REMOVING HEAVY METALS FROM OFF-GAS FOR CARBON CAPTURE

Abstract

Carbon capture method for an industrial plant off-gas involves removing mercury and/or other heavy metals from the off-gas by contacting the off-gas with an at least partially hydrated cement paste as sorbent in a scrubber before a concentration step of the off-gas to concentrate CO.sub.2. The at least partially hydrated cement paste is simultaneously carbonated and loaded with the mercury and/or other heavy metals during contacting with the off-gas and is used as supplementary cementitious material in composite cement. The at least partially hydrated cement paste is used as sorbent in a scrubber before a concentration step of the off-gas to concentrate CO.sub.2.

Claims

1. A carbon capture method for an off-gas comprising the steps of: removing mercury and/or other heavy metals from the offgas by contacting the off-gas with an at least partially hydrated cement paste as sorbent in a scrubber; and subsequently concentrating the off-gas to concentrate CO.sub.2 in a concentration step, wherein the at least partially hydrated cement paste is simultaneously carbonated and loaded with the mercury and/or other heavy metals during contacting with the off-gas and is used as supplementary cementitious material in composite cement.

2. The method according to claim 1, wherein the heavy metal is mercury, lead, chromium, copper, manganese, molybdenum, titanium or any mixture thereof.

3. The method according to claim 1, wherein the at least partially hydrated cement paste is obtained from waste or byproducts, by mixing aged cement with water and/or by retrieving recycled cement paste from a recycling process of cement containing waste.

4. The method according to claim 3, wherein the recycled cement paste is retrieved from suspensions resulting from cleaning concreting equipment, filter cakes thereof, discarded concrete or mortar, fine fractions from concrete demolition waste, and any mixture of two or more of them.

5. The method according to claim 1, wherein the hydration degree of the at least partially hydrated cement paste ranges from 50 to 100% of the hydratable phases in the contained cement paste.

6. The method according to claim 1, wherein the sorbent contains no more than 60 wt.-% substances other than partially hydrated cement paste, calculated as solids with respect to the total solids.

7. The method according to claim 1, wherein the sorbent has a particle size distribution with a d.sub.50250 m, and/or a d.sub.10 from 1 to 50 m, and/or a d.sub.90 from 60 to 200 m.

8. The method according to claim 1, wherein a relative humidity of 70 Vol.-% is provided during contacting the sorbent with the off-gas by spraying water into a device for contacting and/or by providing the sorbent in the form of a slurry with a solid concentration ranging from 100 to 250 g/l.

9. The method according to claim 1, wherein a temperature during contacting the sorbent with the offgas is set to range from 50 to 100 C.

10. The method according to claim 1, wherein a contact time of contacting the sorbent with the off-gas is set to 5 minutes, and/or until at least 20 wt.-% of the heavy metals are removed from the off-gas, and/or until a carbonation degree of the sorbent of 70 to 100% of the carbonatable phases in the sorbent is reached.

11. The method according to claim 1, wherein the CO.sub.2 content in the off gas is reduced by not more than 30% with respect to the initial CO.sub.2 content and/or the CO.sub.2 content in the off-gas after the scrubber is at least 8 Vol.-%.

12. The method according to claim 1, wherein the concentration step comprises chemical or physical absorption of CO.sub.2 in a solvent, physical absorption in an absorbent bed comprising silica or zeolites, a pressure or temperature swing absorption process, or extraction using a membrane.

13. The method according to claim 12, wherein the concentrated CO.sub.2 is subjected to a series of compression steps and cooling using cryogenic treatment for further CO.sub.2 purification.

14-15. (canceled)

16. The method according to claim 2, wherein the heavy metal is mercury, lead, copper, titanium, or any mixture thereof.

17. The method according to claim 16, wherein the heavy metal is mercury.

18. The method according to claim 5, wherein the hydration degree of the at least partially hydrated cement paste ranges from 75 to 100% of the hydratable phases in the contained cement paste.

19. The method according to claim 6, wherein the sorbent contains no more than 30 wt.-% substances other than partially hydrated cement paste.

20. The method according to claim 9, wherein the temperature during contacting the sorbent with the off-gas is set to range from 65 to 80 C.

