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Natural smoulder in cigarettes
Natural Smoulder in Cigarettes K. Gtro,~r Department of Chemical E~gineeving and Chemical Technology, Imperial College, Pvinc~ Consort Road, London, S.W.7 {Received July 1965;revised SelJtt;nber1965) This paper contains results which are complementary to those published earlier on combustion in puffed cigarettes. Theory is presented which predicts the combustion zone shape on a diffusion controlled model. Comparison of volumetric rates of combustion in the interval between puffs and in cigarettes allowed to smoulder uithout puffing shows that rates of .smoulder are lower between puffs. The reason for this in the phenomenon described earlier, namely "choking'. This is only encountered in puffed cigarettes and reduces both the rate of supply of oxygen and the volumetric rate of combustion. Experiments to determine the calorific value of tobacco and tobacco/air stoichiometry have yielded values (3 7aO callg and 1/7.5 g / g respectively) making possible the calculation of the smouldering zone temperature. Comparison between the values of stoichiometry for cigarettes during the, puff and during natural smoulder indicates that some 90 per cent of tobacco combustibles evades combustion during the puff.
Introduction EARLIER work I has given results of the influence of puffing parameters on the temperature attained, and the volumes consumed in cigarettes. In addition to this, some work has been done on the process of natural smouldering which differs in some important respects from forced smouldering during the puff. This later work is complementary to the earlier paper and is part of a complete picture of this combustion process. Combustion Zone Shape Experiments on the combustion of cigarettes under conditions of natural smoulder have shown that, after an initial (unsteady state) period prior to attainment of steady state conditions, the rate of smoulder becomes constant with time and position. The constancy with time was observed by measuring the rate of progress of the combustion zone periphery, and the constancy with position by random extinction experiments, performed as was described earlierL It seems logical to suppose that the mass-rate of consumFtion must be proportional to the rate of oxygen supply, since the amount of oxygen occluded in the interstices is an insignificant part of the stoichiometric requirement. This supply of oxygen is caused by di~usion and by
natural convection although only one of these processes may prove important in practice. In both of them the rate of transport would be expected to vary with distance from the axis, r. The observed uniform linear propagation velocity therefore implies that the smouldering zone assumes a shape such as to compensate for the radial variation in supply rate, i.e. such that the varying rate of mass consumption results in a uniform linear velocity" parallel to the axis. The purpose of the subsequent discussion is to determine the shape which manifests this property, and to compare it with measurements on quenched cigarettes. Any theoretical treatment of natural convection is so complicated by the varying porosity of the tobacco, of the cigarette paper, and by the varying viscosity and density of air as to be virtually intractable. On the other hand, with diffusion transfer, by employing some simplifying assumptions it is possible to formulate an equation relating the shape of the combustion zone to the properties of the system. This can be generalized, as will be shown, to any transport process in which flow is proportional to gradient. The assumptions are that: the burning zone is axially symmetrical, ash provides a porous sheath about the reaction zone of the same superficial dimensions as those of the cigarette (or else that diffusion in the ash does 161
162
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not differ from that "n the atmosphere), the rate of combustion is proportional to the rate of supply of oxygen, gas phase mass transfer occurs by molecular diffusion, an,:: 'hat the oxygen concentration at the combustion surface is zero (a necessary consequence of control by diffusion). Since it is also a necessary consequence of a combustion surface independent of time that reaction rates at all points should be the same, if the reactions over the whole surface may be assumed to follow the same paths, then it follows that corresponding points in the reaction zone must have the same temperature. Cylindrical coordinates were chosen, ~ along the axis, and r radially. The origin of x is at the top of the combustion surface and the positive direction is towards the tobacco. Then the mass rate of combustion of an annular cylinder i radius r and thickness dr is vp21rr dr
