1. Main harmful gases and hazards
1. 1 SO2
SO2 in waste gas from cement industry mainly comes from sulfur compounds in cement raw materials or fuels and sulfur oxides generated under high temperature oxidation conditions. For the new dry production, sulfur, like potassium, sodium and chlorine, is one of the important factors that cause the crust blockage of preheater and calciner, and it is harmful to production and needs to be limited. Because there is enough calcium and a certain amount of potassium and sodium in the cement rotary kiln, the formed sulfate has poor volatility, and more than 80% remains in the clinker, so the SO2 emitted in the waste gas is much less than that in other industrial kilns (such as electric boilers). For dry hollow kiln, vertical kiln and wet cement production process, SO2 emission is much greater than that of new dry production.
SO2 is one of the most important sulfur-containing air pollutants. SO2 is a colorless toxic gas with pungent smell, nonflammable and easy to liquefy. Sulfur dioxide is the main cause of acid rain in the world.
1.2 nitrogen oxides
Nitrogen oxides emitted during cement production mainly come from the combination of N2 in combustion air and oxidation of fuel during high temperature combustion. The amount of nitrogen oxides produced depends on the temperature of the combustion flame. The higher the flame temperature, the more nitrogen oxides produced by N2 oxidation. In the new dry production system, because 50% ~ 60% of the fuel is burned in the calciner with low temperature, the nitrogen oxides emitted by the new dry production system are far lower than those of the traditional production method. It is estimated that the annual emission of nitrogen oxides in China cement industry is about 1 10,000 tons.
Among nitrogen oxides, NO and NO2 are the two most important air pollutants.
NO is a colorless gas, a light blue liquid or a blue-white solid, which is easily oxidized into NO2 by O3 and photochemical action in the air.
NO2 is a yellow liquid or brownish red gas, which can generate nitric acid and nitrous acid when dissolved in water, so it is corrosive. The affinity of NO to hemoglobin is several hundred times greater than that of CO, and animals exposed to high concentration of NO may have central nervous system diseases. NO2 can irritate eyes and respiratory tract. Acute poisoning with high concentration of NO2 can cause tracheitis and emphysema, and even death in severe cases. NOX can also form photochemical smog, which seriously affects the visual field. See table 1 for the hazard degree of NOX grading concentration.
Table 1? Harm of nitrogen oxides to human body
Nitrogen oxide concentration (ppm)
Harm to human body
0.5
After continuous exposure for 4 hours, the pathological tissue of lung cells changed. After continuous exposure for 3- 12 months, emphysema, infection and weakened resistance appeared in bronchi.
1
Smell a bad smell
five
Smell a strong smell
65438+ 10 month 15
Eyes, nose and respiratory tract are stimulated.
80
Causes chest pain within 3-5 minutes.
100- 150
People will die of emphysema within 30 minutes at most 1 hour.
& gt200
People die instantly.
1.3 SO2, nitrogen oxides and acid rain
Acid rain is one of the chief culprits of global environmental pollution. Acid rain contains many inorganic and organic acids, most of which are sulfuric acid and nitric acid. Sulfur dioxide and nitrogen oxides are absorbed by droplets condensed by water vapor in the process of atmospheric rain formation, forming sulfuric acid raindrops and nitric acid raindrops; In the process of falling, the droplets containing acid rain continue to merge, adsorb and wash away other droplets containing acid rain and acid-containing gas, forming larger raindrops and finally falling to the ground, forming acid rain. When the acidity of acid rain is too high and the PH value drops below 5.6, it will cause serious harm. It can directly kill large areas of forests and make crops wither; It will also inhibit the decomposition of organic matter and the fixation of nitrogen in the soil, leach nutrients such as calcium, magnesium and potassium from soil particles, and make the soil barren; It can also acidify lakes and rivers, dissolve heavy metals in soil and water sediments, make them enter the water and poison fish; Accelerate the corrosion and weathering process of buildings and cultural relics; It will also endanger human health.
