ELSEVIER Journal of Analytical and Applied Pyrolysis

35 (199.5) 181-197

JOURNAL 01 ANALYTICAL and

APPLIED PYROLYSIS

Polycyclic aromatic compounds in oils derived from the fluidised bed pyrolysis of oil shale

Paul T. Williams *, Jamal M. Nazzal I

Received 16 February 1995: accepted 30 May 1995

Abstract

Oil shale was pyrolysed in a 10 cm diameter x 100 cm high fluidised bed reactor with nitrogen as the fluidising gas at pyrolysis temperatures of 400. 450. 520. 570 and 620°C. The gases were analysed by packed column gas chromatography. The condensed pyrolytic oils were analysed for their content of polycyclic aromatic compounds (PAC), including poly- cyclic aromatic hydrocarbons (PAH), sulphur-PAH (PASH) and nitrogen-PAH (PANH). The oils were fractionated into chemical classes using mini-column liquid chromatography followed by analysis using capillary column gas chromato&raphy with flame ionisation detection (GCiFID) and capillary column GC with mass spectrometry (CC/MS) for identifi- cation and quantification of PAH. PASH and PANH were identified in the chemical class fractions using capillary column GC with selective dctcction and GC:‘MS. The pyrolytic shale oils were found to contain significant concentrations of PAH. PASH and PANH. The concentrations of PAC were increased with increasing reactor temperature and residence time. The PAH consisted mainly of naphthalene. fluorene and phenanthrene and their

alkylated homologues, and lower concentrations of fluoranthene. pyrene and chrysene. The PASH identified included benzothiophene. and dibenzothiophcne, and the PANH identified includmg indole and carbazole and their alkylated derivatives. Some of the PAC identified have been reported to be mutagenic and/or carcinogenic.

Ke,wwds: Fluidised bed reactor: GC/FID; GC/MS: Liquid chromatography fractionation:

Oil shale; Polycyclic aromatic compounds; Pyrolysis: Pyrolytic oil analysis

* Corresponding author. ’ On leave from the Natur-al Resources Authority. Amman. Jordan

0165-23701’95/$09.50 0 1995 Elsevier Science B.V. All rights reserved

SSDI 0165.2370(95)00908-6

182 P. T. Williams, J.M. Na~zal ! J. Anal. Appl. Pyrolysis 35 (199.5) 181- I97

1. Introduction

It has been reported that oil shale deposits occur in over 50 countries world-wide, representing an extractable hydrocarbon reserve of 2.72 x lOI t of oil [l]. Many countries are considering the future large-scale development of an oil shale retorting industry to mitigate against predicted long term shortages of crude petroleum oil.

The environmental impact of the large-scale exploitation of such a large energy resource should be examined in terms of the extraction, refining and end use of the oil. The chemical composition of the oil has consequences for its safe handling and transport and also its end use in combustion systems. Oil derived from the pyrolysis of oil shale has been shown to contain polycyclic aromatic compounds (PAC) [2-61. PAC represent a group of chemicals over which there is some concern because of associated health hazards, since among the environmental chemical

groups, PAC comprise the largest group of carcinogens [7-lo]. Not all of the very large number of PAC known to exist are carcinogenic and/or mutagenic and many have not been tested either individually or as they occur in complex mixtures. The identity and concentration of individual PAC is therefore of importance, and in particular, how the process conditions of oil shale pyrolysis may be influenced to control their concentration.

The presence of carcinogenic and/or mutagenic PAC in the derived oil shale pyrolysis oils, particularly if they are present in high concentration, might limit the use of this fuel as a direct substitute for petroleum-derived fuels, since the fuel would represent a health hazard. The PAC of interest in fuels chemistry include polycyclic aromatic hydrocarbons (PAH), sulphur-containing PAH (PASH) and nitrogen-containing PAH (PANH). Some PASH and PANH are toxic and/or mutgenic [ 1 1 - 131 and fuels which contain these compounds give rise to toxic and corrosive SO.,. and NO,. on combustion. The presence of PAC in the derived shale oil as a potential fuel also has further significance, in that increased concentrations of PAC in the fuel may lead to increases in the emissions of PAC when the fuel is combusted. Work relating the emissions of PAC from diesel engines and domestic oil burners has shown a direct link between the identities and concentrations of PAC in the fuel and in the emissions of PAC from the combustion unit [13-161.

