Implications of Fuel Choice and Burner Settings for Combustion

Feb 7, 2017 - The combustion process applied in the grate-kiln process for iron ore pellet production employs air-to-fuel equivalence ratios in the ra...
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Implications of fuel choice and burner settings for combustion efficiency and NOx formation in PF-fired iron ore rotary kilns Rikard Edland, Fredrik Normann, Christian Fredriksson, and Klas Andersson Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03205 • Publication Date (Web): 07 Feb 2017 Downloaded from http://pubs.acs.org on February 11, 2017

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Implications of fuel choice and burner settings for

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combustion efficiency and NOx formation in PF-

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fired iron ore rotary kilns

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Rikard Edland1*, Fredrik Normann1, Christian Fredriksson2, Klas Andersson1

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1

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University of Technology, SE-412 96, Göteborg

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2

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*Corresponding author, Tel. +46 31 722 1000. [email protected]

Division of Energy Technology, Department of Energy and Environment, Chalmers

LKAB, Box 952, SE-971 27 Luleå

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ABSTRACT:

The combustion process applied in the grate-kiln process for iron ore pellet

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production employs air-to-fuel equivalence ratios in the range of 4–6, typically with coal as

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fuel and high-temperature air (>1000°C) as oxidant. The NOx emissions from these units are

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in general significantly higher than those in other combustion systems and the large flows of

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flue gases make the implementation of secondary measures for NOx control costly. Therefore,

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it is of importance to investigate NOx formation under combustion conditions relevant for iron

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ore production, in order to control the emissions from these units. The present work examines

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NOx formation during the combustion of four pulverized coals, as well as during co-firing

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with biomass in a pilot-scale kiln (580 kWfuel) based on a two-week experimental campaign.

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The influence of burner settings was also included in the investigation. Based on the 1 ACS Paragon Plus Environment

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presented experimental results and the results of previous modelling and experimental studies,

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we suggest that the NOx emissions are mainly the result of a high conversion of fuel-bound

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nitrogen (fuel-N) to NO. In particular, char-bound nitrogen (char-N) conversion appears to be

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higher than in conventional pulverized fuel flames, presumably due to the high levels of

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oxygen present in the char-burnout region. The temperatures in the kiln varied between the

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test cases, but thermal NO formation is estimated to be of low importance.

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1. Introduction

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Iron ore is typically processed into pellets for easy transportation and handling in steel

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making. To achieve the high temperatures required for sintering the hematite (Fe2O3) (the

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pellet temperature should be >1300°C1), a rotary kiln may be used in the so-called ‘grate-kiln

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process’. The kiln is heated by a flame and large volumes of highly preheated air. In the

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rotary kiln, the molar air-to-fuel equivalence ratio, λ, is in the range of 4–6, which is high

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compared to conventional pulverized fuel (PF) burners with λ-values in the range of 1.2–1.5,

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and the inlet air temperature is around 1000°C (300°C in conventional burners). The process

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is dependent upon an efficient combustion process, so current operation is often restricted to a

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specific coal or oil. However, with the increasing volatility of the fossil fuel market and

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increasing concerns regarding global warming, fuel flexibility and increased use of biomass

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are of interest. Increasingly stringent regulations designed to reduce NOx emissions have also

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intensified the desire to mitigate NOx emissions from the iron ore industry. Due to the high-

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volume flue gas flow from the process, secondary mitigation strategies are costly, so primary

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measures are preferred. However, when modifying the combustion process it is critical to

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consider the pellet-making process, given that the sintering of the pellets requires controlled

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heat transfer conditions and the oxidization of magnetite to hematite requires a high oxygen

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content in the flue gas. 2 ACS Paragon Plus Environment

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The combustion conditions in the kiln used for iron ore processing are thus considerably

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different from those in conventional coal flames for heat and power generation, and it is

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unclear how these conditions influence NOx formation. During the combustion of coal, the

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main source of NOx is the nitrogen bound to the fuel (fuel-N), as long as the temperature is

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kept low enough to avoid significant amounts of air-borne nitrogen to react with oxygen

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(thermal NO formation). Although thermal NO may have some importance for solid fuel

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combustion, fuel-NO usually contributes with at least 75% of the total NOx formation2. The

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prompt NO mechanism, whereby hydrocarbon radicals attack the air-bound nitrogen, is

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considered to play a negligible role in solid fuel combustion3. The properties of the fuel,

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burner design, and combustion conditions are known to be important factors for controlling

