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Energies 2020, 13, 420 50 of 96 Table 2. Methane and hydrogen properties evaluated at NTP (normal temperature and pressure) and stoichiometric air. Parameter Higher heating value [MJ/m3] Relative density [m3/m3] Wobbe index [MJ/m3] Stoichiometric air requirement [mol/mol] Laminar flame velocity [cm/s] Adiabatic flame temperature [K] Flammability limits in air by volume [%] Diffusion coefficient [cm2/s] CH4 39.82 0.5548 53.54 9.55 36.7 1950 5.3 ÷ 15 0.21 H2 12.75 0.0695 48.37 2.39 275 2210 4.1 ÷ 74 0.63 The NATURALHY project, supported by the European Commission, investigated the effect of hydrogen addition to natural gas in terms of feasibility, safety and critical issues. The goal was to establish the conditions under which hydrogen can be injected into the natural gas grid. Results demonstrate that safety is generally ensured with a hydrogen concentration of 20% if measures are adopted to overcome the criticalities observed [630]. Below are summarized the main considerations on the possible effects of hydrogen and natural gas blend on sensitive components. Burner technologies are sensitive to gas composition and low tolerant of gas composition fluctuation. The gas composition could affect the residential appliance operation resulting in soot formation, a higher level of polluting emissions and safety issues (i.e., flame stability). Atmospheric burner adaptation depends on the air excess and the initial Wobbe index [631]. In the gas engines, a low fraction of hydrogen (<5%) enhances flame stability and combustion efficiency. Nevertheless, the hydrogen admixture provokes a higher knocking (detonation) propensity reducing the engine performance and lifetime. The methane number denotes the resistance of fuel gas to knock. Pure methane represents the knock resistance reference fuel with methane number 100, while the methane number of pure hydrogen is 0 [632]. Thus, the addition of hydrogen into the natural gas blend reduces the methane number. The Gas Infrastructure Europe (GIE) suggests a methane number above 80 since lower values could reduce the efficiency and increase the emissions of gas engines [633]. Anyway, a concentration of up to 10% in volume is feasible with additional control systems. Similarly, gas turbines are susceptible to gas quality variations due to the lean premixed combustion processes. Many gas turbines are guaranteed for a blend with 1%–5% of hydrogen in volume. However, simple modification can increase the acceptable concentration of up to 15% [634]. Finally, non-combustion applications in several industrial processes that adopt natural gas as a chemical and manufacturing feedstock (i.e., ammonia fertilizers, reforming, fuel cells) are sensitive to the gas composition that could impact on safety and efficiency [631]. A possible solution is the downstream extraction of hydrogen that concerns the separation of hydrogen and methane that are mixed in the gas pipeline. Gas separation is a feasible way to supply natural gas to industrial facilities that cannot accept hydrogen for manufacturing processes. Moreover, it can also be used to provide pure hydrogen to applications such as fuel cells for transportation [635,636]. The pressure swing adsorption system is a well-established technology for gas separation. However, it is economically and energetically non-affordable. An alternative is the selective membrane separation technology that is very efficient with relatively high hydrogen concentration, but at a low level, the high pressure required is a challenge [637]. The safety issue associated with hydrogen is not more critical than that related to other fuels. Hydrogen is colorless, odorless and difficult to detect. Moreover, the flammability range is broad (4%–75% by volume), and the minimum ignition energy is low (0.017 mJ), resulting in an easy ignition. Anyway, the low density (0.0873 kg/m3 at normal temperature and pressure) makes it buoyant in air. So, in an open environment, hydrogen is easily and quickly dispersed. Though, in enclosed spaces, the hydrogen accumulation near the ceiling has to be prevented to avoid gas ignition and deflagration [638,639].PDF Image | Green Synthetic Fuels
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