Simulation study on catalytic oxidation of low concentration mine gas in an oxidation device

Abstract

Anthropogenic emissions of non-CO₂ greenhouse gases, such as low-concentration coal mine methane (cCH₄ < 30 vol%), have a significant impact on global warming. The main component of coal mine methane is methane (CH₄), which is both a greenhouse gas and a high-quality clean energy gas. To study the combustion and heat transfer reactions of low-concentration coal mine methane in a catalytic oxidation device, a numerical simulation approach was employed to establish a model of the catalytic oxidation device that includes periodic boundary conditions, methane combustion mechanisms, and turbulent-laminar flow characteristics. The core focus of this study is on the dynamic changes in the bed temperature of the oxidation device, the temperature of the extracted hot air, and the methane conversion rate. By varying parameters, the study explored the effects of factors such as methane concentration, switching time, and the amount of hot air extraction on the combustion efficiency and safety within the oxidation device. Furthermore, the optimal placement of the catalyst within the device was refined. The results indicate that the methane concentration in the oxidation device should not exceed 1.8 vol% to avoid equipment damage and potential safety risks due to excessively high methane concentrations. Under conditions where the methane concentration is between 1.6 and 1.8 vol%, the appropriate switching time is 30–60 s, and the amount of hot air extraction should be maintained within the range of 15–20% to achieve efficient combustion and heat transfer performance. Additionally, the placement of the catalyst needs to be finely adjusted according to the changes in the internal temperature field of the oxidation device to ensure the maximization of catalytic effects. This study not only provides theoretical basis and technical support for the efficient utilization of low-concentration coal mine methane (LC-CMM) but also offers references for its widespread promotion and application in the industrial field.

Introduction

Coal mine methane (CMM), as an inevitable by-product in the process of coal mining, is not only regarded as a safety hazard in the coal industry but also recognized as a potential unconventional natural gas resource^1–3. According to authoritative statistics, methane emitted during global coal mining accounts for 8% of the total methane emissions from human activities, of which low-concentration coal mine methane (LC-CMM, cCH₄ < 30 vol%) accounts for more than half of the total^4,5. However, unfortunately, most LC-CMM is not effectively utilized but directly emitted into the atmosphere, which not only exacerbating global warming but also constituting a significant waste of valuable energy resources. Reducing greenhouse gas emissions in the atmosphere is crucial to mitigating the environmental issue of global warming. Strategies to reduce atmospheric CO₂ concentrations are mainly divided into two categories: one is emission reduction, including improving energy efficiency and shifting to low-carbon or zero-carbon fuel sources; the other is deploying negative emission technologies (NETs) to remove and sequester carbon from the atmosphere⁶. The catalytic oxidation technology for LC-CMM discussed in this study not only helps reduce this greenhouse gas emission source from coal mine methane but also converts the methane in the gas into thermal or electrical energy, achieving effective energy utilization.

Utilization status of low-concentration coal mine methane

China, as one of the countries with rich coal resources in the world, has a significant energy structure characterized by "lean oil, limited gas, and relatively abundant coal”. This energy structure has led to China’s rich coalbed methane resources. According to detailed geological surveys and exploration data, the gas resource reserves in coal seams with a burial depth of less than 2000 m amount to a staggering 36.8 trillion cubic meters. This immense resource reserve undoubtedly provides a solid foundation for the development of unconventional coalbed methane resources in China.

However, the recovery and utilization of coal mine methane during coal mining face numerous challenges. According to statistical data from the coal industry in China, the methane concentration extracted from underground coal mines is generally low, with only about 43.58% of the methane having a concentration exceeding 30%. This is attributed to the unsatisfactory performance of coal seam methane extraction, significant fluctuations in methane concentration during extraction, and an excessively high proportion of low-concentration methane and ventilation methane. In terms of the utilization of low-concentration coal mine methane (LC-CMM), China has obvious shortcomings. Statistics indicate that in 2018, the utilization rate of low-concentration methane with a concentration ranging from 6 to 30% was only about 28%, while the utilization rate of ultra-low-concentration methane with a concentration below 6% was as low as 2%. Such low utilization rates not only waste valuable energy resources but also exacerbate the issue of global warming.

