Types of Pyrolysis and Their Applications
It is the chemical decomposition of organic (carbon-based) materials through the application of heat. It is a thermochemical treatment, which can be applied to any organic product. In this treatment process, materials are exposed to a very high temperature, and in the absence of oxygen, it goes through a chemical and physical separation into different molecules. The rate of pyrolysis increases with an increase in temperature.
The most important point to be noted is that the process of pyrolysis brings about a chemical change in the substance subjected to it (the chemical compositions of the initial reactant and the final product are different). The term ‘pyrolysis’ is derived from a Greek word that means “fire separating”.
Generally, the substances which are subjected to pyrolysis undergo a chemical decomposition process and are broken down into multiple product compounds. The thermal decomposition process leads to the formation of new compounds. This allows receiving products with a different, often more superior character than original residue. In industrial applications, the temperatures used are often 430 °C or even higher, whereas in small-scale operations the temperature may be much lower.
The process is commonly used to convert organic materials into a solid residue containing ash and carbon, small quantities of liquid, and gases. On the other hand, extreme pyrolysis yields carbon as the residue, and the process is termed carbonization. Other high-temperature processes including hydrolysis and combustion, pyrolysis process does not involve reacting with water, oxygen, or any other reagents. However it is not possible to achieve an oxygen-free environment every time, So a small amount of oxidation process always occurs in any pyrolysis system. Pyrolysis is also considered to be the initial step for other related processes like combustion and gasification. The pyrolysis of an organic substance can produce multiple products which are volatile and will also leave behind a solid residue that is highly enriched with carbon.
Eg: The charring of wood (or the incomplete combustion of wood) resulting in the formation of charcoal involves the process of pyrolysis. The well-known products created with the help of pyrolysis are a form of charcoal also called biochar, which is created by heating wood, and coke (which can be used as an industrial fuel and a heat shield), created by heating of coal. The pyrolysis process produces condensable liquids (called tar) and non-condensable gases.
In this topic we have discussed pyrolysis definition, Now we will discuss the uses and types of Pyrolysis.
Uses of Pyrolysis
Utilization of renewable resources.
Self-sustaining energy.
Conversion of low energy in biomass into high energy density liquid fuels, potential to produce chemicals from bio-based resources.
It is a simple, inexpensive technology that can help in processing a wide variety of feedstocks.
It reduces waste going to landfills and greenhouse gas emissions.
It reduces the risk of water pollution.
It can reduce the country’s dependence on other energy resources by generating energy from domestic resources.
Waste management done with the help of pyrolysis technology is inexpensive compared to disposal in landfills.
The construction of a pyrolysis power plant is a fast process.
It can create new jobs for low-income people based on the quantity of waste generated in the region, which in turn provides public health benefits through waste clean-up.
Pyrolysis is one of the sustainable solutions that are economically profitable on very large scales and can minimize environmental problems especially in terms of waste minimization.
Types of Pyrolysis
There are generally three types of Pyrolysis:
Slow Pyrolysis
Fast Pyrolysis
Flash Pyrolysis
Slow Pyrolysis: It is characterized by lengthy solids and gas residence times, low temperatures, and slow biomass heating rates. It is used to modify the solid material and minimize the oil produced. On the other hand, fast pyrolysis and ultra-fast (flash) pyrolysis maximize the gases and oil produced.
Temperature: Med-high (400-500 °C)
Residence time: Long (5-30 min)
Fast Pyrolysis: It is a rapid thermal decomposition of carbon-containing materials in the absence of oxygen in moderate to high heating rates. It is the most common method used in research and in practical use. The major product is bio-oil. Pyrolysis is an endothermic process. Char is accumulated in very large quantities and is to be removed frequently.
Temperature: Med-high (400-650 °C)
Residence time: Long (0.5-2 s)
Flash Pyrolysis: It is a very rapid thermal decomposition pyrolysis process, the heating rate is also very high. The main products are gases and bio-oil. Flash pyrolysis produces a very less quantity of gas and tar as compared to slow pyrolysis.
Temperature: high (700-1000 °C)
Residence time: Long (less than 0.5 sec)
The feedstock subjected to pyrolysis is exposed to temperatures above its decomposition temperature. At this point, the chemical bonds holding the molecules of the feedstock together are broken down. This process results in the fragmentation of the molecules of the feedstock into smaller molecules.
The process of pyrolysis is carried out in the absence of oxygen and water in some cases, a very small quantity of water and oxygen is allowed to enter the pyrolysis setup. This is done to facilitate other important processes such as combustion and hydrolysis, Certain chemical substances may also be mixed with the feedstock in order to obtain specific products from the pyrolysis process.
Applications of Pyrolysis:
The heat-facilitated browning of sugar (also known as caramelization) is an example of the pyrolysis process.
Destructive distillation is an important application of pyrolysis. In this process, unprocessed material (organic products) are subjected to large amounts of heat in relatively inert atmospheres to facilitate them breaking down into smaller molecules. The extraction of coke and coal ash from coal is achieved with the help of this technique.
Many common cooking techniques involve pyrolysis like grilling, frying, toasting, and roasting.
It is widely used in the chemical industry to produce methanol, activated carbon, charcoal, and other substances from wood.
Synthetic gas produced by the conversion of waste materials using the pyrolysis process can be used in gas or steam turbines to produce electricity.
A mixture of stone, ceramics, soil, and glass obtained from pyrolytic waste can be used as a building material or for filling landfill cover liners.
