Radiative Heat Transfer Conditions in a Rotary Kiln Test Furnace

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Radiative heat transfer conditions in a rotary kiln test furnace using coal, biomass and co-firing burners Adrian Gunnarsson, Daniel Bäckström, Robert Johansson, Christian Fredriksson, and Klas Andersson Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 12 Jun 2017 Downloaded from http://pubs.acs.org on June 13, 2017

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Radiative heat transfer conditions in a rotary kiln test

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furnace using coal, biomass and co-firing burners

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Adrian Gunnarssona*, Daniel Bäckströma, Robert Johanssona,

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Christian Fredrikssonb, Klas Anderssona

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a

Department of Energy and Environment, Chalmers University of Technology, SE-41296

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Göteborg, Sweden b

Luossavaara Kiirunavaara Aktiebolag (LKAB), Box 952, SE-971 28, Luleå, Sweden

*Corresponding author. Tel.: +46 31 772 1442 e-mail address: [email protected]

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Abstract

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This work studies the radiative heat transfer in a 580 kWth pilot scale test furnace that resembles a

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full-scale rotary kiln for iron ore pellet production. The aim is to quantify the radiative heat transfer

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in coal and co-firing flames, but also to study the possibility to model the radiative heat transfer

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for such combustion conditions. Three combustion cases of coal and co-firing are studied and an

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evaluation is made using a detailed radiation model. The test furnace is cylindrical and refractory

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lined but does not rotate and no iron ore pellet bed material is included. In-flame measurements of

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temperature, gas composition, particle concentration, radiative intensity and radiative heat flux are

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conducted for the different fuels and fuel combinations. Overall, the differences in measured

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radiative intensities and heat fluxes between the three studied fuel cases are minor, which implies

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that introduction of renewable fuels by co-firing in a full-scale rotary kiln should be feasible with

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respect to heat transfer conditions. In the model, the furnace is treated as an axisymmetric and

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infinitely long cylinder and gas properties are calculated with a Statistical Narrow-Band model

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while particle properties are calculated using Mie theory. The modeling results show reasonable

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to good predictivity compared to the measured intensity data. This implies a good quality of the

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collected experimental data, but also indicates the potential use of the model in full-scale rotary

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kiln calculations in future work.

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Keywords: radiative heat transfer, co-firing, rotary kiln, radiation measurements, particle radiation,

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gas radiation

1. Introduction

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Iron ore is a widely used resource with an estimated global production of 3320 million tonnes in

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2015 according to the 2016 U.S. geological survey on mineral commodity summaries1. The

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primary user of the iron ore is the steel industry and the large volume produced by the mining

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companies has to be delivered in a suitable form for easy handling and often long transportations.

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After crushing and refinement of the mined ore, it is common to deliver the iron ore in a pelletized

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form. A common process to produce such pellets is the grate kiln process: raw iron ore pellets are

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fed onto a travelling grate where they are partially dried and oxidized with hot air and flue gases.

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Then, from the grate, the iron ore pellets enter a rotary kiln where they are further heated, oxidized

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and sintered. The heated pellets finally leave the rotary kiln as they fall down onto a cooler, see

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Figure 1. A portion of the air that is used to cool the pellets is introduced as heated secondary air

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to the rotary kiln. Compared to a more conventional combustion process, the secondary air flow is


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considerably larger to allow oxidation of the iron ore pellets in the kiln, resulting in an oxygen

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concentration in the stack of about 16%. For a more detailed description of a typical grate-kiln

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process, see e.g. Jonsson et al.2.

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Figure 1. Simplified schematic of the grate-kiln process showing the grate, rotary kiln and cooler

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sections. The burner is located in a center position and is designed to create a jet-type flame. Gas

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flows are indicated with dashed arrows.

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During the recent COP 21 conference meeting in Paris 2015, a global target was set to keep the

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increase of the mean temperature on earth below 2°C. In order to meet this target, the emission of

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greenhouse gases from the stationary industrial sector need to be drastically reduced. A problem

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connected to the production of iron ore pellets is the substantial emission of carbon dioxide that it

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generates since heat is supplied to the process by a flame that is today commonly fueled using

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fossil coal3. The use of coal in the iron ore pellets industry is mainly due to the availability,

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composition, cost and high heating values. One way to meet the reduction target in the iron ore

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pellet industry is to switch to less carbon intensive fuels. However, this may affect the heat transfer

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conditions in the rotary kiln, thereby impacting the quality of the product.

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This work focuses on the radiative heat transfer within the rotary kiln, used for iron ore pellets

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production, and how it might be affected by a fuel switch and by co-firing. A change of fuel may

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have large effects on the heat transfer within the kiln and it is important to consider possible effects

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on the product quality. The heat transfer in the freeboard of the rotary kiln is dominated by


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radiation that makes up for about 90% of the total heat transfer3, primarily due to the high

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temperature of the flame. Thus, the radiative heat transfer in the rotary kiln has a large impact on

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the product quality of the iron ore pellets. A number of different elements need to be considered

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in the rotary kiln: heat is exchanged between the kiln wall and pellet bed as well as the present

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gases and particles in the furnace that emits, absorbs and scatter radiation. Since a solid fuel is

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used, the radiation emitted from the flame is anticipated to be dominated by hot particles being

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present, mainly char but also some soot and ash particles. However, there is today little quantitative

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knowledge of the heat transfer conditions in rotary kilns using any fuel, mainly due to the rotation

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of the kiln, a fact that clearly complicates in-flame measurements.

