Thermal Effects in Cellulose Pyrolysis ... - ACS Publications

Ind. Eng. Chem. Res. 1996, 35, 653-662

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Thermal Effects in Cellulose Pyrolysis: Relationship to Char Formation Processes Ivan Milosavljevic, Vahur Oja, and Eric M. Suuberg* Division of Engineering, Brown University, Providence, Rhode Island 02912

The thermochemistry of cellulose pyrolysis has been studied by a combination of differential scanning calorimetry and thermogravimetric analysis. Additionally, the vapor pressure and heat of vaporization of levoglucosan have been determined by an effusion method. The cellulose pyrolysis has been carried out under inert gas at heating rates from 0.1 to 60 K/min. The main cellulose thermal degradation pathway is endothermic, in the absence of mass transfer limitations that promote char formation. The endothermicity is estimated to be about 538 J/g of volatiles evolved. It is concluded that this endothermicity mainly reflects a latent heat requirement for vaporizing the primary tar decomposition products. Pyrolysis can be driven in the exothermic direction by char-forming processes that compete with tar-forming processes. The formation of char is estimated to be exothermic to the extent of about 2 kJ/g of char formed. Low heating rates, in concert with mass transfer limitations, serve to drive the pyrolysis in this direction. The enthalpy of cellulose pyrolysis is thus seen to be a sensitive function of the pyrolysis conditions. Pyrolysis appears to initially follow a common thermal pathway (in terms of enthalpy required per mass of volatile loss), irrespective of heating rate. Only at some finite level of conversion does the “thermal trajectory” of the process follow a heating rate dependent path, as significant char formation begins to occur. Introduction The pyrolytic behavior of cellulose is of interest in the context of fire and combustion research because this material serves as a good model of a charring solid and it is also obviously important in its own right. Cellulose has also been used as one of the model materials for biomass, in the context of “renewable energy” research. As a result, the literature on the pyrolysis of cellulose is extensive. Many articles have appeared concerning the chemical mechanisms of pyrolysis, the products released during pyrolysis, the kinetics of pyrolysis, and the role of transport processes in pyrolysis. Relatively less has been published concerning the thermodynamics of the pyrolysis process, and this is the subject of the present study. There are also implications of this work for establishing the global kinetics of the pyrolysis process. The “heat”, or enthalpy, of cellulose pyrolysis is a quantity of interest in the above applications areas. The paucity of reliable enthalpy of pyrolysis information is partly attributable to the fact that a simple calculational approach, based upon the enthalpies of formation of the starting material and of the products of the process, is still impractical. As is the case with many pyrolysis processes, there is such a broad range of products that the full accounting of products that would be necessary for obtaining a precise calculated value remains too difficult. Even the accurate experimental characterization of the enthalpy of pyrolysis of the starting cellulose can be difficult, as will be evident below. How critical the accurate knowledge of the enthalpy of pyrolysis is depends upon the particular application. One application involves modeling fire phenomena. It has been pointed out (Sibulkin, 1986) that, in such analyses, an accurate heat of cellulose pyrolysis reactions is not the key to correctly modeling the thermal * Author to whom correspondence should be addressed.

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sink represented by the solid. Rather, it is a “heat of gasification” that is of greater interest in these situations. This effective heat of gasification takes into account the fact that there is also a significant sensible enthalpy sink within the bulk solid and that this sensible heat sink can be as important as the pyrolysis reaction enthalpy in determining the energy-transportlimited kinetics of volatiles release into the vapor phase. A goal of this study has been to establish both the sensible and reaction enthalpy effects accompanying cellulose pyrolysis. In this paper, only the results on the enthalpy of pyrolysis will be considered. Energy conversion processes often involve use of small particles, as opposed to the bulk samples of interest in fire situations. Thus, there is a difference in modeling heat transfer in such small and finite, as opposed to semi-infinite geometries. The heating rates of fine particulate solids may be much higher than those experienced in large burning solids. Consequently there also arises a question about potential effects of heating rate on the enthalpy of pyrolysis. We have recently presented the results of a study on the global kinetics of cellulose pyrolysis in which two apparently distinct kinetic regimes of decomposition were identified (Milosavljevic and Suuberg, 1995). With the high purity cellulose that we examined, there is an apparent shift in global decomposition kinetics at around 600 K, from a higher activation energy process (218 kJ/mol) to a lower activation energy process (140 kJ/mol). The heating rate determines which kinetic pathway is followed, because this rate determines the extents of decomposition occurring above and below 600 K. Our interest here is in establishing whether the shift in kinetics gives rise to a shift in the thermodynamics of the decomposition process as well. Finally, we are also concerned about the factors that influence char formation during pyrolysis, and how these, in turn, influence the thermodynamics of decomposition. © 1996 American Chemical Society

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Table 1. Values of ∆Hpyrolysis Employed or Deduced from Modeling Studies on Pyrolysis/Combustion of Cellulosics ∆Hpyrolysisa (J/g) -360 -220 750 -84 to -126 (low T) -840 to -2100 (high T) -1016 -314 to -1700 203 0, -210, -420 0 1255 to 2510 418 (low heat flux) -393 to -1090 (high heat flux) 300 419 to 837 418 1256 500* 360 1464 (tars) -301 (char, gases) 0 600 20 (low conversions) -255 (high conversions)

source Bamford et al. (1946), Weatherford et al. (1965) (recalculated by Roberts (1975)) (recalculated by Kung (1975)) Tinney (1965) (averaged by Roberts (1975)) Roberts and Clough (1963) (recalculated by Kung and Kalekar (1973)) Roberts (1971) Kanury (1971) Kung (1972) Lee et al. (1977) Kanury and Holve (1982) Holve and Kanury (1982) Chan et al. (1985) Miller and Ramshalli (1986) Sibulkin (1986) Gandhi and Kanury (1988) Curtis and Miller (1988) Atreya and Wichman (1989) Bennini et al. (1991) Koufopanos et al. (1991)

a All values on a per unit mass of volatiles basis, unless noted by *, which indicates a unit mass of solids basis.