1.4 fluoride
When fluorite (CaF2 _ 2) and other fluorine-containing minerals are used as raw materials, one or more volatile fluorine-containing inorganic compounds will be discharged into the atmosphere at high temperature. If silicate compounds exist, it will form SiF4, which will be discharged into the atmosphere, and further hydrolyzed to generate hydrogen fluoride (HF), a colorless gas, which is a colorless liquid below 19.54℃, extremely volatile, smoky in the air, toxic, irritating to eyes and corroding skin. Anhydrous hydrogen fluoride is one of the strongest acidic substances, which is highly corrosive to ordinary steel.
Silicon tetrafluoride (SiF4) is a colorless, nonflammable gas, which is highly toxic and has a suffocating odor similar to hydrogen chloride. When hydrolyzed in humid air, silicic acid and hydrofluoric acid are produced, and smoke is produced at the same time.
When the residual concentration of fluorine-containing compounds in the atmosphere exceeds the allowable concentration, it will have a great impact on animal and plant life and even the climate.
1.5 carbon monoxide (CO)
In the process of cement calcination, due to the incomplete combustion of carbon, a small amount of CO will be produced, which is a flammable substance. When electrostatic precipitator is used to treat kiln tail exhaust gas, it often causes explosion because of the high concentration of CO in exhaust gas. CO is a colorless and odorless gas, which is extremely toxic! Not easy to liquefy and solidify, slightly soluble in water. When carbon monoxide burns, it shows a blue flame in the air, which can react with various metals or nonmetals, and react with chlorine to generate highly toxic phosgene (COCL2). Poisoning incidents occur from time to time when the shaft kiln surface or dust collector is overhauled.
1.6 carbon dioxide (CO2)
1.6. 1 CO2 gas generated in cement production process
In the process of cement production, CO2 gas mainly comes from cement clinker calcining kiln and drying equipment.
1. CO2 emitted by cement calciner comes from carbonate decomposition and fuel combustion in cement raw materials. At present, the main raw material for producing cement clinker is limestone. Ordinary Portland cement clinker contains about 65% calcium oxide. According to the chemical reaction equation (CaCO3=CaO+CO2), for every 1 ton of cement clinker produced, 0.5 1 1 ton of CO2 is generated.
2. CO2 produced by fuel combustion is related to the calorific value and quantity of fuel consumed.
When the calorific value of fuel coal used in cement plant is 22000 kJ/㎏, it contains about 65% fixed carbon. According to the chemical reaction equation:
C+O2=CO2 shows that when carbon is completely burned, 2.38 tons of CO2 are produced per ton of coal.
The fuels used in cement production are divided into clinker combustion fuel and raw fuel dry fuel. The fuel consumption of clinker burning is related to the production technology and scale of cement clinker. See Table 2 for the relationship between cement production technology, scale and heat consumption in China. The amount of fuel used for drying is related to the utilization degree of waste heat and the natural moisture of raw fuel, regardless of the utilization of waste heat by dry materials. According to the calculation that the natural moisture of the raw fuel is 18% and the raw fuel needs to be dried to produce 1 ton clinker, the coal used for drying is about 0.02 ton.
Table 2 clinker unit heat consumption corresponding to different cement production processes and scales
Technology and scale
Ordinary machine shaft kiln
Li Boer kiln
Wet kiln
Hollow kiln
Preheating kiln
Small and medium-sized precalciner kiln
Large precalciner kiln
Heat consumption/(kilojoule)
4400
3762
6072
5280
3762
3400
3 100
Coal for combustion/(ton)
0.2
0. 17 1
0.276
0.24
0. 17 1
0. 155
0. 14 1
Note: The low calorific value of coal is 22000KJ/Kg.
It can be seen that with the different production processes, it takes 0. 1 ~ 0.296 tons of coal to produce1~ 0.296 tons of clinker, that is, the CO2 produced by coal combustion and material drying varies within the range of 0.383 ~ 0.704 tons.
When the above two items are added together, for every 1 ton of cement clinker produced, 0.894 ~ 1.2 15 ton of CO2 will be emitted. According to the current average level of cement production in China, for every 65,438+0 tons of cement clinker produced, about 65,438+0 tons of CO2 will be emitted.
