Flue gas dew point formula

Based on the SO3 content, consult a chart for the water vapor partial pressure. The usual values are often empirical values.

Generally, we set the temperature of flue gas emissions controlled by our equipment to be above 150 degrees, while the inlet temperature of the economizer is kept at least at 140 degrees

Reply to 2# dongliangsir: Could you give me that table?

The cost of burning natural gas is generally 140℃

This post was last edited by dongliangsir on 2010-7-1 at 17:09. Reply to 4#: *awenjun – It’s mentioned in books; I don’t have a scanner. Many books on heating furnaces cover this topic. Additionally, I think that diagram is quite basic and not necessarily accurate. If precise measurements are required, instruments might be considered. A dew point meter works by inserting a probe (about φ50 in diameter) into the convection chamber or air preheater of the heating furnace. Coolant air is passed through the dew point meter, and the instrument automatically adjusts the amount of coolant air used. As a result, the temperature at the probe gradually decreases. When dew forms at the probe, the current flowing through it changes, and the instrument automatically records this change along with the temperature at which it occurs. This corresponding value is the dew point temperature. In petrochemical plants, if the fuel gas is well desulfurized, the dew point is usually around 90 degrees; otherwise, it may be higher. For heating furnaces used to burn low-sulfur gas, the dew point is generally around 97–100 degrees, while for those burning oil (or residue oil), it’s around 130–140 degrees. Of course, this is closely related to the sulfur content.

There is no problem if the flue gas outlet temperature reaches over 180 degrees

180 degrees is no problem for the dew point, but the thermal efficiency of the furnace becomes very low. We burn a mixture of fuel oil (with 1.3% sulfur content) and desulfurized gas, and the flue gas temperature is kept at 160–180°C; if we burned only gas, it would be around 140°C.

