Refine emissions adjustments for CHP

[grgmiller]

There are several ways that the CHP adjustment (used to calculate emission_mass_lb_for_electricity could be improved.

Assumed values

• When calculating useful_thermal_output, data_cleaning.calculate_electric_allocation_factor() uses an assumed efficiency factor of 0.8, because this is what is used in the eGRID methodology. We should investigate whether this assumption can be improved.
• When calculating the electric_allocation_factor, data_cleaning.calculate_electric_allocation_factor() uses an additional assumed efficiency factor of 0.75, because this is what is used in the eGRID methodology. We should investigate whether this assumption can be improved.

Adjust calculation for bottoming cycles

The eGRID techincal support document notes regarding their CHP adjustment methodology that:

This assumes that the CHP units generate electricity first and use the waste heat for other purposes, also
known as “topping.” While there aresome units that generate and use heat first and then use the waste heat
to generate electricity, also known as “bottoming,” data from the EIA shows that the vast majority of CHP
facilities are topping facilities

However, the EIA-860 generator table contains information about whether each plant uses a topping or bottoming cycle, so we could incorporate this information to create a different calculation for bottoming cycle plants.

According to the data for 2020, of the 72, 337 MW of operable capacity for CHP generators, 67,571MW (93%) uses a topping cycle, while the remaining 7% uses a bottoming cycle.

Refine methodology for adjusting CEMS data

Because CEMS reports data by the unit, our current understanding is that each unit either only produces steam (heat), or only produces electricity, but not both. If this is the case, it simplifies the calculation because we can simply exclude steam-only units from the calculation of emissions for electricity production. However, we need to investigate this further to understand whether this is the case, and whether there would be any reason that any emissions from these plants should be allocated to electricity generation.

[grgmiller]

Also, EIA-923 collects a CHP Efficiency field. As per the EIA-923 instructions:

CHP Plant Efficiency: If the CHP checkbox is “YES”, enter the efficiency of the combined heat and
power plant. Report the annual average total CHP plant efficiency.

To calculate the total plant efficiency, apply the following formula by dividing the sum of the energy
outputs in Btu, including gross generation and useful thermal output, by the sum of the energy inputs
(total fuels consumed converted to Btu).

If gross generation is not available, use net generation as an approximation. Total Fuel Consumption
(MMBtu) is found using Schedule 3 fuel quantities multiplied by their average heat content

Could we use this reported value instead of calculating this ourselves? This formula looks similar to some of the calculations we perform in our CHP allocation step.

[grgmiller]

I have confirmed that the reported steam load data in CEMS is not necessarily related to CHP: #216

[grgmiller]

One paper that explores CHP allocation methods is https://ieeexplore.ieee.org/document/8352048/