- Email: [email protected]
Modeling NOx emission of coke combustion in iron ore sintering process and its experimental validation
Fuel 179 (2016) 322–331
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Modeling NOx emission of coke combustion in iron ore sintering process and its experimental validation Hao Zhou ⇑, Mingxi Zhou, Zihao Liu, Ming Cheng, Jianzhong Chen State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou 310027, PR China
a r t i c l e
i n f o
Article history: Received 14 October 2015 Received in revised form 27 March 2016 Accepted 28 March 2016 Available online 1 April 2016 Keywords: Coke combustion Iron ore sintering NOx emission model
a b s t r a c t NOx emission of coke combustion in iron ore sintering was modeled by overall reaction rate equations of NOx formation and reduction. Incorporating into a previous sintering heat treatment model, overall NOx emission can be predicted and the simulated results were well agreed with four sinter pot tests under varying conditions which are similar to actual production. In sintering, NOx emission is significantly related to fuel combustion. Due to heat input by ignition and smaller airflow in the initial stage of sintering, the predicted NOx emission has a higher value of about 350 ppm first then it decreases a little and keeps at a relatively constant level of about 300 ppm until the burn-through point, and decreases rapidly as a result of the accomplishment of coke combustion. Simulation results indicate that fuel NOx is the main NOx emission in sintering while thermal NOx is rarely produced since the bed temperature is much lower than 1800 K. The generated NOx could be reduced not only on the surface and in the pores of coke but also by CO around coke particles, about 50% and 10% of the generated NOx could be reduced by char and CO, respectively. Increasing coke rate and decreasing coke size promote NOx generation by accelerating the coke combustion. The reduction extent by char is greatly influenced by contact between NOx and char while the reduction extent by CO is mainly determined by the combustion atmosphere. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Iron ore sintering is a basic agglomeration process providing sinter for blast furnace. From environmental protection point of view, iron ore sintering is also a key pollution emission process in integrated steelworks. Diverse pollutants including SOx, NOx, dust and dioxins are produced from sintering, accounting for around 50% of total pollutant release in an integrated steelworks. Crude steel production in China during calendar year 2014 was 822.7 million tons, it was estimated that the produced sinter should be as high as around 900 million tons. About 0.1 million tons of NOx is emitted annually in sintering process due to its huge production capacity and poor emission control [1]. With stricter emission control regulations, controlling NOx emission in iron ore sintering has been paid much more attention to in recent years. Most researchers have concerned for NOx controlling methods in sintering. Those methods to reduce NOx emission contain SCR (selective catalytic reduction) treatment of flue gas [2], modification of fuel [3], flue gas recirculation [4], as well as the utilization of biomass fuel [5]. However, little literature was published on the
⇑ Corresponding author. Tel.: +86 571 87952598; fax: +86 571 87951616. E-mail address: [email protected] (H. Zhou). http://dx.doi.org/10.1016/j.fuel.2016.03.098 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
mechanisms of NOx production in sintering by far. It is acknowledged that NOx is mainly produced from fuel combustion in sintering [6], though the NOx emission of fuel combustion in pulverized coal boiler or fluidized bed has been investigated sufficiently [7–9], the traditional fuel used in sintering such as coke is much different from fuels in coal fired system. Coke size in sintering is 0–5 mm, which is much coarser than pulverized coal with size of micrometers. On the other hand, coke combusts with flame front moves down sinter bed in a complex zone consisted of gas, solid and melted liquid, resulting a large distinction of combustion atmosphere from coal fired system. Zhou et al. [10] investigated the factors of coke combustion influencing NOx emission in sintering using a novel three-layer sinter bed. Compared to its experimental way which has the disadvantages of large operating cost and quite time-consuming, mathematical model is more economic to be applied to practical situations and can give a more fundamental understanding of the problem. Most models developed for simulating iron ore sintering in recent years have been focused on the thermal pattern and combustion characteristics in sinter bed [11–15], rather than pollutant emission in flue gas. Only Cavaliere [16] built a data base and employed a multiobjective optimization tool to evaluate the effect of some processing parameters on pollutant emission and sinter productivity. However, this machine
323
H. Zhou et al. / Fuel 179 (2016) 322–331
learning method based on large amount of data is not analytical enough. Models simulating NOx emission in sintering need to be developed further. The aim of the present study is to model the formation and reduction of NOx in sintering process by overall reaction rate equations. Submodels of NOx emission are incorporated into a previous sintering heat treatment model, then the predicted overall NOx emission is validated by four sinter pot tests. Moreover, influences of various mechanisms on the overall NOx emission are quantified. 2. Modeling sintering heat treatment In iron ore sintering process, as shown in Fig. 1, raw materials including iron ores, fluxes, return fines and fuel are blended and mixed first. The blended mix is further granulated in drum with the addition of water to get a coarser particle size and narrower size distribution. The granules are then charged onto a moving strand forming a porous packed bed with proper bed permeability. The upper bed is ignited for around 90 s and the fuel in blended mix supplies about 80% of the whole heat to partially melt granules and convert it to sinter. The formed sinter would be further crushed and sized, usually the +5 mm sinter is used as the product for ironmaking in blast furnace while the 5 mm part is returned to sinter again. After ignition, a flame front forms at the top bed and traverses downward along the bed. Many complex exothermic and endothermic reactions happen in flame front and change the characteristics of flame front in turn. Since all the iron ores, fluxes and return fines have nitrogen rarely, NOx emission during sintering should be mainly produced from the release of nitrogen in the combusted fuel and conversion of nitrogen in air, which is greatly influenced by the heat conditions in sinter bed. A 1-D transient sintering heat treatment model was developed by our group before [17]. The sintering process was described by governing equations including mass, species, energy and
momentum conversion for both solid and gas phases. The general form of the partial differential equation is
@ @ @ @n ðe q nÞ þ ðe q u nÞ ¼ þ Sn C @t @y @y @y
ð1Þ
where the general variable n can be velocity, mass, enthalpy, etc. C and Sn are the relevant diffusion coefficient and source term respectively. Many complicated physical and chemical interactions were considered, such as coke combustion, calcination of limestone and dolomite, drying and condensation of water, melting and solidification. Specific to the coke combustion, unreacted core model shown in Fig. 2 considering the resistance of ash layer to gas was used and coke oxidation rate was calculated as follows.
Rcoke ¼
2AC O2 M c þ k1 þ kk1c kcoating 1
ð2Þ
f
where Rcoke is the combustion rate of coke (kg/s/m3), A is volumetric surface area of coke particle (m2/m3), C O2 is oxygen mole concentration at coke particle surface (mol/m3), Mc is carbon molecular weight (kg/mol), kcoating and kf are mass transfer coefficient of ash layer and gas boundary layer respectively (m/s), kc is chemical reaction rate constant (m/s), k is a factor of particle area. In the previous heat treatment model, it was assumed that CO is the main product of coke oxidation because the bed temperature in combustion zone is greater than 1000 K. Then the CO produced from coke would be further oxidized in gas phase. Table 1 gives the kinetic parameters for coke oxidation and CO oxidation. The oxidation kinetics of fuel do vary greatly with the characteristic of fuel and the combustion atmosphere. Based on the previous 1D sintering model, the heat conditions of gas and solid phases could be predicted correctly, indicating that the referred kinetic data could represent the combustion of the particular coke used in present study. Combining with submodels of NOx formation and reduction, the overall NOx emission could be determined.
Fig. 1. Schematic of a typical sintering process.
324
H. Zhou et al. / Fuel 179 (2016) 322–331 Table 2 Reactions considered in sintering NOx emission model. Reaction
Formula
Fuel NOx formation
Char N ! NO
Thermal NOx formation
O2
k1
N2 þ O NO þ N k2 k3
N þ O2 NO þ O k4
k3
N þ OH NO þ H k4
NOx reduction by char NOx reduction by CO
Char
NO ! N2 þ . . . NO þ CO ! 12 N2 þ CO2
3. Modeling NOx formation and reduction 3.1. NOx formation
Fig. 2. Schematic of unreacted core model of coke combustion.
Table 1 Kinetic parameters for coke oxidation and CO oxidation. Reaction
A (s1)
E (kJ/mol)
Reference
2C + O2 ? 2CO 2CO + O2 ? 2CO2
5.03 105 3.25 107
180 15.1
[18] [19]
Fig. 3 shows the overall feature of the sintering NOx emission model and the reactions considered are given in Table 2. Details of NOx submodels are reported below.
3.1.1. Fuel NOx Coke and anthracite are generally used as fuel in sintering, which have little volatile. For coke, its volatile content averages as small as 1–2 wt.% and its ignition temperature is more than 500 °C. In recent years, some pilot-scale experiments have been carried out to investigate the effects of replacing coke with some potential substitution fuels in sintering such as charcoal and biomass char. However, charcoal and biomass char have higher porosity than coke particles as well as higher volatile matter, resulting in higher fuel reactivity [20]. The combustion of charcoal/biomass char would start earlier than coke and promotes the flame front speed during sintering, resulting in smaller bed temperature and less melt formation, which are unfavorable to the tumbler strength of sinter [21]. Therefore, the utilization ratio of charcoal and biomass char is limited while the coke is still widely used in industrial sinter strand [22]. As the main aim of present paper is focused on
Fig. 3. Overall feature of the sintering NOx emission model.
325
H. Zhou et al. / Fuel 179 (2016) 322–331
the NOx emission of coke combustion in actual production, it is assumed that all the nitrogen in coke is char-N. Only the conversion of char-N to NOx is considered for fuel NOx formation and the conversion of volatile-N to NOx is neglected. With the progress of coke combustion, it has been reported that carbon and nitrogen in coal are not released in any particular order in most cases [9,23,24]. Though there has been different views on the oxidation mechanisms of char-N to NOx, some researchers agreed the conversion is directly happened without homogeneous reaction [9,25,26], for example, Cen et al. [25] indicated that charN is oxidized to NO directly without homogeneous reaction and the release rate of nitrogen is proportional to that of carbon. The conversion of char-N to NO should increase with increasing burn-out rate of coal [25]. Other researchers hold the opinion of the oxidation is happened through homogeneous reactions [7,27–29]. However, direct oxidation is widely used in modeling [9,12–17,23–26,30]. Nelson et al. [31] pointed out the conversion rate of char-N to NO in single char particles ranges from 75% to 100% with the condition of 1000–1400 K and 5–20 vol% O2. Referring to previous research results, it is assumed in present model that the release of nitrogen is linked to the char-C oxidation rate proportionally, and the char-N is oxidated to NOx directly without other intermediate product. Typical flue gas emission in our sinter pot tests before showed that the ratio of NO/NO2 is as high as 31. NO accounts more than 90% for the overall NOx emission in sintering. Therefore, it can be assumed that char-N in coke is oxidated to NO only. The reaction rate of char-N to NO in sintering is calculated as Eq. (3) [24].
