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

# Thermo-chemical conversion of biomass

Energy Environment and Society · Chapter 3: Renewable Energy Sources

- Asked in: 2071 Shrawan, 1.5+2.5 marks; year not given
- Video: [Biomass Energy](https://www.youtube.com/watch?v=Cux0Xwvy0cU) — Najam Academy
- 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/thermochemical-conversion-biomass
- Whole chapter: https://www.bctengineer.com/md/semester_7/energy_environment_and_society/chapter-3

## Question

What is biomass? Describe any thermo-chemical conversion process of biomass.

## Short answer

Biomass is organic material from plants and animals storing solar energy, renewable if sustainably grown. Gasification, a thermo-chemical process, partially burns biomass at 800–1000 °C with limited air to produce syngas (CO, H₂), which can run engines or be converted to fuels.

## Answer

**Biomass** is organic material from plants and animals (wood, crop residues, dung, municipal and food waste, energy crops) that stores solar energy chemically and can be used as fuel. It is renewable and carbon-neutral if grown sustainably.

#### Thermo-chemical conversion
These processes use heat (and sometimes limited oxygen) to convert biomass into fuel gas, liquid or solid. The main ones are combustion, gasification, pyrolysis and liquefaction.

#### Gasification (explained in detail)
Gasification partially burns solid biomass at 800–1000 °C with a limited supply of air, oxygen or steam to give a combustible **producer gas (syngas)**: carbon monoxide, hydrogen, methane, with CO₂ and N₂.

Zones of a downdraft gasifier:
```text
   Biomass in
      |
  [Drying]        100-200 C
  [Pyrolysis]     200-600 C
  [Oxidation]     1000+ C   C + O2 -> CO2
  [Reduction]     800 C     C + CO2 -> 2CO
                            C + H2O -> CO + H2
      |
  Producer gas + ash out
```
- Main reactions: C + O₂ → CO₂ (heat); C + CO₂ → 2CO; C + H₂O → CO + H₂.
- Gas heating value is about 4–6 MJ/Nm³ (with air).
- Uses: running gas engines and generators, boilers, kilns, and as raw material for methanol or Fischer–Tropsch fuels after cleaning.
- Benefits: more efficient than direct burning for electricity, can use various wastes. Problems: tar in the gas and gas cleaning cost.
- In Nepal, small gasifiers have been used for rural cooking and power in the Tarai.

#### Pyrolysis (brief)
Heating biomass to 400–600 °C in the absence of oxygen gives bio-oil (liquid), charcoal and gases. Fast pyrolysis gives mostly bio-oil.

## Check yourself

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

1. What happens in gasification?
   A. Biomass is fully burnt in excess air
   B. Biomass is partially burnt at 800-1000 °C with limited air, oxygen or steam to give producer gas
   C. Bacteria digest biomass
   D. Biomass is heated at 100 °C
   Answer: B. Limited oxidant gives combustible CO, H2 and CH4 rather than complete combustion. (Key point: Gasification definition)

2. What is the order of zones in a downdraft gasifier from top to bottom?
   A. Reduction, oxidation, pyrolysis, drying
   B. Drying, pyrolysis, oxidation, reduction
   C. Oxidation, drying, reduction, pyrolysis
   D. Pyrolysis, drying, reduction, oxidation
   Answer: B. Biomass is dried, pyrolysed, then oxidised (heat), and the hot gas is reduced to CO and H2. (Key point: Gasifier zones)

3. Which reactions in the reduction zone produce fuel gas?
   A. C + O2 → CO2 only
   B. C + CO2 → 2CO and C + H2O → CO + H2
   C. 2H2 + O2 → 2H2O
   D. CH4 + 2O2 → CO2 + 2H2O
   Answer: B. The Boudouard and water-gas reactions produce combustible CO and H2. (Key point: Reduction reactions)

4. What is a main problem of gasification?
   A. Tar in the gas and cleaning cost
   B. It gives no usable gas
   C. It works only with coal
   D. It needs no heat
   Answer: A. Tar contaminates engines, so gas cleaning is needed. (Key point: Gasification problem)

## Related in Energy Environment and Society

- [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 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