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Chapter | 7  Gasification Theory                             205


             reactivity and the reaction potential of the gasifying medium. For example,
             amongst gasification medium, oxygen is most active, followed by steam and
             carbon dioxide. The rate of the char oxygen reaction (R4: C 1 0.5O 2 -CO)
             is the fastest among the four reactions listed in Table 7.2 (R1, R2, R3, and
             R4). It is so fast that the reaction quickly consumes the entire oxygen, leav-
             ing hardly any for any other reactions.
                The rate of the char steam reaction (R2: C 1 H 2 O-CO 1 H 2 ) is three
             to five orders of magnitude slower than that of the char oxygen reaction.
             The char carbon dioxide reaction (R1: C 1 CO 2 -2CO), known as
             Boudouard reaction, is six to seven orders of magnitude slower (Smoot and
             Smith, 1985). The rate of water steam gasification reaction (R2), known as
             water gas reaction, is about two to five times faster than that of the
             Boudouard reaction (R1) (Blasi, 2009).
                The char hydrogen reaction (hydrogasification reaction) that forms
             methane (C 1 2H 2 -CH 4 ) is the slowest of all. Walker et al. (1959) esti-
             mated the relative rates of the above four reactions at a temperature of
                                                               3
                                                 5

             800 C and one at a pressure of 0 kPa, as 10 for oxygen, 10 for steam, 10 1
             for carbon dioxide, and 3 3 10 23  for hydrogen. The relative rates, R, may be
             shown as:
                                                                        (7.5)
                              R C1O 2  cR C1H 2 O . R C1CO 2  cR C1H 2
                When steam reacts with carbon, it can produce CO and H 2 . Under certain
             conditions, the steam and carbon reaction can also produce CH 4 and CO 2 .


             7.3.3.2 Boudouard Reaction
             The gasification of char in carbon dioxide is popularly known as the
             Boudouard reaction.
                           C 1 CO 2 22CO ðreaction R1 in Table 7:2Þ     (7.6)
                Blasi (2009) described the Boudouard reaction through the following
             intermediate steps. In the first step, CO 2 dissociates at a carbon-free active
             site (C fas ), releasing carbon monoxide and forming a carbon oxygen surface
             complex, C(O). This reaction being reversible can move in the opposite
             direction as well, forming a carbon active site and CO 2 in the second step. In
             the third step, the carbon oxygen complex produces a molecule of CO.
                                               k b1
                            Step 1:  C fas 1 CO 2     ! CðOÞ 1 CO       (7.7)

                                               k b2
                            Step 2:  CðOÞ 1 CO     ! C fas 1 CO 2       (7.8)
                                                k b3
                                  Step 3:  CðOÞ    ! CO                 (7.9)
             where k i is the rate of the ith reaction.
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