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4. Greenhouse Gas and Energy Balances of Biomass Energy Options

4.1 Background

4.1.1 Life Cycle Assessment

There are two main reasons for current interest in greenhouse gas ( GHG) and energy balances of conventional and new sources of energy; global climate change and energy resource depletion. Concern over the impacts and consequences of global climate change means that it is essential to determine the amounts of GHGs emitted by the complete life cycle of current and possible future energy technologies. Most studies which investigate these life cycles concentrate on the three most prominent GHGs; carbon dioxide ( CO₂), methane ( CH₄) and nitrous oxide (N ₂O). These GHG emissions arise from a number of different sources but the most significant are emissions from the combustion of fossil fuels, such as coal, natural gas and petroleum products, and emissions from soils due to their cultivation.

For convenience, CO₂, CH₄ and N ₂O emissions can be aggregated together into "equivalent CO₂" emissions by means of their relative effectiveness at absorbing infra-red radiation, represented by their "global warming potential" ( GWP). Because each GHG is subject to different physical and chemical dynamic processes, it resides in the atmosphere for a different period of time. This means that the GWP of each GHG varies differently over time. As a consequence, the Inter-governmental Panel on Climate Change ( IPCC) recommends values of GWP which can be selected to represent various scientific, policy and decision-making time horizons ( IPCC 2001). A series of GWPs for specific time horizons is reproduced in Table 4.1. Currently, 100 year values for GWPs are commonly adopted in LCA and similar calculations.

Table 4.1: Global Warming Potentials for Prominent Greenhouse Gases

Concerns over energy resource depletion and the related issue of fuel price inflation mean that it is essential to evaluate the amounts of depletable energy, such as fossil and nuclear fuels, that are needed by the complete life cycle of current and possible future energy technologies. The quantification of energy resource depletion depends on the estimation of primary energy which is a measure of the amount of energy available from sources at their original points of extraction. This includes the energy contained in coal and uranium ore at the pithead, and natural gas and oil at the wellhead. The measurement of primary energy inputs is normally based on appropriate calorific values, which express the heat or energy content of the source of energy. Typically, primary energy is expressed in units of Joules (J) or multiples of 10 of these units, such as mega-Joules which equal 1 million Joules (10 ⁶ J or 1 MJ). In this report, the term 'energy balance' refers to the amount of fossil fuel energy necessary to produce one MJ of energy. The energy balance includes both direct inputs due to the consumption of fuels and electricity at particular stages of the biofuel technology and indirect inputs due to the provision of materials and equipments.

The usual method used to calculate total GHG emissions and primary energy inputs is life cycle assessment ( LCA). This is an established approach, specified by International Standard ISO 14040 series (International Standards Organisation 1997, 1999, 2000a, 2000b, 2000c, 2002), which has been developed to provide a consistent framework for evaluating the environmental impacts of the complete life cycle of any given product or service. In theory, LCA can be used to estimate all the natural resource inputs and environmental outputs associated with whole chain of processes needed to provide, use and dispose of a product or service. In practice, actual LCA studies often focus of specific inputs and/or outputs and occasionally restrict their scope to certain parts of the main process chain or life cycle. The reasons for effectively terminating LCA studies in this way include the need to concentrate on specific impacts and the intention of addressing particular environmental and policy concerns.

LCA studies are conducted for a variety of diverse purposes which, by necessity, must define the specific question that will be answered by the results. It essential to appreciate that the precise nature of the question posed affects the results generated by LCA studies. As with most other assessment methods, LCA studies are not capable of producing universal results which answer every possible question. The "goal and scope" of a LCA study determines all aspects of the way it is undertaken. In order to provide meaningful results, it is necessary to specify the exact characteristics of the product or service under investigation as well as the circumstances of its use or application. In LCA studies, this is summarised by the "functional unit" which defines the nature of the product or service under investigation.

This is important because the actual specification of, for example, an energy technology has a fundamental bearing on subsequent LCA results. This is particularly true for biomass energy technologies which can be governed by site-specific factors such as fertiliser application rates, crop yields and distance to processing and utilisation plants for biomass feedstocks. In fact, LCA results for any product or service can be sensitive to a range of basic parameters including the country where the technology is located and the time period under consideration. The reason for this is that results are affected by environmental performance of whole economy which supports any given technology. This varies to between nations and over time (see, for example, Lenzen and Wachsmann 2004). Hence, it is important to specify the "location" and "year" to which LCA results refer.

There are also other considerations which influence the nature and relevance of LCA results. These include specific issues such as "system boundaries", "reference systems" and "allocation procedures". Systems boundaries have to be established in any LCA study as these determine the extent of the process chain under investigation. As it is impractical to take all possible activities related to the process chain into account, LCA studies have to be qualified in terms of systems boundaries which explain, for example, whether the manufacture of capital equipment or the provision of maintenance has been incorporated into the calculations. Reference systems are necessary because many biomass feedstocks are obtained from the cultivation of land which could have been used for other purposes. These other uses and their environmental inputs and outputs have to be taken into account in the LCA calculations. It should, however, be noted that the actual choice of reference system can complicate analysis considerably, especially if the alternative is to grow another crop with a completely different use. For this reason, simple reference systems are often selected, such as no- or low-maintenance set-aside. Another frequently-encountered feature of biomass energy systems is that they produce joint rather than single products. For example, biodiesel production from oilseed rape using esterification also produces rape straw (often treated as a waste product), rape meal (usually sold as an animal feed) and glycerine (mainly sold as a chemical raw material). Allocation procedures are needed to share the environmental burden amongst joint products. However, there is not a single, "correct" way of undertaking such allocation and the choice of allocation procedure can affect the results of a LCA study.

4.1.2 Literature Review Procedures

From the introduction to the basic features of LCA set out in Section 4.1.1, it will be realised that there is no such thing as a unique set of LCA results which represents the environmental inputs and outputs of a given product or service in any situation. LCA does not provide universal answers which are relevant to all circumstances. Instead, it is necessary to qualify, in clear terms, a LCA study and its results in relation to its goal and scope, functional unit, location and year, systems boundary, reference system(s) and allocation procedure(s). In many instances, it may be necessary to modify, adjust or replace basic parameters and assumptions in an existing LCA so that it represents, correctly, the provision of a product or service under very specific circumstances. This can only be achieved if the LCA study is accessible (normally available in the public domain) and transparent (described in sufficient detail to enable changes to be made in parameters and assumptions).

