IE912649A1 - Method and apparatus for reducing the contents of nitrogen¹oxides in combustion effluent gas - Google Patents

Abstract

The invention relates to a method and an apparatus for reducing the contents of nitrogen oxides in oxygen-containing effluent gas developed by the combustion of fuel, by which method an ammonia-containing treatment gas comprising a carrier gas is injected into the effluent gas. The treatment gas is injected as a jet flowing counter-currently to the effluent gas at a velocity sufficient for causing the treatment gas to penetrate the effluent gas as a jet stream over a predetermined length of travel. By means hereof, the treatment gas may, if desirable in combination with possible additives, be injected into the zone with optimum temperature for NOx reduction.

Description

TITLE: Method and Apparatus for Reducing the Contents of Nitrogen Oxides in Combustion Effluent Gas.

This invention relates to a method and an apparatus for reducing the content? of nitrogen oxides in oxygen-containing effluent gas developed by the combustion of fuel. More particularly this invention relates to a method and an apparatus, by which the contents of nitrogen oxides are reduced without the employment of a catalyst by injecting an ammonia-containing treatment gas comprising a carrier gas into the flue gas.

By the combustion of fuel with air in surplus, atmospheric nitrogen is oxidized into various nitrogen oxides, particularly NO and NO2, generally referred to as thermal Ν0χ. Furthermore Ν0χ may be formed by the oxidation of nitrogen containing compounds in the fuel (fuel Ν0χ), e.g. by the combustion of coal, natural gas and biomass. Nitrogen oxides are undesirable, and extensive research is continuously conducted with the purpose of improving the techniques for reducing the contents of nitrogen oxides in the effluent gas emitted to the atmosphere. However, these efforts have not yet led to a completely satisfactory result.

The selective non-catalytic reduction (the SNR process) is based upon the overall reaction scheme of a reduction of Ν0χ with NH^ to form N? and Η£θ, which may be emitted to the atmosphere without causing any undesirable effects. However, the reaction between HNg and ΝΟχ is not proceeding directly under the complex conditions prevailing in a flue gas, in view of which scientists have described as many as 99 possible elementary reactions (cf. Miller et al., Combustion and Flame 43, p. 81-98 (1981), and Combustion Science and Technology, vol. 34, p. 149176 (1983)). As an example of the complexity of possible reaction schemes a theoretical model is depicted below (cf. Lyon et al., Kinetics and Mechanisms of NH^ Oxidation; Nineteenth Symposium (int.) on Combustion; The Combustion Institute (1982)) According to this model, the initial reaction stage is the conversion of NH^ to Nl^ radicals, which reaction is intiated by OH and 0 radicals. This initial reaction stage has later been confirmed by other scientists (cf. Lodder and Lefers, The Chemical Engineering Journal, 30, p. 161 (1985), and thus it is today the prevailing view that both the reduction of NO and the oxidation of NH^ may be accelerated if the formation of NH^ radicals is accelerated.

Furthermore, it is well-known within the art that the SNR process is very temperature-sensitive and only proceeds within a very narrow range of temperatures, which dependent upon among other factors the molar ratio between NH^ and Ν0χ lies within the range of temperature of 950-960°C. In case the temperature is too high, NH^ may be oxidized to Ν0χ, quite contrary to the desired result, and in case the temperature is to low, NH^ may be transported unchanged along with the flue gas as an undesirable slip of ammonia. Accordingly, it is to be understood that the optimalization of the SNR process in a utility boiler or in any other combustion plant demands introduction of the ammonia-containing treatment gas within a very narrow zone of the flue gas duct from the fire box, in which zone the temperature of the flue gas is in the order of 950 - 960’C. In this connection, the varying distance between the optimum temperature zone and the fire box, dependent upon a number of factors, causes complications. The first factor is the load factor, since the optimum temperature zone will be situated further away from the fire box at full load than at partial-load. Another factor is the heat exchange surfaces or the cooling surfaces of the plant, including the inner surface of the flue gas duct, which surfaces cause extraction of heat and thus cause a variation of the temperature profile over an area extending transversely relative to the direction of flow of the flue gas. The third factor is the surplus of air and the content of nitrogen in the fuel, since the location of the optimum temperature zone will vary at varying surplus of air and at varying contents of nitrogen in the fuel. Furthermore, the content of Ν0χ in the flue gas will vary across the flow of the flue gas dependent upon among other things the temperature conditions. In addition to this, the location of the optimum temperature zone is related to the specific plant in question.

By the introduction of various additives into the ammoniacontaining treatment gas, the optimum temperature range may be shifted downwards, cf. e.g. Lodder and Lefers, Loc Cit. According to this reference, it has been found that the introduction of natural gas, C^Hg and CO into NH^ may shift the optimum temperature range downwards by 150-200’C. A number of further additives having a similar effect have been examined by Duo Wenli, cf. the 23rd International Symposium on Combustion, the Combustion Institute, University of Orleans, France, July 22nd-27th, 1991. Such additives may be added to an ammoniacontaining treatment gas with the purpose of adjusting the composition of the treatment gas so that it may be introduced at a place having temperature conditions optimum for the NO reduction.

In case of an actual utility boiler plant, it is generally not possible to inject a treatment gas at any desirable place, since the construction of the plant limits the possible locations of the feeding conduit necessary for the treatment gas. More specifically, the possibilities within an existing plant of selecting zones of introducing the treatment gas in which optimum temperature conditions may be considered to prevail are severely limited.

In US patent no. 4 115 515 some of the above-mentioned complications are discussed, and this patent discloses among other things a device comprisi ng several parallel extending grids having jet orifices or jet nozzles, through which the treatment gas may be introduced into the flue gas. These grids, which may be in a number of two, are positioned in a manner that one grid is positioned at a distance from the fire box, at which distance the temperature of the flue gas is presumed to be within the range of 900-1000’C, whereas a different grid will be positioned at a further distance from the fire box, by which distance the temperature of the flue gas may presumably be in the range of 700-900‘C. Hereby a treatment gas, which contains ammonia as the sole reducing agent, may be introduced at the high temperature zone, whereas a treatment gas, which apart from ammonia contains an additional reducing agent, may be introduced at the lower temperature zone. The treatment gas, which further comprises an inert carrier gas, is introduced flowing counter-currently to the flue gas flow by means of jet orifices so-that a quick mixing in the flue gas over the entire cross section of the flue gas is provided during the formation of a blanket of the treatment gas. In order to compensate for variations in the temperature profile over the cross section of the flue gas in its direction of flow, it is suggested that discharge of the treatment gas through jet orifices where the temperature is higher than 1000‘C or lower than 700‘C may be avoided by employing and controlling tailored manifolds.

