J. Cent. South Univ. (2012) 19: 1359-1363 

DOI: 10.1007/s11771-012-1150-y

Influence of dioxin reduction on chemical composition of sintering exhaust gas with adding urea

LONG Hong-ming(龙红明), LI Jia-xin(李家新), WANG Ping(王平)

School of Metallurgy and Resource, Anhui University of Technology, Ma’anshan 243002, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2012

Abstract: With the addition of urea as an inhibitor, four groups of reducing dioxin emission experiments in sintering pot were conducted. The results show that, adding 0.05%, 0.1% and 0.5% (mass fraction) urea, the emission concentrations of dioxin are 0.287, 0.258 and 0.217 ng-TEQ/m³, respectively. The dioxin emission rates drop substantially compared to 0.777 ng-TEQ/m³ free of    urea. With an increase of the urea content, the concentration of SO₂ emission reduces sharply. (NH₄)₂SO₄, formed by the reaction of SO₂ and NH₃, goes into the dust and part of NH₃ is released before reaction with the emission of exhaust gas. The NO_x emission presents an increasing trend because the reaction of NH₃ and O₂ at high temperature produces NO_x. Based on the consideration of factors such as the effect of reducing dioxin emission, and the chemical composition of exhaust gas, 0.05% is the optimum adding content of urea.

Key words: sintering; dioxin; SO₂; NO_x; urea

1 Introduction

Sintering plant is an important part of the entire production chain in iron and steel industry. It provides a large amount of low cost, stable quality sinter for the blast furnace. In recent years, the increasingly stringent regulatory requirements and market requirements of saving energy as well as reducing costs put double pressure on the sintering plant. The discharged pollutants of sintering plant mainly include particles, SO₂, NO_x, fluoride and dioxin. Among them, dioxin [1-3] is the most toxic persistent organic pollutant (POP) in unintentional synthesized byproducts. It is a tricyclic aromatic compound, composed by one or two oxygen atoms joining with two chloro-substituted benzene rings. It totally has 210 congeners, including polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF), which is collectively known as dioxin (PCDD/Fs). Dioxin detection is on ultra-trace level, lower by several magnitudes compared with other pollutants. More and more researches [4-8] show that the long-term hazards of dioxin are much more serious than we now realize. Therefore, the prevention of dioxin pollution should be paid great attention.

At present, high efficient flittering technology, physical absorbing technology, and catalytic decomposition are mainly adopted worldwide to reduce dioxin produced in the sintering process [9-15]. These methods are effective in some ways, but such problems as the complicated technology, the great renovation of process flow and the huge investment limit their application. Based on the analysis of the present research, the conclusion is drawn that it is more highly effective to develop the new technology inhabiting the release of dioxin in the sintering process than the traditional end treatment. However, study on the influence of new inhabiting technique on the chemical composition of sintering exhaust gas, and comprehensive evaluation of the feasibility of the technology based on emission reduction effect and its impact on the environment are seldom reported.

In this work, emission reduction effect and its impact on the environment are focused on. First, urea with different ratios is melted in water. Then, the water is added to the raw materials when the mixture is blended in the mixer. With the sintering process going on, urea is discomposed by heating, and ammonia is produced in the drying and pre-heating zone. With a series of chemical reactions, the production of dioxin is inhabited. The optimum proportion of adding urea to inhibit the dioxin emission is obtained with the sintering pot experiments. Further, the influence on the chemical composition of exhaust gas in sintering process is studied to ascertain whether it increases the release of other pollutants (SO₂ and NO_x) by addition of urea. This research will provide basic experiment data for developing high efficiency and low cost dioxin emissions reduction technology in sintering process.

2 Experimental

2.1 Material properties

The sintering material came from the raw material of the sintering plant of an iron and steel company. The ferrous raw materials mainly included the ore powder (Newman, Kara, Yankee and FMG) and metallurgy accessories (blast furnace dust, and rolling skin). Flux mainly included lime, limestone, dolomite, and fuel includes coal and coke. The chemical compositions of raw materials are listed in Table 1. In the experiments, a fixed ratio of raw materials was adopted, as listed in Table 2. The urea used in the experiments is the  common industrial urea, in which nitrogen content is larger than 46.3%, and particle size ranges    0.85-2.80 mm. The urea was dissolved and added   with water in the raw material and granulating process. The urea mixing ratio in four experiments is given in Table 3.

2.2 Methods

2.2.1 Parameter of sintering

Sintering process occurred in the sintering pot, of which the diameter is 150 mm, the height of the material bed is 600 mm, the ignition time is 2 min, the ignition temperature is 1 100 °C, the negative pressure is 12 kPa, and the hearth layer mass is 2 kg. A pumping pore of about 30 mm in diameter is opened on the exhaust gas pipeline, and is kept sealed. The schematic diagram of sintering pot is shown in Fig. 1. Each instance of tests in one group was repeated three times, and all data are the average values of them.

