Higher Bed Temperature Research Articles

NOx and N2O emissions from Ca-rich fuel conversion in oxyfuel circulating fluidized bed combustion

A part of Estonia’s power generation is based on oil shale combustion in thermal power plants, resulting in high CO2 and pollutant emissions. The presented paper is a part of long-term experimental work to investigate the combustion of Ca-rich oil shale for air and oxyfuel combustion environments. The experiments were performed in a 60 kWth circulating fluidized bed (CFB) test facility under air, O2/CO2, and O2 with the recycled flue gas (RFG) modes of inlet O2 as vol.% ranging from 21% to 52%, and with RFG of 50% and 87%. The influence of different combustion atmospheres, excess oxygen volumetric ratios in the primary oxidizer (λPr), and dense bed temperatures (TDB), on the total NOx and N2O formations, have been studied in detail.The results show that specific concentrations of NOx (per MJ) were reduced by 14% in O2/CO2 mode, and 22% in O2/RFG compared to air combustion. N2O emissions were increased in OXY21 mode and reduced significantly from 20 mg/MJ to 4 mg/MJ with increasing inlet O2% to 52% under O2/CO2. NOx and N2O emissions were the lowest of all combustion experiments at high inlet O2% with RFG application (OXY87 + RFG). Under all tested atmospheres, NOx emissions were increasing with increasing excess oxygen in the primary oxidizer. N2O enhanced with excess oxygen, accounted for the homogenous gas-phase reaction of volatile-N and the heterogenous reaction of char-N. NOx emissions at low operating TDB were decreasing with increasing bed temperatures and increased at higher TDB > 770 °C in air combustion and TDB > 850 °C in OXY30. N2O emissions were slightly decreased at higher bed temperatures under air and O2/CO2 modes. Combustion efficiency enhanced at higher inlet O2% resulting in lower CO concentrations with reduced fuel burnout, and increased NOx and N2O emissions.Overall, NOx and N2O formations were more affected by the condition of operating parameters under both air and oxyfuel combustion experiments, and the obtained results are giving confidence that oxyfuel technology does not influence the release of nitrogen emissions from oil shale combustion. To lower the cost of CO2 impurities removal, the optimal operating conditions during oil shale CFB combustion can be confirmed after considering the discussed parameters. However, further CO2 purification resulting from oil shale oxyfuel flue gas stream is possibly required.

Investigation of the Interfacial Adhesion Strength of Parts Additively Manufactured on Fabrics

Abstract This research first presents a method of peel testing developed by the researchers to characterize the strength of the interface between fabric and additively manufactured material. Experimentation is next presented that characterizes the interfacial strength relative to a set of parameters which include fabric fiber morphology, thickness of sizing applied to fabric, 3D printer bed temperature, and angle of additive manufacturing relative to the fabric warp direction. The interface strength within the parameter space presented was then searched and found to have a maximum of 5.18 N/mm using a novel set of parameters. This interface strength indicates the method of additive manufacturing direction on fabric may be suitable for use in a broader range of applications than previously proven feasible. Relatively rough, thick, and loose weave fabrics were found to promote interface strength compared to smoother, thinner, and finer woven fabrics. Relatively higher bed temperatures also promoted higher interface strength. Sizings on the fabric were found to promote interface strength with relatively smooth, thin, or fine fabrics which do not themselves promote high mechanical interlocking. Using these research findings, interface strength between fabric and additively manufactured material can be modified to suit the application.

An improved CFD-DEM modelling of raceway dynamics and coke combustion in an industrial-scale blast furnace

