Stanislaw Prusek*,Eugeniusz Krause,Jacek Skiba

Central Mining Institute,Katowice 40-166,Poland

Keywords:Coal exploitation Methane Spontaneous fires Associated hazards Designing coal panels Ventilation

ABSTRACT An increase in methane,spontaneous fire and bumping hazards in Polish hard coal mines,observed in the last two decades,led to the need to elaborate the tools allowing proper selection of a range of preventive measures to fight them at the stage of designing coal extraction.Designing the production of a coal seams in the conditions of associated methane and spontaneous fires hazards in Polish hard coal mines requires elaboration of the design standards for coal panels in gassy coal seams.This paper presents the guidelines on how to design production in the conditions of associated methane and spontaneous fire hazards.Presented tools and methodology since the very first research were many times verified by daily mining operations in the conditions of associated methane and spontaneous fire hazards,which confirms their significant contribution to the development of safe and economical mining operations.

1.Introduction

The issues presented in this paper encompass problems of designing coal mining in the conditions of associated methane and spontaneous fire hazards,considering circumstances resulting from the applied method of ventilating a coal panel.

Natural hazards,which by their co-existence have an impact on the initiation,intensification and occurrence of their symptoms,are called associated hazards [1,2].This concept was proposed for the first time in 1994 [3].Typical examples of associated hazards are co-existence of methane and rock-bump hazards [4–6],dust and methane hazards[7–10],as well as methane and fire hazards [11–14].

Despite the significant progress made in the last 20 years in identifying and controlling methane and spontaneous fire hazards in Polish hard coal mines,the associated hazards are still frequent.It is clearly seen as the depth of coal mining and its concentration increase.The above problems are universal around the world as they affect many countries and their mining sectors.One of the examples is Australian research related to underground hard coal mining with the longwall system,conducted in increasingly more difficult mining and geological conditions,and growing needs to design optimal ventilation system for the coal panel workings[15,16].The increased occurrence of hazards also results from a significant impact of conducting the works under the main haulage level.

Methane content in coal seams,increasing with their depth,and higher coal output concentration result in increased methane emissions into the exploited coal panels.When coal panels located in very gassy coal seams are mined,ventilation methods of fighting methane hazard and methane drainage,in many cases,appear not to be effective enough to decrease methane hazard.Consequently,the daily advance rate of the longwall coal face and coal output have to be lowered.

Broad introduction of high coal output concentration,mainly through increasing the width of the longwalls,has a significant influence on the increased methane emissions.The factors which play role in increasing methane hazard in coal panels located in gassy coal seams are showed in detail in the flow chart (Fig.1).

The essential condition,which has to be met at the stage of designing the coal panels,for proper selection of methane hazard preventive measures and their extent is estimating the hazard prior to mining operations,based on the calculations of forecast methane emission.The absolute methane-bearing capacity forecast for a coal panel ought to be recognized as the basis for determination of ventilation system to be applied,preventive measures including the drainage technique,and the range of coal output(considering impact of the hazards).It has to be emphasized that underestimating methane hazard at the stage of designing a coal panel can be a direct cause of a sudden fall in safety standards while mining.Such a state of affairs was an inspiration for the search for the correlation and solutions aimed at improving the designing methods.The methods which can reduce occurrence of dangerous incidents while mining,and,at the same time,provide optimal output concentration considering the level of existing hazards.Thus,the stage of designing coal output in highly gassy deposits should be preceded by the analysis of mining capabilities after estimating forecast methane emissions into the coal panel[17].

In the last 20 years,in Polish hard coal mines,spontaneous fire hazard,together with dangerously high methane concentrations,often contributed to the occurrence of spontaneous fires in longwalls(in their gobs).The achievements of the Central Mining Institute in Katowice in identifying and forecasting spontaneous fire hazard,and,particularly,elaborating and implementing a method of early fire detection as a preventive measure,allow continuous estimation of methane and spontaneous fire hazards in the longwall environment.It prevents self-ignition of coal and consequently eliminates possibility of methane fires in the gobs.

Estimation of the actual spontaneous fire hazard level at the stage of designing should,first of all,determine the location of the coal left in the longwall gob,which may heat up and get ignited.In most of longwall operations conducted in Polish hard coal mines there is potentially high risk of spontaneous fires.It is estimated that 75% of Polish coal panels are being ventilated with‘‘U” type system along the coal solid.Selection of the above coal panel ventilation method in gassy coal seams results in dangerous methane concentrations in the gobs moving closer to the longwall workings while,at the same time,migration of the oxygen rich air in the longwall gobs is limited.It must be mentioned that explosion hazard in the environment of the longwalls occurs in the gassy coal beds and coal deposits surrounding them when methane content is above 2.5 m3CH4/Mgdaf.Co-occurrence of methane and spontaneous fire hazards requires,at the stage of designing coal panel,consideration of their potential level.

