Influence of Macrocomponents on the Pyrolysis Heat Demand of

May 15, 2017 - The present study investigated the influence of pressure on the thermal effects associated with the pyrolysis of biomass macrocomponent...
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Influence of Macrocomponents on the Pyrolysis Heat Demand of Lignocellulosic Biomass Lucia Basile, Alessandro Tugnoli, and Valerio Cozzani* LISESDipartimento di Ingegneria Civile, Chimica, Ambientale e dei Materiali, Alma Mater Studiorum Università di Bologna, via Terracini 28, 40131 Bologna, Italy S Supporting Information *

ABSTRACT: The present study investigated the influence of pressure on the thermal effects associated with the pyrolysis of biomass macrocomponents: cellulose, hemicellulose, and lignin. The heat demand of the pyrolysis process was measured by differential scanning calorimetry at pressures ranging from 0.1 to 4 MPa, using a specifically developed experimental configuration. An increase of the operating pressure affected both the heat demand and the final char yield. A summative model to describe the overall heat demand of the pyrolysis process was tested, and the results were compared to those obtained for an actual biomass sample and a mixture of cellulose, xylan, and lignin (synthetic biomass). Both samples showed important differences in the thermal demand with respect to that obtained from the summative model, possibly because of interactions among volatiles generated in the pyrolysis process and differences in the amounts of ash and extractives present in the samples.

1. INTRODUCTION With the inexorable growth of global population and energy consumption, lignocellulosic biomass has taken a central role in the global energy strategy by providing sustainable sources for both liquid fuels and chemicals without competing with food and feed supplies. Wood pyrolysis can be assumed as a simultaneous pyrolysis of its three main constituents: namely, cellulose, lignin, and hemicellulose. Lignin may be described as a group of highmolecular-weight, amorphous, chemically related compounds. Cellulose is a long-chain glucose polymer. Hemicellulose is a heterogeneous branched polysaccharide that binds tightly to the surface of each cellulose microfibril.1−3 To establish a large-scale reliable and economic lignocellulosic industry, biomass recalcitrance must be well understood and overcome. The decomposition of wood and its constituents takes place through competitive and parallel reactions. Comparisons of the thermal decomposition and devolatilization of wood with its major constituents have been the subject of a number of earlier publications.4−9 To date, several studies on the pyrolytic decomposition of the biomass main components were carried out.4 However, the wood thermal effects during its pyrolysis have rarely been investigated with the aim of correlating them to the thermal effects of its main constituents. Both heat transfer and chemical reactions vary with the biomass composition because the three polymeric constituents have different internal energies as well as heating values.10 Therefore, to clarify the origin of the wood thermal behavior, it is interesting to study the thermal behavior of wood “building © 2017 American Chemical Society

blocks”. This is a key step of fundamental importance to understand the thermal conversion of a biomass feedstock during pyrolysis. The role of the reaction heat is important in practical biomass conversion systems, which are highly influenced by the interaction of heat- and mass-transfer phenomena with chemical reactions. Actually, the thermal effects may be of utmost importance in processes requiring accurate control of the operating temperature to obtain specific products or properties, such as in catalytic pyrolysis or in torrefaction.11 Pyrolysis is also the first step in combustion and gasification processes, where it is followed by total or partial oxidation of the primary products. For these reasons, the heat of biomass pyrolysis is of interest to improve the design and achieve optimization of processes involving biomass to energy conversion. Strezov et al.12 measured the heat of pyrolysis of cellulose, hemicellulose, lignin, and four different sawdust biomass samples in an IR furnace. They reported that all of the samples showed both endothermic and exothermic reactions during pyrolysis. The decomposition of hemicellulose was completed at 320 °C, a considerably lower temperature with respect to cellulose and lignin. Hemicellulose and lignin exhibited two exothermic peaks, while cellulose showed a strong endothermic Received: Revised: Accepted: Published: 6432

February 9, 2017 May 13, 2017 May 14, 2017 May 15, 2017 DOI: 10.1021/acs.iecr.7b00559 Ind. Eng. Chem. Res. 2017, 56, 6432−6440

