---
title: "Bio-chemical conversion of biomass"
subject: "Energy Environment and Society"
chapter: 3
key: "semester_7/energy_environment_and_society:biochemical-conversion-biomass"
source: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/biochemical-conversion-biomass
site: BCT Engineering
---

# Bio-chemical conversion of biomass

Energy Environment and Society · Chapter 3: Renewable Energy Sources

- Asked in: year not given
- Video: [Types of Biofuels, Energy conversion routes from biomass](https://www.youtube.com/watch?v=2ZG2CyknGCg) — Vidya-mitra
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/biochemical-conversion-biomass
- Whole chapter: https://www.bctengineer.com/md/semester_7/energy_environment_and_society/chapter-3

## Question

Describe the bio-chemical conversion process of biomass.

## Short answer

Biochemical conversion uses microorganisms to transform wet biomass into fuels. Anaerobic digestion: bacteria decompose organic matter in a closed digester (hydrolysis, acidogenesis, acetogenesis, methanogenesis) producing biogas (CH4 55–65%, CO2) and fertiliser. Fermentation: yeast converts sugars to ethanol (C6H12O6 -> 2C2H5OH + 2CO2) for blending with petrol.

## Answer

**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.
```text
 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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What kind of biomass suits bio-chemical conversion?
   A. Wet organic matter such as dung, sewage and sugar crops
   B. Dry wood and straw only
   C. Plastic waste
   D. Coal dust
   Answer: A. Microorganisms work in wet material at low temperature. (Key point: Suitable feedstock)

2. What is the correct order of stages in anaerobic digestion?
   A. Methanogenesis, hydrolysis, acetogenesis, acidogenesis
   B. Hydrolysis, acidogenesis, acetogenesis, methanogenesis
   C. Acidogenesis, hydrolysis, methanogenesis, acetogenesis
   D. Hydrolysis, methanogenesis, acidogenesis, acetogenesis
   Answer: B. Complex organics are broken down, acids form, acetic acid forms, then methane-forming bacteria produce CH4. (Key point: Stages of digestion)

3. What is the typical composition of biogas?
   A. CH4 35-45% and CO2 55-65%
   B. H2 60% and CO2 40%
   C. CH4 55-65% and CO2 35-45%
   D. CH4 95% and CO2 5%
   Answer: C. Biogas is mainly methane (55-65%) with CO2; the slurry is used as fertiliser. (Key point: Biogas composition)

4. What does yeast do in ethanol fermentation?
   A. Converts ethanol to sugar using oxygen
   B. Converts methane to ethanol
   C. Burns sugar to give heat
   D. Converts sugar to ethanol and CO2 in the absence of air
   Answer: D. C6H12O6 gives 2C2H5OH + 2CO2 under anaerobic conditions; starch must first be hydrolysed to sugar. (Key point: Fermentation)

## Related in Energy Environment and Society

- [Thermo-chemical conversion of biomass](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Athermochemical-conversion-biomass) — Energy Environment and Society, chapter 3
- [Synthetic fuel from biomass, bio fuel cells](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Asynthetic-fuel-biofuel-cell) — Energy Environment and Society, chapter 3
- [Biomass and conversion routes](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Abiomass-conversion-routes) — Energy Environment and Society, chapter 3, asked 3×
- [Biomass conversion, IV curve, wind mapping](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Abiomass-iv-curve-wind-mapping) — Energy Environment and Society, chapter 3

## Related in other subjects

- [Biochemical conversion of biomass to fuels](https://www.bctengineer.com/question.md?key=semester_6%2Fenergy_resources%3Abiomass-biochemical-conversion) — Energy Resources, chapter 3
- [Biomass compared with geothermal energy](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Abioenergy-vs-geothermal) — Power Plant Design, chapter 1