Measurement of mass transfer between the bubble and dense phases in a fluidized bed combustor

An experimental study is described on mass transfer between the bubble and dense phases in a fluidized bed, used as a coke combustor. The experimental technique allowed quantification of the mass transfer rate during bubble formation and during a bubble’s rise through the bed. The combustion experiments were performed at 1 atm and 1223 K, in a fluidized bed (i.d. 120 mm) of sand (average diam. 325 μm) with static heights of 0.10–0.21 m. The bubbling flow rate ranged from 2.5 to 5.0 times that at incipient fluidization. The coke particles were 3.0 or 3.5 mm in diameter. Results indicate that the equivalent bed height, L_eq (the height a bubble must rise to transfer to the dense phase the same quantity of oxygen as during its formation) is independent of the bubbling air flow rate. The mean value L_eq = 50 mm suggests that for shallow beds the mass transferred during bubble formation is a significant part of the total mass transferred. The measured mass transfer factor between phases during a bubble’s rise (x′ = X/L_mf) is independent of the bubbling air flow rate and substantially lower than the theoretical predictions of Kunii and Levenspiel [1]. This disagreement is explained by the fact that the theoretical model is for an isolated bubble and does not account for the strong interaction between consecutive bubbles; this increases a bubble’s velocity and induces their coalescence, leading to a decrease in mass transferred between phases.     

Experimental investigation of the two-phase theory in a fluidized-bed combustor

Though the two-phase theory of fluidization is well-accepted, no direct experimental measurements of the different gas concentrations predicted to occur in bubble and particulate phases could be found in the literature. For the first time, theoretical predictions of these different gas concentrations have been validated experimentally, using a combined oxygen/bubble probe. Based on the two-phase theory, a mathematical model was developed for the combustion of a batch of char particles in a fluidized-bed combustor. The experimental oxygen concentration in the particulate phase as a function of time was well predicted by the model. Slight discrepancies for the bubble phase values were eliminated when low-oxygen-concentration bubbles were excluded from the data, attributed to some char combustion occurring in bubbles being contrary to the model assumption. The temperature difference between char and bed particles (ΔT) was the only adjustable parameter in the model. A value of 20°C fitted the burnoff times measured by visual observation of the top of the bed, for both 5 and 10 g char batch masses. Model predictions of the oxygen concentrations were not sensitive to ΔT during the first half of burnoff, when mass transfer controlled the combustion rate, so the mass transfer processes were predicted correctly by the model effectively with no adjustable parameter. The ΔT value of 20°C was significantly lower than experimental measurements of maximum burning char particle temperatures, reported to be 70°C for the small-diameter bed particles used in this work. The discrepancy was attributed to two factors: (i) the decrease in char particle temperature towards the end of the burnoff, when kinetics significantly affected the combustion rate; and (ii) a lower char particle temperature in the particulate phase than in the bubble phase, with experimental char particle temperature measurements biased towards the higher bubble phase values. It was inferred: (i) that the maximum values of ΔT measured experimentally are too high for calculation of the char particle combustion rate during the kinetic-controlled latter stage of burnoff and (ii) that reported values of the heat transfer coefficient from burning char particles to the particulate phase deduced from these particle temperature measurements may have been underestimated.     

Hydrogen impact on the shrinkage behaviors of wustite packed beds above 900 °C

The steel industry is facing increasing pressure and challenges from the environment in recent years. The urgent utilization of clean energy not only reduces greenhouse gas emissions, but also promotes future innovations in blast furnace iron making technology. Hydrogen (H₂) energy is considered to be one of the most promising alternatives to carbon-based fossil energy for the reduction of iron oxides. Therefore, the gaseous reduction of iron oxides by H₂ has been intensively studied for decades. However, the impact of H₂ on the shrinkage behavior of iron oxide packed beds above 900 °C has rarely been studied, and the interaction between H₂ and carbon monoxide (CO) in the shrinking process is not fully understood. Therefore, this study uses H₂, CO and H₂+CO mixture gas for the well-designed high temperature experiments of wustite (FeO) packed beds. The results show that H₂ protects the coke from further damage in the packed bed at 900–1000 °C, and the corresponding shrinkage rate (SR) decreases from 0.31%/°C for CO case to 0.16%/°C. Meanwhile, when the temperature exceeds 1350 °C, the packed bed under the CO atmosphere accelerates shrinkage due to the melting and dripping of the metallic iron after carbonization. By contrast, when CO is replaced by H₂, the carbonization process is controlled by the solid state diffusion of coke carbon rather than the reverse Boudouard reaction. As a result, the lower carbonization efficiency not only increases the transition temperature by up to 100 °C, but also reduces the weight of the melted hot metal by one third, which significantly improves the bed permeability.     