An example of transparency is provided in Annex 1 which demonstrates the evaluation of the GHG and energy balances for the small-scale production of heat using wood chips obtained from woodland management. These illustrative results are based on earlier work (Elsayed et al 2003) which has been modified to reflect Scottish conditions. This demonstrates how accessible and transparent LCA studies can be adapted for use in specific situations. However, it should be noted that not all LCA studies are accessible because some are conducted for private companies or organisations on the basis of commercial confidentiality. Additionally, many LCA studies are either not published with full details or have been conducted using proprietary software and databases which do not reveal the basic calculations.

Accessibility and transparency are essential considerations in any review of LCA literature. In this study, other important considerations for a review of existing LCA literature are the relevance of the technology to Scotland and the relevance of any associated LCA study to Scotland. Hence, the following criteria were adopted in the review of existing LCA literature in this study:

Source: Main reference details for the work.

Summary: Concise description of the work to provide the reader with a quick and simple understanding of its essential details.

Coverage: Coverage of the energy technology and coverage of environmental inputs and outputs (primary energy inputs, and/or CO₂, CH₄ and N ₂O and/or total GHG emissions) with a brief explanation of the key LCA features including systems boundary, reference system(s) and allocation procedure(s).

Transparency: Assessment of the relative transparency of the work (fully transparent, partially transparent or not transparent).

Relevance: Current relevance of the work to Scottish circumstances and, where necessary and if possible, the modifications needed to adjust the work so that it is appropriate for Scottish circumstances.

Based on previous experience (Mortimer et al, 2003; Elsayed et al, 2003; Mortimer et al, 2004), it was decided that the review of existing LCA literature should be conducted in a consistent and standard manner. This assists with identification of existing LCA studies which might be suitable for providing estimates of primary energy inputs and GHG emissions for relevant biomass and other energy technologies in Scotland. Even more significantly, this approach enables important gaps in coverage to be determined. Such literature review work was necessary to establish:

• which existing LCA studies can be used in an unmodified way to represent Scottish circumstances,
• which existing LCA studies can be accessed and modified as necessary for Scottish circumstances, and
• where new LCA studies are required to address gaps that exist with biomass and other energy technologies for Scotland.

There are a variety of parameters and assumptions in LCA studies which have to be consistent with Scottish circumstances in order for the results to be relevant for use in Scotland. Apart from the choice of technology and its specific features, values of fertiliser application rates, crop yields, transport distances, etc., must be appropriate. It is essential that results from existing studies can either reflect these and other parameters and assumptions, or accommodate necessary changes. Similarly, it is necessary to ensure that the treatment and associated uncertainties of soil carbon sources and sinks, and N ₂O emissions from soil can be addressed consistently or modified accordingly (see Section 4.7). It should be appreciated that some of the basic science of the soil carbon and nitrogen cycles is not complete and this has to be taken into account in interpreting subsequent LCA results. Selection of consistent values for GWPs is also required and it is important to realise that, over a period of time, different LCA studies may have adopted different values. Unfortunately, this is sometimes not explicit and, even when it is, the details of calculations are not provided so that necessary alterations cannot be made to a consistent set of GWP values.

Finally, in relation to consistency, it must be understood that extreme caution must be applied when combining the results of LCA studies with other studies, such as economic evaluations. Such combination is often attempted in order to derive estimates of the net cost-effectiveness of saving GHG emission by different technologies. Whilst there is an understandable need for measures of cost-effectiveness, meaningful and reliable estimates can only be derived from studies which adopt the same, consistent parameters and assumptions. LCA and economic studies are rarely conducted together. Hence, when the results of independent LCA and economic studies are inevitably combined, they have the potential to generate unreliable or misleading conclusions.

4.1.3 Outcomes of the Review

LCA studies for review here were selected after an updated literature search. In addition, reviews of known commercial work that has been completed recently were added wherever possible. In total, 25 potentially-relevant LCA studies were identified and the subsequent review summaries, based on agreed criteria (see Section 4.1.2), are presented in Annex 2. The outcomes of this exercise can be presented, in terms of this study, by indicating the overall suitability or otherwise of particular LCA studies as means of representing GHG and energy balances for relevant energy technologies in Scotland. To this end, summary tables outlining the suitability to Scotland of previous work on GHG and energy balances of biomass energy technologies are provided in Annex 3.

For convenience, this review evaluation is divided into the groups of energy technologies. Conventional energy technologies which provide heat and electricity from current fossil fuels and other renewable energy technologies are evaluated in Section 4.2. Biomass energy technologies which produce heat, electricity, combined heat and power ( CHP) and transport biofuels are addressed in Sections 4.3, 4.4, 4.5 and 4.6, respectively. In order to avoid unnecessary evaluation and subsequent complications, this review has concentrated on the most recent and most relevant LCA studies for these particular energy technologies. Such effective pre-selection has been based on knowledge and experience gained from previous literature reviews. However, every attempt has been made to identify and review new, published LCA studies which cover relevant energy technologies.

4.2 Baseline Greenhouse Gas and Energy Balances

Before embarking on an examination of the review evaluation of LCA studies for relevant biomass energy technologies in Scotland, it is necessary to consider conventional energy technologies and other renewable energy technologies. GHG and energy balances are needed for these particular technologies because they form the baseline against which results for biomass energy technologies can be compared. Consequently, literature searches and reviews were conducted for LCA studies on relevant fossil fuel, nuclear and other renewable energy technologies which might be relevant to Scotland. Review summaries for the limited number of LCA studies identified are provided in Annex 3 and review evaluations are presented for conventional energy technologies and other renewable energy technologies in Tables 1 and 2 of Annex 2, respectively.

It may seem rather surprising that no existing, published LCA studies were found that were directly relevant to these particular technologies in Scotland. There are a range of different reasons for this. First, very few truly transparent LCA studies have been published for conventional energy technologies because, historically, there has been more interest in new energy technologies. Indeed, there is a general but incorrect assumption that LCA results for conventional energy technologies based on fossil fuels are commonplace and well-established. In reality, little attention has been directed to such LCA work perhaps because it is known, in advance, that their GHG and energy balances are considerably less attractive than those for most new energy technologies Second, LCA studies for nuclear power are somewhat contentious with little agreement between those produced within and outside the nuclear industry. Such studies would benefit from total transparency but, so far, this is largely lacking, partly due issues of commercial confidentiality. Finally, although substantially more LCA studies have been conducted on renewable energy technologies, it must be realised that these, by their very nature, are rather site-specific. Hence, without the necessary transparency, subsequent LCA results cannot be adopted as representative for the specific conditions encountered in given locations.

