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Collection and Disposal of Landfill Gas
9.3 COLLECTION AND DISPOSAL OF LANDFILL GAS Gerhard Rettenberger
FUNDAMENTALS Landfill gas (LFG) extraction and utilization should be a standard procedure at all municipal solid waste landfills. By these means the emission of climate influencing gases can be significantly reduced, energy can be utilized, odors can be avoided, and explosion risks can be reduced. There are five essential conclusions resulting from experiences with gas collection over the last decades: 1. LFG extraction systems should be installed and operated from the beginning of landfill operation. Fig. 9.3.1 shows the gas volumetric composition (s) at different gas wells from the beginning of the landfill operation (1). For this test, the gas wells had been installed directly on top of the bottom liner. As can be seen in Fig. 9.3.1, the s CH4 to s CO2 ratio of 1 increases after 11e15 months after the landfill operation started. Therefore it is necessary to have the gas extraction system available during the first year of operation. Having in mind that the half-life value of the LFG production is in the early period of a landfill in the range of 3e4 years, means that without operating a gas extraction system almost 50% of the gas may be lost in this time frame. 2. Gas collection can be realized using an active or passive system. Operating an active system the gas is extracted out of the landfill by means of blowers, which generally provide vacuum pressures in the landfill body in the range of 1 to 30 hPa. Passive gas collection systems use the positive pressure in the landfill body to transport the gas out of the landfill under semicontrolled conditions. Passive systems may only be feasible, if the methane gas production is less than 0.5 L/ m2 h and a gastight top cover has been installed. Fig. 9.3.2 shows gradients of LFG and air components over the depth of the landfill body or a cover soil (Rettenberger, 2004). As can be seen in Fig. 9.3.2, the ratio of s CH4 to s CO2 changes with the depth. These results show that methane oxidation occurs. This process is technically used in landfill covers for biological oxidation of residual methane (see Chapter 9.4). 3. When active gas extraction is practiced, the methane concentration may change when air is sucked into the landfill.
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Figure 9.3.1 s(CH4)/s(CO2) over time at four gas wells (K1, K2, K4, K5) since the beginning of
deposition (Rettenberger, 2004).
Figure 9.3.2 s(CH4), s(CO2), s(O2), and s(N2) in different depths of a pervious soil cover or an open
landfill (Rettenberger, 2004). Fig. 9.3.3 shows typical curves when oversucking occurs, which may happen when the gas extraction rate is higher than the gas production rate and/or when the gas wells are not adequately designed (Rettenberger, 2004). The figure shows that the kind of correlation is individual for each well and is corresponding to the mathematical model, which describes the mixing of airddue to air intrusiondwith the LFG (Rettenberger, 2004): sðCH4 Þ ¼ sðCH4;v¼0 Þ 1 þ b$v_2 s(CH4), volume concentration of methane depending from flow; s(CH4,v¼0), volume concentration when flow is 0 m3/h; b, factor calculated by statistical method; v, _ flow.
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Figure 9.3.3 Methane concentrations as a function of gas flow (over sucking) during pumping tests at three wells (sections) (Rettenberger, 2004).
Together with the methane concentrations the O2 and N2 concentrations also vary with different gas flow rates. Fig. 9.3.4 shows the oxygen concentrations at a gas well in relation to the extracted gas volume. Significantly elevated oxygen concentrations in the landfill may be a potential for explosions, fires, and may inhibit methane production. On the other hand, there should be a certain negative pressure in the landfill body to avoid uncontrolled emissions. As a conclusion, LFG extraction should be operated in a way that methane concentrations are in the range of 50%e45% during landfill operation and >50% after completion of the landfill. To achieve this, all wells should be adjusted individually about once per week. This is also due to the fact that the atmospheric pressure (increasing pressure change reduces and decreasing pressure change increases the gas pressure in the landfill body) and the wind (wind may cause a negative pressure inside the landfill) may have a strong influence on the gas extraction rate and/or the gas quality.
Figure 9.3.4 Volumetric concentration s(O2) as a function of the gas flow at a gas well over a period of
3 months (Rettenberger, 2004).
