Bioenerg. Res. (2013) 6:663–677 DOI 10.1007/s12155-012-9281-4
Characterization of North American Lignocellulosic Biomass and Biochars in Terms of their Candidacy for Alternate Renewable Fuels Sonil Nanda & Pravakar Mohanty & Kamal K. Pant & Satyanarayan Naik & Janusz A. Kozinski & Ajay K. Dalai
Published online: 14 December 2012 # Springer Science+Business Media New York 2012
Abstract The use of lignocellulosic biomass as a renewable energy source is becoming progressively essential. Much attention is focused on identifying suitable biomass species that can provide high energy outputs to replace conventional fossil fuels. The current study emphasizes on some commonly available biomasses in North America such as pinewood, timothy grass, and wheat straw for their usage towards next generation biofuels. Fast pyrolysis of the feedstocks was performed at 450 °C to generate biochars that were further characterized to advocate their energy and agronomic relevance. The biomasses were examined physiochemically to understand their compositional and structural characteristics through analytical approaches such as CHNS (carbon–hydrogen–nitrogen–sulfur), ICP-MS (inductively coupled plasma-mass spectrometry), particle size, FTIR (Fourier transform infrared) and Raman spectroscopy, thermogravimetric and differential thermogravimetric, XRD (X-ray diffraction), and high-pressure liquid chromatography. The chemical composition of feedstocks significantly differed from that of biochars and the variations among feedstock composition were also found to be greater than S. Nanda : J. A. Kozinski Lassonde School of Engineering, York University, Ontario M3J 1P3, Canada P. Mohanty : A. K. Dalai (*) Department of Chemical and Biological Engineering, University of Saskatchewan, Saskatchewan S7N 5A9, Canada e-mail:
[email protected] P. Mohanty : K. K. Pant Department of Chemical Engineering, Indian Institute of Technology Delhi, New Delhi 110 016, India S. Naik Centre for Rural Development and Technology, Indian Institute of Technology Delhi, New Delhi 110 016, India
for biochars. The presence of cellulose, hemicellulose, and lignin along with other organic components were identified in the spectroscopic and chromatographic analysis. The FTIR spectra of biochars showed removal of oxygen- and hydrogen-containing functionalities from feedstocks due to pyrolysis at higher temperature, although retaining certain significant cellulose-derived functionalities. A number of crystallographic phases in the XRD of biomass, ash, and biochars were due to minerals commonly Na, Mg, Al, Ca, Fe, and Mn. ICP-MS of biochars demonstrated substantial amount of alkali elements indicating their compatibility towards soil amendment for restoring degraded soils. Keywords Lignocellulosic biomass . Biochar . Characterization . Biofuel potential
Introduction The climate change and diminishing oil supplies are issues of acute concern for most countries in the world today. The global use of petroleum and other liquid fuels was 85.7 million barrels per day in 2008; however, the consumption is assumed to escalate to 97.6 million barrels per day in 2020 and 112.2 million barrels per day in 2035 [50]. This projection throws light on waste plant biomass to view it as a means of achieving international energy security and reducing greenhouse gas emissions caused by excessive use of fossil fuels. On the other hand, the dominant sources for today’s bioethanol are corn and sugarcane and for biodiesel are rapeseed, soy, and palm oil. Since these raw materials are food and feed-based goods, they are often surrounded by the food-versus-fuel controversy. In addition, the resulting inflation in food prices has created an international interest in exercising waste biomass for fuels. The worldwide
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energy consumption in 2008 was 533 EJ and it is projected to increase to 653 EJ by 2020 and 812 EJ by 2030 [50]. It is believed that a major portion of this future energy supply (500 EJ per annum by 2050) will be from waste biomass [21]. The waste plant biomass is generally non-edible and lignocellulosic in nature. Lignocellulosic materials incorporate agricultural residues, energy crops (temperate grasses), wood residues, and municipal paper waste. Chemically, lignocellulosic biomass is composed of 35–55 % cellulose, 20–40 % hemicellulose, and 10–25 % lignin. Above all, the intrinsic properties of biomass determine both the choice of conversion process and any subsequent processing complexities that may arise [32]. The type of biomass (woody or herbaceous) equally explains the amount of energy stored in it. Furthermore, the sugar (pentose and hexose) composition in the biomass decides