I remember reading in a relatively recent issue of Coal Power Magazine that Powder River Basin (PRB) coal now accounts for about 40% of all the coal fired in the U.S. to produce electricity. Although lower in cost per Btu, PRB is unlike any other coal in that its easily crumbled, dusty nature requires special attention to its safe handling and storage; which can only be the result of thoughtful and deliberate attention paid to the details of operation.
Keeping in touch with some of my old firefighter brothers and now actively involved in the Power Generation Industry, we are learning – unfortunately through passed major industrial explosions involving the accumulation of combustible powder – that accidents have mainly occurred when operators underestimate, or dismiss entirely, the devastating potential of combustible particulate solids.
A mechanical failure within the conveyor system can cause enough heat buildup to start a coal fire. Inadequate lubrication in the bearing of a roller or the friction between a seized roller and the conveyor belt can buildup heat sufficient to ignite a coal laiden belt.
Although many such incipient fires may not have been as widely advertised as Russia’s recent Sayano-Shushenskaya hydro-electric castastrophy, they have occurred. (John Cowdrey will also point out that even this devistating catastrophy hasn’t hit the main-stream U.S. media.) Plant Operators throughout our industry speculate that the frequency of incidents may be rising with small contained fires occurring regularly at many plants.
To be explosive, coal dust must have a large enough volatile ratio (determined by laboratory analysis), a small enough particle size, and sufficient quantity. Coals with a volatile ratio greater than 0.12 are considered a dust explosion hazard. This category includes PRB and bituminous coals but not anthracite coals. Bituminous and anthracite coals burn, but only bituminous coals explode.
The National Fire Protection Association’s NFPA 654 standard (Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids) warns that only 1/32 of an inch of dust over 5% of a room’s surface area presents a significant explosion hazard and makes the following recommendations:
-
Minimize the escape of dust from process equipment and ventilation systems.
-
Use dust collection systems and filters.
-
Prefer surfaces that minimize dust accumulation and facilitate cleaning.
-
Provide access to all hidden areas for inspection purposes.
-
Inspect for dust residues in open and hidden areas at regular intervals.
-
Clean dust residues at regular intervals.
-
Use cleaning methods that do not generate dust clouds, if ignition sources are present.
-
Only use vacuum cleaners approved for dust collection.
-
Locate relief valves away from dust hazard areas.
-
Develop and implement a formal program for performing hazardous dust inspections, testing, housekeeping, and dust control using defined methods at defined intervals.
Informative post on the combustible dust fire and explosion hazards of Powder River Basin (PRB) coal. Solely relying on the hazardous classification of 1/32 of an inch of dust is a sure path to disaster. If you leave footprints behind then it’s time to start cleaning or catastrophe is soon to follow. Not mentioned in much of the public literature is bulk density, this is the key. Think of bulk density of solids like vapor density in flammable gases and liquids. The heavier the density then the more likely a facility will have hazards present close to ignition sources. In 2008 the Combustible Dust Policy Institute found through media accounts, over 150 combustible dust related fires and explosions throughout industry.
John,
The fuels handling team at one of our facilities adamantly subscribes to the “no visible footprints” logic in their proactive practices. One of the other facilities, however – not so much. FYI: This pulverized fuel is also quite commonly used in the cement industry for injection-fueling their kilns.
As someone in your position obviously realizes, this article is little more than a primer – the tip of the proverbial iceberg if you will – when it comes to discussing the management & control of this particular particulate hazard. We’ve seen footage where an unrelated primary explosion in a facility caused the accumulated PRB dust to enter into suspension, creating an exponentially larger, secondary explosion of catastrophic proportions.
With the implementation of the Federal Clean Air Act Amendments of 1990, Industry saw many facilities re-engineer to make the switch from higher-sulfur coals to the low-sulfur, sub-bituminous Powder River Basin coal for its reduced sulfur dioxide and NOx emissions. I believe for this reason combined with the many economic benefits – while they last, since demand continues to grow – that we will be visiting with this issue for many years to come.
While some of the hazards such as Oxidative Weathering are managed though specific practices (i.e. Compaction), others such as keeping the bulk density within safe limits – as you’ve brought up – can effectively be managed by good, old-fashioned housekeeping practices; but sadly & all too often are not.
Thank you for your interest and comments.
Respectfully,
John