Pyrolysis or pyrolytic decomposition (from ancient Greek πῦρ pyr 'fire' and λύσις lýsis '(dissolving)') describes various thermo-chemical conversion processes in which organic compounds are split at high temperatures and in the absence of oxygen (λ = 0). The high temperatures split the bonds within the molecules and the exclusion of oxygen prevents combustion. The resulting products are diverse.
Pyrolysis is used in many technical processes. In this way, biomass can be specifically converted into higher-quality products such as fuels or chemicals, but pyrolysis processes also take place in the classic coking of coal and in the production of charcoal . From a chemical point of view, the cracking of petroleum is also a pyrolysis process, but is not called that.
Outdated terms for technical pyrolysis are pyrolysis , dry distillation , decomposing distillation , degassing or carbonization . The root word “pyranic” in the names of chemical compounds such as catechol , pyrucic acid and pyruvic acid goes back to this.
The term pyrolysis is often not used consistently.
On the one hand, it is used to describe technical processes that aim for an incomplete thermo-chemical conversion that does not go beyond this phase of pyrolytic decomposition. On the other hand, the phase of pyrolytic decomposition in the thermo-chemical conversion is often referred to as pyrolysis itself. In this sense, it can be quite undesirable and occur, for example, when overheating or poor heat transfer.
In terms of fire behavior of wood, the term pyrolysis is also used for the point in time at which an insulating and protective layer of charred wood is created over the residual wood.
Differentiation from gasification
Gasification is to be distinguished from pyrolysis . This is also a thermo-chemical conversion, but it goes beyond pyrolysis. In comparison to pyrolysis, gasification uses a gasification agent with oxygen or oxygen atoms, whereby the raw substance is further oxidized and mainly gaseous products are formed. Gases are also produced during pyrolysis, but the target is liquid or solid products.
Wood tar and pitch obtained by pyrolysis are the oldest plastics known to man. The extraction of tar and pitch ( birch pitch ) by pyrolysis was known as early as the European Mesolithic (8300-4000 BC ). This was especially used as an adhesive and for sealing. Birch pitch was also used for gluing the arrows of the so-called Ötzi . From the 18th century onwards, tar was also made from various other raw materials, e.g. B. from coal .
The production of charcoal by means of pyrolysis has also been known for several centuries.
Currently, pyrolysis is being researched further, particularly as a means of energetic and material use of renewable raw materials.
Chemistry of pyrolysis
The pyrolytic decomposition is a phase in the course of a thermochemical conversion of a substance or a mixture of substances, which, depending on the substance, is reached at around 150 - 700 ° C. The heat breaks bonds in the large organic-chemical molecules and creates new, smaller molecules. Decomposition of the substance can be seen visually. Since there is no external oxygen present, there is no combustion and no oxidation. Nevertheless, reactions involving oxygen atoms can take place if these are already present in the starting material. Pyrolytic decomposition is an endothermic process.
During pyrolysis, complex product mixtures are created from solid (e.g. charcoal ), liquid ( pyrolysis oil ) and gaseous ( pyrolysis gas ) products, the exact proportions depending on the specific conditions and the starting material. Basically, it can be said that the higher the temperature and the longer pyrolysis time, more gaseous products are obtained, and the lower the temperature and the shorter the time, more liquid products. When polymers are pyrolyzed, the corresponding monomers are often formed as a product.
The products can be used energetically as a secondary energy carrier, as they have a high energy content, and can also be used further as a material by extracting individual chemicals from them. The pyrolysis of biomass is a possibility to produce organic basic chemicals such as benzene or phenol bio-based, which are currently only produced from fossil sources.
Technical pyrolysis variants
Pyrolysis systems are differentiated according to the type of heating. With direct pyrolysis, hot gases are passed over the substrate, while with indirect pyrolysis the reaction space is heated from the outside.
In addition, there are numerous other differentiation and classification options, for example according to substrate, length of stay or temperature.
