Process and device for carbonating concrete waste and/or sequestering CO.SUB.2

Abstract

A method for manufacturing supplementary cementitious material and sequestering CO.sub.2 by carbonating concrete fines has the following steps: grinding the concrete fines obtained from crushed concrete demolition waste in a mill at a temperature from 1 to 10° C. above the water dew point in a carbonating atmosphere provided by a gas containing from 10 to 99 Vol.-% CO.sub.2, circulating the ground and partially carbonated concrete fines in a fluidized bed reactor in contact with the carbonating atmosphere, and withdrawing decarbonated gas and carbonated concrete fines.

Claims

1. A method for manufacturing supplementary cementitious material and sequestering CO.sub.2 by carbonating concrete fines comprising the steps: grinding the concrete fines obtained from crushed concrete demolition waste in a mill at a temperature from 1 to 10° C. above the water dew point in a carbonating atmosphere provided by a gas containing from 10 to 99 Vol.-% CO.sub.2, circulating the ground and partially carbonated concrete fines in a fluidized bed reactor in contact with the carbonating atmosphere, and withdrawing decarbonated gas and carbonated concrete fines with recirculation of some of the gas and concrete fines to the reactor and/or mill, wherein at least 80 wt.-% of the concrete fines are carbonated.

2. The method according to claim 1, wherein the concrete fines are adjusted to a particle size distribution determined via sieving or laser granulometry with a d.sub.90≤2 mm.

3. The method according to claim 1, wherein the temperature is adjusted to a range from 2 to 7° C. above the water dew point.

4. The method according to claim 1, wherein the carbonating atmosphere contains from 10 to 30 Vol.-% CO.sub.2 or from 50 to 90 Vol.-% CO.sub.2.

5. The method according to claim 1, wherein the carbonating atmosphere is an exhaust gas from a coal fired power plant, a cement plant or a cement plant operating in an oxyfuel mode, or a gas concentrated in CO.sub.2.

6. The method according to claim 1, wherein the ground and partially carbonated concrete fines are recirculated on average 10 to 100 times to the reactor and/or the mill.

7. The method according to claim 1, wherein the carbonating atmosphere is at ambient pressure and fans are used to provide draft and compensate a developed pressure drop.

8. The method according to claim 1, wherein at least 80 wt.-% of the concrete fines are carbonated.

9. The method according to claim 2, wherein the concrete fines are adjusted to a particle size distribution determined via sieving or laser granulometry with a d.sub.90≤1.5 mm.

10. The method according to claim 3, wherein the concrete fines are adjusted to a particle size distribution determined via sieving or laser granulometry with a d.sub.90≤1 mm.

11. The method according to claim 1, wherein the temperature is adjusted to a range from 3 to 6° C. above the water dew point.

12. The method according to claim 4, wherein the temperature is adjusted to a range from 2 to 7° C. above the water dew point.

13. The method according to claim 6, wherein the temperature is adjusted to a range from 2 to 7° C. above the water dew point.

14. The method according to claim 12, wherein the ground and partially carbonated concrete fines are recirculated on average 10 to 100 times to the reactor and/or the mill.

15. The method according to claim 13, wherein at least 95 wt.-%, of the concrete fines are carbonated.

16. A device for sequestering CO.sub.2 and carbonation of concrete fines comprising a mill with separator adapted to receive a concrete fine feed and a gas containing from 10 to 99 Vol.-% CO.sub.2 as carbonating atmosphere, a fluidized bed reactor connected to the mill and adapted to receive partially carbonated and ground concrete fines from the mill and the partially decarbonated gas for carbonation recirculation, and output means adapted to withdraw decarbonated gas and carbonated concrete fines and to recirculate the gas until it is decarbonated and to recirculate the concrete fines until they are carbonated to a desired degree.

17. The device according to claim 16, additionally comprising a crusher with means for separating concrete fines from particles reusable as aggregate and foreign matter contained, for providing the concrete fines from concrete demolition waste.

18. The device according to claim 16, wherein the output means are a fabric filter and a rotary valve or V-slot turning gate.

19. The device according to claim 16, additionally comprising a water supply as cooling means for adjusting the temperature in the mill and/or reactor as well as the water content in the carbonating atmosphere.