the theoretical yield of fuel alcohols and can thereby have a considerable impact on the process economics. Hence, it is the interaction between all these parameters that enables flexibility in utilization of biomass as an economic and efficient energy resource. It is well understood that renewable energy strategies have tendencies to mitigate greenhouse gas emissions and decelerate climate change. One of such approach is biochar which is a major product of pyrolysis (thermochemical conversion) of biomass to biooils and syngas. Fast pyrolysis is one such biomass conversion techniques that yield mostly biooil, for subsequent upgrading to transportation fuels, and smaller quantities of biochar and pyrolysis gas [11]. Fast pyrolysis usually operates under O2 free atmosphere at temperatures ranging from 400 to 550 °C with a residence time as low as 5 s. Both fixed and fluidized bed reactors have been operated for fast pyrolysis [10]. Some recent studies on wood [18], crop stems [46], and coconut shells [42] have demonstrated the use of fixed-bed (batch) reactors for fast pyrolysis. Moreover, the temperature and vapor residence times influence the quality of the biooil [10]. The vapors continue to crack at high temperatures and the longer they remain at high temperatures, the extent of cracking increases. While a typical fast pyrolysis is characterized by a short vapor residence time of 1–5 s, some studies have employed residence times of 38–88 s in the pyrolysis of rice husk [39] and cotton seed [49] in fixed-bed reactors. Although biooil is a major product (∼40–60 wt.%) of fast pyrolysis, biochar is an important byproduct that is typically obtained in the range of 15–40 wt.% of the feedstock. It is collected as a dry and brittle residue with lower amount of tars. Biochar is suitable for fertilization, gasification, or combustion and has promising potentials towards carbon sequestration and reducing the rise in atmospheric CO2. It is rich in carbon and has abilities to retain soil fertility and organic matter.
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The primary aspect in utilizing biomass for fuel is to understand its basic composition and properties. Biomass is a complex heterogeneous mixture of key structural organic components such as cellulose, hemicellulose, and lignin along with accessory organic and inorganic composites. The qualitative and quantitative characterization of such components in the biomass is essential for its application perspectives. Hence, an overall characterization of biomass is indispensable to expand the bioenergy and bioproduct sectors worldwide. With this objective, we aim to characterize lignocellulosic biomass of different origin such as agricultural, forestry, and perennial grass systems, along with their biochars. The biomass in this study are wheat straw, pinewood, and timothy grass; all the three are available in abundance in the North America. It is predicted that the availability of lignocellulosic materials in the USA and Canada is in a range of 6–577 and 64–561 million dry tons per annum, respectively [19]. The agricultural residues and energy crops comprise a larger category of biomass in the USA, whereas the bulk category is reserved to energy crops and wood residues in Canada. The major objective of the research is to characterize the above lignocellulosic materials and their biochars for evaluation of the candidacy towards biofuel production. It is noteworthy that biomass characteristics depend on local climate and soil conditions. However, in this study, Canadian climatic conditions are considered.
Materials and Methods Biomass In this experimental study, Pinus banksiana (pinewood, PW) as forest residue, Phleum pratense (timothy grass, TG) as energy crop and/or perennial grass, and Triticum aestivum (wheat straw, WS) as agriculture residue were used as biomass sources. The biomasses were obtained from the province of Saskatchewan, Canada in the summer of 2011. Wheat straw and timothy grass were procured from a local farm immediately after harvest. During harvest, the maturity of wheat was in the ripening stage, particularly the “kernel hard” phase, while TG was in the later flowering stage. PW was collected from a neighboring forest and the sampling site was at least 1.0 km away from the road. The sampling pine trees were nearly a century old in age. During sampling, the trees were healthy and of distinctive pulp-wood quality. After collection, PW was made free of bark. The feedstocks were collected between 3 and 5 kg in weight, and the visible contaminants such as sand, soil, or shell particles were removed manually. The feedstocks were then air-dried under ambient conditions over a period of 3 months.
