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isooctane flames
Fuel 84 (2005) 691–700 www.fuelfirst.com
Formation of organic acids from propane, isooctane and toluene/isooctane flames E. Zervas* Institut Franc¸ais du Pe´trole, 1 et 4 avenue du Bois Pre´au, F-92500 Rueil-Malmaison cedex, France Received 30 September 2004; received in revised form 17 November 2004; accepted 18 November 2004 Available online 10 December 2004
Abstract Three fuels (propane, isooctane and toluene/isooctane) are used for the study of the formation of organic acids from their flames. Four organic acids have been found in the combustion products: formic, acetic, propionic and isovaleric acid. These acids are formed very quickly; their concentration then generally increases to reach a maximum value, and then decreases to zero. Toluene enhances the formation of organic acids comparing to propane and isooctane. The concentration of these acids depends strongly on the air/fuel equivalence ratio. Some correlations are found between the concentration of the acids and some alcohols or aldehydes. These results are in accordance with those presented in the case of internal combustion engines. q 2004 Elsevier Ltd. All rights reserved. Keywords: Acetic acid; Combustion; Formic acid; Isooctane; Propane; Propionic acid; Toluene
1. Introduction Oxygenated compounds are important intermediate products of the oxidation of hydrocarbons [1]. The most important family of oxygenated compounds is carbonyl compounds, and many authors study their formation in different flames as in the case of n-butane [2–4], 1-butene [5], n-heptane [6,7], isooctane [8–10], benzene [11] and other fuels. The formation of methanol is also presented in the case of n-butane [4], n-heptane [7,12] and isooctane [8]. These compounds are also found in the exhaust gas of internal combustion engines [13–16]. Organic acids is another family of oxygenated compounds found in exhaust gas of internal combustion engines [14,17–20]. These compounds are also found in the products of wood combustion (for higher than C14 acids, [21]). Organic acids are important pollutants of urban and rural atmospheres and contribute to acid rain formation [22]. The formation and reactions of organic acids of urban, rural and marine atmospheres are also presented in literature [23]. * Present Address: Renault-CTL L26 0 60, 1, Alle´e Cornuel, F-91510 Lardy, France. Tel.: C331-76-87-84-77; fax: C331-76-87-82-92. E-mail address: [email protected]. 0016-2361/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2004.11.012
As organic acids are emitted from internal combustion engines, they must also be emitted from burners or other industrial combustion sources, but no article presents their formation in of from flames. The present work studies the emission of organic acids from the combustion of three simple fuels: propane, isooctane and toluene/isooctane. The concentration profiles at stoichiometry, lean and rich conditions are presented and discussed. Some correlations with the other species formed from these flames, and also with the organic acids emitted from internal combustion engines are presented. Based on these results, some probable paths for the formation of these compounds are presented.
2. Experimental section Three fuels were used for this study: propane, isooctane and a blend of 20% of toluene and 80% of isooctane. The last two fuels were also used in a previous work concerning the emission of organic acids from a spark ignition engine [18]. The two liquid fuels were first evaporated in a long inox tube at 120 8C, and then mixed with air. The fuel rate was controlled with a peristaltic pump, and the stability of
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flame temperature was used to verify the evaporation rate of these liquids. This temperature varied less than 2 8C, indicating a constant fuel/air ratio, so a constant evaporation rate. The flames were stabilized on a flat (diameterZ 60 mm), water cooled (TZ20 8C) sintered bronze burner. The cold gas velocity was 6 cm/s, and five l values were used for these experiments: 0.83, 0.91, 1.0, 1.11 and 1.25. These values were also used in a previous work using a spark ignition engine [18]. The temperature was measured with a fast response thermocouple (silica coated 25fm Pt/Pt–13%Rh), and was corrected for the radiation. The combustion products were sampled isokinetically from the centre of the flame in a stainless steel oil cooled probe (internal diameterZ1 mm). The oil temperature was maintained at 110 8C to avoid water condensation, and a heated line at the same temperature was used to lead the gas from the probe to analytical devises. The sampling distance varied from 0.5 to 7 mm from the burner with a path of 0.5 mm. As the water flow rate must be enough to cool the burner but no too much to completely cool the flame, specific experiments were conducted to adjust it, and obtain, at stoichiometry, a temperature profile with a maximum at about 3 mm after the burner. The same flow rate was then used for all l. Organic acids were collected in deionised water and analysed by two methods: ionic chromatography/conductimetric detection for the analysis of formic acid and gas chromatography/flame inonization detector (GC/FID) for the analysis of the other aliphatic acids. Individual hydrocarbons, alcohols and aldehydes/ketones were also analysed, but these results will be presented in a next article. More details about the analytical methods used can be found elsewhere [14,24]. The repeatability of these tests is determined by five identical tests, performed at the first mm from the burner, for the five l used. The other tests were doubled and average value was used. For all l, the relative standard deviation was less than 8% for the CO, CO2, HC, NOx and O2 analysis. The relative standard deviation of all organic acids measurements was less than 19%.
3. Results and discussion 3.1. Profiles of O2, CO, CO2, total HC, NOx and flame temperature As expected, for the three fuels used, the concentration of O2 decreases with the distance from the burner and increases with l, this of HC decreases with both distance and l and this of NOx decreases with distance and presents a maximum value at about lZ1.11. The concentration of CO increases with distance and decreases with l; while this of CO2 increases with distance before reaching a plateau, and presents a maximum value at stoichiometry. Flame temperature initially increases and reaches a maximum
value at about 3 mm, and then decreases with distance; temperature presents a maximum value at stoichiometry. These results are in accordance with those presented in literature in the case of flame experiments [25] and internal combustion engines [26]. 3.2. Profiles of organic acids Four organic acids were found in detectable concentrations in the products of these flames: formic, acetic, propionic and isovaleric acid. 3.2.1. Formic acid Fig. 1 presents the concentration of formic acid versus the distance from the burner for the three fuels and the five l used. In the case of propane, formic acid is formed very quickly, as its concentration is 5–9 ppmv at the first 0.5 mm from the burner. This concentration remains quite constant up to the 3rd mm at rich conditions and stoichiometry. It increases at lean ones for the same distance from the burner to reach 12–13 ppmv, indicating that oxygen excess enhances the oxidation of C1 species to this acid. After the third mm, its concentration falls down very sharply to reach zero ppmv after the fourth mm. The profile of formic acid is quite different in the case of isooctane. This acid is also formed very quickly. Five to seven ppmv are found at 0.5 mm under rich conditions, but more of the double under lean ones (10–18 ppmv). This indicates that the CH3 radicals are found in higher concentrations in isooctane flames, for two reasons: isooctane has more CH3 groups and combustion temperature is a little higher in the case of isooctane, enhancing its decomposition. These initial concentrations increase for each lambda, to reach a maximum value at the third mm (8–30 ppmv). This increase is sharper in the case of lean conditions; oxygen excess continues the oxidation of C1 radicals to formic acid, while lack of oxygen prevents this oxidation at rich conditions. After the third mm, formic acid is oxidised and its concentration falls down to zero at the fourth mm. The addition of toluene to isooctane fuel modifies the profile of formic acid, which is found at slightly higher initial concentrations than in the case of pure isooctane. After this point, all the curves increase to reach a maximum concentration at the second mm, earlier than the other two fuels, for which the maximum concentration is at the third mm. The maximum concentration is also higher than in the case of pure isooctane. After this point, formic acid concentration decreases down to zero ppmv after the fourth mm, but this decrease is less sharp than in the case of the two previous fuels. The comparison between the profiles of the three fuels indicates that this acid is enhanced by tolueneO isooctaneOpropane. As toluene/isooctane has lower combustion temperature than pure isooctane, this cannot be due to the higher production of C1 radicals due to increased temperature, as in the case of isooctane. Toluene can give
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Fig. 1. Left figure: profile of formic acid concentration for the five l and the three fuels used. Right figure: influence of l on the concentration of formic acid. Three distances from the burner for the three fuels used.
