SORBENT COMPOSITION FOR AN ELECTROSTATIC PRECIPITATOR

Abstract

Powdery calcium-magnesium compound, sorbent composition based on calcium-magnesium for being used in flue gas treatment, compatible with electrostatic precipitators and process for reducing the resistivity of a powdery sorbent composition for flue gas treatment installation comprising an electrostatic precipitator.

Claims

1. Process for reducing the resistivity of a powdery sorbent composition for flue gas treatment installations which include an electrostatic precipitator, said powdery sorbent composition having a reduced resistivity under 1E11 Ohms.cm and over 1E07 Ohms.cm at 300 C., wherein said resistivity of said powdery sorbent composition is measured in a resistivity cell in an oven under a stream of air having 10% humidity, said powdery sorbent composition comprising a powdery calcium-magnesium compound comprising at least a calcium-magnesium carbonate content greater than or equal to 80 weight % or a calcium-magnesium hydroxide content greater or equal to 80 weight %, with respect to the total weight of the powdery calcium-magnesium content, the process comprising the steps of: a) providing said powdery sorbent composition in a reactor and; b) adding to said powdery sorbent composition an additive or a mixture of additives, comprising at least one metallic ion M and/or a counter ion X with M being a metallic ion having an atomic number less than or equal to 74, M being a transition metal ion or a post-transition metal ion, or Mg.sup.2+ or Na.sup.+ or Li.sup.+, and X being a counter ion selected from the group consisting of nitrates, nitrites, O.sup.2, OH.sup. and mixtures thereof in an amount calculated to obtain between 0.1 weight % and 5 weight %, of said metallic ion M and/or counter ion X in weight with respect to the total weight of dry sorbent composition.

2. Process according to claim 1 wherein said metallic ion M is selected from the group consisting of Cu.sup.2+, Fe.sup.2+, Fe.sup.3+, Mn.sup.2+, Co.sup.2+, Mo.sup.2+, Ni.sup.2+, Zn.sup.2+.

3. Process according to claim 1, wherein said counter ion X is nitrate.

4. Process according to claim 1, wherein said powdery calcium magnesium compound presents a BET specific surface area by nitrogen adsorption of at least 20 m.sup.2/g.

5. Process according to claim 1, wherein said powdery calcium-magnesium compound presents a BJH pore volume for pores having a diameter lower or equal to 1000 by nitrogen desorption of at least 0.1 cm.sup.3/g.

6. Process according to claim 1, wherein said powdery sorbent composition further comprises an additional additive selected from the group consisting of activated charcoal, lignite coke, halloysite, sepiolite, clays, bentonite, kaolin, vermiculite, fire clay, aerated cement dust, perlite, expanded clay, lime sandstone dust, trass dust, Yali rock dust, trass lime, fuller's earth, cement, calcium aluminate, sodium aluminate calcium sulphide, organic sulphide, calcium sulfate, open-hearth coke, lignite dust, fly ash and water glass.

7. Process according to claim 1, further comprising a step of adding to the said powdery sorbent composition a sodium additive comprising sodium in an amount up to 3.5 weight % with respect to the total weight of the powdery sorbent composition and expressed as sodium equivalent.

8. Process according to claim 1, wherein the said powdery calcium magnesium compound is hydrated lime.

9. Powdery sorbent composition for flue gas treatment installations including an electrostatic precipitator, said powdery sorbent composition comprising a powdery calcium-magnesium compound comprising at least a calcium-magnesium carbonate content greater than or equal to 80 weight % or a calcium-magnesium hydroxide content greater or equal to 80 weight %, with respect to the total weight of the powdery calcium-magnesium content, said powdery sorbent composition having a reduced resistivity under 1E11 Ohms.cm and over 1E07 Ohms.cm at 300 C., wherein the said resistivity of said powdery sorbent composition is measured in a resistivity cell in an oven under a stream of air having 10% humidity, said reduced resistivity being provided by an additive or a mixture of additives, comprising at least one metallic ion M and for a counter ion X with M being a metallic ion having an atomic number less than or equal to 74, M being a transition metal ion or a post-transition metal ion, or Mg.sup.2+ or Na.sup.+ or Li.sup.+, and X being a counter ion selected from the group consisting of nitrates, nitrites, O.sup.2, and mixtures thereof in an amount calculated to obtain between 0.1 weight % and 5 weight % of said metallic ion M and for counter ion X in weight with respect to the total weight of dry sorbent composition.

10. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0105] FIG. 1 presents a schematic embodiment of a flue gas treatment installation carrying out the flue gas treatment process with the sorbent composition according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0106] According to a first aspect, the present invention is related to a sorbent composition for flue gas treatment installation including an electrostatic precipitator, said sorbent composition comprising calcium-magnesium compound, characterized in that it further comprises an additive or a mixture of additives in an amount comprised between 0.1% and 5%, preferably 0.3% to 3% in weight of the dry composition, said additive or additives containing at least one metallic ion M having an atomic number less than or equal to 74 and is a transition metal ion or a post-transition metal ion, and at least one counter ion X chosen amongst nitrates, nitrites, and their mixture.

[0107] In a preferred embodiment, the calcium-magnesium compound is based on hydrated lime.

[0108] Calcium hydroxide sorbents are manufactured by reacting (or slaking) calcium oxide, CaO or quick lime, with water in a so called hydrator, also called slaking unit. Alternatively, calcium magnesium hydroxide sorbents are manufactured by reacting dolomitic lime (also called dolime) or magnesium lime with water in a hydrator. Alternatively, quick lime and dolomitic lime can be mixed together and slaked with water in a hydrator to provide a mixture of calcium hydroxide and calcium magnesium hydroxide. In the following, the process of manufacturing of the sorbent composition will refer to quick lime but the process of manufacturing is not limited to quick lime as a starting material and dolomitic lime or a combination of dolomitic lime and/or magnesium lime and quick lime can also be used as starting materials.

[0109] The process of manufacturing of the said sorbent composition according to the invention comprises a step of slaking quicklime with a predetermined amount of water to obtain hydrated lime with an predetermined amount of moisture, and is characterized in that it comprises a step of adding an additive or a mixture of additives in an amount calculated to obtain between 0.1% and 5%, preferably between 0.3 and 3.5% of said additive or mixture of additives in weight of the dry sorbent composition, said additive or additives containing at least one metallic ion M having an atomic number less than or equal to 74 and is a transition metal ion or a post-transition transition metal ion, and at least one counter ion X chosen amongst nitrates, nitrites, O.sup.2, and OH and their mixture.

[0110] In an embodiment of the process of manufacturing the said sorbent composition, the predetermined amount of water in the said step of slaking is in a water to lime ratio 2:1 by weight or higher.

[0111] In an embodiment of the process of manufacturing the said sorbent composition, the amount of water in the slaking step can be adapted to obtain a hydrated lime with a moisture less than or equal to 10 wt. %, preferably less than or equal to 5 wt. %, preferably less than or equal to 2 w %, more preferably less than or equal to 1 w % with respect to the total weight of the sorbent composition at a powdery state.

[0112] In another embodiment, the amount of water in the slaking step can be adapted to obtain a hydrated lime with a moisture content comprised between 5 wt. % and 20 wt. %. The amount of water in the slaking step can also be higher such as to obtain a hydrated lime with a moisture content above 20 wt. %, all % being expressed with respect to the total weight of the sorbent composition at a powdery state.

[0113] In an embodiment, the hydrated lime obtained after the slaking step is dried in a further step.

[0114] In an embodiment of the process of manufacturing of the sorbent composition according to the invention, the said additive containing at least one metallic ion M and at least one counter ion X is added as an aqueous solution or as a suspension or as a powder before or during the said step of slaking of calcium oxide or calcium magnesium oxide or a combination thereof.

