Published : PROCEEDINGS of the 7TH INTERNATIONAL MINE VENTILATION CONGRESS, June 17-22

Chapter 135

Published : PROCEEDINGS OF THE 7TH INTERNATIONAL MINE VENTILATION CONGRESS, June 17-22, Crakow, Poland

CONTROL OF FIRE IN A LONGWALL PANEL UNDER SHALLOW COVER WITH CHAMBER METHOD OF VENTILATION AND HIGH PRESSURE HIGH STABILITY NITROGEN FOAM - A CASE STUDY

N. Sahay
Scientist
Central Mining Research Institute,
Dhanbad, India / B.C. Bhowmick
N.K. Varma
S.K. Ray
S.M. Verma
Scientist
Central Mining Research Institute,
Dhanbad, India
P.K. Mandal
General Manager
Jhanjra Area
Eastern Coalfields Limited, India

ABSTRACT

Fire in a powered support longwall panel (AW1 ) in RVIIA seam of 1 & 2 Incline mine of Jhanjra Project in Raniganj Coalfields, India was successfully brought under control using a novel ventilation technique and infusion of high pressure high stability nitrogen foam from surface through boreholes. A few years ago in 1994, a similar fire in Kottadih colliery about 10 km away from this mine was successfully tackled with pressure balancing and infusion of gaseous and liquid nitrogen from PSA type nitrogen generator and LN2 from tanker/reservoir (1). Due to complex geo-mining situations at AW1 panel the above method had to be supplemented by Chamber method of ventilation and infusion of high pressure high stability nitrogen foam.

The panel had to be sealed three times within a period of six months due to recurrence of fire after reopening. Finally, Chamber method of ventilation was adopted for making the pressure of affected panel positive with respect to surface atmosphere and caved & sealed longwall panel (W1) in RVII seam which was about 40 m above the fire affected panel. Further high pressure high stability nitrogen foam procured from TECHNOVENT Czech Republic was injected through a number of boreholes in a systematic manner. For this purpose a foam generator of 9.6 m3/ min capacity coupled with trolley mounted PSA type nitrogen generator of 300 Nm3/h capacity was put into service. Foaming agent having an expansion factor of 80 - 160 times and foam stability of about 72 hours was used. In addition to above, to meet any emergency situation LN2 in tanker and a reservoir filled with LN2 were kept in readiness. Above measures supported by thorough monitoring of status of fire by drawing gas samples from goaf through a number of boreholes and its analysis in the field laboratory ultimately led to control of fire in the panel. The panel is running smoothly with an average production of about 3000 tonnes of coal per day.

The paper briefly discusses the genesis of the problem, chronological order of the events and action taken, results obtained, principle & practices of Chamber method of ventilation and high pressure high stability nitrogen foam technology coupled with a trolley mounted PSA type nitrogen generator.

The method seems to be promising and cost effective for controlling fire in a running longwall panel with complex geo-mining conditions. This technique is general enough for successful application in other mine under similar situation.

KEYWORDS

Longwall technology, nitrogen infusion, chamber method of ventilation, dynamic balancing of pressure, sealed off area, foam infusion, goaf

CONTROL OF FIRE IN A LONGWALL PANEL UNDER SHALLOW COVER

INTRODUCTION

In 21st century powered support longwall technology has bright future in Indian coal mines. It has been successfully adopted in 1 & 2 Incline mine, Jhanjra Area of Raniganj Coalfields about 200 km from Kolkata, India. The mine has the reputation of successful completion of 10 (ten) longwall panels at shallow depth (about 40 m). The success at Jhanjra brightened the prospect of Longwall technology in Indian coal mine. However, fire in AW1 longwall panel, RVIIA seam not only crippled the Jhanjra mine by bringing all production activities to a halt but also had a much greater suppressive impact in the entire mining industry of the country as the story of Longwall technology in the country seemed to be leading to a sad end. It was, therefore, imperative to pool the collective wisdom and expertise of CMRI and mine management to work out innovative measures to tackle this fire, reopen the sealed off panel to save the costly equipment like powered support, shearer, AFC etc. and resume production at an early date.