a CH3 radical by dealkylation, but isooctane has more CH3 groups, the concentration of CH3 radicals must be higher in the case of isooctane. This must indicate that other C1 species are also (or main) precursors of formic acid, and toluene can give more easily unsaturated C1 radicals like CH or CH2 (or oxygenated versions) than isooctane. Fig. 2 presents the ratio between the concentrations of formic acid emitted from the toluene/isooctane fuel and the isooctane one. The first fuel emits 1.1–1.6 times more formic acid than the second one in the beginning of the combustion, indicating that it produces more C1 radicals. This ratio increases with the distance and reaches 1.5–3 at the second mm, indicating that toluene enhances more the formation of formic acid in the middle of the combustion, because at this stage toluene is more decomposed to unsaturated C1 species. After this point, the ratio is less than one, as formic acid produced from toluene is oxidised earlier that this produced from isooctane. This ratio is more important at rich conditions, because formic acid is more enhanced by unsaturated C1 radicals, which are probably favourably produced under these latter conditions. These results are in accordance with those observed on a SI engine: formic acid’s concentrations are higher in the case of toluene/isooctane fuel comparing to pure isooctane [18].
Formic acid is completely oxidised at the fifth mm in the case of propane and at the fourth mm in the case of isooctane and toluene/isooctane fuels. Even if the concentrations found in propane flames are lower than those of the other two fuels, the formation of this acid lasts more in time in the case of this fuel. The reason must be that propane gives unsaturated C1 radicals only at the end of its combustion. Fig. 1 shows that for the same distance from the burner, the concentration of formic acid is higher at lean conditions for all three fuels used, indicating that even unsaturated C1 radicals are enhanced at rich conditions, oxygen excess is more important for its formation. The same results are observed in the case of internal combustion engines [18–20]. The results presented here are not sufficient to propose a detailed mechanism for the formation of formic acid. This compound can be product of the HC, aldehydes, alcohols or oxygenated radicals oxidation. As all organic acids are products of low temperature oxidation, they (or a part of them) are probably not formed in the flame, but during the cooling of exhaust gas in the probe, from a rapid combination of different species, as for example CxHyCO and OH or CxHyCOO and H. This is probably the reason for
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– the addition of an H to a HCOO; this is a final reaction that can take place during the cooling phase, – the isomerisation of a hyperoxide H–O–O–H/ HCOOH. No bibliographical reference propose these reactions; a more detailed formation path is difficult to be proposed at this stage of knowledge.
Fig. 2. Ratio between the concentrations of formic, acetic and propionic acids emitted from the toluene/isooctane fuel and the isooctane one, as a function of the distance from the burner for the five l used.
the increased concentration of formic acid from toluene flames: the unsaturated C1 radicals are easily oxidised at the low temperature of the probe to formic acid. The formation of formic acid might go through one of the four following paths: – the addition of a OH to a HCO: the OH attacks the HCO and a H extraction must take place. The reactions: HCOCOH/HCOOH, HCHOCOH/HCOOHCH, HCHOCH2O/HCOOHCH2, can probably be applied in this case. HCO is a common compound in flames [1,12]; a combination with oh can give formic acid. – the addition of an O to a HCOH. The reactions: HCH2 OHCO/HCOOHCH2, HCHOHCO/HCOOHCH, HCOHCO/HCOOH can take place,
3.2.2. Acetic acid Fig. 3 presents the profiles of acetic acid for the three fuels used. In the case of propane, the initial concentration of acetic acid is within 4 and 7 ppmv, which is quite similar to this of formic acid. These concentrations increase up to the third mm to reach 6–9 ppmv, and then decrease sharply to zero ppmv at the fifth mm. The shapes of acetic and formic acids are quite similar indicating that these two compounds follow similar formation paths. Isooctane produces initially 5–10 ppmv of acetic acid (slightly higher than propane), but its concentration increases to reach 7– 17 ppmv at the third mm. This increase is sharper at stoichiometry and lean conditions, due to oxygen excess. These concentrations indicate that isooctane produces more acetic acids precursors, as C2 radicals, than propane. And, as isooctane has more carbons atoms in his structure, the formation of C2 species is longer than this of propane; for this reason the formation of acetic acid is delayed comparing to propane. After the third mm, acetic acid concentration falls sharply to zero ppmv after the fourth mm. The shape of these profiles is quite similar to that of formic acid formed from isooctane, indicating that these two acids follow quite similar formation paths. The addition of toluene into isooctane changes these curves. Acetic acid is still formed very early: it is found at about 5–10 ppmv at the 0.5 mm (like in the case of isooctane), increases sharply to reach about 15 ppmv at the second mm, and then falls down to zero ppmv before the fourth mm. As in the case of formic acid and apparently for the same reasons, the points of the maximum concentration and the complete oxidation are found earlier in the case of this fuel than the other two. The shape of these profiles is similar to that of formic acid, indicating that these two compounds follow quite similar formation paths. At the 0.5 mm, toluene/isooctane emits slightly higher concentration of acetic acid than isooctane (Fig. 2). As in the case of formic acid, this ratio increases with distance to reach a maximum value of 1.3–2 ppmv at the second mm, indicating that, as in the case of formic acid, toluene enhances more the formation of acetic acid at the middle of the combustion. For each distance, this ratio is more important at rich conditions. This is must due to the favourable formation of unsaturated C2 species from toluene than from isooctane. These results are in accordance with those observed on a SI engine: acetic acid is emitted at higher concentrations from toluene/isooctane comparing to isooctane [18]. Acetic and higher acids are not found in detectable concentrations in the exhaust gas of
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Fig. 3. Left figure: profile of acetic acid for the five l and the three fuels used. Right figure: influence of l on the concentration of acetic acid. Three distances from the burner for the three fuels used.
a natural feed SI engine, even if the natural gas used contained 8% of ethane [20]. The acetic acid is completely oxidised after the fifth mm in the case of propane, the fourth mm in the case of isooctane and the 3.5th mm in the case of toluene/isooctane. Even if the concentrations found in propane flames are lower than those of the other two fuels, the formation of acetic acid lasts more in time in the case of this fuel (as in the case of formic acid). This is probably due to the later formation of unsaturated C2 radicals from propane at the end of combustion. Fig. 3 shows that for the same distance from the burner, the concentration of acetic acid is higher at lean conditions, indicating that oxygen enhances its formation. The same results are observed in the case of a SI engine [18] and a Diesel engine [19]. In the case of a SI engine, acetic acid presents a maximum value at lZ1.0 in the case of fuels containing oxygenated compounds, while its concentration increases at lean conditions in the case of fuels containing only hydrocarbons [18]. In the case of oxygenated fuels, this is probably due to the rapid oxidation of intermediate oxygenate species under lean conditions than the production of acetic acid.