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NO formation. A simple way to reduce the NOx emissions is to change to a fuel with a lower

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nitrogen content. In the flame or combustor, a reducing zone can be formed in which N2

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formation is favored over NO formation. Such a zone may be created by staging the

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combustion air, either externally or internally. Fuel-NO can further be divided into NO that is

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formed from the nitrogen in the volatiles (vol-N) and NO that is formed from the nitrogen in

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the char (char-N). It is mainly the conversion of vol-N that is reduced when air-staging is

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applied. NO formation from vol-N is strongly dependent upon the local stoichiometry and

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may reach levels from 10% to 100%4. While the conversion of char-N to NO has been

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investigated in many studies, the results are complex. For single-particle experiments, char-N

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conversion may be 100%, whereas when more particles are introduced (approaching a fixed

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bed system) heterogeneous NO reduction becomes of importance and char-N conversion

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decreases to around 50%5. Furthermore, the initial NO concentration has a strong influence on

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the net NO formation during char burnout, in which a higher initial NO concentration

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decreases the conversion of char-N to NO6, 7. Common NO/char-N ratios found in the

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literature are in the range of 10%–30%2, 4, 8-10. While the importance of oxygen for char-N 3 ACS Paragon Plus Environment

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conversion is debated, at high temperatures it appears that the conversion rate increases

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significantly with increasing amounts of air2, 7, 11. The total conversion of fuel-N to NO is thus

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mainly a reflection of how the fuel-N is distributed between volatiles and char and the local

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stoichiometry during the release of vol-N and char-N, of which vol-N conversion is somewhat

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easier to control. The nitrogen fraction in the volatiles increases with the pyrolysis

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temperature and the oxygen content of the fuel3, 12. With local sub-stoichiometric conditions

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(achieved through low levels of excess air, as well as controlled mixing), it is possible to have

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a total fuel-N conversion of 2000°C29, 30) and the air-to-fuel

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equivalence ratio is lower, which alters significantly the conditions for NO formation.

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The present work is a technical-scale experimental investigation of the influences of fuel

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characteristics and burner settings on the combustion process in general and on NOx

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formation in particular. The aim is to characterize how the combustion process is affected by

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a fuel change given the special conditions of iron ore processing, so as to be able to increase

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fuel flexibility and enable the use of biofuels without risking the efficiency of the iron ore

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process. We evaluate current combustion praxis (i.e., fuel characteristics, influences of λ-

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values and preheating temperatures) for low-NOx combustion under conventional combustion

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to conclude on its applicability to the conditions of iron ore processing.

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2.1. Equipment and fuels

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The kiln is a pilot-scale version of a 40-MWfuel rotary kiln used for iron ore processing. The

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pilot kiln is designed to explore the features of the combustion process, i.e., the product

2. Experimental

The present investigation was performed with the LKAB test kiln (580 kWfuel) (Figure 1).

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stream and the rotation of the kiln are not included. The combustion process is not adiabatic.

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The burner hood (prior to the kiln inlet) and the first 4 m are scaled with regard to the full

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scale kiln using constant velocity scaling. The diameter of the scaled kiln part is 0.65 m. The

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total length of the test facility is 14 m in order to provide further measurement possibilities,

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e.g. ash extraction. Measurement ports are located along the unit (marked as MH0-MH12 in

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Figure 1). The burner has six registers (Figure 2), which are denoted N1–N6. Combustion air

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enters at N1 and N4, while the remaining registers are fuel registers. The air that enters at N4

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is swirled at an angle of 30°, so it is termed ‘swirl air’, while the air that enters N1 is called

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‘axial air’. The burner is placed in the center with large volumes of highly preheated air being

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introduced at two large inlets located above and below the burner (Figure 3). This air will be

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referred to as ‘secondary air’, and the air flows in the burner (N1, N4 and fuel transport air)

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will be referred to as ‘primary air’. The key combustion settings are listed in Table 1.