21. The method according to claim 10, wherein the contact time of contacting the sorbent with the off-gas is set to from 15 minutes up to 120 minutes, and/or until at least 40 wt.-% of the heavy metals are removed from the off-gas.

22. The method according to claim 11, wherein the CO.sub.2 content in the off-gas is reduced by not more than 20% with respect to the initial CO.sub.2 content and/or the CO.sub.2 content in the off-gas after the scrubber is at least 12 Vol.-%.

Description

EXAMPLE

[0029] Filter cake from a pre-cast concrete elements plant was obtained as sorbent. This material arises after filtration of the process and cleaning water in the precast facility, and is characterized by a high concentration of hydrated cement paste, here mainly CEM III/A. The hydration time is limited to 2-6 weeks. The composition of the used RCP is shown in table 1.

TABLE-US-00001 TABLE 1 Chemical composition (wt.-%) LOI (950 C.) 22.44 SiO.sub.2 39.18 Al.sub.2O.sub.3 6.88 Fe.sub.2O.sub.3 1.76 CaO 43.51 SO.sub.3 3.04 MgO 3.90 K.sub.2O 0.41 Na.sub.2O 0.19

[0030] 50 tons of this sorbent were collected, crushed and ground in an impact pin mill to a d.sub.50 around 200 m with 1-2 wt.-% residue above 1 mm. This sorbent was used instead of slaked lime as scrubbing media in the gas suspension absorber of a cement plant. The gas suspension absorber is normally used in conventional clinker production operation to scrub SO.sub.2 from the off-gas of one of the two preheater strings. The semi-dry scrubber system was composed of the reactor itself, two cyclones with two recirculation boxes and a spraying nozzle system. The reactor was 3.65 m diameter and 14.2 m tall, featuring a gas retention time of 2.2 s. It was composed of an inlet bend, a venturi with a velocity of 35 m/s and a riser section with a velocity of 6.4 m/s. Further, the fabric filter was used to collect the fine particles of spent sorbent leaving the cyclones. The sorbent was placed into the recirculation boxes and the system was modified to allow the recirculation of the sorbent captured in the bag filter to the riser section in order to maintain a constant hold-up in the system. The concentration of heavy metals measured with Advanced Mercury Analyzer and Inductively Coupled Plasma Mass Spectrometry in the fresh sorbent and in the spent sorbent collected in the bag 5 filter and the recirculation containers, respectively, is listed in table 2.

TABLE-US-00002 TABLE 2 in carbonated in carbonated sorbent collected in sorbent collected in in sorbent the fabric filter the recirculation Element [mg/kg] [mg/kg] containers [mg/kg] As <5.00 <2.68 <4.76 Ba 258 242 284 Be 1.30 1.00 1.05 Bi <4.69 <2.48 <2.92 Cd 1.56 <0.428 1.30 Co 15.07 <0.431 15.30 Cr 38.72 58.02 46.15 Cu 93.20 179.75 139.00 Ga <2.64 <1.18 <2.03 Hg 0.0038 2.59 2.23 Li 17.67 12.07 16.6 Mn 685 1000 713.5 Mo <2.22 11.37 6.86 Ni 23.42 24.22 25.0 Pb <4.15 198.25 54.25 Se <2.43 <5.00 <3.94 Sn <5.00 <3.83 <5.00 Sr 411.5 366.5 380.0 Te <4.94 <4.7 <5.0 Ti <5.00 69.42 35.85 V 47.2 24.85 42.35 Zn 306 135 248.5

[0031] The substantial increase in concentration found for mercury and other heavy metals proves that those were loaded onto the sorbent. The mercury capture can be estimated based on the registered mercury emission from the typical emission value near 20 g/Nm.sup.3 (dry, 10% O2) of the plant in past years (in indirect mode without raw mill and De-SO.sub.x scrubber operation). According to the values shown in table 2, the carbonated sorbent was loaded with an weighted average of 2.35 mg mercury per kg of sorbent (34% of carbonated RCP was recovered in the fabric filter), totalizing 28 g mercury (12 tons of sorbent were used in the trial). During the carbonation period (3.5 hours)<30 g mercury were transported in the exhaust gas considering historical values of gas measurements at the stack, and therefore it can be assumed that the sorbent provided full mercury removal.