Production of hydrogen

The secondary energy carrier hydrogen, like the secondary energy carrier electricity, can be produced from all forms of energy :

  • from electricity and water by means of electrolysis,
  • from carbonaceous energy carriers by means of thermochemical gasification,
  • as a by-product of the chemical industry, for example in the electrochemical production of chlorine and caustic soda,
  • from carbonaceous energy carriers by separating hydrogen and carbon in an electric arc (Kvaerner process),
  • by light by means of catalytic and biological (blue-green algae) processes,
  • through fermentation of biomass (dark fermentation) as a sub-process of biogas production,
  • by water decomposition through heat (>1200°C) with sunlight or atomic energy in complex cycle processes.

Of the not entirely complete list, only the first two processes are of technical and economic importance. Only these will be explained in more detail.

 

water electrolysis

When discussing the production of hydrogen in connection with the energy industry, the production of hydrogen by means of electrolysis is usually meant. The background is the storage of excess electricity in the form of hydrogen. Therefore, the principle of electrolysis should be briefly presented here.

Electrolysis is the reverse process to the fuel cell . In principle, it can be the same construction.

During electrolysis, water and electricity must be supplied.

With the fuel cell, hydrogen and oxygen (air) must be supplied. A fuel cell produces electricity and water.

Both cell types can be built as large or as small as you like. In the case of fuel cells, sizes in the kW scale are preferred, and in the case of electrolysers, sizes in the MW scale are preferred

Principle of the fuel cell

If you immerse 2 electrodes in an electrolyte and supply hydrogen and oxygen (air) to the electrodes, current begins to flow from around 1V.

Salt solutions, bases or acids are suitable as electrolytes. Shown here is an acidic (proton conducting) plastic membrane that requires porous electrodes

Principle of water electrolysis

If you immerse 2 electrodes in an electrolyte, the splitting of water into hydrogen and oxygen begins at a voltage of 1.23 V. At 1.48V the cell operates at 100% energy efficiency. At higher voltages, the cell heats up. The supplied energy is then not only bound to hydrogen, but is partly released as heat.

Salt solutions, bases or acids are suitable as electrolytes. Shown here is an acidic (proton conducting) plastic membrane that requires porous electrodes

Source: ELT

The picture shows a technical electrolyser like the ones that Lurgi manufactured in the 1950s for the Aswan Dam in Egypt. The hydrogen produced was used to produce nitrogen fertilizer. The further development of electrolysers has just started again. In the future, electrolysers will be needed in the energy industry to store the energy from excess renewable electricity or to bring the electricity to the end customer more cheaply in the form of hydrogen.

Since electrolysers have to be operated with high currents for economic reasons, only calorific value-related efficiencies of 70%-80% are achieved. with a calorific value-related efficiency of 80%, however, the energy yield is already 80%*1.18 = 94.4%. However, 18% of the energy is only released when the reaction water condenses during combustion or when used with fuel cells.

Hydrogen economy with electrolysis hydrogen

Hydrogen from electricity is always more expensive than the electricity from which it was produced. This statement can often be read: It is nonsensical to produce gas from electricity with losses that is then converted back into electricity with losses. That's true when hydrogen is used in a niche and all that matters is electricity. This is different in a hydrogen economy with a systemic surplus of electricity, also because the infrastructure for hydrogen is simpler and in a heat-driven energy economy, electricity is provided without losses. The cost-effective distribution of the hydrogen alone would justify electrolysis.

Economically, the situation today is that electricity from fossil power plants is not competitive with electricity from renewable sources. We only pay part of these electricity costs from fossil power plants via the electricity bill. We pay the social costs of power generation (experts say external costs) as health insurance contributions, illness and death (70,000 deaths per year from particulate matter in Germany alone) or shift the costs to future generations (CO2). We pay another part of the costs with tax deductions, because the subsidies for fossil fuels from the federal budget alone are higher than all the costs caused by the EEG.

hydrogen from biomass

Hydrogen can also be chemically obtained directly from biomass. This happens by reacting with water vapor at 800°C to 1000°C. This thermochemical reaction is called steam reforming. The chemical model equations are:

C 6 H 9 O 4   + 8 H 2 O = 6 CO 2 + 12.5 H 2   (allothermic)

C 6 H 9 O 4 + 1.095 O 2 + 5.81 H 2 O = 6 CO 2 + 10.31 H 2   (autothermic)

It is an endothermic reaction in which no heat is released, but heat must be supplied for the reaction to be possible in the first place. This means that all the energy that is supplied comes out bound to hydrogen at the end. If energy is supplied from outside (e.g. electric heating), one speaks of an allothermic reaction process. If the required heat is generated by partial combustion of the biomass, this is referred to as an autothermal reaction. This procedure requires oxygen. The efficiencies of both methods are the same.

