MEMORANDUM

DATE: December 17, 2012

SUBJECT: Establishing an Op erating Limit for PM CPMS

FROM: Steffan Johnson Measurement Policy Group, SPPD

TO: Boiler MACT Docket

Docket EPA-HQ-OAR-2002-0058

A. INTRODUCTION

The EPA received many comments about the impr acticality of the technique specified for establishing operating limits for the particulat e matter (PM) continuous parameter monitoring systems (CPMS) in the May 2012 proposed amen dments to Air Toxics Standards and New

Source Performance Standards for Portland Ceme nt Manufacturing. Most of the commenters focused on the impact on sources with very low PM emissions who, under the proposed amendments, may be required to conduct multiple retests due to the CPMS response to

fluctuation of the in-stack PM characteristics. In other words, the best performing sources were likely to be triggering the largest number of operating limit exceedances simply because of the tight operating limits imposed by their low emi ssions measurements. A few of the commenters further pointed out that this is punitive on the most well-controlled sources and some offered “scaling” strategies for establishing the PM CPMS operating limit that they felt would alleviate the problem. Other commenters pointed out that under the proposed appro ach to establishing the operating limit it would be possible for a source to tie the operating limit to a test run that showed non-compliance with the PM emission limit. We do not wish to promulgate a regulation that is punitive to well-performing sources, nor do we want sources to set operating limits corresponding to an emissions level above the st andard. Therefore, we thoroughly analyzed the comments and scaling strategies included to determ ine a more workable approach for setting the PM CPMS operating limit. This memorandum presen ts the findings of our analysis and provides guidance and calculations for applying this proce dure in Portland cement and potentially other rulemakings.

B. DISCUSSION

The initial focus of our analysis was to more directly connect the PM CPMS operating limit to the determination of compliance, by requiring it to correspond to the average of three PM test runs rather than the single highe st test run during the performan ce test. This would alleviate the potential for setting an operating limit that co rresponds to an emissions result higher than the

emission standard.

The term “scaling” as used in this memo repres ents the determination of an instrument response slope based on the average PM CPMS output va lue (in milliamps) corresponding to a 3-run EPA2

Method 5 PM performance test and the instrume nt output at zero PM concentration. Once the slope is established, a PM CPMS output theoretically equivalent to a higher PM concentration value may be determined, where an operati ng limit may be estab lished. A significant complication in consideration of scaling options is that PM CP MS are not directly calibrated instruments, with known responses to a series of calibration test runs across a wide measurement range, as is a PM continuous emission monito ring system (CEMS). For PM CPMS, the only known relationship between the instrument signal and the in-stack PM concentration is the average value established during manual complia nce testing using EPA Method 5. The scaling approach presumes a linear relationship between th e instrument and PM concentration, and this is done for two reasons. Firstly, a linear scaling approach is simple for source operators to use in calculating operating limits and is therefore simpler to comply with. Secondly, where a number of PM instruments do have linear responses to PM concentrations, we acknowledge that this approach is likely to develop very accurate results. However, th is presents difficulty in scaling

measured values that lie close to the emission limit, as the linear ity of the instrument response is

not demonstrated.

This concept is contrary to using a calibrated PM CEMS for quantitative determination of a compliance value but holds for qualitative determ inations of in-stack PM concentrations.

We note that a PM CPMS instrument used in this manner must have a sufficient measurement scale to cover not only very low PM concentrat ions but also values well above the actual emission limit, as the operating limit is a long te rm average and brief periods of high emissions must be included in such averages.

