CCPP Stack Temperature Optimization

Combined cycle power plants (CCPPs) are expanding rapidly to meet the growing global demand for electricity. Because they are more efficient than traditional single-stage turbines and solid-fuel combustion plants, their growth is expected to continue, especially with rising demand from AI and data centers.

ACID DEWPOINT MAKEUP

Adjusting the exhaust stack temperature based on the acid dewpoint can boost plant efficiency. The acid dewpoint depends on the sulfur content of the natural gas (and its conversion to SO2), the rate at which SO2 converts to SO3 (influenced by the plant’s NOx reduction technology), and the moisture content of the exhaust gas. Once the sulfur content and conversion rate are set, the only factor that changes regularly is the exhaust gas moisture. Using the Verhoff-Banchero sulfuric acid dewpoint correlation gives a real-time acid dewpoint temperature to use as a control point.

Verhoff-Banchero Sulfuric Acid Dewpoint Correlation

1000/T_ADP,K​=2.276−0.0294ln(P_H2​O​)−0.0858ln(P_SO3​​)+0.0062ln(P_H2​O​)ln(P_SO3​​)

Where:

• $T_{ADP,K}$​ = sulfuric acid dewpoint temperature in Kelvin
• $P_{H_2O}$​ = water vapor partial pressure in mmHg
• $P_{SO_3}$​​ = SO₃ partial pressure in mmHg
• natural logarithm is used.

Then convert:$$T_{ADP,F} = (T_{ADP,K}-273.15)\times\frac{9}{5}+32$$

or:$$T_{ADP,C}=T_{ADP,K}-273.15$$

Installing H₂O’s absolute humidity sensor in the exhaust stack of the CCPP will provide the real-time variable for the equation, allowing the exhaust gas stack temperature to be accurately controlled to the sulfuric acid dewpoint. It’s recommended to include a safety margin of 25°F above the calculated dewpoint to account for cold spots in the HRSG or stack.

Lowering the exhaust temperature as much as possible without risking acidic corrosion lets more energy transfer to the steam system, boosting efficiency. Carefully measuring and controlling stack temperature based on the acid dew point helps prevent tube leaks and avoid forced outages.

The exhaust gas humidity content plays a large role in the acid dewpoint equation. Depending on the time of year, the load (full or partial), duct burner operation and if power augmentation is used, the humidity concentration can vary at CCPP from 8 to 15% volume. For a plant with 0.5 ppmv S and 1% conversion rate, the acid dewpoint can vary from 159 F to 174 F just due to moisture changes. The sensor can also aid in picking up tube leaks.

In this spreadsheet, plants can figure out the stack acid dewpoint based on real-time humidity data from an H2O sensor, along with the constant sulfur content of the gas and a fixed conversion rate. After determining the safe stack temperature range, you can plug in the temperature drop and plant base load to estimate yearly fuel savings and heat rate improvements using the Fuel Savings Calculator.