In a first aspect, the present invention is related to a process for manufacturing a sorbent suitable for a use in a circulating dry scrubber device. In a second aspect, the present invention is related to a premix for use in said process for manufacturing a sorbent suitable for a use in a circulating dry scrubber device. In a third aspect, the present invention is related to a sorbent suitable for a use in a circulating dry scrubber device. In a fourth aspect, the present invention is related to the use of said sorbent in a circulating dry scrubber for a flue gas treatment process. In a fifth aspect, the present invention is related to a process for flue gas treatment using said sorbent. In a sixth aspect, the present invention is related to the use of a premix in a process of flue gas treatment wherein the premix is slaked in a hydrator upstream of a circulating dry scrubber device.

By the term "hydrator" in the meaning of the present invention, it is meant a conventional hydrator single or multi-stage or a mixer.

State of the art

The combustion flue gases contain substances considered harmful to the environment and flue gases treatment is more and more often performed in order to remove or neutralize those harmful substances and pollutants. Various processes are used for flue gas treatment, including the scrubbing technology. A first type of such technology is the wet scrubber technology using wet scrubbers which work generally via the contact of target compounds or particulate matter with a scrubbing liquid which can be water for dust or solutions or suspensions of reagents for targeting specific compounds. A second type of scrubbing technology includes the dry scrubbing systems and the semi-dry scrubbing systems, also called semi-wet scrubbing systems. Those systems in comparison to the wet scrubbers do not saturate the treated flue gas with moisture. In some cases, no moisture is added, while in other cases only the amount of moisture that can be evaporated in the flue gas without condensing is added. The main use of dry or semi-dry scrubbing devices is related to the capture and removal of acid gases such as sulfur oxides and hydrochloric acid primarily from combustion sources. In the present disclosure, the terms "circulating dry scrubber device" or "circulating dry scrubber installation" or "circulating dry scrubber systems" refers to either circulating dry scrubber systems or circulating semi-dry scrubber systems.

Circulating dry scrubber (CDS) technology was first developed for S0 ₂ removal in coal-fired power plants. Today it is also used in flue gas treatment for industrial furnaces and boilers that use biomass, industrial or municipal waste as fuels. The CDS process is based on the recirculation of residues collected from particulate control device, comprising unreacted sorbent, reaction products and fly ash.

A CDS unit generally comprises a reactor for receiving flue gases and sorbents which are generally calcium-based sorbents. The reactor is followed by a particulate control device which filters the solids (also called residues and comprising unreacted sorbent, reaction products and fly ash) from the gas released. These solids are partially recycled into the reactor afterwards through a recycling loop. Some fresh sorbent can be periodically or continuously added to the reactor, before or after. In most cases water is injected into the reactor and/or onto the solids for temperature control, to improve the pollutants removal performances and to re-activate the residues. Some CDS facilities may comprise a hydrator (also called slaking unit) and use quicklime CaO that is hydrated prior to entering the CDS process. Some other CDS facilities do not comprise any hydrator and the fresh sorbent injected is hydrated lime.

In a first way to handle a CDS process, the residues are wetted before reinjection in the reactor. In a second way to handle a CDS process, water is directly injected in the reactor.

Unfortunately, even if the CDS technology is effective in terms of removal of pollutants, limitations exist regarding the amount of water which can be added, while water addition remains a key factor for removal of these pollutants. Indeed, it is known that higher capture levels of acid gases can be achieved by increasing the flue gases moisture, while keeping in mind that going below the dew point may cause corrosion issues especially in the reactor.

In the case wherein the residues are wetted before reinjection in the reactor, the maximum water content relative to the mass of dry recirculated residue observed at commercial scale is 10 weight %, more often between 2 and 7 weight %. Above 10% of water content, sticky behavior and clogging phenomena occur on duct walls both in the recycling loop and in the reactor, bringing operational instability up to a complete stop of the flue gas cleaning unit.

In the case wherein water is directly injected in the reactor, even though water is not carried by the recycled material, clogging phenomena appearing in the reactor are still observed, thereby impacting negatively the flue gases treatment process.

There is also a need to provide a sorbent or a process allowing the operation of a CDS process wherein the water content can be increased without impacting negatively the circulating dry scrubbing process. It is particularly desirable to at least reduce the sticky behavior and the clogging phenomena of the recycled materials on duct walls, in the recycling loop and in the reactor.

Summary of the invention

According to a first aspect of the invention, a process for manufacturing a sorbent suitable for a use in a circulating dry scrubber device comprises the steps of: - providing quicklime and water in an hydrator;

- slaking said quicklime via a "non-wet route" in the hydrator;

- collecting a lime based sorbent at an exit of the hydrator.

The process according to the present invention is further characterized in that it comprises a further step of adding at least one compound comprising silicon or aluminum or a combination thereof before or during said slaking step at a molar ratio between silicon or aluminum or a combination thereof and the calcium provided to said hydrator equal to or below 0.2 and equal to or above 0.02.

As it can be seen, the process according to the present invention, by slaking the quicklime in presence at least one compound comprising silicon or aluminum or a combination thereof added before or during said slaking step, allows the manufacturing of a sorbent able to provide a residue in a circulating dry scrubber device which is able to carry more water than prior art residues while keeping a good flowability of such residue in the CDS process, thereby preventing sticking in pipes, ducts or other parts of the circulating dry scrubber device. The sorbent according to the invention is able to release its carried water at low temperature, typically at the temperature of the circulating dry scrubber device between 50°C and 350°C. The molar ratio between silicon or aluminum or a combination thereof and the calcium provided to said hydrator being equal to or below 0.2 and equal to or above 0.02 ensure a good compromise between having a benefit from the addition of the compound comprising silicon or aluminum or the combination thereof without increasing too much the material production costs.

The sorbent manufactured in the process according to the present invention provides a residue in a CDS process that presents good flowability properties. Indeed, it is believed that the calcium silicate and/or aluminates formed during the process of manufacturing of such lime based sorbent have a layered structure (like phylosilicates) able to absorb the moisture for carrying it and therefore preventing the free moisture to surround the particles (moisture trapped in porosity), which is usually the phenomenon explaining the poor flowability of wet powders while still being able to release its carried water at low temperature as above mentioned.

Further, as said before, when used in a circulating dry scrubber device, the sorbent manufactured according to the present invention creates residues for which it is also believed that the residues form granulates in presence of water creating an apparent coarser particle size distribution therefore improving the flowability. The presence of silicon or aluminum or a combination thereof in the sorbent therefore ensure a good flowability even with high moistures such as more than 10 weight % in the residue circulating in a circulating dry scrubber device.

With higher water content in the sorbent carrying water, the performance of the flue gas treatment device is thought to be improved significantly because: adding water is believed helping conditioning the gas reducing in particular the reaction temperature and increasing relative humidity;

the added water is believed helping rejuvenating the residues bringing remaining Ca(OH) ₂ available for reaction again;

the added water is believed creating local favorable conditions around the solid in the reactor to boost the activity of the sorbent, the reaction products (the added water may help converting carbonated forms of Ca into reacted species with targeted acid gas removal (SO _x , HCI, HF...) and even possibly the fly ash;

If the same quantity of water can be brought in the reactor on a lower quantity of recycled materials, downsizing the conditioning mixer and all related equipment in particular the conveying devices (screws, airslides...) could be possible at the benefit of investment costs but also utilities and maintenances costs to run a CDS process, which will be reduced as less material would circulate. By quicklime, it is meant within the meaning of the present invention a mineral solid material for which the chemical composition is mainly calcium oxide, CaO. Quicklime is usually obtained by calcination of limestone (mainly CaC0 ₃ ). The quicklime suitable according to the present invention comprises at least 70 weight %, preferably 80 weight %, preferably 90 weight % CaO, preferably at least 92 weight %, more preferably at least 94 weight % CaO with respect to the total weight of quicklime, as measured with the sugar method (available lime according to standard EN 459).

Quicklime may also contain impurities including for example, sulfur oxide, S0 ₃ , silica, Si0 ₂ or even alumina, Al ₂ 0 ₃ . The impurities are expressed herein under their oxide form, but of course, they might appear under different phases. Within the meaning of the present invention, the impurities may be present at a level from 0.5 to 15 weight %, preferably at most 10 weight %, preferably at most 5 weight %, preferably at most 2 weight %, more preferably at most 1 weight % impurities with respect to the total weight of quicklime.

Quicklime contains generally also residual limestone CaC0 ₃ , called unburned residues. The quicklime suitable according to the present invention may comprise CaC0 ₃ at an amount, comprised in the range of 0.5 to 20 weight %, preferably equal to or lower than 10 weight %, preferably lower or equal to 5 weight %, more preferably equal to or lower than 3 weight %, most preferably equal to or lower than 1 weight % with respect to the total weight of the quicklime.

The quicklime suitable according to the present invention may further comprise MgO at an amount, expressed under MgO form, comprised in the range of 0.5 to 10 weight %, preferably equal to or lower than 5 weight %, more preferably equal to or lower than 3 weight %, most preferably equal to or lower than 1 weight % with respect to the total weight of the quicklime.

