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Detailed analysis of many pollutants in flue gas of cement plant

Date:2019-08-20 14:57 Source:cementepcViews:

In recent years, a variety of waste gas pollution has led to the frequent occurrence of serious haze weather, let people suffer greatly.PM2.5 is one of the main causes of smog, and it is no doubt that a large amount of exhaust from thermal power generation, steel, cement and other sources as well as automobile exhaust emissions are the main culprits.Reducing pollution sources and reducing the emission of air pollutants is the fundamental way to solve the haze.
 

1. Analysis of pollutant types and generation mechanism


In 2014, China's cement output reached a historical peak of 2.5 billion tons. In 2017, China's total cement output reached 2.316 billion tons.There are about 3 465 enterprises above the scale, and about 1 700 new dry cement clinker production lines.Even if all cement enterprises fully implement the current national environmental protection emission standards, the industry surplus problem has been solved, the pressure on local environmental capacity is still huge.
 

The smoke pollutants from cement production mainly include the following:


(1) the dust (PM);

(2) harmful gases, such as SO2, NOx, CO2, fluoride and Hg;

(3) HCl, HF, TOC, dioxins (PCDD, PCDF), heavy metals (Tl, Cd, Pb, As, Be, Cr, etc.) generated by collaborative waste disposal.

The main sources of pollutants are analyzed.

 

1.1 dust (PM)


The generation mechanism of dust (PM) is relatively simple. All kinds of raw materials produce dust in the process of crushing, grinding, calcining, transportation, loading and unloading, etc., and are discharged with the process ventilation.Another part of dust is dust generated in the process of reverse transportation, stacking storage and homogenization of materials, which belongs to unorganized discharge.
 

1.2 sulfur dioxide (SO2)


Sulfur dioxide (SO2) mainly exists in the flue gas of kiln tail.Sulfur comes from two main sources: raw materials and fuels.The sulfur in the raw materials is in the form of organic sulfide, sulfide, or sulfate.Most of the sulfides are pyrite and white iron ore (FeS2), and some elemental sulfides (such as FeS).Sulfates mainly include gypsum (CaSO4·2H2O) and anhydrite (CaSO4).Sulfide oxidizes to form SO2 gas at 300~600 ℃, which mainly occurs in the secondary or tertiary cylinder of the preheater.

Sulfate minerals are stable at temperatures below the firing zone, do not decompose in the preheater, and generally enter the kiln system.Sulfur is found in fuels in the same form as in raw materials: sulfur compounds, sulfates, and organic sulfur.The coal is burned in the calciner or rotary kiln, and the calciner has a large amount of active CaO. Meanwhile, the temperature of the calciner is the best range for desulfurization reaction. Therefore, SO2 gas generated in the calciner can be absorbed by CaO or combined with alkali to form sulfate in the transition zone and calciner.That means that normally, the sulfur in the fuel rarely affects sulfur emissions.

 

1.3 nitrogen oxides (NOx)


Nitrogen oxide (NOx) is produced in the combustion process of pulverized coal and mainly exists in the flue gas of kiln tail.There are three types of NOx: thermal type, fast type (also called instantaneous type) and fuel type.

Due to the high temperature conditions in the rotary kiln, no matter what measures are taken, there is still a considerable amount of NOx generated corresponding to the amount of coal injection at the kiln head.There is a strong reducing atmosphere in the kiln tail smoke chamber, and a considerable part (more than 90%) of NOx generated in the rotary kiln enters the kiln tail smoke chamber and is reduced.Therefore, the NOx formation concentration in the decomposing furnace has a greater impact on the final discharge concentration of the cement kiln.Therefore, the focus of the process control is to suppress the NOx formation concentration in the decomposition furnace, but it does not mean that the low nitrogen combustion control technology of the kiln head is not important.

cement industry

2. Analysis of existing emission reduction technologies


In recent years, the fine particulate matter (PM), sulfur dioxide (SO2) and nitrogen oxide (NOx) emissions control technology (hereinafter referred to as desulfurization and denitration technology) research and development work, as countries increasingly strict environmental protection policy, especially the two revised the cement industry air pollutants emission standards as well as national and local governments issued various regulations and measures for the management, greatly promote and promote.However, compared with dust removal technology, desulphurization and denitrification technology is still in the development process, and is not mature in general, or cannot fully meet the technical and market needs.

In the cement industry, compared with various pollutant control and emission reduction technologies, dust removal technology is the earliest, most in-depth, most applied, most mature, and the technical route is clear.There are two main categories: bag dust removal technology and electrostatic dust removal technology.Composite dust removal technology of electric bag is a new technology which integrates and innovates on the basis of the above two kinds of technologies.The technical goal is very clear, is to improve dust removal efficiency, reduce emissions;Reduce filtration resistance;Extend service life.

