Modeling the contributions of volatile and char-bound nitrogen to the

Jan 23, 2018 - Given that more stringent NOx emission limits are expected in the near future, several industrial processes are in need of NOx mitigati...
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Modeling the Contributions of Volatile and Char-Bound Nitrogen to the Formation of NOx Species in Iron Ore Rotary Kilns Rikard Edland,* Fredrik Normann, and Klas Andersson Division of Energy Technology, Department of Space Earth and Environment, Chalmers University of Technology, SE-412 96 Göteborg, Sweden S Supporting Information *

ABSTRACT: Given that more stringent NOx emission limits are expected in the near future, several industrial processes are in need of NOx mitigation measures. The Grate-Kiln process, applied in the iron ore industry, is one such process. NOx formation is inherently high in the process, and due to the combustion conditions, several standard mitigation strategies are impractical. Alternative solutions are thus needed. The current paper aims at developing a model capable of describing the NO formation under conditions relevant in iron ore rotary kilns and to identify governing parameters that may be modified for mitigation purposes. The developed model uses detailed reaction modeling for the homogeneous combustion chemistry combined with simpler modeling with apparent kinetics for the heterogeneous chemistry. The main findings are that thermal NO is of low significance and that the NO formation during char combustion is the main contributor to the high NOx emissions. Attempting to control the partitioning between the volatile nitrogen and the char-bound nitrogen is suggested as a mitigation strategy, since the combustion of char is challenging to control compared to the combustion of volatiles.

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the high flame temperatures. Thus, according to these sources, it should not be possible to decrease the formation of NOx without altering the temperature profile. To the best of our knowledge, only three published studies have looked specifically at NOx formation in rotary kilns for iron ore production, out of which two used natural gas as the fuel4,5 and one applied solid fuels.1 Comparing emissions from natural gas with emissions from solid fuels used in the Grate-Kiln process, natural gas typically generates 2- or even 3-fold higher levels of NOx per tonne of pellets.3 This indicates that, for gaseous fuels, the thermal NOx formation is significantly higher than in conventional combustion systems such as boilers, where the burning of gaseous fuels typically yields less NOx than the burning of solid fuels. This does not necessarily mean that thermal NOx is dominating when solid fuels are used. As shown in our previous work,1 the high levels of NOx in the Grate-Kiln flue gas can just as well be attributed to the fuel-bound nitrogen, and an increase in temperature does not directly lead to increased emissions of NOx for solid fuels. Furthermore, substituting a part of the coal (that contains nitrogen) with biomass (which contains almost no nitrogen) resulted in a proportional decrease in NOx formation. We therefore concluded1 that the high NOx emissions are most likely a consequence of a higher conversion of the char-bound nitrogen (char-N) than that typically observed for suspension-fired systems. The mechanisms that govern NO formation from solid fuels include the release of volatile nitrogen (vol-N) during pyrolysis, vol-N conversion, and char-N conversion. Much of the work performed on these mechanisms is relevant to pulverized fuel

INTRODUCTION NOx formation from solid fuel combustion has been studied extensively due to its adverse effects on human health and the environment. Several mitigation strategies have been developed, and significant reductions in NOx levels have been achieved. Most of these studies have focused on combustion systems for boilers, with industrial combustion systems receiving less attention. However, there is currently a lot of interest in industrial emission sources, i.e., those that do not involve heat and power generation, given that more stringent NOx emission limits are expected in the near future. For example, there is a need for improved techniques for NOx control in the Grate-Kiln process for iron ore production,1 which is the focus of this work. In the Grate-Kiln process, iron ore pellets are heated on a moving grate until the pellets are of sufficient strength to enter a rotary kiln. In the kiln, heat is provided by combustion with large volumes of preheated air (at >1000 °C), which ensure adequate oxidation of the pellets. The amount of air is 4−6-times that needed to oxidize the fuel, i.e., an equivalence air-to-fuel ratio (λ) of 4−6. High temperatures and an excess of oxygen are known to promote NOx formation. Most of the air (referred to as “secondary air”) is introduced into the kiln via two large channels, one above and one below the centrally positioned burner. Thus, the air that is introduced through the burner (“primary air”) corresponds only to a small fraction of the total air flow. The large volumetric gas flow in the process dilutes the NOx in the flue gases and makes secondary NOx reduction measures, such as SCR (selective catalytic reaction), both expensive and inefficient.2 In addition, commonly used primary measures, such as external air and fuel staging, are impractical due to the rotation of the kiln. Reference documents on the Grate-Kiln process2,3 that discuss emissions in a general sense suggest that thermal NOx is the predominant form of NOx formation in the kiln owing to © XXXX American Chemical Society