3. In addition, every 65,438+0 tons of cement produced in the cement production process consumes an average of 65,438+000 kWh of electricity. If the CO2 emitted by electric energy generated by coal combustion is calculated into cement production, the CO2 emitted by electric energy consumed in cement production 1 ton is 0. 12 ton. In 2007, China produced 65.438+35 billion tons of cement, including 972 million tons of cement clinker (estimated by 0.72 tons of 654.38+0 tons of cement clinker). According to this calculation, the CO2 emitted into the atmosphere by cement production in China in 2007 was about 654.38+065.438+034 million tons. The number is amazing.
Carbon dioxide and greenhouse effect
Solar short-wave radiation penetrates the atmosphere and shoots to the ground. While receiving short-wave radiation from the sun, the ground is constantly radiating long-wave electromagnetic waves to cool down. The carbon dioxide and other substances in the atmosphere can strongly absorb the long-wave radiation on the ground, and at the same time radiate longer long waves outward. The part that goes down to the ground is called inverse radiation, and the ground will heat up after receiving inverse radiation, which is the atmospheric greenhouse effect. The greenhouse gases in the atmosphere are carbon dioxide, methane, ozone, nitrogen oxides, freon and water vapor. Scientific research shows that with the continuous increase of human activities, there are more and more greenhouse gases in the atmosphere, which will make the temperature of the earth higher and higher. According to the prediction of the United Nations Environment Programme, if urgent measures are not taken to limit the emission of greenhouse gases, the global economic loss will reach more than 300 billion US dollars every year due to the sea level rise caused by frequent tropical cyclones and global warming in the 50 years from 2000 to 2050.
Global warming will also make animals and plants face survival crisis. If the migration speed of a species can't keep up with the speed of environmental change, the species is in danger of extinction.
Climate warming also directly or indirectly affects human health. People living in mid-latitude areas are most sensitive to global warming, and long-term hot summer days and high temperature and humidity weather directly threaten their health. At the same time, the warming temperature, the "urban heat island" effect and air pollution are more obvious, which provides a more suitable hotbed for the reproduction and spread of many diseases.
2. Prevention and control of main harmful gases
2. 1 sulfur dioxide pollution control technology
The measures to reduce SO2 emission in cement production are: replacing raw materials; Concentrate on raw material grinding; Adding hydrated lime-calcium hydroxide; Set D-SOx cyclone separator; Set up a water washing tower. At present, China's cement industry only adopts the method of minimizing SO2 production in the production process. Among them, the simplest and most effective method is to choose the appropriate ratio of sulfur to alkali in the new dry production line, and at the same time adopt the integrated operation of kiln and mill and bag filter.
The waste gas treatment method of kiln grinding integration is adopted to introduce the waste gas from kiln tail into the raw meal grinding system. In the raw meal mill, a large number of new interfaces are produced due to the action of external forces. The CaCO3 activity of the new interface is high, which can absorb SO2, and the temperature of the kiln tail gas is low. At the same time, due to the evaporation of water in raw materials, a large amount of water vapor exists in the raw material mill, which accelerates the absorption of SO2 by CaCO3, transforms SO2 into CaSO4, and fixes 20% ~ 70% of SO2 in kiln tail gas.
The concentration of acid gas can be reduced by 30% ~ 60% because the alkaline substances trapped on the surface of the filter bag of the bag filter combine with the acid substances of SO2 and NO2 that try to pass through the filter bag to form salt. It can be seen that the bag filter can become a multifunctional equipment for controlling dust and harmful gases in the cement industry.
2.2 Prevention and control of nitrogen oxide pollution
The main measures to prevent and control Nox are to optimize the combustion system of kiln and calciner, keep proper flame temperature and shape, control excessive air, ensure uniform and stable feed rate and coal feed rate, ensure the grate cooler to run well, and adopt low nitrogen oxide compound coal injection pipe. After taking these measures, nitrogen oxides may be reduced to below 1000mg/m3. However, in order to implement the revised new emission standard (GB49 15-2004) and consider the abnormal operation of the kiln, it is necessary to set up special nitrogen removal measures. Here is a brief introduction to ammonia reduction and denitrification.
It uses NH3 to eliminate NO in waste gas in a non-contact way. Developed by American Exxon Research Engineering Company, and patented in the former Federal Republic of Germany on 1974. Since then, it has been further developed.
