Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Sulfur or sulfur containing component
Reexamination Certificate
1998-09-25
2001-10-09
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Sulfur or sulfur containing component
C423S243080, C423S244070
Reexamination Certificate
active
06299848
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semi-dry scrubbing process for the removal of sulfur dioxide from gases and to a filter cake used therein.
2. Description of the Related Arts
A variety of industrial and commercial plants generate pollutant-containing gas streams. Sintering and coal burning operations typically generate unacceptably high levels of sulfur dioxide (SO
2
). These gas streams must be cleansed or “scrubbed” before being released to the atmosphere. One of the known techniques for removing SO
2
and other similar pollutants is the “semi-dry” scrubbing process.
Generally the semi-dry scrubbing process comprises contacting the gas with an aqueous solution or slurry of a reagent to neutralize the pollutant, under conditions that evaporate the liquid water carrier, for example, contacting in a spray drier. The resulting gas stream, which is cooler and more humid, will contain particulates of unreacted reagent and reaction product(s). These particulates are normally removed from the gas by a particle separator such as a baghouse to yield a scrubbed, pollutant-reduced gas.
Such a system is a compromise between the wet scrubbing process where high pollutant removal efficiencies are achieved but with high maintenance and liquid disposal costs, and the dry scrubbing process, where lower removal efficiencies are obtained. In the semi-dry process, the initial presence of water or other liquid catalyzes a more rapid pollutant transfer (capture) and neutralization (reaction) than in simply passing the gas over a solid reactant as in the dry scrubbing process. However, because the liquid is dried or evaporated, no liquid waste stream is formed as in the wet scrubbing process, thereby reducing the maintenance and disposal costs. Another disadvantage associated with wet scrubbing is the common requirement that the gas be cooled and water-saturated in order to avoid evaporation of the absorbent. The gas often must then be reheated to burn-off excess moisture in the gas before being discharged to the atmosphere so that the plume is not overly dense and offensive. As a result of these demands, the wet scrubbing process usually requires substantial equipment and energy consumption.
Suitable reagents in the semi-dry scrubbing process include lime compounds such as hydrated lime, limestone, etc., as well as alkali compounds such as sodium carbonate. The lime reagents are generally less expensive than the alkali reagents, but the alkali reagents are generally more effective. Because of these trade-offs, lime is normally used unless the required removal efficiency/reliability requires an alkali reagent or it otherwise becomes more cost effective.
While the initial aqueous contacting step generally provides the main pollutant removal effect, the subsequent particulate filter can provide supplemental pollutant removal efficiency. In a typical SO
2
semi-dry scrubbing process where the gas is contacted in a spray drier with a lime slurry, about 70-80% of the SO
2
is removed in the spray drier and up to about 20% of the SO
2
is additionally removed in the baghouse (overall removal is about 80-90%). This additional removal is possible in the baghouse because of the contact of the gas with the accumulated filter cake present on the filter. The filter cake contains the particulates removed from the gas including the unreacted lime. Thus, the filter cake can operate as a dry scrub reactor. However, the removal efficiency is inconsistent, ranging from near zero to 40% (efficiency based on the residual amount of pollutant), and is generally independent of the ratio of the residual pollutant in the gas stream to the unreacted lime in the cake (the residual “&agr;”).
U.S. Pat. No. 4,581,210, the entire contents of which are incorporated herein by reference, teaches that introducing a sorbent such as limestone via a dry venturi into the humidified gas between the quench reactor and the baghouse increases SO
2
removal efficiency in the baghouse. According to this patent, the dry venturi facilitates formation of an improved filter cake which acts as an improved secondary fixed-bed reactor in the baghouse to further purify the gas. The cake is substantially non-tacky and porous, thus allowing much greater cake buildup over the prior art systems before removal is required.
However, the gases being scrubbed in this patent generally contain HCl, as is commonly found in municipal waste incinerators. Further, the patent requires doping of the calcium based slurry used in the spray drier or quench reactor with a hygroscopic agent. Either one of these features increases the adsorption of water in the filter cake which in turn is believed to catalyze the SO
2
transfer and reaction. Accordingly, such a method is not expected to be suitable for treating other types of gas streams such as those derived from sintering operations, or without the dopant.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an efficient semi-dry scrubbing process for removing SO
2
from a gas stream.
It is another object of the invention to provide a semi-dry scrubbing process that effectively utilizes the particulate filter for SO
2
removal.
A further object of the present invention is to provide a filter cake useful in pollutant removal from gaseous streams.
Preferred forms of the present invention accomplish at least one of the above objects. One embodiment of the present invention is a semi-dry process for removing sulfur dioxide from a gas stream. The process comprises (1) combining a gas stream containing SO
2
with an aqueous solution or slurry stream containing a first reactant to evaporate the water in the solution or slurry and form a humidified gas stream; (2) entraining a second reactant in the humidified gas stream; and (3) subsequently passing the humidified gas stream through a filter; wherein one of the first and second reactants is a lime compound and the other is an alkali carbonate compound.
Another embodiment of the present invention relates to a filter cake comprising a lime compound, an alkali carbonate, and an alkali sulfate in sufficient admixture to provide a water adsorbing lime-alkali sulfate complex.
Yet another form of the invention contemplates a process for removing SO
2
from a gas stream, which comprises passing a humidified gas containing SO
2
through a filter cake supported on a filter to reduce the amount of SO
2
in the gas by about 60 to 90%. The filter cake comprises a lime compound, an alkali carbonate, and an alkali sulfate in sufficient admixture to provide a water adsorbing lime-alkali sulfate complex.
The present invention is based on the discovery that the use of two specified reagents in a semi-dry scrubbing process increases the SO
2
scrubbing efficiency in general and that an improved filter cake can be formed having good removal efficiencies in particular. While not wishing to be bound by any theory, it is believed that the filter cake contains the alkali sulfate reaction product in close proximity/touching the unreacted lime and that this “complex” forms a water adsorbing site. The water present in the gas is thus adsorbed onto this site forming a liquid micro-environment. The water catalyzes the absorption of the SO
2
out of the gas and facilitates rapid neutralization with the adjacent unreacted lime. This system allows for high SO
2
removal efficiencies even without the use of a hygroscopic dopant and even without HCl in the gas stream.
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patent: 3987147 (1976-10-01), Guerrieri
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Griffin Steven P.
Hamon Research-Cottrell
Howrey Simon Arnold & White , LLP
Vanoy Timothy
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