Process for removing SO2 from flue gases using liquid sorbent injection

Abstract

Finely atomized alkaline sorbent salt solutions are injected into a hot flue gas stream to remove SO.sub.2. Flash evaporation of the droplets produces very fine dried sorbent particles, which react efficiently with SO.sub.2 in the flue gas. The liquid sorbent may be sodium carbonate, sodium hydroxide, sodium sulfite, potassium carbonate, potassium hydroxide or the like. In a coal-fired boiler, the liquid sorbent may be injected after the economizer section, where the flue gas temperature is below 850 F., and upstream of a particulate collection device. The dried sorbent particles react with SO.sub.2 and then are removed from the flue gas stream in the particulate collection device, producing a cleaned flue gas stream.

Claims

1. A method for removing sulfur dioxide from a flue gas stream, comprising: passing a flue gas stream through an emission control system having a boiler, an economizer, an air heater, a particulate control device and a stack; injecting into the flue gas stream a liquid sorbent solution having a liquid fraction and a sorbent fraction, wherein the liquid fraction evaporates upon contact with the flue gas stream leaving the sorbent fraction as dry sorbent particles; chemically reacting the dry sorbent particles with sulfur dioxide in the flue gas stream to form a particulate; and collecting the particulate in a particulate collection device.

2. The method of claim 1, wherein the temperature of the flue gas stream at the point the liquid sorbent solution is injected is less than 850 F.

3. The method of claim 1, wherein the temperature of the flue gas stream at the point the liquid sorbent solution is injected is between 600 F. and 800 F.

4. The method of claim 1, wherein the liquid sorbent solution is injected into the flue gas stream after the flue gas stream exits the economizer.

5. The method of claim 1, wherein the liquid sorbent solution is injected into the flue gas stream after the flue gas stream passes through the economizer and is also injected into the flue gas stream before the flue gas stream enters.

6. The method of claim 1, wherein the liquid sorbent solution is injected into the flue gas stream as a finely atomized mist.

7. The method of claim 1, wherein the particle size of droplets of the liquid sorbent solution when injected into the flue gas stream is between 5 and 100 microns.

8. The method of claim 1, wherein the liquid sorbent solution is injected into the flue gas stream using a dual fluid nozzle that mixes a liquid stream with an air stream at high pressure to atomize the liquid sorbent solution.

9. The method of claim 1, wherein the size of the dry sorbent particles ranges from 1 micron to 40 microns.

10. The method of claim 1, wherein the concentration of the sulfur dioxide in the flue gas stream is less than 1000 ppm.

11. The method of claim 1, wherein the sorbent fraction of the liquid sorbent solution is sodium carbonate.

12. The method of claim 1, wherein the liquid sorbent solution is sodium carbonate.

Description

DESCRIPTION OF THE DRAWINGS

(1) For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

(2) FIG. 1 shows a schematic diagram of one embodiment of the flue gas desulfurization system of the present invention, including the locations where the liquid sorbent solution is injected into the flue gas stream.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(3) The present invention is directed to improved methods and systems for, among other things, removing sulfur dioxide from flue gas. The configuration and use of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of contexts other than the specific types of liquid sorbent solution injection described herein. Accordingly, the specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. In addition, the following terms shall have the associated meaning when used herein:

(4) flue gas means an exhaust gas that is produced from an industrial process and includes both gas that will be used in connection with the process from which it is produced or even another related process (e.g., to produce heat), which will exit into the atmosphere via a stack for conveying waste exhaust gases from an industrial process. The flue gas can be produced from any industrial process such as a power generating process, metal smelting process and the like; and

(5) injecting means the introduction of a material into a flue gas from a point external to the duct work containing the flue gas and includes the introduction of a liquid phase solution or a powder into the flue gas, and the placement of a solid in the flue gas stream.

