A novel landfill design and system for landfill gas utilization

Transcription

1 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 CH 4 is 21 times higher than the GWP of CO 2. In 1991, the atmospheric concentration of CH 4 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

2 430 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 O 2, N 2 and CO 2 from CH 4, 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.

3 Waste Management and the Environment II 431 Figure 1: Simplified cross section of a simplified landfill design for LFG extraction and utilization (P at 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 CH 4 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 CH 4 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 N 2 rejection, which amounts to more than one third of the total cost of: collection and drying, feed compression, CO 2 rejection, N 2 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.

4 432 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. 5 The new design and system for LFG confinement To create an airtight capping for a landfill would be too expensive. The author believes that a way exists to prevent air influx into landfills even under high negative extracting pressures and still to keep the costs within acceptable level. In the following text the new landfill design and system where the LFG is used for energy production will be described. The design of the new landfill is shown in Figure 2. The following notation is used: 1 extracting pipe network; 2 control valves; 3 LFG collection wells; 4 carbon dioxide injection pipe network; 5 & 6 low permeability capping layers, made from one or several different layers; 7 highly permeable layer; 8 the pumping system, the LFG treatment system and the LFG utilisation system;

5 Waste Management and the Environment II control pipes; and 10 liner of the repository, made from one or several different layers. Figure 2: Simplified sketch of the new repository design for LFG extraction and utilization (P at atmospheric pressure) A system for preventing a flow of gas from a gas-containing region to an adjacent region, comprising a composite barrier and means to create a pressure differential across the permeable layer is not a new idea [7]. However, in the past this idea has been mainly used in order to prevent the LFG from escaping outside the landfill or to protect a building from volatile gases in the soil. In this case a similar idea is used in order to prevent the LFG from leaving the landfill, and air from entering the landfill. The system relies on availability of gas, which would be more suitable for pumping in the permeable layer than air. The most suitable gas would be CO 2, as it does not affect the anaerobic digestion, and it is already present in the LFG, therefore injecting CO 2 would not increase the costs of LFG treatment. Also, when LFG is treated for pipeline quality gas, CO 2 is separated and the quality of this CO 2 is normally such that it can only be used for industrial purposes (e.g., oil recovery). The new system would provide good use of the separated CO 2. In the present system, in the layer 7 a sufficient supply of carbon dioxide is pumped in through the injection system 4 to keep the pressure of the gas in the permeable layer above atmospheric. In this way flow of carbon dioxide is provided from the permeable layer through the two barrier layers 5 and 6 into the atmosphere and the repository, respectively. As a result of this flow a decrease of influx of air into the repository as well as decrease of escape of LFG from the site is expected. This can be understood if one takes into account that the carbon dioxide will escape from the layer 7 mainly through the routes that air would use to flow into the repository, as these routes (mainly faults in the capping system) represent the lowest resistance for the flow of gases. Therefore, the air would flow in the opposite direction to the flow of carbon dioxide when entering

