Boiler Flue Gas Acid Dew Point: Affecting Equipment Corrosion And Environmental Emissions

Flue gas acid dew point is the temperature at which any gaseous acid in the flue gas begins to condense into liquid acid under a given pressure.

The fuels used in boilers, such as coal, heavy oil, and natural gas, all contain S. During the combustion process, S and O₂ generate SO₂. A small amount of SO₂ is converted into SO₃ under the catalysis of catalysts such as Fe₂O₃ and V₂O₅. Under normal circumstances, the volume content of SO₃ in boiler flue gas is 1×10-6～50×10-6, and water vapor is about 10%. When the flue gas temperature is below 200℃, SO₃ and water vapor are completely combined to form H₂SO₄ vapor. A small amount of H₂SO₄ vapor will significantly increase the acid dew point. When the surface temperature of the boiler heat exchange equipment is lower than the flue gas acid dew point, H₂SO₄ vapor will condense on the surface into a sulfuric acid solution with a concentration of about 80%, which adheres to the surface of the heat exchanger and causes acid corrosion. Typical equipment susceptible to corrosion include air preheaters, induced draft fans, and flue walls.

Corrosion of such equipment can lead to system downtime and expensive maintenance, directly affecting the plant's production efficiency and increasing maintenance costs. In addition, sulfuric acid can also lead to the emission of acid fumes, which can harm the local environment or surrounding facilities. Advanced burner manufacturing and combustion control technology have greatly improved the thermal efficiency of boilers. The increase in efficiency is mainly due to the reduction of exhaust gas temperatures; however, lower flue gas temperatures increase the risk of acid content formation.

The SLDA2100 is a H2SO4 acid dew point temperature detector that can directly, accurately and reliably measure flue gas acid dew point.

The SLDA2100 acidity dew point transmitter uses high-voltage electric heating sensor technology to monitor the acid dew point of fuel gas. When the probe is inserted into the pipe or flue, the probe tip temperature will increase due to the high flue gas temperature. Then slowly open the air valve to cool the probe tip. Due to the increase in electrode current passing through the acidity dew point transmitter, condensation will appear on the probe surface. When the temperature drops, condensation at the probe tip will increase. If the cooling air flow rate is reduced, the probe temperature will increase and the acid will evaporate.

At the acid dew point temperature, the deposition rate and evaporation rate tend to be consistent, and the condensation thickness and current readings are constant. When the readings remain unchanged, the temperature is the flue gas acid dew point under this condition.