Article pubs.acs.org/EF
Influence of Coke Combustion on NOx Emission during Iron Ore Sintering Hao Zhou,* Zihao Liu, Ming Cheng, Mingxi Zhou, and Ruipeng Liu State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou, 310027 P.R. China ABSTRACT: This paper explores NOx emission during iron ore sintering on a pilot-scale pot. A novel technique of using a three-layered bed structure was adopted to study the role of coke level, properties and combustion behavior on NOx emission. In addition, the lime content of the mix was also altered to understand the effect of the melt formation process. Thirteen sinter pot tests were employed to analyze the influence of coke combustion on NOx emission. As the flame front descended down the pot, NOx emission decreased because of increasing bed temperatures. For this reason, it is to be expected that increasing coke rate will result in higher sinter bed temperature and lower conversion of coke-N to NOx. However, overall NOx emission is little changed because more coke means higher N availability. Increasing basicity (CaO/SiO2) from 1.9 to 2.4 in raw mix results in the decrease of NOx emission, about 5%. Increasing coke size may result in the decrease or increase of NOx emission. Placing coke particles on the outside of granules can lower NOx emission in the middle and bottom layers, by 4−15%. NOx emission in the iron ore sinter process is determined by the conversion of coke-N to NOx and nitrogen source in sinter mix. The atmosphere around coke particles is very important for NOx reduction, and NOx emission will increase by around 15% when oxygen increases by 1 vol.%. During iron ore sintering, temperature has great influence on NOx emission, with levels decreasing by 15−25% for a 100 K temperature increase. standing of NOx formation in a coal fired system is applicable to iron ore sintering. In recent years several papers have investigated NOx emission in the iron ore sintering. Pan et al.14,15 and Morioka et al.16 studied the influence of calcium ferrites formed in the high temperature zone on NOx emission and found that they catalyzed NOx decomposition, resulting in reduced emission levels. They14−16 proposed that increased calcium ferrites levels will reduce NOx emission, which could be achieved by the preferential pregranulation of limestone and ores. Probably because of NOx decomposition, Chen et al.2,17,18 found that their levels decreased during iron ore sintering when a specially prepared coke containing CeO2 was used. Xiong et al.19,20 studied the reaction between iron ores and NOx in a tube furnace and proposed that iron ores containing TiO2 or V2O5 are effective catalysts in the decomposition of NOx to N2. It also appears that under certain conditions CO present can reduce NOx. Using a fixed bed quartz reactor, Chen et al.21 proposed that this reduction reaction is catalyzed by CaO, sinter, Fe2O3, and MgO. More general studies on the effect of sintering conditions on NOx levels have also been carried out. Fuel types were shown by Mo et al.22 and Gan et al.23 to influence NOx emission. NOx levels could be decreased by 20−30% when biomass fuels such as charcoal and rice straw were used to replace coke as sinter fuels. Pan et al.14,15 studied the impact of some sinter parameters on NOx emission and found that NOx emission increased with coke rate and moisture increasing. Studies on
1. INTRODUCTION In 1995 China’s crude steel production was 95 million tonnes but in 2012 has increased to 700 million tonnes.1 Clearly the corresponding large increase in NOx emission has placed significant pressure on the environment. It is well-established that the sintering plant is the main source of NOx emission in steel plants, accounting for about 50% of the total NOx emission.2,3 For this reason, there are great benefits in understanding sinter plant NOx emission and determining techniques to reduce its level in the waste gas. NOx emission in coal fired systems has been widely investigated. Char combustion contributes about 20−30% of the total NOx generated; mechanistic studies indicate that formed NOx could be reduced by char or CO on the surface of char.4−12 Iron ore sintering involves coke combustion, but the environment is very different from those encountered in a pulverized coal fired boiler or circulating fluidized bed. Coke particles during iron ore sintering are coarser (0 to 5 mm) than those used in a pulverized coal fired furnace. During iron ore sintering, coke particles could be incorporated in the adhering fine layer and encapsulated by fine iron ore or flux particles after granulation.13 Minus 0.25 mm coke particles would be well dispersed in the adhering fine layer, plus 1 mm coke particles are mostly free particles, and medium fine coke particles (plus 0.25 minus 1 mm) are located in the outside of adhering fine layer.13 Within the flame front, the combusting coke particles are surrounded by a gas−liquid−solid mixture and the atmosphere contains a lot of CO, which can lead to the reduction of NOx. Generally the sintering waste gas contains 0.5−2.0 vol % CO, 10−15 vo1 % O2, and 10−15 vol % CO2. Meanwhile, the mixture surrounding coke particles contains high content of Ca, Fe oxides, which have important influence on coke combustion. It is, therefore, unlikely that under© XXXX American Chemical Society
Received: November 10, 2014 Revised: January 16, 2015
A
DOI: 10.1021/ef502524y Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
convert to NO directly but to HCN, which then oxidized to NO. Notwithstanding the lack of clarity on the NOx formation mechanism, there is sufficient information to indicate that homogeneous reactions are important. NOx emission decreases when the atmosphere is more reducing and NOx could be deoxidized to N2 by coke and CO on the coke or other catalysts surfaces.4−6,10−12 Within the flame front, a chemically and structurally complex three-phase solid−liquid−gas mixture is generated and the CO content in the flowing gases can be in the region of 0.5−2%. For this reason, it is to be expected that the NOx formation and reduction mechanisms are very different from those encountered in a pulverized coal furnace and in fluidized bed combustion.
the impact of lime/limestone on emissions in the combustion of coke by Lee et al.24 showed that SO2 emission could be decreased when lime/limestone was added to coke but there was little change on NOx emission. Although significant research has been carried out to understand NOx emission during sintering, it is clear that much more work is necessary because the NOx formation and breakdown/reduction processes are very complex. The aim of this paper is to explore NOx emission in a pilot-scale sinter pot. The sinter mix is altered by changing coke and limestone addition levels and also changing the combustion characteristic of the coke by placing them on the outside of granules to increase the access to oxygen. However, the experimental technique is novel in which two sinter mixes are considered in a test. The bed is composed of three layers of equal heights. The same sinter mix is used for the top and bottom layer. The middle layer is composed of a different mix (e.g., increased coke levels), which means that the descending flame front encounters two clear step-change points. Determined gas compositions over the test period can be divided into the three corresponding regions and analyzed. This technique gives much more definitive results about the influence of sinter mix properties on NOx emission because the properties of the flame front also change as it moves down the bed.
