Soot Oxidation Rate Research Articles

Effect of Cs-Ce-Zr Catalysts/Soot Contact Conditions on Diesel Soot Oxidation.

Cs–Ce–Zr catalysts with various weight ratios are prepared by the sol–gel method in this paper. The main crystalline phases were identified by X-ray diffraction. The activities of catalysts during soot combustion were tested by thermogravimetric and differential scanning calorimetry. The contact conditions of soot/catalysts (sintered at 450 and 380 °C, respectively, under loose and tight contact conditions) were observed by scanning electron microscopy to study the effect of contact conditions on catalytic activity, and it was determined that the catalytic activities under tight contact conditions are superior to those under loose contact conditions. However, the soot oxidation rate speeds up after the peak temperature of about 450 °C under loose contact conditions, which is due to the fact that the contact condition is enhanced by melting CsNO3. The soot onset ignition temperature is lower for the catalysts with more Cs content under loose contact conditions. The minimum gaps of the soot onset ignition temperature and soot oxidation rates under the two contact conditions are 32 and 7 °C, which shows that the gap of catalytic activities under the respective contact conditions can be decreased by the formation of different crystalline phases.

Effects of variable O2 concentrations and injection pressures on the combustion and emissions characteristics of the petro-diesel and hydrotreated vegetable oil-based fuels under the simulated diesel engine condition

This experimental research investigates the effects of variable O2 concentrations and injection pressures on the combustion and emissions characteristics of the diesel (B7) and the hydrotreated vegetable oil (HVO)-based fuels. The O2 concentrations included 21%, 15% and 10% O2, while the injection pressures were 80 and 120 MPa. The experimental fuels were the diesel fuel (B7), the neat HVO, the 20%, 50% and 80% HVO (by mass fraction) blended with the diesel. The experiments were carried out in a rapid compression-expansion machine (RCEM) under the direct injection (DI) diesel combustion condition. The analysis was undertaken using the two-color method. The experimental results indicated that the ignition delay, the heat release rate, the flame temperature, the soot density-KL factor, the NOx and soot-out emissions were inversely correlated to the HVO fraction in the blend. In addition, the findings revealed the similar flame profiles in which the higher flame temperature region and the darker KL density were concentrated around the spray flame upstream, regardless of the HVO mixing ratio. Besides, the decrease in the O2 concentration resulted in the lower heat release rate, integral heat release, flame temperature, KL factor and NOx emissions but the longer ignition delay and higher soot concentration, with the highest soot concentration observed under the 15% O2 environment. Nevertheless, the higher pressure differential (i.e. between the injection pressure and the ambient pressure) contributed to the shorter ignition delay, higher heat release rate, early peak of the flame temperature, wider combustion area, faster soot oxidation rate and higher NOx production.

Photocatalytic soot oxidation on TiO2 microstructured substrate

Photocatalytic soot oxidation was investigated using TiO2 microstructured substrates. Plain and micropillar substrates of sintered TiO2 nanoparticles were prepared using a soft lithography process. Photon absorption analysis revealed substantial photon transmission for plain substrate within the current substrate thickness range (<2.5μm). The porous structure of substrate was confirmed by electron microscopy and by dimensional changes due to sintering, allowing gaseous species and radicals to diffuse across micrometer-thick substrates. It was found that the soot oxidation rate increased with the thickness of the plain TiO2 substrate; this was related to the increase in electron-hole pair generation associated with increased photon absorption. The soot oxidation rate increased further with the use of TiO2 micropillars, enhancing photon absorption through multiple scattering. These multiscale phenomena in the photocatalytic soot oxidation on TiO2 microstructured substrates were analyzed.

Numerical Simulation of Effects of Swirl Ratio on Flow and Emissions of Diesel Engine

With the increase of EGR, soot emission increases firstly (because of the lower oxidation rate of soot) and then decreases (because of the lower formation rate of soot), that is the so‐called soot‐bump. Previous papers mainly attempted to reduce soot emission at low temperature combustion condition by optimizing and altering some parameters, few papers clarified the effects of these parameters on the shape, peak value and the corresponding EGR rate of soot‐bump as well as CO emissions. The effects of swirl ratio on flow and emissions of a direct injection diesel engine were studied by numerical simulation method in this paper. The results indicate that with the increase of swirl ratio, the soot‐bump peak value decreases, and the corresponding EGR rate of peak value increases, the soot‐bump curve will decline rapidly after the peak value. In the same EGR rate, soot emission decreases with the increase of swirl ratio. The CO emission decreases firstly and then increases with the increase of swirl ratio, the corresponding EGR rate of soot‐bump peak value is almost the same as the EGR rate when CO emission increases drastically.

