Research Institute,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201999,China

Abstract: Based on an analysis of the chlorine elements in each component of the sintering mixture,industrial tests of the use of dioxin reduction techniques in the sintering process were found to reduce the chlorine sources and inhibit the formation of dioxins.The dioxin reduction effect obtained in the industrial experiment was better than that in the sintering pot experiment,but their patterns were consistent.When urea is used as an inhibitor,the dioxins can be significantly reduced.When a 0.02% ratio of urea was added,a maximum dioxin emission reduction of 79% was obtained.Reducing the chlorine sources also had an obvious dioxin reduction effect,enabling a 69.49% reduction.In addition,when these two technologies were applied simultaneously,a significant emission reduction rate of 92.23% was achieved.The concentration of dioxins in flue gas dropped to 0.242 1 ng TEQ/m3 before desulfurization,which meets the emission standard for dioxins in the final exhaust gas.

Key words: dioxins; flue gas; sintering process

1 Introduction

According to a 2004 national survey of dioxin pollution sources in China,the dioxin emissions from metal production accounted for 45.6% of China’s total dioxin emissions.Dioxin emissions from iron and steel enterprises account for 56% of all dioxin emissions from metal production,and iron ore sintering accounts for 57% of dioxin emissions from iron and steel enterprises.Therefore,iron-ore sintering represents a major source of dioxin emissions in China.In the standard GB 28662-2012 “Emission standard of air pollutants for sintering and pelletizing of iron and steel industry”,the emission concentration requirements regarding particulate matter and sulfur dioxide have been made more stringent,and the emission limit of dioxins on the sintering flue gas has been added,which is 0.5 ng TEQ/m3.On April 28,2019,the Ministry of Ecology and Environment,the National Development and Reform Commission,and five other ministries and commissions jointly released the “Opinions on pro-moting the implementation of ultralow emissions in the iron and steel industry”.This report emphasized that notable progress must be made in the upgrade of steel enterprises in key regions to achieve ultralow emissions,with a goal of reaching an approximately 60% transformation of their capacity by the end of 2020.As the environmental protection policy for sintering flue gas emissions becomes increasingly strict,a large number of sintering plants must add processing equipment or facilities to their original desulfurization treatment equipment to meet the new ultralow emission requirements for SO2,NOx,dioxins,and heavy metals.However,owing to the complex pollutants generated in the sintering pro-cess,a large amount of flue gas produced,and large fluctuation in the operating parameters,purification treatment is difficult to achieve.Therefore,devel-oping various dioxin emission reduction technologies in the sintering process can not only significantly reduce the end treatment cost,but also provide more options for the subsequent cooperative treatment of the other pollutants emitted by sintering flue gas.

2 Dioxin formation mechanism in the sinter-ing process

The dioxin formation mechanism in the sintering process is complex and is considered to occur in one of three main ways[1]:(1) Dioxins can be present in the sintering raw material,which is not completely decomposed during sintering.(2) Dioxin is formed by condensation,oxidation,or other organic reactions of chlorine-containing precursor compounds.(3) Dio-xin is formed viadenovosynthesis reactions,whereby the macromolecular carbon and organic or inorganic chlorine in the sintering exhaust gas are catalyzed by copper,iron,and other metals or their oxides to form dioxins(i.e.,polychlorinated dibenzo-p-dioxins and polychlorinated dibenzo-p-furans,i.e.,PCDD/ FS) at low temperatures(250-450 ℃).

Some studies[2]reported that the dioxins in the sintering process were mainly generated bydenovosynthesis in the dry zone of the sintering bed.As sintering raw materials contain a variety of components,including iron(Fe),carbon(C),chlorine(Cl2),copper(Cu),and other solid wastes from each process of steel production,and there is plenty of oxygen available in the process of sintering,these C,Cl2,and O2sources and their catalysts are available for thedenovosynthesis of dioxins.As such,thedenovosynthesis of dioxins can occur over a wide temperature range of 250-450 ℃.

Given the above scenario,Cl2,Cu,Fe,and other transition metal catalysts are important factors that affect the formation of dioxins.The element chlorine in the sintering raw material mainly exists in the form of an alkali metal chloride,such as KCl or NaCl.These chlorides form HCl,Cl2,and other gaseous compounds during the sintering process at high temperatures and participate in thedenovosynthesis reaction within the reaction temperature range to produce dioxins.Therefore,an effective means for controlling and reducing the formation of dioxins is adding some inhibitors to remove the HCl generated during the sintering process.

Urea can decompose to produce a large amount of ammonia(NH3) in the high-temperature process,which is beneficial for the removal of HCl.At the same time,the NH3molecule has a lone pair of electrons,which can react with Cu,Fe,and other transition metals to form stable Cu-N compounds;thus,reducing the possibility of catalyzing the formation of dioxins.In addition,urea is a preferred inhibitor of the formation of dioxin because of its low cost and convenient operation.