By the prior art disclosed in US patent no. 4 115 515 it is not possible to change the actual location of the feed-in points for the treatment gas, and accordingly it is uncertain whether the grids built into the flue gas duct are located within the area of the optimum temperature range, which may vary dependent on the above-mentioned factors. It has been suggested that these kinds of grids or other might be displaced upwards or downwards in order to adjust the location relative to optimum temperature zones, but in actual practice it is very complicated to design durable, movable devices able to sustain the temperatures and the environment prevailent in the flue gas duct from a combustion chamber. An increase of the number of grids or of the number of other feeding means would make it easier to adapt the feed-in areas for the treatment gas to the optimum temperature conditions, but this kind of solution is very expensive. Furthermore, the use of the additives that may be used in order to compensate for a non-optimum placement of the feed-in areas for the treatment gas increases the costs, and the injection of such additives complicates the technical process and may not bring the NO reduction obtained abreast of the NO X X reduction obtained by the injection of ammonia alone at optimum temperature conditions.

DE published application no. 36 06 535 discloses a method and an apparatus for the reduction of the contents of nitrogen oxides in an effluent gas stream, wherein a treatment gas containing ammonia is injected concurrently with the effluent gas streams through jet pipes having their nozzles arranged in the effluent gas stream at a location where the temperature is below 950’C, this temperature being assumed to be the optimum one for the reduction of nitrogen oxides. This arrangement contains no facility for adjusting the zone of introduction into the effluent gas stream to adapt it to a possible shift in the location of the optimum temperature zone.

The present invention provides a method for the reduction of the contents of nitrogen oxides in oxygen containing effluent gas developed by the combustion of fuel, wherein a treatment gas comprising ammonia and a carrier gas is injected into the effluent gas stream, said method being characterized by the injection of the treatment gas as a jet stream counter-currently to the effluent gas stream and at a velocity sufficient to make the treatment gas jet stream penetrate the effluent gas stream over a predetermined length of travel in the form of a beam.

Oxygen containing effluent gas is used here to designate any effluent gas developed by the combustion of fuel with a surplus of air. The effluent gas typically contains between 4 and 10, more specifically between 5 and 7, and more specifically about 6 per cent oxygen by volume. The effluent gas may be developed by any kind of combustion, regardless whether carried out in a mobile or a stationary plant and regardless of the purpose of the combustion. The fuel may be any kind of matter that will burn in air, such as solid, liquid and gaseous fuels including coal, oil, lignite, peat, biomass and natural gas.

A treatment gas comprising ammonia is here.understood to comprise a treatment gas containing ammonia, ammonia precursors or ammonia derivatives capable of liberating ammonia under the relevant conditions in a form able to produce NH2 radicals upon suitable initiation.

Examples of such derivatives of ammonia are ammonium sulfate and ammonium sulfite, which may be introduced by atomizing a solution in the treatment gas.

By injecting the treatment gas as a jet“stream counter-currently to the flow of the effluent gas, the treatment gas may be delivered as a jet beam penetrating the effluent gas stream in a controlled fashion so as to reach the zone where the opttmunr temperature range reigns under the prevailing conditions. Upon reaching this zone, the jet stream will i have losts it momentum so that it will be mixed into the effluent gas allowing the desfred reduction of the nitrogen oxides to take place. Should the zone where the optimum temperature range prevails be displaced due to any of the above-mentioned factors, the jet stream may be introduced at a different velocity so as to penetrate the effluent gas stream over a different length of travel before loosing its momentum to be mixed with the effluent gas. A jet stream is here used to designate a concentrated narrow, beam-like jet able to flow over a predetermined length of travel counter-currently to the flow of the effluent gasses in a stable flow pattern.

According to a preferred embodiment of the invention, the treatment gas is injected by a number of jet streams distributed over a cross section of the effluent gas stream perpendicularly to the direction of flow of the effluent gas stream. This makes it possible to distribute the treatment gas uniformly all over the effluent gas cross section, each of the jet streams being introduced at a velocity, at which it will penetrate the effluent gas to reach the desired zone, where each of the jet streams will have losts its momentum to be distributed in the effluent gas stream in a homogenous manner. According to another embodiment, it is possible to introduce the jet streams at different velocities so that each of them will penetrate the effluent gas stream over a different length. This makes it possible to tailor the streams so as to adapt them to the varying temperatures prevailing in the effluent gas stream across a cross section of the flow, where the temperature profile is varying due to heat dissipation to the inner surfaces of the flow duct and to any heat exchanger surfaces arranged inside the duct. Injection of the treatment gas by several jet streams at different velocities may also be of interest in order to adapt these streams to the profile of ΝΟχ concentration across any cross section of the effluent gas flow. According to a different embodiment of the invention, the jet streams may be injected from various stages along the flow of the effluent gas in order that the stages may allow compensation for variations in temperature and NO concentration across a cross section Λ — — of effluent gas flow, while injecting all of the jet streams at substantially equal velocities. Details about these adjustment facilities are elaborated in the detailed description of preferred embodiments of a plant according to the invention to be given below.

The method according to the invention eliminates the disadvantages inherent to the methods of the prior art to a great extent, as it should be evident from the explanation. Due to the velocity of the jet streams, the treatment gas may be delivered to different levels in order to reach just the zones with optimum conditions for the reduction of nitrogen oxides.

As already mentioned, the jet stream should have a velocity sufficient to make the treatment gas penetrate the effluent gas stream as a beam over a predetermined length of travel. This velocity should be sufficiently high to form a stable, narrow, beam-like jet. The velocity should also be sufficient for the treatment gas to be introduced with a momentum sufficient to penetrate the counter-currently flowing effluent gas as a beam over the intended length of travel so that the major part is only intermixed therein upon reaching the zone where the desirable temperature prevails. The velocity of the effluent gas may vary depending upon the plant, typically in the range from 20 to 30 m/s. In order to achieve a sufficient penetration, the treatment gas should preferably be injected at a velocity of at least 75, preferably at least 100, and more preferably at least 200 m/s. With the method according to the invention satisfactory results have been achieved by injecting the treatment gas at a supersonic velocity, such as in the order of 344-700 m/s. At a velocity of 370 m/s an approximately cylindrical beam of treatment gas will be able to penetrate 4 to 5 meters into an effluent gas stream flowing counter-currently at a velocity of the order of 20 to 30 m/s. By providing the jet streams from jet nozzles, the velocity may easily be adjusted by adjusting the pressure, as it will be explained in greater detail below.