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Industrial & Engineering Chemistry Research peak at 330 °C, followed by an exothermic one at 370 °C. The decomposition of cellulose was completed at 400 °C, while in case of lignin, the process was completed at 470 °C. It should also be noted that in the literature13−15 the overall lignin and cellulose thermal degradation processes are considered to be respectively exothermic and endothermic. The exothermic decomposition of lignin is presumed to be responsible for the exothermic thermal behavior in the final stage of wood pyrolysis. For hemicellulose decomposition, both exothermic and endothermic values of the enthalpy of pyrolysis are reported.12,16−18 This can be due to the fact that hemicellulose is present in different forms in real biomasses, and it can be modified during the extraction process.1,2 Moreover, hemicellulose is a heterogeneous group of polysaccharides whose composition differs between biomass species, affecting the thermal behavior.18 Furthermore, it has to be mentioned that minerals could influence the thermal behavior of extracted hemicellulose, whose isolation techniques result in a significant amount of mineral residues.19 Scarce experimental data are available concerning the variation with pressure of the thermal effects of the pyrolysis process of wood constituents. The implementation of highpressure conversion processes for biomass pyrolysis and gasification may have a number of potential advantages, such as reduced reactor sizes, increased reaction rates, a higher throughput, and lower compression costs of product gases.20,21 The assessment of the thermal behavior of the pyrolysis process under pressure is thus of high interest for the development of such technologies. Mok and Antal22 report that, in experimental runs where the pyrolysis of cellulose was carried out, when the operating pressure was increased, the overall heat demand decreased and the yields of CO2 and CO, of all hydrocarbons formed in the process, and of char were reduced. Several other studies evidence a relationship among secondary reactions of volatiles and the overall heat demand of the pyrolysis process even at atmospheric pressure.5,23−27 To our knowledge, there are limited studies about the heat of pyrolysis of hemicellulose and lignin, and no results are reported in the open literature concerning the influence of pressure on the lignin and hemicellulose heat of pyrolysis. The aim of the present study is to explore the influence of pressure on the heat of pyrolysis of wood constituents, addressing slow pyrolysis conditions. The interactions among the constituents during the biomass pyrolysis were investigated. A synthetic biomass was prepared by mixing each of the individual biomass macrocomponents. The heat of pyrolysis of the synthetic biomass was analyzed and compared with that of the real biomass having the same composition. High-pressure differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to assess the effects of pressure on the overall heat demand of the pyrolysis process of the biomass main components, also accounting for the influence of limitations in the mass transfer of the volatiles from the decomposing solid to the gas phase.

was used as a reference biomass. The composition of cellulose, hemicellulose, and lignin in poplar has been experimentally determined, based on the analysis of thermogravimetric pyrolysis data according to the method developed by Cozzani et al.,32 and it is presented in Table 1. The table reports the Table 1. Analysis and Composition Dry Basis of the Biomass Materials Used in This Study properties volatiles fixed carbon ash C H N Oa cellulose hemicellulose lignin a

cellulose

xylan

Proximate Analysis 94.8 73.9 4.9 21.8 0.3 4.2 Ultimate Analysis 42.2 43.3 6.3 6.1 51.5

50.6 Composition

lignin

poplar

55.7 40.5 3.7

81.8 14.8 3.4

63.4 5.8 0.7 30.1

47.8 5.9 0.5 45.8

100

48 22 30

100 100

By difference.

results of proximate and ultimate analysis of the biomass macrocomponents and the compositions and results of proximate and ultimate analysis of the reference biomass. It can be noticed from proximate analysis that cellulose has the highest content in volatiles and the lowest content in ash, while lignin has the highest content in fixed carbon, as confirmed by the high carbon content (63.4%). Experimental runs were carried out also on a mixture of cellulose, lignin, and xylan (respectively 48, 30, and 22 wt %), having the same composition of poplar in terms of macrocomponents and referred to as synthetic poplar in the following. 2.2. Techniques and Procedures. Experimental runs were carried out using TGA and DSC. Details on the experimental devices and procedures, calibration methods,21,22 and specific features of the experimental device allowing DSC runs at high pressure33−35 are reported in the Supporting Information.

