RS56483B2 - Method for optimising the burnout of exhaust gases of an incinerator - Google Patents

Description

Description

<The proposed invention relates to a process for optimizing the afterburning of combustion plant gases according to the main claim 1. A process of this type is described for example in EPA-1508745. Combustion plants for burning solid fuels such as municipal waste, fuel substitutes, biomass and other materials are well known to the expert. Such plants include a combustion chamber in which solid matter is burned with the supply of primary air, which is referred to as primary combustion. At the same time, the solid matter, from the entrance to the combustion chamber to the exit, goes through different parts of the process, which can roughly be divided into drying, ignition, combustion and ash combustion.>

<In each of these parts of the process, gases of different compositions are generated. While in the drying phase the primary air takes only moisture along with the substances to be burned, in the ignition phase there are pyrolytic decomposition products. Contrary to the drying phase, in the ignition phase the introduced oxygen often reacts completely, so that in this phase the generated gas stream contains only little or no oxygen. In the combustion phase, gases with typical contents of CO, CO2, O2, H2O and N2 are produced, while finally there is practically unconsumed air above the ash combustion.

As a rule, these different streams of gases arrive after the primary combustion in the post-combustion chamber placed in the downward direction, where they burn with the supply of primary air, which is referred to as secondary combustion.

A process involving combustion of solid matter and afterburning of incompletely burned gas components is known to some extent from WO 2007/090510, which aims to decompose the primary nitrogen compounds NH3 and HCN, in order to minimize the build-up of nitrogen oxides (NOx) in the afterburner chamber.

EP-A-1077077 refers to a similar process as WO 2007/090510, whereby the SNCR process is used to extract nitrogen from the flue gases, where no catalyst is used but a reducing agent is sprayed into the flue gases. Such SNCR processes operate at temperatures from 850 to 1000°C and require sophisticated regulation.>

<Reduction of nitrogen oxides is the subject of WO 99/58902. According to the procedure described in it, the gases coming out of the combustion chamber are homogenized with the addition of an oxygen-free or oxygen-poor medium in one mixing stage, after which the gas stream passes through a stationary zone where the already created nitrogen oxides should be reduced. According to the operating conditions, it can happen that the amount of gas produced by pyrolysis is so large that the amount of local secondary air standing on the atmosphere is not sufficient for complete combustion. This leads to unburned gases coming out of the afterburner chamber, which for example settle in the combustion chamber as soot.>

<As a result of the different combustion zones, a temperature jump appears in addition to the difference in the composition of the gas streams. Thus, the temperature in the ignition and combustion zone is significantly higher than in the ash combustion zone. This jump in the post-combustion chamber increases even more, since the gases generated in the ignition and combustion zone have a higher proportion of primary combustible gases than the gas generated in the ash combustion zone, so the combustion of these combustible gases further raises the temperature.

In boiler 6 of the waste incineration plant in Düsseldorf Flingern (DE), air is injected via side nozzles transverse to the direction of the combustion material to achieve a diversion of the rising exhaust gases (described in the dissertation of Ferdinand Krüll, 2001).

Precisely in the area on the entrance side, the peripheral wall surrounding the combustion chamber or the afterburner chamber can suffer damage on one side due to the high temperatures prevailing there. On the other hand, burns or coking may occur in this area due to high temperatures, which must be removed by expensive maintenance work.

The methods described in EP-A-1081434, EP-A-1382906 and US-B-5313895 try to approach the problem of reducing the amount of unburned substance and especially CO. Thus, according to US-B-5313895, a mixing fluid is introduced, which causes the outgoing gases from the combustion chamber to swirl. EP-A-1081434 describes a special position of the nozzles for the introduction of fluids which produce a rotating current in the current channel by the nozzles lying in the plane of the flame blanket area. <However, the method described in particular in US-B-5313895 takes into account only in an unsatisfactory way the problem of the temperature jump that exists in the combustion chamber. Thus, the temperature is reduced in the area on the side of the entrance to the combustion chamber according to the mentioned document by means of the injection of water drops, i.e. water vapor. This is however bad as far as the energy recovery balance is concerned. The aim of the proposed invention is thus to make available a procedure for the optimization>combustion of combustion plant flue gases, which on the one hand ensures high reliability of exploitation and which on the other hand allows a large amount of return energy from combustion.

The task is solved by the procedure according to claim 1. Favorable forms of execution are presented in the dependent claims.