The flue gas temperature should generally be kept 20 degrees above the dew point temperature to prevent dew point corrosion. Calculation of Acid Dew Point Temperature for Waste Heat Boilers by Xin Quzhen, Kang Yengji, and Wang Donghao. Abstract: The techniques and principles for calculating the acid dew point temperature of waste heat boilers, as well as methods for determining the safe operating pressure of such boilers. Keywords: waste heat boiler; acid dew point temperature; low-temperature corrosion. Introduction: The flue gases emitted from many industrial furnaces often contain corrosive gases and substances, such as sulfur oxides, vanadium oxides, and sulfate complexes. These substances cause severe corrosion to the waste heat boiler; in serious cases, it can damage the boiler within a very short period of time. When flue gas entering the waste heat boiler contains sulfur dioxide, a portion of it is converted into sulfur trioxide, which then combines with water vapor in the flue gas to form sulfuric acid vapor. This raises the dew point temperature of the flue gas significantly, allowing sulfuric acid solution to condense on low-temperature metal surfaces. This solution reacts with alkaline ash as well as with the metal itself, thereby causing corrosion. It is called low-temperature corrosion because it often occurs on the low-temperature heating surfaces of boilers. Low-temperature corrosion is characterized by uniform corrosion, which gradually reduces the wall thickness of the tubes until they rupture, posing a serious threat to the safe operation of waste heat boilers. To effectively prevent the occurrence of low-temperature corrosion, it is necessary to calculate the acid dew point temperature of sulfuric acid vapor in order to determine the wall temperature of the heat-exchanging surfaces in the waste heat boiler as well as the operating pressure of the boiler. 　　Apart from sulfur trioxide, gases such as chlorine and sulfur dioxide can also cause low-temperature corrosion, but this occurs below the dew point of the water vapor in the flue gas. Since the dew point temperature is very low (usually between 30°C and 60°C), it need not be considered in waste heat boilers. 1 Generation of sulfur trioxide and determination of conversion rate The mechanism for the generation of sulfur trioxide in flue gas is extremely complex. It is generally believed that part of it is generated during the manufacturing process, while part is produced in flue ducts and waste heat boilers. 　　During the industrial production process, sulfur trioxide is primarily formed due to the action of atomic oxygen, which is mainly generated in combustion reactions. For example: CO + O2 → CO2 + O; H + O2 → OH + O. These atomic oxygen species are very reactive and can easily convert sulfur dioxide into sulfur trioxide. Furthermore, under the high-temperature radiation of the furnace, oxygen molecules, carbon dioxide, and metal oxides also cause some of the atomic oxygen to be released, thereby converting sulfur dioxide into sulfur trioxide. 　　The equilibrium curve for the conversion of sulfur dioxide to sulfur trioxide at a constant pressure is shown in Figure 1. It can be seen from this graph that low temperatures are favorable for the conversion to sulfur trioxide. At high temperatures above 850°C, almost no sulfur trioxide is produced. At the same temperature, an increase in pressure increases the conversion to sulfur trioxide. But in reality, it becomes more complex due to the effects of atomic oxygen, sulfur trioxide catalysts, and fly ash. 　　During the industrial production process, when sulfates are present in the materials being processed, they also decompose directly to form sulfur trioxide. In the pyrometallurgical processing of heavy non-ferrous metal sulfide ores, sulfur trioxide in the high-temperature flue gases generated typically accounts for 2% to 6% of sulfur dioxide. http://china-heatpipe.net/up_files/image/2007-1-28/56692531.gif Figure 1: Equilibrium state of SO2 and SO3. In waste heat boilers, under the action of a catalyst, some of the sulfur dioxide in the flue gas is converted into sulfur trioxide. Common catalysts in the flue gas of waste heat boilers include iron oxide trioxide, aluminum oxide trioxide, vanadium pentoxide, silicon dioxide, and soot. As can be seen from Figure 2, the catalytic effect generally occurs only between 500°C and 800°C. When ash accumulates in the waste heat boiler, the surface temperature of the ash increases as the ash thickness increases, which facilitates the conversion of sulfur dioxide into sulfur trioxide. Among them, iron oxide and soot have the most significant impact on the conversion. Due to the differences in raw materials and fuels used in industrial furnaces, as well as variations in the manufacturing processes, the conversion rate of sulfur dioxide to sulfur trioxide in the flue gases also varies. For chemical processes, the conversion rate is generally between 3.2% and 8.7%, while in heavy non-ferrous metallurgy it is usually between 6% and 10%. To ensure the safety of waste heat boilers, a conversion rate of 10% is recommended when calculating the acid dew point temperature of flue gases. http://china-heatpipe.net/up_files/image/2007-1-28/56692532.gif Figure 2 Relationship between the conversion of SO2 to SO3 under the action of various catalytic substances and temperature; X—Fe2O3 ； O—Dust ； □—SiO2 ； △—Al2O3 2　Calculation of the acid dew point temperature of sulfuric acid vapor　The flue gas contains not only sulfur trioxide but also water vapor, and these interact to produce sulfuric acid vapor. If the tube wall temperature falls below a certain value, sulfuric acid vapor will condense on the tube wall, causing corrosion. This value is known as the acid dew point temperature of sulfuric acid vapor. The acid dew point temperature of sulfuric acid vapor mainly depends on the contents of sulfur trioxide and water vapor in the flue gas, and can generally be determined using the following method. 　　The mass concentration of sulfuric acid in the flue gas is calculated using the following formula: C = 98 × VSO3 / (80 × VSO3 + 18 × VH2O). Where: (1) VSO3 = K × VSO2. In this formula, VSO3 represents the volume fraction of SO3 in the flue gas, expressed as a percentage ； VH2O—Volume fraction of H2O in flue gas, % ； VSO2—Volume fraction of SO2 in flue gas, % ； K—Conversion rate of SO2 to SO3 in flue gas, % ； C—Mass concentration of sulfuric acid in the flue gas, %. 　　The sum of the partial pressures of water vapor and sulfur trioxide in the flue gas is calculated using the following formula: PH2O + SO3 = (B – P/13.6) × (VSO3 + VH2O) / 100 (3) Where: B is the atmospheric pressure at the location where the boiler is installed, in Pa ； P—Negative pressure of flue gas at the boiler inlet, Pa ； PH2O+SO3—The sum of the partial pressures of water vapor and sulfur trioxide in the flue gas, in Pa. 　　The acid dew point temperature can be found from Figure 3 based on the values of C and PH2O+SO3. http://china-heatpipe.net/up_files/image/2007-1-28/56692533.gif Figure 3 Relationship between sulfuric acid concentration and dew point in the vapor phase 3 Example: Composition of flue gas from a roasting furnace: SO2: 9.075% ； CO2: 0.45% ； H2O: 10% ； O2: 5.48% ； N2: 75% ； Calculate the acid dew point temperature and the operating pressure of the boiler. It is known that the negative pressure of the flue gas at the boiler inlet is –30 Pa. Solution: Let the conversion rate of sulfur dioxide to sulfur trioxide be K = 10%. Then, VSO3 = K × VSO2 = 0.1 × 9.07 = 0.907 (%). The mass concentration of sulfuric acid in the flue gas is calculated using formula (1): C = 98 × VSO3 / (80 × VSO3 + 18 × VH2O) = 98 × 0.907 / (80 × 0.907 + 18 × 10) = 35.2 (%). The sum of the partial pressures of water vapor and sulfur trioxide in the flue gas is calculated using formula (2): PH2O + SO3 = 9.8 × (b – Pr/13.6) × (VSO3 + VH2O) / 100 = 9.8 × (760 – 30/13.6) × (0.907 + 10) / 100 = 810 Pa. Using C = 35.2% and PH2O + SO3 = 810 Pa, the acid dew point temperature can be determined from Figure 1 as Tld = 214°C. By referring to the saturated steam table at Tld = 214°C, it can be concluded that the minimum operating pressure for the boiler should be 2.0 MPa. 4 Conclusion Since there are many factors that affect low-temperature corrosion, for safety reasons, in both domestic and international contexts, for waste heat boilers subject to low-temperature corrosion, the operating pressure is generally set at above 2.5 MPa, taking into account the steam requirements of users as well as the thermal parameters required by the waste heat generator sets. Author profile: Xin Quzhen, born in 1961, is an assistant engineer who has been engaged in the design and development of industrial boilers. She has participated in the design and development of CPC boilers and *-type boilers. Affiliation: Xin Quzhen, Kang Yingxi, Industrial Boiler Development Center of Ha Boiler Co., Ltd ； Wang Donghao, Harbin Third Power Plant

Reply to 9# DragonLee: Could you send me another copy of the chart? It seems that Figure 3 is missing the pressure data

Figure 3 in this document does not show the voltage division, so it is impossible to obtain the data from the figure