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Utilization of Landfill Gas and Safety Measures
9.4 UTILIZATION OF LANDFILL GAS AND SAFETY MEASURES Gerhard Rettenberger
INTRODUCTION Landfill gas (LFG) might be a considerable source for energy and should be extracted and used whenever this would be convenient environmentally, technically and economically. About 60e80 m3 of LFG per tonne of wet municipal solid waste (MSW) can be utilized over a period of approximately 15e20 years. Where LFG collection systems have been installed in the most cases, the LFG is used for energy production, but there are also examples where huge amounts of gas are flared. The reason in those cases is often that the cost of electricity in these countries is relatively low, so that the investment and operation of a gas utilization plant is not economical. Also for this reason in several cases, gas extraction and flaring is only practiced due to the financial support by a carbon credit program. This often happens in economically developing countries where there is the noncomprehensible situation that on one side energy is strongly needed and on the other side energy is wasted. Another obstacle may also be the utility companies, whichefor whichever reasonemay not be interested in feeding the electricity produced from LFG into their grit. In other cases there is no grit nearby. In these situations, island solution should be considered where the energy is used on site, e.g., in form of electricity or as a fuel to substitute relative expensive diesel fuel in already existing engines. But it is especially a political task to provide the legal basis to improve this situation and support LFG utilization. As the gas production curve is not constantdincreasing during the operation phase and decreasing in the aftercare phasedit is very important that the capacity of the gas utilization plant can be relatively easy adapted to changing gas extraction rates (see also Chapter 9.1). Otherwise, too high amounts of the gas have to be flared. The kind of utilization depends very much on the specific situation. In many cases the conditions for using LFG on site or in the near vicinity are not existing because there is no energy user. Therefore in most plants the gas is used for producing electricity also due to the fact that especially in industrialized countries the public grid is usually nearby and available. In Europe, the production of electrical energy in gas engines is the most common practice and technical standard.
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The upgrading of LFG natural gas quality may be due to its trace constituents and its content of oxygen and nitrogen is quite costly. This concept may be especially interesting when large amounts of LFG are produced over long periods of time. But there are also smaller treatment plantsdbuilt in containersdin use for upgrading biogas to natural gas quality, but these are not used for LFG yet. It is also technical standard to use LFG for heat production in boilers using burners that are adapted to thedcompared with natural gasdrelatively low methane content. In addition, also the low temperature heat (max. 100 C) from the cooling system of the gas engine can be used. Often the heat cannot be used over the entire year so that the energy production plant is not always running at full capacity. Only in a few cases the gas can be used in a nearby power or industrial plant where the gas is provided via a gas pipeline. Gas storage is in general costly, but there may be in the future more cases where gas engines also are operating mainly during the peak hours where higher revenues from the produced electricity can be gained. This can be achieved by storing the LFG for a couple of hours in gas storage tanks. This concept becomes more and more interesting in countries where a high amount of electricity is produced by wind power plants, whichddue to changing wind conditionsddo not operate constantly. Large gas storage tanks for LFG storage over longer periods (several days or weeks) may not be economical. In general before constructing a gas utilization plant, the technical and economical conditions should be investigated very carefully. But the environmental effects also have to be discussed, because LFG emissions contribute significantly to global warming. REQUIREMENTS As LFG contains trace components (see Chapter 9.1), which may cause damages (e.g., corrosion) in the technical equipment such as gas engines or boilers concentrations of trace constituents should be low. The manufactures provide only a guarantee for their plants if certain limit values of trace gas concentrations will be met (see the following example from one manufacturer) (Anon, 2013): • • • • • • •
total chlorine 0e18 mg/m3, total fluorine 0e9 mg/m3, total chlorine and fluorine 0e18 mg/m3, total sulfur 3e400 mg/m3, total silicon 0e3.5 mg/m3, dust < 0e2 mg/m3, and water vapor < 50% humidity.
Other manufacturers may require different limit values. If those are not met LFG treatment may become inevitable. In addition to the raw LFG, the concentrations of several components in the exhaust gases of flares, gas engines, and boilers are also regulated. These limit values differ from country to country. As an example, some of the threshold values that are required in Germany for reciprocating gas engines are presented (Anon, 2002).
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Figure 9.4.1 Unburned methane emissions as CO2 e equivalents dependent on the installed power of a
gas engine (full-scale data). Courtesy of SGS-RUK GmbH
• CO < 1000 mg/m3 if <3 MW (650 mg/m3, if >3 MW) • NOx < 500 mg/m3 (lean gas Otto engine, diesel engine >3 MW); <250 mg/m3 (normal Otto engine); <1000 mg/m3 (diesel engine < 3 MW) • CH2O < 60 (40) mg/m3 • SOx < 350 mg/m3 • Dust < 20 mg/m3 (only diesel engine) In general it should be kept in mind that the operator has the duty to minimize the emissions beyond the target values due to the state of the art. It should be considered that there might be (diffusive) CH4 emissions due to gas leakage; in addition, there may also be unburned methane residues in the exhaust gas of LFG engines. As may be concluded from Fig. 9.4.1 the unburned methane is not neglectable. As a consequence additional requirements to control these emissions in the future may come up. GAS PRETREATMENT, GAS STORAGE TANKS As indicated earlier, treatment of gas may be necessary either for emission control or damage prevention. All treatment methods of course add to the cost of the LFG utilization. In several cases, pretreatment is substituted by changing the oil in the gas engines more often because the oil acts as a sorbent. Dewatering, drying In most cases LFG leaves the landfill at temperatures between 35 C and 45 C. The gas is almost vapor saturated. As mentioned above, the moisture content in the gas should not exceed 50% if the gas is used in gas engines. In many cases this is achieved, e.g., by cooling the gas down to 10 C and heating it up to 20 C. In most plants in moderate or cold climate (in the latter case, freezing of the condensate in the
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Figure 9.4.2 Siphon as a condensate knock-out in a landfill gas pipe. Courtesy of Ingenieurgruppe
RUK GmbH
pipes or other technical devices has to be avoided) there is no separate technical moisture reduction necessary because this temperature change happens “naturally” during gas collection. The situation may be different in, e.g., tropical countries where separate cooling devices may have to be considered. In any case, it is necessary to remove the condensate from the pipes. Therefore passive dewatering units have to be installed. As already mentioned this can be achieved, e.g., by installing a siphon at the pipe system at their lowest points. Fig. 9.4.2 shows an example of a siphon installation emplaced in the landfill body (designed for negative pressure in the pipes of up to 100 hPa). Providing adequate slopes condensate can also be redirected back into the landfill through the gas wells. If additional dewatering is necessary, mostly cooling systems are used. In these facilities, the gas temperature is lowered down to a temperature of around 4 C. A further dewatering action will be necessary if the gas has to be stored under high pressure in gas storage tanks made of steel. This may be necessary if the gas shall be used, e.g., in vehicles or shall be supplied into the public gas grid. In those cases, drying systems have to be used. Often adsorption filter filled with silica pellets or other appropriate sorption materials are used. These systems are operated using the pressure swing technology.
