Cashew nut shells are one of the most abundant forms of tropical biomass waste which can be used for energy generation. The cashew nut tree (Western Anacardium) grows both naturally and through planned agricultural practices in subtropical countries, such as: Brazil, Nigeria and Ghana.
The cashew comprises of approximately 30% nut and 70% shell. While the nut is eaten throughout the world, the Cashew nut shells is very hard, with a thickness of 1 to 2mm is commonly discarded. Cashew shells typically contain: 10.8% water and 2.6% ash. Cashews shells have a net calorific value of 18.9 MJ/kg, placing them right in the middle of the biomass fuel mean range (14 - 21 MJ/kg).
Cashew Nut Shell Liquid (CNSL)
25 to 35% of this hard cashew is comprised of Cashew Nut Shell Liquid (CNSL), a viscous and caustic substance. CNSL has a similar quality and heat content to Light Fuel Oil (LFO). Both the CNSL and CNS are used as biomass fuels for clean power generation.There is a growing demand and for this CNSL due to its other multiple uses (including for: paints, laminating resins and intermediates for chemical industry). The scope and varied opportunities for the development of other tailor-made polymers the CNSL offerse is subsequently increasing its value.
Biochar as a Cashew Nut Shell By-product
Biochars are created in the processing of cashew nut shells along with other bioproducts. Biochars have a high carbon content (70–75 wt%) and high heating values in the range of 25–28 MJ kg−1 and can therefore be used as an energy source. Biochars also contain significant quantities of Potassium along with smaller amounts of Magnesium, Calcium, Copper, Zinc and Iron. These minerals result in biochar being a potential fertilizer.
Bio-oil as a Cashew Nut Shell By-product
Bio-oils have been shown to have a higher heating value than biochar with 32 MJ kg−1. Bio-oil is not suitable for use in pure form but can be used in mixtures with Diesel. The gas phase of bio-oil is predominantly CO2 and CO below 400 °C with a greater formation of H2.
Issues with unprocessed cashew shells
The shell by-product created through cashew processing cause various issues. Cashew nut shells are naturally toxic which is why cashew nuts must be processed to remove toxins before selling. Discarded cashew nut shells from cashew processing plants can weigh hundreds of tonnes and can be a fire hazard or toxic, with toxins from the shell waste leeching into rivers or local residential areas during heavy rains.
Cashew Nut Shell Gasifier Stove
Gasification is thermo-chemical process that converts carbon-containing fuels into a combustible gas through incomplete combustion and reduction. The combustible gas can then be used in internal combustion engines or gas turbines, burned directly or used in the production of methanol or hydrogen. There are three types Gasifiers which are classified according to the way air or oxygen is bought into the system: Downdraft, Updraft, and Crossdraft. The Cashew Nut Shell Gasifier Stove was found to have a thermal efficiency of 35.5 % and an energy output of 2.19 kW (Singh et al. 2006). While the corresponding values for LPG and Kerosene stoves higher with 53.5 3% (0.69 kW) and 39.3 % (0.6 kW) respectively, cashew nut shells are still a viable alternative.
Case Studies: Examples
Using Waste from Cashew Nut Processing in Burkina Faso
Burkina Faso, is a landlocked country in Africa with a population of almost 18 million, produces 35,000 tonnes of cashew products in annually with a third being processed locally .
Dakoro is an industrial cashew nut production facility that utilizes LPG (butane) and non-renewable wood fuel as power sources. However, recently Dakoro installed a biogas-powered water-steam system as a pilot project. This pilot project was coordinated by SNV and was hosted by WOUOL, the association of agricultural producers that owns Dakoro .
Separating the cashew nut from the shell requires two energy-intensive steps: weakening and drying. Conventionally, wood is used as a source of energy for the weakening step, while butane gas is used for the drying. However, the biogas-powered water-steam system uses cashew nut shell waste to produce the energy necessary for both steps. he system consists of a gasifier, a storage tank or boiler and three dryers. The gasifier and the storage boiler are equipped with fans and pumps powered by a solar photovoltaic system. The steam produced by the boiler weakens the shells from the nuts and is a source of heat in the drying phase.
The system resulted in: an 83% lower energy cost compared to the baseline, reduced time to soften the nuts, reduced smoke, and a better quality product as a result of the introduction of new components, such as a heat exchanger. WOUOL established a system to sustain the Dakoro pilot by setting aside 20% of reduced energy costs for maintenance . Potential improvements to the system were also identified, such as: including double piping to allow for continuous operation during maintenance. It was also found that a larger heat exchanger would allow the boiler to heat up faster resulting in a shorter turnover time .
The initial capital costs and lack of access to credit are the main sources of limitation to expanding the Dakoro pilot project further.
Environmental and Social Issues
The replacement of fossil-based sources and the productive use of the cashew shell waste reduce emissions while also creating less smoke and heat – thereby improving the working environment. The biogas-powered water-steam system increased the facilities processing capacity, increasing the employment of local women. On the other hand, the lower maintenance requirements resulted in plant technicians seeing a reduction in their working hours .
Similar technology is available in other countries (mostly Asia and the US), however, it eas the first of its kind in Burkina Faso. As of 2010 three different agro-businesses have replicated the technology for the drying and processing of cassava, rice and mango. Cotton stalks, rice husks and peanut shells have all been suggested as addition biomass fuel for the system .
This project utilized cashew nut shells to generate energy using a High Calorific Cashew Pyrolyser H2CP. A H2CP transforms waste from cashew nut shells into two types of fuel:
- Pyrolysis gas that can be used to fuel a boiler, and
- Biochar that can be sold to local population as a substitute for charcoal.
When cashew nut shells are burnt to produce energy the phenolic oil in the shells produces acid fumes that are polluting and irritating. Cajouvalor proposes to alleviate this while also creating jobs selling a transformed product .
The gas and biochar can then go on to be used in other small scale industries such as in mango drying, in cotton oil factories or even by local soap producers. There is also potential to produce electricity from the gas and the biochar to supply local households with electricity .
The Cajouvalor project recently received the SUEZ – Institut de France Award in the “Access to essential services” category .