How to make biochar
A guide to feedstock and pyrolysis production equipment
To make biochar you need feedstock and pyrolysis equipment. For optimum conditions to produce biochar, your pyrolysis process should use slower heating, as faster heating tends to produce Bio-Oil.
There are many reports and descriptions of fast pyrolysis reactors sometimes called gasifiers. These are based on fluidised beds, rotating cones, ablation systems with cyclone separators and electrostatic precipitators. Some of these designs have progressed to pilot and some commercial stages.
Fast pyrolysis focuses on producing pyrolysis liquids (bio-oil) with yields of liquids up to 75% of dry mass. The significant mechanical actions of these processes combined with small particulate feedstocks typically 2mm mean that they may be less suitable for producing the types of biochar needed for agriculture and horticulture.
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.
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
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.
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
To make biochar at any commercial-scale you will need pyrolysis equipment, a machine or continuous feed technology that can produce large volumes.
Typically used in forestry to burn trees and brush on site in difficult locations.
It is a batch/continuous process in terms of feedstock, but once the container is full it needs to cool and its contents removed manually.
Typically a reactor system uses a continuous feed in/out. It uses the pyrolysis gas from the combustion zone to provide heat for the process. Some systems use a horizontal rotary kiln under vacuum with indirect heating.
In the context of biochar a generator uses the waste heat from the system for another purpose. This could be electricity generation via steam, hot water and carbon dioxide storage and distribution in greenhouses.
There are a few manufacturers who provide electrothermal units with a mechanised feed that also serves as the heater element. They are adaptable to a variety of feedstocks including sewage and can include heat recovery, plus vapour/liquids handling.
Investigators have tried to develop a microwave powered unit with EU funding support. They planned to demonstrate a mobile unit for use in forestry settings to produce bio oil/fuel and biochar. They did make progress but sadly they were not able to demonstrate a fully functional system.
Final Report Summary – MICROFUEL (Mobile microwave pyrolysis plant turns biomass into fuel locally)
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?
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.
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.