In order to improve the safety of coal mine production, the Ventilation Air Methane (VAM) is usually exhausted by a great deal of ventilation. Methane, the main component of VAM, is the second largest greenhouse gas after carbon dioxide. However, it is also a kind of clean gas energy. China is one of the largest coal producers and the annual emission of pure methane through VAM exceeds 150 billion cubic meters. It causes not only huge waste of the renewable energy resource but also serious pollution of the atmospheric environment [1I. Karakurt, G. Aydin, and K. Aydiner, "Mine ventilation air methane as a sustainable energy source", Renew. Sustain. Energy Rev., vol. 15, no. 2, pp. 1042-1049, 2011.
[http://dx.doi.org/10.1016/j.rser.2010.11.030] -5C.Ö. Karacan, F.A. Ruiz, M. Cotè, and S. Phipps, "Coal mine methane: A review of capture and utilization practices with benefits to mining safety and to greenhouse gas reduction", Int. J. Coal Geol., vol. 86, no. 2, pp. 121-156, 2011.
[http://dx.doi.org/10.1016/j.coal.2011.02.009] ].

It is difficult to burn the VAM directly by the traditional burner for its low concentration (generally 0.1% ~ 0.75%) and large fluctuation of volume concentration and flow rate [6K. Warmuzinski, "Harnessing methane emissions from coal mining", Process Saf. Environ., vol. 86, no. 5, pp. 315-320, 2008.
[http://dx.doi.org/10.1016/j.psep.2008.04.003] -10K. Gosiewski, A. Pawlaczyk, and M. Jaschik, "Thermal combustion of lean methane–air mixtures: flow reversal research and demonstration reactor model and its validation", Chem. Eng. J., vol. 207-208, no. 5, pp. 76-84, 2012.
[http://dx.doi.org/10.1016/j.cej.2012.07.044] ]. According to the character of the VAM, Shandong University of Technology of China proposed a preheating catalytic oxidation technology, which could effectively oxidize the VAM by a honeycomb ceramic oxidation bed loaded by the noble metal Pd catalyst [11Y.Q. Liu, R.X. Liu, and Y.X. Wang, "A Preheating Catalytic Oxidation Reactor for Coal Mine Ventilation Air Methane", China Patent CN202113840U, 2012.-15B. Zheng, Y.Q. Liu, R.X. Liu, S. Chen, M.M. Mao, and J. Meng, "Starting characteristics of ventilation air methane preheating catalytic oxidation reactor", J. Chin. Coal Soc., vol. 39, no. 6, pp. 1084-1088, 2014.]. In addition, the heat of the exhaust can be recycled by a recuperative heat exchanger to achieve the thermal equilibrium state to maintain the self-heating maintenance operation. This technology brings a stable and reliable temperature field and high heat recovery efficiency with compact structure and small flow resistance, so it is very economical and feasible.

Because the methane content of the inlet stream is very low, the key to the successful operation of preheating catalytic oxidation device is whether a thermal equilibrium state during the catalytic oxidation reaction can be reached. The thermal equilibrium is a balance between the released heat by the catalytic oxidation reaction and the heat taken away by the exhaust and dissipated on the device surface.

In the operation of the device, the operating parameters have a great influence on the catalytic oxidation heat release, the exhaust heat loss and the surface heat dissipation loss. The influence plays an important role for the design guidance of the heat exchanger and the insulation layer. Previous studies about the VAM catalytic oxidation focused on methane conversion rate, stability of temperature field and heat extraction performance [16B. Zheng, Y.Q. Liu, R.X. Liu, and J. Meng, "Catalytic oxidation of coal mine ventilation air methane in a preheat catalytic reaction", Int. J. Hydrogen Energy, vol. 40, no. 8, pp. 3381-3387, 2015.
[http://dx.doi.org/10.1016/j.ijhydene.2015.01.020] -20Z.L. Gao, Y.Q. Liu, and Z.Q. Gao, "Heat extraction characteristic of embedded heat exchanger in honeycomb ceramic packed bed", Int. Commun. Heat Mass., vol. 39, no. 10, pp. 1526-1534, 2012.
[http://dx.doi.org/10.1016/j.icheatmasstransfer.2012.09.010] ], while the analysis on the device overall thermal equilibrium and the research of the effects of the operating parameters on the heat loss are very lacking. Thereby the thermal equilibrium is analyzed in the running process of a coal mine VAM preheating catalytic oxidation device, and the influence of several important operating parameters, including inlet stream methane volume concentration, space velocity and oxidation bed inlet temperature on the energy balance terms to maintain the thermal equilibrium is investigated. The above research can provide an important theoretical guidance for the optimal design and the stable self-heating maintenance operation of the oxidation device.