Therefore, an understanding of the formation of PAC in the pyrolysis of oil shale in relation to process conditions is required to minimise the formation of PAC in the derived shale oil. In this paper oil shale has been pyrolysed in a fluidised bed reactor at various temperatures and the derived shale oil has been analysed for PAH, PASH and PANH in relation to the fluidised bed pyrolysis temperature.

2. Materials and methods

2.1. Oil shale

The oil shale was of Jurassic, Kimmeridge Clay age obtained from the Golden Hill Pit at Marton, North Yorkshire, UK, and its lithology and organic geochem-

istry has been described in detail [17]. The oil shale was crushed to a grain size ot I.2 -3.3 mm and dried at 105°C. The oil shale had a Fischer assay of IO. I wt.Y [i 71.

2.2 Fluitlisetl hcd plw)l_vsis reuc’tor

The fluidised bed pyrolysis reactor was 10 cm diameter x 100 cm high, including a 20 cm diameter x 17 cm high expander section at the top of the freeboard to reduce both the gas velocity and the elutriation of fine material. The reactor was constructed from stainless steel, with full temperature and gas-flow control. The fluidising gas was nitrogen, which was preheated to 400°C. The temperature throughout the reactor was maintained at the pyrolysis temperature by the use of separate external heaters for the fluidised bed, freeboard and cyclone of the reactor. and the temperature was monitored at several points throughout the system. The bed material was silica sand of mean size 250 /lrn with a static bed depth of 8 cm. The fluidising velocity was maintained at I. 15 units of minimum fluidising velocity, and the flow rate of nitrogen was altered to maintain a constant fluidising velocity at the

selected pyrolysis temperature. The oil shale was gravity fed, via a rotary vlalve feeder, to the fluidised bed at an accurately metered rate of 0.75 kg h ‘_ The pyrolysis vapours were cooled in a series of water-cooled condensers, followed by condensation of the oils in a series of cold traps, maintained at different temperatures with the aid of ice-water or solid COzpmethanol mixtures. The superficial residence time of the gases in the fluidised bed reactor, freeboard and cyclone was between 25 and 30 s depending on the temperature, which included gas flow through the heated fluidised bed, freeboard and cyclone. The residence time would vary as pyrolysis vapours were evolved from the oil shale. The duration of each experiment was approximately 60 min. Fig. 1 shows a schematic diagram of the fluidised bed reactor.

1.3. GUS uncr!,~si.s

The evolved gases were sampled using a gas syringe at intervals throughout the duration of the pyrolysis of the oil shale and were analysed off-line by packed

column gas chromatography. The gases analysed were CO, H,, CH, and 0, using a molecular sieve SA 60-80 column with argon as the carrier gas and a thermal conductivity detector. Nitrogen, which was the purge gas used in the reactor. was also determined on this column and the volumetric flow rates of the other gases were calculated by comparison with the nitrogen flow rate. CO, was determined separately using a silica gel column with argon as the carrier gas and a thermal conductivity detector. Gaseous hydrocarbons up to C, were determined on a Porasil C 80 I00 column with nitrogen as the carrier gas, using a flame ionisation detector.

2.4. Oil anci!l~sis

The oils consisted of an aqueous phase and an oil phase. The oil was separated from the aqueous phase by centrifuging, decanting and removal of the aqueous phase by pipette.

184 P. T. Williams, J.M. Naxal I) J. Anal. Appl. Pwo1.wi.T 35 (1995) 181~ 197

Gas Chromatography

Condenser and Cold Trap System

Gas Pre-Heater

Fig. I. Schematic diagram of the fluidised bed reactor.

Asphaltenes were precipitated with n-pentane. The pentane-soluble fraction was analysed for PAC by liquid chromatography to separate the oils into chemical class fractions, followed by detailed analysis of the fractions. The liquid chromatography consisted of 10 cm x 1 cm glass columns packed with a silica, Bondesil (sepralyte), sorbent, pretreated at 105°C for 2 h prior to use. To prevent the formation of a solid phase with the addition of the pentane mobile phase, and to improve solvent

contacting with the oil, the oil was intimately mixed with Chromosorb G/AW/ DMCS 60-80 support and packed in the column above the analytical phase. The column was then sequentially eluted under vacuum with pentane, benzene, ethyl acetate and methanol to produce, aliphatic, aromatic, ester and polar fractions, respectively. The pentane fraction was further subdivided into the first eluted

pentane fraction (pentane-1), to fractionate the aliphatic compounds, and into the extended pentane elution fraction (pentane-2) to distinguish between lower and

higher molecular weight PAC. Each fraction was evaporated to dryness, weighed and the percentage mass in each fraction calculated. The evaporation of the solvent would inevitably lead to some loss of volatile material; consequently, this step in the analytical procedure was carefully carried out to minimise these losses. The main aim of the paper was to characterise the pyrolysis oils in terms of their PAC content, and it was deemed that since the PAC of most interest had higher boiling points, the loss of these compounds would be insignificant.

