Practices to eliminate smoke emission from the exhaust gases of solid sulfur acid production plants

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1 Smoking from the exhaust gases of the sulfur-based acid production plant was once considered inevitable. In 1972, China introduced an 180kt/a sulfur-based acid production plant from the Maoa Plant in Cuba, which was installed at the Nanjing Chemical Industry Company. Within 10 days after operation, significant smoke was emitted from the exhaust gases; this gradually decreased over time, but it remained higher and more noticeable than that of the nearby sulfur iron ore-based acid production plants. Subsequently, our country built another 8 sulfur-based acid production units at places such as the Beijing Dye Factory and the Tianjin Sulfuric Acid Plant; all of these units emitted smoke, in a situation similar to that of Nanhua Company. Therefore, it is generally believed that it is normal for sulfur-based acid production plants to produce more smoke than those using pyrite as a raw material, as sulfur contains soluble hydrocarbon organic compounds, making smoke emission an inevitable consequence. 2 Not all exhaust gases from sulfuric acid production plants emit smoke. In April 2005, to assist Egypt’s SHOTMED company in building a new sulfuric acid production plant with a capacity of 400 kt/a, our company, along with six other members of the team including those from China National Import and Export Equipment Corporation, went to Egypt for pre-design site inspections and technical discussions. $HOTMED is one of the largest phosphate producers in Egypt, located on the banks of the Nile on the outskirts of Cairo. It originally had two sets of solid sulfur acid production units, one with a capacity of 200 kt/a, which were designed and constructed by the American company Monsanto (now Monomock) in 1998 ； Another unit with a capacity of 100 kt/a was designed and built in Romania in 1989 by the German company Lurgi (now Otto Kumpmann). It was shut down for 10 years before being sold to Egypt in 2001; it operates on a one-turn-one-suction process, and its actual capacity can reach 120 kt/a. The raw materials used in these two sets of installations are bulk powdered sulfur imported from Saudi Arabia, and their process flows and equipment configurations are basically the same as those in China. The exhaust chimneys of the two units are not very tall, at 60m and 50m respectively; no smoke could be seen from a distance or up close. Only upon approaching did one feel a stream of air moving (with acidic mist), causing a noticeable itching sensation on the face. We worked on the factory for 5 days in reality, gaining a fairly thorough understanding of the situation. Generally speaking, they have the following 3 characteristics in terms of sulfur management and sulfur melting and refining: a) All the sulfur produced by the plant is stored in one large sulfur warehouse (a cement-based arc-shaped storage facility), with a storage capacity of up to 60 kt. Sulfur is very dry, with an ω(H2O) of generally 0.1%-0.5% ； Among them, fine particles ranging from 0.5 to 1.0 mm account for about 50%. The warehouse has open ends without retaining walls or doors and windows; sulfur is stored in separate areas for stacking and for use. b. Before melting the sulfur, a certain amount of diatomite and soda ash is mixed into it; in terms of mass ratio, the amount of diatomite is approximately 0.5% and that of soda ash is approximately 0.3%. Use a crane inside the warehouse to carry out “one mixing and two stirring” operations. c. The melting and refining process of sulfur takes a long time. The effective depth of their sulfur melting tanks, filter aid tanks, and refined sulfur tanks is similar to ours, at 1,600 mm each. The liquid sulfur storage tank is 6000 mm high, with a total effective volume of 640 m3 for a plant with a capacity of 200 kt/a, and 280 m3 for a plant with a capacity of 100 kt/a. The sulfur melting and refining time is 5.7 days (136.8 hours) for a 200kt/a plant, and 5 days (120 hours) for a 100kt/a plant. In the technical negotiations, Egypt requested that the sulfur melting and refining time for the new 400kt/a plant be 6 days, with a minimum of 5 days. According to them, when the output of the existing 100kt/a acid production plant is increased to 120kt/a, slight amounts of smoke and droplets are emitted from the exhaust gases due to the shortened sulfur melting and purification time (from 5 days to 4.2 days); however, no smoke is produced when the output is reduced back to 100kt/a. To assess the level of our design, manufacturing, installation, and production technologies, a delegation of 7 people led