LCA Food 2012 in Saint Malo, France! - Manifestations et colloques ...

PARALLEL SESSION 2A: LAND USE 8 th Int. Conference on LCA in the
Agri-Food Sector, 1-4 Oct 2012
assumed to be sold and the rest used as coolant in the upgrading process (Johansson, 2008). Biogas was used
also for the grain dryer.
Scenario A2 was based on straw, which was converted to ethanol via hydrothermal pretreatment, to enzymatic
hydrolysis and fermentation. The lignin is separated out during hydrolysis and was assumed to be
used in an integrated CHP plant for production of process steam and electricity, as well as surplus electricity
to cover the power demand of the farm (51 GJ). The plant was assumed to be a single pressure combined
cycle gas turbine (CCGT) and the flue gases recovered in a heat recovery steam generator (HRSG). Straw
furnaces supplied heat for farm buildings and the grain dryer.
2.6. Greenhouse gas emissions
Methane emissions from storage of manure and digestion residues were calculated according to the IPCC
Guidelines (IPCC 2006). Methane is formed in the digestive system of dairy cows when cellulose is decomposed
via anerobic microbial activity and depends on milk production rate, feed and the cow’s body weight.
For a dairy cow producing 8000 kg milk/yr, assuming a metabolic weight of 600 kg/cow the emissions are
120 kg CH4 /cow and yr. For heifers, 50 kg CH4 /animal and yr was assumed (Cederberg et al., 2007).
Direct emissions of nitrous oxide (N2O) from soil and fertilisers were calculated based on IPCC methodology
(1% of added nitrogen and 1% of nitrogen content in crop residues). Indirect emissions in the form of
leached nitrogen (which volatizes downstream) were set to 0.75% of leaching (IPCC 2006) and the amount
leached was assumed to be 11 kg N/ha (Wivstad et al., 2009). Emissions of N2O from storage of manure and
digestion residues were set at 0.1% of the nitrogen content available in the substrate, based on emission factors
for liquid manure (IPCC 2006).
The soil carbon balances of the cultivation systems studied were simulated with the ICBM (Andrén and
Kätterer 2001). The model assumes two carbon pools (a ‘young’ pool, Y, and an ‘old’ pool, O) of which
carbon entering the old pool is considered a carbon sink, as the decomposability is 100 times lower than in
the young pool, where rapid mineralisation of carbon as CO2 takes place. Equations 3a and 3b describe the
dynamics of the carbon balance for the young and old pool, respectively:
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dY e
dt i kY
r Y
(kg) Eq 1
dt hkY
reY
kOr
O
(kg) Eq. 2
dO e
where i is the input of crop residues or manure, ky and ko are the decomposition rates of Y and O respectively,
re is the external influence component (including for example soil climate, crop type and frequency of
soil tillage, and is lower for ley and grazing fields), and h is the humification coefficient (dimensionless).
The humification rate depends on the composition of the respective material. Values are described further in
Kimming et al., (2012). The annual mineralisation of carbon to carbon dioxide released to the atmosphere is
a mean over 21 years (3 crop rotations).
4. Results
4.1 Self-sufficiency potential and energy balance
The renewable energy supply systems in Scenario M1, M2a and M3 can make the farm self-sufficient in
energy under the given assumptions. In scenario M2b, only two thirds of the electricity requirement on the
farm was covered by the available rapeseed oil, so Scenario 2b was excluded from further calculations of
energy balance and GWP.
On the arable farm, Scenario A1 requires that ley is harvested from 25 ha, which is 13% of the total farm
area. A2 requires 49 ha of straw, i.e., 25% of the total farm area. This biomass is available in both scenarios,
since ley is planted on 29% of the farm area and cereals on 44% in the given crop rotation.
In the self-supply scenarios, 2.63 MJ of fossil energy (primary energy) per kg ECM was saved in the milk
production system and 755 for the whole 200 ha farm in the arable system
4.2 GWP
The GHG emissions from the milk production reference scenario (962 g CO2/kg ECM) were similar to
those in previous studies of organic milk production with conventional energy supply systems (e.g. Ceder-