Thermal degradation of Pinus halepensis pine-needles using various analytical methods

Journal

of Analytical

Thermal

II5

Pyrolysis

I23

degradation of Pinus halepensis pine-needles using various analytical methods

M. Statheropoulos National

and Applied

43 (1997)

ELSEVIER

JOURNAL .a, ANALYTICAL and APPLIED PYROLYSIS

Technical

*, S. Liodakis,

Unicersiry

q/ Athens

9 Iroon

Received

(.YTL:A).

Polytrclmiou

28 February

N. Tzamtzis, S/r.,

1997; received

Deparwnenf 157

A. Pappa, of Chwnicul

73, Athmr.

in revised

form

S. Kyriakou

En~ineerrn~.

Srctor.

1.

Grrrc~c 14 August

1997

Abstract The analytical methods of DSC. TG, DI-MS, Py-GC-FID, Py-GC-MSD were used to study the thermal degradation of Pinus halepensis pine-needles. As was shown by DSC measurements endotherm peaks could be attributed to the desorption of high volatility compounds, moisture, softening and/or melting of the waxy constituents of pine-needles: as well as to the degradation of hemicellulose and cellulose. Exotherm peaks could be attributed to the pyrolysis of lignin and char recombination. These results were reconfirmed by DTG curve. In addition, the DI-MS measurements showed. through the presence of certain mass peaks, the existence of volatile degradation products which can be related to the degradation pathways observed by DSC and TG. Py-GC-FID proved that the evolution of organic degradation products commences at 200~~250°C and has its maximum evolution rate between 150 and 450°C. Py-GC-MSD analysis of the flash pyrolysis products at 400°C identified a number of organic compounds and CO,. C 1997 Elsevier Science R.V. Keywords:

Pinus

hakpmis

pine-needles:

Pyrolysis:

DSC; TGA:

DI-MS:

Py-GC-FID:

Py-

GC-MSD

1. Introduction Pinus halepensis is one of the most important forest species in the Mediterranean The study of the thermal degradation of Pinus hulepensis pine-needles is of special significance for forest fires. Among others, it can provide information on the region.

* Corresponding chemeng.ntua.gr 0165-2370

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author.

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stathero(rrorfeas.

116

M. Statheropoulos

et al. /I J. Anal. Appl. Pyrolysis

43 (1997) I15- 123

composition of the volatile products evolved during a forest fire. This is important from the environmental point of view, as well as for forest fire retardation studies in which chemicals are used [l]. Various analytical methods can be used for the thermal degradation study of Pinus halepensis pine-needles. The pyrolytic ones are quite useful, especially when their results are combined. DSC, TGA, DI-MS, Py-GC-FID and Py-GC-MSD have been successfully used to study the thermal behavior of various forest fuels. Susott [2] has reported a number of DSC data on different forest fuels to estimate the pyrolysis heat required in each case. Significant changes between the DSC data of wood and those of foliage were recorded. Minor changes between cured ponderosa pine-needles and green ones were observed. Rogers et al [3] related the thermal behavior of forest material with its chemical composition. This was achieved by fractionating several plant tissues to isolate specific chemical components. Each tissue was then analyzed by TG before and after treatment. In this manner, it was shown that certain components such as arabinogalactan, suberin and cutin are responsible for the unusual thermal behavior of some of the forest fuels examined. Corder0 et al. [4] used the TG and DTG methods to study the kinetics of thermal decomposition of eucalyptus sawdust and its components (cellulose, lignin). Boon et al [5] used combined Curie point pyrolysis with high resolution GC-MS for the characterization of beech wood. Schulten et al have analyzed spruce needles by temperature-programmed pyrolysis in combination with field ionization MS [6,7]. In previous works, the effect of fire retardants on the pyrolysis of Pinus halepensis pine-needles has been studied by thermal methods [1,8] and DI-MS [9]. In the present work, a study of the thermal degradation of Pinus halepensis pine-needles is attempted which is based on correlating the results of DSC, TG, DI-MS, Py-GCFID and Py-GC-MSD. This work is mainly focused on analyzing the thermal degradation products of Pinus halepensis pine-needles. These products were correlated with the thermal degradation pathways of Pinus halepensis pine-needles monitored by thermal analysis methods.