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Air-dried feedstocks were crushed using a Wiley mill with a sieve screen of 1.18 mm. The crushed biomass particles passed the screen of sieve #16 (1.18 mm) and sieve #200 (0.075 mm). Particles unable to pass through sieve #16 were regarded as oversized, while those passing through it were used in the characterization and pyrolysis experiments due to their smaller particle diameter. The pulverized feedstock samples were stocked in glass jars at room temperature and used as necessary. The age of the feedstocks after collection, air drying, and milling was around 5–6 months until the initial analysis and pyrolysis were performed. All the three feedstocks have varying morphological properties, e.g., pinewood is more fibrous and rigid than timothy grass and wheat straw. In order to study the percent distribution of biomass particles (diameter WS. Similar to the C trend, H content in woody biomass (PW) and its biochar (PWB) was higher than that of herbaceous biomasses (TG and WS) and their biochars, respectively. The analogous variation of C and H indicates their association and occurrence in biomass as carbohydrates and in biochars as hydrocarbons. TG (1.3 wt.%) and TGB (1.9 wt.%) showed noticeable levels of N among the biomasses and biochars which is in agreement with the findings of Obernberger et al. [36]. The O/C fraction within the biomass samples did not show major variation; however, the H/C, N/C, and O/C in TG and WS had an analogous inclination in TGB and WSB, respectively. From the atomic ratios (H/C, O/C, and N/C), the empirical formulae for PW, TG, and WS in this study are established as CH 1.5 O 0.7 N 0.001 , CH 1.7 O 0.8 N 0.03 , and CH1.6O0.8N0.01, respectively. Likewise, the empirical formulae for PWB, TGB, and WSB are ascertained as CH0.6O0.2N0.002, CH0.7O0.3N0.03, and CH0.6O0.3N0.01, respectively. A van Krevelen diagram comparing atomic ratios of H/C and O/C was constructed using the data in Table 1 to estimate the degree of aromaticity and carbonation in biomass and biochars (Fig. 2). Atomic H/C and O/C ratios are indices of aromaticity and carbonation of biochars. The H/C and O/C ratios in biochars were lower than those in their respective feedstocks which were due to dehydration,
Fig. 2 van Krevelen diagram of atomic ratios for biomass and biochars
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decarboxylation, and decarbonylation [25]. van Krevelen diagram also demonstrates the significance of O/C and H/ C ratios on the heating value of feedstocks and biochars. From the van Krevelen diagram (Fig. 2), it is well illustrated that biochars had a high calorific value than that of their feedstocks, which was further explained using the Dulong’s formula for HHV. The calorific values of the biomass samples were estimated both experimentally through bomb calorimeter and theoretically (HHV) from their respective C, H, and O contents. Values from both the sources matched with an error percent of ±5. Both experimental and theoretical calorific value of PW and PWB was high in comparison to the respective biomasses and biochars (Table 1). It is noteworthy that calorific value of a feedstock is equivalent to its maximum value of the energy produced or available. In general, biochars had high calorific value compared to biomass samples because of their lower proportion of H and O than C. This considerably increases the energy value of the fuel as more energy is contained in C–C bonds than in C–O and C–H bonds. Irrespective of the conversion process used such as combustion, gasification, pyrolysis, fermentation, or mechanical extraction, the total available energy will always be the same; however, the form and amount of energy obtained will vary within different conversion processes [32]. Table 2 gives the inorganic metal composition of biomass, ash, and biochar samples. Compared to PW and TG, significant amount of elements such as Li, Na, K, Ca, and Zn were detected in WS. In the same way, Mg, P, Ni, and Cu were detected in significant amounts in TG. Ca content was found to be high in TG (3,056 ppm) and WS (3,586 ppm) compared to PW (1,459 ppm). In plants, Ca is accumulated in the aerial parts [54], especially in the bark and parenchyma cells of leaves through precipitation of calcium oxalate crystals as raphides. Since PW was made free of bark, it showed a lower amount of Ca than TG and WS that have most of their aerial parts used in the analysis. The level of heavy metals such as Cr, Fe, Co, Ag, Cd, and Pb was highest in case