Oxygen Mole Fraction Research Articles

A statistical model to relate pyrite oxidation and oxygen transport within a coal waste pile: case study, Alborz Sharghi, northeast of Iran

This paper presents a statistical model for predicting pyrite fraction remaining in a coal waste pile at the Alborz Sharghi coal mine located at northeast of Iran. This model calculates the fraction of pyrite remaining using mole fraction of oxygen diffused into the pore spaces of the pile and the pile depth. Comparison of the statistical outputs revealed that a second-order polynomial expression with respect to oxygen mole fraction and depth provides a better correspondence to the field measurements for the fraction of pyrite remaining with a RMSE of 0.089231. Besides, two statistical relationships have been proposed for the remaining pyrite fraction and the mole fraction of oxygen versus the pile depth. A quadratic polynomial shows the best correlation of the field measured data. The suggested models were successfully validated with the acceptable confidence levels of 92 and 90 % for remaining pyrite and oxygen using a new data set which revealed the fact that they can be applied in similar situations. Both statistical analysis and field data indicate that the pyrite oxidation process is limited to the shallower depths of the waste pile where the mole fraction of oxygen decreased rapidly.

Three-Dimensional Design Optimization of an Anode-Supported SOFC Using FEM

Solid oxide fuel cells (SOFCs) are promising as energy producing devices, which at this stage of development will require extensive analysis and benefit from numerical modeling. A 3D model is developed based on the FEM for a single cell planar SOFC design optimization. Ion, electron, heat, gas-phase species and momentum transport equations are implemented and coupled to the kinetics of electrochemical reactions. High current density spots are identified, where the electron transport distance is short and the oxygen concentration is high. The relatively thin cathode results in a significant oxygen mole fraction gradient in the direction normal to the main flow direction. The electron transport especially within the cathode is found to be limiting for the electrochemical reactions at positions far from the channel walls (interconnect ribs). It is concluded that an increased pore size in the cathode support layer increases the current density more than an increased pore size in the anode support layer.

Multi-variable optimisation of PEMFC cathodes based on surrogate modelling

A two-dimensional, steady state, isothermal agglomerate model for cathode catalyst layer design is presented. The design parameters, platinum loading, platinum mass ratio, electrolyte volume fraction, thickness of catalyst layer and agglomerate radius, are optimised by a multiple surrogate model and their sensitivities are analysed by a Monte Carlo method based approach. Two optimisation strategies, maximising the current density at a fix cell voltage and during a specific range, are implemented for the optima prediction. The results show that the optimal catalyst composition depends on operating cell voltages. At high current densities, the performance is improved by reducing electrolyte volume fraction to 7.0% and increasing catalyst layer porosity to 52.9%. At low current densities, performance is improved by increasing electrolyte volume fraction to 50.0% and decreasing catalyst layer porosity to 12.0%. High platinum loading and small agglomerate radius improve current density at all cell voltages. The improvement in fuel cell performance is analysed in terms of the electrolyte coating thickness, agglomerate specific area, conductivity, overpotential, volumetric current density and oxygen mole fraction within the cathode catalyst layer. The optimisation results are also validated by the agglomerate model at different cell voltages to confirm the effectiveness of the proposed methodologies.

Process simulation of oxy-fuel combustion for a 300 MW pulverized coal-fired power plant using Aspen Plus

This work focuses on the amounts and components of flue gas for oxy-fuel combustion in a coal-fired power plant (CFPP). The combustion process of pulverized coal in a 300MW power plant is studied using Aspen Plus software. The amount of each component in flue gas in coal-fired processes with air or O2/CO2 as oxidizer is obtained. The differences between the two processes are identified, and the influences of temperature, excess oxygen ratio and molar fraction of O2/CO2 on the proportions of different components in flue gas are examined by sensitivity analysis. The process simulation results show that replacing atmospheric air by a 21%O2/79%CO2 mixture leads the decrease of the flame temperature from 1789°C to 1395°C. The equilibrium amount of NOx declines obviously but the SOx are still at the same level. The mass fraction of CO2 in flue gas increased from 21.3% to 81.5%. The amount of NOx is affected sensitively by the change of temperature and the excess oxygen ratio, but the change of O2/CO2 molar fraction has a little influence to the generation of NOx. With the increasing of O2 concentration, the flame temperature and NOx emission enhance rapidly. When the molar fraction of O2 increases to 30%, the flame temperature is similar and the mass fraction of NOx is about 1/8 of that air atmosphere.

