Systems and method for removal of acid gas in a circulating dry scrubber

Abstract

Systems and methods for the use of highly reactive hydrated lime (HRH) in circulating dry scrubbers (CDS) to remove sulfur dioxide (SO.sub.2) and other acid gases from the flue gas.

Claims

1. A system for removal of acid gas from a flue gas, the system comprising: a flue gas duct having a flow of flue gas, said flue gas duct including: a circulating dry scrubber (CDS) including a reactor section; an injection system for injecting a highly reactive lime hydrate (HRH) into said flue gas in said reactor section; HRH positioned within said flue gas duct and said injection system; and a particulate collection system for returning collected ash to said reactor section; wherein said reactor section is positioned upstream of said particulate collection system in said flow of flue gas; and wherein said HRH is fed from said injection system to said flue gas duct at a rate of less than 1000 lb./hr.

2. The system of claim 1 wherein water droplets are also injected into said reactor section with said HRH.

3. The system of claim 1 wherein said HRH comprise particulate lime hydrate wherein: 90% percent of particles in said particulate lime hydrate are less than or equal to about 10 microns and greater than or equal to about 4 microns in size; a ratio of a size of particle said 90% of said particles are below to a size of particle 10% of said particles are below is less than 8; and said particles have a BET surface area of about 18 m.sup.2/g or greater.

4. The system of claim 3 wherein said ratio is less than 6.

5. The system of claim 3 wherein said ratio is between about 4 and about 7.

6. The system of claim 3 wherein the ratio is between about 5 and about 6.

7. The system of claim 3 wherein 90% percent of said particles are less than or equal to about 8 microns and greater than or equal to about 4 microns.

8. The system of claim 3 wherein 90% percent of said particles are less than or equal to about 6 microns and greater than or equal to about 4 microns.

9. The system of claim 3 wherein 90% percent of said particles are less than or equal to about 5 microns and greater than or equal to about 4 microns.

10. The system of claim 3 wherein said particles have a BET surface area of about 20 m.sup.2/g or greater.

11. The system of claim 3 wherein 50% of said particles are less than or equal to about 4 microns.

12. The system of claim 3 wherein 50% of said particles are less than or equal to about 2 microns.

13. The system of claim 1 wherein said HRH is fed at a rate of less than 750 lb./hr.

14. The system of claim 1 wherein said HRH is fed at a rate of less than 600 lb./hr.

15. The system of claim 1 wherein said HRH has citric acid reactivity of less than 10 seconds.

16. The system of claim 1 wherein said HRH has citric acid reactivity of less than 7 seconds.

17. The system of claim 1 wherein said acid gas comprises sulfur dioxide (SO.sub.2).

18. The system of claim 1 wherein said acid gas comprises hydrochloric acid (HCl).

19. The system of claim 1 wherein said acid gas comprises sulfur trioxide (SO3).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 provides a conceptual block diagram of an embodiment of a flue gas duct system as may be used is, for example, in a coal fired power plant including common components.

(2) FIG. 2 provides a conceptual block diagram of a circulating dry scrubber (CDS).

(3) FIG. 3 provides a table illustrating increased reduction of hydrate use with a high reactivity lime hydrate (HRH) compared to a more traditional hydrated lime composition to control stack SO.sub.2 emissions.

(4) FIG. 4 provides a graph showing average feed rates of HRH and traditional lime hydrate at a full load.

(5) FIG. 5 provides a bar chart showing relative feed rates of HRH and traditional lime hydrate at different load conditions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

(6) Most flue gas scrubbing systems are commonly focused on making sure that certain materials do not leave the flue stack and disperse in the air for environmental reasons. FIG. 1 shows a loose block diagram of an arrangement of a flue gas duct system such as can be used in a coal fired power plant or an industrial facility that generates sulfur dioxides as part of its production process. Generally, major components include the boiler (101), a selected catalytic reduction (SCR) system for reducing NO, emissions (103), an air preheater (APH) (105), a bag house or electrostatic precipitator (ESP) (107), a Flue Gas Desulfurization (FGD) unit (109), and then the exhaust stack (111).

(7) Traditionally the FGD system (109) has utilized wet flue gas desulfurization (WFGD) which provides wet lime or limestone scrubbers where the calcium source (lime or limestone) is mixed in a slurry and introduced to the gas stream in a large reactor to scrub the SO.sub.2 from the gas stream.

