Evaluation of the efficiency of a heat recovery steam generator via computational simulations of off-design operation

This work investigates the flow and heat transfer on the gas side of a heat recovery steam generator. During the operation of a power plant, off-design conditions that significantly impact the steam generator efficiency, caused by variations in the gas flow, corrosion and fouling of tube walls, may be encountered. In addition, passages around the heat exchanger modules develop along the equipment lifetime, allowing the bypass of flowing gas into zones of the boiler of low heat transfer effectiveness. These issues are here addressed via computational simulations, using a commercial software application for solving the mass, momentum, and energy conservation equations. The boiler geometry and the boundary conditions for the simulations are taken from a real, typical industrial heat recovery steam generator fed by gas turbines exhaust gases. Flow turbulence is modeled using the k-ϵ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\epsilon}$$\end{document} model, and heat exchanger modules are considered as porous media, through which the exhaust gases flow while supplying heat to the water. The water flow properties are specified from process data. Numerical results for thirteen realistic off-design conditions are compared to those for the boiler normal operation point. It is found that the cases considering deviation of gas to the trapdoors and lateral passages have the greatest impact on the boiler efficiency. Results indicate an efficiency drop from 86% for the base case to 48% for the case with gas deviations. In addition, a reduction in the gas flow or the plugging of the low-pressure economizer tubes may decrease the boiler efficiency by more than 10% points.