Selection of sulfur recovery units in the coal chemical industry

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This post was last edited by liaifeng on 2018-8-11 at 19:59. With the rapid development of China’s economy and the sharp rise in global energy prices, China’s coal chemical industry has entered a period of rapid growth. Current coal chemical projects mainly involve the gasification of coal to produce syngas, which is then further processed to manufacture synthetic ammonia, methanol, dimethyl ether, synthetic oils, synthetic olefins, and synthetic natural gas. The scale of most units (converted to methanol) ranges from 300 to 1200 kt per year, with coal consumption typically ranging from 2000 to 5000 t/day. Considering a sulfur content in coal of 0.5% to 1.5%, the design capacity of the sulfur recovery units for the vast majority of projects is an annual sulfur production of 5 to 40 kt, or a daily sulfur production of 15 to 120 t. As environmental protection concepts become increasingly widespread, along with the strict enforcement of environmental regulations and the continuous expansion of the scale of coal chemical plants, the conventional Claus process or other outdated treatment methods that were suitable for small-scale plants can no longer meet environmental requirements. Therefore, it is essential to select a sulfur recovery process suitable for the coal chemical industry. 1 Characteristics of sulfur recovery units in coal chemical industry: Unlike those used in the natural gas and refining sectors, sulfur recovery units in the coal chemical industry have the following characteristics. 1.1 The scale of sulfur recovery units is relatively small. Current refinery units have a refining capacity of around 10 Mt of crude oil per year, while natural gas processing units can handle more than 10 million cubic meters of gas per day; the corresponding sulfur recovery units have an annual sulfur production capacity of 50–250 kt, and this capacity is continuing to increase. Coal chemical plants are different; even for those with large planned capacities (such as 1,800 kt/a of methanol), the daily coal consumption for production processes does not exceed 10,000 tons. Based on a higher sulfur content, the maximum annual sulfur production is also below 50 kt. In practice, the annual sulfur production is usually between 10 and 30 kt. 1.2 Lower concentration of acidic gases: Unlike in refineries and natural gas processing plants, where acidic gases originate from the absorption using alkanolamines (such as MDEA), acidic gases in coal chemical plants generally come directly from syngas purification (such as low-temperature methanol washing); as a result, the H2S concentration is low, typically ranging only from 20% to 30%. 1.3 Complex composition of acidic gases: The complex composition of coal leads to a complex composition of acidic gases. In addition to the hydrocarbons, ammonia, and organic sulfur commonly found in refineries and natural gas processing plants, it also contains impurities such as methanol, COS, and HCN. And these impurities have a significant impact on the design of sulfur recovery units. 1.4 Large fluctuations in acidic gas concentration: Due to the diversity of coal types, the range of variations in acidic gases in coal chemical plants is much greater than in other petrochemical industries. High requirements are placed on operational flexibility. 1.5 Insufficient hydrogen source but sufficient oxygen: Refineries require hydrocracking, which is why they all have large-scale hydrogen production facilities, ensuring an adequate supply of hydrogen. Coal chemical plants generally do not have large-scale hydrogen production units, so there is a shortage of hydrogen supply. On the contrary, coal chemical plants generally have air separation units, providing an ample supply of pure oxygen; this feature is conducive to the oxygen-enriched combustion process, making the hydrogen reduction process less suitable. 2 Available sulfur recovery processes in the coal chemical industry There are many sulfur recovery processes available on the market that meet environmental requirements, which can be roughly classified into the following three categories. 2.1 Claus extended process: This type of process builds upon the Claus process by adding a selective catalytic oxidation stage; it increases the recovery rate to 99.2%–99.5% while retaining the advantages of the Claus process, such as simplicity, reliability, and low investment costs. SUPERCLAUS, EUROCLAUS, CLINSULF, the MARA process, and others all fall into this category. See Figure 1. http://www.nmtech.com.cn/jishuwang/upload1/0805191004249615.jpg Figure 1: Claus extended sulfur recovery process. This type of process is simple and reliable, with a high recovery rate (up to 99.5%). The total investment in the facility and its operating costs are low, as there is no need for hydrogenation or alcohol-amine (such as MDEA) absorption units. It has been widely applied in the coal chemical industry. 