Does Biomass emit more CO2 than coal?

Paul Homewood: asked DECC

I asked DECC to supply comparative figures CO2 produced/MWh, for coal, gas and biomass (specifically wood pellets). I specifically requested that the biomass figures should purely relate to emissions from the power stations, and not to include “whole life” calculations.

He quotes the reply he got, which I have truncated here:

We have identified a calculation of the biologically derived CO2 emitted from a typical power station converted to biomass combustion. The calculation assumed that 47% of the dry weight of the wood pellet is carbon and that large biomass conversions are typically 35.5% efficient at converting the energy content of the wood pellets into electricity. The value calculated was 920 kg CO2/MWh of electricity generated.
… (snip) … The BEAC report and spreadsheet referred to above contain values for the comparable emissions of carbon dioxide from typical coal and natural gas fired power stations of 1018 and 437 kg CO2 equivalent/MWh electricity generated respectively.

This reply is wrong. First the figure for CO2 they give is for dry wood. In fact wood-pellets in the BEAC report contain 7% moisture and produce rather less energy (see Biomass Energy Centre). Also the BEAC documents/spreadsheet they refer to assume a powerstation efficiency of 36% not 35.5%.

Where did their 1018 kgCO2/t come from? It’s paragraph 3 of the executive summary of the BEAC report to which a footnote adds that the figure is not from the chimney only but includes other “life-cycle” emissions. That is, it includes what Paul Homewood asked to be excluded. In fact Table 5D of DUKES 2015 gives 906 kgCO2/MWh for 2013 (for 2014 its only 903, better coal I supopose).

The table below shows some values for the amount of CO2 that goes up the chimney when generating electricity from wood and the correponding figure for powerstation coal. The way thes entrie are calculated is described below:

CO2 from electricity generation with wood fuel
  kgCO2/MWh at efficiency
Moisture %  Calorific Value
GJ/t 
  36% 35.5%
Wood
    0 19   907 920
  Drax 7 17.5   916 929
    10 16.8   923 936
    25 13.6   950 964
Coal
    - 25   906 918
†Not confirmed. But BEAC report uses this value after consultation

So the answer is “Yes”, burning wood sends more CO2 up the chimney than burning coal (but not much more).

Calculations

From Biomass Energy Centre and the spreadsheet ‘Calorific Value as a function of moisture content’ we get:

1 tonne hardwood pellets at 7% moisture produces 17.5 GJ

Using the default value of 47% carbon for dry wood from the BEAC document:

1 tonne hardwood pellets at 7% moisture contains 93% × 47% = 43.7% carbon

Powerstations vary in effciency. I believe that the latest coal fired stations can achieve 40%, old stations can be as low as 25%. The BEAC document “in discussion with users” uses 36% efficency.

Convert GJ to MWh and use thermal efficiency of powerstation to get MWh (electricity)

\[ (17.5\;GJ/t) \times (1/3.6\; MWh/GJ) \times 0.36 = 1.75\; MWH(e)/t\]

12 kg Carbon produces 44 kg CO2 when burned so a tonne of of 7% wood produces:

\[ (0.437\;tCarbon/t) \times (44/12\; tCO2/tCarbon) \times (1000\; kg/t) = 1602.7\; kgCO2/t\]

so the CO2 emitted per MWh generated is:

\[ 1602\; kgCO2/t \times 1/1.75\; t/MWh = 915\; kgCO2/MWh \]

Now for coal. The 2015 issue of DEFRA/DECC carbon Factors for Company Reporting of GHG emissions has both the calorific value of powerstation coal and its emission factor (so the figures will have - should have - been chosen to reflect the same assumptions):

Burning 1 tonne power station coal produces 25.0 GJ and 2.265 tonnes CO2

So a tonne of coal produces 2.5 MWh(e):

\[ (25\;GJ/t) \times (1/3.6\; MWh/GJ) \times 0.36 = 2.50\; MWH(e)/t\]

and the emissions per MWh are:

\[ 2265 \; kgCO2/t \times 1/2.5\; t/MWh = 906 \;kgCO2/MWh \]

Note the calculation is for the CO2 that wood sends up the chimney, but for coal the figure refers to CO2e - it includes other GHG. So the table is biased to wood.

References

Biomass emits double the CO2 of gas, Paul Homewood

DUKES Digest of Energy Statistics 2015

Life Cycle Impacts of Biomass Electricity in 2020, Anna Stephenson & David MacKay July 2014 (BEAC report)

BEAC Model spreadsheet

Calorific Value vs Moisture Content. Biomass Energy Centre