Oxidant Flow Rate Research Articles

Combustion characteristics of methane–oxygen enhanced air turbulent non-premixed swirling flames

Application of oxygen-enhanced combustion to existing fossil fuel energy systems to facilitate CO2 capture presents several challenges. This work investigates the combustion characteristics of methane oxygen enriched air turbulent non-premixed swirling flames. It focuses on the stability of flames, NOx, CO2 and CO emissions and the flow field dynamics. The burner configuration consists of two concentric tubes with a swirler placed in the annular part for the oxydant. The experiments are conducted using a 25kW water cooled combustion chamber. The exhaust gas compositions are measured using gas analyzers. OH chemiluminescence experiments are conducted to investigate the structure and the stability of the flames without and with oxygen enrichment. Flame liftoff heights, fluctuations of the flame base and flame lengths are determined. Particle Image Velocimetry is used to analyze the dynamics of swirling flows. The measurements are performed for oxygen concentrations ranging from 21% to 30% by volume, with swirl numbers from 0.8 to 1.4 and global equivalence ratios from 0.8 to 1. The results show that the addition of oxygen to air, while keeping the oxidant flow rate constant, enhances the combustion efficiency and flame stability. It is observed that increasing oxygen concentration leads to lower lift-off heights and reduces flame height fluctuations. Increasing the swirl number significantly improves the flame stability. The results demonstrate that the CO2 emissions in the exhaust gases linearly increase with increasing O2 content in the oxidant. The CO emissions are shown to decay exponentially, whereas the NOx emissions, mainly produced through the thermal pathway, increase strongly with oxygen enrichment. The PIV results illustrate that increasing the swirl intensity increases the reverse flow velocities close to the burner exit. The decay of axial velocity presents favorable flow patterns for the stabilization of the flame.

Maximum Power Point Tracking With Reactant Flow Optimization of Proton Exchange Membrane Fuel Cell

In this paper, a fuel cell (FC) maximum power point tracking (MPPT) with fuel flow optimization is presented. The aim of this study is to extract the maximum power from a FC at different fuel flow rates and to protect the FC from over-current and voltage collapse across terminals. The system is composed of a tracker with a buck converter able to change the output impedance of the FC and therefore its power. In order to illustrate our approach, the tracker is simulated by using both static and dynamic FC models to describe the FC response. Simulation results show the behavior of the tracker at different fuel and oxidant flow rates and verify the concept of maximum power tracking.

Oxy-fuel combustion of selected solid fuels under atmospheric and elevated pressures

Oxy-fuel combustion is suggested as a clean technique for energy conversion with respect to the reduction of CO2 and NOx emissions. The positive effect of NOx reduction can be enhanced by an increase of the pressure inside the combustion chamber. The oxy-fuel combustion of bituminous and lignite coals was investigated. The influence of process parameters, such as the type of fuel, the type of oxidant and the pressure, on the emissions of NOx, N2O, and other exhaust gas compounds was investigated. The experiments were performed using a prototype, laboratory-scale (fuel input of up to 3 kg/h) pressurised fluidised-bed combustor (BFB). The main process parameters were a maximal pressure of 6 bar, a maximal temperature of 910 °C and an oxidant flow rate of 39 kg/h. The results of the experiment performed with the pressurised Flexi-Burn™ procedure are presented and discussed. In particular, the emissions of NO and N2O from air-fired and oxy-fuel processes were compared. The NO emission was significantly reduced under higher pressures. The maximal NO reduction (due to an increase in the pressure) was as high as 50%, which can be explained by the promotion of the char + NO reaction.

Limiting Factors for a Planar Solid Oxide Fuel Cell Under Different Flow and Temperature Conditions

AbstractThe work investigates the performance of an anode supported solid oxide fuel cell under relevant conditions at different flow and temperature settings with the aim to identify performance limiting factors through impedance spectroscopy. Impedance spectroscopy is used to deconvolute impedance spectra of an in‐operating SOFC and identify limiting overpotentials. Those measurements are made under a wide range of flow and temperature conditions. In particular, oxidant flow rate is varied yielding fuel cell operation with 20, 40, 60, 80% oxidant utilization; fuel is instead changed in terms of flow and composition: fuel utilization factors in the range of 20–80% are investigated as well as the dilution with nitrogen. The operating temperature is varied in the range between 650 and 800 °C with steps of 50 °C. Results show that charge and mass transport can lead to a performance limitation according to the selected operating range for the investigated cell design. For the investigated anode‐supported design, a major improvement of performance could arise by reducing ohmic resistances (i.e. employing a thin electrolyte) and by an improvement of the anode geometry aiming at enhancing mass transport. In particular, at low temperature and high fuel utilization, fuel oxidation seems to be a relevant performance limiting factor.

