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Carbon Capture

The first  technology to separate CO2 from other gases was developed more than 80 years ago to remove CO2 from  town-gas. Today we can draw on the extensive experiences from this and other CO2 uses, and the technology to build full scale carbon capture plants is already commercially available.

CO2 can be captured from any large stationary emission source. Fossil fuel power plants and certain industrial processes are particularly viable for CO2 capture. About 40 percent of the total global greenhouse gas emissions derives from large stationary sources.

Capture technologies are usually distinguished into three main categories, depending on where in the process the CO2 is removed, namely post-combustion, pre-combustion and oxyfuel. Of these, post-combustion technologies are the most developed, and some post-combustion technologies are commercially mature. Pre-combustion is based on technology that is commercial mature, but needs further development for this application. Oxy-fuel technologies are used in some industrial processes but is still in the demonstration phase in power generation.

Contents

  1. Stationary point sources of CO2
  2. Capture Technologies 
  3. Industrial captures processes
  4. Bio-CCS
  5. Efficiency
  6. References

 

Stationary point sources of CO2

Main articles: Stationary point sources of CO2

Regional emission clusters with a 300 km buffer relative to world geological storage prospectivity

Regional emission clusters with a 300 km buffer relative to world geological storage prospectivity (IPCC, 2005)

A stationary point source of CO2 is any source that is a single localized emitter, such as fossil fuel power plants, oil refineries, industrial process plants and other heavy industrial sources. Stationary point sources can be distinguished from mobile sources, such as automobiles, ships, aircraft and other non-stationary sources.

While there are millions of stationary point sources of CO2 in the world, total emissions are dominated by a limited number of large point sources. A survey conducted by IEA showed that out of more than 14 thousand identified emission sources, the 7500 largest of these accounted for 99 percent of the these sources' total emissions, with the top 25 percent of this fraction accounting for 85 percent of the emissions. (IEA GHG, 2000)

The power and industry sectors combined account for about 60 percent of total global CO2 emissions. While future projections indicate that this share will decline, these sectors will still account for around 50 percent of all CO2 emissions in 2050 (IEA, 2008). Coal is currently the dominant fuel in the power sector, accounting for 38% of electricity generated in 2000, with hydro power accounting for 17.5%, natural gas for 17.3%, nuclear for 16.8%, oil for 9.8% and other renewables for 1.6%. Coal is projected to still be dominant fuel for power generation in 2020, whilst natural gas generation will surpass hydro to be the second largest (IEA, 2008).

Capture Technologies

Main Articles: Post-combustion, Pre-combustion, Oxyfuel, Novel designs and technology

Aminrensing / Nyhetsgrafikk.no

Simplified model of a post- combustion capture unit. 

(Source: Nyhetsgrafikk.no / ZERO)

There are a number of capture technologies available, but only a few technologies are currently commercially mature or close to commercial maturity. The technologies that have reached maturity are based on well tested technologies for industrial CO2 production, natural gas processing, ammonia production and other well industrial applications.

Most capture technologies can be sorted within three general categories; post-combustion, pre-combustion and oxyfuel. Post-combustion technologies use solvents to absorb CO2 from the flue gas after combustion. Pre-combustion technologies separates CO2 from the feed fuel, using well known processes such as hydrocarbon gasification and water-shift reaction, and uses the remaining hydrogen gas as fuel. Oxyfuel plants replace air with pure oxygen in the combustion chamber. When burned with pure oxygen, hydro carbons emits an almost pure stream of CO2 and steam, facilitating end separation of CO2.

Other technologies are generally in an early research phase. Among these are membrane technologies and natural gas fuel cells. While many of these technologies have been promising in small scale testing and may have high theoretical potential, commercial maturity applications shouldn't be expected in the near future.

Novel power plant process designs that integrates CO2 separation in the power generation process are being tested. Early tests suggest that such designs may increase overall plant efficiency significantly over existing fossil fuel power plants with CO2 capture. However, these designs are mostly still in a theoretical or early miniature testing phase, and are probably still many years from commercial maturity.

Industrial captures processes

Main article: Industrial captures processes

Norcem Brevik

The Norcem cement plant at Brevik, Norway (Image source: Foto: Vetle Houg / Wikipedia Commons)

CO2 from industrial processes  have yet to be tested on a large scale, apart from in natural gas sweetening. Different processes produce flue gas with widely different pressure and CO2 content. Different processes must therefore be considered individually for viability of CO2 capture.

