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Energies 2020, 13, 420 52 of 96 Furthermore, in the transmission line, gas compression stations are installed to increase the pressure of the gas mixture. Indeed, the pressure of the gas flowing in the pipeline drops due to distributed and concentrated losses. The pressure losses of gas blend with hydrogen are lower than that of natural gas due to the lower friction coefficient. Thus, the power required for compression is reduced [659]. Reciprocating compressors are not sensible to gas admixture composition, while centrifugal compressors operation is affected by the higher rotational speed required to compress a higher volume of gas. Therefore, the rotational velocity is increased by 1.75 times and suitable materials are required [625]. Contrariwise, pressure-reduction stations reduce the gas pressure for the injection of the blend into a pipeline with lower operating pressure. The gas expansion in a throttle valve reduces the pressure. The gas temperature is changed due to the Joule-Thompson effect. At room temperature, the Joule-Thompson coefficient of natural gas is positive (0.5 K/bar) resulting in a temperature decrease with isenthalpic expansion and therefore, is preheated to avoid condensation. Whereas, the hydrogen coefficient is negative (−0.035 K/bar) and does not need preheating treatment [660]. The Joule-Thomson effect also occurs during hydrogen injection into the natural gas grid. When pressurized hydrogen is introduced into the pipeline, the temperature increases for the Joule-Thomson expansion but is balanced by the temperature decrease due to intermolecular forces which reduce the kinetic energy–the overall effect results in a decrease in temperature [661]. Finally, leakage is a critical issue in hydrogen transportation since the hydrogen diffusion coefficient is up to four times higher than that of natural gas. Indeed, the hydrogen easily permeates seals and plastic pipes due to its small size, high diffusivity and low viscosity. Fugitive emissions occur from valves, seals, gasket and also through the pipeline walls. On a volume basis, if the leak is tiny and the predominant mechanism of transport is the molecular diffusion, the relative leak rate for two gases is inversely proportional to the square root of their molecular weight [662]. Therefore, the rate of loss of pure hydrogen through pipeline orifices or leaky seals is up to three times higher than that of natural gas. However, on energy basis hydrogen loses are lower. The diffusion and leakage through the pipeline walls are mainly related to plastic (polyethylene) pipelines used in the distribution grid. Although the annual loss of hydrogen due to leakage is negligible compared to the total volume transported (0.0005%–0.001%) [663]. 5.3. Combustion Combustion parameters such as the Wobbe index, methane number and laminar flame speed are crucial indicators of the interchangeability of different fuel gases. The Wobbe index (WI) is defined as the ratio of the calorific value (higher or lower heating value) on a volumetric basis to the square root of the relative density (HHV/ d) at specified reference conditions [664]. The hydrogen addition to the natural gas blend changes the Wobbe index non-linearly due to the linear decrease in heating value and the non-linear decrease in the square root of density (see Figure 22). A natural gas blend with the high Wobbe index accepts a hydrogen concentration of approximately 55% before achieving the lower limit, and the minimum value of the Wobbe index is reached at a hydrogen concentration of approximately 80% in natural gas. However, when the hydrogen concentration of 20%, the higher heating value is significantly reduced [665]. √PDF Image | Green Synthetic Fuels
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