The energy carrier hydrogen can not be produced like coal or mineral oil; it has to be made of other chemical compounds. Therefore it is called a secondary energy carrier (similar to electricity).
Naturally, the best example for a hydrogen-compound is water. Water is built on two hydrogen atoms and one oxygen atom. There are, however, many other substances which contain hydrogen.
Besides carbon, organic compounds normally contain hydrogen. One example: Methane, (essential part of natural gas) is built on one carbon atom and four hydrogen atoms.
Plants consist of organic compounds which contain hydrogen and oxygen. Bio-waste, plant waste, residual wood, or especially for this purpose grown plants like rape or special grasses- biomass in general - consists of carbon, hydrogen, and oxygen as their main components.
Independently from the original source material hydrogen can be obtained by a special manufacturing process. One needs energy for this.
The advantage of using hydrogen is that the energy used does not have to derive from fossil energy carriers. Wind energy, solar energy or water-power are primary energies as well! The production of hydrogen is not really new. At present all over the world more than 500 billions cubic metres of hydrogen are produced, stored, transported and applied, especially in the chemical and petrochemical industry.
Chemical Hydrogen Production
The biggest part of today´s 500 billion cubic metres hydrogen sold world-wide is generated from fossil sources (natural gas, oil) or is obtained as by-product-hydrogen in chemical processes. A lot of hydrogen is obtained by chlor-alkali electrolysis and crude-oil-refinery-processes.
All in all hydrogen generation as a by-product amounts to around 190 billion cubic metres world-wide.
There are the following processes to generate hydrogen from fossil fuels:
Hydrogen Production by Electrolysis
- Small Reformer
To be able to use hydrogen in fuel cell applications in the near future small reformers are being developed. These systems are especially intended for small stationary applications, to produce hydrogen from natural gas.
As far as mobile applications are concerned, the development of reformers on board of vehicles has considerably decreased. Only special applications like on-board power supplies are still being further developed. For this purpose the reforming of petrol or diesel is particularly important.
On the other hand, steam reformers supplied with natural gas installed at filling stations for example, have become more and more efficient in the last years. Especially at highly frequented filling stations like along highways they could become an interesting option.
- Steam Reforming
Steam reforming is the endothermic catalytic conversion of light hydrocarbons (from methane to naphtha) into synthesis gas (a mixture of carbon monoxide and hydrogen). This large-scale process normally takes place at a temperature of 850°C and pressures of about 2 to 3 MPa (20-50 bar).
For the production of pure hydrogen, carbon monoxide reacts with steam to generate carbon dioxide and hydrogen in the so called "shift reaction".
The carbon dioxide and other unwelcome constituents (i.e. methane and carbon monoxide) are removed from the gas mixture by adsorption or membrane-separation later on. The separated residual gas which contains about 60% of combustible components (H2, CH4, CO) is used as a fuel in the reformer, together with a part of the input gas.
The large-scale generation of hydrogen is done in steam reformers with production capacities of usually 100.000 cubic metres of hydrogen per hour. These plants are built by companies like Linde, Lurgi and Foster Wheeler.
- Partial Oxidation
Partial oxidation is the thermal conversion of hydrocarbons with oxygen into synthesis gas (a mixture of carbon monoxide and hydrogen). In case of natural gas the main purpose of this generation process is to produce a synthesis gas with a H2/CO relation which is suitable for the synthesis of fluid hydrogen (Fischer-Tropsch-Synthese). Furthermore it is applied to the conversion of heavy hydrocarbons (e.g. residues from oil refining).
This hydrogen generation process works with coal as well. The coal is ground very fine and mixed with water into a pumpable suspension with 50-70% of solid matters and then turned into a hydrogen-rich gas with oxygen.
In case hydrogen is to play an important role in the energy economy in the medium or long-term it is not recommendable to base its generation on conventional steam reforming or partial oxidation from natural gas, oil or coal in view of the environmental requirements (CO2 reduction).
In an energy industry which will be more and more based on renewable energies, electric current will become an important energy carrier. Hydro power, wind energy and photovoltaics can produce electricity directly and the conversion of biomass and biogas into electricity can be seen as an useful complementation as well. Although the most practical way would be to use current directly, the dominance of the energy carrier "electricity" makes storage necessary in order to be able to balance supply and demand. In addition a lot of available renewable energy does not yet solve the problem of the fuel for vehicles. Hydrogen can be helpful here. Electricity can be transformed into storable hydrogen by the electrolysis process.
The conventional process is the alkaline electrolysis which has been in commercial use for more then 80 years.
Water decomposition via electrolysis takes place in two partial reactions at both electrodes, which are separated by an ion conducting electrolyte.
At the negative electrode (cathode) hydrogen is produced and on the positive electrode (anode) oxygen is produced. The necessary charge exchange works via ion conduction. To keep the product gases separated the two reaction compartments are separated by an ion conducting separator (diaphragm).
The energy for the splitting of the water is provided by electric power. The following types of electrolysis exist:
The research for this technology is still in the elementary phase.