In addition, the quicklime suitable according to the present invention may comprise Ca(OH) ₂ , resulting from the reaction of CaO with ambient moisture during handling and storage periods, at an amount comprised in the range of 0.5 to 10 weight %, preferably equal to or lower than 5 weight , more preferably equal to or lower than 2 weight %, most preferably equal to or lower than 1 weight % with respect to the total weight of the quicklime as measured by the loss on ignition method at 550°C.

Typically, to form slaked lime, also sometimes called hydrate or hydrated lime, quicklime is provided in presence of water. Calcium oxide from the quicklime reacts quickly with water to form calcium di-hydroxide Ca(OH) ₂ , under the form of slaked lime or hydrated lime, in a reaction called hydration or slaking reaction which is very exothermic. In the following, calcium di- hydroxide will be simply called calcium hydroxide.

The slaked lime may therefore contain the same impurities than the quicklime from which it is produced.

The slaked lime may also comprise calcium oxide, which might not have been entirely hydrated during the slaking step, or calcium carbonate CaC0 ₃ . The calcium carbonate can be originated from the original limestone (unburned) from which said slaked lime is obtained (via calcium oxide) or being the result of a partial carbonation reaction of slaked lime through the contact with an atmosphere containing C0 ₂ .The amount of CaCC>3 in the slaked lime can be equal to or lower than 20 weight %, preferably equal or lower than 10 weight %, preferably equal to or lower than 5 weight %, more preferably equal to or lower than 3 weight %, and most preferably equal or lower than 1 weight %, with respect to the total weight of the slaked lime according to the present invention.

In the process of manufacturing according to the invention, the step of slaking is a slaking mode via a "non-wet route" which designates slaking modes via a dry route, via a quasi-dry route or via semi-dry route. In a non-wet route, a hydrated lime product may be obtained with a moisture comprised between 0.5 and 35 weight %, as measured on the raw hydrate taken at the outlet of the hydrator. The expression "non-wet route" excludes the two slaking modes via a wet route and via a putty route. Each of these slaking routes is defined herein after.

In a dry hydration of quicklime, meaning a slaking mode "via a dry route", the amount of added water corresponds to what is required for the slaking reaction of quicklime, increased with the amount lost as steam because of the exothermic nature of the reaction, typically, the double of the stoichiometric quantity of water is added to the hydrator. Upon exiting the hydrator, the obtained product is powdery and generally comprises both at maximum 2% of residual non-hydrated CaO and at most 2 % of moisture, with preferably a maximum of 1 % of moisture. It may be packaged and sold directly, after optional steps for controlling grain size.

When some installations have a hydrator connected to the CDS unit, those hydrators may produce a hydrated lime with a moisture inferior or equal to 4% but eventually with more remaining quicklime. This remaining quicklime is hydrated afterwards during its passage in the CDS unit. The percentage of moisture is measured under atmospheric pressure by measuring the weight loss during heating at 150°C of 20 g of lime product until the weight of the lime product does not vary of more than 2 mg for at least 20 seconds. In a quasi-dry hydration of quicklime, being another slaking mode, the hydration may be achieved with a larger excess of water according to WO 97/14650. In this case, the obtained hydrate contains moisture of the order of 15 to 35% by mass when exiting the hydrator. Because of this humidity, the hydrated lime requires a drying and de-agglomeration step before storage and transport.

In a semi-dry hydration of quicklime, one referred to any amount of water added for the slaking reaction between the dry hydration of quicklime and the quasi-dry hydration of quicklime. In a slaking mode « by a wet route », the amount of added water is in very large excess as compared with the amount strictly required for the slaking reaction. A « lime milk » is then obtained, i.e. an aqueous suspension of slaked lime particles. In a slaking mode "via a putty route", the amount of water used for the slaking reaction is a little lower than the amount of water used for the slaking "by the wet route" and the obtained product is pasty (lime putty).

Advantageously, in the process of manufacturing according to the invention, said compound comprising silicon or aluminum or a combination thereof is provided at least partially in a solution or in a suspension and added to said water and/or said compound comprising silicon or aluminum or a combination thereof is provided at least partially under solid form and added to said quicklime.

In an embodiment of the process of manufacturing according to the invention said compound comprising silicon, aluminum or a combination thereof comprises at least 4 weight % of silicon or aluminum or of a combination thereof, preferably at least 7 weight % of silicon or aluminum or of a combination thereof, preferably at least 10 weight % of silicon or aluminum or of a combination thereof, preferably at most 53 weight % of silicon or aluminum or of a combination thereof, preferably at most 40 weight %, preferably at most 30 weight % of silicon or aluminum or of a combination thereof with respect to the total weight of said dried compound.

In the context of the present invention, the amounts of silicon and aluminum in the compound comprising silicon, aluminum or a combination thereof can be measured by XRF on a sample dried at 150°C until constant weight with a temperature ramp of 5°C/min under a flow of nitrogen and are expressed in weight % under their elemental form in the said compound, but aluminum and silicon are not in their elemental form in the said compound, the result of the XRF being corrected by the TGA measure. In an embodiment of the process of manufacturing according to the invention said quicklime and said compound comprising silicon or aluminum or a combination thereof are provided in a premix containing at least 50 weight % of quicklime, preferably at least 70 weight % of quicklime, preferably at least 80 weight % of quicklime, preferably at least 90 weight % of quicklime, preferably at least 98,5 weight % of quicklime and at least 0.7 weight % of silicon or aluminum or a combination thereof, preferably at least 0.8 weight % of silicon or aluminum or a combination thereof, preferably at least 0.9 weight % of silicon or aluminum or a combination thereof and at most 10 weight % of silicon or aluminum or a combination thereof preferably at most 7 weight % of silicon or aluminum or a combination thereof preferably at most 5 weight of silicon or aluminum or a combination thereof preferably at most 3 weight % of silicon or aluminum or a combination thereof with respect to the total weight of said premix under a dry form.

The amounts of silicon and aluminum in the premix can be measured by XRF on a dried sample at 150°C until constant weight and are expressed in weight % under their elemental form in the said premix, but silicon and aluminum are not in their elemental form in the said premix, the result of the XRF being corrected by the TGA measure.

In an embodiment of the process of manufacturing according to the invention that said compound comprising silicon or aluminum or a combination thereof further comprises sodium.

In an embodiment of the process of manufacturing according to the invention, a further step of adding a second compound comprising sodium is performed.

Preferably, the second compound comprising sodium is soluble in water, such as for example sodium hydroxide, sodium carbonate, sodium hydrogenocarbonate, sodium nitrate, sodium phosphate, sodium persulfate or sodium acetate. Preferably, the second compound has a solubility in water at 20°C superior or equal to 50 g/dm ³ , preferably superior or equal to 100 g/dm ³ , preferably superior or equal to 200 g/dm ³ , preferably superior or equal to 300 g/dm ³ , preferably superior or equal to 500 g/dm ³ .

Advantageously, in the process of manufacturing according to the invention, the said second compound comprising sodium is provided at least partially in a solution or in a suspension and added to the said water and/or the said second compound comprising sodium is provided at least partially under solid form and added to the said quicklime.

The said second compound comprising sodium may be added in the process before or during or after the step of slaking, whereas the said compound comprising silicon or aluminum or a combination thereof must be added before or during said slaking step.

In an embodiment of the process of manufacturing according to the invention the molar ratio between silicon or aluminum or the combination thereof and sodium is equal to or above 0.5 and of maximum 20. Preferably, the molar ratio between silicon or aluminum or the combination thereof and sodium is of maximum 10, preferably of maximum 5, more preferably of maximum 2.

In an embodiment of the process of manufacturing according to the invention, a step of drying said lime based sorbent or classifying said lime based sorbent or grinding said or milling said sorbent or a combination thereof is performed.

In an embodiment of the process of manufacturing according to the invention the said compound comprising silicon, aluminum or a combination thereof is a pozzolan material. In an embodiment of the process of manufacturing according to the invention, the each of the said at least one compound comprising silicon, aluminum or a combination thereof is selected from the group comprising silicates, silicates of sodium, aluminosilicates, aluminosilicates of sodium, metasilicates, metasilicates of sodium, aluminates, aluminates of sodium, fly ash, diatomite, kieselguhr, diatomaceous earth, precipitated silica, silica fume, blast furnace slag, metakaolin, perlites, paper ash, rice husk ash, silicic acid, amorph silica and tobermorite.

In an embodiment of the process of manufacturing according to the invention, the each of the said at least one compound comprising silicon, aluminum or a combination thereof is selected from the group consisting of silicates, silicates of sodium, aluminosilicates, aluminosilicates of sodium, metasilicates, metasilicates of sodium, aluminates, aluminates of sodium, fly ash, diatomite, kieselguhr, diatomaceous earth, precipitated silica, silica fume, blast furnace slag, metakaolin, perlites, paper ash, rice husk ash, silicic acid, amorph silica and tobermorite

Advantageously, in the process according to the present invention, the residence time of quicklime being slaked inside the hydrator is comprised between 5 and 45 minutes, preferably between 20 and 40 minutes and more preferably between 25 and 35 minutes.

