Over the last two decades, our understanding of soot formation has evolved from an empirical, phenomenological description to an age of quantitative modeling for at least small fuel compounds. In this paper, we review the current state of knowledge of the fundamental sooting processes, including the chemistry of soot precursors, particle nucleation and mass/size growth. The discussion shows that though much progress has been made, critical gaps remain in many areas of our knowledge. We propose the roles of certain aromatic radicals resulting from localized π electron structures in particle nucleation and subsequent mass growth. The existence of these free radicals provides a rational explanation for the strong binding forces needed for forming initial clusters of polycyclic aromatic hydrocarbons. They may also explain a range of currently unexplained sooting phenomena, including the large amount of aliphatics observed in nascent soot formed in laminar premixed flames and the mass growth of soot in the absence of gas-phase H atoms. While the above suggestions are inspired, to an extent, by recent theoretical findings from the materials research community, this paper also demonstrates that the knowledge garnered through our longstanding interest in soot formation may well be carried over to flame synthesis of functional nanomaterials for clean and renewable energy applications. In particular, work on flame-synthesized thin films of nanocrystalline titania illustrates how our combustion knowledge might be useful for developing advanced yet inexpensive thin-film solar cells and chemical sensors for detecting gaseous air pollutants.

Formation of nascent soot and other condensed-phase materials in flames

Hierarchical and comparative kinetic modeling of laminar flame speeds of hydrocarbon and oxygenated fuels

Oxygen defects and surface chemistry of ceria: quantum chemical studies compared to experiment.

An updated H2/O2 kinetic model based on that of Li et al. (Int J Chem Kinet 36, 2004, 566–575) is presented and tested against a wide range of combustion targets. The primary motivations of the model revision are to incorporate recent improvements in rate constant treatment and resolve discrepancies between experimental data and predictions using recently published kinetic models in dilute, high-pressure flames.

Attempts are made to identify major remaining sources of uncertainties, in both the reaction rate parameters and the assumptions of the kinetic model, affecting predictions of relevant combustion behavior. With regard to model parameters, present uncertainties in the temperature and pressure dependence of rate constants for HO2 formation and consumption reactions are demonstrated to substantially affect predictive capabilities at high-pressure, low-temperature conditions. With regard to model assumptions, calculations are performed to investigate several reactions/processes that have not received much attention previously. Results from ab initio calculations and modeling studies imply that inclusion of H + HO2 = H2O + O in the kinetic model might be warranted, though further studies are necessary to ascertain its role in combustion modeling. In addition, it appears that characterization of nonlinear bath-gas mixture rule behavior for H + O2(+ M) = HO2(+ M) in multicomponent bath gases might be necessary to predict high-pressure flame speeds within ∼15%.

The updated model is tested against all of the previous validation targets considered by Li et al. as well as new targets from a number of recent studies. Special attention is devoted to establishing a context for evaluating model performance against experimental data by careful consideration of uncertainties in measurements, initial conditions, and physical model assumptions. For example, ignition delay times in shock tubes are shown to be sensitive to potential impurity effects, which have been suggested to accelerate early radical pool growth in shock tube speciation studies. In addition, speciation predictions in burner-stabilized flames are found to be more sensitive to uncertainties in experimental boundary conditions than to uncertainties in kinetics and transport. Predictions using the present model adequately reproduce previous validation targets and show substantially improved agreement against recent high-pressure flame speed and shock tube speciation measurements. Comparisons of predictions of several other kinetic models with the experimental data for nearly the entire validation set used here are also provided in the Supporting Information. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 44: 444–474, 2012

