---
title: "Biochemical conversion of biomass to fuels"
subject: "Energy Resources"
chapter: 3
key: "semester_6/energy_resources:biomass-biochemical-conversion"
source: https://www.bctengineer.com/q/semester_6/energy_resources/biomass-biochemical-conversion
site: BCT Engineering
---

# Biochemical conversion of biomass to fuels

Energy Resources · Chapter 3: Bio energy

- Asked in: year not given, 5 marks
- Video: [The Conversion of Biomass to Ethanol](https://www.youtube.com/watch?v=jlbBZYZ2Tro) — kathergaga
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/energy_resources/biomass-biochemical-conversion
- Whole chapter: https://www.bctengineer.com/md/semester_6/energy_resources/chapter-3

## Question

What is biomass? Explain the biochemical conversion processes used to produce fuels from biomass, with the chemical reactions where relevant.

## Short answer

Biomass is renewable organic matter from recent biological origin. Biochemical conversion uses microbes/enzymes on wet biomass: anaerobic digestion (bacteria without air) produces biogas via hydrolysis, acidogenesis, acetogenesis, methonogenesis; alcohol fermentation of sugars by yeest yields ethanol: C6H12O6 → 2C2H5OH + 2CO2.

## Answer

**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.

## Check yourself

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

1. Biochemical routes suit biomass of what kind?
   A. Wet biomass with moisture above 50%
   B. Dry wood only
   C. Coal
   D. Biomass of any moisture needing high temperature
   Answer: A. Drying wet biomass for thermochemical routes would waste energy. (Key point: Biochemical vs thermochemical)

2. Which reaction describes alcohol fermentation?
   A. C6H12O6 → 6CO2 + 6H2O
   B. CH3COOH → CH4 + CO2
   C. C6H12O6 → CH4 only
   D. C6H12O6 → 2C2H5OH + 2CO2 by yeast
   Answer: D. Yeast converts glucose to ethanol and carbon dioxide. (Key point: Fermentation reaction)

3. What is done to starch before yeast fermentation?
   A. It is burnt
   B. It is mixed with methanol
   C. It is converted to glucose by enzymes (hydrolysis)
   D. It is distilled
   Answer: C. Yeast ferments sugars, not starch directly. (Key point: Starch crops)

4. What does anaerobic digestion of dung produce?
   A. Ethanol and CO2
   B. Methane-rich biogas and a good fertiliser slurry
   C. Bitumen
   D. Charcoal
   Answer: B. Bacteria decompose wet biomass without air. (Key point: Anaerobic digestion)

## Related in Energy Resources

- [Hydrogen production processes](https://www.bctengineer.com/question.md?key=semester_6%2Fenergy_resources%3Ahydrogen-production-methods) — Energy Resources, chapter 6

## Related in other subjects

- [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
- [Bio-chemical conversion of biomass](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Abiochemical-conversion-biomass) — Energy Environment and Society, chapter 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
- [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×