RNO;f ¼
Rcoke gY N M NO w MN
1=2
RNO;t ¼ 3 1014 C N2 C O2 expð542; 000=RTÞ
ð7Þ
where RNO,t is the release rate of thermal NO during coke combustion (kg/s/m3), C N2 is the mole concentration of N2 (mol/cm3), T is the coke particle temperature (K), R is universal gas constant (J/mol/K). 3.2. NOx reduction 3.2.1. Reaction of char and NOx Glarborg et al. [8] reviewed that, even though the details of char-N oxidation mechanism was not very clear by far, it has been widely accepted that the overall reaction should be the char-NO formation first and subsequent reduction of NOx by char. At first, the nitrogen C(N) on the char surface reacts to form NO,
CðNÞ þ O2 ! NO þ CðOÞ
ð8Þ
Then a portion of the formed NO diffuses to bulk gas while some may be absorbed on the char surface again. Here Cf is a free carbon site, which can react with NO producing a new nitrogen surface species C0 (N).
2Cf þ NO ! C0 ðNÞ þ CðOÞ
ð9Þ 0
The reactivity of this new C (N) is different from the char-N site C(N). C0 (N) and NO in the bulk gas may react to form free nitrogen. Besides, two C0 (N) species may recombinate to form free nitrogen also. In the temperature range of 900–1200 K, the former mechanism dominates the formation of N2 while the latter one is less important.
ð3Þ
C0 ðNÞ þ NO ! N2 þ CðOÞ
ð10Þ
where RNO,f is the release rate of fuel NO during coke combustion (kg/s/m3), g is the burn-out rate of coke (%), YN is the fraction of nitrogen on a mass basis in coke (wt.%), MNO and MN are the molecular weight of NO and N respectively (kg/kmol), w is the conversion rate of char-N to NO during coke oxidation.
C0 ðNÞ þ C0 ðNÞ ! N2 þ 2Cf
ð11Þ
3.1.2. Thermal NOx Atmospheric nitrogen could be oxidated to thermal NOx in fuel-lean environments with relatively high temperature [9]. And this kind of thermal NOx is really sensitive to the combustion temperature. As so far, Zeldovich mechanism is the most widely accepted model to explain the formation of thermal NOx by three reversible elementary reactions [9,25]. The overall reaction rate of three reversible thermal NO reactions is showed in Eq. (4). When initial concentration of NO is low, Eq. (4) can be simplified to Eq. (5) because the forward reactions of these mechanisms dominate during combustion. Assuming oxygen atoms are in equilibrium with O2 further, Eq. (5) could be written to Eq. (6) [9,25].
d½NO ¼ 2½O dt
( ) 2 ½NO k1 ½N2 2k1k2k½O 2 ½NO 1 þ k2 ½Ok1 2 þk3 ½OH
ð4Þ
d½NO ¼ 2k1 ½N2 ½O dt
ð5Þ
d½NO 0 ¼ 2k ½N2 ½O2 1=2 dt
ð6Þ
For fuel-lean combustion process with oxidizing atmosphere, Eq. (7) is a simplified form of Eq. (6) based on experimental results of k0 by Zeldovich to model the thermal NOx production. In iron ore sintering, air is sucked into the packed bed continuously and coke is combusted in a relatively strong oxidizing atmosphere. So Eq. (7) is used to calculate the thermal NOx.
Increasing char particle size and pressure may lead to longer residence time of NO in the pores, increasing the reactivity of char and concentration of NO in gas phase would increase the reaction probability, a larger reduction of NO by char would be obtained [8]. Schonebeck et al. [32] and Zhang [33] combined experiments and simulations to study the reaction between char and NO in high temperature. They found that NO-char reaction is in first order and the NO would be reduced more with increasing NO concentration in gas phase. The reaction rate in single coke particle is [33,34]:
RNO;c ¼ kSP NO g
ð12Þ
E k ¼ k0 exp RT
ð13Þ
1 3 p d q Ai 6 P P 3 1 1 g¼ / tan h/ / S¼
ð14Þ ð15Þ
where RNO,c is the reaction rate of NO reduction by char during coke combustion (kg/s/m3), k is the intrinsic reactivity (kg/cm2/s/Pa), S is the internal surface area of coke (m2), PNO is the partial pressure of NO in bulk gas (Pa), g is the effectiveness factor, namely the ratio of actual reaction rate to the theoretical maximum reaction rate, k0 and E are the pre-exponential factor and activation energy of the reaction between NO and char with the value of 0.0045 and 144,000 respectively (kg/cm2/s/Pa, J/mol), dP is the coke particle diameter (m), qp is the apparent density of coke (kg/m3), Ai is the specific internal surface area of coke particle (m2/kg), / is the Thiele modulus, can be calculated according to Eq. (16) [33,34].
326
H. Zhou et al. / Fuel 179 (2016) 322–331
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi dp cAi qp kPox /¼ 2 C ox De
ð16Þ
Test number
where c is the stoichiometric coefficient of reaction C(s) + O2(g) ? CO2(g), Cox is the oxidant density in bulk gas (kg/m3), De is the effective diffusion coefficient in coke particle pores (m2/s), which is determined by the Knudsen diffusion coefficient Dk and the bulk molecular diffusion coefficient D0 [33,34]:
De ¼
ec 1 1 þ s Dk D0
Table 3 Conditions of the sinter pot tests.
Blend details (% dry total basis)
1 ð17Þ
where ec is the porosity of coke particle, s is the tortuosity of pores, pffiffiffi a typical value of 2 was used. The bulk molecular diffusion coefficient D0 is [33]:
D0 ¼ C
½ðT þ T 1 Þ=2 dp
0:75
ð18Þ
where C is a rate constant of diffusion process with the value of 6.26 1011 (m3/K0.75/s), T 1 is the bulk gas temperature (K). The Knudsen diffusion coefficient Dk is [33,34]:
Dk ¼ 97rp
sffiffiffiffiffiffiffiffiffiffi T MNO
Basicity (CaO/SiO2) SiO2 (%) MgO (%) Target moisture (%) Bed height (mm) Ignition suction (kPa) Sinter suction (kPa)
Case 2
Case 3
Case 4
19.00 22.34 18.69 0 0 0 7.44 19.94 0 6.30 1.50
18.99 22.34 18.69 0 0 0 8.51 20.01 1.07 4.28 1.51
19.56 23.01 19.25 0 0 0 5.95 20.00 0 6.36 1.50
10.26 10.26 0 20.53 10.26 10.26 9.11 20.00 0.14 5.11 0
0.43 4.35 2.0 5.15 2.0 6.35 600 6 16
0 4.60 2.0 5.15 2.0 6.35 600 6 16
0 4.35 2.0 4.60 2.0 6.35 600 6 16
0 4.05 1.9 5.00 1.7 6.50 500 5 13.3
ð19Þ Table 4 Ultimate and proximate analysis and key parameters of coke.
where r p is the mean pore radius of coke particle (m). 3.2.2. Reaction of CO and NOx The formed NO can not only be reduced by char itself, but also can react with CO on the surface of coke or some catalysts, which has been confirmed by many experiments [19,35–37]. Chen et al. [38] found that the reduction reaction of NO and CO could be catalyzed by CaO, Sinter, Fe2O3 and MgO in blend, decreasing the overall NOx emission. In most studies, the reaction between NO and CO is first order in NO. For the simplicity of the model, only the reaction between CO and NO on the surface of single coke particle is considered, the reaction rate is calculated as Eq. (20) [36,39].
RNO;co ¼ kNO—CO C NO C CO
Iron ore A Iron ore B Iron ore C Iron ore D Iron ore E Iron ore F Limestone Return fine Serpentine Dolomite Hydrated lime Silica sand Coke
Case 1
Item Cad Had Nad St, ad Oad
83.3 1.1 1.1 0.7 0.4
Proximate analysis (mass %)
Mad Aad Vad FCad
1.1 13.4 1.6 83.9
Size distribution (mass %)
6.3–8 mm 3.2–6.3 mm 1.18–3.2 mm 0.6–1.18 mm 0.3–0.6 mm 0.15–0.3 mm 0.106–0.15 mm 0.075–0.106 mm 0.063–0.075 mm 0.034–0.063 mm 0.034 mm
1.2 1.3 23.9 31.7 15.1 11.8 7.4 2.4 1.6 0.8 3.0
ð20Þ
where RNO,co is the reaction rate of NO reduction by CO during coke combustion (kg/s/m3), kNO–CO = 3.68 107 exp (108,889/RT). 4. Model validation In practice, granulation moisture, coke rate, basicity (CaO/SiO2) as well as sinter suction are key controlled parameters to get the desired sinter productivity and strength. The model of NOx emission was validated by sinter pot tests focusing on the cases which are similar to actual plant production. Testing details are listed in Table 3. Case 1 to case 3 were sintering an Australia-ore blend with high aluminum content. In case 1, the SiO2 content was adjusted by silica sand to the value of 5.15% while in case 2 the SiO2 content was adjusted by serpentine and coke rate was increased from 4.35% to 4.60%. In case 3, neither silica sand nor serpentine were used and the SiO2 content was decreased to 4.60%. Case 4 was sintering a typical ore blend used in Asia–pacific region. However, the sinter bed height and sinter suction were five-sixths of the level of former three cases. The input parameters of model are obtained from sinter pot test including bulk density, bed voidage, airflow rate after ignition, and the composition of sinter mixture (the content of water, coke, limestone, dolomite, etc.). The important parameters of coke used in simulation are summarized in Table 4. As the unreacted-core model was applied to model the combustion of coke particle, the particle size of coke definitely influences the combustion rate due to different gas diffusion potential. To correct
Coke
Ultimate analysis (mass %)
Apparent density (kg/m3) True density (kg/m3) Mean pore radius of coke particle (nm)
1200 1900 0.2
the reaction rates of coke combustion for model validation, the size distribution of coke was measured and dispersed into total 13 sizes in simulation. The governing equations of solid and gas phases were numerically solved by Finite-Volume Method while SIMPLE algorithm was used to solve the relationship between gas pressure and gas velocity of the gas momentum equation. The computational domain was discretized into grids with the space step of 1.2 mm bed height and the time step of 2 s, sufficient to capture the physicochemical changes in sinter bed. For conducting the sinter pot test, all raw materials were dry mixed for 1 min first and then wet granulated for 10 min in a rotary drum with the diameter of 1.1 m to get a steady-state of granules. Around 90 kg granules were poured into a sinter pot with the height of 600 mm and the diameter of 330 mm. Fig. 4 shows the schematic view of sinter pot used. The sinter bed was ignited by a natural gas burner with 1400 K temperature, after 90 s
H. Zhou et al. / Fuel 179 (2016) 322–331
327
Fig. 4. Schematic view of sinter pot.