20. The device according to claim 16, additionally comprising a gas-to-gas heat exchanger and/or burner for adjusting the temperature of the carbonating atmosphere.

21. The device according to claim 16, additionally comprising one or more induced draft fans to provide a gas flow in the device and/or to compensate a developed pressure drop.

22. The device according to claim 17, wherein the output means are a fabric filter and a rotary valve or V-slot turning gate.

23. The device according to claim 22, additionally comprising one or more of: a water supply as cooling means for adjusting the temperature in the mill and/or reactor as well as the water content in the carbonating atmosphere, a gas-to-gas heat exchanger and/or burner for adjusting the temperature of the carbonating atmosphere, one or more induced draft fans to provide a gas flow in the device and/or to compensate a developed pressure drop.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the figures

(2) FIG. 1 illustrates a first carbonation grinding process, and

(3) FIG. 2 illustrates a second carbonation grinding process.

DETAILED DESCRIPTION OF THE INVENTION

(4) FIG. 1 illustrates a first carbonation grinding method and device directly using exhaust gas as gas containing carbon dioxide. The pipelines transferring gas and/or particles are shown as lines, wherein the flow direction of gas and particles, respectively, is indicated with an arrow.

(5) In FIG. 1 the kiln exhaust gas from a cement plant is conveyed in a so-called tail-end configuration to the device according to the invention. At choice the entire kiln exhaust gas or only a portion 1 is utilized for the carbonation and forms the gas containing carbon dioxide. During carbonation the SO.sub.2, HCl and HF present in the exhaust gas will also be absorbed by the concrete fines along with the CO.sub.2.

(6) If a portion of kiln gas 2 is bypassing the carbonation grinding it is preferably used for heat exchange raising the water dew point in the mill 8/reactor 18 and vented through the same stack 14 as the gas portion 1 being decarbonated in the mill 8. Mill 8 can be a vertical roller mill, a horizontal ball mill, an impact mill or a roller press.

(7) For start up and in the event of a heat deficiency due to excess moisture of the concrete fines feed 9, the deficient heat can be provided by a burner 23 via heat exchangers 20 and 21 or by the heat contained in the portion 2 of the kiln exhaust gas which is bypassing the carbonation grinding. For this a gas-to-gas heat exchanger 4 can be advantageously utilized.

(8) To control the temperature and the water dew point in the mill 8 and fluidized bed reactor 18 a portion of the treated kiln gas is to taken to a flue gas cooler and condenser 3. The condenser is air-cooled and the conditioning temperature is controlled by a fan cooling the air feed. The to be recycled gas can also be mixed with the some of the by-pass gas to adjust the temperature above the water dew point again. The heat for this is provided by the inherent heat of the bypass gas. It has to be understood, that a main cost driver for the operation is if additional heat has to be provided. Therefore can be more economical to bypass a large portion of the gas and extract the CO.sub.2 than trying to absorb all the CO.sub.2.

(9) The water which condenses during cooling of the flue gas is removed via siphon 5 and taken away as condensate 24 via a pump. The condensed water can be utilized for other applications, e.g. for concrete manufacturing or any other industrial process tolerable or in need of carbon dioxide containing water.

(10) The concrete fines feed 9 is prepared by means of a crusher 10, including a separator 27 for extraction of foreign objects 11 such as metal and plastic pieces and withdrawal of the coarse but low-Ca portion 12 for use as concrete aggregate, e.g. by screening. The mill 8 and the crusher 10 as well as the gas (water and eventual burner exhaust gas) and air drafted or developed within the device are vented to the system fabric filter 17 for de-dusting and collection of the (partially) carbonated concrete fines 13.

(11) The mill 8 is fed with concrete fines 9 from the crusher 10, which have a maximum particle diameter from 100 μm to 4 mm, preferably to 2 mm. The fines 9 are fine ground in the mill 8 to an average particle diameter in the range from 10 to 50 μm. The mill 8 has a static or dynamic separator 35 to control the fineness of the concrete fines. The separator 35 passes too coarse particles back into the mill 8 via the separator 27 of the crusher. Of course, it is also possible to convey too coarse particles directly into the mill or the concrete fines feed.

