by Hilary Haskell
Post-closure, landfills require after-care and remediation. As waste decomposes in anaerobic landfill environments, methane emissions are released as a component of landfill gases. Landfill gases can be recovered through combined heat and power generation, district heating coupled with asphalt production, or incorporation of landfill gas into biomethane. Each method has a different collection and utilization efficiency. Niskanen et al. (2013) studied Kymenlaakso Jate Oy landfill in Finland, and found that the combined heat and power recovery scenario has the highest landfill gas utilization efficiency, resulting in greenhouse gas emissions reductions of 8,000 tCO2−eq/year. Landfill gas recovery serves as an economical landfill remediation option that decreases pollution of the surrounding environment.
Landfills are the cheapest and hence most common method of waste disposal, but they landfills pose a threat to the surrounding environment through groundwater pollution, soil contamination, and greenhouse gas emissions. In 2005, waste and wastewater accounted for approximately 1.5 GtCO2−eq of greenhouse gas emissions globally. The Intergovernmental Panel on Climate Change stresses the importance of greenhouse gas mitigation through waste management. Finland and the European Union have stringent regulations for waste management that decrease landfill gas emissions. Finland’s working group entitled “Bioenergy from Waste” emphasized the importance of more efficient landfill gas recovery in 2010. However, the number of landfills with gas collection systems did not increase between 2005 and 2008. These findings are important in Finland, as landfill gas emissions account for approximately 84% of all emissions from the Finnish waste management sector.
Although landfills emit gases both before and after closure, the highest emission rates occur while the landfill is still in operation. High levels of cumulative landfill gas emissions and landfill mining make post-closure landfills an energy resource. Landfill waste typically has energy values of 11–20 kJ/kg, depending on overall landfill waste composition.
Waste extraction and landfill gas collection prevents greenhouse gas emissions from decomposition of organic waste and combustion of carbon-intensive fossil fuels. Other environmental benefits include prevention of soil and groundwater contamination. Generating energy from landfills decreases the amount of time required to remediate a landfill, and thus decreases overall landfill operation costs. Landfill waste recovery allows for recovery of precious metals.
The authors considered landfill gas recovery and utilization improvements for the Kymenlaakso Jate Oy’s municipal solid waste landfill in South-East Finland, at both its old and new sites. This landfill generated 1.12 MtCO2−eq in 2008. The new landfill has experienced high landfill gas emissions since its opening in 2001, the same year that the older landfill closed. Using this landfill as a case study, the authors estimated yearly greenhouse gas emission mitigation potential for three landfill gas management scenarios. The new landfill has a bottom lining that complies with the European Directive on landfilled waste while the old landfill does not. Approximately 0.88 x 106 m3 year−1of landfill gas with an average methane content of 33% was collected from the old landfill. Each year, between 65 x 103 and 72 x 103 t of municipal solid waste is deposited in the new landfill, resulting in 0.80 x 106 m3 year−1 of landfill gas emissions. In 2010, emissions increased to 4.5 x 106m3 year−1.
The authors considered three landfill gas utilization scenarios. The first scenario involves combined heat and power production with a gas engine. The second uses heat generation for asphalt production in the summer and district heat production by a water boiler in the winter. The third scenario upgrades landfill gas to biomethane. While the first two scenarios have already proven to be feasible, the third has yet to be proven in Finland. The authors compared the scenarios to determine whether they would result in a net decrease in greenhouse gas emissions from energy generation, compared to fossil fuel combustion. The baseline for gas collection efficiency was set at 75%, per the United States Environmental Protection Agency standard. This efficiency level amounts to 21,300 Mwh of landfill gas energy production potential. Any landfill gas not utilized was assumed to be flared, with a treatment efficiency of 99%.
Each scenario has its own efficiency. Gas engine efficiency is 44% for heat and 39% for electricity, while asphalt and district heating is 90%. Upgrading landfill gas to biomethane has an internal energy consumption of 9.1%. For conventional electricity production, the authors assumed greenhouse gas emissions of 207 kg/Mwh. Finally, methane oxidation efficiency of landfill cover was assumed to be 10%.
Capture technology at both the new and old landfill sites results in collection of 4.14 x 106 m3 year−1 of landfill gas. Scenario 1 has the greatest greenhouse gas emissions savings of 8,000 tCO2−eq/year, which is mainly the result of avoided conventional electricity generation. Scenario 3 results in the next highest emissions savings of 6,800 tCO2−eq/year, and Scenario 2 results in the lowest greenhouse gas emissions savings of 4,300 tCO2−eq/year. These savings are based on a comparison with the 75% landfill gas collection efficiency of 11,400 tCO2−eq/year. However, this baseline collection efficiency may vary considerably.
A positive relationship exists between landfill gas collection efficiency, amount of gas utilized, and greenhouse gas emissions avoided, because landfill gas emissions are typically the largest contributor to greenhouse gas emissions from waste management. The authors found that by varying collection efficiency, overall utilization and greenhouse gas emissions savings differ. If Scenario 1 has collection efficiency higher than 75%, then the greenhouse gas emissions balance could actually result in negative net greenhouse gas emissions of 2,220 tCO2−eq/year.
Variation in emission factors can also greatly impact overall gas collection, utilization, and greenhouse gas emission avoidance. This study bases emissions savings on marginal electricity production from coal-fired power plants because Finland’s average electricity production mainly comes from non-carbon intensive resources. However, if this study were to be conducted in a nation with a fuel supply that is more carbon-intensive, emissions savings would increase. Country-specific emission data can vary, resulting in different emissions savings. Despite these uncertainties, as well as those posed by the amount of landfill gas available for collection, the rank order of the landfill gas recovery efficiencies is not affected.
Other considerations are also important in evaluating which scenario is best implemented on a site-specific basis. For some existing technologies, there must be a certain quantity and quality of landfill gas emissions available for capture. Existing infrastructure must be suitable for implementation of landfill gas recovery. Long distance transport of recovered gases decreases both economic viability of landfill mining and the potential greenhouse gas emissions savings. For Scenario 1 to attain maximum efficiency, landfills must be in close proximity to gas utilization sites. This criterion is also important for landfill after-care, when landfill gas emissions begin to decline. At this point, material recovery may take place so that valuable deposits can be extracted and residues can be incinerated, thus providing additional energy recovery potential. This material recovery process can also decrease the risk of other potential environmental impacts, such as soil and groundwater pollution.
Niskanen et al. found that combined heat and power landfill gas collection and utilization results in the greatest greenhouse gas avoidance. Material recovery serves as a means of maximizing energy recovery after landfill closure. Although greenhouse gas emission avoidance and power generation were the main objectives in this study, economic and environmental benefits also result from landfill gas capture and utilization.
Niskanen, A., Varri, H., Havukainen, J., Uusitalo, V., Horrtanainen. M., 2013. Enhancing Landfill Gas Recovery. Journal of Cleaner Production 55, 67–71.