Solutions to corrosion problems in the waste heat recovery system of heating furnaces

As long as the flue gas temperature is kept above 160 degrees, dew point corrosion will not occur

Since the flue gas temperature affects the thermal efficiency of the heating furnace, it is necessary to find a balance between thermal efficiency and equipment lifespan. The dew point temperature of the flue gas depends on the sulfur content in the fuel and the amount of water vapor in the flue gas. The metal temperature, on the other hand, is related to the temperature of the medium inside the pipes, the amount of ash accumulation outside the pipes, as well as the flow conditions and temperature distribution. If heat pipe technology is used, it is recommended to employ a load-regulating system using water as the medium in the low-temperature section. Additionally, enamel-coated pipes should be used in the low-temperature section to prevent corrosion caused by low dew points. My rather extreme view is to use plain pipes or pipes with a low fin ratio, as the heat transfer efficiency drops significantly when finned pipes become clogged with ash. It seems that the flue gas temperature has little to do with various devices; it is determined by the configuration of energy recovery systems and the properties of the fuel. There is empirical data showing that the flue gas temperature is 30 degrees higher than the lowest metal temperature

An air preheater with light tubes can be installed at the end; depending on the load, the amount of combustion air also changes accordingly. When determining the temperature of the exhaust smoke, it is essential to consider whether the available energy from the smoke is being utilized effectively; in other words, the heat utilization efficiency in the convection section should be improved first, before focusing on corrosion prevention. Otherwise, an excessive focus on corrosion prevention will result in a significant loss of energy.

We usually keep the temperature at 160–165°C. The management has asked us to lower it further, but I haven’t dared to do so – mainly because there is no guarantee regarding the hydrogen sulfide level in the fuel gas; it can be very low, below 10 ppm, and that’s fine, but sometimes it rises to very high levels, exceeding 10,000 ppm. During this year’s maintenance, we will check it by using heat pipes before making any decisions.

The flue gas temperature of the boilers (gas-oil hybrid combustion) in our plant is controlled at 135–160°C, and heat pipes are used for air preheating; no pitting corrosion has been observed during previous maintenance activities. Controlling the flue gas temperature should be considered from the perspective of overall economic benefits.

In power plants, AL-6XN is commonly used as a lining; C276 is chosen for areas where greater resistance is required.

I have a question regarding the use of AL-6XN as a lining material. Could you please provide some information on the properties of AL-6XN?

The flue gas from boilers burning high-sulfur fuels often contains large amounts of SO2 and SO3, meaning that the flue gas contains sulfuric acid gases. In the low-temperature areas of the boiler (such as economizers, air preheaters, dust collectors, flues, chimneys, etc.), when the temperature is below the dew point of sulfuric acid, sulfuric acid condenses, resulting in what is known as \"sulfuric acid dew point corrosion.\" This type of corrosion is actually a form of corrosion caused by dilute sulfuric acid. In recent years, the newly developed ND steel (09CrCuSb) in our country exhibits excellent resistance to sulfuric acid dew point corrosion. ND steel is a new type of low-alloy structural steel; compared with other steel grades such as low-carbon steel, Cortert, and CRIA, it possesses excellent corrosion resistance and mechanical properties. The test results show that the corrosion resistance of ND steel in aqueous solutions such as sulfuric acid, hydrochloric acid, and sodium chloride is higher than that of carbon steel. The most prominent feature among them is its strong resistance to sulfuric acid dew point corrosion ； The mechanical properties of ND steel remain stable from room temperature to 500°C. And it has good weldability. ND steel is generally used in the manufacture of economizers, heat exchangers, air preheaters, etc. Since 1990, it has been widely applied in industries such as petrochemicals and power generation.

Enameled heat pipes can also be used… but they are extremely expensive and prone to damage

For the new project, the required exhaust temperature is 120 degrees. The pilot furnace used by Sinopec at its upstream processing facility seems to use enamel-coated heat pipes – what’s the performance like? I heard it has just been overhauled; please let me know if you have any information on that.

Set the flue gas temperature control at a higher level, or add an air preheater at the rear

ND steel is useful as it is resistant to low-temperature corrosion