Each fraction was analysed for PAH, PASH and PANH. PAH were analysed by capillary column gas chromatography/mass spectrometry (GC!MS), together with retention indices. to identify the PAH. The system used was a Carlo-Erba, Vega HRGC with cold on-column injection, coupled to a Finnigan Mat ion trap detector (ITD) via a heated transfer line. The column used in the gas chromatograph was a 25 m x 0.3 mm fused-silica capillary coated with DB5, and the temperature programme was 60°C for 2 min followed by a 5 K min ’ heating rate to 270°C. The carrier and make-up gas was helium with a carrier flow rate of 2 cm’ min ’ at 270°C. The ITD has a mass range from 20 to 650 a.m.u. with scan times of between 0.125 and 2 s. The ITD was linked to an IBM PCXT computer with an NBS’EPA mass spectral library containing 38 752 mass spectra. Single ion monitoring (SIM) was also carried out to confirm the presence of PAH and also to examine the samples for a series of substituted PAC. for example, naphthalene. methylnaph- thalene and dimethylnaphthalene. Quantification of the PAC was performed sepa-

rately by capillary column GC with cold on-column injection. but with a flame ionisation detector (GC!FID) instead of an ITD. The GC.‘FID system was found to give much better resolution of peaks than the GC/ITD system; hence its USC fog quantification. Internal PAH standards together with the identifications found from the GC:‘ITD allowed quantification of the PAH in the oil shale pyrolysis oils. The system used was a Carlo-Erba Mega Series HRGC 5300 gas chromatograph. and the capillary column, temperature programme. carrier gas and flow rate are identical to those used in the GClITD system. PASH were identified with the aid of GC/MS and retention indices, and also by using GC with flame photometric sulphur-selective detection. The gas chromatograph used in this case was a Hewlett- Packard 5890 capillary column system with on-column injection on to a DB5 column. The temperature programme, carrier gas and flow rate conditions were identical to those used for the determination of PAH. PANH were identified with the aid of GC:‘MS and retention indices and also using GC with alkali salt nitrogen-selective detection. The gas chromatograph used uas the Carlo-Erba Mega Series 5300, and gas chromatographic conditions were as for the determination of

PAH. The molecular weight range of the derived shale oils was determined by size

exclusion chromatography. The system incorporated two 1.50 mm x 4.6 mm i.d. columns with Polymer Laboratories 5 /tm RPSEC 100 A-type packing. A third column of the same material was placed in line between the pump and the injection valve, to ensure presaturation of the solvent with the column packing material and also to avoid analytical column dissolution and hence loss of performance. The solvent used for the mobile phase was tetrahydrofuran (THF). The calibration system used was based on polystyrene samples of low polydispersity in the molecular weight (MW) range 800&860 000 also included was benzene for low MW calibra- tion. The detectors were a UV detector from Merck--Hitachi. and an RI detector from Varian. The UV detector will be sensitive to the detection of aromatic compounds and the RI detector will measure the elution of all compounds. The system was operated at O”C, and an ice--water mixture was used to maintain the column temperature at 0°C throughout the work. Previous work [18] using the same

186 P. T. Williams, J. M. Nazzal / J. Anal. Appl. Pyrolysis 35 (1995) 181~ 197

Table I Production yield from the fluidised bed pyrolysis of oil shale in relation to pyrolysis temperature

Product Yield

(wt.%)

Fluidised bed temperature (“C)

400 450 520 570 620

Oil 1.78 5.26 8.68 7.79 6.46

Water 1.92 2.35 2.36 2.72 3.24

Gases 1.68 4.20 3.56 4.10 5.53

Spent shale 87.40 80.20 77.90 76.82 76.00

Total 92.78 92.01 92.50 91.43 91.23

system has shown systemaic variations in the measured MW of model compounds of n-alkanes, n-alkenes, PAH and alkylated cyclic hydrocarbons, as has also been shown previously by other workers j19-211.

