Experimental study of homogeneous Hg oxidation in air and oxy-simulated flue gas

This study investigated the effects of Cl₂, SO₂, and NO on the mercury (Hg) speciation during oxy-combustion and compared it with the Hg speciation in air-simulated flue gas with Cl₂. Experiments were conducted in a bench-scale device at 200 °C. The results of Hg oxidation in an N₂ and CO₂ atmosphere with Cl₂ showed that CO₂ indirectly restrained Hg oxidation. Oxy-simulated flue gas promoted Hg oxidation more than air-simulated flue gas promoted that, because the oxygen in oxy-simulated flue gas indirectly promoted Hg oxidation using Cl₂. The percentage of Hg oxidized in both oxy-simulated flue gas and air-simulated flue gas with NO decreased as the concentration of Cl₂ increased because NO restrained Hg oxidation with Cl₂ through the elimination of the O and ClO radicals. SO₂ inhibited Hg oxidation with Cl₂ by consuming the O radicals. Moreover, when both NO and SO₂ were present in oxy-simulated flue gas with Cl₂, the effect of SO₂ on Hg oxidation was related to the NO concentration.     

Effects of CO and H2 on the formation of N2O via catalytic NO reduction

The formation of N₂O in a mixture of NO, CO, H₂, O₂ and N₂ was investigated experimentally in a tubular-flow reactor containing a catalyst.It was found that the reduction of NO is enhanced by the presences of H₂, and to a lesser extent CO, and that N₂O is formed as a by-product of NH₃ decomposition and NO reduction in the presence of H₂, and through NO reduction in the presence of CO. The main product of NH₃ oxidation is N₂ in addition to the products of NO and N₂O, and the rate of conversion for NH₃ to N₂O is about 10%. The conversion of NO to N₂O is higher in the presence of H₂ than in the presence of CO at lower temperature, and there is a range of temperature in which the formation of N₂O is enhanced in the presence of either H₂ or CO, whereas CO enhances N₂O production from catalytic NO reduction more than H₂ in a CO/H₂/NO/O₂/N₂ gas system.     

Modeling study of homogeneous NO and N2O formation from oxidation of HCN in a flow reactor

To investigate the reaction chemistry of HCN oxidation, a modeling study was performed. The plug flow calculation code was used at atmospheric pressure in the temperature range from 1000 to 1400 K. The effect of initial H₂O concentrations and that of other components were discussed. The oxidation of HCN is controlled primarily by the HCN+OH reaction in case of increasing H₂O concentration. The oxidation of HCN starts at lower temperatures and the conversion of HCN to NO is inhibited by increase in H₂O concentration. N₂O formation by the NCO+NO reaction is inhibited by increase in H₂O concentration because of the small amount of NO and NCO. In the presence of initial NO, NCO acts as a reducing agent for NO. NCO mainly reacts with initial NO, so N₂O formation is not affected by H₂O concentration. In case of adding CO, CO oxidation chemistry acts as a source of a radical pool, and HCN oxidation shifts to lower temperatures. Increasing H₂O affects, the consumption of O radical and inhibits NO formation. The effect of H₂O concentration on N₂O formation is small because of the number of O and H radicals formed by CO oxidation.     

Predictions of the impurities in the CO2 stream of an oxy-coal combustion plant

Whilst all three main carbon capture technologies (post-combustion, pre-combustion and oxy-fuel combustion) can produce a CO₂ dominant stream, other impurities are expected to be present in the CO₂ stream. The impurities in the CO₂ stream can adversely affect other processes of the carbon capture and storage (CCS) chain including the purification, compression, transportation and storage of the CO₂ stream. Both the nature and the concentrations of potential impurities expected to be present in the CO₂ stream of a CCS-integrated power plant depend on not only the type of the power plant but also the carbon capture method used. The present paper focuses on the predictions of impurities expected to be present in the CO₂ stream of an oxy-coal combustion plant. The main gaseous impurities of the CO₂ stream of oxy-coal combustion are N₂/Ar, O₂ and H₂O. Even the air ingress to the boiler and its auxiliaries is small enough to be neglected, the N₂/Ar concentration of the CO₂ stream can vary between ca. 1% and 6%, mainly depending on the O₂ purity of the air separation unit, and the O₂ concentration can vary between ca. 3% and 5%, mainly depending on the combustion stoichiometry of the boiler. The H₂O concentration of the CO₂ stream can vary from ca. 10% to over 40%, mainly depending on the fuel moisture and the partitioning of recycling flue gas (RFG) between wet-RFG and dry-RFG. NO_x and SO₂ are the two main polluting impurities of the CO₂ stream of an oxy-coal combustion plant and their concentrations are expected to be well above those found in the flue gas of an air-coal combustion plant. The concentration of NO_x in the flue gas of an oxy-coal combustion plant can be up to ca. two times to that of an equivalent air-coal combustion plant. The amount of NO_x emitted by the oxy-coal combustion plant, however, is expected to be much smaller than that of the air-coal combustion plant. The reductions of the recirculated NO_x within the combustion furnace by the reburning mechanism and the char-NO reactions are the main reason for a smaller amount of NO_x emitted by the oxy-coal combustion plant. The concentration of SO₂ in the flue gas of an oxy-coal combustion plant can be up to six times to that of an equivalent air-coal combustion plant if the recycling flue gas is not desulphurized. The flue gas volume flow rate of an oxy-coal combustion plant is much smaller (<20%) than that of an equivalent air-coal combustion plant, which is a significant advantage for the purification of the flue gas.     

