Thermal Flow Reversal Reactor Research Articles

Strategies for heat recovery in a reverse flow reactor designed to lessen coal mine methane emissions test

ABSTRACT Thermal pollution resulting from the release of ventilation air methane (VAM) has emerged as a pressing concern in coal mining. This study aims to address this issue by harnessing the waste VAM energy and converting it into reusable resources, employing an efficient thermal flow reversal reactor. The research investigates five distinct heat extraction strategies and compares their performance. The numerical analysis focuses on transient heat recovery performance by employing a two-step reaction mechanism. Furthermore, the study uncovers the heat transfer characteristics of heat exchange tubes and conducts an analysis of construction costs and profit periods for the different strategies. The findings indicate that the central heat recovery strategy exhibits inferior recovery performance, resulting in higher outlet temperatures and lower extracted power. Conversely, the end recovery strategy successfully reduces heat loss by 56%. The two-sided strategy demonstrates better recovery performance, maintaining an average outlet temperature of 311 K. Additionally, the heat recovery efficiency of the heat exchange tube is approximately 85.6%. Although the combination strategy appears to reduce outlet heat loss, it results in an extended investment payback period.

Combustion Characteristics of Ventilation Air Methane Blending with Dimethyl Ether in a Thermal Reverse Flow Reactor

Combustion Characteristics of Ventilation Air Methane Blending with Dimethyl Ether in a Thermal Reverse Flow Reactor

Experimental and simulation research on heat extraction characteristics from a pilot-scale reverse flow reactor for low-grade energy

This work presents heat extraction characteristics in a pilot-scale thermal flow reversal reactor dedicated to the mitigation of ventilation air methane escaping from coal mining and the recovery of residual heat. The heat extraction mode arranged on both sides of the central zone was applied. The influence of flowrate, concentration and reversing time on comprehensive heat transfer performance were investigated through experiments. The temperature evolutions and the quantitative relationship between temperature evolutions and heat extraction were predicted through simulations. The experimental results show that low flowrate is susceptible to establishing rapidly initial temperature field. The heat recovery rate and generated superheated steam increase with concentration and varies parabolically with switch time. The optimum heat extraction is that heat recovered is slightly greater than or close to the stored heat. Therefore, the heat extraction research involved in this paper will provide a reference for other low-grade waste fuels in industrial applications.

Numerical analysis of oxidation performance of basalt fiber bundle thermal flow-reversal reactor

• Using low-dimensional devices and mixed runners to enhance heat transfer. • Reducing the self-sustained concentration to 0.1%. • Gas processing capacity per unit volume increased by 1.03–2.5 times. • The benefit of carbon emission reduction is increased by at least 1.95. • The efficiency of waste heat utilization is increased by 2.5 times. This paper uses Ansys Fluent software to conduct a two-dimensional numerical study on the oxidation performance of basalt fiber bundle thermal flow-reversal reactor (TFRR). The influence of inlet methane concentration, inlet flow rate and circulation period on the oxidation performance of TFRR is analyzed. The results show that the maximum temperature in the reactor increases significantly with the increase of inlet concentration and flow rate. At the same time, the lower limit of the concentration that can be handled by the reactor was studied, and it was found that the basalt fiber bundle TFRR can completely treat the gas with the concentration of 0.1%, under the operating parameters of t c = 80 s, v in = 0.65 m/s or t c = 60 s, v in = 1.0 m/s. In addition, the device was compared with the honeycomb ceramic TFRR in terms of economic efficiency. It was found that the clean development mechanism(CDM) emission reduction benefit of the former was 1.95 times than that of the latter, and the economic benefit of waste heat utilization was 2.5 times than that of the latter.

