Add biochar to a garden or farm to improve the soil’s ability to hold and retain water and increase the crop yield. Biochar can also filter contaminants out of drinking water, including agricultural chemicals that may have leached into the water supply. For more on the benefits and the science of making biochar, please see our article, An ancient filtration material removes pesticides from drinking water.

You make biochar by burning wood at high temperatures in a low-oxygen chamber. The process is similar to making charcoal, except that the biochar material burns at a much higher temperature. The biochar gasifier that we show here burns at 750 to 900 C and can yield 6 to 12kg of char.

This top-lit, updraft gasifier design, including the photos and diagrams, come courtesy of Josh Kearns the science director at Aqueous Solutions, a non-profit water, sanitation and hygiene development organization.

The design is open architecture, so feel free to improve upon it and adapt it to your needs. For questions, and to offer tips and comments, please contact Josh Kearns at josh (at) aqsolutions (dot) org.

For more information, see Aqueous Solutions‘ instructional videos in English and Thai, and downloadable handbooks in English, Thai, Burmese and Spanish.

How to make a 200L (55gal) top-lit updraft gasifier

*Two 200L (55gal) steel drums
*Square tubular steel or angle iron scrap
*Sheet metal or flue pipe (not tin, aluminum or thin galvanized steel)
*Concrete blocks or bricks or something comparable
*Bolts, nuts, washers

*Angle grinder
*Drill with bits
*Cold chisel
*Basic welding setup

Photo by Lyse Kong

The image above shows most of the construction process. Select one of the two drums for the reactor body. In that drum, cut a circle out of the top leaving a 2in (5cm) lip around the edge (upper left in the image above).

Photo by Lyse Kong

Drill roughly 300 holes, evenly spaced, each just shy of 1/2 in (10mm) in diameter into the bottom (bottom left). Or, if you prefer, cut radial slits into the bottom of the drum (see the photo on the left).

To make the handles, cut two lengths of angle iron or square tubular steel at least 50in (125cm) long, then weld or bolt them to the sides of drum.

To make the chimney, either use a flue pipe or roll a piece of sheet steel into a cylinder, clamp the edges and weld them closed.

Now, onto the second drum. This will become the lid and the crown placed on top of the reactor (the first drum), and it will support the chimney. The crown is where the combustion will happen.

Photo by Lyse Kong

To make the crown, measure 10in (25cm) from the bottom and, from that point, cut evenly around the perimeter to make a cylinder. Cut eight triangular vents, four in the top and four in the bottom. The four upper vents should be 6x8in (15x20cm) and the four lower vents should be 4x5in (10x13cm). Stagger the vents so that the lower four fall between the upper four.

To attach the chimney, cut a tabbed hole in the top of the crown, right in the center. Bend the tabs outward and attach the chimney with bolts or welds. The photo on the left shows the tabbed attachment in a view through the top of the chimney looking down.

Cut the lid out of the other end of the second drum. The lid should have a diameter of about 22in (55cm), large enough to place snuggly onto the lip that you left in the first drum, plush against the inside rim.

Now cut two 6ft (2m) lengths of steel tubing or angle iron that you will use to remove the crown when it is hot after combustion.

Arrange the cement blocks, bricks, or other material into a stable base for the gasifier. Leave a large enough gap between the bottom of the reactor and the ground to allow for airflow.

Set the crown with the chimney attached onto the lip that you left around the inside top edge of the reactor. Try for a snug fit. Make sure it is stable and will not wobble or tip during the burn.

Image courtesy of Josh Kearns

See the diagram below for more information.

How to use the gasifier

Pack the reactor full of biomass. That can be any woody or plant-based cellulosic material. Kearns recommends materials such as corn cobs, lumber scrap and coconut husks. For details on what works best, please see the handbook.

Photo by Lyse Kong

Photo by Lyse Kong

Photo by Lyse Kong

Place the crown and chimney on top, then light the biomass through one of the vents in the crown. You can use straw or other kindling, but no lighter fluid or gasoline or anything like that is necessary. As the material pyrolizes (releases and burns off gases) it will burn with an orange flame that you can seen through the vents in the crown, as shown in the photo on the left. When the gases are burned off, the orange flame will fade to a clear, bluish color. At that point, the material is combusting and you need to shut down the reactor before it burns up.

Remove the crown by inserting the poles into the vents and set it aside. This is a two-person job, as you can see in the photo above.

Place the lid on the reactor and, using the handles, move it to a nearby mud pit. Seal the edge of the lid with mud. Wait a couple of hours for it to cool and then enjoy your biochar!

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