2.2.2 Analysis methods of dioxin, ammonia and exhaust gas

Dioxin sample collection started from sintering ignition and ended at the BTP (Burn Through Point), which is the top point of the exhaust gas temperature curve. Samples were collected in the filter membrane and adsorption materials. The sample was removed and saved. The isotope dilution high resolution gas chromatography-high resolution mass spectrometry (HRGC-HRMS) was employed to detect the dioxin. The collected sample was added with isotope-labeled internal sign. Sample extraction liquid was acquired for dealing with filter membrane and adsorption materials, and then it was purified and concentrated into final analytical sample.

The method of Nessler spectrophotometry was used to detect ammonia, with precision of 0.03 mg/m³. The portable flue gas analyzer KM9406E produced by British Kane Company was used to analyze the contents of O₂, CO, SO₂, NO_x in exhaust gas. A set of data was recorded automatically every 20 s.

Table 1 Chemical compositions of raw materials (mass fraction, %)

Table 2 Ratio of raw materials (mass fraction, %)

Table 3 Urea mixing ratio (mass fraction, %)

Fig. 1 Schematic diagram of sintering pot

3 Results and analysis

3.1 Effect on dioxin reduction

The results show that in 17 kinds of homologues of dioxin, the discharge concentration of dioxin in the exhaust gas of sintering pot without adding urea is 0.777 ng-TEQ/m³; with adding 0.05%, 0.1%, 0.5% urea, it is 0.287, 0.258 and 0.217 ng-TEQ/m³, respectively, as listed in Table 4. The reducing effect is remarkable. The dioxin emissions are decreased by 63.1%, 66.8% and 72.1% compared with free urea, which illustrates that urea has a significant inhibitory effect on the formation of dioxin in sintering process. With increasing the ratio of urea, the reduction also increases, but the increasing trend gradually decreases. For instance, the urea content of the fourth group is 10 times of the second one, but the reduction is increased by only 9.0%. The reason is that the function of inhibiting the formation of dioxin by urea mainly depends on the low-temperature synthesis reactions. Stable nitride forms through the reaction of ammonia and Cu²⁺, reducing the catalytic activity of Cu²⁺, and through the reaction of ammonia and HCl, reducing the chlorine source of dioxin formation. For the high temperature gas synthesis reaction, urea has no obvious inhibition effect; in contrast, the HCl forming in the high-temperature reaction may lead to the increase of dioxin concentration from the low-temperature synthesis reactions [16].

Table 4 Analyzing results of dioxin emission

3.2 Analysis of O₂ and CO contents

The contents of O₂ and CO in the sintering exhaust gas in the experiments are shown in Fig. 2 and Fig. 3, respectively. With an increase of the urea ratio, O₂ presents an increase trend while the concentration of CO slightly decreases. The phenomenon displays the improvement of the fuel utilization efficiency. Combined with sintering technical index, it also proves the efficiency improvement. Vertical sintering speed gradually increases from 23.08 to 26.09 mm/min, sinter utilization factor rises from 1.49 to 1.70 t/(h·m²), but drum strength decreases from 62.19% to 58.54%. Therefore, in the view of the negative influence of sinter drum strength, the suitable ratio of urea should not be higher than 0.05%.

Fig. 2 O₂ content in sintering exhaust gas

Fig. 3 CO concentration in sintering exhaust gas

3.3 Analysis of SO₂ concentration

The concentration of SO₂ in sintering exhaust gas is shown in Fig. 4. The concentration of SO₂ varies obviously. In the sintering process, there are two clear peaks. The first peak appears when igniting, where the SO₂ comes from ignition gas. The second peak emerges when wet material zone disappears in sintering material layer and the temperature in lower part of material bed rises quickly. It is the critical point to release SO₂ in sintering exhaust gas. With an increase of urea adding content, the two peaks decrease gradually especially for the second one, which even disappear in S3 and S4 samples. The results indicate that urea does have remarkable effect on SO₂ emission reduction.

Fig. 4 SO₂ concentration in sintering exhaust gas

The above analysis indicates that with the temperature rising in material layer, urea and SO₂ may react in the way [17]:

(NH₂)₂CO+H₂O=NH₄COONH₂                  (1)

NH₄COONH₂=2NH₃+CO₂                      (2)

SO₂+2NH₃+H₂O+0.5O₂=(NH₄)₂SO₄                    (3)

The reaction of SO₂ and NH₃ produces (NH₄)₂SO₄. Gibbs free energy increases with the temperature rising. Under standard conditions, when the temperature is above 800 K (527 °C), the reaction cannot proceed to the right. This illustrates that in sintering process, reaction (3) mainly occurs in the middle-lower part of material layer bed during the period before the sintering BTP arrives. During this period, the relatively higher negative pressure and the slow flow speed of exhaust gas make the reaction fully-reacted. It is shown that SO₂ concentration varies with time, which coincides with the experiment results.