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4. Due to decreasing gas production rates in the older parts of a landfill or in the aftercare phase, more air may be sucked into the landfill that results in a so-called lean gas, i.e., methane concentrations < 20%e25%. Systematical oversucking can improve the collection efficiency rate considerably. When the gas flow is in the range of 2%e5% of the initial flow rate, air intrusion happens and a lean gas occurs. Owing to the oxygen content in the landfill, methane oxidation processes may occur. The gas extraction system has to be designed in a way that it can operate in a flow range of 1:20 (start of gas extraction) to 1:40 (final phase). As a consequence, the gas extraction system has to be continuously adapted during the operation of the plant. 5. Owing to the fact that the landfill body is very inhomogeneous and each landfill is different as well as the construction of the gas extraction system, the efficiency of a gas collection system (how much of the produced gas is captured) cannot be predicted. The design of a gas collection system is therefore not ultimate and has to be controlled either by comparing the extracted gas flow rates with the results of the gas prognosis and/or by detecting uncontrolled emissions around the wells and at the landfill surface. Gas emissions at the landfill surface can be detected using the flame ionization detector (FID) method. Using this measuring device means to walk over the entire area of the landfill placing a little cap on top of the surface at different points and pumping small amounts of gas into the FID detector. In general the concentration range that can be measured with the detector is between 0e10,000 ppm CH4. Using this method, most of the spots where gas is emitting can be detected. To reduce uncontrolled gas emissions, the gas extraction rates have to be adjusted, more wells may be installed, and/or the areas where the gas is emitting may be ceiled. Investigations in the last years show that most of the LFG emissions at landfills occur in a concentrated way at relatively few small areas (hot spots). Therefore, in Germany, landfill operators should identify those hot spots to take actions to avoid these emissions (Anonymous, 2016). An alternative measurement to the FID method is the control of diffusive emissions measuring methane concentrations outside of the landfill and the meteorological data to calculate the emission rate using gas distribution models. The disadvantage of this method is that the hot spots where the main emissions occur cannot be identified. More methods to measure gas emissions from a landfill are under investigation (Hrad and Huber-Humer, 2015), see also Chapter 9.5.
GAS EXTRACTION AND TRANSPORTATION Fig. 9.3.5 shows the general system of an active gas collection system under the German regulation (Anonymous, 2001). The gas is extracted by a blower, which can be to a high degree adjusted to different flow rates. It should provide vacuum pressures between 1 and 50 (100) hPa. The vacuum pressure should be distributed into the landfill body as homogeneous as possible. To reach this aim, a certain number of vertical
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Figure 9.3.5 Scheme of an active gas collection system (Anonymous, 2001).
or horizontal gas wells have to be installed in the landfill body. There should be nearly no pressure loss inside the well, which can be achieved by choosing adequate gas pipe diameters (typical diameter 100 mm). If the landfill has a depth >25e30 m, two gas extraction pipes of different length (with a gas seal underneath the shorter pipe) may be installed that may be operated with different vacuum pressures (lower and higher level). It is standard in Germany that each well is connected via a pipe to a distribution station, where the transportation pipe of several wells is connected to a main header pipe that transports the gas to the blower station. In the distribution station a valve can adjust the flow rate. In the pipe just in front the valve technical facilities for measuring in the pipe pressure, flow rate, gas composition, and temperature should be installed. In the gas distribution station, it may make sense to distinguish between lean gas and “normal” gas as it may, e.g., occur in landfills, which consist of an older and a younger part. In this case, the distribution station may be connected to two header pipes; as a result two separate gas collection systems exist. In most cases this is not necessary as long as the gas collection system can be adjusted in a way that high methane concentrations can be reached. As the LFG is in general water saturated (at 40 C approx. 60 g/m3, at 10 C approx 10 g/m3), and cools down when it leaves the landfill it is necessary to remove the condensate from the pipes at their deepest points. These may be located at the gas wells, the distribution stations, or the
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utilization unit. Of course if necessary more knock out facilities have to be installed, e.g., in the header pipes. In cases where there is more gas extracted as utilized, the gas has to be flared. Flaring may also be necessary when the gas utilization plant has a break down or is in maintenance. As the gas extraction rates vary with time, utilization units may not use all the extracted gas. Lean gas that is not used for energy production should also be flared until low methane concentrations. Standard flares can burn gas until methane concentrations reach 20%e25%; recently developed special flares may use gas with methane concentrations down to 8%e10% or even lower. If the gas cannot be thermally treated, it emits into the atmosphere and odor problems may occur; in addition, methane adds to the climate gas load. For all these reasons, flares should be standard at all landfills.
DESIGN AIDS FOR A GAS COLLECTION SYSTEM Gas wells For the design of gas wells, the following aspects should be considered: • • • • •
Inside the wells almost no pressure loss should occur. Landfill settlements should not cause pipe damages. Pipe stability should be proven. High temperatures (50e60 C) should not influence the pipe stability. Wells should be constructed in a way that air is not sucked in through the landfill surface, i.e., gas wells should be gastight against air intrusion especially at the well head and the upper part of the well.
Fig. 9.2.6 shows a typical vertical gas well; in this case with a flexible pipe in a pipe system for compensating settling, which is not used in any cases. A vertical gas well is in most cases drilled into the landfill by using an auger. The well should have the following characteristics: • The diameter should be between 1.0 and 1.2 m, dependent on the (expected) water level in the landfill. • They should be filled with lime-deficient course gravel (e.g., 16/32 mm). • The central pipe should be made of PE and meet at least the pressure level of SDR 11 (Anonymous, 2005). • The perforation should cover not more then 5% of the pipe surface. The openings can either be slots or holes. Slots and holes should be large enough to avoid clogging but smaller than the diameter of the gravel. It should also to be considered that: • To compensate settling the connection between the gas, extraction pipe and the horizontal pipe should be flexible. • The upper part of the well has to be sealed (e.g., using low permeable clay)ddependent of the height of the landfilldat length from the landfill surface of at least 3 m. The gas extraction pipedwhere it is slotteddshould be surrounded by course gravel to avoid clogging.