Effect of the equivalence ratio on the concentration of CH4/NO2/O2 combustion products

A chemical kinetic model for determining the mole fractions of stable and intermediate species for CH₄/NO₂/O₂ flames is developed. The model involves 30 different species in 101 chemical elementary reactions. The mole fractions of the species are plotted as a function of the distance from the surface of the burner. The effects of the equivalence ratio on the concentrations of CO, CO₂, N₂, NH₂, OH, H₂O, NO and NO₂ for lean CH₄/NO₂/O₂ flames in the post flame zone at 50 Torr are obtained. The flames are flat, laminar, one dimensional and premixed. The calculated concentration profiles as a function of the equivalence ratio and distance from the surface of the burner are compared with the experimental data. The comparison indicates that the kinetics of the flames are reasonably described by the developed model. The mole fraction of N₂, NH₂, OH, H₂O, CO₂ and CO increase while the mole fractions of NO and NO₂ decrease by increasing the equivalence ratio for lean flames. Copyright © 1999 John Wiley & Sons, Ltd.     

NO mechanisms of syngas MILD combustion diluted with N2, CO2, and H2O

The NO mechanism under the moderate or intense low-oxygen dilution (MILD) combustion of syngas has not been systematically examined. This paper investigates the NO mechanism in the syngas MILD regime under the dilution of N₂, CO₂, and H₂O through counterflow combustion simulation. The syngas reaction mechanism and the counterflow combustion simulation are comprehensively validated under different CO/H₂ ratios and strain rates. The effects of oxygen volume fraction, CO/H₂ ratio, pressure, strain rate, and dilution atmosphere are systematically investigated. For all the MILD cases, the contribution of the prompt and NO-reburning routes to the overall NO emission is less than 0.1% due to the lack of CH₄ in fuel. At atmospheric pressure, the thermal route only accounts for less than 20% of the total NO emission because of the low reaction temperature. Moreover, at atmospheric pressure, the contribution of the NNH route to NO emission is always larger than 55% in the N₂ atmosphere. The N₂O-intermediate route is enhanced in CO₂ and H₂O atmospheres due to the increased third-body effects of CO₂ and H₂O through the reaction N₂ + O (+M) ? N₂O (+M). Especially in the H₂O atmosphere, the N₂O-intermediate route contributes to 60% NO at most. NO production is reduced with increasing CO/H₂ ratio or pressure, mainly due to decreased NO formation from the NNH route. Importantly, a high reaction temperature and low NO emission are simultaneously achieved at high pressure. To minimize NO emission, the reactions should be operated at high values of CO/H₂ ratios (i.e., >4) and pressures (e.g., P > 10 atm), low oxygen volume fractions (e.g., X_O2 < 15%), and using H₂O as a diluent. This study provides a new fundamental understanding of the NO mechanism of syngas MILD combustion in N₂, CO₂, and H₂O atmospheres.     