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The research on radiative heat transfer in combustion systems has been intense since the 1960’s

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until today with large contributions by combustion scientists such as Hottel & Sarofim4, Siegel &

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Howell5, Özisik6 and Modest7. From Modest7 we have used theory to build a model to study flame

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radiation including both gases and particles. Our latest work investigates the radiative heat transfer

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in coal and gas flames in cylindrical furnaces, using different measurement methodologies and

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modeling work for different applications including oxy-fuel flames and rotary kilns8–11. Some

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earlier modeling work of the heat transfer in a rotary kiln can also be found in the literature12–14.

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These models have focused on the radiative and convective heat transfer of the gases in the

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freeboard and the surfaces of the kiln, but neglecting the particles present in the freeboard

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consisting of unburned fuel, soot and ash. However, flame radiation in cylindrical furnaces has

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been investigated in works related to coal combustion15–17. These studies have investigated the

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importance of different parameters for pulverized coal-fired furnaces such as scattering

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efficiencies for different particle types, particle concentrations and size distributions15 as well as

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the temperature distribution, extinction coefficients and single scattering albedo16. Measurements


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and modeling of the radiative heat flux in a pulverized coal-fired furnace were performed while

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also evaluating the use of separate temperatures for gases and particles17. In these studies it was

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found that the coal and char particles were the dominant contributors to the heat flux at the wall

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and the importance of scattering was stressed15. However, the impact of soot particles on the

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radiative heat flux to the wall was also stressed as well as the importance of using separate

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temperatures for gases and particles17. It was further concluded that an accurate knowledge of the

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temperature and particle concentration distributions were more critical when predicting the

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radiative heat transfer in a coal fueled furnace than detailed information of gas concentration or

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index of refraction16.

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In the present work, measurements and modeling of radiation in a 580 kWth cylindrical test

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furnace is conducted. The test furnace is built to resemble a full-scale rotary kiln and the burners

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used are down- scaled models of those employed in the full-scale process. The specific aim of this

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work is to quantify the radiative heat transfer in coal and co-firing flames, with a specific focus on

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the influence from the fuel on flame radiation. The main interest is to obtain a better understanding

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of the possibilities to apply biomass through co-firing in the great-kiln process. In addition, effort

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is made to examine how well the radiative heat transfer can be modeled in this kind of combustion

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process by using a detailed axisymmetric discrete transfer model. This model will later be used to

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get a better understanding of the heat transfer conditions in full scale rotary kilns. In a previous

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experimental campaign11, significant differences were observed with respect to the emitted flame

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radiation when different fuels were burned in a test furnace, but the cause of those differences

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proved difficult to assess due to lack of flame data. This was mainly due to problems in achieving

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accurate flame temperatures since the suction pyrometer used was clogged with particles and a

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high suction velocity could therefore not be achieved. Also, particle data was missing in positions


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close to the burner. However, it was possible to conclude that the heat transfer from the flame was

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clearly dominated by particle radiation for both coal and co-firing. In this work, a greater focus

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has been on co-firing flames to better examine differences in temperature conditions and flame

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structure when compared to a coal flame. Temperature measurements were improved since extra

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care was taken to reach and sustain a sufficient suction velocity using a new suction pyrometer

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and ejector system. Particles were sampled in the flame, close to the burner and radiative heat flux

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was measured. The radiative intensity was also measured and, by using all the gathered data, the

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modeling was conducted with a higher precision. Also, the test matrix is increased with respect to

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the number of fuels and fuel combinations tested as well as introducing new measurement

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techniques such as an FTIR for gas analysis and an infrared camera for temperature estimations.

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To further improve the quality of the experiments, the test furnace was modified for better process

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control and a new burner was installed to achieve more stable conditions with respect to biomass

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feeding. The conducted measurement campaign included one reference coal and three additional

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coals as well as co-firing of the reference coal and two different biomasses. In this paper, we have

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selected three different fuels and fuel combinations for evaluation.

2. Experiments

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A measurement campaign was conducted over two weeks during which different fuels and fuel

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combinations were tested. The experiments were run in a cylindrical, refractory lined test furnace,

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constructed as a down-scaled version of a full-scale rotary kiln used in a grate-kiln process, see

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Figure 2. Throughout the campaign different measurements were performed to collect and

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compare data considering the radiative heat transfer within the furnace between different fuels.

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Compared to the previous campaign performed in 2013 significant changes regarding the furnace

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and operating conditions were carried out. The same type of furnace was used in both campaigns,


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but a new facility was built with modifications to better represent the full scale rotary kiln. The

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inner diameter in the first section (port MH1 to MH12) of the furnace was reduced from 0.8 m to

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0.65 m, while keeping the second section unchanged. The furnace was tilted to achieve a slope of

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3° and a cooling system was installed at the bottom of the first section of the furnace to achieve a

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heat sink resembling the pellet bed. Six additional measurement ports were installed along the axis

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of the furnace and all ports were numbered, with MH0 corresponding to the position closest to the

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burner. All ports allow horizontal in-flame measurements using probes. The burner has six

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registers where two of these were used for primary air, the first (axial air) and fourth (swirled air

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with an angle of 30°) counted from the rim, and the other four were available for fuel. Secondary

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air was introduced to the furnace in two large registers, one above and one below the burner, see

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the works of Bäckström et al.11 and Edland et al.18 for descriptive figures of the burner orifice and

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secondary air registers. In both campaigns the burner axis was the same as the furnace axis, but

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the axial position of the burner was changed and the fuel feed was increased from 400 kWth to 580

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kWth. In contrast to the full scale rotary kiln, the test furnace did not rotate as this would have

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complicated the construction and operation of the test furnace, but, primarily since this would

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prevent any in-flame measurements using probes. However, with the burner located at the center

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of the furnace diameter and by aiming for an axisymmetric flame, the rotation of the kiln itself is

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thought to have a minor effect on the radiative heat transfer in the furnace.


