Low-concentration Coal Mine Methane Research Articles

Performance evaluation of methane separation from coal mine gas by gas hydrate formation in a stirred reactor and in a fixed bed of silica sand

Coal mine methane (CMM) gas is a sort of unconventional natural gas that can be used as a clean supplementary energy resource when methane is purified. In this work, methane separation from a low-concentration CMM gas (30mol% CH4, 60mol%N2, and 10mol% O2) was investigated using gas hydrate formation in a stirred reactor and in a fixed bed of silica sand with the presence of tetrahydrofuran (THF) and sodium dodecyl sulfate (SDS). The concentration of THF and SDS was fixed at 1.0mol% and 500ppm, respectively. The experiments were carried out in batch operation with the initial pressure fixed at 3.6MPa, and the temperature was set at 277.15K. The results indicated that gas hydrate formation in the stirred reactor and in the fixed bed of silica sand were both promoted by adding SDS in THF solutions. It was found that the hydrate-based separation process for CH4 recovery from the CMM gas mixture performed better in the stirred reactor as compared to the fixed bed of silica sand employed in this work. The CH4 recovery obtained at 1.0mol% THF and 500ppm SDS in the stirred reactor was about 40.5%, which was much higher than those obtained in TBAB solutions (26.2%) and cyclopentane/SDS solutions (33.3%).

Scale Effect of Ceramic Foam Burner on the Combustion Characteristics of Low-Concentration Coal Mine Methane

The apparatus of low-concentration coal mine methane (LCM) combustion, from experimental scale to industrial scale, is necessary for the large volume of LCM. In this paper, an experimental system has been set up for the LCM combustion to validate the corresponding numerical model. Following this, the revised burner, with increased inlet diameter and burner length, was used to study the combustion characteristics numerically based on the reliable model. Results indicated that the temperature distribution exhibits a one-dimensional nature evidently and the temperature discrepancy among different inlet diameters is negligible with the adiabatic wall. However, the temperature discrepancy is apparent when the heat loss of the wall is considered, which shows the importance of heat preservation outside the burner wall. As the burner diameter increases, the total temperature increases and appears to be more homogeneous with the heat loss wall. In addition, NO emissions decrease gradually and CO emissions increase...

Combustion characteristics of low concentration coal mine methane in divergent porous media burner

Low-concentration methane (LCM) has been one of the biggest difficulties in using coal mine methane. And previous studies found that premixed combustion in porous media is an effective method of low calorific gas utilization. This paper studied the combustion of LCM in a divergent porous medium burner (DPMB) by using computational fluid dynamics (CFD), and investigated the effect of gas initial temperature on combustion characteristic, the distribution of temperature and pollutant at different equivalence ratios in detail. Besides, the comparison of divergent and cylindrical burners was also performed in this paper. The results show that: the peak temperature in DPMB increases as the increasing of equivalence ratio, which is also suitable for the outlet NO discharge; the linear correlation is also discovered between peak temperature and equivalence ratios; NO emission at the initial temperature of 525K is 5.64times, larger than NO emission at the initial temperature of 300K. Thus, it is preferable to balance the effect of thermal efficiency and environment simultaneously when determining the optimal initial temperature range. The working parameter limits of divergent burner are wider than that of cylindrical one which contributes to reducing the influence of LCM concentration and volume fluctuation on combustion.

Safe Separation of the Low-Concentration and Oxygen-Bearing Coal Mine Methane by Vacuum Pressure Swing Adsorption

Pressure swing adsorption (PSA) is a suitable method to enrich the concentration of methane in coal mine methane (CMM) separation processes. However, traditional PSA processes for low-concentration CMM can easily increase the concentration of methane to explosive limits, which subsequently increases the risk of explosion. In this paper, a novel PSA process for low-concentration CMM was studied. Using a mixture of activated carbon (AC) and carbon molecular sieve (CMS) as the adsorbent, this method ensures that the gaseous mixture adsorbed does not reach the explosive limits by adsorbing methane and part of the oxygen simultaneously. Using a two-bed vacuum PSA experimental apparatus, the low-concentration CMM was safely enriched from 20% to more than 30%, with CMS and AC mass ratio of 3.4. The results of experimental studies indicate that the mixture adsorbent (AC and CMS) has explosion-suppression and flameproof characteristics. The igniting resource (methane) will not explode even if it appears in the ads...

