Potential till energiutvinning från deponigas på Tagenedeponin norr om Göteborg

Sammanfattning: When organic material is deposited at a landfill it is decomposed over time by microorganisms to form a gas mixture consisting of mainly methane, carbon dioxide and nitrogen called landfill gas. To limit the environmental impact from this gas there is a requirement in Sweden today that all landfills which have accepted or are accepting organic material to be equipped with a gas collection system and at least a flare to combust the methane. One such landfill is Tagene north of Gothenburg with is owned and managed by Renova AB Miljö. Renovas policy states that greenhouse gases from handling of waste and to utilise the full potential of waste to generate benefits are prioritised climate aspects. As such there is a wish from the company to replace the flare used today with a system that utilises the energy in the landfill gas and to quantify the diffuse emissions from the landfill. Diffuse emissions compile all greenhouse gases that leaks to the atmosphere through advection or diffusion. The techniques available to measure diffuse emissions can be divided into direct techniques that measures for example the concentration of methane and the gas flow out of the landfill (chamber techniques, eddy covariance techniques and trace gas methods) and indirect methods that model the landfill gas potential. This makes it possible to calculate the diffuse emissions as the difference between the collected gas and the total production. To assess the current and future diffuse emissions a simulation was carried out. The simulation showed that approximately 35% of landfill gas is collected today which in carbon dioxide equivalents corresponds to 1440 metric tons of CO2ekv per year. The result from the simulation is similar to other studies. Further validation through direct measurement av diffuse emissions through trace gas or EC would increase the reliability of the model. When modelling future emissions two parameters were identified as critical, gas collection efficiency and landfilled organic material. Three scenarios were employed for the collection efficiency, 15, 35 and 70%. Deposition was assumed to be the same as 2019. Seen over a 25-year period this resulted in emissions corresponding to 30500, 23300 and 10800 metric tons respectively. A simulation is assessed to be the most sustainable alternative with the added benefit that it gives substratum to and investment in a gas utilisation technique. To use direct techniques is not advised. Chamber techniques would interfere with the management of the landfill to much while the investment cost for the equipment required for EC or trace gas is to high which requires it to be loaned which makes it difficult to create continuous measurements. The available methods to utilise the landfill gas can be summarised in direct techniques which utilises the heat in the gas (boiler, hot water boiler), combined heat and power (gas engine, stirling engine, gas turbine, ORC) and upgrading techniques to produce pure methane/syngas/hydrogen. A comparison for the techniques in the three different collection scenarios showed that there are only 3 pertinent techniques for Tagene. A boiler, a stirling engine and a gas turbine. These three techniques were then further studied in a case study where investment and management cost were calculated through quotations and literature sources. Net present value method was used to calculate profitability of investment using a return of investment of 3% and an economic lifespan of 10 years. For the 15% collection scenario no technique was profitable whilst for the 35% and 70% scenarios a boiler gave return after 7-8 years. Cost of electricity and heat was set to 0.25 SEK and 0.05 SEK, respectively. The profitability analysis prerequisites that the cost to utilize the heat in the scale house debits other projects. The result of the case study leads to the recommendation for Renova to invest in a boiler as it brings a small economic risk whilst it meets the company incentive to utilise the landfill gas. Furthermore, the study identified improved management and maintenance and automatization of gas extraction to be key factors to improve the gas extraction and subsequently the environmental and economic performance of the site.