METHOD OF PROCESSING EXHAUST GAS FROM A CEMENT PLANT

Abstract

A method of processing exhaust gas from a cement plant, the cement plant including one or several raw meal mills for milling a cement raw meal, one or several preheater strings, each having a plurality of preheater stages, and a rotary kiln, wherein the one or several preheater strings are for preheating the cement raw meal in counter-current flow with an exhaust gas from the rotary kiln. The method includes extracting the exhaust gas from a second highest or highest stage of at least one of the preheater string(s), cooling the exhaust gas to a temperature of <250 C., dedusting the exhaust gas, introducing the cooled and dedusted exhaust gas into a carbon capture installation, in which CO.sub.2 is separated from the exhaust gas for obtaining a CO.sub.2 lean gas, wherein the exhaust gas, when being introduced into the carbon capture installation, has a CO.sub.2-content of >25 vol.-%.

Claims

1. A method of processing exhaust gas from a cement plant, the cement plant comprising one or several raw meal mills for milling a cement raw meal, one or several preheater strings, each having a plurality of preheater stages, and a rotary kiln, wherein the one or several preheater strings are for preheating the cement raw meal in counter-current flow with an exhaust gas from the rotary kiln, wherein the method comprises: extracting the exhaust gas from a second highest or highest stage of at least one of the one or several preheater strings, cooling the exhaust gas to a temperature of <250 C., dedusting the exhaust gas, introducing the cooled and dedusted exhaust gas into a carbon capture installation, in which CO.sub.2 is separated from the exhaust gas for obtaining a CO.sub.2 lean gas, wherein the exhaust gas, when being introduced into the carbon capture installation, has a CO.sub.2-content of >25 vol.-%.

2. The method according to claim 1, further comprising introducing the CO.sub.2 lean gas into at least one of the one or several raw meal mills and using the CO.sub.2 lean gas in the at least one of the one or several raw meal mills to dry the cement raw meal.

3. The method according to claim 2, further comprising extracting the CO.sub.2 lean gas from the at least one of the one or several raw meal mills and discharging it through an exhaust stack of the cement plant.

4. The method according to claim 1, wherein the entire flow of exhaust gas leaving the at least one of the one or several preheater strings is subjected to the cooling step and introduced into the carbon capture installation.

5. The method according to claim 1, wherein said step of cooling the exhaust gas comprises causing the exhaust gas to flow through a heat exchanger, in which the exhaust gas is cooled in heat exchange with the CO.sub.2 lean gas that is being heated.

6. The method according to claim 5, wherein the CO.sub.2 lean gas is subjected to an additional heating step before being introduced into the at least one of the one or several raw meal mills.

7. A cement production unit for carrying out a method according to claim 1, comprising: one or several preheater strings for preheating cement raw meal, each preheater string comprising a plurality of preheater stages, a rotary kiln, and optionally a pre-calciner, for calcining the pre-heated cement raw meal and obtain cement clinker, a cooling unit, a dedusting unit and a carbon capture installation that are connected to at least one of the one or several preheater strings in order to extract exhaust gas from a second highest or highest stage of the at least one of the one or several preheater strings, to cool and dedust the exhaust gas and to introduce the cooled and dedusted exhaust gas into the carbon capture installation, in which CO.sub.2 is separated from the exhaust gas for obtaining a CO.sub.2 lean gas.

8. The cement production unit according to claim 7, further comprising one or several raw meal mills for milling a cement raw meal, wherein an outlet line of the carbon capture installation is connected to at least one of the one or several cement raw mills for introducing the CO.sub.2 lean gas from the carbon capture installation into the at least one of the one or several raw meal mills.

9. The cement production unit according to claim 7, wherein the cooling unit is designed as a heat exchanger, in which the exhaust gas is cooled in heat exchange with the CO.sub.2 lean gas that is being heated.

10. The cement production unit according to claim 9, wherein an additional heat exchanger is provided for heating the CO.sub.2 lean gas before being introduced into the at least one of the one or several raw meal mills.

11. The method according to claim 1, wherein the exhaust gas, when being introduced into the carbon capture installation, has a CO.sub.2-content of >30 vol.-%.

12. The method according to claim 5, wherein the exhaust gas is cooled in heat exchange with the CO.sub.2 lean gas that is being heated, before being introduced into the at least one of the one or several raw meal mills.