3. Results and discussion

3.1. Product yield

Table 1 shows the product yield for the fluidised bed pyrolysis of oil shale in relation to fluidised bed temperature. The oil yield at the lowest fluidised bed temperature of 400°C is low, only 1.78 wt.%. The residual spent shale at 87.40 wt.% represented the highest figure suggesting that there was incomplete pyrolysis at the 400°C fluidised bed temperature. The oil yield shows a progressive increase to reach a maximum oil yield of 8.68 wt.% at the 520°C pyrolysis temperature, followed by a decrease in yield as the temperature was increased to 620°C. The gas yield shows a general increase with pyrolysis temperature and the residual oil shale decreases with increasing pyrolysis temperature. The water yield from the pyrolysis of the oil shale was small, but showed an increase with increasing pyrolysis temperature. This phenomenon has also been observed by Campbell et al. [22] who suggest that water evolved at higher temperatures is formed during the kerogen thermal decomposi- tion process. As the pyrolysis temperature is increased, more hydrocarbons are evolved from the oil shale kerogen; however, above 520°C the oil hydrocarbons are cracked in the reactor, producing more gas.

3.2. Gas anulysis

The gases evolved from the pyrolysis of oil shales were mainly CO,, CO, H,, CH, and lower concentrations of other hydrocarbon gases, as has been reported by many workers for example Ekstrom et al. [23] and Campbell et al. [24]. Fig. 2 shows the total yield of the alkane gases methane, ethane, propane, isobutane and n-butane. Fig. 3 shows the total yield of the alkene gases ethene, propene and

P.T. Williams, J.M. Na-_:al J. Ad. Appl. P~ro!l~.vi.~ .15 11995) 181~ 197 187

butene. Fig. 4 shows the ratio of alkene to alkane gases. The ethene to ethane ratio has been suggested as an indicator of oil shale retorting conditions, in particular the pyrolysis temperature [25]. The method has been applied to other oil shale pyrolysis research and extended to propane/propene and butane,‘butene ratios [26628]. The alkenejalkane ratio shows an increase with increasing pyrolysis temperature and also heating rate [25528]. Carter and Taulbee [28] have suggested that the increase in these ratios with increasing pyrolysis temperature reflects the increase in sec- ondary cracking reactions. Raley [27] has suggested a mechanism to explain the correlation of the ratio with heating rate in terms of a free radical, chain reaction scheme. The dependence of alkanelalkene ratios on heating rate is attributed to competition between carbon-carbon bond cleavage and hydrogen atom-transfer processes. Fig. 4 confirms that the alkenejalkane ratio, including etheneiethane, propenejpropane and butene/butane ratios can be used as an indicator of pyrolysis temperature, increasing alkeneialkane ratios occurring with increasing pyrolysis

temperature.

Chemical Claus jkctionation

Table 2 shows the chemical class fractionation of shale oil generated in relation to the fluidised bed pyrolysis temperature. The pentane-1, pentane-2, benzene, ethyl acetate and methanol eluants represent, aliphatic. low molecular weight aromatic.

50

350 400 450 500 550 600 650

Pyrolysis Temperature (C)

Fig. 2. Total yield of the alkane gases, methane, ethane. propane. [sobutane and n-butane

188 P. T. Williams, J.M. Nazsal ! J. Anal. Appl. Pyrolysis 35 (1995) 181- 197

30

350 400 450 500 550

Pyrolysis Temperature (C)

600 650

Fig. 3. Total yield of the alkene gases ethene, propene and butene.

higher molecular weight aromatic, ester and polar fractions, respectively. The

aliphatic material showed a decrease with increasing temperature of pyrolysis in the fluidised bed reactor, and the higher molecular weight material represented by the benzene fraction showed a corresponding increase. The pentane-2, ethyl acetate and methanol fractions showed only small changes with increasing pyrolysis tempera- ture. The asphaltene fraction also showed little change with increasing pyrolysis temperature, although there was a significant reduction in the asphaltene fraction at the highest pyrolysis temperature of 650°C. The relative decrease in aliphatic

concentration and relative increase in aromatic concentration of the shale oils suggests aromatisation of the oils with increasing temperature of pyrolysis in the fluidised bed reactor.

