Multifaceted Analysis of Landfill Gas Use for Energy Purposes

Abstract

This paper assesses the use of gas from a landfill located in Kiełcz for energy purposes, considering energy, environmental and socioeconomic aspects. The literature review contains information on waste management methods in various regions of the world and the legal acts regulating them. The reference was also made to the methods of degassing landfills and the methods of reducing pollutants from this process. The research methodology describes, among other things, the method of measuring exhaust gas quality and temperature or gas composition. This section also presents the catalyst that was used as a reducer of flue gas emissions. The use of a catalyst in the flue gas duct allowed for the reduction in CO emissions by 85%, NO_X by 53%, SO₂ by 95%, and PM by 82%. For the socio-economic analysis, formulas and quantities were given that allow for the parameterization of profits. Three leading indicators were selected, i.e., the impact of the energy produced on the producer, the reduction in greenhouse gases, and the responsibility for the product. The section of results graphically presents the efficiency of using the catalyst. This part also contains a summary table of the socioeconomic analysis on the basis of which the social profit index SROI was determined, which amounted to 7.57. This analysis may be helpful for landfill managers/owners who may consider the potential commercial use of landfill gas for energy purposes, as well as for governments, which, amid the current global fuel crisis, can benefit from this analysis and include the use of landfill gas in their energy transition strategies.

1. Introduction

Annually, about 2.01 billion tons of waste are produced in the world. One person’s daily amount of waste varies between 0.11 and 4.54 kg. It is estimated that by 2050, with a doubling of the population, the amount of waste worldwide will increase to approximately 3.4 billion tons [1,2].

The country with a characteristic way of waste management is the United States. It is a highly developed country and, at the same time, belongs to the group of the largest waste producers. Half of the municipal waste management in this country is based on landfilling, and the percentage of waste recycled in 2000–2018 is relatively stable and oscillates between 28–35% [3]. In this country is located the largest waste landfill in the world—Apex Regional, with an area of 890 ha [2]. The regions with different waste management are countries belonging to the Association of Southeast Asian Nations (ASEAN). Indonesia is the largest waste producer there, producing 64 million tonnes of municipal waste annually. The dominant trend in these countries is transferring waste to open landfills and incineration in open spaces. The raw material recycling of waste containing valuable and desired secondary raw materials on the production market is popular, but mainly in an unorganized and ineffective form. ASEAN countries have established a framework and strategies for waste management corresponding to the broadly understood sustainable development and global policy on climate change. The Association of Southeast Asian Nations is struggling with the rapid increase in the growth of waste amount, which determines the development of technologies for its effective treatment [4].

As for the area of Europe, in 2010, the European Union implemented Directive 2008/98/EC on waste. The document set out the legal framework for waste treatment to emphasize the importance of the recovery, recycling, and more intensified re-use of raw materials. The directive contains provisions on the increased responsibility of waste producers and the need to develop a waste management plan for each member state. The directive also sets out the recycling levels to be achieved by 2020. Household waste was to be recycled at a rate of 50% and construction and demolition waste at 70% [5].

At the beginning of July 2020, the new Directive (EU) 2018/851 entered into force. It was introduced as part of the circular economy package. The current legal act defines the minimum operational requirements for extended producer responsibility schemes. The introduction of the new directive resulted in stricter provisions for waste generation. Sustainable production and consumption patterns are currently being supported, as well as the reuse of waste and the creation of repair networks. The directive requires the promotion of recycling and the reuse of products containing critical raw materials, which are the most important materials needed by the modern economy. These activities are to prevent the formation of waste from these materials. The directive currently in force also specifies new weight levels for recycling municipal waste. By 2025, it is to be 55% of municipal waste; by 2030, the level is to increase to 60%, and by 2035, it is to reach the level of 65%. In addition, by 2025, EU Member States are to introduce a selective collection of hazardous waste in households. In addition, from the end of 2023, biowaste must be collected selectively or recycled at the site of its production [6].

