Comparison Of Three Types of NH3-SCR Catalysts Shuhua Su, Shiye Feng, Yuanfang Zhao, Qiang Lu, Weiliang Cheng,
Changqing Dong*
National Engineering Laboratory for Biomass Power Generation Equipment,North China Electric
Power University, Beijing, China, 102206
* Corresponding author. Fax: +86-10- 61772031
E-mail address:
Keywords: SCR catalysts, processing comparison, characterization comparison, factors analysis.
Abstract. The lective catalysis reduction (SCR) is one of the most promising technologies for NOx reduction at prent. There are three types of NH3-SCR catalysts in the market, honeycomb catalyst, p
late-types catalyst and corrugated catalyst. This paper firstly describes the preparation of the three types of catalysts, and then analyzes their performance. The analysis indicates the catalyst structure plays an important role on their performance. The honeycomb catalyst and plate-type catalyst are widely utilized in world’s coal power station, which should be due to their excellent capabilities of ash prevention, wear resistance and anti-poisoning.
Introduction
Emission of nitrogen oxides in coal-fired power station is the main sources of air pollution, accounting for 40% of total NOx emission, therefore NOx reduction of thermal power plant is the important task of energy saving during the Twelfth five years. EPA issued Emission standards of air pollution of power plant(the cond Draft) in February 2011. The new De-NOx standards will be raid substantially. The lective catalytic (SCR) of NOx by NH3is to date the best-proven and world-wide ud method of removing NOx from stationary sources of power plants due to its efficiency, lectivity and economy[1]. The key of SCR flue gas denitrification technology is the catalyst, currently there are three main forms of domestic and foreign commercial catalyst for the SCR reaction: honeycomb catalyst, plate-type catalyst and corrugated catalyst .
Becau flue gas of coal-fired power plant has high fly ash content, catalysts will be worn and poisoning.Therefore monolithic catalyst requires not only activity and lectivity, but also mechanical strength, wear and performance of anti-poisoning[2].
Most coal-fired power station around the world (about 95%) us honeycomb and plate catalysts. Honeycomb catalysts are widely ud due to its excellent features such as strong durability, high corrosion resistance, high reliability, high repeated utilization and low droput [3].
Compared with the honeycomb catalysts, becau of their advantage of strong ability to adapt to the high dust and small probability of block, plate catalysts is more suitable for the high-ash coal power plant and are more and more attention in the commercial market in recent years. Processing and characterization of catalysts
Table 1 prent the structural properties of three forms of the catalyst. Honeycomb Catalyst ud titanium dioxide as a reinforcing material. Plate catalyst ud a metal mesh plate as the support material. Corrugated catalyst ud glass fiber or ceramic plate as a substrate.
Table 1 The structural properties of three catalysts
Type Honeycomb Catalyst Plate catalyst Corrugated catalyst
Structural properties the whole catalyst is full of
active ingredients
only surface coating has报告封面
active ingredient
only surface coating
has active ingredient
Features
Large surface area, high
activity, activity material is
50%-70% more than other
types, long life, catalytic
prrsv
regeneration remains lective
Small surface area,
production simple , the
degree of automation is
high, conducive to
smoke through
Surface area is between
honeycomb and plate
catalyst, light weight,
easy to plug between
the upper and lower
module
Scope Apply to both high dust and
离娄上low dust董事会秘书职责
Apply to both high dust
and low dust
Mainly apply to low
dust
(1)Honeycomb catalyst is homogeneous catalyst. It was made through mixing and string the support
s and V2O5, WO3 or MoO3 adding other additives, adhesives according to certain proportion, then kneading uniform molding materials and extrusion. Its characteristics:
1) Honeycomb catalyst has a honeycomb structure with long, parallel and usually straight channels or cells through which the gas flows. The channels’wall can be coated with a high surface area washcoat that contains disperd catalyst[4]. Therefore the effective area per unit volume is so large that if reached the same De-NOx efficiency honeycomb catalyst contains less catalyst. Compared to the plate-type catalyst, honeycomb catalyst could easily regulate cellular aperture by replacing the extruder to increa surface area, accordingly it has a wider range of applications such as coal-fired boiler, oil and gas boiler.
2) The disadvantage of honeycomb catalyst is: the process of adding additives and adhesives is complex. The conditions of extrusion, drying and sintering have greater impact on the performance of the catalyst.
(2)Plate-type catalyst is heterogeneous catalyst. The plate catalysts are made by depositing the catalytic paste onto a stainless steel net or a perforated plate coating activity composition in the two-sided of stainless steel net[5]. Generally, plate-type catalyst ud a metal mesh plate as the support
material coated with a catalyst composition, the adhesion of the catalyst composition to the support material is a substantial. A filamentary organic material is usually to be add to the catalyst composition to increa the plasticity of the catalyst[6]. Plate-type catalyst formed through bilateral extrusion combined active materials with the metal plate. It shape is similar to air preheater. Its characteristics:
1) Compared with honeycomb catalyst, plate-type catalyst has smaller surface area and demands more catalysts if reaching the same effect. Meanwhile, becau of the metal frame, plate-type catalyst has high strength and needs smaller catalyst layer if reaching the same effect. That is mean SCR reactor can be more compact.
2) Plate-type catalyst has less pressure loss, high corrosion resistance and higher mechanical, thermal stability. It is also can not easily be dust pollution[7].
3) Compared with honeycomb catalyst, plate-type catalyst has a stronger anti-fouling performance, which is not only associated with the large flow area and little risk of large particles of fly ash block but more important is plate-type catalyst’s special structure with a thin stainless steel mesh as the support structure. In view of wide variety of coal power with the different coal quality in our country, and usually with large amount of ash, plate-type catalyst is especially for such a high ash flue gas[8].