18O/16O isotope exchange for yttria stabilised zirconia in dry and humid oxygen

The oxygen surface kinetics and mechanism of oxygen interaction between oxygen from the gas phase and yttria-stabilised zirconia nano-sized powder have been studied by pulse ¹⁸O/¹⁶O isotope exchange (PIE) in dry (O₂) and humid (mixture O₂ + H₂O with pH₂O = 2.6 kPa at T = 22 °C) oxygen atmospheres in comparison with micro-sized powder. Dependences of the heterogeneous oxygen exchange rate (r_H) in the temperature range from 550 to 900 °C, and oxygen partial pressure range in the carrier gas and pulses of 5–19% have been obtained. It has been established that the presence of water causes a decrease in the heterogeneous oxygen exchange rate due to the presence of strongly bound hydroxyl groups in the nanopores of the sample. Differences between the mechanisms of isotopic exchange for dry and humid atmospheres have been found in this temperature range (550–700 °C). The observed differences are associated with the interaction of the gas phase and hydroxyls in the adsorbed layer of the oxide. An original method for the separation of the contributions of three types of oxygen exchange for dry and humid atmospheres has been proposed.     

Phase development in ZnO varistors

Abstract

Based on a typical ZnO varistor composition (97·0 mol.-% ZnO, 1·0 mol.-% Bi₂O₃, 1·0 mol.-% Sb₂O₃, 0·5 mol.-% MnO and 0·5 mol.-% Co₃O₄), phase development of the ZnO varistor during sintering has been investigated using in situ high temperature X-ray diffraction up to 900°C, and conventional ambient X-ray diffraction for samples sintered at 900°C to 1250°C. The results indicate that α-Bi₂O₃ can be detected until 700°C; the pyrochlore phase can be detected in the samples heat treated at 700°C and up to 1250°C; the spinel phase is present at and >900°C. However, the main phases in the varistor are established by 950°C. By this temperature, the essential microstructure features are formed, and the varistors exhibit non-linear electrical properties, with a non-linear coefficient α of 35 and breakdown field of 8000 V cm^?1. With increasing sintering temperature, both the α value and breakdown field decrease.     

Enhancing the oxygen reduction activity of PrBaCo2O5+δ double perovskite cathode by tailoring the calcination temperatures

In this study, the oxygen reduction activity of PrBaCo₂O_5+δ (PBC) double perovskite is remarkably enhanced by rationally tuning the calcination temperatures of the cathode precursor for solid oxide fuel cells (SOFCs). Effects of the calcination temperatures on the phase structure, microstructure, surface area and oxygen reduction reaction (ORR) activity of PBC cathode is systematically investigated. The cathode with optimized calcination temperature (900 °C, PBC-900) shows excellent activity and stability for ORR at 600 °C in terms of area specific resistances (ASRs). A distinctive low ASR of 0.068 Ω cm² is obtained at 600 °C for PBC-900, which is 92.6%, 34.6% and 15.0% lower than PBC-800, PBC-1000 and PBC-1100, respectively. After operating for 250 h in air at 600 °C, the ASR value of PBC-900 is not significantly reduced. Furthermore, a single cell with PBC-900 cathode delivers attractive peak power density of 1.60 W cm⁻² at 600 °C. The present study suggests that the ORR activity of PBC cathode can be greatly boosted by rationally tailoring the calcination temperatures, which may bring new avenue for the design of highly active cathodes for SOFCs.     

