Process for Treating Flue Gases in CDS Flue Gas Treatment

Abstract

Process for treating flue gases in a circulating dry scrubber device wherein flue gases containing pollutants pass to a reactor where said flue gases contact a sorbent comprising high pore volume and high specific surface area slaked lime or hydrated lime being further sent to a particulate control device where said sorbent particles are separated from said flue gases substantially depleted in pollutants and form respectively a flow of flue gases substantially depleted in pollutant and a flow of sorbent particles being recycled and returned to said reactor.

Claims

1. Process for treating flue gases in a circulating dry scrubber device wherein flue gases containing pollutants pass from a flue gases source to a reactor where said flue gases contact a sorbent injected to said reactor and form with said sorbent a suspension of sorbent particles in said flue gases wherein at least a fraction of said pollutants are captured by said sorbent, said suspension being further sent to a particulate control device where said sorbent particles are separated from said flue gases and form, respectively, a flow of flue gases substantially depleted in pollutants and a flow of sorbent particles, said flow of flue gases substantially depleted in pollutants being evacuated from the particulate collection device towards a chimney, said flow of sorbent particles being at least partially recycled and returned to said reactor, wherein said sorbent injected to said reactor consists essentially of slaked fine having a BJH total pore volume of at least 0.15 cm.sup.3/g and a BET specific surface area of at least 25 m.sup.2/g.

2. Process according to claim 1, wherein said sorbent injected to said reactor is fresh sorbent or a mixture of fresh sorbent and said flow of sorbent particles recycled and returned to said reactor.

3. Process according to claim 1, wherein said sorbent injected to said reactor is injected from a sorbent mixing zone to said reactor and wherein said flow of sorbent particles recycled from said particulate control device and returned to said reactor is recycled from said particulate control device and returned to said sorbent mixing zone before being sent to said reactor.

4. Process according to claim 2, wherein said sorbent injected to said reactor is a mixture of fresh sorbent and said flow of sorbent particles recycled and returned to said reactor, further injected from a sorbent mixing zone to said reactor and wherein said flow of sorbent particles recycled from said particulate control device and returned to said reactor is recycled from said particulate control device and returned to said sorbent mixing zone before being sent to said reactor.

5. Process according to claim 1, wherein said flow of flue gases substantially depleted in pollutants exiting the particulate collection device is partially withdrawn to be recycled back to the reactor.

6. Process according to claim 1, wherein water is further injected to said reactor or to said sorbent mixing zone through water injection means.

7. Process according to claim 1, wherein said sorbent is injected such as the normalized stoichiometric ratio NSR divided by the conversion x is superior or equal to 1,
wherein: NSR=(n.sub.Ca/n.sub.P)/N, wherein n.sub.Ca is the number of moles of Ca(OH).sub.2 from the fresh sorbent; wherein n.sub.P is the number of moles of pollutant from the raw flue gas; wherein N is the stoichiometric number of moles of Ca(OH) required according to the theoretical chemical reaction to completely convert one mole of pollutant;
wherein x=(P.sub.inP.sub.out)/P.sub.in; wherein P.sub.in is the number of moles of pollutant entering into the circulating dry scrubber device via the raw flue gas and; wherein P.sub.out is the number of moles of pollutant in the flue gas substantially depleted in pollutants being evacuated from the particulate collection device.

8. Process according to claim 1, wherein said sorbent is injected such as the normalized stoichiometric ratio NSR is a maximum of 4.

9. Process according to claim 1, wherein said pollutant comprises SO.sub.2 content and/or wherein the said sorbent is injected such as the normalized stoichiometric ratio is a maximum of 2.

10. Process according to claim 1, wherein said suspension stays in movement inside said reactor for a predetermined residence time preferably greater than 1 second and preferably less than 20 minutes.

11. Process according to claim 1, wherein said slaked lime has a BJH partial pore volume of at least 0.1 cm.sup.3/g for pore diameters ranging from 100 to 400 Angstrom.

12. Process according to claim 1, wherein said slaked lime has a BJH total pore volume inferior or equal to 0.30 cm.sup.3/g.

13. Process according to claim 1, wherein said slaked lime has a BET specific surface area inferior or equal to 50 m.sup.2/g.

14. Process according to claim 1, wherein said slaked lime presents an alkali metal content of at least 0.2% and of maximum 3.5% based on the total weight of the said slaked lime.

15. Process according to claim 1, wherein said alkali metal is selected from the group consisting of sodium, potassium, lithium and a combination thereof.

16. Process according to claim 2, wherein said sorbent is recycled at a recycling ratio comprised between 0.5 and 300, said recycling ratio being defined as the injection rate of sorbent particles recycled and returned to said reactor divided by the injection rate of fresh sorbent.

