Inlet O2 Concentration Research Articles

The Combined Effect of the Oxidizing Agent and its Concentration on the Oxidative Coupling of Methane

AbstractThe combined use of O2 and CO2 as oxidizing agents in the oxidative coupling of methane (OCM) has been assessed over two representative OCM catalysts, i. e., La−Sr/CaO and NaMnW/SiO2, under a wide range of inlet O2 and CO2 concentrations. The overall impact of CO2 was found to depend on the operating conditions and the catalyst used. At O2‐rich conditions, a negative effect of CO2 was observed on C2 selectivity. At O2‐lean conditions, a positive effect of CO2 on both catalysts was observed, likely originating from an enhanced dehydrogenation of C2H6 to C2H4 and limited deep oxidation of the involving hydrocarbons and reaction intermediates. The behavior induced by CO2 is attributed to its mild oxidizing ability and interactions with the catalysts. The enhanced dehydrogenation of C2H6 was observed and confirmed by specific tests with a CO2 and C2H6 feed. Well‐controlled CO2 addition improved the value of the OCM product mixture, mainly due to a 9 % increase in C2H4 and a 19 % increase in CO selectivity, resulting in a 25 % reduction of CO2 at the outlet over the La−Sr/CaO catalyst. These findings provide new insight into the CO2 effect on OCM and the data to translate this research into OCM process alternatives.

A methodology for structure dependent global kinetic models: Application to the selective catalytic reduction of NO by hydrocarbons

Global kinetic models capable of capturing experimentally observed features of catalytic reactions are vital in design and optimization studies. In this work, a detailed analysis of the selective catalytic reduction of NO and NO2, particularly in automotive exhaust control is undertaken. The prominent metal based catalysts for this reaction range from Pt, Au & Rh to cheaper options including Cu, Ag & Co supported on Al2O3, SiO2, and occasionally, zeolites. Here, we focus on Ag & Co supported on Al2O3 catalysts, and propose a computationally tractable kinetic model capable of capturing the observed SCR features across a range of catalyst synthesis and reactor operating conditions. In particular, the effects of metal loading on catalyst performance are closely examined. In SCR, an important aspect is the catalytic activity at higher inlet O2 concentrations. The global kinetic model proposed here is shown to predict the trends with respect to reactor temperature and inlet feed compositions (including O2), well, for both catalysts.

Study on Co-firing Characteristics and NOx emission of Ammonia/Propane

Reducing the use of carbon fuel is one of the important measures to achieve zero carbon emission. As a carbon-free and high-hydrogen fuel, ammonia has promising application prospects because of its high energy density and low transportation and storage cost. Due to the characteristics of high ignition temperature, low flame propagation combustion speed and high NOx concentration in the combustion process, its wide application needs further research. In this paper, the co-combustion and the NOx emission characteristics of ammonia mixed with propane are experimental investigated for pure ammonia, 5% propane and 10% propane mixtures. The effects of ammonia/propane ratio, air excess coefficient and inlet gas oxygen concentration on combustion and NOx emission characteristics are explored. The results obtained show that the highest NOx emission is formed by 10% propane ratio case, and the lowest NOx is for pure ammonia. With the increase of propane, the concentrations of NOx, NO and NO2 increase and the concentration of oxygen decreases. With the increase of air excess coefficient from 15% to 21%, NOx, NO and NO2 also increase, and the concentration of oxygen goes up. With the increase of inlet oxygen concentration from 15% to 21%, the concentrations of NOx and NO at the exit of furnace changes less for pure ammonia, and 75% increase for 5% propane; while for 10% propane, the concentration of NO first decreases rapidly from 1498ppm to 1071ppm with O2 decreasing from 15% to 18%, then increases to about 1700ppm again when the inlet O2 concentration increases from 18% to 21%.

