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Ind. Eng. Chem. Res. 2006, 45, 4486-4493
Devolatilization of Biomass Fuels and Biomass Components Studied by TG/FTIR Technique Enrico Biagini,* Federica Barontini, and Leonardo Tognotti Dipartimento di Ingegneria Chimica, Chimica Industriale e Scienza dei Materiali, UniVersita` di Pisa, Via DiotisalVi 2, 56125 Pisa, Italy
Biomass fuels represent a renewable energy source, they are CO2 neutral fuels, and their use reduces the consumption of fossil fuels and limits the emissions of SOx, NOx, and heavy metals. They are used in pyrolysis, gasification, combustion, and co-combustion. The devolatilization is a fundamental mechanism in all these processes, especially for high volatile matter fuels. In this work, the devolatilization of biomass fuels (of different origin, properties, and composition) and biomass components is studied coupling thermogravimetric (TG) analysis with infrared spectroscopy. The characteristic temperatures are determined for the main devolatilization steps and compared for all fuels. A bituminous coal and a paper sludge are also studied for comparison. Light gases released (CO, CO2, H2O, CH4, CH3OH, HCOOH) are detected, whereas more complex organic (hydrocarbon and oxygenated) compounds are grouped because of the large variety of volatile species released in a narrow range of temperature. The weight loss of biomass fuels is related to their chemical composition (i.e., considering the devolatilization behavior of cellulose, hemicellulose (xylan), and lignin in the same operating conditions). The aim of the work is to apply a summative law for the TG results (validated in previous experimental and literature works) to obtain the chemical composition of biomass fuels and to validate and extend a summative law for the FTIR profiles of volatile species released. Calculated values obtained using this method are in good agreement with the experimental results. Therefore, the validation of this correlation allows the prediction of the devolatilization of biomass fuels considering the initial chemical composition. This is useful for practical applications, plant designing, handling, and modeling. Introduction Alternative and renewable energy is of current importance in satisfying environmental concerns over fossil fuels. Biomass fuels are CO2 neutral fuels, and this is one of the most important features of such fuels. In fact, the amount of CO2 produced during the combustion of these fuels is the same amount absorbed during the growth of the plants. This is particularly the case of energy crops, agricultural residues, and alimentary wastes, which are produced with continuity. The use of such materials as fuels limits the consumption of fossil fuels and reduces the CO2 in the atmosphere. The latter point is a crucial objective of international treaties because of the rising concern about the greenhouse effect. Biomass is a low-cost fuel today only when available as waste or byproduct of a higher-value activity or product. The higher costs related to the fuel and its utilization are only partially recovered by the benefits, which can even include credits for reduced CO2, SOx, and heavy metal emissions. The direct combustion of biomass fuels is problematic and scarcely applied, especially because of technological (low heating value, flame instability due the high reactivity and volatile matter content, fouling, slagging, and corrosion phenomena) and economical problems.1 Other processes usually find a practical application, namely, co-combustion, pyrolysis, and gasification.2-6 In all cases, a fundamental characterization is required for these fuels, which exhibit very different properties with respect to traditional fossil fuels. In particular, lignin-cellulosic materials are more reactive and have a higher volatile matter content than coals. Devolatilization is the first step in all thermal processes and produces a large amount and variety of volatile species and a solid residue (char). * Corresponding author. Fax: +39050511266. E-mail: e.biagini@ ing.unipi.it.