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Figure 2. The pilot scale test furnace used in the experimental campaign. Measurement ports are displayed in the figure including the distance [mm] to the burner. 147

The furnace was operated continuously during the campaign and operating conditions were

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changed during night time so that the system had reached stable conditions in the morning of each

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day. Seven different set-ups of fuels and fuel combinations were tested in total. This work is

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focused on three of these cases: a reference coal and co-firing of the reference coal with two

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different biomasses using a composition of 70% reference coal and 30% biomass. The percentage

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numbers are based on the lower heating values. The reference coal (RC) is the coal normally used

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on a daily basis in the full scale rotary kiln. It is a carbon rich coal which was milled to a volume

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weighted mean diameter of 54 µm, similar to what is used in the full scale process. The two

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biomasses are here referred to as biomass A (BA) and biomass B (BB). Biomass A was based on

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wood treated with steam explosion while biomass B was untreated but grinded wood and delivered

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in a pelletized form. The pellets were grinded before introduced to the burner with volume

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weighted mean diameters of 210 µm and 443 µm for biomass A and B, respectively. A proximate

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analysis of each fuel is presented in Table 1. Coal and biomass were fed in two different annular

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registers in the burner, namely registers two and three from the rim; the biomass was fed in the

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outer register. The total flow of primary air and fuel transport air to the burner was about 200


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Nm3/h at a temperature of about 20°C. The secondary air flow was about 2300 Nm3/h and was

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preheated to about 1050°C. For more precise air flows see Edland et al.18.

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Table 1. Proximate fuel analysis and lower heating values for the fuels studied

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RC

BA

BB

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Moisture

(as received) 0.9%

4.4%

7.8%

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Volatiles

(dry basis)

21.6%

76.2%

83.9%

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Fixed carbon

(dry basis)

65.1%

22.7%

15.7%

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Ash

(dry basis)

13.3%

1.1%

0.37%

Lower heating value

(as received) 29.4 MJ/kg

19.3 MJ/kg

17.5 MJ/kg

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For the three cases the thermal input was kept constant at 580 kWth. The mass flow of fuel and

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the carbon dioxide emissions are shown in Table 2 for the three cases, calculated on the

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composition as the fuels were received. It should be noted that, although the total amount of

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emitted carbon dioxide was slightly increased from the furnace in the co-firing cases, the portion

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originating from fossil coal was reduced by 30%.

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Table 2. Mass flow of fuel and carbon dioxide emissions for the different cases Case

Fuel [g/s]

CO2 [mole/kg]

CO2 [g/MJ]

RC

19.7

62.8

93.9

RC + BA

22.8

54.7

94.6

RC + BB

23.7

52.9

95.2

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3. Measurements

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During the experimental campaign gas and wall temperatures, gas composition, particle

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concentration, particle size distribution, radiative intensity and radiative heat flux were measured.


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Most of these in-flame measurements were performed using probes to traverse the flame. All

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probes described below were constructed in either stainless steel or titan and were water cooled.

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For gas temperature measurements a triple shielded suction pyrometer of IFRF type with a Type

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B thermocouple was used. The probe was traversed along the furnace diameter and the temperature

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was measured at 7 positions distributed along the furnace diameter for ports MH0, MH1, MH3

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and MH7, see Figure 2. The shields at the tip of the probe were constructed in ceramics (not cooled)

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with a length of about 15 cm. To perform temperature measurements by means of using a suction

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pyrometer is challenging in environments with high particle concentrations and high temperatures

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since this might cause problems due to melting of ashes with subsequent blocking of the probe.

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Severe such problems were encountered at some occasions during the campaign and the ceramic

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tip was therefore regularly checked to see if it had been clogged and, if so, it was exchanged. The

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opening in the ceramic tip was directed perpendicular to the probe and to decrease the particle load

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in the suction line, it was directed towards the stack. Keeping it directed towards the burner caused

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instant clogging and thus destruction of the ceramics. Further, a too low suction velocity or large

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local temperature gradients may result in under- as well as over-predictions of the temperatures

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due to radiative heat exchange with the surroundings and gas extraction from a too large

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measurement volume.

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For measurements of the gas composition, a gas extraction probe connected to a FTIR gas

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analyzer was traversed through the flame in the same manner as the suction pyrometer and the

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measurements were performed in the same positions as the temperature measurements.

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Particle concentration and size distribution were measured by extracting samples in the flame.

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This was done by using a probe connected to a low pressure impactor with thirteen size steps

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ranging from 30 nm to 10 µm. Particles of a size larger than 10 µm were captured in a cyclone


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placed prior to the impactor. The opening of the particle sampling probe was directed towards the

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burner, perpendicular to the port axis. The sample gas was diluted with nitrogen in the tip of the

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probe, with a flow of about 10 times the sampling flow. The overall dilution rate was then

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estimated by comparing the oxygen concentration in the diluted sampling gas with the oxygen

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concentration in the measurement position. These particle measurements are practically

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challenging, and with a limited number of low pressure impactor set-ups also time consuming.

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This made it necessary to restrict the particle sampling to the center position in ports MH3 and

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MH7, with one repetition at each position and fuel. To better resolve the particle size distribution,

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particles collected in the cyclone were further analyzed after the campaign with laser diffraction.

