FI92102C - Method of removing NOx gases from flue gases - Google Patents

Description

92102 5

Method for removing NOx gases from flue gases Forfarande for att avlågsna NOx gaser från rokgaser

The invention relates to a method for removing NOx gases from flue gases lost in the temperature.

As is generally known, nitric acid is produced by burning ammonia. In this case, the intake of nitrogen oxides requires a very rapid cooling of the NOx gases of the combustion gases in order to obtain the best possible intake. Therefore, a platinum network is used directly to cool the post-combustion gas.

15 At physicochemical equilibrium, NO decomposes 100% when the låm patient is below 700 ° C. At temperatures below 500 ° C NO is metastable. At a temperature of about 1700 ° C, the free air contains 0.3-0.4% by volume of NO, i.e. about 3000 mg NO / m3. Thus, in normal combustion of oils (1200-1500 ° C), 1200-2000 mg NO / m3 of flue gases are always generated. In practice-20 weather with as little overhead as possible, somewhat lower NOx emissions are released.

Normal combustion most commonly serves heat production, ie the aim is to obtain the most complete and efficient heat energy from the combustion gases to the boiler's fire surfaces and from there to the water or similar medium on the other side of the wall. Flue gases should be flushed. the temperature should be as full as possible and preferably close to 130-150 ° C, because the lower the temperature of the flue gases, the more efficiently the thermal energy is recovered. In larger boilers, however, no attempt is made to allow the temperature to be just below 160 ° C, because flue gases begin to condense e.g. SO3 gas, which is usually always i: slightly in the flue gases in addition to SO2, to the cold parts of the boiler. Condensation results in corrosion. SO2 also begins to condense as the humidity in the combustion air increases and the temperature drops too much in the smoke-35 gases (below 170 ° C). In general, in boilers, the flue gases cool down rapidly to a temperature below 500 ° C, in which state the NO stabilizes, i.e. it does not decompose to the elements according to the state diagram, although according to the state diagram NO should decompose to the starting materials. This is due to the fact that the acidification reaction is very slow and almost non-existent, i.e. the lower the temperature, the lower the decomposition.

Current technologies have sought to eliminate NOx emissions from flue gases. in phased combustion, in which case the aim is to keep the combustion temperature below 1000 ° C, because here, according to the state diagram, the share of NOx gases in the flue gases is substantially lower than in normal direct combustion (above 1200 ° C). However, this has proved to be a very demanding task. Only a few burners have achieved sufficiently good results, ie the proportion of NOx gases has been reduced, e.g.

50% compared to normal combustion.

A solution in which flue gases are recycled back to combustion in order to achieve a low combustion temperature is also known from the prior art.

15 The solution has its own disadvantages, especially if the fuel contains e.g.

chlorine, dioxin-generating components, the combustion of which requires a combustion temperature well above 1000 ° C, even above 1200 ° C. At that time, according to the equilibrium diagram, a significant amount of NOx gases is generated, not to mention the fact that the nitrogen content in the fuel burns 20 to NOx gases.

Thus, currently known techniques try to get as little NOx gas as possible from the combustion process by lowering the combustion temperature, but this also causes annoyances other than those found above. Exceptional methods, except that they are expensive and involve a lot of automation, cause incomplete combustion and generally achieve a NOx reduction of up to 50-60% compared to conventional combustion.

30 The formation and concentration of NOx gases in flue gases depends not only; the nitrogen content of the fuel also depends on the temperature at which the combustion takes place. The nitrogen in the fuel burns to almost 100% NOx gases. With regard to the state of the art, reference is made to the article: Holleman-Wiber, Lehrbuch der Anorganischen Chemie, Walter de Gruyter, Berlin-New York 35 1976, page 395, which shows how NOx gases are in equilibrium in the free atmosphere at different temperatures and how NO at different temperatures 3 92102 diverges. It is generally known that NOx gases begin to form above about 730 ° C and the amount of NOx gases increases almost exponentially as the temperature rises. Below the temperature of 500 ° C, NOx gases are in turn exposed to so-called to a metastable state, i.e., NOx gases tend to disperse, but the kinetics of the reaction are so slow that below a temperature of 500 ° C, NO can be considered to be in an almost stable state. It is also well known that NO2 in gases above 650 ° C is no longer possible, but by 650 ° C NO2 is already decomposed into NO and oxygen. Thus, science-10 today NOx gases (NO2 and NO) decompose into elements oxygen and nitrogen in the temperature range of 500-730 ° C. The lower the upper limit of the temperature, the faster the decomposition takes place and vice versa. On the other hand, if the temperature exceeds the upper limit of the temperature range, the more gases in the gases also start to contain NOx gases.

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In ordinary boilers, NOx gases cannot be decomposed at all due to the transfer of the flue gas radiant heat to the cold boiler wall, whereby the NOx gases enter a metastable state. It is known that the gas, which always cools to a temperature of 500 ° C, releases considerably more radiation at temperature than convection. Thus, the cold wall in the boilers prevents the diffusion of NOx gases, even if the NOx gases are in the temperature range of 500-730 ° C on the basis of the temperature measured by a thermometer, because NO. in the temperature range, the radiant heat is transferred to a cold surface by Stefan-Boltzmann's formula 25 W - C A [(Tl / 100) * - (T2 / 100) *], where TI - gas temperature 30 T2 - boiler wall temperature.

It can be seen from the formula that when TI - 973 ° K (700 ° C) and T2 - 523 ° K (250 ° C), the heat transfer is 8214 times the heat transfer as if TI - T2.

35 4 92102

With regard to the state of the art, reference is made to SE patent publication 466814, which discloses raising the temperature of flue gases with a separate bulb from a temperature of 850 ° C upwards to a temperature of 900-1100 ° C for the purpose of decomposing NO 2 gas.

