The influence of catalyst type on the composition of upgraded biomass pyrolysis oils

Journal of Analytical and Applied Pyrolysis 31 (1995) 39 61

JOURNALOI ANALYTICALand APPLIED PYROLYSIS

The influence of catalyst type on the composition upgraded biomass pyrolysis oils Paul T. Williams

*, Patrick

of

A. Horne

Received IO July 1994: accepted 31 September 1994

Abstract The composition of oils derived from the on-line, low pressure upgrading of biomass pyrolysis oils from a fluidised bed pyrolysis unit have been investigated in relation to catalyst type. Na-ZSM-5 (partially exchanged), H-ZSM-5 and Y type zeolite catalysts, and activated alumina were used. In addition, a blank run was undertaken in which the catalyst bed was replaced by a bed of stainless steel ball-bearings to determine the influence of thermal cracking. The composition of the oils before catalysis and after catalyst upgrading were analysed by liquid chromatography fractionation, followed by coupled gas chromatography/ mass spectrometry analysis of each fraction. In particular, the aromatic and oxygenated aromatic species were identified and quantified. There were only small differences in the product yields and compositions from the catalysis of biomass derived pyrolysis oils for the Na-ZSM-5 and H-ZSM-5 catalysts. All of the catalysts were effective in deoxygenating the biomass pyrolysis oils; however. there were still significant concentrations of oxygonatcd compounds in the upgraded oils. The ZSM-5 catalysts gave the highest yields of hydrocarbon products when compared to the Y-zeolite and activated alumina catalysts. Significant concentrations of polycyclic aromatic hydrocarbons (PAH) were formed by all the catalysts. Some of the PAH have been shown to be carcinogenic in biological studies. The formation of coke was increased for Y-zeolite and alumina compared to the Na-ZSM-5 and H-ZSM-5 catalysts. Kc~~u’ora’s: Biomass;

* Corresponding

Catalysts:

Composition:

Oils; Pyrolysis

author.

0165-2370/95/$09.50 SSDI 0165-2370(

4: 1995 ~ Elsevier 94)00847-7

Science

B.V. All rights reserved

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1. Introduction The pyrolysis of biomass produces an oil product which may be used as a fuel or a chemical feed stock. However, the oils may be highly oxygenated, viscous, corrosive, relatively unstable and chemically very complex [ 1,2]. Consequently, catalytic upgrading of the oils to produce a premium quality fuel or high value chemical feed stock has received increasing attention. Two main routes to catalytic upgrading have been investigated: high pressure catalytic hydrotreatment [3] and low pressure catalysis using shape selective catalysts of the zeolite type [2]. As a result of the large volume of work that has been carried out on the use of zeolite catalysts in relation to the petroleum oil industry. there is increasing interest in their potential in the low pressure upgrading of biomass pyrolysis oils. In addition, the high oxygen content and chemical complexity of these oils has led to research into alternative catalyst types which might improve the product slate of upgraded biomass pyrolysis oils. The zeolite ZSM-5 catalysts have a strong acidity, high activities and shape selectivities which convert the oxygenated oil to a light hydrocarbon mixture in the C,-C,,, range by dehydration and deoxygenation reactions [2,4]. The oxygen in oxygenated compounds of biomass pyrolysis oils is mainly converted to CO, CO2 and H,O, and the resultant oil is highly aromatic with a dominance of single ring aromatic compounds [5]. It is preferable for the oxygen to be eliminated as CO or CO, rather than H,O, so that the hydrogen may participate in hydrocarbon forming reactions. The majority of the work undertaken on the low pressure catalysis of biomass oils has involved the ZSM-5 type of zeolite catalyst, mostly in its hydrogen form, H-ZSM-5. For example, Evans and Milne [6] used zeolite H-ZSM-5 in a fast pyrolysis batch reactor system coupled to a molecular beam mass spectrometer to investigate the influence of catalyst upgrading on the composition of biomass pyrolysis oils. Scahill et al. [7] used zeolite H-ZSM-5 in a fixed bed catalytic reactor to study the upgrading of biomass oil produced in a vortex reactor, and Chantal et al. [S] used a H-ZSM-5 catalyst for upgrading pyrolytic oil fed in a flow of helium over a fixed bed microreactor containing the catalyst. Zeolite H-ZSM-5 catalysts have also been used to upgrade oxygenated oils derived from biomass by liquefaction rather than pyrolysis. For example, Sharma and Bakhshi [9] upgraded oil, obtained from the liquefaction of Aspen wood, using H-ZSM-5 in a fixed bed microreactor, and Mathews et al. [lo] upgraded the thermally fractionated components of an oil prepared by the liquefaction of Aspen poplar wood using H-ZSM-5. Other catalysts have been used to investigate the low pressure catalytic upgrading of biomass pyrolysis oils. The replacement of the hydrogen cation in zeolite ZSM-5 catlysts by a metal has been shown to considerably improve the selectivity of the catalyst, leading to production of a more aromatic product for the catalytic conversion of lower alkanes [ 111. In addition, Dao et al. [ 121 investigated the catalysis of a range of model compounds typical of those found in biomass pyrolysis oils over H-ZSM-5, Zn-ZSM-5 and Mn-ZSM-5 zeolite catalysts. Catalysts have also been used to reduce the tar content of biomass pyrolysis gases used for

gas production in engines. For example, Magne et al. [ 131 investigated the use of dolomite, calcined dolomite, alumina, silica and aluminosilicate catalysts in decreasing the tar in gas phase pyrolysis gases from the fluidised bed pyrolysis of biomass. When the targeted upgraded product is a premium grade fuel or high value chemical feed stock, the influence of the catalyst type on the chemical composition of the upgraded oil should be determined in addition to other factors pertaining to the catalytic process, such as hydrocarbon yield, catalyst coking, etc. The chemical compositions of the oils are important, as they have been shown to contain high value chemicals in significant proportions [2,14]. In addition, the oils have been [ IS], some of which are carcinoshown to contain polycyclic aromatic hydrocrbons genic and/or mutagenic [ 16- 191. In this work. biomass was pyrolysed in a fluidised bed reactor at 5.50 C and the evolved pyrolysis vapours passed over a fixed bed of different catalyst types placed in the freeboard of the reactor. The catalysts used were zeolite H-ZSM-5. Na-ZSM5 (partially exchanged) and Y-zeolite, and activated alumina. Stainless steel ballbearings of the same dimensions as the catalysts were placed in the freeboard of the reactor to determine the influence of the thermal reactions. The percentage mass yield and chemical composition of the derived gases and oils were determined in relation to catalyst type.

Table I Proximate

and ultimate

analysis

Wood

Proximate

Volatilcs Moisture Ash

Yl.0 1.5 I.5

Table 2 Properties

of the biomass analysis

(wt’h. as rcccivcd) Element

Illtimate

C‘iU-bOl1

35.M

Hydrogen Oxygen Nitrogen

5.71 46.64 0. IX

analyG

of the catalysts

Property

Particle siTe (mm) Pore size (rn~ “‘I Pore volume (m ’ kg ‘) Surface area ( m2 g ’) Bulk density (kg m ‘) Support

Catalyst Na-ZSM-5

H-ZSM-5

Y-leolitc

Alumina

2 5.5 0.48 300 0.72 Clay binder

? 5.5 0.48 300 0.72 <‘lay binder

7 7.4 0.64 440 0.61 (‘lay hinder

I 2 20 (average) 0.X 270 0.90

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safety Valve

Water Cooled Condenser

JCU

Fluid&d Bed ermcouplc

u

N2-&+.(l i$

Dry Ice/Acetone

Preheater Fig, I. Schematic

diagram

of the combined

fluidised

bed/catalyst

bed reactor.

