MEMORANDUM

DATE: December 17 , 2012

SUBJECT: Establishing an Operating Limit for PM CPMS

FROM: Steffan Johnson Measurement Policy Group, SPPD

TO: Portland Cement Reconsideration Docket

Docket EPA

- HQ

- OAR

- 2011

- 0817

A. INTRODUCTION

The EPA received many comments about the impracticality of the technique specified for establishing operating limits for the particulate matter (PM) continuous parameter monitoring systems ( CPMS ) in the May 2012 proposed amendments to A ir T oxics S tandards and N ew S ource P erformance S tandards for Portland C ement M anufacturing

. 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 emissions measurements. A few of the comment er s further pointed out that this is punitive on the most well

- controlled sources and some offered “scaling” strategies for establishing the PM CPMS operating lim it that they felt would alleviate the problem

. O ther comment er s pointed out that under the proposed approach 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 w ant source s to set operating limits corresponding to an emissions level above the standard. Therefore, we thoroughly analyzed the

comments and scaling strategies included to determine a more workable approach for setting the PM CPMS operating limit

. This memorandum presents the findings of our analysis and provides guidance and calculations for applying this procedure in Portland ce ment and potentially other rulemaking s.

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 highest test run during the performance test

. T his w ould alleviate the potential for setting an operating limit that correspond s to an emissions result higher than the

emission standard

.

The term “scaling” as used in this memo represents the determination of an instrument response slope based on the average PM CPMS output value (in milliamps) corresponding to a 3

- run EPA2

Method 5 PM performance test and the instrument output at zero PM con centration. Once the slope is established, a PM CPMS output theoretically equivalent to a higher PM concentration value may be determined, where an operating limit may be established. A significant complication in consideration of scaling options is that P M CPMS 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 monitoring system (CEMS). F or PM CPMS, the only known relationship between the instrum ent signal and the in

- stack PM concentration is the average value established during manual compliance testing using EPA Method 5 . The scaling approach presumes a linear relationship between the instrument and PM concentration

, and this

is done for two rea sons. Firstly, a linear scaling approach is simple for source operators to use in

calculat ing operating limits and is therefore simpler to comply with. Secondly, where a number of PM instruments do have linear responses to PM concentrations, we acknowled ge that this approach is likely to develop very accurate results. However, this presents difficulty in scaling measured values that lie close to the emission limit, as the linearity of the instrument response is

not demonstrated.

This concept is contrar y to using a calibrated PM CEMS for quantitative determination of a compliance value but holds for qualitative determinations 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 concentrations but also values well above the actual emission limit, as the operating limit is a long term 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 belo w:3

The above graphic shows a PM CEMS calibration trial using PS 11 and illustrates a linear relationship between PM CPMS instrument response and in stack PM concentration. While several data points are required to establish this relationship, we note 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 certification 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 p roblematic to assume a linear relationship and try to predict the PM CPMS instrument response at any other point when only given a zero and single

non

- zero data point. Several data points are needed to establish the curve and relate the instrument signal to actual PM concentrations in the stack.

Both linear and exponential relationships are acceptable when correlating PM CEMS with PS 11; as the number of data points required (15) is sufficient to characterize the inst rument

response curve. With a PM CP MS 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

a ssuming a linear relationship, but it is also not possible to determine the curve and thereby

calculate an accurate estimate. Instruments with either exponential or non

- linear type s of signal output correlation provide sufficient sensitivity to gauge smal l PM concentration changes in the stack; they just have different ways of presenting what they detect. As we do not know what instrumentation will be employed at any given facility 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.

W e reviewed the scaling alternatives provided by commenters as well as several other5

alternatives developed internally, in an attempt to fin d a workable solution to avoid triggering the most operating limit exceedances on the cleanest sources. W e were looking for an approach to “scaling” that leveraged an inverse relationship between PM concentration and the operating limit whereby a source wi th very low PM concentrations would be provided greater operational flexibility than a source with higher PM concentrations and those closest to the PM emission limit would be provided the least amount of flexibility with respect to the operating limit. Th is approach was undertaken in an attempt to provide the greatest operational flexibility to the cleanest sources. We were also aware of potential issues with non linear instrument responses and wanted to investigate options that would provide this flexibi lity 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 performance 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 underestima te the actual PM concentration when such a calculation is conducted. Likewise, calculated values below the measured emissions value on the same graph would be overestimated. While the far right portion of the graph represents a great deal of risk in

estima ting 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 closer 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 professional judgement have determined that the 75 percent scaling cap is appropriate for protecting the emission standard in t his regard.

