Method for scrubbing exhaust gas from CO.SUB.2 and/or SO.SUB.x

Abstract

A method of exhaust gas scrubbing includes providing recycled concrete fines as a waste material rich in carbonatable Ca and/or Mg phases and with d.sub.90≤1000 μm and a Rosin-Rammler slope n from 0.6 to 1.4 , injecting the waste material into an exhaust gas stream containing CO.sub.2 and/or SO.sub.x for reaction with CO.sub.2 and/or SO.sub.x at a relative humidity of 50 to 100 Vol.-% and a temperature from 40 to 130° C. in an amount of dry waste material ranging from 5 to 30 kg/m.sup.3, withdrawing a partly carbonated and/or sulphurized waste material and purified exhaust gas, and recycling a part of the partly carbonated and sulphurized waste material while the remainder is discharged, as well as use of a waste material slurry for exhaust gas cleaning of CO.sub.2 and/or SO.sub.x.

Claims

1. A method for scrubbing an exhaust gas from CO.sub.2 and SO.sub.x with x from 1 to 3, comprising the steps: providing recycled concrete fines with d.sub.90≤1000 μm and a Rosin-Rammler slope n from 0.6 to 1.4 in dry form or as a slurry, injecting the recycled concrete fines into the exhaust gas for reaction with CO.sub.2 and SO.sub.x to provide in the exhaust gas stream an amount of dry recycled concrete fines ranging from 5 to 30 kg/m.sup.3, and adjustung a relative humidity from 50 to 100% and a temperature from 40 to 130° C., withdrawing partly carbonated and/or sulphurized recycled concrete fines and cleaned exhaust gas recycling a part of the partly carbonated and sulphurized recycled concrete fines while the remainder is discharged.

2. The method according to claim 1, wherein the recycled concrete fines are from recycling and reuse of used concrete.

3. The method according to claim 1, wherein the recycled concrete fines are added as a slurry containing from 4 to 28% by weight solids and in an amount from 20 to 50 l/m.sup.3.

4. The method according to claim 3, wherein the temperature during reaction of the recycled concrete fines with CO.sub.2 and SO.sub.x in the exhaust gas is adjusted to from 55 to 75° C. for injection of the recycled concrete fines as a slurry at a relative humidity of 100%.

5. The method according to claim 1, wherein at least 12.5 wt.-% of the recycled concrete fines calculated as oxides is CaO and MgO and at least 80 wt.-% of the CaO and MgO are in carbonatable phases before carbonation.

6. The method according to claim 5, wherein at least 20 wt.-% of the recycled concrete fines calculated as oxides is CaO and MgO and at least 85 wt.-% of the CaO and MgO are in carbonatable phases.

7. The method according to claim 5, wherein at least 30 wt.-% of the recycled concrete fines calculated as oxides is CaO and MgO and at least 90 wt.-% of the CaO and MgO are in carbonatable phases.

8. The method according to claim 1, additionally comprising mechanical pretreatment of the recycled concrete fines.

9. The method according to claim 1, wherein the recycled concrete fines have a particle size distribution having a d.sub.90≤500 μm and/or a Rosin-Rammler slope n from 0.7 to 1.2.

10. The method according to claim 9, wherein at least 20 wt.-% of the recycled concrete fines calculated as oxides is CaO and MgO and at least 85 wt.-% of the CaO and MgO are in carbonatable phases.

11. The method according to claim 10, wherein the recycled concrete fines are added as a slurry containing from 4 to 28% by weight solids and in an amount from 20 to 50 l/m.sup.3.

12. The method according to claim 11, wherein the temperature during reaction of the recycled concrete fines with CO.sub.2 and SO.sub.x in the exhaust gas is adjusted to from 55 to 75° C. for injection of the recycled concrete fines as a slurry at a relative humidity of 100%.

13. The method according to claim 11, wherein the weight ratio between recycled and discharged recycled concrete fines ranges from 6000 to 200.

14. The method according to claim 10, wherein the temperature during reaction of the recycled concrete fines with CO.sub.2 and SO.sub.x in the exhaust gas is adjusted to from 55 to 85° C. for injection of the recycled concrete fines as dry material with or without injection of water and/or slurry at a relative humidity from 50 to 95%.

15. The method according to claim 1, wherein the exhaust gas is taken from a plant burning fuel to generate heat for a material or energy production process.

16. The method according to claim 15, wherein the exhaust gas is taken from a coal fired power plant, a ceramics manufacturing plant, the magnesia industry, a lime kiln, or a cement plant.

17. The method according to claim 16, wherein before or after cleaning the exhaust gas is further subjected to one or more of: dust removal, gas cooling, gas conditioning, heat recovery, and/or the cleaned exhaust gas is used for drying of wet materials.

18. The method according to claim 17, wherein the weight ratio between recycled and discharged recycled concrete fines ranges from 6000 to 200.

19. The method according to claim 1, wherein sulfur is removed from or diminished in the exhaust gas prior to scrubbing with the recycled concrete fines.

