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Effects of fire retardants on the pyrolysis of Pinus halepensis needles using microscopic techniques
Journal of Analytical and Applied Pyrolysis 63 (2002) 147– 156
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Effects of fire retardants on the pyrolysis of Pinus halepensis needles using microscopic techniques N. Tzamtzis a,*, A. Pappa a, M. Statheropoulos a, C. Fasseas b a
National Technical Uni6ersity of Athens (NTUA), Department of Chemical Engineering, Sector I, 9 Iroon Polytechniou Str., 157 -73 Athens, Greece b Agricultural Uni6ersity of Athens, Department of Agricultural Biotechnology, Electron Microscopy Laboratory, 75 Iera Odos, 118 -55 Athens, Greece Received 2 October 2000; accepted 29 March 2001
Abstract Two microscopic techniques, transmission light microscopy (LM) and stereoscopy, were used for studying the effect of fire retardants on the pyrolysis of Pinus halepensis needles. Pure (NH4)2HPO4 and a commercial product (Fire Trol), based on polyphosphates as the active chemical retardant, were tested as fire retardants. With increasing pyrolysis temperature, the loss of the cuticle layer, the cracking of the epidermal cells, the charification of the inner part and the degradation of the lignin content of the needles were observed by LM. LM showed that all these phenomena were accelerated (shifted to lower temperatures) in those treated with fire-retardant needles compared with the untreated ones. Stereoscopy was used for monitoring changes on the external surface of the pyrolyzed needles such as changes concerning the retardant itself and the tar and char formation. It appeared that, in the presence of the retardants, tar and char formed at lower pyrolysis temperatures and at a greater extent compared with the untreated needles. In all cases studied, the commercial product appeared to be more effective than the pure (NH4)2HPO4. The recorded observations were associated with the chemical and physical phenomena occurring during the pyrolysis procedure and the theory of the fire retardation action of the chemicals. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Pyrolysis; Pinus halepensis; Pine needles; Fire retardants; Light microscopy; Stereoscopy
* Corresponding author. Tel.: + 30-1-7723194; fax: +30-1-7723188. E-mail address: [email protected] (N. Tzamtzis). 0165-2370/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 2 3 7 0 ( 0 1 ) 0 0 1 4 7 - 4
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1. Introduction Pinus halepensis is one of the most important forest species in the Mediterranean region. Its foliage, while on the trees or after falling on the ground, is very flammable, especially during the hot and dry summer season. This is mainly due to its low water and high resin content [1,2]. Some of the important chemical phenomena that occur during a forest fire can be determined by studying the mechanism of thermal degradation in forest fuel (wood, foliage, etc.) and/or its components. It is known that the combustion of a forest fuel follows two steps. First, pyrolysis takes place (which seems to be the most significant step) and then, in the presence of oxygen, it is followed by combustion. One of the main components of forest fuels is cellulose. It is generally agreed that pyrolysis of cellulose is carried out by two competing decomposition pathways. The first is the dehydration of cellulose, which occurs at low temperatures and slow heating rates, and yields ‘char fraction’, gases (mainly CO and CO2) and water. The second pathway is a depolymerisation process that yields primarily levoglucosan and becomes important at higher temperatures and high heating rates. Levoglucosan is an intermediate product and finally forms volatile products, which burn with flaming combustion. The flame combustion of a forest fuel is what is seen as flame front. Hemicellulose, which is also a forest fuel component, seems to follow the same pyrolysis mechanism as that of cellulose. However, lignin, the other forest fuel component, is pyrolyzed, mainly with the production of a ‘char fraction’. Other forest fuel components are the ‘extractives’ that play an important role in the combustion process. They are, mostly, readily evaporated and burnt in the gas phase with flaming combustion. (NH4)2HPO4 is widely used as prime chemical of the fire retardants used for fighting forest fires. This retardant acts by producing phosphoric acid prior to the flaming temperatures, which changes the decomposition profile of forest fuels. It is believed that it favors the dehydration process. This results in the increased quantities of char, water vapour, CO2 and the reduction of the volatile organic products. The char is a graphite-like carbon residue that burns with glowing combustion without flame. In the presence of the retardant, pyrolysis commences at a lower temperature, probably due to the catalytic action of the acids. Consequently, the presence of the retardant has, as a result, to inhibit fire promotion by producing less flammable volatiles and more char that burns with no flame. In addition, the decrease of pyrolysis temperature influences the fire propagation, because most of the combustible volatiles are liberated and dissipated in temperatures lower than their ignition temperatures [3]. The effects of (NH4)2HPO4 on the pyrolysis of P. halepensis needles and their main components (cellulose, hemicellulose, lignin, extractives) have been studied in previous works using various analytical methods such as differential scanning calorimetry (DSC), thermogravimetric analysis, pyrolysis gas chromatography– mass spectrometry (Py-GC– MS), direct inlet mass spectrometry (DI-MS), and microscopic methods [3– 7]. In this work, pine needles treated with (NH4)2HPO4
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and Fire Trol (a commercial product based on polyphosphates) were pyrolyzed at various temperatures and then examined by light microscopy (LM) and stereoscopy to investigate microscopic and macroscopic changes during pyrolysis. The purpose of this work was: (a) to monitor changes in the structure of pine needles, and/or on their surface, during pyrolysis in the presence of retardants, for comparison with untreated needles, (b) to correlate the microscopic findings with those obtained by other analytical studies, and (c) to contribute to the existing knowledge on fire retardation.
