Thermal Decomposition of Bagasse: Effect of Different Sugar Cane

Ind. Eng. Chem. Res. 2011, 50, 791–798

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Thermal Decomposition of Bagasse: Effect of Different Sugar Cane Cultivars Vanita R. Maliger,† William O. S. Doherty,*,† Ray L. Frost,‡ and Payam Mousavioun† Sugar Research & InnoVation, Centre for Tropical Crops and Biocommodities and Inorganic Materials Research Program, School of Physical & Chemical Sciences, Queensland UniVersity of Technology, GPO Box 2343, Brisbane, Australia

Sugar cane fiber (i.e., bagasse) is the residue from sugar cane milling during sugar manufacture. This study uses chemical analysis, thermogravimetry analysis (TG), derivative thermogravimetry (DTG), X-ray powder diffraction (XRD), and energy-dispersive spectroscopy to investigate the composition and thermal decomposition of bagasse from various origins. The results indicate that bagasse from different varieties of sugar cane have different proportions of carbohydrates, lignin, and ash contents and different degrees of crystallinity. TG thermograms show four distinct stages of mass losses instead of three stages reported for bagasse decomposition. This is due to the presence of residual sucrose. The thermal decomposition profile of bagasse is independent of origin, though minor differences exist in the temperatures at the maximum rate of weight losses for the hemicellulose and cellulose components of bagasse as well as on the residue yield. The main phases in ashes of the bagasse chars are quartz, acranite, and langbeinite, with slight shifts in the d values among the samples probably related to differences in the concentrations of inorganic ions in the crystal lattices. The results are further discussed in terms of the activation energy of the devolatilization process obtained using Friedman’s method. Introduction The collapse in world sugar prices together with a series of poor seasons and a strengthening Australian dollar have resulted in the Australian sugar industry making concerted efforts to develop new products derived from sugar cane. The residue from sugar cane milling (i.e., bagasse) is currently used to fuel the boilers of sugar factories. As there is excess bagasse in many sugar factories due to improved efficiencies of both factory processes and boilers, a comprehensive study is being undertaken by our group to develop economically viable technologies to produce biofuels, platform chemicals, and other value-added products by the thermochemical processing of bagasse. Devolatilization of biomass is a first and vital step in thermal chemical processes since the chemistry of the process is highly dependent, apart from temperature and pressure, on the biomass composition. Bagasse is a mixture of lignocellulose, inorganic matter (mainly insoluble, ‘ash’), water-soluble matter, and water. Dry bagasse consists of cellulose (32-48%), hemicellulose (23-32%), lignin (19-24%), and ash (1.5-5%).1 The differences in the composition of bagasse are due to cane variety and age, soil type, amount of fertilizer, and harvesting conditions.2 The combustion and devolatilization of bagasse have been studied by many workers.2-8 When biomass (e.g., bagasse) is heated in a gaseous environment the following processes take place: (a) drying, (b) devolatilization, and (c) combustion of volatiles and residual char.5,9 The devolatilization profile of biomass materials is dependent on the reactivities of the individual components: cellulose, hemicellulose, lignin, and ash.7,10-12 Bilba and Ouensanga7 using infrared analysis reported that structural modifications appeared at 200 °C and intensified between 300 and 400 °C. These changes were characterized by a decrease of hydroxy, C-O, and C-C peak intensities and formation of alkyl bonds. At intermediate temperatures (300-320 °C) dehydration of carbohydrates is favored, while at slightly * To whom correspondence should be addressed. Tel.: 61 7 3138 1245. Fax: 61 7 3138 4132. E-mail: [email protected]. † Centre for Tropical Crops and Biocommodities. ‡ School of Physical & Chemical Sciences.

