Production practice of particulate matter emission from activated carbon desulfurization and denitration

　　Abstract: activated carbon desulfurization and denitration system can simultaneously remove SO2, NOx, particulate matter, heavy metals and other pollutants in sintering flue gas. At present, it is widely used in the process of sintering flue gas treatment. Due to the low removal rate of particles by the system, the concentration of particles at the inlet, the characteristics of activated carbon and the process operation will have a great impact on the emission of particles. When the inlet particulate matter is high, the outlet particulate matter will be high; The change of activated carbon desulfurization and denitration system and sintering process parameters will also have an important impact on the emission of particulate matter. Through long-term production practice, the emission characteristics of particulate matter are analyzed, and the factors affecting particulate matter emission and control methods are found.

　　Key words: Production Practice of activated carbon desulfurization and denitration particulate matter emission

　　With the improvement of national environmental protection standards, activated carbon desulfurization and denitration has been widely used for its collaborative treatment of SO2, NOx, particulate matter, heavy metals, dioxin, etc. in sintering flue gas. However, due to the limited removal capacity of activated carbon for particles and the particles generated by its own crushing and friction in the adsorption tower, there are bottlenecks in the emission control of particles in this process. How to reduce particulate emissions, or even achieve ultra-low emissions, has become the main direction of the current technological research.
 

　　1. Mechanism and characteristics of particulate matter removal by activated carbon

　　The dust removal principle of the activated carbon moving layer is the same as that of the ordinary filter dust removal. The dust is removed through collision, shielding and diffusion capture effects. The micro particles contained in the sintering flue gas filtered by the electrostatic precipitator are mainly deposited in the activated carbon holes, depressions and surfaces through inertial collision, interception, diffusion and sedimentation. Normally, diameter 1 μ Particles above M can be captured by collision effect. Less than 1 μ Particles of M should be captured through occlusion and diffusion capture effects. [1]

　　The particles discharged from the activated carbon desulfurization and denitration system along with the flue gas can be roughly divided into two categories: one is the particles brought in by the sintering flue gas and escaped after being adsorbed and removed by the activated carbon; the other is the particles generated by the activated carbon itself during operation, collision, friction, etc. Through the composition analysis of the emitted particles, the particles produced by activated carbon itself account for about 18% ~ 40% of the total particulate emissions. Based on the principle of activated carbon desulfurization and Denitration Process for removing particles from sintering flue gas and the characteristics of particles generated by itself, in order to reduce the emission of particle concentration at the outlet of desulfurization and denitration system, it is necessary to start from two aspects: 1) reduce the inlet particle concentration; 2) Reduce particles produced by activated carbon itself.
 

　　2. Effect of inlet particulate concentration on particulate emission

　　2.1 influence of sintering process on inlet particles

　　Appropriate sintering production parameters are not only the key to ensure the quality of sintered minerals, but also the main parameters to reduce the emission of pollutants from sintering production. When the sintering production process parameters are matched, the sintering combustion speed is consistent with the heat transfer speed. At this time, the temperature of the combustion zone will reach the highest, the combustion zone will be narrowed, some refractory sinters will be bonded together, and the particle concentration in the sintering flue gas will also be reduced. When the sintering parameters fluctuate, the vertical sintering speed and sintering air volume will fluctuate, resulting in the increase of particulate emissions.

　　2.2 particle adsorption saturation theory

　　2.2.1 definitions

　　The saturation value of activated carbon adsorbed particles refers to the maximum value of activated carbon adsorbed (removed) particles when the flue gas reaches the emission standard.

　　It is worth explaining that the adsorption saturation value of particles is negatively correlated with the flow rate of flue gas in the adsorption tower. If the duration (T) of particulate matter reaching the standard discharge at a certain flow rate is used to represent the saturation value (t saturation) of activated carbon adsorption on particulate matter, the relationship between it and flow rate (V) is as follows:

　　Tsaturation =k/v

　　Where k is the proportion coefficient, and the minimum value of T saturation Tmix ≥ d/v (D is the thickness of activated carbon bed).

　　That is to say, the saturation value of adsorption (removal) of particles by activated carbon is relative to different flue gas flow rates. The higher the flue gas flow rate, the smaller the saturation value.

　　2.2.2 serious high inlet particle concentration

　　When the inlet particle concentration is higher than a certain value, although most of the particles are intercepted and removed when the flue gas passes through the activated carbon bed, some particles are still directly discharged with the flue gas, resulting in the outlet data exceeding the emission limit. Therefore, we should strictly control the inlet particulate matter in actual production to ensure that the inlet data does not exceed this value, otherwise the particulate matter will not be controlled.