3. RESULTS AND DISCUSSION 3.1. Baseline Data at Atmospheric Pressure. Data at atmospheric pressure were first collected and validated by a comparison to the previous results reported in the literature, to obtain a set of baseline data, allowing a more effective analysis of the effect of pressure. Figure 1a compares the curves obtained for the thermal effects of the three wood building block samples considered during heatup in pure nitrogen at 10 °C/min at atmospheric pressure. As is evident from the figure, cellulose behaves differently from xylan and lignin: at atmospheric pressure, the former shows endothermic effects, while the latter two samples show exothermic effects. Such findings are in agreement with the previous results reported in the literature. Mok and Antal22 in a DSC study of cellulose and levoglucosan pyrolysis established that the formation of char and gas is an exothermic process, whereas levoglucosan (tar) formation and evaporation are endothermic processes. The role of exothermic char formation is counterbalanced by an endothermic heat of volatiles release.11,24 Similarly, Ball et al.36 pointed out that the

2. EXPERIMENTAL SECTION 2.1. Materials. Commercial samples of hemicellulose, cellulose, and lignin were used in the present study. Details on the origin of the samples and a short discussion concerning the representativeness of such samples7,28−31 are reported in the Supporting Information. Poplar supplied by the Department of Agro-Environmental Science and Technology of the University of Bologna (Italy) 6433

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Figure 1. Heat flow for cellulose, xylan, and lignin at different pressures: (a) cellulose, lignin, and xylan at 0.1 MPa; (b) cellulose at different pressures (max std. dev. = 5); (c) xylan at different pressures (max std. dev. = 15); (d) lignin at different pressures (max std. dev. = 22). Conditions: pure nitrogen, 10 °C/min, crucibles without lids, and initial sample weight = 7.5 mg.

charring process is highly exothermal, whereas volatilization is endothermal. Table 2 summarizes the results in terms of the heat demand and char yield at atmospheric pressure as a function of the sample weight during the pyrolysis of cellulose, xylan, and lignin. The approach of Basile et al.33 was used to obtain quantitative data for the overall heat demand of the process from the experimental DSC curves. As is evident from the table, the values of heat demand during cellulose pyrolysis obtained in the present study, ranging between 345 and 595 J/g, are in good accordance with those reported in the literature. Milosavljevic et al.5 reported a heat demand of 538 kJ/kg of volatiles evolved in conditions where the formation of char was limited. Stenseng et al.37 measured the heat of pyrolysis for Sigma cellulose to be 450 J/g. A similar value is reported by Fisher et al.38 The heat of pyrolysis obtained in the present study varies from −240 to −265 J/g for xylan and from −221 to −489 J/g for lignin. Bilbao et al.13 found an exothermic heat of lignin pyrolysis of −353 J/g. The exothermic character of the lignin relies on the liberation of hydrocarbons and of phenolic and neutral oils.12 The influence of the sample weight evidenced in Table 2 is in accordance with the findings of Stensteng et al.37 on Avicell cellulose, evidencing that small samples show the largest heat of reaction. For xylan and lignin,

Table 2. Heat Demand and Char Yield up to 550°C of the Pyrolysis Process of the Biomass Macrocomponents Resulting from DSC Runs in Pure Nitrogena biomass cellulose

xylan

lignin

initial weight (mg)

char yield (% daf)

measured heat (J/g daf)

3.5 8.0 10.0 17.0 3.5 9.5 14.0 16.5 3.8 8.0 10.0 17.5

4.4 5.7 6.8 9.5 14.0 17.6 19.1 17.6 43.7 48.3 48.5 49.3

595 444 375 345 −240 −240 −250 −265 −489 −355 −282 −221

Conditions: 110−550 °C, 10 °C/min, 0.1 MPa operating pressure, crucibles without lids, and initial sample weight = 3−18 mg. Negative values of the heat demand indicate exothermic behavior of the process.

a

a slight increase in the heat of reaction with increasing sample mass was recorded. An additional set of atmospheric DSC runs was carried out using pierced lids to cover the DSC crucible, in order to obtain 6434

