Demonstration and its validation for ventilation air methane (VAM) thermal oxidation and energy recovery project

Applied Thermal Engineering 90 (2015) 75e85

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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng

Research paper

Demonstration and its validation for ventilation air methane (VAM) thermal oxidation and energy recovery project Qingzhao Li a, *, Baiquan Lin a, Desheng Yuan a, b, Genma Chen c a

School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China Shaan xi Binchang Mining (Group) Co., Ltd., Xianyang 721000, China c Shaan xi Binchang Dafosi Mining Co., Ltd., Xianyang 721000, China b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Demonstrated TFRR for VAM and drainage methane co-oxidation was analyzed.
Running stability is improved by premixing part of low concentration methane.
36 h is needed for TFRR completing its preheating process.
Electricity generation was possible only for the methane concentrations above 0.6%.
Thermal efficiency various from 31.61% to 46.82% at the experimental conditions.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 January 2015 Accepted 22 June 2015 Available online 9 July 2015

In the present work, a preliminary technical and demonstration running assessment had been conducted in the Dafosi coal mine, China. First commercially available technologies for both mitigation and utilization of coal mine methane (VAM) combined with drained low concentration methane by premixing and co-oxidation was briefly described. Based on the methane emission characteristics, the running properties of thermal flow reversal reactor (TFRR) and gas mixing unit have been analyzed systemically. Under the experimental condition, TFRR should be preheated for about 27 h until the central temperature of monolith bed reaching to about 1000  C. Another 9 h are needed to make the temperature evolutes to the similar profiles under the stable operation conditions with the help of electric heater and fed methane gas oxidation. For the TFRR, about 31.61%e46.82% energy can be recovered and used for electricity generation at the stable running conditions. Methane concentration corresponding to TFRR stable and self-maintained running should not be less than 0.25%. However, the effective and economical operation with heat recovery for electricity generation is possible only for the methane concentrations higher than 0.6%, and recommended at 1.0 vol.% methane concentration. © 2015 Elsevier Ltd. All rights reserved.

Keywords: VAM Low concentration methane TFRR Co-oxidation Energy recovery Demonstration

1. Introduction

* Corresponding author. Tel./fax: þ86 516 83884401. E-mail address: [email protected] (Q. Li). http://dx.doi.org/10.1016/j.applthermaleng.2015.06.089 1359-4311/© 2015 Elsevier Ltd. All rights reserved.

China is rich in coal-related methane resources. According to the latest evaluation on coal and coal mine methane by the National Development and Reform Commission (NDRC) and the Ministry of

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Land and Resources, Chinese coal-related methane resources stored in a depth of 2000 m are over 34 trillion cubic meters, about 12.5% of the world's in total, ranking the third in the world [1,2]. Coal mine methane (CMM) refers to methane emitted from the coal seam and surrounding rock strata due to coal mining activities. And now, China has become the world's leading emitter of coal mine methane, accounting for about 15% of anthropogenic methane emissions [3]. Furthermore, methane is the second most important greenhouse gas (GHG) after carbon dioxide over a 100year period in trapping heat in the atmosphere according to the Intergovernmental Panel on Climate Change (IPCC, 2007) [4]. Therefore, coal mine methane mitigations would produce important near-term progress toward climate changes [5e7]. In addition, methane is a hazard source to underground coal safety mining since it is explosive in concentrations ranging from about 4.5% to 15% in air. So, underground coal production progresses must employ large-scale ventilation systems to dilute the methane and remove the methane from the mining environments for improving safety. Typical concentration of methane in the underground coal mine air is always below 1%. Although this concentration is very dilute, ventilation air methane (VAM) is the largest methane emissions source since the exhaust gas volume is huge. Approximately 64% of methane emissions from coal mining activities are the ventilation air methane (VAM) [8]. Methods to mitigate VAM are desirable from a global climate perspective and it is economical if the VAM energy can be captured. However, VAM energy is very difficult to use because of its low methane concentration and high volumetric flow. Almost all the ventilation air methane (VAM) in Chinese coal mine is released directly to the atmosphere [9]. Among all the different technologies for mitigating ventilation air methane, approach of oxidizing methane to carbon dioxide is the most effective method. Due to global warming potential of methane is 20 times higher than that of carbon dioxide, there is a clear environmental advantage for oxidizing methane to carbon dioxide before releasing it to the atmosphere [10]. Generally, there are two basic methods for the mitigation and utilization of VAM: ancillary use and principal uses [11]. Ancillary use is referred to use of VAM as an oxidizer instead of ambient air in the combustion engines, gas turbines or rotary kilns to improve combustion performance [12,13]. However, VAM ancillary use is restricted by the local combustion equipments. The principal uses of VAM as a primary fuel can be found in thermal flow reversal reactor (TFRR), catalytic flow reversal reactors (CFRR), lean burn gas turbines, and regenerative thermal oxidation apparatus [11,14e16]. The most appropriate combustion technology depends on the concentration of methane in the emitted mine gas. For the lower concentration methane, auto-thermal oxidation by TFRR technology (e.g., without the need of additional fuel) is feasible, but it requires heat exchange between the inlet and outlet streams. During the TFRR oxidation process, the heat of the outlet flue gas is first stored in the inert porous medium bed, which is later used to preheat the inlet VAM stream. So, the TFRR device operates under the forced unsteady-state conditions by periodically reversing the feed flow direction. Therefore, the heat released by the exothermic reaction is trapped inside the porous medium bed between two consecutive flow reversals, being used to preheat the cold VAM gas up to the reaction temperature. Moreover, with the integrated heat recovery system, the excess thermal energy in the TFRR can be recovered for power generation [17]. Therefore, TFRR technology has the potential to reduce coal mine low concentration emissions at relatively low cost [11,18e20]. In all, reducing coal mine methane emissions may provide much clean energy, safety mining conditions and beneficial environment. However, the spread of the VAM mitigation and utilization project