ignition, the natural gas burner was removed and a hood was put on the sinter bed. The suction value was raised from 6 kPa during ignition to 16 kPa for sintering except in case 4. Air was sucked into the sinter bed by suction fan and its velocity was monitored by a hot wire anemometer with the resolution ratio of 0.01 m/s. The flue gas was sampled and its NOx and O2 content was analyzed by a Testo 350 continuously. The resolution ratio is 1 ppm for NOx and 0.01 vol% for O2. When the flue gas temperature measured just below the sinter pot got a maximum value and began to decrease, the experiment was done. Three thermocouples could be inserted into sinter pot at the location of 100, 300, 500 mm from top of the bed to record bed temperature profiles. Since the previous heat treatment model was validated by a large amount of pot tests results [17], the accuracy of its prediction results of heat conditions can be reliable and thermocouples were not used in this study. Only the flue gas composition were applied to validate the present NOx emission model. Fig. 5 compares the NOx emission from experimental measurement and model prediction [40]. It can be seen that the predicted NOx agrees well with the measured NOx in most cases. There is a distinction between the measured and predicted NOx in case 3. However, the average error is about 18.8%, which is still acceptable from the industrial point of view. Neither serpentine or silica sand was used in case 3 to adjust the SiO2 content of raw materials’ chemical composition, so calcium ferrite in the formed melt would contain less SiO2 and would be more evenly distributed, resulting in a greater catalytic effect on the NOx reduction reactions [38]. The results of four sinter pot tests are listed in Table 5, it needs to be noted that the flow rate of flue gas varies much under different conditions. High concentration of NOx in flue gas is not necessarily to mean high conversion efficiency of char-N to NOx in coke. In all cases, it can be observed from Fig. 5 that the predicted NOx emission has a higher value at the beginning of sintering then decreases a little and keeps at a relatively constant level until the burn-through point. Pahlevaninezhad et al. [12] tested some kinetic parameters on combustion characteristics in sintering bed by their unsteady-2D axisymmetric sintering model and their simulation results showed that, the maximum bed temperature increases rapidly due to the heat input of ignition first, then
decreases a little bit when cold air begins to flow into sintering bed and increases again as a result of the heat accumulation in sintering bed. Therefore, the possible reason why this changing trend of predicted NOx happened is the oxidation rate of char-N to NOx closely relates to the maximum bed temperature. Higher coke combustion rate and higher char-N oxidation rate due to heat input by ignition leads to the initial high NOx emission value. In addition, whatever the condition is, the measured O2 in flue gas increased earlier than the model predicted and the measured NO decreased earlier also than predicted in the late stage of sintering. The reason why the mismatching happen should be an unflat flame front exists in experimental sintering bed, which differs from the hypothesis of flat flame front in model. Due to the shrinkage of raw materials, air velocity in the edge of sinter pot should be larger than the center of sinter pot. A faster flame front speed would be got in the edge of sinter pot than the center, resulting in an earlier burnt-through in the edge. 5. Discussion 5.1. Quantification of NOx formation and reduction on overall NOx emission To quantify the NOx formation and reduction on overall NOx emission, case 1 which reaches return fine balance was adopted to perform simulation considering different NOx formation and reduction mechanisms. Fig. 6a compares the total formed NOx without reduction during sintering and the NOx emission considering reduction by char only, therefore the reduction extent of the generated NOx by char can be determined. The concentration of generated NOx by coke combustion is around 680 ppm and when reduction of NOx by char is considered, the NOx emission decreases to about 320 ppm. Almost 50% formed NOx is reduced in the pores and on the surface of coke particles. As discussed in introduction, coke combustion in sintering happens in a hightemperature zone consisted of gas–liquid–solid mixture. Though the atmosphere around coke particles is much different from coal fired system, it is the same that the generated NOx could be reduced by char itself significantly.
H. Zhou et al. / Fuel 179 (2016) 322–331
O2 measured
O 2/vol.%
500
NOx predicted NOx measured
400 300 200 100
600
&DVH
600
O2 predicted
NO x /ppm
40 35 30 25 20 15 10 5 0 1200
1800
0 3000
2400
40 35 30 25 20 15 10 5 0
600
O2 predicted O2 measured
500
NOx predicted NOx measured
400 300 200
NO x /ppm
&DVH
O 2/vol.%
328
100 1200
600
1800
0 3000
2400
O2 measured
O 2/vol.%
NOx measured
400 300 200
NO x /ppm
500
NOx predicted
100 600
&DVH
600
O2 predicted
1200
1800
0 3000
2400
40 35 30 25 20 15 10 5 0
600
O2 predicted O2 measured
500
NOx predicted NOx measured
400 300 200
NO x /ppm
&DVH
O 2/vol.%
Time/s 40 35 30 25 20 15 10 5 0
100 600
1200
1800
2400
0 3000
Time/s Fig. 5. Comparasion of predicted and measured NOx emission [40].
Table 5 Sintering parameters of the four sinter pot tests. Test number
Case 1
Case 2
Case 3
Case 4
Experimental moisture (%) Sintering time (min) Maximum windbox temperature (°C) Flame front speed (mm/min) Sinter air flow (N m3/h) Sinter yield (%, +5 mm) Productivity (t/m2/d) Tumbler index (%, +6.3 mm)
6.18 32.92 688.7 18.2 113.8 72.7 28.7 65.9
6.14 30.07 681.9 19.9 125.5 75.0 32.1 65.8
6.45 25.93 674.0 23.1 146.8 72.3 35.1 62.7
6.55 21.30 601.7 23.5 126.3 69.0 31.7 60.6
The total generated NOx without reduction during sintering and the NOx emission considering reduction by CO only are compared in Fig. 6b. Compared to total formed NOx with the value of 680 ppm, NOx emission in the case of considering reduction of NOx by CO only becomes about 580 ppm. It is consistent with previous work that the reaction between CO and NO is also effective and cannot be neglected.
Fig. 7 illustrates the predicted temperature profile of the 100 mm, 300 mm, 500 mm bed height respectively. Due to heat accumulation in sinter bed, the maximum bed temperature and thickness of high-temperature zone increase with flame front descends down sinter bed. It can be seen that the bed temperature is much lower than 1800 K, therefore, negligible thermal NOx is produced in sintering process [9]. In the submodel of coke combustion, Loo [41] and Zhao et al. [17] assumed that CO is the main product when temperature is higher than 1000 K. Therefore, the CO generated from coke oxidation is split into two fractions: a fraction diffuses into bulk gas and reacts with O2 to form CO2 while the rest spreads around coke surface and react with NO to reduce formed NOx. Generally, the concentration of CO in flue gas during sintering is in the range of 0.5–1.5%. The heat condition changes gradually down a bed due to preheating of the combustion air, a more reducing atmosphere may be formed in lower bed compared to upper bed. As shown in Fig. 6b, with flame front descends down sinter bed, the reduction of NOx by CO keeps at a relatively constant level. Possible explanation is that, when CO concentration reaches
329
H. Zhou et al. / Fuel 179 (2016) 322–331
(a)
NOx /ppm
1500
Fuel NOx + Thermal NOx Considering reduction of NO by char only
1000
500
0 600
1200
1800
2400
3000
Time/s 1500
Fuel NOx+Thermal NOx
NOx /ppm
Considering reduction of NOx by CO only 1000
CO
110 ppm 500
0 600
1200
1800
2400
10 9 8 7 6 5 4 3 2 1 0
CO/vol.%
(b)
3000
Time/s
16
900
100mm 300mm 500mm
0
600
1200
1800
2400
3000
t/s Fig. 7. Predicted bed temperature along the sinter bed of different heights [40]. Note: 100/300/500 mm are the distance from the top of sinter pot.
800 700
Reduction by Char Reduction by CO NOx emission O2
14
600
57.0%
500
45.0%
50.9%
12 10
400 300
10.1%
11.3%
11.8%
O2 /%
1600 1400 1200 1000 800 600 400 200 0
NOx concentration/ppm
T/K
Fig. 6. Quantification of NOx reduction in iron ore sintering [40]. (a) NOx reduction by char and (b) NOx reduction by CO.
8
200 6 100
a particular value of 0.6 vol%, the rate of reaction between NO and CO would not increase anymore [23].
4
0 4.0
4.5
5.0
coke rate (%)
5.2. Influence of coke parameters on NOx emission in sintering
Fig. 8. Effects of coke rate on NOx reduction, NOx emission and O2 concentration during sintering.
It needs to be mentioned that changing coke parameters such as coke rate and coke size may lead to changes in characteristics of granules and the following packed bed properties. When the flame front is set in after ignition, the resistance to airflow in sinter bed may change also, resulting in distinction of sintering airflow rate. However, the interest lies in understanding typical fuel parameters on the extent of NOx formation and reduction in this study. Therefore, a series of simulations were carried out keeping all other parameters the same as case 1. Charges with coke rate of 4.0%, 4.5%, 5.0% were performed to investigate coke rate effects on NOx emission. Since the ignition stage and burnt-through stage were too short compared to the whole sintering period, only the NOx emission during the steady sintering state was discussed. In Fig. 8, the NOx reduction and NOx emission are plotted in stacked column graph, and the O2 concentration with various coke rate is also plotted together for easily comparing the combustion atmosphere. Taking the 4.5% coke rate as an example, the total generated NOx is 695 ppm, of which
329 ppm is reduced by char, 73 ppm is reduced by CO and 293 ppm is emitted in flue gas, accounting for 50.9%, 11.3% and 37.8% respectively. Raising coke rate would increase bed temperature and combustion rate of coke significantly [12], resulting in much more NOx generation. It can be seen from Fig. 8 that, 45.0% generated NOx is reduced by char and 10.1% by CO when coke rate is 4.0% while 57.0% generated NOx is reduced by char and 11.8% by CO when coke rate increases to 5.0%. Increasing the number of coke particles in sinter bed means more chances of the interaction between the generated NOx and char. Besides, under the condition of fixed inlet airflow, more oxygen is consumed with the increase of coke rate, resulting in a more reducing atmosphere which promotes the NOx reduction both by char and CO. The values of the NOx emission are 285 ppm, 293 ppm and 287 ppm when the coke rate are 4.0%, 4.5% and 5.0%, respectively. The overall NOx emission values do not show much variation with the change of coke rate. Though the nitrogen source increases with
330
H. Zhou et al. / Fuel 179 (2016) 322–331
16
900
Reduction by Char Reduction by CO NOx emission O2
700
12
600 500 400
52.4%
48.9%
10.4%
11.8%
67.0%
300 200
14
10
O2 /%
NOx concentration/ppm
800
NOx could be reduced on the coke surface or in the pores of coke effectively. About 50% and 10% generated NOx would be reduced by char and CO respectively. The generated NOx increases with the increase of coke rate and the decrease of coke size. Increasing coke rate and decreasing coke size can promote the NOx generation because the coke combustion rate is enhanced. The reduction extent by char is mainly influenced by contact between NOx and char while the reduction extent by CO is mainly determined by the combustion atmosphere in sintering process.