by the general manager and chief engineer of the Egyptian company visited our country in June 2005. They visited six sulfuric acid production plants using solid sulfur in China, including those operated by Yunnan Sanhuan Company, Hubei Yihua, and Rizhao in Shandong, as well as the relevant design, manufacturing, and installation firms, and unanimously affirmed the scale of China’s sulfuric acid industry as well as the reliability of its technology and equipment. But Mr. Tadros Georgui, the Egyptian CEO, said, “Your sulfuric acid plants are emitting smoke, while our plants do not…” Despite the explanations, they still have doubts about our technology. Finally, we temporarily invited them to visit two liquid sulfur acid production units at Nanhua Company, whose exhaust gases do not produce smoke. They were very pleased to see this; they said on several occasions both on site and at meetings, “These two sulfuric acid production units do not emit any smoke from their exhausts – just like the units in our country; they are very clean. We hope that the new 400kt/a unit will be similar to these…” After seeing off the Egyptian guests, the author deliberately inspected another two sulfuric acid production plants in our country. It was found that the exhaust gases from the plants that use solid sulfur to produce sulfuric acid all emitted smoke to varying degrees, whereas the plants that use liquid sulfur did not emit any smoke. Why is this? And why don’t the Egyptian plants that use solid sulfur produce any smoke? 3 Analysis of the reasons for smoke emission from plants using solid sulfur to produce sulfuric acid: Similar to the case of sulfuric acid production from pyrite or smelting flue gases, smoke emission in plants that use sulfur to produce sulfuric acid is caused by the combination of moisture in the gases with SO2, resulting in acid mist. Practical production experience tells us that smoke emission increases when sulfur becomes damp, when there are leaks in the boiler or economizer, and when a filter is first used or has low resistance. All these phenomena repeatedly demonstrate that moisture is the fundamental cause of exhaust smoke. In the sulfuric acid production process, regardless of the raw materials used, the moisture content in the gas exiting the drying tower (air or furnace gas) remains the same, at less than or equal to 0.1 g/m3. In fact, sulfur-based acid production plants generally meet this standard, with values ranging from 0.05 to 0.1 g/m3; therefore, the discussions below do not cover cases of inadequate drying or boiler leaks, but only the moisture brought in by sulfur. The moisture in sulfur generally comes from: a. free water in sulfur. According to China’s GB/T2449-2006 standard, the ω(H2O) value for solid sulfur should be ≤2.0%, but in practice this value is often higher; in some cases, water flows out from the sulfur storage areas, and the sulfur can be compressed into clumps when picked up. The soluble hydrocarbon organic compounds in sulfur decompose into CO2 and H2O during combustion. c. H2SO4 in sulfur decomposes into SO2, SO3, and H2O during combustion. Therefore, if a certain evaporation surface and evaporation time can be provided for the sulfur melting and refining system during the design phase, based on the mass of solid sulfur used as raw material, to facilitate the evaporation of moisture in sulfur at a melting temperature of 135–150°C, and if measures are taken to remove hydrocarbon compounds and sulfuric acid, it may be possible to eliminate smoke emission from the exhaust gases. For example, the two solid sulfur acid production plants in Egypt extend the refining time of molten sulfur to 5–6 days, which is about twice the usual time in China, allowing sufficient evaporation of water. Additionally, diatomaceous earth and lime are used to remove hydrocarbons and sulfuric acid; hence it makes sense that no smoke is emitted from the exhaust gases. As for the plants for producing acid from liquid sulfur, in China liquid sulfur is currently imported mainly from Japan and South Korea; the storage and transportation process takes more than a month. For the small amount of liquid sulfur produced in China, such as that manufactured by Qilu Petrochemical, it is transported by road to the Zibo Cobalt Factory, and the storage and transportation time is also around 20 days, providing sufficient time for the moisture to evaporate. Therefore, the exhaust gas from liquid sulfur acid production plants is generally smoke-free. 4 Improving device design and operation to eliminate exhaust smoke 4.1 Appropriately extending the sulfur melting and refining time In most of the existing solid sulfur-based sulfuric acid production