Investigation on the interaction of NH3&CH4 co-activated char under reburning conditions.

In the current global context, there is a pressing need to curtail greenhouse gas emissions, making the utilization of a coal and zero-carbon energy blend an imperative strategy for reducing carbon emissions from coal-fired power generation. The planar flame burner serves as a tool to simulate the temperature and atmospheric conditions within the reburning zone, facilitating extensive examination of the physical and chemical structural alterations, as well as the nitrogen oxide reduction potential, during NH3/CH4 activation for reburning pulverized coal. Experimental results underscore that blending high-activity fuels optimizes the combustion performance of coal char. Through the addition of NH3 and CH4, the NO reduction capability of coal char is bolstered by approximately 0.67 times compared to sole reliance on recirculating flue gas transport. Furthermore, NH3 introduction facilitates the conversion of C]O double bonds into C-O single bonds, rendering them more amenable to reduction by NO. While the joint influence of NH3 and CH4 does not significantly impact char particle size, it does foster the evolution of N-Q to N-5 and N-6 on the char surface. Furthermore, there was a significant increase in the BET-specific surface area, which rose by 50%. Additionally, the total pore volume increased by approximately 21.43%. The comprehensive understanding of NH3 and CH4 modified pulverized coal reburning technology holds significant promise for optimizing power plant operations and mitigating carbon dioxide and nitrogen oxide emissions.

Association of Cl with C2H2 by unified variable-reaction-coordinate and reaction-path variational transition-state theory.

Barrierless unimolecular association reactions are prominent in atmospheric and combustion mechanisms but are challenging for both experiment and kinetics theory. A key datum for understanding the pressure dependence of association and dissociation reactions is the high-pressure limit, but this is often available experimentally only by extrapolation. Here we calculate the high-pressure limit for the addition of a chlorine atom to acetylene molecule (Cl + C2H2→C2H2Cl). This reaction has outer and inner transition states in series; the outer transition state is barrierless, and it is necessary to use different theoretical frameworks to treat the two kinds of transition state. Here we study the reaction in the high-pressure limit using multifaceted variable-reaction-coordinate variational transition-state theory (VRC-VTST) at the outer transition state and reaction-path variational transition state theory (RP-VTST) at the inner turning point; then we combine the results with the canonical unified statistical (CUS) theory. The calculations are based on a density functional validated against the W3X-L method, which is based on coupled cluster theory with single, double, and triple excitations and a quasiperturbative treatment of connected quadruple excitations [CCSDT(Q)], and the computed rate constants are in good agreement with some of the experimental results. The chlorovinyl (C2H2Cl) adduct has two isomers that are equilibrium structures of a double-well C≡C-H bending potential. Two procedures are used to calculate the vibrational partition function of chlorovinyl; one treats the two isomers separately and the other solves the anharmonic energy levels of the double well. We use these results to calculate the standard-state free energy and equilibrium constant of the reaction.

Direct dynamics in a proton transfer reaction of isomer product competition. Insight into the suppressed formation of the isoformyl cation.

Proton transfer between HOCO+ and CO produces the formyl cation HCO+ and isoformyl cation HOC+ isomers initiating multiple astrochemical reaction networks. Here, the direct chemical dynamics simulations are performed to uncover the underlying atomistic dynamics of the above reaction. The simulations reproduce the measured product energy and scattering angle distributions and reveal that the reaction proceeds predominantly through a direct stripping mechanism which results in the prominent forward scattering observed in experiments. The reaction dynamics show propensity for the HCO+ product even at a collision energy larger than the threshold for HOC+ formation. This is a consequence of the larger opacity and impact parameter range for HCO+. In accordance with the revealed direct mechanistic feature, the reaction can be controlled by orienting the reactants into a reactive H-C orientation that also favors HCO+ formation. Considering the lack of equilibrated reactant complexes and the on the fly migration of the proton, the CO2-catalyzed isomerization is assumed to have insignificant impact on the isomer ratios. This work provides insights of dynamical effects besides energetics into the interesting finding of strongly suppressed formation of the metastable isoformyl cation for related proton transfer reactions in the measurements.

