The cement industry is a typical energy and resource-consuming industry. Coal and cement raw materials will produce SO2 during the high-temperature calcination process. The results of national sampling in 2013 showed that the average SO2 emission concentration of cement companies was 59.0 mg/Nm3, with a maximum of 310 mg/Nm3 . my country's cement production has been ranked first in the world in recent decades, accounting for about 50% of the world's total cement. As of the end of 2019, my country has 1,624 new dry process production lines, and the national cement output is 2.33 billion tons. Therefore, the SO2 emission problem of my country's cement industry cannot be ignored. In order to strengthen the SO2 emission control of the cement industry, the country has formulated the corresponding emission standard GB 4915-2013 "Cement Industry Air Pollutant Emission Standard", which requires that the SO2 emission in cement production does not exceed 200 mg/Nm3, Guangdong, Shandong, Beijing , Tianjin, Hebei, Zhejiang and other places require SO2 emissions not to exceed 100 mg/Nm3, and ultra-low emissions control to 35 mg/Nm3. The further tightening of pollutant emission standards in the cement industry is an inevitable trend for future industry development, which directly affects the benefits of cement companies and even threatens their survival. Therefore, increasing the innovation of desulfurization technology in the cement industry, improving desulfurization efficiency, and reducing desulfurization costs are issues that cement companies need to solve urgently.

1 Sources of SO2 emissions from cement companies

The SO2 emissions of cement companies are mainly sulfur brought in from raw materials (see Figure 1). The sulfur in raw meal exists in the form of organic sulfur, sulfide and sulfate, and the sulfides are mainly pyrite and marcasite (both pyrite and marcasite). For FeS2), sulfates mainly include gypsum (CaSO4·2H2O) and anhydrite (CaSO4). Sulfur in the form of sulfide and organic sulfur is oxidized to SO2 at 300~600 ℃, which mainly occurs in the second-stage cyclone of the five-stage preheater or the third-stage cyclone of the six-stage preheater; as sulfate The sulfur in the form will be decomposed in the calciner and rotary kiln, and most of the generated SO2 will be absorbed by the calcium oxide in the calciner. Therefore, the SO2 emission level of cement production mainly depends on the content of sulfide and organic sulfur in the raw meal, and has nothing to do with the content of sulfate. In the preheater of the cement precalcining kiln system, when the temperature is lower than 600 ℃, the absorption efficiency of CaO to SO2 is much higher than that of CaCO3. In the first and second stage preheaters, the decomposition rate of CaCO3 is low, the flue gas contains less CaO, and the absorption efficiency of SO2 is very low, which leads to the increase of SO2 emission concentration. Therefore, how to oxidize SO2 in the preheater to SO3, and at the same time promote the absorption of SO3 by the raw meal in the preheater, and ultimately reduce the concentration of SO2 in the primary and secondary preheaters is the key to the desulfurization agent technology.

2 Current status of desulfurization technology

At present, the preferred method of reducing SO2 emissions commonly used by cement companies is to minimize the production of SO2 during the production process. For example, using low-sulfur raw materials and coal, low-sulfur raw materials and coal bring less sulfur into the production line system, which can reduce SO2 from the source. produce. The second is the integrated operation of the kiln and mill. During the cement production process, the raw mill and the kiln operate at the same time. The waste gas generated in the firing system enters the raw mill system, and the raw meal (limestone) always produces a fresh surface during the grinding process. At the same time, the raw meal and waste gas have a certain residence time in the mill, and the waste gas heats the raw meal to a certain extent, so that most of the SO2 in the waste gas is adsorbed on the surface of the raw meal. In addition, the bag filter removes dust and desulfurizes. The gas in the bag dust is in close contact with the material, and the relative humidity is high, which can better adsorb SO2. The cement kiln system inherently has a complete desulfurization system. Only when the sulfur content in the raw materials and coal is too high or the raw meal mill stops grinding, additional desulfurization measures are required. The currently used desulfurization methods mainly include wet, semi-dry, dry and ammonia desulfurization, as shown in Table 1.

Wet desulfurization is a kind of desulfurization technology that uses liquid absorbents such as alkaline solutions to scrub the tail gas to remove SO2 in the tail gas. The desulfurization efficiency can reach about 95%. Many domestic cement plants have adopted wet desulfurization technology. The problems with this technology are: large area and high investment; equipment is easy to wear, corrode and produce secondary waste. This desulfurization method is suitable for cement companies with very high long-term sulfur emissions.

