WO1991005205A1

WO1991005205A1 - PROCESS FOR REDUCING THE NOx CONTENT IN FURNACE FLUE GASES

Info

Publication number: WO1991005205A1
Application number: PCT/EP1990/001669
Authority: WO
Inventors: Heinz KÖSER; Wolfgang Schreier; Jürgen ROSENTHAL
Original assignee: L. & C. Steinmüller Gmbh
Priority date: 1989-10-05
Filing date: 1990-10-05
Publication date: 1991-04-18
Also published as: DE3933286A1; EP0452468A1; DE3933286C2

Abstract

Process for reducing the NOx content in furnace flue gases. To reduce the content of dinitrogen oxides (N2O)in the flue gases (11) of a fluidized bed combustor (1), the flue gas temperature is increased during combustion by 50-150 °C to a final combustion temperature of 800-950 °C.

Description

Process for reducing the nitrogen oxide content in the

Smoke gases from a furnace

The invention relates to a method for reducing the nitrogen oxides in the flue gases of a furnace.

In dust firing, nitrogen monoxide NO and nitrogen dioxide NO2 are formed on the one hand from the molecular nitrogen in the air or a fuel gas and on the other hand from the fuel chemically bound nitrogen.

In order to reduce the formation of such nitrogen oxides summarized under the term "NOx", it has been proposed in dust firing to supply a so-called reducing fuel above the burner and to supply additional burn-out air above the supply of the reducing fuel or to supply reducing fuel to the burner flame in the individual burner the secondary flame leads to a reducing atmosphere and thus to NOx reduction. In a subsequent burnout zone, slow combustion takes place in a low-O2 atmosphere. With the very high combustion chamber temperatures of the dust furnaces, however, only a small proportion of di-nitrogen oxide was found.

A fluidized bed is known from DE-OS 25 39 546, in which the solid separation in one Separator takes place, which is downstream of the fluidized bed reactor. The separated solid is returned to the combustion system via a return line. From DE-OS 36 40 377 a fluidized bed combustion has become known in which the solid separation from the flue gases takes place directly in the upper area of the spinal reactor and the solids are freely returned to the combustion on the wall of the reactor or in return channels in the reactor. In addition to these fluidized bed furnaces with solids recycling, there are also the stationary fluidized beds.

Examinations of the exhaust gases from these fluidized bed furnaces, which are fed with nitrogenous fuels, have shown, however, that significantly higher N2θ concentrations can occur there. These can contribute to the greenhouse effect and the weakening of the stratospheric ozone belt.

It is therefore the object of the present invention to provide a method by means of which a reduction in the content of nitrogen oxide in the flue gases of a fluidized bed combustion, in particular a fluidized bed combustion with solid separation from the flue gases and recycling of the solids into the combustion, is possible.

To solve the problem, it is proposed that the flue gas temperature be increased by 50-150 ° C at the end of the combustion with a combustion temperature of 800-950 ° C.

Experiments have shown that a significant proportion of the N2O can be broken down by this temperature increase.

The residence time in the raised temperature range is preferably 0.2-1 sec, more preferably 0.2-1 0.5 sec. The raised temperature should not exceed 1000 ^* C.

Depending on the stoichiometric conditions at the end of the combustion, the temperature can be increased by adding an additional fuel or by adding an oxygen-containing gas, in particular air. If the oxygen content at the end of the combustion is still sufficient to bring about the desired rise in temperature, no oxygen-containing gas has to be added with the additional fuel. The fuel-oxygen content ratio is preferably set such that, in addition to the temperature increase, local reducing agents such as H2 and CO also occur, which support the thermal degradation of the N2O. In this case, it is expedient that after the N2θ degradation has taken place, an oxygen-containing burnout gas, in particular burnout air, is added to the exhaust gas stream for complete oxidation of the reducing agent. However, it is possible that burnout air must be added to certain furnaces in full-load operation, but this is no longer the case at partial load because the oxygen content of the exhaust gas generally rises at partial load and oxidation of the locally occurring reducing agent is ensured in this way .