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In the analysis the particle diameter was estimated for 100 size steps between 0.010 and 10000

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µm. During the particle extraction in the co-firing cases, condensation of tars caused blocking of

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the probe suction line and data for these cases are therefore lacking. Due to these problems, the

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particle data had to be discarded for the co-firing cases and only measurements on the reference

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coal could be used.

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Radiative intensity was measured using a narrow angle radiometer, see Figure 3. This instrument

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measures in the line-of-sight of the probe, extinguishing incoming light that is not in the axis of

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the probe, and has been used in several of our previous studies10,11,19,20. Rays of light reaches a

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thermopile in the detector house of the probe (a). The detector was calibrated before and after the

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measurement campaign using a black body oven. The direction of the probe was aimed towards a

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quartz glass on the opposite wall of the furnace acting as a cold background to remove background

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radiation from the hot wall. This allows us to isolate the flame radiation. However, due to

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scattering, some radiation emitted from the wall will still reach the detector. The radiative intensity

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was measured in 15 positions distributed along the furnace diameter for ports MH1, MH3 and


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MH7. The radiative heat flux was measured at the inner wall of the furnace, at the port entrance,

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using an ellipsoidal radiometer with a viewing field of 2π sr placed in the tip of a probe for ports

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MH0, MH1, MH3 and MH7.

Figure 3. Schematic of the narrow angle radiometer which was used to measure the total radiative intensity. The probe includes the following components: a thermopile, b focusing lens, c shutter, d PT-100, e water cooled sensor housing, f collimating tube and g water cooled probe. 230

The inner wall temperatures were studied using an infrared camera. The camera used was a FLIR

231

A655SC with the possibility to measure temperatures up to 2000°C at a sampling frequency of

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200 Hz and for 640x120 pixels. The camera was placed outside the ports, looking into the furnace,

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and videos were recorded for ports MH0-MH10. By analyzing the recorded videos in the software

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FLIR Research IR Max, the wall temperatures could be estimated using temperature field

235

resolution. Boundaries for such temperature fields were set from observing the presence and

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absence of particles; streaks of particles could be observed when passing the measurement ports.

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The average temperature within this field was then estimated using a set emissivity for the wall of

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0.80 and a set reflected temperature to match the wall temperature. Flame temperatures could

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possibly also be estimated from the infrared camera but are not considered in this work.


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Energy & Fuels

4. Modeling

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The radiation model used in this work is an axisymmetric discrete transfer model for cylindrical,

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infinitely long furnaces considering the contribution from both gases and particles. Gas properties

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are modeled on a spectral basis using the Malkmus Statistical Narrow-Band model for gases21,

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with parameters from the work of Rivière and Soufiani22, and particle properties using Mie

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theory23. Complex indices of refraction are required using Mie theory, and data presented by Foster

246

and Howarth24 was used in this work, and a representative particle diameter was calculated from

247

the size distribution of the fuel. Scattering of radiation due to the presence of particles was

248

considered to be isotropic, but this assumption is also tested in a sensitivity analysis. The

249

assumption of the infinitely long furnace leads to neglected gradients along the axis of the furnace,

250

parameters such as temperature or gas concentration. However, considering the radial and axial

251

dimensions of the furnace, axial gradients are estimated to have a limited effect on a radial cross

252

section of the furnace. Required inputs to the model are mainly radial profiles of temperature, gas

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composition and projected surface area of particles. The output from the model of main interest in

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this work was the radiative intensity from the gases and particles separately, as well as a total

255

intensity, which corresponds to the intensity measured with the narrow angle radiometer. The

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model is described in more detail in the work of Johansson et al.25.

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Measurements from the suction pyrometer were used for the temperature profile. The

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surrounding wall temperature was estimated based on data from the infrared camera, but a cold

259

background was used at the position directly opposite of the ports. Gas profiles were obtained from

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the FTIR data for the gases considered in the model i.e. water vapor and carbon dioxide. Particle

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measurements were in this work performed in the center position of the flame where the largest

262

portion of the sampled particles was unburnt fuel. Ash particles were also present, but to a much

263

lower degree and are thus not considered in the model. Any particles measured with the low-


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pressure impactor with a size smaller than 0.2 µm were assumed to be soot and emit and scatter

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radiation according to Rayleigh theory10. All particles larger than 0.2 µm were considered to be

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unburnt fuel particles and from here, the term “particles” refers to these unburnt fuel particles. The

267

surface area of the particles within the furnace is required to calculate the absorption coefficient

268

of the particles and therefore a projected particle area per surrounding furnace volume was

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calculated23, based on the particle size distributions for the different cases and ports. For these

270

calculations some assumptions had to be made. Since no particles were extracted from port MH1,

271

and due to the short distance from the burner, the fuel was assumed to be oxidized to some degree.

272

For most of the results shown in this paper it was set to be 20% based on a good fit to the measured

273

radiative intensity. That is, the total projected surface area of the particles at port MH1 accounted

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for 80% of the total projected surface area of the fuel inserted through the burner, as calculated

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from the fuel analysis. To test this assumption, the degree of oxidation was varied in a sensitivity

276

analysis and the impact on the modeled radiative intensity was studied. Based on the extractive

277

particle measurements in ports MH3 and MH7 the total projected surface area of the particles was

278

estimated. Since particle data in the co-firing cases was not available, the projected surface area

279

was assumed similar for all three cases. This was based on the fact that the co-firing cases still

280

contained 70% reference coal. Since the coal particles are smaller, the largest portion of the total

281

projected surface area will arise from these particles. It was assumed that the total projected surface

282

area was decreased at the same rate for all three cases along the axis of the furnace. That is, a total

283

projected surface area was calculated separately for each fuel at port MH1, based on fuel analyses,

284

but the relative decrease between downstream ports is assumed to be the same for the three cases

285

in this study. This assumption leads to higher uncertainties in the co-firing cases compared to the

286

reference coal case.


