The forms of hydrogen production

It is increasingly evident that the world is suffering from climate change, with increasingly extreme temperatures and increasingly frequent and severe natural disasters. These negative effects come from the pollution produced by humanity for centuries, such as the continuous emission of greenhouse gases through the use of fossil fuels.

Greenhouse gases (GHG) are a very broad group, but the most abundant is carbon dioxide (CO2) and it is used as a proportioning factor for others. For example, one ton of methane has the same effect on the atmosphere as the effect of approximately 20 to 25 tons of carbon dioxide, so this is where the greenhouse effect potential of methane is 20 to 25 tCO2e (tons of carbon dioxide equivalent). for mass decarbonization by the 2030s and with a focus on zero carbon equivalent emissions by 2050.

To achieve these goals, it is necessary to understand the energy transition, which deals with a paradigm shift involving the generation, consumption and reuse of energy, especially considering that energy generation in its most varied forms is responsible for a large part of global GHG emissions.

This concept starts from the protection of energy matrices, such as coal or oil-based fuels, for renewable energy sources, such as solar, hydroelectric, wind and biomass. This energy transition also extends to energy efficiency, digitalization, the environment, waste management and other means, so that the common goal of reducing GHG emissions is achieved.

In this context, hydrogen (H2) produced through low or zero CO2 emission processes emerges as an alternative energy source capable of decarbonizing the so-called intensive producers of GHGs, released when burning fossil fuels, as occurs, for example, in the cement and steel industries.

Although H2 is not an unknown substance and has already been widely used in various processes, its use as a decarbonization alternative is based on the possibility of obtaining the molecule through renewable energy, as well as its use, without the emission of polluting gases, for example, the combustion of hydrogen releases only Water (H2O) and Oxygen (O2). However, it is still necessary to evolve the processes of production, transportation and storage of H2, considering both safety aspects and technical and economic forecasts.

The hydrogen production process can be divided into three main routes: electrolytic, thermal and photolytic, being divided into seven main processes, providing various resources, both biomass and fossil fuels. Processes that use fossil fuels include reforming (partial oxidation, steam reforming and partial oxidation and autothermal reforming) and pyrolysis of hydrocarbons.

The production processes from renewable resources can be classified according to the raw material: biomass or water. The processes that use biomass can be divided into two subcategories: thermochemical and biological. The first involves pyrolysis, gasification, combustion and liquefaction of biomass, while the main biological processes are biophotolysis, dark fermentation and photofermentation. The second category of renewable technologies involves the production of hydrogen from water, through electrolysis, pyrolysis (thermolysis) and photolysis (photoelectrochemical decomposition) processes.

Source: Hydrogen Storage Investment Tax Credits (Resources for the Future, 2020)

Depending on the hydrogen production route, it is categorized into groups according to its characteristics. There is still no consensus regarding this categorization, however, some classifications are being widely disseminated by institutions such as IEA, EPE, Hydrogen Council, etc., and can be summarized as follows:

Source: Adapted from Bases para a Consolidação da Estratégia Brasileira do Hidrogênio (EPE, 2021)

Brown/Black Hydrogen

Produced from the gasification of mineral coal (lignite/hard coal – brown hydrogen, anthracite – black hydrogen) without capturing, using and sequestering carbon dioxide from the process.

Lignite has in its composition a lot of volatile matter, which makes it easier to convert to a gas and to products petroleum products than some coals that have a better quality. However, its high humidity and susceptibility to spontaneous combustion cause transportation and storage problems, which makes its use more difficult, meaning that companies that use this biomass are generally located near the mining area of this material. The important point is that due to the high humidity and low calorific value of Lignite, emissions of carbon dioxide are generally much higher per megawatt (potential energy) generated compared to black coal and "superior" coals.

Anthracite is created by metamorphism and is associated with metamorphic rocks, in the same way that bituminous coal (compacted peat) is associated with sedimentary rocks. Anthracite releases high energy per kilogram and burns cleanly with little soot, also used as a filter medium, which makes it a more sought after and therefore higher value variety of coal. Fossil coal was formed by the buried remains of tropical and subtropical plants, especially during the Carboniferous and Permian periods.

The gasification process occurs when carbon sources, in the case of coal, are exposed to air, or pure oxygen, and water vapor in a pressure vessel at very high temperatures (over 1800°C) and pressure. These high pressures and temperatures cause several reactions to occur, generating a mixture of gases called synthesis gas, generally with carbon monoxide (CO) and hydrogen (H2) in greater abundance, in addition to ash and slag in processes that use mineral sources. It is possible to apply a steam reforming process, such as the one described in gray hydrogen below, to convert CO, which is highly harmful, into CO2.

Grey Hydrogen

This hydrogen in question is produced from the reforming of natural gas or coal without CCUS (carbon capture, utilization and sequestration). This form of production releases large amounts of carbon dioxide into the atmosphere, contributing to global warming and climate change.

