Discussion on mercury emission reduction measures

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Discussion on mercury emission reduction measures in the cement industry

the 2013 global mercury assessment report released by the United Nations Environment Programme (UNEP) [1] pointed out that the global anthropogenic mercury emissions in 2010 were about 1960t, of which the mercury emissions from the cement industry were the fourth largest source of mercury emissions after manual small-scale gold mining, coal combustion and metal smelting, accounting for 9% of anthropogenic mercury emissions. According to the analysis of the research results of the Chinese Academy of Environmental Sciences, it is estimated that the annual mercury emission of China's cement industry is about 89-144t, which is the third largest mercury emission source after coal-fired and non-ferrous metal smelting

due to the serious harm caused by mercury pollution to human beings and the environment, many countries and regions in the world have formulated corresponding laws and regulations for the mercury emission of cement industry used to evaluate the long-term anti-wear function of smooth oil and grease. In August, 2010, the national emission standard of hazardous pollutants (NESHAP) of the United States was officially enacted. This standard takes the 30 day average mercury emission of cement kilns as the benchmark, and stipulates that the emission standard of cement plants built before 2010 is 8-13 μ G/m3 the emission standard of cement plants built after 10 years is 3-5 μ G/m3[2], the mercury emission limits of cement plants in EU, South Korea, India and Australia are 50 respectively μ g/m3、100 μ g/m3、200 μ G/m3 and 1000 μ g/m3[3]。 At the end of 2013, China issued the emission standard of air pollutants for cement industry (gb4915-2013) and the pollution control standard for collaborative disposal of solid wastes in cement kilns (gb30485-2013), which listed Hg and its compounds, Sox, NOx and particulate matter as one of the pollutants restricted by the cement industry, and the maximum allowable emission concentration is limited to 50 μ g/m3[]。

at present, the global cement industry mainly takes the following measures to control mercury: the first is to control raw fuels; The second type is to use the kiln ash discharge technology for control; The third is to use the existing pollutant control measures for collaborative control; The fourth category is the use of specialized technology for control. This paper will introduce them respectively, and put forward the best feasible technology to provide reference for mercury emission reduction in the cement industry

1 control raw fuels

raw fuels are the source of mercury pollution. Careful screening and control of raw fuels entering the cement kiln is of great significance to reduce mercury emissions [6]. Raw materials for cement production mainly include calcareous raw materials, clayey raw materials, correction raw materials and additives, and low-grade raw materials and industrial waste residues can also be used. The fuel is mainly coal. China's technical code for environmental protection of solid waste collaborative disposal in cement kilns (hj662-2013) stipulates that the maximum allowable dosage of mercury in kiln materials (including conventional raw materials, fuels and solid waste) should not be greater than 0.23mg/kg clinker [7]. However, the data of mercury content in raw materials and fuels for cement production in China are still relatively few. Zhang Le [8] tested the mercury content of limestone, raw meal and clinker of a production line, which are 20ng/g, 15ng/g and 1ng/g respectively; Wang Qichao [9] et al. Tested that the average mercury content in Chinese coal is about 0.22mg/kg, which is higher than the world average mercury content of 0.13mg/kg. See Table 1 for mercury content of raw fuel for cement production abroad

it can be seen from table 1 that the mercury content of different raw materials varies greatly, and the mercury content of the same raw materials also varies greatly. Cement industry calcareous raw materials to 75% of cement raw materials 2 Rotate the screw on the zigzag rod - 80%. It can be seen from table 1 that the mercury content of calcareous raw materials varies greatly. Therefore, the mercury content in limestone should be regularly monitored during the site selection of cement plant and limestone mining. The mercury content in sewage sludge is very high, so 3 is to stimulate the innovation ability of small and medium-sized enterprises. When the cement plant cooperates to dispose of sludge, the disposal amount of sludge should be limited. The mercury content of different types of coal varies greatly, so low mercury coal should be selected as fuel as far as possible, and high mercury coal should be pretreated to be used as fuel for cement production. Other raw materials are generally purchased from the origin and transported to the cement plant. Low mercury materials can be used. Some materials, such as kiln ash, have great changes in mercury content, so they should be carefully selected during use. Usually, the cement plant will give a detailed description of the raw fuels it uses, including the restrictions on mercury, and usually specify the analysis cycle of raw fuels to ensure that they are within the limits when replacing raw fuels

