Pyrolysis is the chemical decomposition of an organic substance by heating in the absence of oxygen. The word is derived from Greek word ‘pyro’ meaning fire and “lysis” meaning decomposition or breaking down into constituent parts. In practice it is not possible to create a completely oxygen free environment and as such a small amount of oxidation will always occur. However, the degree of oxidation of the organic matter is relatively small when compared to combustion where almost complete oxidation of organic matter occurs, and as such, a substantially larger proportion of the carbon in the feedstock remains and is not given off as CO2. However, with pyrolysis much of the C from the feedstock is still not recovered in charcoal form, but converted to either gas or oil.
Simplified depiction of pyrolysis chemistry.
How it occurs?
Pyrolysis occurs spontaneously at high temperatures (generally above approximately 300°C for wood, with the specific temperature varying with material). It occurs in nature when vegetation is exposed to wildfires or comes into contact with lava from volcanic eruptions. At its most extreme, pyrolysis leaves only carbon as the residue and is called carbonization. The high temperatures used in pyrolysis can induce polymerization of the molecules within the feedstock, whereby larger molecules are also produced (including both aromatic and aliphatic compounds), as well as the thermal decomposition of some components of the feedstock into smaller molecules.
The flame provides heat for pyrolysis, and the resulting gases and vapors burn in the luminous flame in a process called flaming combustion. After the flame passes a given point, the char may or may not continue to burn (some matches are chemically treated to prevent the charcoal from smouldering). When the match is extinguished, the remaining wood continues to undergo residual pyrolysis, generating a visible smoke composed of the condensed tar droplets.
The process of pyrolysis transforms organic materials into three different components, being gas, liquid or solid in different proportions depending upon both the feedstock and the pyrolysis conditions used. Gases which are produced are flammable, including methane and other hydrocarbons which can be cooled whereby they condense and form an oil/tar residue which generally contains small amounts of water. The gasses (either condenses or in gaseous form) and liquids can be upgraded and used as a fuel for combustion.
The remaining solid component after pyrolysis is charcoal, referred to as biochar when it is produced with the intention of adding it to soil to improve it. The process of pyrolysis has been adopted by the chemical industry for the production of a range of compounds including charcoal, activated carbon, methanol and syngas, to turn coal into coke as well as producing other chemicals from wood. It is also used for the breaking down, or ‘cracking’ of medium-weight hydrocarbons from oil to produce lighter hydrocarbons such as petrol.
A range of compounds in the natural environment are produced by both anthropogenic and non-anthropogenic pyrolysis. These include compounds released from the incomplete burning of petrol and diesel in internal combustion engines, through to particles produced from wood burned in forest fires, for example. These substances are generally referred to as black carbon in the scientific literature and exist in various forms ranging from small particulate matter found in the atmosphere, through to a range of sizes found in soils and sediments where it makes up a significant part of the organic matter (Schmidt et al., 1999; Preston et al., 2006; Hussain et al., 2008).
Biochar improves soil health and is a carbon “sink” that sequesters carbon and reduces greenhouse gas emissions. Pyrolysis is a unique alternative energy source because it produces heat and power from the burning of woody products while creating biochar with its rich sequestering properties.
The following table shows that different pyrolysis conditions lead to different proportions of each end product (liquid, char or gas). This means that specific pyrolysis conditions can be tailored to each desired outcome. For example, the IEA report (2007) stated that fast pyrolysis was of particular interest as liquids can be stored and transported more easily and at lower cost than solid or gaseous biomass forms.
Mean of post-pyrolysis feedstock residues resulting from different temperatures and residence times (IEA, 2007)
However, with regard to the use of biochar as a soil amendment and for climate change mitigation it is clear that slow pyrolysis, would be preferable, as this maximizes the yield of char, the most stable of the pyrolysis final products.