Research and analysis

Low carbon transport fuels: DfT Science Advisory Council position paper

Published 5 June 2023

Background

The Department for Transport (DfT) Science Advisory Council (SAC) convened on 16 March 2022 to provide an independent perspective on the role of low carbon fuels in delivering reductions in transport system greenhouse gas emissions. DfT is currently developing a low carbon fuels strategy, for which a ‘call for ideas’ consultation ran February to April 2022, and the SAC’s input contributed to this activity.

The focus of the meeting was to identify major challenges and uncertainties that need to be resolved to accelerate cost-effective decarbonisation while optimising other possible co-benefits. The session included input from additional industry experts^{[footnote 1]} and government officials, but the content of this paper is the responsibility of the SAC alone.

Introduction

Aside from the immediate impacts of the COVID-19 pandemic, the quantity of fossil fuels consumed by the transport sector has changed little in recent years and transport is the largest contributor to UK territorial greenhouse gas emissions, responsible for around one quarter of the total.

Given national commitments to reduce these emissions in the fifth and sixth carbon budgets (2028 to 2032, and 2033 to 2037, respectively), it will be a necessity in the short and medium term to deliver some fraction of decarbonisation using low carbon fuels^{[footnote 2]} that can be used by the existing transport fleet.

This is in addition to implementing strategies that may introduce new fleet using different propulsion systems, alternative energy storage and energy efficiency measures. For some sectors, such as passenger cars, conversion to full battery electric propulsion seems almost a certainty. However, battery energy storage may not deliver the most sustainable solution in an appropriate time frame for other modes, such as heavy goods vehicles, aviation and shipping, so we may need to adopt alternative fuel strategies.

An immediate challenge is the breadth and complexity of possible future low carbon fuels, feedstocks and production routes that might plausibly play a role in transport decarbonisation.

Liquid biofuels such as ethanol and diesel, produced from biomass feedstocks, are already in daily use in the UK. Other synthetic hydrocarbon fuels (such as sustainable aviation fuel) made from recycled CO2, or created from waste materials, are technically feasible and commercially available. However, these are not always economically competitive, and being produced currently on only a limited scale.

Renewable electricity can be used to produce fuels such as hydrogen and ammonia; while the use of these in transport is very limited at present, demand is projected by some industry observers to grow rapidly^{[footnote 3]}.

Some of these fuels may deliver their main impact through supporting a transitional period towards final battery or fuel cell electrification, others may be needed for the long term. The future fuels landscape is likely to be more complex than past transport systems built around gasoline, kerosene and diesel.

It is imperative that the mixture of future low carbon fuels, supported by UK policy interventions, deliver timely greenhouse gas reductions – sooner is better since it is cumulative CO2 emissions that matter. However, fuel production and use must avoid unintentional negative economic, environmental or public health impacts when integrated at a system level. The adoption of fuels that are derived from feedstocks other than fossil carbon must also consider the resilience and diversity of the supply chain and how national infrastructure may need to adapt to support secure storage and distribution.

The SAC focused on the challenges and opportunities associated with developing and using different fuels, including potential synergies across modes and sectors, and the resultant impact on the wider energy system if those fuels were adopted. The key issues identified are described below.

Environmental Issues

Carbon Emissions

The primary rationale for switching to low carbon fuels is greenhouse gas emission reduction, and so any such fuels supported by policy must deliver demonstrably real reductions. Confidence in this can be undermined by sometimes contradictory figures for the greenhouse gas savings that are achieved from using different types of fuels. These differences can arise from the adoption of different methodologies^{[footnote 4]}, often to simplify calculations or address data gaps.

This can lead to inconsistency in how different fuels are evaluated including by different regulatory instruments. Different sectors (electricity, heat, transport fuel) may use the same biomass/biofuel but report their greenhouse gas emissions in different ways depending on which regulation applies to their particular application – Renewable Energy Directive II (REDII), Renewable Transport Fuels Obligation (RTFO), Renewables Obligation (RO) or Renewable Heat Incentive (RHI).

This results in a lack of comparability between sectors that potentially limits fair competition, since producers will deliver their fuel to the sector where they obtain highest financial reward, but that may not correspond to highest carbon savings^{[footnote 5]}. It is also currently difficult to compare greenhouse gas emissions reductions deriving from use of low carbon fuels versus alternative uses of the same feedstock resources, or the use of different energy and propulsion systems (for example, battery storage, fuel cells).

