Analysis of the Efficiency of Landfill Gas Treatment for Power Generation in a Cogeneration System in Terms of the European Green Deal

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Analysis of the Efficiency of Landfill Gas Treatment for Power Generation in a Cogeneration System in Terms of the European Green Deal


1. Introduction

The economic activities of man over the past centuries, including industrial development and the extraction and use of the Earth’s energy resources, have resulted in visible climate change and progressive environmental degradation. As a result, measures are needed to transform the economies of individual EU countries into modern, resourceful, economical and competitive economies, aiming for zero net greenhouse gas emissions by 2050. These actions included in the European Green Deal (EGD) [1] include, among other things, energy transformation and moving toward a closed-loop economy, including in the municipal waste sector [2,3]. Reducing the generation of municipal waste is a major challenge for individual countries, while the ways to manage it challenge technologists in the processes of its treatment and disposal [4,5]. These processes must be implemented in accordance with the waste hierarchy, within which landfilling is the least desirable process [6]. Modern legal requirements in the field of municipal waste management force the need for comprehensive measures to implement a closed-loop economy that is fully scalable and can respond to current raw material needs [7,8]. The legal acts of the European Union, being the directives and the conclusions on Best Available Techniques (BATs) for municipal waste, define measures to prevent and reduce waste generation, along with recycling [9,10].
Municipal waste collected for disposal in landfills undergoes physical and chemical processes throughout the period of deposition. The result of these processes is the “products of the landfill”, among other things, the formation of wastewater, which, captured using drainage, is discharged and treated before it enters sanitary sewer facilities [11]. Proper water and wastewater management during the operation of a landfill prevents the migration of wastewater into the ground layer and surface- and groundwater, which consequently reduces the negative environmental impact of the landfill as a facility [12,13]. Another “product” is landfill gas (LFG), which is formed during anaerobic digestion of the biodegradable fraction in a bed of waste subjected to appropriate compression. LFG, containing large amounts of methane and carbon dioxide, is a source of fugitive emissions from the landfill surface into the atmospheric air, which are greenhouse gases that contribute to climate change on Earth [14]. Therefore, a key issue during the operation of landfills where bio-waste has been stored is to reduce the air emissions of the biogas generated [15,16].
One effective way to reduce and control LFG emissions is to perform landfill degassing, capture the gas and use it for energy purposes, provided it has an optimal calorific value [17,18]. The use of landfill biogas as a fuel for energy generation requires a detailed analysis in terms of quality parameters and the efficiency of the thermodynamic processes carried out [19,20]. Examples of the capture and energy management of LFG, mainly focus on the combustion of gas in gas engines integrated with combined heat and power (CHP) systems [21,22]. This way of managing the gas guarantees the optimal use of the chemical energy contained in LFG, the products of which are electricity and heat, produced in a combined system [23,24]. Since raw LFG sourced directly from the landfill contains harmful impurities, including hydrogen sulfide, ammonia and silicon compounds, it is required to undergo a purification process as a target gas fuel [25,26]. The LFG cleaning techniques used throughout the world are mainly wet and dry technologies, which effectively reduce contaminants and allow gas engines to operate properly. Dry technology, which is increasingly used in LFG conditioning plants, is the use of activated carbon to remove mainly sulfur and silicon compounds [27,28].
The efficiency of removal of compounds contaminating the gas mainly depends on the size of the compounds and the moisture content. Therefore, in installations with high gas moisture content, intermediate heating and cooling devices are used before feeding the gas to the gas engine [29,30]. The most commonly used method for drying biogas is condensation dehydration by gravity or using waste heat from the engine cooling block, providing a lower heat source for the heat pump. The cooled biogas is directed to the next point to avoid further condensation. In most biogas plants, a water condenser is set at the lowest point in the pipeline to avoid condensation [31,32]. The process by which hydrogen sulfide is removed from biogas is carried out mainly to prevent corrosion of equipment involved in the gas transmission process, to protect the mechanical parts of the gas engine and to reduce the sulfur compounds emitted into the atmosphere in the flue gas [33,34]. LFG generated in the landfill waste bed is a renewable source of energy, while electricity and heat that are the product of cogeneration are counted from the so-called “green energy”, which is part of the closed-loop economy and green energy transition [35,36]. In recent years, the European Union has been placing a heavy emphasis on the energy transition, pointing to the need for intensive development of renewable energy generation technologies, reducing the use of fossil fuels [37,38].
The European Green Deal includes initiatives in a number of closely related areas such as climate, environment, energy, transportation, industry, agriculture and sustainable financing [39,40]. The implementation of this program envisages a thorough energy transformation implemented through investments in renewable energy sources to achieve EU energy independence and security of energy supply [41]. Projects and investments aimed at curbing climate change as a result of reducing fugitive methane emissions into the air from landfill surfaces are part of these principles. Capturing landfill gas and purifying it of sulfur, silicon, ammonia, chlorine and ammonia compounds before it undergoes combustion reduces the amount of gas and dust emissions into the air as a result of its conversion into energy [42]. The revised Renewable Energy Promotion Directive [43] sets a target for the EU of 32% in 2030, which was agreed upon by EU leaders and represents some compromise between the European Parliament and some member states [44]. Further developments in this regard include new support mechanisms, a regulatory framework adapted to the actual consumption of the energy sector and precisely defining sustainability criteria for biofuels and bioenergy [45]. Of particular importance is the fact of support for small-scale installations using, for example, landfill gas, which are key to increasing public acceptance and ensuring the implementation of renewable energy projects, particularly at the local level.