Direct coherent switching with decay of mixing for intersystem crossing dynamics of thioformaldehyde: The effect of decoherence.

We evaluate the effect of electronic decoherence on intersystem crossing in the photodynamics of thioformaldehyde. First, we show that the state-averaged complete-active-space self-consistent field electronic structure calculations with a properly chosen active space of 12 active electrons in 10 active orbitals can predict the potential energy surfaces and the singlet-triplet spin-orbit couplings quite well for CH2S, and we use this method for direct dynamics by coherent switching with decay of mixing (CSDM). We obtain similar dynamical results with CSDM or by adding energy-based decoherence to trajectory surface hopping, with the population of triplet states tending to a small steady-state value over 500 fs. Without decoherence, the state populations calculated by the conventional trajectory surface hopping method or the semiclassical Ehrenfest method gradually increase. This difference shows that decoherence changes the nature of the results not just quantitatively but qualitatively.

Correction: Electronic spectrum and characterization of diabatic potential energy surfaces for thiophenol.

Correction for 'Electronic spectrum and characterization of diabatic potential energy surfaces for thiophenol' by Linyao Zhang et al., Phys. Chem. Chem. Phys., 2018, 20, 28144-28154.

Dynamics of Cl-(H2O) + CH3I Substitution Reaction: The Influences of Solvent and Nucleophile.

The study of microsolvation provides a deeper understanding of solvent effects on reaction dynamics. Here, the properties of the SN2 reaction of hydrated chloride with methyl iodide are investigated by direct dynamics simulations, and how the solute-solvent interactions and the basicity of nucleophiles can profoundly affect the atomic level dynamics is discussed in detail. The results show that the direct-rebound mechanism dominates the substitution reaction, and the roundabout mechanism, which prevails in the indirect unsolvated counterpart reaction, still accounts for a high proportion of the indirect mechanisms. The involvement of a solvent water molecule does not significantly reduce the cross section and rate constant compared to the unhydrated reaction at high collision energy. By varying solvated Cl- to F-, the dominant mechanisms are totally different and in contrast, the dynamics of water does not show much difference, and the departure of H2O tends to occur prior to the substitution reaction because of the facile breakage of the hydrogen bond at high collision energy.

Full-dimensional three-state potential energy surfaces and state couplings for photodissociation of thiophenol.

An analytic full-dimensional diabatic potential energy matrix (DPEM) for the lowest three singlet states of thiophenol (C6H5SH) at geometries accessible during photodissociation is constructed using the anchor points reactive potential (APRP) scheme. The data set used for modeling is obtained from electronic structure calculations including dynamic correlation via excitations into the virtual space of a three-state multiconfiguration self-consistent field calculation. The resulting DPEM is a function of all the internal coordinates of thiophenol. The DPEM as a function of the S-H bond stretch and C-C-S-H torsion and the diabatic couplings along two in-plane bend modes and nine out-of-plane distortion modes are computed using extended multiconfigurational quasidegenerate perturbation theory followed by the fourfold way determination of diabatic molecular orbitals and model space diabatization by configurational uniformity, and this dependence of the DPEM is represented by general functional forms. Potentials along 31 tertiary internal degrees of freedom are modeled with system-dependent, primary-coordinate-dependent nonreactive molecular mechanics-type force fields that are parameterized by Cartesian Hessians calculated by generalized Kohn-Sham density functional theory. Adiabatic potential energy surfaces (PESs) and nonadiabatic couplings are obtained by a transformation of the DPEM. The topography of the APRP PESs is characterized by vertical excitation energies, equilibrium geometries, vibrational frequencies, and conical intersections, and we find good agreement with available reference data. This analytic DPEM is suitable for full-dimensional electronically nonadiabatic molecular dynamics calculations of the photodissociation of thiophenol with analytic gradients in either the adiabatic or diabatic representation.

