

conditions. If a substantial portion of the methane problem is eliminated, then a bleederless entry ventilation 

system may be easier to implement and an atmosphere in 

the gob less conducive to spontaneous combustion easier 

to maintain. 

The basic technology for methane drainage in advance of mining from underground has been discussed 

previously. These techniques consist of drilling methanedrainage holes into the strata identified as gas-bearing 

zones that will contribute methane to the longwall or 

development mining. The most common techniques applied in the United States are the drilling of multiple 

horizontal holes from the tailgate entries into the virgin 

coal adjacent to the panel currently being mined or developed, and the drilling of holes into the panel ready for 

mining (fig. 19). These holes generally require a year or 

more of drainage to provide si&icant reduction in methane content and emission upon mining (44, 54). 

Horizontal holes can successfully drain gas from the 

coalbed to be mined, but they do not address the problem 

of gas in the surrounding strata. It is this gas that in most 

cases accumuiates in the gob and causes the methane 

emission problems underground (36). While the coal 

mines in other countries have used horizontal and crossmeasure-type holes underground to drain methane from 

other gas-bearing horizons, the gas production from such 

systems has generally been limited because of the lack of 

permeability in the surrounding strata. In fact, the standard cross-measure holes were found to generally be effective only when they were drilled into the "destressed" zone 

behind the longwall face. 

The technology most amenable to drainage from multiple horizons in advance of mining is the use of hydraulically stimulated vertical boreholes drilled from the surface 

(fig. 20). These holes have the advantage over underground drainage techniques in that they can be drilled 

many years in advance of mining to drain gas from large 

areas. Multiple gas-bearing horizons can be completed for 

production, and the gas can easily be captured for sale to 

help defray the cost of the methane-drainage system. An 

important advantage of these vertical wells is that individual gas-bearing zones can be stimulated to enhance their 

permeability and increase gas production. The technology 

is successfully being used in the mining district of the 

Black Warrior Basin of Alabama (42, 55-56), the Central 

Appalachian Basin of Virginia, the San Juan Basin of 

Colorado, and in New Mexico in the West (57). This 

technology is quickly gaining popularity in many U.S. coal 

basins, including some mining areas, but primarily for 

commercial production of the coalbed methane (56). 

There is one cautionary note relative to the use of stimulated vertical wells. Existing planes of weakness, prirnarily the coal cleat, but also bedding planes and rock joints, 

are enhanced and propped with sand as a result of the 

stimulation treatments (58). As discussed previously, with 

woveraggressivew methane drainage, mine air could be 

drawn into a spontaneous-combustion-prone area through 
Advance 

drilling with 

development 

Main entries -/ 

Figure 19.--Schematic plan view of short horizontal holes used for draining gas from longwall panels. A, On 

active panel in advance of face; B, on developing panel adjacent to active panel; C, from advancing entries. 

naturally occurring or mining-induced fractures and joints 

in pillars and roof and floor strata. 

There is some possibility (although no published literature suggests that this has ever occurred) that mine air 

leakage could also occur through the enhanced permeability of the propped fractures of stimulated vertical wells. 

The most likely problem area would be a propped fracture 

that extended from the gob into a mine airway, either 

through a pillar or through the surrounding roof or floor 

strata, in association with a bleeder less ventilation system. 

Provisions would have to be made to seal such exposed 

fractures that are encountered with an impermeable 

coating. 

Mine air could also into the solid coal if wells 

were left on production after mining had occurred close to 

their location. This potential problem could be dealt with 

by plugging the hole with cement prior to mining if it was 

in a location such as a pillar or rib, where it would provide 

no further benefit. If the hole was located in the panel 

where the intent was to use it for gob gas venting, then 

production could be suspended, or the exposed strata infused with water to block gas flow during any critical time 

identified by experience as hazardous for mine air leakage. 

One final concern relative to methane drainage in a 

spontaneous-combustion-prone coalbed was raised by 

Humphreys (10). He states that "coal which has been 

drained of gas is susceptible to oxidation" and "the reduction of the natural moisture content accompanying gas 

drainage increases the coal's susceptibility to oxidation." 

The concern appears to be generally related to "overaggressive" methane drainage that may pull mine air through 

solid coal that has previously been drained of gas. While 

no examples of spontaneous heatings of solid coal resulting 

from the effects of methane drainage were given, the 

potential is probably plausible enough to warrant vigdance. 
Water 

sand 

A Coo l bed A 

Figure 2O.Schematic view of open hole (A) and cased hole 

(B) completions for vertical methane-drainage well in advance of 

mining. 

METHANE-DRAINAGE CONSIDERATIONS FOR USE 

OF BLEEDERLESS VENTILATION SYSTEMS 

IN THE UNITED STATES 

Controlling methane emissions in longwall mines prone 

to spontaneous combustion is a complicated problem. In 

other countries where considerable experience with this 

problem has been gained, excluding or minimizing the 

infitration of air into the gob by some type of bleederless 

entry ventilation system is generally the preferred method 

of spontaneous combustion control. Since bleederless 

entry ventilation systems can result in high methane emissions on the face, methane drainage of the gob directly 

through cross-measure holes or by open-ended pipes extending into the gob through sealed entries is commonly practiced. Every effort is made to ensure that the 

methane-drainage system is not "overaggressive," thereby 

pulling mine air into a spontaneous-combustion-prone 

zone. The methane-drainage holes themselves are drilled 

and completed so that they are not a conduit for mine 

airflow. Additionally, the seals, pillars, and ribs in common with the gob and the mine ventilation system must be 

of sufficient integrity to resist an exchange of atmospheres 

through them. Finally, the underground atmosphere is 

fine tuned by pressure balancing, and comprehensive monitoring of the methane-drainage system and the atmosphere behind sealed gob areas is required to detect and 

monitor the early stages of oxidation. 

If bleederless entry ventilation systems are used in the 

United States for spontaneous combustion control it is 

most likely that methane-drainage systems similar to those 

already being practiced will be the technology of choice. 

This would most probably be vertical gob gas vent holes. 

. . r"iob:eiiis with cross-measire drakage nay dsz m:se tF 

access is required to any active drainage holes. To be 

effective, cross-measure holes should be on the return side 

of the panel. Most bleederless entry ventilation systems 

actually allow mine air to ventilate the gob directly behind 

the face on the return corner to control methane emissions 

in that area. Maintaining accessible and ventilated entries 

on the return side of a panel that is commonly adjacent to 

the previous panel's gob is difficult because of abutment 

loading. In fact, to do so may require substantial additional roof support and ventilation capacity. Even if an 

entry on the return side was maintained, and not progressively sealed as with the back return bleederless entry 

system practiced outside the United States, it would in 

effect become a bleeder entry. 