- Alkaline Water Electrolysis
This process works with alkaline, aqueous electrolytes. Anode compartment and cathode compartment are separated by a microporous diaphragm to avoid the mixing of the product gases. The formerly used asbestos diaphragms have been replaced by other materials in the meantime. Over-outlet pressure up to 3,0 MPa will result in an efficiency factor of 65 to 70 % in relation to the lower caloric value.
On the market you will find electrolysers which work at ambient pressure as well as pressure electrolysers. As hydrogen is usually stored at pressures higher than ambient pressure, pressure electrolysers are advantageous. (This saves electricity for the compression process, the required space is less and the investment needs are lower because of less compressor steps).
Modern electrolyser systems allow fluctuating operation and are suited for a combination with renewable energy generating plants.
- PEM - Water Electrolysis
Opposed to the alkaline electrolysers which use caustic potash, a proton-conducting membrane serves as electrolyte (proton-exchange-membrane). The as yet offered PEM-Electrolysers of Distributed Energy Systems in the USA reach efficiency factors of about 50%. The purity of the produced hydrogen is more than 99,999%. Hydro indicates an efficiency factor of about 68% (4,4kWh/Nm3 ) in relation to the lower caloric value of the produced hydrogen for its PEM-Electrolyser. The purity of the produced hydrogen is 99,9 %.
The outlet pressure of the produced hydrogen is 1,6 MPa (absolute) with Distributed Energy Systems and 3.1 MPa (absolute) with Hydro.
Electrolysers with a pressure level of 13,8 MPa and more are still being developed.
- High-Temperature Electrolysis
High-temperature electrolysis has been discussed as an interesting alternative for some years. It would be advantageous to bring part of the energy needed for dissociation as high-temperature heat at around 800-1000°C into the process and then to be able to run the electrolysis with reduced electric power. Considerations like these aim at using the heat set free in a solar-concentrator. Also thinkable would be solar thermal power plants for power generation (Parabolrinnenkraftwerk) in combination with a solar tower power plant in which temperatures of more than 1000°C could be created. This way the electrical efficiency factor of the electrolysis could be increased up to 90%. Of course this is only possible in countries with a lot of direct solar radiation.
Biological Hydrogen Production
Hydrogen from Biomass
Technologies for the generation of hydrogen from biomass are in principle practicable today. As there is no commercial market for hydrogen as an energy carrier except for petrochemical usage, gasification plants for the production of pure hydrogen have not yet been built. Instead the synthesis gas rich in hydrogen from the gasification plant is converted into electricity in a gas engine which is sold together with the excess of heat. The same applies to biogas by fermentation. At present commercial usage of hydrogen produced from biomass is not cost competitive to hydrogen produced from natural gas. Would hydrogen produced from biomass be used for vehicles, however, it would be as competitive as the liquid bio fuels of the second generation called Biomass to Liquid (BtL)
Experts differentiate between the following methods for the generation of hydrogen by gasification: Conversion of firm biomass (e.g. pellets from cultivation, residues consisting of biomass), fermentation of biomass like liquid-manure and biological generation of hydrogen.
In any case, the efficiency factor for generating hydrogen directly from biomass is higher than the detour of converting the energy contained in the biomass to electric power needed for the electrolysis.
These methods of generating hydrogen are still in the development stage but they are a complementing option for a future hydrogen economy.
- Biomass Gasification
Suitable organic solids can be transformed into a gaseous product by gasification. Known is the gasification of coal or wood. Many other kinds of biomass for example grass, straw or residues consisting of biomass are suited for gasification as well.
Before the actual gasification process the organic substances decompose into coke, condensate and gases while heat is supplied. This process is called thermal decomposition or pyrolysis. Because of the oxygen in the reactor the intermediate products are not reformed but partial oxidation is taking place instead.
The water vapour gasification process of biomass generates a gas mixture (depending on the biomass and the gasification technology used) which consists of
- 47% hydrogen
- 15% carbon monoxide
- 10% methane
In a second phase, the shift-reaction, the carbon monoxide together with water vapour is converted into hydrogen and carbon dioxide. After that the gas mixture is dissociated in a pressure-swing-absorption process into pure hydrogen and residual gas. The residual gases are converted into electricity in a gas engine. Heat for heating the heat carrier ( e.g. metal balls or sand) is gained from the coke which is a result of the pyrolysis. The heat necessary for the continuation of the gasification process is implemented by the heat carrier.
- Fermentation of Biomass
Biogas can be generated by anaerobe methane fermentation. It contains a high percentage of carbon dioxide (30-50%) and methane (50-70%). This gas mixture can be used as fuel in advanced high-temperature fuel cells (MCFC). Because of the high process temperatures (~ 650°C) the reforming of the methane takes place directly at the electrode. Before it can be used in membrane fuel cells (PEM) the gas has to be converted into hydrogen in a reformer.
- Biological Hydrogen Production
There are different biological processes in which hydrogen is set free or is produced as an intermediate product. In principle two different types of processes can be distinguished: The photosynthesis which requires light and the fermentation which takes place in darkness. Hydrogen is produced by algae in the first case and by micro-organisms in the latter case.