Study on Ultra-low Emission Technology of nitrogen oxides from Sintering flue Gas

1. High efficiency comprehensive sealing technology of sintering machine. 

Among the air leakage parts of the sintering machine, the air leakage at the head and tail is the most serious and the most difficult to control. 
The negative pressure adsorption end sealing technology (as shown in figure 3) uses negative pressure as the sealing power to force the bellows sealing plate to fit with the side of the bottom plate of the sintering machine, so as to achieve the sealing of the joint, and skillfully solve the contradiction between pressure difference and sealing. 
The top sealing plate is changed from split type to whole plate type, which completely eliminates the air leakage caused by the gap between the traditional split floating seals and saves the ash box. 
This technology has been widely used at home and abroad, in which Baosteel Zhanjiang No.2 550m2 sintering machine initial test air leakage rate is only 17.8%, Japan Wakeshan 185m2 sintering machine test air leakage rate is only 16.7%. 
The reduction of the air leakage rate of the sintering machine is helpful to reduce the power of the main exhaust fan of the sintering machine and the treatment load at the end of the sintering flue gas. 

2. Sintering flue gas circulation technology. 

The sintering flue gas circulation process is a method to return part of the flue gas produced in the sintering process to the material surface of the sintering machine for cyclic sintering. 
On the one hand, this method can obviously reduce the emission of waste gas, thus reducing the investment and operation cost of desulphurization and denitrification equipment; on the other hand, part of the sensible heat and latent heat of sintering flue gas in the process of cyclic sintering will be recycled, which can reduce the ratio of coke powder to a certain extent. 
In addition, part of the dust in the circulating flue gas will be adsorbed and retained in the sinter layer; when PCDD/Fs and NOx pass through the sinter layer, part of the emission can be reduced by thermal decomposition; SO2 can be enriched, which is conducive to improve the desulfurization efficiency of the desulfurization system. 
According to the source of sintering flue gas, the circulation process can be divided into internal circulation and external circulation. 
The internal circulation is to take the air from the bellows branch pipe in front of the main exhaust fan for circulation, and the outer circulation is to take the air from the flue behind the main exhaust fan for circulation. 
In view of the fact that the O2 content in the sintering flue gas does not meet the requirements of sintering production (usually not less than 18%), and it is difficult to develop and utilize the cooling waste gas in the low temperature section of the ring cooler (the O2 content is similar to the air), oxygen enrichment in the sintering flue gas can be achieved by mixing part of the cooling waste gas. 
At present, sintering flue gas circulation technology has been listed as the key promotion technology of cleaner production in iron and steel industry in China. 
This technology is adopted in the new sintering project of Baowu No.2 600m2. In the design process, in addition to fully considering the air volume balance, pressure balance and oxygen balance of the sintering system, the modeling, flow field simulation and structure optimization of the flue gas mixer, distributor and circulation cover of the circulation system are carried out with the help of ANSYS software. after the project is put into operation, it is expected that the sintering flue gas emission will be reduced by 25%, no _ x and dioxin emissions by 20%. 
The energy consumption of per ton ore process is reduced by 1-2kgce. 

3. Clean gas surface top blowing low C and low NOx sintering technology. 

The gas surface top blowing technology is that a certain amount of gas is injected into the top of the sinter surface at the appropriate position after the sinter ignition furnace, so that it is pumped into the material layer under the action of sintering negative pressure and burns heat in the upper part of the combustion layer. 
First of all, from the point of view of calorific value equivalent replacement, the process uses gas instead of part of solid fuel. 
Secondly, through the control of the jet position, the cascade heating mode of ignition gas + solid fuel consumption at the top of the sinter layer, gas blowing at the top of the sinter layer + solid fuel consumption, and self-heat storage + solid fuel consumption in the lower layer is realized, so that the heat distribution in the whole sinter layer is more reasonable and the heat utilization efficiency is higher, thus the solid fuel consumption is reduced as a whole, and the corresponding sintering COx, SOx, NOx and other pollutants are reduced accordingly. 
The heat distribution in the sinter layer is more reasonable, which will produce more high-quality compound calcium ferrite, which will also help to restrain the production of NOx. 

4. Flue gas purification technology by activated carbon method. 

Industrial flue gas treatment at home and abroad has experienced a process from single dust removal, to dust removal and desulfurization compound control, and finally to dust removal and multi-pollutant cooperative treatment. 
After examining the multi-pollution and efficient cooperative removal efficiency of various technical routes, the degree of resource utilization of by-products, operation reliability and performance-to-price ratio, it is generally considered that activated carbon flue gas purification technology and medium and low temperature SCR technology are more suitable for ultra-low emission of iron and steel sintering flue gas. 
Of course, some other methods, such as oxidation, are also being explored. 
The principle of activated carbon desulfurization is as follows: using the adsorption and catalytic characteristics of activated carbon, SO2 in flue gas reacts with water vapor and oxygen in flue gas to form H2SO4 adsorbed on the surface of activated carbon, and the activated carbon adsorbed SO2 is heated and regenerated to release high concentration SO2 gas. The regenerated activated carbon is recycled, and the high concentration SO2 gas can be processed into sulfuric acid, elemental sulfur and other chemical products. 
The principle of denitrification is to realize the removal of nitrogen oxides through the reaction of nitrogen oxides and ammonia catalyzed by activated carbon. 
The principle of mercury removal is to remove heavy metals such as mercury in flue gas by using the adsorption properties of activated carbon. 
The principle of dust removal is: like the conventional filter dust collection, the activated carbon layer realizes the dust removal function through collision, occlusion and diffusion capture. 
The principle of removing dioxins is that solid and foggy dioxins will be attached or adsorbed on the surface of dust particles in the waste gas, and will be filtered and removed when passing through the activated carbon layer. gaseous dioxins can be removed from the flue gas through chemical adsorption of activated carbon layer; then, when activated carbon desorption at high temperature, the adsorbed dioxins will desorb and split into non-toxic substances.

 

5. Typical process of flue gas purification by activated carbon method. 

Activated carbon process has the advantages of simultaneous removal of sulfur dioxide, nitrogen oxides, dust and dioxins in flue gas. 
According to the different adsorption methods, it can be divided into cross-flow process and countercurrent process, in which cross-flow refers to the movement direction of flue gas and activated carbon perpendicular to each other; countercurrent refers to flue gas from bottom to up, activated carbon moves from top to bottom. 

6. Stratified adsorption technology. 

Based on the study of the adsorption law of activated carbon to pollutants, the adsorption rate of SO2 and dust is higher than that of NOx, that is, about 80% of SO2 and dust are adsorbed in the front of the activated carbon bed along the air flow direction, because SO2 adsorption is an exothermic reaction, which will lead to a large amount of heat accumulation in the front of the bed; at the same time, the adsorption of dust will reduce the air permeability of the bed and increase the resistance of the system, so the activated carbon in the front of the bed needs to be discharged as soon as possible. 
Through the study of the effect of the blanking speed of activated carbon at the back of the bed on the outlet dust concentration and denitrification rate, it is found that the slow blanking speed is beneficial to reduce the outlet dust concentration, but it will lead to the sharp increase of system pressure loss and the decrease of denitrification rate. 
In order to solve the above contradictions, a stratified cross-flow adsorption technology is developed, that is, the adsorption layer is divided into three layers: front layer desulfurization + dust removal, activated carbon rapid discharge; the middle layer further desulfurization + dust removal + denitrification, followed by activated carbon discharge speed; the rear layer deep desulfurization + denitrification + dust suppression, activated carbon slow discharge, so as to achieve efficient and cooperative removal of different pollutants with different operating parameters.