ARTICLE pubs.acs.org/EF
Surface Functionality and Carbon Structures in Lignocellulosic-Derived Biochars Produced by Fast Pyrolysis Pyoungchung Kim,† Amy Johnson,‡ Charles W. Edmunds,† Mark Radosevich,‡ Frank Vogt,§ Timothy G. Rials,† and Nicole Labbe*,† †
Center for Renewable Carbon, ‡Department of Biosystems Engineering and Soil Science, and §Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States
bS Supporting Information ABSTRACT: Switchgrass- and pine wood-derived biochars produced by fast pyrolysis were characterized to estimate the degree of thermochemical transformation and to assess their potential use as a soil amendment and to sequester carbon. The feedstocks were pyrolyzed to biochars in an auger reactor at 450, 600, and 800 °C with a residence time of 30 s. Ash contents of switchgrass and pine wood biochars varied from 13 to 22% and from 1.3 to 5.2%, respectively. Nutrients, such as N, P, K, S, Mg, and Ca, in switchgrass biochars ranged from 0.16 to 1.77%. Under combustion conditions, switchgrass chars were decomposed at lower temperatures than pine wood biochars because of the structural differences between the two feedstocks. Principal component analysis of the Fourier transform infrared (FTIR) spectra allowed for the discrimination of all biochars by significant contributions of cellulose-derived functionality at low pyrolysis temperatures, while the same analysis of the Raman spectra presented apparent separation of all biochars by two broad bands at 1587 and 1350 cm 1. These two broad peaks were deconvoluted into pseudo-subpeaks, which revealed that the number of aromatic rings linearly increased with the pyrolysis temperature. Cross-linkages between aromatic rings were also found to increase with thermal treatment, and switchgrass biochars contained a higher number of aromatic rings and cross-linkages than pine wood biochars, which was consistent with turbostratic carbon crystallites in the X-ray diffraction (XRD) pattern.
1. INTRODUCTION The desire to reduce the dependence upon fossil fuels and mitigate greenhouse gas emissions has resulted in increased efforts to develop technologies to acquire more energy from renewable resources. An integrated agricultural biomass bioenergy system offers the potential to mitigate greenhouse gas emissions, substitute for fossil fuels, and improve soil quality.1 Various options and technologies, such as thermochemical conversion and biotechnologies, including pyrolysis, gasification, digestion, and fermentation, exist for producing bioenergy from biomass.2,3 Of particular interest is pyrolysis, a process that converts biomass at elevated temperatures and in the absence of oxygen into bio-oil, syngas, and biochar fractions. The relative amounts and characteristics of these fractions are influenced by the processing conditions, such as temperature, pyrolysis time, and feedstock types and characteristics.4 Bio-oil and syngas are used as substitutes for all transportation fuels, such as diesel, jet, and gasoline fractions.4 As a co-product of the pyrolysis process, biochar is also a potential energy product. Previous studies examining the effects of landapplied biochar suggest that biochar incorporation improves the physical and chemical properties of soil and may enhance nutrient cycling and plant growth.5,6 Additionally, most studies indicate that remineralization of soil-incorporated biochar is slow, and therefore, the extended persistence may provide carbon-sequestration potential.6 If correct, long-term sequestration of biochar C suggests that pyrolysis of biomass may be a C-negative technology with respect to the amount of carbon dioxide returned to the atmosphere.7 r 2011 American Chemical Society
Biochar is high in C, and most, if not all, of that C is in recalcitrant forms.7 The physical and chemical properties of biochar depend upon the characteristics of the biomass used and the operating parameters of the pyrolysis process. Gaskin et al.8 pyrolyzed various types of feedstocks and determined that biomass type has a significant influence on the elemental composition of the resulting biochar. Tang and Bacon9 observed that, with an increasing pyrolysis temperature, the thermochemical conversion of biomass to biochar leads to the formation of aromatic ring structures while simultaneously removing chemical functionality. Industrially available charcoals are produced by slow pyrolysis under conditions of long char residence times from hours to days.10 Fast pyrolysis involves 0.5 30 s of residence time to maximize bio-oil production, with biochar as a co-product.11 Physical and chemical features of charcoals produced by slow pyrolysis have been wellcharacterized, but few studies have examined biochar produced by fast pyrolysis. As interest in the use of biochar increases, for example, as a soil amendment as part of an integrated bioenergy crop production system, it will be necessary to assess the impact of process variables on the final characteristics and quality of the resulting biochar. In particular, surface characteristics of biochars have been measured using Fourier transform infrared (FTIR) spectroscopy, Received: June 23, 2011 Revised: September 5, 2011 Published: September 09, 2011 4693
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Figure 1. Schematic of the single-auger pyrolysis system for producing biochars at 450, 600, and 800 °C.
X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), solid-state 13C nuclear magnetic resonance (NMR), and Raman spectroscopy.12 15 Among the instrumental techniques, FTIR spectroscopy has been widely used to detect nonaromatic functionality on the surface of biochars.15 XRD has been used to measure the degree of crystallinity in the carbonaceous materials,14 while the Raman spectroscopic technique has been applied to characterize the structural and reactive features of graphitic carbonaceous structures using two bands at 1580 1620 cm 1 (IG peak) and 1355 1380 cm 1 (ID peak) because Tuinstra and Koenig16 revealed the correlation of structural crystallinity measured by Raman scattering and XRD. Therefore, Raman spectroscopy is an essential tool for the characterization of biochars produced from lignocellulosic biomass. Recently, using Raman spectra of highly disordered brown coals and their resulting charcoals, Li et al.17 reported that broad Raman peaks, IG and ID peaks of amorphous charcoals, are not correlated to the structural crystallinity of graphitic materials measured by XRD. They also suggested that the large overlap between the two broad Raman IG and ID peaks of amorphous charcoals conceals many structural defects that contain valuable information about the characteristics of amorphous C materials. The objective of this study was to compare the characteristics of biochars produced from two different types of biomass, switchgrass (Panicum virgatum L.) and pine wood, under fast pyrolysis at different temperatures. While pine wood is an important feedstock for the forest and now the bioenergy industry, switchgrass is a perennial herbaceous crop and a promising source of cellulosic biomass for the production of biofuel because of its high productivity and potentially low requirements for agricultural inputs.18,19 Understanding thermal transformation of biomass during pyrolysis will allow for optimization of process variables to maximize oil yield while generating a biochar co-product suitable as a soil amendment in agricultural applications. In this study, biochars from switchgrass and pine wood were produced and characterized by elemental analysis, thermogravimetric analysis (TGA), and spectroscopic techniques, including FTIR, XRD, and Raman spectroscopy. Principal component analysis (PCA), a multivariate statistical method, was used to investigate the chemical changes that occurred during the thermal treatments and the classification of the biochars produced under different experimental conditions. This study also demonstrated the semi-quantitative interpretation of thermochemical transformation of biochars using the deconvolution of Raman spectra into pseudo-subpeaks.
