Fate of Higher-Mass Elements and Surface Functional Groups during

Nov 23, 2015 - The goal of this study was to understand the fate of surface functional groups and higher-atomic-mass elements during the pyrolysis of ...
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Fate of Higher-Mass Elements and Surface Functional Groups during the Pyrolysis of Waste Pecan Shell Keith Jones,†,‡ Girish Ramakrishnan,† Minori Uchimiya,*,†,§ Alexander Orlov,† Marco J. Castaldi,∥ Jeffrey LeBlanc,∥ and Syuntaro Hiradate⊥ †

Department of Material Science and Engineering, State University of New York, Room 314 Old Engineering, Stony Brook, New York 11794, United States ‡ Biological, Environmental, and Climate Sciences Department, Brookhaven National Laboratory, 53 Bell Avenue, Upton, New York 11973, United States § USDA-ARS Southern Regional Research Center, 1100 Robert E. Lee Boulevard, New Orleans, Louisiana 70124, United States ∥ Department of Chemical Engineering, The City College of New York, Room 307, Steinman Hall, 160 Convent Avenue, New York, New York 10031, United States ⊥ National Institute for Agro-Environmental Sciences, 3-1-3 Kan-nondai, Tsukuba, Ibaraki 305-8604, Japan S Supporting Information *

ABSTRACT: Thermochemical conversion of agricultural wastes to bioenergy has a potential to play forefront roles within the context of the food, energy, and water nexus. The biochar solid product of pyrolysis is a promising tool to manage food crop production and water resources by means of soil amendment. The goal of this study was to understand the fate of surface functional groups and higher-atomic-mass elements during the pyrolysis of pecan shell, which is known to accumulate calcium oxalate. Pecan shell feedstock and biochars were analyzed ex situ using X-ray computed microtomography and solid-state 13C cross-polarization and magic-angle-spinning NMR spectroscopy; the pyrolysis kinetics was monitored in situ by thermogravimetric analysis−gas chromatography (TGA−GC). The NMR spectra indicated the greatest (i) reduction in O/N alkyl functionality and (ii) increase in the aromatic peak between 300 and 500 °C. Primary physical transformation was observed near 400 °C in the tomography slice images and corresponding attenuation coefficients. Key changes in physical structure (microtomography) as well as chemical constituents (solid-state NMR) of pecan shell at 300−500 °C coincided with the evolution of gaseous products (hydrogen, methane, carbon monoxide, carbon dioxide, ethylene, and ethane, as monitored in situ by TGA−GC) occurring at 200−500 °C. These observations followed the reported (i) formation and removal of carboxyl surface functional groups of biochar and (ii) conversion of calcium oxalate to carbonate, both occurring at the key transition temperature near 400 °C. Combined with the mass balance (99.7%) obtained for gas-, liquid-, and solid-phase products, these findings will facilitate reactor design to optimize syngas and bio-oil yields and manipulate the surface reactivity of biochar soil amendment.

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INTRODUCTION Thermochemical conversion of agricultural wastes to bioenergy has the potential to play a forefront role within the food, energy, and water nexus. In particular, pyrolysis transforms biomass into tar and noncondensable gases (including CO2, CO, H2, CH4, C2H4, C2H6, C3H6, and C3H8) at 250−400 °C,1 which can be represented by eq 1:

states and other countries having legislative renewable energy targets. Char(coal) solid product, popularly called biochar, is a promising food crop production and water management tool when used as an amendment to agricultural soils. Of its complex physicochemical properties, (1) carboxyl surface functional groups and (2) speciation of higher-atomic-mass elements (e.g., Ca) are expected to control the surface charge,2 complexation of transition metals,3 cation exchange capacity,4 water uptake and release, and life span of amended biochar in soil.5 Carboxyl surface functional groups of biochar have been intensively investigated using Fourier tranform infrared (FTIR) (1700 cm −1 ) 6 and synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) (288 eV peak) 7 techniques. Both approaches indicated the enrichment of the carboxylate functionality on biochar surfaces near 400 °C as a