Methane drainage in the presence of potential spontaneous combustion will require greater preplanning, and 

monitoring and control after implementation, than is 

generally practiced in the United States. Instead of 

waiting until a methane problem is encountered, the probable sources of gob gas should be determined early in the 

design stages of a new mine (or a mine contemplating 

switching to a bleederless entry ventilation system) so that 

effective methane-drainage systems can be incorporated 

into the overall mine design. To successfully utilize a 

bleederless entry ventilation system to control spontaneous 

combustion in high-gas-emission longwall mines, some 

form of methane drainage in advance of mining will probably have to be practiced. By reducing the volume of 

methane contained in the coalbed being mined as well as 

that in the surrounding strata, control of the underground 

atmosphere should be considerably less ciacult. Once 

methane drainage and mining are underway, monitoring 

and control of the underground environment is essential to 

ensure a safe and productive mining operation. 
GROUND CONTROL ASPECTS OF BLEEDERLESS DESIGN 

FOR SPONTANEOUS COMBUSTION CONTROL 

INITIAL PLANNING AND EVALUATION 

Once a bleederless ventilation system has been chosen 

as the primary means to control spontaneous combustion, 

a comprehensive geotechnical evaluation of the mine property needs to be made. In addition to the geotechnical 

data directly related to spontaneous combustion, such as 

coal rank, self-heating susceptibility, seam pitch (23), 

and the proximity and susceptibility of accompanying rider 

seams (9, Id), the evaluation should also include the vital 

information that will serve as the basis for an integrated 

ground control plan. The ability to control the ground, 

especially to limit leakage through seals, strata, and pillars, will govern or at the very least impact the success 

and safety of bleederless ventilation as a spontaneous 

combustion control measure. 

The information assembled needs to be as complete as 

possible so that a ground control plan that has a high 

chance for success may be devised. Collecting the required data is easier if the property is in operation. In 

this case, much of the information may already be available and only need to be compiled and molded into a 

comprehensive control plan. However, if the operation 

is a greenfield site, especially if there are few or no 

neighboring operations in the same or similar conditions, 

the gathering of the information will involve greater effort 

and expense. Also, the inability to observe the ground 

control plan in practice will require some assumptions 

that may need to be modified once actual conditions are 

encountered and can be documented. 

The geotechnical evaluation should include information 

on a number of major areas that need to be assessed for 

their impact on ground control's role in a bleederless 

design for spontaneous combustion control and resultant 

issues such as leakage; stress concentrations; coal friability 

(9); and entry, pillar, rib, and floor stability. Some of the 

areas that should be addressed are as follows: 

1. Floor, roof, and coal stratigraphy, strength of units, 

and general uniformity. The roof should be classified by 

an accepted system. 

2. The direction and magnitude of the principal 

stresses of the horizontal stress field. 

3. Depth of cover over the property and its impact on 

ambient temperature (19, stresses, pillar behavior, entry 

stability, pillar fracturing (23), rib sloughage, and potential 

surface leakage (if depth is shallow). 

4. Discontinuities of the roof, floor, and/or seam, such 

as bedding, joints, cleats, faults (a particular problem that 

may need to be avoided (9, 23)), clay veins, and other 

unfavorable geologic conditions. 

5. Caving characteristics of the immediate and main 

roof: impact on methane liberation, profile in the gob, 

and variance of gob line direction with cleat and the horizontal stress field. (The British feel that cleat direction is 

important, but recent research suggests that the observed 

relationship may have been more a result of the direction 

of the principal horizontal stress field than cleat direction.) 

Also important are the length of gob line, gob consolidation characteristics, and the impact on room-and-pillar 

mining method (interplay of pillar design). 

6. Multiple coalbed effects-which should be minimized 

through the overall design. These include mining codbeds 

systematically in a downward direction (16), considering 

impact of gob and barrier areas in other coalbeds, superimposing pillars and gate roads, and considering the direction and angle of approach to upper coalbed gobs and 

barriers. 

7. Quantification of the ground movements of selected 

pillar, entry, and support designs in anticipated and/or 

actual geological and stress environments. This information would be used to design these mine structures as well 

as to match the critical seal design(s) to the ground control plan. 

While many of the above are required or prudent ingredients for a mine plan for any underground operation, 

many of these areas assume roles of greater importance 

when spontaneous combustion and bleederless design as a 

control measure are considered. For instance, other coalbeds above or below the mined coalbed that would be exposed by caving or bottom heave could contribute to or be 

the source and fuel for spontaneous combustion events. 

Likewise, short- and possibly long-term rib, entry, pillar, 

and floor stability during development and retreat mining 

will be very critical if the spontaneous combustion control 

plan includes limiting the amount of coal as a fuel or minimizing leakage paths to a gob. 

Once all required geotechnical information has been 

accumulated, the comprehensive ground control plan can 

be developed. If the operation is existing, past ground 

control experience at that mine will be the cornerstone of 

the plan. Experience from neighboring mines or mines 

with similar conditions would also be helpful inputs, especially if the operation is a greenfield site. Because of the 

impact that the ground control plan has on a major safety 

issue of the bleederless ventilation system (i.e., assisting to 

maintain the integrity of the sealed gobs by minimizing or 
eliminating leakage paths through floor or roof strata, 

through pillars, or through seals stressed by ground movements), accepted ground control techniques and designs 

should be utilized in the plan. 

The ground control plan should revolve around a focused design philosophy as to how the ground will be 

controlled in and around the bleederless gobs. There are 

two basic choices: to design a stiff, stable system that will 

carry anticipated loads with little or no ground movement, 

or to employ a yielding design that permits a controlled 

amount of ground movement. There are also hybrids of 

these two extremes. The choice made will then define the 

roles of gate roads and barrier and chain pillars (in the 

case of room-and-pillar mining). Within the ground control plan, pillar and opening dimensions should be derived 

using accepted design methods and should complement the 

overall concept, whether yielding or stable. 