2. EXPERIMENTAL SECTION 2.1. Materials and Production of Biochars. The feedstocks, air-dried switchgrass and pine wood, were obtained from a local producer in eastern Tennessee and from American Wood Fibers (Columbia, MD), respectively. Switchgrass, as received, contained 7 8% moisture and was chopped and then milled to a particle size of ∼4 mm before use. Pine wood, as received, was less than 0.85 mm in particle size and contained 7.0 8.5% moisture. From these two types of biomass, biochars were produced at 450, 600, and 800 °C with a residence time of 30 s and a feed rate of 5 kg/h in the presence of N2 using a continuous auger pyrolysis unit. Single-walled carbon nanotubes (CNTs), purity > 90% (Carbon Nanotechnologies, Inc.), were used as representative graphitic materials to compare to the produced biochars. The characteristics of the CNTs are provided elsewhere.20 2.2. Description and Operation of the Continuous Auger Pyrolysis System. Figure 1 presents the continuous auger pyrolysis system designed and manufactured by Proton Power, Inc. (Lenoir City, TN), featuring a feeding system, cylindrical auger reactor, bio-oil condenser, and biochar collector. The feeding system includes a feedstock hopper with a lid and a single-auger configuration with a variablespeed drive assembly. The auger pyrolysis reactor features a cylindrical reactor (8.9 cm in diameter 3 m in length) with an internal single auger (7.6 cm in diameter and 10 cm in pitch). The auger speed can vary from 10 to 100 revolutions per minute (rpm). The single auger transfers the feedstock to the biochar collector through the heated zone. The reactor is enclosed by a rectangular chamber [80 cm (H) 80 cm (W) 250 cm (L)]. The heated zone, in the middle of the rectangular chamber, is heated with a 1 m long electrical resistance furnace system. Thermocouples are installed to monitor the real-time temperature at 1 m intervals in the reactor. N2 gas is inserted into the front of the reactor. A bio-oil condenser is used for condensation of effluent gases using flowing tap water. The biochar collector (approximately 15 L) is located at the end of the reactor and purged with N2 gas. Before biochar was produced, the pyrolysis reactor was equilibrated to 450, 600, or 800 °C for 1 h, purging with 10 L/min of N2 gas. The ground feedstock in the feed hopper was transferred to the cylindrical auger pyrolysis reactor at 5 kg/h. The auger speed in the pyrolysis reactor was set at 60 rpm, which corresponded to 30 s of residence time in the 1 m long heated zone. During pyrolysis, the collected biochar was purged with 10 L/min of N2 gas to cool the biochar and to prevent the condensation of bio-oil in the biochar collector.
2.3. Characterization of Feedstocks and Resulting Biochars. 2.3.1. Chemical Composition of Feedstocks. Moisture, ash, extractives, lignin, and saccharide contents in both feedstocks, switchgrass and pine wood, were determined by following the standard method described by the National Renewable Energy Laboratory (NREL).21 In brief, the total extractives fraction was sequentially extracted with water 4694
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Table 1. Chemical Composition (% Dry Weight) of Switchgrass and Pine Wooda hemicellulose
a
cellulose
xylan
galactan
arabinan
mannan
switchgrass
34.1 (0.9)
21.5
1.3
2.9
0.0
pine wood
41.7 (0.2)
9.9
2.0
1.3
9.3
total
lignin
extractives
25.7 (0.6)
18.8 (0.5)
14.2 (0.8)
22.5 (0.3)
25.8 (0.2)
2.7 (0.3)
ash 2.7 (0.1) 0.3 (0)
All samples were average values calculated from N = 3 replicate measurements, with standard deviation values in parentheses.
