Thermal cracking is cheap but the degree of conversion is small. Sometimes catalytic elements escape from FCCUs into the residue they produce, typically aluminum, and are what the bunker trade refers to as catalytic fines . Aluminum is a contaminant, hence the specification of a maximum aluminum content in many bunker orders. Although effective and environmentally friendly, these units are expensive and require the input of hydrogen, which is costly. While short residue from atmospheric distillation and visbreaker residue are used to blend fuel oils, including bunker fuel, on their own they are too heavy for fuel purposes.
Short residue has the consistency of molasses.
Visbreaker residue is only slightly more pleasant. To produce fuel oil, lighter products need to be blended in. Light cycle oil has the consistency of diesel fuel and paraffin. Heavy cycle oil has a consistency similar to golden syrup. When combined, these various refinery materials make fuel oil.