Condensers

2. Materials

and methods

The biomass used was a mixture of wood types obtained as waste shavings from a wood working company. Table I shows the proximate and ultimate analyses of the biomass waste material pyrolysed. 7 7 C’ulrrll~.st.s -.-. The catalysts used were zeolite Na-ZSM-5, H-ZSM-5. Y-zeolite and activated alumina, and Table 2 shows their properties, The Na-ZSM-5 and Y-zeolite catalysts were obtained from BDH, Ltd. (UK). The Na-ZSM-5 zeolite was in the partially hydrogen-exchanged form with a Na content of 0.03%. The zeolite Na-ZSM-5 form was converted to the H-ZSM-5 form using the recommended procedure of the supply company: Na-ZSM-5 was ion-exchanged with an NH,NO, solution. followed by washing and drying to produce NH,-ZSM-5. The NH,-ZSM-5 catalyst was then heated to 350 C in a dry stream of nitrogen to produce the hydrogen form of ZSM-5. After conversion the Na content of the H-ZSM-5 form was <0.003%. The Y-zeolite has a larger pore size and surface area than ZSM-5 catalysts. The pore size is significant in determining the size of the reactants and products which can enter and leave the active sites of the catalyst. The cation present in the Y-zeolite structure was hydrogen. The zeolite catalysts consisted of 0.2 cm diameter spheres and the binder was clay. This grain size was chosen to prevent tluidisation of the fixed catalyst bed and elutriation of the catalyst from the reactor. The silica/alumina ratio was 50 and the elliptical pore aperture was 5.4- 5.6 mm I” in diameter. The activated alumina was obtained from Alcan Chemicals ( UK) in the form of 0. I5 cm spherical particles. with a high surface of 270 m2 g-‘. However, the pore structure is not as homogeneous as that of the zeolite type catalyst. and the average pore size was significantly larger for the former. The catalysts were used new. rather than in regenerated form. OUI work has shown that substantial differences in the product slate can be obtained depending on whether the catalyst is new or regenerated. This work will be reported later. Stainless steel ball-bearings were also used to determine the influence of thermal cracking reactions in the reactor. The ball-bearings used were 0.2 cm diamctcr spheres packed into 1.5 times the volume of the other catalysts.

The experimental system used was a combined fluidised bed pyrolysis unit, in which the freeboard of the fluidised bed was packed with the catalyst to form a tixed catalyst bed ( Fig. I). The reactor was IO cm diameter x 100 cm height. constructed of stainless steel with full gas flow and temperature control. The reactor was heated externally and the temperatures of the fluidised bed and freeboard could be controlled separately. The incoming fluidising gas was nitrogen and was preheated to a temperature of 400 C. The bed material was silica sand of mean size 250 /lni diameter with a static bed depth of 8 cm. The fluidising velocity was three times

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the minimum fluidising velocity. The mass of catalyst used was 200 g and the bed depth 6.5 cm. The biomass was fed via a screw feeder and nitrogen gas stream to the top of the fluidised bed at a feed rate between 0.216 and 0.228 kg h-‘. The weight hourly space velocity (WHSV) representing weight of reactant per hour/ weight of catalyst was between 1.05 and 1.14 h-‘. This value of the WHSV was chosen to give a high degree of conversion to the upgraded product. WHSV values used by other workers have been, for example, 0.6-J [6], 2.5-2.8 [9] and 0.992.4 [20]. The approximate ratio of carrier gas to pyrolysis vapour was 18: 1, calculated from the mass of vapours and gases evolved during pyrolysis and assuming an average molecular mass. The primary fluidised bed temperature was maintained at 550°C and the fixed bed catalyst temperature was kept at 500°C. In the experimental system used in this work, the char was continuously accumulating in the fluidised bed primary reactor and the amount of coke on the zeolite catalyst was continuously increasing in the catalyst bed. The outlet from the fluidised bed was split; a portion of the effluent gas passed to a series of condensers to trap the derived oils and the remainder of the split gas was passed to waste. Samples of pyrolytic oil without catalyst upgrading were taken by sampling the pyrolysis vapours immediately below the catalyst bed and condensing the oils using the condensation traps as before. The primary bed uncatalysed oils consisted of a single liquid phase. However, after catalysis the condensed phase consisted of distinct oil and aqueous phases, which were separated by removal of the oil layer. The primary bed oil was separated into oil and water fractions by benzene reflux, using the standard ASTM D244 and IP 29.1 standard methods. The oil sample after catalysis was easily separated from the water phase. The extracted oil samples were then analysed in detail as described below. 2.4. Gas analysis The evolved gases were sampled at intervals by means of a gas syringe and were analysed off-line by packed column gas chromatography. The gases were analysed for CO, H,, and CH, using a molecular sieve SA 60-80 column with argon as the carrier gas and a thermal conductivity detector. The amount of carbon dioxide was determined using a silica column and argon as the carrier gas with a thermal conductivity detector. Gaseous hydrocarbons up to C5 were determined on a Porosil C 80-100 column with nitrogen as the carrier gas, using a flame ionisation detector. The total mass of gases was determined by calculations based on the analysis of the concentration of individual gases and their corresponding molecular weights. The range of analytical systems used for the gas analysis covered the majority of gases generated by the pyrolysis and catalytic upgrading of the biomass. 2.5. Oil analvsis 2.5. I. Molecular weight range The molecular weight range of the oil was determined using a mini-column size exclusion chromatography (SEC) system. The SEC system consisted of a pump,

ultraviolet (UV) detector and refractive index (RI) detector in series. Two 4.6 mm x I50 mm, 5 [tm, RPSEC 100 A type columns were used; these were placed in series and maintained at 0 C. Tetrahydrofuran was used as the mobile phase and the system was calibrated with polystyrene standards up to 100 000 mass units. The UV detector was fixed at a wavelength of 254 nm to optimise the detection of aromatic compounds. whilst the RI detector indicated all eluting compounds.

Chemical class fractionation of the oils was performed by liquid chromatography. Glass columns of IO cm x 1 cm were packed with silica, Bondesil (sepralyte) sorbent. and pretreated at I05 C for 2 h before use. The mini-columns were conditioned by washing with n-pentane. A 250 mg sample of the oil was placed on the column. The samples were adsorbed onto an inert Chromosorb G/AW/DMCS 60.- 80 support, mixed and then packed above the silica section of the column. This approach is necessary for polar oils which may produce a solid phase precipitate with the n-pentane solvent and block the column. and also to improve solvent contact with the oil. The column was then eluted with jr-pentane. benzene, ethyl acetate and methanol (polarity relative to Al,O, was 0.00. 0.32, 0.58 and 0.95 respectively), to produce aliphatic, aromatic, oxygenated-aromatic and polar chemical class fractions respectively. The fractions were analysed by Fourier transforminfrared spectroscopy (FT-IR) to determine the efficient separation of the chemical classes. A Perkin-Elmer 1750 model with spectral library search facility was used for the FT-IR. In addition, each fraction was analysed by gas chromatography/

580

v

??

m

v

A A

v

A

A

??

0

0

5

IO

30

15

25

Time (mms) Fig. 7. Temperature--time

profiles

of the catalyst

beds dunng

catalytic

upgrading.

