Process and apparatus for eliminating NOX and N2O

Abstract

A process for removing N.sub.2O and NO.sub.x from offgases by catalytic decomposition of N.sub.2O by means of iron-containing zeolite catalysts and catalytic reduction of the NO.sub.x by means of reducing agents, the deNO.sub.x stage connected downstream of the deN.sub.2O stage being operated at inlet temperatures of T<=400 C., and the inlet gas for the deN.sub.2O stage comprising water and having a selected N.sub.2O/NO.sub.x ratio, and the operating parameters of temperature, pressure and space velocity of the deN.sub.2O stage being selected so as to result in an N.sub.2O degradation of 80 to 98%. Under these conditions, a degree of NO.sub.x oxidation of 30-70% is established at the outlet of the deN.sub.2O stage, which is defined as the ratio of the molar amounts of NO.sub.2 to the total molar amount of NO.sub.x. The result of this is that the downstream deNO.sub.x stage can be operated under optimal conditions. Also provided is an apparatus for carrying out the process.

Claims

1. A process for reducing the NO.sub.x and N.sub.2O contents in gases comprising NO.sub.x and N.sub.2O, comprising the steps of a) passing a gas stream comprising N.sub.2O, NO.sub.x and water into a deN.sub.2O stage comprising an iron-laden zeolite catalyst to reduce the N.sub.2O content by decomposing the N.sub.2O to nitrogen and oxygen, such that said gas stream entering the deN.sub.2O stage and a gas stream exiting the deN.sub.2O stage are characterized by compositional requirements selected from (i) or (ii): (i) the gas stream entering the deN.sub.2O stage has a water content between 1.0 and 10% by volume and the ratio of the molar amount of N.sub.2O in the gas stream entering the deN.sub.2O stage to the molar amount of NO.sub.x in the gas stream exiting the deN.sub.2O stage (N.sub.2O/NO.sub.x) is at least 1.0; or (ii) the gas stream entering the deN.sub.2O stage has a water content between 0.1 and less than 1.0% by volume and the ratio of the molar amount of N.sub.2O in the gas stream entering the deN.sub.2O stage to the molar amount of NO.sub.x in the gas stream exiting the deN.sub.2O stage (N.sub.2O/NO.sub.x) is at least 1.5; wherein the temperature of the gas stream in the deN.sub.2O stage having been adjusted to a value between 400 C. and 650 C., the pressure in the deN.sub.2O stage having been adjusted to a value between 1 and 50 bar abs, and the space velocity in the deN.sub.2O stage having been adjusted to such a value as to result in an N.sub.2O degradation of 80% to 98% in the deN.sub.2O stage, with the additional proviso that the degree of NO.sub.x oxidation at the outlet of the deN.sub.2O stage is at least 30%, b) supplying the gas stream leaving the deN.sub.2O stage to a cooling apparatus and cooling the gas stream, as it flows through this apparatus, to a temperature below 400 C., and c) supplying the gas stream leaving the cooling apparatus to a deNO.sub.x stage for catalytic reduction of NO.sub.x with a reducing agent in the presence of a deNO.sub.x catalyst, with addition of such an amount of reducing agent which is sufficient to reduce the desired proportion of NO.sub.x to the gas stream, viewed in flow direction, after it leaves the deN.sub.2O stage and before it flows through the deNO.sub.x catalyst.

2. The process as claimed in claim 1, wherein the ratio of the molar amount of N.sub.2O which enters the deN.sub.2O stage to the molar amount of NO.sub.x which leaves the deN.sub.2O stage is at least 1.5.

3. The process as claimed in claim 1, wherein the molar ratio of N.sub.2O and NO.sub.x in the gas stream comprising NO.sub.x, N.sub.2O and water, even before it enters the deN.sub.2O stage, is at least 1.5, or wherein a reducing agent for NO.sub.x is added to the gas stream comprising NO.sub.x, N.sub.2O and water before or on entry thereof into the deN.sub.2O stage in such an amount that the NO.sub.x present in the gas stream is partly degraded, such that the molar ratio of N.sub.2O and NO.sub.x, immediately after the entry of the gas stream comprising NO.sub.x and N.sub.2O in the deN.sub.2O stage, is at least 1.5.

4. The process as claimed in claim 3, wherein the water content of the gas stream comprising N.sub.2O, NO.sub.x and water, before it enters the deN.sub.2O stage, is adjusted by addition of water vapor and/or by addition of water in liquid form.