also faces many challenges, such as technology and operation costs [21]. And, there is no other commercial VAM mitigation demonstration running in China. The present research work gives some analysis on the performance of the first VAM utilization project in Dafosi coalfield, China, which may provide a clear and feasible reference for the future of coal mine VAM mitigation and utilization in China. 2. General information about ventilation air methane mitigation project in Dafosi coal mine 2.1. Methane emission characteristics in Dafosi coal mine Dafosi Jurassic coalfield is located in the southern fringes of the Ordos Basin, northern edge of Weibei flexure, Shaanxi province, China. The coal vitrinite reflectance in the mine field is about 0.5%e 0.75%, belonging to long flame coal according to GB/T 17608-2006 (Coal quality technology, 2007). Methane contents of major coal seam in Dafosi coal mine varies from 6.29 m3/t to 9.24 m3/t. The averaged gas saturation degree of coal seam is about 69.45%. According to the geological reserves of coal and measured coal mine methane contents, the total gas resource in the major mining coal seam is about 15.3 billion cubic meters. Therefore, it can be predicted that coal mine methane exploitation and utilization may have a bright prospect in Dafosi coal mine. Dafosi coal mine has developed and implemented surface drainage wells and suitable underground gas drainage pumps according to its geologic and geographic characteristics. Drainage gas with higher methane concentrations (higher than 65%) from the surface wells is used as fuels for civil and chemical plants in the Binxian, China. For the gas drained from in-seam and adjacent coal seams, it could be used for power generation directly with internal combustion engine if the methane concentration is higher than 10% (shown in Fig. 1(a)e(c)). However, drainage gas with methane concentration lower than 8% (shown in Fig. 1(d)) is always directly vented to the atmosphere before TFRR system operated in Dafosi coal mine due to the lack of necessary and appropriate using technology. But in Dafosi coal mine, about 23.71% of total emitted gas is low concentration (8%) methane mixtures. For the venting of ventilation air methane (VAM), two ventilation shafts are operated in Dafosi coal mine. During measurement period from January to June 2013, the VAM concentration was remained substantially constant, from 0.15% to 0.30% (average of 0.23%) and the average VAM flow rate was about 20,466 m3/min (shown in Fig. 2). Compared with the total methane emitted from coal mining, pure methane capacity in the VAM has reached about 53.6% in Dafosi coal mine. Therefore, the present demonstration of coal mine methane mitigation and utilization, including drain of low concentration methane (lower than 8%) and ventilation of air methane, may has great significance not only for Dafosi coal mine but for most of Chinese coal mines. 2.2. General information about the on-site VAM and low concentration methane co-utilization projects Among the typical VAM mitigation method, TFRR may be a suitable selection for the actual practice at the coal mine site. Based on the thermal oxidation principle, MEGTEC, VOCSIDIZER and SHENGDONG are having the demonstrated ability of offering TFRR unit commercially. Based on emitted methane gas properties of Dafosi coal mine and economical evaluation, five on-site TFRR VAM oxidation systems had been demonstrated and operated by SHENDONG Corporation. The TFFR system contains an inert porous medium bed for thermal regeneration exchanger, which should be preheated to the temperature about 1000  C initially by the