The main principle of this method is that NH3 reacts with OH- to generate NH2 and H2O, and NH3 reacts with NO to generate various intermediate products and molecular nitrogen, water and other compounds, thus eliminating NO.
In addition, in the reducing atmosphere, due to the existence of reducing gases such as CO and H2, the oxidized nitrogen oxides can be reduced to harmless N2 under the catalytic action of Fe2O3 and Al2O3 existing in the raw materials, thus greatly reducing the emission of nitrogen oxides. This reaction mechanism of nitrogen oxides points out the direction of efforts for emission reduction measures of nitrogen oxides in cement kilns.
2.3 prevention and control of fluoride pollution
Fluoride produced in the process of clinker burning comes from raw materials and fuels. Some clay contains fluorine, especially in some domestic vertical kiln factories, in order to reduce heat consumption, fluorine-containing minerals (fluorite) are mixed into raw materials. During the sintering process, most fluoride, CaO and Al2O3 form calcium fluoaluminate, which is solid in clinker and a few are discharged with waste gas.
The reliable way to prevent fluoride pollution is not to use substances with high fluorine content as raw materials, let alone fluorite to reduce the firing temperature.
2.4 Carbon dioxide emission reduction
2.4. 1 emission reduction methods
1. replace other high energy consumption cement production lines with large and medium-sized new dry cement production lines.
The CO2 emission per ton of clinker produced by this kiln is 68.2%, 79.8%, 49.9%, 56.8%, 68.2% and 88% of that of ordinary vertical kiln, vertical kiln, wet kiln, hollow kiln, preheater kiln and small precalciner kiln respectively.
According to the calculation, if all the production lines with heat consumption of more than 3,400 kJ/kg clinker are transformed into large and medium-sized new dry production lines with heat consumption of less than 3,400 kJ/kg clinker, which will be used to burn cement clinker into carbon dioxide greenhouse gas, the emissions will be reduced and 9% of the total fuel will be consumed.
2. Waste heat utilization and emission reduction
(1) Dry the raw fuel. Using waste gas heat to dry raw fuel can save coal for drying, and each ton of cement clinker can save 0.02 tons of coal for drying and reduce CO2 emission by 0.0476 tons.
(2) Low-temperature waste heat power generation. At present, the new dry cement production process uses kiln tail gas to dry raw meal, which makes the raw meal mill and kiln work integrally. Generally, raw meal mill only uses 70% kiln tail gas, the rest is used for waste heat power generation, and the clinker cooled by tail gas can all be used for waste heat power generation. The average data of low-temperature waste heat power generation in some cement plants confirmed that the production of 1 ton cement clinker was 30 kwh. Although it is estimated that a new dry production line with an annual output of 6.5438+0.5 million tons (5,000 t/d) of cement clinker can reduce carbon dioxide emissions by more than 50,000 tons per year.
3. Use alternative fuels to reduce emissions
Burning cement clinker with combustible waste such as municipal solid waste instead of coal, under the condition of providing the same heat, the total amount of carbon in combustible waste is less than that of coal, and the total amount of CO2 emitted after combustion is also less than that of coal. According to the experience of using combustible waste in British and American cement industry in recent years, under the same unit heat consumption, the greenhouse gas CO2 produced by burning 1 ton clinker is generally only about half of that produced by coal.
Change the chemical composition of raw materials or clinker to reduce emissions
(1) Use substances that do not produce CO2 but contain CaO as raw materials. For example, the main chemical component of carbide slag in chemical industry is Ca(OH)2, and 1 ton anhydrous carbide slag contains 0.54 ton of CaO. If carbide slag is used as raw material for cement production, CO2 will not be emitted. Compared with using limestone containing 65%CaO as raw material for cement production, using 1 ton anhydrous carbide slag is equivalent to reducing CO2 emission by 0.425 ton; For example, blast furnace slag, fly ash and slag all contain more CaO than clay, which can reduce the proportion of limestone in the ingredients. These waste residues calcined at high temperature will not emit CO2 when producing cement. For every 1 ton of CaO provided by the above waste residue, the emission of CO2 is reduced by 0.7857 ton. If calcium carbide slag is used to provide CaO in cement clinker, the emission reduction per ton of cement clinker is 0.5 1 1 ton of CO2. A 2000t/d new dry process production line completely replaces limestone with carbide slag, which can reduce CO2 emission by 306,600 tons per year. In addition, using waste residue as raw material to produce cement can also reduce the sintering temperature of clinker, thus reducing coal consumption and CO2 emission.