It is apparent from Annex 3 (Tables 1 and 2) that there are no existing LCA results for conventional energy technologies and other renewable energy technologies which can be used without significant modification or entirely new work as representative baseline GHG and energy balances for Scotland. A number of specific considerations would have to be taken into account in the preparation of suitable baseline GHG and energy balances for Scotland. For example, data are available to determine the primary energy inputs and GHG emissions associated with the production of natural gas and oil from the North Sea along with subsequent treatment and processing into heating fuels. However, such data would have to be combined with other information on the construction and operation of the natural gas transmission and distribution systems, and oil refining and distribution systems in either Scotland or the United Kingdom. Additionally, new work would be required on the manufacture and operation of natural gas- and oil-fired heating systems. This would be necessary to ensure consistent comparison with heat-producing renewable energy technologies since they incorporate, inherently, the means of providing heat for consumers.

Most LCA studies of fossil fuel-fired electricity generation have been conducted outside the United Kingdom. Hence, it is necessary to appreciate the significant modifications may be required to ensure that subsequent GHG and energy balances reflect the characteristics, operation and performance of these conventional energy technologies in Scotland or the United Kingdom. In terms of electricity generation from nuclear power, most LCA studies focus on the construction and operation of pressurised water reactor ( PWR) power plant and their associated fuel cycles. However, the nuclear power plants which currently operate in Scotland are based on the advanced gas-cooled reactor ( AGR). Consequently, even if it was possible to resolve differences over basic data, assumptions and methods of calculation between existing LCA studies, subsequent GHG and energy balances for nuclear power would not represent current circumstances in Scotland.

The situation for GHG and energy balances for petrol and diesel produced from crude oil is somewhat clearer since there are a number of published LCA studies of these conventional transport fuels. However, relative consensus only exists for production but not for utilisation. This is because many factors, including vehicle type and performance, engine set-up and maintenance, driving conditions, etc., effect the direct emissions from fuel combustion. It is, of course, possible to select a representative level of direct emissions for given circumstances, ideally based on actual road tests. However, problems arise when attempts are made to compare subsequent GHG and energy balance with those of alternative biofuels such as biodiesel and bioethanol. Meaningful comparison can only be achieved if direct emissions from these biofuels are measured under comparable test conditions. Unfortunately, there appears to be no complete consensus over the results of testing which, ideally, should be conducted by independent organisations with standard procedures for comparative purposes. Given the current lack of agreement, it is normal to limit comparison of GHG and energy balances for transport fuels to production only.

The main concern over GHG and energy balances for other renewable energy technologies is that most relevant LCA studies have been conducted on examples outside the United Kingdom. Obviously, where possible, these would have to be adjusted to Scottish renewable energy resource conditions. Even so, it must be appreciated that the GHG and energy balances for most of these technologies can only be represented by a range of results. For example, the results for electricity generated from wind power depend on the average annual wind speed of the site, the size and number of wind turbines, and the distance of the connection to the electricity network. Similar considerations apply to all these renewable energy technologies. Furthermore, the actual country of origin of any equipment incorporated into these renewable energy technologies can alter their GHG and energy balances due to national differences in economic structure and dependence on fossil fuels. Apart from these general considerations, no recent, published LCA studies with adequate transparency could be found for solar water heating and wave power. Whilst a number of LCA studies were conducted on solar water heating in the 1970's (see, for example, Evans 1970), these do not reflect modern systems. Additionally, no LCA studies seem to have been published on current designs of wave power devices in the United Kingdom.

This dearth of suitable LCA results raises the question of whether it is possible to provide reliable baseline GHG and energy balances for subsequent comparison with those for biomass energy technologies. Strictly speaking, a complete set of baseline GHG and energy balances is not currently available for Scotland nor, in fact, for the United Kingdom. Instead, it is only possible to adopt results from existing LCA studies and related work (see, for example, Eyre 1990; Eyre and Michaelis 1991) to provide illustrative results. In this study, conventional heating is assumed to consist of individual oil-, liquefied petroleum gas- ( LPG) and natural gas-fired central heating systems and illustrative results for these were derived from earlier work (quoted in Sustainable Development Commission, 2005 and Country Landowners' Association, 2006). Illustrative results were available for average grid electricity in the United Kingdom in 1996 which reflect the mix of power plants operating at that time ( NETCEN 1999). A study in Germany (Pehnt 2006) provided illustrative results for solar water heating, small-scale hydro power (300 kW), medium-scale hydro power (3,100 kW), solar photovoltaics (polycrystalline SOG-Si PV cells), onshore wind power (1,500 kW) and offshore wind power (2,500 kW). Illustrative results for unleaded petrol and ultra low sulphur diesel derived from crude oil in the United Kingdom were obtained from a combination of a standard source ( NETCEN 1999) and an earlier study of biofuels (Mortimer et al 2003). Illustrative GHG and energy balances for conventional and other renewable energy technologies which produce heat and electricity, and for conventional transport fuels are shown in Figures 4.1 to 4.6, respectively. It must, again, be emphasised that these results do not constitute representative baseline GHG and energy balances for conventional and other renewable energy technologies in Scotland. Instead, they are only provided here to indicate the comparative magnitude of GHG emissions and primary energy inputs.

Figure 4.1: Illustrative GHG Balances for Heat-Producing Conventional and Other Renewable Energy Technologies (Not Necessarily Representative for Scotland)

Figure 4.2: Illustrative Energy Balances for Heat-Producing Conventional and Other Renewable Energy Technologies (Not Necessarily Representative for Scotland)

Figure 4.3: Illustrative GHG Balances for Electricity-Producing Conventional and Other Renewable Energy Technologies (Not Representative for Scotland)

Figure 4.4: Illustrative Energy Balances for Electricity-Producing Conventional and Other Renewable Energy Technologies (Not Representative for Scotland)

Figure 4.5: Illustrative GHG Balances for Conventional Transport Fuels

Figure 4.6: Illustrative Energy Balances for Conventional Transport Fuels

4.3 Greenhouse Gas and Energy Balances of Heat Production from Biomass

The review evaluation of heat-producing biomass technologies is summarised in Annex 3 (Table 3), while details of the review of individual, published LCA studies are provided in Annex 2. In particular, this relates to the production of heat by the combustion and gasification of wood chips from forestry residues and the combustion of straw. For these technologies, there is one particular study, sometimes referred to as the Biofuels Report, is in the public domain (Elsayed et al 2003). The Biofuels Report was prepared jointly by the Resources Research Unit of Sheffield Hallam University and Forest Research for Future Energy Solutions (formerly ETSU) on behalf of the Department of Trade and Industry. One essential feature of the Biofuels Report is that it adopts a transparent approach to the calculation of GHG and energy balances. An example of this approach is demonstrated in Annex 1.

