Process for removal of sulfur oxides from flue gas with regenerable active carbon.

0001The invention relates to a process for removing sulfur dioxide and sulfur trioxide from flue gas containing oxygen and water vapor, which is passed over regenerable activated carbon in an adsorption zone, on which the sulfur oxides are separated out as sulfuric acid, and regeneration of the loaded activated carbon, which is mixed with reducing gas containing sulfur (H₂S and / or CS₂ and / or COS) is treated, whereby elemental sulfur is formed, from which the sulfur-containing reducing gas is formed with hydrogen or hydrocarbons and used for regeneration.

0002Similar processes are described in German Patent 19 66 711 and German Patent Application 19 45 090. The activated carbon loaded with sulfuric acid is treated with hydrogen sulfide so that the gaseous S0₂ arises, which is rinsed and fed for further processing.

0003The invention has for its object to achieve a high loading of the activated carbon in sulfuric acid in the adsorption zone with the shortest possible residence time of the flue gas. At the same time, the regeneration of the loaded activated carbon should be able to be carried out in a simple and inexpensive manner, with the aim being to obtain a product which is easy to use, in particular salable and storable. In the process mentioned at the outset, this is done according to the invention by passing the flue gas into the adsorption zone at a temperature of about 50 to 80 ° C and a relative water vapor saturation of 35 to 90% for the regeneration of the loaded activated carbon in a first reduction zone with circulated sulfur-containing reducing gas at temperatures of 120 to 180 ° C the H₂S0₄ completely converted into elemental sulfur, about 20 to 30% of the elemental sulfur loading with inert gas removed in a desorption zone at temperatures in the range from 300 to 500 ° C. and the sulfur removed is recovered, then in a second reduction zone, the remaining elemental sulfur loading of the activated carbon with hydrogen or hydrocarbons at temperatures in the range from 400 to 550 ° C to the sulfur-containing reducing gas and this reducing gas is fed to the first reduction zone, and that the activated carbon is cooled and reused in the adsorption zone .

0004This way of working brings significant improvements over the known methods. First of all, due to the rather low temperatures of the flue gas and its incomplete saturation with water vapor, sulfuric acid loads in the adsorption zone with dwell times of only a few seconds can reach up to about 50% by weight, based on the weight of the dry, unloaded activated carbon . The sulfuric acid loading of the activated carbon is usually in the range from 20 to 5 with residence times of about 1 to 3 seconds₀ % By weight. Due to this high effectiveness of the activated carbon, its quantity in the adsorption zone can be kept low, which leads to cost reductions both in the use of coal and on the plant side.

0005The high load also has the advantage that the activated carbon has to be promoted less often for regeneration, which reduces the mechanical stress. If the flue gas is passed through the activated carbon under the conditions of other known processes, namely at about 130 to 160 ° C, the S0 settles₂due to the low relative humidity to sulfuric acid much more slowly, and only sulfuric acid loads of about 1 to 3% by weight, based on the weight of the coal, are obtained. Under the conditions of the process according to the invention, the flue gas experiences a temperature increase of approximately 15 to 20 as a result of the heat of reaction^OC, so that reheating is generally not necessary before being introduced into the fireplace.

0006For regeneration, the loaded activated carbon is first fed to a first reduction zone, where H₂S0₄ is completely converted to elemental sulfur at temperatures of 120 to 180 ° C. It is deliberately avoided here, as well as S0₂ to generate, since this would require a further treatment step. The elemental sulfur with which the activated carbon is loaded at the exit of the first reduction zone can be desorbed with a hot inert gas and recovered by cooling, resulting in an easy-to-handle product. The sulfur can be stored without much effort, it is not aggressive, takes up only a relatively small volume and is usually a salable product.

0007In the second reduction zone, the sulfur-containing reducing gas required in the first reduction zone, for example hydrogen sulfide, is generated and the sulfur content of the activated carbon is removed completely. After passing through a cooling zone, the regenerated activated carbon can then be returned to the adsorption zone for cleaning flue gas.

0008A high-quality activated carbon can be used for the process, since the carbon is used in the chemical reactions that take place during _Rgenerate, is not consumed, but acts as a catalyst. It is therefore recommended to use an activated carbon with a BET surface area of ​​at least 1000 m²/ g, a micropore volume of 0.5 to 0.7 cm³/ g and a macro pore volume of 0.45 to 0.60 cm³/G. A particularly suitable activated carbon has a hydrophobicity ratio of 1.5 to 2.5. The hydrophobicity quotient is calculated from QB: QW, where Q_B the wetting heat for benzene and QW is the wetting heat for water with respect to the activated carbon used. The wetting heat is determined in the following way:<ul id="ul0001" list-style="none"><li>Pipette 100 cm³ Benzene or water in a Dewar jar containing a Beckmann thermometer, seals it with a rubber stopper and leaves it to stand for a few hours for temperature compensation. At the same time, a certain amount of powdered activated carbon is weighed in (3 g for benzene, 10 g for water), the previous one for 120⁰C was dried. The coal should also remain closed for some time until the temperature equilibrates in the test tube. Is the temperature of the benzene or water for 3 min. constant, one enters the coal quickly and measures the temperature rise with constant gentle shaking of the vessel at intervals of 30 seconds. The temperature rise ΔT on which the calculation is based is determined from the maximum of the temperature-time curve.</li></ul>

