NOx emission characteristics of counterflow syngas diffusion flames with airstream dilution

Syngas is produced through a gasification process using variety of fossil fuels, including coal, biomass, organic waste, and refinery residual. Although, its composition may vary significantly, it generally contains CO and H₂ as the dominant fuel components with varying amount of methane and diluents. Due to its wide flexibility in fuel sources and superior pollutants characteristics, the syngas is being recognized as a viable energy source worldwide, particularly for stationary power generation. There are, however, gaps in the fundamental understanding of syngas combustion and emissions, as most previous research has focused on flames burning individual fuel components such as H₂ and CH₄, rather than syngas mixtures. This paper reports a numerical investigation on the effects of syngas composition and diluents on the structure and emission characteristics of syngas nonpremixed flames. The counterflow syngas flames are simulated using two representative syngas mixtures, 50%H₂/50%CO and 45%H₂/45%CO/10%CH₄ by volume, and three diluents, N₂, H₂O, and CO₂. The effectiveness of these diluents is characterized in terms of their ability to reduce NO_x in syngas flames. Results indicate that syngas nonpremixed flames are characterized by relatively high temperatures and high NO_x concentrations and emission indices. The presence of methane in syngas decreases the peak flame temperature, but increases the formation of prompt NO significantly. Consequently, while the total NO formed is predominantly due to the thermal mechanism for the 50%H₂/50%CO mixture, it is due to the prompt mechanism for the 45%H₂/45%CO/10%CH₄ mixture. For both mixtures, CO₂ and H₂O are more effective than N₂ in reducing NO_x in syngas flames. H₂O is the most effective diluent on a mass basis, while CO₂ is more effective than N₂. The effectiveness of H₂O is due to its high specific heat that decreases the thermal NO, and its ability to significantly reduce the concentration of CH radicals, which decreases the prompt NO. The presence of methane in syngas reduces the effectiveness of all three diluents.     

The effect of oxygen and carbon dioxide concentration on soot formation in non-premixed flames

The influence of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation was studied by Time Resolved Laser Induced Incandescence (TIRE-LII) and TEM photography in non-premixed co-flowing flames. TIRE-LII method was used to measure the distribution of two-dimensional soot volume fraction and primary particle size. The soot was directly sampled by the thermophoretic method, and its diameter was examined by TEM photography. Two suitable delay times of the TIRE-LII method affecting measurable range and sensitivity were determined by comparing TEM photographs with the TIRE-LII signal. The effects of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation were investigated with these calibrated techniques. An O₂+(CO₂, N₂, and [Ar+CO₂]) mixtures in co-flow were used to isolate carbon dioxide effects systematically. The primary particle number concentration and soot volume fraction were abruptly decreased by the addition of carbon dioxide to co-flow. This suppression was resulted from the short residence time in inception region because of the late nucleation and the decrease of surface growth distance by the low flame temperature due to the higher thermal capacity and the chemical change of carbon dioxide. The increase of oxygen concentration in the co-flow caused an enhancement of soot nucleation and thus the residence time increase, but the specific growth rate showed almost the same value regardless of the co-flow mixture in the growth region. This result suggests that the specific growth rate has a weak dependence on the relative change of co-flow conditions in non-premixed co-flowing flames.     

Experimental determination and simulation of explosion limits of benzene-inert gas-air mixtures

In this work, the influences of inert diluents, N₂ and CO₂, on the explosion limits of benzene-air mixtures were investigated at two different initial temperatures. The model of constant flame temperature was tested for the simulation of the experimental data. It was seen that the model fitted the data of the lower explosion limits, but didn't correlate with the data of the upper explosion limits.     

Simulation of NOx gas absorption under adiabatic condition and comparison with plant data

A mathematical model has been developed for the absorption of NO_x gases in aqueous solutions of NaOH. The present model is for adiabatic operation, which is an extension of the previous isothermal model of Pradhan and Joshi (Chem. Eng. Sci. 55(7) (2000) 1269). Heat effects have been considered for gas phase reactions such as the oxidation of NO, dimerisation of NO₂, and formation of N₂O₃, HNO₂ and HNO₃. Absorption of NO₂, N₂O₃, HNO₂(g), HNO₃(g), into NaOH solutions is accompanied by large heat effects which have been incorporated in the model. Evaporation/condensation of water has also been considered. A comparison between isothermal and adiabatic operation has been presented. The model was validated using data from an operating small-scale plant which uses the process of absorption of NO_x in NaOH and a good match between plant and simulation results was obtained. Minor modifications were suggested in the existing plant which would improve the selectivity of the product NaNO₂.     

