Process and device for the purification of waste gas

Abstract

For the purification of waste gas containing carbon compounds and nitrogen oxides by means of a regenerative post-combustion system, at least two regenerators (A, B, C) filled with heat accumulator bodies (7a, 7b, 7c) and connected by a combustion chamber (10) are provided, wherein the waste gas is alternately heated in a regenerator (A, B, C), the carbon compounds are oxidised in the combustion chamber (10), and, with the addition of a nitrogen-hydrogen compound, the nitrogen oxides are reduced in the combustion chamber (10) thermally and thus not catalytically. Remaining nitrogen oxides are removed by means of a catalytically active heat accumulator layer (6a, 6b, 6c) and the addition of a further nitrogen-hydrogen compound in the regenerator (A, B, C) from which the clean gas exits.

Claims

1. Process for the purification of waste gas containing carbon compounds and nitrogen oxides in a regenerative post-combustion system which has at least two regenerators (A, B, C) filled with heat accumulator bodies (7a, 7b, 7c) and connected by a combustion chamber (10), wherein the waste gas is heated alternately in at least one regenerator (A, B, C) to which it is supplied, the carbon compounds are oxidised in the combustion chamber (10), and, with the addition of a nitrogen-hydrogen compound as a reducing agent, a simultaneous reduction of the nitrogen oxides takes place in the combustion chamber (10), and the hot clean gas formed is drawn off by means of at least one further regenerator (A, B, C), characterized in that a catalytically active heat accumulator layer (6a, 6b, 6c) reducing remaining nitrogen oxides to nitrogen using a nitrogen-hydrogen compound is each provided as a lower part of the regenerator (A, B, C), wherein said catalytically active heat accumulator layer (6a, 6b, 6c) is separated from the heat accumulator bodies (7a, 7b, 7c), which are separated above the heat accumulator layer by a space (10a, 10b, 10c), and wherein the catalytically active heat accumulator layer (6a, 6b, 6c) can be separately purified or removed from the regenerator (A, B, C).

2. Process according to claim 1, characterized in that the catalytically active heat accumulator layer (6a, 6b, 6c) is configured in the form of a honeycomb block having prismatic channels.

3. Process according to claim 1, characterized in that the catalytically active heat accumulator layer (6a, 6b, 6c) is used simultaneously as a heat exchanger in which part of the heat of the gas flowing out is accumulated and is available to the raw gas flowing in after the switch-over.

4. Process according to claim 1, characterized in that the nitrogen-hydrogen compound supplied to the combustion chamber (10) is supplied in a hyperstoichiometric manner, wherein the catalytically active heat accumulator layer (6a, 6b, 6c) uses the additionally available nitrogen-hydrogen compound for reducing the nitrogen oxides.

5. Process according to claim 1, characterized in that at least part of the nitrogen-hydrogen compound for reducing the nitrogen oxides is already supplied with the waste gas.

6. Process according to claim 1, characterized in that aqueous solutions of ammonia, carbamic acid or urea are used as a nitrogen-hydrogen compound for reducing the nitrogen oxides in order to decrease the reaction temperature required.

7. Process according to claim 1, characterized in that the catalytically active heat accumulator layer (6a, 6b, 6c) also reduces dioxins and furans in the waste gas.

8. Process according to claim 1, characterized in that, during the course of a partial cycle as the combustion chamber temperature decreases, the amount of a nitrogen-hydrogen compound supplied is continuously increased in the third zone (11a, 11b, 11c) of the combustion chamber (10) of the regenerator (A, B, C) from which the clean gas is drawn off.

9. Process according to claim 1, characterized in that the temperature required for the selective catalytic reduction in the area of the catalytically active heat accumulator layer (6a, 6b, 6c) is between 150 and 300 C., which is reached by dissipating the heat of the gases drawn off from the combustion chamber (10) to the heat accumulator layer (6a, 6b, 6c) passed through.

10. Process according to claim 1, characterized in that the nitrogen-hydrogen compound of the third zone (11a, 11b, 11c) of the combustion chamber (10) of the regenerator (A, B, C) from which the clean gas is drawn off is supplied in a hyperstoichiometric ratio.

11. Process according to claim 1 for the purification of the waste gases arising during the production of cement clinker.

12. Process according to claim 11, characterized in that secondary fuels/raw materials are used for the production of cement clinker so that the waste gas has a sufficient carbon monoxide content for the autothermal operation of the regenerative thermal post-combustion system.

13. Process according to claim 11, characterized in that the nitrogen oxides in the waste formed mainly by the primary combustion of the rotary kiln are partially degraded gas by supplying a nitrogen-hydrogen compound through a selective non-catalytic reduction before the waste gas is supplied to a heat exchanger for preheating the raw meal.