Likewise, the artificial support system(s) used should 

also complement the overall ground control goals. In addition to the design methods used to accomplish the basic 

philosophy, the ground control plan should also contain 

sufficient information detailing how known or anticipated 

special problem areas will be addressed. For example, 

bottom heave or rib control measures would be detailed if 

pillar sloughage or floor leakage paths were viewed as presenting abnormal safety risks. 

GENERAL GROUND CONTROL CONSIDERATIONS 

As noted earlier, ground control considerations may 

change remarkably when designing for spontaneous combustion control through implementation of a bleederless 

ventilation plan. The following is a discussion of some of 

the major issues that should be considered in the ground 

control plan. 

Number of Openings 

A general goal of the plan should be to limit the number of openings into and around the gob areas that will be 

isolated and the number of openings between intake and 

return airways where associated leakage paths may present 

a potential problem. Obviously, the fewer the number of 

openings, the less the number of leakage paths, the less 

the chances for incidents of abnormally high leakage, and 

the lower the sum of the total volume of leakage. Limiting the number of openings is also beneficial when sealing 

areas or panels according to the mine plan or if necessitated during a heating. Using fewer openings reduces 

the number of seals required and speeds the sealing 

process in emergencies. To limit the number of openings, 

extensive use of barrier pillars and long pillars should be 

made in the mine design (59). 

Coal Barriers 

Coal barriers can be left between main intake and main 

return entries, with only a few interconnections between 

the two. Usually, this requires that these entries be driven 

as separate sets of entries and then converted to an intake 

use or return use after their development is complete. 

Coal barriers should also be extensively used to separate 

longwall or room-and-pillar panels from main and submain 

entries. Openings through these barriers should be minimal and only made for emergency or other practical 

measures. 

Coal barriers can also be used to separate longwall or 

room-and-pillar panels into districts of a limited number. 

Reasons why this may be done include the following: to 

maintain districts that meet methane control capabilities; 

for groiind control considerations, siich as to control 

periodic large abutments; or as insurance to limit the area 

that would or could be sealed in the event of a heating 

(11, 35). In all cases coal barriers should be large enough 

to carry the expected loading and provide long-term stability (10). Where used to isolate bleederless gobs, barriers 

should also be explosion proof. 

Long Pillars 

Where barrier pillars are impractical to separate intakes 

and returns such as gate roads, panels, or some mainsubmain drivages, long pillars can be used to at least 

minimize the number of openings and seals. To develop 

long pillars to separate intakes and returns in these 

drivages, the mining and ventilation plans need to be 

integrated to create the longest distance prior to holing 

through without outstripping the mining or ventilation 

system capability. 

Number of Entries 

A corollary to limiting t3e number of openings is to 

limit the number of entries. With fewer entries there are 

fewer openings to leak when sealing entire panels. Also, 

when difficult ground conditions are encountered, a limited 

number of entries may permit the use of different ground 

control techniques than are presently possible or practical. 

As U.S. mines increase in depth, corresponding increases 

in stress will likely lead to the same ground control 

problems currently experienced in the rest of the world. 

Most older coal mining countries limit the amount of 

drivage, commonly to one entry on each side of a panel. 

However, current regulations, especially those regarding 

escape, prohibit the use of these ground control designs in 

the United States. 
Floor Heave 

While a minimal amount of floor heave may not have 

a great impact on the operation of some mines, it can 

present a major problem in coal mines susceptible to 

spontaneous combustion. Floor heave can provide leakage 

pathways through the floor or damage seal integrity, 

allowing oxygen to contaminate gob areas. It can also 

expose reactive coalbeds in the floor to the atmosphere (7, 

60). 

Where these conditions exist, the control of floor heave 

may be a major consideration and, therefore, dictate the 

general approach of the overall ground control plan. This 

is especially true if mine structure control techniques are 

used to control floor heave. Mine structure control techniques are those where the parameters of the major mine 

LIL. ctrllrtur~c UCI. IVU, UUIY cllrh 2s p__n_try npa~ing geometry and pillar 

geometry, are varied to accomplish specific ground control 

objectives. One example of this would be using a larger 

size pillar to provide more bearing surface area if "pillar 

punching" into a weak floor was the source of the floor 

heave (61). Another would be using small, yield pillars to 

decrease the level of stresses in the immediate floor when 

high stresses and a weak floor were the cause of the heave 

(62-63). Narrowing entry width may also help to control 

floor heave (64). 

Other floor heave control techniques that may be employed in the ground control and mine plan are minimizing 

water on the floor by using the minimal amount required 

for dust suppression, and a well-maintained sump and 

pumping system for ground water inflows; considerations 

made for the orientations of the mine workings to the 

direction of the principal horizontal stress; rib softening 

techniques; and supplemental support techniques such as 

floor bolting, cementitious lining, and polyurethane injection (#). The results of the geotechnical evaluation 

would provide insights into the source(s) of the floor heave 

and may suggest appropriate control techniques. Some of 

the control techniques listed may be rejected because of 

their inability to be practically incorporated into the mine 

plan. 

Rib Sloughage 

Rib sloughage, existing or anticipated, needs to be 

assessed as to whether it should be a consideration in the 

ground control plan. Excessive rib sloughage can detrimentally impact a coalbed susceptible to spontaneous combustion by providing ventilation leakage paths through 

pillars or by providing a ready fuel that increases heating 

potential. Since rib sloughage is a result of perimeter 

yielding of the coal pillar, one control method is to vary 

the geometry of the mine structures as discussed in the 

section "Floor Heave." The goal would be to lessen the 

magnitude or location of stress loading on the pillar. One 

problem with this approach is that there is a practical limit 

in a given physical environment to the extent that pillar 

perimeter yielding can be minimized. Another problem 

with this approach is that this technique may require the 

exact opposite parameters to other more important aspects 

of a ground control plan aimed at facilitating bleederless 

ventilation for the control of spontaneous combustion. 

For instance, control of floor heave, which may be more 

important, may require the use of thin yield pillars. 

Another rib sloughage control approach is to orient 

pillar edges as near as possible to 45" to the cleat (12). 

Other methods to reinforce the rib include the use of rib 

bolting or banding, meshing, posts, and other passive 

restraints, such as cementitious linings. As with the floor 

heave control measures, the applicability, practicality, and 

contribution to minimizing the sloughage hazards need to 

be assessed for any of these techniques to be a component 

of the plan. 

Entry Stability 

As with any mining, entry stability will be at the core 

of the ground control plan. In a bleederless design, and 

especially for a mine with high methane emissions, the 

importance of entry stability is even greater. This is due 

to its impact on leakage, stability and integrity of seals, 

methane drainage, and monitoring. 