A simulation study of coal combustion under O2/CO2 and O2/RFG atmospheres in circulating fluidized bed

The Euler–Euler modeling approach was employed to simulate the coal combustion in a circulating fluidized bed combustor. The unsteady processes of gas–solid two phase flow, heat transfer, mass transfer and chemical reactions were considered. By numerical simulation, distributions of pressure, solid volume fraction, temperature and gas concentrations (O2, CO2, H2O, CO, NO and SO2) were studied in O2/CO2 combustion mode (mixture of O2 and CO2 from gas bottles as oxidant) and O2/RFG combustion mode (mixture of O2 and recycled flue gas as oxidant) with O2 concentration range of 21–40%. Calculations were made for two cases, maintaining the same coal feed rate and maintaining the same oxidant volume flow rate. Simulated temperature distribution, pressure distribution and outlet gas concentrations were validated by the experimental data in 30% O2/70% CO2 atmosphere.

Salen‐Based Coordination Polymers of Manganese and the Rare‐Earth Elements: Synthesis and Catalytic Aerobic Epoxidation of Olefins

Treatment of N,N'-bis(4carboxysalicylidene)ethylenediamine (H(4)L), with MnCl(2)⋅(H(2)O)(4), and Ln(NO(3))(3)⋅(H(2)O)(m) (Ln = Nd, Eu, Gd, Dy, Tb), in the presence of N,N-dimethylformamide (DMF)/pyridine at elevated temperature resulted (after work up) in the formation of 1D coordination polymers {[Ln(2)(MnLCl)(2)(NO(3))(2)(dmf)(5)]⋅4 DMF}(n) (1-5). In these coordination polymers the rare earth ions are connected through carboxylate groups from Mn-salen units in a 1D chain structure. Thus, the Mn-salen complex acts as a "metalloligand" with open coordination sites. All compounds were used as catalysts in the liquid-phase epoxidation of trans-stilbene with molecular oxygen, which resulted in the formation of stilbene oxide. Since the choice of the lanthanide had virtually no influence on the performance of the catalyst, only the manganese-gadolinium was studied in detail. The influence of solvent, catalyst concentration, reaction temperature, oxidant, and oxidant flow rate on conversion, yield, and selectivity was analyzed. A conversion of up to 70%, the formation of 61% stilbene oxide (88% selectivity), and a TON of 84 were observed after 24 h. A hot filtration test confirmed that the reaction is mainly catalyzed through a heterogeneous pathway, although a minor contribution of homogeneous species could not be completely excluded. The catalyst could be reused without significant loss of activity.

Laminar oxy-fuel diffusion flame supported by an oxygen-permeable-ion-transport membrane

A numerical model with detailed gas-phase chemistry and transport was used to predict homogeneous fuel conversion processes and to capture the important features (e.g., the location, temperature, thickness and structure of a flame) of laminar oxy-fuel diffusion flames stabilized on the sweep side of an oxygen permeable ion transport membrane (ITM). We assume that the membrane surface is not catalytic to hydrocarbon or syngas oxidation. It has been demonstrated that an ITM can be used for hydrocarbon conversion with enhanced reaction selectivity such as oxy-fuel combustion for carbon capture technologies and syngas production. Within an ITM unit, the oxidizer flow rate, i.e., the oxygen permeation flux, is not a pre-determined quantity, since it depends on the oxygen partial pressures on the feed and sweep sides and the membrane temperature. Instead, it is influenced by the oxidation reactions that are also dependent on the oxygen permeation rate, the initial conditions of the sweep gas, i.e., the fuel concentration, flow rate and temperature, and the diluent. In oxy-fuel combustion applications, the sweep side is fuel-diluted with CO2, and the entire unit is preheated to achieve a high oxygen permeation flux. This study focuses on the flame structure under these conditions and specifically on the chemical effect of CO2 dilution. Results show that, when the fuel diluent is CO2, a diffusion flame with a lower temperature and a larger thickness is established in the vicinity of the membrane, in comparison with the case in which N2 is used as a diluent. Enhanced OH-driven reactions and suppressed H radical chemistry result in the formation of products with larger CO and H2O and smaller H2 concentrations. Moreover, radical concentrations are reduced due to the high CO2 fraction in the sweep gas. CO2 dilution reduces CH3 formation and slows down the formation of soot precursors, C2H2 and C2H4. The flame location impacts the species diffusion and heat transfer from the reaction zone towards the membrane, which affects the oxygen permeation rate and the flame temperature.

