Comparison of steam-assisted versus microwave-assisted treatments for the fractionation of sweet sorghum bagasse
© Kurian et al. 2015
Received: 20 May 2015
Accepted: 25 June 2015
Published: 9 July 2015
The fractionation of lignocellulosic materials can be applied to get each of the components (cellulose, hemicellulose and lignin) of biomass in its maximum purity and yield. The individual component can be further processed to high-value products such as fuels and biomaterials at existing or newly developing biorefineries. The steam-assisted and microwave (MW)-assisted processes were used to fractionate sweet sorghum bagasse into hemicellulosic sugars, cellulose-rich and high-density residue and solid lignin. The treatment temperature evaluated for the fractionation process was 121 °C for 30 to 120 min. The substrate was autohydrolysed to extract the hemicellulose, and the residue was delignified using lime solution to obtain cellulose-rich residue. The lignin and lime from the liquor obtained after the lime treatment of the autohydrolysed sweet sorghum bagasse (SSB) was precipitated using carbon dioxide gas.
Under optimum conditions, the steam-assisted autohydrolysis extracted 72.69 (±0.08) % by weight of the hemicelluloses while the MW-assisted autohydrolysis extracted 70.83 (±0.49) % of the hemicelluloses from the sweet sorghum bagasse. The steam-assisted lime treatment resulted in 69.67 (±1.26) % of the lignin extraction the MW-assisted lime treatment resulted in 68.27 (±1.19) % of the lignin extraction from the corresponding autohydrolysed sweet sorghum bagasse samples. The CO2 treatment precipitated 58.85 (±3.2) % of the lignin dissolved in the yellow liquor of the steam-assisted process while 60.26 (±2.11) % of the dissolved lignin was recovered from the yellow liquor of the MW-assisted process.
The two methods have not exhibited significant differences in overall recovery of the solids, hemicellulose extraction, delignification, residual concentration of cellulose and ash or in the recovery of lignin and lime. The difference was significant (p value <0.05) only in the concentration of total reducing sugars in the hydrolysate and the yellow liquor. The MW-assisted process increased the total crystallinity index (TCI) of the cellulose in the treated SSB and also increased the concentration of guaiacyl lignin content in the recovered lignin which was thermally more stable than the lignin produced in the steam-assisted process.
Innovative research on the fractionation of lignocellulosic biomass is essential for the sustainable production of intermediary materials and bioproducts for the direct industrial applications. Multi-step processes producing multi-products need to be established for the effective conversion of lignocellulosic materials in large-scale biorefineries . The fractionation of lignocellulosic materials is applied to get each of the components (cellulose, hemicellulose and lignin) of biomass in its maximum purity and yield. The individual component can be further processed to high-value products such as fuels and biomaterials at existing or newly developing biorefineries [1, 2].
Fractionation of lignocellulosic materials can be achieved with the use of chemicals such as acids, alkalis, water and ionic liquids . Hydrothermal fractionation and obtaining the hemicellulosic products from lignocellulosic biomass is considered as the first process in the biorefinery operations. Hydrothermal treatment of biomass is highly dependent on the behaviour of water at subcritical conditions. The ionization constant of water increases with increase in temperature and provides an acidic condition for the hydrolytic reactions in the medium. The decrease in dielectric constant of water with increase in temperature at subcritical conditions facilitates the dissolution of organic compounds from biomass . The increase in the severity in treatment conditions would lead to increase in degradation of hemicellulosic sugars. Studies have shown that lower reaction temperatures for longer residence times favour the pentose sugar recovery .
Extraction of lignin from the hemicellulose extracted (autohydrolysed) lignocellulosic materials can be achieved through the use of alkaline materials like calcium hydroxide (lime). The effectiveness of the delignification using lime would be higher in autohydrolysed lignocellulosic materials than in the untreated biomass [4, 5]. Extraction of lignin from the autohydrolysed lignocellulosic materials can be applied to produce a solid residue rich in cellulose and with higher bulk density than untreated and autohydrolysed biomass. Moreover, the use of lime for the extraction of lignin from the autohydrolysed biomass has the advantage of recovering the lime and lignin using carbon dioxide gas . Thus, hydrothermal treatment followed by lime treatment can be applied for the industrial fractionation of lignocellulosic materials into multiple products in an economic and eco-friendly way .
