Avaliação de eficiência e desempenho de uma caldeira de recuperação de calor via simulações computacionais fora do ponto de projeto

Abstract

A heat recovery boiler without supplementary burning, inserted in the context of electricity generation, was simulated in this thesis. Some cases have been elaborated that will be compared with the base case, which represents the operational conditions of the boiler. For description of the reported problem, were considered the equations of continuity, conservation of amount of movement and energy, as well as the turbulence model equations 𝑘 − 𝜀, the heat transfer model 𝜀 − 𝑁𝑈𝑇 and the porous media approach to represent the set of tubes. The program used to solve the set of equations was Ansys Fluent, version 18.0. The conditions studied outside the design point were: 1) Load variation (reduction and increase in exhaust gas flow); 2) Fouling on the walls of the evaporator tubes; 3) Corrosion in tubes (partial and total plugging); 4) Restriction of gas flow through lateral passages; 5) Gas flow in the boiler’s inactive areas, that is, regions where there is not termal exchange between gas and water. The cases that considered the deviation of the gas to the trapdoors (upper and lower openings) and to the lateral passages had a greater impact on the efficiency of the steam generator. The results showed a reduction in the efficiency of the steam generator from 93%, when there is no gas passing through the inactive areas or through the lateral passages, to 48% when the gas deviations are present. In addition to these results, the reduction in exhaust gas flow at the boiler inlet reduces efficiency from 86% (base case) to 71% and the total plugging of the low pressure economizer reduces efficiency to 73%.