METHOD FOR OPTIMIZING THE BURNOUT OF EXHAUST GASES OF AN INCINERATOR

Abstract

A method for optimizing the burnout of exhaust gases of an incinerator includes introducing a solid material via an inlet into a combustion chamber defining a primary combustion space, combusting the solid material in the primary combustion space, in the form of a combustion bed conveyed over a combustion grate, with admission of primary air, discharging the combusted solid material from the primary combustion space via an outlet arranged opposite the inlet in the conveying direction, combusting the primary combustion gases released during the combustion of the solid material, with admission of secondary air, in a secondary combustion chamber defining a secondary combustion space and arranged downstream of the combustion chamber in the flow direction of the combustion gases, and homogenizing the exhaust gases containing the primary combustion gases in a mixing zone by means of a fluid introduced via a nozzle before entry into the secondary combustion space.

Description

BACKGROUND

The present invention relates to a method for optimizing the burnout of exhaust gases of an incinerator according to the preamble of claim 1 and also to a combustion chamber for carrying out the method and to a waste incinerator comprising such a combustion chamber.

Incinerators for combusting solid fuels, such as municipal waste, substitute fuels, biomass and other materials, are best known to a person skilled in the art. Such facilities comprise a combustion chamber, in which the solid material is combusted with admission of primary air, which is referred to as primary combustion. Here, the solid material passes through different sub-processes from the inlet into the combustion chamber to the outlet, said sub-processes being divided roughly into drying, ignition, combustion and ash burnout.

In each of these sub-processes, exhaust gases of different composition are generated. Whereas in the drying phase the primary air merely absorbs moisture from the solid material to be combusted, pyrolytic decomposition products are found in the ignition phase. In contrast to the drying phase, the oxygen fed in the ignition phase is often converted fully, such that the exhaust gas flow generated in this phase includes only very little oxygen or even no oxygen. Exhaust gases with typical compositions of CO, CO₂, O₂, H₂O and N₂ are produced in the combustion phase, whereas practically unconsumed air is ultimately present above the ash burnout.

These different exhaust gas flows, after the primary combustion, generally reach a secondary combustion chamber arranged downstream in the flow direction, where they are burned out with admission of secondary air, which is referred to as secondary combustion.

A method comprising a combustion of the solid material and a secondary combustion of the incompletely burned exhaust gas constituents is known for example from WO 2007/09510, which has the objective of breaking down the primary nitrogen compounds NH₃ and HCN in order to minimize the formation of nitrogen oxides (NO_x) in the secondary combustion chamber.

EP-A-1077077 concerns a method similar to that in WO 2007/090510, wherein the SNCR method is used for NO_x removal from flue gases, in which no catalyst is used, but instead a reducing agent is injected into the flue gases. Such SNCR methods operate at temperatures from 850 to 1000° C. and require elaborate regulation.

The reduction of nitrogen oxides is additionally addressed in WO 99/58902. In accordance with the method described therein, the gases exiting from the combustion chamber are homogenized in a mixing stage with addition of a medium free from oxygen or low in oxygen, after which the homogenized exhaust gas flow passes through a steady-state zone, in which the nitrogen oxides already formed are to be reduced. Depending on the operating conditions, it may be that the quantity of accumulating pyrolysis gas is of such a size that the quantity of locally available secondary air is not sufficient for complete burnout. As a result, unburned gases escape from the secondary combustion chamber and precipitate for example in CO peaks in the flue.

As a result of the various combustion zones, a temperature imbalance is also produced in addition to the differences in the composition of the exhaust gas flows. A much higher temperature is thus present in the ignition and the combustion zone compared for example to the ash burnout zone. This imbalance is intensified further in the secondary combustion chamber, since the exhaust gases generated in the ignition and combustion zone have a higher proportion of combustible primary combustion gases compared to the exhaust gas generated in the ash burnout zone, and the combustion of these combustible gases increases the temperature additionally.

Particularly in the inlet-side region, the peripheral wall surrounding the combustion chamber or the secondary combustion chamber can be damaged by the prevailing high temperatures. In addition, caking or coking may occur in this region due to the high temperatures and has to be removed in complex maintenance procedures.

