Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Sulfur or sulfur containing component
Reexamination Certificate
1999-03-31
2001-06-12
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Sulfur or sulfur containing component
Reexamination Certificate
active
06245308
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART STATEMENT
This invention relates to a method for decreasing sulfuric acid (H
2
SO
4
) and sulfuric anhydride (SO3) contained in combustion exhaust gas from combustors (e.g., boilers) using fossil fuels, and to a combustion exhaust gas flow system therefor.
FIG. 6
is a flow diagram showing the flow of combustion exhaust gas in one example of a conventional boiler. In
FIG. 6
, fuel fed to a burner
2
is burned in a furnace
1
. The resulting combustion exhaust gas is passed through a heat exchanger
3
consisting of a reheater, a secondary superheater, and a primary superheater and a fuel economizer installed in a back pass
4
, and then conducted through a flue
5
, where it is mixed with NH
3
supplied by a denitrating NH
3
feeder
10
and denitrated with the aid of a denitration catalyst
6
. Subsequently, the combustion exhaust gas is subjected to heat exchange in an air preheater
7
, dedusted in an electrostatic precipitator or the like, desulfurized in a desulfurizer
8
or the like, and then discharged into the atmosphere through a stack
9
. The detailed explanation of the electrostatic precipitator, heat exchanger and other apparatus installed downstream of air preheater
7
is omitted.
When a fossil fuel containing sulfur (S) is burned in such a combustor (e.g., a boiler), the combustion exhaust gas produced in furnace
1
contains, for example, sulfur dioxide (SO
2
) formed by the combustion of S, sulfuric anhydride (SO
3
) formed by the oxidation of a portion of the sulfur dioxide, sulfuric acid (H
2
SO
4
) and water vapor (H
2
O).
In flue
5
, air preheater
7
, stack
9
and other apparatus installed downstream of the combustor (e.g., boiler), these SO
3
, H
2
O and H
2
SO
4
] tend to condense and deposit, in the form of highly concentrated H
2
SO
4
, on low-temperature parts whose surface temperature is lower than the acid dew-point, and thereby cause troubles such as low-temperature corrosion and ash deposition.
In order to prevent troubles such as low-temperature corrosion and ash deposition, it is an effective measure to reduce the concentrations of H
2
SO
4
and SO
3
in combustion exhaust gas so as to lower the acid dew-point and thereby decrease the amount of H
2
SO
4
condensed.
As one means therefor, attempts have been made to decrease the amount of H
2
SO
4
condensed by adding a neutralizer, such as calcium carbonate (CaCO
3
), slaked lime [Ca(OH)
2
] or magnesium hydroxide [Mg(OH)
2
], to combustion exhaust gas within the flue so as to neutralize and remove H
2
SO
4
and SO
3
while they exist in gaseous form, and thereby lower the acid dew-point (see, for example, Japanese Patent Provisional Publication Nos. 9-75661 and 58-36623). However, these attempts have failed to achieve sufficient efficiency of desulfurization reaction and neutralizer utilization because the concentrations of gaseous H
2
SO
4
and SO
3
are low and the efficiency of their contact with a solid neutralizer such as CaCO
3
is low. Moreover, problems concerning operation and equipment construction (e.g., those associated with the handling of a neutralizer), problems concerning the treatment of dust [comprising calcium sulfate (CaSO
4
) or magnesium sulfate (MgSO
4
)] which is produced as a result of neutralization and may deposit in the equipment, and like problems remain unsolved. In the present situation, this technique has not yet been put to practical use because its overall economic merit has not been established.
Thus, when a solid SO
3
neutralizer such as CaCO
3
is added to the flue in order to prevent troubles (e.g., low-temperature corrosion and ash deposition) due to the formation of H
2
SO
4
and SO
3
in combustion exhaust gas and the ensuing condensation and deposition of highly concentrated H
2
SO
4
on low-temperature parts of the flue, heat exchanger, air preheater and other apparatus installed downstream of the boiler, various problems may arise. As described above, they include low efficiency of desulfurization reaction and neutralizer utilization, poor handleability of the SO
3
neutralizer, difficulty in the treatment of dust (e.g., CaSO
4
) formed as a result of neutralization, trouble due to the deposition of dust in the flue, and the like.