The injection of the treatment gas in the form of a beam-like jet penetrating the effluent gas stream over a predetermined length of travel is believed to have an advantageous effect upon the possibilities of developing NH^ radicals believed to be important for the achievement of an efficient reduction of the nitrogen oxides. During the short dwell time of the treatment gas in the jet beam, surrounded by hot, effluent gasses, the treatment gas will be heated so that it will reach approximately the temperature of the effluent gas, whereby it may form free radicals capable of initiating the conversion of NH3 to NH2 radicals, even before the treatment gas has contacted any Ν0χ molecules Examples of free radicals capable of initiating this reforming are OH and 0 radicals, the most important reactions believed to be the following: (1) 02 + M - 20 + M >800°C 10 (2) H2O + M - OH + H + M >900*C (3) NH3 + 0 - NH2 + OH >700’C (4) OH + NH3 - NH2 + H20 >700°C M designating a so-called third body, i.e. an inert molecule capable of absorbing momentum and energy. These reaction schemes make it clear that the presence of oxygen in the treatment gas makes it possible to form 0 radicals according to scheme (1) able to combine with ammonia to form NH2 and OH radicals according to scheme (3). Such OH radicals may react with ammonia forming NH,, radicals and H20. An ammonia carrier gas containing oxygen and a third body is therefor believed to be favourable for the formation of free NH2 radicals before the treatment gas contacts the flue gas. This is supported by the experimental finding that air from the ambient atmosphere, wherein the nitrogen may serve as the third body, is well suited as carrier gas. According to reaction scheme (2) above, H2O may also form free OH radicals, which may then react with ammonia forming NH2 radicals, evidencing that also steam will be a suitable carrier gas for exercising the method according to the invention.

At temperatures in excess of approximately 1050‘C ammonia may be oxidized according to the reaction scheme 4NH3 + 5O2 - 4N0 + 6H20.

Thus, it is essential to deliver the treatment gas in a zone where the temperature in the effluent gas is safely below 1050eC. By the_ method of the invention this may be ensured.

The above-listed theories for the possible formation of free radicals should not be understood as limiting the invention in any way, but are believed to explain the possibility of achieving a particularly efficient reduction of nitrogen oxides by the method according to the invention. Comparing this to the method .according to the above-mentioned US patent no. 4 115 515, this piece of prior art teaches the use of an inert carrier gas as treatment gas to be intermixed with the effluent gas immediately upon the introduction herein. It is therefor likely that the various NH^ radicals will only be formed after the injection of the treatment gas into the effluent gas, i.e. at a stage at which competing reactions between NH^ and various components in the effluent gas are also possible.

In case the power load is reduced, the zone of optimum temperature moves closer to the fire box, i.e. in an upstream direction relative to the direction of flow of the effluent gas. By the method according to the invention it will be possible to compensate for this displacement by injecting the jet streams at an increased velocity. Another compensation strategy may be the injection of additives, such as natural gas, dimethylamine, ethane, methane or any other additive known from the prior art (cf. Duo Wenli, loc. cit.) causing a lowering of the optimum temperature range equivalent to a displacement of the optimum zone concurrently with the direction of effluent gas flow. These additives may be introduced along with the treatment gas or as separate jet streams along with a carrier gas. By the method according to the invention, it is generally preferred not to use additives, as they tend to complicate the process while the Ν0χ reduction achievable is inferior to what can ideally be achieved.

Although the method according to the invention is described as comprising the injection of the treatment gas counter-currently to the effluent gas flow, this should not be construed as only comprising injection of the treatment gas in the direction exactly opposite to the direction of effluent gas flow. The injection of the treatment gas at an obligue angle, which is only partially counter-currently to the effluent gas flow, may also be used and may also achieve the objects and advantages according to the invention and explained above, and such forms of injection are also considered to be comprised by the invention as being counter-currently to the effluent gas flow. According to the invention, it is also possible to inject supplementary treatment gas in other directions relative to the effluent gas flow, e.g. perpendicularly across or wholly or partially concurrently hereto, in case this is found advantageous for the achievement of an optimum result.

The content of ammonia in the treatment gas may be tuned or controlled depending upon the desired degree of Ν0χ reduction, the flow quantity of effluent gas, the content of Ν0χ within the effluent gas, and the load factor, as the most relevant factors. The Ν0χ reduction increases with an increasing content of ammonia in the treatment gas, until a molar ratio of NHg to Ν0χ about 2, higher molar ratios yielding only a marginal improvement of the Ν0χ reduction. The injection of increased quantities of NHg implies, on the other hand, the risk of unreacted ammonia slipping out. The method according to the invention may be controlled to keep the slip of ammonia into the ambient environment to a minimum by keeping the molar ratio of NHg to Ν0χ within a range of approximately 1 to 3, preferably between 1.5 and 1.7, and more preferably about 1.6. If maximum Ν0χ reduction is considered the primary objective, ΝΗ3/Ν0χ molar ratios near the upper ends of the ranges will be preferred, e.g. a molar ratio of between 2 and 3, and in this case unreacted ammonia may be removed to avoid any slip into the ambient atmosphere, e.g. by cooling the effluent gas and allowing the ammonia to react with SO^ or S03 in the effluent gas forming ammonium sulfite or ammonium sulfate, which may be removed in a washing tower or filtered out together with any excess of ammonia by methods well-known within the art, whereby it is recovered to be available for recycling.

In case of a reduced load, the jet stream should travel longer, meaning that the jet stream velocity should be increased. This implies an increased rate of the introduction of treatment gas, which may be compensated by reducing the relative content of ammonia in the treatment gas. The effluent gas may in a typical case contain 500 to 600 ppm ΝΟχ.

In this case, the treatment gas should preferably contain about 3 to 9, and more preferably about 6, per cent ammonia by volume. The preferred additive is natural gas, and preferably in a quantity of about 2 to 6, and more preferably about 5, per cent by volume. The molar ratio between the introduced quantities of natural gas and ΝΟχ should preferably be between 0 and 3, and more preferably between 0.9 and 1.1, and most preferably about 1.0.

The invention also provides an apparatus for the combustiqp of fuel comprising a device for reducing the contents of nitrogen oxides in an oxygen containing effluent gas and comprising means for injecting an ammonia-containing treatment gas comprising a carrier gas into said effluent gas, said apparatus being characterized by said means of injection comprising jet nozzles adapted to inject the treatment-gas as a jet stream counter-currently to the fl.ow of the effluent gas, said jet nozzles being connected with feeding conduits for ammonia-containing treatment gas.

Hereby the effective zone of reaction, i.e. the zone where the treatment gas is intermixed with effluent gas, will be located at a distance from the nozzles. The nozzles may therefor be arranged at a section of the flue gas ducts where the temperatures are lower than they are in the zone of reaction.

According to a preferred embodiment of the invention, means are provided for controlling the pressure of the carrier gas, and means are provided for controlling the pressure of the ammonia. Hereby the dosage of ammonia may be tuned exactly to the desired value independently of the quantity of carrier gas introduced. By providing separate means for controlling the carrier gas pressure, it is possible to produce jet streams of different lengths depending upon the velocities developed through the nozzles in accordance with the pressure selected. This provides a very expedient possibility of controlling the geometric location of the zone where the treatment gas is intermixed with the effluent gas. It is also possible to displace the zone of mixing so as to locate it always to the zone where the most favourable range of temperature prevails, even though this zone will be located differently depending upon the current load factor in the combustion apparatus.