Entry stability is dependent upon the physical nature 

and the interactions and interrelationships of the surrounding mine structures--the roof, floor, and coal pillarrelative to the imposed stress field. The magnitude and 

pattern of the stresses, the absolute and relative strength 

and the geometry of the mine structures, and the installed 

support are the elements that will dictate entry stability 

(61). Those con trollable parameters will be varied within 

practical limits in the ground control plan to create serviceable entries for their required life span. As a result, 

some of the mine structures may behave undesirably because of the higher priority given to entry stability, because 

of their own physical inadequacies, or because of practical 

mining considerations such as entry width. 

The resultant stability of the entry will then impact the 

particular bleederless design concerns. Leakage paths and 

the resultant leakage through the mine structures will be 

a function of entry stability. Entry stability will also dictate 

the loading and stresses on seals. If cross-measure holes 

or another type of in-seam methane drainage is used, 

those involved entries must remain accessible for a time 

period required by the methane-drainage scheme (25). 
Similarly, the entries must provide and maintain access to 

key points of the bleederless system to meet the maintenance requirements of methane, carbon monoxide, or 

any other monitoring system(s). This access may be required for people, instruments, conduits, or all three. 

Because of the time interval required by these functions 

and their physical locations within the system, entry 

stability will need to be maintained in areas subjected to 

gob abutment loading. 

Seal Design 

A special concern of the ground control plan is the 

interrelationship of the seals. Since the intention of the 

bleederless design is to limit airflow through the gob as a 

spontaneous combustion control, the success and safety of 

this control technique is measured by how weii this is 

accomplished (18, 60). Control of the airflow through the 

gob becomes even more critical in gassy mines, where an 

accumulating methane body must also be flawlessly controlled. Consequently, the hermetic sealing of the gob is 

of paramount importance, and seals must perform with a 

high degree of reliability (65). The loading and ground 

movements in and around the seal locations must be anticipated to select seal strength, flexibility, size, material, and 

need for sealants (65). The seal performance and design, 

therefore, must be a major consideration of the ground 

control plan (10). 

THE GROUND CONTROL PLAN 

Once the geotechnical information has been gathered 

and evaluated, and the general considerations for ground 

control contemplated and assessed, the ground control 

plan can be devised. This plan, of course, would need to 

complement the general mine plan and the plan to control 

spontaneous combustion through bleederless mine design. 

In many cases, it will not be possible to maximize all of 

the desirable aspects of bleederless design in the ground 

control plan. Some features may even conflict. In such 

instances, decisions need to be made as to the ground 

control approach that best addresses the most salient 

requirements of the overall plan. The highest priority 

requirements will be those tied most directly to the most 

likely and major hazards of spontaneous combustion and 

the bleederless control approach. 

Of these ground control decisions, probably the major 

decision is whether to use a stiff design or yielding design 

in and around the gob areas. Experience around the 

world acknowledges that small ground movement is much 

preferred (11, 18, 65). Rigid ground eliminates many of 

the major concerns of bleederless design and spontaneous 

combustion control. Rib sloughage, floor heave, leakage 

paths through strata and pillars, lack of seal stability and 

integrity, inaccessibility of methane-drainage and various 

monitoring systems, exposure of roof and floor coalbeds, 

and coal remnants in the gob are eliminated or minimized 

if there is little ground movement (10). Therefore, rigid 

ground should be the first goal of the ground control plan 

and every effort should be made to investigate methods to 

utilize a stiff, stable design. This includes exploring various mine structure geometries, support types or methods, 

and mining methods. 

Where it is not possible to design or maintain stiff m-ine 

structures because current control techniques are either 

ineffective or impractical, an approach that permits one or 

more of the mine structures to yield is the alternative. 

However, whether the focus of the ground control plan is 

stiff or yielding, aii eiements of the ground control phi 

should be directed to accomplishing predictable ,ground 

movements. Those elements not totally controlled by the 

overall design approach that still represent substantial 

concerns or hazards may be addressed by specifics of the 

plan. As an example, if an unacceptable amount of rib 

sloughage exists, rib bolting could be incorporated into the 

plan. There is, of course, a wide variety of primary and 

secondary support schemes and methods that can be used 

to add stability to the ground. These techniques are generally known to the industry or can be easily researched if 

not known. 

Stiff Design 

In a stiff design the goal is little or no movement of the 

roof, pillars, or floor. Pillars should be large enough to 

carry the original development loads and the later abutment loads due to pillar mining. As a result, there would 

be minimal pillar crushing, rib spalling, convergence of the 

ground on the seals, and floor heave. The primary and 

secondary support should also be stiff to complement and 

assist the overall design. Additional support should be 

added in the seal construction area if needed to keep the 

mine structures stable with little movement (18). 

The seals used in this design should also be stiff and 

rigid, perhaps even providing some of the support to the 

immediate area. They should be able to withstand the 

rather small ground movements and still preserve their 

hermetic seal. However, even with a stiff, stable design, 

the mere proximity of seals to the gob area will expose 

them to ground movements. If these movements are of 

such a magnitude that a rigid seal will fail, some flexibility 

can be built into the design through elements that readily 

crush yet remain airtight. However, ground movements 

may be so extreme that, even in the case of a conceptually 
stiff ground control design, the seal may need to be 

flexible to allow convergence of the roadway (18). This 

flexible philosophy is discussed more fully under "Yielding 

Design." 

The length of time that the seal must maintain its 

integrity would also affect its design. Therefore, seals to 

isolate a panel, mined-out areas, or a mains or gob cut 

through, which are usually more long term in nature, may 

be required to be substantially constructed. Seals used to 

separate a longwall panel gob from an entry used initially 

for methane drainage and monitoring on a current panel, 

and then as a return on the next longwall panel, would 

only need to successfully function until the successive panel 

is mined and may not need to be as substantial. However, 

if these seals are subjected to large abutment loading and 

severe ground movements as a result, the opposite case 

couici exist. If the opporiuniiy exists, the seds slior;!d 52 

tested in the same or similar conditions prior to their 

being employed in a bleederless plan. 