3. EXPERIMENTAL SECTION 3.1. Test Philosophy. Pilot-scale studies involving small cylindrical pots are very widely used to simulate the sintering process on a commercial strand. NOx formation during sintering can be assessed by the continuous monitoring of the waste gas during a test. Very simply, a test involves filling a pot with granulated sinter feed, igniting the top of the bed and drawing the formed flame front through the bed by the application of suction at the bottom of the bed. At the end of sintering the sintered bed is shattered and crushed. The coarser fraction is the product sinter from the test. This product will contain a large proportion of material from the lower regions of the pot because sintering temperatures are higher there. At the upper regions of the bed the air used for coke combustion is at ambient or close to ambient temperatures. Before reaching the lower layers the air for combustion has to pass through recently formed hot sinter, prior to reaching the flame front. The preheating of this air greatly increases flame front temperature. The generation of more melt of lower viscosity, material coalescence is enhanced and this results in the formation of larger, denser and stronger sinter particles. The changes in sintering conditions down a bed due to preheating of the combustion air are gradual. Because thermal NOx is likely to be unimportant in sintering, measured NOx values may not change over the bed sintering period. With increased sintering reactions leading the formation of more melt, it is possible that more calcium ferrites could form, leading to some decreases in NOx values. Another factor which could have some effect on NOx level is the increase in CO levels down the bed. As the contribution of thermal NOx and the reduction of the formed NOx by catalytic reaction and CO reduction could be small, a novel experimental technique was used in this study. The main aim of this technique is to introduce a major change in mix composition in the middle of the bed. For example, if placing a layer of higher basicity (increased Ca content) in the middle of the bed results in a sharp change in NOx emission then one can be very confident that the influence of this variable is significant. Of course the same information can be obtained from two separate sinter pot tests, one with standard basicity value and the other with the higher basicity value. However, the proposed methodology is preferred because more confidence can be placed on analyzing a step change that occurs during at test to reach a conclusion rather than through comparing two sets of data. In addition, for the same number of sinter pot tests it is possible to confirm the test results obtained for a particular sinter mix for the following reason. The first three-layer test will have the different middle layer−for example, mix used in the top and bottom layers have standard basicity while the middle layer mix has a higher basicity. For the complementary second three-layer test the standard basicity mix is used for the middle layer and the high basicity mix for the top and bottom layers. Step changes that occurs when the flame front moves from the top layer to the middle layer and then from the middle layer to the bottom layer should be in the opposite direction in the two tests. The effect of basicity changes obtained in the first test is confirmed by the second test. 3.2. Procedure. The pilot-scale facility used in this work is shown in Figure 1.28 Sinter pot has a height of 600 mm and an internal
2. NOX FORMATION DURING IRON ORE SINTERING PROCESS In combustion systems, NOx essentially originates from three sources, simply termed the following: thermal NOx, prompt NOx, and fuel NOx.4,25 Thermal NOx is formed from oxidation of nitrogen in the atmosphere at relatively high temperature. The Zeldovich model, widely adopted to simulate and predict thermal NOx, shows that little thermal NOx forms if temperature is below 1800 K.4,26 Even though flame front temperature increases with depth because the bed is increasingly preheated, the maximum temperature reached is still around 1400 °C.26,27 Therefore, it is unlikely that thermal NOx contributes toward NOx levels in iron ore sintering. Prompt NOx is mainly formed by the reactions between carbon−hydrogen radicals and nitrogen in a fuel-rich environment.4,25,26 Coke is the main fuel used in iron ore sintering, and its volatile matter content is low, indicating low carbon− hydrogen radical levels. Consequently, it is most likely that in the iron ore sintering process NOx emission mainly comes from fuel NOx. As iron ores and fluxes do not contain significant nitrogen, the combustion of coke and the other solid fuels (if any) is the dominant contributor to iron ore sintering NOx levels. While it is clear that coke combustion is the main generator of NOx, its formation mechanism is a point of debate. De Soete et al.4−6 proposed that char-N can be oxidized directly to NOx, involving no homogeneous reactions. However, results from other researchers indicated the presence of homogeneous NOx reactions in the NOx forming process.7−10 Winter et al.8 added iodine as a tracer to investigate the mechanism of the reaction between NO and N2O, and found that NO was mainly the product of heterogeneous reactions while N2O was mainly from HCN oxidation. Liu et al.9 studied the influence of limestone/ CaO on NOx and N2O formation, proposed that homogeneous reactions existed during char-N oxidization, and found limestone/CaO could increase NO emission. Molina et al.10 added HBr into the char combustion system to study the charN oxidized mechanism, and they found that char-N did not B
DOI: 10.1021/ef502524y Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 1. Sinter pot flowchart28
Table 1. Test Conditions test
variation
upper layer
middle layer
bottom layer
remarks
1 2 3 4 5 6 7
coke level coke level coke particles size coke particles size coke particles size coke particles size coke addition method
4.85 wt % 4.05 wt % +1 mm coke full size +0.5 mm coke full size add-8
coke addition method
4.05 wt % 4.85 wt % full size +1 mm coke full size +0.5 mm coke normal addition method add-8
suction 8 kPa
8
4.05 wt % 4.85 wt % full size +1 mm coke full size +0.5 mm coke normal addition method add-8
9
basicity (lime to silica ratio) lime to silica ratio lime to silica ratio lime to silica ratio melt/coke level
10 11 12 13
1.9
normal addition method 2.4
2.4 1.9 1.4 4.05 wt %
1.9 1.4 1.9 4.85 wt %
add-8:coke addition after 8 min granulation normal: coke addition with fluxes and iron ores
1.9 2.4 1.9 1.4 4.05 wt %
suction 8 kPa iron ores were replaced by sand
Table 2. Coke Particles Size Distribution Coke Dry Sizing (Cumulative Mass % Passing) full size plus 0.5 mm coke plus 1 mm coke
