Much of the sulfur contained in the gas delivered by PG&E consists of compounds that naturally occur in the gas, but PG&E also adds sulfur compounds to odorize the gas. The gas is odorized as a safety measure so that leaks can be detected by consumers.
Common sulfur compounds that may be found in the gas supply are Tetrahydrothiophene (THT), Tertiary Butyl Mercaptan (TBM), Dimethyl Sulfide (DMS), and Hydrogen Sulfide (H2S). A more comprehensive listing of typical sulfur compounds that may be found in the PG&E gas supply is shown at the end of this page. These compounds are harmless at low levels but can cause customer complaints due to excessive gas odor or in the case of Hydrogen Sulfide become dangerous at very high levels.
PG&E continuously monitors the gas stream for specific sulfur compounds at several points on its system as part of its program to ensure that the gas is properly odorized and that sulfur levels are within established limits. The monitoring equipment works well for the intended purpose but it does not provide comprehensive data on all potential sulfur compounds flowing in the gas supply. To supplement the continuous monitoring equipment PG&E has started a program to collect and analyze samples from representative parts of the system to provide an estimate of the total sulfur contained in the PG&E gas supply expressed as parts per million by volume or grains per 100 Standard Cubic Feet.
PG&E's Gas Rule 21, Section C contains specifications on the quality of the gas received into the system and these specifications include limits for sulfur compounds as well as for other constituents contained in the natural gas. Gas Rule 21 can be found in the PGE.com Web tariff book and the sulfur limits are summarized below.
Rule 21, Section C
Section C of Gas Rule 21 provides quality specifications for gas delivered into the PG&E pipeline system from California gas wells and generally governs the gas quality received from interconnecting pipelines. However, gas quality specifications, contained in the interconnection agreement may supersede the Gas Rule 21 C specifications.
- Total Sulfur: The gas shall contain no more than one grain (17 ppm) of total sulfur per one hundred standard cubic feet.
- Mercaptan Sulfur: The gas shall contain no more than 0.5 grain (8 ppm) of mercaptan sulfur per one hundred standard cubic feet.
- Hydrogen Sulfide: The gas shall contain no more than 0.25 grain (4 ppm) of hydrogen sulfide per one hundred standard cubic feet.
Pipeline Quality Gas
Many air quality compliance reporting requirements can be met by certifying that the fuel used in the process was "pipeline quality natural gas." The EPA criteria for pipeline quality natural gas from [40CFR72.2] is as follows:
"Pipeline natural gas means a naturally occurring fluid mixture of hydrocarbons (e.g., methane, ethane, or propane) produced in geological formations beneath the Earth's surface that maintains a gaseous state at standard atmospheric temperature and pressure under ordinary conditions, and which is provided by a supplier through a pipeline. Pipeline natural gas contains 0.5 grains or less of total sulfur per 100 standard cubic feet. Additionally, pipeline natural gas must either be composed of at least 70 percent methane by volume or have a gross calorific value between 950 and 1100 Btu per standard cubic foot."
One criterion is that the total sulfur be less than 0.5 gr/100 scf which is a lower amount than in PG&E's tariff. To help customers understand PG&E gas quality in relation to their reporting requirements, the total sulfur measured in the gas stream as total sulfur is expressed as grains per 100 standard cubic feet (gr/100 scf). PG&E's gas supply typically qualifies as "pipeline quality natural gas" as shown in the Gas System Survey Results table below.
Gas System Sulfur Survey Results
PG&E has a program of periodically surveying representative locations on the system to determine the sulfur containing compound concentrations in the gas stream. PG&E tests for sulfur using ASTM D 5504 "Standard Test Method for Determination of Sulfur Compounds in Natural Gas and Gaseous Fuels by Gas Chromatography and Chemiluminescence." Our detection limit is approximately 10 PPBv for each compound. PG&E odorizes our gas with a 50/50 blend of Tetrahydothiophene (THT) and Tertiary Butyl Mercaptan (TBM) and they are typically present at approximately 1 PPMv each in the natural gas on our system.
Click here for a table providing a representative sample of the gas flowing on the PG&E system.
Typical Sulfur Compounds Contained in Natural Gas
|Compound||Molecular Weight (MW)|
Grams per mole
|Tertiary Butyl Mercaptan (TBM)||90.00|
|Dimethyl Sulfide (DMS)||62.00|
|Hydrogen Sulfide (H2S)||34.00|
|Methyl Mercaptan (MTM)||48.00|
|Ethyl Mercaptan (ETM)||62.00|
|Isopropyl Mercaptan (IPM)||76.00|
|Normal Propyl Mercaptan||76.00|