Metrology for Monitoring Emissions

from Coal-Burning Power Plants

NIST Greenhouse Gas & Climate Science 

Measurements Workshop

August 1, 2014 (9:00-9:30)

Michael R. Moldover

Fluid Metrology Group, NIST
U.S CO2 Emissions, By Source (EPA, 2012)

2

What is the Problem? Why is it Important?
World-Wide, Coal is Most Important, IEA (2013)

3

Gas

Gas
Power Plants Fueled With Natural Gas 

4

• Volume of natural gas flowing into a power plant can be 

metered to better than 1 % with traceability to NIST (and SI)

• Energy content of natural gas is determined within ~1 % from 

composition analysis via gas chromatography. Can be checked 

by calorimetry. 

• If required, on-line composition metering could be adopted 

(Rhurgas) with a reasonable cost

• Only minor measurement problems, if combustion is complete
1

Fuel:

Amount & quality

of coal (EIA data)

1

Emissions:

Flow and CO2

concentration (EPA data)

2

2

Coal-Fired Plants: Two methods to determine CO2
Coal-Fired Plants: Two methods to determine CO2

Do they agree?

-40

-20

0

20

40

60

1 10 100 1,000 10,000

100 

 (Emissions/Fuel 

- 1) 

Emissions (1,000’s tons of CO2 / year)
Coal-Fired Plants: Two methods to determine CO2

Do they agree?

Non-Gaussian

Mean ~1.5%

FWHM ~ 20%

~20%

0.00

Probability Density

0.02

0.04

0.06

-40 -20 0 20 40

% Difference: Emissions – Fuel 

No!
8

Measurement Need

• Carbon controls (carbon tax, cap and trade) require 

accurate, SI-traceable, measurements of the CO2flux 

emitted by coal-burning power plants. 

• Coal is too heterogeneous to serve as a surrogate for CO2

• Current CO2flux measurements may be biased too high. 

• NIST Objective: SI-traceable, CO2-flux measurements with 

1 % uncertainty at a reasonable cost, 

to provide the technical basis for carbon control 

in US and internationally
9

Why are Emissions Measurements Difficult?

• Stacks are big: cannot calibrate a 10 m diameter meter in any lab

• Flow is fast: 5 m/s to 25 m/s

• High Reynolds number ~ 107; cannot be simulated. 

• Nasty conditions: 

– Access via outside cat-walk on older stacks

– Noisy

– Gas is either “hot” (no scrubber 90+ C) or “ambient & raining” (scrubber)

– Gas is asphyxiating: composition (by volume)

13.7 % CO2

3.4 % O2

74.8 % N2

8.0 % H2O

• Flow is complicated
Flow is Complicated

Real stacks have swirl
Flow is Complicated

Real stacks have skew
12

How are Emissions Measurements Made Today?

1) Using EPA-approved protocols, flue gas 

flux is continuously monitored

composition is continuously analyzed for O2, Hg, S, NOx

to comply with emission controls for Hg, S, NOx

2) The instruments used for 1) comprise the 

CEMS = Continuous Emissions Monitoring System

3) Typical CEMS uses two ultrasonic meters to monitor flow

4) Annual “Relative Accuracy Test Audit” (RATA) “calibrates” 

ultrasonic CEMS flow monitors. Typically, the flow is surveyed 

with a S-probe, that is temporarily installed on the stack. 

5) As the name suggests, the EPA protocols provide only relative 

accuracy, not uncertainty relative to primary standards. 
What is NIST Doing?

1) Tie EPA-CEMS instruments and protocols to primary standards

(Essential for International Recognition)

A. Calibrate Pitot probes under realistic conditions

B. Measure sensitivity of ultrasonic flow meters to complex flows

C. Understand/model results to generalize and scale up 

2) Invent alternative flow standards for flue gas stacks

(to check entire measurement chain)

A. Long Wavelength Acoustic Flow Meter

B. Tracer Dilution
S-probe: workhorse for stack flow measurements

Flow

2. Rotate

1. Measure Pressure Difference Dp

NEEDS CALIBRATION
S-probe: workhorse for stack flow measurements

Cheap

Stable

Rugged

Passive

Can be calibrated
NIST’s wind tunnel generates well-defined airspeeds

to calibrate anemometers
NIST Wind Tunnel: BTW Parameters

• Large test volume  small wall effects

• k =2 uncertainty of 0.42%, 5 m/s to 25 m/s

• Low (0.1 %) turbulence intensity

• Uniform flow along tunnel axis (1-dimensional flow)
Automated stage for changing pitch angle and yaw angle