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Industrial & Engineering Chemistry Research different char yields in the pyrolysis process.33,39,40 Heat-flow curves are highly influenced by the lids on the crucibles.34,40 In the case of experimental runs without lids, the partial pressure of the volatiles around the primary char surface is lower, and the more refractory components may decompose. Thus, the pyrolysis process would end at higher temperatures, and lower char yields would be obtained.41 As shown in Table S1, the char yield from cellulose pyrolysis in crucibles with lids is 18.9% with respect to 5.7% in crucibles without lids. This is in well accordance with the findings of Mok and Antal42 and Varhegyi et al.,34 confirming that the char formation from cellulose can be increased remarkably by pyrolysis within a sealed reactor. For xylan and lignin, the same trend is observed. 3.2. Effect of the Pressure on the Heat Demand of Wood Macrocomponents. The influence of the pressure on the macrocomponent pyrolysis is evident in Figure 1, which shows the effect of increasing pressure on each of the biomass macrocomponents. In the case of cellulose, the heat demand of the pyrolysis process decreases with increasing pressure (Figure 1b). As is evident from the figure, the observed behavior of cellulose is very peculiar. At atmospheric pressure, cellulose pyrolysis is completely endothermic, but at 0.5 MPa, it has a shift at 351 °C from endothermic to exothermic behavior of the pyrolysis phenomenon. At 4 MPa, the shift occurs in the early stage of the pyrolysis process at 327 °C, resulting in a small endothermic peak, followed by a major exothermic peak. As is evident from Table 3, the shifting temperature results are Table 3. Shifting Temperature of the Cellulose Pyrolysis Process Analyzed at Different Operating Pressures (Initial Sample Weight = ∼7.5 mg) pressure (MPa)

temp shift endo−exo (°C)

0.1 0.5 1

351 336

pressure (MPa)

temp shift endo−exo (°C)

2 4

329 327

dependent on the operating pressure, showing the presence of two separate reaction steps: with the first endothermic peak corresponding to the main decomposition of cellulose into volatiles and the second exothermic peak due to char formation reactions.22 The operating pressure has on xylan the same effect as that observed on cellulose, although to a lesser extent. An increasing exothermicity of the pyrolysis process, denoted by an accentuation of the exothermic peak, is observed (Figure 1c). Lignin exhibits a different behavior: pressure has no significant effect on the heat flow (Figure 1d). In Figure 2, the values of char yield (Figure 2a) and overall heat demand (Figure 2b) were reported as a function of the pressure for all three biomass macrocomponents. As shown in Figure 2b, with increasing pressure, a clear decrease in the heat demand of the pyrolysis process can be observed for cellulose and slightly for xylan. When the pressure is increased from 0.1 to 4 MPa, the heat of pyrolysis shifts from +504 to −120 J/g for cellulose and from −240 to −403 J/g for xylan.22 Mok and Antal22 showed that increasing pressure causes the total heat of cellulose pyrolysis to shift from values near 230 J/g at 0.1 MPa to values near −130 J/g at 2.5 MPa. The pyrolysis of hemicellulose is also affected by an increase in the pressure. A broad exothermic peak within 195 and 235 °C was observed by

Figure 2. Dependency of the heat of reaction and char yield with pressure: (a) final char yield as a function of the operating pressure obtained from DSC data for cellulose, xylan, and lignin (max exp. error = 2%); (b) overall heat demand as a function of the operating pressure obtained from DSC data for cellulose, xylan, and lignin (max exp. error = 6%); (c) heat of pyrolysis curves calculated by eq 1 using the best-fit parameters reported in Table S2 for cellulose and xylan compared to corn stalks, poplar, switchgrass Alamo, and switchgrass trailblazer studied elsewhere.39 Conditions: pure nitrogen, 10 °C/min, and crucibles without lids.

Mok and Antal during the pyrolysis of a xylan extract in a flowing inert gas at 0.1 MPa. Experimental values of the lignin heat of pyrolysis range between −343 and −471 J/g, without any clear trend with respect to the pressure. This is, in part, coherent with the study of Mok and Antal, where increasing pressure at low flow rates was also shown to cause lignin to undergo exothermic pyrolysis with a small increase in yield, although with very limited effects.22 6435

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Figure 3. Heat demand with respect to the final char yield: (a) obtained in the pyrolysis of cellulose, xylan, and lignin in comparison with cellulose by Mok and Antal;22 (b) obtained for corn stalks, poplar, switchgrass Alamo, switchgrass Trailblazer, cellulose, xylan, and lignin. Conditions: pure nitrogen, 10 °C/min, pressure = 0.1−4 MPa, crucibles with and without lids, initial sample weight = 3−14 mg, and purge gas flow rate = 5−50 N mL/min.

