Economic Analysis of Methanating CO₂ and Hydrogen-Rich Industrial Waste Gas in Depleted Natural Gas Reservoirs

Abstract

This study explored underground biomethanation as a means to achieve carbon neutrality and promote carbon circular utilization by methanating CO₂ and hydrogen-rich industrial waste gas in depleted natural gas reservoirs (MECHIG). This approach not only aids the development of carbon capture, utilization, and storage (CCUS) technologies, but also effectively processes industrial waste gas, thereby reducing pollutant emissions. In order to verify the feasibility of the MECHIG concept, this study builds upon the analysis of the MECHIG process overview and employs the net present value (NPV) analysis method to investigate its economic viability. Additionally, the study conducts a sensitivity analysis on six factors, namely methanation efficiency, facility site investment, hydrogen content in waste gas, natural gas prices, operation and maintenance (O&M) investment, and CO₂ capture and injection prices. The results indicate the following: (1) Under the baseline scenario, the NPV of the MECHIG concept is approximately CNY 5,035,100, which suggests that the concept may be economically viable. (2) The fluctuation in natural gas prices has the most significant impact on NPV, followed by facility site investment and methanation efficiency. In contrast, the variations in hydrogen content in waste gas, O&M investment, and CO₂ capture and injection prices have relatively smaller effects on NPV. (3) To ensure the economic feasibility of the concept, the acceptable fluctuation ranges for the factors of methanation efficiency, facility site investment, hydrogen content in waste gas, natural gas prices, O&M investment, and CO₂ capture and injection prices are −16.78%, 5.44%, −32.14%, −4.70%, 14.86%, and 18.56%, respectively.

1. Introduction

Amidst the escalating severity of global climate change, greenhouse gas emissions and industrial waste gases have imparted significant detrimental effects on the environment [1]. CCUS technologies are regarded as one of the pivotal strategies for mitigating greenhouse gas emissions, with biological carbon sequestration technologies currently in their growth stage and displaying immense potential for development [2,3]. In this context, the exploration of innovative techniques and approaches, such as the MECHIG technology, offers a promising avenue for addressing this pressing issue. MECHIG technology not only aids in reducing greenhouse gas emissions but also effectively harnesses waste gas resources, thereby enhancing energy utilization efficiency. However, prior to the widespread adoption of this technology, it is crucial to thoroughly comprehend aspects such as its economic feasibility and market potential. Such understanding will facilitate the determination of the likelihood and potential profitability of implementing MECHIG on an industrial scale, ultimately laying a foundation for future sustainable energy development.

CO₂ hydrogenation to methane technology is a process that transforms carbon dioxide and hydrogen into methane, offering a promising approach to achieving greenhouse gas reduction and energy transition [4,5]. Over the past several decades, the development and application of this technology have made significant progress, involving various chemical and biological pathways, such as the Sabatier reaction and biological methanation [6,7,8]. The Sabatier reaction is the most common method of methanation, typically conducted at elevated temperatures (approximately 300–400 °C) and pressures, employing metallic catalysts to facilitate the generation of methane from hydrogen and carbon dioxide. However, the thermodynamic conditions of this reaction are rather stringent, necessitating catalysts and appropriate operating conditions to achieve high methane yields [9,10]. In contrast, biological methanation is a process that utilizes microbial catalysts, such as methanogenic archaea, to convert carbon dioxide and hydrogen into methane [11]. This approach is typically performed under milder temperature and pressure conditions compared to the Sabatier reaction, potentially exhibiting greater economic viability and feasibility in specific scenarios [12,13].