According to a preferred embodiment of the invention, means are provided for separately controlling the pressure of at least one additive. As mentioned above, additives may be desirable, as they may, under some conditions, lower the optimum temperature range for the Ν0χ reduction. Such additives may be introduced together with the treatment gas or separately with a carrier gas from separate nozzles of a type similar to the type of the main-treatment gas nozzles.

According to a preferred embodiment, the jet nozzles are formed with a conically shaped zone of restriction, the apex angle being in the order from 5 to 20 degrees, and preferably from 8 to 14 degrees. The conically shaped restriction zone can have a length of from 50 to 100 mm, and more preferred from 60 to 80 mm. By this design, gas forced through the nozzles will, due to the different pressures at the inlet and at the outlet side, be gradually accelerated while travelling through the zone of restriction, whereby ejection velocities in the jet stream exceeding the velocity of sound in the gas medium, e.g. up to about 700 m/s in air, may be achieved by a.pressure difference in the order of between 1 and 3 bar.

The jet nozzle orifice formed at the narrowest portion of the nozzle opening comprises a short section, e.g. from 2 to 10 mm long, and preferably from 6 to 8 mm long, in which the aperture may be cylindrically shaped. A cylindrically shaped jet nozzle aperture has the advantage of a simpler manufacture. However, it is disadvantagous with respect to performance. Outside the jet nozzle aperture the gas jet will tend to expand, which may lead to whirl formation, eddy currents and irregular pressure fluctuations in case the difference between the pressure at the jet nozzle inlet and at the outlet exceeds a critical ratio, generally about 1.7 for gasses of practical interest. These instabilities in the flow pattern may be avoided by forming the jet nozzle aperture slightly conically expanded with an apex angle of the order of between 1 and 10 degrees, and preferably from 1 to 2 degrees.

This makes the jet stream more stable with sharp edges, whereby it will also achieve the maximum length of travel before getting mixed with the surrounding medium. It is particularly important that the mixing of the jet stream along its surface is kept to a minimum.

According to a preferred embodiment, the jets are mounted in jet support pipes, serving also as conduits for a carrier gas and enveloping or incorporating narrower pipes serving as lead lines for the ammoniacontaining gas. Carrier gas and ammonia-containing gas are mixed at the nozzles. This provides the optimum intermixing of these agents immediately before the injection and allows an optimum control response for carrier gas pressure and ammonia dosage, and also ensures that the different jets eject uniformly equal dosages. The enveloped pipes carrying ammonia are shielded against being heated through the side walls. The jet support pipes and the jets are preferably manufactured from a material which is stable at elevated temperatures, e.g. steel of the type Avesta 253 MA.

According to a preferred embodiment, one jet support pipe envelopes several smaller pipes, each feeding one jet or one subgroup of jets, and each being provided with a separately controllable supply of ammoniaIE 912649 containing gas. This permits sectioning of the jets in a comparatively simple fashion so that the dosage of ammonia may be controlled in the separate sections, making it possible to tune the dosage so as to suit varying conditions over the flue gas duct, e.g. variations in the content of Ν0χ. According to a preferred embodiment, the jets are arranged so that the jet stream from each of the jets, when running the combustion apparatus at full load, may penetrate the effluent gas stream countercurrently in order to reach a zone where the effluent gas has a temperature in the range of 950*C to 960’C. This ensures the optimum conditions for the Ν0χ reduction and simultaneously makes it possible to arrange the jets in a section of the flue gas duct where the temperatures are somewhat lower than the temperatures most favourable for the Ν0χ reduction, which means that the thermal strains imposed upon the jets and the jet support pipes are somewhat reduced.

According to a preferred embodiment, the jets are arranged along a contour or a profile across the flue gas duct adapted to a contour of constant temperature within the effluent gas stream. Hereby optimum conditions of reaction are achieved all over the cross section of the flue gas duct, even in cases where the temperature profile varies across the flue gas duct, as it will generally be the case.

The invention will be described in greater details below with reference to the accompanying drawings, wherein figure 1 shows a vertical, longitudinal section through portions of a power plant boiler, figure 2 shows a horizontal section along the line 2-2 of figure 1, figure 3 shows a vertical, transverse section along the line 3-3 of figure 2, figure 4 shows a vertical, longitudinal section through a jet support pipe, figure 5 shows a partial side view of components for a jet pipe, and figure 6 shows a vertical, transverse section of a jet support pipe.

All figures are schematic and not to scale, and illustrate only parts essential for the understanding of the invention, other parts being omitted. Throughout the drawings, identical references are used for identical components. - ' ' · Referring first to figure 1, this illustrates a boiler designated by reference numeral 1 as a whole for use in an electric power plant or in a similar plant. In the lower, left-hand portion a fire box 2 with a burner 3 for the introduction of a mixture of fuel and air is shown. The fire box 2 is defined by heat resistant outer walls 5 and by a partition wall 7 usually being built with refractory bricks and panels containing cooling pipes in a manner well-known within the art. Effluent gasses developed by the combustion move upwards through a generally vertical flue gas duct 4 to pass from an upper portion of the flue gas duct 4 through an aperture 9 in the partitioning wall 7 into a downwards flue gas duct 10 to proceed then through a generally horizontal flue gas duct section 11. The partitioning wall 7 separates the upwards flue gas duct 4 from the downwards flue gas duct 10, both ducts being surrounded by outer walls 5 and by a ceiling 50 constructed in refractory and heatinsulating materials and provided with internal cooling panels in a manner well-known within the art.

The effluent gasses meet on their way a number of heat exchanger panels, whereby the heat of the flue gasses may be transferred to other media, such as steam or water, before the flue gasses reach the horizontal duct 11. At the upper portion of the upwards flue gas duct 4, a number of heat exchanger panels are arranged in a pendant fashion, these heat exchanger panels serving generally as superheater panels 12, 13. Within the downwards flue gas duct 10, the effluent gas passes a number of heat exchanger surfaces, figure 1 illustrating symbolically an evaporator loop 14 arranged after the superheater panels, then a preheater loop 15 arranged after the evaporator loop 14. As mentioned above, the walls 5 and the partitioning wall 7 also comprise a number of integrated heat exchanger pipes serving the dual purpose of protecting the walls by keeping the temperatures within the walls down to a level which is safe for these elements, and of recovering the heat dissipated into these surfaces.

A portion of the partitioning wall 7 in a region below the flue gas aperture 9 comprises a grid of pipes, a so-called screen 8, constituted by pipes carrying a cooling medium with through holes in between. In a practical plant, the number of heat exchanger surfaces and panels is much larger, the drawings illustrating just symbolically as much as necessary to understand the basic principles of the invention.

The component details described so far are all considered to belong to the state of art.