The behavior of biomass materials during devolatilization has often been referred to the behavior of chemical components (cellulose, hemicellulose, and lignin), and this was validated with thermogravimetric (TG) analysis.7,8 Experimental investigations revealed no interaction among components in biomass materials as well as in composite materials (e.g., municipal solid wastes) and fuel blends for co-combustion processes.9-12 The mineral matter present in the fuels influences devolatilization behavior, and this may cause some differences in the behavior of natural biomasses and synthetic fuels (obtained as a blend of cellulose, hemicellulose, and lignin).13 Although a large number of studies have been carried out to evaluate the effect of mineral matter on biomass pyrolysis, a detailed understanding has not been obtained yet.14 TG analysis coupled with Fourier transformed infrared (FTIR) spectroscopy has been applied to study the compounds evolving in the pyrolysis of biomass fuels.15-17 It provides important information on the devolatilization of materials, that is, the identification of major volatile species and the typical temperature range of release, with a continuous measurement (unlike gas chromatography, which requires sampling and allows a discontinuous analysis). On the other hand, homodiatomic species (H2, N2, O2) cannot be measured using FTIR spectrometry, and this should be taken into account to perform a global balance of volatile matter released. The quantification of volatile species is fundamental to establish the quality of the gas in energetic terms in the gasification process. The individuation of the optimal range of temperature for the release of a specific volatile compound allows for the optimization of a pyrolysis process with the aim of chemical recovery. Also, a quantitative analysis can be carried out to provide kinetics for each volatile species and basic parameters for design and modeling purposes (combustion and gasification processes).
10.1021/ie0514049 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/24/2006
Ind. Eng. Chem. Res., Vol. 45, No. 13, 2006 4487 Table 1. Ultimate and Proximate Analyses and Heating Value of Fuels
fuel pine wood wood pellets olive residue hazelnut shells paper sludge Kema coal
ultimate analysis (% daf) C H N S 53 49.4 51.2 51.0 24.3 71.4
6.0 6.1 6.7 5.4 3.4 4.5
0.2 1.0 0.8 1.3 0.5 1.1
0.08 0.7 0.05 0.01 0.8
proximate analysis (% dry) VM FC ash 80.6 76.9 78.4 78.5 50.0 30.4
17.7 20.8 20.4 20.2 3.0 55.8
1.7 2.3 1.2 1.3 47.0 13.8
HV (MJ/kg) 18.1 19 20.1 nd 5.1 28.7
In this work, the results of TG-FTIR analysis of lignincellulosic materials (of different origin, properties, and composition) are studied, the main volatile compounds are identified, and the main devolatilization steps are characterized. A bituminous coal and a paper sludge are also studied for comparison. Light gases (CO2, CO, H2O, CH4, CH3OH, HCOOH) are easily detected, even though the temperature range of the main devolatilization is quite narrow and several organic species are released simultaneously. More complex organic compounds are hardly distinguished but can be grouped in classes of organic species (hydrocarbons and oxygenated compounds). A quantitative comparison is made with results obtained on the chemical components. A simple correlation that allows the prediction of the devolatilization of biomass fuels considering the initial chemical composition is desirable for practical applications, plant designing, handling, and modeling. The aim of the work is to apply a weighted sum law for the TG results (obtaining the fractions of cellulose, hemicellulose, and lignin, which are supposed to devolatilize independently from each other in the initial biomass) and to validate and extend a weighted sum law for the FTIR profiles of volatile species released. Experimental Section Materials. Four biomass fuels of different origin are studied in this work, namely, pine wood (from pruning in the parks), wood pellets (residues of the wood industry), and olive stones and hazelnut shells (residues of the food industry). These materials have similar ultimate and proximate analyses, as listed in Table 1. A paper sludge, residue of the paper industry, is studied because of the large availability on specific geographic sites and the possibility of recovering energy from this waste. The very high moisture and ash content of this material compromises its heating value, making the direct combustion prohibitive. Nevertheless, the lignin fraction of this material is significant, and the comparison of the devolatilization behavior with the other biomass fuels is interesting. A high volatile bituminous coal (Kema coal) is also studied for comparison. Finally, commercial chemical components are studied, namely, cellulose Avicel, lignin from olive residue, and xylan from oat spelts, representative of the hemicellulose component. Each material is crushed and sieved, and the 0.125-0.300 mm fraction is studied in this work. The sample is dried in a ventilated oven at 105 °C before every experiment. Equipment. Thermogravimetric data are obtained using a Netzsch STA 409/C thermoanalyzer. A constant heating rate of 20 °C/min is used in all experimental runs, from 105 to 1000 °C. Typical sample weights of 15 mg are studied. Experimental runs are carried out using a purge gas flow (60 mL/min) of pure nitrogen. FTIR measurements are carried out using a Bruker Equinox 55 spectrometer. TG-FTIR simultaneous measurements for the on-line analysis of volatile compounds formed during TG runs are carried out
Table 2. Devolatilization Characteristics fuel
Ton a (°C)
Tmax b (°C)
Tpr c (°C)
VMpr d (%)
pine wood wood pellets olive residue hazelnut shells cellulose xylan lignin paper sludge Kema coal
261 267 250 264 319 253 259 284 397
371 366 344 363 354 299 362 372 475
392 391 372 385 368 308 482 403 528
83.2 79.5 72.0 76.7 79.6 76.3 74.7 42.1 57.7
a Onset temperature, correspondent to a weight loss of 5% respect to the final weight loss. b Temperature for the maximum rate of devolatilization. c Final temperature of primary devolatilization, defined as the temperature correspondent to the intersection of the tangent lines to the dtg curve in the primary and secondary devolatilization. d Volatile matter released during the primary devolatilization, calculated as percentage of the final volatile matter released at 1000 °C.