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Measurements of CO2, CO, NO, NO2, O2, and SO2 were performed at the kiln outlet. The

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levels of CO, CO2 and SO2 were measured by NDIR (ABB URAS), O2 by paramagnetism

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(ABB MAGNUS), and NO and NO2 by NDUV (ABB LIMAS). In-flame gas measurements

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were performed with a filter-equipped cooled probe connected to an FTIR-system (MKS

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Multigas 2030) and a paramagnetic O2-measuring instrument (Maihak SIDOR). It is

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estimated that the gas experienced instantaneous quenching due to the excessive cooling in

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the probe. The probe was traversed horizontally in the kiln at four ports (MH0, MH1, MH3,

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and MH7 in Figure 1). Seven radial measurement positions were used for each port to map

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the radial concentration profiles. The central position was not included for MH0 due to the

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high density of the fuel particles and potential interference with the flame. Temperature

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measurements were performed using a suction pyrometer with a thermocouple type B at the

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same locations as the gas measurements. The thermocouple was protected by two ceramic

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housings, so as to minimize the effects of radiation, and mounted on the tip of a water-cooled 6 ACS Paragon Plus Environment

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probe with a central pipe connected to an ejector, which provided the required suction

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velocity.

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Four pulverized coals and two types of biomass were used. Biomass A was a pulverized

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wood treated with steam explosion, and Biomass B was an untreated but pulverized mixture

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of fir and pine. The coals were pulverized in a roller mill to the size distribution of a full-scale

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process. The biomass was instead treated in a hammer mill due to technical constraints of

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roller mills with respect to the fibers in biomass (it might have been possible to mill Biomass

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A in the roller mill but this was not tested). During the milling process, the biomass pellets

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were partly pulverized into their original particle size (prior to pelletization), which was

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smaller for Biomass A than for Biomass B. The size of the biomass particles was larger than

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the coals but probably smaller than they would have been in a full scale process. The fuel

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compositions and volume weighted mean diameter of the particle distribution are presented in

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Table 2. The fuels were introduced through register N6 during the coal firing. Co-firing of

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coal and biomass was performed with the burner inserted 220 mm into the kiln and with the

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coal fed through register N3 and biomass fed through register N2. This was done in order to

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represent two different burner types seen in full scale. The biomass provided 30% of the total

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fuel energy input, which corresponded to approximately 40% on a mass basis.

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For comparison, the outlet NOx concentration is presented as ppm NOx corrected to 6% O2

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and is calculated according to Eq. (1). The actual outlet concentration was around 15-16% in

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all cases, which is somewhat lower than the theoretically calculated concentration of 16%

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(Table 3). Considering the flows that need to be taken into account (secondary and primary air

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flows as well as the fuel feeding rate), this is an acceptable discrepancy. The ratio of NOx

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emissions to fuel-N (ߟே ) is calculated according to Eq. (2). This value is equal to or higher

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than the actual conversion ratio depending on the contribution from the thermal-NO formation

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from the air born nitrogen. 7 ACS Paragon Plus Environment

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‫ݔ‬ேைೣ ሺ6%ܱଶ ሻ = ሺ‫ݔ‬ேை + ‫ݔ‬ேைమ ሻ ∗

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ߟே =

଴.ଶ଴ଽି଴.଴଺ ଴.ଶ଴ଽି௫ೀమ

೛ ൫௫ಿೀ ା௫ಿೀమ ൯௏ሶ೑೗ೠ೐ ቀ బ ቁ

௠ሶ೑ೠ೐೗ ௬ಿ /ெಿ

೅బ ೃ

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Eq. (1)

Eq. (2)

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Where:

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‫ݔ‬௜

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ሶ = volumetric flue gas flow (in m3n/h) ܸ௙௟௨௘

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‫݌‬଴

= normal pressure (101.3 kPa)

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ܶ଴

= normal temperature (273.15 K)

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R

= 8314 ௞௠௢௟௄

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݉ሶ௙௨௘௟ = fuel mass flow (in kg/h)

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‫ݕ‬ே

= mass fraction nitrogen in the fuel

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‫ܯ‬ே

= molar mass of nitrogen (14 kmol/kg)

= fraction of component i



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2.2 Fuel test cases

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into two sections, one for the coal comparison and one for the biomass co-firing. The

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secondary air temperatures varied somewhat during the experiments and the measured values

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are displayed in one of the columns.

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2.3 Parameter study

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involved changing the primary air flow and observing the outlet NOx concentration, although

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flame measurements were performed on one occasion where the total amount of primary air

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was significantly lower. The influence of the air-to-fuel equivalence ratio, λ, on the

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concentration of NOx was investigated by decreasing the oxygen content of the secondary air

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flow. The effect of secondary air temperature was also examined. Table 4 provides an

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overview of the ranges and settings for the investigated parameters.

Table 3 presents the combustion settings for each of the different fuels. The table is divided

A study of how the air flows influence NOx emissions was carried out. These tests mainly

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This chapter presents the measurements from the experimental campaign. The results are

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discussed and interpreted with respect to implications on combustion and NOx formation in

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direct connection to the results.

