Biomethane – Degrees of decarbonisation

Recently a POST note was produced on decarbonising the gas network. An early draft had a mistake in the number quoted for the carbon footprint for biomethane and finding the source of this error turned up a number of interesting issues which some may find disagreeable.

In short, biomethane is probably not as green as it is painted. It has a useful role, especially (possibly only) if some defects are rectified; but the greener it is, the less there is of it. Expansion of biomethane options needs to take account of these intrinsic difficulties. Capture of the vented CO2 may become necessary and profitable.

[This post written 29th Nov. 2017 and updated on 7th Dec. 2017 and 20th Jan.2018]

Conversion Factors for GHG reporting

The POST team used official statistics from BEIS designed for companies reporting their emissions “Government emission conversion factors for greenhouse gas company reporting“. Unfortunately those numbers are not actually used for any real purpose except as one of the inputs to the UK emissions report to the UNFCCC and to track the UK progress against the carbon budget process. So while they should be accurate, nobody gets upset if they are only approximate. Any fallout from inaccuracy would come years  in the future and only on an aggregated basis: nobody’s job is on the line. So I would expect that they are the last set of data to be updated. If we want the best and most recent data we should look elsewhere.

Nevertheless it is instructive to look at those official “conversion factors” as this will set the context for obtaining better numbers elsewhere. This is a bit of a treasure hunt along a chain of official publications and illustrates why it is sometimes so hard to get a simple, correct answer to a simple question. The most recent 2017 reports include spreadsheets of data and a methodology paper. (Reading the spreadsheets makes these all relate to “pure” biomethane, before it has the calorific value corrected to make it identical to natural gas.) The data for biomethane is given in the methodology paper in Table 42 on page 83, and we see a footnote that the number for biomethane Total Lifecycle is 10.11 gCO2e/MJ and actually comes from a different report, from the Department of Transport:


This indicates that we are in trouble. This data is implicitly relevant for biomethane used for road fuel, which is not typical of the biomethane injected into the grid. Indeed there is only one plant supplying biomethane road fuel in the UK but there are now over 80 AD plants in the UK injecting biomethane into the gas grid. Note also that the CH4 and N2O emissions – which increase the value by 10% – are presumably typical of combustion in a road vehicle and not combustion in a boiler. So these too will also be incorrect if we want to know the carbon footprint of injected biomethane.

Road fuel biomethane

The original version of this data is published by Dft “Biofuel statistics: Year 10 (2017 to 2018), report 1” and the relevant part of the spreadsheet is:

Now this is a bit odd. We know there is only one plant putting biomethane into vehicles (Leyland, Lancs.), and we are pretty sure that they are using UK food waste from Widnes, not sugar beet waste imported from Hungary or food waste from Austria. So this is pretty certainly corrupt data. (One has to be suspicious that may be copied from data provided by Czechia or Slovakia.) It is also misquoted: BEIS says 10.00* (plus CH4 and N2O emissions) whereas this is clearly 10.5 gCO2e/MJ (i.e. 37.8 gCO2e/kWh presumably LHV calorific value**).

Now the Leyland filling station uses the green gas certificate mechanism (see footnote #), so it is actually filling trucks with fossil natural gas. So to find the correct feedstock for DfT to use, they should be using the  AD sites which are issuing those certificates. And indeed it seems to be food waste “by Green Gas Certificates from AD plants that use waste feedstock, such as ReFood in Widnes” but they don’t give a complete list of feedstocks or say precisely which plants these are. We could use the BEIS RHI statistics to finds the exact list of feedstocks at Widnes…  or we could look at the 2017 list provided by Green Gas Certificates but this road-fuel data is all beside the main point.

What we want is the mean GHG saving for all the biomethane injection in the UK – not the corrupt data copied by DfT from a single plant or sub-set of plants which may be atypical. So we shouldn’t be using this corrupt data from DfT at all. The master reference document for how this is all calculated is issued by Ofgem.

[It is worth noting that where RHI subsidy is claimed for heat produced by burning biogas, the sustainability regulations apply to the heat (page 8), not the biogas calorific value. So the GHG criteria are significantly stricter for this than they are for biomethane injection.]

UK-wide biomethane injection carbon accounting

We could  use the Sustainable Gas Institute (SGI) report for biomethane where pp70-71 of that report says less than zero up to 450 gCO2e/kWh for biomethane, depending on feedstocks and counterfactuals. This is repeated in the conclusions: “Biomethane CO2  emissions range from -50 to 450 gCO2e/kWh” on page 91. But in the UK, the RHI will be taking a dim view of anything more than 125 gCO2e/kWh as that is the legal maximum for RHI (Renewable Heat Incentive) subsidy.  [125.28gCO2 equivalent per kWh  is 34.8gCO2e/MJ]. So that gives us a more restricted likely range of -50 to +125 gCO2e/kWh.