Previous Conclusions Concerning the Enthalpy of Cellulose Pyrolysis Reactions Table 1 shows the wide variety of reaction enthalpies assumed by various workers who have modeled pyrolysis and/or combustion processes in cellulosics (especially wood). It may be noted that in no case is the reaction enthalpy a very large value. For example, if one uses as a crude approximation a constant solid heat capacity of around 2 J/g‚K, then the process of heating the sample up to a typical degradation temperature of 350 °C (623 K) would involve a sensible enthalpy increase of about 650 J/g, comparable to the highest reported endothermic pyrolysis enthalpies. This also helps to explain why there is basic disagreement as to whether the heat of pyrolysis is significant. In determining heats of pyrolysis from bulk pyrolysis or combustion tests, the magnitude of the heat of pyrolysis is not a clearly dominant factor in determining a sample’s temperature history. It contributes, but its effect is comparable to that of the sensible enthalpy effects. Adding to this the fact that actual direct measurements by differential scanning calorimetry (DSC) or similar techniques have revealed enthalpies of pyrolysis that range from endoto exothermic, it is little wonder that modelers have often sought to simplify thermal calculations by entirely disregarding the heat of pyrolysis. Still, the importance of the reaction energy sink in pyrolysis of bulk cellulosic solids has been recognized for some time. McCarter (1972) reported that an endotherm associated with pyrolysis was significant below 623 K (350 °C), in pyrolysis experiments in inert gas as well as in bulk combustion tests. Similarly, “energy sinks” were reported by Murty Kanury (1972), Martin (1965), and Milosavljevic and Suuberg (1992) in combustion or combustion simulation tests. McCarter noted that the endothermicity could, to significant extents, be reduced or eliminated by pretreating the

sample quickly at 623 K, by finely grinding the cellulose (to particle sizes below 5 µm), or by swelling the sample in ammonia and quickly water washing and drying prior to pyrolysis. The reasons for the observed effects are not fully understood, but these results suggest that great care must be exercised in reporting the condition of the starting materials prior to any measurements of heat effects. It should also be noted at the outset that reliable experimental evidence has been presented by several groups to the effect that the pyrolysis process can actually exhibit both endo- and exothermic character, depending upon the reaction conditions (e.g., Tinney, 1965; Roberts and Clough, 1963; Kung and Kalekar, 1973). In the case of the work by Tang and Neill (1964), it was shown that the process becomes less endothermic in the presence of char-promoting inhibitors. In the case of the work performed by Chen et al. (1991), the process was seen to shift from a strongly endothermic character if the cellulose was pure to a strongly exothermic character if the cellulose contained a large amount of retardant. Both of these results can be understood in terms of the fact that as the pyrolysis process shifts from one that favors the release of tars to one that favors the release of oxide gases (accompanied by significant char formation), then the process shifts toward exothermicity. An actual correlation between heat of pyrolysis and char yield was presented by Mok and Antal (1983), and showed clearly that increased char formation drives the process toward exothermicity. This issue will be explored further below. Experimental Section The thermochemistry of cellulose pyrolysis was examined using a standard differential scanning calorimeter (DSC). These measurements were performed under a variety of heating rate conditions, such as are of relevance in fire situations. The DSC measurements were performed under conditions identical to those used for measurements of mass loss kinetics obtained using previously described thermogravimetric analyzer (TGA) techniques (Milosavljevic and Suuberg, 1995). Cellulose Samples. The samples selected for study were all prepared from Whatman CF-11 ashless filter paper pulp, used without any further treatment. This cellulose is prepared from high purity cotton of 99% R-cellulose content. The ash content is 0.009% by mass. The molecular weight of the cellulose is in the range 36 000-45 000 daltons. The diameter of the fibres is 10-30 µm. Differential Scanning Calorimetry. The DSC device was a TA Instruments System 2910. The sample was placed into a standard aluminum DSC pan, which had a cover that was lightly seated (not crimped on) and into which two or more approximately 1 mm diameter holes were poked with a pin. The intent was to allow escape of evolving volatiles during pyrolysis, but at the same time to maintain a well-defined environment for the DSC measurement. As it turned out, the mass transfer aspects of the process are key to the observed kinetics and thermochemistry. This will be discussed further in connection with the TGA results. Generally, between 0.5 and 15 mg of powdered cellulose sample was used in an experiment. A series of experiments was designed to look for the influence of sample loading in this range. The significance of the size of sample on char yield and kinetics will be discussed below, in connection with the role of mass

