[Chemistry Class Notes] on What is Pyrolysis? Pdf for Exam

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:

1. Slow Pyrolysis

2. Fast Pyrolysis

3. 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.

• Destructiv
  e 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.