INTRODUCTION

Liquid spray combustion of hydrocarbon fuels is used in modern society in many processes of energy utilisation, such as diesel engines, gas turbines, heating furnaces, and hoilers. Over the pilst few years. research efforts have fo- cused on the improvement of the performance of liquid spray combustion systems, and also on the reduction of soot as its environmental impact is causing increasing concern. This in- terest has resulted in the application of stricter legal limits to the permitted emission levels of particulates from engines, etc. The survival of any industrial processes is now dependent on its ability to satisfy such prospective tough en- vironmental legislation. The future use of lower grade fuels and the enforcement of stricter controls make it imperative to obtain a better understanding of the combustion chemistry, the formulation of fuels from components, and the influence of fuel additives. It has been reported [l-4] that many parameters, such as fuels, temperature, air-to-fuel ratio, and pres- sure, significantly influence soot emissions, al- though the formation mechanisms are not yet fully understood.

Combustion systems are the major sources of PAH emission [51. A range of PAHs can be found in the gaseous phase products, and also absorbed on soot as a pollutant from combus-

* Corresponding author.

COMBUSTION AND FLAME 112:35Y-370 (199X) 0 1998 hy The Combustion Institute Published by Elsevier Science Inc.

tions. Soot and PAHs are not physiologically inert. Soot particles are one of the major sources of PM10 (particulate matter, less than IO pm in diameter) which are presently cans- ing health concerns. Some of PAHs, in particu- lar those having four or five aromatic rings, are known to be mutagenic and carcinogenic [6-81. PAHs in the gaseous phase or condensed on soot particles can be inhaled by humans, and hence can be harmful to health. These eom- pounds are also formed in combustion at the same time as soot, and are intrinsically in- volved in the soot formation chemistry. The process of PAH mass growth could lead to the inception and growth of soot, and PAHs as well as acetylene fC,H,I are possible soot growth species [9-121. Such a possible mecha- nism is given later in Fig. 12 and discussed below.

Chemically different fuels have different sooting tendency in flames [131, and may utilise differences in the formation mechanism for soot. Soot formation in gas-phase combustion has been extensively investigated using both various pure components and commercial bt- els. However, differences in both the chemical and physical properties exist between hetero- geneous and homogeneous reaction systems. Fuel spray combustion involves the stages of spray injection, dispersion, vapourisation (which influence air-fuel mixing during the last burning stage), and gas-phase pyrolysis, all of which may also influence soot formation. Fuel droplets can also be pyrolysed or oxidased directly to form soot.

tW10-2180/‘XI//s19.MJ PII suu1ll-21Klu97)w134-x

360 R. WANG AND P. CADMAN

These considerations led to this study of soot production from the combustion of fuel sprays, particularly in conjunction with the as- pect of PAH formation. Primarily, three dif- ferent single hydrocarbons, toluene, n-heptane, and propanol-I, were chosen to represent fuel types of aromatic-, aliphatic-, and oxygen-con- taining fuels for this study, and their combus- tion was investigated over a range of tempera- tures and pressures behind reflected shock waves. Alcohols have been found to give lower soot yields [14, 151 than other hydrocarbons in combustion so that an alcohol was chosen as the oxygenated fuel.

EXPERIMENTAL

A stainless steel shock tube of 6.4 cm id. was used. The driver section, 6 m long, was sepa- rated from the driven section of 5 m length by mylar diaphragms. This configuration was found to give up to 3.5 ms reaction time in the reflected wave with constant conditions. After this time, the pressure and temperature de- creased due to the arrival of the contact sur- face/rarefaction waves from down the tube. The driver gas was helium, and the driven gas was prepared mixtures of oxygen (O-10%0) in argon. The shack was trigged by melting the two layers of a plastic diaphragm using a 12 V/4 A pulse through a hot wire loop placed between tnem. This method was found to be a clean reproducible way to initiate the shock. At the end of driven section, a small fuel pulse injection system (constructed in the author’s laboratory and based upon a Bosch injector) was used to give short pulses of small fuel droplets (typical size lo-30 pm1 injected in a narrow angle up the shock tube and operated in synchronisation with the shock wave. The droplet sizes were measured separately using with a Malvern Sizer (kindly loaned by Shell Research, Thornton). The mass of injected fuel was about 75-80 mg. The incident and re- flected shock velocities were determined by PCB pressure transducers mounted on the endplate and at 10 and 30 cm distances from the endplate. The reflected shock tempera- tures and pressures were calculated from the incident and reflected velocities using a com- puter code via the normal methods. The den-

