User:Maudesquad

Project topic: Metal-organic frameworks.

We choose this topic because we found there isn't a lot of information on Wikipedia about the carbon capture application of MOFs. Also, we felt like there's a lot of room to improvement in articles related to this topic, especially in the Zeolitic imidazolate framework (a class of MOFs) one, which is a very short article. UC Berkeley conducts research about MOFs in relation to carbon capture, so we are in close proximity to many sources of information on this topic. To help general understanding the importance of carbon capture and sequestration, we can improve the readily available information about carbon capture technologies like MOFs.

Related articles to edit: Carbon capture and storage, Metal-organic framework, Carbon dioxide removal, Carbon dioxide scrubber and Zeolitic imidazolate framework.

Sources: http://www.osti.gov/scitech/servlets/purl/1091874, http://digitalassets.lib.berkeley.edu/etd/ucb/text/Swisher_berkeley_0028E_12933.pdf, http://www.osti.gov/scitech/servlets/purl/1003992-YRfi3u/, http://digitalassets.lib.berkeley.edu/etd/ucb/text/Sumida_berkeley_0028E_12601.pdf, http://www.chem.tamu.edu/rgroup/zhou/PDF/095.pdf and the Carbon Capture and Sequestration Textbook used in class.

Carbon Capture and Storage Edits (Maude)

For next two sections, underlined parts are the additions to original article. The whole capture technologies section is an addition to the original article.

Lead Section

Carbon Dioxide can be captured out of air or fossil flue power plant flue gas using absorption (or carbon scrubbing), membrane gas separation, or adsorption technologies. Amines are the leading carbon scrubbing technology. Capturing and compressing CO₂ may increase the energy needs of a coal-fired CCS plant by 25–40%. These and other system costs are estimated to increase the cost per Watt energy produced by 21–91% for fossil fuel power plants. Applying the technology to existing plants would be more expensive, especially if they are far from a sequestration site. However, recent industry reports suggest that with successful research, development and deployment (RD&D), sequestered coal-based electricity generation in 2025 may cost less than unsequestered coal-based electricity generation today.

Capture Section

Main articles: carbon dioxide scrubber, carbon dioxide removal, clean coal, amine gas treating, membrane gas separation, metal-organic framework

Capturing CO₂ is most effective at point sources, such as large fossil fuel or biomass energy facilities, industries with major CO₂ emissions, natural gas processing, synthetic fuel plants and fossil fuel-based hydrogen production plants. Extracting CO₂ from air is also possible, but not very practical because the CO₂ is not concentrated.

Flue gas from the combustion of coal in oxygen has a large concentration of CO2, about 10-15% CO₂ whereas natural gas power plant flue gas is about 5-10% CO₂ . Therefore, it is more energy and cost efficient to capture CO2 from coal-fired power plants. Impurities in CO₂ streams, like sulfurs and water, could have a significant effect on their phase behaviour and could pose a significant threat of increased corrosion of pipeline and well materials.[10] In instances where CO₂ impurities exist, especially with air capture, a scrubbing separation process would be needed to initially clean the flue gas. According to the Wallula Energy Resource Center in Washington state, by gasifying coal, it is possible to capture approximately 65% of carbon dioxide embedded in it and sequester it in a solid form.

Broadly, three different configurations of technologies for capture exist: post-combustion, pre-combustion, and oxyfuel combustion:

• In post combustion capture, the CO₂ is removed after combustion of the fossil fuel — this is the scheme that would be applied to fossil-fuel burning power plants. Here, carbon dioxide is captured from flue gases at power stations or other large point sources. The technology is well understood and is currently used in other industrial applications, although not at the same scale as might be required in a commercial scale power station. Post combustion capture is most popular in research because existing fossil fuel power plants can be retrofitted to include CCS technology in this configuration.
• The technology for pre-combustion is widely applied in fertilizer, chemical, gaseous fuel (H₂, CH₄), and power production. In these cases, the fossil fuel is partially oxidized, for instance in a gasifier. The resulting syngas (CO and H₂) is shifted into CO₂ and H₂. The resulting CO₂ can be captured from a relatively pure exhaust stream. The H₂ can now be used as fuel; the carbon dioxide is removed before combustion takes place. There are several advantages and disadvantages when compared to conventional post combustion carbon dioxide capture. The CO₂ is removed after combustion of fossil fuels, but before the flue gas is expanded to atmospheric pressure. This scheme is applied to new fossil fuel burning power plants, or to existing plants where re-powering is an option. The capture before expansion, i.e. from pressurized gas, is standard in almost all industrial CO₂ capture processes, at the same scale as will be required for utility power plants.
• In oxy-fuel combustion the fuel is burned in oxygen instead of air. To limit the resulting flame temperatures to levels common during conventional combustion, cooled flue gas is recirculated and injected into the combustion chamber. The flue gas consists of mainly carbon dioxide and water vapour, the latter of which is condensed through cooling. The result is an almost pure carbon dioxide stream that can be transported to the sequestration site and stored. Power plant processes based on oxyfuel combustion are sometimes referred to as "zero emission" cycles, because the CO₂ stored is not a fraction removed from the flue gas stream (as in the cases of pre- and post-combustion capture) but the flue gas stream itself. A certain fraction of the CO₂ generated during combustion will inevitably end up in the condensed water. To warrant the label "zero emission" the water would thus have to be treated or disposed of appropriately. The technique is promising, but the initial air separation step demands a lot of energy.