2. Experimental

2.1. Instruments The differential scanning calorimeter (DSC) used was a Stanton Redcroft instrument, model HT 1500. The thermobalance (TG) was a TA Instruments model 2050. The mass spectrometer used was a MAT-44 with an electron impact (EI) ion source. The pyrolytic unit was a CDS instrument, model Pyroprobe 1000. The gas chromatograph (GC) used in connection with the pyrolytic unit was a Hewlett Packard instrument, model HP 5890, series I, equipped with a flame ionization detector (FID). The chromatographic column was capillary with dimensions 50 x 0.32 mm i.d. and 0.3 urn phase thickness (HP 101). The MSD detector was Hewlett Packard. model HP 5972.

M. Statheropoulos et al. : J. Anal. Appl. Pyrolysis 43 (1997) 11% 123

117

2.2. Methods

2.2.1. DSC Approximately 50 mg of the sample were placed in a platinum crucible and heated from ambient temperature to 500°C at the rate of 10°C min - ‘, in nitrogen atmosphere. Precured a-A&O, was used as reference material. 2.2.2. TG Each sample weighed 34-36 mg, the heating rate was 10°C min ~ ’ from ambient to 500°C and the carrier gas was nitrogen with a flow rate of 90 cm3 min - ’ through the furnace and 10 cm3 min ~ ’ through the balance chamber. 22.3. DI-MS Approximately 0.5 mg of the sample were brought directly into the ion source where they were heated at a rate of 10°C min ~ ’ from 70 to 400°C. Mass spectra were recorded under the following operating conditions: voltage of electrons, 70 V; emission current, 0.7 mA; temperature of source. 200°C; scan duration, 1 s for mass range 40-200 amu. 2.2.4. Py-GC-FID About 7.5 mg of sample was placed into the quartz tube of the pyrobrobe, which then introduced into the injection port of the chromatograph. This quantity was selected in order to acquire sufficient GC peak signals. The samples were flash pyrolyzed for 20 s, first at 200°C and the same sample was pyrolyzed consequently at 250, 300, 350, 400, 500, 600, 700, 800, 900 and 1000°C. The carrier gas was helium with a column and auxiliary flow 1.2 and 40 cm3 min ‘, respectively. A splitless-split mode was used with a split ratio 1:40 after 20 s. The GC inlet temperature and FID detector temperature were 200 and 250°C respectively. The GC oven program temperature started at 60°C for 4 min, followed by a heating rate of 4°C min - ’ with a final temperature at 250°C for 30 min. 2.2.5. Py-GC-MSD About 7.5 mg of sample was flash pyrolyzed at 400°C. The GC operating conditions were as described previously. The MS transfer line was kept at 280°C. The electron impact source was tuned at 70 eV. The scan duration was 2.5 scan ‘. The mass range was selected between 18 and 300 amu. S 2.3. Sumplrs

The pine-needles were collected from a forest near an urban area. They were washed with deionized water, dried in an oven at 35°C for 24 h and then they were ground. A fraction between 200 and 500 urn was separated and used for the preparation of samples.