2. Experimental
2.1. Instruments [7] 2.1.1. Pyrolytic unit The pyrolysis unit was a CDS instrument, model Pyroprobe 1000. An Olympus BX40 fluorescent microscope, equipped with a mercury vapor lamp as the source of UV radiation, was used for transmission LM. For stereoscopy, an Olympus SZX12 stereomicroscope was used. 2.2. Materials Needles were collected from a 30-year-old P. halepensis tree in a forest near an urban area. The composition per weight on dry basis of the needles concerning the main components was found: 21% extractives in ethanol–toluene solvent (using the method ASTM D-1107/96), 24% cellulose (by a method reported in the literature [8]) and 18% lignin (using the method ASTM D-1106/96). The pure (NH4)2HPO4 used is a BDH analytical grade reagent. The commercial fire retardant has the trade name Fire Trol 931 French formulation and was used in liquid red concentrated form 20% per weight, equivalent to 15% in (NH4)2HPO4. The commercial product contains polyphosphates and additives (thickener, color pigments, stabilizers, etc.).
2.3. Samples The needles were cut to 3 cm in length. A preweighed sample of needles was submerged in a stock aqueous solution of known (NH4)2HPO4 concentration and in liquid concentrated form of Fire Trol. The volume completed to 10 cm3 with distilled water. Then they were dried in an oven at 40°C for 48 h. The aforementioned procedure results in mixtures of needles with pure (NH4)2HPO4 at a concentration about 20% and also in mixtures of needles with Fire Trol at a concentration, equivalent to (NH4)2HPO4, of 15% per weight. Untreated control needles were submerged in distilled water only and manipulated as already described for comparison.
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2.4. Methods 2.4.1. Pyrolysis Two pieces of needles, about 3 cm in length, were placed into the quartz pyroprobe tube. Samples were flash pyrolyzed at 100, 200, 250, 300, 350, 400 and 500°C (new items were used at each pyrolysis temperature) for 20 s while helium was passed through at 80 cm3 min − 1 as described elsewhere [7]. The pyrolyzed items were then carefully removed from the quartz tube and kept refrigerated in glass vials for microscopic examination. 2.4.2. Light microscopy Transverse sections of pyrolyzed needles were cut, mounted on microscope slides without a mounting medium or coverslip and examined by fluorescent LM. The average transverse sections of the needles were about 0.9 mm wide and 0.7 mm thick (maximum). The excitation filter used was 330–385 nm, the dichroic mirror was 400 nm, and the barrier filter was 420 nm. The same magnification was used in all photographs except Fig. 1c at 250 and 300°C, which are 50% of the others. 2.4.3. Stereoscopy External morphology of the needles was examined by the stereoscope. Specimens were illuminated from above with a Highlight 3100 (halogen lamp with twin fiber optics). The same magnification was used in all photographs.
3. Results and discussion
3.1. Untreated pine needles The anatomy of needles has been described in the literature [9,10]. The presence of lignin in cells, tissues, and resin ducts is observed as a blue coloration by LM, whereas the photosynthetic tissue is a brick-red color because of the chlorophyll. The major changes of untreated needles during flash pyrolysis at various temperatures as observed by LM and stereoscopy have been presented elsewhere [7].
3.2. Light microscopy 3.2.1. Pine needles treated with (NH4)2HPO4 Fig. 1a,b show the transverse sections of untreated needles and needles treated with (NH4)2HPO4, respectively, as examined by LM, after pyrolysis at various temperatures. Up to 200°C, the retardant does not significantly affect the thermal process. At around 200°C, (NH4)2HPO4 evolves NH3 but this does not have a retardation effect (i.e. dilution effect on flammable degradation gas products because they have not yet start to evolve). Degradation of the needle lignin content can be monitored by the loss of blue fluorescence that takes place at about 250°C, that is 50°C lower than that of the untreated needles (compare Fig. 1a at 300°C for
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untreated needles and Fig. 1b at 250°C for treated needles with (NH4)2HPO4). This agrees with the results of DSC studies examining the effects of this chemical on pyrolysis of the needle lignin content [4]. Also, the external epidermic cells are exposed as a result of progressive cuticle loss (compare Fig. 1a at 300°C and Fig. 1b at 250°C). In addition, mild cracking of the epidermal cell layer appears in treated needles. At 300°C, these phenomena become more intense. As a result, the cuticle is removed completely, the epidermic cells are destroyed, and there is no blue
Fig. 1. Transverse sections of (a) untreated needles, (b) treated needles with (NH4)2HPO4 at a concentration of about 20% per weight, and (c) treated needles with Fire Trol at a concentration equivalent to 15% per weight on (NH4)2HPO4 contained, after flash pyrolysis at 200, 250, 300 and 350°C, as observed by light emission microscopy.