The essential approach and most of the data and assumptions in the Biofuels Report were subsequently incorporated into the Biomass Environmental Assessment Tool ( BEAT) which is owned by the Environment Agency (Environment Agency 2005). Whereas the Biofuels Report is presented in paper-based format, BEAT consists of Excel spreadsheets in electronic format. These spreadsheets were prepared by North Energy Associates Ltd under sub-contract to Future Energy Solutions. The scope of BEAT is considerably wider than the Biofuels Report but, since it is only intended for internal use within the Environment Agency, it is not available for direct public access. Despite this, the approach used in both the Biofuels Report and BEAT could be adapted to derive GHG and energy balances for the majority of heat-producing biomass energy technologies for Scotland. This would mainly involve modifying key parameters, data and assumptions so that they reflect circumstances in Scotland.

he only heat-producing biomass energy technologies for which existing, published LCA studies could not be identified where those involving the combustion, gasification and pyrolysis of wood chips and wood pellets from short rotation forestry and the combustion of biogas from the anaerobic digestion of slurry. In both these cases, new work would have to be undertaken to obtain relevant data on short rotation forestry and biogas production and utilisation so that GHG and energy balances can be calculated accordingly. Whilst there appear to be no specific published LCA studies on biomass co-combustion or co-firing with fossil fuels in existing equipment, it might be possible to adopt the Biofuels Report and/or BEAT to evaluate subsequent GHG and energy balances for these options. However, basic data on equipment performance and combustion emissions would be needed to do this.

Using Biofuels Report for relevant energy technologies, it is possible to provide illustrative results. On this basis, GHG balances are shown in Figure 4.7 and energy balances are demonstrated in Figure 4.8. For convenience, these results are compared with the GHG and energy balances of heat produced from conventional energy technologies and other renewable energy technologies (see Section 4.2). However, it must be noted that, in most instances, these results cannot be regarded as representative for Scotland since some will require varying degrees of modification which is beyond the scope of this study.

Figure 4.7: Illustrative GHG Balances for Heat-Producing Biomass Energy Technologies (Not Necessarily Representative of Scotland)

Figure 4.8: Illustrative Energy Balances for Heat-Producing Biomass Energy Technologies (Not Necessarily Representative of Scotland)

4.4 Greenhouse Gas and Energy Balances of Electricity Production from Biomass

Based on examination of the appropriate reviews in Annex 2, the relevance of existing, published LCA studies to electricity-producing biomass energy technologies for Scotland is evaluated in Annex 3 (Table 4). The outcome of this evaluation is very similar to that for heat-producing biomass energy technologies (see Section 4.3). Only a few LCA studies provide GHG and energy balances which might be directly relevant to Scotland. In particular, these consist of the results from the Biofuels Report (Elsayed et al 2003) and BEAT (Environment Agency 2005) for the generation of electricity by combustion, gasification and pyrolysis of wood chips for forestry residues and by the combustion of straw. However, as previously suggested, suitable results could be derived for most other electricity-producing biomass energy technologies by modifications to either the publicly-available Biofuels Report or, if accessible, the BEAT software of the Environment Agency. This would require the collection and incorporation of specific data for these biomass energy technologies to ensure that GHG and energy balances were relevant to Scotland. In some cases, existing, published LCA studies for certain electricity-producing biomass energy technologies could not be identified. Hence, it is suggested that new work would have to be undertaken to determine the GHG and energy balances for electricity generation by combustion, gasification and pyrolysis of wood chips and wood pellets from short rotation forestry, poultry litter, and meat and bonemeal. lustrative results for relevant electricity-producing biomass energy technologies can be obtained from the Biofuels Report. GHG balances are presented in Figure 4.9 and energy balances are provided in Figure 4.10. These results are compared with the GHG and energy balances of electricity produced from conventional energy technologies and other renewable energy technologies (see Section 4.2). However, as before, it must be emphasised that, in most instances, these results cannot be regarded as representative for Scotland since some will require varying degrees of modification which is beyond the scope of this study.

Figure 4.9: Illustrative GHG Balances for Electricity-Producing Biomass Energy Technologies (Not Necessarily Representative of Scotland)

Figure 4.10: Illustrative Energy Balances for Electricity-Producing Biomass Energy Technologies (Not Necessarily Representative of Scotland)

4.5 Greenhouse Gas and Energy Balances of Combined Heat And Power Production from Biomass

Similar conclusions to those drawn previously (see Sections 4.3 and 4.4) were found for the review evaluation of existing LCA studies for combined heat and power ( CHP) biomass energy technologies based on the details presented in Annex 2 and summarised in Annex 3, Table 5. Once again, it was concluded that the Biofuels Report (Elsayed et al 2003) and BEAT (Environment Agency 2005) provide suitable results which could be used to represent the GHG and energy balances for CHP with combustion, gasification and pyrolysis of wood chips from forestry residues, and with combustion of straw. Modifications to either the Biofuels Report or BEAT would be needed to represent most of the other CHP biomass energy technologies. Due to the apparent absence of existing, published LCA studies, new work would be needed to derive GHG and energy balances for CHP with combustion, gasification and pyrolysis of wood chips and wood pellets from short rotation forestry, poultry litter, and meat and bonemeal.

