The purpose of the third phase of an LCA, the life-cycle impact assessment (LCIA), is to assess a product system’s life-cycle inventory results, to better understand their environmental significance (ISO 14042 2000). The impact assessment is achieved by translating the environmental loads from the inventory results into environmental impacts.

For this study the so-called CML 2001 method (normalisation factors from November 2009) was applied to translate the environmental loads of the 37 lignocellulosic bioenergy systems into environmental impacts. CML 2001 is a collection of impact assessment methods that restricts quantitative modelling to the relatively early stages in the cause-effect chain, to limit uncertainties and to group LCI results into midpoint categories, according to themes (Guinee et al. 2002).

Only one of the global impact categories, the global warming potential (GWP100 years), calculated as t CO2-equivalent, will be further discussed in this chapter. The results in terms of other environmental impacts of the assessed lignocellulosic bioenergy systems, such as the abiotic depletion potential (ADP, measured in gigajoules), acidification potential (AP, t SO2-equivalent), eutrophication potential (EP, t phosphate-equivalent) and photochemical ozone creation potential (POCP, t ethene-equivalent), can be found in Von Doderer (2012).

Climate change may lead to a broad range of impacts on ecosystems and our societies, but greenhouse gases (GHG) have one property in common, which is useful for characterisation in an LCA. Characterisation of GHGs is based on the extent to which they enhance the radiative forcing in the atmosphere, i. e. their capacity to absorb infrared radiation and thereby heat in the atmosphere (Baumann and Tillman 2004: 149).

The mechanism of the greenhouse effect can be observed on a small scale, as the name suggests, in a greenhouse. These effects also occur on a global scale. Short­wave radiation from the sun reaches the earth’s surface and is partially absorbed and partially reflected as infrared radiation. The reflected fraction is absorbed by greenhouse gasses (GHGs) in the troposphere and is re-radiated in all directions, including back to earth. This results in a warming effect on the earth’s surface (PE International 2010).

This effect is amplified by human activities, in addition to the natural mechanism. Carbon dioxide is not the only gas that causes climate change. Methane, chlorofluo — rocarbons (CFCs), nitrous oxide and other trace gases also absorb infrared radiation. Compared with CO2, they absorb much more effectively. The potential contribution of a substance to climate change is expressed as its global warming potential (GWP) (Baumann and Tillman 2004: 149).

The LBSs’ overall performance in terms of global warming potential is presented in Fig. 11.8, above. Significantly, different results can be seen for LBSs 5, 13, 21, 29 and 37. These alternatives have bioenergy system V in common, where only

■ Unstablebio-charin soil

Fig. 11.9 Global warming potential of selected lignocellulosic bioenergy systems subdivided into production phases bio-oil produced in mobile fast-pyrolysis units is used for electricity generation. The other product from the fast-pyrolysis process, bio-char, is assumed to be sold to the fertilising industry for application to soil. Eighty percent of the bio-char is assumed to be stable in the soil, resulting in negative GWP levels of more than 32,000 t CO2-equivalent. For the other LBSs, a similar observation can be made as for the acidification and eutrophication potential impact categories: the greater the overall-conversion efficiency of the bioenergy conversion system applied, the fewer up-stream activities are required and the lower the GWP. Other LBSs also show negative GWPs, which can be explained by the positive effects of carbon stock changes when introducing SRC plantations. In these cases the increase in carbon stock compensates for the GHG emissions caused during harvesting, forwarding, pre-processing and secondary transportation. In comparison, the South African power-grid mix shows a GWP of more than 44,000 t/CO2-equivalent, assuming the same functional unit.

Figure 11.9, above, shows the GWP performance of five selected LBSs, sub­divided in production phases. LBS 14 uses bioenergy conversion system I, while LBS 2, 20, 27 and 37 deploy BCS II, IV, III and V respectively. The relatively large fraction of GWP for the harvesting phase for LBS 14 can be explained by the 30 % of unutilised biomass remaining infield. Although there is no direct relation between the harvesting and decomposition of the unutilised biomass, it is during the harvesting phase that the trees are felled, de-branched and cross-cut, leaving the tops and branches behind. LBS 37 entails an unstable carbon fraction. When using biochar as additive to soil around 20 % are assumed to be unstable, resulting in the decomposition thereof.