118

M. Statheropoulos et al. /J.

Anal. Appl. Pyrolysis 43 (1997) I IS- 123

3. Results and discussion The DSC curve of pyrolysis of pine-needles (Fig. 1) showed two broad endotherm peaks at 88 and 171°C respectively. Two other less distinct endotherms at about 242 and 302°C as well as an exotherm peak at 348°C were also recorded. The endotherm at 88°C might be attributed to the desorption of high volatility compounds, moisture and/or softening, melting of some of the waxy constituents of the pine-needles. The endotherm at around 302°C could be correlated to the pyrolysis of hemicellulose and/or cellulose and the exotherm at 348°C to the lignin’s pyrolysis [8]. A similar observation has been reported for the DSC curve of Ponterosu foliage [2]. The TG profile of pine-needles is presented in Fig. 2. The mass loss started at about 50°C. This mass loss in the range of 50- 150°C can be correlated with the first endotherm of DSC curve. No significant mass loss is observed around 170°C. Consequently, the respective small endotherm of the DSC curve appeared at 170°C might be attributed to the softening or melting of the waxy constituents and/or to the softening of lignin of the pine-needles. Mass losses in the range of 200-300°C as shown in the DTG curve (Fig. 2) can be attributed mainly to the desorption of the low volatility extractive constituents. The mass loss around 350°C is correlated to the exotherm at 348°C of the DSC curve. The minor mass loss at around 410°C is due to the recombination that might takes place in the char and results in evolution of various products. This mass loss is probably correlated to a not so clear exotherm at 410°C of the DSC curve.

end0

I

I

I

I

I

100

300

200 Temperature

Fig. 1. DSC curve in nitrogen

400

(“C)

of Pinus halepensis pine-needles.

M. Statheropoulos

et ul.

, .I. Anul.

Temperature Fig. 2. TG and DTG curves

Appl.

( ‘C)

in nitrogen

P~roly.~i.v 4.3 (1997)

115-11.3

Universal VlSB of Pinus ldqwnsis

119

TA Instruments

pine-needles.

In order to correlate the evolved degradation products with the degradation pathways, DI-MS was run in the temperature range 70-400°C. It should be noticed that due to the low pressure used in the ion source of MS the correlations are indicative as regards to the temperatures given. The DI-MS spectra of pine-needles at 240°C is presented in Fig. 3. Some of the mass peaks recorded, i.e. 44, 57, 60 and 73 could be correlated with the pyrolysis of hemicellulose and/or cellulose, while the mass peaks 91, 93, 105, 107, 119, 121, 133 could be attributed to the extractive

mi

240°C

Fig. 3. DI-MS

spectra

of PInus ha/rpen.~i.t pine-needles

at 240°C

120

M. Statheropoulos et al. /J.

Fig. 4. Py-GC-FID chromatograms

Anal. Appl. Pyrolysis 43 (1997) I15- 123

of Pinus halepensis pine-needles at various temperatures

of pine-needles. These results can be correlated with the region of 200-400°C of the DSC curve which describes mainly the desorption of heavy pine-needles extractives and the degradation of hemicellulose and/or cellulose. It was found that the masses 91, 93, 105, 107, 119, 121, 133 were most abundant at 16O”C, whilst the masses 44, 57, 60 and 73 were most abundant at 320°C. Some of the peaks which are attributed to the lignin pyrolysis and char recombination (124, 138, 140, 150, 152, 164, 166, 180) were present in the DI-MS spectrum at 400°C. The last peaks were not so abundant in the DI-MS spectrum and this is mainly due to the ionization mode used in this work (EI) [9]. In order to monitor the volatile organic pyrolysis products in a broad temperature range, usually encountered in forest fires, Py-GC-FID was run in the range of 200- 1000°C. These measurements were used to determine the pyrolysis temperature range with the highest evolution rate of the organic pyrolysis products. The Py-GC-FID chromatograms of pine-needles in the temperature region 200- 1000°C is presented in Fig. 4. The evolution of organic degradation products of pineneedles commences at 200-25O”C, while the maximum evolution rate of organic volatiles occurs between 350 and 450°C. Certain GC peaks were recognized in all pyrograms having different intensities in each pyrolysis temperature. Also the same peak pattern was found in the range of 350&45O”C possibly pointing to the same components. Therefore, it is clear that Py-GC-MSD analysis of the pyrolysis products of Pinus halepensis pine-needles could be quite representative at a temperature around 400°C. The resolution of the Py-GC-MSD pyrogram (Fig. 5) was relatively low due to the nature of the sample examined. However, the pyrograms were very reproducible. The compounds identified in the volatile pyrolysis products of Pinus halepensis pine-needles are presented in Table 1. The compounds identified had retention times from 3.35 to 42.3 min and Mws within U-166. The compounds presented in Table 1 were selected among those proposed by the automate searching and retrieval system of the MSD. When necessary mass spectra were interpeted manually using mass spectra data collections. The compounds identified constituents

hf. Statheropoulos

Table I The volatile

pyrolysis

products

et ul. ; J. And.

of Pinus halrprnsis

Appl. Pyrolysis

pine-needles

33 (1997) 115

identified

123

by Py-CC-MSD

No.