As in the case of formic acid, the results presented here are not sufficient to propose a detailed mechanism for the formation of acetic acid. A part of this acid is probably not formed in the flame, but during the cooling of exhaust gas in the probe. The formation of acetic acid might go through one of the five following paths: – the addition of a OH to a CH3CO: the OH attacks the CH3CO at the O containing carbon; a H extraction must also take place. The reactions: CH3COCOH/CH3 COOH, CH3CHOCOH/CH3COOHCH, CH3CHOC H2O/CH3COOHCH2, can probably be applied in this case. The formation of CH3CO from CH3CHO is already presented in literature [1,27], – the addition of an O to a CH3COH: the O attacks the CH3COH at the OH containing carbon. The reactions: CH3CH2OHCO/CH3COOHCH2, CH3CHOHC O/CH3COOHCH, CH3COHCO/CH3COOH can probably take place, – the addition of an H to a CH3COO: this is a final reaction that can take place during the cooling phase, – the isomerisation of a hyperoxide CH3–O–O–H/ CH3COOH.
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– the combination of two C1 radicals, like: CH3C COOH/CH3COOH. This is a final reaction and must take place during the cooling phase. As in the case of formic acid, the above reactions does not exist in bibliographical references; a more detailed formation path is difficult to be proposed at this stage of knowledge. 3.2.3. Propionic acid Fig. 4 presents the profiles of propionic acid. Propane produces low initial concentrations of this acid: from 0.3 to 0.6 ppmv, which corresponds to 10–25 times less than formic and acetic acids, indicating that the C1 and C2 radicals are found in higher concentrations than the C3 ones. Another probable reason is that propionic acid comes from the combination of two adequate C1 and a C2 radicals; this combination seems to be quite difficult. These concentrations increase slightly at stoichiometry to reach a maximum value at the third mm and then decrease to zero ppmv at the sixth mm. The concentration of this acid presents a plateau up to the third mm for all the other l. This plateau is shorter in the case of rich conditions and at lZ0.83 is practically
inexistent. The shape of these curves is very similar to that of formic acid, indicating that these two acids follow parallel paths. The formation of propionic acid from isooctane is delayed comparing to propane, indicating that propane produces easier C3 radicals than isooctane. Isooctane produces this acid only after the 0.5 mm, but its concentration increases rapidly to reach 2–4 ppmv at the second mm; then it decreases to zero ppmv at the 4th mm. The shape of the stoichiometric curve is the sharpest one; this of lean conditions is less sharp and this of the rich ones even less sharp. The shape of these curves is quite different from those of formic and acetic acids, indicating that propionic acid follows a different formation path in the case of isooctane. The beginning is delayed, indicating that the C3 radicals are produced later or that the propionic acid is formed from a combination of a C1 and a C2 radical which adds a supplementary path or that the this combination is difficult because the adequate radicals are found in low concentrations; the increase is sharper while the decrease is less sharp, indicating that its precursors are formed quite late, but the formation of propionic acid is then very fast. The addition of toluene into isooctane changes the profiles as it produces high initial concentrations of
Fig. 4. Left figure: profile of propionic acid for the five l and the three fuels used. Right figure: influence of l on the concentration of propionic acid. Three distances from the burner for the three fuels used.
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propionic acid: 10–20 ppmv, 30–40 times more than those produced from propane, indicating that toluene is broken into two unsaturated C3 radicals that can be oxidised to propionic acid, while the C3 radicals produced from propane give lighter products. Another probable reason is that toluene gives higher concentrations of unsaturated C1 and C2 radicals that can give propionic acid after their combination. These concentrations increase to reach a maximum value of 25–33 ppmv at the second mm and then decrease to zero ppmv before the fourth mm. The shape of these curves is very similar to that of formic and acetic acids, indicating that these acids follow parallel paths in the case of this fuel. The toluene/isooctane emits 100–150 times more propionic acid at the 0.5 mm than isooctane, and this ratio falls with distance to reach the values of 2–4 at the 4th mm (Fig. 2), indicating that, contrary to the other two acids, toluene enhances the formation of this acid more at the beginning of the combustion. This must indicate that toluene can break almost immediately in two unsaturated C3 radicals which are oxidised to propionic acid. This ratio is more important at rich conditions, indicating that toluene breaks into two C3 radicals easily under these conditions, while it is relatively more easily oxidised to lighter products at lean ones. This ratio decreases with the distance from the burner indicating that the difference at the formation of propionic acid precursors is lower at the latter stages of the combustion. These results indicates also that toluene gives also high concentrations of unsaturated C1 and C2 radicals (that combined and form propionic acid) in the beginning of its combustion, while propane and isooctane gives these radicals after several stages. The formation of propionic acid from fuel aromatics is already observed in the case of a SI engine [18], where a model correlating the exhaust emission of this acid with the aromatic content of the fuel is proposed. These results are also in accordance with those presented in the case of a Diesel engine, where the exhaust concentration of this acid increases with a fuel containing more mono-aromatics [19]. This acid in not detected in a natural gas feed SI engine [20]. As in the case of the two other acids, propionic acid is completely oxidised at the 3.5th mm in the case of toluene/isooctane, the fourth mm in the case of isooctane and later, at the sixth mm in the case of propane. Even if the concentrations found in propane flames are lower in the case of this fuel comparing to the other two, the formation of propionic acid lasts more in time in the case of isooctane/toluene indicating that the radicals produced from propane are easily oxidised to lighter products, while isooctane or toluene produce these precursors later. Another reason must be that the C1 and C2 unsaturated radicals produced from toluene/isooctane are found also at the end of the combustion and give propionic acid during the cooling phase. Contrary to the two previous acids, propionic acid presents a maximum concentration at stoichiometry for
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the three distances from the burner in the case of propane and isooctane (Fig. 4). The probable reasons for these profiles must be [18]: – the formation of this acid is enhanced by the increased temperature at stoichiometry, – the lack of oxygen prevents its formation at rich conditions, – the excess of oxygen oxidizes quickly propionic acid or its precursors formed at lean conditions. Contrary to the other two fuels, propionic acid has a more complex behaviour in the case of toluene/isooctane fuel. This acid increases with l in the beginning of its formation (1st mm), and presents a maximum at stoichiometry at the other distances from the burner. This indicates that propionic acid is enhanced by oxygen excess at the beginning of its formation; when oxygen concentration decreases at the second mm or higher, the precursors of propionic acid are preferably oxidised to CO. In the case of a SI engine, propionic acid presents a maximum value at stoichiometry [18], while it decreases with l in the case of a Diesel engine operating under lean conditions [18]. These results are in accordance with the ones presented here (except for the positions close to the flame, corresponding to the combustion beginning, in the case of toluene/isooctane fuel). As in the case of formic and acetic acids, the results presented here are not sufficient to propose a detailed mechanism for the formation of propionic acid. Its formation might go through one of the five following paths: – the addition of a OH to a CH3CH2CO: the OH attacks the CH3CH2CO at the O containing carbon; a H extraction must also take place. The reactions: CH3CH2COC OH/CH3CH2COOH, CH3CH2CHOCOH/CH3CH2COOHCH, CH3CH2CHOCH2O/CH3CH2COOHC H2, can probably be applied in this case, – the addition of an O to a CH3CH2COH: the O attacks the CH3CH2COH at the OH containing carbon. The reactions: CH 3CH 2CH 2OHCO/CH 3CH 2COOHCH2 , CH 3CH 2 CHOHCO/CH 3CH 2COOHCH, CH 3CH 2 COHCO/CH3CH2COOH can probably take place, – the addition of an H to a CH3CH2COO: this is a final reaction that can take place during the cooling phase, – the isomerisation of a hyperoxide CH3CH2–O–O–H/ CH3CH2COOH. –the combination of a C1 and a C2 radical, like: CH3C CH2COOH/CH3CH2COOH, or CH3CH2CCOOH/ CH3CH2COOH This is a final reaction and must take place during the cooling phase. As in the case of the two previous acids, the above reactions does not exist in bibliographical references; a more detailed formation path is difficult to be proposed at this stage of knowledge.