The relative concentration of aliphatic compounds and aromatic compounds shown in Table 2 are respectively lower and higher than those reported for the slow pyrolysis of oil shales. For example, Ekstrom et al. [23] reported a combined alkane and alkene concentration in shale oil produced at a heating rate of 3 K min- ’ of 46.7 wt.% and an aromatic concentration of 21.3 wt.%. Similarly, Yanik et al. [29] reported an aliphatic concentration of 42.8 wt.% and an aromatic concentration of 19.1 wt.% in shale oil produced at a pyrolysis heating rate of 5 K min-‘. However, fluidised bed pyrolysis of oil shales where flash pyrolysis with high heating rates occur produces oils with higher aromatic concentrations and lower aliphatic concentrations. In this work, an aliphatic concentration of between 7.8 and 13.6 wt.% and a combined low and high molecular weight aromatic concentration of

P. T. Williums, J.M. Nu:xl J. Anal. Appl. qlrol~si, _li (1995) I$\- 197 1x9

6

350 400 450 500 550

Pyrolysis Temperature (C)

600 650

Fig. 4. Ratio of alkene to alkane gases

between 31.8 and 39.2 wt.% was found in the shale oils. This compares with analyses for flash pyrolysis shale oils by Yanik et al. [29] where the aliphatic concentration was 41.7 wt.% and the aliphatic concentration was 18.7 wt.‘%,. Carter and Taulbee [28] have also shown that fluidised bed pyrolysis of oil shales gives higher oil aromatic yields than slow pyrolysis (Fisher assay).

Table 3 shows the absolute concentrations of PAH, PASH and PANH found in shale oils derived from the fluidised bed pyrolysis of oil shale. The data represent not all the PAC found in the shale oils but only those that could be positively

Table 2

Chemical class fractionation of the shale oil in relation to pyrolysis temperature (data shown are relatl\e

weight per cent)

Elucnt Fraction Fluidised bed temperature (“C)

400 43) 520 570 620

Pen tane- I Pentane-2

Benzene

Ethyl acetate

Methanol

Aliphatics I.36 I I.3 ‘I.8 7.9 7.8

Low MW aromatics 8.8 8.3 5.3 7.8 7.8

High MW aromatics 23.0 25.1 x.0 17.0 31.4

Esters 29.7 30.7 32.6 31.3 32.6

Polars 5.2 5.3 i.3 7.0 4.6

Asphaltenes 19.7 19.3 IS.0 19.0 15.8

190

Table 3

P.T. Williams, J.M. Nazzal /J. Anal. Appl. Pyrolysis 35 (1995) 181-197

Polycyclic aromatic compounds in shale pyrolysis oils (mg kg-’ oil) in relation to pyrolysis temperature

PAC Fluidised bed temperature (“C)

400 450 520 570 620

PAH

Naphthalene

Methylnaphthalenes

Biphenyl

Ethylnaphthalene

Dimethylnaphthalenes

Acenaphthalene

Methylbiphenyl

Acenaphthene

Trimethylnaphthalenes

Fluorene

Methylfluorene

Phenanthrene

Anthracene

Methylphenanthrenes

Fluoranthene

Pyrene

Methylpyrene

Chrysene

PASH

Benzothiophene

3-Methylbenzo[b]thiophene

Dibenzothiophene

Benzo[b]naphthothiophene

PANH

Indole

Methylindoles

4-Azabiphenylindole

Dimethylindoles

&Methoxyquinoline

4-Azafluorene

2-Methylquinoline

2-Hydroxyquinoline

Carbazole

4-Nitrobiphenyl

Methylcarbazoles

<IO <IO 395 340 830

515 995 2695 2310 4230

60 60 170 120 365 160 245 670 520 485

340 1275 2110 2290 4305

235 255 800 890 2150

585 635 1080 1050 1410

<IO <IO 30 <IO <IO

140 210 235 235 370

210 240 360 790 980 < 10 < IO <IO < IO 40

145 145 450 570 270

<IO 30 100 100 100

30 130 100 200 255

<IO 100 120 65 190 <IO <IO 85 140 240 <IO <IO <IO 15 140 <IO <IO 105 350 260

<IO

IO

< 10

<IO

<lO

35

25

<IO

<lO 35 95 35 45 160 40 185 325

<IO 20 80

<IO

130

60

215

<lO

< IO

<IO

<IO

< IO

10

210

< 10 20 50 65 100 235 490 575 60 130 155 235

330 450 640 690 40 40 80 110 55 40 250 140

<IO <IO 70 60 < 10 20 25 100

I10 133 150 100 < 10 <lO 150 100 280 345 370 495

identified, since many more PAC have been identified in oil shale pyrolysis oils [2-61. The concentrations of those PAC positively identified are used to illustrate the influence of the temperature of pyrolysis on the concentration of PAC in the fluidised bed reactor. The PAH found consist mainly of naphthalene, biphenyl, fluorene, phenanthrene and their alkylated derivatives. Some of the PAH have been

shown to have some carcinogenic and/or mutagenic activity, for example the PAH chrysene has been shown to be carcinogenic [8], and phenanthrene and the methylphenanthrenes [9], and the methylfluorenes [lo] have given positive results in carcinogenicity and/or mutagenicity tests. These PAH are present in significant concentrations in the shale oil, for example phenanthrene ranges from 145 to