Yielding Design 

A yielding design usually employs small pillars that are 

unable to carry the total imposed load and begin to fail 

and deform. As a result, they then pass some of their load 

onto surrounding structures such as longwall panels, barriers, or larger pillars. Ideally, in a bleederless design, 

these pillars are small enough to yield, thereby relaxing the 

immediate roof and floor and providing the desired stress 

relief, but not so small as to become excessively crushed 

and create leakage paths through the pillar and/or excessive rib spalling. The goal of the yielding approach is 

also stability. However, unlike the stiff concept, yielding 

permits deformation of the mine structures to obtain this 

goal. 

Because of the inherent benefits of using a stiff design 

already noted, the use of a yielding design should be limited to those areas where a stiff design cannot be practically maintained. Generally, the most severe problems 

with stiff design will occur upon retreat abutment loading. 

In these cases, if a yielding concept is required, it should 

be limited to those areas subjected to this type of loading, 

such as longwall gate roads. However, high vertical loads, 

high horizontal stresses, physical inadequacies of the mine 

structures, or combinations of these could render stiff designs incapable or impractical on a broad scale (66). This 

would require a more widespread use of yielding design 

throughout the mine (60). Still, because of the benefits, a 

stiff design should be implemented to the greatest extent 

practical where it impacts the bleederless system. 

As with stiff, stable design, it is extremely critical that 

the seal design be harmonious with the ground control 

design. The yielding ground control design should be 

aimed at a consistent, predictable, and fairly uniform 

ground convergence on the seal that in turn could be 

incorporated into the seal design (26). Most current seal 

designs of this type allow for and generally require 

consistent and uniform ground convergence for their own 

design to be an effective deterrent to leakage. For 

instance, a U.S. coal mine that uses bleederless design in 

deforming ground conditions has used seals made from 

3-ft wooden blocks stacked and filled with rock dust or 

sand. The ground convergence compresses the wood and 

that, combined with the rock dust or sand filler, creates 

the seal (6, 9). 

An Australian mine, also in deforming ground conditions, reports successful performance with a flexible seal 

design built around anchored steel straps to which expanded steel mesh is attached. A fi;,ber&iss rekfcrced cp.- 

ment grout is then sprayed onto the mesh. The cured 

grout and the surrounding 1-m perimeter of mine entry 

are then oversprayed with a 3-in covering of polyurethane 

foam. These flexible seals have bulged out up to 500 mm 

without losing their effectiveness (18). However, large 

ground movements, such as the pillar deformation and 

yielding, may create leakage paths around these types 

of flexible seals. Therefore, more extensive sealing and 

grouting techniques may be necessary for acceptable seal 

performance. 

However, the meshing of an acceptable seal design with 

deforming ground is probably the biggest obstacle for 

yielding approaches for bleederless design. Because of the 

limited experience in the United States with seals requiring 

these design parameters, this topic should be investigated 

further for bleederless applications. If possible, like the 

stiff seals, the yielding seal design(s) should be constructed 

and tested in ground conditions similar to those anticipated. This will permit seal integrity to be checked and 

design improvements, if needed, to be made prior to reliance on their performance. 

Combination Design 

In some cases, the best ground control plan may be to 

combine the two design approaches, for example, by using 

yield-abutment and yield-abutment-yield schemes. Experience in Great Britain with the small pillar used to create 

the back return ventilation system has indicated that a 

yield pillar next to the gob may be beneficial if crossmeasure methane drainage is practiced (13, 29). When 

yield-abutment and yield-abutment-yield designs are used, 

seal design problems should be eased if the seals are 

located between the more stable abutment pillars (10, 23). 
MONITORING 

The revised Federal regulations require that, in mines 

with a demonstrated history of spontaneous combustion, 

the ventilation plan provide a description of the measures 

that will be used to detect methane, carbon monoxide, and 

oxygen concentrations in worked-out areas (2). This is in 

addition to those requirements for air quality and quantity 

in work or travel areas. The use of a bleederless system 

places extra demands on the monitoring plan, especially in 

terms of spontaneous combustion detection, maintaining 

large-scale experiment using 11,000 kg of coal. Although 

all the ratios indicated a heating after thermal runaway 

was underway, only the C02/A02 ratio gave an early warning of the heating in the coalbed (71). 

Continuous monitoring of carbon monoxide levels is the 

preferred method of spontaneous combustion detection 

worldwide. The most popular methods are telemetry or 

tube bundle systems (10, 12, 65). With the telemetry system, electrochemical sensors located at key sites in the 

safe air quality at the face, pressure balancing, methane- mine are wired to a computer on the surface for analysis, 

drainage monitoring, and seal integrity. A thorough un- alarming, and recordkeeping (10,23 65). The tube bundle 

derstanding of the methods currently used in these areas, system collects samples underground and delivers them to 

and of the additional monitoring requirements imposed by a central analyzer, usually aboveground. The main advanthe use of a bleederless system with respect to methane tage of the telemetry system is that the information is 

and dust, 1s required. gathered instantaneousiy. ow ever, these systems are 

costly, must be relatively durable, require calibration, 

SPONTANEOUS COMBUSTION DETECTION and are dependent on the mine's power system. The main 

Early detection is critical for the prevention and control 

of spontaneous combustion. The analysis of gaseous products of combustion is the primary method used in underground coal mines for early fire detection. The use of gas 

analysis and various ratios of these gases in the detection 

of gob self-heatings was recognized and evaluated early in 

this century. However, only in cases of advanced heatings 

were gas analysis data able to identify heatings (67). Considering the state of gas analyses at the time, this is 

understandable. More recently, Chamberlain (68) showed 

that carbon monoxide is the most sensitive indicator of the 

early stages of self-heating and recommended that continuous monitoring of this gas would provide the earliest 

detection of spontaneous combustion. Figure 21 shows the 

evolution of carbon monoxide and some other gases as a 

function of temperature. 

Other gases have been investigated as indicators of 

spontaneous combustion, such as carbon dioxide, methane, 

hydrogen, and higher hydrocarbons. However, carbon 

dioxide and methane are usually present from other 

sources, and hydrogen and higher hydrocarbons are usually 

not produced until much higher temperatures are reached, 

making the use of these gases unreliable. 

Various ratios have been investigated, including 

CO/A02, CO/C02, and C02/A02. Thorough discussions 

of the use and advantages and disadvantages of the 

CO/A02 ratio are given by Humphreys and Greuer, 

among others (10, 69). Recently, the USBM developed 

the R ratio to evaluate the atmosphere of sealed mines for 

safe reentry (70). This ratio compares the carbon monoxide concentration with respect to the residual gas concentration (all gases except ambient air, methane, and 

ethane). These ratios were evaluated by the USBM in a 

0 

50 

Figure 21 .--Evolution of detector gas in laboratory tests (68). 