14

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287

As the radiative emission of the particles is strongly dependent on the flame temperature it is

288

important to consider how the particles may be distributed over a cross section of the furnace. The

289

fuel stream was introduced at the burner orifice in the center of the furnace and particles are then

290

spread over the furnace diameter. How they are spread is indicated by the radiative intensity

291

measurements. In port MH1, relatively close to the burner, the particle stream was assumed to be

292

widened in the form of a cone growing from the burner, with a constant particle concentration in

293

the whole cross section of the particle cone. This distribution may be described using a step

294

function of the projected surface area. That is, the projected surface area was set to a constant value

295

for a certain radial distance between the furnace centerline and the furnace wall, after which it is

296

set to zero; this distance is referred to as the flame radius. Downstream, in port MH3, particles

297

were assumed to be spread over a larger radius but also with concentration gradients over the cross

298

section of the furnace. This distribution was assumed to follow a cosine profile with a peak value

299

at the center of the furnace, and, set to zero at Rf, i.e. the radius from the center within which

300

particles are present. The projected particle surface area per furnace volume at a distance r from

301

the furnace center is, in port MH3, given by: 0 ≤ 𝑟 ≤ 𝑅𝑓 𝑅𝑓 < 𝑟 ≤ 𝑅

𝐴𝑝 (𝑟) = 𝑓𝑝 ∙

𝑅2 𝜋∙𝑟 2 ∙ 1.681 (cos ( 𝑅 ) + 1) 𝑅𝑓 𝑓

(1)

𝐴𝑝 (𝑟) = 0

302

where fp is the projected surface area per reactor volume when the particles are distributed equally

303

over a disk with radius R, as in port MH1. The total projected surface area from the particles is

304

kept constant in the cross section of the furnace using this profile when assuming a cosine profile

305

instead of a constant value. The peak value assumption of the projected surface area, located at the

306

center position of the furnace, was assessed in a sensitivity analysis. The peak value was relocated


15

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Page 16 of 34

307

to appear at some distance from the center axis, to study the effect on the modeled radiative

308

intensity.

309

Since the projected surface area of the particles was calculated from the size distribution of the

310

fuel at port MH1, the total projected particle surface area was fixed and a smaller flame radius

311

resulted in a higher peak value, see Figure 4 a. At ports downstream of MH1 the projected surface

312

area was instead calculated from the extractive particle measurements using the collected masses

313

in the low pressure impactor and cyclone. Since particles were collected at the center line of the

314

furnace, the peak value of the projected surface area was a fixed value and a particle distribution

315

was calculated for a smaller flame radius resulted in a smaller total projected particle surface area,

316

as in Figure 4 b for port MH3. In port MH7 particles were assumed to have developed a more even

317

distribution, but throughout the whole furnace diameter and a plug flow of particles was assumed

318

in the model. The plug flow concentration of particles in port MH7 can be seen in Figure 4 c.

319

Figure 4. Distributions of the projected surface area per volume within the furnace for the three

320

ports. The flame radius (Rf) is varied for different appropriate values considering the studied cases.

321

a) Port MH1, assuming a fixed total projected surface area, b) and c) using a fixed peak value as

322

measured with a low-pressure impactor and cyclone but different types of distribution profiles for

323

ports MH3 and MH7 respectively.


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Energy & Fuels

5. Results and Discussion

324 325

Interpolated temperature maps for the axial cross section at the center of the furnace, seen from

326

above, are shown in Figure 5 for the three studied cases. Temperature measurements were

327

performed at four axial positions, i.e. ports MH0, MH1, MH3 and MH7, indicated with black

328

dashed lines in the figure with the burner to the left and port MH7 to the right. To achieve the

329

temperature maps, the temperature was first extrapolated to reach all the way to the opposite wall

330

of the port entrance in the furnace, i.e. at 0.65 on the diameter axis, using the two temperature

331

measurements performed closest to the wall. Between the measurement ports, temperatures were

332

calculated at axial positions corresponding to all ports between MH1 and MH7, i.e. with an equal

333

axial distances of 250 mm. These values were linearly interpolated using measured temperatures

334

at the same radial positions for axially upstream and downstream ports of the evaluated position.

335

The gas temperature of the secondary air entering at the axial position of the burner was set to

336

1045°C, since the air was preheated to that temperature. At the center position, 0 m from the

337

burner, cold fuel enters the furnace with primary air not being preheated, and the temperature at

338

this position was set to 20°C. It can be seen that the flames were tilted slightly towards the furnace

339

wall located opposite of the port entrances, i.e. the flames were not perfectly centered in the

340

furnace. It should be noted that some temperature measurements in the center, and/or next to the

341

center position, in port MH0 and MH1 appears to be unexpectedly low. This was due to high

342

concentrations of particles clogging the suction pyrometer. However, the measured radial

343

temperature profiles were used as input to the radiation model assuming the flame to be

344

axisymmetric using an average value of temperatures, measured at equal distances on the right and

345

left side of the furnace center, according to:


17

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Page 18 of 34

1

4 4 4 𝑇𝑅𝑖𝑔ℎ𝑡 + 𝑇𝐿𝑒𝑓𝑡 ̅ 𝑇=( ) 2

(2)

346

The measured gas compositions were also not radially symmetric, and gas composition profiles

347

were introduced to the model using a simple averaging for positions on equal distances from the

348

center.