(6) Various embodiments of the present invention include methods and systems for the injection of a liquid sorbent solution into a flue gas stream for the removal of SO.sub.2. Typically, the temperature gas in a flue gas stream will be between 1000 F. and 2000 F. In coal-fired boilers, the liquid sorbent solution may be injected after the economizer section following the boiler, where the flue gas temperature is below 850 F., and generally in the range between 600 and 800 F. In most embodiments, the liquid sorbent solution is also injected upstream of a device designed to collect particulate from the flue gas, such as an electrostatic precipitator, a baghouse, or a fabric filter. The dried sorbent particles react with SO.sub.2 and are subsequently removed from the flue gas stream in the particulate collection device, producing a cleaned flue gas stream.

(7) Because of the relative cost of the injected sorbents and limitations in the amount of liquid that can be practically injected into the flue gas, the invention is typically applied where the SO.sub.2 concentration in the flue gas is less than 1000 ppm. This would be the case, for example, where coal-fired boilers burn low-sulfur fuels resulting in flue gas SO.sub.2 concentration ranging from approximately 200 to 500 ppm.

(8) In some embodiments of the invention, the liquid sorbent solution is injected into the flue gas as a finely atomized spray. The mean particle size of the injected droplets may be between approximately 5 and 100 microns. When the liquid sorbent solution is injected into the hot flue gas stream, the liquid in the sorbent solution is fully evaporated to produce very fine solid particles of the injected sorbent that are smaller than the injected liquid droplets from the nozzle. The mean particle size of the dried sorbent may range from approximately 1 micron to 40 microns. As a result, the injected sorbent is effectively divided into a large quantity of particles, possessing very high specific surface area. The quantity of particles and resulting surface area of the particles act to promote rapid and complete reaction with SO.sub.2 in the flue gas.

(9) As will be appreciated by those skilled in the art, the invention requires that the liquid fraction of the injected liquid sorbent solution be fully evaporated to produce the fine dried sorbent particles. If the liquid fraction of the injected liquid sorbent solution is not fully evaporated, the droplets will not fully react with the SO.sub.2 which may cause undesirable buildup of the sorbent in the downstream ductwork or downstream equipment. Suitable liquids include water and other liquids in which the sorbent is soluble and which evaporate sufficiently quickly and efficiently.

(10) To atomize the injected liquid sorbent solution, the solution may be injected using a dual-fluid nozzle that introduces and mixes the liquid stream with an air stream at high pressures to achieve atomization of the liquid. In some cases, the pressure at which the fluid is injected is between approximately 50 and 150 psi. Alternatively, a single-fluid nozzle may be used that injects only the liquid stream, but at a much higher pressure, typically 500 to 3000 psi.

(11) To promote effective reaction with SO.sub.2, the liquid sorbent solution should be well dispersed in the flue gas stream. In some embodiments, this may be achieved by injecting the liquid sorbent solution through a multitude of injection lances, each equipped with a multitude of injection nozzles.

(12) Various sorbents may be used in various embodiments of the present invention. For example, the sorbent fraction of the liquid sorbent solution may be a chemical or salt solution that is alkaline relative to the acidity of SO.sub.2, or sulfurous acid (H.sub.2SO.sub.3) in an aqueous solution. In some embodiments, the sorbent will have a pH greater than 6. A preferred sorbent is sodium carbonate (Na.sub.2CO.sub.3) which reacts with sulfur dioxide to produce sodium sulfite and carbon dioxide. Other chemicals or solutions are also effective and may also be used, including, without limitation, sodium hydroxide (NaOH), sodium sulfite (Na.sub.2SO.sub.3), potassium carbonate (K.sub.2CO.sub.3), and potassium hydroxide (KOH). These chemicals and salts are highly soluble in water, and thus can be injected in a concentrated form. In some embodiments, for example, the concentrations are greater than 10 weight percent.

(13) Referring now to FIG. 1 which depicts a typical emission control system in a plant configured to burn fossil fuels to produce energy. Coal is conveyed to a coal-fired boiler 10 from an external location (a coal pile or barge, etc.). The steam produced in the boiler is used to produce electrical energy using a system of turbines and ancillary equipment, and condensate produced from the steam is recycled to the boiler 10 beginning at the economizer 20. The energy extracted into the boiler water causes the temperature of the flue gas to decrease, and at the point where the flue gas leaves the economizer 20, the temperature is typically within the range of 600 to 800 F.