[0115] In another embodiment of the process of manufacturing of the sorbent composition according to the invention, the said additive or mixture of additives containing at least one metallic ion M and at least one counter ion X is added as aqueous solution or as a suspension or as a powder after the said step of slaking. The said additive or mixture of additives containing at least one metallic ion M and at least one counter ion X is preferably added to calcium hydroxide or calcium magnesium hydroxide before injection in an injection zone of the flue gas treatment installation. Alternatively, the said additive or mixture of additives containing at least one metallic ion M and at least one counter ion X can be added during injection in an injection zone of the flue gas treatment installation, separately from the calcium hydroxide or calcium magnesium hydroxide and upstream the electrostatic precipitator.

[0116] In a preferred embodiment of the process of manufacturing of the sorbent composition, the said step of slaking quicklime is performed in the conditions such as to obtain hydrated lime with a BET specific surface area from nitrogen adsorption of at least 20 m.sup.2/g and a BJH pore volume obtained from nitrogen desorption of at least 0.1 cm.sup.3/g. Various processes are available to the man skilled in the art to obtain an hydrated lime with such properties, and are disclosed for example in documents U.S. Pat. Nos. 6,322,769 and 7,744,678 of the applicant and incorporated by reference.

[0117] In the process of manufacturing the sorbent composition according to the invention, particles of quicklime are advantageously used having a particle size distribution of less than 5 mm, in particular quicklime particles of particle size distribution 0-2 mm.

[0118] Other processes for obtaining hydrated lime with high specific area and/or high pore volume can be found for example in U.S. Pat. No. 5,492,685 wherein an amount of alcohol such methanol or ethanol is added prior and/or the step of slaking quicklime and is removed after drying, in patent DE3620024 wherein sugar is added in the step of slaking for increasing the specific surface area and wherein glycols or amines are added to increase the flowability, in U.S. Pat. Nos. 5,277,837 and 5,705,141 wherein additives such as ethylene glycol, diethylene glycol, tri ethylene glycol, monoethanolamine, diethanolamine, triethanolamine or a combination thereof is added in the step of slaking for increasing the surface area of hydrated lime.

[0119] In the process of manufacturing the sorbent composition, the said additive or mixture of additives containing at least one metallic ion M and at least one counter ion X can be added before the said step of slaking, during the step of slaking or after the step of slaking without substantially changing the BET specific surface area nor the BJH pore volume for pores having a diameter lower than or equal to 1000 of the sorbent composition. Moreover the BET specific surface area and the BJH pore volume of the sorbent composition according to the present invention is substantially the same as for calcium hydroxide sorbent prepared by the known methods such as the one described in U.S. Pat. Nos. 6,322,769 and 7,744,678 incorporated by reference. Therefore, the properties of the sorbent ensuring the efficiency of SO.sub.2 removal are preserved.

[0120] In the said process of manufacturing the sorbent composition according to the invention, if a hydrated lime composition is prepared according to the method described in U.S. Pat. No. 7,744,678, such method comprises a step of adding a quantity of an alkali metal, preferably sodium in an quantity to the quicklime or to the slaking water or to the hydrated lime, sufficient to obtain in the hydrated lime an alkali metal content that is equal to or greater than 0.2% and equal or less than 3.5% by weight based on the total weight of the dry sorbent composition. According to this embodiment, the said additives or mixture of additives containing at least one metallic ion M and at least one counter ion X is further added to the quicklime or to the slaking water or to the hydrated lime with an amount such as to obtain a content in additive or in a mixture of additives containing at least one metallic ion M and at least one counter ion X between 0.1% and 5%, preferably 0.3% to 3% in weight of the dry sorbent composition.

