Desulfurization and decyanidation of coke oven gas

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The YST process for desulfurization and decyanidation of coke oven gas: Since the 1980s, China has introduced various foreign methods such as the Thaweesin method, A-S method, MEA-sulfuric acid method, FRC method, and VS-WSA method. In addition, there are domestic methods developed in China such as the HPF method (along with methods like ZL method and PDS method), the OPT method, and the improved ADA method. However, all these methods have one flaw or another: they involve long processing procedures, result in low-quality sulfur, high production costs, and present difficulties in dealing with the waste liquids resulting from desulfurization. As a result, they have been gradually phased out. 1. Development of the YST process technology: The YST method was developed by Ningbo Kexin Chemical Engineering Technology Co., Ltd. and Dalian Chemical Research and Design Institute. It was put into operation at Hangzhou Iron and Steel Co.’s coking plant in September 2005. Through four years of hard work and the evaluation of the YST unit, this desulfurization and decyanation process technology has been brought to maturity. The production process of YST can be divided into two main parts: the first part is YS desulfurization and decyanidation ； The second part involves extracting high-value chemical products from the desulfurization liquid, namely ammonium thiocyanate, thiosulfuric acid, and sulfuric acid. The absorption liquid used for desulfurization and decyanidation in YST is water used to absorb ammonia from gas, resulting in ammonium hydroxide; its catalyst is mainly the remaining ammonium hydroxide, with a trace amount of 1,4-naphthoquinone-2-sulfonate (abbreviated as NO) possibly added if necessary. Coke oven gas enters the lower part of the desulfurization absorption tower, where it comes into full contact with the absorption liquid sprayed from the top of the tower. This leads to the absorption of ammonia, hydrogen sulfide, hydrogen cyanide, and a series of chemical reactions take place. The absorption liquid exiting the desulfurization absorption tower is pressurized using a pump and then sent together with compressed air to the bottom of the oxidation tower, where further chemical reactions occur, converting all the absorbed ammonia, hydrogen sulfide, and hydrogen cyanide into ammonium thiocyanate, thiosulfate, a small amount of sulfuric acid, and trace amounts of sulfur, thereby achieving the purpose of desulfurization and decyanidation. The principle behind salt extraction is based on the ternary phase diagram of ammonium thiocyanate – thiosulfate an – water. The desulfurization solution is evaporated; when it reaches a certain concentration, it enters the crystallization zone for thiosulfate an. By maintaining a specific temperature, both thiosulfate an and ammonium sulfate crystallize out. Thiosulfate an and ammonium sulfate are then separated by centrifugation. The filtrate resulting from this centrifugation process is cooled again to allow ammonium thiocyanate to crystallize out, which is subsequently separated by centrifugation to yield the ammonium thiocyanate product. The filtrate left after the separation of ammonium thiocyanate is sent back to the evaporation system, thus creating a closed-loop cycle. The YST desulfurization and decyanation process is carried out at normal temperature and pressure. All of its equipment is basically made of carbon steel; salt extraction from the desulfurization solution is achieved through vacuum evaporation and crystallization. Due to the high concentration of ammonium thiocyanate, which gives it strong corrosive properties, most of the equipment is made of stainless steel. 2 Characteristics of desulfurization and decyanidation by the YST method ⑴ The YST method belongs to pre-desulfurization; it is installed after the electrostatic tar catcher and before the ammonia removal unit. This allows for the early removal of highly toxic and harmful acidic substances such as hydrogen sulfide and hydrogen cyanide, thereby reducing environmental pollution and equipment corrosion. ⑵ The YST desulfurization agent enables the ammonia in gas to be automatically absorbed into the desulfurization solution in the desulfurization absorption tower; as a result, there is no need to incur costs associated with desulfurization alkalis (such as KOH, NaOH, NaCO3, or MEA), nor are facilities like dissolution tanks and alkali storage areas