Pyrolysis (; ) is a process involving the separation of covalent bonds in organic matter by thermal decomposition within an inert environment without oxygen.
Applications Pyrolysis is most commonly used in the treatment of organic materials. It is one of the processes involved in the charring of wood. In general, pyrolysis of organic substances produces volatile products and leaves char, a carbon-rich solid residue. Extreme pyrolysis, which leaves mostly carbon as the residue, is called carbonization. Pyrolysis is considered one of the steps in the processes of gasification or combustion. Compared to syngas, pyrolysis gas has a high percentage of heavy tar fractions, which condense at relatively high temperatures, preventing its direct use in gas burners and internal combustion engines.
The process is used heavily in the chemical industry, for example, to produce ethylene, many forms of carbon, and other chemicals from petroleum, coal, and even wood, or to produce coke from coal. It is used also in the conversion of natural gas (primarily methane) into hydrogen gas and solid carbon char, recently introduced on an industrial scale. Aspirational applications of pyrolysis would convert biomass into syngas and biochar, waste plastics back into usable oil, or waste into safely disposable substances.
Terminology Pyrolysis is one of the various types of chemical degradation processes that occur at higher temperatures (above the boiling point of water or other solvents). It differs from other processes like combustion and hydrolysis in that it usually does not involve the addition of other reagents such as oxygen (, in combustion) or water (in hydrolysis). Pyrolysis produces solids (char), condensable liquids (heavy and light oils, and tar), and non-condensable gasses.
Pyrolysis is different from gasification. In the chemical process industry, pyrolysis refers to a partial thermal degradation of carbonaceous materials that takes place in an inert (oxygen free) atmosphere and produces both gases, liquids and solids. The pyrolysis can be extended to full gasification that produces mainly gaseous output, often with the addition of e.g. water steam to gasify residual carbonic solids, see Steam reforming.
Types Specific types of pyrolysis include: Carbonization, the complete pyrolysis of organic matter, which usually leaves a solid residue that consists mostly of elemental carbon. Methane pyrolysis, the direct conversion of methane to hydrogen fuel and separable solid carbon, sometimes using molten metal catalysts. Hydrous pyrolysis, in the presence of superheated water or steam, producing hydrogen and substantial atmospheric carbon dioxide. Dry distillation, as in the original production of sulfuric acid from sulfates. Destructive distillation, as in the manufacture of charcoal, coke and activated carbon. Charcoal burning, the production of charcoal. Tar production by destructive distillation of wood in tar kilns. Caramelization of sugars. High-temperature cooking processes such as roasting, frying, toasting, and grilling. Cracking of heavier hydrocarbons into lighter ones, as in oil refining. Thermal depolymerization, which breaks down plastics and other polymers into monomers and oligomers. Ceramization involving the formation of polymer derived ceramics from preceramic polymers under an inert atmosphere. Catagenesis, the natural conversion of buried organic matter to fossil fuels. Flash vacuum pyrolysis, used in organic synthesis.
Other pyrolysis types come from a different classification that focuses on the pyrolysis operating conditions and heating system used, which have an impact on the yield of the pyrolysis products. Pyrolysis Operating conditions Pyrolysis product yield (wt%)Slow low temperature pyrolysisTemperature: 250–450 °C
Vapor residence time: 10–100 min
Heating rate: 0.1–1 °C/s
Feedstock size: 5–50 mmBio-oil ~30
Biochar~35
Gases~35Intermediate pyrolysisTemperature: 600–800 °C
Vapor residence time: 0.5–20 s
Heating rate: 1.0–10 °C/s
Feedstock size: 1–5 mmBio-oil~50
Biochar~25
Gases~35Fast low temperature pyrolysisTemperature: 250–450°C
Vapor residence time: 0.5–5 s
Heating rate: 10–200 °C/s
Feedstock size: <3 mmBio-oil ~50
Biochar~20
Gases~30Flash pyrolysisTemperature: 800–1000 °C
Vapor residence time: <5 s
Heating rate: >1000 °C/s
Feedstock size: <0.2 mmBio-oil ~75
Biochar~12
Gases~13Hydro pyrolysisTemperature: 350–600 °C
Vapor residence time: >15 s
Heating rate: 10–300 °C/sNot assignedHigh temperature pyrolysisTemperature: 800–1150 °C Vapor residence time: 10–100 min
Heating rate: 0.1–1 °C/sBio-oil ~43 Biochar~22
Gases~45
History
Pyrolysis has been used for turning wood into charcoal since ancient times. The ancient Egyptians used the liquid fraction obtained from the pyrolysis of cedar wood in their embalming process.
The dry distillation of wood remained the major source of methanol into the early 20th century. 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.
Wood distillation components
Primary thermal decomposition: When resinous woods undergo traditional pyrolysis, distillation forms three physical states: Gas: Volatile vapors often used for fuel. Charcoal: The solid carbon residue remaining in the kiln. Liquid distillate: The condensed vapors that form the basis for further refinement.
