Environmental Engineering Reference
In-Depth Information
The obtained configuration for energy recovery was evaluated also from an
environmental point of view, estimating the overall GHE produced, considering the
contribution due to the presence of both carbon dioxide and methane in the escaped LFG and
considering the contribution due to carbon dioxide (originally present in LFG and obtained
from methane combustion) in collected and combusted LFG.
Moreover, it is possible to consider that the amount of electric energy produced by the
engines fed with LFG, is no more produced by conventional energy system, this term can be
considered as an avoided effect of GHE emissions and then subtracted from the overall
balance. Being the specific emission of about 0,551 kg of equivalent CO
2
/kWh for electric
energy production (with reference to the Italian situation (ENEL, 1999)) the overall avoided
effect can be calculated. Table 9 summarises the above mentioned contributions.
Table 7. Investment costs and depreciation cost for the selected engines
Engine
cost [€]
Additional
cost [€]
Total investment
cost [€]
Depreciation
[€/year]
Engine size
kWe 143
252.000
282.500
534.500
74.351
kWe 625
377.000
282.500
659.500
91.740
kWe 1413
689.300
282.500
971.800
135.182
Table 8. Costs and earnings for the LFG energy recovery system
kWe143
kWe625
kWe1413
Produced EE [kWh]
6.513.868
42.315.774
139.652.646
Total produced EE [kWh]
188.678.178
EE selling earnings [€]
846.803
5.501.051
18.154.844
Engine costs + maintenance costs [€]
751.249
2.927.351
2.826.306
Net profit [€]
95.553
2.573.700
15.328.538
Total net profit [€]
17.997.791
Table 9. Contributions to GHE during the landfill life time
GHE from escaped LFG [t
CO2eq
]
1.221.175
GHE from collected LFG [t
CO2eq
]
234.839
Total produced GHE [t
CO2eq
]
1.456.014
Avoided GHE form EE production [t
CO2eq
]
- 103.962
Net produced GHE [t
CO2eq
]
1.352.052
The simple collection and combustion of LFG (for example by means of flaring) offers
the possibility of reducing GHE with respect to uncontrolled emission of all LFG to
atmosphere (in that case the GHE production would be 2.199.035 t
CO2eq
and the reduction by
means of collection and combustion is about 34%). When EE is produced the contribution of
avoided GHE allows a further decrease of about 7%. From an energetic point of view, it is
possible to calculate an overall energy conversion efficiency, dividing the amount of
generated electric energy (Table 8) by the energy content of the LFG (considering the
