High-Temperature WGS Reaction

Abstract

The chapter mainly deals with the various catalysts investigated for the high-temperature WGS reaction. This chapter explains the various iron oxide based catalysts investigated for the high-temperature WGS reaction. The complete description about Fe-Cr catalysts such as synthesis, activation and deactivation has been given. The various Cr-free iron oxide based catalysts investigated for the high-temperature WGS reaction have also been presented. This chapter also presents the various Ce and perovskite catalysts investigated for the high-temperature WGS reaction. The complete information about integrated gasification combined cycle technology is also given in the present study.

Keywords

Iron oxide

Chromium

Ceria

Perovskite

Magnetite

Catalyst activation

Fe-Al

2.1 Fe-Cr Catalysts

The catalysts operate between 310 and 450 °C temperatures are called high-temperature shift catalysts. Mond [1], one of the greatest chemist-industrialists, first time discovered a high-temperature WGS catalyst, i.e., finely divided nickel for converting CO into CO2 in the coal gasification process. In 1914, BASF scientists Bosch and Wild tested various catalysts for high-temperature WGS reaction and found out that iron oxide stabilized with Cr is the best catalyst [2]. Until the invention of Cu-based catalyst which operates at lower temperatures, industries were using Fe-Cr for more than 70 years for commercial use. Conventional Fe2O3-Cr2O3 catalyst contains 80-90% of Fe and 8-10% of Cr2O3. In Fe-Cr catalysts iron oxide is the active phase for the high-temperature WGS reaction and Cr2O3 acts as a stabilizer. In the absence of chromium oxide the effective lifetime of the catalyst is severely restricted because of rapid thermal sintering. The extended activity of the iron oxide/chromium oxide catalyst results from the presence of Cr2O3 which prevents the sintering of neighbouring iron oxide crystallites. During the initial stages of the catalyst use, rapid catalyst deactivation occurs, but after 1500 h on steam the catalyst activity stabilizes. The catalysts typically operate in plants for 2-5 years before relatively slow thermal sintering leads to a sufficiently large decrease in activity to warrant catalyst replacement. In addition to being a textural promoter preventing the sintering of iron oxide crystallites, Cr2O3 also functions as a structural promoter to enhance the intrinsic catalytic activity of Fe2O3.

Chinchen et al. [3] suggest that as the reaction progresses, discrete Cr2O3 grains grow and become dispersed over Fe3O4 domains, thereby blocking the thermal agglomeration of Fe3O4 particles. A different opinion is that Cr3 + ions enter into the inverse-spinel lattice of Fe3O4 and form a solid solution. Robbins et al. [4] found that Cr3 + ions dissolve into the Fe3O4 lattice and occupy the octahedral site and the displaced Fe2 + and Fe3 + ions (from the octahedral sites) are transferred to the tetrahedral sites. Edwards et al. [5] claimed that the dissolved Cr3 + is enriched at the surface region of the catalyst and that the Cr-enriched surface shell, being more thermodynamically stable than the Fe-rich core, reduces ion diffusion and sintering effects. Natesakhawat et al. [6] reported that the Cr3 + in Fe/Cr was oxidized to Cr6 + during WGS catalysis. The r3+Cr6+ oxidation-reduction cycle was expected to enhance the redox rate of magnetite and promote the WGS activity of the catalyst. Today, people believe that Cr acts as both a stabilizer (resist the sintering of magnetite particles) and a promoter (improve the redox efficiency of e2+Fe3+ redox couple).

2.1.1 Fe-Cr Activation

The synthesis of iron oxide catalysts leads to the hematite (Fe2O3) phase after calcination. Before the WGS reaction the hematite phase is reduced to magnetite phase (Fe3O4), since magnetite is the active phase for the WGS reaction. Usually, the reduction is taking place in the presence of process gas between 350 and 450 °C. Process gas is a mixture of CO, CO2, H2 and water vapour. The representative reactions are shown as follows:

Fe2O3+H22Fe3O4+H2O?H=-16.3kJ/mol

  (2.1)

Fe2O3+CO2Fe3O4+CO2?H=+24.8kJ/mol

  (2.2)

