High-temperature Drop-tube Furnace Research Articles

Synergistical reduction of PM and NO formation in preheating co-firing of coal and biomass

Preheating co-firing of coal and biomass was proposed for synergetic mitigation of pollutant source emissions. The effects of different operating parameters on particle matter (PM) and nitric oxide (NO) emissions were investigated, including excess air ratio (αp), temperature (Tp), residence time (tp) in the preheating zone and temperature (Tc) in the combusting zone. Coal and biomass particles were fed separately by two feeders, mixed uniformly in a mixing device and then inserted in a high-temperature drop-tube furnace. PM and NO emissions at the furnace outlet were measured by ELPI+ and the infrared gas analyzer. The results showed that as αp increased from 0 to 0.5, PM10 emission declined by 10.65% while NO emission went down first and then rose up with a turning point at αp = 0.3 of 247.93 mg/m3. When Tp increased from 1400 K to 1800 K, PM10 emission increased but NO emission decreased. The increase in the former was more than twice the decrease in the latter. More interestingly, the forming mechanism of ultrafine mode PM0.3 was dominated by “solid-vapor” process at low combusting temperature but transformed into “vapor-particle” process at high temperature. When Tc ascended from 1700 K to 1800K, the dominant “vapor-particle” process was inhibited, leading to a decrease of 13.5% in PM0.3 emission. PM0.3-2.5, PM2.5-10 and NO emissions were boosted by 64.99%, 36.40% and 78.20% respectively with Tc ascending from 1400 K to 1800 K. As tp rose from 0.1 to 0.5, the production of PM0.3 and NO decreased by 29.50% and 44.55% respectively, but that of PM0.3-2.5 and PM2.5-10 increased by 20.04% and 33.83% continuously. Under optimum parameters (Tp = 1300 K, αp = 0.3, Tc = 1400 K and tp = 0.5 s), PM10 and NO emissions could be reduced by 30.88% and 48.01%, respectively.

Mechanistic study of PM2.5 reduction by kaolin-limestone compound additive in pulverized coal combustion

A novel compound additive prepared from kaolin and limestone shows promising prospects for in-furnace abatement of combustion PM2.5 (particulate matter with aerodynamic diameters equal to or less than 2.5 μm). However, the mechanisms need further clarification. These were investigated by combustion of a low-alkali coal on a high-temperature drop tube furnace (DTF). Combustion experiments with the kaolin-limestone additive were carried out at 1300 °C and in simulated air. Baseline tests of individual additives (i.e., kaolin and limestone) were also performed for comparison. Particulate samples were collected and classified by a Dekati low pressure impactor (DLPI). Heat-treated additive samples were also collected by DLPI and filter. They were characterized by various techniques such as laser diffraction particle sizing, X-ray diffraction (XRD), and field emission scanning electron microscopy equipped with energy dispersive spectrometry (FESEM-EDS). The results show that the addition of kaolin or limestone suppresses the emission of aerosols between 0.3 and 2.5 μm (PM0.3-2.5), but unexpectedly increases that of ultrafine aerosols less than 0.3 μm (PM0.3). The latter is primarily attributed to the carryover of ultrafine additive particles. Due to different effects on the two particle size fractions, the individual additives are found to have overall insignificant effects on PM2.5 emission. In striking contrast, the addition of the kaolin-limestone additive leads to an apparent reduction of both PM0.3 (∼46%) and PM0.3-2.5 (∼37%). As a result, a significantly high reduction of PM2.5 by ∼ 40% is achieved. The superior performance of the compound additive, compared with the individual additives, is accounted for by enhanced interactions between kaolin-limestone, additive-vapor, and additive-ash. These mechanisms, together with the collision-induced particle capture mechanism proposed previously, are believed to play key roles in PM2.5 abatement in real boilers when the compound additive and fuel are premixed.