2. Object of Research

The object of this study was a biogas extraction, purification and energy utilization facility located at the landfill. LFG is extracted from the landfill’s vertical and horizontal degassing wells by means of a gas suction device with a capacity of up to 200 m3 per hour and is burned in a gas engine which, in a cogeneration system, drives a generator with an electrical output of 320 kW. Annually, the installation extracts about 1.2 million m3 of biogas from the landfill, which, before being fed into the gas engine, is purified mainly from sulfur and silicon compounds in a filter containing activated carbon with a capacity of 1.5 cubic meters. The installation for the energetic use of landfill gas, including measurement points, is shown in Figure 1.

Landfill gas is extracted from the waste bed using a gas squeegee installed at the biogas extraction station and is then subject to regulation by a maintenance worker with manual valves using rotameters and measurement at point (1). Subsequently, the gas undergoes primary treatment with a particulate filter, is metered using a gas flow meter and is subject to quality measurements at point (2). Secondary purification of biogas is carried out in an activated carbon filter in which reduction takes place, among other things, of sulfur compounds, and measurement of biogas quality parameters is carried out at measuring point (3). The purified biogas is thus directed as gaseous fuel to a reciprocating engine that drives an electric generator.

Electricity produced at the generator is metered with a gross energy meter (4) at the generator terminals and is primarily used for the facility’s own needs, while excess electricity is sent to the external power grid and metered with a two-way electricity meter (5). Heat recovered from the cooling of the engine block and from the exhaust gas heat exchanger is recovered in the main heat exchanger in the form of hot water. The heat is used for social purposes and technological processes and is metered with a heat meter (6).

4. Results and Discussion

4.1. Landfill Gas Research

In the case of operating a distributed renewable energy source such as landfill gas as a fuel for cogeneration in a cogeneration unit, a key issue is the quality of the gas. Therefore, in the first measurement campaign, the results of measurements of untreated (raw) gas, representing a total of 42 measurements taken as part of six series of seven measurements every 5 days, were analyzed. Table 1 shows the basic parameters of the “raw” untreated gas obtained directly from the landfill as averages of the six-measurement series.
Performed at the landfill gas energy utilization facility, measurements of basic gas parameters during the first measurement campaign showed a high methane content averaging 58.2 [%] as the main fuel for the gas engine, which is a good prognosis for this gas fuel. Figure 2 shows the interrelationship of methane and hydrogen sulfide as a result of the first measurement campaign and analysis.