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3.1 Coal comparison

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are presented in Figure 4 in the form of interpolated contour maps, in which the white fields

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correspond to areas where measurements were not performed due to technical issues.

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Temperatures of >1500°C were reached for Coal D and >1600°C for Coal C, while the peak

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temperatures were significantly lower for Coal A and Coal B. The O2 concentrations were

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high throughout the flame and the CO concentrations were relatively low (usually 2%) relative to the

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other coals and since it is unlikely that the levels of O2 and CO would be high simultaneously,

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the O2 levels for Coal C are assumed to be lower than those for the other coals.

3. Results and discussion

The in-flame measurements of temperature and selected gas components for the coal tests

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It is clear that the oxygen-lean zone in front of the burner is negligible for any of the three

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coals with O2 data. This means that there is a potential to enhance burner design to decrease

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mainly the vol-N conversion to NO. From the NO maps, it appears that the concentrations are

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higher further into the kiln for Coal B than for the other coals, and that the highest

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concentration is measured for Coal C, although this concentration drops to levels similar to

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those for Coal A shortly afterwards. NH3 was generally present at around 0–20 ppm, and is

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therefore not presented; however, an ammonia peak at 200 ppm was observed in the central

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position in MH1 for Coal C.

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Figure 5 presents the NOx conversion for each coal. With the exception of small variations

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in the secondary air temperature, burner settings were kept constant when testing the four 9 ACS Paragon Plus Environment

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coals. Coal A generated the lowest NOx emissions and Coal B the highest emissions. As

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anticipated, the level of NOx formation correlates with the amount of fuel-bound nitrogen in

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the coals. An exception to this is Coal C, which exhibits a comparatively low fuel-N to NOx

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conversion. However, Coal C has a high volatile content relative to the other coals, which

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according to literature12 favours a low conversion of fuel-N to NOx (ߟே ). From Figure 6, it

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can further be seen that the fuel-N to NOx conversion correlates well with the inverse volatiles

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content.

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The relative importance of fuel-nitrogen and volatile content is, however, case-specific and

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difficult to predict without experimentation. Other factors, such as how the nitrogen is bound

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in the fuel and local temperature profiles, will obviously also affect the formation of NOx in

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the grate-kiln flames, which, taken together, makes modelling predictions uncertain. Full-

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scale experiments are thus of great interest for future studies.

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Although the NOx emissions correlate well with the fuel characteristics, they are

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substantially higher than the levels seen in conventional PF flames. As a reference point,

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using the best available technology (BAT) for large combustion plants gives emissions around

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45-150 ppm (corrected to 6% O2) for pulverized coal using combined primary reduction

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measures31. Although certain primary measures are not applicable to rotary kilns - due to the

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rotation - significant improvements should be possible. The emissions seen in the test kiln are

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also somewhat higher than the ones typically observed in full scale. The high level of

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emissions (>1000 ppm at 6% O2) may be explained by either higher conversion of the volatile

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nitrogen or the char-bound nitrogen to NO or thermal NO formation. Thermal NO formation

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is dependent on O, N, and OH radicals, and is therefore coupled to the fuel combustion,

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although decoupling is common due to the slower kinetics of thermal-NO formation. To get

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an estimation of the thermal NO contribution, the nitrogen mechanism by Glarborg and

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Mendiara32 was used to determine the NO-formation from N2 and O2 at 1700°C, i.e. 100°C 10 ACS Paragon Plus Environment

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above the highest temperature measured during the campaign. Based on the measurements,

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the high-temperature zone can, as a rough estimate, be assumed cylindrical with a diameter of

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0.3m and a length of 1m (i.e. a volume of 0.07 m3). In this case, with a constant temperature

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profile of 1700°C, the thermal NO formation rate becomes 0.0005 mole/s. This should be

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compared to the total outlet NOx flow, which was about 0.015 mole/s in the case with the

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highest temperature (Coal C). Thus, according to these rough calculations, the thermal NO

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formation contributes with maximum 3% of the total NOx emissions.

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In summary, vol-N conversion to NO appears to be high due to the high oxygen fraction in

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the proximity of the burner outlet. Furthermore, char-N conversion is likely to be higher than

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is usually seen in conventional combustion, since the high level of excess air leads to a high

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level of oxygen in the char-burnout zone. The way in which the secondary air is added

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probably inhibits external recirculation zones, which further promotes an oxidizing

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environment.

