Simulação numérica do pré-aquecimento de ar em calcinadores de alumina através de técnicas de fluidodinâmica computacional (CFD).

Abstract

The application of computational fluid dynamic (CFD) method was used to simulate the pre-heating of air in a combustion system found in alumina fluidized bed calciners. When talking about energy efficiency and product quality, as well as production and maintenance costs, the calciner combustion system corresponds to the most important system in the alumina refining process. Among the negative impacts related to this system, this work highlights the high temperatures found in the area of the air pre-heater dome of the calciner, represented by the primary combustion zone, which indicates a premature stress of this equipments refractory material, and thus leaving the calciners integrity at risk. The main objective of this work was to describe the fluid dynamic behavior of the air in the pre-heater through computational fluid dynamics (CFD) method, in order to predict the location of the premature failure presented, as well as to identify possible causes of the problem. Modeling and numerical simulation of the process were carried out using the CFX software. The geometry of the pre-heater was generated according to the technical design of the available equipment. The use of an unstructured mesh was adopted, being it more refined in the input burner area. The operating conditions were adjusted according to the conditions found in industry. For simulation purposes, CH4 was inputted as fuel. The conservation equations were solved numerically through the finite volume method, using the generated mesh. The results were evaluated by analyzing the profiles of temperature and air speed vectors in the pre-heater. According to those profiles visualization, it was possible to identify the formation of a vortex close to the side air inlet, promoting a whirlpool moving in a preferential course in the direction of the vessel wall of the pre-heater. In region of the vortex a high concentration of energy can be observed, justified by the greatest mixture between the air's oxygen and the fuel, which explains the excessive increase in temperature in this region. The area where this high concentration of energy hits the wall is consistent to the region where the premature failure of the refractory occurs. A better homogenization of the mixture of air with fuel, avoiding preferential pathways, may be the solution to the problem.