Dry desulfurization and semi-dry desulfurization are also used by some manufacturers. The process is relatively simple, but the desulfurization effect is limited and unstable.

For ammonia water desulfurization, from the national spot check results, it can be known that the SO2 emission concentration of most domestic cement kilns ranges from 0 to 500 mg/Nm3. Usually, there is no need to choose a wet desulfurization process. The ammonia water desulfurization process is more appropriate. Ammonia water desulfurization is to atomize ammonia water with a high-pressure spray gun at the position of the secondary cylinder of the preheater and spray it into the preheater to react with SO2 in the preheater to produce ammonium sulfate and other products. The flue gas is removed, and part of it flows to the rotary kiln along with the material. The advantage of this method is that the equipment cost is low, the use is convenient and flexible, and the dosage can be adjusted according to the level of sulfur content, which can effectively solve the problem of excessive sulfur emissions in cement companies with sulfur emissions within 500 mg/Nm3. The disadvantage is that the ammonia water is alkaline and heavy. , It is highly corrosive to equipment, and it is easy to cause ammonia to escape and form secondary pollution.

3 Desulfurization mechanism of desulfurizer

Through the analysis of the production of SO2 and the current desulfurization technology, it is proposed that the technical scheme of the project should meet the requirements of flexible and simple use, high desulfurization efficiency, low investment cost, and harmless to humans and equipment. Based on the formation mechanism and emission process of SO2 in cement kilns, desulfurization is achieved by catalytic activation, high-efficiency adsorption, etc. The catalytic reaction can reduce the activation energy of the reaction, and the adsorption not only provides the basis for the adsorption and oxidation of SO2, but also Catalytic activation provides a solid phase medium for the reaction. Therefore, the synthesis of unique polymer materials has abundant groups on the main chain, which in turn can provide more adsorption sites.

In summary, the desulfurizer should be composed of three parts: oxidant, surfactant and polymer activated adsorbent. The sulfide in the raw meal is oxidized at high temperature to produce SO2. The oxidant improves the reaction efficiency of the conversion of SO2 to SO3 and the conversion of sulfite to more stable sulfate. The polymer activator can reduce the reaction temperature of these two processes, and at the same time, the polymer Certain groups can directly capture SO2 to generate organic sulfates. Surfactants can promote the absorption of sulfur dioxide by carbonates or metal oxides. The sulfur in the raw materials is oxidized at 300~600 ℃ to produce SO2 gas. When the temperature near the secondary cylinder is lower than 600 ℃, the absorption of SO2 mainly depends on CaO. However, the decomposition rate of CaCO3 in the second and third-stage cyclones is extremely low, and sufficient CaO is not generated, so the absorption efficiency is not high, causing SO2 to flow to the second-stage cyclones. Adding desulfurizer near the secondary cylinder can effectively improve the material's absorption efficiency of SO2. The desulfurizer is sprayed into the preheater through a high-pressure atomizing nozzle and then mixes with the raw meal thoroughly along with the rotating airflow. The desulfurizer is adsorbed on the surface of the suspended raw meal and promotes the following reactions. The main reaction equation of the process is as follows:

(1) High temperature oxidation of sulfide in raw meal to SO2

2S2- 2 +5O2 → 4SO2+2O2-

(2) Speed up the reaction rate of SO2 oxidation by oxidant

2SO2 + O2 → 2SO3

(3) Increase reaction activity and increase reaction rate through polymer

XmO+SO3 Xm SO4

(4) Polymer captures SO2 directly

mSO2 +(CxHyOz)n+mO2 →(CxHyOz)n(SO4)m

4 Usage plan

The desulfurizer is a transparent liquid (slightly yellow) with a density of 1.18~1.19 kg/L, added according to the mass ratio of raw meal 0.03%~0.05%. The way to add is as follows:

(1) Adding at the conveyor belt (see Figure 2): The desulfurizing agent is evenly sprayed on the limestone through a metering pump. After the raw meal mill, the milling efficiency of the raw meal mill can be promoted, and the desulfurizing agent and the raw meal can be fully mixed at the same time. Improve the ability to adsorb SO2. This addition method is suitable for enterprises with high sulfur content in raw materials and long-term high SO2 emissions.