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3.2 Biomass co-firing

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reference case with Coal A (all three cases used similar burner settings, see Table 3). Overall,

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the temperature and concentration maps resemble each other, which means that substituting

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30% of the coal energy with biomass does not appear to affect significantly (i.e. relative to

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differences between coals and the requirements of the iron ore pelletization) the combustion

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process. The oxygen levels were relatively high throughout the kiln for all three cases,

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although slightly lower levels were found for the co-firing case with Biomass A, which also

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exhibited a higher CO concentration. The zone with lower oxygen concentration was only

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slightly longer for the biomass cases, although the diameters of the biomass particles are

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substantially larger than those of the coal particles and the former are likely to burn later

Figure 7 presents the flame measurements for the two co-firing cases, as compared to a

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within the kiln. The temperatures measured during these experiments never reached more than

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1500°C, and thermal NO formation was presumably negligible.

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Figure 8 presents the NOx emissions and ߟே values for the coal plus biomass co-firing

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cases. Co-firing coal and biomass emitted less NOx than when coal was fired alone, as

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expected since the biomass have lower nitrogen content. The reduction in NOx emissions due

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to co-firing (23% relative to Coal A) is of the same magnitude as the reduction in coal flow

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(30%) in the co-firing cases. This indicates that the reduction in NOx is primarily an effect of

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reducing the incoming fuel nitrogen, and that there are no significant effects on NO formation

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owing to interactions between the coal and biomass. The NOx/fuel-N ratios are slightly higher

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for the biomass cases, which may reflect the fact that the added transport air required for

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biomass feeding (Table 3) augments the early oxidizing environment for nitrogen released

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with the volatiles.

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Since the NO maps in Figure 7 are strikingly similar, and the case without biomass has

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significantly higher NOx emissions (Figure 8), it is likely that a substantial share of NO is

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formed from char-N in the coal particles after the last measurement port (MH7). Assuming

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that the velocity of a coal particle is constant at 9 m/s (the speed of the secondary air), it

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would take about 240 ms for it to reach MH7, which may be insufficient for complete

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burnout.

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3.3 Parameter study

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combustion settings on the combustion process and NO formation.

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The parameter study represents an investigation of the influences of the different

3.3.1 Primary air flow

The influence of different primary air flows in the burner on NOx emissions was

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investigated by altering the swirl, axial, and transport air flows. It was found that the levels of

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NOx correlated more with the total amount of primary air (see Figure 9) than with specific

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swirl/flow settings. The results from the coal experiments are consistent with the finding of 12 ACS Paragon Plus Environment

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Vaccaro24 that the levels of NOx in rotary kilns decrease with decreasing primary air flow.

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However, the cases with biomass co-firing showed the opposite trend, albeit much more

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weakly than for the coals.

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In-flame measurements were performed for one case in which the primary air flow was low,

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and it is compared to the reference case with Coal A (same as Coal A in Figure 4), in which

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the primary air flow was high. These two data-points are marked in red in Figure 9, and the

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specific flame measurements are shown in Figure 10. The measured oxygen concentration

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right in front of the burner is lower and the CO concentration is higher when the primary air

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flow is low. Thus, the change in primary air flow has affected the flame and created a

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reducing zone at the flame root. This flame type should produce less NO, which is in line with

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the observation that NOx emissions are decreased from 1230 ppm to 930 ppm. We propose

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that a decrease in vol-N conversion to NO is the most important factor, since less oxygen is

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available during the release of volatiles. The temperatures are significantly higher when the

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primary air flow is low, although this does not appear to contribute to any significant level of

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thermal NO formation.

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3.3.2 Secondary air flow

One of the most pronounced differences between combustion in rotary kilns for iron ore

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processing and conventional PF combustion units (i.e., those used mainly for utility purposes)

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is the amount of oxygen that is present during the process. To correlate the increased presence

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of oxygen to NO formation, parts of the secondary air flow were replaced with a nitrogen

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flow during one test sequence. Figure 11 shows how the ߟே value relates to the total

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stoichiometric ratio of oxygen to fuel. The conversion of fuel-bound nitrogen to NO is

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decreased from around 0.67 at λ=4.2 to around 0.59 at λ=2.8. As expected, the decreased

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oxygen-to-fuel ratio leads to lower NOx emissions. The reason for this is that less oxygen is

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available for both vol-N and char-N oxidation. The decrease in NOx emissions is, however, 13 ACS Paragon Plus Environment

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319

limited at these levels of excess air, as decreasing the oxygen supply by roughly 30% reduces

320

NOx formation by about 10%. The effect of decreased λ is expected to be significant first

321

when it approaches unity. Such low levels of oxygen are not feasible in rotary kilns for iron

322

ore processing, since oxygen is required to oxidize the product, i.e., in addition to being

323

needed for fuel oxidation.