Hydrogen plays a pivotal role in CO₂ hydrogenation to methane technology. Hydrogen sources are diverse, encompassing pathways such as natural gas reforming, coal gasification, biomass conversion, and water electrolysis [14,15]. However, some of these methods may lead to additional carbon emissions or exhibit high energy consumption [16,17], necessitating the development of cheaper and cleaner hydrogen sources to address current challenges [18]. It is noteworthy that China is one of the world’s largest industrial producers, with abundant hydrogen content present in industrial waste gases. Utilizing the hydrogen from these waste gases for CO₂ hydrogenation in methane technology offers distinct advantages [19,20]. On the one hand, it facilitates the resourceful utilization of waste gases, reducing the cost of methanation. On the other hand, it helps to decrease industrial waste gas emissions, promoting the development of a circular economy, and providing support for China’s energy structure adjustments and low-carbon development objectives.

Economic analysis holds significant importance in assessing the implementation of methanation projects. By conducting in-depth research into the influencing factors of costs, benefits, and other aspects, it is possible to evaluate the feasibility and potential profitability of the methanation process on an industrial scale [21,22]. Building upon the authors’ previously proposed novel concept of CO₂ capture, circular utilization, and storage (CCCUS) through underground biological methanation [13,23], the innovative MECHIG concept is introduced. To validate the feasibility of this concept, the present study employs the NPV method for a detailed economic assessment. Based on this foundation, sensitivity analyses were conducted concerning six key factors: methanation efficiency, facility site investment, hydrogen content in waste gases, natural gas prices, O&M investment, and CO₂ capture and injection prices. Through this economic methodological framework, the aim is to provide a comprehensive and in-depth economic rationale and data support for the effective implementation of the MECHIG concept.

2. MECHIG Process Overview

To promote a carbon circular economy, underground energy storage, and an increased scale of CO₂ utilization, the authors have proposed a new concept of CCCUS: underground biological methanation [13,23]. In this approach, H₂ and CO₂ are mixed or sequentially injected into depleted natural gas reservoirs, where the injected gases are biologically transformed into water and renewable methane under the catalysis of methanogenic microbes. The methanation reaction equation is as follows:

$${\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2\to {\mathrm{C}\mathrm{H}}_4+{2\mathrm{H}}_2\mathrm{O};∆\mathrm{G}=-165 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(1)

Compared to the hydrogen source in the proposed CCCUS concept, which originates from renewable energy hydrogen production, the MECHIG technology fully exploits hydrogen-rich industrial waste gases, converting CO₂ and hydrogen-containing waste gases into methane (CH₄), presenting advantages in both environmental impact and cost. The specific MECHIG process is illustrated in Figure 1.

China produces a significant volume of industrial waste gases, with the production and composition of hydrogen-containing waste gases presented in

Table 1. Production and composition of major hydrogen-containing industrial waste gases in China.

No.	Gas Type	Production (×108 m3/a)	Composition (%)	Hydrogen Amount (×108 m3/a)
1	Coke oven gas	1114	H2: 57, CH4: 25.5, CO: 6.5, CnHm: 2.5, CO2: 2, N2: 4	635
2	Blast furnace gas	12,000	H2: 1.5~3, CH4: 0.2~0.5, CO: 25~30, CO2, 9~12, N2: 55~60, O2: 0.2~0.4	270
3	Calcium carbide furnace waste gas	120	H2: 0~10, CH4: 0.5, CO: 70~90, CO2: 1~10, N2: 1~8, O2: 0.2~0.6	6
4	Refinery gas	1193	H2: 14~90, CH4: 3~25, C2+: 15~30	620
5	Synthetic ammonia waste gas	124	H2: 20~70, CH4: 7~18, Ar: 3~8, N2: 7~25	86
6	Methanol purge gas	329	H2: 60~75, CH4: 5~11, CO: 5~7, CO2: 2~13, N2: 0.5~20	161
7	Semi-coke waste gas	290	H2: 26~30, CO: 12~16, CH4: 7~8.5, CO2: 6~9, N2: 35~39	81.2
8	Sodium chlorate byproduct gas	5.7	H2: 95, O2: 2.5, others	5
9	Polyvinyl chloride (PVC) waste gas	12.86	H2: 50~70, C2H2: 5~15, C2H3Cl: 8~25, N2: 10~15	6
10	Caustic soda waste gas	99.17	H2: 98.5, N2: 0.5, O2: 1, others	97.7
11	Propane dehydrogenation (PDH) waste gas	3.8	H2: 80~92, C2H6: 1~2, C3H8: 0.5~1, N2: 1~2	3.1

