Specializing in providing assistance to power plants to resolve operational and maintenance challenges.
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Mill Inerting Systems Presentation to the ASBCUG

Mill Inerting and

Pulverizer/Mill Explosion Mitigation

Richard P. Stormrichardstorm@innovativecombustion.com (205) 453-0236

2013 Annual Meeting | WWW.PRBCOALS.COM

Coal Mills are the Heart of a Coal Fired Plant

Maximum capacity, reliability and performance of your operation rely on the critical roles that your coal mills perform:

  • Conditioning coal for proper combustion. (Fineness, fuel distribution, throughput)
  • Delivering 100% of fuel to the boiler.
  • Profoundly influencing ability to generate power economically.

There are risks…..

  • Firing high-moisture & highly-reactive sub-bituminous coals exposes your plant to risks of:
    1. Unsafe working conditions, possible threat to worker safety.
    2. Damage & repair cost (Primary air inlet ducts, PA fans, mill internals, feeders, etc.)
    3. De-rates, forced outage, shut down.
  • Loss of availability of certain mills can cause slagging, problems with environmental compliance, high exit gas temperatures, non-optimum steam temperatures and other adverse consequences.
  • Downtime may extend to weeks or months, backup generation can be sparse or expensive.2013 Annual Meeting | WWW.PRBCOALS.COM

Mill inerting is regarded as the principal approach or engineering control to prevent mill fires & explosions

  1. People working in our industry use the term “inerting” loosely and sometimes incorrectly when describing systems that prevent mill explosions, puffs or fires.
  2. Systems often named as “inerting systems” by coal fired power plant personnel include:• Steam Inerting Systems
    • Water fogging or deluge systems
    • CO2 inerting systems
    • N2 Inerting (Not common in power plants)
    • Explosion Suppression Systems

Steam, CO2 and N2 Inerting Systems

  • Technicallyspeakingitisgenerallyacceptedthatachievingan “inert” environment inside a coal mill, the O2 level should fall below 14% O2.
  • Technically speaking, only Steam, CO2 and N2 (Rare) systems are true inerting systems.
  • Steam is the most common inerting media. To know with confidence that an inerting environment is achieved, an O2probe is needed. They’re easily plugged, high maintenance and/or unreliable.
  • Alternativeischaracterizationtestingtodetermineflowand time required to achieve an inerting environment and maintain it as steam condenses inside the mill.
  • Not effective at washing away coal residues, fines and dust that can be combustible or explosive.
  • Inertingsystemsareexpensive
    2013 Annual Meeting | WWW.PRBCOALS.COM

Water Fogging/Deluge

  • Waterfogginganddelugesystemsareessentiallymillinternal combustible dust suppression systems and they also suppress or extinguish burning or smoldering coal.
  • Themajorityofburningorsmolderingcoalmoreoftenoccurs under the grinding zone where the hot primary air flow enters the mill. This area is often described as the under-bowl, under-table, reject area, pyrite area or primary air inlet wind box depending on the type of mill. Fogging and deluge systems can be very effective at “washing” away hazards under the grinding zone. Flooding this zone is also effective at extinguishing burning coal.
  • Water fogging can be temperature driven to enhance effectiveness.

All of these systems can be effective in reducing the risk of mill fire/explosion or limiting damage (explosion suppression systems). Each has strengths and weaknesses.

Some plants have operated safely with and without these systems.

  • Coal characteristics profoundly influence risk.
    1. Lower rank coals are generally more easily ignited inside the coal mill.
    2. High volatile matter and highly reactive coals encourage pre-ignition inside the mill.
    3. Higher moisture content requires temperature extremes inside the mill.
    4. High moisture, high volatile matter and highly reactive coals are prone to self heating and can spontaneously ignite due to rewetting.
      • Rehydration is exothermic and dried coal that is stagnate anywhere in the coal mill can begin to smolder and burn.
      • Smoldering coal inside an idle mill can evolve into a big problem when the mill is started, agitating these accumulation combined with a high air to fuel ratio environment
  • PRB is one of the most difficult coals with respect to mill fires and explosions.
  • Mill internal temperatures are generally high enough somewhere to ignite any type ofcoal. Some bituminous coals ignite in the range of 315° to 350°F.

All of these systems can be effective in reducing the risk of mill fire/explosion or limiting damage (explosion suppression systems). Each has strengths and weaknesses.

• Some plants operate with and without these systems. Coal characteristics profoundly influence risk, PRB coal is one of the most difficult coals with respect to mill fires/explosions with greater magnitude explosions.