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Figure 9.3.6 Typical vertical gas well. Courtesy of Ingenieurgruppe RUK GmbH.
The distance between the wells should be in the range of 35e50 m. At high landfills the spacing can be wider than at shallower landfills. The radius of influence can roughly be approximated using the following empirical equation (dimensions are not fitting, for details see Rettenberger, 1988): qffiffiffiffiffiffiffiffi Rw1:2 1=p R, radius of influence, m; P, peak gas production, m3/m3 h (m3 gas per m3 landfill body per hour). Peak gas production is the highest value in the gas prognosis curve starting from the beginning of waste disposal. If the landfill already exists, it is strongly recommended to install one or more test wells in the landfill and make some gas extraction tests. In addition, the potential water levels in the wells should be measured, and waste samplesdgained during drillingdshould be taken to estimate the gas production potential. The samples should be analyzed especially for AT4, GB21, OTS, org. C, etc. Gas extraction tests should be carried out at actual or specifically built gas extraction wells. It is recommended to make the gas extraction test using at least three wells. In the surrounding of the wells, several gas probes should be installed in different depths to measure the pressure distribution. By these
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means the radius of influence can be determined more exactly. The gas extraction test should last over periods of 1e3 months starting with a low extraction rate. This should be slightly increased every 2e3 days until the methane concentration is lower than 35%. As a next test phase, the methane concentration should be kept constant at 50% tonne over a period of about 2 months. After this phase, the initially practiced test procedure with the varying extraction rates and periods should be repeated. If the gas shall be extracted from the start of landfill operation, wells are necessary that can “grow” with the landfill height, i.e., the wells can be elongated. A typical example of such a well type is presented in Fig. 9.3.7. When the landfill height increases, the steel pipe is pulled out of the landfill for some meters using an excavator or similar equipment. It is very important that the gas well stays gastight. Only during the period when the wells are enlarged in height, the gas extraction has to be interrupted.
Figure 9.3.7 Extensible gas well. Courtesy of Ingenieurgruppe RUK GmbH.
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Figure 9.3.8 Open gas wells at a landfill (inacceptable solution). Photo by G. Rettenberger.
Horizontal gas extraction pipes have proven their long-term functionality when they are adequately built and operated. These systems can be installed from the beginning of landfilling; they are influencing landfill operation much less than the “growing” vertical wells. Designing such systems it has to be respected that they may be partly filled up with water, which might result in clogging; in addition, settling has to be compensated. Also for these reasons PE pipes 250 [SDT 17,6, (4)] with slots or holesdas prescribed for the vertical wellsdshould be placed in, e.g., 1m2 profile trenches filled with coarse gravel or other coarse materials with low lime content and low solubility. The horizontal distances should be less then 20e40 m, and the vertical distances should be in the range of 8 m. An unacceptable solution of a gas extraction system is shown in Fig. 9.2.8. LFG may emit directly into the atmosphere causing high methane gas emissions. In addition, it creates a dangerous situation because there is a high potential of fires and explosions. Also due to the trace compounds and the high CO2 concentrations in the LFG, people at the landfill may inhale toxic gases. Header pipes and gas distribution stations To allow condensate flow, gas transportation pipes on the landfill have to be installed with a constant slope in a depth that is not affected by frost. At the lowest points, water traps have to be installed. The diameter of the pipes should be at least da 90, in combination with a sufficient slope of preferably 5%. By these means a certain degree of settling may be compensated. PE is the favorable material, and the pressure strength should at least meet SDR 17.6. Dependent on the size of the landfill, different separate collection systems may be suitable (Fig. 9.3.9): decentralized gas collection with several distribution stations and several gas flares, central ramification gas collection system with distribution stations and central flaring, central gas collection
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Figure 9.3.9 General layout of a gas collection system (Rettenberger, 1994).
system as ring pipeline, several distribution stations and central flaring. In the case of a separate lean gas and normal gas collection system, two collection header pipes will be necessary. The central gas collection system has advantages concerning safety measures, quality, and flow control, and in most cases the number of different analytical equipment; in addition only one flare will be necessary, which all together will save costs. Fig. 9.3.10 shows a distribution station for lean and “normal” gas quality pipes. Gas extraction and flaring As gas blowers have to face varying gas flow rates, it is necessary that they can be adequately adjusted without losing efficiency. Eccentric rotor blowers have proven successfully in practice. (see Fig. 9.3.11). Conventional blowers as, e.g., side channel blowers are only efficient if they work on the designed flow rate. In case a frequency transformer is used, the maximum flow rate can be reduced by about 50%. Meanwhile, in Germany conventional radial blowers are frequently in use. Using gas flares, methane has to be “completely” oxidized to CO2; in addition, the trace constituents in the LFG including the odor-producing constituents have to be destroyed. By adjusting the operation of the flare adequately, the production of dioxins will be minimized. This aim can only be achieved if the temperature in the combustion chamber of the flare is >950 C, and a retention time of >0.3 s exists. For keeping the temperature high, the combustion chamber has to be insulated. To meet safety prescriptions, the flare has to be equipped with a flame control and an electronic ignition system. Fig. 9.3.12 shows a typical flare for LFG. It should be mentioned that in most cases LFG flares have to operate constantly over the year. Therefore permanent control and maintenance are imperative.