The methods described in EP-A-1081434, EP-A-1382906 and U.S. Pat. No. 5,313,895 for example attempt to overcome the problem of reducing the quantity of unburned substances and in particular CO. For example, in accordance with U.S. Pat. No. 5,313,895, a mixing fluid is introduced which causes the gases exiting from the combustion chamber to be swirled in an eddy current. In addition, to introduce the fluid, a special nozzle arrangement is described for example in EP-A-1081434, as a result of which a rotating flow is generated in the flow channel in an injection plane arranged in the region of the flame cover. However, the method described in particular in U.S. Pat. No. 5,313,895 only takes into account unsatisfactorily the problem of the temperature imbalance present in the combustion chamber. In accordance with said document, the temperature in the inlet-side region in the combustion chamber is to be reduced by means of injection of water droplets or water vapor. This is disadvantageous however in view of the energy recovery balance. The objective of the present invention is therefore to provide a method for optimizing the burnout of exhaust gases of an incinerator, said method on the one hand ensuring high operational reliability and on the other hand allowing a high energy recovery from the combustion process.

SUMMARY

A method relating to an embodiment of the invention consequently includes the steps of introducing the solid material to be combusted via an inlet into a combustion chamber defining a primary combustion space, combusting the solid material in the primary combustion space, in the form of a combustion bed conveyed over a combustion grate, with admission of primary air, and discharging the combusted solid material from the primary combustion space via an outlet arranged opposite the inlet in the conveying direction.

The primary combustion gases released during the combustion of the solid material are combusted, with admission of secondary air, in a secondary combustion chamber defining a secondary combustion space and arranged downstream of the combustion chamber, that is to say generally above the combustion chamber, in the flow direction of the combustion gases.

Before entry into the secondary combustion space, that is to say upstream in the flow direction and therefore generally below the combustion space, the exhaust gases containing the primary combustion gases are homogenized in a mixing zone. This occurs by means of a fluid introduced via a nozzle.

In this context “a” (nozzle) is to be understood as the indefinite article; the term includes both a single nozzle and also a plurality of nozzles.

In this context, the term homogenization is understood to mean that the exhaust gases or the individual exhaust gas flows of different composition are mixed in such a way that a gas mixture that is as homogeneous as possible is obtained. In accordance with the invention, the mixing zone then adjoins the combustion bed at least approximately directly in the flow direction of the exhaust gases. It is therefore in other words generally arranged at least approximately directly above the combustion bed. This allows very hot exhaust gas flows, for example as may be produced in the ignition or combustion zone, to mix practically directly above the combustion bed with the cooler exhaust gas flows from the drying and ash burnout zones and therefore to compensate for or to reduce temperature peaks in good time. At the same time, the method prevents the energy recovery balance from being impaired, for example as would be the case with cooling by means of a cooling medium.

In addition, a gas mixture is obtained as a result of the homogenization of the exhaust gas flows generated in the individual combustion zones and is optimally preconditioned for the secondary combustion in the secondary combustion space. As a result, it is possible to ensure optimal burnout of the exhaust gases, even with low (secondary) air excess; the emission of harmful substances, such as CO or unburned hydrocarbons, can thus be kept very low, even with small quantities of admitted secondary air.

It has also been found that the mixture of the reduced nitrogen-containing combustion gases (nitrogen oxide precursor substances) generated in the combustion zone with the oxygen present above the drying or burnout zone does not result in an increase in nitrogen oxides. This can be explained by the fact that, as the exhaust gas flow from the combustion zone is mixed with the oxygen-rich exhaust gas flows accumulating in the drying and burnout zones, the temperature of said gas flows is simultaneously reduced, which suppresses the formation of thermal NO_N.

As mentioned above, the fluid is introduced via one or more nozzles.

The exit speed of the fluid from the nozzle is approximately 40 to approximately 120 m/s, wherein, within the meaning of the present invention, the nozzle is oriented at an angle from −10° to +10° relative to the inclination of the combustion grate.

In addition to the above-defined nozzles, further nozzles can be provided which are not aligned relative to the inclination of the combustion grate at the above-defined angle.

In this context, the inclination of the grate is understood to mean the total inclination of the grate (and not the orientation of any individual grate steps present).

Due to the orientation of the nozzle, it is ensured that excessive swirling of solid materials by the grate is avoided, even with the arrangement of the mixing zone directly above the combustion bed.

The injection speed of the fluid from approximately 40 to approximately 120 m/s also helps to avoid a swirling of solid materials.

The discovered combination of nozzle arrangement and injection speed therefore on the whole enables the mixing zone to adjoin the combustion bed at least approximately directly in the flow direction of the exhaust gases without resulting in an excessive undesired swirling of the solid materials by the combustion grate.

