Converting between MMBTU and MWh

Question

a quick question:

I want to convert between MMBTU and MWh to get at emission factors from energy.

I have the following information:

I use the data given in 40 CFR 98 (see EPA source here) by the EPA and gives me the Stationary Combustion Emission Factors by Energy content (in MMBTUs) for CO2, CH4 and N2O emissions. I convert the CH4 and N2O emissions using the global warming potentials (21 for CH4 and 310 for N20) to CO2eq.

This gives me 94.007 kgCO2eq/MMBTU for coal and 53.112 kgCO2eq/MMBTU for natural gas.

Now that I have CO2eq emissions by a common unit of energy, I use the information I have on a kWh to convert this to kg CO2eq emissions by kWh. A kWh contains 3412 BTUs or 0.003412 MMBTUs of energy. Therefore, a kWh of coal produces 0.3207519 kg CO2eq and a kWh of gas produces 0.1812181 kg CO2eq. (or Therefore, a coal produces 0.32 tCO2eq/MWh and a kWh of gas produces 0.181 tCO2eq/MWh ).

BUT: I have also found information, like here where I get values for a kWh of 0.882 tCO2eq/MWh for Coal or 0.440 tCO2eq/MWh for Gas. These two pieces of information clearly don't square up!!

In essence, I have the information that a country produced 9,028,638 MWh of Coal electricity. With an emission factor of e.g. 0.91 tCO2eq/MWh that would be 8.76 MtCO2eq. But using the other methodology, 9,028,638 MWh would be 28,369,254 MMBTUs and so would be 28,369,254 * (94.007/1000) = 2.6 MtCO2eq

So clearly using the two methodologies, I get two very different results - but I have no clue where I'm going wrong...

Is the emission factor just so different in individual cases?

Thanks for the help!

Answer 1

IMHO, the difference is that you are trying to compare different things. I'll try to sum it up in the following table:

	coal	gas
EPA	0.32 tCO2eq/MWh	0.181 tCO2eq/MWh
e-control	0.880 tCO2eq/MWh	0.440 tCO2eq/MWh
ratio of EPA to e-control	0.36	0.41

It appears to me that when you are using the data in the EPA report you are considering the primary energy content (so its MWh of primary energy).

However, the values from the e-control report consider MWh of electricity generation. Therefore in order to produce 1 MWh of electricity you need to consume about 2.5 to 4 MWh of primary energy depending on the type of fuel and the efficiency of the generator.

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Although there is variability in the efficiency, the interpretation above is supported by the following data:

• According to this the efficiency of gas fired electrical generators is about 42%, and coal is about 33%.
• According to this the efficiency of coal electrical generators is about 37%. (In "Basic concepts: heat into mechanical energy" it states Typical thermal efficiency for utility-scale electrical generators is around 37% for coal and oil-fired plants)