The raceway dynamics and combustion by lateral injection of high-temperature gas into a packed bed of solid fuel are widely practised in various industries such as ironmaking blast furnace (BF), yet the particle-scale simulation and understanding were hindered by low resolution and huge computational cost. In this work, an advanced reactive computational fluid dynamics-discrete element method (rCFD-DEM) featuring two state-of-the-art techniques - coarse-grained method and smoothing method is developed to describe raceway dynamics and coke combustion in a three-dimensional (3D) industrial-scale BF. The model is validated against the measurements in the open literature. The simulation results show that the smoothing method can more reliably capture raceway morphology and the coarse-grained treatment reduces 78.14 % computational costs under the given conditions. The typical phenomena are illustrated in terms of raceway shape and size, temperature field and gas species distributions. Then, the effect of the key operating parameters is quantified in terms of raceway size, gas temperature and species distributions. The comparisons indicate increasing the blast rate from 2.78 Nm3/s to 3.05 Nm3/s and oxygen mass fraction from 0.17 to 0.27, the raceway volume expands by 61.11 % and 47.62 %, respectively; moreover, lower blast rate, higher bed temperature and higher oxygen mass fraction lead to higher temperature distribution and higher CO concentration. The present work provides a cost-effective tool for understanding and optimizing raceway operation in practical BFs.

Experimental study of liquid vaporization in a fluidized bed

AbstractLiquid injection into a gas–solid fluidized bed has been applied in various industries, such as coating and granulation processes in pharmaceutical and food industries, reactor cooling in polyolefin production, fluid catalytic cracking, and fluid coking in the petroleum industry. A new experimental method has been successfully developed to monitor the vaporization rate of a liquid injected into a fluidized bed. In addition, it can be used to determine the mass of liquid accumulated in the bed at a steady state. With this new method, measurements have identified three phenomena that may increase the amount of liquid accumulated at steady state in a fluidized bed. (1) Gas mixing affects vaporization when the bed temperature is lower than the liquid boiling point. In liquid‐rich regions of the bed, local vapour may build up, limiting the vaporization rate. Consequently, suitable emulsion to bubble gas transfer reduces the amount of accumulated liquid. (2) Solids mixing: hot particles from the rest of the bed must mix with the wetted particles to provide enough heat for vaporization. Good solids mixing reduces the amount of accumulated liquid. (3) Wet agglomerates formation: liquid trapped within wet agglomerates takes much longer to vaporize. The amount of accumulated liquid can be reduced by injecting the liquid in a well‐agitated bed region, operating at a higher bed temperature, or increasing the flowrate of atomization gas.

Simulation on the Electric and Thermal Fields of a Microwave Reactor for Ex Situ Biomass Tar Elimination

Microwave treatment is an emerging technique for biomass tar elimination. The electric and thermal fields of the microwave reactor are the key to high elimination efficiency and energy utilization. In this work, we simulated the electric and thermal fields of a microwave reactor with various parameters including irradiation feed position, microwave power, silicon carbide length and flow velocity. Results show that the irradiation feed position that locates 5 mm vertically to the central plane can obtain the highest electric intensity and silicon carbide temperature (ca. 1100 K) after wave absorbing. Both the electric and thermal fields are strengthened when microwave power is increased. Extending the silicon carbide bed length will decrease the bed temperature and heating rate. A high flow velocity leads to non-uniform temperature distribution of the silicon carbide. For the purpose of achieving a high microwave energy utilization and uniform bed temperature, suitable irradiation feed position (zi = 5 mm), high microwave power (P = 1000 W), short silicon carbide bed length (lSiC = 100 mm) and low flow velocity (v = 0.02 m/s) are preferred, but the chemical kinetics of biomass tar elimination should also be considered in the practical application.

Numerical analysis of heat and mass transfer during hydrogen absorption in metal hydride beds with a novel peridynamic model

Numerical modelling and study of metal hydride bed with heat and mass transfer is a key to identify and predict the state of the storage device during absorption and desorption process. It is a great challenge to numerically study this problem considering cracks and volumetric expansion for conventional approaches due their limitations in treating discontinuities and large deformations. To effectively predict the hydrogen charging performance with the influences of cracks and volumetric expansion, a novel peridynamic (PD) model considering hydrogen absorption was proposed, which has not been used in most existing studies. The developed PD model was applied to simulate metal hydride reaction including depleted uranium (DU) and zirconium cobalt (ZrCo) in a thin-layer bed. The simulation results agreed well with both existing numerical solutions and experimental observations. The cracks were introduced to identify their influences on the performance of hydrogen absorption in a DU bed as a prototype. It is demonstrated that crack length and location had a significant influence on the absorption rate and average temperature evolution. With the increase in the crack length, the hydrogen absorption efficiency of the metal hydride bed (MHB) decreases dramatically due to the high bed temperature. The crack approaching the cooling boundary would greatly affect the temperature evolution, which causes the average H/M value to reach saturation later. Finally, the effects of the volumetric expansion were taken into consideration in the present PD model. Unlike ZrCo, the hydrogen kinetics performance would be overrated due to the neglection of the volumetric expansion in a DU bed.

