Presented at the 1997 EPRI CEM Users Group Meeting
Denver, Colorado
May 14-16, 1997

ABSTRACT

Since installing continuous emissions monitoring systems (CEMS) as required by the Acid Rain Rule (40 CFR Part 75), many utilities have noted that the CEMS are recording consistently higher heat input and SO2 emissions than conventional methods (input/output and output loss). The apparent CEMS bias is causing utilities to report more heat input and SO2 and CO2 emissions than are believed to be justified. Many believe that the major problem is Method 2, the Environmental Protection Agency (EPA) standard measurement method for stack volumetric flow rate. It has been clearly shown by previous work in this project that Method 2 will be biased high in the presence of "swirling stack flow" and, since all of the stack volumetric flow monitors are calibrated to Method 2 measurements, a high bias in Method 2 will be directly transferred to the flow monitors. In addition to the potential high bias in stack flow measurement, any high bias in SO2 or CO2 stack gas concentration measurement will also bias the overall mass (lb/hr) emission rate.

In order to better understand the problem, the Electric Power Research Institute (EPRI) initiated a project to identify the cause(s) for the high heat input and SO2/CO2 emissions measurements. This project has three basic objectives: (1) to understand the potential error in Method 2 under stack flow conditions by the review of existing literature and data, (2) to demonstrate the validity of the literature assessments by conducting flow measurements in a specially designed "swirl tunnel," and, (3) to verify the flow and heat input errors, and identify the cause(s) in full scale field tests. During the course of this project several technical issues have been identified that dictate further investigation.

This paper contains the summary results of the swirl tunnel measurements and full scale field tests and also describes future project plans to further investigate additional issues that have been discovered during the course of the project. The swirl tunnel tests were performed in a precision flow facility with custom fabricated swirl vanes for inducing different tangential flow components, variable speed fan control for flow rate adjustment and a venturi section for total flow measurement. Summary results are presented from tests designed to systematically assess the effect of non-axial flow components on EPA Methods 1 and 2, the relative suitability of alternative multidimensional pitot probes and the relative accuracy of pressure reading instrumentation. The full-scale field tests were conducted under tightly controlled conditions so that the error sources could be identified and quantified. Almost every possible source of heat rate discrepancy was simultaneously evaluated. Flue gas flow measurements were made using different 2-D and 3-D pitots. Independent gas concentration measurements were made. Unit heat rate was determined using the conventional input/output method. Multiple fuel sampling and analysis approaches were used. The results of this simultaneous, multiple methodology approach helped shed light on the sources of heat input error. 

INTRODUCTION

Virtually all electric utility power plants were required to install CEMS as a result of the Acid Rain Program mandated by the Clean Air Act of 1990. Monitors were required for SO2, NOX, CO2 and stack volumetric flow rate. In addition to measuring emissions, CEMS have been used (using fuel F-factors, CO2 concentration and the volumetric flow rate) to obtain boiler heat inputs and, subsequently, unit heat rates. Since the installation of the CEMS, many plants have found that the heat input/rate, as determined by the CEMS, was higher (by 5-25%) than determined by conventional heat rate methods (input/output or output loss). This discrepancy is disconcerting since all of the methods should give equivalent results. The heat input value from the CEMS was immediately suspect since there is more than a 50-year history with the conventional methods and, in many cases, the heat input from the CEMS was simply thermodynamically improbable. The individual component of the heat input measurement that was suspected to be the major cause of the problem was the volumetric flow measurement. The flow measurement instruments were new and unproven while the CO2 instruments and F-factors had been used for a number of years.

If, in fact, the flow measurement was in error, the problem became more than disconcerting; it became a matter of money. The flow measurement is a fundamental component of the SO2 tonnage emission calculation and, if high, results in excess SO2 allowances being used. A SO2 allowance permits a utility to emit one ton of SO2 and the allowances can be bought and sold on the open market. Therefore, using excess allowances can have a multimillion dollar impact on a large utility. In addition, many utility boilers have operating permits that contain heat input and SO2 tonnage limits and, in the past, compliance with these limits has been demonstrated with fuel analysis. Some state agencies and EPA regional offices have begun using the CEMS data to evaluate compliance with the permit limits and have started applying pressure on utilities that are showing "excess" emissions and heat input. 

