Fire risk from solid fuel commercial cooking

Creosote joins grease as flammable fuel in kitchen exhaust systems, creating high risk for fires that can start in ducts above conventional fire-suppression-system detectors.

12/08/2014


Learning objectives

  • Understand the characteristics of creosote deposits and related fire risk in commercial kitchen exhaust hoods and ducts.
  • Identify issues related to creosote and grease fires, including ignition, spread, detection, and likelihood of suppression.
  • Know recommendations for improved fire prevention, detection, and suppression though new technologies, improved equipment, compliance with codes and standards, and owner/employee practices.

Though codes and standards specify Type I hoods and approved automatic fire suppression systems for cooking operations that emit smoke and grease, recent fire reports suggest that risk of fires from solid-fuel cooking, mostly using wood, are not fully understood by commercial kitchen designers, engineers, and owners. Important findings are that solid fuel cooking often leads to flammable creosote deposits in exhaust systems and conventional fire suppression systems sometimes do not extinguish resulting fires. Because solid-fuel cooking is increasingly common in restaurants, related fire risks are increasing. Prudent design requires more robust construction, full code and standards compliance, state-of-the-art fire suppression systems, and other preventive measures.

Gas flame display at entrance to restaurant with solid fuel cooking. Courtesy: Doug HortonCreosote: A highly flammable product of solid-fuel cooking

The role of grease as fuel in commercial kitchen fires is well known. Solid-fuel cooking with wood causes creosote to deposit in hoods and ducts as additional fuel. Creosote is well known for its fire threat in residential chimneys above wood-burning fireplaces, and it's also a growing cause of restaurant fires. What is creosote? According to online source InspectAPedia:

"Creosote is a black, oily wood-tar condensate...that forms inside of chimneys and flues when burning wood[...].Creosote tar is carried into the chimney as a vapor[...] but this creosote tar vapor condenses as a liquid onto the cooler chimney sides as smoke passes up the chimney and flue. As the creosote deposits cool they harden to a shiny black coating that can be difficult to remove, especially if the creosote deposits are left over a period of time. Creosote will continue to accumulate on chimney surfaces until it is removed[...]. Once ignited, the deposits of creosote burn at very high temperature, so hot, in fact, that depending on the amount of creosote that is burning, a runaway...fire occurs, making a sound like a roaring freight train."
 
Thus, solid-fuel cooking adds creosote as a source of flammable fuel for fires in commercial kitchen hoods and ducts, in addition to grease. The combination of creosote and grease in exhaust hood plenums and ducts is assumed to ignite and burns hotter than creosote alone.

Extremely low flash point

The most surprising property of wood-based creosote is its extremely low flash point-the lowest temperature at which a liquid or solid gives off vapor that can form a flammable mixture with air. According to the New Jersey Dept. of Health's report, the flash point of wood tar creosote is 165 F. (Yes, that low.) Combustion at the flash point requires a source of ignition, and with solid-fuel cooking the source of ignition can be airborne sparks or embers. If the temperature is much higher, on the order of 600+ F, creosote can auto-ignite.

Creosote fire risk comes with appliances that burn any form of wood, generally to add flavor, such as charbroilers, rotisseries, pit barbeques, ovens (including brick ovens), and smokers. Some appliances burn wood and natural gas simultaneously. Wood fuel is available in various species, shapes, and sizes: chips, bricks (compressed chips and sawdust), and logs. In some cases, wetted wood is used, which burns at a lower temperature and is known to produce more creosote.

Recent fires from high-risk solid-fuel cooking

Reports of fires with solid-fuel cooking confirm that creosote increases risk, and surprisingly, conventional fire suppression systems are being reported as not detecting or extinguishing related fires in hoods and ducts. Below are brief descriptions of three recent fires from solid-fuel cooking that highlight risk issues.

Wood-fired rotisserie above which fire started in hood and duct. Courtesy: Doug Horton1. Duct fire above wood-fired rotisserie: This fire ignited in the exhaust duct over a wood-fired rotisserie in a full-service restaurant. After the fire, a service technician assessed the condition of the water-wash hood and associated wet-chemical fire system and found several problems. Most significantly, the hood's water valves had been closed because of leaks above the ceiling.

With water valves shut, the continuous water mist in the hood was disabled and unable to quench airborne sparks and burning embers from the wood fire. These in turn ignited grease and creosote in the duct above the conventional suppression system's fusible links detector, and the suppression system did not activate. The duct fire was extinguished by workers with a portable extinguisher and confirmed out by firefighters. Though damage to the building was not extensive, there was significant cost for restoring operation of the older water-wash hood and fire suppression system.

2. Fire in exhaust duct: Fire occurred in the exhaust duct above a solid-fuel charbroiler. The fire spread quickly from the hood to the duct. The conventional wet chemical fire-suppression system activated but failed to extinguish the fire. Firefighters needed to disassemble the exhaust fan and cut into the duct to extinguish the fire. As reported in a local newspaper, there was creosote buildup in the wood-burning grill system. The fire damaged the restaurant's hood system, attic, and roof. There was water damage in the main dining room from fire hoses.

The damaged equipment was replaced with state-of-the-art components: a new hood with an electronically controlled and monitored fire-suppression and hood-cleaning system connected to an unlimited supply of a water/surfactant solution; factory-built, listed ductwork; and a newly designed curb-mounted "utility set" fan. The replacement equipment decision was made in concert with the ventilation-system supplier, which, according to some, for solid-fuel cooking will only sell state-of-the-art equipment.


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