sity of 0, was calculated to be in the range of 0.4-0.7 x lo-” mol/cm3 in the reflected shock.

Soot and PAHs were collected by trapping them in filters (Whatman type 31 through which the shock-tube contents were vented and pumped immediately after each test. These filters, containing the soot and other products, were kept in a desiccator at constant humidity, and the yield of soot was determined by weigh- ing the filters before and after each test. About 90% of soot produced from combustion was recovered. After each test, the shock tube was cleaned by a cloth and, regularly, by a high- temperature and high-oxygen blank test to bum out any remaining residues.

The soot samples were Soxhlet-extracted with dichloromethane (DCMI for about 2-3 h until the solvent in the extraction chamber was clear. The total PAH yield was defined as extracts by DCM from the Scot, and was deter- mined by weight difference after drying the extracted soot. A DCM solution of PAHs in concentration of about 0.1-0.5 wt% was anal- ysed by a Waters h.p.1.c. equipped with a ChromSpher 3 PAH (100*4.6 mm) column from Chrompack UK Ltd. using a mixture of acetonitrile and water (gradient from 53.47 to 9O:lO v/v) as mobile phase. This Waters h.p.1.c. has a photodiode array detector which can scan and record the UV-VIS spectrum of each peak of chromatogram as it appears, as an aid to their identification. Some selected samples were further analysed by a Kratos MS25 Mass Spectrometer (m.s.1 equipped with a heated probe or a g.1.c. and electron impact ionisation detector or by a fast atom bombardment ffab) m.s. carried out at EPSRC Mass Spectrometry Service Centre, Swansea, UK. Standard PAHs, used for h.p.1.c. analysis, were obtained from various commercial sources, such as Fisons Ltd. and Aldrich Ltd., UK.

RESULTS AND DISCUSSION

Soot Formation

Pyrolysis and combustion of toluene, heptane, and propanol were carried out in both argon and in 10% O,/drgon mixtures. The results arc shown in Figs. l-3. The temperatures

SPRAY COMBUSTION OF HYDROCARBONS

shown refer to the prcccmbustion conditions calculated for real gas mixtures. After the start of combustion. the temperatures can rise due to the exothermicity of the reaction. Among the three single fuels, toluene was found to be the most strongly sooting fuel. The tempera- ture strongly influenced the soot formation. At temperatures below 1800 K and higher than 2400 K, there was low soot production. A maxi- mum occurred at a temperature between 1900 and 2200 K. Pyrolysis of toluene produced a maximum amount of soot at a temperature above 2000 K.

The soot yield produced from heptane com- bustion was lower (about 50% of the maximum of toluene), but gave a similar bell-shaped curve (Fig. 2). This indicated that the maximum a nount of soot was produced at a temperature of about 1700 K, lower than for toluene. A temperature shift of the maximum of 200 K higher for the pyrolysis was again observed. Pyrolysis of heptane gave a higher soot produc- tion than did its combustion in 10% Oz.