Other embodiments of the process according to the first aspect of the present invention are mentioned in the appended claims

According to a second aspect, the present invention is related to a premix for a process for manufacturing a sorbent suitable for use in a circulating dry scrubber device, said premix comprising quicklime and at least a compound comprising silicon, aluminum or a combination thereof with a molar ratio between the silicon or aluminum or the combination thereof and the sodium is of at least 0.5 and of maximum 20.

As it can be seen, the premix according to the present invention is providing quicklime and at least one compound comprising silicon or aluminum or a combination thereof to be slaked for example on site, just before using it for example in a CDS process. The premix according to the present invention ensures the presence of said at least one compound comprising silicon or aluminum or a combination thereof when slaking the quicklime and allows the manufacturing of a sorbent able to provide a residue in a circulating dry scrubber device which is able to carry more water than prior art residues while keeping a good flowability of such residue in the CDS process, thereby preventing sticking in pipes, ducts or other parts of the circulating dry scrubber device.

The sorbent resulting from hydration of the premix according to the invention is able to release its carried water at low temperature, typically at the temperature of the circulating dry scrubber device between 50°C and 350°C. The molar ratio between silicon or aluminum or a combination thereof and the calcium provided to said hydrator being equal to or below 0.2 and equal to or above 0.02 ensure a good compromise between having a benefit from the addition of the compound comprising silicon or aluminum or the combination thereof without increasing too much the material production costs.

Indeed, for installations comprising a circulating dry scrubber device and a hydrator on the same site, it can be advantageous to provide a premix comprising quicklime and at least a compound comprising silicon or aluminum or a combination thereof. Such a premix can be provided to the hydrator for slaking in the process of manufacturing the sorbent according to the present invention. In this case, fresh sorbent according to the invention can be manufactured on site just before its use in the flue gas treatment process.

In an embodiment of the premix according to the invention, the said at least a compound comprising silicon, aluminum or a combination thereof further comprises sodium and/or the said premix further comprises a second compound comprising sodium.

In an embodiment, the premix according to the invention comprises at least 50 weight % of quicklime preferably at least 70 weight % of quicklime, preferably at least 80 weight % of quicklime, preferably at least 90 weight % of quicklime, preferably at least 98,5 weight % of quicklime and at least 0.7 weight % of silicon or aluminum or a combination thereof, preferably at least 0.8 weight % of silicon or aluminum or a combination thereof, preferably at least 0.9 weight % of silicon or aluminum or a combination thereof and at most 10 weight % of silicon or aluminum or a combination thereof preferably at most 7 weight % of silicon or aluminum or a combination thereof preferably at most 5 weight of silicon or aluminum or a combination thereof preferably at most 3 weight % of silicon or aluminum or a combination thereof with respect to the total weight of said premix under a dry form. The amounts of silicon and aluminum in the premix can be measured by XRF on a sample dried at 150°C until constant weight with a temperature ramp of 5°C/min under a flow of nitrogen and are expressed in weight % under their elemental form in the said premix, the result of the XRF being corrected by the TGA measure, but silicon and aluminum are not in their elemental form in the said premix.

In an embodiment of the premix according to the present invention, the molar ratio between the silicon or aluminum or the combination thereof and the sodium is of at least 0.5 and of maximum 20.

Preferably, the molar ratio between silicon or aluminum or the combination thereof and sodium is of maximum 10, preferably of maximum 5, more preferably of maximum 2.

Other embodiments of the premix according to the second aspect of the present invention are mentioned in the appended claims

According to a third aspect of the present invention, a sorbent suitable for use in a circulating dry scrubber device comprises at least 50 weight % of Ca(OH) ₂ , preferably at least 70 weight % of Ca(OH) ₂ , at least 80 weight % of Ca(OH) ₂ , at least 90 weight % of Ca(OH) ₂ , at least 95 weight % of Ca(OH) ₂ ,and at least 0.5 weight % of silicon or aluminum or a combination thereof, preferably at least 0.6 weight % of silicon or aluminum or a combination thereof, preferably at least 0.7 weight % of silicon or aluminum or a combination thereof, preferably at least 0.8 weight % of silicon or aluminum or a combination thereof, and at most 8 weight % of silicon or aluminum or a combination thereof, preferably at most 5 weight % of silicon or aluminum or a combination thereof, preferably at most 3 weight % of silicon or aluminum or a combination thereof, preferably at most 2 weight % of silicon or aluminum or a combination thereof expressed under its elemental form with respect to the total weight of said sorbent under a dry form.

Said sorbent according to the present invention is further characterized in that it comprises 1 to 12 weight % of bound water with respect to the total weight of said sorbent under a dry form.

The amounts of silicon and aluminum in the sorbent can be measured by on a sample dried at 150°C until constant weight with a temperature ramp of 5°C/min under a flow of nitrogen and are expressed in weight % under their elemental form in the said sorbent, the result of the XRF being corrected by the TGA measure but silicon and aluminum are not in their elemental form in the said sorbent.

The bound water can be measured by thermogravimetric analysis, by introducing in an oven or a furnace a sample of sorbent according to the present invention, the sample being first dried until constant weight at 150°C to remove the moisture and then heated until 350°C until constant weight to remove the bound water, typically with a temperature ramp of 5°C/min under a flow of nitrogen. The loss of weight of the dried sample (i.e. between 150 and 350 °C) is related to the percentage of bound water in the sample.

In an embodiment of the sorbent according to the invention, at least 1 mol % of calcium is neither under the form of Ca(OH) ₂ nor CaC0 ₃ nor CaO, preferably at least 2 mol % of calcium is neither under the form of Ca(OH) ₂ nor CaC0 ₃ nor CaO, preferably at least 2.5 mol % of calcium is neither under the form of Ca(OH) ₂ nor CaC0 ₃ nor CaO, preferably at least 3 mol % of calcium is neither under the form of Ca(OH) ₂ nor CaC0 ₃ nor CaO and at most 40 mol % of calcium is neither under the form of Ca(OH) ₂ nor CaC0 ₃ nor CaO, preferably at most 25 mol % of calcium is neither under the form of Ca(OH) ₂ nor CaC0 ₃ nor CaO, preferably at most 15 mol % of calcium is neither under the form of Ca(OH) ₂ nor CaC0 ₃ nor CaO, preferably at most 6 mol % of calcium is neither under the form of Ca(OH) ₂ nor CaC0 ₃ nor CaO. In an embodiment of the sorbent according to the invention, the molar ratio between silicon or aluminum or the combination thereof and calcium is of at least 0.02 and of maximum 0.2.

The molar ratio between silicon or aluminum or the combination thereof relative to calcium in the sorbent of the invention is low relative to prior art sorbents for S0 ₂ removal which are disclosed in the prior art documents "Highly active absorbent for 50 ₂ removal prepared from coal fly ash", Tsuchiai et al., Ind. Eng. Chem. Res. 1995, 34, 1404-1411 and "Fly ash recycle in dry scrubbing" Jozewicz et al. Environmental Progess vol. 5, No 4, November 1986 219-224. Those prior art documents are related to activation of fly ash to be able to use and recycle this by-product. In those prior art documents, the aim is to obtain high yields of calcium aluminate or calcium silicate which are being known to capture S0 ₂ . In those prior art documents, the calcium aluminate or calcium silicate is formed with a very high specific surface area as the silicates or aluminates have been activated in order to create porosity. Calcium aluminate and calcium silicate are compounds presenting a sheet structure inside which the S0 ₂ can be inserted and thereby captured. However, the production of calcium silicates or calcium aluminates as main compounds for S0 ₂ removal requires several steps and high concentrations of silicon or aluminum based compounds as starting materials which is not cost effective.

In an embodiment, the sorbent according to the present invention further comprises at least 0.1 weight % of sodium expressed under its equivalent Na ₂ 0 oxide form, preferably at least 0.3 weight % of sodium expressed under its equivalent Na ₂ 0 oxide form, preferably at least 0.5 weight % of sodium expressed under its equivalent Na ₂ 0 oxide form, preferably at least 0.7 weight % of sodium expressed under its equivalent Na ₂ 0 oxide form, preferably at most 15 weight % of sodium expressed under its equivalent Na ₂ 0 oxide form, preferably at most 7 weight % of sodium expressed under its equivalent Na ₂ 0 oxide form, preferably at most 5 weight % of sodium expressed under its equivalent Na ₂ 0 oxide form, preferably at most 2 weight % of sodium expressed under its equivalent Na ₂ 0 oxide form with respect to the total weight of said sorbent under a dry form.

In an embodiment of the sorbent according to the invention, the molar ratio between silicon or aluminum or a combination thereof and sodium is of at least 0.5 and of maximum 20. Preferably, the molar ratio between silicon or aluminum or the combination thereof and sodium is of maximum 10, preferably of maximum 5, more preferably of maximum 2.

Preferably, the sorbent according to the present invention when comprising sodium, has a BET specific surface area comprised of at least 3 m ² /g and of maximum 25 m ² /g measured by manometry with adsorption of nitrogen after degasing in vacuo at 190°C for at least 2 hours and calculated according the multipoint BET method as described in the ISO 9277/2010E standard.

Advantageously, the sorbent according to the present invention, when comprising sodium has a total BJH pore volume of at least 0.01 cmVg and of maximum 0.15 cm ³ /g determined manometry with adsorption of nitrogen after degasing in vacuo at 190°C for at least 2 hours and calculated according the multipoint BJH method as described in the ISO 9277/2010E standard.