The problem of high heat input was so pervasive throughout the utility industry and the costs of excess allowances were so great that the Electric Power Research Institute (EPRI) initiated a project with RMB Consulting & Research, Inc. to better understand the high heat input measurement problem. The objectives of the project are; (1) to identify the source(s) of the heat input discrepancy, (2) to quantify the errors, and (3) to suggest ways to reduce the error. In order to accomplish these objectives a three-task project was developed. Since flow measurement errors were suspected as the primary problem, the first task was to conduct a literature/technology survey on flow measurement methods and potential errors. This work has been completed and a report has been published (EPRI TR-106698, available to EPRI members only). The results of Task 1 were also reported in a paper presented at the May 1996 EPRI CEM Users Group Meeting in Kansas City. Task 2 was to construct a "swirl tunnel" and to test various pitot configurations under controlled yaw swirl conditions. This work has been completed and is summarized in this paper. Task 3 was to confirm the results of the swirl tunnel work and to define and quantify error sources in a series of tightly controlled field tests at power plants that were experiencing CEMS heat input measurement errors. This paper also summarizes the results of those field tests.

As we progressed through the project several new issues were discovered that bear further study. Virtually all of the CEMS CO2 analyzers appear to be reading slightly high relative to reference test measurements. The readings are not high enough to cause the CO2 analyzers to fail a RATA but are contributing to the high bias in the heat input measurements. Also, a considerable amount of pitch flow (flow from the center of the stack toward the wall or from the wall toward the center of the stack) was observed at both field test sites. It is unclear how this pitch flow may be impacting flow measurement instrumentation.

SWIRL TUNNEL TESTS

One of the reasons for fabricating a tunnel where controlled swirl could be induced was to demonstrate whether error estimations based on straight-flow wind tunnel testing were indeed comparable to the measurement errors associated with true cyclonic flow. A number of studies suggest that S-type pitot errors due to non-axial flow can be significant, even with an average resultant angle of less than 20o as allowed by Method 1. (Muzio, L.J, et al.; Flue Gas Flow Rate Measurement Errors, Interim Report TR-106698 2819-32; Electric Power Research Institute, June 1996.) This error relates to a design characteristic of the S-Type probe, its moderate insensitivity to misalignment. This characteristic, while minimizing bias associated with probe misalignment in the tangential (yaw) or radial (pitch) flow directions, contributes to bias when measuring flow with non-axial components because the velocity head includes non-axial components when only the axial component is desired. Figure 1 shows error estimations for flows with both tangential and radial components as simulated in a straight flow wind tunnel with combined yaw and pitch misalignments of the pitot.

The "swirl tunnel" also allowed for more complete testing of the relative suitability of alternative two- and three-dimensional pitot probe designs. If cyclonic flow is indeed a contributor to the high bias with Method 2, one approach for more accurate flow measurement is the use of directionally sensitive probes rather than the S-type probe. If the pitch component does not play a significant role, then, perhaps, a two-dimensional probe or a "yaw nulling" technique might suffice. As for 3-D probes, the probe should ideally exhibit a high degree of angular sensitivity with well-behaved responses to pitch and yaw while the velocity measurement should be insensitive to yaw and pitch. 

"Swirl Tunnel" Design

Figure 2 shows a layout of the "swirl tunnel" along with its sampling ports. The tunnel was constructed and located at Fossil Energy Research Corporation's (FERCo) facility in Laguna Hills, California. Due to space constraints, a two-level construction, as shown in the figure, was used. The upper section contained the swirl generating vanes and the test stations while the lower section consisted of the venturi and the fan. 

Air entered the tunnel via a bell inlet after which it passed through either a six-inch deep honeycomb flow straightener (L/d=12, L/D=0.17 and d/D=0.014) or one of a set of removable fixed vane sections used to generate the non-axial swirl flow components. Leaving the swirl vanes, the flow entered the test section, housing two test stations with four ports each and plexiglas windows for visualization. Upon leaving the test section, the flow passed through a 180o bend incorporating turning vanes to reduce the pressure drop and help straighten the flow, i.e., keep the flow from "hugging" the bottom of the duct as it exits the turn. Leaving the turn, the flow passed through another honeycomb flow straightener, the venturi section and then through the fan. The venturi, which conformed to ASME specifications, had an inlet diameter of 35.96 inches and a throat diameter of 23.45 inches. The fan was followed by a three-foot diffuser.