where v denotes linear (axial) combustion velocity (shown by experiment to be constant) and p is the density of the cigarette, Also at po.~ition ~ (corresponding to r) the radial rate of transfer of oxygen is given by 2¢r dx DCo / ln(R / r)
where D is the coefficient of molecular diffusion of oxygen in tobacco and product gases, R is ihe radius of the cigarette, and Co is the concentration of oxygen in air. By the third of the assumptions it can be written that vp2crr d r = 12~- dx DCo/In (R/r)] K where K is a constant of proportionality. Therefore t i n ( R / r ) dr = k dx where k = K C , , D / v p which may be integrated, if k is constant, to give
Figure ! shows the comparison between the theoretical profile and an actual profile. The
Actuat / /
profite--j 1 //
or
o4-
// ;: //
1.0 0'8 06 0"4 0.2 =
~r" In (R/r) + tr "~= kx
\
0 0'2 0'4 0.6 0'8 10 r/R
Figure 1. Comparison between theoretical and actual combustion zone pro]iles
agreement between the two curves is fair, their shapes being similar. The experimental agreement with this derivation is obviously not to be regarded as proof that diffusion of oxygen-with a constant diffusion coefficient--is the ratecontrolling step, but, rather that it fortuitously describes the physical process and is adequate until a more comprehensive theory is available. Three sizes of cigareites were available for study and were supplied accurately sized, within a certain weight and draw-resistance range and of the same moisture content. Rates of consumption for the large, the standard and the small cigarette are shown in Table !, in comparison with rates of natural smoulder that exist between 'standard' puffs (i.e. puffs of 25 cm:' w)lume, of two seconds duration once per minute). The method of measuring volume Table I
Put] duration, sec
Pug volume, cm"
cnPI58 sec
Volume consumed during natural smoulder. cm=158see
2 2 2
25 25 25
0"108 0.127 0"196
0"III 0.156 0.215
o Cigarette
dl....... em
which becomes dimensionless on the substitutions r / R = o" and 4kx / R °-= X -Hence o'2(1 - I n o "=) =X
\)
o,
f_rlnRdr-~frlnrdr=:kx o
~" 0'3-
0.7o o.82 0.9')
Volume consumed between
OUJ~$,
Results taken from Table 2, ref. I
June 1966
Natural smoulder in cigarettes
coasumed differed from that adopted for the 'p~lffing' profiles described in the earlier reference. Since the profiles remained similar to ea~:h other an accurate measure, within the linfits of statistical distribution, of volumes consu:,'ned was available by noting the linear rates of combustion. The number of cigarettes used for each smoulder-rate determination was only between six and eight. Consequently these results are not on the same statistical basis as those of the consumption work described earlier. Nevertheless a surprising trend appears. It seemed reasonable to think that purely natural smoulder combustion represented the lowest intensity natural r6gime on the basis that heat and mass transfer were functions only of diffusion and natural aerodynamics. Any shape change tended to distort these, and hence increased the intensity of combustion where the surface/volume ratio was increased, and vice versa. However, the results show the reverse effect, that the rate of consumption after distortion by the puff is, in fact, lower than for purely natural smoulder. This effect is unlikely to be statistical in view of its size and occurrence in the three cases. A very probable mechanism which may account for the existence of this phenomenon is 'choking'. This was postulated earlier to explain results on the volumes consumed during the puff as well as the occurrence of peaks in 'radiation temperature' at the onset of each puff. Choking, the swelling of tobacco filaments on heating and reduction in flow area by condensing tars, occurs at the periphely during the puff by the action of hot produzt gases or~ the previously unpyrolvsed tobacco. The result of this process is that the combustion rate falls due to the air bypassing the combustion zone (by virtue of the porosity of cigarette paper). The magnitude of this effect is related to the initial combustion intensity, i.e. the more powerful puff, it being found that for very strong puffs, the volume of cigarette consumed is actually less than that for weaker puffs. Durin~ the natural smoulder period, between puffs, it is not tobacco of normal consistency or densitv of packing that is being burnt, but a material of fairly high calorific value, closely packed. Hence one wor,ld expect, as observed, a lower combustion rate.