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Fig. 1. Pure methane flow rate and concentration of different typical gas drainage systems.

mounted electric heating elements. VAM enters into thermal oxidizers and will be oxidized if the bed materials have been preheated to the temperature of methane auto oxidation. With VAM oxidizing and heat releasing, the temperature of the bed materials will maintain at 1000  C or even higher. Thereby, the heat storage

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Date (yy-mm-dd) Fig. 2. Characteristics of ventilation air methane (VAM) emissions from Dafosi coal mine.

materials will sustain the auto-oxidation process over time without any additional fuel inputs. When the far side of the porous medium is sufficiently hot, the gas flow direction should be automatically reversed to keep the hot zone in the center of the oxidizer. The technical specifications to the VAM oxidizer are shown in Table 1. Thermal Flow-reversal reactor (TFRR) employs the principle of regenerative heat exchange between gas and solid heat storage medium. To start the operation, the bed should be preheated to the temperature required to initiate methane oxidation. Ventilation air methane (VAM) at ambient temperature enters and flows through the reactor in one direction, gas temperature increases until methane oxidation takes place near the center of the bed. Hot products of oxidation continue through the bed, losing heat to the solid heat storage medium in the process. When the far side of the bed is sufficiently hot, gas flow direction should be reversed to keep the hot zone in the center of the oxidizer. The overall thermal

Table 1 TFRR and power generator set specifications. Specifications of TFRR and power generator VAM abatement principle TFRR unit capacity Minimum VAM concentration required Steam parameter Steam engine capacity

Thermal oxidation 60,000 Nm3/h 0.25% 1.2 MPa & 280  C 4.5 MWe

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oxidation mechanism of methane in the TFRR may be represented by the following equation,

CH4 þ 2O2 /CO2 þ 2H2 O

DHð298Þ ¼ 802:7KJ=mol

However, this is a gross simplification, since the actual reaction mechanism involves many free radical chain reactions [22]. Methane thermal oxidation may produce CO or CO2 depending on the air/methane ratio by the following reactions,

parameters and thermal recovery efficiency actually. Based on this purpose, one gas mixing system is specially designed and installed for the mixing of drained low concentration methane (CCH4 8%) with ventilation air methane (VAM). Therefore, the present thermal oxidizer in Dafosi coal mine is actually operated for the cooxidizing processes of the drained low concentration methane (CCH4 8%) and VAM. 3. Results and discussion

CH4 þ 2O2 /CO2 þ 2H2 O CH4 þ 1:5O2 /CO þ 2H2 O And, other reactions may also be presented, such as:

CH4 þ H2 O/CO þ 3H2 2H2 þ O2 /2H2 O CO þ H2 O/CO2 þ H2

2.3. Approach of low concentration methane and VAM co-oxidation by TFRR and power generation The simplified flow sheet of low concentration methane and VAM co-oxidation and power generation plant are given in Fig. 3, while the general views of on-site methane drainage and utilization project is shown in Fig. 4. In order to recover the oxidized thermal energy and improve economical performance of TFRR system, one set of special designed coil heat exchanger is embedded in the porous medium to recover heat and generate power in result. Furthermore, on-site TFRR oxidation and power generation plant also includes one set of gas mixing system, five parallel arranged thermal oxidizers and one set of steam turbine power generator. The generated electricity is first supplied for the self-utilization of coal mine. Although the self-sustaining operation methane concentration can be lower to about 0.25%, the methane concentration of inlet gas is set at 1.0% in order to improve the generated water steam