8
6.1%
Acknowledgments 6
100 0 0.0
0.1
0.2
0.3
0.4
0.5
4 0.6
This work was supported by National Basic Research Program of China (2015CB251501) and National Natural Science Foundation of China (51476137).
coke size (mm) Fig. 9. Effects of coke size on NOx reduction, NOx emission and O2 concentration during sintering.
higher coke rate, the conversion ratio of coke-N to NOx decreases due to higher bed temperature [10]. Effect of coke size on NOx reduction, NOx emission and O2 concentration during sintering was studied for coke sauter mean diameter of 0.1 mm, 0.3 mm and 0.5 mm, results are showed in Fig. 9. Small coke particles have better access to oxygen and its combustion process tends to be controlled kinetically. For large coke particles, the oxygen and combustion product have to diffuse through larger formed ash layer and its combustion rate seems to be controlled by mass transfer. Therefore, the 0.1 mm coke particles have the highest NOx generation of 707 ppm due to its higher combustion rate. When coke size is increased from 0.1 mm to 0.5 mm, the reduction amount by char decreases from 67.0% to 48.9% while the reduction by CO increases from 6.1% to 11.8%. The specific surface area is so large for small coke particles that the combustion efficiency is higher and rarely CO is existed around coke particles, causing a much more oxidizing atmosphere. Therefore, only a small fraction of the generated NOx is reduced by CO when coke size is small. The values of the NOx emission are 219 ppm, 283 ppm and 299 ppm when the coke size are 0.1 mm, 0.3 mm and 0.5 mm, respectively. The overall NOx emission tends to increase with the increase of coke size. However, by three-layer sintering bed method, it is found that when removing 0.5 mm coke particles, NOx in flue gas decreases about 5–10% while oxygen content remains relatively constant [10]. A possible reason is that the flame front thickness vary with different coke size and this changes the sintering airflow remarkably. To conclude, the generated NOx could be reduced on the surface or in the pores of coke and the extent of reduction is relatively influenced by coke characteristics. 6. Conclusion Submodels of NOx formation and reduction were developed in present study and incorporated into a previous sintering heat treatment model, the overall NOx emission can be predicted well with experimental NOx emission of four sinter pot tests under different conditions. In sintering process, NOx emission is significantly related to fuel combustion. At the initial stage, a higher NOx emission can be got due to higher coke oxidizing rate caused by ignition heat input. After ignition, NOx emission remains almost constant and decreases rapidly with the accomplishment of coke combustion. As the sinter bed temperature is much lower than 1800 K, thermal-NOx formation could be neglected, the overall NOx emission is mainly produced from fuel NOx. The generated
References [1] Fan XH, Yu ZY, Gan M. Energy saving and emission reduction status and development direction in iron ore sintering. In: Proceeding of the 2011 annual conference on sintering and pelletizing technology of China. Changsha; 2011 [2] Goemans M, Clarysse P, Joannes J, De Clercq P, Lenaerts S, Matthys K, et al. Catalytic NOx reduction with simultaneous dioxin and furan oxidation. Chemosphere 2003;50:489–97. [3] Chen YG, Guo ZC, Wang Z. Application of modified coke to NOx reduction with recycling flue gas during iron ore sintering process. ISIJ Int 2008;48:1517–23. [4] Chen YG, Guo ZC, Feng GS. NOx reduction by coupling combustion with recycling flue gas in iron ore sintering process. Int J Miner Metal Mater 2011;18:390–6. [5] Fan XH, Ji ZY, Gan M, Chen XL, Yin L, Jiang T. Characteristics of prepared cokebiochar composite and its influence on reduction of NOx emission in iron ore sintering. ISIJ Int 2015;55:521–7. [6] Morioka K, Inaba S, Shimizu M, Ano K, Sugiyama T. Primary application of the‘‘In-Bed-deNOx” process using Ca–Fe oxides in iron ore sintering machines. ISIJ Int 2000;40:280–5. [7] Molina A, Murphy JJ, Winter F, Haynes BS, Blevins LG, Shaddix CR. Pathways for conversion of char nitrogen to nitric oxide during pulverized coal combustion. Combust Flame 2009;156:574–87. [8] Glarborg P, Jensen AD, Johnsson JE. Fuel nitrogen conversion in solid fuel fired systems. Prog Energy Combust Sci 2003;29:89–113. [9] Hill SC, Smoot LD. Modeling of nitrogen oxides formation and destruction in combustion systems. Prog Energy Combust Sci 2000;26:417–58. [10] Zhou H, Liu ZH, Cheng M, Zhou MX, Liu RP. Influence of coke combustion on NOx emission during iron ore sintering. Energy Fuels 2015;29:974–84. [11] Yang W, Choi S, Choi ES, Ri DW, Kim S. Combustion characteristics in an iron ore sintering bed-evaluation of fuel substitution. Combust Flame 2006;145:447–63. [12] Pahlevaninezhad M, Emami MD, Panjepour M. The effects of kinetic parameters on combustion characteristic in a sintering bed. Energy 2014;73:160–76. [13] De Castro JA, Nath N, Franca AB, Guiherme VS, Sasaki Y. Analysis by multiphase multicomponent model of iron ore sintering based on alternative steelworks gaseous fuels. Ironmaking Steelmaking 2012;39:605–13. [14] Ahn H, Choi S, Cho B. Process simulation of iron ore sintering bed with flue gas recirculation part 1-modelling approach. Ironmaking Steelmaking 2013;40:120–7. [15] Ahn H, Choi S, Cho B. Process simulation of iron ore sintering bed with flue gas recirculation part 2-parametric variation of gas conditions. Ironmaking Steelmaking 2013;40:128–37. [16] Cavaliere P, Perrone A. Analysis of dangerous emissions and plant productivity during sintering ore operations. Ironmaking Steelmaking 2013;40:9–24. [17] Zhou H, Zhao JP, Loo CE, Eills BG, Cen KF. Numerical modeling of the iron ore sintering process. ISIJ Int 2012;52:1550–8. [18] Fu WB, Zhang BL, Zheng SM. A relationship between the kinetic parameters of char combustion and the coal’s properties. Combust Flame 1997;109:587–98. [19] Jensen A, Johnsson JE, Andries J, Laughlin K, Read G, Mayer M, et al. Formation and reduction of NOx in pressurized fluidized bed combustion of coal. Fuel 1995;74:1555–69. [20] Murakami K, Sugawara K, Kawaguchi T. Analysis of combustion rate of various carbon materials for iron ore sintering process. ISIJ Int 2013;53:1580–7. [21] Lovel Roy R, Vining Keith R, Dell Amico Mark. The influence of fuel reactivity on iron ore sintering. ISIJ Int 2009;49:195–202. [22] Gan M, Fan XH, Chen XL, Ji ZY, Lv W, Wang Y, et al. Reduction of pollutant emission in iron ore sintering process by applying biomass fuels. ISIJ Int 2012;52:1574–8. [23] Wang CA, Du YB, Che DF. Investigation on the NO reduction with coal char and high concentration CO during oxy-fuel combustion. Energy Fuels 2012;26:7367–77. [24] Karlstrom O, Brink A, Hupa M. Biomass char nitrogen oxidation-single particle model. Energy Fuels 2013;27:1410–8.
H. Zhou et al. / Fuel 179 (2016) 322–331 [25] Cen KF, Yao Q, Luo ZY. Advanced combustion science. Hangzhou: Zhejiang University Press; 2002. [26] De Soete GG, Croiset E, Richard JR. Heterogeneous formation of nitrous oxide from char-bound nitrogen. Combust Flame 1999;117:140–54. [27] Jones JM, Harding AW, Brown SD, Thomas KM. Detection of reactive intermediate nitrogen and sulfur species in the combustion of carbons that are models for coal chars. Carbon 1995;33:833–43. [28] Winter F, Loffler G, Wartha C, Hofbauer H, Preto F, Anthony JE. The NO and N2O formation mechanism under circulating fluidized bed combustor conditions: from the single particle to the pilot-scale. Can J Chem Eng 1999;77:275–83. [29] Liu H, Gibbs BM. The influence of calcined limestone on NOx and N2O emissions from char combustion in fluidized bed combustors. Fuel 2001;80:1211–5. [30] Shimizu T, Sazawa Y, Adschiri T, Furusawa T. Conversion of char-bound nitrogen to nitric oxide during combustion. Fuel 1992;71:361–5. [31] Nelson PF, Nancharrow PC, Bus J, Prokopiuk A. Fractional conversion of char N to NO in an entrained flow reactor. Proc Combust Inst 2002;29:2267–74. [32] Schonebeck C, Gadiou R, Schwartz D. A kinetic study of the high temperature NO-char reaction. Fuel 2004;83:443–50. [33] Zhang JW. Kinetic study on NO-char reaction at high temperature PhD Thesis. Harbin: Harbin Institute of Technology; 2009.
331
[34] Schaffel N, Mancini M, Szlek A, Werber R. Mathematical modeling of MILD combustion of pulverized coal. Combust Flame 2009;156:1771–84. [35] Kilpinen P, Kallio S, Konttinen J, Barisic V. Char-nitrogen oxidation under fluidized bed combustion conditions: single particle studies. Fuel 2002;81:2349–62. [36] Geol S. Environmental problems: fundamental studies and global ramifications, Academic Dissertation. USA: Massachusetts Institutes of Technology; 1996. [37] Aarna I, Suuberg EM. The role of carbon monoxide in the NO-carbon reaction. Energy Fuels 1999;13:1145–53. [38] Chen YG, Guo ZC, Wang ZJ. Simulation of NO reduction by CO in sintering process. J Iron Steel Res 2009;21:6–9. [39] Jensen A, Johnsson JE, Andries J, Laughlin K, Read G, Mayer M, et al. Formation and reduction of NOx in pressurized fluidized bed combustion of coal. Fuel 1995;74:1555–69. [40] Liu ZH. Investigation of the factors influencing high-temperature zone of sinter bed flow resistance and NOx emission characteristics during iron ore sintering PhD Thesis. Zhejiang: Zhejiang University; 2015. [41] Loo CE. Effect of high-temperature reactions on the iron ore sintering process. In: The 6th Int Symp on agglomeration Japan; 1993.
