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Discussion on the sulfur evolution problem of Baosteel's Takahaks process gas desulfurization liquid Peng Youshan (Shanghai Baosteel Chemical Co., Ltd., Shanghai 200942) 1 Production status Baosteel Chemical Company's first phase of coke oven gas desulfurization and decyanization adopts the Takahaks process. The maximum gas treatment capacity is 105,000 m3/h, and the desulfurization efficiency is very high. The desulfurization waste liquid is sent to the Shiloh Hakes device for processing and used as raw material for the ammonium sulfate process. Since the coke oven uses high-sulfur coal for coking and the desulfurization device operates under high load, sulfur evolution often occurs in the desulfurization liquid. In recent years, sulfur evolution has occurred frequently. A small amount of sulfur is released from the desulfurization liquid almost every month, and a large amount of sulfur is released at least once a year. In June and November 2002, due to the large amount of sulfur released from the desulfurization liquid, the desulfurization unit was shut down, and hundreds of tons of sulfur released were cleaned up. In November 2003, several hundred tons of sulfur was released during the annual maintenance of the desulfurization unit. In 2005, the desulfurization solution still had multiple sulfur releases. It can be seen that the phenomenon of sulfur precipitation from the Takahakes desulfurization liquid has had a significant impact on production, not only affecting the quality of gas, but also affecting the safe operation of production. 2 Principle of desulfurization and decyanization by Takahaks method The desulfurization liquid is an ammonia solution containing a small amount of sodium 1,4-naphthoquinone-2-sulfonate (referred to as 1,4NQ) and organic phenol. After absorbing the hydrogen sulfide, hydrogen cyanide and ammonia in the coal gas in the desulfurization tower, it enters the oxidation tower, reacts with oxygen in the air to generate sulfide, and regenerates the catalyst. This is a continuous cycle to achieve the purpose of gas desulfurization and decyanation. The reaction equation is as follows: NH3+H2O → NH4OH (1) NH3+H2O → NH3·H2O (2) NH4OH+H2S → NH4HS+H2O (3) NH3·H2O +H2S →(NH4)2S+H2O (4) NH4OH+HCN → NH4CN+H2O (5) (NH4)2S+S(x-1)→ (NH4)2Sx (8) Organophenol + O2 → Organoquinone (9) (NH4)2Sx+NH4OH+O2 → (NH4)2S(x-1)+(NH4)2SO3 (10) (NH4)2SO3+(NH4)2Sx → (NH4)2S2O3+(NH4)2S(x-1) (11) (NH4)2SO3+O2 →(NH4)2SO4 (12) (NH4)2S(x-1)+S → (NH4)2Sx (13) NH4HS+1/2O2 → NH4OH+S (14) NH4HS+O2 → (NH4)2S2O3+H2O (15) S+O2+NH4OH →(NH4)2SO4+H2O (16) NH4CN+S → NH4SCN (17) In the oxidation tower, the regenerated desulfurization liquid returns to the top of the desulfurization tower for circulation spraying. Due to the reaction with H2S and HCN, the salt concentration in the desulfurization liquid gradually increases, which directly affects the desulfurization efficiency. For this reason, part of the circulating desulfurization liquid is sent to the Shilohax unit as waste liquid for processing into ammonium sulfate mother liquor. 3 Generation and transformation of sulfur From the above principle of desulfurization and decyanation, it can be known that after the Takahaks method desulfurization liquid absorbs H2S in the coal gas, under the action of catalyst and air, it is converted into elemental S and sulfides such as SCN-, S2O3-2, SO4-2, HS-, (NH4)2Sx. The way sulfur is produced is: (1) NH4HS+1/2O2 → NH4OH+S↓ (2) The conversion pathways of sulfur are: S+3/2O2+2NH4OH → (NH4)2SO4+H2O (3) NH4CN+S → NH4SCN (4) (NH4)2S(x-1)+S → (NH4)2Sx (5) At the same time, through 2NH4HS+2O2 →The (NH4)2S2O3+H2O reaction pathway can directly convert HS- into (NH4)2S2O3 under the catalysis of organic quinone, avoiding the formation of elemental sulfur. Based on the production data in September 2005 and the sulfur balance of each component in the system, the calculations are shown in Table 1. Table 1 Composition of coke oven gas and desulfurization liquid in September 2005. Composition of coke oven gas desulfurization liquid. Gas volume per 10,000 m³/h. Inlet H2S content, g/m³ outlet H2S content, g/m³ inlet HCN content, g/m³ outlet HCN content. g/mSCN¯g/LS2O3-2g/LSO4-2g/LSg/LHS-g/L9.84.960.121.070.0739.5569.0813.740.120.03 (1) The total amount of sulfur entering the desulfurization system. The total amount of H2S in coal gas is: 98000×4.96／1000 = 486.08 kg/h which contains total sulfur content: 486.08×32/34 = 457.49 kg/h (2) Sulfur taken away by the gas leaving the desulfurization tower. First, the sulfur taken away by the gas at the outlet of the desulfurization tower: 98000×0.12×32/34 = 11.07 kg/h The second is the sulfur taken away by the externally