Chemistry: electrical and wave energy – Processes and products – Processes of treating materials by wave energy
Reexamination Certificate
2002-02-12
2004-08-10
Langel, Wayne A. (Department: 1754)
Chemistry: electrical and wave energy
Processes and products
Processes of treating materials by wave energy
C204S157490, C423S243020
Reexamination Certificate
active
06773555
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a treatment of flue gas containing sulfur oxides, more particularly to a flue gas treatment method and apparatus for removing sulfur oxides by injecting ammonia into flue gas containing sulfur oxides.
BACKGROUND ART
As economy develops, more and more energy is demanded. In such circumstances, an energy source is still dependent on fossil fuels such as coal and petroleum. However, harmful products or pollutants generated by burning of fossil fuels are responsible for environmental pollution. To prevent the diffusion of pollutants such as sulfur oxides and nitrogen oxides into the atmosphere and the progress of environmental pollution, development work of a fuel gas treatment system is being carried out for a fuel combustion plant such as a power plant. However, in the conventional flue gas treatment system, there are still many areas of improvement to meet problems such as the need for the equipment requiring complicated control and the indispensability for large-scale waste water treatment systems.
In order to solve these problems, a flue gas desulfurizing process in which flue gas discharged from the combustion facility such as a boiler is treated by injecting ammonia into the flue gas has been developed.
In the flue gas desulfurizing process in which ammonia (NH
3
) is injected into flue gas containing sulfur oxides (SO
X
) such as combustion gas discharged from a boiler (hereinafter referred to as an ammonia injection process), NH
3
reacts with SO
X
to produce a powder of ammonium compounds containing ammonium sulfate. Among reactions in the ammonia injection process, a chemical reaction in which sulfur dioxides (SO
2
) as a main component of SO
X
reacts with NH
3
, oxygen (O
2
) and water (H
2
O) contained in flue gas to produce ammonium sulfate [(NH
4
)
2
SO
4
] as by-product is expressed in the following formula (1).
SO
2
+2NH
3
+H
2
O+1/2O
2
→(NH
4
)
2
SO
4
+550.1KJ/mol (1)
As expressed in the formula (1) representatively, because the reaction in which SO
X
reacts with NH
3
to produce ammonium compounds is an exothermic reaction, the lower the temperature of flue gas, the more accelerated the reaction is. Therefore, in the ammonia injection process, flue gas is cooled before injection of NH
3
, or water is sprayed into flue gas before, or simultaneously with, or after injection of NH
3
; or after mixing with NH
3
. In this case, the injected water is consumed by the desulfurizing reaction representatively expressed in the formula (1), and evaporated by the heat of reaction, and the sensible heat possessed by flue gas before injection of ammonia. Thus, if the amount of sprayed water is adjusted appropriately, the recovery of the produced ammonium compounds in the form of powder is not hindered. The recovery of the powder is normally carried out in an electrostatic precipitator, and the recovered powder comprises ammonium compounds such as ammonium sulfate and can be utilized as a fertilizer.
However, in the ammonia injection process, generally, efficiency of removing SO
X
(hereinafter referred to as desulfurization rate), especially efficiency of removing SO
2
is not high. Residual NH
3
which has injected and has not reacted with SO
X
is released together with the treated flue gas to the atmosphere. In order to lower leakage of NH
3
, it is necessary to reduce the amount of injected ammonia. However, reduction of the amount of injected ammonia lowers further a desulfurization rate, particularly the efficiency of removing SO
2
. As a result, unreacted NH
3
is additionally released by the amount corresponding to the lowered desulfurization rate. As a result, the problem of leakage of NH
3
which is not lowered by the amount corresponding to the reduced amount of injected ammonia arises.
On the other hand, the desulfurizing reaction may be accelerated by increasing the amount of water sprayed together with NH
3
to lower the temperature of flue gas. In this case, the temperature of the flue gas reaches around a water saturation temperature even in the vicinity of an outlet of a process vessel, and hence it becomes difficult to recover the produced powder in a dry state.
Thus, in order to achieve a high desulfurization rate, normally, NH
3
is sprayed and injected, and irradiation of electron beam of several kGy to dozen kGy is carried out (the flue gas desulfurizing method in which injection of NH
3
and irradiation of electron beam are performed is hereinafter referred to as electron beam process). The purpose of this method is that residual SO
2
which has not been removed in the above formula (1) is oxidized to sulfur trioxide (SO
3
) or sulfuric acid (H
2
SO
4
) by radicals such as O, OH or HO
2
produced from gas molecules such as oxygen and water vapor in flue gas by irradiation of electron beam, and then the produced SO
3
or H
2
SO
4
reacts with water (water vapor originally contained in the flue gas, and water sprayed and injected together with NH
3
) and NH
3
by the following formulas (2) and (3) to recover ammonium sulfate.
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)
In order to irradiate flue gas of weight flow Q (kg/s) with electron beam having 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)
where &eegr; is the ratio of energy of electrons absorbed by the flue gas to the supplied electric power, and this &eegr; is normally in the range of 50 to 80%.
However, generally, in the electron beam irradiating method, when a high desulfurization rate is required on the condition that leakage of NH
3
is controlled to a lower level, the required absorbed dose becomes large, and hence electric power consumption becomes large as expressed by the formula (4).
Therefore, the inventors of the present application have proposed a flue gas desulfurizing method and apparatus in which flue gas is cooled in the range of a water saturation temperature to 80° C., aqueous ammonia is sprayed and injected into the cooled flue gas, and the aqueous ammonia is pulverized into droplets having a Sauter mean diameter of 0.5 &mgr;m to 30 &mgr;m and sprayed, whereby a high desulfurization rate is achieved while keeping low leakage of NH
3
without irradiation of electron beam or with a relatively small absorbed dose.
However, even in such method, in order to obtain 90% or more of the desulfurization rate while leakage of NH
3
is suppressed to about 10 ppm or less, a large amount of absorbed dose of not less than about 5 kGy is required, or aqueous ammonia is required to be pulverized into droplets having a Sauter mean diameter of not more than about 5 &mgr;m. In the latter as well as the former, a large amount of energy such as energy for generating a large amount of compressed air for pulverization is necessary.
DISCLOSURE OF INVENTION
In view of the above, it is therefore an object of the present invention to provide a flue gas desulfurizing method and apparatus which can achieve a high desulfurization rate and can lower leakage of NH
3
while reducing the cost of energy.
In order to achieve the above object, according to the present invention, there is provided a flue gas treatment method for removing sulfur oxides in flue gas using ammonia, characterized in that: ammonia is injected into the flue gas containing sulfur oxides to react sulfur oxides with ammonia to produce ammonium compounds containing ammonium sulfate, and after recovering the produced ammonium compounds from the flue gas, the flue gas is brought into contact with an absorption liquid to remove residual sulfur oxides and/or ammonia contained in the flue gas.
In the absorption liquid, sulfate ions (SO4
2−
) and/or sulfite ions (SO
3
2−
) and/or ammonium ions (NH
4
+
) are dissolved.
In the flue gas treatment method of the present invention, the residual SO
X
, contained in the flue gas, which has n
Aoki Shinji
Hayashi Kazuaki
Izutsu Masahiro
Saku Daisuke
Suzuki Ryoji
Ebara Corporation
Langel Wayne A.
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