4) The disadvantage of honeycomb catalyst is: the active materials of catalysts’ surface are easy to fall off in the role of mechanical or thermal stress and are aslo susceptible to wear[9].
(3)Corrugated catalyst is heterogeneous catalyst. It takes a soft fiber as its support, coating the active substance and glass fiber or ceramic fiber as a skeleton, therefore its structure is very hard. Corrugated catalyst has lower market share, ud to gas units. Its characteristics:
1) This catalyst is relatively small aperture. The catalytic efficiency per unit volume is similar compare to honeycomb catalyst. It has relatively small number of load and generally small size of the reactor and low-load support structure. Accordingly, the interchangeability with other types of catalysts is poor.门口风水
杀人如麻的意思2) As the catalyst ud flexible fiber structure, so it can run at high temperatures, with very good resistance to thermal stress capacity.
3) The disadvantage of corrugated catalyst is that it is less able to anti-clogging and wear fly ash. It c
an not maintain its catalytic properties after the catalyst surface was wear and tear. Corrugated catalyst can not regeneration. It is generally applicable to the lower ash content of flue gas environment.
(4) Performance comparison of three forms of the catalyst
Table2 Performance comparison of three forms of the catalyst
Performance Analysis Honeycomb Catalyst Plate-type catalyst Corrugated catalyst support overall Extrusion stainless steel net glass fiber
Processing Uniform extrusion
calcined
Bilateral extrusion Coating type
Catalytic activity General Low High
Oxidation rate High High High
Pressure loss High General Low Corrosion resistance General High General Anti-toxic Low Low High Blocking probability Genera Low Genera
Comprehensive cost Low Low Genera
Analysis of factors of Catalyst properties
Reaction temperature. The typically required operating temperature of the industrial catalyst is 573K-673K[11]. If temperature is too low(below 503K), SO3 in smoke gas will combination with ammonia generating hydrogen sulfate, an extremely viscous material, which will be attached to the catalyst, adsorption fly ash, and prevent smoke contact with catalyst, and finally make the catalyst failure. While high temperature will sinter the catalyst and result in the condary reaction of oxidation NH3 to NO[12]. Conquently, the SCR unit should be placed between the economizer and the air pre-heater where the high concentration of dust will decrea the catalyst activity[13], especially in China, becau of coal with large ash, the situation is more rious. Accordingly the catalysts require a higher corrosion resistance of fly ash.
Flue gas velocity. The SCR unit was placed between the economizer and the air pre-heater, Higher gas flow rate will help alleviate the situation of catalyst fouling. However, if the catalyst flue gas veloc
ity is too high, the catalyst erosion and flue gas resistance in catalyst unit will increa. In general, the flue gas velocity is not uniform in the catalyst gap. The average rate of about 8 m/s. In the region of speed less than 3 m/s, fly ash is likely attached to the catalyst, prevetning flue gas contact catalyst. According to a study of BHK, velocity zone below 3m/s of flat catalyst is about 13%, while Honeycomb catalyst about 22%, so the fouling conditions of Honeycomb catalyst is rious than plate-type catalyst.
Content of fly ash. Coal ash contains alkali metal, alkaline earth metal and other heavy metals.Under normal operation, Catalyst poisoning is the main reason for catalyst failure. DeNOx catalyst poisons mostly derived from fly ash,such as CaO, K2O, Na2O and so on.Catalyst deactivation rate depends on fly ash deposition rate on the catalyst surface. As plate-type catalyst’s flow area is larger and us thin stainless steel mesh panels as support, When the gas flows through the catalyst surface,catalyst will be continuous vibrating, fly ash is not easily deposited on the catalyst. Conquently, the ability of anti-poisoning of plate-type catalyst is much better than honeycomb.
Fly ash block. China’s coal is rich and varied, Many of which have the properties of high ash,Therefo
re, the Layout of high-dust environment SCR system congestion.
Catalyst’s block mainly due to ammonium salt and small particles in fly ash stay in the holes of catalyst, resulting flue gas could not floe smoothly, which will hinder the NOx, NH3 and O2 to reach the catalyst surface Cauing catalyst passivation. And ash deposition and bypass will reduce the activity of the catalyst reaction area, At the same time result in greater pressure drop, In the anti-blocking capability, since plate-type catalyst has less geometric corner, while honeycomb catalyst has more low-velocity zone corner, under the same catalyst pitch, the capability of preventing the ash of plate catalyst is stronger than that of honeycomb catalyst.
Conclusion
(1)The three types of catalysts posss the same components and catalytic reaction mechanism. They only differ in structures which will affect their catalytic performance and application greatly.
(2)The corrugated plate catalyst is large in surface area, but light in weight, which limits its application. Hence, it is usually ud for gas-fired units with low ash.
(3)The plate-type catalyst is capable of ash prevention, wear resistance and anti-poisoning. Under th
e conditions of high ash, the plate catalyst requires smaller placing space than the honeycomb catalyst, which is helpful to reduce investment capital. In China, coal is complex and generally with high ash, and thus, the plate-type catalyst has great market potential.
Acknowledgements
The authors thank the financial support for this work provided by the Fundamental Rearch Funds for the Central Universities (09ZG03) and Projects of China Huaneng Group.
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到底的近义词Mechanical and Electronics Engineering III
10.4028/www.scientific/AMM.130-134
Comparison of Three Types of NH<sub>3</sub>-SCR Catalysts 10.4028/www.scientific/AMM.130-134.418平板划线
