Biochar is carbon residue from pyrolysis of biomass. Biochar is desirable as a way to preserve carbon that has been "fixed" by photosynthesis of carbon dioxide to biomass, in a form that will not decompose readily. It has also been found to have beneficial properties for soil - increasing moisture and nutrient retention. It is not known to have harmful effects in soil. A form of biochar found in "terra preta" soil of the Amazon region, shows that indigenous communities of centuries ago found the mixture of biochar and soil advantageous, and that the carbon mixed into soil in this way is stable for centuries.
This is a DIY simple biochar production reactor with efficient combustion and no smoke - it can be scaled to larger sizes for greater capacity - it is a batch process. The instructions here are for a one-gallon paint can fuel chamber. It will hold approximately one kilogram of wood splinters. Biochar yield will be about 15% by mass.
TLUD signifies top-light-up-draft; a term of art in common use. It can also be called a top-lit updraft gasifier. The concept is that flame on the top of a burning load of biomass will act to complete combustion of pyrolysis gasses and smoke particles, whereas a conventional fire at the bottom of a load of biomass is delivering heat upward so that biomass can be pyrolyzed - but the conditions may not support combustion. This is why a new-lit campfire produces smoke, for example. The simple design of the TLUD reactor incorporates features to collect heat from combustion and redirect it to the unburned biomass, and also maximize contact between combustion plasma (flame) and inlet air so smoke and particulate emissions are minimized.
The reactor system features three components:
The FUEL CHAMBER, where biomass is loaded, where combustion occurs, and where biochar accumulates. Pre-heated air enters the fuel chamber from the bottom and pre-heats biomass.
The DRAFT SHELL, which encloses the fuel chamber below and on the sides - the updraft induced by convection over the flame zone creates low pressure which draws air into the bottom of the fuel chamber. This air must pass between the outer wall of the fuel chamber and the inner wall of the draft shell where it is heated by the fire chamber.
The CHIMNEY, which is placed over the fuel chamber and induces higher velocity gas flow as the combustion gas rises in a confined stream through surrounding dense ambient air.
Reactor fabrication is described in the project wiki and included files: 'Materials List' and 'TLUD funnel pattern' (which includes pattern for the chimney and draft shell)
Reactor operation consists of six steps: assembly, loading, ignition, chimney placement, burning, and quenching. More detail is given in the project wiki.
Use of biochar is discussed in the project wiki.