Topotactic transformation of (T-T’) La1.8Nd0.2CuO4: Synthesis,structure, electrical properties and oxygen diffusion pathways simulation

The starting sample T-La_1.8Nd_0.2CuO₄ (SG: Bmab) has been synthesized by the conventional solid-state reaction at 900 °C. The T′-La_1.8Nd_0.2CuO₄ (SG: I4/mmm) sample has been obtained via a topotactic reduction reaction of T-La_1.8Nd_0.2CuO₄ (SG: Bmab) using CaH₂ (as reductor) followed by an oxidation at 400 °C in air. The temperature of the phase transition (T-T′) has been determined using the differential thermal analysis (DTA) and the temperature-dependent powder X-ray diffraction (TDXD). The crystal structures of the title compounds have been refined by using the Rietveld method and confirmed by the means of the charge distribution model (CHARDI). The electrical properties of the title compounds have been studied by impedance complex spectroscopy between 200 °C and 650 °C. This study shows two slopes in the Arrhenius plot with experimental activation energies 1.033 eV and 1.657 eV which correspond to the reduced phase and the T′ phase respectively. The simulation of oxygen diffusion in structures by using the Bond Valence Site Energy (BVSE) method shows three-dimensional pathways of oxygen diffusion. The calculated activation energies of the T′ and T structure are 1.619 and 2.369 eV, respectively.     

Thermophilic versus mesophilic dark fermentation in xylose-fed fluidised bed reactors: Biohydrogen production and active microbial community

Dark fermentative biohydrogen production in a thermophilic, xylose-fed (50 mM) fluidised bed reactor (FBR) was evaluated in the temperature range 55–70 °C with 5-degree increments and compared with a mesophilic FBR operated constantly at 37 °C. A significantly higher (p = 0.05) H₂ yield was obtained in the thermophilic FBR, which stabilised at about 1.2 mol H₂ mol^?1 xylose (36% of the theoretical maximum) at 55 and 70 °C, and at 0.8 mol H₂ mol^?1 xylose at 60 and 65 °C, compared to the mesophilic FBR (0.5 mol H₂ mol^?1 xylose). High-throughput sequencing of the reverse-transcribed 16S rRNA, done for the first time on biohydrogen producing reactors, indicated that Thermoanaerobacterium was the prevalent active microorganism in the thermophilic FBR, regardless of the operating temperature. The active microbial community in the mesophilic FBR was mainly composed of Clostridium and Ruminiclostridium at 37 °C. Thermophilic dark fermentation was shown to be suitable for treatment of high temperature, xylose-containing wastewaters, as it resulted in a higher energy output compared to the mesophilic counterpart.     

Emissions of organic hazardous air pollutants during Chinese coal combustion

This paper provides a general investigation of the emissions of organic hazardous air pollutants (HAPs) during the combustion of several typical Chinese coals. First, the distribution of four types of HAP, i.e., aliphatics, cyclic hydrocarbons, monoaromatic compounds and PAHs, in the CH₂Cl₂ extracts of six Chinese coals were studied and the influences of the extractive times and coal varieties were also evaluated. Second, the partitioning of these HAPs in the flue gas during coal combustion in a small-scale reactor were investigated, depending on oven temperatures (500 °C, 600 °C, 700 °C, 800 °C, 900 °C) and coal varieties. The behaviors of HAP in the combustion flue gas were compared with those in the CH₂Cl₂ extracts. Finally, combustion was conducted at given conditions in two laboratory-scale reactors: a fluidized bed and a fixed bed. Two coals (Shengmu bituminous coal and Xunhuan anthracite coal) and one coke were considered in this case. The HAP partitioning both in flue gases and in ashes were evaluated and compared between the two combustors.     

