Flue gas scrubbing apparatus

Details Flue gas scrubbing apparatus Flue gas scrubbing apparatus

1994-11-08

2001-04-10

Spitzer, Robert H. (Department: 1724)

Gas separation: apparatus

Gas and liquid contact apparatus for gas separation...

Gravitational separator for contact liquid

C096S265000, C096S267000, C096S356000

Reexamination Certificate

active

06214097

ABSTRACT:

This invention generally relates to gas-liquid contactors used in the removal of particulate matter and gases, such as from utility and industrial flue gases. More particularly, this invention is directed to a gas-liquid contactor which is configured so as to eliminate the requirement for a pump to deliver a contact liquid to the contact section of a gas-liquid contactor, and further configured to have a high velocity section which promotes the absorption of gases and matter by the contact liquid, such that the efficiency of the gas-liquid contactor is increased while simultaneously reducing its operating and maintenance costs.
BACKGROUND OF THE INVENTION
Gas-liquid contactors are widely used to remove substances such as gases and particulate matter from combustion or flue gases produced by utility and industrial plants. Often of particular concern are sulfur dioxide (SO
2
) and other acidic gases produced by the combustion of fossil fuels and various industrial operations. Such 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 such gases are removed with a spray tower or other type of gas-liquid contactor is known as wet flue gas desulfurization (FGD).
The cleansing action produced by a gas-liquid contactor is generally derived from the passage of gas upwardly through a tower countercurrently to a descending liquid which cleans the air. Wet flue gas desulfurization processes typically involve the use of calcium-based slurries or sodium-based or ammonia-based solutions. As used herein, a slurry is a mixture of solids and liquid in which the solids content can be any desired level, including the extreme condition in which the slurry is termed a moist solid. Examples of calcium-based slurries are limestone (calcium carbonate; CaCO
3
) slurries and hydrated lime (calcium hydroxide; Ca(OH)
2
) slurries formed by action of water on lime (calcium oxide; CaO). Such slurries react with the acidic gases to form precipitates which can be collected for disposal or recycling. Intimate contact between the alkaline slurry and acidic gases which are present in the flue gases, such as sulfur dioxide, hydrogen chloride (HCl) and hydrogen fluoride (HF), result in the absorption of the gases by the slurry. Thereafter, the slurry is accumulated in a tank.
A known type of gas-liquid contactor is a spray tower
10
shown in cross-section in FIG.
1
. The spray tower
10
generally is an upright structure composed of a tower
14
equipped with an inlet duct
12
through which combustion gases enter the tower
14
. Above the inlet duct
12
is a lower bank of spray headers
16
which introduce a spray
20
of an alkaline slurry into the tower
14
. A second, upper bank of spray headers
18
is typically provided above the lower bank of spray headers
16
, with additional banks of spray headers being used as required for a given application. One or more pumps
26
are required to recycle the alkaline slurry by pumping the slurry from a tank
30
to the banks of spray headers
16
and
18
. Each bank of spray headers
16
and
18
may be individually equipped with a pump
26
for the purpose of promoting the flexibility of the pumping and spraying operation to accommodate varying demands by the scrubbing operation.
Intimate contact between the alkaline slurry spray
20
and the flue gases rising through the tower
14
results in a cleansing action, by which the slurry and the entrapped or reacted gases are collected at the bottom of the tower
14
in the tank
30
. The cleansed gases which continue to rise through the tower
14
then typically pass through a mist eliminator
22
, and thereafter are either heated or passed directly to the atmosphere through a chimney
24
.
Due to the large quantity of slurry which must be pumped to scrub the flue gases, a significant cost in the construction, operation and maintenance of gas-liquid contactors is attributable to the pumps
26
. Yet, the pumps
26
also constitute a significant limitation to the scrubbing operation, in that the quantity of slurry pumped by the pumps
26
cannot be readily adjusted to accommodate changes in the scrubbing operation, such as the amount of flue gas which must be scrubbed or the amount of contaminants present in the flue gases.
Another limitation of prior art gas-liquid contactors is the relatively low solids content permitted when using a slurry as the cleaning liquid. Typically, the solids content of such slurries must be limited to about ten to about fifteen weight percent. However, higher concentrations would allow the use of a smaller tank
30
, since its size is generally dictated by, among other things, the residence time for crystallization of solids within the slurry. Higher solids contents would also eliminate the requirement for primary dewatering devices such as thickeners or hydrocyclones, which are well known devices employed in the art to remove solids and/or byproducts from a slurry. However, high solids contents significantly increase erosion within the tower
14
, tank
30
, fluid conduit, spray headers
16
and
18
and pump
26
, while also increasing the power required to pump the slurry due to the higher specific gravity of the slurry.
Finally, it would be advantageous to maximize the flue gas velocity within the tower
14
from the standpoint of improving contact between the slurry and the flue gases, so as to enable a reduced slurry flow to the tower
14
. Higher flue gas velocities would also allow for the use of a tower
14
having a smaller cross-sectional area, such that the cost of constructing the spray tower
10
is reduced. However, conventionally-accepted design practices typically limit the flue gas velocity within the tower
14
to about ten feet per second (about three meters per second) in order to assure the proper operation of the mist eliminator
22
. Higher flue gas velocities within the tower
14
tend to increase the gas pressure drop within the tower
14
, and therefore increase the likelihood of liquid particles being carried to and flooding the mist eliminator
22
.
Those skilled in the art will appreciate that, in view of the considerations noted above, it would be desirable if a flue gas scrubbing apparatus were available which overcame the above-noted disadvantages associated with the use of slurry pumps, yet could employ slurries having higher solids concentrations and higher flue gas velocities.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a flue gas scrubbing apparatus for the removal of particulate matter, sulfur dioxide and other acidic gases, such as from flue gases produced by utility and industrial facilities.
It is a further object of this invention that such a scrubbing apparatus eliminate the requirement for a device to pump a contact liquid which serves to remove gases and particulate matter from the flue gases, and thereby enable the use of high concentration levels of solids within the contact liquid.
It is still a further object of this invention that such a scrubbing apparatus be constructed and configured so as to maximize the velocity of the flue gases while in contact with the contact liquid.
It is another object of this invention that such a scrubbing apparatus operate in a manner which does not adversely effect the operation of devices used to remove liquid particles from the flue gases.
Lastly, it is yet another object of this invention that such a scrubbing apparatus be configured such that its construction, operation and maintenance costs are minimized.
The present invention provides a gas-liquid contactor of the type suitable for removing gases and particulate matter from flue gases produced by utility and industrial plants. The gas-liquid contactor is generally composed of a passage having a lower end and an upper end. The passage may be formed by a tower equipped with an inlet adjacent its lower end through which flue gases are introduced into the tower. The gas-liquid contactor further includes a device which sprays

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