The pyrolysis is the thermal decomposition of materials at high temperatures under an atmosphere inert. This type of atmosphere does not contain oxygen, and is mainly made up of nitrogen, which ensures that decomposition is only caused by the effect of heat.
In pyrolysis processes, large and complex organic compounds are broken down into much simpler and smaller ones. Some of these compounds evaporate and turn into gases, others turn into viscous, dark-colored liquids, and a good portion of them turn into carbon.
Today, pyrolysis is used as a means to transform different types of waste and solid waste into fuels that can then be used to generate energy.
An example of pyrolysis is what happens to food when it is fried in oil. Oil helps heat food without air touching it, thus allowing pyrolysis to occur. This is what produces the flavor and color in fried foods. In fact, when we say that something was burned, what happened is that the pyrolysis turned it into carbon.
Pyrolysis is a complex process that involves many chemical reactions. This complexity is determined by several factors:
- It depends on the type of substance that is being pyrolyzed. It is not the same to break down sugar than an oil, since the reactions will be different.
- Different pyrolysis reactions begin and end at different temperatures.
- It varies depending on how fast the substance heats up.
- It depends on whether it is a pure substance or a mixture of different types of substances.
- It depends on the humidity of the sample.
To carry out the pyrolysis process, it is necessary to use a reactor. This is nothing more than some kind of closed container (similar to a pressure cooker) that can be heated without air entering.
Regardless of how complicated it may be, pyrolysis of most organic compounds almost always yields the same types of products. These can be grouped as follows:
It is the solid residue from pyrolysis. It is a black solid that is composed mainly of different forms of carbon. This is what remains when our food burns.
It is a viscous liquid and dark in color, almost black. Contains tar, pitch and other liquid organic compounds such as acids, esters, alcohols, etc.
The gases produced during the pyrolysis of materials such as wood, paper and other organic waste are called biogas . It contains a mixture of various gases including carbon monoxide and the gas we use for cooking, among others.
Pyrolysis curves are graphs chemists use to study the pyrolysis process. With them it is possible to determine at what temperature the pyrolysis begins and ends, if heat is absorbed or released and other characteristics of the process.
These curves are obtained through two important techniques called Thermogravimetric Analysis (ATG) and Differential Scanning Calorimetry (CDB), which are complemented by Gas Chromatography (GC), as described below:
Thermogravimetric Analysis (ATG)
This technique consists of weighing a substance as it heats up, and then plotting mass versus temperature.
When pyrolysis occurs and the chemical compounds in a sample break down and evaporate, the sample becomes lighter, as the mass decreases.
Differential Scanning Calorimetry (CDB)
This technique is used to measure how much heat a substance absorbs or releases during the various stages of pyrolysis. If you absorb heat, the process is said to be endothermic , and if you release it, it is said to be exothermic .
Gas chromatography (GC)
In most cases, the above two techniques are used in conjunction with Gas Chromatography. This serves to separate and identify all the products of the pyrolysis.
Types of pyrolysis
Pyrolysis is classified according to the speed with which substances are heated:
Slow pyrolysis or charring
This is the type of pyrolysis that has been used for hundreds of years to produce charcoal from wood. It is a slow process that can take up to 48 hours and heats up to around 500 ° C.
Intermediate or conventional pyrolysis
In this case, heating is faster and pyrolysis is carried out for 5 to 10 minutes. Intermediate pyrolysis is often used to convert waste such as sawdust, food, and even sewage waste into secondary fuels.
It is one in which the samples are quickly heated to 425-600 ° C in just a couple of seconds for pyrolysis to occur, and then the products are quickly cooled with water. It is used to produce as much bio-oil as possible.
Flash or very fast pyrolysis
It is very similar to the previous one, only much faster. In less than half a second the substance is heated to more than 1000 ° C, pyrolysis occurs in a very short time and producing much more bio-oil than in all the previous cases.
As the name suggests, it takes place in a vacuum and lasts between 2 and 30 seconds. The heating rate is medium, the maximum temperature is usually 400 ° C and its main product is bio-oil.
Examples of pyrolysis reactions
- Glucose pyrolysis: Also known as caramelization and occurs at 160 ° C.
- Cellulose pyrolysis: starts at 280 ° C and ends above 500 ° C. At first the cellulose dehydrates and some bonds are broken, then the sugar chain breaks and forms a tarry mixture.
- Pyrolysis of vegetable oils: Vegetable oils and also animal fats that contain triglycerides can be converted to biodiesel through pyrolysis.
- Pyrolysis of used rubbers: degradation begins at 152 ° C and ends at approx. 490 ° C. It involves a total loss of 67% of the original mass of the rubber.
- Plastic pyrolysis: carried out at temperatures between 300 and 900 ° C. Catalysts are almost always used to improve the conversion of the polymer to fuel oils.