The goals adopted in the legal acts are to reduce the negative impact of waste on the environment. One of the threats is fires in landfills. The most common cause is the biological and chemical reactions of the waste, the product of which is landfill gas consisting of approximately 40% carbon dioxide and 60% methane, which is a combustible gas [7]. Landfill gas differs in composition depending on the waste it is generated from and external factors. The methane content may be in the range of 35–75% [8]. In 2021, studies were carried out on the composition of gas obtained at Polish municipal waste landfills. The highest average methane yield in the studied gases was 60%, carbon dioxide 37.9%, and nitrogen 2.1% [9]. Methane generated in landfills is also responsible for the degradation of the Earth’s atmosphere. Due to its emission, the air quality deteriorates, directly impacting the increase in the greenhouse effect. Moreover, methane harms plant vegetation. The top layer of wild and controlled landfills is covered with various vegetation types. The landfill gas released from the waste limits the access to oxygen in the root zone, disrupting the proper vegetation of plants [10]. As an outdated form of waste management, a landfill imposes the necessity to use it in the most useful way possible.

Degassing systems allow the extraction of gas from the landfill. Depending on the use of the obtained gas, two types of systems can be distinguished (passive and active). The passive system consists mainly of a flare where gas is burned without heat recovery or energy production. On the other hand, the active degassing system is focused on generating electricity and heat (e.g., using motors coupled with electricity generators). Gas extraction requires a system consisting of vertical or horizontal wells. The type of well depends on the age of the landfill—horizontal wells are used in new quarters, where waste is accepted, and vertical wells are used in already-closed landfills. The entire active degassing system consists of installing gas transmission pipes, a process control system, suction fans, a gas engine coupled with a generator, and an emergency combustion flare. The flares, which are the basis of the passive system, can be divided into structures with an open combustion chamber and a closed chamber. Open flares are cheap and uncomplicated, with an estimated efficiency of approximately 50%. Closed flares can achieve an efficiency of 98% [11].

The obtained landfill gas after the purification process can be used for energy purposes. An example of such a solution is the Waste Utilization Plant in Gdańsk-Szadółki. The active degassing installation was built there at the municipal landfill site. Landfill gas was obtained from 26% of the enclosed area, consisting of 63% methane, 36% carbon dioxide, and trace amounts of water vapor. After several expansions of the landfill degassing system at the Utilization Plant in Gdańsk from 2011–2017, gas recovery amounted to nearly 35 million m³, of which in 2017 alone it was over 7 million m³ of landfill gas. The generated electricity was allocated to the current demand of the plant, and the surplus was sent to the power grid [12,13].

Until 2018, 68 landfills were operating in Poland, producing 84.8 TJ of thermal energy and 105.4 GWh of electricity annually from the thermal conversion of landfill gas [14]. By comparison, three landfills in India, Ghazipur, Bhalswa, and Okhla, have a heat generation potential of 5783.62 TJ [15]. The energy potential of municipal and industrial organic waste is 5690 MW. Currently, installations with a total capacity of 0.2 GW are in operation, but an extension is planned, thanks to which, the total capacity of the installation will reach 0.5 GW [16].

In order to clean the exhaust gases resulting from the combustion of liquid and gaseous fuels, catalysts are used, i.e., substances that influence the course of a chemical reaction. By adding them to the system, you can most often observe an increase in the speed of the reaction.

The exhaust gases from the thermal conversion contain significant amounts of carbon monoxide (CO), nitrogen oxides (NO_X), polycyclic aromatic hydrocarbons (PAH), and volatile organic compounds (VOC). Research conducted at Wroclaw University of Environmental and Life Sciences proves that using catalysts influences the reduction in the harmful substances’ emissions into the atmosphere [17]. In order to diminish the concentration of individual pollutants contained in the flue gases, active substances with reducing and oxidizing properties are used. In the case of non-selective catalytic reduction, very active catalysts are transition metal oxides deposited on silicon, aluminum, or aluminosilicates and are reduced by the reaction mixture. Low oxygen content in the exhaust gas effectively reduces NO_X to N₂ [18]. Selective catalytic reduction is used by SCR catalysts, which reduce the concentration of nitrogen oxides, converting them into nitrogen and oxygen [19]. The SCR method is used on a large scale, including in diesel engines [20].