In another embodiment of the sorbent according to the present invention, the mean particle size d ₅₀ ranges between 5 and 20 μιη. The notation dx means a particle size distribution of a sample of particles wherein x % of the particles have a size under a certain value expressed in μιτι. The particle size distribution can be measured by laser granulometry of a sample in methanol after sonication. In another embodiment of the sorbent according to the present invention, the particle size d ₉₀ ranges from 15 μηι and 1mm, preferably from 15 μιτι to 100 μιη when measured after sonication.

Other embodiments of the sorbent according to the third aspect of the present invention are mentioned in the appended claims

According to a fourth aspect, the present invention is related to the use of a sorbent such as disclosed herein or obtained from a process for manufacturing a sorbent according to the present invention in a circulating dry scrubber for a flue gas treatment process.

Other uses according to the fourth aspect of the present invention are mentioned in the appended claims

According to a fifth aspect, the present invention is related to a process of flue gas treatment using a circulating dry scrubber device characterized in that it comprises a step of recirculating a sorbent such as disclosed herein or obtained from a process for manufacturing a sorbent according to the present invention into the said circulating dry scrubber.

In the process of flue gas treatment using a circulating dry scrubber device, the sorbent particles will enter in contact with flue gas and form a suspension of reacted sorbent particles, unreacted sorbent particles and eventually other by-products. The suspension is filtered by a particulate control device. The flue gas depleted in pollutants is directed to the chimney whereas residues R formed by reacted sorbent particles, unreacted sorbent particles and eventually other by-products are redirected and recycled in the CDS device for another cycle. The said residues can be recirculated and recycled several times. Some fresh sorbent can also be introduced at any time in the CDS installation. Water is added to reactivate the reacted sorbent.

With the sorbent according to the present invention, it is foreseen to add water on said residues circulating in the circulating dry scrubber (CDS) device such as to have a water content relative to the dry mass of residues of at least 5 weight %, preferably at least 7 weight %, preferably at least 10 weight %, preferably at least 12 weight %, preferably at least 15 weight %.

In function of the ratio of sulfur oxide to HCI in the flue gas treated in a circulating dry scrubber device, the amount of water added on the residues circulating in the circulating dry scrubber device can be adapted.

For ratios of sulfur oxide relative to HCI superior to 20, the amount of HCI is generally low and it is possible to add water on the residues circulating in the circulating dry scrubber device such as to have a water content relative to the dry mass which can go up to maximum 20 weight % without risk of clogging of residues in the circulating dry scrubber device.

For ratios of sulfur dioxide relative to HCI inferior to 20, the amount of HCI is generally considered as high and may cause more problem of clogging of the residues in the circulating dry scrubber device. Therefore for such ratios of sulfur oxide to HCI inferior to 20, the water on the residues circulating in the circulating dry scrubber device can be such as the water content relative to the dry mass of residues is only of at least 2 weight %.

In an embodiment, the process of flue gas treatment according to the invention comprises a step of introduction in the said circulating dry scrubber device of a sorbent according to the present invention or obtained from a process of manufacturing such as disclosed herein.

Other embodiments of the process according to the fifth aspect of the present invention are mentioned in the appended claims

In a sixth aspect, the present invention is related to the use of a premix such as disclosed herein in a flue gas treatment process wherein the premix is slaked in a hydrator upstream of a circulating dry scrubber device.

Other uses according to the sixth aspect of the present invention are mentioned in the appended claims. Other characteristics and advantages of the present invention will be derived from the non-limitative following description, and by making reference to the drawings and the examples.

Brief description of the drawings

Figure 1 shows a schematic embodiment of a circulating dry scrubber installation used in a process of flue gas treatment according to the present invention.

Figure 2 shows an alternative schematic embodiment of a circulating dry scrubber installation used in a process of flue gas treatment according to the present invention.

Figure 3 shows a XRD pattern of a sample of a sorbent according to example 4 of the present invention.

Figure 4 shows a XRD pattern of a sample of a metasilicate in the same measurement condition than for the sample of sorbent of example 4 according to the XRF measurement of figure 3.

Figure 5a presents the Si cartography of particles from a sample of sorbent according to an embodiment of the invention. Figure 5b presents the calcium cartography of particles from the same sample.

Figure 6 presents a termogravimetric analysis (TGA) of the percentage of loss of weight of three samples of sorbents according to the present invention and of a hydrated lime as comparative example in function of the temperature.

Figure 7 shows two curves of the ratio of the content of S0 ₂ in a treated gas flow in a CDS pilot unit relative to the content of S0 ₂ set up initially in the synthetic gas flow in function of a molar ratio of calcium under any form relative to sulfur. Figure 8 presents the evolution of temperature at the top of the reactor in function of time for a sorbent according to the present invention and for a hydrated lime as comparative example.

In the drawings, the same reference numbers have been allocated to the same or analog element.

Description of the invention

The figure 1 shows a schematic embodiment of a circulating dry scrubber for flue gas treatment. A circulating dry scrubber installation 100 (also referred as circulating dry scrubber device) comprises a loop through which residues and flue gas are circulated, said loop comprising: a reactor 102 comprising:

o a flue gas inlet 102a;

o a treated flue gas and residues outlet 102c; and o a residues inlet 102b;

a particulate control device 103 comprising

o a treated flue gas and residues inlet 103a connected by a first duct 201 to the said treated flue gas and residue outlet 102c of said reactor 102;

o a residues outlet 103b connected by a second duct 202 to the said residues inlet 101b of the said reactor 102 o a treated flue gas outlet 103c connected to a chimney 104; o a separation means (not illustrated) between a zone for accommodating the suspension of treated flue gas and residues and the treated flue gas outlet 103c, said zone communicating with said treated flue gas and residues inlet 103 and the second residues outlet 103b. The separation means separating the suspension of treated gas and residues in a treated gas depleted of residues and the residues for allowing the particulate control device to filter the treated gas from residues and

a fresh sorbent inlet 101a which can be arranged at any location on the loop formed by the reactor 102, the first duct 201, the zone of the particulate control device 103 and the second duct 202.

In the non-limitative embodiment of fig. 1, the fresh sorbent inlet 101a is arranged on the reactor 102.

In a process for flue gas treatment using such a circulating dry scrubber device, a fresh sorbent FS is injected in the said loop, a flue gas FG containing pollutants flows through the reactor 102 entering by the said flue gas inlet 102a such as to form a suspension of residues in the said flue gas. The residues R comprises reacted sorbent particles, unreacted sorbent particles and eventually other by-products. The said suspension TFG + R is filtered by separation means of the said particulate control device 103 from which the said flue gases depleted in pollutants TFG are directed to the said chimney 104 whereas residues R are redirected and recycled to the said reactor 102 for another cycle. The said residues can be recirculated and recycled several times. Some fresh sorbent can also be introduced at any time in the CDS installation through the fresh sorbent inlet 101a. The figure 2 shows a schematic embodiment of another embodiment of a circulating dry scrubber for flue gas treatment which further comprises a mixing zone 101. For example, a circulating dry scrubber installation 100 (also referred as circulating dry scrubber device) can comprise: - a mixing zone 101 comprising:

o a fresh sorbent inlet 101a;

o a first residues inlet 101b; and

o a first residues outlet 101c;

a reactor 102 comprising:

o a flue gas inlet 102a; o a second residues inlet 102b connected by a first duct 301 with the said first residues outlet 101c of the mixing zone; and

o a treated flue gas and residues outlet 102c; and a particulate control device 103 comprising:

o a treated flue gas and residues inlet 103a connected by a second duct 302 to the said treated flue gas and residues outlet 102c of said reactor 102,

o a second residues outlet 103b connected by a third duct

303 to the said first residues inlet 101b of the mixing zone; and

o a treated flue gas outlet 103c connected to a chimney 104 o a separation means (not illustrated) between a zone for accommodating the suspension of treated flue gas and residues and the treated flue gas outlet 103c, said zone communicating with said treated flue gas and residues inlet 103a and the second residues outlet 103b. The separation means separating the suspension of treated gas and residues in a treated gas depleted of residues and the residues for allowing the particulate control device to filter the treated gas from residues.

In this embodiment of the CDS installation, the mixing zone 101, the first duct 301, the reactor 102, the second duct 302, the zone for accommodating the suspension of treated flue gas and residues of the particulate control device 103 and the third duct 303 form a loop through which residues can be recirculated and recycled several times. Some fresh sorbent can be introduced at any time in the CDS installation through the fresh sorbent inlet 101a.

In a process for flue gas treatment using such a circulating dry scrubber device, a fresh sorbent FS is injected to the said sorbent mixing zone 101. The fresh sorbent FS can be mixed with residues already present in the loop and then sent to the said reactor 102. A flue gas FG containing pollutants flows through the reactor 102 entering by the said flue gas inlet 102a such as to form a suspension of residues in the said flue gas. The residues R comprises reacted sorbent particles, unreacted sorbent particles and eventually other by-products. The said suspension TFG + R is filtered by the separation means of the said particulate control device 103 from which the said flue gases depleted in pollutants TFG are directed to the said chimney 104 whereas the said residues R are redirected to the said mixing zone 101 to be recycled and to be injected again in the reactor for another cycle. The rate of injection of sorbent and of residues is generally adapted in function of the size of the CDS device and of the flow of flue gas to be treated and the amount of pollutants to remove from the flue gas. Two important factors are the stoichiometric ratio between fresh sorbent and S0 ₂ contained in flue gas, and a predetermined recycling ratio : injection rate of residues/injection fresh sorbent.