A 50 hp variable speed axial vane fan capable of maintaining five inches H2O of pressure powered the tunnel. The unit was operated in an induced draft mode to minimize fan effects on the flow. The tunnel duct had a three-foot diameter with a flow of 38,000 scfm at a velocity of 90 ft/sec.

Since only four swirl conditions were included in the test plan (no swirl and three different levels of swirl), removable fixed swirl vanes were opted over variable swirl vanes to belay concerns over repeatability with manually adjusted variable vanes. The vanes, fastened together by a hub, were constructed with a constant angle over the radius with 50% overlap so that no straight flow would pass, (i.e., longer vanes were used to generate smaller yaw angles). The vanes were designed to generated yaw angles of approximately 10o, 20o and 30o. The vanes produced very constant swirl angles across the duct with very little pitch. The hub had little effect on the flow beyond the very center of the tunnel.

Probe Comparison Testing

The swirl tunnel was used for comparison testing of the following five probes, three supplied by EPRI and two provided by EPA (other probes were also tested but are not included with this paper):

•1.25" S-Type Probe
•1.25" DAT 3-D Probe
•1.25" Spherical 3-D Probe
•1.25" EPA DAT 3-D Probe
•An EPA modified S-Type 

All EPRI supplied probes were new probes. The S-Type and the DAT probes are standard, commercially available probes that were manufactured by United Sensors. The spherical 3-D probe was custom built for this project. Prior to the testing, all of the EPRI supplied probes were calibrated in the University of Alabama (Birmingham) wind tunnel.

Four series of tests were performed. Tests were conducted at three different velocities of approximately 50, 70 and 90 ft/s. The fourth test series was a duplicate of the 70 ft/s series to demonstrate repeatability. For each test, the variable speed fan was adjusted until the approximate desired velocity was indicated by the venturi. This approximate velocity was maintained while traverses for each of the various probes were tested in conjunction with various swirl conditions. It was seen that the swirl vanes designed to generate approximately 10o, 20o and 30o of yaw actually yielded about 13o, 24o and 35o of yaw, respectively. While slightly different from the anticipated values, the actual swirl generated for a particular vane was seen to be constant across the test section. 

Figure 3 shows the measurement error associated with the various probes at 90 ft/s, where error is defined in terms of percent difference with the venturi-based flow value. The results of the EPRI DAT and Spherical 3-D probes were very similar and demonstrated good repeatability. During the initial tests, the response of the EPA DAT probe did not match that of the EPRI DAT probe and performed much more poorly than expected. An incorrect calibration was suspected, and a misplaced 0° scribe line has been subsequently confirmed. After correction, the EPA DAT data agree with the EPRI DAT data.

Both the standard S-type pitot tube probe and modified probe supplied by EPA displayed degradation of performance with increased swirl. It is interesting to note, however, that the effect of swirl on the standard S-Type probe was considerably less than expected. As shown by Figure 1, earlier work suggested an axial velocity bias of about 25% for yaw angles approaching 30o, while the swirl tunnel test results indicate a bias of only about 15%. 

As part of the test series, a S-type pitot tube "yaw nulling" technique (or "null+90o" method) was evaluated for its potential for more accurately measuring axial velocity in the presence of swirl. Using the null+90o method involves finding the total velocity vector by first rotating the S-type probe to an angle (the "null angle") where the pressures measured at tubes A and B of the pitot are equal, i.e., the differential pressure across the two pitot heads equals zero. The total velocity vector is assumed to be moving perpendicular to the probe plan when it is in the null position, and rotating the probe 90o will point the probe in the direction of the flow. Multiplying the cosine of the angle of the probe with respect to the axial direction by the total velocity provides the axial velocity. It is important to realize, however, that this technique can only account for the yaw component of the flow and cannot compensate for pitch. Considerable pitch flow was observed during both field tests and the effect cannot be measured, or even detected, without a 3-D pitot. In addition, performing a yaw null pitot traverse in the field is just as difficult as performing a 3-D traverse.