Electronic spectrum and characterization of diabatic potential energy surfaces for thiophenol.

The electronic spectrum of thiophenol was simulated by a normal-mode sampling approach combined with TDDFT in the Tamm-Dancoff approximation (TDA). The vertical excitation energies were compared with electronic structure calculations by completely renormalized equation-of-motion coupled cluster theory with single and double excitations and noniterative inclusion of connected triples (CR-EOM-CCSD(T)) and by multi-reference perturbation theory. The spectrum was computed both with and without solvation effects, and these spectra are compared to each other and to experiment. Using multireference-perturbation-theory adiabatic wave functions and model-space diabatization by the fourfold way, diabatic potential energy surfaces of the lowest three singlet states (1ππ, 1ππ*, and 1nπσ*) were constructed along the S-H stretching coordinate, the C-C-S-H torsion coordinate, and the v16a and v16b normal coordinates. The first two of these two are primary coordinates for the photodissociation, and the diabatic crossing seams of the three states were calculated and plotted as functions of the two coordinates. The other two coordinates are out-of-plane ring distortion modes studied to assess the extent of their role in coupling the states near the first conical intersection, and the v16a mode was shown to be an important coupling mode there. The current study is the first step toward a detailed mechanistic analysis of the photoinduced S-H fission process of thiophenol, a test system to understand 1nπσ*-mediated reactions, at the same time already providing a better understanding of the thiophenol electronic excitations by clarifying the assignment of the experimental results.

Effects of Water Molecule on CO Oxidation by OH: Reaction Pathways, Kinetic Barriers, and Rate Constants.

The water dilute oxy-fuel combustion is a clean combustion technology for near-zero emission power; and the presence of water molecule could have both kinetic and dynamic effects on combustion reactions. The reaction OH + CO → CO2 + H, one of the most important elementary reactions, has been investigated by extensive electronic structure calculations. And the effects of a single water molecule on CO oxidation have been studied by considering the preformed OH(H2O) complex reacts with CO. The results show little change in the reaction pathways, but the additional water molecule actually increases the vibrationally adiabatic energy barriers (VaG). Further thermal rate constant calculations in the temperature range of 200 to 2000 K demonstrate that the total low-pressure limit rate constant for the water assisted OH(H2O) + CO → CO2 + H2O + H reaction is 1-2 orders lower than that of the water unassisted one, which is consistent with the change of VaG. Therefore, the hydrated radical OH(H2O) would actually slow down the oxidation of CO. Meanwhile, comparisons show that the M06-2X/aug-cc-pVDZ method gives a much better estimation in energy and thus is recommended to be employed for direct dynamics simulations.

Research on NO generation characteristics of ammonia-premixed flame.

Ammonia is a promising fuel with high energy density, accessible storage, and no CO2 production by combustion, but its combustion produces the pollutant NO. In this study, a Bunsen burner experimental bench was selected to investigate the concentration of NO generated by ammonia combustion at different initial oxygen concentrations. Further, the reaction pathways of NO were analyzed in depth, and sensitivity analysis was performed. The results show that the Konnov mechanism has an excellent predictive effect on NO generated by ammonia combustion. In the ammonia-premixed laminar flame at atmospheric pressure, the NO concentration peaked at an equivalence ratio of 0.9. The high initial oxygen concentration enhanced the combustion of ammonia-premixed flame and increased the conversion of NH3 to NO. NO was not only a product but a contribution to the combustion of NH3. As the equivalence ratio increases, NH2 consumes a large amount of NO and reduces NO production. The high initial oxygen concentration enhanced NO production, and the effect was more pronounced at low equivalents. The study results provide theoretical guidance for the utilization of ammonia combustion and pollutant reduction and help to drive the process of ammonia combustion toward practicality.