According to one embodiment of the invention, the upper portion of the downwards flue gas duct 10 is provided above the evaporator loop 14 with a transversely extending pipe, a so-called carrier lance 20,21 supported as shown in figure 1 by braces 22 suspended from the ceiling. This carrier lance is provided with a number of jet support pipes 16,17,18 extending through holes in the screen 8 so as to reach into the region of the superheater panels 12,13. Each of the jet support pipes 16,17,18 is provided with a number of jets 23 in order that a fluid may be introduced through the carrier lance 20,21 to be ejected through the jets 23 into the upwards flue gas duct 4 in the region of the superheater panels 12,13.

Reference is now made to Figure 3, illustrating a vertical, transverse section through the upper portion of the upwards flue gas duct 4, in order to illustrate in greater details the spatial relationship between the carrier lance 20,21 and the superheater panels 12,13. In figure 3, the superheater panels are schematically illustrated as a number of thin plates suspended from the ceiling within the upwards flue gas duct 4 mutually parallel with essentially equal spacings. This figure illustrates how two of the superheater panels 13 arranged near opposite sides of the upwards flue gas duct 4 are slightly taller, i.e. the lower edges are located at a lower level than the lower edges of the other panels 12 arranged between them. This design is advantageous because the temperature of the effluent gas will be comparatively higher at the mid portion of the duct and comparatively lower near the side surfaces, so that the superheater panels are better adapted to the prevailing temperature profile by arranging the superheater panels adjacent to the side walls so as to reach to a slightly lower level than the other of the superheater panels. Figure 3 shows six of the shorter panels 12 and two of the taller panels 13, it being understood that a practical boiler plant will comprise a much greater number of superheater panels. The effluent gas may flow upwards between the superheater panels and may transfer heat into them, whereafter the flue gas flows on through the aperture 9 as explained earlier.

In figure 3, the screen 8 has been omitted entirely from the drawing in order to illustrate clearly the design of the carrier lance 20,21. Figure 3 illustrates more particularly how the carrier lance comprises two cylindrical tube elements arranged one after the other along a common center line, the tube element 20 comprising a first part of the carrier lance being extended through the wall in the flue gas duct illustrated to the left in figure 3, the tube element 21 comprising a second part of the carrier lance 21 being extended coaxially relative to the first carrier lance through the opposite side wall in the flue gas duct 4. The carrier lance elements 20,21 are supported by braces 22 suspended from the ceiling 50 in order to provide a mechanically strong design. As mentioned above, a number of jet support pipes, in figure 3 four on the left hand carrier lance 20 and five on the right hand carrier lance 21, are mounted in a cantilever fashion so as to extend into the spaces between the superheater panels essentially along the midplanes thereof. Figure 3 further shows how five of the jet support pipes 16 generally around the mid section are arranged at level with the carrier lances 20,21, the next two jet support pipes 17 (one to either side) being arranged at a slightly lower level relative to the carrier lances 20,21, while the outermost jet support pipes 18 are arranged at a further lowered level.

Reference is now made to figure 2, showing a horizontal section through the flue gas duct 4 and the flue gas duct 10 at the level of the carrier lances 20,21. Figure 2 shows to the left-hand portion an upwards flue gas duct 4 and to the right hand portion a downwards flue gas duct , said ducts being separated by the partitioning wall 7 and being surrounded by the outer, high-temperature resistant wall 5. Within the upwards flue gas duct 4, the superheater panels 12 and 13 are arranged as explained above, while the downwards flue gas duct 10 contains panels of the evaporator loop 14. Figure 2 further shows the two carrier lances ,21 arranged consecutively extending from either one of two opposite sides of the downwards flue gas duct 10, respectively. The carrier lances 20 and 21 are provided with a number of jet support pipes 16,17,18, figure 2 illustrating how they extend perpendiculary from the carrier lances and through the openings 40 in the partitioning wall 7, and more specifically through the portion of the partitioning wall 7 designated the screen 8 (cf. figure 1) to extend into the spacings between the superheater panels 12,13 in order that each space is provided with one jet support pipe extending almost all the way to the opposite side wall in the upwards flue gas duct 4. Each jet support pipe 16,17,18 is provided with five uniformly spaced jets 23 in order that essentially the entire area of the upwards flue gas duct 4 may be uniformly blanketed by a fluid introduced through the jets.

The outer ends of the lancet extending outside the heat resistant wall 5 are provided with a number of pipe connections illustrated symbolically in figure 2, in order that fluid may be introduced into the lances. The lances are designed as essentially cylindrical tubes of metal having a comparatively large diameter in order that a number of narrower pipes may be incorporated therein. In a preferred embodiment, one narrower pipe is extended into each jet support pipe. Thus, the carrier lance at the upper portion of figure 4 carrying four jet support pipes envelopes four internal pipes of a smaller diameter, whereas the carrier lance 21 in the lower portion of figure 2 is provided with five internal pipes, each of the internal pipes being extended into a respective one of the jet support pipes fixed to this lance.

Outside the downwards flue gas duct 10, the narrower pipes extend out from the lances and into individual conduit lines 39, figure 2 illustrating how each of these lines extends to a junction point 41, each of these junction points 41 being connected with two feed lines, each of these feed lines being provided with control valves and connected to pipe manifolds 31,36, respectively, in order that each of the conduit lines 39 may be provided with fluid from either of these manifolds in a fully controllable fashion. The space inside the carrier lances, but outside the narrower pipes, may be provided with compressed air by means of an air compressor 25 illustrated in the lower portion of figure 2 and connected with the carrier lance through a compressed air conduit 27 and an air-control valve 26.

In the lower portion of figure 2, a tank 28 for an agent, preferably ammonia, is schematically shown, this tank being connected through an ammonia conduit 29 through a main control valve 30 to the ammonia manifold 31. Thereby ammonia may be provided to the manifold 31 and through line control valves 32 to each of the lines 39 in a controllable fashion so that ammonia may be dosed in a controlled fashion into each of the jet support pipes 16,17,18. The lower portion of figure 2 also illustrates a second tank 33 for other additives, such as natural gas, this tank being connected through the gas conduit 34 and through a gas control valve 35 to the gas manifold 36 in order that each of the lines 39, and thereby the jet support pipes in a similar manner may be provided with additive from the tank 33, the flow of this additive into each of the.lines also being controllable by gas line control valves 37.

Reference is now made to figure 4, showing a longitudinal section r through a jet support pipe. Similarly to the design of the lances, the jet support pipe as shown in figure 4 also comprises a pipe of a comparatively large diameter, in order that a number of tubes of a smaller diameter may be arranged inside the jet support pipe. The left hand portion of figure 4 illustrates how the above-mentioned pipe conduit 39 extending through one of the lances is extended into the jet support pipe so as to reach a distribution box 41 inside the jet support pipe where the flow is distributed into a number of jet pipes 42, in this case five, each extending to a respective jet 23. By this arrangement, the flow of fluid introduced through the conduit 39 is uniformly distributed among the five jets 23 arranged at the lower side of the jet support pipe.