Effects of iron on coal pyrolysis-derived soot formation

Soot formation is one major challenge in developing clean and efficient utilizes based on hydrocarbon fuels. In this work, ferric nitrate was selected as an iron additive to be loaded into pulverized coal, and the pyrolysis experiment with an inert atmosphere was conducted in a high-temperature drop tube furnace (DTF). The physical and chemical character of the soot obtained from coal samples with different iron content were analyzed to explore the effect of the soot format process. The results showed that the soot yield was reduced 7% and 23%, respectively, at the same conditions when the content of iron in the coal sample was 0.51% and 0.87%. After iron was added, the diameter of soot primary particles was reduced from 50.8 nm to 37.4 and 37.8 nm, and the Df of soot aggregates increased, indicating that iron inhibited the growth process of soot primary particles and promoted the agglomeration between particles. Thus, the degree of graphitization of soot increased. In addition, the content of C–O functional groups in soot derived from coal with iron addition increased significantly, which implied that the participation of iron enhanced the oxidation reaction of soot.

Separate physicochemical effects of CO2 on the coal char combustion: An experimental and kinetic study

The overall impact of CO2 on the coal char combustion is complicated due to the interplay between the CO2 thermal effect and chemical effect (including the gasification endothermicity, and its direct additional carbon consumption). To assess the impact of these factors on the carbon consumption process, we conduct a quantitative study on the separate physicochemical effects of CO2 through experimental and kinetic research. Two oxygen concentrations (15% and 21% O2), representing industrial boiler and air combustion environments, were selected. The experiment research on its thermal effect (evaluated by carbon conversion ratios) is performed at 1773 K in a high-temperature drop tube furnace. The chemical effect of CO2 is predicted for a 100 μm coal char particle using a self-developed char burning kinetics model. Results indicate that the carbon conversion ratios show a V-type distribution with higher CO2 concentrations, as a result of its complex physicochemical effects, with minimal carbon conversion points around the CO2 concentrations of 5–10 vol.%. The thermal effect initially increases with higher CO2 concentrations from 0 to 10 vol.%, but it declines with a further increase in CO2 concentrations, while the endothermicity effect of the gasification reaction increases with higher CO2 concentrations. The relative contribution of the endothermicity effect on the char consumption is 20.4% in the 21%O2/10CO2/N2 environment, lower than the thermal effect (29.9%), but it continuously increases and becomes the most influential inhibitory factor at higher CO2 concentrations. The strongest suppression effect of CO2 corresponds to the CO2 concentrations of 10 vol.%.

Catalytic mechanism of Na on coal pyrolysis-derived carbon black formation: Experiment and DFT simulation

Coal-derived carbon black is the main composition of PM2.5, used in the rubber industry and energy storage fields. The formation and structure control of carbon black have attracted significant attention. Alkali metals have influence on the dynamic formation of coal-derived carbon black. To analyze the mechanism of Na in the formation of carbon black, a high-temperature drop tube furnace was used to analysis of carbon black from the pyrolysis of acid-washed coal and Na-containing coal at 1250 °C. DFT was applied to simulate the stepwise inhibitory effect of Na on the polymerization of PAHs into carbon black. The results show that Na promotes the oxidative cracking of large PAHs into small PAHs during the formation of coal-derived carbon black (reaction energy barrier is reduced by 17.7%), and inhibits the condensation of small PAHs molecules to form carbon black molecules (reaction energy barrier is increased by 74.7%), which causes the length of carbon black is shortened by 14.3%, and its curvature is increased by 1.7%. Na could change the spatial structure of formed carbon black-graphite crystallites, which increases the spacing of carbon black by approximately 6.2% to 16.3%. Na enhances the electrochemical performance of carbon black by 2 to 3 times.

Reduction Mechanism of Fine Hematite Ore Particles in Suspension

In order to understand the pre-reduction behaviour of fine hematite particles in the HIsarna process, change of morphology, phase and crystallography during the reduction were investigated in the high temperature drop tube furnace. Polycrystalline magnetite shell formed within 200 ms during the reduction. The grain size of the magnetite is in the order of magnitude of 10 µm. Lath magnetite was observed in the partly reduced samples. The grain boundary of magnetite was reduced to molten FeO firstly, and then the particle turned to be a droplet. The Johnson-Mehl-Avrami-Kolmogorov model is proposed to describe the kinetics of the reduction process. Both bulk and surface nucleation occurred during the reduction, which leads to the effect of size on the reduction rate in the nucleation and growth process. As a result, the reduction rate constant of hematite particles increases with the increasing particle size until 85 µm. It then decreases with a reciprocal relationship of the particle size above 85 µm.