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fluorescence visible (compare Fig. 1a at 350°C and Fig. 1b at 300°C). It should be noted that, at this temperature, the treated needles become fragile, which may be correlated to lignin degradation. At 350°C, the treated needles are completely black and no further investigation is possible by LM. This indicates at this temperature that charification has proceeded more extensively throughout the inner part of the treated needles than in the untreated ones. Phenol derivatives, which are referred to as marker compounds of lignin degradation products, have been detected extensively at this temperature and this is an indication that lignin pyrolysis has been accelerated in the presence of the chemical. It should be emphasized that various analytical methods have recorded maximum evolution rates of degradation volatiles at temperatures around 350°C for treated needles with (NH4)2HPO4 [6]. Therefore, the main action of (NH4)2HPO4 as observed by LM and previous analytical studies [3,6] is the acceleration of the pyrolysis by lowering the temperature at which pyrolysis commences.
3.2.2. Pine needles treated with Fire Trol The transverse sections of needles examined by LM after treatment with Fire Trol and pyrolysis at various temperatures are presented in Fig. 1c. It appears that this product also accelerates pyrolysis by lowering the temperature at which pyrolysis commences even more than (NH4)2HPO4. The loss of blue fluorescence (indicating lignin degradation) in the samples begins at about 200°C (compare Fig. 1a at 300°C for untreated needles and Fig. 1b at 250°C for treated needles with (NH4)2HPO4). In addition, at this temperature, the external epidermic cells start to be exposed due to the loss of cuticle (compare Fig. 1a at 300°C and Fig. 1b at 250°C). Needle fragility develops at 250°C. At 300°C, the samples are completely black and no further investigation is possible by LM. 3.3. Stereoscopy 3.3.1. Pine needles treated with (NH4)2HPO4 Fig. 2a,b shows the external morphology of untreated needles and needles treated with (NH4)2HPO4, respectively, as examined by stereoscopy, after pyrolysis at various temperatures. The easily observed white crystals (Fig. 2b at 200°C) covering almost all the needle surface are pure (NH4)2HPO4. The tar (dark brown liquid drops) and char (gray– black debris) formation become visible in the treated needles at 250 and 300°C, respectively. At this temperature range (250–300°C), LM has monitored lignin degradation as well as DSC studies, whereas Py-MS studies have not monitored the evolution of lignin degradation marker compounds at a great extent. This may be attributed to the fact that lignin pyrolysis, at these temperatures, produces low enough quantities of volatiles. Consequently, the tar and char residue formed at this temperature range can be attributed mainly to cellulose pyrolysis [5]. It appears that the char forms some bubble shaped agglomerates at about 350°C in the treated samples. This may result from volatile pyrolysis components continuing to evolve rapidly from the inner part of the needles and becoming entrapped below the surface char. In addition, at this temperature,
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Fig. 2. External surfaces of (a) untreated pine needles, (b) treated needles with (NH4)2HPO4 at a concentration of about 20% per weight, and (c) treated needles with Fire Trol at a concentration equivalent to 15% per weight on (NH4)2HPO4 contained, after flash pyrolysis at 200, 250, 300, 350 and 400°C as observed by stereoscopy.
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significant changes were found in the composition of the pyrolysis volatiles; high evolution of levoglucosenone (originated from cellulose content) and the presence of new compounds, such as aromatic nitriles and nitro-phenyl compounds (originating from lignin content) [6]. It should be emphasized that Py-GC-flame ionization detection indicates that, around 350°C, there is a maximum evolution of degradation volatile products from treated needles with (NH4)2HPO4 [6]. Therefore, the main action of (NH4)2HPO4 as observed by stereoscopy is the acceleration of the pyrolysis by lowering the temperature at which pyrolysis commences.
3.3.2. Pine needles treated with Fire Trol Fig. 2c shows the external morphology of needles treated with Fire Trol, as examined by stereoscopy, after pyrolysis at various temperatures. The brick red muddy substance, clearly observed at 200°C, is the colored pigment used in the commercial product. At 250°C, a brown –black mud is observed and this is an indication that tar is mixed with the solid retardant on the surface. At 300°C, the formation of char agglomerates is visible. The quantity of the char formed on the needle surface seems to be greater than that on untreated needles, and in the treated ones with (NH4)2HPO4. It should be noted that the retardant seemed to fully cover (paint) the needle surface and possibly mask any early occurring phenomena (e.g. tar and char formation). For this reason, the experiments were repeated using lower concentrations of (NH4)2HPO4 (10% per weight) and Fire Trol (equivalent to (NH4)2HPO4, 5% per weight). The new stereoscopy results confirmed the previous findings.
