OXIDATIVE REFORMING OF METHANE : THERMODYNAMIC AND MODELING STUDY

Abstract

Energy is the pivot upon which our modern society is hinged. Everything in this world depends on energy. Traditionally fossil fuels found in the nature have been the relied and trusted sources of energy. All our modern technologies are developed to run on fossil fuels, be it a thermal power generation plant running on coal or the engine of a car, designed to draw power from crude oil derivatives. Now we know that excessive use of fossil fuel has a huge environmental cost attached with it. The rapidly changing climatic scenario, melting of polar ice caps and rise in sea levels are offshoots of global warming. Our over dependence on fossil fuel adds to it. Hydrogen has been touted as the fuel of the future owing to its large calorific value and harmless by products of its combustion. Reforming of natural gas is a process by which industrial scale production of hydrogen can be carried out. Syngas is another fuel of a similar origin. It is a majorly a mixture of hydrogen and carbon monoxide in varying proportions. It finds its applications in Gas to liquid (GTL) fuel production. Syngas is an important feedstock for a large number of industrial products including ammonia, methanol, urea etc. Oxidative reforming involves addition of steam and oxygen to methane (natural gas) at a sufficiently high temperature in presence of a catalyst to produce hydrogen and carbon monoxide along with other side products. It comprises of both endothermic reforming reactions and exothermic combustion reaction. In a perfect autothermal system, the amount of added oxygen is fixed in such a way that the net AH = 0 for the system. This point acts as the lower bound for the amount of oxygen considered in this study. For an estimate on the temperature range over which the considered reactions occur, the region where L\G < 0 is evaluated, which comes out to 900- 1100 K. Based on these values as a pointer, a thermodynamic analysis by minimizing the Gibb's free energies is conducted to find out the equilibrium moles of each species including carbon. The results of thermodynamic analysis act as a starting point for the simulation study. It is found that at temperatures exceeding 1000K the theoretical conversion is sufficiently high and coke deposited is in miniscule amount. A large range of steam to methane ratio was evaluated and ultimately a range of 0.5-3.0 was selected for simulation study on the basis of conversion, coke formation and H2/CO ratio. The large values of H2/CO ratio were not considered relevant since xlii the primary motive was to study the syngas production. At those high ratios, it is beneficial to go for pure hydrogen production. 021CH4 ratio was upper bounded by considering the fact that the number of hydrogen moles produced drops off with an increase in oxygen supplied. The steady state one-dimensional, non-isothermal model was developed. Nickel based catalyst was selected and a fixed tubular reactor system was considered. The kinetic model was based on the experimental works of Xu and Froment (on steam reforming reactions), and Trimm and Lam (on methane combustion). The industrial data of De Groote and Froment was used for validation. On the basis of simulation of model for a low pressure operation, it was concluded that a temperature of 1000K- 1100K with an 02/CH4 ratio of 0.8 is suitable for syngas production, with varying H2/CO ratios. The minimum H2/CO for this configuration was found to be 2.7 when the steam to methane ratio was taken as 0.5 and a peak value of H2/C09. 1 was found with steam to methane ratio3.0. This ratio can be further altered as desired by using another separation unit. It is further stated that this operation requires low pressure and reasonable temperature. Thus, it is preferable to be used in practice.