100 150 200 250 

I 

COAL TEMPERATURE, "C 

I I 

- I - KEY 

Carbon I -- monoxide 

- I ..- Hydrogen - 

Ethylene 

----- - I Propy lene 

- 

- I 

I - 

- I 

I 

- 1 

/ 

>/ -.y 
advantages of the tube bundle system are the lower cost, 

lack of dependence on the mine power system, and relative 

ease of calibration. The main disadvantage is the time 

delay in sampling. Humphreys and Bacharach provide 

thorough discussions of these systems (10, 12). 

A sigmficant problem in the detection of spontaneous 

combustion is in recognizing when a heating is developing. 

U.S. regulations require mines that utilize a bleederlesstype ventilation system to monitor for the detection of selfheating. The monitoring requirements will probably be 

based on carbon monoxide measurements and require action plans or alert levels for self-heatings. The monitoring 

plans should be proposed and reviewed on a mine-by-mine 

basis. 

The ambient levels of carbon monoxide in a mine depend on the type of coal, the rate of mining, and many 

ether factnrc- Trmsi~nt spikes in carbon monoxide can be 

caused by diesel machinery and changes in ventilation, as 

well as other factors. A new method that has been used 

by some mines with spontaneous combustion problems is 

that of trending. Data are accumulated over a period of 

time to establish normal ambient carbon monoxide background levels. Averaging methods are employed to compare the continuous monitoring analyses to the ambient 

background levels. A Wyoming mine that relies heavily on 

the use of diesel equipment has developed a method to 

deal with transient spikes called wave trend crossing. This 

method uses two carbon monoxide sensors sampling the 

same gas stream at different locations, along with computer analysis, to constantly update the ambient carbon 

monoxide background level (72). 

METHANE DRAINAGE 

Monitoring of methane-drainage systems in the presence of a spontaneous combustion potential has been 

touched on previously, primarily relative to automated 

underground methane-drainage pipeline control and pressure balancing systems described by Deguchi (53). This 

system monitored methane and carbon monoxide concentrations in the various methane-drainage pipelines, the 

flow rate, and the pressure on both sides of seals in real 

time. 

Computer-based expert systems were used to evaluate 

the many variables from multiple data collection sites and 

to adjust automated valves to control the amount of suction applied to the methane-drainage pipelines. With the 

system in operation, the atmosphere in the gob behind the 

seals of a bleederless ventilation system was maintained at 

optimum conditions (low pressure differential and high 

methane concentration) for spontaneous combustion 

control. 

McKensey (18) describes an automated control system 

that operates in conjunction with the surface production 

facilities connected to the various underground pipelines. 

This system is fully automatic and controlled by a Programmed Logic Control (PLC) unit and automatic sensors. 

A predetermined operating pressure is set (0 to -50 kPa) 

and then the PLC controls "the total plant outflow by 

either controlling the number of exhausters operating or 

altering the quantity of gas being recirculated through the 

plant." Monitoring includes pressure, methane concentration, gas flow, and temperature at the inlet of the underground pipeline and methane concentration, temperature, 

and pressure at the outlet on the surface. In addition to 

the monitoring of the methane pipelines, a "tube bundle 

system" is used to monitor gas concentrations in return 

airways and inside gob seals. This monitoring system is 

interfaced with the PLC unit to "assist in the control of the 

mine environment both inside and outside sealed goaf 

areas." For spontaneous combustion control and avoidance of explosive mixtures, total plant outflow is automatically adjusted whenever low methane concentrations 

(less than 40 pct) are detected at the inlet monitor. If the 

methane concentration falls below 30 pct, the plant is 

"isolated" from the methane-drainage system. The PLC 

system in conjunction with gas monitoring via the tube 

bundles is also used to detect and monitor spontaneous 

combustion incidents and balance the pressures around 

sealed gob areas for additional control. 

Gas compositional monitoring of individual methanedrainage holes or at least monitoring of several locations 

along pipelines to which drainage holes are connected is 

recommended by many authors (11, 14 45). Monitoring 

can be beneficial in detecting increasing levels of oxygen 

that may indicate "overaggressive" methane drainage that 

is pulling mine air into a potential spontaneous combustion 

zone. Swift provides examples of case studies that document such occurrences (26). 

Monitoring of carbon monoxide levels (or the CO/O, 

deficiency ratio) (11), or carbon monoxide, oxygen, hydrogen, and methane for calculation of the fire index (4.9, 

combined with sufficient background "norms" can provide 

early warning of a developing problem. In fact, Highton states that gas compositional monitoring of methanedrainage pipelines give "the fist indication of an increase 

in oxidation" (13). With a sufficient number of monitoring 

sites, it may be possible to locate the area of infiltration or 

oxidation. The more specific the identification of the 

problem area or methane-drainage holes causing the problem, the more quickly and efficiently remedial actions can 

be initiated. 
SEALS 

Another important place for monitoring for the detection of spontaneous combustion in a bleederless ventilation 

system, in addition to monitoring returns and working 

areas, is behind seals. Continuous monitoring behind each 

seal constructed in the headgate entry is impractical, 

because of the large number of seals. In the two U.S. 

mines that are currently using a bleederless ventilation 

system, gas sampling tubes are installed in each seal so 

that periodic gas samples can be withdrawn and analyzed 

by gas chromatography. Samples are analyzed for oxygen, 

carbon dioxide, carbon monoxide, methane, hydrogen, and 

some higher hydrocarbons. A recent paper by Koenning 

describes the use of these samples at one of the mines to 

detect two spontaneous heatings behind seals (23). Information o1.1, the techAquec zn_d ga zm!yses at the other 

mine employing a bleederless system is, at this time, 

unpublished. 

As bleederless ventilation systems become more accepted and utilized by mines, the importance of detection 

of spontaneous combustion will become greater. This is 

particularly true in methods to monitor behind seals to 

SELECTION CRITERIA FOR BLEEDER 

An important issue that will need to be considered is 

determining when the spontaneous combustion hazard 

potential in a mining operation is severe enough to 

warrant the use of a bleederless ventilation system as a 

control measure. This section will review the use of risk 

analyses available to assess the spontaneous combustion 

potential of coal. 