. The hydrogen content in industrial waste gases can reach approximately 2 × 10¹¹ m³ per year [20], accounting for about 20% of the total hydrogen production [24]. The injection of hydrogen-containing industrial waste gases into depleted natural gas reservoirs utilizes the hydrogen for methanation reactions, while the denser gases serve as a cushion for enhanced gas recovery (EGR). CO₂ is captured from industrial waste gases or the atmosphere, achieving the circular utilization of carbon dioxide and continuous underground storage. The regenerated methane can be utilized for gas grids, gas vehicles, gas-fired power plants, and other applications, fostering the development of a circular economy. Concurrently, the heat generated by the MECHIG technology can be used for geothermal power generation, and the gas storage facilities can also serve as peak-shaving and frequency regulation resources.

In addition to the economic benefits outlined above, MECHIG technology also offers significant social and ecological advantages. The utilization of waste gases for energy production reduces the reliance on fossil fuels, which in turn mitigates greenhouse gas emissions and contributes to cleaner air quality. This improvement in air quality positively impacts public health, reducing respiratory issues and other pollution-related health problems. Furthermore, the circular approach to carbon dioxide utilization and storage supports global efforts to combat climate change, fostering a more sustainable and environmentally friendly energy landscape.

3. Methodology

3.1. Economic Analysis Method

In the technoeconomic evaluation of engineering projects, the NPV method and the internal rate of return (IRR) method are commonly used. In some projects, the NPV method is more effective than the IRR method, particularly in energy-related projects where the NPV method is often employed to determine if the project is worth investing in [25,26]. Therefore, this study adopts the NPV method to assess the feasibility of the methane conversion project involving CO₂ and hydrogen-rich industrial waste gases in depleted oil reservoirs. The NPV represents the sum of the differences between all cash inflows and outflows, discounted to the present value [27]. The calculation formula is as follows:

$$NPV={\sum }_{t=1}^n{(CI-CO)}_t{(1+r)}^{-t}$$

(2)

In the formula, NPV represents the net present value (in currency units), CI represents the cash inflow (in currency units), and CO represents the cash outflow (in currency units). n represents the calculation period (in years), and for this study, n was set to 20 years. r denotes the designated discount rate (also known as the benchmark return rate). According to the regulations set by China’s National Development and Reform Commission, the benchmark return rate for natural gas pipeline transportation prices is 8% [28]. The risk associated with natural gas extraction projects is relatively high, and the benchmark return rate is generally higher than that of natural gas pipeline transportation and distribution sectors. Based on data from the Forward Database, the return on net assets for listed companies in China’s natural gas extraction industry in 2021 was 11.50% [29]. Therefore, in this study, the benchmark return rate ‘r’ was set at 11.50%.

Typically, a positive NPV indicates that a project is expected to generate a return that exceeds the expected return and investment in the project should therefore be pursued. Conversely, a negative NPV suggests that the project will fail to meet the expected return, and investment in the project should be declined. On the other hand, an NPV equal to zero indicates that the project is expected to generate exactly the expected return, and investment in the project may be acceptable or declined based on the decision-maker’s preference.

3.2. Key Assumptions

(1) Evaluation Period: Considering long-term capital recovery and investment benefit assessment, the evaluation period was set at 20 years.

(2) Region: The study area was designated as a depleted natural gas reservoir in Sichuan Province, with a total volume of 100 million cubic meters. It was assumed that the extraction rate was 40%, meaning that 40 million cubic meters of gas had already been extracted for subsequent methanation reactions.

