AU683242B2 - Recovery of carbon dioxide from kiln gas - Google Patents

Description

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AUSTRALIA

PATENTS ACT 1990 CO0M PLERTER S P RC TPT rAT T QN FOR A STANDARD PATENT S S S @5 S S Sr *5 S S

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ORIGINAL

Name of Applicant; Actual Inventors: Address for Service: Invention Title: THE BOC GROUP, INC.

Ramachandran KRISHNAMVURTHY and Akhilesh KAPOOR SHELSTON WATERS Clarence Street SYDNEY NSW 2000 "RECOVERY OF CARBON DIOXIDE FROM K~ILN' GAS" The following statement is a full description of thin invention, including the best method of performing it known to us:- -la- RECOVERY OF CARBON DIOXIDE FROM KILN GAS FIELD OF THE INVENTION This invention relates to the recovery of carbon dioxide from gas streams, and more particularly to the recovery of 5 carbon dioxide from gaseous effluents from kilns.

BACKGROTUD OF THE INVENTION Certain inorganic substances, such as limestone (calcium carbonate) and dolemite (magnesium carbonate) contain significant quantities of carbon dioxide in the form of 10 carbonates. When these substances are subjected to high temperatures, such as in lime and cement manufacturing processes, tL, carbon dioxide is liberated from the mineral and is generally released to the atmosphere. As part of the manufacturing process the feed material is heated, usually by directly firing a fuel, such as coal or coke or a liquid or gaseous hydrocarbon or mixture of hydrocarbons into a kiln containing the mineral substance, thereby raising the temperature of the mineral substance to as high as 3000c F.

(about 16500 or more, and the carbon dioxide formed during the combustion is released to tUe atmosphere with the carbon dioxide liberated from the mineral. Since the release of carbon dioxide to the atmosphere contributes to the already overburdening "greenhouse effect", it would be desirable from an environmental standpoint to reduce the volume of carbon dioxide discharges, such as those described above, to the atmosphere.

-2- Since high purity carbon dioxide is a valuable commodity, recovery of carbon dioxide from kiln exhaust gases can provide an economic benefit, provided that the value of the recovered carbon dioxide exceeds the recovery cost. In general, it is economically feasible to recover carbon dioxide from waste gas streams only when the concentration of carbon dioxide in the waste gas streams is fairly high, such as in the neighborhood of 40% by volume or more. The exhaust gas stream exiting furnaces such as lime kilns contains up to 40% carbon dioxide when the oxidant is air, and up to 50% or more by volume carbon dioxide when oxygen-enriched air or pure oxygen is used as the oxidant. Accordingly, it is worthwhile to undertake recovery of carbon dioxide from these gas streams.

ge .Exhaust gases from kilns contain ash and other fine solids 15 which must be removed from the gases prior to their discharge from the plant. The fine solids are typically removed from the exhaust gas by filtration using fine filtering equipment such as baghouses. The baghouses contain glass fiber bags through which the exhaust gases pass, which results in the removal of fine particulates from the gases. However, because the kiln exhaust gase.s are very hot, typically as hot as 10000 C. or more, it is n icessary that they be cooled prior to introduction into the filtering equipment. The hot kiln gases are usually cooled by injecting cool air into them soon after they exit the 25 kiln. This rapidly cools the waste gases to temperatures at which they can be further cooled by passage through a water spray or directly introduced into a baghouse. Unfortunately however, the introduction of air into the kiln exhaust gas dilutes the carbon dioxide therein, often to concentrations of 20% by volume or less. Cooling the exhaust gas with water is possible but is typically more expensive than dirept gas cooling.

Since direct injection of a cool gas into a hot gas is an economical procedure for rapidly cooling the hot gas it would be desirable to continue to use this cooling method, bt~i ,i c -I -3without decreasing the concentration of carbon dioxide in the stream, so that recovery of the carbon dioxide from the waste gas stream will remain economically attractive. The present invention accomplishes this goal by using cool carbon dioxide as the heat exchange medium.

