In Denmark, many former gravel pits have been used as residual waste landfills. Organic material in the waste degrades over time and converts into landfill gas (LFG), which contains the greenhouse gasses methane and carbon dioxide. When residential buildings are located near a landfill, the risk of gas migration into the buildings may cause methane to accumulate, which poses a risk of explosion at concentrations of 5-15 vol.%. Previously, the Danish Regions focused mainly on eliminating the threat of choking and explosion. However, within the last years a larger focus has also been on reducing the climate impacts by allowing the emission of methane to the atmosphere.
From gravel pit to landfill
In a former gravel pit in Hvalsoe a landfill was established in 1972. During the following 12 years, the landfill received residual waste covering the gravel pit with up to 18 meters of waste. Investigations have shown that LFG, produced at the landfill, is in risk of migrating into a nearby residential building.
As the LFG represents a hazard for the people living in the building, Geo has participated in the installation of a remediation plant, which collects the hazardous gasses.
82% of the energy produced at the remediation plant originate from combusting methane in the LFG.
Did you know that former landfills, containing organic waste, produce LFG? LFG is considered a significant source in the overall methane emission and is thought to be a significant factor in the Earth’s climate changes.
Methane (CH4) is a severe greenhouse gas with a heating effect 25 times that of CO2. The climate therefore benefits immensely, when the methane is oxidised to CO2.
257.883 m3 LFG was pumped through the remediation plant during the first year of running. (March 2014 to May 2015.)
Ca. 167.465 kWh electricity were sold to the electricity network during the first year of running. (March 2014 to May 2015.)
To prevent LFG from migrating into a neighbouring residential building, Region Zealand requested Geo to assist in establishing a remediation plant and afterwards document the effects of the remediation. It was discovered that a temporarily established pumping station could turn the pressure gradient away from the building and instead towards the established remediation boreholes, preventing the LFG from migrating into the building.
Following the documented effect of the temporary remediation plant, the plant was converted into a permanent plant.
After documenting the effect of the temporary remediation plant, the permanent remediation plant was established by covering the pipes in the ground and establishing a bio filter treatment facility.
Initially, the collected LFG was treated in a bio filter, in which the methane was converted into CO2. The Region of Zealand wanted the permanent remediation plant to be eco-friendly and cost neutral. Geo was requested to investigate the possibilities of establishing a gas generator to combust the methane in the LFG, which was about 22 vol.% at the remediation treatment facility. The gas generator would use the energy from combusting the methane, in order to produce electricity.
For the project, Geo has applied its expert knowledge in technical remediation, providing consultancy assistance in the choice of remediation and for the establishment of an energy producing gas treatment solution. As the methane content in the LFG was too low for a normal gas generator, it was necessary to use a dualfuel machine type. The dualfuel machine supplements the methane with a secondary fuel, in order to utilise the methane in the LFG, to produce electricity. The electricity produced at the remediation plant is sold to the electricity network. Data from the gas generator has shown that 82% of the electricity, produced at the dual fuel remediation plant, originate from the LFG. Only 18% of the electricity produced at the remediation plant originate from the supplementary fuel. Therefore, the electricity produced at the remediation plant must be regarded as green energy, as the methane would otherwise be treated in a bio filter without energy production or emitted directly to the atmosphere.
By combusting the methane in the LFG in a dualfuel gas generator and selling the produced electricity to the electricity network, the overall emission of greenhouse gasses is reduced. According to IPCC Fourth Assessment Report: Climate Change 2007, the global warming potential factor (GWP-factor) for methane is 25 times the CO2-equivalent. This implies that combusting 1 kg of methane corresponds to saving the environment from emission of 25 kg in CO2-equivalents. The combustion of the methane in the collected LFG provides a significant reduction in the emission of greenhouse gasses. Furthermore, Geo expects the profit, from selling the produced electricity to the electricity network, will be able to repay the installation expenses within 5 to 10 years.