In a process of flue gas treatment using a circulating dry scrubber device according to the present invention, the fresh sorbent introduced in the CDS installation is a lime based sorbent characterized in that it comprises at least 50 weight % of Ca(OH) ₂ , preferably at least 70 weight % of Ca(OH) ₂ , at least 80 weight % of Ca(OH) ₂ , at least 90 weight % of Ca(OH) ₂ , at least 95 weight % of Ca(OH) ₂ , and at least 0.5 weight % of silicon or aluminum or a combination thereof, preferably at least 0.6 weight % of silicon or aluminum or a combination thereof, preferably at least 0.7 weight % of silicon or aluminum or a combination thereof, preferably at least 0.8 weight % of silicon or aluminum or a combination thereof, and at most 8 weight % of silicon or aluminum or a combination thereof, preferably at most 5 weight % of silicon or aluminum or a combination thereof, preferably at most 3 weight % of silicon or aluminum or a combination thereof, preferably at most 2 weight % of silicon or aluminum or a combination thereof as well as from 1 to 12 weight % of bound water with respect to the total weight of said sorbent under a dry form. The amounts of silicon and aluminum in the sorbent can be measured by X F on a sample dried at 150°C until constant weight with a temperature ramp of 5°C/min under a flow of nitrogen and are expressed in weight % under their elemental form in the said sorbent, the result of the XRF being corrected by the TGA measure, but silicon and aluminum are not in their elemental form in the said sorbent.

Preferably, the said sorbent comprises from 1 to 40 mol % of calcium which is neither under the form of Ca(OH) ₂ nor CaC0 ₃ nor CaO.

In the said sorbent, the molar ratio between silicon or aluminum or the combination thereof and calcium is of at least 0.02 and of maximum 0.2.

The sorbent according to the present invention is able to provide a residue which is able to carry more water than prior art residues while keeping a good flowability of such residue in the CDS process, thereby preventing sticking in pipes, ducts or other parts of the circulating dry scrubber device. The sorbent according to the invention is able to release its water at low temperature, typically at the temperature of the circulating dry scrubber device between 50°C and 350°C.

The said sorbent is obtained by a process of manufacturing according to the invention comprising the steps of: providing quicklime and water in a hydrator;

- slaking said quicklime in the hydrator; and

collecting a lime based sorbent at an exit of the hydrator.

The said process of manufacturing being characterized in that it further comprises a step of adding at least a compound comprising silicon or aluminum or a combination thereof before or during said slaking step with a molar ratio between silicon or aluminum or the combination thereof and calcium is of at least 0.02 and of maximum 0.2. It is essential that the said step of slaking is performed "via a non-wet route" such as disclosed herein above.

Preferably, in the process of manufacturing of the sorbent of the invention, the said compound comprising silicon or aluminum or a combination thereof can be provided at least partially in a solution or in a suspension in said water which is used for the step of slaking and/or the said compound comprising silicon or aluminum or a combination thereof can be provided at least partially under solid form and added to said quicklime.

Preferably, in the process of manufacturing of the sorbent according to the invention, said compound comprising silicon, aluminum or a combination thereof comprises at least 4 weight % of silicon or aluminum or of a combination thereof with respect to the total weight of said compound. The amounts of silicon and aluminum in the compound comprising silicon, aluminum or a combination thereof can be measured by XRF on a sample dried at 150°C until constant weight with a temperature ramp of 5°C/min under a flow of nitrogen and are expressed in weight % under their elemental form in the said compound, the result of the XRF being corrected by the TGA measure, but aluminum and silicon are not in their elemental form in the said compound. For installations comprising a circulating dry scrubber device and a hydrator on the same site, it can be advantageous to provide a premix comprising quicklime and at least a compound comprising silicon or aluminum or a combination thereof with a molar ratio between silicon or aluminum or the combination thereof and calcium is of at least 0.02 and of maximum 0.2.

Such a premix can be provided to the hydrator for slaking in the process of manufacturing the sorbent according to the present invention. In this case, fresh sorbent according to the invention can be manufactured on site just before its use in the flue gas treatment process. The premix can be introduced into a hydrator, for example in a single stage hydrator and hydrated with water with an amount of water leading to moisture of the raw hydrate ranging between 0.5 and 35 weight %, preferably at least 5 weight % and most preferably at least 10 weight %, particularly at most 25 weight % and most particularly at most 15 weight % with respect to the total weight of said raw hydrate. The water/solid ratio can be varied depending on the targeted moisture of the sorbent at the outlet of the hydrator.

Preferably, the said premix comprises at least 50 weight % of quicklime, preferably at least 70 weight % of quicklime, more preferably at least 80 weight % of quicklime, preferably more than 85%, preferably more than 90% of quicklime and at least 0.7 weight % and at most 10 weight % of silicon, aluminum, or a combination thereof with respect to the total weight of said premix under a dry form.

The amounts of silicon and aluminum in the premix can be measured by XRF and are expressed in weight % on a sample dried at 150°C until constant weight with a temperature ramp of 5°C/min under a flow of nitrogen under their elemental form in the said premix, the result of the XRF being corrected by the TGA measure, but silicon and aluminum are not in their elemental form in the said premix.

For installations comprising a circulating dry scrubber device without any hydrator on the same site, the sorbent according to the present invention is manufactured at another site according to the process of manufacturing of the present invention and is provided for example as a ready-to-use sorbent for use in the flue gases treatment process according to the invention.

The raw lime based sorbent coming out of the hydrator can be optionally deagglomerated and/or milled (without being required) and/or dried before being used in a circulating dry scrubber device (also called CDS unit). Deagglomeration could be performed using a soft mill, typically a cage mill used only as a mill in this case and not for the drying of the sorbent. The sorbent according to the present invention can also be optionally classified with an air classifier.

The coarse fraction from the air classifier can be either separated and valorized independently from the fine fraction, or milled and blended with the fine fraction.

There might be some drying during the deagglomeration and classification steps and some percentages of moisture may be lost.

Therefore, the final product (the sorbent) has a moisture content between 0.5 and 25 weight %, preferably at least 5 weight % and most preferably at least 10 weight %, particularly at most 20 weight % and most particularly at most 15 weight % with respect to the total weight of said sorbent.

Such moisture content can be measured under atmospheric pressure by measuring the weight loss during heating at 150°C with a temperature ramp of 5°C/min under a flow of nitrogen of said sorbent until the weight of said sorbent does not vary of more than 0,1 weight % for at least 20 seconds

In the process of manufacturing of the sorbent, the molar ratio between the silicon or aluminum or the combination thereof relative to the calcium is ranging from 0.02 to 0.2, preferably between 0.02 and 0.10, and most preferably between 0.02 and 0.05. Such ratios ensure a good compromise between having a benefit from the addition of the compound comprising silicon or aluminum or the combination thereof without increasing too much the material production costs. From the targeted molar ratio of silicon, aluminum or the combination thereof relative to the calcium in the sorbent, the amount of compound comprising silicon, aluminum or the combination thereof to be blended with the quicklime can be calculated. Depending on the molar ratio between the silicon or aluminum the combination thereof relative to the calcium used in the process of nufacturing of the sorbent, and depending on the compound comprising icon, aluminum or the combination thereof, the sorbent may contain:

- at least 50 weight % of Ca(OH) ₂ , preferably at least 55 weight % and preferably 92 weight % or less, more preferably 90 weight % or less of Ca(OH) ₂ determined by thermogravimetric analysis between 350°C and 600°C with a temperature ramp of 5°C/min under a flow of nitrogen;

- at least 1 weight % but maximum 10 weight %, preferably 8% or less, more preferably 5% or less of silicon, aluminum or a of combination thereof, determined by X F on a sample dried at 150°C until constant weight with a temperature ramp of 5°C/min under a flow of nitrogen (X ray fluoroscopy) analysis with respect to the total weight of said sorbent under a dry form, the result of the XRF being corrected by the TGA measure;

- some calcium which is neither under the form of Ca(OH) ₂ nor CaC0 ₃ , the amount of which, expressed by default in its oxide equivalent form CaO, ranging between 1 to 40 mol % and calculated by the formula : (mol total Ca - mol Ca(OH) ₂ - mol CaC0 ₃ ) x 100 /mol total Ca, wherein the mol total Ca is measured by XRF on a dried sample at 150°C until constant weight, the mol Ca(OH) ₂ is measured by TGA between 350°C and 600°C with a temperature ramp of 5°C/min under a flow of nitrogen, and the mol CaC0 ₃ is measured by TGA between 600°C and 900°C with a temperature ramp of 5°C/min under a flow of nitrogen;

- 0.5 weight % of bound water, preferably 10 weight % or less, such bound water being released between 150°C and 350°C typically with respect to the total weight of said sorbent under a dry form. - The rest being CaC0 ₃ or other impurities.