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Figure 9.4.3 Activated carbon filter for landfill gas treatment placed in front of a gas engine. Photo by G. Rettemberger.
Removal of chlorine, fluorine, and silica compounds To protect the gas equipment and meet the requirements of the manufacturers, the removal of chlorine, fluorine (chlorinated, fluorinated hydrocarbons), and organic silica compounds (siloxanes) from the raw gas may become necessary. Quite common is the application of adsorption processes using activated carbon (AC) pellets, which can be loaded with these compounds up to 15% of their original weight. There is a great variety of AC qualities and the contamination of the LFG is also site specific, for this reason, it might be viable to carry out tests to find out the most efficient/less costly material. After reaching the maximum loading capacity, the AC will be transported to a regeneration plant and will be substituted by unloaded AC. This might not be possible in all countries. In these cases the carbon can also be regenerated on site, which takes of course more efforts. When handling loaded AC, safety measures have to be followed. As adsorption systems show a typical breakthrough behavior, it is common to operate 2 filters in sequence. Fig. 9.4.3 shows an AC plant for LFG treatment. Other sorption materials that may be used are molecular sieves and silica gels. Removal of sulfur In general the concentration of hydrogen sulfide in the LFG is rather low. Therefore sulfur removal is in most cases not necessary. But there may be some exceptions especially when significant amounts of construction and demolition waste or other wastes containing gypsum have been codisposed with MSW. In these cases, sulfates will be biologically reduced to H2S. H2S is up to a certain concentration very odorous but cannot be smelled at higher concentrations. In addition, it is very toxic at low
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Figure 9.4.4 Gas storage tank. Photo by G. Rettemberger.
concentrations also and very corrosive. For these reasons the codisposal of biologically degradable waste with gypsum-containing waste has to be strictly avoided. H2S can be biologically oxidized, but for its removal from LFG more often specially coated AC is used. These filters have a high efficiency. But biological processes also have been used in full-scale plants successfully. Gas storage tank As mentioned above, in only a few cases gas storage tanks have been installed at landfills. As the typical pressure range in landfills is between 10 hPa and 30 hPa, membrane gas storage tanks with or without steel casing may be used. Flexible membranes provide varying gas storage volumes. The LFG is supplied to or taken out from the storage facility by pumps. The pressure in the tank can be kept constant by means of an appropriate weight on top of the tank. Otherwise a feeding pump, which provides the necessary pressure for the user is needed. Fig. 9.4.4 shows a gas storage tank made of plastic membrane that is placed in a metal casing.
LANDFILL GAS UTILIZATION TECHNOLOGIES Combined heat and power gas engines LFG is mainly used in gas engines for producing electricity and heat (combined heat and power-CHP plants). The electricity can be supplied into the public grid or may be used on site as an island solution. The low temperature heat (100 C) can be used for heating houses or supplying heat for industrial or commercial purpose (heating green houses, fish ponds, etc.). Often the heat cannot be used constantly
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or at all, in these cases the cooling water of the engine has to be cooled using, in most cases, an air cooler. The advantages of CHP engines are as follows: • Proven and well-developed technology, in general no or only minor LFG treatment necessary, operation of more than 8200 h/a possible. • Electrical efficiency even more than 40%. • Operation with lean LFG down to 30% methane concentration in many cases possible. • In most cases the electricity is supplied into the public grid providing constant supply. • Relatively cost-effective, in some countries price per kWh isddue to state regulationdfinancially subsidized by the utility companies. Gas engines can be operated as stand-alone operation with asynchronous generator (island solution) or with synchronous generator for supplying electricity to the public grid. Gas engines need constant maintenance at different levels (oil exchange, engine inspection, etc.). Under these conditions, gas engines can be operated for more than 10 years (more than 80,000 h). During this period the gas production varies to a high degree (after the operation phase of the landfill, the gas production reduces by more than 60%); the capacity of the gas engine has to be adjusted. This is necessary because the efficiency of the gas engine running at half capacity is significantly decreasing. As a consequence, in the most cases an appropriate solution may be to: • Install several gas engines and add or remove one or more engines due to available gas volumes. • Install gas engines completely in containers as a unit for simple exchange. • Rent gas engines which can be exchanged with engines that have a lower capacity. With these measures it can be avoided that lower gas volumes cannot be used and have to be flared instead (reducing the extraction rate cannot be accepted in any case). Fig. 9.4.5 shows container units and Fig. 9.4.6 presents an installation of a CHP plant in an industrial building. To meet the gas emission standards for exhaust gasesdespecially for NO2dand to increase the electrical efficiency of the engine, in most cases turbo charged lean gas engines are used. In the future decreasing limits for formaldehyde and unburned methane in the exhaust gas can be expected and may ask for additional measures. Lean gas engines need a special gaseair mixing system to operate in a window where relatively low N2O will be produced. In some cases diesel engines are modified so that they can use LFG as a fuel. The disadvantage of this engine type is the necessity to inject continuously diesel of up to 15% of the total fuel consumption into the cylinder. Diesel engines are self-priming; compared with gas spark-ignited engines they can be operated at lower methane concentrations and have a high electrical efficiency. The emissions of N2O are higher in diesel engines, which is taken care of granting higher N2O target values in the exhaust gas. The advantages of diesel engines are more relevant for smaller plants. There are developments on the way to further improve this engine type for LFG utilization.