18
Modify wind tunnel: add

Grid to Generate Turbulence
Measure Effects of Grid

FLOW
Measure Effects of Grid

turbulence intensity 

2 ( ) u u

u

   

 
S-Probe, (used in EPA protocol 2)

22

Calibration Factor is a Function of 4 variables

1. Air speed

2. Pitch angle

3. Yaw angle

4. Turbulence intensity

EPA protocol assumes calibration factor = 0.84

(literature shows small, linear dependence on air speed)
S-probe: cannot detect pitch

Flow Flow


Results for “typical” S-probe at 10 m/s

0.84 = EPA value

10 %
Typical CEMS Ultrasonic Flow Meter

Typically, 2 crossing paths

Measures time of flight of ultrasonic waves

“sound beam” moving with and against flow 

 velocity component along beam and sound 

speed

Does not detect swirl or velocity profile 

distortions. If these change between 

calibrations the results will be biased
“Calibrate” CEMS using Calibrated Pitot Tube

d1

d2

d3

d....

d....

dN/2-1

dN/2

dN/2+1

dN/2+2

dN

dN-1

d....

d.... Pitot

Probes

Access

Port 

Cross section of Smokestack

θ = 

0

o

θ = 270o

θ = 90o

θ = 

180o

q

Access

Port 

• Pitot Probe traverse along two diameters in stack cross section

• Traverse Protocol Based on EPA Documents

– 40 CFR Part 60

– 40 CFR Part 75 (2F, 2G, 2H)

Measurement problems:

• Pitot probes not calibrated for pitch

• Velocity measured at only 2 angles 

• Integration errors
Facility to study/solve measurement problems 

must generate known, turbulent, swirling flows 

Reference

Section

(Dref = 3ft)

Fiber Glass

Cone Test Section

(Dtest = 4ft)

Automated Pitot

Traverse

Air Intake

Module

Horizontal orientation for cost and safety

Smokestack Simulator is 1/10th the diameter of typical stack
Smokestack Simulator

Reference Section (D = 3 ft)

Flow Direction

8 Path USM

Air

Intake

Air Exhaust

Outside Building

Fans

Test Section (D = 4 ft)

Flow Direction

CEMS USM

Corner
8 Path ultrasonic flow meter

Installed after 19 D of straight pipe (good flow)

Calibrated against NIST flow standards

Determines bulk flow to 0.5%

Design of Smokestack Simulator
Calibration of USM at CEESI in Colorado

8 path USM

Flow from

Fiber Glass Bell 

Pressure

Measurements

12 Temperature

RTDs

USM

NIST

v

v

φ 

Calibration Factor
Calibration Data

 Excellent Reproducibility < 0.075 %

 Expanded Uncertainty: 0.45 % to 0.58 %

 Best-ever calibration in air in this size 
Loudspeakers

Microphones

distorted wave-fronts 

approach a plane wave

Outgoing Wave

Radiated

Wave

Reflected Wave

FLOW

Open End

Velocity of plane wave averages over flow distortions

Long Wavelength Acoustic Flowmeter
Long-Wavelength Acoustic Flowmeter

LWAF

1/100th Scale

It works with

• obstacles

• complex bends

• water spray

Can we scale up to 1/10th ?
Plans

• Document performance and uncertainty of existing flow

measurements in swirling and skewed flows

- EPA Pitot traverse method

- CEMS flow meters (Ultrasonic Flow Meters)

• Develop alternative/improved stack flow measurement techniques

- Multi-chord and 3-D pitot traverses, advanced integration 

- Multi path ultrasonic flow meters

- Long Wavelength Acoustic Flow Meter

- Differential absorption LIDAR ?

- Tracer Dilution Methods 

• Develop benchmark data to validate Computational Fluid Dynamic 

(CFD) models to facilitate scale-up
Thank You

Joey Boyd

Rodney Bryant

Chris Crowley

Jim Filla

Keith Gillis

Lee Gorny

Mark Khalil

Aaron Johnson

Kyle Lee

Zhang Liang 

Jacob Ricker

Dan Sawyer

Iosif Shinder

John Wright