CFD modelling of the air preheater fired with gas mixture to find the cause of the preheater damage

Cracow University of Technology, Department of Thermal Processes, Air Protection and Waste Utilisation, ul. Warszawska 24, 31-155 Cracow, Poland

^* Corresponding author: tomasz.sobota@pk.edu.pl

Abstract

Since various fuels, such as natural gas, blast furnace gas, coke oven gas, and a mixture of the three, are burned in the boiler, there is a danger of corrosion caused by sulphuric acid. Sulphur dioxide produced after combustion and condensed water vapour from flue gases on cold tube surfaces form sulphuric acid. Sulphuric acid solution flowing down the vertical tubes of the air heater causes corrosion not only of the air heater tubes but also of the horizontal water heater tubes located below. A simulation of the flow and heat exchange on the flue gas and air sides was performed to determine the areas of the air heater where the temperature of the air heater tubes is low and condensation may occur. Based on the measured air and flue gas mass flows, the measured flue gas temperatures behind the heater, and the air temperature at its inlet, the flue gas temperature before the air heater was determined using the ε-NTU (Effectiveness-Number of Transfer Units) method. Based on CFD modelling and experimental research, the areas in the heater where sulphuric acid can form at given outside air temperatures were determined.