The high hydrogen sulfide content, averaging 593.8 [ppm], disqualifies LFG as a fuel for the gas engine powering the cogeneration unit. This raises the need to treat the landfill gas to reduce the hydrogen sulfide content to a maximum of 100 [ppm], as recommended by the equipment supplier. Therefore, it was decided to apply dry landfill gas treatment technology in the form of a filtration device using activated carbon. The proposed filter material is molded impregnated activated carbon with enhanced adsorption properties for hydrogen sulfide. The carbon is produced by steam activation from selected grades of anthracite to ensure consistent quality and a high degree of hardness and is then impregnated with chemicals to achieve optimal chemisorption properties.

The efficiency of removal of siloxanes in biogas using an activated carbon filter was carried out by [47,48], comparing the efficiency with the gas engine manufacturer’s data. The results of the obtained tests showed more than 98% removal efficiency of organic sulfur compounds and thus the lowest level of hydrogen sulfide at the biogas outlet. Activated carbon also showed high removal efficiency of silicon compounds including siloxanes at 95%. The high efficiency of the process of reducing sulfur compounds and siloxanes contained in biogas, using activated carbon, was confirmed in their study [49,50], performing tests on 12 different types of activated carbon. The results showed that the reduction in carbon adsorption capacity in biogas purification is affected by the level of relative humidity, as well as the percentage of methane in the LFG. Industry consultation and a literature analysis confirmed the high efficiency of hydrogen sulfide removal from the gas, which is a good recommendation for this solution.

4.2. Research on Activated Carbon

In order to check the surface and grain condition of the “fresh” activated carbon, tests were performed using a scanning electron microscope with an EDS system in order to have comparative material in the future. Figure 3 shows the state of the surface and grains of the new activated carbon at magnifications of 100, 500, 1000 and 2000 times.

The test results revealed the high quality of the adsorption material, which is the proposed activated carbon. The coal used does not contain dust impurities that often occur in low-quality coal, which adversely affects the gas purification process.

In order to check the elemental composition of the “fresh” activated carbon, with a perspective of further studies of the adsorption rate of sulfur compounds planned in the future, tests and elemental balances were carried out using a scanning electron microscope with an EDS system. Figure 4 shows the surface condition of the washed activated carbon sample for a magnification of 500 times, along with the chemical composition for the three points analyzed.

The test results showed that activated carbon as a porous material has been used for several years in the purification of propellant and waste gases. The activated carbon used is characterized by optimal physicochemical properties. It has a very large specific surface area and a large pore volume. From the point of view of the processes occurring at the solid–gas interface, the surface area available to the gas molecules, it includes both its outer surface and the inner surface active in the adsorption process.

The elemental composition shown during the test shows the highest carbon content as the main medium where excess substances accumulate. The size of the adsorber’s specific surface favors the accumulation of particles due to the action of physical and chemical forces. Figure 5 shows the percentage elemental composition of activated carbon at the three points analyzed.
Analysis of the composition of the activated carbon sample showed an average elemental content of carbon of 92.89%, while the other elements accounted for 7.11% including an average content of sulfur of 0.25% and silicon of 0.96%. The research confirmed the correct elemental composition of activated carbon, which was used in the landfill gas treatment installation. Figure 6 presents the results of the analysis in the form of a dendrogram, or binary tree, for the elemental balance of activated carbon used in the landfill gas treatment plant.

Analysis of the results of the study revealed that the objects form clusters resulting in four main groupings (concentrations). The first contains calcium and sulfur, the second includes clusters of aluminum and iron, the third includes oxygen and silicon content, while the fourth involves carbon content. The smallest bond distance occurs at the level of the first cluster for the agglomeration distance (x = 1.0; y = 6.49), while within the second cluster, it is the distance (x = 2.0; y = 4.50) between aluminum and iron. The third cluster has parameters (x = 3.0; y = 1.52), while the largest distance is in the fourth cluster for carbon of (x = 5.0; y = 4.25). All clusters are connected by a bond with parameters (x = 6.0; y = 2.62). This state of bonds within the dendrogram indicates the highest content and carbon in the samples and the lowest contents of sulfur, calcium and silicon, which proves the very good adsorption parameters of activated carbon used in the landfill gas treatment plant.

