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A company’s 5000t/d cement clinker production line was completed and put into operation in 2008. In order to achieve the goal of ecological and environmental protection control, the company built a SNCR denitration system in 2012, and used ammonia water with a concentration of 20% to remove NOx formed in the cement kiln. SN-CR denitration has the advantages of simple system, less investment, little influence on kiln system and small system area. However, due to the low denitration efficiency of SNCR, the ammonia consumption of SNCR is large. In 2019, the ammonia consumption of the company was 7,126 t, and the cost per unit clinker increased by 3.16 yuan/t. At the same time, a large amount of ammonia was sprayed, which caused problems such as crust in the calciner, high coal consumption in the system and internal corrosion of equipment. In order to reduce the production cost, during the overhaul in 2020, the company organized and implemented the technical transformation of denitration and nitrogen reduction.
01 Ideas of technical reform
NOx generated in the cement kiln mainly for thermal type of NOx and NOx fuel type, type thermal NOx combustion air for the N in ₂, generated at above 1500 ℃ high temperature oxidation, fuel type of nitrides contained in NOx as fuel oxidation and generated in the combustion process, the two kinds of NOx were associated with the combustion process, the rotary kiln in high temperature above 1500 ℃, and decomposition furnace temperature under 1000 ℃, usually visible NOx mainly comes from inside the kiln, combining with the characteristics of NOx formation and main source, its control technology can be divided into before burning control, process control and combustion control. The main method of pre-combustion control is to control the N content in fuel, but this technology has not been well developed. The main combustion process control methods are graded combustion and low oxygen combustion. The main methods of post-combustion control are SCR and SNCR. In general, pre-combustion control and post-combustion control require additional equipment, large investment and increased operating costs. The combustion process control method is from the perspective of optimal combustion, so it only needs to transform existing equipment without increasing daily operating costs.
In order to reduce ammonia consumption and save production and operation costs, the company combined two ways of staged combustion and low oxygen combustion to carry out denitration transformation. The transformation is mainly composed of four parts: kiln tail burner transformation, tertiary air pipe transformation, fourth-stage feeding pipe transformation and kiln head burner transformation.
02 Technical Improvement Program
2.1 Improvement of kiln tail burner
By optimizing the local structure of the calciner, a reduction area is formed in the calciner, and reducing substances such as CO and C-H compounds produced by pulverized coal under the condition of anoxia are used to react with NOx generated in the rotary kiln to achieve the purpose of denitration. The company adopts the way of classification of coal combustion denitration renovation, the increase above the smoke chamber reducing denitration 4 burner, symmetrical distribution, and the original cone of decomposing furnace is the location of the preheater burner with three duct relative position of two remain the same, but with three duct up, because of the smoke chamber reducing gas for pulverized coal combustion with air in rotary kiln to produce the flue gas, oxygen content is usually about 1% ~ 2%, thus reducing the smoke chamber increase 4 root denitration burner, coal powder, after reducing feeding under the anoxic condition, sufficient combustion, produce a large amount of CO, C – H compounds for NOx reduction.
Fig. 1 Site drawing of denitrification and nitrogen reduction
2.2 Three air duct renovations
The third air duct of the company is connected to the calciner at the cone part of the calciner. Although it enters the calciner tangentially, it is inclined about 15° downward (i.e. in the direction of the smoke chamber). Points before coal transform, 2 root preheater burner located above three duct of 300 mm, grading after reducing the smoke chamber above 4 root preheater burner has been added, because three times duct tip down about 15 °, the pulverized coal charging ways lead to reducing smoke chamber under anoxic conditions generated CO with oxygen content 21% the left and right sides of wind met ahead of time, three times in the reduction of NOx is equivalent to shorten the duration of stay. In order to ensure the denitration effect and prolong the NOx in the residence time of reduction of area, the company for three times in the duct is reformed, the transformation contents as follows: the duct starting preheater platform raised three times and three times in the duct into the position by decomposing furnace cones to straight tube and cone junction of decomposing furnace, charging position of 1800 mm, three tangential wind vertically into decomposition furnace, because three times duct after modification, 2 preheater burner located at three times the wind charging of root area, if you use the coal pipe, easy cause local high temperature crust, therefore, the preheater burner of 2 root up to 1800 mm.
2.3 Modification of four-stage blanking pipe
After the renovation of the kiln tail burner, the tailing coal is basically fed above the shrinking orifice of the smoke chamber, which will inevitably lead to the temperature rise of the shrinking orifice and the cone of the calciner. If no raw materials absorb heat and decompose here, it will cause the scaling of the shrinking orifice and the cone of the calciner, affecting the output and quality of the system. So on level 4 tremie pipe is reformed: the company level 4 tremie pipe, a total of 4 root, symmetrical distribution in the centre of decomposing furnace, unable to achieve the purpose of reduce cone of temperature of decomposing furnace, so the preheater burner after modification, the pyramidal decomposition furnace add 2 root level 4 tremie pipe, a material valve, two symmetrical distributed tremie pipe, located 1500 mm above the smoke chamber reducing burner, points the material valve opening adjusted according to the lower temperature of decomposing furnace, control in 900 ~ 950 ℃, avoid the cones of decomposing furnace is reducing and local high temperature.
2.4 Transformation of kiln head burner
Low oxygen combustion is to reduce the excess air coefficient of pulverized coal combustion, and then reduce the oxygen concentration around the fuel, to achieve the purpose of reducing the peak combustion temperature and reducing the generation of thermal NOx. The kiln burner head before modification for DBC – 220-550-8 burner, the characteristics of the burner is mainly manifested in high volume, low air pressure at a time, kiln head equipped with ARF – 295 – a fan, air volume after 145.83 m/min, the wind pressure of 29.4 kPa, is conducive to form stable flame of coal adaptability is stronger, but a large number of low temperature in the primary air into rotary kiln, rise, not only cause the heat consumption also increase type thermal NOx generation. In order to reduce coal consumption and NOx production, the company has made technical changes to the kiln burner. DJGX-5000T/D-T low-nitrogen burner is used to replace the original burner, and YG150 magnetic suspension blower is used to replace Roots fan. The primary air volume is 90m³/min and the air pressure is 72kPa.
03 Effect after modification
By means of grading and low oxygen combustion preheater burner, three times of two kinds of methods for duct, level 4 tremie pipe, the kiln head combustor denitration reduced nitrogen after modification, the company denitration significantly lower dosage of ammonia, ammonia consumption about 850 t/month before modification, and modification after the ammonia consumption of about 352 t/month, denitration nitrogen dosage of modified ammonia lower about 498 t/month, ammonia calculated at 800 yuan/t, is expected to cost savings of about 3.984 million yuan a year. The total investment of denitrification and nitrogen reduction transformation of the company is about 3 million yuan, the cost can be recovered within a year, and the return on investment is high. The reduction of ammonia water consumption is equivalent to reducing the water entering the decomposition furnace, so the reduction of ammonia water consumption is conducive to reducing coal consumption. At the same time, considering that SNCR denitration efficiency is generally low and ammonia escape is large, which will not only cause corrosion of pipes and equipment shells but also pollute the environment. Therefore, reducing the amount of ammonia water can also play a role in protecting pipes, equipment shells and the ecological environment.
In conclusion, denitrification and nitrogen reduction transformation of the company is beneficial to reduce production and operation costs, protect pipes and equipment shells, and improve the ecological environment, which is a technical transformation worthy of promotion in the cement industry.