Therefore, vigorously developing the utilization technology of LC-CMM has important practical significance for energy transition and environmental protection under the increasingly serious global warming^7–10. Compared with traditional fossil fuels such as oil and coal, CMM produces much lower carbon emissions during combustion, which undoubtedly provides the possibility for optimizing energy structure and developing low-carbon economy^11,12. However, LC-CMM, as a combustible gas with a large flow rate and extremely low concentration, faces significant challenges in terms of processing and utilization technologies in energy utilization technology and practical application^13–15. In recent years, with the continuous progress of technology, thermal and catalytic oxidation technologies have become the main technical means for processing LC-CMM. Especially catalytic oxidation technology, which has received widespread attention in the industry due to its lower oxidation temperature and less NOx emissions¹⁶. The core of this technology lies in the regenerative thermal oxidizer (RTO), which utilizes the difference between the fuel gas flow rate and the reaction heat front moving speed in an unstable state through the built-in porous media heat storage filler and catalyst layer, reasonably controls the cycle duration, and enables the released and stored heat inside the device to achieve self-heat balance while achieving the extraction of high-temperature flue gas and waste heat utilization^17,18.

The reaction of LC-CMM in the oxidation unit is a complex process, encompassing gas flow, combustion, and heat transfer. To promote the utilization of LC-CMM, various scholars have conducted in-depth studies on the factors affecting the oxidation reaction and the optimization of the oxidation unit’s performance from different perspectives, focusing on the catalytic oxidation process. Unger et al.¹⁹ was the first to define the Cyclic Steady State (CSS), which refers to a state where the process repeats cyclically. In the context of thermal countercurrent oxidation, the system is considered to have reached the Cyclic Steady State when all state variables, including temperature, concentrations of gas components, chemical reaction products, and so forth, begin to recur cyclically. Additionally, the time interval between two adjacent changes in gas flow direction is defined as the reversal time. Mei et al.²⁰ systematically analyzed the effects of gas inlet velocity, temperature, concentration, and catalyst loading on methane conversion efficiency. By simulating the catalytic oxidation process of the entire reactor, they provided valuable research foundations for subsequent industrial applications. Marín et al.^21,22 focused their research on the effects of inlet methane concentration and gas flow rate on methane conversion efficiency and system stability. They further simulated the performance of different heat recovery methods and found that extracting gas from the catalytic bed end is a more stable heat recovery strategy compared to returning cooled gas. Wang et al.²³ designed and operated a pilot-scale central fluted heat exchanger (CFRR) and experimentally studied the effects of cycle time, methane inlet concentration, and heat exchange flux on temperature fluctuations and temperature distribution, effectively achieving the elimination and utilization of low-concentration coal mine methane. Lan et al.²⁴ explored in detail the specific effects of channel length, inlet methane concentration, inlet velocity, and cycle time on reactor performance, especially focusing on the minimum methane concentration required for a specific channel length. Dai et al.²⁵ reported on the effects of steam concentration in low-concentration coal mine methane on temperature distribution, flame stability limits, and chemical reactions. They found that increasing steam concentration significantly affects combustion stability, thereby significantly impacting subsequent heat utilization. Mollamahdi et al.²⁶ studied the effects of porous media wall length and inner diameter on flame stability, temperature, methane conversion efficiency, and pressure drop through numerical simulations, providing theoretical guidance for the structural optimization of porous media burners. Chen et al.²⁷ focused on the catalytic stable combustion characteristics of methane-air mixtures in microscale thermal cycling systems. They innovatively introduced a thermal cycling structure into the catalytic oxidation system, providing new ideas for designing more stable oxidation systems. Li et al.²⁸ analyzed the self-heating operation boundary of a low-concentration methane thermal oxidation flow reversal reactor and deeply explored the impact of hot air export volume on the behavior of the flow reversal reactor, providing important research references for reactor waste heat recovery. In summary, numerical simulation studies, small-scale experimental studies, and medium to large-scale experimental studies have all made significant progress in the field of LC-CMM utilization. Not only have they revealed the effects of different factors on the oxidation reaction, but they have also provided valuable theoretical and practical guidance for the design optimization of oxidation units and subsequent industrial applications.

However, most of the current studies on concentration focus on the minimum concentration required to maintain self-heating reactions, which can even be reduced to around 0.2 vol%²⁹. For the industrial application of this technology, too low a methane concentration imposes high demands on equipment operation and results in poor economic performance during actual operation^30–32. This study focuses on the industrial application of LC-CMM and conducts numerical experiments by establishing a dual oxidation unit model. It investigates the catalytic oxidation process within the oxidation unit when the methane concentration ranges from 1.2 to 2.8 vol%. The study obtains the effects of methane concentration, reversal time, and hot air export volume on the operation of the oxidation unit and discusses the placement positions of the catalyst under different conditions. The research results provide a reference for numerical simulation studies on the oxidation utilization of LC-CMM and also provide a basis for the industrial application of LC-CMM.

