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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 Small Combustion Units 

Summary: This memorandum describes the analysis that was conducted to set an equipment 

threshold on small external combustion units. 
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Equipment Threshold for Small Combustion Units 

In the final rule, EPA has set an equipment threshold on sources with a rated heat capacity equal 

to or less than 5 million British thermal units per hour (MMBtu/hr) on external combustion 

devices, below which only activity data is required to be reported. This equipment threshold was 

developed in response to several comments that suggested small combustion devices with 

minimal fuel use should not be required to report emissions using rigorous and/or burdensome 

quantification methods. This memorandum describes the analysis that was conducted to set an 

equipment threshold on small external combustion units. This threshold does not apply to 

internal combustion engines or turbines. 

Comments 

Commenters from onshore production and natural gas distribution have described the burden on 

owners and operators who must comply with the metering requirements under the Tier 3 

methodology in subpart C of the Greenhouse Gas Reporting Rule, for minor combustion units 

including heat tracers, glycol dehydrator reboilers, separator heaters, crude oil heater-treaters, 

and small tank heaters. Commenters suggested that these small combustion devices typically 

have an average heat rating of approximately 0.3 MMBtu/hr. In addition, EPA has proposed 

amendments to subpart C that would ease the Tier 3 requirements for combustion units less than 

30 MMBtu/hr. 

Expert Opinion 

EPA consulted experts to gather opinions on the distribution of equipment heat ratings in the 

onshore petroleum and natural gas production segment. Opinion was that a 5 MMBtu/hr 

equipment threshold would exclude 50% of external combustion devices, while a 10 MMBtu/hr 

equipment threshold would exclude nearly all external combustion units. Additionally, most 

heater-treaters consume less than 5 MMBtu/hr and glycol dehydrator reboilers typically consume 

0.4 MMBtu/hr. 

Quantitative Analysis of Floor Options 

The objective in determining the equipment threshold is to not require all of the small emission 

sources listed by the commenters as minor combustion devices. To test the thresholds suggested 

by experts and commenters, calculations were performed on glycol dehydrator reboilers, which 

commenters listed as a small external combustion unit. The suggested equipment thresholds for 

the external combustion devices were converted to dehydrator throughputs. Hence, the number 

of dehydrator reboilers excluded was determined. 

Methodology

In the Natural Gas STAR Lessons Learned documentation titled “Replacing Glycol Dehydrators 

with Desiccant Dehydrators1,” partners are able to quantify the fuel gas used by a glycol reboiler 

unit using Equation 1. 

Hv cf Mcf

F W Qr R days yr FGR

1000 /

365 .

⋅

⋅ ⋅ ⋅ ⋅ = Equation 1

 

1

 http://www.epa.gov/gasstar/documents/ll_desde.pdf 
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where, 

FGR = Fuel gas for reboiler (Mcf/yr) 

F = Gas flow rate (MMcf/d) 

W = Inlet minus outlet water content (lb/MMcf) 

Qr = Heat duty of reboiler (Btu/gal TEG) 2

Hv = Heating value of natural gas (Btu/scf) 3

R = Glycol-to-water ratio (gal TEG/lb water) 4

This equation can be rearranged as shown in Equation 2. 

hour/day

F W Qr R

days/year hour/day

FGR Hv cf Mcf Heat Rate

365 24 24

1000 / ⋅ ⋅ ⋅ = ⋅

⋅ ⋅ = Equation 2

where Heat Rate is the combustion heat delivered by the fuel gas used in the reboiler each hour. 

Equation 2 can be further manipulated to estimate the throughput of a glycol dehydrator with 

knowledge of the heat rate. 

W Qr R

Heat Rate hour/day F ⋅ ⋅

⋅ = 24 Equation 3

Equation 3 was used to estimate the natural gas throughput of a glycol dehydrator unit that 

requires a reboiler with a heat rate of 0.3 MMBtu/hr, 5 MMBtu/hr, and 10 MMBtu/hr. The 

percentage of dehydrators excluded was estimated using the calculated size of glycol dehydrators 

from the memo “Equipment Threshold for Dehydrators.” 

The definition for external combustion is as follows: 

External combustion means fired combustion in which the flame and products of 

combustion are separated from contact with the process fluid to which the energy is 

delivered. Process fluids may be air, hot water, or hydrocarbons. External combustion 

equipment may include fired heaters, industrial boilers, and commercial and domestic 

combustion units. 

The definition for internal combustion is as follows: 

Internal combustion means the combustion of a fuel that occurs with an oxidizer (usually 

air) in a combustion chamber. In an internal combustion engine the expansion of the 

high-temperature and –pressure gases produced by combustion applies direct force to a 

component of the engine, such as pistons, turbine blades, or a nozzle. This force moves 

the component over a distance, generating useful mechanical energy. Internal combustion 

 

2

 Based on calculation in Engineering Data Book, Volume II, 11th edition, Gas Processors Supply Association, 1998, 

Section 20-Dehydration.

3

 Energy Information Administration (EIA), Monthly Engineering Review, Table A4. 

4

 From EPA's Lessons Learned: Optimize Glycol Circulation and Install Flash Tank Separators in Glycol 

Dehydrators. 
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equipment may include gasoline and diesel industrial engines, natural gas-fired 

reciprocating engines, and gas turbines. 

Result

A reboiler with a 0.3 MMBtu/hr heat rate is associated with glycol dehydrator with a throughput 

of 152.52 MMcf/day. This throughput is larger than approximately 99% of all glycol dehydrators 

in the onshore petroleum and natural gas production segment, based on the distribution of 

dehydrator sizes estimated in the memo “Equipment Threshold for Dehydrators.” A 5.0 or 10.0 

MMBtu/hr heat rate is associated with glycol dehydrators having throughputs of 2,542 

MMcf/day and 5,084 MMcf/day, respectively. Both these heat rates do not include all onshore 

production glycol dehydrators. In addition, based on comments from the natural gas distribution 

segment, this threshold would also not include small combustion units in distribution. 

Table 1: Estimated throughput of glycol dehydrators 

and percentage of dehydrators below each threshold. 

Heat Rate 

(MMBtu/hr) 

Throughput 

(MMcf/day) 

Percentage of 

Dehydrators Below 

Threshold 

0.3 152.52 99% 

5.0 2,541.94 100% 

10.0 5,083.88 100% 

Conclusion 

An equipment threshold of 0.3 MMBtu/hr will not include 99% of all glycol dehydrators but will 

likely not eliminate from reporting all heater-treaters as per expert opinion. EPA is aware that 

there are large external combustion equipment in some onshore production operations, , such as 

large steam boilers used for secondary recovery which can range from under 10 MMBtu/hr up to 

40 MMBtu/hr, and in distribution internal combustion, such as compressors. EPA does not want 

to not include such equipment. As a result, the equipment threshold on external combustion 

devices was set at a rated heat capacity equal to or less than 5 MMBtu/hr.