17. Process according to claim 6, wherein water is injected with a sufficient amount for having a moisture content in the sorbent injected to said reactor between 0.1 and 10 weight % based on the total weight of said sorbent injected or for having a gas moisture content gas/water ratio between 5 and 35 vol %.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0081] FIG. 1 is a graph showing the relationship between normalized stoichiometric ratio and SO.sub.2 conversion for three different sorbents at different normalized stoichiometric ratio.

[0082] FIG. 2 is a graph showing the concentration of SO.sub.2 in flue gases before and after the gas treatment with a standard slaked lime.

[0083] FIG. 3 is a graph showing the concentration of SO.sub.2 in flue gases before and after the gas treatment with a sorbent composition according to the present invention.

[0084] In the drawings, the same reference numbers have been allocated to the same or analogue element.

DESCRIPTION OF THE INVENTION

[0085] This invention relates to the use of high pore volume and high specific surface area slaked lime as a sorbent in circulating dry scrubber units.

[0086] While the belief in the literature mentions no benefit with enhanced materials due to high recycling conditions, the higher pore volume and the higher surface area of those products brings surprisingly additional performances.

[0087] The process for treating flue gases in flue gas treatment units according to the present invention is a process wherein flue gases containing polluting components pass from a flue gases source to a reactor. In the reactor, said flue gases contact a high pore volume and high specific surface area slaked lime as a sorbent injected to a reactor, optionally from a sorbent mixing zone or injection zone to said reactor. The high pore volume and high specific surface area slaked lime present a BJH pore volume equal to or greater than 0.15 cm.sup.3/g, preferably equal to or greater than 0.17 cm.sup.3/g, preferably equal to or greater than 0.19 cm.sup.3/g, advantageously equal to or greater than 0.20 cm.sup.3/g and presents a BET specific surface area calculated according to the BET method (ISO 9277:2010E standard) and obtained from nitrogen adsorption equal to or greater than 25 m.sup.2/g, preferably equal to or greater than 30 m.sup.2/g, more preferably equal to or greater than 32 m.sup.2/g, advantageously equal to or greater than 35 m.sup.2/g.

[0088] The flue gases form with said sorbent a suspension of sorbent particles in said flue gases which stays in movement inside said reactor for a residence time which depends on the size of the circulating dry scrubber device, and on the gas flow rate, before being sent to a filter unit where said sorbent particles are separated from said flue gases and form respectively a flow of flue gases and a flow of sorbent particles. The filter unit can be a bag house filter or an electrostatic precipitator, or any other suitable particle collection device.

[0089] Said flow of flue gases being depleted in polluting component exits the filter unit to enter a chimney. Part of the flue gas can also be recycled back to the reactor. Of course, between the filter unit and the chimney, other devices can be present.

[0090] A part of the flow of sorbent particles is recycled with a predetermined recycling ratio (injection rate of recycled sorbent divided by the injection rate of fresh sorbent) and returned to said sorbent mixing zone or injection zone for being afterwards reinjected inside said reactor.

[0091] In the process according to the present invention, the high pore volume and high specific surface area slaked lime, optionally doped with an alkali metal is injected at a normalized stoichiometric ratio NSR typically comprised between 1 and 4, preferably between 1.5 and 3, more preferably below or equal to 2.

[0092] Said sorbent injected from a sorbent mixing zone or injection zone is a mixture of fresh sorbent fed from a sorbent storage device and said flow of (spent) sorbent particles being recycled.

[0093] Water is further injected in said sorbent mixing zone or injection zone or in said reactor.

[0094] Indeed, two ways to handle CDS process exist. In a first way, the residues and the sorbent are wetted before reinjection at the bottom of the reactor in a said sorbent mixing zone. In the second way, water is injected in the reactor and not directly on the residues and sorbent.

[0095] Said water is injected at a sufficient amount for having a moisture content in the sorbent injected from said sorbent mixing zone between 0.1 and 10 weight %, in particular between 0.5 and 8 weight %, based on the total weight of said sorbent injected or for having a gas moisture content 5 and 35 vol. %, preferably between 7 and 30 vol. %, more preferably between 10 and 25 vol. %.

[0096] Preferably, said predetermined recycling ratio is comprised between 0.5 and 300, preferably between 2 and 150, more preferably between 10 and 60.

[0097] More particularly, the reactor presents a temperature at a reactor outlet between 70 C. and 220 C.