[0121] Various sorbent compositions have been prepared according to the method of the present invention and measurements of the resistivity of dry powders of said sorbent compositions have been carried out in following the procedure outlined by IEEE (Esctcourt, 1984). Basically, a resistivity cell of a determined volume is filled by a dry powder of sorbent composition and the powder is then compacted with a weight such as to obtain a flat surface. An electrode with a guard is placed over the surface of the powder and the resistivity of the powder is measured in an oven under a stream of air comprising 10% of humidity at various temperatures comprised between 150 C. (302 F.) and 300 C. (372.2 F.). The resistivity of comparatives examples have been measured in the same conditions. For each measurement, a maximum resistivity Rmax and a resistivity at 300 C. (372 F.) has been determined. The resistivity measurements are presented herein after:

Example Set A

[0122] Example 1 is a comparative sample of calcium hydroxide sorbent designed for the removal of acid gas pollutants manufactured according to U.S. Pat. No. 6,322,769 B1. No sodium nor additive of general formula MX have been added.

[0123] Example 2 is a comparative sample of a calcium hydroxide sorbent designed for the removal of acid gas pollutants manufactured according to U.S. Pat. No. 7,744,678 B2. This sample comprises 1 wt % of sodium as Na.sub.2CO.sub.3. No further sodium or additive of general formula MX has been added.

[0124] Example 3 is a calcium hydroxide sorbent manufactured according to the present invention using iron nitrate as dopant.

[0125] Table 1 shows the measured resistivity parameters R.sub.max and R.sub.300.

TABLE-US-00001 TABLE 1 Resistivity parameters of calcium hydroxide sorbents doped with sodium and iron salts. R.sub.max R.sub.300 Exam- Compo- Na.sub.2CO.sub.3 Fe(NO.sub.3).sub.3 Cu(NO.sub.3).sub.2 ( ( ple sition (wt. %) (wt. %) (wt. %) cm) cm) Ex. 1 Ca(OH).sub.2 0 0 0 5 E12 3 E12 Ex. 2 Ca(OH).sub.2 + 1 0 0 4 E11 1 E11 Na.sub.2CO.sub.3 Ex. 33 Ca(OH).sub.2 + 0 0.5 0 1 E12 2E10 Fe(NO.sub.3).sub.3

[0126] From Table 1, it is clear that the both the R.sub.max value and the R.sub.300 value of Example 1 are high at and above the preferred range of resistivity values comprised between 10E7 ohms.cm and 2E10 ohms.cm.

[0127] Addition of 1 wt % of sodium in Example 2 reduces the R.sub.max and R.sub.300 values by more than one order of magnitude. Surprisingly the addition of a small amount of iron nitrate at 0.5 wt % reduces the R.sub.max value by nearly one order of magnitude and by nearly two orders of magnitude for R.sub.300. Surprisingly the addition of iron nitrate is more effective than the addition of sodium.

Example Set B

[0128] A set of sorbents was prepared by taking the sorbents manufactured according to U.S. Pat. No. 7,744,678 B2 and adding iron and copper salts according to the method of present invention to said sorbents.

[0129] Example 4 is a sample of a calcium hydroxide sorbent designed for the removal of acid gas pollutants manufactured according to U.S. Pat. No. 7,744,678 B2 wherein an amount of iron nitrate has been added. According to the manufacturing method presented in U.S. Pat. No. 7,744,678 an amount of sodium has been added.

[0130] Example 5 is a sample of a calcium hydroxide sorbent designed for the removal of acid gas pollutants manufactured according to U.S. Pat. No. 7,744,578 B2 wherein an amount of copper nitrate has been added. According to the manufacturing method presented in U.S. Pat. No. 7,744,678 an amount of sodium has been added.

