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Whereas, the stoichiometric hydrogen-air requirement is four times lower than that of natural gas [669]. For rich premixed combustion, the higher air ratio results in an increase in burning velocity and therefore, in lower flame stability. Whereas, in a lean gas, the flame velocity slightly decrease due to the air ratio growth that compensates the increase in burning velocity [670]. Moreover, the flashback occurs more likely at a ratio near the stoichiometric air point where is reached the Energies 2020, 13, 420 53 of 96 maximum flame velocity. Energies 2020, 13, x FOR PEER REVIEW 51 of 95 Whereas, the stoichiometric hydrogen-air requirement is four times lower than that of natural gas [669]. For rich premixed combustion, the higher air ratio results in an increase in burning velocity and therefore, in lower flame stability. Whereas, in a lean gas, the flame velocity slightly decrease Figure 22.. Changes of higher heating value (HHV), lower heating value (LHV), the Wobbe index of a due to the air ratio growth that compensates the increase in burning velocity [670]. Moreover, the hiighWoobbbeeggasas(W(WobobebHe)Ha)nadntdhethWeoWbboebibnedienxdoefxaolfowalWowobWbeogbabse(Wgaosb(bWeoLb).bReeLp)r.inRtepdrfirnotmed[6fr6o5m]. flashback occurs more likely at a ratio near the stoichiometric air point where is reached the The hydrogen addition causes the increase in the laminar flame velocity and reactivity. The hydrogen flame velocity is higher than that of methane and typical natural gas injected in the gas network. When the burning velocity in the primary flame front exceeds the velocity of the unburned mixture, the flame becomes unstable. The flame propagates upstream into the burner, causing the so-called flame flashback [666]. The flame instability could also lead to flame-lift lifting, flame blow-off, escape of explosive mixture and excess of CO emissions [667]. Typically, the laminar flame velocity increases of approximately 5% with a hydrogen concentration of 10% in natural gas [648]. Moreover, the laminar flame velocity of atmospheric burners is also affected by primary excess air ratio (see Figure 23). The excess air ratio is defined as the ratio of the actual air supplied to the burner to the stoichiometric air needed for the complete theoretical combustion [668]. Therefore, the excess air ratio rises with hydrogen addition since the amount of intake air for combustion in atmospheric gas burner depends only on fuel pressure that is not affected by fuel composition. Whereas, the stoichiometric hydrogen-air requirement is four times lower than that of natural gas [669]. For rich Figure 23. Laminar flame velocity as a function of the air excess ratio of methane and hydrogen. [665]. maximum flame velocity. premixed combustion, the higher air ratio results in an increase in burning velocity and therefore, Reprinted with permission from [671]. in lower flame stability. Whereas, in a lean gas, the flame velocity slightly decrease due to the air ratio Figure 22. Changes of higher heating value (HHV), lower heating value (LHV), the Wobbe index of a growth that compensates the increase in burning velocity [670]. Moreover, the flashback occurs more hTighhe Whyodbrboeggeans a(WddoibtbioenHt)oanadtuthrealWgoabsbredinudceexsotfhae lCoOw2Wemobibsseiogans(aWsothbbeereLi)s. Raelporwinetredqufraonmtity of likely at a ratio near the stoichiometric air point where is reached the maximum flame velocity. carbo[6n65i]n. the blend to oxidize (see Figure 24). However, since the hydrogen heating value is lower than that of natural gas, the CO2 emission decrease is non-linear due to the higher amount of fuel 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]. Finally, the addition of hydrogen to natural gas blend increases the formation of NOx. Indeed, at a constant equivalence ratio, the hydrogen combustion increases the flame temperature. Thermal NOx generation occurs at a temperature above 1900 K and is enhanced by temperature increase. Moreover, the hydrogen addition promotes the formation of intermediate compounds (e.g., OH and H) that support the reactions of thermal NOx generation. Anyhow, the overall NOx formation increase is slightly smoothed by the lower production of prompt NOx. Prompt NOx derives from the reaction between N2 and hydrocarbon radicals [661]. Figure 23. Laminar flame velocity as a function of the air excess ratio of methane and hydrogen. Figure 23. Laminar flame velocity as a function of the air excess ratio of methane and hydrogen. Reprinted with permission from [671]. The hydrogen addition to natural gas reduces the CO2 emissions as there is a lower quantity of carbon in the blend to oxidize (see Figure 24). However, since the hydrogen heating value is lower than that of natural gas, the CO2 emission decrease is non-linear due to the higher amount of fuel 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]. Finally, the addition of hydrogen to natural gas blend increases the formation of NOx. Indeed, at a constant equivalence ratio, the hydrogen combustion increases the flame temperature. ThermalPDF Image | Green Synthetic Fuels
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