Numerical simulation on the deposition characteristics of MSWI fly ash particles in a cyclone furnace.

In melting municipal solid waste incineration (MSWI) fly ash by cyclone furnace, the deposition characteristics of particles affect the slag flow and the secondary MSWI fly ash formation. In this study, the composition mechanism based on critical viscosity is selected as the particle deposition model to predict the deposition and rebound of particles on the furnace wall. The Riboud model with an accurate viscosity prediction performance is selected, then the particle deposition model is integrated into a commercial computational fluid dynamics (CFD) solver through the user-defined function (UDF) to realize the coupling of particle motion and deposition process. The results show that under the same case, the deposition rate decreases obviously with the increase of MSWI fly ash particle size. And the escape rate reaches a maximum at particle size 120 µm. Controlling the particle size of fly ash particles within 60 µm can effectively reduce the generation of secondary MSWI fly ash. During the forward movement of the fly ash inlet position, the escape of MSWI fly ash particles with large particle sizes has been significantly weakened. This measure not only lowers the post-treatment cost but also dramatically reduces the pretreatment step of MSWI fly ash before the melting and solidification process. In addition, the deposition rate and quality will reach the maximum values, respectively, along with gradually increasing MSWI fly ash input flow. Overall, this study has the guiding significance for reducing the pretreatment steps and post-treatment costs of MSWI fly ash by melting in the cyclone furnace.

New Gradient Correction Scheme for Electronically Nonadiabatic Dynamics Involving Multiple Spin States.

It has been recommended that the best representation to use for trajectory surface hopping (TSH) calculations is the fully adiabatic basis in which the Hamiltonian is diagonal. Simulations of intersystem crossing processes with conventional TSH methods require an explicit computation of nonadiabatic coupling vectors (NACs) in the molecular-Coulomb-Hamiltonian (MCH) basis, also called the spin-orbit-free basis, in order to compute the gradient in the fully adiabatic basis (also called the diagonal representation). This explicit requirement destroys some of the advantages of the overlap-based algorithms and curvature-driven algorithms that can be used for the most efficient TSH calculations. Therefore, although these algorithms allow one to perform NAC-free simulations for internal conversion processes, one still requires NACs for intersystem crossing. Here, we show that how the NAC requirement is circumvented by a new computation scheme called the time-derivative-matrix scheme.

Recommendation of Orbitals for G0W0 Calculations on Molecules and Crystals.

The many-body GW approximation, especially the G0W0 method, has been widely used for condensed matter and molecules to calculate quasiparticle energies for ionization, electron attachment, and band gaps. Because G0W0 calculations are well-known to have a strong dependence on the orbitals, the goal of the present work is to provide guidance on the choice of density functional used to generate orbitals and to recommend a choice that gives the most broadly accurate results. We have systematically investigated the dependence of G0W0 calculations on the orbitals for 100 molecules and 8 crystals by considering orbitals obtained with a diverse set of Kohn-Sham (KS) and generalized KS (GKS) functionals (63 functionals plus Hartree-Fock). The percentage of Hartree-Fock exchange employed in density functionals has been found to have strong influence on the predicted molecular ionization energy and crystal fundamental band gaps (with optimum values between 40 and 56%), but to have less effect on predicting molecular electron affinities. The low cost of the Gaussian implementation, even with hybrid functionals in periodic calculations, the better performance of global hybrids as compared to range-separated hybrids of either than screened exchange or long-range-corrected type, and the relatively low cost of global-hybrid-functional periodic calculations using Gaussians means that one can employ global-hybrid functionals at a very reasonable cost and obtain more accurate band gaps of semiconductors than are obtained by the methods currently widely employed, namely local gradient approximations. We single out three global-hybrid functionals that give especially good results for both molecules (100 in the test set) and crystals (8 in the test set, for all of which our benchmark data are the proper band gap rather than an optical band gap uncorrected for exciton effects).