Low-nitrogen combustion technology as process control technology mainly includes low-nitrogen burner and decomposing furnace classification combustion technology.Low nitrogen burner can reduce the residence time of fuel in high temperature area or adjust the ratio of fuel and combustion air to produce partial reducing atmosphere and reduce nitrogen oxides by 5%~20%.

The application of mature SCR technology in the power industry has encountered resistance and incompatibility in the cement industry. The main reason is that the dust concentration in the flue gas of cement kiln tail is high, and it also contains heavy metal ions, which is easy to cause the catalyst blockage, wear and poisoning failure.Low dust, low temperature SCR technology because of low temperature catalyst is not mature, does not have the industrial application conditions.The measures to reduce SO2 emission in cement production line can be divided into two categories: to strengthen the desulfurization function of cement production process and special desulfurization technology.

 

At present, desulfurization technologies adopted by the cement industry mainly include the following:


Dry powder desulfurization process: refers to the injection of hydrated lime into the appropriate position of the preheater system.Some foreign companies (RMC Pacific, Polysius, etc.) developed and applied in the 1990s.Domestic application is very few, there are a few cement plants try to use raw material into the kiln lift adding lime Ca(OH)2 dry powder.The desulfurization efficiency is 40%~50%, the process is simple, the land use is small, but the operation cost is high (the material cost is high).

Hot and raw material injection process: take out the decomposed raw material and feed it to the appropriate position of the preheater system.Fuller's de-sox system sets up a cyclone dust collector near the connecting pipe between the upper two stages of the cyclone. Some of the waste gas from the decomposition furnace outlet enters the cyclone, and the collected dust is fed into the exhaust pipe between the upper two stages of the cyclone.The hot raw materials contain a large number of active CaO, and the desulfurization efficiency can reach 25%~30% when the calcium-sulfur ratio is 5~6.For cement plants with high pyrite content in raw materials, about 5%~10% decomposition furnace exhaust gas can meet the requirements.RMC feeds the decomposed raw materials at the cutting end of the kiln into the connecting pipes between the two stages of cyclone barrels or the waste gas pipes after feeding the top preheater. The calcium-sulfur ratio is about 30, and the desulfurization efficiency can reach 30% and 40% respectively.Domestic have individual cement factory to also undertook similar attempt.Desulfurizer comes from its own process, the system process and operation maintenance is simple, the investment and operation cost is low.But the desulfurization efficiency is low, suitable for low concentration production line.

Spray drying process (semi-dry method) : the grout formed after the digestion of lime is sprayed into the absorption tower by the spray device, and the original system humidification tower is used as the absorption reaction tower to reform the original spray system.It is reported that foreign companies have carried out similar work, and the desulfurization efficiency can reach 90%.It is reported that domestic cement factories have been tried and some results have been achieved.The process of high efficiency, collect the kiln ash of sulfur-containing compounds into the raw meal grinding, there is no waste disposal problem, but the system is complex, containing pulping system, lime slurry injection process piping, valves, nozzle, preheater fan congestion is more serious, maintenance workload is bigger, investment and operation cost is high, suitable for discharge of background value is higher production line.

Ammonia desulfurization process: China materials international engineering co., ltd. has developed a simple high temperature ammonia desulfurization process. Ammonia water with a concentration of 20%~25% is used as desulfurization agent.Zhejiang zhuji eight cement company in 1 400 t/d precalcining kiln production line adopts ammonia desulphurization technology, the use of denitration with ammonia water nozzle is set in the humidifying tower, initial concentration of SOx 600 mg/Nm3 flue gas, 20% ammonia consumption about 150 L/h, has been able to control the SOx in the exhaust gas emission values always below 200 mg/Nm3, ammonia consumption cost is about 110 yuan/h, the running cost is not high.In the above cases, the desulfurizer and the reducing agent of denitrification both adopt ammonia water of the same concentration, and there is no need to set up a storage tank separately. The system process is simple.The technology is faced with the following problems that need further research and analysis: ammonia volatilization and ammonium sulfate aerosol corrosion of equipment and insulation materials;The thermal stability of the desulfurization product ammonium sulfate is poor (ammonium sulfate begins to decompose into ammonia and ammonium hydrogen sulfate at 280 ℃, and ammonium sulfate is completely decomposed into ammonia, nitrogen, sulfur dioxide and water at 513 ℃).

All the above technologies are faced with a difficult problem, that is, the secondary decomposition of desulfurization products leads to the cyclic enrichment of SO2 in the kiln, and whether SO2 can be continuously and stably reached the standard, etc., especially when the sulfur content of raw materials is high, it needs to be further practiced and summarized.

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