Last reviewed 1 Oct 2026 · Facts as of 1 Oct 2026 · 8 min read
Liquid biofuels — overview
| Biofuel | Replaces | Source | Process |
|---|---|---|---|
| Bio-diesel | Diesel | Vegetable oils, animal fats, used cooking oil | Transesterification |
| Bio-ethanol | Petrol | Sugar, starch, cellulose | Fermentation + distillation |
| Bio-methane / CBG | CNG | Biogas | Anaerobic digestion + upgrading |
| Green diesel / SAF (HVO) | Diesel, jet fuel | Oils, fats | Hydro-treatment |
Generations: 1st generation — from food crops (sugarcane, maize, edible oils); 2nd generation — from non-food biomass (straw, bagasse, non-edible oilseeds, waste); 3rd generation — algae.
Bio-diesel
Feedstocks
- Non-edible oilseeds (India's preference to avoid food competition): Jatropha (ratanjot), Pongamia/Karanja, neem, mahua, simarouba, castor, rubber seed, cottonseed.
- Edible oils (soybean, rapeseed, palm, sunflower) are used widely abroad.
- Used cooking oil (UCO) and animal fats — cheap, waste-derived; India's RUCO (Repurpose Used Cooking Oil) initiative of FSSAI collects UCO for bio-diesel.
- Oil content: Jatropha seed about 30–40 %, Pongamia about 30–35 %, neem about 20–30 %.
Transesterification
A triglyceride (oil or fat) reacts with three molecules of alcohol (methanol or ethanol) in the presence of a catalyst to give three molecules of alkyl esters (FAME — fatty acid methyl esters = bio-diesel) and one molecule of glycerol:
Triglyceride + 3 CH₃OH → 3 FAME + Glycerol
The reaction is reversible, so excess alcohol is used (about 6:1 molar ratio to the oil, i.e., ~20 % by volume).
| Condition | Typical value |
|---|---|
| Alcohol | Methanol (cheapest, most reactive) |
| Catalyst | NaOH or KOH, about 0.5–1 % of the oil mass (alkali route; sodium methoxide is common) |
| Temperature | About 55–65 °C (below methanol's boiling point of 65 °C) |
| Time | About 1–2 hours, with stirring |
| Yield | About 90–98 % of the oil converts to esters; glycerol ≈ 10 % by mass |
Free fatty acids (FFA)
- FFA less than about 1–2 % → direct alkali transesterification.
- FFA above about 2 % (Jatropha, Karanja, UCO) → alkali reacts with FFA to form soap, lowering the yield and hampering separation; the oil first undergoes acid-catalysed esterification (H₂SO₄ + methanol) to convert FFA into esters, and then the alkali step (two-step process).
- Water must be minimised (it also causes soap).
Process steps
- Oil extraction (expeller/solvent) and degumming/filtering.
- Pre-treatment (esterification if FFA is high).
- Transesterification in a stirred reactor.
- Settling/centrifuging — the lower layer is glycerol, the upper layer is crude bio-diesel.
- Washing with warm water (and mild acid) to remove soap, catalyst and methanol; drying.
- Methanol recovery by distillation and recycling.
- Glycerol is purified and sold (soap, cosmetics, pharmaceuticals).
Properties and use
| Property | Bio-diesel vs petro-diesel |
|---|---|
| Cetane number | Higher (about 50–60) |
| Calorific value | About 10 % lower (about 37–40 MJ/kg) |
| Sulphur | Almost nil |
| Flash point | Higher (safer, about 150 °C) |
| Lubricity | Better |
| Cold flow | Poorer (higher cloud and pour points) |
| Oxidation stability | Poorer; absorbs moisture |
| Emissions | Lower CO, HC, particulates, SOx; NOx may be slightly higher |
| Solvent action | Can attack rubber seals and hoses |
- Blends: B5, B10, B20 (the number shows the % bio-diesel); B100 is pure. Existing diesel engines can run on low blends (up to B20) without modification.
- Indian standard: specifications are laid down by BIS (for B100 and the blends).
- Policy: the National Policy on Biofuels (2018) (amended later) encourages non-edible oils and UCO; the blending target for bio-diesel is 5 % by 2030 (check the current notifications).