Dilution effects analysis on NOx emissions of opposed-jet H2/CO syngas diffusion flames

Syngas is a promising alternative fuel for stationary power generation due to cleaner combustion than convectional fossil fuels. During the gasification processes, the by-products of CO₂, H₂O, or N₂ may be present in the syngas mixture to control the flame temperature and emissions. Several studies indicated that syngas with dilutions is capable of reducing pollutant emissions such as NO_x emissions. This work applied a numerical model of opposed-jet diffusion fames to explore the dilution effects on NO_x formation and differentiate the inert effect, thermal/diffusion effect, chemical effect, and radiation effect from CO₂, H₂O, or N₂ dilutions. The numerical study was performed by a revised OPPDIF program coupling with narrowband radiation model and detail chemical mechanism. The dilution effects on NO_x formation were analyzed by comparing the realistic and hypothetical cases. Regardless the diluent types, the inert effect is the main cause to reduce NO production, followed by chemical effect and radiation effect. The thermal/diffusion effect may promote NO formation because the preferential diffusion due to different diffusivities between diluents and syngas magnifies the reaction rate locally. CO₂ dilution reduces NO by radiation effect at low strain rate, and contributes NO reduction by chemical effect at high strain rate. At the same dilution percentage, CO₂ dilution reduces NO production the most, followed by H₂O and N₂. Besides the thermal/diffusion effect, the chemical effect of H₂O enhances NO production through thermal route and reburn route.     

Effects of low pressure exhaust gas recirculation on regulated and unregulated gaseous emissions during NEDC in a light-duty diesel engine

Regulated and unregulated gaseous emissions with high pressure and low pressure EGR (exhaust gas recirculation) system were tested in a 4-cylinder, light-duty diesel EURO IV engine typically used in European vehicles. Four different engine calibrations with the low pressure EGR system were studied. Regulated emissions of NO_X, CO, HC and CO₂ were measured for each configuration. Unburned Hydrocarbon Speciation, HCHO (formaldehyde), HCOOH (formic acid) and N₂O (nitrous oxide) were also measured in order to determine the MIR (maximum incremental reactivity) of the gaseous emissions. Pollutants were measured without the DOC (diesel oxidation catalyst) to gather data about raw emissions. When the low pressure EGR system was used, decreases in NO_X, N₂O and fuel consumption were observed and significant increases HC, CO and unregulated emissions; this is the result of a lower intake manifold temperature, which provides a higher gas density which modifies the combustion process. The potential of tropospheric ozone production was higher in all cases when the low pressure EGR was used.     

Study of Ce-Pt/γ-Al2O3 for the selective oxidation of CO in H2 for application to PEFCs: Effect of gases

In order to supply pure hydrogen to proton exchange membrane (PEM) fuel cells and avoid CO poisoning, selective CO oxidation in H₂ was studied over Ce-Pt/γ-Al₂O₃. Adding the Ce promoted the CO conversion and selectivity of Pt/γ-Al₂O₃ with changing loading weights of Pt and Ce, oxygen concentration, residence time, and the composition of gases (H₂O, CO₂, and N₂). At 250 °C, adding H₂O to the feed gas enhanced the CO conversion due to the water–gas shift reaction. While, adding CO₂ to the feed gas suppressed the CO conversion due to the reversible water–gas shift reaction. In situ BET and XRD tests showed that well-dispersed metallic Pt particles (−2 nm) existed on the Ce oxide over the alumina support, which helps to supply oxygen to the Pt for a high activity of CO oxidation and selectivity.     

Impact of nitrogen oxides (NO,NO2, N2O) on the formation of soot

The emission of both nitrogen oxides and soot from combustion processes is still a matter of concern. When a flue gas recirculation (FGR) technique is applied, the presence of a given nitrogen oxide in the recirculated mixture can affect the emissions of other pollutants, such as soot, and be used for its control in a combustion process. In this context, the present work is focused on the identification of the effect of the main nitrogen oxides (NO, NO₂ and N₂O) present in combustion systems on soot and main product gases formation from the pyrolysis of ethylene, at atmospheric pressure and in the 975–1475 K temperature range. The experimental results are examined to assess the effectiveness of each nitrogen oxide in suppressing or boosting soot formation, to achieve the possible nitrogen oxides reduction, and to identify the elementary steps involved in the nitrogen oxides and ethylene conversion as function of the different nitrogen oxides. This analysis is supported on model calculations.     

