FI90913C - Combustion to eliminate NOx gases - Google Patents

Description

90913 NOx gas eliminating combustion Nox gaser eliminerande brånning 5

The invention relates to a method for preventing NOx gas emissions contained in combustion flue gases from entering the flue gas exhaust stream.

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

15

At physico-chemical equilibrium, NO decomposes 100% when the temperature 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 (1200-1500 ° C: -20), 1200-2000 mg NO / m3 of flue gases are always generated. In practice, with somewhat as little over-air as possible, we get somewhat lower NOx emissions.

Normal combustion most commonly serves heat production, i.e. the aim is to obtain the most complete and efficient heat energy from the combustion gases 25 to the boiler hot surfaces and from there to the water on the other side of the wall or a similar medium. The aim is to cool the flue gases as completely as possible and preferably to a temperature 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 slightly present in the flue gases in addition to SO2, to the cold parts of the boiler. Condensation results in corrosion. SO2 begins to condense as the humidity of the myds burn-35 increases and the temperature drops too much in the flue gases (below 170 ° C). In general, in boilers, the flue gases cool rapidly to a temperature below 500 ° C, in which state NO stabilizes, i.e. it does not decompose into elements according to the state diagram, although according to state diagram 2 NO should decompose into starting materials. This is due to the fact that the decomposition reaction is very slow and almost non-existent, i.e. the lower the temperature, the lower the decomposition.

5 Current technologies have sought to eliminate the so-called NOx emissions from flue gases. in phased combustion, in which case the temperature of the combustion is kept below 1000 ° C, because here, according to the state diagonal, the share of NOx gases in the flue gases is substantially lower than in normal direct combustion (above 1200 ° C). However, this has proven to be 10 very demanding tasks. Only a few burners have achieved sufficiently good results, ie a reduced share of NOx gases, e.g. 50% compared to normal combustion.

A solution is also known from the prior art in which flue-15 gases are recycled back to combustion in order to achieve a low combustion temperature. 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. In this case, according to the equilibrium diagram, a significant amount of NOx gases 20 is generated, not to mention the fact that the nitrogen content in the fuel burns in its entirety to NOx gases.

Thus, the currently known techniques try to obtain as little NOx gases as possible in the combustion process by lowering the combustion temperature, 25 but this also causes annoyances other than those found above. Known 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

It is an object of the invention to provide an improvement on the currently known methods for preventing NOx gas emissions from combustion flue gases in the flue gas exhaust stream.

35 3 90913

It is a more detailed object of the invention to provide a method which enables NOx gases to be dispersed into the elements as completely as possible, whereby the exhaust stream of the cooled flue gases is substantially free of NOx flue gas emissions.

5

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

10

In the method according to the invention, combustion is possible even at high temperatures, but after that the lamp space of the flue gases is gently dropped into the temperature range according to the invention and delayed there, whereby the N0x> gases are completely dispersed as possible.

15

The invention is based on the realization that the flue gas lamp space falls as smoothly as possible between 400-750 ° C. According to Talld's state diagram, NOx fasts decompose into elements, oxygen and nitrogen, because the lamp state of NOx gases is high enough to the dispersion reaction 20 is sufficiently rapid and thermally possible.

The invention will be explained in detail with reference to some preferred embodiments of the invention shown in the figures of the accompanying drawings, to which, however, the invention is not intended to be exclusively limited.

Figure 1 shows a schematic side view of a preferred embodiment of the method according to the invention.

Figure 2 shows a schematic side view of another preferred embodiment of the method according to the invention.

Figure 3 shows a schematic side view of a third preferred embodiment of the method according to the invention.

35 4

In the embodiment according to Figure 1, the method according to the invention is applied to a combustion boiler, generally indicated by reference numeral 10. The body of boiler 10 is denoted by reference numeral 11, burner by reference numeral 12 and furnace by reference numeral 13. by means of the fan 21, the combustion air flow B. In the furnace 13 of the boiler 10, the combustion is assumed to be as complete as possible, i.e. there is sufficient combustion air.

10 The temperature of the flue gases in the firebox 13 can vary, for example, in the range from 1000 to 1700 ° C. In this case, NOx gases are also generated during combustion. According to the basic understanding of the invention, the flue gases after combustion are cooled to a temperature of 400-750 ° C and delayed at this temperature for at least 0.1 second, preferably for at least 0.5-5 seconds, whereby the NOx gases decompose into 15 elements.

In the embodiment according to Figure 1, the cooling of the hot combustion gases is carried out in a so-called with a pre-cooler, which can be, for example, a gas-cooled tube heat exchanger or a liquid-cooled tube heat exchanger.

In this embodiment, the heat exchanger 14 is shown as a gas-cooled tube heat exchanger. If the walls of the heat exchanger 14 are preferably at a temperature above 400 ° C, the contacting flue gas cools to a maximum temperature of 500 ° C. In this embodiment, the boiler 10 has, after the heat exchanger 14, a so-called a radiating section 15, in which hot flue gas, e.g. in a lamp room, exceeds 25,700 ° C by transmitting heat to the radiating wall 16 by rain.

