Bioenergy utilization for a low carbon future in the UK: the evaluation of some alternative scenarios and projections

Energy sources and climate change

Energy services underpin human development, but they also put at risk the quality and longer-term viability of the biosphere as a result of unwanted or ‘second order’ effects [40]. Arguably the most significant of these side-effects emanate from changes in atmospheric concentrations of ‘greenhouse gases’ (GHGs) that affect the energy balance of the global climate system, and are arguably the key environmental burden constraining moves towards global sustainability. Carbon dioxide (CO₂) emissions represent the principal GHG having an atmospheric residence time of about 100 years [40]. Human activities have led to quite dramatic increases since 1950 in the ‘basket’ of GHG originally incorporated in the Kyoto Protocol; concentrations rising from 330 ppm to about 430 ppm presently [53]. The most recent (2013) scientific assessment by the Intergovernmental Panel on Climate Change (IPCC) asserts that it is ‘extremely likely’ that humans are the dominant influence on the observed global warming since the mid-twentieth century [53]. Thus, human activities lead to the emission of CO₂ (and other GHGs) that, in turn, trap long-wave thermal radiation from the Earth’s surface in the atmosphere. The IPPC suggest that these are the main cause of rises in climatic temperatures [53]. The subsequent 2015 Paris Agreement following the COP21 meeting in that city aims to keep temperatures “well below 2°C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5°C above pre-industrial levels” (see, for example, [5]). However, bottom-up pledges received by countries prior to the Paris Conference [the so-called ‘Intended Nationally Determined Contributions’ (INDCs)] for national GHG mitigation efforts are expected by analysts of the United Nations Framework Convention on Climate Change (UNFCCC) to result in a warming of around 2.7 °C. So the world still faces a significant challenge of reducing GHG emissions further in order to bring global warming into line with the aspirations in the Paris Agreement.

End-use energy demand from the domestic, service, industrial and transport sectors of the United Kingdom of Great Britain and Northern Ireland (UK) economy give rise to approximately 61% of CO₂ emissions [22]. [The transport sector is often separated from the energy system, but contributes around 25% of these emissions.] In contrast, some 37% of CO₂ emissions can presently be attributed to energy and power supply-side. The UK Government’s Committee on Climate Change [10] – an expert, independent statutory public body – recommended the adoption of a target of an 80% reduction in GHG emissions (against 1990 levels) by 2050 in order to militate against anthropogenic climate change from such human activities. This was incorporated in the Climate Change Act 2008 [51], and is broadly consistent with 2 °C of global warming. Recently the UK Government asked the CCC to give it advice on possible tightening of the 2050 target in light of the Paris Agreement [5], and the aspiration of restricting global warming to 1.5 °C above pre-industrial levels. The CCC argued, in any event, that the steepest reductions in GHG emissions must occur before 2030.

Looked at from a global perspective, human beings were almost completely dependent on finite fossil and nuclear fuels for energy resources at the turn of the Millennium [see Fig. 1 [47]]; amounting to about 77 and 7% of primary energy needs respectively [27]. ‘Traditional’ renewable energy sources, such as burning fuelwood and dung or using water and windmills, accounted for 11% of these worldwide requirements. Large-scale hydroelectric power contributed 3%, and other renewables (including modern wind turbines and liquid biofuels) contributed just 2%. Sustainable development in a strict sense requires a reversal of these roles, but it is unlikely that renewable energy technologies (RET) could meet a high proportion of industrial countries’ energy demand before at least the middle of the twenty-first century [79]. This is partly due to the conflict between the needs of environmental sustainability and the downward economic pressures on energy prices arising from moves towards energy market liberalisation in the industrialised world. However, the British policy incentives for RET have recently been significantly weakened with a result that Geels et al. [30] believe it is unlikely that the UK will meet its current renewable electricity target of 30% by 2020 under the (pre-Brexit) European Union agreement. Likewise, the uptake of new nuclear power stations and carbon capture and storage (CCS) facilities coupled to fossil-fuelled power stations and industrial process plants have been significantly delayed in comparison with what was envisaged in the original version of the UK Carbon Plan [22]; superseded by the UK Government’s recent Clean Growth Strategy [14, 52].

