# An Overview of Hydrogen's Application for Energy Purposes in Lithuania
## 3 Hydrogen Value Chain
Additionally, catalyst-based technology TDM (methane pyrolysis) is gaining the momentum as one of the cleaner and more economical options to produce COx-free H\({}_{2}\) as it does not require an additional step to separate H\({}_{2}\), which significantly reduces the cost and complexity and creates only structured solid carbon nanomaterial as a byproduct. The methane conversion efficiency can reach 60-90%, while H\({}_{2}\) yield is between 71-77% [32]. Researches show that TDM has proven to be one of the most reliable and beneficial method for the production of pure green hydrogen for various applications, particularly for fuel cells [33,34]. Although great laboratory-scale achievements have been accomplished so far, TDM and biomass fermentation technologies for green hydrogen production are still in a very early stage of commercial development. Hydrogen storage has become the key component of the whole value chain due to its flammable and explosive properties. Green hydrogen can be stored in three different states (gas, liquid or solid) depending on the various conditions (storage duration, pressure and volumes):  
* Gaseous. The most widely-used hydrogen storage method currently. It must be compressed up to 700 bars and kept as a pressurized gas in suitable cylinders, tanks or underground caverns [22]. As hydrogen energy density by volume is low, it has to be compressed at high pressure and stored ideally in underground salt caverns (so far, the best storage option as much larger volumes can be stored);
* Liquid. This method requires a vacuum insulated storage tank and a considerable amount of energy for the cooling process of hydrogen due to its boiling temperature \(-253~{}^{\circ}\)C [22]. Due to the boil-off effect, heat absorption and vaporization, it is not economical to store hydrogen for long periods due to gradual decrease in the volume [35];
* Solid. It is considered as the safest mode of storing hydrogen in either a form of metal hydrides (magnesium metal has been proven to be a good choice) or as a LOHC (ammonia, methanol or toluene). Hydrogen can be extracted from solid form by applying heat at atmospheric pressure, thus reducing the costs [22]. The most economically-viable hydrogen applications are considered those where hydrogen is consumed at the point of production avoiding hydrogen transportation. However, due to geographical differences between large-scale renewable energy generation and energy consumers' concentration areas hydrogen transportation component is inevitable. This sector is categorized as transmission (hydrogen transportation from its production sites to the suppliers) and distribution (hydrogen transportation to the fuelling stations or the end users) [36]. If distances are less than 1500 km, then the pipeline transportation is the most advantageous option; however, trucks are used to transport small volumes [22]. The most promising and economically-viable way is using natural gas pipelines for moving hydrogen [37]. Technical possibility has already been proven with tests that keep a mix of 20-25% hydrogen blended into natural gas [38]. It is vital to notice that the injection of hydrogen into gas networks is related to several technical risks, which require additional consideration and upgrade of gas network infrastructure. According to [39], due to the density and concentration differences between hydrogen and methane or natural gas, poor gas mixing may occur following the hydrogen injection into the pipeline; this may lead to increased risk of steel embrittlement and consequently pipeline rupture. Moreover, in order to maintain the same calorific value of the mixed gas flow (hydrogen density is nine times lower than that of natural gas), the blend flow rates or pressure must be increased. Possible solutions to this issue could be introduction of steel pipes with thicker walls, using fiberglass pipe system design or adding passive inline mixers, e.g., porous plates in the hydrogen inlet points.