570 ppm, methylphenanthrenes from 30 to 255 ppm, and chrysene ranges from below 10 to 350 ppm depending on the fluidised bed pyrolysis temperature. The PASH identified included benzo[b]thiophene, dibenzothiophene, methylben- zo[b]thiophene and benzonaphthothiophene, which occurred in significant concen- trations. The PASH identified in the shale oils have either not been assessed for their mutgenic and/or carcinogenic activity or there are limited data available.

However. dibenzothiophene and the methyldibenzothiophenes have been assessed for their mutagenic activity by McFall et al. [l I]. They showed that this group of compounds exhibited no mutagenic activity. Similarly. other PASH which have been detected in shale oils appear to exhibit no carcinogenic and/or mutagenic activity [7.12]. The PANH identified in the shale oils consisted mainly of indole. quinoline and carbazole and their alkylated derivatives. The PANH idenihed in the shale oils have been shown to exhibit carcinogenic activity. for example, quinoline and the methylquinolines are carcinogenic [8]. Examination of the literature shows that various PAC have been shown to be present in derived oil shale pyrolysis oils including. for example PAH including naphthalene, phenanthrene, fluorene. pyrene. chrysene and their alkylated derivatives. PASH including benzo[b]thiophene. alky- lated benzo[b]thiophenes and dibenzothiophenes and PANH including quinolines. carbazoles and their alkylated derivatives [2- 61.

Table 3 shows that as the temperature of pyrolysis was increased from 400 to 620°C in the fluidised bed reactor. the concentration of PAC in the shale oils also increased. This increase was observed for PAH. PASH and PANH. For most individual PAC there was an increase in concentration in relation to increasing pyrolysis temperature. The formation of aromatic and polycyclic aromatic com- pounds via secondary reactions during pyrolysis has been attributed to either Diels-Alder type reactions [30-341 or to the gas-phase cracking of ahphatic compounds leading to the selective concentration of aromatic compounds.

The reactions involving the formation of monoaromatic and polyaromatic species via a Diels-Alder type reaction to form aromatic and polycyclic aromatic com- pounds are well known [30&34]. For example. Cypres [30] pyrolysed model aliphatic compounds and confirmed the Diels-Alder route to PAH formation in pyrolysis cracking reactions. Pyrolysis of n-decane produced alkenes by thermal degradation. and the post pyrolysis cracking of the alkenes between 600 and 900°C

with a 2 s residence time showed a decrease in light alkenes and the formation of

single ring aromatic compounds, such as benzene. toluenc and alkylaromatics by Diels-Alder reactions. In addition. naphthalene and alkylnaphthalenes are formed and condensation reactions may continue to produce higher PAH. There was no evidence of the direct cyclisation of alkanes to cycloalkanes followed by dehydro- genation to aromatic compounds. Fairburn et al. [31] examined the flash pyrolysis of n-hexadecane in a micro-reactor at temperatures between 576 and 842°C. They

192 P.T. Williams, J.M. Nazal J. Anal. Appt. P_vrolysis 35 (1995) 181-197

showed that hexadecane initially pyrolyses to form light alkanes in the range C,-C, and alkenes in the range (Z-C,,. Further decomposition at higher temperatures (or increased reaction time) produces a decomposition of the heavier alkenes to lighter alkenes (C-C,) and subsequently the formation of dialkenes such as butadiene via dehydrogenation of the lighter alkenes. The butadiene then immediately combines with available light alkenes such as ethene and propene to form aromatic com-

pounds such as benzene, toluene, ethylbenzene and styrene via the Diels-Alder reaction. Higher molecular weight PAH are then formed by further reaction between aromatic compounds and alkenes. Depeyre et al. [32] have also suggested the Diels-Alder reaction mechanism as the formation route for aromatic and polycyclic aromatic compounds in the pyrolysis of n-hexadecane and n-nonane at

temperatures between 600 and 900°C. It can be suggested that the formation of PANH and PASH would also involve

a Diels-Alder type reaction involving sulphur and nitrogen. The formation of PANH and PASH would therefore be dependent on the amount of sulphur and nitrogen in the oil shale kerogen, and consequently different oil shales will produce different proportions of PANH and PASH. The chemical class fractionation results presented in Table 2 shows that the pentane fraction, representing the aliphatic compounds, decreases in relative percentage mass fraction with increasing pyrolysis temperature. A consequence of the aromatisation reactions are increased concentra- tions of alkenes [30-321. The increased concentration of alkenes, and the alkene to alkane ratio for the gases produced in the fluidised bed pyrolysis of the oil shales of this work (Figs. 3 and 4) also suggest that the formation of PAH occurs via the Diels-Alder aromatisation reaction.

