Fate of Higher-Mass Elements and Surface Functional Groups during the Pyrolysis of Waste Pecan Shell

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 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.

Metal Immobilization on Wood‐Derived Biochars: Distribution and Reactivity of Carbonate Phases

“…This conversion takes place during pyrolysis between 400 and 500°C. as shown using XRD measurements in several studies (Shinogi and Kanri, 2003; Singh et al, 2010; Perez‐Rodriguez et al, 2011; Kloss et al, 2012; Al‐Wabel et al, 2013; Wang et al, 2014, 2015; Jones et al, 2015). Here, a full conversion of calcium oxalate into calcite (calcium carbonate) was clearly shown in biochars produced from poplar bark at 450°C.…”

Structural Transformation of Biochar Black Carbon by C60 Superstructure: Environmental Implications

“…The disintegration of biochar pellets was not observed at lower biochar loadings (Figure S7 top), when low pyrolysis temperature biochar was employed (PS350 in Figure S7 , bottom), or when nC 60 -sonicate was employed instead of nC 60 -stir. Higher temperature biochars have higher attrition 41 to form smaller particles by mechanical forces 42 , 43 . Higher biochar loading could enhance the mechanical crushing of biochar pellets during the end-over-end rotation in the presence of nC 60 -stir.…”

“…3 ), and thus could originate from CaCO 3 in pecan shell biochar (Fig. 1 ) 41 . It is inherently challenging to distinguish two carbon materials (char and C 60 ) by TEM because of low contrast and overlapping projection 30 , 45 .…”

Potential applications of pecan residual biomasses: a review

“…Considering the importance of the product generated from pyrolysis, studies have found suitable conditions of this technique to evaluate the behavior of elements and functional groups present in pecan shells. It was found that in a temperature range between 300 and 500 ºC, changes occurred and the removal of functional groups from the surface of the pyrolysis product generated, as well as a change in the shell structure, and an increase of gaseous products, such as methane, carbon dioxide, and ethane [47]. In addition, other studies have reported the adsorptive capacity of pecan waste materials from the application of pyrolysis.…”