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Williams,

Table 3 Product yield for the fluidised (a)

(b)

of biomass

Pyrol)Cs

31 (1995) 39 61

(wt%)

Gases

Oil

Aqueous

Char

15.2 37.x 36.9 29.6 28.2 25.3

40.41 6.01 5.47 I.13 3.12 I I .80

14.8 19.3 18.0 23.9 23.0 27.0

17.1 17.1 16.8 17.5 17.1 17.8

In terms of oil passed

H-ZSM-5 Na-ZSM-5 Y-zeolite Alumina Stainless steel

bed pyrolysis

/ J. And. A&.

feed (as received)

In terms of biomass

Primary bed H-ZSM-5 Na-ZSM-5 Y-zeolite Alumina Stainless steel

P.A. Home

Coke

I I.9 12.5 19.1 IX.4

Total 87.5 92. I 94.7 91.3 89.9 Xl.9

over the catalyst

Gases

Oil

Aqueous

Coke

Total

53.7 51.8 34.4 31.0 23.9

14.3 13.0 2.7 7.4 28.0

14.5 II.3 25.9 23.8 33.4

28.4 29.8 45.5 43.7

110.9 105.9 108.5 105.8 85.3

mass spectrometry using ion trap detection (GC/ITD) to verify the fractionation scheme. When the catalytically upgraded oil was analysed it was found to be highly aromatic, consisting of over 50% single ring and PAH by mass of oil. The highly aromatic nature caused the low molecular mass aromatic compounds to elute in the pentane fraction. The percentage mass in each class fraction was determined by analysis of the total gas chromatogram in relation to known masses of sample, internal standards and blends of chemical class fraction standards analysed by the system. This avoided the need to nitrogen evaporate (blow-down) the solvent elutant which can result in losses of light hydrocarbons. 2.5.3. Drtded oil unalysis The concentrations of aromatic and polycyclic aromatic species in the pentane and benzene fractions, and of oxygenated species in the ethyl acetate fraction. were determined using coupled gas chromatography/mass spectrometry (GC/MS). The GC/MS system was a Carlo-Erba, Vega HRGC, with cold on-column injection, coupled to a Finnigan Mat ion trap detector via a heated transfer line. A 25 m x 0.3 mm fused silica capillary column coated with DB5 was used in the gas chromatograph; the temperature programme was 60’C for 2 min followed by a 5°C min’ heating rate to 270°C. The carrier and make-up gas was helium with a carrier flow rate of 2 cm3 min’ at 270°C. The ion trap detector had a mass range from 20 to 650 u with scan times of between 0.125 and 2 s. It was linked to an IBM PC/XT computer with an NBS/EPA mass spectral library of 38 752 mass spectra. Identification was determined by GC/MS. Single ion monitoring was carried out to detect the species present, as well as retention indices.

3. Results and discussion The temperature of the catalyst bed was recorded throughout the 30 min experimental run period. The results are shown in Fig. 2. For all the catalysts used there was an initial large rise in temperature during the first five minutes. The temperature rise in the catalyst bed was due to exothermic catalytic reactions. The activated alumina gave the largest temperature rise from 500 to 580 C. After the initial temperature rise the catalyst temperature decreased steadily over the experimental period. The temperature of the stainless steel ball-bearings remained fairly constant throughout the experimental period. Consequently, whilst the initial experimental set-up was for a 550’C primary pyrolysis temperature and a 500 C catalytic bed temperature, the temperature profiles showed that the catalytic temperature varied over the experimental period.

Table 3(a) shows the percentage mass yield in relation to the mass of biomass, fed for the gases, oil, aqueous phase, residual char in the primary fluidised bed and coke formed on the catalyst. The primary bed oil represented a single liquid phase, whereas the catalysed product represented two distinct oil and aqueous phases. The primary bed oil was separated into aqueous and oil fractions using the standard ASTM D244 and IP 291 .I methods. Table 3(b) shows the product yield from the catalytic upgrading of biomass in terms of the oil vapours passed over the catalyst, based on the primary bed results. The oil yield from the primary bed in relation to the mass of biomass fed was 40.41 wt’%; after passing through the catalyst, the yield of oil decreased markedly. The ZSM-5 catalysts were found to give similar oil yields, whilst the Y-zeolite and activated alumina oil yields were reduced. Although the oil has a much lower oxygen content (Table 5). the yield in relation to the mass of biomass fed was low. The aqueous phase contained a significant percentage mass of organic material as evidenced by the elemental analysis (Table 5). Comparison with the yield of upgraded oil from the literature express the conversion as a shows higher conversions; however, many authors percentage yield from the oil phase only. On this basis, Table 3(b) shows a conversion of 2.7 and 7.4 wt% for the Y-zeolite and alumina respectively. but much higher conversions, up to 14.3 wt%, for the ZSM-5 catalysts. Also. these percentage conversions of oil do not include the aqueous phase and gas phase hydrocarbons. Scahill et al. [7] presented their results for the upgrading of pyrolysis oil over a H-ZSM-5 zeolite catalyst in relation to the mass of biomass fed, and found similar mass yields of oil in the gasoline distillation range. The results were a 7 wt% yield of gasoline range oil at a zeolite temperature between 445 and 46O’C, decreasing to 4.4 wt% at a catalyst temperature between 493 and 548 C. Evans and Milne [6] obtained hydrocarbon yields of about I8 wt’/;, in relation to the biomass fed for the upgrading of biomass pyrolysis oil with H-ZSM-5 catalyst.

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Table 4 Composition

Williams.

of the gases derived

(a) In terms of biomass co Primary bed H-ZSM-5 Na-ZSM-5 Y-zeolite Alumina Stainless steel (b)

6.1

16.7 15.9 13.3 12.7 14.5

In terms of oil passed

H-ZSM-5 Na-ZSM-5 Y-zeolite Alumina Stainless steel

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Appl.

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bed pyrolysis

31 (1995)

of biomass

39-~ 61

(wt’%)

(as received) CO,

CH,

C,H,

CZH,

C,H,

C,H,

HZ

6.8 13.4 12.3 9.1 9.7 8.2

0.69 1.58 1.71 2.64 2.40 0.65

0.16 0.21 0.23 0.47 0.87 0.19

0.26 2.15 3.41 1.86 0.84 0.34

0.04 0.34 0.45 0.25 0.27 0.18

0.45 2.43 2.64 1.64 1.01 0.76

0.02 0.08 0.07 0.13 0.30 0.05

over the catalyst

co

CO?

CH,

CzHh

C,H,

C,H,

C,H,

H2

23.9 21.9 15.8 14.3 18.5

15.6 13.0 5.4 6.9 3.4

2.12 2.43 4.64 4.07

0.11 0.16 0.73 1.68 0.07

5.93 7.50 3.81 3.81 0.05

0.70 0.97 0.49 0.54 0.32

4.14 5.22 2.86 1.35 0.75

0.14 0.11 0.25 0.66 0.04

The coke on the catalysts for the Y-zeolite and activated alumina were 19.1 and 18.4% respectively, compared to approximately 12% for the ZSM-5 catalysts. The increase in coke formation would have contributed to the lower oil yields observed for the Y-zeolite and activated alumina. The larger pore size of these catalysts allows larger coke precursors to enter the pore structure of the catalyst, initiating and contributing to the increased formation of coke. The pore structure of the ZSM-5 catalysts is size selective and only allows molecules approximately the size of a C,,, molecule to enter and leave the catalyst structure. The coking of the stainless steel ball-bearings was negligible, although a very thin layer of coke was observed to form on the surface of the steel. Coke formation has been reported in the literature for the H-ZSM-5 catalyst at less than lo”/;, when the results are expressed as coke formation in relation to mass of biomass fed [6]. However, if the results are expressed as formation of coke in relation to the oil arising from the primary bed, then Table 3(b) shows that coke formation is in the order of 28-31 wt”Al for the ZSM-5 and activated alumina catalysts, and 45 wt% for the Y-zeolite. In comparison, coke formation yields as high as 27 and 30 wt% have been reported in the literature for H-ZSM-5 [8,9]. The yields of aqueous products, which would also contain dissolved organic species, were highest for the Y-zeolite and activated alumina. The yield of the aqueous phase for the steel ball-bearings was also high, indicating significant thermal cracking of the oil in the steel bed. The gas yield from the primary bed pyrolysis of biomass in the absence of catalyst was 15.2’s, but showed a marked increase after catalysis. The highest yields were found for the ZSM-5 catalysts. The

