Diesel autothermal reforming with hydrogen peroxide for low-oxygen environments
Introduction
Fuel cells are energy conversion devices that convert chemical energy into electrical energy with relatively high efficiency and low pollutant emissions. Moreover, fuel cells have less noise emission and require lower oxygen supply than traditional combustion-based power systems [1], [2]. Therefore, there has been much interest in applying fuel cells to low-oxygen environments such as submarines, UUVs (unmanned underwater vehicles), and other applications [3], [4], [5], [6]. For submarine applications in particular, fuel cells can be the most attractive option for APUs (auxiliary power units) or AIP (air independent propulsion) due to their silent and lower oxygen-consuming characteristics and high efficiency [7], [8]. To drive fuel cells, a continuous hydrogen supply is required. Metal hydride is currently the main hydrogen source for submarine applications. However, metal hydride has a low hydrogen storage density and requires a relatively longer recharge time. Therefore, the mission range and underwater endurance of submarines (or UUVs) has been limited [9]. Some studies have sought to apply hydrocarbon reforming to supply hydrogen into fuel cells effectively in low-oxygen environments [10], [11].
Diesel fuel has a higher hydrogen density than other hydrocarbon fuels, suggesting diesel reforming as a promising hydrogen source for fuel cells [12], [13], [14], [15], [16]. Diesel fuel also has advantages in terms of storage and accessibility for low-oxygen environment applications due to its liquid phase and well-equipped infrastructure [17], [18], [19]. If hydrogen production via diesel reforming could be performed in low-oxygen environments, diesel reforming could be applied to submarine applications for successful missions and objectives.
The three available reforming methods are classified by the types of reactants: POX (partial oxidation, Eq. (1)), SR (steam reforming, Eq. (2)) and ATR (autothermal reforming, Eq. (3)) [20].
SCR (steam-to-carbon ratio) and OCR (oxygen-to-carbon ratio) are each defined by the ratio of steam, and oxygen to carbon number n.
By varying SCR, OCR and the reaction temperature, the reaction sets can be net exothermic as in POX, net endothermic as in SR, or thermally neutral as in ATR [21]. There are advantages and disadvantages of each reforming method that must be considered when selecting the most suitable method for the desired application. ATR combines POX and SR; fuel, water, and oxygen are fed to the reactor simultaneously. ATR shares several advantages with both POX and SR, such as rapid thermal response and relatively high hydrogen yields. Furthermore, ATR requires no external heat source due to its slight exothermic reaction, enabling simple reformer configurations [21], [22], [23]. Based on these advantages, ATR may be the most suitable method for subsea submarine applications.
ATR has been considered the most appropriate method for diesel reforming because the steam supply suppresses coke formation over the catalyst and aromatic compounds in the diesel fuel can be effectively decomposed by oxygen [24], [25], [26], [27], [28], [29]. Yoon et al. successfully suppressed carbon deposition by controlling SCR and OCR and successfully tested long-term operation of ATR for 2500 h by considering the fuel delivery method [30], [31]. Samsun et al. developed 5 kWe high-temperature polymer electrolyte fuel cell system operated with diesel ATR and other components for an auxiliary power unit [22]. However, in low-oxygen environments such as the subsea, conventional diesel ATR using oxidants such as water and air (or oxygen) could not be used without additional oxygen sources.
In this study, aqueous hydrogen peroxide solution (H2O2(aq)) was proposed for diesel ATR in low-oxygen environments. H2O2(aq) is used not only as a source of oxygen but also as a source of water; both are acquired by decomposition of hydrogen peroxide according to the following Eq. (4):
In subsea applications, both high oxygen density and safety issues should be considered. Hydrogen peroxide is in liquid form, and the oxygen per unit volume is high, which could reduce the required size of storage tanks. The oxygen density of pure hydrogen peroxide is approximately 21 mol O2/liter at 1 bar and 25 °C. In comparison, pure O2 gas has to be stored at 485 bar and 5 °C to attain comparable oxygen density [10]. According to Eq. (4), during decomposition, heat is released due to the exothermic nature of the reaction. If the concentration of H2O2(aq) is 67 wt.% or greater, all decomposition heat is used to vaporize water included in the solution, increasing the potential for spontaneous decomposition [32]. Decomposition of a high concentration of H2O2(aq) would be hazardous in extreme situations such as submarine applications and should therefore be prohibited. In addition to decomposition heat, purchasing commercial H2O2(aq) as a solution more concentrated than 50 wt.% is difficult. For these reasons, the concentration should be less than 50 wt.%.
In this paper, diesel ATR using H2O2(aq) as an alternative oxidant was comprehensively studied. First, experiments comparing different oxidants (H2O2(aq), water, and air) were conducted to assess the characteristics of hydrogen peroxide as an oxidant for diesel reforming. Second, operating conditions such as SCR, OCR, and the reaction temperature were analyzed using thermodynamic calculations to discover which enables ATR reaction, when H2O2(aq) is less than 50 wt.%. Third, the reforming catalytic activity and stability of our in-house prepared catalysts were evaluated for various diesel compounds. The catalytic performance assessment was performed for ATR with n-dodecane and two synthetic diesels that each contained sulfur and aromatics. Finally, long-term testing was conducted using Korean commercial diesel.
Section snippets
Catalysts
For the experiments comparing different oxidants, the ATR reforming catalyst of platinum (Pt) on gadolinium (Gd)-doped CeO2, Pt (0.5 wt.%)/CGO (Ce0.8Gd0.2O3−δ) was used. This catalyst was synthesized using combustion spray method, and it was supplied by Praxair Co. in the form of fine powder. To test diesel reforming performance, Ni–Ru (19.5–0.5 wt.%)/CGO (Ce0.9Gd0.1O2−δ), which was originally developed for diesel pre-reforming, was used [33], [34]. Ni–Ru (19.5–0.5 wt.%)/CGO catalyst was
Hydrogen peroxide characteristic tests as an oxidant
The presence of H2O2 is thermodynamically unfavorable at all temperatures. Consequently, H2O2 can easily decompose into O2 and H2O when it is exposed to high temperature above the boiling point or reacted with catalysts [38], [39]. In our previous study, it was confirmed that H2O2(aq) was completely decomposed into O2(g) and H2O(g) when the pre-decomposition supply method was selected. The pre-decomposition test was conducted using a blank reactor at approximately 200 °C [40]. However, when H2O
Conclusion
To produce hydrogen effectively in low-oxygen environments, such as submarines and UUVs, diesel ATR with hydrogen peroxide was investigated. The conventional oxidants water and air were compared to H2O2(aq) as an alternative oxidant. When the pre-decomposition supply method was used, H2O2(aq) exhibited the same reforming performance as steam and air (or oxygen) in diesel reforming. Thermodynamically feasible operating conditions were investigated when safety issues related to hydrogen peroxide
Acknowledgments
This work was supported by the Agency for Defense Development in South Korea (ADD). The authors wish to thank ADD for financial assistance and permission to publish the results.
In addition, this work also supported by the Global Frontier R&D Program on Center for Multiscale Energy System funded by National Research Foundation under the Ministry of Science, ICT&Future, Korea (2011-0031569) and BK21 Plus Project.
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