higher temperatures (340-380 °C) dehydration of lignin is predominant. Further heat treatment leads to saturation of the aromatic rings, rupture of the C-C linkages present in lignin, CO and CO2 release, and rearrangement of carbohydrates and lignin structures. Luo and Stanmore8 using the kinetics of the mass loss for a first-order reaction to calculate the activation energy of the devolatilization process obtained a value of 88 kJ mol-1 in a nitrogen atmosphere, while Nassar et al.6 obtained a higher value of 128 kJ mol-1 in air. The higher value in air implies that bagasse is thermally more stable in an oxidizing atmosphere than in an inert one. However, Garcia-Perez et al.5 using Friedman’s method13 obtained higher values of 150-200 kJ mol-1 over a wide range of conversions. The values of Garcia-Perez et al. are of the same order of magnitude as other cellulosic materials and low-rank coals.5 The catalytic effect of ash and metal ions on biomass combustion has been reported in several publications.14-20 However, there is very little direct information on factors affecting the rate of bagasse combustion. The work by Nassar21 on two similar lignocellulosic materials, bagasse and rice straw, which differ only in silica content, showed differences in their activation energies for the thermal decomposition process. Nassar21 suggested that the lower activation energy obtained for rice straw was because of the significantly higher silica loading which may have acted as a flame retardant. Further work conducted by Nassar21 on the effect of inorganic constituents on the thermal decomposition of a lignin-carbohydrate complex confirmed that salts such as potassium bicarbonate and calcium chloride affected the decomposition profile of lignocellullosics. On the basis of the foregoing it was envisaged that since bagasse originates from different sugar cane cultivars and contains different proportions of inorganic constituents, carbohydrate, and lignin contents, the thermal decomposition profile may vary between bagasse samples. This study uses a two-step acid hydrolysis process for determination of structural carbohydrates and lignin contents of bagasse samples from sugar cane cultivars. X-ray powder diffraction was used to determine the degree of crystallinity of the cellulose component. Thermogravimetric

10.1021/ie101559n  2011 American Chemical Society Published on Web 12/01/2010

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Ind. Eng. Chem. Res., Vol. 50, No. 2, 2011

Table 1. Bagasse Sugar Cane Cultivars and Soil Types sample no. 6987 7087 7098 7170 7212

variety GQ185R GQ157 GQ197P GQ197R GQ190P

soil type wagoora wollingford sunnyside benholme mytle

Australian soil classification brown dermosol mesonatric gray sodosol subnatric gray sodosol gray vertosol aquic vertosol

(TG) analysis and differential thermogravimetry (DTG) were used to study bagasse thermal decomposition reactions, while energy-dispersive spectroscopy was used to determine the elemental composition of the residue remaining after the dynamic heating process. Experimental Section There are two main methods of extracting sugar juice from sugar cane. A set of roller mills is used exclusively in Australia raw sugar factories, while diffusers are commonly used elsewhere. Bagasse is the residue obtained after the extraction process. Significant damage of bagasse occurs during the sucrose extraction process. Hammermills are used to break up whole fibers into short fibers and for opening up the parenchyma cells. Some additional shear forces are also used on the fibers in the roller mills (tyically, six roller mills in series), which is located in between the hammermills and the dewatering mill in a factory that uses a milling train. In factories where diffusers are used, after cane preparation in the hammermill and the first roller mill, the fibers pass through a diffuser for the extraction process. The differences in mechanical treatment between milled bagasse and diffuser bagasse produce materials of different physical properties.22 Bagasse samples of five different varieties of sugar cane (Table 1) that originates from different soil types were obtained from Proserpine in North Queensland, Australia. These varieties constitute the main sugar cane varieties in the mill area. The sugar extraction process in the factory involves the use of a set of roller mills. The samples were not washed to remove residual sugars because in an industrial setting the samples would be used as received. However, for comparisons, a portion each of two bagasse samples was washed prior to analysis. This involved placing 5 g of the sample in a Buchner filter funnel containing Whatman No. 52 filter paper and washed under vacuum with Milli-Q water. The filtrate was concentrated in a freeze dryer and the sugar content determined by standard high-performance liquid chromatography (HPLC). The air-dried bagasse samples were ground to pass through 5 mm screen sieve. The samples were then dried at 45 °C overnight to constant mass and stored in desiccators. The procedures used for compositional analyses of the bagasse samples were based on those reported by the National Renewable Energy Laboratory (NREL).23 The moisture contents of the samples were determined from the amount of total solids remaining after drying at 105 °C to a constant mass. The error in the analysis was (2% based on triplicate analysis. The ash content (with error (2%) of a bagasse sample is expressed as the percentage of residue remaining after dry oxidation from 550 to 600 °C, the result reported for the 105 °C oven dry mass of the sample. The NREL procedure23 for structural carbohydrates and lignin determination uses a two-step acid hydrolysis process to fractionate bagasse into glucose, xylose, arabinose, organic acids, acid-insoluble lignin, and acid-soluble lignin. The sugars were quantified using HPLC, the acid-insoluble lignin determined by gravimetry, and the acid-soluble lignin determined by UV-vis