Data-driven prediction of flame temperature and pollutant emission in distributed combustion

The flame temperature and pollutant emission (of NO and CO) characteristics in distributed combustion were examined using data-driven artificial neural network (ANN) approach. Experimental results collected from swirl-assisted distributed combustion using methane fuel at an equivalence ratio 0.9 were utilized for dataset preparation. The distributed combustion condition was created in a swirl-assisted burner (at thermal intensity of 5.72 MW/m3-atm) by diluting the main airstream with carbon dioxide. Experimental results of exhaust NO and CO concentrations and adiabatic flame temperature (AFT) derived from Chemkin-Pro® simulation were selected as the target output. The ANN model was developed with inlet airflow rates and O2 concentrations as the input, and pollutant emission and AFT as the output variables. The ANN possessed one hidden layer with variable number of neurons (10, 15, and 20). Tangent sigmoid and Log sigmoid transfer functions were tested along with feed forwards and cascade forward network schemes having Levenberg–Marquardt backpropagation training algorithms. Different learning rates of model training were investigated to determine the optimized training model. The best model (with learning rate 0.2) was selected based on the mean square error (MSE) and the strength of correlation (R) predicted for individual outputs. The model predicted very well the target outputs in both conventional swirl combustion and distributed combustion regime for wider applicability in a range of applications. A very strong correlation was predicted by the current ANN model for the overall data as indicated by the R value of 0.9842. Furthermore, the results obtained with Bayesian Regularization backpropagation method demonstrated better prediction than those from Levenberg–Marquardt algorithm.

Oxy-Fuel Combustion of Hard Coal, Wheat Straw, and Solid Recovered Fuel in a 200 kWth Calcium Looping CFB Calciner

The fluidized bed combustion (FBC) of biomass and solid recovered fuel (SRF) is globally emerging as a viable solution to achieve net-negative carbon emissions in the heat and power sector. Contrary to conventional fossil fuels, alternative fuels are highly heterogeneous, and usually contain increased amounts of alkaline metals and chlorine. Hence, experimental studies are mandatory in order to thoroughly characterize the combustion behavior and pollutant formation of non-conventional fuels in novel applications. This work gives an overview of experimental investigations on the oxy-fuel combustion of hard coal, wheat straw, and SRF with a limestone bed in a semi-industrial circulating fluidized bed (CFB) pilot plant. The CFB combustor was able to be operated under different fuel blending ratios and inlet O2 concentrations, showing a stable hydrodynamic behavior over many hours of continuous operation. The boundary conditions introduced in this study are expected to prevail in carbon capture and storage (CCS) processes, such as the oxy-fuel combustion in the CFB calciner of a Calcium Looping (CaL) cycle for post-combustion CO2 capture.

Process optimization of S (IV) oxidation in flue gas desulfurization scrubbers

At present, the control of oxidation process is imprecise in the wet flue gas desulfurization system. In this study, a modified oxidation model is established to investigate convective mass transfer of O2 and the oxidation of S (IV) under natural and forced oxidation conditions. By comparison, the model results are in good agreement with experimental results. Based on the motion of droplet in the scrubber and bubble in the slurry pool, natural oxidation rate is calculated to provide guidance for forced oxidation and recommended oxidation air flow rate. Droplet accelerates until it reaches terminal velocity. Mass transfer resistance mainly occurs in the liquid film. The mass transfer flux of the droplet with diameter of 1.5 mm increases continuously until it remains stable. The natural oxidation rate increases with the increasing of inlet O2 concentration while the forced oxidation rate increases with the increasing of oxidation air flow rate. Several gas flow rates are taken into consideration to obtain the optimal oxidation air flow rate, showing that smaller drop diameter has positive impact on oxidation process in flue gas desulfurization scrubbers.