0009The wetting heat is calculated as follows:<chemistry id="chem0001" num="0001"><img file="EP0158748A2_D0001.tif" /></chemistry>Where:<ul id="ul0002" list-style="none"><li>q = wetting heat based on sample weight</li><li>m_L= the mass of the benzene or water used (in g) m_K= the weight of the coal (in g)</li><li>c_L= the specific heat of benzene or water (in _{J /}g) c_K= the specific heat of the coal (about 0.8 _{J /}G).</li></ul>

0010If you divide by the weight, you get the wetting heat Q in J / g.

0011Other characteristics of preferred activated carbons are as follows:<ul id="ul0003" list-style="none"><li>_Benzole loading at 20 ° C and a relative saturation of 90%: 44 to 52% by weight, and with a relative saturation of 10%: 38 to 44% by weight. The activated carbon carbon density can range from about 350 to 400 g / l. In principle, granular or shaped activated carbon is suitable, the grain size being selected accordingly, depending on whether one wishes to work with a fixed bed, fluidized bed or, for example, with a sliding bed (see drawing). Above all, this selection will also take into account the permissible pressure drop during flue gas desulfurization. A cylindrical molded carbon with a diameter of approximately 4 mm has proven itself for fixed bed and sliding bed adsorbers.</li></ul>

0012An embodiment of the procedure is shown in the drawing.

0013Hot flue gas, for example from a power plant, is introduced in line 1. The flue gas usually has temperatures in the range of about 130 to 160 ° C, it contains water vapor and more oxygen than stoichiometric to convert SO₂ with H₂0 in H₂SO₄ is necessary. While injecting water from line 2, the flue gas is cooled to temperatures in the range of approximately 50 to 80 in an injection cooler 3^OC, preferably 60 to 70 ° C. At the same time, the relative water vapor saturation of the flue gas is set to 35 to 90% and preferably 45 to 60%. This relatively cold and dry flue gas is fed to the adsorber 5 in the line 4. The adsorber contains a layer 6 of granular activated carbon slowly moving down between two gas-permeable, louver-like walls 5a and 5b. Fresh activated carbon comes from inlet 8. The residence time of the gas in the activated carbon layer 6, which is flowed across, is about 1 to 3 seconds. In deviation from the drawing, the adsorber 5 can also be designed, for example, as a fluidized bed or a horizontal moving bed.

0014S0 becomes on the surface of the activated carbon₂ together with the oxygen and the water vapor present in the flue gas under the catalytic effect of the coal to sulfuric acid, which is adsorbed on the coal. Also S0₃ is in the form of H₂S0₄ bound on the coal. Desulfurized flue gas with an S0₂Content of at most about 100 to 150 mg / m³ leaves the adsorber 5 through the chimney 9.

0015Loaded activated carbon is fed from the outlet 7 via a cellular wheel sluice 10 on the transport path 7 to a regeneration tower 11. The activated carbon is about 35 to 40% by weight H₂S0₄ (based on the weight of the unloaded coal). If you leave that_Ak- Active carbon run faster through layer 6 and is satisfied with an H₂SO₄Loading of only about 20% by weight, the flue gas in the chimney 9 is completely free of SO₂. In tower 11, the activated carbon passes through various treatment zones essentially in the form of a bed 11a. First, the complete conversion of the H takes place in the first reduction zone 12₂S0₄- Loading the activated carbon with sulfur _Rinduction gas, eg hydrogen sulfide, to elemental sulfur, for example according to the reaction equation<chemistry id="chem0002" num="0002"><img file="EP0158748A2_D0002.tif" /></chemistry>

0016For this purpose one leads H₂S with temperatures of approximately 400 to 550 ° C. in line 15 into a ring line 16, to which circulating gas is also supplied from line 17. Through gas distribution nozzles 18, this is essentially derived from H₂S existing _Gas upwards through the activated carbon bed, temperatures in the range from 120 to 180 ° C. being maintained in the first reduction zone. The gas drawn off in line 19 is first fed to cooler 20, which condenses the water vapor carried along and discharges the water in line 21. The circulation fan 22 returns the cooled gas in line 17, which is then in the ring line 16 with hot H₂S is mixed from line 15. At the lower end of the first reduction zone 12, the activated carbon is only loaded with elemental sulfur.

0017About 20 to 30% of the elemental sulfur loading of the activated carbon is in the desorption zone 24 at temperatures in the range from 300 to 500⁰C removed with inert gas. This inert gas, which can be nitrogen, for example, is introduced in line 25, in gas heater 26 to 400 to 450⁰C heated and pressed into the desorption zone 24 via a ring line 27 with distributor nozzles 28. The sulfur-containing inert gas emerges again via discharge shafts 29, which open into a collecting line 30, and reaches a cooler 31. The sulfur is condensed there and drawn off in line 32. The inert gas is returned via the circulation blower 33.