In the selected space 17 between the radiation wall 16 and the body 11, either outdoor air or a cooled flue gas stream flows. In the embodiment according to Figure 1, the outdoor air flow D flows in the space 17. It should be noted that in the rain space 15 the flue gas flow and the cooling outdoor air flow D 30 are opposite to each other. At the lower end of the radiation wall 16, the cooled outdoor air flow D coincides with the flue gas flow.

In the method according to the invention, the lamp space of the flue gases is in any case caused to fall to a temperature below 750 ° C, preferably to a temperature below 700 ° C. In female lamp rooms, the decomposition of NO is energetically sufficiently rapid, as a result of which flue gases are obtained, the NOx content of which is non-existent.

Once the flue gases have cooled to the lamp space below 700 ° C, they pass through a duct 18 where the cooled flue gases remain for at least 0.1 seconds, preferably 0.5-5 seconds. Such a residence time at a temperature of 500-700 ° C leads to the diffusion of NOx gases to the initial elements in the composition corresponding to the state diagram (2 NO -> N2 + 02). After the duct 18, the flue gases are directed to the contactor part 19 of the boiler 10, where the flue gases generate their thermal energy to the boiler 10 selector. The flue gases exit the boiler 10 outlet duct 20 as a flue gas flow C in a manner known per se.

The embodiment shown in Fig. 2 is otherwise the same embodiment shown in Fig. 15, but in the embodiment according to Fig. 2 the hot flue gases are cooled in addition to the heat exchanger 14 in the heating section 15 by means of a flow D of cooled flue gases. Some of the flue gases flowing in the exhaust duct 20 are led by means of a fan 22 as a flow D to a select space 17 for cooling the hot flue gases.

20

In the embodiment of Fig. 3, the cooling effect of the lamp exchanger 14 is completely replaced by cooling the hot flue gases in the rain section 15 with the flow of cooled flue gases D. In the embodiment of Fig. 3, the fan 22 controls the flow downstream and mixes with the countercurrent flow of hot flue gases in the lower parts of the pipes 24. The mixed gases flow up the pipes 24 into the duct 18, where the residence time is similar to that in the embodiments of Figures 1 and 2. The solution according to Figure 3 achieves the mixing of hot flue gases and cold but already circulating flue gases in the pipes 24, while the rain lamp can be moved to the walls of the pipes 24 and thus to contact with the circulating flue gas flow D. The length of the pipes 24 and the size of the selection space 17 can be varied as necessary so that the circulating flue gas flow D can be distributed as evenly as possible to meet the hot flue gases flowing from the furnace 13.

6

The method according to the invention, in particular the embodiments according to Figures 2 and 3, does not waste energy, but nevertheless achieves a very good removal of NOx gases. The method according to the invention is thus characterized in that the hot flue gases are cooled as smoothly as possible, either by cooling the gas flow on cooling surfaces whose temperature is preferably above 300 ° C, most preferably above 400 ° C. In this way, the possibility of NOx gases being stabilized is avoided, as is the case with the current rapid cooling on the boiler transfer walls of the boilers. In these previously known solutions, the only way to reduce NOx emissions has been to reduce the combustion temperature, which is not necessary in the method according to the invention.

The principle of the invention 15, i.e. a method for decomposing NOx gases generated during combustion, has been illustrated above in light of three different embodiments. It will be clear to a person skilled in the art that the method according to the invention, i.e. the delayed flue gas flow in the lamp space area according to the invention, can be achieved by numerous different technical solutions. The lamp space range according to the invention can be achieved or reduced by certain catalysts. The residence times are not limited to 5 seconds, but the residence time can also be longer, depending on the embodiment.

Il

Claims (8)

90913 Claim et A method for preventing NOx gas emissions from combustion flue gases from escaping from the cooled flue gas outlet, characterized in that the flue gases after combustion are cooled to a temperature of at least 400-750 ° C and delayed at a temperature of 400-750 ° C and delayed. the gases disperse into the elements. 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. Method according to Claim 1 or 2, characterized in that the flue gases after combustion are cooled by a gas-cooled heat exchanger (14). Method according to Claim 1 or 2, characterized in that the flue gases after combustion are cooled by a liquid-cooled lamp exchanger (14). 20 Method according to Claim 1 or 2, characterized in that the flue gases after combustion are cooled by both a lamp exchanger (14) and the cooled flue gases by passing at least part of the cooled flue gas outlet stream (C) as a circulating stream (D) to the hot flue gases. Method according to Claim 5, characterized in that the flow (D) of the cooled flue gases is conducted upstream of the space (17) between the boiler body (10) and the radiating wall (16) upstream of the flow direction of the hot flue gases. Method according to Claim 1 or 2, characterized in that the hot flue gases are cooled only by passing at least part of the cooled flue gas flow (C) in a circulating flow 35 (D) to the boiler section (15) of the boiler (10), where the cooled flue gas flow (D) is mixed . Method according to Claim 7, characterized in that the flow (D) of the cooled flue gases is directed countercurrently to the flow along the pipes (24) of the hot flue gases (24) in the radiating section (15), the pipes (24) being ) at the bottom, the flow of cooled flue gases (D) mixes as efficiently as possible with the hot flue gases. I: 90913