Which is better: the DDS desulfurization method, the 888 method, or the tannin desulfurization method?

Each method has its advantages; in terms of actual operational performance, normal-pressure desulfurization using 888 is better than DDS, while pressurized desulfurization using DDS is superior to 888. Based on the actual operating results in our plant, both DDS and 888 perform better than tannin-based desulfurization. The desulfurization process at normal pressure uses the DDS-01 catalyst, while that at increased pressure uses the DDS catalyst; in terms of performance during desulfurization under increased pressure, 888 is not as effective as the method used at normal pressure!

Personally, I believe that using tannin extract together with 888 yields better results. In reality, the effectiveness of wet flue gas desulfurization largely depends on the level of management. Among many enterprises with essentially the same production and desulfurization processes, there are significant differences in the actual application results. For example, poor catalyst preparation and improper sulfur recovery operations have a significant impact. This post was last edited by wycasia on 2008-7-15 10:33]

Reply: The three desulfurization methods each have their advantages and disadvantages: 1. Tannin-based desulfurization has lower costs, but the varying quality of tannins affects the desulfurization efficiency. Additionally, it requires a large amount of raw materials, results in high pressure differences in the tower, leading to low desulfurization efficiency – especially when dealing with gases containing high levels of sulfur. It also causes corrosion to the equipment and results in a low sulfur recovery rate. 2. The 888 desulfurization catalyst meets the process requirements and improves the desulfurization capacity by about 20%. 3. The DDS desulfurization technology belongs to the category of biochemical desulfurization methods; it features high desulfurization efficiency, simple operation, low overall energy consumption, and no environmental pollution. However, the sulfur particles formed during desulfurization regeneration are very fine, which imposes special requirements on regeneration and sulfur recovery processes. Using a combined desulfurizing agent such as tannin + 888 for desulfurization is a good option, and many enterprises have achieved good results from its use.

All three processes are wet flue gas desulfurization processes. Each has its own advantages and disadvantages; it’s impossible to say which is better than the other. 1. The tannin desulfurization process is mature and stable; the regeneration, flotation, and separation of elemental sulfur are straightforward, making it widely used in China. However, in production, there are issues such as low desulfurization efficiency for high-sulfur gas, easy blockage of towers, equipment corrosion, as well as higher operating costs. The same problems exist for its use in transformed gas desulfurization due to the high carbon dioxide partial pressure. To address the aforementioned issues, some manufacturers are taking further measures such as adjusting the ratio of glue to vanadium, increasing the gas-to-liquid ratio in the absorption solution, and controlling the temperatures during absorption and regeneration. They also add desulfurization aids such as 888 to improve desulfurization efficiency and reduce the pressure difference in the absorption tower. 2. 888 desulfurization was first used for desulfurizing coke oven gas. Since it can remove some organic sulfur, its overall desulfurization efficiency is relatively high; moreover, the preparation and management of the solution are quite simple. However, the 888 desulfurization catalyst exhibits strong oxidation and elemental sulfur stripping capabilities, which requires a relatively high absorption liquid-to-gas ratio. The regeneration, flotation, and separation of elemental sulfur are somewhat inferior to those in the tannin extract desulfurization process. 3. DDS desulfurization belongs to the category of biochemical desulfurization technologies; therefore, it features high desulfurization efficiency, stable operation, and low environmental pollution. However, microbial control of the solution presents certain difficulties; the regeneration, flotation, and separation of elemental sulfur are challenging, requiring some modifications to the equipment. However, it performs better than the above two processes in desulfurizing shifted gas.

