**1. Introduction**

Are the existing infrastructures ready for the decarbonized energy systems of the future, or do they need to be adapted through design and development of new solutions? In the last years investments in renewable power plants have grown rapidly worldwide moving towards a "renewable electrical networks scenario" [1,2]. However, due to the production unpredictability of some renewable power sources (i.e., wind and solar) and the possible mismatch between production and demand, energy storage solutions are essential to avoid grids' instability [3].

Among the available technological solutions, power-to-gas (P2G), based on chemical energy storage concept, is considered as one of the most interesting for energy system decarbonization [4]. In fact, through P2G the power surplus is stored as renewable fuel, i.e., a fuel produced by converting

renewable energy sources into chemical molecules for use in various applications with minimum greenhouse emissions or without adding net CO2 to the atmosphere [5], that can be used for different purposes, like feedstock for in industrial processes [6], energy carrier [7], fuel in residential/district heating and cooling [8] and in the transport sector [9].

ENTSOG, i.e., the European Network of Transmission System Operators for Gas, proposed the "2050 roadmap for gas grids" in which several recommendations and actions are suggested to implement a European P2G strategy [10]. Particularly, three configurations are proposed for the energy grid of the future, i.e., the grids towards a close to carbon neutral gas system: (i) the use of biomethane and synthetic natural gas (SNG) that ensure no adaptation of end-user applications; (ii) an increasing hydrogen blending percentage into the existing natural gas networks; and (iii) the retrofitting of the natural gas networks to transport only hydrogen. From an environmental point of view, the first option should be preferred to the other two, since a neutral or a net negative balance of CO2 could be obtained in the production of biomethane and SNG, respectively. However, several resources and long times could be required to put in place such approach: (i) plants for CO2 capture from flue gases emissions should be realized, and (ii) infrastructures dedicated to CO2 storage and transport to the final users should be implemented. For these reasons, the first configuration is considered for a long-term energy strategy. The third option seems as well to be not feasible in the short-medium term for the same reasons of the first option, i.e., high infrastructural costs. Therefore, hydrogen blending into the natural gas grids appears to be the most viable solutions in the short-medium terms [10].

Among possible P2G configurations, power-to-hydrogen (P2H) is the simplest, the most reliable and energy efficient. In addition, renewable hydrogen production, i.e., "green hydrogen", produced from renewable or nuclear sources [11], is an essential topic in the recent "Hydrogen strategy for a climate-neutral Europe" promoted in 2020 by the European Commission [12]. The strategy includes the natural gas sector as a key driver for the effective implementation of a hydrogen economy, since the existing natural gas infrastructures can play a relevant role in the early stage of the hydrogen strategy development as a way to transport and store green hydrogen [13].

Nevertheless, in the literature several technological limitations have been identified to hydrogen blending in the existing natural gas networks. First of all, safety concerns have to be considered since metallic pipelines shows a higher risk of failure in case of operation with hydrogen and compressed natural gas (HCNG) blend. Several authors investigated the interaction of high and low pressure hydrogen in metallic and plastic pipelines [14]. Assuring the highest safety condition in gas infrastructures should be the first aim of gas operators [15,16]. Particularly, higher leakage rate, i.e., a greater hazardous distance in case of failure, is expected for HCNG for high pressure systems [17] even if they are comparable for low pressure distribution systems [18]. Secondly, HCNG quality, i.e., the energy content, supplied to final end-users has to be controlled and correctly measured. In fact, since hydrogen concentration could change with time, smart metering is crucial to monitor the hydrogen percentage and to measure the effective energy content of the HCNG flow [19]. In addition to metering issues, the hydrogen concentration in HCNG is limited by existing end-users' devices and equipment that are designed and certified only for NG supply. Based on a literature review, [20] reported a maximum concentration up to 20% for vehicle engines, burners and boilers while higher concentration, i.e., up to 50%, could be considered for gas cookers and CHP application. HCNG quality also affects the performances of equipment installed in the transportation and distribution networks. Particularly, a maximum hydrogen concentration of 10% is suggested for the operation of existing compressors installed along the natural gas network [21]. Furthermore, since hydrogen percentage increasing causes a reduction of the low heating value (LHV) of the HCNG [22], higher mass flowrates, and so possible congestion, are expected in the network to convey the same quantity of energy.

Among the non-technological barriers, it is relevant that only qualitative evaluations have been carried out about the potential of green hydrogen blending in the existing natural gas networks [23]. In particular, a fundamental question for the development of a long-term strategy is "how much green hydrogen could be yearly produced and blended in the existing natural gas networks without

any relevant impact on the infrastructure and the end-users?" In fact, without the assessment of the nominal capability of the network to transport HCNG, insufficient information would be available also for the proper localization, planning and design of P2H plants.

The aim of the paper is to propose a methodology for the quantitative estimation of the Italian natural gas network capacity to accept green hydrogen and transport HCNG with low hydrogen concentration. Moreover, the paper includes a first assessment of the Italian P2H plants capacity and location.

#### **2. Methodology**

The following section reports the description of the methodology followed by the Authors to quantitatively estimate the HCNG transportation potential of Italian natural gas infrastructure in the case of low percentage blending of hydrogen. The Italian natural gas network and the main technical operative conditions are firstly presented. Then, the main concepts of the paper's methodology are introduced. After that, the assumptions for the following calculation are described, discussed and justified.