Effect of preparation parameters on performance of dense homogeneous polycarbonate gas separation membranes

The aim of this study is to determine the effect of preparation parameters on the membrane permeation mechanism and separation performances of dense homogeneous poly(bisphenol A carbonate) (PC) membranes. Blade‐casting and drop‐casting techniques are used for casting films from solutions with varying concentrations. Chloroform and methylene chloride are used to determine the effect of the properties of the casting solvent. Permeation measurements are done with Ar, N₂, O₂, CH₄, CO₂, and H₂ gases. The selectivity values of dense homogeneous membranes with a PC composition of 7% (w/v chloroform) are 1.7 for Ar/N₂, 1.1. for CH₄/N₂, 18 for CO₂/N₂, 26 for H₂/N₂, and 9.7 for O₂/N₂. These values with a PC composition of 15% (w/v methylene chloride) are 2.7, 1.3, 25, 44, and 13 for the same gases, respectively. In addition to the importance of the solvent type and composition, the casting type and thermal history also affects the performance of the membrane. Increasing the annealing period enhances the selectivity while decreasing the permeability for both casting solvents, yet this thermal history effect strongly depends on the solvent type because of solvent–polymer interactions. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 776–785, 2003     

NO x Reduction by Ethanol on Pd/Sulphated Zirconia

The NO reduction by ethanol was studied on palladium catalyst supported on sulphated zirconia. Temperature programmed desorption of NO and ethanol (TPD) and temperature programmed surface reaction (TPSR) analyses as well as catalytic tests in reducing and oxidizing conditions (O₂ presence), besides diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed the formation of intermediate species during the reaction, such as ethoxy species that reacted forming ethylene. Besides dehydrogenate formed adsorbed acetate species, which than decompose and/or react with hydroxyls of the support. The sulphated zirconia support increased the acid sites with the formation of strong Brönsted sites, favoring the formation of ethoxy species. Acetate species also react with NO adsorbed on Pd forming N₂, N₂O, CO and CO₂. The excess of O₂ favored ethanol oxidation to CO₂, consequently less ethanol was available to react with NO _x .     

Effect of H2O and CO2 on NOx emission control for lean-burn engines by electrochemical-catalytic cells

Lean-burn engines can offer superior fuel efficiency but require advanced technology for NO_x emission control. Electrochemical-catalytic cell has been proposed for lean DeNO_x. This work demonstrates that the DeNO_x rate can be enhanced by the presence of H₂O and/or CO₂, and can increase with increasing H₂O and CO₂ concentrations, although the increased extent is quite small. In the low NO_x concentration range, relatively constant DeNO_x rates were observed and can result in zero NO_x emissions, where the presence of H₂O and CO₂ has important enhancement effect. Higher temperature generally results in larger N₂ selectivity in the low NO_x concentration region.     

NOx precursors evolution during coal heating process in CO2 atmosphere

The pyrolysis/gasification experiments of Xuzhou bituminous coal (XZ) and Longyan anthracite (LY) were carried out in a tube furnace under Ar or CO₂ atmosphere, and the effect of CO₂ on the evolution of NO_x precursors, NH₃ and HCN, was studied using a Fourier transform infrared (FTIR) spectrometer. Results show that CO₂ influences NH₃ and HCN evolution process in two main ways: one is blocking the contact of the N-sites and the H-radicals by absorbed on the coal matrix surface at low temperature, and the other is opening the N-sites from the coal matrix by gasification at high temperature. For both XZ and LY coals, CO₂ atmosphere suppresses NH₃ yield and enhances HCN yield due to the gasification effect compared with that in Ar atmosphere. But the impact is not the same. The HCN/NH₃ ratio is elevated in CO₂ atmosphere compared with that in Ar atmosphere.     

