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Energy Assessment: How much Carbon Dioxide per Kwh of Gas?

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    Posted By: tedThe SAP 2005 (Apr 2008 updated) figure is 0.194 kg CO2 per kWh and Annex A of the Digest of UK Energy Statistics 2005 states natural gas is 11 kWh/m3 so that's 2.134 kg CO2 per m3.


    But that's the input kWh, not the output of the appliance. If the appliance is rated at 90% efficiency and an output of 5kW, then the input is 5.55kW and so the CO2 emission is also correspondingly 11.1% higher too. It is often easy to confuse the input and output power ratings of fossil fuel appliances. No such problem with anything electric - despite what many people might think they're all 100% efficient (not counting heat moving devices such as heatpumps or air conditioners).

    So for KimiK, it could be that the actual CO2 emission is 2.37kg as they'll use 1.11m3 to get the heat output of 1m3 of raw natural gas.

    Paul in Montreal.
    •  
      CommentAuthorted
    • CommentTimeSep 4th 2008
     
    Surely if the boiler consumes 1m3 of gas then it will have liberated 2.134 kg of CO2. Burning 1m3 of gas will do this irrespective of the appliance that uses it as this is a property of the fuel.

    The amount of useful heating that this then generates in the house is another matter and is based on the boiler efficiency. If this is 90% then only 90% of the 11kWh will be useful; i.e. 9.9kWh heating and 1.1kWh wasted.

    Please correct my thinking on this if I am wrong.
  3.  
    Posted By: tedPlease correct my thinking on this if I am wrong.


    No you are correct - I just wanted it to be clear that the output efficiency has to be considered in the kg CO2/kWh figures since it is the input rating that counts, not the output. Burning 1m3 gas will always liberate the same amount of CO2 (everything else being equal like delivery pressure and such like).

    Paul in Montreal.
    • CommentAuthorKimiK
    • CommentTimeSep 5th 2008 edited
     
    I am much obliged to both of you, Paul and Ted.

    This figure of 2.134 kg CO2 / m3 is very close to what is presented on the CRAG website without explanation (i.e. 2.2 kg CO2 / m3).
    Link to the CRAG website: http://www.carbonrationing.org.uk/wiki/carbon-conversion-factors/
    I have sent along a query in the hope of reconciling or explaining the difference between 2.134 and 2.2. On the face of it, it appears to be the result of rounding: .194 rounded up to .2 gives .2x11=2.2.
    I shall report back here when/if they respond.

    Your posts have also clarified to me that a "90% efficient" boiler is not less than 100% efficient because of any characteristic of the burn itself, but rather because of the less-than-100% efficiency in capturing and transmitting the heat liberated in the burn. This is very helpful to me, because I was under the impression that there may be variation in the efficiency/completeness/cleanliness of the burn itself (and the implication of your posts is that such variation, if it exists, isn't factored into the CO2 calculation).

    k.
    •  
      CommentAuthorted
    • CommentTimeSep 5th 2008
     
    Some of the CO2 figures available in different places do vary a bit. I've also seen .19 quoted for natural gas.

    But if the purpose of your calculation is in any way related to SAP then 0.194x11 is the right figure to use.
    • CommentAuthorKimiK
    • CommentTimeSep 5th 2008
     
    The 2008 update of Defra's GHG Conversion Factors has recently become available, and in this report the figure of 0.194 has been updated to 0.206 kg CO2 / kWh.

    http://www.defra.gov.uk/environment/business/envrp/pdf/ghg-cf-guidelines-annexes2008.pdf

    k.
    •  
      CommentAuthorted
    • CommentTimeSep 5th 2008
     
    Hmm 6%. That's a big change. I'm wondering if the figure that has been used in SAP includes the equivalence values for the methane and N2O. I'll try and cross check.

    Incidentally that figure of 11 kWh/m3 for natural gas looks like it may have been the subject of averaging as well.

    Annex A1 of the BRE Methodology for Environmental Profiles of Construction Materials gives a gross calorific value for natural gas of 39.1 MJ/m3 which converts to 10.86111 kWh, but this is based on 1996 data.

    Wikipedia (!) quotes UK natural gas as being 39,710 kJ/m3 which is 11.03055 kWh.

    I suspect that this is a value that is subject to periodic revision as the actual chemical composition of natural gas varies slightly over the lifetime of its extraction.

    Copy of the BRE doc is available here: http://cig.bre.co.uk/envprofiles/downloads/BRE%20Environmental%20Profiles%20Methodology.pdf?id=0000000000000000000000000000000000000041&type=.pdf
    •  
      CommentAuthorted
    • CommentTimeSep 5th 2008
     
    The DTI use 39.6 MJ/m3 which happens to be exactly 11 kWh. And it comes with an interesting footnote:

    The gross calorific value of natural gas can also be expressed as 10.992 kWh per cubic metre. This value represents the
    average calorific value seen for gas when extracted. At this point it contains not just methane, but also some other
    hydrocarbon gases (ethane, butane, propane). These gases are removed before the gas enters the National Transmission
    System for sale to final consumers. As such, this calorific value will differ from that readers will see quoted on their gas bills.

    http://www.berr.gov.uk/files/file19273.xls
    •  
      CommentAuthorted
    • CommentTimeSep 5th 2008
     
    Kimi, just had a look at the DEFRA doc you posted a link to.

    That figure of 0.206 is the Net Calorific Value. For a condensing boiler you should be using the Gross Calorific Value which is quoted there as 0.185.