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Figure 9.4.5 Gas engines built in container. Photo by G. Rettemberger.
Figure 9.4.6 Gas engine in an industrial building. Photo by G. Rettemberger.
Owing to the mostly abandoned locations of most of the landfills, the use of waste heat is limited. Investigations had been done using technologies to convert the heat from the engine cooling system into electrical power. Organic rankine cycle uses organic liquids that vaporize at lower temperatures than water to produce vapor that drives a steam engine, which is connected to a generator. The vapor is cooled and the organic liquid is recovered and used again. Most of the organics that have this characteristic are toxic or not environmental friendly, for this reason synthetic silicon-based organic liquids are under development. This process operated successfully at full scale and needs some further development to be more stable. It may be an option in special situations.
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A proven technology is the so-called Stirling engine that may provide for smaller or older landfills electrical power of 5e8 kWel, which, e.g., may be used for the electricity supply for facilities in the aftercare phase on site. One manufacturer produces only one type (5e8 kWel), which can be bought from the shelf (Anon, 2016a). Gas turbines In a few cases in Germany (there may be more in other countries), gas turbines (20e65 kW) that use LFG have been installed. In the United States also, larger turbines (producing several hundreds kWel) are in operation (Bischoff, 2008). Gas turbines need relative clean gas with very low trace compound concentrations, and the LFG has to be pressurized up to 5 bar. As an advantage that gas turbines can operate with lean gases (>25% CH4), they need only little maintenance and meet exhaust standards quite easily. On the other hand the electrical efficiency compared with gas engines is lower (26%e32%, Bischoff, 2008). Gas turbines could close the gap between CHP engines and lean gas flares for gas utilization of landfills in the aftercare phase. Methane supply in to a public or private gas net For the operation of a private gas net, e.g., in Germany the national building code has to be respected. The operation itselfde.g., to supply a power unit with LFGdrequires removing water/vapor from the gas and to install the pipes in the soil at a depth where freezing does not occur. Of course safety measures have to be fulfilled. Owing to the content of toxic trace compounds in the LFG, the utilization should be restrictive. These gas nets are mainly used to transport the gas to an industrial plant where it is used on site. Owing to the content of trace compounds, it is not allowed in Germany to supply LFG into a public grid (in contrast to biogas from agricultural sources). Internationally there are several plants where LFG is supplied into a national grid. LFG treatment technologies such as molecular sieves as a pressure swing process, the “Selexol” process or membranes have been used successfully to treat the LFG to natural gas quality (Bilitewski and Härdtle, 2013). Fig. 9.4.7 shows a “Selexol” plant, which was operated in California/United States. In general this kind of technology may only be economical at huge landfills under special economical conditions. Burner and boiler The direct burning of LFG to produce heat and steam is quite common and a standard technology. If the LFG gas can be used, e.g., in a plant all year around, the efficiency of the utilization of the energy content in the LFG can be more than 80%e90%, but the economical problem of these kind of plants arises when the LFG cannot be used all year around (e.g., when heat or cold is used). In the latter cases only a certain part of the totally produced gas can be used. In addition, during the period when no or less gas is used, other disposal units (such as flares) have to be installed. Fig. 9.4.8 shows a burner/boiler unit in a garden center for heating green houses in Germany.
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Figure 9.4.7 Landfill Gas treatment for supplying into the national grit using the “Selexol” process in
California/USA. Photo by G. Rettemberger.
Figure 9.4.8 Landfill gas burner in a garden center. Photo by G. Rettemberger.
Using LFG in boilers has also the advantage that there are, e.g., in Germany less strict requirements regarding the raw gas quality, but of course the exhaust gas threshold values have to be met. With special burners, lean gases can also be used down to methane concentrations of 4.5% (Anon, 2016b). It is recommended to make sure that no corrosion will occur inside the boilers; it should also be checked on a routine basis if depositsde.g., caused by siloxanesdat the inside walls of the boiler have been built up.
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Figure 9.4.9 Diagram to assess a methane, air and inert gas mixture due to their explosion potential (Müller and Rettenberger, 1986).
SAFETY MEASURES LFG containing oxygen or getting in contact with air can cause explosive mixtures with considerable effects to human beings as well as to the environment. If deflagration happens explosions can reach a pressure around 7 bar, but much higher pressures occur if a detonation has developed. An explosion may occur if three premises exist as follows: • A combustible gas with good dispersion behavior. • A gas mixture in an explosive condition (between upper and lower explosion limits). • An igniting source. To find out if a gaseair mixture can be ignited the diagram presented in Fig. 9.4.9 can be used. To ignite an explosive mixture, a source containing sufficient energy has to be available. Sources can be electrical sparks, hot surfaces, electrostatically loaded materials, or lightning. For the safe operating of a gas extraction and utilization plant, a safety concept has to be developed. It consists of the following steps: • Identification of any dangers regarding the actual or potential development of explosive mixtures and assessment of the risks of an explosion as well as potential effects on the complete plant (danger and risk assessment). • Identification of potential explosion hazards to workers and the environment. • Selection of measures against the identified dangers and risks.