Where possible, a common metric/measure should be used for evaluating overall greenhouse gas emission reductions arising from using different feedstocks, fuel types and propulsion systems.

Such a metric needs to reflect and inform multiple national objectives, including across climate, energy and environmental policy, and use a calculation methodology that suitably accounts for life-cycle greenhouse gas emissions and/or savings.

The rationale for this is similar to that now being adopted for national carbon budgets: to have a science-based target that transparently defines the carbon objective, facilitating cross-comparison across sectors. No metric will ever be perfect in all regards, instead performance needs only to be sufficient to support the timely progression of low-regret beneficial actions.

The SAC noted that many different life cycle analysis (LCA) tools have been developed already for alternative fuels, but there is limited evidence to support decision-makers in how they compare these methods or judge their appropriateness for particular tasks.

Multiple fuel pathways can be conceived of that support decarbonisation of transport, however these need to consider the availability of feedstocks and the conversion of these into useable fuels.

The efficiency of conversion of input resources into fuels should continue to be quantitatively assessed using methods such as life cycle analysis, to avoid inherently wasteful systems or adoption of pathways where decarbonisation may be limited by feedstock supplies.

This may be acutely important for future fuels that may be derived from limited resources such as biomass or waste streams, and where other sectors may compete for the same resources.

The “feedstock-embodied” carbon (carbon emissions associated with the upstream activities that actually produce the feedstock) is best managed by the producer, who currently declares carbon intensity to regulators. Disseminating this information more broadly would support decision-making for purchasers focused on carbon performance and build trust and public confidence that the wider impacts of producing low carbon fuels were being considered.

Well-evidenced and articulated scenarios of anticipated technology adoption and market penetration would help industry to better plan the commercialisation of CO2 emission reduction technologies that best supported the trajectory of the overall sector. As an example, the rate of uptake of battery technology in different sectors may affect where fuels such as biodiesel would be better prioritised.

Non-CO2 emissions

In addition to consideration of greenhouse gas emissions, adopting different low carbon fuels and propulsion systems gives rise to changes in other atmospheric emissions of relevance to climate, air quality and public health.

Nitrogen oxides (NOx) emissions occur as a consequence of combustion and are influenced by the fuel composition and the temperatures generated during the combustion phase. Indeed when any fuel is used in an internal combustion engine or turbine the potential for NOX emissions exists; this is due to NOX being formed from high temperatures that dissociate nitrogen from the air. NO­x­ emissions can be higher for some low carbon fuels if higher combustion temperatures are experienced; hydrogen flames for example can burn hotter than hydrocarbon fuels in the absence of improved combustion.

Pollutants such as carbon monoxide (CO), hydrocarbons and particulate matter (PM) are also found in exhaust gases with the amount varying with fuel composition and type of power plant.

For some low carbon fuels such as hydrogen and ammonia there are potentially beneficial reductions in CO, SO2 and PM emissions when used in compression ignition engines and gas turbines^{[footnote 6]}. NOx however is known to increase when burning ammonia due to the additional nitrogen contained in the fuel. Overall, however, the evidence base is limited and variable in terms of non-CO2 impacts^{[footnote 7]} so more comprehensive real-world data on emissions is needed to establish the likely long-term impacts of future changes to fuel supply and associated changes to engine technologies.

Lower aromatic content associated with certain novel types of sustainable aviation fuel (SAF) may lead to reduced emissions of PM, which would be a benefit for air quality^{[footnote 8]}. However, low aromatic fuels have been reported to impact some elastomer seals in aero engines with maintenance consequences and a potential need to modify seal materials^{[footnote 9]}. Fuel producers and aircraft/engine manufacturers do not however consider aromatics in fuel to be fundamental to aviation fuel performance and are currently proving such fuels are compatible with existing products^{[footnote 10]}. To move forward confidently pan-stakeholder agreement on any fuel specification modifications will be needed that may require additional evidence in line with industry protocol.

Specific to the aviation sector, the use of either SAF or hydrogen (a longer-term option) in turbines result in emissions of water vapour at high altitude, as today’s fuels do. This contributes to a range of complex radiative forcing effects which can partly offset the climatic benefits of using a low carbon fuel, although there are large uncertainties in magnitude of effect especially when scaled globally^{[footnote 11]}.