4. Conclusions The two microscopy techniques successfully monitored the pyrolytic changes occurring in both the inner and outer parts of needles in the presence of fire retardants, giving complementary information. Both methods reveal that the presence of (NH4)2HPO4 lowers the pyrolysis temperatures about 50°C compared with untreated samples. This is in agreement with the results of other analytical methods, such as TG, DSC, DI-MS and Py-GC. In the case of Fire Trol, there are indications that the pyrolysis temperatures are lowered even more than (NH4)2HPO4 at about 50°C. LM shows visual evidence of the accelerated thermal degradation of lignin in the presence of retardants. It should be noted that monitoring the degradation of lignin during the needle pyrolysis is quite complicated with other methods. Stereoscopy monitors tar and char formation on the needle surface. It shows that, in the presence of retardants, tar and char are formed at lower temperatures. Stereoscopy also reveals that the retardants mode of action is to enhance char formation, which is in agreement with existing retardation theory. Based on these observations, the following mechanistic model of untreated and treated with fire retardands pine needles pyrolysis is proposed. 1. From ambient temperature to 200°C in both cases of pure and treated with (NH4)2HPO4 needles, only physical changes occur; such as moisture loss,
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softening and melting of resinous acids, and evaporation of high-volatility extractive constituents. These changes can only result in the shrinking of the cross-section of the needles. In the presence of Fire Trol, the cuticle is lost at about 200°C and the external epidermic cells start to be exposed. Some cracks of the external surface, around the resin ducts, are observed. The retardants, in all the presented cases, remain unchanged on the surface of the needles. 2. At about 250°C, the untreated needle shape remains the same as previously. Some blackening on the external surfaces around stomata and some opaque droplets on the external surfaces are indications of condensate beginning to be formed, due to desorption and/or degradation of the low-volatility extractives. In the presence of (NH4)2HPO4, the cuticle is completely lost, the external epidermic cells are revealed and a severe breaking of the external surface is occurred. The lignin content starts to degrade (starting gradually loss of blue fluorescence). In the presence of Fire Trol, all the aforementioned phenomena are more intense, especially in the case of lignin degradation. In all the cases of the treated needles, tar formation starts on the external surface and a gradually blackening of the inner part occurs simultaneously. This is an indication of the beginning of charification. The needles become brittle due to lignin degradation. 3. At about 300°C, the untreated needles start to become brittle. Crackings in the outer surface and more intense lignin degradation take place. This results in tar formation on outer needle surface. In the presence of (NH4)2HPO4 and Fire Trol, all the blue fluorescence is gone (no lignin present at the original form). The external epidermic cells are destroyed, resulting in a mixture of tar and char in the needle surface. The tar and char come from cellulose degradation. 4. At about 350°C, the untreated needle cuticle is completely lost and the external epidermic cells are revealed. The intense cracking on the external surface and the charification of the inner part of the needle result in a mixture of tar and char deposit on the needles surface. In the case of (NH4)2HPO4, tar and char are still visible on the outer surface. In the case of Fire Trol, only char is observed. In these cases of treated needles, full blackening (completely charification) is observed throughout the inner part. 5. At about 400°C, the untreated needles are fully black (complete charification) throughout the inner part. On its external surface, tar and char are still observed (char is formed on the expense of tar). From this temperature and above, the treated needles consist of only char. 6. Above 500°C, the untreated needles also consist of only char. In conclusion, first, the retardants attack the outer needle surface (cuticle breaking). Cuticle breaking allows, second, the retardants to penetrate into the inner part of the needles (evidence of lignin degradation at lower temperatures), facilitating the exit of combustible volatiles (pyrolysis degradation products) at lower temperatures. This has a negative effect on fire propagation, because the volatiles are dissipated in the atmosphere before their ignition temperatures are reached. The earlier char coverage of external needle surfaces at lower temperatures in the presence of the retardants may prohibit the fire spread and give rise to glowing combustion. Finally, the commercial retardant seems to have a greater
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impact on needle pyrolysis than pure (NH4)2HPO4. This may be due to the presence of specific additives, resulting in better adhesion on the needles surface compared with pure (NH4)2HPO4.
Acknowledgements This work was carried out in the framework of the research project ACRE (Project ENV4-CT98-729 ‘Additifs Chimiques Rheologie Evaluation’, Environment and Climate Program-E.C./D.G.XII).
