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and Ir and bimetallic Co-Ir supported on a ZnO support. Out of these three catalysts, Co-Ir/ZnO catalyst indicated a better hydrogen production of ~70% mol in the exit gas compared to Co/ZnO and Ir/ZnO. The improved activity of Co-Ir/ZnO was attributed to better contact between Co and Ir and lower coke deposition on the catalyst surface. The coke deposition was determined by a Raman spectroscopy and TPO tests. The tests confirmed the following trend for the graphitization of carbon deposits: Ir/ZnO > Co/ZnO > Co-Ir/ZnO [30]. 2 Oxidative steam reforming 2.1 General aspects The advantage which OSR has over the conventional SR process is that the heat required for endothermic SR reac- tions can be taken from in situ generated by exothermic POX reactions. Moreover, the presence of oxygen can help in the continuous removal of carbonaceous deposits on the catalyst surface, which in turn improves catalyst sta- bility [51]. There is also a disadvantage of a lower hydrogen yield due to the POX reaction [8]. Thermoneutral/auto- thermal conditions can be achieved by maintaining appro- priate OCMR. The thermoneutral/auto-thermal condition is defined as the condition at which overall heat duty of the reformer is zero [5]. In terms of energy savings, ther- moneutral conditions are an attractive option but have not been widely explored using butanol as feed. Limited stud- ies have been documented based on auto-thermal reform- ing (ATR) of butanol [5, 41]. Due to the renewable nature of the feedstock, only a few studies on OSR of bio-butanol have been reported [51–54]. 2.2 Auto-thermal reforming of butanol Hartley et al. [5] predicted thermoneutral conditions in OSR of butanol using Gibbs-free energy minimization method. With the help of Aspen plus software, reactions were simulated and product compositions were predicted. The results of the thermoneutral conditions are tabulated in Table 4. It was found that thermoneutral conditions were a function of reaction temperature, oxygen to butanol molar ratio (OBMR) and SBMR. At identical temperature, increase in SBMR increases the requirement of oxygen to achieve thermoneutral conditions and also increases hydrogen and carbon dioxide yields. A result of the increase in SBMR was a decreased yield of carbon monoxide [5]. The endo- thermicity of the process increases with a higher propor- tion of steam in feed and also requires a higher proportion of oxygen in feed to overcome the overall heat require- ments of the process. A higher proportion of steam in feed favors the activity of the WGS reaction, which ultimately leads to higher production of hydrogen and carbon dioxide with a reduction of carbon monoxide production. Harju et al. [41] predicted thermoneutral conditions based on a thermodynamic study using HSC Chemistry 5.11 software. The thermoneutral conditions with 0.8L OCMR, 4I SCMR and the temperature between 450 and 730 °C showed almost a constant higher hydrogen production. Experimental studies based on ATR were also conducted over Rh/ZrO2 catalyst at 500, 600 and 700 °C temperatures. It was found that conversion and hydrogen yields were stable over a period of 23 h at 700 °C and decline after a cer- tain period at 500 and 600 °C. It was concluded that reac- tion mechanism plays an important role in activity of the catalyst. The presence of Rh enhances formation of coke by dehydration of n-butanol at lower temperatures, i.e. at 500 and 600 °C. The reforming and gasification reactions were found to be fast enough to reduce carbon deposits on the catalyst at 700 °C. TPO tests also confirmed more car- bonaceous deposits on catalyst surface at lower tempera- tures, while the magnitude of carbonaceous deposits on catalyst surface was very small after auto-thermal reform- ing at 700 °C [41]. From these studies, it can be concluded that formation of hydrogen and activity of the catalyst are strong functions of temperature, SCMR and OCMR. 2.3 Oxidative steam reforming of butanol/ bio-butanol Temperature, SCMR and OCMR govern the yield of hydrogen. Out of these three factors, temperature and SCMR should have a similar impact on conversion and hydrogen yields as in the case of SR. A literature review indicated limited research and scope of future studies for optimization of temperature and SCMR in the case of OSR. There are few studies that investigate the role of OCMR in OSR of butanol [5, 41]. Evidence suggests the stability of the catalyst and improved conversion with an increase in OCMR [5, 41]. Longer stability and higher conversion are attributed to increased oxidation of coke over the catalyst surface. TPO of the spent catalyst also confirmed the same result [5, 41]. Table 4 Thermoneutral conditions in oxidative steam reforming of butanol [5] Patel and Patel | 7 Serial number 1 2 3 4 Temperature (°C) SBMR 700 9J 700 12J 800 9J 800 12J OBMR Yield of H2 2.7M 5.37F 2.8M 5.56F 2.65M 5.13F 2.75M 5.33F Yield of COa 1.11 0.92 1.36 1.17 Yield of CO2a 2.88 3.07 2.63 2.83 aYield was calculated by the following equation: moles of component/moles of butanol fed. Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkx008/4743500 by guest on 15 December 2017PDF Image | Renewable hydrogen production from butanol
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