324 325

3.3.3 Secondary air temperature

326

and the air flow in the upper inlet is on average about 40°C warmer than the air flow in the

327

lower inlet, due to differences in heat loss. The influence of air temperature on NOx emissions

328

was examined by raising the temperatures of the two air flows by about 80°C when Coal A

329

was combusted with low amounts of primary air. The result is displayed in Figure 12. The

330

level of NOx was not affected to a significant degree. If anything, the levels of NOx emissions

331

decreased during the course of the temperature increase. This supports the earlier assessment

332

of the weak importance of thermal NO formation, since the thermal-NO mechanism is

333

sensitive to temperature changes once activated.

The secondary air enters the rotary kiln at inlets above and below the fuel inlet (Figure 3),

334 335

This work analyzes at the implications of fuel choice and burner settings for combustion

336

efficiency and NOx formation in PF-fired rotary kilns used for iron ore production. Four coals

337

and two biomass co-firing cases were assessed. In general, the NOx emissions were

338

substantially higher than those usually detected for pulverized fuel flames. The main reason

339

for this is a high conversion of fuel-N to NO, as the importance of thermal NO is estimated to

340

be limited or even negligible. We show that the primary air flows in the burner have a

341

significant effect on the flame and that low primary air flows lead to a flame with an oxygen-

342

lean zone in front of the burner, thereby decreasing the conversion of volatile nitrogen. The

343

lowest measured NOx emission for such a flame was 900 ppm (corrected to 6% O2), which is

4. Conclusions

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344

still exceptionally high. NO that originates from the char is anticipated to be the main

345

contributor to NOx in the studied burner configuration. When the amount of excess air is

346

decreased the NOx emissions are reduced, although this is not expected to be feasible in a full-

347

scale setup due to the importance of oxygen with respect to the product quality. Co-firing coal

348

with biomass reduces the NOx emissions, mainly as a result of the lower nitrogen content in

349

the biomass. Thus, in order to reduce NOx emissions using primary measures in iron ore

350

production rotary kilns, the choice of fuel is critical. An appropriate choice is a fuel with low

351

nitrogen content and high volatile content need to be assessed carefully. If the chosen fuel

352

combusts efficiently with low amounts of primary air, this will facilitate more stringent

353

control of both the thermal and fuel-related reactions routes.

354

Acknowledgment

355

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

356

Figures

357 358

Figure 1. The test kiln viewed from the side. MH0–MH12 indicate the measurement ports.

359

The distances are given in mm.

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360 361

Figure 2. Schematic of the burner orifice. The registers are denoted N1–N6. N4 is the swirl

362

air register and N1 is the axial air register. The other registers (white) are for fuel feeding.

363 364 365

Figure

3.

Cross-sectional

view

of

the

test

kiln

inlet.

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Figure 4. Contour maps created by interpolation between measurement points for each test coal. The y-axis in each figure reflects a distance of 0 mm to 650 mm and the black dots represent the measurement points. Blank regions represent sites where no measurements were performed 17

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Figure 5. Nitrogen contents and NOx concentrations for each of the four test coals.

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Figure 6. Ratio of outlet NOx to inlet nitrogen plotted against the percentage of volatile matter for each of the four fuels.

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Figure 7. Contour maps created by interpolation between measurement points for the three cases. The black dots represent measurement points.

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Figure 8. NOx emission levels and conversion ratios of outlet NOx to inlet nitrogen (ߟே ) for the coal- and biomass-firing cases.

Figure 9. NOx emissions for cases of combustion with different amounts of primary air. 21 ACS Paragon Plus Environment

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Figure 10. Contour maps created by interpolation between measurement points for the two cases. The black dots represent measurement points 23

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Figure 11. Nitrogen to NOx conversion as a function of global stoichiometry. Coal B was used as fuel and the primary air flow was 60 m3n/h.

Figure 12. Effects of secondary air temperature on concentrations of NOx in a rotary kiln. Coal A was used as fuel and the primary air flow was 60 m3n/h.