References [1] P. Panov, Analysis of pyric properties of Mediterranean type vegetation, MAICh, M.Sc. Thesis, Chania, 1994. [2] P. Matli, Measurements of bulk density and fuel chemical properties affecting flammability of Mediterranean species, MAICh, M.Sc. Thesis, Chania, 1997. [3] A.A. Pappa, N.E. Tzamtzis, M.K. Statheropoulos, S.E. Liodakis, G.K. Parissakis, J. Anal. Appl. Pyrolysis 31 (1995) 85. [4] A.A. Pappa, N.E. Tzamtzis, M.K. Statheropoulos, G.K. Parissakis, Thermochim. Acta 261 (1995) 165. [5] M. Statheropoulos, S.A. Kyriakou, Anal. Chim. Acta 409 (2000) 203. [6] N. Tzamtzis, A. Pappa, A. Mourikis, Polym. Degradation Stability 66 (1999) 55. [7] A. Pappa, N. Tzamtzis, M. Statheropoulos, C. Fasseas, J. Anal. Appl. Pyrolysis 55 (2000) 195. [8] H. Pereira, R. Sardinha, Lab. Appita 37 (1984) 661. [9] A. Fahn, Plant Anatomy, 4th ed., Pergamon Press, New York, 1990, p. 250. [10] T.W. Goodwin, E.I. Mercer, Introduction to Plant Biology, 2nd ed., Pergamon Press, New York, 1988, pp. 73 –77.
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Effects of fire retardants on the pyrolysis of Pinus halepensis needles using microscopic techniques N. Tzamtzis a,*, A. Pappa a, M. Statheropoulos a, C. Fasseas b a
National Technical Uni6ersity of Athens (NTUA), Department of Chemical Engineering, Sector I, 9 Iroon Polytechniou Str., 157 -73 Athens, Greece b Agricultural Uni6ersity of Athens, Department of Agricultural Biotechnology, Electron Microscopy Laboratory, 75 Iera Odos, 118 -55 Athens, Greece Received 2 October 2000; accepted 29 March 2001
Abstract Two microscopic techniques, transmission light microscopy (LM) and stereoscopy, were used for studying the effect of fire retardants on the pyrolysis of Pinus halepensis needles. Pure (NH4)2HPO4 and a commercial product (Fire Trol), based on polyphosphates as the active chemical retardant, were tested as fire retardants. With increasing pyrolysis temperature, the loss of the cuticle layer, the cracking of the epidermal cells, the charification of the inner part and the degradation of the lignin content of the needles were observed by LM. LM showed that all these phenomena were accelerated (shifted to lower temperatures) in those treated with fire-retardant needles compared with the untreated ones. Stereoscopy was used for monitoring changes on the external surface of the pyrolyzed needles such as changes concerning the retardant itself and the tar and char formation. It appeared that, in the presence of the retardants, tar and char formed at lower pyrolysis temperatures and at a greater extent compared with the untreated needles. In all cases studied, the commercial product appeared to be more effective than the pure (NH4)2HPO4. The recorded observations were associated with the chemical and physical phenomena occurring during the pyrolysis procedure and the theory of the fire retardation action of the chemicals. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Pyrolysis; Pinus halepensis; Pine needles; Fire retardants; Light microscopy; Stereoscopy
* Corresponding author. Tel.: + 30-1-7723194; fax: +30-1-7723188. E-mail address: [email protected] (N. Tzamtzis). 0165-2370/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 2 3 7 0 ( 0 1 ) 0 0 1 4 7 - 4
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1. Introduction Pinus halepensis is one of the most important forest species in the Mediterranean region. Its foliage, while on the trees or after falling on the ground, is very flammable, especially during the hot and dry summer season. This is mainly due to its low water and high resin content [1,2]. Some of the important chemical phenomena that occur during a forest fire can be determined by studying the mechanism of thermal degradation in forest fuel (wood, foliage, etc.) and/or its components. It is known that the combustion of a forest fuel follows two steps. First, pyrolysis takes place (which seems to be the most significant step) and then, in the presence of oxygen, it is followed by combustion. One of the main components of forest fuels is cellulose. It is generally agreed that pyrolysis of cellulose is carried out by two competing decomposition pathways. The first is the dehydration of cellulose, which occurs at low temperatures and slow heating rates, and yields ‘char fraction’, gases (mainly CO and CO2) and water. The second pathway is a depolymerisation process that yields primarily levoglucosan and becomes important at higher temperatures and high heating rates. Levoglucosan is an intermediate product and finally forms volatile products, which burn with flaming combustion. The flame combustion of a forest fuel is what is seen as flame front. Hemicellulose, which is also a forest fuel component, seems to follow the same pyrolysis mechanism as that of cellulose. However, lignin, the other forest fuel component, is pyrolyzed, mainly with the production of a ‘char fraction’. Other forest fuel components are the ‘extractives’ that play an important role in the combustion process. They are, mostly, readily evaporated and burnt in the gas phase with flaming combustion. (NH4)2HPO4 is widely used as prime chemical of the fire retardants used for fighting forest fires. This retardant acts by producing phosphoric acid prior to the flaming temperatures, which changes the decomposition profile of forest fuels. It is believed that it favors the dehydration process. This results in the increased quantities of char, water vapour, CO2 and the reduction of the volatile organic products. The char is a graphite-like carbon residue that burns with glowing combustion without flame. In the presence of the retardant, pyrolysis commences at a lower temperature, probably due to the catalytic action of the acids. Consequently, the presence of the retardant has, as a result, to inhibit fire promotion by producing less flammable volatiles and more char that burns with no flame. In addition, the decrease of pyrolysis temperature influences the fire propagation, because most of the combustible volatiles are liberated and dissipated in temperatures lower than their ignition temperatures [3]. The effects of (NH4)2HPO4 on the pyrolysis of P. halepensis needles and their main components (cellulose, hemicellulose, lignin, extractives) have been studied in previous works using various analytical methods such as differential scanning calorimetry (DSC), thermogravimetric analysis, pyrolysis gas chromatography– mass spectrometry (Py-GC– MS), direct inlet mass spectrometry (DI-MS), and microscopic methods [3– 7]. In this work, pine needles treated with (NH4)2HPO4
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and Fire Trol (a commercial product based on polyphosphates) were pyrolyzed at various temperatures and then examined by light microscopy (LM) and stereoscopy to investigate microscopic and macroscopic changes during pyrolysis. The purpose of this work was: (a) to monitor changes in the structure of pine needles, and/or on their surface, during pyrolysis in the presence of retardants, for comparison with untreated needles, (b) to correlate the microscopic findings with those obtained by other analytical studies, and (c) to contribute to the existing knowledge on fire retardation.