P. T. Willicmr,

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39

5

Elemental

Primary

analysis of the biomass derived pyrolysis oils

(wt”:, oil)

Gases

Oil

39.7

59.6 ,’

38.1

2.4

6.5 I’

8.5

2.6

52.x

24.3

bed

Carbon Hydrogen Oxygen

71.6

51.9

33.s “

44.7

86.1

2.3

72 Y

3.5

1.9

10.X

4.3

5.i

51.3

5.5

X6.7

23.x

55.0

46.6

87.6

2.1

4.0

7.4

12.0

73

48.9

4.6

85.6

4Y.2

46.5

81.1

2.x

4.8

7.3

IO.5

5.0

48.7

8.4

86.0

37 6

43.0

81.1

1.5

50 7

5.0

8.4

IO.6

51

48.Y

9.2

X6.6

44 0

43.7

45.5

26.1

2.9

8.0

8.9

56.5

46.4

64.9

H-EM-5 Carbon Hydrogen Oxygen

ix.9

Na-ZSM-5 Carbon Hydrogen Oxygen

42.7

Y-Leolite Carbon Hydrogen Oxygen

57

I

Alumina Carbon Hydrogen Oxygen Stainless steel Carbon Hydrogen Oxygen

I’Oil analysis after removal

of water.

stainless steel ball-bearings produced an increase in gas yield, indicating significant thermal cracking of the pyrolysis vapours. The char results shown in Table 3 reflect the reproducibility of the fluidised bed. For each experiment the pyrolysis was undertaken in the primary bed under identical conditions: hence, the percentage weights of char should have been identical. 3.2. Gus anu1ysi.s

Table 4(a) shows the individual yields of the gaseous products in terms of the percentage mass of biomass fed into the reactor. Table 4(b) shows the results in terms of the percentage mass of pyrolysis oil vapours passed over the catalyst. where the gases formed on catalysis are minus the primary bed gases and corrected for the mass of oil passing over the catalyst bed. Table 4(b) shows that the main gaseous products were CO and CO,, and the main hydrocarbons were CH, and the alkene gases C,H, and C,H,. There were also minor concentrations of other

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hydrocarbon gases. Alkane and alkene gases have been reported to be the main hydrocarbon gases from the zeolite catalyst upgrading of oils derived from the pyrolysis of biomass [6-S] and also from the zeolite catalytic upgrading of biomass liquefaction oils [9,10]. The conversion of the oxygen in the pyrolysis oil to CO, CO, and H20 has been reported as the primary conversion oxygen species for zeolite catalytic upgrading [ 6,8]. The main aim of the catalysis of biomass pyrolysis oils is to upgrade the highly oxygenated pyrolysis vapours by removal of the oxygen to produce a hydrocarbon product. The three main routes for removing oxygen from the vapours are via water, CO and CO1 formation. For the ZSM-5 catalysts, the CO, CO? and aqueuous product yields account for approximately 46-54 wt% of the organic pyrolysis vapours. This coupled with the formation of coke on the ZSM-5 catalyst, which Table 3 shows was between 28.4 and 29.9 wt%, represents 75-80 wt’% conversion of the pyrolysis vapours to non-oil products. For the Y-zeolite and activated alumina catalyst, the aqueous product was the main result of oxygen removal from the biomass pyrolysis vapours. The combination of CO. CO, and aqueous products accounts for 47 and 45 wt’% of the pyrolysis vapours for the Y-zeolite and activated alumina catalysts respectively, figures similar to those for the ZSM-5 catalysts. However, the formation of coke on the Y-zeolite and activated alumina catalysts was markedly higher than on the ZSM-5 catalysts, representing 45.5 and 43.7 wt’% respectively. Churin [21] has suggested that the maximum yield of hydrocarbons from the upgrading of a typical biomass pyrolysis oil over ZSM-5 catalyst would be achieved if all the oxygen present in the oil was removed as CO,. The removal of the oxygen as CO produced lower hydrocarbon yields, and removal as H,O produced less than one third of the potential hydrocarbon yield, compared to the removal of the oxygen as COz. This is confirmed in the work presented here, where high hydrocarbon gas and oil yields for the ZSM-5 catalysts are coupled with a lower production of the aquoues phase, and lower hydrocarbon gas and oil yields are coupled with a higher production of the aqueous phase. 3.3. Elemental

analysis

of the products

Table 5 shows the elemental analysis for the products of biomass pyrolysis, for the primary bed oil and after catalytic upgrading. The primary bed uncatalysed oil had a high oxygen content of 33.5 wt% but after catalysis this was markedly reduced, the ZSM-5 catalysts being most effective in removing the oxygen from the pyrolysis oils. Even so, the oils still clearly contain a certain proportion of oxygenated compounds. The carbon content of the oils from all the catalysts was high, above 80 wt%. The elemental analysis of the coke on the catalyst showed that the coke contained a high proportion of oxygen, representing unconverted or partially converted pyrolysis oils. The char also contained a significant quantity of .oxygen. The carbon and a proportion of the hydrogen content of the aqueous phase represents the dissolved hydrocarbon compounds found in the aqueous phase. The elemental analysis of the oil when stainless steel was used in place of the catalysts shows that the derived oil is still highly oxygenated.

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16

3.4. Arzalysis qJ’the oils

Fig. 3 shows the molecular weight (MW) range measured by size exclusion chromatography (s.e.c.) using the RI detector of the uncatalysed pyrolysis oil and after catalytic upgrading for the various catalysts. The MW range of the uncatalysed oil was broad, from a nominal 50 to 1300 MW units. However, after catalysis, the MW range of the ZSM-5 and Y-zeolite catalysed oils decreased to 50-600 MW units. with the majority of the oil components in the oils in the range 50-200 MW units. The Y-zeolite showed a slight increase in high MW compounds compared to ZSM-5 catalysts. In contrast to the zeolite catalysed oils. the activated alumina catalysed oil had a broad MW range from 50 to 750 MW units, and there was no dominance of low MW compounds as found in the zeolite catalysed oils. The pore structure of the catalysts determines the MW range of the upgraded oils. The ZSM-5 catalysts have a pore structure which allows compounds of molecular the size approximately that of a C,,, molecule to enter and leave the structure. Y-zeolite has a slightly larger pore size, whilst the activated alumina does not have a definite pore structure, but an average pore size of 20 rn-‘“. The higher MW material detected in the upgraded zeolite catalyst oils could either be unconverted pyrolysis vapours or catalysis products formed on the outer surface of the catalyst. The oil produced when stainless steel ball-bearings replaced the catalyst showed very little difference in MW range when compared to the uncatalysed oil. The oil also showed an absence of the low MW compounds which were detected in the catalysed oils.

52

P.T.