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3.2.4. Isovaleric and other organic acids Isovaleric acid is found only in the flames of toluene/ isooctane (Fig. 5), where it is produced very early: it is found at 0.1–0.3 ppmv at the 0.5 mm. These concentrations, which are very low comparing to the other acids produced from this fuel, increase to reach a maximum value at the second mm and then decrease to zero ppmv before the fourth mm. This acid is found in the exhaust gas of a SI engine, but not in the exhaust gas of pure isooctane or toluene/isooctane [18], indicating that it is oxidised before the exhaust gas sampling. The results presented in this previous work are very scattered, but this acid is enhanced by o-xylene and ETB. The present results confirm that this acid is also formed from toluene. For every distance from the burner, isovaleric acid presents a maximum concentration at rich conditions (Fig. 5). The same results are obtained in the case of a SI engine in the case of other fuels [18]. The slope of these curves decreases with the distance from the burner. The formation paths proposed for the previous organic acids can also applied in the case of the isovaleric acid. Two other acids, butyric and acrylic acids, are also found in the exhaust gas of a SI engine, but they are their formation is enhanced, the first one from o-xylene, and the second one from 1-hexene, cyclohexane and octane [18]. These acids are not detected in our flames, confirming that they are not produced by isooctane or toluene.
3.3. Comparison with the organic acids emitted by internal combustion engines Fig. 6 presents the concentration ranges of the organic acids emissions from different engines [18–20] and from the burner of this study. The natural gas engine emits only formic acid in very low concentration comparing to the others. For all acids, the Diesel engine emits lower concentrations comparing to the SI one. The concentrations found in the exhaust gas of the SI engine, when only isooctane or toluene/isooctane fuels are used, are lower for all acids detected. Even if some differences are observed, the concentration ranges found in the burner are of the same order of magnitude as those found in SI engine exhaust gas. 3.4. Links with flame temperature In some previous articles, some links between the exhaust concentration of organic acids and exhaust temperature were presented: – exhaust formic acid decreases with exhaust temperature at lean conditions [18–20], while it is quite independent of temperature at rich conditions [18]. – exhaust acetic and propionic acids decrease with exhaust temperature at lean conditions in the case of a Diesel engine [19], while they have no correlation at lean or rich conditions in the case of a SI engine [18]. In the case of the burner experiments, no correlation is found between flame temperature and the concentration of one of the four acids detected, using all the experimental points, or only lean or rich conditions, or those of each l or
Fig. 5. Profile of isovaleric acid for the five l used and influence of l toluene/isooctane fuel).
Fig. 6. Concentration range of the organic acids emissions of different engines and the burner used in this study. NG: SI natural gas fed engine; diesel: diesel engine; gasoline: SI gasoline fed engine; gasoline iC8, iC8T: SI engine fed with isooctane or toluene/isooctane; burner: burner experiments of the present work.
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3.6. Ratios between the concentration of organic acids and this of other combustion products with the same number of carbons The maximum concentrations of formic acid found in the combustion products are very low comparing to these of methanol and formaldehyde (Fig. 7). This acid is a secondary combustion product comparing to formaldehyde, which is the main C1 oxygenated intermediate product. Acetic acid concentrations are higher than those of ethanol, but lower than those of acetaldehyde. The concentrations of propionic acid are very low in the case of propane and isooctane, but quite important in the case of the toluene/ isooctane fuel. This last acid is a secondary product in the case of the first two fuels, but the main C3 oxygenated one in the case of toluene.
4. Conclusions Fig. 7. Maximum concentrations of organic acids, alcohols and aldehydes. C3: propane, iC8: isooctane, iC8T: toluene/isooctane.
even those of each distance from the burner. The results of the internal combustion engines seem to correspond to a more complex process; a part of exhaust organic acids is formed during the combustion, but the rest is probably formed in the exhaust manifold after the rapid cooling of the exhaust gas. 3.5. Links with other exhaust compounds 3.5.1. Oxygen In the case of engine experiments, formic acid increases with exhaust oxygen at lean conditions [18–20]. Acetic and propionic acids increase with exhaust oxygen in the case of a Diesel engine [19], but no correlation is found in the case of a SI engine [18]. In the case of the present results, no correlation is found between the concentration of oxygen and this of organic acids, because the oxygen concentration measured in the exhaust gas does not correspond to this measured in the burner flame. 3.5.2. Alcohols and carbonyl compounds No link between exhaust organic acids and alcohols or carbonyl compounds is found in the case of the engine exhaust gas [16,18]. Concerning the burner results, two linear correlations are found in the case of propane and two in the case of toluene/isooctane. These correlations are the following (units in ppmv): MethanolZ9.308!Formic AcidC7.75 (r2Z0.883), and AcroleineZ0.768!Acetic AcidK0.257 (r2Z0.907) for propane and AcetoneZ 0.404!Propionic AcidC2.3 (r2Z0.840) and n-PropanolZ0.0791!Propionic AcidC0.57 (r2Z0.804) in the case of toluene/isooctane. These equations indicate that the above compounds follow parallel formation paths.
Four organic acids were found in the combustion products of propane, isooctane and toluene/isooctane flames: formic, acetic, propionic and isovaleric acid. All these acids are formed very quickly; their concentration generally increases to reach a maximum value, and then decreases to reach zero ppmv. Toluene enhances more the formation of these acids than isooctane. The concentration of formic and acetic acids increases at lean conditions, this of isovaleric decreases and this of propionic presents generally a maximum at stoichiometry. These results are in accordance with those presented in the case of internal combustion engines. The concentrations of organic acids produced from these flames are of the same order of magnitude to those emitted from internal combustion engines. Some links are found between organic acids and some alcohols or carbonyl compounds, indicating that these compounds are probably formed in parallel. According to the above results, some probable paths for the formation of these acids are proposed. The organic acids can be formed in the flames, but also in the probe during the cooling process.
References [1] Warnatz J. 20th Symposium on Combustion 1984 p. 843–50. [2] Chakir A, Cathonnet M, Boettner JC, Gaillard F. Comb Sci Technol 1989;65:207–30. [3] Corre C, Dryer FL, Pitz WJ, Westbrook CK. Twenty-Fourth Symposium on Combustion 1992 p. 843–50. [4] Warth V, Stef N, Glaude PA, Battin-Leclerc F, Scacchi G, Come GM. Combust Flame 1998;114:81–102. [5] Chakir A, Cathonnet M, Boettner JC, Gaillard F. Twenty-second Symposium on Combustion 1998 p. 873–81. [6] Cavaliere A, Ciajolo A, D’Anna A, Mercogliano R, Ragucci R. Combust Flame 1993;93:279–86. [7] Dagaud Ph, Reuillon M, Cathonnet M. Combust Flame 1995;101: 132–40.
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[8] Dagaud P, Reuillon M, Cathonnet M. Comb Sci Technol 1994;95: 233–60. [9] Ranzi E, Faravelli T, Gaffuri P, Sogaro A, D’Anna A, Ciajolo A. Combust Flame 1997;108:24–42. [10] Curran HJ, Gaffuri P, Pitz WJ, Westbrook CK. Combust Flame 2002; 129:253–80. [11] Schobel A, Glass AG, Krebs L, Braun-Unkhoff M, Ahl C, Frank P. Chemosphere 2001;42:591–9. [12] Chakir A, Bellimam M, Boettner JC, Cathonnet M. Int J Chem Kinet 1992;24:385–410. [13] Kaiser EW, Siegel WO, Henig YI, Anderson RW, Trinker FH. Environ Sci Technol 1991;25:2005–12. [14] Zervas E, Montagne X, Lahaye J. J Air Waste Manag Assoc 1999;49: 1304–14. [15] Poulopoulos SG, Samaras DP, Philippopoulos C. Atm Environ 2001; 35:4399–406.
