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Energies 2020, 13, 420 54 of 96 required to generate the same heat of combustion [672]. Further, the addition of hydrogen reduces the number of unburned hydrocarbons and CO due to displacement of carbon-containing fuel, and high flammability limits and rapid flame propagation that favor complete combustion [652,673]. Energies 2020, 13, x FOR PEER REVIEW 52 of 95 Figure 24. Reduction of the CO2 emissions at constant energy input as a function of the mole fraction Figure 24. Reduction of the CO2 emissions at constant energy input as a function of the mole fraction of of H2 and NG (natural gas) and energy fraction delivered by H2. Reprinted with permission from H2 and NG (natural gas) and energy fraction delivered by H2. Reprinted with permission from [672]. [672]. Finally, the addition of hydrogen to natural gas blend increases the formation of NOx. Indeed, at a5.c4o.nFsutealnCtelqlsuGivaaslQenucaelirtaytio,thehydrogencombustionincreasestheflametemperature.ThermalNOx generation occurs at a temperature above 1900 K and is enhanced by temperature increase. Moreover, Hydrogen can be used in fuel cells both for transport and heat and power generation (CHP) for the hydrogen addition promotes the formation of intermediate compounds (e.g., OH and H) that domestic and larger commercial systems [674]. support the reactions of thermal NOx generation. Anyhow, the overall NOx formation increase is PEM fuel cells are an emerging technology in the transport and small power generation sectors slightly smoothed by the lower production of prompt NOx. Prompt NOx derives from the reaction mainly due to their fast start-up time and high efficiency. However, contaminants including carbon between N2 and hydrocarbon radicals [661]. monoxide, hydrogen sulfide and ammonia affect fuel cell degradation and durability [675]. Contaminants mainly deteriorate the performance of catalyst materials (typically platinum) due to 5.4. Fuel Cells Gas Quality dissolution and sintering, the carbon supports and the membranes [676]. The normative ISO 14687-2 Hydrogen can be used in fuel cells both for transport and heat and power generation (CHP) for specifies the quality requirements and maximum limits of contaminant compounds in the fuel [677]. domestic and larger commercial systems [674]. Carbon monoxide is a critical contaminant due to carbon formation and deposition on the PEM fuel cells are an emerging technology in the transport and small power generation sectors catalyst active surface. Addition of dopants or suitable supported materials decreases the effects of mainly due to their fast start-up time and high efficiency. However, contaminants including CO impurities [678,679]. High operating temperature and introduction of oxygen into the gas flow carbon monoxide, hydrogen sulfide and ammonia affect fuel cell degradation and durability [675]. are methods to reduce the CO poisoning [680]. Contaminants mainly deteriorate the performance of catalyst materials (typically platinum) due to Furthermore, hydrogen sulfide and ammonia poison the catalyst surface reducing the activity dissolution and sintering, the carbon supports and the mem®branes [676]. The normative ISO 14687-2 [681]. Further, ammonium ions could migrate to the Nafion membrane and compete with hydrogen specifies the quality requirements and maximum limits of contaminant compounds in the fuel [677]. ions reducing the cell conductivity [682]. PSA units and quench efficiently remove sulfurs and Carbon monoxide is a critical contaminant due to carbon formation and deposition on the catalyst ammonia contaminants [683]. active surface. Addition of dopants or suitable supported materials decreases the effects of CO Finally, carbon dioxide, as well as inert gases (e.g., N2) provoke a dilution effect of fuel, limiting impurities [678,679]. High operating temperature and introduction of oxygen into the gas flow are the diffusion of the hydrogen [684]. However, CO2 also increases the CO content through the reverse methods to reduce the CO poisoning [680]. water gas shift reaction increasing losses due to carbon deposition [685]. Furthermore, hydrogen sulfide and ammonia poison the catalyst surface reducing the activity [681]. Contrariwise, solid oxide fuel cells and molten carbonate fuel cells are used for industrial power Further, ammonium ions could migrate to the Nafion® membrane and compete with hydrogen ions generation (CHP) since they operate at high temperatures. High temperature fuel cells (HTFC) are reducing the cell conductivity [682]. PSA units and quench efficiently remove sulfurs and ammonia not suitable for the transport sector since they operate with a slow start up and suffer thermal cycles contaminants [683]. [686]. However, the main advantage is fuel flexibility. Indeed, HTFC can operate with hydrocarbons, Finally, carbon dioxide, as well as inert gases (e.g., N ) provoke a dilution effect of fuel, limiting but fuels are typically reformed in an external unit to avoi2d coke formation [687]. thediffusionofthehydrogen[684].However,CO alsoincreasestheCOcontentthroughthereverse Hydrogen sulfides and halogens are adsorbe2d on Ni/YSZ active surface inhibiting the catalytic water gas shift reaction increasing losses due to carbon deposition [685]. reactions [688]. H2S and HCl are removed by adsorption in materials, such as activated carbon, metal Contrariwise, solid oxide fuel cells and molten carbonate fuel cells are used for industrial power oxides (ZnO, MnO, CuO), silica gel and zeolites [689], or chemical absorption in water [690]. The generation (CHP) since they operate at high temperatures. High temperature fuel cells (HTFC) are not hydrogen sulfide solubility is improved by alkaline compounds or by H2S catalytic oxidation [690]. Furthermore, siloxanes are dangerous since they decompose to silica that deposits on catalyst surface [691]. Siloxanes are removed by adsorption in activated carbon, silica gel or zeolites, absorption in water or more effective organic solvents such as SelexolTM, that is a blend of polyethylene glycol or dimethyl ethers. Siloxanes are also removed through the cryogenic separation or biological removal [692].PDF Image | Green Synthetic Fuels
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