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transport limitations. Under comparable heating rate and mass transfer conditions, however, variation of sample loadings in this range did not appear to significantly influence the observed thermodynamics. Experiments were performed in the presence of a flowing inert purge gas, generally nitrogen or helium, at a flow rate of 50 mL/min through the sample cell. Which inert gas was chosen made no difference in the observed results. This suggests that heat transfer through the gas was not an important factor in supplying heat to the sample, since the thermal conductivity of helium is about a factor of 5 greater than that of nitrogen. This is not surprising, because the DSC instrument is designed to provide a good heat transfer surface between the sample pan and the heater/temperature sensor plate that it rests upon. The DSC system was itself calibrated with indium, zinc, and water samples. Thermogravimetric Measurements. The measurements were performed with a DuPont Model 951 TGA system. There have often been concerns raised about the accuracy of temperature measurement in TGA systems, particularly at some of the high heating rates of interest in this study. Most TGA systems (including the one used here) rely on a thermocouple placed close to, but not in contact with, the sample whose kinetics are of interest. Unlike the situation in the DSC, in which the sample pan is in good contact with a temperature sensor, there is a possibility in the TGA that the sample temperature is significantly lower than the temperature reported by a thermocouple in the gas phase. The latter receives heat from the furnace wall and gas phase, as does the sample pan, but it might not sense a significant endotherm in the cellulose. Such an endotherm would thus cause the sample pan to lag the thermocouple in temperature. To address the above concern, a series of tests were performed in the TGA, in which a second thermocouple was brought into direct physical contact with the sample pan. The method has been described earlier (Milosavljevic and Suuberg, 1995). The contact of the thermocouple with the sample pan naturally precluded obtaining accurate mass loss information, but the recorded sample temperatures were used to correct the results of the actual mass loss experiments through reference to the gas phase thermocouple, which was in a constant location throughout the procedures. This calibration procedure, using the measured sample pan temperature to correct the gas phase thermocouple reading, was used for all experiments, and showed good reproducibility. Thus we believe that our TGA results will accurately correspond to the the DSC results. This temperature correction procedure revealed that, at a heating rate of 60 K/min, “normally” measured TGA temperatures (with the thermocouple in the gas phase) could be as much as 20 K too low. At lower heating rates, the correction proved to be much smaller, to the extent that at 1 K/min, there was no correction needed to the gas phase TGA temperatures. Combustion Calorimetry. The oxygen bomb combustion calorimetry was performed using a Parr Model 1421 semimicro calorimeter. Samples of dry Whatman CF-11 cellulose and 99% pure levoglucosan were examined. Samples of 20-40 mg were employed. Vapor Pressure Measurements. The vapor pressure of levoglucosan was determined using the so-called Knudsen effusion method (Hollahan, 1962). The method employed here will be described elsewhere (Oja and

Figure 1. Integrated thermal demand for cellulose pyrolysis at 1 K/min heating rate and corresponding mass loss from a DSC pan. The thermal results are expressed as enthalpy per original mass of cellulose.


Suuberg, 1995). Briefly, a sample is enclosed in a capsule which contains a pinhole leak. When the capsule is placed in a high vacuum environment, the sample’s mass loss rate (the pinhole leak rate) is proportional to its vapor pressure. The mass loss rate is recorded using a Cahn recording electrobalance. Sample temperatures are recorded to 0.1 K accuracy. Results and Discussion General Behavior of Cellulose in DSC and TGA Experiments. The pyrolysis behavior of the cellulose samples was examined at heating rates ranging from 0.1 to 60 K/min. This range was chosen so as to bracket the range of heating rates observed in other experiments designed to simulate the behavior of cellulose samples under fire-level heat fluxes (Milosavljevic and Suuberg, 1992). It had earlier been concluded that the effect of heating rate on the kinetics of pyrolytic mass loss was significant (Milosavljevic and Suuberg, 1995). A crude dividing line between “high” and “low” heating rate behavior was seen to be about 10 K/min. Thus heating rates both above and below this value were examined. Figures 1 through 4 illustrate the DSC and TGA results obtained on samples pyrolyzed at heating rates of 1, 6, 15, and 60 K/min. The DSC curves have been integrated to show the cumulative heat effect as a function of temperature, so as to give a curve that may be conveniently compared with the TGA curve of remaining mass. Figures 1-4 show some superficially unremarkable features of the pyrolysis process. The shift of the TGA

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and DSC decomposition curves to higher temperatures with increasing heating rate would be qualitatively expected from Arrhenius kinetics in a nonisothermal environment. As will be described below, however, a simple kinetic picture does not emerge from these results. The TGA and DSC results were both obtained with the powder samples enclosed in standard aluminum DSC sample pans. The pans were prepared as described above, with loose covers containing two pinholes. This procedure was followed to obtain as close a correspondence between the DSC and TGA conditions as possible, since it was feared that a difference in mass transfer conditions could lead to different kinetics in the two different experiments. There was basis for such a concern, as illustrated in Figure 5. Consequences of Mass Transfer Limitations. Figure 5 shows a comparison between the TGA results obtained in the “normal” manner, in which the sample is placed into an open TGA sample pan, compared with those from samples in a DSC pan. The differences appear small at 1 K/min heating rates, but are significant enough at 60 K/min to make a difference in kinetic studies. Since the temperature of the DSC pans was measured by direct contact in all cases, including in the TGA work, the difference in mass loss results can only be ascribed to the mass transport limitation out of the DSC sample pans. Presumably, the rate of mass loss from the pans was, to some degree, limited by the ability of the volatiles to escape from the openings in the DSC pan. Therefore we believe that the DSC and TGA

Figure 5. Loss of mass during cellulose pyrolysis from an open TGA pan (solid lines) and from a DSC pan containing 2 holes (dashed lines) at 1 and 60 K/min heating rates.

results can be compared only when both are taken in the same configuration, i.e., with the samples inside the DSC pans. The TGA results of Figures 1-4 reveal typical patterns of mass loss. There is an initially relatively rapid period of “primary” mass loss, followed by a much slower period of slow, secondary “outgassing” of a char intermediate. This is most easily visible in Figure 1, for the 1 K/min case. The apparent char yield at the end of the primary phase is about 30%, near 600 K, whereas the actual final char yield is slightly less than 20%, due to the outgassing (which actually involves continued pyrolysis of a nonvolatile residue of primary pyrolysis). The contribution of the secondary outgassing period becomes progressively less important with increasing heating rate, although it still accounts for about 5% of the total mass loss at 60 K/min. This slow loss rate period is quite similar to that reported by McCarter (1972), which he also interpreted in terms of slow “char decomposition” processes. The char yields obtained here at the lowest heating rates (1 and 6 K/min) are higher than those obtained during pyrolysis in open pans. In our earlier work (Milosavljevic and Suuberg, 1995), we obtained between 5 and 10% char yield under these heating rate conditions. Clearly, the mass transport limitations affect char yield. This effect was systematically examined by performing a number of additional studies, using both the TGA and DSC devices. The results are shown in Figure 6. The legend of Figure 6 indicates whether the experiment was performed in a TGA or a DSC. Regardless of which device was used, unless otherwise noted (by “open pan”), samples were enclosed in a standard DSC type of pan, containing the number of roughly 1 mm diameter holes indicated in the legend. For comparison, the results from a series of experiments performed in an open TGA pan are also shown. The mass of sample in the pans is also indicated in the legend. Generally speaking, the higher the heating rate, the less was the effect of a mass transport limitation. This came as somewhat of a surprise, in light of Figure 5, which showed that the escape of volatiles out of the pan was significantly delayed at the higher heating rates. Low heating rates, combined either with large amounts of sample or with significant limitations to volatiles escape (low numbers of holes), assured significantly higher yields of char at the end of the period of active mass loss. Low heating rate alone was not sufficient to assure “high” char yields, although the widely accepted trend toward higher char yields with lower


