Approach Temperature plays a key role in the performance and reliability of circulating dry scrubber systems. Operating too far above the optimal range can limit hydrated lime efficiency and reduce SO₂ capture, while operating too close to saturation can increase the risk of downstream corrosion in ductwork and equipment. Despite its importance, obtaining a reliable, real-time measurement of Approach Temperature in harsh process environments has historically been a challenge.
Approach Temperature is often referenced as the approach to adiabatic saturation temperature (AAST), a value that is difficult to measure directly and impractical for use in real-time control. Prior research has shown a strong correlation between AAST and acid dewpoint temperature, which can be measured more reliably in ductwork. As a result, acid dewpoint has become a practical surrogate for understanding and controlling Approach Temperature.
More recent research suggests that the traditional definition of dewpoint may underestimate the temperature at which acid condensation begins. Instead of occurring only at equilibrium saturation, condensation may initiate at higher temperatures, at the first onset of liquid formation. This distinction is important when evaluating corrosion risk and deposit formation in downstream equipment.
To better understand this behavior in a real operating system, Ohio Lumex applied the Ei4200 Dewpoint Monitor to directly measure in-situ acid dewpoint in a circulating dry scrubber. The Ei4200 uses a high-sensitivity detection method that identifies condensation by monitoring small changes in electrical current as acid vapor condenses on a glass sensor surface. This approach allows detection of condensation at much lower levels than earlier-generation instruments.
During a multi-week field demonstration, the instrument operated reliably in the duct between the reactor vessel and baghouse. The sensor remained clean, and the data collected was consistent with expected process behavior. Analysis showed that the reported dewpoint value is influenced by the instrument’s sensitivity setting, but further evaluation of the sensor temperature versus delta current relationship made it possible to project the actual onset temperature where condensation begins.
The data also demonstrated how Approach Temperature is affected by key operating variables, including SO₂ load, water injection rate, and hydrated lime feed rate. Under relatively constant load conditions, increases in water flow were associated with lower dewpoint values. As SO₂ loading increased, dewpoint generally increased when other variables remained stable. These trends highlight the dynamic nature of Approach Temperature and the importance of understanding how process conditions interact.
Overall, the results indicate that direct, high-sensitivity dewpoint measurement can provide valuable insight into true condensation behavior within a circulating dry scrubber. By better identifying the onset of acid condensation, operators may be able to run closer to optimal conditions, improving SO₂ removal efficiency while minimizing corrosion risk and making more effective use of hydrated lime.
For those interested in the full technical discussion, including methodology, data analysis, figures, and supporting references, the complete paper is available here: Direct Measurement of Approach Temperature in a Circulating Dry Scrubber