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Figure 9.3.10 Gas distribution station with connecting pipes. Photo by G. Rettenberger.
Figure 9.3.11 Eccentric rotor blower. Photo by G. Rettenberger. CHAPTER 9 j Collection and Disposal of Landfill Gas
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Figure 9.3.12 Gas flare with gas inlet and combustion chamber. Photo from Lambda GmbH.
MONITORING, CONTROL, AND CONCLUSIONS Owing to varying conditions in the landfill body, atmospheric pressure fluctuations, increasing landfill height and age, and changing operation modes, the LFG collection system has to be permanently adjusted. Automatically operating monitoring systems may be helpful. As an example, the following monitoring program may be followed: Once a week: • Measuring gas composition at each well (e.g., in the distribution station), if methane concentrations divert from the set value 50%, the extracted flow rate should be adjusted (preferably continuous gas quality monitoring at least before the blower). • Gas flow rate at the blower station (preferably continuous flow monitoring). Two times a year: • Measuring the water level in the wells, if the water level is too high and affects gas extraction, water has to be pumped out. • Measuring the temperature in the wells. • Analyzing the gas flow rate and gas composition including trace gases at the central blower. If the concentrations of trace gases are too high, additional gas treatment measures may be necessary; if the gas composition shows air intrusion leaks, it should be identified and repaired; if the gas flow is less then 5e7 times lower then the original flow rate, the installation has to be investigated, repaired, and if not feasible diminished and redesigned. If there is a certain tendency to
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lean gases, the gas collection concept has to be revised. In this case aerobization of the landfill, passive gas venting, or lean gas treatment may be the options. • Detecting gas emissions at the surface of a lined landfill (FID monitoring would help to identify leaks). If emissions are detected the operation mode and the gas collection system should be investigated and appropriate measures have to be taken. Every 1e5 years: • Controlling gas wells and pipes using camera systems, if failures are determined, actions such as cleaning or repairing have to be started. • Safety control installations: maintenance and repairing may be necessary.
PASSIVE GAS DISPOSAL/VENTING Passive gas venting may become relevant at old landfills where due to very low gas production rates there is no active gas extraction anymore. If a landfill is lined at the surface, a certain positive pressure inside the landfill may build up. This positive pressure may be used to control and treat gas emissions. Owing to the low pressure and flow rate, thermal treatment is not an option. At certain collection pointsde.g., former gas wellsdthe gas may migrate out of the landfill body by passing through the landfill cover. The methane shall biologically oxidize in the soil cover before reaching the atmosphere (see also Chapter 9.5). To achieve this technically, the gas has to migrate into a kind of gas distribution layer (layer filled with sand or gravel) where the gas quality and flow is more or less equalized. From this layer, the gas migrates into the cultivation layer (top soil) where the methane shall be oxidized. Fig. 9.3.13 shows an example of a full-scale application.
Figure 9.3.13 Passive gas venting using methane oxidation processes in the top cover. Courtesy of
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References Anonymous, 2001. DGUV Regel 114e004, 2001, Deponien, BG Bau Berufsgenossenschaft der Bauwirtschaft. Deutsche Gesetzliche Unfallversicherung e.V., Berlin. Anonymous, 2005. GDA-Empfehlungen 2005, E 2e18: Geotechnische Belange der Deponieentgasung, Bautechnik 9. Anonymous, 2016. VDI 3790, Blatt 2, 2016, Emissionen von Gasen, Gerüchen und Stäuben aus diffusen Quellen. Beuth Verlag, Berlin. Hrad, M., Huber-Humer, M., 2015. Innovatives monitoring tool zur Bewertung von Methanemissionen. In: von Stegmann, R., Rettenberger, G. (Eds.), Trierer Berichte zur Abfallwirtschaft, herausgegeben, Band 22. Verlag Abfall aktuell, Stuttgart. Rettenberger, G., 1988. Planerische, bauliche und sicherheitstechnische Gesichtspunkte bei der Erstellung von Entgasungsanlagen mit dem Ziel der Emissionsminderung. In: von Stegmann, R., Rettenberger, G. (Eds.), Hamburger Berichte, Band 1. Economica Verlag GmbH, Bonn. Rettenberger, G., 1994. Deponie. In: Kranert, M., Cord-Landwehr, K. (Eds.), Einführung in die Abfallwirtschaft, 4. Auflage 2010. ViewegþTeubner Verlag, Wiesbaden. Rettenberger, G., 2004. Untersuchungen zur Charakterisierung der Gasphase in Abfallablagerungen. In: Stuttgarter Berichte zur Abfallwirtschaft, Band 82. Kommissionsverlag Oldenbourg Industrieverlag GmbH, München NOTE- s is used in this Chapter for indicating the volume concentration of a given gas X:: s(X) ¼ V(X)/V. It can be expressed as ratio or as percentage. - m3, if not alternatively specified, stays for 1 cubic meter of gas volume under normal conditions (temperature: 0 C, pressure: 105Pa).
