BKI REC Biochar Retort

BIOCHAR KILNS INTERNATIONAL (PTY) LTD is in the business of providing small-scale famers, the forestry and sawmilling industry as well as other interested parties with carbonizing equipment to suit their needs. The latest model in our range is the BKI REC Biochar Retort that underwent trials in 2018. The purpose of the trial was to test:

  • loading/offloading of 55 gal drums
  • duel lid/chimney system (sealing, draft, removal)
  • heat stress and longevity projections on all components
  • Bolt/nut configuration
  • Ignition techniques
  • Flat-packing/Crating and transport options
  • Modular design options

Some of our clients that have purchased the Commercial Licence expressed that the plans for the design were too complex for manufacturing in their respective countries and once built, the kilns would not last very long, even if painted with high temperature resistant paint. This criticism raised against the CIRCULAR / PORTABLE RING KILN / 3-DRUM BIOCHAR RETORT made us re-think the design. A simple expanded metal screen welded on the inside of the curved panel plastered with a specially developed fibre plaster paste, solved the material longevity problem. Increasing the diameter of the kiln to increase production is possible, but only practical for kilns up to 2 metre diameter in size.


We have devised a ‘jib crane’ to pivot out the lid which is insulated with kaowool and too heavy to lift by hand.

We understand a circular design cannot be made modular and allow additional add-on compartments to be added. The circular kilns still have relevance however as we have demonstrated with our “Faecal Sludge Carbonization Plant” in East London, South Africa.

With the REC KILN we are trying to address the following aspects: CAPACITY, MOBILITY, LONGEVITY, LOADING/OFF-LOADING OF RETORT DRUMS, PRACTICALITY OF CONSTRUCTION and EFFICIENCY. The REC KILN improves on these points, except MOBILITY (obviously we cannot roll it), but it is still a DIY assembly, can be flat-packed – and it can be disassembled for re-positioning.

Let’s address each point separately:
•CAPACITY: Original 3 drums increased to 6 drums (150kg/day).
•MOBILITY: Still mobile once disassembled. Still portable. Flat-packed and pallet-wrapped into a crate for shipping.
•LONGEVITY: Double-walled, inner panel now 3CR12 – which can be replaced, once worn out. Thin stainless steel outer cladding provides a permanent aesthetic appeal. Insulative fibre plaster applied to expanded metal mesh / kaowool inserted into double-walled panels to act as insulation to protect the metal of both the RING and REC kilns going forward. Standard insulative fire bricks are also available. Special ceramic paint can be used to paint over cracks and residually damaged areas, as they emerge.
•LOADING OF RETORT DRUMS: 55 gal drums are loaded (and removed) through a side swivel door negating the need for an overhead gantry.
•PRACTICALITY OF CONSTRUCTION: Plasma-cut plate with pre-cut holes, bent over at the edges to form panels – easier to build. To build a circular kiln we cannot exceed a gauge thickness of 3mm to be able to cold roll it which caused limited lifetime.
•EFFICIENCY: Increase thermodynamic efficiency by over 60% due to addition of insulation, by needing less firewood material.

The rest of this blog post explains the development process with illustrations and photos.

Photo 1 – The BKI REC kiln underwent extensive Research and Development work on a smallholding outside Johannesburg. This is an earlier prototype.

ILLUSTRATION 1 – VIEW OF FOUNDATION INFRASTRUCTURE and REC KILN in STAINLESS STEEL CLADDING. A total of about 240 standard bricks set with mortar is required (not included in the kit). The bricks are mortared in to create four AIR INLETS. The AIR INLETS are designed to allow air to enter underneath the kiln without creating hotspots on the metal components that could cause the kiln to deteriorate prematurely. The AIR INLETS can be sealed off with bricks, and then covered with sand.

Illustration 1

ILLUSTRATION 2 – VIEW OF FOUNDATION INFRASTRUCTURE and REC KILN in STAINLESS STEEL CLADDING lifted off the ground. Note that the base of the kiln is open-ended and the walls of the kiln rests on top of the layer of bricks. The base of the kiln is filled with sand or soil. If the kiln is to be operated on a concrete surface then the kiln will sit on top of a four-brick layer and filled with soil.