2C42H60O28 → 3C16H10O2 + 28H 2O + 5CO2 + 3CO + C28H34O9

(1)

where approximate stoichiometries are used for wood feedstock (C42H60O28), charcoal solid product (C16H10O2), and condensable coproducts (C28H34O9).1 This C28H34O9 is to be understood as an aggregate C/H/O ratio that is actually composed of numerous individual chemical species. Products in all phases (gas, liquid, and solid) of biomass waste-to-energy conversion are candidate fossil fuel substitutes for heating, electricity generation, and engine operation, with many U.S. © 2015 American Chemical Society

Received: October 15, 2015 Revised: November 19, 2015 Published: November 23, 2015 8095

DOI: 10.1021/acs.energyfuels.5b02428 Energy Fuels 2015, 29, 8095−8101

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Energy & Fuels

Figure 1. X-ray computed microtomography images of pecan shell samples: (A, B) unpyrolyzed feedstock [2D slice in (A) and 3D reconstruction in (B)] and (C−H) biochars after pyrolysis at (C) 300, (D) 350, (E) 400, (F) 500, (G) 600, and (H) 700 °C.

pores began to form at low pyrolysis temperature (350 °C) on the low-density cottonseed hull feedstock composed primarily of cellulose.12 Our present study focuses on pecan shell feedstock, a ligninrich13 and dense14 waste biomass known to accumulate crystalline calcium oxalate, also called whewellite (CaC2O4· H2O).15,16 Calcium oxalate crystals (KSO = 10−8.75) are sitespecifically produced in the idioblasts of plant tissues in varying shapes: crystal sand, raphide, druse, styloid, and prismatic.17 Accumulated Ca could constitute as much as 10% (dry weight) of the plant material.18 Proposed functions of calcium oxalate crystals include tissue calcium regulation, defense mechanism against predators, and buffering of pH and the nicotinamide adenine dinucleotide (NAD/NADH) redox pool.17 Crystal nucleation and growth, morphology, and distribution are regulated by the complex cellular pathways involving metalcoordinating proteins, cytoskeletal components, and the intravacuolar matrix.17 Pyrolysis of calcium oxalate (CaC2O4) forms calcium carbonate (CaCO3) above 400 °C, which is transformed to calciate (CaO) above 600 °C:19

result of their formation at 100−400 °C and removal at 400− 700 °C.8 Lesser efforts have been made to understand the fate of higher-mass elements such as Ca during the thermochemical conversion of agricultural wastes. Scanning electron microscopy (SEM) is routinely employed in biochar research with the goal of visualizing the mineral components and micrometer-sized pores, especially in situ under reaction conditions.9 A complementary approach is the use of X-ray computed microtomography to produce a volume image of the biochar structure. X-ray microtomography is a noninvasive and nondestructive three-dimensional (3D) technique that produces a series of radiographic images for reconstruction of a 3D map of X-ray attenuation coefficients.10 The X-ray attenuation depends on the elemental composition and density of the sample along the beam path, which are related to the microstructure and mineralogical composition.10 The samplespecific linear attenuation coefficient (μ) is a function of the energy of the X-ray radiation and the elemental composition and bulk density of the sample. Voxel intensity histograms, which plot the frequencies of voxels of particular intensities, can be used to segment the image into different phases such as pores, solid phases, and aggregates.11,12 Our previous study focused on the X-ray computed microtomography imaging of cottonseed hull biochars.12 In that study, micrometer-sized

CaC2O4 ·H 2O → CaC2O4 + H 2O CaC2O4 → CaCO3 + CO 8096

(100−200 °C)

( >400 °C)

(2) (3)

DOI: 10.1021/acs.energyfuels.5b02428 Energy Fuels 2015, 29, 8095−8101

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Figure 2. Log attenuation of pecan shell (A) before and (B−F) after pyrolysis at (B) 300, (C) 400, (D) 500, (E) 600, and (F) 700 °C.

CaCO3 → CaO + CO2

(600−800 °C)

spinning (CP-MAS) NMR spectroscopy and to quantify released gas and condensable liquid coproducts by thermogravimetric analysis−gas chromatography (TGA−GC).