　　2.2.3 long term high inlet particle concentration

　　When the inlet particle concentration does not reach the above limit value, but is still in a high state, the particle emission will not exceed the standard in a short time. However, if this state is maintained for a long time, the emission data will gradually increase and eventually exceed the emission limit value. However, the duration of particulate matter at a certain value is inversely proportional to the concentration of particulate matter. The higher the concentration of particulate matter, the shorter the duration of achieving standard emission.

　　2.2.4 hysteresis of particulate emission

　　The lag of particulate emission refers to that when the concentration of particulate matter at the inlet continues to be high for a period of time and then decreases significantly, the particulate emission does not exceed the standard within the high inlet period, but exceeds the standard after the concentration decreases (as shown in Figure 1 and Figure 2). The main reason is that the flue gas enters from the bottom (or one side) of the adsorption tower, and the activated carbon that first contacts the flue gas adsorbs the most particles. During the discharge process, some particles attached to the surface of the activated carbon separate from the activated carbon and are adsorbed, desorbed and desorbed in the activated carbon layer with the flue gas for many times. Finally, some particles are discharged with the flue gas, resulting in excessive discharge of outlet particles.

 

　　3. Influence of desulfurization and denitration system on particulate matter emission

　　3.1 causes and influencing factors of particulate matter produced by activated carbon

　　In the process of material circulation, the physical friction between activated carbon and activated carbon, extrusion, wear on the material circulation path and other factors. In addition, in the process of desulfurization and denitrification, C element participates in the chemical reaction to consume, which leads to the deterioration of the strength of activated carbon and the intensification of physical wear. The main factors affecting the particulate matter produced by activated carbon are as follows:

　　3.1.1 physical properties of activated carbon

　　There are many physical properties of activated carbon, such as moisture, filling density, particle size, compressive strength, wear resistance, etc. The particle size reflects the crushing degree of the new coke, and the wear resistance and pressure resistance is mainly reflected in the difficulty of the activated carbon to produce particles in the transfer and moving bed. If the abrasion resistance and pressure resistance strength is unqualified, it will lead to gradual crushing due to the inability to bear the material pressure and the mutual extrusion during the moving process.

　　3.1.2 influence of activated carbon material transportation

　　During the design of desulfurization and denitration system, the activated carbon will be seriously damaged due to many times of activated carbon transfer, large drop, extrusion of discharge valve, uneven unloading of adsorption tower, etc. The damage of activated carbon will inevitably lead to an increase in the number of particles produced by the activated carbon itself, which will have an impact on the emission of particles. The following measures shall be taken to reduce the damage of activated carbon:

　　1) Adsorption tower and analytical tower adopt uniform discharge device to ensure uniform discharge of activated carbon and prevent extrusion and crushing of activated carbon under the dual influence of material pressure and movement;

　　2) Reduce the number of chain bucket machines in the material circulation system and reduce the number of reshipment;

　　3) Select the discharge valve with good sealing performance to reduce the extrusion caused by the large gap between the valve core and the valve shell during the discharge of the discharge valve.

　　4) Each buffer bin in the system adopts medium and high material level control to reduce the impact and crushing of activated carbon entering the buffer bin.

　　3.2 influence of material circulation

　　To maintain the continuous removal of SO2, NOx, particulate matter and other pollutants by the adsorption tower, the saturated activated carbon in the adsorption tower must be discharged in time and sent to the analysis tower for analysis and regeneration, so as to restore the activity. Therefore, the circulating amount of activated carbon is an important parameter to maintain the pollutant emission of activated carbon desulfurization and denitrification system.

　　Increasing the circulating amount of activated carbon will not only increase the consumption of activated carbon, but also increase the particulate matter emission at the outlet of the system. The main reason is that with the increase of carbon circulation, the flow speed of activated carbon in the adsorption tower increases, and the friction, extrusion and collision between activated carbon intensifies, resulting in the damage of activated carbon. The resulting particles will be discharged with the flue gas, resulting in the increase of particles.

　　As the adsorption of activated carbon on particles is saturated, when the removal of particles reaches the limit, the activated carbon must be discharged in time, so that the new activated carbon can continue to remove particles. Therefore, in order to ensure the stable emission of particles in the system, the circulation amount of activated carbon must be ensured.

　　However, as mentioned above, the increase of carbon circulation will lead to the increase of particulate matter produced by activated carbon itself. There is a certain contradiction between the two. Therefore, there is an equilibrium point in controlling the concentration of particulate matter at the outlet. When the concentration of particulate matter at the inlet is high, the carbon circulation must be appropriately increased so that the particulate matter adsorbed on the activated carbon can be discharged from the system as soon as possible; When the concentration of particulate matter at the inlet is low, increase the carbon circulation amount, and the particulate matter produced by the activated carbon itself increases. In order to reduce the particulate matter emission, the carbon circulation amount can be appropriately reduced.