and ethanol using an accelerated solvent extractor (ASE 350, Dionex Corp.), freeze-dried, and then gravimetrically determined. Extractivesfree biomass was then subjected to two-stage sulfuric acid hydrolysis (1 h at 30 °C, 72% sulfuric acid, followed by 1 h at 121 °C, 4% sulfuric acid in an autoclave). The resulting hydrolysate was filtered. The acid-insoluble lignin fraction was gravimetrically determined, and the acid-soluble lignin fraction was quantified by ultraviolet (UV) measurement (Lambda 650, PerkinElmer, Shelton, CT). The monosaccharides within the soluble liquid fraction were analyzed by high-pressure liquid chromatography (HPLC, Flexar, PerkinElmer). Chromatographic separation was carried out using an Aminex HPX-87P column (300 7.8 mm inner diameter, 9 μm particle size, Bio-Rad) and a refractive index detector (Series 200a, PerkinElmer) with a constant flow rate of 0.25 mL/min using deionized water (Milli-Q, Millipore) and a temperature of 85 °C.22 The ash content was gravimetrically determined by combusting the biomass at 575 °C for 24 h. All compositional experiments were performed in triplicate. 2.3.2. Biochar Yield and Elemental Analysis. The biochar yield in the pyrolysis process at different temperatures was calculated as a percentage of the feedstock input and biochar output (produced biochar/feedstock wt %).23 The pH of the feedstocks and their corresponding biochars was measured after mixing 10 g of material in 200 mL of deionized water (Milli-Q, Millipore) and then shaking (180 rpm) for 24 h. The pH was then measured in the supernatants. Proximate analysis, including moisture content, volatile matter, ash content, and fixed C, was measured by ASTM D1762-84.24 Ultimate analysis of carbon, hydrogen, oxygen, and nitrogen of switchgrass and pine wood, and the resulting biochars was performed using a CHN analyzer (Costech Analytical Technologies). Inorganic elements were analyzed by inductively coupled plasma optical emission spectroscopy with an Optima 7300 DV spectrometer (ICP OES, PerkinElmer) after microwave digestion (U.S. EPA 3502).25 In brief, 0.3 0.5 g of samples were microwavedigested (Multiwave 3000, Anton Paar) with 8 mL of HNO3 (67 70%), 3 mL of HCl (35%), and 0.1 0.2 mL of HF (51%) at 180 210 °C for 100 min. After digestion, 1 mL of boric acid (4%) was added and the sample was reheated at the same digestion condition to complex any remaining HF and to facilitate dissolution of the precipitated fluorides.26 The cooled solutions were transferred to 50 mL polypropylene centrifuge tubes and made up to the mark with deionized water. The samples were filtrated (0.2 μm, Millipore), acidified by 2% HNO3, and stored at 4 °C before the determination of inorganic elements by ICP OES. 2.3.3. TGA. TGA was used to measure thermal decomposition of biomass, the resulting biochars, and CNTs under air atmosphere using a thermogravimetric analyzer (Pyris 1 TGA, PerkinElmer). Samples of 5 6 mg were heated from 50 to 105 °C at a rate of 20 °C/min and kept at 105 °C for 10 min to remove the moisture. Samples were then heated to 800 °C at 20 °C/min (air flow rate of 20 mL/min) to collect thermal decomposition curves. The treatment of thermal curves involved the analysis of TG and differential TG (DTG) combustion thermograms. The sample weight loss at a specific temperature and the rate of weight loss were determined in accordance with the TG combustion curve and the DTG combustion curve, respectively.
2.3.4. Spectroscopic Characterization. FTIR spectroscopy was applied to investigate the functional groups in the feedstock and in the derived biochars, using an attenuated total reflectance (ATR) attachment. Spectra were recorded in the range of 4000 600 cm 1 with a resolution of 4 cm 1 using a PerkinElmer Spectrum One spectrometer. The collected spectra were ATR-corrected prior to analysis (see the Supporting Information). XRD and Raman spectroscopy were used for the structural characterization of the biochars. XRD diffractograms were collected using XRD (Bruker AXS, Germany). The ground sample was loaded into a circular cavity holder, and the XRD was operated at 40 kV and 40 mA using Cu Kα radiation. Diffractograms were obtained from 5° to 50° (2θ scale) at a scan rate of 0.05° and a counting time of 40 s steps. Raman spectra were collected at room temperature with a SENTERRA spectrometer (Bruker), equipped with a thermoelectrically cooled charge-coupled device (CCD). A laser energy of 532 nm wavelength was used as an excitation source. The laser was focused on the sample at 10 mW with an Olympus 20 objective to prevent irreversible thermal degradation. The integration time was 30 s. Spectra were collected in the range of 2750 50 cm 1 with 3 5 cm 1 of resolution. Raman spectra at 1800 1100 cm 1 were deconvoluted into mainly seven pseudo-subpeaks in the Gaussian mode using XPSPeak 4.1 software (Freeware available at http://www.phy.cuhk.edu.hk/∼surface). 2.3.5. Multivariate Statistical Analysis. For each sample, 15 spectra were collected by FTIR ATR and Raman spectroscopy and analyzed using the multivariate method, principal component analysis (PCA), to classify the samples by their spectral features.27 Prior to analysis, the spectral data were pre-processed in the Unscrambler software (version 9.0, CAMO, Woodbridge, NJ). FTIR ATR (1800 800 cm 1) and Raman (1800 1100 cm 1) spectra were pretreated using baseline correction, mean normalization, and multiplicative scatter correction (MSC). MSC is a transformation method used to compensate for additive and/or multiplicative effects in spectral data. The results from the PCA are displayed in scores and loadings plots. The scores plot describes the relation between samples and helps visualize any clustering or trends in the data set in the new system of axes of principal components (PCs). The loadings plot presents the relationship between the wavenumbers and determines which spectral region contributed the most to the separation and/or classification of the samples.
3. RESULTS AND DISCUSSION 3.1. Chemical Composition of Biomass. Table 1 presents the chemical composition summary of both feedstocks, switchgrass and pine wood. Switchgrass contains more hemicellulose, extractives, and ash and less lignin and cellulose than pine wood, demonstrating the structural differences between the two feedstocks. 3.2. Biochar Yield and Elemental Analysis. The pH and proximate and ultimate analyses of feedstocks and the corresponding biochars produced at 450, 600, and 800 °C are presented in Table 2. As the pyrolysis temperature increased from 450 to 800 °C, biochar yields decreased from 31.3 to 11.4% 4695
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Table 2. Properties of Feedstocks and the Resulting Biochars Produced at Pyrolysis Temperatures of 450, 600, and 800 °Ca proximate analysis (wt %) water
volatile
ash
fixed
content
matter
content
carbon
produced biochar/
Ob
atomic ratio
H/C
O/C
9.70 (0.30) 45.58 (0.17) 5.45 (0.08) 45.65 0.59 (0.02)
1.43
0.75
9.1 (0.4) 1.84 (0.19) 26.34 (2.32) 13.44 (0.91) 58.38 (1.70) 66.54 (0.71) 3.43 (0.24) 15.31 1.28 (0.03)
0.62
0.17
feedstock (wt %)
pH
switchgrass
ultimate analysis (wt %)
6.1 (0.2) 8.60 (0.30) 79.00 (0.40)
2.73 (0.13)
C
H
N
SW450
31.3 (2.8)
biochar (°C) SW600 SW800
16.9 (3.4)
10.6 (0.1) 0.89 (0.21) 11.15 (0.86) 19.43 (0.72) 68.54 (0.97) 71.52 (0.99) 2.53 (0.05)
5.39 1.13 (0.03)
0.42
0.06
11.4 (2.4)
11.2 (0.1) 0.53 (0.31)
4.85 0.86 (0.03)
0.19
0.05
pine wood
3.26 (1.25) 21.52 (0.99) 74.69 (1.40) 71.62 (1.74) 1.16 (0.23
4.7 (0.1) 6.62 (0.03) 83.44 (0.58)
0.3 (0.01)
9.64 (0.56) 48.50 (0.10) 5.92 (0.11) 45.16 0.12 (0.02)
1.46
0.70
P450
26.6 (0.6)
5.1 (0.7) 1.77 (0.68) 44.65 (5.15)
1.37 (0.28) 52.22 (4.47) 71.80 (0.47) 3.94 (0.80) 22.66 0.23 (0.01)
0.66
0.24
biochar (°C) P600 P800
15.2 (1.1)
6.5 (0.1) 0.99 (0.29) 19.68 (1.22)
2.05 (0.03) 77.29 (1.02) 84.66 (0.26) 2.81 (0.36) 10.25 0.23 (0.03)
0.40
0.09
5.19 (1.69) 91.55 (1.40) 89.70 (1.03) 1.24 (0.03)
0.17
0.03
9.5 (0.7)
10.4 (0.4) 0.64 (0.07)
2.61 (0.87)
3.61 0.26 (0.02)
a
All samples were average values calculated from N = 3 replicate measurements, with standard deviation values in parentheses. b Calculated by difference (O % = 100 ash content C H N).
