Molten carbonate 3V Button batterys are one of the mainstreams of 3V Button battery research at present

3V Button batterys directly convert chemical energy (fuel) into electrical energy, with the advantages of high efficiency and low pollution, and have been highly valued by all parties in recent years. Molten carbonate 3V Button batterys (MCFC) work at high temperatures (about 650C), can use the exhaust gas waste heat and gas turbine to generate electricity, so they have higher efficiency and are one of the mainstreams of 3V Button battery research at present.

In the past, many achievements have been made in the research of MCFC 3V Button batterys, such as the 2.85MW MCFC demonstration project of Proect, the 15kW MCFC successfully studied by Shanghai Jiaotong University, and the 100kW MCFC being studied. Nevertheless, as a new generation of energy system, many aspects of the working mechanism of 3V Button batterys, whether it is the electrochemical reaction process, the heat and mass transfer process, the flow process of the oxidant and fuel inside the battery, or the steady-state and dynamic characteristics of the 3V Button battery, need to be further studied. Only on the basis of mastering the laws can these processes be organized well, so that 3V Button batterys can truly become efficient and clean energy systems.

The focus of this paper is to study the dynamic characteristics of MCFC cells. The exploration of dynamic characteristics is not only necessary for revealing the temperature distribution, flow state, and performance change law of the 3V Button battery itself, but also provides essential basic data for combining 3V Button batterys with gas turbines to form a hybrid system. Many physical parameters of the MCFC model studied in this paper are obtained from the 15kW molten carbonate 3V Button battery of the 3V Button battery Research Institute of Shanghai Jiaotong University.

Internal characteristics of MCFC Monomer molten carbonate 3V Button batterys are generally flat-plate type, consisting of electrode-electrolyte, fuel flow channel, oxidant flow channel and upper and lower partitions, see.

The working process of the 3V Button battery is as follows: H2 in the fuel flow undergoes oxidation reaction at the anode, reacts with C3- ions in the electrolyte to generate H2O and C2, and releases electrons: +2e. O2 in the oxidant flow reacts with CO2 at the cathode (Cathode), and captures electrons to generate CO3- and enter the electrolyte: (1/2) O2+CO2+2eCO3-. Then CO32- is free and diffuses to the Anode of the fuel flow to replenish the consumed CO3. The electrons generated by the Anode pass through the external circuit junction ICathode, thus forming a complete loop including electron transfer and ion movement. The intensity of the electrochemical reaction can be expressed by the number of moles of substances participating in the electrochemical reaction per unit area on the electrode plate-electrolyte per unit time, that is, the electrochemical reaction rate is. It can be seen that the electrochemical reaction process is accompanied by a strong mass transfer process. The above working process has explained the flow direction of O2, CO2, CO3-, and H2O. For every 2g of substance (Ma) consumed in the fuel flow, 60g (1/2O2 and CO2) of substance enters the electrode from the oxidant side to form CO3-, and passes through the electrolyte and enters the fuel flow to become CO2 and H2O. This strong mass transfer process has a significant impact on the thermodynamic characteristics of the 3V Button battery. The mass transfer intensity can be expressed by the mass transfer rate. The heat generation and transfer process in the 3V Button battery can be analyzed as follows. The heat generated inside the battery includes the heat generated by the electrochemical reaction and the resistance heat generated by the current. The electrochemical reaction heat is mainly the heat of water generation, and the unit area electrochemical reaction heat is /mol; DS is the entropy change of water generation, / (mol.K); 7; is the average temperature of the electrode plate-electrolyte, K. The resistance heat generated by the current per unit area is In addition to the electrochemical reaction heat and resistance heat, there is also heat brought into and out of the 3V Button battery by the fuel flow and the oxidant flow. The next section will establish the heat and mass balance equations to form a mathematical model of the dynamic process of the 3V Button battery.

Schematic diagram of micro-element mass transfer and heat transfer of single MCFC Dynamic process mathematical model of MCFC 3.1 Overview The ways of heat transfer inside the 3V Button battery include: heat transfer caused by mass transfer, mutual convection heat transfer between the fuel and oxidant flow, electrode-electrolyte and separator, radiation heat transfer between the electrode-electrolyte and separator, etc.

In mathematical modeling, it is assumed that the electrolyte substrate has the same temperature as the two electrodes; there is no heat transfer between the solid outer surface of the 3V Button battery and the surrounding environment, which is an adiabatic boundary. Let Pi, Cf, Wf be the density, mass heat capacity and gram molecular weight of the i-th substance respectively, /=1, 7 corresponds to the seven substances H2, CH4, CO, CO2, H2O, N2 and O2 respectively. At the 3V Button battery x, take the microelement dx for analysis (see,), according to the principles of mass conservation and energy balance, there are the following basic equations for the fuel channel, oxidant channel, electrode-electrolyte and separator.

3.2 Fuel flow microelement equation The mass conservation equation is expressed as: The mass of the fuel flow G2a flowing out of the microelement dx is equal to the fuel flow flowing into dx minus the Ma of the diversion into the electrode, plus the CO2 and H2O from the anode, that is: Based on the conservation equation, the energy conservation equation can be established. The input energy of the microelement dx is composed of the heat brought in by the fuel flow, the heat brought in from the anode by the mass transfer of C2 and H20, the convective heat exchange between the anode and the fuel flow, and the convective heat exchange between the partition and the fuel flow: W2 is composed of the heat brought out by the fuel flow and the heat taken away by H2 to the anode: ydx%. The density and mass heat capacity of the fuel flow microelement equation are obtained from the energy balance; ea, are the convective heat transfer coefficients between the fuel flow and the electrode-electrolyte and the partition; Sa is the height of the fuel flow channel.