Direct N2H4/H2O2 Fuel Cells Powered by Nanoporous Gold Leaves

Dealloyed nanoporous gold leaves (NPGLs) are found to exhibit high electrocatalytic properties toward both hydrazine (N2H4) oxidation and hydrogen peroxide (H2O2) reduction. This observation allows the implementation of a direct hydrazine-hydrogen peroxide fuel cell (DHHPFC) based on these novel porous membrane catalysts. The effects of fuel and oxidizer flow rate, concentration and cell temperature on the performance of DHHPFC are systematically investigated. With a loading of ~0.1 mg cm−2 Au on each side, an open circuit voltage (OCV) of 1.2 V is obtained at 80°C with a maximum power density 195 mW cm−2, which is 22 times higher than that of commercial Pt/C electrocatalyst at the same noble metal loading. NPGLs thus hold great potential as effective and stable electrocatalysts for DHHPFCs.

Performance Analysis of PEM Fuel Cell with Varying Oxidant Supply Rates

In this work, the performance of proton exchange membrane (PEM) fuel cells has been analyzed for different oxidant (air and oxygen) flow rates (supplied at the cathode side). The performance analysis of a single PEM fuel cell coupled with a direct current (DC) permanent magnet motor (as load) was analyzed for air and oxygen. With an increase in the air flow rate, the maximum current values of the PEM fuel cell increase due to the reduction in the concentration of polarization losses (in the polarization curve) and the voltage regulation for the load increases. The maximum power production in a PEM fuel cell is a logarithmic function of different air flow rates. In this system, a DC permanent magnet motor with a reduction gear was used as a load on the PEM fuel cell. With the increase in air flow supply rates, the starting torque of the DC permanent magnet motor load improves. Also, a performance analysis was conducted for pure oxygen, and it has been observed that the PEM fuel cell provides a very good performance, even at the reduced supply flow rate of pure oxygen. For this work, a test bench was developed at Kocaeli University (Turkey), which consists of a PEM electrolyzer, a PEM fuel cell, and a DC permanent magnet motor with an air pump. The results of this work will be very useful for comparing the performances of the PEM fuel cell for different air and oxygen (as oxidants) flow rates for the dynamic loads (e.g., fuel cell vehicles with a DC permanent magnet motor).

Infrared Radiation and Acoustic Characteristics of Combustion Instabilities in Turbulent Premixed Flames