Electrochemical performance and stability of nano-structured Co/PdO-co-impregnated Y2O3 stabilized ZrO2 cathode for intermediate temperature solid oxide fuel cells

Palladium (Pd) is an attractive cathode catalyst component for solid oxide fuel cells (SOFCs) that has high tendency to agglomerate during operation at around 800 °C. This work shows that such agglomeration can be inhibited by alloying Co into Pd. PdO, Pd_0.95Co_0.05O, Pd_0.90Co_0.10O, and Pd_0.80Co_0.20O were synthesized and characterized. Powder X-ray diffraction patterns at 750 and 900 °C confirmed that PdO decomposition to Pd which normally occurred at 840 °C was suppressed for Co containing Pd alloys while thermal gravimetric analyses indicated improved redox reversibility of PdO ? Pd conversion for alloys during the thermal cycling between 600 and 900 °C. Scanning electron microscopy images supported these arguments. Pd_0.90Co_0.10+yttria stabilized zirconia (YSZ) electrode (i.e., 10 mol % Co containing PdO-impregnated YSZ electrode) displayed the highest oxygen reduction reaction (ORR) performance and stability. The polarization resistance for ORR on Pd_0.90Co_0.10+YSZ cathode is only 0.088 Ω cm² at 750 °C. During polarization test at 750 °C, Pd_0.90Co_0.10+YSZ cathode showed stable performance for 30 h while the performance of Pd+YSZ cathode degraded after 10 h.     

Mn-rich SmBaCo0.5Mn1.5O5+δ double perovskite cathode material for SOFCs

SmBaCo_0.5Mn_1.5O_5+δ oxide with Sm-Ba cation-ordered perovskite-type structure is synthesized and examined in relation to whole RBaCo_0.5Mn_1.5O_5+δ series (R: selected rare earth elements). Presence of Sm and 3:1 ratio of Mn to Co allows to balance physicochemical properties of the composition, with moderate thermal expansion coefficient value of 18.70(1)·10⁻⁶ K⁻¹ in 300–900 °C range, high concentration of disordered oxygen vacancies in 600–900 °C range (δ = 0.16 at 900 °C), and good transport properties with electrical conductivity reaching 33 S cm⁻¹ at 900 °C in air. Consequently, the compound enables to manufacture catalytically-active cathode, with good electrochemical performance measured for the electrolyte-supported laboratory-scale solid oxide fuel cell with Ni-Gd_1.9Ce_0.1O_2-δ|La_0.4Ce_0.6O_2-δ|La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-δ|SmBaCo_0.5Mn_1.5O_5+δ configuration, for which 1060 mW cm⁻² power density is observed at 900 °C. Furthermore, the tested symmetrical SmBaCo_0.5Mn_1.5O_5+δ|La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-δ|SmBaCo_0.5Mn_1.5O_5+δ cell delivers 377 mW cm⁻² power density at 850 °C, which is a promising result.     

Steam reforming of hot gas from gasified wood types and miscanthus biomass

The reforming of hot gas generated from biomass gasification and high temperature gas filtration was studied in order to reach the goal of the CHRISGAS project: a 60% of synthesis gas (as x(H₂)+ x(CO) on a N₂ and dry basis) in the exit gas, which can be converted either into H₂ or fuels. A Ni-MgAl₂O₄ commercial-like catalyst was tested downstream the gasification of clean wood made of saw dust, waste wood and miscanthus as herbaceous biomass. The effect of the temperature and contact time on the hydrocarbon conversion as well as the characterization of the used catalysts was studied. Low (<600 °C), medium (750°C–900 °C) and high temperature (900°C–1050 °C) tests were carried out in order to study, respectively, the tar cracking, the lowest operating reformer temperature for clean biomass, the methane conversion achievable as function of the temperature and the catalyst deactivation. The results demonstrate the possibility to produce an enriched syngas by the upgrading of the gasification stream of woody biomass with low sulphur content. However, for miscanthusthe development of catalysts with an enhanced resistance to sulphur poison will be the key point in the process development.     

Generalized Prediction of Maximum Heat Transfer to Single Cylinders and Spheres in Gas-Fluidized Bed

A simple dimensionless correlation is presented and compared with data from 35 experimental studies. Data include spheres, horizontal and vertical cylinders, cylinder diameters from 0.13 to 220 mm, particle diameters from 104 to 15,000 μm, bed temperatures up to 900° C, and bed pressures up to 9.25 bars. Fluidizing gases include air, helium, CO₂, H₂, and R-12. Particle densities range from 1986 to 11,340 kg/m³ and p_sC_s from 1474 to 4173 kJ/m² °C. A total of 142 data points are correlated with a mean deviation of 17%. Complete tabulations are provided for all data analyzed. Application to tube bundles is discussed briefly.     

