Cassava roots serve as a valuable resource for bioethanol production, particularly during the abundant and cost-effective harvest season. However, these roots have high moisture content, making them susceptible to spoilage during storage. To address this issue, cassava roots are often converted into a dried form known as cassava chips near the plantation areas.
The process involves manually or mechanically cutting the harvested roots into pieces, which are then sun-dried. The resulting dried chips have low moisture content (< 14%), making them more compact, cost-effective for transportation, and capable of being stored for up to a year in warehouses. Dried cassava chips share similarities with corn grains and can be processed using corn grain conversion technology.
However, precautions must be taken during storage to prevent heat generation, which can lead to overheating and the potential for chip combustion. Adequate air ventilation in warehouses is essential to mitigate these risks. When using the chips, a first-in, first-out approach is recommended for efficient processing.
Nevertheless, the use of chips comes with challenges such as dust production, resulting in starch loss and air pollution. Contamination with soil and sand, introduced during the root drying process, poses a risk of machine corrosion and a shortened machine shelf life. Therefore, thorough removal of contaminants is necessary during production.
In Thailand, where cassava chips are utilized for various applications, including animal feed and bioethanol production, farmers are encouraged to produce high-quality chips that meet established standards. Bioethanol production from cassava can involve different forms, including fresh roots, chips, and starch.
Effective feedstock planning is crucial for bioethanol plants, as feedstock costs can account for up to 70% of total ethanol production costs. The choice of feedstock depends on factors such as production capacity, plant location, proximity to cassava growing areas, feedstock availability, and processing technology.
Ethanol plants located away from cassava farms often prefer dried chips to reduce transportation and storage costs. Conversely, plants situated near cassava fields can use both chips and roots, requiring some adjustments to the processing.
The preparation of cassava feedstock involves two main processes: dry milling and wet milling. Dry milling, similar to corn grain bioethanol production, involves milling and sieving dried chips, followed by slurring with water and subsequent cooking and enzyme hydrolysis. Wet milling, on the other hand, requires modifying the cassava starch production process, resulting in starch slurry preparation from dried chips.
While wet milling yields more valuable products, it is capital and energy-intensive, with lower ethanol yield compared to dry milling. A modified dry-milling process has been developed to enhance cost reduction and value addition.
For cassava roots, two processes are used: the ‘With Fiber’ process, which involves grinding and mixing fresh roots with cassava chips, and the ‘De-fiber’ process, where starch slurry is prepared from fresh roots. Each process has its advantages and drawbacks, with considerations for equipment, investment cost, and suitability for batch or continuous fermentation.
In summary, the diverse methods of cassava feedstock preparation for bioethanol production require careful consideration of factors such as moisture content, storage, contamination risks, and processing technology to optimize efficiency and minimize costs.