Table 2. Compositional Analysis of Predried Bagasse Samples sample no.

bagasse component (mass %)

6987

7087

7098

7170

7212

moisture ash glucan xylan arabinan lignin (acid insoluble) lignin (acid soluble) crystallinity

3.65 2.43 43.10 14.12 0.70 11.22 4.55 42.1

3.39 3.56 40.24 12.29 0.76 10.23 4.82 39.3

3.06 3.52 39.20 10.76 0.72 8.34 3.43 54.1

3.20 4.43 47.64 12.46 0.71 10.69 3.83 47.4

3.29 3.86 41.62 15.24 0.78 14.19 4.18 53.3

spectroscopy. The results were expressed as an average of three measurements, and the error in the analysis was (4%. The organic acid contents of the hydrolyzed samples were not analyzed. Thermal decomposition studies were carried out in a TA Instruments Q500 in a flowing nitrogen atmosphere at 60 cm3 min-1. Approximately 18 mg of sample was used, and heating was carried out at 5 °C min-1 from 40 to 800 °C. The heating program of the instrument allowed the furnace temperature to be regulated precisely to provide a uniform rate of decomposition. Duplicate analysis was performed for each sample, and the error in the analysis was (0.2%. For the kinetic studies, samples were heated at 2, 5, and 10 °C min-1 from 40 to 800 °C. For these studies ∼5 mg of samples was used so that the decomposition process was controlled by chemical kinetics. The ash contents of the samples obtained after thermal decomposition (i.e., the residue) were analyzed by energydispersive spectroscopy. The samples were carbon coated and examined using a JEOL JXA-840 (20 kV, 1.0 nA, T3). The elemental compositions of the residue were obtained the v3.30 JEOL Analysis Program. The error in the analysis by this method was (4%. The crystallinity of the bagasse samples, prior to thermal treatment, was measured by XRD. The XRD pattern was taken using a PANalytical X’Pert MPD, Cu KR radiation with nominal conditions. The crystallinity index proposed by Segal et al.24 was used to determine the crystallinity of the samples. The error in the analysis was (5%. The phases present in the bagasse residue obtained after thermal heating in the TA Instruments was also determined by XRD. Results and Discussion Bagasse Composition. The composition of the bagasse samples is given in Table 2. The unaccounted components included extractives (e.g., wax and lipids), organic acids (e.g., acetic acid and formic acid), furfural, and hydroxy methyl furfural. The general distribution of the main components in the bagasse samples is typical of lignocellulosics, though the xylan content is lower than that typical of bagasse.1 As the moisture contents of the bagasse samples were similar, it can be inferred that the distribution of ‘free’ and hygroscopic water in the fibers are similar for this group of cultivars. Biagini et al.12 and others10,11 studied the devolatilization of biomass components, i.e., cellulose, hemicellulose, and lignin, and established that the different reactivities of the components and the amounts and types of volatile matter released determined the devolatilization profile of the biomass materials. Thus, differences in the proportions of cellulose, hemicellulose, and lignin will influence the combustion properties among biomasses, even with biomasses from the same origin. Furthermore, Kollman and Topf25 found that the temperature at which wood


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Figure 1. Thermal decomposition curve of bagasse sample 6987 performed under nitrogen atmosphere.