It is known to recover carbon dioxide from waste gas streams from kilns. For example, in Romanian Patent No. 91001, Barau et al. describes the recovery of carbon dioxide from the exhaust gas of a rotary lime kiln. Steps to prevent .l 10 unnecessary dilution of carbon dioxide produced in lime kilns are also known. For instance, in Hungarian Patent No. T36257, Body et al. discloses a lime kiln furnace with a feed throat containing alternately operated locks.

9** SUMMARY OF THE INVENTION According to a broad embodiment of the invention high purity carbon dioxide is produced by calcining a carbonate-containing mineral by combusting a high carbon content fuel with an oxidant in the presence of the mineral, thereby producing a hot carbon dioxide-rich exhaust gas; 20 cooling the hot exhaust gas by injecting a cool carbon dioxide-rich gas into the hot exhaust gas; optionally further cooling the gas by passing it through a water spray or by indirect heat exchange with a cool fluid; purifying the cooled exhaust gas to remove particulate impurities therefrom and separating carbon dioxide from the other components of the purified gas stream.

The invention is most advantageously applied to the production of carbon dioxide by the calcination of a calcium carbonate-containing material, such as limestone or dolemite, in a kiln by burning a high carbon content solid, liquid or gaseous fuel, such as coal or coke, liquid hydrocarbons, or natural gas. Preferred fuels are coal and natural gas.

Any oxygen-inert gas mixtures, such as air, oxygen-enriched air, or oxygen-inert gas mixtures can be used in the invention. The preferred oxidant is oxygen-enriched air.

Recovery of the high purity carbon dioxide from the purified exhaust gas may be accomplished by any procedure effective for separating carbon dioxide from gas streams.

Typical carbon dioxide separation procedures are adsorption, absorption, condensation and membrane separation. Preferred separation procedures are adsorption and absorption, and the most preferred separation technique is adsorption, particularly pressure swing adsorption, using a zeolite adsorbent such as a 13 X zeolite.

In a preferred embodiment moisture is removed from the exhaust gas prior to carbon dioxide recovery. The method of moisture removal is not critical, uut in preferred embodiments moisture is removed by passing the exhaust gas through an adsorbent such as silica gel or activated alumina, or mixtures of these.

According to the most preferred embodiment of the invention 20 high purity carbon dioxide is produced by calcining limestone or dolemite, either alone or in the presence of other minerals, in a kiln using coal or natural gas as fuel and oxygen-enriched air as oxidant; the hot exhaust gas issuing from the kiln is cooled by injecting previously cooled and purified exhaust gas into the hot exhaust gas; the cooled gas is purified to remove particulate impurities therefrom; and high purity carbon dioxide is separated from the cooled and purifued gas stream by pressure swing adsorption using a zeolite adsorbent. If desired, moisture can be removed from the high purity carbon dioxide product stream by, for example, passing it through a bed of silica gel or activated alumina adsorbent.

I BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic representation of one embodiment of a system for producing high purity carbon dioxide by the process of the invention; and Fig. 2 is a schematic representation of an alternate embodiment of a system for producing high purity carbon dioxide by the process of the invention.

DETAILED DESCRIPTION OF THE INVENTION The invention can be practiced with any lime or cement 10 manufacturing process in which an inorganic carbonateontaining substance is calcined to remove carbon dioxide by heating the substance by combusting a carbon-containing fuel with an oxidant in the presence of the inorganic substance.

Such processes include calcination of limestone or dolemite in 15 the manufacture of lime and cement.

9* S The process of the invention can be more complet¢ely understood from the following description considered with the appended drawings. Vrlves, lines and equipment not necessary for an understanding of the invention have been omitted from 20 the drawings. Like reference characters designate like or corresponding parts in different views.

Turning now to Fig. 1, illustrated therein is a system for processing a carbonate-containing mineral substance by a preferred embodiment of the invention. The equipment comprising the system of Fig. 1 includes kiln 2, secondary exhaust gas cooler 4, gas purifier 6 and carbon dioxide separator 8.

Kiln 2 may be any type of furnace in which mineral substances are calcined, such as a rotary or vertical kiln.