Table 3. Inorganic Elements (mg/kg) Contained in Switchgrass and Pine Wood and the Resulting Biochars Produced at Pyrolysis Temperatures of 450, 600, and 800 °Ca inorganic elements (mg/kg of dry biochar)b P switchgrass
S
992 (20)
814 (14)
SW450 1963 (240)
1257 (34)
Ca
Na
2890 (114)
Mg
122 (7)
10439 (994) 248 (67)
K
3429 (62)
Al
Ba
3007 (122) 58.8 (3.8)
15.6 (0.5)
9637 (344) 11309 (371) 411 (55.5)
67.3 (2.9)
Si
Ni
6653 (302) 0.3 (0.03) 22840 (947)
Sr 8.7 (0.2)
7.9 (2.4) 31.2 (0.8)
biochar (°C) SW600 2400 (179) 1338 (36) 14611 (1260) 208 (12) 11802 (449) 16452 (429) 533 (85) 75.8 (2.6) 29383 (890) 11 (8.6) 44.7 (1.3) SW800 4046 (235) 1917 (105) 17513 (1116) 220 (14) 15022 (653) 16734 (638) 427 (51.2) 135.3 (7.5) 37177 (2108) 15.4 (5.9) 55.4 (2.9) pine wood P450 biochar (°C) P600 P800
44 (2) 162 (12)
79 (5) 153 (8)
725 (53) 2318 (162)
23 (1) 56 (2)
219 (15) 730 (60)
281 (24)
214 (14)
4158 (282)
104 (6)
1281 (85)
439 (19)
270 (11)
4915 (272)
129 (7)
1952 (188)
472 (12) 24.3 (1.0) 1684 (112) 77.2 (6.4)
11.6 (0.9) 37.9 (4.6)
1063 (144) 1159 (100)
NA 4.9 (0.6) 6.4 (1.9) 15.9 (1.2)
155 (9.3)
65 (4.7)
1624 (53)
8.7 (0.1) 27.0 (1.6)
4237 (158) 212 (19.3)
81.5 (5.1)
2889 (212)
1944 (89) 22.1 (3.7) 36.8 (1.8)
inorganic elements (mg/kg of dry biochar) Fe switchgrass
Cr
Cu
Mo
Se
Ge c
Ti
Pb
V
sum (%)
41 (4)
0.8 (0.0)
ND
ND
7.7 (0.6)
ND
ND
1.82 (0.07)
246 (20) 102 (10)
2.3 (0.3)
4.4 (0.8)
ND
52 (5.7)
ND
0.8 (0.1)
5.91 (0.3)
24.7 (5)
61 (7)
341 (30)
115 (7)
5.7 (1.7)
5.6 (1.5) 33.1 (4.6) 68.6 (13.4) 2.6 (0.4) 1.3 (0.2) 7.84 (0.37)
26.8 (1) ND
79 (5) 374 (24) ND 54 (4)
155 (7) 13 (2)
6.0 (1.0) 5.1 (0.4) 36.1 (1.9) ND ND ND
457 (57) 9.5 (1.2)
biochar (°C) SW600 797 (162) SW800 633 (80) pine wood 24 (2)
Zn
46 (3)
64 (6) 0.8 (0.0) 14 (0.4) SW450
Mn
ND
66 (2)
60 (5) 2.8 (0.5) 0.6 (0.1) 9.46 (0.51) 37 (2.6) ND ND 0.28 (0.02
P450
202 (54)
8 (1)
195 (13)
49 (8)
0.5 (0)
3.8 (0.2)
ND
108 (10.5)
ND
ND
0.70 (0.06)
biochar (°C) P600 P800
167 (27)
16 (4)
6 (1)
324 (21)
71 (2)
1.9 (0)
7.2 (0.4)
ND
238 (41)
ND
ND
1.16 (0.08)
164 (32)
23 (2)
18 (1)
488 (18)
45 (6) 4.6 (0.2)
5.7 (0)
ND
244 (23)
ND
ND
1.52 (0.09)
a
All samples were average values calculated from N = 3 replicate measurements, with standard deviation values in parentheses. b Concentrations were calculated on the basis of the dry weight of biochar (mg/kg). c ND = not detected.