Modeling

Computation module

This paper establishes a model of the catalytic oxidation device to simulate the catalytic oxidation process of LC-CMM inside the device. Since the gas activities within the channel involve the intercoupling of processes such as fluid flow, heat transfer, and chemical reactions, it is extremely difficult to accurately simulate them. To simplify the model for simulation, the following assumptions are made in this paper:^33,34.

1. The velocity, temperature, and concentration of the premixed gas are uniform.

2. The premixed gas flows uniformly through the device model, and the flow, heat transfer, and chemical reaction states within the model are the same.

3. The reactor is adiabatic, and heat dissipation losses are approximately negligible.

Figure 1 presents a simplified schematic diagram of the Catalytic oxidation device. This device adopts a symmetrical layout, divided into two regenerative catalytic chambers separated by partition walls. The maximum treatment capacity of the oxidation device is 10,000 m³/h. For better catalytic performance, Gao et al.³⁵ designed the regenerative catalytic chambers with a mixture of honeycomb ceramics and ceramic spheres. From bottom to top, the regenerative catalytic chambers are filled with 400 mm of honeycomb ceramics, 200 mm of catalyst, and 400 mm of ceramic spheres. The flow area of the regenerative chamber is 1.8 m², with an empty tower velocity of 1.543 m/s. Both the ceramic spheres and honeycomb ceramics are made of corundum. The walls of the oxidation chamber are lined with 350 mm of lightweight thermal insulation material (asbestos fiber modules with a thermal conductivity less than 0.153 W/m °C). The temperature inside the catalytic oxidation chamber is calculated at 1050 °C, and assuming all walls are insulated, the outer surface temperature remains below 70 °C. Uniformity in inlet airflow distribution is a crucial factor in ensuring the treatment efficiency of the oxidation device. To guarantee uniform airflow distribution, a deflector is installed in the inlet channel, and a high-temperature flue gas outlet is provided to extract hot air.

Fig. 1.

Catalytic oxidation device.

This device diagram is based on previous experimental and numerical simulation results conducted by the author’s team³⁶, and incorporates literature³² by dividing the regenerative material into spherical and honeycomb shapes. The model diagram aims to visually demonstrate the internal structure and working principle of the catalytic oxidation equipment (Table 1).

Table 1.

The operating conditions of catalytic oxidation of LC-CMM in oxidation device.

Item	Unit	Value
Methane concentration	vol%	1.2–2.8
Feed flow rate	m3/h	10,000
Inlet temperature	℃	293.15
Switching time	s	30–70
The amount of hot air exported		15–30%

The simplified diagram of the catalytic oxidation device model is as follows:

Thermal oxidation rate:

First and foremost, a detailed numerical model of ventilation air methane thermal oxidation incorporating mechanisms such as reaction, mass transfer, and heat transfer should be established based on the aforementioned conditions, including key design parameters like reactor size, methane concentration, and material flow rate. Subsequently, through numerical simulation, the bed temperature distribution, appropriate reversal time, and hot air output of the flow reversal reactor can be obtained when the methane concentration ranges from 1.2 to 2.8 vol%. Additionally, the catalyst loading position can be determined, providing theoretical support for the design and operation of the reactor.

Control equations and reaction kinetics

Based on the assumptions of the calculation model in this article, the governing equations of the premixed gas in the channel are as follows:

Gas continuity equation:

1

Gas momentum equation:

2

Gas phase energy conservation equation:

3

In the equation, represents the gas density(kg/m³); t is time(s); are positional coordinates(m); are velocity vectors(m/s); p is pressure(pa); is the shear stress tensor(N/m²); the internal energy of the gas(J); is the enthalpy of the gas(J/kg); is the thermal conductivity of the gas(W/m · k); is the airflow temperature(k); is the enthalpy of component (J/kg); is the mass diffusion rate of component (m²/s); is the source term for the gas-phase chemical reaction.

Regenerator energy equation:

4

Gas phase composition conservation equation:

5

in the equation, R represents the ideal gas constant; denotes the mass fraction of component ; and stands for the net chemical reaction production rate of component .

