Biomass Conversion Technologies: Thermochemical, Biochemical and Chemical Routes

Biomass conversion technologies are the processes that turn organic matter into heat, power or fuels, and they fall into three families: thermochemical (combustion, gasification, pyrolysis, torrefaction, hydrothermal liquefaction), biochemical (anaerobic digestion and fermentation) and chemical (transesterification to biodiesel). The biggest single factor in choosing between them is moisture: dry woody biomass suits the heat-driven routes, while wet waste such as dung and food waste suits microbes.

This page is the map of those routes. For what biomass is, its sources and India’s installed capacity, see biomass energy.

The routes at a glance

Pyrolysis and torrefaction conditions are from IEA Bioenergy Task 34; biogas figures from the FAO biogas manual.

Thermochemical conversion

Heat drives these routes. Their products depend mostly on how much oxygen is let in, how hot it gets and how long the material stays hot. Pyrolysis is always the first step: any biomass heated first loses its volatiles, and combustion and gasification then oxidise what pyrolysis releases.

Combustion

Burning biomass with excess air releases all its chemical energy as heat. It is the oldest and still the largest route: household chulhas, industrial boilers on rice husk or wood chips, and sugar-mill bagasse boilers that raise steam for a turbine and for process heat (cogeneration). Efficiency to electricity is modest in small plants, and high-ash fuels such as rice straw cause slagging. A worked example on electricity from straw is on our biomass energy page; boiler types are covered in boiler classification.

Gasification

A gasifier burns part of the fuel with a limited supply of air (or oxygen, or steam). The heat drives drying, pyrolysis and then reduction reactions in which hot char reacts with CO₂ and water vapour to make carbon monoxide and hydrogen. The result is a combustible gas:

• Producer gas (air-blown): diluted by nitrogen from the air. FAO data for commercial downdraught wood gasifiers give roughly 50 to 54% N₂, 17 to 22% CO, 12 to 20% H₂, 9 to 15% CO₂ and 2 to 3% CH₄, with a heating value of 5.0 to 5.9 MJ/m³, about a sixth of natural gas.
• Syngas (oxygen- or steam-blown): mostly CO and H₂, without the nitrogen, and suitable for making liquid fuels, methanol or hydrogen.

Gasifier types: updraught (counter-current, tolerant of moist fuel but tarry gas, for heat), downdraught (co-current, low tar, the usual choice for engines), cross-draught and fluidised bed (large scale, even temperature, handles ash better). Small downdraught gasifiers running on wood chips or husk power rice mills and village grids in India.

Tar is the chronic problem: it condenses in pipes and ruins engines, so gas cleaning (cyclone, scrubber, filters) is as important as the gasifier itself.

Pyrolysis

Pyrolysis is thermal decomposition without oxygen. Temperature and time decide what comes out. IEA Bioenergy Task 34 gives these typical yields from dry wood:

Fast pyrolysis needs finely ground feed (about 2 mm for fluidised beds) dried to below about 10% moisture, very rapid heating and rapid quenching of the vapours. The product, bio-oil, is a dark brown liquid with about half the heating value of fuel oil; it is acidic and unstable, so it needs upgrading before use as a transport fuel. Slow pyrolysis is the traditional charcoal kiln; today its char is also sold as biochar, a soil amendment that locks up carbon.

Torrefaction

Torrefaction is mild pyrolysis at about 290 °C for about 30 minutes, which keeps about 82% of the mass as solid (IEA Task 34). The product is dark, brittle and water-repellent: it grinds like coal, does not rot in storage, and packs more energy per tonne than raw biomass. It is a pretreatment for pellets used in co-firing with coal.

Hydrothermal liquefaction (HTL)

HTL cooks wet biomass in hot, pressurised liquid water, typically about 250 to 370 °C, just below the critical point of water (374 °C, 22 MPa), with pressure high enough to keep the water liquid. Because the water is the reaction medium, feeds such as algae, sewage sludge and food waste need no drying. The product is a bio-crude that is denser and lower in oxygen than pyrolysis bio-oil. HTL is at pilot and demonstration scale rather than widespread commercial use.

Biochemical conversion

Anaerobic digestion

Bacteria break down wet organic matter without air in four stages (hydrolysis, acidogenesis, acetogenesis, methanogenesis) to give biogas of about 50 to 70% methane and a digested slurry used as manure. It suits the wettest feeds: cattle dung, food waste, sewage sludge, press mud and distillery spent wash. Household plants, their types and sizing are covered on biogas generation plants. Scrubbed and compressed, biogas becomes compressed biogas (CBG) for vehicles.

Fermentation to ethanol

Yeast converts sugars to ethanol and CO₂:

C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂

• First generation (1G): sugarcane juice, B-heavy and C-heavy molasses, and grains (broken rice, maize), whose starch is first broken into sugar with enzymes. The fermented “wash” is distilled and dehydrated to fuel-grade ethanol.
• Second generation (2G): lignocellulosic residues such as rice straw and bagasse. Cellulose must first be released by pretreatment and enzymatic hydrolysis, which is the costly step.

India: under the Ethanol Blended Petrol Programme, blending rose from under 1.5% in 2013-14 to 20% (E20) in ethanol supply year 2025-26, five years ahead of the original 2030 target, according to a PIB backgrounder of July 2026. Ethanol procurement was projected at over 1,200 crore litres for 2025-26, with production capacity of about 2,000 crore litres.

