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How Much Can Biofuels Reduce Greenhouse Gas Emissions?

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How Much Can Biofuels Reduce Greenhouse Gas Emissions? 1 HOW MUCH CAN 2 BIOFUELS REDUCE GREENHOUSE GAS 3 EMISSIONS? Karin Pettersson Maria Grahn Department of Energy and Environment, Chalmers University of Technology* * Divisions of Heat and Power Technology (K. Pettersson) and Physical Resource Theory (M. Grahn). Chapter reviewer: Björn Sandén, Environmental Systems Analysis, Energy and Environment, Chalmers INTRODUCTION When evaluating the greenhouse gas emission 4 The transport sector is today totally dominated balances or overall energy efficiency of introduc- by fossil oil-based fuels, above all gasoline and tion of new biomass-based technologies, it is diesel. In order to decrease the fossil greenhouse important to adopt a life cycle perspective and gas (GHG) emissions from the transport sec- consider the impact of all steps from feedstock to tor, and the dependency on crude oil which is final product(s). There are a number of different a scarce resource, one option is to introduce approaches that can be used for this purpose, 5 biomass derived motor fuels, here called biofuels. and different choices can be made for each step However, biomass is also a limited resource from feedstock to product. Thus, different studies which makes efficient resource utilization essen- can come to very different conclusions about, for tial. Therefore, the usage of biomass for biofuel example, the climate effect for a given product production will have to be compared to other pos- and feedstock. These issues have been heavily sible ways to use the limited biomass resource. debated, particularly regarding evaluation of different biofuel routes. Parameters identified as 6 The biomass derived transportation fuels that responsible for introducing the largest variations are available today includes, for example, ethanol and uncertainties are to a large part connected to from sugar or starch crops and biodiesel from system related assumptions, for example system esterified vegetable oil. Biofuels based on ligno- boundaries, reference system, allocation meth- cellulosic feedstock are under development. The ods, time frame and functional unit. The purpose two main production routes are gasification of of this chapter is to discuss a selection of these solid biomass or black liquor followed by syn- issues, in order to give the reader an improved 7 thesis into, for example, methanol, dimethyl ether understanding of the complexity of evaluating (DME), synthetic natural gas (SNG) or Fischer- GHG emission balances for different biorefinery Tropsch diesel (FTD), and ethanol produced from products, with biofuels used as an example. lignocellulosic biomass. Potential lignocellulosic feedstocks include forest residues, waste wood, ASSESSING GHG EMISSIONS FROM black liquor and farmed wood. What feedstock BIOFUEL SYSTEMS will come to predominate in a country or region 8 The evaluation of energy efficiency and climate will very much depend on local conditions. impact of biofuels and other transportation options is usually done from a well-to-wheel 9 72 1 (WTW) perspective. A WTW study is a form of CO2 in the process (see further below). The life cycle analysis (LCA) that is normally limited produced biofuel is then distributed to refueling to the fuel cycle, from feedstock to tank, together stations. The final step includes the vehicle opera- with the vehicle operation, and that typically tion where the biofuel is used to fuel the vehicle’s focuses on air emissions and energy efficiency1 powertrain. A well-to-tank (WTT) analysis (see also discussion in Chapter 1 and Figure 1.2). includes the steps from feedstock to tank, and 2 A WTW analysis generally does not consider the thus does not include the vehicle operation stage. energy or the emissions involved in building facili- This type of analysis could be used for example ties and vehicles, or end of life aspects. The main when comparing different ways to produce a reason for this simplified life cycle analysis is that specific biofuel. Most studies are focused on the fuel cycle and vehicle operation stages are direct effects from physical flows in the WTW the life cycle stages with the greatest differences chain, but some studies also include an estima- 3 in energy use and GHG emissions compared to tion of contributions to system change2 (see also conventional fuels. In this chapter, WTW analysis discussion in Chapter 1). will be used to illustrate different methodological approaches and issues regarding the different CO-PRODUCTS AND ALLOCATION steps from feedstock to product. However, the PROBLEMS discussion can easily be generalized to apply to How to allocate the distribution of environmental other products as well. burdens between the different outputs of a 4 process producing more than one product has Figure 7.1 illustrates possible main energy and been one of the most controversial and heavily material flows between the main steps in a WTW debated issues of LCA methodology, as it can analysis of