20. The method according to claim 1, wherein the temperature during reaction of the recycled concrete fines with CO.sub.2 and SO.sub.x in the exhaust gas is adjusted to from 55 to 85° C. for injection of the recycled concrete fines as dry material with or without injection of water and/or slurry at a relative humidity from 50 to 95%.

21. The method according to claim 1, wherein the weight ratio between recycled and discharged recycled concrete fines ranges from 10000 to 100.

22. The method according to claim 21, wherein the weight ratio between recycled and discharged recycled concrete fines ranges from 6000 to 200.

23. The method according to claim 1, wherein before or after cleaning the exhaust gas is further subjected to one or more of: dust removal, gas cooling, gas conditioning, heat recovery, and/or the cleaned exhaust gas is used for drying of wet materials.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically shows an embodiment of the method according to the invention using a slurry, i.e. a wet process,

(2) FIG. 2 schematically shows an embodiment of the semi-dry or semi-wet process.

DETAILED DESCRIPTION OF THE INVENTION

(3) In FIG. 1 a typical cement plant is shown. It comprises a preheater and calciner 1, rotary kiln 2 and clinker cooler 3 for manufacturing cement clinker from raw materials provided in a silo 4. Cooling air supplied to the cooler 3 is mostly passed into the kiln 2 and preheater/calciner 1, a part can be vented directly to the stack 5 via filter 6. The plant comprises a gas conditioning tower 7 for cooling the exhaust gas by injected water with or without limestone or calcium hydroxide addition. An induced draft fan 8 provides the necessary gas flow. As usual in the art, the conditioned exhaust gas from gas conditioning tower 7 is normally utilized to dry the raw material in mill 9, where also a part of the acid gases is absorbed onto the raw material. The range of SO.sub.2 collection in the raw mill system varies between 10 and 95% depending on the gas, the raw material moisture and the raw mill type. Mill 9 is provided with a separator 10 to recycle coarser particles back to the mill 9 and pass the raw meal to silo 4. Exhaust gas from the mill 9 and separator 10 is passed to a filter 11 with the help of induced draft fans 12 and 13. In direct mode, i.e. when the mill 9 is not working, the exhaust gas from gas conditioning tower 7 is passed directly to the filter 11.

(4) A wet scrubber reactor 14 is provided to clean the exhaust gas from CO.sub.2 and/or SO.sub.x with injected waste material slurry. The slurry is provided from dry RCF in silo 15 and water from tank 16 in a mixing device 17 and stored in a hold tank 18. The mixing ratio of RCF to water usually ranges from 1:3 to 1:10, preferably from 1:4 to 1:6, most preferably it is about 1:5. A slurry pump 19 injects the slurry into the scrubber reactor 14. If needed, additional water can be injected with the help of water pump 20 to adjust the relative humidity to 100%. Exhaust gas (from filter 11) and waste material slurry are guided in counter current through the scrubber 14, cleaned exhaust gas leaving it at the top and being passed through line 21 to the stack 5. In the wet scrubber 14 the gas, which can enter at any temperature up to 250° C., is cooled by the injected RCF slurry and if needed by water, provided by the fresh water pump 20, to a wet bulb temperature of usually from 50 to 75° C., mostly from 55 to 65° C. as a function of the gas matrix exiting the kiln 2. The slurry has 10 to 28 wt.-% solids. The proportion of water injected via the pumps 19 and 20 is a function of the temperature reduction achieved by water quenching and the amount of slurry containing the RCF required to achieve the desired SO.sub.2 collection efficiency. The slurry is fed at a rate of 20 to 50 l/m.sup.3. This ensures intensive contact with the gas constituents SO.sub.2 and CO.sub.2. Usually the SO.sub.2 absorbing efficiency is 70% to 99% and dependent on the amount of unreacted RCF fed into the system. In the slurry usually a pH from 3 to 8, preferably from 4 to 6.5, and ideally of about 6 is adjusted. As the water is evaporated during the quenching of the gas with water make-up water is required and provided in the form of fresh slurry fed by pump 19 and by pump 20. The make-up water at the same time flushes the droplet separators in the exit of the wet scrubber 14 so that the particle carry over into the main gas stream is reduced to usually 5 to 20 mg/m.sup.3. The density of the slurry transferred by pump 23 and within the scrubber 14 is usually 10 to 18% solids, but not more than 28%. The total retention time of the RCF in the scrubber 14 is usually between 8 and 48 hours, but not less than 8 hours, to allow a full consumption of the active centers of the RCF of 95 to 99.9%, usually about 97%. The rate of CO.sub.2 collection depends on the amount of RCF added. Once the gas has been cleaned and the droplet collected, it is released to the main stack 5. Optionally the stack gas can be reheated with clinker cooler off air so that the escaping gas does not develop water droplets once released to the atmosphere. The slurry with the partly reacted waste material leaves the scrubber 14 at the bottom and is divided in a splitter 22 into a first, usually major, part recirculated into the reactor 14 via pump 23 and a second, usually minor, part extracted into further processing device 24. The extracted part is mainly calcium sulfate and carbonate here. Other exhaust gas cleaning devices/measures can be foreseen, like catalytic or non-catalytic NO and NO.sub.2. Further, it is foreseen that exhaust gas from filter 11 can also be vented directly to the stack 5.