Nonadiabatic Dynamics Algorithms with Only Potential Energies and Gradients: Curvature-Driven Coherent Switching with Decay of Mixing and Curvature-Driven Trajectory Surface Hopping.

Direct dynamics by mixed quantum-classical nonadiabatic methods is an important tool for understanding processes involving multiple electronic states. Very often, the computational bottleneck of such direct simulation comes from electronic structure theory. For example, at every time step of a trajectory, nonadiabatic dynamics requires potential energy surfaces, their gradients, and the matrix elements coupling the surfaces. The need for the couplings can be alleviated by employing the time derivatives of the wave functions, which can be evaluated from overlaps of electronic wave functions at successive time steps. However, evaluation of overlap integrals is still expensive for large systems. In addition, for electronic structure methods for which the wave functions or the coupling matrix elements are not available, nonadiabatic dynamics algorithms become inapplicable. In this work, building on recent work by Baeck and An, we propose new nonadiabatic dynamics algorithms that only require adiabatic potential energies and their gradients. The new methods are named curvature-driven coherent switching with decay of mixing (κCSDM) and curvature-driven trajectory surface hopping (κTSH). We show how powerful these new methods are in terms of computation time and accuracy as compared to previous mixed quantum-classical nonadiabatic dynamics algorithms. The lowering of the computational cost will allow longer nonadiabatic trajectories and greater ensemble averaging to be affordable, and the ability to calculate the dynamics without electronic structure coupling matrix elements extends the dynamics capability to new classes of electronic structure methods.

Nonadiabatic Dynamics of 1,3-Cyclohexadiene by Curvature-Driven Coherent Switching with Decay of Mixing.

The photoinduced ring-opening reaction of 1,3-cyclohexadiene to produce 1,3,5-hexatriene is a classic electrocyclic reaction and is also a prototype for many reactions of biological and synthetic importance. Here, we simulate the ultrafast nonadiabatic dynamics of the reaction in the manifold of the three lowest valence electronic states by using extended multistate complete-active-space second-order perturbation theory (XMS-CASPT2) combined with the curvature-driven coherent switching with decay of mixing (κCSDM) dynamical method. We obtain an excited-state lifetime of 79 fs, and a product quantum yield of 40% from the 500 trajectories initiated in the S1 excited state. The obtained lifetime and quantum yield values are very close to previously reported experimental and computed values, showing the capability of performing a reasonable nonadiabatic ring-opening dynamics with the κCSDM method that does not require nonadiabatic coupling vectors, time derivatives, or diabatization. In addition, we study the ring-opening reaction by initiating the trajectories in the dark state S2. We also optimize the S0/S1 and S1/S2 minimum-energy conical intersections (MECIs) by XMS-CASPT2; for S1/S2, we optimized both an inner and an outer local-minimum-energy conical intersections (LMECIs). We provide the potential energy profile along the ring-opening coordinate by joining selected critical points via linear synchronous transit paths. We find the inner S1/S2 LMECI to be more crucial than the outer one.

Reburning pulverized coal with natural gas/syngas upgrading: NO reducing ability and physicochemical structure evolution of coal char.

The lifting gas activates the coal particles, which increases their ability to reduce NO. This technique overcomes the oxygen consumption of large pulverized coal in the early stages of re-firing during air/flue gas transport of pulverized coal. This study conducted experiments on a planar flame burner bench to analyze the physicochemical structure evolution of coal coke after natural gas and syngas activation using FTIR, XPS, and BET. The NO reduction capacity was tested on a micro fluidized bed reaction test bench. The results show that natural gas's upgrading effect is better than syngas. Hydrogen and hydrocarbon radicals generated by the reaction of natural gas with oxygen play a significant role in activation. After upgrading by natural gas, the specific surface area of carbon increased by about 54.2 %, the total pore volume increased by about 51.2 %, the whole oxygen-containing groups decreased by nearly 4.4 %, the total amount of alkyl complexes increased by about 3.6 %, and the nitric oxide reducing ability increased by almost 75 %. The technology minimizes expensive reactive gases while ensuring less reburned coal is used to reduce NOx emissions.