(3) Hydrogen content in industrial waste gas: As shown in Table 1, blast furnace gas is the highest-yielding type of industrial waste gas. However, its low hydrogen content is not conducive to methanation reactions, making it less suitable for such processes. In contrast, coke oven gas and refinery gas exhibit higher hydrogen contents, making them relatively appropriate industrial waste gases for methanation. This study first analyzed hydrogen content at 50%, and then conducted dynamic simulations for varying hydrogen contents. The consideration of industrial waste gases containing harmful substances and their carbon dioxide content is not included in this study.

(4) Methanation efficiency and reaction time: This study considered only the natural reaction process and did not involve the catalytic effect of artificially cultivated bacteria. Referring to related research [13,30], the reaction period was set at one year. At the same time, this study first analyzed methanation efficiency at 80%, and then conducted dynamic simulations for varying methanation efficiency.

(5) In this economic analysis, fluctuations in electricity, water, and natural gas prices, as well as factors such as fiscal subsidy policies, were temporarily disregarded.

The amount of natural gas regenerated in each MECHIG reaction, and the required hydrogen, carbon dioxide, and industrial waste gas quantities, are presented in

Table 2. The amount of natural gas regenerated and gas demand in each MECHIG reaction.

Methanation Efficiency (%)	Reactive Natural Gas (×104 m3/a)	Hydrogen Demand (×104 m3/a)	CO2 Demand (×104 m3/a)	Industrial Waste Gas Demand (×104 m3/a)	Storage Capacity Demand (×104 m3/a)
80%	356	1780	445	3560	4005

.

4. Economic Benefit Analysis of MECHIG

4.1. Cost Analysis

4.1.1. Facility Site Investment

(1) Civil Engineering Investment

The main device area includes production wells, auxiliary facilities, processing equipment, and construction projects. Production wells utilize existing old wellheads in depleted oil and gas reservoirs. The area occupied by auxiliary facilities includes mixing stations, material storage areas, and others. The area occupied by processing equipment depends on factors such as production capacity and processing methods. Construction projects include central control rooms, office spaces, and other facilities. The main device area and construction project investments are influenced by various factors and require specific design proposals for accurate estimates. This study estimated a land area of approximately 6000 m², with general land requisition costs of about CNY 60,000 per acre, resulting in a land cost of approximately CNY 540,000. Construction project investment was assumed to be approximately CNY 10 million, totaling CNY 10.54 million for civil engineering investment.

(2) Equipment Investment

This project utilized existing old wellheads in depleted oil and gas reservoirs, eliminating the need to consider expenditures related to drilling. The project primarily required the following categories of equipment: ① injection wellhead equipment, such as injection pipelines, valves, and pressure sensors; ② pipeline transportation equipment, including oil and gas pipelines, oil pumps, and gas compressors; ③ extraction equipment, such as separators, separation tanks, expanders, and others; ④ collection equipment, including gas trees, gas valves, and others; ⑤ processing equipment, comprising separation, dehydration, desulfurization, and decarbonization devices; and ⑥ monitoring equipment, including flowmeters, pressure sensors, temperature sensors, and data collection transmission systems. Referring to the prices of natural-gas-collection-related equipment [31] and adding a 10% contingency for unforeseen expenses, approximately CNY 80 million was estimated for equipment investment.

4.1.2. Waste Gas Transportation

The average transportation cost for industrial waste gas in China is approximately CNY 0.2–0.6 per ton of waste gas transported over one kilometer. This study assumed a cost of 0.4 CNY/(t·km) and a transport distance of 50 km. According to Table 2, the annual waste gas requirement was 35.6 million cubic meters. Calculating waste gas density based on coke oven gas at 1.16 kg/m³, the annual waste gas transportation expense was approximately CNY 825,900.