Table 6 Chemical (a)

Williams,

class fractionation

P.A. Hornr 1 J. And.

of the biomass

derived

Appl. Pyroly.sis 31 (1995) 39-61

pyrolysis

oils

(wt%)

In terms of oil composition Soluble

Primary bed H-ZSM-5 Na-ZSM-5 Y-zeolite Alumina Stainless steel

Pentane

Benzene

Ethyl acetate

Methanol

Total

<0.2 40.2 46.2 48.9 32.5 0.8

0.5 10.2 6.2 6.2 16.7 I.8

38.4 45.4 41.6 40.3 45.6 77.8

55.2 I.1 I.3 I.9 2.8 13.7

94.3 96.9 95.3 97.3 97.6 94.1

Pentane

Benzene

Ethyl acetate

Methanol

Total

0.1 2.4 2.5 0.5 1.0 0.1

0.2 0.6 0.3 0.1 0.5 0.2

15.2 2.7 2.3 0.5 I.4 9.2

23.1 0. I 0. I
38.6 5.8 5.2 I.2 3.0 II.1

(b) In terms of biomass

feed

Soluble

Primary bed H-ZSM-5 Na-ZSM-5 Y-zeolite Alumina Stainless steel

in:

in:

3.4.2. Chemical class fractionation of the oil Table 6 shows the percentage weight chemical class fractionation of the oils, in terms of the oil analysed (Table 6(a)) and in terms of the biomass fed to the reactor (Table 6(b)). The pentane elution corresponded to aliphatic compounds for the uncatalysed oil, but owing to the high aromatic content of the catalysed oils, aromatic compounds were found by GC/MS analysis to have eluted in the pentane fraction. The benzene fraction was found to contain aromatic and polycyclic aromatic compounds, the ethyl acetate fraction, oxygenated compounds, and the methanol fraction, polar compounds. The total analytical recoveries are also presented in Table 6(a). The uncatalysed oil shows a low percentage mass fraction of aliphatic and aromatic compounds, but a high concentration of oxygenated and polar material. After catalysis the methanol fraction was markedly reduced with a corresponding increase in the other, less polar fractions. Oxygenated compounds will be present in both the ethyl acetate and methanol fractions, but of different polarities. The results shown in Table 6(a) suggest that the compounds found in the methanol fraction are converted to aromatic compounds but the compounds found

Table

7

Aromatic

and polycyclic aromatic

Name

compounds Primary

in biomass pyrolysis oils (mgikg Catalyst

011)

type

bed H-ZSM-5

4

Y-Kohte

Alumina

Steel

Benzene

97

15840

43415

239x0

5995

508

Toluene

67

70050

98645

63360

30 I60

670

Ethylbewene

24

34125

32470

2 I 770

I2405

?YO

Dimethylbewenes

1’)

21130

22815

I5 I30

I3300

355

Trimethylbewenes

Y

15.525

I4570

15575

I SOY5

330

1778

2320

707

35’5

2

X785

7660

2565

1775

I51

Methylindene

2

5474

7X80

I Y47

I I730

ho

Dihydroindene

4

5590

5704

I330

6475

50

1464

III5

I593

1058

12

Tetramethylbenzene


Indene

x

Na-ZSM-5

X5

I0 II

Benzofuran

IU

Methylbenrofuran

I3

I398

?300

3012

l-14

12

Naphthalene

II

2x170

‘3780

51239

6570

3YX

I3

Methylnaphthalenes

I4

475X7

33310

60443

lY935

75.1

I4

Dimethylnaphthalenes

IS

25490

22030

86 I05

I7660

355

15

Trimcthylnaphthalenes

7

4758

4552

X053

37x1

I51

I6

Tetramethylnaphthalenes

‘450

50

I7

Biphenyl

IX I9


1353

715

5I0

973

2

50

35

90

11x

Accnaphthcnc

3

50

55

90

I’X

0

Fluorene

I

3045

1553

1770

~~10

25

0

20

Methylfluorene


10465

XXI7

6902

12532

ho

31

Dimethylfluorene


1313

I590

2Y20

51’-0 _.

2’

Anthracenc

535X

46’5

2035

246X

Ii \

23

Phcnanthrene

II

5008

4315

IXX50

21 I5

1’

24

Methylphenanthrenes

I6

23045

I6745

37433

10417

I35

25

Dimethylphenanthrenes

I2

XXIX

7440

6X I5

X525

36

Trimethylphenanthrenes

2

965

118X

1415

403X

76 i,

27

Tetramethylphenanthrene

2

2196

1700

I’30

Xl40

ho

1x

Pyrene

I

IX30

1645

9470

3565

17

1’)

Methylpyrcne


3677

3655

5665

6335

17

30

Dimethylpyrene


3744

3785

39x2

5575

X

31

Chrysene


550

676

973

610

0

32

Methylchrysenc


33

Benzopyrenes


Total

347

6

730

7Y5

3x0

0

I680

I YY2

256.5

1315

X

36242 I

3YO67 I

X4X

150079

22836X

5OIll

in the ethyl acetate fraction increase in quantity. However, Table 6(b) shows the data of Table 6(a) in relation to the total biomass pyrolysed, which reflects the reduced percentage mass of pyrolysis oil produced on catalysis. The results show that both the methanol and ethyl acetate fractions decrease as the pyrolysis oil is catalysed, the methanol fraction showing the most marked reduction. It is also seen that the ZSM-5 catalysts gave the highest conversion of pyrolysis oil to aromatic compounds. The stainless steel experiment shows that the oil produced had a low aromatic content and high concentration of oxygenated and polar compounds.

54

P.T.

Williums.

P.A. Home 1 J. And.

Appl. Pyrolysis

31 (1995) 39~~61

However, the methanol fraction was markedly reduced when stainless steel ballbearings replaced the catalysts. The methanol fraction contains large amounts of cellulose and hemicellulose derived material, such as levoglucosan and other anhydrosugar monomers and polymers [9]. The reduction in the methanol fraction demonstrates the potential instability of the pyrolysis vapours at the temperatures used in this work of 500°C. The instability of the pyrolysis vapours at 500°C suggests that the catalysts could initially act as the stainless steel in providing a surface to thermally crack the larger molecular weight material present in the pyrolysis vapours as they pass over the catalyst bed. The intermediate products formed from the thermal cracking of the pyrolysis vapours would then be more reactive with respect to the catalyst pore structure and active sites to form aromatic compounds. 3.4.3. Detailed oil analysis Table 7 shows the detailed analysis of the benzene fraction of the uncatalysed oil, and the pentane and benzene fractions of the catalysed oils for aromatic and polycyclic aromatic hydrocarbons (PAH). The table shows that the uncatalysed oil contains low concentrations of aromatic compounds and PAH. However, after catalysis there was a marked increase in the concentration of single ring aromatic compounds and PAH. The former compounds consisted of benzene, toluene and alkylated benzenes. The PAH mainly consisted of naphthalene, phenanthrene and their alkylated homologues, whilst fluorene, pyrene and chrysene, and their alkylated homologues, were also present in significant concentrations. The concentration of single ring aromatic compounds in the oils totalled 15.9 wt% for the H-ZSM-5 catalyst, 21.3 wt% for the Na-ZSM-5 catalyst, 14.0 wt% for the Y-zeolite and 8.0 wt’% for the activated alumina catalyst. The Y-zeolite produced the highest concentration of two-ring naphthalene compounds in the oils. The total concentrations of naphthalene and alkylated homologues in the oils were 10.7 wt% for the H-ZSM-5 catalyst, 9.3 wt% for the Na-ZSM-5 catalyst, 20.7 wt% for the Y-zeolite and 5.0 wt% for the activated alumina catalyst. The concentrations of phenanthrene and anthracene were approximately equal for the ZSM-5 catalysts and the activated alumina catalysts. However, for the Y-zeolite catalyst, the phenanthrene concentration was approximately 10 times higher than that of anthracene. The Y-zeolite pore structure appeared to favour the formation of the phenanthrene aromatic ring structure to that of anthracene. The Y-zeolite also favoured the formation of fourand five-ring PAH, producing higher oil concentrations of pyrene, and benzo[e]pyrene and benzo[a]pyrene, than the other catalysts. The oils produced with stainless steel had much lower concentrations of single ring aromatic compounds and PAH than the catalysed oils. The formation of single ring aromatic compounds and PAH by thermal rather than catalytic reactions was therefore not deemed as a significant reaction in this work compared to the formation of these compounds by catalysis. The formation of mono- and polycyclic aromatic species by a Diels-Alder type reaction is well known [22-251, and has been suggested as a mechanism for the formation of PAH in biomass pyrolysis oils subjected to secondary or tertiary cracking reactions [26]. The reactions involve the pyrolysis of alkanes to produce

