Woody biomass is being considered a potential co-firing fuel to reduce coal consumption in iron-ore pelletizing rotary kilns. An important consideration is the slagging inside the kiln caused by ash deposition that can lead to process disturbances or shutdowns. In terms of ash chemistry, co-firing woody biomass implies the addition of mainly Ca and K to the Si- and Al-dominated coal-ash (characteristic of high-rank coals) and Fe from the iron-ore that are both inherent to the process. An alkali-laden gaseous atmosphere is also present due to the accumulation of alkali via the recirculation of flue gas in the system. The slagging propensity of blending woody biomass with coal in the grate-kiln process was studied based on the viscosity of the molten phases predicted by global thermochemical equilibrium modeling. This was carried out for variations in temperature, gaseous KOH atmosphere, and fuel blending levels. Results were evaluated and compared using a qualitative slagging indicator previously proposed by the authors where an inverse relationship between deposition tendency and the viscosity of the molten fraction of the ash was established. The results were also compared with a set of co-firing experiments performed in a pilot-scale (0.4 MW) experimental combustion furnace. In general, the co-firing of woody biomass would likely increase the slagging tendency via the increased formation of low-viscosity melts. The fluxing behavior of biomass-ash potentially reduces the viscosity of the Fe-rich aluminosilicate melt and intensifies deposition. However, the results also revealed that there are certain conditions where deposition tendency may decrease via the formation of high-melting-point alkali-containing solid phases (e.g., leucite).© 2019 Elsevier

Iron-ore pelletizing plants use high-rank coals to supply the heat necessary to process ores. Ash material from coal, in combination with iron-ore dust originating from the disintegration of the pellets, can cause deposition/slagging which often leads to severe production losses and damage. Deposition/slagging is most prominent in the hot areas of the grate-kiln setup and is more severe at the inlet of the rotary-kiln, i.e., the transfer-chute. Following on from our previous work, high-rank bituminous coals with potential for use in the pelletizing process were combusted in a pilot-scale (0.4 MW) pulverized-coal fired experimental combustion furnace (ECF). The fly-ash particles and short-term deposits were characterized to shed light on the observed difference in slagging/deposition tendencies of the coals. Global thermodynamic equilibrium modeling, in combination with viscosity estimates, was used to interpret the experimental findings and investigate the effect of the coal-ash composition upon deposition/slagging. This approach was carried out with and without the presence of Fe₂O₃-rich pellet-dust under oxidizing conditions within the temperature range at the transfer-chute of iron-ore pelletizing rotary-kilns. Based on the findings, a Qualitative Slagging Indicator (QSI) was proposed that can help pre-screen new solid fuels for potential slagging issues. The proposed QSI highlights the following: (1) an inverse relationship between viscosity and slagging/deposition tendency of the coals was observed (2) as viscosity decreases (either with increasing temperature or due to the change in the coal-ash composition), stronger deposits will form that will complicate the mechanical removal of the deposited layer. It was therefore inferred that low viscosity molten phases facilitate deposition/slagging, which is exacerbated by the presence of fluxing agents (e.g., CaO, MgO, K₂O, Na₂O, and Fe₂O₃) in the deposits. The low viscosity coal-ash-induced molten phases are also more likely to interact with the Fe₂O₃-rich pellet-dust that results in further decreases in viscosity, thereby intensifying depositions. The results from this work complement the on-going research by our group to elucidate and alleviate ash-related problems in industrial grate kilns.

Fly ash particles from the combustion of solid-fuels together with disintegrated particles arising from iron-ore pellets result in accumulation of deposits on the refractory linings of the grate-kiln induration machine during the iron-ore pelletizing process. The deposits amass in the high-temperature regions of the induration furnace thus disturbing the flow of gas and pellets. Therefore, to tackle the above-mentioned issues, an understanding of deposit formation mechanism is of crucial importance. This study was conducted with the objective of addressing the effect of disintegrated iron-ore pellet dust on deposit formation and the mechanisms behind deposition (slagging) in the grate-kiln process. A comprehensive set of experiments was conducted in a 0.4 MW pilot-scale pulverized-coal- fired furnace where three different scenarios were considered as follows; Case 1 (reference case): Coal was combusted without the presence of pellet dust. Case 2: Natural gas was combusted together with simultaneous addition of pellet dust to the gas stream. Case 3: Coal was combusted together with the addition of pellet dust simulating the situation in the large-scale setup. Fly ash particles and short-term deposits were characterized and deposition was addressed in Part I of this study. In light of the experimental observations (Part I) and the thermochemical equilibrium calculations (Part II), a scheme of ash transformation during the iron-ore pelletizing process was proposed. The dissolution of hematite particles into the Ca-rich-aluminosilicate melt (from the coal-ash constituents) decreased the viscosity and resulted in the formation of stronger (heavily sintered) deposits. Overall, this pilot-scale work forms part of a wider study which aims at deepening the understanding of ash transformation phenomena during the large-scale pelletizing process.

The iron ore pelletizing industry utilizes the grate-kilnprocess to dry and sinter the pellets into finished product.The grate-kiln process has a known deposit formation issuethat needs to be further understood. Combustion ofpulverised coal in the rotary kiln generates fly ash particles;in addition to that, particles generated from disintegratediron ore pellets are also entrained in the process gas stream.The combined effect of both sources of particles cantherefore contribute to the deposit formation in the process.In this work, particle- and deposit formation were studiedboth from a full-scale grate-kiln plant (40 MW) and from apilot-scale pulverised coal fired furnace (400 kW). Particleswere collected with a water-cooled probe with nitrogen gasas dilution medium at the tip of the probe. The particleswere separated simultaneously with a pre-cyclone and a 13stages low-pressure impactor during samplings. Depositswere collected with a refractory plate which was attachedat the tip of a water-cooled probe, exposed to the hightemperature (>1100 °C) process gas stream. Particles anddeposits were characterized with an environmentalscanning electron microscope and a scanning electronmicroscope that equipped with energy dispersivespectroscopy detector. A comparison of particle and depositcharacteristics between the grate-kiln plant and the pilotscale pulverised coal fired furnace is presented in this paper,with focus on the potential influence of disintegrated ironore pellets on the particle- and deposit formation process.

