Agro-biofuels are expected to reduce the emissions of greenhouse gases because CO2
emitted during the combustion of the biofuels has recently been taken from the
atmosphere by the energy crop. Thus, when replacing fossil fuels with biofuels we
reduce the emission of fossil fuel-derived CO2 into the atmosphere. However, cultivation
of the soil results in emission of other greenhouse gasses, especially nitrous oxide (N2O).
Agricultural activity is the dominant source of N2O, which is produced by microbes in
the soil when the nitrogen availability is high, for instance following fertilization or
incorporation of crop residues.
In this study we relate measured field emissions of N2O to the reduction in fossil
fuel-derived CO2, which is obtained when energy crops are used for biofuel production.
The analysis includes five organically managed crops (viz. maize, rye, rye-vetch, vetch
and grass-clover) and three scenarios for conversion of biomass to biofuel. The scenarios
are 1) bioethanol production, 2) biogas production and 3) co-production of bioethanol
and biogas, where the energy crops are first used for bioethanol fermentation and
subsequently the residues from this process are utilized for biogas production. The net
reduction in greenhouse gas emissions is calculated as the avoided fossil fuel-derived
CO2, where the N2O emission has been subtracted. This value does not include farm
machinery CO2 emissions and fuel consumption during biofuel production. Thus, the
actual net greenhouse gas reduction will be lower than indicated by our data. We
obtained the greatest net reduction in greenhouse gas emissions by co-production of
bioethanol and biogas or by biogas alone produced from either fresh grass-clover or
whole crop maize. Here the net reduction corresponded to about 8 tons CO2 per hectare
per year. The worst result was obtained for bioethanol produced from vetch straw where
high N2O emissions outweighed the avoided fossil fuel-derived CO2.
|Conference||Risø International Energy Conference 2011|
|Period||10/05/2011 → 12/05/2011|
|Series||Denmark. Forskningscenter Risoe. Risoe-R|
- Environment and climate