Ethanol from cassava or molasses: what really drives yield and cost

For a new ethanol plant in Nigeria, picking cassava or molasses as the main feedstock decides the process design, the capital cost, and the running economics. Four things separate them: fermentable sugar, processing complexity, energy, and supply security.

Fermentable sugar comes first

Yeast ferments simple sugars. Molasses is already sugar (sucrose, glucose, fructose) and only needs diluting to a workable strength, with pH and nutrient adjustment, before fermentation. Cassava is starch, which yeast cannot use. It has to be cooked and converted with enzymes, first liquefaction with alpha-amylase, then saccharification with glucoamylase, to release fermentable sugar. That conversion step is the main difference between the two routes.

The cassava route

Cassava's strengths are agronomic. It grows across Nigeria, supports smallholder supply, and does not depend on another industry's cycle. The difficulties are in processing.

• A cook-and-convert stage, with its own vessels, heat load, and enzyme cost.
• Higher solids and viscosity, which make pumping, mixing, and heat transfer harder.
• Variable input. Starch content shifts with variety, crop age, and handling after harvest, so the process has to cope with a moving target.
• Short shelf life. Fresh cassava spoils within days, so you either solve the supply logistics or dry it to chips.

The molasses route

Molasses removes the front end. No cooking, no enzymes, so lower capital cost and a simpler process. The trade-offs are commercial.

• Supply and price follow the sugar industry. Availability is seasonal, and when sugar output is low, molasses is short and expensive.
• Quality moves between mills and batches: sugar content, ash, and unfermentable material.
• Domestic volume is limited, so a plant of any size may rely on imports, which adds currency and logistics risk.

Where the cost sits

A few items dominate operating cost on both routes.

• Feedstock cost per litre of ethanol, not per tonne. A cheaper feedstock with less fermentable sugar can work out dearer.
• Distillation energy. Separating ethanol from water takes a lot of steam and is usually the biggest energy cost in the plant. Heat integration, and at larger scale multi-effect distillation or vapour recompression, change this figure a lot.
• Enzymes. A recurring cost on cassava, close to nothing on molasses.
• Water and effluent. Stillage (vinasse) is very strong waste. Handling it is a real cost and a regulatory requirement either way.
• Downtime. Contamination, weak temperature control, and mechanical stoppages all eat into fixed costs.

The yield levers you control

Set the feedstock aside and yield is decided in fermentation and distillation.

• Fermentation efficiency: how close you get to the theoretical sugar-to-ethanol conversion. Yeast choice, pitching rate, nutrients, and oxygen at the start all count.
• Temperature control. Fermentation gives off heat. Without enough cooling the temperature climbs, the yeast struggles, and yield and speed both drop. Thermotolerant strains help where cooling is limited.
• Contamination control. Lactic and acetic acid bacteria eat sugar that should have become ethanol. Cleaning discipline, clean propagation, and antimicrobial dosing where needed protect the yield.
• Distillation. Run the column outside its stable range and ethanol goes out with the stillage. Steady feed, reflux, and steam keep recovery high.

Deciding

A short study settles it for a given site: delivered price for each feedstock now and projected, fermentable content, the capital gap between the two designs, energy cost, and how reliable local supply is. Some Nigerian plants are built to run mainly on one feedstock and take the other when price or supply shifts. That costs more up front and lowers the risk of depending on a single supply chain.