CAPTURE AGENT FOR THE TREATMENT OF FLUE GASES

Abstract

The present invention relates to a capture agent for the treatment of gases, having an active phase that comprises a calcium silicate hydrate of (CaO).sub.x(SiO.sub.2).sub.y(H.sub.2O).sub.z type with a Ca/Si molar ratio between 1.55 and 1.72, preferably between 1.65 and 1,72 and an H.sub.2O/Ca molar ratio between 1 and 1.4, preferably between 1.1 and 1.3, z being between 0.3 and 0.8, the capture agent having a specific surface area greater than 120 m.sup.2/g, preferably greater than 150 m.sup.2/g and particularly preferably greater than 200 m.sup.2/g and a pore volume greater than 0.4 cm.sup.3/g, preferably greater than 0.6 cm.sup.3/g and particularly preferably greater than 0.8 cm.sup.3/g.

Claims

1-12. (canceled)

1. Sorbent for the treatment of gases, having an active phase comprising a calcium silicate hydrate of (CaO).sub.x(SiO.sub.2).sub.y(H.sub.2O), type with a Ca/Si molar ratio of between 1.55 and 1.72, preferably between 1.65 and 1.72, and a H.sub.2O/Ca molar ratio of between 1 and 1.4, preferably between 1.1 and 1.3, z being between 0.3 and 0.8, the sorbent having a specific surface area larger than 150 m.sup.2/g, and preferably larger than 200 m.sup.2/g, with a pore volume greater than 0.6 cm.sup.3/g, and preferably greater than 0.8 cm.sup.3/g.

2. The sorbent as in claim 1, wherein the mean particle size (D50) is less than 1000 m, preferably less than 500 m, and more preferably less than 300 m.

3. The sorbent as in claim 1, further comprising sodium chloride, calcium chloride or iron chloride hydrate in its pores.

4. The sorbent as in claim 1, further comprising a fluidifying agent selected from among monoethanol amine, diethanol-amine, triethanol-amine, monoethylene-glycol, diethylene glycol and triethylene-glycol.

5. Method for preparing a sorbent as in claim 1, wherein the calcium silicate hydrate is obtained by: preparing an aqueous suspension of silica and lime, from colloidal silica, silica fume or diatomaceous earth, the aqueous silica suspension comprising a proportion of 1 to 5%, preferably 2 to 4% of colloidal silica freshly synthesised by causing a sodium silicate solution to react with a dilute acid until a milky suspension of a precipitate of colloidal silica is obtained within a few minutes, before adding the amorphous silica via the silica fume or diatomaceous earth and quicklime to obtain said calcium silicate hydrate; drying under heat.

6. The preparation method as in claim 5, wherein the silica fume or diatomaceous earth or mixture of these ingredients is previously milled to obtain particles having a d50 diameter of less than 30 m.

7. The preparation method as in claim 5, further comprising a step to add a chlorine salt, preferably sodium chloride, calcium chloride or iron chloride.

8. Process for treating gases by means of a sorbent, wherein a sorbent as in claim 1 is placed in contact with the gases to be treated.

9. The treatment process as in claim 8, wherein it is a dry process in which the gases are placed in direct contact with the sorbent.

10. The treatment process as in claim 8, wherein that the gases to be treated pass through an electro-filter or bag filter containing the sorbent.

11. The treatment process as in claim 8, wherein the concentration of SO.sub.2 is measured, as indicator compound, in the gases leaving the electro-filter or bag filter, and wherein the sorbent is replaced when the concentration exceeds a previously-set limit value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0036] It is the aim of the present invention to provide a sorbent based on calcium silicate hydrate (CSH), or on a composition with calcium silicate hydrate in powder form for the treatment of flue gases, and a method for manufacturing this product. The invention also discloses a process to purify flue gases using the sorbent of the present invention.

[0037] Calcium silicate hydrates (CSH) are generally characterized by CaO/SiO.sub.2 and H.sub.2O/CaO molar ratios, and by their structural characteristics such as microstructure (CSH of type , or ), Ca(OH).sub.2 content, water molecule stability, pore volume (PV), pore size, specific surface area (BET) and CO.sub.2 content. Low uptake capacity of CO.sub.2 is a highly desired property insofar as the gases to be purified are generally combustion gases with much higher contents of CO.sub.2 than SO.sub.2 or HCl for example (10% CO.sub.2 compared with 0.2% SO.sub.2 for example).

[0038] Besides, some properties are only obtained under specific synthesis conditions involving T, time, pressure and the additives used.

[0039] To obtain maximum uptake efficiency of SO.sub.2, SO.sub.3, HCl, HF and even of some heavy metals, and optimal stability of the product, it is also generally desired to obtain frost resistance properties in spite of its high content of residual water (3-day test at 20 C.) and optimal flow (measured by the cohesion index at increasing and decreasing speeds in the Granu-Drum by Aptis).

[0040] To reach these characteristics, the particle size of the CSHs as in the invention must not exceed a mean (D50), measured in volume, of 1000 m, preferably 500 m, more preferably 200 m. Particle size is measured by laser diffraction where all particles are considered to be spheres. The apparatus used is the Sympatec HELOS/KR sensor using the Fraunhofer method.

[0041] One particularly advantageous manner for preparing CSH is to replace 2 to 4%, preferably about 3% of silica by freshly prepared colloidal silica. To do so, a dilute acid (H.sub.2SO.sub.4, HCl, . . . ) is reacted with a sodium silicate solution. This way of proceeding is called the amplified method as in the present invention.

[0042] A comparative table between the CSH disclosed in WO 00/48710 and that of the present invention shows the following main differences:

TABLE-US-00001 Ca/Si H.sub.2O/Ca Specific Param. (molar ratio) (molar ratio) Particle size surface area WO 1 to 5 0.1 to 2 0.5 to 30 mm BET > 120 m.sup.2/g 00/48710 1.54 to 5 (preferred) 0.1 to 1 (preferred) 1.54 to 2.5 (+preferred) 0.25 to 1 (+preferred) Present 1.55 < Ca/Si < 1.72 0.1 to 2 <1000 m BET > 120 m.sup.2/g invention preferred 0.1 to 1 preferred preferred 1.65 < Ca/Si < 1.72 0.25 to 1 <500 m >150 m.sup.2/g <300 m PV > 0.4 cm.sup.3/g

[0043] The fresh colloidal silica used in small amount (1 to 5%) in the silica mixture allows to increase the BET up to 200 m.sup.2/g and a pore volume PV>0.5 cm.sup.3/g. The pore volume is measured using the BJH method (barret-Joyner-Halenda).

[0044] Ca solely represents the calcium content that may react with silica. If one of the reagents (lime or silica) contains calcium carbonate that does not take part in hydrothermal synthesis of CSH, this calcium is not taken into consideration for calculating the Ca/Si ratio. This calcium carbonate is assayed by thermogravimetry.

[0045] The highly specific Ca/Si ratios in the CSH gels as in the present invention have the advantage that they release Ca(OH).sub.2 which, in an aqueous medium, ionises to Ca++ and hydroxyl (OH.sup.) ions neutralising the acidic gases.

[0046] It was possible to show that for Ca/Si molar ratios < or =1.72, only CSH is formed. With higher ratios, a mixture of CSH and calcium hydrate is obtained. With a Ca/Si ratio>1.72, CSH is therefore diluted with calcium hydrate and the performance level drops.

[0047] The CSH gels comprise water in three different forms: [0048] 1) capillary contact water between CSH particles: We [0049] 2) water contained in CSH pores: Wp [0050] 3) constituent water of calcium silicate gel: Wg [0051] Total water Wt=We+Wp+Wg.

[0052] When said product is subjected to thermogravimetric analysis, four regions are observed: [0053] 1) from 25 to 150 C., the capillary contact water and water contained in the pores are evaporated [0054] 2) from 350 to 500 C., Ca(OH).sub.2 is dehydrated to CaO and H.sub.2O [0055] 3) from 550 to 800 C., constituent CSH water is released [0056] 4) from 800 to 1000 C., CaCO.sub.3 is decarbonated, possibly having three origins: [0057] a. impurity from amorphous silica [0058] b. quicklime impurity [0059] c. carbonation of CSH and decalcification thereof.

[0060] The uptake of acidic gases (SO.sub.2, SO.sub.3, HCl, HF) by a porous solid only truly performs well when the pores of this solid are partly or totally filled with water and dissolved salts. These gases dissolve in pore water where the calcium hydrate has also dissolved. The acid-base reaction between Ca(OH).sub.2 and the acidic gases occurs in a medium dissolved in the pores, and then the formed gypsum and/or calcium chloride are deposited on the inner surface of the pores.

[0061] In dry calcium hydrates with pore volumes of between 0.08 and 0.2 cm.sup.3/g, water is provided by the flue gases and preferably condenses in pores via capillary effect. In the scenario of the present invention, water is already contained in the pores as from the manufacture of the porous solid and Ca(OH).sub.2 is already dissolved therein ready to react with the acidic gases.

[0062] By adding a chloride salt during CSH synthesis (e.g. sodium chloride, calcium chloride or iron chloride), chlorine forms calcium chloride hydrates in the pores which progressively release crystallization water during contact with the hot gases. They thereby release water that is available for the dissolution of the acidic gases: [0063] CaCl.sub.2.6H.sub.2O stable below 30 C.; [0064] CaCL.sub.2.4H.sub.2O stable from 30 to 45 C.; [0065] CaCl.sub.2.2H.sub.2O stable from 45 to 87 C.
Performance tests showed a most beneficial effect of chlorine in the reagent to treat HCl-depleted gases.

[0066] The following table compares the efficiency of different sorbents tested in an incinerator. The specific surface area (BET-Brunauer-Emmett-Teiler) of the powders was measured in accordance with ISO standard 9277, second Edition, Sep. 1.sup.st 2010. Calculation of pore distribution was based on the step-by-step analysis of the isotherm adsorption branch using the BJH method by Barret, Joyner and Halenda (1951) conventionally used with 77K nitrogen as adsorbent gas. The method is described in DIN standard 66134.

[0067] Sorbent Chemical Reactions

1) Ca(OH).sub.2+SO.sub.2+1/2 6O.sub.2=>CaSO.sub.4+H.sub.2O.

2) (CaO).sub.x(.SiO.sub.2).sub.y.(H.sub.2O).sub.z+x SO.sub.2+x/2 O.sub.2=>CaSO.sub.4+y SiO.sub.2+z H.sub.2O.

1.6 <X/Y<1.72

0.25<Z/X<1

The capture reaction of pollutants such as sulfur oxide by CSH releases silica and CSH constituent water. Only the lime contained in the CSH molecule reacts with the pollutant. CSH therefore has the drawback of containing a larger amount of material that does not participate in the capture reaction of the pollutant, than calcium hydrate. Nevertheless, this drawback is largely offset by the greater reactivity of CSH towards the pollutant on account of its large specific surface area and high pore volume.

TABLE-US-00002 CSH CSH (without fresh (with fresh precipitated precipitated Standard Improved silica) of the silica) of the Sorbents Ca(OH).sub.2 Ca(OH).sub.2 invention invention Access to alkalinity 34% 50% 87% 95% i.e. Ca(OH).sub.2* BET in m.sup.2/g 22 40 >120 >150-(200) PV in cm.sup.3/g 0.08 0.2 >0.4 >0.6 % alkalinity 90% Ca(OH.sub.)2 95% Ca(OH).sub.2 63% Ca(OH).sub.2 63% Ca(OH).sub.2 Kg alkalinity i.e. 34*0.9 = 30.6 kg 50*0.95 = 47.5 kg 87*0.63 = 54.8 kg 95*0.63 = 60 kg effective Ca(OH).sub.2 (i.e. reacting with SO.sub.2) per 100 kg of product *Access to alkalinity is obtained by analysing the sorbent after its exposure to synthetic flue gases containing O.sub.2, N.sub.2, SO.sub.2, HCl and CO.sub.2. The % of Ca(OH).sub.2 derived from a hydrate or from a CSH combined with SO.sub.2 and/or HCl, relative to the total available hydrate, expresses access of SO.sub.2 and HCl polluting gases to the alkalinity of the Ca(OH).sub.2 used. The CSH as in the invention contains more accessible alkalinity per 100 kg of product and therefore generates less waste per kg of captured SO.sub.2; which is a major advantage since disposal costs are lower.

Modes for Synthetizing CSH Milky Slurries

[0068] CSH synthesis may be conducted at atmospheric pressure at about 95 C. for about 3 hours, or at high pressure (between 5 and 10 bars corresponding to saturating vapour temperatures of between 150 and 180 C.). Since the synthesis times are shortened under these conditions (about 30 minutes), synthesis can be carried out in batch mode or continuous mode in a thermostat-controlled reactor of coil type or simply insulated against heat loss.

[0069] Numerous syntheses performed in laboratory and on semi-industrial scale (from 0.5 m.sup.3 to 25 m.sup.3) show that the surface properties of CSH are not dependent on the surface properties of the amorphous silicas used for the production thereof; in contrast, the addition of a small amount of freshly synthesised colloidal silica (about 3% of total silica) has a considerable impact on surface quality.

[0070] The synthesis of colloidal silica is performed by reacting dilute sulfuric acid with sodium silicate in solution. The colloidal silica is left to stand a few minutes until it precipitates and forms a milky suspension. Amorphous silica (diatomaceous earth, silica fume, . . . ) and quicklime are then added to obtain synthesis of the CSH suspension.

Drying Modes of CSH Milky Slurries as in the Invention

[0071] The purpose of drying is to reduce the humidity percentage of the sorbent from about 78% free water to 5-20% free water, to obtain a powder sorbent having adequate flow properties.

Drying of CSH Milky Slurry at Atmospheric Pressure and Temperature Below 500 C. (to Prevent Deterioration of CSH Hydration)

[0072] Calories may be obtained by burning a fossil fuel or by recovering lost calories (lime rotary furnaces without preheater, cement kilns, etc.) via a heat exchanger.

[0073] The calories can be conveyed by: [0074] 1) CO.sub.2-depleted air (to prevent carbonatation of the CSH gel); [0075] 2) nitrogen (costly solution); [0076] 3) water vapour which has the advantage of having twice the specific heat of air and therefore capable of conveying twice more calories at the same temperature.

Drying the CSH Milky Slurry Under Pressure

[0077] When CSH is produced under pressure, e.g. at 150 C. and at a pressure higher than 5 bar, by expansion at atmospheric pressure, the CSH free water evaporates when the paste is spray dried.

Measuring the Performance of the CSH of the Invention

[0078] Essentially three systems are distinguished to measure the performance of a sorbent: [0079] 1) Breakthrough method on 10 g of granulate powder or 250 mg of fine powder. This method is performed on a dry powder and therefore does not reflect industrial reality. In this method, a breakthrough time is defined which is the time required for the concentration of pollutants leaving the bed to be equal to the concentration of incoming pollutants. This breakthrough time is the image of sorbent performance. [0080] 2) In-flight uptake method
A powder sorbent is poured into a vertical cylinder a few metres high. Recomposed flue gases pass through the cylinder and meet the sorbent in counter flow. Reacted sorbent deposits at the bottom of the cylinder. A filter collects the fine powder particles entrained by the flue gases. This method has the drawback that uniform distribution of the powder throughout the entire cross-section of the cylinder is not certain. [0081] 3) Reduced-scale simulation of the operation of an industrial bag filter used to depollute flue gases
This system was chosen to test the performance of the sorbents of the present invention since it comes closest to true conditions of use. [0082] The bag filter has a filtering surface of 35 m.sup.2, i.e. 12 rows of 5 bags per row. One bag therefore has a lateral surface area of 0.58 m.sup.2, a perimeter of 0.58 m and length of 1 m. As in any industrial filter, the sorbent is continuously directed into the bags and the twelve rows of bags are regularly pulsed with compressed air, row after row, with an adjustable cycle time of 30 to 60 minutes. The filter rate of flue gases is 1 m/minute and the flow of recomposed flue gases may be adjusted depending on filtering temperature to take heed of this filter speed.

EXAMPLES

[0083] CSH milk was synthetized in a laboratory PARR reactor. CSH was synthetized for three hours at different temperatures. For amplified CSH, 3% fresh colloidal silica was added during synthesis.

[0084] The variation in structural characteristics depending on temperature of CSH synthesis, accelerated and non-accelerated, are given in the table below.

[0085] Diatomite from Cekesa (Spain) was used having a specific surface area of 103 m.sup.2/g and pore volume of 0.29 cm.sup.3/g containing 72% SiO.sub.2; 27.2% CaCO.sub.3 and 0.8% (Al.sub.2O.sub.3+MgO).

[0086] Examples 1 to 6 were conducted with a Ca/Si ratio of 1.7; Examples 7 to 9 with a Ca/Si ratio of 1.55 and Examples 10 to 12 with a Ca/Si ratio of 1.72. Tests 7 to 12 were conducted in the region of temperatures that were considered to be the most favourable in tests 1 to 6.

TABLE-US-00003 BET PV BET PV (m.sup.2/g) (cc/g) (m.sup.2/g) (cc/g) T non-amplified with amplified with fresh Example ( C.) Ca/Si fresh colloidal silica colloidal silica 1 95 1.7 120 0.42 180 0.6 2 120 1.7 130 0.40 185 0.6 3 140 1.7 160 0.50 200 0.9 4 150 1.7 198 0.64 220 1.1 5 160 1.7 170 0.52 200 0.9 6 180 1.7 130 0.40 180 0.6 7 140 1.55 142 0.48 190 0.9 8 150 1.55 150 0.59 210 1.0 9 160 1.55 138 0.50 185 0.8 10 140 1.72 160 0.45 192 0.7 11 150 1.72 195 0.51 212 0.9 12 160 1.72 165 0.47 205 0.8

[0087] It is noted that in the region of 150 C., the specific surface areas and pore volume are the largest and hence the most favourable for depolluting flue gases.

[0088] Performance Comparison

[0089] Comparison between pollution uptake performances by reduced-scale simulated operation of an industrial bag filter used to depollute flue gases

[0090] The performance of the CSH as in the invention was compared with Ca(OH).sub.2 products. The CSH synthesis conditions were those conducted at 150 C. and at 5 bars for three hours. The CSH milky slurry was spray dried in an atomizer without direct contact with fumes from the hot-air generator operating on natural gas. There remained 15% residual water after drying. The indication kg of acid means total weight of SO.sub.2 and HCl.

[0091] Different flue gas compositions were tested and the results are given in the following table.

[0092] Flue gas composition No 1: [0093] 1000 mg/Nm.sup.3SO.sub.2 and 1000 mg/Nm.sup.3HCl at 160 C., 10% H.sub.2O, 5% CO.sub.2

TABLE-US-00004 % uptake of acid in flue gases 2 kg sorbent/ 3 kg sorbent/ 4 kg sorbent/ Type of sorbent kg acid kg acid kg acid CSH of the SO.sub.2 = 74%/ SO.sub.2 = 83%/ SO2 = 90%/ invention HCl = 96% HCl = 99% HCl = 99.5% CSH amplified SO.sub.2 = 78%/ SO.sub.2 = 86%/ SO.sub.2 = 76%/ with fresh silica HCl = 98% HCl = 100% HCl = 100% Ca(OH).sub.2 SO.sub.2 = 62%/ SO.sub.2 = 70%/ SO.sub.2 = 76%/ BET = 40 m.sup.2/g & HCl = 93% HCl = 96% HCl = 98% PV = 0.2 cm.sup.3/g Ca(OH)2 SO.sub.2 = 38%/ SO.sub.2 = 43%/ SO.sub.2 = 51%/ BET = 22 m2/g& HCl = 60% HCl = 70% HCl = 74% PV = 0.1 cm3/g

[0094] Flue gas composition No 2: [0095] 250 mg/Nm.sup.3SO.sub.2 and 1000 mg/Nm.sup.3 HCl at 160 C., 10% H.sub.2O, 5% CO.sub.2

TABLE-US-00005 % uptake of acid in flue gases 2 kg sorbent/ 3 kg sorbent/ 4 kg sorbent/ Type of sorbent kg acid kg acid kg acid CSH of the SO.sub.2 = 86%/ SO.sub.2 = 92%/ SO.sub.2 = 99%/ invention HCl = 91% HCl = 96% HCl = 99% CSH amplified SO.sub.2 = 90%/ SO.sub.2 = 94%/ SO.sub.2 = 100%/ with fresh silica HCl = 94% HCl = 98% HCl = 100% Ca(OH).sub.2 SO.sub.2 = 74%/ SO.sub.2 = 83%/ SO.sub.2 = 94%/ BET = 40 m.sup.2/g & HCl = 83% HCl = 94% HCl = 98% PV = 0.2 cm.sup.3/g Ca(OH).sub.2 SO.sub.2 = 64%/ SO.sub.2 = 68%/ SO.sub.2 = 69%/ BET = 22 m.sup.2/g & HCl = 60% HCl = 5% HCl = 69% PV = 0.1 cm.sup.3/g

[0096] Flue gas composition No 3: [0097] 1000 mg/Nm.sup.3SO.sub.2 and 0 mg/Nm.sup.3HCl at 160 C., 10% H.sub.2O, 5% CO.sub.2

TABLE-US-00006 % uptake of acid in flue gases 2 kg sorbent/ 3 kg sorbent/ 1 kg sorbent/ Type of sorbent kg acid kg acid kg acic CSH of the SO.sub.2 = 50% SO.sub.2 = 52% SO.sub.2 = 60% invention CSH amplified with SO.sub.2 = 55% SO.sub.2 = 60% SO.sub.2 = 65% fresh silica Ca(OH).sub.2 SO.sub.2 = 42% SO.sub.2 = 50% SO.sub.2 = 55% BET = 40 m.sup.2/g & PV = 0.2 cm.sup.3/g

[0098] Comparing the performance tests shows the advantage of the CSH as in the invention, in particular when it is amplified with fresh silica, compared to the two Ca(OH).sub.2 versions used for comparison in the comparative tests.