Difference Between Combustion and Pyrolysis

Main Difference – Combustion vs Pyrolysis

Combustion and pyrolysis are thermochemical reactions. Combustion is an exothermic chemical reaction; the combustion of a fuel can form light and heat as forms of energy. Pyrolysis is a decomposition reaction; here, organic materials are decomposed when provided with heat. Although both these processes are thermochemical reactions, there are differences between the two processes. The main difference between combustion and pyrolysis is that combustion is done under the presence of oxygen whereas pyrolysis is done under the absence (or near absence) of oxygen.

Key Areas Covered

1. What is Combustion
      – Definition, Complete and Incomplete Combustion, Uses
2. What is Pyrolysis
      – Definition, Process, and Uses
3. What is the Difference Between Combustion and Pyrolysis
      – Comparison of Key Differences

Key Terms: Burning, Combustion, Complete Combustion, Decomposition, Exothermic Reaction, Incomplete Combustion, Pyrolysis, Thermochemical Reactions

What is Combustion

Combustion is a chemical reaction that involves the reaction between substances and oxygen, producing light and heat as forms of energy. This process is also called burning. Heat and light are the energy forms given as the outcome of the reaction. Light energy appears as a flame. However, most of the energy is released as heat energy.

Combustion reaction can be found in two types as complete combustion and incomplete combustion.

Complete Combustion

Complete combustion occurs when an excess of oxygen is present for the reaction. This gives a limited number of products. When fuel is burnt, it gives carbon dioxide and water as the end products. When an element is burnt, it gives the most stable oxide of that element.

Incomplete Combustion

Incomplete combustion occurs in the presence of a low amount of oxygen. This gives more products at the end. When fuels undergo incomplete combustion, it gives carbon monoxide along with carbon dioxide and water. Sometimes, even unburnt carbon element is also produced. Here, carbon is released as soot.

Figure 1: Fire is a Result of Combustion

There are many uses of combustion reaction. One major use is the production of energy by burning fuels. This is important in industries, automobiles, etc. Producing fire is another use. Fire is used for many needs such as cooking. Sometimes, combustion of elements is used to identify elements. Different elements give different colored flames when combusted. By observing these flames, we can identify different elements.

What is Pyrolysis

Pyrolysis is the chemical decomposition of organic materials in the absence of oxygen. This process requires the application of heat. Therefore, the rate of pyrolysis can be increased by increasing the temperature.

Pyrolysis typically occurs at temperatures above 430^oC. However, since it is very difficult to obtain an oxygen-free atmosphere for pyrolysis, it is done under conditions with near absence of oxygen. Pyrolysis gives end products in gaseous phase, liquid phase, and solid phase. Most substances are converted into gases. Sometimes a little amount of liquid is formed. This liquid is called tar. The solid residue typically includes charcoal and biochar.

Figure 2: Products of Pyrolysis

The process of pyrolysis transforms organic material into their gaseous components, a solid residue of carbon and ash, and a liquid called pyrolytic oil. In pyrolysis, there are two methods that are used to remove any contaminants from a substance. They are destruction and removal. In destruction, contaminants are broken down into smaller compounds. But in the process of removal, rather than breaking compounds, contaminants are separated.

Pyrolysis is useful in industries for the production of charcoal, activated carbon, methanol, etc. Apart from that, pyrolysis treats and destroys semi-volatile organic compounds, fuels, and pesticides in soil. This process can also be used to treat organic waste coming out from factories.

Difference Between Combustion and Pyrolysis

Definition

Combustion: Combustion is a chemical reaction that involves the reaction between substances and oxygen producing light and heat as forms of energy.

Pyrolysis: Pyrolysis is the chemical decomposition of organic materials in the absence of oxygen.

Atmosphere

Combustion: Combustion is done under the presence of oxygen in the atmosphere.

Pyrolysis: Pyrolysis is done under the absence of oxygen.

End Products

Combustion: Combustion produces gaseous end products.

Pyrolysis: Pyrolysis produces gaseous components along with trace amounts of liquid and solid residues.

Types

Combustion: Combustion may occur as complete combustion or incomplete combustion.

Pyrolysis: Pyrolysis can be done in a vacuum or in an atmosphere with near absence of oxygen.

Conclusion

Combustion and pyrolysis are thermochemical reactions. These reactions are used in industries and daily needs. These processes are different from each other in several factors. However, the main difference between combustion and pyrolysis is that combustion is done under the presence of oxygen whereas pyrolysis is done under absence (or near absence) of oxygen.

References:

1. Kondratiev, Victor Nikolaevich. “Combustion.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 22 Apr. 2016, Available here.
2. Boslaugh, Sarah E. “Pyrolysis.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 31 Mar. 2016, Available here.

Image Courtesy:

1.”9334″ (CC0) via PEXELS
2. “Pyrolysis” By Mintrick (talk) –  based on File:Pyrolysis.gif (CC BY-SA 3.0) via Commons Wikimedia