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Nitrogen oxide emission reduction technology in cement production process

Date:2019-08-28 16:16 Source:cementepcViews:

1. Cement flue gas denitrification technology

 

At present, the following denitrification technologies can be adopted for flue gas denitrification in cement industry:

 

(1) low-nitrogen burner technology:


The denitration efficiency of low nitrogen burner is 10%~15%.In new type of low nitrogen kiln head with large thrust burner, reduce a air volume in the first burning zone form of oxygen combustion flame to reduce nitrogen oxides produced, at the same time through the flame shape control to prevent high temperature to produce reducing NOx generate high temperature type, generally can make the initial concentration is less than 800 milligrams of nitrogen oxide, from the source to reduce nitrogen oxide method reduces the high temperature thermal type nitrogen oxides, and reduce the first step of cement kiln NOx, no running costs.
 

(2) fractional combustion technology:


Divided into fuel classification and grading air combustion, mainly by using the method of grading burning in calciner cones form of oxygen combustion environment, produce a reduction of carbon monoxide in the local area, to use its produce CO, CH4 and H2, HCN and fixed carbon reductant and preheater reaction of nitrogen oxide in flue gas, the nox reduction into the pollution-free inert gas such as nitrogen.
 

Air fractional combustion technology:


Decomposition furnace combustion with three times the wind classification into two parts into the decomposing furnace, the part of the wind (wind - three times into the upper primary decomposition furnace combustion zone (i.e., through all the way from three duct duct into the upper decomposition furnace), the rest of decomposing furnace is still three times according to the original entry into the wind, the decomposition furnace bottom coal burning under anoxic conditions, can generate CO reducing NOx in reducing atmosphere, most of fuel nitrogen decomposed into HCN, HN, CN, CH2, such as reaction with NOx, and NOx reduction part, also prevents the decomposition furnace fuel type NOx formation.After combustion, the flue gas rises along the decomposing furnace and remixes with the fractional wind and burns the fuel in the decomposing furnace.
 

Fuel fractional combustion technology:


Through the computer simulation of flow field and temperature field (CFD), the optimal feeding point and amount of fuel fractional combustion were determined, and a strong reducing atmosphere was formed in the cone of the decomposition furnace to maximize the reduction of NOx in the flue gas of the kiln tail.
 

(3) SNCR denitration technology:


It is adopted widely in cement enterprise at home and abroad mainstream denitration technology, used in 850 ℃ to 900 ℃ temperature range into ammonia, spray gun set to the appropriate location, calciner is usually in the upper or export, gun pressure to ensure that the penetration of the airflow by the mist ammonia and the whole injection profile, using the reaction of ammonia and nitrogen oxides form of nitrogen and water, denitration efficiency can reach above 60%;The existing problem is that the use of ammonia as a reducing agent increases the operating cost of enterprises, while some ammonia escapes and produces new pollution.The best way to use this method is to set the instrument to detect ammonia escape and NOx at the outlet of the preheater, and to automatically adjust the amount of ammonia spraying according to the detection results, so as to reduce the amount of ammonia escape and reduce the operating cost under the premise of ensuring that NOx meets the emission requirements.
 

(4) SCR denitrification technology:


Mainly in the temperature range of 250℃ to 300℃ using catalyst as the carrier to spray ammonia water and nitrogen oxide reaction to form ammonia gas and water. At present, it is rarely used in cement enterprises, and only three cement enterprises in Europe use it.The total investment is high, but the denitrification efficiency can reach more than 80%.The problem is that the cement kiln dust concentration is too high, the catalyst is easy to block, poisoning, resulting in high operating costs, and the catalyst failure after treatment is difficult, the catalyst failure is toxic and harmful waste.The technology is still in the experimental stage in cement kilns.
 

(5) air non-catalytic oxidation absorption denitrification technology (wet denitrification technology) :


In the exhaust gas less than 100℃, nitric oxide is oxidized to nitrogen dioxide, and then sprayed into sodium carbonate solution to absorb nitrogen dioxide to produce sodium nitrate (and other similar methods). It is a mature technology in the chemical industry, but has not been used in cement enterprises.At present, small-scale industrial trials are being carried out, and sodium nitrate or other products can be produced if the trials are successful through denitrification.So the operating cost might be positive, unlike SNCR or SCR which just adds cost, not benefit.

denitration technology

2. Cost calculation

 

(1) use low nitrogen burner:


The investment is about 500,000 yuan with no operating cost.
 

(2) fractional combustion technology:


The investment of 5000t/d line is about 2 million yuan (including refractory materials), and the investment of 500 thousand yuan to 900 thousand yuan, with no operating cost.
 

(3) SNCR technology:


A. The main equipment is imported: the total investment of 5000t/d line is about 8 million yuan, the total investment of 2500t/d line is about 6.5 million yuan, and the operating cost is 3-5 yuan/ton of clinker;

B. The main equipment is domestic: the total investment of 5000t/d line is about 4.5 million yuan, and the total investment of 2500t/d line is about 4 million yuan;Operating cost 3~5 yuan/ton clinker;

All of the above include a set of ammonia escape tester and automatic flue gas analyzer and a complete SNCR denitrification device for air classified combustion, decomposition furnace outlet and chimney outlet.

 

(4) SCR technology:


The total investment of 5000t/d line is more than 40 million yuan;The total investment of 2500t/d line is more than 20 million yuan;The operating cost is about 10 yuan/ton of clinker.
 

(5) air non-catalytic oxidation absorption denitrification technology:


Currently, there is no production line. It is estimated that the total investment of 5000t/d line is about 24 million yuan, which may have certain benefits.
 

3. Technical selection and matters needing attention

 

(1) denitration technology selection in cement industry


According to the above analysis and the application of denitrification technology at home and abroad, it is suggested to adopt low-nitrogen burner + fuel classification or air classification combustion technology +SNCR technology, so that the denitrification effect can meet the new national standards and greatly reduce the operating cost (about 1 ~ 2 yuan/ton of clinker).

First, the use of low-nitrogen burner to reduce thermal NOx generation, and then the use of fuel classification combustion or air classification combustion technology, using CO reduction to further reduce NOx, can reduce the initial nitrogen oxide concentration to 550 mg per standard cubic meter or so, if the national standard

No change, this concentration fully meets the current standard, if the national standard is 450 or 320 mg per standard cubic meter, then using SNCR technology, NOx can be less than 320 mg per standard cubic meter.

 

(2) safety measures


Liquid ammonia is dangerous goods, 20% ammonia solution is not dangerous goods, but also pay attention to the safety of use, for example, a cement factory in fujian province, explosion incident, is because the original stored in the pipe when welding ammonia solution evaporated ammonia gas emptied clean caused ammonia gas explosion accident.Strict personnel training and management regulations should be formulated to prevent accidents.
 

(3) detection system problems


The NOx concentration detection system and ammonia escape monitoring system should be ensured to operate stably, and the operating cost should be reduced on the premise of ensuring the emission concentration reaches the standard, so as to prevent excessive ammonia overflow from generating new pollution.
 

(4) at present, the market competition in the cement industry is fierce. Due to the increased operating cost of denitrification projects, the government and the competent department of the industry should treat cement enterprises as equals to achieve fair competition.


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