2.2 Claus off-gas treatment process: This type of process involves hydrogenating the Claus off-gases, followed by absorption using a solvent such as MDEA, and then returning H2S to the Claus unit, as shown in Figure 2. The sulfur recovery rate is high (up to 99.9%). Scot, RAR, SSR, and others all fall under this category of processes. http://www.nmtech.com.cn/jishuwang/upload1/0805191005102720.jpg Figure 2: Claus exhaust gas treatment process. This type of process is reliable and features a high recovery rate. But the process is long, it includes a solvent treatment stage, and the setup is complex. The total investment in the facility and its operating costs are very high. It is widely used in the natural gas and refining industries, and is suitable for large-scale plants (with an annual sulfur production of over 50 kt). 　 2.3 Alkali absorption (biological desulfurization or complexed iron method) process: This process removes hydrogen sulfide by using a solution (alkaline solution) for absorption. Then the alkaline solution is regenerated through biological methods or iron valence change methods. Shell BioDesulfurization and SULFEROX all fall under this process. The sulfur recovery rate of this process can exceed 99.99%. See Figure 3. http://www.nmtech.com.cn/jishuwang/upload1/0805191006369328.jpg Figure 3: Alkali absorption desulfurization process. This process is characterized by its simplicity and high recovery rate; it can directly treat syngas with very low H2S concentrations. The disadvantage is that the regeneration reactor is relatively large in size ； The operating cost (alkali consumption) is high. Suitable for small-scale plants (with an annual sulfur production of less than 5 kt). A simple comparison of the above three processes is shown in Table 1. http://www.nmtech.com.cn/jishuwang/upload1/0805191009229038.jpg 3 Main factors to consider when selecting a sulfur recovery process and process selection 3.1 Meeting **environmental requirements: Complying with the emission standards for newly built sulfur production facilities (GB16297-1996). And leave some room for future development. 3.2 Reliability of technology: Given the characteristics of coal chemical industry, a sulfur recovery process that is suitable for low concentrations of acidic gases, operates within a wide range of elasticities, and can handle complex gases should be selected. Reliable processes such as those with oxygen-enriched or pure oxygen combustion systems, ammonia and HCN combustion systems, organic sulfur hydrolysis and methanol pretreatment systems, etc. 3.3 Plant investment and consumption: Under the premise of meeting the above two points, plant investment and operating costs should be kept as low as possible. At the same time, if the main unit does not have an alcohol amine absorption unit, efforts should be made to avoid introducing new solvent absorption systems (such as MDEA), thereby preventing unnecessary increases in the complexity of the entire plant. The above factors should also be considered in conjunction with the scale of the installation. The size of the device has a significant impact on meeting environmental standards, as well as on investment and operating costs. Figure 4 shows the general relationship between plant scale and technical choices. http://www.nmtech.com.cn/jishuwang/upload1/0805191010332448.jpg Figure 4: Influence of sulfur treatment capacity on the selection of desulfurization and sulfur recovery processes. In summary, regarding the scale of sulfur recovery units in coal chemical projects, under the conditions of meeting environmental requirements, having advanced and reliable technology, as well as keeping investment and operating costs low, the Claus extended process is clearly the most suitable option. In the Claus extended process, Super Claus and Ultra Super Claus are highly favored for their advantages such as simple process flow, advanced and reliable technology, high performance, as well as lowest investment and operating costs. It is undoubtedly the best process option for sulfur recovery units in the field of coal conversion in our country at present. Globally, this process is employed in over 150 industrial installations in the fields of natural gas, refining, and syngas. It is also widely used in the fields of natural gas, oil refining, and coal chemical industry in our country. 4 Super/Ultra-EUROCLAUS process: The Ultra-EUROCLAUS process (Figure 5) was developed on the basis of the Super EUROCLAUS process. The core of this technology is to reduce SO2 in the Claus off-gases to H2S through a catalytic hydrogenation section located within the Claus reactor. The off-gas containing only H2S is then selectively catalytically oxidized and reduced to elemental sulfur in the super Claus reaction section. Unlike conventional exhaust gas treatment processes, this hydrogenation process does not require a separate reactor; therefore, the process gas does not need to be heated or cooled. Hydrogen is generated by the process itself, so no external hydrogen supply is required. Furthermore, H2S in the exhaust gas does not require solvent absorption, thus eliminating the need for solvent absorption and regeneration systems that incur high investment and operational costs. http://www.nmtech.com.cn/jishuwang/upload1/0805191013408152.jpg Figure 5: Process flow of the EUROCLAUS® technology. 