The total electricity consumed by the AD process was determined
to be 5913 kWh per year. By using the same procedure,
the corresponding GHG emission was 5.21 tonnes CO2-eq per year
(35.58 kg CO2-eq/tonne food waste/yr). The rates of fugitive emission
(Table S8 in the supplemental material) for individual GHGs
were obtained from the EcoInvent database (2009). The biogas
generated from the AD process can also be used to generate heat
and electricity through CHP. It was assumed that over 99.9% of
the methane from biogas was consumed during energy production
and the resulting CO2 was released to atmosphere along with the
rest of the CO2 in the biogas. Although CO2 emission from burning
food waste-derived methane is usually considered as carbon neutral
(biogenetic CO2) (Sedjo, 2013), a factor of 20% was used in this
study to account for some of the food waste being derived from
animal sources (Lee et al., 2007; Dahlén, 2010). Similar to the previous
calculations, all of the GHG emissions were converted into
CO2-eq (Table 8). The total emission from the AD process was estimated to be 16.62 tonnes of CO2-eq per year (113.84 kg CO2-eq/tonne food waste/yr). The energy generated from this
biogas was approximately 355,134 MJ per year. Since the energy
content of natural gas (NG) is 36.6 MJ/m3 (Pradhan et al., 2011)
and assuming the same CHP efficiency of 76%, UC could save
12,767 m3 of NG annually.
The total electricity consumed by the AD process was determinedto be 5913 kWh per year. By using the same procedure,the corresponding GHG emission was 5.21 tonnes CO2-eq per year(35.58 kg CO2-eq/tonne food waste/yr). The rates of fugitive emission(Table S8 in the supplemental material) for individual GHGswere obtained from the EcoInvent database (2009). The biogasgenerated from the AD process can also be used to generate heatand electricity through CHP. It was assumed that over 99.9% ofthe methane from biogas was consumed during energy productionand the resulting CO2 was released to atmosphere along with therest of the CO2 in the biogas. Although CO2 emission from burningfood waste-derived methane is usually considered as carbon neutral(biogenetic CO2) (Sedjo, 2013), a factor of 20% was used in thisstudy to account for some of the food waste being derived fromanimal sources (Lee et al., 2007; Dahlén, 2010). Similar to the previouscalculations, all of the GHG emissions were converted intoCO2-eq (Table 8). The total emission from the AD process was estimated to be 16.62 tonnes of CO2-eq per year (113.84 kg CO2-eq/tonne food waste/yr). The energy generated from thisbiogas was approximately 355,134 MJ per year. Since the energycontent of natural gas (NG) is 36.6 MJ/m3 (Pradhan et al., 2011)and assuming the same CHP efficiency of 76%, UC could save12,767 m3 of NG annually.
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