A similar type of fire in a longwall panel of Kottadih colliery(1), a neighbouring mine about 10 km from Jhanjra project was successfully controlled by application of pressure balancing and nitrogen infusion. However, the difficult geo-mining conditions at Jhanjra, explained later in the paper, made the fire problem more complicated and the technique of pressure balancing with nitrogen infusion was found effective in controlling fire in the longwall panel as long as it remained sealed. However, on reopening the panel the fire would erupt again.

It was obvious that an improved technique of fire control was needed for this mine which led to introduction of chamber method of ventilation along with high pressure nitrogen foam technology that ultimately proved successful in keeping the fire under control not only when the panel was sealed but also when the panel was reopened and put under normal operation. The paper briefly deals with the problem, various measures taken to control the fire, their impact on status of fire and highlight the important lessons learnt which would be beneficial to other mines.

PARTICULARS OF THE MINE

Jhanjra Area extends to about 12 Sq km which is encircled by faults with throws varying from 10 to 30 m. Jhanjra block has nine viable seams with total extractable reserves of 212 million tonnes of coal. It is being extracted by two mines viz 1&2 Inclined mine and Main Industrial Complex. Depth of the seams varies from 38 to 380 m. Details of thickness and depth of the five top seams are given in Table below:

S.No / Seam number / Thickness, m / Depth, m
RVII / 1.27 - 4.23 / 76
RVIIA / 0.45 - 4.11 / 109
RVI / 2.15 - 5.60 / 201
RV / 2.90 - 6.64 / 231
RIV / 3.75 - 11.27 / 257

The seams are degree II gassy. In Incline 1 & 2 mine workings are spread in two seams viz. RVII and RVIIA. In RVII seam a manual section is in operation and ten longwall panels have already been successfully completed, whereas in RVIIA seam developments by Road header machine in east and west sides of the property are in progress and one longwall panel (AW1) in west side of the mine has just started. Layout of panels and developments in RVII and RVIIA seams are shown in Figure 1. Entries into the mine are through Incline 1 & 2 and ‘D’ shaft sunk upto RVII seam. No 1 & 2 Incline mine was producing about 3500 tonnes of coal per day deploying 1200 personnel before the onset of fire in AW1 panel.

INCLINE 1INCLINE 2

MANUEL DIST

1D 2D

AW2

AW2 AE1

AW1

AE1

W1 D

D Shaft

RVII seam circuit

RVIIA seam circuit

Figure 1. Schematic diagram of 1 & 2 Incline mine, Jhanjra area

Ventilation circuit

General Ventilation layout of the mine is shown in Figure 1. The mine is being ventilated by an Axial flow fan (make - Voltas, Model - VF 2500) installed at Incline No 1. The fresh air reaches the mine through Incline No. 2 and Shaft ‘D’ situated at extreme boundary of the property. Development of the mine is between these openings viz. No 2 Incline and ‘D’ shaft along RVII and RVII A seams. No 1 dip having conveyor belt installations all along its length is connected with incline no. 1 serving the purpose of main return airways and transportation of coal of the total mine. The air flowing down through incline no. 2 is mostly utilised for ventilation of workings of RVII seam and developments of RVIIA seam whereas air entering through ‘D’ shaft is used for ventilation of isolation stoppings of sealed Longwall Panel of RVII seam and Longwall panel AW1 of RVIIA seam. Air enters through ‘D’ shaft into RVIIA seam flows along shaft level, 1 L north sump, 0 dip and reaches to bottom gate of AW1 longwall panel. Return air of AW1 flows along top gate, 0 rise and joins the main return flowing along 1st rise and No 1 incline.

Some details of AW1 panel

AW1 longwall panel with single entries, retreat and U-system of ventilation is the first panel in RVIIA seam. This panel is located below the caved and sealed longwall panels W1 & W2 in RVII seam. Details of the panel are given below.

Name of the Panel: AW1
Name of seam:RVIIA
Seam Thickness:3.2 m
Length of panel:850 m
Depth from surface:97 m
Parting between RVII:40 m

& RVIIA seams

Length of face:120 m
No. of chocks:82
Chock resistance:(4 × 550) tonnes
Distance of installat-

ions chamber between:40 m

AW1 & W1 panels

Date of Starting of :08.06.1999

the panel

Thickness of RVIIA seam at installation chamber is 3.2 m, but due to limitation in support resistance (4 x 550 tonnes) only 2.4 m height of the seam could be extracted along the floor. About 0.8 m thick coal layer along the roof was left in the goaf. After progress of 40 m of the face it was observed that there was no periodic weighting and goaf was not even half filled. Apprehending the danger of air blast, Induced blasting was resorted to after every 10.0 m mark to fill up the goaf. After 40.5 m progress of the face local fall occurred on 27.7.99 and as a result the goaf was filled to about 90%. Main fall occurred after 80.5 m progress of the face on 18.8.99. After occurrence of main fall water from upper seam goaf started percolating down at the rate of about 200 GPM, although a borehole was drilled earlier from lower seam to upper seam for dewatering purpose.