Hydrogen gas yield and trace pollutant emission evaluation in automotive shredder residue (ASR) gasification using prepared oyster shell catalyst

This work examines the hydrogen gas yield and trace pollutants partitioning in automobile shredder residue (ASR) catalytic gasification by fixed bed and fluidized bed gasifier with controlling at equilibrium ratio (ER) 0.2, temperature 900 °C, and 5%–15% prepared catalyst addition. Oyster shell (OS) is a valuable resource due to its higher calcium content that it could prepare as a catalyst for enhancing the hydrogen production in ASR gasification. In the case of the fixed bed gasifier experiments, the highest lower heating value (LHV) and syngas production were found at 900 °C and 10% OS catalyst addition. The maximum H₂ and CO composition were 6.57% and 5.97%, respectively. The LHV of syngas was approximately 4.43 MJ/Nm³. The fluidized bed gasifier could provide a good ASR decomposition and heat transfer behavior. The syngas yield results indicated the maximum H₂ and CO composition were 12.12% and 10.59%, respectively. It was obviously showed that the syngas production and energy conversion efficiency were enhanced by applying fluidized bed gasifier. The maximum produced gas LHV was 10.77 MJ/Nm³ as well as the cold gas efficiency (CGE) of produced gas was 71.62%. On the other hand, the volatile sulfur and chlorine speciation formed in ASR gasification were mainly partitioned in the solid and/or liquid phase. It implied that tested OS catalysts could inhibit the volatile sulfur and chlorine speciation emission in the produced gas as well as enhance the produced gas quality. In summary, this research could provide basic insight into enhanced syngas production and quality in ASR catalytic gasification using the prepared OS catalyst.     

Effect of temperature on the gasification of olive bagasse particles

A semi-batch fluidized-bed gasifier was used to investigate the experimental gasification process of olive bagasse particles, an olive oil industry residue. The effect of bed temperature was studied, in the range of 750 to 900 °C. The oxidant agent was air, fed at constant flowrate, and sand particles were used as bed material. The bagasse particles used had diameters within 1.25–2 mm and the biomass was characterized in terms of its higher heating value and ultimate analysis. During each run, several gaseous samples were collected to be further analysed by gas chromatography allowing the quantification of CO, CO₂, H₂, CH₄, O₂ and N₂. The reaction mechanism of the gasification process is determinant on the composition of the producer gas. Experimental results showed that higher bed temperatures favoured gas production as well as other gasification performance parameters. Best results were obtained for a bed temperature of 850 °C.     

Steam methane reforming on LaNiO3 perovskite-type oxide for syngas production,activity tests,and kinetic modeling

The kinetic experiments at various working conditions (ie, temperature: 500°C-900°C, pressure: 1-15 bar, H₂O/CH₄ ratio [S/C ratio] of 1-2.5, and gas hourly space velocity of 900-1700 1/h) in the presence of LaNiO₃ perovskite-type oxide were done to consider and assess the outcome of steam methane reforming (SMR) and to build up its kinetic models depending on Langmuir-Hinshelwood method in a fixed bed reactor. The outcomes demonstrate, the methane conversion, H₂ and CO yields and formed CO₂ are affected by the working parameters. Elevated temperature is profitable for more methane conversion, H₂ and CO yield. While high temperature has a negative effect on mol% of CO₂ in outlet products. The high working pressure will not profit SMR respect to CH₄ conversion and products distribution. The efficacy of S/C ratio on the CH₄ conversion and CO yield depended on temperature range. H₂ yield considerably diminishes with an increment in S/C ratio, while the trend was reverse for CO₂ value in outlet gas. The accuracy of suggested kinetic model was evaluated by correlation and statistical tests. Outcomes exhibited that the obtained data were well anticipated through the suggested model, owning to presumption of nonideal gas phase and by utilizing reasonable equation of state of PPR78.     

