Characterization of Pecan Shells for Value-added Applications

Abstract

There is a growing need in the United States to decrease dependence on fossil fuels because of energy security and environmental concerns associated with their use. Energy derived from biomass is especially important to the southeast due to availability of vegetation and optimum climate conditions. Pecan shells, the by-product from the Pecan (Carya illinoinensis) shelling process, are a potential biomass feedstock. In this study some of the physical characteristics of pecan shells (such as bulk, particle and tap densities, compressibility and flowability) that are important for its storage and process design were investigated. It was found that particle size and moisture content significantly affected the bulk, tap and particle densities. The porosity of pecan shells was significantly affected by particle size and moisture content as well. Hausner ratio was affected by particle size but not by moisture content. Mechanical compressibility was found to increase as particle size decreases and as moisture content increases. The mechanical compressibility of the samples increased with pressure and decreased particle size or increased moisture content. The flow behavior of pecan shells was not affected by particle size but moisture content indicated that lower moisture contents exhibited better flowability than pecan shells at higher moisture contents. Rate of moisture sorption was determined using the Page model and equilibrium moisture content and equilibrium relative humidity (EMC-ERH) relationships for pecan shells were sigmoidal in shape and best predicted by the Henderson and Chung-Pfost equations. The thermal decomposition characteristics of pecan shells were examined in nitrogen and air atmospheres at heating rates of 5, 10, 20, 30, and 40 °C/min. Four main stages of mass loss were observed during the thermal decomposition of pecan shells: moisture evaporation, hemicelluloses decomposition, cellulose decomposition and lignin degradation. The moisture evaporation stage occurred within the temperature range of 30-150°C in both atmospheres. The thermal decomposition of pecan shells demonstrated mass loss rate peaks attributed to hemicelluloses decomposition (275-330°C and 270-331°C) and cellulose decomposition (348-386°C and 315-339°C) for nitrogen and air thermal decomposition, respectively, increasing with increased heating rate. The thermal decomposition of pecan shells was considered essentially complete at 600°C. Volatilized gases during thermal decomposition of pecan shells were analyzed using a Fourier-transform infrared spectrometer. Gaseous products volatilized during thermochemical conversion processes were identified and quantified by concentration. The major gases produced from nitrogen thermal decomposition of pecan shells were carbon dioxide (CO2), carbon monoxide (CO), ethanol (CH3CH2OH) and acetic acid (CH3COOH). The major gases produced from air thermal decomposition of pecan shells were carbon monoxide, carbon dioxide and methyl isocyanate (C2H3NO). A differential scanning calorimeter (DSC) was used to determine energy requirements at two temperature zones: moisture evaporation and thermal decomposition. It was found that they energy required to drive off moisture was more than the energy required to raise the pecan shells to thermal decomposition temperatures. The energy requirements at the two stages were not affected by heating rate. It was also found that the energy required in both temperature zones was about 30% of the energy contained in raw pecan shells.