C OMBUSTION stability in turbulent premixed flames is desired for many applications including aircraft engines, heat recovery steam generators, and industrial boilers. Anchoring the flames to a bluff body is one technique for stabilizing these flows [1]. The interaction between pressure and heat release fluctuations can destabilize turbulent premixed flames and result in two distinct phenomena: combustion oscillations or localized extinctions near lean blowout limits. Combustion instabilities have been the focus of much research, as reviewed by Lee and Santavicca [2] and Shanbhogue et al. [3] Infrared radiation characteristics of turbulent premixedflames [4] and their relationship to combustion instabilities have received limited attention and are the focus of the present investigation. Combustion oscillations are typically driven by both flame-vortex interactions and equivalence ratio fluctuations from feed-system coupling [2,5]. During flame-vortex interactions the flame is stretched as vortices periodically shed from theflame holder and pass through the flame front [2,6]. The flame stretching increases the flame area (increases heat release rate) or leads to localized extinction (decreases the heat release rate) depending on the flame stretch rate and local equivalence ratio [2]. Feed-system coupling occurs when equivalence ratio fluctuations, resulting from fluctuations in either the fuel or oxidizer flow rate, are advected downstream to the combustion region [2]. If the equivalence ratio and pressure fluctuations arrive at the flame front in phase, the resulting heat release fluctuation amplifies combustion oscillations [2]. Outof-phase equivalence ratio and pressure fluctuations result in the heat release fluctuations damping the combustion oscillations [2]. Localized extinction near lean blowout is another phenomenon that contributes to instabilities in premixed flames. As the equivalence ratio is decreased, localized holes in the flame sheet occurwhere extinction stretch rates are exceeded [3]. Flame straining is the primary mechanism by which localized extinction occurs with secondary contributions by heat losses to the bluff body [3]. Blowout occurs when localized extinctions result in the flame being sufficiently cooled by the reactants entrained into the flow such that reactions are no longer self-sustaining. Acoustic and optical emission fluctuations increase sharply near blowout [1,3,7,8]. Power spectral densities of acoustic measurements showed strong peaks for = LBO 1:02 but not for = LBO 1:19 [1]. Nair and Lieuwen [1] experimentally observed that the fraction of visible images demonstrating localized extinction events was 0% at = LBO 1:12, 8% at = LBO 1:08, and 40% at = LBO 1:02. Localized reductions in OH chemiluminescence have been observed near blowout conditions ( = LBO 1:08), indicating extinction events [7]. The frequency of the extinction events monotonically increased as the lean blowout limit was approached. Motivated by the importance of radiation heat transfer and combustion stability in turbulent premixed combustors, the specific objectives of this work are 1) to measure the narrowband infrared radiation intensity emitted from the recirculation zone of bluff-body stabilized turbulent premixed flames for a range of equivalence ratios; 2) to characterize the combustion stability regimes using highfrequency acousticmeasurements and the infrared radiation intensity measurements; and 3) to observe and report plausible relations between the measured narrowband infrared radiation intensity statistics and measured acoustics. The results show that the midinfrared images yield time-dependent multipoint radiation intensities in emission bands of major combustion products (carbon dioxide, carbon monoxide, and water vapor). Turbulent statistics of the radiation intensity fluctuations can be used in designing infrared sensors for use in active control schemes to suppress combustion instabilities.

Stability of spray combustion for water/alcohols mixtures in oxygen-enriched air

The feasibility of burning fuels that are heavily diluted with water was evaluated. This concept could potentially be utilized to directly generate steam for a variety of applications, including power generation and enhanced oil recovery. Two water-soluble fuels, ethanol and 1-propanol, were heavily diluted with water and sprayed into a swirl-stabilized burner and the blow-off limits were obtained to identify the flame stability. The blow-off limit was experimentally determined as the lowest O2 concentration where a flame could exist under a given oxidizer flow rate. For the present experimental system, stability maps were determined as a function of fuel concentration and oxidizer flow rate for different types of nozzles and both alcohols. Contours of temperature and overall fuel mass fraction in the droplets were obtained for a flame burning 15wt% ethanol in an oxidizer of 50% O2 and 50% N2. The results revealed that preferential vaporization of ethanol over water plays an important role in enhancing flame stability. To further understand this, a model of bi-component droplet vaporization was employed for both the diffusion limit and distillation limit modes of liquid mass transfer. The results confirmed the preferential vaporization of ethanol and demonstrated that it is necessary to account for the non-ideal solution behavior of the alcohol–water mixture to appropriately model the vaporization process. The model also demonstrated that for this experimental system, droplet vaporization was better characterized by the distillation limit mode with enhanced mass transfer by convection.

The effect of oxidant flow rate on a coaxial oxy-fuel flame

The effect of oxidant flow rate on temperature, heat transfer, and NOx formation of an oxy-fuel flame is investigated using numerical simulation. The finite volume approach is used to solve the governing equations. The realizable k-e turbulence model and β-PDF model are adopted to simulate the turbulence-combustion interaction. There is a good agreement between the present numerical results and the reference experimental data. The exhaust temperature is decreased with decreasing the oxidant inlet flow rate. As an important result, the adiabatic flame temperature can be considered a key parameter in the oxy-fuel flame applications. Also, it is observed that increasing oxygen inlet angle causes a decrease in NO formation.

