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
Milk or
Cash crops
Figure 1. Schematic figure of the energy and material flows on the arable and milk farms respectively. Substituted
production refers to the impact on the external market of reduced or increased supply of goods from
the farm to the respective market. LBG = liquid biogas.
2.5 Renewable energy supply systems
Manure/straw/rapeseed/ley
Electricity/LBG/CO 2
In the milk production system biogas from manure was the main energy supply in all scenarios. In Scenario
M1, biogas produced from manure and cut straw was assumed to cover the entire energy demand. A
fraction of the gas was fed to a combined heat and power (CHP) system (gas engine) with electric output 14
kW. The rest was pumped via low pressure pipelines to a cryogenic upgrading facility for liquid biogas
(LBG) production. LBG was assumed to be delivered in trucks to the farm, where it was pumped to the tractors
via a pumping station in which the LBG first vaporized.
In Scenario M2a, the manure on the farm was utilised to produce biogas, assumed to be combusted in a
CHP system (gas engine) with electric output 13 kW. The rapeseed oil was assumed to be used to produce
rapeseed methyl ester (RME) in a small-scale production unit at the farm. The tractors ran on RME with
minor modification of the original diesel engines, such as replacing components with others of more resistant
materials (Ahlgren et al., 2010).
In Scenario M2b, RME was assumed to be produced in the same way as in Scenario 2a, but used in a
combustion engine for CHP production, with electric output 13 kW. Tractors are assumed to run on biogas
from manure, from which LBG is produced in an external production facility as in Scenario 1.
In Scenario M3, it was assumed that a wind turbine, owned by a farm cluster, supplies electricity and heat
to the farm (via heat pumps). Required capacity is 70 kW, embedded in a larger wind tower. Tractors run on
LBG produced from manure, upgraded to LBG as in Scenario 1 and 2b.
The CHP systems in scenario M1, M2a and M2b were dimensioned to work on full load 95% of the time.
The heat produced was used to heat buildings on the farm and for the anaerobic digestion process.
Grain produced on the farm was assumed to be dried in a straw-fuelled furnace in all M-scenarios (capacity
232 kW). Total energy consumption was 115 GJ which came from approximately 4 ha of wheat straw.
The biogas production took place under anaerobic conditions at mesophilic temperature, in a one-stage
anaerobic digester of 250 m
145
3 in Scenario M1 and 150 m 3 in Scenario M2 and M3. The raw biogas was assumed
to be upgraded to vehicle fuel quality in a large-scale cryogenic upgrading plant separating off methane
(CH4), carbon dioxide (CO2), water vapor and impurities at their different condensing temperatures (Johansson
2008). The process requires 1.62 MJ electricity/Nm 3 and the methane losses in existing facilities are
0.5%. Here, the plant was assumed to be located 50 km from the farm and LBG was delivered back to the
farm in vacuum-insulated trucks.
The biogas production in each scenario was dimensioned based on the availability of substrate and not on
the demand for energy on the farm, as this was considered the most feasible solution. This might result in
overproduction of LBG and electricity. Surplus LBG was assumed to be sold and replace use of diesel. Surplus
electricity was assumed to be sold to the market and electricity produced in natural gas-fired condensing
plants.
In the arable system, one scenario (A1) was based on biogas from ley. Assumptions were largely the same
as in the milk system. The upgrading process produces liquid CO2 (LCO2) as a by-product, which can be sold
as a refrigerant. In this case, it was assumed that the LCO2 replaced the HFC type R404a as a refrigerant in
grocery shops. However, this market is currently rather undeveloped and therefore only 1% of the LCO2 was