The mode of energy supply for the fractionation of lignocellulosic biomass is important, and it may also affect the yield and overall efficiency of the fractionation process. Generally, steam is used to supply energy for myriad of industrial processes . However, microwave heating is an efficient method of energy supply for the treatment of biomass for the production of high-value products from it [8, 9]. Microwave-assisted pretreatment processes are based on the selective heating of the lossy components in the lignocellulosic materials that leads to the disruption in the original structure of lignocelluloses. Cellulose component has low loss tangent factor (tan δ) and generate less heat whereas the lignin and water have high loss tangent factor (tan δ) and can generate high heat when the lignocelluloses are treated with microwaves .
The effect of microwave on the depolymerization of the lignocellulosic components is attained through the reduction in glass transition temperature. The added water for the treatment of biomass reduces the glass transition temperature of polymers in biomass and thus facilitates the interaction of the individual molecules with the microwaves to generate heat. The lignocelluloses heated in an aqueous medium releases acetic acid that leads to the hydrothermal treatment of the substrate . Presence of ionic compounds like calcium hydroxide further increases the loss tangent factor (tan δ), which measures the heating ability of the substance under microwave and of the aqueous solutions, and facilitates the reactions in biomass mixture . Moreover, microwave heating can reduce the reaction times from hours to minutes with minimum side reactions and improve yields and reproducibility of the reactions .
In this work, a comparison was made between the outcomes of the fractionation of sweet sorghum bagasse (SSB) in an autoclave and in a microwave apparatus. The SSB was fractionated into hemicellulosic sugars, cellulose and lignin at 121 (±0.5) °C in both cases. The isothermal treatment time was 90 min for the extraction of hemicellulose through hydrothermal treatment and 106 min for the extraction of lignin from the autohydrolysed residue. The comparison was made on the efficiency of each process on the extraction of hemicellulose as reducing sugars, extraction of lignin from the autohydrolysed biomass, recovery and purity of cellulose and lignin, and the physicochemical properties of the products produced through the two processes.
Substrates and the treatments
SSB powder was used as the substrate for the steam-assisted hydrothermal treatment (SAHT) and microwave-assisted hydrothermal treatment (MAHT) studies. Sweet sorghum (Sorghum bicolor L. Moench, CSSH-45) was collected from the experimental field (45° 26' 01" N, 73° 55' 53" W) of the Department of Plant Science of McGill University. The harvested plants were defoliated, and the sweet juice was extracted by crushing the stems. The free sugars present were removed by washing the bagasse using cold water . The bagasse obtained was then dried in a hot air oven at 50 °C for 72 h to obtain constant dry mass. The dried bagasse was chopped to 1–2-cm-long pieces using scissors and pulverized using a blender-grinder (Homeland Housewares, LA, USA). The powder was stored in plastic air-tight containers and was used after drying at 105 °C for 24 h for treatments and analyses [14, 15].
The SSB powder was initially subjected to the hydrothermal treatments at 121 (±0.5) °C for 30 to 120 min with substrate concentration varying from 10 to 20 % by weight of the mixture. The optimum conditions to get maximum hemicellulose extraction and solids recovery were found to be 13 % substrate concentration and 90 min of isothermal treatment . The residue obtained from the SAHT was dried and then subjected to the steam-assisted lime treatment (SALT) at 121 (±0.5) °C for 30 to 120 min with 10 % of the mixture substrate concentration and calcium hydroxide dosage varying from 10 to 20 % by weight of the substrate. The optimum conditions for the maxima of lignin extraction and solids recovery were found to be 10 % of the substrate calcium dosage and 106 min of isothermal treatment residence time . Similarly, the residue obtained from the MAHT was dried and then subjected to the MW-assisted lime treatment (MALT) at 121 (±0.5) °C for 30 to 120 min with 10 % by weight of the mixture substrate concentration and calcium hydroxide dosage varying from 10 to 20 % by weight of the substrate. The optimum conditions for the maxima of lignin extraction and solids recovery were found to be 10 % of the substrate calcium dosage and 78 min of isothermal treatment residence time.
