What is Pyrolysis? And, Its Pros and Cons Versus Biogas for Energy

Pyrolysis is the thermal decomposition of a material by heating it without oxygen. Applied to biomass, it produces a mixture of solid char, condensable vapours and gases. The vapours can be cooled to produce a liquid commonly called pyrolysis oil or bio-oil.

Unlike combustion, pyrolysis does not rely on burning the feedstock with oxygen inside the reaction chamber. Unlike anaerobic digestion, it uses heat rather than microorganisms to break the material down.

Here we explain the meaning of pyrolysis, how the process works, its advantages and disadvantages, and how it compares with anaerobic digestion for recovering value from biomass and waste.

Originally published on 13 November 2019. Updated on 20 October 2026 with revised technical explanations, a clearer comparison with anaerobic digestion, and additional questions and answers. The original video and illustrations have been retained.

What Is Pyrolysis? A Simple Definition

Pyrolysis means breaking a substance down through heat in the absence of oxygen. In biomass processing, that heat changes the chemical structure of the material and divides it into solid, liquid and gaseous products.

Pilot units for biomass gasification, fast pyrolysis and fluidised-bed combustion. Original credit: VTT Technical Research Centre of Finland, CC BY-NC-ND. Photograph retained from the original article; original attribution link.

Charcoal production from wood is a familiar example. Heating wood with oxygen excluded releases vapours and gases while leaving a carbon-rich solid.

The US Department of Agriculture’s explanation of pyrolysis describes this thermal conversion and the recovery of useful products from biomass.

Is Burning Wood an Example of Pyrolysis?

Pyrolysis occurs within the sequence of events when wood burns. Heat releases volatile substances from the wood, and those substances can burn where they mix with oxygen.

However, an open fire also involves combustion. It is not the same as a controlled pyrolysis reactor designed to exclude oxygen and collect products.

Likewise, charring food can illustrate thermal decomposition, but frying, baking and roasting involve several physical and chemical changes. They should not all be described simply as industrial pyrolysis.

Watch Our Original Pyrolysis Video

The following video accompanied the original article. It provides historical context; the updated explanations on this page clarify product differences and qualify the earlier cost and technology comparisons.

How Does the Pyrolysis Process Work?

Although equipment designs differ, a biomass pyrolysis system typically involves the following stages:

1. Feedstock preparation: suitable material is sorted, sized and, where necessary, dried.
2. Controlled heating: the material enters a reactor designed to exclude oxygen.
3. Thermal decomposition: heat breaks down the feedstock, releasing vapours and gases and leaving solid char.
4. Product separation: solids are separated and vapours may be cooled to recover liquid products.
5. Product use and treatment: gas, oil and char are managed according to their properties, quality and intended outlets.

Reactor temperature, heating rate, residence time and feedstock composition influence the product mixture. There is no single temperature or product yield that describes every pyrolysis installation.

Heat can come from an external source or from controlled combustion of some process products in a separate heating arrangement. Excluding oxygen from the pyrolysis reaction does not mean the complete installation needs no combustion equipment.

What Does Pyrolysis Produce?

1. Char

Char is the solid carbon-rich residue. Depending on the feedstock, it also contains ash-forming minerals and potentially unwanted constituents.

Char is not the same as ash. Ash is predominantly the inorganic residue remaining after combustion; pyrolysis char retains substantial carbon.

Nor is every char automatically suitable for sale as biochar or application to soil. Feedstock suitability, contaminants, product properties and the applicable requirements must be assessed.

2. Pyrolysis Oil or Bio-Oil

Cooling condensable vapours produces a liquid fraction. For biomass, this is commonly called bio-oil.

It is a chemically complex mixture and should not be assumed interchangeable with diesel, petrol or conventional heating oil. Depending on the application, further processing and suitable equipment are needed.

The US Department of Energy discusses these processing requirements in its guidance on synthesis and upgrading of bioenergy intermediates.

3. Non-Condensable Gas

The gas composition depends on the material and operating conditions. It can include carbon monoxide, carbon dioxide, hydrogen, methane and other compounds.

Some constituents are combustible, so the gas may supply process heat after appropriate handling and treatment. However, pyrolysis gas is not the same as methane-rich biogas from anaerobic digestion.

Gasification commonly aims to produce a carbon monoxide- and hydrogen-rich synthesis gas. Describing every pyrolysis gas stream simply as “syngas” can obscure important differences in composition and use.

Slow Pyrolysis and Fast Pyrolysis

Slow pyrolysis generally uses slower heating and longer residence times, often with an emphasis on char production.

Fast pyrolysis uses rapid heating and short vapour residence times, commonly with rapid cooling to recover liquid products. Biomass fast pyrolysis is often carried out around 450–500°C, but the appropriate conditions depend on the process.

These are broad descriptions rather than guarantees of a particular yield. Feedstock moisture, particle size, mineral content and reactor operation affect performance.

The Department of Energy’s thermochemical conversion overview explains the distinction between pyrolysis routes producing bio-oil and gasification routes producing gaseous intermediates.

Pyrolysis vs Gasification vs Combustion

Gasification is therefore more than pyrolysis with a little air added. It includes further reactions that change the gas and solid products.

Steam gasification should also be distinguished from hydrothermal liquefaction, which processes wet material in hot, pressurised water. These terms should not be used interchangeably.

Advantages of Pyrolysis

It Can Process Materials Poorly Suited to Biological Digestion

Dry woody biomass is generally difficult to digest in a conventional anaerobic plant. Pyrolysis provides a thermal conversion option that does not depend on microorganisms accessing its biodegradable components.

Different feedstocks require different equipment and product assessments. A reactor proven on clean wood is not automatically proven on mixed municipal waste.