2. Experimental
2.1. Instruments [7] 2.1.1. Pyrolytic unit The pyrolysis unit was a CDS instrument, model Pyroprobe 1000. An Olympus BX40 fluorescent microscope, equipped with a mercury vapor lamp as the source of UV radiation, was used for transmission LM. For stereoscopy, an Olympus SZX12 stereomicroscope was used. 2.2. Materials Needles were collected from a 30-year-old P. halepensis tree in a forest near an urban area. The composition per weight on dry basis of the needles concerning the main components was found: 21% extractives in ethanol–toluene solvent (using the method ASTM D-1107/96), 24% cellulose (by a method reported in the literature [8]) and 18% lignin (using the method ASTM D-1106/96). The pure (NH4)2HPO4 used is a BDH analytical grade reagent. The commercial fire retardant has the trade name Fire Trol 931 French formulation and was used in liquid red concentrated form 20% per weight, equivalent to 15% in (NH4)2HPO4. The commercial product contains polyphosphates and additives (thickener, color pigments, stabilizers, etc.).
2.3. Samples The needles were cut to 3 cm in length. A preweighed sample of needles was submerged in a stock aqueous solution of known (NH4)2HPO4 concentration and in liquid concentrated form of Fire Trol. The volume completed to 10 cm3 with distilled water. Then they were dried in an oven at 40°C for 48 h. The aforementioned procedure results in mixtures of needles with pure (NH4)2HPO4 at a concentration about 20% and also in mixtures of needles with Fire Trol at a concentration, equivalent to (NH4)2HPO4, of 15% per weight. Untreated control needles were submerged in distilled water only and manipulated as already described for comparison.
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2.4. Methods 2.4.1. Pyrolysis Two pieces of needles, about 3 cm in length, were placed into the quartz pyroprobe tube. Samples were flash pyrolyzed at 100, 200, 250, 300, 350, 400 and 500°C (new items were used at each pyrolysis temperature) for 20 s while helium was passed through at 80 cm3 min − 1 as described elsewhere [7]. The pyrolyzed items were then carefully removed from the quartz tube and kept refrigerated in glass vials for microscopic examination. 2.4.2. Light microscopy Transverse sections of pyrolyzed needles were cut, mounted on microscope slides without a mounting medium or coverslip and examined by fluorescent LM. The average transverse sections of the needles were about 0.9 mm wide and 0.7 mm thick (maximum). The excitation filter used was 330–385 nm, the dichroic mirror was 400 nm, and the barrier filter was 420 nm. The same magnification was used in all photographs except Fig. 1c at 250 and 300°C, which are 50% of the others. 2.4.3. Stereoscopy External morphology of the needles was examined by the stereoscope. Specimens were illuminated from above with a Highlight 3100 (halogen lamp with twin fiber optics). The same magnification was used in all photographs.
3. Results and discussion
3.1. Untreated pine needles The anatomy of needles has been described in the literature [9,10]. The presence of lignin in cells, tissues, and resin ducts is observed as a blue coloration by LM, whereas the photosynthetic tissue is a brick-red color because of the chlorophyll. The major changes of untreated needles during flash pyrolysis at various temperatures as observed by LM and stereoscopy have been presented elsewhere [7].
