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12.4  Carbon Capture Processes                                  357

                        C sðÞ þ CO 2 ! 2CO        Boudouard reaction    ð12:10Þ

              Partial oxidation of methane can be described as

                         1
                   CH 4 þ O 2 ! CO + 2H 2   Methane partial oxidation   ð12:11Þ
                         2
                                             Steam methane reaction
                   CH 4 þ H 2 O ! CO + 3H 2                             ð12:12Þ
              The steam-methane reaction is effective in the temperature range of 700–950 °C
            at 1.4–4 MPa. Under these conditions, 70–80 % of the methane can be converted
            into hydrogen in one single reactor.
              The syngas can be directly burned for energy production where CO is converted
            into CO 2 by oxidation, but water–gas shift (WGS) is necessary to facilitate pre-
            combustion carbon capture. The CO produced in partial oxidation process even-
            tually is converted into CO 2 by the WGS reaction

                                   CO + H 2 O ! H 2 þCO 2               ð12:13Þ
              Typical syngas composition after WGS is shown in Table 12.3. The high con-
            centration of CO 2 favors effective CO 2 capture.
              By the WGS reaction over 90 % of the carbon exist as CO 2 . For the processes
            aiming at high purity H 2 ,CO 2 removal efficiency has to be improved. This is
            achieved commonly by multiple CO 2 removal units.



            Table 12.3 Composition of typical gases subjected to pre-combustion and post-combustion CO 2
            separation
            Gases         Mole fraction
                          Pre-combustion syngas after WGS reaction  Post-combustion flue gas
            CO 2          37.7 %                            10–15 %
            H 2 O         0.14 %                            5–10 %
                          55.5 %
            H 2
            NO x                                            <1,000 ppm
            SO x                                            <1,000 ppm
                                                            3–4%
            O 2
            CO            1.7 %                             20 ppm
            N 2           3.9 %                             70–75 %
            H 2 S         0.4 %
            Temperature   40 °C                             40–75 °C
            Pressure      30 atm                            1 atm
            Sources D’Alessandro et al. [17] and Ramdin et al. [49]
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