SAN JOAQUIN VALLEY AIR POLLUTION CONTROL DISTRICT 

MEMO 

DATE: November 13, 2009 

TO: Dave Warner 

FROM: Jim Swaney/Brian Clements 

SUBJECT: Estimate of SOx Impacts of Rule 2020 (Exemptions) Change 

I. Purpose

This memo supports and further details the January 10, 2008, District Rule 2020 

(Exemptions) update to revise the natural gas and liquefied petroleum gas (LPG) fuel 

sulfur content permit-exemption threshold for indirect-fired combustion devices such as 

boilers, steam generators, and process heaters. 

II. Rulemaking Background

Rule 2020 Section 6.1.1 exempts indirect heat transfer units with a heat input rating 5 

MMBtu/hr or less if they are fired exclusively on natural gas, LPG or any combination 

thereof. LPG is a general term that includes propane, butane, and propane-butane 

blends. Previous versions of Rule 2020 requires that LPG have no more than 5% by 

weight hydrocarbons heavier than butane and sulfur content no more than 0.75 grains 

per 100 standard cubic feet (0.75 gr S/100 scf) of gas. 

Stakeholders requested amendments to Rule 2020 because the actual sulfur content of 

LPG is typically higher than the current rule’s sulfur content limit. District staff reviewed 

other California air districts’ rules to determine the sulfur content limit of LPG and 

natural gas for permit-exempt small gas-fired units. Unlike Rule 2020, other air districts 

do not have sulfur-content limits for LPG or for natural gas. Gas Processors 

Association (GPA) liquefied petroleum gas specifications for total sulfur content of LPG 

ranged from 123 parts per million by weight (ppmw) to 185 ppmw. Specifically, the total 

sulfur content specifications are as follows: (a) 185 ppmw for commercial propane; (b) 

140 ppmw for commercial butane; and (c) 140 ppmw for commercial propane/butane 

mixtures. A survey of LPG distributors indicates odorant (typically mercaptan) 

containing between 1 and 3 gr S/100 scf is added to LPG. 

The District had modified Rule 2020, Section 6.1.1, to codify how the District has 

historically applied the exemption to small indirect heat transfer units. The following 

changes were made in the latest rulemaking decision: 

• For natural gas, the total sulfur content limit would be changed from 0.75 grains 

per 100 scf to 1.0 grain per 100 scf of gas. This change was adopted as natural 
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gas suppliers in the District can legally deliver gas with a sulfur content up to 1.0 

gr/100 scf. 

• For LPG, the total sulfur content limit would be no more than 15 grains per 100 

scf of gas and no more than two (2) percent by volume hydrocarbons heavier 

than butane. This change was adopted since, as mentioned above, a survey of 

LPG distributors indicates odorant (typically mercaptan) containing between 1 

and 3 gr S/100 scf is added to LPG, which was higher than the rule allowed (15 

grains per 100 scf is allowed according to LPG standards). 

III. Impacts of Rulemaking Changes

As detailed above, the rule was updated to reflect the actual sulfur content of 

commercially available natural gas and LPG. The rule amendment was necessary to 

correct an error, in that the intent was to allow permitting exemption for units burning 

natural gas or LPG. 

A. LPG Impacts

Strict interpretation of the previous exemption would have required permits for 

nearly all indirect-fired combustion devices fired on LPG, regardless of heat 

input, since the fuel sulfur content specification was lower than normal 

commercially available LPG. The intent of the District has never been to regulate 

and permit nearly all of these types of units, which would include such units as 

travel trailer water heaters and home water heaters in areas without natural gas 

availability, which is common in the foothills of the San Joaquin Valley. 

The rule change now allows the maximum amount of sulfur to be present in LPG 

(15 gr S/100 scf). As such, there is a potential increase in combustion SOx 

emissions (difference between 15 gr S/100 scf and the previous permitexemption limit of 0.75 gr S/100 scf). However, as mentioned above, there is no 

actual increase in emissions since the sulfur content of San Joaquin Valley (SJV) 

LPG has always been 1-3 gr S/100 scf. 

B. Natural Gas Impacts

SJV natural gas suppliers (PG&E, SoCal Gas, etc) regulate the quality of the gas 

supplied to customers and identify maximum allowable fuel sulfur contents as 

follows: 

• Pacific Gas & Electric (PG&E) has a transportation agreement to deliver gas 

with a maximum total sulfur content of 1.0 gr/100 scf (actual: 0.3 to 0.5 gr/100 

scf, based on source testing). 