The thermal oxidation of low-concentration coal mine methane (LC-CMM) is a complex process comprising hundreds of consecutive fundamental reactions, accompanied by the generation and consumption of numerous radicals. Wang et al.'s research broadly categorizes the combustion of CMM into four stages:²³ preheating, heterogeneous reaction, homogeneous reaction, and post-combustion. This study primarily focuses on the homogeneous reaction stage. Furthermore, to reduce computational costs and accurately predict methane combustion products, it is necessary to simplify the mechanisms involved. In numerical simulation studies, there are three reaction mechanisms to choose from: the methane-air one-step reaction mechanism, the methane-air two-step reaction mechanism, and the SKEL detailed mechanism (with 16 reaction components and 41 elementary reactions). Although the detailed mechanism can provide excellent simulation results, its complexity and computational cost hinder direct application in industrial reactor design. Considering both computational time and accuracy, the methane-air two-step reaction mechanism is adopted. Research indicates that this two-step reaction mechanism can describe the methane oxidation process in a honeycomb ceramic reactor with sufficient accuracy³⁷. The two-step reaction mechanism for methane oxidation is shown in Table 2.

Table 2.

The two-step reaction mechanism for methane oxidation.

No	Reactions
1	2CH4 + 3O2 → 2CO + 4H2O
2	2CO + O2 → 2CO2
3	2CO2 → 2CO + O2

During the calculation, a fluid–solid coupling method is employed, considering the influence of boundary layer effects. Periodic boundary conditions are applied to the solid wall. The main numerical models used are: the Standard k-epsilon turbulence model, the EDC combustion model, the two-step reaction mechanism for methane-air, the DO radiation model, the Simple algorithm, and the second-order upwind differencing scheme.

Results and discussion

Methane concentration

The temperature value in the high-temperature zone of the oxidation device bed (hereinafter referred to as the bed) is approximately considered equal to the sum of the ignition temperature of the premixed gas and the temperature change³⁸. The level of methane concentration in the premixed gas represents the magnitude of the input calorific value. Excessively high concentration leads to excessively high internal temperature, which can cause damage to the oxidation device and catalyst or even danger. On the other hand, excessively low concentration can lead to the blowing out of the flame, resulting in the cessation of the oxidation reaction. To determine the appropriate methane concentration for a double-oxidation device, considering a material flow rate of 10,000 m³/h and under suitable conditions for hot air extraction, the situation where the methane concentration increases from 1.2 to 2.8 vol% in the calculation conditions is investigated.

Figures 2 and 3 show the maximum temperature of the bed and the temperature distribution of the bed at steady state after the catalytic oxidation reaction of methane with different concentrations under the appropriate hot air outlet flow rate. As can be observed in Fig. 2, with the increase in methane concentration, the maximum temperature of the bed rises from 1300 to 1800 °C. The temperature increase is most significant when the methane concentration exceeds 1.8 vol%. Once the maximum temperature of the bed exceeds 1500 °C, even exporting more hot air cannot reduce the maximum temperature. Furthermore, Fig. 2 demonstrates that higher methane concentration leads to a higher maximum temperature of the bed, a more pronounced temperature change trend, a narrower temperature peak, less effective utilization of the regenerative layer, and an increase in the amount of hot air required to be exported, resulting in poorer stability of the oxidation device. This is because the increase in methane concentration directly leads to an increase in the calorific value inside the device, causing a rapid rise in temperature and an acceleration in gas flow rate. This reduces the time for gas mixing and reaction, and the combined effect of temperature rise and flow rate increase impacts the stability of the oxidation device. Therefore, based on the simulation results, from the perspective of stable operation of the reactor, heat resistance of the reactor material, and safe operation, the methane concentration in the dual oxidation device should not exceed 1.8 vol%.

Fig. 2.

Different methane concentrations, appropriate hot air output, and the maximum bed temperature at quasi-steady state.

Fig. 3.

Different methane concentrations, Bed temperature distribution in quasi-steady state.

Reversing time

During the instant of airflow reversal, some gas may not flow through the reaction zone and is directly discharged, resulting in incomplete oxidation of methane in this portion of the gas. This leads to partial energy loss and impacts the operation of the system. As the cycle duration shortens, the impact of energy loss caused by the incomplete oxidation of the escaping gas on the system operation increases, resulting in a decrease in the average outlet temperature. The influence of the reversal time also includes the position of the flame front, the internal gas pressure of the device, and the tolerance of the porous media and catalyst. More frequent reversal times can cause instability in internal pressure and damage to the heat storage packing. Conversely, longer reversal times increase the probability of flame blow-off. Based on the previous conclusion that the optimal methane concentration for this device is 1.8 vol%, we selected 1.6 vol% methane concentration as a comparison. We investigated the cases with methane concentrations of 1.8 vol% and 1.6 vol%, a material flow rate of 10,000 m³/h, a fixed hot air output of 25%, and reversal times of 30 s, 50 s, and 70 s.

