Hydrogen or deuterium gas when mixed with air or oxygen forms a highly flammable mixture over a wide range of proportions; they also form flammable mixtures with chlorine and the oxides of nitrogen, further, they will also react spontaneously with fluorine and chlorine trifluoride.
Because it is impossible to guarantee that any system will be completely leak free every effort should be made to exclude all sources of ignition. The designer and the user should give careful consideration to the risk from a wide variety of ignition sources e.g. smoking, flames, hot surfaces, electrical and other sparking, static electricity, shock, impact, catalytic and chemical action.
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Thus petrol vapour 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 mixtures’
Flammability
For flammable liquids with flash point temperatures above normal ambient, e.g., kerosene, white spirit 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 delay
A further factor in the mechanism of ignition of gases and vapours 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 dependent on temperature and are reduced with an increase in temperature.
Cigarette composition and combustion
Composition
The most commonly encountered, manufactured cigarette consists of a cylindrical packed bed of 1 g of shredded tobacco enclosed in paper and is generally 8 mm in diameter and 65 mm to 85 mm long. The strands of tobacco 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 burning characteristics is that hand-rolled cigarettes tend 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 discussion relates to manufactured cigarettes.
The smoke itself consists of mainstream smoke, from the maximum gas phase temperature (850°C) 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 ‘puffing’ of the cigarette. One puff of a cigarette is, there- the fore, one suction or draw by the smoker.
There are two main regions at the tip on the ‘coal’, namely the combustion zone A and the pyrolysis and distillation zone B. Combustible vapours are produced in zone B prior to ignition in zone A
In the interval between puffs, the natural convection flow of air around the combustion coal in an upwards direction 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 representative of the temperatures encountered inside a burning cigarette tip.
smoking machine, taking a 35 cm3 volume of 2 sec duration, once per minute
of both the gas and solid phases, were determined. These temperatures can, therefore, be regarded as approaching the maximum attained during the drawing of a cigarette
surprisingly, the maximum solid phase temperature in the area region of 900°C-950°C was attained 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 distinct pale red glow can be seen when a cigarette is puffed. The maximum gas phase temperature (850°C) was at the centre of the cigarette coal.
Ignition of gases and vapours by hot surfaces
Although 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 parameters 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.
Some 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 cigarettes
Experiments carried out at the US Bureau of Mines found that a smoked (puffed) cigarette would only ignite methane air mixture if the latter 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 spirit
Laboratory studies of ignition by a lighted cigarette were carried out by Yockers and Segal testing the liquids carbon disulphide, ethyl ether, benzene, toluene, xylene, acetone,
methylethylketone, amylacetate, ethylacetate, ethyl benzene, ethanol, methanol, petroleum ether, gasoline and Stoddard solvent (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 cigarette. Carbon disulphide was readily 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 concentration of this substance could give rise to a hot spot as described in Yocker’s and Segal’s paper.) They also tried 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 Division 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 mixtures
Flammability limits
The 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 (700°C to 950°C), 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 virtually depleted of oxygen and in other areas less than 10% oxygen by volume was measured.

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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 energy both to strip off vapour from the liquid surface and to ignite the air vapour mixture produced.
Auto-ignition temperature
For 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 700°C to 775°C) ignition of methane and gasoline by a hot surface in this temperature range is highly unlikely. For methane, temperatures in excess of 1,000″C are needed for ignition. Temperatures in the region of 900°C to 950°C are generated 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 continuous 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 distance
The 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 flammable 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 wire gauze (e.g., as in the Davy
Lamp). 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 quenching distance of the fuel and these distances for stoichiometric mixtures of fuels (at 20°C 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 reacting 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.