RECOVERY STEAM GENERATOR

DESCRIPTION

[0001]The invention relates to a heat recovery steam generator for the cooling of hot flue gases from metallurgical processes and a method for operating a heat recovery steam generator.

[0002]The main problem of a heat recovery steam generator or heat recovery boiler used in metallurgy is that of cooling the flue gases of, for example, a melting or converter furnace and producing steam through the transferred heat.

[0003] In some metallurgical processes, moreover, it is necessary for process- related reasons to cool the process or flue gases below a specific temperature in order to enable further process-related processing of the flue gases. Pyrite and zinc-blende roasting, sulphur combustion and acid cracking, for example, are examples of such processes.

[0004] Such heat recovery steam generators or heat recovery boilers usually comprise a radiation section and a convection section. After-reactions, such as oxidation of sulphur and metals, take place on a large scale in the radiation section of the heat recovery steam generator. The oxygen required for such reactions is blown in the form of oxygen-containing gas, usually air or oxygen-enriched air, for the most part with additional nozzles into the radiation section or radiation chamber (a much smaller part of the air flows through leakage openings into the heat recovery steam generator, since a partial vacuum prevails in the heat recovery steam generator). The nozzles serve not only to feed oxygen-containing gas, but also ensure the increase in the flow turbulence inside the radiation chamber and thus improve the mixing of the flue gases from the metallurgical process furnace with the oxygen-containing gas, in order to accelerate the required chemical reaction. In the case of large heat recovery steam generators with radiation chamber dimensions of for example 4 m width, 6 m height and 10 m length, a greater number of nozzles usually has to be installed in order to achieve uniform mixing. The installation of additional platen heating surfaces is often also required in order to achieve uniformity of the flow and mixing over the height of the heat recovery steam generator.

[0005] It is the task of the invention to create a heat recovery steam generator and a method for the operation of such a heat recovery steam generator, wherein the mixing of the process or flue gases with oxygen-containing gas inside the radiation chamber of the heat recovery steam generator and thus after- reactions at the constituents of the flue gas are improved with, at the same time, an unchanged total quantity of the injected oxygen-containing gas.

[0006]The aforementioned task is solved by the features of claim 1 with regard to the heat recovery steam generator and by the features of claim 8 with regard to the method for operating such a heat recovery steam generator.

[0007]Advantageous embodiments of the invention can be deduced from the dependent claims.

[0008] Through the solution according to the invention, a heat recovery steam generator and a method for operating such a heat recovery steam generator are created, having the following advantages:

- Increase in efficiency of the heat recovery steam generator.

[0009]An advantageous embodiment of the invention is that the nozzles, viewed over width B of the heat recovery steam generator, are disposed at a distance of 200 to 1000 mm from one another. This thus ensures that sufficient feeding of oxygen-containing gas for the efficient after-reaction of constituents of the flue gas takes place over the width of the heat recovery steam generator. [0010]The nozzles are preferably disposed in at least one plane. Optimum feeding of oxygen-containing gas is thus ensured.

[0011] It is also advantageous to constitute respective clear cross-section Q,, of a nozzle with 1 to 200 cm ² . The cross-sectional area of the nozzle is in the first place dependent on the required medium inflow quantity, i.e. the quantity of oxygen- containing gas to be fed in, as well as on the admission pressure. Larger gas quantities and therefore also larger nozzle cross-sections are of course worked with and operated in the case of large geometrical dimensions of the heat recovery steam generator,.

[0012] In an advantageous development of the invention, the clear cross- section of the nozzles is round. In the case of certain geometrical conditions of the heat recovery steam generator, it is further advantageous to constitute the cross- section of the nozzles not round, but oblong. The form of the gas outflow cone can thus be influenced and the achievable width of the mixing can thus be improved.

[0013]An advantageous development of the invention makes provision to use a quick-opening shut-off valve as a means for effecting a pulsating feeding of the oxygen-containing gas. The pulse duration, i.e. the duration of the feeding of the oxygen-containing gas, can thus be controlled as desired.

[0014] It is advantageous to select the time periods or cycle times of the pulsating injection of the oxygen-containing gas, i.e. the time from the start of one injection to the start of the next injection, between 1 and 10 seconds, wherein furthermore the duration of the injection itself, i.e. the pulse duration, lies between 0.5 and 5 seconds in an advantageous embodiment. The duration of the cycle times and of the pulse duration is dependent on the size and the geometry of the heat recovery steam generator. It also influences the turbulence and the effective mixing of the oxygen-containing gas with the flue gas. [0015] In an advantageous development of the invention, the exit speed of the oxygen-containing gas brought in through the nozzles amounts to 20 to 100 m/s. The effect of this is that the penetration depth of the oxygen-containing gas into the flue gases becomes as great as possible and a correspondingly high turbulence in the flue gas is generated.

[0016] It is expedient to use air as an oxygen-containing gas. The use of air as an available operating medium is cost neutral and thus easy to manage. In order, if possible, to make the after-reactions at the constituents of the flue gas more efficient, it may be advantageous also to enrich the air with oxygen.

[00 7] A further advantageous embodiment of the invention makes provision to use water vapour of a water-vapour/air mixture as an oxygen-containing gas. Water vapour or a water-vapour/air mixture usually has a higher pressure compared to atmospheric pressure. Means for increasing the pressure of this medium can thus be dispensed with.

[0018] Examples of embodiment of the invention are explained below in greater detail with the aid of the drawings and the description.