2 the new dry process cement kiln is a typical thermal kiln. The flue gas temperature in the rotary kiln and preheater is much higher than the volatilization temperature of mercury. In the process of cement firing, almost all mercury brought in by raw fuels is converted into elemental mercury, which is volatilized in the preheater, rarely brought out of the system with clinker, and all enter the waste gas in the form of mercury vapor. When the raw mill is turned on, the waste gas is used to dry the raw material. Because the temperature of the raw mill system is far lower than the volatilization temperature of mercury, mercury vapor condenses and is adsorbed by dust particles, so mercury is mainly adsorbed by dust in the dust collector. The dust collected by the dust collector is returned to the kiln system as raw material, resulting in the circulation and enrichment of mercury in the kiln system and fly ash. When the raw material mill is closed, the waste gas is no longer used to dry the raw material, but is directly discharged from the chimney through the dust collector. Therefore, when the raw material mill is closed, the contact time between mercury in the flue gas and dust particles is short, the mercury emission in the flue gas increases, and the mercury content in the kiln ash decreases [18]

in order to limit the circulation and enrichment of mercury, the kiln ash rich in mercury can be removed regularly. After being taken out of the kiln, this part of ash can be introduced into the cement mill for cement production, but the mercury concentration in the kiln ash needs to be limited. In order to achieve high efficiency of kiln ash discharge technology, the temperature of flue gas should be lower than 140 ℃, because at this time, the adsorption of mercury on particles is significantly higher than that at higher temperature. Under the condition of fixed ash amount, more mercury can be removed. When the raw mill is turned on, the flue gas temperature in the dust collector is between 90-120 ℃. When the raw mill is shut down, the flue gas temperature in the dust collector is between 140-170 ℃, even as high as 200 ℃. Therefore, when the raw mill is shut down, the flue gas needs to be cooled to 120-140 ℃ through the cooling tower. Through this technology, mercury emissions can be reduced by 10% - 35% [19]

3 collaborative control using existing pollutant control measures

3.1 wet desulfurization technology

wet desulfurization technology is the most widely used desulfurization technology in the world. Its main principle is to add limestone powder into slurry as absorbent, pump it into the absorption tower and fully contact and mix it with the flue gas. The sulfur dioxide in the flue gas reacts with calcium carbonate in the slurry and the air blown from the bottom of the tower to produce calcium sulfate, When calcium sulfate reaches a certain saturation, it crystallizes to form dihydrate gypsum. The gypsum slurry discharged from the absorption tower is concentrated and dehydrated to make its water content less than 10%, and then sent to the gypsum silo by conveyor for stacking. These gypsum can be used for cement production again

mercury in flue gas is mainly divided into gaseous elemental mercury, divalent mercury and particulate mercury. The mercury released when the raw meal is calcined in the kiln is gaseous elemental mercury. When there are other elements (chlorine, bromine, iodine, sulfur) in the flue gas, the gaseous elemental mercury is converted into divalent mercury (HgCl2, HGO, HgBr2, HgI2, HgS and HgSO4) at high temperature, and the mercury condensed on the particle surface in the flue gas becomes granular mercury. Among them, divalent mercury has good water solubility and can be absorbed by the aqueous slurry in the absorption tower, so that a part of divalent mercury can be effectively removed. However, gaseous elemental mercury has poor water solubility, so it cannot be removed by this technology. Gaseous elemental mercury can be oxidized to divalent mercury, thereby improving the removal efficiency [20]

wet desulfurization technology is mainly used in thermal power plants, but less in cement plants. At present, five cement plants in the United States have used this technology. According to research, the removal efficiency of this technology for divalent mercury can reach more than 80% [21]. However, the use of this technology will increase energy consumption, increase waste output, increase CO2 emissions, increase water consumption, increase operating costs, and may cause mercury pollution to water

