Method of scrubbing combustion gases in order to reduce so2 concentration and gas scrubbing apparatus as well as method of and apparatus for reducing sox concentration in combustion gases by scrubbing

The combustion of sulfur-containing materials such as oil, kerosene and coal produces a variety of sulfur oxides, grouped as SO2. Legal regulations impose an obligation to reduce SO2 emissions. An example of an effective flue gas treatment method to remove SO2 is wet scrubbing using limestone. The limestone wet rinse technology has been well developed and is effective. However, large-scale equipment is necessary for its use. The costs are also proportionately high. It would be desirable to be able to use high velocity gases for a number of reasons explained in the pending patent identified as US Patent application SN 08/257, 160 filed on behalf of GE Bresowar,

J. Klingspor and E. Bakke June 9, 1994 (attomey's number 1930-P0004).

The use of gas-liquid sorbers with high gas velocities, such as empty spray towers, creates problems when removing liquid droplets from gases, makes it difficult to clean mist suppression devices, and causes inconvenience of droplets hitting walls, roofs, and other devices. The baffle mist eliminators used in such processes tend to clog as the deposited liquid becomes saturated with calcium sulfate. Crystallized calcium sulphate settles on walls, roofs and other equipment and can lead to pressure drops inside the scrubber, and the deposited solids from time to time can break off in large pieces and fall off and damage scrubber equipment such as heads, nozzles, reinforcements, etc. In addition, the deposited solids that break off in this form can enter the liquid processing system and the sorption solution, leading to clogging of the nozzles and lower system reliability. It would be desirable to improve the removal of droplets entrained by this gas, for example sorbent droplets, from a rapidly moving gas stream, for example exhaust gas. In order for the liquid trapping separator to function effectively, the liquid droplets must be made to impinge on one or more surfaces to reduce the amount of liquid in the gas. The surfaces must be washable and easy to drain off. At the same time, they should not contribute to the secondary production of drops. In many cases, it is preferable to use vertically oriented fog eliminators, but they are not practical when the vertical direction of the gas flow must be altered through the walls the gas passes through. The consequence is the crystallization of the deposits and other related problems.

It would be desirable to improve gas scrubbing and other gas-liquid contact technologies in the capture and removal of liquid behind the flue gas by increasing the liquid droplet capture and removal efficiency with high throughput, low pressure drops, and low liquid-borne solids tendency.

Single pass unpacked scrubber systems using calcium carbonate to react with SO2 are the simplest in terms of design and mode of operation. These systems are preferable to use as they operate with low pressure drops and do not tend to flake or clog. Their advantages in terms of simplicity and reliability are, however, diminished by the large size of the devices related to the reactivity of the calcium carbonate suspensions used. Since they are towers without fillers, grates, screens etc., the heights of these towers are usually large to ensure the necessary contact between the flue gas and the scrubbing liquid and multiple levels of spray nozzles must be used. Another source of problems is the need to ensure adequate sorption capacity of the scrubbing liquid to absorb SO2 from gases, a feature which depends on the alkalinity of the liquid. However, the solubility of calcium carbonate decreases as alkalinity increases, so towers must be high to facilitate good SO2 absorption even with high system loads.

It would be advantageous to improve single-pass wet scrubbing in unpacked towers using calcium carbonate to process SO2-containing flue gases by increasing the linear velocity of the exhaust gas upstream of the tower and lowering the tower height to reduce space requirements and facilitate the use of a vertically oriented rotating gas - gas heat exchanger between the gases.

Wet flue gas scrubbing to remove SO2 is provided by a variety of gas liquid contact systems. One of the best of these is the system based on either countercurrent unfilled or packed countercurrent spray towers. In the vast majority of constructions of this type, the gas flows upwards and the liquid flows downwards under the influence of gravity. A variety of reagents are recommended, but the most preferred are those obtained at low cost and stored and transported without any special precautions. Calcium carbonate access4

182 It is commercially available in a variety of forms, including limestone, as it is an exemplary material as it meets the above requirements and produces by-products that can be easily disposed of as landfilled waste or sold as gypsum.