The equations given above are gross equations. In practice, synthesis gas is first generated at approx. 850°C, which still contains carbon monoxide (CO). This is reacted at temperatures of 200°C to 400°C with steam in the presence of a catalyst in so-called shift reactors to form CO2 and H2.

Proven systems

Gasification plants have a long history. It started with the production of charcoal 40,000 years ago and continued 200 years ago with the production of coke and town gas. Without charcoal and coke, the production of metals would not have been possible. After the first oil crisis, many wood gasifiers were built in the USA, but these were shut down when the oil price fell. In Europe, the construction of wood gasifiers began about 15 years ago, some of which are still in operation. One of the best-known plants is in Güssing (southern Austria).

The wood gasification plant in Güssing is shown. The power is 8 MW. The plant has been in operation since 2001. It is an unpressurized fluidized bed gasifier with a diameter of 1.6 m. Pressurized at 30 bar, the plant would have an output of approx. 200 MW with the same size. The reactor is heated by a surrounding bed of sand, which is heated in a second reactor by partial combustion of the biomass. The generated synthesis gas is converted into electricity with two gas engines of 2 MW each. The city of Güssing is supplied with the waste heat. The facility is temporarily used as a research facility for various syntheses under the leadership of the Vienna University of Technology. The production of synthetic natural gas and the production of FT fuel were carried out on a larger scale. The production of pure hydrogen from biomass has so far only been investigated in computer simulations and scientific reports. Of theCold gas efficiency was given as 60% for this simulation . The cold gas efficiency is the ratio of the heating energy of the cold hydrogen to the heating energy of the biomass actually used. Since the synthesis gas is produced without pressure, it has to be compressed to the required pressure of the pressure swing adsorption system (8-20 bar), which requires a large amount of electrical energy.

In Germany there is a similar plant of 20 MW in Ulm. The synthesis gas is also converted into electricity here. The heat is coupled into an existing heating network.

There are other plants in Scandinavia. The city of Gothenburg plans to be fully supplied with synthetic natural gas from thermochemical wood gasifiers by 2020.

Problems in the production of hydrogen from biomass

Processes must be developed with the desired end product in mind. Hydrogen suitable for fuel cells is required under high pressure. The pressure is required for feeding into a pipe network (natural gas network) and is necessary for cleaning the hydrogen using a pressure swing adsorption system. Another challenge is the removal of tar components. In existing plants, the tar content is the biggest annoyance. The known types of gasifiers of industrial size produce synthesis gas with a tar content of 2000 to 20000 ppm. Elaborate tar scrubbing with biodiesel achieve tar levels of 50 ppm. This is sufficient for gas engines, but not for fuel cells, which require a tar content of less than 1 ppm. The widespread use of such carburetors also requires a closed material cycle.

New technical approach to the production of hydrogen from biomass

For the above reasons, the following requirements should be met:

  1. The carburetor should be operated under pressure
  2. A fluidized bed reactor is advantageous because it makes it easier to prevent the ash from melting.
  3. The tar content should be brought to <10 ppm by primary measures (in the reactor).

The first requirement suggests an improvement in the entry system for the biomass. The use of locks has so far proved to be complex and expensive. A self-sealing screw has been patented, but it still has to be tested.

The problem with operating fluidized bed reactors lies in introducing the required heat without overheating ash particles. This is critical for systems with a reactor and burner (Güssing) because individual particles can reach temperatures of 1100°C. This is already above the ash softening point of annual plants. The only thing left is wood. The use of nickel-containing particles to reduce the tar as a fluidized bed material, as is done in Güssing, is also critical in this respect because spreading ash containing nickel on the field is not permitted.

Energy chain of an innovative process

Based on the calorific value of the biomass of 100%, the gasification plant theoretically supplies usable energy of approx. 107%. With a higher investment in gasification, more than 99% reaches the end user. The possible use of heat in the system is then correspondingly smaller. The high efficiency of a hydrogen economy becomes clear here. Since the potential of biomass is greater than the demand, efficiency is not that important here. Poor efficiency only increases the proportion of heat. In a hydrogen economy, there is a systemic lack of heat.