discharged desulfurization waste liquid. The discharge volume of the desulfurization waste liquid is determined based on the concentration of SCN-. Usually the SCN-concentration 45g/L is used as the standard. Theoretical liquid discharge volume = SCN - generated volume/extracted concentration = [98000×1.07 ×93.5% ×58/(27×1000)]/45 = 4.6 m3/h In actual production, the discharge volume of waste liquid is generally greater than the theoretical discharge volume, usually around 6.0 m3/h. According to the data in Table 1, the amount of sulfur taken away by the desulfurization liquid is calculated as: The amount of sulfur taken away in the form of SCN- is: 39.55×32/58×6 = 130.9 kg/h The amount of sulfur taken away in the form of S2O3-2 is: 69.08×64/112×6 = 236.85 kg/h The amount of sulfur taken away in the form of SO4-2 is: 13.74×32/96×6 = 27.48 kg/h The amount of sulfur taken away in the form of elemental sulfur is: 0.12×6 = 0.72 kg/h The amount of sulfur taken away in the form of HS- is: 0.03×32/33×6 = 0.17 kg/h The total amount of sulfur taken out of the system is: 11.07+130.9+236.85 +27.48+0.72+0.17 = 407.19 kg/h The remaining sulfur is discharged from the system in the form of SO3-2, (NH4)2Sx and other forms. According to the above calculation, the order of bringing out the sulfur content of the system is:: S2O3－2＞SCN－＞SO4－2＞COG＞S＞HS－. Among them, S2O3-2 accounts for 58.17%, SCN- accounts for 32.15%, SO4-2 accounts for 6.7%, elemental sulfur accounts for 0.17%, HS- accounts for 0.042%, and the proportion of H2S taken away by gas is 2.77%. From the above sulfur balance calculation, it can be seen that most of the sulfur entering the system is brought out of the system in the form of sulfide, and the amount of elemental sulfur only accounts for a small part. To control the concentration of sulfur, it is necessary to control the entry, transformation and discharge of sulfur. 4 Factors affecting sulfur evolution In summary, the factors affecting sulfur evolution in the Takahakes desulfurization liquid mainly come from two aspects. One is the composition of the coke oven gas, and the other is the control of the desulfurization liquid (that is, the redox status of the desulfurization liquid). 4.1 Gas components In order to improve the efficiency of gas decyanization, H2S and HCN in the gas must reach a certain ratio. Under normal circumstances, the H2S/HCN molar ratio is preferably greater than 2. This is because: HCN+NH4OH → NH4CN+H2O NH4CN+S → NH4SCN NH4CN+(NH4)2Sx → NH4SCN+(NH4)2 S (x-1) When the hydrogen sulfide content in the gas is high, the large H2S/HCN molar ratio is beneficial to the removal of HCN and improves the decyanization efficiency. ； However, when the H2S content is high and the H2S/HCN molar ratio is large, even if the CN- ions generated after HCN is absorbed completely react with sulfur to form SCN-, a large amount of elemental sulfur will remain in the desulfurization liquid, which will lead to the presence of a large amount of suspended sulfur in the desulfurization liquid. In severe cases, sulfur will be released from the desulfurization liquid. In recent years, due to changes in coke oven coal blending, the composition of gas has also undergone great changes. According to laboratory analysis data, the gas composition and H2S/HCN ratio from 1998 to 2005 are shown in Table 2. Table 2 Gas composition and H2S/HCN molar ratio H2S content in years, g/m3 HCN content, g /m3H2S/HCN than 19984.2141.0544.0019994.1411.0833.8220004.6611.1274.1420015.1051.0 744.7520025.0831.1124.5720034.4631.054.2520044.8640.9535.1020055.0311.03154.88 It can be seen from Table 2 that from 1998 to 2005, the average content of H2S in coal gas and the molar ratio of H2S/HCN generally showed an upward trend. At the same time, the phenomenon of sulfur evolution in the Takahakes desulfurization liquid gradually became more frequent in actual production. In 2004 alone, 19 sulfur evolutions occurred, of which the H2S/HCN ratio was too large (greater than 5) in 12 of them, accounting for 63.16% of the occurrences of sulfur evolution. ； In 2005, 18 sulfur evolutions occurred, of which the H2S/HCN ratio was greater than 5 in 77.8% of the cases. Therefore, it can be determined that when the H2S content in coal gas is too high and the H2S/HCN ratio is too large, it will have a great impact on the normal production of Takahakes desulfurization. This is an important factor leading to the Takahakes sulfur precipitation accident. However, the composition of the gas is beyond our control and depends on the coal blending and production operations of the coke oven. What we can do is to promptly adjust the status of the desulfurization liquid according to the gas composition and gas treatment capacity. 