# Technology for Green Hydrogen Production: Desk Analysis
## 2 Background of Analysis
### Types of Hydrogen  
A concept illustration of grey, blue, green, and pink hydrogen production is presented in Figure 5. Grey hydrogen is a term used to describe hydrogen that is produced from fossil fuels in high-emission processes, primarily through SMR (Figure 5a) [83]. Figure 4: Hydrogen production mix, 2020 and 2021 [82]. The SMR process involves reacting natural gas (methane, CH\({}_{4}\)) with steam (H\({}_{2}\)O) to produce hydrogen (H\({}_{2}\)) and carbon monoxide (CO) at a temperature of 700-1000 \({}^{\circ}\)C in the presence of an appropriate metal catalyst according to the following chemical Reaction (1) [87; 88]:  
\[\mathrm{CH_{4}+2H_{2}O\to CO_{2}+4H_{2}}. \tag{1}\]  
The SMR process produces high emissions of carbon dioxide (CO\({}_{2}\)), a greenhouse gas, as a by-product of 9-12 kg CO\({}_{2}\)/kg H\({}_{2}\). Although methane is a raw material that ensures high efficiency, resource sufficiency, and relatively low costs compared to other methods using fossil fuels, the SMR process requires significant energy inputs and, due to its emissivity, contributes to climate change and environmental concerns. Grey hydrogen is the most common form of hydrogen produced today, accounting for the majority of the  
Figure 5: Type of hydrogen production: (**a**) grey hydrogen [83]; (**b**) blue hydrogen [84]; (**c**) green hydrogen [85]; (**d**) pink hydrogen [86]. hydrogen used globally. The production of grey hydrogen in the world already exceeds 55 Mt per year. Grey hydrogen is widely used in various industries, including chemical production, refining, and as a feedstock for the production of ammonia and methanol. To address the environmental impact of grey hydrogen production, there are efforts to develop and implement CCUS technologies [89; 90]. When these technologies are applied to the SMR process, the resulting hydrogen is often referred to as blue hydrogen as shown in Figure 5b [84]. The goal of CCUS is to capture the CO\({}_{2}\) emissions from the SMR process and store them underground or use them in other applications, thereby reducing the net carbon footprint of hydrogen production. Implementing CCUS technologies for grey hydrogen production involves several challenges, such as high costs, energy penalties, transportation, and storage, among others [82]. The initial capital expenditure for CCUS infrastructure is significant. This includes the cost of capture technologies, transportation systems, and storage facilities. Additionally, the operational costs can be high due to the energy requirements for capturing and compressing CO\({}_{2}\). Capturing CO\({}_{2}\) from industrial processes typically requires a lot of energy, which can reduce the overall efficiency of the hydrogen production process. This energy penalty can increase the cost of hydrogen and may lead to higher greenhouse gas emissions if the additional energy comes from fossil fuels. Transporting large volumes of CO\({}_{2}\) to storage sites can be challenging and costly. It requires pipelines, ships, or trucks, and the routes must be carefully planned. Capturing CO\({}_{2}\) from the blue gases of hydrogen production facilities is technically challenging, especially if the concentration of CO\({}_{2}\) is low. The capture process must be highly efficient to be economically viable, and it must not significantly disrupt the existing industrial processes. Demonstrating CCUS at a pilot or demonstration scale does not guarantee that it can be effectively scaled up to meet the needs of large industrial hydrogen production. Scaling up requires overcoming engineering challenges and ensuring the economic viability of the process. As RESs and technologies advance, green hydrogen produced from electrolysis using renewable electricity may become more competitive. This could reduce the incentive for investing in CCUS for grey hydrogen production.