Contributor: Daryl Bessa, President of F.E. Moran Special Hazard Systems
Writer: Sarah Block, Marketing Director of The Moran Group
Powder River Basin (PRB) coal has given coal-fired plants an environmentally conscious, inexpensive alternative to traditional bituminous coal since the 1980s. The lower NOx and SO2 in PRB coal reduced power plant emissions, decreasing pollution, and appeasing the 1990 Clean Air Act. At the same time, the low cost and availability made PRB coal not only a viable option, but a fuel of choice. While PRB coal was the predominant emission efficient energy source in 1990, in 2012, it no longer reins as the most environmentally friendly fuel. With the current government crack-down on coal-fired power plants, the existing plants will likely be the last.
Coal-fired power plants are extremely volatile. After all, there is a reason the industry saying is, "It isn't if a fire occurs, but when." With the prospect of aging coal-fired power plants and their propensity for combustion, it is essential to understand their fire hazards and insure the fire protection preparedness of coal-fired power plants with extensive fire protection solutions.
What Causes Coal-Fired Power Plant Explosions?
Throughout a twenty-five year (1980-2005) study of PRB coal-fired power plants, there were an average of 11 fires or explosions, 29 injuries, and 5 deaths per year. Another study conducted by the United States Department of Labor during the 1996-2009 time period noted 437 workplace coal power-related deaths, averaging 33 deaths per year in the United States. To understand what fire protection is necessary to guard against mishaps, it is crucial to first understand why explosions occur.
Coal Dust Hazards
For a fire to occur, the fire triangle needs to be present - oxygen, fuel, and heat. An explosion happens when two other elements are added to the equation - dispersion of dust and confinement of dust, as shown in diagram A. Oxygen and fuel cannot be avoided in a PRB coal-fired power plant, but the heat source can originate from several different sources. A common cause is the conveyor belt. As the coal is being transported from storage to use, the coal-dust begins to fall off the belt and accumulate. Once the dust accumulates to 1/32 of an inch, or about the breadth to leave a footprint, it becomes a fire hazard. NFPA 654 defines combustible dust as, "any finely divided solid material that is 420 microns or smaller in diameter and presents a fire or explosion hazard when dispersed and ignited in the air." If a conveyor belt is not in impeccable condition, and one moving part stops, the friction can create a heat source for combustion. Other causes of heat are friction through mixing operation, electrical shortage, tool usage, or storage bin transfer. The fire triangle is difficult to avoid.
Two additional elements are added to the fire triangle to create an explosion. The dispersion of dust happens naturally as the coal is being moved. The sub-bituminous coal is high in oxygen and moisture, making it more susceptible to deteriorate into powder than standard bituminous coal. It easily creates a dust and disperses over pipes, conveyor belts, floors, ceilings, and machinery. The confinement of coal dust happens just as easily. The dust spreads in unseen areas, like coal silos or chutes. A Kansas City coal-fired power plant witnessed this first hand when, on April 4, 2012, an explosion rocked the plant. Dust accumulated in a chute, completely unseen, and caused the fire. Often, it is the hidden dust that causes the devastation, carrying the explosion or causing secondary explosions throughout the plant.
Oil Fire Hazards
Coal dust is not the only cause of fires in a PRB Coal-Fired Power Plant. Both the turbine and transformer are insulated by oil, making them flammable. There are three different types of oil fires that can take place in or near the turbine or transformer: spray, pool, and three-dimensional. Spray fires happen when highly pressurized oil is released; 50% of the time, this fire happens because of malfunctioning bearings. If there is an unpressurized leak, plants could see a pool fire when the oil catches fire after it has accumulated on the floor or a three-dimensional fire if it catches fire while flowing downhill.
Hydrogen Fire Hazards
Hydrogen cools generators in coal-fired power plants. Hydrogen is an invisible threat with the capability to catch fire and/or explode. The gas is odorless, colorless, tasteless, and the flames are invisible. It will not be detected without the use of hydrogen sensors. Fire fighting should not commence until after the hydrogen source has been shut off. If hydrogen is still present, it is likely to re-ignite or explode.