4.2 Condition of desulfurization liquid The condition of desulfurization liquid refers to its oxidation-reduction degree and its ability to convert H2S and elemental sulfur into sulfides. The redox performance of the desulfurization liquid can be reflected by the ORP value. The ORP value is a comprehensive index, which has a great relationship with the components of the solution, but mainly depends on the properties and content of HS-, S2O3-2, SCN-, catalyst and the dissolved oxygen content in the solution. HS-, S2O3-2, SCN- plasma are all reducing, while catalysts, dissolved oxygen, etc. are all oxidizing. Therefore, the greater the ORP value, the better the oxidation performance of the solution and the better the desulfurization and decyanization effect. In order to improve the desulfurization and decyanization efficiency, it is necessary to increase the ORP value, that is, to reduce the concentration of HS-, S2O3-2, and SCN- plasma in the desulfurization liquid. However, if the ORP value is too large or too small (that is, the solution is over-oxidized or under-oxidized), it may lead to sulfur evolution accidents in the desulfurization liquid. Therefore, to control the condition of the desulfurization liquid, the key is to control the ORP value of the solution within an appropriate range based on the gas volume, gas composition, and HS-, S2O3-2, SCN-plasma concentration in the solution, so that the solution condition is optimal. Table 3 Operating parameters of the Takahakes method when sulfur is released from the desulfurization liquid Time H2S content in the tower gas g/m3 HCN content in the tower gas g/m3 S content in the tower desulfurization liquid g/L Entering tower desulfurization liquid HS-content g/L SCN-content in the raw material tank g/L Outlet gas flow m3/hORP value mV2006 02135.04-0.530.0329.5791565.91-8420060214--0.430.0530.4792914.24-99200602154.430.760.410.04-94575.78-17 620060216--0.390.06-97055.68-72200602174.78-0.310.05-92640.93-85200602204.54-0.340.0430.689453.47-24620 060221--0.350.0631.1390009.96-21120060228--0.280.04-91752.91-105200603295.41.190.610.0433.594120.16-180 It can be seen from Table 3 that in several sulfur evolution accidents that occurred from February to March 2006, the gas treatment capacity was not very large, and the H2S content in the gas was not high. The other parameters were similar. The only big change was the ORP value of the solution. It can also be seen from Table 3 that the ORP value of the solution was either very large or very small. This means that when these sulfur evolutions occur, the desulfurization liquid is either under-oxidized or over-oxidized. Production practice shows that if the ORP value of the desulfurization liquid can be controlled between -90 and -120mV, the Takahakes desulfurization liquid will not be prone to sulfur precipitation accidents. In other words, when the gas treatment capacity is large and the H2S content is high, as long as the ORP value of the desulfurization liquid is controlled to be appropriately large, there will be no sulfur precipitation accident in the desulfurization liquid. On the contrary, when the gas processing capacity becomes smaller and the H2S content is low, as long as the ORP value of the desulfurization liquid is controlled to a smaller value, sulfur precipitation accidents will not occur. This can also be proven from the actual production data in 2004 listed in Table 4. Table 4 Actual production data of Takahakes process gas desulfurization in 2004 Monthly composition of coke oven gas desulfurization liquid Gas volume 10,000 m³/h Average H2S content entering the tower g/m³ Desulfurization efficiency % Average HCN content entering the tower g/m³ Desulfurization efficiency % H2 S/HCN ratio SCN¯g/LS2O3-2g/LSO4-2g/LS0mg/LORP value mv18.74.6799.30.9195.45.1330.7574.89.781 87-14128.54.3999.60.9195.54.8233.1161.513.358-13239.24.5699.30.91696.54.9833.445 6.7811.3113-13048.84.9699.50.9596.45.2224.350.99.7119-15059.15.499.11.06595.95.0 730.562.911.37-10868.25.6799.20.9596.65.9727.162.36152-11278.705.1998.10.9494.55 .5239.666.1512.7142-10089.25.1397.40.9893.55.2339.278.614.2146-11098.44.9697.90. 