Experimental study on effects of operating conditions and fuel quality on thermal efficiency and emission performance of a 300-MW boiler unit firing Thai lignite

This paper presents the results of an experimental study on a 300-MW boiler unit fired with Thai lignite. Effects of operating conditions (excess air ratio and unit load) and fuel quality on the boiler heat losses and thermal efficiency as well as on the gaseous (CO₂, CO, NO_x and SO₂) and particulate matter (PM) emissions from the boiler unit are discussed. The boiler thermal efficiency was weakly affected by the excess air ratio, unit load and fuel lower heating value, varying from 90.3 to 92.3% for wide ranges of the above variables. In all the tests, the NO_x, SO₂ and PM emissions were below the national emission standards for these pollutants. Quite low level of the SO₂ emission was secured by the high-efficiency flue gas desulphurization system. The CO emissions of rather small values were detected only at extremely low excess air ratios. The emission rate and specific emission (i.e. per MWh of electricity produced) for NO_x, SO₂ and CO were quantified using experimental emission concentrations of the pollutants. Meanwhile, the emission characteristics for CO₂ were determined with the use of fuel-C and fuel consumption by the boiler. In addition, the emission rate and specific emission for PM were estimated by taking into account the actual fuel-ash content and fuel consumption by the boiler, as well as the effects of SO₂ adsorption by fly ash in the boiler gas ducts and overall ash-collecting efficiency of the electrostatic precipitators and flue gas desulphurization system. Elevated CO₂ and NO_x emissions from the 300-MW boiler units firing Thai lignite are of great concern.     

Experimental investigation of NOx emissions in oxycoal combustion

This work presents the results of an experimental investigation on NO_x emissions from coal combustion in a pilot scale test facility. Three oxidiser atmospheres have been compared, namely air, CO₂/O₂, and O₂ enriched recirculated flue gas. NO_x emissions from two different combustion modes have been studied, swirl flame and flameless combustion. The influence of the burner oxygen ratio and the oxidiser O₂ concentration on NO_x formation and reduction have been analysed. With increasing burner oxygen ratio, an increase of NO_x emissions has been obtained for air and CO₂/O₂ in both, swirl flame and flameless combustion. In case of the swirl flame, flue gas recirculation leads to a reduction of NO_x emissions up to 50%, whereas in case of flameless combustion this reduction is around 40% compared to CO₂/O₂. No significant impact of the oxidiser O₂ concentration in the CO₂/O₂ mixture on NO_x emissions is observed in the range between 18 and 27 vol.% in swirl flames. An analysis of NO_x formation and reduction mechanisms showed, that the observed reduction of NO_x emissions by flue gas recirculation cannot be attributed to the reduction of recirculated NO_x alone, but also to a reduced conversion of fuel-N to NO.     

Modeling of NOx conversion during combustion under high CO2 concentration using detailed chemical kinetics

Combustion of fuels under enhanced oxygen atmospheres has been well investigated over the past decades in various types of combustors, varying from diesel engines to coal-fired boilers. Most studies have found significantly lower NO_x emissions during Oxy-coal combustion. In this paper, NO_x combustion chemistry under O₂/CO₂ atmosphere as well as air atmosphere was studied using detailed kinetic model. A suitable reaction mechanism was chosen based on the comparison between the calculation result and the experimental data. The influence of various parameters (temperature, CO₂ concentration) on NO_x conversion was investigated. The chemical effects of high CO₂ concentration on NO formation and destruction process was studied. On the basis of investigations through elementary chemical reactions, it can be concluded that high CO₂ concentration play a pronounced role on NO_x conversion process. Moreover, the dominant reaction steps contribution to production and destruction of NO as well as the most important reactions for NO reduction under different atmospheres were discussed.     

Formation of NOx precursors during wheat straw pyrolysis and gasification with O2 and CO2

The release behavior tests of NO_x precursors from wheat straw during pyrolysis in argon and gasification in 5%O₂/95%Ar and 5%CO₂/95%Ar were performed using a thermogravimetric analyzer (TGA) coupled with a Fourier transform infrared (FTIR) spectrometer. The results show that heating rate and particle size have substantial effects on the selectivity of N-conversion due to the selectivity of cracking of cyclic amides and the secondary reaction influencing the formation of NH₃, HCN and HNCO. The atmosphere influences the N-selectivity to HCN, NH₃, NO and HNCO. The formation of HCN and NH₃ in 5%O₂/95%Ar is a result of competition among the opposing effects of O₂. The presence of O₂ promotes the yields of HCN and HNCO evidently, and HNCO seems to be a favourable product from biomass-N compared in Ar atmosphere although HCN yield is a little bigger than that of HNCO. The use of CO₂ reduces the formation of HCN, the yield of NH₃ keeps essentially constant compared in Ar, and the emission of HNCO is suppressed. NH₃ seems to be a favourable product from biomass-N in 5%CO₂/95%Ar.     