Different fuels have different properties and characteristics e.g. ammonia is toxic, hydrogen is lighter than air and particularly susceptible to leakage. These characteristics will need to be taken into account to ensure that future fuel transportation and delivery systems are safe, sustainable and do not result in atmospheric leakage.

Real-world verification of non-CO2 emissions from low carbon fuels is essential to help maximise possible air quality co-benefits. Improved quantification of climate effects arising from water vapour emissions and from fuel leakage are also needed.

Material resource depletion

Fuels such as hydrogen may be converted to electrical power in devices that themselves have significant material resource demands e.g. platinum or rare earth metals for fuel cells. Other fuels, e.g. SAF, may need expensive or supply-constrained catalysts for their production. Rare or specialised metal resources required for manufacturing may also need considering from a security of supply perspective^{[footnote 12]}, where no domestic source is available. Beyond the fuels themselves, there may well also be resource constraints related to materials needed for retrofitting or adapting current fleet, or for new vehicles.

Assessment of future fuel production and use should be assessed on a supply chain and life cycle basis to identify resource constraints and comparisons made to any equivalent resource limitations related to battery production.

Appropriate comparison points should be used, e.g. the material resource demand may still be lower than the life cycle impact of an equivalent battery system, which is likely to have significant demands for lithium, nickel, manganese, cobalt (for batteries) and copper for electrical power transmission.

Carbon management

In the future economically viable and readily recyclable carbon atoms may become a limited resource. It will be necessary to consider where best to prioritise use of that carbon feedstock for conversion to material products, e.g. in plastics production, fibre optics in communications, carbon electrodes in batteries, or as carbon-containing fuels.

A sectoral prioritisation is needed to develop a hierarchy of different uses of recyclable carbon, considering the full range of manufacturing end uses and including as a feedstock for fuels in the transport sector.

The government’s current Biomass Policy Statement and planned Biomass Strategy aim to provide such a prioritisation for biomass. Such prioritisation should consider the impacts on broader UK decarbonisation and manufacturing and the effects of earlier or later decarbonisation of road transport. It is important to understand how the choice to adopt fuels from carbon feedstocks such as waste CO2 may impact on, and be impacted by, other sectors with similar feedstock requirements.

System Issues

System trajectories

Low carbon hydrocarbon-based fuels (e.g. ethanol, SAF, biodiesel) already provide valuable near-term greenhouse gas reductions which deliver a greater integrated climate benefit than equivalent reductions achieved further into the future. So, for most current fossil fuel transport applications, blending in a lower carbon fuel delivers some immediate emissions reductions using an existing infrastructure.

In contrast, in the short-term, adding new battery electric propulsion systems may at some times of the year lead to emission increases (both CO2, and others such as NOx) since marginal demand in the UK is currently met by fossil fuelled dispatchable thermal generators. However, as the UK electricity grid carbon intensity continues to fall, and grid supply adapts to a changing demand profile (including that created by battery electric vehicles), a tipping point will be reached when adding battery electrical propulsion will likely lead to lower greenhouse gas emissions than continued use of low carbon liquid fuels. Retaining system-wide agility in fuelling and propulsion options will therefore be critical, and thresholds will be crossed where previously optimal policies for transport decarbonisation will need to be revised.

Strategies for decarbonising transport will be dependent on how decarbonisation of the wider UK energy system develops, and integration of DfT fuel policies with those of other government departments is crucial.

The energy demands and decarbonisation impacts from each transport modality (HGVs/private cars/alternatives) needs to be considered in order to support planning for infrastructure and investment in low carbon electricity, energy storage and related R&D. It is likely that systems analysis techniques will play a major role in supporting these assessments. The transition between different fuels and propulsion systems will need to be thought through holistically with early steps well considered, and buy-in to a systems approach needs to be secured across government. The complexity of this is immense when the number of stakeholders, local authorities, contracts, mechanisms etc. are taken into account.

Surface transport generates most UK territorial transport-related greenhouse gas emissions^{[footnote 13]} and so there is a compelling case for low carbon fuels and/or decarbonisation strategies to focus on that sector in the short term. Aviation and maritime fuels and propulsion are undoubtedly more challenging to decarbonise due to power and range requirements, but there may be value in considering synergies between these two sectors. Both may potentially adopt hydrogen (or a vector such as ammonia, dimethyl ether or methanol) as a medium to long-term fuel source and will have common requirements for a secure supply and effective distribution.