349

Figure 5. Linearly interpolated temperature maps [°C] for the three cases studied using

350

measurements from the suction pyrometer. Each contour line corresponds to a temperature step of


18

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351

50°C. Top - reference coal, middle - co-firing with biomass A and bottom - co-firing with biomass

352

B. Black dashed lines indicates port axis where measurements where performed with port MH7

353

located at the right end of the figures.

354

Throughout the campaign the infrared camera was used to estimate the inner wall temperature

355

of the furnace, but also to study the flame and presence of particles along the axis of the furnace.

356

For the three cases studied in this work a narrow and dense stream of particles was observed close

357

to the burner at MH0 where the particles had yet not ignited. The particle stream was broadened

358

downstream of port MH0, but still consisted of “cold” particles, which had not yet fully ignited in

359

MH1. However, it should be noted that single particles could not be studied, while fields of

360

particles could be observed. Downstream of MH1 the particle concentration appeared to be

361

reduced for each port farther from the burner, see Figure 6 a – d. a)

b)

c)

d)

362

Figure 6. Representative snapshots for the reference coal case showing the interior of the furnace

363

with hot gases and particles from recorded videos using the infrared camera. The suspension flow

364

moves from the left to the right in the figures. Particles can be observed as more "dense fields"

365

passing the ports. a) Port MH0 with a high density of particles, b) port MH1 with considerable


19

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Page 20 of 34

366

amounts of particles, c) port MH3 and d) port MH7 with fewer particles. The ports on the opposite

367

side of the furnace acting as cold backgrounds can be seen as the dark circles located close to the

368

center of figures b)-d).

369

After analyzing the extracted particles, the total projected surface area from the particles were

370

estimated and could be compared to the total projected surface area as calculated from the fuel

371

analyses. This was done under the assumptions that particles are spread according to a cosine

372

profile in port MH3 but equally spread along the furnace diameter in port MH7, and all three fuel

373

cases to be combusted at the same rate. The total projected surface area was estimated to be reduced

374

to about 50% and 45% of the total projected surface area introduced to the burner for ports MH3

375

and MH7 respectively. This reduced area is due to fewer particles being present for each

376

downstream port. It should be noted that the particle amount close to the wall at port MH7 is

377

probably overestimated due to the plug flow assumption, but this has a minor impact on the

378

radiation analysis.

379

Particles collected in the cyclone were studied for the different cases in a microscope and it could

380

be observed that the collected biomass particles were considerably larger than the coal particles,

381

and, that the collected biomass particles had not yet ignited, Figure 7. Figure 7 a and b shows

382

particles extracted during co-firing with biomass A in ports MH3 and MH7 respectively; the red-

383

brown particles being biomass A. Particles collected from the co-firing case with biomass B can

384

be seen in Figure 7 c and d in ports MH3 and MH7 respectively. Biomass B particles were much

385

larger than the coal particles. Although it could be observed that the particles were not spherical,

386

especially the biomass particles, all particles were still assumed to be spherical for all three cases

387

and ports studied in the model. This assumption is for example justified by the work of Gronarz et

388

al.26, which shows that the particle shape has a negligible influence on the scattering and absorption


20

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Energy & Fuels

389

coefficients. Further, the large biomass particles contained a lot of mass but a relatively small

390

emitting surface area. a)

b)

c)

d)

391

Figure 7. Particles extracted from the flame and collected in the cyclone studied in a light

392

microscope. a) & b) co-firing with biomass A at ports MH3 & MH7 respectively, c) & d) co-firing

393

with biomass B at ports MH3 and MH7 respectively.

394

Figure 8 shows the measured and modeled radiative intensity along the furnace diameter for the

395

three cases in ports MH1, MH3 and MH7. The narrow angle radiometer was traversed along the

396

diameter with incremental steps of 5 cm with an extra position at the center line. Measurements

397

are indicated with triangles in the figure. The inner wall of the furnace, at which the probe entered

398

the furnace, is positioned at the 0 m position. At the port entrance, high intensities were measured

399

as the line-of-sight of the detector was the entire furnace diameter. Measured intensities appearing

400

to show on a flat profile, close to the port entrances for ports MH1 and MH3, is mainly an effect

401

of the absence of particles close to the furnace wall. As the probe was traversed closer to the center

402

the radiative intensity decreased as the amount of particles in the line-of-sight between the probe


21

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Page 22 of 34

403

and the cold background decreased. At the opposite wall, located at the distance 0.65 m, the

404

intensity was close to zero due to the quartz window that was used as a cold background. The

405

measured intensities increased moving from port MH1 to MH3 and decreased again moving to

406

port MH7. This was an effect of cold fuel particles that had yet not been ignited at port MH1 while

407

particles had been ignited to a larger extent at port MH3, which resulted in higher flame

408

temperatures. In port MH7 the particle concentration was lower due to the progression of the

409

combustion and the contribution from particles to the radiative emission decreased. The gradient

410

of the radiative intensity plots decrease along the furnace axis as the particles become more evenly

411

distributed, i.e. when moving from port MH1 to MH3 and MH7. The size of the flame radius is

412

indicated when observing the distance between the furnace center and to where the measurement

413

profile flattens out. Based on this, the flame radius (where particles exist) was estimated to 0.15 m

414

in MH1 and 0.325 in MH3. In port MH7 a plug flow distribution of particles was assumed.