163
Calot,ific Value and Stoichiometry of Tobacco To calculate the combustion temperature in the absence of forced convection (and hence heat recirculation), experiments on heat of combustion of tobacco and on tobacco/air stoichiometry in a bomb calorimeter have been performed. The mean of three experiments, using a bomb calorimeter in the standard manner, gave the heat of combustion of tobacco as 3 7e~0 cal/g. An approximate experiment on the stoichi,~ merry of tobacco was carried out as described below. The bomb calorimeter wa~ modified so that the pressure variation within it couhi be observed, by attaching a mercury manometer. The maximum pressure this system could withstand was about four atmospheres absolute. This restricted the amount of oxygen that could be contained within the bomb, and consequently the amount of tobacco, so that pressure changes were necessarily small and the efficiency of oxidation lower. Large excesses of anhydrous phosphorus pentoxide and sodium hydroxide (partitioned from one another) were included within the bomb to absorb acidic, aqueous and alkaline products. The tobacco was weighed before and after burning, the residue, in excess of ash, being largely carbonized tobacco. The volume of the reaction system was measured by pressure sharing with a vessel of known volume. Before burning, a good vacuum was applied to the bomb, prior to oxygen admittance to minimize the amount of residual nitrogen. Sufficient time was allowed after the experiment to ensure that no further pressure change due to absorption occurred. The pressure drop was noted and the equivalent weight of oxygen calculated. The residual matter, after allowing for the known ash content, was assumed to be carbon. It was also assumed that no permanent neutral gases were formed. It was found that the approximate amount of air required to burn 1 g of tobacco completely is 7.$ g. A knowledge of the heat of reaction and the stoichiometry of the process enables a calculation of the steady state temperature of combustion to be made, assuming values for the various specific heats. This process is entirely analogous to the calculation of final flame temperatures% though in this case refinements such as the allowance for variation of specific heats with temperature are not justifiable because of
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164
the approximate values of the 'stoichiometry' and heats of combustion. First, an equation il~ general terms was derived for the fina" flan~e temperature in which the following symbols appear: AH Heat of combustion, cal/g tobacco V Volumetric rate of combustion of the cigarette, cm ~/ s p~ Density of the cigarette, g / c m 3 po Density of a~;h, g / c m 3 c~ Suecific be~.~~ f ~ h , cal/g °C cp~ Specific heat of product gases, cal/g °C d~ Equivalence ratio, g~J,/g, ob....... required for complete combustion A Radiation area, cm -° o" Stefan'sconstant, cal/cm°'.s.°K * Therefore, approximately Vp~AH-o'Tb4A r~ = V[poc,,,, + (p,~ + (p~- po))c,A where T~, is the 'final flame temperature' and the enthalpy datum is ambient temperature. Values for the specific heats were taken from the literature,, that of the products being estimated. The volumetric consumption rate and radiation surface area were derived from the experiment with the 'standard' cigarette, the radiating area being taken as zero (in the case of complete shielding by the ash). The densities of the cigarette and of the ash were found by burning a cigarette in an ash determination furnace. The density of ash was calculated on the basis that it assumed the same superficial dimensions as those of the cigarette. Therefore taking p~=0.311 g/cm a, /),,=0.034 g/cm a, and cp~=0.25 and ct,p=0.30 T~ = 1 473°K, or 1 200°C In the case where all the ash has been removed, a radiation correction is required and if the radiating area is taken as that of a whole cone, of height equal to that of the combustion surface, standing on the same base, then with A = l - 1 3 cm -~ T,,=I 093°K, or 820°C. These two calculated temperatures r~;present the extremes between which normal smouldering temperatures lie. Experiments indicate that temperatures lie much closer to the second value despite the layer of ash which remained undisturbed. There are two reasons for this
namely: cooling by conductive heat transfer through the ash to file surface and subsequent loss b y radiation and convection, and convective flow of air supplying more than the stoichiometric air requirements--this effectively: 'dilutes' the reaction and lowers its temperature. It should be noted that the weight i:atio of air to tobacco combusted, calculabl!; from Table 2 of ref. 1, for the puffed cigarett':; varie~: frem 0.021 to 0.153 of the stoichiometric ratio found in the bomb calorimeter experiments (7.5). These results were calculated on the basi.~: of a 25 cm ~ puff drawing pure air. Of course, the metering device was downstream of the cigarette end, although all the smoke had been precipitated prior to metering, part of the metered flow consisted of permanent gases and vapours generated within the cigarette. Thus the 25 cm:' puff was equivalent to the induction of rather less thait 25 cm 3 of air. The actual volume of air can be calculated on, say, a gaseous nitrogen balance if the composition of the smoke gases is known. In a private communication ~, the weight fraction of nitrogen in smoke free gas was shown to be 0.67. This is equivalent to an induced air volume of 22.2 cm '~ for a 25 cm :' metered puff. Hence the true air flow involves a correction to the calculated weight ratio of air to tobac,:o burnt which is small in comparison with the difference that exists with the value determined in the bomb calorimeter experiments. This would appear to indicate that most of the tobacco in a cigarette, puffed in the usual manner, evades combustion. This is so, since much of the tobacco is distilled ahead of the combustion zone bv the hot gases drawn through it in the puff. With truly, natural smoulder there is little or no distillation of the volatiles. It is therefore probable that the stoichiometry here agrees with the bomb calorimeter determination. The foregoing calculation may consequently be considered t~ have generated a realistic temperature. I wou~d like to mention m y gratitude to Dr F. J. Wei~tberg [or encouragement in this w o r k References
J E~.~:wro.~, Sir A. C., GU~A.~0 K. and WEINBERG, F. J. Combustion <5, Flame, 1.069, 7, 6,q W~txm:ac,, F. J. Optics o] Flames, p 44. Batterworths: I.ondon, 1963 :~ AmericanTobacco Co. : Private communicatioa.