(5) The usual entry operation temperature of the scrubber 14 is 80 to 150° C. By this the operation temperature in a fully water saturated environment is 50 to 70° C. At this temperature, though the rate of RCF active center utilization is 10 to 30% and the SO.sub.2 collection efficiency 20 to 90%, the CO.sub.2 collection would be <5%. By recycling the RCF collected by the separator 22 back to the reactor 14, the utilization of the RCF in terms of SO.sub.2 collection can be enhanced to a level in the range from 80% to 95%. The matured slurry has to be finally dewatered in either centrifuges, band filters or hydro clones. Usually, the residual moisture to be attained is 5 to 50%. But if dry residual RCF is needed, 5 to 25% can be provided, but mostly though a moisture of 10 to 15% can be expected whilst using a centrifuge or a vacuum band filter. The matured RCF can be reused for cement finish grinding or for concrete manufacturing.

(6) FIG. 2 illustrates a semi-wet process. Analogous parts of the plant are assigned the same reference numbers as in FIG. 1. Like in FIG. 1 the cement plant comprises the usual devices and the scrubber reactor 14 for cleaning exhaust gas is added after the dedusting filter 11. However, in contrast to FIG. 1 water is always additionally injected into scrubber 14 here. The humidity in the scrubber 14 is adjusted to below saturation and cleaned exhaust gas as well as partially carbonated/sulphurized waste material particles are passed out of the scrubber 14 at the top. The separator 22 extracts desired parts of cleaned gas and waste material and also recirculates the predetermined parts thereof to the scrubber 14. The product here is a mixture of calcium sulfite, sulfate, and carbonate. A fan 25 is installed to transfer the exhaust gas from separator 22 to the stack 5 and to assist extracting gas and particles from the scrubber 14. In the scrubber 14 the RCF slurry is dried by the inherent heat and by this the water dew point rises and the operation temperature is reduced at the same time to ideally 60 to 85° C. Once the gas is cleaned from SO.sub.2 and CO.sub.2 to a sufficient degree, it is conveyed via the fan 25 to the main stack 5 where it joins in with other gas or air streams such as the clinker cooler 3 off air. However, it does not have to be joined if it is not necessary to reheat the rather high moisture of gas streams. In the scrubber 14 the gas is cooled from usually 100° C. in compound mode down to 60 to 85° C. by means of injection of water via the water pump 20 and the RCF slurry via the slurry pump 19. In direct mode the temperature on the entry can be as high as 250° C. In order to have a good water spray atomization, the water droplets are furnished by a nozzle system, which works with compressed air from compressor 26 as an atomization agent. Usually 95% of the droplets produced have to be smaller than 800 μm. A finer droplet distribution will be of no negative impact to the reactor 14 but for the RCF utilization and collection of SO.sub.2 and CO.sub.2 in the filter 11 95% shall not be finer than 70 μm.

(7) The slurry produced from the RCF from storage 15 and mixed in the slurry preparation tank 17 before passing into a hold tank 18 has a density of 10 to 28% solids. The proportion of water injected via the pump 20 is a function of temperature reduction requirements to achieve a low approach temperature differential between the final temperature in the reactor 12 and the water dew point. The approach temperature is usually 1 to 10° C., ideally 3 to 7° C., but a differential has to always remain to avoid water condensation on or in the related equipment as well as the equipment for dust/solid transports. In the scrubber reactor system 14, 22 and dependent of the amount of RCF slurry injected, the SO.sub.2 collection efficiency can be as high as 95%, for example from a content of 1.500 mg/m.sup.3 down to 30 mg/m.sup.3. With the recycling of not fully carbonated/sulphurized RCF the reactor solid density can be controlled between 10 and 100 kg/m.sup.3. The loss of RCF out of the reactor, which has to be replenished with fresh slurry, is usually 1 to 10 kg/m.sup.3. Therefore, the recycle rate or passes back to the reactor is 10 to 1000 times, usually about 100 times. Once the dried RCF leaves the separator 22 they are favorably utilized up to 95% and can be used either for cement finish grinding or for concrete admixing. If service of the semi-wet scrubber 14 is necessary, the kiln exhaust gas can be bypassed directly to the filter 11 and stack 5. The rate of RCF utilization is typically 80 to 90%, but should not exceed 95% in order to avoid corrosion of the equipment.

(8) List of Reference Numbers

(9) 1 preheater and calciner 2 rotary kiln 3 cooler 4 raw meal silo 5 stack 6 clinker cooler filter 7 gas conditioning tower 8 induced draft fan 9 raw material mill 10 mill separator 11 main system filter 12 mill induced draft fan 13 induced draft fan 14 scrubber reactor 15 waste material silo 16 water tank 17 slurry mixing device 18 waste material slurry hold 19 slurry pump 20 water pump 21 cleaned gas line to stack 22 separator 23 slurry recirculation pump 24 carbonated/sulphurized waste material processing 25 scrubber induced draft fan 26 compressor