## 2 Methodology
### Process Modelling
#### 2.2.3. GSR H\({}_{2}\) Plant  
The GSR H\({}_{2}\) plant is modeled following the guidelines provided in [PERSON] et al. [22]. Due to the higher temperatures reached (up to 1100 \({}^{\circ}\)C at the end of the reduction and beginning of the reforming stage) in the flameless combustion attained with the chemical looping principle, a lower S/C ratio is needed to achieve high methane conversion relative to the SMR models. Precisely this parameter is tuned to obtain sufficient heating value in the PSA off-gas, which is routed to the reduction step after compression and heating to satisfy the cluster energy balance. Somewhat higher H\({}_{2}\) efficiencies are obtained in the present work relative to the original work [22] as a result of improved heat integration. Thus, a higher fraction of the fuel heating value is preserved as H\({}_{2}\) and not combusted to maintain reactor operating temperatures. The elements of which the GSR-H\({}_{2}\) plant consists are practically the same as the GSPOX-H\({}_{2}\) plant described in the next section and illustrated in Figure 8. The base case reactor operating pressure is the same as in the SMR plants (32.7 bar) and it was assumed that the air and PSA-off-gas stream pressurization was carried out in a single adiabatic compressor stage. The PSA separation performance was represented using the correlation derived by [PERSON] [22], assuming 100% purity of the H\({}_{2}\) product stream. As represented in Equation (A1) in Appendix A, the hydrogen recovery  
Figure 7: Process diagram of the SMR H\({}_{2}\) plant with MDEA absorption CO\({}_{2}\) capture.  
is directly related to the pressure relation between the adsorption and desorption stages, which is linked to the off-gas compressor auxiliary power for the plants integrating GS clusters. H2 recoveries beyond 80% are achieved for all the plants investigated.  
Some considerations will be made regarding the heat integration for the benchmark and GS-based plants. For the fired tubular reforming SMR plants, on the combustion gases side, the low pressure of the stack gases will result in a small heat transfer coefficient, which leads to large temperature differences with respect to the incoming natural gas feed, achieving a relatively low preheating temperature. On the syngas side, the steam generation network is arranged in such a way as to avoid steam superheating with the high-temperature reformer outlet syngas, preventing an environment that would cause exchanger tube failure due to metal dusting [45]. On the other hand, the temperature approaches considered in the GS plants allow for a considerably higher heat recovery performance. The pressurized conditions of the streams found in the GSR and GSPOX clusters maximize the heat transfer coefficient relative to the reference plants, enabling closer temperature approaches. Special precautions may be required for the exchangers which preheat the mixed steam and gas streams to the GS clusters with the high-temperature syngas product, despite the fact that the steam pressures (25-40 bar) are notably below the design values of the conventional SMR processes (>100 bar). The PSA off-gas recuperator with the GS reduction outlet is an item that will also require advanced heat exchanger design [46]. The potential to achieve higher preheating temperatures inherent to the GS cluster operation represents an important feature to accomplish the high H2 efficiency conversions attained in the concepts integrating this technology.