The cashew processing industry generates substantial biomass waste in the form of cashew shells, accounting for approximately 67.5% of the cashew fruit. These shells, traditionally considered waste, possess significant energy potential with a calorific value of 4800 Kcal/kg, higher than many conventional biomass fuels. However, their direct combustion presents challenges due to the presence of Cashew Nut Shell Liquid (CNSL), which contains acidic compounds that can damage equipment and generate toxic emissions. This white paper examines pyrolysis-based technologies that enable clean and efficient energy recovery from cashew shells, focusing on advanced pyrolysis systems and their implementation in cashew processing facilities.

The Cashew Shell Burner is a cutting-edge pyrolysis burner that utilizes a unique process called pyrolysis to convert cashew shells into a highly efficient and eco-friendly fuel source. Pyrolysis is a thermal decomposition process that occurs in the absence of oxygen, transforming the cashew shells into a combustible gas and a carbon-rich solid residue called biochar.

The Cashew Shell Burner consists of a combustion chamber where the pyrolysis process takes place, and a gas collection and filtration system that captures the combustible gases released during the process. These gases are then used as a fuel source for various applications, such as heating systems, industrial processes, or even electricity generation.

1. Environmental Sustainability: By converting cashew shells, which were previously considered waste, into a valuable fuel source, the Cashew Shell Burner significantly reduces the environmental impact of cashew processing. It eliminates the need for open burning or landfilling, minimizing air pollution and greenhouse gas emissions.
2. Renewable Energy Source: The combustible gases produced by the Cashew Shell Burner serve as a renewable and sustainable energy source, reducing our reliance on fossil fuels and contributing to a cleaner, greener future.
3. Economic Benefits: The Cashew Shell Burner not only reduces waste disposal costs for cashew processing facilities but also generates a valuable fuel source, potentially leading to cost savings and additional revenue streams.
4. Biochar Production: In addition to the combustible gases, the Cashew Shell Burner also produces biochar, a highly porous and carbon-rich material that can be used as a soil amendment, improving soil fertility, water retention, and plant growth.

The cashew processing industry represents a significant agricultural sector globally, with Vietnam, Nigeria, and India leading production. The industry faces two interrelated challenges: managing substantial quantities of shell waste and meeting high energy demands for processing operations.

For every 1000kg of raw cashew processed, approximately 550-650kg of shells are generated as waste. These shells contain 15-25% CNSL, a viscous, acidic substance that complicates disposal. Direct combustion of untreated shells produces:

• Toxic and corrosive fumes containing anacardic acid
• Black smoke emissions due to high carbon credit
• Damage to furnace equipment and refractory materials
• Environmental pollution and worker health hazards

Cashew processing facilities require significant thermal energy for operations such as steaming, drying, and roasting. Processing 1000kg of raw cashew requires approximately 2000MJ of energy, predominantly thermal energy for steaming. This creates an opportunity to address both challenges simultaneously by converting shell waste into process energy through advanced thermal technologies.

Understanding the characteristics of cashew shells is critical for developing effective energy recovery systems. Cashew shells have a complex structure consisting of three layers: an outer green smooth layer, a middle honeycombed porous layer containing CNSL, and a hard inner gray shell.

Proximate analysis of cashew shells reveals:

• Moisture content: 4.4% (wet basis)
• Volatile matter: 82.6 (dry basis)
• Ash content: 1.9% (dry basis)
• Fixed carbon: 11.4% (dry basis)
• Heating Value: 20.2 Mj/kg (4824.69 kcal/kg)

The high volatile content (82.6%) makes cashew shells particularly suitable for pyrolysis and gasification processes. The low ash content (1.9%) reduces operational problems related to ash handling and disposal.

CNSL constitutes 15-25% of cashew shell weight and presents both challenges and opportunities:

• Dark, reddish-brown viscous liquid with acidic properties (pH 4.5-5.2)
• High calorific value of approximately 11,380 Kcal/kg, similar to light furnace oil
• Contains valuable components (anacardic acid, cardol) with applications in polymer-based industries, including paints, brake linings, and epoxy resins

The presence of CNSL necessitates specialized approaches to energy recovery that can manage its corrosive properties while harnessing its energy content.

Pyrolysis represents a thermochemical decomposition process that occurs in the absence of oxygen. When applied to cashew shells, it offers significant advantages over direct combustion by managing the problematic CNSL component while generating valuable products.

During cashew shell pyrolysis, several temperature-dependent transformations occur:

• 55-200°C: Moisture removal phase
• 200-270°C: Decomposition of hemicellulose begins; CNSL starts to be gradually removed
• 270-450°C: Violent decomposition of cellulose occurs; approximately 67% of volatiles are released
• At 400°C: About 90% of CNSL is removed, and shells transform into charcoal
• At 600°C: Charcoal undergoes significant structural modification

This temperature-controlled process enables the management of CNSL while producing three valuable outputs: biochar, bio-oil, and syngas.

The pyrolysis of cashew shells generates multiple valuable products:

• High carbon content (70-75% by weight)
• Heating value of 25-28 MJ/kg, making it an excellent solid fuel
• Can be used similarly to conventional charcoal or for soil amendment applications

• Higher heating value than biochar (32 MJ/kg)
• Can be used in mixtures with diesel as a liquid fuel
• Contains valuable chemical compounds with industrial applications

• Predominantly composed of CO₂ and CO at temperatures below 400°C
• Increasing H₂ formation at higher temperatures
• Can be combusted directly for thermal applications

Advanced cashew shell pyrolysis systems represent specialized technology developed specifically for converting cashew shells into energy within processing facilities.

Modern pyrolysis systems for cashew shells consist of several integrated components:

Pyrolysis Reactor: A specially designed chamber that heats cashew shells to temperatures ranging from 400°C to 1000°C in an oxygen-limited environment.

Gas Transport System: Engineered channels that direct pyrolysis gases from the reactor to the combustion chamber through natural convection or chimney effect.

Combustion Chamber: Often integrated with an existing boiler, where pyrolysis gases mix with oxygen and combust to produce heat.

Biochar Collection System: Mechanisms to recover the biochar produced during the process for further use or sale.

Heat Exchange System: Components that transfer the thermal energy from combustion gases to process applications such as steam generation or drying.

Advanced pyrolysis systems operate through the following sequence:

• Cashew shells are fed into the high-temperature environment of the reactor (typically 400-1000°C).
• In the absence of oxygen, shells decompose through pyrolysis, releasing volatile gases and forming biochar.
• The released pyrolysis gases are conducted by natural convection or mechanical means to the combustion chamber, typically located in the boiler hearth.
• Upon contacting oxygen in the combustion chamber, the gases ignite, producing thermal energy for the production unit.
• The biochar remaining in the reactor can be recovered and used as charcoal for various applications.

Modern pyrolysis technologies demonstrate remarkable adaptability:

• Compatible with various biomass feedstocks beyond cashew shells, including shea cakes, cocoa pods, and rice husks
• Applicable across multiple industrial sectors with thermal energy needs, including food processing, agricultural drying operations, and oil extraction
• Adaptable to different processes, including slow pyrolysis, fast pyrolysis, carbonization, and charcoal briquette production

This versatility enables the technology to serve diverse applications while maintaining its core functionality of converting biomass waste into energy.