coupling the FTIR spectrometer to the TG balance. The transfer line and the head of the TG balance are heated at a constant temperature of 200 °C to limit the condensation of volatile decomposition products. FTIR measurements are carried out in a specifically developed gas cell heated at a constant temperature of 250 °C. Further details can be found elsewhere.18,19 Most runs are repeated three times to verify the reproducibility of the analyses. Variations in TG results are negligible. IR profiles are obtained with maximum variations of 6% with respect to the average values reported in the following section. Results TG Results. The weight loss and derivative (dtg) curves of biomass fuels and biomass components are shown in Figure 1. The characteristic parameters of devolatilization, defined and commented on in this section, are listed in Table 2. The devolatilization behavior is very similar for all lignin-cellulosic materials studied. The onset temperature of devolatilization (correspondent to a weight loss of 5% respect to the final weight loss) is in the range of 250-270 °C for all biomass fuels studied. The onset temperature of paper sludge is slightly higher, whereas it is extremely higher for Kema coal (approximately 400 °C). The main weight loss ends at 370-400 °C for all biomass fuels and is followed by a slow and continuous weight loss. The former step is due to the primary devolatilization, whereas the latter is attributed to the degradation of heavier chemical structures in the solid matrix, which can also be produced during the previous thermal devolatilization. This is the reason some authors called this weight loss step “secondary thermolysis”.20 A final temperature of primary devolatilization can be defined as the temperature correspondent to the intersection of the tangent lines to the dtg curve in the primary and secondary devolatilization. Quantitatively, the volatile matter released in the primary devolatilization is more important than in the secondary devolatilization, accounting for more than 70% of the final volatile matter released at 1000 °C. A long tail of devolatilization is also observed in the weight loss of paper sludge and Kema coal. The former material actually shows a deeper step in the weight loss, at relatively high temperature (temperature for the maximum rate of devolatilization 764 °C). The narrow range of temperature denotes the decomposition of a specific component. Such temperature in these conditions is typical of the degradation of carbonates.10 In fact, calcium carbonate is a filler in the paper industry. The analysis of gas from the devolatilization of paper sludge will give a further confirmation of this supposition in the next subsection.
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Figure 1. Weight loss and dtg curves of biomass fuels and biomass components (W is the actual weight of the sample; W0 is the initial weight).