====Capture Technologies==== Carbon dioxide can be separated out of air or flue gas with absorption, adsorption, or membrane gas separation technologies. Absorption, or carbon scrubbing, with amines is currently the dominant capture technology. Membrane and adsorption technologies are still in the developmental research stages, initiating primary pilot plants in the near future. Metal-Organic Frameworks (MOFs) are a novel, promising adsorption carbon capture technology. Carbon dioxide adsorbs to a MOF through physisorption or chemisorption based on the porosity and selectivity of the MOF leaving behind a Greenhouse gas poor gas stream that is more environmentally friendly. The carbon dioxide is then stripped off the MOF using temperature swing adsorption (TSA) or pressure swing adsorption (PSA) so the MOF can be reused. Adsorbents and absorbents require regeneration steps where the CO₂ is removed from the sorbent or solution that collected it out of the flue gas in order for the sorbent or solution to be reused. Monoethanolamine (MEA) solutions, the leading amine for capturing CO₂, have a heat capacity between 3-4 J/g K since they are mostly water. Higher heat capacities add to the energy penalty in the solvent regeneration step. Thus, to optimize a MOF for carbon capture, low heat capacities and heats of adsorption are desired. Additionally, high working capacity and high selectivity are selected for in order to capture as much CO₂ as possible from the flue gas. However, there is an energy trade off with selectivity and energy expenditure. As the amount of CO₂ captured increases, the energy, and therefore cost, required to regenerate increases. A large drawback of MOFs for CCS is their chemical and thermal stability. Current research is looking to optimize MOF properties for CCS, but it has proven difficult to find these optimizations that also result in a stable MOF. Metal reservoirs are also a limiting factor to the potential success of MOFs.

Capture is attributed to about two thirds of the total cost of CCS, making it limit the wide-scale deployment of CCS technologies. To optimize a CO₂ capture process would significantly increase the feasibility of CCS since the transport and storage steps of CCS are rather mature technologies.

Organisms that produce ethanol by fermentation generate cool, essentially pure CO₂ that can be pumped underground. Fermentation produces slightly less CO₂ than ethanol by weight.

An alternate method under development is chemical looping combustion (CLC). Chemical looping uses a metal oxide as a solid oxygen carrier. Metal oxide particles react with a solid, liquid or gaseous fuel in a fluidized bed combustor, producing solid metal particles and a mixture of carbon dioxide and water vapor. The water vapor is condensed, leaving pure carbon dioxide, which can then be sequestered. The solid metal particles are circulated to another fluidized bed where they react with air, producing heat and regenerating metal oxide particles that are recirculated to the fluidized bed combustor. A variant of chemical looping is calcium looping, which uses the alternating carbonation and then calcination of a calcium oxide based carrier as a means of capturing CO₂.

A few engineering proposals have been made for the more difficult task of removing CO₂ from the atmosphere – a form of climate engineering – but work in this area is still in its infancy. Capture costs are estimated to be higher than from point sources, but may be feasible for dealing with emissions from diffuse sources such as automobiles and aircraft. The theoretically required energy for air capture is only slightly more than for capture from point sources. The additional costs come from the devices that use the natural air flow. Global Research Technologies demonstrated a pre-prototype of air capture technology in 2007.

Metal-organic Framework Page Edits (Jesika)

Additional potential applications

Carbon Capture

Main article: carbon capture and storage

Because of their small, tunable pore sizes and high void fractions, MOFs are a promising potential material for use as an adsorbent to capture CO₂.  MOFs could provide a more efficient alternative to traditional amine solvent-based methods in CO2 capture from coal-fired power plants.  

MOFs could be employed in each of the main three carbon capture configurations for coal-fired power plants: pre-combustion, post-combusiton, and oxy-combustion. However, since the post-combustion configuration is the only one that can be retrofitted to existing plants, it garners the most interest and research. In post-combustion carbon capture, the flue gas from the power plant would be fed through a MOF in a packed-bed reactor setup. Flue gas is generally 40 to 60 °C with a partial pressure of CO2 at 0.13 - 0.16 bar. The main component CO2 must be separated from is nitrogen, but there are small amounts of other gasses as well. A typical flue gas composition is: 73-77% N2, 15-16% CO2, 5-7% H2O, 3-4% O2, 800 ppm SO2, 10 ppm SO3, 500 ppm NOx, 100 ppm HCl, 20 ppm CO, 10 ppm hydrocarbons, 1ppb Hg.

