An Analysis Of Cigarette Ignition Environmental Sciences Essay

An Analysis Of nates Ignition Environmental Sciences Essay hydrogen or deuterium float when heterogeneous with air or oxygen forms a highly inflammable mixture over a wide dress of proportions they also form flammable mixtures with chlorine and the oxides of nitrogen, further, they provide also react spontaneously with fluorine and chlorine trifluoride.Because it is impossible to guarantee that any system bequeath be completely leak free every effort should be made to exclude all sources of excitation. The designer and the user should give careful rumination to the risk from a wide variety of dismission sources e.g. smoking, flames, intense surfaces, electrical and other sparking, static electricity, shock, impact, catalytic and chemical action.,Thus petrol vaporisation mixed with air has a lower flammability limit of just over 1% and an upper limit of 6% by volume petrol vapour in air, at normally encountered temperatures. Concentrations below the lower limit are said to be lean mixtures and those above the upper limit rich mixturesFlammabilityFor flammable liquids with cremate point temperatures above normal ambient, e.g., kerosene, white genius and diesel oil, an ignition source has to ignite not only the flammable mixture of fuel vapour but to generate this mixture in the first place by heating the bulk liquid.Ignition delayA further factor in the mechanism of ignition of splashes and blue devils is the ignition delay time or induction period, that is the time period between bringing a potentially flammable mixture to a condition where it will ignite,Ignition delay times are dep abateent on temperature and are reduced with an increase in temperature.Cigarette composition and combustionCompositionThe most commonly encountered, manufactured buttocks consists of a cylindrical jam-packed bed of 1 g of shredded baccy enclosed in paper and is generally 8 mm in diameter and 65 mm to 85 mm hanker. The strands of baccy are non-uniformly packed and the volume of the cigarette consists of about 75 percent free space. Often a smoke filter is attached and this is typically 20 mm long and contains cellulose acetate, paper fibres and sometimes charcoal.The Virginia tobacco typically used in the manufacture of British cigarettes contains less than 0.1 % of nitrates.One obvious difference in their electrocution characteristics is that hand-rolled cigarettes persist to go out unless puffed by the smoker, whereas a cigarette with tightly packed tobacco will smoulder for about twenty minutes with no assistance from the smoker. The experimental work reported here and the backchat relates to manufactured cigarettes.The smoke itself consists of mainstream smoke, from the maximum gas phase temperature (850C) was at the centre butt end of the cigarette during suction by a smoker, and. sidestream smoke during the smoking cycle. The term used here for the suction or drawing process during smoking is smoke of the cigarette. One puff of a ci garette is, there- the fore, one suction or draw by the smoker.There are two main expanses at the purpose on the coal, namely the combustion zone A and the pyrolysis and distillation zone B. Combustible blues are produced in zone B prior to ignition in zone AIn the interval between puffs, the natural convection flow of air around the combustion coal in an upwards circumspection sustains burning, and forms the sidestrearn smoke.Baker 33 employed two different methods of measurement, one for the solid phase and one for the gas phase, and his results are generally accepted as being legate of the temperatures encountered inside(a) a burning cigarette tip.smoking machine, taking a 35 cm3 volume of 2 sec duration, once per minuteof both the gas and solid phases, were determined. These temperatures can, therefore, be regarded as approaching the maximum attained during the drawing of a cigarettesurprisingly, the maximum solid phase temperature in the area region of 900C-950C was attai ned at the points of maximum air flow, i.e., 0.2 mm to 1 mm forward of the paper burn line. This is the area of the cigarette where a distinguishable pale red glow can be seen when a cigarette is puffed. The maximum gas phase temperature (850C) was at the centre of the cigarette coal.Ignition of gases and vapours by hot surfacesAlthough the tip of a cigarette cannot be treated simply as a hot surface in a discussion about its potential to ignite substances, it is useful to consider such an ignition source and the combustion controversys involved. Powell has reviewed much detailed experimental work on hot surface ignition 42, and the results provide an insight into some of the reasons for ignition or otherwise of flammable gas and vapour mixtures by a lighted cigarette. nigh of the factors involved in the ignition of flammable mixtures of vapour or gas by hot surfaces are temperature of heated surface contact time of gas and surface movement of gas composition of surface shape and size of surface chemical nature of substance ignition delay time.Ignition of gases and vapours by lighted cigarettesExperiments carried out at the US Bureau of Mines put that a smoked (puffed) cigarette would only ignite methane air mixture if the last mentioned were caused to flow across the glowing cigarette at 1000 ft per min. Attempts were made to ignite propane, petrol and butane with lighted cigarettes without success and similar results were obtained with white spiritLaboratory studies of ignition by a lighted cigarette were carried out by Yockers and Segal testing the liquids atomic number 6 disulphide, ethyl