(12) The system fabric filter 17 collects the ground and (partially) carbonated concrete fines 13 leaving the fluidized bed reactor 18. At demand and in the event that the concrete fines are not fully carbonated, the partially carbonated concrete fines 15 are fed back to the circulating fluidized bed reactor 18. When these have to be further ground to activate the material, they are fed back as particles 16 to the mill 8.

(13) A significant portion of the CO.sub.2 is already absorbed in the mill 8 and to a smaller extent in the crusher 10. The remaining portion is absorbed in the circulation fluidized bed reactor 18. As the concrete fines have to be brought into contact with the gas containing CO.sub.2 multiple times, a continuous returning of the fines to the mill 8 is arranged by storing the concrete fines in the hopper of the filter 17. It is also possible to recycle the partially carbonated concrete fines into the fluidized bed reactor 18 or into the mill 8 and the reactor 18.

(14) The back-dosing of the stored fines to the mill 8 (or to the reactor 18 orifice—not shown) and the withdrawal as carbonated concrete fines 19 is enabled by e.g. a rotary valve or V-slot turning gate arrangement 30 or any other suitable means. The rate of recirculation to the mill 8 or reactor 18 is on average 10 to 100 dependent on the milling intensity and achieved surface activity. The usual solid material load in the reactor 18 is 10 to 100 kg/m.sup.3 (S,T,P). To facilitate the correct hold-up in the reactor 18, the feed grain size is adjusted so that 80 wt.-% of the feed has a aprticle size in the range from 25 to 100 μm, which is also a function of the chosen vertical speed, which is typically 5 to 9 m/s.

(15) The temperature of the carbonating atmosphere in the mill 8 and reactor 18 is kept within a range of 2 to 7° C. above water dew point. This usually ranges from 58 to 83° C. Thus, the circulating gas, mainly comprising H.sub.2O, CO.sub.2, N.sub.2 and O.sub.2 should have a temperature from 60 to 90° C. and a water content from 17 to 55 Vol.-%.

(16) In the event of excess heat in the mill 8 e.g. caused by hard material, the required cooling is arranged by injection of water in the mill 8 via water control valve 31. The added water also raises the water dew point and facilitates dissolving of CO.sub.2 into the water rich micro pores of the RCF.

(17) The main purpose of the induced draft fans 6 and 26 is to provide the necessary gas flow in the device and overcome a developed pressure drop. In particular, the heat exchanger 4, the flue gas cooler and condenser 3, the mill 8, the reactor 18, and the filter 17 typically develop an overall combined pressure drop of 8 to 16 kPa. The specific particle load of the system is 1 to 5 kg/m.sup.3 in the routing from the mill 8 to the filter 17. After the filter 17 the particle load to the heat exchanger 4 and cooler and condenser 3 should ideally be <5 mg/Nm.sup.3 to prevent scaling at the heat exchanger 4 and the heat exchange surfaces of the cooler and condenser 3.

(18) The distribution of cycle gas split to the mill 8 and the crusher 10 is controlled by a gas splitter 32 and by trimming the fans 6 and 26, which results in the required lift speed in the mill 8 as well as in the crusher 10.

(19) An induced draft fan 33 is also foreseen in the line of the bypassed gas portion 2 of exhaust to compensate the pressure drop of the heat exchanger 4. Alternatively or additionally, the gas pressure at the splitting point 34 is positive to overcome the drag developed by the heat exchanger 4 in that line. Usually a surplus pressure of 5 to 20 mbar is sufficient to make an additional induced draft fan 33 superflous. This is also the usual pressure surge requirement of the induced draft fan 33.

(20) For start-up of the system a secondary heat exchanger 21 is installed to pre-heat the device parts. The necessary heat is provided by burning of oil or gaseous fuels in the burner 23. Ideally, natural gas or any other gas composed of hydrocarbons is used as fuel. The burner off gas generated during the heating up is vented as waste gas 28.

(21) FIG. 2 illustrates a method and device using liquefied carbon dioxide to provide the gas containing carbon dioxide. Many parts of the device are identical to the embodiment shown in FIG. 1 and have the same reference numbers, therefore.