Principal component analysis and partial least square regression models to understand sorption-enhanced biomass gasification

Gasification represents a potential technology for the conversion of biomass into usable energy. The influence of the main gasification parameters, i.e. the type of biomass used and its composition, as well as the composition of the outlet gas, were studied by a multivariate statistical analysis based on principal component analysis (PCA) and partial least square (PLS) regression models in order to identify the main correlations between them and to the contents of methane, ethylene and tar in the outlet gas. In this work, the experimental data used as input for the multivariate statistical analysis came from a TRL-4 gasification plant running under sorption enhanced conditions, i.e. using steam as the gasifying agent and CaO as the bed material. The composition of the biomass feed played an important role in the quality of the outlet gas composition. In fact, biomasses with high ash and sulphur contents (municipal solid waste) increased ethylene content, while those with high-volatile matter content and fixed C content (wood pellets, straw pellets and grape seeds) mainly increased CO and CO2 formation. By increasing the gasification bed temperature and the CaO/C ratio, it was possible to reduce the methane and the collected tar contents in the outlet gas. Other light hydrocarbons could also be reduced by controlling the Treactor and TFB. Methane, ethylene and tar contents were modelled, cross-validated and tested with a new set of samples by PLS obtaining results with an average overall error between 8 and 26%. The statistically significant variables to predict methane and ethylene content were positively associated to the thermal input and negatively to the CaO/C ratio. The biomass composition was also remarkable for both variables, as mentioned in the PCA analysis. As far as the tar content, which is undesirable in all gasification processes, the decrease in the tar content was favoured by high bed temperature, low thermal input and biomass with high-volatile matter content. In order to produce an outlet gas with adequate quality (e.g. low tar content), a compromise should be found to balance average bed temperature, sorbent-to-mass ratio, and ultimate and proximate analyses of the biomass feed.Graphical abstract

High efficient thermochemical energy storage of methane reforming with carbon dioxide in cavity reactor with novel catalyst bed under concentrated sun simulator

Thermochemical energy storage performance of methane reforming with carbon dioxide in cavity reactor under concentrated sun simulator has been experimentally and numerically studied. Novel catalyst bed with Ni/Al2O3 particles and perforated quartz encapsulation is proposed to perform high bed temperature for greenhouse effect, and then chemical energy storage efficiency and total energy utilization efficiency of experimental system can respectively reach as high as 41.1% and 80.3%. As concentrated heat flux increases or reactant flow rate decreases, methane conversion rises with bed temperature increasing, while chemical energy storage efficiency first rises and then falls for larger heat loss. As inlet mole fraction of methane increases, chemical energy storage efficiency firstly increases and then drops. Numerical model of cavity reactor under concentrated heat flux is established and validated. In catalyst bed, maximum reaction rate occurs near focal point, while reverse reaction occurs near its side. With the improvement of porosity in catalyst bed, methane conversion and thermochemical storage efficiency firstly increase and then decrease, while they gradually increase with heat conductivity of bed rising. In addition, optimal concentrated heat flux, flow rate, mole fraction of methane (0.5) and bed porosity (0.6) are derived for maximum chemical energy storage efficiency.

Modified Polylactic Acid with Improved Impact Resistance in the Presence of a Thermoplastic Elastomer and the Influence of Fused Filament Fabrication on Its Physical Properties

The standard polylactic acid (PLA), as a biodegradable thermoplastic polymer, is commonly used in various industrial sectors, food, and medical fields. Unfortunately, it is characterized by a low elongation at break and low impact energy. In this study, a thermoplastic copolyester elastomer (TPCE) was added at different weight ratios to improve the impact resistance of PLA. DSC analysis revealed that the two polymers were immiscible. A good balance of impact resistance and rigidity was reached using the formulation that was composed of 80% PLA and 20% TPCE, with an elongation at break of 155% compared to 4% for neat PLA. This new formulation was selected to be tested in a fused filament fabrication process. The influence of the nozzle and bed temperatures as printing parameters on the mechanical and thermal properties was explored. Better impact resistance was observed with the increase in the two thermal printing parameters. The crystallinity degree was not influenced by the variation in the nozzle temperature. However, it was increased at higher bed temperatures. Tomographic observations showed an anisotropic distribution of the porosity, where it was mostly present between the adjacent printed filaments and it was reduced with the increase in the nozzle and bed temperatures.

