EP2691701B1 - Method for optimising the burnout of exhaust gases of an incinerator - Google Patents

Description

The present invention relates to a method for optimizing the combustion of exhaust gases of a combustion plant according to the preamble of claim 1. Such a method is described for example in the EP-A-1508745 described. Incinerators for burning solid fuels such as municipal waste, refuse derived fuels, biomass and other materials are well known to those skilled in the art. Such plants include a combustion chamber in which the solid is burned with supply of primary air, which is referred to as primary combustion. The solid from the inlet into the combustion chamber to the outlet through various sub-processes, which can be roughly subdivided into drying, ignition, combustion and ash burning.
In each of these sub-processes, exhaust gases of different composition are generated. While in the drying phase, the primary air only absorbs moisture from the solid to be burned, can be found in the ignition phase pyrolytic decomposition products. In contrast to the drying phase, the oxygen supplied in the ignition phase is often completely converted, so that the exhaust gas flow generated in this phase has very little or no oxygen. In the combustion phase, exhaust gases are formed with typical compositions of CO, CO ₂ , O ₂ , H ₂ O and N ₂ , while finally over the ash ash practically unused air is present.

As a rule, after the primary combustion, these different exhaust gas streams pass into a secondary combustion chamber arranged downstream in the direction of flow, where they are burnt out with the supply of secondary air, which is referred to as secondary combustion.

A method comprising a combustion of the solid and an afterburning of the incompletely combusted exhaust gas constituents is known, for example, from US Pat WO 2007/090510 The aim of this invention is to reduce the primary nitrogen compounds NH ₃ and HCN in order to minimize the formation of nitrogen oxides (NO _x ) in the post-combustion chamber.

EP-A-1077077 relates to a similar process as that of WO 2007/090510 , wherein the denitrification of the flue gases, the SNCR method is used in which no catalyst is used, but a reducing agent is injected into the flue gases. Such SNCR processes operate at temperatures of 850 to 1000 ° C and require sophisticated control.

The reduction of nitrogen oxides is also in WO 99/58902 addressed. According to the method described therein, the gases emerging from the combustion chamber are homogenized with the addition of an oxygen-free or oxygen-poor medium in a mixing stage, after which the homogenized exhaust gas flow passes through a steady zone in which the nitrogen oxides already formed are to be reduced. Depending on the operating conditions, it may happen that the amount of accumulating pyrolysis gas is so large that the locally available secondary air quantity is insufficient for complete burnout. This causes unburned gases to escape from the post-combustion chamber, resulting in, for example, CO peaks in the chimney.

As a result of the various combustion zones, apart from the differences in the composition of the exhaust gas streams, there is also a temperature imbalance. Thus, a much higher temperature is present in the ignition zone and the combustion zone than in the ash combustion zone, for example. This imbalance is exacerbated in the post-combustion chamber because the exhaust gases generated in the ignition and combustion zones have a higher proportion of combustible primary combustion gases than the exhaust generated in the ash combustion zone, and the combustion of these combustible gases additionally increases the temperature.

Especially in the inlet-side region, the peripheral wall surrounding the combustion chamber or the afterburning chamber can be damaged on the one hand by the prevailing high temperatures. On the other hand, because of the high temperatures, caking or coking may occur in this area, which must be removed in time-consuming maintenance work.

The problem of reducing the amount of unburned substances and in particular CO, try about in EP-A-1081434 . EP-A-1382906 and US-B-5313895 approached procedures. So is about according to US-B-5313895 a mixed fluid is introduced, the gases leaving the combustion chamber in a Eddy current offset. In addition, approximately in the EP-A-1081434 for the introduction of the fluid, a special nozzle arrangement is described, by means of which a rotating flow in the flow channel is produced in a plane of injection of the flame in the region of the flame ceiling. However, especially in US-B-5313895 described method the problem of present in the combustion chamber temperature imbalance only unsatisfactory way. Thus, according to said document in the combustion chamber, the temperature in the inlet-side region should be reduced by means of injection of water droplets or water vapor. However, this is disadvantageous in terms of the energy recovery balance. The aim of the present invention is thus to provide a method for optimizing the burnout of exhaust gases of a combustion plant, which on the one hand ensures a high degree of operational reliability and which, on the other hand, allows a high energy recovery from the combustion to be obtained.

The object is achieved by a method according to claim 1. Preferred embodiments of the invention are given in the dependent claims.

Thus, the method according to the invention comprises the steps of introducing the solid to be combusted via an inlet into a combustion chamber defining a primary combustion chamber, combusting the solid in the primary combustion chamber in the form of a combustion bed conveyed via a combustion grate by supplying primary air and the combusted solid via a combustion chamber in the conveying direction the inlet opposite outlet is discharged from the primary combustion chamber.