plants in China, the sulfur melting and refining time is 3 days or a little more than that. If the solid sulfur has a high moisture content, the time during which it remains in liquid form can be appropriately extended; generally, 6–8 days can be considered as a starting point. The best approach is to use liquid sulfur storage tanks with a storage depth of 4–6 meters. Firstly, there is no need to make major changes to the existing sulfur melting and refining equipment, making the modification relatively simple and easy ； Secondly, it does not **increase steam consumption before driving or during long periods of idling of the sulfur burning furnace; only a small amount of steam is used for heating during normal operation, resulting in low energy consumption. For newly built sulfuric acid production plants, when the ω(H2O) of solid sulfur is 0.5%-1.0%, it is appropriate to design the melting and liquid sulfur purification times at 6-8 days on an economically viable basis (the time should be extended as appropriate if the water content is high). In 2006, we designed two sets of solid sulfur acid production units for Myanmar, with the melting and refining times set at 7 days each (the effective depth of the sulfur melting tank and the sulfur refining tank is 1,650 mm). Seven days after operation began, no smoke could be seen coming from the chimneys, and the exhaust gases remained clean and clear throughout prolonged operation. 4.2 Use diatomaceous earth to adsorb hydrocarbon organic compounds in sulfur As mentioned earlier, the Egyptian companySHOTMED mixes diatomaceous earth into solid sulfur at a ratio of 0.5% by weight in order to adsorb hydrocarbon organic compounds, which are then removed through filtration along with the diatomaceous earth. At present, in sulfuric acid production plants in our country, diatomaceous earth is added only once before the filter is started up, with the aim of forming a filtering layer on the filter screen to improve the efficiency of filtering out solid particles. Of course, the diatomaceous earth in the filter layer does have a certain adsorptive effect on hydrocarbon organic compounds, but due to the thinness of the filter layer and the short contact time, its efficiency cannot be very high. Additionally, the small amount of diatomaceous earth used in the filter layer limits its ability to remove hydrocarbon organic compounds. Therefore, we can follow the approach of SHOTMED Company by mixing some diatomaceous earth into sulfur to remove more hydrocarbon organic compounds. This is a different concept from using diatomaceous earth as a filtering layer on the filter screen of a filter; due to two different mechanisms of action, the effects are naturally quite different as well. The diatomite to be used should be of good quality, with the following composition: ω(SiO2) greater than 90%, loss on ignition less than 5.0%, ω(Fe2O3) less than 2.5%, and ω(Al2O3) less than 3.5%. Due to the small amount added, it is difficult to add it continuously and evenly; therefore, the “batch mixing method” provided by Egypt’sSHOT-MED company is a better option. The amount of mixed diatomite can be determined based on the content of soluble hydrocarbon organic compounds in the batch of sulfur and experience. 4.3 Addition of lime or soda ash: At present, most sulfur-based acid production plants in China use the method of adding lime and soda ash to neutralize sulfuric acid, employing a \"surface dispersion method\". Due to the irregular manual operation, the addition amount is inaccurate and uneven; as a result, the neutralization effect is poor, and it is easy for a layer of sulfuric acid with a concentration of 20%-50% ω(H2SO4) to form on the surface of liquid sulfur, causing corrosion, or an excess amount may lead to the formation of a hard crust. The improved method is to use the same batch mixing approach as that for adding diatomite, continuously adding a certain amount of lime or soda ash to ensure the removal of sulfuric acid from liquid sulfur. 4.4 Strengthen the management of solid sulfur: During storage and transportation, strict control should be exercised over the addition of water to solid sulfur; in particular, efforts must be made to prevent any increase in weight under the pretext of preventing the sulfur from being blown away by the wind ； It also needs to be covered to prevent sunlight and rain from accelerating oxidation and increasing the sulfuric acid content. Good ventilation must be maintained during loading, unloading, and stacking; the concentration of liquid sulfur vapor in the sulfur melting plant and the concentration of sulfur dust in the sulfur storage area must not exceed the specified limits.