Page 1 of 4 UNITED STATES ENVIRONMENTAL PROTECTION AGENCY WASHINGTON, D.C. 20460

OFFICE OF
AIR AND RADIATION

Petroleum and Natural Gas Systems (Subpart W) of the Greenhouse Gas Reporting Rule Docket: EPA-HQ-OAR-2009-0923

Title: Equipment Threshold for Dehydrators

Summary: This memo discusses the development of a minimum throughput of well pad gas dehydrators, below which emissions could be calcul ated using emission fact ors rather than rule specified equilibrium process models.Page 2 of 4 Equipment Threshold for Dehydrators

This memo discusses the development of a mi nimum throughput of well pad gas dehydrators,

below which emissions could be calculated usin g emission factors rather than rule specified equilibrium process models. Industry comment ers pointed out correctly that many wellhead dehydrators have very small gas throughput an d small methane vent emissions, for which application of the April 2010 propos ed rule glycol dehydrator em issions model such as GRI’s GlyCalc® would be a substantial burden for lit tle contribution to emissions. EPA determined that glycol dehydrators at well pads with gas th roughput less than 0.4 million standard cubic feet per day (mmscf/day) represent a large fraction of well pad dehydrators wh ile contributing a small fraction of total emissions from dehydrators. Th is memorandum describes the analysis that was conducted to determine this threshold. The dehydrat or threshold developed is applicable to all dehydrators in subpart W.

EPA examined thresholds from existing regula tions and potential other equipment thresholds. Subsequently, EPA evaluated an acceptable equipm ent threshold using aggregated data from the HPDI database. As part of the analysis, it estimated the number of dehydrators that are above and below the threshold and their ag gregate emissions contributions.

Precedence for Glycol Dehydrator Thresholds

The National Emission Standards for Hazardous Ai r Pollutants (NESHAP) regulations require operators to follow standards on installation, ope ration, monitoring of control devices for major sources of hazardous air pollutants. These standa rds are set forth in 40 CFR Part 63 Subpart HH 1
for petroleum and natural gas production faci lities, and in 40 CFR Part 63 Subpart HHH 2 in petroleum and natural gas transmission and stor age facilities. Under Subpart HH, owners or operator are exempt from sta ndard set forth in §63.764 if

(i) The actual annual average flowrate of na tural gas to the glycol dehydration unit is

less than 85 thousand standard cubic meters [3 million standard cubic feet] per day, as determined by the procedures specifie d in §63.772(b)(1) of this subpart; or

(ii) The actual average emissions of benz ene from the glycol dehydration unit process vent to the atmosphere are less than 0.90 me gagram per year, as determined by the procedures specified in §63.772( b)(2) of this subpart.

This exclusion clause sets a threshold of 85,000 standard cubic meters per day (3 mmscf/day) throughput, below which it is not consider ed a major source of HAP emissions.

Under Subpart HHH, standards set fort h in §63.1275 only apply if the:

1 EPA. 40 CFR Part 63 63.760 – 63.779. Subpart HH—National Emission Stan dards for Hazardous Air Pollutants

From Oil and Natural Ga s Production Facilities. http://ecfr.gpoaccess. gov/cgi/t/text/textidx?c=ecfr&sid=2c6a4334a98427e866966423764af7f6&rgn=div6&view=text&node=40:10.0.1.1.1.8&idno=40

2 EPA. 40 CFR Part 63 63.1270 – 63.1289. Subpart HHH—National Emission Standards for Hazardous Air Pollutants From Natural Gas Transmission and Storage Facilities . http://ecfr.gpoaccess.gov/cgi/t/text/textidx?c=ecfr&sid=be33069d80bdecef68193057a216b329&rgn=div6&view=text&node=40:11.0.1.1.1.4&idno=40Page 3 of 4 (1) The actual annual average flow of gas to the glycol dehydration unit is less than

283.0 thousand standard cubic meters [10 million standard cubic feet] per day, as determined by the procedures specified in §63.1282(a)(1); or

(2) The actual average emissi ons of benzene from the glycol dehydration unit process vents to the atmosphere are less than 0.90 megagram per year as determined by the procedures specified in §63.1282( a)(2) of this subpart.

This exclusion clause sets a threshold 283,000 st andard cubic meters per day (10 mmscf/day) throughput, below which it is not consider ed a major source of HAP emissions.

Quantitative Analysis of Threshold Options EPA applied the 80:20 criteria used for subpart W to select the top emission sources that contribute 80% of the emissions in a sector, wh ile not including or minimizing the burden for

sources that contribute the re maining 20% of emissions.