USBM research on the prediction of the spontaneous 

combustion potential of coal dates back to 1928. Davis 

concluded that two available methods, the rate at which 

coal absorbs oxygen and the rate of heating of coal under 

adiabatic conditions, were reliable for distinguishing relative self-heating tendencies with respect to coal. He also 

concluded that all coals, with the exception of anthracite, 

can self-heat under favorable conditions and that, in general, lower rank coals have higher self-heating susceptibilities (73). These findings have not changed over the 

years, but the methods have improved, and as recently as 

1987 and 1991, Smith and Miron published reports on the 

use of an adiabatic oven and oxygen adsorption apparatus 

to evaluate the relative self-heating potential of coal 

(74- 75). 

In 1972, Nandy published a paper describing the use of 

the crossing point method to classify Indian coals with 

respect to their relative self-heating potential. In this test, 

check seal integrity and to minimize both the spontaneous 

heating potential and the explosion potential. Research 

is strongly needed in the areas of improved and novel 

detection methods and guidelines. 

ADDITIONAL CONSIDERATIONS 

The use of a bleederless ventilation system will require 

additional monitoring in the face and travel areas, especially in gassy mines. Current U.S. regulations stipulate 

that measures to detect oxygen, methane, and carbon 

monoxide must be provided in the ventilation plan (2). 

Because the gob is sealed, combustion gases and methane 

can flow from the gob into the work areas and returns 

because of massive caving, barometric changes due to 

weather, and unexpected methane emissions leading to an 

ove.rl_oa&ng of the methane-drainage system, This places 

an increased reliance on the monitoring systems. 

It is anticipated that additional monitoring along the 

face for methane and carbon monoxide emissions, and 

around the seals for pressure balancing of the gob, will be 

required. Research is necessary to develop more reliable 

criteria and guidelines for these areas. 

-BLEEDERLESS VENTILATION SYSTEM 

the temperature of a coal sample is monitored with respect to a constantly increasing oven temperature, and the 

temperature at which the sample temperature overtakes 

the oven temperature is measured. The lower the crossing 

point temperature, the higher the self-heating potential 

( 76). 

Up to that time, no methods to classlfy self-heating 

risks took into account the role that mining factors or 

other mining conditions play in self-heating. In 1973, Feng 

developed a risk index that considered a coal's liability to 

self-heat, as well as the mine environment. The liability 

index was a variation of the crossing point method that 

assigned a risk based on the heating rate and ignition temperature. The mine environment index assigned risk based 

on the coal loss, fissuration, and ventilation pressure differential (77). Canada still uses the liability index to characterize the self-heating potential of coals (78), but reference to the environmental index has disappeared from 

recent publications. 

In 1978, an extensive study of spontaneous combustion 

was made under a U.S. Department of Energy contract by 

Bacharach (12). Included in this study was a risk analysis 

that considered the rank of the coal, pyritic sulfur, hardness, humidity, strata temperature, and unworked roof coal 

thickness. The analysis was called "Rapid Appraisal of 
Spontaneous Combustion Assessed Liability," or RASCAL, 

and claims to give "only a very generalised indication of 

susceptibility and should never be used as the sole measure of risk." This risk analysis was unique in that it did 

not require laboratory tests. 

In 1982, Bhattacharya described a classification system 

based on a coal factor, a geological factor, and a mining 

factor (79). The coal factor was dependent on the crossing 

point temperature, while the geological factor considered 

coalbed gradient, thickness, depth, faulting, and structure, 

and the mining factor was determined from ventilation, 

depillaring time, and type of explosives used. No information was given on how the factors were determined, and 

no follow-up publications were found. 

At about the same time, Banerjee published the first 

description of a risk analysis that is still being used and 

rehed (8G-82). The iiieihod considers 22 parazieters *itE 

subjective assessment of the influence of 8 factors that 

affect spontaneous combustion. This method provides criteria for assigning high or low risk values for each of the 

22 parameters. 

Banerjee also refers to a Polish risk index, the "Oplinski 

Method," that considers the spontaneous heating susceptibility of the coal, the amount of coal left in the gob, the 

mining method, ventilation method, amount of leakage in 

the gob, wetness of the coalbed, depth, and ventilation 

pressure. However, all references are to private communications, and no literature was found for this risk analysis. 

Researchers in Great Britain have also been actively 

involved in the development of a risk index throughout the 

1980's (8, 59, 83). They were among the first to attempt 

to computerize the method by using an expert system format (84-85). The index is based on laboratory adiabatic 

oven tests to assess the spontaneous combustion potential 

of the coal, combined with a modified risk classification 

system of Bystron and Urbanski (86) that evaluates the 

contributions of some geologic and mining factors. 

The USBM developed a risk assessment method to 

evaluate the self-heating potential of coal, based on the 

coal's minimum self-heating temperature, a measure of the 

coal's relative reactivity, in adiabatic heating experiments 

(74). These results were correlated with oxygen adsorption experiments, and a prototype sealed-flask test apparatus was developed to evaluate the relative self-heating 

tendencies of coals in the field (87). 

A statistical analysis of the adiabatic oven results also 

showed that the relative reactivity of a coal was strongly 

dependent on the coal's dry ash-free oxygen content, a 

number readily available from a routine coal analysis. 

These results were incorporated into an expert system 

computer program that allows for the prediction of a 

coal's relative self-heating potential without the need for 

laboratory experiments (88). 

While the USBM's methods are widely used in the 

United States to assess the relative reactivity, or suscepti- bzt-. r of cod t~ SCX-heat, they do zot corsider =the: fzctors that contribute to self-heating, such as geologic factors, mining conditions, and mining practices. A prototype 

expert system is currently being developed to take into 

account these other factors. 

As described in a previous section, there are currently 

two mines in the United States operating with a bleederless ventilation method, and one other mine has used a 

bleederless system at least temporarily. In these instances, 

the case for the bleederless ventilation system as a spontaneous combustion control measure depended primarily on 

establishing a spontaneous combustion risk based on a 

history of self-heating events (6). Until a ranking criteria 

or risk analysis is developed that accurately describes the 

self-heating risk of a mining operation, it is likely that 

spontaneous combustion history will remain as a primary 

determinant in the justification for the use of a bleederless system. Since the successful use of a bleederless 

ventilation system is more likely when the system is designed into the mining plan from the start of an operation, 

it is essential that the development of a ranking criteria be 

a priority. 