Separation of the liquid: The liquid distillate naturally separates (or is processed) into two distinct layers: Pyroligneous acid: An aqueous (water-based) layer containing acetic acid and methanol. Crude tar: A thick, dark, oily mixture that contains the heavier organic compounds.
Fractional distillation of crude tar: The crude tar is then further distilled based on density and boiling points to produce: Light oils (turpentine): Oils lighter than water that rise to the top. Pitch or tar: The heavy, nearly solid residue left at the bottom. Heavy oils (creosote): These are the "oils heavier than water." This specific fraction is the medical or industrial creosote, valued for its preservative and antiseptic properties.
+Product by wood pyrolysis temperatureSlow pyrolysisFast pyrolysisGasificationProductCharcoal (solid fuel)Bio-oil (liquid fuel)Synthesis gasTemperature250–450 °C450–600 °C700–1000+ °CLiquid yieldLow (~30%)High (~50–75%)Very Low (<5%)Solid yieldHigh (~35–40%)Low (~15–20%)Minimal (ash only)Gas yieldModerate (~35%)Low (~15–20%)High (~85%+)
General processes and mechanisms
Pyrolysis generally consists of 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 many settings, some amounts of oxygen, water, or other substances may be present, so that combustion, hydrolysis, or other chemical processes may occur besides pyrolysis proper. Sometimes those chemicals are added intentionally, as in the burning of firewood, in the traditional manufacture of charcoal, and in the steam cracking of crude oil.
Conversely, the starting material may be heated in a vacuum or in an inert atmosphere to avoid chemical side reactions (such as 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 about 100 °C, volatiles, including some water, evaporate. Heat-sensitive substances, such as vitamin C and proteins, may partially change or decompose already at this stage. At about 100 °C or slightly higher, any remaining water that is merely absorbed in the material is driven off. This process consumes a lot of energy, so the temperature may stop rising until all water has evaporated. Water trapped in crystal structure of hydrates may come off at somewhat higher temperatures. Some solid substances, like fats, waxes, and sugars, may melt and separate. Between 100 and 500 °C, many common organic molecules break down. Most sugars start decomposing at 160–180 °C. Cellulose, a major component of wood, paper, and cotton fabrics, decomposes at about 350 °C. Lignin, another major wood component, starts decomposing at about 350 °C, but continues releasing volatile products up to 500 °C. The decomposition products usually include water, carbon monoxide and/or carbon dioxide , as well as a large number of organic compounds. Gases and volatile products leave the sample, and some of them may condense again as smoke. Generally, this process also absorbs energy. Some volatiles may ignite and burn, creating a visible flame. The non-volatile residues typically become richer in carbon and form large disordered molecules, with colors ranging between brown and black. At this point the matter is said to have been "charred" or "carbonized". At 200–300 °C, if oxygen has not been excluded, the carbonaceous residue may start to burn, in a highly exothermic reaction, often with no or little visible flame. Once carbon combustion starts, the temperature rises spontaneously, turning the residue into a glowing ember and releasing carbon dioxide and/or monoxide. At this stage, some of the nitrogen still remaining in the residue may be oxidized into nitrogen oxides like and . Sulfur and other elements like chlorine and arsenic may be oxidized and volatilized at this stage. Once combustion of the carbonaceous residue is complete, a powdery or solid mineral residue (ash) is often left behind, consisting of inorganic oxidized materials of high melting point. Some of the ash may have 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.
Safety challenges Because pyrolysis takes place at high temperatures which exceed the autoignition temperature of the produced gases, an explosion risk exists if oxygen is present. Careful temperature control is needed for pyrolysis systems, which can be accomplished with pyrolysis controller. Pyrolysis also produces various toxic gases, such as carbon monoxide. The greatest risk of fire, explosion, and release of toxic gases comes when the system is starting up and shutting down, operating intermittently, or during operational upsets.
Inert gas purging is essential to manage inherent explosion risks. The procedure is not trivial and failure to keep oxygen out has led to accidents.
Occurrence and uses
Clandestine chemistry
Conversion of CBD to THC can be brought about by pyrolysis.
Cooking
Pyrolysis has many applications in food preparation. Caramelization is the pyrolysis of sugars in food (often after the sugars have been produced by the breakdown of polysaccharides). The food goes brown and changes flavor. The distinctive flavors are used in many dishes; for instance, caramelized onion is used in French onion soup. The temperatures needed for caramelization lie above the boiling point of water. Frying oil can easily rise above the boiling point. Putting a lid on the frying pan keeps the water in, re-condensing some and keeping the temperature too cool to brown.
Pyrolysis of food can also be undesirable, as in the charring of burnt food (at temperatures too low for the oxidative combustion of carbon to produce flames and burn the food to ash).
Coke, carbon, charcoals, and chars