## 2 Methodology
### Process Modelling
#### 2.2.5 GSPOX H\({}_{2}\)-Power Plant  
As described earlier, the GSPOX H\({}_{2}\)-power plant can simultaneously produce H\({}_{2}\) and electricity, by operating the reforming step of the cluster with a water sweep. A similar option is possible with the GSR, withdrawing some H\({}_{2}\) product for extra firing, but this hydrogen would need to be separated from the syngas stream via larger WGS and PSA units instead of being directly produced via water splitting as in the GSPOX process. In addition, if the oxidation step is operated with excess air, the reactor cool-down taking place after carrier full oxidation will reduce the subsequent reforming step temperature, thereby increasing the steam requirement for comparable methane conversion. The basic layout of this concept is represented in Figure 9. The essential differences with respect to the H\({}_{2}\) only production plant are (1) the use of a water sweep in the reforming step to produce a semi-pure H\({}_{2}\) stream for extra firing, (2) the avoidance of a WGS step after the partial oxidation and heat recovery sections, and (3) a more complex power cycle in the oxidation section. By removing the WGS units, a lower amount of the heating value originally present in the natural gas is withdrawn as H\({}_{2}\) in the PSA unit. Consequently, a larger flow rate of air is heated in the GSPOX reactor, producing more electricity upon expansion. From an efficiency perspective, removal of the WGS reactors also avoids degrading a significant fraction of the fuel heating value to relatively low-grade heat via the exothermic WGS reaction. Since the PSA off-gas is now much richer in volatile components (CO, H\({}_{2}\)) relative to the H\({}_{2}\) plant (with a large presence of CO\({}_{2}\) and H\({}_{2}\)O from the shift), the PSA-off-gas compressor was split into two intercooled stages, to reduce the safety concerns of an excessive adiabatic temperature rise. The power cycle indicated in Figure 9 is specifically integrated with the rest of the plant. It integrates hot water/LP steam produced from low-temperature heat recovery units of the reduction and partial oxidation streams to effectively use them for extra electricity generation. Moreover, the cycle is designed to satisfy the internal steam demand, delivering IP steam to the reforming step and pre-reformer as required. In this way, the potential disadvantage in the case of no suitable end-use for LP steam production in the H\({}_{2}\)-only plants is entirely avoided. Given the uncertainty of the scale of the devised concepts using GS technology, several power cycles may prove competitive, depending on different factorssuch as plant location, natural gas heat input, price of electricity and H\({}_{2}\) etc. However, for this study, only a combined power cycle is considered to evaluate the potential benefits of the H\({}_{2}\)-power GSPOX configuration relative to the H\({}_{2}\)-only configuration. In such a combined power cycle, the gas turbine integrated with the GSPOX oxidation and water splitting steps delivers a high-temperature exhaust gas stream to a steam cycle, to produce additional electricity. This configuration is depicted in Figure 10, where only the oxidation and water splitting sections of the GSPOX cluster unit are shown and process equipment within the dotted lines refer to the units appearing in the block \"power cycle\" in Figure 9.  
Figure 10: Process diagram of the combined cycle for the oxidation section of the GSPOX H\({}_{2}\)-power plant. Figure 9: Process diagram of the generic GPSPOX H\({}_{2}\)-power concept. The gas turbine elements associated with the GSPOX H\({}_{2}\)-power plants are representative of two industrial turbomachines employed for power production in combined cycles. It is noted that the power systems integrated into these processes are not specifically the actual GTs, but rather incorporate several defining technological features to represent that technology level.