Using the results presented in the Biofuels Report, illustrative GHG and energy balances for certain CHP biomass energy technologies can be provided and compared with those of heat production and electricity generation from conventional energy technologies and other renewable energy technologies. However, such comparison is slightly more involved than previous (see Sections 4.3 and 4.4) because, obviously, CHP technologies produce two important outputs, namely, heat and electricity, rather than one output as in the technologies considered so far. As in the case of any process that generates more than one product, GHG emissions and primary energy inputs have to be allocated between the joint products (see Section 4.1.1). Conventionally, this is achieved for CHP technologies by, effectively, placing relative values on the energy available in the heat and electricity produced (Defra 2005). Specifically, electricity is attributed with twice the value of heat from a CHP technology. This means that twice as much GHG emissions and primary energy inputs are allocated to the electricity as to the heat. On this basis, separate meaningful comparison can be made between the heat and the electricity produced by CHP biomass energy technologies and from both relevant conventional and other renewable energy technologies. In particular, GHG and energy balances for heat produced from CHP biomass energy technologies are presented and compared in Figures 4.11 and 4.12, respectively, and for electricity produced from CHP biomass energy technologies in Figures 4.13 and 4.14, respectively. As stressed previously, in most instances, these results cannot be regarded as representative for Scotland since some will require varying degrees of modification which is beyond the scope of this study.

Figure 4.11: Illustrative GHG Balances for Heat from CHP Biomass Energy Technologies (Not Necessarily Representative of Scotland)

Figure 4.12: Illustrative Energy Balances for Heat from CHP Biomass Energy Technologies (Not Necessarily Representative of Scotland)

Figure 4.13: Illustrative GHG Balances for Electricity from CHP Biomass Energy Technologies (Not Necessarily Representative of Scotland)

Figure 4.14: Illustrative Energy Balances for Electricity from CHP Biomass Energy Technologies (Not Necessarily Representative of Scotland)

4.6 Greenhouse Gas and Energy Balances of Transport Biofuel Production

A significant number of LCA studies have been undertaken and published on the production of transport biofuels, as is indicated by the reviews presented in Annex 2. This provided a good basis for the review evaluation which is summarised in Annex 3, Table 6. In the United Kingdom, transparent LCA studies have been conducted for the production of biodiesel from oilseed rape and recycled vegetable oil in the national context (Elsayed et al, 2003; Environment Agency 2005) and in a regional setting (Mortimer 2006), and for the production of bioethanol from wheat grain in the national context (Elsayed et al 2003; Punter et al 2004; Environment Agency 2005). The results for biodiesel production from recycled vegetable oil could probably be used directly in the Scottish context. However, results from these studies for biodiesel production from oilseed rape and bioethanol production from wheat grain cannot be regarded as specific to Scotland, as they are based on average UK data and do not represent one part of the UK in particular. It would be necessary to adjust them accordingly, with Scottish data on fertiliser application rates, crop yields and transport distances to provide representative GHG and energy balances. Whilst there is a published LCA study which includes results for biodiesel production from tallow (Beer et al 2002), this represents circumstances in Australia and is not sufficiently transparent to allow modification to Scottish conditions. No suitable, published LCA studies of bioethanol production from barley or potatoes could be identified in this literature review. Hence, new work would be required to determine the GHG and energy balances for these particular methods of biofuel production in Scotland.

It is possible to present illustrative GHG and energy balances of transport biofuel production and to compare these with those of conventional transport fuels. However, it is essential to appreciate at least three important considerations which affect these results and their comparison. First, the results are not necessarily representative of circumstances in Scotland. In particular, some of these results require adjustment to Scottish conditions which is beyond the scope of this study. Second, these results do not include the CH₄ and N ₂O emissions of fuel combustion in vehicles (see Section 4.2). This is mainly due to a lack of consistent and agreed basic data on tailpipe emissions from comparable tests for using biofuels and conventional fuels in vehicles. Third, GHG and energy balances for biofuels can be fundamentally influenced by the specific types of production technologies and joint product treatments chosen. Evidence has been presented elsewhere to demonstrate that the choice of sources of fuels and electricity used in biofuel processing can alter the GHG and energy balances significantly (Mortimer, Elsayed and Horne 2003). Indeed, as is shown in Figure 4.16, 'negative primary energy inputs' may result. This situation arises when by-products are co-fired, displacing coal for electricity production. This additional fossil fuel displacement may be greater than the total fossil energy inputs used to produce the fossil diesel and petrol with which biodiesel and bioethanol are compared to in the life cycle assessments and is thus termed 'negative'. Replacing fossil fuels with renewable energy for process electricity production strengthens this effect.

In particular, earlier LCA studies indicated relative high values of primary energy inputs and GHG emission due to the assumption that natural gas or oil would be used as main process heating and that all electricity supplies would be obtained from the national grid. However, most developers are adopting CHP plants to provide process heat and electricity in their current biofuel plants. The higher overall thermal efficiencies achieved by utilising CHP reduces both primary energy inputs and GHG emissions. As most CHP plants are designed to match the heat demands of the biofuel production plants, surplus electricity can often be generated. When sold for export to other consumers directly or via the national grid, this produces effective "credits" which further improve the GHG and energy balances (see Section 4.1.1). In some instances, developers may consider using renewable energy sources, such as straw, to fire the CHP plant which has obvious benefits for the GHG and energy balances. Finally, the way in which the joint products are treated can have a further beneficial effect on the overall GHG and energy balances of the biofuel plant. The traditional market for major by-products such as rape meal and distillers' dark grains ( DDGS) is animal feed. However, due a combination of falling demand for animal feed in the United Kingdom, possible future over-supply and the emergence of alternative markets, other options for using such joint products now have to be taken into account. Specifically, these options include sending joint products, such as rape meal, glycerine and DDGS, to existing coal-fired power stations for co-firing. The effective "credits" can dramatically reduce the GHG and energy balances of biofuel production.

With all these considerations in mind, illustrative GHG and energy balances for the production of biofuels and conventional transport fuels are shown in Figures 4.15 and 4.16 respectively. It will be noted that there are substantial differences in the comparative results for biodiesel and bioethanol depending on the assumed methods of providing energy in the production processes and on the assumed subsequent use of joint products. It is important to realise that the relevance of particular results depends on decisions made by individual developers and operators of biofuel production plants which, in turn, is mainly influenced by the over-arching policy framework. As stated previously, these results are only illustrative and cannot be regarded as representative for Scotland without further adjustment which is beyond the scope of this study.