for switchgrass and from 26.6 to 9.5% for pine wood. This is due to the significant loss of volatile matter in proximate analysis and indicates the significant loss of CH4, H2, and CO, the dehydration of hydroxyl groups, and the thermal degradation of the lignocellulose structure.23 Fixed C increased from 9.7 to 74.7% for switchgrass biochars and from 9.6 to 91.6% for pine wood biochars. The ash content of switchgrass biochars increased from 2.7 to 21.5%, while that of pine wood slightly increased from 0.3 to 5.2% with increasing temperatures. Ultimate analysis showed a higher C content for pine wood biochars (49 90% C) compared to switchgrass biochars (46 72% C), which is attributed to the difference in the ash content. The pH of switchgrass and pine wood biochars increased from 6.1 to 11.2 and from 4.7 to 10.4,
respectively. Table 3 presents the inorganic composition of the feedstocks and corresponding biochars determined by ICP OES. It was observed that switchgrass biochars contained 2 10 times higher concentration of inorganic elements than pine wood biochars, and the concentration of most inorganic elements increased with the pyrolysis temperature. In particular, plant nutrients, such as N, P, K Ca, K, and Mg, and trace minerals were more prevalent in switchgrass biochars. This indicates that increasing inorganic elements and decreasing acidic functionalities related to volatile matter may affect pH in soil amended with biochars. Furthermore, biochar containing inorganic nutrients can play an important role in soil fertility and crop production. Brewer et al.12 also suggested that switchgrass biochar containing high ash 4696
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Energy & Fuels content and low surface area is applicable as a soil amendment depending upon the economic circumstances and the local soil properties.12 A van Krevelen diagram (comparing atomic ratios of H/C and O/C) was constructed using the data in Table 2 to estimate the degree of aromaticity and carbonation in materials (Figure 2). Atomic H/C and O/C ratios (an index of aromaticity and carbonization) of biochars decreased with increasing pyrolysis temperatures. Moving from feedstock to 450 and 600 °C in Figure 2, both H/C and O/C ratios decreased comparably, which
Figure 2. van Krevelen diagram of atomic ratios for switchgrass and pine wood and the resulting biochars produced by pyrolysis at 450, 600, and 800 °C calculated from data in Table 2.
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is due to dehydration, decarboxylation, and decarbonylation.9 From 600 to 800 °C, H/C ratios decreased more significantly than O/C ratios, resulting from dehydrogenation and demethanation.9 Interestingly, H/C ratios of e0.2 for biochars at 800 °C are defined as black carbon, which represents a partial continuous transformation from aromatic biochars to graphitic structures.5 These results indicate that an elevated pyrolysis temperature produces more recalcitrant carbon structures in both switchgrass and pine wood biochars. 3.3. TGA. TG and DTG combustion curves under air combustion conditions were analyzed for thermal decomposition of feedstocks and the corresponding biochars and are shown in Figure 3. Combustion temperatures producing significant mass loss (wt %) of feedstocks and the resulting biochars in TG thermograms were higher with the biochars produced at greater pyrolysis temperatures (panels a and c of Figure 3). Interestingly, pine wood and its resulting biochars showed larger shifts toward higher combustion temperatures compared to switchgrass and its resulting biochars. This behavior can be explained by considering the structural difference between the two feedstocks (Table 1). Keiluweit et al.14 suggested that wood-derived biochar requires higher activation energy for thermochemical transformation than grass-derived biochar because wood contains a more complex ligneous polymer structure and less thermally labile hemicellulose than grass. Pine wood contained much more lignin, which is more difficult to volatilize. In addition, pine wood had less alkali metals than switchgrass (Table 3). It is known that alkali metals in biomass and biochars play a catalytic role in reducing the temperature for thermochemical decomposition.28 However, the thermograms of both biochars at 800 °C were not shifted closer
Figure 3. TGA of feedstocks, biochars produced by pyrolysis at 450, 600, and 800 °C, and CNTs at a heating rate of 20 °C/min and air combustion. (a) Switchgrass TG curve, (b) switchgrass DTG curve, (c) pine wood TG curve, and (d) pine wood DTG curve. 4697
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Figure 4. FTIR and Raman spectra of feedstocks and biochars produced by pyrolysis at 450, 600, and 800 °C. (a) FTIR spectra of switchgrass and corresponding biochars, (b) FTIR spectra of pine wood and corresponding biochars, (c) Raman spectra of biochars from switchgrass, and (d) Raman spectra of biochars from pine wood.
Table 4. FTIR and Raman Wavenumbers and Their Assignmentsa intensity wavenumber (cm 1)
a
assignment
FTIR
Raman
reference 30, 32, and 34
1220 1160
υC
O,
υCdO, υC
C
medium
medium
1270 1230
υC
O,
υCdO, υC
C
medium
medium
30, 34, and 35
1350 1325
δO
H,
υC
medium
strong
32 35
1470 1430 1540 1510
δCH2, δCH3 υCdC
medium medium
medium medium
17, 30, and 34 17, 30, and 34
1605 1580
υCdC
medium
strong
32, 34, and 35
1700 1690
υCdO
strong
medium
17 and 34
C
υ, stretching; δ, deformation.
to that of the CNTs, implying that the structure of both biochars at 800 °C are not similar to the graphitic structures of the CNTs. Panels b and d of Figure 3 show the DTG curves presenting the thermal decomposition peaks corresponding to the temperatures at which the weight loss rate is maximum. The DTG peaks at 200 420 °C were associated with thermal decomposition of feedstock components, such as cellulose, hemicellulose, and lignin, that volatilized and produced the resulting biochars. The DTG peaks at 370 550 °C were assigned to thermal degradation of biochars derived from cellulose, hemicellulose, and lignin of the feedstocks. Barneto et al.29 reported that the first sharp peaks at 350 480 °C are associated with the autocatalytic reaction of cellulosic and hemicellulosic components of biomass
Figure 5. XRD analysis of feedstocks and biochars produced by pyrolysis at 450, 600, and 800 °C. (a) XRD patterns of switchgrass and corresponding biochars and (b) XRD patterns of pine wood and corresponding biochars. 4698
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Figure 6. PCA of FTIR spectra extracted from feedstock, biochar produced at 450, 600, and 800 °C, and CNT samples. (a) Scores plot of PC1 (92%) and PC2 (6%) derived from feedstock, biochar, and CNT samples, (b) corresponding loadings plot of PC1 extracted from feedstock, all biochar, and CNT samples, (c) scores plot of PC1 (90%) and PC2 (5%) extracted from all biochars, and (d) corresponding loadings plot of PC1 extracted from biochars.