<Accordingly, the procedure according to the invention includes the stages of combustion of the solid matter that burns and its introduction through the entrance to the primary combustion space defined by the combustion chamber, the solid matter in the primary combustion space burns in the form of embers that are pushed over the combustion grate with the supply of primary air, and the burned solid matter is taken out of the primary combustion space by a drain that lies in the direction of pushing opposite to the entrance.>

<The primary combustion gases that are released during the combustion of solid matter are burned in one, in their direction of flow, flow upwards, which means, as a rule, above the combustion chamber, placed in the combustion chamber that defines the secondary combustion space, with the supply of secondary air.>

<Before entering the secondary combustion space, which means in the direction of the upward flow and thus as a rule below it, the primary combustion gases containing smoke are homogenized in the mixing zone. This takes place by means of a fluid which is introduced through a nozzle.>

In this connection, "one" (nozzle) should be understood as an indefinite article; thus the term covers both a single and multiple nozzles.

Under homogenization in this regard, it is understood that gases or individual streams of gases of different compositions are mixed in such a way as to obtain a mixture of gases that is as homogeneous as possible. According to the invention, the mixing zone in the direction of gas flow is connected at least approximately directly to the embers. As a rule, in other words, it is placed approximately directly above the embers. This allows mixing very hot gas streams that can occur in the ignition or combustion zones practically directly above the embers with colder gas streams from the drying zone and the ash combustion zone and thus the temperature peaks are equalized or lowered in a timely manner. At the same time, the procedure allows that the energy recovery balance is not disturbed, as would be the case when cooling with a cooling medium.

In the rest, the homogenization of the generated gas streams in the individual combustion zones results in a gas mixture that is optimally preconditioned for subsequent combustion in the secondary combustion space. The proposed invention allows this, with the result that even with low (secondary) excess air, optimal combustion of gases is ensured; The emission of harmful substances such as CO and unburned hydrocarbons can thus be kept low even with a small amount of introduced secondary air.>

<Furthermore, it was determined that mixing in the combustion zone of the generated, combustion gases> reduced nitrogen content (precursor substances of nitrogen oxides) with oxygen found in the drying or combustion zones does not result in an increase in nitrogen oxides. This can be explained by the fact that during the mixing of the gas stream from the combustion zone with the oxygen-rich streams created in the drying or afterburning zones, their temperature drops at the same time, which <prevents the formation of thermal NOx.>

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

<The velocity of the fluid exiting the nozzle according to the invention is approximately 40 to 120 m/s, whereby the nozzle in the sense of the proposed invention is directed at an angle of -10° to 10° relative to the inclination of the combustion grate, so that in the mixing zone the primary combustion gases from the drying and ash combustion zones are mixed with the primary combustion gases from the ignition and combustion zones.>

<In addition to those nozzles defined above, there may be other nozzles that are not directed at an angle defined above relative to the combustion grate slope.>

In this connection, the inclination of the combustion grate means the total inclination of the grate (and not the orientation in the given case of the existing individual grate levels).

By directing the nozzles according to the invention, it is ensured that, even when the mixing zone according to the invention is placed directly above the embers, excessive lifting by swirling of solids from the grate is avoided.

Avoiding the lifting of the solid matter also contributes to the speed of blowing the fluid from about 40 to 120 m/s.

The found combination of the setting of the nozzles according to the invention and the speed of blowing <enables the total connection of the mixing zone in the direction of the flow of gases at least approximately directly to the embers, without excessive unwanted lifting by the swirling of solid matter from the combustion grate.>

<Since a good homogenization can already be achieved with a blowing speed according to the invention of 40 to 120 m/s, it is so surprising, when significantly higher values are taught in the state of the art.> Thus in EP-A-1508745, for example, an output speed of at least 1 MACH is published. The MACH-number of 1 has the same meaning as the speed of sound, which for air at 20°C is usually stated as 343 m/s and at higher temperatures, which are found in fire areas, it takes on even higher values. The distance between the mixing zone and the embers can be a maximum of 1.5 meters, preferably a maximum of 0.8 meters. This distance indicates the maximum distance between the upper border of the embers and the beginning of the mixing zone in the direction of gas flow. Said maximum distance still falls under the term "approximately above the embers" with respect to the usual>dimensions of the combustion plant. Since the upper boundary typically lies about 0.3 to 1 meter above the surface of the combustion grate, the mixing zone is appropriately offset from the combustion grate. Furthermore, the mixing zone can extend up to a maximum of 2 meters away from the embers. Observed in the direction of gas flow, the mixing zone ends after at least 2 meters, and thus still at a sufficient distance before the secondary air is blown. In the case of the mixing zone, which according to the invention is at least approximately directly connected to the embers, the specified upper limit is sufficient to obtain the desired homogenization of the gases.>

A particularly good homogenization is achieved when, according to a favorable design, the velocity of the fluid exiting the nozzle is approximately 90 m/s.