Coal Mine Methane Gas Recovery by Hydrate Formation in a Fixed Bed of Silica Sand Particles

In the present work, the separation of CH4 from low-concentration coal mine methane gas (30 mol % CH4/N2) through hydrate crystallization was investigated in a fixed bed of silica sand particles. The influence of the additive tetrahydrofuran (THF) on hydrate equilibrium conditions and kinetics of CH4 separation was studied as well. The incipient hydrate equilibrium conditions at 1 mol % THF were determined using the isothermal pressure search method. It was found that the presence of THF significantly reduced the hydrate equilibrium conditions as compared to those obtained in liquid water with the same gas mixture. CH4 recovery in the water-saturated silica sand bed was considerably low (∼12.0%) because N2 molecules might compete with CH4 molecules to enter the hydrate crystals under high pressure conditions. The addition of THF to the bed of silica sand particles reduced the nucleation time of gas hydrate formed from the 30 mol % CH4/N2 gas and increased the CH4 recovery (∼21.4%) significantly. The compa...

Proportion Pressure Swing Adsorption for Low Concentration Coal Mine Methane Enrichment

Using traditional Pressure Swing Adsorption (PSA) with a single adsorbent for low concentration coal mine methane (LCCMM) at a concentration of 30% or less can result in a final CH4 concentration very close to the explosion limit, increasing the risk of explosion. Proportion Pressure Swing Adsorption (PPSA) is a new and safer enrichment method suggested for LCCMM enrichment that uses a mixture of active carbon (AC) and carbon molecular sieves (CMS) as adsorbents. With this method, CH4 and O2 in LCCMM can be adsorbed simultaneously because CH4 is mostly adsorbed by active carbon and O2 is mostly adsorbed by the CMS. Therefore, the concentration of CH4 and O2 is well controlled and does not exceed the explosive limit during the adsorption and desorption processes. We have demonstrated the safety and feasibility of PPSA for obtaining 30% CH4 from LCCMM, with 20% CH4 in air as a feed stock. Our results show that the O2 concentration can be controlled well and does not exceed the explosive limit in both adsorption and desorption, and the CH4 concentration in the desorption gas can be increased to more than 30% by adjusting the bed length and mass ratio of the AC and CMS. Taking these results together, it appears that PPSA is a safe method for LCCMM enrichment.

Impact of Bed Length on Low-Concentration Coal Mine Methane Enrichment by Proportion Pressure Swing Adsorption

The enrichment and utilization of low-concentration coal bed methane (LCCMM) will provide a fuel source, will ensure safety from flammable gas release and will prevent significant greenhouse gas emissions. One of the special challenges in dealing with LCCMM originates from the presence of O2, which is difficult to process economically and explodes easily. In this work, we studied oxygen-bearing LCCMM with a CH4 concentration of 20% and developed a safe and effective enrichment method for LCCMM: Proportion Pressure Swing Adsorption (PPSA), which used a mixture of active carbon (AC) and carbon molecular sieves (CMS) as adsorbents. With this method, CH4 and O2 in LCCMM can be adsorbed simultaneously because CH4 is mostly adsorbed by active carbon and O2 is mostly adsorbed by the CMS. Therefore, the concentration of CH4 and O2 is well controlled and does not exceed the explosive limit during the adsorption and desorption processes. The impact of bed length on LCCMM by PPSA was investigated. We have demonstrated the safety and feasibility of PPSA for obtaining 30% CH4 from LCCMM, with 20% CH4 in air as a feed stock. Our results showed that the O2 concentration can be controlled well and does not exceed the explosive limit in both adsorption and desorption, and the CH4 concentration in the desorption gas can be increased to more than 30% by adjusting the bed length and mass ratio of the AC and CMS.

Experimental Investigation of Methane Separation from Low-Concentration Coal Mine Gas (CH4/N2/O2) by Tetra-n-butyl Ammonium Bromide Semiclathrate Hydrate Crystallization

In this work, tetra-n-butyl ammonium bromide (TBAB) semiclathrate hydrate was employed in a batch operation to separate CH4 from low-concentration coal mine methane (CMM) gas with a mole composition of 30% CH4, 60% N2, and 10% O2. TBAB semiclathrate hydrate formed from the low-concentration CMM gas has more favorable incipient equilibrium conditions than gas hydrates formed from the same gas mixture. The experiments were carried out at a fixed pressure of 4.0 MPa, three TBAB mole concentrations of 0.29, 0.62, and 1.38 %, and in the temperature range of 274.75–283.35 K. The effects of TBAB concentrations and subcoolings (ΔTsub) on CH4 separation from the low-concentration CMM gas were investigated. The results indicated that CH4 was preferentially incorporated into the hydrate crystals rather than N2 and O2 in the presence of TBAB. The experimental conditions of the TBAB concentration of 1.38%, 4.0 MPa, and ΔTsub = 7.0 K were most favorable for TBAB semiclathrate hydrate to separate CH4 from the low-concentration CMM gas. The CH4 recovery obtained at these conditions was approximately 27%. Compared with CH4 content in the low-concentration CMM gas, the mole fraction of CH4 in TBAB semiclathrate hydrate was increased to 41%.

