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To Improve GHG Emissions Monitoring, PML Gets the Flue

November 27, 2013

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Contact: Aaron Johnson
(301) 975-5954

researchers with section of smokestack pipe
Joey Boyd (left) and Aaron Johnson of the Fluid Metrology Group with a section of pipe to be used for a multi-path ultrasonic flow meter in the test section of the new facility.
PML is known for devising, developing, and employing state-of-the-art technologies. So why is a team from the Sensor Science Division building a smokestack on NIST’s Gaithersburg, MD campus?

The answer is that, for a typical coal-fired power plant in the United States, current measurements of carbon dioxide flux in smokestacks have uncertainties that may be as large as 10% to 20% -- leaving actual emissions totals up in the air.

“For both current mitigation efforts and future emissions monitoring, the nation needs to improve CO2 measurement in smokestacks of coal-burning power plants,” says Aaron Johnson of PML’s Fluid Metrology Group. “And if you improve measurement of total flow, you will improve measurements of other pollutants as well, such as sulfur compounds or nitrous oxide. So we’re constructing a scale-model smokestack simulator here at NIST to better understand and monitor gas flow. The goal of the project is to find a way to get CO2 flux measurements with a 1% uncertainty at a reasonable cost.”

It’s a big issue. Collectively, electricity generation accounts for about one-third of total U.S. CO2 emissions, greater than any other economic sector. There are more than 500 coal-fired power plants in the United States, ranging in capacity from a few megawatts (MW) to 2 gigawatts or more. According to the Environmental Protection Agency, coal-fired plants’ average emission rates are 2,249 lbs/MWh of carbon dioxide, 13 lbs/MWh of sulfur dioxide, and 6 lbs/MWh of nitrogen oxides.

"At present, facilities are required to state their total emissions," says James Whetstone, head of NIST's Greenhouse Gas and Climate Science Measurements program, which is funding the SMSS project. "But in the absence of a requirement to estimate the uncertainty associated with those measurements, it is very difficult to quantitatively assess the quality of those data."

Determining exactly what the flow rate is within an individual smokestack (some are 30 feet in diameter), is “a tough measurement,” Johnson says, both because of present metering technologies and because of an imperfect understanding of how gas velocity can vary by position in the stack. 

Each smokestack contains a Continuous Emission Monitoring System (CEMS), about two-thirds of which are ultrasonic systems made up of two transmitter/receiver units placed at different elevations in the stack. A sound signal traveling from one to the other is speeded up if it travels in the direction of the gas flow, or slowed down if it travels against the flow. The difference in transit times across that single path is used to calculate the velocity in the stack; multiplying that by the area yields flow volume.

diagram of commercial smokestack
Diagram illustrating numerous measurement challenges in determining smokestack flow at coal-fired power plants. Click on image for enlarged view.
However, the flow in plant stacks is turbulent and inhomogeneous – caused in part by the fact that the gases usually take a sharp 90-degree turn as they travel out of the scrubber and into the smokestack. “All the flow is not moving in the axial direction,” Johnson says. “Swirling flow contributes a horizontal component to the gas velocity, and thus introduces error into velocity measurements. In general, asymmetric flows and rotating flows are bad for accurate measurement.”

It is possible that placing one or more additional pairs of ultrasonic units that sample different paths could produce more accurate measurements. One of the goals of the new Scale-Model Smokestack Simulator (SMSS) project, which includes a section fitted with four separate ultrasonic transmitter/receiver units is “to understand how much bang for the buck you get by using multi-path metering,” Johnson says.

EPA requires that the CEMS apparatus be calibrated at intervals according to a protocol based on velocity measurements made by two pitot probes inserted horizontally into the smokestack from orthogonal ports on the side. “The trouble is, we don’t really know how to put uncertainties onto those measurements,” Johnson says. “There are different sorts of pitot tubes, differences in precision between practitioners, and no real traceability back through every step to the SI unit of flow, which is cubic meters per second.

“We need to study different kinds of pitot tube designs and sizes and placement. Introducing a pitot tube affects the velocity you’re measuring and the smaller the flow cross-section, the larger the effect would be. These blockage effects are generally negligible during use in a large smokestack, but are introduced when pitot tubes are calibrated in situ in relatively small-diameter apparatus.

"At present, those effects are poorly understood. So in the SMSS, we’ll experiment with different sizes until we’re satisfied that we’re not changing the flow pattern that we’re trying to measure. We need to assess the EPA method against reliable flow standards, as well as examine the performance of CEMS flow meters in distorted gas-flow profiles.”

All this and more will take place in the new SMSS facility (see image at bottom of page), presently under construction in a repurposed building. Instead of a vertical smokestack, the SMSS will be a horizontal arrangement. That is possible because hot gases are not used, and thus buoyancy effects are not significant: Most modern coal-fired power plants are equipped with wet scrubbers to remove SO2. After passing through the scrubber, the flue gas exhaust temperature is not substantially above the ambient temperature.

ultrasonic flow meter section

The 8-path ultrasonic flow meter to be installed in the reference section of the SMSS, shown during calibration at a private-sector facility. This calibration was the first time that flow traceability was established at uncertainties better than 0.5 % for a gas ultrasonic flow meter of this size (D = 91.5 cm) calibrated at high velocities (46 m/s) and ambient pressure. Click on image for enlarged view of calibration.

Air, a reliable proxy for flue gas, is drawn into the SMSS at controlled speed by large fans and passes through two connected sections of pipe, each about 80 feet long and set at right angles to one another to echo the path that gases in commercial plants travel from the scrubber to the base of the smokestack.

In the first (reference) section, air will enter via a bell-shaped intake port and travel through a 3-foot diameter channel which includes an 8-path ultrasonic flow meter (shown at right during calibration) that is traceable to NIST primary flow measurement standards and has an expanded uncertainty of 0.5 % at a 95 % confidence level. Air speed in the reference section will range from 11 to 44 meters per second with near-ideal flow conditions -- that is, a symmetric velocity profile with no swirl.

The moving air then takes a 90-degree turn and enters the 4-foot diameter test section, where speed will vary from 6 to 25 m/s, a range common in plant smokestacks. Different “de-conditioning plates” will be inserted into the pipe to create controllably different asymmetric flow profiles simulating a variety of smokestack configurations. From there, the air moves into the test area that can be fitted with pitot tubes and contains a multi-path ultrasonic flow test fixture. (Shown above.)

Data collected in this facility will also be compared to computational fluid dynamics (CFD) results to evaluate the accuracy and uncertainty of this type of computer analysis. “Comparison of the CFD results to the results measured in the smokestack simulator will allow us to scale up from our four foot diameter test section to the thirty foot diameter section of real smokestacks” says Ricker. “This method allows us to validate our computer models, and then use that to predict measurement uncertainties of a real smokestack without building a full sized model.”

In addition to measuring the effects of different flow profiles and sensor configurations and providing benchmark data for computational fluid dynamics, Johnson says, “the SMSS will also allow us to look at alternative flow measurement techniques, putting tracers in flow, using long-wavelength acoustics, and using LIDAR.” Construction is scheduled for completion by spring of 2014.

“In order to understand the effects of pollution or to control it, you have to be able to measure it,” says John Wright, a Project Leader in the Fluid Metrology Group. “The pollution control and monitoring programs established in the United States are a model for developing economies, and our stack simulator has attracted attention from researchers from other countries, including China. We are planning a workshop to coordinate our work. Our goal is to improve stack flow measurements and put solid uncertainty analyses in place for them. We need reliable stack flow data to make science-based decisions.”

diagram of planned facility
Diagram of the SMSS facility under construction on NIST's Gaithersburg campus.