The recent transition to zero-carbon fuels imposes the scientific community to perform physics-based studies on the properties of new fuels. In particular, this study focuses on the spray dynamics of diluted ammonia spray jets. Direct numerical simulations are performed in the OpenFOAM framework at different pre-heating conditions and qualitatively compared. In particular, the differences in terms of penetration length and jet width are analyzed and related to the relatively low saturation temperature of ammonia at ambient pressure.

Global warming, overpopulation, rampant pollution, and resource depletion are all significant environmental challenges that the world is facing. Like other countries all over the world, Saudi Arabia is also battling these issues. These issues are exacerbated by increasing human population, industrialization, urbanization, and transport development [1]. There is rising economic and environmental pressure to mitigate greenhouse gas (GHG) emissions, consisting of carbon dioxide (CO2) (74.4%), methane (CH4) (17.5%), nitrous oxide (N2O) (6.2%), and other emissions (HFCs, CFCs, SF6) (2.1%) [2], which are the principal contributors to global warming. The Kingdom of Saudi Arabia (KSA) heavily relies on fossil fuels such as crude oil and natural gas as its main energy provider [3]. The total CO2 emissions reached about 588.8 million tons in 2020 [1, 4], with the transport sector accounting for about 25% [1] of this emission. A number of strategies have been developed to mitigate GHG emissions and diversify energy sources, with the goal of reducing 4% in the total CO2 emissions by 2030 and achieving 3.45 gigawatts from renewable energy by 2020, 9.5 gigawatts by 2030, and 54 gigawatts by 2040 [5, 6]. KSA government has committed to a net-zero emissions target for 2060 aligned with the Paris Agreement [7]. Saudi Arabia seeks to alleviate the power sector's dependence on fossil fuels and will vigorously develop technologies that contribute to global decarbonization [8]. Reducing the transport sector's emissions is also a key requirement of the net-zero pledge. The increasing attention to combat climate change in Saudi Arabia has boosted the development of highly energy-efficient and alternative fuel vehicles (AFVs), such as hydrogen proton-exchange membrane (PEM) fuel cell vehicles (FCV) and battery electric vehicles (BEVs). BEVs, zero-emission vehicles in the operation phase, play a critical role in the direct coupling between the transport sector and electric utilities [9, 10]. However, the long charging time, limited driving range, high battery cost, and the lack of charging stations make BEVs less competitive [11]. Further, because of the obvious larger cell size and weight requirements, BEV technology is particularly difficult to be utilized for heavy-duty transport. Hydrogen PEM fuel cell vehicles, with their only products being water and heat, offer a promising alternative solution for decarbonizing the transport sector. Currently, three forms of hydrogen are used to power the PEM fuel cell vehicle - 'grey' hydrogen produced from natural gas, 'blue' hydrogen that is also from natural gas but with CO2 emissions captured using carbon capture and storage (CCS), and 'green' hydrogen made from water electrolysis powered by zero/ low carbon energy sources [12]. In the short to medium term, grey and blue hydrogen are more cost-competitive than green hydrogen due to the technology availability and cost advantage [13]. Further, only 2% of global hydrogen was green hydrogen in 2018, whereas 76% was grey hydrogen [13]. As the sixth largest natural gas reserve, with 333 trillion cubic feet (Tcf), Saudi Arabia has tremendous potential for natural gas development [14]. During the last two decades, natural gas production and consumption in KSA have been growing consistently and are expected to continue to grow due to the KSA government's initiative to utilize natural gas and renewable energy sources instead of fossil fuels to generate electricity [14]. Therefore, grey and blue hydrogen sources are considered to be more accessible and feasible for PEM fuel cell vehicle development in Saudi Arabia. Inspired by this, we propose to assess the life-cycle GHG emissions of 100 PEM fuel cell (FC) buses operating in Makkah using grey and blue H2 produced in KSA. For this, to compare, 100 BE buses using electricity from the KSA s grid and 100 internal combustion engine (ICE) buses using diesel as the fuel will also be examined across their entire life.