4.1.3. CO₂ Capture and Injection

The cost of CO₂ capture in China is roughly between CNY 200 and 600 per ton of CO₂, while injection costs depend on factors such as underground reservoir conditions and injection technology, typically ranging between CNY 50 and 200 per ton of CO₂ [32,33]. This study calculated CO₂ capture and injection costs at 400 CNY/ton. Disregarding the carbon dioxide content in waste gas, this study calculated 4.45 million cubic meters required annually, amounting to approximately CNY 3.5191 million.

4.1.4. Bacterial Cultivation

This study primarily considered naturally occurring methanogenic bacteria, with a complete reaction cycle requiring approximately one year. The large-scale cultivation for subsequent production requires specific analysis and assessment.

4.1.5. O&M Investment

(1) Electricity Consumption

According to a report in the China Petrochemical News, a typical well site (including refueling stations, gas transmission stations, and substations) in a large natural gas field in the eastern Sichuan Basin requires approximately 0.06 kWh of electricity consumption per cubic meter of natural gas production [34]. Calculating electricity costs at 0.5 CNY/kWh, the annual electricity consumption expense was approximately CNY 240,000.

(2) Water Consumption

Water consumption varies greatly among different natural gas fields. In the Sichuan Basin of China, approximately 0.9–2 tons of water are needed for the production of every cubic meter of natural gas [35,36]. This study assumed 1.5 tons of water consumption per cubic meter of natural gas produced. Industrial water prices are calculated at 4.1 CNY/m³, resulting in an annual water consumption cost of approximately CNY 492,600.

(3) Labor

Personnel allocation included factory manager, safety management staff, accounting personnel, and operational personnel. The numbers of operational personnel were calculated based on three shifts with ten people per shift, totaling thirty people. Assuming an annual salary expenditure of CNY 80,000 per person, CNY 2.4 million were required. Assuming labor costs grow annually at China’s 2023 policy benchmark interest rate of 2.75%, for a total of CNY 62.8738 million over 20 years.

(4) Maintenance

Maintenance costs were assumed to be a fixed percentage of equipment costs, calculated at 1%.

The distribution of various MECHIG costs over the 20-year lifecycle is shown in

Table 3. The distribution of various MECHIG costs over the 20-year lifecycle.

Year of Operation	Civil Engineering Investment/104 CNY	Equipment Investment/104 CNY	Waste Gas Transportation/104 CNY	CO2 Capture and Injection/104 CNY	Electricity Fee/104 CNY	Water Fee/104 CNY	Labor/104 CNY	Maintenance/104 CNY
0	1054	8000	-	-	-	-	-	-
1	-	-	82.59	351.91	24	49.26	240	80
2	-	-	82.59	351.91	24	49.26	246.60	80
3	-	-	82.59	351.91	24	49.26	253.38	80
4	-	-	82.59	351.91	24	49.26	260.35	80
5	-	-	82.59	351.91	24	49.26	267.51	80
6	-	-	82.59	351.91	24	49.26	274.87	80
7	-	-	82.59	351.91	24	49.26	282.42	80
8	-	-	82.59	351.91	24	49.26	290.19	80
9	-	-	82.59	351.91	24	49.26	298.17	80
10	-	-	82.59	351.91	24	49.26	306.37	80
11	-	-	82.59	351.91	24	49.26	314.80	80
12	-	-	82.59	351.91	24	49.26	323.45	80
13	-	-	82.59	351.91	24	49.26	332.35	80
14	-	-	82.59	351.91	24	49.26	341.49	80
15	-	-	82.59	351.91	24	49.26	350.88	80
16	-	-	82.59	351.91	24	49.26	360.53	80
17	-	-	82.59	351.91	24	49.26	370.44	80
18	-	-	82.59	351.91	24	49.26	380.63	80
19	-	-	82.59	351.91	24	49.26	391.10	80
20	-	-	82.59	351.91	24	49.26	401.85	80
Total	1054	8000	1651.84	7038.12	480	985.2	6287.38	1600

.