Steam-assisted treatments were conducted in an autoclave (DSE-8000, NAPCO, USA), and the microwave-assisted treatments were conducted in a microwave apparatus (MiniWave, SCP Science, Canada).
The response surface experimental design used in the studies
Substrate OR Ca(OH)2 concentration (%)
Treatment residence time (min)
Each treatment including the two centre points (15 % substrate concentration (in hydrothermal treatment) or 15 % calcium hydroxide dose (in lime treatment) and 75 min of treatment time) was conducted in triplicates, and the solid residues obtained after the treatment were analysed in triplicates for cellulose, hemicellulose, lignin and mineral concentrations. Similarly, the liquid samples (hydrolysate) were analysed for hexose and pentose sugars, total carbohydrates, sugar degradation products (hydroxymethyl furfural (HMF) and furfural) and dissolved lignin. The process conditions were optimized to obtain the maximum hydrolysis of hemicellulose (in hydrothermal treatment) and maximum extraction of lignin (in lime treatment) and recovery of solids with minimum sugar degradation.
The carbon dioxide treatment for the yellow liquor was applied by following a completely randomized design of experiment with one factor (final pH of the yellow liquor) at multiple levels, from pH 2.5 to 12 with a pH value interval of 0.5 .
The particle size distribution of the SSB powder was estimated using sieve analysis , and the bulk density of the solid samples produced through the steam-assisted and microwave-assisted methods was estimated using a 100-mL measuring cylinder . The dielectric properties of the SSB slurries with water and lime solution at 2450 MHz were measured by using a network analyser equipped with a probe (Agilent 8722 ES s-parameter, Agilent USA). The measurements were controlled, and the results were obtained using the software (Agilent 85070D E01.02, Santa Clara, USA) integrated with the network analyser .
Gravimetric methods for the analysis of forage fibers were used to estimate the concentration of cellulose, hemicellulose, lignin and minerals in the solid SSB samples before and after the treatments . UV-Vis spectroscopy was used for measuring the concentration of hexose and pentose sugars, sugar degradation products and lignin in the liquid samples. The para-bromoaniline method was used for the estimation of pentose sugars. Heating of the sample at 70 °C for 10 min with para-bromoaniline reagent converts pentose sugars in the sample into furfural which in turn formed a pink coloured complex with para-bromoaniline. The pentose sugar concentration was estimated by measuring the absorbance at a wavelength of 520 nm. Concentration of pentose sugars in the samples was estimated from a standard curve prepared using D-xylose sugar .
The concentration of reducing sugars were estimated by the 2,4,6-tripyridyl-s-triazine (TPTZ) method. It is based on the oxidation of sugars using Fe3+(K3[Fe(CN)6]) in alkaline media which leads to the formation of Fe2+. TPTZ reacts with Fe2+ to form a violet-coloured complex that has a maximum absorption at a wavelength of 595 nm . The total carbohydrate present in liquid samples was estimated by using the anthrone reagent method. It uses sulphuric acid to dehydrate the sugars into furfurals which in turn condenses with the anthrone reagent to form a green-coloured complex showing maximum absorbance at a wavelength of 620 nm .
Ultraviolet (UV) spectroscopy was used for the estimation of furfural and hydroxyl-methyl furfural (HMF) in liquid samples . Furfural shows specific absorption maximum at 277 nm, and HMF shows specific absorption maximum at 285 nm. Lignin has a characteristic absorption at 280 nm which may interfere with the estimation of HMF and furfural. Sodium boro-hydride (NaBH4) was used to reduce furfural and HMF so that their absorption maxima at 277 and 285 nm respectively would disappear. The difference in absorption before and after the reduction was used to estimate the concentration of furfural and HMF in the samples . Similarly, lignin solubilized in the liquid was estimated by UV-spectroscopy by measuring the absorbance at 205 nm .