Ret. time

.Mw

Compound

Possible origin

Reference

1 2 3 4 5 6 I 8 9 IO II I2 I3 I4 I5 I6 17 I8 I9 20 21 22 23 24 25

3.35 3.74 4.26 4.71 5.38 5.84 6.17 7.45 8.56 9.90 IO.26 10.65 10.92 I I .93 13.08 13.30 14.07 14.74 15.1 I 15.16 Il.06 18.19 19.95 20.67 21.22

44 44 60 74 I04 84 88 96 98 96 II0 96 98 I10 112 94 II2 I12 136 I36 I24 122 144 I38 I38

C“

[IO.1 II [IO.1 II [IO.1 II [IO.1 I]

26 27 28 29

21.53 22.61 23.52 24.40

122 120 II0 152

30 31 32

25.58 25.86 27.15

150 150 I64

33 34

29.05 30.48

I64 I64

35

42.30

I66

Carbon dioxide Acetaldehyde Acetic acid I Hydroxy, 2 propanone or acetol Ethanol, 2 propoxy Cyclopropane. I.2 dimethyl, 3 methyiene I Hydroxy, 2 butanone 2 Furancarboxaldehyde or furfural 2 Furanmethanol 2 Cyclopenten-l-one, 3 methyl Ethanone-I-(2-furanyl) Z(H)-pyran-2-one Cyclohexanone 2 Furancarboxaldehyde, 5-methyl 2-Octene Phenol Bicyclo[2,2,l]heptan-7-01 2 Cyclopenten-l-one, 2 hydroxy, 3 methyl Cyclohexene, 1-methyl4-( I -methylethylenyl) DL-limonene 2 methoxy-phenol or guaiacol Benzeneethanol 4H-Pyran-4-one, 2,3-dihydro-3.5 dihydroxy. 6 methyl 4-Methoxymethylphenol 2-Methoxy, 4 methyl phenol or 4- guaiacyl-methane or 4-methyl-guaiacol Benzoic acid Benzofuran, 2. 3-dihydroI .2 Benzenediol Phenol, 4-ethyl-2-methoxy or guaiacyl-ethane or 4-ethyl-guaiacol 2 Methoxy, 6 vinyl phenol or guaiacyl-ethene Phenol, 2-( I ,I -dimethylenyl)Phenol, 2-methoxy-4-(2-propenyl-) or guaiacyl-prop-2-ene A guaiacyl derivative Phenol, 2-methoxy+( I -propenyl-) or guaiacyl-prop-I-ene 3.4.5,6 Tetramethyl-2,5-octadiene _

* Cellulose. b Unknown origin. ’ Lignin. a Extractives

c C C Ih

[lOI [IO,1 I]

[l(U [IO.1 I] C

IlO,l

II

‘>

L’ ? C

[IO~~l’]

Ed L

[I31

C L L

][ IOIl

L

[ll.l2.14]

L L L

[[l l.l2.14]]

L L

[[I l,IZll [[I 131211

[I()1

[[11.12.1411

[U211 [[11,12.14]]

]]l

III

M. Statheropoulos et al. /J.

122

Anal. Appl. Pyrolysis 43 (1997) 115- 123

Time lmin

Fig. 5. Py-GC-MSD chromatogram

of Pinus halepensis pine-needles at 400°C.

were related to their ‘possible’ origin. This origin could be extractives, cellulose and lignin. It should be emphasized that thermal properties of various types of pure cellulose and lignin, as well as chromatographic characteristics as referred in the literature, are not preserved in pine-needles. This could be explained by the specific form in which these components exist in pine-needles as well as the way that they are combined. Consequently, correlation of the compounds presented in Table 1 with their ‘possible’ origins is indicative. Literature reference was given when this was possible.