Experimental Studies on Thermal Effect of H2O on the Combustion of Pulverized Coal Char

ABSTRACT The differences in the gas thermophysical properties (including heat capacity, thermal conductivity and diffusivity) between char combustion in wet and dry environments have remarkable influences on the combustion process. We conducted a detailed experimental study on the thermal effects of H2O on the coal char combustion at 1773 K in a high-temperature drop tube furnace (HTDTF). An appropriate portion of Ar was introduced into the bulk gas (O2/H2O/N2) during combustion to eliminate the opposed thermal effect of H2O. Results indicate that the H2O thermal effect initially increased with higher H2O concentrations from 0 to 10 vol.%, but it declined with a further increase in H2O concentrations in the bulk gas. Meanwhile, its effect was dominant in the preliminary and middle stages of char burning at low H2O concentrations. The increased H2O concentration and residence time promoted the gasification reaction and the water-gas shift reaction so as to weaken the negative thermal effect of H2O and improve the carbon conversion. In the 15%O2/5%H2O/80%N2 environment, the relative contribution of thermal effects to carbon conversion ratios ranked up to −36% at the residence time of 0.3 s, but it decreased to −0.3% at 21%O2/30%H2O/49%N2 atmosphere and the residence time of 0.4 s.

The role of H2O(g) in the formation of ultrafine PMs under O2/N2/H2O atmosphere

To explore the effect of water vapor (H2O(g)) on the formation characteristics of ultrafine particulate matter (PMs), Huangling coal char reacted in a high-temperature drop tube furnace (DTF) at 1800 K under various O2/N2/H2O atmospheres, with the content of H2O(g) varied at 0, 5%, 10%, 20% and 30% respectively. The results showed that both the number-based and mass-based particle size distributions (PSDs), the bimodal distribution of ultrafine PMs remained almost unchanged with increasing H2O(g) content, while the peak of ultrafine PMs shifted and the magnitude of the main peak fluctuated. H2O(g) elevated the mass fraction of ultrafine PMs with a larger particle size (D50 = 0.2134 μm) and the number fraction of ultrafine PMs with a smaller particle size (D50 = 0.0093 μm and D50 = 0.0167 μm) significantly. With the increase of H2O(g) content, both the number and mass of ultrafine PMs decreased first and then increased with the inflection point at 5% H2O(g) content, indicating that lower H2O(g) content inhibited the formation of ultrafine PMs while higher H2O(g) content promoted its formation. Meanwhile, with the increase of H2O(g) content, the average mass of single ultrafine particle, being higher in wet combustion atmospheres than that in dry combustion atmosphere, increased first and then decreased with the inflection point at the content of 5%. In addition, when the H2O(g) content was 5%, the average particle size of ultrafine particle was the largest, which increased by 9.07% compared with that under dry combustion atmosphere. Finally, the influence mechanism of H2O(g) on the formation of ultrafine PMs during char combustion was clarified.

Experimental and kinetics studies on separate physicochemical effects of steam on coal char combustion

The synthetic effect of H2O on the combustion characteristics of coal char is complicated due to its thermal (high volumetric heat capacity), chemical, and dilution (thereby low oxygen concentration) effects. To assess the impact of these factors on the carbon consumption process, we conduct a quantitative study on the net effect of H2O gasification through experimental and kinetic research. The experiment research is performed at 1773 K in a high-temperature drop tube furnace. The thermal effects of H2O are stripped off the net effect by introducing Ar into the bulk gas to neutralize its high heat capacity. We assess the chemical effects of gasification (gasification endothermicity, and its additional carbon consumption) for a 100 µm coal char particle reacting in environments of varying O2, H2O, and Ar using a self-developed char burning kinetics model. The relative contributions of H2O gasification and oxidation to carbon consumption are also discussed. Results indicate that the reduced carbon consumption through the negative effect of gasification (e.g., thermal effects and the gasification endothermicity) exceeds the additional carbon consumption from gasification reaction in the early combustion stages (< 0.3 s), but it is surpassed by the latter with char consumption. Increasing both the concentrations of H2O and O2 favor the gasification endothermicity. The reduced carbon conversion ratio caused by the endothermicity effect increases as high to around 0.4 then drops down with char consumption. Besides, in the 15 vol.% O2 case, an increase in H2O concentration from 5 vol.% to 30 vol.% improves the gasification contribution from 22.9% to 46.6% in the middle stage of combustion. It also widens the particle temperature gap by endothermicity effect to over 60 K, but the gap narrows in the burnout stage. Similar results were found in the air-fired case. The relative contribution from H2O gasification reaction is less influential upon the rise of oxygen concentration, which decreases by over 15% than in 15 vol.% O2 case.