It is also important to understand the di ffering instrument responses to varying PM concentrations, as may be found by examining the cross section of technologies employed by

various instrument manufacturers. PM CPMS instrument output generally conforms to one of two basic patterns, linear non linear (exponential) . Data showing these two correlations are displayed below:3

The above graphic shows a PM CEMS calibration trial using PS 11 and illustrates a linear relationship between PM CPMS instrument resp onse and in stack PM concentration. While several data points are required to establish this relationship, we not e that if linearity is assumed with such an instrument, PM CPMS instrument output at any PM concentration may be closely predicted based on the known location of zero and another non-zero data point.4

This graphic, again using data from a PS 11 certif ication of a PM CEMS instrument, illustrates an exponential or non-linear relationship between PM CPMS instrument response and in stack PM concentration. In this example it is problem atic to assume a linear relationship and try to predict the PM CPMS instrument response at an y other point when only given a zero and single non-zero data point. Several data points are n eeded to establish the curve and relate the instrument signal to actual PM concentrations in the stack.

Both linear and exponentia l relationships are accep table when correlating PM CEMS with PS 11; as the number of data points required (15) is sufficient to characterize the instrument response curve. With a PM CPMS installation, we do not have the luxury of data with which to establish such relationships. It is important to note that not only is it difficult to accurately calculate a PM concentration value from just two data points (1 at zero and 1 non zero) by assuming a linear relationship, but it is also no t possible to determine the curve and thereby

calculate an accurate estimate. Instruments with either exponen tial or non-linear types of signal output correlation provide sufficient sensitivity to gauge small PM concentration changes in the stack; they just have different ways of presen ting what they detect. As we do not know what instrumentation will be employed at any given faci lity we recognize that we must be sufficiently conservative in our approach of leveraging a single, non zero data point to demonstrate compliance with an operating limit.

We reviewed the scaling alternatives provi ded by commenters as well as several other5

alternatives developed internally, in an attempt to find a workable solution to avoid triggering the most operating limit exceedances on the cleanest sources. We were looking for an approach to “scaling” that leveraged an i nverse relationship between PM c oncentration and the operating limit whereby a source with very low PM concentr ations would be provide d greater operational flexibility than a source with higher PM concentrations and t hose closest to the PM emission limit would be provided the least amount of flexib ility with respect to the operating limit. This approach was undertaken in an attempt to prov ide the greatest operational flexibility to the cleanest sources. We were also aware of poten tial issues with non linear instrument responses and wanted to investigate options that would pr ovide this flexibility without putting the emission

limit at risk in our effort to try and make the operating limit more flexible for clean sources.

With scaling, we are concerned primarily about uncertainty in values that lie above and below the instrument response noted during the pe rformance test. Note the above illustration (p. 3) displaying a non linear instrument response . Errors made in calculating values higher than the measured emissions value, where we assume a linear relationship, are always going

to underestimate the actual PM concentrati on when such a calc ulation is conducted. Likewise, calculated values below the measured emissions value on the same graph would be overestimated. While the far right portion of th e graph represents a great deal of risk in estimating an emission value, errors due to the non linear relationship are mitigated when we focus on the left half of the graph; an area where the relationship of signal to PM

concentration is much clos er to a linear relationship. We recognize that non linear instruments provide increased uncertainty in estimating PM concentrations above the performance test data point and in our best pr ofessional judgement have determined that the 75 percent scaling cap is appropr iate for protecting the emissi on standard in this regard.

The options considered were:

1. Establish a linear scale, determine the emissi on limit on that scale, and allow sources to operate below the emission limit on a 30- day rolling average (as suggested by

Commenter 0261 from the Portland cement ru le Response to Comment Document);

2. Establish a linear scale and set the source operating limit at a point where that scale intersects with the PM CPMS average plus so me percentage (x) of the “unused” emission limit where x is equal to the percent of the emission limit not consumed by source emissions as established by a compliance test (an in house EPA suggested approach);

3. Establish a linear scale and set the source ope rating limit at a point equal to the average PM CPMS value measured during the complian ce test plus one-half of the remaining scale up to the emission limit (an in house EPA suggested approach);

4. Establish a linear scale and set the source ope rating limit at a point equal to the average PM CPMS value measured during the complian ce test plus one-third of the remaining scale up to the emission limit (an in house EPA suggested approach);

5. Use Option 3, but also roll back the operating limit a distance equivalent to one standard deviation of the three PM CPMS average va lues recorded during the three performance tests(as suggested by Commenter 0261 from the Portland cement rule Response to Comment Document); and6

6. Establish a linear scale and set the source ope rating limit at 75 percent of the emission limit if a source showed compliance below that level, and set the operating limit at the average PM CPMS values for sources demons trating compliance above that limit (an in house EPA suggested approach).