A frequently chosen classification, especially in the pyrolysis of biomass, is the classification according to reaction time. Often in slow pyrolysis (Engl. Is slow pyrolysis ), medium speed (Engl. Intermediate pyrolysis and fast pyrolysis) (Engl. Almost pyrolysis or flash pyrolysis ) divided, but there are also coarser or finer subdivisions.
|Type of pyrolysis||Temperature [° C]||Length of stay||Heating rate||Share of solid products [%]||Share of liquid products [%]||Share of gaseous products [%]|
|Fast pyrolysis||≈500||<2-3 s||high||≈ 12||≈ 70||≈ 13|
|Medium-speed pyrolysis||≈ 500||10-30 s||medium to high||≈ 25||≈ 50||≈ 25|
|carbonization||≈ 400||h – d||low||≈ 35||≈ 30||≈ 35|
|Torrefaction||≈ 250||10-60 min||low||≈ 80||≈ 5||≈ 20|
This process has been intensively researched since the 1990s and is aimed at the production of liquid pyrolysis oil. There are different types of systems, but they all have in common that the process is divided into three parts. First the biomass has to be prepared, e.g. B. by drying and mechanical crushing. This is followed by brief pyrolytic decomposition at around 500 ° C. The product is then condensed and purified and, if necessary, further refined. The process energy can be partially covered by the combustion of the unwanted solid and gaseous reaction products.
In order to obtain as much pyrolysis oil as possible, it is important that the biomass particles are heated very quickly and then cool down again very quickly. This goes hand in hand with very special technical systems. In addition, the biomass particles must be sufficiently small for this.
The medium-fast pyrolysis takes place at approx. 500 ° C. The pyrolysis mass is heated up medium-quickly and for approx. 10 to 30 seconds. This form of pyrolysis is currently still in the pilot phase.
The aim of slow pyrolysis is the production of solid secondary energy sources. It can be further subdivided into charring and torrefaction.
Charring or carbonization (complete slow pyrolysis) has been known as a method of making charcoal for millennia and is still carried out commercially to this day.
Applications of technical pyrolysis
Pyrolysis of renewable raw materials
Pyrolysis processes are classified as promising technologies in order to use renewable raw materials - especially those based on lignocellulose - and to displace fossil fuels, which is why massive research has been carried out on them for a long time. However, the processes are currently not yet economically and economically of no great importance. The pyrolysis of biomass is a step in the production of a wide variety of biofuels and platform chemicals .
The pyrolytic production of products offers a great potential for greenhouse gas reduction compared to conventional production on a fossil basis, the exact balance particularly depends on the use of the pyrolysis products and the type of biomass.
The pyrolysis of biomass in the form of pyrogenic CO 2 separation and storage (PyCCS) is also seen as a means for CO 2 fixation.
Pyrolysis of waste
Pyrolysis is an important alternative to incineration for the recycling of waste such as old tires, waste wood or plastic. Many such systems are operated in Asia, especially Japan, and have also been tested in Germany. For Germany, the Federal Environment Agency rates waste pyrolysis rather critically and at most considers pyrolytic pre-treatment of waste to be sensible under certain circumstances.
In addition, pyrolysis can also be used as a thermal type of soil remediation for soils with oil, mercury and Dixon pollution.
Activated carbon production and regeneration
Consumed, d. H. Activated carbon contaminated with the pollutant is heated in an oxygen-free atmosphere and the pollutants are expelled at temperatures of around 800 ° C and sometimes cracked.
Other pyrolysis processes and fields of application
- Acetylene production by the HTP process
- the cracking as a process of the petrochemical industry for the production of aromatenreichem gasoline that is characterized by good knock resistance distinguished (pyrolysis gasoline)
- Flash vacuum pyrolysis for the synthesis of complex structures such as fullerenes
- Carbon black production
- Carbon fiber reinforced carbon : In the production of such materials, pyrolysis is an essential process step.
- Coking plant ( coke production from brown or hard coal )
- Pyrolysis in combination with gas chromatographic methods (e.g. GC / MS ) for the investigation and determination of polymeric materials
- Hydrogen production from water using plasma torches
- Self-cleaning ovens also work with a technology based on pyrolysis
If, for example, oxygen is sucked in through faulty seals, an explosive mixture can form if the temperature is too low. From approx. 450 ° C this is no longer possible, as the oxygen then immediately reacts with the combustible reactor contents (gas, carbon) in the sense of partial combustion.
Another problem is that gaseous products (e.g. tar oil ) condense on cold bridges and can then drip out of leaks.
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- Flexible material analysis using pyrolysis and standard GC / MS
- Basel-based energy supplier IWB is building a pyrolysis plant that will turn green waste into biochar.
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