Complexometric titration with ethylene-diamine-tetra-acetic acid (EDTA) was used for the estimation of calcium concentration in solid and liquid samples [28, 29]. In this method, 500 mg of the oven dried solid sample was burned to ash in a muffle furnace (Barnstead Thermodyne 48000, Thermo Scientific, USA) at 600 °C for 4 h, and the ash was completely dissolved in 10 mL of 0.5 N HCl and then diluted to 50 mL with distilled water. The solution was kept at a room temperature for 5 min after adding 6 mL of 1 M NaOH to make the pH of the solution to 13. Two drops of Eriochrome Black T (EBT) (Sigma–Aldrich, Canada) indicator solution were added, and the solution was titrated with 0.1 M EDTA (Sigma–Aldrich, Canada) until the colour changed from blue to red. Calcium concentration was estimated from the titrated values which were taken from the average of three titrations of a sample.
The solid samples were analysed using a differential scanning calorimeter (DSC) (Q100 V9.8, TA Instrument, Waters LLC), and the data was analysed using Universal Analysis 2000, for thermal property changes. For the DSC analyses, oven-dried samples between 9 and 10 mg were weighed and sealed in aluminum hermetic pans. The heating range was from 20 to 500 °C with a ramp up temperature of 10 °C/min with a Nitrogen flow rate of 50 mL/min to purge the heating chamber. The DSC thermograms of the SSB treated with the steam-assisted and microwave-assisted methods were compared against each other. Similarly, the DSC thermograms of the lignins precipitated from the yellow liquor produced through the two treatment methods were compared against each other for thermal properties such as glass transition temperature, crystallization temperature, melting temperature and decomposition temperature.
The solid samples were also analysed for functional group changes using the Fourier transform infrared (FTIR) spectroscopy (Nicolet iS5, Thermo Scientific, USA). The spectra of the SSB samples produced through the steam-assisted and microwave-assisted methods were compared to each other. Similarly, the spectra of the lignins precipitated from the yellow liquor produced through the two treatment methods were also compared against each other.
Characteristic bond and movement
Phenyl ring stretching
C–O of guaiacyl ring
C–O of guaiacyl ring
C–O of syringyl ring
Guaiacyl and syringyl condensed
Aromatic skeletal vibration
C–O stretching at C6
C–O stretching at C3
The physical changes in the solid samples were also analysed using scanning electron microscopy (SEM) (TM3000, Hitachi, USA), and the surface changes in the SSB samples treated with steam-assisted and microwave-assisted methods were compared to each other. The oven-dried samples were kept under vacuum in the SEM microscope without any treatments or modifications.
Analysis of variance (ANOVA) was conducted to determine the statistical significance (p value ≤0.05) of the responses and to estimate the fit of the polynomial prediction equation to the observed responses. The difference in the response by the SSB towards steam-assisted and microwave-assisted treatments were analysed using the Student’s t-test .
Results and discussion
Physicochemical properties of the sweet sorghum bagasse used for the treatments
The composition of SSB powder used for the treatments was estimated on a mass percent basis to be 45.03 (±0.03) % cellulose, 30.17 (±2.06) % hemicellulose, 24.40 (±3.02) % lignin and 0.78 (±0.22) % ash. The particle size distribution of the samples used for the treatment was 75–1000 μm estimated through the sieve analysis . The bulk density of the raw SSB powder was 85.79 (±5.73) (kg/m3) estimated using a 100-mL measuring cylinder .
Dielectric properties of the mixtures used in the microwave-assisted treatments
The tan δ was increased with increase in temperature when the lime solution was used to prepare the mixture. The increase in the tan δ value for the calcium hydroxide solutions can be attributed to the increase in ions from calcium hydroxide and the dissociation of calcium hydroxide molecules to ions with the increase in temperature . The increase in loss factor increases the heat generation in lime solutions with increase in temperature. The increase in tan δ value with increase in temperature will help in reduced energy input for obtaining higher temperatures. However, the energy input required for the MW-assisted treatments of SSB was not considered in this study.
