Feasibility of biogas and oxy-fuel combustion in steam cracking furnaces : experimental and computational study

This work evaluates the feasibility of biogas air-fuel combustion and natural gas oxy-fuel in steam cracking furnaces. Four cases, namely air-fuel combustion of pure natural gas, 20% CO2, 40% CO2 diluted natural gas, and oxy-fuel combustion of natural gas are investigated both experimentally and numerically. The John Zink Hamworthy Combustion test furnace, representing a section of a steam cracking furnace, is used for experimental studies. A three-dimensional steady-state CFD model is also developed to simulate the test furnace. The simulation results of the air-fuel combustion scenarios are in good agreement with the experimental data, with the maximum and average relative errors of furnace temperature of 3.86% and 1.78%, respectively. The reduction of flame length with increasing CO2 mole fraction in the fuel is observed in both experiments and simulations. It is shown that CO2 dilution has minor effect on the overall heat flux profile, which is beneficial for retrofitting existing furnaces. On the other hand, the oxy-fuel combustion simulation using default EDC model predicts a significant flame lift-off and incident radiative heat flux shift towards the higher elevations which was not observed in the experiments. This can be mainly attributed to the reduced reaction rate in a CO2 and H2O enriched combustion environment. Adjusting the EDC model parameters helps to achieve better agreement between simulation results and experimental data, while additional lab-scale experiments are essential for further validation of the numerical model. Moreover, it is of particular interest to study the optimal mole fraction of O2 in oxy-fuel combustion scenario

Computational fluid dynamics-based study of a high emissivity coil coating in an industrial steam cracker

To assess the effect of applying a high emissivity coating to the reactor coils in a steam cracking furnace, a complete energy balance was made for two cases based on simulations of the radiant section, reactors, convection section, and transfer line exchanger. A base case with a typical emissivity spectrum for a generic high-alloy steel was compared to a case with an artificially increased emissivity corresponding to a high emissivity coating. At the same cracking severity, coating the radiant coils increases the radiant section efficiency by 0.70% absolute, reduces the required furnace firing rate by 1.73%, and reduces the flue gas bridge wall temperature by 14 K. Minor changes to the convection section layout are required to compensate for the shift in duty to the radiant section: the reactor feed is still fully preheated to the targeted crossover temperature, but the production of high pressure steam is reduced