In order to analyze the viability of using landfill gas for energy purposes, a helpful tool is the SROI (Social Return on Investments) indicator. This indicator can be defined as the ratio of social incomes in a given period to the incurred investment expenditures. The analysis formed using the SROI indicator allows for effective investment planning, considering the impact on society. This translates into predicting the effects of introducing a given idea to the market, with the possibility of overcoming some barriers on the way to its implementation. In order to prepare an appropriate analysis, it is necessary to identify all factors operating in the studied area and then select the most important ones and present the results in a monetary form while maintaining the transparency of operations. A similar analysis has already been carried out for the energetic use of biomass of agricultural origin [21]. However, for the combustion of landfill gas, no one has carried out such an analysis, and even more so for a plant equipped with an exhaust gas cleaning system. Introducing the social aspect to the analysis enables people interested in using this raw material as fuel to explore the subject more widely and also illustrates the impact of this type of investment on the local society, which, through its attitude, can play a significant role in its development.

The following paper consists of four parts. Section 1 is a literature review containing information on waste management methods in various regions of the world and the legal acts regulating them, ways of degassing landfills, and the purification of exhaust gases from this process. Section 2 refers to methods of measuring exhaust gas quality and temperature and gas composition. It also contains a methodology of socio-economic analysis. Section 3 presents the obtained research results. Section 4 summarizes the research outcomes and presents the future plans of the research team for a follow-up study in this field.

2. Materials and Methods

2.1. Description of the Test Stand

The research was carried out on an active municipal landfill located in Gać in the Oława county in the Dolnośląskie Voivodeship. The basic test stand parameters are given in

Table 1. Parameters of the test stand in Gać.

Parameter	Unit	Value
Total capacity	m3	780,000
Total capacity within the crown of the landfill	m3	30,000
Mass of deposited waste	Mg	1,000,000

.

The facility consists of three landfill cells: cell No. 1—closed, after exploitation and reclamation; cell No. 2—closed, after exploitation and during reclamation; cell No. 3—actively operated since 2014. The choice of this facility was prompted by the proximity to Wrocław and its character (active degassing system). Figure 1 shows a top view of the active municipal waste landfill in Gać.

The installation will be equipped with four Flexi 350 cogeneration units manufactured by TEDOM a.s. (Třebíč, Czech Republic). Currently, only one of them is working in the company. The rest of the landfill gas is flared. It is assumed that all produced electricity and heat will be sold.

Table 2. Technical specification of Flexi 350 cogeneration unit.

Parameter	Unit	Value
Maximum electrical output	kW	354
Maximum heat output	kW	398
Electrical efficiency	%	40.1
Heat efficiency	%	45.1

presents the basic data of the cogeneration unit.

All measurements were taken on the flue gas discharge line belonging to the cogeneration system. A specially fabricated stainless steel cover was placed on the end of the exhaust system (Figure 2). The installed cover had to measure stubs and mounting brackets for catalytic systems.

2.2. Applied Catalytic Additives

For research purposes, the use of a platinum catalyst on the ceramic carrier was selected. It was installed at a height of 85 cm below the exhaust chimney outlet. This type of catalyst has been shown in Figure 3. The catalyst lifetime is estimated from one to three years depending on the chemical composition of the exhaust gases.

2.3. Measurement of Landfill Gas Composition

In order to determine the basic properties of landfill gas, its composition was determined in terms of the content of CH₄, CO₂, O₂, NH₃, and H₂S. The landfill gas sample was previously taken into a 1 dm³ Tedlar bag. The secured sample was transported to the laboratory. The gas composition was tested with the BIOGAS 5000 gas analyzer (Figure 3) produced by Geotech Instruments, Inc. (Hainesport, NJ, USA). The analysis of the content of individual compounds in the gas made it possible to design appropriate catalytic systems adapted to the generated emission.