Evolution of In-Cylinder Diesel Engine Soot and Emission Characteristics Investigated with Online Aerosol Mass Spectrometry

To design diesel engines with low environmental impact, it is important to link health and climate-relevant soot (black carbon) emission characteristics to specific combustion conditions. The in-cylinder evolution of soot properties over the combustion cycle and as a function of exhaust gas recirculation (EGR) was investigated in a modern heavy-duty diesel engine. A novel combination of a fast gas-sampling valve and a soot particle aerosol mass spectrometer (SP-AMS) enabled online measurements of the in-cylinder soot chemistry. The results show that EGR reduced the soot formation rate. However, the late cycle soot oxidation rate (soot removal) was reduced even more, and the net effect was increased soot emissions. EGR resulted in an accumulation of polycyclic aromatic hydrocarbons (PAHs) during combustion, and led to increased PAH emissions. We show that mass spectral and optical signatures of the in-cylinder soot and associated low volatility organics change dramatically from the soot formation dominated phase to the soot oxidation dominated phase. These signatures include a class of fullerene carbon clusters that we hypothesize represent less graphitized, C5-containing fullerenic (high tortuosity or curved) soot nanostructures arising from decreased combustion temperatures and increased premixing of air and fuel with EGR. Altered soot properties are of key importance when designing emission control strategies such as diesel particulate filters and when introducing novel biofuels.

Effect of potassium-doping and oxygen concentration on soot oxidation in O2/CO2 atmosphere: A kinetics study by thermogravimetric analysis

In oxy-fuel combustion, switching from nitrogen to carbon dioxide atmosphere with higher oxygen concentration will affect the oxidation rate of soot, and it was observed in our recent study on soot emission from biomass pyrolysis that potassium (K) crystals are embedded in soot and its precursor clusters. In this study, the effects of K-doping (KCl and K2SO4) and O2 concentration on soot oxidation in O2/CO2 atmosphere are studied using thermogravimetric analysis (TGA), and the extent of catalysis is compared with that in O2/N2 atmosphere. The delays on start, peak and end temperatures of soot oxidation are observed in O2/CO2 atmosphere. However, increase in O2 concentration which promotes oxidation significantly reduces the delay. All the K-doping cases results in accelerated soot oxidation rate, but catalytic role of the K-doping in O2/CO2 is significantly lower than that in O2/N2 because the CO2-enriched environment inhibits the performance of potassium as oxygen carrier. The accelerating degree from K-doping is also affected by the potassium type, doping mass and oxygen concentration. KCl acts as a better, more efficient doping agent than K2SO4 with the increase in doping mass. The catalytic effect of K2SO4 will not change and even decrease at 375μmol(K)/g(soot) for K2SO4 while the catalytic role of KCl keeps increasing even at 600μmol(K)/g(soot) for KCl. In O2 concentration range of 5–30%, the accelerating degree from K-doping presents the minimum value around 15%. This phenomenon strongly approves the hypothesis that potassium as the oxygen carrier and accelerating the oxygen transportation, because in the cases of without K-doping and at a high O2 concentration there is no additional active site for more O2 adsorption thus inducing the slow accelerating degree. The kinetic analysis indicates the first order reaction for soot oxidation and also a good compensation relation between apparent activation energy E and logarithmic frequency factor A. E is generally reduced with the atmosphere changing from O2/N2 to O2/CO2, with K-doping, and with O2 concentration decreasing. This study is beneficial to demonstrate the mechanism of how potassium doping and oxygen concentration affect soot oxidation rate in oxy-fuel combustion environment.