The primary combustion gases liberated upon combustion of the solid become downstream in the flow direction, i. usually arranged above the combustion chamber, a secondary combustion chamber defining afterburner chamber burned under supply of secondary air.

Before entering the secondary combustion chamber, i. upstream of, and thus typically below, the exhaust gases containing the primary combustion gases are homogenized in a mixing zone. This is done by means of a fluid introduced via a nozzle.

In this context, "a" (nozzle) is to be understood as an indefinite article; Thus, the term includes both a single and multiple nozzles.

Homogenization is understood in this context to mean that the exhaust gases or the individual exhaust gas streams of different composition are mixed in such a way that the most homogeneous possible gas mixture is obtained. According to the invention, the mixing zone now at least approximately immediately adjoins the combustion bed in the flow direction of the exhaust gases. As a rule, it is thus arranged in other words at least approximately directly above the combustion bed. This allows very hot exhaust gas streams, such as may occur in the ignition or combustion zone, practically immediately above the combustion bed with the cooler exhaust gas streams from the drying and ash combustion zone to mix and thus compensate for temperature peaks early or lower. At the same time, the method allows the energy recovery balance to be unaffected, as would be the case with cooling by means of a cooling medium.

Incidentally, the homogenization of the exhaust gas streams generated in the individual combustion zones results in a gas mixture which is optimally preconditioned for afterburning in the secondary combustion chamber. As a result, the present invention thus allows to ensure an optimal combustion of the exhaust gases even at low (secondary) excess air; the emission of pollutants, such as CO or unburned hydrocarbons, can thus be kept very low even with small amounts of supplied secondary air.

It has also been found that the mixture of the reduced nitrogenous combustion gases (nitrogen oxide precursor substances) generated in the combustion zone with the oxygen present above the drying zone or the burnout zone does not result in an increase in nitrogen oxides. This can be explained by the fact that in the course of mixing the exhaust gas stream from the combustion zone with the resulting in the drying and Ausbrandzone oxygen-rich exhaust gas streams at the same time its temperature is lowered, which prevents the formation of thermal NO _x .

As stated above, according to the invention, the fluid is introduced via one or more nozzles.

According to the invention, the exit velocity of the fluid from the nozzle is about 40 to about 120 m / s, wherein the nozzle is aligned in the sense of the present invention at an angle of -10 ° to + 10 ° relative to the inclination of the combustion grate.

In addition to the nozzles defined above, there may be other nozzles that are not oriented at the angle defined above relative to the inclination of the combustion grate.

Inclination of the combustion grate in this context is understood to mean the total inclination of the grate (and not the orientation of possibly existing individual grate steps).

The inventive alignment of the nozzle ensures that even with the inventive arrangement of the mixing zone immediately above the fuel bed excessive swirling of solids is avoided by the rust.

To prevent fluidization of solids, the inventive injection rate of the fluid also contributes from about 40 to about 120 m / s.

The found combination of nozzle arrangement according to the invention and injection rate thus makes it possible in total to connect the mixing zone in the direction of flow of the exhaust gases at least approximately directly to the combustion bed, without resulting in excessive unwanted swirling of the solids from the combustion grid.

That good homogenization can already be obtained with the injection rate of about 40 to about 120 m / s according to the invention is all the more surprising since significantly higher values are taught in the prior art. So will be in the EP-A-1508745 For example, discloses an exit velocity of at least 1 MACH. A MACH number of 1 is equivalent to the speed of sound, which is usually 343 m / s for air at 20 ° C, and is even higher at higher temperatures found in combustion chambers. The distance between the mixing zone and the fuel bed may be at most 1.5 meters, preferably at most 0.8 meters. This distance thus refers to the maximum distance between the upper limit of the fuel bed and the beginning of the mixing zone viewed in the direction of flow of the exhaust gases. Said maximum distance still falls under the term "approximately above the fuel bed" in view of the usual dimensions of an incinerator. Since the upper limit of the fuel bed is typically about 0.3 to 1 meter above the surface of the combustion grate, the mixing zone is appropriately spaced from the combustion grate. Furthermore, the mixing zone may extend at most to a distance of 2 meters measured from the fuel bed. When considered in the flow direction of the exhaust gases, the mixing zone thus ends after a maximum of 2 meters and thus still at a sufficient distance before the secondary air injection. In the invention At least approximately immediately adjacent to the combustion bed subsequent mixing zone, said upper limit is sufficient to obtain the desired homogenization of the exhaust gases.
A particularly good homogenization is achieved if, according to a preferred embodiment, the exit velocity of the fluid from the nozzle is about 90 m / s.
The exit velocity refers to the velocity that the fluid has on exit from the nozzle opening. The nozzles used as standard usually have a circular nozzle cross-section of 60 mm to 200 mm. It is conceivable that the nozzle cross-section continuously tapers in the direction of the nozzle orifice so that the diameter of the outlet opening of the nozzle is 60 mm to 90 mm.
In order to minimize swirling of the solids caused by the introduction of the fluid, the respective nozzle is preferably oriented at an angle of -10 ° to + 5 °, more preferably -5 ° to + 5 °, relative to the inclination of the combustion grate. The respective nozzle may be oriented at an angle of -10 ° to 0 ° relative to the inclination of the combustion grate. According to a further preferred embodiment, the fluid comprises a flue gas returned from a downstream zone of the secondary combustion chamber. In conventionally configured waste incineration plants, the recirculation preferably takes place from one zone between the steam generator and the fireplace. In general, the amount of introduced flue gas is about 5 to 35% of the amount of primary air supplied, preferably about 20%. Alternatively or in addition 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, according to a preferred embodiment, the injection of the fluid takes place via a nozzle or nozzle row arranged in this region. Thus, a very pronounced temperature imbalance and thus damage or contamination of the peripheral wall surrounding the combustion chamber can be effectively prevented.