SOLID WASTE LANDFILLING j Concepts, Processes, Technologies j R. Cossu, R. Stegmann
FUNDAMENTALS Landfill gas (LFG) extraction and utilization should be a standard procedure at all municipal solid waste landfills. By these means the emission of climate influencing gases can be significantly reduced, energy can be utilized, odors can be avoided, and explosion risks can be reduced. There are five essential conclusions resulting from experiences with gas collection over the last decades: 1. LFG extraction systems should be installed and operated from the beginning of landfill operation. Fig. 9.3.1 shows the gas volumetric composition (s) at different gas wells from the beginning of the landfill operation (1). For this test, the gas wells had been installed directly on top of the bottom liner. As can be seen in Fig. 9.3.1, the s CH4 to s CO2 ratio of 1 increases after 11e15 months after the landfill operation started. Therefore it is necessary to have the gas extraction system available during the first year of operation. Having in mind that the half-life value of the LFG production is in the early period of a landfill in the range of 3e4 years, means that without operating a gas extraction system almost 50% of the gas may be lost in this time frame. 2. Gas collection can be realized using an active or passive system. Operating an active system the gas is extracted out of the landfill by means of blowers, which generally provide vacuum pressures in the landfill body in the range of 1 to 30 hPa. Passive gas collection systems use the positive pressure in the landfill body to transport the gas out of the landfill under semicontrolled conditions. Passive systems may only be feasible, if the methane gas production is less than 0.5 L/ m2 h and a gastight top cover has been installed. Fig. 9.3.2 shows gradients of LFG and air components over the depth of the landfill body or a cover soil (Rettenberger, 2004). As can be seen in Fig. 9.3.2, the ratio of s CH4 to s CO2 changes with the depth. These results show that methane oxidation occurs. This process is technically used in landfill covers for biological oxidation of residual methane (see Chapter 9.4). 3. When active gas extraction is practiced, the methane concentration may change when air is sucked into the landfill.
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Figure 9.3.1 s(CH4)/s(CO2) over time at four gas wells (K1, K2, K4, K5) since the beginning of
deposition (Rettenberger, 2004).
Figure 9.3.2 s(CH4), s(CO2), s(O2), and s(N2) in different depths of a pervious soil cover or an open
landfill (Rettenberger, 2004). Fig. 9.3.3 shows typical curves when oversucking occurs, which may happen when the gas extraction rate is higher than the gas production rate and/or when the gas wells are not adequately designed (Rettenberger, 2004). The figure shows that the kind of correlation is individual for each well and is corresponding to the mathematical model, which describes the mixing of airddue to air intrusiondwith the LFG (Rettenberger, 2004): sðCH4 Þ ¼ sðCH4;v¼0 Þ 1 þ b$v_2 s(CH4), volume concentration of methane depending from flow; s(CH4,v¼0), volume concentration when flow is 0 m3/h; b, factor calculated by statistical method; v, _ flow.
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Figure 9.3.3 Methane concentrations as a function of gas flow (over sucking) during pumping tests at three wells (sections) (Rettenberger, 2004).
Together with the methane concentrations the O2 and N2 concentrations also vary with different gas flow rates. Fig. 9.3.4 shows the oxygen concentrations at a gas well in relation to the extracted gas volume. Significantly elevated oxygen concentrations in the landfill may be a potential for explosions, fires, and may inhibit methane production. On the other hand, there should be a certain negative pressure in the landfill body to avoid uncontrolled emissions. As a conclusion, LFG extraction should be operated in a way that methane concentrations are in the range of 50%e45% during landfill operation and >50% after completion of the landfill. To achieve this, all wells should be adjusted individually about once per week. This is also due to the fact that the atmospheric pressure (increasing pressure change reduces and decreasing pressure change increases the gas pressure in the landfill body) and the wind (wind may cause a negative pressure inside the landfill) may have a strong influence on the gas extraction rate and/or the gas quality.
Figure 9.3.4 Volumetric concentration s(O2) as a function of the gas flow at a gas well over a period of
3 months (Rettenberger, 2004).
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4. Due to decreasing gas production rates in the older parts of a landfill or in the aftercare phase, more air may be sucked into the landfill that results in a so-called lean gas, i.e., methane concentrations < 20%e25%. Systematical oversucking can improve the collection efficiency rate considerably. When the gas flow is in the range of 2%e5% of the initial flow rate, air intrusion happens and a lean gas occurs. Owing to the oxygen content in the landfill, methane oxidation processes may occur. The gas extraction system has to be designed in a way that it can operate in a flow range of 1:20 (start of gas extraction) to 1:40 (final phase). As a consequence, the gas extraction system has to be continuously adapted during the operation of the plant. 5. Owing to the fact that the landfill body is very inhomogeneous and each landfill is different as well as the construction of the gas extraction system, the efficiency of a gas collection system (how much of the produced gas is captured) cannot be predicted. The design of a gas collection system is therefore not ultimate and has to be controlled either by comparing the extracted gas flow rates with the results of the gas prognosis and/or by detecting uncontrolled emissions around the wells and at the landfill surface. Gas emissions at the landfill surface can be detected using the flame ionization detector (FID) method. Using this measuring device means to walk over the entire area of the landfill placing a little cap on top of the surface at different points and pumping small amounts of gas into the FID detector. In general the concentration range that can be measured with the detector is between 0e10,000 ppm CH4. Using this method, most of the spots where gas is emitting can be detected. To reduce uncontrolled gas emissions, the gas extraction rates have to be adjusted, more wells may be installed, and/or the areas where the gas is emitting may be ceiled. Investigations in the last years show that most of the LFG emissions at landfills occur in a concentrated way at relatively few small areas (hot spots). Therefore, in Germany, landfill operators should identify those hot spots to take actions to avoid these emissions (Anonymous, 2016). An alternative measurement to the FID method is the control of diffusive emissions measuring methane concentrations outside of the landfill and the meteorological data to calculate the emission rate using gas distribution models. The disadvantage of this method is that the hot spots where the main emissions occur cannot be identified. More methods to measure gas emissions from a landfill are under investigation (Hrad and Huber-Humer, 2015), see also Chapter 9.5.
