Bio-chemical conversion of biomass

Bio-chemical conversion uses microorganisms (bacteria, yeast) or enzymes to break down wet biomass into useful fuels, at low temperature. It suits wet organic matter such as dung, sewage, food waste and sugar crops. Two main processes:

1. Anaerobic digestion (biogas production)

Bacteria decompose organic matter in the absence of oxygen at 30–40 °C (mesophilic) in a closed digester. Stages:

1. Hydrolysis: complex organics (carbohydrate, protein, fat) are broken into sugars, amino acids and fatty acids.
2. Acidogenesis: acid-forming bacteria produce volatile fatty acids, alcohols, CO₂ and H₂.
3. Acetogenesis: conversion into acetic acid, CO₂ and H₂.
4. Methanogenesis: methane-forming bacteria produce methane: CH₃COOH → CH₄ + CO₂; CO₂ + 4H₂ → CH₄ + 2H₂O.

Product: biogas (CH₄ 55–65%, CO₂ 35–45%) and digested slurry as fertiliser.

 Dung+water -> [Inlet] -> [Digester] -> gas holder -> stove
                              |
                          [Outlet] -> slurry pit

Factors: temperature (optimum 30–35 °C), pH 6.8–7.5, C:N ratio about 25–30:1, retention time (40–60 days), water content (1:1 dung to water) and absence of oxygen. Nepal's GGC 2047 fixed-dome design is common.

2. Fermentation (ethanol production)

Yeast converts sugars to ethanol and CO₂ in the absence of air: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ Feedstock: sugar-cane juice or molasses, maize, cassava (starch is first hydrolysed to sugar). The product is distilled to 95% ethanol and dehydrated to 99.5% for blending with petrol (E10 and so on).

Advantages: low temperature, renewable, waste treatment. Limits: slow process, sensitive to conditions, and low gas yield in cold areas.