required. This reduces the capital expenditure and simplifies the production process. Based on the experience of Hangzhou Iron and Steel Co.’s coking plant, the YST method consumes some of the chlorine in the gas during the desulfurization and decyanidation processes, thereby reducing the production load in the subsequent ammonium sulfate production stage by about 30%. This results in a reduction of annual losses in this process by around 1 million yuan. ⑶It has a very high desulfurization efficiency. Under normal circumstances, after desulfurization using a single tower, the H2S level is ≤200 mg/m3; whereas after desulfurization using two towers in series, the H2S level is ≤20 mg/m3, which can meet the needs of different users. Taking the coking plant of Hangzhou Iron and Steel as an example, the data for September and October 2005 correspond to the period right after it started operation, while the data from May 2009 onward are those obtained after YST evaluation (Table 1). Table 1 Comparison of H2S content in gas: Early operation period, September 2005 and October 2005. Average content before desulfurization: mg/m3 – 496, 244, 344. Average content after desulfurization: mg/m3 – 8.97, 58. Desulfurization efficiency: % – 99.82, 98.69. May to December 2009: Average content before desulfurization: mg/m3 – 510, 546, 124, 086, 443, 843, 984, 725, 467, 741, 30. Average content after desulfurization: mg/m3 – 562, 426, 231, 534, 2. Desulfurization efficiency: % – 98.99, 9.48, 99.36, 99.48, 99.66, 99.94, 99.91, 99.95. The average desulfurization efficiency was 99.26% during the early operation period, and it rose to 99.59% after the evaluation session, indicating that the YST method achieves a high level of desulfurization efficiency. ⑷ The efficiency in removing hydrogen cyanide is also high. Hydrogen cyanide is more toxic than hydrogen sulfide. When coke oven gas is burned, hydrogen cyanide is oxidized to NOx, which is a potent carcinogen; therefore, it poses an even greater threat to humans. Among the various desulfurization methods used in the last century, the efficiency of cyanide removal was not given much attention, and the removal of hydrogen cyanide was considered a drawback. For example, in the vacuum potassium carbonate method, hydrogen sulfide is removed along with some hydrogen cyanide; moreover, during acid production, an SCR reactor must be installed to remove nitrogen oxides from the gases emitted from the incinerator. In this case, stoichiometric amounts of ammonia must be injected upstream of the SCR, which is a catalyst reactor, and the reaction equation is as follows: it converts NO into harmless nitrogen gas. For example, to treat 65,000 m3/h of coke oven gas using the vacuum potassium acid method, 127 tons of liquid ammonia are required per year; this not only increases the investment in equipment and complicates operations but also raises production costs. Another example: In the desulfurization process, the HPF method requires the oxidation of hydrogen sulfide to sulfur, thereby producing sulfur products. However, due to the presence of hydrogen cyanide in coke oven gas and the presence of oxygen during the desulfurization process, large amounts of ammonium thiocyanate and ammonium thiosulfate are generated, resulting in a sulfur yield of only about 50%-60%. The high levels of ammonium thiocyanate and ammonium thiosulfate in the desulfurization solution inevitably lead to a decrease in desulfurization efficiency. To ensure normal production for desulfurization, it is necessary to discharge desulfurization liquid with extremely high COD levels and toxic properties, which poses serious environmental problems. Otherwise, a waste liquid treatment system would have to be installed, thereby increasing the investment required for the HPF method and consequently raising production costs as well. The YST method regards hydrogen cyanide as an important raw material; during the desulfurization and decyanidation process, ammonium thiocyanate of high value is produced, and ammonium thiocyanate products with high added value are obtained in the salt extraction unit. The average decyanidation efficiency at Hangzhou Iron and Steel Co.’s coking plant is 91.60%. ⑸The desulfurization tower has low resistance, enabling stable operation over the long term; it also offers great operational flexibility, allowing stable production within a load range of 25%–100%. The YS method converts hydrogen sulfide and hydrogen cyanide into sulfur-containing compounds such as ammonium