5. Reasons for choosing the EUROCLAUS process for sulfur recovery projects in the coal chemical industry. 5.1 Meeting environmental requirements: The recovery rate achieved with EUROCLAUS is 99.2%–99.5%, and the SO2 emission concentration can be kept below 550 mg/m3. It not only fully meets the **environmental requirements but also leaves room for future development. As can be seen from Table 2, for sulfur recovery units in the coal chemical industry with an annual sulfur production of less than 50 kt, the ultra-high Claus process is fully capable of meeting environmental emission requirements. http://www.nmtech.com.cn/jishuwang/upload1/0805191014388516.jpg 5.2 The device is stable and reliable, taking into account the characteristics of the coal chemical industry. Chaoyou Klaus has taken the following measures to ensure the stable and reliable operation of the device. (1) In coal chemical enterprises, the acid gas concentration typically ranges from 20% to 30% in terms of H2S; using air cannot ensure stable combustion, and problems such as failure to ignite and carbon deposition are likely to occur. This process generally uses oxygen-enriched or pure oxygen (O2 ≥ 28%–100%) instead of air for combustion in order to solve this problem. Oxygen supply is generally not a problem for coal chemical enterprises. At the same time, investments have been significantly reduced due to the smaller size of the equipment. (2) The acid gas concentration in coal chemical enterprises is complex; typical gases contain impurities such as olefins, COS, HCN, NH3, and CH3OH. If methanol and higher olefins are not treated, severe carbon deposition will occur; in mild cases, this reduces the catalyst’s lifespan and results in sulfur products that do not meet quality standards ； In severe cases, it will block the catalyst bed, preventing the unit from operating. If NH3 and HCN are not treated, it will cause corrosion and blockage of the equipment as well as catalyst failure. Various sulfur compounds, if left untreated, may fail to meet emission requirements. Jacobs Company has effectively resolved these issues by pre-washing methanol, using a special burner system, and selecting appropriate catalysts to handle organic sulfur, ammonia, and hydrogen cyanide. Ensuring the long-term stable operation of the equipment has guaranteed the sustained and stable functioning of all the main units in the plant. 5.3 Low equipment investment: Due to the absence of complex hydrogenation and amine absorption systems, the investment required for ultra-optimized Claus processes is only 70% to 50% of that required for Claus exhaust treatment processes of similar scale. For a sulfur recovery unit with an annual production capacity of 20 kt, the total investment in the Claus flue gas treatment process is generally over 80 million RMB, whereas the ultra-high-efficiency Claus process requires only around 50 million RMB in total investment. 5.4 Low operating costs: The Claus flue gas treatment process requires the use of a solvent (MDEA), and the regeneration of this solvent necessitates the use of large amounts of steam. Ultra-Klaus, on the other hand, requires only a small amount of heating steam, and the entire system still has excess steam available for output. Therefore, the overall consumption is less than 50% of that of the Claus flue gas treatment process. Due to the simplicity of the ultra-optimal Claus process, the overall maintenance costs are also much lower than those of exhaust gas treatment processes. Furthermore, this process does not require external hydrogen for hydrogenation, thereby solving the problem of a lack of hydrogen supply in coal chemical projects. Due to the obvious technical and economic advantages of super/ultra-high Claus technology, it has been widely applied in China’s coal chemical industry. China has chosen the super/ultra-high-efficiency Claus coal chemical processing technology: Coal-to-methanol production at Inner Mongolia Mengda New Energy Chemicals Base Development Co., Ltd ； Shaanxi Yanchang Petroleum (Group) Co., Ltd. Coal-based Acetic Acid ； Coal-to-methanol production at Gansu Huating Zhongxu Coal Chemical Co., Ltd ； Shenhua Ningxia Coal Industry Group Co., Ltd. Coal-to-Dimethyl Ether ； Anhui Huaihua Group Co., Ltd. Coal-to-Methanol ； Guizhou Kaiyang Chemical Co., Ltd. – Coal-based synthetic ammonia production ； Shandong Tengzhou Phoenix Fertilizer Co., Ltd. – Coal-based synthetic ammonia production ； Yulin Energy Chemical Company of Shandong Yanzhou Coal Industry produces methanol from coal.