Figure 4.15: Illustrative GHG Balances of Biofuels for Transport (Not Necessarily Representative of Scotland)

Figure 4.16: Illustrative Energy Balances of Biofuels for Transport (Not Necessarily Representative of Scotland)

4.7 Key Uncertainties

4.7.1 Time Frames

The issue of time is of crucial importance when performing assessments of GHG mitigation potential of different biomass energy technologies. Besides the changes in global warming potentials of individual greenhouse gases, discussed in Section 4.1, a variety of other variables are sensitive to changes over time.

A critical factor that may affect calculations of GHG benefits of different biomass energy technologies is changes in baseline levels over time. As noted earlier, there is much uncertainty regarding total GHG emissions of fossil fuel and nuclear energy systems, meaning that more transparent studies which are specifically relevant to Scotland could lead to results which differ markedly from existing and strictly indicative results shown here. Development of cleaner fossil technologies such as clean coal and carbon capture systems should be expected to reduce the relative GHG emissions savings of renewable energy systems such as biomass energy technologies.

Another factor influenced by appropriate time frames is the market dynamics of biomass energy technologies, which is heavily coupled to policy measures. Such measures, themselves, may evolve over time as awareness of the practicality and potential of biomass energy technologies expands. This could favour the uptake of one biomass energy technology over another, which would have a direct effect on the overall GHG emission levels. There could also be changes in join product allocation where the development of markets for co- and by-products could also alter the energy and GHG balances. This is shown clearly in Figures 4.15 and 4.16, where marked improved are seen in the energy and GHG balances, respectively, of biodiesel and bioethanol when joint products are used for co-firing instead of animal feed.

Finally, climate may influence the future of the biomass energy market in Scotland. Temperature can play a crucial role in shaping soil emissions, for example, increasing both CO₂ and N ₂O emissions (Knorr et al 2005; Flynn et al 2005), negatively impacting on greenhouse gas balances. Increasing temperature can have an impact on crop yields which can also alter GHG balances. Climate change may even eventually affect the choice of crops that are grown for biomass energy utilisation. It could be, for example, that increasing temperatures could favour the adoption of miscanthus in Scotland, which does not seem to be likely at present due to climatic incompatibility (Schroter et al 2005).

4.7.2 Carbon Source/Sink Dynamics

4.7.2.1 General Background

Carbon sequestration can be defined as the accumulation of carbon in a particular ecosystem or land use system. Those ecosystems where carbon is accumulating are termed sinks while those that are losing carbon are termed sources. Ecosystems can store carbon either in aboveground vegetation or in the soil. In most countries, forests under sustainable management act as strong carbon sinks while arable ecosystems tend be characterised as carbon sources. A recent study suggested that for most European countries, the overall ecosystem carbon balance is small due to the similar strengths of these opposing effects (Janssens et al 2005). Biomass energy systems can act as either sources or sinks, depending on the land use system that they replace.

Carbon sequestration effects are constrained by permanence issues. Carbon stored in vegetation is released when wood is burned for energy or when it decomposes. A comparatively small fraction of this enters long-term storage in wood products, which can be regarded as temporary carbon pools (Matthews and Robertson 2002). Different systems have different equilibrium carbon pools, beyond which the effects of carbon sequestration are negligible. Gains accrued by carbon sequestration effects can be easily reversed by land use changes. Reversion of land use back to arable cropping, for example, will result in a loss of soil carbon which could negate any increases under perennial grass or short rotation coppice systems (Smith 2005).

The soil and vegetation carbon pools are closely linked to each other. Essentially, the rate of change in soil carbon is the result of the relative rates of the processes of organic matter addition to the soil and the rates at which organic matter is lost from the soil through erosion or respiration. Actual sequestration rates, however, depends on a host of factors, both edaphic and climatic. Carbon sequestration rates are very slow and many measurements over periods of several decades are required for significant changes to be quantified. Besides its CO₂ mitigation value, increased soil carbon can result in a host of benefits to soils including greater water holding capacity, greater resistance to mechanical damage and improved aeration (Jarecki and Lal 2003).

Due to fossil fuel carbon substitution, however, the overall emission benefits of biomass energy production greatly exceed those of other mitigation strategies. The maximum carbon mitigation potential of biomass energy production has been compared with other carbon mitigation strategies, such as amendment of arable soils with manure, cereal straw incorporation, no-till farming, agricultural extensification and natural woodland regeneration (Smith et al 2000). The results, summarised in Table 4.8, indicate that biomass energy production could result in a maximum carbon mitigation potential (in the 0-30 cm soil layer) of 3.5 million tonnes of carbon (mt C yr ^-1) in the United Kingdom, compared with 2.6 mt C yr ^-1 for woodland regeneration and 1.6 mtC/a for extensification. On a European scale, it has been estimated that biomass energy crops could mitigate as much as 75 mt C yr ^-1 (Smith et al 2000b). It is important to emphasize that there is an element of uncertainty associated with these figures, as they are crucially dependent on the quality of the data on soil carbon sequestration rates for different practices and on average soil carbon values for the UK.

Table 4.8: Estimated Maximum Carbon Mitigation Potential of Different Land Use Options for Arable Land in the United Kingdom

More recently, the total amount of carbon that could be sequestered in United Kingdom soils over the next 50 - 100 years, as well as the amount of emitted carbon that could be avoided through substitution of fossil fuels with biomass energy production has been estimated (Cannell, 2003). It was found that a realistic mitigation potential of 3.4 - 13.6% of United Kingdom fossil fuel emissions in 2000 could be mitigated by biomass energy production, while the realistic potential for carbon sequestration was 2.0 - 3.4%.

4.7.2.2 Agricultural and Forestry Residues

Biomass energy systems based on the removal of biomass from an existing land use may deplete soil carbon content through reduction in organic matter addition to soil, export of plant nutrients from the site and reduced plant productivity over time caused by declining soil carbon levels (Cowie et al 2006). It has been estimated that incorporation of crop residues into soil would sequester an average of 0.7 t C ha ^-1 yr ^-1 (Smith et al, 2005). Although this benefit would be lost through removal, the straw would still have a mitigating effect if used for biomass energy by substituting for fossil fuel use. By the same token, increased harvesting of forestry residues would be expected to cause additional disturbance and to have a detrimental effect on soil carbon, possibly resulting in a new equilibrium with a lower carbon content (Schlamadinger et al 2001). In Scotland, this effect may be accentuated due to the higher soil carbon stocks under Scottish forests. The average soil carbon stock under woodland in Scotland stands at 580 t C ha ^-1, approximately twice that in England (Matthews and Broadmeadow 2003). Effects of forestry residue removal are site-specific, however, with largest effects expected in peaty soils with high organic matter content. Literature reviewed on the effects of forest management on soil carbon suggest that whole-tree harvesting resulted in an average decrease of 6% in soil carbon in managed forest systems (Johnson and Curtis 2001). It has also been shown that depletion occurred only in the top soil horizon, with lower horizons being unaffected.