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Modeling NOx emission of coke combustion in iron ore sintering process and its experimental validation Hao Zhou ⇑, Mingxi Zhou, Zihao Liu, Ming Cheng, Jianzhong Chen State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou 310027, PR China
a r t i c l e
i n f o
Article history: Received 14 October 2015 Received in revised form 27 March 2016 Accepted 28 March 2016 Available online 1 April 2016 Keywords: Coke combustion Iron ore sintering NOx emission model
a b s t r a c t NOx emission of coke combustion in iron ore sintering was modeled by overall reaction rate equations of NOx formation and reduction. Incorporating into a previous sintering heat treatment model, overall NOx emission can be predicted and the simulated results were well agreed with four sinter pot tests under varying conditions which are similar to actual production. In sintering, NOx emission is significantly related to fuel combustion. Due to heat input by ignition and smaller airflow in the initial stage of sintering, the predicted NOx emission has a higher value of about 350 ppm first then it decreases a little and keeps at a relatively constant level of about 300 ppm until the burn-through point, and decreases rapidly as a result of the accomplishment of coke combustion. Simulation results indicate that fuel NOx is the main NOx emission in sintering while thermal NOx is rarely produced since the bed temperature is much lower than 1800 K. The generated NOx could be reduced not only on the surface and in the pores of coke but also by CO around coke particles, about 50% and 10% of the generated NOx could be reduced by char and CO, respectively. Increasing coke rate and decreasing coke size promote NOx generation by accelerating the coke combustion. The reduction extent by char is greatly influenced by contact between NOx and char while the reduction extent by CO is mainly determined by the combustion atmosphere. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Iron ore sintering is a basic agglomeration process providing sinter for blast furnace. From environmental protection point of view, iron ore sintering is also a key pollution emission process in integrated steelworks. Diverse pollutants including SOx, NOx, dust and dioxins are produced from sintering, accounting for around 50% of total pollutant release in an integrated steelworks. Crude steel production in China during calendar year 2014 was 822.7 million tons, it was estimated that the produced sinter should be as high as around 900 million tons. About 0.1 million tons of NOx is emitted annually in sintering process due to its huge production capacity and poor emission control [1]. With stricter emission control regulations, controlling NOx emission in iron ore sintering has been paid much more attention to in recent years. Most researchers have concerned for NOx controlling methods in sintering. Those methods to reduce NOx emission contain SCR (selective catalytic reduction) treatment of flue gas [2], modification of fuel [3], flue gas recirculation [4], as well as the utilization of biomass fuel [5]. However, little literature was published on the
⇑ Corresponding author. Tel.: +86 571 87952598; fax: +86 571 87951616. E-mail address: [email protected] (H. Zhou). http://dx.doi.org/10.1016/j.fuel.2016.03.098 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
mechanisms of NOx production in sintering by far. It is acknowledged that NOx is mainly produced from fuel combustion in sintering [6], though the NOx emission of fuel combustion in pulverized coal boiler or fluidized bed has been investigated sufficiently [7–9], the traditional fuel used in sintering such as coke is much different from fuels in coal fired system. Coke size in sintering is 0–5 mm, which is much coarser than pulverized coal with size of micrometers. On the other hand, coke combusts with flame front moves down sinter bed in a complex zone consisted of gas, solid and melted liquid, resulting a large distinction of combustion atmosphere from coal fired system. Zhou et al. [10] investigated the factors of coke combustion influencing NOx emission in sintering using a novel three-layer sinter bed. Compared to its experimental way which has the disadvantages of large operating cost and quite time-consuming, mathematical model is more economic to be applied to practical situations and can give a more fundamental understanding of the problem. Most models developed for simulating iron ore sintering in recent years have been focused on the thermal pattern and combustion characteristics in sinter bed [11–15], rather than pollutant emission in flue gas. Only Cavaliere [16] built a data base and employed a multiobjective optimization tool to evaluate the effect of some processing parameters on pollutant emission and sinter productivity. However, this machine
323
H. Zhou et al. / Fuel 179 (2016) 322–331
learning method based on large amount of data is not analytical enough. Models simulating NOx emission in sintering need to be developed further. The aim of the present study is to model the formation and reduction of NOx in sintering process by overall reaction rate equations. Submodels of NOx emission are incorporated into a previous sintering heat treatment model, then the predicted overall NOx emission is validated by four sinter pot tests. Moreover, influences of various mechanisms on the overall NOx emission are quantified. 2. Modeling sintering heat treatment In iron ore sintering process, as shown in Fig. 1, raw materials including iron ores, fluxes, return fines and fuel are blended and mixed first. The blended mix is further granulated in drum with the addition of water to get a coarser particle size and narrower size distribution. The granules are then charged onto a moving strand forming a porous packed bed with proper bed permeability. The upper bed is ignited for around 90 s and the fuel in blended mix supplies about 80% of the whole heat to partially melt granules and convert it to sinter. The formed sinter would be further crushed and sized, usually the +5 mm sinter is used as the product for ironmaking in blast furnace while the 5 mm part is returned to sinter again. After ignition, a flame front forms at the top bed and traverses downward along the bed. Many complex exothermic and endothermic reactions happen in flame front and change the characteristics of flame front in turn. Since all the iron ores, fluxes and return fines have nitrogen rarely, NOx emission during sintering should be mainly produced from the release of nitrogen in the combusted fuel and conversion of nitrogen in air, which is greatly influenced by the heat conditions in sinter bed. A 1-D transient sintering heat treatment model was developed by our group before [17]. The sintering process was described by governing equations including mass, species, energy and
momentum conversion for both solid and gas phases. The general form of the partial differential equation is
@ @ @ @n ðe q nÞ þ ðe q u nÞ ¼ þ Sn C @t @y @y @y
ð1Þ
where the general variable n can be velocity, mass, enthalpy, etc. C and Sn are the relevant diffusion coefficient and source term respectively. Many complicated physical and chemical interactions were considered, such as coke combustion, calcination of limestone and dolomite, drying and condensation of water, melting and solidification. Specific to the coke combustion, unreacted core model shown in Fig. 2 considering the resistance of ash layer to gas was used and coke oxidation rate was calculated as follows.
Rcoke ¼
2AC O2 M c þ k1 þ kk1c kcoating 1
ð2Þ
f
where Rcoke is the combustion rate of coke (kg/s/m3), A is volumetric surface area of coke particle (m2/m3), C O2 is oxygen mole concentration at coke particle surface (mol/m3), Mc is carbon molecular weight (kg/mol), kcoating and kf are mass transfer coefficient of ash layer and gas boundary layer respectively (m/s), kc is chemical reaction rate constant (m/s), k is a factor of particle area. In the previous heat treatment model, it was assumed that CO is the main product of coke oxidation because the bed temperature in combustion zone is greater than 1000 K. Then the CO produced from coke would be further oxidized in gas phase. Table 1 gives the kinetic parameters for coke oxidation and CO oxidation. The oxidation kinetics of fuel do vary greatly with the characteristic of fuel and the combustion atmosphere. Based on the previous 1D sintering model, the heat conditions of gas and solid phases could be predicted correctly, indicating that the referred kinetic data could represent the combustion of the particular coke used in present study. Combining with submodels of NOx formation and reduction, the overall NOx emission could be determined.
Fig. 1. Schematic of a typical sintering process.
324
H. Zhou et al. / Fuel 179 (2016) 322–331 Table 2 Reactions considered in sintering NOx emission model. Reaction
Formula
Fuel NOx formation
Char N ! NO
Thermal NOx formation
O2
k1
N2 þ O NO þ N k2 k3
N þ O2 NO þ O k4
k3
N þ OH NO þ H k4
NOx reduction by char NOx reduction by CO
Char
NO ! N2 þ . . . NO þ CO ! 12 N2 þ CO2
3. Modeling NOx formation and reduction 3.1. NOx formation
Fig. 2. Schematic of unreacted core model of coke combustion.
Table 1 Kinetic parameters for coke oxidation and CO oxidation. Reaction
A (s1)
E (kJ/mol)
Reference
2C + O2 ? 2CO 2CO + O2 ? 2CO2
5.03 105 3.25 107
180 15.1
[18] [19]
Fig. 3 shows the overall feature of the sintering NOx emission model and the reactions considered are given in Table 2. Details of NOx submodels are reported below.
3.1.1. Fuel NOx Coke and anthracite are generally used as fuel in sintering, which have little volatile. For coke, its volatile content averages as small as 1–2 wt.% and its ignition temperature is more than 500 °C. In recent years, some pilot-scale experiments have been carried out to investigate the effects of replacing coke with some potential substitution fuels in sintering such as charcoal and biomass char. However, charcoal and biomass char have higher porosity than coke particles as well as higher volatile matter, resulting in higher fuel reactivity [20]. The combustion of charcoal/biomass char would start earlier than coke and promotes the flame front speed during sintering, resulting in smaller bed temperature and less melt formation, which are unfavorable to the tumbler strength of sinter [21]. Therefore, the utilization ratio of charcoal and biomass char is limited while the coke is still widely used in industrial sinter strand [22]. As the main aim of present paper is focused on
Fig. 3. Overall feature of the sintering NOx emission model.
325
H. Zhou et al. / Fuel 179 (2016) 322–331
the NOx emission of coke combustion in actual production, it is assumed that all the nitrogen in coke is char-N. Only the conversion of char-N to NOx is considered for fuel NOx formation and the conversion of volatile-N to NOx is neglected. With the progress of coke combustion, it has been reported that carbon and nitrogen in coal are not released in any particular order in most cases [9,23,24]. Though there has been different views on the oxidation mechanisms of char-N to NOx, some researchers agreed the conversion is directly happened without homogeneous reaction [9,25,26], for example, Cen et al. [25] indicated that charN is oxidized to NO directly without homogeneous reaction and the release rate of nitrogen is proportional to that of carbon. The conversion of char-N to NO should increase with increasing burn-out rate of coal [25]. Other researchers hold the opinion of the oxidation is happened through homogeneous reactions [7,27–29]. However, direct oxidation is widely used in modeling [9,12–17,23–26,30]. Nelson et al. [31] pointed out the conversion rate of char-N to NO in single char particles ranges from 75% to 100% with the condition of 1000–1400 K and 5–20 vol% O2. Referring to previous research results, it is assumed in present model that the release of nitrogen is linked to the char-C oxidation rate proportionally, and the char-N is oxidated to NOx directly without other intermediate product. Typical flue gas emission in our sinter pot tests before showed that the ratio of NO/NO2 is as high as 31. NO accounts more than 90% for the overall NOx emission in sintering. Therefore, it can be assumed that char-N in coke is oxidated to NO only. The reaction rate of char-N to NO in sintering is calculated as Eq. (3) [24].