GAS EXTRACTION AND TRANSPORTATION Fig. 9.3.5 shows the general system of an active gas collection system under the German regulation (Anonymous, 2001). The gas is extracted by a blower, which can be to a high degree adjusted to different flow rates. It should provide vacuum pressures between 1 and 50 (100) hPa. The vacuum pressure should be distributed into the landfill body as homogeneous as possible. To reach this aim, a certain number of vertical
SOLID WASTE LANDFILLING j Concepts, Processes, Technologies j R. Cossu, R. Stegmann
Figure 9.3.5 Scheme of an active gas collection system (Anonymous, 2001).
or horizontal gas wells have to be installed in the landfill body. There should be nearly no pressure loss inside the well, which can be achieved by choosing adequate gas pipe diameters (typical diameter 100 mm). If the landfill has a depth >25e30 m, two gas extraction pipes of different length (with a gas seal underneath the shorter pipe) may be installed that may be operated with different vacuum pressures (lower and higher level). It is standard in Germany that each well is connected via a pipe to a distribution station, where the transportation pipe of several wells is connected to a main header pipe that transports the gas to the blower station. In the distribution station a valve can adjust the flow rate. In the pipe just in front the valve technical facilities for measuring in the pipe pressure, flow rate, gas composition, and temperature should be installed. In the gas distribution station, it may make sense to distinguish between lean gas and “normal” gas as it may, e.g., occur in landfills, which consist of an older and a younger part. In this case, the distribution station may be connected to two header pipes; as a result two separate gas collection systems exist. In most cases this is not necessary as long as the gas collection system can be adjusted in a way that high methane concentrations can be reached. As the LFG is in general water saturated (at 40 C approx. 60 g/m3, at 10 C approx 10 g/m3), and cools down when it leaves the landfill it is necessary to remove the condensate from the pipes at their deepest points. These may be located at the gas wells, the distribution stations, or the
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utilization unit. Of course if necessary more knock out facilities have to be installed, e.g., in the header pipes. In cases where there is more gas extracted as utilized, the gas has to be flared. Flaring may also be necessary when the gas utilization plant has a break down or is in maintenance. As the gas extraction rates vary with time, utilization units may not use all the extracted gas. Lean gas that is not used for energy production should also be flared until low methane concentrations. Standard flares can burn gas until methane concentrations reach 20%e25%; recently developed special flares may use gas with methane concentrations down to 8%e10% or even lower. If the gas cannot be thermally treated, it emits into the atmosphere and odor problems may occur; in addition, methane adds to the climate gas load. For all these reasons, flares should be standard at all landfills.
DESIGN AIDS FOR A GAS COLLECTION SYSTEM Gas wells For the design of gas wells, the following aspects should be considered: • • • • •
Inside the wells almost no pressure loss should occur. Landfill settlements should not cause pipe damages. Pipe stability should be proven. High temperatures (50e60 C) should not influence the pipe stability. Wells should be constructed in a way that air is not sucked in through the landfill surface, i.e., gas wells should be gastight against air intrusion especially at the well head and the upper part of the well.
Fig. 9.2.6 shows a typical vertical gas well; in this case with a flexible pipe in a pipe system for compensating settling, which is not used in any cases. A vertical gas well is in most cases drilled into the landfill by using an auger. The well should have the following characteristics: • The diameter should be between 1.0 and 1.2 m, dependent on the (expected) water level in the landfill. • They should be filled with lime-deficient course gravel (e.g., 16/32 mm). • The central pipe should be made of PE and meet at least the pressure level of SDR 11 (Anonymous, 2005). • The perforation should cover not more then 5% of the pipe surface. The openings can either be slots or holes. Slots and holes should be large enough to avoid clogging but smaller than the diameter of the gravel. It should also to be considered that: • To compensate settling the connection between the gas, extraction pipe and the horizontal pipe should be flexible. • The upper part of the well has to be sealed (e.g., using low permeable clay)ddependent of the height of the landfilldat length from the landfill surface of at least 3 m. The gas extraction pipedwhere it is slotteddshould be surrounded by course gravel to avoid clogging.