Illustration 3


ILLUSTRATION 5 – GIF animation file showing how BKI REC kiln is assembled

Illustration 5

ILLUSTRATION 6 – REPRESENTING REC KILN to scale using a 1.82m tall man

Illustration 6


ILLUSTRATION 7- DIY REC kiln can be flat-packed into a crate measuring 1.9 x 1.6 x 2.9 metres for DIY assembly


Illustration 7


DIY Assembly

Site preparation consists of laying down bricks as a foundation on which the kiln rests, making space for air inlets and the single rocket stove. The inner panels and lid are made from a durable chrome-based metal called 3CR12 to withstand high heat without warping. The panels are all the same size with laser-cut hole-positions corresponding with adjacent panels, to allow them to be bolted together. Kaowool insulation is loosely placed inside the panels, door and the two loose fitting lids. The sealing furrow is lowered down onto the panels that are now fully assembled. Stainless steel cladding is attached on to the outside of the panels and over the sealing furrow. The fibre plaster mix is applied over the inside of the panels, post-assembly, containing a mesh screen, up to the desired thickness and allowed to dry. Special instructions will be given on how to seal the door. The two kiln lids have handles and one person on each side of the kiln can lift it off by hand. 55 gal open top drums are fitted with ‘full curl lids’ to seal off the feedstock inside the drums and are rolled into the kiln manually through the side door. Entire kiln can be dissembled for translocation.

ILLUSTRATION 8 – FULLY ASSEMBLED REC kiln (at an extra fee) can be shipped in the same crate measuring 1.9 x 1.6 x 2.9 metres. Note that the kiln is fitted with lifting lugs to allow for easy offloading with offloading equipment, machinery or overhead gantry.

Illustration 8

ILLUSTRATION 9 – VIEW OF DOUBLE REC KILN in STAINLESS STEEL CLADDING. The inner panels are removed to allow free flow of gases from one end to the other.

Illustration 9



Operating procedure

Firewood consisting of low quality material such as timber offcuts or used pallets is packed around the drums and once lit, creates the heat into the drum retorts. Air into the kiln is regulated with the rocket stove and other air openings created into the bricks below it. The retort lids need to be punched with holes to allow gases to escape and for the process to become exothermic. Complete pyrolysis occurs within a few hours but allow 24 hours or longer for contents to cool. Lids can be removed to accelerate the cooling process. Once the 55 gal drums have cooled off sufficiently open the side door and remove them / load the next batch. It will not be necessary to use water to quench the char, simply rotate the drums through 180° and place the drums with the lids and holes facing down to seal off air.

CLICK on the image below to open the CALC SHEET:

calc sheet

The CALC SHEET includes INPUT areas for each BIOCHAR kiln model (RING17, RING20, REC) listed on separate worksheet tabs to calculate their respective costs.

The options available to you can be quite overwhelming and this worksheet aims to simplify it. Some cells will autocomplete as you choose from drop-down menus. Some costs had to be adjusted upwards especially the INSULATED FIRE BRICKS and FIBRE PLASTER for the larger RING20 model. Also note that if your project is outside of KWAZULU-NATAL, SOUTH AFRICA, the delivery cost and training (flights, as opposed to own transport) will have be adjusted upwards on a case by case basis.

We are making many items optional in an attempt to render these kilns more affordable to all. The INSULATED FIRE BRICK for instance is more expensive but you will get a lifetime of usage out of it. Using FIBRE PLASTER creates more space inside the kilns and it is cheaper BUT you will need to replace/patch it up periodically as it is not as durable. Note that FIBRE PLASTER is only obtainable in JOHANNESBURG (JHB), but there are INSULATED FIRE BRICK manufacturers in JHB, CATO RIDGE & PORT ELIZABETH (PE) so the closest facility will be sourced. Any others I should know of? International clients may want to buy BRICKS locally as they are very heavy and expensive to ship.

Hope this helps you in your decision-making. Overseas clients will have to convert from ZAR into their own currencies depending on the prevailing exchange rate. The CALC SHEET will be updated once a month.

We hope this blog post was useful. Contact us for more information.

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