What is Pyrolysis?

Pyrolysis can be defined as the process of subjecting substances to highly elevated temperatures in relatively inert atmospheres in order to facilitate their thermal decomposition. It is important to note that the process of pyrolysis brings about a chemical change in the substance subjected to it (the chemical compositions of the initial reactant feedstock and the final product are different). The term ‘pyrolysis’ has Greek roots and can be roughly translated as “fire separating”. Generally, substances that are subjected to pyrolysis undergo a chemical decomposition reaction and break down into multiple product compounds.

The process of pyrolysis is widely used to break down organic substances. For example, the charring of wood (or the incomplete combustion of wood) that results in the formation of charcoal involves the process of pyrolysis. Typically, the pyrolysis of an organic substance will produce multiple volatile products and will also leave behind a solid residue which is often highly enriched with carbon. It can be noted that extreme pyrolysis, which often leaves only carbon as a residue, is commonly known as carbonization. It can also be noted that pyrolysis is widely considered as the initial step for other related processes such as combustion and gasification.

The chemical industry makes extensive use of the process of pyrolysis.

One of the most important applications of this process is in the production of ethylene and other important carbon compounds from coal and petroleum. Furthermore, it can be noted that these compounds can also be extracted from certain organic matter such as wood. Coke can be obtained from coal (along with a wide spectrum of different compounds) via the process of pyrolysis.

What Happens to the Feedstock During Pyrolysis?

  • The feedstock that is subjected to pyrolysis is exposed to temperatures above its decomposition temperature. At this point, the chemical bonds that hold the molecules of the feedstock together are broken. This results in the fragmentation of the molecules of the feedstock into smaller molecules. It is not uncommon for these smaller molecules to participate in chemical reactions with each other to form other larger molecules within the pyrolysis setup. In fact, some of these molecules even go on to form covalently bonded amorphous solids.
  • In most cases, the process of pyrolysis is carried out in the absence of oxygen and water. This is done to make the atmosphere as inert as possible. However, in some cases, a very small quantity of water and oxygen is allowed to enter the pyrolysis setup. This is done in order to facilitate other important processes such as combustion and hydrolysis (which can play a role in altering the types of products that are obtained from the pyrolysis of the feedstock). Furthermore, certain chemical substances may also be mixed with the feedstock in order to obtain specific products from the pyrolysis process. Common examples of such practices can be observed in the steam cracking of crude oil and the production of charcoal from the pyrolysis of firewood.
  • When some organic feedstock is subjected to pyrolysis, the following processes are known to take place.
  • When the temperature of the environment is below 100 degrees Celsius, certain volatile compounds in the feedstock mixture (such as water) undergo evaporation. Furthermore, the components of the feedstock that are sensitive to heat (such as proteins and vitamin C or ascorbic acid) undergo partial changes in their chemical compositions. It is not uncommon for these proteins and vitamins to begin going through the decomposition process at this stage.
  • When the temperature of the environment crosses the 100 degrees Celsius mark, all remaining traces of water that was absorbed by the feedstock are removed. However, the water that is trapped inside hydrated crystals may still require higher temperatures to break free and evaporate. A lot of energy is absorbed by the feedstock at this point in order to convert all the liquid water into water vapour. At this temperature point, the sugars, fats, and waxes that are present in the organic feedstock may undergo melting and separate from the feedstock.
  • In the temperature range that lies between 100 degrees Celsius and 500 degrees Celsius, most of the common organic molecules that are present in the organic feedstock undergo a decomposition reaction and break down into smaller molecules. For sugars, the ideal temperature range under these conditions for a decomposition reaction lies in between 160 degrees Celsius and 180 degrees Celsius. If there is any cellulose present in the feedstock, it will undergo decomposition when the temperature of the environment reaches 350 degrees Celsius. Another common compound that begins to undergo decomposition at such temperatures is lignin – a vital component of wood. However, lignin is known to continue releasing volatile products during pyrolysis processes up to temperatures of 500 degrees Celsius. Other notable products of this decomposition reaction include carbon dioxide, carbon monoxide, and water.
  • Once the combustion of all the carbonaceous residues are finally completed, the product left behind is a powdery substance which is usually referred to as ash. This ash is known to be made up of inorganic materials that are oxidized. These inorganic substances have very high melting points, which is the reason why they retain their solid nature despite the extreme temperatures that they had to endure during the pyrolysis process. It is important to note that in the scenario where certain metals were present in the initial organic feedstock subjected to pyrolysis, then the metals are usually concentrated in the ash in the form of metal oxides or metal carbonates.

Examples of Pyrolysis

Several processes are known to involve pyrolysis. Some common examples of the different types of processes in which pyrolysis can occur are listed below.

  • The process of dry distillation, which involves the heating of solid substances in order to obtain gaseous processes, is known to be a type of pyrolysis of solids. One common application of such processes is in obtaining sulphuric acid from sulfates.
  • The process of destructive distillation is another important application of pyrolysis. In this process, unprocessed material (usually organic in nature) is subjected to large amounts of heat in relatively inert atmospheres to facilitate their breaking down into smaller molecules. The extraction of coke and coal ash from coal is achieved with the help of this technique.
  • The heat-facilitated browning of sugar (commonly known as caramelization) is another important example of pyrolysis.
  • Many other common cooking techniques involve pyrolysis. Notable examples include grilling, frying, toasting, and roasting.
  • Another notable application of pyrolysis is when wood is placed in tar kins and subjected to high temperatures in order to obtain tar.
  • The process of oil refining also exploits the process of pyrolysis. In this industry, heat is employed to create cracks in relatively large hydrocarbons, facilitating their breaking down into smaller hydrocarbons.
  • The natural processes that are involved in the creation of fossil fuels also involve pyrolysis. It can also be noted that the process of catagenesis, which is the process through which buried organic matter is subjected to high temperatures and pressures over extremely long durations of time (to finally convert the organic matter into coal and other fossil fuels) is a specific type of pyrolysis.

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