3.2. Light microscopy 3.2.1. Pine needles treated with (NH4)2HPO4 Fig. 1a,b show the transverse sections of untreated needles and needles treated with (NH4)2HPO4, respectively, as examined by LM, after pyrolysis at various temperatures. Up to 200°C, the retardant does not significantly affect the thermal process. At around 200°C, (NH4)2HPO4 evolves NH3 but this does not have a retardation effect (i.e. dilution effect on flammable degradation gas products because they have not yet start to evolve). Degradation of the needle lignin content can be monitored by the loss of blue fluorescence that takes place at about 250°C, that is 50°C lower than that of the untreated needles (compare Fig. 1a at 300°C for
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untreated needles and Fig. 1b at 250°C for treated needles with (NH4)2HPO4). This agrees with the results of DSC studies examining the effects of this chemical on pyrolysis of the needle lignin content [4]. Also, the external epidermic cells are exposed as a result of progressive cuticle loss (compare Fig. 1a at 300°C and Fig. 1b at 250°C). In addition, mild cracking of the epidermal cell layer appears in treated needles. At 300°C, these phenomena become more intense. As a result, the cuticle is removed completely, the epidermic cells are destroyed, and there is no blue
Fig. 1. Transverse sections of (a) untreated needles, (b) treated needles with (NH4)2HPO4 at a concentration of about 20% per weight, and (c) treated needles with Fire Trol at a concentration equivalent to 15% per weight on (NH4)2HPO4 contained, after flash pyrolysis at 200, 250, 300 and 350°C, as observed by light emission microscopy.
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fluorescence visible (compare Fig. 1a at 350°C and Fig. 1b at 300°C). It should be noted that, at this temperature, the treated needles become fragile, which may be correlated to lignin degradation. At 350°C, the treated needles are completely black and no further investigation is possible by LM. This indicates at this temperature that charification has proceeded more extensively throughout the inner part of the treated needles than in the untreated ones. Phenol derivatives, which are referred to as marker compounds of lignin degradation products, have been detected extensively at this temperature and this is an indication that lignin pyrolysis has been accelerated in the presence of the chemical. It should be emphasized that various analytical methods have recorded maximum evolution rates of degradation volatiles at temperatures around 350°C for treated needles with (NH4)2HPO4 [6]. Therefore, the main action of (NH4)2HPO4 as observed by LM and previous analytical studies [3,6] is the acceleration of the pyrolysis by lowering the temperature at which pyrolysis commences.
3.2.2. Pine needles treated with Fire Trol The transverse sections of needles examined by LM after treatment with Fire Trol and pyrolysis at various temperatures are presented in Fig. 1c. It appears that this product also accelerates pyrolysis by lowering the temperature at which pyrolysis commences even more than (NH4)2HPO4. The loss of blue fluorescence (indicating lignin degradation) in the samples begins at about 200°C (compare Fig. 1a at 300°C for untreated needles and Fig. 1b at 250°C for treated needles with (NH4)2HPO4). In addition, at this temperature, the external epidermic cells start to be exposed due to the loss of cuticle (compare Fig. 1a at 300°C and Fig. 1b at 250°C). Needle fragility develops at 250°C. At 300°C, the samples are completely black and no further investigation is possible by LM. 3.3. Stereoscopy 3.3.1. Pine needles treated with (NH4)2HPO4 Fig. 2a,b shows the external morphology of untreated needles and needles treated with (NH4)2HPO4, respectively, as examined by stereoscopy, after pyrolysis at various temperatures. The easily observed white crystals (Fig. 2b at 200°C) covering almost all the needle surface are pure (NH4)2HPO4. The tar (dark brown liquid drops) and char (gray– black debris) formation become visible in the treated needles at 250 and 300°C, respectively. At this temperature range (250–300°C), LM has monitored lignin degradation as well as DSC studies, whereas Py-MS studies have not monitored the evolution of lignin degradation marker compounds at a great extent. This may be attributed to the fact that lignin pyrolysis, at these temperatures, produces low enough quantities of volatiles. Consequently, the tar and char residue formed at this temperature range can be attributed mainly to cellulose pyrolysis [5]. It appears that the char forms some bubble shaped agglomerates at about 350°C in the treated samples. This may result from volatile pyrolysis components continuing to evolve rapidly from the inner part of the needles and becoming entrapped below the surface char. In addition, at this temperature,
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Fig. 2. External surfaces of (a) untreated pine needles, (b) treated needles with (NH4)2HPO4 at a concentration of about 20% per weight, and (c) treated needles with Fire Trol at a concentration equivalent to 15% per weight on (NH4)2HPO4 contained, after flash pyrolysis at 200, 250, 300, 350 and 400°C as observed by stereoscopy.
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significant changes were found in the composition of the pyrolysis volatiles; high evolution of levoglucosenone (originated from cellulose content) and the presence of new compounds, such as aromatic nitriles and nitro-phenyl compounds (originating from lignin content) [6]. It should be emphasized that Py-GC-flame ionization detection indicates that, around 350°C, there is a maximum evolution of degradation volatile products from treated needles with (NH4)2HPO4 [6]. Therefore, the main action of (NH4)2HPO4 as observed by stereoscopy is the acceleration of the pyrolysis by lowering the temperature at which pyrolysis commences.
