Biochemical conversion of biomass to fuels

Biomass is organic matter of recent plant or animal origin (wood, crop residues, animal dung, sewage, energy crops, municipal organic waste) that stores solar energy through photosynthesis. It is renewable and nearly carbon-neutral.

Conversion routes are thermochemical (combustion, gasification, pyrolysis) and biochemical, in which microbes or enzymes act on wet biomass at low temperature.

1. Anaerobic digestion (biogas)

Bacteria decompose wet biomass such as dung and sewage sludge in the absence of air, giving methane-rich biogas through four stages: hydrolysis, acidogenesis, acetogenesis and methanogenesis. Overall, for a simple sugar:

$$C_6H_{12}O_6 \xrightarrow{\text{bacteria}} 3CH_4 + 3CO_2$$

The main methane-forming steps are $CH_3COOH \rightarrow CH_4 + CO_2$ and $CO_2 + 4H_2 \rightarrow CH_4 + 2H_2O$. The digested slurry is a good fertiliser.

2. Alcohol fermentation (ethanol)

Sugar and starch crops (sugarcane, molasses, maize, cassava) are fermented by yeast. Starch is first converted to glucose by enzymes (hydrolysis), then:

$$C_6H_{12}O_6 \xrightarrow{\text{yeast}} 2C_2H_5OH + 2CO_2$$

The dilute product (8–12 % ethanol) is distilled up to about 95 % and dehydrated to anhydrous ethanol for blending with petrol (E10). Cellulosic biomass needs pre-treatment and cellulase enzymes (second-generation ethanol). The theoretical yield is 0.51 kg of ethanol per kg of glucose.

3. Other biochemical routes

• Biodiesel by transesterification of vegetable oil with methanol and a catalyst (a chemical process, often grouped with these).
• Biohydrogen by dark fermentation or photo-fermentation.
• Composting to produce a soil conditioner.

Biochemical routes suit biomass with moisture above 50 %, which would waste energy in drying for thermochemical routes.