Landfill Gas (LFG) Emission Adjustments

[grgmiller]

The eGRID 2020 technical guide notes:

Emissions adjustments for NOx , SO2 , CH4 , and N2O emissions are only conducted for landfill gas in eGRID. This adjustment is based on the assumption that in many cases landfills would flare the gas if they did not combust it for electricity generation. Therefore, we assume that, at a minimum, the gas would have been combusted in a flare and would have produced some emissions of NOx , SO2 , CH4 , and N2O anyway.

Potential Methodological issues

• Is this a good assumption? Is there data about landfill flaring?
• Even if this is an appropriate assumption, should these emissions be adjusted to zero? As opposed to other biomass fuels, where the argument for adjusting them is that there is no net addition of GHG to the atmosphere because of the carbon that was sequestered in the biomass in the first place, landfill gas energy production still leads to net emissions to into the atmosphere. This starts to border on a consequential accounting approach of emissions since it uses a baseline to make this adjustment. If we are taking a consequential approach, what about emissions from LFG that would have otherwise been vented instead of flared? Relative to the baseline of venting, this reduces emissions, but we are still adding emissions into the atmosphere. This could be a slippery slope. Also, selectively applying a consequential emissions approach to landfill gas emissions is not necessarily appropriate for inventorying attributional emissions from the power sector.

Emission factor assumption

It also notes:

For NOx emissions from landfill gas, an emission factor for flaring of landfill gas, 0.02 tons per MMBtu, is used (EPA, 1995). Note that this factor was converted from units of lb/standard cubic foot (scf) to tons/MMBtu based on a value of 500 Btu/scf (EPA, 2016).

• [x] Need to add this 0.02 emission factor into the data pipeline (we are currently using 0)
• [ ] Look into whether there is a more recent emission factor available, and whether the heating value used is reasonable.

[grgmiller]

Regarding the NOx adjustement for LFG

According to AP-42,

Converting 39 lbNO2 / million scf to lb / MMBtu using a factor of 500 BTU/scf results in a value of 0.078 lb/MMBtu.

As a side note, the existing value of 0.02 tons / MMBtu appears to be incorrect. Converting 39 lb/million scf to tons/MMbtu results in a value of 0.000039 tons / MMBTu. For 0.02 tons/MMBtu to be correct, the lb/million scf value would have to be approximately (0.02 x 500 x 2000=) 20,000 lb / million scf.

The methodology then involves subtracting the baseline NOx emissions from flaring from the unadjusted NOx emissions

[grgmiller]

For now, we are implementing this fix using the value of 0.078 lbNOx/MMBtu.

To prevent negative emissions, we use the formula adjusted emissions = MAX(0, unadjusted_emissions - (fuel_consumed_mmbtu * 0.078))