The Kst (Explosibility Constant or Deflagration Index) value for sub-bituminous coals are higher than for bituminous coals. Sub-bituminous coals have a higher rate of pressure rise if an explosion (deflagration) occurs.


Type of Coal



200 bar-m/sec


55-154 bar-m/sec


123 bar-m/sec

1 Options for Biomass Fuels Utilization in Power Plants, 2011, Don Koza 2 NFPA 68 Guide for Venting of Deflagration, 1998
3 Dust Explosions in the Process Industries, 1991, R.K. Eckhoff

Some units without these systems have prevented or managed fire and explosion risks by:

  • Mechanical Blue-printing and optimization (maintenance practices)
  • Proper and optimum airflow management.
  • Proper operation and plant staff awareness. Ability to identify common causes of mill fires and understanding of coal characteristics that promote fires.
  • Coal mill system design and configuration.Protective systems provide forgiveness and additional protection by addressing risks of mill fire explosion that develop that are not operator controlled or when maintenance and operation control measures fail.
  • High inlet or outlet temperatures
  • Foreign Material (trash, wire, rope, rags, wood, etc.)
  • Coal accumulations – below or above the grinding zone.
  • Abnormal conditions
  • Component failures; dampers, slide gates, feeder belts, etc.

• Feed interruptions caused by wet or frozen coal.

  • PRBisoneofthemostdifficultcoalswithrespecttomill fires and explosions.
  • PRBcoalsarehighmoistureandhighlyreactivecoals.
  • Utilizationofsometypeofprotectionsystemisprudent and widely accepted as a necessary best practice when firing PRB and other sub-bituminous coals.

An understanding of what causes mill fires and explosions is required for safe operation and optimal results with any type of system that inhibit mill fires or explosions.

  • It is important to remember that a mill fire, whether it be flaming, smoking or smoldering coal inside the mill means a mill explosion could be occur at any time.
  • Any burning or smoldering material inside the mill can evolve into an explosion because they provide an ignition source if the air to fuel (coal) ratio is elevated or higher than desired.
  • Air to fuel ratio inside the mill is always elevated during mill start-up, shutdown, mill trips and interruptions in raw coal feed.– Itswellknownthetheriskofmillexplosionishighestduringthese times of high air to fuel ratio.
    – Because of this, these events are the “triggers” that start mill inerting systems.

Most utility boiler coal mills do and should operate at primary air to coal ratios of 2:1 or less.

Coal Mill Air to fuel ratios are higher during:

  • Start-up.
  • Shutdown.
  • Interruptions in raw coal feed.
  • Lower feeder speeds.
  • This occurs because a minimum airflow through the coal mill must be maintained to insure that burner line velocities are not too low to allow settling of coal in the fuel lines or burner components.
  • Not uncommon to achieve air to fuel ratios of 3 to 5 pounds of air per pound of coal.

In the example below, which is typical, Mill air flow is held constant at 140,000 Lbs./Hr. at coal flows <78,000 Lbs./Hr. to maintain minimum burner line of 3,500 Fpm.


A common assumption is that at full load coal flows, air to fuel ratio us too low for pre-ignition of coal or below the explosive or combustible range inside the coal mill.

  • Coal and air is being actively mixed inside the mill and localized areas of high air to fuel ratio.
  • Air to fuel ratio can suddenly increase to explosive levels with an interruption in raw coal feed.


Hot Temperatures always exist inside the coal mill while firing PRB Coal

  • Sub-bituminous Coal is 15-30% moisture. PRB usually 25% to 30% moisture.
  • Very hot mill inlet temperatures are required to:
    • Dry coal
    • Achieve mill outlet temperatures.
    • Heat required is determined by coal moisture.
    • Heat supplied is product of the temperature and quantity of airflow at mill inlet.

135° to 145°F

150° to 160°F

500° to 700°F


Typical temperature inside the mill with Coal Moisture of 30% Higher Moisture = Higher temperatures and higher risk of mill fire


Typical temperature inside the mill with Coal Moisture of 3% Less Moisture = Lower Mill Inlet Temperatures
– despite the Higher Outlet Temperature

100,000 LBS/HR Coal X 3% Moisture
= 3,000 Moisture


Typical Temperature of a Pulverizer – Inlet Temperatures will high regardless of pulverizer manufacturer with high moisture coal.


130°F – 140°F

140°F – 160°F



Between 500°F & 700°F


  • Temperature and air-to-fuel ratio is very high under the bowl.
  • Rejected coal quickly dries and ignites.
  • Most common location of mill fires.

Air-to-Fuel Ratio by Design


Air-to-Fuel Ratio

Very High