Environmental Engineering Reference
In-Depth Information
conventional landfills - and reproduced by means of adapting the landfill gas production
model. The landfill gas production and energy recovery for the conventional landfill and
the landfill with leachate recirculation were compared from different points of view:
economic evaluation, energy conversion and environmental impact. The economic
analysis showed that the specific disposal cost is lower for the landfill with leachate
recirculation with respect to the conventional landfill. Moreover, the landfill with
leachate recirculation shows better indicator values both for the overall energy
conversion efficiency and for Greenhouse Effect specific emission.
1.
I
NTRODUCTION
Energy recovery from landfill gas (LFG) is strongly recommended as a means to reduce
the environmental impact, in terms of Greenhouse Effect (GHE), arising from landfills
containing biodegradable wastes (Lombardi et al., 2006). As a matter of fact, biodegradable
organic matter contained in Municipal Solid Wastes (MSW) is degraded by anaerobic
biological processes in landfills giving place to a LFG, which is approximately composed of
50% methane and 50% carbon dioxide. Hence the LFG has two main features: it is composed
of two of the main Greenhouse gases and it has a not negligible heating value (approximately
16.000-17.000 kJ/Nm
3
).
Simple flare combustion of LFG allows reducing landfill GHE contribution converting
methane to carbon dioxide, since Global Warming Potential (GWP) of methane is twenty-one
times larger (on mass basis) than GWP of carbon dioxide. When LFG is combusted with
energy recovery, both thermal and electric energy can be delivered and landfill GHE is
further reduced, considering the avoided emissions from conventional sources of energy, in
place of which LFG is exploited.
The proper design of LFG collection and energy recovery system needs to be based on an
adequate LFG production estimation during the landfill operation and post-closure phases,
this can be done through LFG production modelling.
2.
L
ANDFILL
G
AS
C
HARACTERISTICS
AND
G
ENERATION
M
ECHANISMS
LFG is composed of a number of gases that are present in large amounts (the principal
gases) and a number of gases that are present in very small amounts (the trace gases). The
principal gases are produced from the decomposition of the organic fraction of municipal
solid waste (OFMSW) and they include mainly carbon dioxide (CO
2
) and methane (CH
4
), but
also ammonia (NH
3
), carbon monoxide (CO), hydrogen (H
2
) and oxygen (O
2
) can be present.
Trace constituents mainly belong to the VOCs family. The typical percentage composition of
LFG is reported in Table 1.
The generation of LFG takes place in different steps. At the beginning, as OFMSW is
placed in the landfill, it undergoes microbial decomposition in aerobic conditions, because
certain amount of air is trapped within the waste (phase I). When oxygen is depleted,
anaerobic conditions begin to develop and after a transition phase (phase II), during which the
oxidation/reduction potential decreases, the acid phase (phase III) starts, consisting in
