Condensation of water vapor and sulfuric acid in boiler flue gas

Condensation of vapors in flue gas is a complicated phenomenon since heat and mass transfer of water vapor and various acids simultaneously occur in the presence of non-condensable gases such as nitrogen and oxygen. Design of a condenser depends on the knowledge and understanding of the heat and mass transfer processes. A computer program for analytical modeling of water and sulfuric acid condensation in a flue gas condensing heat exchanger was developed using MATLAB. Governing equations based on mass and energy balances for water vapor condensation were derived to predict variables such as flue gas exit temperature, cooling water outlet temperature, mole fraction and condensation rates. The equations were solved using an iterative solution technique with calculations of heat and mass transfer coefficients and physical properties. An experimental study was carried out in order to yield data for validation of modeling results.;To evaluate the performance of a condensing heat exchanger, the term 'condensation efficiency' was defined as the ratio of total mass flow rate of condensed water from the heat exchangers to the incoming mass flow rate of water vapor on the flue gas inlet. Parametric studies for both modeling and experiments were performed to investigate the effects of parameters such as flue gas flow rate, cooling water flow rate, inlet cooling water temperature and tube configurations (bare and finned tubes) on condensation efficiency. Predicted results of water vapor condensation were compared with experimental data for model validation, and this showed agreement between experimental data and predictions to within a few percent. The most important parameters affecting performance of the condensing heat exchangers was the ratio of cooling water to flue gas flow rates, since this determines how much heat the cooling water can absorb.;Modeling results for prediction of sulfuric acid vapor concentration in the flue gas were compared with measured data obtained by the controlled condensation method. An analytical model for oil-firing showed two trends - steep reduction within high temperature HX and smooth reduction within lower temperature HX, which is in agreement with experimental data. An analytical modeling for coal-firing showed a larger discrepancy between predicted and measured sulfuric acid concentration in flue gas than the oil-firing analysis.;A full scale modeling developed for large scale condensing heat exchangers for application to a 625 MWe power plant in the range of m˙ c/m˙g,in from 0.5 to 1.0 with inlet cooling water temperature of 90 °F and heat transfer surface area of 50,000 ft 2 showed the condensation efficiency from 10.2 to 28.4 wt%. The investigation for effects of design parameters showed that the condensation efficiency was improved up to 15 %. The condensing heat exchanger had the largest effectiveness with a given heat transfer area when the flue gas flow rate was equal to the cooling water flow rate.