Introduction EARLIER work I has given results of the influence of puffing parameters on the temperature attained, and the volumes consumed in cigarettes. In addition to this, some work has been done on the process of natural smouldering which differs in some important respects from forced smouldering during the puff. This later work is complementary to the earlier paper and is part of a complete picture of this combustion process. Combustion Zone Shape Experiments on the combustion of cigarettes under conditions of natural smoulder have shown that, after an initial (unsteady state) period prior to attainment of steady state conditions, the rate of smoulder becomes constant with time and position. The constancy with time was observed by measuring the rate of progress of the combustion zone periphery, and the constancy with position by random extinction experiments, performed as was described earlierL It seems logical to suppose that the mass-rate of consumFtion must be proportional to the rate of oxygen supply, since the amount of oxygen occluded in the interstices is an insignificant part of the stoichiometric requirement. This supply of oxygen is caused by di~usion and by
natural convection although only one of these processes may prove important in practice. In both of them the rate of transport would be expected to vary with distance from the axis, r. The observed uniform linear propagation velocity therefore implies that the smouldering zone assumes a shape such as to compensate for the radial variation in supply rate, i.e. such that the varying rate of mass consumption results in a uniform linear velocity" parallel to the axis. The purpose of the subsequent discussion is to determine the shape which manifests this property, and to compare it with measurements on quenched cigarettes. Any theoretical treatment of natural convection is so complicated by the varying porosity of the tobacco, of the cigarette paper, and by the varying viscosity and density of air as to be virtually intractable. On the other hand, with diffusion transfer, by employing some simplifying assumptions it is possible to formulate an equation relating the shape of the combustion zone to the properties of the system. This can be generalized, as will be shown, to any transport process in which flow is proportional to gradient. The assumptions are that: the burning zone is axially symmetrical, ash provides a porous sheath about the reaction zone of the same superficial dimensions as those of the cigarette (or else that diffusion in the ash does 161
162
Vol. 10
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not differ from that "n the atmosphere), the rate of combustion is proportional to the rate of supply of oxygen, gas phase mass transfer occurs by molecular diffusion, an,:: 'hat the oxygen concentration at the combustion surface is zero (a necessary consequence of control by diffusion). Since it is also a necessary consequence of a combustion surface independent of time that reaction rates at all points should be the same, if the reactions over the whole surface may be assumed to follow the same paths, then it follows that corresponding points in the reaction zone must have the same temperature. Cylindrical coordinates were chosen, ~ along the axis, and r radially. The origin of x is at the top of the combustion surface and the positive direction is towards the tobacco. Then the mass rate of combustion of an annular cylinder i radius r and thickness dr is vp21rr dr
where v denotes linear (axial) combustion velocity (shown by experiment to be constant) and p is the density of the cigarette, Also at po.~ition ~ (corresponding to r) the radial rate of transfer of oxygen is given by 2¢r dx DCo / ln(R / r)
where D is the coefficient of molecular diffusion of oxygen in tobacco and product gases, R is ihe radius of the cigarette, and Co is the concentration of oxygen in air. By the third of the assumptions it can be written that vp2crr d r = 12~- dx DCo/In (R/r)] K where K is a constant of proportionality. Therefore t i n ( R / r ) dr = k dx where k = K C , , D / v p which may be integrated, if k is constant, to give
Figure ! shows the comparison between the theoretical profile and an actual profile. The
Actuat / /
profite--j 1 //
or
o4-
// ;: //
1.0 0'8 06 0"4 0.2 =
~r" In (R/r) + tr "~= kx
\
0 0'2 0'4 0.6 0'8 10 r/R
Figure 1. Comparison between theoretical and actual combustion zone pro]iles