Methodology

HPDI database contains oil a nd gas production data from thou sands of reporting properties across the U.S. EPA aggregated reporting propert ies by field, and the petroleum and natural gas production rates and well count s were estimated for each.

The number of glycol dehydrators was estimated in the U.S. Inventory of Greenhouse Gas

Emissions and Sinks (U.S. Inventory). EPA calculated the ratio of glycol dehydrators per well by dividing the total number of glyc ol dehydrators by the total number of gas producing wells. It

used this ratio to determine th e number of glycol dehydrators in each field, as characterized by HPDI. This analysis distributed dehydrators evenly to production properties by gas production

to determine a size dist ribution of dehydrators.

To fully characterize the uncontrolled emissions fr om glycol dehydrator units, emission factors for the absorbed methane released in the reboiler, and the natural gas used to power the glycol chemical circulation pneumatic pump were need ed. Emission factors for both of these sources

are available in the EPA/GRI 3 study Methane Emissions from the Natural Gas Industry , June 1996. Under NESHAP regulations, onshore produc tion operators of glycol dehydrators with throughputs greater than 3 mmscf/ day are required to reduce their HAP emissions by 95%. As a result, to calculate the actual methane emi ssions, EPA scaled down the uncontrolled methane emissions from glycol dehydrators with a throughput greater than 3 mmscf/day by 95%.

Results Table 1 shows the results of the quantitative analysis. To obtain 20% of the emissions determined by calculating with an emission fact or, then the threshold should be set at approximately 0.386 mmscf/day. At the subpart HH NESHAP threshold of 3 mmscf/day, 97% of total emissions from glycol dehydrator vents woul d be calculated using an emission factor. At the 10 mmscf/day threshold, used in subpart HHH of NESHAP, 98% of total emissions from glycol dehydrator vent s would be calculated us ing emission factors.

3 EPA/GRI. Methane Emissions from the Natural Gas Industry, Vol. 14. June 1996. Available online at: <http://epa.gov/gasstar/tools/related.html>.Page 4 of 4 Table 1: Options for thresholds on glycol dehydrator units.

Percentage of

Population
Emissions Emissions
Covered (scf/yr) Dehydrator Throughputs (scf/day) Dehydrator
Count

20% 1,620,755,208 386,909 22,644

21% 1,689,438,814 400,000 23,022

97% 7,851,805,696 3,000,000 33,423

98% 7,973,038,944 10,000,000 37,035

Total 8,114,537,522 37,816

Emission Factor Development

EPA reviewed several data sources for emission f actors suitable for glycol dehydrators with gas throughput less than 0.4 mmscf/day (rounded from 0.386 mmscf/day). It determined that the same emission factors it used to develop th e threshold would be adequate to account for emissions from those dehydrators that fall below the threshold. The EPA/GRI 3 emission factors which were used for the analysis require the i nput for each dehydrator and its configuration (gas assisted glycol pum p, electric pump, etc.)

EPA determined it could reduce burden on repor ters for determining emission from dehydrators falling below the threshold by developing a per-de hydrator emission factor, rather than requiring reporters to monitor the conf iguration and throughput. It divided the methane and carbon

dioxide emissions covered at th e 0.4 mmscf/day threshold in th e above analysis by the total number of dehydrators that fall below that thre shold. When adjusted for standard conditions (defined as 68  ̊ F and 14.7 psia in subpart A), the resu lting emission factors were 74.5 thousand standard cubic feet (mscf) per dehydrator per year of methane and 3.26 mscf/dehydrator/year of

carbon dioxide.

Conclusions EPA determined that use of the glycol dehydr ator throughput thresholds currently used under subpart HH or subpart HHH of the NESHAP regula tions would result in collecting 97% or more of emissions data from this source by calculating with emission factors. This would be no better than the U.S. Inventory and not su itably provide data adequate to inform future policy. Using the principles of the 80:20 criteria, EPA dete rmined that a minimum throughput (floor) of 0.4 mmscf/day would require the la rgest 60% of wellhead dehydr ators, representing 80% of emissions, to report using simulation software. This will allow nearly 40% of dehydrators, representing 20% of emissions, to report using an emission factor per dehydrator. This would be a substantial reduction in burden to reporters whil e maintaining the targeted data quality of this

rule.

EPA determined that the most appropriate em ission factors for dehydrators below 0.4 mmscf/day throughput, are 74.5 mscf/dehydrator/year for meth ane and 3.26 mscf/de hydrator/year of carbon

dioxide. These emission factors would provide sufficient estimates of emissions for these small dehydrators while diminishing burden to reporters.