In the case of fuels with a high content of volatile constituents, ie with good burnout behavior, the temperature can also be raised solely by adding an oxygen-containing gas, in particular air, at the end of the combustion. The combustion then takes place initially under low-oxygen conditions, so that with the addition of z. B. Air is reached at the end of the combustion zone. Due to the air grading, the combustion chamber is enlarged in the combustion, in which the N2O-containing release can react with the reducing agent. Due to the staged combustion there is a delay in fuel sales, and by Adding the oxygen-containing combustion gas at the end of the combustion achieves the desired temperature increase and thus thermal decomposition of the N∑O.

When separating solids in the fluidized-bed reactor, it is advantageous to supply the additional fuel or the oxygen-containing gas upstream of the separator in order to use it as a mixing device.

In some cases, however, it may also make sense to add the additional fuel to the separator in the fluidized bed reactor as well as to the downstream separator only after the solid separation, because the specific heat of the flue gas is reduced by the solid separation and thus only a lower for the necessary temperature increase in the exhaust gas Additional amount of fuel may be required. In the case of external cyclones, which are generally bricked up, the cyclone is not subjected to any thermal stress when the additional fuel is added after the separator.

In this case, no solid fuel should be used as the additional fuel, but a gaseous or liquid one.

The additive fuel should have a good burnout behavior for all applications; For this reason alone, gaseous additional fuels are particularly suitable. In principle, however, liquid or highly reactive solid fuels, such as. B. Lignite can be used.

The object of the invention can also be achieved in that the N∑O-containing flue gases are passed over a catalyst for the catalytic decomposition of N2O. DE-OS 35 43 640 describes a catalyst for the catalytic decomposition of N2O in connection with industrial exhaust gases.

The invention will now be explained in more detail with reference to the accompanying figures.

It shows:

FIG. 1 shows a diagram of the thermal N∑O decomposition as a function of the temperature,

FIG. 2a shows a schematic representation of a fluidized bed combustion with an internal separator (cf., for example, DE-OS 36 40 377) to explain the measures for increasing the temperature during full-load operation,

FIG. 2b shows a representation comparable to FIG. 2a to explain the measures during partial load operation,

FIG. 3 shows a representation comparable to FIG. 2a and FIG. 2b to explain the temperature increase during the combustion of fuels with a high content of volatile constituents and to illustrate the process alternative with catalytic treatment of the flue gas.

FIG. 4 shows a schematic illustration of a

Fluidized bed firing with outside the

Fluid bed reactor arranged separator and one

Return line for the return of the separated

Solids for combustion (see, for example, DE-OS 25 39 546 with outside cyclone and WO 83/03294 with outside bar separator).

The curves according to FIG. 1 for the N∑O degradation depending on the residence time and the temperature were with the help 6

a flow apparatus made of quartz. The quartz tube reactor was heated from the outside.

Since the usual combustion temperatures for fluidized bed furnaces are in the range of 800 - 950 "C, the temperatures 890, 955 and 1000 ^* C were chosen as test temperatures.

The FIG. 1 shows that with an increase in the dwell time and with an increase in temperature, there is a substantial breakdown of the N2O. The test gas composition is shown in FIG. 1 specified.

As can be seen from the table below, the thermal degradation is increased by the presence of reducing substances.

No. N∑O degradation in%

+2800 vpm +2800 vpm CO H2.

1) 850 0.25 0 10 60 2 7

2) 900 0.24 0 19 68

0.9 10

2 7

3) 950 0.23 12 60 88

The thermal N∑O breakdown is compared with the N∑O breakdown after the addition of 2800 vpm carbon monoxide or hydrogen. As can be seen from the table, carbon monoxide and, to an even greater extent, hydrogen lead to an improvement in thermal degradation.

However, the table clearly shows that this N2O degrading effect of hydrogen and carbon monoxide is primarily is effective for low-oxygen combustion gases. Such low-oxygen combustion gases are not present in the vast majority of cases, so that the primary factor to be considered is thermal decomposition.