Received: September 11, 2017 Revised: January 19, 2018 Published: January 23, 2018 A

DOI: 10.1021/acs.energyfuels.7b02707 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 1. Illustration of the model structure with inputs and outputs.

(PF) flames. In this context, the reviews of van der Lans,6 Glarborg,7 and Molina8 are recommended to interested readers. The release of nitrogen during pyrolysis is important for NO formation, as the possibilities for NO x control differ substantially for volatile and char-bound nitrogen. The conditions that prevail during pyrolysis affect the yields of volatile products. There exists a consensus that higher temperatures during pyrolysis will result in higher levels of nitrogen-release via volatiles, although considerable differences have been reported in the literature.9−13 Blair et al.10 concluded that nitrogen release is more sensitive than mass release (which also increases with temperature) to the process temperature. The nitrogen-containing volatiles evolve mainly as lowmolecular weight nitrogen species (HCN or NH3), either directly from the char matrix or from the tar. The conversion of vol-N to NO is strongly dependent upon the availability of oxygen, and the conversion may vary from 0% to 100% depending on the local stoichiometry.11,14 Miller and Bowman15 have provided a thorough assessment of the major reaction pathways for HCN and NH3, which have generally been confirmed and accepted: nitrogen atoms in HCN or NH3 eventually end up as N radicals, which react either with OH to form NO or with NO to form N2. Nitrogen-containing volatiles that are not directly released from the coal matrix or from the tar are incorporated into the soot (soot-N) formed by the tar. The fraction of volatile nitrogen components trapped in the soot is usually low, although it may reach up to 30%.16,17 The fate of soot-N is not well understood. Soot itself can effectively reduce NO,18,19 although incorporated soot-N leads to lower amounts of nitrogen gas species being susceptible to primary NOx reduction measures. Although the conversion of char-N has been extensively studied, the results obtained are inconclusive. It is a challenge to differentiate between the intrinsic conversion of char-N, i.e., the selectivity of char-N toward NO (prior to reduction by the char), and the apparent conversion of char-N, i.e., the net formation of NO after reduction of NO by char has occurred. The apparent conversion of char-N is easier to measure, and such data are more commonly used. The values reported for the apparent conversion of char-N vary within the range of 10%−100%.8,11,20−26 Jensen et al.20 discovered that, at high temperatures, the conversion of char-N to NO is close to 100% when combusting small amounts of char (1773 K are reached temporarily). A sensitivity analysis was conducted in this work to evaluate the uncertainty related to the extrapolation in temperature. C

DOI: 10.1021/acs.energyfuels.7b02707 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 3. Chosen Parameters for the Sensitivity Analysis and Their Investigated Ranges parameter

a

unit

reference case

investigated range

initial temperature

°C

25

temperature

°C

profile (Table4)

mixing of secondary aira amount of primary air volatile composition (H2/CO/CH4)

mn3/h mass %

arbitrary profile 60 3.9/40.8/55.2 (Coal A)

vol-N/fuel-N R1 and R2 A factor

cm3/mol/s

0.5 2.24 × 1011

R3 A factor

cm3/mol/s

1.46 × 1011

R1 and R2 activation energy

cal/mol

29 400

R3 activation energy

cal/mol

29 400

200 600 1000 −200 −100 +100 +200 +300 ±50% ±100% 100/0/0 0/100/0 0/0/100 0.25−0.75 ±50% ±90% ±50% ±90% ±10% ±50% ±10% ±50%