biofuels. If biofuel is produced inte- have significant impact on the results.3 Several grated with an industrial process, such as a pulp reviews of WTW studies of various biofuels show mill, the flows represented are net differences that co-product allocation is one of the key issues compared to a reference case representing the that influence the GHG and energy efficiency industrial process as it would have been non- 5 4 results. (See also examples in Chapters 6 and 8 integrated with the biofuel plant. and the general discussion Chapter 1.) The first step in a WTW chain includes opera- Allocation can be done on the basis of physical tions required to extract, capture or cultivate properties (mass, energy content, volume, etc.) the primary energy source, in this case biomass feedstock. Thereafter, the biomass needs to 2 See for example Sandén, B. A. and M. Karlström (2007). 6 «Positive and negative feedback in consequential life-cycle be transported to the biofuel production plant. assessment.» Journal of Cleaner Production 15(15): 1469- At the biofuel production plant, the biomass is 1481 and Hillman, K. (2008). Environmental Assessment processed into biofuel and possibly also other and Strategic Technology Choice – The Case of Renewable Transport Fuels. PhD Thesis. Department of Energy and products such as electricity, heat or other co- Environment, Division of Environmental Systems Analysis, products. The biofuel production plant may have Chalmers University of Technology, Göteborg, Sweden. a deficit of electricity. The biofuel production 3 See for example Finnveden, G. et al. (2009). Recent developments in Life Cycle Assessment. Journal of Environ- 7 process may also have a net deficit of steam. mental Management 91(1):1-21. However, this is usually handled within the plant 4 Börjesson, P. (2009). Good or bad bioethanol from a greenhouse gas perspective – What determines this? by firing additional fuel, or by using internal co- Applied Energy 86(5):589-594. products. Thus, the biofuel plant will not have a Delucchi, M. (2006). Lifecycle analyses of biofuels. Draft heat deficit. It could also be possible to capture report. Institute of Transportation Studies, University of California, Davis. Larson, E. (2006). A review of life-cycle analysis studies on 1 MacLean, H.L and Lave, L.B. (2003). Evaluating automo- liquid biofuel systems for the transport sector. Energy for bile fuel/propulsion system technologies. Progress in Energy Sustainable Development 10(2):109-126. 8 and Combustion Science 29(1):1-69. And Edwards, R. et al. Fleming, J.S., et al. (2006). Investigating the sustainability (2007). Well-to-wheels analysis of future automotive fuels of lignocellulose-derived fuels for light-duty vehicles. and powertrains in the European context, version 2c. JRC, Transportation Research Part D-Transport and Environment EUCAR and CONCAWE. 11(2):146-159. 9 73 1 2 3 Figure 7.1 Simplified illustration of possible main energy and material flows between the main steps in a well-to- 4 wheel (WTW) analysis of biofuels, where also the well-to-tank (WTT) and tank-to-wheel (TTW) parts are illustrated. or on the basis of economic value. Allocation REFERENCE SYSTEM can also be avoided through system expansion In systems analyses with the purpose of assess- or substitution, that is, expansion of the system’s ing global fossil GHG emissions, a baseline boundaries to include the additional functions of or reference system must be defined, based all co-products. Co-product credits can some- on an estimation of what would have occurred times also be handled by recalculating co-prod- in the technology’s absence. The reference 5 uct streams into the same raw material as used system should include alternative pathways for for the main product and then subtracting the the production of transportation fuel as well as calculated amount from the raw material usage. for electricity, heat, and other coproducts. If the feedstock production results in land-use change, Using physical or economic allocation, or recal- an alternative land use must also be included in culation of co-product streams, to handle copro- the reference system. Similarly, when the same 6 duced electricity, heat or other co-products, may feedstock is in demand for other purposes an hide wider system implications. Furthermore, the alternative biomass use should be included, as size of certain co-product markets are limited and the increased use of a resource with constrained this also needs to be taken into consideration, production volume results in less of that resource especially for large scale technology implemen- being available for other parts of the system, tation.5 Therefore, to fully see the impact of a which can cause important effects that may biofuel technology one has to estimate the impact significantly affect the results.7 of the co-products by using system expansion, as 7 6 recommended by for example the ISO standard. The choice of reference system depends largely on the aim and time frame of the study. The refer- ence system should constitute a close alternative 5 Hillman, K.
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