[16] Zervas E, Montagne X, Lahaye J. Environ Sci Technol 2002;36: 2414–21. [17] Kawamura K, Ng LL, Kaplan IR. Environ Sci Technol 1985;19: 1082–6. [18] Zervas E, Montagne X, Lahaye J. Environ Sci Technol 2001;35: 2746–51. [19] Zervas E, Montagne X, Lahaye J. Atm Environ 2001;35:1301–6. [20] Zervas E, Tazerout M. Atm Environ 2000;34:3921–9. [21] Schauer JJ, Kleeman MJ, Cass GR, Simoneit BRR. Environ Sci Technol 2001;35:1716–28. [22] Lawrence JE, Koutrakis P. Environ Sci Technol 1994;28:957–64. [23] Chebbi A, Carlier P. Atm Environ 1996;30:4233–49. [24] Zervas E, Montagne X, Lahaye J. Atm Environ 1999;33(29):4953–62. [25] Inal F, Senkan SM. Combust Flame 2003;131:16–28. [26] Degobert P. Automobile et pollution. Paris: Technip; 1992. [27] Davis SG, Law CK. Combust Flame 1999;119:375–99.
Formation of organic acids from propane, isooctane and toluene/isooctane flames E. Zervas* Institut Franc¸ais du Pe´trole, 1 et 4 avenue du Bois Pre´au, F-92500 Rueil-Malmaison cedex, France Received 30 September 2004; received in revised form 17 November 2004; accepted 18 November 2004 Available online 10 December 2004
Abstract Three fuels (propane, isooctane and toluene/isooctane) are used for the study of the formation of organic acids from their flames. Four organic acids have been found in the combustion products: formic, acetic, propionic and isovaleric acid. These acids are formed very quickly; their concentration then generally increases to reach a maximum value, and then decreases to zero. Toluene enhances the formation of organic acids comparing to propane and isooctane. The concentration of these acids depends strongly on the air/fuel equivalence ratio. Some correlations are found between the concentration of the acids and some alcohols or aldehydes. These results are in accordance with those presented in the case of internal combustion engines. q 2004 Elsevier Ltd. All rights reserved. Keywords: Acetic acid; Combustion; Formic acid; Isooctane; Propane; Propionic acid; Toluene
1. Introduction Oxygenated compounds are important intermediate products of the oxidation of hydrocarbons [1]. The most important family of oxygenated compounds is carbonyl compounds, and many authors study their formation in different flames as in the case of n-butane [2–4], 1-butene [5], n-heptane [6,7], isooctane [8–10], benzene [11] and other fuels. The formation of methanol is also presented in the case of n-butane [4], n-heptane [7,12] and isooctane [8]. These compounds are also found in the exhaust gas of internal combustion engines [13–16]. Organic acids is another family of oxygenated compounds found in exhaust gas of internal combustion engines [14,17–20]. These compounds are also found in the products of wood combustion (for higher than C14 acids, [21]). Organic acids are important pollutants of urban and rural atmospheres and contribute to acid rain formation [22]. The formation and reactions of organic acids of urban, rural and marine atmospheres are also presented in literature [23]. * Present Address: Renault-CTL L26 0 60, 1, Alle´e Cornuel, F-91510 Lardy, France. Tel.: C331-76-87-84-77; fax: C331-76-87-82-92. E-mail address: [email protected]. 0016-2361/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2004.11.012
As organic acids are emitted from internal combustion engines, they must also be emitted from burners or other industrial combustion sources, but no article presents their formation in of from flames. The present work studies the emission of organic acids from the combustion of three simple fuels: propane, isooctane and toluene/isooctane. The concentration profiles at stoichiometry, lean and rich conditions are presented and discussed. Some correlations with the other species formed from these flames, and also with the organic acids emitted from internal combustion engines are presented. Based on these results, some probable paths for the formation of these compounds are presented.
2. Experimental section Three fuels were used for this study: propane, isooctane and a blend of 20% of toluene and 80% of isooctane. The last two fuels were also used in a previous work concerning the emission of organic acids from a spark ignition engine [18]. The two liquid fuels were first evaporated in a long inox tube at 120 8C, and then mixed with air. The fuel rate was controlled with a peristaltic pump, and the stability of
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flame temperature was used to verify the evaporation rate of these liquids. This temperature varied less than 2 8C, indicating a constant fuel/air ratio, so a constant evaporation rate. The flames were stabilized on a flat (diameterZ 60 mm), water cooled (TZ20 8C) sintered bronze burner. The cold gas velocity was 6 cm/s, and five l values were used for these experiments: 0.83, 0.91, 1.0, 1.11 and 1.25. These values were also used in a previous work using a spark ignition engine [18]. The temperature was measured with a fast response thermocouple (silica coated 25fm Pt/Pt–13%Rh), and was corrected for the radiation. The combustion products were sampled isokinetically from the centre of the flame in a stainless steel oil cooled probe (internal diameterZ1 mm). The oil temperature was maintained at 110 8C to avoid water condensation, and a heated line at the same temperature was used to lead the gas from the probe to analytical devises. The sampling distance varied from 0.5 to 7 mm from the burner with a path of 0.5 mm. As the water flow rate must be enough to cool the burner but no too much to completely cool the flame, specific experiments were conducted to adjust it, and obtain, at stoichiometry, a temperature profile with a maximum at about 3 mm after the burner. The same flow rate was then used for all l. Organic acids were collected in deionised water and analysed by two methods: ionic chromatography/conductimetric detection for the analysis of formic acid and gas chromatography/flame inonization detector (GC/FID) for the analysis of the other aliphatic acids. Individual hydrocarbons, alcohols and aldehydes/ketones were also analysed, but these results will be presented in a next article. More details about the analytical methods used can be found elsewhere [14,24]. The repeatability of these tests is determined by five identical tests, performed at the first mm from the burner, for the five l used. The other tests were doubled and average value was used. For all l, the relative standard deviation was less than 8% for the CO, CO2, HC, NOx and O2 analysis. The relative standard deviation of all organic acids measurements was less than 19%.