4.2. Revenue Analysis

4.2.1. Regenerated Natural Gas (RNG)

Using Chengdu City in Sichuan Province as an example, the average price for gas consumption by users is 2.60 CNY/m³. Based on Table 2, the annual production of RNG in our study was 3.56 million m³, resulting in an estimated annual revenue of approximately CNY 9.256 million.

4.2.2. Carbon Trading

China officially launched its national carbon trading market in July 2021. Since July 2021, the average listing price on China’s National Carbon Emissions Trading Market (ETS) has ranged between CNY 41 and 61 [37]. In this paper, we performed calculations assuming 50 CNY/ton. Without considering the CO₂ content in the waste gas, the annual consumption of 4.45 million cubic meters of carbon dioxide amounts to approximately CNY 439,900.

4.2.3. Savings on Waste Gas Treatment Costs

According to market research, waste gas treatment prices in China generally range between 0.5 and 2.5 CNY/kg, which requires the consideration of factors such as treatment scale, process type, pollutant types, and region. In this paper, since we did not consider harmful gases in the waste gas, we reduced the waste gas treatment price and calculated it at 0.1 CNY/kg. According to Table 2, the annual waste gas consumption is 35.6 million cubic meters, and we calculated the waste gas density based on coke oven gas at 1.16 kg/m³. This resulted in annual savings on waste gas treatment costs of approximately CNY 4.1296 million.

4.2.4. EGR

The increase in recovery from depleted oil and gas reservoirs due to the use of cushion gases, such as carbon dioxide, depends on several factors, such as reservoir type, reservoir permeability, rock pore structure, injection pressure, and temperature. According to data from the United States Geological Survey (USGS), approximately 0.4 tons of additional oil and gas production can be obtained for every ton of injected carbon dioxide. Based on the main assumptions of this paper and Table 2, after the reaction of 35.6 million cubic meters of industrial waste gas per year, approximately 17.8 million cubic meters can be used as cushion gas. Considering the different waste gas components, we calculated a 10% increase in methane production by volume, resulting in an annual increase of 1.78 million cubic meters of methane, generating approximately CNY 4.628 million in revenue.

4.2.5. RNG Storage

RNG storage can be used for peak shaving and frequency regulation in the power system to better adapt to the fluctuations and intermittency of renewable energy sources (such as wind and solar energy). In this paper, the economic benefits generated by the RNG storage were calculated based on the difference in natural gas prices between winter and summer. Taking Sichuan Province as an example, the price difference between winter and summer natural gas for industrial and commercial users is about 0.4 CNY/m³. The economic benefits generated by peak shaving and frequency regulation amounted to approximately CNY 2.136 million.

4.2.6. Geothermal Utilization

The thermal efficiency of geothermal power plants is typically between 10% and 20%. In this paper, we calculated it based on an efficiency of 15%. The energy required to generate 1 kWh of electricity through a geothermal power plant is approximately 24,000 kJ. According to Formula 1, the heat release of 1 mole of methane is 165 kJ, and the heat release of 1 cubic meter of methane is approximately 7393.65 kJ. The annual regeneration of 3.56 million cubic meters of natural gas releases approximately 263.22 billion kJ, which can generate 1,096,720 kWh of electricity. With an electricity price of 0.5 CNY/kWh, this generates economic benefits of approximately CNY 548,400.

The annual revenue and total revenue over the 20-year life cycle are shown in

Table 4. The annual revenue and total revenue over the 20-year life cycle.

Period	RNG Benefits/104 CNY	Carbon Trading Benefits/104 CNY	Savings on Waste Gas Treatment Costs/104 CNY	EGR Benefits/104 CNY	RNG Storage Benefits/104 CNY	Geothermal Utilization Benefits/104 CNY
Per year	925.60	43.99	412.96	462.80	213.60	54.84
Total	18,512.00	879.77	8259.20	9256	4272	1096.72

.

4.3. NPV Calculation

The NPV calculation process is shown in

Table 5. NPV calculation table.

