Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
2000-09-08
2002-07-23
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C423S351000
Reexamination Certificate
active
06423283
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a process for removing nitrogen oxides (NOx), and more particularly to a process for removing NOx by selective reduction from gases having a low temperature (e.g., of up to 300° C.) and a nitrogen dioxide (NO
2
)/NOx ratio in excess of 0.5, such as combustion exhaust gases produced in starting up gas turbines, regeneration exhaust gases containing removed NOx and resulting from the regeneration of NOx adsorbents by heating, and process exhaust gases in various modes of chemistry of nitric acid.
BACKGROUND ART
It is conventional practice to use a vanadium-tungsten supporting titania catalyst and a reducing agent, such as ammonia or urea, for reducing and decomposing NO and/NO
2
in the gas to be treated for the removal of NOx.
However, this denitration process has the problem that the catalytic activity is lower when the component molar ratio of NOx in the gas to be treated is NO
2
>NO than when the ratio is NO
2
≦NO as will be described below.
FIG. 1
shows the relationship between the NO/NOx ratio and the denitration efficiency.
Denitration conditions:
Areal velocity (AV) 35 Nm
3
/m
2
·h
Composition of the gas to be treated
Air+H
2
O (about 3%)
NOx 90 ppm
NH
3
90 ppm
Reaction temperature 250° C.
The graph shows that the denitration efficiency becomes maximum when the NO/NOx ratio is 0.5 (No:No
2
=1:1) and lowers as the NO/NOx ratio decreases from 0.5. One of the causes of the reduction in the catalytic activity is thought attributable to a diminution in NH
3
adsorption sites due to an excess of oxygen on the catalyst active sites as will be described below.
1) In the case of removal of NO
NO+NH
3
+1/4O
2
→N
2
+3/2H
2
O
Although the catalyst active sites are reduced to result in a deficiency of oxygen, the active sites are reoxidized with the oxygen in the gas to be treated and are thereby replenished with oxygen. If the reaction temperature has a low value of up to 200° C., difficulty is encountered in oxidizing the catalyst with this gaseous-phase oxygen to result in markedly impaired denitrating properties.
2) In the case of removal of NO
2
NO
2
+NH
3
→N
2
+3/2H
2
O+1/4O
2
When the gas to be treated contains oxygen in a high concentration, the oxygen produced on the catalyst active sites is not readily releasable into the gaseous phase. An excess of oxygen on the catalyst active sites therefore inhibits the adsorption of ammonia, consequently impairing the denitrating properties of the catalyst.
3) In the case of denitration of NO+NO
2
(1:1 in molar ratio)
NO+NO
2
+2NH
3
→2N
2
+3H
2
O
There is no excess or deficiency of oxygen, permitting the catalyst to exhibit the highest denitrating properties.
An object of the present invention is to provide a process for removing NOx which is free of impairment in denitration efficiency at a reaction temperature of up to 300° C. even when the component molar ratio of NOx in the gas to be treated is NO
2
>NO.
DISCLOSURE OF THE INVENTION
In removing NOx from the gas to be treated and containing NO
2
in a larger amount than NO, that is, having a (NO
2
)/NOx ratio in excess of 0.5, by selective reduction with use of ammonia serving as a main reducing agent in the presence of a denitration catalyst, a process for removing NOx which is characterized by adding to the denitration reaction system a substance for removing an excess of oxygen accumulating on catalyst active sites by selectively reducing the oxygen at not higher than 300° C., for example, at 300 to 150° C., in other words, a substance which reacts with the excess of oxygen on the catalyst active sites and becomes oxidized at not higher than 300° C. (the substance will be referred to as an “auxiliary reducing agent”).
The auxiliary reducing agent is a substance which reacts with the excess of oxygen on the catalyst active sites and becomes oxidized at not higher than 300° C., irrespective of gaseous-phase oxygen. Preferably, the agent is an organic compound.
It is desired that the auxiliary reducing agent or a liquid containing the agent (e.g., aqueous solution, to be used in the same meaning hereinafter) be present in the form of a vapor or gas before reaching the denitration catalyst, as uniformly diffused. Accordingly, it is desired to introduce the auxiliary reducing agent into the system, for example, by:
injecting the agent or the liquid containing the agent directly into the flow of gas to be treated, or
injecting the agent or the liquid containing the agent into a stream of air for diluting ammonia as the main reducing agent and forcing the agent or liquid into the flow of gas to be treated along with the ammonia.
In the case where the auxiliary reducing agent is a liquid, the amount of injection may be controlled by feeding the agent or the liquid containing the agent to the NOx removal apparatus by a metering pump, detecting the concentration of NOx (NO, NO
2
) at the inlet of the apparatus, and controlling the pump with the resulting detection signal so as to alter the operating conditions such as the stroke, pitch, etc. of the pump.
When the auxiliary reducing agent or the liquid containing the agent is injected into the ammonia diluting air stream, it is likely that the agent or liquid will not be evaporated completely. It is then desirable to preheat the ammonia diluting air before the agent or liquid is injected. Instead of preheating the ammonia diluting air, it is also desirable to admix a portion of the gas of high temperature to be treated with the ammonia diluting air.
Aqueous ammonia or aqueous solution of urea is also usable as the ammonia supply source. In this case, it is desired to dissolve the auxiliary reducing agent in the aqueous solution first to add the agent and NH
3
to the denitration reaction system at the same time.
The auxiliary reducing agent is a substance which is not oxidized with gaseous-phase oxygen at a low temperature (up to 300° C.) but selectively reacts with an excess of oxygen on the catalyst active sites.
The preferred auxiliary reducing agents include hydrocarbons and alcohols.
Examples of hydrocarbons are lower alkanes having 1 to 10 carbon atoms, such as ethane, propane, butane, pentane and hexane; lower alkenes having 2 to 10 carbon atoms, such as ethylene, propylene, butene, pentene and hexene; and saturated or unsaturated hydrocarbons such as derivatives of these compounds.
Alcohols are useful insofar as they are compounds having one or at least two hydroxyl groups. Examples of these alcohols are primary alcohols, secondary alcohols or tertiary alcohols having 1 to 10 carbon atoms, such as methanol, ethanol, propanol, butanol, pentanol and hexanol; and alcohols such as derivatives of these alcohols. Useful alcohols may be monohydric alcohols, dihydric alcohols or polyhydric alcohols. Aromatic alcohols are also usable. Especially desirable are monohydric alcohols having 1 to 10 carbon atoms.
It is desired that the amount of the auxiliary reducing agent to be injected be as small as possible in view of the occurrence of unreacted substances and formation of by-products. Stated more specifically, the useful amount of injection is at least an amount capable of consuming by an oxidation reaction ½ mole of excessive oxygen to be produced when 1 mole of nitrogen dioxide (NO
2
) is removed. Further in the presence of nitrogen monoxide (NO), ½ mole of excessive oxygen is consumed by 1 mole of NO, so that the amount of the auxiliary reducing agent to be injected is not smaller than is capable of consuming the excessive oxygen resulting from the difference of [amount of NO
2
−amount of NO]. For example, in the case where isopropanol is used as the auxiliary reducing agent and when the component molar ratio (NO/NOx)=0 (i.e., NO
2
only), the amount is preferably at least {fraction (1/9)} mole to not greater than ½ mole per mole of NO
2
. When the component molar ratio is in the range of 0>(NO/NOx)<0.5 in this case, the amount is preferably up to {fract
Akiyama Masaki
Fukuju Atsushi
Ichiki Masayoshi
Armstrong Westerman & Hattori, LLP
Griffin Steven P.
Hitachi Zosen Corporation
Johnson Edward M.
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