Finally, some thought needs to be given to the possible consequences of phasing out existing petrochemical refineries which typically generate a range of products. These do not just provide fuels, but also materials such as graphite, lubricants, and chemical feedstocks that act as the base for a range of other materials and products. In the UK, fuel refining is often co-located with chemical manufacturing, because of this interdependency.

Phasing out of fossil fuels for transport may therefore affect the supply of other materials, including plastics and asphalt, and affect their wider viability for other industries. Biorefineries may provide a pathway to producing some of these feedstock fractions, but not necessarily in the same quantities as at present, so careful consideration of effects on chemical and material supply for manufacturing, construction and other industries is needed.

There is a need to evaluate the wider systems impact of fossil fuel phase-out on the supply of non-fuel hydrocarbon-based chemicals and derived products.

Future fuel / electrification interplay

Delivering decarbonisation of transport will require a flexible strategy that mixes a combination of low carbon fuels and battery electrification. The optimal blend of energy sources will depend on multiple and interlinked factors including the effectiveness of future battery storage systems (alongside the economics and resource supply to produce them), the evolution of UK low-carbon electricity production capacity and distribution, and the cost effectiveness of low carbon fuel production.

Layered on top of these largely technological constraints will come considerations around diversity and security of energy supply (see section 5). The relative balance of low-carbon fuels and electrification will co-evolve over time and are intrinsically difficult to predict with certainty. Regardless, however, of which comes to dominate the landscape, it is inescapable that significantly increased low carbon energy production will be needed to service the transport sector.

Setting aside current practical feasibility in individual transport modes, longer term there may be insufficient resource supply to support a solely “electrification” or “liquid fuel” pathway for transport, and so retaining a mixed fuelling strategy could well be a practical necessity. For some classes of road vehicles with long lifetimes, the policy die has already been cast – for passenger cars, 2030 will see the last sales of internal combustion engine vehicles, and 2035 for hybrid vehicles. Technology trajectories for other classes, such as vans, buses and HGVs are likely to emerge in the next decade, as will other sectors such as maritime and aviation.

Transport energy demand, and the impact of demand changes or demand management, should be considered as part of a national transport decarbonisation strategy. Electrification of the transport fleet, along with greater vehicle autonomy, should enable some energy efficiency improvements and so can further contribute to carbon emissions reduction.

Government policies for decarbonising transport will need to be flexible and responsive; the optimal mix of low carbon fuels and electrification will co-evolve in the coming decades as individual technologies develop alongside the national capacity for low carbon electricity generation.

Security and resilience of supply

The development and integration of alternative lower carbon fuels presents disruptive risks to transport and mobility, but also to related logistics and supply chains and in other energy critical sectors ranging from heating to manufacturing. Production methods for current fossil fuels are based on centuries-old chemical engineering know-how, whereas low carbon fuels may come from processes that have yet to be tested at scale. The supply of feedstock will also likely pivot, away from crude oil producing nations, to those with abundant biomass or renewable resources, those that may have excess nuclear energy supply or potential for carbon capture and storage. A move to low carbon fuels is not inherently more risky or less resilient than the present-day reliance on fossil fuels, but it is untested during periods of global instability and may have additional exposure to climate-related risk. There is manufacturing uncertainty layered on top, along with limited clarity on long-term technological and commercial pathways for some transport sectors like aviation.

Mitigation of risk may require initial investments in a diverse range of low carbon fuel options while assessing pathways to net zero in 2050, and by integrating into decision-making the possible changed resilience and security of supply.

The emergence of a range of new fuel types will demand timely and coherent compliance with emerging international standards and certification schemes. This will help support future resilience in the transport system, opening up options for both supply and exports. Given the comparatively small size of the UK market compared to Indian, Chinese, South-East Asian and North American counterparts, it would be unwise to assume that the UK will have significant influence on future fuel market changes. New feedstock supply chains might result in international dependencies that are different from our current reliance on overseas crude oil and gas, but not necessarily more positive.

The UK should aim to build security and resilience of future fuel supplies by taking multiple global lines of approach and pioneering schemes that leverage existing assets, position and expertise to enable UK innovation and secure position.