One sunny day in the early summer, Grandma heard a commotion4（，） in the front yard and looked out the window to see a frightening sight a gigantic5 swarm6 of bees filled the air between two tall trees. There was thousands of bees in the air, so many that the swarm reached the tree-tops!The buzzingsound was tremendous. Grandma watched as the bees made their way into a hole up in one of the trees. Before long, everyone of those bees had disappeard into its new home.

The principal criterion to classify coal panels as prone to methane explosion and not prone to it is the value of their methane content–coal bed methane content.The classification of coal panels to be designed into ones not prone to and ones prone to methane fires in the gobs due to self-heating of coal is shown in the simplified drawings of a coal panel in Figs.2 and 3,respectively.In panels located in coal deposits of methane content lower than or equal to 2.5 m3CH4/Mgdaf,methane concentration in gobs does not exceed 2%,which means there is no methane explosion hazard(Fig.2).In the panels where methane content is higher than 2.5 m3CH4/Mgdaf,methane concentration in gobs may make them prone to explosion (Fig.3).The cause of self-ignition of methane in coal panel gobs is self-heating of coal and,as a consequence,a spontaneous fire.Considering the above,guidelines for designing coal exploitation in the conditions of methane and spontaneous fire hazards ought to recognize the last mentioned a priori as the most important one,the one which has the biggest influence on the emerging explosion hazard.

Fig.3.Distribution of methane concentration in the gobs of the coal panel ventilated with ‘‘U” type system along coal solid for the coal content < 2.5 m3CH4/Mgdaf (prone to methane explosion).

2.Factors influencing the increase in spontaneous fire hazard when coal exploitation goes deeper

Practice shows that when coal panels in Polish hard coal mines are exploited,the increased spontaneous fire hazard which can be observed there is associated with mining deeper and deeper coal seams.The coal occurring in deeper seams is much better metamorphosed than coal in shallower ones and has much less nonflammable parts,which makes deeper coal seams more prone to self-ignition.Additionally,an increase in the original temperature of deeper rocks makes the time of coal self-heating shorter before it reaches critical temperature,which in consequence results in shortening the time needed to reach the coal ignition temperature.The influence of the original rock temperature on the process of self-ignition of coal is shown in Fig.4.

Fig.4 shows the periods of coal self-ignition process for two original rock temperatures Tp1(at smaller depth)and Tp2(in deeper seams),which depend on the coal seam occurrence.It can be concluded that the higher the original coal temperature (resulting from the original rock mass temperature) the shorter the process of coal self-ignition.Breaking the self-ignition process by starting fire prevention actions results in decreasing coal temperature in the location of its self-heating.If applied prophylaxis is effective the process of self-ignition stops or rather slow down and the bed temperature is decreasing.In Fig.4 stopping the self-heating process is shown by separation between solid and dashed lines,which means transition from higher temperature to lower one.The shape of dashed line depends on the process inertia.If applied prophylaxis is not effective one can observe increase of coal bed temperature up to its self-ignition(Tz).In Polish conditions it varies from 360 to 390 ℃.

Fig.4.Influence of original temperature of rocks on the process of coal self-ignition in gobs.

The damage to coal panel workings resulting in decrease of their cross section,causing uncontrolled migration of coal mine air into their surroundings,is a significant factor which influences an increase in spontaneous fire hazard in fissures.The results of the research and calculations conducted by the Central Mining Institute in Katowice in the years 2010–2012,covering simulations of the strata destruction and changes in the coal panel working cross-sections ventilated with the‘‘U”type system along coal solid and the‘‘Y”system,are shown in Fig.5[18].Above research,based on applied ‘‘Phase2” software,resulted in elaboration forecasted method allowing prediction of individual workings’ cross-section changes in time.It was many times validated by field measurements and is presently used by Central Mining Institute of Katowice as an auxiliary tool when designing coal panel exploitation and selecting proper ventilation method.The development of spontaneous fire hazard is influenced by the concentration of longwall coal output,due to their increasing width and advance rate.An increase in the exploitation advance rate has a positive influence on minimizing possibility of initiating a spontaneous fire in gobs.An increase in the width of a coal panel to be exploited automatically makes the volume of gobs much bigger,increasing the risk of coal self-heating.Additionally,the applied coal panel exploitation method and the ventilation systems have a direct impact on the area and volume of gobs with high oxygen content and on the level of spontaneous fire hazard.The volume of the oxygen zone in the gobs of a coal panel ventilated with ‘‘U” type system along coal solid,in the transverse,diagonal and longitudinal coal exploitation systems is shown in Fig.6a–c,respectively.