RNO;f ¼
Rcoke gY N M NO w MN
1=2
RNO;t ¼ 3 1014 C N2 C O2 expð542; 000=RTÞ
ð7Þ
where RNO,t is the release rate of thermal NO during coke combustion (kg/s/m3), C N2 is the mole concentration of N2 (mol/cm3), T is the coke particle temperature (K), R is universal gas constant (J/mol/K). 3.2. NOx reduction 3.2.1. Reaction of char and NOx Glarborg et al. [8] reviewed that, even though the details of char-N oxidation mechanism was not very clear by far, it has been widely accepted that the overall reaction should be the char-NO formation first and subsequent reduction of NOx by char. At first, the nitrogen C(N) on the char surface reacts to form NO,
CðNÞ þ O2 ! NO þ CðOÞ
ð8Þ
Then a portion of the formed NO diffuses to bulk gas while some may be absorbed on the char surface again. Here Cf is a free carbon site, which can react with NO producing a new nitrogen surface species C0 (N).
2Cf þ NO ! C0 ðNÞ þ CðOÞ
ð9Þ 0
The reactivity of this new C (N) is different from the char-N site C(N). C0 (N) and NO in the bulk gas may react to form free nitrogen. Besides, two C0 (N) species may recombinate to form free nitrogen also. In the temperature range of 900–1200 K, the former mechanism dominates the formation of N2 while the latter one is less important.
ð3Þ
C0 ðNÞ þ NO ! N2 þ CðOÞ
ð10Þ
where RNO,f is the release rate of fuel NO during coke combustion (kg/s/m3), g is the burn-out rate of coke (%), YN is the fraction of nitrogen on a mass basis in coke (wt.%), MNO and MN are the molecular weight of NO and N respectively (kg/kmol), w is the conversion rate of char-N to NO during coke oxidation.
C0 ðNÞ þ C0 ðNÞ ! N2 þ 2Cf
ð11Þ
3.1.2. Thermal NOx Atmospheric nitrogen could be oxidated to thermal NOx in fuel-lean environments with relatively high temperature [9]. And this kind of thermal NOx is really sensitive to the combustion temperature. As so far, Zeldovich mechanism is the most widely accepted model to explain the formation of thermal NOx by three reversible elementary reactions [9,25]. The overall reaction rate of three reversible thermal NO reactions is showed in Eq. (4). When initial concentration of NO is low, Eq. (4) can be simplified to Eq. (5) because the forward reactions of these mechanisms dominate during combustion. Assuming oxygen atoms are in equilibrium with O2 further, Eq. (5) could be written to Eq. (6) [9,25].
d½NO ¼ 2½O dt
( ) 2 ½NO k1 ½N2 2k1k2k½O 2 ½NO 1 þ k2 ½Ok1 2 þk3 ½OH
ð4Þ
d½NO ¼ 2k1 ½N2 ½O dt
ð5Þ
d½NO 0 ¼ 2k ½N2 ½O2 1=2 dt
ð6Þ
For fuel-lean combustion process with oxidizing atmosphere, Eq. (7) is a simplified form of Eq. (6) based on experimental results of k0 by Zeldovich to model the thermal NOx production. In iron ore sintering, air is sucked into the packed bed continuously and coke is combusted in a relatively strong oxidizing atmosphere. So Eq. (7) is used to calculate the thermal NOx.
Increasing char particle size and pressure may lead to longer residence time of NO in the pores, increasing the reactivity of char and concentration of NO in gas phase would increase the reaction probability, a larger reduction of NO by char would be obtained [8]. Schonebeck et al. [32] and Zhang [33] combined experiments and simulations to study the reaction between char and NO in high temperature. They found that NO-char reaction is in first order and the NO would be reduced more with increasing NO concentration in gas phase. The reaction rate in single coke particle is [33,34]:
RNO;c ¼ kSP NO g
ð12Þ
E k ¼ k0 exp RT
ð13Þ
1 3 p d q Ai 6 P P 3 1 1 g¼ / tan h/ / S¼
ð14Þ ð15Þ
where RNO,c is the reaction rate of NO reduction by char during coke combustion (kg/s/m3), k is the intrinsic reactivity (kg/cm2/s/Pa), S is the internal surface area of coke (m2), PNO is the partial pressure of NO in bulk gas (Pa), g is the effectiveness factor, namely the ratio of actual reaction rate to the theoretical maximum reaction rate, k0 and E are the pre-exponential factor and activation energy of the reaction between NO and char with the value of 0.0045 and 144,000 respectively (kg/cm2/s/Pa, J/mol), dP is the coke particle diameter (m), qp is the apparent density of coke (kg/m3), Ai is the specific internal surface area of coke particle (m2/kg), / is the Thiele modulus, can be calculated according to Eq. (16) [33,34].
326
H. Zhou et al. / Fuel 179 (2016) 322–331
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi dp cAi qp kPox /¼ 2 C ox De
ð16Þ
Test number
where c is the stoichiometric coefficient of reaction C(s) + O2(g) ? CO2(g), Cox is the oxidant density in bulk gas (kg/m3), De is the effective diffusion coefficient in coke particle pores (m2/s), which is determined by the Knudsen diffusion coefficient Dk and the bulk molecular diffusion coefficient D0 [33,34]:
De ¼
ec 1 1 þ s Dk D0
Table 3 Conditions of the sinter pot tests.
Blend details (% dry total basis)
1 ð17Þ
where ec is the porosity of coke particle, s is the tortuosity of pores, pffiffiffi a typical value of 2 was used. The bulk molecular diffusion coefficient D0 is [33]:
D0 ¼ C
½ðT þ T 1 Þ=2 dp
0:75
ð18Þ
where C is a rate constant of diffusion process with the value of 6.26 1011 (m3/K0.75/s), T 1 is the bulk gas temperature (K). The Knudsen diffusion coefficient Dk is [33,34]:
Dk ¼ 97rp
sffiffiffiffiffiffiffiffiffiffi T MNO
Basicity (CaO/SiO2) SiO2 (%) MgO (%) Target moisture (%) Bed height (mm) Ignition suction (kPa) Sinter suction (kPa)
Case 2
Case 3
Case 4
19.00 22.34 18.69 0 0 0 7.44 19.94 0 6.30 1.50
18.99 22.34 18.69 0 0 0 8.51 20.01 1.07 4.28 1.51
19.56 23.01 19.25 0 0 0 5.95 20.00 0 6.36 1.50
10.26 10.26 0 20.53 10.26 10.26 9.11 20.00 0.14 5.11 0
0.43 4.35 2.0 5.15 2.0 6.35 600 6 16
0 4.60 2.0 5.15 2.0 6.35 600 6 16
0 4.35 2.0 4.60 2.0 6.35 600 6 16
0 4.05 1.9 5.00 1.7 6.50 500 5 13.3
ð19Þ Table 4 Ultimate and proximate analysis and key parameters of coke.
where r p is the mean pore radius of coke particle (m). 3.2.2. Reaction of CO and NOx The formed NO can not only be reduced by char itself, but also can react with CO on the surface of coke or some catalysts, which has been confirmed by many experiments [19,35–37]. Chen et al. [38] found that the reduction reaction of NO and CO could be catalyzed by CaO, Sinter, Fe2O3 and MgO in blend, decreasing the overall NOx emission. In most studies, the reaction between NO and CO is first order in NO. For the simplicity of the model, only the reaction between CO and NO on the surface of single coke particle is considered, the reaction rate is calculated as Eq. (20) [36,39].
RNO;co ¼ kNO—CO C NO C CO
Iron ore A Iron ore B Iron ore C Iron ore D Iron ore E Iron ore F Limestone Return fine Serpentine Dolomite Hydrated lime Silica sand Coke
Case 1
Item Cad Had Nad St, ad Oad
83.3 1.1 1.1 0.7 0.4
Proximate analysis (mass %)
Mad Aad Vad FCad
1.1 13.4 1.6 83.9
Size distribution (mass %)
6.3–8 mm 3.2–6.3 mm 1.18–3.2 mm 0.6–1.18 mm 0.3–0.6 mm 0.15–0.3 mm 0.106–0.15 mm 0.075–0.106 mm 0.063–0.075 mm 0.034–0.063 mm 0.034 mm
1.2 1.3 23.9 31.7 15.1 11.8 7.4 2.4 1.6 0.8 3.0
ð20Þ
where RNO,co is the reaction rate of NO reduction by CO during coke combustion (kg/s/m3), kNO–CO = 3.68 107 exp (108,889/RT). 4. Model validation In practice, granulation moisture, coke rate, basicity (CaO/SiO2) as well as sinter suction are key controlled parameters to get the desired sinter productivity and strength. The model of NOx emission was validated by sinter pot tests focusing on the cases which are similar to actual plant production. Testing details are listed in Table 3. Case 1 to case 3 were sintering an Australia-ore blend with high aluminum content. In case 1, the SiO2 content was adjusted by silica sand to the value of 5.15% while in case 2 the SiO2 content was adjusted by serpentine and coke rate was increased from 4.35% to 4.60%. In case 3, neither silica sand nor serpentine were used and the SiO2 content was decreased to 4.60%. Case 4 was sintering a typical ore blend used in Asia–pacific region. However, the sinter bed height and sinter suction were five-sixths of the level of former three cases. The input parameters of model are obtained from sinter pot test including bulk density, bed voidage, airflow rate after ignition, and the composition of sinter mixture (the content of water, coke, limestone, dolomite, etc.). The important parameters of coke used in simulation are summarized in Table 4. As the unreacted-core model was applied to model the combustion of coke particle, the particle size of coke definitely influences the combustion rate due to different gas diffusion potential. To correct
Coke
Ultimate analysis (mass %)
Apparent density (kg/m3) True density (kg/m3) Mean pore radius of coke particle (nm)
1200 1900 0.2
the reaction rates of coke combustion for model validation, the size distribution of coke was measured and dispersed into total 13 sizes in simulation. The governing equations of solid and gas phases were numerically solved by Finite-Volume Method while SIMPLE algorithm was used to solve the relationship between gas pressure and gas velocity of the gas momentum equation. The computational domain was discretized into grids with the space step of 1.2 mm bed height and the time step of 2 s, sufficient to capture the physicochemical changes in sinter bed. For conducting the sinter pot test, all raw materials were dry mixed for 1 min first and then wet granulated for 10 min in a rotary drum with the diameter of 1.1 m to get a steady-state of granules. Around 90 kg granules were poured into a sinter pot with the height of 600 mm and the diameter of 330 mm. Fig. 4 shows the schematic view of sinter pot used. The sinter bed was ignited by a natural gas burner with 1400 K temperature, after 90 s
H. Zhou et al. / Fuel 179 (2016) 322–331
327
Fig. 4. Schematic view of sinter pot.