3. Results and discussion 3.1. Profiles of O2, CO, CO2, total HC, NOx and flame temperature As expected, for the three fuels used, the concentration of O2 decreases with the distance from the burner and increases with l, this of HC decreases with both distance and l and this of NOx decreases with distance and presents a maximum value at about lZ1.11. The concentration of CO increases with distance and decreases with l; while this of CO2 increases with distance before reaching a plateau, and presents a maximum value at stoichiometry. Flame temperature initially increases and reaches a maximum
value at about 3 mm, and then decreases with distance; temperature presents a maximum value at stoichiometry. These results are in accordance with those presented in literature in the case of flame experiments [25] and internal combustion engines [26]. 3.2. Profiles of organic acids Four organic acids were found in detectable concentrations in the products of these flames: formic, acetic, propionic and isovaleric acid. 3.2.1. Formic acid Fig. 1 presents the concentration of formic acid versus the distance from the burner for the three fuels and the five l used. In the case of propane, formic acid is formed very quickly, as its concentration is 5–9 ppmv at the first 0.5 mm from the burner. This concentration remains quite constant up to the 3rd mm at rich conditions and stoichiometry. It increases at lean ones for the same distance from the burner to reach 12–13 ppmv, indicating that oxygen excess enhances the oxidation of C1 species to this acid. After the third mm, its concentration falls down very sharply to reach zero ppmv after the fourth mm. The profile of formic acid is quite different in the case of isooctane. This acid is also formed very quickly. Five to seven ppmv are found at 0.5 mm under rich conditions, but more of the double under lean ones (10–18 ppmv). This indicates that the CH3 radicals are found in higher concentrations in isooctane flames, for two reasons: isooctane has more CH3 groups and combustion temperature is a little higher in the case of isooctane, enhancing its decomposition. These initial concentrations increase for each lambda, to reach a maximum value at the third mm (8–30 ppmv). This increase is sharper in the case of lean conditions; oxygen excess continues the oxidation of C1 radicals to formic acid, while lack of oxygen prevents this oxidation at rich conditions. After the third mm, formic acid is oxidised and its concentration falls down to zero at the fourth mm. The addition of toluene to isooctane fuel modifies the profile of formic acid, which is found at slightly higher initial concentrations than in the case of pure isooctane. After this point, all the curves increase to reach a maximum concentration at the second mm, earlier than the other two fuels, for which the maximum concentration is at the third mm. The maximum concentration is also higher than in the case of pure isooctane. After this point, formic acid concentration decreases down to zero ppmv after the fourth mm, but this decrease is less sharp than in the case of the two previous fuels. The comparison between the profiles of the three fuels indicates that this acid is enhanced by tolueneO isooctaneOpropane. As toluene/isooctane has lower combustion temperature than pure isooctane, this cannot be due to the higher production of C1 radicals due to increased temperature, as in the case of isooctane. Toluene can give
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Fig. 1. Left figure: profile of formic acid concentration for the five l and the three fuels used. Right figure: influence of l on the concentration of formic acid. Three distances from the burner for the three fuels used.
a CH3 radical by dealkylation, but isooctane has more CH3 groups, the concentration of CH3 radicals must be higher in the case of isooctane. This must indicate that other C1 species are also (or main) precursors of formic acid, and toluene can give more easily unsaturated C1 radicals like CH or CH2 (or oxygenated versions) than isooctane. Fig. 2 presents the ratio between the concentrations of formic acid emitted from the toluene/isooctane fuel and the isooctane one. The first fuel emits 1.1–1.6 times more formic acid than the second one in the beginning of the combustion, indicating that it produces more C1 radicals. This ratio increases with the distance and reaches 1.5–3 at the second mm, indicating that toluene enhances more the formation of formic acid in the middle of the combustion, because at this stage toluene is more decomposed to unsaturated C1 species. After this point, the ratio is less than one, as formic acid produced from toluene is oxidised earlier that this produced from isooctane. This ratio is more important at rich conditions, because formic acid is more enhanced by unsaturated C1 radicals, which are probably favourably produced under these latter conditions. These results are in accordance with those observed on a SI engine: formic acid’s concentrations are higher in the case of toluene/isooctane fuel comparing to pure isooctane [18].
Formic acid is completely oxidised at the fifth mm in the case of propane and at the fourth mm in the case of isooctane and toluene/isooctane fuels. Even if the concentrations found in propane flames are lower than those of the other two fuels, the formation of this acid lasts more in time in the case of this fuel. The reason must be that propane gives unsaturated C1 radicals only at the end of its combustion. Fig. 1 shows that for the same distance from the burner, the concentration of formic acid is higher at lean conditions for all three fuels used, indicating that even unsaturated C1 radicals are enhanced at rich conditions, oxygen excess is more important for its formation. The same results are observed in the case of internal combustion engines [18–20]. The results presented here are not sufficient to propose a detailed mechanism for the formation of formic acid. This compound can be product of the HC, aldehydes, alcohols or oxygenated radicals oxidation. As all organic acids are products of low temperature oxidation, they (or a part of them) are probably not formed in the flame, but during the cooling of exhaust gas in the probe, from a rapid combination of different species, as for example CxHyCO and OH or CxHyCOO and H. This is probably the reason for
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– the addition of an H to a HCOO; this is a final reaction that can take place during the cooling phase, – the isomerisation of a hyperoxide H–O–O–H/ HCOOH. No bibliographical reference propose these reactions; a more detailed formation path is difficult to be proposed at this stage of knowledge.
Fig. 2. Ratio between the concentrations of formic, acetic and propionic acids emitted from the toluene/isooctane fuel and the isooctane one, as a function of the distance from the burner for the five l used.
the increased concentration of formic acid from toluene flames: the unsaturated C1 radicals are easily oxidised at the low temperature of the probe to formic acid. The formation of formic acid might go through one of the four following paths: – the addition of a OH to a HCO: the OH attacks the HCO and a H extraction must take place. The reactions: HCOCOH/HCOOH, HCHOCOH/HCOOHCH, HCHOCH2O/HCOOHCH2, can probably be applied in this case. HCO is a common compound in flames [1,12]; a combination with oh can give formic acid. – the addition of an O to a HCOH. The reactions: HCH2 OHCO/HCOOHCH2, HCHOHCO/HCOOHCH, HCOHCO/HCOOH can take place,
3.2.2. Acetic acid Fig. 3 presents the profiles of acetic acid for the three fuels used. In the case of propane, the initial concentration of acetic acid is within 4 and 7 ppmv, which is quite similar to this of formic acid. These concentrations increase up to the third mm to reach 6–9 ppmv, and then decrease sharply to zero ppmv at the fifth mm. The shapes of acetic and formic acids are quite similar indicating that these two compounds follow similar formation paths. Isooctane produces initially 5–10 ppmv of acetic acid (slightly higher than propane), but its concentration increases to reach 7– 17 ppmv at the third mm. This increase is sharper at stoichiometry and lean conditions, due to oxygen excess. These concentrations indicate that isooctane produces more acetic acids precursors, as C2 radicals, than propane. And, as isooctane has more carbons atoms in his structure, the formation of C2 species is longer than this of propane; for this reason the formation of acetic acid is delayed comparing to propane. After the third mm, acetic acid concentration falls sharply to zero ppmv after the fourth mm. The shape of these profiles is quite similar to that of formic acid formed from isooctane, indicating that these two acids follow quite similar formation paths. The addition of toluene into isooctane changes these curves. Acetic acid is still formed very early: it is found at about 5–10 ppmv at the 0.5 mm (like in the case of isooctane), increases sharply to reach about 15 ppmv at the second mm, and then falls down to zero ppmv before the fourth mm. As in the case of formic acid and apparently for the same reasons, the points of the maximum concentration and the complete oxidation are found earlier in the case of this fuel than the other two. The shape of these profiles is similar to that of formic acid, indicating that these two compounds follow quite similar formation paths. At the 0.5 mm, toluene/isooctane emits slightly higher concentration of acetic acid than isooctane (Fig. 2). As in the case of formic acid, this ratio increases with distance to reach a maximum value of 1.3–2 ppmv at the second mm, indicating that, as in the case of formic acid, toluene enhances more the formation of acetic acid at the middle of the combustion. For each distance, this ratio is more important at rich conditions. This is must due to the favourable formation of unsaturated C2 species from toluene than from isooctane. These results are in accordance with those observed on a SI engine: acetic acid is emitted at higher concentrations from toluene/isooctane comparing to isooctane [18]. Acetic and higher acids are not found in detectable concentrations in the exhaust gas of
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Fig. 3. Left figure: profile of acetic acid for the five l and the three fuels used. Right figure: influence of l on the concentration of acetic acid. Three distances from the burner for the three fuels used.