(2) Adding at the cyclone of the secondary preheater: the desulfurization rate reaches more than 90%. The desulfurizing agent is added to the cyclone through an atomizing nozzle, and is adsorbed on the surface of the suspended raw meal. The materials and airflow in the cyclone move in a swirling motion, and the desulfurizing agent directly reacts with SO2 gas. This method has the advantages of short desulfurization time and quick effect.

5 Industrial applications

5.1 Guizhou HPJF Cement Company

The raw materials of Guizhou HPJF Cement Company contain a large amount of sulfides, resulting in high SO2 emissions. Without the use of desulfurizers, the average emissions are 550 mg/Nm3. The emission is relatively high when the raw mill is turned on, and the peak SO2 emission even exceeds 1 000 mg/Nm3 when the raw mill is shut down. The purpose of this industrial application is to verify the desulfurization range and desulfurization rate of the desulfurizer. The data collected in the company's industrial test in September 2020 are shown in Table 2.

It can be seen from Table 2 that under the condition of normal and stable operation, continuous test data collection shows that the overall desulfurization efficiency of the desulfurizer is relatively high, which is reduced from 550 mg/Nm3 to 30 mg/Nm3, and has no adverse effects on kiln operation and clinker. . Figure 3 shows the data change between the desulfurization agent content and SO2 emissions when the raw meal mill starts and stops.

It can be seen from Figure 3 that the raw meal mill has a great impact on the desulfurization of the kiln system. When the raw meal mill shuts down, the SO2 emission value rises to 585 mg/Nm3 and has a rising trend. In the case of increasing the amount of desulfurization agent, a large amount of SO2 emissions can be effectively controlled, and the effect is rapid, which well verifies that the desulfurization range of the desulfurization agent is above 600 mg/Nm3, and the effect is rapid and the effect is significant.

5.2 Zhejiang XKNF Cement Co., Ltd.

The SO2 emission of Zhejiang XKNF Company basically meets the requirements of the national standard when the raw meal is milled, but the SO2 exceeds the standard when the raw meal is stopped. This industrial application aims to solve the problem of SO2 exceeding the standard when the raw mill is shut down and to achieve ultra-low SO2 emissions when the raw mill is turned on. The data collected in the company's industrial test in April 2020 are shown in Table 3.

It can be seen from Table 3 that after adding the desulfurizer, SO2 is reduced from 330 mg/Nm3 to 25 mg/Nm3, the desulfurization effect is obvious, it has no influence on the operation of the kiln, and will not increase other burdens on the operation of the kiln, meeting the functional design goals of the product .

Figure 4 lists the data changes between the desulfurization agent content and SO2 emission when the raw meal mill starts and stops. It can be seen that with the shutdown of the raw meal mill, the SO2 emission rises rapidly; the desulfurization effect of the desulfurizer increases with the admixture. The amount increases. It achieves ultra-low emission effect when the raw mill is turned on, and can effectively control SO2 emissions to meet national standards when the raw mill is shut down. It is simple and flexible to use. According to the level of SO2 emissions, the amount of desulfurizer can be adjusted at any time to effectively control the cost of desulfurization and desulfurization. The effect has reached the goal of industrial application.

6 Conclusion

Through the study of the desulfurization mechanism of the polymer environmentally friendly desulfurizer, the analysis of industrial application data and the comparison with the current desulfurization process adopted by cement companies, the source and path of SO2 emissions from cement companies are clarified, and the feasibility and science of this desulfurization product process are verified It has opened up a new way for cement enterprises’ desulphurization work. The advantages of this project are summarized as follows:

(1) Compared with the traditional end desulfurization treatment technology, based on the production reasons and mechanism of SO2 emission, the front-end treatment has been independently developed, and the polymer environmental protection desulfurization technology (oxidation, catalysis, capture) is adopted to directly adsorb SO2 and improve the SO2 absorption of the original system ability.

(2) The introduction of polymer materials during the reaction greatly improves the desulfurization efficiency (over 90%), and the desulfurization range is above 600 mg/Nm3. The addition amount is small, and the use cost is low.

(3) It takes effect quickly, it can take effect 15~20 minutes after adding, and it is convenient to dosing and flexible to use. It can be added when the SO2 rises due to the shutdown of the raw mill (or the use of high-sulfur raw materials), and it can be used less or stopped when the raw mill is started (using low-sulfur raw materials).

(4) Compared with wet desulfurization, it basically does not require hardware investment. Compared with ammonia desulfurization, this product process is non-corrosive to equipment, safe and environmentally friendly, and does not cause secondary pollution.