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Suppose I have a cogeneration plant that runs on natural gas and produces heat and electricity. The heat effeicieny is $\eta_{th} = 0.4$ and the electricity efficiency is $\eta_{el} = 0.4$.

The burning of natural gas produces 215 g/kWh of CO2.

For the calculation of the CO2 emission of the generated electricity from the cogeneration plant, one could now calculate $\frac{215}{0.4} = 537.5$ g/kWh.

However, I would like to calculate the CO2 emissions for the electricity generation of the cogeneration plant, taking into account that CO2 emissions are saved because another heat generator has to generate less heat.

A natural gas boiler with an efficiency of $\eta_{th} = 0.9$ is taken as a reference for heat generation. So the 1 kWh of heat is generated with $\frac{215}{0.9} = 238.88$ g of CO2.

How do I calculate the CO2 emissions for the electricity generation in this case?

My calculation attempt:

First I would calculate how much CO2 emssions the boiler has for 0.4 kWh:

$238.88 \cdot 0.4 = 95.55$ g of CO2

Then I would subtract this value from the previously calculated co2 emission of the electricity:

$537.5 - 95.55 = 441.95$

The electricity generated by the cogeneration plant would then have a CO2 emission of 441.95 g/kWh.

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    $\begingroup$ Hi and welcome to this site! This is not a we do your homework for you site so please xplain what you've tried so far and where you are stuck. I feel something is missing from the question, to what purpose is the efficeincy of the boiler given? the 215g correspond to lower or higher heating value? $\endgroup$
    – mart
    Commented Jan 6, 2020 at 14:27
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    $\begingroup$ @mart Thanks for your answer. I have now edited my question with a few details and a caluclation attempt. $\endgroup$
    – Emma
    Commented Jan 7, 2020 at 13:12

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Your basic logic is sound: You compare the case of the cogeneration plant + boiler vs. boiler alone. Your math looks correct to me.

Where did you get the value 215 g/kWh from? All (german language!) sources I found give 202g/kWh for natural gas, which may well be because actual of differences in grid gas composition. When I do the math myself I arrive at 197 g/kWh if using the lower heating value, only 178 g/kWh for the HHV.

The difference between HHV and LHV could be crucial since many boilers today are condensing boilers (that utilize the heat of evaporation of the exhaust water) but few to none cogeneration plants are, so you should possibly use different carbon intsities for both.

Lastly, the effiencies for the cogeneration plant don't look like real life numbers but are not too far of (I'm just now looking at a datasheet for a small CHP with 38% and 43% respectivly, in my experience this is a rather high $\eta_{el}$)

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  • $\begingroup$ Thank you very much for your detailed answer. Actually, the values like natural gas emissions that I used were from my memory, I was mainly interested in the calculation. The point about taking HHV and LHV into account is a good hint, thank you very much. Could you perhaps add your calculation for 197 g/kWh and 178 g/kWh? (I'm trying to figure out how you came up with that) $\endgroup$
    – Emma
    Commented Jan 9, 2020 at 18:57
  • $\begingroup$ I did it in an excel sheet that I since deleted but It follows from the stochiometry of the combustion and the molar HHV/LHV. I hop I didnt make a mistake :) $\endgroup$
    – mart
    Commented Jan 9, 2020 at 20:36

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