What is Pyrolysis?

Introduction

Pyrolysis is the process of chemical degradation that involves heating organic material at high temperatures in the absence of oxygen. It differs from other processes like combustion and hydrolysis which do not involve addition of other reagents such as oxygen (combustion) or water (hydrolysis). Pyrolysis produces solids (char), condensable liquids (tar), and permanent gases.

Pyrolysis is considered the first step in the processes of gasification or combustion. The word is derived from the Greek where pyro means "fire", "heat", "fever" and lysis "separating". It is also used in the conversion of natural gas (methane) into hydrogen gas and solid carbon char, recently introduced on an industrial scale. The applications of pyrolysis are it converts biomass into syngas and biochar, waste plastics back into usable oil, or waste into safely disposable substances.

Pyrolysis has been used for turning wood into charcoal since ancient times. The ancient Egyptians used methanol, which they obtained from the pyrolysis of wood, in their embalming process. Pyrolysis was instrumental in the discovery of many chemical substances, such as phosphorus from ammonium sodium hydrogen phosphate in concentrated urine, oxygen from mercuric oxide, and various nitrates.

Pyrolysis of biomass produces three products a liquid, a gas and a solid. Bio-oil can be used as a low-grade diesel oil. Biochar is being promoted for its potential to improve soil properties and fertility as well as sequester carbon. Syngas can be used in place of natural gas or converted with catalysts to ethanol.

Types of Pyrolysis Systems

There are two types of pyrolysis systems.

• Fast Pyrolysis optimizes bio-oil production by increasing the rate of pyrolysis temperature to 1000°C/second. This process yields approximately 60-70% bio-oil, 15-25% biochar, and 10-15% syngas.

• Slow Pyrolysis uses slower heating rates and biochar is the major by-product. In both processes, the system is self-sustaining by harnessing the syngas to drive the reaction.

General Processes and Mechanisms

Pyrolysis generally consists in heating the material above its decomposition temperature, breaking chemical bonds in its molecules. The fragments usually become smaller molecules but may combine to produce residues with larger molecular mass, even amorphous covalent solids. In the presence of oxygen and water, combustion, hydrolysis, or other chemical processes take place besides pyrolysis proper.

The starting material may be heated in a vacuum or in an inert atmosphere to avoid chemical side reactions (combustion or hydrolysis). Pyrolysis in a vacuum also lowers the boiling point of the byproducts, improving their recovery. When organic matter is heated at increasing temperatures in open containers, the following processes generally occur, in successive or overlapping stages.

Below 100°C, volatiles like some water evaporate. Vitamin C and proteins partially change or decompose at this stage.

At 100°C, the remaining water is absorbed or driven off. At slightly higher temperature water trapped in crystal structure of hydrates is driven off. Some solid substances, like fats, waxes, and sugars, may melt and separate.

Between 100-500°C, many common organic molecules break down. Sugars start decomposing at 160-180°C.

At 350°C, cellulose, a major component of wood, paper, and cotton fabrics, decomposes. The decomposition products usually include water, carbon monoxide CO and/or carbon dioxide CO2, as well as many organic compounds. Gases and volatile products leave the sample as smoke. Some volatiles burn creating a visible flame. The non-volatile residues are said to be "charred" or "carbonized” and contain carbon with colors ranging between brown and black.

At 200-300°C, if oxygen has not been excluded, the carbonaceous residue may start to burn with little or no visible flame. Once carbon combustion starts, the temperature rises releasing carbon dioxide and/or monoxide. At this stage, some of the nitrogen remaining in the residue may be oxidized into nitrogen oxides like NO2 and N2O3. Sulfur and other elements like chlorine and arsenic may be oxidized and volatilized at this stage.

Once the combustion of the carbonaceous residue is complete, a powdery or solid mineral residue (ash) is often left behind, consisting of inorganic oxidized materials of a high melting point. Some of the ash may have been left during combustion, entrained by the gases as fly ash or particulate emissions. Metals present in the original matter usually remain in the ash as oxides or carbonates, such as potash. Phosphorus, from materials such as bone, phospholipids, and nucleic acids, usually remains as phosphates.

Occurrence and Uses

Cooking

Caramelization is the pyrolysis of sugars in food. The food goes brown and changes flavor. Pyrolysis has many applications in food preparation.

Waste Management

Pyrolysis is used to treat municipal solid waste and plastic waste. The main advantage is the reduction in the volume of waste. Tire pyrolysis is a well-developed technology.

Thermal Cleaning

Pyrolysis is used for thermal cleaning in an industrial application to remove organic substances such as polymers, plastics and coatings from parts, products or production components like extruder screws, spinnerets, and static mixers.

Coke, Carbon, Charcoals, And Chars

Carbon and carbon-rich materials have desirable properties but are nonvolatile, even at high temperatures. Pyrolysis is the reaction used to coat a preformed substrate with a layer of pyrolytic carbon. Pyrolytic carbon coatings are used in many applications, including artificial heart valves.

Liquid and Gaseous Biofuels

Pyrolysis is the basis of several methods for producing fuel from biomass, i.e., lignocellulosic biomass. Syngas is usually produced by pyrolysis. Fuel bio-oil can also be produced by hydrous pyrolysis.

Methane Pyrolysis for Hydrogen

Methane pyrolysis is an industrial process operating around 1065°C for "turquoise" hydrogen production from methane by removing solid carbon from natural gas.

Ethylene

Pyrolysis is used to produce ethylene, the chemical compound industrially produced on the largest scale.

Semiconductors

Silicon chips are produced by the pyrolysis of silane. Gallium arsenide, another semiconductor, forms upon co-pyrolysis of trimethylgallium and arsine.

Conclusion

In ancient times pyrolysis was used to turn wood into charcoal. In recent times, pyrolysis is being developed as a waste of energy technology to convert biomass and plastic waste into liquid fuels. The pyrolysis products depend on several process parameters such as temperature, catalyst choice, heating rate, carrier gases, retention time, kind of plastics, reactor, and pressure.

Safety is one of the primary concerns during the construction and design of the pyrolytic reactors. Reactors should be designed in such a way that they perform under high temperatures and pressures without rupture. The quality of the bio-oil is also low grade and cannot be used in all applications where fossil fuels are used.