Carbon capture flue gases

The idea here is to capture carbon dioxide from power plants and then inject it into natural gas hydrate reservoirs assumed to contain primarily methane hydrate. Thus one achieves the simultaneous sequestration of carbon dioxide with the production of natural gas. Lee et al. (2003) presented laboratory data that showed the replacement of methane molecules by C02. Yoon et al. (2004) and Ota et al. (2005) confirmed these laboratory findings. Park et al. (2006a) used a CO2/N2 mixture containing 20 mol % carbon dioxide (flue gas) instead of pure C02 and noticed that the methane recoveiy increased from 64 to 85 %. A similar idea for sequestering captured C02 is to use it as cushion gas for natural gas storage in reservoirs (Oldenburg, 2003). 

The reactivities of pure NaHCOa solid. Sorb NHR, NHR5, and NX30 sorbents were examined in a fast fluidized bed reactor. The CO2 removal of the pure NaHCOa solid increased from 3 % to 25 % when the variables were altered. Removal increased as gas velocity was decreased, as the carbonation temperature was decreased, or as the solid circulation rate was increased. The CO2 removal of Sorb NHR and NHR5 was initially maintained at 100 % for a short period of time but quickly dropped to a 10 to 20 % removal. However, the Sorb NX30 sorbent showed fast kinetics in the fast fluidized reactor, capturing all of the 10 % of the CO2 in the flue gas within 3 seconds in the fast fluidized reactor. 

Nelson, T.O., P.D. Box, D.A. Green, and R.P. Gupta, Carbon Dioxide Recovery from Power Plant Flue Gas using Supported Carbonate Sorbents in a Thermal Swing Process, Sixth Annual Conference on Carbon Capture and Sequestration, Pittsburgh, PA, May 2007. 

The absorption technique using hot potassium carbonate has also been developed to capture C02 (Probstein and Hicks, 1990). The chilled ammonia process is another solvent-based C02 capture technology where ammonia carbonate slurries are used to capture 90% of the C02 in the gas stream mixture gas forming ammonia bicarbonate in the process. A pilot-scale chilled ammonia unit for 5 MW equivalent flue gas capture is under construction by ALSTOM and EPRI. Although this process is developed for a combustion system, the results will provide valuable information for the future development of such a process for hydrogen production. According to ALSTOM, commercial products on chilled ammonia process will be available by 2010 (Alstom, 2007). 

Like natural gas, the producer gas from coal is a clean fuel. Additionally, it is a rich source of chemicals. Coal-derived gas can also be recombined into liquid fuels, including high-grade transportation fuels, and a range of petrochemicals that serve as feedstock workhorses in the chemicals and refining industries. In contrast to conventional combustion, carbon dioxide exits a coal gasifier in a concentrated stream rather than diluted in a high volume of flue gas. This allows the carbon dioxide to be captured more effectively and then used... 

Compared to conventional combustion, carbon dioxide exits a coal gasifier in a concentrated stream instead of a diluted flue gas. This allows the carbon dioxide to be captured more easily and used for commercial purposes or sequestered. 

Existing capture technologies, however, are not cost-effective when considered in the context of sequestering C02 from power plants. Most power plants and other large point sources use air-fired combustors, a process that exhausts C02 diluted with nitrogen and excess air. Flue gas from coal-fired power plants contains 10%-12% C02 by volume, while flue gas from natural gas combined cycle plants contains only 3%-6% C02. For effective carbon sequestration, the C02 in these exhaust gases must be separated and concentrated. 

Radosz M, Hu X, Krutkramelis K et al (2008) Flue-gas carbon capture on carbonaceous sorbents toward a low-cost multifunctional carbon filter for green energy producers. Ind Eng Chem Res 47(10) 3783-3794... 

As was mentioned earlier, it is probably wise to separate the carbon dioxide from the flue gas and inject only a C02-rich stream. This is the so-call "capture" part of the carbon capture and storage. 