The research developments in diagnosing and forecasting spontaneous fire hazard in coal panels clearly indicate the need for locating the remaining coal (from coal beds or/and seams) in the gobs and its impact on the occurrence of the hazard.It must be mentioned that coal may be left in the gobs of coal panels due to its occurrence in the roof of the coal seams or layers,covered by the zone of direct or primary caving.Thin pillars dividing adjacent coal panel galleries are very often left in the gobs of coal panels mined with caving roof.In most cases,they get crushed and cracked.It creates favourable conditions for coal mine air migration due to the difference in aerodynamic potentials at the edges of the gobs–very often contributing to self-heating of coal in crushed coal pillars and,in consequence,to creating a possibility to start a spontaneous fire in coal fissures.The factors increasing spontaneous fire hazard in gobs ought to be recognized as the critical ones when deciding on the scope and selection of preventive measures at the stage of designing coal panel exploitation.

Fig.5.Changes in cross-section of the coal panel workings associated with the influence of exploitation pressure in the vicinity of the longwall result in unfavourable migrations of coal mine air through coal pillars and gobs and thus increasing spontaneous fire hazard (based on numerical modelling performed in ‘‘Phase2” software).

Fig.6.Development of oxygen zone in gobs of caving coal panels ventilated with ‘‘U” type ventilation system along coal solid in the transverse,diagonal,and longitudinal coal exploitation systems,respectively.

When designing coal mining,the location of coal left in gobs and in the surrounding area of the coal panel has to be considered as it has direct influence on the level of spontaneous fire hazard.The unmined coal,depending on its location,in most cases undergoes self-heating processes contributing to its self-ignition and development of a spontaneous fire.

The process of self-heating of the coal left in the gobs of a coal panel is influenced by the following factors:(1) location of coal left in gobs of a coal panel,(2) applied system of coal panel ventilation,which influences the oxygen content distribution in gobs,(3) location and direction of the coal mining operations,related to the occurrence of the faults within a coal panel plot,(4) location of coal panel gobs and of other gobs,related to the return shaft of the ventilation grid,and (5) methane content in the coal panel gobs.

The location of coal left within coal panels allows determination of the hazard level for the coal panel to be designed (small,medium,high) (Fig.7).

Designing coal panels in seams prone to spontaneous fire hazard ought to consider location of the coal left in the gobs in the first place.The method of coal panel ventilation and the direction of mining ought to be selected a priori,after the initial estimation of potential spontaneous fire hazard.The assessment of potential spontaneous fire hazard at the stage of coal panel designing ought to minimize possibility of mistakes and allow making right assumptions for the coal panel to be designed.

Fig.7.Influence of location of the coal left within coal panels on the occurrence of spontaneous fire hazard.

Possibility of occurrence of spontaneous fire hazard in the zone of primary caving depends on the thickness of the remaining coal layer and its location in coal panel gobs (Fig.8).

When in the roof of direct caving(up to 5-fold longwall height)there is remaining coal of thickness above 0.4 m along the whole coal panel,the coal panel will be prone to spontaneous fire hazard regardless of the coalface advance.At a very low advance rate,it is necessary to consider an increase in spontaneous fire hazard.If the coalface advance rate is at least 60 m/month and‘‘U”type ventilation system along coal solid is applied,the possibility of selfignition of coal remaining in the gobs is limited.If the thickness of coal left in the gobs is above 0.4 m,and the coalface advance rate is below 60 m/month,then even the application of‘‘U”type ventilation system along coal solid cannot completely prevent coal selfignition and spontaneous fire.Leaving coal in a coal panel in the zone of disturbances (faults),depending on the thickness of the exploited seam and throw of the faults and method of passing the longwall through them,may also mean initiating a spontaneous fire.

Designing coal panels in the conditions of spontaneous fire occurrence ought to consider the following features:(1) location of coal left within a coal panel,(2) tendency of coal to self-ignite,(3)advance rate of the longwall face of 60 m/month,and(4)selection of a mining system and a ventilation method.

Fig.8.Influence of location of the undermined coal seam or layer on the development of spontaneous fire hazard in the coal panel gobs.

3.Guidelines for designing coal mining in the conditions of associated methane and spontaneous fire hazards

The effect of elaborating the guidelines for designing coal mining in the state of associated hazards is a consistent and clear-cut designing sequence and criteria,which,as a consequence,ought to increase the work-place safety.The principal step when elaborating the guidelines for designing coal mining in the state of associated hazards is determining criteria relationships while their occurrence.When both hazards occur,there are cases of their mutual exclusion and contradictory criteria which the coal mining design should be based on.