Combustion of propanol with 10% Oz pro- duced much less soot than that of toluene and heptane, about 10 times less than heptane and 20 times less than toluene. This is perhaps to be expected because of the leaner mixtures used. Over the temperature range of 100&3000 K, the yield of soot from propanol remained virtually unchanged at 0.2-0.5 mg. However.

j i, I 0 500 1000 1500 2cml 2500 3mo 3soa

Temperature. K Fig. 2. Smt yields at various temperature from heptane comhuslion.

pyrolysis of propanol yielded much more soot (four-ten times higher than its combustion). Below 1700 K, the pyrolysis of propanol gave low soot yields (about 0.2-0.5 me, but these increased to about 2.0 mg about 1700 K, and held this range up to the maximum tempera- ture studied (2900 K). Addition of 10% te- tradecane to propanol-1 increased the soot production markedly, to a value more than tenfold higher than without it (Fig. 3). These results indicated that alcohols, generally re-

Fig. 3. Soot production from combustion of propand and in its mixture with tetradrcane.

362 R. WANG AND P. CADMAN

TABLE I

PAW ldcntified from Soot Extract of Ueptane and Toluenc Combustion

Naphlhalene Biphenyl Phenanthrene

m-Terphenyl p-Terphenyl

Acenaphthalene FlUOR3WhenE Chlysene wene

Benza[a]anlhracene Eenzo[blflf laranlhene Benzolklfluoranthene Oibenzo[a.hjanthracene

q enzo[a]pyrene Indeno[l.Z3cdjpyrene Eenzo[ghi]petylene C0r0lWl.2

garded ns clean furls, or a soot-reducing agent, have a very limited effect in fuel mixtures. In the tests with propanol-1 and 10% tetradecane mixtures, soot production behavior was domi- nated by the presence of the tctradecane. The average soot yield over the temperatures of 1400-2200 K was about 3.5 mg, and the high- est yields of soot were observed at a tempera- ture of about 1900 K.

Tests of the influence of pressure, in a range of 2-25 bar (Pj), on a soot production used toluene at 1500-1600 K. The change in tem- perature over this region did not markedly affect soot production (Fig. 1). The influence of pressure on soot production was found to be minimal over the pressure range of 2-25 bar,

only B slight increase with incrcasc in pressure was found (Fig. 4).

PAH Formation

The chromatographic analysis revealed that there exist more than 50 PAHs in the soot extract (Fig. 5). Most of the PAHs have a 2-7 fused benzene ring in structure. Among the 21 PAHs identified by h.p.1.c. or mass spectrome- try were those listed in Table 1. Some carcino- genic PAH, namely bcnzolalpyrenr, was found in the soot extract. (Identification of pen- ta[c,d]pyrene, one of the prevalent PAH in combustion products, has not yet been found, but may be present in low yield). Acenaphtha-

SPRAY COMBUSTION OF HYDROCARBONS 363

1 DChI (ruiven1) 2 Kaphthalene 3 Fluorene 4 Phenanthrtm 5 *n*raccne 6 Fluarnnthene 7 Pyene S BenzoWmthracene 9 Chryrene

10 Benzo[b]fluaranlhene 11 Benro[klfluoranthene 12 Benzo[%3,pyrene 13 Dibemo[a,hlanthracenr 14 Indena[l,l.3.cdlpyrene

I , 0 20 40 60 90

Fig. 5. Chmmatogram of soot DCM c‘strilct from toluenc comhustkm.

lcnc was found ip some samples in small amounts. It was ohserved that similar types of PAH were identified in the heptane ind toluene soot samples, regardless of tentpera- ture. pressure, and in the presence of oxygen. The quantity of PAHs from propanol soot was

c‘rv small. and less tha. the amount necessary for n.p.l.c. analysk. However, the mixture’ of propanol and tetradccane (10% by volume) produced similar amounts of PAHs to :oluene, and as much as did heptane under comparable conditions.

Figure 6 shows the results of PAH yields at different temxratures for the toluene and heptane. For example, a high yield of PAH was obtained with toluene combustion in the tem- perature rang of Iti!-2500 K. Maximum im~ounts of PAHs were obtainrd betwern 1900 and 2000 K for toluene combustior but at IhW K for heptane. These were the tempera- tures at which totuene and heptane also pro- duce : the highest amount of soot. A further increase in temperature resulted in a decrease in PAH production for both of the hydrwar- bons. The temperature did not have much ef- fect on the production of PAH from the pyrol- ysis OC heptane, but the latter Gelded much smaller amounts of PAHs than its combuztion in 0,. Oxygen can promote hydrocarbon breakdown, and can interact by attacking the chemical bonds in the structure of the fuel.