Infrastructure considerations

Low carbon fuels have some potential advantages compared to battery electrification, particularly when infrastructure can be reused, including for storage, distribution and sale/supply. Reuse of existing infrastructure would likely be a preferred option when planning both nationally and by individual businesses. However, while low carbon fuels can be designed to mimic closely the properties of the fossil fuels they replace, in-use compatibility needs comprehensively evaluating. There may be issues associated with materials compatibility when using existing pipelines with new fuels (e.g. gas permeability of metal pipes to hydrogen, metals corrosion from methanol).

Consideration needs to be given as to how current infrastructure might support a transition to blends of fuels with an increasing percentage of non-fossil origin. Too large a step increase in the non-fossil proportion may risk making large numbers of vehicles unusable. Use of high fraction or indeed pure low carbon fuels in suitably compatible vehicles could require further investment in both storage and retail distribution infrastructure, increasing the range of fuels available to the public via pump selection. Any major data gaps identified should be prioritised for practical studies because of the need for cumulative hours of testing to provide convincing evidence of long-term effects.

The evidence base on fuels compatibility and issues arising needs further developing including not only materials and engineering aspects, but also legal consequences, for example around warranties.

Geographical factors are extremely important for infrastructure planning, e.g. centres of population, industrial clusters (hydrogen demand, CO2 offtake opportunities), coastal locations (marine demand). Consideration needs to be given to transport distances/pressures etc., including differences between infrastructure for heavy goods vehicles (HGV) and cars. Transport operators could be required to submit plans to indicate their fuel pathways to a net zero position in 2050, e.g. local authority plans on readiness for 2050 transport systems could aid co-ordination and support positive action through the planning system.

The retained use of gaseous/liquid fuels and their related storage and distribution networks requires continued vigilance related to security of that infrastructure and protection from malicious attacks. Arguably a future decarbonised transport system may have increased resilience from a security perspective since it may use a more diverse range of fuels, and from a diversified production base. The adoption of some fuels, notably ammonia, may require additional protections, given inhalation toxicity if uncontrollably released.

Future costs and economics

Economic factors and constraints apply to future fuels in the same way as fossil fuels – global supply and demand; regional and national resource advantage; scale, capital intensity and longevity of conversion/distribution systems; contract models/policy/taxation. So, while a UK perspective can be developed (which could start with UK-focused techno-economic evaluations) ultimately fuels will be globally traded. Nations with access to reliable low-cost renewable power, excess nuclear supply, cost effective carbon capture and storage (CCS), or biomass/land/geological resources will likely be advantaged.

There is a need to carefully evaluate wider economic impacts for transport modes facing more limited fuel options (such as long-haul aviation) within any cost-benefit analyses and include economic impacts arising from supply/demand from non-transport sectors which could act as an economic enabler (large scale H2 production for industry). There is a need to also consider the economics of scale up in terms of real-world flow rate per annum. Production of low carbon fuels may be limited by supply (for example, conversion of municipal solid waste (MSW) to fuel may be limited by reducing levels of supply as circular economy policies develop) or suitability of feedstock (for example, non-homogeneity of MSW requiring additional separation leading to lower yield value/higher cost for remaining waste disposal).

Skills availability may impact on production economics in a disproportionate way, particularly where there is call upon the same skills in other sectors. This might be particularly the case where new-build is needed with a range of engineering skill bases in high demand. There will be a need to develop skills through the full supply chain, including in sectors such as agriculture and waste.

Requirements for certain skills could become more pressing given the ambition of the Net Zero timeline, a constrained labour force and challenging UK demographics. Significant skills related progress is being made around future fuels with academic-industry collaboration e.g. the Aviation Impact accelerator and Supergen Bioenergy Hub.

Continued collaborative academic-industry initiatives are needed to support the innovation and training that will underpin future fuels for transport, and this will be enhanced with investment in public awareness and communications.

Summary of SAC recommendations

Successful decarbonisation of transport systems in the UK will require flexible and adaptive government strategies that support the use of low-carbon fuels alongside widespread battery electrification, where that is possible.

A diverse range of low carbon fuels are available and so developing the evidence base on i) potential constraints on feedstocks and supply, ii) competing demands from other energy intensive sectors, and iii) the carbon emissions saved over the whole life cycle of a fuel from production to point of use, are critical to supporting DfT decision-making. The resilience of national and global supply chains for low carbon fuels should remain a key consideration where the extent and nature of the associated risks may differ from those of fossil fuel supply.