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• Installation of the necessary safety equipment and development of an operation and behavior mode. • Audit the designed and installed safety concept/measures by an authorized expert. Based on the above mentioned mentioned criteria the potential dangers and hazards of an explosion at LFG plants can be identified. Where vacuum pressure occurs (e.g., in the gas pipes from the landfill to the blower) air can be sucked into the pipe/plant and may create explosive mixture inside. In those areas where an overpressure exists (mostly in the area from the blower to the utilization facility or flare) gas may leave the pipe/plant and create explosive mixtures outside the plant. This can especially happen if the leakagede.g., in a pipe connectiondis in a building where the methane can accumulate and where there are potential ignition sources such as the blower or the electrical switch. During the risk and hazard assessment, different scenarios have to be identified where and under which conditions an explosion may occur. Based on the gained results the safety measures have to be developed. The elaborated safety measures can be divided into three categories, which are ordered in a certain hierarchy: Primary measures: all measures to avoid the formation of explosive mixtures (e.g., gas tight pipes, (continuous) monitoring methane and/or oxygen concentration in the gas, inertization of LFG, ventilation of buildings so that no explosive mixtures can build up, etc.). Secondary measures: all measures that avoid that potential ignition sources may become effective (explosion proven machines, switches, etc.). Tertiary measures: all measures that avoid damage after an explosion had happened (casings around blowers and other potential equipment that has the potential to explode). For the development of safety measures, the first step would be to select all the primary measures that are necessary to avoid the development of explosive mixtures. Owing to experiences, not all dangers/risks may be avoided. As additional measures, the following two aspects should be investigated: Locating where potential danger still may occur. • Probability of the existence of a dangerous mixture. This is realized by characterizing areas (zones) where explosive mixtures may occur with a certain probability. A detailed definition can be found in Anon (2015a): • zone 0: where explosive mixtures exist constantly or often, • zone 1: where explosive mixtures exist occasionally, and • zone 2: where explosive mixtures exist seldom and only for a short time. The different zones can be identified using handbooks containing examples (Anon, 2001a). The zones should be marked on a map (map of zones) and posted at the plant. When additional zones are identified during operation, additional measures have to be taken. In any case it has to be avoided that electrical equipment may become an ignition source. Only those electrical installations may be used that meet the requirements for the specific identified zone and the published
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Figure 9.4.10 Tertiary explosion measures in a landfill gas collection scheme (Anon, 2001b). (A) collection pipes, (B) transportation pipe, (C) methane and oxygen concentrations control, (D) flame arrestors, (E) temperature control, (F) blower, (G) high speed valve, (H) flare, (I) flame control.
technical rules. Owing to European law, the manufacturer of a device has to give a declaration of conformity with the required standards and has to equip the product with a label showing the CE sign and the technical standards (Anon, 2015b). If these measures are not sufficient, tertiary measures have to be taken. In that case the plant has to be constructed in a way that even if an explosion happens, the equipment will not be destroyed so that the surrounding areas will stay intact. It is important to prevent that the explosion influences other areas so that a detonation may occur. Beyond many other explosion control measures, it is important that a flame arrestor is installed in the extraction pipedwhere there is a vacuum pressuredbefore and after the blower to prevent a backward and forward ignition. This concept is shown in Fig. 9.4.10. The example in Fig. 9.2.10 shows the following situation: In the landfill collection pipes (A) it has to be expected that air is sucked in. Therefore zone 1 exists. This zone still exists in the transportation pipe (B) until the flare is reached (H). The blower (F) might be an ignition source. Therefore measures have to be taken. Flame arrestors (D) are installed before and after the blower and in front of the flare. The total system up to the flame arrestors is designed for explosion stability. In addition, the flame arrestor has to be temperature controlled (E), and the entire system has to be controlled with regards to methane and oxygen concentrations (C). If the upper explosion limit is reached, the plant will be shut down automatically and the closure of a high speed valve (G) will occur. The plant will also shut down in case the flame control (I) sends a signal that there is no fire. After the installation of all necessary equipment, an audit of the plant by an authorized expert is imperative. The complete safety measures have to be documented together with a description of the plant, the assessments and the measures, the control and audit plan, the servicing and maintenance concept, and the instruction and directive activities and descriptions. All the actions have to be documented in a manual.
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References Anon, 2001a. DGUV Information 213-015, Beispielsammlung Explosionsschutzmaßnahmen bei der Arbeit auf Deponien. Deutsche Gesetzliche Unfallversicherung e.V. (DGUV), Berlin. Anon, 2001b. DGUV Regel 114-004 Deponien. Deutsche Gesetzliche Unfallversicherung e.V. (DGUV), Berlin. Anon, 2002. Erste Allgemeine Verwaltungsvorschrift zum Bundes-Immissionsschutzgesetz (Technische Anleitung zur Reinhaltung der Luft e TA Luft). Carl Heymanns Verlag KG, Köln. Anon, 2013. MWM Technisches Rundschreiben 0199-99-03017/05 DE. Caterpillar Energy Solutions GmbG, Mannheim. Anon, 2015a. Verordnung zum Schutz vor Gefahrstoffen (Gefahrstoffverordnung e GefStoffV), Bundesgesetzblatt I S 49. Bundesanzeiger Verlag, Bonn. Anon, 2015b. Gesetz über die Bereitstellung von Produkten auf dem Markt (Produktsicherheitsgesetz e ProdSG), Bundesgesetzblatt I S 1474. Bundesanzeiger Verlag, Bonn. Anon, 2016a. Lambda Stirling BHKW im Land Brandenburg. Lambda Gesellschaft für Gastechnik mbH, Herten. www. lambda.de. Anon, 2016b. Flox e Brenner für die Energietechnik. e-flox GmbH, Renningen. www.e-flox.de. Bilitewski, B., Härdtle, G., 2013. Abfallwirtschaft, Handbuch für Praxis und Lehre. Springer-Verlag, Berlin, Heidelberg. Bischoff, V., 2008. Deponie Eichelbuck Verwertung von Deponieschwachgas in einer Mikrogasturbinenanlage. Abschlussbericht des Ingenieurbüros Roth & Partner, Karlsruhe. www4.lubw.baden-wuerttemberg.de. Müller, K., Rettenberger, G., 1986. Gasabsauge- und Gasverwertungsanlagen an Mülldeponien e Anleitung zur Entwicklung sicherheitstechnischer Konzepte. BMFT, Bonn.
