Need for a marginal methodology in assessing natural gas system methane emissions in response to incremental consumption

Figures & data

Table 1. Drivers of methane emissions from sources in the natural gas system. scf: standard cubic feet, CH₄: methane.

Category	Description	Potential equations	NG system examples
Time	Continuous emissions via leakage or venting. Events occurring at a known frequency	Emissions (scf CH4) = Time interval (time) * Magnitude (scf CH4/time)	Continuous pneumatic controllers; Fugitive leaks
Random event	Emissions related to discrete events or occurrences with no systematic or causal dependence on system flow	Emissions (scf CH4) = Event probability (event/time) * Magnitude (scf CH4/event)	System damage; Intermittent pneumatic controllers (IPC)
Throughput	Event or occurrence-based emissions whose frequency and magnitude depends upon system flow rate or cumulative flow over a time period	Emissions (scf CH4) = Volume throughput (scf gas) * Magnitude (scf CH4/scf gas)	Compressor operations emissions; Manual liquid unloadings

Table 2. Potential methane emission sources from the natural gas system. From U.S. EPA (U.S. EPA 2016).

Main sector	Emission source				Potential methane emissions, kt	Percentage (%)	Potential for nonmarginal emissions*
Production	Gas wells (nonassociated gas wells and gas wells with hydraulic fracturing)				51.4	1.09	✓
	Well pad equipment (heaters, separators, dehydrators, meters/piping, compressors)				291.2	6.18	✓
	Gathering and boosting (stations, pipeline leaks)				2134.2	45.31	✓
	Drilling, well completion, and well workers				28.11	0.60	✓
	Produced water from coal methane (Powder River and Black Warrior)				60.9	1.29	✓
	Pneumatic device vents				1065.2	22.62	✓
	Dehydrator vents				21.7	0.46	✓
	Pumps (chemical injection, Kimray)				196.5	4.17	✓
	Condensate tank vents				33.7	0.72	✓
	Compressor exhaust vented (gas engines)				248.6	5.28
	Liquid unloading (vent using plungers and vent without using plungers)				218.4	4.64
	Blowdowns (vessel and compressor blowdown, compressor starts, etc.)				207.1	4.40	✓
	Upsets (pressure relief valves and mishaps)				2.1	0.04	✓
	Offshore (shallow and deep water gas platforms; Gulf of Mexico and Pacific)				150.6	3.20	✓
Total production					4709.7	100.00
Processing	Plant fugitives, reciprocating compressors, centrifugal compressors, flares, dehydrators				156.3	35.13	✓
	Compressor Exhaust (gas engines, gas turbines, AGR vents, pneumatic devices)				253.1	56.89	✓
	Blowdowns/venting				35.5	7.98	✓
Total processing					444.9	100.00
Transmission & Storage	Stations venting	Transmission and storage			183.3	10.86	✓
	Stations fugitives	Transmission and storage			142.2	8.43	✓
	Dehydrator vents	Transmission and storage			6.5	0.39	✓
	Pneumatic venting	Transmission and storage			52.1	3.09	✓
	Wells	Storage			92.6	5.49	✓
	M&R (Trans. Co. Interconnect, farm taps, direct sales)				92.5	5.48	✓
	Compressors	Compressors fugitive	Reciprocating	Transmission	339.4	20.11	✓
				Storage	104.2	6.17	✓
			Centrifugal	Transmission	115.7	6.86	✓
		Compressors exhaust	Engines, turbines, generators		288.3	17.08
	Pipeline venting				183.6	10.88	✓
	Pipeline leaks/fugitive				3.3	0.20	✓
	LNG operation (storage, compressor exhaust, import terminal)				83.92	4.97	✓
Total transmission and storage					1687.6	100.00
Local Distribution	Underground pipelines	Mains	Cast iron		32.1	6.69	✓
			Unprotected steels		48.1	10.02	✓
			Protected steels		46.9	9.77	✓
			Plastic		20.4	4.25	✓
		Services	Unprotected steels		47.8	9.96	✓
			Protected steels		18.6	3.87	✓
			Plastic		12.5	2.60	✓
			Copper		4.4	0.92	✓
	Metering and regulating (M&R) facilities	M&R	M&R (>300 psi)		8.6	1.79	✓
			M&R (100–300 psi)		14.6	3.04	✓
			M&R (<100 psi)		5.7	1.19	✓
		Regulators	Regulators (>300 psi)		3.8	0.79	✓
			Regulators (100–300 psi)		1.9	0.40	✓
			Regulators (40–100 psi)		6.5	1.35	✓
			Regulators (<40 psi)		0.4	0.08	✓
		Regulator-vaults	Regulator-vaults (>300 psi)		0.2	0.04	✓
			Regulator-vaults (100–300 psi)		0.6	0.12	✓
			Regulator-vaults (40–100 psi)		0.4	0.08	✓
	Upsets (mishaps/dig-ins)				67.1	13.98	✓
	Routine maintenance (pressure relief valve releases and pipeline blowdown)				5.5	1.15	✓
	Customer meters (residential and commercial/industrial)				134	27.91	✓
Total local distribution					480.1	100.00

Notes. *Preliminary assessment by authors based on survey of the literature cited. Identifies categories of emissions in which some portion may arise from sources independent of throughput. It should be noted that the majority likely exhibit some degree of both independence and dependence, i.e., partial dependence.

Figure 1. A representative meter set assembly (MSA) with major components highlighted.

Figure 2. Methane emissions from MSA with simulated external leak systems in response to inlet pressure variation.

Figure 3. Methane emissions from MSA with simulated external leak systems in response to flow rate variation.

U.S. EPA. 2016. Annex 3.6: Methodology for estimating CH4 and CO2 emissions from natural gas systems. Annexes to the Inventory of U.S. GHG Emissions and Sinks., United States Environmental Protection Agency. https://www.epa.gov/sites/production/files/2017-02/documents/3._6_natural_gas_systems_annex_2017-2-10_.pdf (accessed June 6, 2018).

 Google Scholar