A new breakthrough in cement kiln dry desulfurization technologyDate: 2019-08-20 Views:
SO2 is one of the main air pollutants in cement production.SO2 emission level of cement industry has been strictly limited at home and abroad.With the large-scale use of low-quality raw materials and alternative fuels with higher sulfur content in the future, more sulfur will be brought into the cement production process, which will inevitably lead to the continuous increase of SO2 emission level.Adopting proper desulfurization technology to control SO2 emission concentration is an important problem facing cement enterprises.
1.Overview of cement kiln dry desulfurization technology
Desulfurization technology is usually divided into dry desulfurization, semi-dry desulfurization and wet desulfurization.Due to the special environment of cement production, the emission concentration of SO2 in most cement production lines is not high [2-3]. Therefore, dry desulfurization technology is widely used in cement industry to control the emission level of SO2.Compared with the traditional high temperature dry desulfurization (> 800℃), the existing dry desulfurization technology of cement kiln usually refers to feeding the hot raw materials of the decomposition furnace or the desulfurization agents such as CaO and Ca (OH) 2 purchased by others through the bucket elevator to the preheater together with the raw materials of the kiln.Cement kiln dry desulfurization technology usually has a certain desulfurization effect, but there are significant differences in desulfurization parameters between different enterprises.
Some enterprises use the hot raw materials produced by the decomposition furnace as dry desulfurizer.Some enterprises use Ca (OH) 2 as dry desulfurizer.The desulfurization efficiency of hot raw materials in the decomposing furnace is generally about 30%.The desulfurization efficiency of Ca (OH) 2 can reach about 70%, but the molar ratio of Ca/S varies greatly in different enterprises.In general, the desulfurization efficiency of hot raw materials is low, and the operating cost is generally low.The purchased Ca (OH) 2 has high desulfurization efficiency, but its operation cost is often high.
2.The main factors affecting the dry desulfurization efficiency of cement kiln
Cement kiln dry desulfurization reaction belongs to non - catalytic gas - solid reaction.Take CaO as a desulfurizer for example. In addition to the reaction on the surface of CaO particles, the reaction gas will react with the pore surface in the process of diffusion to the internal pores of CaO.As the molar volume of CaSO4 (46.0 cm3/mol) is 2.72 times that of the reactant CaO (16.9 cm3/mol), product layer will gradually form on the CaO surface as the reaction progresses, and the growth of the product layer will gradually clog the pores, causing the resistance of gaseous reactant SO2 to diffuse to the unreacted CaO surface to increase sharply, thus seriously affecting the reaction to continue.Therefore, cement dry desulfurization reaction is affected by temperature, CaO pore structure, diffusion of other gas phase components, reaction time, etc.
(1) influence of temperature
(2) the effect of CaO pore structure
(3) influence of CO2 concentration of flue gas
(4) the effect of reaction time
For the traditional cement kiln dry desulfurization technology, the reaction has the following characteristics: the temperature is low, the main reaction area is C1~C2 preheater, the temperature is about 300~500℃;CaO has limited specific surface area, which is usually the raw material of commercial CaO or decomposing furnace without special treatment.CO2 concentration is high, and the CO2 volume concentration in cement preheater is usually more than 30%, which has a significant impact on desulfurization reaction.The reaction time is short. As the desulfurization reaction mainly occurs in the rising flue of C1~C2 preheater, its effective reaction time is usually less than 1~2 s.Based on the above reasons, the dry desulfurization reaction efficiency of cement kiln is usually low, so it is only applicable to cement enterprises with low initial emission concentration of SO2.When the SO2 emission concentration of cement enterprises is high, the traditional dry desulfurization technology of cement kilns often cannot reach the desulfurization target.
3.High efficiency desulfurization technology for hot raw material in cement kiln
Aiming at the problems existing in the traditional dry desulfurization technology of cement kilns, a high efficiency desulfurization technology for hot raw materials in cement kilns was developed.On the basis of the traditional dry desulfurization with hot and fresh materials in decomposing furnace, this technology optimizes the dry desulfurization system of cement kiln by means of computational fluid dynamics (CFD) numerical simulation, and significantly improves the desulfurization reaction efficiency.Specifically, the technology has the following characteristics:
(1) the mixing effect of SO2 between hot raw material and flue gas is strengthened
Through CFD simulation and other advanced means, CFD simulation optimization was carried out on the position of hot raw material taking, system optimization arrangement, and feeding position of hot raw material, which promoted the mixing of hot raw material and SO2 in flue gas and helped to improve the desulfurization reaction efficiency.
(2) the desulfurization reaction time is prolonged
Through the optimization of the traditional cement kiln dry desulfurization system, the effective desulfurization reaction area is greatly expanded and the desulfurization reaction time is prolonged.The desulfurization reaction time is extended from the traditional 1~2 s to 8~10 s, which significantly improves the desulfurization reaction efficiency.
(3) improved the reactivity of desulfurizer
Direct extraction of hot raw materials from the decomposition furnace contains uncooled oxides such as CaO, K2O, Na2O, etc., which has the advantages of high reactivity, developed pore structure inside particles, large specific surface area, etc., so the reaction rate with SO2 is higher.
The application in many cement enterprises shows that this technology has the following advantages:
(1) high desulfurization efficiency
(2) low operating cost
(3) high operational reliability
(4) low investment cost
(5) almost no effect on kiln system
Through optimization of kiln system and desulfurization system, the amount of thermal biomass used by this technology is very small, which has no obvious impact on the output and quality of the kiln system, and almost no impact on the overall heat consumption of the kiln system, while ensuring the desulfurization efficiency.
4. To summarize
Dry desulfurization has the advantages of low investment cost, high operational reliability and simple technological process. However, the traditional dry desulfurization technology of cement kiln is affected by temperature, desulfurizer pore structure, flue gas CO2 concentration, reaction time and other factors, resulting in low desulfurization efficiency.In this paper, a cement kiln raw thermal efficient desulfurization technology, it, on the basis of the principle of dry desulfurization via strengthening the desulfurization reaction mixture effect, the longer reaction time, improve the activity of desulfurization agent, which can realize SO2 reduction efficiency reached 80%, at the same time has a low investment cost, operation cost is almost zero, the high reliability is outstanding advantage, for the cement enterprise to carry out the SO2 emission reduction provides a new way.
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