Figure 4 shows the maximum bed temperature at a maximum hot air output of 25% with methane concentrations of 1.6 vol% and 1.8 vol% at reversal times of 30 s, 50 s, and 70 s. It can be observed that the maximum bed temperatures are relatively close, fluctuating between 1500 and 1700 ºC. As the reversal time increases, the amplitude of temperature fluctuation also increases. Figures 5 and 6 display the temperature profile of the exported hot air and the methane conversion rate in the exported hot air, respectively, at reversal times of 30 s, 50 s, and 70 s. It is evident that as the reversal time increases from 30 to 50 s and then to 70 s, the minimum temperature of the exported hot air drops significantly, with the lowest temperature reaching around 300 °C. Moreover, the methane conversion rate in the exported hot air approaching 0% indicates a significant amount of unreacted methane. Therefore, with longer reversal times, the temperature fluctuation of the exported hot air increases, accompanied by a large amount of unreacted methane. Conversely, shorter reversal times result in narrower temperature peaks and smaller heat accumulation intervals.

Fig. 4.

Different reversing time, the maximum temperature of the bed is the highest when the maximum amount of hot air is 25%.

Fig. 5.

Different reversing time, derived hot air temperature (local).

Fig. 6.

Different reversing time, derived methane conversion rate in hot air (local).

The study found that the reversal time does not affect the maximum bed temperature but has a significant impact on the methane conversion rate, temperature, and residual heat quality of the exported hot air. Shorter reversal times are beneficial for increasing the temperature and conversion rate of the exported hot air. In this study, with methane concentrations of 1.6 vol% and 1.8 vol%, a reversal time within the range of 30-60 s is considered suitable.

Catalyst placement position at appropriate concentration of methane

This study utilizes the perovskite catalyst LaCoO₃ among non-noble metal catalysts, which is mounted between ceramic spheres and honeycomb ceramics. The honeycomb ceramic is in the shape of 20 mesh and measures 200 mm. The catalytic reaction process employing LaCoO₃ as the catalyst follows the basic principles of catalytic reactions, including reactant diffusion, adsorption, surface reaction, desorption, and product diffusion.

In the catalytic oxidation device, the catalyst needs to be loaded in an appropriate position. The loading position should be chosen between the ignition temperature and the maximum tolerable temperature of the catalyst. The maximum tolerable temperature of the catalyst used in this simulation does not exceed 650 ℃.

Figure 7 shows the temperature distribution at different positions of the device under appropriate hot air output when the methane concentration is 1.6 vol% and 1.8 vol%. According to the results of numerical simulation, when the methane concentration is 1.6 vol%, the catalyst needs to be loaded at a position of about 0.75 m from the reactor inlet; when the methane concentration is 1.8 vol%, the catalyst needs to be loaded at a position of about 0.8 m from the reactor inlet. It should be noted that the higher the methane concentration in the mixed gas, the narrower the temperature peak of the bed, and the more heat accumulates towards the center of the reactor, requiring the catalyst position to be shifted towards the center.

Fig. 7.

Different hot air output, temperature distribution at different positions.

The amount of hot air exported

Hot air extraction is an effective method for heat recovery and bed temperature adjustment. However, for bed temperature adjustment, extracting a portion of hot gas from the reactor will affect the downstream bed temperature, so additional measures must be taken simultaneously to avoid overheating of the upstream catalyst. Additionally, as hot air extraction significantly impacts the stability of the reactor, the proportion of extracted hot air needs to be carefully adjusted. The thermal energy from the oxidation unit is designed to be used for power generation or heating, improving the economic efficiency of the operating system. However, the temperature curve of the oxidation unit is highly sensitive to the extracted heat, so determining the appropriate amount of hot air extraction for methane oxidation at different concentrations has a significant impact on the stable operation of the oxidation unit, methane conversion rate, and the quality of waste heat.

For methane concentrations of 1.6 vol% and 1.8 vol%, with a material flow rate of 10,000 m³/h and a fixed reversing time of 50 s, the extracted hot air volumes were investigated at 15%, 20%, 25%, and 30%. Figure 8 shows the maximum bed temperatures for different extracted hot air volumes at methane concentrations of 1.6 vol% and 1.8 vol%. It can be observed that the maximum bed temperature increases with the increase in extracted hot air volume. This result differs from common sense. Generally speaking, the more hot air extracted, the more heat it carries away from the oxidation unit, and the bed temperature should be lower. However, analysis reveals that a portion of the extracted hot air contains unreacted methane, and the proportion of unreacted methane increases with the extracted hot air volume, ultimately leading to a limited amount of heat in the actual extracted hot air and a higher bed temperature in the oxidation unit.

