WO1993014348A1

WO1993014348A1 - CALCINATION ELIMINATING NOx GASES

Info

Publication number: WO1993014348A1
Application number: PCT/FI1993/000005
Authority: WO
Inventors: Viljo JÄRVENPÄÄ
Original assignee: Wiser Oy
Priority date: 1992-01-10
Filing date: 1993-01-07
Publication date: 1993-07-22
Also published as: FI90913B; AU3353193A; FI90913C; FI920103A; FI920103A0

Abstract

The invention relates to a method for preventing NOx gas releases contained in flue gases occurring in the calcination from entering into an outlet flow of cooled flue gases. In the method, the flue gases after the calcination are cooled to a temperature of 400-750 °C and they are delayed at this temperature for at least 0.1 seconds, whereby the NOx gases decompose into initial materials.

Description

Calcination eliminating NOx gases

The present invention relates to a method for preventing NOx gas releases contained in flue gases occurring in the calcination from entering into an outlet flow of cooled flue gases.

As is commonly known, the preparation of nitric acid occurs by calcinating ammonia. Obtaining nitric oxides then requires a very rapid cooling of NOx gases of flue gases in order to make the yield as good as possible. Therefore, a platinum net is utilized immediately after calcination for cooling the gas.

In a physical-chemical equilibrium state, NO decomposes up to 100%, when the temperature is below 700°C. At a temperature below 500°C, NO is in a metastable state. At a temperature of ca. 1700°C, there is in a free air space 0.3- 4% by volume of NO, i.e. ca. 300 mg of NO/m³. In practise, somewhat lower NOx releases may be obtained by means of overair as low as possible.

Normal calcination most generally serves heat production, i.e. the intention is to cause the heat energy to transfer as completely and efficiently as possible from flue gases into the fire surfaces of the boiler and therefrom into water or a corresponding medium on the other side of the wall. The intention is to cool the flue gases as completely as possible and most preferably close to a temperature of 130-150°C, since the lower the temperature of the flue gases reached is, the more efficiently the heat energy is recovered. However, in connection with larger boilers, the intention is not essentially to reach a temperature below 160°C, since e.g. an S0₃ gas starts to condensate from the flue gases, which is generally always slightly contained in the glue gases in addition to S0₂, into the cold portions of the boiler. The condensation results is corrosion. S0₂ also starts to condensate, as the moisture of the calcination air increases and the temperature drops too much in the flue gases (below 170°C). In general, the flue gases in boi- lers rapidly cool to a temperature below 500°C, in which state NO stabilizes, i.e. it does not decompose into initial materials according to the equilibrium dia¬ gram, although NO should decompose into initial materials according the equilibrium diagram. This results from the fact that the decomposition reaction is very slow and nearly non-existent, i.e. the lower the temperature reached is, the lower the decomposition is.

By means of current techniques, attempts have been made to eliminate the NOx releases of flue gases by so-called phased calcination, whereby the intention is to keep the temperature of the calcination below 1000°C, since at this temperature the share of the NOx gases in the flue gases is according to the equilibrium diagram essentially lower than in a normal direct calcination (over 1200°C). However, this has proved to be an extremely demanding task. Only few boilers have reached sufficiently good results, i.e. it has been possible to drop the share of the NOx gases e.g. to 50% in comparison with normal calcination.

Referring to prior art, a solution is also known, in which flue gases are recirculated back to calcination in order to reach a low calcination temperature. The solution has its own disadvantages, especially if the flue contains e.g. chlorine, components producing dioxines, whose calcination requires a calcina¬ tion temperature considerably over 1000°C, even over 1200°C. Then, substantial amounts of NOx gases occur even according to the equilibrium diagram, not to mention the fact the nitrogen content contained in the flue totally calcinates into NOx gases.

Thus, attempts are made by means of techniques known currently to produce as low an amount as possible of NOx gases in the calcination by reducing the calcination temperature, but this causes also other disadvantages than previously mentioned. The known methods, except that they are expensive and contain plenty of automatics, cause insufficient calcination, and max. a 50-60% NOx reduction is reached by means thereof in comparison with normal calcination. The object of the invention is to provide an improvement on the methods known currently for preventing NOx gas releases contained in flue gases occurring in the calcination from entering into an outlet flow of cooled flue gases.

A more detailed object of the invention is to provide a method, which possi- bilitates the decomposition of NOx gases into initial materials as completely as possible, whereby the outlet flow of the cooled flue gases is essentially free from NOx flue gas releases.