6 434 Waste Management and the Environment II layer 7 and the flow of air would be only due to diffusion, as the pressure driven flow is in the direction layer 7 air. The flow of air from layer 7 into the repository would be in the same direction as the flow of carbon dioxide, and would be due to both, the concentration and pressure gradients. However, the presence of air in layer 7 would be largely reduced for three reasons: there would be fresh supplies of carbon dioxide in the permeable layer that would dilute the air; part of the air, which entered layer 7 will flow back in the atmosphere due to the bulk flow since the pressure in the layer 7 is higher than atmospheric; and finally, the mechanism by which air enters the permeable layer 7 is only due to diffusion. Therefore, from the fluid mechanics point of view, it can be concluded that the new type of repository would reduce the ingress of air into the repository, and also would reduce the escape of LFG from the landfill. The reduction of LFG escape is not explained explicitly, but the argument follows the same logic as for the reduction of air ingress. The amount of available CO 2 for use in the containment system can be estimated. If the LFG is composed of 50% CH 4 and 50% CO 2 by volume, and if the LFG treatment would successfully separate at least 80% of the CO 2, which is initially present in the LFG, then 40% of the initial volume of extracted LFG would be converted into a CO 2 supply available for use in the system. This CO 2 used within the new capping system would effectively replace the air that would otherwise enter the landfill if the system does not operate. Another benefit of the use of the system is that higher negative extraction pressures can be applied without a danger of significant air influx, since higher negative extraction pressures would produce higher LFG yield, which in turn would provide more CO 2 for extra supply, if necessary, in the permeable layer. The system may be applied to all the sides and the bottom of the landfill as well, but this would significantly increase the construction costs. The author expects that the application of the system only to the top or capping of the landfill should be sufficient to bring the air ingress to insignificant levels, since it is well known that deeper in the landfill the LFG virtually does not contain any air which suggests that the air influx comes through the top of the landfill. At the start of the use of the new system in a landfill certain amount of CO 2 may be needed in order to eliminate the aerobic and leave just anaerobic digestion inside the landfill. Before the system is implemented, the LFG would normally contain some air and until the air is completely removed from the landfill, the anaerobic digestion will be accompanied with aerobic digestion. Only after the air ingress has been prevented, by using the initial quantities of CO 2 for a certain period of time, the anaerobic process would prevail in the whole landfill. There are some questions that need to be considered before the new design is implemented. For example: (i) What should be the permeability of the layers 5 7 in the capping system; (ii) What should be the distance between wells for the injection of carbon dioxide; (iii) What is the maximum negative extraction pressure that can be applied in the extraction pipe network considering the available CO 2 for the system; etc. The author s initial guess would be that: (i) the permeability of layers 5 and 6 would be of the same order of magnitude as the

7 Waste Management and the Environment II 435 permeability of capping used usually in landfills where LFG utilization is implemented, while for the layer 7 the permeability should be as high as possible in order to enable unrestricted flow of CO 2 ; (ii) the injection wells should be placed in vicinity of each extraction well, where the pressure drop of LFG due to extraction is most significant; (iii) answering this would require some prior research. These and some other questions of importance can be answered in two ways. The first is to build an experimental repository where the performance of several different designs could be tested and the optimal parameters of the system can be defined. The other one is to use efficient and reliable computer simulation tools, as a first stage of the investigation. Computer simulations for gas flow in landfills have been done in the past, for example Popov and Power [8-12], however, such analysis is beyond the scope of this paper. The computer simulation would give initial estimation, which would largely reduce the need for testing various designs and would also define the permeabilities of the capping system negative extraction pressure and the injection pressure for CO 2, while the fine tuning of the system would be done on site through measurements. 6 Advantages of the new strategy The new system has several advantages in respect to the traditional landfills, which make the LFG utilization more cost effective. The new design virtually eliminates the problems related to air ingress due to LFG extraction. By reducing the air ingress to insignificant levels, the new system secures that the LFG is virtually composed of only CO 2 and CH 4. This would largely simplify the purification of the LFG and would make it less costly. Separation of CO 2 from CH 4 is relatively straightforward and inexpensive, while the price for N2 rejection is the most expensive part of the LFG treatment [5]. There are landfills where the LFG utilization for heat and energy purposes has not been implemented because the landfill has not been of sufficient size or with sufficient production of methane. These landfills can be upgraded using the new design and thus their efficiency for methane production and extraction can be increased sufficiently for the LFG utilization to become profitable. When evaluating the benefits of the new system it must be taken into account that there are expenses related to the construction of the new capping system, and this costs depend on the type of material used. Pumping of CO 2 is also linked with costs for electricity and maintenance of the pumping system, however the overpressure required is not high and therefore would not add much to the total costs. The CO 2, which is used in the system, does not have to be of high quality, and would be supplied from the LFG treatment process. The extra costs of the system must be compared to the costs of removing of O 2 and N 2 from the LFG. Therefore, the costs of implementing and running the new system need to be compared to the costs of establishing a line and running the process for removal of O 2 and N 2. Even in the case that both systems involve similar costs for construction and operation, the new system should be the one that offers more benefits since a larger range of landfills, including smaller and