(4)

Carbonate in biochar contributes to the CO2 emission during short-term incubation studies on the biochar mineralization rate; this approach is often used to estimate the stability of biochar’s recalcitrant organic carbon fraction in amended soils.20 The present study had two objectives. One was to understand the role played by high-atomic-mass elements (e.g., Ca) in the development of microstructures as a function of the pyrolysis temperature. This was done using X-ray microtomography, which identified a key transformation temperature near 400 °C. The second was to determine the surface functional groups remaining on the biochar product using solid-state 13C cross-polarization and magic-angle-

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MATERIALS AND METHODS

Slow Pyrolysis. Pecan shells were obtained from a sheller and were ground (SM 2000 cutting mill, Retsch GmbH, Haan, Germany) and sieved to 10 cm−1 in Figure 2) increased at 300−400 °C and then slightly decreased at 400−600 °C. Near 400 °C, calcium oxalate is converted to calcium carbonate (eq 3) and carboxyl surface functional groups form on biochar.8 Solid-State 13C CP-MAS NMR Spectroscopy. Figure 3 presents solid-state 13C CP-MAS NMR spectra of pecan shell feedstock and biochars (300, 500, and 700 °C). As widely reported for slow-pyrolysis biochars,28 the spectra for the 500 and 700 °C biochars are dominated by the aromatic (−C

(12.9 keV). The beam was 8 mm wide and 5 mm high. X-rays passing through the sample were detected with a CsI(Tl) scintillator. Light from the scintillator was detected with a 1340 × 1300 pixel CCD camera with a pixel size of 4 μm. Samples were mounted on a wooden stirring rod. A total of 1200 radiographs were collected as the sample was rotated 180° in 0.15° increments. The results were converted to a 3D tomographic volume and visualized using open-source software.22 The volume voxel (in X-ray attenuation per voxel) was converted to linear attenuation (in cm−1). Linear attenuation coefficients for specific elements were estimated from the mass attenuation coefficients and densities.23,24 Solid-State 13C CP-MAS NMR Spectroscopy. The 13C signals of finely ground pecan shell feedstock and biochars in a KEL-F spinning tube (6 mm diameter, JEOL, Tokyo, Japan) were recorded at 75.45 MHz with a magic-angle spinning rate of 6 kHz, a contact time of 1 ms, and a pulse interval of 3 s with a 100 Hz broadening factor for the Fourier transform. The chemical shifts were interpreted with respect to tetramethylsilane and adamantane external reference (29.50 ppm). The resulting 13C NMR spectra were interpreted on the basis of the following chemical shift regions (in ppm): aliphatic C (0−45), O/N alkyl C (45−110), aromatic C (110−160), and carbonyl C (160− 190).25 TGA−GC Determination of Gas-Phase Products and Carbon Mass Balance. Pecan shells were pyrolyzed using a close-coupled TGA−GC instrument to analyze the decomposition products during a constant temperature ramp rate. As described in detail previously,26 the effluent of the thermogravimetric analyzer (Netzsch Luxx STA 409PC) was transferred through a passivated, heated sample line to a condensing train of impingers followed by a micro gas chromatograph (micro-GC) (Inficon 3000). The TGA exhaust was connected to a quarter-inch SilcoNert-coated stainless steel tube maintained at 315 °C. This heated transfer line routed the TGA effluent to a series of SKC Glass Midget impingers in a methanol/dry ice slurry at −80 °C. Pyrolysis (10 °C min−1 up to 500 °C) was performed in triplicate using ultrahigh-purity nitrogen as the sweep gas (40 mL min−1) to transfer vapor species through the transfer line to the condensers and micro-GC. Hydrogen, methane, carbon monoxide, carbon dioxide, ethylene, and ethane were quantified by the micro-GC. The solid product (residual mass by TGA) and condensable materials in the impingers were determined gravimetrically.