of PW and PW ash (PWA) than in TG ash (TGA) and WS ash (WSA) and the rest of the samples. In a biofuel industry, volatile toxic heavy metals such as Pb and Cd are enriched in fly ash fractions, whereas Hg which is present in very low concentrations in the fuel escapes almost completely with the gas fraction [40]. This supports the trend of Pb, Cd, and Hg in our findings. A considerable amount of alkaline metals such as Na, Mg, P, K, and Ca in biochar samples indicate their compatibility as soil amendments for enhancing soil fertility, increasing soil organic carbon, and reducing soil acidity. Among biochars, WSB had their highest levels (P, 17,675 ppm and K, 78,499 ppm). Nevertheless, higher levels of Na, Mg, P, K, and Ca were noticed in herbaceous
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Table 2 ICP-MS for common elements present in biomass, ash, and biochar samples (ppm) Element
PW
TG
WS
PWA
TGA
WSA
PWB
TGB
WSB
Li Na Mg Al P K Ca Cr
0.1 17 256 113 94 591 1,459 2
0.3 54 1,973 57 1,819 18,254 3,056 1
0.3 2,720 1,021 113 1,195 25,747 3,586 2
17 24,634 26,275 4,242 17,676 78,499 312,633 106
13 4,481 19,253 429 27,887 219,701 59,816 11
11 36,443 12,841 891 13,586 208,550 141,891 9
2 164 5,227 110 4,685 46,351 8,352 8
1 7,878 2,993 270 3,620 76,563 11,594 19
1 24,634 26,275 4,242 17,675 78,499 312,633 106
Mn Fe Co Ni Cu Zn As Rb Sr Ag Cd Hg Pb Th U
78 654 0.2 1 2 24 0.1 1 7 0.1 0.1 1 59 1 0.1
54 123 0.1 3 3 23 – 4 13 0.01 0.1 0.4 2 – 0.01
15 125 0.1 1 1 32 – 5 14 – 0.1 1 1 – –
4,874 21,592 10 294 1,302 1,610 1 2 20 15 6 – 287 – 1
640 3,137 1 37 511 257 – 11 35 0.1 1 – 62 – 0.2
176 1,681 1 16 615 106 – 13 39 0.1 1 – 56 – 0.3
149 254 0.2 44 24 62 2 50 990 0.1 0.3 1 14 0.04 0.3
51 668 0.4 93 13 65 2 57 130 0.1 1 1 8 0.04 0.4
4,874 21,592 10 294 1,302 1,610 1 44 221 15 6 4 6 0.1 1
residues than in PW. This might be due to the fact that the feedstocks with high annual growth rates are rich in alkaline elements as they are readily taken up from the soil [51]. The presence of alkali metals is another important considering factor for the thermochemical conversions as their reaction with silica present in ash can form slag-like liquid that blocks the pipelines in the furnace and boiler plants as discussed earlier. This drastically contributes to the processing cost. The enrichment of feedstocks with alkaline metals has a possibility to contribute significantly in the capture and reduction of harmful volatile components in the vehicular emissions by modification of the feed fuel composition through tailored and self-cleaning fuel mixtures during cofiring or co-gasification of biomass [52]. The ICP-MS results also suggest a minimal sensitivity for some transition metals such as Rb, Sr, Th, and U in most of the samples. The particle size distribution for the biomass samples is shown in Fig. 3. From the distribution, it is clear that the particle size for PW, TG, and WS were in a range of 145– 1,147, 101–1,159, and 185–1,375 μm, respectively. It was observed that a total of 10 % of PW particles were under the size of 145 μm, 50 % under the size of 525 μm, and 90 % under the size of 1,147 μm. Similarly, 10 % of TG and WS particles were under 101 and 185 μm, 50 % under 421 and
609 μm, and 90 % under 1,159 and 1,375 μm, respectively. Smaller the particle size of the feedstock higher is its surface area which makes the activity of fungal hyphae and air to penetrate to the vicinity of the particles during fermentation to higher alcohols [28]. As discussed before, a significant fraction of biomass consists of inorganic constituents in form of ash which
Fig. 3 Particle size distribution of biomass samples
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cannot be converted into energy. The inorganic material are commonly associated with O-, S-, and N-containing functional groups. These organic functional groups can provide suitable sites for the inorganic species to be associated chemically in the forms of cations or chelates. The FTIR spectra of biomass samples, their extracts (water, ethanol, and hexane), raffinate materials, and biochars were studied to determine the vibration frequencies in the functional groups. The spectra displayed a number of sorption peaks indicating the complex nature of biomass, extractives, and biochar (Fig. 4). The major peaks are tabulated in Table 3 indicating different compounds associated and the type of bonds available in these fractions. Peaks with intensities of strong, medium, broad, and weak were due to different C– H, C–O, C–O, O–H, N–H, C–N, and NO2 bonds available at different frequency levels. The spectra of biomass (Fig. 4a) revealed that most prominent peaks in the spectrum originated from –OH stretching vibration (3,300–3,400 cm−1) and CH2 and CH3 asymmetric and symmetric stretching vibrations (2,915– 2,935 cm−1) [43]. Very intense peaks for biomass in the region 1,620–1,745 cm−1 originated from the stretching of carbonyls, mainly ketones and esters. These bands were predicted from waxes, fatty acids, fatty esters, high