In this study,industrial experiments were conducted to verify the dioxin emission reduction effect in the sintering process.Compared with the experimental results obtained using the sintering pot,adding an inhibitor was confirmed to signifi-cantly reduce the dioxin concentration in the sinter-ing flue gas,which provides a technical basis and data support for applying a dioxin treatment in the sintering flue gas.In addition,the effect of reducing the chlorine sources on reducing the dioxin emission was verified in industrial tests,which means that the introduction of chlorine in various miscellaneous auxiliary materials significantly increases the gener-ation of dioxins in the sintering process.

3 Experimental procedure

3.1 Material preparation

In the industrial tests,commercial-grade urea powder was used as the inhibitor.In the sintering pot test,the analytical pure urea reagent produced by Sinopharm Chemical Reagents(Shanghai) Co.,Ltd.was used as the inhibitor,and the raw materials of the sintering plant were also used,including the iron ore mixture,sintering powder,and return ore.The flux mainly contained quicklime,limestone,ser-pentine,and dolomite,and the fuel contained coke and pulverized coal.

3.2 Experimental apparatus and methods

3.2.1 Dioxin emission reduction experiment in sintering pot test

An experiment to reduce the dioxin emitted from the sintering pot was conducted using a pilot device in a sintering pot test,and a dioxin sampling hole was drilled into the exhaust gas pipeline.A diagram of the main body of the sintering pot and the dioxin sampling location is shown in Fig.1.The diameter of the main sintering pot is 800 mm and its height is 300 mm.

Fig.1 Sintering pot test equipment and dioxin sampling location

As shown in Fig.1,in the test,iron ore and other sintered raw materials were loaded into a homemade laboratory drum mixer at a certain proportion for mixing and granulation,and a pre-prepared urea solution was evenly sprinkled into the cylinder.During sintering mixture granulation,the urea was well mixed into the mixture,which was then moved into the pre-paved hearth layer of the sintering pot.

At the time of sintering surface ignition,many types of detection and sampling devices were turned on.When the temperature of the sintering exhaust gas reached the highest point,the gas sampling and analysis devices were turned off.The dioxin samples were obtained at a certain time in the sintering pot test.After collection,the dioxin samples were sent to the laboratory for analysis and the total toxicity equivalent of 17 types of dioxins(hereinafter referred to as the dioxin concentration) was calculated.This analytical method is based on that presented in the standard HJ 77.2-2008 “Ambient air and flue gas Determination of polychlo-rinated dibenzo-p-dioxins(PCDDs) and polychlo-rinated dibenzofurans(PCDFs) Isotope dilution HRGC-HRMS ”.Other flue gas components(NOx,SO2,CO,CO2,O2,etc.) were directly determined using a portable online flue gas analyzer(MRU,Germany).

The ratios of the amounts of urea added(mass fraction) are 0.01%,0.02%,0.05%,and 0.10%,respectively.Three parallel tests were conducted for each ratio experiment,from which the average value of the experimental results was obtained.

3.2.2 Industrial test apparatus and methods

The purpose of the industrial experiment was to verify the reduction in the dioxin formation in the sintering process when using urea as an inhibitor to reduce the chlorine sources.During the test,the urea solution was prepared outside the plant.After the solid urea was prepared as a 20% urea solution,it was transported to the sintering plant by tank truck and pumped into the blocker reserve tank for standby application.

The amount of urea solution added was controlled by a flow control valve.The solution was sent from the conveying pipe to the top of the conveyor belt of the sintering burden and added into the sintering raw material at a uniform speed by a spraying device.Then,the solution moved into the primary and secondary mixers in succession and participated in the sintering production after having been fully mixed with the sintering mix material.A flow chart of the experimental procedure is shown in Fig.2.To prevent interference from the desulfurization system in the analysis of the dioxin emission reduction effect,the dioxin sampling point was located before the sintering desulfurization system,and the variations in the dioxin concentration in the flue gas at different urea dosages were determined.

Fig.2 Flow chart of the industrial test process for suppressing dioxin formation by adding urea

4 Results and discussion

4.1 Effect of chlorine on dioxin emission in flue gas

4.1.1 Chlorine distribution in sintering raw materials

The components of the sintering raw materials are complex.In addition to the iron ore mixture,sintered powder,return ore,quicklime,limestone,serpentine,dolomite,and coal or coke powder,there are also many kinds of dust,mud,and other solid wastes generated by other process sections in the factory(called miscel-laneous auxiliary materials in this paper).The con-centrations of chlorine in these components are shown in Table 1.As the content of chlorine in most samples was less than 0.01%,the total chlorine in various samples was analyzed by high-temperature cleavage-ion chromatography using a Mitsubishi’s AQF-100 high-temperature cleavage analyzer and Diane’s ICS-100 ion chromatography analyzer.