3.3.2. Pine needles treated with Fire Trol Fig. 2c shows the external morphology of needles treated with Fire Trol, as examined by stereoscopy, after pyrolysis at various temperatures. The brick red muddy substance, clearly observed at 200°C, is the colored pigment used in the commercial product. At 250°C, a brown –black mud is observed and this is an indication that tar is mixed with the solid retardant on the surface. At 300°C, the formation of char agglomerates is visible. The quantity of the char formed on the needle surface seems to be greater than that on untreated needles, and in the treated ones with (NH4)2HPO4. It should be noted that the retardant seemed to fully cover (paint) the needle surface and possibly mask any early occurring phenomena (e.g. tar and char formation). For this reason, the experiments were repeated using lower concentrations of (NH4)2HPO4 (10% per weight) and Fire Trol (equivalent to (NH4)2HPO4, 5% per weight). The new stereoscopy results confirmed the previous findings.
4. Conclusions The two microscopy techniques successfully monitored the pyrolytic changes occurring in both the inner and outer parts of needles in the presence of fire retardants, giving complementary information. Both methods reveal that the presence of (NH4)2HPO4 lowers the pyrolysis temperatures about 50°C compared with untreated samples. This is in agreement with the results of other analytical methods, such as TG, DSC, DI-MS and Py-GC. In the case of Fire Trol, there are indications that the pyrolysis temperatures are lowered even more than (NH4)2HPO4 at about 50°C. LM shows visual evidence of the accelerated thermal degradation of lignin in the presence of retardants. It should be noted that monitoring the degradation of lignin during the needle pyrolysis is quite complicated with other methods. Stereoscopy monitors tar and char formation on the needle surface. It shows that, in the presence of retardants, tar and char are formed at lower temperatures. Stereoscopy also reveals that the retardants mode of action is to enhance char formation, which is in agreement with existing retardation theory. Based on these observations, the following mechanistic model of untreated and treated with fire retardands pine needles pyrolysis is proposed. 1. From ambient temperature to 200°C in both cases of pure and treated with (NH4)2HPO4 needles, only physical changes occur; such as moisture loss,
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softening and melting of resinous acids, and evaporation of high-volatility extractive constituents. These changes can only result in the shrinking of the cross-section of the needles. In the presence of Fire Trol, the cuticle is lost at about 200°C and the external epidermic cells start to be exposed. Some cracks of the external surface, around the resin ducts, are observed. The retardants, in all the presented cases, remain unchanged on the surface of the needles. 2. At about 250°C, the untreated needle shape remains the same as previously. Some blackening on the external surfaces around stomata and some opaque droplets on the external surfaces are indications of condensate beginning to be formed, due to desorption and/or degradation of the low-volatility extractives. In the presence of (NH4)2HPO4, the cuticle is completely lost, the external epidermic cells are revealed and a severe breaking of the external surface is occurred. The lignin content starts to degrade (starting gradually loss of blue fluorescence). In the presence of Fire Trol, all the aforementioned phenomena are more intense, especially in the case of lignin degradation. In all the cases of the treated needles, tar formation starts on the external surface and a gradually blackening of the inner part occurs simultaneously. This is an indication of the beginning of charification. The needles become brittle due to lignin degradation. 3. At about 300°C, the untreated needles start to become brittle. Crackings in the outer surface and more intense lignin degradation take place. This results in tar formation on outer needle surface. In the presence of (NH4)2HPO4 and Fire Trol, all the blue fluorescence is gone (no lignin present at the original form). The external epidermic cells are destroyed, resulting in a mixture of tar and char in the needle surface. The tar and char come from cellulose degradation. 4. At about 350°C, the untreated needle cuticle is completely lost and the external epidermic cells are revealed. The intense cracking on the external surface and the charification of the inner part of the needle result in a mixture of tar and char deposit on the needles surface. In the case of (NH4)2HPO4, tar and char are still visible on the outer surface. In the case of Fire Trol, only char is observed. In these cases of treated needles, full blackening (completely charification) is observed throughout the inner part. 5. At about 400°C, the untreated needles are fully black (complete charification) throughout the inner part. On its external surface, tar and char are still observed (char is formed on the expense of tar). From this temperature and above, the treated needles consist of only char. 6. Above 500°C, the untreated needles also consist of only char. In conclusion, first, the retardants attack the outer needle surface (cuticle breaking). Cuticle breaking allows, second, the retardants to penetrate into the inner part of the needles (evidence of lignin degradation at lower temperatures), facilitating the exit of combustible volatiles (pyrolysis degradation products) at lower temperatures. This has a negative effect on fire propagation, because the volatiles are dissipated in the atmosphere before their ignition temperatures are reached. The earlier char coverage of external needle surfaces at lower temperatures in the presence of the retardants may prohibit the fire spread and give rise to glowing combustion. Finally, the commercial retardant seems to have a greater
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impact on needle pyrolysis than pure (NH4)2HPO4. This may be due to the presence of specific additives, resulting in better adhesion on the needles surface compared with pure (NH4)2HPO4.
Acknowledgements This work was carried out in the framework of the research project ACRE (Project ENV4-CT98-729 ‘Additifs Chimiques Rheologie Evaluation’, Environment and Climate Program-E.C./D.G.XII).
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