FIELD TESTS

Description of Units Tested

Field testing was performed at two large coal-fired units. The first series of tests was performed at Wisconsin Power & Light’s Columbia Unit 2. Columbia Unit 2 is a conventional 560 MW (gross) unit that burns Wyoming low-sulfur, sub-bituminous coal. The CEMS is a typical dilution extractive system equipped with an ultrasonic flow monitor. The CEMS and test ports are located in the stack approximately 2.5 diameters up from the entrance of two opposed entry ducts. The stack diameter at the test location is 21 feet. The unit is equipped with gravimetric coal feeders/scales using the latest load cell-based technology. It also has an on-line heat rate monitoring system. 

The second series of tests was performed at Cooperative Power’s Coal Creek Unit 2. Coal Creek Unit 2 is a 560 MW (gross) that burns lignite from an on-site mine (mine-mouth). The unit is equipped with a lime-based SO2 scrubber. Lignite is supplied to the unit by six gravimetric feeders/scales which were a mix of load cell-based and an older technology. The CEMS is a typical dilution extractive system equipped with an ultrasonic flow monitor. The CEMS and test ports are located in the stack approximately 7.7 diameters up from the single entry duct. The stack diameter at the test location is 25.6 feet. This unit is also equipped with an on-line heat rate monitoring system.

Columbia Unit 2 Field Test

Five tests were conducted at Columbia Unit 2. These tests were based on preliminary findings on Unit 2 that revealed flow with moderate yaw and pitch components. Also, since the yaw components varied from port to port, four additional test ports were added at a 45-degree offset from the original ports. The five tests included runs designed to compare the results of S-type and 3-D pitot measurements and to evaluate the effect of test port variation. During each test CEMS, coal flow and reference method measurements were taken. Tests 1 and 2 were conducted with the unit operating at about 520 MW (gross); Tests 3-5 were conducted with the unit operating at about 550 MW (gross).

•Test 1: S-type/3-D comparison, No Test Port Variation
•Test 2: S-type Only, No Test Port Variation
•Test 3: S-type/3-D comparison w/Test Port Variation (Full Load)
•Test 4: S-type/3-D comparison, No Test Port Variation (Full Load)
•Test 5: S-type/3-D comparison w/Test Port Variation (Full Load)

Figure 4 shows comparisons of stack flow data for the tests performed at full load. The S-type pitot measurements averaged 4.1% higher (2.9%-5.2%) than the 3-D probe. This small difference demonstrated the effect of the slight yaw swirl on the S-type pitot.

To illustrate the bias effects in terms of unit heat rate, the results of the full load tests (Tests 3-5) are shown in Figure 5. For consistency, only flow traverse values taken from the original test ports are included. Except as indicated by the 3-D corrected bar, the data in Figure 5 are uncorrected and based on manual Delta P readings. The standard FC-factor of 1800 for subbituminous coal was used and no corrections were made for wall effects. The unit heat rate based on S-type pitot flow measurements and reference method CO2 values was an average of 10.8% higher and the heat rate based on CEMS data was an average of 9.8% higher than the heat rate calculated using the input/output method. The uncorrected heat rate based on 3-D probe flow measurements and reference method CO2 values was an average of 7.1% higher than the input/output method. Figure 5 also shows that, with the proper corrections, the 3-D-based heat rate agrees well (within 1.9%) with the input/output method. (Discussions of the corrections are included in a following subsection -- See Heat Rate Bias Components) 

Coal Creek Unit 2 Field Test

Seven tests were conducted at Coal Creek Unit 2. During each test run, separate but simultaneous measurements were taken by test teams from FERCo, RMB’s stack test subcontractor and Climax, EPA’s stack test subcontractor. (While many measurements were duplicated, not all measurements for all tests were performed by both test teams given the physical limitations of the test site.) All tests were conducted with the unit operating at full load, 560-565 MW (gross). During each test CEMS, coal flow and reference method measurements were taken.

•Test 1: FERCo S-type and 3-D; Climax S-type and 3-D
•Test 2: FERCo 3-D; Climax S-type
•Test 3: FERCo S-type and 3-D; Climax S-type and 3-D
•Test 4: FERCo 3-D; Climax S-type and 3-D
•Test 5: FERCo S-type and 3-D; Climax S-type and 3-D
•Test 6: FERCo 3-D; Climax S-type and 3-D (w/near wall measurements) •Test 7: FERCo 3-D; Climax S-type and 3-D

For simplification, various aspects of the tests are included in the preceding list. A "French" probe was also tested as a potential alternative to the S-type pitot and, while favorable, those results are not included in this paper. Yaw nulling techniques were also investigated but the results are not included herein. 