Performance of Ti6Al4V fabricated by electron beam and laser hybrid preheating and selective melting strategy

Electron beam selective melting (EBM) and selective laser melting (SLM) are regarded as significant manufacturing processes for near-net-shaped Ti6Al4V components. Generally, in the conventional EBM process, preheating is necessitated to avoid “smoke” caused by the charging of electrons. In the conventional SLM process, laser as an energy source without the risk of “smoke” can be employed to melt metal powder at low temperatures. However, because of the low absorption rate of laser, the powder bed temperature cannot reach a high level. It is difficult to obtain as-built TiAl4V with favorable comprehensive properties via conventional EBM or SLM. Hence, two types of electron beam and laser hybrid preheating (EB-LHP) combined with selective melting strategies are proposed. Using laser to preheat powder allows EBM to be performed at a low powder bed temperature (EBM-LT), whereas using an electron beam to preheat powder allows SLM to be performed at a high powder bed temperature (SLM-HT). Ti6Al4V samples are fabricated using two different manufacturing strategies (i.e., EBM-LT and SLM-HT) and two conventional processes, i.e., EBM at a high powder bed temperature (EBM-HT) and SLM at a low powder bed temperature (SLM-LT). The temperature-dependent surface quality, microstructure, density, and mechanical properties of the as-built Ti6Al4V samples are characterized and compared. Results show that EBM-LT Ti6Al4V exhibits a higher ultimate tensile strength (981±43 MPa) and a lower elongation (12.2%±2.3%) than EBM-HT Ti6Al4V owing to the presence of α′ martensite. The SLM-HT Ti6Al4V possesses the highest ultimate tensile strength (1,059±62 MPa) and an elongation (14.8%±4.0%) comparable to that of the EBM-HT Ti6Al4V (16.6%±1.2%).

Optimization of fused deposition modeling process parameters using the Taguchi method to improve the tensile properties of 3D-printed polyether ether ketone

The interesting mechanical properties of polyether ether ketone give the material a place among the foremost competitors when it comes to replacing metal. Fused deposition modeling has been recognized as an alternative method to process polyether ether ketone parts. In this study, the effect of different process parameters such as nozzle, bed, and radiant temperatures as well as printing speed and layer thickness on the tensile properties of three-dimensional printed polyether ether ketone was investigated. The optimization of the tensile properties of PEEK were studied by performing a reduced number of experiments, using the experimental design method based on the Taguchi approach which limits the number of experiments to 8 instead of 32. Results showed that a decent Young’s modulus was found by setting the nozzle temperature, print speed, and bed temperatures to their high levels and by setting the layer thickness and radiant temperature to their low level. Using these parameters, a Young’s modulus of 3.5 GPa was obtained, which represents 87.5% of the value indicated in the technical sheet. With these settings, we also found a tensile strength of 45.5 MPa, which corresponds to 46.4% of the value given by the studied polyether ether ketone material. A scanning electron microscopic investigation of the porosity and interlayer adhesion, confirmed that a higher bed temperature also tended to promote adhesion between layers.

CFD-DEM modelling of raceway dynamics and coke combustion in an ironmaking blast furnace

In this study, a CFD-DEM reacting model featuring thermochemical sub-models is developed for describing gas–solid reacting flow related to raceway dynamics and coke combustion in an ironmaking blast furnace (BF). The simulation results show that a ring-type zone is formed around the cavity where the coke combustion and gasification occur with a large amount of heat released, leading to intense momentum and thermochemical exchanges in this zone. Heat fluxes contributed from radiation and reaction dominate total heat transfer during the coke combustion, while conductive heat transfer is insignificant. Then, the impacts of key operating parameters on raceway evolution are quantified. The simulation results indicate increasing the blast gas velocity from 210 m/s to 230 m/s and oxygen mass fraction from 0.17 to 0.27, the cavity depth is increased by 42.3% and 20.8%, respectively; lower inlet velocity, higher bed temperature, higher oxygen mass fraction leads to more CO production and the resulting better BF performance.

