1. Principles of Explosion Prevention in Underground Coal Mines
Rao Balusu
CSIRO ENERGY
09 March 2017
International Workshop on Best Practices in Methane Drainage and Use in Coal Mines, 9-10 March 2017, Ranchi, India
(Organised by UNECE Group of Experts on CMM in cooperation with Ministry of Coal, CMPDI, GMI and US EPA)

2. Presentation Outline
Introduction - Fires and Explosions issues in coal mines
Explosion prevention – fundamental principles
Gas drainage and control – safety measures to minimise risks
Proactive measures to minimise explosion risks in goaf areas
Risk assessments and Management systems
Conclusions
Rao Balusu | Page 2

3. After mine fire/explosion
Introduction
Mine Fire
After mine fire/explosion
Fires and explosions – highly critical issue
Severe consequences – for people & mine/industry
Very high cost of dealing with explosions

4. Fires and explosion issues in coal mines (1)
Coal seam gas – mostly methane (CH4)
Coal mining – methane gas released and mixes with air
Explosive range (5% - 15% CH4) and higher % in goaf areas
Gas explosions can lead to devastating dust explosions
Higher risk in gassy coal mines
Explosion risk in low to medium gassy mines (due to reduced attention)
Explosion risk in sealed/sectionalized areas
(Fires can generate high CO and H2 and form explosive mixtures in sealed areas
– nose point for oxygen becomes 5% if CO and H2 are the dominant gases; 12% for CH4)

5. Fires and explosions issues in coal mines (2)
Gas in general body of air – due to outbursts/ accumulations/ air-blasts
Goaf areas of working panels
Sealed/sectionalized areas
Ventilation systems – inadequate or failure
Gas drainage systems – inadequate or failure
Management systems – inadequate or failure
External ignition sources – electrical, belts, contrabands, ..
Inherent ignition sources – spontaneous combustion, cutting
Inadequate/lack of gas monitoring

6. Fires and explosions issues in coal mines (3)
Not just an issue related to Ventilation officer/ Safety officer
Fires and explosions in goaf areas – very complicated issue, depends on
Coal left in the goaf, location of rider/split seams
Spontaneous combustion (sponcom) propensity of coal seams
Ventilation system and other mining parameters
Geological structures – seam contours, faults, dykes, ..
Geotech/ Caving characteristics – strong roof areas
Production schedules/stoppages
Oxygen ingress patterns into the goaf areas
Power failures/ barometric pressure changes
Adjacent sealed panel conditions (geometry, heatings, ...)
Fundamental understanding of all inherent risk parameters is critical for prevention of explosions in coal mines

7. - Fundamental Principles
Explosion Prevention
- Fundamental Principles

8. Explosion prevention – fundamental principles (1)
Avoiding explosive gas accumulations in general body of air
Control of potential ignition sources
Gas drainage and control – and safety measures
Minimising explosion risk in goaf/sealed areas
Comprehensive gas monitoring
Proactive measures
Explosion proof seals
Just following regulations won’t be enough – requires additional site specific measures
Extensive risk assessments & proactive control measures

9. Explosion prevention – fundamental principles (2)
Avoiding explosive gas accumulations
To prevent pockets of gas from forming, the work area must be properly ventilated at all times – with good ventilation standards
Gas to be diluted below 1% to 2% in roadways with ventilation
Ventilation flow modifications/devices near mining machines/cutting zones to minimize gas accumulations
Maintain minimum air velocities to minimize gas layering
Well ventilated belt roadways in gas seams (to minimize accumulation of gas emissions from cut coal on the belt)
Gas drainage – either pre-drainage or post-drainage, if required

10. Explosion prevention – fundamental principles (3)
Control of potential ignition sources
Flame-proof and intrinsically safe systems
Make sure the equipment used in coal mines does not have defects that can create sparks and set off explosion
Electrical cables maintenance – good standards
Methane gas and dust explosion can also be triggered by frictional ignition/heat and sparks produced by mining equipment
- maintain sharp picks
- avoid cutting roof/floor
Zero tolerance to contrabands
Spontaneous combustion – prevention and control

11. Gas drainage and control (for explosion prevention)

12. Gas drainage and control (1)
Gas drainage may be necessary in gassy mines to minimize excessive gas emissions into mine’s general body of air
However, gas drainage may also create additional explosion risks, if not carried out appropriately & safely
Gas drainage systems require safety measures to minimise explosion risks in UG mines
Pre-drainage
Post-drainage (also known as Goaf gas drainage)
Gas drainage infrastructure – Underground & Surface