and the resulting biochars, while the second peaks at 370 550 °C are associated with the ligneous component of the biomass and biochars. They also suggested that shifting maximum peaks indicates the removal of cellulose- and hemicellulose-derived functionalities and functionality grafted upon the ligneous component. 3.4. Spectroscopic Analyses. 3.4.1. Infrared and Raman Spectroscopy. FTIR and Raman spectra of all samples, switchgrass and pine wood and their resulting biochars produced at 450, 600, and 800 °C, are given in Figure 4, and peak assignments are shown in Table 4. FTIR involves the absorption of infrared light to sense dipole vibrations and measures dangling functional groups in materials, while Raman spectroscopy applies emission of scattered laser light to sense polarizable vibrations in CdC and aromatics and measures the molecular backbone, making them complementary techniques for analyzing biochars. The FTIR spectra of the feedstocks and biochars showed the removal of functionality with increasing temperatures, whereas the Raman spectra showed a significant increase in two broad bands at 1610 1580 and 1380 1325 cm 1 (Figure 4). It is accepted that overlaps between the two broad bands of the Raman spectra can be deconvoluted into several pseudo-subpeaks, which correspond to peaks of the FTIR spectra with similar wavenumbers but different intensities.17,30 Examples of deconvoluted Raman spectra of the biochar produced at 450 °C are shown in panels c and d of Figure 4. All other Raman spectra from all biochars investigated in this
study were successfully deconvoluted into seven pseudosubpeaks. A comparison of two very broad Raman peaks in the ranges of 1605 1585 cm 1 (IG peak) and 1360 1334 cm 1 (ID peak) was used to determine C structural characteristics in purified graphitic carbons.31 However, Li et al.17 and Schwan et al.32 suggested that the IG peak in amorphous coals produced at ∼1000 °C is mainly due to the aromatic quadrant ring breathing rather than the fundamental vibration of the graphite structure. The ID peak has also been assigned to highly ordered C C between benzene rings in amorphous C rather than the defects in the graphite structure.17,33 They also reported that, with increasing pyrolysis temperatures, the IG and ID peaks shift because of the fact that the IG and ID peaks may be overlapping with their neighboring ordered carbonaceous peaks associated with cellulose- and lignin-derived carbon structures. 3.4.2. XRD Patterns. XRD patterns of feedstocks and the resulting biochars are shown in Figure 5. XRD patterns showed that increasing pyrolysis temperatures produced a greater amount of small, sharp peaks in the biochars, indicating the presence of miscellaneous minerals, such as quarts (2θ = 26.5°) and calcites (2θ = 29.3°). In particular, XRD patterns of switchgrass biochars contained predominant small peaks, which is consistent with a higher ash content. Peaks at 2θ = 15.3°, 16.1°, and 22.4° are assigned to crystallographic planes of the crystalline regions of cellulose.14 In comparison to the raw biomass, at a pyrolysis temperature of 450 °C, a significant loss of these strong peaks and a peak shift of 2θ = 22.4 24.5° are 4699
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Figure 7. PCA of Raman spectra collected on biochars produced at 450, 600, and 800 °C and CNT samples in the range of 1800 1100 cm 1. (a) Scores plot of PC1 (66%) and PC2 (25%) derived from biochar and CNT samples, (b) corresponding loadings plot of PC1 extracted from all biochar and CNT samples, (c) scores plot of PC1 (77%) and PC2 (11%) extracted from all biochars, and (d) corresponding loadings plot of PC1 extracted from all biochars.
Table 5. Summary of Pseudo-subpeak Assignments Deconvoluted from Raman Spectra at 1800 1100 cm
1
peak name
peak position (cm 1)
II
1220 1160
C H on aromatic rings
17 and 32
IS
1270 1230
sp3-rich alkyl-aryl C C structure and methyl carbon attaching to an aromatic ring
17 and 36
ID
1350 1325
sp2-bonded highly ordered carbon; aromatics with 6 or more fused
17, 32, and 33
peak assignment
reference
benzene rings but less than graphite IC
1490 1430
C H; semicircle ring stretch or condensed benzene rings
17 and 32
IG*
1540 1510
amorphous sp2-bonded carbon; aromatics with 3 5 rings
17
IG
1605 1580
graphite crystalline structure; aromatic ring breathing of CdC
17 and 33
IO
1700 1690
carbonyl group CdO
17
observed in the biochar SW450, whereas pine wood showed a gradual loss of the strong peaks and no shift of any signal, which indicates that, at a pyrolysis temperature of 450 °C, cellulose crystallinity was significantly lower in switchgrass biochar (SW450) and decreased more gradually in pine wood biochar (P450). This result is due to the structural difference between switchgrass and pine wood, which is consistent with the results of TGA. It is interesting to note that the two peaks at 2θ = 15.3° and 16.1° are absent in switchgrass and pine biochars at 600 and 800 °C, indicating the complete loss of the crystalline structure of cellulose in the material. As the pyrolysis temperature increased from 450 to 600 and 800 °C, broad peaks around 2θ = 24.5° (002 planes) and 43.5° (100 planes) developed in both switchgrass and pine wood biochars. This result indicates the formation of turbostratic carbon crystallites.