Table X Oxygenated

xomatic

compounds

in biomass pyrolysis

NUllC

Primaq

011s (mg/kg oil)

(‘aalyst

type

bed H-ZSM-5

Na-ZSM-5

Y-/colitc

Alumina

Cyclopenranonc

1380

233

1500

442

121X

Cyclopentenonc

55X0

4hY7

XX65

5133

767X

Furanmethanol

2467

50

710

X45

2145

4863

Methylcyclopentenone Furanonc Merhylfurfurvl

IO013 330x

Phenol

I x3’)

Hydronymethylcyclopcntenone

2470

Methylphcnol

2261

Methoxyphenol

2930

XX 2655

I ‘JZ 4100

a00

I I70

177

I I53

450

_.17?J

7%

25%

I0565 7.33 I4775

1713

I5064

lO2XX

713

354

70164

16360

545

I?160

1964

?hSO

I946

linkflown

1032

1335

2111

12X2

IJnknown

1664

13X’)

1681

I XYO

Y303

X2.37

Dimethylphenol

121x

436

663’)

Methylmethoxyphenol

1412

1280

I YY.?

IN?

Henzenediol

402x

317x

3766

3OY7

Mcthylbenrenediol

2675

19x0

2oio

23X!,

Ii45

229

2878

6174

3x05

36X5

0

20x0

475

1504

317

750

I ‘9X

050

1123

370

0

0

0

1335

l5Y’

‘):o

Trimethylphenol Tetramethylphenol Ethylmcthoxyphenol Hydroxymcthylphenylethanone

3097

Dimethoxyphenol

476

0

I4425

YiO 3-w I

I

I680

Dimethyl/ethylbcnzenediol

I414

621

6I 0

673

Methoxypropenylphenol

3542

4808

3405

54X6

1304

Hydroxymethoxyphenylethanone

46.5

1775

X4X

X95

706

352

Dihydroxymethoxyphenylethanone

807

565

530

256

Hydroxymethoxyphenylpropanone

913

IO0

566 >5

I 76

06

676

70x

310

3’00

3x03

I ouo

MW

IX0

Dimrthoxypropenylphenol MW

I80

5346

III5

2157

4076

465

0

0

0

0

Hydroxydimethoxybenzaldehyde

I6hY

465

I IO

354

160

Hydroxymethoxypropenylphenol

9203

0

0

0

0

Hydroxydimethoxyphenlethanone

IO16

0

0

0

0

0

0

0

0

0

MW

210

MW

210

3146

0

0

Naphthol

54

41 IO

351.5

6775

2372

13X1

4753

5x40

2660

5x2

16X0

300

I 13)

IX3

475

Methqlnaphthol Dimethylnaphthol

YOh

II6 0

Trimethylnaphthol Total

X25? I

77714

I I2302

0

44’

352

97154

76X4Y

alkenes which are subsequently aromatised by a Diels-Alder type reaction to form single ring aromatic compounds, reacting further to give PAH. The reaction has been shown to be influenced by high temperatures or long residence times [22S25]. However, the oils produced with stainless steel in place of the catalysts and under

56

P. T. Williams.

P.A.

Home

1 J. Anal.

Appl.

Pyrolysis

31 (1995)

39

61

identical reaction conditions to the experiments with the catalysts, showed that the formation of single ring aromatic compounds and PAH were much lower. The formation of the aromatic compounds and PAH were, however, significant and consequently their formation in the catalysed oils would represent both a catalytically formed fraction and a thermally formed Diels-Alder fraction. The concentrations of aromatic compounds and PAH in the catalysed oils are high; however, it should be noted that the yields of the upgraded oils in terms of the weight of biomass fed to the reactor were low, as shown in Table 3. Consequently, the yield of the aromatic compounds and PAH in terms of the weight of biomass pyrolysed would also be low. The detailed analysis highlights the presence of certain high value chemicals found in the oil, e.g. benzene, toluene, xylene and alkylated benzenes, and naphthalene are well known chemical feed stocks. These compounds are present in high concentration in the oils, along with PAH which have been shown to be carcinogenic and/or mutagenic. For example, Lee et al. [ 161 list the relative carcinogenicities of certain PAH and show that chrysene, tri- and tetramethylphenanthrene, benzo[a]pyrene and benzo[e]pyrene [ 161, phenanthrene and the methylphenanthre[ 191 have been shown to be biologically active in nes [ 181, and the methylfluorenes carcinogenicity and/or mutagenicity tests. Single ring aromatic compounds and PAH have been detected by other workers in biomass pyrolysis oils and in upgraded catalysed oils. The uncatalysed biomass pyrolysis oils contain low concentrations of PAH [27,28], e.g. benzene, naphthalene, phenanthrene and fluorene, and thier alkylated homologues have been detected in low concentrations in biomass vacuum pyrolysis oil [28]. In addition, some of the PAH present were biologically active, such as benzo[a]pyrene, chrysene and benzo[ klfluoranthene. Catalytic upgrading of biomass derived oils has also been shown to produce increased concentrations of aromatic compounds. H-ZSM-5 catalyst has been shown to produce marked increases in the concentration of benzene, toluene and alkylated benzenes, naphthalenes and alkylated naphthalenes from the catalytic upgrading of biomass oils [6,7,9,10]. Dao et al. [ 121 found that replacing the hydrogen cation in zeolite H-ZSM-5 catalyst with Zn or Mn cations produced a reduction in the aromatic concentration, but an increase in the PAH concentration of the upgraded oil for the model biomass compounds, furfural and glycerol. Ono [ 111 showed for low molecular weight alkanes that replacing the hydrogen cation in H-ZSM-5 with Zn and Ga cations produced increased total concentrations of aromatic compounds. Table 8 shows the concentration of oxygenated aromatic compounds in the ethyl acetate fraction determined using GC/MS for the uncatalysed and catalysed oils. The uncatalysed oil contains oxygenated species in high concentrations, as shown in Table 5; however, after catalysis the oxygen content of the oils was markedly reduced. Table 8 shows that after catalysis, although some oxygenated species increased in concentration in the oil, because the oil percentage mass had decreased significantly, the overall influence of the catalysts was to decrease the yield of the oxygenated species. The data shown in Table 6(a) for the concentration of species in the oil suggest this was due to the reduction in the methanol fraction; in addition,