Table 1 Content of chlorine in components of the sintering raw materials %

Based on the data in Table 1 and the annual dosage of a sinter machine,the distribution of chlorine in the sintering raw materials can be calculated,as shown in Fig.3.In the figure,we can see that in the sintering mixture,the element chlorine mainly enters the sintering process through miscellaneous auxiliary materials and iron ore,which account for 88% of the total chlorine content.In addition,the chlorine content in miscellaneous auxiliary materials is also closely related to their sources.In general,if the miscellaneous auxiliary materials contain a large amount of blast furnace dust and dust from the electrostatic precipitator of the sintering machine,the chlorine content in the miscellaneous auxiliary materials will be higher.

Fig.3 Distribution of chlorine in each component of the sintering mixture

4.1.2 Effect of reducing chlorine content on dioxin emission reduction

In the industrial test,to reduce the amount of chlorine in the sintering mixture,the mixing of miscellaneous auxiliary materials with the highest concentration of the element chlorine in the sintering mixture was omitted.When the production was stable,dioxin samples were collected before desulfurizing the sintering flue gas.The collected data were then com-pared with those of a benchmark experiment(including miscellaneous auxiliary materials),as shown in Fig.4.The results show that the concentration of dioxins in the flue gas is significantly reduced,and 69.49% dioxin emission reduction efficiency can be achieved by simply taking this measure.This result indicates that reducing the chlorine content in the sintering mixture is an effective measure for reducing the concentration of dioxin in the exhaust gas.In contrast,if a large amount of chlorine is mixed into the sintering mixture,the concentration of dioxin in the exhaust gas is significantly higher.

Fig.4 Results of industrial test of dioxins emission reduction by reducing chlorine sources

4.2 Effects of adding urea on dioxin emission reduction

4.2.1 Experimental results of the sintering pot

Fig.5 shows the experimental results obtained in the sintering pot by the addition of urea on the formation of dioxins.The amount of urea added is defined as a percentage of the sintering mixture.In the figure,it can be seen that after adding 0.01%,0.02%,0.05%,and 0.10% urea,compared with the benchmark test(without urea),the dioxin emission was reduced by 53.77%,67.74%,57.86%,and 56.35%,respectively,which means the inhibitory effect is significant.With the addition of 0.02% urea,the dioxin emission reduction rate was the highest.However,the efficiency of the dioxin emission reduction decreased when the amount of urea added continued to increase.This result indicates that the effect of adding urea as an inhibitor in reducing the generation of dioxins is limited and cannot be improved indefinitely.

Fig.5 Sintering pot test of the effect of adding urea on the formation of dioxins

The above results are consistent with the experimental results reported by ANDERSON et al.[3],who found that the optimal addition ratio of solid urea was between 0.020% and 0.025%.The authors also reported that the inhibitory effect of dioxins did not increase with the addition of more urea.However,LONG et al.[4]reported that increas-ing the urea dosage resulted in a constant increase in the dioxin inhibition effect.In that study,when 0.05%, 0.10%, and 0.50% urea was added,the dioxin concentration decreased by 63.1%, 66.8%,and 72.1%, respectively.Urea is recognized as hav-ing a significant inhibition effect on the formation of dioxins in the sintering process,but the amount of urea added and the rules regarding urea inhibition remain controversial.

4.2.2 Industrial test results

The results of two successive industrial tests(T1 and T2) are shown in Figs.6 and 7,respectively.According to the inhibitory effect of urea on the formation of dioxins shown in the figure,the dioxin emission reduction law in the two groups of experi-ments is similar to the experimental results obtained in the previous sintering pot experiment(Fig.5),which confirms again that when using urea as an inhibitor,the emission reduction effect of dioxin is signifi-cant,but does not increase indefinitely.When the ratio of urea solution was 0.02%,the inhibitory effect on dioxin formation was the best.

Fig.6 Industrial test of the effect of adding inhibitor on the reduction of dioxin emissions(T1)

In addition,at the same ratio,the inhibitory effect of adding urea in the industrial experiment was better than that in the sintering pot experiment.When 0.02% urea was added,the dioxin emission reduction rates in the two industrial tests were 79% and 77%,respectively,which were 18% and 15% higher than those in the sintering pot experiment.This can be because,in the large-scale production,the urea solution is first uniformly poured onto the conveyor belt of sinter materials and then gradually moves into the first and second sinter mixers for granulation with the sintering mixture.Thus,the urea and sintering mixture are fully mixed,which further promotes the inhibition effect of urea.How-ever,under the same sintering conditions,the emis-sion reduction rate reached a maximum when the amount of urea was 0.02%.In addition,when the amount of urea solution added was 0.01%,the dioxin reduction rate reached 65%.Considering the operational cost involved,this ratio is the most appro-priate for industrial applications.

