Scottish Government

Executive Summary

1. The aims of this study are to review the literature and summarise the information available on the energy generation potential, greenhouse gas and energy balances, air quality and associated environmental impacts and economics of a wide range of biomass energy technologies. The specific objectives of the study are as follows:

• to review existing studies on the aspects of biomass energy mentioned above and assess them in terms of their relevance to Scotland,
• to highlight information deficiencies and indicate the level of uncertainty associated with the current results,
• to make recommendations on any further studies that may be needed and what information they are likely to yield, and
• As far as possible, based on the available information, to make recommendations on the best options for a sustainable bioenergy industry in Scotland.

2. The study is set in the context of biomass energy development as part of a portfolio of renewable energy systems that could be exploited in Scotland to reduce GHG emissions and contribute to future energy security. In relation to other renewable energy sources, biomass has several advantages including its use as a potential source of heat as well as electricity, its ability to produce energy continuously without problems of intermittency and its ability to stimulate the rural economy, due to more extensive supply chains than most other renewable energy technologies. Potential drawbacks in relation to other renewable energy technologies include the costs associated with the production of certain feedstocks, the often dispersed nature of biomass resources which can make supply chains more challenging to manage, and the low energy density of biomass feedstocks (relative to fossil fuels) which increases transportation and storage requirements.

3. A range of policy measures, at European Union, United Kingdom ( UK) and Scotland scales have encouraged the growth of the renewable energy sector in Scotland, although biomass has played a relatively minor role so far. There is increasing recognition however, that biomass could make a significant contribution to energy supply in the UK, generally, and Scotland, specifically. This is reflected in the number of reports and studies published on this topic in recent years.

4. Updated estimates of Scotland's available wood resources for bioenergy, including those from forestry and the timber-processing industry, have recently been published ( FREDS, 2005; SDC, 2005) and continue to be refined ( FCS 2006). There is also a potentially significant resource that could be available from secondary processing industries (recycled wood), but estimates of the amount that would be available for biomass energy use still need refinement. The information on the availability of other feedstocks for bioenergy, such as straw and animal slurry, is less precise, although these are expected to play only minor roles in the development of Scotland. Among the purpose-grown energy crops, short rotation coppice ( SRC) is believed to hold the most potential, but limited commercial experience with its cultivation in Scotland means that it is difficult to predict yields with any great accuracy and there is still much doubt surrounding the economics of SRC in Scotland.

5. Biomass feedstocks are generally regarded as carbon neutral in that they emit the same amount of carbon dioxide ( CO₂) during combustion as they absorb during their growth cycle. During the utilisation cycle of a biomass feedstock, however, a range of processes, from the production and application of fertiliser used in cultivation to the transportation of the final product, emit CO₂ and other GHGs, such as nitrous oxide (N ₂O) and methane ( CH₄), both of which have stronger global warming effects than CO₂. Additionally, there are energy costs associated with biomass energy technologies since fossil fuel resources are utilised throughout the lifecycle of the fuel. Life Cycle Assessment ( LCA) is used to calculate total primary energy inputs from depletable energy resources (energy balances) and the total GHG balance during the complete life cycle of a biomass energy feedstock.

6. The results of any given LCA studies are specific to the individual questions addressed by each study. In the case of biomass energy technologies, they are specific to the particularities of the biomass feedstock chain in question and, thus, cannot be applied universally. Evaluation of the relevance to Scotland of existing LCA studies which determine GHG and energy balances for biomass energy technologies indicates that there is a considerable body of existing work that could be modified without major difficulty for Scottish conditions. There are, however, some key uncertainties in LCA studies that need to be addressed. These include the lack of transparency of GHG and energy balance data of baseline fossil fuel systems, which hinders comparison with biomass energy technologies, as well as uncertainties associated with N ₂O emissions from fertiliser and carbon sink/source dynamics of bioenergy systems. To enable meaningful comparison, GHG and energy balances for conventional and other energy technologies would have to be prepared for Scotland by modifying existing LCA studies.

