Henning Richter

Bio: Henning Richter is an academic researcher from Massachusetts Institute of Technology. The author has contributed to research in topics: Combustion & Soot. The author has an hindex of 23, co-authored 50 publications receiving 3538 citations. Previous affiliations of Henning Richter include Northeastern University & New Jersey Institute of Technology.

Formation and consumption of single-ring aromatic hydrocarbons and their precursors in premixed acetylene, ethylene and benzene flames

Abstract: Kinetic modeling is becoming a powerful tool for the quantitative description of combustion processes covering different fuels and large ranges of temperature, pressure and equivalence ratio. In the present work, a reaction mechanism which was developed initially for benzene oxidation, and included the formation of polycyclic aromatic hydrocarbons was extended and tested for the combustion of acetylene and ethylene. Thermodynamic and kinetic property data were updated. If available, data were taken from the recent literature. In addition, density functional theory as well as ab initio computations on a CBS-Q and CBS-RAD level, partially published previously, were carried out. Quantum Rice–Ramsperger–Kassel analysis was conducted in order to determine pressure-dependent rate constants of chemically activated reactions. The model was developed and tested using species concentration profiles reported in the literature from molecular beam mass-spectrometry measurements in four unidimensional laminar premixed low-pressure ethylene, acetylene and benzene flames at equivalence ratios (ϕ) of 0.75 and 1.9 (C2H4), 2.4 (C2H2) and 1.8 (C6H6). Predictive capabilities of the model were found to be at least fair and often good to excellent for the consumption of the reactants, the formation of the main combustion products as well as the formation and depletion of major intermediates including radicals. Self-combination of propargyl (C3H3) followed by ring closure and rearrangement was the dominant benzene formation pathway in both rich acetylene and ethylene flames. In addition, reaction between vinylacetylene (C4H4) and vinyl radical (C2H3) contributed to benzene formation in the ϕ = 1.9 ethylene flame. Propargyl formation and consumption pathways which involve reactions between acetylene, allene, propyne and singlet and triplet methylene were assessed. Significant overpredictions of phenoxy radicals indicate the necessity of further investigation of the pressure and temperature dependence and the product distribution of phenyl oxidation. The possible formation of benzoquinones, the ratio of the ortho and para isomers and their degradation pathways are of particular interest.

Detailed modeling of PAH and soot formation in a laminar premixed benzene/oxygen/argon low-pressure flame

Abstract: Combustion-generated polycyclic aromatic hydrocarbons (PAH) and soot particles are of significant environmental concern whereas controlled combustion is of increasing interest for the synthesis of carbonaceous nanostructures such as fullerenic material. Improved understanding of chemical and physical processes involved in PAH and soot formation is required to correlate operating conditions with emission characteristics. A detailed kinetic model describing the formation and consumption of PAH and soot in fuel-rich hydrocarbon combustion has been developed. Using a sectional approach, large PAH and carbonaceous particles with diameters of up to ≈70 nm are defined as classes (BINs) covering given mass ranges. Numbers of carbon and hydrogen atoms corresponding to their average masses are assigned to each BIN, accounting for a decrease in H/C ratios with increasing particle size. The model has been successfully tested for a rich premixed benzene/oxygen/argon flame ( ϕ = 2.4, 10% argon, v = 25 cm s −1 , 5.33 kPa). Model predictions are compared with published experimental data including mole fraction profiles of individual PAH and concentration as well as number density profiles of soot. Reactions of PAH radicals with PAH and between PAH radicals were found to be the dominant pathway to soot nuclei. Surface growth contributes ≈75% to the final particle mass, and reaction of acetylene with particle radicals is the major growth pathway. Surface growth reactions are involved in PAH depletion in the postflame zone. Particle coagulation involving BINs and BIN radicals significantly contributes to the formation of progressively larger particles whereas oxidation by OH plays a non-negligible role in their depletion.