ignition, the natural gas burner was removed and a hood was put on the sinter bed. The suction value was raised from 6 kPa during ignition to 16 kPa for sintering except in case 4. Air was sucked into the sinter bed by suction fan and its velocity was monitored by a hot wire anemometer with the resolution ratio of 0.01 m/s. The flue gas was sampled and its NOx and O2 content was analyzed by a Testo 350 continuously. The resolution ratio is 1 ppm for NOx and 0.01 vol% for O2. When the flue gas temperature measured just below the sinter pot got a maximum value and began to decrease, the experiment was done. Three thermocouples could be inserted into sinter pot at the location of 100, 300, 500 mm from top of the bed to record bed temperature profiles. Since the previous heat treatment model was validated by a large amount of pot tests results [17], the accuracy of its prediction results of heat conditions can be reliable and thermocouples were not used in this study. Only the flue gas composition were applied to validate the present NOx emission model. Fig. 5 compares the NOx emission from experimental measurement and model prediction [40]. It can be seen that the predicted NOx agrees well with the measured NOx in most cases. There is a distinction between the measured and predicted NOx in case 3. However, the average error is about 18.8%, which is still acceptable from the industrial point of view. Neither serpentine or silica sand was used in case 3 to adjust the SiO2 content of raw materials’ chemical composition, so calcium ferrite in the formed melt would contain less SiO2 and would be more evenly distributed, resulting in a greater catalytic effect on the NOx reduction reactions [38]. The results of four sinter pot tests are listed in Table 5, it needs to be noted that the flow rate of flue gas varies much under different conditions. High concentration of NOx in flue gas is not necessarily to mean high conversion efficiency of char-N to NOx in coke. In all cases, it can be observed from Fig. 5 that the predicted NOx emission has a higher value at the beginning of sintering then decreases a little and keeps at a relatively constant level until the burn-through point. Pahlevaninezhad et al. [12] tested some kinetic parameters on combustion characteristics in sintering bed by their unsteady-2D axisymmetric sintering model and their simulation results showed that, the maximum bed temperature increases rapidly due to the heat input of ignition first, then
decreases a little bit when cold air begins to flow into sintering bed and increases again as a result of the heat accumulation in sintering bed. Therefore, the possible reason why this changing trend of predicted NOx happened is the oxidation rate of char-N to NOx closely relates to the maximum bed temperature. Higher coke combustion rate and higher char-N oxidation rate due to heat input by ignition leads to the initial high NOx emission value. In addition, whatever the condition is, the measured O2 in flue gas increased earlier than the model predicted and the measured NO decreased earlier also than predicted in the late stage of sintering. The reason why the mismatching happen should be an unflat flame front exists in experimental sintering bed, which differs from the hypothesis of flat flame front in model. Due to the shrinkage of raw materials, air velocity in the edge of sinter pot should be larger than the center of sinter pot. A faster flame front speed would be got in the edge of sinter pot than the center, resulting in an earlier burnt-through in the edge. 5. Discussion 5.1. Quantification of NOx formation and reduction on overall NOx emission To quantify the NOx formation and reduction on overall NOx emission, case 1 which reaches return fine balance was adopted to perform simulation considering different NOx formation and reduction mechanisms. Fig. 6a compares the total formed NOx without reduction during sintering and the NOx emission considering reduction by char only, therefore the reduction extent of the generated NOx by char can be determined. The concentration of generated NOx by coke combustion is around 680 ppm and when reduction of NOx by char is considered, the NOx emission decreases to about 320 ppm. Almost 50% formed NOx is reduced in the pores and on the surface of coke particles. As discussed in introduction, coke combustion in sintering happens in a hightemperature zone consisted of gas–liquid–solid mixture. Though the atmosphere around coke particles is much different from coal fired system, it is the same that the generated NOx could be reduced by char itself significantly.
H. Zhou et al. / Fuel 179 (2016) 322–331
O2 measured
O 2/vol.%
500
NOx predicted NOx measured
400 300 200 100
600
&DVH
600
O2 predicted
NO x /ppm
40 35 30 25 20 15 10 5 0 1200
1800
0 3000
2400
40 35 30 25 20 15 10 5 0
600
O2 predicted O2 measured
500
NOx predicted NOx measured
400 300 200
NO x /ppm
&DVH
O 2/vol.%
328
100 1200
600
1800
0 3000
2400
O2 measured
O 2/vol.%
NOx measured
400 300 200
NO x /ppm
500
NOx predicted
100 600
&DVH
600
O2 predicted
1200
1800
0 3000
2400
40 35 30 25 20 15 10 5 0
600
O2 predicted O2 measured
500
NOx predicted NOx measured
400 300 200
NO x /ppm
&DVH
O 2/vol.%
Time/s 40 35 30 25 20 15 10 5 0
100 600
1200
1800
2400
0 3000
Time/s Fig. 5. Comparasion of predicted and measured NOx emission [40].
Table 5 Sintering parameters of the four sinter pot tests. Test number
Case 1
Case 2
Case 3
Case 4
Experimental moisture (%) Sintering time (min) Maximum windbox temperature (°C) Flame front speed (mm/min) Sinter air flow (N m3/h) Sinter yield (%, +5 mm) Productivity (t/m2/d) Tumbler index (%, +6.3 mm)
6.18 32.92 688.7 18.2 113.8 72.7 28.7 65.9
6.14 30.07 681.9 19.9 125.5 75.0 32.1 65.8
6.45 25.93 674.0 23.1 146.8 72.3 35.1 62.7
6.55 21.30 601.7 23.5 126.3 69.0 31.7 60.6
The total generated NOx without reduction during sintering and the NOx emission considering reduction by CO only are compared in Fig. 6b. Compared to total formed NOx with the value of 680 ppm, NOx emission in the case of considering reduction of NOx by CO only becomes about 580 ppm. It is consistent with previous work that the reaction between CO and NO is also effective and cannot be neglected.
Fig. 7 illustrates the predicted temperature profile of the 100 mm, 300 mm, 500 mm bed height respectively. Due to heat accumulation in sinter bed, the maximum bed temperature and thickness of high-temperature zone increase with flame front descends down sinter bed. It can be seen that the bed temperature is much lower than 1800 K, therefore, negligible thermal NOx is produced in sintering process [9]. In the submodel of coke combustion, Loo [41] and Zhao et al. [17] assumed that CO is the main product when temperature is higher than 1000 K. Therefore, the CO generated from coke oxidation is split into two fractions: a fraction diffuses into bulk gas and reacts with O2 to form CO2 while the rest spreads around coke surface and react with NO to reduce formed NOx. Generally, the concentration of CO in flue gas during sintering is in the range of 0.5–1.5%. The heat condition changes gradually down a bed due to preheating of the combustion air, a more reducing atmosphere may be formed in lower bed compared to upper bed. As shown in Fig. 6b, with flame front descends down sinter bed, the reduction of NOx by CO keeps at a relatively constant level. Possible explanation is that, when CO concentration reaches
329
H. Zhou et al. / Fuel 179 (2016) 322–331
(a)
NOx /ppm
1500
Fuel NOx + Thermal NOx Considering reduction of NO by char only
1000
500
0 600
1200
1800
2400
3000
Time/s 1500
Fuel NOx+Thermal NOx
NOx /ppm
Considering reduction of NOx by CO only 1000
CO
110 ppm 500
0 600
1200
1800
2400
10 9 8 7 6 5 4 3 2 1 0
CO/vol.%
(b)
3000
Time/s
16
900
100mm 300mm 500mm
0
600
1200
1800
2400
3000
t/s Fig. 7. Predicted bed temperature along the sinter bed of different heights [40]. Note: 100/300/500 mm are the distance from the top of sinter pot.
800 700
Reduction by Char Reduction by CO NOx emission O2
14
600
57.0%
500
45.0%
50.9%
12 10
400 300
10.1%
11.3%
11.8%
O2 /%
1600 1400 1200 1000 800 600 400 200 0
NOx concentration/ppm
T/K
Fig. 6. Quantification of NOx reduction in iron ore sintering [40]. (a) NOx reduction by char and (b) NOx reduction by CO.
8
200 6 100
a particular value of 0.6 vol%, the rate of reaction between NO and CO would not increase anymore [23].
4
0 4.0
4.5
5.0
coke rate (%)
5.2. Influence of coke parameters on NOx emission in sintering
Fig. 8. Effects of coke rate on NOx reduction, NOx emission and O2 concentration during sintering.
It needs to be mentioned that changing coke parameters such as coke rate and coke size may lead to changes in characteristics of granules and the following packed bed properties. When the flame front is set in after ignition, the resistance to airflow in sinter bed may change also, resulting in distinction of sintering airflow rate. However, the interest lies in understanding typical fuel parameters on the extent of NOx formation and reduction in this study. Therefore, a series of simulations were carried out keeping all other parameters the same as case 1. Charges with coke rate of 4.0%, 4.5%, 5.0% were performed to investigate coke rate effects on NOx emission. Since the ignition stage and burnt-through stage were too short compared to the whole sintering period, only the NOx emission during the steady sintering state was discussed. In Fig. 8, the NOx reduction and NOx emission are plotted in stacked column graph, and the O2 concentration with various coke rate is also plotted together for easily comparing the combustion atmosphere. Taking the 4.5% coke rate as an example, the total generated NOx is 695 ppm, of which
329 ppm is reduced by char, 73 ppm is reduced by CO and 293 ppm is emitted in flue gas, accounting for 50.9%, 11.3% and 37.8% respectively. Raising coke rate would increase bed temperature and combustion rate of coke significantly [12], resulting in much more NOx generation. It can be seen from Fig. 8 that, 45.0% generated NOx is reduced by char and 10.1% by CO when coke rate is 4.0% while 57.0% generated NOx is reduced by char and 11.8% by CO when coke rate increases to 5.0%. Increasing the number of coke particles in sinter bed means more chances of the interaction between the generated NOx and char. Besides, under the condition of fixed inlet airflow, more oxygen is consumed with the increase of coke rate, resulting in a more reducing atmosphere which promotes the NOx reduction both by char and CO. The values of the NOx emission are 285 ppm, 293 ppm and 287 ppm when the coke rate are 4.0%, 4.5% and 5.0%, respectively. The overall NOx emission values do not show much variation with the change of coke rate. Though the nitrogen source increases with
330
H. Zhou et al. / Fuel 179 (2016) 322–331
16
900
Reduction by Char Reduction by CO NOx emission O2
700
12
600 500 400
52.4%
48.9%
10.4%
11.8%
67.0%
300 200
14
10
O2 /%
NOx concentration/ppm
800
NOx could be reduced on the coke surface or in the pores of coke effectively. About 50% and 10% generated NOx would be reduced by char and CO respectively. The generated NOx increases with the increase of coke rate and the decrease of coke size. Increasing coke rate and decreasing coke size can promote the NOx generation because the coke combustion rate is enhanced. The reduction extent by char is mainly influenced by contact between NOx and char while the reduction extent by CO is mainly determined by the combustion atmosphere in sintering process.