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Figure 9.3.6 Typical vertical gas well. Courtesy of Ingenieurgruppe RUK GmbH.
The distance between the wells should be in the range of 35e50 m. At high landfills the spacing can be wider than at shallower landfills. The radius of influence can roughly be approximated using the following empirical equation (dimensions are not fitting, for details see Rettenberger, 1988): qffiffiffiffiffiffiffiffi Rw1:2 1=p R, radius of influence, m; P, peak gas production, m3/m3 h (m3 gas per m3 landfill body per hour). Peak gas production is the highest value in the gas prognosis curve starting from the beginning of waste disposal. If the landfill already exists, it is strongly recommended to install one or more test wells in the landfill and make some gas extraction tests. In addition, the potential water levels in the wells should be measured, and waste samplesdgained during drillingdshould be taken to estimate the gas production potential. The samples should be analyzed especially for AT4, GB21, OTS, org. C, etc. Gas extraction tests should be carried out at actual or specifically built gas extraction wells. It is recommended to make the gas extraction test using at least three wells. In the surrounding of the wells, several gas probes should be installed in different depths to measure the pressure distribution. By these
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means the radius of influence can be determined more exactly. The gas extraction test should last over periods of 1e3 months starting with a low extraction rate. This should be slightly increased every 2e3 days until the methane concentration is lower than 35%. As a next test phase, the methane concentration should be kept constant at 50% tonne over a period of about 2 months. After this phase, the initially practiced test procedure with the varying extraction rates and periods should be repeated. If the gas shall be extracted from the start of landfill operation, wells are necessary that can “grow” with the landfill height, i.e., the wells can be elongated. A typical example of such a well type is presented in Fig. 9.3.7. When the landfill height increases, the steel pipe is pulled out of the landfill for some meters using an excavator or similar equipment. It is very important that the gas well stays gastight. Only during the period when the wells are enlarged in height, the gas extraction has to be interrupted.
Figure 9.3.7 Extensible gas well. Courtesy of Ingenieurgruppe RUK GmbH.
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Figure 9.3.8 Open gas wells at a landfill (inacceptable solution). Photo by G. Rettenberger.
Horizontal gas extraction pipes have proven their long-term functionality when they are adequately built and operated. These systems can be installed from the beginning of landfilling; they are influencing landfill operation much less than the “growing” vertical wells. Designing such systems it has to be respected that they may be partly filled up with water, which might result in clogging; in addition, settling has to be compensated. Also for these reasons PE pipes 250 [SDT 17,6, (4)] with slots or holesdas prescribed for the vertical wellsdshould be placed in, e.g., 1m2 profile trenches filled with coarse gravel or other coarse materials with low lime content and low solubility. The horizontal distances should be less then 20e40 m, and the vertical distances should be in the range of 8 m. An unacceptable solution of a gas extraction system is shown in Fig. 9.2.8. LFG may emit directly into the atmosphere causing high methane gas emissions. In addition, it creates a dangerous situation because there is a high potential of fires and explosions. Also due to the trace compounds and the high CO2 concentrations in the LFG, people at the landfill may inhale toxic gases. Header pipes and gas distribution stations To allow condensate flow, gas transportation pipes on the landfill have to be installed with a constant slope in a depth that is not affected by frost. At the lowest points, water traps have to be installed. The diameter of the pipes should be at least da 90, in combination with a sufficient slope of preferably 5%. By these means a certain degree of settling may be compensated. PE is the favorable material, and the pressure strength should at least meet SDR 17.6. Dependent on the size of the landfill, different separate collection systems may be suitable (Fig. 9.3.9): decentralized gas collection with several distribution stations and several gas flares, central ramification gas collection system with distribution stations and central flaring, central gas collection
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Figure 9.3.9 General layout of a gas collection system (Rettenberger, 1994).
system as ring pipeline, several distribution stations and central flaring. In the case of a separate lean gas and normal gas collection system, two collection header pipes will be necessary. The central gas collection system has advantages concerning safety measures, quality, and flow control, and in most cases the number of different analytical equipment; in addition only one flare will be necessary, which all together will save costs. Fig. 9.3.10 shows a distribution station for lean and “normal” gas quality pipes. Gas extraction and flaring As gas blowers have to face varying gas flow rates, it is necessary that they can be adequately adjusted without losing efficiency. Eccentric rotor blowers have proven successfully in practice. (see Fig. 9.3.11). Conventional blowers as, e.g., side channel blowers are only efficient if they work on the designed flow rate. In case a frequency transformer is used, the maximum flow rate can be reduced by about 50%. Meanwhile, in Germany conventional radial blowers are frequently in use. Using gas flares, methane has to be “completely” oxidized to CO2; in addition, the trace constituents in the LFG including the odor-producing constituents have to be destroyed. By adjusting the operation of the flare adequately, the production of dioxins will be minimized. This aim can only be achieved if the temperature in the combustion chamber of the flare is >950 C, and a retention time of >0.3 s exists. For keeping the temperature high, the combustion chamber has to be insulated. To meet safety prescriptions, the flare has to be equipped with a flame control and an electronic ignition system. Fig. 9.3.12 shows a typical flare for LFG. It should be mentioned that in most cases LFG flares have to operate constantly over the year. Therefore permanent control and maintenance are imperative.