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molecular mass aldehydes, or ketones [20]. The 1,000– 1,280 cm−1 stretch in the biomass spectra was due to alcohols, ethers, carboxylic acids, and esters. The bands due to aliphatic C–O–C (1,097 cm−1) and alcohol C–O stretches (1,175 cm−1) represented oxygenated functional groups of cellulose, whereas the peaks at 1,516 and 1,616 cm−1 are assigned to C–C and C–O stretching in the aromatic ring signifying lignin [13]. The C–O–C symmetric stretching at 1,097 cm−1 in the biomasses is characteristic of cellulose and hemicellulose (pyranose rings and guaiacyl monomers). The C–H and O–H bending frequencies indicating lignin mostly centered at 1,230–1,270 cm−1 and 1,370–1,430 cm−1 [43]. According to Moore and Owen [34], the most characteristic bands of lignin were at 870, 1,263, 1,506, and 1,601 cm−1. The biomass peaks at 2,860 and 2,928 cm−1 are assigned to CH2 stretching bands that were due to the presence of waxes (Fig. 4a). However, due to the series of solvent (hexane and ethanol) washing during Soxhlet extraction, most of the waxy components were removed in the raffinate biomass (Fig. 4b). However, these waxes were noticed again in the hexane extract of the three biomasses (Fig. 4c) confirming the washing out phenomenon. Similar washing out effect was noticed in case biomass peaks at 782 cm−1 (C–H alkynes bends and phenyl ring substitution
Fig. 4 FTIR spectra of a biomass, b raffinate biomass (PWR, TGR, WSR), c biomass extract, and d biochar samples
Bioenerg. Res. (2013) 6:663–677 Table 3 Functional groups and components in biomass, extractives, and biochars
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Component
Biomass 480, 554 625 674 782, 815 903 1,097 1,175 1,280 1,442 1,516, 1,616 1,668 1,745 2,860, 2,928
Aromatic rings, C–C stretching C–H alkenes stretching C–H alkynes bends, C–H alkenes, C–H phenyl ring substitution bands C–H alkynes bends, C–H phenyl ring substitution bands C–H alkynes bends C–O–C symmetric stretching Alcohol C–O stretches, ethers, carboxylic acids Aromatic C–O stretching Aliphatic C–H stretching, alkanes C–H scissoring and bending Aromatic C–C ring stretching, N–H amines C0C alkene stretching, C–H phenyl ring substitution overtones Aldehydes C0O stretches, esters, ketones, carboxylic acids Alkanes/aliphatic C–H stretching
3,450 Extractives 450, 545 800, 880 1,051, 1,070 1,220 1,254 1,380 1,410 1,630 2,095 2,910 2,970 3,450 3,670 Biochar 480, 592, 652 782, 840, 885 1,097 1,618 1,709 2,950 3,544 3,642
bands) and 2,860 cm−1 (alkanes/aliphatic C–H groups) and their absence in raffinate fractions. In Fig. 4c, prominent broad peaks centered at 3,200– 3,600 cm−1 originating from the stretching of H-bonds available with alcohols or phenols. In case of hexane extract, the CH2 and CH3 asymmetric and symmetric stretching vibrations occurred in biomass spectra in the range of 2,910–2,973 cm−1. These vibrations were due to the extraction of some hemicellulosic and cellulosic materials. Different transmitted peaks
–OH stretching Aromatic rings, C–C stretching C–H, aromatic hydrogen Aliphatic ether, alcohol and ester C–O stretching C–O stretching, phenols C–N amines, alcohol C–O stretching, ethers, carboxylic acids esters Aliphatic CH3 deformation NO2 nitro compounds symmetrical stretching C–H alkanes scissoring and bending Aromatic C–C ring stretching C≡C alkynes stretching Alkanes/aliphatic C–H stretching Alkyl/aliphatic C–H stretching –OH stretching –OH stretching, alcohols, phenols Aromatic deformation rings, C–C stretching C–H, aromatic hydrogen C–O–C symmetric stretching Aromatic C–C ring stretching Phenyl ring substitution overtones Alkyl/aliphatic C–H stretching –OH stretching –OH stretching, alcohols, phenols
at 1,620–1,760 cm−1 in water and ethanol extracts were due to the stretching of C–O available with aldehydes, ketones, carboxylic acids, and esters [37]. Extractives with hexane and ethanol exhibited less intense –OH peaks (3,450– 3,670 cm−1) indicating the partial loss of waxy materials from –OH groups into the organic solvents. In the spectra of extractives, peaks between 1,056 and 1,603 cm−1 represented C–H and O–H bending frequencies which were due to lignin. Peaks at 1,516–1,616 cm −1 in biomasses and 1,254 cm –1 in