It Offers Several Potential Products

Char, oil and gas provide different opportunities for material recovery and energy use. A project can be designed around the intended product rather than treating every output as fuel for immediate combustion.

Those opportunities depend on meeting product specifications and securing reliable customers.

The Reaction Can Be Rapid

Thermal conversion can take place much faster than biological digestion. However, short reaction time does not automatically mean a smaller complete site, lower capital cost or easier commissioning.

Feed preparation, drying, heating, gas cleaning, product storage and emissions control remain part of the installation.

Process Products Can Supply Heat

Using some gas or other products for process heating can reduce purchased energy. The useful energy exported must nevertheless be calculated after accounting for drying, heating and other plant demands.

Disadvantages of Pyrolysis

Feedstock Quality Matters

Moisture, contamination and inconsistent composition can affect operation and product quality. Mixed wastes may need substantial sorting and preparation.

Drying Can Consume Significant Energy

Water must be managed before conventional dry-feed pyrolysis. Very wet material can therefore impose a substantial drying burden unless suitable heat is available.

Products May Need Further Treatment

Gas can require cleaning, oil can need upgrading, and char requires characterisation. Producing a material is not the same as producing an approved, marketable product.

Emissions and Residues Still Need Managing

Excluding oxygen from the reactor does not make the installation emissions-free. Combustion equipment, fugitive releases, condensates and solid residues all need appropriate controls.

Commercial Performance Is Application-Specific

Established charcoal production and carefully controlled biomass processing should not be assessed as though they have the same risks as a new mixed-waste installation.

Developers should seek evidence from comparable operating plants using similar feedstocks, capacities and product routes.

Pyrolysis vs Anaerobic Digestion

This comparison was central to the original article. Both technologies can recover value from organic materials, but their strongest applications often involve different feedstocks.

For wet, readily biodegradable materials, anaerobic digestion often deserves consideration before a thermal route requiring extensive drying. For dry, lignified material such as wood, conventional AD is usually a poor fit.

However, moisture alone does not determine the decision. Contamination, available heat, scale, product markets and total costs must also be assessed.

Read our guides to anaerobic digestion, high-solids digestion and biomass anaerobic digestion for the biological alternatives.

Can Pyrolysis Treat Municipal Solid Waste?

Some systems target selected waste fractions, but mixed municipal solid waste is not a uniform fuel.

Food, paper, plastics, metals, glass and other materials have different properties. Sorting changes the feedstock entering the reactor and the contaminants reaching its products.

The resulting fuel is not automatically wholly renewable: fossil-derived plastics do not become renewable biomass merely because they pass through pyrolysis.

Comparisons with AD should therefore specify the fraction being treated. Anaerobic digestion targets biodegradable material; it does not digest the entire mixed waste stream.

Can Pyrolysis and Anaerobic Digestion Work Together?

They can potentially serve complementary roles. Digestion may recover biogas from wet biodegradable material, while a thermal stage processes an appropriate separated and dried residue.

Such integration needs a complete energy and material balance. Drying requirements, heat availability, contaminants, nutrient management and product outlets determine whether the combination is worthwhile.

It should not be assumed that every digestate is suitable for producing marketable biochar.

Conclusion: Which Process Is Better?

There is no universal winner between pyrolysis and anaerobic digestion.

Pyrolysis offers thermal conversion of suitable feedstocks into char, oil and gas. Anaerobic digestion uses biology to produce biogas and digestate from suitable biodegradable material.

The sensible choice starts with the material available and ends with dependable outlets for the products. Compare the complete installation, including preparation, energy consumption, emissions control and residual material management, rather than choosing on reactor speed alone.

Our view is that at present pyrolysis and gasification is not yet a significant competitor to the anaerobic digestion of MSW (in either dry or wet process types).

However, while it may not be competitive for anything other than dry woody wastes at this time (2026), improving heat treatment control technology could change this conclusion at any time.

Frequently Asked Questions

What Does Pyrolysis Mean in Simple Terms?

It means using heat to break a material down without oxygen. Wood converted into charcoal is a familiar example.

What Are the Three Main Products of Biomass Pyrolysis?

Solid char, condensable vapours that can form bio-oil, and non-condensable gas. Their proportions depend on the feedstock and process conditions.

Is Pyrolysis the Same as Burning?

No. Burning requires oxygen for combustion. Pyrolysis excludes oxygen from the decomposition stage, although separate combustion equipment may provide heat.

Is Pyrolysis an Anaerobic Process?

It occurs without oxygen, but it is not anaerobic digestion. Pyrolysis is thermal decomposition; anaerobic digestion is biological conversion.

Does Pyrolysis Produce Biogas?

It produces a gas mixture, but that should not be confused with the methane-rich biogas made by anaerobic digestion. Gas composition and treatment requirements differ.

What Is the Main Disadvantage of Pyrolysis?

There is no single disadvantage for every application. Important challenges include feed preparation, drying demand, product quality, emissions control and finding reliable markets.

Is Pyrolysis Environmentally Friendly?

Its environmental performance depends on feedstock sourcing, energy inputs, emissions, product use and the alternative treatment route. The absence of oxygen in the reactor does not establish a low-emissions lifecycle.

Is Pyrolysis Char a Permanent Carbon Store?

Not automatically. Carbon storage depends on the feedstock, char properties, stability, final use and accounting method. Burning the char releases its stored carbon rather than retaining it as a long-term store.

Sources and Further Reading

• USDA Agricultural Research Service: What is pyrolysis?
• US Department of Energy: Thermochemical conversion processes
• US Department of Energy: Synthesis and upgrading
• US Energy Information Administration: Biomass explained

[Published on 13 November 2019. Updated on 20 October 2026.]