Oxy-fuel

Oxy-fuel refers to burning a fuel with pure oxygen (O2) instead of air. Air has a high (78%) content of nitrogen. In combustion with air, most of this nitrogen passes through the process unchanged, with only a small fraction being oxidies to form NOx gases. Typical power plant exhaust gases contain about 75% nitrogen, which must be removed to create a CO2 stream for storage.

When burned with O2, hydro-carbon fuels generate a stream of almost pure CO2 and steam. The steam can easily be removed by condensation, leaving a CO2 stream ready for compression and storage.

In practical applications of oxy-fuel technology, the flue gas will have a CO2 content of 80-98%, depending on the fuel used and the particular oxy-fuel process used. If the content of inert gases or other impurities is to high for transport and storage, the CO2 stream might need to be purified. As the CO2 content is very high, further removal of impurities can be done at comparatively low cost. At present, oxy-fuel combustion is the only technology that has an immediate potential of achieving capture rates very close to 100%

Oxy-fuel technology has industrial applications today, primarily in welding and cutting of metals, as fuels burn at a significantly higher temperature in O2 than in air. Glass furnaces usually use oxy-fuel combustion to obtain the high temperatures needed in to produce glass.

Contents

1. Oxy-fuel in a steam cycle
2. Gas turbine cycle
3. Oxygen production
4. Novel designs and technology
5. See also

Oxy-fuel in a steam cycle    

The Vattenfall oxyfuel pilot plant at Schwarze Pumpe, Germany. (Image: Vattenfall)

In a coal fired oxy-fuel steam cycle, the basic design of the boiler is the same as with conventional goal-fired steam cycles. However, coal burned with pure oxygen has a combustion temperature of about 3500°C, while conventional boilers are design to operate at about 1900°C. It may be possible to design boilers that can withstand higher temperatures, taking greater advantage of the high combustion temperature. Still, the combustion temperature will need to be controlled by mixing the oxygen with steam or recycled flue gas. This may complicate the retro-fitting of oxy-fuel combustion to existing emission sources. Flue gas recycling may also increase CO2 concentration of the final flue gas stream.

The oxygen purity may be as high as 99.5%, but due to the relatively high cost of producing oxygen at that level of purity, lower concentrations may be an option. The increased level of impurities in this case could make post-combustion purification of the CO2 stream necessary before storage. However, since purification of a gas stream with such high CO2 conten consumes comparatively little energy, the lower cost of oxygen production may offset the extra cost of a second CO2 separation stage. This is particularly true if the fuel used has a high level of impurities or if case of retro-fitting oxy-fuel on existing boilers, where more leakage of air into the boiler might be expected, thus making a secondary separation stage necessary anyway.

Illustration of an oxyfuel CO2 capture plant. Click image to enlarge. (Source: Vattenfall)

An advantage of oxy-fuel combustion is that the very low content of nitrogen in the boiler reduces the levels of NOx produced. However, if combustion temperature is higher, this increases the rate of NOx production, and this may reduce the positive effect of lower nitrogen levels. To avoid this, strict temperature control is necessary unless high oxygen purity and minimal air contamination can be achieved.

Gas turbine cycle    

Oxy-fuel combustion may be well suited in gas turbine cycle, but requires fundamental changes in the design of the gas turbine. The thin blades in a gas turbine are particularly vulnerable to high temperatures, as they are subject to deforming or melting at tempeartures above 1400°C. This requires extensive recycling of flue gas to control combustion temperature.

The change in working fluid from air to a CO2-rich gas has significantly changes the properties that are of importance in turbine design. The working gas would have a density 50% higher than air, and it would also cool less with expansion. The advantage of the latter property is that the flue gas leaving the turbine has a higher temperature, increasing the efficiency of the steam cycle in a combined cycle power plant.

Oxygen production        

The major determining factor in cost and efficiency of oxy-fuel CO2 capture is the production of oxygen. The methods in use for oxygen production today are quite energy demanding. Developing more efficient ways of producing oxygen would reduce the cost and increase the efficiency of oxy-fuel CO2 capture.

The most common method today is cryogenic oxygen production. After removing impurities such as water, CO2, N20 and trace hydro carbons, the air is cooled under pressure until it turns into liquid air and oxygen and nitrogen is separated in a distillation column. Typical power consumption to deliver 95% O2 is 200 to 240 kWh/tO2. To achieve higher levels of purity energy costs can be considerably higher.

A promising future technology is using high temperature ceramic membranes to separate oxygen from air. This technology may produce high purity O2 at a lower cost than cryogenic oxygen production. This technology is currently in the pilot plant stage, and large scale production may be available in a few years.

Novel designs and technology

Main articles: Clean Energy Systems / ZENG, Chemical looping combustion

Several novel oxyfuel solutions have been proposed and tested in experimental scale. 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.

See also