Chemical conversion: transesterification to biodiesel

Vegetable oils and animal fats are triglycerides: three fatty-acid chains on a glycerol backbone. Too viscous to use directly in diesel engines, they are reacted with methanol in the presence of an alkali catalyst (NaOH or KOH):

Triglyceride + 3 CH₃OH → 3 fatty acid methyl esters (biodiesel) + glycerol

• Methanol is used in excess (a 6:1 methanol-to-oil molar ratio is common, typical) to push the reaction to completion.
• The feed oil must be dry and low in free fatty acids; water and free fatty acids react with the alkali to form soap. Used cooking oil is often given an acid-catalysed esterification step first.
• The glycerol settles out as a heavier layer and is sold as a by-product.

Feedstocks include non-edible oils such as jatropha and karanja, and used cooking oil.

Which route for which feedstock? The moisture guide

Moisture ranges other than those credited are typical, not fixed limits. The reason behind the table is simple: every kilogram of water fed to a thermal process has to be evaporated, using about 2.3 MJ that never becomes useful energy, while microbes need water to live.

Worked example 1: cold gas efficiency of a gasifier

Given (typical values for a small downdraught wood gasifier running an engine): wood chips with a lower heating value of 14 MJ/kg (FAO gives 13 to 15 MJ/kg at 20 to 25% moisture), fuel consumption 100 kg/h, gas yield 2.0 m³ per kg of wood (assumed), gas heating value 5.0 MJ/m³ (FAO range 5.0 to 5.9), gas engine efficiency 25% (assumed).

Step 1, energy in

• 100 kg/h × 14 MJ/kg = 1,400 MJ/h

Step 2, energy in the cold, cleaned gas

• Gas flow = 100 × 2.0 = 200 m³/h
• Gas energy = 200 × 5.0 = 1,000 MJ/h

Step 3, cold gas efficiency

• η_cg = gas energy / fuel energy = 1,000 / 1,400 = 71%. The FAO manual quotes 60 to 75% for engine-grade gasifiers; the rest is lost as sensible heat, tar and unburnt char.

Step 4, electricity

• Gas power = 1,000 MJ/h / 3.6 = 277.8 kW
• Electrical output = 277.8 × 0.25 = 69.4 kW
• Specific fuel use = 100 kg/h / 69.4 kW = 1.44 kg of wood per kWh
• Overall, wood to electricity = 69.4 / (1,400 / 3.6) = 69.4 / 388.9 = 17.9%

If the same gas is burned hot, straight in a furnace, its sensible heat also counts and FAO notes thermal efficiency can reach 93%. That is why gasifiers for process heat are much more efficient than gasifier power plants.

Worked example 2: ethanol from sugarcane feedstocks

NITI Aayog’s ethanol blending roadmap gives these yields: 70 L per tonne of sugarcane (juice route), 300 L per tonne of B-heavy molasses and 225 L per tonne of C-heavy molasses.

Feed needed for a 100 kL/day distillery (100,000 L/day)

• C-heavy molasses: 100,000 / 225 = 444 t/day
• B-heavy molasses: 100,000 / 300 = 333 t/day
• Cane juice: 100,000 / 70 = 1,429 t of cane/day

Stoichiometry check on C-heavy molasses

• From the equation, 180 g of glucose gives 2 × 46 = 92 g of ethanol: a theoretical maximum of 92 / 180 = 0.511 kg ethanol per kg of sugar.
• 225 L of ethanol × 0.789 kg/L = 177.5 kg of ethanol.
• Minimum sugar needed = 177.5 / 0.511 = 347 kg. So each tonne of C-heavy molasses must supply at least about 350 kg of fermentable sugar. Real fermentations reach less than the theoretical 0.511, so the actual sugar content is somewhat higher.

B-heavy molasses gives more ethanol because less sugar has been crystallised out of it, which is why diverting B-heavy molasses and cane juice to ethanol was central to reaching E20.

Comparing the routes

Biomass conversion is part of the non-conventional energy sources unit in B.Tech syllabi. Useful references: IEA – Bioenergy, IRENA – Bioenergy, and national CO2 emissions data for context. The renewable vs non-renewable resources page explains where biomass fits.

FAQs

What are the three main types of biomass conversion technology?

Thermochemical (combustion, gasification, pyrolysis, torrefaction, hydrothermal liquefaction), biochemical (anaerobic digestion and fermentation) and chemical (transesterification of oils to biodiesel).

What is the difference between gasification and pyrolysis?

Pyrolysis heats biomass with no oxygen and gives mainly liquid bio-oil or char, depending on speed and temperature. Gasification lets in a limited amount of air, oxygen or steam at about 800 degrees Celsius or more and turns most of the biomass into a combustible gas of carbon monoxide and hydrogen.

Which biomass conversion method is best for wet waste?

Anaerobic digestion, because the bacteria need water and no drying energy is wasted. Hydrothermal liquefaction also handles wet feeds such as sludge and algae, but it is still at pilot scale.

What is cold gas efficiency?

It is the chemical energy in the cooled, cleaned producer gas divided by the energy in the fuel fed to the gasifier. Small engine gasifiers typically reach about 60 to 75%.

How much ethanol comes from a tonne of molasses?

NITI Aayog’s roadmap uses about 225 litres per tonne of C-heavy molasses and 300 litres per tonne of B-heavy molasses. Sugarcane juice gives about 70 litres per tonne of cane.

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