a natural feed SI engine, even if the natural gas used contained 8% of ethane [20]. The acetic acid is completely oxidised after the fifth mm in the case of propane, the fourth mm in the case of isooctane and the 3.5th mm in the case of toluene/isooctane. Even if the concentrations found in propane flames are lower than those of the other two fuels, the formation of acetic acid lasts more in time in the case of this fuel (as in the case of formic acid). This is probably due to the later formation of unsaturated C2 radicals from propane at the end of combustion. Fig. 3 shows that for the same distance from the burner, the concentration of acetic acid is higher at lean conditions, indicating that oxygen enhances its formation. The same results are observed in the case of a SI engine [18] and a Diesel engine [19]. In the case of a SI engine, acetic acid presents a maximum value at lZ1.0 in the case of fuels containing oxygenated compounds, while its concentration increases at lean conditions in the case of fuels containing only hydrocarbons [18]. In the case of oxygenated fuels, this is probably due to the rapid oxidation of intermediate oxygenate species under lean conditions than the production of acetic acid.
As in the case of formic acid, the results presented here are not sufficient to propose a detailed mechanism for the formation of acetic acid. A part of this acid is probably not formed in the flame, but during the cooling of exhaust gas in the probe. The formation of acetic acid might go through one of the five following paths: – the addition of a OH to a CH3CO: the OH attacks the CH3CO at the O containing carbon; a H extraction must also take place. The reactions: CH3COCOH/CH3 COOH, CH3CHOCOH/CH3COOHCH, CH3CHOC H2O/CH3COOHCH2, can probably be applied in this case. The formation of CH3CO from CH3CHO is already presented in literature [1,27], – the addition of an O to a CH3COH: the O attacks the CH3COH at the OH containing carbon. The reactions: CH3CH2OHCO/CH3COOHCH2, CH3CHOHC O/CH3COOHCH, CH3COHCO/CH3COOH can probably take place, – the addition of an H to a CH3COO: this is a final reaction that can take place during the cooling phase, – the isomerisation of a hyperoxide CH3–O–O–H/ CH3COOH.
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– the combination of two C1 radicals, like: CH3C COOH/CH3COOH. This is a final reaction and must take place during the cooling phase. As in the case of formic acid, the above reactions does not exist in bibliographical references; a more detailed formation path is difficult to be proposed at this stage of knowledge. 3.2.3. Propionic acid Fig. 4 presents the profiles of propionic acid. Propane produces low initial concentrations of this acid: from 0.3 to 0.6 ppmv, which corresponds to 10–25 times less than formic and acetic acids, indicating that the C1 and C2 radicals are found in higher concentrations than the C3 ones. Another probable reason is that propionic acid comes from the combination of two adequate C1 and a C2 radicals; this combination seems to be quite difficult. These concentrations increase slightly at stoichiometry to reach a maximum value at the third mm and then decrease to zero ppmv at the sixth mm. The concentration of this acid presents a plateau up to the third mm for all the other l. This plateau is shorter in the case of rich conditions and at lZ0.83 is practically
inexistent. The shape of these curves is very similar to that of formic acid, indicating that these two acids follow parallel paths. The formation of propionic acid from isooctane is delayed comparing to propane, indicating that propane produces easier C3 radicals than isooctane. Isooctane produces this acid only after the 0.5 mm, but its concentration increases rapidly to reach 2–4 ppmv at the second mm; then it decreases to zero ppmv at the 4th mm. The shape of the stoichiometric curve is the sharpest one; this of lean conditions is less sharp and this of the rich ones even less sharp. The shape of these curves is quite different from those of formic and acetic acids, indicating that propionic acid follows a different formation path in the case of isooctane. The beginning is delayed, indicating that the C3 radicals are produced later or that the propionic acid is formed from a combination of a C1 and a C2 radical which adds a supplementary path or that the this combination is difficult because the adequate radicals are found in low concentrations; the increase is sharper while the decrease is less sharp, indicating that its precursors are formed quite late, but the formation of propionic acid is then very fast. The addition of toluene into isooctane changes the profiles as it produces high initial concentrations of
Fig. 4. Left figure: profile of propionic acid for the five l and the three fuels used. Right figure: influence of l on the concentration of propionic acid. Three distances from the burner for the three fuels used.
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propionic acid: 10–20 ppmv, 30–40 times more than those produced from propane, indicating that toluene is broken into two unsaturated C3 radicals that can be oxidised to propionic acid, while the C3 radicals produced from propane give lighter products. Another probable reason is that toluene gives higher concentrations of unsaturated C1 and C2 radicals that can give propionic acid after their combination. These concentrations increase to reach a maximum value of 25–33 ppmv at the second mm and then decrease to zero ppmv before the fourth mm. The shape of these curves is very similar to that of formic and acetic acids, indicating that these acids follow parallel paths in the case of this fuel. The toluene/isooctane emits 100–150 times more propionic acid at the 0.5 mm than isooctane, and this ratio falls with distance to reach the values of 2–4 at the 4th mm (Fig. 2), indicating that, contrary to the other two acids, toluene enhances the formation of this acid more at the beginning of the combustion. This must indicate that toluene can break almost immediately in two unsaturated C3 radicals which are oxidised to propionic acid. This ratio is more important at rich conditions, indicating that toluene breaks into two C3 radicals easily under these conditions, while it is relatively more easily oxidised to lighter products at lean ones. This ratio decreases with the distance from the burner indicating that the difference at the formation of propionic acid precursors is lower at the latter stages of the combustion. These results indicates also that toluene gives also high concentrations of unsaturated C1 and C2 radicals (that combined and form propionic acid) in the beginning of its combustion, while propane and isooctane gives these radicals after several stages. The formation of propionic acid from fuel aromatics is already observed in the case of a SI engine [18], where a model correlating the exhaust emission of this acid with the aromatic content of the fuel is proposed. These results are also in accordance with those presented in the case of a Diesel engine, where the exhaust concentration of this acid increases with a fuel containing more mono-aromatics [19]. This acid in not detected in a natural gas feed SI engine [20]. As in the case of the two other acids, propionic acid is completely oxidised at the 3.5th mm in the case of toluene/isooctane, the fourth mm in the case of isooctane and later, at the sixth mm in the case of propane. Even if the concentrations found in propane flames are lower in the case of this fuel comparing to the other two, the formation of propionic acid lasts more in time in the case of isooctane/toluene indicating that the radicals produced from propane are easily oxidised to lighter products, while isooctane or toluene produce these precursors later. Another reason must be that the C1 and C2 unsaturated radicals produced from toluene/isooctane are found also at the end of the combustion and give propionic acid during the cooling phase. Contrary to the two previous acids, propionic acid presents a maximum concentration at stoichiometry for
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the three distances from the burner in the case of propane and isooctane (Fig. 4). The probable reasons for these profiles must be [18]: – the formation of this acid is enhanced by the increased temperature at stoichiometry, – the lack of oxygen prevents its formation at rich conditions, – the excess of oxygen oxidizes quickly propionic acid or its precursors formed at lean conditions. Contrary to the other two fuels, propionic acid has a more complex behaviour in the case of toluene/isooctane fuel. This acid increases with l in the beginning of its formation (1st mm), and presents a maximum at stoichiometry at the other distances from the burner. This indicates that propionic acid is enhanced by oxygen excess at the beginning of its formation; when oxygen concentration decreases at the second mm or higher, the precursors of propionic acid are preferably oxidised to CO. In the case of a SI engine, propionic acid presents a maximum value at stoichiometry [18], while it decreases with l in the case of a Diesel engine operating under lean conditions [18]. These results are in accordance with the ones presented here (except for the positions close to the flame, corresponding to the combustion beginning, in the case of toluene/isooctane fuel). As in the case of formic and acetic acids, the results presented here are not sufficient to propose a detailed mechanism for the formation of propionic acid. Its formation might go through one of the five following paths: – the addition of a OH to a CH3CH2CO: the OH attacks the CH3CH2CO at the O containing carbon; a H extraction must also take place. The reactions: CH3CH2COC OH/CH3CH2COOH, CH3CH2CHOCOH/CH3CH2COOHCH, CH3CH2CHOCH2O/CH3CH2COOHC H2, can probably be applied in this case, – the addition of an O to a CH3CH2COH: the O attacks the CH3CH2COH at the OH containing carbon. The reactions: CH 3CH 2CH 2OHCO/CH 3CH 2COOHCH2 , CH 3CH 2 CHOHCO/CH 3CH 2COOHCH, CH 3CH 2 COHCO/CH3CH2COOH can probably take place, – the addition of an H to a CH3CH2COO: this is a final reaction that can take place during the cooling phase, – the isomerisation of a hyperoxide CH3CH2–O–O–H/ CH3CH2COOH. –the combination of a C1 and a C2 radical, like: CH3C CH2COOH/CH3CH2COOH, or CH3CH2CCOOH/ CH3CH2COOH This is a final reaction and must take place during the cooling phase. As in the case of the two previous acids, the above reactions does not exist in bibliographical references; a more detailed formation path is difficult to be proposed at this stage of knowledge.