The reforming of natural gas, composed mostly of methane (CH4), begins with the gas entering a reactor and receiving pre-filtration to remove the fire. Then, the methane, with the help of a discovery, reacts with water vapor inside a reactor at high temperature, forming hydrogen (H2) and carbon monoxide (CO). Then, to improve hydrogen production in the process, another discovery is added where carbon monoxide reacts with steam and forms carbon dioxide (CO2), thus making the final separation stage more effective, where the mixed gases are separated and the pure hydrogen is stored. Currently, this is the most common form of hydrogen production in the world, but there is no capture of the greenhouse gases produced in the process, although some of these gases are reused for the heating process. It is known as “decarbonized gas” or “low-carbon gas” and is considered by some to be a clean energy source. There is controversy over this consideration, as carbon capture and storage technologies are not always free from environmental problems. This specific protection is produced from fossil fuels and natural gas (usually by the reforming method), but in this case the CCUS (carbon capture, utilization and sequestration) method also occurs. The CCUS process consists of storing carbon through liquids with specific connections that, after heating, release the gas; or there is simply a direct capture of carbon dioxide (CO2). In both cases, the CO2 is transported through pipelines to buried storage pockets (“buried”) or stored for transport. Some industries use this gas, such as the fertilizer industry, the chemical industry and the fuel methanol industry.

Pink Hydrogen

Pink Hydrogen is produced from the electrolysis of water using nuclear energy. Water electrolysis is the chemical optimization of water (H2O) generating the products oxygen (O2) and hydrogen (H2), through the application of an electric current (energy) to the water. In this specific case, the electric current comes from the energy of nuclear reactors.

Nuclear reactors are thermoelectric, that is, units that generate energy from heating water, but that use the high energy resulting from reactions that reach our atomic nuclei as a source of energy for this heating. There are two ways to produce nuclear energy, through nuclear fission or nuclear fusion. Today, only nuclear fission is commercially applicable, generally using radioactive Uranium atoms. There is research to make nuclear fusion commercially viable, and in 2022 it was the first time in history that it was possible to achieve a positive energy balance in this process. This type of energy can have several applications, mainly in the generation of electrical energy.

Uranium, in turn, is a finite resource, although there are large reserves of this material, which means that the hydrogen from this energy source is not renewable. Despite the positive point of not producing polluting gases in the atmosphere, there are still great risks, both from the radioactive waste generated by the fission process, which is harmful to nature, and from the danger that a plant in this sector brings with it (for example, the risk of leaks). If, in the future, nuclear fusion becomes viable, these problems will not exist.

Turquoise Hydrogen

Produced through the pyrolysis of methane from natural gas, and is itself used as an energy source for the thermal process. Its residue is solid carbon (coal), and is therefore considered an emission-free production.

Turquoise Hydrogen is produced when methane (CH4) enters a reactor heated to over 1000ºC (Celsius), which operates using energy from renewable sources. Further on, inside the reactor, the methane undergoes pyrolysis (heat division), resulting in solid carbon, or coal, (C) and hydrogen (H2). In the last stage, the hydrogen in gas form is collected at the top of the reactor and the second product (coal) exits the bottom of the reactor in solid form, making its storage simpler. Some experts call this hydrogen “low-carbon hydrogen”.

Yellow Hydrogen

Produced by electrolysis of water with the energy to carry out the process coming from any available source.

Like Pink Hydrogen, Yellow Hydrogen is also produced from the electrolysis of water using electrical energy. The difference in this H2 color is the energy source used in the electrolysis, or in reality “the energy sources”, after all this hydrogen is produced with a mixture of energy from sources such as renewables and fossil fuels, and it is not possible to trace the origin of the energy used. This energy comes from the National Interconnected System networks, which are supplied by various types of energy sources. Therefore, this type of hydrogen is not considered clean and renewable.

White Hydrogen

This type of hydrogen is found in its natural form, as a free gas, either in layers of the continental crust (gas pockets), at the bottom of the oceanic crust, in volcanic gases, in geysers or in hydrothermal systems. Natural/White hydrogen is present in a wide range of rock formations and geological regions. It arises from a variety of natural sources such as diagenetic origin (iron oxidation) in sedimentary basins, from radiolysis (natural electrolysis) or bacterial activity, also from hyperalkaline sources that contain dihydrogen emissions (2H2), among other forms. However, there are currently not many strategies and plans to explore this type of hydrogen, as the other “nuclei” are much more advantageous and practical to acquire and use.

Hydrogen Moss

The gasification process, in the case of biomass, occurs in 4 stages: the first is dry, in which the biomass loses the retained water; in the second stage, pyrolysis is used to start the installation of the biomass and prepare it for the next stage; in the third stage, the combustion of this product occurs; and in the fourth stage, the reduction of the material occurs where the carbon and hydrocarbons of the fuels used partially react with oxygen and generate carbon monoxide (CO), which is highly harmful, and hydrogen gas (H2). This process occurs at very high temperatures, starting at 900°C.

This mixture of gases, produced in the fourth stage, can subsequently be converted to nothing more than hydrogen and carbon dioxide (CO2), by adding steam and reacting on one found in a water gas placement reactor, facilitating the use of CCUS technology, if it is included in the process.

Green Hydrogen

Also produced from the electrolysis of water using electrical energy, as well as Pink and Yellow. In this case, the electricity used in the process must necessarily be advised from renewable sources that do not emit GHGs, mainly hydro, wind and solar. It is a cleaner alternative for producing hydrogen, as it is free of greenhouse gas (GHG) emissions, and thus has received strong support from governments and companies around the world. Ships.

There is still a long way to go, since green hydrogen currently represents a small percentage of the global hydrogen market and is not yet economically viable. However, as an example of what has happened with the introduction of renewable energy sources into energy matrices, the trend is for the production cost of green hydrogen to decrease as it gains economies of scale and becomes a safer and more mature process from a technological and financial point of view.