The primary devolatilization of biomass fuels shows a composite shape (as observed from the dtg curves in Figure 1). This is ascribed to the decomposition of the chemical components. TG results of cellulose, hemicellulose, and lignin are also compared in Figure 1. The different reactivity and volatile matter release determine the devolatilization of biomass materials. Xylan, which is representative of hemicelluloses, is the most reactive among all components, the onset temperature being 253 °C. The temperature range of devolatilization is very narrow, with the temperature for the maximum rate being 299 °C. The volatile released after the primary devolatilization is 76.3% of the final volatile matter released (at 1000 °C). Cellulose exhibits the maximum weight loss among all components, with the final value being 86.8%. The onset temperature is 319 °C; the temperature for the maximum rate of devolatilization is 354 °C. The volatile released after the primary step of devolatilization is 79.6% of the total. Lignin decomposes in a wider range of temperature than the other components. The onset temperature is comparable to xylan. However, the primary devolatilization can be considered complete only at 482 °C, with a barely defined slope change in the dtg curve. The maximum rate of weight loss occurs at 361 °C. The final weight loss is the lowest among the components (60.2%). FTIR Results. TG-FTIR analysis of biomass fuels and biomass components may be difficult because most volatile species are released in a very narrow range of temperature. In fact, the variety of compounds released makes the individuation of single species quite complicated. The separation of steps producing different chemical species is barely obtained, varying the thermal history of the sample, at least in the range of heating rate studied preliminarily (5-60 °C/min). The largest quantity and variety of gases are released during the primary devolatilization. This can be verified in Figure 2 for a biomass fuel (wood pellets) and all biomass components. Also the results for Kema coal and paper sludge are reported for comparison. In each graph, the derivative weight loss curve is compared to the IR profiles of some gaseous species released as function
of the temperature. The gas evolution profiles in TG-FTIR runs are obtained from the experimental data following the procedure described extensively elsewhere.21 The procedure uses an integral form of the Lambert-Beer relation. The integrated absorbance (IA, measured in cm-1) is the integral value of the spectral absorbance over a selected wavenumber interval characteristic for the compound of interest. The procedure requires that the compound of interest has a wavenumber absorption interval that is free of additional contributions from other substances. The simultaneous formation of a wide number of volatile compounds complicates the selection of such intervals. During the devolatilization of biomasses, light gases (namely, CO2, CO, H2O, CH4, CH3OH, and HCOOH) can be easily detected in the specific wavenumber intervals listed in Table 3. IA gives a quantitative measure of the gaseous species detected, being directly proportional to the concentration c of the chemical species in the gas released:
IA ) Kc
(1)
The proportional constant K differs from one compound to another, depending on the characteristics of the experimental apparatus and the operating conditions. Therefore, a calibration procedure in similar conditions is necessary to relate IA to the concentration of each gaseous species.19 More complex organic compounds are barely distinguished because of the enormous number of chemical compounds with similar IR spectra released in a narrow temporal range. The definition of most volatile species is beyond the aim of this work. As a matter of fact, it requires a deeper investigation and further analysis (e.g., sampling systems and gas chromatography).22 Complex compounds can be obtained from the material balance and classified as tar.14 However, a direct balance on volatile species may be inaccurate (for instance, homodiatomic species are not considered). In this work, the organic compounds released during the devolatilization are grouped in classes. Compounds with characteristic C-H bond (wavenumber interval 2645-3042 cm-1) are classified in the hydrocarbon group and called CHn
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Figure 2. Derivative weight loss curves and FTIR integrated absorbance profiles of main gaseous species evolved in the devolatilization of a biomass fuel, biomass components, paper sludge, and Kema coal. Table 3. Wavenunber Intervals of Different Gaseous Species for the Definition of the Integrated Absorbance gas species CO2 CO H2O CH4 HCOOH CH3OH CHn group CdO group
wavenumber interval (cm-1) 2240 2143 3792 3001 1101 1026 2645 1660
2400 2236 4025 3026 1111 1040 3042 1820
compounds in the following. Compounds with characteristic carbonyl bond typical of acids, esters, aldehydes, and ketones (wavenumber interval 1660-1820 cm-1) are called CdO compounds. The definition of the concentration is meaningless in these cases because they represent a family of different compounds, and a calibration procedure is inappropriate. Nevertheless, the integrated absorbance can give a quantitative measure of each group of organic compounds because it represents the sum of the contributions of all organic compounds. This approach allows the comparison of IA of the single volatile species released by different materials. In particular, every profile is normalized to the same initial amount of the sample (15 mg). However, profiles of different gases released by the devolatilization of a single material can be compared
only qualitatively, giving information on the temperature range and the evolution behavior for each compound. The release of CO2, CO, H2O, CHn, and CdO for wood pellets has the same behavior of the weight loss, that is, the largest amount can be observed during the primary devolatilization (with a correspondence between the maximum rate of weight loss and the maximum of IR profiles for most gas species) although a small, slow, and continuous release can be observed also in the secondary thermolysis step. Also profiles of gases released during the devolatilization of cellulose and xylan have a good accordance with the weight loss (Figure 2). Only the release of CO2 and CO during lignin devolatilization shows different evident steps during the secondary devolatilization. However, these peaks can explain the small shoulder in the dtg curve at temperatures just above the temperature for the maximum rate of devolatilization. The release of other light gases is reported in Figure 3, where the IR profiles of different fuels are compared. In particular, for wood pellets, CH3OH and HCOOH are released in the same temperature range of the primary devolatilization, with a composite shape of the profile. This is also the case of the other biomass fuels. Vice versa, CH4 exhibits a characteristic temperature range of release at higher temperature. Two sub-steps can be recognized in the composite shape of the CH4 profile of biomass materials. The first sub-maximum is in the range of 430-440
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Figure 3. FTIR integrated absorbance profiles of CH3OH, HCOOH, and CH4 for biomass fuels and biomass components.