1st t f--l I

Temperalure. K Fig. 6. Total PAH yields from hcptane and wlurne cum- bustiun ;md pyrolysis.

361 R. WANG AND P. CADMAN

Combustion conditions: 10% 0, in Argon at 1600 K

Fig. 7. ~ompwiaon of individual PAH yields found in swt from heptanc and t~tucnc comhustiw and pyrolysis.

therefore giving smaller chemical radicals. Hcncc, more PAHs may be generated in its presence. In an inert atmosphere (pure argon), “polymerization” of PAH type wdicals to form soot may predominate, and this may inhibit the net formation of the PAHs.

The typical distribution of nine PAHs identi- fied in soot samples is shown in Figs. 7 and 8.

These nine PA&, together with naphthalene, Buorene, benzofslanthracene, dibenz[a,h]an- thracene, and benz[ghilperylene in Table 1, are among the list of 16 “priority pollutant PAlIs” issued by the U.S. Environmental Protection Agency [161.

Higher yields of fluoranthene and pyrene compared to the others were found in soot

250

Combustion Temperature: 1710 K Fig. 8. Comparison of PAHs from combustion and pyrolysis of heptane.

SPRAY COMBUSTION OF HYDROCARBONS 365

extracts. Large PAHs, such as those having five or more benzene rings in the structure. were present in much smaller quantities. This re- duced production may indicate the extent to which large PAHs were consumed in soot for- mation. For the PAHs compared in Fig. 7, similar quantities were gcncrally observed from both toluene and heptane. except for fluoran- theme and pyrene which were present in much higher amounts from toluene combustion. The smaller PAHs (three-four benzene rings) were found in larger quantities from heptane com- bustion than in pyrolysis. However, the reverse was true for five- or six-ring PAHs (Fig. S).

Temperature was found to influence signiti- cantly the individual PAH formation (Figs. 9- 11). Generally, toluene combustion (Fig. 9) gave

yields of fluoranthene and pyrene which varied only slightly at temperatures between 1350 and 1977 K, but reduced dramatically from 1977 to 2088 K. They remained virtually unchanged up to a temperature of 2300 K. The chrysene yield, though, was only slightly reduced by this temperature increase. Phenanthrene and an- thracene production decreased as the tempera- ture increased from 1350 to 2088 K, and then stabilised between tempelatures of 2085-2306 K. The yields of large PAHs, benzo[a]pyrene, benzo[bl, and [klfluoranthene and indenoll.2. 3,cdIpyrene (those having five- or six-numbered

benzene ring in a molecule), were found to reduce slightly as temperature increased. Simi- lar trends were obtained in heptane combus- tion (Fig. IO). In heptane pyrolysis, though, any increase in temperature was found to result in a decreased production of the individual PAH. In particular, the production of pyrene and fluoranthene was found to decline markedly (from 140 to IO ppm of soot for fluoranthene and from 70 to 15 for pyrenej when the tem- neraturr was raised from 1700 to 2500 K (Fig. ii,.

Fig. I I. PAH yields from hcptane pyrolysis in AI.

366 R. WANG AND P. CADMAN

RING FORMING MASS GROW-W

ACETYLENE PAW

MASS GROWTH SOOT WJCLEUS

MASS GROWTH Fig. 12. Possible swt formation mechanism.

Relationship of Soot and PAHs

The extent that the detailed role that the PAHs play in the mechanism of formation of soot has not been definitely elucidated, although the importance of PAH participation in the forma- tion has been recognised for a long time. There is one consensus regarding soot production, that the route involves, first, the formation of aromatic rings from small alphatic units, and then mass growth into higher PAHs or soot “nuclei.” Then growth occurs via the addition of acetylene or PAHs to these “nuclei” to produce the end form of soot, as shown in Fig. 12. In this work, it is not intended to account for all the steps of the soot formation mecha- nism from the small hydrocarbons. However, the results from this investigation were found to positively support such a close relationship between PAHs and soot during the formation of the latter.