The rate was increased/decreased by decreasing/increasing the distance until complete mixing was achieved. conventional PF combustor36 (100 kW fuel), for comparison. Schematics of the two furnaces are presented in Figure 2. More details about the burners can be found elsewhere.1,37 Secondary air is added to the kiln through the two open channels above and below the burner, while for the 100 kW combustor, it is swirled through a burner register. Fuel and primary air enter through the burner in both setups. The primary air is swirled in the 100 kW combustor, whereas in the pilot-scale kiln it is split into two burner registers, one of which is swirled while the other introduces the air in the axial direction. The model does not distinguish between axial air and swirl air. Table 5

parameters. Table 3 summarizes the sensitivity runs. An arbitrary mixing profile for the secondary air was chosen for the reference case, i.e., a fit with the experiments was not attempted. As the secondary air enters the combustor through separate channels rather than through the burner, the mixing rate was set to be slow initially and faster subsequently. A measured temperature profile1 (Table 4) was used for

Table 4. Temperature Profile for the Reference Case distance [mm] temperature [°C]

0 25

328 1365

653 1573

1153 1519

2153 1396

9000 1312

Table 5. Modeled Cases of the Performed Experimentsa

the reference case and was kept constant for all sensitivity runs (except when investigating the sensitivity of temperature). The ranges in Table 3 for each variable were chosen to be representative for the process conditions and to give a significant yet reasonable impact. Evaluation of the Experiments. The model was used to interpret previously presented measurement data.1,36 The main aim was to evaluate the results from our previous study using a pilot-scale plant kiln1 (580 kWfuel), although the model was also applied to a more

cases

FC (wet%)

N (wet%)

conditions

Coal A_1 Coal A_2 Lignite

63.9 63.9 34.9

1.4 1.4 0.5

pilot kiln, λtot = 4.3, λprim = 0.11 pilot kiln, λtot = 4.4, λprim = 0.24 combustor, λtot = 1.2, λprim = 0.30

FC, fixed carbon; N, fuel nitrogen; λtot, total air-to-fuel equivalence ratio; λprim, primary air-to fuel equivalence ratio. a

Figure 2. Schematics of (a) the 580 kW pilot-scale kiln and (b) the 100 kW combustor. The dark discs represent the measurement ports used for the in-flame measurements. Only the 1st, 2nd, 4th, and 8th ports were used for the pilot-scale kiln. Fuel and primary air enter through the burner in the pilot-scale kiln, while secondary air enters through the two open channels above and below the burner. In the 100 kW combustor, secondary and primary air enter through designated burner registers; that is, no air enters the combustor separate from the burner. D

DOI: 10.1021/acs.energyfuels.7b02707 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels presents the cases modeled in the present work (termed: Coal A_1; Coal A_2; and Lignite), and Table 6 shows the measured temperature

of the secondary air with the fuel (i.e., adjusting how much secondary air was injected at certain distances in the PFR), as well as the shares of vol-N and char-N, so that the O2, CO, and NO profiles matched. The mixing profile was primarily fitted to the measured O2 concentrations and then validated by comparing the resulting CO profile with the CO measurements. If the CO profiles differed significantly, the mixing rate was modified. This process was repeated with the constraint that the O2 profile in the model should lie within the range of the three central in-flame measurements. The NO profile was mainly fitted by adjusting the partitioning of fuel-N to vol-N and char-N such that the outlet NO levels matched. NOx formation was then evaluated by analyzing the contributions of the different reactions, as described above. Similar approaches have been used successfully in previous studies.31,38 Impacts of Local and Global Stoichiometry. An evaluation of the potential for reducing NO formation (primarily via the contribution of char) through altering the local and global stoichiometry was performed by modifying the concentrations of O2 and excess air. Four cases were set up and compared to the Coal A_1 case (Table 7). To isolate the factors impacting char-N conversion, the

Table 6. Temperature Profiles Measured in the Experiments1,36 (in °C) 0

328

653

1153

Coal A_1 Coal A_2 distance [mm]

