---
title: "Biogas routes and climate change"
subject: "Energy Environment and Society"
chapter: 3
key: "semester_7/energy_environment_and_society:biogas-routes-climate-change"
source: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/biogas-routes-climate-change
site: BCT Engineering
---

# Biogas routes and climate change

Energy Environment and Society · Chapter 3: Renewable Energy Sources

- Asked in: 2073 Shrawan
- 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/biogas-routes-climate-change
- Whole chapter: https://www.bctengineer.com/md/semester_7/energy_environment_and_society/chapter-3

## Question

What is biogas? List any four major routes for the conversion of biomass to energy and other useful products. How does it reduce climate change effect?

## Short answer

Biogas is methane-rich gas from anaerobic digestion of organic waste. Four major biomass conversion routes are direct combustion, thermo-chemical (gasification/pyrolysis), bio-chemical (digestion/fermentation), and chemical (biodiesel). Biogas reduces climate change by replacing fossil and wood fuels, capturing methane that would otherwise escape, and supplying organic fertiliser instead of chemical fertiliser.

## Answer

**Biogas** is a combustible gas produced when organic matter such as animal dung, human waste, kitchen waste and crop residue is decomposed by bacteria in the absence of oxygen (anaerobic digestion). Composition: methane (CH₄) 55–65%, carbon dioxide 35–45%, traces of H₂S, H₂ and N₂. Calorific value is about 20–25 MJ/m³. The by-product, slurry, is a rich organic fertiliser.

#### Four major routes for conversion of biomass
1. **Direct combustion** (heat, steam, power; briquettes and pellets).
2. **Thermo-chemical conversion:** gasification and pyrolysis, giving producer gas, bio-oil and charcoal.
3. **Bio-chemical conversion:** anaerobic digestion (biogas) and fermentation (ethanol).
4. **Chemical conversion / extraction:** transesterification of oils to biodiesel; Fischer–Tropsch synthesis of liquid fuels from syngas.

#### How biogas reduces climate change effects
- **Replaces firewood, kerosene and LPG:** less CO₂ from burning fossil fuel, and fewer trees cut (forests are carbon sinks).
- **Captures methane:** dung left in the open releases methane, which has a global warming potential about 25–28 times that of CO₂ over 100 years. In a digester, methane is collected and burnt to CO₂, which reduces the warming effect greatly.
- **Biogenic carbon:** the CO₂ released on burning was recently absorbed from the air by plants, so it is nearly carbon-neutral.
- **Slurry replaces chemical fertiliser**, whose production emits N₂O and CO₂.
- **Reduces black carbon and smoke** from traditional stoves.
- Nepal's biogas programme (supported by AEPC; first CDM-registered activity of Nepal) earns carbon credits for the avoided emissions.

## Check yourself

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

1. What is the composition of biogas?
   A. CH4 55-65% and CO2 35-45%
   B. CO 55-65% and H2 rest
   C. CO2 55-65% and CH4 rest
   D. Pure H2
   Answer: A. Biogas is mainly methane with carbon dioxide and traces of H2S, H2 and N2. (Key point: Biogas composition)

2. How does a digester reduce climate impact compared with leaving dung in the open?
   A. It increases methane release
   B. It captures methane and burns it to CO2, which has far lower warming potential
   C. It converts methane to N2O
   D. It stops plants absorbing CO2
   Answer: B. Methane has a GWP about 25-28 times CO2, so burning it to CO2 reduces warming greatly. (Key point: Methane capture)

3. Which set lists the four conversion routes of biomass?
   A. Combustion, thermo-chemical, bio-chemical and chemical/extraction
   B. Wind, solar, hydro and tidal
   C. Gasification only, four types
   D. Fermentation, four types
   Answer: A. The four routes are direct combustion, thermo-chemical, bio-chemical and chemical conversion. (Key point: Four routes)

## Related in Energy Environment and Society

- [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
- [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
- [Climate change and renewable mitigation](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Aclimate-change-renewables-mitigation) — Energy Environment and Society, chapter 4

## Related in other subjects

- [Climate change and renewable energy](https://www.bctengineer.com/question.md?key=semester_7%2Ftechnology_environment_and_society%3Acc-renewable-energy) — Technology, Environment and Society, chapter 6
- [Short note: Biomass to biogas](https://www.bctengineer.com/question.md?key=semester_7%2Ftechnology_environment_and_society%3Abiomass-to-biogas) — Technology, Environment and Society, chapter 6
- [Biochemical conversion of biomass to fuels](https://www.bctengineer.com/question.md?key=semester_6%2Fenergy_resources%3Abiomass-biochemical-conversion) — Energy Resources, chapter 3
- [Climate change gases and symptoms in Nepal](https://www.bctengineer.com/question.md?key=semester_7%2Ftechnology_environment_and_society%3Acc-gases-symptoms-nepal) — Technology, Environment and Society, chapter 6