NIST turns to new sensor to measure smokestack emissions

The flue gases of coal-fired power plants must be continuously monitored (WikipediaCommons)
The flue gases of coal-fired power plants are continuously monitored by built-in sensors (WikipediaCommons)

Coal-fired plants undergo smokestack emissions testing on an annual basis, a process that can become lengthy and time consuming. To speed up this process, researchers at the National Institute of Standards and Technology (NIST) have completed the first real-world test of a potentially improved way to measure smokestack emissions in coal-fired power plants. The process involves using new sensor probes to measure flow emissions and a new measurement method that could potentially speed up on-site audits by a factor of 10.

NIST's Joey Boyd (left) and Aaron Johnson conducting a test of the new flue gas sensors at a power plant. Credit: Tom Martz/EPRI
NIST's Joey Boyd (left) and Aaron Johnson conducting a test of the
new flue gas sensors at a power plant. Credit: Tom Martz/EPRI

NIST conducted this work as part of a cooperative research and development agreement (CRADA) with the Electric Power Research Institute (EPRI), an independent nonprofit organization whose members include electric utility companies, businesses and government agencies.

To monitor coal-fired power plant emissions, technicians need to measure the rates at which flue gas is emitted from the smokestack. The flow inside the smokestack contains eddies and swirls but generally travels upwards. The NIST tests involve inserting four probes—called pitot tubes—horizontally into the smokestack.

Smokestacks at coal-fired power plants incorporate monitors that continuously measure the concentration of flue gas emissions, which include carbon dioxide, mercury, sulfur dioxide and nitrogen oxides, as well as the flow rate of the flue gas. By federal law, the built-in flow-rate sensors need to be calibrated during the annual audit.

The four sensor probes each take a flow measurement at four different spots, for a total of 16 measurements. With this information, NIST scientists could test the precision and accuracy of a new pilot tube design and measurement method.

"Coal-fired electric generating units may benefit from the current NIST work by having improved standards and techniques to measure mass emissions more accurately, with increased confidence that all entities are reporting on a uniform basis," says EPRI program manager Tom Martz, in a statement. He adds that the potential time savings "is not something we can accurately quantify at this time, but this will be a key objective of future work."

The researchers envision the EPA someday developing a new standard for smokestack emissions calibration. "Our goal is to get it written as an EPA standard," says Johnson. "But it's still up to industry members to decide whether they would want to use it."

Smokestacks at coal-fired power plants incorporate monitors that continuously measure the concentration of flue gas emissions, which include carbon dioxide, mercury, sulfur dioxide and nitrogen oxides, as well as the flow rate of the flue gas. By federal law, the built-in flow-rate sensors need to be calibrated during the annual audit.

This calibration is performed by inserting small portable devices called pitot tubes into the stack.  The most common kind of sensor used for this work is an "S-probe," which has two holes, or ports. One port faces directly into the flow of gas and detects the pressure that builds up in the tube. The other port faces the opposite direction. The faster the flow, the higher the pressure difference between the two ports; measuring this difference in pressure allows auditors to calculate the flow's speed.

Using traditional S-probes is labor-intensive, because the technician must manually rotate the sensor until one side is facing directly into the flow to take the measurement. Because the flue gas flow is not necessarily traveling directly upward at the point being tested, an on-site annual calibration can take a day or more to complete.

To speed up this process, the NIST scientists have made three innovations. First, they have created two new models of pitot tubes, with five holes instead of two, which perform better than S-probes and may offer advantages over other five-hole models of pitot tubes currently in use.

The probes, designed by NIST physicist Iosif Shinder, come in two shapes: hemispherical and conical.

The scientists also developed a calibration scheme for their new sensors that does not require a technician to rotate the probe inside a smokestack to find the true direction of the flow for each measurement. So, while the sensors would have to be calibrated before use, they would take much less time to use during an actual audit.

The coal-fired plant had an enclosed platform where the pitot tubes were inserted into the smokestack. But the natural gas plant's platform was open to the elements. And at roughly 45 meters (145 feet) in the air, "things shake," said NIST technician Joey Boyd. "While you're working, the stack is swaying, and the floor beneath you is moving."

When NIST researchers analyzed the data, their results were promising, agreeing to within 2% with their laboratory findings.

"The probes performed equally well in the smokestack as they did at NIST's test facility," Johnson says.

Future field tests will help the researchers solve the biggest problem they had: sensor clogging, in which the pitot tubes' ports get gummed up with water and particulate matter and have to be flushed before a test can continue.

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