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Tables

Table 1. Operating conditions of the LKAB test kiln Fuel input

580 kW

Primary air flow

60–200 m3n/h

Primary air temperature

20°C

Secondary air flow

≈2250 m3n/h

Secondary air temperature

950°–1050°C

Total λ

4–5

Primary air λ

0.24–0.45

Table 2. Compositions and properties of the fuels used. Parameter Unit

Coal A

Coal B

Coal C

Coal D

Biomass A

Biomass B

LHV

MJ/kg, dry

29.7

31.2

28.5

26.7

20.3

19.2

H2 O

Mass-%

0.9

1.3

1.5

2

4.4

7.8

Ash

Mass-% dry

13.3

8.4

9.2

15.2

1.1

0.37

Volatiles

Mass-% dry

21.6

22.6

36.5

26.9

76.2

83.9

C

Mass-% dry

76.1

79.8

72.1

69.6

52.9

50.8

H

Mass-% dry

3.9

3.8

4.7

3.5

5.8

6.2

N

Mass-% dry

1.38

2.09

2.35

1.77

0.11

0.1

O

Mass-% dry

5.1

5.7

11.2

9.4

40

42.5

Diameter

µm (average)

43

44

47

46

210

443

LHV, Lower heating value.

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Table 3. Combustion settings for the different fuels used. The air flows are set values and not actual averages.

Fuel(s)

Register

Secondary Axial air T [°C] air flow [m3n/h]

Swirl air flow [m3n/h]

Transport Total λ gas flow (calculated [m3n/h] O2% out)

Coal A

N6

990

60

70

10 (N2)

4.4 (16.1%)

Coal B

N6

965

60

70

10 (N2)

4.5 (16.1%)

Coal C

N6

995

60

70

10 (N2)

4.5 (16.0%)

Coal D

N6

1000

60

70

10 (N2)

4.5 (16.0%)

Coal A

N3

1015

45

30

100 (Air)

4.5 (16.2%)

Coal A + Bio A

N3 + N2

1020

30

30

140 (Air)

4.6 (16.2%)

Coal A + Bio B

N3 + N2

1020

30

30

140 (Air)

4.5 (16.1%)

Table 4. Combustion settings used during the parameter study Investigated parameter