self-ignites during heat treatment decreases with increasing lignin content. They found that a drop in lignin content by 5 wt % in wood resulted in a drop of the self-ignition temperature by up to 10 °C. Thus, the differences that exist in glucan, xylan, and lignin (acid soluble and acid insoluble) contents among the bagasse samples obtained in the present study (Table 1) will result in differences in the devolatilization profiles. As the combustion of cellulose is highly susceptible to catalysis and autocatalysis, the same though to a lesser extent should occur with bagasse. Studies on the various components of Mallee biomass, i.e., leaf, bark, and wood, showed that the differences in ash content impacted significantly the combustion properties.26 The ash content of the bagasse samples varied from 2.4% to 4.4%, though the catalytic effect of ash chiefly depends on the composition of its mineral constituents.14-18,21 Work conducted by previous researchers27-31 has shown that the heat of combustion of cellulose is related to its crystallinity because the latter is associated with the number of moles of hydrogen bonds in the matrix. The more hydrogen bonds present, the more energy is required to cleave the bonds. Shafizadeh32 has also shown that the physical properties of

cellulose have a significant effect on the thermal decomposition of cellulose. As such, the differences in the degree of crystallinity among the bagasse samples (as shown in Table 2) may contribute to differences in the overall decomposition profile among the samples. Thermogravimetry (TG) and Derivative Thermogravimetry (DTG) Analyses. Generally, the decomposition reaction of a lignocellulose material can be divided into a number of processes including bond breaking (e.g., cleavage of glycosidic bonds), dehydration, decarboxylation, decarbonylation reactions, and bond formation (e.g., double bonds between carbon and oxygen bonds).5 While the individual contribution of hemicellulose, cellulose, and lignin to the combustion process can be computed,5,11,12 studies have shown that the thermal decomposition process is not a simple additive function of each contributing fraction and that more realistic information is obtained by looking at the overall devolatilization process. The thermograms of the bagasse samples from the temperatue range 40-800 °C are given in Figures 1-7 showing unresolved overlapping peaks. The mass losses (as shown by the TG thermograms) appear to occur in four distinct stages instead of three stages reported for

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Figure 3. Thermal decomposition curve of washed bagasse sample 7087 performed under nitrogen atmosphere.


bagasse decomposition.5,8,9 Stage I of the process is the evaporation of residual water and is in the range 0.34-1.75%. The second mass loss occurs between 120 and 230 °C. However, there was no mass loss recorded for samples 7087 and 7170 after they were washed prior to analysis. HPLC analysis of the filtrates obtained after washing samples 7087 and 7170 indicated the presence of 5.5 and 7.1 mass % of sucrose, respectively. This suggests that the mass loss recorded for the samples in stage II is associated with the decomposition of residual sucrose not removed during the milling process.33 The stage III mass loss is caused primarily by the decomposition of hemicellulose, while the stage IV mass loss is caused by the combined decomposition of cellulose and lignin.34,35 For the unwashed samples the magnitude of the mass losses recorded for stages III and IV are similar. However, there was a greater mass loss for stage IV for the washed samples. This may be due to the fact that the washing removed some soluble ash components that altered the devolatilization process. The amount of residue remaining after thermal treatment ranged from 25% to 39%. The highest value (39%) was obtained

for sample 7170. This may simply may be related to the fact that the sample contains the highest glucan and ash values (see Table 2). Figures 1-7 show the DTG of the samples. There are differences in the temperature at a maximum rate of weight loss (i.e., peak temperature) for each decomposition stage as well as in the intensities of these peaks among the samples. While most of the samples have peak maxima for stage I at 73/78 °C, the peak maximum for sample 7098 occurs at a slightly lower temperature of 67 °C. This peak is associated with water loss during the drying process. The peak maximum that occurs between 207 and 211 °C for the unwashed samples, as has been mentioned earlier, is associated with the maximum rate of sucrose degradation.34 The peak maximum associated with hemicellulose decomposition occurs at 283/287 °C for all samples, except for sample 7098 (which contains the lowest proportion of xylan and lignin) where it occurs at the lower temperature of 273 °C. These values are lower than that reported by Garcia-Perez et al.,5 who obtained a value of 299 °C for the devolatilization process.