25 25 0

1365 1260 215

1573 1470 384

1519 1315 553

1396 1312 1280 1209 800 1400 2400a

Lignite

25

1222

1115

1044

943

a

2153

9000a

distance [mm]

845

708

Stack measurement.

profile for each case. Coal A_1 and Coal A_2 are rotary kiln cases, with both using the same fuel (a bituminous coal), although less primary air was used in the Coal A_1 case (10% of the stoichiometric air-to-fuel ratio compared to 26% for Coal A_2), which resulted in large differences between the flames in the experiments. The Lignite case was performed in the 100 kW combustor (primary air accounted for 28% of the stoichiometric air-to-fuel ratio). The evaluation of NOx formation was performed by examining the NO concentration profiles and analyzing the contributions of vol-N and char-N to the total formation of NO. The model outputs the reaction rates (in mol/cm3/s) at each step in the PFR. It also calculates the net formation rate of any species at all positions. The rates can thereafter be integrated either over time or volume, depending on the desired outcome. The net formation of NO (in mol/s) in the PFR is obtained simply by assessing the molar flow of NO at the outlet or by integrating the net NO formation rate over the volume of the reactor. Both methods yield the same result. Likewise, the level of NO production from char-N and the level of NO reduction by char are calculated by integrating R2 and R3, respectively. The thermal NO formation is usually described by the reactions: N2 + O ↔ NO + N

(R5)

N + O2 ↔ NO + O

(R6)

N + OH ↔ NO + H

(R7)

Table 7. Four Cases (Plus Coal A_1 as Reference) for Estimating the Importance of Stoichiometry Coal A_1 Case 1 Case 2 Case 3 Case 4

λglob

O2,oxidizer

4.3 4.3 4.3 1.3 1.1

21% 40% 15% 21% 21%

temperature profile of Coal A_1 was maintained for the four cases, while the mixing profile was adjusted so that the mixing of oxygen and fuel was similar during the early stages of combustion for all cases. The level of primary air from Coal A_1 was also kept the same.

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RESULTS AND DISCUSSION First, the results from the sensitivity analysis are presented, and the effects of the combustion conditions on NOx formation are discussed. The model is then fitted to experimental data (O2 and CO) by adjusting the key parameters to interpret the experimental results, followed by a discussion of the contributions of vol-N and char-N. Finally, the model is applied to evaluate emissions mitigation options for the rotary kiln. The discussion will revolve around thermal NO formation, vol-N formation, and the contributions of char-N and NO reduction on char. Sensitivity Analysis. The sensitivities of the NO concentration profiles to the investigated parameters are shown in Figure 3. Table 8 provides a summary of the maximum influence of the different parameters on the peak and outlet NO concentrations. From the temperature analyses (Figure 3a,b), it appears that an increase in temperature in the furnace does not increase notably the level of NO emissions, unless the entire temperature profile is increased by 300 °C. These results indicate that thermal NO formation is not important under the investigated conditions, even though local temperatures may exceed the temperature threshold for the activation of thermal formation. As argued by Glarborg7 with reference to Pershing and Wendt,39 the high levels of NO formed by fuel-N may inhibit the formation of NO through the thermal pathway by reversing the reaction: N2 + O ↔ NO + N. Therefore, exceptionally high temperatures (>2200 K) may be required for thermal NO to contribute significantly in coal dust flames.