0018Of the H in the first reduction zone 12₂S0₄ Molecule according to the equation<chemistry id="chem0003" num="0003"><img file="EP0158748A2_D0003.tif" /></chemistry>An S atom, ie approximately a quarter of the sulfur load, is removed in the desorption zone 24 from the resulting four sulfur atoms. The remaining sulfur loading of the activated carbon is then completely desorbed in the second reduction zone 35 and into the sulfur-containing reducing gas, for example in H_2S, transformed. For this purpose, hot hydrogen is passed through line 36 into the ring line 37 and through nozzles 38 through the sulfur-laden activated carbon. Temperatures in the range from 400 to 550 ° C. are maintained in the second reduction zone 35, with S + H₂ Hydrogen sulfide is formed If one leads to the generation of the sulfur-containing reducing gas hydrocarbons (eg CH₄) through line 36 into the desorption zone 36, so they settle with the sulfur and possibly water vapor according to the equations<chemistry id="chem0004" num="0004"><img file="EP0158748A2_D0004.tif" /></chemistry>or<chemistry id="chem0005" num="0005"><img file="EP0158748A2_D0005.tif" /></chemistry>around. C._S2 and COS are also able to use H₂S0₄ to form elemental sulfur in the first reduction zone, according to the formula<chemistry id="chem0006" num="0006"><img file="EP0158748A2_D0006.tif" /></chemistry>or<chemistry id="chem0007" num="0007"><img file="EP0158748A2_D0007.tif" /></chemistry>

0019The resulting C0₂ is easy to separate and does not cause environmental pollution. The sulfur-containing reducing gas formed in the second reduction zone 35, for example H₂S_' is led to the first reduction zone 12 via the exhaust ducts 39, the collecting line 40 and the line 15.

0020At the lower end of the second reduction zone 35, the activated carbon is freed of its load again and is only guided through the cooling zone 41. The cooling zone has the cooling gas lines 42 and 43, the cooler 44, the circulation blower 45, the ring line 46 with distributor nozzles 47 and the gas extraction shafts 48. For example, nitrogen or also the hydrogen subsequently introduced into the second reduction zone 35 can be used as the cooling gas. At temperatures of approximately 40 to 70 ° C., the cooled activated carbon leaves the regeneration tower 11 via the cellular wheel sluice 50 and is returned to the inlet 8 via the transport path 51.

Example:

0021The example, the data of which is partially calculated, is based on the desulfurization of a flue gas quantity of 1.9. 1₀⁶_Nm³how it generates a coal-fired large-scale power plant with 600 MW per hour. The flue gas contains 0.15 vol.% S0₂₁ which corresponds to 8350 kg / h sulfur. The further explanation of the example is carried out with the aid of the procedure shown in the drawing.

0022Through the inlet 8, the adsorption zone 5 is fed 40,000 kg of activated carbon per hour. The same amount of activated carbon is used over the_Z.ellenradschleuse 10 passed into the regeneration tower 11 together with the load of 11 500 kg / h H₂S0₄ and 11,500 kg / h of water. Through line 15 8 100 Nm per hour³ H₂S introduced, the temperature in the first reduction zone is about 150^OC, and the recycle gas in line 19 has a temperature of 110 ° C. In the cooler 20, which the gas leaves at a temperature of approximately 80 ° C., 20 500 kg of water are discharged through the line 21 per hour. In the desorption zone 24, a quarter of the desorption takes place_SSulfur loading of the activated carbon, for example, at a temperature of 450 ° C. with nitrogen as the carrier gas, the recycle gas in line 30 has a temperature of 350 ° C. and becomes 130 in cooler 31⁰C cooled. This results in 3 850 kg of sulfur per hour in line 32. After heating in the heat exchanger 26, the gas passes through the ring line 27 at a temperature of 550 ° C. back into the desorption zone 24.

0023In the second reduction zone 35 8 100 per hour _Nm³ H₂ used, which are introduced in line 36. This creates the same amount of H₂S that with a temperature of 500⁰C reaches the first reduction zone 12 in line 15.

0024For cooling in the cooling zone 41, hydrogen is used, which is partly circulated and of which a partial flow in an hourly amount of 8 100 Nm³ branched off from line 42 and fed to hydrogen line 36. The hydrogen in line 42 has a temperature of 400 ° C and that in line 43 has a temperature of about 50 ° C. 40,000 kg of activated carbon are returned per hour on the transport path 51 for reuse in the adsorption zone 5.

0025The activated carbon used, a cylindrical shaped carbon with a diameter of 4 mm, has the following specification:<tables id="tabl0001" num="0001"><img file="EP0158748A2_D0008.tif" /></tables>

0026Benzene loading at 20 ° C and a relative saturation of 90%: 48% by weight; with relative saturation of 10%: 42% by weight.