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Novel designs and technologies
Many technologies require considerably more research and development to be considered mature for full scale implementation. The following presentation highlights a few possible future technologies for carbon capture in power production.
Contents
- Clean Energy Systems / ZENG
- Chemical looping combustion
- Hydrogen membrane reactor
- Solid Oxide Fuel Cells (SOFC)
- Carbon Black
- See also
Clean Energy Systems / ZENG
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Illustration of the ZENG hybrid cycle oxy-fuel technology. Click image to enlarge. (Source: ZENG) |
In Risavika in Stavanger several Norwegian companies (Lyse Energi, CO2-Norway, Nebb Engineering AS and Procom Venture AS) have co-formed the company ZENG, which is dedicated to building a pilot plant for oxy-fuel technology. The pilot is based on technology delivered by Clean Energy Systems (CES). CES has developed a gas generator based on combustion technology from space industry, where gas is burned at high temperatures with water as a coolant.
Flue gas from such a generator contains as much as 90 percent steam from the cooling process, making it possible to run it through a high pressure steam turbine, thus running a gas/steam hybrid cycle. Having gone through the first turbine the gas is reheated before being run through a secondary turbine, the CO2 /steam mixture finally being expanded in a low pressure steam turbine. CO2 can then easily be separated using water condensation. ZENG aims to make the investment decision for a 50-70 MW pilot plant by the end of 2008.
Chemical looping combustion
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Schematic of chemical looping combustion (Source: ZERO) |
In this process pure oxygen for combustion is supplied by cycling metals that bind the oxygen from air.
Two separate reactors are used. In the first one the metal reacts with oxygen under pressure and a temperature of 400-500 °C. The oxygen is bound to the metal, which oxidises to form a metal oxide. This in turn is transported to the next reactor, where the oxygen is released at 500-900 °C in a reaction with the natural gas. The metal is then recycled and transported back to the first reactor. In stead of moving large volumes of metal from one reactor to another, it is possible to have each reactor play a dual role, alternating between supplying air and fuel to each reactor, while the metal stays and is not cycled.
From the reactor the exhaust gas is led through a turbine to generate power. As in any other oxy-fuel process CO2 is separated, by cooling the exhaust gas and condensing the water. In addition, some power may be generated in a turbine for the warm oxygen lean air from the oxidation process.
In a pre project report (Brandvoll & Bolland 2002) a power generation efficiency of 51-53 per cent was estimated. The combination of high efficiency and a CO2 capture potential of 99 percent renders this technology environmentally advantageous. Chemical looping combustion can also be used with coal, but only with gasification.
Hydrogen membrane reactor
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Cylindrical membrane reactor for steam reformation (Source: Aasberg-Petersen) |
In hydrogen membrane reactors hydrogen is produced using a membrane that separates hydrogen from a given gas mixture. This reaction is therefore more complete than in conventional reactors, and the technology can improve efficiency of pre-combustion carbon capture.
With this membrane natural gas and steam is transported through a pipe filled with small spheres of catalytic material. CO and H2 are formed in the reforming process, through which hydrogen is continuously tapped through the membraned pipe wall. Concentration of hydrogen therefore remains low. The reaction is more complete and can take place at a lower temperature with lower steam surplus and at a higher pressure – all in all reducing the energy loss. The CO2 Capture Project (CCP) has concluded that this is a promising future technology.
This technology has the potential for high efficiency, low CO2 emissions and a lower cost than present day solutions. However, acquiring adequate materials and a practical design for integrated high temperature membranes is very difficult. Substantial technological research is required to succeed. It is hard to predict when, or even if, these efforts will be successful.
Solid Oxide Fuel Cells (SOFC)
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| Schematic of SOFC (Source: ZERO) |
Solid oxide fuel cells operate at very high temperatures. They are made of ceramic materials requiring temperatures as high as 1000 °C to acquire the necessary conductivity in the cell.
Natural gas and air enters through separate channels on different sides of a membrane made from thin ceramic material. In a fuel cell electric power is generated directly from a reaction on each side of the membrane rather than by combustion. Power generation efficiency using SOFC technology is about 48-50 per cent. The fuel cell will leave an excess stream of around 15 per cent that does not react. This could be used to fuel an afterburner to provide extra power generation in a turbine. Such a hybrid solution may yield 60-70 per cent generation efficiency.
Carbon Black
Carbon Black is highly pure carbon, which is used in metallurgical industries and in manufacture of tires, among other things. Carbon Black can be manufactured from natural gas by separating hydrogen and carbon (the Kværner process).
Carbon black can be viewed as a way of separating carbon from natural gas before combustion or process use. The separated hydrogen may be used as a fuel in a gas turbine or in fuel cells. There is a conciderable energy loss in the process, and by not using the carbon in combustion. Therefore, usefulness of the technology for carbon capture purposes is likely limited to to situation where the produced carbon has use as a product in it self.