3.5. Multivariate Analysis. 3.5.1. FTIR Spectral Analysis. FTIR spectra of feedstocks and the corresponding biochars were analyzed using the PCA method, and the results are shown in Figure 6. In the scores plot (Figure 6a), the samples were noticeably classified by the first and second principal components (PC1 and PC2), accounting for 93 and 6% of the total spectral variance, respectively. The feedstocks were discriminated from all biochars and the CNTs. The biochars produced at 800 °C and the CNTs are close to each other, suggesting that the biochars produced at 800 °C are composed mainly of condensed aromatic structures. The corresponding loadings plot of the PCA, which provides information on the major variables (wavenumbers) in the spectra that significantly contribute to clustering of the samples along PC1, is presented in Figure 6b. The PC1 loadings plot shows that feedstocks were separated 4700
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Figure 8. Peak area ratios between deconvoluted pseudo-peaks extracted from Raman spectra of biochars at 450, 600, and 800 °C. The error bars represent the standard deviations of the peak area ratios. The lined curves show the trend. (a) Raman ID/IG ratio, (b) Raman ID/(IG* + IC) ratio, and (c) Raman IS/IG ratio.
from their corresponding biochars by positive contributions of FTIR bands that originate from feedstock components, such as cellulose, hemicellulose, and lignin. The bands were assigned to bonds, such as CdC, CdO, and C O, in the regions of 1710 1030 cm 1. To better understand the thermochemical transformation of the biochars produced at 450, 600, and 800 °C, PCA was performed with only the biochars, excluding feedstocks and CNTs (Figure 6c). All biochars produced at 450, 600, and 800 °C were classified by two PCs (PC1 and PC2), accounting for 90 and 7%, of the variance in the data, respectively. The corresponding loadings plot of PC1 (Figure 6d) shows that the biochars produced at 450 °C were separated from the biochars produced at 600 and 800 °C by positive contributions of the FTIR peaks assigned to CdC stretching (1593 and 1513 cm 1), CdO stretching (1425 cm 1), and C O stretching (1260 and 1121 cm 1). This result suggests that the biochars produced at lower pyrolysis temperatures contain more undecomposed organic matter, including cellulose- and hemicellulose-type biochars, while elevated pyrolysis temperatures result in biochars with an increasing degree of condensed aromatic rings. Furthermore, all biochars produced from
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switchgrass and pine wood were discriminated, indicating that various fractions of cellulose, hemicellulose, lignin, and extractives and the structural differences between the two feedstocks produce different types of biochars. However, at the highest thermal treatment (800 °C), switchgrass and pine wood biochars possessed similar FTIR chemical signatures, making the discrimination between the two biochars impossible. 3.5.2. Raman Spectra Analysis. Raman spectra of each biochar produced at 450, 600, and 800 °C and the CNTs were also analyzed by PCA, and the results are presented in Figure 7. The scores plot (Figure 7a) shows that the biochars were noticeably separated from the CNTs by PC1 with variances of 66%. The corresponding loadings plot of PC1 (Figure 7b) shows a sharp positive peak at 1587 cm 1 (IG), a negative peak at 1625 cm 1, and a broader ID peak, including peaks at 1410 cm 1 (CH3) and 1286 cm 1 (C C stretching) significantly contributing to differentiate the CNTs from all biochars. The positive IG peak is associated with the graphitic carbon structure of the CNTs, and the negative peaks at 1625 cm 1 and broad ID peaks are associated with amorphous carbonaceous structures in the biochars. To compare only the biochars, Raman spectra of the CNTs were excluded and biochar spectra were reanalyzed using the PCA method. PC1 and PC2 accounted for 89% of the variation in the data, with 77% for PC1 and 11% for PC2 (Figure 7c). Interestingly, switchgrass-derived biochars produced at 600 and 800 °C clustered in the positive quadrant of PC1, whereas pine wood-derived biochars produced at 450 and 600 °C were located on the negative quadrant of PC1. This result implies that these different feedstocks were transformed to various types of carbonaceous structures even at the same pyrolysis temperature because of the different structural compositions of the starting material (Table 1). The loadings plot of PC1 (Figure 7d) shows that the broad IG and ID peaks significantly contributed to separation of the biochars. Switchgrass biochars at 450 °C and pine wood biochars at 450 and 600 °C contained more CdC (negative band at 1533 cm 1), more CdO (negative band at 1660 cm 1), and less C C (positive band at 1340 cm 1) than switchgrass biochars at 600 and 800 °C and pine wood biochars at 800 °C. This trend suggests that an increasing pyrolysis temperature results in a more pronounced ID peak associated with a highly ordered carbonaceous structure in biochars. 3.5.3. Raman Spectra Deconvolution (Curve-Fitting). Pseudo-subpeaks deconvoluted from the overlaps between the two broad Raman IG and ID peaks at 1800 1100 cm 1 have been used to semi-quantitatively estimate pyrolytic transformation of coals.17 The corresponding assignments used for the pseudosubpeaks are listed in Table 5. The peak (IO) at 1700 1690 cm 1 is assigned to the carbonyl CdO structure in biochars. Both peaks IG* at 1540 1510 cm 1 and IC at 1440 1420 cm 1, between the IG and ID peaks, represent aromatic semi-quadrant ring breathing for rings containing more than two fused aromatic rings. The peak IS at 1250 1230 cm 1 is assigned to the sp3-rich structures as an alkyl-aryl C C structure and methyl carbon dangling to an aromatic ring. The small peak II at 1190 1180 cm 1 corresponds to C H on aromatic rings. Figure 8 represents the peak area ratios between the seven deconvoluted pseudo-subpeaks. The ratio between the deconvoluted ID and IG peaks, the ID/IG ratio, in switchgrass and pine wood biochars delineates the concentration of aromatic rings containing 6 or more fused benzene rings. The ID/IG ratios in both switchgrass and pine wood biochars increased linearly 4701
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Energy & Fuels with increasing pyrolysis temperatures from 450 to 800 °C (Figure 8a). This trend is inversely related to the atomic H/C ratio. These results indicate that increasing pyrolysis temperatures initiate the growth of aromatic rings as dehydrogenation of hydroaromatics occurs.9,17 Furthermore, ID/IG ratios in switchgrass biochars showed higher values than those in pine wood biochars, which implies lower thermal stability of switchgrass because of structural differences and much more catalytic alkali metals than pine wood. This agrees with the TGA presented in Figure 3. The ratio between the ID and (IG* + IC) peaks, denoted the ID/(IG* + IC) ratio, represents the ratio between more than 6 fused benzene rings and 2 8 or more fused aromatic rings, typically found in amorphous C. The ID/(IG* + IC) ratio of the biochars of both feedstocks gradually increased with the pyrolysis temperature, implying that small aromatic rings were gradually integrated to larger aromatic rings with the condensation of aromatic rings (Figure 8b). We believe that these aromatic rings that increase with the temperature would be resistant to subsequent mineralization in the environments such as soil and aquatic systems, and can thus offer a C-negative benefit to pyrolysisderived biofuels upon sequestration of biochar in soil. The ratio between IS and IG peaks, the IS/IG ratio, represents the formation of alkyl-aryl C C bonds as a result of the cross-linking breakdown of oxygen-containing functionalities. The IS/IG ratios of the biochars of both feedstocks significantly increased from 450 to 600 °C with a decreasing atomic O/C ratio. This is due to the fact that oxygen-containing functionalities are decomposed by decarbonylation and decarboxylation, followed by transformation to the alkyl-aryl C C bonds as a cross-linking between small aromatic rings. Furthermore, between 600 and 800 °C, the IS/IG ratio of switchgrass biochars did not increase with a slightly decreasing atomic O/C ratio. On the other hand, the IS/IG ratio of pine wood biochars increased gradually with a decreasing atomic O/C ratio. Therefore, switchgrass biochars contain higher aromatic condensation compared to pine wood biochar under a 30 s residence time in a pyrolysis reactor. This is consistent with the results of the XRD patterns. Keiluweit et al.,14 however, suggested that wood biochars produced by slow pyrolysis (more than 1 h of residence time) contain more turbostratic crystallite than grass biochars. This implies that, under the fast pyrolysis process, switchgrass biochars are likely to be more recalcitrant than pine wood biochars in soil, suggesting that they may provide greater carbon sequestration potential.