9695.15.1739.785.712.8153-98108.14.6499.10.9594.94.8838.0285.210.527-101119.84.3 298.90.9394.74.6538.1787.59.1832-961210.14.4898.20.9894.74.5738.677.1512.479-105 It can be seen from Table 4 that when the gas volume is large and the H2S/HCN molar ratio is roughly the same, the higher the ORP value, the higher the S2O3-2 in the solution, the sulfur concentration is low, and the concentration of SO4-2 does not change much. From March to April 2004, the ORP value was controlled at -140mV, the sulfur concentration was 113-119mg/L, and S2O3-2 was about 56g/L. However, from November to December, the ORP value was around -102mV, the sulfur concentration dropped to about 50mg/L, and S2O3-2 rose to about 83 g/L. The amount of conversion from sulfur to S2O3-2 increased significantly. There are three main means to adjust the ORP value, namely increasing the concentration of 1.4NQ in the solution, increasing the amount of regeneration air and adding remaining ammonia to the solution. In the current production operation, the air volume remains unchanged, so the ORP value of the solution can only be adjusted by adjusting the amount of catalyst added and the remaining ammonia supplement amount. (1) Catalyst addition amount. Catalyst 1,4NQ has good oxidation properties and can oxidize HS- and elemental sulfur into sulfides. It is easily regenerated by air oxidation and has good catalytic oxidation properties. However, the addition amount must be controlled according to the HS- content and ORP value in the desulfurization liquid. Actual production practice has also proven that, with other conditions being the same, as the catalyst concentration increases, the desulfurization effect can significantly increase, but after reaching a certain value, the effect increases slowly. When the hydrogen sulfide and hydrogen cyanide content in the gas behind the tower is low, the catalyst concentration increases, and the amount of suspended sulfur generated will * * increase, so on the basis of meeting the indicators of hydrogen sulfide and hydrogen cyanide after the tower, it is better to have a lower catalyst concentration. In order to study the impact of the addition amount of 1,4NQ on the desulfurization liquid, a simple experiment is conducted below. Take 500mL of desulfurization liquid, ventilate 50L of gas, add 1,4NQ to the solution, and ventilate 33L of air again. Observe the condition of the solution and measure the ORP value of the solution under various conditions. The ORP value of the desulfurization liquid in the blank experiment was -126mV. The ORP value of the desulfurization liquid after 50L of gas flow is -410 mV. It can be seen from the experiment that when other conditions are the same, although both solutions have sulfur precipitated after aeration, the solution with a large amount of 1,4NQ begins to become turbid 1 to 2 minutes after aeration, and the amount of precipitated sulfur is also large, resulting in sulfur foam. (2) Air volume at the inlet of the oxidation tower. Air plays two roles in the oxidation tower. One is to oxidize sodium 1,4-naphthohydroquinone-2-sulfonate into sodium 1,4-naphthoquinone-2-sulfonate; the other is to oxidize ammonium thiocyanide and ammonium sulfide in the desulfurization liquid into thiosulfate an and sulfate an. Since the remaining ammonia water is now added to the desulfurization tower, another function of the air is to oxidize the organic phenol in the remaining ammonia water into organic quinones, and then sulfur and HS- are converted into SO4-2 and S2O3-2 under the action of organic quinones and oxygen. However, the amount of air entering the oxidation tower must be strictly controlled. If the amount of air is too small, the oxidation performance of the desulfurization liquid will be insufficient, affecting the desulfurization efficiency, and the sulfur precipitated in the solution cannot be converted into sulfide in time. ； If the air volume is large, S2O3-2 will tend to be generated. However, the air volume cannot be too high, otherwise it will cause excessive oxidation of the solution, and a large amount of HS- in the solution will be oxidized into elemental sulfur, causing a large amount of sulfur precipitation in the desulfurization liquid. It can be seen that both insufficient and excessive air volume are extremely detrimental to the Takahakes desulfurization operation. The amount of air entering the oxidation tower must be controlled based on the HS- content and ORP value in the desulfurization liquid. Now, the air volume entering the oxidation tower is generally controlled at around 5800m3/h. It can also be seen from the above experiments that as the amount of air increases, the amount of sulfur precipitated also increases. This shows that when the catalyst concentration is constant and the amount of air reaches a certain value, the rate at which the desulfurization liquid generates elemental sulfur is much greater than the conversion rate of sulfur, which causes a large amount of sulfur to appear in the desulfurization liquid. (3) The amount of remaining ammonia added. In order to determine the sulfide formation rate (oxidation selectivity) when using ammonia water as a catalyst, add a certain amount of catalyst NQ or ammonia water to the bubbling tank in Figure 1, use ammonium hydroxide