Fig.7 Industrial test of the effect of adding inhibitor on the reduction of dioxin emissions(T2)

4.3 Effect of urea on NOx,NH3,and other pollutant emissions in flue gas

Another widely expressed concern is whether NH3or NOx,which are produced in the high-temperature decomposition process when adding amino inhibitors,will cause secondary environ-mental pollution.LONG et al.[4]tracked and moni-tored the NH3in the flue gas in an experiment involving the addition of urea and found that when the ratios of urea were increased to 0.10% and 0.50%,NH3was detected at emission concentra-tions of 0.07 and 0.11 mg/m3,respectively,whereas when the ratio of urea was 0.05%,no ammonia emission was detected.In the process of monitoring the concentration of NOxin flue gas after adding urea,the Corus Group[3]found that the concentra-tion of NOxin flue gas did not change significantly when adding urea within the range of 0.020%-0.025%,but it did not track or monitor the changes in the NH3in the flue gas.In this study,a portable Fourier transform infrared gas analyzer was used to monitor the composition of flue gas pollutants in industrial tests,the monitoring results of which are shown in Fig.8.

Fig.8 Effect of adding urea on emissions of NOx,NH3,and other pollutants in flue gas

As shown in Fig.8,the addition of urea has different effects on the emission of various pollutants in the flue gas.With an increase in the urea content,acid gases in the flue gas,such as SO2and HCl,decreased slightly.In contrast,the concentrations of NOxand NH3in the flue gas increased to different degrees.In particular,the con-centration of NH3in the exhaust gas increased expon-entially,with a correlation coefficient of 0.993 3.When the amount of urea added was 0.01%,the concentration of NH3in the exhaust gas was 16.91 mg/m3,and when the amount of urea added was increased to 0.02%,the concentration of NH3in the flue gas increased sharply to 44.98 mg/m3,repre-senting an increase of 166%.Therefore,considering the need for a balance between cost and the increased NH3concentration in the flue gas,the optimal proportion of urea added is 0.01%.

4.4 Comprehensive reduction effect of reducing chlorine sources and adding urea on dioxins removal in flue gas

Fig.9 shows the industrial test results obtained after adding urea to further inhibit the formation of dioxins on the basis of reducing the chlorine sources.The proportion of urea added in the test was 0.02%.The experimental results reconfirm the significant inhibition effect of urea on the removal of dioxins.Compared with the test in which miscellaneous auxiliary materials were reduced,the dioxin emission reduction efficiency in this test reached 74.5%.Compared with the results of the benchmark experiment,the reduction rate of dioxin reached 92.23% after combining the measures of reducing the sources of chlorine and adding urea.The dioxin concentration in the flue gas dropped to 0.242 1 ng TEQ/m3,which meets the final dioxin emission standard.The adoption of these dioxin emission reduction technologies through the source and process can greatly reduce the end treatment cost of flue gas and also greatly reduce the risk of exposure from the secondary transfer of dioxin.

Fig.9 Industrial test on the reduction of dioxin emissions(minus miscellaneous auxiliary materials+urea)

5 Conclusions

(1) Reducing the chlorine content is an effective approach for reducing the concentration of dioxin in the sintering flue gas.The results of industrial tests show that a 69.49% reduction in dioxin emissions was achieved after the removal of miscellaneous auxiliary materials containing a large amount of chlorine.

(2) The results of both the industrial test and sintering pot experiment show that when urea was used as an inhibitor,the dioxin emission reduction effect was significant,with the effect in the industrial test better than that in the sintering pot experiment.There exists an optimal urea addition ratio above which the effect does not increase indefinitely.In our tests,when the ratio of urea added was 0.02%,the dioxin emission reduction effect was the best,up to 79%.

(3) The concentration of NH3in sintering flue gas increased exponentially after urea was added.Considering the industrial operational cost and the need to prevent secondary pollution caused by a high concentration of NH3in the flue gas,the optimal and recommended amount of urea to add is 0.01%.

(4) A dioxin reduction rate of 92.23% was realized by combining the measures of reducing the chlorine content and adding urea,which reduced the concentration of dioxin in the flue gas to 0.242 1 ng TEQ/m3,which meets the required dioxin emission standard in the flue gas.

In conclusion,the use of technology to reduce dioxin emissions during the sintering process can not only reduce the cost of the end treatment of flue gas,but also greatly reduce the risk of exposure caused by the secondary transfer of dioxin in subsequent flue gas treatment measures such as desulfurization.However,the amount of urea added must be strictly controlled to prevent secondary pollution caused by an increase in the NH3concentration in the flue gas.