char yield increases from 5.7% to 27.6% for cellulose, from 17.6% to 29.8% for xylan, and from 48.7% to 56.0% for lignin. It can be argued that cellulose exhibits the greatest effect because it contains the higher content in volatiles: volatiles evolving during pyrolysis react with char and influence the thermal behavior to a great extent. Cellulose is generally considered to be a “charring solid”, meaning that during pyrolysis a solid, highly carbonaceous residue is usually a significant product. On the other hand, lignin undergoes pyrolysis to form much higher yields of carbon (char) than either pure cellulose or isolated hemicellulose. The enrichment in carbon relative to the carbohydrate components is, at least in part, the reason for the high yield of carbon from lignin. The formation of polyhydric nonvolatile phenols by saponification of ether bonds between the lignin fragments and the decrease of the temperature needed for decomposition of lignin are both promoted by water vapor at high pressure.41 Figure 3a shows values of the overall heat demand of the pyrolysis process obtained for the different macrocomponents under various pyrolysis conditions, leading to char yields that are compared with the values obtained from Mok and Antal.19 There is good agreement between the data obtained in the present study and those of Mok and Antal concerning the linear decrease in the endothermic heat of pyrolysis as the char yield increases.22 Figure 3b displays the relationship that exists between the heat of reaction and the observed char yield for biomass components and lignocellulosic biomass samples studied in a previous study.33,40 The relationship is linear for cellulose and the biomass samples,5,11,42,44 for which the heat evolved by pyrolysis is proportional to the amount of charcoal produced by the primary and secondary carbonization reactions. The linear correlation between the char yield and heat of pyrolysis in the biomass has the same shape as that of the cellulose but is translated toward lower values because of its content of hemicellulose and lignin. For hemicellulose, the relationship is linear but the change in the char yield is very limited, whereas the values obtained for lignin do not fit any straight line. Lignin exhibits in general the most exothermic behavior and the highest char yield compared to the other biomass macrocomponents. The yield of char thus is confirmed as a main factor determining whether the overall pyrolysis process is endo- or exothermic. This is an important global feature to build into

Figure 2c reports the heat of pyrolysis as a function of the pressure for cellulose and xylan, as well as for four biomass samples analyzed in a previous study using the same experimental conditions.33 The shape of the curve for all of the samples is well fitted from an empirical relationship derived from the Langmuir adsorption model, as observed in a previous study33,39 HP = H1 + H2

αyv P 1 + αyv P

(1)

where H1 is the heat due to the primary degradation of biomass, H2 is the heat due to the volatile decomposition, yv is the molar fraction of the volatile pseudocomponent, α is the Langmuir adsorption constant, and P is the total pressure of the system. Table S2 reports the values calculated for the fitting parameters of the curve. It has to be remarked that the above remarks do not apply to lignin, for which the overall heat demand of the pyrolysis process is almost unaffected by pressure. A further important observation is that, in Figure 2c, all of the values of the overall heat demand of the pyrolysis process for the biomass samples considered fall between the curve of cellulose (which always shows a higher heat demand) and that of xylan (which always shows a lower heat demand). 3.3. Relationship of the Heat Demand with the Char Yield. As is already known from the literature,20−22,43 the operating pressure of the pyrolysis process also influences the yield in char. A tubular flow reactor embedded in a differential scanning calorimeter was used by Mok and Antal21 to investigate the effect of the operating pressure on cellulose pyrolysis. They reported an increase of the char yield from 12 to 22% when the operating pressure was increased from 0.1 to 2.5 MPa. An increase in the char yield from 6% at 0.1 MPa to 15% at 4 MPa was reported by Blackadder and Rensfelt20 from cellulose pyrolysis in a pressurized thermogravimetric analyzer. They also observed an increase from 21 to 28% of the char yield over the same pressure range during the investigation of wood pyrolysis. Richard and Antal43 reported an increase in the char yield for cellulose from 19 to 41% when the pressure was increased from 0.1 to 2.0 MPa. Data obtained from analysis of the DSC runs are reported in Figure 2b and confirm the literature data. Upon an increase in the operating pressure, a definite increase in the total yield of char can be observed. The 6436

