How is the dew point temperature of flue gas from burning materials calculated?

In the design and operation of boilers, flue gas dew point is an indicator that clearly shows whether corrosion will occur, and to some extent it also reflects the degree of corrosion. For boilers burning high-sulfur coal, flue gas dew point is one of the important factors affecting technical and economic indicators, as well as a factor influencing the efficiency of dust collectors. One of the key technologies for flue gas desulfurization in circulating fluidized beds is to strictly control the flue gas temperature at the inlet of the fluidized bed. The closer this temperature is to the dew point, the better the desulfurization efficiency. However, this temperature must also be maintained above the dew point; otherwise, condensation will occur, leading to equipment blockage and corrosion, and preventing the system from functioning properly. The most commonly used empirical formula for calculating the dew point of flue gas at present is: tsld = B (SarZS) 1/3 / 4396afhAarAS] + tld (1), where tsld represents the acid dew point of the flue gas, in ºC ； tld -- Water vapor dew point of the flue gas, °C ； B--a constant related to the excess air coefficient; when at=1.4~15, B=208 ； At at=1.2, B=195 ； SarZS, AarAS--Base-based conversion values (value per 1000 kJ) for sulfur and ash content, % ； afh--the proportion of fly ash in the total ash content. SO3 has a significant impact on the dew point; even with just a trace amount of sulfuric acid vapor present, the dew point rises above 373 K. The effect of SO2 on the dew point is much smaller; within a fairly wide range of concentrations, the fluctuation in the dew point does not exceed 1K. Near the dew point temperature, SO3 is almost completely dissolved in water vapor within the flue gas, and the partial pressure of sulfuric acid vapor, PH2SO4, is equal to the partial pressure of SO3, PSO3 ； Although the partial pressure of SO2, PSO2, is much higher than that of PH2SO4, very little SO2 dissolves in water vapor within the flue gas to form sulfurous acid vapor; thus, the partial pressure of sulfurous acid vapor, PH2SO3, is close to 0, and this prevents an increase in the dew point temperature of the flue gas. It is analyzed based on the dissociation equilibrium constants K298 and KT of SO2 in air and water. At K298, its equilibrium expression is: SO2 + H2O ⇌ H2SO3; the equilibrium constant for this reaction is K298 = 1.3×10-2, with DH298 = 16.3. For the reaction HSO3- ⇌ H+ + SO3²-, the equilibrium constant at K298 is 6.4×10-8, with DH298 = 11.9. At other temperatures, the equilibrium constant is given by KT = K298·exp(DH298/T – 298). Since KT remains very small, the solubility of SO2 in water is extremely low, and therefore it does not have a significant effect on the dew point temperature of flue gases. If attempting to control the flue gas dew point, it is necessary to control the formation of SO3, that is, the conversion of SO2 to SO3, in order to regulate the conversion rate of SO2. The amount of SO3 generated by flue gas is related not only to the sulfur content of the coal, but also to the flame temperature, the amount of combustion air, the properties and quantity of fly ash, as well as the catalytic effect of the boiler’s heating surfaces. The following measures can be taken to control the generation of SO3: (1) Fuel desulfurization, to reduce the sulfur content in the fuel to a very low level. (2) Improve the combustion method to reduce the SO3 content in flue gases. (3) The additive is added and reacts with SO3 in the flue gas. The flue gas dew point is closely related to the SO3 content in the flue gas, and the formation of SO3 is closely linked to the combustion equipment and conditions. Therefore, there is no quantitative relationship between the flue gas dew point and the sulfur content of the fuel; hence, using an equivalent sulfur content in equation (1) to account for the effect of sulfur on the flue gas dew point is insufficient from a mechanistic perspective, and it does not adequately correspond to various combustion devices, combustion methods, and combustion conditions in practical applications. Based on the dew point values at different sulfuric acid vapor partial pressures (Table-1), the relationship between the difference between the sulfuric acid dew point and the water vapor dew point and the sulfuric acid vapor partial pressure was fitted as follows: tsld – tld = 0.1855 + 0.5651 PH2O – 0.0009382 PH2SO4 (4). There is a relationship between the sulfuric acid vapor partial pressure and the SO3 partial pressure given by: PH2SO4 = X PH2O (5), where X represents the fraction of SO3 that converts into sulfuric acid vapor. X is related to temperature; it is defined as: X = PH2SO4/PSO3 × 100% (6). Assuming that all the sulfur in the fuel is converted into SO2, and a portion of this further converts into SO3, then the sum of the partial pressures of SO2 and SO3 in the exhaust gases is given by PSO2 + PSO3 = Sar/100 · (22.4/32 · 1/Vy) (Pa) (7). Here, Sar represents the sulfur content on a received basis in the fuel, while Vy denotes the amount of exhaust gas generated per unit mass of fuel [in m³/kg (fuel)]. The generation rate of SO3 is defined as: R = PSO3/(PSO2 + PSO3) · 100% (8). In all the equations above, PSO3 represents the partial pressure of SO3 before sulfuric acid vapor is generated. Table-1: Dew point at different sulfuric acid vapor partial pressures. Sulfuric acid vapor partial pressure PH2SO4, Water vapor partial pressure PH2O: 5000, 8300, 245000; 0334, 364; 1040, 4870; 5063, 6887; 1008, 691105; 2001, 116121130. Table-2: Rate of SO3 formation. Fuel sulfur content (%), Equipment temperature conditions, Excess air percentage in flue gas, Rate of SO3 formation R (volume ratio of SO3 to SO2). Economizer, Air preheater, Flue gas temperature, Water temperature, Air temperature: Inlet ºC, Outlet ºC; Inlet ºC, Outlet ºC; Inlet ºC, Outlet ºC, Outlet ºC. Coal: 4.45, 3051, 881491, 991881, 43104680.0294; Coal: 3.18, 3071, 729317, 717211679570.02; Coal: 2.10, 34321611, 6166216112148490.0123; Coal powder: 4.67, 568177962181, 77110138360.00755. The influence of fly ash in the flue gas on the flue gas dew point has two aspects. Components in fly ash such as Fe2O3 and V2O5 act as catalysts for the conversion of SO2 to SO3, whereas unburned carbon particles, oxides of calcium and magnesium, as well as Fe3O4, can absorb or neutralize SO2 in the flue gas. The quantity, composition, and morphology of fly ash are related to the fuel and combustion method; the absorption and neutralization effects of fly ash are greater than its catalytic effect. That is, the presence of fly ash in the flue gas reduces the partial pressure of SO3 in it, thereby increasing the dew point of the flue gas. By modifying equation (4), we obtain: tsld-tld = +0.5651PH2SO4 – 0.0009382(PH2SO4)²]/Kafh·AarAS. In equation (9), K is a constant related to fly ash. By making slight modifications to the above equation, the final formula for calculating the dew point of flue gas is obtained: tsld + 0.5651·PH2SO4 – 0.0009382·(PH2SO4)²] / (Kafh·AarAS + tld) (10)

This post was last edited by dongjiang1001 on 2011-9-15 09:25. Great expert, I admire you greatly; thank you so much.

This post was last edited and replied to by ssun*nhong on 2011-7-9 at 12:35. Reply to 2# zgj2405: Calculate the water content, find the vapor pressure, and refer to the saturated steam table to determine the corresponding temperature

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Expert: May I ask how to calculate the dew point of flue gas from gas boilers? Note: Both sulfur content and ash content are very low.

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