Biogas from cashew apples

The main product of cashew tree is the nut, with cashew apples being a byproduct. Cashew apples are sweet and similar in flavor to mangoes. Cashew apples are edible and even healthy, but they fragile and spoil quickly, making them difficult to transport and market. Cashew apples can be used to produce juice, marmalade, alcohol, liqueur, and biogas. Producing biogas from cashew apples is particularly interesting because it can provide energy for food processing plants that process cashew apples, and, most importantly, biofertilizer for cashew plantations.

The cashew season lasts from 40 to 70 days. Cashew harvesting occurs at different times on different plantations. Even from the same tree, the fruits don't fall to the ground simultaneously, but at different times. A biogas plant must operate year-round. Therefore, cashew apples must be ensilaged to create a year-round supply. Ensilaging is done in the same way corn is ensilaged at cattle farms. Similar processes also occur when ensilaging cucumbers, cabbage, and regular apples. The fruit is fermented by natural lactic acid bacteria in the silo. 35-40 days is sufficient for ensilaging cashew apples. The silo is best made of concrete, but buried under the ground. Unlike corn, cashew apples release juice during ensilaging. To prevent juice leakage, the silo is built below ground level as a concrete pit. To add lactic acid bacteria, juice can be taken from a dairy farm silage storage. When storing cashew apples, spray the silo with 1-2% juice and cover with a PVC film.

Ensilaging not only preserves cashew apples but also improves biogas yield. Pre-fermentation with lactic acid bacteria occurs in the silo. Biogas yield increases by 5-10% compared to fresh cashew apples.

Ensilaging cashew apples can be challenging due to the high moisture content 80-85%. Therefore, instead of the cashew apple itself, pressed cashew apple pulp, a left-over from juice and puree production, can be used for biogas. Pressed cashew apple pulp contains 25-30% dry matter and is easily ensilaged like pressed sugar beet pulp. Ensilaging can be done in concrete silos or in a polyethylene bags. The same machines used for ensilaging corn silage into bags can be used for ensilaging pressed cashew apples pulp.

Ensilaged cashew apple pulp can be stored for up to 3-4 years. The biogas yield from 1 ton of ensilaged cashew apple pulp is 150-170 m³ with a methane content of 58%. Fermentation period is 28 days. 

Biogas yield is 110 m³ per ton of cashew apple silage or 90 m³ of biogas from 1 ton of fresh cashew apples. Methane content is 53%. Fermentation period is 35 days. A biogas plant processing 100 tons of cashew apples produces approximately 1 MW of gross electrical power (900 kW net). This translates to 24,000 kWh of gross electrical energy per day.