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Figure 9.3.10 Gas distribution station with connecting pipes. Photo by G. Rettenberger.
Figure 9.3.11 Eccentric rotor blower. Photo by G. Rettenberger. CHAPTER 9 j Collection and Disposal of Landfill Gas
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Figure 9.3.12 Gas flare with gas inlet and combustion chamber. Photo from Lambda GmbH.
MONITORING, CONTROL, AND CONCLUSIONS Owing to varying conditions in the landfill body, atmospheric pressure fluctuations, increasing landfill height and age, and changing operation modes, the LFG collection system has to be permanently adjusted. Automatically operating monitoring systems may be helpful. As an example, the following monitoring program may be followed: Once a week: • Measuring gas composition at each well (e.g., in the distribution station), if methane concentrations divert from the set value 50%, the extracted flow rate should be adjusted (preferably continuous gas quality monitoring at least before the blower). • Gas flow rate at the blower station (preferably continuous flow monitoring). Two times a year: • Measuring the water level in the wells, if the water level is too high and affects gas extraction, water has to be pumped out. • Measuring the temperature in the wells. • Analyzing the gas flow rate and gas composition including trace gases at the central blower. If the concentrations of trace gases are too high, additional gas treatment measures may be necessary; if the gas composition shows air intrusion leaks, it should be identified and repaired; if the gas flow is less then 5e7 times lower then the original flow rate, the installation has to be investigated, repaired, and if not feasible diminished and redesigned. If there is a certain tendency to
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lean gases, the gas collection concept has to be revised. In this case aerobization of the landfill, passive gas venting, or lean gas treatment may be the options. • Detecting gas emissions at the surface of a lined landfill (FID monitoring would help to identify leaks). If emissions are detected the operation mode and the gas collection system should be investigated and appropriate measures have to be taken. Every 1e5 years: • Controlling gas wells and pipes using camera systems, if failures are determined, actions such as cleaning or repairing have to be started. • Safety control installations: maintenance and repairing may be necessary.
PASSIVE GAS DISPOSAL/VENTING Passive gas venting may become relevant at old landfills where due to very low gas production rates there is no active gas extraction anymore. If a landfill is lined at the surface, a certain positive pressure inside the landfill may build up. This positive pressure may be used to control and treat gas emissions. Owing to the low pressure and flow rate, thermal treatment is not an option. At certain collection pointsde.g., former gas wellsdthe gas may migrate out of the landfill body by passing through the landfill cover. The methane shall biologically oxidize in the soil cover before reaching the atmosphere (see also Chapter 9.5). To achieve this technically, the gas has to migrate into a kind of gas distribution layer (layer filled with sand or gravel) where the gas quality and flow is more or less equalized. From this layer, the gas migrates into the cultivation layer (top soil) where the methane shall be oxidized. Fig. 9.3.13 shows an example of a full-scale application.
Figure 9.3.13 Passive gas venting using methane oxidation processes in the top cover. Courtesy of
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References Anonymous, 2001. DGUV Regel 114e004, 2001, Deponien, BG Bau Berufsgenossenschaft der Bauwirtschaft. Deutsche Gesetzliche Unfallversicherung e.V., Berlin. Anonymous, 2005. GDA-Empfehlungen 2005, E 2e18: Geotechnische Belange der Deponieentgasung, Bautechnik 9. Anonymous, 2016. VDI 3790, Blatt 2, 2016, Emissionen von Gasen, Gerüchen und Stäuben aus diffusen Quellen. Beuth Verlag, Berlin. Hrad, M., Huber-Humer, M., 2015. Innovatives monitoring tool zur Bewertung von Methanemissionen. In: von Stegmann, R., Rettenberger, G. (Eds.), Trierer Berichte zur Abfallwirtschaft, herausgegeben, Band 22. Verlag Abfall aktuell, Stuttgart. Rettenberger, G., 1988. Planerische, bauliche und sicherheitstechnische Gesichtspunkte bei der Erstellung von Entgasungsanlagen mit dem Ziel der Emissionsminderung. In: von Stegmann, R., Rettenberger, G. (Eds.), Hamburger Berichte, Band 1. Economica Verlag GmbH, Bonn. Rettenberger, G., 1994. Deponie. In: Kranert, M., Cord-Landwehr, K. (Eds.), Einführung in die Abfallwirtschaft, 4. Auflage 2010. ViewegþTeubner Verlag, Wiesbaden. Rettenberger, G., 2004. Untersuchungen zur Charakterisierung der Gasphase in Abfallablagerungen. In: Stuttgarter Berichte zur Abfallwirtschaft, Band 82. Kommissionsverlag Oldenbourg Industrieverlag GmbH, München NOTE- s is used in this Chapter for indicating the volume concentration of a given gas X:: s(X) ¼ V(X)/V. It can be expressed as ratio or as percentage. - m3, if not alternatively specified, stays for 1 cubic meter of gas volume under normal conditions (temperature: 0 C, pressure: 105Pa).
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