However, the fuel cell is generally at a slightly higher temperature level, so the equilibrium of the reformate will shift in the direction of hydrogen production adjusting to the higher temperature. No further reactions will occur only in the case in which the reformate leaving the reformer is already at the temperature level of the fuel cell. In the case of external reforming, it can be assumed that the reformate leaving the reformer unit is already in thermodynamic equilibrium. The difference with operation on hydrogen will be a temperature drop due to the heat taken from the fuel cell for the reforming reaction to occur. In the case of internal reforming, the rate of the reforming reaction at the Ni/YSZ cermet anode is high enough to reach equilibrium. Because of the incomplete conversion of methane, less hydrogen is produced and less water is consumed, leading to lower hydrogen to steam ratios in the reformate and thus lower Nernst voltages. At temperatures below 700 ☌, the differences become larger. The potential for the electrical power output of the cell is thus higher in the case of operation on hydrogen than in the case of operation on (reformed) methane. Nernst voltages for a methane/steam mixture with a steam to carbon (S/C) ratio of 2 and for a mixture of hydrogen and 10 vol% steam as a function of temperature.įigure 4 shows that for the methane/steam mixture at equilibrium, the Nernst voltage is always 30–45 mV lower than for the hydrogen/steam mixture. 108 However, it is important to note that the information obtained from such calculations is valid only if equilibrium was reached, which is the underpinning assumption as well as the fundamental limitation of F*A*C*T.įigure 4.
In conjunction with a slag viscosity model, F*A*C*T can also be used to predict the viscosities of homogeneous (completely molten) liquid slag systems and heterogeneous (partially crystallised) slag systems as a function of the bulk slag composition and operating temperature over a range of conditions. The information obtained from F*A*C*T can provide an explanation for the variations in AFTs between different coal ashes and the processes taking place during AFT tests. 108 With the application of the newly developed F*A*C*T thermodynamic database for the system Al-Ca-Fe-O-Si, Jak 108 demonstrated the validity of the AFTs predictions with the F*A*C*T equilibrium calculations, showing good agreement with the measurements determined using AFT tests. This approach is based on the phase equilibrium science rather than relationships with the bulk composition and takes into account the relative stabilities of the liquid and solid phases. 107 In the coal ash study, this tool will provide information on AFTs to predict the melting behaviour of coal (or blended coal) ash.
27, 94, 106 Facility for the Analysis of Chemical Thermodynamics (F*A*C*T) is a thermodynamic computer package used to predict chemical equilibria, the proportions of liquid and solid phases as a function of temperature, composition and atmospheric conditions.
Thermodynamic equilibrium calculations or equilibrium phase diagrams are also used to provide information on the properties of coal ash or ash deposits, such as mineralogy, ash fusion temperatures (AFTs) and viscosity, or to predict the behaviour of coal ash during combustion. Zhang, in Ultra-Supercritical Coal Power Plants, 2013 6.5.3 Thermodynamic equilibrium calculations
0 kommentar(er)
