The Conservation Burn Technique

When done properly “conservation burning” is a technique for burning woody material that greatly reduces the production of smoke particulates and reduces the amount of Carbon released into the atmosphere. Biochar can be produced from the material left over after the burn and spread with compost to increase soil organic matter and sequester Carbon in the soil.

Tools and Equipment
The Technique
Small piles are better than big ones. A large pile makes it difficult to light, is harder to stack loosely, and makes it less likely the conservation burn process will work as desired.
Pieces in the pile should be about the same size. The exception is kindling. Collect kindling size pieces from the material going into the stack. Place them on top once the pile is built. Pieces of material of varying sizes will burn at different rates. Smaller pieces will turn to ash before the larger pieces have burned to char. Cut up large pieces to match the size of average pieces.
Stack piles somewhat loosely. The conservation burn process requires air flow.  In particular, air flow to the kindling on top of the pile. 
Piles need to season. This is the same principle as seasoning firewood - to reduce moisture. The material should be less than 20% moisture content. A firewood moisture meter can be an inexpensive way of determining if the pile is ready for a successful burn.
Light the pile from the top, not the bottom. If it is not windy, light the entire circumference of the top of the pile. If it is windy, light the pile from the downwind side of the pile. If the pile is difficult to light try adding more kindling to the top before resorting to additional accelerant.
Put out the fire before everything turns to ash. The fire is ready to be extinguished when you can still tell what type of material was burned (part of a trunk, part of a cordon, a big spur position) and the pile is covered with fine white ash.  To reduce the amount of Carbon released into the atmosphere it is crucial to put out the fire before the material turns completely to ash. The more black material left at the end of the burn, the less Carbon that has been lost.
Put out the fire with water from the upwind side. Slowly remove the charred material. This is the biochar. Hose down the area ahead of the workers. Worker safety is critical. Be sure water is supplied to the area around and ahead of the workers’ feet. This process is repeated across the pile until the fire is completely out. 
Rake out the charred material. Spray down an area where charred material can be raked out and any unburned material can be removed. 
Crush the charred material. Once the material has cooled, run over the charred material with a piece of heavy equipment to pulverize it. The greater the surface area, the better. Crush the biochar to a size that is easy to work in a compost pile and spread.
Blend with compost. The ideal time to add biochar to compost for spreading is early in the composting processing. 

Content courtesy of Raymond Baltar of the Sonoma Biochar Initiative