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INTRODUCTION Landfill gas (LFG) might be a considerable source for energy and should be extracted and used whenever this would be convenient environmentally, technically and economically. About 60e80 m3 of LFG per tonne of wet municipal solid waste (MSW) can be utilized over a period of approximately 15e20 years. Where LFG collection systems have been installed in the most cases, the LFG is used for energy production, but there are also examples where huge amounts of gas are flared. The reason in those cases is often that the cost of electricity in these countries is relatively low, so that the investment and operation of a gas utilization plant is not economical. Also for this reason in several cases, gas extraction and flaring is only practiced due to the financial support by a carbon credit program. This often happens in economically developing countries where there is the noncomprehensible situation that on one side energy is strongly needed and on the other side energy is wasted. Another obstacle may also be the utility companies, whichefor whichever reasonemay not be interested in feeding the electricity produced from LFG into their grit. In other cases there is no grit nearby. In these situations, island solution should be considered where the energy is used on site, e.g., in form of electricity or as a fuel to substitute relative expensive diesel fuel in already existing engines. But it is especially a political task to provide the legal basis to improve this situation and support LFG utilization. As the gas production curve is not constantdincreasing during the operation phase and decreasing in the aftercare phasedit is very important that the capacity of the gas utilization plant can be relatively easy adapted to changing gas extraction rates (see also Chapter 9.1). Otherwise, too high amounts of the gas have to be flared. The kind of utilization depends very much on the specific situation. In many cases the conditions for using LFG on site or in the near vicinity are not existing because there is no energy user. Therefore in most plants the gas is used for producing electricity also due to the fact that especially in industrialized countries the public grid is usually nearby and available. In Europe, the production of electrical energy in gas engines is the most common practice and technical standard.
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The upgrading of LFG natural gas quality may be due to its trace constituents and its content of oxygen and nitrogen is quite costly. This concept may be especially interesting when large amounts of LFG are produced over long periods of time. But there are also smaller treatment plantsdbuilt in containersdin use for upgrading biogas to natural gas quality, but these are not used for LFG yet. It is also technical standard to use LFG for heat production in boilers using burners that are adapted to thedcompared with natural gasdrelatively low methane content. In addition, also the low temperature heat (max. 100 C) from the cooling system of the gas engine can be used. Often the heat cannot be used over the entire year so that the energy production plant is not always running at full capacity. Only in a few cases the gas can be used in a nearby power or industrial plant where the gas is provided via a gas pipeline. Gas storage is in general costly, but there may be in the future more cases where gas engines also are operating mainly during the peak hours where higher revenues from the produced electricity can be gained. This can be achieved by storing the LFG for a couple of hours in gas storage tanks. This concept becomes more and more interesting in countries where a high amount of electricity is produced by wind power plants, whichddue to changing wind conditionsddo not operate constantly. Large gas storage tanks for LFG storage over longer periods (several days or weeks) may not be economical. In general before constructing a gas utilization plant, the technical and economical conditions should be investigated very carefully. But the environmental effects also have to be discussed, because LFG emissions contribute significantly to global warming. REQUIREMENTS As LFG contains trace components (see Chapter 9.1), which may cause damages (e.g., corrosion) in the technical equipment such as gas engines or boilers concentrations of trace constituents should be low. The manufactures provide only a guarantee for their plants if certain limit values of trace gas concentrations will be met (see the following example from one manufacturer) (Anon, 2013): • • • • • • •
total chlorine 0e18 mg/m3, total fluorine 0e9 mg/m3, total chlorine and fluorine 0e18 mg/m3, total sulfur 3e400 mg/m3, total silicon 0e3.5 mg/m3, dust < 0e2 mg/m3, and water vapor < 50% humidity.
Other manufacturers may require different limit values. If those are not met LFG treatment may become inevitable. In addition to the raw LFG, the concentrations of several components in the exhaust gases of flares, gas engines, and boilers are also regulated. These limit values differ from country to country. As an example, some of the threshold values that are required in Germany for reciprocating gas engines are presented (Anon, 2002).
SOLID WASTE LANDFILLING j Concepts, Processes, Technologies j R. Cossu, R. Stegmann
Figure 9.4.1 Unburned methane emissions as CO2 e equivalents dependent on the installed power of a
gas engine (full-scale data). Courtesy of SGS-RUK GmbH
• CO < 1000 mg/m3 if <3 MW (650 mg/m3, if >3 MW) • NOx < 500 mg/m3 (lean gas Otto engine, diesel engine >3 MW); <250 mg/m3 (normal Otto engine); <1000 mg/m3 (diesel engine < 3 MW) • CH2O < 60 (40) mg/m3 • SOx < 350 mg/m3 • Dust < 20 mg/m3 (only diesel engine) In general it should be kept in mind that the operator has the duty to minimize the emissions beyond the target values due to the state of the art. It should be considered that there might be (diffusive) CH4 emissions due to gas leakage; in addition, there may also be unburned methane residues in the exhaust gas of LFG engines. As may be concluded from Fig. 9.4.1 the unburned methane is not neglectable. As a consequence additional requirements to control these emissions in the future may come up. GAS PRETREATMENT, GAS STORAGE TANKS As indicated earlier, treatment of gas may be necessary either for emission control or damage prevention. All treatment methods of course add to the cost of the LFG utilization. In several cases, pretreatment is substituted by changing the oil in the gas engines more often because the oil acts as a sorbent. Dewatering, drying In most cases LFG leaves the landfill at temperatures between 35 C and 45 C. The gas is almost vapor saturated. As mentioned above, the moisture content in the gas should not exceed 50% if the gas is used in gas engines. In many cases this is achieved, e.g., by cooling the gas down to 10 C and heating it up to 20 C. In most plants in moderate or cold climate (in the latter case, freezing of the condensate in the
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Figure 9.4.2 Siphon as a condensate knock-out in a landfill gas pipe. Courtesy of Ingenieurgruppe
RUK GmbH
pipes or other technical devices has to be avoided) there is no separate technical moisture reduction necessary because this temperature change happens “naturally” during gas collection. The situation may be different in, e.g., tropical countries where separate cooling devices may have to be considered. In any case, it is necessary to remove the condensate from the pipes. Therefore passive dewatering units have to be installed. As already mentioned this can be achieved, e.g., by installing a siphon at the pipe system at their lowest points. Fig. 9.4.2 shows an example of a siphon installation emplaced in the landfill body (designed for negative pressure in the pipes of up to 100 hPa). Providing adequate slopes condensate can also be redirected back into the landfill through the gas wells. If additional dewatering is necessary, mostly cooling systems are used. In these facilities, the gas temperature is lowered down to a temperature of around 4 C. A further dewatering action will be necessary if the gas has to be stored under high pressure in gas storage tanks made of steel. This may be necessary if the gas shall be used, e.g., in vehicles or shall be supplied into the public gas grid. In those cases, drying systems have to be used. Often adsorption filter filled with silica pellets or other appropriate sorption materials are used. These systems are operated using the pressure swing technology.