8.0 mm
5.6 mm
4.0 mm
2.0 mm
1.0 mm
0.5 mm
0.25 mm
0.125 mm
0.063 mm
99.6 99.5 99.3
97.1 96.3 95.4
85.3 81.1 76.4
59.4 47.8 34.5
38 20.2
22.3
11.5
5.9
2.8
diameter of 335 mm. About 110 kg of sinter mix is prepared to provide sufficient material to fill the sinter pot. The weighed ores, fluxes and coke are blended in a drum for 1 min before transferring into a granulated drum of 1.1 m internal diameter for 10 min. About 90 kg of the granulated mix is used in a sinter pot. An ignition hood using natural gas is then pulled over the top of the pot to ignite the bed for 90 s at a suction of 6 kPa across the bed. On turning off the burners in the ignition hood, the PID controller raises the suction at the bottom of the bed to 16 kPa or 8 kPa. The flow of air into the bed is determined by a hot wire anemometer placed in a specially designed hood which rests on top of the sinter pot and results are logged by a PC. A small portion of the flue gas is extracted, cleaned and fed to the Testo 350 gas analyzer for the determination of NOx, O2 and CO. In this work NOx is assessed at 6.0% O2 level. The bulk of the flue gas is cleaned in a wet scrubber before venting to the atmosphere. Windbox temperature is recorded continuously using a K type thermocouple. When this value falls below 200 °C, the sinter fan is shut off. On
further cooling the content of the pot is emptied. For this particular program the product sinter is not processed. 3.3. Scope. Twelve sinter pot tests were conducted to investigate the influence of coke rate, coke size, coke addition method and basicity on NOx emission. A 13th test using sand in place of a sinter mix was also carried out and details of all the tests are shown in Table 1. The mixes in tests 1−2 have different coke levels, while tests 3−6 involved removing the minus 1 mm or 0.5 mm fractions of the coke. Details about coke particles size are shown in Table 2. Coke particles are located in different positions within a granule depending on their size and this affects their combustion behavior, which could influence NOx generation. It is also well-established that fine coke increases CO levels and a consequence of this is to lower flame front temperatures. Tests 7−8 involved changing coke addition method. In a normal or standard test, the coke is blended with all the other blend components and granulated. For the late coke addition tests, the coke is only introduced into the mix just before the end of the granulated i.e., after 8 min granulation. This preferentially places all the coke particles on the C
DOI: 10.1021/ef502524y Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 3. Raw Materials Chemical Assay/wt % item
Fe
Al2O3
SiO2
MgO
CaO
P
S
LOI1000
AUS1 BRA1 AUS2 AUS3 BRA2 serpentine limestone dolomite coke
58.07 64.98 60.62 62.39 64.31 5.55 2.91 0.35 1.07
1.26 1.26 2.25 2.23 0.79 1.37 0.79 0.58 4.32
5.09 2.36 4.45 4.28 5.42 37.70 2.16 2.26 6.11
0.07 0.07 0.08 0.13 0.17 36.50 0.36 19.45 0.04
0.08 0.06 0.05 0.15 0.09 1.49 51.32 31.45 0.62
0.04 0.02 0.07 0.08 0.03 0.01 0.01 0.01 0.03
0.02 0.01 0.03 0.02 0.01 0.04 0.07 0.03 0.01
10.20 2.03 5.91 3.51 1.09 14.10 40.80 45.50 86.85
outer surfaces of formed granules, which improves oxygen access and increases the combustion rate of the coke particles. The effect of altering basicity (CaO/SiO2) is explored in tests 9−12. Basicity has a large influence on melt generation and, of particular reference to this work, the formation of calcium ferrites. In test 13 the use of sand to replace the sinter mix means that no calcium ferrites are formed to catalyze the reduction of NOx. For this study, the blend used is composed of five types of iron ore, limestone, dolomite, serpentine and coke. Table 3 shows the chemical compositions of these materials. The last column of the table shows the loss-on-ignition (LOI) values of these materials, determined at 1000 °C. LOI values are strongly dependent on the goethite and carbonate contents of the ores and fluxes, respectively. For coke it is a reflection of its ash value. With the exception of the increased basicity tests, the blends used had a fixed basicity (CaO/SiO2) of 1.9, MgO of 1.7 wt % and SiO2 of 5.0 wt %. The coke addition level was kept at 4.05 wt % except in tests 1 and 2 where some layers had the higher value of 4.85 wt %. The aim mix moisture of the granulated material charged into the pot was fixed at 6.5 wt %. Suction was kept at 16 kPa except for tests 1−2 and 13 where suction is 8 kPa. 3.4. Additional Analysis. Two areas were identified which could assist the interpretation of measured NOx results. The first is flame front temperatures, which was not measured in this work except for tests 7−8. They can be obtained by inserting thermocouples into different regions of the bed. As the flame front passes through the thermocouple, a temperature−time profile is obtained and properties such as the maximum temperature at the flame front can be determined. However, significant effort is necessary to obtain meaningful results. The position of the thermocouple tip−whether it is in close proximity to a burning coke particle or a flux particle undergoing calcination−has a large impact on results. In addition, the formation of melt and shrinkage accompanying the transformation of the particulate bed to a sintered bed means that thermocouples are bonded to the material, often fractures and cannot be reused. This adds very significant additional cost to the project. Proven computer models which simulate the sintering process can also provide this information. The sinter model developed by Zhao is a 1-D dimensional sintering model, which incorporates most of the phenomena and processes involved in sintering process, such as coke combustion, melting and solidification processes, the evaporation of water in the preheating zone and condensation in the wet zone, limestone and dolomite decomposition.29 Several important sintering areas in the model, such as coke combustion, the melting and solidification processes, have been improved on the basis of previous scholars. The model can simulate the sintering process well when input parameters are given, which has been validated against the sinter pot. The input parameters are bed voidage, bulk density, specific heat of iron ores, airflow rate in sintering and the content of moisture, coke, limestone, dolomite. In this study the sinter model developed by Zhao29 is used to simulate the sinter bed temperatures to help us understand the combustion process. Sinter bed temperatures in tests 7−8 were measured by the thermocouple because sinter model cannot simulate the influence of late coke addition well. Information on the level of calcium ferrites formed in the bed is the other area which could help the interpretation of NOx results. During sintering it is believed that the first phase formed from solid−solid