Investigation of late-cycle soot oxidation using laser extinction and in-cylinder gas sampling at varying inlet oxygen concentrations in diesel engines

This study focuses on the relative importance of O2 and OH as oxidizers of soot during the late cycle in diesel engines, where the soot oxidation is characterized in an optically accessible engine using laser extinction measurements. These are combined with in-cylinder gas sampling data from a single-cylinder engine fitted with a fast gas-sampling valve. Both measurements confirm that the in-cylinder soot oxidation slows down when the inlet concentration of O2 is reduced. A 38% decrease in intake O2 concentration reduces the soot oxidation rate by 83%, a non-linearity suggesting that O2 in itself is not the main soot oxidizing species. Chemical kinetics simulations of OH concentrations in the oxidation zone and estimates of the OH-soot oxidation rates point towards OH being the dominant oxidizer.

Parameters Influencing Soot Oxidation Rates in an Optical Diesel Engine

Soot emissions from diesel engines are the net result of two competing processes: soot formation and soot oxidation. Previous studies have shown poor correlation between soot formation rates and the soot emissions. This article presents a systematic study of a number of parameters affecting soot oxidation rate and how it correlates with the soot emissions. An optical heavy-duty engine is used in conjunction with a laser extinction setup in order to collect time resolved data of the soot concentration in the cylinder during the expansion stroke. Laser extinction is measured using a red (685 nm) laser beam, which is sent vertically through the cylinder and modulated to produce 10 pulses per crank angle degree. Information is obtained about the amount of soot formed and the soot oxidation rate. The parameters studied are the motored density at top dead center (TDC), motored temperature at TDC, injection pressure, engine speed, swirl level and injector orifice diameter. A central composite design is employed to assess the importance of the parameters as well as identifying potential interaction effects. A single exponential decay function is fit to the extinction data to describe the oxidation process and the half-life of the decay is used as a measure of the oxidation rate. The half-lives are compared with engine out emissions and the importance of each parameter is studied using regression analysis. The results suggest that the injection pressure has the strongest effect on the late-cycle soot oxidation rate, while the temperature at TDC has the weakest effect of the parameters studied. (Less)

Optical soot measurement of bio-butanol upstream product, ABE (Acetone–Butanol–Ethanol), under diesel-like conditions

ABE (Acetone–Butanol–Ethanol), the upstream product of bio-butanol, is considered as a promising alternative fuel for IC engine. Two-dimensional soot distribution and soot mass of spray combustion of ABE and diesel was measured by forward illumination light extinction (FILE) technology. The ambient temperature and oxygen concentration of injection was varied from 1200K to 900K and 21% to 11% to represent normal temperature combustion and low temperature combustion modes respectively. It was found that soot distribution area and intensity of ABE, especially ABE with a high acetone fraction, is much lower than that of diesel. The soot clouds area and intensity increase with increasing ambient temperature and decreasing oxygen concentration, but the increased scale of soot for ABE is much lower than that of diesel. Quantitative results of soot mass for ABE show that the soot increases rapidly with increasing ambient temperature at lower oxygen concentration conditions because the soot producing rate is largely increased when elevating temperature, while the soot oxidation rate remained slow due to the insufficient oxygen. Based on analysis, it was found that clean combustion of ABE benefited from a lean air–fuel mixture because of the great volatility and low stoichiometric ratio of the fuel, and the competition of Carbon atom between CO and C2H2 in the pyrolysis reactions that reduced the soot source materials. Overall, as an alternative IC fuel, ABE is able to achieve much lower soot than diesel and is able to maintain clean combustion in a large range of environmental conditions.

Catalytic oxidation of diesel soot particulates over Ag/LaCoO3 perovskite oxides in air and NOx