agreement between the two curves is fair, their shapes being similar. The experimental agreement with this derivation is obviously not to be regarded as proof that diffusion of oxygen-with a constant diffusion coefficient--is the ratecontrolling step, but, rather that it fortuitously describes the physical process and is adequate until a more comprehensive theory is available. Three sizes of cigareites were available for study and were supplied accurately sized, within a certain weight and draw-resistance range and of the same moisture content. Rates of consumption for the large, the standard and the small cigarette are shown in Table !, in comparison with rates of natural smoulder that exist between 'standard' puffs (i.e. puffs of 25 cm:' w)lume, of two seconds duration once per minute). The method of measuring volume Table I
Put] duration, sec
Pug volume, cm"
cnPI58 sec
Volume consumed during natural smoulder. cm=158see
2 2 2
25 25 25
0"108 0.127 0"196
0"III 0.156 0.215
o Cigarette
dl....... em
which becomes dimensionless on the substitutions r / R = o" and 4kx / R °-= X -Hence o'2(1 - I n o "=) =X
\)
o,
f_rlnRdr-~frlnrdr=:kx o
~" 0'3-
0.7o o.82 0.9')
Volume consumed between
OUJ~$,
Results taken from Table 2, ref. I
June 1966
Natural smoulder in cigarettes
coasumed differed from that adopted for the 'p~lffing' profiles described in the earlier reference. Since the profiles remained similar to ea~:h other an accurate measure, within the linfits of statistical distribution, of volumes consu:,'ned was available by noting the linear rates of combustion. The number of cigarettes used for each smoulder-rate determination was only between six and eight. Consequently these results are not on the same statistical basis as those of the consumption work described earlier. Nevertheless a surprising trend appears. It seemed reasonable to think that purely natural smoulder combustion represented the lowest intensity natural r6gime on the basis that heat and mass transfer were functions only of diffusion and natural aerodynamics. Any shape change tended to distort these, and hence increased the intensity of combustion where the surface/volume ratio was increased, and vice versa. However, the results show the reverse effect, that the rate of consumption after distortion by the puff is, in fact, lower than for purely natural smoulder. This effect is unlikely to be statistical in view of its size and occurrence in the three cases. A very probable mechanism which may account for the existence of this phenomenon is 'choking'. This was postulated earlier to explain results on the volumes consumed during the puff as well as the occurrence of peaks in 'radiation temperature' at the onset of each puff. Choking, the swelling of tobacco filaments on heating and reduction in flow area by condensing tars, occurs at the periphely during the puff by the action of hot produzt gases or~ the previously unpyrolvsed tobacco. The result of this process is that the combustion rate falls due to the air bypassing the combustion zone (by virtue of the porosity of cigarette paper). The magnitude of this effect is related to the initial combustion intensity, i.e. the more powerful puff, it being found that for very strong puffs, the volume of cigarette consumed is actually less than that for weaker puffs. Durin~ the natural smoulder period, between puffs, it is not tobacco of normal consistency or densitv of packing that is being burnt, but a material of fairly high calorific value, closely packed. Hence one wor,ld expect, as observed, a lower combustion rate.
163
Calot,ific Value and Stoichiometry of Tobacco To calculate the combustion temperature in the absence of forced convection (and hence heat recirculation), experiments on heat of combustion of tobacco and on tobacco/air stoichiometry in a bomb calorimeter have been performed. The mean of three experiments, using a bomb calorimeter in the standard manner, gave the heat of combustion of tobacco as 3 7e~0 cal/g. An approximate experiment on the stoichi,~ merry of tobacco was carried out as described below. The bomb calorimeter wa~ modified so that the pressure variation within it couhi be observed, by attaching a mercury manometer. The maximum pressure this system could withstand was about four atmospheres absolute. This restricted the amount of oxygen that could be contained within the bomb, and consequently the amount of tobacco, so that pressure changes were necessarily small and the efficiency of oxidation lower. Large excesses of anhydrous phosphorus pentoxide and sodium hydroxide (partitioned from one another) were included within the bomb to absorb acidic, aqueous and alkaline products. The tobacco was weighed before and after burning, the residue, in excess of ash, being largely carbonized tobacco. The volume of the reaction system was measured by pressure sharing with a vessel of known volume. Before burning, a good vacuum was applied to the bomb, prior to oxygen admittance to minimize the amount of residual nitrogen. Sufficient time was allowed after the experiment to ensure that no further pressure change due to absorption occurred. The pressure drop was noted and the equivalent weight of oxygen calculated. The residual matter, after allowing for the known ash content, was assumed to be carbon. It was also assumed that no permanent neutral gases were formed. It was found that the approximate amount of air required to burn 1 g of tobacco completely is 7.$ g. A knowledge of the heat of reaction and the stoichiometry of the process enables a calculation of the steady state temperature of combustion to be made, assuming values for the various specific heats. This process is entirely analogous to the calculation of final flame temperatures% though in this case refinements such as the allowance for variation of specific heats with temperature are not justifiable because of