An alternative to the Diels-Alder reaction formation route for PAC in shale oil pyrolysis has been suggested. It has been proposed that the increasing aromatic content in relation to increased pyrolysis temperature is due to the higher-tempera- ture (above 500°C) gas-phase cracking of aliphatic compounds (free and attached to aromatic rings), resulting in the selective concentration of aromatic compounds [35-381. Gas-phase oil cracking reactions occur where long-chain aliphatic hydro- carbons are converted to low molecular weight hydrocarbons [35]. Burnham [35] has suggested that since aromatic compounds are more thermodynamically stable than aliphatic compounds and are therefore more resistant to thermal cracking reactions, then the gas-phase cracking reactions will selectively produce oils with increased concentrations of aromatic hydrocarbons. Comparison of naphthalene concentrations with concentrations of C,, and C,, alkanes and alkenes in thermally cracked oil shale samples showed that as the degree of thermal cracking increased, the relative concentration of naphthalene/(C,, + C,z) increased markedly [35]. It was suggested that less severe levels of oil cracking produced primarily alkenes and that further cracking produced primarily aromatic compounds. Burnham 135,371 has also shown that nitrogen-containing aromatic compounds are also increased in shale oil with increasing pyrolysis temperature and suggests a similar selective concentration process. In addition, increased mutagenic activity of shale oil pro- duced with increasing temperature of pyrolysis was attributed to increases in nitrogen-containing aromatic compounds.

The data on PAC concentrations in shale oil from this work cannot categorically assign PAC formation to either the Diels-Alder route or the selective concentration route. Indeed, the routes are not mutually exclusive. In addition, because of the different bond strengths within the kerogen, the composition of the oil derived from the pyrolysis may change as the extent of conversion of the kerogen increases with increasing temperature of pyrolysis. Consequently. more severe thermal degrada- tion of the kerogen at higher temperatures may release PAC. increasing the relative aromatic concentration in the oil. Table 1 shows that at the lower fluidised bed pyrolysis temperatures, not all of the potential oil was liberated from the oil shales. Clearly future work is required before an unequivocal formation route for PAC is confirmed.

The formation of PAH in oil shale pyrolysis oils in this work is clearly related to the temperature of pyrolysis (Tables 2 and 3). In addition to increased temperature. increased residence times of the pyrolysis vapours at high temperature will also lead to increased concentration of PAH [33.34]. It is generally agreed that the fast pyrolysis which occurs in a fluidised bed reactor usually involves short residence times which maximise the liquid product and minimises char formation. As a result. it might be expected that the pyrolysis oils derived from the fluidised bed pyrolysis of oil shales would lead to low concentrations of PAH. This is most probably due to the fluidised bed operating at the minimum fluidising velocity, producing an extended vapour residence time in the reactor. The freeboard, freeboard-expander section and cyclone of the reactor used in this work were heated externally and maintained at the set pyrolysis temperature throughout. Consequently, the resi- dence time was of the order of 45- 58 s in the hot zone. The extended residence time allowed increasing production of PAH. The results shown in this paper suggest that the conditions prevailing in the reactor are important in defining the oil yield and composition of the products. In particular, the temperature of pyrolysis and the residence time of the products in the hot zone should be specified. This work has shown that fast pyrolysis in a fluidised bed reactor can produce pyrolysis oils containing significant concentrations of PAC due to increased secondary reactions which can occur during the long residence times in the hot freeboard and cyclone

of the reactor used. The presence of PAC in shale oil also has significance if shale oil is to be used as

a substitute petroleum fuel in combustion systems. PAC associated with the soot emissions from combustion systems such as diesel engines and domestic oil heaters have been shown to be largely derived from the PAC present in the fuel, as an unburnt fuel fraction [13- 161. The fuel PAC survive the combustion process and are deposited on the soot as an unburned fuel fraction. Whilst other sources of PAC from combustion system are known, including pyrosynthesis, the unburned fuel route has been shown to be dominant [15,16]. The presence of carcinogenic and/or mutagenically active PAC absorbed on the soot emission from combustion systems may represent a health hazard. Consequently. the presence of high concen- trations of PAC in shale oil pyrolysis oils will lead to high concentrations of PAC in the combustion emissions from the fuel and may limit the use of shale pyrolysis oil as a directly combusted fuel. Petroleum-derived fuels also contain PAC. in