It is also used in carbon-14 dating and mass spectrometry.
Wood placed in tar kins and subjected to high temperatures in order to obtain tar is also an example of the pyrolysis process.
This process is also used in several cooking procedures like grilling, frying, and baking.
FAQs on What Is Pyrolysis in Chemistry?
1. What is pyrolysis in chemistry? Provide an example.
Pyrolysis is the thermal decomposition of organic materials at high temperatures (typically above 400°C) in the complete or near-complete absence of oxygen. Instead of burning, the heat breaks down complex organic molecules into simpler, smaller molecules. The term itself comes from Greek, meaning "fire separating". A common example is the production of charcoal, where wood is heated in a low-oxygen environment, resulting in charcoal (a solid), pyrolysis oil (a liquid), and syngas (gaseous products).
2. What is the main difference between pyrolysis and combustion?
The primary difference between pyrolysis and combustion lies in the presence of oxygen and the final products.
Pyrolysis occurs in the absence or near-absence of oxygen. It is a thermochemical decomposition process that breaks down materials into valuable solids (biochar), liquids (bio-oil), and gases (syngas).
Combustion is a chemical reaction that occurs in the presence of sufficient oxygen. It is an exothermic process (releases heat and light) that results in the complete oxidation of the material, primarily producing carbon dioxide (CO₂) and water (H₂O).
3. What are the different types of pyrolysis?
Pyrolysis is generally classified based on the heating rate and the time the material spends at the reaction temperature. The main types are:
Slow Pyrolysis: Characterised by very slow heating rates and long residence times (hours to days). This process is optimised to maximise the yield of biochar (solid charcoal).
Fast Pyrolysis: Involves very high heating rates and short residence times (typically less than 2 seconds). This method is used to maximise the production of liquid bio-oil.
Flash Pyrolysis: An extremely rapid version of fast pyrolysis with even higher heating rates and shorter residence times, also aimed at maximising liquid and gas yields.
4. What are the main products obtained from pyrolysis?
The pyrolysis of organic materials, such as biomass or plastic, yields three main types of products. The proportion of each depends on the specific process conditions:
Biochar (Solid): A stable, carbon-rich solid residue similar to charcoal. It can be used as a fuel, for carbon sequestration, or as a soil amendment.
Bio-oil/Pyrolysis Oil (Liquid): A dark, viscous liquid composed of a complex mixture of oxygenated organic compounds. It can be upgraded into biofuels or other valuable chemicals.
Syngas/Pyrolysis Gas (Gas): A mixture of non-condensable gases, primarily hydrogen (H₂), carbon monoxide (CO), carbon dioxide (CO₂), and methane (CH₄). This gas can be burned to provide energy for the pyrolysis process itself.
5. Why is pyrolysis considered an important process in modern industry and environmental science?
Pyrolysis is a critically important process because it provides a sustainable pathway to convert waste materials into valuable resources, addressing both energy and environmental challenges. Its importance stems from:
Waste Valorisation: It can effectively process diverse waste streams like agricultural residue, non-recyclable plastics, and old tyres, converting them from landfill problems into valuable commodities.
Renewable Energy Production: It produces biofuels (bio-oil and syngas) and solid fuel (biochar) from biomass, offering a carbon-neutral alternative to fossil fuels.
Environmental Benefits: The production of biochar allows for long-term carbon sequestration in soil, helping to mitigate climate change while also improving soil fertility and water retention.
6. Is there a single chemical equation to represent the pyrolysis reaction?
No, there is no single chemical equation for pyrolysis. This is a common misconception among students. Pyrolysis involves the simultaneous thermal breakdown of thousands of different complex organic molecules (like cellulose and lignin in wood) through numerous parallel and sequential reactions. Therefore, it is impossible to represent the entire process with one balanced chemical equation. Instead, chemists describe it in terms of product distribution and yields (e.g., % biochar, % bio-oil, % syngas) which are dependent on process conditions.
7. How is the concept of pyrolysis applied in the CBSE Class 11 and 12 Chemistry syllabus?
In the CBSE syllabus for the 2025-26 session, the principle of pyrolysis is a key concept in organic chemistry, particularly in:
Class 11, Chapter: Hydrocarbons: Pyrolysis is discussed under the specific name 'cracking'. It is defined as the process of breaking down large hydrocarbon molecules of higher alkanes into smaller, more useful lower alkanes and alkenes by heating in the absence of air. An example is the cracking of hexane to produce ethene, propene, and other smaller hydrocarbons.
Class 12, Applied Chemistry: The principles of pyrolysis are fundamental to understanding biomass conversion, plastic recycling, and the production of synthetic fuels, which are often discussed as applications of chemical principles in various units.
8. What key factors influence the products of a pyrolysis reaction?
The final yield and composition of pyrolysis products (biochar, bio-oil, syngas) are not random; they are heavily influenced by several controllable factors:
Temperature: This is the most critical factor. Lower temperatures (around 400-500°C) combined with slow heating favour the production of solid biochar. Conversely, higher temperatures (above 500°C) and rapid heating favour the production of liquid bio-oil and syngas.
Heating Rate: How quickly the material is heated. A slow heating rate allows for secondary reactions that form more char. A very fast heating rate quickly vaporises the material, maximising liquid oil yield before it can be converted to char.
Feedstock Composition: The type of organic material (e.g., wood, plastic, agricultural waste) drastically changes the outcome due to its unique chemical structure (e.g., high lignin content in wood vs. long polymer chains in plastic).
