What is thermal cracking in oil refining?

Thermal cracking is any refining process that uses heat (and/or catalysts) to break ('crack') large, heavy hydrocarbon molecules in crude oil into smaller, lighter, more valuable molecules. Crude oil contains a wide range of molecular sizes: small molecules (gases, LPG, naphtha) are inherently valuable as transport fuels; large molecules (residual fuel oil, bitumen) have low value. Cracking converts the low-value heavy end into high-value light products. The main cracking processes in modern refineries are: fluid catalytic cracking (FCC) for gasoline production; hydrocracking for diesel/jet fuel production; and delayed coking for maximum residue conversion. Thermal cracking (in the strict historical sense, using only heat without catalysts) is largely obsolete in modern refineries.

What is the difference between FCC and hydrocracking?

FCC (fluid catalytic cracking) and hydrocracking are the two main catalytic cracking processes: (1) FCC — uses zeolite catalysts at 500–550°C and low pressure; optimised for gasoline (petrol) production; feedstock is vacuum gas oil; produces gasoline, LPG, light cycle oil (diesel blendstock), and coke; operates at low hydrogen consumption; characteristic of US refineries optimised for gasoline. (2) Hydrocracking — operates at high pressure (100–200 bar) with hydrogen feed; produces high-quality diesel and jet fuel as primary products, plus naphtha; feedstock is vacuum gas oil or heavy residues; very low sulphur products (ULSD specification); more capital-intensive than FCC; characteristic of European refineries where diesel demand exceeds gasoline. The choice between FCC-heavy and hydrocracking-heavy configurations determines whether a refinery maximises gasoline or diesel yield.

What is delayed coking and what does it produce?

Delayed coking is a severe thermal cracking process that converts the heaviest, lowest-value crude oil residues (vacuum residue, atmospheric residue) into lighter products plus petroleum coke ('petcoke'). In delayed coking, residue is heated to ~500°C and fed into large coke drums where thermal cracking reactions occur over 16–24 hours. Outputs: gas (~5%), naphtha (~10%), light gas oil (~25%), heavy gas oil (~30%), and petroleum coke (~30%) by weight. Coker naphtha and gas oils are fed to hydrotreaters and other units for further upgrading. Petcoke can be sold as fuel (power stations, cement kilns) or as calcined coke for aluminium anode production. Delayed coking is the preferred choice for refineries processing heavy high-sulphur crude oils, particularly in the US Gulf Coast and India (Reliance Jamnagar). It maximises liquid yield from the cheapest possible feedstocks.

Thermal Cracking

The family of refinery processes — fluid catalytic cracking (FCC), hydrocracking, and delayed coking — that convert low-value heavy crude residues into high-value petrol, diesel, and jet fuel by breaking large hydrocarbon molecules into smaller ones.

Definition

Thermal cracking (broadly, "cracking" in modern usage) refers to refinery processes that apply heat, pressure, and/or catalysts to break large, heavy hydrocarbon molecules into smaller, lighter ones with higher commercial value as transport fuels. The term "thermal cracking" technically refers only to heat-driven processes (now largely obsolete in primary refining) but is commonly used to describe all secondary conversion processes.

Crude oil's natural distillation yields a mixture of light fractions (gas, LPG, naphtha, kerosine — collectively ~20–35% of a typical barrel) and heavy fractions (gas oil, vacuum gas oil, residue — ~65–80% of a barrel). Simple distillation refineries leave much of this barrel as low-value fuel oil. Cracking processes upgrade the heavy fraction into valuable light products, dramatically improving refinery economics.

For refinery economics context, see our oil refinery economics guide. For refinery complexity measure, see the Nelson Complexity Index discussed in that guide. For crude oil price benchmarks that feed into cracking economics, see our Brent crude glossary entry.

