ASSESSMENT OF ENGINEERING SOLUTIONS FOR AN OIL-FIRED AIR PREHEATER USING NUMERICAL SIMULATIONS

This paper describes the CFD (Computational Fluid Dynamics) analysis of the combustion chamber of a horizontal oil-fired air preheater. The combustion chamber was originally designed to operate with low excess air at 15%. Upon start-up, it experienced refractory overheating issues on its roof. Following the initial failure, there were questions about the apparent non-uniformity of the shell temperatures with the roof being at higher temperatures. The unknown was whether the "hot-spot" was a combustion related issue or due to refractory failure. The purpose of this project was to determine the root causes of the refractory overheating and develop solutions for the customer. Several cases with different excess air levels were investigated using CFD simulation. The simulations showed that although no flame impingement on the refractory wall was found, the flame shape was short and wide. Since the combustion chamber was operated close to adiabatic condition, the shape of the flame and high overall flue gas temperatures are believed to have contributed to the verheating of the roof refractory. The installations of 8, 12, and 16 bussle pipes around the burner in the firing end were simulated and evaluated based on combustion performance, flame shape, and wall temperature distribution in order to provide a practical solution for the customer. Air supply to the combustion chamber was increased by transferring additional air from a portion of the downstream process. The total air supply was distributed between the main burner and bussle pipes. Several conditions of different air split ratios were also evaluated. Finally, a solution was found that was both satisfactory to the air preheater operation and economical in implementation.