solution to control the pH value, and then use the method of increasing or decreasing ammonium hydrosulfide to control the ORP value of the solution. Determine the sulfide formation ratio at different ORP values. The results are shown in Figure 2. Figure 1 Schematic diagram of the bubbling tank Figure 2 Sulfide production ratio when adding ammonia water and catalyst respectively. Compared with NQ, the production rate of elemental sulfur when using ammonia water is reduced to 1/3 to 1/4 of NQ, while the production rates of S2O3-2 and SO4-2 increase. In other words, using ammonia as a catalyst can convert HS- and elemental sulfur into sulfide, inhibiting the generation of sulfur. Thereby reducing or avoiding sulfur evolution in the desulfurization liquid. Although the remaining ammonia can inhibit the production of sulfur, the more you add, the better, because the remaining ammonia contains catalysts with similar properties to NQ, such as polyvalent phenol derivatives. If too much remaining ammonia is added, the desulfurization liquid will be peroxidized under the action of air and these excess catalysts, making the sulfur generation rate much greater than the sulfur conversion rate, resulting in sulfur evolution in the desulfurization liquid. Table 5 Effect of ammonia addition amount on sulfur evolution in desulfurization liquid Time of sulfur evolution Number of ammonia additions 2004 195% (about 20m3) 2005 185% (about 20m3) 2006 January to February 35% (about 20m3) 2006 March to July 13% (about 15m3) It can be seen from Table 5 that the number of sulfur evolutions in 2006 was much lower than that of the same period in the previous two years. The reason for this is that in addition to the different gas composition and ORP value control in the coal gas, another important factor is the amount of ammonia added. After the Takahakes desulfurization unit adjusted the ammonia feed amount in March, the number of sulfur releases has been significantly reduced. Therefore, an appropriate amount of remaining ammonia must be added according to the HS- content and ORP value in the solution. 4.3 Sulfur discharge at the bottom of the tower In addition to converting HS- and sulfur into sulfides through catalytic oxidation, elemental sulfur can also be separated from the absorption liquid through the sulfur discharge at the bottom of the absorption tower. This can reduce the sulfur content in the absorption liquid and reduce the sulfur conversion pressure. Production practice shows that this is very effective. It can not only reduce the sulfur content in the solution, but also reduce the sulfur accumulation at the bottom of the tower, thereby extending the shutdown and maintenance cycle of the desulfurization tower. 5 Conclusion (1) High gas volume and high H2S content in coal gas will have a great impact on the Takahakes desulfurization operation, which is one of the reasons for the sulfur release from the desulfurization liquid. A better way is to promptly adjust the desulfurization liquid conditions as the gas volume and gas composition change. (2) Controlling the ORP value of the solution is an important means to adjust the condition of the desulfurization liquid. The main means are to control the amount of air entering the oxidation tower, the amount of catalyst added, and the amount of remaining ammonia added. When the gas treatment capacity is large and the H2S content in the gas is high, the ORP value needs to be controlled larger, and vice versa. Generally, when it is controlled at -90~-120mV, the probability of sulfur evolution in the desulfurization liquid is smaller. (3) Insufficient regeneration air volume will lead to a decrease in desulfurization and decyanization efficiency. Large air volume will tend to generate S2O3-2, but it must be controlled appropriately, otherwise it will cause sulfur precipitation accidents in the desulfurization liquid. The regeneration air volume is generally maintained at around 5800m3/h. (4) The remaining ammonia water contains substances with catalytic properties similar to NQ. The addition of ammonia water can inhibit the production of elemental sulfur and convert more HS- and sulfur into sulfides such as S2O3-2. However, the amount of ammonia added should also be controlled according to the ORP value of the solution. Adding too much ammonia will also cause sulfur to evolve from the solution. In daily production, the amount of ammonia added should not exceed 3%. (5) Discharging sulfur through the bottom of the tower can reduce the elemental sulfur content in the solution, reduce the load of sulfur conversion, reduce the occurrence of sulfur evolution accidents, and also extend the shutdown and maintenance cycle of the desulfurization tower.