Analysis of the Efficiency of Landfill Gas Treatment for Power Generation in a Cogeneration System in Terms of the European Green Deal
2. Object of Research
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
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.
4.2. Research on Activated Carbon
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.
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.
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.
4.3. Hydrogen Sulfide Removal Efficiency
—effectiveness of hydrogen sulfide reduction (%),
HSp—value of the parameter in treated biogas (ppm),
HSu—value of the parameter in untreated biogas (ppm).
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.
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.
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.
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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