°C, whereas the second is between 530 °C for olive residue and 557 °C for wood pellets, with a different predominance of the relative area. In all cases, the characteristic range is at very higher temperature than the maximum rate of devolatilization (345-368 °C). The shape of the IR profiles is determined by the decomposition of the chemical components contained in the biomasses. Lignin releases the largest quantity of CH3OH, at higher temperature than xylan. A negligible amount of CH3OH is detected during the thermolysis of cellulose. Vice versa, xylan releases the largest quantity of HCOOH, at a lower temperature than cellulose. A negligible amount of HCOOH is detected during the thermolysis of lignin. The different chemical structure and characteristic functional groups of the components determine the volatile species released. For instance, the production of CH3OH may be attributed to the decomposition of the methoxyl group.14 Lignin produces the largest quantity of CH4, cellulose produces the smallest quantity, and xylan produces an intermediate value. In all cases, the typical temperature range for the release of CH4 is shifted with respect to the primary devolatilization. A similar behavior for lignin decomposition was studied by Jensen et al.14 with characteristic temperatures of the IR profile of CH4 in good agreement with the results of this work. The main gaseous products of the primary devolatilization of Kema coal are hydrocarbons, released in the same temperature range of the main devolatilization step (Figure 2). No CdO species are detected. The lower oxygen content of Kema coal with respect to biomass fuels is released as CO2 and CO in a very wide range of temperature, comprising both primary and secondary devolatilization. The maximum rate of CH4 release is at 540 °C (against 476 °C for the maximum of the weight loss rate). This value is in the same range of biomass fuels. This leads to the supposition that the thermochemical mechanism producing CH4 is the same for both classes of fuels, at least from an energetic point of view. The FTIR analysis of gas released in the thermolysis of paper sludge gives results similar to biomass fuels in the primary and
secondary devolatilization. The accordance of FTIR profiles with the dtg curve motivates the first macro-step of weight loss followed by a long tail in the dtg curve. This is actually interrupted by a steep weight loss, starting at approximately 680 °C. Correspondingly, the FTIR spectrum analysis indicates that only CO2 is released, with profiles of other volatile species being negligible at this temperature. So that, the supposition announced above is verified, the macro-step in the weight loss of paper sludge at this temperature and in these conditions is due to the decomposition of a carbonate. The origin of the material suggests that it is calcium carbonate. The TG analysis of calcium carbonate in the same conditions confirmed this supposition.10 The importance of this analysis is in the characterization of the fuel for a practical use. The decomposition of a carbonate is endothermic and thus absorbs heat in the combustion system, decreasing the efficiency. Even so, it produces CO2, which makes the quality of the produced gas in the gasification process worse. On the other hand, it can capture sulfur when a sulfurcontaining fuel is used, thus lowering the SOx emissions of the process. Hence, this information is fundamental for the use of paper sludge in combustion, co-combustion, and gasification processes. Discussion The aim of this section is to verify the possibility to deduce the profile of a gas evolved during the devolatilization of a biomass fuel from the analysis of gases released by cellulose, hemicellulose, and lignin. The biomass material is supposed to devolatilize as the blend of a fraction xc of cellulose, xh of hemicellulose (xylan), and xl of lignin, respectively. The weight loss of the biomass material Wb is obtained considering the weighted sum law of the weight loss curve of components according to the following equation:
Wb ) Wcxc + Whxh + Wlxl
(2)
Basically, no interactions are supposed to exist during the devolatilization of chemical components, each one acting
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Figure 4. Comparison of the experimental weight loss curve and the weight loss curve obtained applying the weighted sum law (eq 2) for wood pellets and paper sludge. The curves for paper sludge are optimized up to 650 °C, thus excluding the carbonate decomposition. Symbols: experimental results; continuous line: calculated curve.