The design and operation of single-circuit counterflow limestone spray towers are described by Rader and Bakke in Incorporating Full - Scale Experience Into Advanced Limestone Wet FGD Designs at IGCI Forum '91, September 12, 1991, Washington, DC Unpacked spray towers, that is without grates, screens or solid fillings to facilitate the contact between the liquid and gas, are presented as simple in construction, allowing high flue gas desulphurization (FGD) for reasons of reliability. The authors do not give a broad discussion of the trap, but present a two-stage mist eliminator and a top-down and bottom-up washing method.

A thorough discussion of conventional fog eliminators on the market was conducted by Jones, McIntush, Lundeen, Rhudy and Bowen in Mist Elimination System Design and specification for FGD Systems presented August 26, 1993 at the SO2 Control Symposium, Boston, MA. The authors demonstrate, through the results of the exhaustive text, that by employing large, vertical towers and upwardly directed gas velocities greater than about 4.5 meters per second, it is difficult to achieve effective mist removal in the spray zone due to the "choking" phenomenon involved. "Slipping" occurs with insufficient dehydration as the mist eliminator becomes flooded with liquid. Depending on the different designs of the mist eliminators, this problem occurs at different gas velocities, which in turn depends on other elements of the individual structures. However, it is a general rule that there is no mist eliminator that works satisfactorily with vertical upward flow of gases at velocities greater than 4.5 meters per second. Each of them has a risk of flooding ("choking").

It would be desirable to use a fog eliminator operating efficiently at gas velocities greater than 4.5 meters per second as reported by Rader and Bakke. One manufacturer of limestone-based vertical flue gas desulphurization systems provides a system that has proven to be suitable for gas velocities greater than 4.5 meters per second in the spray zone. This structure is illustrated in the form of a fog suppression system in N. V. Provinciale Zeeuwse Energie - Maatschappij's Borssele power Station, Unit 12, located in the Netherlands and reasonably well described by Rosenberg and Koch on July 10, 1989 in the Batelle's Stack Gas Emissions Control Coordination Center Group report. This design is based on a horizontal - flow mist eliminator located approximately and above the vertical spray flow zone. The sorbent entrained from the spray zone must travel up and then turn radially outward to pass through the mist eliminator. The mist eliminator operates at a much lower surface velocity than the tower spray zone velocity, in fact about 20% of the spray zone velocity. In addition, the upper regions of the mist eliminator are not used sufficiently, while the lower regions closest to the sputtering zone process most of the exhaust gas and entrained sorbent. The unit presented at Borsselle is expensive to build and maintain. If the Borsselle structure were to be shortened to reduce costs, then problems with the build-up of solids on the roof would be expected due to the impact of droplets coming vertically from the spray zone and not returned to the mist eliminator. Consequently, although it is widely accepted to use a horizontal mist eliminator in flue gas desulfurization systems based on limestone and operating at gas velocities ranging from

4.5 to 6.0 meters per second, it would not be possible with the Borsselle construction due to the high risk of deposits on the roof.

The impact of sorbent droplets on the surfaces at the bottom or top of the mist eliminator in limestone-based flue gas desulphurization systems is an undesirable phenomenon.

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Droplets that strike a surface and do not recirculate, e.g. are not rinsed out during washing or fall off by themselves, lead to the formation of flaking deposits as dissolved calcium ions oxidize to sulphate together with the absorbed sulfite. These flaking plaster deposits build up on their own and fall off under the action of sufficiently high mechanical forces and their own weight. This phenomenon is very disadvantageous as it leads to a series of severe damage to the internal spray tower devices and other equipment.

The prior art does not address the need to make any improvements in methods for removing entrained liquid droplets appearing in scrubbed gas streams in flue gas desulfurization processes running at gas velocities greater than

4.5 meters per second.