Slag formation in the grate-kiln process is a major problem for iron-ore pellet producers. It is therefore important to understand the slag formation mechanism in the grate-kiln production plant. This study initiated the investigation by in situ sampling and identifying particles in the flue gas from a full-scale 40 MW grate-kiln production plant for iron-ore pelletizing. Particles were sampled from two cases of combustion with pulverized coal and heavy fuel oil. The sampling location was at the transfer chute that was situated between the traveling grate and the rotary kiln. The particle-sampling system was set up with a water-cooled particle probe equipped with nitrogen gas dilution, cyclone, and low-pressure impactor. Sub-micrometer and fine particles were size-segregated in the impactor, while coarse particles (>6 μm) were separated with a cyclone before the impactor. Characterization of these particles was carried out with environmental scanning electron microscopy (ESEM), and the morphology of sub-micrometer particles was studied with transmission electron microscopy (TEM). The results showed that particles in the flue gas consisted principally of fragments from iron-ore pellets and secondarily of ashes from pulverized coal and heavy fuel oil combustions. Three categories of particle modes were identified: (1) sub-micrometer mode, (2) first fragmentation mode, and (3) second fragmentation mode. The sub-micrometer mode consisted of vaporized and condensed species; relatively high concentrations of Na and K were observed for both combustion cases, with higher concentrations of Cl and S from heavy fuel oil combustion but higher concentrations of Si and Fe and minor P, Ca, and Al from coal combustion. The first fragmentation mode consisted of both iron-ore pellet fines and fly ash particles; a significant increment of Fe (>65 wt %) was observed, with higher concentrations of Ca and Si during heavy fuel oil combustion but higher concentrations of Si and Al during coal combustion. The second fragmentation mode consisted almost entirely of coarse iron-ore pellet fines, predominantly of Fe (90 wt %). The particles in the flue gas were dominantly iron-ore fines because the second fragmentation mode contributed >96 wt % of the total mass of collected particles.

Buildup of deposit material in chunks on refractory linings caused by combustion of various fuels is a well-known problem. This study characterizes the short-term deposits on refractory material in a grate–kiln process, carried out through in situ measurements using a water-cooled probe with a part of a refractory brick mounted in its end. Sampling was carried out during combustion of both oil and coal. A significant difference in deposition rates was observed; deposition during oil firing was negligible compared to coal firing. The deposits are mainly hematite particles embedded in bonding phase, mainly comprising Si, Al, Fe, Ca, and O. Moreover, it was found that the prevailing flue-gas direction determines the formation of the deposits on the probe and that inertial impaction controls the deposition rate. However, this rate can also be affected by the amount of air-borne particles present in the kiln.

To be able to create a model that accurately describes a district heating system, it is important to identify the thermal losses and how they are distributed. However, general methods targeting the determination of losses are scarce in the literature. In the current case the losses for a district heating system in Kiruna, a town in northern Sweden, has been estimated in the year 2010 to be 12%, which is in the range for a typical Swedish network. Unfortunately, detailed information of the thermal losses is lacking.In this paper two methods to determine loss distribution in a district heating system are presented.Two databases of pipe lengths and diameters have been compiled for two piping categories, loops and feeds. Any missing data regarding pipe diameters in the map has then been determined with the two different methods.In the first method average pipe diameters for loops and feeds are calculated. All pipes with unknown diameter are then assumed to have the average one. The second method considers a percentage based distribution of known diameters and assigns the same distribution to the missing pipe diameters. The losses were estimated in the whole system according to the data from a pipe producer catalogue, in which losses are calculated according to current European standard. The results show that the losses in the system are similar to the losses caused by pipes with the lowest insulating capacity.By using the two methods two fictitious pipe series reproducing exactly the losses in the system are created by scaling the calculated losses of the catalogue pipe series which would give the most similar losses (the one with the lowest insulating capacity). This adjustment was +3.1% by using the first method, and +4.9% by using the second method.The major conclusion of this study is that, both methods can be used for calculating the distribution of thermal losses in the district heating system of Kiruna; moreover, this kind of analysis can be an important tool for analyzing investments in the district heating network in Kiruna.

Due to the mining of iron ore by the local mining industry (LKAB) a deformation zone is approaching the town of Kiruna, which is situated in the north of Sweden. The town is going through an urban transformation and parts of the town is about to be relocated. The relocating of the town is affecting all the infrastructure of the town, one of those is the district heating system. The aim of this paper is to apply a process integration method to study how the existing district heating system is behaving and to create a model for the whole district heating system.The district heating system in the town has a complex design with several loops. When dealing with modeling of a district heating system there are two methods that are most common, the Danish- and the German methods. These methods allow for simplification of the network shape, still with good accuracy in system behavior compared to real system behavior. A disadvantage with these two methods is that they cannot handle a district heating system that contains loops without making changes compared to the real physical system.A method that allows modeling of loops in the district heating system has been developed, using the remind-software. This method makes it possible to analyze how loops in the district heating system are behaving, without making physical changes when modeling the district heating system. The model will be used as a default model for redesigning the district heating system when moving the town. It will also be used to optimize heat deliveries from LKAB district heating system and for analyzing the possibility for seasonal storage of waste energy from LKAB and municipality energy company TVAB or how new heat production plants should optimized.In this paper the method is described and applied in order to model the whole present district heating system for the town.