OBJECT AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for decreasing sulfuric acid and sulfuric anhydride present in combustion exhaust gas which can solve the above-described problems of the prior art, can prevent the occurrence of troubles (e.g., low-temperature corrosion and ash deposition) arising from H
2
SO
4
and SO
3
contained in combustion gas, and is easy to operate.
In order to accomplish the above object, the present invention provides:
(1) A method for decreasing sulfuric acid and sulfuric anhydride present in combustion exhaust gas which comprises adding an SO
3
-decreasing agent to combustion exhaust gas and thereby reducing sulfuric acid (H
2
SO
4
) and sulfuric anhydride (SO
3
) contained in the combustion exhaust gas;
(2) A method for decreasing sulfuric acid and sulfuric anhydride present in combustion exhaust gas as described above in (1) wherein the SO
3
-decreasing agent is added at one or more positions in a region which lies downstream of a combustor and in which the temperature of the combustion exhaust gas is in the range of 300 to 1,000° C.; and
(3) A method for decreasing sulfuric acid and sulfuric anhydride present in combustion exhaust gas as described above in (1) or (2) wherein the SO
3
-decreasing agent is hydrogen gas.
Moreover, the present invention also provides a combustion exhaust gas flow system which is installed downstream of a combustor, wherein the combustion exhaust gas flow system is equipped with at least one SO
3
-decreasing agent feeder at a position in a section extending from a heat exchanger just behind the furnace of the combustor to an air preheater.
In the present invention, H
2
SO
4
and SO
3
present in combustion exhaust gas are decreased by adding an SO
3
-decreasing agent (the term “SO
3
-decreasing agent” as used herein means an additive for reducing H
2
SO
4
and SO
3
) to the combustion exhaust gas and thereby reducing H
2
SO
4
and SO
3
contained in the combustion exhaust gas to form H
2
SO
3
and SO
2
.
As the SO
3
-decreasing agent, hydrogen (H
2
) gas is especially preferred from the viewpoint-of reactivity and handleability.
The amount of SO
3
-decreasing agent added may suitably be determined according to the type of the SO
3
-decreasing agent, the properties of combustion exhaust gas, and the like. However, when H
2
gas is used as the SO
3
-decreasing agent and the combustion exhaust gas comprises ordinary boiler-exhaust gas, it will be sufficient to use H
2
gas in such an amount as to give an H
2
gas concentration of not greater than 2,000 ppm or a molar H
2
/SO
3
ratio of not greater than 15.
The SO
3
-decreasing agent is preferably added at a position in a region which lies downstream of the combustor and in which the temperature of the combustion exhaust gas is in the range of 300 to 1,000° C. The reason for this is that the aforesaid temperature range is favorable for the reduction reaction of SO
3
to SO
2
, the reduction reaction of H
2
SO
4
to H
2
SO
3
, and the reduction reaction of oxygen (O) adsorbed to the catalytically active sites of deposited ash and responsible for the formation of SO
3
. Although the SO
3
-decreasing agent may be added at one position, the reduction reactions can be made to proceed more efficiently by dividing the SO
3
-decreasing agent into a plurality of portions and adding them at different positions.
In the flow diagram of
FIG. 6
showing the flow of combustion exhaust gas in a boiler, a section extending from behind the secondary superheater within heat exchanger
3
to air preheater
7
corresponds to the region where the temperature of the combustion exhaust gas is in the range of about 300 to 1,000° C., and hence serves as a site suitable for the addition of the SO
3
-decreasing agent
Fujioka Yuichi
Hishida Masashi
Ichinose Toshimitsu
Ohkubo Toshiaki
Tokunaga Kikuo
Griffin Steven P.
Mitsubishi Heavy Industries Ltd.
Myers Bigel & Sibley & Sajovec
Vanoy Timothy C
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