The coal mine VAM preheating catalytic oxidation device is shown in Fig. (1). The whole system mainly includes a reaction chamber, the gas supply system, the preheating system, the startup/auxiliary heating system, and the measurement and control system. The catalytic oxidation bed in the reaction chamber is the core part of the device, which is packed by the cordierite honeycomb ceramic blocks. The catalytic oxidation bed has a length of 1050mm and the cross section is 600mm × 600mm square. The honeycomb ceramic blocks have a large number of square channels with hole density of 17.3 PPI. The block size is 150mm×150mm×150mm and the honeycomb porosity is about 60%. The inner wall of the honeycomb ceramic hole is coated by the catalyst with the active ingredient of noble metal palladium. The air supply system, mainly including a fan, a fan inverter, a compressed natural gas cylinder and a pressure regulating box, can supply the methane-air mixture with a volume concentration of less than 1%. The preheating system uses a recuperative heat exchanger to recycle the exhaust gas to heat the inlet stream. There is a two-way corrugated plate for heat transfer enhancement inside of the recuperative heat exchanger. The startup/auxiliary heating system is composed by an electric heater and a control cabinet, which can heat the oxidation bed to the oxidation temperature of methane in the initial operation of the device. The measurement and control system mainly conducts the oxidation bed temperature measurement, the gas flow rate measurement, the pressure difference measurement and the gas composition analysis, and adjust the inlet flow rate and volume concentration.

Fig. (1) Diagram of preheating catalytic oxidation reactor.

The total flow rate of the gas is measured by the orifice plate flow meter, whose measuring range is 0~1500Nm³/h, and the measurement accuracy is 1%. The temperature and pressure difference of the cold side and the hot side of the preheater are both measured by the KIMO TPS-08-500-T type pitot tube and the CP200 type micro differential pressure transmitter, and the measurement accuracy is 4%. Methane volume concentration at the inlet and outlet of the device is real-time monitored by the GJG10H(C) type pipeline infrared methane concentration sensor manufactured by Chongqing branch of China Coal Research Institute, with the measurement range 0 to 1%, error within ± 0.07%, and response time 20s. To acquire more accurate experimental data when the energy balance state is achieved, the methane volume concentration in the inlet and outlet of the device is measured by the J2KN type flue gas analyzer of German RBR company, and the accuracy is 1ppm.

The oxidation bed structure is shown in Fig. (2a), which is composed of the first layer of the ceramic without catalyst and the back seven layers of the catalyst supported ceramic. The thickness of each layer is 150mm. The first ceramic layer has a K type thermocouple in the center to measure the inlet temperature of the oxidation bed and evens the inlet flow and temperature. In the catalytic oxidation bed, four temperature measurement cross sections are arranged, with five K type thermocouples at each section for temperature measurement. The arrangement of measuring points is shown in Fig. (2b). The temperature measuring point is assigned respectively in the cold and hot side of the heat exchanger to obtain the inlet stream temperature and the exhaust temperature, which is shown in Fig. (2c).

Fig. (2a) Temperature measurement points arrangement of the reactor.

Fig. (2b) Temperature measurement points arrangement of the reactor.

Fig. (2c) Temperature measurement points arrangement of the reactor.

The operation principle of the preheating catalytic oxidation device is shown in Fig. (1). Firstly, the electric heater turns on under a small flow rate to heat the oxidation bed rapidly until the fresh VAM oxidation temperature is achieved. Then the methane gas is mixed into the inlet stream. The methane-air mixture is catalytically oxidized in the oxidation bed and the oxidation heat is released and divided into two parts. One part is absorbed by the oxidation bed and the other part preheats the unreacted inlet stream through the heat exchanger. The cooled exhaust is finally discharged into the atmosphere. When the preheated inlet stream temperature rises high enough for the methane-air mixture catalytic oxidation in the oxidation bed, the electric heater is turned off and the device achieves self-heating maintenance operation relying on the exothermic oxidation heat and the heat feedback of the heat exchanger.