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3.2.4. Isovaleric and other organic acids Isovaleric acid is found only in the flames of toluene/ isooctane (Fig. 5), where it is produced very early: it is found at 0.1–0.3 ppmv at the 0.5 mm. These concentrations, which are very low comparing to the other acids produced from this fuel, increase to reach a maximum value at the second mm and then decrease to zero ppmv before the fourth mm. This acid is found in the exhaust gas of a SI engine, but not in the exhaust gas of pure isooctane or toluene/isooctane [18], indicating that it is oxidised before the exhaust gas sampling. The results presented in this previous work are very scattered, but this acid is enhanced by o-xylene and ETB. The present results confirm that this acid is also formed from toluene. For every distance from the burner, isovaleric acid presents a maximum concentration at rich conditions (Fig. 5). The same results are obtained in the case of a SI engine in the case of other fuels [18]. The slope of these curves decreases with the distance from the burner. The formation paths proposed for the previous organic acids can also applied in the case of the isovaleric acid. Two other acids, butyric and acrylic acids, are also found in the exhaust gas of a SI engine, but they are their formation is enhanced, the first one from o-xylene, and the second one from 1-hexene, cyclohexane and octane [18]. These acids are not detected in our flames, confirming that they are not produced by isooctane or toluene.
3.3. Comparison with the organic acids emitted by internal combustion engines Fig. 6 presents the concentration ranges of the organic acids emissions from different engines [18–20] and from the burner of this study. The natural gas engine emits only formic acid in very low concentration comparing to the others. For all acids, the Diesel engine emits lower concentrations comparing to the SI one. The concentrations found in the exhaust gas of the SI engine, when only isooctane or toluene/isooctane fuels are used, are lower for all acids detected. Even if some differences are observed, the concentration ranges found in the burner are of the same order of magnitude as those found in SI engine exhaust gas. 3.4. Links with flame temperature In some previous articles, some links between the exhaust concentration of organic acids and exhaust temperature were presented: – exhaust formic acid decreases with exhaust temperature at lean conditions [18–20], while it is quite independent of temperature at rich conditions [18]. – exhaust acetic and propionic acids decrease with exhaust temperature at lean conditions in the case of a Diesel engine [19], while they have no correlation at lean or rich conditions in the case of a SI engine [18]. In the case of the burner experiments, no correlation is found between flame temperature and the concentration of one of the four acids detected, using all the experimental points, or only lean or rich conditions, or those of each l or
Fig. 5. Profile of isovaleric acid for the five l used and influence of l toluene/isooctane fuel).
Fig. 6. Concentration range of the organic acids emissions of different engines and the burner used in this study. NG: SI natural gas fed engine; diesel: diesel engine; gasoline: SI gasoline fed engine; gasoline iC8, iC8T: SI engine fed with isooctane or toluene/isooctane; burner: burner experiments of the present work.
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3.6. Ratios between the concentration of organic acids and this of other combustion products with the same number of carbons The maximum concentrations of formic acid found in the combustion products are very low comparing to these of methanol and formaldehyde (Fig. 7). This acid is a secondary combustion product comparing to formaldehyde, which is the main C1 oxygenated intermediate product. Acetic acid concentrations are higher than those of ethanol, but lower than those of acetaldehyde. The concentrations of propionic acid are very low in the case of propane and isooctane, but quite important in the case of the toluene/ isooctane fuel. This last acid is a secondary product in the case of the first two fuels, but the main C3 oxygenated one in the case of toluene.
4. Conclusions Fig. 7. Maximum concentrations of organic acids, alcohols and aldehydes. C3: propane, iC8: isooctane, iC8T: toluene/isooctane.
even those of each distance from the burner. The results of the internal combustion engines seem to correspond to a more complex process; a part of exhaust organic acids is formed during the combustion, but the rest is probably formed in the exhaust manifold after the rapid cooling of the exhaust gas. 3.5. Links with other exhaust compounds 3.5.1. Oxygen In the case of engine experiments, formic acid increases with exhaust oxygen at lean conditions [18–20]. Acetic and propionic acids increase with exhaust oxygen in the case of a Diesel engine [19], but no correlation is found in the case of a SI engine [18]. In the case of the present results, no correlation is found between the concentration of oxygen and this of organic acids, because the oxygen concentration measured in the exhaust gas does not correspond to this measured in the burner flame. 3.5.2. Alcohols and carbonyl compounds No link between exhaust organic acids and alcohols or carbonyl compounds is found in the case of the engine exhaust gas [16,18]. Concerning the burner results, two linear correlations are found in the case of propane and two in the case of toluene/isooctane. These correlations are the following (units in ppmv): MethanolZ9.308!Formic AcidC7.75 (r2Z0.883), and AcroleineZ0.768!Acetic AcidK0.257 (r2Z0.907) for propane and AcetoneZ 0.404!Propionic AcidC2.3 (r2Z0.840) and n-PropanolZ0.0791!Propionic AcidC0.57 (r2Z0.804) in the case of toluene/isooctane. These equations indicate that the above compounds follow parallel formation paths.
Four organic acids were found in the combustion products of propane, isooctane and toluene/isooctane flames: formic, acetic, propionic and isovaleric acid. All these acids are formed very quickly; their concentration generally increases to reach a maximum value, and then decreases to reach zero ppmv. Toluene enhances more the formation of these acids than isooctane. The concentration of formic and acetic acids increases at lean conditions, this of isovaleric decreases and this of propionic presents generally a maximum at stoichiometry. These results are in accordance with those presented in the case of internal combustion engines. The concentrations of organic acids produced from these flames are of the same order of magnitude to those emitted from internal combustion engines. Some links are found between organic acids and some alcohols or carbonyl compounds, indicating that these compounds are probably formed in parallel. According to the above results, some probable paths for the formation of these acids are proposed. The organic acids can be formed in the flames, but also in the probe during the cooling process.
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