The percentage of Ca which is neither in the form of Ca(OH) ₂ , nor CaC0 ₃ increases with the initial molar ratio between the silicon or aluminum or the combination thereof relative to the calcium used in the process of manufacturing, for example in presence of silicate or metasilicate or aluminate or a combination thereof, and is attributed to the formation of Ca silicate or Ca aluminate or a combination thereof from the reaction between the quicklime and the silicate or metasilicate or aluminates or a combination thereof. The formation of these phases is favored when large amounts of compound comprising silicon or aluminum or a combination thereof are used.

Depending on the conditions used such as the time of hydration, the amount of water provided in the step of slaking, the origin of quicklime, the nature of the compound comprising silicon, aluminum or a combination thereof, some unreacted compound comprising silicon or aluminum or a combination thereof and some intermediate reaction products may remain in the final sorbent product.

The sorbent has preferably a d ₅₀ between 5 and 20 μιη and a dgo between 15 and 100 μητι (when measured with sonication).

The sorbent obtained by the process of manufacturing according to the invention may contain large soft agglomerates that can be broken by sonication.

The sorbent according to the present invention provides a residue in a CDS process that presents good flowability properties. For the sorbents comprising silicon, or aluminum or a combination thereof, it is believed that the calcium silicate and/or aluminates formed during the process of manufacturing of such lime based sorbent have a layered structure (like phylosilicates) able to absorb the moisture and therefore preventing the free moisture to surround the particles (moisture trapped in porosity), which is usually the phenomenon explaining the poor flowability of wet powders.

In a CDS process, it is also believed that the residues form granulates in presence of water creating an apparent coarser particle size distribution therefore improving the flowability.

The presence of Si or Al or a combination thereof in the sorbent could therefore ensure a good flowability even with high moistures also called carried water such as more than 10 weight % in the residue circulating in a circulating dry scrubber device with respect to the total weight of said sorbent under a dry form.

The said compound comprising silicon, aluminum or a combination thereof is a pozzolan material. Preferably each of the said at least one compound comprising silicon, aluminum or a combination thereof is selected the group comprising silicates, silicates of sodium, aluminosilicates, aluminosilicates of sodium, metasilicates, metasilicates of sodium, aluminates, aluminates of sodium, fly ash, diatomite, kieselguhr, diatomaceous earth, precipitated silica, silica fume, blast furnace slag, metakaolin, perlites, paper ash, rice husk ash, silicic acid, amorph silica and tobermorite.

In an embodiment, the sorbent further comprises at least 0,1, preferably at least 0.3 to 15 weight % of sodium expressed under its equivalent Na ₂ 0 oxide form with respect to the total weight of said sorbent under a dry form.

Preferably, in the sorbent, the molar ratio between silicon or aluminum or a combination thereof and sodium is of at least 0.5 and of maximum 20.

Such a sorbent may be produced from a process of manufacturing according to the invention comprising the steps of: providing quicklime and water in a hydrator; slaking said quicklime via a "non-wet route" in the hydrator; and - collecting a lime based sorbent at an exit of the hydrator;

The said process of manufacturing being characterized in that it further comprises a step of adding at least a compound comprising silicon or aluminum or a combination thereof before or during said slaking step with a molar ratio between silicon or aluminum or the combination thereof and calcium is of at least 0.02 and of maximum 0.2, and wherein the said compound further comprises sodium.

Alternatively, a first compound comprises silicon or aluminum or a combination thereof and a second compound comprising sodium is added in the process. When a second compound comprising sodium is added in the process, such second compound comprising sodium can be added before or during the step of slaking but also after the step of slaking in a further step of mixing. Preferably, the said second compound comprising sodium is hydrosoluble and can be selected amongst sodium hydroxide, sodium carbonate, sodium hydrogenocarbonate, sodium nitrate, sodium phosphate, sodium persulfate or sodium acetate. Preferably, the second compound at 20°C has a solubility in water superior or equal to 50 g/dm ³ , preferably superior or equal to 100 g/dm ³ , preferably superior or equal to 200 g/dm ³ , preferably superior or equal to 300 g/dm ³ , preferably superior or equal to 500 g/dm ³ .

Preferably, said second compound comprising sodium may be provided at least partially in a solution or in a suspension and added to the said water and/or said second compound comprising sodium may be provided at least under solid form and added to the said quicklime.

Preferably, the molar ratio between silicon or aluminum or the combination thereof relative to sodium is above 0,5 and of maximum 20. In function of the molar ratio between the silicon or aluminum the combination thereof relative to the calcium used in the process of nufacturing, the sorbent may contain :

- at least 50 weight % of Ca(OH) ₂ , preferably at least 55 weight % and preferably 92 weight % or less, more preferably 90 weight % or less of Ca(OH) ₂ determined by thermogravimetric analysis between 350°C and 600°C with a temperature ramp of 5°C/min under a flow of nitrogen;

- at least 1 weight % but maximum 10 weight %, preferably 8% or less, more preferably 5% or less of silicon, aluminum or a of combination thereof, determined by XRF on a sample dried at 150°C until constant weight with a temperature ramp of 5°C/min under a flow of nitrogen (X ray fluoroscopy) analysis with respect to the total weight of said sorbent under a dry form, the result of the XRF being corrected by the TGA measure;

- at least 0,3 weight % and 15 weight % or less of sodium expressed in Na ₂ 0, determined by XRF analysis on a sample dried at 150°C until constant weight with a temperature ramp of 5°C/min under a flow of nitrogen with respect to the total weight of said sorbent under a dry form, the result of the XRF being corrected by the TGA measure;

- some calcium which is neither under the form of Ca(OH)2 nor CaC03, the amount of which, expressed by default in its oxide equivalent form CaO, ranging between 1 to 40 mol % and calculated by the formula : (mol total Ca - mol Ca(OH) ₂ - mol CaC0 ₃ )/mol total Ca, wherein the mol total Ca is measured by XRF on a dried sample at 150°C until constant weight, the mol Ca(OH) ₂ is measured by TGA between 350°C and 600°C with a temperature ramp of 5°C/min under a flow of nitrogen, and the mol CaC0 ₃ is measured by TGA between 600°C and 900°C with a temperature ramp of 5°C/min under a flow of nitrogen;

- 0.5 weight % of bound water, preferably 10% or less, such bound water being released between 150°C and 350°C typically with respect to the total weight of said sorbent under a dry form.

- The rest being CaC03 and/or other impurities.

The percentage of calcium which is neither in the form of Ca(OH) ₂ , nor CaC0 ₃ increases with the initial molar ratio between the silicon or aluminum or the combination thereof relative to the calcium used in the process of manufacturing, for example in presence of silicate or metasilicate or aluminate and a second compound of sodium or in presence of sodium silicate or sodium metasilicate or sodium aluminate or a combination thereof, and is attributed to the formation of Ca silicate or calcium aluminate or calcium and sodium silicate or calcium and sodium aluminate or a combination thereof from the reaction between the quicklime and the silicate or metasilicate or aluminate and a second compound of sodium or between the quicklime and the sodium silicate or sodium metasicliate or sodium aluminate or a combination thereof. The formation of these phases is favored when large amounts of compound comprising silicon or aluminum or a combination thereof are used. This reaction may produce in a first step NaOH and finally Na ₂ C0 ₃ by consuming the CaC0 ₃ initially present as unburnt in the quicklime. The reaction is uncomplete depending on the conditions used. Unreacted compound comprising silicon or aluminum or a combination thereof, unreacted compound comprising sodium and intermediate reaction products (NaOH for example) may remain in the final sorbent product.

The sorbent comprising sodium according to an embodiment of the invention has a specific surface area calculated according to the BET method as mentioned before comprised between 3 and 25 m ² /g and a total pore volume calculated according to the BJH method ranging between 0.01 and 0.15 cm ³ /g- The sorbent has preferably a d ₅₀ between 5 and 20 μιη and a d ₉₀ between 15 and 100 μηη (when measured after sonication).

The sorbent obtained by the process of manufacturing according to the invention may contain large soft agglomerates that can be broken by sonication.

The sorbent according to the present invention presents good flowability properties. For the sorbents comprising silicon and/or aluminum, it is believed that the calcium and sodium silicate and/or calcium and sodium aluminate formed during the process of manufacturing of such sorbent have a layered structure (like phylosilicates) able to absorb the moisture and transform it in carried water and therefore to prevent the free moisture to surround the particles (moisture trapped in porosity), which is usually the phenomenon explaining the poor flowability of wet powders.

The presence of a compound comprising silicon and or aluminum in the product could therefore ensure a good flowability even with high moistures. On the other hand, the presence of Na may be the reason explaining the good activity of the product for S0 ₂ removal in spite of very low specific surface area and pore volumes.

Each of the said at least one compound comprising silicon, aluminum or a combination thereof is selected from the group comprising silicates, silicates of sodium, aluminosilicates, aluminosilicates of sodium, metasilicates, metasilicates of sodium, aluminates, aluminates of sodium, fly ash, diatomite, kieselguhr, diatomaceous earth, precipitated silica, silica fume, blast furnace slag, metakaolin, perlites, paper ash, rice husk ash, silicic acid, amorphous silica and tobermorite.