(8) While WFGD is a very effective technology for scrubbing SO2, The WFGD purge stream is an aqueous solution and generally contains a wide variety of pollutants making it a rather toxic material to handle and dispose of. It, includes gypsum, along with heavy metals, chlorides, magnesium and dissolved organics. In many applications, WFGD purge water is first treated by dewatering to separate synthetic gypsum cake which can be a valuable secondary product. The remaining WFGD purge water is then recycled back to the scrubber. A portion of this water (still containing dissolved chlorides) is removed from the recycle stream (the chloride purge), and is subjected to various forms of water treatment (as required) to reduce or eliminate dissolved metals and other contaminants of concern prior to discharge back into the environment in accordance with the applicable permits and laws. As should be apparent, this process is both resource intensive and as regulations on allowed discharge tighten, increasingly difficult to use.

(9) Recently, there has been an interest in the use of circulating dry scrubbing (CDS) technology. A CDS is also referred to as circulating fluid bed (CFB). The technology of a CDS is relatively straightforward and is illustrated in FIG. 2. Essentially flue gas (200) is directed into the reaction vessel (201) where hydrated lime (203) (possibly with additional water (205)) is directed into the flue gas stream. This causes the hydrated lime to react with the SO.sub.2 to produce calcium sulfite according to Equation 1.
Ca(OH).sub.2+SO.sub.2.fwdarw.CaSO.sub.3+H.sub.2O
The calcium sulfite may further react with available oxygen to produce calcium sulfate (CaSO.sub.4). Hydrated lime can also react with other acid gases in the CDS as well.

(10) Some neutralization occurs in the reactor portion of the CDS (in-flight capture). After passing the flue gas through the reactor section, unreacted hydrate, sulfates, sulfites, and fly ash in the flue gas stream collect on electrostatic precipitator plates or, more commonly, fabric filters (207), where additional acid gas neutralization may occur with remaining hydrate. Clean flue gas (209) comes through the ash mixture bed and is discharged to the stack (111). The remaining material, including the bulk of the fly ash and remaining hydrated lime, are returned (211) to the inlet of the reaction vessel (201) to provide for reuse while a portion of the reaction products are removed and disposed of (213) as waste. Thus, hydrated lime is rarely wasted as unreacted sorbent is cycled back into the process for reuse. However, hydrated lime is also available to react with other components of the flue gas, the majority of which is carbon dioxide.

(11) This reaction, which forms calcium carbonate (CaCO.sub.3), reduces the efficiency of the hydrated lime both initially and as it is being recirculated and necessitates use of a molar excess of hydrated lime for sufficient reaction with SO.sub.2.

(12) One problem with use of hydrated lime in Circulating Dry Scrubber (CDS) systems is the collection of the used lime. Used lime is commonly collected as a particulate (along with fly ash) in a baghouse or other particulate collecting system. Because of the way a CDS works with the spent sorbent being interacted without water, the spent sorbent is pushed further down the flue stream and the amount of particulate generated and that needs to be removed is a multiple of the amount of hydrate originally added. That multiple is commonly around 2 and is often around 1.8.

(13) Without being limited to any particular theory of operation, it is believed that much of this factor is due to the increase in molecular weight of the byproducts when compared to hydrate. As baghouses have to be cleaned to remove captured ash, CDS systems using hydrated lime often result in a substantial increase in the amount of waste to be removed. For smaller plants that have small baghouses, where the waste collection processes are not particularly efficient, or where for industrial processes that generate sulfur oxides as an off-gas and need to remove it, this increase can result in a substantial bottleneck which can substantially reduce the production of the plant.

(14) With the development of a highly reactive lime hydrate (HRH) with properties designed to significantly improve the speed of reaction with acid gases present in flue gas, it became a possibility that HRH may be useable in CDS systems and specifically in-flight in such systems. The ability of HRH to capture acid gases in-flight more effectively than conventional hydrated limes is believed to be potentially sufficiently effective to allow for only fresh hydrate to enter the reaction section of the scrubber, eliminating the need to recycle used sorbent. HRH can be manufactured in accordance with a number of processes. In an embodiment, it may be manufactured in accordance with, and/or have the properties discussed in, U.S. patent application Ser. Nos. 13/594,538, 14/180,128, 14/289,278, and 15/344,173 the entire disclosure of all of which is herein incorporated by reference. HRH is notably different from other hydrated lime as it has an improved removal rate of acidic pollutants present in the flue gas where the sorbent is delivered and the rate of removal is generally substantially higher. The use of a high purity, highly reactive hydrated lime such as HRH will have faster neutralization of acidic species.