415

The narrow angle radiometer was calibrated using a black body oven before and after the

416

campaign and an averaged deviation of the measured radiative intensity from the two calibrations

417

was found to be 1.8 kW/m2sr in the signal interval 0.2-75 kW/m2sr. In the modeling of the radiative

418

heat transfer an extra distance of 0.285 m was added representing the measurement port on the

419

opposite side of the furnace. No particles were assumed to be present in the measurement ports

420

and the concentrations of water vapor and carbon dioxide were low, which results in an

421

insignificant contribution to the radiative intensity in the measurement ports. When comparing the

422

modeled contributions from gases and particles to the total radiative intensity, it becomes clear

423

that the particle radiation (dashed line) dominates and the portion arising from the gases (dash-

424

dotted line) was small for all cases and ports. In total, the agreement between measured and


22

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Energy & Fuels

425

modeled radiative intensity was satisfactory for the different ports and cases. This implies that the

426

measurements of temperature and particle concentration were accurate in this campaign.

427

The radiative intensities for the case with the reference coal are shown in Figure 8 a – c. Overall,

428

the modeled data agrees well with the measurements, in particular with respect to ports MH3 and

429

MH7. Figure 8 d – f shows satisfactory agreement between modeled and measured intensities, for

430

the co-firing with biomass A, in ports MH1 and MH7, while the temperature or particle

431

concentration for port MH3 appeared to be underestimated. Comparing the reference coal and the

432

biomass A co-firing cases port by port, the radiative intensities were in the same order of

433

magnitude for all three ports, which indicates that the combustion process and the resulting heat

434

transfer conditions are similar for these fuel mixes. The modeled radiative intensity, with the

435

reference coal co-fired with biomass B are shown in Figure 8 g – i. For ports MH1 and MH7, the

436

model over-predicts the radiative intensity. The likely explanation is the projected surface area of

437

the particles being different than assumed. Smaller particles, with a high projected surface area per

438

mass, are likely combusted in a shorter time, while the large particles of biomass B are still

439

unburned, resulting in a lower total projected surface area. From a comparison of the reference

440

coal and the co-firing case with biomass B, the radiative intensities are lower for the co-firing case

441

in all three ports. However, the axial trends are similar, which indicates that the combustion

442

processes is similar to the reference coal case.


23

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Page 24 of 34

443

Figure 8. Modeled total radiative intensity as well as the contributions from gas and particles

444

represented with lines. Measured intensities are represented with triangles. a) – c) the reference

445

coal, d) – f) co-firing with biomass A and g) – i) co-firing with biomass B for ports MH1, MH3

446

and MH7 respectively.

447

During the campaign the radiative heat flux to the inner wall of the furnace was measured for

448

ports MH0, MH1, MH3 and MH7 for the three studied cases, Figure 9. As the heating of the iron


24

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Energy & Fuels

449

ore pellets is largely influencing the product quality, the radiative heat flux directed to the wall of

450

the furnace is an important parameter. Figure 9 shows only small differences in the measured

451

radiative heat flux between the different fuel combinations at the different ports. In addition, it

452

should be noted that a large portion of the measured heat flux stems from the hot inner walls of

453

the furnace and the increased measured values are in line with the increased wall temperature as

454

estimated from the infrared camera. Although it was shown in Figure 8 that the radiative intensity

455

from the flame is due to particles, the dominating portion of the flux is emitted by the hot

456

surrounding wall.

457

Figure 9. Measured radiative heat flux to the inner wall of the furnace for the three studied cases

458

at ports MH0, MH1, MH3 and MH7.

459

As discussed in the modeling section, the projected surface area from the particles was not

460

known in port MH1. This parameter was tested in the model for the reference coal case by varying

461

the fuel portion that is not combusted at this location, see Figure 10. It appears as the particle load

462

has to be reduced to 60% of what was introduced to the burner to achieve a good fit between

463

modeled and measured radiative intensities. However, this is not a likely explanation as the

464

distance between the burner and port MH1 is rather short. A more probable explanation is that for

465

the particles in the center not yet have ignited and therefore still have a low temperature, compared

466

to the surrounding gas.


25

Energy & Fuels

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Page 26 of 34

467

Figure 10. Modeled total radiative intensity for the reference coal case in port MH1. The portion

468

of the fuel introduced to the burner not being combusted at the port is varied. Measurement data

469

is represented by triangles.

470

The axisymmetric assumption (see modeling section) was assessed in the modeling, see Figure

471

11. When comparing the modeled total radiative intensity profiles, it can be seen that the largest

472

deviation was found for the co-firing case with biomass A in port MH1. The modeled total

473

radiative intensities along the furnace diameter using the different temperature profiles for the co-

474

firing case with biomass A for the three ports are shown in Figure 11 a-c together with the

475

measured radiative intensity. The larger differences observed at port MH1 is mainly due to

476

problems with clogging of the suction pyrometer while measuring the temperature in positions

477

close to the center position of the port, as discussed previously. These problems resulted in

478

temperature differences of more than 300 K for the same radial distance but at different sides of

479

the furnace centerline. The differences were found to be smaller for the downstream ports MH3

480

and MH7. From these findings it is clear that the assumption of an axisymmetric flame does not

481

introduce significant errors in the modeling.


26

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Energy & Fuels

482

Figure 11. Measured and modeled total radiative intensity along the furnace diameter for the co-

483

firing case with biomass A using three different temperature profiles in ports, a) MH1, b) MH3

484

and c) MH7.

485

The location of the peak value for the particle concentration was also assessed, see Figure 12.

486

Based on the particle extraction, the maximum value was relocated with 0.075 m from the center.

487

The new maximum value was set to be 120% of the value at the center position. It should be noted

488

here, that increasing the peak value increases the total projected surface area and hence increases

489

the modeled radiative intensity that originates from the particles. For a better comparison, it was

490

decided to change the flame radius to keep the total projected surface area in the cross section of

491

the furnace. This was tested for the reference coal case at port MH3, Figure 12. As shown, the

492

modeled radiative intensity is slightly increased as the projected surface area at the hot

493

temperatures close to the center line is increased, but the effect is small.