3.2 selective catalytic reduction technology

selective catalytic reduction technology is the most mature flue gas denitration technology, which uses NH3 or urea as reducing agent to selectively react with NOx on the catalyst surface at 300-400 ℃ to generate N2 and H2O, thereby reducing NOx emissions. Catalysts are generally made into three types: honeycomb, plate or corrugated, with TiO2 as the carrier and V2O5 or V2O5-WO3 or v2o5-moo3 as the active ingredient. On the surface of the catalyst, a part of gaseous elemental mercury will be oxidized to divalent mercury, which can be removed by subsequent wet desulfurization technology, which means that selective catalytic reduction technology can convert gaseous elemental mercury into a form that is easy to capture [22]. This technology is the mainstream technology of flue gas denitration in power stations. However, due to the high construction and operation costs, this technology is rarely used in the cement industry. At present, only a few cement plants in the world adopt this technology, and this technology is in the stage of demonstration and application in China

3.3 dust collector technology

dust collector technology includes bag type dust collector technology and electrostatic precipitator technology. The particulate mercury condensed on the dust surface in the flue gas is also collected into the dust as the dust is removed by the dust collector, thereby reducing the emission of mercury in the atmosphere. The study of dust collectors in different power stations shows that [23], the efficiency of removing mercury from flue gas by dust collectors is not only related to the adsorption performance of mercury by fly ash particles, but also related to many factors such as carbon content of fly ash and flue gas composition. The content of chlorine and sulfur in coal will also affect the mercury removal performance of dust collectors. The removal efficiency of mercury in the flue gas of two power stations equipped with bag type dust collectors is about 80% and 20%, and the removal efficiency of mercury in the flue gas of three power stations equipped with electrostatic precipitators is about 4%, 6% and 20%. ICR tests of 84 different coal-fired power plants in the United States showed that the mercury capture rates of electrostatic precipitator for bituminous coal and sub bituminous coal smoke were 46% and 16% respectively, and bag filter for smoke 2 Meng shuaiqi, et al: Discussion on mercury emission reduction measures in cement industry mercury capture rates in coal and sub bituminous coal flue gas are 83% and 72% respectively [24]. It can be seen that bag filter is more effective in removing mercury from flue gas. At present, dust collector technology is widely used in China's cement industry, which effectively reduces the emission of particulate mercury

4 special technology for control

4.1 brominated activated carbon injection technology

this technology mainly sprays brominated activated carbon adsorbent before the dust collector. The gaseous elemental mercury in the flue gas is first oxidized by the bromide on the activated carbon, then adsorbed by the activated carbon, and finally captured in the dust collector. This technology makes full use of dust removal devices to jointly remove mercury, which is the most mature mercury removal technology. If the activated carbon dust collected by the dust collector is used in cement production, attention should be paid to their impact on the quality of cement. In most cases, it is not necessary to spray adsorbent when the raw mill is running, because the capture of mercury by the raw mill will control the emission of mercury to a reasonable level. The adsorbent is usually injected when the raw mill stops running, so as to reduce the peak emission. The mercury removal efficiency of this method can reach 80%, and it can remove SO2, organics, HCl, HF. Therefore, at present, the best-selling fixture in the fixture industry is the manual fixture, which has a certain removal effect, but the operation cost is high. In order to avoid the mixing of mercury loaded activated carbon and dust, brominated activated carbon is sometimes injected into the flue behind the main dust collector and the injected adsorbent is captured by the secondary dust collector, which requires additional costs, so it is not common in the cement industry [25]

4.2 mercury calciner technology

mercury calciner technology [] refers to the use of mercury rich dust collected by the dust collector to clean the mercury calciner process, and then return the fly ash to the kiln system. In this patented mercury

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