An open above ground fire can increase the amount of BioChar by simply  lighting a full pile from the top and watching it burn down engulfing  BioMass until the flames are gone and a pile of red hot coals remain developing a thin layer of grey ash, then apply as much water as needed to fully quench to cold black BioChar. Rake out the pile of  char as you quench to assure they are fully out & to avoid them  reigniting from any residual heat. Now you have clean empty stable  Carbon BioChar ready for enriched inoculation. If water is not abundant  or available the coals can be smothered with dirt. 

Using a pit will increase the efficiency producing more BioChar than an open above ground fire. There are dangers with starting a file pit below the surface, as root balls nearby can ignite and cause a bigger issue such as a forest fire. Therefore, its best to utilize a Kiln or Pyre, if the desire to make biochar arises, of which we primely advocate..
 
Dig a cone shaped pit 6 feet deep & 6.5 feet wide at the top. Once the initial pile in the pit burns down more fuel  can be added at the rate it burns down until the pit is full and the  flames burn out. Then fill the pit with water to fully quench. More water may  be needed when you dig out the pit full of BioChar, as the pit itself  can retain and radiate heat which could cause the char to reignite days  later. If water is not abundant or available the pit can be covered with dirt to smother the coals. 

Pit dug over 7 feet deep 3 feet wide at the base and over 7 feet wide at the rim. After one afternoon burning we filled the pit full of BioChar made from burning 8 large piles of  Scotch Broom in the pit. The pine log that was added on top was not  completely burned and will be added in the next burn to complete char. 

Portable Kilns are the most efficient in that they convert a high percentage of biomass to make BioChar.  Links for more information about various size  transportable open air kiln designs:

• Wilson Biochar 
• Paul Anderson’s RoCC 

Air curtain burners operate on Diesel motor to spin a air blower, they are portable units that require a CDL to operate in some states, including California.  These units can fit large pieces of wood that convert a percentage of the biomass into biochar. These units fit in to large 1000+ acre project but have a smaller conversion ratios verses the above methods. We see the value in these machines to scale up reduction to conversion for those looking to make an impact verses normal slash to burn procedures. Similar apparatuses such as the Tigercat (Carbonizer) perform similar conversion percentages.