5. The process as claimed in claim 1, wherein the water content of the gas stream comprising N.sub.2O, NO.sub.x and water before it enters the deN.sub.2O stage is adjusted by introducing an offgas stream from a combustion stage in which hydrogen and/or hydrogen-containing compounds are combusted, and/or wherein the water content of the gas stream comprising N.sub.2O, NO.sub.x and water is adjusted before it enters the deN.sub.2O stage by passing it through a water loading apparatus selected from a group comprising saturators and absorption towers.

6. The process as claimed in claim 1, wherein the gas stream comprising N.sub.2O, NO.sub.x and water, before it enters the deN.sub.2O stage, is heated by means of a heating apparatus to a temperature between 400 C. and 650 C.

7. The process as claimed in claim 1, wherein the resulting gas stream from the deN.sub.2O stage is supplied to a cooling apparatus which is a recuperator, the heat released being transferred to the gas stream comprising N.sub.2O, NO.sub.x and water before it enters the deN.sub.2O stage, thus heating it to a temperature between 400 C. and 650 C.

8. The process as claimed in claim 1, wherein the gas stream comprising N.sub.2O, NO.sub.x and water, viewed in flow direction, is divided into two substreams upstream of a heating apparatus, a first substream, after bypassing the heating apparatus, being combined again with the second substream which has passed through the heating apparatus, or wherein the gas stream bypasses the heating apparatus, the amount of the sub streams being regulated by means of a valve.

9. The process as claimed in claim 1, wherein the deN.sub.2O stage has an upstream guard bed which comprises random packings or structured packings of shaped bodies comprising alumina.

10. The process as claimed in claim 1, wherein the deNO.sub.x stage comprises a deNO.sub.x catalyst based on V.sub.2O.sub.5TiO.sub.2.

11. The process as claimed in claim 1, wherein the process in the deN.sub.2O stage is performed at space velocities of 2000 to 50 000 h.sup.1.

12. The process as claimed in claim 1, wherein the reducing agent for NO.sub.x is ammonia.

13. The process as claimed in claim 1, wherein space velocity, temperature and pressure in the deNO.sub.x stage are adjusted such that NO.sub.x conversions between 80% and 100% are attained.

14. The process as claimed in claim 1, wherein the ratio of the molar amount of N.sub.2O which enters the deN.sub.2O stage to the molar amount of NO.sub.x which leaves the deN.sub.2O stage is at least 2.

15. The process as claimed in claim 1, wherein the ratio of the molar amount of N.sub.2O which enters the deN.sub.2O stage to the molar amount of NO.sub.x which leaves the deN.sub.2O stage is at least 5.

16. The process as claimed in claim 6, wherein the heating apparatus is a heat exchanger.

17. The process as claimed in claim 1, wherein the iron-laden zeolite catalyst of the deN.sub.2O stage is based on a BEA or WI type zeolite.

18. The process as claimed in claim 1, wherein the iron-laden zeolite catalyst of the deN.sub.2O stage is based on a ZSM-5 zeolite.

19. The process as claimed in claim 1, wherein the process in the deN.sub.2O stage is performed at space velocities of 2500 to 25 000 h.sup.1.

20. The process as claimed in claim 1, wherein the process in the deN.sub.2O stage is performed at space velocities of 3000 to 20 000 h.sup.1.

21. The process as claimed in claim 1, wherein space velocity, temperature and pressure in the deNO.sub.x stage are adjusted such that NO.sub.x conversions between 90% and 100% are attained.

22. The process as claimed in claim 1, wherein the iron-laden zeolite catalyst of the deN.sub.2O stage is based on a zeolite type selected from the group consisting of MFI, BEA, FER, MOR, MEL, and mixtures thereof, and the deNO.sub.x stage comprises a transition metal-comprising SCR catalyst.

23. The process as claimed in claim 1, wherein the N.sub.2O degradation in the deN.sub.2O stage is 90 to 95%.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The invention is described in detail above and below in connection with numerous examples and in connection with the attached Figures. In the Figures:

(2) FIG. 1 is a graph illustrating the mole fractions of NO and NO.sub.2 in thermodynamic equilibrium at 1 bar abs proceeding from 500 ppm of NO, 500 ppm of NO.sub.2, 2% by volume of O.sub.2 and remainder N.sub.2;

(3) FIG. 2 is a schematic diagram of the process according to the invention and of the plant for reducing the content of NO.sub.x and N.sub.2O in gases, such as process gases or offgases; and

(4) FIG. 3 is a schematic diagram of a preferred variant of the process according to the invention and of the inventive plant.

