I had a fun introduction to this material last July. Driving along Highway 29 in Oakville, where a vineyard had just been removed and the vines piled for burning, I saw what looked like a large dumpster out in the bare field, with heatwaves and a few flames rising out of it and a big loader feeding material into it.
Too curious to pass it by, I pulled onto the nearest side road, drove out onto the field and had a chat with the operator, Davie Piña of Piña Vineyard Management. He explained they were operating an air curtain incinerator, manufactured by Air Burners. The device blows a strong current of fresh air over the burning contents of the fire box, forcing the partially burned smoke back into the inferno.
It is not the least expensive way to dispose of dead grapevines, but it greatly reduces the environmental costs of open-air burning with respect to heavy smoke and lung-damaging fine particulates. Its main purpose is not the production of biochar, but some is generated in the process.
Most of the woody material is reduced to white ash and CO2, but some of it, where the ash prevents oxygen from mixing with the wood, become biochar. There was a big pile of it stockpiled in the field. I could not resist saving a lump about the size of a briquette. The material is really nothing more than charcoal.
About the CO2 released from burning grapevines, or burning forests for that matter, keep in mind, this is carbon extracted from the atmosphere by plants, not carbon from coal and oil stored deep in the Earth. And biochar is a very durable component of soil, serving plant and soil health as well as long term storage of carbon. The net effect is worth a detailed study.