thiocyanate and thiosulfate, thereby removing them. As a result, the suspended sulfur content in the solution at the inlet of the desulfurization tower is extremely low, remaining between 0.02–0.03 g/l over time. The solution appears brown, and no suspended particles are visible to the naked eye; hence, the resistance in the desulfurization tower does not increase significantly. From its commissioning in September 2005 until the end of 2009, the resistance of the desulfurization tower at Hangzhou Iron and Steel Co.’s coking plant, using the YST method, remained between 300–400 Pa, and the packing inside the desulfurization tower was not cleaned throughout those over four years of operation. ⑹The product has a high added value and wide range of applications. The sulfur pastes and sulfur obtained through the HPF method, ZL method, and PDS oxidation method face certain difficulties in sales due to quality issues. In contrast, the YST process produces products free of sulfur; they are all high-value chemical products such as ammonium thiocyanate and thiosulfuric acid. The price of ammonium thiocyanate in China is between 6,000 and 7,000 yuan per ton; therefore, the value of products produced using the YST method is much higher than the added value of other desulfurization products. ⑺Lower energy consumption. Due to the shorter process flow of the YS method, which operates at normal temperature and pressure, and because the salt extraction process employs advanced techniques involving single evaporation and double crystallization, its energy consumption is also low; overall, its energy consumption is lower than that of both the HPF method and the vacuum potassium carbonate method. ⑻The costs of chemical raw materials such as desulfurizers and catalysts are the lowest. The desulfurization agent in the YST method is ammonia present in gas, and the catalyst is excess ammonia water; basically, no cost is involved. Even when some sodium 1,4-naphthoquinone-2-sulfonate is used as an auxiliary catalyst, the cost of raw materials remains low. ⑼Savings in infrastructure investment. Due to the simplicity of the YST process, YS desulfurization is carried out at normal temperature and pressure; the equipment used is mainly made of carbon steel for corrosion protection, while the salt extraction units are mostly constructed from stainless steel. Thanks to the adoption of this new process technology, the process flow is shortened and the amount of equipment required is reduced, resulting in lower capital investment costs. ⑽The production cost is the lowest. Due to the high level of automation in the YST process, the need for fewer operators, lower labor intensity, stable production, reduced maintenance requirements, as well as lower consumption of various energy sources and raw materials, the HPF process requires 31.1 million yuan per year to handle the same volume of 65,000 m3/h of coke oven gas ； The vacuum potassium carbonate-sulfuric acid method requires 24.67 million yuan ； The YST method only requires 19.06 million yuan, which is 61.3% of the HPF method and 77.3% of the vacuum potassium carbonate-sulfuric acid method. ⑾The production cost is low; it is calculated by dividing the production expenses by the annual volume of coke oven gas processed, with the price per 1000 m3 of gas as a reference. The HPF method results in a cost of 54.622 yuan, the vacuum potassium carbonate-sulfuric acid method yields 43.476 yuan, while the YST method costs only 33.465 yuan. Therefore, the YST method offers better economic efficiency. ⑿ The YST method is environmentally friendly. —— The exhaust gas is discharged after treatment. In the YST method, the oxidized and regenerated exhaust gas is treated first, and ammonia in the exhaust gas is recovered before it is discharged; as a result, there are basically no abnormal odors at the production site using the YST method. —— No waste liquid discharge: The YST method features a closed-loop system in which all evaporated condensate returns to the system, eliminating the need to dispose of any waste liquid. —— No waste residue emission: In the entire production process of the YST method, apart from ammonium thiocyanate and crude thiosulfate as solid products, no other solids are generated; therefore, there is no solid waste residue emitted. ⒀It has independent intellectual property rights. The YST process technology is the latest technique invented on the basis of summarizing previous desulfurization and decyanation processes both at home and