Ag/LaCoO3 perovskite catalysts for soot combustion were prepared by the impregnation method. The structure and physicochemical properties of the catalysts were characterized using X-ray diffraction, N2 adsorption-desorption, H2 temperature-programmed reduction, soot temperature- programmed reduction, and X-ray photoelectron spectroscopy. The catalytic activity of the catalysts for soot oxidation was tested by temperature-programmed oxidation in air and in a NOx atmosphere. Metallic Ag particles were the main Ag species. Part of the Ag migrated from the surface to the lattice of the LaCoO3 perovskite, to form La1-xAgxCoO3. This increased the amount of oxygen vacancies in the perovskite structure during thermal treatment. Compared with unmodified LaCoO3, the maximum soot oxidation rate temperature (Tp) decreased by 50–70 °C in air when LaCoO3 was partially modified by Ag, depending on the thermal treatment temperature. The Tp of the Ag/LaCoO3 catalyst calcined at 400 °C in a NOx atmosphere decreased to about 140 °C, compared with that of LaCoO3. Ag particles and oxygen vacancies in the catalysts contributed to their high catalytic activity for soot oxidation. The stable catalytic activity of the Ag/LaCoO3 catalyst calcined at 700 °C in a NOx atmosphere was related to its stable structure.

Diesel flame imaging and quantitative analysis of in-cylinder soot oxidation

This study concerns a quantitative analysis of late-cycle soot oxidation in diesel engines that focuses on two-dimensional KL factor images obtained by the two-color method. The spatially integrated KL factor was converted into the in-cylinder soot mass using a new formula of diesel soot emissivity. This methodology was applied to two combustion systems: a heavy-duty optical engine which was tuned for a higher fuel–air mixing capability and a rapid compression and expansion machine which had a lower mixing performance. The in-cylinder soot mass history during the last stage of soot oxidation phase was converted into a normalized soot mass history and was used for comparison with simulated soot mass history. A model calculation of in-cylinder soot mass history which was based on oxidation of a primary soot particle was performed with the surface-specific soot oxidation rate as a parameter. A value of the surface-specific soot oxidation rate was specified from the curve fitting approach between the experime...

Numerical Simulation of Diesel Particulate Filter Regeneration Considering Ash Deposit

Non-combustible ash can be deposited on channel walls through the full length of a diesel particulate filter (DPF). This type of ash can affect the exhaust condition and heat transfer process during the periodical soot regeneration of DPF. A model of soot regeneration is established in this paper to describe the effects of ash deposits on exhaust condition and heat transfer. Mass, momentum, and energy balances are considered for the multiphase system of gas, soot, and wall. The good agreement with experimental data confirms that the model is able to describe the processes. The axial patterns of wall temperature and soot regeneration rate during DPF regeneration are investigated by numerical simulation. Results indicate that the deposited ash layer reduces the exhaust flow rate and increases the heat conduction resistance during DPF regeneration, thus leading to high wall temperature and soot oxidation rate. When 5 g/L of ash is deposited, the complete oxidation of soot can be achieved 90 s faster with a 60 ∘C increase in wall temperature. The gain on soot oxidation rate increases with increasing amounts of deposited ash and reaches a plateau when the deposited ash approaches 15 g/L. Owing to the sintering and melting of ash when the temperature reaches 900 ∘C and the consequent uncontrolled regeneration, the soot carrying capacity should be identified based on the amount of deposited ash within DPF.

Optimized rate expressions for soot oxidation by OH and O2

The two principal soot oxidizers in flames are the hydroxyl radical (OH) and molecular oxygen (O2). Many soot oxidation rate expressions exist for these oxidizers, but they have considerable disparity and have not been sufficiently validated. To address this, twelve published experimental studies in diffusion flames, premixed flames, thermogravimetric analyzers, and flow reactors are examined. These are all the known studies that measured all of the following quantities at discrete locations: soot oxidation rate, temperature, OH concentration (if nonzero), and O2 concentration. This yielded 160 measured soot oxidation rates spanning seven orders of magnitude. Optimized soot oxidation rate expressions for OH and O2 are developed here by maximizing the coefficient of determination between measured and modeled oxidation rates. Oxidation of soot by OH is found to have a negligible activation energy and a collision efficiency of 0.10. The activation energy for O2 oxidation of soot is 195kJ/mol, which is higher than previous models. The new expressions for OH and O2 match the measurements with a regression coefficient of 0.98, compared to 0.79 for the most widely used models. The optimized models indicate that soot oxidation in flames by OH generally dominates over that by O2.

