What is the Use and Future of Hydrogen as Fuel?

In order to meet human needs, the increasing demand for energy in the production and consumption stages in many sectors has led to an increase in the use of fossil fuels. This factor has led to the deterioration of environmental dynamics, especially air pollution, and the search for alternative fuels. Especially in the automotive industry, it has become necessary to limit the emission of greenhouse gases that cause global warming, especially CO2, due to the fuels used by vehicles. Thanks to new technologies, emissions caused by the fuels used are reduced, and the search for new fuels also serves this purpose. One of the fuel types that is clean and has high energy potential is obtained from hydrogen, the most abundant element in the universe. In this article, the properties, purposes and areas of use, advantages and disadvantages of hydrogen fuel are stated.

Hydrogen Fuel Definition

Hydrogen is the simplest and most abundant element in the universe and occurs naturally on Earth only when combined with other elements. The density of hydrogen is very low and is 7% of the density of air [1]. Like hydrogen, electricity; It is an energy carrier that can be used to store, transport and distribute energy. When we look at the fuel content of hydrogen, it is a preferred fuel type because it has no carbon content, high flame speeds and wide combustion limits. Compared to other fossil fuels, hydrogen has a high specific energy density, providing approximately three times more energy by mass and allowing for a lower heating value. Hydrogen fuel can be produced from various sources and methods without creating a carbon footprint. The Hydrogen Council, a global initiative established to promote hydrogen energy, including various energy and transportation companies, predicts that by 2050, approximately 25% of passenger vehicles and 20% of non-electric rail transport will be filled with hydrogen, potentially reducing daily fuel consumption by up to 20%. predicts that it will reduce

Hydrogen Production Methods.

In order for hydrogen to be used in fuel cell electric vehicles, it must be in the form of pure hydrogen (H2). Therefore, hydrogen, which exists in nature as part of a compound, must be separated by various methods. These methods have their own different environmental impacts, production costs and energy efficiency. Hydrogen production methods can be examined in three categories: fossil fuels, H2O separation and biological processes.
1. Fossil Resources

This method involves steam reforming, in which steam reacts with a hydrocarbon-containing fuel (syngas) at high temperature to produce hydrogen. The fuels used include: Diesel, gasified coal, renewable liquid fuels, biomass and natural gas [6]. Carbon monoxide reacts with water and produces additional hydrogen. It is the cheapest, most efficient and widely used method among other methods. However, since the fuel contains carbon, it causes carbon dioxide emissions. Hydrogen with lower emissions can be produced by incorporating carbon capture and storage into the production phase.

2. Decomposition of H2O
Electrolytic Processes
Water is separated into oxygen and hydrogen by electrolysis. Separation takes place in the electrolyzer, which functions in reverse, like a fuel cell. On a commercial scale, this process uses electricity as power and hydrogen as gas. In the electrolysis process, no by-products or emissions occur other than the formation of hydrogen and oxygen. However, renewable resources or fossil fuels can be used to produce electricity. Therefore, emissions from electricity generation must also be taken into account.

Solar Powered Processes

The basic material for hydrogen production is sunlight, and there are three systems that produce hydrogen: photobiological, photoelectrochemical and solar thermochemical systems. Special semiconductors are used to produce hydrogen in photoelectrochemical processes. In solar thermochemical systems, solar energy is used to separate water with metal oxides. Natural photosynthetic activity of bacteria and green algae is used in photobiological processes
3. Biological Processes
It is based on the principle of producing hydrogen using the metabolic processes of organisms such as bacteria and microalgae. Organisms produce hydrogen by breaking down organic materials such as wastewater and biomass. The energy needed to carry out this process is provided by the sun [6]. In addition to biomass, hydrogen can be produced from liquid forms derived from biomass (such as ethanol). Close to final use, these fuels are reacted with steam at high temperatures, thus producing hydrogen.
Comparison of Hydrogen with Other Fossil Fuels

The auto-ignition temperature of H2 is quite high compared to other fuels. When looking at the thermal energy of hydrogen per unit weight, it is three times that of other hydrocarbon fuels. However, from a volumetric perspective, the thermal energy of hydrogen is two-thirds of that of other fuels. Because hydrogen burns very quickly, its mass explosion potential is quite high compared to other gases. The flammability limits of hydrogen in air vary between 4% and 75% by volume, and this flammability limit of hydrogen is higher than other fuels. The temperature required to ignite hydrogen is approximately twice that required to ignite gasoline and is only slightly higher than methane.

What are the Advantages of Using Hydrogen?

When we look at air pollution, which significantly reduces the quality of life of those living in big cities, it is stated that traffic-related emissions increase air pollution. Air pollutants from traffic include nitrogen oxides, hydrocarbons, particulate matter and carbon monoxide. Hydrogen fuel is a type of fuel that has the potential to create near-zero greenhouse gas emissions. When hydrogen burns with oxygen, only water vapor is formed. Therefore, only hot air and water vapor are emitted into the atmosphere from the fuel cell, where electrical power is obtained. However, due to high temperatures during combustion, oxygen and nitrogen in the air can combine to form nitrogen oxide. The amount of nitrogen oxide formed; It depends on features such as air/fuel ratio, engine compression ratio and engine speed. Additionally, trace amounts of carbon monoxide and carbon dioxide may be released in the exhaust gas due to rapid combustion of fuel (depending on the operating principle of the machine). The use of renewable energy or fossil fuels with high carbon capture properties allows minimizing the emissions that will occur in hydrogen production. The energy consumption and greenhouse gas emissions of hydrogen storage, transportation and raw material processing systems are shown in Figure 4.
Effects of Hydrogen Fuel Properties on the Internal Combustion Engine

Hydrogen can easily spread into the air thanks to its high diffusion coefficient, and the homogeneous fuel-air mixture formed thanks to this diffusion increases combustion efficiency. Compared to other hydrocarbon-derived fuels, the minimum energy required to ignite hydrogen is one-tenth of that of hydrocarbon-derived fuels. Hydrogen's very high burning rate increases its explosion potential compared to other gases.

Disadvantages and Precautions in Hydrogen Use

Premature Ignition and Flame Back Problem

Hydrogen has a much lower ignition energy than other fuels and its use as fuel in internal combustion engines causes premature ignition. The ignition energy of hydrogen is 0.02 [mJ], while that of gasoline is 0.24 [mJ] and that of natural gas is 0.29 [mJ]. If ignition occurs too early, power loss occurs. The fact that hydrogen has a high flame speed means that the hydrogen fuel-air mixture will burn much faster than gasoline. Under stoichiometric conditions (the amount of air required to optimally burn 1 kg of fuel), hydrogen tends to burn 8.3 times faster than gasoline.