DETAILED DESCRIPTION

(5) The inventive system will be explained in detail by way of example hereinafter with reference to two figures, without any intention of a restriction thereby. The figures show:

(6) FIG. 2: an outline of the process according to the invention and of the plant for reducing the content of NO.sub.x and N.sub.2O in gases, such as process gases or offgases;

(7) FIG. 3: a preferred variant of the process according to the invention and of the inventive plant.

(8) FIG. 2 shows an outline of the process according to the invention/of the apparatus according to the invention. What is shown is an apparatus (2) for adjusting the water content of the gas (1) comprising NO.sub.x and N.sub.2O. The water-laden offgas then enters a deN.sub.2O stage (3) which comprises an iron-laden zeolite catalyst. After passing through the deN.sub.2O stage (3), the gas stream (5) is passed through a cooling apparatus (4) and then enters a deNO.sub.x stage (6), and subsequently leaves the inventive cleaning plant. At the start of the deNO.sub.x stage (6), a feed line (7) is provided for introduction of reducing agent for NO.sub.x into the gas stream (5) leaving the deN.sub.2O stage (3). In the outline, this feed line (7) is arranged immediately upstream of the deNO.sub.x stage (6); it may also be arranged in the region between the outlet of the deN.sub.2O stage (3) and the position shown in FIG. 2. This feed line (7) may also open into the deNO.sub.x stage (6) itself, but upstream of the entry of the gas stream into the catalyst bed of the deNO.sub.x stage (6).

(9) FIG. 3 shows an outline of a preferred variant of the inventive system. A gas stream (1) comprising NO.sub.x and N.sub.2O is passed into a heat exchanger (18). Heat is supplied to the gas stream (1) therein, resulting in a heated gas stream (21). A substream (13) of the gas stream (1) can bypass the heat exchanger (18) and is subsequently introduced together with the other substream of the gas stream (1) and with an offgas stream (15) from a burner (9) into a deN.sub.2O stage (3). In burner (9), air (11) and hydrogen as combustion gas (10) are combusted. The hot offgas (15) heats the gas stream (1) further, and the water content in this gas stream (1) is also increased. The latter subsequently passes, as gas stream (12), into the deN.sub.2O stage (3) which comprises an iron-containing zeolite preferably surrounded by an Al.sub.2O.sub.3 bed. The Al.sub.2O.sub.3 bed protects the iron-containing zeolite from, for example, phosphate which may additionally be present in the gas stream. At the start of the deN.sub.2O stage (3) is a feed line (17) for introduction of NH.sub.3, which serves as a reducing agent for partial degradation of the NO.sub.x present in gas stream (12), which establishes an optimal N.sub.2O/NO.sub.x ratio. The gas stream (5) leaving the deN.sub.2O stage (3), which has an optimal degree of NO.sub.x oxidation of approximately NO:NO.sub.2=1:1, is then passed through the heat exchanger (18) for heat exchange. At the same time, the gas stream releases the stored heat in the cooling apparatus (4) (here: part of the heat exchanger (18)) to the gas stream (1) which comprises NO.sub.x and N.sub.2O and is to be heated, and is itself cooled. In the next cleaning step, the gas stream thus cooled passes through the deNO.sub.x stage (6), into which NH.sub.3 is introduced through line (7) in addition to the NO.sub.x degradation. The gas stream which has thus been depleted of N.sub.2O and NO.sub.x leaves the unit (6), is passed into a turbine (19) and is then released to the environment (20).

(10) In order to ensure optimal startup of the inventive apparatus, the substream (13) is provided in this illustrative embodiment. The volume of this substream can be controlled via the valve (14). This can ensure that the deN.sub.2O stage (3) in particular is brought to the temperature of the NO.sub.x- and N.sub.2O-containing residual gas stream (1) within a short time. This eliminates the sluggishness of the system during startup.

(11) In the case of startup from the cold state, the valve (14) is thus opened such that a substream bypasses the heat exchanger (18). As soon as a sufficient exit temperature of the deN.sub.2O stage (3) has been attained, the burner (9) is lit in order to further raise the temperature of the gas stream to be cleaned in the deN.sub.2O stage (3). The water concentration in the gas stream (12) also increases as a result of the supply of the offgas (15). When the optimal operating conditions for the deN.sub.2O stage (3) have been attained, the valve (14) is closed and the bypassing of the heat exchanger (18) by the substream (13) is prevented. During operation, the opening of the valve (14) can also be adjusted so as to result in an optimal combination of inlet temperature and water content in the deN.sub.2O stage.