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RESULTS AND DISCUSSION X-ray Computed Microtomography. Figure 1 presents 2D slices of tomography images for pecan shell (A) before pyrolysis [(B) presents a 3D reconstruction of the 2D slices in (A)] and (C−H) after pyrolysis at 300, 350, 400, 500, 600, and 700 °C, respectively. Each pixel (4 μm) has a unique linear attenuation coefficient that depends on the density and the elemental mass (primarily C, O, and Ca). Small pieces at 600 °C indicate attrition (formation of smaller particles by mechanical forces)27 at higher temperatures. In all of the images, bright spots (high-attenuation voxels) indicate highermass elements such as Ca. However, bright spots in the image for one sample can have different attenuation coefficients than those for another sample. The ash contents of pecan shell biochars are 2−5% dry weight.21 Calcium is by far the most abundant higher-mass element in pecan shell feedstock as well as biochars (see the elemental composition in Table S1 in the Supporting Information). Microtomography images of calcium oxalate monohydrate crystals (bright spots) in pecan shells have been reported.15 The authors observed an increase in the volume fraction of calcium oxalate crystals toward the external surface of the pecan shell.15 At 12 keV, the attenuation coefficient was estimated to be 66.1 cm−1 for calcium carbonate and 38.6 cm−1 for calcium oxalate.24 Figure 2 presents a histogram of attenuation coefficients for each sample using a log scale (the y axes have arbitrary scales).

Figure 3. Solid-state 13C CP-MAS NMR spectra of pecan shell feedstock and slow-pyrolysis biochars pyrolyzed at 300, 500, and 700 °C. The aromatic (−CC−) peak at 110−160 ppm is dominant in >300 °C biochars. 8098

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products containing levoglucosan, other anhydroglucoses, randomly linked oligosaccharides, and glucose decomposition products.32 Lignin is a randomly linked phenolic macromolecule that undergoes chain fragmentation primarily at 300− 480 °C to release monomeric phenol units into the vapor phase; lignin also undergoes a secondary degradation at >500 °C,32 but that was not tested here. Lignin is thermally more stable and affords a higher char yield than (hemi)cellulose, and the weight loss occurs over a wider temperature range (160− 900 °C).33 These temperature trends for the hemicellulose, cellulose, and lignin components allowed deconvolution of differential thermogravimetric (DTG) curves for wood pyrolysis.34 The major mass loss at 210−370 °C was accompanied by two sharp peaks at 278 °C (hemicellulose) and 345 °C (cellulose) in the mass loss rate curve (open circles in Figure 4). As illustrated in eq 1, the mass loss rate reaches a maximum in this temperature range because of evaporation of tar, which contains heavy hydrocarbons and oxygenated hydrocarbons.35 The remaining weight of the solid stabilized at 400−500 °C, and the mass loss rate plateaued approximately 10 min after the peak temperature of 500 °C was reached. Over a 4 h period at 500 °C, only 3.5% of the initial mass was lost, indicating that 96.5% of the reactions were complete within 60 min (Figure 4). These observations are in agreement with other reported TGA and DTG analyses of pecan shell decomposition.16 Hemicellulose decomposes at 150−350 °C, and cellulose decomposes at a narrower temperature range of 275−350 °C.16 Whewellite was observable in raw pecan shell by X-ray diffraction and was converted to calcite at 500 °C.16 Dolomite (CaMg(CO3)2) is another form of inorganic carbon observed in biochar.36 Hydrogen, methane, carbon monoxide, carbon dioxide, ethylene, and ethane were measured as a function of temperature by GC as described in detail in Materials and Methods. As shown in eq 1, the primary gas products are expected to be CO2, CO, and CH4, with lesser amounts of H2 and C2 hydrocarbons.34 Figure 5 presents the gas product evolution rate (in mL min−1) and temperature as functions of time. It can be seen that the onsets of CO and CO2 evolution occurred at 170 and 255 °C, respectively. Within this temperature range, cellulose and hemicellulose form monomers