The world has undergone various cycles or energy transitions between differing energy sources since the start of the Industrial Revolution (c. 1760–1840). These so-called Kondratieff long-waves [66, 71] are depicted in terms of world primary energy shares in Fig. 1 [47], alongside future pathways out to 2050 as suggested by the Shell ‘Dynamics as Usual’ Scenario [15]. ‘Traditional’ energy sources include animal manure, fuel wood, water wheels, and windmills. Over the forthcoming 30 years or so there could be a significant growth in energy demand, resulting mainly from the economic development of rapidly industrialising countries (such as China and India). The depletion of finite fossil fuel resources (like oil and natural gas), and the need for climate change mitigation, will therefore require to be offset by a portfolio of countervailing energy strategies: energy demand reduction and energy efficiency improvements, CCS from fossil fuel power plants, and a switch to other low or zero carbon energy sources; various sorts of renewables (including, potentially, liquid biofuels for transport) or nuclear power [see again Fig. 1 [47]].

Bioenergy resources for use in the UK energy and transport sectors

In order to achieve the targets set out in the UK Climate Change Act [51], it is necessary to drastically reduce energy demand, whilst also moving away from a fossil-fuelled energy system and towards the use of a much greater proportion of low carbon or renewable energy sources. One such renewable source is bioenergy; defined as energy from any fuel that is obtained from biomass resources [58]. The latter are organic matter derived from living, or recently living, organisms that could include both animal and plant or vegetable-derived material, including animal waste [61, 73]. The combustion of this biomass is sometimes considered to be ‘carbon neutral’, because the release energy is assumed to be equal to that sequestered from the atmosphere during its cultivation [1, 73]. Biomass provides two main routes to mitigating climate change: its growth removes CO₂ from the atmosphere, and then stores it over long time periods in soils, trees and other plants [12]. In reality, this mitigation potential depends on whether or not the biomass is managed appropriately (i.e., ‘sustainably’). Only then can it deliver significant net reductions in CO₂ emissions when compared to fossil fuels [1, 67]. Likewise, bioenergy has the potential to contribute to future UK energy services for heat, electricity and transport. These can constitute solid and liquid biofuels, as well as biogas. The so-called first generation biofuels (FGB) are produced mainly from food crops. They are restricted in terms of their ability to meet targets for oil-product substitution (without threatening food supplies and biodiversity), as well as reductions in GHG emissions [42, 73]. In contrast, more advanced or second generation biofuels (SGB) are generally produced from agricultural or crop ‘wastes’ [such as wheat straw [46]] and from non-food energy crops, which significantly reduces these negative impacts [42, 73]. Potential feedstocks and conversion routes [44] therefore need to be assessed against the full range of sustainability considerations and over the full life-cycle of the biofuel supply chain [25, 43, 81]: from ‘field-to-forecourt’ or ‘seed-to-wheel’. Only in this way will the true consequences of a given biofuel – environmental, economic and social - be determined [43].

Biomass electricity generation accounted for 6% of the Britain’s power supply in 2017, whilst the corresponding heat generated from biomass was around 8% of overall heat demand, with liquid biofuels accounting for some 3% of the UK’s road transport fuel [17, 78]. Electricity generation from renewable sources in Britain was incentivised via the introduction of the Renewables Obligation, which obliges UK electricity suppliers to source a fixed percentage of their electricity from renewable sources [1, 72]. Bioenergy producers have been remunerated for supplying electricity to the distribution grid via a ‘Feed-in Tariff’ (FiT) since April 2010, and an analogous ‘Renewable Heat Initiative’ (RHI) from April 2011 [22]. The British Government set up the ‘Office for Renewable Energy Deployment’ (ORED) in July 2009 to co-ordinate actions aimed at achieving its 2020 renewable energy targets [1, 20]. ORED aims to stimulate investment and develop supply chains in all RETs, and has a specific objective to encourage and enable more use of ‘sustainable bioenergy’ [20]. The European Union (EU) have viewed adoption of liquid biofuels in the transport sector [44] as a policy intervention for meeting climate change mitigation targets, enhancing regional energy or fuel security, and contributing to rural development. The latter could be aided by the provision of an alternative source of income for, otherwise depressed, agricultural communities from the production of biomass. Such biomass resources can be converted into premium-quality liquid biofuels and biochemicals [36, 88]. Thus, bioethanol and biodiesel hold out the prospect of retaining the existing transport infrastructure (e.g., refuelling or ‘petrol’ stations), in contrast to other potentially low carbon options, such as hydrogen-fuelled or electric vehicles. That has significant benefits in terms of limiting capital expenditure and the potential speed of take-up. Nevertheless, the deployment of bio-based products may have significant deleterious impacts in terms of direct and indirect land use change, loss of biodiversity and the impairment of eco-system services [78, 81, 88], and competition with food production.