Figure 6. Influence of heating rate and sample confinement conditions on yields of char from cellulose pyrolysis. All experiments were performed with the samples contained inside of a DSC pan, except those experiments marked “TGA open”, which were performed in open TGA pans. The legend indicates the amount of sample contained in the pans, and the number of holes in the DSC pan’s lid.

heating rates was observed (and here magnified by the choice to use a “large” sample of cellulose in the open pans). It may be concluded that transport limitations impede outflow of volatiles from the sample pans, and that this shows itself more clearly at higher heating rates for two reasons. One reason is that the volatiles are more quickly formed at the higher temperatures, and the limitation offered by the holes in the DSC pans plays a greater role, the more mass per unit time that must pass through them. The second feature of relevance in examining the data of Figure 5 is that a particular difference in abscissa temperatures signifies shorter times at higher heating rates. Therefore, while it might appear from Figure 5 that the transport limitation is more “severe” at 60 K/min than at 1 K/min, in fact, the actual situation is not as clear. Taking as an example the data at a conversion of 50%, there is about a 3 K difference between the DSC and open pan results at 1 K/min (corresponding to 3 min difference to achieve comparable mass losses), and about a 20 K difference at 60 K/min (corresponding to only a 0.3 min difference). Whether additional residence time or higher temperature is more important in determining an increase in char yield depends upon the kinetics of the char-forming processes. We believe that char formation is an inherently low activation energy process, and that the residence time of tar in a char-forming environment is key to determining char yields. This aspect of the pyrolysis process will be more fully explored in a future publication. It should be noted that Varhegyi et al. (1988) have shown for various materials that mass transfer limitations can influence both the kinetics and thermochemistry observed in DSC measurements. We see clear evidence of such effects in the cellulose system as well. Thermochemistry of Pyrolysis. Returning to the calorimetric data in Figures 1-4, it was noted that the primary period of cellulose pyrolysis gave very sharp endothermic and/or exothermic peaks which were straightforward to integrate. The only exception to this was seen in the results obtained at 0.1 K/min heating rate, in which baseline drift made accurate integration difficult (these results are thus not shown here). The measurement of heat effects of pyrolysis during the slow, secondary outgassing period was not nearly

as reliable as that during the primary phase of pyrolysis. This is because the very long time scale of the process again made it difficult to establish accurately a baseline for integration. The outgassing could be qualitatively judged to be exothermic, but the magnitude of the heat effect was small, because there was only a small residual mass at that point. It is for this reason that the main emphasis here will be given to the heat of primary pyrolysis, and only those portions of the DSC curves are shown in integrated form on Figures 1-4. All of the integrated results are shown on an original cellulose mass basis. Examination of the DSC results of Figures 1-4 shows more complexity than might be inferred from the TGA data. The processes begin endothermically at all four heating rates. At the lower heating rates, there is evidence of a subsequent exothermic process which, to some extent, overlaps and overwhelms the endothermic process (see Figure 1). The pyrolysis process is, at an intermediate stage, more endothermic than it is at the conclusion of the primary pyrolysis stage. Even in the results obtained at 6 K/min (Figure 2), it is possible to see that the reactions, overall, become nearly thermoneutral before the end of the primary pyrolysis period. We hypothesize that this thermoneutrality is due to a “balancing” of endo- and exothermic decomposition reactions. The hypothesis, at this point, is based upon an extension of the observations at other heating rates, and the fact that char formation appears to have been established as an exothermic process, vide infra. In the case of the 60 K/min heating rate (Figure 4), the pyrolysis process appears as though it were entirely endothermic, and there is good correspondence of the enthalpy and mass loss curves throughout. Comparison of the present DSC results with other published results shows the sensitivity to conditions of measurement. For example, at a heating rate of 5 K/min, the enthalpy of pyrolysis has been earlier reported to be 336 J/g (Rogers and Ohlemiller, 1980), as opposed to the value near 200 J/g obtained here at 6 K/min. The difference might have to do with a difference in char yields in the two studies (20.4% was reported by Rogers and Ohlemiller, whereas at the end of primary pyrolysis our “char yield” was almost 30%). Another study reported an overall enthalpy of pyrolysis to be 368 J/g at 12 K/min, with a char yield estimated at about 15% (Tang and Neill, 1964). This may be compared with the value of about 280 J/g obtained here at 15 K/min, with a char yield of about 19%. In another study, an enthalpy of pyrolysis of 220 J/g was found, using a heating rate of 40 K/min, but a char yield was not reported (Chen et al., 1991). Comparison with a more extensive study, in which the role of char yield on thermochemistry was clearly demonstrated, will be shown below. This sampling of results illustrates part of the difficulty in comparing results on the “heat of pyrolysis” from the literature. First, it is important that the results be taken at similar heating rates. Then it is also important that similar conditions of measurement were employed, such that char yields were comparable. It is little wonder that the literature contains so many different values of the heat of pyrolysis, since the process is of interest is quite complex, and the significance of certain variables has not been fully recognized. Combination of DSC and TGA Results. The results from the DSC and TGA experiments are crossplotted in Figure 7. What is somewhat striking in this