8 436 Waste Management and the Environment II shallower landfills can be used for LFG purification. This would have impact on reducing the CH 4 emission and increasing the use of renewable energy. The new system gives opportunity to the landfill operators to use higher negative extraction pressures for higher yield without danger of compromising the composition of the extracted LFG by air ingress. Higher yield would produce more CO 2, which would be used in the system to prevent the higher air ingress potential, by increasing the CO 2 pressure inside the permeable layer of the capping. Another benefit that the new system offers is linked to the observed effects of barometric pressure variation on the LFG emission [13, 14]. It is not clear whether the barometric pressure variation influences only the emission of LFG or the production as well. In the case that the LFG production is affected as well, the new system eliminates such influence on the LFG yield since the pressure inside the permeable layer could be kept constant, or adjusted in such way that the barometric pressure variation does not have any influence on the LFG yield. In other words, the new system brings the landfills closer to the idea of a controlled bioreactor. 7 Conclusions A new design and confinement system for landfills where the LFG is used for treatment for obtaining pipeline grade gas is presented. The new design employs multi-layer capping, in which a permeable layer is sandwiched between two low permeability layers. In the permeable layer a supply of CO 2 is provided in order to prevent the air from entering and the LFG from leaving the landfill. The main advantage of the new system is that it would provide LFG virtually free of air, which would simplify the treatment process, decrease the costs, and make the LFG treatment a more attractive renewable energy option. It is expected that the new system would enable large number of landfills where at the moment the LFG utilization is not economically justified to be converted to landfills where profitable LFG utilization would be possible, by implementing the new system. At existing landfill sites it would be costly to implement the containment system anywhere else apart from the top of the landfill, however, it is known that deeper in the landfill the LFG virtually does not contain any air which suggests that the air ingress comes from the top of the landfill. This is why the author believes that many landfills where the LFG utilization is not viable at the moment could be converted to landfills for LFG utilization by implementing the new containment system at the top of the landfill. This would in turn reduce the greenhouse gas emission and would increase the use of renewable energy resources. References [1] Department of the Environment, UK. Landfill Gas - Waste Management Paper No. 27. Crown Copyright, London, 1991.

9 Waste Management and the Environment II 437 [2] Department of the Environment, UK. Landfill Design, Construction and Operational Practice - Waste Management Paper No. 26B. Crown Copyright, London, [3] IPCC. Climate change 1992, Supplementary Report to the IPCC Scientific Assessment. Houghton JT, Callander BA and Varney SK, editors. Published for the Environmental Panel on Climate Change, Cambridge University Press, [4] Desideri U, Di Maria F, Leonardi D, Proietti S. Sanitary landfill energetic potential analysis: a real case study. Energy Conversion and Management 2003; 44: [5] Knaebel KS, Reinhold HE. Landfill Gas: From rubbish to resource. Adsorption 2003; 9: [6] Martin S, Maranon E, Sastre H. Landfill gas extraction technology: study, simulation and manually controlled extraction. Bioresource Technology 1997; 62: [7] Stevens et al. Unites States Patent No , May 23, [8] Popov V, Power H. BEM solution for the problem of flux of a multicomponent mixture of gases out of a multi-layered landfill. International Journal for Numerical Methods in Fluids 1996; 23: [9] Popov V, Power H, Baldasano JM. BEM solution of design of trenches in a multi-layered landfill. Journal of Environmental Engineering ASCE 1998; 124(1): [10] Popov V, Power H. Landfill Emission of Gases into the Atmosphere- Boundary Element Analysis. Computational Mechanics Publications/WIT Press, Southampton; [11] Popov V, Power H. DRM-MD approach for the numerical solution of gas flow in porous media, with application to landfill. Engineering Analysis with Boundary Elements 1999; 23(2): [12] Popov V, Power H. Numerical analysis of the efficiency of landfill venting trenches. Journal of Environmental Engineering ASCE 2000; 126(1): [13] Boltze U, de Freitas MH. Monitoring gas emissions from landfill sites. Waste Management and Research 1997; 15: [14] Czepiel PM, Shorter JH, Mosher B, Allwine E, McManus JB, Harriss RC, et al. The influence of atmospheric pressure on landfill methane emissions. Waste Management 2003; 23: