Radiative Heat Transfer in Suspension-Fired Systems

Radiation is the dominating heat transfer mechanism in most furnaces, and the radiative properties of combustion products represent important aspects regarding its suitability as a heat source. However, fuel and energy markets are undergoing rapid changes due to increasing concerns related to the security of supply and increased global warming. Industrial processes that have traditionally relied on a specific fuel must increase their flexibility in terms of their energy supply. Therefore, an increased understanding and improved modeling capacity of radiative heat transfer are required to facilitate a more rapid evaluation and implementation of novel fuels in large furnaces that have reduced emissions and sustained efficiency. For such studies, digital twins of processes may be a useful and economical alternative.This PhD thesis on radiative heat transfer focuses on: (i) the application to one industrial process, i.e., rotary kilns for heat treatment of iron ore pellets; and (ii) radiative heat transfer from soot particles in various propane flames. In-flame measurements of the combustion conditions, radiative intensities and radiative heat fluxes were performed during several measurement campaigns with different burners, furnace geometries and fuels, including gaseous, coal, and co-firing fuels. The radiative heat transfer was modeled using a discrete transfer model and a 3D-modeling tool that applies a discrete ordinates method, with the latter being developed within this thesis.The 3D-modeling tool was used to study the heat transfer conditions within the rotary kiln, as well as the heat treatment of the bed material, and a first attempt to validate the model in relation to actual measurements was made. Radiation was shown to account for more than 80% of the total heat transferred to the bed material, and the flame radiation was dominated by the particles present. Nevertheless, the possibility of using co-firing, including up to 30% biomass, was found to be feasible, and was not expected to have any significant impact on the radiative heat transfer within the process.The soot volume fraction and the radiative properties of soot particles were measured following gas extraction and using a laser-induced incandescence system for the various propane flames, while altering the combustion conditions and oxidizer composition. The modeled radiative intensities of the flames correspond well with the measured values, indicating that the soot volume fraction was accurately measured by either technique. Furthermore, very high soot volume fractions were observed when there were high concentrations of oxygen in the oxidizer, revealing the potential for promoting the radiative heat transfer in such furnaces

Discrete-Ordinates Modelling of the Radiative Heat Transfer in a Pilot-Scale Rotary Kiln

This paper presents work focused on the development, evaluation and use of a 3D model for investigation of the radiative heat transfer in rotary kilns. The model applies a discrete-ordinates method to solve the radiative transfer equation considering emission, absorption and scattering of radiation by gas species and particles for cylindrical and semi-cylindrical enclosures. Modelling input data on temperature, particle distribution and gas composition in the radial, axial and angular directions are experimentally gathered in a down-scaled version of a rotary kiln. The model is tested in its capability to predict the radiative intensity and heat flux to the inner wall of the furnace and good agreement was found when compared to measurements. Including the conductive heat transfer through the furnace wall, the model also satisfactorily predicts the intermediate wall temperature. The work also includes a first study on the effect of the incident radiative heat flux to the different surfaces while adding a cold bed material. With further development of the model, it can be used to study the heat transfer in full-scale rotary kilns

Heat Transfer Conditions in Hydrogen-Fired Rotary Kilns for Iron Ore Processing

This work analyzes the heat transfer conditions in a rotary kiln used for the heat treatment of iron ore pellets in the grate-kiln process. The analysis concerns conditions relevant to fuel switching from coal to hydrogen gas. A modeling assessment of the radiative heat transfer in the kiln is conducted including the pellet bed and inner kiln wall temperature conditions. The results show that the heat transfer rate to the iron ore pellets under conditions of a pure hydrogen flame is comparable to the conditions relevant to coal firing. However, it is higher at the kiln wall surfaces near the burner region and lower in the remaining parts of the kiln. Increasing the particle concentration in the hydrogen flame represents a practical implication of co-firing coal with hydrogen. By adding particles, the emittance of radiation from the flame is significantly influenced, leading to further increased kiln surface temperatures closer to the burner position. Increased flame length also showed enhanced heat transfer rates to the kiln wall, although further away from the burner region

Recirculation of NOx and SOx Scrubber Effluent to an Industrial Grate Fired MSW Boiler - Influence on Combustion Performance, Deposition Behavior, and Flue Gas Composition

The concept of scrubber effluent recirculation has recently received attention in connection to NOx emission control. Here, we present data from an industrial-scale MSW-fired plant, where effluent from a combined NOx and SOx scrubber was recirculated and injected into a grate-fired boiler. The combustion characteristics were carefully studied during the injections to observe the potential effects on burnout and flue gas composition. In addition, deposition measurements were performed to observe effects on growth rate and chemical composition of deposits, which are critical factors for any solid fuel-fired heat and power plant. The recirculation of the nitrogen-rich waste streams was performed via pre-existing liquid injection equipment, and the results show that the N-containing compounds in the scrubber effluent were efficaciously reduced to inert nitrogen gas. Furthermore, the recirculation of the scrubber effluent may reduce ammonia demand for selective non-catalytic reduction systems by inhibiting the formation of ammonium chloride. Sulfur and alkali components in the effluent increased the deposition growth rate and also changed the chemical composition of the deposits. Understanding how the local conditions at the injection point influence the distribution and speciation of the injected compounds is essential for a successful recirculation strategy

Learn2Reg: comprehensive multi-task medical image registration challenge, dataset and evaluation in the era of deep learning