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extractives are dominated by aliphatic and aromatic Nheterocyclics as amine groups that are available in the form of N–H and C–N functional groups. The FTIR spectra showed the removal of O- and Hcontaining functionalities in biochars as compared to feedstocks due to pyrolysis at higher temperature (Fig. 4d). The absence of these functional groups in the biochars is an indication of evolution of CO2, CH4, and H2 from the pyrolysis of cellulose, hemicelluloses, and lignin at temperatures above 400 °C. While CO2 was generated by cracking and reforming of C0O bonds, CO was produced from cracking of C–O–C and C–O bonds, and H2 was produced by the breaking of C–H bond and formation of H–H bond [42]. It is clear from the FTIR spectra of biochars that with increased temperature there was a remarkable decrease in features associated with –OH stretch (3,544–3,642 cm−1), aromatic C–C ring stretching (1,618 cm−1), alkyl/aliphatic C–H stretching (2,950 cm−1), and C–O–C symmetric stretching (1,097 cm−1). The occurrence of these features was because of the dehydration of lignocellulosic components as well as increase in the degree of condensation above 400 °C. Absorption bands arising from aromatic C–H groups (782, 840, and 885 cm−1) were found in biochars; however, the intensity of peak at 480 cm−1 (aromatic C–C stretching) was higher than in feedstocks. The components present in biomass within the spectral region of 903– 1,516 cm−1 diminished upon heating the materials to 450 °C. Due to thermal destruction of cellulose and lignin, aliphatic C– H alkyl (2,950 cm−1), and aromatic (480–885 and 1,618 cm−1) and –OH (3,544–3,642 cm−1) groups were exposed and remained at high charring temperatures. FTIR precisely identifies the O-containing functional groups in carbonaceous materials, but it has restricted application in exploring the less-polar aromatic structures. On the other hand, Raman spectroscopy is widely used to evaluate the structural features of such materials because it is sensitive to both crystalline and amorphous constructions. In the Raman spectrum of biomasses (Fig. 5), major vibrations in the region 1,595–1,650 cm−1 were due to lignin [1], whereas cellulosic and hemicellulosic components occurred in the spectral range of 973–1,183 cm−1. Li et al. [30] reported the presence of aromatic skeletal lignin at 1,510 cm−1 and syringyl and guaiacyl condensed lignin at 1,329 cm−1 in switchgrass. In this study, two main lignin features were observed that correspond to lignin aromatic ring stretch at 1,595 cm−1 and ring-conjugated C0C bond at 1,626 cm−1 (1,650 cm−1 in case of PW), respectively. The Raman peak at 900 cm−1 is reflective to amorphous cellulose, whereas the one at 1,098 cm−1 is assigned to crystalline cellulose. Moreover, the bands at 1,056 cm−1 represented C–O stretch in cellulose and hemicelluloses, while the 1,375 cm−1 band represented C–H deformation in cellulose and hemicellulose, respectively. Studies have shown that the characteristic broad peaks in Raman spectra of biomass samples at 1,580–1,610
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Fig. 5 Raman spectra for biomass samples
and 1,325–1,380 cm−1 represent the graphite (G band) and defect (D band) structures in biochars, respectively [24, 25]. Thermogravimetric analysis in the inert atmosphere of N2 was carried out to evaluate the pyrolytic degradation and decomposition behavior of the three lignocellulosic biomass species (Fig. 6a). The devolatilization process started between 200 and 250 °C with the maximum weight loss occurring in
Fig. 6 a Thermogravimetric and b differential thermogravimetric analysis of biomass samples
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the range of 300–350 °C. A characteristic change in weight loss of the biomass samples was found in the temperature range of 370–400 °C following a slower weight loss which is an indication of biochar generation. The investigation of the weight loss for the three feedstocks indicated the liberation of more than 60 wt.% of the final volatile matter between 150 and 550 °C. A very similar trend of devolatilization was found in case of poplar wood by Carrier et al. [12], where the primary weight loss of the biomass started at 150 °C with a slow and constant material loss between 370 and 400 °C. The initial weight loss of the feedstocks was due to their moisture and volatile components, whereas the later weight loss was attributed by the degradation of fibrous material from cellulosic and hemicellulosic contents [16]. Figure 6b gives the differential thermogravimetric spectra of PW, TG, and WS. The spectra revealed the major peak along with some shoulder peaks. Carrier et al. [12] have assigned the major peak as per the degradation of hemicellulose (200–300 °C) and the shoulders with the degradation of cellulose (250–350 °C) and lignin (200–500 °C). The intensities of these peaks could be