The measurement campaigns No. 2 and No. 3, performed in the study, had as their main objective to check the effectiveness (efficiency) of hydrogen sulfide removal from LFG by the activated carbon bed in order to reduce the value of hydrogen sulfide to the permissible value specified by the equipment supplier to 100 ppm. Table 2 shows the results of measurements of LFG parameters before entering the gas purification device and after the gas purification process before feeding into the gas engine.
After the landfill gas treatment process, its basic parameters, such as methane, carbon dioxide and oxygen, underwent little change, confirming the proper operation of the filtration unit and its tightness. On the other hand, the hydrogen sulfide content of the LFG was significantly reduced, confirming the proper response of the activated carbon in contact with sulfur compounds. The average hydrogen sulfide content in the treated gas was 16.8 ppm, while the average content of the treated gas was 566.1 ppm. Figure 7 shows the reciprocal relationship of methane and hydrogen sulfide for the purified gas, which is the result of the third measurement campaign and the analyses performed.

4.3. Hydrogen Sulfide Removal Efficiency

In order to determine the degree of reduction in the parameter (hydrogen sulfide removal efficiency) by activated carbon from LFG, calculations were made using expression (1) for six measurement series in campaigns No. 1 and No. 2.

η H S = H S u H S p H S u · 100 %

where

η H S —effectiveness of hydrogen sulfide reduction (%),

HSp—value of the parameter in treated biogas (ppm),

HSu—value of the parameter in untreated biogas (ppm).

The resulting values are summarized in Table 3 and represent average values for each measurement series.

Calculations based on the results of the second and third measurement campaigns showed an average hydrogen sulfide removal efficiency of 97.05%, which confirms the practical effectiveness of using activated carbon in a landfill gas treatment plant. In order to maintain high hydrogen sulfide removal efficiency, the parameters of landfill gas should be monitored on an ongoing basis, including, in particular, its relative humidity and temperature. The average annual stream of landfill gas feeding the cogeneration installation that was the object of the research was 150.35 m3 per hour. The pressure of gas injected into the engine was on average 125 mbar per year. The above parameters affect the efficiency of hydrogen sulfide removal. This mainly concerns the gas stream that flows through the purification device, which determines the capacity of the activated carbon charge and the efficiency of hydrogen sulfide adsorption.

The effect of relative humidity on the adsorption process of activated carbon, used to remove hydrogen sulfide compounds contained in biogas, was analyzed in their work [51,52]. The results showed that an increase in relative humidity in the range of 60 to 80% causes the adsorption capacity of activated carbon to begin to decrease, which is due to the interference of water directly into the pores of activated carbon. The second parameter affecting the removal rate of contaminants in biogas is temperature. When the operating temperature of the gas treatment plant increases, the adsorption capacity of the bed shows a decreasing trend. The optimal adsorption capacity of the bed was obtained in the temperature range from +5 °C to +20 °C [53,54]. Measurements of the relative humidity and temperature of the biogas are provided by the LFG monitoring system as part of the plant’s operation at the landfill. The average annual relative humidity of the gas was 50.7%, while the average annual gas temperature was 12.7 °C. The humidity parameters testify to the efficient operation of the landfill’s leachate removal system, which excludes flooding of degassing wells and dehydrators in the waste bed. On the other hand, the gas temperature is directly related to the temperature prevailing in the waste bed where biogas is produced [55,56]. The maintenance of optimal gas temperatures is due to the high compression (compaction) of the waste by the machine (compactor) used to evenly spread and compact the waste on the surface of the landfill. Figure 8 shows the reciprocal correlation between hydrogen sulfide removal efficiency and hydrogen sulfide content resulting from the second and third measurement campaigns.