[0019] In the figures:

Fig. 1 shows, represented diagrammatically, a radiation chamber of a heat recovery steam generator in longitudinal cross-section,

Fig. 2 shows, represented diagrammatically, a cross-section through the radiation chamber according to cross-section A-A in figure 1 .

[0020] Figure 1 shows radiation chamber 2 of a heat recovery steam generator, into which the hot process or flue gases 5 from a metallurgical process, for example from a melting or convection furnace not represented (in which a reduction process takes place), are introduced in its front part. Radiation chamber 2 is constituted with radiant heating surfaces not represented, through which a working fluid, usually water or steam, flows. The working fluid takes up heat from hot flue gas 5 and thereby cools down flue gas 5. After flowing through radiation chamber 2, flue gas 5 then flows out of rear part 9 of radiation chamber 2 into the convection section (not represented) of heat recovery steam generator 1 , in which further heat is removed from the flue gas, which is taken up by the working fluid flowing in the heating surfaces of the convection section. The cooling of flue gas 5 below a specific temperature is necessary in order to enable further process-related processing of flue gases 5.

[0021]Apart from the cooling of flue gas 5 in radiation chamber 2, there also takes place in the latter an after-reaction of flue gas 5 originating from a metallurgical process, since flue gas 5 still comprises constituents such as for example sulphur, carbon, hydrogen or hydrocarbons, i.e. reducing agents from the preceding reduction process. Oxygen-containing gas is required to react out these constituents (reducing agents). For the required after- reaction, oxygen-containing gas 6, preferably air, is introduced or injected via nozzles 4, which are disposed in front part 3 of radiation chamber 2, said oxygen-containing gas mixing with flue gas 5 and effecting a reacting-out of the constituents or reducing agents. Feed lines 7 are provided for feeding oxygen-containing gas 6 to nozzles 4. Nozzles 4 are disposed approximately at 90° to upper enclosing wall 10 of radiation chamber 2 and in enclosing wall 10 (i.e. nozzles 4 penetrate enclosing wall 10), wherein enclosing wall 10 is constituted at an angle in front part 3 of radiation chamber 2. Furthermore, nozzles 4 are preferably installed in upper enclosing wall 10, it also being possible for them to be disposed in lower enclosing wall 10.

[0022] In order to be able to carry out efficiently these after-reactions at the constituents of flue gas 5, effective mixing of flue gas 5 with oxygen-containing gas 6 is required. According to the invention, this is achieved by the fact that the injection of oxygen-containing gas 6 does not take place continuously, but in an intermittent or pulsating manner with short and pulse-intense oxygen-containing gas flow surges. A flow pulse leads to a periodic acceleration and subsequent slowing-down of injected oxygen-containing gas 6 and thus causes greater turbulence inside radiation chamber 2 and a better mixing, i.e. a better after-reaction, of the reducing agents or constituents contained in flue gas 5 compared to a continuously injected oxygen- containing gas flow 6. Depending on the time lapse and the intensity of the pulsed flow, both the degree of turbulence and the mixing region of hot flue gases 5 and oxygen-containing gas 6 can be increased considerably.

[0023] In the preferred example of embodiment, the injection of oxygen- containing gas 6 takes place via a plurality of nozzles 4, which are distributed uniformly over width B of radiation chamber 2. Nozzles 4 are preferably disposed at a distance A of 200 to 1000 mm from one another, wherein nozzles 4 lie on a plane as represented in figure 2. It is however also possible to design the distances of nozzles 4 from one another differently, for example to select closer distances in the centre than at the outside. A further variant, not represented, makes provision to dispose nozzles 4 on two or more planes. Furthermore, nozzles 4 preferably have in each case a clear cross-section Q of approximately 1 to approximately 200 cm ² , wherein the cross-sections of nozzles 4 can be constituted round or oblong.

[0024]The exit speed of oxygen-containing gas 6 brought in through nozzles 4 preferably amounts to 20 to 100 m/s. Optimum penetration of oxygen-containing gas 6 into flue gas 5 is thus ensured. Furthermore, the time periods or cycle times of the pulsating injection of oxygen-containing gas 6, i.e. the time from the start of one injection to the start of the next injection, are preferably between 1 and 10 seconds, wherein the duration of the injection itself, i.e. the pulse duration, lies between 0.5 and 5 seconds.

[0025] According to the invention, feed line 7 to nozzle 4 is provided with a means 8 for effecting a pulsating feeding. According to a preferred example of embodiment, means 8 is a quick-opening shut-off valve. The cycle time and pulse duration of the injection of oxygen-containing gas 6 can be fixed as required by means of this quick-opening shut-off valve 8, which is controlled by a control device not represented. It is possible for a feed line 7 with a means 8 to be provided for each nozzle 4. It is however also possible to serve a plurality of nozzles 4 with one feed line 7 and to provide this feed line 7 with a means 8, for example one feed line 7 serves all nozzles 4. Feed line 7 thereby branches before nozzles 4 corresponding to the number of nozzles 4 to be served.

[0026]According to a further example of embodiment of the present invention, water vapour or a water-vapour/air mixture can be used as an oxygen-containing gas. Water vapour or a water-vapour/air mixture usually has a higher pressure compared to atmospheric pressure. A means for increasing the pressure of this medium can thus be dispensed with and a reduction in efficiency can thus be avoided.

[0027] Processes, e.g. pyrite and zinc-blende roasting, sulphur burning or acid cleavage, can be carried out with the present heat recovery steam generator and the method for operating such a heat recovery steam generator.