In the FIG. 2a and 2b, a fluidized bed combustion according to DE-OS 36 40 377 is shown, the disclosure content of which is expressly referred to. A vortex shooting reactor 1 is fuel 2, as well as fluidizing air 3 and secondary air in z. B. two levels 4a and 4b supplied.

A gutter separator 5 is arranged in the upper end of the fluidized bed reactor 1. Additional fuel 6 is added in the vicinity of the gutter separator 5. 90-98% of the total fuel is fed into the lower end of the reactor and 6-2-10% into the combustion chamber. Preferably about 5%. By adding the additional fuel 6 below the gutter separator 5, a good mixing of the additional fuel is achieved. If the combustion takes place in the combustion chamber at full load with an air ratio of n = 1.1, the addition of around 5% additional fuel below the gutter separator results in a combustion zone with an air ratio n = 1.05. By adding the additional fuel and the low specific heat of the low-solids flue gas behind the gutter separator 5, a temperature increase of 100 ° C above the temperature prevailing in the combustion chamber of 850 ^* C is achieved. The oxygen content in the flue gas behind the gutter separator is reduced to a volume percent, so that reducing agents such as H2 and CO can also occur at least locally, which support the thermal degradation of N2O. If such reducing agents occur, it is advantageous if additional burnout air is added at 7 in order to completely oxidize the reducing agents.

At partial load (see FIG. 2b), the oxygen content (n = 1.3) in the flue gas generally rises, so that after the addition of the additional fuel at 6, combustion air 7 no longer needs to be supplied (air number in the flue gas after temperature increase n = 1, 24).

The above statements apply to the combustion of sufficiently reactive fuels.

In the case of less reactive fuels, the excess air in the fluidized-bed reactor 1 must also be chosen higher at full load in order to ensure good burnout. In this case, the addition of burnout air, also at partial load, may also be an option. However, the N2O reduction due to the additional fuel is lower with the same amount of fuel. This means that a higher dosage may be required.

Using FIG. 3 is the procedure according to the invention for fuels with a high content of volatile constituents, i. H. very good burnout behavior. In addition to the secondary air 4a and 4b, additional air 8 is supplied below the separator 5, so that there is an air gradation in the combustion chamber area below the separator 5. While in the

Main combustion zone of the combustion chamber the air ratio is between n = 1.0 and 1.1, an air ratio between 1.2 and 1.3 is reached in the supply area of the additional air. This staged combustion causes a delay in the fuel turnover in the combustion chamber, which leads to a significant increase in the temperature of the flue gases after the additional air 8 has been added. With this combustion management First, the combustion chamber area is enlarged, in which the N2O-containing exhaust gases can react with the reducing agents that occur as a result of the low-oxygen combustion. This is followed by an increase in temperature, which ultimately leads to the desired reduction in the NaO content.

In FIG. 4 shows a circulating fluidized bed with a circulation system consisting of a fluidized-bed reactor 1, an external separator connected downstream of it in the form of a cyclone 9 and a return line 10. The additional fuel 6 'is added to the line 11 discharging the flue gas from the cyclone. As a result of deposition of solids in the cyclone 9 is reduced, the specific heat of the flue gas, the necessary increase in temperature that is, for example, 900 ^* C in Wirbelschiehtreaktor 1 to 950 ^* C in the exhaust pipe 11 can be realized by a smaller amount of added fuel. The temperature load of the circulating bed material returned via the cyclone 9 and the return line 10, which may lead to sintering, can be avoided in this way.

In FIG. 4 is shown in dashed lines that additional air 8 'can be supplied solely for increasing the temperature even with such a fluidized bed combustion. Since there is no internal separator, the supply of additional air 8 ′ can preferably have taken place below a swirl installation 12. Such a vortex installation can be constructed from baffles.