Year of Operation	Cash Inflow/104 CNY	Cash Outflow/104 CNY	Cash Flow/104 CNY	Discount Factor	Present Value of Cash Flow/104 CNY
0	0	9254	−9254	1	−9054
1	2113.78	827.76	1286.03	0.897	1153.39
2	2113.78	834.36	1279.43	0.804	1029.12
3	2113.78	841.14	1272.64	0.721	918.08
4	2113.78	848.11	1265.68	0.647	818.89
5	2113.78	855.27	1258.52	0.580	730.27
6	2113.78	862.62	1251.16	0.520	651.12
7	2113.78	870.18	1243.60	0.467	580.44
8	2113.78	877.95	1235.84	0.419	517.32
9	2113.78	885.93	1227.86	0.375	460.97
10	2113.78	894.13	1219.66	0.337	410.67
11	2113.78	902.55	1211.23	0.302	365.77
12	2113.78	911.21	1202.57	0.271	325.70
13	2113.78	920.11	1193.68	0.243	289.94
14	2113.78	929.25	1184.54	0.218	258.05
15	2113.78	938.64	1175.15	0.195	229.60
16	2113.78	948.29	1165.50	0.175	204.23
17	2113.78	958.20	1155.58	0.157	181.61
18	2113.78	968.39	1145.40	0.141	161.44
19	2113.78	978.85	1134.93	0.126	143.47
20	2113.78	989.61	1124.17	0.113	127.45

. Calculated using Equation (2), the NPV of the MECHIG concept is CNY 5,035,100. The concept’s cash inflows exceed its cash outflows, indicating that the concept may be economically feasible. Furthermore, the concept can utilize waste resources and reduce dependence on fossil fuels, promoting the development and realization of a carbon circular economy.

5. Sensitivity Analysis

Considering the variations in certain uncertain factors during the concept implementation, a sensitivity analysis was conducted on the effects of single-factor changes in methanation efficiency, facility site investment, hydrogen content in waste gas, natural gas prices, O&M investment, and CO₂ capture and injection prices by increasing them by 40%, 30%, 20%, and 10% and decreasing them by 10%, 20%, 30%, and 40% on NPV. A 3D Smoother was employed for visualization, as shown in Figure 2. The following conclusions can be drawn:

(1) Fluctuations in natural gas prices have the most significant impact on NPV, followed by facility site investment and methanation efficiency. The effects of changes in hydrogen content in waste gas, O&M investment, and CO₂ capture and injection prices on NPV are relatively small.

(2) An increase in methanation efficiency leads to an increase in NPV, while a decrease in methanation efficiency results in a decrease in NPV. When the methanation efficiency decreases by more than 16.78%, the NPV is less than 0, rendering the concept infeasible. In other words, under the premise of other factors remaining unchanged, the concept becomes infeasible when the methanation efficiency is below 63.22%.

(3) An increase in facility site investment leads to a decrease in NPV, while a decrease in facility site investment results in an increase in NPV. When facility site investment exceeds 5.44%, the NPV is less than 0, rendering the concept infeasible. In other words, under the premise of other factors remaining unchanged, the concept becomes infeasible when the facility site investment exceeds CNY 95,465,376.

(4) An increase in hydrogen content in waste gas leads to an increase in NPV, while a decrease in hydrogen content results in a decrease in NPV. When the hydrogen content in waste gas decreases by more than 32.14%, the NPV is less than 0, rendering the concept infeasible. In other words, under the premise of other factors remaining unchanged, the concept becomes infeasible when the hydrogen content in waste gas is below 17.86%.

(5) An increase in natural gas prices leads to an increase in NPV, while a decrease in natural gas prices results in a decrease in NPV. When natural gas prices decrease by more than 4.7%, the NPV is less than 0, rendering the concept infeasible. In other words, under the premise of other factors remaining unchanged, the concept becomes infeasible when natural gas prices are below 2.4778 CNY/m³.

