Non-nulling protocols for fast, accurate, 3-D velocity measurements in stacks

Area of Science:

• Environmental Engineering
• Fluid Dynamics
• Measurement Science

Background:

• Continuous Emissions Monitoring Systems (CEMS) in coal-fired power plants rely on accurate flue gas flow rate measurements for greenhouse gas (GHG) emissions reporting.
• Current methods using S-type pitot probes for velocity profiling have limitations in accuracy due to flow angle variations (pitch and yaw) when not nulling the probe.
• Existing probes can be susceptible to clogging from ash and water droplets in harsh stack environments.

Purpose of the Study:

• To develop and validate a novel 5-hole hemispherical differential-pressure probe for fast, accurate, 3D velocity measurements in coal-fired power plant stacks.
• To provide a measurement protocol traceable to international standards for rigorous emissions regulation.
• To overcome the limitations of existing non-nulling pitot probes regarding flow angle dependency and potential clogging.

Main Methods:

• Utilized novel, five-hole, hemispherical, differential-pressure probes optimized for non-nulling measurements.
• Developed a wind tunnel calibration procedure for 130 strategic (V_a, α, β) values across relevant stack conditions (Reynolds number range: 7,600–45,000).
• Integrated differential pressures for 5 seconds to determine axial velocity, pitch, yaw angles, and static pressure.

Main Results:

• Achieved axial velocity (V_a) measurements with expanded relative uncertainty ≤ 2% (V_a range: 4.5–27 m/s).
• Determined pitch (α) and yaw (β) angles with expanded uncertainties of 1° (α range: ±20°, β range: ±40°).
• Measured static pressure (p_s) with expanded relative uncertainty ≤ 0.1%.

Conclusions:

• The developed 5-hole hemispherical probe offers fast, accurate, and traceable 3D velocity measurements in stacks, independent of flow angles.
• The probe's design resists clogging from ash and water droplets, enhancing reliability in power plant environments.
• This technology provides a more rigorous basis for calibrating CEMS and improving the accuracy of reported GHG emissions from coal-fired power plants.