AECOM Peak-to-Mean Paper Machine HAP Emissions Ratios 

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Peak-to-Mean HAP Emissions Ratios for Paper 

Machine Sources 

Prepared by David W. Heinold, CCM 

AECOM Technical Services, Westford, Massachusetts 

 on behalf of the American Forest and Paper Association 

February 15, 2011 

In characterizing acute inhalation risk, the method outlined in EPA’s Risk Assessment and 

Modeling - Air Toxics Risk Assessment Reference Library: Volume 2 - Facility-Specific 

Assessment (EPA-453-K-04-001B, http://www.epa.gov/ttn/fera/risk_atra_vol2.html) is to apply 

HEM-3 to estimate maximum off-site concentrations due to a facility’s emissions. Because 

emissions from some sources are more variable than others, EPA’s guidance indicates that to 

provide a conservative estimate of acute exposure and risk, the upper-limit short-term emissions 

from all hazardous air pollutant (HAP) sources should be simultaneously modeled. The resultant 

maximum 1-hour modeled off-site concentrations are then compared to acute health effects 

benchmarks. In the absence of direct estimates of maximum hourly emissions EPA’s guidance 

provides a default peak-to-mean emissions ratio of 10 to all HAP sources at a facility. This means 

that the annual average emission rates which are used in HEM-3 to estimate cancer risks and 

chronic hazard indices are all multiplied by ten for the acute risk assessment along with the 

maximum modeled 1-hour average concentration. Especially for many sources at paper mills this 

is a highly conservative method of estimating acute risk because it implies that operating 

conditions that result in a 10-fold increase in emissions occur simultaneously for all mill sources 

and that these conditions are also coincident with worst-case dispersion meteorology. Given that 

modeling conducted by EPA has indicated that emissions from paper machines sources may 

substantially contribute to maximum 1-hour average HAP concentrations, an investigation has 

been undertaken by AF&PA to identify a peak-to-mean emission ratio that is more appropriate for 

paper machines than the generic factor of 10. 

In the absence of direct measurements, it is standard practice for HAP emissions from paper 

machines sources to be computed by multiplying the air dried ton of finished product (ADTFP) by 

an emission factor, e.g. those developed by NCASI for various types of paper products. Using 

emission factors to estimate emissions, inherently assumes that emissions are proportional to the 

production rate, which is a reasonable assumption based on available sampling data. This implies 

hourly emission rates are determined by the production rate. 

In addition to considering the variability in paper machine emission rates associated with 

production rate fluctuations, the possibility that emission rates of gaseous organic compounds may 

vary due to changes in process-related conditions has been considered. It would be desirable to 

have continuous emission monitoring data or repeated manual stack sampling data to address this 

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possibility, but obtaining such data is extremely difficult because paper machines have multiple 

vents, and the concentrations of organic compounds of concern to acute risks (acetaldehyde, 

acrolein, formaldehyde) are typically near or below instrumental or manual method detection limits 

in the majority of these vents. 

To help evaluate the degree to which paper-machine emissions vary due production rate and 

other factors NCASI has provided data for a mill that it had in its possession in which methanol 

sampling was conducted on multiple days on a paper machine to support the mill’s Clean 

Condensate Alternative demonstration. In the Cluster Rule, methanol is considered to be the 

surrogate for all volatile organic hazardous air pollutants emitted from pulping, bleaching, 

papermaking and wastewater treatment operations (MACT I sources). Tests were conducted on 

five days for the 18 paper machine vents. Table 1 provides the results of this testing on this paper 

machine. 

Table 1. Measurements of Paper Machine Methanol Emissions and Production 

Date 

Production 

 (tons/hr) 

Methanol Emission 

Rate 

 (lb/hr) 

Methanol Emission 

Factor 

(lb/ton) 

09/06/08 35 28.3 0.81 

09/10/08 43.5 28.1 0.65 

09/12/08 49.9 31.5 0.63 

09/18/08 54.8 30.8 0.56 

09/20/08 34.8 18.4 0.53 

Mean 43.6 27.4 0.64 

Peak/Mean 1.26 1.15 1.26 

The peak-to-mean production ratios for this machine for the five-day period were similar to those 

for other machines over much longer periods of time. For this set of measurements, the data 

indicate that the methanol emissions are highly correlated with production (correlation coefficient 

of 0.73) and the computed emission factors are slightly negatively correlated with production 

(correlation coefficient of -0.36). Thus, the peak-to-mean ratio for methanol emission rate (1.15) is 

less than the peak-to-mean ratio for paper machine production rate, indicating that using paper 

machine production rate to evaluate variability of HAP emissions from paper machines is suitably 

conservative. 