(14) The flue gas path between the economizer 20 and the stack 70 typically contains emission control equipment to remove various flue gas contaminants. Equipment typically found upstream of the air preheater 40 can include a selective catalytic reduction (SCR) system 30 to reduce NOx emissions. Equipment typically found downstream of the air preheater 40 can include particulate removal equipment 50, such as a dry or wet electrostatic precipitator (ESP or WESP), a fabric filter (bag house) and a scrubber 60 or other flue gas desulfurization (FGD) system.

(15) The composition of the flue gas leaving the boiler 10 will usually consist of mostly nitrogen (typically more than two-thirds) derived from the combustion air, carbon dioxide (CO2), and water vapor as well as excess oxygen (also derived from the combustion air). The flue gas also typically contains a small percentage of a number of pollutants, such as particulate matter, carbon monoxide, nitrogen oxides, sulfur oxides and mercury. In coal-fired boilers, most of the sulfur in the fuel is converted to sulfur dioxide.

(16) Referring back to FIG. 1, which shows points of injection 80 of a liquid sorbent solution into the flue gas path. The liquid sorbent solution may be injected 80 after the flue gas leaves the economizer 20 and, either in addition or as an alternative, the liquid sorbent solution may be injected 80 before the particulate collection device 50. In each case, the temperature of the flue gas is typically below around 850 F.

(17) Those skilled in the art will appreciate that embodiments of the present invention have an advantage over dry sorbent injection because the reagents may be injected directly in the existing flue gas ductwork, thereby avoiding the expense and downtime associated with the installation of a large vessel for contacting dry sorbent with the flue gas. In addition, the liquid sorbent solution is finely atomized to produce very small liquid droplets that quickly flash evaporate in the hot flue gas, producing even smaller dried particles that are much smaller than the dry sorbent powders that are injected with dry sorbent injection. As a result, the number of dried particles produced, and the surface area of said particles, is far greater than that achieved with dry sorbent injection. Therefore, the reaction rates with SO.sub.2 are significantly improved, allowing the invention to achieve measurably higher SO.sub.2 removal at measurably lower sorbent injection rates, compared to dry sorbent injection. The invention is particularly attractive for boilers that that burn low-sulfur fuels (<1 weight % sulfur) where moderate to high SO.sub.2 removal is required (50-80%).

(18) While the present system and method has been disclosed according to the preferred embodiment of the invention, those of ordinary skill in the art will understand that other embodiments have also been enabled. Even though the foregoing discussion has focused on particular embodiments, it is understood that other configurations are contemplated. In particular, even though the expressions in one embodiment or in another embodiment are used herein, these phrases are meant to generally reference embodiment possibilities and are not intended to limit the invention to those particular embodiment configurations. These terms may reference the same or different embodiments, and unless indicated otherwise, are combinable into aggregate embodiments. The terms a, an and the mean one or more unless expressly specified otherwise. The term connected means communicatively connected unless otherwise defined.

(19) When a single embodiment is described herein, it will be readily apparent that more than one embodiment may be used in place of a single embodiment. Similarly, where more than one embodiment is described herein, it will be clear that a single embodiment may be substituted for that one device.

(20) In light of the wide variety of methods for removing sulfur dioxide from flue gas known in the art, the detailed embodiments are intended to be illustrative only and should not be taken as limiting the scope of the invention. Rather, what is claimed as the invention is all such modifications as may come within the spirit and scope of the following claims and equivalents thereto.

(21) None of the description in this specification should be read as implying that any particular element, step or function is an essential element which must be included in the claim scope. The scope of the patented subject matter is defined only by the allowed claims and their equivalents. Unless explicitly recited, other aspects of the present invention as described in this specification do not limit the scope of the claims.