However, the forward reaction of R7 is the main formation route for nitrogen that originates from vol-N species (such as HCN), and the reverse reaction of R5 is the main pathway for homogeneous NO reduction. Thus, these two reactions are central to the conversion of vol-N, and they cannot simply be integrated to obtain the thermal NO formation. Therefore, R7 and the reverse reaction of R5 are not considered to be part of the thermal NO mechanism. The production of thermal NO is consequently defined as the integral of only the positive values for R5 and R6. Although it is possible to integrate the remainder of the homogeneous NO reactions to obtain a value for NO formation from vol-N, it becomes slightly misleading due to circular reactions (such as NO → HONO → NO2 → NO), which make volatile formation and reduction reactions appear more prominent than they actually are. Instead, the net amount of NO formed from the volatiles is calculated as the difference between the total net NO formation and the net level of NO formation from the processes of char-N formation, thermal NO formation, and NO reduction by char, which are calculated as described above. Since the 580 kW pilot-scale kiln and the 100 kW PF combustor are different sizes and use different air-to-fuel ratios, the reaction rates are scaled by the fuel input (i.e., divided by 580 kW for Coal A_1 and Coal A_2, and by 100 kW for Lignite). The resulting values are in units of mol/MJ. The reaction rates are further scaled by the nitrogen content of the fuel (kgN/kgfuel) since the facilities use different fuels with different nitrogen contents. The final unit is displayed as mol/ MJ/N, where N stands for the nitrogen content. Fitting Procedure. To evaluate the data from the previously performed experiments, the modeled concentration profiles were fitted to the actual measurements. This was achieved by adjusting the mixing E

DOI: 10.1021/acs.energyfuels.7b02707 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 3. Sensitivities of NO formation to (a and b) temperature; (c and d) secondary and primary air; (e and f) the compositions of the volatiles; and (g−j) the kinetic parameters. All concentrations are given on a wet basis.

The conversion of vol-N to NO is sensitive to the levels of available oxygen in the early stages of the furnace, which can be assessed by observing the influence of secondary mixing in Figure 3c and level of primary air in Figure 3d; the NO levels are significantly higher early on in the flame in the case of increased mixing and primary air. In particular, it is the amount of oxygen that is available at the onset of volatile combustion (i.e., ignition) that is important for vol-N conversion. The onset

of combustion is determined partly by local stoichiometry and partly by temperature. If the temperature is high at an early stage, combustion will be initiated earlier and less oxygen will be available when vol-N conversion begins. To illustrate this, a simulation was conducted in which the concentration of primary air was high and the initial temperature was increased significantly. Figure 4 presents the model result compared with the reference case, as well as the case with a high concentration F

DOI: 10.1021/acs.energyfuels.7b02707 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 8. Summary of the Sensitivity Analysisa parameter initial temperature temperature profile mixing of secondary air amount of primary air volatile composition share of vol-N R1 and R2 A-factor R3 A-factor R1 and R2 activation energy R3 activation energy

ΔNOmax −55 −42 0 to −38 −34 −30 −20 −27 −36 −80

to −25% to 37% 84% to 184% to 254% to 29% to 20% to 65%b to 139% to 75%b

The temperature-dependence is eliminated, since the activation energies are assumed to be the same. If the N/C ratio in the char is assumed to be constant during combustion, the apparent yield of NO from the char becomes a function of the ratio of O2 to NO in the gas mixture. Similarly, relating the consumption rate of char by O2 with the reduction rate of NO by char indicates the effectiveness of the NO reduction by char (i.e., how fast char is consumed by O2 compared to how fast it can reduce NO):

ΔNOout −2 to 0% −2 to 59% −4.5 to 24% 2 to 20% 2 to 10% −46 to 46% −27 to 20% −11 to 22% −86 to 0% −85 to 26%

r1 A [O2 ] = 1 r3 A3 [NO]

ΔNOmax is defined as the largest difference in max NO concentration compared to reference case, and ΔNOout as the largest difference in outlet NO concentration compared to thte reference case. bMax NO concentration is reached at an early stage and is the same for all investigated cases; that is, the actual difference in max NO concentration is 0%. The displayed values are for the second peak. a

