19 Nov 2012
By Rob Cruickshank
Wood pellet production in North America is predicted to double over the next five years, according to The Biomass Energy Resource Center (www.biomasscenter.org). With our growing interest in renewable energy sources, it’s really no surprise. Wood and other biomass pellets are a sustainable source of power that can reduce our reliance on fossil fuels, such as oil.
Wood, much like other forms of biomass, is now often pressed into pellet form. Because of their shape, density, and low moisture content, pellets are not only more easily to transport, but they also burn quite efficiently and can be used for heating or electricity generation. Costing about 60% less than heating oil biomass, pellets offer an economical and one-of-a-kind opportunity for industries to incorporate renewable energy solutions in their operations.
As this booming industry continues to gain momentum, however, there are a number of safety concerns that require attention, including the risk of fire and explosion. The combination of wood and heat in production make fires in biomass pellet plants increasingly common. The good news is that the appropriate fire protection system can have a significant impact on mitigating these risks.
Playing it Safe
Here are a few common fire hazards and tips for reducing the associated risks…
• Furnace Unit
Ensure there are sprinkler heads over the furnace burner heads to protect against fuel spills that could ignite and cause an explosion. If the furnace unit includes a fossil fuel burner head, it must include proper combustion controls such as flame-failure detection, air-purge controls, and fuel double-valve isolations.
• Utility Systems
Cooling towers can pose a fire risk if wooden construction loses moisture during a maintenance period. The cooling tower should be made of non-combustible steel or concrete. And, it should be separated by either a spatial separation of approximately 15 meters, or by a four-hour firewall. Automatic fire-protection systems are also recommended.
Additionally, oil-filled transformers, which are usually part of turbine generators and older units, are more hazardous than dry resin units. Therefore, the oil is more likely to burn or explode. It’s recommended that fire protection systems include blast walls or, in some cases, appropriate spatial separation. A complete water spray impingement on all exposed exterior surfaces should provide a water density of 10.2mm/m².
Diesel generators can catch fire, too, often because of crank failure. It’s advisable that the generator is installed in a location where a potential fire could easily be contained, such as in a fire-resistant enclosure.
Fires in electrical, control, and cable rooms are often isolated and might only result in small material losses. But, even small fires can have a negative impact on business operations. To prevent small-scale fires from happening, fire protection systems should always include adequate compartmentalization of cables and installation of cable sealing through the walls. The material used for sealing should be fire-resistant cement. Furthermore, inspections using a thermographic camera are important to perform annually, so as to pinpoint hot spots on cables or in electrical cabinets.
Air-aspirated early warning smoke detection systems are an option for control and electrical rooms. This type of system is very effective at detecting smoke particles before a fire fully develops, giving an operator the chance to extinguish a potentially devastating fire before it starts. Automatic sprinkler systems, designed to a density of 12.2mm/min, are also recommended for high-value cable rooms. Fire extinguishers located in strategic areas of these plants provide further protection.
Fuel gas handling systems are vital to the combustion process, and flammable components are usually minimal. If the incineration operation and the fuel gas handling system have been properly designed, users shouldn’t experience any problems.
• Waste Reception & Handling
Often stored near the incineration plant, large quantities of solid or liquid fuel can ignite as heat accumulates. The easiest way to prevent this is to ensure that the fuel bunker is constructed using fire-resistant, steel-reinforced concrete, and is located more than 15 meters away from the central processing building. These precautions ensure that a fire in the fuel bunker will cause minimal localized damage, and won’t spread to the main processing plant. Operating procedures and stock control measures should include turning the biomass fuel over at least once a week to keep the fuel aerated and cooled, which reduces the risk of self-ignition.
Shredders are also at risk for explosion caused by dust, so these and their enclosures should be designed to resist back-ups in the chute where waste can soak up water and swell.
• Steam Turbine
The lube oil to maintain the temperature and conditions of the bearings can spray over hot steam turbine casings and cause a fire. Isolating the lube oil in the turbine is often what causes the most damage, but installing fire-resistive cables for the electrical lube oil supply pumps is one way to reduce risk. Other methods to decrease lube oil sprays are to use pipe standards that include minimum flanged pipelines, spray guards over flanges to control any lube oil spray, and installing high-pressure (supply) lube oil pipelines within low-pressure (return) lube oil pipelines. Areas underneath the turbine generator operating floor, which receive oil flow, spray, or accumulation, should be safeguarded with an automatic sprinkler or foam-water sprinkler system.
Overall Construction Standards
Using non-combustible construction materials and composite panel insulation are first choices when it comes to wood and biomass pellet production plants. Composite panels should have a minimum guarantee of two hours of fire resistance, and be made with mineral wool rather than fire-retarding foam, which can be highly toxic. Walls separating fuel storage facilities from the rest of the plant should be concrete and provide at least two hours of fire separation.
Overall, fire prevention strategies in pellet production plants are two-fold: passive fire protection for containing and slowing the spread of fire; and active fire protection including automatic fire sprinklers or gaseous fire suppression.
In all cases, an insurance broker and one’s insurance company will be able to provide further advice on how to minimize fire risks, and can work with an organization to create a fire risk control plan that best suits the facility’s needs.
Rob Cruickshank is the practice director for Construction & Renewable Energy at RSA Canada.