Thermally cracking, or pyrolysis, of chemical bonds within petroleum, coal and biomass is convenient route to produce additional liquids for use in chemicals and fuels, and/or to produce coke for fuel or the refining of iron ore to produce steel. The sources and conditions for this thermal cracking is controlled by various processes to tune the yields and properties of the products. Eventually thermal cracking will yield a highly aromatic, polymerized and carbonaceous material generally referred to as coke. Many different types of coke can be made to serve different industry needs, each with a different set of requirements.
As petroleum undergoes pyrolysis, covalent bond are broken and intermediate polarity molecules become depleted. As the bridge in solubility between the least polar molecules and most polar molecules is lost, the most polar molecules are forced to aggregate and polymerization reaction dominate to form coke.
During early pyrolysis in coking units, mild pyrolysis such as in visbreaking, and sever processing as in hydrocracking asphaltenes play an important role in sediment and coke formation. As asphaltenes (island-type is only shown for clarity) undergo pyrolysis causes aliphatic side chains to be cleaved from the aromatic core increasing its polarity and aromaticity.