In particular, when a recirculated flue gas is used as the fluid, the respective nozzle preferably has an outer tube and an inner tube extending in the axial direction of the outer tube and enclosed by the inner tube, wherein the inner tube is intended for guiding the flue gas and the outer tube for guiding air , Preferably, the inner diameter of the inner tube is about 70 mm, while the inner diameter of the outer tube, i. the outer diameter of the present between inner tube and outer tube annular gap, about 110 mm.

The air flow in this embodiment serves as a shield which protects the nozzle from the accumulation of contaminants entrained in the flue gas. Especially in the inlet side area Temperatures such deposits could easily lead to caking, which can lead to failure of the nozzle in extreme cases; This is effectively prevented according to the described embodiment.
It has proven to be advantageous if at least 1 nozzle per meter of the combustion chamber width is provided. Preferably, the introduction of the fluid via at least two nozzles, more preferably at least six nozzles. This ensures as complete as possible homogenization with a relatively small amount of injected fluid. A combustor of an incinerator for carrying out the method may include a peripheral wall enclosing a primary combustion chamber, an inlet for introducing the solid to be combusted into the primary combustion chamber, a combustion grate for combustion of the solid, an outlet disposed opposite the inlet in the conveying direction of the solid for discharging the combusted solid from the primary combustion chamber and a nozzle for homogenizing the exhaust gases released during the combustion of primary combustion gases include. The nozzle is arranged 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.
In general, the nozzle is arranged in the peripheral wall of the combustion chamber, preferably in the region of the inlet or the outlet.

Fig. 1

eine schematische Darstellung einer Brennkammer und einer teilweise dargestellten Nachbrennkammer zur Durchführung des Verfahrens gemäss der vorliegenden Erfindung; und

Fig. 2

eine graphische Darstellung der gemessenen O₂-Konzentration (in Vol-%) bzw. CO-Konzentration (in mg/m³ im Normzustand) über die Zeit in einem in der Verbrennungszone generierten Abgasstrom, wobei die Düsen in einzelnen Zeitintervallen ein- bzw. ausgeschaltet sind.

In order to avoid that the homogenization is associated with a Aufwirbelung of the present in the fuel bed solid, the nozzle is at an angle of -10 ° to + 10 °, preferably from -10 ° to + 5 °, more preferably from -5 ° to +5 °, oriented relative to the inclination of the combustion grate. Furthermore, the respective nozzle may be oriented at an angle of -10 ° to 0 ° relative to the inclination of the combustion grate. The invention is illustrated by the attached figures. Of these shows

Fig. 1

a schematic representation of a combustion chamber and a partially shown Nachbrennkammer for carrying out the method according to the present invention; and

Fig. 2

a graphical representation of the measured O ₂ concentration (in% by volume) or CO concentration (in mg / m ³ in the standard state) over time in an exhaust gas stream generated in the combustion zone, the nozzles at individual time intervals on or are turned off.

As in Fig. 1 shown, the solid to be burned 2 is filled in a hopper 4 and of this usually introduced by means of a Dosierstössels via an inlet 6 into the combustion chamber 8. The combustion chamber 8 comprises a peripheral wall 10, which encloses an upwardly tapering, primary combustion chamber 12.

The solid 2 is conveyed in the form of a fuel bed 14 via a (advancing) combustion grate 16 through which primary air flows and thereby burned. In the conveying direction F, there are successively a drying zone, an ignition zone, a combustion zone and an ash combustion zone, before the burnt solid is discharged via an outlet 18 arranged opposite the inlet 6 and subsequently fed to a slag conveyor via a purifier. The distribution of the primary air takes place in the embodiment shown via individual sub-wind chambers 20a, 20b, 20c, 20d, which are fed via separate primary air lines 22a, 22b, 22c, 22d.

