Optimizing syngas production for chemicals and/or hydrogen using 2 and 3-cycle Unmixed Reforming: A comparative thermodynamic study

Unmixed Reforming (UMR) offers a promising alternative to simultaneously address the dual problem of energy and environment prevalent in the industrially adopted Steam Methane Reforming (SMR) process for syngas (a mixture of CO + H2) production. UMR is powered by a novel form of combustion called Unmixed or Chemical Looping Combustion in which air and fuel alternately pass over an Oxygen Storage and Release Material (OSRM) bed in a reactor. The OSRM undergoes cyclic oxidation (with no NOx formation) and reduction, enabling the net energy released from both reactions to be used for reforming. This makes UMR relatively less energy intensive and more environment friendly. In this study, two UMR process variants, based on 2 and 3-cycle operation were simulated in Aspen Plus using the Peng-Robinson thermodynamic model. The conventional SMR process was also simulated for comparison purposes. The effect of relevant operating parameters such as temperature, pressure, steam to carbon (S/C) and NiO/CH4 ratios in respective cycles in each variant was investigated. The effect of air/Ni ratio and recycling a fraction of the reformer exit gas on the oxidation and reduction cycles respectively was also studied. Optimal operating conditions were identified and a comparative energy analysis was carried out. The reformer exit gas composition was maximized at 850 °C, 1 bar and an S/C ratio of 3 in both variants. Under these conditions, CH4 conversion was found to be greater than 99 %. The H2 and CO compositions at the reformer exit were 55.2 % and 11.3 % in the 3-cycle UMR as against 41.3 % and 8.6 % in the 2-cycle UMR. The net energy requirement associated with the 2 and 3-cycle UMR variants, comprising of oxidation, reduction and reforming (as appropriate) was estimated to be 23.3 and 10.8 kJ/molH2 respectively, as against 36.3 kJ/molH2 in the conventional SMR process. Overall, the UMR offers advantages over the SMR process in terms of efficient feedstock utilization, higher methane conversion, reduced energy demand and lower emissions. When compared to the 2-cycle variant, the 3-cycle variant is superior in terms of steam utilization, operational flexibility and energy consumption.