Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Sulfur or sulfur containing component
Reexamination Certificate
2000-06-30
2003-05-27
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Sulfur or sulfur containing component
C204S157300
Reexamination Certificate
active
06569395
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and an apparatus for flue gas desulfurization, and more particularly to a method and an apparatus for flue gas desulfurization by converting sulfur oxides typically comprising sulfur dioxide into powdery ammonium compound and collecting the ammonium compound.
2. Discussion of the Background
In a method for flue gas desulfurization, aqueous ammonia is sprayed and injected into flue gas containing sulfur oxides (SOx) such as combustion flue gas emitted from a boiler, and aqueous ammonia reacts with sulfur oxides to produce ammonium compound. Particularly, sulfur dioxide (SO
2
) which is a main component of SOx reacts with aqueous ammonia (m·NH
3
+n·H
2
O) and oxygen (O
2
) to produce ammonium sulfate ((NH
4
)
2
SO
4
) as by-product. This chemical reaction is expressed in the following formula (1).
SO
2
+2NH
3
+H
2
O+1/2O
2
→(NH
4
)
2
SO
4
+437.7 kJ/mol (1)
As typically indicated in the above formula (1), the reaction in which aqueous ammonia reacts with sulfur oxides to produce ammonium compound is exothermic reaction, and water contained in aqueous ammonia sprayed and injected is consumed by reaction as shown in the left side of the formula (1) and is evaporated while removing the heat of the reaction. In general, the desulfurizing reaction as shown in the above formula (1) is liable to proceed as the temperature of flue gas is lowered, and hence removal of the heat caused by vaporization of water contained in aqueous ammonia prevents the temperature of flue gas from increasing due to the heat of the reaction and therefore prevents the desulfurizing reaction from not proceeding. Therefore, if water sprayed and injected as aqueous ammonia is controlled to a certain amount which is the sum of the amount consumed by reaction and the amount required for preventing the temperature of flue gas from increasing over a temperature suitable for desulfurizing reaction, then it is possible to evaporate water contained in aqueous ammonia completely in a process vessel.
However, if the sum of water required, the amount of ammonia required for desulfurizing reaction as shown in the above formula (1), and the concentration of aqueous ammonia are not well-balanced, then gaseous ammonia is injected or water is sprayed and injected, separately from aqueous ammonia. Besides aqueous ammonia, the sprayed and injected water and/or gaseous ammonia react with SO
2
according to the above formula (1).
In this manner, the reaction product such as ammonium sulfate is converted into dry powder in the process vessel, and this power is collected by a by-product collector such as a dry-type electric precipitator. The powdery by-product collected is ammonium compound such as ammonium sulfate which can be utilized as a fertilizer. Further, this desulfurizing process has such an advantage that no waste water is generated differently from a wet-type desulfurizing method in which sulfur oxides are absorbed by a slurry of lime.
However, in the method for flue gas desulfurization in which aqueous ammonia is sprayed and injected into flue gas containing sulfur oxides, and ammonium compound is collected as dry powder, the removal efficiency of SOx, particularly SO
2
is not generally high. Further, the remaining ammonia, in the injected aqueous ammonia, which has not reacted with SOx becomes gaseous state along with evaporation of water, and this remaining ammonia is discharged together with the remaining ammonia, in gaseous ammonia injected separately from aqueous ammonia, which has not reacted with SOx, and with the treated flue gas to the atmosphere. In order to suppress this ammonia leak, the amount of ammonia to be injected (the sum of ammonia sprayed and injected as aqueous ammonia and gaseous ammonia injected separately from the aqueous ammonia) is required to be reduced, and then the removal efficiency of SOx, particularly that of SO
2
is further lowered. Since unreacted ammonia corresponding to the lowered removal efficiency is discharged, the amount of ammonia which is leaked is not significantly lowered, although the amount of injected ammonia is reduced.
On the other hand, it is possible to accelerate the desulfurizing reaction by increasing the amount of water to be injected (the sum of water sprayed and injected as aqueous ammonia and water sprayed and injected separately from aqueous ammonia) and lowering the temperature of flue gas. In this case, the temperature of flue gas is equal to approximately saturation temperature of water (less than saturation temperature of water plus 5° C.) in the vicinity of the outlet of the process vessel, and therefore water in aqueous ammonia and/or water sprayed separately from aqueous ammonia are difficult to be evaporated only by the heat of reaction. Thus, a huge process vessel is required to evaporate water completely, or waste water is generated because water is not evaporated completely in the process vessel.
Therefore, normally, after aqueous ammonia is sprayed and injected into flue gas, the mixed gas is irradiated with electron beam in the range of several kGy to over a dozen kGy, and the removal efficiency of SOx, particularly that of SO
2
is improved even in case the temperature (not less than saturation temperature of water plus 5° C.) of flue gas at the outlet of the process vessel is higher than that of flue gas when it is not irradiated by electron beam. This is for the following purpose. The remaining SO
2
which has not been removed in the above formula (1) is oxidized to produce sulfur trioxide (SO
3
) or sulfuric acid (H
2
SO
4
) by radicals such as O, OH, or HO
2
generated from gas molecular such as oxygen or water vapor in the flue gas by irradiation of electron beam, and the generated SO
3
or H
2
SO
4
reacts with water (water contained in aqueous ammonia, water vapor evaporated from aqueous ammonia, or water vapor originally contained in flue gas) and ammonia (ammonia dissolved in aqueous ammonia, gaseous ammonia evaporated from aqueous ammonia, or gaseous ammonia injected separately from aqueous ammonia) to produce ammonium sulfate which is recovered according to the following formula (2) and (3).
SO
3
+2NH
3
+H
2
O→(NH
4
)
2
SO
4
(2)
H
2
SO
4
+2NH
3
→(NH
4
)
2
SO
4
(3)
However, in order to irradiate flue gas of weight flow Q (kg/s) with electron beam in absorbed dose of D (kGy), electric power P (kw) calculated by the following formula (4) is consumed.
P(kW)=Q (kg/s)×D(kGy)/(&eegr;)(%)/100) (4)
In the above formula (4), &eegr; is the ratio of energy of electron beam absorbed by the flue gas to the supplied electric power, and this &eegr; is normally in the range of 50 to 80%. Thus, a large amount of electric power is consumed for desulfurizing flue gas, and therefore it is necessary to reduce electric power consumption, increase the desulfurizing efficiency under the condition of the flue gas having a high temperature at the outlet of the process vessel (saturation temperature of water plus 5° C. or more), and reduce ammonia leak.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method and an apparatus for flue gas desulfurization which can reduce the cost of energy, and lower the amount of ammonia which is leaked while maintaining a high desulfurizing efficiency.
In order to achieve the above object, the inventors of the present application have studied intensively and found that the desulfurizing reaction as expressed by the above formula (1) is accelerated by cooling flue gas to a suitable temperature, and spraying aqueous ammonia which is atomized into fine droplets having a small Sauter mean diameter, and have accomplished the present invention.
The above object is achieved by the following means.
According to the present invention, there is provided a method for the flue gas desulfurization in which aqueous ammonia is sprayed and injected into flue gas containing sulfur oxides, and
Fujimura Hiroyuki
Izutsu Masahiro
Ebara Corporation
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Silverman Stanley S.
Vanoy Timothy C.
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