　　3.3 influence of analysis effect on particulate emission

　　The resolution effect of activated carbon not only affects the adsorption and removal of SO2 and NOx in flue gas by activated carbon, but also plays a crucial role in the emission of system particle concentration. According to the removal principle of activated carbon for particles, the adsorption of activated carbon for particles is also one of the main ways to remove particles. Ensure a good analytical effect, so that the chemical substances and other macromolecular substances adsorbed in the micropores of activated carbon can be separated out, the adsorption capacity of activated carbon for particles will be greatly increased, and the removal capacity of the system for particles will be improved.

　　Figure 3 shows the trend chart of particulate emission data from August to October 2018. From the end of July to the beginning of August, due to a series of problems, such as frequent failures of the upper and lower rotary valves of the analytical tower, sulfur rich fans, insufficient pressure of blast furnace gas, etc., the analytical effect of activated carbon became worse, and the particulate matter at the outlet continued to rise. As the above problems are gradually solved, the resolution effect of activated carbon is gradually improved, and the particles show a gradually stable downward trend.

 

　　4. Influence of other factors on particulate emission

　　4.1 impact of sintering overburning on emission

　　The sintering end point is advanced or delayed due to the fluctuation of sintering production. When the sintering end point is advanced, the sinter is cooled on the trolley, and the sensible heat carried by the sinter enters the sintering flue gas, causing the flue gas temperature to rise. Due to the harsh requirements of the activated carbon desulfurization and denitration system on the inlet flue gas temperature, a certain amount of air must be added to reduce the temperature when the flue gas temperature is high. At this time, the amount of flue gas processed by the system will increase, and the fluctuation of flue gas amount will lead to the turbulence of flue gas flow in the adsorption tower module, resulting in the increase of particle fluctuation.

　　In addition to the increase of air volume caused by sintering overburning, the cooling of sinter on the trolley will cause a large amount of air to enter the flue gas. In addition, the mixed air will cause the rapid increase of oxygen content entering the adsorption tower. At present, the conversion value will be further increased when the oxygen content based on 16% is adopted for conversion, which is not conducive to the standard emission of particles.

　　4.2 influence of flue gas circulation

　　Sintering flue gas circulation is based on the difference of flue gas emission characteristics (temperature, oxygen content, flue gas volume, pollutant concentration, etc.) of sintering bellows, and on the premise of not affecting the quality of sinter, the flue gas of four groups of bellows in high-temperature section and low-temperature section is selected to circulate back to the trolley surface of sintering machine for hot-air sintering. The process not only realizes the purpose of hot air sintering, but also ensures that the recycled flue gas has a high O2 content.

　　The application of sintering flue gas circulation plays a significant role in reducing the emission of sintering flue gas pollutants while reducing the proportion of sintering fuel, and has effectively achieved the goal of energy conservation and emission reduction. In addition, the flue gas circulation system plays a significant role in reducing the flue gas air volume, temperature and oxygen content at the inlet of the desulfurization and denitration system, which is conducive to the realization of ultra-low emission of the desulfurization and denitration system.

　　Since the flue valve of the flue gas circulation system is installed on the vertical pipe connecting the bellows branch pipe and the large flue, it is necessary to switch and discharge materials regularly, which has a great impact on the amount of flue gas, flue gas temperature and oxygen content entering the desulfurization and denitration system. The desulfurization and denitration system was forced to make appropriate adjustment according to the change of sintering flue gas, resulting in the fluctuation of flue gas entering the adsorption tower.

　　4.3 influence of forward and backward modules of desulfurization and denitration system

　　A stable flue gas flow rate is the key to maintain the stable and up to standard discharge of particulate matter at the outlet of the desulfurization and denitrification system. When some modules of the desulfurization and denitration system need to exit due to hot spots, maintenance and other reasons, it will cause rapid changes in the amount and pressure of flue gas entering other modules, resulting in large fluctuations in particulate matter in a short time. If the flue gas flow and pressure remain high after the module exits, the particulate matter may be high for a long time and the hourly average value may exceed the standard.
 

　　5. Conclusion

　　Through the above analysis, we draw the following conclusions:

　　1) The low flow rate of flue gas in the adsorption tower is conducive to ultra-low emission of particles, so a certain amount of flue gas margin should be considered in the design;

　　2) Appropriate activated carbon circulation amount is an important parameter to reduce the emission of activated carbon desulfurization and denitration particles, and too large or too small is not conducive to the realization of ultra-low emission;

　　3) The analytical effect of activated carbon affects the removal effect of particles, so great attention should be paid to the analytical effect of activated carbon desulfurization and denitrification;

　　4) Sintering process operation also plays a key role in particulate emission.