7. Despite the lack of LCA studies designed specifically to reflect Scottish conditions, the following broad conclusions are drawn:

• Biomass heat, electricity and combined heat and power ( CHP) technologies result in considerable GHG emissions and depletable energy savings relative to fossil-fuel based systems (savings can be over 90%, depending on the systems being compared).
• GHG emissions and depletable energy savings from other renewable electricity and heat technologies, such as heat from solar heating panels and electricity from wind and hydro, are generally in the same order as those from biomass energy technologies. Differences between renewable technologies are small relative to the substantial savings gained by replacing fossil fuel technologies.
• Estimated GHG emissions and depletable energy savings of more advanced gasification and pyrolysis systems are greater than those of combustion-based systems but are not yet proven commercially.
• The GHG and energy balances of transport biofuels are heavily influenced by the source of the energy used in the production process, and by the end-use of by-products (rape meal from biodiesel production and distillers' dark grains from bioethanol production). The use of straw-fired CHP systems and by-products in co-firing, results in significantly improved GHG and energy balances than the conventional approach to biofuel production based on fossil fuels and the sale of by-products for animal feed.

8. Air quality impacts of biomass energy technologies were addressed by reviewing both combustion and life cycle emissions, and by analysis based on projected changes in combustion emissions under two simple scenarios for heat/electricity, and two simple scenarios for transport biofuel consumption to 2020. The following conclusions were drawn:

• The fossil fuel that biomass energy technologies replace is very important in determining whether air pollution emissions increase or decrease. Displacement of coal results in significant reductions in SO₂, as well as reductions in CO, PM, NO_x and NMVOCs emissions, whereas displacement of oil tends to lead to decreases in SO₂ emissions, but increases in other emissions such as PM or NO_x. Substitution of natural gas with biomass, on the other hand, generally leads to increases in emissions of all major pollutants.
• Although emission of some pollutants is determined by fuel
  characteristics, the choice of electricity/heat generation technology, including abatement systems, can also have a significant impact on non-greenhouse emissions and, in some instances, the technology can be more relevant than fuel characteristcs.
• There are substantial gaps in reliable emission data for biomass combustion for energy. This is especially true of PM2.5, PAH, VOC, ultra-fine and trace element emissions.
• Compared to LCA work on GHG and energy balances, very few studies on air pollution life cycle emissions have been conducted. Moreover, there have been no comparative studies that employed a fully transparent methodology for estimating these emissions and there are no studies that can be taken as being representative of Scotland.
• LCA studies that present results in terms of summed eutrophication and acidification impacts, often report that biomass systems based on energy crops are at a disadvantage to equivalent oil and gas-based systems, although there is less difference with forestry residue systems.
• There is much inconsistency regarding combustion emissions of transport biofuels in the literature. For biodiesel, the main trends in light duty vehicles are reduced particulate ( PM), carbon monoxide ( CO) and hydrocarbon ( HC) emissions and slightly increased NO_x emissions in relation to fossil diesel. For low-blend bioethanol, there appear to be no significant changes in emissions of NO_x and HC, although PM emissions are significantly decreased while acetaldehyde emissions are greatly increased in relation to petrol.
• LCA studies for transport biofuels produced from purpose-grown crops invariably report that biodiesel and bioethanol have greater eutrophication and acidification impacts than fossil diesel and petrol. Fertiliser emissions of NH₃ contribute heavily to this trend.

9. A review of impacts on water quality, soils and biodiversity was also undertaken. These impacts can be beneficial, neutral or negative, according to the crops grown and the land-use they replace (in the case of energy crops), and may also depend on the intensity of extraction, in the case of agricultural residues

10. The early stage of biomass market development means that there are relatively few studies available on the economics of biomass energy technologies in Scotland, although there are some noteworthy exceptions for the biomass heat sector. There have been a number of UK-wide studies published in recent years, from which the following conclusions were derived:

• Heat production is, on the whole, more favourable economically than electricity production and small-scale commercial and industrial heat (although not domestic-scale), in particular, seems to provide the best economic returns.
• In areas away from the gas supply, wood fuel already competes well with fossil fuel alternatives.
• Biomass electricity is currently more expensive than wind electricity, but less expensive than less-mature technologies such as wave and solar power.
• The carbon abatement costs of biomass heat systems are much lower than those of biomass electricity systems and than those from other renewable heat technologies.
• The carbon abatement costs of transport biofuels are high in comparison to other biomass end-uses. The carbon abatement costs are, however, dependent upon carbon balances, and improvement of carbon balances by using renewable sources to provide necessary energy inputs, and using co-products to provide further energy, will result in lower carbon abatement costs.

11. Integrated LCA and economic studies of the biomass energy technologies likely to be of greatest relevance to Scotland are required for different biomass uptake scenarios, involving different allocations for heat and electricity schemes of various scales. Such work would provide data on economic aspects, such as carbon abatement costs, which are specific to Scottish conditions.