Under certain circumstances, it is also possible to dispense with such a mixed installation 12 and to introduce the additional air as additional air 8 ″ into the deflection area of the flue gases from the vortex shooting reactor 1 into a transverse train 13 leading to the separator 9. As a result of the redirection, sufficient interference is achieved. 10

In the two methods of combusting fuel in a fluidized bed (FIG. 2a, 2b, 3 and FIG. 4), smoke gases can be partially recycled in the part-load range to stabilize the fluid dynamic conditions. Insofar as the flue gas N2O should be contained, the recirculation takes place via the fluidizing air 3 and in this case the N AnteilO portion has the greatest dwell time in the reducing combustion chamber zone, which favors the degradation in these combustion chamber zones and the expenditure for the thermal Degradation is reduced.

In the embodiments according to FIG. 2a, 2b and 3, of course, the additional fuel or the additional air can be added above the separator if, for. B. when using gaseous or liquid additional fuels sufficient injection is achieved solely by the injection or the deflection in a - as in the embodiment according to. FIG. 4 already described- transverse pull 13 takes place.

Both in the fluidized bed combustion with internal separation and in the fluidized bed combustion with external separation for the return of solids, but also in the stationary fluidized bed, the furnace is followed by a fine dust separator 14, the z. B. can be constructed from multi-cyclones and / or electrostatic filters. Such a separator is shown in FIG. 3 is shown schematically. Furthermore, the furnace for removing the heat has heating surfaces, as is shown schematically in FIG. 3 is shown.

In addition to the previously described temperature increase to reduce the N∑O content in the flue gases, it can also be useful to pass the flue gas cooled by the removal of heat over a catalyst 16, as is shown in the right part of FIG. 3 is shown. Preferably the flue gas should have cooled to a temperature in the range of 300-500 ^* C. The separated fine dust can - as in FIG. 3 shown - be returned to the furnace.

As the examples show, air is preferably used as the oxygen-containing gas. However, oxygen-enriched air or air-flue gas mixtures can also be used. Air does not necessarily have to be used as the fluidizing gas.

(3 sheets of drawings)

Claims

1.) Process for reducing the content of nitrogen oxides in the flue gases of a furnace, characterized in that to reduce the content of di-nitrogen oxide in the flue gases of a fluidized bed furnace, the flue gas temperature at the end of the combustion with a combustion temperature of 800-950 ^* C 50 - 150 ^* C is raised.

2.) Method according to claim 1, so that the dwell time in the raised temperature range is preferably 0.2-1 sec., More preferably 0.2-0.5 sec.

3.) Method according to claim 1 or 2, d a d u r c h g e k e n e z e i c h n e t that the temperature increase is carried out by adding an additional fuel.

4.) Method according to claim 1 or 2, d a d u r c h g e k e n e z e i c h n e t that the temperature increase takes place by adding an oxygen-containing gas, in particular air.

5.) Method according to at least one of claims 1-4, characterized in that at the end of the combustion the fuel-oxygen content ratio is adjusted so that in addition to the temperature increase, reducing agents also occur.

6.) Method according to claim 5, d a d u r c h g e k e n n z e i c h n e t that after the N2θ decomposition has taken place, an oxygen-containing burnout gas is added to the exhaust gas stream for complete oxidation of the reducing agent.

7.) Method according to at least one of claims 1-6, d a d u r c h g e k e n n z e i c h n e t that the additional fuel or the oxygen-containing gas is fed upstream of the separator in the case of solids separation in the fluidized-bed reactor.

8.) Method according to at least one of claims 1-6, d a d u r c h g e k e n n z e i c h n e t that the additional fuel is supplied after the solid separation.

9.) Method according to claim 8, d a d u r c h g e k e n n z e i c h n e t that a gaseous or liquid additional fuel is added.

10. A process for reducing the nitrogen oxide content in the flue gases of a furnace, so that the nitrogen dioxide-containing flue gases are passed over a catalyst for the catalytic decomposition of N2O in order to reduce the nitrogen oxide content of a fluidized bed furnace.