A comparison was made of simultaneous stack flow measurements recorded by FERCo and Climax. The measurements showed excellent agreement with an average S-type flow value disagreement of less than 1.2% and an average 3-D measurement disagreement of less than 0.8%. (Since such good agreement was found, the average of any duplicate measurements taken by FERCo and Climax was used in any subsequent analysis contained in this paper.)

Figure 6 shows a comparison of simultaneous stack flow measurements recorded during the seven tests. The S-type pitot measurements were an average of 15.0% higher (12.3%-19.9%) than the 3-D probe. This large difference demonstrated the effect of the significant amount of swirl (the average yaw angle was ~ 21o ) on the S-type. The CEMS flowmeter reported values an average of 6.6% higher (2.4%-9.9%) than the 3-D probe.

To illustrate the bias effects in terms of heat rate, the results of the full load tests are shown in Figure 7. The data in Figure 7 are uncorrected and based on manual readings. Except as indicated by the 3-D corrected bar, the standard FC-factor of 1910 for lignite was used and no corrections were made for wall effects.

The heat rates based on S-type pitot flow measurements and reference method CO2 values were an average of 23.7% higher than heat rate values calculated using the input/output method and values based on CEMS data were an average of 18.6% higher. The heat rates based on uncorrected 3-D probe flow measurements and reference method CO2 values were an average of 8.1% higher. Figure 7 also shows that, with proper corrections, the 3-D-based heat rate agrees well (within 3.4%) with the input/output method. 

Curves depicting the yaw/pitch variation measured during the Coal Creek tests are shown in Figure 8 and Figure 9. While there was some variability in the angles measured from test-to-test and fairly significant difference from port-to-port, the pitch and yaw patterns remained relatively consistent during the tests. This is not to suggest that these "swirl" patterns are consistent over long time periods.

Heat Rate Bias Components

Figure 10 and Figure 11 illustrate the discrepancies observed between heat rates based on 3-D measurements and the input/output method at Columbia and Coal Creek. As the figures show, once a few biases were corrected, there was excellent agreement between the two measurements. The corrected 3-D-based heat rate agreed within 1.9% of the input/output-based heat rate at Columbia and within 3.4% at Coal Creek. Corrected biases to the 3-D-based heat rate values included:

•FC-factor. At Columbia, the FC-factor calculated based on the average coal percent as-fired carbon and average gross calorific value as determined from as-fired samples collected during the tests was 1839 scf CO2/mmBtu. Using the standard FC-factor of 1800 would result in an overestimation of the heat rate by approximately 2.2%. At Coal Creek, the FC-factor determined from the as-fired coal analysis was 1942 scf CO2/mmBtu. Using the standard FC-factor of 1910 for lignite would result in an overestimation of the heat rate by approximately 1.6%. 
•Wall effects. The S-type and the 3-D flow values were based on equal area traverses which do not take into account the fact that the stack velocity goes to zero at the stack wall. To account for this effect, near wall measurements were taken at Coal Creek. Based on numerical integration, not taking into account the wall effects introduces a bias of 1.9%. Similar effects were also seen in the "swirl" tunnel tests. (Although near wall flow measurements were not taken at Columbia, it is assumed that the wall effect at Columbia is approximately equal to that seen at Coal Creek based on the similarity of the stacks.)
•Manual pressure reading bias. During the field tests, pitot Delta P readings were made both manually using calibrated magnehelics and automatically using a data logger equipped with precision pressure transducers. Subsequent analysis revealed a consistent bias in the manual readings when compared to the readings collected automatically using the data logger. This bias appears to be related to a tendency of individuals to overestimate when doing "eyeball averaging" of fluctuating readings. At Columbia, the manual readings resulted in flow values 1.1% higher than those based on the automatic readings. At Coal Creek, the manual readings resulted in flow values 0.9% higher than those based on the automatic readings.

