## 2 Research Method
### Hydrogen Production Technologies Being Evaluated  
The production technologies being evaluated are as follows:  
* Conventional via steam-methane reforming (SMR)  
The conventional method for H\({}_{2}\) production is steam-methane reforming (SMR), which accounts for around 50% of H\({}_{2}\) production [5]. This method converts natural gas (primarily methane) and steam to H\({}_{2}\) and CO\({}_{2}\), a byproduct that contributes to global warming. The second conventional method of producing gray hydrogen is coal gasification, but it is not included in this research because it is less common than SMR. Autothermal reforming (ATR) of natural gas is yet another conventional technology for H\({}_{2}\) production. ATR is similar to SMR in that it produces H\({}_{2}\) and CO\({}_{2}\), but it has the advantage of producing more exothermic reactions from the partial oxidation that occurs [8]. The H\({}_{2}\) produced from SMR, coal gasification, and ATR is denoted as \"gray\" H\({}_{2}\). The integration of carbon capture and storage (CCS) with any of these conventional technologies produces the so-called \"blue\" H\({}_{2}\). While integration with CCS results in CO\({}_{2}\) emissions reduction, both production cost and energy consumption are shown to increase compared to the case without CCS. Figure 1: Technology readiness levels (TRL). The utilization of various waste streams for hydrogen production is examined in food waste fermentation (FWF), waste plastics gasification (wPG), and biomass gasification with and without CCS. The valorization of biodegradable food waste by dark fermentation can help alleviate the waste management crisis and reduce the production of carbon emissions and toxic pollutants in overcrowded landfills, while also producing more eco-friendly H\({}_{2}\)[49; 50; 51]. Data-driven interpretation, comparison, and optimization of H\({}_{2}\) production from supercritical water gasification of biomass has been carried out. Gasification is the high-temperature partial oxidation of waste feedbacks, like plastics or biomass, to produces syngas, which is a mixture of CO and H\({}_{2}\). Syngas can be further converted to make other useful chemicals using the Fischer-Tropsch (FT) process or combusted to generate steam for electricity generation [52]. Waste byproduct reforming (glycerol from biodiesel production)  
A promising waste stream to utilize for H\({}_{2}\) production is the glycerol byproduct that results from biodiesel production via the transesterification of agricultural crops [17]. The technologies studied that convert glycerol into hydrogen are supercritical water reforming (SCWR), aqueous-phase reforming (APR), autothermal reforming, and steam reforming (SR). Greener hydrogen production via water electrolysis  
The water electrolysis technologies considered herein are alkaline electrolyzer (AKE), proton exchange membrane electrolyzer (PEME), anion exchange membrane electrolyzer (AEME), and solid oxide electrolyzer (SOE) via solar photovoltaic (PV) power, wind power, hydropower, grid mix electricity, and nuclear power. Hydrogen produced via water electrolysis with renewable energy is called green H\({}_{2}\) and is characterized by its low global warming impact. Electrolysis powered by grid mix electricity does not produce green H\({}_{2}\), but it benefits from utilizing the current electricity grid. The US grid electricity mix is roughly as follows: 38% natural gas, 17% coal, 20% nuclear, 11% wind, 7% hydropower, 5% solar, and 2% other [53]. Various emerging clean H\({}_{2}\) production technologies were evaluated in this comparative analysis. The high-temperature processes of biomethane pyrolysis and solar methane cracking (SMC) produce the so-called \"turquoise\" H\({}_{2}\) and carbon black. SMC utilizes concentrated solar energy to power the high-temperature process, resulting in even cleaner hydrogen [38].