reactions is calcium ferrite. With increasing temperature they decompose and as silica, alumina and FeO enters into the melt a complex form of calcium ferrites−termed silico ferrite of calcium and aluminum (SFCA) − forms. Strictly, the only form of calcium ferrite in sinter is SFCA. The volume percent SFCA in sinter can be estimated using microscopy techniques. Sinter particles from different parts of the pot were composited and crushed to minus 6.3 mm plus 4.0 mm. A number of the particles were mounted in epoxy resin and sectioned. The selected face for microscopy study was polished using a Struers Tegrapol-35. Under a Zeiss Axioskop, images were captured by an Axiocam MRc 5 CCD camera and transferred into a PC. In this work a 20x objective lens was used to capture about 70 images for each sample. Based on gray scale values the phases and minerals could be differentiated, and the volume of SFCA was determined. For pilot-scale tests, it is very important to keep good repeatability of tests. In this work when raw materials arrived at the lab, they were divided by rotate sample divider (RSD) to ensure homogeneity for every test. Then standard operation procedure (SOP) was carried out in order to maintain the repeatability for every sinter pot test. Also for the sake of assessing sinter pot tests repeatability, parallel tests were conducted during debugging stage. The error of important sintering parameters, such as sintering time, sintering airflow, moisture and sinter tumble strength is less than 5%. In this study a Testo 350 gas analyzer was used to determine O2 and CO, NOx. The resolution ratio is 0.01 vol % for O2 and 1 ppm for CO and NOx. A hot wire anemometer from TSI was used to measure the airflow rate during iron ore sintering, and its resolution ratio is 0.01 m/ s. The resolution ratio of the S type thermocouple for tests 7−8 is 1 °C. As shown in Table 1, the raw material of tests 3, 5, 7, 9, and 11 and the sintering suction in the top layer are the same, so we choose the NOx emissions of these tests in the top layer to estimate the experimental error. Table 4 shows that the maximum relative error is less than 5% and the mean relative error is 3.1%. This suggests that the results of the sinter pot tests are useful and acceptable.
Table 4. Experimental Error Estimation test ID
3
5
7
9
11
mean value
NOx/ppm error/%
489 −2.36
497 −0.76
477 −4.75
520 3.83
521 4.03
500.8
4. RESULTS The results involving varying coke addition level (Tests 1 and 2) are given in Figure 2. The start positions of the top, middle and bottom layers are indicated. Flame front burnt-through comes after the bottom layer and, as expected, Figure 2 shows sharp rises in oxygen levels. During sintering, waste gas oxygen level is dependent on the level of coke in the sinter mix, the coke combustion rate and also the combustion efficiency of the coke. Burning to CO rather than CO2 will increase the amount of oxygen in the surrounding gas. Down a sintering bed, combustion efficiency decreases slightly, most probably because of increasing bed temperaturecaused by the preheating of the D
DOI: 10.1021/ef502524y Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 2 also suggests that NOx emission in the bottom layer is lower than that in the upper layer, by about 20−35%. The properties of the sinter mix of the bottom layer are the same as that in the upper layer. However, combustion conditions are very different, being much higher at the bottom layer. High temperature accelerates coke combustion, leading to reduced oxygen and increased CO content of the waste gases.6,29 A lower oxygen content indicates a more reducing atmosphere, which promotes NOx reduction and decreases NOx formation. Increasing CO content can also promote the reaction between CO and NOx.21 Table 5 indicates that the residence time of Table 5. Sintering Time in Different Layers/min
Figure 2. NOx emissions under different coke rates.
air as it flows through the hot sinterincreasing the coke combustion rate. Therefore, down a defined uniform bed, it is to be expected that there will be marginally more oxygen in the gas. While combustion rate also increases along the sinter bed height, this can reduce oxygen in the waste gas. In practice, oxygen is determined on a volume percent basis and is strongly influenced by the gas flow rate through the bed. If the flame front enters the middle layer and this results in increased flame front resistance (e.g., higher temperatures) and reduced gas flow rate, then it is to be expected that oxygen levels in the waste gas will drop sharply. On the other hand, if the middle layer has a lower flame front resistance and gas flow volume increases significantly, then waste gas oxygen level will increase for the same amount of oxygen consumed in the coke combustion process. For these reasons, it is not easy to interpret waste gas oxygen results. Likewise, waste gas NOx results are also strongly dependent on gas volumes. In this study gas volume was also monitored and used to assist the interpretation of oxygen and NOx results. 4.1. Coke Addition Level. Figure 2 shows that when the middle layer has more coke (Test 1), there is a significant drop in oxygen level, which is consistent with expectations and previous results.29 Our results and previous results30 have shown that airflow rate is relatively constant until flame front reaches to the bottom of sinter bed. Figure 2 indicates that the oxygen content of the waste gas in the middle layer is lower by about 9.0 vol % compared to values obtained for the top and bottom layers. Increasing the number of coke particles in the sinter mix means higher oxygen requirement during coke combustion. For Test 2, where the top and bottom layers have the higher coke level of 4.85 wt %, the oxygen content is marginally lower in the top layer and relatively constant in the middle and bottom layers. Bed temperature in the bottom layer will be very high with 4.85 wt % coke rate, and more melts will be formed in the flame front, which can hinder the diffusion of oxygen to the coke particle surface. The resulting decrease in combustion rate will have the effect of raising oxygen levels in the waste gas. Data from a hot wire anemometer shows that airflow rate decreases from 85 m3/h to 65 m3/h when the flame front descends down from the middle layer to the bottom layer in Test 2. This confirms that the drop in airflow rate is the result of very high temperatures at the flame front. The difference of airflow rate between Test 1 and Test 2 explains the difference in oxygen levels.