the ethyl acetate fraction after catalysis would also have contained high concentrations of oxygenated compounds. Other analyses of biomass pyrolysis oils using different analytical techniques to those in this work have identified a variety of oxygenated compounds such as levoglucosan. carboxylic acids such as acetic. propionic and butanoic acids, benzoic acid, alcohols, such as furfuryl, coumaryl and coniferyl alcohols, aldehydes such as hydroxy, coniferyl and syringyl aldehydes. etc. [26,28-311. The analytical techniques used in this work specified the oxygenated aromatic compounds and did not analyse the full range of oxygenated species. Table 8 shows that after catalysis there was a change in composition of the oxygenated aromatic species. For example. furanone. furanmethanol. hydroxymethylcyclopentanone, hydroxymethylphenylethanone. hydroxymethoxypropenylphenol, hydroxydimethoxyphenylethanone and unidentified compounds of MW 180 and 210 are in high concentration in the uncatalysed oil, but this is markedly reduced after catalysis. Other compounds such as naphthols and alkylated naphthols, and phenol and the alkylated phenols arc absent or in low concentration in the uncatalysed oil. but reach high concentrations in the catalysed oil. The oil produced with stainless steel in place of the catalysts was similar in composition to the uncatalysed oil, showing that thermal reactions in the presence of stainless steel had a minor influence on the composition of the upgraded oils. Fat example, the compounds such as furanone, furanmethanol. hydroxymethylcyclopentanone, hydroxymethylphenylethanone. and hydroxymethoxypropenylphenol which were eliminated during the catalytic process survived in the stainless steel reacted oil. Similarly, compounds such as naphthols and alkylated naphthols. and phenol and the alkylated phenols which were formed or increased in concentration after catalysis were not found in the stainless steel reacted oil. It should be noted that Tables 7 and 8 show the concentration of the single ring aromatic compounds. PAH and oxygenated aromatic compounds in the oil, and conversion can be made to the concentration of these compounds in relation to the weight of biomass fed into the reactor using the oil yield data of Table 3.

4. Mechanism

of the catalytic

upgrading of biomass

The catalysts used in this work had a defined pore size of 5.5 m I” for the ZSM-5 catalysts. 7.4 m -“’ for the Y-zeolite, and a range for the activated alumina. but with an average pore size of 20 mu”‘. Consequently, reactants which are too large to enter the catalyst pores will remain unconverted unless they can react on the catalyst surface. For example, the maximum size of the product which can pass in and out of the ZSM-5 type catalysts is equivalent to a C,,, molecule such as naphthalene [ 321. Therefore, the larger molecular weight material in the uncatalysed pyrolysis oil would first be required to thermally decompose before catalytic reactions can occur, owing to the specific pore size of the catalysts, particularly the ZSM-5 and Y-zeolite types. The larger molecular weight material may also decompose on the surface of the catalyst followed by pore catalysed reactions. The

58

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WiNiums.

P.A.

Hortw

/ J.

And.

Appl.

P~w/~‘.si.s 31 (1995)

39

61

formation of higher molecular weight material in the pyrolysis oils, such as three-, four- and five-ring PAH which were found in substantial concentrations in the catalysed oils, could be formed by surface reactions on the catalyst. The aromatic compounds and PAH found in the catalysed pyrolysis oils could be formed from two reaction pathways. The catalytic process could form low molecular weight hydrocarbon products which undergo polymerisation and aromatisation to form aromatic hydrocarbons, or the oxygenated constituents in the pyrolysis vapours could be directly converted to non-oxygenated aromatic hydrocarbons. The catalytic process has been shown to produce large amounts of the alkenes, C,H, and CjHb. The formation of these alkene hydrocarbons increased as the formation of aromatic and polycyclic aromatic hydrocarbons increased. It may be necessary for the catalysis process to initially form these alkene gases which can then react further to form aromatic and polycyclic aromatic hydrocarbons. Ono [ 1 l] investigated the catalysis of the alkenes C,H, and C,H, over zeolite ZSM-5 catalyst and discovered the production of a highly aromatic oil. Similarly, Chang and Silvestri [33] investigated the catalysis of methanol over zeolite ZSM-5 catalyst and suggested that an initial reaction step was the conversion of the methanol to dimethyl ether; this was followed by conversion to C,-C, alkenes which then reacted on the catalyst to form aromatic hydrocarbons. Alternatively, Evans and Milne [6] proposed that a large fraction of the aromatic hydrocarbons in zeolite ZSM-5 catalysed pyrolysis oils are derived more directly by the deoxygenation of primary pyrolysis products followed by aromatisation, rather than through an alkene route. Indeed, Chen et al. [34] passed acetic acid over zeolite ZSM-5 catalyst and detected large amounts of aromatic hydrocarbons in the product. Similarly, Dao et al. [ 121 passed a range of model biomass compounds over H-ZSM-5 and cation modified ZSM-5 catalysts, and found high concentrations of aromatic and polycyclic aromatic hydrocarbons. However, Chantal et al. [35] passed many different phenolic compounds over zeolite ZSM-5 catalyst and found that the phenolic compounds were only partially converted during the catalysis process to other phenolic compounds and large amounts of coke, and that the formation of aromatic compounds was low. Sharma and Bakhshi [36] passed the separated non-phenolic fraction of a biomass oil over zeolite ZSM-5 catalyst and found that the conversion of the non-phenolic fraction of the oil to aromatic hydrocarbons was approximately 15 wt%. The formation of coke when the non-phenolic fraction was pased over the catalyst was much lower than when the phenolic fraction was catalysed. Consequently, the formation of aromatic compounds and PAH during the catalytic upgrading of biomass pyrolysis oils is most probably a dual mechanistic route. The formation of low molecular weight hydrocarbons on the catalyst is increased due to thermal degradation of the pyrolysis vapours at a catalyst temperature of over 500°C. These low molecular weight hydrocarbons then undergo aromatisation reactions to produce aromatic compounds and PAH. Additionally, deoxygenation of oxygenated compounds found in the non-phenolic fraction of the pyrolysis oils may lead to the direct formation of aromatic compounds.

5. Conclusions t I) Biomass in the form of mixed wood waste has been pyrolysed in a fluidised bed reactor and catalytically upgraded using different catalysts: zeolite H-ZSM-5, Na-ZSM-5 (partially exchanged), Y-zeolite and activated alumina. (2) The oil produced on pyrolysis was highly oxygenated with a significant aqueous content. After catalysis, the percentage mass yield of oil decreased markedly with a consequent increase in the gas and aqueous phases. Coke formation on the catalyst in terms of biomass fed to the reactor represented between I 1.9 and 12.5 wt’!/;, for the ZSM-5 catalysts. but increased to IO. I wt’% for the Y-zeolite and IX.4 wt’% for the activated alumina. The coke formation in terms of the pyrolysis vapours passed over the catalyst represented over 30 wt’% conversion of the vapours to coke. (3) Analysis of the gas composition after catalysis showed an increase in CO and CO,. Hz, alkane and alkene hydrocarbons compared to the uncatalysed pyrolysis. (4) Chemical class fractionation and elemental analysis of the oils showed that oxygenated species decreased and aromatic species increased in concentration after catalysis. (5) Size exclusion chromatography analysis of the oils showed a shift to a Iovv;cr molecular weight range after catalysis. (6) Detailed analysis of the uncatalysed oils showed that they contained low concentrations of aromatic and polycyclic aromatic hydrocarbon (PAH) species which markedly increased in concentration after catalysis. The single ring aromatic species were mainly benzene, toluene and alkylated benzenes. The PAH were mainly naphthalene, phenanthrene, fluorene and their alkylated homologues. In addition, the oils contained some PAH which are known to be carcinogenic and/or mutagenic and were in significantly high concentration. (7) Detailed analysis of the oxygenated aromatic species in the oil showed that before catalysis they consisted mainly of phenols and benzenediols, and theit alkylated homologues. After catalysis some of the oxygenated species decreased and some increased in concentration. (X) The formation of aromatic and polycyclic aromatic compounds during the catalytic upgrading of biomass pyrolysis oils was suggested as a dual mechanistic route. Low molecular weight hydrocarbons are formed on the catalyst. which then undergo aromatisation reactions to produce aromatic and polycyclic aromatic hydrocarbons. Additionally, deoxygenation of oxygenated compounds found in the non-phenolic fraction of the pyrolysis oils may lead to the direct formation ot aromatic compounds.

