NIST Develops 3D Measurement Device for Coal Plant Smokestack Emissions Audits

A device developed at the National Institute of Standards and Technology (NIST) enables three-dimensional (3D) stack flow measurements at coal-fired power plants. Compared to existing technologies, NIST’s design improves the accuracy and turnaround time of auditing smokestack emissions. 

 

A coal-fired power station. Image used courtesy of Pixabay/by Denny Franzkowiak

 

Emissions at the U.S.’s 200-plus coal plants are audited annually by independent testers. Smokestacks are lined with monitors to measure the continuous concentration and flow rate of flue gas emissions, which include carbon dioxide, nitrogen oxides, sulfur dioxide, and mercury. By law, sensors must be calibrated for accuracy during each audit using small portable probes known as pitot tubes. 

NIST’s pitot tube design offers a faster, more accurate method for measuring smokestack flow. Auditors can insert the probes horizontally into the smokestack to gauge the flow velocity vector across several areas. The hollow cylinders feature five holes (pressure ports) with a large diameter to prevent interference from water droplets or particulates. NIST says they’re more accurate and don’t require manual adjustments by auditors, thus speeding up the measurement process. 

With the design patented and the protocols published, NIST is looking to take the technology to its next phase: standardized use. The American Society of Mechanical Engineers is drafting a new stack flow measurement standard for the Environmental Protection Agency (EPA) to adopt and recommend to auditors. 

The improved pitot tubes come after the EPA proposed carbon emission caps at coal- and gas-fired power plants earlier this year. 

 

NIST researchers inserted four probes into a smokestack to measure the accuracy of their five-hole pitot tube design. Image used courtesy of NIST 

 

Overcoming Limitations in Existing Probes

Most auditors measure smokestack emissions with S-probes, featuring two ports facing opposite directions. Technicians can calculate gas flow speeds by looking at the pressure differences between the two holes. However, these devices only support one- or two-dimensional measurements and often produce erroneous data when eddies and swirls cause gas flows to evade the testing point. 

By contrast, 3D probes produce more reliable data but come with another challenge, as their small ports often get clogged with debris. This means testers must manually rotate the probe for each measurement to point in the right direction, adding time to the process. These issues have led audit technicians to opt for S-probes, which are easier to use but less accurate than their 3D counterparts. 

 

NIST’s hollow pitot tube hosts five ports to measure three-dimensional gas flow, providing a more accurate measurement of complex flows than two-port S-probes covering only one or two dimensions. Image used courtesy of NIST

 

So arises the basis of NIST’s alternative pitot tube design, featuring a hemispherical face and five holes. Unlike incumbent 3D measurement tools, the probe is calibrated to measure flow accurately without needing rotations. 

 

NIST’s New and Improved Pitot Tube

The probe marks an improvement upon an earlier design NIST unveiled in 2019 – real-world testing in smokestacks exposed two major design issues with the prototype. While the probes were five times faster than conventional 3D technologies requiring manual rotation, ash and water droplets could easily clog the five ports. This would slow down the process since auditors needed to purge the tubes. But NIST’s improved model addresses the problem by upsizing the ports from 2-3 millimeters (mm) to 6.5 mm. 

 

A NIST mechanical engineer holds the five-port pitot tube. Image used courtesy of NIST/by Jennifer Lauren Lee

 

The second issue involved calibration. Unlike S-probes, NIST’s probes had to be calibrated with 3,000 separate measurements in a wind tunnel before use in the field. Knowing this was more calibration than feasible in the industry, the engineers found they could still get high-accuracy measurements with 130 carefully selected points informed by a combination of angles and air velocities in a wind tunnel. 

The changes make NIST’s pitot tube design a strong candidate for widespread adoption over existing EPA-sanctioned probes.