14. A regenerative post-combustion device for carrying out the purification of waste gas containing carbon compounds and nitrogen oxides process according to claim 1, comprising: at least two regenerators filled with heat accumulator bodies and connected by a combustion chamber wherein the waste gas is heated alternately in at least one regenerator to which it is supplied, the carbon compounds are oxidised in the combustion chamber, and, with the addition of a nitrogen-hydrogen compound as a reducing agent, a simultaneous reduction of the nitrogen oxides takes place in the combustion chamber, and the hot clean gas formed is drawn off by means of at least one further regenerator; characterized in that the catalytically active heat accumulator layer (6a, 6b, 6c) contains titanium oxide, tungsten oxide and vanadium oxide as a catalyst.

15. Device for carrying out the process according to claim 14, characterized in that the catalytically active layer (6a, 6b, 6c) consists of elements which have a height from 100 to 1000 mm.

16. Device according to claim 14, characterized in that instead of three regenerators (A, B, C) which the waste gas enters and which the clean gas exits while the third one is purged, a plurality of parallel inlet and outlet regenerators is available.

17. Device according to claim 14, characterized in that a separate regenerator is provided for purging out the raw gas.

18. Use of the device according to claim 14 for the purification of waste gases arising during the production of cement clinker, nitric acid, adipic acid, fertiliser or uranium trioxide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be explained in more detail below by way of example with reference to the enclosed drawing, in which:

(2) FIG. 1 schematically shows an enlarged view of the operating position A-B of the regenerative thermal post-combustion system, in which the waste gas is supplied to the regenerator A, the clean gas is drawn off from the regenerator B and the regenerator C is purged, and

(3) FIG. 2 schematically shows also the two other operating positions B-C and C-A, wherein the waste gas is supplied to the regenerator B, the clean gas is drawn off from the regenerator C and the regenerator A is purged, and the waste gas is supplied to the regenerator C, the clean gas is drawn off from the regenerator A and the regenerator B is purged, respectively.

DETAILED DESCRIPTION

(4) Each regenerator A, B, C, with its end facing away from the combustion chamber 10 of the regenerative thermal post-combustion systems, is connected via an inlet shut-off device 1a, 1b, 1c to the waste gas duct 1, through which the waste gas to be purified is supplied to the regenerator A, B, C, via an outlet shut-off device 2a, 2b, 2c to the clean gas duct 2, by which the clean gas is released into the atmosphere via a stack, for example, and via a purge gas shut-off device 3a, 3b, 3c to a purge gas duct 3a, 3b, 3c. The shut-off devices can be configured in the form of valves or flaps.

(5) The main fan 4 for generating a negative pressure in the regenerators A, B, C is provided in the clean gas duct 2 downstream of the regenerative thermal post-combustion system. The purge gas duct 3 is connected to the waste gas duct 1 via a shut-off device 4 and an auxiliary fan 5.

(6) In the operating position A-B according to FIGS. 1 and 2, the waste gas is supplied to the regenerator A, the clean gas is drawn off from the regenerator B and the regenerator C is purged, whereas in the next cycle in the operating position B-C, the waste gas is supplied to the regenerator B, the clean gas is drawn off from the regenerator C and the regenerator A is purged; in the subsequent cycle according to the operating position C-A, the waste gas is supplied to the regenerator C, the clean gas is drawn off from the regenerator A and the regenerator B is purged, whereupon the operating position A-B is taken again in the next cycle.

(7) The waste gas which is supplied via the waste gas duct 1 has a carbon monoxide content of e.g. 0.2 to 1 percent by volume, a nitrogen oxide content of e.g. 100 to 1000 mg/Nm.sup.3 and an oxygen content of e.g. 8 to 13 percent by volume, the remainder being substantially nitrogen, water and carbon dioxide.

(8) Each regenerator A, B, C has a catalytically active heat accumulator layer 6a, 6b, 6c at its lower area facing away from the combustion chamber 10 as well as heat accumulator bodies 7a, 7b, 7c at a distance above the catalytically active heat accumulator layer 6a, 6b, 6c on the side facing the combustion chamber 10.

(9) The catalytically active heat accumulator layer 6a, 6b, 6c is separated from the heat accumulator bodies 7a, 7b, 7c arranged above by a space 10a, 10b, 10c.

(10) This means that the catalytically active heat accumulator layer 6a, 6b, 6c can be separately purified or removed from the regenerator A, B, C, for example.

