# Prospects and Technical Challenges in Hydrogen Production through Dry Reforming of Methane
## 3 Alternative Processes for Increased H\({}_{2}\)/CO Ratio
### Combined Steam and Dry Reforming of Methane (CSDRM)  
A combination of steam and dry reforming of methane (CSDRM) has been investigated as an alternative for modulating the H\({}_{2}\)/CO ratio with a focus on increasing H\({}_{2}\) production and allowing for a higher versatility on the chemical and fuels production steps [132]. The use of steam on the feed for DRM may account for a reduced coke formation by oxidation of adsorbed CH\({}_{\text{X}}\) that could otherwise lead to carbon deposits [133]. In addition, studies have shown that the convenient adjustment of the CH\({}_{4}\)/H\({}_{2}\)O ratio on feed composition leads to the desired modulation of H\({}_{2}\)/CO ratio [132; 134].  
Figure 4 presents the thermodynamic equilibrium of CSDRM at different pressures, considering a CH\({}_{4}\):CO\({}_{2}\):H\({}_{2}\)O feed ratio of 3:1:2. As both SRM and DRM have an endothermic aspect, the conversion of all reagents is favored at higher temperatures. Lower pressures are related to a decrease in both CH\({}_{4}\) and CO\({}_{2}\) conversion and lower H\({}_{2}\) selectivity. H\({}_{2}\)/CO ratio decreases with the temperature increase and stabilizes around 2.0 in the range of 800-1000 \({}^{\circ}\)C [135].  
[PERSON] et al. [136] investigated the performance of 10 wt% Ni/Al2O3 catalyst, which is a classical reforming catalyst, on CSDRM conditions. The study has shown that a higher hydrogen production can be achieved by the CSDRM operation at 700 \({}^{\circ}\)C with 100% conversion of CH4 and a high H2O/CO2 ratio on the inlet feed. For H2O/CO2 ratios of 2.5, 7.3 and 12.7, the H2/CO ratios achieved on this study were 2.2, 5.0 and 8.0, respectively. In addition, the stability of the 10 wt% Ni/Al2O3 catalyst was evaluated for 24 h at CSDRM conditions and 100% CH4 conversion was maintained for the whole test when H2O/CO2 = 7.3 and 12.7 were used at 700 \({}^{\circ}\)C. Moreover, analysis of spent catalyst showed no oxidation or size changes of Ni sites, indicating that deactivation by catalyst sintering or oxidation was not triggered on the reaction conditions, and no carbon deposits were detected.  
[PERSON] et al. [137] reported the influence of feed composition on the CSDRM activity and final hydrogen yields over Ni- and Pd-based catalysts supported on Al2O3. Mono- and bimetallic catalysts were prepared by sol-gel method and tested for CSDRM at 500-1000 \({}^{\circ}\)C with various (CO2 + H2O)/CH4 ratios (1-3) and CO2/H2O ratios (1-3) under atmospheric pressure. A central composite experimental design was used to evaluate the statistical significance of the influence of each reaction condition on the final CH4 conversion and H2 yield. Based on the ANOVA results, the authors showed that higher temperatures (T \(>\) 700 \({}^{\circ}\)C) were crucial to achieve higher CH4 conversions (>75%) due to the endothermic character of both SRM and DRM reactions. As for the influence of feed composition, the authors confirmed that the CO2/H2O ratio had no significant impact on the final CH4 conversion and H2 yields. However, when the amount of CO2 was higher in the inlet feed, lower H2/CO ratios were obtained. Finally, higher H2 yields were favored by lower (CO2 + H2O)/CH4 ratios at temperatures lower than 1000 \({}^{\circ}\)C, when competitive RWGS reaction takes place and reduces the H2 final production.