Acknowledgements This work was supported by the UK Science under grant numbers GR/F/06074, GR/F/87837 the authors gratefully acknowledge.

and Engineering Research Council and GR/H/83355, whose support

60

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1 J. And.

A&.

Pwo!,~,si.s 31 (1995)

39-61

References [I] A.V. Bridgwater and S.A. Bridge, in A.V. Bridgwater and G. Grassi (Ed%). Biomass Pyrolysis Liquids, Upgrading and Utilisation, Elsevier, London, 1991. [2] T. Stoikos, in A.V. Bridgwater and G. Grassi (Ed%), Biomass Pyrolysis Liquids, Upgrading and Utilisation, Elsevier. London, 1991. [3] E.G. Baker and D.C. Elliott. in J. Soltes and T.A. Milne (Eds.). Pyrolysis Oils from Biomass. Producing. Analysing and Upgrading, American Chemical Society, Washington, DC, 1988. [4] E. Costa, J. Aguado, G. Ovejero and P. Canizares Ind. Eng. Chem. Res., 31 (1992) 1021~~1025. [S] J.P. Diebold, H.L. Chum, R.J. Evans, T.A. Mime, T.B. Reed and J.W. Scahill. Energy, Biomass Wastes, IO ( 1987) 801-830. [6] R.J. Evans and T. Milne, in J. Soltes and T.A. Mime (Eds.). Pyrolysis Oils from Biomass, Producing, Analysing and Upgrading, ACS Symposium Series 376. American Chemical Society, Washington. DC, 1988. [7] J. Scdhill. J. Diebold and A. Power. in A.V. Bridgwater and J.L. Kuester (Ed%). Research in thermochemical biomass conversion, Elsevier, London, 1988. [8] P. Chantal, S. Kaliaguine. J.L. Grandmaison and A. Mahay, Appl. Catal.. IO (1984) 317 332. [9] R.K. Sharma and N.N. Bakhshi. Bioresour. Technol.. 35 (1991) 57766. [IO] J.F. Mathews, M.G. Tepylo. R.L. Eager and J.M. Pepper. Can. J. Chem. Eng.. 63 ( 1985) 686 689. (I I] Y. Ono. Catal. Rev. Sci. Eng.. 34(3) (1992) 1799226. [ 121 L.H. Dao. M. Hanitf. A. Houle and D. Lamothe, in J. Soltes and T.A. Milne (Eds.). Pyrolysis Oils from Biomass, Producing, Analysing and Upgrading. ACS Symposium Series 376, American Chemical Society, Washington. DC. 1988. [ 131 P. Magne, A. Donnot and X. Deglise. in G. Grassi, G. Gosse and G. dos Santos (Ed%). Biomass for Energy and Industry, Elsevier, London, 1990. [ 141 A.V. Bridgwater. in A.V. Bridgwater and G. Grassi (Eds.). Biomass Pyrolysis Liquids. Upgrading and Utilisation. Elsevier, London, 1991. [ 151 P.T. Williams and P.A. Horne. Characterisation of oils from the fluidised bed pyrolysis of biomass with zeolite catalyst upgrading, Symposium on Thermochemical Conversion; Fast pyrolysis Paper No. 59, ACS meeting, Denver, CO, March 1993, American Chemical Society. Cellulose, Paper and Textile Division. Washington, DC, 1993. [ 161 M.L. Lee, M. Novotny and K.D. Bartle. Analytical Chemistry of Polycyclic Aromatic Compounds. Academic Press, New York, 1981. [ 171 P.T. Williams, The sampling and analysis of polycyclic aromatic compounds from combustion systems: A review, J. Inst. Energy, 63 (1990) 22 30. [ 181 J.P. Longwell. Polycyclic aromatic hydrocarbons and soot from practical combustion systems. in J. Lahaye and G. Prado (Eds.), Soot in Combustion Systems and its Toxic Properties, Plenum. New York. 1983. [ 191 T.R. Barfnecht, B.M. Andon, W.G. Thilly and R.A. Hites, Soot and mutation in bacteria and human cells. in M. Cooke and A.J. Dennis (Eds.). Chemical Analysis and Biological Fate: Polycyclic Aromatic Hydrocarbons, Battelle Press. Columbus. OH. 1980. [20] J. Diebold and J. Scahill, in J. Soltes and T.A. Milne (Eds.). Pyrolysis Oils from Biomass, Producing, Analysing and Upgrading. American Chemical Society, Washington, DC, 1988. [2l] E. Churin. Energy: Catalytic treatment of Pyrolysis Oils, Commission of the European Communities, Cat. No. CD-NA-I2480-EN-C, Luxembourg, 1990. [22] R. Cypres, Fuel Process. Technol.. I5 (1987) I. [23] J.A. Fairburn. L.A. Behie and Y. Svrcek. Fuel, 69 (1990) 1537. [24] D. Depeyre. C. Fhcotedux and C. Chardaire, Ind. Eng. Chem., Process Des. Dev., 24 (1985) 1251. [25] P.T. Williams and D.T. Taylor, Fuel. 72 (1993) 146991474. [26] R.J. Evans and T.A. Milne, Energy and Fuels, I (1987) 1255137. [27] D.C. Elliott, in J. Soltes and T.A. Milne (Eds.), Pyrolysis Oils from Biomass. Producing Analysing and Upgrading American Chemical Society. Washington, DC, 1988. [28] H. Pakdel and C. Roy, Energy and Fuels, 5 ( 1991) 4277436.

1291 0. Beaumont. Wood Fibre Sci.. V, 17(2) (1985) 238 39. [30] D.S. Scott, J. Piskorz and D. Radlein. Ind. Eng. Chem., Procea Des. Da,.. 24 (19X5) 5x1 5Xx. 1311 H. Pakdel, H.G. Zhang and C. Roy. in A.V. Bridguatcr (Ed.). Advances in Thermochemical Riomash Conversion. Blackie Academic and Professional. London. 1994. 1331 S. Bhatia. Zeolite Catalysis Principles and Application. Chemical Rubber Compan) CR<‘ Pre\x Boa Raton. FL. 1990. [33] C.D. Chug and A.J. Silvestri. J. Catal.. 47 ( 1977) 249 259. 1341 N.Y. Chen. D.E. Walsh and L.R. Koenig. Preprints of lY3rd American Chemical Society MeetIn&. Di\. Fuel Chem.. Denver, CO. April 5 IO. Vol. 32. 2. Washington. DC. 1987. p. 264. 1351 P.D. Chantal. S. Kaliaguine and J.L. Grandmaison. Appt. Catal.. I8 ( 1985) I33 135. 1361 R.K. Sharma and N. Bakhshi, Biorrsour. Technol.. 45 ( 1993) 195 203.