PDF - Institut national polytechnique de Toulouse

2. Theoretical backgrounds
2.1 Pyrolysis
Pyrolysis and thermolysis, commonly referred to as destructive distillation, are defined
as an irreversible chemical change brought about by the action of heat in an oxygendeficient
(less than 2 %), inert-gas environment. The pyrolysis process requires
temperatures ranging from 400 °C to 900 °C. Pyrolysis systems use a source of heat to
drive the endothermic pyrolysis reactions in the complete absence of oxygen. The only
difference between pyrolysis and thermolysis is that the former employs a direct heat
source within the reactor (retort), while the latter employs an indirect external source of
heat to the reactor (retort). Depending on particular reaction conditions (temperature,
partial pressure of oxygen, total pressure), the organic fraction decomposes into gas, oil,
and solid carbonaceous residues. Mild temperatures favour production of oils against
gases. These products are recuperated at the end of the process, with the intention of
being valorised.
Low oxygen content can be obtained under partial vacuum, by reinjecting one part of
produced gases in the reactor or by injecting gaseous nitrogen into input riddle. The
thermolysis technologies differ from each other by the reactor type, methods of reactor
heating and conversion operating conditions.
Pyrolysis leads to decomposition of matter into various by-products (gas, oil, char,
etc.). Yields of particular products are very variable, according to pyrolysis technology
used. Some processes lead only to gases, others will produce a great quantity of oil.
Pyrolysis products are gas, liquid, and char, the relative proportion of which depend
very much on the pyrolysis method and process. Pyrolysis products are synthetic gas, oil,
and carbonaceous residue. Synthetic gas produced in pyrolysis consists generally of
a mixture of volatile organic compounds, some more heavy than others, methane,
hydrogen, carbon dioxide and monoxide, and water coming largely from a humid fraction
of the waste. After its treatment, gas can contain yet more volatile organic compounds (oil
or tar). Most often, it is valorised in a boiler or directly in a thermolysis reactor,
producing energy necessary for the thermal dissociation.
— 12 —