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Figure 7. Data on the enthalpy of pyrolysis as a function of remaining sample mass, taken from Figures 1-4.

figure is the nearly linear relationship between mass loss and heat absorption, observed at 60 K/min heating rate. The processes responsible for net heat absorption are apparently quite constant throughout the process of rapid heating. In contrast to this, the results obtained at lower heating rates show a deviation from a steady heat absorption curve, at some point. This deviation occurs at progressively lower extents of mass loss, the lower the heating rate. It has already been suggested that the heats of pyrolysis depend upon the extent of char formation, and the exothermic nature of the char formation process will be explored further below. The conclusion from Figure 7 is that a large portion of the exothermic char formation is delayed to progressively later times during the pyrolysis process, the higher the heating rate. As noted above, char formation appears to be a process with a lower activation energy than the main process of mass loss, since the influence of the former process wanes as more of the decomposition is driven to occur at higher temperatures by the increase in heating rate. There is, of course, no obvious period of char formation clearly evident in the 60 K/min results of Figure 7, but it is clear from the TGA results that char formation does occur. The conclusion is that the char formation must be distributed over the entire period of pyrolysis. Thus even in this most extreme case, competing processes of pyrolysis to volatiles and char formation must be present, and they completely overlap. The results of Figure 7 are of interest as much for what they do not reveal as for what they do reveal. During the time before the apparent initiation of extensive char-forming reactions (indicated by the deviation from a linear relationship between mass loss and heat of pyrolysis), there were no differences in the pathways traced by the high heating rate and low heating rate experiments. This was true despite the fact that the reactions were believed to be following different kinetic pathways at high and low heating rates (Milosavljevic and Suuberg, 1995); the heats of pyrolysis gave no clue to this. It is concluded that the heat of pyrolysis reflects a process that is similar in both kinetic regimes, regardless of the kinetic pathway itself. An enthalpy of pyrolysis that is dominated by the latent heat of evaporation of tar and oil products of pyrolysis might be one explanation for this behavior, and this will be considered in light of other measurements below. Enthalpy of Pyrolysis and Char Yield. Cellulose is generally termed a “charring solid”, meaning that during pyrolysis a solid, highly carbonaceous residue is usually a significant product. The yields of char may range from almost zero, when the cellulose is heated at

Figure 8. Overall enthalpy of pyrolysis (per unit mass of starting cellulose) as a function of char yield at the end of pyrolysis. The data of Mok and Antal (1983) are shown for comparison (crosses). Also shown are data points (open symbols) from Rogers and Ohlemiller (1980) and Tang and Neill (1964), as described in the text.

rates of a few hundred to a few thousand degrees per second (Lewellen et al., 1977), to several tens of percent of original mass, under slow heating conditions such are of interest here. The formation of a char product plays an important role in determining the enthalpy of pyrolysis. A major problem in defining an actual “enthalpy of cellulose pyrolysis” is made apparent in Figure 8. This figure, adapted from Mok and Antal (1983), shows values of the enthalpy of pyrolysis obtained under various pyrolysis conditions, leading to various char yields. There is good agreement between us and Mok and Antal on the conclusion that there is a linear decrease in the endothermic heat of pyrolysis as char yield increases. There is not as good agreement on the absolute magnitudes of the values. There is, at present, no definite explanation as to why this should be. In addition to the difference in absolute values of enthalpy, the slopes of the two regression lines in Figure 8 are also a bit different: we obtain -2.0 kJ/g char increase, and Mok and Antal obtain -3.6 kJ/g char increase. These results illustrate how sensitive heat of pyrolysis values are to char yields. If the heat of pyrolysis were measured to be 400 J/g cellulose (endothermic) in a case in which char yield is 10%, then a 1% additional yield of char would decrease the measured heat of pyrolysis by 5%. It is thus easy to understand the wide range of reported results in light of this sensitivity. Figure 8, again, illustrates that the yield of char is the main factor determining whether the overall pyrolysis process is endo- or exothermic. This is an important global feature to build into any model of pyrolysis. Enthalpy of Volatiles Release Processes. It is quite plausible that the measured “heat of pyrolysis” is actually a composite of several different contributions. The role of exothermic char formation is counterbalanced by an endothermic heat of volatiles release. In this latter case, there may be enthalpy effects associated with actual degradation reactions, and there is also a latent heat of evaporation of many primary decomposition products (tars and oils). The latter two effects are often lumped into a single enthalpy of reaction, but it is convenient for the moment to keep them separate. The slope of the nearly straight-line 60 K/min heating rate curve of Figure 7 provides a rough estimate of a “heat of volatiles release”. The value so calculated from


the slope is approximately 430 J/g of volatiles evolved (endothermic). It should be recognized that, even at 60 K/min heating rate, there is concurrently with volatiles evolution some char formation (the char yield is not zero at the end of pyrolysis). Therefore, since char formation is exothermic, the estimate from Figure 7 would represent an underestimate of true volatiles release endothermicity. A different, better, experimental estimate of the enthalpy of volatiles release is available from Figure 8. The extrapolation of the results in Figure 8 to zero char yield provides an estimate of the enthalpy of pyrolysis, were there no char formation at all. This yields an estimate of about 538 J/g volatiles. Extrapolation to zero char yield in Figure 8 represents an idealization not realized in the experiments of Figure 7, but provides a clearer picture of volatiles release enthalpy. Thus it would be expected that if char formation could be experimentally suppressed, the actual heat of pyrolysis would be closer to the extrapolated value of 538 J/g. The following calculations may be compared with this value. As noted earlier, estimates of the thermochemistry of pyrolysis, based upon product composition data, are difficult because of the large numbers of products that must be considered. A crude estimate is nevertheless available by employing simple assumptions. Since product yields were not measured in the DSC experiments performed here, this information had to be indirectly obtained. In our TGA experiments under comparable conditions, the yields of CO2, CO, and CH4 were approximately 1.2%, 0.45%, and 0.01% when char yields were approximately 10% (tar and water yields were not separately measured). These primary pyrolysis yields are comparable to those reported by Hajaligol et al. (1982), at 400 °C (Table I of that study). Using, therefore, the complete yield information from Hajaligol et al., it is possible to approximate the complete set of products of primary pyrolysis. The main products were 83.35% tar, 1.45% CO2, 6.49% water, and 6.17% char. The stoichiometry of pyrolysis in this case may then be reasonably approximated by