Fig. 8.

The maximum temperature of the bed with different hot air output.

Figures 9 and 10 respectively show the temperature diagram of the exported hot air and the conversion rate of methane in it under the methane concentrations of 1.6 vol% and 1.8 vol%. It can be obviously found that with the increase of the amount of exported hot air, the temperature of the exported hot air will fluctuate sharply, resulting in dramatic changes in the methane conversion rate in the exported hot air. Moreover, the change patterns of all three indicate that an increase in the amount of exported hot air will lead to a decrease in the temperature and methane conversion rate of the exported hot air. As mentioned earlier, when the hot air is exported, some unreacted methane gas will also be exported, and the proportion increases as the amount of exported hot air increases. Therefore, more exported hot air results in less heat exported and a significant reduction in the methane conversion rate.

Fig. 9.

Different hot air derived quantity, derived hot air temperature.

Fig. 10.

Different hot air derived quantity, the methane conversion rate of hot air is derived (local).

According to the results of the array simulation, when the methane concentration is 1.6%, the reasonable amount of exported hot air is around 15%; when the methane concentration is 1.8%, the reasonable amount of exported hot air is around 20%. This study found that with a lower amount of exported hot air, the temperature peak of the bed layer is wider, allowing more heat to accumulate in the heat storage layer, thus improving the shock resistance of the bed layer. Therefore, an appropriate and lower amount of exported hot air can ensure the temperature stability of the exported hot air and the almost complete conversion of methane in the exported hot air. This is conducive to improving the overall methane conversion rate, reducing the temperature of the bed layer, improving the quality of waste heat recovery, and enhancing the stability of the bed layer.

Conclusions

Using the method of numerical simulation, the catalytic oxidation process of LC-CMM was simulated and studied under the conditions of a material flow rate of 10,000 m³/h, methane concentration ranging from 1.2 to 2.8 vol%, reversing time ranging from 30 to 70s, and hot air outlet ranging from 15 to 30%. Focusing on the temperature changes in the bed layer of the oxidation device, the temperature in the exported hot air, and the methane conversion rate, the study analyzed the influence of methane volume fraction, reversing time, and hot air outlet on the internal combustion conditions of the oxidation device, and investigated the appropriate position for catalyst placement. The main conclusions of the study are:

1. When the methane concentration rises from 1.2 vol% to 2.8 vol%, the maximum temperature of the bed rises from 1300 to 1800 °C. From the perspective of stable and safe operation of the oxidation device and heat resistance of the thermal storage material, the methane concentration should not exceed 1.8 vol%.

2. The reversing time does not affect the maximum temperature inside the oxidation device, but it affects the methane conversion rate and the quality of the residual heat in the exported hot air. When the methane concentration is 1.6 vol% and 1.8 vol%, a reversing time within 30–60 s is considered suitable.

3. When the methane concentration is 1.6%, the reasonable export volume of hot air is around 15%; when the methane concentration is 1.8%, the reasonable export volume of hot air is around 20%. The increase in the export volume of hot air will lead to a decrease in the temperature of the exported hot air and the methane conversion rate. A suitable hot air export volume is conducive to waste heat recovery and safe operation of the device.

4. When the methane volume fraction is 1.6 vol%, the catalyst needs to be filled at a position about 0.75 m from the reactor inlet; when the methane volume fraction is 1.8%, the catalyst needs to be filled at a position about 0.8 m from the reactor inlet. The higher the methane volume fraction in the mixed gas, the narrower the peak temperature of the bed, and the more heat accumulates towards the center of the reactor, requiring the catalyst position to move towards the center as well.

The research results provide a reference for the numerical simulation study of LC-CMM oxidation utilization and a basis for the industrial application of LC-CMM. These findings offer new insights into the operation and heat recovery potential of ultra-low methane concentration heat flow reversal reactors.

Author contributions

X.J., X.F., W.L. and X.Y. made suggestions on the design and change parameters of the oxidation device. With the help of X.J. and Q.X., Y.H. completed the numerical simulation experiment and data analysis. Y.H. drafted the manuscript. All authors contributed to commenting and writing on the draft manuscript.

Data availability

All data generated or analysed during this study are included in this published article.

Competing interests

The authors declare no competing interests.