Experimental study on control of flame inclination in a thermal flow reversal reactor with extra lean premixed methane/air intake

Combustion instability is a basic obstacle to the wide application of porous media combustion devices in industry. However, the investigation about it with extremely dilute gas fuel issorelylacking due to the limitation of laboratory test conditions. In this study, inclinational instability of extra lean-premixed flame (with volume fraction below 1%) in a thermal flow-reversal reactor with honeycomb ceramic packings is investigated experimentally. The effects of inlet methane concentration on the flame inclination are studied. Then the flame inclination is restrained through electronically controlled adjustment on the valve opening and switch time. The experimental results show that the intake methane concentration has a remarkable effect on the flame inclination angle variation in each semi cycle. When the intake concentration decreases from 0.6 vol% to 0.2 vol% and the flow direction is downward, the growth of the flame inclination angle in each semi cycle gets lower. The adjustment on the scale of the high temperature zones in two half parts of the packed bed through the valve opening has a great influence on the flame front position and flame inclination angle. The setting of asymmetric switch time for the reverse flow also has obvious effect on the evolution of the flame inclination. Finally, the flame inclination angle drops firmly for approach of the two high temperature zones in size and height. The synchronous adjustment of valve opening and switch time fastens the convergence of the two high temperature zones in two half parts of the packed bed. This work is beneficial to the development of thermal flow-reversal reactor technique for better dealing with ventilation air methane and other lean gaseous fuel streams.

A Multi-Scale Numerical Model for Investigation of Flame Dynamics in a Thermal Flow Reversal Reactor

In this paper, the flame dynamics in a thermal flow reversal reactor are studied using a multi-scale model. The challenges of the multi-scale models lie in the data exchanges between different scale models and the capture of the flame movement of the filtered combustion by the pore-scale model. Through the multi-scale method, the computational region of the porous media is divided into the inlet preheating zone, reaction zone, and outlet exhaust zone. The three models corresponding to the three zones are calculated by volume average method, pore-scale method, and volume average method respectively. Temperature distribution is used as data for real-time exchange. The results show that the multi-scale model can save computation time when compared with the pore-scale model. Compared with the volumetric average model, the multi-scale model can capture the flame front and predict the flame propagation more accurately. The flame propagation velocity increases and the flame thickness decreases with the increase of inlet flow rates and mixture concentration. In addition, the peak value of the initial temperature field and the width of the high-temperature zone also affect the flame propagation velocity and flame thickness.

Investigations on the mitigation of ventilation air methane and energy recovery in site trial thermal flow-reversal reactor

In this paper, the key influencing factors on the ventilation air methane (VAM) utilization site trials were discussed. A new model of pressure loss and heat transfer between gas and solid phase in regenerative bed was innovatively developed. The factors of inlet gas flow rate, methane concentration fluctuations, the channel size in the monolith blocks and the energy recovery on the behaviors of site trial thermal flow-reversal reactor (TFRR) were studied systematically. Results show that the predicted temperature profile matched well compared with the site trial results. The shape of temperature profile in the reactor gradually evolves from M-shaped to ∩-shaped with the increase of the channel size of the honeycomb monolith bed. The residual methane concentration in the exhaust gas can be lower than that of 0.00079 vol% when the TFRR is filled with optimal monolith blocks with channel size of 3 ×3 mm. Using the heat exchanger in the reactor, about 40% to 44% thermal energy can be recovered from the TFRR site pilot plant.

Heat transfer analysis of H-type finned tube embedded in packed bed for gasification to produce hydrogen

Gasification for hydrogen production by the H-type finned tube embedded in the packed bed is considered to be more economical. Based on the thermal flow reversal reactor, the heat transfer characteristics of H-type finned tube embedded on the both sides of high-temperature zone of the packed bed under the quasi-steady were researched. The contribution of radiation heat transfer rate on the H-type finned tube is about 60.5% at most and radiation mainly occurs on the central tube. Within the range of inlet temperature and Re, the fin-assisted degree is both decreased gradually and the maximum is 48.9%. When the temperature is above 906 K or within the range of Re, the heat transfer rate of the fin is mainly from the radiation heat transfer rate. Under the condition of high inlet temperature and high Re, the fin-assisted degree is minimum, which indicates the operation stability of the H-type finned tube is better.