4. Conclusions The thermal degradation of Pinus hulepensis pine-needles results in various volatile organic products. In temperatures higher than 400°C it produces char residue. As is shown in the TG curve the char residue at 410°C is approximately 35% w/w. This char residue undergoes recombination leading to more stable, possibly polycondensation, products (see DSC exotherm at 410°C). It should be emphasized that the char residue of biomass is greatly depended on trace amounts of naturally occurring inorganic salts. However, the char residue of 31.4% w/w referred in literature for Ponterosa pine-needles at 500°C [2], is very close to the one measured in this work at 500°C which is 27% w/w. The organic volatile pyrolysis products of Pinus halepensis pine-needles have a special interest in forest fires because of their high flammability, their contribution to the spread of the fire, as well as for environmental reasons. As it was shown by DSC and TG measurements in combination with DI-MS spectra, in temperatures lower than 150°C compounds such as extractives are evolved. In the range of 200-400°C organic volatiles are

eluted which are pyrolysis products of the basic components of pine-needles (e.g. extractives. hemicellulose, cellulose. lignin). Based on the Py-GC-FID measurements, it was found that the pyrolysis products evolved are very rich in organic volatiles in the range of 3506450°C. The flash pyrolysis GC-MSD at 400°C showed that the volatile products evolved are related to cellulose and lignin structures. It should be noted that by using soft ionization techniques (i.e. field ionization MS) heavier constituents can be recognized. This technique could also be used for the characterization of the pine-needles as forest species because it can provide with more detailed information compared with the techniques of DSC and TG. In addition the use of hyphenated methods such as TG-GC-MS could provide on line information on the above.

Acknowledgements The authors acknowledge PENED 95, for financially

the General Secretariat supporting this work.

of Research

and Technology.

References [I] SE. Liodakis, M.K. Statheropoulos. N.E. Tzamtzis. A.A. Pappa. G.K. Parisssakis. Thermochim. Acta 278 (1996) 99. [2] R.A. Susott, Forest Sci. 28 (1982) 839. [3] J.M. Rogers, R.A. Susott, R.G. Kelsey. Can. J. For. Res. I6 (19X6)721. [4] T. Cordero. J.M. Rodriguez-Maroto. J. Rodriguez-Mirasol. J.J. Rodriguez. Thermochim. Acta 164 (1990) 135. [5] J.J. Boon. A.D. Pouwels, G.B. Eijkel, Biochem. Sot. Trans. 15 (1987) 170. [6] H.R. Schulten, N. Simmleit, R. Muller. Anal. Chem. 61 (1989) 221. [7] R. Hempfling, N. Simmleit. H.R. Schulten, Biogeochemistry 13 (1991) 27. [8] A. Pappa. N. Tzamtzis, M. Statheropoulos. G. Parissakis, Thermochim. Acta 261 (1995) 165. [9] A. Pappa. N. Tzamtzis, M. Statheropoulos. S. Liodakis. G. Parissakis. J. Anal. Appl. Pyrolysis 31 (1995) 85. [IO] A.D. Pouwels, G.B. Eijkel, J.J. Boon, J. Anal. Appl. Pyrolysis 14 (1989) 237. [I I] R. Alert. E. Kuoppala, P. Oesch, J. Anal. Appl. Pyrolysis 36 (1996) 137. [I21 J.J. Boon. A.D. Pouwels, G.B. Eijkel, Biochem. Sot. Trans. 15 (I ) (1987) 170. 1131 F. Tazerouti. A.Y. Badjah-Hadj-Ahmed, B.Y. Meklati, S. Alamercery, Riv. Ital. EPPOS I I (1993) [I41 M. Kleen. G. Gellerstedt,

J. Anal.

Appl.

Pyrolysis

19 (1991)

139.