Description
Onsite, direct measurements, downwind tracer flux, and other test method (OTM) measurements were made at 9 renewable natural gas (RNG) plants located in the central US states of Kansas, Oklahoma, and Texas between 8/17/2021 and 9/10/2021. CH4 emissions originating between the inlet and outlet of each plant were quantified. Plant-level, throughput-normalized emissions (emission intensity) based on plant inlet CH4 flows, emission factors developed from measurements and observations, coupled with estimates for emissions from flaring and waste gas streams range from 0.4% to 5.7%. RNG plants without direct venting all show emission intensities less than 1%, while those that vent some portion of waste streams directly to atmosphere show emission intensities of 4-6%. Concurrent downwind measurements at two plants employing direct venting support modeled results, but indicate slightly higher emission intensities of 5-7%.
In aggregate, the 1.8% CH4 emission rate from the 9 plants, including the direct venting, is comparable to that of flaring (≈2%). If all 9 plants exhibited similar emissions as those without direct venting, CH4 rates would be approximately reduce by 2/3rds, to ≈0.6%. Considering total equivalent carbon emissions, expressed as CO2e, for the 9 plants studied, including direct venting at some of the plants, the RNG gas has approximately one half of the carbon intensity of natural gas. If direct venting of gas can be eliminated, resulting in a CH4 emission rate of 0.6% of methane handled, RNG gas has approximately one third the carbon intensity of the equivalent amount of natural gas.
Report by Timothy Vaughn, Cody Ross, Clay Bell, Chiemezie Ilonze, Winrose Mollel, Daniel Zimmerle
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