(C6H10O5)(s) f 0.8589C6H10O5(v) + 0.0053CO2(g) + 0.584H2O(g) + 0.83C (1) where the yields of some trace products (e.g., CO, methanol, acetaldehyde) have been neglected, and the “missing” mass has been assigned to the tar. The char is treated as essentially pure carbon, for the purposes of this analysis. The heats of formation of all of the products on the right hand side of eq 1, with the exception of the tar, are known (taking the carbon to be the pure elemental form with nearly zero enthalpy of formation). Note that the tar is assumed to be present as a vapor product, reflecting the fact that in escaping the DSC sample pan most of it must presumably vaporize. Unfortunately, the enthalpy of formation of cellulose is not a universally accepted value, since it depends upon many features of the cellulose (e.g., its crystallinity). An estimate of the enthalpy of formation can be made from the heat of combustion values that have been reported for cellulose. The values that have been reported include 16.7 kJ/g (Chen et al., 1991), 17.3 kJ/g (Susott et al., 1975), and 16.9 kJ/g (Tang and Neill, 1964). The measured value obtained in this laboratory is 17.1 kJ/g. Since these values were obtained by oxygen bomb calorimetric methods, they correspond to the

higher heat of combustion (water as a liquid product). The actual stoichiometric reaction for combustion in the bomb calorimeter would be

(C6H10O5)n + 6nO2 f 6nCO2(g) + 5nH2O(l) (2) The heat of formation of cellulose would then follow from

∆Hcomb [kJ/g] × 162 g/equiv ) ∆Hf(6 mol of CO2) + ∆Hf(5 mol of H2O) - ∆Hf (1 equiv of cellulose) where “1 equiv” of cellulose corresponds to 1 formula weight equivalent of 162 g per cellulose repeat unit. The enthalpy of formation of CO2 is -393.51 kJ/mol, and that of liquid water is -285.84 kJ/mol, from which ∆Hf of cellulose is -1020.1 kJ/equiv or -6.3 kJ/g (using the value of -17.1 kJ/g for the enthalpy of combustion). The properties of the tar, including its enthalpy of formation, are also unknown. The tar has therefore been assumed to be similar to levoglucosan, a wellknown major constituent of “pure” cellulose pyrolysis (e.g., Shafizadeh et al., 1979; Mok and Antal, 1983; Arseneau, 1971). Taking the tar as levoglucosan is in reasonable agreement with the reported elemental analysis of the tar obtained by Hajaligol et al. The tar formation part of the pyrolysis process in eq 1 can by itself be thermodynamically represented as

(C6H10O5)n(s) f C6H10O5(v) cellulose “tar”

(3)

We are aware of no direct measurements of the enthalpy of this reaction, as written. However, it may be estimated from a knowledge of measured properties. If it were acceptable to take tar to be levoglucosan, the enthalpy change for the reaction 3 to solid levoglucosan product may be estimated from the heats of combustion of cellulose and levoglucosan. The measured values are -17.1 kJ/g for cellulose and -17.9 kJ/g for levoglucosan. This yields ∆Hpyro ) 0.8 kJ/g ) 129.6 kJ/mol (to the solid product). The heat of combustion values obtained by us are a bit higher than have been reported elsewhere, -16.8 and -17.5 kJ/g, for cellulose and levoglucosan, respectively (Bains, 1974), but the calculated heat of pyrolysis is still the same from either data pair. The assumption that tar is represented by levoglucosan turns out to not be particularly good, as discussed below. The above value of ∆Hpyro must be combined with a value of the heat of vaporization of solid levoglucosan, ∆Hvap, to yield the total enthalpy change, ∆Htot, of process 3. Again, the enthalpy of this vaporization process has not been reported. The direct measurement of this latent heat by DSC proved to be impossible, because both the cellulose tar and levoglucosan were observed to thermally degrade in such experiments. Thus the enthalpy was estimated from vapor pressure data measured in this laboratory under very low temperature conditions, at which levoglucosan and cellulose tars would not degrade. Here, only the work with levoglucosan will be reported. The enthalpy of vaporization of a substance can be estimated from the Clausius-Clapeyron equation, d[ln P]/d[1/T] ) -∆Hvap/R, given suitable vapor pressure data. These data are shown for levoglucosan in Figure 9. The correlation derived from the data is

ln P [Torr] ) 32.34 - 14354/T [K]

(4)

660


(C6H10O5)n f 6nC + 5nH2O

Figure 9. Vapor pressure measurements for levoglucosan, obtained using the effusion method.