ether, benzene, toluene, xylene, acetone,methylethylketone, amylacetate, ethylacetate, ethyl benzene, ethanol, methanol, petroleum ether, gasoline and Stoddard consequence (white spirit) 50. They suggested that it might be thought that a cigarette would ignite these substances as their ignition temperatures were lower than those encountered in the burning tip of a c igarette. Carbon disulphide was quick ignited and ignition also occurred during one of the tests with toluene. Ignition of toluene could not be achieved, however, in several subsequent tests under the conditions which prevailed during the one ignition. It was speculated that this one ignition was perhaps owing to a hot spot when a solid chunk of tobacco seems to spit or sizzle. (It was later suggested that as some tobaccos in the USA contain nitrates, a assimilation of this substance could give rise to a hot spot as described in Yockers and Segals paper.) They also seek without success to ignite natural gas (mostly methane), butane and acetylene.Carbon disulphide, acetylene, ethylene oxide, hydrogen sulphide and hydrogen phosphide were readily ignited.Diethyl ether was ignited but only in a closed vessel and after a long delay. Included in the substances which were not ignited were methane, butane, benzene, hexane and cyclohexane.In 1989 scientists at the Research and Technology element of British Gas studied the effects of lighted cigarettes on methane and ethylene. Six tests were carried out wit smoked cigarettes in natural gas-air mixtures over a concentration range of 5.6% (v/v) to 9.6% (v/v) natural gas in air tests were also carried out with ethylene-air mixturesFlammability limitsThe low values of the upper flammable limits of methane, gasoline and propane suggest that in the burning tip of the cigarette or indeed at the surface, oxygen is not present in a sufficient concentration to allow ignition. At the temperatures encountered (700C to 950C), the upper limit would be much higher than those quoted for ambient temperatures but insufficient oxygen molecules are present. Some regions of the cigarette coal are near depleted of oxygen and in other areas less than 10% oxygen by volume was measured.In addition to this, carbon dioxide is produced during the combustion of the tobacco and it has been shown by Jones et a1 54 that gasoline vapour-air-carbon dioxide mixtures are non-flammable when the concentration of carbon dioxide in the mixture exceeds 28.9% by volume. Although some combustible gases such as carbon monoxide and hydrogen are produced, inside the cigarette coal there is a reducing atmosphere depleted in oxygen.A cigarette tip does not, therefore, have sufficient heat ability both to strip off vapour from the liquid surface and to ignite the air vapour mixture produced.Auto-ignition temperatureFor a burning cigarette, a given parameter involved in the combustion process cannot be discussed in isolation from the others but for ignition by hot surfaces the number of these factors is reduced. It has been found that at the temperatures encountered in a cigarette where the smoulder is stable (ca 700C to 775C) ignition of methane and gasoline by a hot surface in this temperature range is highly unlikely. For methane, temperatures in excess of 1,000C are needed for ignition. Temperatures in the region of 900C to 950C are gen erated when a cigarette is puffed, so even at this elevated temperature methane would not be expected to be ignited.During the puff of a cigarette in a flammable atmosphere there will be a unremitting flow of fuel and air drawn into the combustion zone. Only substances with short ignition delay times, of around 1 millisec or less, at the temperatures encountered in the cigarette, will be expected to be ignited. For substances with long ignition delay times cooler reactants would be swept into the path of reacting fuel and oxygen molecules, before a flame could propagate through the mixture.Namely that methane and gasoline constituents are not ignited, diethyl ether is but after a long delay and hydrogen and carbon disulphide are ignited.Quenching distanceThe ignition of flammable gases and vapours will probably take place in the airlgas space between the glowing tobacco fibres. It is possible that some substances are ignited in this region but the flame does not propagate to the fl ammable gas outside the cigarette. Guest 44 found that some surfaces, which formed a layer of scale or ash, e.g., iron or impure carbon, did not ignite natural gas mixtures with ease, and that surfaces which were strongly catalytic or possessed an interstitial structure required higher temperatures. This latter effect could be similar to the flame arresting property of a fine-mesh fit gauze (e.g., as in the DavyLamp). Heat is conducted away by the gauze from the reacting gaslair mixture, lowering the temperature of the reactants and preventing the progression of flame to unburned fuellair mixture. This phenomenon is related to the defunctness distance of the fuel and these distances for stoichiometric mixtures of fuels (at 20C and one atmosphere) tested by cigarettes are given in Table 2 57 and 58. The substances which are ignited by a cigarette have the shortest quenching distances. These are the substances where, because of their reactivity, the loss of heat or destruction of re acting molecules at a boundary is not significant until the dimensions of the boundary are small. As with the other parameters discussed, this alone cannot provide an explanation why some substances are ignited and others are not. For example diethyl ether, which is ignited by cigarettes, has a greater quenching distance than hexane, which is not ignited.