Experimental tests on co-firing coal and biomass waste fuels in a fluidised bed under oxy-fuel combustion

The co-firing of coal and biomass waste in oxy-fuel fluidised beds is an important oxy-fuel combustion technology for CO2 capture and waste disposal. However, practical experience regarding co-firing in an oxy-fuel fluidised bed is still limited. In this study, a 10 kWth oxy-fuel fluidised bed with continuous fuel-feeding and real-time flue gas measurements was developed. The combustion stability and dynamic response during the switch in combustion states were examined. The effect of the combustion atmosphere (air or oxy-fuel combustion), inlet O2 concentration (V0o2), combustion temperature, fuel type, and biomass blending mass ratio (Mb) on the combustor temperature distribution, CO2 enrichment, carbon conversion, and combustion residues were systematically investigated. The results indicated that the combustion state can switch quickly during the co-firing process (similar to the rapid switch in combustion states during oxy-coal or oxy-biomass combustion), and stable combustion in the fluidised bed can be maintained when V0o2 is increased to 30% or more. The optimal range of Mb, which is directly related to the fuel type and combustion mode, contributes to a highly uniform temperature distribution, increased CO2 generation, and decreased amount of unburnt carbon in the fly ash. The bottom slag had a honeycomb structure, which became larger than quartz sand (bed material) after 120 h of combustion. Furthermore, no agglomeration of bottom slag, quartz sand, and fly ash was detected during the tests.

3D simulation on pressurized oxy-fuel combustion of coal in fluidized bed

Pressurized oxy-fuel combustion technology is considered as a perspective carbon capture technology in industrial process. A computational fluid dynamics (CFD) model based on Multi-Phase Particle-In-Cell (MP–PIC) method was developed to predict pressurized oxy-coal combustion process in fluidized bed. The heterogeneous and homogeneous combustion reactions of coal were considered in this model. The predicted results were validated the accuracy of this model with experimental data from a 15 kWth pressurized fluidized bed combustor in terms of the gas component and temperature characteristics. The characteristics of gas–solid flow and combustion under different pressure (0.1–2 MPa) and oxygen atmosphere were studied in this work. The predicted results show that the intensity of particle motion and the expansion degree in the fluidized bed was gradually decreased with an increase in pressure. A correlation was proposed based on the simulation results to maintain suitable fluidization conditions in pressurized circulating fluidized bed at different pressures. The temperature of particle phase region gradually increased with combustion pressure and inlet O2 concentration increased. In addition, the CO2 concentration in outlet increased while the emission of CO and NOx decreased as the combustion pressure increased.

Ash Formation and Deposition in Oxy-fuel Combustion of Rice Husk, Coal, and Their Blend with 70% Inlet O2

Managing the risk of ash deposition is still a priority for oxy-fuel combustion of biomass–coal blends. This is especially a concern when high inlet O2 concentrations are adopted to reduce the amou...

Evaluating aeration and stirring effects to improve itaconic acid production from glucose using Aspergillus terreus

The effects of the bioreactor conditions, in particular the mode and intensity of aeration and mixing were studied on itaconic acid (IA) fermentation efficiency by Aspergillus terreus strain from glucose substrate. IA was produced in batch system by systematically varying the oxygen content of the aeration gas (from 21 to 31.5 vol% O2) and the stirring rate (from 150 to 600 rpm). The data were analyzed kinetically to characterize the behavior of the process, and besides, the performances were evaluated comparatively with the literature. It turned out that the operation of the bioreactor with either the higher inlet O2 concentration (31.5 vol% O2) or faster stirring (600 rpm) could enhance biological IA generation the most, resulting in yield and volumetric productivity of 0.31 g IA/g glucose and 0.32 g IA/g glucose and 3.15 g IA/L day and 4.26 g IA/L day, respectively. Overall, the significance of fermentation settings was shown in this work regarding IA production catalyzed by A. terreus and notable advances could be realized by adjusting the aeration and stirring towards an optimal combination.Graphic abstract

Study on oxy-fuel combustion behaviors in a S-CO2 CFB by 3D CFD simulation

The supercritical CO2 (S-CO2) power cycle CFB with oxy-fuel combustion is considered as a perspective carbon capture technology with high net efficiency. A three-dimensional (3D) CFD simulation using MP-PIC method was developed to predict oxy-fuel combustion in a 12 MWth S-CO2 CFB. This model couples the gas-solid flow, the heat transfers with combustion reactions, and takes into account of the particle size distribution. The effects of the different thermal wall boundary conditions and various O2 concentration on the particle motion, combustion characteristics and gaseous pollutant emissions were systemically studied in this work. Results show that the combustion efficiency and the emission of CO, NO and SO2 in the S-CO2 boiler were higher than those in the conventional steam boiler. The increase in the inlet O2 concentration increased the temperature of gas phase, the combustion efficiency and the desulfurization efficiency in the S-CO2 boiler.

