Fluent material handling – with receiver or receiver coacting mea – With material treatment – With fluid contact
Reexamination Certificate
2002-06-10
2004-02-24
Maust, Timothy L. (Department: 1754)
Fluent material handling, with receiver or receiver coacting mea
With material treatment
With fluid contact
C141S069000, C141S011000, C141S091000, C141S198000, C096S297000, C096S326000, C095S224000, C423S243010, C261SDIG009
Reexamination Certificate
active
06695018
ABSTRACT:
FIELD AND BACKGROUND OF INVENTION
The present invention relates generally to wet flue gas desulfurization (FGD) scrubbers and in particular to systems employing bleed streams or ex situ forced oxidation to oxidize sulfites to sulfates.
Typical wet FGD scrubbers, sometimes referred to as absorbers, consist of two major components: the scrubbing zone in which the actual gas scrubbing takes place and a reaction tank to allow efficient utilization of the reagent. The liquid reagent sprayed in the scrubbing zone captures sulfur dioxide (SO
2
) forming sulfites and bisulfites. These systems run free of scale if the oxidation of sulfites to sulfates is kept below about 15% (inhibited oxidation) or above 98% (forced oxidation). One means of controlling scale formation in an FGD system is to force oxidization of the sulfites to sulfates by bubbling air through the recirculated reagent.
Many present-day wet FGD scrubbers are single loop forced oxidation systems in which the scrubbing zone and the reaction tank are combined into one structure within the wet scrubber in what is referred to as in situ forced oxidation. A known single loop, in situ forced oxidation wet FGD scrubber
100
is shown in FIG.
1
. Flue gas enters the scrubber at an inlet
12
located above the internal reaction tank and passes through a scrubbing zone consisting of a series of spray header levels
14
having a plurality of nozzles
16
which spray liquid reagent recirculated from the internal reaction tank by pumps
18
. In the internal reaction tank, air is introduced to promote oxidation of the sulfites to sulfates. Other reactions such as reagent dissolution also occur. Sulfur oxides, produced in significant quantity by the combustion of coal, fuel oil or other fossil fuels, are removed by the liquid spray from the flue gas before the flue gas is exhausted to a stack (not shown) through an outlet
2
. The liquid reagent is usually an alkaline slurry of lime, limestone, alkaline fly ash with supplemental lime, magnesium-promoted lime or a solution of sodium carbonate. The liquid reagent sprayed in the scrubbing zone captures SO
2
, forming sulfites and bisulfites. The pH of the partially reacted liquid reagent leaving the scrubbing zone falls to as low as about pH 4.5 depending on the reagent, stoichiometry, SO
2
concentration and other design parameters. The low pH scrubbing liquid then falls into the reaction tank. Fresh liquid reagent is added to bring the pH of the liquid reagent in the tank back up to a preset level, for example from 5.8 to 6.2.
To maintain the reaction tank free of scale, the sulfites are oxidized to sulfates. This oxidation is typically accomplished by forcing air to a header
22
from a pump (not shown) which is distributed to a series of perforated sparger pipes
24
located in the reaction tank to allow air to be bubbled therein to force oxidation of the sulfites to sulfates in the reaction tank.
Older systems were designed to oxidize sulfites by bubbling the air through the reagent in a separate, external reaction tank. The formed sulfates were separated and disposed of. These systems were referred to as ex situ forced oxidation systems. Other systems bled a slip stream of reagent from the internal reaction tank, bubbled air through the reagent to oxidize the sulfites, and then returned the reagent back to the reaction tank, in an arrangement intermediate between the ex situ and in situ oxidation systems.
In some ex situ oxidation systems, the partially reacted liquid reagent is captured in the internal reaction tank, where fresh alkaline reagent is added to replace the reacted reagent and readjust the pH. A first stream is removed from the internal reaction tank and recirculated to the scrubbing zone. A second stream is removed from the internal reaction tank and sent to a separate external reaction tank, sometimes referred to as an oxidizer or oxidation tank. The preferred pH of the reagent in the oxidation process in the external reaction tank is 5 or lower. The pH of the readjusted reagent removed from the internal reaction tank, however, is about 5.8 and may be higher, due to the addition of fresh alkaline reagent. Therefore sulfuric acid is added to the oxidation stream or the separate external reaction tank to neutralize the alkali, and adjust the pH to the range preferred for promoting the oxidation process.
In some other older systems, a semi-in situ forced oxidation process was used in which a scoop collected nearly all of the sprayed liquid reagent, and sent it to an external oxidation tank. The contents of the tank were then pumped directly back to the FGD tower, rather than to a de-watering system. Only a bleed stream containing fresh alkali was removed from the process stream for dewatering.
Yet another FGD system employed a bowl in a double-loop operation to collect all of the liquid reagent from the absorber stage of the FGD system, however no attempt was made to minimize the use of sulfuric acid.
In a double-loop system, there are two loops, which are virtually separate from each other. The scrubbing loop contains fresh alkaline liquid reagent. The liquid reagent is then sprayed over several layers of packing to enhance the SO
2
removal capability of the system. The contact between the reagent liquid on the packing and the flue gas causes the pH of the liquid reagent leaving the packing to drop, similar to the single-loop system. Then, the partially reacted liquid reagent, which is fairly low in pH, is collected in a bowl and sent to an external reaction or oxidation tank.
In a double-loop system, fresh liquid reagent is added to the external oxidation tank to maintain the process set point pH, and the adjusted pH liquid reagent is recirculated from the oxidation tank to the spray zone over the packing. Liquid reagent from the external oxidation tank overflows into the bottom of the scrubber, under the bowl, and is recirculated through a second loop to a set of headers, also located under the bowl. The function of these headers is to humidify the flue gas entering the scrubber so that wet/dry interface deposits do not form and assist in the scrubbing process. Partially reacted reagent liquid is usually drawn from the bottom of the scrubber based on a preset level. The lower loop usually runs at a lower pH than the upper loop to improve limestone utilization and reduce operating costs.
FIG. 2
illustrates a known double-loop flue gas desulfurization system comprising a housing, generally designated
5
, having an inlet
12
near the bottom of the housing
5
for incoming flue gas. An outlet
2
is located at the top of housing
5
for the exit of flue gas after undergoing a scrubbing process within the housing
5
.
Fresh liquid reagent is fed into feed tank
10
and pumped by pumps
30
to a plurality of upper level spray headers
20
located near the top of the housing
5
through a feed line
32
which is connected to the feed tank
10
, the pumps
30
and the upper level spray headers
20
. The liquid reagent sprayed by the upper level spray headers
20
has a high pH and is sprayed onto a packing
40
, which is usually layered and has a depth of 2 to 3 feet, to enhance absorption of the SO
2
in the high pH liquid reagent and filter exiting flue gas before the flue gas is channeled through outlet
2
.
The sprayed liquid reagent from the upper level spray headers
20
trickles over and through the packing
40
, and is diverted by a shroud ring
50
which is disposed concentrically around an inner diameter of the housing
5
for channeling liquid reagent through its inner opening to a bowl
60
located directly beneath the shroud ring
50
which collects the liquid reagent. The shroud ring
50
and the bowl
60
prevent the upper loop liquid reagent from being channeled into the internal reaction tank
70
located at the bottom of the housing
5
. The liquid reagent collected in bowl
60
is directed back into the feed tank
10
by a return line
62
which is connected to the bowl
60
and the feed tank
10
.
As the level of liquid reagent rises in feed tank
10
Johnson Dennis W.
Murphy David W.
Myers Robert B.
Grant Kathryn W.
Marich Eric
Maust Timothy L.
The Babcock & Wilcox Company
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