CO2 can bind to the MOF surface through either physisorption or chemisorption, where physisorption occurs through van der Waals interactions and chemisorption occurs through covalent bonds being formed between the CO2 and MOF surface. Once the MOF is saturated with CO2, the MOF would then be regenerated (CO2 is removed from the MOF to be transported elsewhere for sequestration or enhanced oil recovery) through either a temperature swing or a pressure swing. In a temperature swing, the MOF would be heated up until CO2 desorbs; temperature swing regeneration is usually used in post-combustion configurations where the heat is supplied from heat exchangers with the power plant. To achieve working capacities comparable to the amine solvent process, the MOF must be heated up to around 200 C. In a pressure swing, the pressure would be decreased until CO2 desorbs; this would usually be used in a pre-combustion carbon capture configuration.

The Yaghi group tested various MOFs for their performance in absorbing CO₂ and found their highest CO₂ capacity achieved of 33.5 mmol CO₂ / g MOF at ambient temperature and 35 bar, using MOF-177.  For comparison, zeolite 13X has a CO₂ capacity of 7.4 mmol/g at 32 bar and ambient temperature, and MAXSORB has a capacity of 25 mmol/g at 35 bar and ambient temperature.  MOF-177 has pore sizes of 11 and 17 Angstroms and is comprised of zinc complexes connected by benzene rings, with the repeating structure being Zn₄O(1,3,5-benzenetribenzoate)_2.  

In addition to their tunable selectivities for different molecules, another property of MOFs that makes them a good candidate for carbon capture is their low heat capacities.  Monoethanolamine (MEA) solutions, the leading method for capturing CO₂ from flue gas, have a heat capacity between 3-4 J/g K since they are mostly water.  This is one of the main factors contributing to the energy penalty in the solvent regeneration step— when the absorbed CO₂ is removed from the MEA solution so that it can be reused.  MOF-177, on the other hand, has a heat capacity of 0.5 J/g K at ambient temperature.  The low heat capacities of MOFs could significantly reduce the energy penalty of carbon capture, which is currently around 30% of plant power for MEA solution-based methods.

In a project sponsored by the DOE and operated by UOP LLC in collaboration with faculty of four different universities, MOFs were tested as possible carbon dioxide removal agents in post-combustion flue gas. They were able to separate 90% of the CO₂ from the flue gas stream using a vacuum pressure swing process. From extensive investigation, they found out that the best MOF to be used was Mg(dobdc), which has a 21.7 wt% CO₂ loading capacity. Estimations showed that, if a similar system would be applied to a large scale power plant, the cost of energy would increase by 65%, while a NETL baseline amine based system would cause an increase of 81% (the DOE goal is 35%). The cost of capturing CO₂  would be $57 / ton CO2 captured, while for the amine system the cost is estimated to be $72 / ton CO2 captured. The project estimated that the total capital required to implement such project in a 580 MW power plant would be $354 million.

Carbon Dioxide Scrubber Edits (Gustavo)

MOFs

Metal-organic frameworks are one of the most promising materials for carbon dioxide capture and separation in adsorption processes. Although no large-scale commercial technology exists nowadays, several research findings have indicated the great potential that MOFs have for CO₂ removal. Their characteristics such as pore structure and surface functions can be easily tuned to improve CO₂ adsorption selectivity over other gases.

A MOF could be specifically designed as a CO₂ removal agent in post-combustion power plants. In this scenario, the flue gas would pass through a bed packed with a MOF material, where CO₂ would be adsorbed. After saturation is reached, CO₂ could be desorbed by doing a pressure or temperature swing. Carbon dioxide could then be compressed to supercritical conditions in order to be stored underground or utilized in enhanced oil recovery processes. However, this is not possible in large scale yet due to several difficulties, one of those being the production of MOFs in great quantities.

In a project sponsored by the DOE and operated by UOP LLC in collaboration with faculty of four different universities, MOFs were tested as possible carbon dioxide removal agents in post-combustion flue gas. They were able to separate 90% of the CO₂ from the flue gas stream using a vacuum pressure swing process. From extensive investigation, they found out that the best MOF to be used was a Mg/DOBDC one, which has a 21.7 wt% CO₂ loading capacity. Estimations showed that, if a similar system would be applied to a large scale power plant, the cost of energy would increase by 65%, while a NETL baseline amine based system would cause an increase of 81% (the DOE goal is 35%). Also, each ton of CO₂ avoided would cost $57, while for the amine system this cost is estimated to be $72. The project ended in 2010,estimating that the total capital required to implement such project in a 580 MW power plant was 354 million dollars.