Two zone thermodynamic model for prediction of particulate matter emission from direct injection diesel engine

In this work, two zone thermodynamic models have been developed for prediction of particulate matter (PM) from direct injection Diesel engine. Two key compositions of PM were considered in this simulation model developed, it consists of soot and soluble organic fraction (SOF). Soot formation model were developed initially and then coupled with SOF model to get overall PM formation rates. Primary soot formation rate was obtained by using Hirosysu model and Nagle and Strick-land-Constable model was adopted to get soot oxidation rate. The oxidation and formation rate difference gives overall soot formation value. Unburned hydrocarbons were considered as key factor for SOF formation so the formation and oxidation rate of hydrocarbon was determined. Then the difference between these two gives overall SOF formation rate. At last soot formation and SOF model was integrated to get overall PM formation rate. Various submodels like ignition delay, heat release rate, and combustion model were involved in this study to predict PM formation rate. Validation of this simulation model developed were carried out on single cylinder, naturally aspirated water cooled direct injection Diesel engine. Simulation results matched well with the experimental results and it clearly shows that model developed is an accurate one. Results obtained shows that soot formation increases at higher loads and SOF formation rate increase at lower loads. Simulation model developed is very useful for understanding the PM formation mechanisms and also useful for control of PM formation.

A comparison of diesel soot oxidation rates measured with two different isothermal set-ups

Soot aerosol particles which are emitted by incomplete combustion e.g. in diesel engines are known to have a big influence on the environment and the human health. A detailed understanding of their elimination through trapping and oxidation in diesel soot filters is therefore of great importance. Isothermal experiments of diesel soot oxidation are very advantageous for establishing detailed reaction kinetics. Nevertheless, most soot oxidation studies in the literature rely on non-isothermal burn-off experiments. In this study we present and compare two different experimental set-ups for determining isothermal soot oxidation rates. Such experiments gain importance in order to identify engine operation conditions and/or diesel fuel compositions, which produce more reactive soot, which is easier to oxidise. The first set-up, the so-called flat bed reactor (FBR), was originally developed for determining exhaust purification kinetics, using segments of exhaust monoliths with realistic through-flow under reproducible isothermal conditions. It has been adapted to study soot oxidation in segments of commercial diesel soot filters. The second set-up was originally designed for studying diesel soot oxidation in temperature programmed oxidation (TPO) experiments and requires a lower experimental effort. Although the two set-ups are very different, basic results of the oxidation experiments obtained turn out to be quite comparable, in particular under active regeneration conditions. This allows us to reduce the experimental effort by replacing some of the isothermal FBR experiments by isothermal TPO.

Effect of Y on improving the thermal stability of MnOx-CeO2 catalysts for diesel soot oxidation

A series of MnOx-CeO2-Y2O3 catalysts with different Y loadings (0, 1, 3, 6, and 10 wt%) were prepared by a co-precipitation method and investigated for NOx-assisted soot oxidation. The thermal stabilities of these catalysts were evaluated by treating them at 800 °C for 12 h under dry air flow. The catalysts were characterized by X-ray diffraction, N2 adsorption-desorption, Raman spectroscopy, H2 temperature-programmed reduction, oxygen storage capacity, NO temperature-programmed oxidation, X-ray photoelectron spectroscopy, and soot temperature-programmed oxidation. The addition of Y led to decreased BET surface areas and poor low-temperature reduction abilities and oxygen storage capacities, which affected NO and soot oxidation activities. However, after aging, the doping of Y had effectively enhanced the stability of the catalytic activities for NO and soot oxidations, where the addition of 6%–10% Y achieved the optimum result because the maximal soot oxidation rate temperature was increased by only 34–35 °C. Additionally, the catalytic activity and deactivation of MnOx-CeO2-containing catalysts were closely related to the presence of Mn4+ and oxygen species on the surface.

