CFD analysis of the combustion process in a boiler of a 160MWe power plant: leakage influence

This paper presents a developed and validated CFD model of a boiler with the objective of studying a presumed defective actual operating condition, that is, air leakage into the furnace. Three cases were assessed, concerning the amount of air leakage across the boiler water seal. The base case considered the actual operational condition of 5% air leakage. Case 1 assumed 10%, and Case 2 simulated a more extreme leakage of 25%. The influence of the air leakage was analyzed with emphasis on temperature profiles, flow, wall heat flux, O-2, CO, NOx, Soot and CO2 formation. Eddy dissipation together with finite rate chemistry models were used to model the combustion process, while the k- approach was considered to predict the turbulence effect. Brazilian pulverized coal type CE3100 was adopted as fuel, burning in atmospheric air. The conservation equations of mass, momentum, energy and chemical species were solved by Finite Volume Method using Ansys CFX commercial software. As main results, it has been observed that air leakage across the water seal has influence over the profiles of temperature and concentration of oxygen and carbon dioxide. Significant amounts of air leakage can induce the plant staff to change the amount of air to be fed through the burners, changing the burning condition, thus impairing energy generation. Although the simulated case concerns a specific operational condition of a given thermal power plant, the results demonstrate the capability of CFD as a decision support tool. It has been found that even a small drop in the flue gas temperature in the combustion chamber due to air leakage in the water seal significantly affects the radiation heat transfer from flue gases to the chamber walls. The overall reduction in average wall heat flux was - 0,3% and - 1,81% of base case for cases 1 and 2, respectively. This situation alters the boiler behavior related to the formation of pollutants, especially NOx, also reduces the burning efficiency, increasing the concentration of CO and CH4 in the exit region.