Please refer to the fact that over the past 30-plus years, China has developed more than 30 types of desulfurization catalysts and methods based on the wet oxidation process. Some of these were not adopted by users because of high desulfurization costs, severe sulfur blockage in the towers, or unresolved issues related to safety and environmental protection; as a result, they were eliminated from use. The most widely used modification methods since the 1970s, namely the ADA method and the tannin method, still fail to resolve the issue of sulfur blockage. As a result, users prefer cobalt phthalocyanine-based desulfurization catalysts; in particular, the 888 desulfurization catalyst (abbreviated as “888”) does not suffer from sulfur blockage or safety/environmental issues, and it also has the function of cleaning sulfur from towers. For these reasons, it has received praise from experts and favor among users, leading to an increasing range of applications and market share. “The most common application of “888” is in the semi-water gas desulfurization systems of nitrogen fertilizer plants, as it is suitable whether using soda ash solution or ammonia water as the absorbent. It can be used in any sulfur recovery system, whether there is a continuous sulfur melting system or not. Impurities present in the gas, such as coal tar, as well as by-products in the solution, such as sodium thiosulfate, do not impair the activity of “888”. It can also remove organic sulfides and hydrogen cyanide from most gases ； Low dosage, low cost ； Easy to operate and manage ； The sulfur particles obtained through flotation are large and easy to separate ； The regenerated lean solution has a low sulfur suspension content, low resistance, and low energy consumption. According to statistics, about 80% of the total sales of “888” are used in semi-water gas desulfurization systems, while the remainder is used for desulfurizing coke oven gas, natural gas, waste gases from pharmaceutical factories, and shift gas from nitrogen fertilizer plants. The various products produced by nitrogen fertilizer plants, such as ammonium bicarbonate, urea**, synthetic ammonia, methanol, etc., have different requirements regarding the purity of the desulfurized shift gas as well as other process parameters during production; moreover, the selection of desulfurization catalysts is quite stringent. The desulfurization of shift gas is usually carried out under pressure using soda ash solution as the absorbent; in some cases, ammonia water is used as the absorbent at atmospheric pressure. The desulfurization methods and catalysts used for this purpose are not very suitable for shift gas desulfurization systems. “888” is suitable for desulfurizing transformed gas in any situation. Factories that use \"888\" for desulfurization of transformed gas include Jilin Tonghua Fertilizer Factory, Shandong Pingyuan Fertilizer Factory, Fujian Longyan Ammonia Synthesis Factory, Jiangsu Wuxian Fertilizer Factory, Henan Hui County Fertilizer Factory, and Zhengzhou Crystal Chemical Company (formerly Zhengzhou Fertilizer Factory), among others. Some of these factories produce ammonium bicarbonate, while others manufacture ammonia synthesis products, methanol, and urea; in all cases, the use of \"888\" as a desulfurization catalyst yields good results. In recent years, Peking University has developed a desulfurization method and catalyst called DDS, which it believes is suitable for desulfurizing shift gas and could be promoted for wider use. But we believe it has not yet reached a stage where it can be promoted. Judging from the two technical papers on DDS published this year, there are still many issues that need to be resolved. An article titled “Summary of Production Experiments on the Iron-Alkali Solution Catalytic Method for Gas Decarburization, Desulfurization, and Decyanation (abbreviated as DDS decarburization, desulfurization, and decyanation technology)” was published in Issue 2, 2000 of Nitrogen Fertilizer Technology Improvement. Another article titled “Technical Principles, Theoretical Mathematical Models, and Industrial Applications of DDS for Removing Acidic Gases (Sulfides, Cyanides, CO2)” is included in the “Proceedings and Materials from the 2000 Technical Exchange Conference” compiled by the National Chemical Ammonia Production Design Technology Center in October this year. The author of the former article is Wei Xionghui from Peking University; the author of the latter article is Wei Xionghui along with six other individuals. The previous report stated that the standard DDS catalyst is used for desulfurization of shifted gas, while it serves as an additive to the DDS catalyst for the desulfurization of semi-water gas. In the latter paper, the DDS-O type catalyst and the DDS-Z type catalyst appear. These names are inconsistent between the two articles, making it difficult to gain a clear understanding of DDS. The previous article suggested that by using DDS catalyst additives for the desulfurization of semi-water gas, the desulfurization efficiency could be increased by more than 1.5 times compared to the original method, while the subsequent article reported that a chemical company attempted to use DDS desulfurizing solution instead of tannin solution in its semi-water gas desulfurization equipment to remove H2S from the gas, but failed to achieve the desired results. This is because a large amount of tar and Na2S2O3 generated as a by-product accumulate in the desulfurization solution; these substances can damage the DDS-O type catalyst and render it inactive. The DDS-Z catalyst, developed specifically for desulfurizing gases with high sulfur content such as semi-water gas and natural gas, is expected to achieve satisfactory results. It can be seen that whether satisfactory results will be achieved by using DDS-Z type is still uncertain; let alone the claim that it will definitely improve the technical and economic efficiency when it is applied to the desulfurization of semi-water gas. In transformed gas desulfurization, the latter article presents a successful application example, suggesting that by using DDS to reduce outlet H2S levels, subsequent dry desulfurization processes and protection decarburization systems can be eliminated, resulting in significant economic benefits. However, the previous article indicated that 3 out of the 4 applications encountered some problems: in one case, when the concentration of Na2S2O3 in the desulfurization solution reached 270 g/l, it caused the DDS catalyst to be completely damaged, requiring the desulfurization solution to be replaced; at the same time, issues with the equipment also necessitated modifications ； One company experiences large fluctuations in H2S emissions, which is attributed to an unreasonable design of the desulfurization unit for semi-water gas and the regeneration oxidation tank used for desulfurizing shift gas ； Another company requires that the H2S level at the desulfurization outlet after gas conversion not be lower than 0.08 g/Nm3, as it is concerned that the ammonium carbonate product might turn black; therefore, it is requested that the