Formation of polycyclic aromatic hydrocarbons and their radicals in a nearly sooting premixed benzene flame

Abstract: Polycyclic aromatic hydrocarbons (PAH) are associated with health hazardous effects, and combustion processes are major sources of their presence in atmospheric aerosols. In the present work, chemical reaction pathways of PAH formation have been investigated by means of the modeling of a nearly sooting, low-pressure, premixed, laminar, one-dimensional benzene/oxygen/argon flame (equivalence ratio =1.8, 30% argon, gas velocity at burner at 298 K v =50 cm s −1 , pressure=2.67 kPa). This flame has been investigated by Bittner and Howard using molecular-beam sampling coupled to mass spectrometry. More recently, Benish extended the set of available data for radicals up to 201 amu and for stable species up to 276 amu using nozzle-beam sampling followed by radical scavenging with dimethyl disulfide and subsequent analysis by gas chromatography-mass spectrometry. An existing kinetic model has been refined. Density functional theory computations were used to update the thermodynamic database, while transition state theory followed by a bimolecular quantum Rice-Ramsperger-Kassel analysis allowed for the determination of kinetic data relevant for the present study. The reaction of phenylacetylene radicals with acetylene is shown to be limiting for the concentration of 1-naphthyl radicals, while naphthalene is formed mainly by self-combination of cyclopentadienyl. The insufficient consumption of PAH as well as acetylene beyond the reaction zone gives some evidence of the need of additional PAH growth pathways involving acetylene but thermodynamically more favorable than subsequent hydrogen-abstraction/acetyleneaddition reactions. A new pathway for acenaphthylene formation is suggested and consists of benzyne recombination followed by hydrogen attack and isomerization.

ReaxFF Reactive Force Field for Molecular Dynamics Simulations of Hydrocarbon Oxidation

Abstract: To investigate the initial chemical events associated with high-temperature gas-phase oxidation of hydrocarbons, we have expanded the ReaxFF reactive force field training set to include additional transition states and chemical reactivity of systems relevant to these reactions and optimized the force field parameters against a quantum mechanics (QM)-based training set. To validate the ReaxFF potential obtained after parameter optimization, we performed a range of NVT−MD simulations on various hydrocarbon/O2 systems. From simulations on methane/O2, o-xylene/O2, propene/O2, and benzene/O2 mixtures, we found that ReaxFF obtains the correct reactivity trend (propene > o-xylene > methane > benzene), following the trend in the C−H bond strength in these hydrocarbons. We also tracked in detail the reactions during a complete oxidation of isolated methane, propene, and o-xylene to a CO/CO2/H2O mixture and found that the pathways predicted by ReaxFF are in agreement with chemical intuition and our QM results. We o...

Small molecule semiconductors for high-efficiency organic photovoltaics

Abstract: Organic photovoltaic cells (OPVs) are a promising cost-effective alternative to silicon-based solar cells, and possess light-weight, low-cost, and flexibility advantages. Significant progress has been achieved in the development of novel photovoltaic materials and device structures in the last decade. Nowadays small molecular semiconductors for OPVs have attracted considerable attention, due to their advantages over their polymer counterparts, including well-defined molecular structure, definite molecular weight, and high purity without batch to batch variations. The highest power conversion efficiencies of OPVs based on small molecular donor/fullerene acceptors or polymeric donor/fullerene acceptors are up to 6.7% and 8.3%, respectively, and meanwhile nonfullerene acceptors have also exhibited some promising results. In this review we summarize the developments in small molecular donors, acceptors (fullerene derivatives and nonfullerene molecules), and donor–acceptor dyad systems for high-performance multilayer, bulk heterojunction, and single-component OPVs. We focus on correlations of molecular chemical structures with properties, such as absorption, energy levels, charge mobilities, and photovoltaic performances. This structure–property relationship analysis may guide rational structural design and evaluation of photovoltaic materials (253 references).

Reaction mechanism of soot formation in flames

Abstract: Chemical reactions and physical processes responsible for the formation of polycyclic aromatic hydrocarbons and soot in hydrocarbon flames are reviewed. The discussion is focused on major elements in the present understanding of the phenomena, clarification of concepts central to the present state of the art, and a summary of new results.