(6) An increase in O&M investment leads to a decrease in NPV, while a decrease in O&M investment results in an increase in NPV. When O&M investment exceeds 14.86%, the NPV is less than 0, rendering the concept infeasible. In other words, under the premise of other factors remaining unchanged, the concept becomes infeasible when the first-year O&M investment exceeds CNY 4,516,984.

(7) An increase in CO₂ capture and injection prices leads to a decrease in NPV, while a decrease in CO₂ capture and injection prices results in an increase in NPV. When CO₂ capture and injection prices exceed 18.56%, the NPV is less than 0, rendering the concept infeasible. In other words, under the premise of other factors remaining unchanged, the concept becomes infeasible when CO₂ capture and injection prices exceed 474.24 CNY/ton.

6. Conclusions and Prospects

6.1. Conclusions

Based on the analysis of the MECHIG process overview, this study employed the NPV method to explore the economic feasibility of the MECHIG concept. Sensitivity analyses were conducted for six factors: methanation efficiency, facility site investment, hydrogen content in waste gas, natural gas prices, O&M investment, and CO₂ capture and injection prices. The specific conclusions are as follows:

(1) Under the baseline assumption scenario (in a depleted natural gas reservoir with a total capacity of 100 million cubic meters, 40% has been extracted. The project has a lifecycle of 20 years. The methanation efficiency is 80%, industrial waste gas contains 50% hydrogen, and the natural gas price is 2.6 CNY/m³), the NPV of the MECHIG concept is CNY 5,035,100, suggesting that the concept may be economically feasible.

(2) Natural gas price fluctuations have the greatest impact on NPV, followed by facility site investment and methanation efficiency, while hydrogen content in waste gas, O&M investment, and CO₂ capture and injection prices have relatively smaller impacts on NPV.

(3) To ensure the concept’s economic feasibility, the tolerable fluctuation ranges for methanation efficiency, facility site investment, hydrogen content in waste gas, natural gas prices, O&M investment, and CO₂ capture and injection prices are −16.78%, 5.44%, −32.14%, −4.70%, 14.86%, and 18.56%, respectively.

(4) Under the premise of other factors remaining unchanged, the concept becomes infeasible when the following situations occur: methanation efficiency is below 63.22%; facility site investment exceeds CNY 95,465,376; hydrogen content in waste gas falls below 17.86%; natural gas prices drop below 2.4778 CNY/m³; the first year’s O&M investment surpasses CNY 4,516,984; or CO₂ capture and injection prices exceed 474.24 CNY/ton.

6.2. Limitations and Future Research Possibilities

This paper presents a preliminary exploration of the economic aspects of the MECHIG concept, acknowledging certain limitations. Firstly, the analysis is based on a range of assumptions, such as the cost of land, equipment, and transportation, which may not accurately reflect real-world conditions. Secondly, this study did not consider fluctuations in the prices of electricity, water, and natural gas, or the effects of government subsidies. Additionally, the analysis did not account for the variations in CO₂ content in waste gases, which may significantly affect the results. Lastly, this study’s findings are largely based on a specific case study in the Sichuan Basin of China, limiting the generalizability of the results to other regions or reservoir types.

The parameters employed in this study for the economic analysis of the MECHIG concept are relatively conservative. For instance, while the typical lifecycle of natural gas extraction projects generally exceeds 30 years, this study assumed a 20-year timeframe. In the future implementation of such projects, the large-scale cultivation of methanogenic archaea could occur, resulting in a reduced reaction cycle time. These adjustments, among others, may further enhance the economic feasibility of the MECHIG concept when considered in practical applications.

Consequently, considering the potential technical challenges and market risks associated with the MECHIG concept, a detailed risk assessment and management process should be implemented to ensure long-term, stable operation. In future studies, a multifactor sensitivity analysis can be conducted based on specific concept examples, and the feasibility of the MECHIG technology under different backgrounds and conditions can be explored to address the aforementioned limitations.