In single-pass counter-current towers without packing of the type discussed by Rader and Bakke, the calcium carbonate sorbent flows downward while the SO2-containing volatile exhaust gas flows upward. This allows the sum of many parameters including the velocity of the absorbed gas ranging from 2 to less than

4.5 meters per second noting that the velocity of the absorbed gas has little effect on the liquid-to-gas (L / G) ratio - a key factor in terms of both investment and operating costs. For the towers described, the height of the spray contact zone is not given, but is typically in the order of 6 to 15 meters, which was a prerequisite for achieving the expected 95% SO2 reduction in the exhaust gas.

SO2 is absorbed by the descending downward rinsing sorbent collected in the reaction vessel, where calcium sulfite and calcium sulfate are formed. Ideally, the reaction vessel is oxygenated to increase sulfate production. When the sulfate crystals have grown to a sufficient size, they are separated from the sorbent in the reaction vessel. Such scrubber towers are relatively efficient in terms of their operation, however their size and overall unit costs may be a limiting factor for the usefulness of this type of scrubber in existing power plants.

A method of scrubbing gases to reduce the concentration of SO2 in the exhaust gas according to the invention, in which the exhaust gas is contacted countercurrently with an aqueous suspension, which, after contact with the exhaust gas, is collected and discharged, and part of the collected suspension is returned to further contact with the exhaust gas, while the washed exhaust gas is processed defogging is characterized by the fact that the vertical stream of exhaust gas is capable of a speed greater than 4.5 meters per second, and upon contact with said suspension, the flow direction of the exhaust gas stream is simultaneously changed by at least 45 ° to the previous, vertical direction of the exhaust gas stream and the number of droplets with a diameter smaller than 100 µm, according to Sauter, is reduced by consolidation or precipitation by at least 40% according to Sauter. in the stream of flushed exhaust gas, with a pressure drop of not less than 3.8 mm of the water column.

A gas scrubbing device for reducing the concentration of SO2 in the flue gas according to the invention, comprising a scrubber provided at the bottom with a channel for supplying flue gas to the vertical scrubbing section, the vertical flue gas scrubbing section terminating at the top with sprinkler heads and having a reaction tank at the bottom. when a mist eliminator is located above the flushing section at the flue gas outlet, it is characterized by the tower scrubber has at least one trap separator positioned above and across the vertical scrubber section of the tower scrubber at an angle ranging from 10 ° to 45 ° from the horizontal, and at least one horizontal flow and vertical orientation mist eliminator is positioned downstream of the collecting separator. The catching separator is formed of a single-tier group of lamellas, with adjacent lamellae units at an angle of 120 to 150 ° to each other, and the lamellae of each unit being parallel to each other, while the individual lamellae relative to the vertical are oriented at an angle of 120 ° to 150 °. an angle from 35 ° to 55 °, and the distance between the individual lamellae is from 40% to 70% of the width of a single lamina. Each of the lamellae assemblies is provided with a collector located above and below it, provided with nozzles for periodic flushing of the lamellae with water.

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The invention is explained in more detail in the working examples by means of a drawing, in which Fig. 1 shows schematically a preferred embodiment of the process of the invention using a single-pass countercurrent limestone flushing scrubber empty, Fig. 2 is an axonometric view of the trap shown in the spray tower. in Fig. 1, Fig. 3 is an axonometric view of an alternative embodiment of the catching separator.

The description of an embodiment relates to the preferred embodiment shown in Fig. 1, which relies on the operation of a single pass packed scrubber with a countercurrent limestone wash to remove sulfur oxides, mainly SO2, from the flue gas. However, the technology of the invention can be used in other gas-liquid contact processes and other types of scrubbers.

In the method according to the invention (Fig. 1), the flue gas, for example from coal-fired industrial or utilization boilers, is introduced into the scrubber 1 through the introducing channel 2. The gas is led up through the vertical scrubbing section 3, countercurrent to the sprayed aqueous suspension containing limestone fines, discharged through rows 4 and 4 'of spray nozzles. The atomized suspension appears as drops in the tower.

According to the invention, the gas stream is suited to a speed of greater than 4.5 meters per second, preferably 5 to 6 meters per second. These gas velocities are desirable for unpacked single pass scrubbers based on wet limestone scrubbing as they facilitate flue-gas treatment using relatively smaller amounts of slurry, i.e., a lower liquid-to-gas ratio (L / G). Limestone is the preferred form of calcium carbonate, but can be replaced if necessary with another form of calcium carbonate. Besides limestone, other forms of calcium carbonate are oyster shells, aragonite, calcite, chalk, marble, marl and travertine. It can be mined or produced industrially. In this specification, the terms calcium carbonate and limestone are used interchangeably. It is preferable to use finely ground limestone, preferably in. form particles with a weight average diameter less than 8 µm at the time of insertion. The sulfur oxides in the exhaust gas are absorbed into the aqueous phase of the slurry and react with the available calcium to form mainly calcium sulfite, which can then be oxidized to calcium sulfate. The reaction takes place to some extent in the falling droplets, but mainly takes place in the reaction vessel 5 collecting the slurry.

The washed exhaust gas then passes through a single-stage collecting separator 6 which simultaneously changes the direction of the exhaust gas flow by at least 45 ° to the vertical flow direction of the exhaust gas stream and reduces the number of droplets smaller than 100 µm in diameter by about 40% according to Sauter no reducing the pressure drop by more than 3.8 mm water column.

The diversion of the flue gas purified in the scrubbing process offers a number of benefits, including a reduction in the size of the slurry impact against the roof of the tower scrubber 1. It also allows the efficient removal of mist from the high velocity volatile flue gas by using a substantially 8 horizontal flow mist eliminator for defogging. vertical orientation. The high gas velocities in the vertical scrubbing section 3 have the great advantage of allowing better scattering of the scrubbing slurry droplets thus giving each droplet a longer contact time with the flue gas at a given tower height. Clean and fog-free exhaust fumes are then discharged via pipe 9.

The high flue gas flow rates desirable for a more efficient flue gas desulfurization method in the prior art pose problems with sedimentation on the roof of the scrubber and the conventional mist eliminator. Accumulation of deposits can occur whenever the gas entrained slurry builds up and is not rinsed or removed from the surface. The gas entrained slurry can quickly become saturated with calcium sulfate and further precipitate into a gypsum flaky coating that grows rapidly, leading to severe fouling. In the case of large overgrowth of the cross-section of the scrubber, the pressure drop in the scrubber may increase and this may lead to the pieces of tarnish being detached and falling into the spraying part of the scrubber.

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This leads to potential damage to the heads, nozzles or support parts. Once the fallen pieces enter the reaction vessel 5, they can be transported further by the recycle pump to the spray heads 10 and 10 'and cause the nozzles 11 to clog.

More efficient mist eliminators could be used as a substitute for trap 6, however, for the reasons previously described, mist eliminators have a very low capacity at vertical gas velocities of 4.5 to 6.0 meters per second.

In the known solutions, high gas velocities are not practiced because of the deposits that will accumulate on the roof inside the scrubber.

The method according to the invention makes it possible to use high exhaust gas flow velocities without negative effects, as is the case in the known art.

The gas scrubbing device for reducing the SO2 concentration in the exhaust gas according to the invention has a single-stage catching separator 6 located above and across the vertical scrubbing section 3. The design and operation of trap separator 6 are sufficiently effective to reduce the appropriate amount of mist droplets and to divert the exhaust gas flow into. position to efficiently use a high-power, horizontal flow mist eliminator. The catch separator 6 shown in Fig. 2 is at an angle γ with respect to the level of tower scrubber 1. This angle is preferably between 10 ° and 45 °, for example 20 °. A preferred form of separator 6 is shown in Fig. 2.