Numerical investigation of MSW combustion influenced by air preheating in a full-scale moving grate incinerator

Air preheating is generally used to improve moisture evaporation and boiler efficiency in waste incineration systems. This study employs FLIC-FLUENT coupling procedure to investigate the influence of primary air preheating on municipal solid waste (MSW) combustion characteristics in a moving grate incinerator. The MSW properties, incinerator specifications and operating parameters were acquired from an incineration plant in Zhejiang Province, China. The MSW combustion modelling was carried out with air preheating temperatures in a range of 453–513 K. The simulation results were compared with measurements data from the incinerator and literature. It was found that higher primary air temperature caused a larger layer of evaporation at the bottom of the bed. When the evaporation layer comes in contact with the ignition front, the remaining mass burned instantaneously, resulting in a faster complete combustion. Peak bed temperature was observed during the complete combustion, it was highly influenced by heat flux from the primary air and the combustion. Higher primary air temperature also resulted in increased devolatilization rate and peak flame temperature. The peak flame temperature reached 1961 K under 513 K air temperature. Boiler efficiency increased by 5.6% when remaining flue gas heat was utilized to preheat the air to 453 K compared to the unutilized. However, high primary air temperatures also have negative influences, for example, accelerated thermal NOx generation and locally high bed temperature. Condensation of acid gases could also occur if the remaining flue gas heat was over-utilized. Therefore, the determination of the suitable primary air temperature is essential.

Pressurized oxy-fuel combustion of a char particle in the fluidized bed combustor

Pressurized oxy-fuel combustion of coal in fluidized bed (FB) holds the potential to realize low-cost CO2 capture. However, the fundamental study in this manner is still rare due to the difficult access to the pressurized oxy-FB combustion tests. In this work, the experimental study of single char combustion was firstly conducted in a visualized pressurized FB combustor under various operating conditions. Then an experimentally verified particle-scale char combustion model was developed to reveal the dependence of char combustion on parameters. Results showed that the char conversion was accelerated with the increase of pressure, mainly due to the high oxygen diffusion and char gasification. The gasification played a non-negligible role in pressurized oxy-fuel combustion, especially under high oxygen concentration and bed temperature. Increasing oxygen concentration and bed temperature not only promotes the char oxidation rate and particle temperature, but also increases the gasification rate and the share of char conversion via gasification, resulting in shortening the burnout time of char. In addition, a higher fluidization number lowered both the burnout time and peak temperature of char particle, due to the simultaneous improvement of mass and heat transfer. The influences of char size and fluidization number on char gasification conversion ratio are very weak. In addition, the quantitative analysis of the influence of different operating parameters on the combustion process was obtained by model sensitivity analysis.

Acrylonitrile Butadiene Styrene and Polypropylene Blend with Enhanced Thermal and Mechanical Properties for Fused Filament Fabrication.

Acrylonitrile butadiene styrene (ABS) is the oldest fused filament fabrication (FFF) material that shows low stability to thermal aging due to hydrogen abstraction of the butadiene monomer. A novel blend of ABS, polypropylene (PP), and polyethylene graft maleic anhydride (PE-g-MAH) is presented for FFF. ANOVA was used to analyze the effects of three variables (bed temperature, printing temperature, and aging interval) on tensile properties of the specimens made on a custom-built pellet printer. The compression and flexure properties were also investigated for the highest thermal combinations. The blend showed high thermal stability with enhanced strength despite three days of aging, as well as high bed and printing temperatures. Fourier-transform infrared spectroscopy (FTIR) provided significant chemical interactions. Differential scanning calorimetry (DSC) confirmed the thermal stability with enhanced enthalpy of glass transition and melting. Thermogravimetric analysis (TGA) also revealed high temperatures for onset and 50% mass degradation. Signs of chemical grafting and physical interlocking in scanning electron microscopy (SEM) also explained the thermo-mechanical stability of the blend.