Simplified flow chart of the innovative process

In the upper part of the reactor, the tar is destroyed catalytically on coke particles . A low tar content is required for the catalytic shift reaction and the pressure swing adsorption system. The supply of the necessary energy is only indicated here. Innovative devices are provided here that do not lead to overheating of ash particles. Initially, partial combustion with oxygen is preferred because, in the medium term, oxygen from electrolysers will be available free of charge.

The specified dimensions and performance are an indication that such devices cannot be built arbitrarily small. The plant in Güssing shown above would have been pressure-charged with the same apparatus dimensions and an output of approx. 200 MW. Pressurized industrial systems are therefore much more compact and cheaper. The innovative process should therefore not fall significantly below 50 MW with regard to technical functionality. 50 MW requires an input of approx. 10 t/h biomass (related to dry matter). The hydrogen produced can supply a small town with 30,000 inhabitants with electricity, heat and mobility. According to a rough estimate, the cost of such a system is around €30 million. A pilot plant will therefore have to budget around €60 million.2 -Patent GmbH.

Several inventions by H2-Patent GmbH are used in the process shown. Affected is the introduction of the biomass against the system pressure, the series connection of at least 2 fluidized bed reactors, which actually make the synthesis gas tar-free and heating with oxygen without melting the ash.

The suitability of these innovations has now been confirmed by the Fraunhofer Institute UMSICHT. With computer simulation, an exceptionally high cold gas efficiency of over 83% was determined even with simplified heat recovery. However, this value cannot be taken as an absolute value because it can be improved through additional investment. As with all process engineering systems, a compromise must always be found between efficiency and investment.

Investment costs for a hydrogen factory with air separation plant

The graph shows that costs do not increase linearly with size. Experts agree that 50 MW is the smallest size that is technically manageable. Plants larger than 500 MW require a siding or port. An air separation plant (ASU) is included here, which accounts for about 30% of the investment costs. For small plants, the supply of oxygen is more cost-effective. In the long term, oxygen from electrolysers can be used.

Wood pellets cost around 4.8 ct/kWh

Wind hydrogen currently costs around €7/kg

The experts who make suggestions for renewable energy systems to the federal government calculate according to VDI 2067, as shown here. Large corporations calculate with higher profits. Experts from this field can determine the costs themselves with a modified evaluation of the investment costs.

Manufacturing costs of bio-hydrogen

Shown are the production costs for hydrogen from biomass for mature plants at a biomass price of 100 €/t (related to dry matter) free plant, which corresponds to approx. 2 ct/kWh. However, wood prices could fall back to the level before the financial crisis. In 2005, the wood price was around €50/t. When using residual materials, the question of costs still has to be clarified. Usually the supplier has to pay disposal costs. How this works can be seen in the graphic. Residues account for about half of the biomass potential . The hydrogen prices here are based on the calorific value and are therefore comparable with the natural gas bill.

The graphic contains two messages:

  1. The hydrogen price is essentially determined by the biomass.
  2. A hydrogen factory can offer hydrogen at 2.5-3.5 ct/kWh on the outskirts cheaper than natural gas traders (approx. 4 ct/kWh)

Regarding point 1, it can be seen that the investment is not so crucial. This is even more true for large systems. The still uncertain calculation basis for the investment is therefore not that important.

Regarding point 2, it should be noted that the price of natural gas will increase faster than the price of biomass. This shows the historical development of the border crossing price over the last 15 years. The lower energy price level in a hydrogen economy and the possibility of producing all fertilizers entirely from biomass also have a price-dampening effect. Natural gas from fracking does not endanger this situation because the production costs in Europe are around 6 ct/kWh

As already explained in the description of the concept , the distribution of gas to households costs around 1 ct/kWh and to industry < 0.3 ct/kWh. With an H2 price of 3 ct/kWh ex works, electricity and heat could be generated in households for 4 ct/kWh. Even if taxes were still to be paid here, that is significantly less than the current price for household electricity of around 30 ct/kWh. Even if hydrogen were initially produced from natural gas and offered in households for 6 ct/kWh, it would still be worthwhile in today's situation.

updated: 07/04/2014

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