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The object of the invention is to provide a method which makes it possible to remove NOx gases from flue gases which have already been lost in the heat content.

The object of the invention is achieved by a method characterized in that the flue gases are heated to a temperature of 400-750 ° C and delayed at this temperature for at least 0.1 second, whereby the NOx gases decompose into elements.

The invention makes it possible to remove NOx gases from the flue gases by raising the flue gases after the boiler to a temperature range according to the invention of 400-750 ° C, preferably to a temperature range of 500-730 ° C, with separate heating and delaying the heated flue gases in at least 0.1 , preferably for at least 0.5-5 seconds, 20 before any cooling of the flue gases. The heating of the flue gases which have lost their lamp contents is preferably carried out by means of a heating gas flow.

The invention will be described in detail with reference to the figure of the accompanying drawing, which shows a preferred embodiment of the method according to the invention as a schematic schematic side view.

In the figure of the drawing, the flue gas treatment unit used in the method according to the invention is generally indicated by reference numeral 10.

The body of the treatment unit 10 is denoted by reference numeral 11. The flue gas stream B lost in the heat contents flows from the boiler either before or after the filter (not shown) to the thermally insulated treatment unit 10 according to the invention through the inlet duct 12. The heating gas flow A is led to the treatment unit 10 via the inlet-35 duct 13. In the embodiment according to the figure of the drawing, the heating gas flow A is led below the perforated plane 15 located above the inlet duct 13. Below the perforated plane 15, at a distance, a second perforated plane 17 is placed and the perforated planes 15 and 17 are connected to each other by pipes 16. The lower ends of the pipes 16 extend below the lower edge of the inflow channel 13.

The pipes 16 form a space 14 between the planes 15 and 17. The hot gas flow A flows in the space 14 as a flow ax downwards and discharges through the annular holes in the plane 17 at a desired rate as a flow a2 towards the heated flue gas flow B. The flue gas flow A very well 10 and thereafter the mixed gas flow flows through the tubes 16 to the delay space 18. The tubes 16 are preferably arranged as evenly as possible with respect to the cross-sectional area of the treatment unit 10. The cross-sectional area of the tubes 16 may be selected such that the flow rate of the mixed gases A and B flowing in the tubes 16 is desired and so rapid that it creates a sufficient pressure difference before the tubes 16 to equalize the velocities in each tube 16 substantially uniformly. The heat caused by the heating gas flow A on the outer surfaces of the pipes 16 passes through the walls of the pipes 16 to the flow formed inside the pipes 16 by the mixed gas flows A and B. The heat transfer by means of the hot kaa-20 flow A is controlled at a temperature state in the delay state 18, which according to the invention is preferably desired close to a temperature state of 700 ° C. In the delay state 18, the flue gases are delayed for at least 0.1 second, preferably 0.5-5 seconds, whereby the NOx gases decompose into elements.

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From the delay space 18, the flue gases are led to a convection section 19, where their thermal energy is recovered and the temperature of the flue gases decreases to the temperature characteristic of the exhaust flue gases. After the convection section 19, the cooled flue gases flow as an outlet flow C to the outflow channel 30 20.

In the invention, it is essential that the flue gases enter the delay state 18 in the temperature state according to the invention and stay in the delay state 18 until the NOx gases disperse into the elements. It should be noted that the walls of the delay space 18 are in the temperature state according to the invention, i.e. the walls of the delay space 18 are not boiler walls, the temperature of which 6 92102 is generally below 300 ° C. The invention thus solves the treatment of gases which have already lost their heat content in the boiler once and which contain NOx gases in a separate treatment unit 10 by raising the temperature of the flue gas again in can be cooled, ie heat can be recovered from them.

In the temperature zone 11a according to the invention, the NOx gases decompose into elements without the actual chemical additive. If desired, of course, an additional flow containing e.g. carbon can be added to either flow B or A or to the delay space 18.

The pipe mixing solution shown in the figure of the drawing is a preferred embodiment. Naturally, the heating flow A and the heating flow B can be mixed with one another, e.g. by controlling through a fan or by another mixing method known per se. Of course, the flow directions of flows A and B may be opposite to those shown in the figures of the drawing.

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Only some preferred embodiments of the invention have been described above, and it will be apparent to those skilled in the art that numerous modifications may be made to them within the scope of the inventive idea set out in the appended claims.

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Claims (6)

92102 Patent t i vaa t intuks e t A method for removing NOx gases from flue gases lost in heat, characterized in that the flue gases are heated to a temperature of 400-750 ° C and delayed at this temperature for at least 0.1 second, whereby the NOx gases decompose into elements. A method according to claim 1, characterized in that the flue gases are delayed in said temperature for at least 0.5-5 10 seconds before the flue gases are discharged. Method according to Claim 1 or 2, characterized in that the flue gases are heated by a heating gas flow (A). Method according to one of Claims 1 to 3, characterized in that the hot medium flow (A) is led upstream of the intermediate spaces (14) between the pipes (16) to the flow along the flue gas pipes (16), the lower end of the pipes (16) being a hot medium flow. A) mixes as efficiently as possible with the flue gas flow (B). Method according to one of Claims 1 to 4, characterized in that the heating and delaying of the flue gases is carried out in a separate treatment unit (10), the temperature of the delay space (18) of which is maintained substantially in the temperature range of 400 to 750 °. Method according to one of Claims 1 to 5, characterized in that the carbon-containing stream is introduced either into a hot medium flow (A) or into a flue gas flow (B) or into a delay space (18) of the treatment unit (10). • i 92102