    This is rapidly turning into a tail chasing exercise. Just how 'accurate' does the figure need to be? It seems possible to justify anything between 2.0 and 2.2.
    • CommentAuthorMike George
    • CommentTimeSep 7th 2008 edited
     
    I have been using the Part L figure of 0.194kg/kWh. See the attached document for an explanation from first principles.

    Is it possible that any variations are due to molecular differences in the gas? Does this vary by source? Ie North Sea/Russia for example?
    •  
      CommentAuthorted
    • CommentTimeSep 7th 2008
     
    There are certainly a wide range of MJ/m3 figures quoted for natural gas based on geography.

    But I wonder where your author got his 800 kJ/mol figure for Methane from? The purveyors of truth at Wikipedia [1] have a figure of 809 which would give 195.8g CO2. Whereas this [2] shows the calculation to get a figure of 803.

    Also natural gas as delivered to homes is not 100% methane, as there are odour additives to enable detection of leaks, but I guess in such small quantities as to make no material difference to the CO2 calculation.

    1 http://en.wikipedia.org/wiki/Methane
    2 http://www.physics.ohio-state.edu/~wilkins/energy/Resources/Lectures/combenergy.html
  12.  
    He did the calculation for me when I was a student. I think he just grabbed a reference text from his shelf for the 800kJ/mol. It's his field though, so I doubt he would be far wrong.
    • CommentAuthorCWatters
    • CommentTimeSep 8th 2008
     
    Notts say..

    http://www.nottenergy.com/energy-costs-comparison3

    "Lower emissions factors are published by DEFRA, for company emissions reporting; these do not include indirect emissions from fuel supply chain."

    Humm, so companies are allowed to under report their emissions?
    • CommentAuthorKimiK
    • CommentTimeSep 9th 2008 edited
     
    CWatters -- do you know of a recognised reliable study (i.e. published in a respected journal) that augments the Defra figures with either Life-Cycle Analysis or Input-Output-estimates of the indirect emissions?

    ...I would be keen to read it...

    k.
    • CommentAuthorKimiK
    • CommentTimeSep 9th 2008 edited
     

    Posted By: ted
    That figure of 0.206 is the Net Calorific Value. For a condensing boiler you should be using the Gross Calorific Value which is quoted there as 0.185.


    Ted -- sorry to reveal my ignorance, but would you (or anyone else) mind helping me understand why in this case the gross calorific value is lower than the net calorific value?

    Several Defra publications (dated 2008) include the following footnote:

    "Energy and emissions are currently calculated on a Gross Calorific Value basis in the UK, however it is anticipated that in the near future calculations will be moved to a Net Calorific Value basis, which is also consistent with the European Union Emission Trading Scheme (EUETS) for CO2 emissions."


    k.
    •  
      CommentAuthorted
    • CommentTimeSep 9th 2008
     
    The actual MJ/m3 Gross Calorific Value is higher than the Net CV figure. But here we are not looking directly at the GCV and NCV but at the CO2 kg/kWh figures that are based on those and which have an inverse relationship so the Gross is a smaller value than the Net.
    •  
      CommentAuthorted
    • CommentTimeSep 9th 2008
     
    Posted By: CWatters
    "Lower emissions factors are published by DEFRA, for company emissions reporting; these do not include indirect emissions from fuel supply chain."

    Humm, so companies are allowed to under report their emissions?


    Not quite. There are defined boundaries to prevent double counting.

    When you fill up with petrol at your local BP garage do you want a proportion of all of BPs 'overhead' CO2 added to the CO2 in the petrol or should BP be required to account for their own separately?
    • CommentAuthorKimiK
    • CommentTimeSep 10th 2008 edited
     
    According to my calculations, regardless of whether one starts with a net or gross CV basis and working with three significant digits gives the result that the CO2 externality of natural gas is 2.05 kg CO2 / m3.

    My work follows:

    <> 0.206 kg CO2 / kWh for natural gas (net calorific basis) from Source#1, Annex 1.
    <> 9.94 kWh / m3 average (net) calorific value of natural gas from Source#2, Annex A.1, p. 208.
    calculated as (35.8 MJ/m3) / (3.6 MJ/kWh) = 9.94 kWh / m3
    <> 0.206x9.94=2.05

    <> 0.185 kg CO2 / kWh for natural gas (gross calorific basis) from Source#1, Annex 1.
    <> 11.1 kWh / m3 average (gross) calorific value of natural gas from Source#2, Annex A.1, p. 208.
    calculated as (39.8 MJ/m3) / (3.6 MJ/kWh) = 11.1 kWh / m3
    <> 0.185x11.1=2.05

    Source#1: Defra (April 2008) Guidelines to Defra's GHG Conversion Factors: Annexes.
    Source#2: Defra (2007) Digest of UK Energy Statistics.

    Comments welcome.

    k.
  19.  
    A belated comment that the UK emissions coefficients are roughly

    gas 0.195 kg/kWh
    LPG 0.235
    oil 0.27
    electricity 0.55

    Electricity is somewhat higher in winter when many plants in use are coal-fired and similarly is lower in summer. DEFRA now uses a figure of 0.53 and has disowned the BERR figure of 0.42 which used to be standard in govt literature. However the figure is tending to rise further as more coal fired plants are brought back into service.

    Germany uses higher figures again of 0.68 for electricity and 0.25 for gas.

    The UK figures don't account for issues which appear to be associated with particular fuels, and should arguably be allocated to those fuels, such as methane leaks from pipelines and methane leaks from coal mines.

    HTH.

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