Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture
Reexamination Certificate
1999-09-23
2001-08-21
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
C423S243010, C423S243030, C423S243060, C423S24000R, C422S168000, C422S234000, C422S236000
Reexamination Certificate
active
06277343
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to gas-liquid contactors used in the removal of acidic gases, such as from utility and industrial flue gases. More particularly, this invention is directed to a wet flue gas desulfurization process and apparatus that uses an ammonia-containing scrubbing solution to remove sulfur dioxide and other acidic gases from flue gases, promotes the oxidation rate of the scrubbing solution to produce ammonium sulfate, and reduces the presence of free ammonia in the scrubbed flue gases.
2. Description of the Prior Art
Gas-liquid contactors are widely used to remove substances such as acidic constituents and particulate matter from combustion or flue gases produced by utility and industrial plants. Often of particular concern is sulfur dioxide (SO
2
) produced by the combustion of fossil fuels and various industrial operations. Acidic gases are known to be hazardous to the environment, such that their emission into the atmosphere is closely regulated by clean air statutes. The method by which acidic gases are removed with a gas-liquid contactor or other type of flue gas scrubber is known as wet flue gas desulfurization (FGD).
The cleansing action produced by gas-liquid contactors and absorbers is generally derived from the passage of gas through a tower cocurrently or countercurrently to a descending liquid that absorbs the acidic gases. A known configuration for a gas-liquid contactor
10
is shown in
FIG. 1
as including an absorber tower
12
equipped with an inlet duct
14
through which combustion gases enter the tower
12
. Shown above the inlet duct
14
are two banks of spray headers
16
which introduce a contact medium, e.g., an alkaline slurry or solution, into the tower. Calcium-based slurries, sodium-based solutions and ammonia-based solutions are typical alkaline scrubbing liquids used in flue gas scrubbing operations. Additional banks of spray headers can be provided as may be required for a given application. A pump
20
recycles the contact medium from a tank
18
at the bottom of the tower
12
to the spray headers
16
. Intimate contact between the contact medium and the flue gases rising through the tower
12
results in a cleansing action, after which the contact medium and the entrapped or reacted gases are collected in the tank
18
at the bottom of the tower
12
. The cleansed gases continue to rise through the tower
12
, then typically pass through a mist eliminator
22
and thereafter are either heated or passed directly to the atmosphere through an outlet duct
24
.
While gas-liquid contactors and absorbers utilizing calcium-based slurries generally perform satisfactorily, their operation results in the production of large quantities of wastes or gypsum, the latter having only nominal commercial value. In contrast, ammonia-based scrubbing processes have been used in the art to produce a more valuable ammonium sulfate fertilizer, as taught by U.S. Pat. Nos. 4,690,807 and 5,362,458, each of which are assigned to the assignee of the present invention. In these processes, as the flue gases flow upward through the tower
12
, acidic gases present in the gases are absorbed by an ammonium sulfate solution containing ammonia. Afterwards, the solution is accumulated in the tank
18
, where the absorbed sulfur dioxide reacts with the ammonia to form ammonium sulfite (NH
4
)
2
SO
3
and ammonium bisulfite (NH
4
HSO
3
), which are oxidized in the presence of sufficient oxygen to form ammonium sulfate and ammonium bisulfate (NH
4
HSO
4
), the latter of which reacts with ammonia to form additional ammonium sulfate. As shown in
FIG. 1
, oxygen and ammonia for these reactions are injected together into the tank
18
via a single conduit
26
. A suitable source
28
for oxygen is air, and a suitable source
30
for ammonia is an anhydrous or aqueous ammonia solution. A portion of the ammonium sulfate solution and/or ammonium sulfate crystals that form in the solution can then be drawn off to yield the desired byproduct of this reaction. A sufficient amount of ammonium sulfate is preferably removed from the ammonium sulfate solution prior to delivery to the tower
12
in order to maintain ammonium sulfate at a desired concentration in the solution.
In addition to being required to react with sulfur dioxide to produce ammonium sulfate, ammonia also serves to increase the efficiency of sulfur dioxide removal by reducing the acidity of the ammonium sulfate solution introduced into the tower
12
. With the absorption of sulfur dioxide in the tower
12
, the ammonium sulfate solution becomes more acidic and its ability to absorb sulfur dioxide is reduced. For example, without added ammonia the pH of the ammonium sulfate solution is generally in the range of about 4 and 5.5, but with added ammonia the solution generally has a pH of around 5 to 6, depending on control set points and operating conditions, including the SO
2
concentration in the flue gas. However, oxidation of an ammonium sulfite solution is slower with higher pH levels.
Higher pH levels are also associated with the release of free ammonia from the solution, often termed “ammonia slip.” In addition to incurring an economic loss because of lost ammonia, free ammonia in the scrubbed flue gases reacts with uncaptured sulfur dioxide and trioxide to create an ammonium sulfate aerosol that is visible as a blue or white plume in the stack discharge, leading to secondary pollution problems. Controlling the amount of free ammonia in the desulfurization process is in part a function of the ammonia vapor pressure, which results from a combination of pH and levels of unoxidized ammonium sulfite that remain in the absence of sufficient oxygen. Therefore, high pH values and high levels of unoxidized ammonium sulfite promote ammonia slip.
In view of the above, an ongoing demand of desulfurization processes using ammonium sulfate scrubbing solutions is the ability to achieve efficient oxidation rates while reducing the release of free ammonia.
SUMMARY OF THE INVENTION
The present invention provides an apparatus and process for removing acidic gases from flue gases produced by processing operations of the type carried out in utility and industrial plants. The apparatus is generally a gas-liquid contactor whose operation uses an ammonium sulfate-containing scrubbing solution to absorb acidic gases from flue gases, and into which oxygen and ammonia are then injected to react with the absorbed sulfur dioxide to produce ammonium sulfate as a valuable byproduct. According to the invention, the oxygen and ammonia are not introduced together into the scrubbing solution as done in the prior art, but instead are introduced sequentially and in a manner so that the oxidation first occurs in a relatively low pH solution as a result of the absorbed acidic gases. The ammonia is then added to the solution in a manner that inhibits or prevents intermixing of the ammonia with the majority of the solution, but is present in the solution when recycled for further absorption of acidic gases.
The gas-liquid contactor for carrying out this invention generally entails an inlet through which flue gases are introduced into a passage, and an ammonium sulfate-containing scrubbing solution that is introduced into a contact region of the passage, where the solution contacts and absorbs sulfur dioxide and other acidic gases from the flue gases. A vessel is fluidically connected to the passage so that the scrubbing solution containing the absorbed acidic gases accumulates in the vessel. Defined within the vessel is a volume from which the scrubbing solution is drawn for recirculation to the passage. An oxygen-containing gas is introduced into the scrubbing solution within the vessel, but separated from the volume so that oxidation occurs primarily in the vessel outside the volume. Finally, an ammonia-containing fluid is introduced into the scrubbing solution prior to being recirculated to the passage. The ammonia-containing fluid is not introduced into the scrubbing solution with
Gansley Raymond R.
Mengel Michael L.
Griffin Steven P.
Hartman Domenica N.S.
Hartman Gary M.
Marsulex Environmental Technologies, LLC
Vanoy Timothy C.
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