Bio-Briquettes From Processed Cashew Nut Shell Waste: A Review

Introduction

To begin with, Cashew (Anacardium occidentale) is a pseudo-fruit bearing tropical evergreen tree, indigenous to North-eastern Brazil and South-eastern Venezuela that was later spread across the globe during late sixteenth century with an intention of conserving soil. This tree is mainly cultivated for its nuts (cashew seed) which grows at the base of its peduncle or pseudocarp (cashew apple); and are consumed as direct snacks, as an ingredient in cuisines, and are even used as the raw material for producing cashew oil and butter (Mgaya et al., 2019). Apart from this, these nuts (either in roasted or dried form) are used for making pastries, confectionaries, ice creams and chocolates (Tuates Jr et al., 2020); and contribute substantially in international and foreign exchanges market in many countries (Rabany et al., 2015; Maia et al., 2000). Presently, many countries are cultivating these exotic nuts, with United States and European Union countries being established well in this market (Mubofu & Mgaya, 2018). Infact, India is also a leading contributor in this well established market, and is known to be the largest producer, processor, exporter and second largest consumer of cashew in the world (Raghavendra Prasada, 2014; Sanger et al., 2011).

In general, these nuts are deshelled from their pseudocarp, and are then roasted or dried before consumption; hence, producing a large volume of cashew nut shell (CNS) wastes (Sawadogo et al., 2018; Sanger et al., 2011). These wastes are usually discarded into the environment; and causes acid pollution to the soil, besides possessing threats in form of fire risks citing their high organic content (Godjo et al., 2015). As a primary waste from cashew nut processing industries, they are often disposed quickly at very low prices (Singh et al., 2006; Huko et al., 2015); however, these shells hold certain calorific content, which makes them suitable for energy applications. Infact, these CNS wastes have been identified as most versatile organic renewable material available in abundance, and are observed as a raw material for numerous renewable energy resources that can replace fossil-based petroleum, value-added chemicals, and polymers (Kimutai & Kimutai, 2019; Mgaya et al., 2019; Pandiyan et al., 2020).

Though, these wastes do not pose any food competition threats, direct use or burning them produces toxic emissions and releases highly corrosive chemicals, which have adverse effect on the boilers, combustors and heating equipments. Hence, these chemicals in form of cashew nut shell liquid (CNSL) must be removed from these CNS, thus leaving behind residual shell cakes that can be processed into biofuels (Sawadogo et al., 2018). However, these CNS cakes are low density residues, and occupy large areas during their transportation and storage which increases their handling costs (Ngusale et al., 2014; Chen et al., 2015). As a result, the CNS cakes are compacted into briquettes that introduce their superior energy characteristics like low moisture content, and high density and calorific content (Stolarski et al., 2013). Apart from the de-oiled or pressed cakes, CNS wastes are also carbonized into bio-chars using pyrolysis or carbonization process, which later on are compacted into charcoal briquettes. These thermochemical treatments vaporise the harmful and volatile hydrocarbons responsible for toxic emissions from these shells, thus leaving behind significant amount of bio-char and mild traces of CNSL, depending upon the process adopted (Ifa, 2019; Sanger et al., 2011). Hence, briquetting of these CNS wastes helps in developing cheap source for clean energy compatible for both domestic and industrial applications, and is seen as an effective solution for disposing these wastes in a sustainable manner (Lubwama and Yiga, 2017; Mousa et al., 2019; Tuates Jr et al., 2020).