Oxidation-derived maturing process of soot, dependent on O2–NO2 mixtures and temperatures

Oxidizer- and temperature-dependent soot properties were investigated to better understand soot oxidation process. For this work, partially oxidized Printex-U samples as surrogate soot, under three different O2–NO2 mixtures, were analyzed by using the high resolution-transmission electron microscope, Raman microscope and Fourier transform infrared spectroscopy- attenuated total reflectance. The results show that the maturing process was much dependent on NO2 content in the O2–NO2 mixture: with no NO2 added, soot oxidized through the internal-burning out process, whereas with increased NO2 in the mixture, soot tended to oxidize through the external burning process. As the NO2 content increased, the preferential oxidation of less-ordered carbon crystallites decreased and as a result the maturing process was delayed. Internal burning-out process by O2 only was also verified at various temperatures and with actual engine soot such as diesel soot and gasoline direct-injection (GDI) soot by TEM observations. Despite similar internal burning-out process, however, it was shown that oxidation at increased temperature resulted in relatively less-ordered soot, implying that soot maturing process was delayed with temperature. Since soot oxidation rate increased with temperature, the increase in oxidation temperature seems to diminish preference on short-ranged crystallites like more efficient O2–NO2 cases.

Numerical Investigation on the Effect of Using Mixtures of Natural Gas and Aviation Fuel on Flame Characteristics of Swirl Gas Turbine Combustor

The present paper is directed to study aviation fuel combustion compared to that of natural gas in a gas turbine swirl Combustor. Finite-volume based CFD commercial package ANSYS Fluent 12 is used for the present investigation. The SST k-ω model is used to simulate turbulence. The non-premixed combustion model is used to simulate the reacting flow. Combustion of natural gas is sooty in nature. Thus, soot distribution in the combustor is investigated numerically using the two step soot model. Simple expressions for the soot formation and oxidation rates were employed. The radiation heat transfer equation was solved using the P-1 radiation model. The formation of thermal NO from molecular nitrogen was modeled and the prompt NO was also included in the computations. CFD modeling is validated against experimental temperature measurements for a swirl combustor when firing natural gas as referred to a previous paper. Using the SST-K-ω for predicting gas temperature distributions satisfy reasonable agreement with the experimental temperature measurements, except near the combustor upstream and in the vicinity of the combustor axis.

Activity and thermal stability of Pt/Ce0.64Mn0.16R0.2Ox (R = Al, Zr, La, or Y) for soot and NO oxidation

In this study, Al2O3, ZrO2, La2O3, and Y2O3 were separately introduced into Pt/CeO2-MnOx catalyst to improve the thermal stability of catalysts and obtain a better soot and NO oxidation activity after thermal treatment. CeO2-MnOx-ROx (R=Al, La, Y, or Zr) mixed oxides were prepared by a co-precipitation method. The catalysts were characterized by XRD, BET, H2-TPR, CO chemisorption, XPS, TG analysis, soot–TPO, and NO–TPO. It is found that the introduction of Rn+ into CeO2-MnOx markedly increases the thermal stability of the catalysts for soot and NO oxidation, remaining a better catalytic oxidation activity. On the one hand, after high-temperature aging, for the modified catalyst Pt/CeMnROx, the ignition temperature of soot oxidation (Ti) and maximal soot oxidation rate temperature (Tm) increase by 30–50°C and 4–32°C, respectively, whereas Al3+-modified catalyst shows the best thermal stability. On the other hand, due to a key role of NO2 as a strong oxidizing agent in soot oxidation process, the NO oxidation activity is often considered as an important aspect. This work shows that Rn+-modified catalysts display a better stability on NO oxidation activity after aging. Some factors leading to the thermal deactivation of catalysts are also discussed in this paper.

Preparation and NOx-assisted soot oxidation activity of a CuO–CeO2 mixed oxide catalyst

A CuO–CeO2 mixed oxide catalyst is synthesized via the urea-nitrate combustion method. The catalytic activity in the reaction of NOx-assisted soot oxidation is evaluated with a TG/DTG technique. The sample Cu1Ce9 possesses the lowest temperature (Tm=383°C) of the maximum soot oxidation rate and the largest specific surface area (65.1m2/g) among the samples examined. The XRD results show that the fluorite-type CeO2 exists in all the samples forming a solid solution with CuO. Crystalline copper oxide phases are not detectable if the Cu/(Cu+Ce) atomic ratio is lower than 0.25. The activity and NOx-TPD results prove that NO2 formed from NO and the nitrate species play crucial roles for soot oxidation in NOx atmosphere. Kinetic parameters, such as the activation energy and reaction order, corresponding to the oxidation of soot are calculated with a differential method.