Landfill Gas (LFG) SO2 emissions factors

[grgmiller]

Related to LFG emissions factors and issue #69 and #150: eGRID uses two sources of data for their SO2 emisisons factors: EPA's AP-42 document, and the EIA electric power annual (which also uses AP-42 as a source).

EIA assumes that LFG has the same SO2 emissions factor as natural gas (0.0006 lb/thousand ft3), but eGRID uses an SO2 emissions factor for LFG of 0.045 lb/mmbtu, which are calculated based on the 1995 version of AP-42. However, I think that this emission factor may be out of date and overestimating LFG emissions. For example, based on the EIA emission factor, the average SO2 emission factor for all LFG generators in 2020 is about 0.003 lb/mmbtu.

I also tried to slog through the calculation that would have been used to calculate the default emission factor from AP-42, but using the updated draft version of section 2.4 published in 2008. I definitely had to brush off some chemistry knowledge, so the following likely needs to be double checked, but the default number I arrived at was 0.025 lbSO2/mmbtu (which is still way less than the default eGRID value).

Note that the concentration of dimethyl sulfide is highlighted in red because it is unclear in the draft whether the value should be 5.66 or 0.566. The value listed in table 2.4-1 is 5.66E+0, but the lower value of the default C_S mentioned at the top of page 2.4-11 is 33 ppmv, which only works if the concentration of dimethyl sulfide is 5.66E-1.

Note that if we use the default lower value of 33 ppmv S (for newer landfills since 1992) the EF is 0.022 lb SO2/mmbtu, and if we use the higher value of 47 ppmv S (for older landfills pre 1992) the EF is 0.031 lb SO2/mmbtu).

References

• 2008 draft version of AP-42 Section 2.4: https://www.epa.gov/sites/default/files/2020-10/documents/d02s04_0.pdf
• 1998 (Fifth Edition) version of AP-42 section 2.4: https://www.epa.gov/sites/default/files/2020-10/documents/c02s04.pdf

[grgmiller]

It looks like this issue is causing a large discrepancy in SO2 emissions between OGE and eGRID.