The Pyrolysis Arts

Just as "biochar" sounds like something you don't want to have happen to your steak; "pyrolysis" sounds like something you don't want to catch. The connotations are unfortunate. Because "pyrolysis" of biomass is how "biochar" is made.

Pyrolysis is defined as thermal degradation in a restricted-oxygen atmosphere. When you pyrolyze biomass, you wind up with a mix of combustible gases, volatiles, and solid residue. The nature and relative proportion of these co-products depends on the pyrolysis environment and the type of biomass you started out with.

(There isn't yet universally accepted terminology in this field, so here's my take: The generic solid residue of pyrolysis can be referred to as "char". When this char is burned as fuel, as it has been for millennia, call it "charcoal". When we apply it as a carbon-sequestering soil amendment, call it "biochar". OK?)

The biomass, or "feedstock", is typically crop residue or agroforestry waste; but just about anything that was once living or derived from living material can be pyrolyzed, including sorted municipal waste and de-wetted sewage. As for the pyrolysis environment, that's all over the map--maximum temperature reached, heating rate, dwell time at different levels, presence of small amounts of oxygen, suffusing with certain gases, size of the biomass particle, etc. Larger scale engineered plants operate more like refineries, and the pyrolysis environment can be manipulated to maximize the yield of particular products; bio-oil for processing into biodiesel, combustible gases for resale or direct power production, and other commercially valuable products. Alchemy comes to mind. Given the huge range of inputs and variables and possible co-products, referring to the "Pyrolysis Arts" seems well justified.

Our interest is the solid stuff, the lowest common denominator. In the most basic char-making kilns, the gases and volatile compounds produced are consumed in the process, with the excess either released to the atmosphere or burned. Since many of these compounds can be potent greenhouse gases, atmospheric release is a no-no; if nothing else, we'll want to flare them off. Better still would be to capture some of the heat and use it (for drying more biomass, for example).

Many future posts will be devoted to experimentation with and optimization of char-making kilns.