# Systematic Analysis of the Hydrogen Value Chain from Production to Utilization
## 4 Hydrogen Transportation and Distribution Technologies
### Transport and Distribution by Pipelines  
A pipeline system is composed of inlet stations, transmission and distribution pipelines, compression stations, valve stations, pig launcher and receiver stations and pipeline monitoring and control systems [79; 84]. Pipelines are constructed from different materials, depending on their application. Most transmission pipelines are made of carbon steel or stainless steel, due to their exceptional strength and cost-effectiveness compared to other materials, with diameters typically ranging from 4 to 48 inches [85]. Distributionpipelines are usually built with either low-strength steel or high-strength polyethylene, with diameters typically ranging between 0.5 and 8 inches [85]. The steel used to build the pipelines can be classified into four categories based on its microstructure, namely ferrite-pearlite steels (typically grades X70 and below), acicular ferrite steels (grades X60 to X100), bainite-martensite steels (such as X100 and X120), and tempered sorbite steels [86]. This transportation and distribution approach is considered the most attractive option as the volume of hydrogen increases, since increasing the diameter of the pipe results in a capacity increase greater than the increase in the amount of material needed. It presents a cost below USD 1/kgH\({}_{2}\) for pipelines with a length of 5000 km and a diameter of 120 cm [82], and is considered the most environmentally friendly option, has a long service life (between 30 and 50 years [85]), and low maintenance costs. There are two options to build a hydrogen pipeline system [87], either building a completely new hydrogen network, which presents high investment costs, or adapting existing natural gas pipeline systems to accommodate hydrogen through blending with natural gas [86] or fully replacing it. Nevertheless, transporting hydrogen in systems of pipelines presents various technical challenges, as presented below. Hydrogen embrittlement: Hydrogen molecules may dissociate at the surface of the surrounding material, forming hydrogen atoms that may then diffuse to the interior of the material. These atoms accumulate at defects or discontinuities in the crystal lattice of the material and form localized pressure zones which can cause cracks [85]. This results in a decrease in the material's ductility and tensile resistance. The severity of hydrogen embrittlement depends on such factors as exposure time, operating temperature and pressure, diffusion velocity, and defects on the microstructure of the pipeline material, among others [84,86,88]. Hydrogen embrittlement is more severe in high-stress steels with high carbon content, while low-grade steels, like API 5L Grade A, Grade B, X42, and X46, are less vulnerable to this phenomenon [85,88,89]. Nevertheless, this second type of steel withstands lower operating pressures, so thicker walls are needed to match the operating conditions of high-stress steel. The pipeline joints are most susceptible to this phenomenon, especially when defects are present on welds [85]. This degradation phenomenon also occurs in polymers and composite materials used in sealing applications or lining applications (e.g., epoxy resin composites), as hydrogen penetrates the macromolecular network of polymer chains [84]. Hydrogen leakages and safety: The smaller molecular size of hydrogen, when compared to natural gas, increases the leakage potential of the gas transported by the pipelines, especially through fittings, joints, and seals [87]. This increased leakage raises safety concerns. There are three major risk factors to consider. Hydrogen presents wide flammability limits (from 4% to 76% by volume at 1 bar and 20 \({}^{\circ}\)C), it requires a low ignition energy of only 0.017 mJ ignition energy (about 16 less than methane), and hydrogen burns in an almost invisible flame [85]. Together, these characteristics make hydrogen leaks not only more likely but also more difficult to detect and control.