Feed or bioenergy production from agri-industrial residues? An overview of the environmental consequences including indirect land-use change impacts

Davide Tonini, DTU
Lorie Hamelin, SDU
Thomas Astrup, DTU

Second generation biofuels produced from “residual” biomasses are considered promising ways of producing bioenergy for mitigating global warming. However, many studies tend to forget that these biomasses are today used for specific purposes, such as feeding, bedding, and soil amelioration. This means that their use for energy would induce cascading consequences on the food/feed market, or on the carbon balance of the soil. These effects are called land use changes, as they cause an increase in the international demand of a food/feed product that finally induces an expansion of cropland into other ecosystems. Failing to account for these consequences may lead to results that misrepresent the actual environmental impacts [1-3].

This study quantified the GHG emissions associated with a number of bioenergy scenarios involving eight agricultural and industrial residues namely: I) straw, II) grass, III) whey, IV) brewers grains, V) sugar beet molasses, VI) sugar beet pulp, VII) sugar beet tops, and VIII) potato pulp. Three relevant conversion pathways were considered: combustion, fermentation to ethanol, and to biogas. An innovative approach to estimate indirect land use changes (iLUC) was developed, combining economic and GIS (geographic Information system) analyses. The aim was to inform decision makers on the environmental consequences of producing biofuels from these biomasses. Consequential life cycle assessment (cLCA) was used to quantify the impacts. The functional unit of the cLCA was 1 unit of wet biomass.

The LCA results revealed that GHG emissions from indirect land use changes were the major contributor to the total GHG impact (up to ca. 40-60% of the total induced GHG emissions). All in all, the use of biomasses that are today used as animal feed induced significant GHG emissions through indirect land-use changes. These were quantified at between 400-1000 kg CO2/t agri-industrial residue. However, for selected biomasses (straw, grass, and beet tops) the benefits associated with substitution of fossil fuels were higher than the GHG emissions generated through iLUC and/or carbon depletion (e.g. for straw). This was true also for other substrates (e.g. beet pulp) when electricity and heat (instead of transport fuel) were generated.

All in all, this study provides a new approach to estimate iLUC and applies it to selected bioenergy scenarios. The recommendation is to avoid the use for bioenergy of those substrates having a significant nutritional value (e.g. molasses, beet pulp, whey, and brewers’ grains). Conversely, the energy use of straw and grass, though leading to non negligible soil carbon turnover, may provide considerable GHG savings.


References

[1] Tonini, D. and Astrup, T. Life-cycle assessment of biomass-based energy systems: A case study for Denmark. Appl. Energy 2012, 99, 234-246.

[2] Tonini, D.; Hamelin, L.; Wenzel, H.; Astrup, T. Bioenergy Production from Solid Biomass: a Consequential LCA of 12 Bioenergy Scenarios including Land Use Changes. Environ. Sci. Technol. 2012, 46(24), 13521-13530.

[3] Hamelin, L.; Joergensen, U.; Petersen, B.M.; Olesen, J.E.; Wenzel, H. Modelling the carbon and nitrogen balances of direct land use changes from energy crops in Denmark: a consequential life cycle inventory. GCB Bioenergy 2012, 4(6), 889-907.