In an embodiment of the invention, more particularly if the said compound comprising silicon, aluminum or a combination thereof does not comprise sodium, a second compound comprising sodium can be added in the process of manufacturing the sorbent, such a second compound can be selected from sodium hydroxide, sodium carbonate, sodium hydrogenocarbonate, sodium nitrate, sodium phosphate, sodium persulfate or sodium acetate. Preferably, the second compound at 20°C has a solubility in water superior or equal to 50 g/dm ³ , preferably superior or equal to 100 g/dm ³ , preferably superior or equal to 200 g/dm ³ , preferably superior or equal to 300 g/dm ³ , preferably superior or equal to 500 g/dm ³ .

In a non-limitative example of the process of manufacturing of a sorbent according to the present invention, a compound comprising silicon and sodium is used, namely sodium metasilicate pentahydrated Na ₂ Si0 ₃ .5H20 corresponding to 28 weight % Si0 ₂ , 29 weight % Na ₂ 0 and 43% of water.

For installations comprising a circulating dry scrubber device and a hydrator on the same site, it can be advantageous to provide a premix comprising quicklime and at least a compound comprising silicon or aluminum or a combination thereof and possibly sodium or a first compound comprising silicon or aluminum or a combination thereof and a second compound comprising sodium. Such a premix can be provided to the hydrator for slaking in the process of manufacturing the sorbent according to the present invention. In this case, fresh sorbent according to the invention can be manufactured on site just before its use in the flue gas treatment process. The premix can be introduced into a hydrator, for example in a single stage hydrator and hydrated with water with an amount of water leading to carried moisture of the raw hydrate ranging between 2 and 30 weight %, preferably between 5 and 25 weight % and most preferably between 10 and 15 weight % with respect to the total weight of said raw hydrate. The water/solid ratio can be varied depending on the targeted moisture of the product at the outlet of the hydrator.

Preferably, the said premix comprises at least 50 weight % of quicklime, preferably at least 70 weight % of quicklime, more preferably at least 80 weight % of quicklime and at least 0.7 weight % and at most 10 weight % of silicon, aluminum, or a combination thereof with respect to the total weight of said premix under a dry form.

Preferably the said premix further comprises a compound comprising sodium or the compound comprising silicon or aluminum or a combination thereof further comprises sodium.

Preferably, the molar ratio between the silicon or the aluminum or the combination thereof relative to sodium is comprised between 0.5 and 20.

For installations comprising a circulating dry scrubber device without any hydrator on the same site, the sorbent according to the present invention is manufactured at another site according to the process of manufacturing of the present invention and is provided for use in the flue gases treatment process according to the invention.

Examples Process for flue gas treatment using a circulating dry scrubbing device

Comparatives samples of hydrated lime and samples of the sorbent according to the present invention have been tested separately in a CDS pilot unit. The comparative samples of hydrated lime have been produced by a slaking mode in a dry route as defined above, in which a milled quicklime is hydrated in a single stage hydrator producing a raw hydrate with a moisture below 2 % when exiting the hydrator, followed by a classification step giving a coarse fraction and a natural fine fraction. The coarse fraction from this classification is milled with a ball mill and joined with the natural fine fraction in the finished product silo. The CDS pilot unit comprises three main units connected together: a reactor, a filter means and a mixing zone. The reactor is a Venturi reactor and comprises a vertical tube forming an inner cylinder (~7 m long, 4 cm diameter) which is externally enveloped by a concentric tube for the upper half forming the external cylinder. A synthetic gas flow containing acid gas (N ₂ , 0 ₂ , H ₂ 0, C0 ₂ , S0 ₂ ) (20-30 Nm ³ /h) enters the reactor from the bottom of the inner cylinder, goes up and, reaching the top, comes down in the external cylinder and then enters a Fabric Filter. The temperature of the synthetic gas flow is set at 130°C. The injection of fresh hydrated lime and recycled material takes place at the bottom of the reactor. The range of injection rates are respectively 0 to 200 g/h for the fresh sorbent and 0 to 2000 g/h for the recycled material. Those solids particles are entrained by the gas flow to the fabric filter. The fabric filter (filter means) separates the residues formed by the freshly converted hydrated lime and the recycled material from the treated gas.

The solid residues are then sent to a hopper before conditioning and reinjected in the system via a Conditioning Drum (mixing zone). In the conditioning drum, also called ribbon mixer, a given quantity of water is thoroughly mixed with the recycled material. The water content carried by the recycled material can vary from 0.1 weight % up to 25 weight % with respect to the total weight of the sorbent under a dry form.

Table 1 presents four premix compositions and the compositions of the starting materials for preparing those premix compositions. All the premix compositions of table 1 are prepared starting from quicklime and from a compound comprising silicon and sodium, which is Na ₂ Si0 ₃ .5H ₂ 0.

Table 1.-

Premix 1 Premix 2 Premix 3 Premix 4

Quicklime Quicklime Quicklime Quicklime

Quicklime source

1 2 3 3

Quicklime Available CaO in

quicklime (weight 93.2 92.9 93.0 93.0 %)

Si and Na source Na metasi icate pentahydrated (Na ₂ Si0 ₃ .5H ₂ 0)

Si source Weight % Si in Si

13.2 13.2 13.2 13.2 and Na source Theoretical molar

0.03 0.03 0.05 0.20 ratio Si/Ca

Weight %

89.7 89.7 84.0 56.7 quicklime

Weight % Si and

10.3 10.3 16.0 43.3 Na source

Weight % CaO* 83.6 83.4 78.1 52.7

Weight % Si0 ₂ * 2.9 2.9 4.5 12.1

Composition

Weight % Na ₂ 0* 3.0 3.0 4.6 12.6 Premix

Weight %

others*(unburned

(CaC0 ₃ ), water in 10.5 10.7 12.8 22.6 metasilicate,

impurities...)

Weight % Si* 1.3 1.3 2.1 5.7

Weight % Na* 2.2 2.2 3.5 9.3

Si/Na (mol)* 0.5 0.5 0.5 0.5

Calculated values from quicklime and Si and Na source weight % in Premix

The conditions of slaking of those premixes are detailed here below and the compositions and properties of the sorbents obtained from the slaking of those premixes are presented in table 2. The premix is manufactured in such a way that the molar ratio between Si and Ca (Si/Ca) is comprised between 0.02 and 0.2 and is calculated according to the following formulae:

Sif Ca(moI) = ^X ¾ =A

\00x M _StO1 x ¾

Wherein;

Wsi source represents the weight of the compound comprising silicon;

%Si0 ₂ si source represents the % Si0 ₂ in the said compound comprising silicon;

Mcao represents the molar weight of CaO, i.e. 56.1 g/mol

Msio ₂ represents the molar weight of Si0 ₂ , i.e. 60.0 g/mol WQ _L represents the weight of quicklime used in the premix in the approximation that quicklime is only made of CaO while it is not the case in reality as aforementioned. Therefore, if the quicklime contains naturally Si0 ₂ , the actual molar ratio Si/Ca in the product will be larger than the expected one. This is the case of the quicklime 2 that contains about 0.7 % Si0 ₂ .

Example l.-slaking of Premix 1

The Premix 1 was introduced in a laboratory scale hydrator with a feeding rate of 223 g/min. Water (tap water at room temperature) was also introduced in this reactor with a flow of 200 g/min. No additional additive was used during the slaking. Both the Premix and the water were fed into the reactor at the same point (first third of the reactor length) and they were mixed and slaked before going out of the reactor after a retention time in the reactor close to 25 minutes. At the outlet of the hydrator, the moisture level carried by the lime based sorbent collected was 22.5 weight % with respect to the total weight of the raw hydrate. This sorbent has been further air classified and milled. For this purpose, a Hosokawa Alpine ATP 50 - AFG 100 has been used. This equipment is a classification mill, using a jet mill to grind the particles down to the right size. The wet sorbent was introduced in this equipment, in which the rotation speed of the classification wheel was fixed at 2000 rpm and the pressure of the milling air was fixed at 3 bars. Due to contacts with large amount of ambient air, the moisture of the sorbent went down from 22.5 weight % to 18.1 weight % during the classification and milling step with respect to the total weight of the sorbent. The main properties of this obtained sorbent are presented in Table 2 (expressed on the total weight or mole of equivalent dry material except for the residual moisture being based on the sorbent weight).