test
upper layer
midlayer
lower layer
1 2 3 4 5 6 7 8 9 10 11 12
13.1 14.2 9.8 8.1 9.3 9.7 10.3 8.6 9.0 8.6 10.1 10.8
12.1 8.8 6.5 6.6 7.1 6.7 7.0 7.2 6.7 7.0 7.5 7.1
11.2 13.7 7.1 6.3 7.3 7.3 6.0 5.9 5.1 6.8 6.7 9.2
high temperature is shorter in the bottom layer, which can lower NOx formation.22 Increasing temperature favors melt generation31 and increases calcium ferrites formation.32 Table 6 Table 6. Calcium Ferrites in Different Layers/vol % test
upper layer
midlayer
lower layer
1 2
16.9 16.9
19.1 23.4
25.6 22.4
also shows that the calcium ferrites content in the bottom layer is higher than that in the top layer, which would mean increased catalytic conversion of NO x to N 2 and reduced NO x emission.14−16 On the one hand, high temperature can promote NOx reduction by carbon; on the other hand, it can increase the release of nitrogen.33,34 Results suggest that the overall impact of increasing temperature leads to the decrease of NOx emission in the iron ore sintering. The changes in NOx emission are very clear when a threelayered structure is used in sintering. In Figure 2 the influence of increasing coke rate in Test 1 can also be confirmed by Test 2. Figure 2 shows that NOx emission decreases in three layers, by 21% in the top layer, 7% in the middle layer, and 12% in the bottom layer when coke rate is increased from 4.05 wt % to 4.85 wt %. Table 7 indicates that the conversion ratio of coke-N to NOx decreases with the increase of coke rate. Available nitrogen for NOx formation increases with increasing coke level Table 7. Conversion Ratio of Coke-N to NOx test
upper layer
midlayer
lower layer
1 2
14.78 11.62 (13.91a)
11.74 (14.06a) 12.62
11.25 9.87 (11.71a)
a
Data in parentheses is the global conversion ratio of coke-N to NOx combined with coke rate. E
DOI: 10.1021/ef502524y Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
In Test 1, the coke rate in the midlayer is 4.85 wt % and higher than that in the bottom layer. But Figure 2 shows that NOx emission in the midlayer is similar to that in the bottom layer. This suggests that although the reaction between C and NOx is very important for NOx emission, other factors are also important. Sintering time is slightly shorter and there are more calcium ferrites in the bottom layer. Computer model simulation results for Test 1, given in Figure 3, show that the maximum bed temperature in the bottom layer is higher than that in the upper layer. This shows that the influence of temperature on NOx reduction is stronger if temperature is higher. 4.2. Coke Particles Size. Altering coke particles size has a significant impact on the iron ore sintering process. Increasing coke particles size results in lower combustion rate and reduced oxygen consumption rate;6 as a result, there is more oxygen for coke-N oxidation on the coke surface. More NOx should form. Figure 4 shows that the oxygen content in the flue gas increases by about 2% in the bottom layer when plus 1 mm coke particles are used for sintering. When coke particles are coarser, combustion rate decreases6,29 and combustion efficiency increases.35 They both influence sinter bed temperature29 and consequently the level of NOx reduction.33,34 Figure 5a shows that maximum bed temperature decreases slightly when minus 1 mm coke particles are removed. Figure 5b indicates that maximum bed temperature decreases in the top layer and increases slightly in the bottom layer when minus 0.5 mm coke particles are removed. This shows that sinter bed temperature is more dependent on heat generation than on combustion efficiency at low temperature, and combustion efficiency plays a more important role at high temperature. Figure 6 suggests that when coke particles size is coarser, the CO content in the flue gas decreases, which reduces the reaction between CO and NOx.2,34 This means that when coarser coke particles are used as sinter fuel, NOx levels should be higher. On the other hand, the probability of NOx reducing in the pores of coke particle and on the coke surface should increase with increasing coke particles size.12,21 In reality, the impact of altering coke particles size is much more complex. Increasing coke particles size will improve the granulation process, increase sinter bed permeability, and decrease sintering timeall of which would favor a reduction in NOx emission, as shown in Table 5. Figure 4 shows that, on removing minus 1 or 0.5 mm coke particles, NOx emission
in the sinter mix. The overall NOx emitted is determined by both positive and negative contributions during iron ore sintering. In Table 7 the data given in parentheses represents the overall NOx emission in the sinter bed with high coke level. They indicate that the overall NOx emission at high coke level decreases in the top layer and increases in the middle and bottom layers. In Test 2, NOx emission in the upper layer is lower than that in the midlayer, by about 14%. Figure 3 shows that temperature
Figure 3. Sinter bed temperatures in different layers 100/300/500 mm referring to the top of the sinter bed.
in the midlayer is higher than that in the upper layer due to preheating, although the coke level in the upper layer is higher. More calcium ferrites are formed in the middle layer in Test 2 compared to the top layer. The sintering time in the midlayer is shorter compared to that in the upper layer, as shown in Table 5, while the oxygen content in the upper is similar to that in the midlayer. All these can act to reduce NOx emission. On this premise, NOx emission in the midlayer should be lower, but NOx emission for the upper layer with a higher coke level is lower. This is because the higher temperatures in the upper layer have enhanced the reaction between C and NOx and lowered NOx emission.12 This indicates that the reaction between C and NOx has a large effect on NOx reduction.
Figure 4. NOx emissions with different coke particle sizes. F
DOI: 10.1021/ef502524y Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 5. Sinter bed temperatures in different layers with coarser coke particles.
Figure 6. CO curves with different coke sizes.