Comparison of solids recovery during steam-assisted and microwave-assisted processing of sweet sorghum bagasse
The MALT resulted in higher dissolution of solids into the yellow liquor than during the SALT, but the difference in quantity of solids recovered after the two treatments was not found to be significant. The SALT of the SAHT-SSB at 121 (±0.5) °C for 30 to 120 min with 10 % substrate concentration and 10–20 % by weight of the substrate Ca(OH)2 dosage resulted in 77.84 (±8.09) % solids recovery. Under optimum conditions, the SALT (10 % Ca(OH)2 dosage and 106 min of isothermal treatment at 121 (±0.5) °C) of the SAHT-SSB resulted in 76.75 (±0.15) % solids recovery. The MALT of the MAHT-SSB at 121 (±0.5) °C for 30 to 120 min with 10 % (g/g mixture) substrate concentration and 10–20 % of Ca(OH)2 dosage resulted in 75.12 (±7.86) % solids recovery. Under optimum conditions, the MALT of the MAHT-SSB (10 % Ca(OH)2 dosage and 78 min of isothermal treatment at 121 (±0.5) °C) resulted in 71.76 (±1.28) % solids recovery. The increased dissolution of solids during the MALT can be attributed to the higher energy absorption during the MW-assisted treatment and the accompanied higher dissolution of biomass components into the yellow liquor.
Comparison of steam-assisted versus microwave-assisted treatments on hemicellulose extraction from sweet sorghum bagasse
The MALT resulted in higher extraction of hemicellulose into the yellow liquor than during the SALT of the autohydrolysed SSB. The average quantities of hemicellulose extracted during the SALT and the MALT were 19.79 (±7.31) % and 27.38 (±4.10) %, respectively. The difference in quantity of hemicellulose extracted between the two treatments was found to be significant (p value <0.05). Under optimum conditions, the SALT of the SAHT-SSB resulted in 24.19 (±1.06) % hemicellulose extraction, whereas under optimum conditions, the MALT of the MAHT-SSB resulted in 26.11 (±2.79) % hemicellulose extraction. The higher extraction of hemicellulose during MALT can be attributed to higher energy absorbed by the mixture and the resultant dissolution of the dry matter  within the total duration of the treatment .
Comparison of steam-assisted versus microwave-assisted treatments on lignin extraction from the sweet sorghum bagasse
The MALT resulted in less extraction of lignin into the yellow liquor from MAHT-SSB than during the SALT of the SAHT-SSB. The quantities of lignin extracted during the SALT and the MALT were 67.66 (±9.89) % and 63.10 (±12.85) %, respectively. However, the difference in quantity of lignin extracted between the two treatments was found to be not significant. Under optimum conditions, the SALT of the SAHT-SSB resulted in 69.67 (±1.26) % lignin extraction, whereas under optimum conditions, the MALT of the MAHT-SSB resulted in 68.27 (±1.19) % lignin extraction. The study has shown that, in comparison to the steam-assisted fractionation, the microwave-assisted fractionation process require less duration of isothermal treatment to effect similar quantities of lignin extracted from SSB. Moreover, it was also shown that the microwave-assisted and steam-assisted methods have similar effect on the extent of extraction of lignin from the hemicellulose-extracted SSB within the range of treatment conditions investigated.
Comparison of steam-assisted versus microwave-assisted treatments on cellulose extraction from the sweet sorghum bagasse
The SALT resulted in less cellulose hydrolysed into the yellow liquor from SAHT-SSB than during the MALT from the MAHT-SSB. The quantities of cellulose hydrolysed during the SALT and the MALT were 3.90 (±1.45) % and 7.70 (±2.71) %, respectively. The difference in quantity of cellulose hydrolysed between the two treatments was found to be significant (p value <0.05). Under optimum conditions, the SALT of the autohydrolysed SSB resulted in 4.70 (±0.22) % cellulose hydrolysis, whereas under optimum conditions, the MALT of the MAHT-SSB resulted in 10.07 (±0.18) % cellulose hydrolysis. In comparison to the steam-assisted fractionation, the microwave-assisted fractionation process resulted in more cellulose hydrolysed from SSB. This can be attributed to the higher energy absorbed by the mixture during microwave treatment and the accompanied physicochemical changes to the cellulose component of SSB . However, further studies are required to know the exact nature of the cellulose fractions dissolved during the MW-assisted treatments. The results have shown that the MW-treatments have specific effect on the cellulose component of SSB.