3.1. Preheating process of large scale TFRR In principle, TFRR is an auto thermal reactor integrated with regenerative heat exchanger, which allows by periodic reversal flow to ensure an efficient reaction heat recovery to maintain the auto oxidation process without any additional fuel [23]. However, the inert porous medium of the TFRR need to be initially preheated to the temperature to make the entered methane gas be oxidized automatically. Therefore, the preheating process of inert porous medium may have great significance for the running performance of TFRR systems [24]. For the demonstrated TFRR, one set of electric heaters mounted in the center section of inert porous medium are used for preheating the monolith bed to enable the start of auto thermal operation. Due to the large heat capacity of the inert monolith bed, at least a couple of hours are necessary to obtain the cyclic steady state. In order to analyze the evolution process of temperature distributions in the reversal flow thermal regenerator, eight movable thermocouples are equipped along the axial gas channels to measure temperature profiles and evolutions in the TFRR. The 2D and 3D temperature profiles during TFRR preheating process (without heat withdrawal) are plotted in Fig. 5. Previous researches show that the threshold for methane auto thermal operation in the TFRR requires about 50e90  C [25] to 45e70  C [26] higher than the adiabatic burning temperature of methane. So, the present electric preheating temperature has been set to preheat the inert monolith bed up to about 1000  C. From the experimental results, it is shown that the demonstrated monolith bed needs to be preheated for about 27 h until the central temperature reaches to about 1000  C. At this moment, the mixed methane gas could be fed

Fig. 3. Diagram of ventilation air methane (VAM) oxidation and energy recovery system at Dafosi coal mine.

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Fig. 4. On-site VAM & drainage low concentration methane mitigation and energy recovery integrated project in Dafosi coal mine, Binchang, Shaanxi, China.

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into the TFRR to speed up the temperature profiles evolution process with methane oxidation. About another 9 h (36 h for accumulated heating time) are needed with the supplement of electric heater and methane gas oxidation, the temperature profiles will present similar evolutes and the TFRR runs will be stable operated. During the electric heating process, the peak of temperature profiles is much narrower initially. However, it will be great wider until

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designed TFRR are stable, the structure is reasonable and the settings of controlling parameters are effective under the designing and testing conditions. For the present completely designed TFRR system, it is clear that the goal of stable and safety running can be an achievable target depending on the applied circumstances (controlling systems availability, VAM and drainage methane gas quality, the ratio of heat withdrawn).

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the similar and stable profiles with continue heating input and methane gas oxidization supply (Fig. 5b). And, temperature profiles during the consecutive reversal half-cycles move back and forth along the axial direction of inert porous medium bed, which causes the temperature records show repeatable saw shapes in time (shown in Fig. 5a).

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3.3. Performance of TFRR energy recovery system 3.2. Stability of TFRR oxidation system running For the present demonstrated TFRR, oxidized thermal is recovered through one set of central cooling heat exchangers, which includes a water heater (economizer) and a steam super-heater. When amount of heat generated by the reactions is high enough, some part of the heat can be withdrawn from the reactor and be utilized outsider. From the view of thermal recovery, higher reactor temperature is contributed to the heat recovery. However, the temperature profile of TFRR is very sensitive to the quantity of heat withdrawn to surroundings because the oxidized thermal energy is designed to generate superheated water steam used for electricity generation to improve the economy of running systems. Increasing the ratio of heat loss to surroundings will result in forming a visible concavity (saddle) in the central part of the temperature profiles. And indeed, measured temperature profiles concavity is also clearly visible. Considering adjustment of industrial equipment and designing parameters of present demonstration, the measure condition is set as follows: Gas volume capacity60,000 Nm3/hour, half cycle period-150 s and fed water mass flow rate: 9.0 ton/hour. Generally, adequate heat recovery does not change TFRR performance much more except for the possibly larger heat losses [27,28]. Under all the stability testing condition, fed gas volume flow rate is controlled at about 60,000 m3/h and the methane concentration is adjusted at 1.0 vol.%. The half cycle period of gas reversal flow is set at 150 s. During the testing period of each TFRR unit, about 60 h temperature data had been recorded for TFRR stabilized operation performance evaluation. At the experimental running situation, about 31.61%e46.82% (thermal efficiency is calculated by Eq. (1) and methane oxidation efficiency is calculated by Eq. (2)) of energy has been recovered and used for electricity generation. The tested results of temperature distributions and evolutions are shown in Figs. 6 and 7. It can be seen that TFRR temperature profiles present stable and similar distributions and the central temperature maintains constant substantially under the experimental conditions. Although the peak temperature values show some fluctuation to some extent, the temperature profiles present similar “saddle” distributions (Figs. 6a and 7a). The stable temperature distributions indicate that the operation properties of