where a low value indicates a high effectiveness of the NO reduction by char. Thus, a high concentration of NO and a low concentration of O2 leads to a high effectiveness of the NO reduction, and to a low apparent formation of NO from char-N. However, the high air-to-fuel ratio in iron ore rotary kilns counteracts this effect, as the high flow rate of the gas dilutes the NO and increases the concentration of O2. This dependence also means that high conversion of vol-N to NO reduces the apparent conversion of char-N by increasing the NO concentration, although the net conversion of fuel-N might not be reduced. This can be seen in Figure 4b, where the reduction of NO by char is substantially higher when the net NO formation from vol-N is increased (as compared to the reference case), although the total net formation of NO is in fact higher compared to the reference case. This interaction between the different conversion ratios has been discussed by others, e.g., Spinti and Pershing.25 Using the data that they obtained for the apparent conversion of char-N and some theoretical calculations, they showed that minimal conversion of fuel-N was achieved when most of the nitrogen was partitioned to the volatiles and vol-N conversion was low, even though this resulted in a higher apparent conversion of char-N to NO. The importance of the partitioning of nitrogen between volatiles and char can be seen in Figure 3f. Of all investigated combustion parameters (excluding the kinetics and an extreme increase in temperature), this parameter has the largest influence on the outlet NO concentration (Table 8). As discussed in the Methodology section, heterogeneous kinetics were obtained in experiments performed at low temperatures (850−1150 °C) and extrapolated to higher temperatures typically used in rotary kilns. It should, however, be noted that the temperature during postflame conditions in rotary kilns, where the majority of char conversion typically occurs, corresponds reasonably well to the upper range of the

of primary air in Figure 3d. Figure 4a displays the NO profiles, and Figure 4b shows the contributions of the homogeneous and heterogeneous reactions. It is clear that a high concentration of primary air results only in high levels of NO emissions when the initial temperature is low. Increasing the initial temperature results in the same level of NO emission as the reference case. The importance of controlling the ignition phase is also evidenced by the effect of the volatiles composition on the early NO concentration (Figure 3e). Thus, the NO concentration is significantly higher when the volatiles consist entirely of CO, as CO requires higher temperatures for ignition than does CH4 or H2. As can be seen in Table 8, the volatile composition has a large impact on the peak NO concentration. The resulting outlet NO concentration is, however, almost the same as that in the reference case, due to significant reduction of NO by char. It should be noted that the investigated sensitivity range is large for the volatile composition and that the resulting plots in Figure 3e indicate the impact of the volatile components rather than typical variations in volatile composition. The apparent conversion of char-N to NO can be examined by calculating the ratio of the formation rate of NO from charN (R2) to the rate of reduction of NO by char (R3): r2 A [N2(s)][O2 ] = 2 r3 A3 [C2(s)][NO]

(6)

(5)

Figure 4. Illustration of the effects on fuel-N conversion of the interactions between oxygen availability and temperature. Left panel: NO profiles for two cases with increased primary air (plus the reference case), one with an initial temperature of 25 °C and one with an initial temperature of 1000 °C. Right panel: Mechanisms for the formation/reduction of NO. Net NO: Net NO formation in the process. Char-NO: Formation of NO from char-N. Char red: Reduction of NO by char. Vol-NO: Net NO formation from vol-N. G

DOI: 10.1021/acs.energyfuels.7b02707 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 5. Molar concentration profiles for O2, NO, CO, and CO2 (wet basis) for the three investigated cases. The length of the combustion facility on the x axis has been normalized. *Not rotary kiln conditions.

experiments. The sensitivities to the pre-exponential factor and activation energy of the char reactions are both included in Figure 3g−j, mainly to provide an overview of how the reactions influence the formation and reduction of NO. It is clear that the kinetics play an important role, and deriving the kinetics for the specific conditions in an iron ore rotary kiln may be important. Model Fitting. The measured concentration profiles of O2, CO, and NO were used to fit the model to the specific experimental conditions. The modeling results postfitting are depicted in Figure 5. Since the dimensions of the furnaces are different, the distance on the x axis is normalized. The comparisons to the experimental data are presented in the Supporting Information. Satisfactory NO profiles were obtained by setting the share of nitrogen released with the volatiles to 50% for the two rotary kiln cases (Coal A_1 and Coal A_2) and to 55% for the more conventional case (Lignite). Even though they were combusted in the same facility and with similar configurations (the only difference being less primary air for Coal A_1), there are significant differences between Coal A_1 and Coal A_2, especially in terms of the O2 and CO2 concentrations. A clear O2-lean zone is observed for Coal A_1. Significantly faster mixing of secondary air was required in the model to achieve the high O2 levels early on in the Coal A_2 case. In real process conditions, it is not likely that the mixing is faster but rather that a delayed ignition occurs. The delayed ignition results in a significant amount of air being available when combustion starts, generating a strongly oxidizing environment during vol-N conversion. As expected, the lack of an O2-deficit zone in the Coal A_2 case results in a higher local concentration of NO than in the Coal A_1 case. The outlet NO concentration from the two cases differ by about 100 ppm (note that the rotary kiln gas flow is high relative to conventional conditions and an increase in concentration requires a larger amount of NO). The higher NO peak for Coal A_2 is due to a higher conversion of vol-N to NO. This is confirmed in Figure 6, where the formation/ reduction mechanisms are presented for the three cases. When comparing the rotary kiln case to the conventional furnace in Figure 6 (i.e., comparing the Coal A_1 and A_2 cases to the Lignite case), it is seen that the NO reduction by

