This article describes a quick and clean way to make biochar without polluting the environment especially if you live in a valley which tends to trap the smoke until the next rain. The trick is to burn from the top of the pile, not bottom.
Top lighting is the key essential. As your fire starts from above, the heat releases the volatile gasses and there is full combustion. When you light from below, all the pile smokes as it begins to slowly heat up and smolder. Top lighting is faster as the fire moves down, until in one moment, it consumes the whole pile in a clean burn, and the charcoal will fall below. When the fire slows down, separate burning material to one side for more air, and a more clean burn.