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Sustainability 2021, 13, 1213 11 of 34 Apart from conversion to a hydrogen-rich mixture, there are requirements for hydrogen purity, especially for low temperature fuel cells. 2.3.1. Hydrogen Pre-Processing for Low Temperature Fuel Cells Low temperature fuel cells are very sensitive to CO. High purity hydrogen from land industry is required to be supplied onboard ships. Otherwise, CO clean-up processes are required. Commonly, if the hydrogen is from the conversions of hydrocarbon fuels, the CO concentrations in hydrogen after the water gas shift reaction probably exceed the allowable limits (e.g., 0.2 ppm) of low temperature fuel cells [59]. At that point, selective oxidation, selective methanation, membrane separation or pressure swing adsorption could be employed to remove CO or purify H2 [34]. Selective oxidation: Selective methanation: 2CO + O2 → 2CO2 CO+3H2 CH4 +H2O 2.3.2. Ammonia Pre-Processing for SOFC Ammonia fuel cells include direct ammonia alkaline and alkaline membrane fuel cells, direct hydrazine and ammonia borane fuel cells and direct ammonia SOFC [40]. However, an ammonia-fed SOFC has better performance and direct catalytic thermal decomposition of ammonia over catalysts at the anode is possible. Direct thermal cracking of ammonia occurs at 400–1000 ◦C and the equilibrium reaction can be expressed as follows: 2NH3 N2 + 3H2 The rate of NH3 decomposition is influenced by different catalysts and different operating conditions. High partial pressure of NH3 and high operating temperatures could increase the decomposition of NH3 [41]. However, both the SOFC-H and SOFC-O might achieve a lower efficiency and change of external voltage as the temperature increases [40]. Therefore, there is an optimal operating temperature relating to NH3 decomposition, theoretical efficiency and external voltage of a SOFC. 2.3.3. NG Pre-Processing for High Temperature Fuel Cells When NG is fed to fuel cells, steam reforming is commonly used to convert it into hydrogen. The reforming reactions of NG are as follows: Steam reforming: Water gas shift: CH4 +H2OCO+3H2 CO+H2OCO2 +H2 Total reaction from reforming: CH4 +2H2OCO2 +4H2 Steam reforming usually takes place at 500–1000 ◦C in the presence of a catalyst, e.g., nickel. Following the reforming reaction, a water gas shift reaction is usually conducted to improve hydrogen yield and lower CO concentration. An external reforming system is viable by using a suitable catalyst, external heat and steam. However, due to the availability of waste heat from the electrochemical reaction in a high temperature fuel cell, NG can be reformed internally by an independent reforming unit, known as indirect internal reforming. By mixing a part of the anode tail gas with the fresh fuel, heat and steam are supplied to sustain the reforming reaction. However, decreased fuel utilization reduces the overall system efficiency. To increase the fuel-to-electricity conversion efficiency, NG is reformed directly on the anode, known as direct internal reforming. The high temperature released and water vapor produced in the anode promote the reforming reactions, andPDF Image | Fuel Cell Power Systems for Maritime Applications
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