Figure 6. Mechanisms for the formation/reduction of NO. Net NO: Net NO formation in the process. Char-NO: Formation of NO from char-N. Char red: Reduction of NO by char. Vol-NO: Net NO formation from vol-N. *Not rotary kiln conditions.

char is substantially lower under rotary kiln conditions. As discussed earlier, the ratio of O2 to NO (eqs 5−6) governs the effectiveness of the NO reduction by char, and it is clear from Figure 5 that, although the NO levels are similar in all three cases, the O2 level is significantly lower in the lignite case, thus leading to a lower apparent conversion of char-N. The modeling confirms that the high emissions of NOx observed in iron ore-producing rotary kilns can be attributed to a high apparent conversion of char-N, as proposed in our previous paper. Thermal NO contributes only slightly. Even though the peak flame temperatures are higher than in most combustion systems, the effective volumes of these high-temperature zones appear to be too small relative to the total gas flow for thermal NO formation to be of significance. Mitigation Possibilities. In summary, two ways to efficiently reduce the NO formation in iron ore rotary kilns are identified based on the results discussed in this paper: A. promote the NO reduction by char B. control and increase the share of nitrogen released together with the volatiles Option A may be realized by increasing the concentrations of NO and decreasing the concentrations of O2 in those zones where substantial areas of active char sites are available. In theory, this may be achieved by a reduction in the global air-tofuel ratio. This is an obvious solution from a NOx perspective, although the measure is restricted by the need to oxidize the H

DOI: 10.1021/acs.energyfuels.7b02707 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 7. Mechanisms for the formation/reduction of NO (left), and the ratio of O2 to NO plotted along the reactor (right). Net NO: Net NO formation in the process. Char-NO: Formation of NO from char-N. Char red: Reduction of NO by char. Vol-NO: Net NO formation from vol-N.

iron ore. Recirculation of flue gases would achieve a similar effect, although rather large volumes of flue gases would have to be recirculated to change substantially the NO and O2 concentrations, since the outlet O2 concentration typically corresponds to around 15−16 vol %. A low NOx burner that could ignite and convert the fuel with very limited entrainment of secondary air into the fuel stream would also promote the NO reduction by char. Altering the mixing conditions post the devolatilization stage is, however, difficult to achieve with burner design and, due to the rotation of the kiln, external staging is unlikely to be implemented. An alternative would be to partly replace the combustion air with O2, thereby providing sufficient oxygen for oxidation of the fuel and the product without introducing high levels of N2 (which dilutes the NO). However, this would increase the local O2 levels and might lead to an increase in the conversion of both vol-N and char-N. Moreover, O2 production is rather expensive. To investigate the influences of the local and global stoichiometries, four cases with different oxygen contents in the oxidizer and different stoichiometries were set up (Table 7). The resulting NO formation/reduction mechanisms are shown in Figure 7a. Increasing the inlet oxygen concentration while retaining the original global stoichiometry (case 1), reduced the NO emissions due to a slight increase in NO reduction by char and a decrease in the conversion of vol-N to NO. A reduction in the inlet oxygen concentration while retaining the global stoichiometry (case 2) increased the NO emissions. These results may be attributed to the volumes of the gas flows. Reducing the oxygen content in the oxidizer means that significantly larger gas flows are required to maintain the stoichiometry, and this leads to a shorter residence time in the volatile reduction zone, as well as a reduction of the effectiveness of the NO-char reaction; that is, the O2/NO ratio is increased during combustion (see Figure 7b). That the O2/NO ratio increases when a lower O2 content is used means that the effect of diluting the NO is more important than the decrease in O2 concentration. Since the O2/NO ratio controls the effectiveness of the NO reduction by char, one would expect the amount of NO reduced by char to be lower in case 2 than in the Coal A_1 case. However, as more NO is formed from vol-N, more NO is also available for reduction. The decreased effectiveness of the NO reduction by char and the extra NO formed by the volatiles counteract each other, and a similar amount of NO is reduced on the char as in the Coal A_1 case. Cases 3 and 4 explore the theoretical potential of reducing the global stoichiometry. The effect of the stoichiometry on NO reduction is most pronounced close to unity; the difference between Coal A_1 (λ = 4.3) and case 3 (λ = 1.3) is of similar magnitude to the difference between cases 3