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These results are in agreement with those of several previous studies, which suggests that the mechanism of wood pyrolysis can be approximated as a superposition of those of its macrocomponents. Antal48 proposed that a mathematical superposition of the components’ TGA curves should explain their interaction adequately. Predictions from these models correctly reproduce the experimental TGA curves during the pyrolysis of several biomasses.36,38,49 Cellulose, hemicellulose, and lignin are reported to undergo pyrolysis independently in wood samples, without relevant interactions.4,50 According to Ranzi et al.,46 because the components differ significantly in their decomposition behavior,28 it is possible to determine clear trends in the decomposition time, reaction enthalpy, gas yield, and composition of any biomass according to its composition expressed as cellulose, hemicellulose, and lignin. Hosoya et al.51 contradicted this assumption, demonstrating the existence of cellulose−hemicellulose and cellulose−lignin interactions in wood pyrolysis at gasification temperature. Significant interactions were observed in cellulose−lignin pyrolysis. Lignin inhibited thermal polymerization of levoglucosan formed from cellulose, enhanced the formation of low-molecular-weight products from cellulose with changes in the secondary degradation mechanisms of volatile species, and reduced the yield of char. Cellulose reduced the secondary char formation from lignin and enhanced the formation of some lignin-derived products including guaiacol, 4-methylguaiacol, and 4-vinylguaiacol. The interaction between cellulose and lignin significantly influenced the char and liquid yields, decreasing the char yield and increasing the liquid yield.51 Comparatively weak interactions were also observed in cellulose−hemicellulose pyrolysis.51,52 Their interaction weakly affects the liquid and gas yields, which are higher than expected, while the char yield is lower.29,51 It is therefore interesting, also on the basis of the results reported in Figure 2c, to understand whether the macrocomponent summative approach may be applied also to the heat demand of the pyrolysis process or whether interactions among macrocomponents inhibit this approach. The correlation proposed by Raveendran et al.4 for TGA data was thus modified to obtain an additive model to predict the heat demand during the pyrolysis process obtained in DSC runs:

any thermal model of the pyrolysis process. The role of exothermic char formation is counterbalanced by an endothermic heat of volatiles release. Several other studies evidence a relationship among secondary reactions of volatiles and the overall heat demand of the pyrolysis process even at atmospheric pressure.5,23−27,45 3.4. Interactions among Macrocomponents during Pyrolysis. Several correlations have been proposed to determine clear trends in the gas composition, decomposition time, and temperature of any biomass according to its composition of cellulose, hemicellulose, and lignin.46 Branca and Di Blasi developed a theoretical model for the biomass pyrolysis heat demand based on the reaction kinetics of macrocomponents.47 However, to our knowledge, no experimental study addressed the overall heat demand of the pyrolysis process of biomass with the aim of correlating it to the pyrolysis heat demand of its main constituents. In order to investigate whether the effects of the individual components of a biomass on the heat of pyrolysis are simply additive, a mixture of cellulose, xylan, and lignin has been prepared with the same composition as that of poplar. The mixture is named synthetic poplar in the following. Its thermal behavior has been compared to that of poplar. Figure 4 presents the derivative

Figure 4. Differential thermal gravimetric curves for poplar and synthetic poplar. Conditions: pure nitrogen, 10 °C/min, initial sample weight = 7.5 mg, pressure = 0.1 MPa, and crucibles without lids.

qwood ̇ (T ) = qcell ̇ (T ) %wtcell + qxylan ̇ (T ) %wt xylan

weight loss curves for synthetic poplar compared to that obtained for actual poplar wood. The curve of synthetic poplar matches quite well with the curve of poplar. TGA results show correspondence between the real biomass and the mixture of macrocomponents.

+qlignin ̇ (T ) %wtlignin

(2)

where % wtcell, % wtxylan, and % wtlignin are the contents of respectively cellulose, xylan, and lignin in poplar and q̇(T) is the

Figure 5. Heat flow for poplar, synthetic poplar, and calculated by eq 2 at 0.1 MPa (a) and 0.5 MPa (b). Conditions: pure nitrogen, 10 °C/min, crucibles without lids, and initial sample weight = 7.5 mg. 6437

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Industrial & Engineering Chemistry Research heat flow recorded in DSC runs at temperature T. The results obtained from the application of the model at two different operating pressures (0.1 and 0.5 MPa) are reported in Figure 5, where the experimental data recorded for poplar wood and synthetic poplar are also shown. The contents of cellulose, xylan, and lignin were assumed to be equal to 48%, 22%, and 30%, respectively. These values are those corresponding to the macrocomponents present in the poplar wood used in the present study and were also used to prepare the synthetic poplar wood sample (see section 2.2). Figure 5, on the one hand, clearly shows that the heat flow calculated by a summative approach strongly differs from those of both poplar and synthetic poplar at both operating pressures. Thus, important interactions among macrocomponents influencing the overall heat demand seem to be present during the pyrolysis process. The summative correlation between the three major biomass constituents and the selected lignocellulosic biomass samples is found to be inadequate to describe the overall heat demand of the pyrolysis process. However, on the other hand, the qualitative behavior of the heat demand of poplar and synthetic poplar as a function of the temperature during the pyrolysis process is quite similar, even if the quantitative values of the overall heat demand are different. As shown in Figure 5, the shape of the heat flow curve with respect to the temperature and the peak temperatures recorded for the two samples at both pressures are almost coincident. The difference between synthetic and native poplar can be explained by the fact that, for the native one, macrocomponents are localized into cell walls and more energy is needed to break the bonds in the native structure. Furthermore, the pressure inside the cell walls could be more important compared to the synthetic mixture. This is confirmed by Figure 6, where the heat demand for poplar and synthetic poplar is reported as a function of the char