What can I do to reduce the likelihood of an explosion?
The key to reducing the probability of a coal-fired power plant fire or explosion is preparation. Fires generate from several different sources: coal dust, oil, or hydrogen. It is necessary to be knowledgeable about fire ignition in order to avoid it. The main causes of plant fires and explosions are coal dust, equipment error, and human error. Training plant personnel on proper housekeeping and machinery maintenance along with proper fire protection will greatly reduce the chances of a fire or explosion.
Without a stringent housekeeping regimen, even the most advanced fire suppression system will not be able to stop an explosion from happening. A documented housekeeping routine is necessary to reduce the odds of a fire or explosion. According to the Mine Safety and Health Administration, with a robust housekeeping schedule, the fuel source would be eradicated, eliminating secondary explosions. Secondary explosions have the largest death toll of all coal-fired power plant combustions.
Dust collectors alone will not adequately dispose of dust; in fact, 40% of fires and explosions were caused by the dust collectors. An effective option is to wet the dust to weigh it down so it does not float into hidden crevices. Because the dust is microscopic, microscopic water spray must be used. Plants should use a wash down system to keep coal dust at a minimum. Industry surveys have shown that plant personnel who have utilized wash down systems have been happy with the results.
During an outage, it is essential to clear dust completely from bunkers, silos, and conveyor belts. Idle dust can explode. When preparing for the outage, wash down all walls of the bunkers or silos to eliminate the source for explosions.
If dust cannot be completely cleared, another option is to pump carbon dioxide into a sealed bunker or silo. The carbon dioxide would eliminate the possibility of dust combustion by taking away its oxygen.
A bunker or silo should be designed as if a fire is imminent. Access points should be installed on several levels to allow for entrance of fire extinguishment tools. It is important for the water to directly contact the source of the fire in a bunker or silo. Another design choice that will reduce the chances of a fire or explosion is a cone shaped floor or a free flow bottom cone. Many bunkers or silos have a funnel-flow pattern that occurs when the walls inhibit the coal from flowing freely. Most coal will flow down the center, while the remaining coal that has accumulated on the sides will linger stagnantly. Stagnant coal can create a heat source. The key to reduce the likelihood of a bunker or silo fire is in the design.
What are my fire protection options?
Several different detectors are needed throughout the facility, depending on the location. Silos, bunkers, and dust collectors are at a high risk for explosions due to the congregation of PRB dust. It is necessary to choose the correct detection device. Carbon monitors, infrared scanning, temperature scanning, or linear heat detectors are adequate options. Linear heat detectors, such as Protectowire, can detect heat along a length of space, instead of a singular spot. This works extremely well along conveyor belts, which are a major fire hazard because they easily create heat through movement or from idler or roller bearing failure.
Fire Suppression Systems
Sprinkler systems must be installed throughout a plant. The main fire culprits are silos, bunkers, conveyor belts, crusher buildings, dust collectors, coal pulverizers, turbines, generators, and transformers as seen in Diagram B. Hazard location will determine the best system type. Temperature controlled locations are best protected by a wet-pipe system. Non-temperature controlled areas need a dry-pipe system to avoid frozen pipes. Transformers and other areas where quick suppression is important and water damage is not a concern are effectively protected by deluge sprinkler systems.
Three main suppressants dominate coal-fired power plants: water, CO2, foam and/or f500 solutions. It is essential in coal dust-related bunker/silo fires to use a piercing rod or inerting system to smother the fire at its source. In all other areas of a plant, various types of sprinkler systems will effectively suppress fires.
An integral part of finding a solution to fire protection is choosing a company with experience and expertise to implement a comprehensive system. Fire protection providers must have the design capability to plan custom solutions for site obstructions and plant nuances. Each fire susceptible location of a plant must have a fixed sprinkler system that is designed specifically for that area. High value - high risk facilities are vastly more complicated than other industries; a fire protection solution provider should be experienced in providing fire protection for plant environments to ensure solutions that are suitable for the specific application. With proper housekeeping schedules, diligence, and fire suppression systems, the safety of people, plant, and production is greatly increased.