Pre-combustion Capture

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. 

Because of the growing importance of carbon dioxide sequestration, there is currently a lively debate as to whether future coal-fired power stations should be conventional pulverized fuel, oxy-fuel or gasification designs. This is by no means a straightforward choice and involves considerations of overall fuel efficiency, engineering complexity and capital and operating costs. In addition, there are many types of coal (anthracite, bituminous coal, brown coal) with possibly dissimilar impurity contents, each of which may dictate a different plant design. The jury is still out on whether future coal-fired power stations will employ post-combustion or pre-combustion capture of carbon dioxide this is a crucial issue to decide as the plants have a life of 40—50 years. 

At present, the carbon dioxide formed during the manufacture of hydrogen by the steam reforming of natural gas is vented to the atmosphere. With a world-wide hydrogen production today of 45—50 Mt per year, this implies that about 120 Mt of carbon (440 Mt CO2) are released annually from this source 

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In the field of electricity generation, in particular, intensive research is carried out on capture processes, as this industrial sector contributes heavily to worldwide greenhouse-gas emissions. Basically, three concepts for C02 capture in electricity generation can be distinguished post-combustion capture, pre-combustion capture and oxyfuel (see Fig. 6.1 for a detailed description of the various capture processes, see, for example, BMW A (2003) IPCC (2005)). 

Carbon dioxide capture and storage is especially studied in connection with electricity generation, where different technology routes - post-combustion capture, pre-combustion capture and oxyfuel combustion - are under research and development. Carbon dioxide capture is also crucial in connection with hydrogen production... 

Treated flue gas contains C02, N2, 02. Since N2 and 02 form hydrate crystals at approximately the same conditions the treated flue gas is considered a C02/N2 mixture. Thus in post-combustion capture from power plants the task is to separate C02 from a C02/N2 mixture in which the C02 molar content is approximately 15-20 %. Pre-combustion capture involves separation of C02 from a mixture with H2 in which the C02 molar concentration is approximately 40 % (Klara and Srivastava, 2002 Kang and Lee, 2000 Englezos and Lee,... 

Linga, P. Kumar, R. Englezos P. (2007b). The clathrate hydrate process for post and pre combustion capture of carbon dioxide. J Hazardous Materials, revised submitted May 10, 2007... 

Combined Cycles with coal gasification, pre-combustion capture of COj. 

Pre-combustion capture is when the carbon dioxide is separated from the hydrogen before it is combusted in a gas turbine. Post-combustion capture is the more usual process where the fuel is burnt first and the carbon dioxide is extracted from the exhaust gas. See Section 3.2, Chapter 3. 

M. MoUere, Pre-combustion capture a powerful concept against long-range, trans-boundary pollution, in EGTEI-EmTech50-500 Conference Rome, 2011. 

Each of these capture options has its particular benefits. Post-combustion capture and oxyfuel have the potential to be retrofitted to existing coal-fired power stations and new plants constructed over the next 10-20 years. Pre-combustion capture utilizing IGCC is potentially more flexible, opening up a wider range of possibilities for coal, including a major role in a future hydrogen economy. 

Keywords Pilot plant, CO2 capture, natural gas, pre-combustion capture, reactive solvents... 

The pilot is designed to be able to study the removal of CO2 from high pressure gas streams containing methane or nitrogen. Since in the future, CO2 removal from flue gas, biogas and from hydrogen rich gas streams (pre-combustion capture) is anticipated, the pilot can be able to study these processes after relatively small technical modifications. 

Until now, the reports of adsorptive separation of CO2 and other gases in a mixture by using MOFs, conducted by experimental separation process, are limited. Among various characterization methods in separation, the breakthrough experiment and gas chromatographic separation are simple and straightforward in the evaluation of the separation performance of a MOF toward a gas mixture. On the other hand, as previously mentioned, reported CO2 separation in MOFs mainly includes CO2/N2 separation for post-combustion capture, CO2/H2 from synthesis gas for pre-combustion capture, and O2/N2 and CO2/CO separation for oxy-combustion capture, which will be detailed as following. 

Pre-combustion capture this solution is developed in two phases (i) the conversion of the fuel to a mixture of H2 and CO (syngas mixture) through, e.g. partial oxidation, steam reforming or auto-thermal reforming of hydrocarbons, followed by water-gas... 

As far as the pre-combustion capture is concerned, a fuel is converted into hydrogen and CO2 (after gasification if the fuel is in solid form), then CO2 is separated and hydrogen... 

Anderson, C., Scholes, C., Lee, C. et al. (2011) Novel pre-combustion capture technologies in action - Results of the CO2CRC-HRL Mulgrave capture project. Energy Procedia, 4,1192-1198. 

Applicable mainly to new plants, as only few plants are now in operation Barriers to commercial application of gasification are common to pre-combustion capture ... 

Pre-combustion capture, whereby pressurized syngas undergoes the WGS to convert CO (and H2O) to H2 and CO2, and is then separated... 

Layout of a natural gas combined cycle with CO2 pre-combustion capture based on air-blown ATR reactor. 

As discussed in Section 10.2.1, ATR is used today for large-scale production of syngas from natural gas, and is a promising basis for future power plants with pre-combustion CO2 capture. The same configuration described for IGCC plants (Fig. 10.7) can be adopted to integrate OTM into an 02-blown ATR-based power generation plant with pre-combustion capture. 

The main characteristics of the plants proposed in literature using membranes for H2 separation at temperatures suitable for the WGS reaction are reported in Table 10.3. According to these studies, an efficiency gain between 1.6% and 2.9% points can be obtained for membrane-based IGCC with >90% CO2 capture, compared to plants with standard (solvent absorption-based CO2 separation) pre-combustion capture technology. 

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