# Hydrogen Materials and Technologies in the Aspect of Utilization in the Polish Energy Sector
## 3 Hydrogen Production Methods  
On our planet, hydrogen occurs mainly in the bound state. The sources of hydrogen are water, hydrocarbons, and biomass. In the free state, hydrogen occurs in small amounts in volcanic gases and natural gas. Therefore, it cannot be extracted like conventional fuels are but must be produced by using energy. From the climate policy and environmental protection point of view, the method of hydrogen production and the associated CO2 emissions are important. Table 2 compares the hydrogen production methods with respect to their efficiency, costs, environmental impact, advantages, and disadvantages. Therefore, as can be seen from Table 2, hydrogen is designated with the colours, black, grey, blue, pink, and green, among others. For over half a century, hydrogen has been produced and used for industrial purposes. At present, 48% of produced hydrogen is made in the methane reforming process using steam, 30% is produced from crude oil, mainly in refineries, 18% is made from coal, and the remaining 4% is produced by means of water electrolysis [11]. Reforming methane with steam is currently the most common industrial method of obtaining so-called grey hydrogen. Hydrogen produced by the steam reforming of methane is obtained with an energy efficiency of 74-85% (Table 2). During this process, the following reactions mainly occur:  
\[\mathrm{CH_{4}+H_{2}O\to CO+3H_{2}\;\Delta H^{0}{}_{298}=206\; kJ/mol} \tag{2}\]  
\[\mathrm{CO+H_{2}O\to CO_{2}+H_{2}\;\Delta H^{0}{}_{298}=-41\; kJ/mol} \tag{3}\] \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Fuel** & \multicolumn{2}{c}{**Flammability Limits [vol. \%]**} & \multicolumn{1}{p{113.8 pt}}{**Octane Number**} & \multicolumn{1}{p{113.8 pt}}{**Temperature of Self- Ignition [\({}^{\circ}\)C]**} & \multicolumn{1}{p{113.8 pt}}{**Combustion Speed [m s\({}^{-1}\)]**} \\ \hline Hydrogen & 4–75 & 130 & 585 & 2.65–3.25 \\ \hline Methane & 5–16 & 125 & 540 & 0.37–0.45 \\ \hline Methane + 20\% hydrogen fraction & 4.6–19.9 & & & \\ \hline Methane + 60\% hydrogen fraction & 4.4–26 & & & \\ \hline Methane + 80\% hydrogen fraction & 4.6–47.6 & & & \\ \hline Propane & 2.2–9.6 & 105 & 490 & \\ \hline Petrol & 1–7.6 & 87–98 & 230–480 & 0.37–0.43 \\ \hline Diesel fuel & 0.6–5.5 & 30 & 254–285 & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of physicochemical properties of various fuels [11; 12; 13]. The higher hydrocarbons found in natural gas also react with water vapour and are an additional source of hydrogen and carbon monoxide. \[\mathrm{C_{n}H_{m}+nH_{2}O\to nCO+(n+m/2)H_{2}} \tag{4}\]  
This process is carried out on nickel catalysts at temperatures above 500 \({}^{\circ}\)C. The initial purity of hydrogen obtained in this process is 95-98%. The methane reforming process is more beneficial than its simple combustion, owing to the ability to control CO2 emissions and sequester them. Hydrogen produced from fossil fuels combined with CO2 capture processes is designated with the colour blue. Analogous to the reforming process, by means of the Bosch reaction (Equation (5)), synthetic gas can be produced by reacting coke with steam. Additional hydrogen can be obtained in further processing by reacting the resulting syngas with water vapour (Equation (3)), or it can be utilised as a reagent in organic synthesis or an energetic material. \[\mathrm{C+H_{2}O\to H_{2}+CO}\quad\Delta H^{0}{}_{298}=131.0\ \mathrm{kJ/mol} \tag{5}\]  
It is worth noting that the total production efficiency of so-called black hydrogen is twice as low as in the methane steam reforming process, because coke, unlike hydrocarbons, does not contain hydrogen. The energetic efficiency of the coal-to-hydrogen conversion system is about 30-40% (Table 2).