Low carbon fuels offer notable advantages in potential ease and speed of deployment, the potentially efficient re-use of existing national infrastructure for distribution and supply, and use in existing equipment with little or no modification. However, the retention of low carbon fuels when used in engines and turbines does lead to continued emissions of other air pollutants, albeit sometimes at lower levels than fossil equivalents. Optimising the infrastructure re-use benefits, while suitably managing non-CO2 emissions, should be priority areas for evidence development.

The balance of energy sources used for transport is likely to change significantly over the coming decades. The optimal mix of low carbon fuels vs battery electrification in transport will depend on many different factors, some technological, some supply related, and others linked to the capacity of the UK to generate low carbon electricity.

Whether low carbon fuels support only a transitionary phase to full electrification of most transport sectors, or instead become a long-term solution is not yet clear. However, their ability to deliver effective and immediate beneficial reductions in carbon emissions gives them a central role in meeting UK net zero ambitions by 2050, hence continued support for investment in science and technology in this area remains crucial.

List of recommendations

1. Where possible, a common metric/measure should be used for evaluating overall greenhouse gas emission reductions arising from using different feedstocks, fuel types and propulsion systems.

2. The efficiency of conversion of input resources into fuels should continue to be quantitatively assessed using methods such as life cycle analysis, to avoid inherently wasteful systems or adoption of pathways where decarbonisation may be limited by feedstock supplies.

3. Real-world verification of non-CO2 from low carbon fuels is essential to help maximise possible air quality co-benefits. Improved quantification of climate effects arising from water vapour emissions and from fuel leakage are also needed.

4. Assessment of future fuel production and use should be assessed on a supply chain and life cycle basis to identify resource constraints, and comparisons made to any equivalent resource limitations related to battery production.

5. A sectoral prioritisation is needed to develop a hierarchy of different uses of recyclable carbon, considering the full range of manufacturing end uses and including as a feedstock for fuels in the transport sector.

6. Strategies for decarbonising transport will be dependent on how decarbonisation of the wider UK energy system develops, and integration of DfT fuel policies with those of other government departments is crucial.

7. There is a need to evaluate the wider systems impact of fossil fuel phase-out on the supply of non-fuel hydrocarbon-based chemicals and derived products.

8. Government policies for decarbonising transport will need to be flexible and responsive; the optimal mix of low carbon fuels and electrification will co-evolve in the coming decades as individual technologies develop alongside the national capacity for low carbon electricity generation.

9. Mitigation of risk may require initial investments in a diverse range of low carbon fuel options while assessing pathways to net zero in 2050, and by integrating into decision-making the possible changed resilience and security of supply.

10. The UK should aim to build security and resilience of future fuel supplies by taking multiple global lines of approach and pioneering schemes that leverage existing assets, position and expertise to enable UK innovation and secure position.

11. The evidence base on fuels compatibility and issues arising needs further developing including not only materials and engineering aspects, but also legal consequences, for example around warranties.

12. Opportunities for the repurposing of existing fuel infrastructure should be further explored, optimising efficiency and flexibility of distribution while adapting to changing risks around security.

13. Continued collaborative academic-industry initiatives are needed to support the innovation and training that will underpin future fuels for transport, and this will be enhanced with investment in public awareness and communications.

Authors

All members of the SAC contributed to this paper:

Professor Patricia Thornley (lead author) Aston University

Professor Alastair Lewis (SAC Chair) University of York

Anna-Marie Greenaway University of Cambridge

Dr Dave Smith Rolls-Royce plc

Dr Emma Taylor  Cranfield University and RazorSecure Ltd

James Gaade The Faraday Institution

Professor Nick Pidgeon Cardiff University

Professor Peter Jones University College London

Professor Ricardo Martinez-Botas Imperial College London

Professor Rob Miller University of Cambridge

Dr Siddartha Khastgir  WMG, University of Warwick

Professor William Powrie University of Southampton

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2. In this document ‘low carbon fuels’ refers to fuels that over their full life cycle lead to substantially reduced net greenhouse gas emissions when compared to those that would arise from use of fossil fuel derived equivalents. Low carbon fuels potentially encompass hydrocarbon-based gases and liquids such as biomethane, alcohols, biodiesel, and sustainable aviation fuel, and hydrogen-based fuels such as hydrogen gas or ammonia. ↩

3. Global Energy Perspective 2022, McKinsey & Company ↩

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12. UK Critical Minerals Strategy, July 2022 ↩

13. Final UK greenhouse gas emissions national statistics: 1990 to 2020 ↩