27

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Page 28 of 34

494

Figure 12. Variation of the peak value position of the projected particle surface area along the

495

furnace diameter. The peak value is located 0.075 m from the center with a) the distribution profiles

496

and b) comparison of modeled total radiative intensity for the reference coal case at port MH3.

497

Finally the isotropic scattering assumption was also assessed and results are present in Figure

498

13. In the sensitivity analysis the portion of the incoming radiation that was scattered forward was

499

varied to be between 40 and 80% of the total incoming radiation respectively, while the remaining

500

part was still considered to be isotropic scattering. Large deviations appear as the scattering

501

assumption was changed from being fully isotropic. The reason is that radiation that originates

502

from the hot wall is no longer scattered into the modeled diameter with the cold background. The

503

large differences observed in this parameter study motivates and shows on the importance to

504

further examine the particle scattering within suspension-fired systems as well as other solid fuel

505

combustion systems in future works.


28

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Energy & Fuels

506

Figure 13. Variation of the portion of incoming radiation being forward scattered with 40 or 80%

507

compared to fully isotropic scattering for the reference coal case at port MH3.

6. Conclusions

508 509

The present work aims to examine the radiative heat transfer conditions in a 580 kWth rotary kiln

510

test furnace when burning coal and biomass-coal mixtures in suspension. The test furnace is

511

cylindrical and is constructed as a down-scaled version of a rotary kiln used for iron ore pellets

512

production. The possibility to exchange a portion of the fossil coal to renewable biomass is

513

evaluated as a possible measure to reduce greenhouse gas emissions without affecting the quality

514

of the pellet product in the full-scale process. This work presents measurements for three fuel cases

515

performed during one measurement campaign. These cases include a reference coal and two co-

516

firing cases using 70% (based on the lower heating value) of the same reference coal and 30% of

517

biomass (A: wood treated with steam explosion, and B: untreated but grinded and pelletized

518

wood).

519

Collected data of temperature, gas composition and particle concentration were used in a detailed

520

radiation model. The radiation model accounts for gases using a statistical narrow band model and

521

particles using Rayleigh and Mie-theory for soot and fuel particles respectively. The radiative

522

intensity was measured using a narrow angle radiometer and a good agreement between measured

523

and modeled radiative intensities was achieved. This implies that accurate measurements were


29

Energy & Fuels

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Page 30 of 34

524

performed during the campaign, and that the model is a promising tool to be developed and used

525

to obtain a better understanding of full-scale rotary kiln processes. Based on the measured radiative

526

intensities, there are only small differences between the reference case and the co-firing case with

527

biomass A. The measured values for the co-firing case with biomass B are lower but the overall

528

combustion and heat transfer conditions are similar for all three fuel mixtures.

529

The incident radiative heat flux to the wall of the furnace has a large impact on the heating of

530

the pellets and the product quality and was measured using an ellipsoidal radiometer. The

531

differences between the three cases were minor for the studied ports, which indicates that changing

532

the fuel from the reference coal to co-firing with either biomass A or B is a possible option in the

533

full-scale process.

534

Sensitivity analyses were performed for some parameters of interest to test the impact on the

535

modeled total radiative intensity. The particle distribution was not known in the furnace, but by

536

varying the radial position of the particle concentration peak value it was shown that only minor

537

effects on the modeled radiative intensity was obtained. The scattering of radiation was also

538

assessed by varying the portion of the radiation being scattered in the forward direction, that is

539

instead of being isotropically scattered. Anisotropic scattering was shown to have a significant

540

impact on the modeled intensities and this motivates further studies on this important parameter in

541

particular for systems with high wall temperatures.

542

To conclude, from the results presented here it seems promising to use biomass for co-firing

543

purposes in rotary kilns; the heat transfer is not significantly affected while the carbon dioxide

544

emissions can be substantially reduced.


30

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545

Energy & Fuels

7. Acknowledgement

546

LKAB and the Swedish Energy Agency is acknowledged for the financial support of this work.

547

LKAB is also acknowledged for providing the experimental data from the ECF test facility and

548

for the inspiring experimental collaboration.


31

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549 550

References (1)

U.S. Geological Survey. MINERAL COMMODITY SUMMARIES 2016; 2016.

551

(2)

Jonsson, C. Y. C.; Stjernberg, J.; Wiinikka, H.; Lindblom, B.; Boström, D.; Öhman, M.

552

Deposit formation in a grate-kiln plant for iron-ore pellet production. Part 1:

553

Characterization of process gas particles. Energy and Fuels 2013, 27 (10), 6159–6170.

554

(3)

555 556

2008. (4)

557 558

Hottel, H. C.; Sarofim, A. F. Radiative Transfer; McGraw-Hill, Inc.: United States of America, 1967.

(5)

559 560

Boateng, A. A. Rotary Kilns Transport Phenomena and Transport Processes; Burlington,

Siegel, R.; Howell, J. R. Thermal Radiation Heat Transfer; Clark, B. J., Damstra, D., Eds.; McGraw-Hill, Inc.: United States of America, 1972.

(6)

561

Özisik, M. N. Radiative Transfer and Interactions with Conduction and Convection; John Wiley & Sons, Inc.: United States of America, 1973.

562

(7)

Modest, M. F. Radiative Heat Transfer; Elsevier: United States of America, 2013.

563

(8)

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Radiative Heat Transfer Conditions in a Rotary Kiln Test Furnace

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