TABLE-US-00002 TABLE 2 Resistivity parameters of calcium hydroxide sorbents doped with sodium, iron and copper salts. R.sub.max R.sub.300 Exam- Compo- Na.sub.2CO.sub.3 Fe(NO.sub.3).sub.3 Cu(NO.sub.3).sub.2 ( ( ple sition (wt. %) (wt. %) (wt. %) cm) cm) Ex. 1 Ca(OH).sub.2 0 0 0 8 E12 3 E12 Ex. 4 Ca(OH).sub.2 + 1 0.5 0 1 E11 1 E10 Na.sub.2CO.sub.3 + Fe(NO.sub.3).sub.3 Ex. 5 Ca(OH).sub.2 + 1 0 0.5 2 E10 2E8 Na.sub.2CO.sub.3 + Cu(NO.sub.3).sub.2

[0131] Table 2 shows that for these sorbents, the addition of an iron nitrate result in resistivity value R.sub.max nearly two orders of magnitude lower than that of the comparative Example 1. The addition of copper nitrate results in nearly three orders of magnitude lower resistivity for R.sub.max and more than in three orders of magnitude resistivity drop of R.sub.300.

Example Set C

[0132] A set of sorbent was prepared by taking the sorbents according to U.S. Pat. No. 7,744,678 and various irons salts have been added to measure the influence of the counter ion on the resistivity of the sorbent.

[0133] Example 4 is a sample of a calcium hydroxide sorbent designed for the removal of acid gas pollutants manufactured according to U.S. Pat. No. 7,744,678 B2 wherein an amount of iron nitrate has been added. According to the manufacturing method presented in U.S. Pat. No. 7,744,678 an amount of sodium has been added.

[0134] Example 6 is a comparative sample of a calcium hydroxide sorbent designed for the removal of acid gas pollutants manufactured according to U.S. Pat. No. 7,744,678 B2 wherein an amount of iron sulfate has been added. According to the manufacturing method presented in U.S. Pat. No. 7,744,678 an amount of sodium has been added.

[0135] Example 7 is a comparative sample of a calcium hydroxide sorbent designed for the removal of acid gas pollutants manufactured according to U.S. Pat. No. 7,744,678 B2 wherein an amount of iron acetate has been added. According to the manufacturing method presented in U.S. Pat. No. 7,744,678 an amount of sodium has been added.

TABLE-US-00003 TABLE 3 Resistivity parameters of calcium hydroxide sorbents using different iron salts. Na.sub.2CO.sub.3 Fe(NO.sub.3).sub.3 Fe.sub.2(SO.sub.4).sub.3 Fe(C.sub.2H.sub.3O.sub.2).sub.2 R.sub.max R.sub.300 Example Composition (wt. %) (wt. %) (wt. %) (wt. %) ( cm) ( cm) Ex. 2 Ca(OH).sub.2 + 1 0 0 0 4 E11 1 E11 Na.sub.2CO.sub.3 Ex. 4 Ca(OH).sub.2 + 1 0.5 0 0 1 E11 1 E10 Na.sub.2CO.sub.3 + Fe(NO.sub.3).sub.3 Ex. 6 Ca(OH).sub.2 + 1 0 0.5 0 2E12 2E12 Na.sub.2CO.sub.3 + Fe.sub.2(SO.sub.4).sub.3 Ex. 7 Ca(OH).sub.2 + 1 0 0 0.5 3E12 4E11 Na.sub.2CO.sub.3 + Fe acetate

[0136] Table 3 shows that the use of iron nitrate results in a resistivity value R.sub.max four times lower than that of comparative Example 2 and one order of magnitude lower for R.sub.300. Surprisingly the use of iron salts of different composition such as sulfate and acetate result in an increase of the resistivity, both for R.sub.max and for R.sub.300 compared to the comparative Example 2. Note that the use of iron sulfate resifts in a resistivity that does not show lower values for R.sub.300 compared to R.sub.max.

Example Set D

[0137] A set of sorbent was prepared by taking the sorbents according to U.S. Pat. No. 7,744,678 and various copper salts have been added to measure the influence of the counter ion on the resistivity of the sorbent.

