Gas separation: processes – Solid sorption – Moving sorbent
Reexamination Certificate
2000-06-29
2001-11-27
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Moving sorbent
C095S134000, C095S901000
Reexamination Certificate
active
06322613
ABSTRACT:
BACKGROUND OF THE INVENTION
In view of the expected tightening of federal, state, and local regulations on heavy-metal emissions, it is imperative that effective abatement methods be developed. Since toxic-waste release will be regulated in gas, solid, and liquid effluents, the newly developed technologies must be regenerative. Furthermore, control of air toxics must be integrated with the existing SO
2
and NO
x
abatement techniques to form a comprehensive multi-pollutant control system. The high volatility of mercury makes control of this metal particularly difficult; less volatile trace metals readily condense on ash particles, and thus their capture and removal is simpler.
SUMMARY OF THE INVENTION
Accordingly, it is the broad object of the present invention to provide a novel process for the recovery and disposal of mercury present in combustion (which term, as used herein, is inclusive of incineration) flue gas, or any other gas stream, optionally with the simultaneous removal of sulfur dioxide and/or oxides of nitrogen. More specific objects are to provide such a process wherein activated carbon used for mercury adsorption is regenerated, which process is moreover relatively incomplex, economical to carry out, and ecologically advantageous.
As typically practiced, the present innovation will comprise two primary elements: (1) It will employ, as an adsorption medium, activated carbons that are derived from waste tires or other waste products that inherently contain appreciable and desirable amounts of sulfur. (2) The flue gas that is produced by the combustion or incineration operations, of which the process is comprised, will itself be used for mercury stripping and sorbent-regeneration in the mercury recovery operation.
It has now been found that the foregoing and related objects of the invention are attained by the provision of a novel process for the recovery of mercury from a gaseous mixture, comprising the steps:
(a) providing a carbonaceous feedstock having a substantial inherent sulfur content;
(b) effecting pyrolysis and activation of the carbonaceous feedstock so as to produce an activated carbon sorbent having a sulfur content of at least about 3 percent by weight;
(c) passing a first stream of a gaseous mixture containing mercury over the sorbent, at a relatively low temperature, so as to effect adsorption of mercury by the sorbent from the first stream, and thereby to produce therefrom a mercury-bearing sorbent and a stripped gas stream, respectively; and
(d) passing a regenerating gas stream over the mercury-bearing sorbent, at a relatively high temperature, substantially above the relatively low temperature, so as to produce therefrom a mercury-rich gas stream and a regenerated sorbent, respectively.
In preferred embodiments the carbonaceous feedstock employed will be a product selected from the group consisting of scrap tires and rubber waste; scrap tires will most desirably be used, and to achieve greatest benefit the conditions of pyrolysis and activation will be such that the sorbent produced therefrom will have a sulfur content of at least about 5 weight percent. Activation of the feedstock will advantageously be carried out in an atmosphere that is substantially free of hydrogen and hydrogen compounds, most beneficially consisting essentially of carbon dioxide. The sorption step (c) of the process will usually be effected at a temperature in the range of about 50 to 200° C., and preferably not above 150° C., and the desorption step (d) will usually be effected at a temperature in the range of about 150 to 800° C., and preferably above 600° C. In especially preferred embodiments the regenerating gas used in carrying out the process will contain a substantial amount of sulfur, so as to provide an increased sulfur content to the regenerated sorbent; the regenerating gas may also contain a substantial amount of oxygen so as to provide an increased level of activation to the regenerated sorbent and/or to produce a quantity of activated carbon sorbent from a previously unactivated pyrolyzed carbonaceous material added to the system. Although coal and other mercury-containing materials may be utilized, the gaseous mixture will, as a practical matter, usually be produced by effecting combustion of a solid or liquid waste material. When the gaseous mixture produced contains a substantial amount of SO
2
, a second stream thereof may be used as the regenerating gas stream for step (d) of the process.
In certain embodiments of the invention the process is carried out cyclically, with portions of the gaseous mixture being passed alternately (as the so-called first and second streams) over the sorbent to adsorb mercury thereupon and to desorb mercury therefrom, respectively. The sorbent may, in such a case, comprise a plurality of separate beds, with a portion of the first stream being passed over one of the beds while a portion of the second stream is passed over another bed, during a first phase of the process cycle, and with the relationship being reversed during a second phase of the cycle. Alternatively, the sorbent may be in the form of a moving bed during the adsorption and desorption steps. The process may be carried out with a portion of the spent or disintegrated sorbent being removed and replaced with a quantity of pyrolyzed carbonaceous feedstock, on a continuous or semicontinuous basis; pyrolyzed feedstock added may or may not be activated, depending upon whether or not the process is carried out so as to effect in situ activation, as described.
Greatest benefit is derived from the process when it includes a step of treating the mercury-rich gas stream to effect removal and recovery of mercury, with or without a further step for controlling the emission of at least one other gas (e.g., SO
2
or NO
x
). To do so the carrier gas stream, stripped of mercury, may be admixed with a solid waste supply and subjected to combustion therewith, or the step may be carried out for the recovery of values from the gas (e.g., sulfur in elemental or combined form).
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patent: 5254521 (1993-10-01), Knudson
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patent: 5607496 (1997-03-01), Brooks
patent: 0271618 A1 (1988-06-01), None
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patent: 0625752 (1978-08-01), None
Serio Michael A.
Wojtowicz Marek A.
Advanced Fuel Research, Inc..
Dorman Ira S.
Spitzer Robert H.
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