Experimental characterization and performance study of an ammonia–water–hydrogen refrigerator

We present in this paper an experimental study of a commercial diffusion-absorption refrigeration machine (DAR) operating on the Platen and Munters cycle. The temperatures at the inlet and outlet of every component of the machine, as well as the cabinet and ambient temperature are measured continuously. The tests are repeated for various electric power inputs to the refrigerator. The global heat transfer coefficient of the cabinet (UA)_cab is determined using both theoretical and experimental methods. This coefficient is found equal to 0.2 W/°C. The global heat transfer coefficient of the evaporator (UA)_evap is deduced using dynamic and steady state methods. This global heat transfer coefficient (UA)_evap is found equal to 0.3 W/°C. Finally the cooling capacity of the unit and the coefficient of performance are evaluated. The heating power supply to the generator necessary to ensure the desired state of this machine is found to be in the range of 35 W–45 W.     

Mo-promoted Ni/Al2O3 catalyst for dry reforming of methane

Mo-promoted alumina supported Ni catalysts were prepared through a conventional impregnation method and tested in dry reforming of methane (DRM) at temperatures from 550 to 850 °C. The catalysts were characterized by means of H₂-temperature programmed reduction (H₂-TPR), CO₂-temperature programmed desorption (CO₂-TPD), X-ray diffraction (XRD), N₂ physisorption and Raman spectroscopy. Mo-promotion caused a reduction in the DRM catalytic activity. The weaker interaction between NiO species and the alumina support, the formation of a MoNi₄ phase, and the lower basicity of this Ni-Mo/Al₂O₃ catalyst were identified as the main causes for its lower activity. However, pre-reducing the Ni-Mo/Al₂O₃ catalyst at temperatures lower than 700 °C, instead of 900 °C, resulted in a considerable increase of its catalytic activity. This was mainly due to the formation of a separate Ni⁰ phase that did not interact with Mo and to an increase in medium strength basicity.     

Investigation of Indonesian low rank coals gasification in a fixed bed reactor with K2CO3 catalyst loading

Catalytic steam gasification is considered one of promising technologies for converting solid carbonaceous feedstocks into hydrogen-rich syngas, which is an important source of hydrogen for various industrial sectors. The K₂CO₃-catalyzed steam gasification of low rank coals (LRCs) was conducted in a fixed bed reactor for elucidating the effects of gasifying temperature and catalyst loading amount on hydrogen yield. Hydrogen-rich syngas can be obtained at gasifying temperature of 800 °C and loading amount of 10 wt% K₂CO₃. The loading amount of 10 wt% K₂CO₃ was the saturation point and provided a good gasification reactivity in catalytic steam gasification of three LRCs. The experimental data of these three LRCs were well described by the random pore model (RPM). The RPM fitted the experimental data at 800 °C better than the experimental data obtained at 700 °C and 600 °C. Reactivity index (R_0.5), activation energy (Ea) and reaction rate constant (k) were also used to predict the characteristics of the K₂CO₃-catalyzed steam gasification process. Catalytic steam gasification utilizing the mixture of three LRCs as a feedstock was also investigated and displayed X_C of 86.22% and 0.95 mol mol^?1-C, indicating a good feasibility and potential industrial applications.     

Chromium evaporation and oxidation characteristics of alumina-forming austenitic stainless steels for balance of plant applications in solid oxide fuel cells

The chromium (Cr) evaporation behavior of several different types of iron (Fe)-based AFA alloys and benchmark Cr₂O₃-forming Fe-based 310 and Ni-based 625 alloys was investigated for 500 h exposures at 800 °C to 900 °C in air with 10% H₂O. The Cr evaporation rates from alumina-forming austenitic (AFA) alloys were ~5 to 35 times lower than that of the Cr₂O₃-forming alloys depending on alloy and temperature. The Cr evaporation behavior was correlated with extensive characterization of the chemistry and microstructure of the oxide scales, which also revealed a degree of quartz tube Si contamination during the test. Long-term oxidation kinetics were also assessed at 800 to 1000 °C for up to 10,000 h in air with 10% H₂O to provide further guidance for SOFC BOP component alloy selection.