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associated with the magnitude of cellulose, hemicellulose, and lignin content in the biomass. The leaning of differential thermogravimetric curve signifies the devolatilization trend as WS>TG>PW, where the heat flow was endothermic for the processes and varied in the range of 0 to −8 (W/g) for each biomass. TG/DTG analysis is essential in predicting the pyrolytic behavior of a feedstock as biomass pyrolysis to liquid fuels can be divided into four phases: moisture evolution, hemicellulose decomposition, cellulose decomposition, and lignin decomposition. The XRD patterns of biomass, raffinate biomass, and ash samples are presented in Fig. 7. A few number of crystalline and semi-crystalline, mostly poorly crystallized or cryptocrystalline phases which were close to the XRD detection limit belonging to some minerals commonly Na, Mg, Al, Ca, Fe, and Mn were identified in the ash samples. These elements were detected in considerably higher amounts in ash samples through the ICP-MS analysis as well (Table 2). The XRD pattern of biomass is often characterized by an intensive amorphous halo with a major maximum at 20–23° and a
Fig. 7 XRD analysis of a biomass, b raffinate biomass, c ash, and d biochar samples
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minor maximum at 13–17° which signifies the occurrence of cellulose [52]. In biomass, cellulose is present both in crystalline and amorphous forms. Crystalline cellulose comprises the major proportion, whereas a small percentage of unorganized cellulose chains form amorphous cellulose. Amorphous cellulose is more susceptible to enzymatic degradation [27]. In the XRD pattern of biomass, the peaks at 15.5° (d-space ∼5.51 Å), 21.7° (d-space ∼3.96 Å), and 34.5° (d-space ∼2.59 Å) are assigned to cellulose I, cellulose II, and hemicellulose, respectively (Fig. 7a). The degree of crystallinity of biomass materials at 13–17° and 20–23° is reported to be due to cellulosic polymorphs [14], while some reports suggest the occurrence of lignin at 15° which is characterized by a strong peak [41]. The degree of crystallinity at 20–23° as broad diffusion reflections is dependent on the presence of two cellulosic polymorphs [17], waxes, and the complex nature of bonding between the three structural components. Some crystalline phases originally present in biomass samples (57.4° and 67.7°) were not found in their raffinate portions (Fig. 7b) but surprisingly appeared in the ashes (Fig. 7c). The diffraction peaks of biomass at 57.4° (d-space ∼1.61 Å) was due to the presence of halite [31], whereas 67.7° (d-space ∼1.38 Å) represented sylvite [22]. The absence of halite and sylvite in raffinate biomasses was probably due to their washing out during the Soxhlet extraction cycles. However, these chloride peaks were found in the biomass ashes at 36.6° (d-space ∼2.45 Å) and 66.3° (dspace ∼1.41 Å) for halite and 48.3° (d-space ∼1.87 Å) for sylvite, respectively [53]. With the removal of these minerals in the raffinate portions, the relative intensities of the organic components at 15.5°, 21.7°, and 34.5° increased than the untreated biomass. Biochars presented a similar diffraction shape to that of the biomasses within 15–30° with a major maximum at 28.3° (dspace ∼3.17 Å) and a minor maximum at about 40.5° (d-space ∼2.25 Å) (Fig. 7d). These are due to the characteristic graphite d-spacings which are nearly at 27°, 42°, and 51–60° [52]. The peak at 15.9° (d-space ∼5.49 Å) in PWB might be related to the higher occurrence of lignin in PW. Overall, the major maximum and minor maximum peaks of feedstocks tend to have higher d-spacings in contrast to their biochars due to an increase in aromatization of molecular structures in the biochars along with the crystalline nature of graphite. The diffraction pattern of ash samples suggest their highly crystalline nature which was because of their production conditions (575 °C for 4 h) that caused limited occurrence of Table 4 Extractive and lignocellulosic composition (wt.%) in biomass hydrolysates
