FACTS ABOUT HYDROGEN - PART III

Production of Hydrogen

The energy source hydrogen cannot be extracted like coal or petroleum: it can be produced from other chemical compounds. That is why we speak of a secondary energy source (similar to electricity).

The best example of a hydrogen compound is of course water. Two hydrogen atoms and one oxygen atom together form water. But there are many other substances that contain hydrogen.

In addition to carbon, organic compounds usually also contain hydrogen. An example of this is methane (the main component of natural gas), which consists of one carbon atom and four hydrogen atoms.

Plants consist of organic compounds that contain carbon, hydrogen and oxygen. Organic waste, plant waste, residual wood or specially grown plants such as rapeseed or special grasses - in general biomass - consist largely of carbon, hydrogen and oxygen.

Regardless of the starting material, hydrogen can be obtained through a manufacturing process. This requires energy. The advantage of using hydrogen is that the energy for its production does not necessarily have to come from fossil fuels. Wind energy, solar energy and hydropower are also primary energies!

The production of hydrogen is not really new. Over 500 billion cubic meters of hydrogen are currently being produced, stored, transported and used worldwide. This happens mainly in the chemical and petrochemical industries.

CHEMICAL PRODUCTION

GENERAL

The vast majority of the approximately 500 billion cubic meters of hydrogen used worldwide comes from fossil sources (natural gas, oil) or is obtained in the chemical industry as by-product hydrogen from chemical processes. A lot of hydrogen is generated, for example, in chlor-alkali electrolysis and in crude oil refining processes. All in all, hydrogen production as a by-product amounts to approximately 190 billion cubic meters worldwide.

Various processes are used to produce hydrogen from fossil fuels:

• Little reformers

In order to be able to use hydrogen in systems with fuel cells in the near future, small reformers are being developed. These systems are especially intended for small stationary systems to produce hydrogen from natural gas.

For mobile applications, the development of reformers on board vehicles has taken a back seat. At most for special applications, such as on-board power supply, this path is still being followed. The reforming of gasoline or diesel is particularly interesting here. In contrast, there are small steam reformers supplied with natural gas, e.g. at petrol stations have become increasingly powerful and efficient in recent years. They are an interesting option, particularly for petrol stations with high throughput, such as on the motorway.

• Steam reforming

Steam reforming is the endothermic catalytic conversion of light hydrocarbons (methane to naphtha) to synthesis gas (a mixture of carbon monoxide and hydrogen). On an industrial scale, these processes usually run at temperatures of 850 ° C and pressures of about 2 to 3 MPa (20 to 50 bar).

For the production of pure hydrogen, the carbon monoxide is largely converted in the so-called "shift reaction" with water vapor to carbon dioxide and hydrogen.

The carbon dioxide and the other undesirable components (e.g. unreacted methane and carbon monoxide) are then removed from the gas mixture by adsorption or membrane separation. The separated residual gas with approx. 60% combustible components (H2, CH4, CO) is used together with a part of the feed gas to fire the reformer.

Large-scale hydrogen production is carried out in steam reforming plants with the usual capacities of 100,000 cubic meters of hydrogen per hour. These systems are e.g. built by the companies Linde, Lurgi and Foster Wheeler.

• Partial oxidation

Partial oxidation is the thermal conversion of hydrocarbons with oxygen to synthesis gas (a mixture of carbon monoxide and hydrogen). In the case of natural gas, the method is mainly used to produce a synthesis gas with an H2 / CO ratio suitable for the synthesis of liquid hydrocarbons (Fischer-Tropsch synthesis). It is also used to convert heavy hydrocarbons (e.g. residual oil from petroleum processing).

This process of hydrogen production can also be operated with coal. The coal is finely ground and mixed with water to form a pumpable suspension with a solids content of 50-70% and then reacted with oxygen to form a hydrogen-rich gas. Should hydrogen enter the energy industry to any appreciable extent in the medium to long term, from the given environmental point of view (CO2 reduction), extraction by conventional steam reforming or partial oxidation from natural gas, oil or coal is not a sustainable solution and only makes sense for a transition phase.

HYDROGEN PRODUCTION BY ELECTROLYSIS

GENERAL

In an energy industry that is increasingly based on renewable energies, “electrical power” will become an important source of energy. Hydropower, wind energy and photovoltaics directly produce electricity, and the conversion of electricity into biomass and biogas can also be a useful addition for regulatory reasons. Although it makes the most sense to use electricity directly, the dominance of the energy source “electricity” creates the need for storage in order to be able to compensate for deviations between demand and supply. In addition, the supply of plenty of renewable electricity has not yet solved the fuel problem for vehicles. Hydrogen can do both. To do this, however, electrical current must be converted into storable hydrogen. Technically, this is done by means of electrolysis.

In its conventional form, alkaline electrolysis, water electrolysis has been used commercially for over 80 years.

PRINCIPLE DESCRIPTION

The decomposition of water by electrolysis consists of two partial reactions on the two electrodes, which are separated by an ion-conducting electrolyte.

Hydrogen is generated at the negative electrode (cathode) and oxygen at the positive electrode (anode). The necessary charge balance takes place through ion conduction. To keep the product gases separate, the two reaction spaces must be separated by an ion-permeable separator (diaphragm).