(22) In FIG. 2 externally produced and concentrated liquefied CO.sub.2 is delivered by pipeline, truck or rail to a CO.sub.2 hold 100 forming part of the device. The feed CO.sub.2 has to be evaporated and pre-heated before introducing it into the mill 8 for carbonation purposes. Heat is required to prevent icing of the machinery elements during the decompression and evaporation of the liquefied CO.sub.2. This heat is mainly provided by a heat exchanger 103, to a smaller portion by external heating via burner 23 and heat exchangers 20 and 21, but foremost by power demand and subsequent heat development during crushing in the crusher 10 and grinding in the mill 8 as well as from power demand for the gas circulation from induced draft fans 6 and 26.

(23) For this, the liquefied CO.sub.2 is dosed via a valve 102 to a combined cross flow/counter current type of pre-heater and evaporator forming the heat exchanger 103. Within the heat exchanger 103, the CO.sub.2 is also decompressed to ambient pressure. By heat exchange, the CO.sub.2 is pre-heated, evaporated and heated in a combined step to near actual operation temperature. Thereafter the heated CO.sub.2 101 is mixed into the gas stream 25 at mixing point 7 before it is joined with the bypass-gas guided around the heat exchanger 103 at a mixing point 22.

(24) By the above-described heat recovery and introduction of the heated CO.sub.2 101 at point 22, the water vapor purged via the stack 14 is colder than the circulating gas and by this, the heat loss is minimum. Generally, the heat balance control at the heat exchanger 103 is facilitated by the bypass of cycle gas 25, which has also its own induced draft fan 26. Usually, a temperature reduction is triggered by decompression of the gas in the mill 8, reactor 18 and filter 17. Further, gas temperature losses of 3° C. are possible by convection. But compression at the system fans 6 and 26 reestablishes the temperature level. Therefore, at a total pressure drop of 70 mbar the temperature depression can be 7° C. but the actual temperature loss which has to be compensated is less.

(25) During the heating and evaporation process step of the CO.sub.2, a part of water vapor contained in the partially decarbonated gas coming from the mill 8 and crusher 10 condenses on the heat exchange surfaces of the heat exchanger 103. Additionally, a Chevron type droplet separator 104 located in the outlet of heat exchanger 103 collects droplets, which would otherwise escape along with the vented decarbonated gas. In addition, this condensate 24 is removed via a siphon 5 and drained.

(26) Nevertheless, some droplets remain dispersed in the partially decarbonated gas or develop due to compression. These are separated from the partially decarbonated gas by the centrifugal forces in the induced draft centrifugal fan 6. The high tip speed of the impeller propels the droplets to the fan wall, where these collect on the surface and move by gravitation to the drain located in the lower point of the fan casing.

(27) The condensed water can be utilized for other means, e.g. for concrete manufacturing or any other industrial process tolerable or in need of carbon dioxide containing water. The gaseous and preheated CO.sub.2 101 is mixed into the cycling gas 25 and thereafter taken to the mill 8. Mill 8 can be a vertical roller mill, a horizontal ball mill, an impact mill or a roller press.

(28) The concrete fines feed 9 is prepared by means of a crusher 10, including a separator 27 for extraction of foreign objects 11 such as metal pieces and withdrawal of the coarse but low-Ca portion 12 for use as concrete aggregate, e.g. by screening. The mill 8 and the crusher 10 as well as the gas and air drafted or developed within the mill and reactor by carbonation are vented to the system fabric filter 17 for de-dusting and collection of the (partially) carbonated concrete fines 13.

(29) The mill 8 is fed with concrete fines 9 from the crusher 10, which are fine ground in the mill 8 to an average particle diameter in the range from 10 to 50 μm. The mill 8 has a static or dynamic separator 35 to control the fineness of the concrete fines. The separator 35 passes too coarse particles back into the mill 8 via the separator 27 of the crusher. Alternatively, these particles can be recirculated directly into the mill 8.

(30) The system fabric filter 17 collects the ground and (partially) carbonated concrete fines 13 leaving the fluidized bed reactor 18. At demand and in the event that the concrete fines are not carbonated to the desired degree, the partially carbonated concrete fines 15 are fed back to the circulating fluidized bed reactor 18. When these have to be further ground to activate the material, they can be fed back as particles 16 to the mill 8.

