Numerical Simulation of Heavy Fuel Oil Combustion Characteristics and NOx Emissions in Calciner in Cement Industry

The aim of the present study is to numerically investigate the combustion characteristics of Heavy Fuel Oil (HFO) and NOx emissions inside a calciner used in cement industry. The calciner is a furnace placed before the rotary Kiln its main objectives are the reduction of CO₂ emissions and air pollutions while enhancing the cement quality through separating the calcination and clinkering processes. In order to conduct the present investigations the calciner at CEMEX Egypt Cement Company was considered and real dimensions and operating conditions were applied. The combustion model was based on the conserved scalar (mixture fraction) and prescribed Probability Density Function (PDF) approach. The (RNG) k-ε turbulence model has been used. The HFO droplet trajectories were predicted by solving the momentum equations for the droplets using Lagrangian treatment. The radiation heat transfer equation was solved using P1 method. The formation of thermal NO_x from molecular nitrogen was modeled according to the extended Zeldovich mechanism. The effects of varying the burner’s swirl number and viscosity grade on the combustion performance of HFO and the resulting NO_x emissions were considered. The burner’s swirl number influences the mixing rate of air and fuel. A small swirl number ≤ 0.6 is not desired as it elongates the flame; increases flue gases temperatures and increases the NOx emissions inside the calciner. A swirl number ≥ 0.6 is found optimal for good combustion characteristics and NOx emissions concentration. Meanwhile, it was found that the HFO viscosity has a significant effect on the injection velocity and must be considered as a function of temperature during the analysis as this will significantly affects the combustion characteristics.