4. CONCLUSION Bioenergy feedstocks, switchgrass and pine wood, were thermochemically converted to biochars by the pyrolysis process under conditions of 450, 600, and 800 °C for 30 s of residence time. Biochars were characterized to provide insight as to how they may behave as a soil amendment and for their potential to sequester carbon in soil. Switchgrass biochars, containing high ash content and, thus, plant nutrients, can play a role as a liming agent in soil. Switchgrass has a more thermally labile hemicellulose and less complex ligneous polymer structure than pine wood and, therefore, requires less activation energy for thermochemical decomposition. With elevated pyrolysis temperatures, switchgrass and pine wood biochars lose surface functionality and produce more aromatic condensation. Under fast pyrolysis, switchgrass biochars produce higher numbers of aromatic rings with crosslinkage than pine wood biochars. Thus, switchgrass biochars produced at higher temperatures will likely have greater chemical
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and biological recalcitrance, leading to enhanced potential for carbon sequestration once introduced into the environment.
’ ASSOCIATED CONTENT FTIR spectra (4000 600 cm 1) of the feedstocks and the resulting biochars produced at 450, 600, and 800 °C (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.
bS
Supporting Information.
’ AUTHOR INFORMATION Corresponding Author
*Telephone: (865) 946-1126. Fax: (865) 946-1109. E-mail: nlabbe@ utk.edu.
’ ACKNOWLEDGMENT This publication is based on work supported by the United States Department of Agriculture (USDA) under Project 201038419-20903. The authors thank Dr. Samuel C. Weaver and Mr. Dan Hensley of Proton Power, Inc. for their assistance in the production of biochars and Dr. Samuel Jackson for providing the switchgrass material. ’ REFERENCES (1) Richard, T. L. Challenges in scaling up biofuels infrastructure. Science 2010, 329 (5993), 793–796. (2) Digman, B.; Joo, H. S.; Kim, D. S. Recent progress in gasification/pyrolysis technologies for biomass conversion to energy. Environ. Prog. Sustainable Energy 2009, 28 (1), 47–51. (3) Brethauer, S.; Wyman, C. E. Review: Continuous hydrolysis and fermentation for cellulosic ethanol production. Bioresour. Technol. 2010, 101 (13), 4862–4874. (4) Laird, D. A.; Brown, R. C.; Amonette, J. E.; Lehmann, J. Review of the pyrolysis platform for coproducing bio-oil and biochar. Biofuels, Bioprod. Biorefin. 2009, 3 (5), 547–562. (5) Schmidt, M. W. I.; Noack, A. G. Black carbon in soils and sediments: Analysis, distribution, implications, and current challenges. Global Biogeochem. Cycles 2000, 14 (3), 777–793. (6) Lehmann, J.; Czimczik, C.; Laird, D.; Sohi, S. Stability of biochar in soil. In Biochar for Environmental Management: Science and Technology; Earthscan: Sterling, VA, 2009; pp 183 205. (7) Lehmann, J. Bio-energy in the black. Front. Ecol. Environ. 2007, 5 (7), 381–387. (8) Gaskin, J. W.; Steiner, C.; Harris, K.; Das, K. C.; Bibens, B. Effect of low-temperature pyrolysis conditions on biochar for agricultural use. Trans. ASABE 2008, 51 (6), 2061–2069. (9) Tang, M. M.; Bacon, R. Carbonization of cellulose fibers. 1. Low temperature pyrolysis. Carbon 1964, 2 (3), 211–214. (10) Goyal, H. B.; Seal, D.; Saxena, R. C. Bio-fuels from thermochemical conversion of renewable resources: A review. Renewable Sustainable Energy Rev. 2008, 12 (2), 504–517. (11) Mohan, D.; Pittman, C. U., Jr.; Bricka, M.; Smith, F.; Yancey, B.; Mohammad, J.; Steele, P. H.; Alexandre-Franco, M. F.; Gomez-Serrano, V.; Gong, H. Sorption of arsenic, cadmium, and lead by chars produced from fast pyrolysis of wood and bark during bio-oil production. J. Colloid Interface Sci. 2007, 310 (1), 57–73. (12) Brewer, C. E.; Schmidt-Rohr, K.; Satrio, J. A.; Brown, R. C. Characterization of biochar from fast pyrolysis and gasification systems. Environ. Prog. Sustainable Energy 2009, 28 (3), 386–396. (13) Tilman, D.; Hill, J.; Lehman, C. Carbon-negative biofuels from lowinput high-diversity grassland biomass. Science 2006, 314 (5805), 1598–1600. 4702
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