[0098] If the sorbent is a high pore volume and high specific surface area slaked lime doped with an alkali metal, the alkali metal (sodium or potassium or lithium or a combination thereof) content is equal to or greater than 0.2% and equal to or less than 3.5% based on the total weight of said high pore volume and high specific surface area slaked lime.

[0099] Preferably, said high pore volume and high specific surface area slaked lime presents a residual moisture content, said residual moisture content being equal to or less than 3% by weight, more preferably equal to or less than 2% by weight, in particular equal to or less than 1% by weight.

EXAMPLES

Example 1

[0100] During an industrial trial, a standard hydrated lime (curve A), a high pore volume and high specific surface area slaked lime according to the invention (curve B) and a sodium doped high pore volume and high specific surface area slaked lime according to the invention (curve C) were injected in a circulating dry scrubber unit (CDS).

[0101] The scrubber unit is an Enhanced All-Dry unit from Fives Solios. The temperature at the outlet of the reactor was in the range between 140 C. to 160 C., the amount of water injection was set between 2 and 6 dm.sup.3 per minute and the recycling ratio (injection rate of recycled material divided by the injection rate of fresh product) was varying from 30 to 40.

[0102] Each sorbent was injected during two weeks using two different injection rates (of fresh sorbent) ranging from 150 to 300 kg/h. As a consequence, a minimum of two normalized stoichiometric ratios were used for each sorbent. The variation range of the normalized stoichiometric ratio was between 1.5 and 4. This allows a comparison of performance based on the stoichiometric ratio as seen in the graph of FIG. 1.

[0103] For the same removal performance, the consumption of reagent is divided by up to 2 when using the sodium doped high pore volume and high specific surface area slaked lime versus the standard hydrated lime according to prior art.

[0104] Furthermore, removal rate for SO.sub.2 was 84% with a normalized stoichiometric ratio of 1.5 with high surface and high porosity hydrated lime, 94% for same conditions with the sorbent comprising sodium doped high pore volume and high specific surface area slaked lime while in the same operating conditions SO.sub.2 removal rate was only 70% using standard hydrated lime with the same normalized stoichiometric ratio of 1.5.

Example 2

[0105] The testing conditions of example 1 have been reproduced, but this time with measurements allowing identifying a comparison of SO.sub.2 peak mitigation performance.

[0106] As it can be seen in FIG. 3, a complete absence of peaks and a very low level of SO.sub.2 is shown with the use of high pore volume and high specific surface area slaked lime (according to the invention) downstream of the flue gas treatment unit (Curve Bclean gas) in comparison to upstream of the flue gas treatment unit (Curve Araw gas). In contrast, using standard hydrated lime according to prior art shows in FIG. 2 peaks of SO.sub.2 and higher levels of SO.sub.2 in the clean gas (Curve B downstream of the flue gas treatment unit), in comparison to curve A being upstream of the flue gas treatment unit. As one can see, the maximum peak of SO.sub.2 released at the stack reached less than 20 mg/Nm.sup.3 using high surface and high porosity hydrated lime according to the invention, while regular peaks of SO.sub.2 above 100 mg/Nm.sup.3 have been observed using standard hydrated lime of the prior art in similar conditions.

Example 3

[0107] During an industrial trial, a sodium doped high pore volume and high surface area hydrated lime according to the invention was injected in a Dustex circulating dry scrubber. Performances were compared when using a Flue Gas Treatment grade standard hydrated lime.

[0108] The scrubber unit comprises a circulating fluidized bed reactor where water is sprayed at the bottom, one bag house filter separated into 2 parts and 2 recycling air slides bringing back the residues to the reactor.

[0109] The temperature at the outlet of the reactor was set at 105 C. regulated by injection of water in the range of 8 to 9 m.sup.3/h sprayed at 35 bars. The flow of solids in the reactor was in the range of 1100 to 1200 g/Nm.sup.3. The sorbent was injected over a period of more than 2 weeks to leave enough time for the system to reach a steady state.

[0110] The CDS set point targeted a relative constant removal rate of SO.sub.2 between 72 and 78%. Samples of residues were collected every day for analysing the performance. Same procedure applied one week before the test with incumbent standard hydrated lime allowing a direct comparison. When using according to the invention the sodium doped high pore volume and high surface area hydrated lime according to the invention, the normalized stoichiometric factor (which is equal to the normalized stoichiometric ratio multiplied by the conversion) was found to be 1.45 for an average removal rate of 75.9% to compare with a normalized stoichiometric factor of 1.67 with an average removal rate of 72.7% with incumbent hydrated lime. With similar removal requirement, the consumption of sodium doped high pore volume and high surface area hydrated lime according to the invention is reduced by 18%.

[0111] 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.