Example 2.- Slaking of Premix 2

The Premix 2 was introduced in a pilot scale hydrator with a feeding rate of 150 kg/h. Water (quarry water at 12 °C) was also introduced in this reactor. No additional additive was used during the slaking. Again, the Premix and the water were mixed and slaked before going out of the reactor after a retention time in the reactor close to 25 - 30 minutes. At the outlet of the hydrator, the moisture level in the lime based sorbent collected was ranging between 21 and 22 weight % during a whole day of production with respect to the total weight of the raw hydrate. From the outlet of the hydrator, the lime based sorbent collected fall in a rubber jacket screw and was then de-agglomerated and partially dried by going through a Cage Mill (PSP MKS500) in which the sorbent came in contact with warm air leading to a flash drying of the particles. The air was heated by a gas burner which was set at its minimum level (42 °C only measured in the process filter located downstream the cage mill) in order to ensure an uncomplete drying only. The sorbent had a moisture ranging from 5 to 7 weight % with respect to the total weight of the sorbent during the whole production day. This product has been further air classified. For this purpose, a Hosokawa Alpine ATP 50 - AFG 100 has been used at 177 rpm. The fines from this classification step were directly sent to the finished sorbent storage silo whereas the coarse fraction went through a pin mill before joining the fines in the finished sorbent silo. The main properties of this obtained sorbent are presented in Table 2 (expressed on the total weight or mole of equivalent dry material except for the residual moisture being based on the sorbent weight).

Example 3.- Slaking of Premix 3

The Premix 3 has been introduced in the same laboratory scale hydrator as the one described in Example 1, but with a feeding rate of 238 g/min and with a flow of tap water (room temperature) of 204 g/min. At the outlet of the hydrator, the moisture level in the lime based sorbent collected was 20.7 weight % with respect to the total weight of the raw hydrate. In contrary to Examples 1 and 2, this product was neither dried nor classified nor milled. The main properties of this obtained sorbent are presented in Table 2 (expressed on the total weight or mole of equivalent dry material except for the residual moisture being based on the sorbent weight). Example 4.- Slaking of Premix 4

The same process as the one described in Example 3 has been applied, except that the Premix 4 was used, with a feeding rate of 351 g/min and with 156 g/min of water. The main properties of this obtained sorbent are presented in Table 2 (expressed on the total weight or mole of equivalent dry material except for the residual moisture being based on the sorbent weight).

Table 2.-

The XRD pattern of the sample of the example 4 that has been dried at 150 °C is presented in Figure 3 and shows that this material contains a large amount of amorphous phase, portlandite (Ca(OH) ₂ ), calcite (unburned CaC0 ₃ ) and Natrite (Na ₂ C0 ₃ ). No crystalline calcium silicate nor remaining unreacted Na silicate is visible on this XRD pattern.

For comparison purposes, the XRD pattern of the sodium silicate pentahydrated that has been used as the compound comprising Si in this example is shown in Figure 4. This sample has been dried at 150 °C before the XRD analysis in order to compare it with the product of the Example 4 which had been dried at this same temperature. The XRD pattern shows thus all the peaks of Na ₂ Si0 ₃ (anhydrous), which are however not visible on the XRD shown in Figure 3, indicating thus that there is no remaining unreacted Na ₂ Si0 ₃ in the product prepared according to the Example 4.

Figure 5a presents the Si cartography of particles from the sample produced in the example 4 and Fig. 5b presents the calcium cartography of particles from the same sample. It shows that this sorbent contains particles containing both significant amounts of Si and Ca. These particles could therefore be considered to be composed, at least partially, of a calcium silicate that would have been formed by the reaction between lime and the Si compound.

Fig. 6 presents a termogravimetric analysis (TGA) of three samples of sorbent and a hydrated lime as comparative example (analysis done on samples previously dried at 150 °C):

- the curve A of white diamonds represents the TGA of hydrated lime without any added compound comprising silicon, aluminum nor sodium (hydrate w/o any additional Si, Al or Na);

- the curve B of black triangles represents the TGA of the sorbent of example 1 (Si/Ca = 0.03);

- the curve C of black circles represents the TGA of the sorbent of example 3 (Si/Ca = 0.05); and

- the curve D of black squares represents the TGA of the sorbent of example 4 (Si/Ca = 0.20).

Exannple 5.- Test of sorbent from Example 1

2 kg of the fresh sorbent obtained from example 1 were loaded in CDS pilot as synthesized to generate the residue. A fine dispersion of the sorbent was injected at the bottom of the reactor at a flow of 45 g/h. The synthetic gas flow rate in the process was 20.5 Nm ³ /h, and its composition was 8.1 % C0 _2/ 19.4 % 0 ₂ , 8.2 % H ₂ 0 and 530 ppm S0 ₂ . The residue was filtered in a baghouse filter as filter means; the filter was automatically cleaned with air pulses when the pressure loss reached 15 mbar. The residue was then collected, and fell through a cascade of hoppers to reach a mixer as mixing zone, where it was added at a flow of 1000 g/h to be mixed with 50 ml/h of water to obtain a moisturization of 5%. This mixture was then reintroduced at the bottom of the reactor. The temperature at the top of the reactor (inside the reactor) has been measured in function of time as presented in Figure 8 for the sorbent from example 1 according to the present invention (curve B) and compared to the comparative sample of hydrated lime produced (curve A) as explained above. The performance of S0 ₂ removal was measured after stabilization of the composition of the residue. The moisturization was then increased to 20%, and the temperature and performance were measured after stabilization of the composition of the residue (sorbent according to the present invention). The performance of this sorbent (curve B) was compared with the comparative sample of hydrated lime moisturized at 5% (curve A) in the same conditions and temperature. The fig. 7 shows two curves of the ratio of the content of S0 ₂ in the treated gas flow relative to the content of S0 ₂ in the synthetic gas flow in function of a molar ratio of calcium under any form relative to sulfur. The lower curve A shows the performance of S0 ₂ removal for the standard hydrate moisturized at 5% and the upper curve B shows the performance of the sorbent from example 1 moisturized at 20%.

Example 6 : Test of sorbent from Example 2

1.5 kg of the fresh sorbent obtained from example 2 was loaded in a CDS pilot as synthesized to generate the residue. A fine dispersion of the sorbent was injected at the bottom of the reactor at a flow of 11 g/h. The synthetic gas flow rate in the process was 25.6 Nm ³ /h, and its composition was 6.5% C0 ₂ , 20.8% 0 ₂ , 6.6% H ₂ 0 and 430 ppm S0 ₂ . The temperature at the exit of the reactor was 117°C. The sorbent was filtered in a baghouse filter as filter means; the filter was automatically and continuously cleaned with air pulses. The residue was then collected, and fell through a cascade of hoppers to reach a mixer as mixing zone, where it was added at a flow of 1000 g/h to be mixed with 110 ml/h of water to obtain a moisturization carried by the residues of 11%. This mixture was then reintroduced at the bottom of the reactor. The flow ability behavior of this sorbent was compared with a residue of hydrate lime moisturized at 5% in the same conditions: the comparison was made by measuring the Haussner ratio and Carr index at 1250 taps of each of the residues; results are given in table 3.

Table 3.-

The Haussner ratio and Carr index have been measured by a device GranuPack ^® from the company Granutools ^® being an entirely automated instrument that gives information on diffusion and percolation properties of granular materials. It measures the evolution of the tapped density versus a constant constraint. The measurements made by GranuPack consist to record the density of powders or granular materials after each individual tap.

The data analysis of the density curves gives multiple information about the studied granular material properties such as packing fraction, compaction, compressibility and release of the air trapped between the grains, granules or particles.

First, the measurement cell (glass cylinder from which the tare is known) is filled carefully in order to avoid compaction with 35 mL of a bulk powder. The cylinder is then weighted and the mass of sample is calculated by subtracting the tare of the empty glass cylinder. The weight of the sample divided by its initial volume (i.e. 35 ml) gives the bulk density of the product noted WB. The cylinder is then placed into the GranuPack and tapped 1250 times. The decrease of the volume occupied by the sample in the glass cylinder is recorded vs the number of taps.

At the end of the 1250 taps, the Tapped density (BIT) can be calculated by dividing the sample weight by the final volume recorded at the end of the measurement.

The Hausner ratio (H) can be calculated by dividing 5Π ^" by SB.

The Carr Index (C) is calculated by the following formula:

H = 100/UOO-C).

The closer the Hausner ratio is to 1, the better the flowability of the powder. The smaller the Carr Index, (< 15), the better the flowability.

Example 7: Test ofsorbent from Example 3

The same process that the one described in Example 5 has been applied, except that the sorbent from Example 3 was used. The synthetic gas flow rate in the process was 19.3 Nm ³ /h, and its composition was 8.6% C0 ₂ , 20.5% 0 ₂ , 9.4% H ₂ 0 and 550 ppm S0 ₂ . The temperature at the top of the reactor was 116°C. The moisturization carried by the residue was 17.5%.

Example 8: Test of sorbent from Example 4

The same process that the one described in Example 5 has been applied, except that the sorbent from Example 4 was used. The synthetic gas flow rate in the process was 19.5 Nm ³ /h, and its composition was 8.5% C0 ₂ , 20.7% 0 ₂ , 9% H ₂ 0 and 550 ppm S0 ₂ . The temperature at the top of the reactor was 116°C. The moisturization carried by the residue was 17.5%.

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. The sorbent according to the present invention can be advantageously used in circulating dry scrubber for a flue gas treatment process.

In a process of flue gas treatment using a circulating dry scrubber, the sorbent according to the invention can be advantageously recirculated in the circulating dry scrubber device (CDS). Advantageously, the water content can be increased to more than 10% before injection of the sorbent of the present invention in the reactor of a CDS, thereby preventing sticking and clogging phenomena in the CDS. A substantial decreasing of the temperature at the exit of the reactor can be observed.