Mercury is one of a number of toxic heavy metals that occur in trace amounts in fossil fuels, particularly coal, and are also present in waste materials. During the combustion of fuels or wastes in power plants and utility boilers, these metals can be released to the atmosphere unless remedial action is taken. Emissions from municipal waste incinerators can substantially add to the environmental audit of heavy metals, since domestic and industrial waste often contains many sources of heavy metals. Mercury vapor is particularly difficult to capture from combustion gas streams due to its volatility. Some processes under study for the removal of mercury from flue gas streams are based upon the injection of finely ground activated carbon. The efficiency of mercury sorption depends upon the mercury speciation and the gas temperature. The capture of elemental mercury can be enhanced by impregnating the activated carbon with sulfur, with the formation of less volatile mercuric sulfide [37] this technique has been applied to the removal of mercury from natural gas streams. One of the principal difficulties in removing Hg from flue gas streams is that the extent of adsorption is very low at the temperatures typically encountered, and it is often impractical to consider cooling these large volumes of gas. 

Not all power plant designs fit into an upstream or downstream category. Integrated systems let carbon move through the entire process, but they prevent normal dilution of the output flue gas, so that the effluent is concentrated CO2. While most of the plants in this category are still in an early development phase, they promise to combine high efficiency, virtually zero atmospheric pollution, and complete capture of all CO2. All avoid the intake of air. 

Injection of activated carbon particles into the flue gas to absorb the mercury or operation of the coal combustion to convert a portion of the coal into activated charcoal. The mercuryladen charcoal is then captured by the fly-ash removal system (which may be an electrostatic precipitator. Alter, or dust scrubber). 

Physical sorbents for carbon dioxide separation and removal were extensively studied by industrial gas companies. Zeolite 13X, activated alumina, and their improved versions are typically used for removing carbon dioxide and moisture from air in either a TSA or a PSA process. The sorption temperatures for these applications are usually close to ambient temperature. There are a few studies on adsorption of carbon dioxide at high temperatures. The carbon dioxide adsorption isotherms on two commercial sorbents hydrotalcite-like compounds, EXM911 and activated alumina made by LaRoche Industries, are displayed in Fig. 8.F23,i24] shown in Fig. 8, LaRoche activated alumina has a higher carbon dioxide capacity than the EXM911 at 300° C. However, the adsorption capacities on both sorbents are too low for any practical applications in carbon dioxide sorption at high temperature. Conventional physical sorbents are basically not effective for carbon dioxide capture at flue gas temperature (> 400°C). There is a need to develop effective sorbents that can adsorb carbon dioxide at flue gas temperature to significantly reduce the gas volume to be treated for carbon sequestration. 

All three major processes - post-combustion capture, oxy-fuel combustion, pre-combustion capture - require a step that, variously, involves the separation of carbon dioxide, oxygen or hydrogen from a bulk gas stream (flue gas, air or syngas, respectively). These separations can be accomplished by means of physical/chemical solvents, membranes, solid sorbents or cryogenic processes. 

The selective capture of NO, from combustion flue gas by the use of commercially produced activated carbons at typical stack temperatures is reported, lliis adsorption is independent of whether the NO is NO, NO2 or a mixture. The NO, adsorption capacities can be as great as O.IS g NO,/g carbon if O2 is present as a co-reactant. NO2 is the species stored within the carbon and can be released by temperature induced desorption at 140°C. Adsorption capacities are dependent on the type of activated carbon used. 

Our research has shown that activated carbons can be used to selectively capture NO, (NO and NO2) from flue gases at typical combustion stack temperatures (70-120 C) (1-3). Temperature programmed desorption releases NO2 at temperatures near 140°C. It is necessary for O2 to be present for this selective and large NO, adsorption capacity, but CO2 and H2O do not interfere with adsorption nor are themselves adsorbed to any significant level. The NO, adsorption capacity can be as high as 0.15 g NO,/g carbon using a simulated combustion flue gas containing 5% O2,15% CO2,1% H20,2% NO, balance He... 

The development of membrane materials for CO2 capture from flue gas has received much attention during the last decade, clearly as a function of the concern about climate change and the need for carbon capture and sequestration (CCS). Many papers have been published (here only a few are being referred to [10,188-191]), but the major challenges for this membrane application are durability of the material over time, as there will be exposure to SO and NO, and very high separation performance needed (flux and selectivity) in order to decrease the needed membrane area for the huge volume gas streams. Very few pilots have been tested around the world only two are mentioned here (i.e., from MTR and NTNU). The number of pilots is expected to increase over the next few years. 

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