Detailed information about the landfill BIOGAS 5000 gas analyzer is given in

Table 3. Technical specification of the BIOGAS 5000 gas analyzer.

Component	Type of Sensor	Range	Accuracy
CO2	dual wavelength infrared	0–100%	±0.5%
CH4	dual wavelength infrared	0–100%	±0.5%
O2	internal electrochemical	0–25%	±1.0%
NH3	internal electrochemical	0–1000 ppm	±10.0%
H2S	internal electrochemical	0–10,000 ppm	±5.0%

.

2.4. Measurement of the Exhaust Gases Chemical Composition

The measurement of the exhaust gas composition (using the ISO and EN standards presented in Table 3) was based on using the Testo 350 analyzer. A photochemical method performs the detection of individual compounds in the flue gases. The registration began after the combustion conditions stabilized and the device self-calibrated. The exhaust gas composition was measured at the measurement points upstream and downstream of the catalyst. The recording process lasted for 1 h of uninterrupted gas burner operation with the 1 s recording frequency. The basic technical data of the analyzer are given in

Table 4. Technical specification of the Testo 350 flue gas analyzer.

Component	Measurement Method	Range	Precision	Compl. with Standards
O2	paramagnetic	0–25%	±0.1% abs. or 3% rel.	EN 14789; OTM-13
CO	chemiluminescence	0–10,000 ppm	±3 ppm abs. or 3% rel.	EN 15058; METHOD 10
CO2	chemiluminescence	0–25%	±0.03% abs. or 3% rel.	ISO 12039; OTM-13
NOX	chemiluminescence	0–1000 ppm	±3 ppm abs. or 3% rel.	EN14792
SO2	chemiluminescence	0–800 ppm	±5 ppm abs. or 5% rel.	EN14793

.

2.5. Measurement of the Particulate Matter Concentration in Exhaust Gases

The measurement of the dust concentration in the exhaust gases was performed using the Testo 380 particulate matter analyzer. The measurement carried out in accordance with the PN-EN standard included the determination of the sum of particulate matter in the flue gas without taking into account the division into individual fractions. The analyzer determined the amount of dust in the exhaust gases based on infrared detection. The registration process, as in the case of measuring the exhaust gas quality, started after the analyzer self-calibration and the stabilization of the combustion process conditions. The duration of the registration process and the frequency of recording were also the same as the process of measuring the chemical composition of the exhaust gases. The basic parameters of the Testo 380 analyzer are given in

Table 5. Technical parameters of the Testo 380 analyzer.

Component	Measurement Method	Range	Precision	Compl. with Standards
PM	NDIR	0–300 mg·m−3	±1 ppm abs. or 1% rel.	EN14842
CO2	chemiluminescence	0–20%	±0.03% abs. or 3% rel.	ISO 12039
O2	paramagnetic	0–22%	±0.1% abs. or 3% rel.	EN 14789

.

2.6. Measurement of the Exhaust Gases Temperature

In order to determine the temperature of the exhaust gases in accordance with the PN-EN standard, an APAR AR205 recorder was used, to which four K-type thermocouples were connected, which were placed at different points in the exhaust gas duct (1—placed above the burner, measuring the flame temperature; 2—at the measuring point in front of the applied catalyst; 3—at the measuring point after the applied catalyst; 4—at the measuring point at the top of the exhaust duct). The temperature was recorded every second during the entire period of the torch operation. The exhaust gas temperature registration process lasted throughout the gas burner operation with a recording frequency of 1 s. Based on the records obtained from the thermocouples, mean values were determined taking into account the mean measurement error at the level of ±1.5 °C.

2.7. Socio-Economic Analysis of the Use of Landfill Gas for Energy Purposes

In order to conduct the socio-economic analysis, the following assumptions were made:

-

analysis period: 10 years (duration of the depreciation of the fixed asset in Polish conditions, regulated by legal and accounting regulations),

-

a straight-line amortization of the purchased equipment was adopted, without discounting (without taking into account the change in the value of money over time),

-

the produced heat will power the newly established local energy network.