Carbon nanotube supported Pt-Ni catalysts for preferential oxidation of CO in hydrogen-rich gases

A series of carbon nano-tubes supported platinum-nickel catalysts were prepared and used for CO preferential oxidation in H₂-rich streams. The catalysts were characterized by using N₂-adsorption, XRD, HRTEM, H₂-TPD and H₂-TPR techniques. Effects of platinum and nickel loading amount, CO₂ and H₂O in the feed stream on the activity and selectivity over the catalysts were investigated. The results of catalytic performance tests show that the carbon nano-tubes supported Pt-Ni catalysts are very active and highly selective at low temperature for CO preferential oxidation in 1 vol. % CO, 1 vol. %O₂, 50 vol. % H₂ and N₂ gases. Adding 12.5 vol. % of CO₂ into the feed gases has slight negative influence on CO conversion. Adding 15 vol. % of H₂O leads to a little decrease of CO conversion at the temperature range of 100-120 °C, which is proposed to be caused by capillary wetting of water in the micro-pores of carbon nano-tubes. As the reaction temperature is higher, adding water can improve CO conversion. The characterization results indicate that platinum species are in nano-particles uniformly dispersed on the carbon nano-tubes surface. There are two kinds of nickel species, one is interacted with platinum and likely to form Pt-Ni alloy in reduction process, the other is much highly dispersed on carbon nano-tubes and strongly interacted with the supports. The high activity of the catalysts is attributed to the interaction between Pt and Ni with the formation of Pt-Ni alloy.     

NO emission characteristics in counterflow diffusion flame of blended fuel of H2/CO2/Ar

Flame structure and NO emission characteristics in counterflow diffusion flame of blended fuel of H₂/CO₂/Ar have been numerically simulated with detailed chemistry. The combination of H₂, CO₂ and Ar as fuel is selected to clearly display the contribution of hydrocarbon products to flame structure and NO emission characteristics due to the breakdown of CO₂. A radiative heat loss term is involved to correctly describe the flame dynamics especially at low strain rates. The detailed chemistry adopts the reaction mechanism of GRI 2.11, which consists of 49 species and 279 elementary reactions. All mechanisms including thermal, NO₂, N₂O and Fenimore are taken into account to separately evaluate the effects of CO₂ addition on NO emission characteristics. The increase of added CO₂ quantity causes flame temperature to fall since at high strain rates a diluent effect is prevailing and at low strain rates the breakdown of CO₂ produces relatively populous hydrocarbon products and thus the existence of hydrocarbon products inhibits chain branching. It is also found that the contribution of NO production by N₂O and NO₂ mechanisms are negligible and that thermal mechanism is concentrated on only the reaction zone. As strain rate and CO₂ quantity increase, NO production is remarkably augmented. Copyright © 2002 John Wiley & Sons, Ltd.     

A 0D aircraft engine emission model with detailed chemistry and soot microphysics

Aviation emission of gas phase pollutants and particulate matter contribute to global radiative forcing and regional air quality degradation near airports. It is important to understand the formation and time evolution of these pollutants inside aircraft engines to design strategies for emission reduction. A physics and chemistry based zero-dimensional (0D) gas parcel model with detailed jet fuel chemistry and soot microphysics has been developed to predict the time evolution, formation history, and emission index (EI) of key combustion gases and size-resolved soot particles of an aircraft engine. The model was applied to a CFM56-2-C1 aircraft engine for idle operating condition, for which comprehensive measured data from the Aircraft Particle Emissions eXperiment (APEX) campaign are available. The measured EI data of four major pollutants, including CO, NOx (NO + NO₂), total hydrocarbon (HC) mass, and soot mass, were used to optimize the model parameters. The model predicts the time evolution of concentration of CO, NOx, HC, soot size distribution, CO₂, H₂O, SO₂, NO, NO₂, HONO, HNO₃, SO₃, H₂SO₄, O₂, H, H₂, O, OH, HO₂, H₂O₂, and some HC species in the combustor and turbine. Reasonable agreement was found between the simulations and the measurements. It was found for idle operating condition that, for most of the combustion products, concentrations did not change significantly in the turbine and nozzle, however, HONO, H₂SO₄, and HO₂ concentrations did change by more than a factor of 10, while NOx, NO, NO₂, O, OH, soot particle mass and soot particle number by less than a factor of 2. The developed model is computationally efficient and can be used to, study detailed chemical and microphysical processes during combustion, investigate the effects of different fuel compositions and operating conditions on aircraft emissions, and assist air quality study near aircraft emission sources.     