Evaluation of bed-to-tube surface heat transfer coefficient for a horizontal tube in bubbling fluidized bed at high temperature

Experiments have been conducted in a bubbling fluidized bed to determine the bed-to-tube surface heat transfer coefficient to a horizontal tube at high bed temperatures (400–950 °C). The aim was to study the influence of the bed temperature, the particle size, the bed material and the superficial gas velocity. A tube (do = 6 mm) through which water is flowing was placed close to the middle of the bed with a fixed bed height of 17 cm. Silica sand, crushed and beneficiated ilmenite and ground steel converter slag were used as bed materials. Air was used as fluidization gas. The mean particle diameter of the used bed materials was in the range 123–327 μm. The estimated heat transfer coefficients were compared with correlations from literature describing the convective bed-to-tube heat transfer coefficient (all of which have been derived at temperatures below 400 °C), with the addition of the radiative heat transfer contribution. The heat transfer coefficient increased with increasing bed temperature, decreasing particle size and increasing superficial gas velocity up to a certain value. These trends were observed for all bed materials although significant differences were still observed in the estimated heat transfer coefficients. Ilmenite was the material for which the highest heat transfer coefficient was estimated. The experiments verified high heat transfer coefficients (768–1858 W/(m2K)) to the outside tube surface operating at a superficial gas velocity of 0.15 m/s, which aligned very well with two of the heat transfer correlations and reasonably well with a third. The remaining three correlations were not as accurate at the examined conditions. The contribution from the convective heat transfer is significantly higher than the estimated radiative heat transfer to the tube.

Predicting the Microstructural Evolution of Electron Beam Melting of Alloy 718 with Phase-Field Modeling

Electron beam melting (EBM) is a powder bed additive manufacturing process where a powder material is melted selectively in a layer-by-layer approach using an electron beam. EBM has some unique features during the manufacture of components with high-performance superalloys that are commonly used in gas turbines such as Alloy 718. EBM has a high deposition rate due to its high beam energy and speed, comparatively low residual stresses, and limited problems with oxidation. However, due to the layer-by-layer melting approach and high powder bed temperature, the as-built EBM Alloy 718 exhibits a microstructural gradient starting from the top of the sample. In this study, we conducted modeling to obtain a deeper understanding of microstructural development during EBM and the homogenization that occurs during manufacturing with Alloy 718. A multicomponent phase-field modeling approach was combined with transformation kinetic modeling to predict the microstructural gradient and the results were compared with experimental observations. In particular, we investigated the segregation of elements during solidification and the subsequent “in situ” homogenization heat treatment at the elevated powder bed temperature. The predicted elemental composition was then used for thermodynamic modeling to predict the changes in the continuous cooling transformation and time–temperature transformation diagrams for Alloy 718, which helped to explain the observed phase evolution within the microstructure. The results indicate that the proposed approach can be employed as a valuable tool for understanding processes and for process development, including post-heat treatments.

Catalytic Oxidation of Volatile Organic Compounds by 13X Zeolite Coated with nZnO in Presence of UV and Ozone at High Bed Temperature

Catalytic oxidation of benzene, toluene, and ethylbenzene (BTE) on virgin zeolite, nZnO-coated zeolite, with and without UV + O3, at 300 °C bed temperature was investigated using laboratory experiments. The coating was done at three weight ratios of Zn (estimated Zn content in nZnO) to zeolite (0.2:1.0, 0.5:10, 1.0:1.0). The coated adsorbents were examined by scanning electron microscopy, wavelength dispersive X-ray spectroscopy, and Brunauer Emmett Teller analyses. In the catalytic oxidation experiments, the adsorbents were first saturated with BTE by purging an air stream containing a mixture of BTE at 5 ppm each for 28 min. Introduction of UV + O3 on the coated 13X zeolite (0.5:1.0) bed improved the average removal efficiency (RE) of benzene, toluene, and ethylbenzene (with inlet concentration of 5 ppm each) to 68.7, 90.0, and 99.6%, respectively, from the corresponding values of 57.3, 79.9, and 98.5% when no UV + O3 was used. An increase in the coating weight ratio from 0.2:1.0 to 0.5:1.0 had produced a higher RE for benzene only, while a further increase to 1.0:1.0 witnessed a decrease in RE for all three compounds, and more for benzene. Avg RE of BTE decreased with the increase in their inlet concentrations, more significantly for an increase from 5 to 50 ppm and less noticeable for a further increase to 100 ppm. The final oxidation products and intermediate products in the outlet streams from the oxidation and thermal desorption were analyzed which showed predominant compositions of CO2 followed by BTE and some levels of CO and other volatile organic compounds.