The proposed methodology of socio-economic analysis is based on three basic indicators:

• The impact of landfill gas use on an energy producer (I_P).
• The impact of landfill gas use on the environment (I_E).
• Product Responsibility (PR).

The impact of the use of landfill gas on the energy producer is expressed by the formula:

I_P = (E_G1∙MP_EG) + (E_G2∙MP_TH)

(1)

where: I_P—impact on energy producer, PLN; $E_{G1}$—annual amount of generated energy, GJ∙year⁻¹; MP_EG—average market price of generated electricity sale, (MP_EG = 28 EUR∙GJ⁻¹ was assumed [22]); E_G2—annual amount of generated thermal energy, MWh∙year⁻¹, MP_TH—average market price of generated thermal energy sale (MP_TH = 15 EUR∙GJ⁻¹ [23]).

The amount of annual electricity produced was calculated using the formula:

$$E_{G1}=\frac{Y_{LG}·LH{V}_{LG} · {\eta }_E · t }{1000}$$

(2)

where: Y_LG—average hourly landfill gas yield (Y_LG = 500 m³∙h⁻¹ was adopted [24]); LHV_LG—the lower heating value of the landfill gas (LHV_LG = 17 MJ∙m⁻³ was assumed [25]); η_E—electrical efficiency of the cogeneration unit, %; t—annual operating time of cogeneration unit (t = 8340 h was adopted).

The amount of annual thermal energy produced was calculated using the formula:

$$E_{G2}=\frac{Y_{LG}·LH{V}_{LG} · {\eta }_t · t }{1000}$$

(3)

where: Y_LG—average landfill gas yield, m³∙h⁻¹ (Y_LG = 500 m³∙h⁻¹ was adopted [24]); LHV_LG—the lower heating value of the landfill gas (LHV_LG = 17 MJ∙m⁻³ was assumed [25]); η_t—thermal efficiency of the cogeneration unit, %; t—annual operating time of cogeneration unit (t = 8340 h was adopted).

The sum of energy obtained from the combustion of landfill gas is presented using the formula:

$$E_G=E_{G1}+E_{G2}$$

(4)

The calculation of avoided greenhouse gas emissions to the atmosphere is based on the difference in average emissions in the case of the combustion of fossil fuels (in the Oława commune in which the landfill is located, heating systems based on solid fuel boilers dominate, in which are burning mainly various assortments of hard coal [26]) and the combustion of landfill gas for energy purposes. The analysis included two fuel components that are considered greenhouse gases. For standardization, it was decided to use the Global Warming Potential factors presented in

Table 6. Greenhouse gases from fuel combustion, their GWP, and emission factors [27,28].

Component	GWP20 Factor, -	Fossil Fuel CO2 Emission Factor, Mg∙GJ−1	Landfill Gas CH4 Emission Factor, Mg∙GJ−1
CO2	1	0.096	0.050
CH4	72	0.000010	0.000003

. Greenhouse effect potential for 20 years has been adopted due to the limited lifetime of the landfill.

Annual avoided CO₂ emissions to the atmosphere were determined based on the following formula:

$$A{E}_{CO2}=E_G·GW{P}_{CO2}·(FF{F}_{CO2} -LG{F}_{CO2})+E_G·GW{P}_{CH4}·(FF{F}_{CH4}-LG{F}_{CH4})$$

(5)

where:$A{E}_{CO2}$—annual amount of CO₂ emissions avoided, Mg∙year⁻¹$;E_G$—the sum of the annual energy produced from the landfill, GJ∙year⁻¹; $GW{P}_{CO2}$—global warming potential of carbon dioxide, -; $FF{F}_{CO2}$—fossil fuel CO₂ emission factor, Mg∙GJ⁻¹$; LG{F}_{CO2}$—landfill gas CO₂ emission factor, Mg∙GJ⁻¹$;GW{P}_{CH4}$—global warming potential of methane, -; $FF{F}_{CH4}$—fossil fuel CO₂ emission factor, Mg∙GJ⁻¹$; LG{F}_{CH4}$—landfill gas CH₄ emission factor, Mg∙GJ⁻¹

The social indicator taking into account the impact on the environment resulting from the reduction in greenhouse gases is expressed by the formula:

$$I_E=A{E}_{CO2}·C_{CO2}$$

(6)

where: I_E—impact on the environment, EUR; C_CO2—cost of CO₂ emission allowances, EUR‧Mg⁻¹ (C_CO2 = 71.10 EUR‧Mg⁻¹ was adopted [29]).