Image registration is a fundamental medical image analysis task, and a wide variety of approaches have been proposed. However, only a few studies have comprehensively compared medical image registration approaches on a wide range of clinically relevant tasks. This limits the development of registration methods, the adoption of research advances into practice, and a fair benchmark across competing approaches. The Learn2Reg challenge addresses these limitations by providing a multi-task medical image registration data set for comprehensive characterisation of deformable registration algorithms. A continuous evaluation will be possible at https://learn2reg.grand-challenge.org. Learn2Reg covers a wide range of anatomies (brain, abdomen, and thorax), modalities (ultrasound, CT, MR), availability of annotations, as well as intra- and inter-patient registration evaluation. We established an easily accessible framework for training and validation of 3D registration methods, which enabled the compilation of results of over 65 individual method submissions from more than 20 unique teams. We used a complementary set of metrics, including robustness, accuracy, plausibility, and runtime, enabling unique insight into the current state-of-the-art of medical image registration. This paper describes datasets, tasks, evaluation methods and results of the challenge, as well as results of further analysis of transferability to new datasets, the importance of label supervision, and resulting bias. While no single approach worked best across all tasks, many methodological aspects could be identified that push the performance of medical image registration to new state-of-the-art performance. Furthermore, we demystified the common belief that conventional registration methods have to be much slower than deep-learning-based methods

Experimental and numerical studies of thermal radiation in gas, coal and co-fired pilot test facilities

Throughout the industrialized era, fossil fuels have been used extensively in combustion processes to generate heat and electricity. However, the combustion of such fuels creates emissions of greenhouse gases, mainly carbon dioxide, and other hazardous products. As these gases are released to the atmosphere, they contribute to global warming, which is a global issue of concern. It is therefore of great importance to study combustion processes and explore new ways to reduce their environmental impacts. One way of reducing emissions of greenhouse gases is to replace the energy source: from fossil to renewable. To attain a better understanding of the combustion process experimental and modeling work is needed. Such work can be undertaken to increase the efficiency and demonstrate the possible effects of fuel substitution.The first half of this thesis focuses on the rotary kiln process, which is used in iron ore pellet production, and studies the associated rotating furnace and the radiative heat transfer process. The experimental work was performed during a measurement campaign in a down-scaled version of the furnace, and the results are compared and modeled using a discrete transfer model for different fuels. The focus is on comparing a reference coal with co-firing cases that employ a combination of 70% reference coal and 30% biomass for two different types of biomass. The results reveal the possibility of using co-firing in a full-scale rotary kiln, which is not expected to have any significant impact on the radiative heat transfer within the process. Measurements from a similar but earlier campaign were used together with a radiative model, using the discrete ordinates method to study the process in three dimensions. While still under development, this model predicts trends when the operational conditions of the rotary kiln process are changed.The second half of this thesis work focuses on soot formation in different propane flames. The combustion conditions were varied while the process stoichiometry was maintained, and soot formation was analyzed using two different measurement techniques. In the first campaign, the soot volume fraction was measured by gas extraction using a scanning mobility particle sizer (SMPS), together with a photo-acoustic soot spectrometer (PASS-3), to study the radiative properties of the soot particles. In the second campaign, the soot volume fraction was measured using laser-induced incandescence (LII) together with an extinction laser. In both campaigns, the radiative intensity was measured, and the different flames were modeled using a discrete transfer model of the radiative heat transfer. The differences between the modeled and measured radiative intensities were found to be small in both campaigns

Full-Scale 3D-Modelling of the Radiative Heat Transfer in Rotary Kilns with a Present Bed Material

This work discusses the development and usage of a radiative heat transfer model in 3D of a rotary kiln with a present bed material. Using a discrete-ordinates method to solve the radiative heat transfer equation, radiative properties are calculated using a weighted-sum-of-grey-gases (WSGG) model for gases and Mie and Rayleigh theory for particles including fuel, ash and soot. Measurement data gathered from a pilot scale test furnace, comprising temperature, gas composition and particle concentration, is used to model a pilot-scale rotary kiln and satisfactory agreement to radiative heat flux measurement is shown. Combining measurements, from the same test furnace, with data gathered from an industrial full-scale rotary kiln used for iron ore pelletizing, a full-scale rotary kiln with a present bed material is modelled. Conductive heat transfer within, as well as between, the bed and wall material is included in the model and surface temperatures are calculated within the model. The model also includes heat losses from the outside wall of the rotary kiln due to radiation and convection and a simplified mixing model of the pellet bed. When compared to measurements for the inside and outside wall as well as the bed material, the model predicts the surface temperatures with errors less than 10%. The total heat transfer to the present bed material was also studied showing that more than 90% originated form the radiative heat transfer within the furnace

Heat transfer modelling of industrial rotary kilns for iron ore pelletizing

This work discusses the development and usage of a detailed heat transfer model of a rotary kiln in 3D, applying a discrete-ordinates method to solve the radiative heat transfer equation. The model includes conductive and convective heat transfer as well as heat release due to reactions in the bed and heat losses from the outside of the kiln. The rotation of the wall is considered along with a simplified mixing model of the bed. Measurement data gathered from a test furnace, is used in combination with temperature data and operation parameters gathered from a full-scale rotary kiln to calculate surface temperatures. Compared to measurements, the model predicts the temperatures well with errors less than 10%. Different contributions of the total heat transfer to the bed was studied, revealing that more than 80% originated from radiation