Smoke and combustion gases

Smoke is a by-product of most fires caused by the incomplete oxidation of the fuel supply during the chemical process of combustion. It accounts for a large majority of fatalities of from fire incidents at both onshore and offshore petroleum facilities. In the Piper Alpha incident of 1988, probably the worst petroleum industry offshore life loss incident, the majority of deaths were not from bums, drowning or explosion impacts but from smoke and gas inhalation. The report on the incident concluded that, of the bodies recovered from the incident, 83% were as a result of inhalation of smoke and gas. Most of these victims were assembled in the accommodation awaiting evacuation directions or as they may have thought - a possible rescue. 

Smokes from hydrocarbon fires consist of liquid or solid particles of usually less than one micron in size, suspended in the combustion gases, which are primarily nitrogen, carbon monoxide and carbon dioxide, existing at elevated temperatures. At normal temperatures carbon is characterized by a low reactivity. At high combustion temperatures, carbon reacts directly with oxygen to form carbon monoxide (CO) and carbon dioxide (CO2). 

The precise technical name of HCN is Hydrocyanic Acid. The cyanides are true protoplasmic poisons, combining in the tissues with the enzymes associated with cellular oxidation. They thereby render the oxygen unavailable to the tissues, and cause death through asphyxia. Inhaling concentrations of more than 180 ppm of HCN will lead to unconsciousness in a matter of minutes, but the fatal effects would normally be caused by carbon monoxide poisoning after HCN has made the victim unconscious. Exposure to HCN concentrations of 100 to 200 ppm for periods of 30 to 60 minutes can also cause death. 

Inhaling hot (fire) gases into the lungs will also cause tissue damage to the extent that fatal effects could result in 6 to 24 hours after the exposure. 

Psychologically the sight and smell of smoke may induce panic and disorientation. When this occurs movement of personal to achieve evacuation objectives may be severely inhibited. This is especially critical where personnel are unfamiliar to the facility. Smoke will also hinder fire fighting and rescue efforts. 

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Fire propagation. Combustion is initiated also in adjacent systems, by virtue of heat evolved in the fully developed fire. For this reason, the most important factor in fire-spread, besides the flammability and amount of plastics in the system, is the fire endurance of the system boundaries. Evolved heat, produced smoke and combustible gases are also as important as in the other stages. 

The toxicity of smoke and combustion gases has been comprehensively studied in many countries and is discussed later in this Chapter. 

The smoke and combustion gases are drawn to a sampling point, where the smoke measurement is made with a low intensity helium-neon laser beam projected across the diameter of the duct. The smoke data are reported as the specific extinction area. This is defined as the area (m ) of the smoke generated per mass (kg) of specimen decomposed thus the units are m /kg. Specimens used as 100 mm x 100 mm and up to 50 mm thick. The heat flux can be varied from 1 to lOOkW/m, with horizontal or vertical specimen orientation. 

Formaldehyde is a harmful compound released from walls and furnitures in new houses because adhesives containing HCHO are often used in constmction materials. In addition, H CHO is emitted by tobacco smoke and combustion exhaust gases. Long exposure to HCHO causes serious health problems called sick house diseases. In Japan, the concentration of HCHO in indoor air is regulated [54] to under 0.08 ppm based on the recommendation of the World Health Organization (WHO). 

Hot gases rise by thermal lift. Hence in the open air they will disperse. Within buildings this is a serious cause of fire escalation and toxic/asphyxiation hazards if smoke and hot gases are able to spread without restriction (or venting) to upper levels. A balanced flue can serve to effectively isolate a combustion process in a gas-fired appliance, but must be sound in construction and unrestricted to avoid leaks. 

Security and safety Intruder alarm Security systems Fire detection systems, with sensors for - temperature - toxic gases like CO, C02, exhaust gases, smoke, etc. - combustible gases like CH4, C2H6 flame detection, fire detectors, caravans with gas detectors, etc. 

Pryor, A.J., D.E.Johnson and N.N.Jackson. 1975. Hazards of smoke and toxic gases produced in urban fires. Combust. Toxicol. 2 64-112. 

Research in the field of combustion toxicology is primarily concerned with items 1, all of which are related to the toxic potency of the fire gas effluent. Toxic potency is defined by ASTM as a quantitative expression relating concentration (of smoke or combustion gases) and exposure time to a particular degree of adverse physiological response, for example, death on exposure of humans or animals. This definition is followed by a discussion, which states, The toxic potency of smoke from any material or product or assembly is related to the composition of that smoke which, in turn, is dependent upon the conditions under which the smoke is generated. One should add that the LCso is a common end point used in laboratories to assess toxic potency. In the comparison of the toxic potencies of different compounds or materials, the lower the LC50 (i.e., the smaller the amount of material necessary to reach the toxic end point), the more toxic the material is. 

Smoke and toxic gases are responsible for more fire deaths than the effects of burns. The major toxic hazard is carbon monoxide, produced by incomplete combustion of hydrocarbons. Nitrogen-containing polymers (polyamides, polyurethanes, polyacrylonitrile) can produce hydrogen cyanide. 

Decomposition The chemical bonds of the polymer are progressively broken and decomposition products are formed (residual char, various liquids, smoke, incombustible and combustible gases). 

One of the most important parameters that can be nsed to characterise a fire is the rate of heat release. It provides an indication of the size of the fire, the rate of fire growth and consequently the release of smoke and toxic gases, the time available for escape or suppression, the type of snppressive action that are likely to be effective and other attributes that define the fire hazard. Methods based on the oxygen consumption principle are now available to measnre the rate of heat release reliably and accurately. The principle depends upon the fact that the heats of combustion of organic materials per unit of oxygen consumed are approximately the same. This is because the processes in the combustion of all these prodncts involve the breaking of C-C and C-H bonds (which release approximately the same amonnt of energy) with the formation of CO2 and water. 

An aerosol is a suspension of either a solid or a liquid in a gas. Fog, for example, is a suspension of small liquid water droplets in air, and smoke is a suspension of small solid particulates in combustion gases. In both cases the liquid or solid particulates must be small enough to remain suspended in the gas for an extended time. Solid aerosol particulates, which are the focus of this problem, usually have micrometer or submicrometer diameters. Over time, solid particulates settle out from the gas, falling to the Earth s surface as dry deposition. 

Products of Combustion Heat, hght, smoke, and asphyxiating and toxic gases are produced by fire. In a hot, well-ventilated fire, combustion is usually nearly complete. Nearly all the carbon is converted to carbon dioxide, all the hydrogen to steam, and oxides of various other elements such as sulfur and nitrogen are produced. 

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