Impact of Process Parameters on Oxygen-Bearing Low Concentration Coal Mine Methane Enrichment by Proportion Pressure Swing Adsorption

Conventional PSA methods with a single adsorbent can result in an explosive CH4 concentration of 5 to 15%, making low concentration coal mine methane (LCCMM) or risky in terms of safety at a concentration of 30%. Proportion Pressure Swing Adsorption (PPSA) is a new and safer enrichment method suggested for LCCMM enrichment that uses a mixture of active carbon (AC) and carbon molecular sieves (CMS) as adsorbents. The effect of adsorption Pressure, adsorption time and bed length on the PPSA process were investigated to verify the feasibility of this method. The experiment results show that the methane concentration in desorption gas and the oxygen concentration in exhaust gas increased and decreased, respectively, with an increase in adsorption pressure or bed length. And the methane concentration in desorption gas is increased with the adsorption time expanding, but the recovery of methane is reduced. So the methane concentration in desorption gas can be increased to more than 30% and the oxygen concentration in exhaust gas be decrease to less than 12% by increasing the adsorption pressure or lengthening the bed.

Methane Separation from Coal Mine Methane Gas by Tetra-n-butyl Ammonium Bromide Semiclathrate Hydrate Formation

This work presents new data to further develop the hydrate-based process for the separation of CH4 from the low-concentration coal mine methane gas (30 mol % CH4/N2) through the formation of tetra-n-butyl ammonium bromide (TBAB) semiclathrate hydrate. The TBAB semiclathrate hydrate formed from the 30 mol % CH4/N2 gas mixture has more favorable equilibrium conditions than the gas hydrate formed from the same gas mixture. The incipient equilibrium conditions at 0.17, 0.29, and 0.62 mol % TBAB were experimentally determined and reported. The experiments of forming TBAB semiclathrate hydrate from the 30 mol % CH4/N2 gas mixture were performed in both semibatch and batch operation under a fixed driving force of 3.5 MPa. The results indicate that CH4 is preferentially encaged into the TBAB hydrate. The use of a 0.29 mol % TBAB solution is preferred over the 0.17 and 0.62 mol % TBAB solutions. The semibatch operation is more effective than the batch operation for the separation of CH4 from the 30 mol % CH4/N2 ga...

Adsorbents for Oxygen-Bearing Low Concentration Coal Mine Methane Enrichment by Proportion Pressure Swing Adsorption

Traditional Pressure Swing Adsorption (PSA) approach of using single adsorbent probably makes methane concentration very close to the explosion limit and may result in explosion risk in the adsorption process, when methane concentration in coal min methane is blow 30%. Based on pressure swing adsorption, Proportion Pressure Swing Adsorption (PPSA), a new enrichment method with high safety, was suggested to enrich low concentration Coal mine methane (LCCMM). A mixture of active carbon (AC) and carbon molecular sieve (CMS) was employed as adsorbent in the process of PPSA. The methane and oxygen in CMM were adsorbed simultaneously. The experimental study was carried out in order to verify the feasibility of this method. The equilibrium adsorption characteristic of AC and CMS were tested, the mixture of AC1 and CMS5 are chosen as adsorbents for LCCMM enrichment by PPSA.

Thermodynamic Promotion of Tetrahydrofuran on Methane Separation from Low-Concentration Coal Mine Methane Based on Hydrate

This paper proposed a new hydrate-based gas separation (HBGS) method especially for recovering methane from low-concentration coal mine methane (CMM). The main purpose of this study was to evaluate the thermodynamic promotion of tetrahydrofuran (THF) on methane recovery from low-concentration CMM. Temperature and pressure conditions for incipient hydrate formation (vapor−aqueous liquid−hydrate) were closely examined at various different THF concentrations (1.00, 0.10, and 0.01 mol L−1) in gas hydrate systems (CH4−N2−O2−THF−H2O) and at various initial operating pressures (8, 10, and 14 MPa). Subsequently, the phase equilibrium parameters of hydrate formation for the CH4−N2−O2−H2O system were calculated using the Chen−Guo model and the Patel−Teja equation of state. The difference between experimental and calculated values indicates that the addition of THF significantly reduced the equilibrium formation conditions. The higher the THF concentration and the initial operation pressure, the more significant the...