795


Figure 6. Thermal decomposition curve of washed bagasse sample 7170 performed under nitrogen atmosphere.

While Garcia-Perez et al.5 obtained a value of 351 °C for the maximum cellulose degradation temperature, in the present work the value lies between 322 and 340 °C. Sample 7170 has the lowest maximum cellulose degradation temperature but the highest ash value (see Table 2). To account for these differences the mineral constituents of the ashes obtained after thermal decomposition of the bagasse samples were obtained by energydispersive spectroscopy. The inorganic constituents as the metal oxides are presented in Table 3. The main oxides in the samples are the oxides of Si, S, and K. Sample 6987 contains a reasonable proportion of the oxide of Mg, while sample 7170 contains the highest proportion of the oxide of Al. As well as these oxides, the samples contain small quantities of Na, Ca, and Fe oxides (see Table 3). Various workers have reported that the mineral constituents affect cellulose36-38 and biomass decomposition14-21 profiles. In particular, Nassar6 reported that silica influenced the decomposition profile of bagasse, confirming that mineral constituents have a similar catalytic effect on bagasse as in other biomasses. Khelfa et al.20 reported that the presence of MgCl2 or ZnCl2 in cellulose shifts cellulose degradation reactions to lower temperatures, indicating some

catalytic effect of MgCl2 and ZnCl2. Worth noting is that the ash obtained after thermal decomposition of sample 7170 contains the highest proportion of Al2O3 compared to the other ashes obtained from the other bagasse samples. Also, sample 7170 contains the highest proportion of ash originally present in the samples prior to decomposition. To obtain additional information on the inorganic constituents, X-ray powder diffraction of the bagasse ash samples was conducted. The d values of the strongest intensities are presented in Table 4. The main phases identified in the samples are quartz (4.26, 3.34, 2.28 Å), arcanite, potassium sulfate salt (4.15, 3.00, 2.90 Å), and langbeinite, a potassium iron sulfate salt (3.17, 2.68, 2.42 Å). The X-ray powder diffraction patterns also contained some unidentified peaks. There were minor differences in the X-ray powder spectra of the bagasse samples with only slight shifts in the d values. The differences in the d values could be related to differences in the proportions of K and Fe in langbeinite or/and the existence of different concentrations of inorganic ions in the other phases. In a previous study, carried out in our laboratory, the phases identified in a bagasse ash sample derived from a sugar cane cultivar and obtained from a

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Figure 7. Thermal decomposition curve of bagasse sample 7212 performed under nitrogen atmosphere. Table 3. Energy-Dispersive Spectroscopy Results of the Elemental Composition of the Residues Obtained after Thermal Decomposition of Bagasse sample no.

metal oxide (mass %)

6987

7087

7098

7170

7212

Na2O MgO Al2O3 SiO2 P2O5 SO3 K2O CaO MnO TiO2 Fe2O3

0.79 8.11 0.76 39.20 3.36 32.36 11.46 3.57 0.28 0.01 0.01

0.94 2.23 5.43 42.44 2.45 13.16 7.58 1.63 0.60 0.01 2.50

0.68 3.80 4.66 25.36 1.63 24.16 13.87 2.98 0.28 0.01 2.49

0.89 3.61 6.86 27.31 1.81 20.70 11.83 2.42 0.26 0.01 3.02

0.63 2.72 1.07 42.98 3.93 26.31 19.01 2.35 0.32 0.01 1.00

schemes involving single or multiple constant heating rate methods (i.e., nonisothermal). For this work, Friedman’s method13 has been used since the method does not require knowledge of the form of the kinetic equation for the activation energy (Ea) to be determined. The general rate equation for a reaction can be described as dR dR ≈β ) k(T)f(R) dt dt

where R is the degree of conversion, β the linear heating rate (°Cnmin-1), k(T) the rate constant, and f(R) the reaction rate model, a function which depends on the actual reaction mechanism. In this work