The cost of CO₂ emission allowances should be determined on the basis of market prices. Information can be obtained from stock quotes for trading in allowances.

Product Responsibility (PR) determines the degree of identification of the local community with the analyzed good or service, or as a sense of pride connected with the product or service, it is a typical social indicator. This indicator is determined by the method of conditional valuations, i.e., using questionnaires.

In the case of SROI analyses of landfill gas energy use, this indicator should be calculated as follows:

$$PR=\left(E_{G1}·M{P}_{EG}\right)+\left(E_{G2}·M{P}_{TH}\right) ‧API$$

(7)

where: $PR$—product responsibility indicator, EUR; $API$—acceptable price increase, %.

Acceptable Price Increase is a dimensionless value in the range <0; 1> and should be determined by the questionnaire research method. In the case of the energy use of landfill gas, the survey question may be formulated: “How much more would you be able to pay for the energy produced from landfill gas?” The arithmetic mean of all answers will be used to determine the API index.

$$API=\frac{\Sigma RV}{n}$$

(8)

where: $\Sigma RV$—sum of response values, -; $n$—number of respondents, - ($n=100 \mathrm{was}\mathrm{adopted}$).

3. Results

3.1. Measurement of the Landfill Gas Composition

Table 7. Composition of the gas from landfill with active degassing system in Gać.

Component	Unit	Value
CH4	%	50.2
CO2	%	31.1
O2	%	2.1
NH3	ppm	39
H2S	ppm	326

shows the composition of landfill gas used for energy purposes

After analyzing the elemental composition of the gas samples, it can be concluded that the landfill is in the peak phase of biogas productivity. This is evidenced by the ratio of methane to carbon dioxide. Methane accounts for more than half of the landfill gas volume produced, which translates into a high calorific value. In the tested landfill gas samples, relatively high hydrogen sulfide and ammonia concentrations were also detected. These compounds can directly influence the operational problems of active biogas utilization system devices. The concentration of NH₃ and H₂S in the landfill gas may also be reflected in the NO_X and SO_X emissions generated during the biogas energy conversion process.

3.2. Measurement of the Exhaust Gases Chemical Composition

Figure 4 shows the changes in the concentrations of gaseous pollutants in the tested exhaust gases. All the obtained results were converted to the normative conditions, i.e., 3% O₂ content in the exhaust gas.

Using a catalyst based on platinum particles decreased the carbon monoxide content in the exhaust gas by an average of 65%. The initial emission was on average 634 mg·m⁻³, while after using the catalytic system, the CO concentration dropped to the average level of 224 mg·m⁻³. The most effective catalyst system selected also reduced the level of NO_X emissions. During the regular operation of the cogeneration system, an average NO_X concentration of 376 mg·m⁻³ was recorded. The concentration of nitrogen oxides after applying the catalyst decreased to the average value of 178 mg·m⁻³, which is a 53% reduction. Despite using a landfill gas desulfurization system, small concentrations of SO₂ in the flue gas were recorded before it was fed to the cogeneration engine. The average sulfur oxide emission in the exhaust gas was 18 mg·m⁻³. The use of the catalyst allowed to reduce the average concentration of this compound to the level of 0.75 mg·m⁻³. The tested catalyst showed as much as 95% efficiency in this emission range.

3.3. Measurement of the Particulate Matter Content in Exhaust Gases

Figure 5 shows the change in PM concentrations after using the catalyst concerning the reference measurements. All the obtained results were converted to the normative conditions, i.e., 3% O₂ content in the exhaust gas.

