49
Steam Gasification and Reforming Technologies
These two reactions are generally accompanied by the water–gas shift reaction as [18]
(X )CO + H O CO + H
(4.3)
n
2
2
2
The initial steps in steam reforming are the dissociative adsorption of the hydrocarbons on the metal sites of the catalyst and the reaction of the adsorbed C x H y species
with the adsorbed H 2 O-derived species to produce CO and H 2 (Equation 4.2). With an
active catalyst at temperatures below 600°C, reforming of hydrocarbons is irreversible with no intermediates and the only byproduct is carbon that forms on the catalyst.
Besides the above reactions, the following reactions also occur at varying degrees:
CO + 3H
CH + H O
(4.4)
2
4
2
2CO → CO 2 + C
(4.5)
CH 4 g 2H ( )
g + C s
(4.6)
( )
2
( )
Equation 4.1 is the combination of reforming and water–gas shift reactions.
Equations 4.3, 4.4, and 4.6 are equilibrium-limited reactions. Under normal conditions, Equations 4.5 and 4.6 dominate, and they together produce coke on the
catalyst. In general, both methanation and disproportionation reactions are equilibrium limited. The reformed fuel contains carbon monoxide that must be reduced to
a low level (except for the use in high-temperature FCs [HTFCs]). To reduce carbon
monoxide concentration at the desired level of <10 ppm, the reforming reaction is
followed by a high-temperature and a low-temperature water–gas shift reaction, both
of which are exothermic [18]. The residual carbon monoxide can be further reduced
by its preferential oxidation. The mixture of hydrogen and carbon dioxide coming
out of the preferential oxidation process can then undergo a separation process to
remove carbon dioxide and generate pure hydrogen. The separation process can be a
physical (absorption by molecular sieves), a chemical (absorption in an amine solution), or a membrane separation (usually Pd membrane) at high temperature. The
separated carbon dioxide is used with ammonia to produce urea. The purified hydrogen is used in the production of ammonia and a host of other refining and chemical
production operations.
The catalysts for steam reforming of hydrocarbons are mainly nickel based on
oxide support to obtain high thermal stability. Nickel catalysts are preferred because
of their low cost, reasonable thermal stability, and high activity [19–23]. At low temperatures (425°C–500°C), iron catalyst promoted with chromium oxide is sometimes
used to enhance oxidation reaction. More details on the catalysis of steam and trireforming reactions are given in Sections 4.2.3, 4.3, and 4.4.
Currently, more than 65% of hydrogen production uses steam reforming of fossil
fuel technology because it is a mature and reasonably inexpensive technology (compared to other processes). It does not require a new infrastructure. It also reduces the
need for transport and storage of hydrogen. The disadvantages of the steam reforming
process are as follows: (1) reformers are complex, large, and expensive; (2) reformers
have high warm-up period; and (3) reformers introduce additional losses into the
Steam Gasification and Reforming Technologies
These two reactions are generally accompanied by the water–gas shift reaction as [18]
(X )CO + H O CO + H
(4.3)
n
2
2
2
The initial steps in steam reforming are the dissociative adsorption of the hydrocarbons on the metal sites of the catalyst and the reaction of the adsorbed C x H y species
with the adsorbed H 2 O-derived species to produce CO and H 2 (Equation 4.2). With an
active catalyst at temperatures below 600°C, reforming of hydrocarbons is irreversible with no intermediates and the only byproduct is carbon that forms on the catalyst.
Besides the above reactions, the following reactions also occur at varying degrees:
CO + 3H
CH + H O
(4.4)
2
4
2
2CO → CO 2 + C
(4.5)
CH 4 g 2H ( )
g + C s
(4.6)
( )
2
( )
Equation 4.1 is the combination of reforming and water–gas shift reactions.
Equations 4.3, 4.4, and 4.6 are equilibrium-limited reactions. Under normal conditions, Equations 4.5 and 4.6 dominate, and they together produce coke on the
catalyst. In general, both methanation and disproportionation reactions are equilibrium limited. The reformed fuel contains carbon monoxide that must be reduced to
a low level (except for the use in high-temperature FCs [HTFCs]). To reduce carbon
monoxide concentration at the desired level of <10 ppm, the reforming reaction is
followed by a high-temperature and a low-temperature water–gas shift reaction, both
of which are exothermic [18]. The residual carbon monoxide can be further reduced
by its preferential oxidation. The mixture of hydrogen and carbon dioxide coming
out of the preferential oxidation process can then undergo a separation process to
remove carbon dioxide and generate pure hydrogen. The separation process can be a
physical (absorption by molecular sieves), a chemical (absorption in an amine solution), or a membrane separation (usually Pd membrane) at high temperature. The
separated carbon dioxide is used with ammonia to produce urea. The purified hydrogen is used in the production of ammonia and a host of other refining and chemical
production operations.
The catalysts for steam reforming of hydrocarbons are mainly nickel based on
oxide support to obtain high thermal stability. Nickel catalysts are preferred because
of their low cost, reasonable thermal stability, and high activity [19–23]. At low temperatures (425°C–500°C), iron catalyst promoted with chromium oxide is sometimes
used to enhance oxidation reaction. More details on the catalysis of steam and trireforming reactions are given in Sections 4.2.3, 4.3, and 4.4.
Currently, more than 65% of hydrogen production uses steam reforming of fossil
fuel technology because it is a mature and reasonably inexpensive technology (compared to other processes). It does not require a new infrastructure. It also reduces the
need for transport and storage of hydrogen. The disadvantages of the steam reforming
process are as follows: (1) reformers are complex, large, and expensive; (2) reformers
have high warm-up period; and (3) reformers introduce additional losses into the