Hydrophobic PTFE-coated serpentine flow fields with ladder-structure for fuel cell application

A proton exchange membrane's conductivity increases with increasing water content. However, water on the electrode surface inhibits the inlet of oxidant/fuel, resulting in lower performance of the proton exchange membrane fuel cell (PEMFC). Therefore, water management is a critical issue that needs to be addressed in low-temperature PEMFCs. Two strategies used to manage water are the embedding of hydrophobic materials in the electrode (catalyst or gas diffusion layer), and controlling the fuel/oxidant flow rate. In this study, ladder-structure flow fields were manufactured with hydrophobic PTFE materials coated on different ladders. These were used to investigate the effect of oxidant flow rate and oxidant humidity on PEMFC performance and dynamic variation. We have found that using a flow field with hydrophobic PTFE coated on the second ladder (from the GDL) results in the greatest improvement in performance and durability.

COMBUSTION OF PTFE-BORON COMPOSITIONS FOR PROPULSION APPLICATIONS

An experimental study was conducted to evaluate the potential of solid fuels based on PTFE and boron mixtures for hybrid rocket motor applications. Specifically, a processing technique based on sintering was studied to determine the viability of these fuels. Sintering of the fuels provided reasonable mechanical properties to allow for exploration of these fuels without the addition of performance-robbing ingredients. Linear regression rates of sintered and unsintered fuels were collected in a diffusion flame setting with gaseous oxygen as the oxidizing component demonstrating that the sintering process had no effect. This family of fuels has shown that they will not combust at atmospheric pressure unless pure oxygen is present. However, sintered fuels with boron loadings greater than or equal to 25% by weight do self-propagate at atmospheric pressure once ignited in the presence of oxygen, whereas unsintered fuels do not self-propagate unless they have boron loadings greater than or equal to 30% by weight. At pressures up to 12 MPa, fuels containing 10% by weight boron would not self-propagate in a nitrogen atmosphere, whereas fuels containing 20% boron would self-propagate at pressures greater than about 5.7 MPa. Preliminary lab-scale rocket motor firings demonstrate the viability of a hybrid rocket based on PTFE and boron mixtures. In addition, they demonstrate that the regression rates of these fuels show dependencies on pressure and possibly oxidizer flow rate as well. Thermochemical analysis suggests that these fuels offer a significant performance benefit in terms of density impulse, while also presenting a significant technological challenge due to excessively high flame temperatures for some mixtures.

Simulation of cavity formation in underground coal gasification using bore hole combustion experiments

Studies on the growth of three-dimensional cavity geometries in underground coal gasification (UCG) are important in exploiting the large fraction of coal that is present in underground coal seams. In the present study, the cavity formation in UCG has been simulated using experiments carried out in three configurations: (i) sublimation experiments in camphor simulating primarily the heat transfer aspects, (ii) bore hole combustion in Acacia nilotica wood bringing in chemical reaction into play, and (iii) bore hole combustion a coal block bringing into consideration the effect of ash on the cavity formation. In all the three cases, the time-evolution of the cavity shape has been monitored under constant oxygen flow rate conditions by measuring the cavity shape and size at periodic intervals. Results show that the cavity formation rates as well as the shape of the cavity are significantly affected by the oxidant flow rate. The importance of the ash present in the coal on the cavity growth has also been brought out. A fair amount of gasification leading to the formation of H 2, CO and CH 4 was observed; this is shown to depend both on the inherent moisture as well as on the reaction zone temperature.