NOx adsorption and desorption of a Mn-incorporated NSR catalyst Pt/Ba/Ce/xMn/γ-Al2O3.

This study evaluated the NOx adsorption and desorption performance as well as the casual relationship underlying a Mn-incorporated catalyst (Pt/Ba/Ce/xMn/γ-Al2O3). NOx adsorption and desorption are regarded as a prominent index for the NOx removal performance of NOx storage and reduction; we utilized NOx storage experiments with various inlet NO and O2 concentrations and cycling adsorption/desorption experiments with a couple of adsorption time protocols for performance evaluation. In-suit DRIFT and NOx-TPD tests were implemented to reveal the instant stored species and their thermal stability. Eight percent of Mn catalyst at 350°C was adopted in the described experiments for its desirable NOx adsorption characteristics. The optimal NOx storage performance was found under 10% O2, deteriorating when the concentration was further increased. Furthermore, elevating NO concentration impaired the NOx adsorption due to the low NO2/NOx ratio. It was also found that shorter adsorption time facilitated NOx removal via maintaining an unsaturated state for active storage components in terms of a fixed desorption time. The stored species existed as nitrites and nitrates with a good low-temperature thermal stability which however decayed at higher temperatures as exhibited in the DRIFT and NOx-TPD tests. These findings provided invaluable information for the application of Mn-incorporated catalyst for NOx removal in diesel exhaust purification to relieve the aerial pollution.

Numerical simulation of circulating fluidized bed oxy-fuel combustion with Dense Discrete Phase Model

Computational fluid dynamics (CFD) has become a valuable tool to study the circulating fluidized bed (CFB) combustion process. This work conducts the CFB oxy-fuel combustion process with different O2 concentrations by using the Dense Discrete Phase Model (DDPM) based on the Eulerian-Lagrangian concept, in which the gas is treated as a continuous phase whereas the solid is tracked in a discrete way. The effects of heat and mass transfer, gas-turbulence as well as the homogeneous and heterogeneous reactions are considered. Simulated results are analyzed both qualitatively and quantitatively in terms of particle flow structure, temperature, carbon conversion and gas composition. Results show that the physical and thermo-chemical properties of the coal particles with different diameters vary greatly, which illustrates the significant particle size distribution (PSD) effects on the simulated results. Besides, with the increase of the inlet O2 concentration, the coal particles exhibit a different distribution of the flow structure and combustion characteristics due to both a decrease of the superficial gas velocity and an increase of the chemical reaction rate. The simulated results are well validated by the available experimental data, indicating the feasibility of the established DDPM for the CFB combustion process.

Comparative numerical evaluation of autothermal biogas reforming in conventional and split-and-recombine microreactors

A new microreactor design featuring embedded passive mixing elements was tested as a means to enhance autothermal reforming reaction of biogas over a novel ReNi/γ-Al2O3 catalyst. To determine an optimal condition that would result in completely converted biogas with H2/CO product ratio of around one and minimal hot spot formation inside the reactor, use of various inlet O2 and H2O concentrations and inlet temperatures were numerically investigated. The influence of inlet reactant velocity on the reactor effectiveness was then studied at the optimal condition. Performance of a straight-channel microreactor was also studied and compared with that of the novel microreactor. The O2:H2O:CO2:CH4 ratio of 25:5:28:42% (v/v) and inlet temperature of 730 °C were noted as the optimal condition for the novel microreactor. Complete biogas conversion over a wider range of inlet Reynolds number, lower required catalyst loading to achieve the desired reactor performance, higher H2 and CO selectivities and reduced hot spot formation were noted as the advantages of the novel microreactor.