Influences of different diluents on NO emission characteristics of syngas opposed-flow flame

This paper used the opposed-flow flame model and GRI 3.0 mechanism to investigate NO emission characteristics of H₂-rich and H₂-lean syngas under diffusion and premixed conditions, respectively, and analyzed influences of adding H₂O, CO₂ and N₂ on NO formation from the standpoint of thermodynamics and reaction kinetics. For diffusion flames, thermal route is the dominant pathway to produce NO, and adding N₂, H₂O and CO₂ shows a decreasing manner in lowering NO emission. The phenomenon above is more obvious for H₂-rich syngas because it has higher flame temperature. For premixed flames, adding CO₂ causes higher NO concentration than adding H₂O, because adding CO₂ produces more O radical, which promotes formation of NO through NNH + O = NH + NO, NH + O = NO + H and reversed N + NO = N₂ + O. And in burnout gas, thermal route is the dominant way for NO formation. Under this paper's conditions, adding N₂ increases the formation source of NO as well as decreases the flame temperature, and it reduces the NO formation as a whole. In addition, for H₂-lean syngas and H₂-rich syngas with CO₂ as the diluent, N + CO₂ = NO + CO plays as an important role in thermal route of NO formation.     

Enhancing the NO2/NOx ratio in compression ignition engines by hydrogen and reformate combustion,for improved aftertreatment performance

Enhanced NO₂ production (without raising total NOx) in a diesel engine combustion chamber can improve the performance of several catalytic aftertreatment systems. Thus this can facilitate a further reduction in key regulated emissions such as nitrogen oxides (NOx) and particulate matter (PM) emissions. The oxidation of NO to NO₂ is an important intermediate step involved in all current aftertreatment systems that are designed for NOx and PM catalytic removal. The performance of both NOx control systems and catalysed particulate filters depend highly on the NO₂ concentration. In this work we have examined the influence of using hydrogen (H₂) and simulated reformate (H₂, CO and EGR gases) as a supplement to diesel fuel on NO₂ production. In actual engine applications a reformer will be integrated within the engine EGR system. This will not only provide the engine with recirculated exhaust gas (i.e. EGR), but will enrich it with H₂ and CO.     

A comparative study of combustion performance and emission of biodiesel blends and diesel in an experimental boiler

Recently, biodiesel has become more attractive since it is made from renewable resources and also for the fact that the resources of fossil fuels are diminishing day by day. This study compares combustion of B5, B10, B20, B50, B80 and B100 with petroleum diesel over wide input air flows at two energy levels in an experimental boiler. The comparison is made in terms of combustion efficiency and flue gas emissions (CO, CO₂, NO_X, and SO₂) and influence of air flow at two energy levels 219 kJ/h and 249 kJ/h is studied. The findings show that at higher level energy diesel efficiency was a little higher than that of biodiesel, but at lower level biodiesels are efficient than diesel. Except B10, Biodiesel and other blends emitted less pollutant CO, SO₂ and CO₂ than diesel. B10 emitted lower CO₂ and NO_X, but emitted higher SO₂ than diesel. Despite studies reporting an increase in the NO_X level resulting from burning of biodiesel over conventional petroleum diesel fuels in engines, our findings indicated at the second energy level a reduction in the NO_X level in the flue gases resulting from burning of biodiesel.     

High-temperature corrosion of water-wall tubes in oxy-combustion atmosphere

Oxy-combustion is one of the most promising technology for CO₂ capture in coal-fired power plants. However, under oxy-combustion conditions, the concentrations of acid gas species are significantly increased due to the introduction of the flue gas recycle, which aggravates the high-temperature corrosion of heat exchanger materials in boilers. In this study, the early-stage high-temperature corrosion (0–16 h) of two representative water-wall tube materials (20G, 12Cr1MoV) is experimentally tested in a lab-scale furnace with the simulated oxy-combustion atmosphere. The effects of material, temperature, CO₂, H₂O, SO₂, H₂S and CO atmospheres on high-temperature corrosion behaviors is investigated. The micro-morphologies and compositions of corrosion layers are characterized by scanning electron microscope with energy dispersive X-ray spectra (SEM-EDS) and X-ray diffraction (XRD). Kinetic analysis shows that the high concentration of CO₂ accelerates high-temperature corrosion of water wall materials. In the simulated oxy-fuel combustion atmosphere (CO₂/O₂/SO₂), the mass gain rate can be enhanced by 10%–30% compared to the conventional air combustion atmosphere (N₂/O₂/SO₂), and the major composition of oxide scale is magnetite. In a reducing oxy-fuel atmosphere (CO₂/CO/SO₂/H₂S), the major components of oxide scale are magnetite and ferrous sulfide. The high concentration of moisture in the atmosphere accelerated the corrosion rate by 10–30%. For both model alloys, the corrosion kinetics obey the parabolic law. Water-wall tube material 12Cr1MoV appears superiority in corrosion resistance compared with 20G material.