The exit velocity refers to the velocity of the fluid when exiting the nozzle. As a rule, the nozzles that are used as a standard have a cross-section in the form of a circle from 60 mm to 200 mm. It is conceivable that the cross-section of the nozzle in the direction towards the inlet of the nozzle is constantly reduced so that the diameter of the outlet opening is 60 mm to 90 mm.

<To minimize solids swirl lift caused by fluid introduction, each nozzle is directed preferably at an angle of -10° to 5°, preferably -5° to 5°, relative to the slope of the combustion grid. Each nozzle can be directed at an angle of -10° to 0° relative to the slope of the combustion grate. According to another favorable embodiment, the fluid includes the return flue gas from the zone placed downstream of the secondary combustion space. In conventionally equipped waste incineration plants, return flow conveniently follows from the zone between the steam generator and the combustion chamber. As a rule, the amount of flue gas introduced is approximately 5 to 35% of the amount of primary air introduced, primarily 20%. Alternatively or additionally to the flue gas, any other imaginable fluid can be used, especially air, some inert gas, such as nitrogen, water vapor or mixtures thereof.

Since, as a rule, the highest temperatures are located in the side of the entrance area of the combustion chamber, fluid injection takes place according to one advantageous form of execution via a series of nozzles or series of nozzles placed in this area. This can effectively prevent a pronounced temperature jump and thus damage and contamination of the peripheral wall surrounding the combustion area.

Especially when return flue gas is used as the fluid, each nozzle primarily has an outer tube and one inner tube that passes in the axial direction of the outer tube and is encompassed by it, whereby the inner tube is designated for conducting flue gas and the outer tube for conducting air. The inner diameter of the inner tube is primarily approximately 70 mm, while the inner diameter of the outer tube, which means the outer diameter of the annular opening located between the inner tube and the outer tube, is approximately 110 mm.

The air stream serves in this form of execution as protection that protects the nozzles from the deposition of impurities carried by the flue gas. Precisely at the temperatures that exist in the area of the inlet side, such deposits could easily lead to burning, which in an extreme case could lead to the nozzle falling out; this is effectively prevented according to the described embodiment.>

<It has proven to be advantageous if at least 1 nozzle is provided per meter of the width of the combustion chamber. The introduction of fluid takes place advantageously via at least two nozzles, more advantageously via at least >six nozzles. This ensures as complete homogenization as possible with a relatively small amount of injected fluid. The combustion chamber of the combustion plant for carrying out the procedure may include a peripheral wall surrounding the primary combustion space, an inlet for the introduction of solid matter to be burned into the primary combustion space, a grid for burning solid matter, an outlet lying opposite the inlet in the direction of pushing the solid matter out of the primary combustion space and a nozzle for the homogenization of released gases containing primary combustion gases. In doing so, the nozzle is placed above the combustion grate in an area of no more than 3 meters, preferably 0.5 meters to 3 meters, preferably 0.5 meters to 2 meters.

As a rule, the nozzle is placed in the peripheral wall of the combustion chamber, primarily in the area of the inlet or outlet.

<In order to avoid that homogenization goes along with the swirling uplift of the solid matter in the embers, the nozzle is directed at an angle of -10° to 10°, preferably -10° to 5°, more preferably>from -5° to 5°, relative to the inclination of the combustion grate. Furthermore, the respective nozzle can be placed at an angle of -10° to 0° relative to the inclination of the combustion grid. The invention is explained on the basis of the attached figures. Of these it shows

<Figure 1 schematic representation of the combustion chamber and partially shown>

<the afterburner chamber to perform the procedure according to>

<the proposed invention;>

<i>

<Figure 2 graphical representation of measured O2-concentrations (in vol-%) and CO-concentrations (in mg/m3 under normal conditions) over time in the zone>

<combustion by the generated gas stream, where the nozzles are in some>

switched on or off at time intervals.

As shown in Figure 1, the solid matter 2 that needs to be burned is filled into the hopper 4 for filling and from this, as a rule, it is introduced through the inlet 6 into the combustion chamber 8 by means of a dosing pestle. The combustion chamber 8 includes an outer wall 10, which surrounds the primary combustion space 12, which tapers upwards.