Test range

Primary air

Global stoichiometry

Secondary air temperature

Fuel used

Primary air

60-240 m3n/h

-

≈ 4.5

1050°C

Mixed

Global stoichiometry

2.8-4.2

60 m3n/h

-

1050°C

Coal B

Secondary air temperature

940-1050 °C

60 m3n/h

4.3

-

Coal A

1. Thurlby, J., A dynamic mathematical model of the complete grate/kiln iron–ore pellet induration process. Metallurgical Transactions B 1988, 19, (1), 103-112. 2. Pershing, D.; Wendt, J. In Pulverized coal combustion: The influence of flame temperature and coal composition on thermal and fuel NOx, Symposium (International) on Combustion, 1977; Elsevier: 1977; pp 389-399. 3. Glarborg, P., Fuel nitrogen conversion in solid fuel fired systems. Progress in Energy and Combustion Science 2002, 19, (2), 89-113. 4. Pohl, J. H.; Sarofim, A. F. In Devolatilization and oxidation of coal nitrogen, Symposium (International) on Combustion, 1977; Elsevier: 1977; pp 491-501.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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5. Jensen, L. S.; Jannerup, H. E.; Glarborg, P.; Jensen, A.; Dam-Johansen, K., Experimental investigation of no from pulverized char combustion. Proceedings of the Combustion Institute 2000, 28, (2), 2271-2278. 6. Coda, B.; Kluger, F.; Förtsch, D.; Spliethoff, H.; Hein, K.; Tognotti, L., Coal-nitrogen release and NOx evolution in air-staged combustion. Energy & fuels 1998, 12, (6), 1322-1327. 7. Spinti, J. P.; Pershing, D. W., The fate of char-N at pulverized coal conditions. Combustion and Flame 2003, 135, (3), 299-313. 8. Pershing, D.; Wendt, J., Relative contributions of volatile nitrogen and char nitrogen to NOx emissions from pulverized coal flames. Industrial & Engineering Chemistry Process Design and Development 1979, 18, (1), 60-67. 9. Song, Y. H.; Pohl, J. H.; Beér, J. M.; Sarofim, A. F., Nitric oxide formation during pulverized coal combustion. Combustion science and technology 1982, 28, (1-2), 31-40. 10. Thomas, K. M., The release of nitrogen oxides during char combustion. Fuel 1997, 76, (6), 457-473. 11. Nelson, P. F.; Nancarrow, P. C.; Bus, J.; Prokopiuk, A., Fractional conversion of char N to NO in an entrained flow reactor. Proceedings of the Combustion Institute 2002, 29, (2), 2267-2274. 12. van der Lans, R. P.; Glarborg, P.; Dam-Johansen, K., Influence of process parameters on nitrogen oxide formation in pulverized coal burners. Progress in Energy and Combustion Science 1997, 23, (4), 349-377. 13. Harding, N.; Smoot, L. D.; Hedman, P. O., Nitrogen pollutant formation in a pulverized coal combustor: effect of secondary stream swirl. AIChE Journal 1982, 28, (4), 573-580. 14. Wang, W.; Brown, S. D.; Thomas, K. M.; Crelling, J. C., Nitrogen release from a rank series of coals during temperature programmed combustion. Fuel 1994, 73, (3), 341-347. 15. Harding, A.; Brown, S.; Thomas, K., Release of NO from the combustion of coal chars. Combustion and Flame 1996, 107, (4), 336-350. 16. Wendt, J. O.; Sternling, C.; Matovich, M. In Reduction of sulfur trioxide and nitrogen oxides by secondary fuel injection, Symposium (International) on Combustion, 1973; Elsevier: 1973; pp 897-904. 17. Smoot, L. D.; Hill, S. C.; Xu, H., NOx control through reburning1. Progress in Energy and Combustion Science 1998, 24, (5), 385-408. 18. Kühnemuth, D.; Normann, F.; Andersson, K.; Johnsson, F.; Leckner, B., Reburning of Nitric Oxide in Oxy-Fuel Firing, The Influence of Combustion Conditions. Energy & Fuels 2011, 25, (2), 624-631. 19. Dean, A. M.; Bozzelli, J. W., Combustion chemistry of nitrogen. In Gas-phase combustion chemistry, Springer: 2000; pp 125-341. 20. Normann, F.; Andersson, K.; Leckner, B.; Johnsson, F., Emission control of nitrogen oxides in the oxy-fuel process. Progress in Energy and Combustion Science 2009, 35, (5), 385-397. 21. Davis, R. A., Nitric oxide formation in an iron oxide pellet rotary kiln furnace. Journal of the Air & Waste Management Association 1998, 48, (1), 44-51. 22. Zahl, R. K.; Haas, L. A.; Engesser, J. Formation of NOx in iron oxide pelletizing furnaces; University of Minnesota: 1995. 23. Edland, R.; Normann, F.; Andersson, K.; Fredriksson, C. In Formation of nitrogen oxides in rotary kiln burners: an assessment of pilot scale experiments using gaseous, liquid and solid fuels, INFUB 2015, 2015; 2015. 24. Vaccaro, M. H. In Low NOx rotary kiln burner technology: design principles & case study, Cement Industry Technical Confernece, 2002. IEEE-IAS/PCA 44th, 2002; IEEE: 2002; pp 265270. 25. Nielsen, P.; Jepsen, O. In An overview of the formation of SOx and NOx in various pyroprocessing systems, Cement Industry Technical Conference, 1990., XXXII, Record of Conference Papers., IEEE, 1990; IEEE: 1990; pp 255-276. 26. Nathan, G. J.; Manias, C. G., The role of process and flame interaction in reducing NOx emissions. In The Institute of Energy's Second International Conference on Combustion & Emissions Control, Energy, T. I. o., Ed. Pergamon: 1995; pp 309-318.

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27. Boateng, A. A., Rotary kilns: transport phenomena and transport processes. Butterworth-Heinemann: 2011. 28. Remus, R. Best Available Techniques (BAT) Reference Document for Iron and Steel Production: Industrial Emissions Directive 2010/75/EU Integrated Pollution Prevention and Control; 9279264753; Publications Office: 2013. 29. Mullinger, P. J. J., B. G. , NOx Reduction Techniques for Rotary Kilns American Flame Research Committee, Maui, Hawaii, October 16-20, 1994 1994. 30. Karstensen, K. H., Formation, release and control of dioxins in cement kilns. Chemosphere 2008, 70, (4), 543-560. 31. Reference Document on Best Available Techniques for Large Combustion Plants; JRC, The European Comission's science and knowledge service: 2006. 32. Mendiara, T.; Glarborg, P., Ammonia chemistry in oxy-fuel combustion of methane. Combustion and Flame 2009, 156, (10), 1937-1949.

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