Abstract
While the US nut industry is growing, markets for nut by-products, particularly nutshells and tree prunings, have not kept pace. Torrefaction is a thermochemical process used to improve physicochemical properties of biomass for energy and other applications. The goal of the paper was to characterize the effects of a range of torrefaction conditions on the properties of nut by-product feedstock. The process consists of thermal treatment of biomass at a temperature between 200 and 300°C in the absence of oxygen, where final material properties of the torrefied biomass depend on the temperature, heating rate, and residence time. In general, torrefied biomass exhibits higher hydrophobicity and calorific value with reduced moisture absorption compared to untreated biomass, making it an ideal fuel source for energy applications compared to raw biomass. In this study, almond shells of soft, semi-soft, and hardshell varieties, as well as walnut shells and almond wood, were torrefied at two different temperatures (230 and 290°C) and three different residence times (20, 40, and 60 min) in order to characterize the physicochemical properties. The thermal behavior of raw and heat-treated biomass was investigated by TGA analysis, elemental analysis, pH, helium pycnometry, FTIR spectroscopy, and dynamic vapor sorption analysis.
Highlights
Interest in the application of biomass torrefaction as a method for modifying material is increasing
The diverse range of torrefied biomass material properties depends on its physicochemical properties, which depend on the thermal processing conditions and properties of the original feedstock
The results indicate that torrefaction is a successful alternative to only incinerating the shells
Summary
Interest in the application of biomass torrefaction as a method for modifying material is increasing. The mentioned manuscripts have assessed the properties of resulting torrefaction products of almond and walnut shells and the pretreatments to improve the torrefaction processes. The results indicate that torrefaction is a successful alternative to only incinerating the shells Both almond and walnut shells present high solid and energy yields during torrefaction (Barskov et al, 2019) while maintaining low sulfur content, making it interesting to US power plants (Chiou et al, 2016). Adding torrefied biomass residues to a polymer matrix, especially post-consumer recycled polymers, increases the value of a waste material, reduces the plastic fraction of the composite, and improves the biocomposite’s mechanical and thermal properties (McCaffrey et al, 2018). Raw and treated samples were analyzed using elemental analysis, pH, bomb calorimetry, pycnometry, thermogravimetric analysis (TGA), Fourier transform infra-red (FTIR) analysis, and dynamic vapor sorption (DVS) analysis
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