In a Fluid Catalytic Cracking Unit (FCCU), cracking reactions deposit 4% to 5% carbonaceous coke on the zeolite catalyst, deactivating its acidic catalytic sites. The spent catalyst circulates to the regenerator, where high-pressure combustion air burns off the coke at 650°C to 740°C. This regenerates catalytic activity and releases massive exothermic heat, which is carried by the hot regenerated catalyst back into the reactor riser to vaporize fresh feed and drive the endothermic cracking reactions.

In Complete Combustion mode, excess air and combustion promoters (platinum-based CO promoters) oxidize virtually all carbon completely to CO₂ inside the dense bed (CO < 500 ppm, CO₂/CO > 50). In Partial Combustion mode, air is restricted so that carbon partially oxidizes to CO (CO₂/CO ratio ~ 1.0–1.5), operating at a cooler bed temperature (~670°C vs 720°C). The CO-rich flue gas is then routed to an external CO boiler or incinerator to generate high-pressure utility steam.

Coke consists of ~90–93 wt% carbon, 6–9 wt% hydrogen, and minor sulfur and nitrogen. The combustion of hydrogen releases enormous heat (120 MJ/kg vs 32.8 MJ/kg for C to CO₂) and generates steam (H₂O vapor). At regenerator bed temperatures exceeding 720°C, high partial pressures of steam cause severe hydrothermal deactivation, stripping structural aluminum from the zeolite crystal framework and permanently destroying catalyst unit cell size and cracking activity.

Afterburning occurs when unburned carbon monoxide (CO) mixes with residual or breakthrough oxygen in the upper dilute phase or cyclone separators. Because the dilute phase contains very little catalyst to absorb the heat, the exothermic reaction CO + ½O₂ → CO₂ causes gas temperatures to skyrocket within seconds to over 850°C–900°C, warping cyclone barrels, melting internal instrumentation, and causing structural failure of the vessel head.

Delta coke is the weight percentage of coke on spent catalyst minus coke on regenerated catalyst: ΔCoke = C_spent - C_regen (typically 0.7 to 1.1 wt%). It determines the heat generation per ton of circulating catalyst. If delta coke is too low, the regenerator runs cold, requiring supplemental torch oil firing. If delta coke is too high (e.g. from processing heavy resid feedstocks), the regenerator overheats, exceeding the metallurgical limit of the vessel and catalyst, requiring catalyst coolers or reduced throughput.