The reduction is typically performed during the HTS reactor startup, and it should be carefully controlled because of a significant heat release resulting from the exothermic nature of the reactions involved, which may damage the catalyst. The ratios of water vapour to hydrogen and CO2 to CO for the activation step determine the equilibrium of the Fe2 + and Fe3 + ions in octahedral sites. With common used process gases the catalyst is more reduced than the reduction to reach the state of equilibrium. In this manner more Fe3O4 is created, which forms the stable state. It is important to avoid over-reduction of the magnetite active material during the process to lower oxides, carbides or metallic iron species. In such a case, a damaging hot spot can form during the reaction due to exothermic methanation for which metallic iron is known to be a good catalyst.

e3O4+H2FeO+H2O?H=-63.8kJ/mol

  (2.3)

e3O4+COFeO+H2O?H=-22.6kJ/mol

  (2.4)

eO+H2Fe+H2O?H=-24.5kJ/mol

  (2.5)

eO+COFe+CO2?H=-12.6kJ/mol

  (2.6)

e3O4+4H23Fe+4H2O?H=-149.4kJ/mol

  (2.7)

The Cr phase presented in the catalysts is also reduced during the pre-treatment.

CrO3+3H2Cr2O3+3H2O?H=-684.7kJ/mol

  (2.8)

CrO3+3COCr2O3+3CO2?H=-808.2kJ/mol

  (2.9)

The metallic iron species especially are active catalysts for the Fischer-Tropsch process, thus leading to methanation Boudouard reactions in the WGS reaction.

O+3H2CH4+H2O?H=-206.2kJ/mol

  (2.10)

COC+CO2?H=-172.5kJ/mol

  (2.11)

It is generally known from industrial experience that if the reduction factor (R) for the reformed gas is maintained at less than 1.2, then over-reduction of Fe3O4 does not occur, while over-reduction consistently occurs when R is greater than 1.6. If the ratio is too low (in other words the H2O to H2 ratio is too high) the magnetite reacts with the water and forms hematite again.

Fe3O4+H2O3Fe2O3+H2

  (2.12)

In 1985 Rethwisch et al. [7] reported the effect of pre-treatment condition on the WGS activity of iron catalysts supported on graphite. Initially, when the catalysts reduced in the presence of CO/CO2 (15/85) mixture, a rapid initial deactivation followed by stable activity was observed. When they changed CO/CO2 ratio to 4/96 no change in the activity was observed. Treatment in H2/H2O (40/?60) for 22 h had very little effect on the water-gas shift activity. Treatment in CO/CO2 15/85 mixture for 32 h produced a greater extent of catalyst activation. However, this effect was small compared to the effects of treatments in the CO/CO2 gas mixtures. It was found that treatment for 22 h in CO/CO2 (15/85) mixture followed directly by a 22 h treatment in H2/H2O (80/20) had a similar effect on water-gas shift activity to a treatment for 22 h in H2/H2O (40/60). This indicates that treatments in H2/H2O not only fail to activate the samples to a significant extent but also that these treatments deactivate the catalyst following activation in CO/CO2. They also investigated the effect of oxygen pre-treatment on the WGS activity. A 0.5 h treatment in oxygen followed by 23 h in CO/CO2 15/85 mixture led to an increase in catalytic activity, which was comparable to the extent of catalyst activation caused by treatment in CO/CO2 15/85 mixture alone. Treatment in oxygen for 0.33 h followed by treatment in CO/CO2 15/85 mixture for 20 h also caused an increase in catalytic activity which was comparable to the effect of treatment in CO/CO2.

In 1986 Gonzalez et al. [8] studied the effect of thermal treatments and reduction process on the activity of Fe/Cr catalysts. It was shown that the reduction of Fe/Cr catalysts with hydrogen does not lead necessarily to Fe formation, unless they are submitted to a previous thermal treatment or an activation at temperatures higher than 500 °C is carried out. The addition of vapour in standard conditions of hydrogen reduction is not justified due to the fact that Fe formation is not possible. Carbon monoxide is a more powerful reducing agent than hydrogen, so it may lead to Fe formation at temperatures lower than 500 °C. In this case, it is convenient to add steam to the reducing mixture. Process and hydrogen gas are the best reducing agents in order to obtain higher...