Biomass and conversion routes

Biomass is organic matter of plant or animal origin, such as wood, crop residues, animal dung, food waste, municipal waste and energy crops, that can be used as fuel. Biogas is a mixture of about 55–65% methane (CH₄), 35–45% CO₂ and traces of H₂S, produced when organic matter, for example cattle dung, is digested by bacteria without oxygen (anaerobic digestion).

Four major conversion routes

1. Direct combustion: burning wood, residue or briquettes for heat, cooking, steam and electricity.
2. Thermo-chemical conversion: gasification (producer gas), pyrolysis (bio-oil, charcoal, gas) and carbonisation.
3. Bio-chemical conversion: anaerobic digestion (biogas) and fermentation (ethanol from sugar and starch crops).
4. Chemical / mechanical extraction: transesterification of vegetable oils (jatropha, used cooking oil) to biodiesel; also briquetting / densification.

Biomass --+--> Combustion ------> Heat, power
          +--> Gasification/pyrolysis -> Gas, oil, char
          +--> Digestion ------> Biogas + slurry
          +--> Fermentation ---> Ethanol
          +--> Esterification -> Biodiesel

Potential

• World: biomass is the fourth largest energy source and gives about 10% of world primary energy, mostly in developing countries.
• Nepal: biomass (firewood, dung, residues) supplies about 60–65% of total energy. Biogas potential is estimated at 1.9 million household plants, of which over 400,000 have been installed with support from AEPC (previously the Biogas Support Programme). Other potential: institutional and municipal biogas, briquettes from forest residue and invasive species (Banmara), biodiesel from jatropha, and ethanol from sugar-cane molasses.
• Benefits: reduces firewood use and deforestation, indoor smoke and greenhouse gas emissions (CDM credits), and gives rich organic fertiliser (slurry).
• Biogas is limited mainly by the availability of livestock in the hills and by temperature (it works poorly in cold areas).