In relation to production rate, other process-related factors that might affect emission rates are 

relatively constant – roof vent exhaust gas flow rates, dryer temperatures, pulp characteristics, 

whitewater recirculation rates, as evidenced by with Table 1 which indicates that the variability of 

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the methanol emission rate is comparable and somewhat less than the variability in production 

rate. There is also no reason to suspect emission rates from paper machines would be higher 

during startup or shutdown conditions, and paper machine malfunctions, which are extremely rare, 

require prompt shutdown. Typically, paper machine upsets correspond to lack of production 

(lowering average production value). Worst case scenario is pulp continues to go to machine, 

then back in a loop and no paper is being made. Since HAP emissions from paper machines are 

a function of the water carrying the pulp (HAPs are contained in solution), if there is no pulp 

(water) going to the paper machine, there are no corresponding paper machine emissions. 

Paper machines are typically operated at the maximum production possible for that specific grade 

(unless limited by permit). Given that paper machines typically operate at a steady production rate 

for up to several days, the daily variability of production rate for a mill throughout a year is 

considered a reliable measure of the variability of the hourly production rate. As it is common for 

mills to retain records of daily production, the peak-to-mean HAP emission ratio for each paper 

machine can be readily computed as the ratio of the maximum daily production rate for the paper 

machine to its annual production rate. 

Several paper companies surveyed their production records and computed the peak-to-mean 

production ratio for each paper machine for twenty mills over the past three years (2008, 2009 and 

2010). The results of this survey are provided in Table 2. Figure 1 shows the variation of the data 

over the last three years and the distribution of peak-to-mean ratios among all paper machines is 

provided in Figure 2. The median peak-to-mean ratio is 1.3, the maximum ratio is 2.1 and the 95th

percent of the ratio values is less than 1.6. 

The consistency of these paper machine production data and the evidence provided by NCASI in 

Table 1 that the variability of HAP emissions from paper machines is comparable or less than the 

variability in the production rate, supports a generic application of a peak-to-mean ratio for paper 

machines that is much less than EPA’s default factor of 10. Based on the variability in daily 

production rate AF&PA recommends that EPA use a factor of 1.6 as it represents a highly 

conservative estimate for the peak to mean ratio for the vast majority of paper machines. 

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Table 2 Daily Peak to Annual Average Paper Machine Production Ratios

Mill Year PM1 PM2 PM3 PM4 PM5

1 2008 1.30 1.31         

1 2009 1.39 1.61         

1 2010 1.27 1.20         

2 2008 1.30 1.28 1.39 2.11   

2 2009 1.19 1.32 1.30 1.74   

2 2010 1.34 1.26 1.30 1.76   

3 2008 1.43 1.41         

3 2009 1.55 1.43         

3 2010 1.56 1.97         

4 2008 1.25            

4 2009 1.26            

4 2010 1.26            

5 2008 1.27            

5 2009 1.24            

5 2010 1.32            

6 2008 1.44 1.31         

6 2009 1.37 1.35         

6 2010 1.35 1.30         

7 2008               

7 2009       1.31 1.39 1.49

7 2010 1.96 1.99         

8 2008 1.38 1.33 1.28 1.21   

8 2009 1.28 1.49 1.31 1.49   

8 2010 1.31 1.48 1.30 1.24   

9 2008 1.22 1.27         

9 2009 1.19 1.29         

9 2010 1.16 1.30         

10 2008 1.36 1.35 1.42      

10 2009 1.41 1.30 1.29      

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Table 2 (cont'd) Daily Peak to Annual Average Paper Machine Production Ratios  

10 2010 1.24 1.31 1.27      

11 2008 1.29            

11 2009 1.31            

11 2010 1.24            

12 2008 1.28 1.40         

12 2009 1.35 1.46         

12 2010 1.29 1.32         

13 2008 1.16 1.43         

13 2009 1.24 NA         

13 2010 1.19 1.32         

14 2008 1.34 1.28         

14 2009 1.32 1.27         

14 2010 1.27 1.31         

15 2008 1.33 1.47         

15 2009 1.21 1.30         

15 2010 1.35 1.31         

16 2008 NA 1.18 1.19      

16 2009 1.21 1.24 1.28      

16 2010 1.19 1.24 1.33      

17 2008 1.25            

17 2009 1.32            

17 2010 1.32            

18 2008 1.22 1.15         

18 2009 1.30 1.23         

18 2010 1.26 1.19         

19 2008 1.20 1.22         

19 2009 1.09 1.08         

19 2010 NA NA         

20 2008 1.48 1.41 1.28      

20 2009 1.55 1.49 1.56      

20 2010 1.56 1.40 1.34      

Note: NA indicates data not available

   

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Figure 1:  Peak‐to‐Mean Production Ratios for Each Year

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Figure 2:  Distribution of Peak‐to‐Mean Production Ratios  

0.00

0.50

1.00

1.50

2.00

2.50

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

CumulativeDistribution of Daily

Peak to Annual Average Ratios at 20 Mills

from 2008‐2010

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