Many people ask me what exactly we do at BKI. Much of our work builds on previous commissions, from designing charcoal gasifier camping stoves to developing multi-kiln progressive batch biochar systems. Visit our Patreon for unredacted photos:
We frequently get approached by farmers wanting to convert invasive plant species into valuable products. Once faced with cleared invasives, they start thinking there must be a way to transform that waste into something useful. Many have workshops, tools, and scrap metal, resulting in some very inventive—but often Heath-Robinson style—contraptions!
A common theme is trying to redirect volatiles (offgas/syngas) back to the firebox through piping. However, unless your machine is designed with thermodynamic engineering expertise, handling tar and sludge buildup effectively is unlikely. Unlike the Biolyser Bio Agri Group system—which I train on and which allows easy cleaning of accumulated tars and sludges in cooler downstream sections—these homemade rigs often struggle with maintenance.
We also capture a small amount of wood vinegar and burn tars and impurities in a flare to prevent fouling. Spending 50,000 ZAR (~$2,800) on equipment that will rust away after roughly 30 uses is not ideal. It’s better to invest around 100,000 ZAR to include insulation in your build. This investment extends equipment life and enables higher operating temperatures for cleaner emissions.
Regarding gas handling: my advice is to burn off volatiles in a close-coupled burner—no piping required. The best retort for this is a 55-gallon drum, for several reasons:
- They’re durable and cheap to replace, so no point in piping gas to something you’ll soon replace.
- The thin metal skin allows efficient heat transfer.
- Their modular design lets you swap parts easily; I recommend using a trolley jack to move feedstock and biochar.
- They safely handle pressure, much like our Biolyser, though the latter demands expert engineering.
Burning volatiles in an afterburner reduces greenhouse gas emissions by around 80%, according to the US EPA.
However, just having 55-gallon drums isn’t enough. An outer vessel—whether a metal ring or masonry kiln—and insulation are essential. Insulation options include:
(with a chicken wire layer to prevent disintegration)
Welding stainless steel fiber lock pins, studs, or bolts inside the outer vessel helps attach insulation.
Instead of repurposing underground diesel tanks into kilns—a common practice in South Africa—I’ve converted a few by cutting them lengthwise for use as outer vessels.
One reason many avoid insulation is the added cost and the risk of firewood heating damaging it. The workaround is to use rocket stoves made from refractory firebricks to evolve heat gradually (endothermic) and then drive exothermic reactions. This design makes retort kilns the clear winner if you want durability without complex gas redirection or pressure issues.
A major bonus with retort systems: no need for water quenching. The biochar or charcoal (great for woodland farmers) is ready to bag as is.
Below is an example from a recent build proposal where we encouraged a local farmer to cut five water tanks (2 x 2 m) in half for use as outer vessels. The main design challenge is safely lifting up (more like pivot) the halve-round chambers to allow drums to be wheeled in and out. We suggest using a counterweight and a double-purchase pulley system for easy hand lifting—no overhead gantry or hinges required. The foundation and rocket stoves are constructed with loose fitting high density firebricks. We explained to the farmer that he would now have 10 kilns, each capable of producing 90 kg of biochar per day—operating on 24-hour cycles. At 25 days of production per month, he can then produce 22.5 tons of biochar monthly. The outer vessel acts as the burn chamber. Because it’s not pressurized, it doesn’t need a tight seal, but it does require a chimney to draw air in and vent volatiles.
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BKI 