Pyrolysis, or pyroscission, as it is commonly known today, is a specific thermochemical process that takes place in the absence of an oxidizing agent (in our case, oxygen); it involves the thermochemical decomposition of materials of an organic nature that are present.
The pyrolysis of wood is activated by supplying heat to the material to be decomposed, so as to provide the necessary force to break some chemical bonds within the complex molecules, of wood in our case, but it applies to all biomass, and make them become less complex molecules.
During the process, the total absence of oxygen inhibits any oxidizing reactions, the latter reactions that would lead to oxidized compounds, the last step in the chain of reactions of a normal combustion. In the presence of oxygen we do not have pyrolysis, but traditional combustion.
Dissociation could be called the process, but it would be reductive to be able to describe the set of reactions that occur during the complex course of pyrolysis, they are actually a continuous series of associations and dissociations.
The pyrolysis of wood has always found application, an example for all are the pyres for the production of charcoal, in fact in the charcoal kiln a percentage of oxygen is present, but in such low percentages that traditional combustion is not possible. More recently, pyrolysis has found application in waste-to-energy plants suitable for waste treatment, the advantages and disadvantages of this solution have not always been brought to the attention of the public, at least not as much as they should have been.
How pyrolysis works
Pyrolysis in plants, whether a home furnace or a waste-to-energy plant, are created with the same operating concept.
The process, as we have seen, takes place through chemical dissociation and reassociation, which allows us to fragment a complex molecule into many simpler parts by applying appropriate thermal conditions.
The result of the process is the production of two different products: a volatile part and the gaseous part.
A volatile part, which in turn is divided into:
- A gaseous portion called pyrolysis gas or syn gas (synthesis gas);
- A liquid portion resulting from the condensation of the volatile fractions of the fuel (creosote water, etc.).
And a resulting solid fraction, commonly referred to as char; char consists of carbonaceous residues of organic matter, ash, metals, aggregates, etc.
The gaseous part consists of:
- Hydrogen;
- Methane;
- Ethylene;
- Ethane;
- Carbon oxides;
- Other minor gases.
In the liquid portion finally appear mainly:
- Tar;
- Water;
- Various organic substances (often consisting of oils).
This article is obviously not intended to be a scientific treatise on pyrolysis, but a small aid to understanding a complex phenomenon, so we apologize in advance to everyone for any omissions or inaccuracies that you may point out in the comments at the bottom.