13. Pre-drainage – explosion prevention measures
Underground (UG) or Surface-to-inseam (SIS) technologies
UG pre-drainage – risks and safety measures
Generally high purity gas (generally > 70% to 90% gas)
Minimise air leakages at collar of drainage holes (otherwise, gas % can be explosive)
Making sure holes do not intersect -or- holes end up close to roadways
Making sure drivages do not intersect pre-drainage holes filled with gas
Surface-to-inseam pre-drainage – risks and safety measures
Flame-traps to be installed between drainage holes and pumps
Lightening arrestors to be installed at all drainage holes/pumps
Sealing off all boreholes after completion of drainage operations
Making sure UG drivages do not intersect SIS drainage holes
All holes to be surveyed and marked on the mine plans
All drainage holes to be monitored for gas% and flow rates

14. Post-drainage Underground (UG) or Surface to inseam technologies
UG post-drainage – risks and safety measures
Gas purity can vary significantly (5% to 90% - depending on conditions and designs)
Make sure goaf gas drainage does not induce air into the goaf area
Gas concentration in goaf holes should be above 30% (preferably above 40%)
Do not drain gas at explosive gas mixture composition
Surface goaf holes – risks and safety measures
Gas purity – in general 40 % to 90% (Holes shut-off, if gas purity falls below 40%)
Flame-traps to be installed between drainage holes and pumps/venturi’s
Lightening arrestors to be installed at all drainage holes/pumps
Sealing off all boreholes after completion of drainage operations
All drainage holes to be monitored for gas% and flow rates

15. Gas drainage infrastructure
Underground (UG) or Surface infrastructure
UG pipelines – enough capacity, strength and integrity (no leakage)
Make sure that explosive gas mixtures are not transported in UG pipes
UG pipes installation – to minimize damage from vehicles, collapse, ..etc
Positive pressure pipe lines (gas leak) & negative pressure lines (air leak)
Surface pipes – safety and security
Safety systems to make sure goaf holes are shut, if gas% falls below set limit
Continuous monitoring of all gas drainage lines and pumps

16. Proactive measures to minimise explosion risk in goaf areas
(as an example - all these measures are additional
to what was recommended in regulations)

17. Approach Integrated approach
(i) Extensive field monitoring studies – goaf gas distribution
(ii) Data interpretation
(iii) Computational Fluid Dynamics (CFD) modelling techniques
(iv) Field trials
Fundamental understanding of goaf gas flow patterns
Investigation of the effect of various parameters
Development of optimum strategies
Field implementation

18. (i) Field goaf gas monitoring (typical mine data)
Oxygen > 17% at 400 m behind the face on maingate side
High oxygen ingress into the goaf – may lead to heatings, when
Coal left in goaf is prone to sponcom
Face stops or slows down outbye of geologically disturbed areas

19. Field monitoring – Goaf Heating development
Early stages of heating dev
CO over 250 ppm, low H2
No change in return CO
Advanced stage (after 3 weeks)
High CO and H2 and spread
No significant change in return CO
High CO and H2 in goaf - with no significant change in LW return (so, just monitoring LW return gas % won’t be sufficient to prevent fires and explosions in goaf areas)
Even in B & P panels (which are vey short) - no significant change, until it is too late

20. (ii) Data interpretation critical (example)
CO reading reduced at seal near the heating due to changes in ventilation, but heating not reduced
Heating gases accumulations in goaf
CO readings and changes at different locations depend on goaf flow patterns
CO – to be read in combination with other gases, particularly O2

21. (iii) CFD modelling investigations
Longwall Goaf model development
3D model, Multiple gases – CH4, O2, CO2, N2
Both turbulent and laminar flows, Steady state and transient
Goaf gas flow mechanics
Simulation of complex goaf gas flow patterns
Oxygen ingress patterns
Under different scenarios
Parametric studies
With changes in parameters
Inertisation simulations
With different strategies

22. Oxygen ingress patterns in LW goaf
Base model results for a typical mine
Oxygen ingress depends on a number of parameters:
Panel layout, ventilation, seam gradients, caving characteristics, faults/dykes, seal leakages, gateroads support, goaf gas emissions, gas drainage, .. etc

23. Effect of goaf gas emissions on O2 ingress
High oxygen in low gassy seams – more heating issues
However, just depending on high goaf gas emissions is not enough,
- and may give false confidence and misleading sometimes