Kiln 2 is equipped with carbonate-containing fill port, which may be a hopper or the like, fuel line 12, oxidant feed line 14, carbonate-depleted outlet port 16 and exhaust gas line 18, Exhaust gas line 18 connects to the gas inlet to exhaust gas cooler 4. Cooler 4 may be any type of cooler used for cooling hot gases. Preferred coolers are water spray coolers wherein a spray of cool water washes the hot gas passing through the cooler. During the waterwash some of the particulates entrained in the gas stream are washed from the 1 gas and removed from cooler 4 through a cooling water discharge 10 line (not illustrated). Cooler 4 may also be a heat exchanger through which a coolant, such as water passes in indirect contact with the hot gases that are to be cooled. Cooler 4 is equipped with a cooled gas discharge line 20, which is connected to gas purifier 6.

Gas purifier 6 may be any of the well-known types of filter *means used to remove particulate matter from kiln exhaust gases, such as a bag house or an electrostatic precipitator.

Purifier 6 is provided with a clean gas discharge line 22, which connects to the inlet to carbon dioxide separator 8. In the embodiment illustrated in Fig. 1, a cooling gas recycle °ooo line connects line 22 and line 18.

0 o Carbon dioxide separator 8 may be any unit conventionally used for the separation of carbon dioxide from a gas stream.

Typical carbon dioxide separators include absorption systems containing an absorbent which absorbs carbon dioxide but does not appreciably absorb the other gases present in the effluent stream, such as mono- di- and triethanolamine; adsorbent systems containing an adsorbent which adsorbs carbon dioxide in preference to the other gas in the exhaust gas stream, such as zeolites and carbon molecular sieves, etc.; and condensing systems, which cool the gas sufficiently to condense carbon dioxide but not the other components of the gas stream. When separator 8 is an adsorber or a condenser it is preferable to remove moisture from the gas stream prior to carbon dioxidD -i I I Iseparation; otherwise the water would be separated from the gas stream with the carbon dioxide. Water removal can be effected by means of a dryer (not shown) which contains an adsorbent, such as silica gel or alumina which adsorbs moisture in preference to carbon dioxide and the other components of the gas stream. When carbon dioxide separator 8 is an adsorption system it may comprise a single adsorption bed, but usually comprises two or more adsorption beds operated in parallel and out of phase to provide substantially continous operation.

10 Carbon dioxide separator 8 is provided with line 26 for removing the carbon dioxide-rich stream from the system and line 28 for removing the gas stream containing the other components of the kiln gas effluent, (predominantly nitrogen and argon). If nitrogen oxides or sulfur oxides are present in the carbon dioxide-rich gas stream from the kiln, they can be removed, if desired, prior to separation of carbon dioxide from the exhaust gas stream.

The system of Fig. 1 is a preferred embodiment of the invention in that it permits relatively large quantities of 20 cooled gas to be recirculated without diluting the kiln effluent with respect to carbon dioxide content. The recycled gas can be introduced into line 18 by any suitable esvice, such as a dispersing nozzle.

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In operation, a carbonate-containing mineral is calcined in kiln 2 by burning a fuel with an oxidant in the presence of the mineral. The spent mineral is removed from the kiln through part 16 and a hot carbon dioxide-rich exhaust gas is removed through line 18. Upon exiting kiln 2 the hot exhaust gas is mixed with cool recycle gas from line 24, and is thereby cooled. The cooled exhaust gas is passed through cooler 4 (if such a unit is employed) and then through filLer 6, wherein particulate matter, such as flyash, is removed from the gas stream. The cleaned gas is next divided into a recycle stream and a carbon dioxide separator feed stream. A substantially -8pure carbon dioxide product gas is withdrawn from separator 8 through line 26 and the remaining gas stream is vented to the atmosphere through line 28.

The system illustrated in Fig. 2 is similar to the System of Fig. 1, except that recycle line 30 replaces recycle line 24 of the Fig. 1 system. In the system of Fig, 2 a substantially pure carbon dioxide stream is recycled to line 18 to serve as a coolant for the hot gaseous effluent from kiln 2, This embodiment is particularly applicable when separator 8 is an 10 adsorber or a condenser and has the advantage that the carbon dioxide from separator 8 has additional cooling capacity because it is cooler than the gas in line 22 when separator 8 too* is an adsorber or condenser.