(12) In the case of restart of the inventive plant after a brief shutdown, which means that the plant is still in the warm state, the valve (14) is closed and the entire volume flow of the NO.sub.x- and N.sub.2O-containing residual gas stream (1) passes through the heat exchanger (18). In this case, the burner (9) is lit immediately since the temperature in the deN.sub.2O stage (3) is already sufficiently high from the start. The water concentration increases in accordance with the water content of the offgas (15). The inlet temperature of the deN.sub.2O stage (3) rises further due to the preheating in the heat exchanger (18) and in the burner (9) until the normal operating temperature has been attained.

(13) In addition, the control of the volume of the substream (13) can allow optimal partial load operation to be ensured. Without the possibility of bypassing the heat exchanger (18), the temperature of the gas stream (1) would be too high in partial load operation, since the size of the heat exchanger (18) would be excessive. The burner output would have to be throttled, the energy recovery in the turbine (19) would become less, and NO.sub.x slippage would additionally increase, which would be released to the environment. These are disadvantages which are eliminated by the system described by way of example here, by reducing the area of the heat exchanger (18) in operation with the substream (13).

(14) The invention is illustrated by the examples which follow in tables 1 and 2. The data reproduced result from a kinetic simulation of the NO.sub.x-assisted N.sub.2O decomposition and of the NO.sub.x equilibrium with the aid of the Presto Kinetics software from CiT GmbH for a catalyst bed of cylindrical pellets of Fe-ZSM-5 (diameter 2.6 mm, length 5.7 mm) in a flow tube reactor with axial flow. The reactor model used was developed on the basis of laboratory tests and verified by studies in a Mini-Plant, operated with the abovementioned catalyst extrudates on the liter scale.

(15) TABLE-US-00001 TABLE 1 Example 1a 1b 1c 1d 1e 2 3 4a P in bar abs 1 1 1 1 1 1 1 1 T in C. 480 480 480 480 480 480 480 480 [N.sub.2O].sub.in in ppm 2000 2000 2000 2000 2000 2000 2000 2000 [NO].sub.in in ppm 100 100 100 100 100 200 500 [NO.sub.2].sub.in in ppm 100 100 100 100 100 200 500 [NO.sub.x].sub.out in ppm 200 200 200 200 200 200 200 1000 [H.sub.2O].sub.in in % vol 3 3 3 3 3 3 3 3 [O.sub.2].sub.in in % vol 3 3 3 3 3 3 3 3 [N.sub.2].sub.in in % vol remainder remainder remainder remainder remainder remainder remainder remainder Space velocity 6.1 5.2 4.6 3.9 3.0 4.5 4.7 9.8 in 1000 h.sup.1 N.sub.2O 85% 90% 93% 96% 99% 93% 93% 80% degradation Degree of 50% 50% 50% 50% 50% 100% 0% 50% NO.sub.x oxidation at the inlet Degree of 55.0% 46.4% 40% 32.1% 21.8% 40% 40% 38.6% NO.sub.x oxidation at the outlet Degree of 16.5% 16.5% 16.5% 16.5% 16.5% 16.5% 16.5% 16.5% NO.sub.x oxidation at equilibrium Example 4b 4c 5a 5b 5c P in bar abs 1 1 5 5 5 T in C. 480 480 480 480 480 [N.sub.2O].sub.in in ppm 2000 2000 2000 2000 2000 [NO].sub.in in ppm 500 500 500 500 500 [NO.sub.2].sub.in in ppm 500 500 500 500 500 [NO.sub.x].sub.out in ppm 1000 1000 1000 1000 1000 [H.sub.2O].sub.in in % vol 3 3 3 3 3 [O.sub.2].sub.in in % vol 3 3 3 3 3 [N.sub.2].sub.in in % vol remainder remainder remainder remainder remainder Space velocity 8.3 1.0 14.2 12.4 8.6 in 1000 h.sup.1 N.sub.2O 85% 90% 90% 93% 98% degradation Degree of 50% 50% 50% 50% 50% NO.sub.x oxidation at the inlet Degree of 34.9% 30.5% 44.4% 41.4% 34.9% NO.sub.x oxidation at the outlet Degree of 16.5% 16.5% 30.6% 30.6% 30.6% NO.sub.x oxidation at equilibrium