• Southern California Edison’s (SoCal Gas) Standard Specifications for Natural 

Gas and Substitute Fuel Gases (from SoCal Gas Company 

Procedures,12/26/85) has a recommended total sulfur limit of 0.75 gr/100 scf. 
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• Mojave Gas Company’s FERC Gas Tariff General Terms and Conditions

(12/30/91) requires delivered natural gas to contain no more than 0.75 gr/100 

scf total sulfur. 

Further, the California Air Resource Board (ARB), in their document Guidance for 

Power Plant Siting (Stationary Source Div., 6/99), determined that 1.0 gr S/100 scf 

is the maximum sulfur content of gas supplied by a utility regulated by the Public 

Utilities Commission (PUC). 

As such, the rule change allows the maximum amount of sulfur to be present in 

natural gas (1 gr S/100 scf) which in turn results in a potential increase in 

combustion SOx emissions (difference between 1 gr S/100 scf and the previous 

permit-exemption limit of 0.75 gr S/100 scf). 

However, historical fuel sulfur data indicates there is no increase in actual fuel 

sulfur content allowed, i.e., no increase in actual combustion SOx emissions due 

to the rule change. As shown above, SJV natural gas supplied by PUC regulated 

utilities such as SoCal Gas and Mojave Gas Company continue to meet their 0.75 

gr S/100 scf agreement. As for PG&E, although the maximum fuel sulfur content 

allowed is 1 gr S/100 scf, historic fuel sampling data indicated the sulfur content 

to meet 0.75 gr S/100 scf. As such, previous District permitting actions based 

SOx emissions on a fuel sulfur content of much less than 0.75 gr S/100 scf. So for 

natural gas-fired units, data indicates no additional units were brought under 

District permitting with the 0.75 gr S/100 scf. 

C. Potential Emission Reductions from Other Regulations

Whether or not these units would be subject to District permitting requirements 

does not change the applicability of District prohibitory rule requirements such as 

Rule 4308 (Boilers, Steam Generators, and Process Heaters 0.075 to 2 

MMBtu/hr) and Rule 4307 (Boilers, Steam Generators, and Process Heaters 2 to 

5 MMBtu/hr), so there are no emissions reductions avoided there. 

However, there would be the potential for additional emission reductions from 

permitted units, if Best Available Control Technology (BACT) required it. BACT 

implications for each scenario are as follows: 

1. LPG

 

In the SJV, BACT is triggered with a potential to emit of greater than 2 

lb/day for any pollutant. As such, BACT would only be required for units 

with SOx emissions greater than 2 lb/day. For units above 5 MMBtu/hr, 

BACT is and has always been applicable since those units are subject to 

permitting due to the heat input. For units utilizing the discussed permitexemption (up to 5 MMBtu/hr), calculations indicate that even at the 

maximum allowable fuel sulfur content of 15 gr S/100 scf, the emissions 

do not exceed 2 lb/day. As such, there are no additional emissions 

reductions missed via BACT due to the change in the exemption rule. 
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Further, the actual inventory of emissions for these permit-exempt units 

can be more accurately quantified if based on the actual commercially 

available fuel sulfur content of up to 3 gr S/100 scf. Calculations indicate 

emissions of approximately 0.4 lb/day for units up to 5 MMBtu/hr if based 

on fuel sulfur content of up to 3 gr S/100 scf. 

Finally, even if BACT were triggered, BACT requirements for SOx for the 

category of Boilers, Steam Generators, and Process Heaters has 

historically been the use of LPG or natural gas fuel. Feasibility studies 

indicate that SOx reductions are best achieved by fuel sulfur content 

reductions, i.e. sulfur scrubbers, and that these devices are not cost 

effective per District policy, especially on smaller scale units below 5 

MMBtu/hr. 

 

2. Natural Gas

As shown in Section III.B above, no additional units were brought under 

District permitting with the previous permit-exemption limit of 0.75 gr S/100 

scf, there were no additional emissions reductions due to permitting 

regulations, e.g. BACT. 

IV. Conclusion

The January 10, 2008, District Rule 2020 (Exemptions) update to correct and increase 

the natural gas and LPG fuel sulfur content permit-exemption threshold for boilers, 

steam generators, and process heaters to the actual commercially available maximum 

fuel sulfur contents for natural gas and LPG, has no impact on actual or potential 

emissions in the San Joaquin Valley.