Avaliação de desempenho dos trocadores de calor na operação de caldeira de leito fluidizado

Abstract

Heat exchangers represent one of the most widely used equipment types in modern industry, utilized across sectors like chemical, petrochemical, and energy. They also find extensive applications in everyday appliances such as refrigerators, air conditioners, and automotive cooling systems. Heat exchangers come in many types, ranging from the smallest and simplest, like concentric ones, to larger and more robust versions like shell and tube exchangers. This study aims to evaluate a set of heat exchangers that will be part of a fluidized bed boiler. In this setup, the porous medium will consist of sand, and the fuel used will be a combination of sludge and compressed natural gas (CNG). The fluidization and combustion media will be introduced into the boiler through helical conveyors. One end of these conveyors will be in contact with the combustion zone, while the other end will be connected to a drive motor reducer, which must maintain an operating temperature below 90°C to avoid damage. One of the evaluated heat exchangers is of the concentric type, where its outer part covers the helical conveyor and the sand or sludge. This design aims to remove heat in the region close to the motor reducer, preventing equipment from overheating. The methodology involves using equations and analyzing heat transfers within the system formed by the conveyor, sand, heat exchanger, and motor reducer to analytically determine the temperature to which the helical conveyor drive will be subjected. Four heat exchangers with this configuration will undergo evaluation. The other heat exchanger under investigation is of the coil-type, situated inside the fluidized bed boiler. Pressurized water passes through this coil and needs to be heated to 130°C. The evaluation will focus on calculating its extraction efficiency and determining the percentage of combustion heat that this coil-type heat exchanger can absorb. For this study, we estimated losses through the boiler wall and exhaust gases, conducting the analysis for both a smaller laboratory-scale boiler and a larger one. The operating parameters for the larger boiler were estimated using experimental data obtained from the smaller boiler. The results indicate that the maximum temperature of the helical conveyor in contact with the motor reducer will be approximately 86°C, ensuring the safety of the equipment. As for the analyzed coil-type heat exchanger, its combustion heat removal efficiency was close to 84%, while for the pilot-scale fluidized bed boiler, it exceeded 90%.