Geometrical Parameters Influence on PEM Fuel Cell Performance

In order to model the transport phenomena in the porous materials used in Polymer Electrolyte Membrane Fuel Cells, a two-dimensional PEM fuel cell model has been developed and used to investigate the effects of thickness on cell performance parameters. The governing equations of the overall steady state PEM fuel cell model is represented by the Navier–Stokes equations, consisting of the momentum equations, the continuity equation, the species and energy transport. They were used to model the oxygen molar fraction, electrolyte and solid potential distributions in the flow channels, backing layers, catalyst layers and membrane. The coupled partial differential and electrochemical equations theory are solved basing on the control-volume method. The catalyst layer and membrane thickness involved the performance of the PEM fuel cell. This latter is considered as a reasonable approximation for the effects of geometrical parameters which will be inherently used for further parametric studies.

Mathematical modeling of gasification of high-ash Indian coals in moving bed gasification system

Simulation of gasification of high-ash Indian coal in an updraft moving bed gasification system is presented in this paper. A steady one-dimensional numerical model, which takes into account of drying, devolatilization, combustion and gasification processes, is used to solve the mass and energy balances in the gasification system. The results from the model have been validated against the experimental data available in literature for various types of coals. The predicted product gas composition, its calorific value and the exit temperature are in agreement with the reported results. The validated model is used to study the effect of input parameters such as oxygen content in air stream, steam flow rates and the pressure of the gasification system. Results indicate that the value of oxygen mole fraction around 0.42 in the oxidizer stream can provide optimum performance in oxygen-based gasification systems. There is a range of steam-to-coal ratio that is dependent on the oxygen content in oxidizer stream. For air-based systems, this value is around 0.4 and for oxygen-based systems it is 1.5. The gasification performance improves with operating pressure significantly. An operating pressure of around 8 bar and higher, based on the application, can be used for achieving the required performance with high-ash coals. The model is useful for predicting the performance of high-ash Indian coals in a moving bed gasification system under different operating parameters. Copyright © 2013 John Wiley & Sons, Ltd.

Effects of Stefan Flow on Ignition and Heat Transfer of a Char Particle in O2/CO2 Atmospheres

Stefan flow, occurring at the surface of a carbon particle, is often neglected or simply regarded as a mass transfer phenomenon in pulverized-coal combustion. In this work, considering the Stefan flow and the CO oxidation in the boundary layer, a heat balance equation of the char particle is developed to analyze the effects of Stefan flow on the ignition and heat transfer in oxy-fuel combustion. Better agreement is achieved between the experimental data and the predictions with the novel equation. When the Stefan flow is considered, the ignition delay is observed, and a higher gas temperature and a larger oxygen mole fraction are required for the ignition of the char particle. The neglect of Stefan flow would lead to larger combustion rate, higher surface temperature, and smaller burnout diameter under the same conditions, implying the overestimation of char burnout rate. For the situation of considering Stefan flow, the contribution of CO2 reduction reaction to the total reaction heat flux is smaller since the Stefan flow blocks the diffusion of CO2 toward the particle surface. It results in the increase of total reaction heat flux in the boundary layer with the gas temperature increasing. In addition, the presence of Stefan flow greatly weakens the heat transfer by conduction and accelerates the heat loss from the particle, which makes the particle temperature lower. It can be concluded that the effects of Stefan flow on the heat balance of a char particle cannot be neglected in oxy-fuel combustion.