abroad, and it possesses complete independent intellectual property rights. ⒁ Full utilization of hydrogen sulfide and hydrogen cyanide in gas. The YST method uses ammonia present in gas and excess ammonia water from coking plants as catalysts for desulfurization and decyanidation; ammonia reacts with hydrogen sulfide and hydrogen cyanide to produce ammonium thiocyanate, thiosulfate, and a small amount of ammonium sulfate. Unlike other desulfurization methods (except the Thiele process), hydrogen cyanide is not regarded as an impurity or waste, so removing it is not a problem. ⒂Achieve a circular economy. The YST process technology utilizes hydrogen sulfide, hydrogen cyanide, and ammonia present in coke oven gas to react and produce ammonium thiocyanate, crude thiosulfate, and a small amount of sulfuric acid. Through evaporation and concentration, crystallization, and extraction, crude thiosulfate and ammonium thiocyanate are obtained; the filtrate resulting from centrifugation as well as the evaporated condensate are all returned to the system, thereby achieving a closed-loop economic cycle. This allows for continuous desulfurization and decyanidation of coke oven gas, while also enabling the continuous production of ammonium thiocyanate and crude thiosulfate products. ⒃Suitable for various oxidation-based desulfurization upgrades. The coking plant of Hangzhou Steel originally used an improved ADA method, which was a post-desulfurization process employing sodium carbonate as a desulfurizing agent; it has now been converted to a pre-desulfurization YST method. Most of the equipment used in the original improved ADA desulfurization process remains in use (such as the desulfurization tower and regeneration tower), with additional equipment added for the salt recovery process. This makes it a model for converting oxidation-based desulfurization methods into the YST method. In the future, any desulfurization or decyanidation processes based on oxidation methods (such as HPF method, PDS method, FRC method, T-H method) can be implemented using the YST method developed at Hangzhou Steel’s coking plant, and both the process, the equipment, and the quality of the products will surely be improved. 3 Current problems with the YST method: (1) The product quality is not high enough. The quality of the products produced using the YST method is inadequate; crude thiosulfate contains 69.60% thiosulfate, 13.92% sulfuric acid, 10.59% ammonium thiocyanate, and 5.40% other impurities and water ； The ammonium thiocyanate product contains 46.6% ammonium thiocyanate and 40.47% thiosulfuric acid; the water content has not been determined, and the product quality needs to be improved further. ⑵ It is necessary to further explore new markets. Crude thiosulfuric acid could originally be used to manufacture compound fertilizers together with potassium dihydrogen phosphate and similar substances, but its high NH4CSN content of 10.59% makes it unsuitable for use in compound fertilizers. Therefore, it is essential to reduce the NH4CSN content in crude thiosulfuric acid products to below 7%. At present, the quality of ammonium thiocyanate products is low; it is necessary to improve the quality of both ammonium thiocyanate and crude thiosulfate an products. ⑶The photography industry and reagent-grade thiosulfate require high standards. The thiosulfate solution should have a concentration of 56%–60%, with sulfite content at ≤0.6%, alkalinity between 0.5%–1.5%, heavy metals (measured as Pb) at ≤100 PPm, iron content at ≤10 PPm, residual solids after burning at ≤0.1%, and a specific gravity of 1.311–1.335. Although these are stringent quality requirements, they can still be met through effort. However, the volume of this product used is small, so it is not possible to produce all thiosulfate in photography-grade or reagent-grade purity; therefore, only a portion of the crude thiosulfate can be refined into products suitable for use in the photography industry and as reagents, in order to meet market demands. ⑷ The YST process technology has a limited range of applications; it is generally suitable only for use in coking plants. Moreover, the concentration of NH3 in the gas should be comparable to that of hydrogen sulfide and hydrogen cyanide. In such cases, the YST method is preferable, as it facilitates efficient desulfurization and decyanidation. In short, the YST process technology is a new approach that requires support and efforts from all parties in order to better serve enterprises and society.