Due to its high content of methane, landfill gas can be regarded on the one hand as a useful alternative source of energy, when collected and utilized, whilst on the other hand, when released unused and untreated into the atmosphere, it causes considerable harm to the environment, contributing substantially to the manmade greenhouse effect, and to global climate change, respectively.
Although modern, sanitary landfills in most developed countries usually require landfill operators to operate a gas extraction system, a large amount of gas still escapes into the atmosphere. This fact was clearly indicated by the currently available data concerning national and global landfill methane emissions. In Europe for instance, an estimated 20 - 25% percent of anthropogenic methane emissions have been suggested to be emanating from landfills (EEA, 2206).
In 2003, for example, a calculated total amount of approximately 3.34 kilotons of methane were emitted from waste disposal in the European Union (EEA, 2006). In the US, methane emissions from waste disposal are more than twofold higher than those reported for the European Union (USEPA, 2006).
This means that landfills are among the largest anthropogenic methane sources worldwide, ranking third after agriculture (livestock farming and rice cultivation) and losses from fossil fuel distribution, processing and mining (IPCC, 2001). Moreover, methane is also emitted from older and smaller landfill sites, where the subsequent application of a gas collection system is too costly and inefficient, as well as from open, unauthorized dumpsites, and from closed, sanitary landfills during the aftercare phase, when the existing gas disposal technique is no longer adequate or appropriate.
Engineered gas extraction systems, by means of which the produced landfill gas is actively sucked off due by pipe systems that develop an under-pressure induced in the landfill body are applied during gas collection and capturing procedures, are now routinely installed throughout most of the developed nations. This has become a profitable exercise throughout Europe and wherever environmental regulators require landfill gas extraction systems to be installed for the reduction of greenhouse gas emissions.
In such cases the cost of the extraction system is charged to the waste disposal facility users, and does not appear as a cost of landfill gas utilisation. The same cannot be said for the industrialising nations, where methane extraction and flaring as a minimum requirement, is not in force. In such cases the cost of the gas extraction infrastructure cost must be borne by the revenue from the landfill gas. Given the long run-up period between starting planning an Energy-from-Waste landfill gas power generation scheme, and getting it running, funding such projects even with Carbon Credit is a hard act to implement successfully. However, it is hoped that with carbon credit money increasingly the industrialising nations will collect and utilise their landfill gas as well.
The profitability of these schemes is essentially best when landfill gas is of high calorific value fairly soon after the waste has been disposed. The efficiency of these extraction systems dictates how long they are profitable once gas yields start to decline, and depends on the landfill geometry and design (particularly the capping material properties), the waste compaction, density and water content, the atmospheric pressure and wind conditions, the design of the extraction wells and collectors and the technical management of the extraction system. The main factors in this are essentially governed by the negative suction pressure it is possible to apply, flow rate, spacing distance of the gas wells/collectors, depth of waste, etc.
Following intensive field investigations on different landfills in France, Morcet et al. (2003) and Spokas et al. (2006) concluded, that conservative rates for active gas recovery (including vertical wells and horizontal collectors) are 35% for an operating landfill cell, 65% for a temporary covered cell, 85% for a clay final covered cell, and more than 90% for a geo-membrane final covered cell. Based on their findings, a mean gas recovery rate of about 60% has been reported considering the total life-time of a landfill.
The utilisation of the captured landfill gas with a methane content exceeding 45%, in combined heat and power plants (CHP) is the most feasible state-of-the art treatment method, mainly used in the operational phase or shortly after landfill closure. This method affords the possibility of substituting fossil fuels and obtaining an additional economic gain by feeding the electricity produced into an existing grid (Haubrichs and Widmann, 2006).
References:
European Environment Agency (EEA) (2006). Annual European Community greenhouse gas inventory 1990-2004 and inventory report 2006. Submission to the UNFCCC Secretariat. EEA Technical report 6/2006. ISSN 1725-2237. ( http://reports.eea.europa.eu/tech-nical_report_2006_6/en ) Acc.Feb. 2007.
United States Environmental Protection Agency (USEPA) (2006). US Emission Inventory 2006. Inventory of U.S. Greenhouse gas emissions and sinks: 1990-2004. USEPA #430-R-06-002.
IPCC (2001). Climate change 2001. Third assessment report, WMO/UNEP/IPCC, Intergovernmental Panel on Climate Change.
Spokas K., Bogner }., Chanton J., Morcet M., Aran C, Graff C, Moreau-le-Golvan Y, Bureau N., Hebe I. (2006). Methane mass balance at three landfill sites: what is the efficiency of capture by gas collection systems? Waste Management, 26, 516-525.
Haubrichs R., Widmann R. (2006). Evaluation of aerated biofilter systems for microbial methane oxidation of poor landfill gas. Waste Management 26 (2006) 408-416.
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