inorganic amorphous material and organic matter at this higher temperature. Peaks at 20.9° (d-space ∼4.25 Å), 23.2° (d-space ∼3.78 Å), and 28.4° (d-space ∼3.15 Å) are due to some silicates such as plagioclase, quartz, and feldspar, respectively [53]. Considerable amount of silicates are found in straws and grasses [22]. In wood, silicates primarily occur in the supportive tissues such as bark [38]. In this study, PW was debarked hence a very less intensified feldspar (silica) peak at 28.4° was found in its ash. Peaks at 23.2° (gypsum) and 29.5° (anhydrite) in feedstock ashes are characteristic of sulfate group of minerals [53]. Vassilev et al. [53] reported various carbonate peaks in biomass ashes which are coherent to our findings, e.g., calcite at 26.7°, 43.2°, 50.2°, 57.4°, and 67.9°; anhydrite at 29.5°; fairchildite at 32.3° and 39.5°; and natrofairchildite at 36.6°. Most of the mineral components reported here, particularly silicates, are responsible for maintaining plant’s rigidness and posture, and their introduction into the plants can have both natural and anthropogenic routes. Since the majority of these minerals are abundant in soil, they can be found on plant surfaces during harvest and transportation as the common anthropogenic pathways. Their natural introduction into the feedstock could be during the plant’s growth stage through endocytosis as dispersed mineral grains (hemicellulose (H)>lignin (L). A biochemical conversion is dependent on the lignocellulosic composition of the biomass. Cellulose and hemicellulose have a better
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biodegradability than lignin. Hence, the overall conversion of feedstocks with higher cellulose and hemicellulose levels is superior to those having higher lignin levels.
Conclusions The present work not only explains the carbon-neutral nature of energy from lignocellulosic biomass but also throws some light on the carbon capture effect due to fixation of CO2 in the combustion residues. The fast pyrolysis of PW, TG, and WS gave biochar yield of 24.0±2.1, 22.0±1.9, and 21.0±2.4 wt.%, and biooil yield of 48.0±2.9, 42.0±3.2, and 40.0±2.6 wt.%, respectively. Relatively high yield of biooil and biochar from PW was because of its recalcitrant and rigid morphology. This characteristic feature of PW was also studied from the TG/DTG analysis, which explained its maximum weight loss at ∼380 °C than ∼350 °C as in the case of TG and WS. From the XRD analysis of biomass ashes, the mineral composition demonstrated the formation of various carbonates as a result of reaction between the CO2 (atmospheric and/or released from biomass combustion) and alkaline-earth metals and alkaline oxyhydroxides formed during biomass combustion. The biomass ashes were rich in various species of inorganic components such as carbonates, silicates, and sulfates along with traces of chlorides. The organic matter in biomass comprising of cellulose, hemicellulose, lignin, and extractives had nonor poorly crystalline character with larger d-spacings (between 2.59 and 5.51 Å) in comparison to those of biochars (between 1.41 and 5.49 Å) due to increase in aromatization of molecular structures in biochars along with the crystalline nature of graphite. The XRD of biomasses demonstrated the presence of cellulosic polymorphs and hemicellulose at 15.5°, 21.7°, and 34.5°, respectively. The higher amount of hemicelluloses in TG and WS was because of their herbaceous and fast-growing nature. The similar reason also applies for higher amount of alkaline metals in TG and WS than in PW. In addition, high lignin in PW was due to its characteristic tightly bound fibrous texture in contrast to the loosely bound fibers in herbaceous plants. FTIR and Raman spectroscopy revealed the presence of waxes, fatty acids, aldehydes, alcohols, ethers, carboxylic acids, and esters with cellulose, hemicellulose, and lignin in biomasses. The physiochemical resemblance of TG with WS suggests that perennial grasses (short-rotation crops) could supplement the anticipated demand of agro-residues in future as next generation feedstocks. As a major byproduct of fast pyrolysis of feedstocks, the resulting biochars find tremendous application as a soil amendment for enhancing soil fertility, increasing soil organic carbon and reducing soil acidity due to the richness of alkaline metals such as Na, Mg, P, K, and Ca.
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