Experimental investigation on control of temperature asymmetry and nonuniformity in a pilot scale thermal flow reversal reactor

Temperature asymmetry along flow direction and nonuniformity vertical to flow direction have been the prominent obstacles to the stable operation of thermal flow reversal reactor (TFRR) devices in industry. However, the relevant investigations are sorely lacking especially through the experiment method due to the limitation of laboratory test conditions. In this study, temperature asymmetry and nonuniformity for extra lean CH4/air combustion (feed methane concentration below 1 vol%) in a pilot TFRR were investigated experimentally. The effects of the preheating process and feed methane concentration were investigated. The switching time of 120 s and feed rate of 400 m3/h were determined for the preheating process. In the transition period between preheating completion and cyclic steady state achievement, the operation pattern with preheater off was beneficial to heat storage. Stable combustion for minimum feed methane concentration of 0.2 vol% was confirmed in the experiment. When feed methane concentration was decreased from 0.8 vol% to 0.2 vol%, higher feed methane concentration was easier to intensify temperature nonuniformity. To restrain the increase of temperature asymmetry and nonuniformity, an electronically controlled adjustment was adopted. Valve opening ratio could be monitored to obtain more equivalent average temperature values of two sections of the packed bed. A 55-min process and a 116-min process were picked out from the cyclic steady-state period to eliminate temperature nonuniformity through adjusting valve opening ratio. As a result, the gaps in the high-temperature section width of two sections get narrowed and the average temperature of two sections is almost the same. The switching time ratio could be regulated to shift high-temperature cores of two sections to middle height position. A 58 min long process was selected with a switch time ratio of 1.25 and the temperature center height of both two sections dropped obviously. Synchronous adjustment of the valve opening ratio and the switching time ratio accelerated the adjustment process and double effects could be acquired in the meantime. A 36-min long process and a 28-min long process are chosen for simultaneous adjustment. The average temperature of two sections reach good agreement in the end and the temperature center height of two sections both drop obviously. This work is beneficial to the development of TFRR for better dealing with ventilation air methane and other lean gaseous fuel streams.

Experimental Research on the Thermal Oxidation of Ventilation Air Methane in a Thermal Reverse Flow Reactor.

The thermal reverse flow reactor is an effective technical equipment for dealing with ventilation air methane, which has been causing a significant greenhouse effect. An experimental study on the thermal oxidation of ventilation air methane in a thermal reverse flow reactor was conducted. A mixture of domestic gas and ambient air was used to simulate ventilation air methane in the experiments, and the methane conversion efficiency was analyzed based on the concentration of combustion products determined by gas chromatography equipment. In addition, the effects of the switching time, the inlet methane concentration, the flow rate, and heat extraction were studied. The experimental results show that the reverse flow reactor system can run under a wide range of operating conditions with autothermal operation and high methane conversion. In addition, this system can even work with methane concentrations as low as 0.30% in the autothermal operation mode without NOx emission. Unlike previous studies, this study shows that the flow rate has little effect on the methane conversion rate in the cyclic steady state over a wide range of operating conditions. In addition, methane conversion and reaction zone change as the inlet methane concentration varies during the reaction process in the cyclic steady state. The combined optimization of operating parameters can effectively improve the stability of the reverse flow reactor system and methane conversion efficiency.

Aerodynamic CFD simulations of experimental and industrial thermal flow reversal reactors

The paper presents a simplified CFD model suitable for simulating a Thermal Flow Reversal Reactor (TFRR) aerodynamics. This model may be used for an analysis of flow phenomena in a pilot reactor which is planned to be implemented for Ventilation Air Methane (VAM) combustion in a hard coal mine. Moreover, the model was to be used to compare the designed versions of geometries from the point of view of expected pressure drop of future plant, and also uniformity of flow velocity in the cross-section of the ceramic monolith packing.The ANSYS FLUENT simulations presented in the paper were divided into the two following stages:(a)Development of a simulation method allowing to couple simplified CFD simulations with results of separately performed calculations applying a dynamic model based on spatially one-dimensional partial differential equations of the dynamic mass and energy balances with own thermal combustion kinetics. The aerodynamic model was verified by comparing CFD simulation results with the experimental records taken from the research and demonstration (R&D) plant (cf. Section 2.2). The simulations revealed acceptable accordance with experimental results what shows that the model may be useful in practice. This stage was aimed to get reliability of further, forecasted results for an industrial plant in a larger size scale.(b)CFD simulations for currently designed larger pilot industrial flow reversal reactor with use the flow resistance coefficients obtained and verified during simulations of the much smaller experimental reactor aerodynamic model, referred to in the Section 3.1.