The low yields of soot and PAHs from propanol combustion in 10% OJargon is probably due the higher oxygen/fuel ratio and/or to the involvement of the -OH group existing in the structure of propanol, which upon release can act as an oxidant in sooting flames 1171. Both formation and destruction reactions of soot and PAHs may occur in any combustion process [IS]. The survival of soot and various kinds of PAH is dependent on the competition of such reaction, and the latter is

a function of experimental variables, such as fuel characteristics, oxygen, and temperature.

In pure argon, propanol can also pyrolyse first into other gaseous products such as propy- lene and water, and these primary products can then form soot/PAH from such species.

C,H,OH + C,H, + H,O + soot + etc.

A higher yield of soot from propanol pyrolysis than from its combustion was observed as the latter was investigated in lean mixtures.

The types of PAHs found for both heptane and toluene were similar according to h.p.1.c. and m.s. results. This may indicate similarities in the basic PAH mass growth mechanism for the two compounds. Benzophenone, biphenyl, and m- and t-terphenyl were also identified only from toluene combustion soot, suggesting that other (probably many) different reactions also take place, depending on the combustion conditions. Aliphatic heptane and aromatic toluene may have different reaction rates for soot and PAH formation as heptane produced both lower soot and PAH quantities than toluene. The latter result might be explained, at least partly, by the formation of the first benzene ring being rate-determining for the PAH formation, soot inception, and soot mass growth [191. The existence of a benzene ring in the toluene molecule thus allows it to bypass this primary ring formation. However, with

SPRAY COMBUSTION OF HYDROCARBONS 367

heptane as a fuel, it is necessary to form this first ring (which then undergoes further reac- tions to give the increasingly large PAH molecules), and this limits the amount of so01 produced.

The temperature profiles cf soot and PAHs from toluene and heptane indicate that maxi- mum tempcraturc of soot and PAH production is different for different fuels and for changed combustion conditions. Hence, in practical combustion systems, a broad range of maxi- mum temperatures for soot formation may bt: expected as commercial fuels contain many different hydrocarbons. Above the maximum yield temperature for each hydrocarbon, the decomposition and oxidation reactions of soot and PAHs predominate, resulting in a de- crease in yield. The larger PAHs (contain- ing five- or six-numbered benzene rings) were found to be less sensitive to temperature change, and varied only slightly with increasing temperature. This observation is consistent generally with the increased thermal stability of PAHs as their molecular mass increases. It also showed that the large PAHs may play a more important role for soot formation, partic- ularly at high tempcrdtttres. There was an ap- parent minimum threshold rcmpcraturc for soot formation, below which the yield of soot was low. This was probably due to reaction thermodynamics and/or kinetics effects. Gen- erally, the maximum yield was found to shift to higher temperatures for pyrolysis conditions. The larger PAH were found to be present in higher yields in pyrolytic conditions than in combustion (Fig. 8). All of those results indi- cate the positive role of oxygen in the initial decomposition reactions.

High yields of PAH were usually extracted from samples with a high soot yield (Fig. 13). This was true regardless of fuel type and tcm- perature. This may mean, in practical terms, that reduction of soot yields also reduces those of the PAHs. However, it is suggested that soot formation itself consumed PAHs in the com- bustion of all of the different fuels, as the PAH concentration is soot was found to decrease with the increase in soot yield (Fig. 13). This reduction also suggests that control of PAH formation reactions will also give some control

of soot formation. It also indirectly reinforces thr idea that PAHs are precursors to sOOt.