BEIS don’t seem to be reporting very fully on the RHI feedstocks – each plant has to submit a feedstock report to Ofgem but Ofgem don’t have to publish those very promptly.  We know that the carbon footprint depends very sensitively on the feedstocks used : whether they are wastes or crops, and what would have happened to those wastes if they did not go to AD. So without those detailed reports on each of the 80 plants we have to fall back on the published impact analysis of the injection policy as a whole. Page 64 of that impact analysis is “Greenhouse gas abatement” but it is mostly phrased in the effectiveness of the policy as a whole in MT of CO2, not per kWh of gas, so it is table C15 on page 70 we want:


But (sigh) they have changed the units from g to kg, so these numbers are all 1000x smaller than the ones we usually use: so that is a range from -604 to +145 gCO2e/kWh for food waste and crops respectively. [ See later post about the 113 gCO2e/kWh number. ]

Profit-seeking operators will want to use crops, whereas BEIS wants them to use slurries and food waste. We know this from the studies which re-evaluated the level of subsidy and implicitly showed that crops are more profitable: “RHI biomethane injection to grid tariff review“. (Actually crops are really worse than +145 because they have upstream emissions from fertiliser, esp. N2O emissions, as well as cultivation and harvesting carbon costs.) So we can assume that many AD operators will be working at the maximum legal limit, which is the profit-optimal position, which is 125.28 gCO2e/kWh.

When the published revised RHI regulations become effective early in 2018 (in parliament now, very delayed by Brexit, election etc.) new AD plants will get RHI subsidy on biomethane from crops only up to the amount of gas produced from wastes and residues, i.e. payments are 50:50 for gas from wastes and crops.

If we assume that wastes give -604 gCO2e/kWh and crops give 125 gCO2e/kWh then this means that new AD plants will be operating at -240 gCO2e/kWh (the average of the two extremes) which looks excellent… But that calculation assumes that all the wastes are food wastes and assumes the counterfactual of all food waste going to landfill if it doesn’t go to AD. Landfill is climatically horrible because (as assumed in the impact analysis) it leads to 35.3% of the methane escaping to atmosphere. But even so that -230 gCO2e/kWh number is only valid in England where landfill is an option. In Wales, Scotland and Northern Ireland food waste is not allowed to go to landfill: the alternative to AD is incineration or composting, neither of which lead to methane emissions. So outside England (and in England after landfill for food waste becomes illegal, probably before 2030) we have to  assume that the counterfactual for food waste is municipal composting, not landfill, which would give an upstream correction of zero (biogenic CO2 is counted as zero under current Renewable Energy Directive rules). Then the emissions from AD using food waste would be just column 1 plus column 2 from the table above: 113 + 32 = 145 gCO2e/kWh… oh dear. That’s above the legal limit even without using any crops. [ See later post about the 113 gCO2e/kWh number: it is not what it appears to be and my calculation of 145 gCO2e/kWh is wrong.  I have now done a corrected calculation of the carbon footprint of biomethane from food wastes] Still, farmyard manures for AD look good – until farmyards clean up their current handling of manures and reduce the methane emissions anyway without AD.

If that is the situation for the future, what about the existing 80 sites which were mostly built before the carbon accounting rules came in (“sustainability rules”)? They will be using a lot of crops (in general) to maximise income and so will be rather worse than the current legal limit.

In fact ADBA have published data on farm-based dedicated agricultural AD plants: all 273 of them (which are mostly biogas CHP) in their Winter 2017 News. These plants use only agricultural feedstocks: 55% from dedicated energy crops and 45% farm wastes (page 10). These percentages are presumably by tonnage not by biogas-productivity which means that 55% is a lower bound for source of the gas from crops. This means that the existing AD industry probably has a negative GHG intensity because of the benefit (today) from the conventional (bad) slurry disposal outweighs the emissions from the food crops. (But the on-farm food waste would not otherwise be going to landfill, so the -749 gCO2e/kWh number would not be appropriate for that.)

Annex 3 (pp96 onwards) of the published impact analysis  makes some ad hoc calculations to look at indicative sensitivities with no basis in evidence:

“For our lower bound (optimistic) estimate of carbon cost-effectiveness we have assumed that emissions from AD are 20% lower than the sustainability criteria limit where agricultural feedstocks are used; and 90% lower than the sustainability criteria limit where food waste is used.”

And then say they’d like help improving this

“Our approach to estimating carbon savings from AD serves to provide an indicative estimate of £/tCO2e saved based on the emissions limits under the UK’s biomass sustainability criteria. Insofar as the emissions impact of AD differs from the sustainability criteria limits we welcome further evidence which may help to better understand the carbon cost effectiveness of AD.”