Experimental study on the formation of ultrafine particulate matters (PMs) during pulverized coal (PC) char combustion in O2/N2 and O2/CO2 atmospheres

Under the force of increasingly strict emission standard of particulate matters (PMs) and serious haze in China, further understanding of the formation of ultrafine PMs during coal combustion is crucial. In this work, the formation characteristics of ultrafine PMs during pulverized coal (PC) char combustion in O2/N2 and O2/CO2 atmospheres were investigated through a high-temperature drop tube furnace (DTF) with a two-stage dilution sampling system and a 14-stage electrical low pressure impactor (ELPI+). Results showed that in both number-based and mass-based particle size distribution (PSD) the peaks of ultrafine PMs located nearby 0.2 μm under all experimental atmospheres including O221%/N279%, O221%/CO279%, O227%/CO273% and O233%/CO267% (O21N79, O21C79, O27C73 and O33C67). And the peak values increased with the increase of O2 concentration in O2/CO2 atmospheres because of the high char combustion temperature at elevated O2 level. However, the mass and number concentration of ultrafine PMs in O2/CO2 atmospheres reduced significantly compared with these in O2/N2 atmosphere. When O21N79 atmosphere switched to O21C79, O27C73, O33C67 atmosphere, the mass concentration of ultrafine PMs reduced by 80.90%, 76.58% and 14.31%, and the number concentration reduced by 74.16%, 63.17% and 10.50%, respectively. Furthermore, the total mass of the ultrafine PMs was determined by the mass of the ultrafine PMs with larger particle size, whereas the total number of ultrafine PMs was determined by the number of the ultrafine PMs with smaller particle size. In this work, the ultrafine PMs with the aerodynamic size smaller than 0.3 μm were collected on the 1st to 7th impactor surfaces of the ELPI+. Among the ultrafine PMs, the particles on the 7th impactor (PMs with the superior limit of ultrafine PMs size) accounted for more than 94.8% of the total ultrafine PMs by mass, but less than 2.3% of the total ultrafine PMs by number; The particles deposited on the 1st impactor (PMs with the detectable minimum size of ultrafine PMs) accounted for more than 79.6% of the total ultrafine PMs by number, but less than 0.4% of the total ultrafine PMs by mass. In addition, PM0.1 which accounted for 96% of the total number of the ultrafine PMs was analyzed for chemical composition by Inductively Coupled Plasma Mass Spectrometer (ICP-MS). Results showed that the prominent components of PM0.1 were Ca, Na, and Si in both O2/N2 and O2/CO2 atmospheres. However, compared with O2/N2 atmosphere, Na, Si, Al, and Fe were more enriched in PM0.1 in O2/CO2 atmospheres.