A worldwide literature review of bleederless ventilation 

practices for spontaneous combustion control in coal mines 

was conducted to examine the impact of bleederless ventilation systems on U.S. coal operations. The literature 

study reflected that restricting airflow into mined-out areas 

(bleederless ventilation) is recognized worldwide as a 

spontaneous combustion control measure. However, successful application of this concept involves systematic 

minewide design of ventilation, methane, and ground controls with a reliable monitoring system for detecting 

uncontrolled or unexpected circumstances leading to a 

heating. The literature reflected that bleederless ventilation systems reduce the risk of spontaneous combustion, 

but the potential still exists. This is evidenced by the 

continued occurrence of reported spontaneous combustion 

mine fires associated with these ventilation systems. 

The main difference between implementation of bleederless ventilation in Europe and Asia as compared to the 

United States is in the design of mining operations. 

Europe and Asia commonly develop one-entry longwalls 
(sometimes two entries on the tailgate side of the longwall) with a solid barrier pillar between prior and active 

longwall sections. This type of design isolates gob areas 

and minimizes the number of seals constructed. 

Multiple-entry longwall development in the United 

States is common (two-, three-, and four-entry systems) 

with existing gate road entries reused when mining the 

adjacent panel. Two multiple-entry, low-gas-emission coal 

mines in the United States have used bleederless entry 

ventilation systems to control spontaneous combustion 

occurrences. Seals are constructed in entry crosscuts to 

isolate longwall gob areas. This type of bleederless design 

places its reliance on a large number of seals to isolate the 

gob, and these seals are constructed between coal pillars 

subjected to highly stressed ground control conditions. 

Seal leakages and heatings were reported in one mine that 

has used t'his type of bieederiess design. Mkes with Eigh 

methane emissions have an additional risk of a potentially 

explosive atmosphere developing near a leaking seal where 

an oxygen-enriched atmosphere can initiate a heating that 

becomes an ignition source. 

Ventilation control of spontaneous combustion must 

coincide with minewide planning for ground control, methane control, production, environmental monitoring, and 

compliance with all government regulations. Given the 

diversity in geologic conditions of various coalbeds, mine 

designs have to take into account their unique conditions. 

Plans should optimize a balance between conflicting design 

parameters, such as limiting the number of entries to reduce ground stresses as opposed to creating more entries 

to improve ventilation. Since performance of various technologies is mine dependent and cannot always be universally applied throughout the industry, mine planning should 

also include alternatives to the original designs. 

Methane drainage will be a key factor in the successful 

use of bleederless ventilation systems for spontaneous 

combustion control in mines with high gas emissions. 

Even mines that can control methane by ventilation alone 

may need to use supplemental methane drainage if a 

bleederless ventilation system is adopted. Methane drainage with a bleederless ventilation system will require more 

preplanning, monitoring, and control than most U.S. 

mining companies currently practice. It will be necessary 

to more accurately determine the source of the gas that 

accumulates in the gob so that the most efficient methanedrainage system to handle site-specific conditions is 

employed. If sufficient time is available, methane drainage 

can be started several years in advance of mining to minimize the impact of methane emissions on a bleederless 

ventilation system. 

It is important that a methane-drainage system employed in a spontaneous-combustion-prone mine using 

a bleederless ventilation system not over- or underdrain 

the gob. If the methane-drainage system is not sufficiently 

effective, high methane emissions may be experienced on 

the face. Conversely, "overaggressive" drainage can pull 

mine air into the gob, causing spontaneous combustion 

problems. The composition of the gas produced from a 

methane-drainage system must be sufficiently monitored so 

that changes in the atmosphere in the gob can be detected 

as early as possible. Monitoring of the gases produced 

from the methane-drainage system may also provide an 

early warning of the presence and location of a spontaneous heating. Finally, the methane-drainage system must 

be designed so that it can be sufficiently regulated to 

control the atmosphere in the gob in spite of changing 

conditions underground. 

The ground control aspects of bleederless design for 

spontare~l~ c~blmtizo co~tro! EP. re.ntra! tn a safe cornprehensive mine design. A thorough geotechnical evaluation needs to be done as a basic input to the ground control plan. Once this information is gathered, optimum 

pillar designs such as stiff, yielding, or a combination of 

both can be formulated. Stiff, stable ground minimizes 

many of the concerns of a bleederless spontaneous combustion control system. Taking into account the special 

demands of a bleederless system, such as minimizing number of openings, floor heave, rib sloughage, maximizing 

entry stability, and incorporating a complementary seal 

design, the elements of a ground control plan can be 

formulated. 

The use of a bleederless system places additional demands on the monitoring plan, especially in terms of spontaneous combustion detection, maintaining safe air quality 

at the face, pressure balancing, methane-drainage monitoring, and seal integrity. The analysis of gaseous products of 

combustion, particul-arly carbon monoxide, is the primary 

method used for the detection of spontaneous combustion. 

The determination of when the spontaneous combustion 

hazard potential of a mining operation is severe enough to 

warrant the use of a bleederless ventilation system as a 

control measure is an important issue. Many methods to 

determine the self-heating potential of coals are in use 

throughout the world, including the United States. Some 

methods depend on laboratory tests to determine the liability of a coal to self-heat, while other methods use 

empirical data. Some methods consider mining conditions 

and practices, while others do not. Many of the criteria 

used are subjective in nature. It is apparent that no method at this time provides a clear and definitive evaluation of 

the potential for self-heating. 

When a spontaneous combustion risk is present, critical 

attention must be given to mine design, seal construction, 

methane drainage, regulations, monitoring, and emergency 
procedures. However, several of these areas have limited 

technology that needs further development. The mine can 

be designed to minimize entries and isolate each gob with 

barrier pillars. This reduces the reliance on seals, which 

can leak. Seal construction can vary to accommodate local 

mine conditions. To minimize seal leakage, entry closure 

must be reduced and pressure balancing should be practiced. However, pressure balancing is complicated, since 

the mine resistance is dynamic and is responsive to 

changing barometric pressures. An automated pressure 

balancing system under human surveillance would be an 

improvement; however, research still must be conducted to 

develop reliable alternative systems. Methane drainage 

must be closely monitored when used so that air is not 

drawn into the gob. Automated methane-drainage systems 

need to be developed to control, in real time, the drainage 

rate from the gob. Mine gas monitoring systems exist, but 

research is needed to improve system performance, sampling strategies, and data interpretation. 

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