After a review of these six op tions we noted the following:

Option 1 placed a great deal of confidence in the presumption that the instrument response to in-stack PM concentrations ar e linear beyond the average value determined during the performance test. This approach presumes PM CEMS capabilities from an

uncalibrated instrument and ignores potential errors fr om non linear instruments.

Option 2 provided for a great deal of flexibility for the lowest emitting sources but also resulted in operating limits for these sour ces that were significantly higher than some sources with higher PM concen trations. This approach al so ignored potential errors introduced by non linear instrument response.

Options 3 and 4 provided for flexibility atlow concentrations, but also provided flexibility even very close to the PM limit wh ich did not seem appropr iately protective of

the emission standard when a non linear instru ment may be in use. For example, this approach would “bump up” operating limits that were set where the compliance demonstration showed the source operati ng any small amount below the emissions standard. We do not feel it is appropri ate to measure PM emissions of .0698 lb/ton clinker and adjust the operat ing limit upward to further crowd the emission limit of 0.07

lb/ton clinker.

Option 5 addressed the issue noted with Options 3 and 4, but added a level of complexity that seemed to confuse rather than simplif y the process. Subtracting one standard deviation moves toward being more protective of the emission standard, but is focused at addressing test variability a nd not instrument linearity.

Option 6 provided flexibility for low emitting and well-operated sources, and was determined to be a reasonable compromise of flexibility and protection of the emission standard. Seventy-five percent of the emission limit is an already-established threshold in

the Standards of Performance for New Stati onary Sources and Emission Guidelines for Existing Sources: Commercial and Industria l Solid Waste Incineration Unit (76 FR 15757) to determine the frequency of subsequent compliance testing. In that rule, owners or operators of sources were able to re duce their performance test frequency when emissions were equivalent with or be low 75 percent of the limits. Otherwise, performance testing was to occur at the nor mal frequency prescribed in the rule. We believe this threshold can be used in conj unction within a PM CPMS scaling factor, as results above 75 percent of the equivalent em issions limit would be ineligible for scaling

factor use and could lead to increased performance testing, wh ile results equivalent with or below 75 percent of the equi valent emissions limit would be eligible for scaling factor7

use and provide greater operational flexibility for sources demonstrating compliance at

lower emission rates.

Here is an overview of what scaling looks like considering linear a nd non-linear signals:

This example shows a linear signal output (blue line) and two corresponding non linear signals, one showing decreasing PM concen trations with increased milliamp output (green line) and the other showing rapidly increasing PM concentrations as milliamp signal increases (red line). Toward the left si de of the graph are tw o vertical lines, the left most represents 75 percent of the emission li mit, with the vertical line slightly right of that representing the emission limit. We are pr imarily interested in what is happening to

the left of this emission lim it line, though not entirely.

Of primary interest is just how a scaling approach that assumes a linear instrument response compares both to the emission limit an d to predicted PM concentrations from

the instrument.

Below is a close up of the region of primary interest:8

In this image you see the linear instrument resp onse (straight blue li ne) through the middle of the graph. Each line is marked with the same data points, indicating theoretical concentrations of PM in the flue gas. Data points on the x axis start at zero (represented by 4 milliamps on the instrument) then 0.025, 0.04, 0.0525, 0.06, 0.07, and 0.08. In the Portland cement rule, these values repr esent lb/ton c linker; and you will note the y-axis scale represents the instrument signal output in milliamps. With each instrument slope we have drawn an assumed linearity line from zero through a theoretical emission test result, in this instance that is 0.04. A scaling approach would follow this li ne to its intersection with 75percent of the emission limit and establis h the corresponding milliamp output as the operating limit for that source.

