Understanding the greenhouse gas balances of bioenergy systems.

Bioenergy systems play a key role in the UK’s energy future because they offer the triple benefits of being renewable, sustainable and incurring lower greenhouse gas (GHG) emissions than fossil fuels, such as coal, oil or gas.

When biomass is utilized as an energy source, carbon dioxide (CO2) that was recently captured from the atmosphere by plant growth is re-released. As this has been recently sequestered, it is often ignored as not contributing to net increases in long term atmospheric concentrations. However, that neutrality is dependent on the biomass growth vegetation recapturing the equivalent over a
short time horizon, otherwise the balance between carbon in the atmosphere and biosphere is shifted. In the natural world, every unit of greenhouse gas (GHG) emitted has an impact that needs to be considered and, given the tight carbon budget constraints faced by the UK, this must be considered in assessment.

Bioenergy is expected to deliver 8-11% of the UK’s primary energy demand by 2020 and around 12% by 2050 (DECC 2012), playing a key role in delivering policy commitments on greenhouse gas reductions. See section 1 – Why is bioenergy important in the future UK energy mix?

Bioenergy systems achieve GHG reductions by displacing a relatively high carbon intensity existing fuel with a biomass feedstock that has incurred lower GHG emissions along its supply chain than the (usually fossil fuel) incumbent. Verifying GHG reductions therefore requires consideration of the whole supply chain and awareness of the wider impacts of bioenergy implementation. Techniques such as life cycle assessment (LCA) can be used to verify this. When this is done the yield of usable material produced is nearly always important; fertilizer use is often important for annual crops; changes in carbon stocks may be very significant for forestry systems and land-use change can have very large impacts for perennial crops. A summary of which issues tend to be most important for which crops and why is given in section 2 – What are the key differences between different bioenergy systems?

Every bioenergy system is different and their GHG balances must be independently verified. Nevertheless, there are many examples of UK bioelectricity systems achieving substantial GHG savings, while relatively low carbon intensity natural gas is dominant in the UK heating sector, making substantial reductions more difficult to achieve. Biomass-derived liquid transport fuels with existing technologies offer lower potential for savings and there are many reported examples that do not result in greenhouse gas savings. Section 3 – Can bioenergy systems achieve “real” greenhouse gas reductions? shows that real GHG savings can be achieved, but certain factors,
including land-use and the reference comparison can substantially alter the calculated GHG savings.

Section 4 – How can different reports reach different conclusions about the GHG balances of bioenergy systems? examines and classifies the main drivers of variation in LCA of bioenergy systems. Some variation is “real”, where different systems may actually give rise to different physical levels of GHG emissions. Other sources of variation may be methodological – this can often
be thought of as using LCA to answer a “different question” about the same bioenergy system.

It is therefore absolutely critical that the “LCA question” being asked is clearly and adequately defined. Section 5 –What should be considered when assessing if bioenergy is delivering real greenhouse gas reductions? gives guidance on formulating LCA questions and what needs to be considered by policy makers in defining GHG reduction objectives e.g. it is important to consider which demand is being displaced, from whose perspective “reductions” are framed, when emissions are incurred and whether reduced sequestration can be considered equivalent to increased emissions.

Section 6 – What are the methodological issues that make bioenergy LCA calculations difficult and their results contested? then focuses particularly on the methodological issues that result in different LCA analyses of the same system producing different results and the most appropriate context for applying different methods is outlined. There is particular focus on our understanding of temporal aspects of biomass feedstocks. This issue is most significant for forestry systems and it is noted that often the issue is not one of a carbon debt, but foregone future sequestration, which perhaps should be considered differently when assessing the system GHG balance.

Finally section 7 –What are the implications of our understanding of bioenergy system greenhouse gas balances for policy initiatives or “How can policy frameworks incentivize “real” greenhouse gas reductions? synthesizes the policy implications for assessing GHG balances of bioenergy systems and promoting greenhouse gas reductions. It emphasizes the importance of land-use and land-use change for some systems and recognizes the need to better understand the future food-fuel interface for climate policy development. It also identifies a key gap in knowledge surrounding the impact of forest management on carbon stocks and perceives a need for closer examination of
carbon dynamics. It notes the fact that importing biomass is effectively equivalent to exporting our carbon reduction obligations, but notes that this occurs in many sectors where the UK imports goods.