How to make biochar
A guide to feedstock and pyrolysis production equipment
Biochar can be produced using equipment ranging from simple garden kilns and homemade TLUDs to commercial retorts, carbonisers and automated pyrolysis systems.
The right production method depends on the feedstock available, its moisture content, the quantity of biochar required, operating costs and whether useful heat can be recovered from the process.
What is biochar production equipment?
Biochar production equipment converts organic material into a carbon-rich solid by heating it under oxygen-limited conditions. This process is known as pyrolysis.
Different technologies achieve this in different ways. Some use externally heated chambers, while others use controlled combustion to provide process heat. More advanced systems may also recover useful energy from the gases released during pyrolysis.
Equipment ranges from small-scale systems, including flame-curtain kilns, TLUDs and retorts, through to larger commercial batch and continuous pyrolysis systems.
Slow pyrolysis generally favours the production of solid char, while fast pyrolysis is commonly designed to maximise liquid products such as bio-oil. Actual yields depend on factors including feedstock, temperature, heating rate and residence time.
Fast pyrolysis technologies include fluidised-bed reactors and other specialised systems. These may produce some char, but their operating conditions are often optimised for different products.
For anyone primarily interested in producing biochar, the choice of equipment should therefore begin with the intended use and required properties of the finished material.
Feedstock considerations
The feedstock needs to be an organic material and ideally it should be part of a green waste stream that would otherwise rot or go into landfill.
If you don’t have access to a green waste stream, purchase local feedstock to reduce the carbon footprint of transporting it.
Wood is the best feedstock for biochar because of its vascular structure, in the view of many experts. Studies have shown that many agricultural and forestry products can be used for making biochar. Crop straws, bamboo, sawdust and rice husks are all examples of such materials.
Farmers can char waste green materials from their farms like straw, hemp, manure and tree brush.
Be aware
Making backyard/garden biochar
Creating your own backyard/garden biochar with a DIY approach is very achievable. Many people find this cheaper and more fun than buying it from a producer. You can collect organic materials from your garden including wood brush and woody material from bushes.
However consider the following:
If your feedstock is wet then you will need a lot of heat to burn off the water content from your material. Ideally collect your feedstock in advance and let it dry naturally over months.
Only small volumes of char can be produced each run in these set-ups. It is a batch process and is very time and labour intensive.
Many DIY systems will liberate a lot of carbon dioxide and smoke to create the biochar.
DIY method (no equipment necessary)
Prepare some ground as a makeshift fire-pit by removing surrounding debris and digging a hole. Fill the hole with your feedstock and light a fire on top of this feedstock. This setup reduces the amount of oxygen at the bottom of the fire allowing partial pyrolysis to occur. It is sometimes known as top fed open draft (TFOD). It is a highly inefficient and dirty method.
Charcoal maker + heat source
The cheapest equipment setup is using a charcoal maker and a fire bin purchased easily purchased online and from the local DIY store. Load your charcoal maker with your feedstock material. Create a fire in the fire bin and once going place the charcoal maker in the top of the bin. Add your lid to provide a chimney, drawing oxygen into the bottom to burn hotter. This still needs to be fed with wood, so is time intensive and dirty, but more efficient than the DIY method.
Small scale biochar production equipment
Biochar production equipment comes in many shapes and sizes, and the terminology can be confusing. Terms such as kiln, retort, carboniser, gasifier and pyrolysis reactor are sometimes used interchangeably, although they can describe quite different technologies.
Understanding these differences is useful when comparing equipment. The best choice depends on factors including production scale, feedstock, labour, emissions controls and whether useful heat is required.
One of the simplest is the Flame Cap Trough as described by Kark J. Frogner. This is an open system using a metal trough to contain the process.
A stove is traditionally seen as a heat source used to cook food, heat a room or dwelling or dry material.
Rocket stove
These are stoves with stacks or chimneys for improved draft or air flow.
This is essentially a micro kiln, where off-gases are recycled and burnt for greater efficiency. It goes further than a rocket stove because it is nearly smoke free.
It is made with two concentric metal cylinders. The principal is the internal chamber houses the biochar feedstock. A fire is lit at the top of the cylinder and this draws air to keep the burn going, but not too hot that it burns the material. Once hot enough the syn-gases are then releases from the feedstock and burnt.
A kiln is essentially an oven used to bake material. It’s a thermally insulated container with internal or external heating. Traditional kiln technology has been used to produce charcoal since ancient times. Numerous designs including traditional earth-mound kilns and brick-made kilns have provided charcoal for cooking, smelting and metalworking for millennia.