With S-type pitot measurements, in addition to the bias introduced by the FC-factor, wall effects and manual pressure readings, bias is also introduced by non-axial flow:

•Non-axial flow. The difference between the S-type and the 3-D flow values is related to the non-axial components of the flow that are erroneously included in the velocity head of the S-type measurement. At Columbia where only small non-axial flow components were found, the S-type pitot yielded full-load flow values that were 4.1% higher than the 3-D measurements. Thus, the S-type measurements, and subsequently the CEMS flowmeter data, were biased 4.1% high due to non-axial flow conditions. At Coal Creek, where significant non-axial flow components were found, a 15.0% high bias due to non-axial flow conditions was seen. 

Since CEMS flowmeters are calibrated and certified using S-type pitot reference method flow measurements, any bias in the reference method would be passed on to the certified flowmeter. These "calibration bias" effects include wall effects, non-axial flow effects and manual pressure reading bias. In addition to "calibration bias" and FC-factor bias, any bias in the CEMS CO2 measurement would also be transferred to the CEMS-based heat rate value:

•CO2 Discrepancies. Table 1 and Table 2 show comparisons of average CEMS and reference method CO2 values for Columbia and Coal Creek, respectively. A consistent bias is seen in the CEMS values when compared with the reference method. At Columbia, the CEMS values were an average of 4.5% higher than the reference method values. At Coal Creek, the CEMS values were an average of 3.3% higher than the reference method values. Any bias in the CEMS CO2 measurements would result in a corresponding bias in the CEMS-based heat rate. While some of the CEMS CO2 error may be attributed to calibration drift, we believe there may be a fundamental measurement difference relative to the reference method. Small errors in CO2 are not generally considered to be significant; however, at a nominal 10,000 Btu/kWh heat rate an absolute 0.1% CO2 error is equivalent to about 100 Btu/kWh error in the heat rate. Further study of CO2 error sources may be desirable.

FUTURE ACTIVITIES

As discussed above, a number of "minor" errors were noted during the field tests that are somewhat inexplicable and need further evaluation. In addition, there were fairly large time-dependent errors (drift) observed in CEMs heat rate measurements that need additional study. 

A new EPRI project has been initiated to address these issues. The objectives are:

A.To advise and support follow-on testing by EPA on revisions to the reference methods for flow monitors.
B.Investigate the impact of pitch angle on flow measurement methods and potential temporal changes.
C.Investigate apparent bias in CO2 measurements and causes of non-linearity observed in operating CEMs. 

Observe and Evaluate EPA Heat Rate Study

Since previous EPRI work has shown that Method 2 is seriously flawed, EPA will soon begin a field study to expand the EPRI study with respect to alternative flow measurement probes. EPRI will follow and participate in this EPA study. EPA will also be evaluating a revised 3-D flow measurement method (Method 2F), drafted with industry input, with consideration of promulgating the method for use by electric utilities. EPRI will to continue this interface with EPA to ensure that the method(s) will be reasonably useable by utilities and provide sufficient options for the wide range of industry applications. This study will also evaluate the probe calibration procedures used by the only vendor of 3-D probes because there is some evidence that these procedures are flawed.

Flow Tunnel Pitch Angle Study

There is mathematical evidence that stack flow pitch angle changes over time may be causing the drift observed in ultrasonic flow monitor measurements. (Pitch flow is flow moving from the center of the stack toward the stack wall or visa versa thus the interference with the ultrasonic flow monitor.) In order to evaluate these effects, the EPRI "swirl tunnel" will be modified to create pitch flow. Following the modification, a series of tests will be run to evaluate the impact of pitch flow. (Previous studies had focused on yaw flow because it has the most dramatic effect on Method 2.)

The swirl tunnel will be equipped with a rotatable, T-inlet configuration, designed to generate variable amounts of pitch (and yaw) swirl. Following installation of the new inlet, a series of characterization runs will be made to define inlet setup conditions that generate pitch and yaw swirl conditions that may be generally defined as low, medium and high. After these settings are defined, a series of parametric tests will be performed using the DAT, MS5, S-type and French probes at one tunnel velocity (70 ft/sec.). The tunnel tests will then be evaluated for ultrasonic flow monitor impacts by modifying a previously developed computational flow model (CFD). A CFD model allows for the computation of pitch and yaw angle across every point on the line-of-sight of the ultrasonic flow monitor given some input conditions. The expected flow monitor error can thus be calculated across the total flow monitor path for each of the test conditions from the swirl tunnel. A summary report will then be prepared. 

