Captured CO2 refers to CO2 that is captured from the air via DAC or the RCC process and is considered to be a negative emission, and the energy use for these processes is accounted for separately. Combustion of NG/RNG/SRNG is considered a positive emission, and captured CO2 and SRNG combustion balance each other out since the carbon for SRNG production came from atmospheric CO2. Biogenic CO2, which is only relevant for biomass and waste feedstocks, is considered a negative emission deriving from the natural carbon cycle and photosynthetic activity. Waste carbon streams include vented CO2 from biogas purification and emissions from digestates15 and are only associated with the AD process. Energy consumption refers to any electrical and thermal energy utilized during capture and conversion processes, including energy required for electrolytic hydrogen production for SRNG. The patterned bars illustrate the reduction in energy consumption emissions when shifting from todays electrical grid mix to an all-renewable energy grid. The emission factors used in these calculations are 2023 U.S. grid mix = 418 gCO2/kWh,33 average renewable power (includes biomass, wind, solar, hydro, geothermal, and nuclear power) = 24 gCO2/kWh.33