The options considered were:

1. Establish a linear scale, determine the emission 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 rule 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 some percentage (x) of the “unused” emission limit where x is equal to the percent of the e mission 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 operating 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 operating limit at a point equal to the average PM CPMS value measured during the compliance test plus o ne

- 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 values recorded during the thre e 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 operating limit at 75 percent of the emission limit if a source showed compliance below that lev el, and set the operating limit at the average PM CPMS values for sources demonstrating compliance above that limit (an in house EPA suggested approach)

.

After a review of these six options we noted the following:

Option 1 placed a great deal of confide nce in the presumption that the instrument response to in

- stack PM concentrations are linear beyond the average value determined during the performance test

. This approach presumes PM CEMS capabilities from an

uncalibrated instrument and ignores potential errors from 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 sources that were significantly higher than some sources with high er PM concentrations. This approach also ignored potential errors introduced by non linear instrument response.

Options 3 and 4 provided for flexibility at low concentrations, but also provided flexibility even very close to the PM limi t which did not se em appropriately protective of the emission standard when a non linear instrument may be in use

. For example, this approach would “bump up” operating limits that were set where the compliance demonstration showed the source operating any small amount belo w the emissions standard. We do not feel it is appropriate to measure PM emissions of .0698 lb/ton clinker and adjust the operating limit upward to further crowd the emission limit of 0.07

lb/ton clinker.

Option 5 addressed the issue noted with O ption s 3 and 4

, but added a level of complexity that seemed to confuse rather than simplify the process. Subtracting one standard deviation moves toward being more protective of the emission standard, but is focused at addressing test variability and not instr ument 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 a n already

- established threshold in

the Standards of Performance for New Stationary Sources and Emission Guidelines for Existing Sources : Commercial and Industrial Solid Waste Incineration Unit (76 FR

1575 7 ) to determine the frequency of subsequent compliance testing

. In that rule, owners or operators of sources were able to reduce their performance test frequency when

emissions were equivalent with or below 75 percent of the limits

. Otherwise, performance testing was to occur at the normal frequency prescribed in the rule

. We believe this threshold can be used in conjunction within a PM CPMS scaling factor, as results above 75 percent of the equivalent emissions limit would be ineligible for scaling factor use and could lead to increased performance testing, while res ults equivalent with or below 75 percent of the equivalent 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 s caling looks like considering linear and non

- linear signal s :

This example shows a linear signal output (blue line) and two corresponding non linear signals, one showing decreasing PM concentrations with increased milliamp output (green line) and the other showing rapidly increasing PM concentrations as milliamp sign al increases (red line). Toward the left side of the graph are two vertical lines, the left most represents 75 percent of the emission limit, with the vertical line slightly right of that representing the emission limit. We are primarily interested in wh at is happening to the left of this emission limit 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 and to predicted PM concentrations from

the in strument.

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

In this image you see the linear instrument response (straight blue line) through the middle of the graph. Each line is marked with the same data points, indicating theoretical

concen trations 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. 0 4, 0. 0 525, 0. 0 6, 0. 0 7, and 0. 0 8. In the Portland cement rule, these values represent lb/ton clinker; 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. 0 4. A scaling approach would follow this line to its intersection with

75 percent of the emission limit and establish the corresponding milliamp output as the operating limit for that source.

Of note is the illustration 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 line its sign al is digressing f rom linear but in a direction that will trigger re

- testing via scaling more frequently than its red

- line counterpart. A source using this instrument with test emissions of 0.04 will hit the 75 percent operating limit at approximately 0.05. For the instr ument represented 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 the 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 greater than 75 percent of the emissions limit.9

In the example above, this signal will trigger a retest when the 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 are other parameters in

stack emi ssions that fluctuate and will have an impact on the instrument output. Light scattering PM CPMS are known to vary their instrument response with respect to

characteristics of the PM itself.

