Biomass gasification heats organic material with controlled oxygen or steam to make a combustible gas called syngas, while pyrolysis heats it with almost no oxygen to make bio-oil, char and gas. Temperature and oxygen supply set the two apart.
Both belong to the wider family covered in the guide to algae biomass energy, which explains how living matter becomes heat, power and fuel. This page compares the two thermal routes side by side.
Thermochemical conversion in context
The US Energy Information Administration groups pyrolysis and gasification together as thermochemical conversion of biomass. Direct combustion is the most common method for converting biomass to useful energy, and in 2025 biomass accounted for about 5% of U.S. energy consumption.
The same agency explains that the processes mainly differ in the temperatures and in the amount of oxygen present during conversion. In practice the line between them is blurry, because IEA Bioenergy’s pyrolysis overview notes that pyrolysis is always also the first step in combustion and gasification processes, where it is followed by total or partial oxidation of the products.
That shared starting point is why the two are often discussed together. A gasifier performs pyrolysis as part of its work and then goes further, while a dedicated pyrolysis unit stops early and collects what comes off.
How biomass gasification works
The US Department of Energy describes gasification as a process in which lignocellulosic feedstocks such as wood and forest products are broken down to synthesis gas, primarily carbon monoxide and hydrogen, using heat, as set out on its thermochemical conversion page. The feedstock is partially oxidized, or reformed, with a gasifying agent such as air, oxygen or steam.
According to the EIA, gasification involves heating organic materials to between 800° C and 900° C in a vessel while injecting controlled amounts of free oxygen or steam to produce a carbon monoxide- and hydrogen-rich gas called synthesis gas or syngas.
Inside the reactor the chemistry proceeds in stages. The IEA Bioenergy gasification task describes gasification as an intermediate step between pyrolysis and combustion, and an endothermic process. In the initial stage the volatile components of the fuel are vaporized at temperatures below 600°C. In the later stage the leftover char is gasified through reactions with oxygen, steam and hydrogen, and some of the unburned char is combusted to release the heat needed for those reactions.
The main gasification products are gas, char and tars. The same task explains that the products, their composition and their amount are strongly influenced by the gasification agent, temperature, pressure, heating rate and fuel characteristics. The Department of Energy adds that the makeup of syngas varies with the type of feedstock, its moisture content, the type of gasifier used and the temperature and pressure in the gasifier.
A practical attraction is flexibility. The IEA Bioenergy task notes that gasification can even convert waste, from agricultural and crop residues to wood residues, into a useful energy source, with noxious substances separated from the fuel gas before combustion.
How pyrolysis works
In pyrolysis, the US Department of Energy explains, biomass feedstocks are broken down using heat in the absence of oxygen, producing a bio-oil that can be further refined to a hydrocarbon product. The decomposition occurs at lower temperatures than in gasification and produces liquid oil instead of synthesis gas. The oil varies in oxygen content and viscosity according to the feedstock used.
The EIA puts the typical range at between 400° C and 500° C in the nearly complete absence of free oxygen, and lists charcoal and bio-oil among the products. IEA Bioenergy’s pyrolysis explainer sums up the output as a solid product (char), condensable vapors (bio-oil) and gases, with the interaction between pyrolysis temperature and residence time mainly deciding the yield distribution.
Fast pyrolysis is the variant built to maximise liquid. The pyrolysis explainer describes fast pyrolysis as relying on very rapid particle heating and low residence time, and lists typical steps such as drying the feed to a moisture content of 10% or less and grinding it to around 2 mm for fluid bed reactors. After cooling and condensation, the result is a dark brown mobile liquid with a heating value about half that of conventional fuel oil. The explainer also says bio-oil yields reach up to 75% wt on a dry feed basis, and that char and gas are often used within the process to supply its heat.
Feedstock matters. According to the IEA explainer, the different characteristics of feedstocks other than clean wood often lead to inferior bio-oil yield and quality.
Product yields compared
The clearest way to see how pyrolysis conditions steer the output is a table of typical yields. The figures below are mass yields on a dry basis from clean wood, and gasification is left out because it aims at a gas rather than a split of liquid, char and gas.
| Pyrolysis mode | Temperature | Liquids | Char | Gas |
|---|---|---|---|---|
| Slow (torrefaction) | 290°C | 5% | 80% | 15% |
| Slow (carbonization) | 400°C | 30% | 35% | 35% |
| Intermediate | 500°C | 50% | 25% | 25% |
| Fast | 500°C | 75% | 12% | 13% |
Typical yields are as published by IEA Bioenergy’s pyrolysis explainer (adapted from Bridgwater, 2012), and real units vary with the feedstock and reactor.
The pattern is easy to read. Long residence at low temperature leaves mostly char. Moving to moderate temperature, rapid heating and a short vapor residence time shifts most of the mass into condensable liquid. Gasification pushes in a different direction, using higher temperatures and a controlled supply of oxygen or steam to make syngas.
Tars, gas cleaning and carbon monoxide
Raw syngas is not ready to use. The Department of Energy notes that the syngas undergoes clean-up and conditioning, and that among the contaminants removed are tars, acid gas, ammonia, alkali metals and other particulates. IEA Bioenergy describes gas cleaning as the removal of undesirable contaminants (tars, halogens, particulates) that may foul downstream unit operations and catalysts.
Bio-oil also needs treatment. The Department of Energy says oil from pyrolysis must have particulates and ash removed by filtration, and is then upgraded to hydrocarbon fuels through hydrotreating and hydrocracking, which reduces its total oxygen content.
Safety deserves a plain mention. Syngas is rich in carbon monoxide, and the US EPA describes carbon monoxide as a colorless, practically odorless, and tasteless gas that results from incomplete oxidation of carbon in combustion. The same EPA page lists headaches, dizziness, confusion and nausea among the effects at higher concentrations, and states: “At higher concentrations, CO exposure can be fatal.” That is general background rather than guidance for any particular equipment, and permitting, emissions and safety rules differ by place, so official local and national requirements are the reference for any real project.
Where the products end up
The EIA says syngas can be used as a fuel for diesel engines, for heating and for generating electricity in gas turbines, and that it can be further processed to produce liquid fuels using the Fischer–Tropsch process. IEA Bioenergy also lists synthetic natural gas, consisting of mostly methane, and combined heat and power among the applications of product gas.
That methane route overlaps in purpose with renewable natural gas, although the EIA notes that this gas is the product of biological conversion, produced in anaerobic digesters at sewage treatment plants and at dairy and livestock operations, whereas syngas comes from thermochemical conversion.
Using the gas or the heat on site is a separate topic covered in the page on biomass combined heat and power, which looks at efficiency and typical uses.
Gasification also leaves a solid by-product: IEA Bioenergy defines biochar as a solid carbonaceous residue left after gasification.
Frequently asked questions
What is the main difference between gasification and pyrolysis?
Pyrolysis works in the absence of oxygen and tends to yield liquid oil, while gasification adds controlled oxygen or steam at higher temperatures to produce a fuel gas, according to the US Department of Energy.
Does pyrolysis happen inside a gasifier?
Yes. The IEA Bioenergy overview describes pyrolysis as the initial step of gasification and combustion, followed by total or partial oxidation of the products.
Can gasification handle waste materials?
The IEA Bioenergy gasification task says gasification can convert even waste into a useful energy source, though results depend on fuel characteristics and the gasification agent.
Why does the feedstock matter so much?
Feedstock composition changes both yield and quality. The IEA pyrolysis explainer says feedstocks other than clean wood often lead to inferior bio-oil yield and quality compared with clean wood, and the Department of Energy notes that syngas makeup also varies with the feedstock.
