Environmental Engineering Reference
In-Depth Information
Table 6.3
Results for two Hub and PoD case study scenarios
Scenario 1 Scenario 2
Dairy cows
Percentage housed
300
50
300
100
Slurry produced (tonnes year
1
)
4454
8908
Cattle slurry: food waste in digester feed (wet weight
basis)
1.8
2.9
Food waste required to replace N leaving the farm
(tonnes year
1
)
2453
3115
Digester loading rate (kgVSm
3
day
1
)
4
4
Digester capacity (including 10% gas space) (m
3
)
540
788
Retention time (days)
26
22
Biogas (m
3
year
1
)
394107
542090
Methane (m
3
year
1
)
236464
325254
CHP (continuous electrical output) (kW)
99
136
Avoided CH
4
emissions from manure (tonnes CO
2
equivalent)
126.1
252.1
Avoided CO
2
emissions by replacement of grid
electricity
377.8
508.2
Avoided CO
2
emissions by replacement of mineral
fertiliser
136.3
173.1
Capital cost (£ kW
1
)
4000
4000
Total capital cost (£)
396000
544000
Income from electricity (£ year
1
)
139363
152915
.
60% capture rates for food waste
.
centralised processing using waste heat at the county's three energy-
from-waste plants
.
distribution to farms in a 20 km radius in 25 -tonne loads.
Results for two scenarios are shown in Table 6.3, with a herd of 300 dairy
cows, the average for a large dairy farm in Hampshire, either permanently
housed or housed for 50% of the year.
Given that 76 434 tonnes of food waste is available in the region, this
would be enough to supply 31 large farms based on their N requirements. In
estimating the food waste import requirements in this way it was found that
the proportions of nutrients in the cattle slurry and food waste did not
completely match the crop requirements: the application of some P and K in
mineral form is also required. The total methane generated on the farms is
around 10 millionm
3
with around 29 ktonnes of CO
2
equivalent savings, but
lower if the animals can graze freely in the summer months. The capital
costs for a digester vary according to size: costs in the range £2000-
7000 kW
1
electricity installed have been suggested (MREC, 2008; Redman,
2008). These costs do not include connection to the national grid, planning
or permitting, and the incomes shown are based on the current UK feed-in
tariff scheme (£0.115 kWh
1
). Rolling such a scheme out across the UK to
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