<Solid matter 2 in the form of embers 14 is pushed over the (push) grate 16 for combustion, which is blown with primary air, and burns in the process. In the direction of pushing F, there are one after the other a drying zone, an ignition zone, a combustion zone and an ash combustion zone, before the burnt solid material is taken out via the outlet 18, which is located opposite the inlet 6, and then delivered via the slag separator to the slag outlet. The distribution of primary air in the shown embodiment takes place via individual air sub-chambers 20a, 20b, 20c, 20d which are supplied via separate primary air lines 22a, 22b, 22c, 22d.>

<In the peripheral wall 10 of the combustion chamber in Figure 1, there are nozzles 24a, 24b, 24c indicated by arrows, through which fluid is introduced into the chamber 8.>

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, the nozzle 24a is placed in the zone 8' on the side of the entrance of the combustion chamber 8, specifically in one entrance facing the part 10' of the peripheral wall 10 which extends obliquely upwards. Two nozzles 24b, 24c are placed in the area 8" on the outlet side, one nozzle 24b being placed in the part 10" which extends diagonally upwards and one in the part 10"' which extends vertically and defines the front side 25 of the peripheral wall. However, any other number and setting of the nozzles which are favorable for the purpose of the proposed invention are conceivable.

By means of the nozzles 24a, 24b, 24c, the gases, which contain the gases released during combustion, are homogenized in the mixing zone 26, which is connected in the direction of flow at least approximately directly to the ember 14. This homogenization is indicated in the figure by dashed arrows, where A indicates a schematic area of relatively high temperature and a relatively high concentration of primary combustion gases, and B indicates an area of lower temperature and lower concentration of primary combustion gases. After homogenization, which means above the area marked with A and B, the gases are in the form of a homogeneous gas mixture.>

<This flows into the secondary combustion space 27 defined by the afterburner chamber 28 placed after the combustion chamber 8, in which the gases are burned with secondary air supply. Further nozzles 32a, 32b, on the peripheral wall 30 of the afterburning chamber 28, are provided for this, for the introduction of secondary air.>

As shown in Figure 2, the introduction of fluid with the nozzle in the ON position results in the measured O2-concentration (shown by thick lines) locally in the combustion zone of the generated gas stream approximately corresponding to the global, which means with the total O2-concentration found in the gas generated in the combustion chamber (shown by thin dashed lines). On the other hand, the locally measured O2-concentration with the nozzle not turned on in the OFF position is significantly lower than the one measured globally.

As for the CO-concentration with the nozzle on, a relatively low, approximately constant value is obtained, while with the nozzle not on, relatively high and strongly diverging values are obtained, which further makes the homogenization of gases by the introduction of fluid obvious.

Claims (5)

Patent claims 1. The process of optimizing the combustion of combustion plant gases, which includes the steps the solid material for combustion (2) is introduced through the inlet (6) into the combustion chamber (8) which defines the primary space (12) for combustion, <the solid material (2) is burned in the primary combustion space (12) in the form of an ember (14) which is pushed over the combustion grate (16) with the introduction of primary air and the burned solid material is taken out via the outlet (18) placed in the direction of pushing (F) opposite the inlet (6) from the primary combustion space (12), and> During the combustion of solid material (2), the released combustion gases burn in the afterburning chamber (28) which defines the secondary combustion space (27) placed in the direction of the flow of current upwards in the combustion chamber (8) with the introduction of secondary air, whereby the gases that make up the combustion gases are homogenized before entering the secondary space (27) of combustion in the zone (26) of mixing by means of a fluid introduced through a nozzle (24a, 24b, 24c), whereby the mixing zone (26) is connected in the flow direction at least approximately directly to the ember (14) <indicated by the fact that the exit velocity of the fluid from the nozzle (24a, 24b, 24c) is 40 to 120 m/s and> that the nozzle (24a, 24b, 24c) is directed at an angle of -10° to 10° relative to the inclination of the combustion grid (16), so that in the zone (26) primary combustion gases from the drying zone and ash combustion are mixed mix with the primary combustion gases from the ignition and combustion zone. 2. The method according to one of the previous claims, indicated by the fact that the exit velocity of the nozzle fluid (24a, 24b, 24c) is 90 m/s. 3. The method according to one of the previous claims, characterized in that the nozzle (24a, 24b, 24c) is directed at an angle of -5° to 5° relative to the slope of the combustion grid (16). <4. A method according to one of the preceding claims, characterized in that the fluid comprises the return>flue gas from the zone which is placed downstream from the secondary combustion space (27). 5. The method according to claim 4, characterized in that the amount of flue gas introduced is 5% to 35%, primarily about 20% of the introduced primary air. <6. The method according to one of the previous claims, characterized in that the blowing of the fluid takes place via a nozzle (24a) placed in the area on the side of the entrance of the combustion chamber (8). <7. The method according to one of claims 4 or 5, indicated by the fact that the nozzle (24a, 24b, 24c) has an outer tube and an inner tube that passes through the axial direction of the outer tube and is surrounded by it, wherein the inner tube is determined to conduct flue gas, and the outer tube to conduct air. <8. The method according to one of the preceding claims, characterized in that the introduction of the fluid takes place via at least two nozzles (24a, 24b, 24c), preferably at least six nozzles (24a, 24b, 24c).