The objects of the invention are obtained by means of a method, which is characterized in that the flue gases after the calcination are cooled to a tempera¬ ture of 400-750°C and that they are delayed at this temperature for at least 0.1 seconds, whereby the NOx gases decompose into initial materials.

In the inventive method, the calcination is possible even at high temperatures, but thereafter the temperature of the flue gases is gently dropped to the inven¬ tive temperature range and it is delayed therein, whereby the NOx gases decom¬ pose as completely as possible.

The invention is based on the realization to cause the temperature of the flue gas to drop into the range of 400-750°C as flexibly as possible. Then, according to the equilibrium diagram the NOx releases decompose into initial materials, into oxygen and nitrogen, since the temperature of the NOx gases is sufficiently high such that said decomposition reaction is sufficiently rapid and thermically possible.

The invention is described in detail by reference to certain preferred embodi¬ ments of the invention shown in the figures of the accompanying drawings, to which the invention is yet not solely intended to be limited.

Fig. 1 shows a certain preferred embodiment of the inventive method as a schematic side representation. Fig. 2 shows a second certain preferred embodiment of the inventive method as a schematic side representation.

Fig. 3 shows a certain third preferred embodiment of the inventive method as a 5 schematic side representation.

In the embodiment of Fig. 1, the inventive method is applied to a calcination boiler, which has generally been referred to by a reference number 10. The frame of the boiler 10 has been referred to by a reference number 11, the burner

10 by a reference number 12 and the furnace by a reference number 13. A flame enters into the furnace 13 of the boiler 10 from the burner 12. A flue feed A is introduced into the burner 12 and by means of a blower 21 a flue flow B. In the furnace 13 of the boiler 10, it is assumed that the calcination is as complete as possible, i.e. the amount of calcination air is sufficient.

15

In the furnace 13, the temperature of the flue gases may vary e.g. in the range of 1000-1700°C. Then, also NOx gases occur in the calcination. According to the basic observation of the invention, the flue gases occurring after the calcination are cooled to a temperature of 400-750°C and they are delayed at this tempera- ^'20 ture for at least 0.1 seconds, most preferable 0.5-5 seconds, whereby the NOx gases decompose into initial materials.

In the embodiment of Fig. 1, the cooling of hot flue gases has been realized by means of a co-called front cooler, which may e.g. be a gas-cooled tube heat

25 exchanger or a liquid-cooled tube heat exchanger. In this embodiment, a heat exchanger 14 is shown as being a gas-cooled tube heat exchanger. If the walls of the heat exchanger 14 are most preferably at a temperature of more than 400°C, the contacting flue gas cools to a temperature of max. 500°C. In this embodi¬ ment, the boiler 10 comprises after the heat exchanger 14 a co-called radiation

30 section 15, wherein the hot flue gas, e.g. at a temperature of over 700°C, trans¬ fers heat by means of radiation into a radiation wall 16. In an intermediate space ^{"" '} 17 between the radiation wall 16 and the frame 11 flows either outdoor air or cooled flue gas air. In the embodiment of Fig. 1, an outdoor air flow D flows in the intermediate space 17. It is to be noted that in the radiation space 15 the flow of the flue gases and the cooling outdoor air flow D are opposite relative to each other. At the lower end of the radiation wall 16, the cooling outdoor air flow D communicates with the flue gas flow.

In the inventive method, the temperature of the flue gases is in any case caused to drop to a temperature of below 750°C, most preferably to a temperature of below 700°C. At these temperatures, the NO decomposition is energetically sufficiently rapid, due to which flue gases are obtained as a final result, whose NOx content is practically non-existent.

After the flue gases have cooled to a temperature of below 700°C, they are conveyed into a channel 18, wherein the cooled flue gases are delayed for at least 0.1 seconds, most preferably 0.5-5 seconds. Such a delay at a temperature of 500-700°C results in the decomposition of NOx gases into initial materials into a composition (2 NO -> N₂ + 0-y) corresponding to the equilibrium diagram. After the channel 18, the flue gases are led into a contactor portion 19 of the boiler 10, wherein the flue gases deliver their heat energy into the medium of the boiler 10. The flue gases are removed from an outlet channel 20 of the boiler 10 as a flue gas flow C in a manner known per se.