When designing coal production in the state of associated methane and spontaneous fire hazards,it is necessary to rely on output information required for the analysis,such as:(1) forecast level of spontaneous fire hazard,(2) forecast of absolute methane-bearing capacity,and (3) methane-ventilation criteria determining safety of mining.

Below we will describe the principal assumptions when designing mining in gassy coal seams prone to spontaneous fire hazard.

The very first criterion when the above hazards occur together is objective evaluation of spontaneous fire hazard at the stage of coal panel designing.The output parameters for spontaneous fire hazard evaluation at the stage of coal panel designing are:(1)location of coal left within a coal panel,(2)system of coal panel ventilation,(3) an applied mining system (longitudinal,transverse,and diagonal),(4)designed coal panel width,providing a longwall face advance rate of more than 60 m/month,and (5) proneness of coal to self-ignition.

At the stage of designing the coal panel,proneness of coal to self-ignition is less important for the development of spontaneous fire hazard than the method of ventilation.The planned coal panel mining ought to meet safety criteria,considering,first of all,the occurrence of spontaneous fire hazard and,and secondly,of methane hazard.

Wrongly selected methods of coal panel ventilation and coal mining for the given level of forecast spontaneous fire hazard can cause coal self-ignition in the gobs and create explosion hazard during mining.

After selecting a proper ventilation system and the right method of coal mining (longitudinal,transverse,and diagonal)for a given coal panel to be designed,the longwall width is another output parameter which influences the development of both hazards.If the designed longwall width provides mining advance rate of 60 m/month for the applied‘‘U”type system of ventilation along the coal solid,then the coal panel ought to be designed accordingly to the forecast methane hazard.If there are problems to reach the average monthly advance rate of 60 m,then the longwall width ought to be shortened accordingly.

The proceeding algorithm followed while designing mining of coal panels in the conditions of associated methane and spontaneous fire hazards has to take into consideration:(1) location of coal left within a coal panel,(2) proneness of coal to self-ignition,and (3) longwall face advance rate.

The abovementioned proceeding algorithm,together with a coal mining method and a ventilation system is shown in detail in the monograph [4].The algorithm (Fig.9) in most criteria in its individual blocks refers to the evaluation of spontaneous fire hazard.The selection of a coal mining method and a ventilation system for coal panels which are not prone to spontaneous fire hazard will depend on the actual methane hazard level (block 19 of the algorithm in Fig.9).Designing coal panels in mining areas located in coal seams and the surrounding strata of the methane content of above 2.5 m3CH4/Mgdafis based on the methaneventilation criteria included in‘‘Rules of conducting mining operations in the coal panels prone to methane hazard” [19].

Fig.9.Proceeding algorithm for coal panel designing guidelines in the conditions of methane and spontaneous fire hazards.

The algorithm of multi-criteria methane hazard evaluation for designed coal panels in gassy coal seams and the surrounding strata of methane content above 2.5 m3CH4/Mgdafis based on the method of forecasting methane emission into the environment from a coal panel,elaborated in the Central Mining Institute in Katowice,and‘‘Rules of conducting mining operations in coal panels prone to methane hazard”,which contains methane-ventilation criteria for exploited and designed coal panels.With the abovementioned algorithm of multi-criteria methane hazard evaluation for designed coal panels,it is possible to conduct variant analyses of methane-ventilation conditions to estimate methane hazard level and safe longwall face advance rate.According to the guidelines for designing coal panels in the conditions of associated methane and spontaneous fire hazards,after conducting the analysis of spontaneous fire hazard,it is recommended to conduct variant evaluation of methane-ventilation conditions.

4.Conclusions

(1) Mine planning of coal seams,in the conditions of associated methane and spontaneous fire hazards,ought to focus first on the evaluation of spontaneous fire hazard and then on conducting multi-criteria methane hazard evaluation.

(2) Identification of the actual spontaneous fire hazard level in a designed coal panel(based on the algorithm in Fig.9)determines selection of the ventilation method and parameters,and is the starting point for conducting methane hazard evaluation.

(3) Multi-criteria methane hazard evaluation for designed coal panels in gassy coal seams and the surrounding strata of methane content above 2.5 m3CH4/Mgdafenables selection of an appropriate methane drainage technique,required drainage effectiveness and determination of feasible mining output.

(4) In the conditions of associated methane and spontaneous fire hazards,designing properly coal mining under the main haulage level,requires utilising proper tools to decrease the level of their occurrence.