(11) In addition, each regenerator A, B, C optionally has a necking 8a, 8b, 8c above the heat accumulator bodies 7a, 7b, 7c and below the connecting area V1, V2 at which two adjacent regenerators A, B, C are connected to each other.

(12) The combustion chamber 10 of the regenerative thermal post-combustion system consists of several zones, namely the zones 11a, 11b, 11c between the heat accumulator bodies 7a, 7b, 7c and the connecting area V1, V2 of the respective regenerator A, B, C and the zone 12 above the connecting areas V1, V2.

(13) The first zone is the zone 11a, 11b, 11c between the heat accumulator bodies 7a, 7b, 7c and the connecting area V1, V2 of the regenerator A, B, C to which the waste gas from the waste gas duct 1 is supplied, thus the zone 11a in FIGS. 1 and 2 in the operating position A-B.

(14) In the first zone 11a, 11b, 11c, carbon monoxide and/or organic compounds are combusted in the waste gas.

(15) At the level of the connecting areas V1, V2, one injection device 14a, 14b, 14c each is provided on each regenerator A, B, C.

(16) In the second zone 12, which is formed by the zone above the connecting areas V1, V2, the nitrogen oxides in the waste gas exiting the first zone 11a, 11b, 11c are largely reduced to nitrogen by means of a nitrogen-hydrogen compound injected by the injection device 14a, 14b, 14c arranged above in each case, namely thermally, i.e. not catalytically.

(17) The third zone 11a, 11b, 11c is formed by the zone between the connecting area V1, V2 and the heat accumulator bodies of the regenerator A, B, C from which the waste gas is supplied to the clean gas duct 2, thus the zone 11b in the operating position A-B according to FIGS. 1 and 2.

(18) In the third zone 11a, 11b, 11c, a further nitrogen-hydrogen compound is injected into the waste gas exiting the second zone 12 by means of the injection device 14a, 14b, 14c at the level of the connecting zone V1, V2, thus the zone 11b in the operating position A-B according to FIGS. 1 and 2, in order to catalytically reduce the remaining nitrogen oxides to nitrogen by means of the catalytically active heat accumulator layer 6a, 6b, 6c.

(19) The fourth zone of combustion chamber 10, which is formed by the zone between the connecting areas V1, V2 and the heat accumulator bodies 7a, 7b, 7c, thus the zone 11c in the operating position A-B according to FIGS. 1 and 2, is supplied with purified gas from the second zone 12 in order to purify the heat accumulator bodies 7a, 7b, 7c and the catalytically active heat accumulator layer 6a, 6b, 6c, thus the heat accumulator bodies 7c and the catalytically active heat accumulator layer 6c in the operating position A-B according to FIGS. 1 and 2, from raw gas residues by sucking this gas into the purge gas duct 3.

(20) According to the invention, the reduction of the nitrogen oxides by carbon compounds, such as carbon monoxide, in the waste gas in the operating position A-B in the first zone 11a, for example, and the reduction of the nitrogen oxides by means of the nitrogen-hydrogen compound injected by the injection device 14a in the second zone 12 are thus carried out purely thermally, while after the nitrogen-hydrogen compound has been injected by the injection device 14b and after the regenerator 7b has been passed through, a catalytic reduction of the remaining nitrogen oxide on the catalytically active heat accumulator layer 6b takes place in the third zone 11b.

(21) After this first cycle, the cycle is switched over to the next cycle according to the operating position B-C, then to the cycle according to the operating position C-A and then back to the cycle according to the operating position A-B according to FIG. 2.

(22) The waste gas from the waste gas duct 1 is thus supplied alternately to the regenerators A, B and C, wherein, as can be seen from FIGS. 1 and 2, the waste gas to be purified is supplied to the preheated regenerator A, and the clean gas is drawn off via the regenerator B so that a gas flow according to the arrow 15 is generated in the combustion chamber 10.

(23) If a waste gas containing a nitrogen-hydrogen compound and/or carbon monoxide is supplied to the preheated heat accumulator bodies of the regenerator A, B, C, part of the nitrogen oxides in the waste gas in the first zone 11a, 11b, 11c will be reduced.

(24) The nitrogen-hydrogen compound supplied at the beginning of the second zone 12 via the injection device 14a, 14b, 14c leads to a thermal reduction of the nitrogen oxides in the second zone 12.

(25) On the other hand, by injecting the nitrogen-hydrogen compound via the injection device at the end of the second zone 12 or at the beginning of the third zone 11a, 11b, 11c, further amounts of nitrogen oxide are catalytically reduced in the catalytically active heat accumulator layer 6a, 6b, 6c.