The energy for water splitting is provided by the supply of electrical energy.

The following types of electrolysis are available:

• Alkaline water electrolysis

Alkaline electrolysis works with an alkaline, aqueous electrolyte (potassium hydroxide solution). The cathode and anode compartments are separated by a microporous diaphragm to prevent the product gases from mixing. The asbestos diaphragms previously used are now being replaced by other materials. With output pressures of up to 3.0 MPa, efficiencies of 65 to 70% are achieved based on the lower calorific value of hydrogen.

Both electrolysers working at ambient pressure and pressure electrolysers are available on the market. Since the hydrogen is usually stored at pressures higher than the ambient pressure, pressure electrolysers are advantageous (saving electricity for compression, lower space requirements and lower investment requirements due to fewer compressor stages).

Modern electrolysis systems are suitable for fluctuating operation and can therefore be used in combination with regenerative power generation technologies.

• PEM water electrolysis

In contrast to the alkaline electrolysers, in which potassium hydroxide solution is used, a proton-conducting membrane serves as the electrolyte ("proton exchange membrane"). The PEM electrolyzers previously offered by Distributed Energy Systems in the USA achieve efficiencies of around 50%. The purity of the hydrogen generated is more than 99.999%. Hydro specifies an efficiency of about 68% (4.4 kWh / Nm3) for its PEM electrolyser based on the lower calorific value of the hydrogen generated. The purity of the hydrogen generated is 99.9%.

The output pressure of the generated hydrogen is 1.6 MPa (absolute) for Distributed Energy Systems and 3.1 MPa (absolute) for Hydro. Electrolysers that provide hydrogen at a pressure level of 13.8 MPa and above are under development.

• High temperature electrolysis

High temperature electrolysers have been discussed as an interesting alternative for several years. It would be advantageous to introduce part of the dissociation energy of the water in the form of high temperature heat around 800 - 1000 ° C, in order to then carry out the electrolysis with reduced electrical expenditure. The considerations aim to use the heat produced in a solar concentrator. Solar-thermal power plants for generating electricity (parabolic trough power plants) in combination with a solar tower power plant in which temperatures of over 1000 ° C can be generated would be conceivable. The electrical efficiency of the electrolysis could be increased up to 90%. However, this is only possible in countries with a lot of direct solar radiation.

The technology is at the basic research stage.

HYDROGEN FROM BIOMASS

GENERAL

Processes for the direct production of hydrogen from biomass are already technically feasible in principle. Since there is no commercial market for hydrogen as an energy source apart from the petrochemical use, gasification plants are not built for the production of pure hydrogen. Instead, the hydrogen-rich synthesis gas from the gasification plant is converted directly into a gas engine in order to sell it together with an excess of heat. The same applies to biogas from fermentation. For current commercial use, hydrogen from biomass would not be competitive with that from natural gas. However, if biomass hydrogen were used for vehicle fuels, it would be about as competitive as second-generation liquid biofuels (e.g. BTL).

A distinction is made between methods for the production of hydrogen using gasification from solid biomass (e.g. wood or dry waste biomass), the fermentation of wet biomass with low lignin content and the biological production of hydrogen. The efficiency of direct hydrogen production from biomass is in any case higher than the "detour" via the electricity generation from the biomass with subsequent electrolysis.

Biomass gasification

A suitable organic solid can be converted into a gaseous product by gasification. For example, coal gasification or the gasification of wood are known. But many other types of biomass such as grass and straw or solid biological waste are also suitable.

Before the actual gasification, the organic substance decomposes into coke, condensate and gases when heated. This process is known as thermal decomposition or pyrolysis. The presence of oxygen in the reactor leads to partial oxidation of the intermediate products instead of reforming.

During the steam gasification of biomass (depending on the biomass and gasification technology), a gas mixture of approximately:

• 47% hydrogen,
• 15% carbon monoxide,
• 10% methane.

In a second stage, the shift reaction, the carbon monoxide is converted to hydrogen and carbon dioxide with water vapor. The gas mixture is then separated into pure hydrogen and residual gas in a pressure swing absorption system. The residual gases are converted into electricity in a gas engine. The coke generated in the pyrolysis stage is used to generate heat for heating the heat transfer medium (e.g. Kurund balls or sand). The heat required to maintain the gasification process is coupled into the process via the heat carrier.

Fermentation of biomass

Biogas can be produced by anaerobic methane fermentation. This contains high levels of methane (50-70%) and carbon dioxide (30-50%). This gas mixture can serve as fuel gas for molten carbonate fuel cells (MCFC), whereby the methane reforming can take place directly at the electrode due to the high temperatures (~ 650 ° C). Before being used in membrane fuel cells (PEM), the gas must be converted to pure hydrogen in a reformer.

Biological hydrogen production

There are various biological processes in which hydrogen is released or occurs as an intermediate. There are two main types of processes: photosynthesis, which requires light and fermentation, which takes place in the dark. Algae and microorganisms each take over hydrogen production.

These methods of hydrogen production are state of the art in basic research.

Next time I will give you insights into the storage of H2

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