Table 8. Sinter Bed Temperature in 300 mm Sinter Beda
decreases in the top layer although sinter bed temperature and CO content decrease. This suggests that NOx reduction in the pores of the coke particle and on the coke surface is very important during iron ore sintering. According to the results in the top layer, NOx emission in the bottom layer in Test 4 should also be lower due to increased probability of NOx reducing in the pores of the coke particle and on the coke surface. But Figure 4 shows NOx emission increases in the bottom layer with removing minus 1.0 mm coke particles. Figure 4 indicates that oxygen content increases by about 2 vol.% (from 11.8 vol.% to 13.8 vol.%) in the bottom layer by removing minus 1 mm coke particles. This shows that there is much more NOx formation on the coke surface due to the increase of oxidizing atmosphere in the bottom layer. Figure 4 also shows that, by removing the minus 1 mm coke in the top layer, oxygen content also increases by 1.5 vol.%, but NOx emission decreased. A possible explanation for this is that temperature in the top layer is low (compared to temperature in the bottom layer) and the conversion of coke-N to NOx is not very efficient. 4.3. Coke Positioning. When coke is added late during granulation, coke combustion behavior changes, and this has a large impact on the sintering process. Table 8 shows that sinter bed temperature decreases with late coke addition. As decreasing sinter bed temperature will increase NOx emission, NOx emission should increase with late coke addition. The good access to oxygen and high combustion rate means that
test
max. temp/°C
duration time above 1100 °C/min
7 8
1223 1263
1.97 2.28
a
300 mm refers to the top of the sinter bed (sinter bed height: 600 mm).
sintering time decreases with late coke addition, as shown in Table 5. Consequently, Table 8 suggests that the time the bed spends at high temperature (e.g., >1100 °C) also decreases with late coke addition, which can have an impact on NOx emission. Figure 7 shows that the oxygen content in the upper layer remains relatively unchanged but decreases by 5.3% in the middle layer and 9.2% in the bottom layer. These results show that the combustion condition with late coke addition in the upper layer is comparable to that under normal conditions, but the differences becomes significant in the middle and bottom layers. The most possible reason is that the temperature in the upper layer is relatively low and the combustion rate is controlled by chemical kinetics. Oxygen diffusion plays a more important role in the middle and bottom layers due to higher temperatures and reduced access to oxygen because of the generation of more melt. Figure 8 shows that CO content increases when coke is added late during granulation, which can increase the reaction between CO and NOx.2,34 Figure 7 shows that NOx emission will decrease by 4.5% in the middle and 14.5% in the bottom layer but almost has no change in the top G
DOI: 10.1021/ef502524y Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
facilitates melt formation and the formation of calcium ferrites. As the standard mix basicity is 1.9, these tests involve increasing this value to 2.4 and also decreasing it to 1.4. It is important to note that varying basicity has a large influence on flame front temperature because the calcination of limestone is highly endothermic. Any change in flame front temperature will also alter its resistance to airflow and, therefore, the sintering airflow rate. For Tests 9 and 10, involving layers with basicity of 2.4, results indicate that decreases in NOx emission are not very significant (Figure 9a). But it is easy to find the change of NOx emission when a three layers structure is used and test 10 is carried out. This shows the advantage of the three layers structure. Figure 9b suggests that decreasing basicity can increase NOx emission during iron ore sintering. Figure 10 shows sinter bed temperatures in the different layers with different basicities, and results confirm that maximum bed temperature increases with decreasing basicity. As discussed in section 3.1, down a sintering bed, the increasing bed temperature can result in reduced NOx emission. Figure 9 also shows that the oxygen content of the waste gas is relatively constant, and this suggests that the combustion process is relatively stable with altering basicity. Altering basicity can also influence sinter bed permeability,30 and Table 5 shows sintering time in the different layers. For the high basicity runs (Tests 9− 10), sintering time decreases in the top and middle layers but increases in the bottom layers. For the low basicity runs (Tests 11 and 12), sintering time increases in the three layers. Shortening sintering time can reduce NOx emission.4−6 When basicity increases to 2.4, temperature decreases and sintering time increases in the bottom layer. NOx emission in the bottom layer was expected to increase, but actually decreased. This suggests that there are other factors influencing NOx emission during iron ore sintering. Changing basicity can alter melt formation and mineral phase composition in sinter.31 Table 9 shows the calcium ferrites content of sinter in different layers; calcium ferrites increase with increasing basicity, and this is consistent with previous results.14−16,31 And it is also consistent with the NOx emission variation in Figure 9; this shows that when basicity is altered, variation in the mineral phase has a great influence on NOx emission. Figure 11 shows NOx emission when iron ores were replaced by quartz sand. NOx emission with quartz sand is higher than that during iron ore sintering, by around 60%. When iron ores
Figure 7. NOx emissions with late coke addition.
Figure 8. CO curves with late coke addition.
layer as coke is added late. These indicate that oxygen content has a greater influence on NOx emission than CO content and sintering time. The higher coke combustion rate also means a reduction in oxygen content in the atmosphere becomes stronger, which will reduce NOx emission. 4.4. Sinter Lime Level. Tests 9 to 12 involve changes in the basicity or lime to silica ratio of the mix. Increasing basicity
Figure 9. NOx emissions with different sinter lime levels. H
DOI: 10.1021/ef502524y Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 10. Sinter bed temperatures in different layers with different sinter lime levels.