Fluoranthene and pyrene were identified in all of the soot samples with higher concentra- tion than any others. A higher concentration of four-numbered ring PAHs compared to larger rings, was found to be present in a premixed flat CH,/O: names by Lorenzo et al. [ZO]. The production of these latter compounds was found to be (in descending order) toluene com- bustion > heptane combustion > heptane py- rolysis. respectively. The two four-benzene-ring PAHs may play an important role in forming larger PAHs and soot. From :hc PAHs identi- tied from soot samples, mass growth :o form tluoranthene and pyrene seems to be a process of addition of a C2 or CZHZ unit to an existing smaller PAH molecule. This addition process continues from fluoranthene and pyrene to the formation of larger PAHs, such as coronene and indeno[l,2,3-cdlpyrene (Fig. 14). This mass growth route is a general one, and does not necessarily involve any detailed formation pathways. The actual synthesis in combustion can be much more complicated, and may in- volve various types of reactions. It is interest- ing that as the quantities of fluoranthene and pyrene seem to be proportional to those of soot and PAHs, and as they can also be conve- nicntly determined by h.p.1.c. or other tech- niques. then this process may be used to moni-

3hS R. WANG AND P. CADMAN

tor soot and PAH emission from combustion systems. In forming soot, large PAHs may be essential, and in particular. those with relative molecular mass of 650 < M, < 700 (or empiri- cal formulae of C,,H,,-C,,H,,,) [21]. In this work. large PAHs with a molecular mass from 700 to 070 were found using the FAB m.s. tcchniquc (Fig. 19. In certain benzene-soot samples from shock-tube combustion. C,,,,. fullerene (M = 720) was detected [22]. Study of these large PAHs in combustion will have a significant impact on our understanding of soot formation mechanism.

D’Alcssio [23] has dctcctcd the prcsencc of condensed polybenzenoid hydrocarbon species of about 2 nm in size and mass of 2500 amu which arc intermediate between the small ring number polycyclics and soot. These species were thought to grow by the addition of two ring aromatics to smaller structures. The pro- gressive aromatisation of such structures was thought to be responsible for visible fluores- cence. Whatever the detailed mechanism, the

Fig. !4. Molcculsr mass progress to forming soot and fullcrcnr.

involvement of polycyclic aromatics in the soot formation process is now well established.

CONCLUSION

Chemically different hydrocarbons have been found to give different soot and PAH produc- tion. The order of sooting tendency in the spray combustion is toluene > n-heptane > propanal, which represent aromatic, aliphatic, and alcohols, respectively. Propanol produces a very low yield of soot (and PAH) under the lean combustion conditions used here. The temperature range studied strongly affected both soot and PAH formation, but to a differ- ent extent for different fuels and different con- ditions. Depending on the type of hydrocarbon, the temper&x< *;here the maximum yield was obtained varied and shifted to higher tempera- tures for the pyrolysis of both toluene and n-heptane. There seems to be a minimum threshold temperature for soot formation for these fuels. At much higher temperatures (generally over 2200 K), the soot yield reduced, probably due to oxidation by the excess oxy- gen. Under pyrolysis condition in argon, a dif- ferent mechanism of fragmentation probably takes place, as a similar reduction in soot oc- curred at these higher temperatures in the absence of Oz. PAHs (in particular, the larger PAHs) must be involved in the formation of soot as concentrations of PAHs also reduce with increasing soot yield. Fuel type and com- hustion conditions intlllence both the amount of PAH and its distribution. Fluoranthene and pyrene are observed in higher yield than any other PAH. Similar types of PAHs were identi- fied regardless of the chemical properties of the fuel and the combustion conditions. This suggests that similar PAH mass growth mecha- nisms from combustion in different fuels may lead to soot formation

The authors would like to thank Miss N. Huq, Mr. A. Kunz, and Mr. J. He&d jim some erpen’- mental assistance and ms. analysis, and Dr. J. A. Ballantine of the EPSRC Mass Spectrometry Ser- vice Centre, Swansea, UK, for conducting rhe FAB m.s. analysis. Financial suppon for this project by EPSRC, UK, is gratefully acknowl- edged.

SPRAY COMBUSTION ‘F HYDROCARRONS 369

R. WANG AND P. CADMAN 370

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