According to another embodiment, not shown in the drawings, separate pipe conduits for each of the jets are extended through the lances and through a jet support pipe. Each pipe conduit is connected with valves for the controlled introduction of ammonia and of an additive. This embodiment has a more complicated design and requires a larger diameter of the lances, while allowing a more flexible strategy since the jet beams may be dosed with the agents in an individually controllable manner.

In the embodiment illustrated in the drawings, a common system with compressed air is connected to provide all jets with air at the same pressure. The compressed air system may obviously be sectionalized in sections provided with separate pressure control valves in order to allow control of the pressure in the jet beams individually or in subgroups, in order to allow a more flexible dosage strategy.

Reference is now made to figure 5, which illustrates some components belonging to one of the jet pipes 42 to a larger scale, the figure showing how the jet pipe is bent into a shape approximately as an L, and further showing how a small portion of the pipe near the outer end has been slit so as to form a number of lugs, each extending over a quarter of the circumference. The lugs are flared slightly outwards to extend along a conical surface with open slits 44 between them. A ring with a diameter and a wall thickness matching the outer edges of the flared lugs is arranged below the outwardly flared lugs. Keeping in mind that figure 5 illustrates the components before the final assembly, the jet is completed by welding the ring 46 to the outer edges of the outwardly flared lugs 43, in order to provide a conically expanded or flared end of the jet pipe with outwardly open slits.

Reference is now made to figure 6, which shows a cross section through a jet support pipe. The jet support pipe is provided at the lowest section with a hole matching the ring 45 to be welded hereto, and the lower portion of figure 6 illustrates the jet in the shape of an essentially cylindrical body having an axial, through aperture comprising a conical restriction zone 47 and an approximately cylindrical aperture or nozzle opening section 46. The conical restriction zone may extend for a length of from 50 to 100 mm, and more preferred from 60 to 80 mm, while the approximately cylindrical aperture may extend for a length of from 2 to 10 mm, and more preferred from 4 to 6 mm. The apex angle of the conical restriction zone is around 10 degrees, this form of jet making it possible, by providing gas at a comparatively moderate overpressure, e.g. of 2 bars, to achieve gas velocities through the aperture in the supersonic range.

The gas jet beam ejected through the jet aperture will tend to expand, and whirl formation, eddy currents and irregular fluctuations in the pressure may occur, in particular when the pressure at the jet inlet side exceeds about 1.7 times the pressure at the outlet side. These undesirable effects may be avoided by forming the jet aperture with a slightly conical expansion. In case the pressure differential is not too high, or in case some whirl formation may be acceptable, it is preferred to form the aperture 46 cylindrically. At greater pressure differentials, and in cases where a longer travel of the jet beam is desired, the jet aperture should be formed with a slightly conical, outward taper, e.g. with an apex angle of between 1 and 10 degrees, and more preferred between 1 and 2 degrees. Hereby, a jet beam with sharp limits and a minimum of turbulence along the surface may be ejected.

Through the slits 44 at the end of the jet pipe 42, fluid media may be fed into the jet nozzle not only from the inner conduits 39, but also from the space outside the conduits 39, but inside the jet support pipe. By introducing gasses from both sources, the gasses will be mixed in the jet aperture.

According a practical embodiment compressed air from the air compressor 25 is provided within the carrier lances 20,21 and within the jet support pipes at a pressure which is controllable within the range of 1 to 3 bars over the atmospheric pressure, and more preferred about 1.7 bar over pressure. The pressure may be adjusted or controlled so as to vary the gas-mixture ejection velocity from the jets 23, in order that jet streams 24 may be ejected from the jets with a controlled momentum.

The ammonia tank 28 contains ammonia in a cooled state and at a pressure of 6 bars over the atmospheric pressure, the ammonia main valve 30 and the individual ammonia valves 32 in the ammonia conduits 29 being controlled so as to discharge a quantity rate of ammonia in the order of 10 per cent of the quantity rate of compressed air. The tank for additives may contain natural gas at a pressure of 3 bars, the gas main valve 35 and the gas control valves 37 in the respective gas conduits allowing the control of the additive, e.g. to a quantity rate corresponding to 5 per cent of the quantity rate of compressed air introduced.

In the upwards flue gas duct 4, the effluent gas in a practical embodiment flows upwards at velocities of the order of 20 to 30 m/s absolute value. The mixture of compressed air, ammonia and additive is introduced through the jets counter-currently to the main direction of effluent gas flow and at a velocity of e.g. between 300 and 400 m/s at a jet pressure of 1.7 bars over the atmospheric pressure. Hereby, each jet 23 ejects a jet stream 24 travelling at this velocity about 4.5 m before it has lost its momentum. Obviously, the jet stream 24 has a surface in contact with the effluent gas stream right from the aperture of the jet 23, but the major part of the gas mixture introduced will not be contacting the effluent gas and will not be mixed to any substantial extent with the effluent gas before reaching the zone where the jet stream has lost its momentum and thereby disintegrates and looses its character.

By controlling the pressure in the compressed air, it is possible to control the effective reach of the jet stream in order that the zone where the gas mixture is effectively intermixed with effluent gas may be located at different distances from the jets in a controlled manner. As the quantity of added ammonia and_additive is small compared to the jet stream, the mixture ratio may be Controlled independently of the control of the quantity of compressed air, in order to maintain e.g. an unchanged quantity of added ammonia and additive, even though the quantity of added compressed air may vary in cases where the jet stream velocity is varied.

Although the above description mentions air as carrier medium or carrier gas, other media may equally well be used, e.g. steam.

Ideal conditions for the reaction could probably be achieved by designing the boiler plant so that the effluent gas passes a comparatively small area of superheater surfaces, cooling the effluent gas in a controlled manner so as to reach a temperature in the range of 950°C to 960“C, whereafter a portion of the flue gas duct should ideally be arranged with no cooling surfaces, in which portion the effluent gas would flow at essentially constant temperature, and in which portion the treatment gas could be injected so as to react with ΝΟχ in the effluent gas in order to complete the reaction before the effluent gas meets other cooling surfaces lowering the gas temperature to below the temperature range suitable for the reaction. In most cases it will, however, not be economically feasible to reconstruct existing boiler plants in this fashion, and accordingly there is a need for methods of NO reduction that may be applied to existing boiler plants in a simpler Λ manner. This is achieved by the method according to the invention.

In existing boiler plants it is necessary to identify the zones where the effluent gas has just the temperature suitable for the desired reaction between treatment gas and NO , and then to design systems to A inject treatment gas within these zones. These suitable zones are typically located between the superheater surfaces, where the effluent gas temperature gradient, however, is comparatively steep, meaning that the flue gas temperature changes rapidly, implying that the geometric extent of the critical zone is rather narrow. An even more troublesome factor is that this critical zone is displaced depending upon the load operated in the boiler, implying that at any fixed zone the applicable conditions for the reaction will only prevail within a narrow range of load factors.