independently from the others through the different reactivity and nature of the gas released. All materials are considered dry and ash free. Although the moisture is indeed removed from the sample before every experimental run, the effect of ash is assumed to be negligible on the devolatilization. Finally, the behavior of extractives in the biomass materials is not discerned from that of hemicellulose.8 The chemical compositions listed in Table 4 are obtained fitting eq 2 with the experimental results on the lignin-cellulosic materials: xc, xh, and xl are adjusted to obtain the best Pearson coefficient (which is 1 for a perfect fitting). It is also reported in the table to appreciate the good agreement between the weight loss curve obtained experimentally on the biomass and that calculated using eq 2. The graphic comparison can be observed in Figure 4 for wood pellets and paper sludge. The former is qualitatively similar for all biomass fuels. In the latter case, the optimization of eq 1 is carried out excluding the carbonate decomposition step and, thus, verified up to 650 °C. Equation 2 is largely accepted and validated in experimental works.7-12 The chemical composition obtained using this method is in good agreement also with experimental analyses reported in the literature. For instance, the chemical composition of olive stones indicated by Lopez et al.23 is 11.8% of cellulose, 50.5% of lignin, and 29.7% of the sum of hemicellulose and extractive fractions. A quite higher content of cellulose is actually reported by Koufopanos et al.7 In general, big differences in the chemical composition of the same biomass material can be found by different researchers, for instance, as for the composition of hazelnut shells reported in two different works.7,24 These discrepancies are clearly imputed to the heterogeneous nature of biomass fuels, the different species, and the different fractions considered. Also the analytical methodology used to obtain the chemical composition can provide different results because there are several different methods.25,26 The application of eq 2 to TG results can be an alternative and suitable method to obtain the chemical composition of biomass fuels.13 In fact, it can be considered a simple, reproducible, and nonintrusive procedure. Once the chemical composition of the materials is obtained, a summative law can also be applied to the release of each gaseous species. In this case, the integrated absorbance IA substitutes the weight loss W:
IAb ) IAcxc + IAhxh + IAlxl
(3)
where IAi indicates the integrated absorbance of a specified gas released during the devolatilization of material i (indexes as defined for eq 2). The experimental IR profiles are compared to the calculated profiles considering the components acting independently. Some examples are reported in Figure 5 for different gases and different biomass fuels.
Table 4. Chemical Composition (% daf basis) of Biomass Fuels Obtained Using Equation 1a
pine wood wood pellets olive residue hazelnut shells paper sludge a
cellulose
xylan
lignin
R2
64.9 53.0 10.7 35.5 0
23.5 21.5 41.0 30.1 3.5
11.6 25.5 48.3 34.4 96.5
0.9983 0.9992 0.9993 0.9997 0.9995
R is the Pearson coefficient.