Impact of Thermal Asymmetry on Efficiency of the Heat Recovery and Ways of Restoring Symmetry in the Flow Reversal Reactors

Abstract The problem discussed in the paper was revealed during experimental and simulation study of the waste fuel utilization (low concentrated CH4) which in coal mines is emitted to the atmosphere as the so-called ventilation air methane (abbreviation: VAM). When an intense heat recovery would be applied to the Reverse-Flow Reactor (RFR) then the reactor becomes susceptible to creation of temperature profiles asymmetry. This may adversely affects the heat recovery efficiency. When a strong asymmetry arises, it can significantly reduce the heat recovery and the system needs immediate counteraction. Commonly used control systems do not always cope with this problem. The paper discusses various RFRs control algorithms from the point of view of the ability to reduce symmetry to the acceptable range. The impact of the loss of reactor symmetry on possible heat recovery efficiency is also briefly discussed. The work is focused on RFRs which operate with significant heat recovery in a heat exchange unit in which heat is retrieved not at the rector outlet, but by cooling the hot gas from the center part of the reactor. Issues discussed in the paper are derived from being carried out in ICE-PAS through many years of simulation research, patent examinations, but primarily from the own experiences on the research & demonstration thermal flow reversal reactor (TFRR) for VAM combustion. Computer control & data recording system process data records of the experiments enabled effectively analyze the problem of creating and preventing thermal asymmetry.

Research advances and trends of the dilute methane thermal reverse flow reactor technology

Based on coal mine methane (CMM) emission policy in China and other main coal-mining countries in the world, a status quo of dilute methane (such as ventilation air methane (VAM) or similar concentration methane) reverse flow reactor (RFR) technology was looked back and the principal of the reverse flow reactor for dealing with dilute methane was introduced in detail. The development of the dilute methane RFR technology was carded and the main research directions and processes in this field were described, including reaction kinetics mechanism of dilute methane in the inert homogeneous channel, reaction heat front reverse movement rule in the RFR, numerical simulation of the RFR mathematical model, NOx emission of the RFR process, large-scale RFR experiments, and so on. The contradictions existed in the relative research of dilute methane RFR technology were discussed, and the evolution of the dilute methane RFR technology was predicted. And it would be necessary guidance for the scientific study, numerical simulation, optimization design and engineering application of the dilute methane RFR technology.

Dehydrogenation characteristics of lean methane in a thermal reverse-flow reactor

In order to study the dehydrogenation reaction mechanism of ultra-low concentration methane in a thermal reverse-flow reactor, the effects of the cyclic period (120s–240s), the lean methane volume flow (90 Nm3/h to 180 Nm3/h), and the methane concentration (0.2 vol% to 0.8 vol%) on the dehydrogenation performance were studied systematically by using a thermal reverse-flow experimental system. When the methane concentration is 0.2 vol%, the reactor can achieve self-heat maintaining operation. With the increase in the methane concentration, the width of the high-temperature zone, the exhaust gas temperature, the methane conversion rate, and the maximum temperature of the heat-accumulator bed increase. With the increase in the lean methane volume flow, the width of the high-temperature zone, the distance between the center of the high-temperature zone and the center of the reactor, the maximum temperature, the exhaust gas temperature, and the methane conversion rate increase. With the increase in the cyclic period, the exhaust gas temperature and the deviation of the high-temperature zone increase, but the methane conversion rate and the maximum temperature decrease slightly.

Numerical study on thermal oxidation of lean coal mine methane in a thermal flow-reversal reactor

This paper presents a three-dimensional numerical investigation on thermal oxidation of lean coal mine methane in a thermal flow-reversal reactor by the finite volume method. The effects of channel length, feed methane concentration, inlet velocity and cycle time on the reactor behavior were analyzed. Results show that the temperature distributions and methane concentration profiles in the reverse-flow semicycle are mirror images of the ones in the forward-flow semicycle. Thus the cycle for the cyclic steady state is symmetrical. The maximum temperature of the reactor rises significantly with the increases of methane concentration and inlet velocity, and it is nearly unchanged with the increases of the channel length and cycle time. Long channel length, high feed methane concentration, low inlet velocity and short cycle time could achieve a wider high temperature zone in the reactor. The minimum feed methane concentration for self-maintained running rises dramatically with the decrease of channel length and the increase of inlet velocity, and it is almost not affected by the change in cycle time. For a desired minimum feed methane concentration of 0.18 vol.% when vin = 1 m/s, the required channel length should not be less than 1.8 m.