According to the invention, the jets are preferably arranged so as to achieve optimum conditions for the reaction at maximum boiler load by introducing carrier gas mixed with ammonia without additives, whereas operation at reduced load and therefor at reduced temperature near the jets is compensated by an increase of the jet stream velocity and optionally also by introducing additives along with ammonia. According to another embodiment, the jet stream velocity is maintained essentially constant, while the reduced temperature at reduced load is compensated solely by introducing additives.

A practical test has been carried out on a power plant boiler of a rated power of 135 MW. This power plant boiler was provided with a set of jets arranged along a horizontal plane extending transversely to the flue gas duct at a level equivalent to the lowest of the jet support pipes 18 illustrated in figure 3, and a further set of jets were arranged at a horizontal plane extending transversely across the flue gas duct at a level corresponding with the level of the mid jet support pipes 17 shown in figure 3. Through the jets compressed air mixed with ammonia was introduced in quantities of the order of 5 tons air per hour and 400 kg NHg per hour. A number of tests were run, and the results achieved are listed in the table below. In all of the tests the contents of NO2 and ^0 was minimal and therefor only the contents of NO before and after the reduction is listed. u- ε O CX CX O. •r- fO oo z CO o LO LO LO V V u Φ 1 ε ι O O o o o o rt O. 1 o o σι co co M- nj ex < 1 CM co CM r-l co Φ u o ε ι o o o o o o <4— CX ι o o o o o z Φ CL t CO CO CO co s** _Q 1 1 s- ru Φ ε -rt M- '«“) 1 u s- i- C nJ O C C l Φ Φ Φ c ·»- O · 5 * CX S- S- o s- Φ •Γ— 1 o o CX Φ Φ Φ -r— Φ -rt CO -rt 1 —J —1 Z3 ~O ex σι CX •r— Z 1 o -r— cx φ ex 00 Z 1 —i V) zz> u TABLE Φ -rt N ι N ι Ο ι Ζ ι 1 nJ •rt c Φ Φ 1 Φ ε Ό U ι E φ © Φ 1 S- ο O o o -rt o CL 1 O σ LO LO LO to tV CX ι c ω cn σι σι σ< =D ι Φ s- zs Φ Φ » -rt f— r~— I nJ N SI i SI SI ι Φ O o · CL c C 1 o o o o o ε 1 o LO LO LO 00 φ S- ι o »—I rrt rrt f— Φ 1 «-Η »—1 r—H r—H ·—1 5 « O I 1 Ό nJ ι -σ Ο ι nJ f— 1 O 1 r— M— E < O =3 ι o LO LO to o S- E » co r- r*. o Φ r-1 X J o ω nJ ι Q_ nJ E < Φ CL X UJ CM CO rf in In all experiments, compressed air was introduced through a carrier lance at a pressure of 1.7 bars above the atmospheric pressure mixed with ammonia to a quantity so that the molar ratio between ammonia and NO was approximately 2.0. The oxygen content within the effluent gas was approximately 7 per cent. The results of the experiments should be interpreted with some caution, partially because of inaccuracies in the measurements, and partially because of other non-controllable variations in The combustion process, which may haveaffected the results. Since the number of jets in the power plant was rather limited, the mixing or blanketing may have been at least partially incomplete, meaning that there may have been regions with overdosages or underdosages of the reductive agent.

In experiment no. 1, the power plant was operated at 60 per cent of full load injecting carrier gas with NHg through the jets at the lower level corresponding to the level of the jet support pipes 18 in figure 3. The Ν0χ content of the effluent gas was reduced by approximately two thirds, and the slip of NH^ was at a very low level, implying that these results were satisfactory.

In experiment no. 5, the power plant was operated at full load, and the temperatures at the jets were substantially higher and perhaps too high compared to the temperature range in which the Ν0χ reduction according to this process may take place. In this case, carrier gas with NH^ was injected through the jets at the upper level corresponding to the level of the jet support pipes 17 in figure 3. The slip of NHg was ppm, which is acceptable, whereas the effluent gas content of Ν0χ was more than 50 per cent of the contents without Ν0χ reduction, which is considered not quite satisfactory. It is believed that the temperature in this case, at at least a portion of the jets, was too high, meaning that a portion of the added NH^ was oxidized or burned without reacting with Ν0χ.

In experiments nos. 2, 3 and 4, the boiler plant was operated at 75 per cent of full load, and experiments were made of introducing NH^ solely at the lower jet level (experiment no. 2) and solely at the upper level (experiment no. 3). The Ν0χ reduction achieved in either case was considered acceptable as such, but not up to the result expected in view of the quantity of NH^ added. The slip of NH^ in experiment no. 3 was 46 ppm*, which is considered unacceptable. In the attempt to improve these results, another experiment was made, in wFieh NH^ was introduced through jets at the lower level adjacent the side walls and through jets at the higher level in the intermediate region. This produced a clear improvement in the Ν0χ reduction. However, the slip of NHg was higher than acceptable. The slip of NH^ is expected to be due to a non-uniform dosage of NH^ across the cross-sectional area of the flue gas duct, which could probably be remedied by a rearrangement of the jet positions. In view of the very substantial reduction of Ν0χ achieved in experiment no. 4, it is expected that the dosage of NH^ could be reduced, whereby the slip of NH^ would be expected also to be reduced.

The apparatus according to the invention differs from the plant in which the experiments were carried out in that the jets are arranged at three levels as shown in figure 3 in order to provide a better adaptation to the approximately dome-like temperature profile across the cross section of the flue gas duct. The jets at the mid region are further arranged at a higher level than was the case in the pilot plant. All jets are hereby arranged at a distance above the critical temperature zone 19 under all conditions of operation. The critical zone 19, which is the zone where NH^ will react satisfactory with Ν0χ in the effluent gas, will be located at a comparatively high level when operating the power plant at full load, thus the carrier gas should be introduced with a comparatively low pressure, in order that the jet streams 24 with additive will travel a comparatively short length in order to just reach the critical zone before the jet streams 24 have lost their momentum to mix their contents with effluent gas.

Upon a reduction of the boiler load, the critical zone is displaced downwards to be located at a greater distance from the jets. In order to compensate for this increased distance, the jet air pressure is increased in order that the jet streams 24 will travel over a longer distance. The amount of NHg is adapted to the effluent gas flow implying that the content of NH^ relative to the compressed air will be lower at partial load, kith jets as illustrated in figure 6 and at an air pressure of 1.7 bars above atmospheric pressure, the jet streams may travel 4.5 meters from the jet nozzles. In case the boiler is operated at even lower load and at a load so that the jet streams are unable to reach the critical zone, the treatment gas will be mixed with effluent gas at a location where the temperature is lower than the temperature range ideal for reaction with NH^. It is, however, still possible to achieve satisfactory reduction of Ν0χ by .introducing additives together with the NH3 as explained above. By the introduction of natural gas in a quantity of 50 per cent by weight of the quantity of NH^, an acceptable Ν0χ .reduction may be achieved in a temperature range from 700*C to 900*C. .