complex compounds, such as calcium ferrites, can reduce NOx emission. This agrees with previous results well.14−16
Table 9. Calcium Ferrites in Different Layers/vol % test
upper layer
midlayer
lower layer
9 10 11 12
17.1 20.2 19.8 9.4
34.4 20.1 10.1 23.3
21.5 22.5 21.5 19.2
5. DISCUSSIONS 5.1. Influence of Temperature and CO on NO x Reduction in Iron Ore Sintering. There has been a debate on the influence of temperature on NOx reduction. Hill4 believed that reaction of char-N oxidation to NOx is a heterogeneous reaction, so NOx formation was not sensitive to temperature but sensitive to coke particle size and oxygen content surrounding coke particles. Sun33 proposed that when temperature increased in oxyfuel combustion, under a fuel-rich combustion system, NOx reduction would increase, under a fuel-lean combustion system, NOx reduction would decrease, and under both conditions, the conversion of char-N to NO would increase with increasing temperature. Wang34 found that NOx reduction by char and CO increased with increasing temperature. Bed temperature is about 1200 °C in the top layer and 1300 °C in the bottom layer during iron ore sintering.26 In this study all sinter pot tests show that NOx emission decreases with flame front descending, and NOx emission in the bottom layer decreases compared to that in the top layer. NOx emission decreases by 20% as coke rate increases to 4.85% in the top layer. This shows although increasing temperature will result in more coke-N release, NOx reduction also will increase and the conversion ratio of coke-N to NOx will decrease in the iron ore sintering process. Because temperature has a big influence on O2 and CO content in the flue gas and NOx reduction by C and CO, the specific mechanism of action is very complicated. About the influence of CO on NOx reduction, Wang34 found that if CO content increased to a certain value, about 0.6%, increasing CO further has no effect on NOx reduction. Results show that NOx emission decreases and CO content decreases with increasing coke particle size; this also proves that NOx reduction is not very sensitive to CO content. 5.2. Influence of Reduction Atmospheres on NOx Formation in Iron Ore Sintering Process. With late coke addition, NOx emission in the top layer is similar to NOx emission of base conditions, but NOx emission in the middle and bottom layers, especially in the bottom layer, decreases by 14.5%. Decreased sintering time and temperatures in all the three layers happens when coke is added in late. Late coke addition increases waste gas CO content. As mentioned in section 5.1, NOx reduction is not very sensitive to CO content. And during iron ore sintering, the overall effect of the decrease
Figure 11. NOx emission and O2 content of coke and sand blend in the flue gas.
were replaced by quartz sand, the maximum bed temperature is around 800 °C, much lower than the 1200−1300 °C encountered in iron ore sintering. Lowering temperature will increase NOx emission. As discussed above, NOx in the top layer decreases by around 20−35% compared to that in the bottom layer. Predicted temperature results indicate that temperature in the bottom layer is about 150 K higher than that in the top layer. This shows NOx decreases by about 15−25% when temperature increases by 100 K. Furthermore, for the sand test, no catalyst such as calcium ferrites exists to decompose NOx to N2, or accelerate the reaction between CO or carbon and NOx.14−16 With the flame front moving to the middle layer in test 13, bed temperature increases by around 100 °C and NOx emission in the middle layer decreases by 16%. And in the NO.1 test bed, temperature also increases by around 100 °C and NOx emission decreases by 20% when the flame front descends down to the middle layer. Probably, I
DOI: 10.1021/ef502524y Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels of bed temperature will result in the increase of NOx emission. Therefore, Results indicate that other factors are having a greater impact on NOx levels. The oxygen content in the top layer is similar to normal coke addition but decreases in the middle and bottom layers. The change of oxygen content in the waste gas agrees with the variation of NOx emission. These show that reduction atmosphere has a great influence on NOx emission in the iron ore sintering, and it is consistent with previous results in the coal pulverized system.4−6,10−12 If the reduction potential of the atmosphere increases, it will suppress the oxidation of coke-N to NOx and promote NOx reduction to N2. When coke increases to 4.85% in the middle layer, the oxygen content in the flue gas decreases to 9.0% and is lower than that in the bottom layer. Due to the increase of the coke level, the number of coke particles in the raw material increases; this will benefit NOx reduction by carbon.12 But NOx emission in the bottom layer is lower than that in the middle layer. This shows that, in the bottom layer, there is more reduction of the formed NOx compared to the middle layer. The most possible reason is that differences in temperature between the layers have a greater influence on NOx emission. These results clearly show that temperature is probably the most important factor determining NOx emission. High temperature in iron ore sintering can lower the conversion ratio of coke-N to NOx; if the source of nitrogen in sinter fuels is constant, NOx emission decreases. According to this, when flue gas recirculation is adopted, not only is sinter strength improved, but also NOx emission decreases. 5.3. Influence of Coke on NOx Reduction in Sintering. In the iron ore sintering process, a gas−liquid−solid mixture exists in the flame front and the combustion rate of coke particles will decrease significantly when it is encapsulated by other fine particles and melts. The encountered combustion system is quite different from the pulverized furnace and the fluidized bed. Many researchers have considered NOx reduction by coke in the pulverized coal combustion system and fluidized bed,4−12 but there are few reported studies on NOx reduction by coke in the iron ore sintering. The coke particles used are much coarser and the combustion rate slower, which will increase the oxygen partial pressure and increase NOx formation. Results using cokes of different size distributions show that NOx emission decreases by about 5% in the three layers and that the oxygen content in the flue gas is relatively constant as minus 0.5 mm coke particles are removed. And in the middle layer if minus 1.0 mm coke particles are removed, NOx emission will decrease by 8.9% and oxygen content is relatively constant. From the above results NOx can be reduced on the coke surface or in the pores of coke and coke size has a relatively significant influence on NOx reduction. Peter12 found that the conversion of char-N to NOx would decrease with the increase of the number of coal particles in raw materials, because NOx could be reduced as it diffused toward surrounding coal particles. This is consistent with the results in this work. As discussed in section 3.1, when the number of coal particles in raw materials increases, the reaction between NOx and carbon increases and NOx emission decreases.
2.
3.
4.
5.