from the different macrocomponents in the pyrolysis process may be present, influencing the overall heat demand of the process. This assumption is coherent with both the limited influence of interactions on mass loss during pyrolysis, evidenced in a number of studies reporting mostly TGA results, and the findings of several previous studies evidencing the importance of secondary interactions on the overall heat demand of the pyrolysis process. Moreover, it has to be specified that lignin (especially extracted lignin) is almost viscous at low temperature,53 and it could spread and create a binder between macrocomponents, changing the heat demand during pyrolysis.

4. CONCLUSIONS The overall heat demand of the pyrolysis process of cellulose, hemicellulose, and lignin was investigated as a function of the operating pressure. The influence of the pressure on the heat demand and on the char yield was evidenced, as well as the correlation of the latter parameters. The heat demand of the different biomass samples was shown to be, in general, comprised between that of cellulose and those of lignin and hemicellulose. However, the results of a summative model applied to describe the overall biomass pyrolysis heat demand as a function of the biomass constituents evidenced that the heat of pyrolysis cannot be predicted on the basis of the individual behaviors of macrocomponents. Considering the experimental results obtained for the synthetic biomass samples and the obtained mixing of the macrocomponent materials in the same proportions as those in the actual biomass, at least a qualitative correspondence of the heat requirement was obtained. Thus, relevant interactions among generated volatiles and ash and extractives seem to play a nonnegligible role in determining the overall heat demand of biomass during pyrolysis.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b00559. Details on the materials used in the experimental runs; devices, and procedures; heat demand and char yield up to 550 °C of the pyrolysis process of the biomass macrocomponents resulting from DSC runs using pure nitrogen (Table S1); and best-fit parameters for eq 2 calculated from experimental DSC runs (Table S2) (PDF)

■

Figure 6. Heat demand with respect to the final char yield of cellulose, xylan, and lignin in comparison with poplar and synthetic poplar. Conditions: pure nitrogen, 10 °C/min, pressure = 0.1−4 MPa, crucibles with and without lids, initial sample weight = 3−14 mg, and purge gas flow rate = 5−50 N mL/min.

AUTHOR INFORMATION

Corresponding Author

*Tel.: (+39)-051-2090240. Fax: (+39)-051-2090247. E-mail: [email protected]. ORCID

Valerio Cozzani: 0000-0003-4680-535X

yield. The figure shows that the trend is almost coincident for the two samples. From a chemical point of view, the difference between poplar and synthetic poplar is the presence of ash and extractives in the real biomass. Thus, quantitative differences in the heat flow suggest that the ash present in biomass has an important influence on the thermal behavior. Nevertheless, the different behaviors among the summative model and the synthetic poplar (where ashes and extractives are not present) suggest that secondary interactions among volatiles generated

Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Ebringerová, A.; Heinze, T. Xylan and Xylan Derivatives Biopolymers with Valuable Properties, 1: Naturally Occurring Xylans Structures, Isolation Procedures and Properties. Macromol. Rapid Commun. 2000, 21, 542.

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DOI: 10.1021/acs.iecr.7b00559 Ind. Eng. Chem. Res. 2017, 56, 6432−6440

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DOI: 10.1021/acs.iecr.7b00559 Ind. Eng. Chem. Res. 2017, 56, 6432−6440

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NOTE ADDED AFTER ASAP PUBLICATION The version of this paper that was published ASAP on May 23, 2017, contained an incorrect graphic for Figure 6. The corrected version was reposted May 24, 2017.

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DOI: 10.1021/acs.iecr.7b00559 Ind. Eng. Chem. Res. 2017, 56, 6432−6440