1000 kg of Jatropha oil is reacted with methanol; the conversion is 95 %, and glycerol is about 10 % of the oil mass.
Bio-diesel kg of esters (a rule of thumb: about 1 litre of bio-diesel per litre of oil, with a loss of a few per cent). Volume at a density of 0.88 kg/L . Glycerol kg. Methanol at a 6:1 molar ratio: the oil (molar mass about 880 kg/kmol) is kmol, so methanol kg (about 275 L) — most of the excess is recovered and reused.
Bio-ethanol
Feedstocks
| Type | Examples |
|---|---|
| Sugar crops | Sugarcane juice, sugar-beet, sweet sorghum |
| By-products | Molasses (B-heavy and C-heavy) — the traditional Indian source |
| Starch crops | Maize, damaged/surplus rice and wheat, cassava, potato |
| Lignocellulosic (2G) | Rice straw, wheat straw, bagasse, corn stover, wood |
Fermentation chemistry
Glucose (hexose): .
Theoretical yield: kg ethanol per kg glucose.
Sucrose (hydrolysed to glucose + fructose by invertase first): ; theoretical yield kg/kg.
Starch (hydrolysed to glucose, a gain of water by 180/162): theoretical yield kg/kg.
- Practical yield is about 90–95 % of theoretical (some sugar goes to yeast growth and by-products such as glycerol and acids).
- Microbe: yeast (Saccharomyces cerevisiae); Zymomonas mobilis bacterium is also used.
- Conditions: 30–35 °C, pH 4–5, 48–72 hours (anaerobic); the fermented wash (beer) contains about 8–12 % ethanol by volume (higher alcohol kills yeast).
- Starch feedstocks need liquefaction (alpha-amylase, ~90 °C) and saccharification (glucoamylase) before fermentation.
- Lignocellulose (2G) needs pre-treatment (steam explosion, dilute acid, alkali) to release cellulose, enzymatic hydrolysis (cellulases) to glucose, then fermentation (and of the pentose sugars) — costlier; India has commissioned second-generation bio-ethanol plants (e.g., at Panipat) — check current status.
Recovery and dehydration
- Distillation (beer column, rectifier) concentrates ethanol up to the azeotrope of about 95.6 % by volume (rectified spirit); further simple distillation cannot go beyond this.
- Dehydration to anhydrous (fuel-grade, 99.5 % or higher) — molecular-sieve adsorption (zeolite, now the standard), azeotropic distillation (benzene/cyclohexane), or membrane pervaporation.
- Denaturing with small amounts of additives (for non-potable fuel).
- By-products: CO₂ (dry-ice or beverages), distillery spent wash (vinasse) — a high-BOD effluent treated by biomethanation, concentration and incineration/composting (zero liquid discharge) — and DDGS (dried distillers grain with solubles) (cattle and poultry feed) from grain.
Fuel properties and blending
| Property | Ethanol vs petrol |
|---|---|
| Octane number | Higher (about 108) — allows higher compression |
| Calorific value | About 30 % lower (about 27 MJ/kg vs 44) |
| Latent heat of vaporisation | Higher (cooling, cold-start problems at low blends) |
| Oxygen content | About 35 % — cleaner burn, lower CO and HC |
| Water tolerance | Hygroscopic, phase separation with water |
| Corrosive/solvent | Attacks some rubber and plastic parts; E20-compatible materials are required |
- Blending: E10, E20 (20 % ethanol + 80 % petrol); flex-fuel engines run on up to E85 or E100.
- Policy: the Ethanol Blended Petrol (EBP) programme reached E10 in 2022, and the E20 target (advanced to 2025–26 under the 2018 policy as amended); check the current blend level and the notified feedstock rules.
One tonne (1000 kg) of molasses contains 45 % fermentable sugars (as sucrose).
- Sugar kg.
- Theoretical ethanol kg.
- Practical yield at 90 % of theoretical kg.
- Volume at a density of 0.789 kg/L (about 270–280 L per tonne is in the usual industrial range for molasses).
A litre of E20 holds 0.2 L ethanol and 0.8 L petrol. Using 21.2 MJ/L for ethanol and 32 MJ/L for petrol (approximate): energy MJ/L, about 6.8 % lower than petrol's 32 MJ/L — the reason a small mileage drop is seen at E20 (partly offset by the higher octane).