But it’s better than that. Page 97 quotes several bits of evidence for emissions from food waste in landfill, from which they conclude:

we consider that, for the purposes of our analysis, a sensible assumption is that food waste AD produces an upstream emissions abatement effect of between -450 and -900gCO2e/kWh of energy produced, with a central assumption of –790gCO2e/kWh.

[ I don’t know why table C15 has 748.6 instead of 790.0 but I regret to observe that editing consistency is not as reliable as it should be in any of these reports.]

So Fig. A3.1 of the impact analysis summarises all this in terms of the cost of carbon abatement at the proposed subsidy levels. [ For reference, a figure of more than £100/tCO2e is generally considered “rather expensive” for UK carbon abatement policies and is only justifiable if the policy is aggressively encouraging deeper decarbonisation and there is a realistic downwards trajectory. ]


which basically says that AD for anything but food waste in England for the next few years is not a good use of public funds (for an unrealistically small methane leak## from the biogas upgrading unit of 0.5%)  and implicitly that public funds should not be spent on AD plants using any crops.

Some biogas upgrading units emit 3% of methane to atmosphere by design so would struggle to be better than £100/tCO2e even for 100% manures and slurries. The  “RHI biomethane injection to grid tariff review” lists several options that could be used in policy revisions to improve this in future.

Comparison with biomethane in Europe

What about EU data?

Page 134 (injected biomethane) of the Joint Research Centre publication  gives -103 to +58 g/MJ i.e. -371 to 209 gCO2e/kWh. Table 102 on page 136 gives the breakdown and we can see the big emissions contribution made by fertilizers (N2O emissions) and cultivation when maize is used. This confirms that the UK data from the BEIS publications is probably in the right range, and also confirms that the carbon footprint of biomethane depends very significantly on the individual plant and the feedstock it uses.

Recent work in the alluvial Po river valley in northern Italy proposes a system of double-cropping providing biogas at -335 to 25 gCO2e/kWh (page 6 of their February 2017 summary report). (This also confirms that 100% maize-fed AD produces 202 gCO2e/kWh which is only a 22% decarbonisation compared with natural gas.) However whether this sequential cropping method is applicable to the very different climate in the UK and to soils with nitrogen-limits  requires UK-specific studies. There is as yet no published detail on these Italian numbers yet so we don’t know if they include the effect of methane emissions from the upgrading system or not.

So there is no simple answer: it depends on farmers’ current handling of slurry and whether a new AD plant properly manages the liquid digestate or not (which continues to give off methane for weeks). We don’t even have any numbers for “current best practice”.

What to do to improve the data

Biomethane injected into the gas grid comes from

  • Sewage
  • Agricultural manures
  • Agricultural wastes: unsellable food and crop residues
  • Food industry processing wastes
  • Food wastes from restaurants and hotels

Note that there is a concern about food wastes from households because of potential  contamination from collections from the public and the implications this has for disposal of the digestates. About half of all food waste is already collected, so the public campaigns to improve food collection in England will make a useful but not dramatic improvement. The November 2017 report of the ADBA covers this in some detail (pages 36-44).

In gas volume terms, not much of it comes from manures either: although a lot of manures are used in tonnage terms, manure doesn’t produce as much useful gas as food waste: only about a tenth as much.

There are some aspects which mean that biomethane is somewhat better in GHG emissions terms than the current accounting system allows. The use of a break crop or cover crop in AD improves the productivity of later crops. The solid digestate is a soil improver and creates better crops in the next season. The liquid digestate is a nitrogenous fertiliser and offsets the energy required to make artificial fertiliser. (But the digestate does not offset the N2O emissions as these are intrinsic to all fertilisers and depend largely on the timing of their application.) Unfortunately there are limits to the amount of nitrogen that is allowed to be applied in many UK farming areas so this benefit is not available to some AD plants.

The most profitable feedstocks tend to be crops: the ones that emit most CO2e during cultivation (including especially N2O from fertiliser application). This is especially true now that AD is established because gate fees for wastes have reduced. So a profit-seeking operator will choose feedstocks that just meet the legal limit: just below 34.8 gCO2e/MJ. So one can expect that that will be the actual emission, not just the maximum emission, from AD. But we will have to wait for audited reports from Ofgem before we know for sure.

Note that nearly all AD plants injecting biomethane also have CHP biogas engines on site providing heat for the digesters and power for the handling equipment. These engines get FITS subsidies for the power and RHI subsidies for the heat, but there is as yet no planned requirement to use 50:50 wastes feedstocks for FITS subsidies. Thus one could expect slightly more crops to be used than expected and all that biogas would be accounted for under FITS. A full carbon accounting for biomethane injection should ideally take these CHP engines into account.