(26) By means of the shut-off devices 16a, 16b, 16c in the supply duct 16 for the nitrogen-hydrogen compound to the injection devices 14a, 14b and 14c, the supply of the nitrogen-hydrogen compound can be regulated in each case in such a way that a continuous increase in the added nitrogen-hydrogen compound takes place over the course of one cycle of operation.

(27) Since the heat accumulator layer 6a, 6b, 6c and the heat accumulator bodies 7a, 7b, 7c cool down in the course of one cycle of operation due to the waste gas supplied, thereby decreasing the temperature in the zone 12 of the combustion chamber 10, the non-catalytic thermal reduction rate is decreased for reducing the nitrogen oxides in the combustion chamber 10.

(28) The decreased reduction rate due to non-catalytic thermal reduction in the zone 12 can thus be compensated for by an increased supply of a nitrogen-hydrogen compound into the third zone 11a, 11b, 11c, thus by means of the injection device 14b in the operating position A-B according to FIGS. 1 and 2, which is supplied to the catalytically active heat accumulator layer 6b, i.e. by increasing the reduction rate due to catalytic reduction.

(29) Especially in the case of autothermal operation of the post-combustion system, the burner 18 serves to start the system.

(30) The example below, which was carried out using a system for the production of cement clinker and a system for the purification of waste gas according to FIGS. 1 and 2, serves the purpose of further explaining the invention.

EXAMPLE

(31) A waste gas from a rotary kiln for the production of clinker has the following composition:

(32) 15 percent by volume of carbon dioxide

(33) 0.5 percent by volume of carbon monoxide

(34) 10 percent by volume of oxygen

(35) 500 mg/Nm.sup.3 of nitrogen oxides

(36) 30 mg/Nm.sup.3 of ammonia

(37) 100 mg/Nm.sup.3 of organic carbon.

(38) The waste gas with a volume of 300,000 Nm.sup.3/h reaches the regenerative thermal post-combustion system via the duct 1 with the fan 4. The heat accumulator bodies, for example of the regenerator A, heat the waste gas to a temperature of 900 C., at which, in the first zone 11a of the combustion chamber 10, the nitrogen oxides are reduced by the still existing excess ammonia introduced into the waste gas in the system during the previous cycle and by part of the carbon monoxide with the formation of nitrogen. The excess amount of carbon monoxide is oxidised to carbon dioxide by the existing oxygen of the waste gas and contributes to the autothermal mode of operation of the post-combustion system. The volatile organic pollutants and the odour-active substances in the waste gas also combust into carbon dioxide and water vapour in the first zone 11a of the combustion chamber 10.

(39) After leaving the first zone 11a of the combustion chamber 10, the waste gas has the following composition:

(40) 15 percent by volume of carbon dioxide

(41) 0.1 percent by volume of carbon monoxide

(42) 9.6 percent by volume of oxygen

(43) 400 mg/Nm.sup.3 nitrogen oxides

(44) 25 mg/Nm.sup.3 of ammonia

(45) 0 mg/Nm.sup.3 of organic carbon.

(46) At the beginning of the second zone 12 of the combustion chamber 10, 150 kg/h of a 25 percent ammonia solution is injected into water in the direction of flow according to the arrow 15 in order to reduce further amounts of still existing nitrogen oxides. The clean gas is drawn off via the regenerator B, for example. At the end of the second zone 12 of the combustion chamber 10, i.e. at the beginning of the third zone 11b, a further 80 kg/h of a 25 percent by weight ammonia solution is injected into water in the direction of flow in order still to reduce additional amounts of existing nitrogen oxides and to produce an excess of ammonia.

(47) At the end of the third zone 11b of the combustion chamber 10, the purified waste gas has the following composition:

(48) 15 percent by volume of carbon dioxide

(49) 0 percent by volume of carbon monoxide

(50) 9.5 percent by volume of oxygen

(51) 250 mg/Nm.sup.3 nitrogen oxides

(52) 95 mg/Nm.sup.3 of ammonia

(53) 0 mg/Nm.sup.3 of organic carbon.

(54) After passing through the catalytically active layer 6b of the clean gas regenerator B, the waste gas has the following composition:

(55) 15 percent by volume of carbon dioxide

(56) 0 percent by volume of carbon monoxide

(57) 9.5 percent by volume of oxygen

(58) 150 mg/Nm.sup.3 nitrogen oxides

(59) 5 mg/Nm.sup.3 of ammonia

(60) 0 mg/Nm.sup.3 of organic carbon.

(61) If the direction of flow is reversed according to the arrow 17, the dosing of the injected ammonia solution is reversed. The direction of flow is reversed approximately every two to three minutes. The clean gas leaves the post-combustion system at a temperature averaging 40 C. above the inlet temperature.