13. The cement production unit according to claim 9, wherein the exhaust gas is cooled in heat exchange with the CO.sub.2 lean gas that is being heated, before being introduced into the at least one of the one or several raw meal mills.

Description

[0058] The invention will now be described in more detail with reference to the attached drawings. Therein,

[0059] FIG. 1 shows a layout of a cement plant as commonly used and

[0060] FIG. 2 shows a layout of a cement plant for carrying out the method of the invention.

[0061] FIG. 1 schematically illustrates a cement plant, in which raw meal 2 is ground in a raw meal mill 3 and the ground raw meal is charged into a preheater string 4, where it is preheated in counter-current to the hot exhaust gases 9 coming from a rotary clinker kiln 5. The preheater string 4 comprises a plurality of interconnected preheaters, such as cyclone suspension-type preheaters. The preheated and optionally pre-calcined raw meal is then introduced into the rotary kiln 5, where it is calcined to obtain cement clinker. The clinker leaves the rotary kiln 5 and is cooled in a clinker cooler 6. The cooled clinker is charged into a cement mill 7, where the clinker is ground to a desired fineness, optionally together with other components of the final product, such as supplementary cementitious substances and gypsum.

[0062] In FIG. 1, the flow of solid material is shown with solid lines, while the flow of gas is shown with dotted lines. It can be seen that cooling air 8 is introduced into the clinker cooler 6, where the air is heated in heat exchange with the clinker. The heated air leaving the clinker cooler 6 is introduced into the rotary kiln 5, where the preheated raw meal is calcined, i.e. decarbonated, while releasing CO.sub.2. The CO.sub.2 enriched exhaust gas 9 is introduced into the preheater string 4 in order to preheat the raw meal. The exhaust gas withdrawn from the preheater string 4 is introduced into a gas conditioning tower 10 via the kiln ID (induced draft) fan 15, where water may be injected in order to cool the exhaust gas. In a typical operation mode, the cooled exhaust gas may be introduced into the raw meal mill 3 via the line 13 for preheating the raw meal and further cooling the exhaust gas. The exhaust gas leaving the raw meal mill is loaded with fine particles of raw meal and is introduced into the main filter 11 for separating said fine particles from the exhaust gas. The exhaust gas is withdrawn from the main filter 11 and directed through an exhaust stack 12.

[0063] If the raw meal mill 3 is not in operation, the cooled exhaust gas coming from the gas conditioning tower 10 is directly led to the main filter 11 via the line 14, where cement kiln dust entrained from rotary kiln 5 is separated from the exhaust gas.

[0064] FIG. 2 shows a cement plant layout for carrying out the invention. In FIG. 2, the cement plant 1 of FIG. 1 has been adapted with respect to the exhaust gas that leaves the preheater 4 via the kiln ID fan 15. Instead of directing this exhaust gas directly through the conditioning tower 10 and then into the raw meal mill 3, the exhaust gas taken from the uppermost stage of the preheater 4 is first cooled in a heat exchanger 16 to a temperature of <180 C., and then introduced via line 17 and a filter 21 into a carbon capture installation 18, in which CO.sub.2 is separated from the exhaust gas for obtaining an essentially CO.sub.2 lean gas and substantially pure CO.sub.2 19. The CO.sub.2 lean gas is directed to the raw meal mill 3 via line 20 and the heat exchanger 16, and used in the raw meal mill 3 to dry the cement raw meal. In the heat exchanger 16 the CO.sub.2 lean gas is heated in heat exchange with the exhaust gas that comes from the preheater 4.

[0065] In an exemplary embodiment, the carbon capture installation 18 comprises a Direct Contact Cooler 22 to further cool down the exhaust gas and remove the minor impurities like acids (SOx, HCl, HF), a dryer 23, a CO.sub.2 pre-concentrator 24, and a CO.sub.2 purification unit 25. The CO.sub.2 pre-concentrator 24 may be configured to separate CO.sub.2 from the exhaust gas by pressure swing adsorption (PSA), vacuum swing adsorption (VSA) or membranes.

[0066] The CO.sub.2 lean gas leaving the carbon capture installation 18 is a gas that contains substantially no CO.sub.2 and no H.sub.2O. It substantially consists of N.sub.2 and O.sub.2.

[0067] Optionally, an additional heater 26 may be installed upstream or downstream of the heat exchanger 16, in order to further increase the temperature of the CO.sub.2 lean gas before introducing it into the raw meal mill 3.