Hybrid femtosecond/picosecond rotational coherent anti-Stokes Raman scattering temperature and concentration measurements using two different picosecond-duration probes

A hybrid fs/ps pure-rotational CARS scheme is characterized in furnace-heated air at temperatures from 290 to 800 K. Impulsive femtosecond excitation is used to prepare a rotational Raman coherence that is probed with a ps-duration beam generated from an initially broadband fs pulse that is bandwidth limited using air-spaced Fabry-Perot etalons. CARS spectra are generated using 1.5- and 7.0-ps duration probe beams with corresponding coarse and narrow spectral widths. The spectra are fitted using a simple phenomenological model for both shot-averaged and single-shot measurements of temperature and oxygen mole fraction. Our single-shot temperature measurements exhibit high levels of precision and accuracy when the spectrally coarse 1.5-ps probe beam is used, demonstrating that high spectral resolution is not required for thermometry. An initial assessment of concentration measurements in air is also provided, with best results obtained using the higher resolution 7.0-ps probe. This systematic assessment of the hybrid CARS technique demonstrates its utility for practical application in low-temperature gas-phase systems.

Shock tube measurements and model development for morpholine pyrolysis and oxidation at high pressures

Fuel decomposition time-history measurements during morpholine pyrolysis and ignition delay time measurements during morpholine oxidation were carried out behind reflected shock waves in a high-pressure shock tube. For the pyrolysis studies, morpholine concentrations of 5000ppm in argon were employed, with experiments covering the temperature range 1086–1404K and pressure range 20–23.6atm. For the ignition delay time measurements, experiments covered the temperature range 866–1197K with pressures near 15 and 25atm, oxygen mole fractions of 4% and 21%, and equivalence ratios of 0.5, 1, and 2. A morpholine reaction set developed to simulate low-pressure flames was updated based on the current shock tube data, recently published rate constants for relevant reactions, and newly calculated thermochemistry. The simulations from the updated set were in good agreement with the current shock tube experiments.

A MOLECULAR DYNAMICS STUDY OF OXYGEN GAS IN WATER AT DIFFERENT TEMPERATURES

Molecular dynamics simulations of a binary mixture of oxygen gas and SPC/E water, with oxygen gas ( O 2) as solute and water as solvent, at oxygen mole fraction of 0.019 have been accomplished at different temperatures 288, 293, 298, 302 and 306 K using Groningen Machine for Chemical Simulations. The solvent–solvent, solute–solute and solute–solvent radial distribution functions (RDFs) have been estimated. The solvent–solvent (water–water) RDF has been found to agree with that obtained from NMR/X-ray data within 7%. Self-diffusion coefficients of both the solvent and the solute have been determined by means of mean-squared displacement curves using Einstein's relation. They are found to agree with experimental results very well. Darken's relation has also been invoked for the determination of mutual diffusion coefficients at the respective temperatures. The analysis of temperature dependence of the diffusion coefficients has revealed that they follow Arrhenius equation to a very good extent and are consistent with the nature of RDF's at the respective temperatures. The estimated activation energies are in excellent agreement with the available experimental data.

Rapid and accurate quantitative analysis of fermentation gases by Raman spectroscopy

Raman spectroscopy was employed for the precise quantitative analysis of a gaseous mixture. The band ratio between the Raman band of the target gas and that of the external standard was calculated and found to be proportional to the pressure of the target gas. The linearity of the calibration curves was very good (higher than 0.999). The mole fractions of atmospheric nitrogen and atmospheric oxygen were determined from their partial pressures. The average molar fraction of atmospheric nitrogen was calculated to be 0.790, similar to the literature value. Those of methane, hydrogen, and carbon dioxide, which are produced by fermentation, were also determined from the Raman spectrum. The values were identical to those obtained from the measured volume of each gas. The possibility of quantifying gas molecules by Raman spectroscopy is demonstrated.

On the Self-extinction Time of Pool Fire in Closed Compartments

The self-extinction time of pool fires in closed compartments was studied. Experiments were conducted in two bench-scale compartments with volumes of 0.75 m3 and 17.55 m3. It was found that the fire self-extinction time was proportional to the compartment volume but inversed to the pool area for n-heptane. The fuel mass loss rate in closed compartments was lower than that in the open space. The fire self-extinction occurred when the local oxygen mole fraction in the flame vicinity descended to a level of 10.7-15.3%. The mean remaining oxygen mole fraction at the self-extinction moment was about 14.1%. Based on the mass conservation of the oxygen, a model for predicting the self-extinction time of pool fires in closed compartments was developed. By defining a concept of the dimensionless fire volume, the dimensionless self-extinction time was proposed. The dimensionless self-extinction time is proportional to the difference between the initial and remaining oxygen mass fraction, fuel properties, such as heat of combustion and stoichiometric ratio etc., but inverses to the dimensionless fire volume and the integrated combustion coefficient. The predicted results showed a good agreement with the experimental results. The model also provides a good prediction for the results of NRL's t sts.