HISTORY OF FIRE IN THE MINE

Presence of CO to the extent of 80 PPM was noticed in the tail gate during extraction of W1 panel in 1990 - 91 which ultimately vanished after 400 m progress of the face. The measure taken was only dozing on the surface above W1 panel .

On 28.8.99 traces of CO gas at top gate of AW1 panel in RVIIA seam was noticed after ocurrence of main fall which increased gradually to 400 PPM even after further progress of the face by 30 m. Finally, the panel was sealed on 8.9.99 by erecting stoppings at top and bottom gates. At this juncture CMRI was called for scientific support for assessment of status of fire and extinguishment of fire quickly.

A team of scientists of CMRI visited the mine. Details of investigations2 carried out by the authors, approaches adopted for control of fire and result obtained are discussed briefly phase wise in the following paragraphs.

The entire period of fighting the fire are divided into eight phases. List of the action taken and their results with observation are furnished in Table 1.

From the account of action taken to control fire during the first seven phases of fire fighting operation and their results it is clear that:

The conventional model of packing of longwall goaf viz. free zone, critical zone and compact zone as propounded by various authors (3) are not applicable in this mine.
Location of seat of fire was not known.
Heat could not be dissipated even after keeping the panel in sealed condition more than 90 days and maintaining its temperature below 10°C by pouring LN2 through different boreholes.
Requirement of Nitrogen gas is very high to control fire in open longwall panel due to uncontrolled leakage of air .
Rate of emission of CO increased rapidly within a few days after getting an indication of its presence in the face.

Finally, during 8th phase of fire fighting operation a new model for packing of goaf in this situation considering:

bolted roof of top & bottom gate,
panel located below caved panel of upper seam,
shape of the goaf being almost square.

was conceived. Conceptual model alogwith location of bore holes is shown in Figure 2.

Figure 2. Conceptual of the goaf

CONTROL OF FIRE IN A LONGWALL PANEL UNDER SHALLOW COVER

Table 1. Summary of results of action taken during different phases of controlling fire with observations

Ph. No / Period / No of
days/ panel
condition / Bore Hole avail-
able / Action taken / Goaf environment at end of the Day / Amount of LN2 / Observations
O2 % / CO ppm / Tem. 0C / flushed, ltrs.
I / 01.09.99 to
10.09.99 / 9
Sealed / 1, 2 & 3 / Injection of LN2 through BH Nos. 1, 2 & 3 / 12 / Nil / 26 / 51,123 / Environment of goaf behind stoppings and bore holes i.e. only closed to the face could be assessed
II / 11.09.99 to 12.09.99 / 2
Opened / -do- / - / 19 / 400 / - / - / Ventilation of the face could not be controlled because face was abot 90% filled with water
III / 13.09.99 to 27.09.99 / 14
Sealed / 1, 2, 3 & 4 / Injection of LN2 through boreholes and Dynamic balancing of pressure around AW1 panel / 1.9 / 30 / 26 / 1,05,915 / Environment of total goaf was monitored by using number of bore holes
IV / 27.09.99 to
09.10.99 / 12
Opened / - / Chamber method of ventilation and Injection of LN2 through BH4 / - / 1400 / - / 67,046 / Ventilation sys-tem was disturbed on 7.10.99 and LN2 supply was intrupted.
V / 10.09.99 to 28.11.99 / 14
Sealed / 1, 2, 3, 4 & 5 / Dynamic balancing around AW1 and Injection of LN2 through bore-holes. / 1.9 / Nil / 26 / 2,31,965 / Environment of total goaf was monitored by using number of bore holes
VI / 29.11.99 to 08.12.99 / 10
Opened / -do- / Chamber method of ventilation and Injection of LN2 through BH4 & 5 / - / 2200 / - / 35,427 / Chamber method of ventilation could be maintained upto 5.12.99
VII / 9.12.99
to 13.03.99 / 94
Sealed / -do- / Dynamic balancing of pressure and Injection of LN2 through boreholes selected on the basis of O2 in the borehole. Further LN2 inj. rate was controlled so as to keep the goaf only slightly positive with respect to atmosphere / 0.2 / Nil / <10 / 1,74,097 / Environment of total goaf was monitored by using number of bore holes and consumption of LN2 was reduced

