BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 is a schematic representation of an oxy-combustion boiler;
FIG. 2 is a schematic representation of a boiler with a condensing heat exchanger in a flue gas recycle loop;
FIG. 3 is a graphical representation of Spray Flux vs. Delta T;
FIG. 4 is a schematic representation of a combustion process according the present invention;
FIG. 5 is a spray tower embodiment of the present invention;
FIG. 6 is a spray tower embodiment of the present invention;
FIG. 7 is a spray tower embodiment of the present invention;
FIG. 8 is a spray tower embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention generally relates to a method of utilizing a spray tower to regulate the moisture concentration of a flue gas. In embodiments wherein spray towers such as wet scrubbers are used, the present invention provides for a method of duel purpose, wherein contaminants such as SO2 are removed in addition to the regulation of flue gas moisture concentration.
Referring to FIG. 4, a schematic representation of a combustion process according the present invention if shown. Oxygen and coal are supplied to a boiler upstream of a particulate collector and a flue gas desulfurization unit. The flue gas desulfurization unit is equipped with a heat exchanger to cool the liquid reagent. The liquid reagent is cooled to a predetermined point below that of the flue gas inlet temperature water vapor dew point and sprayed into the flue gas. A portion of the cooled flue gas is then reheated and recirculated to the pulverizer and burners to facilitate combustion. The recirculated flue gas may be reheated as necessary with economizer exhaust gases in a gas-gas heat exchanger or by other heating means. Means of flue gas circulation and the process of coal-fired combustion, including the components therein, are discussed in detail in Steam 41, herein incorporated by reference.
While a wet scrubber is generally used in the present application to articulate an exemplary embodiments of the present invention, it is understood that a spray tower according to the present invention may be any tower contactor used in a combustion application wherein a liquid reagent is made to contact a flue gas in either a co-current, a counter-current, or both a counter and co-current manner.
Referring now to FIG. 5, a spray tower 10 is shown. In a method according to the present invention flue gas enters spray tower 10 at flue gas inlet 7, and exits through flue gas outlet 9. Liquid reagent 31 is stored in liquid reagent storage means 11. Heat exchanger means 21 is located within spray tower 10 and at least partially submersed within the liquid reagent 31 contained with the liquid reagent storage means 11. Heat exchanger means 21 cools the liquid reagent 31 in the liquid reagent storage means 11 to a temperature near the desired flue gas outlet temperature. Pump 12 pumps the liquid reagent 31 from the tank liquid reagent storage means 11 upward through piping 13 to a sprayer assembly 14 mounted above the liquid reagent storage means 11. Sprayer assembly 14 has nozzles 15 which spray cooled liquid reagent 31 onto flue gas as it passes through spray tower 10. Upon contact the liquid reagent 31 cools the flue gas causing moisture to condense from the flue gas as fall into the liquid reagent storage means 11. The flue gas then exists the spray tower 10 at flue gas outlet 9 at a lower temperature than the flue gas had at the flue gas inlet 7 and with a reduced moisture content.
In an alternative embodiment, spray tower 10 is also utilized as a wet scrubber for the removal of SO2. Liquid reagent 11, generally comprising an aqueous slurry of gypsum and limestone, is sprayed on flue gas providing the duel benefit of SO2 capture and flue gas moisture control, wherein liquid regent is capable of achieving effective SO2 capture.
In alternative spray tower embodiments, it is understood that the liquid reagent is not limited to gypsum and limestone, but can comprise of any aqueous solution necessary to remove a contaminant from a flue gas or perform a chemical reaction.
In a preferred embodiment a liquid-liquid heat exchange is used for cooling the liquid reagent. Cooling water, generally available at temperatures below 85° F. at power plants, in addition to other common cooling liquids such as glycol may be used to cool the liquid reagent. In one embodiment the liquid reagent is pumped in a loop to and from a shell and tube heat exchanger where the slurry flows through the tube side of the exchanger and the cooling water flows on the shell side. In alternative embodiment, the liquid reagent flows on the shell side.
Alternatively, a liquid/gas heat exchanger may be used, wherein any gas available at a power plant may be used as a cooling medium.
Referring now to FIG. 6, an alternative embodiment of the heat exchanger means 21 is shown. In this embodiment, heat exchanger means 21 is internal to the spray tower 10 but external to the liquid reagent storage means 11. Liquid reagent 31 is maintained in liquid reagent storage means 11 at a level that allows liquid reagent 31 to overflow the wall 5 of the liquid regent storage means 11. Overflow of liquid reagent 31 is then cooled by heat exchanger means as it flows over wall 5 of liquid reagent storage means 11 on its way to the recirculation pumps 12. Cooled liquid reagent 11 is then pumped to sprayer assembly 14 via piping 13, where spray headers 15 sprays the cooled liquid reagent 21 on the flue gas stream as it passes through spray dryer 10 for removal of flue gas contaminants, including SO2, cooling the flue gas, and dehumidifying the flue gas. The condensed moisture from the flue gas and liquid reagent 31 eventually fall via gravity into the liquid reagent storage means 11.
Referring now to FIG. 7, an alternative embodiment of the liquid reagent storage means 11 is shown. In this embodiment, heat exchanger means 21 is located external to the spray tower 10, at least partially immersed in an external liquid reagent storage means 33. Flue gas condensate and sprayed liquid reagent are collected at the bottom of the spray tower 34 and flow to liquid reagent storage means where heat exchanger means 21 cools liquid reagent 11, prior the liquid reagent 11 being sprayed onto the flue gas as the flue gas passes though the spray tower 10.
Referring now to FIG. 8, and alternative embodiment of the heat exchanger means is shown. In this embodiment, heat exchanger means is located external to spray tower 10 and cools the liquid reagent 31 as if passes though piping 13 after being pumped out of the liquid reagent storage means 11. A heat exchanger means 25 can be placed before pump 12. A heat exchanger means 45 could also be placed after pump 12. In an alternative embodiment multiple heat exchanger means could also be used such the there are multiple external heat exchanger means 25, 45, or any combination of both internal and external heat exchanger means.
The present invention can provide several advantages over the known flue gas scrubbing devices. The simultaneous desulfurization and dehumidification in a single device provides a particular advantage to oxy-combustion where the process is served to advantage by the dual benefits of flue gas desulfurization and dehumidification in a single device. Submerging the heat transfer surface into the liquid reagent means further reduces the space required to operate the system and reduces the possibility of surface fouling by slurry solids.
Amine scrubbing is an alternative to oxy-combustion, wherein an amine scrubbing unit removes carbon dioxide from the combustion flue gas. In an alternative embodiment of the present invention, the spray tower may be placed upstream of an amine scrubbing system. To operate effectively, amine scrubbing requires the flue gas to enter the amine scrubber at a lower moisture content and at a significantly cooler temperature than that of flue gases exiting conventional flue gas desulfurization devices. Typically, a separate cooling unit is placed downstream of the conventional flue gas desulfurization device and upstream of the amine scrubber to cool the flue gas prior to entering the amine scrubber. When used in an amine scrubbing application, a spray tower of the present invention placed upstream of the amine scrubber provides a means for removing flue gas contaminants such as sulfur dioxide and the additional benefit of cooling the flue gas to a temperature below the water vapor dew point of the inlet flue gas, thereby eliminating the need for a separate flue gas cooler.
In yet another alternative embody, the spray tower is of a pumpless scrubber design.
While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.
Patent Application
20070243119