The catch separator 6 has one level of lamellas 12 to collect the droplets hitting them and divert the gas in the manner most appropriate for the subsequent trapping elimination, preferably in a manner that does not strike the roof 7 of the scrubber 1. The individual lamellae 12 are mounted in frames 13 to create teams 14 4-14 '. As shown, units 14 4-14 'are constructed from a plurality of lamellae 12, but the actual amount of lamellae in units 14h-14' will vary with the desired weight of the individual units 14 4-14 ', the weight being that so that one or two people dealing with the system can easily install or relocate these assemblies 14 4-14 'if necessary. Typically, a single unit 14-14 'is comprised of three to six lamellae. The assemblies 14 - 14 'are adjacent to each other to form a complete catch separator system. The lower boundaries of the frames 13 define the lower surface 15 of the units 14 - 14 '. The individual lamellae 12 are oriented with respect to the vertical at an angle δ. Typically, this type of plaque is rectangular, 15 cm to 23 cm wide and 6 szerokości to 150 cm long. The distances between the individual laminae usually range from 40% to 70% of the width of a single lamina. Preferably, angle δ is comprised between 35 ° and 55 °, the exact value depending on the direction of the flow of the exhaust gas.

WITH_esp_about14 _- 14 'are constructed and oriented to facilitate the best_withe dehydration. Single units / 14 - 14 'chevron-shaped are shown. Ideally, bands 14 4-14 'sy should be at an angle Θ, usually taking values ​​from 120 ° d_about^p50^about, I will use the enc^sesu from ^after5 ° to 145 °, preferably 140 °. The structure of the separator that catches the flood is through the lower sections 15 which guide the collectors 17 of each M-14 'isespofo. It is not possible to use my devices^y supporting striktorę. However the towers_owa lungsAa 1 shown in the picture has a doggy shape Dreams, it can take p_aboutbe _aboutcircled_ej ', _and with_esp_about14 _- M '' 'b^saszek ^and2 adjacent to the wall will respond^pand iikiształtow ^ e. Stoiktor WyCewyt. Using sepsratos 6 allows for the intermediate washing of M ^ ek 1^and with the help of Zan^rontosss^eThese are spray nozzles 18 on the manifolds 17, which can spray water directly onto the blouses 12 from the bottom and from the bottom. Washing 4lyaonuje is based on the oddztototo k ^ cda gfwtoąwąą_cą i ^sequiv_andfunction in relation to others^y, the eiemljiet are also possible washing strategto. It may be convenient to operate two scales at the same time. The washing water should be of good quality^p be s4ykor4ystywsea in the bearing capacity adequate to reduce the level of salt in the hand and veins, and are located on the surfaces of the separator. Typical water velocities during head operation are from 16 to 60 liters per minute per square meter of surface area. Top wash too^withwozz_andj _ABOUTdbo^rin this s susouu area, however, less often ei^about washing downstairs. It is preferable to wash the top and bottom in frequent cycles to prevent build-up.

182 489 gypsum raids made. The use of good quality water and frequent washing, with a good method of drainage, ensured by arranging the plaque systems in a chevron, practically ensures the operation of the device without the build-up of gypsum deposits. However, the washing sequences may vary depending on the requirements of the particular flue gas desulfurization process.

An advantage of the invention is that the efficiency of the separation performed by the catch separator 6 need not be as high as that of the multilayer separators used in prior art since the change of flow direction from vertical to horizontal allows a high efficiency horizontal flow mist eliminator 8 to be used. Thus, while one might think that the efficiency of the capture activity is lower than that desirable for scrubbers using wet flue gas scrubbing, the capture separator 6 provides very low pressure drops, i.e. about 3.8 mm water column, reduces or consolidates up to 40%. droplets having dimensions below 100 µm, and has other advantages in terms of purification, drainage, ease of maintenance, the reliability and direction of gas flow towards the horizontal flow high efficiency mist eliminator 8, preferably the mist eliminator 8 is of the barrier type, for example the zig-zag type baffle as illustrated in the article by Jones et al. An alternative embodiment is illustrated in Fig. 3, which shows a single set 19 of blades 20 arranged obliquely on a flat surface supported by a frame 21. The blades are oriented at an angle α of 15 ° to 45 °, preferably approximately 20 ° to the frame and at the same angle to the vertical as in the embodiment shown in Figure 2. Importantly, the blade sets shown in Figure 3 do not have chevron shape and are rather flat projections between the support members 16. Good drainage is facilitated by the gravity acting on the captured droplets along the drying paths formed by the angle a and the angle y.