O2/CO2 and O2/N2 combustion of bituminous char particles in a bubbling fluidized bed under simulated combustor conditions

Most of the time char particles in commercial FB units encounter considerably lower oxygen concentrations than that in the entrance, say between 2% and 10%, and the effects of the oxy-fuel atmosphere on the conversion history of char particles need to be clearly treated under these conditions. In the present work, an experimental study of combustion of bituminous coal char was carried out in a laboratory FB in O2/CO2 and O2/N2 atmospheres under simulated FB combustor conditions at O2 concentrations of 4–10% v/v, bed temperatures of 800–900 °C and char particle sizes of 2–8 mm by continuously measuring the concentrations of O2 and CO in the flue gas. The results indicate that the conversion of char is controlled by diffusion of O2 in the boundary layer of the particle in O2/CO2 and O2/N2 environments and that the gasification of char in O2/CO2 is limited by chemical kinetics. A char conversion model, taking into account the mass transfer from the bed to the particle and the gasification kinetics of char, was built, based on the experimental results and the intrinsic reactivity of char obtained in TGA tests. Simulations, carried out under the same conditions as in the FB experiments, give good agreement in terms of burnout time and instantaneous reactivity during the conversion of single particles. Simulated results prove that the low diffusivity of O2 in CO2 is the main reason for the decreased reaction rate of the char particle in O2/CO2 compared with O2/N2. The contribution of gasification to the consumption of char is more notable at high bed temperature (900 °C) and coarse particles (8 mm), particularly at lower oxygen concentration (4–6% v/v).

Oxidative coupling of methane over mixed metal oxide catalysts: Steady state multiplicity and catalyst durability

Exothermic heat effects are a crucial factor in determining the performance and stability of catalysts for the oxidative coupling of methane (OCM). Fixed bed temperature rise, steady state multiplicity, and catalyst durability are investigated over a range of feed conditions for the mixed metal oxides Cs/Sr/MgO, Cs/Ba/MgO, Cs/Sr/La2O3, and Na2WO4-Mn/SiO2. A comparison with previous studies on doped metal oxides catalysts for OCM clearly indicates that doping not only improves the performance but also significantly improves the catalyst stability. We experimentally demonstrate for the first time hysteresis behavior for Cs/Sr/La2O3 powder catalyst. Our results show that the catalyst stability depends on the magnitude of temperature rise in the catalyst bed. At a lower space velocity of 3,600cc/h/g, the catalysts exhibit moderate temperature rise (<50°C) at complete O2 conversion and sustained activity for extended time-on-stream (50–72h). The performance of Cs/Sr/MgO and Cs/Ba/MgO are comparable to Na2WO4-Mn/SiO2 (∼19% C2+ yield). While Cs/Sr/La2O3 activates at lower temperature, its maximum C2+ yield (∼14%) is lower. At a higher space velocity of 14,400cc/h/g, a significant temperature rise of ∼300°C and ignition-extinction behavior is encountered. Under these demanding conditions, improved OCM performance is observed for Na2WO4-Mn/SiO2 (35% methane conversion, C2+ selectivity 60%) but the catalyst deactivates due to the high bed temperature (930°C). In comparison, methane conversion of ∼25% and C2+ selectivity of ∼38% is observed for Cs/Sr/La2O3 at a feed temperature of 395°C. Despite a bed temperature of 830°C, the Cs/Sr/La2O3 catalyst is stable for 50h. The findings show the importance of heat effects in both promoting OCM catalyst performance and leading to deleterious catalyst deactivation.