---
title: "E number and biofuels"
subject: "Energy Environment and Society"
chapter: 3
key: "semester_7/energy_environment_and_society:e-number-biofuels"
source: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/e-number-biofuels
site: BCT Engineering
---

# E number and biofuels

Energy Environment and Society · Chapter 3: Renewable Energy Sources

- Asked in: 2070 Chaitra, 1+3 marks
- 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/e-number-biofuels
- Whole chapter: https://www.bctengineer.com/md/semester_7/energy_environment_and_society/chapter-3

## Question

Define E number. How do biofuels differ from other sources of energy?

## Short answer

E number is the percentage of ethanol blended in petrol by volume (e.g. E10 = 10% ethanol). Bio fuels differ from fossil fuels as they are renewable, near carbon-neutral liquid or gas fuels from recent biomassthat can replace transport fuels, but compete with food and need land.

## Answer

#### E number (biofuels)
The **E number** in the fuel context shows the percentage of **ethanol** blended with petrol by volume. For example E10 = 10% ethanol + 90% petrol; E20 = 20% ethanol; E85 = 85% ethanol + 15% petrol; E100 = pure ethanol. Similarly, B20 means 20% biodiesel in diesel. (Nepal has plans and trials for ethanol blending, with ethanol mainly from sugar-cane molasses; this needs a government policy on the blending ratio.)

#### How biofuels differ from other sources of energy
| Point | Biofuels | Fossil fuels | Other renewables (solar, wind) |
|---|---|---|---|
| Source | Plants, oils, waste (recent biomass) | Ancient buried organisms | Sun, wind, water |
| Renewability | Renewable, if crops are replanted | Non-renewable | Renewable |
| Form | Liquid or gas fuels (ethanol, biodiesel, biogas) | Solid, liquid and gas fuels | Mostly electricity or heat |
| Carbon | Near carbon neutral (CO₂ absorbed by growing the plant) | Adds new CO₂ | Zero operational emissions |
| Use in existing engines | Yes, as blends | Yes | Need conversion or EVs |
| Storage | Easy to store and transport | Easy | Intermittent, need storage |
| Concerns | Food vs fuel competition, land use, water, deforestation | Pollution, climate, imports | Intermittency, cost |

Biofuels are therefore the main renewable source that can directly replace liquid transport fuels, and they can be produced locally, but they need land and care to avoid competing with food crops.

## Check yourself

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

1. What does E20 mean?
   A. 20% petrol and 80% ethanol
   B. 20% ethanol blended with 80% petrol by volume
   C. 20% biodiesel
   D. 20 MJ/kg
   Answer: B. The E number is the percentage of ethanol in the blend. (Key point: E number)

2. Why are biofuels nearly carbon neutral?
   A. They emit no CO2
   B. CO2 released on burning was recently absorbed by growing plants
   C. They are made from ancient organisms
   D. They are burnt without oxygen
   Answer: B. Recent biomass recycles atmospheric carbon, unlike fossil fuels which add new CO2. (Key point: Biofuels vs fossil fuels)

3. Which is a main concern of biofuels?
   A. Intermittency
   B. Food vs fuel competition and land use
   C. They cannot be stored
   D. They need conversion of engines at all blends
   Answer: B. Crops grown for fuel can compete with food and use land, water and forest. (Key point: Concern of biofuels)

## Related in Energy Environment and Society

- [Conventional and non-conventional sources](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Aconventional-nonconventional-sources) — Energy Environment and Society, chapter 2
- [Wind sources, turbines, parks, power control](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Awind-sources-turbines-parks-control) — Energy Environment and Society, chapter 3
- [World energy trend, importance of renewables](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Aenergy-consumption-trend-re-importance) — Energy Environment and Society, chapter 2
- [Wind energy, factors and scope](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Awind-energy-factors-scope) — Energy Environment and Society, chapter 3
- [Development, energy rate, HDI of Nepal](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Adevelopment-energy-hdi-nepal) — Energy Environment and Society, chapter 1

## Related in other subjects

- [Advantages of electrical energy over other forms](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aadvantages-electrical-energy-other-forms) — Utilization of Electrical Energy, chapter 1, asked 2×
- [Energy sources in Nepal and wind scope](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aenergy-sources-nepal-wind) — Power Plant Design, chapter 1, asked 2×
- [Types of renewable energy sources](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Arenewable-energy-types) — Power System Analysis I, chapter 1
- [Energy sources and solar power generation](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aenergy-sources-solar-power) — Power Plant Design, chapter 1