3.3 Oxidant flow microelement equation The mass conservation equation is: The mass Ac of the oxidant flow out of the microelement dx is equal to the oxidant flow into dx minus the heat of the diversion into the cathode, the convective heat exchange between the cathode and the oxidant flow, and the convective heat exchange between the partition and the oxidant flow. The microelement output energy W2 is composed of the heat taken out by the oxidant flow, 1/22 and C2 mass transfer to the cathode to take away the heat. W2=Integrate the above, and set the integral value to zero to obtain a set of ordinary differential equations (20) for solving the unknown coefficients. Using numerical methods and the functional functions of the Matlabsystemfunction environment, a simulation model is established and simulation experiments are carried out.

4.2 Dynamic simulation and analysis 3V Button battery performance calculations generally consist of two types of calculations: electrochemical and heat and mass transfer. Electrochemical calculations can solve the distribution of voltage, internal resistance, current density and fuel (reaction gas) utilization under the condition of known temperature distribution. In the initial calculation, the temperature distribution is unknown, and a constant is often taken first; this article belongs to the calculation of heat and mass transfer, and focuses on dynamic performance calculation. Previous articles are steady-state performance calculations, and there is no detailed report on the dynamic calculation of foreign MCFC. This type of calculation can solve the temperature distribution of working fluid, electrode electrolyte and PT plate under the condition of known internal resistance and fuel utilization distribution. In the initial calculation, the internal resistance and fuel utilization distribution are unknown, and they are taken as constants first, and then the temperature distribution is calculated, and then used for electrochemical calculations. These two types of calculations are carried out alternately, complement each other, and gradually approach each other.

During the calculation, the overall initial temperature of the 3V Button battery is 600C, that is, when operating, the inlet flow Fa of the fuel flow is composed of H260%+CO220%+H2O15%. The entire simulation process can be divided into two stages. The first stage is the startup process. At the 0th second, fuel and oxidant are added, and the 3V Button battery starts to work, and the electrode-electrolyte temperature T rises from 0.6 to 0.9 mol/s and from 1.8 to 2.7 mol/s; - wide (fuel flow temperature Ta-r isolation plate temperature T; (a) 3V Button battery temperature characteristics (x=01) from from 18 to / electrode-electrolyte temperature 7; fuel flow temperature Ta isolation plate temperature 7; the temperature of each point inside the battery is stable, and the simulation dynamic characteristics of the startup process can be obtained (050s); the second stage fuel flow mutation process, at the 50th second, the fuel and oxidant flow rates both increase stepwise, and the simulation dynamic characteristics caused by the fuel step change can be obtained.) curve. The points near the inlet (x=01), mid-point (x=0.5) and outlet (x=0.9) were selected for simulation research, and the corresponding temperature characteristic curves were obtained. From the analysis of the temperature characteristic curves, it can be obtained that: (1) The temperature of each point inside the 3V Button battery is quite different.

During the startup process, as x increases, the temperature of each point increases. At the same x, the temperature of the electrode electrolyte is higher than that of the fuel flow channel, the partition and the oxidant channel. The temperature near the inlet is lower, and the temperature near the outlet is higher. After the startup process stabilizes, the temperature difference near the inlet and outlet of the electrode-electrolyte is about 2030C, and the temperature difference near the inlet and outlet of the fuel and partition is about 515C. The temperature variation range of the oxidant is less than 2C.) As the flow rate of fuel and oxidant increases, the temperature of the electrode electrolyte increases significantly. When the fuel K and oxidant brother increase by 1/3, the temperature of the electrode electrolyte increases by 515C, the temperature of the fuel and partition increases slightly, and varies within a range of about 1C, and the temperature of the oxidant decreases slightly. This is because the step increase in the flow rate of fuel and oxidant causes the electrochemical reaction rate 7 to increase stepwise, the reaction heat increases, and the increase in current causes the increase in resistance heat, reflecting the rapid increase in the temperature of the electrolyte-electrode response process; for the oxidant, the step increase in 7 causes the heat brought out by 1/2O2 and CO2 caused by mass transfer to cause the temperature of the oxidant to drop suddenly; the small range of temperature changes of the fuel and the separator are caused by the combination of factors such as the electrolyte-electrode and the oxidant. ) The temperature in the simulation model is the average value of the x section, and the temperature value on the temperature characteristic curve is also the average value. In fact, some local temperature changes may be much larger. For example, oxidation reaction occurs on the electrode plate, releasing heat, making the temperature of the electrode plate higher than the average temperature. When the amount of fuel increases suddenly, the instantaneous temperature of the electrode plate is much higher than the average temperature of the electrode electrolyte; and because the temperature of the oxidant flow is low, the temperature of the electrode plate is lower than the average temperature. When the amount of fuel and oxidant increases suddenly, due to mass transfer, the instantaneous temperature of the electrode plate is much lower than the average temperature of the electrode electrolyte. This phenomenon has been confirmed in actual experiments.

5 Conclusion The temperature of each point inside the 3V Button battery is quite different. The complex mass transfer inside the 3V Button battery is one of the important factors that cause the change of thermodynamic temperature characteristics.

For the downstream type with the fuel and oxidant input in the same direction, the temperature at the inlet is relatively small. As the inlet distance increases, the temperature of the electrode-electrolyte, fuel flow and separator increases. The temperature of the electrode-electrolyte increases significantly, but the temperature of the oxidant flow remains almost unchanged.

The dynamic characteristics caused by the change of the input amount of fuel and oxidant are complex. As the input amount increases, the temperature of the electrolyte-electrode increases significantly, and the temperature of the oxidant flow decreases.

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