MEMORANDUM

DATE: June, 02, 2015

FROM: Steffan Johnson

Measurement Policy Group, SPPD

TO: Bruce Moore

Fuels and Incineration Group, SPPD

SUBJECT: Background for 60.5412(a)(1)(ii) Total Organic Carbon (TOC) exhaust gas standard for 

combustion control devices to control Volatile Organic Compound (VOC) emissions from oil and gas 

affected facilities. 

INTRODUCTION:

On October 15, 2012 EPA finalized 40CFR 60 Subpart OOOO, setting a standard emission limit of 20 

ppmv on a propane basis, for enclosed combustion devices on affected units with the intent that this 

emissions limit demonstrated appropriate control of the inlet gas stream. On reconsideration the 

petitioners have asserted that 20 ppmv is not indicative of rule-required 95 percent control but rather is 

indicative of control levels well beyond 95 percent and is therefore not the appropriate numerical ppmv 

level at which to set an emissions limit. This memorandum supports our analysis for revising the 20 

ppmv level on reconsideration, to substitute 600 ppmv as propane as the appropriate numerical 

representation of ppmv indicative of 95 percent control by enclosed combustors located at affected 

facilities. 

DISCUSSION:

Field gas is different from “pipeline” natural gas in that it has not been “sweetened”, odorized, or 

otherwise processed to make it suitable for distribution and sale. As such, sources of information that 

speciate the composition of hydrocarbons in field gas that are broadly representative of field gas across 

the country are not readily available. Lacking a pool of data that is representative of the hydrocarbon 

speciation profile of well fields across the nation (our preferred approach for establishing such a value 

would be generated from an average of such a data collection) we are relying on a published average

that appears to generally represent well gas speciation.

 

Most field gas (85% to 95%) removed from the ground at observed well sites is composed of methane, 

ethane and propane. These are largely non-VOC hydrocarbon species, otherwise known as straight 

chain alkane compounds (methane, ethane, and propane) that are readily oxidized in an enclosed 

combustor control device at a destruction rate in excess of 99%. A smaller fraction of the gas making up 

about 4% to 7% of the total are non-combustible gases such as carbon dioxide, oxygen and nitrogen. 

The remaining 3% to 4% are C4 and higher hydrocarbon compounds, are generally categorized as VOC, 

and are more difficult for a control device to fully oxidize. As the methane, ethane and propane 
components of the gas stream are easier to combust, this analysis focuses on removal levels of these 

more difficult-to-oxidize VOC compounds. By determining the level where 95% of these compounds are 

mitigated, we also determine a value representative of appropriate level of control device effectiveness.

Subpart OOOO) requires the use of EPA Method 25A for measurement of hydrocarbon emissions and 

determination of enclosed combustor efficiency. This test method employs a THC analyzer that

measures carbon atoms in gaseous compounds and produces a signal response to each hydrocarbon 

molecule in proportion to the number of carbon atoms present. Methane has one carbon atom, 

propane has three, pentane five, and so on. Where these compounds are present in the emissions

stream, the THC analyzer will not identify the species, but responds with a single value that represents 

the sum of all carbon atoms present in that moment. In other words, the THC analyzer instrument 

response is the sum number of carbon atoms present in the gas, not the specific ppm concentration of 

any particular gas species. A THC analyzer reading 100 ppm “as carbon” would read 33.3 ppm when 

interpreted “as propane” due to the number of carbon atoms in the propane molecule. As these 

instruments are normally calibrated with propane gas, EPA is setting the standard to be on an “as 

propane” basis.

A review of published information on well gas species from: ”Production and Transport of Oil and Gas”, 

2

nd Rev. Ed., Part B. A.P. Szilas, Elsevier 1986. A table on page 55 provided the information presented 

below.

By tallying the hydrocarbon data values C4 and higher from the data provided in this reference, an 

average inlet concentration of 37,100 carbon molecules are likely VOC. 95% control of this value would 

be represented by 1,855 carbon molecules, and when measured on a propane basis the value at 95% 

control would be 618.3 ppmvw, as propane. 

From 18b Production and Transport of Oil and Gas -

Szilas, A. P. Page 55

c1 0.792

c2 0.159

c3 0.0119

c4 0.0075

c4 0.0217

c5 0.0019

c5 0.0022

c6 0.0028

c7 0.001

Total 1

Sum C4-C7 0.0371

Methane 37100

95% 1855

Propane 618.3333 ppm

600 ppm

CONCLUSION:

A combustor with outlet THC concentrations

at or below 600 ppmvw as propane will 

provide adequate demonstration that a 

control device has combusted 95% of the 

inlet gas VOC content.