The embodiment of Fig. 2 is otherwise the same as that shown in Fig. 1, but in the embodiment of Fig. 2 hot flue gases are cooled in the radiation portion 15 in addition to the heat exchanger 14 also by means of a flow D of cooled flue gases. Part of the flue gases flowing in the channel 20 is led by means of a blower 22 as a flow D into an intermediate space 17 for cooling the flue gases.

In the embodiment of Fig. 3, the cooling effect of the heat exchanger 14 is totally compensated for by cooling the hot flue gases by means of the flow D of the flue gases cooled in the radiation portion 15. In accordance with the embodiment of

Fig. 3, the blower 22 controls the cooled flue gas flow D from a perforated transverse plane 24 into the intermediate space 17 formed by through-going tubes 24. The flue gas flow D flows downwards and is admixed at the lower end of the tubes 24 with a flow occurring upstream of the hot flue gases. The admixed gases flow along the tubes 24 upwardly into the channel 18, wherein the delay time corresponds to the embodiments of Fig. 1 and 2. By means of the solution of Fig. 3, the admixing of hot flue gases and cold, but recirculation flue gases heated already in the tubes 24 at the same time as the radiation heat may be transferred into the walls of the tubes 24 and thereby into contact with the recirculation gas flow D. The length of the tubes 24 and the volume of the intermediate space 17 may be varied as needed so that the recirculation flue gas rate D may be distributed as uniform as possible to contact the hot flue gases flowing from the furnace 13.

By means of the inventive method, especially according to the embodiment of Fig. 2 and 3, no energy is lost, but in spite of that, an especially good release of NOx gases is obtained. The inventive method is thus characterized in that hot flue gases are cooled as flexibly as possible either by cooling by means of the gas flow or by cooling surfaces, whose temperature is preferably over 300°C, most preferably over 400°C. In this manner, a possibility is avoided that the NOx gases would stabilize, as is the case in connection with current rapid coolers provided with boiler heat transfer walls. The only way in these solutions known previously for decreasing NOx releases has been the dropping of the calcination tempera¬ ture, which in the inventive method is not necessary at all.

As previously described, the principle of the invention as illustrated by three different embodiments has been shown, i.e a method for decomposing the NOx gases produced in the calcination. It is apparent to those skilled in the art that the inventive method, i.e. a delayed flue gas flow in the inventive temperature range, may be reached by means of various different technical solutions. The inventive temperature range may be reached or it may be reduced by means of certain catalysts. The delay times are not also limited to 5 seconds, but the delay time may be also longer depending on the embodiment.

Claims

1. A method for preventing NOx gas releases contained in flue gases occurring in the calcination from entering into an outlet flow of cooled flue gases, chara- cterized in that the flue gases after the calcination are cooled to a temperature of 400-750°C and that they are delayed at this temperature for at least 0.1 se¬ conds, whereby the NOx gases decompose into initial materials.

2. A method according to Claim 1, characterized in that the flue gases are delayed at said temperature for at least 0.5-5 seconds before further cooling.

3. A method according to Claim 1 or 2, characterized in that the flue gases after the calcination are cooled by means of a gas-cooled heat exchanger (14).

4. A method according to Claim 1 or 2, characterized in that the flue gases after the calcination are cooled by means of a liquid-cooled heat exchanger (14).

5. A method according to Claim 1 or 2, characterized in that the flue gases after the calcination are cooled both by means of the heat exchanger (14) and by cooled flue gases by leading at least part of the outlet flow (C) of the cooled flue gases as a recirculation flow (D) amidst the hot flue gases.

6. A method according to Claim 5, characterized in that the flow (D) of the cooled flue gases is led along the intermediate space (17) between the frame (11) of the boiler (10) and the radiation wall (16) upstream relative to the flow direction of the hot flue gases.

7. A method according to Claim 1 or 2, characterized in that the hot flue gases are cooled by merely leading at least part of the flow (C) of the cooled flue gases as the recirculation flow (D) into the radiation portion (15) of the boiler (10), wherein the flow (D) of the cooled flue gases is admixed with the hot flue gases.

8. A method according to Claim 7, characterized in that the flow (D) of the cooled flue gases is led along the intermediate spaces (17) between the tubes (24) in the radiation portion (15) upstream relative to the flow occurring along the tubes (24) of the hot flue gases, whereby the flow (D) of the flue gases cooled at the lower end of the tubes (24) is admixed with the hot flue gases as efficiently as possible.