Vol. 10
K, Guon
164
the approximate values of the 'stoichiometry' and heats of combustion. First, an equation il~ general terms was derived for the fina" flan~e temperature in which the following symbols appear: AH Heat of combustion, cal/g tobacco V Volumetric rate of combustion of the cigarette, cm ~/ s p~ Density of the cigarette, g / c m 3 po Density of a~;h, g / c m 3 c~ Suecific be~.~~ f ~ h , cal/g °C cp~ Specific heat of product gases, cal/g °C d~ Equivalence ratio, g~J,/g, ob....... required for complete combustion A Radiation area, cm -° o" Stefan'sconstant, cal/cm°'.s.°K * Therefore, approximately Vp~AH-o'Tb4A r~ = V[poc,,,, + (p,~ + (p~- po))c,A where T~, is the 'final flame temperature' and the enthalpy datum is ambient temperature. Values for the specific heats were taken from the literature,, that of the products being estimated. The volumetric consumption rate and radiation surface area were derived from the experiment with the 'standard' cigarette, the radiating area being taken as zero (in the case of complete shielding by the ash). The densities of the cigarette and of the ash were found by burning a cigarette in an ash determination furnace. The density of ash was calculated on the basis that it assumed the same superficial dimensions as those of the cigarette. Therefore taking p~=0.311 g/cm a, /),,=0.034 g/cm a, and cp~=0.25 and ct,p=0.30 T~ = 1 473°K, or 1 200°C In the case where all the ash has been removed, a radiation correction is required and if the radiating area is taken as that of a whole cone, of height equal to that of the combustion surface, standing on the same base, then with A = l - 1 3 cm -~ T,,=I 093°K, or 820°C. These two calculated temperatures r~;present the extremes between which normal smouldering temperatures lie. Experiments indicate that temperatures lie much closer to the second value despite the layer of ash which remained undisturbed. There are two reasons for this
namely: cooling by conductive heat transfer through the ash to file surface and subsequent loss b y radiation and convection, and convective flow of air supplying more than the stoichiometric air requirements--this effectively: 'dilutes' the reaction and lowers its temperature. It should be noted that the weight i:atio of air to tobacco combusted, calculabl!; from Table 2 of ref. 1, for the puffed cigarett':; varie~: frem 0.021 to 0.153 of the stoichiometric ratio found in the bomb calorimeter experiments (7.5). These results were calculated on the basi.~: of a 25 cm ~ puff drawing pure air. Of course, the metering device was downstream of the cigarette end, although all the smoke had been precipitated prior to metering, part of the metered flow consisted of permanent gases and vapours generated within the cigarette. Thus the 25 cm:' puff was equivalent to the induction of rather less thait 25 cm 3 of air. The actual volume of air can be calculated on, say, a gaseous nitrogen balance if the composition of the smoke gases is known. In a private communication ~, the weight fraction of nitrogen in smoke free gas was shown to be 0.67. This is equivalent to an induced air volume of 22.2 cm '~ for a 25 cm :' metered puff. Hence the true air flow involves a correction to the calculated weight ratio of air to tobac,:o burnt which is small in comparison with the difference that exists with the value determined in the bomb calorimeter experiments. This would appear to indicate that most of the tobacco in a cigarette, puffed in the usual manner, evades combustion. This is so, since much of the tobacco is distilled ahead of the combustion zone bv the hot gases drawn through it in the puff. With truly, natural smoulder there is little or no distillation of the volatiles. It is therefore probable that the stoichiometry here agrees with the bomb calorimeter determination. The foregoing calculation may consequently be considered t~ have generated a realistic temperature. I wou~d like to mention m y gratitude to Dr F. J. Wei~tberg [or encouragement in this w o r k References
J E~.~:wro.~, Sir A. C., GU~A.~0 K. and WEINBERG, F. J. Combustion <5, Flame, 1.069, 7, 6,q W~txm:ac,, F. J. Optics o] Flames, p 44. Batterworths: I.ondon, 1963 :~ AmericanTobacco Co. : Private communicatioa.