Condensation of Water Vapor and Acid Mixtures from Exhaust Gases

Boilers, in which condensation takes place, are a very competitive technology in Europe due to energy prices, strict government regulations and a public interest in energy efficiency. They operate at high efficiency by capturing some of the latent heat of condensation, and a large amount of the sensible heat of combustion. When natural gas is combusted, only a very small amount of pollutants are formed. The products of combustion still contain oxides of nitrogen and sulfur as well as carbon dioxide and water vapor. Nitrogen oxides are typical by-products of combustion air, and sulfur is present at very low concentrations as odourant compound added to natural gas. Condensation of these products yields an acidic solution which contains concentrations of nitric and sulfuric acids. Therefore, flue gas condensate becomes increasingly corrosive at surfaces of the condensing heat exchanger after it is concentrated by repeated condensation and evaporation. When the surface temperature is below the dew point of the multicomponent mixture, condensation occurs. Therefore, Vapor-Liquid Equilibrium models such as the Van Laar and Uniquaq models were required in order to determine the dew point of the mixture for a pressure of 0.17 bar. For the binary system HNO3/H2O, the dew point is 56 °C. For the binary system H2SO4/H2O it is 115 °C. In this work, the condensation of nitric acid, sulphuric acid and water vapor in the presence of air on a vertical water-cooled plate has been investigated. A simulation model has been developed and experimentally validated using measurements from a test rig. This simulation model can be applied to a given heat exchanger design in order to perform parametric studies and geometric optimization with the goal of minimizing corrosion. A numerical simulation using the comercial CFD code FLUENT, and a simulation based on empirical correlations using the Engineering Equation Solver EES have been carried out for a 2D vertical water-cooled plate. The numerical model was applied to real 3D geometries including an annular fin heat exchanger and a pin fin heat exchanger. By comparing the numerical simulation with the simulation based on the empirical correlations, the accuracy is very good for almost all range of combustion powers as well as average temperature surfaces. For a combustion power of 8 kW, the deviation between both simulations is about 8 %, whereas for higher combustion powers the deviation decreases until 1-2 %. These discrepancies are due to the way in which the diffusion coefficient has been obtained. In the numerical simulation the diffusion