The issues considered

In the light of the above discussion, the extent to which bioenergy can contribute to future UK energy supply is appraised, given the resources available to the Great Britain and Northern Ireland. Analysis of three notable low or zero carbon energy scenario or pathway sets produced by, respectively, the British Government’s Department of Energy & Climate Change (DECC) [which was merged in 2016 with the then Department for Business, Innovation & Skills (BIS) to form the Department for Business, Energy & Industrial Strategy (BEIS)] (the DECC 2050 Calculator; see [21]), the UK Energy Research Centre (the UKERC Energy 2050 Project; see [83]), and the Centre for Alternative Technology (the Zero Carbon Britain 2030 Project; see [8]) enabled a comparative evaluation to be made of each projection and their realism. They reflect alternative modelling approaches that seek to meet the statutory 2050 carbon reduction target (DECC/BEIS and UKERC) to that of fully decarbonising Britain by 2030 (CAT). The spotlight of the present study is on the use of dedicated energy crops and their implications, with a particular focus on land availability, conversion technologies, and foreign imports. A ‘gap analysis’ leads to recommendations for the improvement of the next generation forecasts, pathways or scenarios in order to provide more realistic projections for bioenergy uptake in the UK, although the lessons learned are applicable across much of the industrialised world. The findings are then analysed in the context of contemporary developments in energy, and particularly bioenergy, policies.

The availability of agricultural land

One of the most critical factors in projecting bioenergy resources to 2050 is the availability of agricultural land and its suitability for the cultivation of bioenergy crops [44, 56, 84, 86]. The definition of ‘availability’ itself is ambiguous and open to interpretation, while there are many uncertainties surrounding the uptake of bioenergy and the implications for land use in Britain. Bioenergy can be derived from a wide variety of biological resources, some of which do not directly require a significant uptake of land. Typically, energy derived from wastes, such as Municipal Solid Waste (MSW) and Commercial & Industrial (C&I) Waste, do not have an associated land use. Similarly, agricultural slurries or farm wastes, such as animal manures, give rise to only indirect land use associated with the grazing livestock (e.g., cattle and sheep). These biogenic wastes will be disregarded here, because the focus is on land use from dedicated energy crops, such as Miscanthus and Short Rotation Coppice (SRC).

Energy crops and crop yields

Potential increases in yields are an important factor in determining the land that can be employed for the cultivation of dedicated energy crops, such as Miscanthus giganteus (hereafter termed ‘Miscanthus’), SRC Willow (Salix spp.), or Poplar (Populous spp.). The former is a woody, perennial, rhizamatous grass hybrid capable of growing between 2.5 m and 3.5 m in height. Its progenitor plants, Miscanthus sinensis and Miscanthus sacchariflorus, are native to Japan. Miscanthus retains nutrients well, and does not require large volumes of nitrogen fertilizer. It is suggested that in the future Miscanthus yields could be in excess of 15 oven-dry tonnes (odt) per hectare [82], although this is on the highest quality arable land that is most likely to be reserved for cultivating food crops. This represents a large improvement on current yield estimates that have been discussed by Bauen et al. [7], where a conservative estimate of baseline yield averages is given as 10 odt per hectare. The broad chemical composition of Miscanthus on a percentage basis is cellulose 44%, hemicellulose 24%, and lignin 17%. It has a net calorific (or ‘lower heating’) value of about 17 GJ per odt. In contrast, SRC are tree species suitable for harvesting on a shortened cycle of between 2 and 5 years. McKendry [68] reviewed a range of European yields for SRC Willow of between 10 and 15 odt per hectare, while research by Aylott et al. [6] suggests figures of between 2 and 13.5 odt per hectare. The rough chemical composition of SRC willow is cellulose 40%, hemicellulose 30%, and lignin 30%, with a net calorific value of around 18.5 GJ per odt. An increase in yield above 15 odt per hectare will clearly be beneficial for future bioenergy production. It is expected that yields for energy crops will increase through biotechnological and genetic engineering advances in the coming decades leading towards 2050 [82]. Whether crop yields would increase indefinitely into the future is difficult to say with any certainty, and is very much dependent on developments in crop genetics. These will give rise to second (and higher) generation biofuels. Obviously with a greater yield of a crop per hectare, more energy can be produced using the same land area or, alternatively, the required land area could be reduced whilst still achieving the same bioenergy production.