Cement production, oil refineries, iron and steel industry, petrochemical industry (predominantly production of ethylene and ammonia) and natural gas processing are (in descending order)  the largest industrial sources of emissions. All these processes have characteristics that make them viable for carbon capture, such as high CO2 partial pressure and typically large emissions pr source. In some cases, CO2 concentration of the flue gas can be further increased by process modifications.

Retro-fitting CO2 capture on existing emission sources may in some cases prove challenging due to complex plant architecture. However, in some cases plant modifications may be combined with process upgrades, increasing the overall efficiency of the plant. This can make extensive modifications economical.


Challenges

The foremost challenge in carbon capture lies in increasing the efficiency of the capture process and reducing overall costs. Developing better solvents may reduce the energy penalty of post-combustion capture plants. Several applications of oxyfuel and pre-combustion technologies have proven to be promising in reducing the overall energy penalty of capture plants. 

A particular short term challenge is the very low number of operative large scale capture plants intended for sequestration purposes. The vast majority of capture projects today are small pilot or demonstration plants, at scales much smaller than required for industrial applications. Testing CO2 separation technologies in full scale is necessary to further improve the technologies and thereby increasing efficiency and reducing cost.

Bio-CCS

Bio mass

CO2 from the combustion of biomass can also be captured and stored. This decreases the net concentration of CO2 in the atmosphere (image source: sxc.hu)

Biomass is a considerable energy source, accounting for almost 10 percent of the total global primary energy use. Most of this is used for small scale cooking and heating. Biomass is not a significant fuel source in power generation and the large manufacturing industries on a global scale. However, biomass has regional significance in certain areas, like Scandinavia and Brazil.

Capturing and storing emissions from biomass combustion has the effect of reducing the net concentration of CO2 in the atmosphere, making it a potentially powerful global warming mitigation measure. Current biomass energy production plants are much smaller than fossil fuel plants. A typical plant has a capacity of 30 MW, with CO2 emissions of less than 0.2 MtCO2. This is too small for CCS is to be practical. Several solutions may exist to make bio-CCS more viable.

One solution is to build a pipeline infrastructure where small amounts of CO2 captured from several sites are combined to generate a CO2 stream with enough volume to be viable for storage. Another solution is to co-fire biomass with fossil fuels. Co-firing of coal and biomass already takes place in a number of countries.

Integration of energy plants based on biofuel with CO2 emitting industry like cement and steel producers will possibly be an interesting option for the future.

There is also possible to build bigger biomass power plants. For this to be a viable option, enough biomass must be available within a short enough distance to avoid to high transport costs. This may be possible in countries with large biomass resources. 

 

Bio-CCS is only a climate change mitigation measure when biomass can be acquired without causing permanent deforestation or otherwise cause land-use changes that reduces the biological storage of carbon. Biomass is currently a limited resource, which also limits potential for bio-ccs. However, alternative biomass sources, like algae, may increase the avilable biomass in the future.

Efficiency

Plant efficiency is often termed as the percentage of fuel energy actually utilized in a given plant. The generated amount can be divided into high-value and low-value energy. Mechanical work is the high-value energy form being used to run machines and perform work tasks, whereas heating and cooling are low-value energy forms. Exergy is the part of a quantity of energy that can be used for mechanical purposes and any other energy purposes. Electricity is pure exergy – it can be completely converted into mechanical work. This means that electrical power is a way of transporting mechanical energy. Even though large volumes of lukewarm water contain massive amounts of energy, this can not be converted into electricity or mechanical work. Consequently the exergy level is zero and for all physical purposes heat is a low-value energy form (Palm et al 1999).

In gas fired plants using a gas turbine and a steam turbine, the former converts almost 40 percent of the energy in the fuel to electricity. The hot exhaust gas is then used to heat water in a boiler creating steam, which in turn drives a steam turbine converting another 20 per cent of fuel energy into electrical power. The added amount of electricity generated from the two turbines is called the electrical generation efficiency of the plant. The surplus energy now exists as heat, and when not used this is called waste heat. A power plant may also deliver heat to industrial processes or district heating. If high temperatures are required, steam will be tapped from the generation cycle, somewhat reducing electrical output. The amount of energy utilized either as electrical power or heat is called total plant efficiency.

References

IEA GHG, 2000: Greenhouse Gas Emissions from Major Industrial Sources III

IPCC, 2005: IPCC Special Report on Carbon Dioxide Capture and Storage

IEA, 2008: World Energy Outlook 2008

 

 





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