24. Inertisation – Simulations and Field studies
Proactive inertisation
Traditional practice - Just behind the face
Current longwall airflow rates m3/s
Comparatively, low inert gas rates m3/s
Optimum inertisation strategies critical
Extensive modelling and field investigations, - with different
Inert gas flow rates
Locations for inert gas injection
Inert gas on MG or TG side
Inert gas types
Panel geometries, ventilation conditions

25. Flow rate - Inert gas injection close to face

26. Location - Inert gas injection

27. (iv) Field results – Proactive inertisation – Mine A
Oxygen ingress into the goaf
– before proactive inertisation
Oxygen ingress into the goaf
– after proactive inertisation

28. Field results – Proactive inertisation – Mine B
Oxygen ingress into the goaf
– before proactive inertisation
Oxygen ingress into the goaf
– after proactive inertisation

29. Oxygen concentration (0.21 = 21%)
Inertisation simulations in B & P panels
Oxygen concentration (0.21 = 21%)
(a) no inert gas injection – base model results
(b) inert gas injection through top roadway, near intake
(c) inert gas injection at inbye locations in the goaf
Effect of inert gas injection location on oxygen distribution in Bord and Pillar panels

30. Field results – Inertisation in B & P panels
Effect of proactive inertisation on goaf gas distribution in Bord and Pillar panels

31. Recommended location of tube bundle monitoring points
Proactive approach (1)
Comprehensive gas monitoring - In addition to extensive mine wide gas monitoring, additional gas monitoring is recommended for goaf areas and sealed areas
Recommended location of tube bundle monitoring points
Extensive goaf gas monitoring recommended
for early detection of heatings in goaf areas
Monitoring locations critical
Tube bundle monitoring and weekly/fortnightly bag sampling

32. Proactive approach (2)
Proactive inertisation may be required to reduce O2 ingress into goaf areas of working panels
High goaf gas emissions alone – may not be sufficient
Inertisation just behind the face – not effective
Inert gas injection at m behind the face
Inert gas flow rates of 0.5 to 1.0 m3/s
Inert gas injection on inbye side of suspected heatings/fire
Continuous inertisation preferred (no intermittent)
Inertisation until control – or face well past heatings
Explosion proof seals after panel seal-up

33. Risk assessments
&
Management systems

34. Risk assessments Comprehensive risk assessments/management processes
Fundamental understanding of all inherent risk parameters
Ventilation, gas drainage, power failure, barometric pressure changes, air-blasts, ..etc
External (electrical, ..) and inherent ignition sources (sponcom, frictional ignition, ..)
Guidelines in the event of gas outbursts, high gas emissions, ..etc
Guidelines in the event of power/fan failures, storms, ..etc
Risk assessments - should involve experienced people and external experts
Hazard identification process – from risk assessment
Trigger Action Response Plans (TARPS) to be developed – site specific
Gas outburst / sudden inrush of gas - risk assessments
All near-miss gas/sponcom/frictional ignition incidents to be investigated
Periodic safety auditing of mines – to identify at-risk behaviour

35. Management systems
Explosion prevention management plans (site specific)
Identification of explosion risk zones on mine plans
Regulations
Education & training
Regular updates on explosion prevention technologies
Safety culture
Dust control management plans (& coal dust explosion prevention plans)
Mine management systems, policies, organisational structure, ..etc
Workers participation (and implementation of contraband measures)
Ensure all contractors follow the mine’s safety systems
Dynamic Emergency Response Management Systems – to be developed

36. Emergency preparedness
Surface and underground infrastructure for fires control
Comprehensive gas monitoring – including goaf/sealed areas
Personal location monitoring systems
Life support systems (and refuge bays with connections to surface)
Emergency escape systems & exercises
Reliable communication systems for emergency management
Inertisation pipe network and inertisation/sampling pipes in seals
Proactive inertisation into the goaf/sealed areas, if required
Emergency management guidelines (incl approaches from other industries)

37. Conclusions Fires and explosions – severe consequences
Fundamental understanding of inherent risks is critical
Good ventilation standards
Adequate gas drainage and control systems
Risk assessments and management processes
Proactive control measures
Site specific explosion prevention plans (in addition to regulations)
Education and training
Safety First culture – and workers participation
Management, policies, organization structure, ..etc

38. Thank you Dr Rao Balusu Mining Research Team Leader t +61 7 3327 4614e w
CSIRO ENERGY