From the perspective of economic and efficient use, quantities of VAM oxidized thermal energy can't be utilized due to its lowquality heat density. Only the “high-temperature & high-pressure” water steam is been produced, the recovered energy can be used to generate electricity further. Besides the previously described influencing factors in Section3.1 and Section3.2, another important influencing factor on TFRR temperatures is the methane concentration in the feeding gas. Therefore, the relationship between the TFRR performance and inlet gas methane concentration had been investigated with the present demonstration. In present experiments, methane concentrations adjusted from 0.2% to 1.2% by mixing drainage methane gas with VAM. The testing results are shown in Figs. 8e11. Fig. 8 shows that there is an approximate linear relation between the generated steam parameters (steam pressure and temperature) and methane concentrations in the feeding gas. And, the average temperature of TFRR also presents a linear relation with methane concentration. Moreover, the methane oxidation efficiency increases with methane concentration by an exponential function. Therefore, considering the requirements of turbine machine to the steam parameters, methane concentration in the feeding gas should not be lower than 0.6%. Under this condition, the average temperature of TFRR is also higher than 800  C and the methane mitigation efficiency is not less than 95% (shown in Fig. 9, methane oxidation efficiency is calculated by Eq. (2)), but the half cycle period of gas reversal flow should be adjusted at about 80e100 s correspondingly. Under present testing conditions, the temperature of exhaust flue gas presents fluctuations due to the gas periodical reversal flowing. Furthermore, differences of the average temperature between the inlet gas and exhaust flue gas are less than 20  C on the whole (shown in Fig. 10), which indicate a higher performance of thermal recovery efficiency and lower flue gas heat losses with the present designed TFRR. Theoretically, VAM oxidation in the TFRR will be auto thermal oxidized if CH4 concentration is above 0.19% [11]. For the tested demonstrations, corresponding to the stable and self-maintained running without additional fuels, the minimum methane concentration should not be less than 0.25%. However, for the present demonstration, the effective and economical running with heat

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recovery and electricity generation is possible only for the methane concentrations exceed 0.6%. Considering the stability and safety of TFRR running systems, methane concentrations in the feed mixed gas is recommended at 1.0 vol.% in the mixed feeding gas. The ultimate goal is to evaluate demonstration's stable running in long term. From the results of Fig. 11, it can be seen that the steam parameters can be remained stable and higher than the expectations (1.5 MPa & 300  C) during the 1000 testing hours, which meet

the requirements (1.2 MPa & 280  C) of turbine for electricity generation. Therefore, the present demonstrations meet the designed targets and achieve remarkable improvements. 3.4. Adaptive property of gas mixing unit for TFRR Due to the coal seam methane reservoir characteristics, drained methane concentration and gas flow rate always fluctuate inevitably with the prolonging of drainage time. However, operation

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performance of TFRR is very sensitive to the methane concentrations in the feeding gas [24]. Therefore, how to keep a constant methane concentration in the feeding gas is a key technical issue needed to be resolved in order to ensure the unit reach the desired operational stability and efficiency. Considering concentration characteristics of drainage methane and ventilation air methane (VAM) as feed forward signal and mixed gas methane concentration as feedback signal, gas mixing systems are operated by one auto adaptive mode adjusting the ratio of VAM and drainage gas flow rate to keep the TFRR inlet gas methane concentration constant substantially or fluctuate within the acceptable ranges to some extent.

The measured results for the gas mixing system during long time operation (about 1000 h) are shown in Fig. 12. Under the condition that VAM and drainage gas methane concentration fluctuates, it is very delighted that the present gas mixing system can adjust the mixed methane concentration among the expected ranges. From the results, it can be seen that methane concentration could be controlled among an acceptable ranges from 0.9% to 1.1% with the help of present gas mixing system, which meets the requirements of TFRR unit safety and economical running. The succeeded gas mixing control technology should also be highlighted because it has assisted the TFRR unit and power generation system to reach its ultimate potential stability.

