Contributor: Daryl Bessa, President of F.E. Moran Special Hazard Systems
Writer: Sarah Block, Marketing Director of The Moran Group
With our world's ever growing energy demands and the pollution produced by the combustion from fossil fuels, we have begun to "re-discover" the usefulness of wood and biomass as energy sources. A major advantage of biomass energy as a source of fuel is it is renewable. With approximately 140 billion metric tons of biomass produced every year, there is a trend in this fuel type being utilized in lieu of fossil fuels as coal burning power plants are being replaced more often due to EPA regulations, increasing the demand for a cheap, efficient renewable fuel source.
What causes biomass fires and explosions?
Self-heating Fuel Piles
Biomass fuel has a wide range of possible refuse items: pellets, chip logs, forestry, sewage sludge, methane, meat and bone, palm kernels, cereal, sawdust, bioenergy crops, or landfill gas. When a biomass fuel is stored in a pile, waiting for transport or use, the biomass can spontaneously heat through oxidation. In order for this to happen, three conditions must sync: rate of heat generation, air supply, and insulation properties of the immediate surroundings. With most biomass material, there is a high moisture content combined with air and/or bacterial fermentation - both of which can cause spontaneous combustion through oxidation.
Oil Fire Hazards
Oil acts as an insulator in both the turbine and transformer, making both susceptible to fires. Three different oil fires are possible on or near the turbine and transformer: spray, pool, or three-dimensional. Spray fires are likely with a pressurized leak. 50% of spray fires in plants are due to malfunctioning bearings. Unpressurized leaks result in pool fires. Pool fires happen when oil accumulates on the floor and then catches fire. Three-dimensional fires can occur when the fire ignites while it is flowing vertically out of a turbine or transformer.
Biomass Fuel Dust
Biomass plants are as prone to explosions as a coal-fired power plant. The dust that plagues a coal plant also infests a biomass plant, but with an additional issue. The dust can be made of a multitude of refuse items, creating the problem of an unknown fuel source, making it more difficult to determine the best fire suppressant, increasing the need for an experienced fire protection contractor. During handling and storage, the various fuel sources - wood pellets, chips, or other refuse - can break down, producing fine, combustible dust.
For a fire to ignite and thrive, the three elements of the fire triangle must be present: oxygen, fuel, and heat. Without one of these elements, the fire could not live. Explosions occur when two more elements are added to the fire triangle, creating the explosion pentagon - disbursement and confinement of dust.
Dust clouds can become confined in bucket elevators, drag chain conveyors, drying and dust extraction systems, mills, and storage silos. It is necessary to not only protect the plant with strong fire protection systems, but to also minimize the dust to reduce the likelihood of an explosion.
What can I do to reduce the likelihood of a fire/explosion?
The key to preventing explosions and fires is preparation. The main causes of plant fires and explosions are dust, equipment failure, and human error. In order to reduce risk, plant owners should have a strict housekeeping regimen to reduce dust. Additional preventative measures include regularly inspecting equipment, training plant personnel on fire hazards, and installing and maintaining proper fire protection throughout the plant. Following these steps will greatly reduce the likelihood of a fire or explosion.
Without a stringent, documented housekeeping routine, even the most robust fire protection system will not prevent an explosion. Biomass power plants should utilize wash down systems and sweep regularly to clear dust build up. Industry surveys show that plants that regularly use wash down systems are satisfied with the results.
Dust explosions have a tendency to cause secondary explosions. The initial explosion frees more dust particles, sending them into the air, causing a secondary explosion. Secondary explosions have the highest death toll of any other type of plant conflagration.
When preparing for an outage, plant personnel should completely clear the fuel source and all dust from bunkers and silos to eliminate the source of an explosion. In addition, a thorough wash down should take place. If all dust cannot be eradicated from the silo or bunker, inert Carbon Dioxide into the sealed space to remove the oxygen.
Silos and bunkers are a common area for fires to ignite due to the collection of combustible materials. To make extinguishment as easy as possible, silos and bunkers should have access points installed on several levels to allow extinguishment tools entrance. Another design choice that may reduce the likelihood of spontaneous combustion is installing a cone shaped or free flow bottom cone floor.
Many silos and bunkers have a funnel-flow bottom that allows the fuel source to flow down the center; however, stagnant fuel can accumulate on the sides. When fuel accumulation occurs, it can cause friction, resulting in a heat source. Designing the silo or bunker in a way that promotes free flow will reduce the chances of a fire event.
What are my fire protection options?
In order to protect the entire biomass facility, several different detectors are needed throughout the plant. Carbon monitors, infrared scanning, temperature scanning, or linear heat detectors are options that can be used throughout the plant. Linear heat detectors will detect heat along a length of space, working well along conveyor belts, which have a tendency to catch fire from idler/roller bearing failure.
Spark detection systems protect chutes from igniting. Spark detectors can sense a spark before it has the opportunity to create an explosion. This system is composed of a control panel, detector, and extinguishing assembly. Spark detectors can sense a spark at speeds of 5,000 feet per minute. The system is mounted upstream from the extinguishing equipment, allowing the extinguishing equipment to suppress the spark before it grows into a larger problem.
Fire Suppression Systems
Sprinkler systems should be installed throughout the biomass plant. Silos, bunkers, conveyor belts, crusher buildings, dust collectors, pulverizers, turbines, generators, and transformers are most susceptible to fires. Hazard location will determine the best fire suppression option. Temperature controlled areas are adequately protected by a wet-pipe system. Non-temperature controlled areas need dry-pipe sprinkler systems to avoid potential freezing. Areas that are outdoors and have hazards that require quick suppression without the fear of water damage should use a deluge sprinkler system.
Three main suppressants are generally used in biomass applications: water, CO2, foam and/or f500 solutions. In bunker/silo fires, a piercing rod or inerting system should be used to smother the fire at its source, using the strategically placed access points. In all other areas of the plant, water or foam will effectively extinguish fires; it is at the discretion of the plant and fire protection contractor to choose the best suppressant for the application.
Due to the non-structured fuel-type, protecting biomass plants can be complicated. Working with a qualified contractor with experience in Biomass Power Plants will keep plant personnel, property, and production safe.