A Note on the Carbon Balance of Afforested Peatland

In the UK, deep peats represent about half of the carbon content of all soils and about 40 times the carbon content in UK vegetation (Hargreaves et al. 2003). In an undisturbed state, peatlands emit methane but accumulate carbon. When peatland is drained for forestry, however, this release of methane and accumulation of carbon stops and peat decomposition is accelerated, releasing CO₂. At the same time, however, there is an uptake in carbon by the vegetation so that the net carbon balance is the difference between that which is released by peat decomposition and that fixed by tree photosynthesis. Hargreaves et al. (2003) suggest that peatland acts as a source immediately following drainage but that within 4-8 years of planting, the afforested peat becomes a sink again. There is still, however, considerable uncertainty regarding the rates of peat decomposition.

4.7.2.3 Energy Crops

As the aboveground biomass of energy crops is removed at harvest for combustion, they only exist as vegetation carbon sinks over the length of their rotation cycle. Energy crops may, however, have a more long-lasting effect as soil carbon sinks and, therefore, strengthen their GHG mitigation potential if they replace annual row crops. The extensive deep rooting systems of perennial biomass energy crops constitutes a substantial belowground carbon pool that adds significant quantities of carbon into the soil. In some cases, the belowground biomass of energy crops can be up to 5 times that of conventional row crops. Several North American studies clearly demonstrate a strong relationship between underground root biomass and soil carbon content of soils under biomass energy crops (as reviewed in Lemus and Lal 2005).

It has been estimated that conversion of cropland area to biomass energy crop plantations would result in an average carbon sequestration rate of 0.6 t C ha ^-1 yr ^-1, approximately 50% greater than the estimated sequestration potential of no-till agriculture (Smith et al 2005). It should be emphasised, however, that there is much uncertainty surrounding these estimates. It has been estimated that replacement of arable systems with short rotation coppice could increase soil carbon stocks by as much as 20 t C ha ^-1 (Schlamadinger at el 2001), whilst miscanthus could increase soil carbon content by up to 38 t C ha ^-1 (Bullard et al, 2003). Experimental trials with miscanthus have demonstrated an increase of 14 t C ha ^-1 after 16 years of cultivation (Hansen et al 2004).

4.7.2.4 Transport Biofuel Feedstocks

It is well known that intensive cropping systems deplete soil carbon (Lal et al 2005). This is thought to result from increased mineralisation arising from cultivation practices that increase aeration, re-distribute the soil and expose physically-protected organic matter (Cowie et al 2006). Conversion of naturally-regenerating set-aside to transport biofuel crop systems, therefore, would be expected to decrease soil organic carbon stocks. Increased use of oilseed rape within existing arable rotations would be expected to have a neutral impact upon soil carbon since oilseeds already from a part of many arable rotational systems. Negative impacts on soil carbon can be mitigated to a certain extent by positive management practices including reduced tillage, fertilisation and use of cover crops (Jarecki and Lal 2003).

4.7.3 Non-Carbon Dioxide Soil Emissions

4.7.3.1 Nitrous Oxide Emissions

N ₂O is produced by natural soil processes in all soil types at low levels. Application of nitrogen fertiliser, however, has the potential to significantly increase N ₂O emissions, although there is much uncertainty surrounding the magnitude of these increases. In fact, N ₂O emissions are considered to be one of the major sources of uncertainty in life cycle assessments of different biomass energy technologies. This is a largely a result of the myriad of factors that can all influence emissions levels, including soil factors, climatic conditions, crop type, method of fertiliser application, season of fertiliser application and type of fertiliser application ( USEPA 2002; Flynn et al 2005; Skiba and Ball 2002). Unfortunately, there are little data from controlled experiments that specifically test the effects of the interaction of different factors on N ₂O emissions, meaning that it is very difficult to predict emissions with a high degree of accuracy. Among the soil factors influencing N ₂O emissions, the soil water-filled pore space ( WFPS) has been found to be particularly important (Dobbie et al 1999). Scottish soils typically have high % WFPS values, leading to enhanced N ₂O emissions (Flynn et al 2005). Soil texture may also influence N ₂O emissions, although the evidence supporting this is less robust (Skiba and Ball 2002). The primary climatic drivers of N ₂O emission are temperature and rainfall, with emissions being higher with higher rainfall levels and higher temperatures (Britt et al 2002; Flynn et al 2005). Emissions of N ₂O are also influence by crop type. Potatoes and leafy vegetables, for example, have been found to emit N ₂O at levels more akin to grasslands than to cereal crops (Flynn et al 2005). Further complicating factors are the method of application, with emissions generally increased by soil compaction, and timing of application. Emission in the summer can be about five times less than in the autumn/winter as crops take up a considerable amount of nitrate from the soil that would otherwise undergo denitrification and result in increased emissions (Stevens and Laughlin 1997). Finally, fertiliser type can also influence emissions, although there is some disagreement as to whether organic or mineral fertilisers result in higher emissions as different studies have produced conflicting results ( USEPA 2002).

Most LCA studies use fixed emission factors for different fertiliser types but clearly the actual emissions are heavily dependent on site-specific conditions, the details of which are often difficult to realistically incorporate in such studies. Often the actual emission factors chosen can vary widely. While the IPCC recommends an emission factor of 1.25% of total nitrogen applied, some studies have used emission factors as low as 0.3% while others have used factors which surpassed 2.0% of applied nitrogen (Britt et al 2002). Recent work has further highlighted the uncertainty associated with simple emission factors such as those recommended by the IPCC (Flynn et al 2005). By incorporating climate and crop-responsive factors as well as a factor to account for soil compaction, this work estimated that a total of 10,662 t N ₂O -N yr ^-1 had been released in Scotland from applied fertiliser and nitrogen deposited by grazing animals, a value 56% higher than estimates based on default IPCC emission factors.

