A Novel Landfill Design and System for Landfill Gas Utilization
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A novel landfill design and system for landfill gas utilization V. Popov Environmental Fluid Mechanics Division, Wessex Institute of Technology, UK Abstract This paper describes a novel landfill design and system for landfill gas containment, air ingress prevention for more efficient landfill gas purification for utilization. The new system offers more control over the processes of anaerobic digestion and biogas extraction than conventional landfills. As the new system reduces the air ingress the landfill gas purification process becomes cheaper. The new system can be implemented on existing landfills, as it is sufficient to modify only the top cover of the landfill, which is the surface with a significant air influx. As the new system makes the landfill gas treatment cheaper and more efficient, the possibility may exist for many landfills to be converted to landfills where the landfill gas utilization would be profitable, increasing in this way the use of renewable energy sources and reducing the greenhouse gas emission. Keywords: Novel landfill design and system, cheaper LFG treatment for fuel. 1 Introduction There are few environmental concerns regarding the waste disposal in landfills [1], [2]. The importance of prevention of uncontrolled emissions of LFG into the atmosphere is obvious if one takes into account that the global warming potential (GWP) of CH4 is 21 times higher than the GWP of CO2. In 1991, the atmospheric concentration of CH4 was about 1.72 ppmv, which is more than twice the pre-industrial level of about 0.8 ppmv [3]. There are two ways in which the problem related to the escape of LFG could be solved. The first one, commonly used in the past, is the extraction and flare of the LFG. In this way the pressure of the LFG within the landfill is decreased which reduces the escape of LFG from the landfill. The flare of the LFG also Waste Management and the Environment II, V. Popov, H. Itoh, C.A. Brebbia & S. Kungolos (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-738-8 430 Waste Management and the Environment II reduces the problem of odour. The main products of flare of LFG are carbon dioxide and water, which means that the GWP of the released gas has been largely reduced. The other way is to follow a similar strategy as in the first case except that the gas is not flared but used in an economical way. Though the flare of LFG reduces the environmental impact of the landfill site on the environment, methane has a high calorific value and the flare of LFG represents waste of valuable resources. This influences the number of landfills where LFG is used as the supplementary or primary fuel for the production of electric power to increase. Other possible uses of LFG include: treatment of LFG for pipeline quality gas and vehicle fuel, supply of heat and carbon dioxide for greenhouses and various industrial processes where the supply of heat is required. 2 Landfill gas extraction LFG is extracted from landfills through extracting wells that are installed throughout the landfill and are connected to the extracting system. When a slight vacuum is applied the LFG migrates towards the extracting wells, see Figure 1. As the cap of the landfill does not seal perfectly, there will be a pressure gradient, especially around the extracting wells, induced by the extraction of the LFG that will cause an air influx into the landfill. This air will mix with the LFG resulting from waste degradation. It is important to keep the depth to which air penetrates into the landfill as low as possible in order to minimize the volume of the landfill in which the anaerobic digestion is reduced by air ingress, and also to reduce the costs of LFG utilization. Figure 1 shows a simplified cross section of a representative landfill used for LFG extraction and utilization. In the drawing the following notation is used: 1 – extracting pipe network; 2 – control valves; 3 – LFG collection wells; 4 – liner of the landfill; and 5 – low permeability capping layer, made from one or several different layers. The pumping system, the LFG treatment system and the LFG utilisation system are all represented as a box – 6. Usually these processes would include: removal of free moisture (droplets) from the LFG, removal of trace contaminants and impurities, in some cases also separation of O2, N2 and CO2 from CH4, and finally utilization of the obtained gas, unless the purified LFG is transported and used as fuel elsewhere. In the figure, for simplicity, only one extracting well is shown, although there would normally be a network of wells usually equally spaced throughout the landfill. Also there could be daily soil covers inside the landfill, which are not shown. Note that the flow of air is more intense closer to the barrier layer at the top and decreases with depth. At the same time the flow of the LFG increases towards the bottom of the landfill, due to the increase in the pressure gradient. In sufficiently deep landfills with sufficient production of LFG, there would be virtually no airflow in the lower part of the landfill. Waste Management and the Environment II, V. Popov, H. Itoh, C.A. Brebbia & S. Kungolos (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-738-8 Waste Management and the Environment II 431 Figure 1: Simplified cross section of a simplified landfill design for LFG extraction and utilization (Pat – atmospheric pressure). 3 Considerations for successful exploitation of landfill gas For shallow landfills the decrease in methane production is mainly due to air ingress, since oxygen concentrations equal or higher than 5% prevent the anaerobic digestion, and therefore the extraction of the LFG for energy utilization in such landfills may not to be economically viable. Usually not all of the wells in a landfill would have sufficiently high CH4 content to be used for energy purposes [4]. Therefore, reduction in air ingress would in some cases make smaller landfills suitable for LFG utilization. If the landfill is very small the LFG yield will be small and therefore it may not be economical to use the LFG for production of electricity. However, in many cases such landfills would still be able to produce heat for industrial processes or residential use, or the collected LFG can be treated and the obtained gas used as fuel. In this paper a new system is considered, which would be beneficial in cases when LFG is treated to separate pipeline-grade methane. However, when purifying LFG for fuel, the LFG must be of higher CH4 content in order to have feasible fuel production. The presence of air in the LFG increases the costs of LFG treatment. A study in which two three-stage flowsheets (involving membrane-pressure swing adsorption (PSA)-PSA; and PSA-temperature swing adsorption (TSA)- PSA) were discussed as means to separate pipeline quality methane from LFG, reported that the most expensive process in the separation scheme is N2 rejection, which amounts to more than one third of the total cost of: collection and drying, feed compression, CO2 rejection, N2 rejection, product compression and transport [5]. This is why usually only LFG from deep wells is used, which eliminates the use of shallower landfills for this purpose. Waste Management and the Environment II, V. Popov, H. Itoh, C.A. Brebbia & S. Kungolos (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-738-8 432 Waste Management and the Environment II 4 Usual practice for extracting the LFG In order to reduce the problems mentioned above related to the presence of air in the LFG, usually an ‘Upper Oxygen Limit’ (UOL) is chosen in the pipe lines in the LFG extraction control systems [6]. This is often achieved through manually or automatically controlled regulation methods that aim to keep the methane concentration constant in the LFG flow and equal to the value chosen as the ‘Methane Objective Value’ (MOV) and the oxygen concentration below the UOL. The energy use intended for the LFG will mark the lower limit for the MOV. The manually controlled LFG extraction normally requires weekly checks for every extracting well. Nevertheless, the composition of the LFG should be monitored on a daily basis at the control station without the need for corrective action, unless strictly necessary. It is recommended that the concentration of oxygen is monitored daily. Figures over 10% would normally indicate a failure in the extracting network because of the penetration of air, and in such case the regulation station should be disconnected from the general system, for safety reasons. There are some problems related to the practice of LFG extraction described above. When the oxygen concentration in the extracting network reaches high values that are still below the UOL, the regulation station will not be disconnected, although there would be a failure of the capping system producing a substantial flow of air into the landfill. This means that in the upper part of the landfill the concentration of oxygen is higher and most likely is affecting the anaerobic digestion inside the landfill and the corresponding production of methane. The reduction in the production of methane will cause the methane concentration to be below the MOV so that the control valve would be proportionally closed thereby reducing even further the yield from the well. The oxygen level will now reduce thereby increasing the methane production. After certain period of time, the control valve can be opened again which will cause the concentration of oxygen to increase. The loop is closed and this particular well will not yield as much methane as it potentially could so long as there is a failure in the capping system.