Industrial-Scale Experimental Study on the Thermal Oxidation of Ventilation Air Methane and the Heat Recovery in a Multibed Thermal Flow-Reversal Reactor

In the present work, an industrial-scale experiment on ventilation air methane (VAM) utilization by a multibed thermal flow-reversal reactor (TFRR) is conducted in China. The influence of the inlet flow rate, feed methane concentration, and cycle time on the temperature distribution of the bed and heat recovery efficiency are investigated. The methane conversion in the studied cases exceeds 97%. The results show that the methane concentration during self-maintained operation of the TFRR without heat recovery should not be less than 0.22 vol % when the inlet flow rate is 103,000 Nm3/h and the cycle time is 300 s. As the inlet flow rate decreases, the lower concentration limit of automatic thermal maintenance increases. The peak temperature of the bed approaches the inlet side as the feed methane concentration increases and the cycle time decreases. The heat recovery efficiency increases linearly with increasing inlet flow rate, rises parabolically with an increasing feed methane concentration, and decreases weakly with increasing cycle time.

Numerical Study on Heat Distribution and Transfer Characteristics of a Manifold in a Coal Mine VAM TFRR Oxidation Bed

A thermal flow-reversal reactor is candidate for utilizing low concentration ventilation air methane. In this paper, a numerical study is performed by using the FLUENT software to explore the details of the transient preheating and starting process of the thermal flow-reversal reactor oxidation bed. The bed was heated by hot gas, which was transported and distributed through the holes of manifolds to the middle of the bed. The homogeneous porous media and coupled heat transfer models were chosen; and the mass and heat flow distributions passing through the holes, the heat transfer on the outer surface of the manifold and the temperature distribution of the bed were calculated. The results indicate that the heat of the hot gas passing through the holes decreases gradually along the direction of the hot gas flowing in the manifold, causing the temperature of the bed decrease accordingly. The calculated temperatures of the oxidation bed are compared with the tested results. The maximum error between the calculation and the test was 8.9%.

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

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%–46.82% 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.

Experimental Investigations of Cold Flow Mass Distribution and Pressure Drop Characteristics in a Structured Honeycomb Monolith Bed

The thermal flow reversal reactor (TFRR) and catalytic flow reversal reactor (CFRR) can utilize the ventilation air methane (VAM) from coal mines effectively. The initial cold feed flow mass distribution uniformity in the monolith oxidation bed, the critical part of both reactors, has a great influence on the stability of methane oxidation process and conversation rate. So the emphases are put on the experimental investigations of the cold flow mechanics in the monolith bed based on a simplified reactor test rig. Firstly, the flow resistance coefficients of the monolith bed were calculated, and then the flow resistance mathematical model was built. It was found that the flow mass distribution uniformity was affected by the pressure drop distribution in the monolith bed, and obviously decreased with increasing inlet velocity. The pressure drop distribution was analyzed through a modified momentum equation. The total pressure loss in the lower header was evidently higher than that in the upper header or monolith bed. Additionally, the local losses at the inlet and outlet of the monolith bed were small, so they could be ignored. Finally, the formula of total pressure losses under different flow conditions was established based on the simplified reactor test rig.

The Numerical Simulation Research on Thermal Flow-Reversal Reactor of Mine Ventilation Gas

The paper uses computational fluid dynamics software FLUENT to build a single-channel numerical simulation model of the mine ventilation gas Thermal Flow-Reversal Reactor (TFRR). Combining with the analysis of orthogonal test, the influences that four factors (Initial temperature distribution, Ventilation gas flow velocity, Volume fraction of methane, Exchange period) act on reactor performance is investigated. An optimal operation condition is proposed by the establishment of the priority sequence of these four factors.