The temperature of flue gas should be controlled to be higher than the acid dew point to avoid fouling and corrosion of heating surfaces in the second pass of boilers. Decreasing the temperature of flue gas in boilers is one of the most effective ways to improve thermal efficiency, enhance electrostatic precipitator efficiency, and decrease water consumption of the desulfurization tower. Accurate knowledge in predicting acid dew point is required. Thus, based on previous research achievements, a measuring device is designed based on the principle of conductive dew point meter, and the acid dew points under different flue gas conditions are measured in this work. An improved thermodynamic correlation formula between the acid dew point and its influencing factors is also derived based on previous research achievements. Then, a semiempirical prediction model that considers the functions of H 2 SO 4 and H 2 O is proposed. The proposed prediction model is validated by field test data, experimental data, and prediction models reported in previous studies. INTRODUCTIONGiven the presence of sulfur, nitrogen, carbon, and other elements in fuel, flue gas typically contains acidic gases, such as SO 2 , SO 3 , NO, NO 2 , CO, and CO 2 . These acidic gases react with water vapor in the flue gas and then generate acid vapors under low-temperature conditions. 1−4 When the outer wall of the heating surface in the second pass of boilers is cooled down to acid dew point, which is dependent on the corresponding saturation temperature of acid vapor partial pressure in flue gas, the acid vapors in flue gas will condense and form acidic droplets on the outer wall of heating surfaces. The condensed acidic droplets will cause serious acidic corrosion to metal heating surfaces. 5,6 More importantly, the condensed acidic droplets will further react with the flaking rust and the fly ash in the second pass of boilers. Fouling will occur under such conditions. Once fouling occupies the flow channel, the burden of induced draft fan will increase significantly. Not only will the content of O 2 in boilers be reduced, the preheated temperature of air will also not be enough. Thus, combustion in boilers will be seriously deteriorated. Moreover, the flow rate of flue gas is increased by the occupied fouling in the flow channel. Thus, the abrasion of the heating surfaces in the second pass of boilers will become severe. Therefore, considering the safety of the power generation unit, the temperature of flue gas must be controlled and maintained higher than the acid dew point of flue gas. 6−22 However, thermal efficiency of boilers could be improved significantly by decreasing the temperature of flue gas. 23−25 According to Li et al., 23 the electrostatic precipitator efficiency could be improved by decreasing the temperature of flue gas because the specific resistance of the fly ash is positively correlated with temperature under low-temperature conditions. Jiang et al. 25 found that the consumption of water in desulfurization tower can be also decre...

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Various damages, including corrosion, fouling, and blue plumes, can be caused by sulfur trioxide (SO3) in the flue gas of a coal-fired boiler on boiler equipment and the atmospheric environment. The increasing demand for energy conservation and emission reduction requires accurate knowledge on predicting and controlling the SO3 concentration. Accurate knowledge on the effect of the factors that influence SO3 in flue gas is necessary as well. Therefore, in this study, various SO3 concentrations under different flue gas conditions are measured with measuring devices built based on controlled condensation and S balance methods. To further study the effect of the factors that influence SO3 in flue gas, an improved SO2/O2/H2O/CO2/CO/NO kinetic mechanism is built based on previously developed mechanisms and validated with previous experimental data. SO3 concentrations under the established flue gas conditions and with increasing residence time are numerically calculated. The SO3 concentration in flue gas is mainly dependent upon SO2, O2, and H2O concentrations as well as the temperature and residence time. The SO3 concentration undergoes three stages of sharp increase, slow increase, and gradual decrease as the residence time increases. In addition, the SO3 concentration increases significantly as CO and NO emerge in the flue gas. However, SO3 is constant as CO2 emerges.

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