1 TSRIA Document 5: PPL Plume Behavior Does Not A ffect High Coburn Readings

During the February 27 meeti ng with EPA, EPA speculated that a possible reason for the higher readings at Coburn Road might be the plume behavior at the PPL stack. EPA conjectured that some hi gh Coburn Road readings migh t have occurred due to, for example, a low flow exhaust rate or a lo wer-than-normal temperatur e at the stack. The reasons for such conditions at the stack c ould include startup, s hutdown, low production and other related causes.

The result of a low flow (and/ or temperature) condition could, theoretically, yield a plume that would not have the same buoyancy as normal. This low-buoyancy plume might, theoretically, impact the ground near Coburn more than during normal buoyancy.

To examine this hypothesis, we compiled data relating to this issue. Under the SIP monitoring program, PPL meas ures a parameter that s pecifically addresses the buoyancy of the plume on an hourly basis. This hourly measurement is required by DEQ, and it is provided to the agency on a quarterly basis.

The parameter in question is known as “buoy ancy flux.” This term is in common use for purposes of dispersion modeli ng. In fact, the dispersion models use this specific parameter in order to estimate how far a pl ume will rise above the top of the stack as it moves in a downwind direction from its origin . For purposes of this discussion, we will not engage in the mathematics or derivati on of the formula that describes buoyancy flux. Suffice it to say that it is an hourly va riable than takes into account stack plume exit velocity, volume flow rate, stack exit temperat ure, ambient temperatur e differential, etc.

Using that parameter, two data sets were obtained. The first is a listing of all hourly buoyancy flux data collected at PPL but ex cluding those hours in which PPL was not operating. The second data set was a list of hourly buoyancy flux data at any time that Coburn road exceeded 60 ppb.

If one assumes that higher Coburn readings ar e related to unusual plume-rise stack conditions (reflected as low buoyancy flux), then there should be a difference between
the two data sets described above. We te sted this hypothesis using a standard statistical technique: an “Unpaired T-Te st” assuming the variances between the two means are not equal. The result s of that investigation ar e found in the table below:2 Buoyancy Flux Comparison and Statistics Normal Operations vs. Periods of Peak Coburn Data

Buoyancy Flux

(PPL Exhaust) Descriptor 2009 – 2013 > 60 PPB Readings at Coburn Mean, BFlux 324 309 Variance 5,584 4,681 Count (operating hours) 34,665 57

Unpaired T-Test (assuming unequal variance) Hypothesis: The unpaired difference betw een the two means is the same.

(There is no difference between ‘normal’ PPL buoyancy flux and the buoyancy flux when Coburn readings are high.) Degrees of
Freedom 56

t-statistic 1.66Critical tstatistic (95%) 2.00

Conclusion: There is not enough evidence to reject the (null) hypothesis. It is
reasonable to conclude that the two dat a sets are, in effect, the same. Thus, conditions that lead to high ppb readings at Coburn are not directly associated with buoyancy flux data from PPL.

The results of the table indica te that there is no statisti cal difference in PPL exhaust plume-rise characteristics (buoyancy flux ) during periods of high Coburn readings versus the rest of the five-y ear period. Plume rise variati on also does not explain the high readings associated with the QA fa ilures identified in the data-set.