Comprehensive H2/O2 kinetic model for high-pressure combustion

New experimental profiles of stable species concentrations are reported for formaldehyde oxidation in a variable pressure flow reactor at initial temperatures of 850–950 K and at constant pressures ranging from 1.5 to 6.0 atm. These data, along with other data published in the literature and a previous comprehensive chemical kinetic model for methanol oxidation, are used to hierarchically develop an updated mechanism for CO/H2O/H2/O2, CH2O, and CH3OH oxidation. Important modifications include recent revisions for the hydrogen–oxygen submechanism (Li et al., Int J Chem Kinet 2004, 36, 565), an updated submechanism for methanol reactions, and kinetic and thermochemical parameter modifications based upon recently published information. New rate constant correlations are recommended for CO + OH = CO2 + H (R23) and HCO + M = H + CO + M (R24), motivated by a new identification of the temperatures over which these rate constants most affect laminar flame speed predictions (Zhao et al., Int J Chem Kinet 2005, 37, 282). The new weighted least-squares fit of literature experimental data for (R23) yields k23 = 2.23 × 105T1.89exp(583/T) cm3/mol/s and reflects significantly lower rate constant values at low and intermediate temperatures in comparison to another recently recommended correlation and theoretical predictions. The weighted least-squares fit of literature results for (R24) yields k24 = 4.75 × 1011T0.66exp(−7485/T) cm3/mol/s, which predicts values within uncertainties of both prior and new (Friedrichs et al., Phys Chem Chem Phys 2002, 4, 5778; DeSain et al., Chem Phys Lett 2001, 347, 79) measurements. Use of either of the data correlations reported in Friedrichs et al. (2002) and DeSain et al. (2001) for this reaction significantly degrades laminar flame speed predictions for oxygenated fuels as well as for other hydrocarbons. The present C1/O2 mechanism compares favorably against a wide range of experimental conditions for laminar premixed flame speed, shock tube ignition delay, and flow reactor species time history data at each level of hierarchical development. Very good agreement of the model predictions with all of the experimental measurements is demonstrated. © 2007 Wiley Periodicals, Inc. 39: 109–136, 2007

A comprehensive kinetic mechanism for CO, CH2O, and CH3OH combustion

We propose a H2–CO kinetic model which incorporates the recent thermodynamic, kinetic, and species transport updates relevant to high-temperature H2 and CO oxidation. Attention has been placed on obtaining a comprehensive and kinetically accurate model able to predict a wide variety of H2–CO combustion data. The model was subject to systematic optimization and validation tests against reliable H2–CO combustion data, from global combustion properties (shock-tube ignition delays, laminar flame speeds, and extinction strain rates) to detailed species profiles during H2 and CO oxidation in flow reactor and in laminar premixed flames.

An optimized kinetic model of H2/CO combustion

For more than two decades [1,2], Corning has served the community with an annual review of global regulatory and technological advances pertaining to emissions from internal combustion engine (ICE) driven vehicles and machinery. We continue with a review for the year 2020, which will be remembered by COVID and the significant negative impact it had on the industry. However, it also provided a glimpse of the possible improvement in air quality with reduced anthropogenic emissions. It was a year marked by goals set for climate change mitigation via reduced fossil fuel use by the transportation sector. Governments stepped up plans to accelerate the adoption of zero tailpipe emitting vehicles. However, any transformation of the transportation sector is not going to happen overnight due to the scale of the infrastructure and technology challenges. A case in point is China, which announced a technology roadmap which envisions half of the vehicles to be hybrids in 2035. The ICE is clearly expected to be part of the powertrain mix for a long time and as such, solutions are needed to attain near-zero emissions, even with conventional engines. The industry is naturally responding to all of these changes and several technology solutions are being advanced, including improved efficiency, advanced aftertreatment systems, hybridization, low carbon fuels and predictive control strategies. It was also a year of heightened regulatory activity on what could perhaps be the last major regulations on criteria pollutants in advanced markets. California adopted the Low NOx Omnibus rule requiring a 90% reduction in NOx from heavy-duty vehicles. Elements of light-duty LEV IV regulations were discussed, which could culminate in a fleet averaged NMOG + NOx limit of 20 mg/mi. Proposals were made for Euro 7/VII, and several major changes put forth for consideration, including tightening of limits, inclusion of sub-23 nm particles, an emphasis on urban driving and an overall shift in certification based on real-world driving emission measurements with limited allowed exclusions. Limits may be imposed on previously non-regulated species such as NH3 which will drive additional content. Technologies are advancing, both on engines and aftertreatment systems. Light-duty gasoline engines are approaching 45% BTE. Heavy duty diesel engines are approaching 55% BTE. We cover some of the major technologies being pursued to extend these gains. Gasoline particulate filters are now rapidly becoming a mature technology for light-duty vehicles in Europe and China, although the next round of regulations will require a significant increase in filtration efficiency. Concept studies show pathways to reduce gas emissions well below the next proposed limits. A major thrust on the heavy-duty side is to analyze systems capable of meeting the low NOx requirements while also extending durability. We cover the various leading approaches and latest advances in de-NOx technologies. We also briefly touch upon fuels, which will play a critical role, whether in improving efficiency of advanced combustion such as gasoline compression ignition or in their role with reducing greenhouse gas emissions through renewable or synthetic fuels. Finally, as we approach near-zero tailpipe emission levels, non-tailpipe emissions could become a significant fraction of the overall particulate inventory. © 2021 SAE International. All rights reserved.