An experimental and numerical study on the combustion of lignites from different geographic origins

Coal combustion involves multi-scale, multi-phase and multi-component aspects, in a process where both transport phenomena and reaction kinetics must be considered. The aim of the work is to investigate the differences between the combustion characteristics of Turkish (Soma lignite, Tunçbilek lignite, Afşin-Elbistan lignite) and German (Rhenish lignite) lignites. Combustion characteristics of these lignites were investigated experimentally and numerically. Experiments were conducted using a high temperature (1000 °C) and high heating rate (~104 °C/s) drop tube furnace (DTF), along with a thermogravimetric analyzer (TGA) at non-isothermal conditions (5, 10, 15, 20 °C/min). Both experimental trials were done in a dry air environment and atmospheric pressure. Additionally, DTF and TGA are the experimental setups used to validate the numerical model used in this work. The numerical part of the study includes the computational fluid dynamic analysis of DTF and the predictive multi-step kinetic model analysis of the fuel particle. TGA experiments showed that fuel ratio has an effect on the ignition times. Moreover, maximum reaction rates obtained by TGA experiments were inversely proportional to the ash contents of the fuels used. High heating rate DTF experiments showed similar combustion behaviours with TGA experiments. According to DTF experiments, RL has the highest reactivity (RL: 7.8 s−1) among all fuels (AEL: 5.3, SL: 4.7, TL: 2.9 s−1). In comparison to experimental data, PoliMi model predictions on high-temperature volatile yields are satisfactory with 5–7% errors. PoliMi model overpredicted the devolatilization rates whereas char oxidation rate predictions seem to be lower than the experimental results.

Experimental study on fouling and slagging behaviors during oxy-fuel combustion of high-sodium coal using a high-temperature drop-tube furnace

Zhundong coalfield occupies an important position in Chinese energy resource. However, the high-sodium content has become the hindrance for the safe use of Zhundong coal, leading to severe fouling and slagging issues. Oxy-fuel combustion is a clean combustion technology and beneficial for capturing CO2. The fouling/slagging mechanisms of Zhundong coal in oxy-fuel combustion were ambiguous and the influences of oxygen content on the slagging behaviors of high-sodium coal have yet to be fully understood. This work employed a drop-tube furnace to explore the effects of flue gas temperature and oxygen content on the fouling and slagging characteristics in oxy-fuel atmosphere, and also conducted the comparison between oxy-fuel and air cases, with the elaboration of mineral constituents, morphologies, and elemental compositions of the deposits. Experimental results showed that the fouling of the high-sodium coal worsened as the flue gas temperature rose from 900 °C to 1200 °C. The element and mineral categories of the slag were similar but the relative contents of the mineral were different between oxy-fuel and air conditions. The diffraction peak of quartz in Wucaiwan slag during oxy-fuel combustion was weaker than that in air, while the condition of Lu’an slag was different. The high sodium content and its compounds promoted the formation of initial layer and the sticky particle surface, worsening the fouling and slagging. More comprehensive understanding of slagging mechanism could be achieved through the present work, which made sense for the optimization of combustion condition and the safe utilization of Zhundong coal.

Gas–Liquid Reduction Behavior of Hematite Ore Fines in a Flash Reduction Process

Novel ironmaking technologies that directly use the raw materials of iron ore fines without preprocessing have greater benefits in environmental protection and energy conservation. In this work, the gas–liquid reduction behavior and kinetic mechanism of hematite ore fines in the flash reduction processes are investigated using a high-temperature drop tube furnace. Morphological observations show the hematite ores to be completely molten at 1700 K and above. The metallic iron is enwrapped in the liquid wustite and the aggregation state of metallic iron inside the wustite is highly dependent on the reaction temperature. Many small irregularly shaped iron particles are found to be scattered in the liquid wustite at 1650 K to 1700 K, while only one big spherical iron particle is observed at 1700 K to 1800 K. The gas–liquid reduction reaction always occurs at the surface of the liquid wustite particle, and the kinetic analysis reveals that the interfacial chemical reaction at the liquid wustite surface is the rate-controlling step during the reduction process. The chemical kinetics limiting rate equation of gas–liquid reduction is determined. The activation energies of the gas–liquid reaction calculated by the model-fitting and the model-free approaches are 148 and 143 kJ/mol, respectively.