Socio-economic implications of bioenergy developments

Future bioenergy uptake will depend heavily on the reaction of the public to adopting new technologies that may have an effect on the landscape in the UK. In order for ambition to become reality, there needs to be a level of public acceptance and understanding and government support for bioenergy technologies [25]. If this fails to materialise, the result will be restricted land availability for cultivating bioenergy crops and increased pressure on other renewable options. One potential concern surrounding the uptake of bioenergy crops in the UK is the associated visual impact and change of landscape aesthetics. Miscanthus is not a crop native to the UK, and can grow to 3.5 m in height in a single year [77], it is suggested that the unfamiliarity of the crop in the UK may result in public objection [89]. However, experience in Austria and Sweden, where bioenergy use is well established, suggests that further uptake will be welcomed by broader society [80].

In addition to the concerns over public reactions to landscape changes and increased uptake of bioenergy, it is important that consideration is given to the incentives for potential suppliers to invest in developing bioenergy feedstocks, especially in the absence of certainty regarding their future. Research by the UK-based International Institute for Environment and Development suggested that it could be compounded by the fact that turning land over to perennial energy crops represents a long-term commitment of around 15–20 years. A supplier is then restricted to one crop throughout this period; as opposed to an arable crop that could be changed year-on-year based on its economic output [59]. Thus, support policies and incentives will inevitably be required in order to encourage bioenergy uptake into the future [80]. Adams et al. [1] also noted that financial considerations - the ability of farmers/producers to ‘make a profit’ - would be the most significant driver for the development of bioenergy, although uncertainty still surrounds the possible return available from biomass crops in the UK.

Background

Bioenergy conversion processes are the methods by which the energy stored within biomass can be released [44, 69]. The complex and varied nature of bioenergy means that unlike other renewable energy sources, which have one set method of energy generation and mode of output, there are a wide range of bioenergy conversion processes which can deliver energy in many ways (see Fig. 3). These can result in solid, liquid and gaseous fuels, and provide energy across the end-uses of heat, electricity and transport [3]. Conversion processes for releasing the energy from biomass range from simple combustion (i.e., burning wood) to the complex formation of liquid biofuels for transport from lignocellulosic biomass feedstocks [57, 60, 70]. The conversion method used will primarily rely on the required end-use of the biomass; whether it is required to provide heat or power in-situ or to generate gaseous or liquid fuels for use elsewhere [69]. The development of increasingly efficient conversion processes into the future will be a key factor in deciding how limited biomass resources are best utilised. Thus, the conversion methods available are outlined below. Such bioenergy conversion processes can be characterised as thermo-chemical, biochemical and physical-chemical methods.

Thermo-chemical conversion

Biochemical conversion

Physical chemical processing

Future developments in conversion technologies

Solid biomass imports

Liquid biofuel imports

Given the strong need to import biodiesel and bioethanol to meet the needs for transport fuels in the UK, it is now necessary to assess the implications of importing liquid biofuels, particularly palm oil produced in Indonesia and Malaysia, and bioethanol production mainly from sugarcane in Brazil and from corn (i.e., maize) in the USA. It is estimated that demand for biofuels is responsible for the 76% increase in UK imports of palm, soya and rapeseed oils since around 2005 [74]. Palm oil production gives rise to similar concerns to those associated with solid palm residues (see Table 3), i.e., deforestation and loss of biodiversity, forest fires, air pollution and associated health impacts, and the abrogation of land and social rights. Likewise, bioethanol production creates burdens on the indigenous producer countries, including environmental degradation, food insecurity, and water profligacy. Many of the feedstocks for biofuel production raise concerns about the ‘carbon debt’ caused by release of CO₂ into the atmosphere through the process of land clearing and subsequent cultivation for energy crops. Research by Fargione et al. [29] suggests, for example, that it takes 423 years to repay palm biodiesel CO₂ emissions associated with peatland rainforest, in contrast to 48 years for corn bioethanol from abandoned cropland.