During the regular operation of the cogeneration system, the PM concentration in the exhaust gas was recorded at an average level of 5 mg·m⁻³. Using a catalyst based on platinum particles reduced PM emissions to an average value of 0.85 mg·m⁻³. Based on the obtained results, the average efficiency of the catalyst in reducing PM emissions was determined, and it amounted to 82%.

3.4. Measurement of the Exhaust Gases Temperature

Figure 6 shows the change in exhaust gas temperature after the use of the Pt catalyst in relation to the reference measurements.

By analyzing the results obtained while recording the exhaust gas temperature, it can be concluded that the catalytic system caused its increase. The average temperature of the exhaust gases recorded during the regular operation of the cogeneration system was 258 °C. The use of the catalytic system increased the average temperature of the exhaust gases to the level of 404 °C. The percentage increase in the temperature value was 57%.

3.5. Socio-Economic Analysis of the Use of Landfill Gas for Energy Purposes

Table 8. Summary of input data and calculation results for the determination of social impacts.

Description	Unit	Value
The impact of landfill gas use on an energy producer	EUR	1,275,521
The impact of landfill gas use on an environment	EUR	199,698
Acceptable price increase	-	0.134
Product responsibility	EUR	170,920
Annual capital expenditure consisting of cogeneration units and technical infrastructure	EUR	217,500
Calculated SROI index	-	7.57

presents summary of input data and results calculated according to the SROI methodology.

The obtained SROI index means that every euro invested in the use of landfill gas for energy purposes will bring 7.57 EUR of social return on investment, the beneficiaries of which will be both:

-

energy producers;

-

local communities that:

• takes care of the CO₂ balance (also reducing emissions throughout Poland, which has signed international commitments to reduce CO₂ and other greenhouse gas emissions),
• is satisfied that the energy is produced in a modern installation from low-emission fuel.

4. Discussion

A multifaceted analysis of purified landfill gas took into consideration measurements of gas chemical composition before and after catalytic reduction but also environmental, economic, and social impact on the local society. To the energetic conclusions belonged analyses of carbon monoxide, nitrogen and sulfur compounds, and dust (PM) emission as well.

Using the selected catalyst decreased the CO content in the exhaust gas by an average of 65%. The obtained result of reducing the CO content correlates with the increase in the exhaust gas temperature, which averaged at 57%. The temperature rose due to the oxidation of CO on the catalyst surface, accompanied by heat release. The described process is related to the structure of the oxidizing properties of the active substance of the catalyst, i.e., platinum. After reaching the appropriate temperature, the catalyst oxidizes CO to carbon dioxide. As a result of this reaction, heat is released, which increases the average temperature in the combustion chamber. Carbon monoxide is a gas with energy potential, the emission of which is one of the components of energy losses in the combustion process.

The selected catalytic system also reduces the emission of nitrogen compounds harmful to the environment. The obtained results confirm the reducing properties of the active substance, which is platinum. Using the catalyst allowed to reduce the amount of NO_X in the exhaust gas by 53% on average due to the relatively low temperature of landfill gas combustion, i.e., approximately 850 °C. The emission mainly results from the high nitrogen content in the landfill gas. This is due to the dominance of the so-called fuel cycle of nitrogen oxide formation in combustion processes of low-power systems [30]. The use of the catalytic system in the selected location was also in line with the appropriate temperature window for the most effective operation of the platinum catalyst. In this case, the temperature window is between 200 and 650 °C [31], while the temperature of the exhaust gases at the installation site ranged between 258 and 404 °C. Therefore, before installing the catalytic system, the team determined the temperature distribution in the exhaust gas duct to determine the optimal location for installing the catalyst. The platinum catalyst used in the research, placed in a place with too low or too high a temperature, would achieve much worse results in NO_X reduction. Therefore, the correlation between the efficiency of the catalytic system and the temperature of the environment in which it operates is very strong.

