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What is pyrolysis?
The pyrolytic process is one in which the degradation of biomass occurs due to the effect of heat without the presence of oxygen, that is, in a completely inert atmosphere.

The generated products have three states:
Solid
Liquids
Fizzy
Belong to:
Coal or char
Tars
Gaseous products or fumed steam
This process can occur individually or jointly within processes such as combustion or gasification, although it does not present the commercial and industrial evolution, as combustion and gasification can.

Classification of pyrolysis
By requiring thermal energy for the process to take place, it allows pyrolysis to be divided into two groups:
Allothermic or indirect systems: the energy source is usually the gas produced itself or the char generated, and it is transmitted by conduction and radiation from the reactor walls.
Autothermal or direct systems: the necessary energy is provided by the combustion of part of the load.
From an operational point of view, pyrolysis can be classified into:
Conventional pyrolysis: rotating or moving bed equipment and even a grill oven are used. In either case, the systems can be direct and indirect.
Fast pyrolysis: known as fast pyrolysis. Suspended powder systems are included.
Instant pyrolysis: also known as flash pyrolysis. In this case, the fluidized bed promises in two stages, in which the combustion of a solid takes place and this is what transfers heat to the system.
If what is taken into account is the temperature from which the process is carried out, pyrolysis could be classified as follows:
Pyrolysis at low temperature and heating rate.
Pyrolysis at high temperature and heating rate.
Special pyrolysis: vacuum pyrolysis, flash pyrolysis and fast pyrolysis.
Pyrolytic process

Given that a set of chemical reactions take place when the thermal decomposition of biomass takes place, the subject has always been approached by studying the different components that make up the material to be pyrolyzed. The material that has generally been studied is wood, so we will proceed to analyze its different components (cellulose, hemicellulose and lignin).

First of all, it must be said that several stages can be distinguished in the pyrolysis of cellulose:
The first takes place at temperatures below 300 ° C, in such a way that depolymerization, oxidation, dehydration and decarboxylation reactions take place. Only the cellulose that is attacked is amorphous, so here only carbon monoxide and water would be generated.
The second is carried out when temperatures are above 300 ºC, when coal, tar (whose main component is levoglucosan) and gaseous products are produced.
Celluloses are more sensitive to heat, so the differentiation between the different stages that are produced takes place at temperatures that are in the range of 200-260 ºC. They give rise to a greater extent to compounds in the gas phase, less tar (without levoglucosan) and less carbon than the pyrolysis of cellulose.

There are also two stages:
Decomposition of the polymer into soluble fragments.
Conversion into monomeric units that quickly become volatile products.
The pyrolysis of wood that takes place is the sum of the pyrolysis of its major components:
Cellulose
Hemicellulose
Lignin
With regard to the pyrolysis of lignin, also a constituent of wood, it generates aromatic compounds and a higher carbon content, approximately 55% in the case of cellulose, while in the case of pyrolithic oils, 20% is generated. %, 15% tarry residue and 10% gas.

In the case that forest biomass is pyrolyzed, its properties have a very notable influence on the resulting products. For example, what humidity does is reduce the performance of the carbonization process, since heat is necessary to evaporate that water, in addition to generating a more fragile carbon than if the biomass had a lower moisture content. For this reason, it is recommended that the biomass has a water content close to 10%.

The density of the initial raw material also influences the quality of the carbon that will form with the pyrolization, being recommended to obtain a quality carbon forest residues. Size is also important: the larger the size, the worse the heat transfer; sizes between 2 and 10 cm are recommended. The chemical composition of the biomass is very important since, the higher the lignin content, the higher the quality of the coal obtained.

In view of the aforementioned, the thermal decomposition of wood takes place in stages:
Cellulose pyrolysis occurs between 200 and 260 ºC.
Between 240 and 350 ºC there is the pyrolysis of hemicellulose.
Between 280 and 350 ºC the pyrolysis of the lignin is carried out.
Therefore, in view of the temperatures mentioned, there are four stages, establishing their limits at 200 ºC, 280 ºC and 500 ºC.

A pyrolytic process that is in the development phase is known as "rapid pyrolysis", in which biomass is transformed into a liquid with a calorific value similar to fuel oil. In order for it to be produced, the biomass must be very fine, the process must be carried out at a temperature of 500 ºC and the vapors must be rapidly cooled to generate mostly “bio-oil”, although they are also generated gaseous and solid fractions that are used as a source of energy in the process.

This process is carried out in fluidized and circulating beds, since it is easy to operate with them. The bio-oil generated has a similar viscosity to that of fuel oil, and can be a perfect substitute for that and diesel, for the generation of electrical and / or thermal energy. Today, its transformation to liquid biofuel is not yet profitable.

As the most outstanding aspects of this technology, which is in the research phase, they highlight that the biomass has to have a humidity of approximately 10% and of small size, the speed of heat transfer must be high, from 500 ºC it is when a maximum production of liquids is carried out from wood, etc.