The release of CH3OH is in good agreement for all biomass fuels considered in this work (see Figure 5, the case of wood pellets and hazelnut shells). In particular, the humps in the IR profiles are attributed to the decomposition of xylan and lignin at low and high temperature, respectively. The correspondence of the characteristic temperature is accurate, whereas some discrepancy can be noticed for the amount of gas released by the xylan fraction. A qualitative good agreement is also verified for HCOOH profiles, as for the characteristic temperatures. As a matter of fact, the amount of gas released by cellulose and lignin allows a poorer fit of the experimental results with respect to the previous case (see Figure 5, the case of hazelnut shells). A very good agreement is instead observed for the profile of CH4 for all biomass fuels studied in this work (see the case of wood pellets in Figure 5). The fit is also good qualitatively for CO, CO2, and H2O, even though in some cases the characteristic temperatures of chemical components differ moderately from those of biomass fuels (see the profile of CO2 for wood pellets, those of CO and H2O for olive residue in Figure 5). A qualitative good fit of the experimental results are also obtained by calculating the FTIR profiles of hydrocarbon (CHn) and carbonyl (CdO) compounds (see the case of pine wood and wood pellets in Figure 5). Finally, the comparison of experimental and calculated curves for paper sludge gives a poorer agreement than that obtained for biomass fuels (see the profile of CO2 in Figure 5). This discrepancy can be ascribed to the predominant ash content of the material (47% on a dry basis), which is expected to produce significant effects on the devolatilization reactions and, thus, on the gaseous species formed. Summarizing, a quite good validation of eq 3 is obtained for FTIR profiles of light gases as well as groups of complex compounds evolved during the devolatilization of biomass fuels. Some discrepancies are noticed for IR signals, which are not completely free of additional contributions of other species (as in the case of HCOOH, which is partially overlapped by signals of other compounds evolving in the same temperature range). In fact, the best agreements are noticed for CH3OH, which
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Figure 5. Comparison of experimental (symbols) and calculated (continuous lines obtained using eq 3) FTIR profiles for different gases and biomass fuels.
exhibits a well-distinct IR signal, and CH4, which is released after the main devolatilization step.
A further reason of discrepancy can be attributed to the choice of the chemical components. Several xylans are used to represent
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the hemicellulose fraction (usually xylans from different wood), which can behave differently and give different amounts of volatile species. Even, extractive compounds, which are usually incorporated in the hemicellulose fraction as for the weight loss behavior,7 may release a different set of gaseous compounds with respect to the three components. Finally, natural biomasses have a higher ash content than synthetic components, influencing the devolatilization behavior and promoting catalytic reactions. So, the yields in volatile species may differ significantly. The quantification of the effect of mineral matter is beyond the aim of this work and should represent a deeper investigation to obtain a more accurate correlation for studying the devolatilization of biomass fuels. Conclusions The devolatilization of biomass fuels and biomass components has been investigated using TG-FTIR analysis. Also, a high volatile bituminous coal and a paper sludge have been studied for comparison. Two steps have been observed: a primary devolatilization, with the release of the largest amount and variety of volatile species, and a secondary thermolysis, with a slow and continuous weight loss. The main light gases released (CO, CO2, H2O, CH4, CH3OH, HCOOH) have been detected, and more complex organic compounds have been grouped (hydrocarbon and oxygenated). The characteristic temperature and parameters of devolatilization have been evaluated in all cases. IR profiles have been related to the weight loss. The weight loss of biomass fuels has been reproduced considering the chemical composition (i.e., considering the devolatilization behavior of cellulose, hemicellulose, and lignin in the same operating conditions). The summative law for TG results (validated in the literature works) has been applied to determine the chemical composition of biomass fuels, using a simple and nonintrusive method. The IR profiles of volatile species released have been obtained considering a weighted sum law, with good agreement of the experimental results and the calculated values. The validation of this correlation allows the prediction of the devolatilization of biomass fuels considering the initial chemical composition. This is useful for practical applications, plant designing, handling, and modeling. In conclusion, the method used in this work represents a valid approach to study the fundamental steps in thermal processes (combustion, gasification, and pyrolysis). FTIR analysis has a good potential to provide valuable information on volatile species. However, more work is needed to optimize the experimental technique, to better utilize the rich information obtained from this analysis, and to evaluate the effect of the operating conditions, the presence of mineral matter, and the gaseous reactants. This would provide more accurate correlations that would be useful for advanced investigations. Acknowledgment This work collects part of the experimental results and the analyses of the EU project BioFlam “Combustion Behaviour of “Clean” Fuels in Power Generation” (Project NNE5-199900449).
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ReceiVed for reView December 16, 2005 ReVised manuscript receiVed April 27, 2006 Accepted April 28, 2006 IE0514049