Source of the 34.8 gCO2e/MJ number

This is documented in a very unusual one-page note written by DECC in 2014 when it came very close to shutting down the entire nascent AD industry by accident. The number 34.8 gCO2e/MJ comes from EU publications and is a 60% decarbonisation of the EU average “fossil fuel counterfactual” (FFC) for heat. So those MJ are MJ of heat after the FFC has been burned in a boiler which is assumed to be only 77% efficient (EU standard number). Representations from the UK AD industry made it clear that this would be an almost impossible target to hit in 2015, so DECC applied the number 34.8 gCO2e/MJ to the calorific value of the biomethane instead, which was an effective relaxation of the target by 30% (i.e. 100/77).

For biomethane injection the real counterfactual is the calorific value of the methane content of the natural gas as the biomethane substitutes for that directly. So this GHG limit should really be changed (reduced by 17%) to 104 gCO2e/kWh [28.9 gCO2e/MJ] for a 60% decarbonisation.

[update 23 Sept.2021: the new Green Gas policy which replaces the RHI is requiring a 24 gCO2e/MJ but not admitting to any previous mistakes.]

If one aimed at a 70% decarbonisation (as the DECC note suggests) which would be approximately appropriate for the 5th Carbon budget period 2027-2032 then this should be 78 gCO2e/kWh [21.7 gCO2e/MJ]. Meeting this in real AD plants will require great care and very careful planning.

The current level of 34.8 gCO2e/MJ corresponds to a decarbonisation of 52%. So if one could substitute half the gas in the grid by this biomethane then this would still only be an overall gas grid decarbonisation of 26%.

These calculations use the same GHG footprint of natural gas – including both upstream emissions and combustion – as the DECC note in 2014. In reality there would be some slight changes in future as the proportion of imported gas changes and as Qatari LNG starts to use decarbonised electricity in the LNG production process.

Solutions for the future

What can we expect AD to be doing in 2025 when we can expect the next wave of policies to be coming into effect?

  • In 2025 we could expect good AD methane monitoring, better practices for accidental leaks and methane emissions from post-AD digestate, proper chemical (amine) systems for biogas upgrading which do not emit methane and better accounting for use of digestate to displace artificial fertiliser: all of which make biomethane better in terms of CO2e GHG footprint.
  • However in 2025 we can also expect that agricultural practices, food waste in landfill and farm slurry handling to also be much improved. All of which destroy the “bad counterfactual” cases for biogas and so make AD appear worse than it does now. Much worse.

The relatively pure biogenic CO2 created by AD is nearly all currently vented. This is assumed under current accounting conventions to have zero climate impact. If this CO2 were collected and sequestered, then a very substantial negative carbon emission would accrue. If there were a market for negative emissions then this would become a very substantial extra income for the AD operator and would also completely transform the GHG footprint of the industry.

Footnote on trading and perverse incentives

When there is such great variation in the carbon intensity of biomethane from different sources it makes it very difficult to construct rational biomethane trading or certificates markets. Indeed premature formation of such markets can freeze carbon intensities at current (poor) levels and create a profound disincentive on all actors in cleaning up the industry – despite that being in the best interests of the industry as a whole.

Footnote on updates

In January 2018 I discovered a misreading of the data in the impact analysis and extended the work to cover the “negligible” emissions from transporting food waste. This is covered in a later article.

*  I had to change the formatting: the downloaded spreadsheet only give zero places of decimals, so I had to change the formatting to show the resolution we need.

** LHV = Lower Heating Value: the calorific value from burning the fuel assuming that all the water vapour is not condensed back to liquid and the latent heat recovered. Ofgem use “net” calorific value in their guidance (page 22)  because they were working from the RHI official regulation on this matter (Schedule 2A) which is copied from EU RED regulations and uses net calorific value as standard for all fuels (para 4.c). “Net” is the same thing as “lower” for dry fuels. However this is not really the right thing to do if the gas is being burned in a condensing boiler for which the gross (i.e. higher) calorific value is appropriate. Using “net” rather than “gross” means that the GHG emission limit is stricter by 11% since that is the ratio between the two numbers for pure methane; see . [I would appreciate someone checking my arithmetic on this issue.]

# The Green Gas Certificates scheme issues certificates for biomethane injected into the UK gas grid which has already been subsidised by the RHI injection scheme: see the page on “additionality” on their website. The level of subsidy in the RHI is intended to exactly compensate for the difference in cost of producing injectable biomethane compared with fossil natural gas, so in principle “green gas” should cost the purchaser no more than the ordinary cost of gas (plus the overhead for operating the certificates mechanism). If any cash finds its was back to the AD operator then that is an argument for reducing the RHI subsidy level in accordance with the EU State Aid Rules and Treasury guidelines for this type of policy.

## IEA Bioenergy Task 37 published a comprehensive study on methane leaks in December 2017 Methane Emissions from Biogas Plants . This report covers a great deal more than leaks – it reviews all the life cycle assessments of greenhouse gas emissions.

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