In the peripheral wall 10 of the combustion chamber are in Fig. 1 Arranged by arrows nozzles 24a, 24b, 24c, via which a fluid is introduced into the combustion chamber 8.

The nozzles are designed such that the exit velocity of the fluid from the nozzles is 40 to 120 m / s.

In the embodiment shown, a nozzle 24a is disposed in the inlet side region 8 'of the combustion chamber 8, specifically in an inlet facing, obliquely upwardly extending portion 10' of the peripheral wall 10. Two nozzles 24b, 24c are shown in FIG outlet side region 8 "is arranged, wherein a nozzle 24b in the obliquely upwardly extending portion 10" and in which the end face 25 defining, perpendicular part 10 '''of the peripheral wall is arranged. However, any other number and arrangement of nozzles suitable for the purposes of the present invention is also conceivable.

By means of the nozzles 24a, 24b, 24c, the exhaust gases, which contain the combustion gases liberated during the combustion, are homogenized in a mixing zone 26 which adjoins the combustion bed 14 at least approximately directly in the flow direction thereof. This homogenization is indicated in the figure by dashed arrows, where A schematically denotes the region of 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, ie in the figure above the areas designated A and B, the exhaust gases are in the form of a homogeneous gas mixture.

This flows into an afterburner chamber 28, which is connected downstream of the combustion chamber 8 and defines a secondary combustion chamber 27, in which the exhaust gases are burned with the supply of secondary air. For this purpose, in the peripheral wall 30 of the afterburner chamber 28 further nozzles 32a, 32b are provided for introducing the secondary air.

As in Fig. 2 illustrated introduces the fluid with actuated nozzle in position to the fact that the O ₂ concentration measured locally in the exhaust gas flow generated in the combustion zone (shown in bold solid lines) approximately corresponds to the global O ₂ concentration (shown in thin dashed lines) in the exhaust gas generated in the combustion chamber , In contrast, when the nozzle is not actuated in the OFF position, the locally measured O ₂ concentration is much lower than that measured globally.

Concerning CO concentration, a relatively low, approximately constant value is obtained when the nozzle is actuated, while relatively high and strongly diverging values are obtained when the nozzle is not actuated, which further illustrates the homogenization of the exhaust gases by the introduction of the fluid.

Claims (8)

1. Method for optimizing the burnout of exhaust gases of an incinerator, said method comprising the steps, that
   the solid material (2) to be combusted is introduced via an inlet (6) into a combustion chamber (8) defining a primary combustion space (12),
   the solid material (2) in the primary combustion space (12), in the form of a combustion bed (14) conveyed over a combustion grate (16), is combusted with admission of primary air and the combusted solid material is discharged from the primary combustion space (12) via an outlet (18) arranged opposite the inlet (6) in the conveying direction (F), and
   the primary combustion gases released during the combustion of the solid material (2) are combusted, with admission of secondary air, in a secondary combustion chamber (28) defining a secondary combustion space (27) and arranged downstream of the combustion chamber (8) in the flow direction of said primary combustion gases,
   the exhaust gases containing the primary combustion gases being homogenized in a mixing zone (26) by means of a fluid introduced via a nozzle (24a, 24b, 24c) before entry into the secondary combustion space (27),
   the mixing zone (26) adjoining the combustion bed (14) at least approximately directly in the flow direction of the exhaust gases,
   characterized in that the exit speed of the fluid from the nozzle (24a, 24b, 24c) is 40 to 120 m/s, and in that the nozzle (24a, 24b, 24c) is oriented at an angle from -10° to +10° relative to the inclination of the combustion grate (16).
2. Method as claimed in one of the preceding claims, characterized in that the exit speed of the fluid from the nozzle (24a, 24b, 24c) is 90 m/s.
3. Method as claimed in one of the preceding claims, characterized in that the nozzle (24a, 24b, 24c) is oriented at an angle from -5° to +5° relative to the inclination of the combustion grate (16).
4. Method as claimed in one of the preceding claims, characterized in that the fluid comprises a flue gas returned from a subsequent zone downstream of the secondary combustion space (27).
5. Method as claimed in claim 4, characterized in that the quantity of introduced flue gas is 5% to 35%, preferably approximately 20%, of the admitted quantity of primary air.
6. Method as claimed in one of the preceding claims, characterized in that the fluid is injected via a nozzle (24a) arranged in the inlet-side region of the combustion chamber (8).
7. Method as claimed in either of claims 4 and 5, characterized in that the nozzle (24a, 24b, 24c) 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.
8. Method as claimed in one of the preceding claims, characterized in that the fluid is introduced via at least two nozzles (24a, 24b, 24c), preferably at least six nozzles.

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