(31) A significant portion of the CO.sub.2 is already absorbed in the mill 8 and to a smaller extent in the crusher 10. The remaining portion is absorbed in a fluidized bed reactor 18. As the concrete fines have to be brought into contact with the feed CO.sub.2 multiple times, a continuous returning of the fines to the fluidized bed reactor 18 and/or the mill 8 is arranged by storing the concrete fines in the hopper of the filter 17.

(32) The back-dosing of the stored material to the mill 8 or to the reactor 18 orifice and the withdrawal as carbonated concrete fines 19 is enabled by e.g. a rotary valve or V-slot turning gate arrangement 30. The average number of recirculation to the mill 8 or reactor 18 is from 10 to 100 dependent on the milling intensity and achieved surface activity. The usual solid material load in the reactor 18 is 10 to 100 kg/m.sup.3 (S,T,P). To facilitate the correct hold-up in the reactor 18, the feed grain size has to be in a certain range, which is also a function of the chosen vertical speed, which is typically 5 to 9 m/s.

(33) The temperature of the carbonating atmosphere in the mill 8 and reactor 18 is kept within a range of 2 to 7° C. above water dew point. This usually ranges from 58 to 83° C. Thus, the circulating gas, mainly comprising H.sub.2O, CO.sub.2, N.sub.2 and O.sub.2 should have a temperature from 60 to 90° C.

(34) In the event of excess heat, the required cooling is arranged by injection of water in the mill 8 via water control valve 31. Alternatively, the water dew point can be adjusted via the valve (31).

(35) The main purpose of the induced draft fans 6 and 26 is to provide the necessary gas flow in the device and overcome a developed pressure drop. In particular, the heat exchanger 103, the mill 8, the reactor 18 and the filter 17 typically develop an overall combined pressure drop of 8 to 16 kPa. The specific particulate load of the system is 1 to 5 kg/m.sup.3 in the routing from the mill 8 to the filter 17. After the filter 17 the particle load to the heat exchanger 103 should ideally be <5 mg/Nm.sup.3 to prevent scaling at the exchange surfaces of the heat exchanger 103.

(36) The distribution of cycle gas split to the mill 8 and the crusher 10 is controlled by a gas splitter 32 and by trimming the fans 6 and 26, which results in the required lift speed in the mill 8 as well as in the crusher 10.

(37) For start-up of the system a secondary heat exchanger 21 is required to pre-heat the device parts. The necessary heat is provided by burning of oil or gaseous fuels in the burner 23. Ideally, natural gas or any other gas composed of hydrocarbons is used as fuel. The generated burner off gas is vented as waste gas 28 during the heating up. In the event of a heat deficiency in the gas cycle after heating up, the burner off-gas can at least partially be guided to the mill 8. However, usually the grinding heat, the crushing heat and the heat developed during gas conveying is sufficient to drive the process.

LIST OF REFERENCE NUMBERS

(38) 1 gas containing carbon dioxide (kiln gas portion to mill) 2 kiln exhaust gas portion bypassing mill 3 gas cooler and condenser 4 gas-to-gas heat exchanger 5 condensate collection siphons 6 induced draft fan 7 mixing point 8 (vertical roller) mill 9 concrete fines feed 10 (pre-)crusher 11 foreign or largely non-mineral portion of the crushed concrete 12 recyclable coarse fraction minerals 13 concrete fines, partially carbonated 14 decarbonated gas to stack 15 concrete fines feed back to the circulating fluidized bed reactor 16 too coarse concrete fines 17 system fabric filter 18 circulating fluidized bed reactor 19 carbonated concrete fines 20 heat exchanger 21 heat exchanger 22 mixing point 23 burner 24 condensate 25 recycle gas 26 induced draft fan 27 three-way screening system 28 burner waste gas 29 burner off gas to mill 30 solid control valve or splitter arrangement 31 cooling water injection 32 crusher feed gas splitter 33 static separator 100 liquid and pressurized carbon dioxide hold 101 evaportaed and heated carbon dioxide 102 carbon dioxide dosing unit 103 heat-exchanger 104 droplet separator