Combustion of Submillimeter Heptane/Methanol and Heptane/Ethanol Droplets in Reduced Gravity

Reduced-gravity experiments were performed on combustion of droplets composed of n-heptane mixed with methanol or ethanol. The initial alcohol mass fraction in a droplet was 0% (pure heptane) or 5%. The experiments were performed at 0.1 MPa and 25°C with air or with ambients of oxygen and helium with oxygen mole fractions of 0.3 or 0.4. Initial droplet diameters were in the range 0.67 mm to 0.92 mm. After considering measurement uncertainties, burning rates decreased appreciably as the initial droplet diameter increased for combustion in air but not for combustion in the oxygen/helium environments. It was also found that addition of either methanol or ethanol did not influence burning rates appreciably and that burning rates were larger for the oxygen/helium environments than for air if initial droplet diameter dependences were accounted for.

Air flow modeling and performance prediction of the integrated-planar solid oxide fuel cell IP-SOFC

Modeling the air flow through an array of the integrated-planar solid oxide fuel cell (IP-SOFC) is very important for the optimization of the cell performance. The finites differences and the lattices Boltzmann methods are used to predict the evolution of the IP-SOFC fuel cell parameters. The effects of the inlet air velocity, the temperature and the pressure on the cell performance are investigated. The results indicate that the oxygen mole fraction decreases and the polarization concentration increases in the x-direction. The voltage losses became more and more important for the module of the next bundle, caused by the oxygen depleting. An increase in the inlet air velocity can reduce the oxygen mole fraction decrease and decrease polarization losses, however the concentration polarizations decrease imperceptibly with increasing temperature.

English

This paper discussed the design and development of the Moderate and Intense Low oxygen Dilution (MILD) combustion burner using Computational Fluid Dynamics (CFD) simulations. The CFD commercial package was used to simulate preliminary designs for the burner before the final design was sent to workshop for the fabrication. The burner is required to be a non-premixed and open burner. To capture and use the exhaust gas, the burner was enclosed within a large circular shaped wall with an opening at the top. An external EGR pipe was used to transport the exhaust gas which was mixed with the fresh oxidant. To control the EGR and exhaust flow, butterfly valves were installed at the top opening as a damper to close the exhaust gas flow at the certain ratio for EGR and exhaust out to atmosphere. High temperature fused silica glass windows were installed to view and capture images of the flame and analyse the flame propagation. The burner simulation shows that MILD combustion was achieved for the oxygen mole fraction between 3-13%. The final design of the burner was fabricated and ready for the experimental validation.

Physical Properties of Particulate Matter Emitted from Combustion of Coals of Various Ranks in O2/N2 and O2/CO2 Environments

This work examined the particulate emissions from pulverized coals burning under either conventional or oxyfuel combustion conditions. Oxyfuel combustion is a process that takes place in O2/CO2 environments, which are achieved by removing nitrogen from the intake gases and recirculating large amounts of flue gases into the boiler; this is done to moderate the high temperatures caused by the elevated oxygen partial pressure therein. In this study, combustion took place in a laboratory laminar-flow drop-tube furnace (DTF) in environments containing various mole fractions of oxygen in either nitrogen or carbon dioxide background gases. A bituminous coal, a sub-bituminous coal, and a lignite were burned at a DTF temperature of 1400 K. Trimodal ash particle size distributions were observed with peaks in the submicrometer region (∼0.2 μm), as well as in the supermicrometer region (∼5 μm and >10 μm). Both submicrometer and supermicrometer particulate emission yields of all three coals were typically lower in O2/CO2 than in O2/N2 environments. Emission yields typically increased with increasing oxygen concentration in the furnace, with an exception noted at moderate oxygen mole fractions (20%–30%) in CO2, where significant amounts of unburned carbon were detected. Submicrometer particulate yields were found to be comparable in the effluents of all three coals, independently of their ash contents, whereas supermicrometer particulate yields were nearly analogous to the ash contents of the three coals. Scanning electron microscopy (SEM) revealed that submicrometer particles were spherical, whereas supermicrometer particles were often of irregular shapes, fractured spheres, and spheres with small particles attached to their surface.