TABLE-US-00004 TABLE 4 Resistivity parameters of calcium hydroxide sorbents using different copper salts Na.sub.2CO.sub.3 Cu(NO.sub.3).sub.2 CuSO.sub.4 CuCl.sub.2 Cu citrate R.sub.max R.sub.300 Example Composition (wt. %) (wt. %) (wt. %) (wt. %) (w %) ( .Math. cm) ( .Math. cm) Example 2 Ca(OH).sub.2 + 1 0 0 0 0 5 E11 1 E11 Na.sub.2CO.sub.3 Example 5 Ca(OH).sub.2 + 1 0.5 0 0 0 2 E10 2 E8 Na.sub.2CO.sub.3 + Cu(NO.sub.3).sub.2 Example 8 Ca(OH).sub.2 + 1 0 0.5 0 0 2E12 3E11 Na.sub.2CO.sub.3 + Cu(SO).sub.4 Example 9 Ca(OH).sub.2 + 1 0 0 0.5 0 3 E12 6 E11 Na.sub.2CO.sub.3 + CuCl.sub.2 Example 10 Ca(OH).sub.2 + 1 0 0 0 0.5 7E12 2E12 Na.sub.2CO.sub.3 + Cu citrate

[0138] It is clear from Table 4 that surprisingly all salts, except copper nitrate, increase the resistivity of the sorbent respective of the comparative Example 2.

[0139] It is to be mentioned that the examples of sorbent compositions presented herein above are not limitative for the present invention, and other additives in the amounts comprised between 0.1 and 5% in weight of the dry sorbent composition can be used to decrease the resistivity of sorbent compositions destined to be used in flue gas treatment processes using an electrostatic precipitator.

[0140] It is to be mentioned that improvements of particulate matter collection on collecting electrodes of an electrostatic precipitators can be observed with the use of the sorbent according to the present invention.

[0141] According to another aspect, the present invention is related to a flue gas treatment installation. FIG. 1 shows a schematic embodiment of a flue gas treatment installation 100 comprising an electrostatic precipitator 101 arranged downstream a first duct portion 102 arranged downstream an air preheater 103, characterized in that an injection zone 104 is arranged upstream said air preheater 103 and comprises a sorbent inlet 105. The said flue gas treatment installation 100 further comprises a reservoir 106 comprising said sorbent composition S to provide said sorbent composition to the said injection zone through the said sorbent inlet. The hot flue gas FG produced by a boiler 10 is flown through the injection zone wherein the sorbent 5 according to the invention is injected to react with SO.sub.2 and other acidic gases from the flue gas, then the hot flue gas crosses the air preheater through which cold air CA is flown to absorb the heat of the hot flue gas and to be injected as hot air HA in the boiler. Then the flue gas flows through the electrostatic precipitator 101 wherein charged collecting electrodes collects the particulate matter including the sorbent composition according to the invention that has reacted with undesired acidic gases. The flue gas treatment installation described herein is relatively simple and is well adapted for the use of the sorbent composition according to the present invention.

[0142] Preferably the said flue gas treatment installation is used for treating flue gas of a power plant using coal or fuel containing sulfur species or other acid gas precursors.

[0143] It should be understood that the present invention is not limited to the described embodiments and that variations can be applied without going outside of the scope of the appended claims.

[0144] For example, in the preferred embodiment, the installation for flue gas treatment was described with an electrostatic precipitator downstream of an air preheater, said air preheater being connected to said electrostatic precipitator by a duct with an injection zone for injecting a sorbent composition according to the present invention arranged upstream of said air preheater. An alternative within the scope of the present may is comprises a particulate collection device upstream of said preheater,

[0145] Another alternative of the flue gas treatment device according to the present invention comprises in sequence an electrostatic precipitator, a preheater followed by optionality a particulate collection device, before reaching the chimney.

[0146] The particulate collection device can be another electrostatic precipitator or any king of filter, such as a bag house filter.

[0147] In all of those embodiments, the sorbent composition according to the present invention is injected in an injection zone located upstream of said electrostatic precipitator, before or after the preheater, depending on the on-site configuration.