(15) While it may be provided in a variety of forms, in an embodiment, the HRH is a dry solid free of excess moisture. The product used may also be described by having available calcium hydroxide concentration of greater than 92% by weight, preferentially greater than 94%, and optimally greater than 95%. The product used may also be described by having at least 90% of particles less than 5 microns (D90), at least 50% of the particles are less than 2 microns (D50), and at least 10% of the particles are less than 1 micron and preferentially less than 0.8 microns (D10). The ratio of the D90/D10 is preferentially less than 3 and preferentially around 2.5 or less. The product may also be described as having a BET surface area of at least 20 m.sup.2/g, at least 30 m.sup.2/g, or at least 40 m.sup.2/g. It will usually have a pore volume of at least 0.2 cc/g.

(16) In an embodiment, 90% percent of the particles are less than or equal to about 10 microns and greater than or equal to about 4 microns and a ratio of a size of particle 90% of the particles are below to a size of particle 10% of the particles are below is less than about 8. The particles preferably have a BET surface area of about 18 m/g or greater or about 20 m.sup.2/g or greater. Depending on embodiment, the D90/D10 ratio is less than 6, between 4 and 7, or between 5 and 6.

(17) In an HRH like the above, 90% percent of the particles may be less than or equal to about 8 microns and greater than or equal to about 4 microns, less than or equal to about 6 microns and greater than or equal to about 4 microns, or less than or equal to about 5 microns and greater than or equal to about 4 microns.

(18) In an embodiment, 50% of the particles (D50) are less than or equal to about 4 microns, less than or equal to about 2 microns, and may be greater than 1 micron.

(19) In order to test reactivity of particular lime hydrate compounds to determine if they are an HRH, in an embodiment, the reactivity to a weak acid (such as, but not limited to, citric acid) provides for a reactivity time that is measurable with commercial instruments. The problem with determining reaction time to stronger acids is that the reaction can be too quick to effectively measure at laboratory scaling. Thus, it is difficult to predict compositions that will function well without performing large scale pilot testing. In order to determine the citric acid reactivity of a particular hydrated lime composition, the amount of time it took 1.7 grams of lime hydrate to neutralize 26 grams of citric acid was measured. As a measurement of effectiveness, it is preferred that this value be less than or equal to 15 seconds, less than 10 seconds, less than 7 seconds, less than 5 seconds, or preferably less than or equal to 3 seconds.

(20) The HRH will generally be used as part of a circulating dry scrubber system of the off gas of an industrial plant, incinerator, or boiler that combusts sulfur and/or halogenated fuels. Hydrated lime (203) is fed from a silo into a conveying line that disperses the fine powder into the bottom of a reactor (201) in the off gas piping. A predetermined amount of recycled ash (211) is also fed into the reactor (201), as is a quantity of spray water (205) designed to wet the solid particles and drop flue gas temperature. At the top of the reactor (201), the flue gas travels through the duct into a baghouse (207)/(107), where ash collects on the bags while clean flue gas (209) flows through the ash/bag layers. Automated mechanical means dislodge the ash from the bag exterior and this ash is either recycled (211) to the reactor (201) or sent (213) to a landfill or other beneficial use as deemed appropriate.

(21) Because of the highly reactive nature of HRH, in a CDS a finer cloud from the sorbent injection lance puts more sorbent particles in the pathway of acid gases, reaching stratified areas and neutralizing more acid components. The use of a high purity, highly reactive hydrated lime such as HRH will have faster neutralization of acidic species which can be problematic when present in flue gas. Such reactivity enhancement is beneficial especially when the finer cloud from the sorbent injection is used as an input into the scrubber reactor portion of a CDS. This puts more sorbent particles in the pathway of acid gases, reaching stratified areas and neutralizing more acid components.

(22) Further, as the HRH is generally more reactive, it will also result in less material being needed to remove the same amount of gas. Thus, for the same level of removal, less material is used. As indicated above, the spent sorbent ash generation is a multiplier (often around 1.8) of the mass of the lime solvent used. Thus, use of an HRH also produces significantly less spent sorbent, generating less ash, and allowing for a smaller baghouse to be used or for process bottlenecks which previously may have existed based on ash generation to be eliminated.

(23) In an embodiment, it is preferred that a plant may reduce its input sorbent use by at least 20%, more preferably by at least 25%, more preferably by at least 30%, more preferably by at least 35%, on even more preferably by at least 40% over the amount of sorbent used if standard lime hydrate was used. Further, even when lime hydrates having high pore volume (e.g. 0.2 cc/g or above) are used but which lack one or more of the other characteristics of an HRH, HRH still will generally result in a 5-15% reduction in input sorbent use compared to such high pore volume hydrates. As indicated above, a reduction of 20% in sorbent use results in an almost 40% reduction in ash generation. It should be clear that these dramatic increases provide dramatic and unexpected process efficiency increases over use of other lime hydrate.