(16) TABLE-US-00002 TABLE 2 Example 6a 6b 7a 7b 8a 8b 9a 9b P in bar abs 1 1 1 1 1 1 1 1 T in C. 480 480 480 480 480 480 480 480 [N.sub.2O].sub.in in ppm 1000 1000 1000 1000 1000 1000 1000 1000 [NO].sub.in in ppm 500 500 1000 1000 500 500 [NO.sub.2].sub.in in ppm 500 500 1000 1000 500 500 [NO.sub.x].sub.out in ppm 1000 1000 1000 1000 1000 1000 1000 1000 [H.sub.2O].sub.in in % vol 0.3 0.3 0.3 0.3 0.3 0.3 3 3 [O.sub.2].sub.in in % vol 3 3 3 3 3 3 3 3 [N.sub.2].sub.in in % vol remainder remainder remainder remainder remainder remainder remainder remainder Space velocity 11.8 10.3 11.8 10.3 11.6 10.1 10.1 8.6 in 1000 h.sup.1 N.sub.2O degradation 90% 93% 90% 93% 90% 93% 80% 85% Degree of 50% 50% 0% 0% 100% 100% 50% 50% NO.sub.x oxidation at the inlet Degree of 23.2% 21.5% 19.8% 19.2% 26.7% 23.9% 30.3% 27.7% NO.sub.x oxidation at the outlet Degree of 16.5% 16.5% 16.5% 16.5% 16.5% 16.5% 16.5% 16.5% NO.sub.x oxidation at equilibrium Example 9c 10a 10b 10c 11a 11b P in bar abs 1 1 1 1 1 1 T in C. 480 480 480 480 430 430 [N.sub.2O].sub.in in ppm 1000 2000 2000 2000 1000 1000 [NO].sub.in in ppm 500 100 100 100 100 100 [NO.sub.2].sub.in in ppm 500 100 100 100 100 100 [NO.sub.x].sub.out in ppm 1000 200 200 200 200 200 [H.sub.2O].sub.in in % vol 3 0.3 0.3 0.3 1 1 [O.sub.2].sub.in in % vol 3 3 3 3 3 3 [N.sub.2].sub.in in % vol remainder remainder remainder remainder remainder remainder Space velocity 6.3 9.1 8.0 5.8 2.6 3.0 in 1000 h.sup.1 N.sub.2O degradation 93% 90% 93% 96% 85% 80% Degree of 50% 50% 50% 50% 50% 50% NO.sub.x oxidation at the inlet Degree of 22.9% 40.5% 35.0% 28.7% 36.7% 39.2% NO.sub.x oxidation at the outlet Degree of 16.5% 16.5% 16.5% 16.5% 27.6% 27.6% NO.sub.x oxidation at equilibrium

(17) As evident in examples 1a-1d, inventive adjustment of the operating parameters of the deN.sub.2O stage, especially of an N.sub.2O/NO.sub.x ratio of 2000/200=10, a water content of 3% by volume and suitable selection of the space velocity at the exit of the deN.sub.2O stage, allows establishment of a degree of NO.sub.x oxidation which differs significantly from the thermodynamic equilibrium position (of only 16.5%) and, in accordance with the invention, approaches the theoretical optimum of 50%.

(18) When the space velocity, as shown in noninventive example 1e, is lowered to such an extent that the N.sub.2O degradation is 99%, the degree of NO.sub.x oxidation is only 21.8%, which would mean an inadequate starting position for operation of a downstream deNO.sub.x stage.

(19) The attainment of the desired degree of NO.sub.x oxidation at the exit of the deN.sub.2O stage depends, in a first approximation, on the degree of oxidation at the inlet of the deN.sub.2O stage, as shown in examples 2 and 3.

(20) Examples 5a-c show the positive influence of an increased operating pressure on the degree of NO.sub.x oxidation.

(21) The high water content of 3% by volume has a positive effect in accordance with the invention, as shown by a comparison of examples 1b-1d with examples 10a-10c. At a water content of 0.3% by volume, the N.sub.2O conversion here should be limited to less than 96% in order to achieve a degree of NO.sub.x oxidation of about 30%.

(22) The lowering of the N.sub.2O/NO.sub.x ratio to a value of 2 under otherwise identical conditions in examples 4a-4c shows the influence of the N.sub.2O/NO.sub.x ratio, which, however, with a value of 2 is also still sufficient to achieve the inventive shift in the degree of NO.sub.x oxidation.

(23) If, in contrast, an N.sub.2O/NO.sub.x ratio of 1 is established (examples 6-9), the inventive effect can be achieved only when the input gas has a sufficiently high water content and, at the same time, a sufficiently high space velocity is established, such that a sufficiently low N.sub.2O conversion is attained (ex. 9a). In noninventive examples 9b and 9c, the space velocity is not high enough, or the N.sub.2O conversion achieved is too high and the desired degree of NO.sub.x oxidation is not attained.