It is worth noting that the vaporization in these experiments actually involved sublimation from the solid phase.These data predict levoglucosan to boil above 560 °C (taking into account condensed phase changes at around 390 and 456 K), and appear to agree with the observation that levoglucosan vigorously evaporates and boils somewhere above 500-510 K (Mok and Antal, 1983; Arseneau, 1971). Our own DSC results would place true boiling behavior nearer 573 K. From these results, if it is assumed that ∆Hvap is constant (a good assumption, given Figure 9), then integrating the Clausius-Clapeyron equation gives ∆Hvap ) 119 kJ/mol ) 735 J/g, for the solid-to-vapor transition. Preliminary experiments have strongly suggested that the enthalpy of vaporization of real cellulose tars is quite close to that obtained for levoglucosan. As a result, the process of tar formation from the cellulose involves a 119 kJ/mol enthalpy of vaporization, and may involve a 129.6 kJ/mol reaction enthalpy (if the tar is like levoglucosan). Using the heats of formation of CO2 and H2O (-393.51 and -241.83 kJ/mol, respectively, and neglecting the small heat capacity correction), eq 1 involves an overall heat of pyrolysis of approximately 1.2 kJ/g cellulose, or 1.3 kJ/g volatiles, using the reaction enthalpy of 129.6 kJ/mol. On the other hand, if the formation of tar is thermoneutral (i.e., ∆Hpyro ) 0) and the main endothermicity associated with tar formation involves the volatilization of tar, then eq 1 would exhibit an endothermicity of about 520 J/g cellulose or 550 J/g of volatiles. This is a bit higher than the values determined from the results of Figure 7 (430 J/g volatiles), but clearly compares better than the values based upon pure levoglucosan. Consequently, we believe that the reactions that form tar precursors might be nearly thermoneutral, and the main enthalpy sink in reaction is vaporization. The above calculation is admittedly crude, and only serves to suggest that the gross heat effects have been reasonably accounted for by a simple model. Independent of the above model, it can be concluded from the comparison of the enthalpy of tar vaporization (735 J/g) and enthalpy of pyrolytic volatiles formation in the limit of no char formation (538 J/g) that the major enthalpy sink during pyrolysis involves the enthalpy of vaporization for the tars. The overall cellulose chain degradation processes cannot be very endothermic, as adding this endothermicity would drive the comparison in an even more unfavorable direction. Thermodynamics of Char Formation. The process of char formation can be thermodynamically idealized as cellulose forming pure carbon by dehydration, in which case the stoichiometry could be modeled as

(5)

The real process of char formation is, of course, not nearly as simple as this equation suggests. The char is generally not pure carbon, and other gases, such as CO2, could equally well be evolved during the charforming step (they are certainly evolved during pyrolysis itself). Nevertheless, the char-forming process has been characterized by some workers as involving mainly a dehydration, in accord with this highly simplified model (Kilzer and Broido, 1965; Broido, 1966; Broido and Weinstein, 1970; Broido and Nelson, 1975; Arsenau, 1971; Arseneau and Stanwick, 1971; Bradbury et al., 1979). Though some features of such simple cellulose pyrolysis models have been the subject of recent challenge (Varhegyi et al., 1994), the basic feature of a competitive pathway leading to differing yields of char has been upheld. In any case, the main focus here is not the mechanism of char formation, but its plausible thermochemistry. The enthalpy of the above char-forming reaction could be calculated from a knowledge of the enthalpies of formation of cellulose and water (since the enthalpy of formation of the mainly carbon char would again be essentially zero, taking the customary zero enthalpy reference state of pure elements in their normal form at 298 K). The heat of charring by pure dehydration may be estimated, taking the enthalpy of the water in a vapor state (-241.83 kJ/mol), since during pyrolysis this is the state of the water. This leads to an enthalpy of pyrolysis of -189.1 kJ/equiv cellulose, or -2.6 kJ/g carbon deposited. It should be noted that this is therefore an exothermic process. Again, it might be argued that the assumption of carbon and water as the sole products of the charring process is unreasonable. Generally, CO2 and H2 are also released during pyrolysis, and the char is not pure carbon, but contains some hydrogen. Assuming for simplicity that the oxygen were to leave as CO2 rather than H2O, and that the residual hydrogen is driven off of the char as H2, the process becomes

(C6H10O5)n f 2.5nCO2(g) + 5nH2(g) + 3.5nC (6) The calculated enthalpy of this char formation process would, in contrast to eq 5, be endothermic, 36.3 kJ/equiv cellulose, or 0.86 kJ/g carbon deposited. Therefore it is obviously important to characterize the products that are formed together with the char, in order to correctly calculate the enthalpy of the char formation process. It is worthwhile noting that in a study in which cellulose was pyrolyzed in a sealed DSC capsule, thus driving the process toward char formation, an exothermic enthalpy of pyrolysis of 650 J/g cellulose was observed (Raemy and Schwiezer, 1983), quite close to a value that may be estimated from the results of another study under similar conditions (Varhegyi et al., 1993). On the basis of these results as well as the results from open systems (such as those shown in Figure 8), it appears that the exothermic dehydration-like routes appear to be most important under most pyrolysis conditions. It is possible to infer from the fact that CO2 and H2 are formed during pyrolysis, that thermodynamically the char formation processes would be a mix of eqs 5 and 6, with eq 5 predominant. Of course other processes not examined here would also contribute significantly under certain conditions, most notably those leading to CO.