Comparison of steam-assisted versus microwave-assisted treatments on ash concentration of the sweet sorghum bagasse residues
SSB contained only 0.78 (±0.22) % ash in it. The hydrothermal treatments resulted in slight increase in the ash concentration because of the relatively high extraction of other biomass components from the substrate. The concentration of ash in the residue obtained from the SAHT and the MAHT were 1.01 (±0.02) % and 0.92 (±0.06) %, respectively. Under optimum conditions, the SAHT resulted in 1.01 (±0.01) % of ash concentration in the SAHT-SSB, whereas under optimum conditions for MAHT, the ash concentration in the SSB residue was found to be 0.88 (±0.01) %. The t-test results have proved that the steam-assisted and microwave-assisted hydrothermal treatments have no significantly different effects on the concentration of ash in the autohydrolysed SSB. It is also clear that the hydrothermal treatments do not significantly affect the ash content in the SSB .
The lime treatment resulted in increasing the ash content of the lime-treated SSB. The increase in ash concentration can be attributed to the binding of calcium ions to the carbohydrate and lignin components during the lime treatment . The concentrations of ash in the residue of SALT and the residue of MALT were 2.62 (±1.38) % and 3.05 (±0.95) %, respectively. The difference in concentration of ash in the residues obtained after the two treatments was found to be not significant. Under optimum conditions, the SALT of the SAHT-SSB resulted in 4.25 (±0.39) % ash concentration in it, whereas under optimum conditions, the MALT of the MAHT-SSB resulted in 4.03 (±0.21) % ash concentration. The similar concentration of ash in the lime-treated residues has shown that, in comparison to the SALT the MALT, it does not affect the binding of calcium ions to the carbohydrates and lignin components in the SSB. However, the increased concentration of ash may have negative effects on further processing of the treated SSB.
Comparison of steam-assisted versus microwave-assisted treatments on isolation of lignin and lime from the yellow liquor
Maximum precipitation of lignin occurred at pH 4.5 (±0.25), and the CO2 treatment resulted in 58.85 (±3.2) % recovery of lignin from the yellow liquor of the SALT. Similarly, the CO2 treatment of the yellow liquor of MALT resulted in 60.26 (±2.11) % lignin precipitation at pH 4.5 (±0.28). There was no effect of the treatment applied to the SSB on the quantity of lignin recovered from the yellow liquor through the CO2 addition because the difference in extent of the precipitation of calcium carbonate from the yellow liquors was found to be not significant. The quantity of precipitates obtained per litre of the yellow liquor of the SALT and the MALT was 3.02 (±1.34) g/L and 3.34 (±0.79) g/L, respectively. This was accounted for 26.57 (±8.68) % and 32.35 (±14.44) % removal of total dissolved solids (TDS) from the respective yellow liquors.
Maximum precipitation of CaCO3 occurred at pH 9.5 (±0.06), and the CO2 treatment resulted in 65.99 (±1.2) % recovery of lime from the yellow liquor of the SALT of the SAHT-SSB. Similarly, the CO2 treatment of the yellow liquor of MALT resulted in 71.85 (±2.48) % precipitation of CaCO3 at pH 9.5 (±0.10). There was no effect of the treatment applied to the SSB on the quantity of lime recovered from the yellow liquor through the CO2 addition because the difference in extent of the precipitation of calcium carbonate from the yellow liquors was found to be not significant.