Because of the limited performance of the thermal insulation layer, the device has surface heat dissipation loss. In addition, the exhaust gas takes away a part of the methane exothermic oxidation heat which could not be recovered and the exhaust heat loss is produced. During the experimental process, in order to adjust the operation parameters conveniently, such as space velocity, inlet concentration and inlet temperature, and study the influence of each parameter on the heat loss, the electric heater stays turned on all the time. The condition for the preheating catalytic oxidation device to reach the thermal equilibrium state is that the sum of the heating power of the methane exothermic oxidation heat and the electric heater equals the sum of the surface heat dissipation loss power and the exhaust heat loss power. When the inlet stream parameter is changed, the temperature field of the oxidation bed varies subsequently and will achieve a new energy equilibrium state after a period of time. This is a slow process for the large thermal inertia of the oxidation bed. If the temperature field keeps essentially the same for at least twenty minutes, the energy equilibrium state is achieved and the measuring data will be collected.

3.1. Calculation of the Methane Exothermic Oxidation Power

For the calculation of the methane exothermic oxidation power, the complete oxidation reaction equation of methane is CH₄+2O₂=CO₂+2H₂O+LHV, and the complete exothermic oxidation heat LHV is 802.7kJ.mol^-1, so the formula of methane exothermic oxidation power is as follows:

(1)

Where, Φ₁ is the methane exothermic oxidation power in kW, q_v is the volume flow rate of methane-air mixture into the device per unit time in the standard condition in L·s^-1, η is the methane conversion rate, x_CH4,in is the methane volume concentration, and V_mol is the gas molar volume in the standard condition with the value of 22.4L·mol^-1.

3.2. Calculation of the Exhaust Heat Loss Power

The high-temperature exhaust gas generated after the methane oxidation reaction preheats the fresh inlet stream through the heat exchanger. Due to the limited heat transfer capability, the temperature of the exhaust gas is still high which leads to the exhaust heat loss. The formula for the calculation of the exhaust heat loss power is as follows:

(2)

Where, Φ₂ is the exhaust heat loss power in kW, ρ is the inlet stream density in the standard condition in kg·L^-1, c_pout is the specific heat at constant pressure of the exhaust gas, c_pin is the specific heat at constant pressure of the inlet stream gas, both in J·kg^-1·K^-1, and T_in is the inlet stream temperature in K. Because the methane concentration of the inlet stream is very low (less than 1%), the specific heat capacity of the inlet stream and exhaust is calculated according to the air.

3.3. Calculation of the Surface Heat Dissipation Power

Under the condition that the preheating catalytic oxidation device achieves the thermal equilibrium state, the surface heat dissipation power can be calculated by the following formula:

(3)

Where, Φ₃ is the surface heat dissipation power, and Φ₄ is the heating power of the starter, both in kW.

• 1) The exothermic oxidation power grows with the increase of the inlet methane volume concentration and the oxidation bed inlet temperature, yet decreases slightly when the space velocity increases to a certain value. This is because the increase of the inlet concentration and temperature improves the methane conversion rate, while the excessive space velocity reduces the methane conversion rate, so the overall methane oxidation heat is affected.
• 2) The inlet stream temperature and space velocity have obviously more influence on the exhaust heat loss power than the inlet stream methane volume concentration. This is due to that the methane volume concentration is in a very low operating range and has small effects on the methane conversion rate. While the rise of the inlet stream temperature improves the methane conversion rate evidently and the increase of the space velocity enhances the heat exchange of the gas and the oxidation bed, thus the exhaust takes more heat away.
• 3) The surface heat dissipation loss power increases significantly with the increase of the inlet stream concentration and temperature, especially at high space velocity, and with the increase of the space velocity firstly increases and then decreases. This is because with the increase of the inlet concentration and temperature the methane oxidation heat release grows and the heat exchange between the reaction chamber and the outside is enhanced. However, the methane conversion rate decreases when the space velocity increases to a certain value, then the oxidation heat release decreases and the heat exchange between the reaction chamber and the external reduces.
• 4) The fraction of the total heat loss accounted by the exhaust heat loss significantly reduces with the improvement of the inlet stream concentration and temperature, and yet obviously increases with the increase of the space velocity. In the whole operating parameter range of the preheating catalytic oxidation device, the heat recovery efficiency of the heat exchanger and the insulation layer performance should be optimized according to the exhaust heat loss and the surface heat dissipation loss under the high inlet concentration, temperature and space velocity condition, so as to reduce the overall heat loss and achieve the self-heating maintenance state.