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Figure 9.4.3 Activated carbon filter for landfill gas treatment placed in front of a gas engine. Photo by G. Rettemberger.
Removal of chlorine, fluorine, and silica compounds To protect the gas equipment and meet the requirements of the manufacturers, the removal of chlorine, fluorine (chlorinated, fluorinated hydrocarbons), and organic silica compounds (siloxanes) from the raw gas may become necessary. Quite common is the application of adsorption processes using activated carbon (AC) pellets, which can be loaded with these compounds up to 15% of their original weight. There is a great variety of AC qualities and the contamination of the LFG is also site specific, for this reason, it might be viable to carry out tests to find out the most efficient/less costly material. After reaching the maximum loading capacity, the AC will be transported to a regeneration plant and will be substituted by unloaded AC. This might not be possible in all countries. In these cases the carbon can also be regenerated on site, which takes of course more efforts. When handling loaded AC, safety measures have to be followed. As adsorption systems show a typical breakthrough behavior, it is common to operate 2 filters in sequence. Fig. 9.4.3 shows an AC plant for LFG treatment. Other sorption materials that may be used are molecular sieves and silica gels. Removal of sulfur In general the concentration of hydrogen sulfide in the LFG is rather low. Therefore sulfur removal is in most cases not necessary. But there may be some exceptions especially when significant amounts of construction and demolition waste or other wastes containing gypsum have been codisposed with MSW. In these cases, sulfates will be biologically reduced to H2S. H2S is up to a certain concentration very odorous but cannot be smelled at higher concentrations. In addition, it is very toxic at low
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Figure 9.4.4 Gas storage tank. Photo by G. Rettemberger.
concentrations also and very corrosive. For these reasons the codisposal of biologically degradable waste with gypsum-containing waste has to be strictly avoided. H2S can be biologically oxidized, but for its removal from LFG more often specially coated AC is used. These filters have a high efficiency. But biological processes also have been used in full-scale plants successfully. Gas storage tank As mentioned above, in only a few cases gas storage tanks have been installed at landfills. As the typical pressure range in landfills is between 10 hPa and 30 hPa, membrane gas storage tanks with or without steel casing may be used. Flexible membranes provide varying gas storage volumes. The LFG is supplied to or taken out from the storage facility by pumps. The pressure in the tank can be kept constant by means of an appropriate weight on top of the tank. Otherwise a feeding pump, which provides the necessary pressure for the user is needed. Fig. 9.4.4 shows a gas storage tank made of plastic membrane that is placed in a metal casing.
LANDFILL GAS UTILIZATION TECHNOLOGIES Combined heat and power gas engines LFG is mainly used in gas engines for producing electricity and heat (combined heat and power-CHP plants). The electricity can be supplied into the public grid or may be used on site as an island solution. The low temperature heat (100 C) can be used for heating houses or supplying heat for industrial or commercial purpose (heating green houses, fish ponds, etc.). Often the heat cannot be used constantly
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or at all, in these cases the cooling water of the engine has to be cooled using, in most cases, an air cooler. The advantages of CHP engines are as follows: • Proven and well-developed technology, in general no or only minor LFG treatment necessary, operation of more than 8200 h/a possible. • Electrical efficiency even more than 40%. • Operation with lean LFG down to 30% methane concentration in many cases possible. • In most cases the electricity is supplied into the public grid providing constant supply. • Relatively cost-effective, in some countries price per kWh isddue to state regulationdfinancially subsidized by the utility companies. Gas engines can be operated as stand-alone operation with asynchronous generator (island solution) or with synchronous generator for supplying electricity to the public grid. Gas engines need constant maintenance at different levels (oil exchange, engine inspection, etc.). Under these conditions, gas engines can be operated for more than 10 years (more than 80,000 h). During this period the gas production varies to a high degree (after the operation phase of the landfill, the gas production reduces by more than 60%); the capacity of the gas engine has to be adjusted. This is necessary because the efficiency of the gas engine running at half capacity is significantly decreasing. As a consequence, in the most cases an appropriate solution may be to: • Install several gas engines and add or remove one or more engines due to available gas volumes. • Install gas engines completely in containers as a unit for simple exchange. • Rent gas engines which can be exchanged with engines that have a lower capacity. With these measures it can be avoided that lower gas volumes cannot be used and have to be flared instead (reducing the extraction rate cannot be accepted in any case). Fig. 9.4.5 shows container units and Fig. 9.4.6 presents an installation of a CHP plant in an industrial building. To meet the gas emission standards for exhaust gasesdespecially for NO2dand to increase the electrical efficiency of the engine, in most cases turbo charged lean gas engines are used. In the future decreasing limits for formaldehyde and unburned methane in the exhaust gas can be expected and may ask for additional measures. Lean gas engines need a special gaseair mixing system to operate in a window where relatively low N2O will be produced. In some cases diesel engines are modified so that they can use LFG as a fuel. The disadvantage of this engine type is the necessity to inject continuously diesel of up to 15% of the total fuel consumption into the cylinder. Diesel engines are self-priming; compared with gas spark-ignited engines they can be operated at lower methane concentrations and have a high electrical efficiency. The emissions of N2O are higher in diesel engines, which is taken care of granting higher N2O target values in the exhaust gas. The advantages of diesel engines are more relevant for smaller plants. There are developments on the way to further improve this engine type for LFG utilization.
