What is Pyrolysis?

Carbon removal
What is Pyrolysis?

Project successes


Seb Megson, Sustainability Consultant at Onnu

Pyrolysis basics

Pyrolysis is the process in which material is heated to very high temperatures without any oxygen present. While this may seem to share similarities to regular burning, the absence of oxygen is a key factor. Rather than combusting, the material undergoes thermal decomposition into a number of combustible gases, as is transformed into biochar.

The majority of these gases can be condensed into a synthetic fuel called bio-oil, however, some permanent gases will remain, such as CO2 and H2. This ‘syngas’ (synthesis gas) is not wasted though, with its combustion actually being used to heat the pyrolysis process, making it self-sustaining.

The technology itself is truly ancient, and has been used throughout history to make charcoal from wood, and enhance soil quality. Only now, however is its value in the fight against climate change being fully understood, and its potential being realised at an industrial scale.

How does pyrolysis work?

Central to the process is the machine itself, known as a pyrolyzer. This workhorse can run 24/7, with a typical plant processing up to 500kg of biomass an hour (12 tonnes a day per machine) with the need for minimal space.

Fuel for the pyrolyzer is called feedstock. Once gathered this is fed into the pyrolyzer, which moves it into the pyrolysis reactor. This is a large container which rotates the feedstock in a corkscrew motion in the central chamber while undergoing pyrolysis.

Once the biochar and bio-oil have been recovered, the syngas moves through a filter into a gas combustor, where it is then used to heat the pyrolysis process. The heat produced through the pyrolysis is circulated around the central chamber and moved through an exhaust gas heat exchanger. This is perhaps the most important step in the process, whereby heat is extracted in the form of thermal capacity and can be used as heat energy or put through a heat exchanger to produce cold energy. It can also power a turbine to generate electricity but this is less efficient.

Finally, all process emissions are passed through an activated carbon filter to remove any possible pollutants.

Can I put anything in it?

Both bio and non-bio material can be used in the pyrolysis process, with biomass being preferred due to its higher efficiency and its superior environmental outcomes. The term biomass is used to describe organic material sourced from living organisms, such as plants and animals. Different sources of biomass when used as feedstocks, hold different properties and results. This can be an increased calorific (heating) value or a change to the soil amendment properties of the produced biochar. Examples of these can be agricultural waste (such as stover and grain husks) and forestry waste.

Controlling the process

What you get out depends, as always on what you put in. Feedstock type and composition, temperature, and speed are the main controls. Slow pyrolysis uses relatively low temperatures (<500°C) with the feedstock spending a longer amount of time in the reactor (residence time). This maximises the yield of biochar, while reducing the production of bio-oil, and so good for carbon capture. Fast pyrolysis utilises a higher heating rate (roughly 10-200°C/s) and reduced time spent in the reactor, allowing a greater yield of bio-oil (50–70%). This is taken even further through flash pyrolysis which makes use of a maximised heating rate (>1000°C/s) and minimised time in reactor (<0.5s), garnering a bio-oil yield of roughly 75–80%.

How does it remove carbon?

Pyrolysis isn't just an effective waste management tool, but if utilised correctly it can also be a game-changing carbon removal and green power generation technology.

Pyrolysis traps the majority of carbon from the biomass in the biochar, a process known as carbon sequestration. This stops the carbon from being released into the atmosphere, whereas regular combustion or decay wouldn’t. The produced biochar has ecological benefits in its own right, and is useful for farming, such as an extremely nutrient-rich soil additive. This reduces the need for chemical fertilisers, ensuring healthier soil ecosystems and better quality crops.

There are few ways of fighting climate change that tick this many boxes.

Why now?

So why all the excitement when it's been around for so long? Firstly, the equipment is improving, making it easier than ever to deploy and manage pyrolysis capacity, but crucially too, the process has been recently approved by the IPCC and the main Carbon Credit registries, such as Puro.Earth, Gold Standard and Verra. This has raised it's profile, and makes it possible to part-finance operations with the revenue from carbon credits.

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