Of note is the illustrati on of how close these lines are to the “true linear” when we make use

of them on the low end of the instrument scale. The further toward the left from the emission limit line we move our scaling mark the tighter these lines become to “true linear”.

For the instrument represented by the green lin e its signal is digressi ng from linear but in a direction that will trigger re-tes ting via scaling more frequently than its red-line counterpart. A source using this instrument with test em issions of 0.04 will hit the 75 percent operating limit at approximately 0.05. For the instrument re presented by the green line we see that its signal is digressing from linear in the opposite direction; PM emissions per milliamp are increasing more rapidly. An instrument with a non linear output such as this is the reason we need to be cautious in our approach. In th e illustration above, a source using this instrument with performance test PM emissions of 0.04 will trigger the 75 percent operating limit retest requirement when actual stack emissions are great er than 75 percent of the emissions limit.9

In the example above, this signa l will trigger a retest when th e average source emissions are at about 0.0625, or 89 percent of the emission limit.

When we review these examples we must also recognize that there ar e other parameters in

stack emissions that fluctuate and will have an impact on the instrument output. Light scattering PM CPMS are known to vary thei r instrument respons e with respect to

characteristics of the PM itself.

In light of such non linear instrument outputs, and the potential for varying signal based on PM characteristics, we cannot allow scali ng to approach 100% of the emission limit. Furthermore, we feel it prudent to allow room beyond the scaling for an instrument to vary in its response, beyond any si gnal linearity issue.

While the example above displays very marked divergence from linearity on both sides, we recognize that we do not have sufficient data to characterize the universe of PM CPMS technology and thus, must exercise cautious judgement in this regard. As such, it is our best technological and engineering j udgement that scaling of PM CP MS signals is appropriate to 75 percent of the emission limit, and no further. While we might work to justify being even more conservative, we also recognize that the affected sources must have already demonstrated compliance well under the emissions standard to qualify for scaling using this approach.

One outcome of this assessment is that we find th is approach to be an appropriate technique to establish operating limits for parametric monitors where the monitor is measuring the pollutant of interest qualitatively . This approach is not appropriate where a parametric monitor (i.e., pH meter, pressure monitor or thermocouple) repres ents an operating condition of a control device (i.e., pH, pressure drop, combusti on temperature) rather than an actual qualitative measurement of the pollutant of interest.

A second outcome of this assessment is its vers atility across production-based standards (e.g., the

Portland cement rule’s PM/ton-clinker limit a nd the MATS rule’s PM/MWh limit). One need only ensure that the appropria te units are used when developing equations and conducting

calculations.

A third outcome is that we find with the Port land cement source category, which includes many

sources with emission levels at or near the method detection levels (MDLs) for PM, measurement imprecision is a significant factor th at should be ameliorated to the extent possible when establishing an operating limit. Measuremen t imprecision is typically highest for values

measured below or near a method’s detection le vel and decreases as values increase above the method detection level. When establishing such a curve based on a single compliance test series, we recognize longer Method 5 test times and/or duplicate Method 5I tests as a best practice; better data will result in a better operating limit. With this in mind, we recommend source operators and testers to consider extended test times and the use of Method 5I, as appropriate, to adequately quantify in-stack PM concentration and more accurately esta blish a PM CPMS output relationship to the compliance test results.10

C. DETERMINING AN OPERATING LIMI T BOTH FLEXIBLE AND PROTECTIVE

After a review of a number of PM CPMS signa l scaling approaches, we arrived at the sixth option above, which is a hybrid approach to a llowing scaling of the PM CPMS measurements upward to a point. We find it reasonable to allow flexibility to sources who have demonstrated compliance below 75 percent of the emission lim it while holding sources whose compliance test results were closer to the emission limit to an ope rating limit with less flexibility and tied directly to their PM compliance demonstration.

