Chapter 4
Deactivation Behavior of Selective Catalytic Reduction DeNO Catalysts x
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 28, 2016 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0587.ch004
Basis for the Development of a New Generation of Catalysts E. Hums and G. W. Spitznagel Siemens AG, Power Generation Group (KWU), Freyesleben Strasse 1, 91058 Erlangen, Germany
Siemens Power Generation Group (KWU) has developed an SCR DeNOx plate-type catalyst without the use of Japanese catalyst technology. This innovative development is particularly suitable for demanding applications with flue gases from slag tap furnaces containing high heavy-metal concentrations (e. g. arsenic oxides). A special feature of this catalyst is that the high SO content in flue gases from combustion of sulfur-rich fuel does not have any significant effects on DeNOx catalysis. The catalytically active phase used for this catalyst was developed in deactivation investigations on MoO -V O compounds dispersed on TiO (anatase). These investigations were conducted in the light of the high arsenic oxide concentrations in German slag tap furnaces with 100% ash recirculation. 2
3
2
5
2
The environmental impact of nitrogen oxides has focused attention on emissions regulations in many countries in recent years. The NOx emission limits imposed by German law cannot be achieved by simply applying primary measures such as staged combustion, over-fire air, etc.; this makes it necessary to apply secondary measures. Up to now, selective catalytic reduction (SCR) has dominated over other combustion control technologies. A key reaction in the catalytic oxidation of ammonia in the presence of N O yielding N 2 and Η2Ο reflects aspects to the well-known Ostwald process for N O production when working unselectively (7). The advantages of this undesired side reaction were recognized and deliberately developed for oxidic catalysts in Japan, first employed in a coal fired power plant at Takehara in 1981 (2). These catalytic materials
0097^6156/95/0587-0042512.00/0 © 1995 American Chemical Society
In Reduction of Nitrogen Oxide Emissions; Ozkan, Umit S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 28, 2016 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0587.ch004
4. HUMS & SPITZNAGEL
Deactivation Behavior of SCR DeNO Catalysts x
do not necessarily differ in composition from those which were used for NO production (3). Although this technique for removing NOx from waste gases using N H 3 as a reducing agent was referred to in an early Germany patent in 1963 (4), the first DeNOx catalytic reactor installed in Europe did not begin operation in Germany until late 1985 (5). In the USA, the technique is still in the pilot plant test phase (6). A direct licensed transfer of the Japanese technology to German power plants with their specifications failed. This quickly became evident when Japanese catalysts tested in catalytic reactors downstream of steam generators with slag tap furnaces and ash recirculation exhibited drastic deactivation. This deactivation was caused by trace elements in the coals used, and affects catalyst lifetime (7, 8). This experience led to the initiation of extensive adaptation efforts, improving the catalyst material as well as engineering measures. Since no advanced technology was available in Japan at that time, Siemens began developing a plate-type catalytic converter in addition to licensed honeycomb-type fabrication. The plate-type design exhibits advantages with regard to dust deposits, mechanical and thermal stability, low pressure drop, etc., which are described elsewhere (9). Therefore this design is primarily used in high-dust flue gas applications where the catalyst is exposed to high concentrations of heavy trace elements from the fired coals. These trace elements, which can act as catalyst poisons, are carried either in the flue gas and/or in the fine dust (JO). Usually they cannot be prevented from depositing on the catalyst surface and they are expected to occur in much higher concentrations when ash is recirculated in the combustion chamber to achieve more complete combustion. This increases the AS2O3 content by a factor of 20. Previous experience had shown that tungsten oxide-based catalysts deactivate at much faster rates in flue gases containing arsenic oxide than molybdenum oxide-based catalysts (77). These differences in behavior gave the impetus to clarify this phenomenon for the purpose of improving catalytic materials. Special attention is given to a T1O2-M0-V composite oxide catalyst, which shows quite different catalytic behavior and differences in other characteristics when compared with those catalysts obtained by monolayer dispersion of the oxides of vanadium and molybdenum onto T1O2 (anatase).
Results and Discussion
Contamination and Deactivation of TÏO2-M0O3-V2O5 Catalysts. When exposed to flue gas of coal-fired power plants with slag tap furnaces, T1O2-M0O3-V2O5 catalysts obtained by formation of a monomolecular dispersion of V 2 O 5 and M 0 O 3 on T1O2 (anatase) display an exponential correlation between exposure time and relative activity constant k/kç. This correlation is shown in Fig. 1 for various power plants. In contrast to furnaces without ash recirculation, the influence of the deactivation process can already be detected after a very short period of operation (72).
Surface analysis of T1O2-M0O3-V2O5 Catalysts. This requires investigation of the degree of contamination of the catalyst surface as a function of time of exposure to flue gas, using X-ray photoelectron spectroscopy (XPS). Because of its detection depth of only a few atomic layers, XPS can yield information on surface contamination. The
In Reduction of Nitrogen Oxide Emissions; Ozkan, Umit S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
43
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 28, 2016 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0587.ch004
44
NO REDUCTION x
0
J
0
1 2000
1 4000
1 6000
1 8000
1 10.000
1
Operating time [h]
12.000
>
Figure 1. Activity profiles of a T1O2-M0O3-V2O5 and T1O2-WO3-V2O5 catalyst downstream of a slag tap furnace at T=350 °C.
930
630
330
30