8
6.1%
Acknowledgments 6
100 0 0.0
0.1
0.2
0.3
0.4
0.5
4 0.6
This work was supported by National Basic Research Program of China (2015CB251501) and National Natural Science Foundation of China (51476137).
coke size (mm) Fig. 9. Effects of coke size on NOx reduction, NOx emission and O2 concentration during sintering.
higher coke rate, the conversion ratio of coke-N to NOx decreases due to higher bed temperature [10]. Effect of coke size on NOx reduction, NOx emission and O2 concentration during sintering was studied for coke sauter mean diameter of 0.1 mm, 0.3 mm and 0.5 mm, results are showed in Fig. 9. Small coke particles have better access to oxygen and its combustion process tends to be controlled kinetically. For large coke particles, the oxygen and combustion product have to diffuse through larger formed ash layer and its combustion rate seems to be controlled by mass transfer. Therefore, the 0.1 mm coke particles have the highest NOx generation of 707 ppm due to its higher combustion rate. When coke size is increased from 0.1 mm to 0.5 mm, the reduction amount by char decreases from 67.0% to 48.9% while the reduction by CO increases from 6.1% to 11.8%. The specific surface area is so large for small coke particles that the combustion efficiency is higher and rarely CO is existed around coke particles, causing a much more oxidizing atmosphere. Therefore, only a small fraction of the generated NOx is reduced by CO when coke size is small. The values of the NOx emission are 219 ppm, 283 ppm and 299 ppm when the coke size are 0.1 mm, 0.3 mm and 0.5 mm, respectively. The overall NOx emission tends to increase with the increase of coke size. However, by three-layer sintering bed method, it is found that when removing 0.5 mm coke particles, NOx in flue gas decreases about 5–10% while oxygen content remains relatively constant [10]. A possible reason is that the flame front thickness vary with different coke size and this changes the sintering airflow remarkably. To conclude, the generated NOx could be reduced on the surface or in the pores of coke and the extent of reduction is relatively influenced by coke characteristics. 6. Conclusion Submodels of NOx formation and reduction were developed in present study and incorporated into a previous sintering heat treatment model, the overall NOx emission can be predicted well with experimental NOx emission of four sinter pot tests under different conditions. In sintering process, NOx emission is significantly related to fuel combustion. At the initial stage, a higher NOx emission can be got due to higher coke oxidizing rate caused by ignition heat input. After ignition, NOx emission remains almost constant and decreases rapidly with the accomplishment of coke combustion. As the sinter bed temperature is much lower than 1800 K, thermal-NOx formation could be neglected, the overall NOx emission is mainly produced from fuel NOx. The generated
References [1] Fan XH, Yu ZY, Gan M. Energy saving and emission reduction status and development direction in iron ore sintering. In: Proceeding of the 2011 annual conference on sintering and pelletizing technology of China. Changsha; 2011 [2] Goemans M, Clarysse P, Joannes J, De Clercq P, Lenaerts S, Matthys K, et al. Catalytic NOx reduction with simultaneous dioxin and furan oxidation. Chemosphere 2003;50:489–97. [3] Chen YG, Guo ZC, Wang Z. Application of modified coke to NOx reduction with recycling flue gas during iron ore sintering process. ISIJ Int 2008;48:1517–23. [4] Chen YG, Guo ZC, Feng GS. NOx reduction by coupling combustion with recycling flue gas in iron ore sintering process. Int J Miner Metal Mater 2011;18:390–6. [5] Fan XH, Ji ZY, Gan M, Chen XL, Yin L, Jiang T. Characteristics of prepared cokebiochar composite and its influence on reduction of NOx emission in iron ore sintering. ISIJ Int 2015;55:521–7. [6] Morioka K, Inaba S, Shimizu M, Ano K, Sugiyama T. Primary application of the‘‘In-Bed-deNOx” process using Ca–Fe oxides in iron ore sintering machines. ISIJ Int 2000;40:280–5. [7] Molina A, Murphy JJ, Winter F, Haynes BS, Blevins LG, Shaddix CR. Pathways for conversion of char nitrogen to nitric oxide during pulverized coal combustion. Combust Flame 2009;156:574–87. [8] Glarborg P, Jensen AD, Johnsson JE. Fuel nitrogen conversion in solid fuel fired systems. Prog Energy Combust Sci 2003;29:89–113. [9] Hill SC, Smoot LD. Modeling of nitrogen oxides formation and destruction in combustion systems. Prog Energy Combust Sci 2000;26:417–58. [10] Zhou H, Liu ZH, Cheng M, Zhou MX, Liu RP. Influence of coke combustion on NOx emission during iron ore sintering. Energy Fuels 2015;29:974–84. [11] Yang W, Choi S, Choi ES, Ri DW, Kim S. Combustion characteristics in an iron ore sintering bed-evaluation of fuel substitution. Combust Flame 2006;145:447–63. [12] Pahlevaninezhad M, Emami MD, Panjepour M. The effects of kinetic parameters on combustion characteristic in a sintering bed. Energy 2014;73:160–76. [13] De Castro JA, Nath N, Franca AB, Guiherme VS, Sasaki Y. Analysis by multiphase multicomponent model of iron ore sintering based on alternative steelworks gaseous fuels. Ironmaking Steelmaking 2012;39:605–13. [14] Ahn H, Choi S, Cho B. Process simulation of iron ore sintering bed with flue gas recirculation part 1-modelling approach. Ironmaking Steelmaking 2013;40:120–7. [15] Ahn H, Choi S, Cho B. Process simulation of iron ore sintering bed with flue gas recirculation part 2-parametric variation of gas conditions. Ironmaking Steelmaking 2013;40:128–37. [16] Cavaliere P, Perrone A. Analysis of dangerous emissions and plant productivity during sintering ore operations. Ironmaking Steelmaking 2013;40:9–24. [17] Zhou H, Zhao JP, Loo CE, Eills BG, Cen KF. Numerical modeling of the iron ore sintering process. ISIJ Int 2012;52:1550–8. [18] Fu WB, Zhang BL, Zheng SM. A relationship between the kinetic parameters of char combustion and the coal’s properties. Combust Flame 1997;109:587–98. [19] Jensen A, Johnsson JE, Andries J, Laughlin K, Read G, Mayer M, et al. Formation and reduction of NOx in pressurized fluidized bed combustion of coal. Fuel 1995;74:1555–69. [20] Murakami K, Sugawara K, Kawaguchi T. Analysis of combustion rate of various carbon materials for iron ore sintering process. ISIJ Int 2013;53:1580–7. [21] Lovel Roy R, Vining Keith R, Dell Amico Mark. The influence of fuel reactivity on iron ore sintering. ISIJ Int 2009;49:195–202. [22] Gan M, Fan XH, Chen XL, Ji ZY, Lv W, Wang Y, et al. Reduction of pollutant emission in iron ore sintering process by applying biomass fuels. ISIJ Int 2012;52:1574–8. [23] Wang CA, Du YB, Che DF. Investigation on the NO reduction with coal char and high concentration CO during oxy-fuel combustion. Energy Fuels 2012;26:7367–77. [24] Karlstrom O, Brink A, Hupa M. Biomass char nitrogen oxidation-single particle model. Energy Fuels 2013;27:1410–8.
H. Zhou et al. / Fuel 179 (2016) 322–331 [25] Cen KF, Yao Q, Luo ZY. Advanced combustion science. Hangzhou: Zhejiang University Press; 2002. [26] De Soete GG, Croiset E, Richard JR. Heterogeneous formation of nitrous oxide from char-bound nitrogen. Combust Flame 1999;117:140–54. [27] Jones JM, Harding AW, Brown SD, Thomas KM. Detection of reactive intermediate nitrogen and sulfur species in the combustion of carbons that are models for coal chars. Carbon 1995;33:833–43. [28] Winter F, Loffler G, Wartha C, Hofbauer H, Preto F, Anthony JE. The NO and N2O formation mechanism under circulating fluidized bed combustor conditions: from the single particle to the pilot-scale. Can J Chem Eng 1999;77:275–83. [29] Liu H, Gibbs BM. The influence of calcined limestone on NOx and N2O emissions from char combustion in fluidized bed combustors. Fuel 2001;80:1211–5. [30] Shimizu T, Sazawa Y, Adschiri T, Furusawa T. Conversion of char-bound nitrogen to nitric oxide during combustion. Fuel 1992;71:361–5. [31] Nelson PF, Nancharrow PC, Bus J, Prokopiuk A. Fractional conversion of char N to NO in an entrained flow reactor. Proc Combust Inst 2002;29:2267–74. [32] Schonebeck C, Gadiou R, Schwartz D. A kinetic study of the high temperature NO-char reaction. Fuel 2004;83:443–50. [33] Zhang JW. Kinetic study on NO-char reaction at high temperature PhD Thesis. Harbin: Harbin Institute of Technology; 2009.
331
[34] Schaffel N, Mancini M, Szlek A, Werber R. Mathematical modeling of MILD combustion of pulverized coal. Combust Flame 2009;156:1771–84. [35] Kilpinen P, Kallio S, Konttinen J, Barisic V. Char-nitrogen oxidation under fluidized bed combustion conditions: single particle studies. Fuel 2002;81:2349–62. [36] Geol S. Environmental problems: fundamental studies and global ramifications, Academic Dissertation. USA: Massachusetts Institutes of Technology; 1996. [37] Aarna I, Suuberg EM. The role of carbon monoxide in the NO-carbon reaction. Energy Fuels 1999;13:1145–53. [38] Chen YG, Guo ZC, Wang ZJ. Simulation of NO reduction by CO in sintering process. J Iron Steel Res 2009;21:6–9. [39] Jensen A, Johnsson JE, Andries J, Laughlin K, Read G, Mayer M, et al. Formation and reduction of NOx in pressurized fluidized bed combustion of coal. Fuel 1995;74:1555–69. [40] Liu ZH. Investigation of the factors influencing high-temperature zone of sinter bed flow resistance and NOx emission characteristics during iron ore sintering PhD Thesis. Zhejiang: Zhejiang University; 2015. [41] Loo CE. Effect of high-temperature reactions on the iron ore sintering process. In: The 6th Int Symp on agglomeration Japan; 1993.