194 P.T. Williams, J.M. Nuzral i J. Anal. Appl. Pyrolysis 35 (1995) 181-197

20

n w4OOc

r l w45oc 16

1 AWS20C

l W62OC

I 2 3 4

Log. Molecular Weight 20

n RI4OOC

l RI45oc

ARI52OC

rRI570c

2 ‘2 l RI62OC c b

j 8

s

I z .I 4

Log. Molecular Weight

Fig. 5. Molecular weight range of oil shale pyrolysis oils in relation to the fluidised bed pyrolysis

temperature.

similar concentrations to those found in the shale oils reported here [14,15]. For example, diesel fuel has been shown to contain 400-700 ppm fluorene, 600-1000 ppm phenanthrene, 50-150 ppm pyrene and less than 10 ppm chrysene depending on the fuel source [14]. PASH and PANH were also found in diesel fuel, for example dibenzothiophene was detected in the range 125- 170 ppm and carbazole in the range 4- 10 ppm depending on the fuel source [14].

P.T. Williams. J.M. .Yaz:al : J. Ad. Appl. qlro(~~,si.c 35 (1995) IRI- 197 195

Fig. 5 shows the molecular weight (MW) range measured using the refractive index (RI) and UV detectors of the oil shale pyrolysis oils pyrolysed at 400, 450, 520. 570, and 620°C. The UV detector will be sensitive to the detection of aromatic compounds and the RI detector will measure the elution of all compounds. The MW range of the oils was from a nominal 50 to over 3000 Da. The MW ranges provided by the LJV detector show a peak MW of approximately 250 Da, whereas the MW range provided by the RI detector shows a peak MW of approximately 500 Da.

Consequently the aromatic compounds present in the oil shale pyrolysis oils generally have a lower MW range than the other compounds present in the oils. As the fluidised bed reactor temperature was increased there was a shift to a lower MW range. This shift to a lower MW range is most marked for the 620°C reactor temperature. The decrease in MW range with increasing pyrolysis temperature reflects the changing composition of the oil shale pyrolysis oils. As the pyrolysis temperature was increased, secondary reactions occur, as discussed previously. Consequently. higher concentrations of single-ring aromatic compounds are formed

in parallel with the thermal degradation of alkanes. some of which will be of high M W. The increased production of single-ring aromatic compounds and the decrease of higher MW aliphatic compounds will shift the measured MW to a lower range.

4. Conclusions

( I ) Oil shale pyrolysed in a fluidised bed reactor with nitrogen as the fluidising gas produced a pyrolysis oil which was found to contain significant concentrations of PAH, PASH and PANH.

(2) The concentrations of most PAH. PASH and PANH were increased with increasing temperature of pyrolysis. The PAH consisted mainly of naphthalene. fluorene and phenanthrene and their alkylated homologues, and lower concentra- tions of fluoranthene, pyrene and chrysene. The PASH identified included benzoth- iophene and dibenzothiophene, and the PANH identified included indole and carbazole and their alkylated derivatives.

(3) The suggested likely mechanism of formation was a Diels-Alder type aroma- tisation involving the cyclisation of alkenes and subsequently to monoaromatic and then polycyclic aromatic compounds, although an alternative formation route via the selective cracking of aliphatic compounds and consequent concentration of aromatic compounds cannot be ruled out.

(4) Some of the PAH and PANH identified have been reported to be mutagenic and/or carcinogenic.

(5) The molecular weight range of the pyrolysis oils was found to decrease with increasing temperature of pyrolysis. This was attributed to a decrease in the concentration of aliphatic compounds and an increase in the concentration of low molecular weight aromatic compounds.

(6) Process conditions that would minimise PAC concentrations in shalt oil would be lower pyrolysis temperatures and short residence times.

196 P.T. Williams. J.M. Naxal /J. Anal. Appl. Pyrolysis 35 (1995) 181-197

Acknowledgements

The authors would like to thank the Arab Students Aid International for the award of a student loan and also the Natural Resources Authority, Amman, Jordan for permission to take extended leave. We would also like to thank Leeds University personnel, Richard Bottrill, Bernard Frere and Peter Thompson.

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