Article pubs.acs.org/EF
Across-Phase Biomass Pyrolysis Stoichiometry, Energy Balance, and Product Formation Kinetics Jeffrey LeBlanc,† Minori Uchimiya,*,‡,§ Girish Ramakrishnan,‡ Marco J. Castaldi,*,† and Alexander Orlov‡ †
Department of Chemical Engineering, The City College of New York, 140th Street, Convent Avenue Steinman Hall, Room 307, New York, New York 10031, United States ‡ Department of Material Science and Engineering, State University of New York, Room 314, Old Engineering, Stony Brook, New York 11794, United States § USDA-ARS Southern Regional Research Center, 1100 Robert E. Lee Boulevard, New Orleans, Louisiana 70124, United States S Supporting Information *
ABSTRACT: Predictive correlations between reactions occurring in the gas, liquid, and solid phases are necessary to economically utilize the thermochemical conversion of agricultural wastes impacting the food, water, and energy nexus. On the basis of an empirical mass balance (99.7%), this study established the overall reaction stoichiometry (C33.42H45.95O20.26N0.22S0.14 = 0.50C20.08H57.21O22.46N0.20S0.22 + 1.72H2O + 0.10H2 + 1.07CH4 + 0.02C2H4 + 0.06C2H6 + 2.21CO2 + 2.05CO + 0.28C63.75H32.47O3.23N0.43S0.12) and energy balance for the slow pyrolysis of lignocellulosic pecan shell waste biomass at 10 °C min−1 up to 500 °C. In situ thermogravimetry−gas chromatography and diffuse reflectance infrared fourier transform spectroscopy (DRIFTs) were used to link the gas-, liquid-, and solid-phase nonisothermal reaction kinetics. Gaussian fit-based deconvolution of individual gaseous product formation rates (hydrogen, methane, carbon monoxide, carbon dioxide, ethylene, and ethane in mg min−1) suggested the relationships between (1) evolved methane and increased aromaticity/energy density of char product at 300−500 °C, and (2) evolved carbon dioxide and decarboxylation of char product near 400 °C. Partial least-squares (PLS) calibrations were obtained between (1) DRIFTs monitoring of the surface functional groups in the solid phase (transition from pecan shell to char) and (2) CO, CO2, CH4, C2H6, C2H4, and tar formation profiles in the gas/condensable phase. Established across-phase PLS calibrations can be used to predict biochar’s surface chemistry based on the fingerprint of volatile products, and vice versa. These new thermodynamic (reaction stoichiometry and energy balance) and kinetic (deconvolution of specific gas formation rates and PLS) predictive methodologies will facilitate the nexus of food, water (designing of biochar soil amendment), and energy (optimization of syngas and bio-oil composition) enabling sustainable agriculture.
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INTRODUCTION
Our previous study reported the empirical mass balance (achieving 99.7% closure) for the slow pyrolysis of a lignocellulosic biomass (pecan shells) in a thermogravimetric analysis−gas chromatography (TGA-GC) system employing a rapid cooling of vapor-phase products.9 A similar approach had been employed to achieve the empirical mass balance for the three phases of products during the fast pyrolysis of cellulose.7 Our present study investigates the reaction stoichiometry and energy balance during the slow pyrolysis of pecan shells, using the mass balance closure as a boundary condition. Furthermore, the linkage between gas-, liquid-, and solid-phase kinetics will be explored using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) and TGA-GC. Gaseous product evolution rates (hydrogen, methane, carbon monoxide, carbon dioxide, ethylene, and ethane) will be fitted by the summation of Gaussian distributions to deduce the contribution of biomolecular components (cellulose, hemicellulose, and lignin) in the formation of specific gaseous species. The existing knowledge in temperature-dependent formation and removal of carboxyl functional groups and condensation of char product9 will allow us to explore the connection to tar and gas composition.
Thermochemical conversion of agricultural wastes has a unique ability to address the food, water, and energy nexus. Bioenergy is produced via syngas1 that is combusted in gas turbines; bio-oil2 is produced as a drop-in fuel or specialty chemicals; and char3 is produced as a solid fuel. In addition, the solid char product is a promising renewable fertilizer/growth enhancer of food crops,4 as well as an water management tool in arid soils.5 To achieve desirable biofuel (syngas and bio-oil) and soil amendment (biochar) characteristics simultaneously, correlations are needed to predict how the pyrolysis kinetics in the gas, liquid, and solid phases are related to one another. Currently, only a few of several hundred elementary reactions occurring during the biomass pyrolysis have been identified.6 As a result, pyrolysis reaction mechanisms are often deduced from the low-temperature, solution-phase chemistry6 and by Diebold’s and related lumped mechanisms.7 For example, Gaussian fitting of differential thermogravimetry (DTG) curves is often employed to estimate the contributions of cellulose, hemicellulose, and lignin pseudocomponents for biomass pyrolysis kinetics.8 Because of unidentified multiphase elementary reactions involving unstable intermediates with heat/mass transfer limitations,7 the linkage between the gas-, liquid-, and solid-product formation kinetics is largely unknown. © 2016 American Chemical Society
Received: June 6, 2016 Revised: July 22, 2016 Published: July 25, 2016 6537
DOI: 10.1021/acs.energyfuels.6b01376 Energy Fuels 2016, 30, 6537−6546
Article
Energy & Fuels
Deconvolution of Gas Product Formation Rate into Pseudocomponents. TGA-GC-based volatile product evolution rate profiles were reported in our previous study,9 and were in agreement with a separate report on pecan shell employing TGA-FTIR.14 In the present study, the gas product evolution rate (in mL min−1) profiles were modeled as the sum of normal distributions:8,15
Partial least-squares (PLS) regression analysis will be performed on (1) gaseous product formation kinetics monitored by TGA-GC and (2) in situ monitoring of the surface functional groups (e.g., carboxyl and hydroxyl) during the transformation from pecan shell to biochar using DRIFTs.10 A recommendation will be provided on the protocols to empirically determine (1) the pyrolysis reaction stoichiometry, (2) the energy balance, and (3) the pyrolysis conditions needed to optimize the syngas and bio-oil yields, and to manipulate the surface reactivity of the biochar to serve as a soil amendment.
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⎛ (T − β)2 ⎞ f(T) = α·exp⎜ − ⎟ 2γ 2 ⎠ ⎝
(1)
where α is the amplitude of the gas rate curve, β the mean temperature of the distribution, γ the standard deviations of temperature from measured gas rate curves, and T the sample temperature during pyrolysis. Most gas species rate profiles in pyrolysis cannot be described by a single normal distribution.16 In such cases, summations of multiple numbers (n) of normal distributions are fitted by varying adjustable parameters α, β, γ, and n to match the measured rate profile. The underlying assumption of eq 1 is that pyrolysis is a series of parallel, firstorder reactions with unique activation energies.16 The individually measured rates at specific temperatures, and areas of the predicted and measured rate profiles were matched by minimizing the percent errors. The collection of tar via the impingers gave an overall aggregate mass that could only be measured at the completion of each test, and was used as a boundary condition. The time-dependent tar rate profile (in mg min−1) was calculated as the difference between DTG and gas product evolution rate (determined by micro-GC):
MATERIALS AND METHODS
Elemental Composition of Pecan Shell Feedstock and Biochar. As described in detail previously,9,11,12 pecan shell feedstock (PS25) was ground and sieved to