Untersuchungen zum Hochdruck‐Permeationsverhalten reiner Gase durch mikroporöse keramische Membranen Teil 1. Messmethode und Einfluss der Adsorption auf den Permeatfluss

The permeation behaviour of single gases (He, H₂, N₂, Ar, CH₄, CO₂) through meso (ZrO₂) and micro (TiO₂) porous ceramic membranes was measured within a pressure range of 1 to 10 MPa and a temperature range of 293 to 373 K, using steady state and dynamic experimental methods. The TiO₂‐membrane shows by adsorption affected permeation, whereas the ZrO₂‐membrane is not influenced by these effects.     

Tensile deformation of polychlorotrifluoroethylene in He,N2, Ar,O2, and CO2 environments

The tensile stress—strain behaviour of quenched polychlorotrifluoroethylene (PCTFE) was measured from 77K to T_g in gaseous environments of He, N₂, Ar, O₂, and CO₂ and in water. The partial pressure of the gas was varied from 0 to 1 atm. N₂, Ar, O₂ and CO₂ produced crazing and a lowering of the tensile strength relative to the intrinsic tensile strength at that temperature. The effect of pressure and temperature on the tensile strength was quantitatively similar to the effect of these gases on PC and PMMA, and could be described by an equation of the form: $\text{σ}{}_{\mathrm{c}}\text{σ}{}_{\mathrm{i}}\text{= [P}\text{exp}\text{(}\text{Q}\text{RT}\text{/P1]}{}^{\mathrm{?0.10}}$. The constants Q and $\text{P1}$ depended on the gas, and σ_i is the intrinsic strength. The Q values were approximately the same as the heat of vaporization for each gas. CO₂ produces crazing below as well as above its sublimation temperature. The intrinsic yield point varied from 0.033 to 0.054 of E, Young's modulus, as the temperature varied from 300 to 77K. The experimental value of the yield point extrapolated to 0.055E at OK and compares favourably with existing theories for the yield point of glassy polymers.     

Global kinetic modelling of the reaction of soot with O2 and NOx on Fe2O3 catalyst

This study deals with the catalytic reaction of NO_x and soot on Fe₂O₃ to yield N₂ and CO₂ in excess of oxygen. Based on the three types of kinetic experiments, i.e. temperature programmed oxidation (TPO), transient examinations and gradient-free loop reactor experiments, as well as mechanistic studies presented recently a global kinetic model is established. The model includes catalytic effect of the iron oxide on soot/O₂ reaction, whereas it is assumed that NO_x reduction occurs on the soot without direct participation of Fe₂O₃. Furthermore, the model implies global kinetic expressions for the CO_x formation and NO_x reduction. These equations include the evolution of the surface area of soot and the correlation of reactive carbon sites (C_f) with those specifically involved in NO_x reduction (C*). The kinetic model is sequentially developed by accounting for the catalytic and non-catalytic soot/O₂ as well as soot/NO_x/O₂ conversion. Kinetic parameters are taken from the literature and are also determined from a fit to experimental data. Comparison of measured and calculated data shows accurate reproduction of the experiments and the model. Finally, the kinetic model is validated by some simulations.     

Performance of Nano-DMA Operated with Different Gases for Sheath and Aerosol Carrier Flows

Two new experimental procedures are proposed to evaluate the performance of DMAs operated with different gases for sheath and polydisperse-aerosol-carrier flows. The first procedure evaluates the potential flow mixing in DMAs. An organic compound vapor of heavy molecular weight is doped, and then its concentration in the DMA aerosol-carrier streams is detected by gas chromatography. DMAs that strongly separate the doped vapor from the carrier are qualified as the separation tools to extract particles of desired sizes from an organic-vapor-rich gas flow. The separation is of importance for the particle composition characterization using existing chemical analytic instruments.