H2S content at the outlet not be reduced too much after using DDS. It can be seen that the use of DDS desulfurization catalysts for the desulfurization of shift gas is also subject to many limitations. When using DDS, soda ash and caustic soda are required as absorbents; the total alkalinity (expressed as Na2CO3) must be ≥30 g/L, with Na2CO3 in the lean solution being ≥5 g/L and NaHCO3 being ≥0.3 g/L. It is a desulfurization fluid with high alkalinity, multiple components, and complex redox reactions; moreover, the weight consumption of hydroquinone and ferrous carbonate is 2–5 times that of the main catalyst DDS. The above components undoubtedly can improve the desulfurization efficiency; however, the preparation and adjustment of many raw materials are rather cumbersome. Moreover, after the total alkalinity is increased, there are significant losses. In contrast, using “888” is much more convenient; only two ingredients are required—soda ash and “888”. In the lean solution, the concentration of Na2CO3 can be as low as about 2.0 g/L, while a concentration of NaHCO3 exceeding 40 g/L poses no problem at all. Plants that use \"888\" can more easily keep the H2S content at the outlet within an appropriate range, and to date, there has been no occurrence of blackening in the ammonium carbonate product. The blackening of ammonium carbonate products is caused by either too high or too low levels of H2S in the shift gas. The high H2S content leads to the formation of abundant black FeS and gray Fe(SH)2, causing the ammonium carbonate to turn black; this is quite easy to understand. However, excessively low H2S content can also cause the contents of the above two substances in the ammonium carbonate mother liquor to increase. This is mainly because when the H2S level is too low, a FeS film does not form on the inner walls of the equipment and pipes in the ammonium carbonate system; this film prevents further corrosion by stopping the further precipitation of Fe+2, thereby avoiding blackening of the ammonium carbonate. Therefore, ammonia bicarbonate production plants have specified ranges for the H2S content in the shift gas; for example, the Jiangxi Ammonia Plant stipulates that the H2S level should be 70–80 mg/Nm3 during low-load operation, and 55–65 mg/Nm3 during high-load operation. It can be seen that for such plants, it is pointless to have too low a H2S level in the transformed gas. Additionally, some plants that produce ammonium carbonate do not carry out desulfurization of the reformate gas; either they only use wet desulfurization without dry desulfurization, or they only use dry desulfurization but lack carbon removal equipment. Therefore, the characteristic that DDS can reduce the H2S level at the outlet of the shift converter is not desirable for all nitrogen fertilizer plants. Furthermore, if a nitrogen fertilizer plant uses two different catalysts for the desulfurization of semi-water gas (referred to as semi-desulfurization) and the desulfurization of shifted gas (referred to as shift-desulfurization), and it is required that these two solutions must not mix at all, then some plants are unable to achieve this without installing additional equipment. This is because some plants share one regenerative oxidation tank or one sulfur melting vessel; in the former case, the same desulfurization solution is used for both semi-desulfurization and modified desulfurization processes, while in the latter case, when the sulfur melting waste liquid cannot be discharged and must be reused, the solutions from semi-desulfurization and modified desulfurization are gradually mixed together. In the semi-dehydration system, a larger amount of soda ash is added; by-products from side reactions such as Na2S2O3 increase rapidly. Additionally, the semi-water gas introduces more coal tar, resulting in a higher concentration of coal tar in the desulfurization solution compared to the desulfurization solution used alone. In such situations, when DDS is used, the DDS-O catalyst added to the desulfurization solution will be degraded and become inactive due to Na2S2O3 and coal tar, which inevitably leads to an increased amount of DDS required, higher costs, and large fluctuations in the H2S content of the exhaust gas, making it difficult to keep it within the specified limits. However, when using “888”, these problems do not occur, because “888” has only one model, which is suitable for both semi-debinding and full debinding. To date, several ammonia synthesis plants use one or two solutions for both semi-dehydration and dehydration; although they are separate, these solutions are sometimes mixed together as well. In such situations, after using “888”, the operation is normal and smooth. In addition, there are five other issues that should be carefully considered when selecting a catalyst: namely, (1) how selective the desulfurization catalyst and method are in absorbing H2S. “888” exhibits good H2S stripping performance and does not catalyze the chemical reaction for CO2 absorption. As is well known, for fertilizer plants that produce ammonium bicarbonate and urea, CO2 is an essential raw material, and losses must be minimized as much as possible. Theoretically, in the process of producing synthetic ammonia using coal coke as a raw material, for every 1 kilomole of ammonia produced, 0.836 kilomoles of CO2 are generated as a by-product. The number of moles of ammonia is about 15% higher than that of CO2; thus, reducing CO2 losses can lead to an increased production of ammonium carbonate fertilizer. Therefore, when selecting desulfurization catalysts, desulfurization methods, and process designs, it is necessary to pay attention to minimizing CO2 absorption while carrying out desulfurization. The reduction in ammonium carbonate production due to excessive CO2 loss is just one aspect. On the other hand, if the solution absorbs too much CO2, it will inevitably lower the pH value, so it is necessary to add more soda ash ； During regeneration, the release of large amounts of CO2 gas reduces the amount of air drawn in by the self-priming air nozzles, thereby affecting the effectiveness of regeneration ； Furthermore, a large amount of CO2 is stirred upward in the regeneration tank, affecting the formation and stability of the sulfur foam layer; this results in incomplete separation of sulfur, reduces the yield of by-product sulfur, and lowers the desulfurization efficiency of the desulfurization tower. Reportedly, the DDS desulfurization method also has a good decarburization effect; this inevitably leads to increased absorption of CO2, making the problem of CO2 interference even more prominent. (II) When DDS is used as a desulfurization and decarburization agent, can the regenerated CO2 gas be used as a raw material for urea? In the article, Wei discusses how DDS solutions absorb carbon dioxide, organic sulfur, inorganic sulfur, and hydrogen cyanide from gases under pressure, and produce CO2 as a by-product during regeneration under reduced pressure and at elevated temperatures. But what to do with the by-produced CO2 has not been clarified. If used as food, H2S content is required