Methane/oxygen combustion in a rapidly mixed type tubular flame burner

An inherently safe technique of rapidly mixed type tubular flame combustion has been applied to pure oxygen combustion. The flame characteristics under various oxygen mole fractions are experimentally investigated with use of burners of different slit widths, and in addition, with optically accessible quartz burners. To fundamentally understand the requirements for rapidly mixed pure oxygen tubular flame combustion, the characteristic reaction times were calculated with Chemkin code, while the characteristic mixing times were estimated based on the width of mixing layer determined. Results show that the rapidly mixed type tubular flame combustion has been obtained under oxygen-enriched and even pure oxygen conditions. At high oxygen mole fraction, diffusion flames are anchored at the exits of the fuel slits, which restrains mixing between fuel and oxidizer, resulting in a failure of tubular flame combustion. By reducing the slit width, however, the diffusion flame is inhibited and a uniform tubular flame can be obtained from 0.11 to 0.18 in overall equivalence ratio. To quantify the mixing process, the flow field was visualized, and it has been found that there exist two types of flows, a boundary layer type flow near the exits of the slits close to the wall and an axisymmetric potential flow around the axis of rotation. Based on the mixing layer width of the boundary layer type flow, the Damkohler numbers are obtained and it is clearly shown that when the Damkohler number is less than unity, the mixing is completed before the onset of reactions, resulting in establishment of tubular flame combustion.

Transient Redox Activity of Vanadyl Pyrophosphate at Ambient and Elevated Pressure

Abstract The transient catalytic activity of vanadyl pyrophosphate (VPP) catalyst was studied at ambient and elevated pressure (4.1 bar) and a wide range of operating conditions. The range included the commercial operating conditions typical of fixed bed, fluidized bed and circulating fluidized beds (CFB) for the partial oxidation of n-butane to maleic anhydride (MA). The maleic anhydride yield improved by increasing the feed oxygen molar fraction, temperature and pressure. When the catalyst was cycled between an oxidizing (synthetic air) and a reducing environment; yield increased with an increase in the catalyst residence time in the oxidizing environment. This effect was more pronounced at higher pressure. At ambient pressure, MA selectivity varied between 50-73 % while it decreased to about 48-54 % at a pressure of 4.1 bar. A strong MA selectivity dependency on feed composition was observed when the oxidation time was in the range of actual industrial reactors (< 1 minute). Selectivity data suggested that different oxygen species might be responsible for CO formation compared to other products such as CO2 and MA. Under oxidizing feed conditions (oxygen/n-butane ≥ than 3.7), an increase in n butane conversion was the main contributor to improved MA yield: n-butane conversion increased by about 70 % when the catalyst oxidation time extended from 0.3 to 10 minute. While, under fuel rich feed conditions, typical of industrial CFB operations, both MA selectivity and n-butane conversion contributed to enhancement in MA yield. Depending on the feed composition, MA selectivity increases by about 16-30 % and n butane conversion increases by about 32-55 % by extending the catalyst oxidation time. These results show the critical importance of catalyst oxidation time on reaction yield improvement especially when operating under fuel rich feed conditions. The surface adsorbed or surface lattice oxygen species were suggested to be the main responsible for n butane activation. While, the contribution of catalyst’s sub-surface lattice oxygen was believed to be very limited at fuel rich feed conditions. Under these conditions, catalyst over-reduction cannot be effectively compensated even after excessive catalyst regeneration and presence of gas phase oxygen is critical to maintain a high catalytic activity. As the reactor pressure increased to 4.1 bar, up to 60 % increase in n butane conversion accompanied by 100 % increase in oxygen conversion was observed. MA selectivity decreased by about 20 % on average but the increase in n-butane conversion resulted in an overall yield improvement of up to 30 %. Data show that the catalytic performance could be enhanced at certain combination of reactor pressure and temperature.