CO2 Analyzer Error and Linearity Study

From evaluation of a number of utility data sets and during both of the field tests, an apparent constant positive bias has been observed in CEM CO2 readings. This error can have a significant impact on heat rates determined from CEM systems. (A 0.1% absolute CO2 reading error nominally equates to a 100 Btu/kWh error in heat rate.) In addition, a number of electric utilities are reporting difficulty in passing EPA required CO2 analyzer linearity tests. This study is designed to address both of these problems because they are likely related to dilution probe or analyzer design effects.

Arrangements have been made with Scott Specialty Gases, Thermo Environmental Instruments and EPM Corp. to host and provide equipment at no cost for this study. Scott will setup a test rig in their laboratory using instruments and equipment provided by the other two vendors. A series of tests will be developed to evaluate the possible source of positive bias in the CO2 analyzer readings. Sources of analyzer linearity problems will also be evaluated.

It is well known that dilution probe sampling systems (which represent the vast majority of utility CEMs) are subject to a number of variations caused by stack pressure, temperature and gas constituents. The pressure and temperature variations are typically small under normal operating conditions and should be random in nature so that they are of little consequence over the long term. In reality, EPA requires daily CEMs calibrations so many of the short-term variations are being observed and adjusted for, only to move in the opposite direction the next day. Pressure and temperature variations can theoretically be compensated and, in fact, a considerable body of information exists on how to make these compensations. Unfortunately, the corrections are not well understood by utility personnel and, in many cases, the equipment and algorithms needed to make the corrections are either not present or are turned off in the CEMS software. Consequently, a primary output of this study will be a guidance document designed to clearly explain the effects and how to implement appropriate dilution probe corrections. 

A test rig will be setup that will enable the introduction of a wide variety of gas samples at various temperatures and pressures to a complete CEMS analyzer system (dilution probe, SO2, NOX, and CO2 analyzers). The response of the analyzers will be evaluated based on temperature, pressure and gas composition changes and appropriate correction equations will be developed. All of the empirically developed relationships will be evaluated relative to theoretical effects. Finally, a clear, concise guidance document will be prepared for use by utility personnel. 

CONCLUSIONS

Both swirl tunnel and field test results clearly demonstrate that swirl introduces high bias in flow measurements taken using S-type pitot tubes. The degree of swirl-induced bias, however, appears to be less than previously estimated from straight-flow tunnel experiments. Earlier work, performed in straight-flow wind tunnels, suggested about a 10% greater bias (at 30o yaw) than seen in the swirl tunnel. 

The spherical and DAT probe measurements showed excellent agreement with the swirl tunnel venturi. A small difference was observed between the manual manometer and automatic electronic pressure transducer readings both at the swirl tunnel and in the field.

Carefully controlled stack tests were conducted at two sites to determine unit heat rate in comparison with conventional input/output heat rate methodology. Closure within 1.9% was achieved at one site and within 3.4% at the other. Error sources impacting CEMS heat input/heat rate measurements were identified and quantified.

It is clear that the EPA Reference Method 2 is biased high in the presence of yaw swirl in the stack. The amount of bias is related to the amount of yaw swirl--the greater the swirl, the higher the bias. This results in a high bias in stack volumetric flow monitors (and all emissions calculated using the flow monitors) because all of the monitors are presently "calibrated" to Method 2. There is also a positive bias from the stack wall effect that is approximately 2%. 

At the two sites tested there was also a small bias (average 1.9%) in the fuel Fc-factor and the present Acid Rain rules allow for this correction. "Eyeball" manual readings of Delta P also appear to have a slight (~ 1%) positive bias relative to computerized instrumental readings.

Both field test sites also showed a positive CO2 CEM bias that was partially due to "within specification" calibration drift; however, there was some indication of an inherent positive bias. This apparent problem will be investigated in more detail in future work.

It is clear from these field tests that well controlled stack tests, using precise test methods, can be made to produce heat rate measurements that agree (within experimental error) with conventional heat rate methodology. Without equivalent EPA Reference Method accuracy and precision, as well as appropriate real, physical corrections, many utilities will continue to report inaccurate, high-biased emissions under the Acid Rain Program. As a follow on to the work discussed in this paper, EPA is conducting further field tests to determine whether to allow the use of 3-D pitots to eliminate the yaw bias and to evaluate the wall effect correction.

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