Computational modeling of microfluidic fuel cells with flow-through porous electrodes

In the current work, a computational model of a microfluidic fuel cell with flow-through porous electrodes is developed and validated with experimental data based on vanadium redox electrolyte as fuel and oxidant. The model is the first of its kind for this innovative fuel cell design. The coupled problem of fluid flow, mass transport and electrochemical kinetics is solved from first principles using a commercial multiphysics code. The performance characteristics of the fuel cell based on polarization curves, single pass efficiency, fuel utilization and power density are predicted and theoretical maxima are established. Fuel and oxidant flow rate and its effect on cell performance is considered and an optimal operating point with respect to both efficiency and power output is identified for a given flow rate. The results help elucidate the interplay of kinetics and mass transport effects in influencing porous electrode polarization characteristics. The performance and electrode polarization at the mass transfer limit are also detailed. The results form a basis for determining parameter variations and design modifications to improve performance and fuel utilization. The validated model is expected to become a useful design tool for development and optimization of fuel cells and electrochemical sensors incorporating microfluidic flow-through porous electrodes.

A Membrane Cell for On-line Hydrogen/Deuterium Exchange to Study Protein Folding and Protein-Protein Interactions by Mass Spectrometry

A membrane cell for hydrogen and deuterium exchange on-line with mass spectrometry has been developed to monitor protein-protein interactions and protein conformations. It consists of two channels separated by a semipermeable membrane, where one channel carries the protein sample and the other deuterium oxide. The membrane allows transfer of deuterium oxide into the sample flow. The labeling time is controlled via the flow rate in the sample channel. This cell was validated against three models commonly used in hydrogen-deuterium exchange mass spectrometry: monitoring of folded and unfolded states in a protein, mapping the protein secondary structure at the peptide level, and detection of protein and antibody interactions. The system avoids the conventionally used sample dilution and handling, allowing for potential automation.

Factors Affecting Thickness Variation of ${\rm SiO}_{2}$ Thin Films Grown by Wet Oxidation

Thin oxide films have been grown by wet oxidation on 200 mm silicon wafers at atmospheric pressure in three different furnaces. The effects of temperature tilt, purge flow rate to oxidation flow rate ratio, reactant mole fraction, and oxidation time on axial and radial thickness distributions have been studied. Two of the furnaces share a similar design while the third furnace has a different fluid flow configuration and torch design. The two similar furnaces show virtually identical axial profiles in average thickness after both the pre-oxidation steps and the combined oxidation-purge steps. Under conditions of a standardized run recipe, the wafer-averaged oxide thickness decreases monotonically by about 3 Å down the axis of the boat. Depletion of water due to oxide formation can account for only about 20% of this decrease in oxide growth. A reduction in the concentration of steam due to adsorption and condensation during the oxidation period, which is then reversed during the purge period, is proposed as a significant cause of those thinner oxide films. Thickness variations within single wafers are also significant, with thickness being highest at the side of a wafer corresponding to the furnace outlet and monotonically decreasing to the opposite side.

Mid-baffle interdigitated flow fields for proton exchange membrane fuel cells

A new design of an interdigitated flow field, called as a mid-baffle interdigitated flow field, was built and tested for its effect on the performance of proton exchange membrane (PEM) fuel cells. The results were compared to the conventional interdigitated flow field. Their performances at different oxidant gas flow rates and operating pressures were also examined and compared by using both O 2 and air as the cathode fuel reactants. The experimental results showed that when air was used as the cathode reactant, the cell with the mid-baffle interdigitated flow field outperformed the conventional one, giving a power output approximately 1.2–1.3 times higher depending on the air flow rates. The polarization curves of the mid-baffle interdigitated flow field showed larger limiting current densities at every air flow rate tested in this work. However, the performances of both flow fields were almost the same when the cathode reactant gas was O 2. The test also demonstrated that the flow field performance could be enhanced by increasing the oxidant gas flow rate and cell operating pressure.

Kinetic study for the reduction of residual char particles using oxygen and air

This study characterizes the chemical kinetics for the reduction and elimination of char particles using air and pure oxygen as the oxidants. Commercial carbon black was used as char for the experimental studies reported here. Different oxidant injection flow rates have been examined at various injection temperatures between 400 and 700 °C under atmospheric pressure conditions to obtain the intrinsic kinetic parameters. The char conversion rates have been measured in a laminar flow hot stream using 0.2 g and 0.3 g mass of initial char samples. The kinetic parameters are obtained by fitting the available experimental data into the derived one-film model. The proposed one-film model is compared and analyzed for consistency and reliability for the calculated intrinsic kinetic parameters.