(2) Reduce the content of CaO in cement clinker. At present, the research and development of low-calcium cement clinker system at home and abroad, that is, reducing the content of CaO in clinker composition, correspondingly increasing the content of low-calcium belite minerals, or introducing new cement clinker minerals, can effectively reduce the sintering temperature of clinker, reduce the consumption of limestone raw materials and reduce the heat consumption of clinker sintering. Low calcium and high belite cement can reduce CaO in clinker to 45%, which is about 10% (about 0. 16 tons) lower than that of existing portland cement clinker.
5. Improve the quality of cement and concrete to improve the strength of clinker and reduce the clinker content in cement.
(1) Reduce the amount of cement clinker. Reducing the amount of cement clinker is manifested in two aspects: one is to mix more additives while grinding cement, and the other is to use alternative cement materials when mixing concrete. At present, about 40% of cement in China has been replaced by ground blast furnace slag, and a foreign research unit has replaced more than 80%. The research on high-volume fly ash cement in China provides a technical way for the cement industry to reduce CO2 emissions.
(2) Vigorously develop green high-performance concrete to replace conventional concrete. In 1994, Academician Wu Zhongwei put forward the concept of green high performance concrete (GHPC). GHPC has the following characteristics: ① A large amount of cement clinker is stored. In GHPC, not clinker cement, but ground water-quenched slag, graded high-quality fly ash and silica fume or their combination become the main components of cementitious materials, thus greatly reducing raw materials, energy consumption and CO2 emissions. (2) A large number of fine admixtures, composite fine admixtures and composite additives, mainly industrial waste residues, are used to replace part of clinker, thus reducing pollution and protecting the environment. Foreign countries have used ground slag and high-quality fly ash to replace more than 50% clinker to prepare high-performance concrete. (3) Give full play to the advantages of HPC, and reduce the cross-sectional area or volume of the structure by improving the strength, and reduce the amount of concrete, thus saving the cement output.
(3) Develop high-grade cement. Establish building quality standards, expand the production of high-quality, high-grade cement and other building materials products, and promote the development of advanced production technology.
6. Introduce and develop more advanced sintering technology.
The theoretical heat consumption of clinker is about 1759kJ/kg. After the invention of cement pre-decomposition technology in 1970s, the preheating system was further improved, and the clinker heat consumption was reduced to 2929 kJ/kg, and the thermal efficiency reached 60%. In order to further reduce the heat consumption of clinker, it is necessary to develop new kiln types, such as fluidized kiln for fluidized bed calcination, and take a series of other auxiliary measures, such as improving preheater system, improving heat exchange efficiency and reducing resistance loss.
Boiling calcination process is considered to be the most advanced technology for calcining cement clinker at present. Its main feature is to cancel the rotary kiln and complete the cement calcination in a fluidized bed with higher heat transfer efficiency. With small floor space and high thermal efficiency, the emissions of nitrogen oxides and CO2 will also be reduced.
2.4.2 Use of clean development mechanism
The clean development mechanism (CDM) originated from the "Clean Development Fund" proposed by Brazil, which was set up by collecting fines submitted by developed countries for failing to fulfill their greenhouse gas emission reduction obligations. The cooperation mechanism currently established in Article 12 of Kyoto Protocol was reached through negotiation, which is one of the three flexible compliance mechanisms introduced by Kyoto Protocol to reduce greenhouse gas emissions. CDM allows parties and non-parties to jointly carry out greenhouse gas emission reduction projects such as carbon dioxide, and the emission reductions generated by these projects can be used by developed countries to fulfill their commitments to limit or reduce emissions. In other words, developed countries help developing countries to reduce greenhouse gas emissions by providing funds and environmental protection technologies, and at the same time buy CERs from developing countries to fulfill their emission reduction obligations under the Kyoto Protocol. For developed countries, the clean development mechanism project implemented in cooperation with developing countries provides a flexible and low-cost implementation method; For developing countries, some financial assistance and advanced technology can be obtained through CDM projects.