The fact that good homogenization can be obtained already with the injection speed from approximately 40 to approximately 120 m/s is all the more surprising since significantly higher values are taught in the prior art. For example, an exit speed of at least 1 MACH is disclosed for example in EP-A-1508745. A MACH number of 1 is synonymous with the speed of sound, which for air at 20° C. is generally specified at 343 m/s, and adopts even higher values at higher temperatures as are to be found in furnaces.

In accordance with a preferred embodiment, the distance between the mixing zone and the combustion bed is at most 1.5 meters, preferably at most 0.8 meters. This distance therefore denotes the maximum distance between the upper limit of the combustion bed and the start of the mixing zone as considered in the flow direction of the exhaust gases. Said maximum distance, in view of the conventional dimensions of an incinerator, still falls within the expression “approximately above the combustion bed”. Since the upper limit of the combustion bed is typically arranged approximately 0.3 to 1 meter above the surface of the combustion grate, the mixing zone is distanced appropriately from the combustion grate.

In accordance with a further preferred embodiment, the mixing zone extends at most up to a distance of 2 meters measured from the combustion bed. As considered in the flow direction of the exhaust gases, the mixing zone in accordance with this embodiment thus ends after 2 meters at most and therefore still at a sufficient distance before the secondary air injection. In the case of the mixing zone adjoining the combustion bed at least approximately directly in accordance with the invention, the mentioned upper limit is sufficient to obtain the desired homogenization of the exhaust gases.

A particularly good homogenization is achieved if, in accordance with a preferred embodiment, the exit speed of the fluid from the nozzle is approximately 90 m/s.

Here, the exit speed refers to the speed that the fluid has as it exits from the nozzle opening. The nozzles used as standard generally have a circular nozzle cross section from 60 mm to 200 mm. It is conceivable for the nozzle cross section to taper continuously in the direction of the nozzle mouth, such that the diameter of the exit opening of the nozzle is 60 mm to 90 mm.

In order to minimize a swirling of the solid materials caused by the introduction of the fluid, the respective nozzle is preferably oriented at an angle of −10° to +5°, preferably from −5° to +5°, relative to the inclination of the combustion grate. In accordance with a further preferred embodiment, the respective nozzle is aligned at an angle from −10° to 0° relative to the inclination of the combustion grate.

In accordance with a further preferred embodiment, the fluid can be a flue gas returned from a subsequent zone downstream of the secondary combustion space. In waste incinerators of conventional design, the return is preferably implemented here from a zone between the steam generator and the flue. The quantity of introduced flue gas is generally approximately 5 to 35% of the admitted quantity of primary air, preferably approximately 20%. Alternatively or additionally to the flue gas, any other conceivable fluid can be used, in particular air, an inert gas, such as nitrogen, water vapor or mixtures thereof.

Since the highest temperatures are generally present in the inlet-side region of the combustion chamber, the fluid is injected in accordance with a preferred embodiment via a nozzle or row of nozzles arranged in this region. A very pronounced temperature imbalance and therefore damage or contamination of the peripheral wall surrounding the combustion space can thus be effectively prevented.

In particular if a returned flue gas is then used as fluid, the respective nozzle preferably has an outer pipe and an inner pipe running in the axial direction of the outer pipe and surrounded thereby, wherein the inner pipe is intended to carry the flue gas and the outer pipe is intended to carry air. The inner diameter of the inner pipe is preferably approximately 70 mm here, whereas the inner diameter of the outer pipe, that is to say the outer diameter of the annular gap present between the inner pipe and outer pipe, is approximately 110 mm.

In this embodiment, the airflow is used as a shield, which protects the nozzle against the attachment of impurities entrained in the flue gas. Particularly at the temperatures present in the inlet-side region, such attachments could easily lead to caking, which in the extreme case could lead to failure of the nozzle; this is effectively prevented in accordance with the presented embodiment.

It has proven to be advantageous if at least 1 nozzle is provided per meter of the combustion chamber width. The fluid is preferably introduced via at least two nozzles, preferably at least six nozzles. This ensures homogenization that is as complete as possible with a relatively small quantity of injected fluid.