. It is evident that the invention provides a plant for Ν0χ reduction of the effluent gas from the combustion plant with excellent flexibility and capable of achieving satisfactory Ν0χ reduction over a wider range of loads in the boiler plant than possible in the prior art.

Claims (26)

1. PATENT CLAIMS

1. A method of reducing the contents of nitrogen oxides in oxygen-containing effluent gas developed by the combustion of fuel, wherein an 5 ammonia-containing treatment gas comprising a carrier gas is injected into the effluent gas as at least one jet stream flowing countercurrently or at least partially counter-currently to the effluent gas at a velocity sufficient for causing the treatment gas to penetrate the effluent gas as an essentially coherent beam over a predetermined length 10 of travel.

2. The method according to claim 1, wherein the treatment gas is injected as a number of jet streams distributed transversely over the cross-sectional area of the effluent gas flow perpendicular to the 15 direction of effluent gas flow, and preferably at different levels along the direction of effluent gas flow.

3. The method according to claim 1 or 2, wherein the treatment gas is injected at a velocity of at least 75, preferably at least 100, 20 especially at least 200 m/s, and in particular at supersonic velocity.

4. The method according to any of the claims 1, 2 or 3, wherein the treatment gas further comprises at least one component forming at a temperature of the order of from approximately 700 to approximately 1000 25 degrees Celsius free radicals of a type which within said temperature range initiates conversion of NH 3 to NH 2 radicals.

5. The method according to claim 4, wherein the carrier gas comprises atmospheric air or steam, and wherein said at least one component is 30 oxygene or H^O.

6. The method according to any of the claims 1-5, wherein together with the treatment gas or as further jet streams one or more additional additives are introduced, serving in combination with ammonia to lower 35 the optimum temperature range for NO x -reduction.

7. The method according to claim 6, wherein the additional additive is methane, ethane, dimethylamine or natural gas.

8. The method according to any of the claims 1-7, wherein the treatment gas contains from 3 to 9, preferab]y 6 per cent ammonia by volume, and optionally from 2 to 6, preferably 4 per cent natural gas by 5 volume.

9. The method according to any of the claims 1-8, wherein the treatment gas is injected in a quantity to feed in from 1 to 3, preferably from 1.5 to 2.0 mol, particularly 1.6 mol ammonia per mol 10. Nitrogen oxides contained in the effluent gas and from 0 to 3, preferably from 0.9 to 1.1 mol natural gas per mol nitrogen oxides contained in the effluent gas.

10. An apparatus for the combustion of fuel having a device for redu15 cing the contents of nitrogen oxides in an oxygen-containing combustion effluent gas and comprising injection means for injecting an ammoniacontaining treatment gas comprising a carrier gas into the effluent gas, said injection means comprising at least one jet nozzle adapted to inject the treatment gas as a jet flowing counter-currently or at least 20 partially counter-currently to the effluent gas flow, said jet nozzle being connected to feeding conduits for the ammonia-containing treatment gas.

11. The apparatus according to claim 10, wherein the injection means 25 comprises means for controlling the pressure of the treatment gas, preferably in the form of means for controlling the pressure of the carrier gas and means for separately controlling the pressure of the ammonia. 30

12. The apparatus according to claim 10 or 11, comprising at least one separate injection nozzle for the injection of at least one additive and a carrier gas, and preferably comprising means for separately controlling the pressure of said at least one additive and said carrier gas.

13. The apparatus according to any of the claims 10, 11 or 12, wherein the jet nozzles have a conically restricted section, in particular with an apex angle in the conically restricted section of the order of from 5 to 20 degrees, and preferably from 8 to 14 degrees.

14. The apparatus according to claim 13, wherein the conically restricted section has a length of from 50 to 100 mm and preferably from 5 60 to 80 mm.

15. The apparatus according to claim 13 or 14, wherein the nozzles are provided with an orifice having a conically expanded discharge section, in particular with an apex angle in the conically expanded discharge 10 section of the order of from 1 to 10 degrees, and more particularly from 1 to 2 degrees.

16. The apparatus according to claim 15, wherein the disharge section has a length of from 2 to 10 mm and preferably from 4 to 6 mm.

17. The apparatus according to any of the claims 10-16, wherein the jet nozzles are arranged on jet support pipings through which the treatment gas is conveyed. 20

18. The apparatus according to claim 17, wherein the jet support pipings are provided with interior, separated conduits in order that the carrier gas and the ammonia-containing gas may be conveyed separately and may be mixed just before the mixture exits through the nozzles, preferably by the jet support pipings being constituted by tubes, 25 through which carrier gas is conveyed, and in which smaller tubes are incorporated, through which ammonia-containing gas is conveyed.

19. The apparatus according to claim 17 or 18, wherein each jet support pipe is arranged substantially perpendicular to the direction of flow of 30 the effluent gas and comprises a number of nozzles spaced substantially uniformly over the length of the pipe.

20. The apparatus according to any of the claims 17, 18 or 19, wherein each jet support pipe incorporates a number of narrower tubes, each 35 feeding one or more nozzles, and each being associated with a separate control for the supply of ammonia-containing gas.

21. The apparatus according to any of the claims T0-20, wherein the nozzles are arranged in a manner that a jet emanating from each of the nozzles during the operation of the combustion apparatus at full load may penetrate the flue gas counter-currently to a region, at which the effluent gas has a temperature in the temperature range of 900 T 1000 5 degrees Celsius, particularly 950-960 degrees Celsius.

22. The apparatus according to any of the claims 10-21, wherein the nozzles are distributed substantially uniformly over the cross section of the flue gas duct, and preferably are arranged along an imaginary 10 contoured surface extending across the flue gas duct, said imaginary contoured surface being adapted to a profile of constant temperature within the effluent gas.

23. The apparatus according to claims 10-22, wherein the nozzles are 15 arranged between cooling panels arranged within the flue gas duct.

24. The apparatus according to claim 23, wherein the jet support pipes are supported within a portion of the flue gas duct, which portion is substantially devoid of internal cooling panels, and extend into the 20 spaces between cooling panels in a cantilever fashion.

25. A method of reducing the contents of nitrogen oxides in oxygen containing effluent gas developmed by the combustion of fuel according to any preceding claim substantially as hereibefore described with reference to and as illustrated in the accompanying drawings.

26. An apparatus for the combustion of fuel according to any preceding claim substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings .