Pyrolysis of cashew nut shells-focus on extractives

Abstract

Cashew nut shells (CNS), as non-edible agricultural residues produced by cashew nut shelling units, are largely abundant in West Africa. They present a strong potential for energy production although the presence of specific extractives known collectively as cashew nut shell liquid (CNSL) is problematic. These extractives can account for up to 40 % of the mass of the shells and are mainly composed of anacardic acids, cardanols and cardols (ACC). The objective of this work was to study the influence of ACC constituents on the pyrolysis of CNS. To this end, an exhaustive characterization of CNS was performed, followed by a series of pyrolysis experiments on CNS, extractives and solid residues. Thermogravimetric analyses revealed intermolecular interactions between the lignocellulosic solid matrix and the extractives of CNS. Experiments carried out in a fixed bed reactor enabled determination of pyrolysis products composition. Only cardanols were present in both CNS and ACC pyrolysis condensates. The degradation of anacardic acids during ACC pyrolysis at 400 °C was investigated and showed that 45 % of anacardic acids produce cardanols as end products, whereas the remaining anacardic acids form thermal cracking products (char, gas and condensates). The distribution of the end products of CNS pyrolysis is illustrated in the graphical abstract.