Table 4. X-ray Powder Diffraction d Values (Å) for Bagasse Ash

R)

sample no. 6987

7087

7098

7170

7212

4.26 4.15 4.08

4.26

4.26 4.15 4.10

3.35 3.16 3.01

3.34 3.17

4.26 4.15 4.08 3.73 3.34 3.16 2.99 2.90 2.77 2.67 2.51 2.42

4.26 4.15 4.10 3.74 3.34 3.17 3.00 2.90 2.77 2.68 2.52 2.42 2.23 2.13

2.77 2.67 2.46 2.13 2.04 1.82

4.09

3.34 3.17 3.00 2.90

2.68

2.68

2.46 2.24 2.13

2.13 2.04

1.98 1.82

(2)

WO - W WO - wf

(3)

where WO is the initial weight, W is the weight during experiment, and wf is the final weight of the investigation determinate from the TG thermograms. The rate constant k(T) can be represented by the Arrhenius equation as Ea

k(T) ) Ae(- RT )

(4)

where Ea is the apparent activation energy (kJ mol-1), R is the gas constant (8.314 J K-1 mol-1), A is the pre-exponential factor (min-1), and T is absolute temperature (K). For a dynamic TGA process, introducing the heating rate β ) dT/dt into eq 4 results

1.96 1.82

dR A (- Ea ) ) e RT f(R) dt β

1.82

different geographical location contained quartz, calcium sulfate anhydrite, albite, microcline, and hematite as the main phases. This implies that significant differences in the inorganic consituents between bagasse samples is more to do with location rather than differences in cultivar. Kinetic Study. Yao et al.35 describe various methods that are used to calculate kinetic parameters for the thermal decomposition of compounds based on weight loss. These include first-order decomposition kinetics with different reaction

()

(5)

Equations 4 and 5 are the fundamental expressions of analytical methods to calculate kinetic parameters on the basis of TGA data. The Friedman method,13 which is a linear differential method of eq 5, is Ea

[ dRdt ] ) - RT + ln(Af(R))

ln β

(6)


797

Figure 8. Friedman’s plot for various R values for sample 6987.

Figure 9. Comparison of the activation energy as a function of the degree of conversion (R) of bagasse samples originating from different sugar cane cultivars.

Then for a given value of R the plots of ln(dR)/(dt) vs 1/T directly lead to -Ea/R from which Ea is calculated. A typical Friedman’s plot is shown in Figure 8 as well as the coefficient of determiation, R2. Plots of R up 0.6 are only given since conversion rates higher than this value may not be meaningful due to higher temperature, considerable sample weight loss, and more complex reaction mechanisms.35 For most cases the fitted lines are nearly parallel, suggesting approximately similar Ea values at various R values. They also suggest similar decomposition mechanisms.35 In other cases where the fitted lines are not parallel with one another, possible multiple/complex mechanisms are occurring.35 Figure 9 shows the plots of Ea as a function of R. For most of the samples, for R ) 0.1-0.6 the Ea values fall in the range 145-185 kJ mol-1. These values are comparable to those reported by Garcia-Perez et al.5 and Yao et al.35 As such, the results suggest that the

decomposition process occurs through cleavage of linkages with similar energy bonds independent of cultivar type.5 The presence of low Ea at R ) 0.1-0.4 for samples 7087 and 7170 implies different decomposition mechanisms from those of the other sampes. This is because of residual sucrose in the samples. Conclusion The thermal decomposition profile of bagasse is independent of the sugar cane cultivar type, though slight differences exist in the peak decomposition temperatures of the various stages. Differences also exist in the amounts of residue remaining after thermal decomposition of the bagasse samples originating from different sugar cane cultivar types. These differences may be related to the sugar, glucan, and ash contents. The differences in the composition of ash components of the bagasse samples

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ReceiVed for reView July 21, 2010 ReVised manuscript receiVed October 18, 2010 Accepted November 10, 2010 IE101559N

Thermal Decomposition of Bagasse: Effect of Different Sugar Cane

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