Low carbon options for the UK

Several notable modelling exercises have been undertaken in recent years that were based on low or zero carbon UK energy scenario or pathway sets. These include those developed by the British Government’s former Department of Energy and Climate Change (the DECC 2050 Calculator; see [21]), the UK Energy Research Centre (the UKERC Energy 2050 Project; see the book-length discussion in [83]), and the Centre for Alternative Technology (the Zero Carbon Britain 2030 Project; see [8]; another book-length contribution, albeit self-published). [DECC was merged with the then Department for Business, Innovation & Skills (BIS) in 2016 to form the Department for Business, Energy & Industrial Strategy (BEIS)]. This enables a comparative evaluation to be made of each projection, which are based on alternative modelling approaches that seek to meet the 2050 GHG reduction target (an 80% fall below 1990 levels in the case of DECC/BEIS and UKERC), to that of fully decarbonising Britain by 2030 (CAT). Nevertheless, Hammond [39] argued that energy projections involve a high degree of uncertainty - forecasting as a “black art”. [Indeed, he suggested that rolling projections using a rather broad, sectoral approach that is continuously updated at not greater than five-year intervals, in a similar manner to econometric forecasts, are more useful for energy planning purposes.] The DECC/BEIS 2050 Calculator is basically an engineering-based, Excel spreadsheet model [inspired by the late DECC/BEIS Chief Scientist, Sir David MacKay; 1967–2016] that is open source and arguably transparent. It is an online platform or tool that allows users to choose their own combination of technologies to attain an 80% reduction in GHG emissions by 2050 against the 1990 baseline, whilst ensuring that energy supply and demand are balanced. The UKERC Energy 2050 Project [83] brought together a wide range of interdisciplinary researchers to explore the possible development of the UK energy system through to 2050. This involved a three-scenario core set that was underpinned by a cost-optimisation model: the UK MARKAL Elastic Demand [MED] model (the details of which are variously described in [4, 24, 83, 87, 91]). UKERC took “an eclectic approach to scenario building” [83] with a backcasting dimension to achieve a combination of UK energy sector resilience and climate change mitigation. In contrast, the Zero Carbon Britain 2030 (ZCB2030) Project [8] examined how to radically ‘PowerDown’ UK energy, heat and electricity demand – what they viewed as ‘high carbon living’ - through the take-up of a combination of new technology and efficient design within society (by motivating behavioural change), while the country will ‘PowerUp’ its economy by way of the use of renewables to supply the residual energy requirements. Forecasts by UKERC and DECC/BEIS both project scenarios out to 2050, which is the year by which the UK Government is legally committed to achieve 80% reductions in GHG emissions below 1990 levels. In contrast, CAT’s ZCB2030 scenario only extends out to the year 2030, by which time it is envisaged that the UK can cut all emissions to zero. It is essentially linked to an ‘ethical construct’ that the per capita GHG emissions should ultimately be shared between the nation states of the world on an equal basis. Thus, Zero Carbon Britain offers a much ‘greener’ and more ambitious perspective than studies by UKERC and DECC, due to the ‘deep’ cuts in domestic emissions incorporated into ZCB2030 and also the ‘rapid’ or ‘constrained’ timeframe in which the target is to be achieved. Nevertheless, it will provide an interesting perspective and basis for comparison with the more conservative work of UKERC and DECC.

The selected UK low carbon scenarios and projections

DECC 2050 calculator

The projected bioenergy contribution under the DECC Base scenario [[21]; see also the outline description given in Table 4] amounted to some 251 TWh, which appears relatively high in comparison to the equivalent scenario by UKERC (70 TWh in its ‘Base’ case). Nevertheless, it is close to the figure produced by the recent UK Government’s UK and Global Bioenergy Resource Model [76], and mentioned in Section 2 above. A majority of this DECC bioenergy resource is presumed to be sourced from waste. This biogenic waste accounts for 196 TWh of bioenergy under the Base scenario, either through combustion of solid wastes, or the collection of landfill gas. The quantity of waste in 2050 increases by 60% in this low ambition scenario, and the total amount of waste to landfill correspondingly increases. In addition to municipal and landfill waste, the remaining bioenergy is sourced from agricultural wastes (37 TWh) and forests and biocrops (18 TWh). It is projected in the DECC Base scenario that livestock numbers rise by 10%, which allows agricultural wastes (primarily manure) to contribute 37 TWh to bioenergy resourcing. In contrast to the increase in livestock numbers, the levels of energy crop and food production are presumed to remain similar to those today. A total area of 350,000 ha would be employed for cultivating energy crops; that amounts to a modest 18 TWh contribution in terms of forestry and biocrops. However, bioenergy imports fall to zero under the DECC Base scenario, as it is assumed that international bioenergy trade does not develop to a significant extent. This scenario performs particularly poorly with regard to GHG emissions, with 2050 emissions being only 1% below 1990 emissions.