Despite the uncertainties surrounding the relative roles of different factors on N ₂O emissions, it is very clear that levels of applied fertiliser will have a considerable bearing on emissions. It is useful, therefore, to compare the fertiliser inputs of different biomass feedstocks as this provides a clear indicator of their impact on N ₂O emissions, assuming all other factors are equal. In general, energy crops require less fertiliser than row crops and, thus, the effects on soil N ₂O emissions tend to be favourable (Bullard et al 2003). Fertiliser is usually required, however, in the establishment year of energy crops (Britt et al 2005). Table 4.9 provides an indication of the fertiliser requirements for different biomass feedstocks of relevance to the United Kingdom, and a comparison with conventional crops. Other management steps also lead to increased GHG emissions including site preparation, herbicide and pesticide application, harvesting and baling when necessary (St. Clair 2006). Table 4.10 provides illustrative results of total differences in pre-harvesting emissions, in CO₂ equivalents, for different biomass feedstocks in relation to conventional land uses (recent review by St. Clair 2006). Benefits in carbon savings were found to result in all instances of conversion of grassland and arable cropland to biomass energy land use systems, including oilseed rape. Conversion of broadleaved forest to biomass energy crop systems, however, is likely to result in unfavourable carbon balances.

Table 4.9: Fertiliser Requirements of Different Biomass Feedstock Options of Relevance to the United Kingdom

Sources
(a) DEFRA 2004.
(b) DEFRA 2004b.
(c) Christian and Haase 2001.

In some instances, however, N ₂O emissions can counteract gains obtained by sequestration (see Section 4.8.2). It has been calculated that application of organic fertiliser every three years to miscanthus could reduce the benefit obtained by sequestration by 24 - 31% (Britt et al 2002). If applied annually, the sequestration benefit was all but cancelled out. Due to its high global warming potential, only small amounts of N ₂O emissions are necessary to undermine benefits gained by increased sequestration. Low fertiliser application is, therefore, the key to positive agricultural carbon balances.

Table 4.10: Carbon Savings Associated with Conversion of Conventional LandUses to Biomass Feedstock Production

Source St. Clair 2006, based largely on Lal (2004).

Notes
Green boxes indicate reduced equivalent CO₂ emissions relative to conventional land use, while red boxes indicate increased equivalent CO₂ emissions relative to conventional land use. Values include emissions from site preparation, planting, pesticide application, herbicide application, fertiliser application and harvesting. Values represent the differences in emissions between the establishment of alternative land uses, rather than emissions of established land uses.

4.7.3.2 Methane Emissions

CH₄ is only emitted from soils under prolonged anaerobic conditions (Britt et al, 2002). As with N ₂O emissions, CH₄ emissions are highest immediately following application. In freely drained cropland, CH₄ emissions are almost insignificant and CH₄ may even be oxidised during aerobic conditions (Smith et al, 2004). The only major crop that emits sizeable amounts of CH₄ is rice, which is not of relevance to the United Kingdom.

4.7.4 Relevance to Scotland

The direct applicability of the results of studies on soil emissions and carbon dynamics to Scotland is not straightforward. With energy crops, this is confounded by several factors, including the uncertainty regarding short rotation coppice and energy grass yields under Scottish conditions. Additionally, there is little information on how soil carbon stocks may change when converting natural regeneration set-aside land to energy crop systems. Conversion of set-aside land to transport biofuel production systems will almost definitely result in soil carbon losses. Forestry residues are expected to play a significant part in the development of the biomass energy market in Scotland. This must be done in a well-planned manner so as to minimise disturbance and reduce soil CO₂ emissions. Little empirical data is available documenting changes in Scottish forests soils following increased removal of residues, but the large post-war increases in forest carbon are broadly attributed to reduced removal of pre-harvest forest products that happened after the war. Presumably, a reversal of this trend would see lower carbon stocks in Scottish forests. This effect could be more pronounced in Scotland than in other parts of the UK due to higher average soil carbon stocks (Matthews and Broadmeadow 2003), although there is still considerable uncertainty in how soil carbon responds to changes in land use and land management (see section 4.7.2.1). In high organic soils such as those commonly found in Scotland, there may be additional feedbacks of nitrogen fertiliser applications, so that additional N ₂O emissions could result (Flynn et al. 2005).

4.8 Greenhouse Gas and Energy Balance Conclusions and Recommendations

This review of GHG and energy balances for biomass energy technologies indicates that there are very few LCA studies which can be used, directly and without any modification, to represent their application in Scotland. Of all the LCA studies that have been published, the so-called Biofuels Report (Elsayed et al 2003) can provide relevant results for Scotland on the production of heat, electricity and combined heat and power by combustion, gasification and pyrolysis of wood chips from forestry residues, the production of heat, electricity and combined heat and power by the combustion of straw, and the production of biodiesel from recycled vegetable oil.

GHG and energy balances for most of the other biomass energy technologies could be derived for Scotland by modifying relevant parameters and assumptions in the Biofuels Report or in the Environment Agency-owned BEAT software (Environment Agency 2005). In order to undertake these modifications, it would be necessary to identify, collect and incorporate appropriate Scottish data on key parameters such as fertiliser application rates, crop yields, transport distances, etc. Additionally, it would be necessary to ascertain the likely sources of process energy and the likely end uses for joint products by developers and operators of future plants which produce biodiesel and bioethanol from oilseed rape and wheat grain, respectively.

To enable meaningful comparison, GHG and energy balances for conventional and other renewable energy technologies would have to be prepared for Scotland by modifying existing LCA studies. Existing LCA studies could be adapted and extended to provide suitable baseline results for Scotland on the production of electricity from coal, natural gas, onshore and offshore wind power, solar photovoltaics, and run-of-river and whole river hydro power, and the production of unleaded petrol and ultra low sulphur diesel from crude oil. New work would have to be conducted on the production of heat from LPG, natural gas and oil, solar water heating, and the production of electricity from nuclear power and wave power in Scotland.

Whilst existing LCA studies could be used extensively to provide suitable results for Scotland, new work would have to be conducted to determine the GHG and energy balances of a number of biomass energy technologies. These would include the production of heat, electricity and combined heat and power by combustion, gasification and pyrolysis of wood chips and wood pellets from short rotation forestry, the production of heat by combustion of biogas from the anaerobic digestion of animal slurry, the production of electricity and combined heat and power from combustion and gasification of poultry litter, meat and bonemeal, the production of biodiesel from tallow and the production of bioethanol from barley and potatoes.

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