Comparison of steam-assisted versus microwave-assisted treatments on composition and properties of the products of sweet sorghum bagasse fractionation
In addition to the delignification, the lime treatment also caused the hydrolysis of a fraction of the hemicellulose and cellulose in the hydrothermally treated SSB. The reducing sugar concentration in the yellow liquor from the steam-assisted lime treatment was considerably less than the reducing sugar concentration in the yellow liquor from the MALT. The SALT of SAHT-SSB resulted in 8.59 (±3.49) g/L reducing sugar concentration while the MALT resulted in 13.50 (±3.75) g/L of reducing sugars in the yellow liquor. Under the optimum conditions, the SALT of the SAHT-SSB resulted in 12.12 (±0.24) g/L reducing sugar concentration in the yellow liquor, whereas under the optimum conditions, the MALT of MAHT-SSB resulted in 15.11 (±0.86) g/L reducing sugars in the yellow liquor. The difference in concentration of reducing sugars in the yellow liquor, over the range of treatment conditions studied, is found to be significant (p value <0.05), and the microwave treatment was found to be increasing the reducing sugar concentration in the yellow liquor from MAHT-SSB. In comparison to the SALT, the increased reducing sugar concentration during the MALT of MAHT-SSB can be attributed to the increased hydrolysis of hemicellulose and cellulose during the MALT.
The concentrations of total carbohydrates, pentose sugars, furfurals and 5-hydroxy methyl furfurals in the hydrolysates and yellow liquors produced by the steam-assisted and MW-assisted treatments were significantly different (p value <0.05), and the pattern in their concentration was like that of reducing sugars.
The HT-SSB contained higher concentration of cellulose and lignin than the untreated SSB. The chemical composition of the SAHT-SSB, produced under the optimum treatment conditions, was 56.36 (±0.06) % cellulose, 11.11 (±0.01) % hemicellulose, 31.42 (±0.01) % lignin and 0.99 (±0.01) % ash, whereas the MAHT resulted in a residue with 53.38 (±0.14) % cellulose, 12.72 (±0.18) % hemicellulose, 30.19 (±0.47) % lignin and 0.88 (±0.01) % ash in it.
The MALT-SSB contained higher concentration of cellulose than the SALT-SSB samples. The chemical composition of the SALT-SSB, produced under the optimum treatment conditions, contained 68.29 (±0.31) % cellulose, 10.77 (±0.15) % hemicellulose, 13.26 (±0.36) % lignin and 4.25 (±0.39) % ash, whereas the MALT resulted in a residue with 69.41 (±1.83) % cellulose, 12.14 (±0.71) % hemicellulose, 12.59 (±0.31) % lignin and 4.03 (±0.21) % ash in it. The difference in composition of the SSB samples treated using the steam-assisted and MW-assisted processes was found to be not significant. Both the steam-assisted treatment and the MW-assisted treatments have similar effects on the fractionation of SSB and the composition of the residual SSB.
Comparison of the FTIR spectra of the SSB samples and lignins produced through the steam-assisted and microwave-assisted treatments
Likewise, hydrothermal treatments lead to increase in concentration of lignin and cellulose, and the intensity of absorption at 900 and 1428 cm–1 by cellulose and 1510 cm–1 by lignin was increased in comparison to the untreated SSB. Intensity of absorption at 1428 cm–1 increased further with decrease in the intensity of absorption at 1510 cm–1 in the two-step treated SSB confirming the increase in the concentration of cellulose and decrease in the concentration of lignin.
The spectra of hydrothermally treated SSB samples produced through the steam-assisted and MW-assisted treatments have shown variations in the intensities of absorption at 900 and 1428 cm–1 confirming the variation in the concentration and nature of cellulose . Intensity of absorption at 900 cm–1 by cellulose in SAHT-SSB was higher than by the cellulose in MAHT-SSB showing the presence of more amorphous cellulose in the former. The higher intensity of absorption at 1428 cm–1 by the cellulose in MAHT-SSB than by the cellulose in SAHT-SSB shows the presence of more crystalline cellulose in the former. The total crystallinity index (TCI) was 0.46 for the SAHT-SSB and was 1.22 for the MAHT-SSB. Thus it is clear that the MAHT produced more crystalline cellulose than the SAHT in the SSB .
The concentration of hemicellulose in the MAHT-SSB was slightly higher than in the SAHT-SSB, and it was confirmed by the relatively high intensity in absorption at 1371 cm–1 by the hemicellulose in the former. Similarly, the intensity in the absorption at 1510 cm–1 by the lignin component of the SAHT-SSB was higher than by the lignin in the MAHT-SSB confirming the high concentration of lignin in the former .