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The present power plant using drainage low concentration methane and VAM has been put into run in August 2012, processing about 300,000 Nm3/h of ventilation air (0.9%e1.1% CH4) to generate about 4.5 MWe electrical power. With the help of gas mixing system blending the drained low concentration methane with ventilation air methane (VAM), inlet gas methane concentration is maintained relatively stable. The measured results about thermal energy and electricity power are shown in Fig. 13. During the testing periods, the generated steam parameters (steam pressure and temperature) could keep constant approximately. Flow rate of superheated water steam is about 9.31 ton per hour and the electricity power generation is about 1299.05 KWe on average. In term of the first low concentration methane power plant demonstration in Dafosi coal mine, China, about 47.5% total ventilation air methane (VAM) combined with part of drainage low concentration methane gas has been mitigated and utilized up to now. According to the statistics data, about 3.33 million KWh electricity has generated in 2012 and 22.42 million KWh electricity has generated in 2013 with the present coal mine methane power plant. And, more than 2.5  107 Nm3 pure methane has been mitigated and utilized, equivalent to about 4.0  107 tons carbon dioxide (vales applied: GWPCH4 ¼ 21tCO2e/tCH4, EffELEC ¼ 99.5%, Weight of 1 Nm3 of

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methane ¼ 0.714 Kg/Nm3). Therefore, the approach of VAM and drainage low concentration methane co-oxidation and utilization meets the coal mine actual situation (especially considered the emission characteristics of coal mine methane), which has practical operability, great environmental and economic values. Additional, co-oxidation method for VAM and drainage low concentration methane greatly improves system's running stability and security. Compared with other technologies for coal mine low concentration methane and VAM mitigation, it can be concluded that the present approach including related technologies is one of the promising programs and worthy to be popularized among similar coal mines.

4. Conclusions In the research work, a preliminary technical assessment had been conducted for the present methane mitigation and utilization project in Dafosi coal mine, China. First commercially available technologies as well as the overall solutions were briefly described as an approach of premixing and co-oxidation method for VAM and drained low concentration methane. Rules about the coal mine methane emissions, performances of gas mixing unit and TFRR systems were analyzed briefly. Methane contents in Dafosi coal mine varies from 6.29 m3/t to 9.24 m3/t. Compared with the total gas emitted from the coal seams, VAM (average concentration 0.23%) accounts for about 53.6% and low concentration (8%) methane is nearly 23.7% in the Dafosi coal mine. In order to improve system's running stability and security, one approach of premixing and co-oxidation low concentration methane with VAM had been designed and demonstrated by TFRR system. With the help of the continuous monitoring system, it proved that the improved TFRR was technically suitable for VAM and drainage low concentration methane mitigation & utilization application. And, the oxidized thermal energy could be recovered by superheated water steam and used for electricity generation. With designed gas mixing unit, TFRR preheating time is about 27 h until the central temperature of TFRR monolith bed reaching about 1000  C under the experimental condition. About another 9 h (36 h for accumulated heating time) are needed with the supplement of electric heater and methane gas oxidation. Under the experimental condition, about 31.61%e46.82% energy can be recovered and used for electricity generation. Steam parameters and average temperature of TFRR present an approximate positive linear relation with the methane concentrations in the feeding gas.

1.6

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Moreover, methane oxidation efficiency increases with methane concentration by an exponential function. For the present TFRR, methane concentration corresponding to stable and selfmaintained running should not be less than 0.25%. However, the effective and economical operation with regard to heat recovery and electricity power generation is possible only for the methane concentrations above 0.6%, and recommended at 1.0 vol.%. The preliminary evaluation analysis shows that the present approach including related technologies is one of the promising and feasible programs, which is worthy to be popularized for low concentration methane utilization and mitigation. It might help coal mine to reduce its methane emissions to meet the Kyoto Protocol's requirements. Therefore, present research work would also have a positive effect both on greenhouse gas mitigation and energy recovery from a so-called wasted methane sources. In this regard, the present research work might have great significance because it provides one economic and feasible options for the mitigation and utilization of coal mine VAM and drained low concentration methane.

Acknowledgements Financial supports from the National Natural Science Foundation of China (Grant No. 51204169), the Natural Science Foundation of Jiangsu Province (Grant No. BK20131115), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20110095120017), the Fundamental Research Funds for the Central Universities (Grant No. 2014QNA04), the Fund for Innovation Team of CUMT, A project funded by the priority academic program development of Jiangsu higher education institutions, and the Program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT13098) are sincerely acknowledged.

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