There is absolutely no (NH₄)₂SO₄ in sinter because when the temperature is higher than 1 200 °C, liquid phase forms consolidated sinter, and (NH₄)₂SO₄ should be completely decomposed. Besides, (NH₄)₂SO₄ may go to dust. Sintering dust in dust catcher and dust left in exhaust fan rotor are collected, and soaked in water. The analysis result of the chemical components in water shows that the sulfate radical contents of dust are 0.59%, 3.98%, 4.52%, and 5.66%, respectively, in S1, S2, S3 and S4 samples. The amount is far lower without adding urea compared with adding urea. With adding more urea, sulfate radical content still presents an increasing trend but with lower speed. Therefore, it can be inferred that reaction (3) occurs. Further analysis indicates that after the reaction of SO₂ and NH₃ in material layer, (NH₄)₂SO₄ is formed and goes into wet material zone in the lower part of sintering bed with exhaust gas. Water is so sufficient that (NH₄)₂SO₄ can be intercepted and dissolved in water. With the temperature rising in material layer, permeability becomes better, and (NH₄)₂SO₄ is precipitated and attached to dust, and then is blown out of material layer with high-speed exhaust gas. Before the high temperature arrives the bottom of sintering bed, it exists in exhaust gas pipeline, avoiding decomposition [18].

Furthermore, NH₃ of exhaust gas in each group is analyzed and the result is listed in Table 4. It can be seen from the result that when the adding ratios of urea are 0 and 0.05%, there is no emission of ammonia in exhaust gas; when the adding ratios of urea are 0.1% and 0.5%, the emission concentrations are 0.07 and 0.11 mg/m³, respectively. So, if the addition of urea is excessive, part of NH₃ before reaction will go right into the exhaust gas pipeline and escape with the release of exhaust gas, which is undesired obviously. As ammonia is a hazardous and harmful gas to human beings, the emission will undoubtedly increase the environment burthen, leading to the secondary pollution.

3.4 Analysis of NO_x content

The concentration of NO_x in sintering exhaust gas is shown in Fig. 5. During middle time of sintering process, the curve presents a regular trend with a platform. The difference is that the platforms of S1 and S2 samples are essentially coincident, while the platforms of S3 and S4 samples present a significant rising trend, from an average of 200 to 230 and 275 mg/m³, respectively. NO_x synthesis is closely related to the temperature. When the temperature rises in the range of 760-840 °C in material layer, reaction (4) instantly occurs. From the perspective of the sintering process, this temperature range just appears in sintering middle time and maintains for a period of time. If O₂ is sufficient, reaction (5) further occurs, from which NO₂ is formed. Thus, the NO_x concentration platform rises obviously, but the emission regularity keeps nearly the same.

4NH₃+5O₂=4NO+6H₂O                        (4)

2NO+O₂=2NO₂                              (5)

As for sintering techniques and environmental protection, the higher concentration of NO_x emission is undesirable. Thus, the amount of urea addition should not be higher than 0.05%.

Fig. 5 NO_x concentration in sintering exhaust gas

4 Conclusions

1) Adding urea is effective to reduce the dioxin emission in sintering process. While adding 0.05%, 0.1% and 0.5% urea, the dioxin emission is 0.287, 0.258   and 0.218 ng-TEQ/m³, respectively. That is, the dioxin emission decreases by 63.1%, 66.8% and 72.1% compared to that free of urea. This illustrates that urea has an extraordinary inhibition effect on the dioxin formation in sintering process.

2) Though addition of urea has little influence on the concentrations of O₂ and CO, it influences SO₂ and NO_x significantly. With the increase of urea ratio, the two peaks of SO₂ concentration curve gradually disappear. (NH₄)₂SO₄ formed by the reaction of SO₂ and NH₃ goes into dust. Part of NH₃ is released with exhaust gas emission before reaction. The curve platform of NO_x emission concentration presents a rising trend because of the reaction of NH₃ and O₂ at high temperature.

3) Based on the consideration of different factors such as reducing dioxin emission effect, the influence of sinter performance parameters and chemical composition of exhaust gas, 0.05% (mass fraction) is the optimum adding ratio of urea.

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(Edited by YANG Bing)

Foundation item: Project(50904001) supported by the National Natural Science Foundation of China; Project(2010SQRL032D) supported by Anhui Provincial Key Science Foundation for Outstanding Young Talent, China; Project(TD200909) supported by Program for Innovative Research Team in Anhui University of Technology, China

Received date: 2011-03-15; Accepted date: 2011-05-11

Corresponding author: LONG Hong-ming, Associate Professor, PhD; Tel: +86-555-2311571; E-mail: yaflhm@126.com