Another embodiment relates to a combustion chamber for carrying out a method. This chamber includes a peripheral wall enclosing a primary combustion space, an inlet for introducing the solid material to be combusted into the primary combustion space, a combustion grate for combusting the solid material, an outlet arranged opposite the inlet in the conveying direction of the solid material for discharge of the combusted solid material from the primary combustion space, and a nozzle for homogenizing the exhaust gases containing the primary combustion gases released during the combustion process. Here, the nozzle is arranged in accordance with the invention in a range of at most 3 meters, preferably 0.5 meters to 3 meters, most preferably 0.5 to 2 meters, above the combustion grate.

The nozzle is generally arranged in the peripheral wall of the combustion chamber, preferably in the region of the inlet or the outlet.

In order to avoid the homogenization process being accompanied by a swirling of the solid material present in the combustion bed, the nozzle is oriented in accordance with the invention at an angle from −10° to +10°, preferably from −10° to +5°, more preferably from −5° to +5°, relative to the inclination of the combustion grate. In accordance with a further preferred embodiment, the respective nozzle is oriented at an angle from −10° to 0° relative to the inclination of the combustion grate.

Besides the described method and the described combustion chamber, an embodiment related to the present invention includes a waste incinerator with a combustion chamber as described.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be illustrated with reference to the accompanying figures, in which

FIG. 1 shows a schematic illustration of a combustion chamber and a secondary combustion chamber (illustrated in part) for carrying out the method according to the present invention; and

FIG. 2 shows a graph of the measured O2 concentration (in vol %) and CO concentration (in mg/m3 in standard ambient conditions) over time in an exhaust gas flow generated in the combustion zone, wherein the nozzles are switched on and off at individual intervals.

DETAILED DESCRIPTION OF THE EMBODIMENTS

As shown in FIG. 1, the solid material 2 to be combusted is poured into a feed hopper 4 and is introduced from here into the combustion chamber 8 generally by means of a dosing tappet via an inlet 6. The combustion chamber 8 comprises a peripheral wall 10, which surrounds an upwardly tapering primary combustion space 12.

The solid material 2 is conveyed in the form of a combustion bed 14 above a (feed) combustion grate 16, through which primary air flows, and is combusted during the process. Here, a drying zone, an ignition zone, a combustion zone and an ash burnout zone are provided in succession in the conveying direction F before the combusted solid material is discharged via an outlet 18 arranged opposite the inlet 6 and is then fed via a slag remover of a slag conveying apparatus. The primary air in the shown embodiment is distributed via individual underblast chambers 20a, 20b, 20c, 20d, which are fed via separate primary air lines 22a, 22b, 22c, 22d.

Nozzles 24a, 24b, 24c, indicated in FIG. 1 by means of arrows, are arranged in the peripheral wall 10 of the combustion chamber and are used to introduce a fluid into the combustion chamber 8.

Here, the nozzles are designed in such a way that the exit speed of the fluid from the nozzles is 40 to 120 m/s.

In the shown embodiment, a nozzle 24a is arranged in the inlet-side region 8′ of the combustion chamber 8, specifically in a part 10′ of the peripheral wall 10, said part facing the inlet and running upwardly at an incline. Two nozzles 24b, 24c are arranged in the outlet-side region 8″, wherein one nozzle 24b is arranged in the part 10″ running upwardly at an incline and one nozzle is arranged in the part 10″ of the peripheral wall defining the end face 25 and running perpendicularly. Any other number and arrangement of nozzles suitable for the purposes of the present invention is also conceivable however.

By means of the nozzles 24a, 24b, 24c, the exhaust gases, which contain the combustion gases released during the combustion process, are homogenized in a mixing zone 26 adjoining the combustion bed 14 at least approximately directly in the flow direction of said gases. This homogenization is indicated in the figure by means of dashed arrows, wherein A schematically denotes the region with relatively high temperature and relatively high concentration of primary combustion gases, and B denotes the region of lower temperature and lower concentration of primary combustion gases. After homogenization, that is to say in the figure above the regions denoted by A and B, the exhaust gases are present in the form of a homogeneous gas mixture.

This flows into a secondary combustion chamber 28 subsequent to the combustion chamber 8 and defining a secondary combustion space 27, the exhaust gases being combusted in said secondary combustion chamber with admission of secondary air. To this end, further nozzles 32a, 32b for introduction of the secondary air are provided in the peripheral wall 30 of the secondary combustion chamber 28.

As illustrated in FIG. 2, the introduction of the fluid with actuated nozzle in the ON position causes the O₂ concentration measured locally in the exhaust gas flow generated in the combustion zone (shown in thick solid lines) to correspond approximately to the global O₂ concentration, that is to say the total O₂ concentration present in the exhaust gas generated in the combustion chamber (shown in thin dashed lines). By contrast, with unactuated nozzle in the OFF position, the locally measured O₂ concentration is much lower than the globally measured O₂ concentration.