It is possible to reexamine to the question of the thermodynamics of char formation, in the context of the results of Figure 8. The values of the slope of the curves in Figure 8 can be compared with the calculation of the thermodynamics of char formation. For each gram of carbon formed from cellulose by a purely dehydration pathway, the process would be expected to exhibit an exothermicity of 2.6 kJ/g, and for each gram by pathway 6 an endothermicity of 0.86 kJ/g is expected. By using the measured ratio of CO2/H2O from eq 1 ()0.053/0.584 ) 0.091), the respective contributions of each pathway can be approximated. This weighting would require that char formation be approximately 2.3 kJ/g carbon exothermic. This value cannot, by itself, be compared with the slope of the curves in Figure 8, because it neglects the fact that for each unit of char formed there is a certain amount of volatiles not formed; the formation of volatiles competes with the formation of char. The formation of volatiles is inherently endothermic, as discussed above. Thus the slope of a line in Figure 8 includes the loss of some of this volatiles formation endothermicity, together with the char-forming exothermicity. An estimate of the endothermicity can be obtained from the data of Figure 8 (538 J/g). Per gram of cellulose, the dehydration reaction (eq 5) yields only 0.44 g of carbon and eq 6 yields only 0.26. Therefore the unrealized endothermicity, due to diverting a part of the cellulose decomposition to char per reaction 1, would be 538/0.41 ) 1.3 kJ/g char formed. This must be added to the exothermicity of the char-forming process itself, yielding 2.3 + 1.3 ) 3.6 kJ/g char increase. This is in good agreement with the slope of the Mok and Antal results in Figure 8, and less satisfactory agreement with our own. Better agreement with our results can be achieved by assuming that the carbon oxide gases represent a larger fraction of char-forming reaction products, as they might, under our conditions. For example, we have seen CO2 + CO yields climb to 2.5%, under only slightly more mass transfer limited conditions in the TGA in which the char yield increased by less than 1%. There is reason to believe from work that we have done in another system that the carbon oxide yields could exceed 5% each, under conditions of char formation more like that in the highly mass transport limited DSC pans. Assuming for simplicity only a small change in CO2/H2O ratio (to 0.18) consistent with the observed small change in product ratios in the TGA, the weighted exothermicity of char formation is already decreased by 13% to 2.0 kJ/g carbon. In addition, it should be recalled that Figure 8 shows our data on the thermochemistry of primary pyrolysis alone, and neglects some additional exothermicity due to the slow outgassing processes. Adding in this extra exothermic period would tend to increase the slope of the line for our results in Figure 8. This would bring these results into better agreement with Mok and Antal’s results, as well as with the above calculations. As noted above, since our measurements were not reliable during the outgassing period, this has not been done. Again, considering the crudeness of the calculation and the number of assumptions that went into it, the reasonable predictions for the slope of the lines in Figure 8 suggest that the essential elements of the model are correct. The sensitivity of the calculations to assumptions concerning product yields is, however, clear. Therefore, calculation of true overall pyrolysis enthalpies from product compositions appears to not be

feasible, beyond the crude levels of accuracy obtained here. When required, this enthalpy will most likely have to remain a measured quantity, with careful attention to performing measurements under conditions designed to simulate the actual usage of the cellulose or biomass. Conclusions The main cellulose thermal degradation pathway is endothermic, in the absence of mass transfer limitations that promote char formation. The endothermicity is estimated to be about 538 J/g of volatiles evolved. It is concluded that this endothermicity mainly reflects a latent heat requirement for vaporizing the primary tar decomposition products. In practical applications, pyrolysis can be driven in the exothermic direction by char-forming processes that compete with tar-forming processes. The formation of char is estimated to be exothermic to the extent of about 2 kJ/g of char formed. Low heating rates, in concert with mass transfer limitations, serve to drive the process in this direction. The enthalpy of cellulose pyrolysis is thus seen to be a sensitive function of the pyrolysis conditions. It appears difficult to predict this enthalpy a priori. Pyrolysis appears to initially follow a common thermal pathway (in terms of enthalpy required per mass of volatile loss), irrespective of heating rate. Only at some finite level of conversion does the “thermal trajectory” of the process follow a heating rate dependent path. There is, therefore, no thermal evidence of a major difference in mechanism of pyrolysis at high and low heating rates. Acknowledgment The main financial support for this work was provided by the Center for Fire Research of NIST, under Grant 60NANB0D1042. Support for the measurements of vapor pressures of tarry decomposition products was provided by the U.S. Department of Energy, under Grant DE-FG22-92PC92544. Experimental assistance was provided by Mr. Indrek Aarna and Mr. William Lilly. Literature Cited Arseneau, D. F. Competitive Reactions in Thermal Decomposition of Cellulose. Can. J. Chem. 1971, 49, 632-638. Arseneau, D. F.; Stanwick, J. J. A Study of Reaction Mechanisms by DSC and TG. Thermal Analysis, Vol. I, Proceedings Third ICTA; Birkhauser Verlag: Basel, Switzerland, 1971; pp 319326. Atreya, A.; Wichman, I. S. Heat and Mass Transfer During Piloted Ignition of Cellulosic Solids. J. Heat Transfer 1989, 111, 719725. Bains, M. S. A Thermochemical Approach to Flame Retardation for Cellulosic Materials. Carbohydr. Res. 1974, 34, 169-173. Bamford, C. H.; Crank, J.; Malan, D. H. The Combustion of Wood. Part I. Proc. Cambridge Philos. Soc. 1946, 42, 166-182. Bennini, S.; Castillo, S.; Traverse, J. P. Effects of an Intense Thermal Flux on a Lignocellulosic Material. J. Anal. Appl. Pyrolysis 1991, 21, 305-314. Bradbury, A.; Sakai, Y.; Shafizadeh, F. A Kinetic Model for Pyrolysis of Cellulose. J. Appl. Polym. Sci. 1979, 23, 3271-3280. Broido, A. Thermogravimetric and Differential Thermal Analysis of KHCO3 Contaminated Cellulose. Pyrodynamics 1966, 4, 243-251. Broido, A.; Weinstein, M. Thermogravimetric Analysis of Ammonia-Swelled Cellulose. Combust. Sci. Technol. 1970, 1, 279285 and 299-304. Broido, A.; Nelson, M. A. Char Yield in Pyrolysis of Cellulose. Combust. Flame 1975, 24, 263-268.

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Received for review July 14, 1995 Revised manuscript received October 25, 1995 Accepted November 14, 1995X IE950438L

X Abstract published in Advance ACS Abstracts, January 15, 1996.

Thermal Effects in Cellulose Pyrolysis ... - ACS Publications

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