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Stuttgart-based ZSW turns wood and electricity into gas

It is rather reassuring to know that fossil energy carriers can be replaced by renewable ones. However, the difficulties are always in the details. For example with regard to the storage capacity of electricity produced with sun and wind; or with regard to the use of biomass to produce natural gas substitutes. The Stuttgart-based Centre for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW) has a number of solutions up its sleeve for overcoming such difficulties. The ZSW researchers are able to produce high-quality natural gas substitutes from wood and electrical power. In addition, the centre has just set a world record in the efficiency of thin-film solar cells.

As the centre’s name suggests, the ZSW researchers mainly focus on solar energy and hydrogen. This chemical compound has a huge advantage in the production of gas from biomass: the more hydrogen the produced gas contains, the more suitable it is to be converted into a natural gas substitute, namely methane.

Absorption is key in the AER process

The researchers call the method to produce hydrogen-rich gas, the AER (absorption enhanced reforming) process. In contrast to a biogas plant where the biomass is degraded in a fermentative process, the AER process is a thermochemical process. Two fluidised bed reactors are coupled for the continuous production of hydrogen-rich gas from biomass. In the first reactor, the biomass is gasified with hydrogen at temperatures between 600 and 800˚C, a process that leads to product gas and residual coke, which is subsequently combusted in the second reactor and provides the process heat required for the steam gasification. The main attraction of this process is the use of sorbent material that absorbs carbon dioxide (CO2). CaO (calcium oxide, burnt lime) circulates between the two reactors and binds unwanted CO2, thereby producing hydrogen-rich gas with a low tar content. This is why the process is called AER - absorption enhanced reforming. The CO2-loaded absorbent material is transported into the second reactor (calcination reaction) for regeneration (i.e. release of CO2 from the material) and is subsequently returned to the gasification reactor.

Principle of absorption-enhanced gas production from biomass using the AER process in two coupled fluidised-bed reactors. CaO is used as circulating sorbent bed material and serves as a “CO2-pump”. CaO removes CO2 from the biomass during the gasification process (first reactor). The carbonate (CACO3) that is formed is regenerated through the combustion of residual coke (second reactor); CO2 is released and returned to the gasification reactor.
Principle of absorption-enhanced gas production from biomass using the AER process in two coupled fluidised-bed reactors. CaO is used as circulating sorbent bed material and serves as a “CO2-pump”. CaO removes CO2 from the biomass during the gasification process (first reactor). The carbonate (CACO3) that is formed is regenerated through the combustion of residual coke (second reactor); CO2 is released and returned to the gasification reactor. © ZSW

Biological material for the production of natural gas substitutes

“The AER process leads to a hydrogen content of 65 to 70 per cent in the product gas,” pointed out Dr. Ulrich Zuberbühler, an expert in AER. He also highlighted that it would be possible to use the product gas produced by the gasification of biomass directly for the production of electricity and heat, or for the production of pure hydrogen. However, the decisive advantage of the AER method is that the product gas can be converted into methane in the methanisation reactor without any additional purification steps. The methane produced can then be fed directly into the natural gas network. Experts refer to the regenerative substitute of fossil gas as substitute natural gas (SNG). In addition, the AER process also has the advantage that it can easily convert lignin-rich biomass such as wood and straw, two materials that can only be fermented in a biogas plant with great difficulty.

Biomass gasification on the commercial scale

Biomass gasification using the AER process leads to a high-quality gas that can be used for the production of electricity, heat and fuel. © ZSW
The construction of the first commercial AER plant is planned in 2011. The 10-MW plant is to be erected in the Baden-Württemberg city of Geislingen close to the “Swabian Mountains” and it will be fuelled with wood and wood-like residues. “The plant can convert around 2.5 t of wood per hour,” said Zuberbühler describing its dimensions. The plant (total costs: 20 million euros) will be operated by the “Bioenergy and Methane Technology Platform” (TBM), a consortium of regional public services, and will receive funding of 3.6 million euros from the German Federal Ministry of the Environment and 500,000 euros from the Baden-Württemberg government. The remaining costs will have to be paid by the shareholders themselves. In order to advance biomass gasification research and development activities, the ZSW established the R&D platform “Biomass to Gas” in 2009, which will be closely linked with the pilot plant in Geislingen.

Power to gas

Besides the ability to convert biomass into gas, the ZSW researchers are also able to convert electrical power into gas. When there are blue skies and strong winds, the regenerative energy market produces excess capacity, so the "power to gas" procedure is able to produce natural gas substitute from electrical power, which can be fed into the gas network. When this happens, the electrical regenerative power is used for the electrolytical production of hydrogen: Methane is produced from hydrogen and CO2 in a methanisation reactor, and subsequently fed into the natural gas network as environment-neutral natural gas substitute. "This enables us to use the excellent infrastructure of the natural gas network and store excess solar and wind energy in underground caverns," said Zuberbühler, pointing out another advantage of SNG production.

The most efficient solar cells in the world

Only recently, the ZSW demonstrated its potential for producing innovative developments in its photovoltaic business area. The ZSW’s solar researchers have developed a thin-film solar cell with the world’s highest degree of efficiency of 20.3 per cent. The insights gained from the increase in efficiency in the laboratory are being used by the ZSW’s industry partner for the production of photovoltaic modules in order to further increase the ROI of photovoltaic plants.

Three sites with an excellent reputation

The three ZSW sites - from left to right: Stuttgart - Widderstall - Ulm
The three ZSW sites - from left to right: Stuttgart - Widderstall - Ulm © ZSW

Stuttgart is only one of three ZSW sites. Besides the three areas of expertise “photovoltaics: materials research”, “photovoltaics: modules, systems, applications” and “regenerative energy carriers and processes”, the Stuttgart site is also home to the “systems analysis” area which offers political consulting and feasibility studies in the area of regenerative energies. The ZSW site in Ulm mainly focuses on research into energy technologies used in electrochemistry, for example accumulators and fuel cells. The eLaB, a new laboratory building for the development of high-capacity batteries and battery safety tests, is currently being constructed in Ulm. Researchers at the ZSW site in Widderstall, located between Ulm and Stuttgart, are testing photovoltaic plants and systems outdoors under real operating conditions.

The ZSW, which was established as a foundation in 1988 by the Baden-Württemberg government, universities, research institutions and companies, employs around 200 scientists, engineers and technicians. It is one of the most renowned research institutions in the fields of photovoltaics, energy system analyses, regenerative fuels, battery technology and fuel cells.

Further information:

Ms. Claudia Brusdeylins
Centre for Solar Energy and Hydrogen Research
Baden-Württemberg (ZSW)
Meitnerstr. 1
70563 Stuttgart
Tel.: +49 (0)711/7870-278
E-mail: claudia.brusdeylins(at)zsw-bw.de

Website address: https://www.biooekonomie-bw.de/en/articles/news/stuttgart-based-zsw-turns-wood-and-electricity-into-gas