With regard to CO concentration, a relatively low, approximately constant value is obtained with actuated nozzle, whereas relatively high and significantly diverging values are obtained with unactuated nozzle, which further illustrates the homogenization of the exhaust gases by the introduction of the fluid.

Claims

1. A method for optimizing the burnout of exhaust gases of an incinerator, said method comprising:

introducing a solid material to be combusted via an inlet into a combustion chamber defining a primary combustion space;

combusting the solid material in the primary combustion space, in the form of a combustion bed conveyed over a combustion grate, arc combusted with admission of primary air;

discharging the combusted solid material from the primary combustion space via an outlet arranged opposite the inlet in a conveying direction;

combusting primary combustion gases released during the combustion of the solid material, with admission of secondary air, in a secondary combustion chamber defining a secondary combustion space and arranged downstream of the combustion chamber in the flow direction of the combustion gases; and

homogenizing the exhaust gases containing the primary combustion gases in a mixing zone by means of a fluid introduced via a nozzle before entry into the secondary combustion space,

wherein the mixing zone adjoins the combustion bed at least approximately directly in the flow direction of the exhaust gases, the exit speed of the fluid from the nozzle is about 40 to about 120 m/s, and

wherein the nozzle is oriented at an angle from about −10° to about +10° relative to the inclination of the combustion grate.

2. The method as claimed in claim 1, wherein the distance between the mixing zone and the combustion bed is at most 1.5 meters.

3. The method as claimed in claim 1, wherein the mixing zone extends at most up to a distance of 2 meters measured from the combustion bed.

4. The method as claimed in claim 1, wherein the exit speed of the fluid from the nozzle is about 90 m/s.

5. The method as claimed in claim 1, wherein the nozzle is oriented at an angle from about −5° to about +5° relative to the inclination of the combustion grate.

6. The method as claimed in claim 1, wherein the fluid comprises a flue gas returned from a subsequent zone downstream of the combustion chamber.

7. The method as claimed in claim 6, wherein the quantity of introduced flue gas is about 5% to about 35%, of the admitted quantity of primary air.

8. The method as claimed in claim 1, wherein the fluid is injected via a nozzle arranged in an inlet-side region of the combustion chamber.

9. The method as claimed in claim 1, wherein the nozzle has an outer pipe and an inner pipe running in the axial direction of the outer pipe and surrounded thereby, the inner pipe being intended to carry the flue gas and the outer pipe being intended to carry air.

10. The method as claimed in claim 1, wherein the fluid is introduced via at least two nozzles.

11. A combustion chamber for carrying out the method as claimed in claim 1, said combustion chamber comprising:

a peripheral wall enclosing the primary combustion space;

the inlet for introducing the solid material to be combusted into the primary combustion space;

a combustion grate for combusting the solid material,

the outlet arranged opposite the inlet in the conveying direction of the solid material for discharge of the combusted solid material from the primary combustion space; and

the nozzle for homogenizing the exhaust gases containing the primary combustion gases released during the combustion process, wherein the nozzle is arranged in a range of at most 3 meters above the combustion grate, and wherein the nozzle is oriented at an angle of about −10° to about +10° relative to the inclination of the combustion grate.

12. The combustion chamber as claimed in claim 11, wherein the nozzle is arranged in the peripheral wall of the combustion chamber.

13. The combustion chamber as claimed in claim 11, wherein the nozzle is arranged in the region of the inlet.

14. The combustion chamber as claimed in claim 11, wherein the nozzle is oriented at an angle from about −5° to about +5° relative to the inclination of the combustion grate.

15. A waste incinerator, comprising a combustion chamber as claimed in claim 11.

16. The method as claimed in claim 2, wherein the distance between the mixing zone and the combustion bed is at most 0.8 meters.

17. The method as claimed in claim 7, wherein the quantity of introduced flue gas is approximately 20% of the admitted quantity of primary air.

18. The method as claimed in claim 10, wherein the fluid is introduced via at least six nozzles.

19. The combustion chamber as claimed in claim 11, wherein the nozzle is arranged in a range of about 0.5 meters to about 3 meters above the combustion grate.

20. The combustion chamber as claimed in claim 19, wherein the nozzle is arranged in a range of about 0.5 meters to about 2 meters above the combustion grate.