Ignition characteristics of single coal particles from three different ranks in O2/N2 and O2/CO2 atmospheres

This work assessed the ignition behavior of single-coal and single-char particles in O2/N2 and O2/CO2 atmospheres, with oxygen mole fractions in the range of 20–100%. Fuels included four pulverized coals from three different ranks (one high-volatile bituminous, one sub-bituminous and two lignites) as well as chars prepared from two of the coals. Particles of 75–90μm were injected in a bench-scale, transparent drop-tube furnace (DTF), electrically-heated to 1400K. Optical access of the furnace allowed the ignition of individual particles to be observed with high-speed cinematography. A particle’s ignition delay was defined as the time lapsed from the instant when the particle exited the injector to the onset of its luminous combustion in the furnace. The experiments were conducted at two different gas conditions inside the furnace: (a) quiescent gas condition (i.e., no flow or inactive flow) and, (b) an active gas flow condition in both the injector and the furnace. In the former case, the axial gas temperatures in the DTF were found to be similar in the N2 and CO2 background cases and, consequently, small differences were observed in the ignition delay and in the ignition behavior of coal particles in O2/CO2 and O2/N2 atmospheres. These small differences were easily accounted for by disparities in the physical properties of the background gases. Increasing the oxygen mole fraction in either N2 or CO2 reduced the ignition delay mildly. To the contrary, in the latter case, i.e., under the active gas flow condition the axial gas temperatures in the DTF were found to be disparate in the N2 and CO2 background cases. The ignition delay of coal particles was drastically prolonged in the slow-heating O2/CO2 atmospheres, relative to the faster-heating O2/N2 atmospheres, particularly at high-diluent mole fractions. Photographic evidence showed that under active gas flow, which is relevant to practical applications in utility furnaces, ignition of volatiles in envelope flames was suppressed in the presence of CO2. As a result, whereas in the quiescent gas condition, bituminous and sub-bituminous coal particles experienced homogeneous ignition in both O2/N2 and O2/CO2 atmospheres, in the active gas flow condition heterogeneous ignition was evident in O2/CO2. Lignite coal particles often fragmented before ignition; this increased the likelihood of heterogeneous ignition in both O2/N2 and O2/CO2, in either active or inactive gas flows.

The effect of oxygen enrichment on incipient soot particles in inverse diffusion flames

To investigate the characteristics of incipient soot particles with respect to oxygen mole fractions in oxidizer streams, experimental measurements of soot zone structure and degree of carbonization were performed for ethene in inverse diffusion flames (IDFs). IDFs are alternative flames used for research with incipient soot particles because the soot in IDFs does not experience the oxidation process significantly. In the present study, polycyclic aromatic hydrocarbons (PAHs)-planar laser induced fluorescence (PLIF) signals and soot scattering images were used to clarify the soot zone structure of IDFs. The flame height was measured by OH-PLIF with respect to the oxygen mole fraction in the oxidizer stream. The flame height decreased slightly as the oxygen mole fraction in the oxidizer increased. The two-color pyrometry showed that soot temperature along the soot growth path below the flame height was between 2000K and 2200K regardless of the oxygen enrichment. Beyond the flame height, soot formation in IDFs remained effective until the temperature maintained a sufficiently high level. The carbon to hydrogen (C/H) ratio, which was obtained through the element analysis of soot sampling, was measured to assess the degree of carbonization. The C/H ratio increased with respect to residence time at which the soot mostly resided in the region below the flame height. In addition, the C/H ratio increased with regard to the oxygen enrichment in the oxidizer. However, regardless of the oxygen enrichment, the C/H ratio increased linearly according to the modified residence time, which considered inception region and high temperature region beyond the flame height. The morphology of the soot sample was investigated by transmission electron microscopy (TEM) micrographs. A micrograph typical of the young soot particles was detected when the soot particles had a C/H ratio of about 2.