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Evaluating net life-cycle greenhouse gas emissions intensities from gas and coal at varying methane leakage rates [1]

['Deborah Gordon', 'Watson Institute For International', 'Public Affairs Brown University', 'Providence', 'Ri', 'United States Of America', 'Rmi', 'Boulder', 'Co', 'Http']

Date: 2024-08

Our baseline analysis considers life-cycle gas and coal emissions from a global perspective derived from previous studies and meta studies. We estimate the parity between gas and coal emissions at varying methane leakage rates. We then conduct a scenario analysis to identify conditions whereby lower methane leakage rates from gas result in parity with coal life-cycle emissions intensities. In these scenarios we factor in different coal sulfur contents, coal flue gas scrubber efficiencies, methane leakage rates, sulfate aerosol climate interactions, and evaluate climate effects over two timeframes using 100- and 20 year global warming potentials (GWPs).

Figure 1. Schematic of coal versus gas warming and cooling effects on the climate. Source: authors' rendition. Note: life-cycle emissions include extraction, processing, transport, waste disposal, infrastructure construction and decommissioning, and end use combustion. Produced gas can also contain sulfur. But, in general, this is removed as elemental sulfur or sulfur compounds (valuable commodities) rather than combusted into SO2.

Burning coal emits CO 2 and SO 2 , while burning natural gas emits CO 2 but no appreciable SO 2 . Both coal and gas can leak methane. As such, the CO 2 from coal has a warming effect but the sulfate from coal has a cooling effect; in contrast natural gas predominantly warms the planet, as shown in figure 1 .

2.2.1. Timeframes

GHGs warm the planet over different time horizons. CO 2 is a long-lived climate pollutant that resides in the atmosphere for centuries [16]. Conversely, methane is a short-lived gas that warms with a lifetime of about a decade [17]. SO 2 is oxidized in the atmosphere to form sulfate particles, which cool (or effectively mask the warming done by other GHGs) but have a lifetime of a few days against deposition [18, 19]. The GWP metric introduced in 1990 indexes the time-integrated warming effect from a mass (1 kilogram) of a given GHG into the atmosphere relative to CO 2 . Climate effects are commonly considered over two timeframes: 20- and 100 year [17]. Recent studies estimate stronger positive GWP for methane compared to a larger negative GWP for sulfur dioxide (SO 2 ), as displayed in table S2. In this study, we evaluate the effects of life-cycle CO 2 -equivalent (CO 2 e) emissions on both a 20- and 100 year timeframe.

2.2.2. Gas content

Produced gas is mostly made up of methane that ranges from <70% to >90% [20]. The remainder of gases in natural gas can include CO 2 , hydrogen sulfide, oxygen, nitrogen, BTEX (benzene, toluene, ethylbenzene, and xylene), radon, and other chemical contaminants [21]. Impurities are removed during natural gas processing [22]. Gas transported to utilities to generate power has a relatively standard composition: mostly methane with natural gas liquids, nitrogen, oxygen, sulfur, and other impurities [22]. Depending where in the supply chain gas is leaked, varying amounts of methane can be released depending on the chemical composition at that point. For the purposes of this analysis, the methane content of gas is uniformly assumed to be 89.3% [23]. Some gas fields are acidic and sour, containing hydrogen sulfide (H 2 S). H 2 S can be deadly when leaked and it is highly corrosive to pipes and equipment. The majority of H 2 S is removed during gas processing (as elemental sulfur and other valuable sulfur-based commodities). No SO 2 is emitted in end uses when consumers burn gas. Minimal amounts may be present in gas plant effluent streams, however, as discussed in the SI.

2.2.3. Methane leakage

Methane can be emitted from both coal and gas operations, including coal mines and conventional and unconventional gas systems. Unconventional gas includes coalbed methane (CBM), a production method that taps coal seams. Coal mine methane (CMM) is attributed to coal production systems, while leakage from CBM is attributable to gas supply chains. Observed methane leakage rates from coal and gas are wide ranging [14, 15, 24]. Table S5 surveys US methane leakage from gas production systems from <1% to >66%. Additional methane leakage occurs across gas value chains. And the growing array of methane-sensing satellites will increasingly measure global methane leakage, especially from super-emitting point sources. Underground coal mines and surface hard coal mines account for 91% and 9% of global CMM emissions, respectively [25]. The IPCC has established a CMM emission factor of 18 cubic meters methane per tonne of coal mined (m3 methane/t) [25]. Other studies reference a range of CMM emission factors, from low methane content mines with 0.74 m3 methane/t, high methane content mines with 11.43 m3 methane/t, and outburst methane content mines with 40.95 m3 methane/t [26, 27]. Super-emitting methane sources from venting coal mines in the US (Pennsylvania) have been detected via aircraft at 6.7 m3 per tonne of coal, which is within this range [28]. We use the IPCC emission factor in our baseline analysis and bound it with low methane and outburst content mines.

2.2.4. Sulfur content

In addition to emitting CO 2 and methane, gas and coal resources contain sulfur in varying amounts. The sulfur in gas is removed during processing before it is consumed because it is caustic and corrodes pipelines. Therefore, burning gas does not produce sulfur dioxide (SO 2 ). The sulfur in coal, however, remains embedded through the life-cycle and is ultimately combusted into SO 2 —a regulated pollutant that forms aerosols and leads to other environmental and health concerns [29]. Coal sulfur content is reported <1% to >12% by mass (%S wt ), with high-sulfur coal containing >3%S wt [30, 31]. The higher the coal sulfur content, the greater the capacity of SO 2 to mask CMM leakage and the more critical scrubber efficiency is to protect public health and the environment.

2.2.5. Sulfur scrubber efficiency

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[1] Url: https://iopscience.iop.org/article/10.1088/1748-9326/ace3db

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