(24) While CDS systems are perceived to be quite efficient due to multiple cycles of sorbent through the reactor section (That the lime sorbent is recirculated (211)), a majority of acid gas pollutant reduction occurs the first cycle through the system (reactor (201) to baghouse (207)/(107) in a common CDS process). A sorbent with improved capability for in-flight capture exhibits better removal in the reactor (201) than standard sorbent, thus providing better pollutant control (typically monitored via SO.sub.2 emission monitoring) with lesser quantities of sorbent.

(25) In an exemplary embodiment, a plant fueled with coal uses a CDS to control stack SO.sub.2 emissions. In this embodiment, the plant compared a traditional hydrated lime against performance of an HRH. The CDS operates under a logic-based controller that adjusts the hydrated lime feed rate in order to maintain a near constant SO.sub.2 emission as determined by a CEMS monitor in the stack flue gas. The unit and scrubber were operated normally over several days with each type of hydrated lime. Results are outlined in FIG. 3.

(26) As shown in FIG. 3, at all load ranges the CDS required significantly less HRH than traditional hydrated lime in order to maintain desired SO.sub.2 emission limits. Scrubber inlet and outlet temperatures are provided to show that temperature variations in the scrubber were not the reason for improved performance with HRH. The experiment was conducted with the CDS first operating on the traditional hydrated lime, then converted to HRH, then converted back. FIG. 4 shows how the plant operated utilizing substantially less sorbent in the HRH window than in either of the others. Further, FIG. 5, which was obtained from a longer term test than that of FIG. 3 shows that the HRH showed dramatic reductions in use at lower loads. Specifically while the reduction at high loads (specifically those of 270 MW or greater in the selected test facility) was good, there is a dramatic reduction at medium loads (between 200-270 MW) and the drop is even more pronounced at low loads (between 125-250 MW). Thus, HRH has shown surprising benefit particularly in low load conditions.

(27) In a still further embodiment, improvements of efficiency were noted by comparing operation of a process similar to the one discussed above using an HRH compared to both a standard lime and a high pore volume lime which did not include the other characteristics of an HRH. In this embodiment, improvements of generally 26-28%, but up to 35% were noted compared to standard hydrates. High pore volume hydrates, which did not meet the requirements to be an HRH, showed improvements of only about 18-20%. Thus, it is clear that an HRH dramatically outperforms its expected level.

(28) The development of a high purity, fast reacting hydrated lime or HRH improves dispersion in the flue gas when the sorbent is delivered to the process. These properties provide a sorbent that increases coverage of the pathway of acid gases and rapidly reacting with those gases to neutralize acidic species in the flue gas.

(29) Without being limited to any theory of operation, increased acid gas capture in-flight improves the sorbent efficiency the first time through the reactor (201) and BH (207)/(107). Since fresh sorbent is believed to capture the most pollutant, there should be differentiation in performance between a traditional hydrated lime and an HRH, but the performance is better than expected. This differentiation evidences itself via reduced hydrated lime requirements to the CDS.

(30) In an embodiment, the use of HRH provides a method of removing SO.sub.x or other acid gases from flue gas of boiler firing sulfur-containing fuel that is advantageous over prior art due to tighter particle size distribution of the hydrated lime sorbent. Advantageous generally refers to reduced sorbent quantities required to achieve similar reduction in concentration of SO.sub.x that end user requires for traditional hydrated lime.

(31) In an embodiment, this also provides a method of removing SO.sub.2 from flue gas of boiler firing sulfur containing fuel that is generally advantageous due to more rapid reactivity of hydrated lime sorbent, as characterized by acid reactivity test. Here, advantageous refers to greater reductions in concentration of SO.sub.2 that end user experiences when using standard hydrated lime.

(32) While the invention has been disclosed in conjunction with a description of certain embodiments, including those that are currently believed to be the preferred embodiments, the detailed description is intended to be illustrative and should not be understood to limit the scope of the present disclosure. As would be understood by one of ordinary skill in the art, embodiments other than those described in detail herein are encompassed by the present invention. Modifications and variations of the described embodiments may be made without departing from the spirit and scope of the invention.

(33) It will further be understood that any of the ranges, values, properties, or characteristics given for any single component of the present disclosure can be used interchangeably with any ranges, values, properties, or characteristics given for any of the other components of the disclosure, where compatible, to form an embodiment having defined values for each of the components, as given herein throughout. Further, ranges provided for a genus or a category can also be applied to species within the genus or members of the category unless otherwise noted.