01 Ideas of technical reform

NOx produced in cement kiln is mainly thermal NOx and fuel NOx. Thermal NOx is nitrogen in combustion air, which is produced when it is oxidized at high temperature above 1500℃. Nitride contained in fuel NOx is oxidized as fuel and produced during combustion. Both NOx are related to combustion process. The temperature of rotary kiln is above 1500℃, and that of decomposing furnace is below 1000℃. Generally, visible NOx mainly comes from kiln, combining the formation characteristics and main features of NOx. The main method of pre-combustion control is to control the nitrogen content in fuel, but this technology has not been well developed. Combustion process control methods mainly include staged combustion and low oxygen combustion. The main methods of post-combustion control are SCR and SNCR. Generally speaking, both pre-combustion control and post-combustion control require extra equipment, large investment and increased operating cost. The control method of combustion process is from the point of view of optimizing combustion, which only needs to transform the existing equipment without increasing the daily operation cost.

In order to reduce ammonia consumption and save production and operational 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 Reconstruction of tertiary air duct

The connection position between the tertiary air duct and the calciner is located in the cone part of the calciner. Although it enters the calciner tangentially, it inclines downward (i.e. in the direction of the smoke chamber) by about 15. Before the coal separation transformation, two kiln-end burners were located 300mm above the tertiary air duct. After the grading transformation, four kiln-end burners were added above the throat of the smoke chamber. Due to the downward inclination of the tertiary air duct of about 15, this feeding mode caused CO generated by the throat pulverized coal in the smoke chamber to meet with the tertiary air with oxygen content of about 21% in advance, which was equivalent to shortening the residence time of NOx in the reduction zone. In order to ensure the denitration effect and prolong the residence time of NOx in the reduction zone, the company reformed the tertiary air duct. The contents of the reform were as follows: the tertiary air duct was raised from the kiln tail platform, and the inlet position of the tertiary air duct was changed from the cone of the calciner to the joint between the straight cylinder and the cone of the calciner, and the inlet position was increased by 1800 mm. The tertiary air entered the calciner tangentially and vertically. After the tertiary air duct was reformed, two kiln tail burners were located in the inlet area of the tertiary air. If this is used,

2.3 transformation of four stage blanking pipe

After the burner at the kiln end is reformed, the tail coal is basically fed from above the necking of the smoke chamber, which will inevitably lead to the necking and the temperature rise of the cone of the calciner. If there is no raw material to absorb heat here, the necking and the cone of the calciner will be crusted, which will affect the output and quality of the system. Therefore, the fourth-level blanking pipes have been reformed: there are 4 in the company, which are symmetrically distributed in the middle of the decomposition furnace, which cannot achieve the purpose of reducing the temperature of the cone of the decomposition furnace. Therefore, after the kiln tail burner is reformed, two new fourth-level blanking pipes, one distributing valve and two symmetrically distributed blanking pipes are added to the cone of the decomposition furnace, which are located 1500mm above the necking burner in the smoke chamber. The opening of the distributing valve is adjusted according to the lower temperature of the decomposition furnace and controlled between 900 and 900.

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. The main parameters of primary fan before and after modification are shown in Table 1.

Table 1 Main parameters of primary fan before and after modification

03 The effect after transformation

After denitration and nitrogen reduction of kiln tail burner, tertiary air duct, four-stage discharge pipe and kiln head burner by two methods of staged combustion and low oxygen combustion, the ammonia consumption of denitration in our company is obviously reduced, which is about 850t/ month before transformation, about 352t/ month after transformation, and about 498t/ month after denitration and nitrogen reduction transformation. Calculated by 800 yuan /t of ammonia, it is estimated that the annual cost can be saved by about 3.984 million The total investment of denitration and nitrogen reduction transformation in the company is about 3 million yuan, and the cost can be recovered within one year. The return on investment is high. The reduction of ammonia consumption is equivalent to reducing the water entering the decomposition furnace, so the reduction of ammonia consumption is beneficial to reducing coal consumption. At the same time, considering that SNCR denitration efficiency is generally low and ammonia escape is large, it will not only cause corrosion of pipelines and equipment shells, but also pollute the environment. Therefore, reducing ammonia consumption can also protect pipelines, equipment shells and ecological environment.

To sum up, the company’s denitrification and nitrogen reduction transformation is conducive to reducing production and operation costs, protecting pipelines and equipment shells, and improving the ecological environment, which is a technical transformation worthy of popularization in the cement industry.

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