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Figure 9.4.5 Gas engines built in container. Photo by G. Rettemberger.
Figure 9.4.6 Gas engine in an industrial building. Photo by G. Rettemberger.
Owing to the mostly abandoned locations of most of the landfills, the use of waste heat is limited. Investigations had been done using technologies to convert the heat from the engine cooling system into electrical power. Organic rankine cycle uses organic liquids that vaporize at lower temperatures than water to produce vapor that drives a steam engine, which is connected to a generator. The vapor is cooled and the organic liquid is recovered and used again. Most of the organics that have this characteristic are toxic or not environmental friendly, for this reason synthetic silicon-based organic liquids are under development. This process operated successfully at full scale and needs some further development to be more stable. It may be an option in special situations.
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A proven technology is the so-called Stirling engine that may provide for smaller or older landfills electrical power of 5e8 kWel, which, e.g., may be used for the electricity supply for facilities in the aftercare phase on site. One manufacturer produces only one type (5e8 kWel), which can be bought from the shelf (Anon, 2016a). Gas turbines In a few cases in Germany (there may be more in other countries), gas turbines (20e65 kW) that use LFG have been installed. In the United States also, larger turbines (producing several hundreds kWel) are in operation (Bischoff, 2008). Gas turbines need relative clean gas with very low trace compound concentrations, and the LFG has to be pressurized up to 5 bar. As an advantage that gas turbines can operate with lean gases (>25% CH4), they need only little maintenance and meet exhaust standards quite easily. On the other hand the electrical efficiency compared with gas engines is lower (26%e32%, Bischoff, 2008). Gas turbines could close the gap between CHP engines and lean gas flares for gas utilization of landfills in the aftercare phase. Methane supply in to a public or private gas net For the operation of a private gas net, e.g., in Germany the national building code has to be respected. The operation itselfde.g., to supply a power unit with LFGdrequires removing water/vapor from the gas and to install the pipes in the soil at a depth where freezing does not occur. Of course safety measures have to be fulfilled. Owing to the content of toxic trace compounds in the LFG, the utilization should be restrictive. These gas nets are mainly used to transport the gas to an industrial plant where it is used on site. Owing to the content of trace compounds, it is not allowed in Germany to supply LFG into a public grid (in contrast to biogas from agricultural sources). Internationally there are several plants where LFG is supplied into a national grid. LFG treatment technologies such as molecular sieves as a pressure swing process, the “Selexol” process or membranes have been used successfully to treat the LFG to natural gas quality (Bilitewski and Härdtle, 2013). Fig. 9.4.7 shows a “Selexol” plant, which was operated in California/United States. In general this kind of technology may only be economical at huge landfills under special economical conditions. Burner and boiler The direct burning of LFG to produce heat and steam is quite common and a standard technology. If the LFG gas can be used, e.g., in a plant all year around, the efficiency of the utilization of the energy content in the LFG can be more than 80%e90%, but the economical problem of these kind of plants arises when the LFG cannot be used all year around (e.g., when heat or cold is used). In the latter cases only a certain part of the totally produced gas can be used. In addition, during the period when no or less gas is used, other disposal units (such as flares) have to be installed. Fig. 9.4.8 shows a burner/boiler unit in a garden center for heating green houses in Germany.
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Figure 9.4.7 Landfill Gas treatment for supplying into the national grit using the “Selexol” process in
California/USA. Photo by G. Rettemberger.
Figure 9.4.8 Landfill gas burner in a garden center. Photo by G. Rettemberger.
Using LFG in boilers has also the advantage that there are, e.g., in Germany less strict requirements regarding the raw gas quality, but of course the exhaust gas threshold values have to be met. With special burners, lean gases can also be used down to methane concentrations of 4.5% (Anon, 2016b). It is recommended to make sure that no corrosion will occur inside the boilers; it should also be checked on a routine basis if depositsde.g., caused by siloxanesdat the inside walls of the boiler have been built up.
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Figure 9.4.9 Diagram to assess a methane, air and inert gas mixture due to their explosion potential (Müller and Rettenberger, 1986).
SAFETY MEASURES LFG containing oxygen or getting in contact with air can cause explosive mixtures with considerable effects to human beings as well as to the environment. If deflagration happens explosions can reach a pressure around 7 bar, but much higher pressures occur if a detonation has developed. An explosion may occur if three premises exist as follows: • A combustible gas with good dispersion behavior. • A gas mixture in an explosive condition (between upper and lower explosion limits). • An igniting source. To find out if a gaseair mixture can be ignited the diagram presented in Fig. 9.4.9 can be used. To ignite an explosive mixture, a source containing sufficient energy has to be available. Sources can be electrical sparks, hot surfaces, electrostatically loaded materials, or lightning. For the safe operating of a gas extraction and utilization plant, a safety concept has to be developed. It consists of the following steps: • Identification of any dangers regarding the actual or potential development of explosive mixtures and assessment of the risks of an explosion as well as potential effects on the complete plant (danger and risk assessment). • Identification of potential explosion hazards to workers and the environment. • Selection of measures against the identified dangers and risks.