The cellulose concentration in the SSB samples produced by the MALT was slightly higher than in the SSB samples produced by the SALT. The intensity of absorption at 1428 cm–1 by the cellulose in SSB sample produced by the MALT was higher than by the cellulose in the SSB sample produced by the SALT. The two-step treatment increases the TCI values of SSB samples produced by the steam-assisted and MW-assisted treatments, and the latter process produced a sample with TCI value of 2.83. Similarly, the TCI value of the SSB samples produced through the SALT of the SAHT-SSB was 1.70. Thus, it is clear that the MW-assisted treatments can be used for the production of crystalline cellulose from lignocellulosic materials. Crystalline cellulose can be used to produce high-value products such as bio-nano-composites having many industrial applications in agriculture, environment, packaging, textiles, etc. [40, 41].
The intensity of absorption at 1500–1600 cm–1 was reduced in the lime-treated SSB samples from that of untreated and HT-SSB samples, confirming the extraction of lignin component from them through the lime treatment. The intensity of absorption was higher in the SSB samples produced through the MALT than by the SALT. It confirms the relatively higher concentration of lignin in the SSB samples produced through the SALT than by the MALT .
Comparison of the DSC thermograms of the SSB samples and lignins produced through the steam-assisted and microwave-assisted treatments
The two-step treated SSB samples from both the methods exhibited difference in thermal decomposition, and the variation can be because of the nature of cellulose component in them. The SSB sample produced through the MALT has more crystalline cellulose when compared to the SSB sample produced through the SALT. The former sample exhibited higher exothermic reactions within the temperature studied .
Comparison of the SEM images of the SSB samples produced through the steam-assisted and microwave-assisted treatments
Comparison of steam-assisted versus microwave-assisted treatments for the scope of on-farm fractionation of SSB
Steam-assisted processing has the potential to be applied for the on-farm processing of biomass to produce intermediary products for biorefineries and other industries. Researchers have also considered the use of microwave reactors for the on-farm processing of biomass to reduce the transportation cost .
The steam-assisted and MW-assisted fractionation processes evaluated in this study have produced a hydrolysate containing pentose sugars, a solid residue rich in cellulose, and solid lignin as products from the SSB. The bulk density of the solid residue obtained after the SALT was about 3.7 times more than that of the raw bagasse. Similarly, the bulk density of the solid residue obtained after the MALT was about 3.6 times more than that of the untreated SSB. The increased bulk density would make the transport of the two-step treated SSB to the distant biorefineries more economical .
An overall comparison of the process results of steam-assisted and MW-assisted fractionation of SSB
Solids recovery (%)
Reducing sugars in the hydrolysate (%)
Cellulose concentration in the residue (%)
Bulk density of the residue (kg/cm3)
Lignin recovery (%)
Lime recovery (%)
Isothermal treatment residence time (min)
The steam-assisted and MW-assisted processes fractionated the SSB into hemicellulosic sugars, cellulose-rich and high-density residue and solid lignin. The processes also recovered a part of the lime used for the fractionation of SSB. The two methods have not exhibited significant differences in overall recovery of the solids, hemicellulose extraction, delignification, residual concentration of cellulose and ash, or in the recovery of lignin and lime. The difference was significant only in the concentration of total reducing sugars in the hydrolysate and the yellow liquor. The MW-assisted process increased the crystallinity of the cellulose in the treated SSB and also increased the concentration of guaiacyl lignin content in the recovered lignin which was thermally more stable than the lignin produced in the steam-assisted process. The steam-assisted process produced denser SSB which was thermally more stable than the SSB produced through the MW-assisted process. The MW-assisted process resulted in increased separation of the fibers in the SSB than by the steam-assisted process. The techno-economic and ecological analyses of the two processes need to be conducted to assess their potential for the on-farm applications.
The authors are acknowledging the financial assistance from the Fonds Québécois de la Recherche sur la Nature et les Technologies (FQRNT) and the Natural Sciences and Engineering Research Council of Canada (NSERC).
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