CONTROL OF FIRE IN A LONGWALL PANEL UNDER SHALLOW COVER

In the Figure the zone marked A is considered well compacted through which no or little leakage of air is possible. Hence it is not very risky from heating point of view. The area marked B is only partially compacted which may allow slight leakage of air. The area under C is by far least compacted zone allowing leakage of air which may lead to spontaneous heating The crushing along the boundary of this zone may further enhance the chances of heating. On the basis of this model a few additional bore holes viz. 11, 12, 13 & 14 were drilled in zone C to facilitate inertisation and plugging this highly sensitive area by injection of high pressure high stability nitrogen foam.

IDENTIFICATION OF LOCATION OF SEAT OF FIRE

To deal with fire identification of its location is very important. However, in most cases identification of exact location of seat of fire is rather difficult. In case of fire in AW1 panel, due to geo-mining condition the problem become very complex. As per example the fire could be in the same (RVIIA) or in the upper seam(RVII). Fire in RVIIA seam may be due to:

Spontaneous heating in the heap of broken coal lying in the goaf.
Non dissipation of heat generated during induced blasting in the panel.
Delay in main fall allowing free path for air in the goaf.

But the possibility of heating in the upper seam (RVII) also could not be ruled out due to following reasons:

The W1 panel in RVII seam having a history of fire is just above the AW1 panel with a parting of only 40.0 m. Toxic gases and even smoke due to fire in this panel may easily leak to AW1 panel giving false indication of fire in the bottom seam.
Leakage of air through surface cracks to the goaf due to suction pressure created by the main fan may lead to heating.
Huge amount of coal lying on the floor in the panels and its possible oxidation after dewatering of the goaf.

Further, the panels W1 , W2 , W3 & W4 in RVII seam may be inter connected and migration of CO from any one of the panels to AW1 panel is possible. Presence of air leakage path from surface through cracks to any segment of the four panels stated earlier may extend upto AW1 panel and seat of fire may be any where in this path.

Considering the possibilities and situations mentioned above in both the seams it was very difficult to decide the exact location of seat of fire.

To locate the seat of fire further investigations were carried out with particular emphasis on following.

Study of concentration gradient of CO in RVII & RVIIA seams. All boreholes samples results showed that CO concentration was always higher in bottom seam than in top seam clearly indicating that location of fire was in bottom seam.
Study of pressure gradient along the panels in both the seams indicated favourable condition for flow of CO from lower to upper seam. The concentration of CO is lower in upper seam than that in lower seam is again a clear indication of existence of fire in the lower seam.

The above analysis more or less confirmed that location of seat of fire was in AW1 panel. Exact location of fire could be established only after assessment of results of foam infusion through different bore holes, discussed in later chapters.

ACTION TAKEN DURING 8th PHASE

Keeping the above arguments in mind following strategies were worked out to control the fire and its quick suppression if it gets aggravated again after reopening on 14.03.2000.

Reduction in air leakage into goaf either from bottom gate side or from upper seam or surface with adequate air quantity at the face. Monitoring of status of fire through boreholes by maintaining pressure of the boreholes slightly positive by adoption of a ventilation arrangement called Chamber method of ventilation, which has been discussed in later chapter in the paper.
Plugging of air path from top and bottom gate sides particularly along the barrier and create N2 bank in the floor of the goaf. For this purpose High pressure high stability Nitrogen foam technology comprising foam generating machines and foaming agent from M/s Technovent PTY Ltd, Czech Republic was used. Another foaming solution from M/s Control system, Kolkata, India was also used when the foaming agent from Czech was exhausted. Details of high pressure high stability foam technology has been discussed in the later chapters.
To keep good environment at face safe and to maintain CO level within permissible limit even when concentration of CO in the goaf became high, air from goaf was discharged to atmosphere through boreholes close to the zone of high CO concentration.

Chamber method of ventilation