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• Installation of the necessary safety equipment and development of an operation and behavior mode. • Audit the designed and installed safety concept/measures by an authorized expert. Based on the above mentioned mentioned criteria the potential dangers and hazards of an explosion at LFG plants can be identified. Where vacuum pressure occurs (e.g., in the gas pipes from the landfill to the blower) air can be sucked into the pipe/plant and may create explosive mixture inside. In those areas where an overpressure exists (mostly in the area from the blower to the utilization facility or flare) gas may leave the pipe/plant and create explosive mixtures outside the plant. This can especially happen if the leakagede.g., in a pipe connectiondis in a building where the methane can accumulate and where there are potential ignition sources such as the blower or the electrical switch. During the risk and hazard assessment, different scenarios have to be identified where and under which conditions an explosion may occur. Based on the gained results the safety measures have to be developed. The elaborated safety measures can be divided into three categories, which are ordered in a certain hierarchy: Primary measures: all measures to avoid the formation of explosive mixtures (e.g., gas tight pipes, (continuous) monitoring methane and/or oxygen concentration in the gas, inertization of LFG, ventilation of buildings so that no explosive mixtures can build up, etc.). Secondary measures: all measures that avoid that potential ignition sources may become effective (explosion proven machines, switches, etc.). Tertiary measures: all measures that avoid damage after an explosion had happened (casings around blowers and other potential equipment that has the potential to explode). For the development of safety measures, the first step would be to select all the primary measures that are necessary to avoid the development of explosive mixtures. Owing to experiences, not all dangers/risks may be avoided. As additional measures, the following two aspects should be investigated: Locating where potential danger still may occur. • Probability of the existence of a dangerous mixture. This is realized by characterizing areas (zones) where explosive mixtures may occur with a certain probability. A detailed definition can be found in Anon (2015a): • zone 0: where explosive mixtures exist constantly or often, • zone 1: where explosive mixtures exist occasionally, and • zone 2: where explosive mixtures exist seldom and only for a short time. The different zones can be identified using handbooks containing examples (Anon, 2001a). The zones should be marked on a map (map of zones) and posted at the plant. When additional zones are identified during operation, additional measures have to be taken. In any case it has to be avoided that electrical equipment may become an ignition source. Only those electrical installations may be used that meet the requirements for the specific identified zone and the published
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Figure 9.4.10 Tertiary explosion measures in a landfill gas collection scheme (Anon, 2001b). (A) collection pipes, (B) transportation pipe, (C) methane and oxygen concentrations control, (D) flame arrestors, (E) temperature control, (F) blower, (G) high speed valve, (H) flare, (I) flame control.
technical rules. Owing to European law, the manufacturer of a device has to give a declaration of conformity with the required standards and has to equip the product with a label showing the CE sign and the technical standards (Anon, 2015b). If these measures are not sufficient, tertiary measures have to be taken. In that case the plant has to be constructed in a way that even if an explosion happens, the equipment will not be destroyed so that the surrounding areas will stay intact. It is important to prevent that the explosion influences other areas so that a detonation may occur. Beyond many other explosion control measures, it is important that a flame arrestor is installed in the extraction pipedwhere there is a vacuum pressuredbefore and after the blower to prevent a backward and forward ignition. This concept is shown in Fig. 9.4.10. The example in Fig. 9.2.10 shows the following situation: In the landfill collection pipes (A) it has to be expected that air is sucked in. Therefore zone 1 exists. This zone still exists in the transportation pipe (B) until the flare is reached (H). The blower (F) might be an ignition source. Therefore measures have to be taken. Flame arrestors (D) are installed before and after the blower and in front of the flare. The total system up to the flame arrestors is designed for explosion stability. In addition, the flame arrestor has to be temperature controlled (E), and the entire system has to be controlled with regards to methane and oxygen concentrations (C). If the upper explosion limit is reached, the plant will be shut down automatically and the closure of a high speed valve (G) will occur. The plant will also shut down in case the flame control (I) sends a signal that there is no fire. After the installation of all necessary equipment, an audit of the plant by an authorized expert is imperative. The complete safety measures have to be documented together with a description of the plant, the assessments and the measures, the control and audit plan, the servicing and maintenance concept, and the instruction and directive activities and descriptions. All the actions have to be documented in a manual.
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References Anon, 2001a. DGUV Information 213-015, Beispielsammlung Explosionsschutzmaßnahmen bei der Arbeit auf Deponien. Deutsche Gesetzliche Unfallversicherung e.V. (DGUV), Berlin. Anon, 2001b. DGUV Regel 114-004 Deponien. Deutsche Gesetzliche Unfallversicherung e.V. (DGUV), Berlin. Anon, 2002. Erste Allgemeine Verwaltungsvorschrift zum Bundes-Immissionsschutzgesetz (Technische Anleitung zur Reinhaltung der Luft e TA Luft). Carl Heymanns Verlag KG, Köln. Anon, 2013. MWM Technisches Rundschreiben 0199-99-03017/05 DE. Caterpillar Energy Solutions GmbG, Mannheim. Anon, 2015a. Verordnung zum Schutz vor Gefahrstoffen (Gefahrstoffverordnung e GefStoffV), Bundesgesetzblatt I S 49. Bundesanzeiger Verlag, Bonn. Anon, 2015b. Gesetz über die Bereitstellung von Produkten auf dem Markt (Produktsicherheitsgesetz e ProdSG), Bundesgesetzblatt I S 1474. Bundesanzeiger Verlag, Bonn. Anon, 2016a. Lambda Stirling BHKW im Land Brandenburg. Lambda Gesellschaft für Gastechnik mbH, Herten. www. lambda.de. Anon, 2016b. Flox e Brenner für die Energietechnik. e-flox GmbH, Renningen. www.e-flox.de. Bilitewski, B., Härdtle, G., 2013. Abfallwirtschaft, Handbuch für Praxis und Lehre. Springer-Verlag, Berlin, Heidelberg. Bischoff, V., 2008. Deponie Eichelbuck Verwertung von Deponieschwachgas in einer Mikrogasturbinenanlage. Abschlussbericht des Ingenieurbüros Roth & Partner, Karlsruhe. www4.lubw.baden-wuerttemberg.de. Müller, K., Rettenberger, G., 1986. Gasabsauge- und Gasverwertungsanlagen an Mülldeponien e Anleitung zur Entwicklung sicherheitstechnischer Konzepte. BMFT, Bonn.
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