The Products of Pyrolysis

To make biochar, we make a fire with biomass. Inside the fire, heat is breaking down the biomass into gases and a solid residue. The flame is the gases burning, which provides the heat that keeps the breakdown (pyrolysis) happening. The solid residue is char.

Vuthisa flame cap kiln, just about ready to quench

By controlling temperature and residence time (how long the heat is applied), we can alter the char’s qualities and obtain additional products. To create biochar, we want a relatively low temperature fire and a long residence time, which consumes the gases and maximizes char production. Short residence times at low temperature result in smoke (unburned gases) that can be condensed into a useful liquid (pyroligneous acid, aka liquid smoke). Higher temperature and longer residence times result in syngas (aka producer gas), through reduction of CO2 and H2O vapor to CO and H2, with a proportional loss of char.

Flame cap kilns and conservation burns provide the right conditions for making good biochar. The reason we wait until the flames disappear to quench the fire is twofold: We don’t want to interrupt the chemical restructuring of carbon bonds into rings, and we don’t want smoke to condense on the char. Carbon rings are what make char biochar, providing adsorption sites and preventing decomposition. Condensed smoke lowers biochar quality because it coats char with tar, which gets in the way of adsorption until microbes can digest it. If we were making charcoal for grilling, we would actually want to allow tar deposit by shortening the residence time, because burning tar gives us the so-called charcoal flavor that we’re after.

Fast pyrolysis is a highly technical process for creating bio-oil, aka bio-crude, which can be refined like fossil fuel into various petroleum substitutes. For a deeper dive into that, read the article from which I sourced this post.