Last reviewed 1 Oct 2026 · Facts as of 1 Oct 2026 · 8 min read
Biomass as energy
Biomass is organic matter of recent biological origin — crop residues (straw, husk, stalks, bagasse), wood, animal dung, food and agro-industry waste, energy crops and the organic part of municipal waste. It is renewable and carbon-neutral over a growth–use cycle (the CO₂ released was absorbed by the plants), and India has very large residue surpluses.
Conversion routes
| Route | Process | Product |
|---|---|---|
| Direct combustion | Burning in stoves, boilers, furnaces | Heat, steam, power |
| Thermochemical | Pyrolysis (heating without air), gasification (partial oxidation), torrefaction, briquetting/pelletising | Char, bio-oil, producer gas, solid fuel |
| Biochemical | Anaerobic digestion (biogas), fermentation (ethanol) | Biogas, ethanol |
| Physico-chemical | Oil extraction and transesterification | Bio-diesel |
Choice depends on moisture: dry biomass (below ~30 % moisture) suits thermochemical routes; wet biomass (dung, sludge, kitchen waste) suits anaerobic digestion.
Anaerobic digestion
Anaerobic digestion is the breakdown of organic matter by bacteria in the absence of oxygen, producing biogas and a nutrient-rich digested slurry.
FormulaFour stages of anaerobic digestion
- Hydrolysis — complex polymers (carbohydrates, proteins, fats) are broken into sugars, amino acids and fatty acids by hydrolytic bacteria.
- Acidogenesis — sugars and amino acids become volatile fatty acids, alcohols, H₂ and CO₂ (acid-forming bacteria).
- Acetogenesis — fatty acids and alcohols become acetic acid, H₂ and CO₂.
- Methanogenesis — methanogens (strict anaerobes) form methane: from acetate (acetoclastic) and from H₂ + CO₂ (hydrogenotrophic).
The first stages are done by acid formers (fast, tolerate a wide pH range); methanogens are slow and sensitive — so a sudden overload that makes acids pile up and drops the pH will "sour" the digester and stop methane formation.
Factors affecting digestion
| Factor | Optimum / effect |
|---|---|
| Temperature | Mesophilic 30–40 °C (about 35 °C) is common; thermophilic 50–60 °C is faster but less stable; gas yield falls sharply below about 15 °C (hence winter problem in north India) |
| pH | 6.8–7.5 (methanogens); below 6.2 is inhibitory |
| C:N ratio | About 20–30 : 1 (cattle dung ≈ 25:1; straw is high (about 80:1), night soil is low (about 8:1)); mix to balance |
| Total solids (TS) | 7–9 % for cattle dung slurry (one part dung to one part water for fresh dung of about 18–20 % TS) |
| Retention time (HRT) | Cattle dung: about 40–55 days in a domestic plant (shorter at higher temperature) |
| Loading rate | Volatile solids (VS) loaded per m³ digester volume per day — overload causes acidification |
| Toxic substances | Antibiotics, detergents, heavy metals, high ammonia, disinfectants inhibit |
| Mixing/agitation | Prevents scum and dead zones |
| Seeding | A starter of digested slurry speeds start-up |
Biogas
- Composition (typical): methane (CH₄) 55–65 %, carbon dioxide 35–45 %, traces of H₂S (corrosive, bad smell), hydrogen, nitrogen and water vapour.
- Calorific value: about 20–25 MJ/m³ (roughly 5000–6000 kcal/m³); pure methane is about 35.8 MJ/m³. It burns with a blue, smokeless flame; its ignition temperature is about 650–750 °C and its density is slightly less than air.
- Rough equivalence: 1 m³ of biogas ≈ about 0.4 kg of LPG ≈ about 0.6 litre of kerosene (approximate; useful for comparison).
- Yield: about 0.035–0.040 m³ of gas per kg of fresh cattle dung (about 25–30 kg dung per m³ of gas); poultry droppings and pig dung give more per kg; crop residues need pre-treatment.
- Cleaning: H₂S is removed by iron-oxide (iron sponge) scrubbers; CO₂ by water or amine scrubbing for upgrading to bio-methane (CBG).
Types of household biogas plants
KVIC floating-drum plant
- Underground cylindrical brick/concrete digester with a central partition wall (guides the flow), over which an inverted steel drum (gas holder) floats on the slurry and rises and falls as gas is produced or used.
- Constant gas pressure (from the drum's weight, about 8–10 cm of water), easy to see the stored gas.
- Disadvantages: expensive steel drum, corrosion, periodic painting, maintenance.
- Gas holder capacity is commonly taken as about 50–60 % of the daily gas production.
Janata (fixed-dome, Chinese type) plant
- Masonry digester with a fixed dome; the gas collects under the dome and displaces slurry into the outlet/compensation tank.
- No moving parts, low cost, long life (20+ years), but gas pressure varies and leaks (cracks in the dome) are hard to repair.
Deenbandhu plant
- A fixed-dome design with a hemispherical (spherical-segment) top and bottom, minimum surface area for the volume, built with bricks and cement (no steel); about 30 % cheaper than the Janata plant and widely promoted, with capacities of 1–6 m³/day.
Other types
| Plant | Use |
|---|---|
| Bag/balloon digesters | Cheap, portable, for small use |
| Continuous stirred tank reactor (CSTR) | Commercial and industrial plants |
| Upflow anaerobic sludge blanket (UASB) | High-strength wastewater, distillery effluent |
| Dry-fermentation / plug-flow | Solid waste, municipal organic waste |
| Community and institutional plants | Large cattle, kitchen waste, human waste |