---
title: "Biogas plants under Clean Development Mechanism"
subject: "Power Plant Design"
chapter: 1
key: "semester_8/power_plant_design:biogas-cdm-ghg"
source: https://www.bctengineer.com/q/semester_8/power_plant_design/biogas-cdm-ghg
site: BCT Engineering
---

# Biogas plants under Clean Development Mechanism

Power Plant Design · Chapter 1: Energy Sources and Electric Power Generation

- Asked in: 2074 Bhadra, 5 marks; 2072 Asoj, 6 marks (asked 2 times)
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/power_plant_design/biogas-cdm-ghg
- Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-1

## Question

Biogas plants of Nepal are registered as Clean Development Mechanism project because of its contribution for reducing greenhouse gas emission. How would you argue that the biogas plants are contributing to reduce greenhouse gas emission?

## Short answer

Biogas plants reduce emissions by displacing firewood and fossil fuels, preventing deforestation, capturing methane from dung (which otherwise would be released) and burning it to biogenic CO₂, substituting chemical fertilizers with bio-slurry. Methane has 25-28 times the GWP of CO₂, so capturing it yields significant emission reductions.

## Answer

**Biogas** is a gas (about 55–65% methane, CH₄, and 35–45% CO₂) produced by anaerobic digestion of cattle dung and other organic waste. Nepal's household biogas programme (Biogas Support Programme, with AEPC) was among the first CDM projects registered with the UNFCCC from Nepal (2005) and earns carbon credits (CERs) for emission reduction.

#### How biogas plants reduce greenhouse gas emissions
1. **Replacing firewood** – each household plant replaces a large amount of firewood each year. Less forest is cut, so the forest keeps absorbing CO₂ (carbon sink protected). Firewood from unsustainable harvest adds net CO₂.
2. **Replacing fossil fuels** – biogas replaces kerosene and LPG used for cooking and lighting, avoiding CO₂ from fossil carbon.
3. **Capturing methane from dung** – dung left in heaps or open pits decomposes and releases methane. Methane has a global warming potential about **25–28 times** that of CO₂ over 100 years. In a sealed digester this methane is captured and burnt:
$$
\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}
$$
   Burning converts a strong GHG into CO₂, which is biogenic (carbon neutral).
4. **Carbon neutral fuel** – CO₂ from burning biogas came from plants eaten by cattle, so there is no net addition.
5. **Bio-slurry as fertiliser** – reduces use of chemical fertiliser, whose manufacture emits CO₂ and N₂O.
6. **Cleaner combustion** – less black carbon and smoke compared to traditional stoves.

#### Cycle
```text
 Grass/fodder --> Cattle --> Dung --> Digester --> Biogas
      ^                                  |          |
      |                               Slurry      Burn
      |                              (manure)       |
      +------------- CO2 absorbed <------ CO2 <-----+
```

#### Co-benefits counted under CDM
- Reduced indoor air pollution and better health.
- Time saved in firewood collection, mainly for women.
- Revenue from CERs helps subsidise more plants.

Hence biogas plants reduce GHG emission both by **avoiding methane release** and by **displacing firewood and fossil fuel**, which justifies their CDM registration.

## Check yourself

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

1. Why does capturing and burning methane from dung reduce GHG?
   A. Methane is converted to CO2, a much weaker GHG (methane GWP about 25-28 times CO2)
   B. Methane is turned into oxygen
   C. Burning creates no gas at all
   D. Methane has lower warming than CO2
   Answer: A. Open dung releases methane; burning it in a digester system gives CO2 that is biogenic and far less warming per molecule. (Key point: Methane capture)

2. How does replacing firewood by biogas help the climate?
   A. Forests are protected and keep absorbing CO2
   B. Forests are cut faster
   C. More smoke is produced
   D. Soil is removed
   Answer: A. Less forest is cut, so the carbon sink is kept and unsustainable harvest CO2 is avoided. (Key point: Firewood replacement)

3. What is the typical composition of biogas?
   A. 55-65% methane and 35-45% CO2
   B. 100% hydrogen
   C. 80% CO and 20% N2
   D. 10% methane and 90% CO2
   Answer: A. Biogas from anaerobic digestion is about 55-65% CH4 and 35-45% CO2. (Key point: Composition of biogas)

4. How does bio-slurry reduce emissions?
   A. It is burnt for heat
   B. It replaces chemical fertiliser whose manufacture emits CO2 and N2O
   C. It produces more methane in the field
   D. It is used as kerosene
   Answer: B. Using slurry as manure avoids emissions from making chemical fertiliser. (Key point: Bio-slurry benefit)

## Related in Power Plant Design

- [Development of hydropower in Nepal](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Ahydro-development-nepal) — Power Plant Design, chapter 1
- [Steps to build a hydro project and sell to NEA](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Ahydro-project-steps-nea) — Power Plant Design, chapter 3, asked 4×
- [Combined cycle gas turbine versus hydro](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Accgt-vs-hydro) — Power Plant Design, chapter 1
- [Project selection with high sediment rivers](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aproject-selection-sediments) — Power Plant Design, chapter 3

## Related in other subjects

- [Clean Development Mechanism in Nepal](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Acdm-nepal-potential) — Energy Environment and Society, chapter 2
- [Floating drum and fixed dome biogas plants](https://www.bctengineer.com/question.md?key=semester_6%2Fenergy_resources%3Afloating-drum-fixed-dome-biogas-plant) — Energy Resources, chapter 3
- [Briquette and biogas in Nepal](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Abriquette-biogas-nepal) — Energy Environment and Society, chapter 3
- [TCTO, crashing and reducing project duration](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Atcto-crashing-reduce-schedule) — Construction Management, chapter 2