Major cracking processes compared

Process	Feedstock	Primary products	Conditions	Key feature
Fluid Catalytic Cracking (FCC)	Vacuum gas oil (VGO)	Gasoline (petrol), LPG, light cycle oil	~500–550°C, low pressure, zeolite catalyst	Maximises gasoline; US refinery preference
Hydrocracking	VGO, heavy residues	Diesel, jet fuel, naphtha	~350–420°C, 100–200 bar H₂, zeolite catalyst	Maximises diesel/jet; ULSD quality; European preference
Delayed Coking	Vacuum residue	Naphtha, gas oil, petcoke	~500°C thermal, atmospheric pressure	Maximum residue conversion; produces petcoke byproduct
Visbreaking	Atmospheric/vacuum residue	Partially upgraded fuel oil, some gas	~450–500°C thermal, low severity	Reduces residue viscosity for fuel oil; minimal conversion
Steam Cracking	Naphtha, ethane, LPG	Ethylene, propylene (petrochemicals)	~800–850°C steam dilution	Petrochemical complex, not fuel production

FCC: the gasoline-maximising unit

The FCC unit is the heart of gasoline-focused refineries (particularly in the United States, where gasoline demand historically exceeded diesel). Key aspects:

Uses a circulating bed of powdered zeolite catalyst; coke deposits on catalyst are continuously burned off in a regenerator — hence "fluid" (catalyst circulates like a fluid)
Produces ~45–55% gasoline from VGO feedstock; also significant LPG and alkylate feedstock for octane enhancement
FCC gasoline has relatively high octane but also high sulphur — requires downstream hydrotreating for ULSG (ultra-low sulphur gasoline) specification
An FCC unit adds ~2–3 NCI to a refinery's Nelson Complexity Index

Why the US is gasoline-centric and Europe is diesel-centric
The historical dominance of FCC in the US and hydrocracking in Europe reflects different vehicle fleet compositions: the US favoured large gasoline-powered V8 vehicles; Europe adopted diesel passenger cars from the 1980s onward (partly due to favourable diesel tax rates). This shaped refinery investment decisions: US Gulf Coast refineries built FCC units to maximise gasoline; European refineries invested in hydrocracking to maximise diesel. The shift toward EVs is now reducing both markets' relevance — but the refinery configuration creates a structural inertia that will persist for decades. See our global refining outlook 2027.

Frequently asked questions

What is thermal cracking in oil refining? Thermal cracking (broadly, "cracking") breaks large heavy hydrocarbon molecules in crude oil residues into smaller high-value molecules (petrol, diesel, LPG). Modern cracking processes use catalysts (FCC, hydrocracking) not just heat. Cracking upgrades the ~65–80% of a crude barrel that simple distillation leaves as low-value residue into valuable transport fuels.: Thermal cracking (broadly, "cracking") breaks large heavy hydrocarbon molecules in crude oil residues into smaller high-value molecules (petrol, diesel, LPG). Modern cracking processes use catalysts (FCC, hydrocracking) not just heat. Cracking upgrades the ~65–80% of a crude barrel that simple distillation leaves as low-value residue into valuable transport fuels.
What is the difference between FCC and hydrocracking? FCC (fluid catalytic cracking) operates at ~500–550°C, low pressure, using zeolite catalyst — maximises gasoline (petrol) yield; preferred by US refineries. Hydrocracking operates at ~350–420°C, 100–200 bar hydrogen pressure — maximises diesel and jet fuel at ULSD quality; preferred by European refineries. FCC is lower capital cost; hydrocracking produces higher-quality products but requires more capital and hydrogen.: FCC (fluid catalytic cracking) operates at ~500–550°C, low pressure, using zeolite catalyst — maximises gasoline (petrol) yield; preferred by US refineries. Hydrocracking operates at ~350–420°C, 100–200 bar hydrogen pressure — maximises diesel and jet fuel at ULSD quality; preferred by European refineries. FCC is lower capital cost; hydrocracking produces higher-quality products but requires more capital and hydrogen.
What is delayed coking and what does it produce? Delayed coking converts the heaviest vacuum residue into lighter products by severe thermal cracking at ~500°C in coke drums. Outputs: ~5% gas, ~10% naphtha, ~25% light gas oil, ~30% heavy gas oil, ~30% petroleum coke (petcoke). Petcoke is sold as fuel (power, cement) or for aluminium anodes. Delayed coking enables heavy sour crude refineries (US Gulf Coast, Jamnagar India) to maximise liquid yield from cheap feedstocks.: Delayed coking converts the heaviest vacuum residue into lighter products by severe thermal cracking at ~500°C in coke drums. Outputs: ~5% gas, ~10% naphtha, ~25% light gas oil, ~30% heavy gas oil, ~30% petroleum coke (petcoke). Petcoke is sold as fuel (power, cement) or for aluminium anodes. Delayed coking enables heavy sour crude refineries (US Gulf Coast, Jamnagar India) to maximise liquid yield from cheap feedstocks.