In the DECC ‘Spread Effort’ (SE) scenario [see the outline description given in Table 4] bioenergy contributes 516 TWh to primary energy; more than double the amount of bioenergy produced in its Base scenario. The quantity of waste is considered to be ‘stable’ and most is recycled; thereby minimising biodegradable waste to landfill. Improved waste management, recycling, and the reduction in the amount of waste to landfill means that the quantity of energy generated from waste in the DECC SE scenario declines to 134 TWh. The land available for cultivating energy crops is greatly increased to some 2.4 Mha. Thus, forests and biocrops account for a significantly higher contribution under the SE scenario, accounting for 182 TWh: making it the largest single contributor to bioenergy. Despite a 10% reduction in the number of livestock in the UK, energy production from agricultural waste is increased to 125 TWh as biogenic waste management is improved. Imports of bioenergy develop to contribute 70 TWh through 35 TWh liquid transport fuels and 35 TWh of solid biomass for thermal generation. This is considered to be 50% of the UK market share. The end-use contribution of liquid biofuels for transport amounts to 12%, while bioenergy for electricity and heat generation account for 47 and 41% respectively. End-uses of bioenergy for heat and electricity are primarily served by solid biofuels with a minor contribution from biogas.

The DECC ‘Solid Biofuel Focus’ (SBF) scenario [see again the outline description given in Table 4] is the highest ambition scenario postulated by the Department (DECC, now BEIS) with the contribution of bioenergy more than double that under the DECC SE scenario. A total of 1062 TWh bioenergy contributes to primary energy in this case: by far the largest amount of bioenergy projected under any of the pathways/scenarios examined here. The maximum possible energy recovery from waste takes place under the SBF projection with the total volume of waste increasing by 30% above 2007 levels. Following improvements in the collection and processing of waste, it is forecast that recycling, energy from waste, and energy from landfill methane and sewage gases all yield greater energy returns; raising the level of energy generation from wastes to 212 TWh. The land made available for energy cropping is increased yet further to approximately 17% of UK land area equivalent to 4.2 Mha. It is assumed that this can yield close to 400 TWh of bioenergy from forests and biocrops; constituting 37% of the total bioenergy contribution to primary energy. A 10% increase in livestock numbers in addition to the land used for food crops will put pressure on land resources, especially when so much is attributed to energy crops. Energy recovery from agricultural wastes contributes 146TWh to the bioenergy make up. Due to the strong SBF emphasis on bioenergy as a renewable resource, the level of imported bioenergy is increased again in solid form for energy generation, as well as liquid biofuel to help decarbonise the transport sector. The total contribution from imports is 259 TWh, which is approximately 200% of the UK’s share of the 2050 international bioenergy market (modelled by DECC). A breakdown of the end-uses of bioenergy available under the SBF scenario is depicted in Fig. 4, along with that for other high ambition scenarios [the UKERC ‘Carbon Super Ambition’ (CSAM) and CAT ‘Zero Carbon Britain’ (ZCB2030) forecasts respectively]. The general breakdown in end-uses is roughly similar to that of the DECC SE scenario, although the proportion of imported liquid biofuel for transport is rather higher. DECC SE and SBF projections diverge during 2010–2020, and SBF achieves greater bioenergy uptake through to 2050.