C−) peak at 110−160 ppm. Spinning sidebands (SSBs) (marked with asterisks in Figure 3) appear on both sides of the peak with equal intensities. The SSBs are artifacts caused when magic-angle spinning is less than the chemical shift anisotropy for a given resonance. 28 The spectrum of unpyrolyzed pecan shell is dominated by the O/N alkyl region (45−110 ppm), in agreement with the literature.13 Sharp peaks in the O/N alkyl region originate from cellulose29 and hemicellulose.30 Lignin forms broad peaks at 100−200 ppm.31 Figure 3 indicates the greatest (i) reduction in O/N alkyl functionality and (ii) increase in the aromatic peak between 300 and 500 °C. This is the critical temperature range for (i) the formation and subsequent removal of carboxyl surface functional groups8 and (ii) the conversion of calcium oxalate to calcium carbonate (eq 3). In conclusion, key changes in the physical structure (microtomography) as well as the chemical constituents (solid-state NMR spectroscopy) of pecan shell occurred at 300−500 °C. TGA−GC Monitoring of the Gas-Phase Product Formation Kinetics. TGA−GC monitoring of the pyrolysis kinetics was undertaken to determine the sum of gas products (determined by the micro-GC connected to the thermogravimetric analyzer), liquid products (condensable in impingers and determined gravimetrically), and solid products (determined by TGA). Complete mass balance (99.7%) was obtained from the pyrolysis of pecan shell, resulting in a total product recovery of 99.3 mg. The initial pecan shell feedstock weight of 99.6 mg resulted in 31.7 mg of solid products (i.e., 31.8% of the total products), 45.6 mg of tar products (i.e., 45.8%), and 22.0 mg of gas products (i.e., 22.1%). On the basis of eq 1, the theoretical charcoal yield is 36.7 wt %,1 in agreement with the present study. Figure 4 shows the percent mass loss with respect to the initial feedstock weight (99.6 mg), the rate of mass loss

Figure 4. Thermogravimetric analysis of pecan shell heated to 500 °C at a rate of 10 °C min−1 and held at 500 °C for 4 h in a N2 atmosphere.

(percent mass loss per minute), and the sample temperature as a function of time for pecan shell feedstock pyrolyzed at 10 °C min−1 and held at 500 °C for 4 h. The majority of the mass loss occurred before the peak temperature of 500 °C was reached. The slight weight loss during the initial 10 min (400− 500 °C with release of CO (eq 3).19 The CO2 evolution at 460 °C is attributable to cellulose and lignin.33 Cellulose, hemicellulose, and lignin can contribute to the CH4 peak at 500 °C in Figure 5. The evolution of methane was initiated at 360 °C, coinciding with the peak mass loss rate (Figure 4) resulting from the condensable liquid products (eq 1). Therefore, methane likely originated from cleavage of methyl groups or the secondary cracking of tars as they were evolved from the surface.35,37 At 415 °C, ethane and ethylene were observed (Figure 5), and they have been reported to form the recombination products of methyl and methylene radicals.38 Hydrogen production began at 500 °C,38 causing the H:C ratio of the biochar product to progressively decrease.8 Lignin is the largest contributor to char and H2 products;33 however, hemicellulose derivatives could produce H2 near 500 °C.37 Portions of some gas- and liquid-phase products may remain on biochar as volatile organic carbon (VOC).39 In amended soils, VOC from biochar could regulate plant/microbial activities, e.g., ethylene and α-pinene as nitrification inhibitors.39 For example, thermal desorption-coupled capillary GC− MS analysis of cottonseed hull biochars showed the maximum emission of ethylene/acetylene, benzene, and methane from 350 °C biochar.39 In contrast, thermally desorbable toluene decreased as a function of pyrolysis temperature.39 In conclusion, 300−500 °C is the critical temperature range in the thermochemical transformation of waste pecan shell. The primary physical transformation was observed near 400 °C in the tomography slice images (Figure 1) and corresponding attenuation coefficients (Figure 2), at 300−500 °C in O/N alkyl and aromatic structures by solid-state NMR spectroscopy, and at 200−500 °C during pyrolysis reactions followed in situ by TGA−GC. These observations followed the reported temperature trends for (i) the formation and removal of carboxyl surface functional groups of biochar as a function of pyrolysis temperature and (ii) the thermal stability of calcium oxalate (eq 3). Mass balance (99.7%) of thermochemical conversion was achieved by the rigorous analyses of solid-, liquid-, and gas-phase products. These findings will facilitate reactor design for syngas and bio-oil adjustments40 as well as desirable physicochemical properties of the solid value-added biochar product. Our future report in this series will investigate the chemical composition of the bio-oil products, the pyrolysis reaction stoichiometry, and the energy contents of the liquid, gas, and solid pyrolysis products.

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Elemental composition of pecan shell feedstock and biochars (PDF)

AUTHOR INFORMATION

Corresponding Author

*Fax: (504) 286-4367. Phone: (504) 286-4356. E-mail: sophie. [email protected]. Notes

Disclaimer: Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer. The authors declare no competing financial interest.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.5b02428. 8100

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