Study of nitric oxide catalytic oxidation on manganese oxides-loaded activated carbon at low temperature

Nitric oxide (NO) is an air pollutant that is difficult to remove at low concentration and low temperature. Manganese oxides (MnOx)-loaded activated carbon (MLAC) was prepared by a co-precipitation method and studied as a new catalyst for NO oxidation at low temperature. Characterization of MLAC included X-ray diffraction (XRD), scanning electron microscopy (SEM), N2 adsorption/desorption and X-ray photoelectron spectroscopy (XPS). Activity tests demonstrated the influence of the amount of MnOx and the test conditions on the reaction. MLAC with 7.5wt.% MnOx (MLAC003) exhibits the highest NO conversion (38.7%) at 1000ppm NO, 20 vol.% O2, room temperature and GHSV ca. 16000h−1. The NO conversion of MLAC003 was elevated by 26% compared with that of activated carbon. The results of the MLAC003 activity test under different test conditions demonstrated that NO conversion is also influenced by inlet NO concentration, inlet O2 concentration, reaction temperature and GHSV. The NO adsorption-desorption process in micropores of activated carbon is fundamental to NO oxidation, which can be controlled by pore structure and reaction temperature. The activity elevation caused by MnOx loading is assumed to be related to Mn4+/Mn3+ ratio. Finally, a mechanism of NO catalytic oxidation on MLAC based on NO adsorption-desorption and MnOx lattice O transfer is proposed.

Nitrogen Oxide Removal from Simulated Flue Gas by UV-Irradiated Sodium Chlorite Solution in a Bench-Scale Scrubbing Reactor

Ultraviolet irradiated sodium chlorite (UV/NaClO2) solution was introduced to remove nitrogen oxide (NOx) from simulated flue gas in a bench-scale scrubbing reactor. Effects of UV irradiation time, NaClO2 concentration, NO inlet concentration, pH value, and O2 concentration were investigated separately. Results showed that NaClO2 solution with UV pretreatment achieved a remarkable promotion in NOx removal efficiency. The ClO2 produced from photodecomposition of NaClO2 in aqueous solution substantially improved the NO absorption process. The NOx removal efficiency by UV/NaClO2 solution increased from 28 to 77% as the UV irradiation time increased from 0 to 600 s. When the NaClO2 concentration increased, the NOx removal efficiency by UV/NaClO2 solution increased, whereas the enhancement factor decreased. The NO absorption rate by UV/NaClO2 solution increased with NO inlet concentration. The NOx removal efficiency by UV/NaClO2 solution was higher than that by NaClO2 solution without UV pretreatment at a pH r...

Elementary kinetics of the oxygen reduction reaction on LSM-YSZ composite cathodes

A multi-physics based transient, continuum model of a solid oxide half-cell comprising of a porous LSM-YSZ composite cathode sintered to a dense YSZ electrolyte was developed to investigate the oxygen reduction reaction kinetics. The model coupled species, electron and ion transport through the porous cathode to surface and electro-chemistry. The electrochemical reduction of O2 was modeled using three candidate elementary kinetic mechanisms. Each mechanism included parallel surface and bulk pathways for O2 reduction and was driven by three different electric phase potentials. The mechanisms were compared against three sets of electrochemical impedance spectra and polarization curves measured by Barbucci et al. (2009), Cronin et al. (2012) and Nielsen and Hjelm (2014) over a wide range of operating temperatures (873–1173K), inlet O2 concentrations (5–100%) and overpotentials (−1V to +1V). Two of the three mechanisms were able to quantitatively reproduce the three sets of experiments by only tweaking the microstructural parameters for each individual set. Yet on analyzing their kinetic and thermodynamic parameters, the mechanism postulating the chemisorption of gas-phase O2 on LSM to form the superoxo-like adsorbate O2- was determined to be the most realistic. A model-based sensitivity analysis revealed that ionic transport in the YSZ phase, O2 dissociation in conjunction with surface to bulk charge transfer in the LSM phase and charge transfer at the three phase boundary were the rate limiting steps throughout the operating space. Additionally, the bulk pathway was found to be insignificant.