That said in the context of making biochar kilns are usually open top to allow for more feedstock to be added. They come in many types including a cone, pyramid or ring (cylinder), sometimes known as Kon Tiki. A Kon Tiki kiln is built to tilt, making it easier to remove the biochar.
A retort is an airtight container, which is heated externally and the gases produced are usually collected as a product or are burnt to heat the process. Using the gases in this way makes the process more efficient.
Commercial scale biochar production equipment
For commercial production, selecting equipment involves more than comparing machine capacity or purchase price. Feedstock, production scale, moisture, labour requirements, heat recovery and the intended use of the biochar can all influence the choice.
Feedstock
Wood chip, forestry residues, agricultural residues and other organic materials behave differently during pyrolysis. Particle size, contamination, consistency and moisture content can all affect equipment suitability.
Production scale
A woodland operation producing occasional batches has very different requirements from a commercial facility processing hundreds or thousands of tonnes of biomass annually.
Feedstock moisture
Drying can represent a significant part of the energy requirement for biochar production. Some systems require relatively dry and consistently sized material, while others may incorporate drying or recover heat that can be used for this purpose.
Useful heat
Pyrolysis can generate recoverable thermal energy. Depending on the system and site, this heat may be used for feedstock drying, buildings, agricultural processes or industrial applications.
Thermal storage can also provide a way of separating when heat is produced from when it is required.
Operation and maintenance
Labour, automation, emissions management, maintenance, char handling and site infrastructure should all be considered alongside headline production capacity.
The intended use of the biochar
Biochar intended for agricultural applications, industrial uses or carbon removal may require different quality controls, testing and production records.
Batch versus continuous pyrolysis
Batch systems process a defined quantity of biomass before the biochar is removed and another production cycle begins.
Continuous systems introduce feedstock and remove biochar during operation, potentially allowing greater throughput and automation.
Neither approach is automatically superior. The most appropriate system depends on production scale, operating hours, feedstock availability, labour, capital budget and the wider objectives of the project.
Pyrolysis and heat recovery
Commercial pyrolysis equipment can form part of a wider energy system rather than operating solely as a biochar production machine.
Recoverable thermal energy can potentially be used for applications such as biomass drying, space heating or agricultural and industrial processes.
Where heat production and demand occur at different times, thermal energy storage can also form part of the overall system.
Comparing biochar production equipment
| Technology | Typical application | How it works |
|---|---|---|
| Flame-curtain kiln / Kon-Tiki | Gardens, woodland and smallholdings | Biomass is added in layers, with a flame curtain helping limit oxygen reaching the char below. |
| TLUD gasifier | Small batches and demonstration systems | A top-lit fuel bed releases combustible gases which burn above the material. |
| Retort | Small to medium production | Biomass is heated in an oxygen-limited chamber, often using an external source of heat. |
| Batch pyrolysis | Farms, estates and commercial projects | A defined quantity of biomass is processed during each production cycle. |
| Continuous pyrolysis | Larger commercial production | Biomass enters and char leaves the system during ongoing operation. |
| Electrically heated pyrolysis | Specialist and commercial applications | Electricity supplies some or all of the process heat. |
Considerations
Pyrolysis location
Many groups are seeking ways to access feedstock which can be in places that are difficult to reach. They are aiming to produce mobile biochar technology for use with trailers or incorporated into shipping containers. These are easier to manoeuvre on site, rather than the feedstock being pulled out and taken to a specific location.
In the process of pyrolysis, CO2 and heat are released, so how can you use these by products to make the whole system even more efficient?
Heat
The heat could be used to heat water or a liquid that could in turn be stored as hot water in large insulated tanks and used for commercial purposes or used with a heat exchanger to heat buildings. The alternative is it could be turned into steam/vapour which could then drive a turbine to produce electricity on site.
CO2
In a clean burn situation the CO2 could either be pumped directly into a greenhouse in a measured way to increase plant growth or collected and sold to industry like carbonated drinks.
Depending on the scale of operation it is likely that using the by products of heat and CO2 would be best in a way to help processes in a specific location.
Making biochar videos from simple to industrial
Looking for biochar production equipment?
Biochar production equipment ranges from small kilns and retorts through to commercial batch and continuous pyrolysis systems.
Choosing suitable equipment starts with understanding the feedstock you have available, the quantity you want to process, your production objectives and how the resulting biochar and heat will be used.
If you are looking for equipment to produce biochar, find out more about the production technologies we are working with and tell us about your project.
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Exploring commercial biochar production? UK technology developer CapChar is developing integrated pyrolysis systems combining biochar production, heat recovery and thermal energy storage. Learn more about CapChar’s pyrolysis technology →