Thermochemical and exergy assessment of coke burning in an olefin cracking furnace using CFD at varying equivalence ratios

Coke formation in olefin cracking furnaces is an inevitable byproduct that diminishes thermal efficiency and raises operational costs. This study explores the potential of utilizing coke particles as supplemental solid fuel, which not only addresses environmental concerns related to coke disposal but also enhances the furnace's energy recovery capabilities. A comprehensive CFD-based thermochemical and exergy analysis is conducted to assess the combustion behavior of coke particles across three equivalence ratios (ER) of 0.77, 0.87, and 1.0. Initial CHEMKIN simulations are performed to establish parameters such as adiabatic flame temperature, ignition delay, and volumetric heat release for each ratio. Following this, a multiphase CFD model using the Euler–Lagrange framework and an intrinsic particle-combustion model validated by experimental data is employed to simulate the internal flow, heat transfer, and reactions within the furnace. The findings reveal that reducing the ER decreases firebox temperature (781.8 to 773.5 K), CO₂ mole fraction, particle temperature (556.2 to 536.7K), residence time (2.59 to 2.37s), and particle burnout (15.07% to 8.49%), while increasing gas velocity, particle velocity, and particle diameter. Exergy analysis shows that decreasing ER slightly reduces total exergy destruction in the firebox from 2081.73 to 2074.00 kW, although the useful exergy transferred to the coils remains essentially unchanged. This integrated CFD–exergy approach provides insight into the coupled combustion and thermodynamic behavior of decoking coke under different operating conditions.