Pyrolysis is the thermal decomposition of materials at high temperatures in the absence of oxygen. Instead of burning, the material breaks down into simpler substances: a liquid (often called bio-oil or pyrolysis oil), a solid residue (char), and a mixture of gases. The process typically occurs at temperatures up to about 700°C and is used to convert biomass, plastics, tires, and other organic waste into usable fuels and chemicals.
How Pyrolysis Works
The defining feature of pyrolysis is the lack of oxygen. When you heat organic material without oxygen, it can’t combust. Instead, the heat breaks chemical bonds within the material, producing vapors from volatile components first. Then the remaining non-volatile substance disintegrates into char, tar, and gases. As temperature rises further, that char undergoes secondary decomposition, releasing hydrocarbons and aromatic compounds into the vapor phase.
Dozens of chemical reactions happen simultaneously during this process, both in sequence and in parallel: dehydration (removal of water), depolymerization (breaking long molecular chains into shorter ones), and charring among them. The exact mix of reactions depends on the feedstock, the temperature, and how quickly the material is heated.
How It Differs From Burning and Gasification
Combustion, gasification, and pyrolysis all involve heat and organic material, but they differ in how much oxygen is present. Combustion uses a full supply of oxygen, completely oxidizing the material and releasing heat and carbon dioxide. Gasification uses a limited amount, typically 20% to 40% of what full combustion would require, and runs hotter (750°C to 1,100°C) to produce a synthesis gas mixture of carbon monoxide, hydrogen, and methane. Pyrolysis uses no oxygen at all and operates at lower temperatures, which is why it yields liquids and solids rather than just gas.
Types of Pyrolysis
The speed at which material is heated changes what you get out of the process. There are three main categories.
Slow Pyrolysis
Slow pyrolysis uses low heating rates and longer processing times, sometimes hours. This maximizes char production with moderate amounts of tar as a byproduct. In one study using a fixed-bed reactor, biochar yield ranged from about 27% at 700°C to nearly 48% at 300°C. Traditional charcoal-making is essentially slow pyrolysis.
Fast Pyrolysis
Fast pyrolysis heats material rapidly and keeps vapor residence times short, favoring liquid production. The goal is to condense those vapors quickly before they break down further into gas. Bio-oil yields from fast pyrolysis can reach 60% to 75% depending on the feedstock.
Flash Pyrolysis
Flash pyrolysis pushes heating rates even higher, up to 10,000 degrees per second, at temperatures below 650°C with rapid quenching of the vapors. This extreme speed maximizes liquid output while minimizing both char and gas formation.
What Pyrolysis Produces
Every pyrolysis run generates three products in varying proportions: a liquid, a solid, and a gas. The ratio depends on temperature, heating rate, and what’s being processed.
In slow pyrolysis of a salt-tolerant plant species at three different temperatures, researchers found that syngas yield climbed from 26% to 46% as temperature increased from 300°C to 700°C, while biochar yield dropped from 48% to 27% over the same range. Bio-oil yield stayed relatively stable at 26% to 30%, peaking at 500°C. Higher temperatures push more material into the gas phase; lower temperatures lock more carbon into solid char.
The liquid product, bio-oil, is a dark, acidic fluid that contains hundreds of organic compounds. In its raw form, it has high oxygen content and is corrosive, which limits its direct use as fuel. Catalytic pyrolysis addresses this problem. Certain catalysts with carefully structured pores strip oxygen from the pyrolysis vapors, releasing it as water or carbon dioxide and converting the remaining molecules into aromatic hydrocarbons and other fuel-like compounds. The result is a higher-quality liquid with fewer corrosive oxygen-containing species.
Common Feedstocks
Pyrolysis can process nearly any carbon-containing material, but moisture and ash content matter. Feedstock is typically dried to below 10% moisture by weight before processing, because excess water absorbs heat that would otherwise drive the chemical reactions. Interestingly, some moisture isn’t always bad. Trials comparing biomass at 1.2%, 9.2%, and 23.6% moisture found that intermediate moisture levels actually produced the largest organic liquid yield.
Plastics are particularly effective feedstocks because of their high carbon and hydrogen content and low moisture. Low-density polyethylene (the material in plastic bags and squeeze bottles) converts to liquid fuel at rates around 76% to 79% by weight. Polystyrene yields are slightly lower, around 65%. Not all plastics are suitable, though. PVC and other chlorine-containing plastics can generate toxic byproducts during thermal decomposition.
Biochar and Carbon Storage
The solid product of pyrolysis, biochar, has drawn significant interest for its ability to lock carbon in the soil for centuries. When biomass grows, it absorbs carbon dioxide from the atmosphere. Pyrolyzing that biomass converts a portion of its carbon into a stable, charcoal-like solid that resists further decomposition. Mixing this biochar into soil keeps that carbon out of the atmosphere while also improving soil structure and water retention.
Life cycle analyses from the USDA Forest Service have estimated that one tonne of biochar can represent the equivalent of up to 10 tonnes of CO₂ sequestered, though this figure varies widely depending on the feedstock, processing conditions, and what energy credits are factored in.
Emissions and Environmental Concerns
Because pyrolysis operates without oxygen, it produces far less combustion pollution than incineration. However, it is not emission-free. When processing waste that contains chlorine (such as certain plastics or treated materials), pyrolysis can generate polycyclic aromatic hydrocarbons, chlorinated compounds, and small quantities of dioxins and furans, which are highly toxic even at trace levels.
Laboratory studies measuring dioxin emissions from pyrolysis of various wastes have found concentrations ranging from single-digit to thousands of nanograms per cubic meter, depending on the feedstock. For context, European Union regulations set a legal limit of 0.1 nanograms per cubic meter for dioxin-like compounds in industrial emissions. Meeting this standard requires careful feedstock screening to exclude chlorinated materials and proper gas cleaning systems on the exhaust side.
Current Applications
Pyrolysis is used commercially across several industries. Tire recycling plants use it to recover carbon black and fuel oil from end-of-life tires. Continuous pyrolysis plants now process 15 to 50 tonnes of feedstock per day, running for 30 to 45 days between maintenance shutdowns. Smaller batch systems handle 100 kilograms to 2 tonnes daily for localized waste processing.
In agriculture, portable pyrolysis units convert crop residue into biochar on-site, giving farmers a soil amendment while reducing open-field burning. In the energy sector, bio-oil from fast pyrolysis is being refined into drop-in replacements for heating oil and, with further upgrading, transportation fuels. Plastic-to-fuel pyrolysis plants are operating in parts of Asia, Europe, and North America, targeting the enormous volume of plastic waste that conventional recycling cannot handle.