Effect of high-temperature and microwave expanding modification on reactivity of coal char for char-NO interaction.

Coal-fired industrial boiler has become a large source of atmospheric pollutants in China, urging to achieve low NOx emissions. This paper adjusts the coal char structure with high-temperature/microwave expanding modification to investigate the char-NO interaction. The results show that after high-temperature or microwave expansion, the pore structure of char is further expanded with more new pore structure of 2-12 nm. The proper expansion temperature/power/treatment-time increases the ablation collapse of char pores and the order of carbon structure. With microwave, COC and CO bands break, forming a large amount of aromatic CC unsaturated carbon atoms, incrseasing the surface active sites of char-NO interaction. The optimum modifications of char-NO reactivity are 800 °C-90 s and 960 W-90 s. The reduction rate of NO by microwave modified char decreases with increase of inlet NO (<1200 ppm), and increases with increase of inlet CO (<8000 ppm). Burnout time of microwave modified char is shortened, with more rapid release of NO and larger conversion rate of char-N to NO. With microwave field, the conversion rate of char-N to NO at 900 °C is more significant than that at 600 °C. The too large microwave power cannot further shorten the char burnout time and the release time of NO.

Time-Derivative Couplings for Self-Consistent Electronically Nonadiabatic Dynamics.

Electronically nonadiabatic dynamics methods based on a self-consistent potential, such as semiclassical Ehrenfest and coherent switching with decay of mixing, have a number of advantages but are computationally slower than approximations based on an unaveraged potential because they require evaluation of all components of the nonadiabatic coupling vector. Here we introduce a new approximation to the self-consistent potential that does not have this computational drawback. The new approximation uses time-derivative couplings evaluated by overlap integrals of electronic wave functions to approximate the nonadiabatic coupling terms in the equations of motion. We present a numerical test of the method for ethylene that shows there is little loss of accuracy in the ensemble-averaged results. This new approximation to the self-consistent potential makes direct dynamics calculations with self-consistent potentials more efficient for complex systems and makes them practically affordable for some cases where the cost was previously too high.

Implementation of Coherent Switching with Decay of Mixing into the SHARC Program.

Simulation of electronically nonadiabatic dynamics is an important tool for understanding the mechanisms of photochemical and photophysical processes. Two contrasting methods in which the electrons are treated quantum mechanically while the nuclei are treated classically are semiclassical Ehrenfest dynamics and trajectory surface hopping; neither method in its original form includes decoherence. Decoherence in the context of electronically nonadiabatic dynamics refers to the gradual collapse of a coherent quantum mechanical electronic state under the scrutiny of nuclear motion into a mixture of stable pointer states. This is modeled in the coherent switches with decay of mixing (CSDM) method by the decay of the off-diagonal elements of the electronic density matrix. Here, we present an implementation of CSDM in the SHARC program; a key element of the new implementation is the use of a different propagator than that used previously in the ANT program.

Conservation of Angular Momentum in Direct Nonadiabatic Dynamics.

Direct nonadiabatic dynamics is used to study processes involving multiple electronic states from small molecules to materials. Compared with dynamics with fitted analytical potential energy surfaces, direct dynamics is more user-friendly in that it obtains all needed energies, gradients, and nonadiabatic couplings (NACs) by electronic structure calculations. However, the NAC that is usually used does not conserve angular momentum or the center of mass in widely used mixed quantum-classical nonadiabatic dynamics algorithms, in particular, trajectory surface hopping, semiclassical Ehrenfest, and coherent switching with decay of mixing. We show that by using a projection operator to remove the translational and rotational components of the originally computed NAC, one can restore the conservation.