Measurements of hydrogen sulfide content in landfill gas and calculations of its reduction efficiency presented in this paper demonstrated that LFG can be reported in a cogeneration installation as a gaseous fuel for eclectic power and heat generation. The permissible values of hydrogen sulfide specified by gas engine manufacturers in the technical documentation by the manufacturer meet the relevant requirements and lie well below 100 ppm, amounting to 16.6 ppm. Such a condition ensures proper and reliable operation of the cogeneration unit and confirms the correctness of the decision to choose activated carbon as a material for reducing the hydrogen sulfide content in landfill gas intended for energy use.

In quantitative terms, representing reductions in hydrogen sulfide with the use of activated carbon in the LFG treatment plant, the values obtained directly correlate with the European Green Deal requirements for the reduction in particulate and gaseous pollutants to ambient air. The results of the measurement series for hydrogen sulfide expressed in mg per cubic meter, along with the reduction value, are shown in Figure 9.

Conversions from ppm to mg were performed for a cubic meter of LFG for hydrogen sulfide, which allowed calculating the average annual reduction in hydrogen sulfide in the case of gas treatment using activated carbon, with an average reduction efficiency of 97.05%. Assuming the actual conditions for LFG, it was calculated that the annual reduction in H2S in activated carbon during the energetic use of gas in the amount of 1.2 million m3∙year−1 might be 929.48 kg on an annual basis. Taking into account the gradual decrease in the efficiency of hydrogen sulfide reduction over the year, it is reasonable to assume that the target reduction may oscillate around 650 kg. Adsorption of hydrogen sulfide in activated carbon results in the reduction in sulfur oxides into the air in the fuel gas, which, in a situation where landfill CHP plants are not equipped to reduce these compounds, activated carbon plays a dual role in this process. Firstly, it provides a high-quality fuel for the CHP unit, and secondly, it reduces organized emissions resulting from the combustion of landfill gas in a gas engine as an energy source.

The use of landfill gas as biofuel in cogeneration units producing electricity and heat requires a detailed analysis in terms of quantitative and qualitative parameters, including the selection of the optimal treatment technology. This form of obtaining and managing biogas contributes to the use of a source of renewable energy, diversification of energy carriers and reduction in gas and dust emissions into the air resulting from the combustion of LFG.

5. Conclusions

The EU directive on the use of renewable energy, including biogas, sets a target for the EU of 32% of energy from renewable sources by 2030. These assumptions are in line with the European Green Deal, which defines sustainability criteria for the energy transition. The capture and use of biogas produced in a landfill whose main component is methane, for energy purposes, is a dedicated way to reduce its negative impact on the environment. Considered in two aspects: energetically, it represents the optimal use of a renewable fuel for energy generation, and environmentally, it represents the abandoned emission of the greenhouse gas methane into the environment. A key aspect in this process is the preparation of biofuel of sufficient quality to power a gas engine operating in a cogeneration unit. The high content of hydrogen sulfide in the tested landfill gas, amounting to an average of 593.8 [ppm], disqualifies LFG as a fuel for the gas engine powering the cogeneration unit. Therefore, the installation operator is forced to purify the biogas in order to reduce the hydrogen sulfide content to a maximum of 100 [ppm], recommended by the equipment supplier. The molded and at the same time impregnated activated carbon used in the biogas purification plant has enhanced adsorption properties for hydrogen sulfide and acidic compounds, as confirmed by tests conducted in this paper. The test results of the activated carbon used showed a high carbon content in the tested sample of 92.78%, while the efficiency of removing hydrogen sulfide from landfill gas by activated carbon, calculated on the basis of measurements, was 97.05%. In order to properly operate landfill gas treatment plants, systematic quality control of activated carbon in the treatment plant is required to ascertain its adsorption capacity. The research results are dedicated to operators of cogeneration installations in order to systematically test the quality of landfill gas and the efficiency of biogas purification devices, which constitute the basis for the reliable operation of gas engines in cogeneration installations and are dedicated mainly to the operators of these installations. Currently, renewable gaseous fuels, such as biogas, biomethane and bio-hydrogen, being a “green” source of energy, in the near future will play an increasingly important role in the energy sectors of individual countries oriented to the so-called bio-economy, reducing the negative impact of energy processes on the environment.


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