How much can biofuels reduce greenhouse gas emissions?
Book chapter, 2012
The transport sector is today totally dominated by fossil oil-based fuels, above
all gasoline and diesel. In order to decrease the fossil greenhouse gas (GHG) emissions from the transport sector, and the dependency on crude oil which is
a scarce resource, one option is to introduce biomass derived motor fuels, here called biofuels. However, biomass is also a limited resource which makes efficient resource utilization essential. Therefore, the usage of biomass for biofuel production will have to be compared to other possible ways to use the limited biomass resource.
The biomass derived transportation fuels that are available today includes, for example, ethanol from sugar or starch crops and biodiesel from esterified veg- etable oil. Biofuels based on lignocellulosic feedstock are under development. The two main production routes are gasification of solid biomass or black liquor followed by synthesis into, for example, methanol, dimethyl ether (DME), synthetic natural gas (SNG) or Fischer-Tropsch diesel (FTD), and ethanol produced from lignocellulosic biomass. Potential lignocel- lulosic feedstocks include forest residues, waste wood, black liquor and farmed wood. What feedstock will come to predominate in a country or region will very much depend on local conditions.
When evaluating the greenhouse gas emission balances or overall energy efficiency of introduction of new biomass-based technologies, it is important to adopt life cycle perspective and consider the impact of all steps from feedstock to final product(s). There are a number of different approaches that can be used for this purpose, and different choices can be made for each step from feedstock to product. Thus, different studies can come to very different conclusions about, for example, the climate effect for a given product and feedstock. These issues have been heavily debated, particularly regarding evaluation of different biofuel routes. Parameters identified as responsible for introducing the largest variations and uncertainties are to a large part connected to system related assumptions, for example system boundaries, reference system, allocation methods, time frame and functional unit. The purpose of this chapter is to discuss a selection of these issues, in order to give the reader an improved understanding of the complexity of evaluating GHG emission balances for different biorefinery products, with biofuels used as an example.