UKERC ENERGY 2050 pathways

The UKERC Energy 2050 pathways [[83]; see again the outline description given in Table 4] appear, in general, to be more conservative in terms of the bioenergy resources used to meet primary energy supply. This is in part due to the reduced overall primary energy supply under the UKERC scenarios, but also due to uncertainty over the role of bioenergy in the 2050 energy mix and competition with other renewables. The UKERC REF scenario, much like the DECC Base scenario, is a continuation of government policies in place around 2010, and leads to very limited reductions in GHG emissions to 2050. The REF scenario achieves only a 2% reduction on 1990 levels by 2050. End-use contributions from bioenergy resources in the UKERC REF scenario suggest that liquid biofuels for transport are the preferred use, although only marginally more so than solid biofuels for electricity generation. In line with the limited ambition for the development of domestically produced biofuels in the REF scenario, all the 20 TWh of bioenergy is used for transport fuels - bioethanol and biodiesel - by 2050. These resources are imported from abroad, as UK land available for bioenergy is assumed to decline to zero. A significant proportion of the final energy arising from biomass is made up from electricity generation through the utilisation of biowaste. It is assumed that this is in the form of methane collection and ‘energy recovery’ from landfill, and the anaerobic digestion (AD) of wet wastes [although an exact breakdown by source is unavailable (in [83])]. This amounts to around 4% of the power generation mix in total under the UKERC REF scenario, with fossil fuels accounting for ~ 80%. In addition to electricity generation and transport, heat provision using bioenergy makes only a minor contribution; providing the service sector with approximately 9 TWh of heat via woodchip fuel for combustion in boilers. Heat supply through biomass use in the residential sector is presumed to decline to zero around 2035. It is assumed that this applies to specific in-house biomass heating systems, rather than the use of woodfuel for heat (for example, ‘log fires’) in the home.

The UKERC CAM scenario [see, once more, the outline description given in Table 4] achieves the early UK GHG emissions target under the 2008 Climate Change Act [51] of an 80% reduction in CO₂ emissions by 2050 [24, 83, 91]. Bioenergy contributes 317 TWh to the UK energy mix at this timeframe; equivalent to around 20% of primary energy supply. In contrast to the UKERC REF pathway, the overwhelming majority of this biomass resources (~ 80%) are converted to liquid biofuels for transport. This increase in the demand for liquid transport fuels is largely met from domestically produced biomass (accounting for 66% of all liquid biofuels) unlike the imported biofuels presumed under the UKERC REF pathway. The motivation for this change appears to be concern surrounding the sustainability of global biomass trade ([83, 84]; and see also Table 3). The increase in domestic biofuel production is made possible by an increase in the area of UK land attributed to bioenergy uses. Projections based on the UKERC CAM scenario suggest that the land take increases to approximately 1.7 Mha by 2050. Finally, a minor portion of the bioenergy resource under the CAM pathway is used for electricity generation. Again, as with the REF scenario above, it was assumed [83] that this is in the form of methane collection and ‘energy recovery’ from landfill, as well as AD processing of wet wastes. However, in contrast to the REF pathway, electricity generation using biowaste actually declines from 18 TWh to 11 TWh in the CAM scenario. This decline, coupled with the increase in other bioenergy end-uses (for heat and transport respectively), means that power supply counts for only 4% of the total bioenergy resource in the UKERC CAM pathway.

The UK Government’s independent Committee on Climate Change [11] believes that if international bunker fuels and non-CO₂ GHG emissions are excluded from UK mitigation targets, then the overall level of decarbonisation required from the remaining sectors of the economy would need to be closer to a 90% reduction [in contrast to 80% under the original 2008 Climate Change Act [51]]. To achieve this target, the most ambitious UKERC pathway [‘Carbon Super Ambition’ (CSAM)], foresees a contribution of 479 TWh from bioenergy to meet UK primary energy supply, as part of a strategy that achieves an overall reduction in CO₂ emissions of 90% compared to 1990 levels. Again, as with the CAM scenario, the majority of the bioenergy resource is presumed to be in the form of liquid biofuels to help decarbonise the transport sector; see again Fig. 4. However, the level of liquid transport fuel is significantly larger under UKERC CSAM pathway [83, 91], representing around 290 TWh, including 10 TWh of biokerosene. In order to accomplish this, domestic production of liquid biofuels is presumed to increase by just over 50% with associated rise in the land required for cultivating the feedstocks. The UKERC book-length text [83] does not provide precise figures for land take associated with the UKERC CSAM pathway. However, an indicative calculation based on land requirements and the domestic production of liquid biofuel and wood pellets under the CAM scenario suggests an approximation for the land required as close to 3.1 Mha: see Fig. 5. Finally, it is interesting to note that under UKERC CAM and CSAM pathways, bioenergy only begins to develop rapidly around the 2025–2030 period, which is in contrast with corresponding DECC projections that indicate a relatively smooth uptake and development of bioenergy from the period 2010–2020.