Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
1999-11-30
2002-04-30
Griffin, Steven P. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S350000, C502S353000
Reexamination Certificate
active
06380128
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates, in general, to a catalyst for denitrification and, more particularly, to a catalyst for removing nitrogen oxides (hereinafter referred to as NO
x
) from flue gas, which is of excellent catalytic activity and of high resistance to SO
2
poisoning as well as is economically favorable in its production. Also, the present invention is concerned with a method for preparing such a catalyst.
2. Description of Prior Arts
NO
x
are generally produced from high-temperature combustion process as a result of the oxidation of nitrogen compounds contained in fuels and/or the reaction of the nitrogen and oxygen excessively fed from the air to the facility. In addition to being a source of photochemical smog, NO
x
, when exhausted into the air, is known as a main cause of acid rain, along with SO
2
. Thus, extensive effort has been made to develop technologies for the removal of the pollutant from flue gas.
In order to reduce NO
x
emitted from the stationary sources, combustion modification methods, such as low NO
x
burners, flue gas-circulating techniques, etc. have been commonly employed. However, combustion modification methods, although NO
x
removal efficiency varies with the technologies applied, can not achieve more than 50% of NO
x
removal efficiency, in general. As an effective method to reduce NO
x
emission, flue gas denitrification has recently attracted the industrial attention.
Flue gas denitrification processes are largely classified into wet methods using absorption solutions and dry methods using adsorption, catalytic decomposition and/or catalytic reduction. Compared with the dry methods, the wet methods are economically and technically unfavorable, since they require large installation and operating costs and can produce secondary waste water which should be treated for discharge. Among the dry methods, the most widely employed technology is the selective catalytic reduction (hereinafter referred to as SCR ), which is the catalytic reduction of NO
x
into harmless N
2
and H
2
O by reductant, NH
3
. The backbone of SCR technology is a highly active and durable catalyst. Various catalysts, including precious metals, metal oxides, zeolites, etc. have been suggested as being useful for SCR. Among them, vanadium pentoxide-based and zeolite-based catalysts are known to be excellent in its performance.
Vanadium pentoxide-based catalysts show the variety of catalytic activity, depending on the supports which include typically titanium dioxide (TiO
2
), aluminum oxide (Al
2
O
3
) and silicon dioxide (SiO
2
). However, titanium dioxide is commonly employed as the support of SCR catalyst due to the catalytic activity for NO
x
removal and durability under actual flue gas conditions (T. Shikada, K. Fujimoto, T. Kunugi and H. Tominaga, J. Chem. Tech. Biotech., 33A, 446(1983); H. Yoshida, K. Takahashi, Y. Sekiya, S. Morokawa and S. Kurita, Proc. 8
th
Conf. On Catal., Berlin, III-649(1984)).
These vanadium pentoxide-based catalysts can be prepared by well-known precipitation methods (H. Miyata, K. Fujji, T. Ono and Y. Kubokawa, J. Chem. Soc. Fara. Trans., 83, 675(1987); M. Sanati, L. R. Wallenbweg, A. Andersson, A. Jansen and Y. Tu, J. Catal. 133, 128(1991)) or by mixing the solution containing vanadium precursor with support materials at desired weight ratios and followed by drying and calcining the mixtures as disclosed in Japanese Pat. Nos. 59-35027, 58-210849 and 58-183946.
In addition to the high performance of NO
x
removal activity, the vanadium pentoxide-based catalysts should be also highly resistant to the poisoning of the catalyst by SO
2
. In general, SCR catalyst can be easily poisoned by SO
2
which is contained at a level of several hundreds of ppm in flue gas from fossil fuel-burning boilers. Therefore, to remove NO
x
from flue gas containing SO
2
, the catalysts are required to be highly resistant to SO
2
poisoning as well as superior NO
x
removal activity.
Conventionally, prior to SCP process, a wet flue gas desulfurization(FGD) has been installed to eliminate sulfur oxides and thereby to prevent NO
x
removal catalysts from being deactivated in catalytic activity due to the existence of SO
2
contained in the flue gas as well as NO
x
. Although it can protect SCR catalyst from the poisoning by SO
2
, it can not be a fundamental measure. The operating cost of SCR process can not be ignored due to the temperature difference between FGD and SCR processes, mainly heating cost of flue gas from the temperature of exhaust stream of FGD process to that of the operating temperature of SCR process. Moreover, large investments are also needed to install and operate FGD process.
The catalyst poisoning by SO
2
is mainly attributed to the following two reasons. The one is the plugging of catalyst pores by the ammonium salts, which are formed through the reaction of SO
3
with the reductant, NH
3
. The other is the pore blocking and/or the decrease of active sites of the catalyst by the reaction of SO
3
with the support and/or the active components of the catalyst. Accordingly, the low conversion of SO
2
oxidation to SO
3
is required for the durability of a catalyst over SO
2
. In addition to the catalyst poisoning, SO
3
produced by SO
2
oxidation, can also react with unreacted ammonia forming ammonium salts which cause the operational problems of the SCR processes such as corrosion and plugging of the downstream of the SCR reactor. Therefore, an useful SCR catalyst should have low catalytic activity for SO
2
oxidation to SO
3
, which largely depends on the chemical composition of the catalyst.
The durability of a catalyst against SO
2
also affects the arrangement of the processes for treating flue gas. For the SCR process where the catalyst is readily poisoned by SO
2
, FGD process should be provided before the SCR process to eliminate SO
2
in advance. In this case, the flue gas at high temperature should be cooled to lower temperature (e.g., 50° C. or lower) adequate for effective SO
2
removal, and then the gas exhausted from a desulfurizing system should be reheated to a temperature (e.g., 300° C. or higher) at which the reaction for NO
x
removal can effectively take place. This is a main disadvantage of the SCR process in terms of energy efficiency. In contrast, SCR catalyst with the high durability against SO
2
allows the direct treatment of the flue gas in the process without the previous removal of SO
2
by FGD. Therefore, additional energy cannot be consumed to reheat the flue gas. Since the sulfur tolerance of the catalyst has a great influence on not only the life span of the catalyst itself, but also the energy efficiency of the SCR process, it is one of the most important catalytic properties determining the economics of the process.
SUMMARY OF THE INVENTION
According to the present invention, the highly active and durable catalyst for NO
x
removal can be prepared by the catalyst preparation methods such as the impregnation and adsorption of vanadium pentoxide on titanium dioxide containing the pore size ranging from 500 to 70,000 Å. The titanium dioxide support could be obtained from a metatitanate (TiO(OH)
2
)-predominating intermediate in a form of slurry which is produced during the course of the production of titanium dioxide for pigment from ilmenite.
Therefore, the objective of the present invention is to overcome the problems mentioned earlier and to provide a highly active catalyst for removing NO
x
from flue gas.
It is another objective of the present invention to provide a catalyst for removing NO
x
from flue gas, which maintains an extended life span without losing NO
x
removal activity.
It is a further objective of the present invention to provide a catalyst for removing NO
x
from flue gas, which can be produced at low cost of the preparation.
It is still a further objective of the present invention to provide a method for preparing such a catalyst.
In accordance with an aspect of the present invention, a V
2
O
5
-based catalyst is provided for
Choo Soo Tae
Eum Hee Mun
Ham Sung Won
Ji Pyung Sam
Kim Dong Hwa
Griffin Steven P.
Ildebrando Christina
Korea Hydro & Nuclear Power Co., Ltd.
Martine & Penilla LLP
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