Due to the high sulfation of landfill gas, all cogeneration systems have gas cleaning systems from this compound. During the combustion process, sulfur forms acidic sulfur compounds, which cause the degradation of friction surfaces in cogeneration engines and thus lead to their failure. In the composition of the exhaust gases, small amounts of sulfur compounds were detected, which may indicate a faulty or insufficient operation of the desulfurization system. Using a platinum catalyst reduced SO₂ emissions in the exhaust gas by as much as 95%. This is an excellent result. Nevertheless, sulfur compounds can negatively affect the operation of the catalytic system through so-called sulfur poisoning. Sulfur compounds can lead to the degradation of the support and catalytic centers, which will gradually reduce their effectiveness.

The PM-suspended particles in the exhaust gas are meconium and unburned solid fuel particles or mineral pollutants. Due to the fact that the engine was powered by gaseous fuel, PM emission was small and amounted to an average of 5 mg·m⁻³. Nevertheless, the catalytic system will also positively affect reducing emissions in this regard. The catalyst efficiency in reducing PM emissions was 82%. Due to the high temperature of the catalyst, carbon deposits were burnt along with the exhaust gases. It also reflects the increase in the exhaust gas temperature noted behind the catalyst, which was the effect of, among others, afterburning the combustible part of the suspended dust. As a result, the use of the catalytic system provides not only an environmental effect but also savings due to the lower amount of soot and meconium deposited in the exhaust system. Often, accumulating soot in the exhaust ducts reduce their capacity, which is associated with the disruption of the system and the need to carry out service work to clean them.

All those measurements were a basis for additional multifaceted analyses. In the next step, all emissions were standardized into CO₂ emissions and monetarized, as well as the other social indexes like product responsibility. The highest impact on energy producers (see Table 8) seems obvious, taking into consideration the high prices of fossil fuels. The possibility of own fuel (cleaned landfill gas) creation made the enterprise profitable, but the other social benefits are worth emphasizing too. The research showed the positive attitude of local society to landfill gas combustion—people are able to pay for energy (on average) 13.4% more when they know it is renewable energy. Additionally, the impact on the environment is positive, although smaller than the benefits of renewable energy production. That proportion could change if the fuel prices return to (normal) lower prices and the emission charges rise (which seems probable).

5. Conclusions

The applied catalytic system has an overall positive effect on the environment resulting from the combustion of landfill gas. Referring to the results obtained in the test cycle, it is possible to reduce the number of harmful compounds emitted in the range of 53–95%. The main task of landfill gas combustion is to avoid methane emissions to the environment, which leads to its significant degradation [32]. Often, with the active (energetic) use of landfill gas, the remaining pollutants emitted after the combustion process are forgotten. Compounds emitted in this way also have a negative impact on the environment. The use of a catalytic system allows to minimize this problem and makes the installation of active landfill gas use more environmentally friendly. An example confirmed in the conducted research is the emission of harmful suspended dust. Thanks to using a platinum catalyst, PM emissions were reduced by 82%. From the perspective of imposing different emission limits on gas-fired installations (the NO_X emission limit for natural gas combustion in cogeneration installations is 110 mg·m⁻³), the use of catalytic systems will help to meet the new requirements imposed on landfill gas-fired installations [33]. To sum up, due to its reducing and oxidizing properties, the solution of the catalytic system developed by the team, which is installed in the exhaust gas pipes, reduces the emission of harmful substances significantly. The developed solution will allow to improve air quality in the immediate working environment of such an installation. The solution will also increase the efficiency of the landfill gas conversion process, which in turn will translate into higher profit for the seller of the produced energy. The developed catalytic system after the installation has a positive effect on the quality of the environment and the economy of the energy production process. Nowadays, when fuel prices are soaring, any installation producing heat or electricity is extremely valuable. The research carried out shows that the use of catalysts not only ensures cleaner combustion, but also energy, environmental, and social gains. The social impact (shown in Table 8), 7.57 EUR, means that 1 EUR spent on the analyzed installation brings 7.57 EUR of social benefits. Such a high level of social profits should inform the rulers (local authorities) that a given venture is even worth subsidizing.

According to the results shown in publication, a thesis can be put forward—correctly performed analysis should take into account all these aspects: energetic, environmental, and social.