The second procedure investigates the sizing accuracy of DMAs operated under the conditions of interest. A tandem DMA (TDMA) setup was applied to achieve the test objective. The first DMA was operated with different gases for sheath and polydisperse-aerosol-carrier flows. The same gas used for the first DMA sheath flow was used for both aerosol-carrier and sheath flows in the second DMA. This study evaluated the performance of Nano-DMA using different gas pairs (Ar, N₂, CO₂, and He). In the first test, at sheath flow rates of 7.5 and 15 lpm, approximately 0.1% or less of the organic vapor in the polydisperse-aerosol-carrier stream reached the DMA monodisperse-aerosol-carrier stream for gas pairings of Ar, N₂, and CO₂. For He with other gases, more organic compound vapor was detected at the Nano-DMA downstream, probably because He has high diffusivity when paired with other gas media. Either N₂ or Ar gas was used as the Nano-DMA sheath flow in the second test to reduce the DMA's operational cost.

The second test shows that the classified particle size can be estimated from the measured electrical mobility using the gas property of sheath flow for cases of N₂-Ar pairs. For the cases of He as polydisperse-aerosol-carrier flow with an other gas (N₂/Ar) as the sheath flow, the classified particle size is less than estimated using the sheath flow gas property, possibly due to the diffusiophoresis effect. The experimental validation of the diffusiophoresis effect was further carried out by switching the roles of He-N₂/Ar as the sheath and aerosol carrier flows in Nano-DMA.     

Comparison of DFT characterization methods based on N2, Ar, CO2, and H2 adsorption applied to carbons with various pore size distributions

Adsorption isotherms of N₂, Ar, CO₂, and H₂ were measured on selected activated carbons representing various pore size distributions. Isotherm data were analyzed using the DFT method. A good agreement was obtained between N₂ and Ar results in a wide range of pore sizes. Both N₂ and Ar results agree with CO₂ analysis at 273 K in the range of small micropores where CO₂ analysis is most applicable. Also, in this range of pores the analysis of H₂ adsorption data at 77 K gives results which are consistent with the results obtained from the more conventional vapor-phase adsorptives. However, the range of PSDs calculated from H₂ data extends to pore sizes smaller than those that can be characterized by other adsorbates.     

Model discrimination by unsteady-state operation: Application to the reduction of no with co on iron oxide

The steady-state catalytic reduction of NO with CO on iron oxide was described with three different models, which all fit the experimental data equally well. By applying the transient method to this reaction, it has been shown that for reduced catalysts without adsorbed CO, the N₂O reduction step is well described qualitatively by two of the models, whereas only one of them can describe the NO reduction step. In the presence of adsorbed CO, however, none of these models can explain the observed transient behaviour. Therefore, a modification of the best model was proposed and tested via computer simulation, which showed good agreement with transient experiments.     

Apparatus for steady-state kinetic measurements of the catalytic reduction of nitric oxide by ammonia on iron oxide

The reduction of nitric oxide with ammonia on an unsupported iron oxide catalyst has been studied in a continuous-flow recycle reactor using simulated flue gas. The responses of the employed reactor system to step and pulse inputs of tracer indicate that the system could be regarded as a continuous stirred tank reactor (CSTR). Preliminary tests were carried out to determine the effect of temperature and particle size on the measured reaction rates. Additional experiments were performed in order to study the influence of oxygen and water concentration on these rates. A gas chromatographic system has been developed to analyze the gas components NO, N₂O, NO₂, NH₃, H₂O, O₂, CO₂ and N₂. In addition, the concentrations of NO and NO₂ were measured with a nondisperse infrared (NDUV/NDIR) analyzer.     

Behaviour of V2O5 — catalyzed CO oxidation in the presence of inert,diluent gases

A study was conducted on the oxidation of CO over an industrial potassium-promoted V₂O₅ catalyst in order to study the influence of He, Ar, Kr, SF₆ and C₄F₈ diluents on the reaction rate. A differential flow reactor was used between 400° and 440°C with partial pressures of CO₂, O₂ and diluents varied from approximately 7 to 150 kPa. All the diluents inhibited the reaction rate to varying degrees. Experiments were also performed in which one diluent gas was displaced by another in the reaction mixture. These experiments indicated that the diluents could be arranged in the following increasing order of sorptive interaction with the catalyst: He or Ar < Kr < SF₆ or C₄F₈ It was concluded that conventional site-blockage was unable to explain the influence of the diluents. An adsorption interaction between the diluent and the surface complex of the rate limiting step was postulated to explain the inhibition of diluents in CO oxidation and the enhancement of diluents in SO₂ oxidation on the same catalyst.