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 13 C 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.

The pyrolysis of agglomerating coal was analyzed by thermogravimetric analysis (TGA) coupled to microgas chromatography (μGC) to determine the effects of reactor confinement on solid product yield, tar evolution, and gas composition. The primary volatile products generated from pyrolysis are studied using two TGA crucibles with height to width aspect ratios of 0.11:1.0 and 2.0:1.0 for heating rates of 1, 3, and 10 K min −1 . Mass balances were determined from measurements of the solid residual, gaseous flow rates, and tar products captured via glass impingers. The measurements resulted in mass balance closures greater than 99%. The higher aspect ratio confinement provided a zone where the residence time of volatile species was extended to 0.35 s from 0.04 s for the low aspect ratio confinement. The extended residence time was found to increase the solid yield by 0.6−5.7% for the low and mid ranged volatile material coals by secondary tar reactions which form coke. There was a trend of increasing solid yield with a decreasing heating rate because of the shorter residence time of the released volatile material at the higher heating rates. A decrease in tar production by 2.1−2.5% was observed in the higher aspect ratio confinement. The differences in product evolution between the two confinements were determined to be due to reactions occurring in the range of 783−848 K, which produce hydrogen, methane, and coke. In the confinement of higher aspect ratio, hydrogen production increased by 10% and 30% for low and mid VM coals, respectively, along with a 40% increase in methane production for both coals. The higher productions of methane, hydrogen, and solid residual between 820 and 1100 K for increased residence time of 0.27−0.35 s are due to demethylation and recombination reactions during the formation of char from recondensed tar products.

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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 (C 33.42 H 45.95 O 20.26 N 0.22 S 0.14 = 0.50C 20.08 H 57.21 O 22.46 N 0.20 S 0.22 + 1.72H 2 O + 0.10H 2 + 1.07CH 4 + 0.02C 2 H 4 + 0.06C 2 H 6 + 2.21CO 2 + 2.05CO + 0.28C 63.75 H 32.47 -O 3.23 N 0.43 S 0.12 ) 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, CO 2 , CH 4 , C 2 H 6 , C 2 H 4 , 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.

Sugarcane mill mud/filter cake is an activated sludge-like byproduct from the clarifier of a raw sugar production factory, where cane juice is heated to ≈90°C for 1–2 hr, after the removal of bagasse. Mill mud is enriched with organic carbon, nitrogen, and nutrient minerals; no prior report utilized 16S rRNA gene sequencing to characterize the microbial composition. Mill mud could be applied to agricultural fields as biofertilizer to replace or supplement chemical fertilizers, and as bio-stimulant to replenish microorganisms and organic carbon depleted by erosion and post-harvest field burning. However, mill mud has historically caused waste management challenges in the United States. This study reports on the chemical and microbial (16S rRNA) characteristics for mill muds of diverse origin and ages. Chemical signature (high phosphorus) distinguished mill mud from bagasse (high carbon to nitrogen (C/N) ratio) and soil (high pH) samples of diverse geographical/environmental origins. Bacterial alpha diversity of all sample types (mill mud, bagasse, and soil) was inversely correlated with C/N. Firmicutes dominated the microbial composition of fresh byproducts (mill mud and bagasse) as-produced within the operating factory. Upon aging and environmental exposure, the microbial community of the byproducts diversified to resemble that of soils, and became dominated by varying proportions of other phyla such as Acidobacteria, Chloroflexi, and Planctomyces. In summary, chemical properties allowed grouping of sample types (mill mud, bagasse, and soil-like), and microbial diversity analyses visualized aging caused by outdoor exposures including soil amendment and composting. Results suggest that a transient turnover of microbiome by amendments shifts towards more resilient population governed by the chemistry of bulk soil.