(λ = 1.3) and 4 (λ = 1.1). It should be borne in mind that in practice it is not possible to isolate a specific change in this way, since the combustion conditions would likely change significantly when the oxygen content of the oxidizer is changed, as well as when the global stoichiometry is reduced. The results of this investigation show, however, that the NO reduction by char reaction is difficult to control at high air-tofuel ratios and is relatively insensitive to mitigation measures. Increasing the share of nitrogen released with the volatiles (option B) might be a more promising approach, and this could be achieved by increasing the pyrolysis temperature. This may, in theory, be achieved by preheating the primary air (and coal) or replacing it with warm recirculated flue gases. The impact of applying these measures relies on a more detailed description of the pyrolysis stage, which is outside the scope of the presently proposed model. However, if the char could be completely depleted of nitrogen after pyrolysis, NO formation would be finished after combustion of the volatiles. The produced NO would then be reduced on the char. In the Coal A_1 case, the vol-N conversion was about 20%. Assuming that this conversion ratio can be maintained and all the nitrogen is released with the volatiles, a total fuel-N conversion rate lower than 20% could be achieved. This can be compared favorably with the current conversion rate of about 56%.

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CONCLUSIONS A model has been developed to describe the formation of NO in rotary kilns used for iron ore induration. The overall aim is to analyze the applicability of primary NO reduction options for such systems. Three major conclusions are drawn from the present work: • Early ignition, before high levels of oxygen become mixed with the fuel, is important for minimizing the conversion of vol-N to NO. This may be achieved by using low levels of (preferably preheated) primary air. • The high-level NOx emissions in rotary kilns (relative to conventional burners) is due to a high apparent conversion of char-N to NO, i.e., a low level of chemical reduction of NO by char, rather than thermal-NO production. • The char-N is relatively insensitive to process conditions and is difficult to control. To control the split between vol-N and char-N is therefore the recommended alternative for primary NOx reduction in rotary kilns. The first point is well-recognized in combustion research. However, given that volatile nitrogen is significantly easier to control than char-bound nitrogen and our observation (point 2 above) regarding the high level of apparent conversion of charI

DOI: 10.1021/acs.energyfuels.7b02707 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

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N to NO, this is an important point that should be noted. If vol-N conversion is not minimized, the last measure (third point above) will be ineffective. The high level of conversion of char-N to NO is mainly the result of the high air-to-fuel ratio, since it leads to high levels of O2, as well as the dilution of NO, which reduces the reduction rate of NO by char. The high temperatures involved are less important. Future work on mitigating NOx from iron ore rotary kilns should focus on controlling the partitioning of nitrogen between volatiles and char.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b02707. Figures 1−3 present the fitting of the model to the experimental measurements. A text that describes and compares the figures is also included. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +46 31 772 1000. E-mail: [email protected]. ORCID

Rikard Edland: 0000-0003-4383-8180 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financed by LKAB and the Swedish Energy Agency.

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REFERENCES

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DOI: 10.1021/acs.energyfuels.7b02707 Energy Fuels XXXX, XXX, XXX−XXX