A comprehensive experimental and modeling study of sulfur trioxide formation in oxy-fuel combustion

This study focuses on the sulfur chemistry occurring within an oxy-combustion system by adopting a combined approach of detailed kinetic modeling and experiments. Influences of variable combustion parameters on SO3 generation have been investigated computationally by implementing different combustion mechanisms. Results indicated significant influences of the equivalence ratio, inlet SO2 concentration and presence of NO on SO3 formation while the inlet O2 concentration exhibited minimal effect. Presence of NO demonstrated different degrees of direct and indirect interactions between NOx and SOx species for different reaction sets. Sensitivity analysis indicated radicals to be playing a dominant role in SO3 formation. Collected temporal profiles of SO3 from the lab-scale combustion setup showed evidence of significant SO3 formation at temperatures as low as 650K under realistic boiler temperature conditions. The equivalence ratio and the concentrations of SO2, O2 and NO were found to influence the SO3 formation. Predictions of final SO3 concentration from different mechanisms were comparable with the experimental data; however, the trend of the temporal profiles could not be anticipated by the models. Experiments conducted in the presence of NO indicated the existence of interactions between NOx and SOx species and also the requirement of modifications to the existing oxy-combustion models.

Oxy-fuel Combustion of Estonian Oil Shale: Kinetics and Modeling

In this work, non-isothermal thermal analysis (TA) methods combined with Fourier transform infrared (FTIR) spectroscopy were applied to investigate specifics of different stages of oxy-fuel (OF) combustion of Estonian oil shale (EOS) and its char. Kinetics of EOS and its char oxidation were analyzed in different atmospheres in order to understand the complex mechanism of oil shale (OS) OF combustion. Additionally, the OF combustion of OS in circulating fluidized bed (CFB) was simulated with the recently released Aspen Plus fluidized bed (FB) reactor which treats bottom zone and freeboard hydrodynamics. Particular attention was given to the determination of the required elutriated mass flows to maintain the heat balance of the system for OF combustion cases. Four case studies were simulated including: Case 1: Air combustion, Case 2: 21%O2/flue gas, Case 3: 23%O2/flue gas and Case 4: 30%O2/flue gas. The results of TA experiments show that the pyrolysis behavior is very similar in Ar and CO2 until 500°C and there is no visible char carbon and CO2 reaction under OF conditions. The emissions of CO2 from mineral part of OS can be diminished as decomposition of calcite takes place at higher temperatures in OF combustion. Activation energies calculated for oxidation of OS and its char in CO2/O2 are notably less than activation energies calculated for Ar/O2 atmosphere. Modeling results show that higher fuel mass flow rate and higher O2 concentration in the oxidizer have to be considered in the set of conditions for OF in order to extract the same amount of heat as in air combustion. The Case 3 with 23% inlet O2 concentration has a similar behavior as compared to air combustion in terms of temperature of the boiler and recirculation rate of the particles. The data obtained from experimental measurements and models is valuable for the possible implementation of OF combustion of EOS in CFB boilers.

High-efficiency removal of NO x by a novel integrated chemical absorption and two-stage bioreduction process using magnetically stabilized fluidized bed reactors

To enhance the bioregeneration of Fe(II)EDTA and to avoid the inhibition of the components in nitrogen oxides (NOx) scrubbing solution, a novel integrated process of metal chelate absorption and two-stage bioreduction was developed. In this process, magnetically stabilized fluidized beds (MSFB) were used as the bioreactors, and the phase diagram for the MSFB operation was determined. Factors including inlet NO, O2 and SO2 concentrations, magnetic field intensity, gas flow rate and liquid circulation rate, were studied experimentally to investigate their effects on NO removal. In addition, a mathematical model for NO removal in this integrated system was developed. The results revealed that the integrated system could be steadily operated with a high NO removal efficiency and elimination capacity, even under the condition of high NO and O2 shock-loading. The established model showed that NO removal efficiency was related to the spray column property and the active Fe(II)EDTA concentration, while the latter depends on the bioregeneration of the disabled absorbent in the MSFB.