In light of such non linear instrument outputs, and the poten tial for varying signal based on PM characteristics, we cannot allow scaling 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 signal linearity is sue.

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 judgement that scaling of PM CPMS 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 affec ted 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 w e find this 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) represents an operating condition of a control de vice (i.e.

, pH, pressure drop, combustion temperature) rather than an actual qualitative measurement of the pollutant of interest.

A second outcome of this assessment is its versatility across production

- based standards (e.g., the

Portland cement rule’s P M/ton

- clinker limit and the MATS rule’s PM/MWh limit)

. One need only ensure that the appropriate units are used when developing equations and conducting

calculations.

A third outcome is that we find with the Portland c ement 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 that should be ameliorated to the extent possible when establishing an operating limit

. Measure ment imprecision is typically highest for values

measured below or near a method’s detection level and decreas es as values increase above the

method detection level

. W hen establishing such a curve based on a single compliance test series, we recognize long er M ethod 5 test times and/or duplicate M ethod 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 appro priate, to adequately quantify in

- stack PM concentration and more accurately establish a PM CPMS output relationship to the compliance test results.10

C

. DETERMINING AN OPERATING LIMIT BOTH FLEXIBLE AND PROTECTIVE

After a review of a number of PM CPMS signal scaling approaches, we arrived at the sixth option above, which is a hybrid approach to allowing scaling of the PM CPMS measurements upward to a point

. We find it reasonable to allow flexibility to sources who have demonstrated compliance be low 75 percent of the emission limit while holding sources whose compliance test

results were closer to the emission limit to an operating limit with less flexibility and tied directly to their PM compliance demonstration.

To establish an operating limit using a PM CPMS a source must conduct a Method 5 or Method 5I performance test

– consisting of at least 3 runs

– annually and establish the relationship between the average milliamp output from the PM CPMS and the average of the PM test runs. Sources with performance tests that show their average PM emissions rate at or below 75 percent of the standard have the flexibility to establish a correlation curve for their CPMS. This correlation curve is based on two data points: th e output of the PM CMPS during the performance tests and the corresponding 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 : the 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 accurate value that represents how the instrument

reads zero PM concentration

. Theoretically

, the z ero 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

establish ing a zero value that is specific to their PM CPMS , preferably in its installed location.

With the PM CPMS output value that represents zero, and the PM CPMS average value s recorded during the compliance test, the scale is delineated in units of the emission standard over the instrument output , e.g., for the Portland cement rule, 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 l b/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 : B ecause of the 4

- 20 milliamp scale we need to subtract the value r epresenting

zero from the average milliamp output so that we may determine the relationship between

PM concentration and the actual milliamp response above zero

. The value representing11

zero is factored back in once this relationship is established (see 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 i ntersects

. This represents the operating limit and is determined by the formula:

Where, continuing with our use of the Por tland 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 milliamps, determined from (1)(i), and

R = the relative lb/ton

- clinker per milliamp for the PM CPMS, from above

.

Note: With other rulemaking applications the units 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 th e importance of being protective of the emission limit

. N ot all PM CPMS instruments are expected to have a linear output in regards to milliamp signal versus PM

concentrati on, we recommend that sources demonstrating compliance at or above 75 percent of emission limit be held to an operating limit representing their average PM CPMS output during

the performance testing.

It is important to note that emission testing is a snap shot and not an indicator of long term emission profiles at a source ; so we recognize that th e relationship established between PM CPMS output and in

- stack PM concentration may be predicated on data that is subject to other operational and seasonable varia bility which may present operational challenges to maintain ing operations within the established operating limit

. Therefore we see the need to provide an allowance for a source to conduct another compliance test and reset this relationship to the most

curr ent data set if the source is unable to meet i t s operating limit

. Sources who show compliance in the range just below

, t o anywhere above

, the 75 percent level are most likely to find themselves facing this mode of operation (i.e.

, more frequent retesting to reset the operating

limit )

. Finally, it bears repeating that we believe a llowing 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 averagin g.