It seems that the gentleman above is quite opposed to DDS, while he highly praises 888. I have no idea what this guy does for a living. And it seems like your opinion was copied from somewhere else; it must be from many years ago. The DDS desulfurization method is performing quite well at present, especially in terms of variable desulfurization; the people upstairs have also gained a deep understanding of this. I hope that when discussing issues, everyone can speak the truth so that we can truly grasp the actual situation. As you mentioned in your text, examples of plants that use \"888\" for gas desulfurization include Jilin Tonghua Fertilizer Plant and Shandong Pingyuan Fertilizer Plant. As for Jilin Tonghua Fertilizer Plant, I needn’t say much – that plant has used many different catalysts over time, with poor results each time. I’m just asking, is 888 still in use in the Shandong Plain nowadays? That should have been in the year 2005. As far as I know, that factory was converted from the original 888 model to the DDS model; currently, it’s operating quite well. I won’t say which one is better – tannin, 888, or DDS – but I fully agree with the opinions of those who commented above me.

Is hydroquinone also added in the DDS method?

DDS does not involve the addition of hydroquinone, but rather a combination of four different medications. They are the main catalyst, auxiliary materials, B auxiliary, and active ferrous carbonate, respectively. Main purpose: 1. Reduce costs. 2. Ensure that the catalyst can remain stable in the solution. This post was last edited by 42936803 on 2008-7-16 13:23]

Using tannin together with 888 yields better results; it can be applied in both normal-pressure and pressure-based desulfurization processes

DDS performs well at low sulfur levels, with a very low decrease in output. If the import volume is too high, it doesn’t work; the manufacturer explains that it’s a problem with the bacteria. 888 has the ability to remove high levels of sulfur, but the sulfur capacity of the desulfurization solution should not be set too high, otherwise side reactions become severe. Like PDS, sulfur foam is thick and highly viscous. I’ve only used the modified tannin, and overall it’s quite good; controlling the ratio of glue to vanadium requires a lot of expertise. This method is not very effective for variable denaturation.