# Calculation of the specific CO2 emission of a cogeneration plant for the produced electricity

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.

• 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? – mart Jan 6 '20 at 14:27
• @mart Thanks for your answer. I have now edited my question with a few details and a caluclation attempt. – Emma Jan 7 '20 at 13:12

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 $$\eta_{el}$$)