Effects of various occurrence modes of inorganic components on the emissions of PM10 during torrefied biomass combustion under air and oxy-fuel conditions

In order to control PM10 (of an aerodynamic diameter of 10 μm or less) emissions from biochar combustion, the effects of various occurrence modes of inorganic components on PM10 formation are investigated. Two typical biochars (torrefied willow and wheat) were processed through sequential extraction procedures followed by combustion experiments in a high-temperature drop-tube furnace at 1400 °C. The generated particulate matter was characterised. The PM1 emissions during air combustion were linearly related to the content of K/Cl/S in water-soluble/ion-exchangeable forms, which are easily vaporised. The nucleation and coagulation of KCl/K2SO4 in gaseous species forming most of the PM1 resulted in the above-mentioned strong correlation. The PM1-10 emissions had a not very good linear relationship with the various forms of inorganic elements, and they were mainly related to the acid-soluble components. The catalysed sintering of CaO and coalescence/fragmentation of Ca-containing minerals formed most of the PM1-10 (torrefied willow). The Ca in torrefied willow mainly exists in an acid-soluble form, which interfered with the effect of other elements. The coalescence/fragmentation of Si-containing minerals in torrefied wheat generated most of the PM1-10. After various washing procedures, the different forms of Ca/K were removed and the silicate decreased during combustion, leading to a weakness in coalescence. This led to the above-mentioned relationship. When the combustion atmosphere was switched to oxy-fuel mode, the great contribution of acid-soluble Ca to PM1-10 (torrefied willow)/water-soluble K to PM1 (torrefied wheat) still exists, indicating that they were less affected by the combustion conditions.

Microstructural characterization and gas-solid reduction kinetics of iron ore fines at high temperature

The detailed reaction mechanism with microstructural changes of iron ore fines in H2 atmosphere has been investigated using a high temperature drop tube furnace. With the model free method, the activation energy of 281 kJ/mol was obtained, indicating that solid-state diffusion of ions in lattice matrix of iron ore particle was the rate-controlling step. The analysis was therefore extended to obtain the microstructural information of the reduction product. It is found that dense product layers were formed and solid-state diffusion of iron and oxygen ions played a key role in the reduction process. The structure evolution of iron ore particles during the transformation from hematite to magnetite is the same as that from magnetite to wüstite, while the transformation from wüstite to metallic iron is clearly different. The microstructural changes and the elementary reaction steps during the two types of transformation are obtained and characterized.

Formation and O2/CO2 combustion characteristics of real-environment coal char in high-temperature oxy-fuel conditions

Oxygen-fuel combustion is a promising technology for CO2 emission reduction. The high-temperature entrained flow reactor and high-temperature drop tube furnace were used to analyses the formation and O2/CO2 combustion characteristics of real-environment coal char in high-temperature oxy-fuel conditions. It proposed “inflection point standard” of high-temperature flame method for the preparation of real-environmental oxy-fuel coal char according to the flame method. The results show that the ratios of C=O/C-O and C=O/Car increase in the coal char compared with the raw coals. The trend of C=O/Car in oxy-fuel condition is opposite to that in the inert atmosphere, due to the effect of high-concentration CO2. To achieve the burnout rate similar to air combustion for coal char, with the increase of coal rank, the O2 concentration should be enhanced. The optimal O2 concentration for the oxy-fuel combustion of JC anthracite is 30%, while that of other low-rank coals could be lower than 30%. The combustion characteristic of JC anthracite is with the highest sensitivity to temperature and O2 concentration.

Gas–Solid Reduction Behavior of In‐flight Fine Hematite Ore Particles by Hydrogen

The ironmaking processes that directly use the raw materials of iron ore fines have more advantages on the aspects of environmental protection and energy conservation. In this work, the gas–solid reduction behavior and kinetic mechanism of the in‐flight fine hematite particles in hydrogen atmosphere are studied with a High‐temperature Drop Tube Furnace (HDTF). The experiments are carried out at 1450–1550 K. The particle size is 53–64 µm and the reaction time is 0.65–1.93 s. With the morphological observation, it is found that the unreacted shrinking core model can describe the in‐flight reduction process of the fine hematite particles in hydrogen atmosphere. According to the analysis based on micro‐kinetics, the rate‐controlling step of the gas–solid reduction process is the solid‐state diffusion of oxygen ions through a dense product layer. This is attributed to the blocking of the tiny pores at high temperature or the generation of dense‐iron layer that nucleate and spread over the oxide. The blocking or the dense‐iron layer region prevent the reducing gas from direct contact with the reaction interface. The activation energies of the gas–solid reaction calculated by model‐fitting and model‐free approaches are 338 and 310 kJ mol−1, respectively.

