Acid dewpoint

The dewpoint for the flue products from gas is a maximum of 60°C and will, in most cases, be lower because of excess air. There is no acid dewpoint because the sulfur content is negligible. It would be thought that condensation would be most unlikely, and this is so for the bulk flue gases. However, the temperature gradient across the flue wall can be such that the skin temperature at the inside wall can be considerably less than the bulk temperature, and condensation will take place. 

Hot water boilers are potentially more susceptible to gas-side corrosion than steam boilers due to the lower temperatures and pressures encountered on low- and medium-temperature hot water boilers. With low-temperature hot water especially, the water-return temperature may drop below the water dewpoint of 50°C, causing vapor in the products of combustion to condense. This, in turn, leads to corrosion if it persists for long periods. The remedy is to ensure that adequate mixing of the return water maintains the water in the shell above 65°C at all times. Also, if medium or heavy fuel oil is to be used for low- or medium-temperature applications it is desirable to keep the heat transfer surfaces above 130°C, this being the approximate acid dewpoint temperature of the combustion gases. It may be seen, therefore, how important it is to match the unit or range of unit sizes to the expected load. 

Although it would be possible to design for an economizer gas exit temperature of 30-50°C above the feedwater temperature, this would result in a temperature too close to the acid dewpoint of the gases. The acid dewpoint is the temperature at which acidic gases begin to condense out of the exhaust gas mixture. This is principally sulfuric acid due to the sulfur contained in the oil or coal. Although the bulk gas temperature may be satisfactory, in practice, the... 

The design of the economizer must be robust enough to survive occasional excursions beneath the acid dewpoint and the effects of the methods used to clean the economizer periodically. This may take the form of rapping equipment to shake off deposits, sootblowing by steam or air and water washing with lances. 

The temperature at which water vapor (steam) condenses. The dewpoint temperature is important in boiler fuel combustion processes, as whenever a metal surface is cooler than flue gas, condensation occurs. If sulfur gases are present and the acid dewpoint (the tem-... 

General References Latest Advances in the Understanding of Acid Dewpoint Corrosion Corrosion and Stress Corrosion Cracking in Combustion Gas... 

Blmodal size distributions of the H-Coals showed little change at reduce air conditions. Emissions of S0a from the coal derived liquids were also quite low (with correspondingly low acid dewpoints) as these chemically cleaned fuels contained only a fraction of the sulfur normally found 1n petroleum liquids (0.15% versus 2.29% typically). 

Reliability A stable temperature control, combined with an excellent heat transfer and a uniform temperature profile (no hot spots) in the fluidized bed, easily achieves an onstream time >99% per year. A specially designed raw-material sparger system allows operation spans of two years without maintenance. Larger heat-transfer area allows a higher steam temperature and pressure in the cooling coils, which improves the safety margin to the critical surface temperature where hydrochloric acid dewpoint corrosion may occur. 

Carbon steel substrates must be sandblasted to a near-white metal finish (SSPC-SP10 or NACE 2) and maintained at least 5°F above the acid dewpoint during installation. Concrete surfaces must be free of any imperfections such as blow holes or honeycombing. Old concrete must be free of oil, grease or chemical contamination. 

Depending on the particular operation, offgases from the incinerator may range from 1100° to 2100°F. Typically, the gases are sent to a conditioning chamber to lower the temperature to 500° to 600°F before they enter a scrubber. While the incinerator is in operation, the gases may remain above the acid dewpoint and corrosion problems will be minimal. But because most incinerators are cyclic operations with frequent periods of idleness, gas condensation in the inlet and outlet scrubber ductwork during shutdowns can cause serious deterioration. 

In some smelters, a reheater is installed to raise the exit gas above the acid dewpoint (generally 350°-450°F) so the acid remains as a vapor and exits with the gas without adversely affecting the ductwork. Even with a reheater, however, during shutdowns the scrubber entry and exit ducts are subject to chemical attack from acid condensates. 

Even though the gases entering a stack may be above the acid dewpoint and safe from a corrosion point of view, if the stack is tall (>200 ft), the temperature may drop below the dewpoint once it reaches the top. This presents a familiar problem to many lining materials. The wet condition at the top of the stack causes lining failure and forces frequent maintenance. With the borosilicate glass block from bottom to top, the stack designer need not try to determine the transition point and then specify different materials for each section of the stack. 

ZareNezhad B (2009) New correlation predicts flue gas sulfuric acid dewpoints. Oil Gas Journal 107(35)... 

Condenser feed gas enters the condenser at 270-290 °C. It is cooled by indirect air cooling through the glass tube walls. Once the gas is cooled below its sulfuric acid dewpoint, liquid acid begins to condense ... 

Homogeneous condensation (c) is a small contributor because the process gas temperature is controlled to always be only slightly below the acid dewpoint temperature. This thermodynamically favors heterogeneous condensation. 

Gases containing much more than 6.5 volume% [S03 + H2S04(g)] cannot be treated by condensation because the acid dewpoint temperature of these gases is quite high, e.g., 270-300 °C. The materials utilized in the WSA condenser are less suitable at these high temperatures. 

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