To establish an operating limit using a PM CP MS a source must conduct a Method 5 or Method 5I performance test – consisting of at least 3 runs – annually and esta blish the relationship between the average milliamp output from the PM CPMS and the average of the PM test runs. Sources with performance tests th at show their average PM emissi ons rate at or below 75 percent of the standard have the flexibility to esta blish a correlation curve for their CPMS. This correlation curve is based on two data point s: the output of the PM CMPS during the performance tests and the corr esponding PM emissions rates, and the PM CPMS instrument value during a period of no PM emissions.

The scale used to determine the operating limit value at 75 percent of the emission limit is developed using two pieces of information: th e PM CPMS output at zero and the average PM CPMS response during the performance test. To simplify the process, we specify a commonly used instrumental output, 4 - 20 milliamps, as the basis for all PM CPMS measurements. To establish the low end of the scale we require an a ccurate value that repres ents how the instrument reads zero PM concentration. Theoretically, the zero PM concentration would be represented by a value near 4 milliamps, but not necessarily 4.0, so we leverage the steps presented in performance specification (PS) 11, section 8.6 (5) to direct the source owner/operator in establishing a zero value that is specific to thei r PM CPMS, preferably in its installed location.

With the PM CPMS output value that repres ents zero, and the PM CPMS average values recorded during the compliance test, the scale is de lineated in units of the emission standard over the instrument output, e.g., for the Portland cement ru le, these units would be in terms of lb PM per ton clinker PER milliamp. One could determine this value for the Portland cement rule using the following formula:



#   z X Y

 

1 1 

### R

Where:

R = the relative lb/ton-clinker per milliamp for a PM CPMS,

Y 1 = the three run average lb/ton-clinker PM concentration from a compliance test,

X 1 = the three run average milliamp output from the PM CPMS, and z = the milliamp equivalent of a PM CPMS instrument at zero.

Note: Because of the 4 - 20 milliamp scale we need to subtract the value representing zero from the average milliamp output so that we may determine the relationship between PM concentration and the actual milliamp re sponse above zero. The value representing11

zero is factored back in once this relations hip is established (s ee equation below).

Once the level of PM concentration relating to a milliamp of signal output from the PM CPMS is established, we may determine where on a slope of milliamp output signals the PM concentration representing 75 percent of the emission limit intersects. This represents the operating limit and is determined by the formula:

൅ ݖൌܧ ሻܮሺ0.75

ܴ

Where, continuing with our use of the Portland cement rule as an example:

E = the operating limit for the PM CPMS, in milliamps. L = the source emission limit expressed in lb/ton clinker, z = the instrument zero in milliamp s, determined from (1)(i), and R = the relative lb/ton-clinker per m illiamp for the PM CPMS, from above.

Note: With other rulemaking applications the uni ts above that are Portland cement specific (lb/ton-clinker) would need to be replaced with appropriate units of production (i.e. lb/Gw Hr, lb/Mmbtu, etc.) depending on the units of compliance determination with the rule.

While this process results in a good deal of flexibility for sources well below 75 percent of the emission limit, we recognize the importance of be ing protective of the emission limit. Not all PM CPMS instruments are expected to have a linear output in regards to milliamp signal versus PM concentration, we recommend that sources demons trating compliance at or above 75 percent of emission limit be held to an operating limit re presenting their average PM CPMS output during the performance testing.

It is important to note that emission testing is a snapshot and not an indicator of long term emission profiles at a source; so we recognize that the relation ship established between PM CPMS output and in-stack PM concentration may be predicated on data that is subject to other operational and seasonable variability which may pr esent operational challenges to maintaining operations within the establishe d operating limit. Therefore we see the need to provide an allowance for a source to conduct another complian ce test and reset this relationship to the most

current data set if the source is unable to m eet its operating limit. Sources who show compliance in the range just below, to anywhere above, the 75 percent level are most likely to find themselves facing this mode of operation (i.e., more frequent retesti ng to reset the operating limit). Finally, it bears repeating that we beli eve allowing a PM CPMS scaling function in excess of 75 percent of the emission limit is not an appropriate approach for compliance assurance through long term operating limit averaging.