■
NOx reduction. NOx reduction is controlled by the reactions between NOx and carbon or CO. Increasing coke rate in raw materials will decrease the conversion ratio of coke-N to NOx, and overall NOx emission is determined by the decrease of the conversion ratio of coke-N to NOx and the increase of nitrogen source. As coke rate increases to 4.85 wt %, overall NOx emission decreases in the top layer and increases in the middle and bottom layers. Atmosphere is very important for NOx reduction, and the influence of atmosphere on NOx emission is influenced by temperature; the oxygen content in the flame front of the bottom layer increases by 1 vol.%, and NOx emission will increase by around 15%. Temperature is double-edged on NOx emission: on the one hand, it can increase the conversion of coke-N to NOx; on the other hand, it can increase NOx reduction. The total influence of increasing sinter bed temperature can lower NOx emission. In the iron ore sintering, increasing temperature by about 100 K can decrease NOx emission by 15−25%. In the iron ore sintering process, NOx can be reduced on the coke surface and in the pores of coke particles effectively. NOx emission decreases by 5−10% with removing minus 0.5 mm of coke.
AUTHOR INFORMATION
Corresponding Author
*Tel: +86-571-87952598; Fax: +86-571-87951616; E-mail address:
[email protected]. Notes
The authors declare no competing financial interest.
■ ■
ACKNOWLEDGMENTS This work was supported by National Basic Research Program of China (2015CB251501). REFERENCES
(1) World Steel Association, Crude steel production, http://www. worldsteel.org/statistics/crude-steel-production.html (Accessed January 8, 2013). (2) Chen, Y. G.; Guo, Z. C.; Wang, Z.; et al. Int. J. Miner. Met. Mater. 2009, 16, 143−148. (3) Mou, J. L. A study of in-plant de-SOx,de-NOx in iron ore sintering process. Ph.D. Thesis, University of Wollongong, Wollongong, 1997. (4) Hill, S. C.; Smoot, L. D. Prog. Energy Combust. Sci. 2000, 26, 417−458. (5) De Soete, G. G.; Croiset, E.; Richard, J.-R. Combust. Flame 1999, 117, 117−140. (6) Cen, K. F.; Yao, Q.; Luo, Z. Y.; et al. Combustion theory and pollutants control; Zhejiang University Press: Hangzhou, 2002. (7) Jones, J. M.; Harding, A. W.; Brown, S. D.; et al. Carbon 1995, 33, 833−843. (8) Winter, F.; Wartha, C.; Loffler, G.; et al. Can. J. Chem. Eng. 1999, 77, 275−283. (9) Liu, H.; Gibbs, B. M. Fuel 2001, 80, 1211−1215. (10) Molina, A.; Murphy, J. J.; Winter, F.; et al. Combust. Flame 2009, 156, 574−587. (11) Harding, A. W.; Brown, S. D.; Thomas, K. M. Combust. Flame 1996, 107, 336−350. (12) Nelson, P. F.; Nancharrow, P. C.; Bus, J.; et al. P. Combust. Inst. 2002, 29, 2267−2274. (13) Teo, C. S.; Mikka, R. A.; Loo, C. E. ISIJ Int. 1992, 32, 1047− 1067.
6. CONCLUSIONS 1. In the iron ore sintering process, coke combustion plays a very important role in NOx emission. The conversion of coke-N to NOx is determined by NOx formation and J
DOI: 10.1021/ef502524y Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels (14) Pan, J. Theoretical and process studies of the abatement of flue gas emissions during iron ore sintering. Ph.D. Thesis, Central South University of Technology, Changsha, 2007. (15) Pan, J.; Zhu, D. Q.; Zou, X. L.; et al. Theoretical studies of the emission rule of SO2 and NOx in iron ore sintering; Proceeding of the 4th Int. Congr. Bioinformatics and Biomedical Engineering (iCBBE); Chengdu, June 18−20, 2010. (16) Morioka, K.; Inaba, S.; Shimizu, M.; et al. ISIJ Int. 2000, 40, 280−285. (17) Chen, Y. G.; Wang, Z.; Guo, Z. C. J. Iron Steel Res. 2009, 21, 8− 11. (18) Chen, Y. G.; Guo, Z. C.; Wang, Z. Fuel Process. Technol. 2009, 90, 933−938. (19) Xiong, W.; Liao, J. Y.; Bi, X. G. Sintering Pelletizing 2007, 32, 12−15. (20) Bi, X. G.; Liao, J. Y.; Xiong, W.; et al. Environ. Eng. 2006, 24, 42−44. (21) Chen, Y. G.; Guo, Z. C.; Wang, Z. J. Iron Steel Res. 2009, 21, 6− 9. (22) Mo, C. L.; Teo, C. S.; Hamilton, I.; et al. ISIJ Int. 1997, 37, 350−357. (23) Gan, M.; Fan, X. H.; Cen, X. L.; et al. ISIJ Int. 2012, 52, 1574− 1578. (24) Lee, M. S.; Shim, S. C. ISIJ Int. 2004, 22, 470−475. (25) Kramlich, J. C.; Linak, W. P. Prog. Energy Combust. Sci. 1998, 20, 149−202. (26) Jin, Y. L. Sintering Pelletizing 2004, 29, 6−8. (27) Zhou, H.; Zhao, J. P.; Loo, C. E.; et al. ISIJ Int. 2012, 52, 1550− 1558. (28) Zhou, H.; Liu, Z. H.; Cheng, M.; et al. Appl. Therm. Eng. 2015, 72, 307−314. (29) Zhao, J. P. Numerical Modelling of the Iron Ore Sintering Process and its Experimental Validation. Ph.D. Thesis, Zhejiang University, Hangzhou, 2012. (30) Loo, C. E.; Hutchens, M. F. ISIJ Int. 2003, 43, 630−636. (31) Loo, C. E.; Leung, W. ISIJ Int. 2003, 43, 1393−1342. (32) Umadevi, T.; Brahmacharyulu, A.; Sah, R.; Mahapatra, P. C. Ironmaking Steelmaking 2014, 41, 410−417. (33) Sun, S. Z.; Cao, H. L.; Chen, H.; et al. P. Combust. Inst. 2011, 33, 1731−1738. (34) Wang, C. A.; Du, Y. B.; Che, D. F. Energy Fuels 2012, 26, 7367− 7377. (35) Loo, C. E. Ironmaking Steelmaking 1991, 18, 33−40.
K
DOI: 10.1021/ef502524y Energy Fuels XXXX, XXX, XXX−XXX