Review of Vehicle Engine Efficiency and Emissions

The atmospheric laminar flame speeds of mixtures of air with ethylene, n-butane, toluene, ethylene-n-butane, ethylene-toluene, and n-butane-toluene were experimentally and computationally investigated over an extended range of equivalence ratios. Binary fuel blends with 1:1, 1:2, and 2:1 molar ratios were examined. Experimentally, the laminar flame speeds were determined using digital particle image velocimetry (DPIV). Since the use of DPIV enables the mapping of the two-dimensional flow field adhead of the flame, the reference speed based on the minimum axial velocity point as well as the imposed strain rate can be identified simultaneously. The latter can now be unambiguously determined by the radial velocity gradient at the minimum velocity point. By systematically varying the imposed strain rate, the corresponding laminar flame speed was obtained through nonlinear extrapolation to zero strain rate. The associated experimental accuracy of the DPIV measurements was also assessed and discussed. Computationally, the laminar flame speeds were simulated for all single-component fuel/air and binary fuel blend/air mixtures with a detailed kinetic model. Comparison of experimental and computed flame speeds shows generally good agreement. A semiempirical mixing rule was developed. The mixing rule which requires only the knowledge of the flame speeds and flame temperatures of the individual fuel constituents, is shown to provide acurate estimates for the laminar flame speeds of binary fuel blends under the conditions tested.

Determination of laminar flame speeds using digital particle image velocimetry: Binary Fuel blends of ethylene, n-Butane, and toluene

Non-catalytic oxidation kinetics of diesel engine soot and more than a dozen commercial carbon black samples was investigated using non-isothermal and isothermal thermogravimetric analysis (TGA) ex...

Experimental Study of Carbon Black and Diesel Engine Soot Oxidation Kinetics Using Thermogravimetric Analysis

The rate coefficient of CO + OH products is analyzed with RRKM/master equation analyses and Monte Carlo simulations. The analyses are based on the recent CCSD(T)/cc-pvTZ potential energy surface of Yu et al. Chem Phys Lett 2001, 349, 547–554). It is shown that the experimental data over the temperature range of 80–2500 K and pressure from 1 Torr to 800 bar can be satisfactorily reproduced by lowering the CCSD(T)/cc-pvTZ energy barrier for the CO2 + H exit channel by 1 kcal/mol and more importantly, by considering an equilibrium factor in the thermal rate constant formulation. This factor accounts for the populations of rovibrationally excited trans- and cis-HOCO, which are then allowed to dissociate only through specific paths that are open to them. By modeling the isothermal but pressure-dependent rate data of Fulle et al. (J Chem Phys 1996, 105, 983–1000) over the temperature range from 98 to 819 K, we obtained an 〈Edown〉 value equal to 150 cm−1 for M = He. The 〈Edown〉 values for M = N2, Ar, CF4, and SF6 were also obtained by fitting the OH and OD data at 298 K. Based on the theoretical analyses, we recommended that the following rate expression be used for CO + OH  CO2 + H in the temperature range from 120 to 2500 K and pressure lower than P(bar) = 9 × 10−17T5.9 exp(520/T): k1b,0(cm3 molecule−1s−1) =1.17 × 10−19T2.053exp(139/T) + 9.56 × 10−12T−0.664 exp(−167/T). Fall-off parameterization is also proposed for the rate coefficient of CO + OH  CO2 + H under extremely high pressures and for CO + OH  HOCO over the temperature range from 120 to 2500 K. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38:57–73, 2006

Ameya Joshi

Papers

An optimized kinetic model of H2/CO combustion

Review of Vehicle Engine Efficiency and Emissions

Determination of laminar flame speeds using digital particle image velocimetry: Binary Fuel blends of ethylene, n-Butane, and toluene

Experimental Study of Carbon Black and Diesel Engine Soot Oxidation Kinetics Using Thermogravimetric Analysis

Master equation modeling of wide range temperature and pressure dependence of CO + OH → products