Emission Behaviors of Inorganic Ultrafine Particles during Zhundong Coal Oxy-Fuel Combustion with Characterized Oxygen Input Fractions Comparable to Air Combustion

Zhundong low-rank coal is very likely to be utilized extensively in oxy-fired boilers in the near future. Its PM10 (particulate matter with an aerodynamic diameter of ≤10 μm) emission behaviors during oxy-fuel combustion need to be carefully studied before its large-scale use. The present study examines the emission behaviors of inorganic ultrafine particles (PM0.5, with an aerodynamic diameter of ≤0.5 μm), as well as PM10 during the combustion of Zhundong coal in air and oxy-fuel conditions (O2/CO2) at three characterized O2 input fractions, i.e., 21, 27 and 32 vol.%. The combustion experiments were carried out in a high-temperature drop-tube furnace (HDTF) at a combustion temperature of 1500 °C. The results show that PM0.5 is composed of Na, S, Mg and Ca, with total fractions of ~90%, while PM0.5–10 (with an aerodynamic diameter between 0.5 and 10 μm) predominantly contains Ca (~50–65%). At three characterized oxygen fractions during oxy-fuel combustion (OXY21, 27 and 32), the promoted O2 fraction was found to increase the yields of both PM0.5 and PM0.5–10. A higher particle-burning temperature and a lower CO2 fraction promote the reactions of both organically bound elements and inorganic minerals, increasing the partitioning of Mg and Ca and causing an increased yield of PM0.5. The yield of PM0.5 from air is high and similar to that from OXY32 while the yield of PM0.5–10 from air is similar to that from OXY27. The high yield of PM0.5 from air is mainly generated by the highest yields of Ca in four conditions.

Thermodynamic evaluation for reduction of iron oxide ore particles in a high temperature drop tube furnace

ABSTRACTMelting and reduction of fine iron ore particles in the gas environment of a HIsarna smelting cyclone is a critically important topic, but very limited information is currently available except for some experimental data from high temperature drop tube furnace (HTDF). This work discusses the equilibrium state of reacting iron ore in the HTDF environment by thermodynamic calculations to strengthen the understanding of the HIsarna process. The limit of reduction termination of the ore particles was estimated in the calculation for the thermal decomposition and topochemical gas reduction. The theoretical calculation results are compared with the experimental data from the previous studies. Furthermore, variation of slag composition and iron valence states were estimated theoretically to understand the effects of post combustion ratio value and hydrogen/carbon ratio on the equilibrium state of the ore particles in the reducing gas.

Reduction effect of co-firing of torrefied straw and bituminous coal on PM1 emission under both air and oxyfuel atmosphere

Straw sample was torrefied at 260 °C and 300 °C in N2, respectively, to prepare torrefied straw named as T-260 and T-300, and the reduction effect of co-firing straw or torrefied straw and steam coal on PM1 is investigated. The combustion experiments were conducted in a high temperature drop tube furnace (DTF) at 1400 °C to collect the inorganic PM10 for further analysis. Combustion atmosphere was air for all cases and 50% O2/50% CO2 (OXY50) for coal, T-260 and their blends of 1:1 and 4:1. The results show that all three biomass fuels show obvious emission reduction of PM with aerodynamic diameters of ≤ 0.3 µm (PM0.3) under both mix ratios. Reduction ratios of co-firing are overall higher at mix ratio of 1:1 than 4:1, and co-firing of T-260 or T-300 with coal shows higher reduction ratio than co-firing of straw. The higher ash content in torrefied straw leads to higher contents of alkali and alkaline earth metals (AAEM), which will further react with both Si and S during co-firing and coagulate into particles of larger sizes, leading to higher reduction ratios of PM0.3 and unconspicuous reduction effects in PM0.3–1 emitted from co-firing. During co-firing in oxyfuel atmosphere, a higher combustion temperature compared to air leads to an intensitive gasification, may resulting in effective and even higher reduction ratio in PM0.3.