The fuel used to provide the heat for calcination of the carbonate-containing material can be any solid, liquid or gaseous fuel. The fuel is preferably a substance that is low in sulfur and nitrogen to avoid the production of sulfur and nitrogen oxides. Preferred fuels are coal, particularly low sulfur coal, and methane, because of their low cost, purity 20 and ready availability.

Preferred oxidants include air, oxygen-enriched air or oxygen-inert gas mixtures. Oxygen-enriched air containing high concentrations of oxygen is preferred because of its ready availability, low cost and low nitrogen content.

Although the invention has been described with reference to specific examples, variations of these are contemplated. For example, other steps, i:ich as sulfur oxide scrubbing, may be included in the process of the invention. Furthermore, supplemental coolin with other gases, such as air, is within the purview of the invention. The scope of the invention is limited solely by the breadth of the appended claims.

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Claims (14)

1. A method of producing high purity carbon dioxide comprising the steps: combusting a carbonaceous fuel with an oxidant in the pre, of an inorganic carbonate-containing substance, thereby producing a hot carbon dioxide-rich gaseous product, cooling the hot carbon dioxide-rich gaseous product by injecting a carbon dioxide-rich gas into the hot gaseous product, purifying the cooled gaseous product by removing particulate impurities therefrom, and to separating high purity carbon dioxide from the purified gaseous product.

2. The method of Claim i, wherein a part of the purified gaseous product of step (c) or the high purity carbon dioxide resulting from step is recycled for use as the carbon dioxide rich cooling gas of step

3. The method of Claim 1, wherein the oxidant is air, oxygen-enriched air or oxygen. 15

4. The method of Claim 1, wherein the carbonaceous fuel is coal, natural gas, a liquid hydrocarbon or mixtures of these.

5, The method of Claim 4, wherein said oxidant is oxygen. o

6. The method of Claim 1, wherein the inorganic carbonate-containing substance is selected from limestone, dolemite or mixtures otifese. 0 20

7. The method of Claim 1, wherein the high purity carbon dioxide is separated from the purified gaseous product by adsorption, absorption, condensation or mixtures of these.

8. A method of producing high purity carbon dioxide comprising the steps: I1rIy Mlono./jap I combusting a carbonaceous fuel with an oxidant selected from air, oxygen-enriched air or oxygen in the presence of a substance selected from limestone, dolemite or mixtures of these, thereby producing a hot carbon dioxide-rich gaseous product, cooling the hot carbon dioxide-rich gaseous product by injecting thereinto part of the purified product of step below, purifying the cooled gaseous product by removing particulate impurities therefrom, removing moisture from the purified cooled gaseous product, and separating high purity carbon dioxide from the moisture-depleted gaseous product. *e 15

9, The method of Claim 8, wherein the carbon dioxide is e* separated from the gaseous product by a pressure swing adsorption process.

10. The method of Claim 9, wherein moisture removal is effected by passage of the purified cooled gaseous product through a bed of adsorbent selected from silica gel, activated alumina and mixtures of these. too""

11. The method of Claim 10, wherein said pressure swing adsorption process is carried out using 13 X zeolite. S e

12. The method of Claim 11, wherein the carbonaceous fuel is selected from coal and natural gas.

13. The method of Claim 12, wherein the oxidant is oxygen.

14. A method of prod'icing high purity carbon dioxide substantially as herein described with reference to Figure 1 or Figure 2 of the accompanying drawings. DATED this 19th Day of April, 1994 TUE BOC GROUP, INC. Attornty: ILE)N K. AEN Fellow Iltnstut 1 t lf R'iint AttniVs rf Austiual of SHIELbSON WAIER'S 0O 7LAC.Q THE DISCLOSUR~E :High purity carbon dioxide-rich gas ig produced from hot, carbon dioxide-rich kiln exhaust gas by corling the exhaust gas 000 by injecting carbon dioxide-rich gas, such as high purity carbon dioxide or prevIously cooled carbon dioxide-rich kiln exhaust gas into the exhaust gas to cool the gas, removing 0:0* particulate impurities from the cooled exhaust gas and separating high pu~rity carbon dioxide from the exhaust gas