Detailed analysis of early-stage NOx formation in turbulent pulverized coal combustion with fuel-bound nitrogen

A carrier-phase direct numerical simulation (CP-DNS) of pulverized coal combustion in a mixing layer is performed, considering three NO_x formation mechanisms (fuel-NO_x, thermal-NO_x and prompt-NO_x). Detailed analyses, including reaction path analysis, chemical timescale analysis, and a priori and budget analyses are conducted to investigate the NO_x production mechanisms and the performance of the flamelet model. Considering the high computational cost of CP-DNS, this work focuses on the early phase governed by devolatilization, where char reactions are less important. The reaction path analyses show that the principal thermal-NO reaction contributes to the net consumption of NO in fuel-bound nitrogen pulverized coal flames, which is essentially different from fuel-nitrogen-free flames. The chemical timescale analyses show that the production rates of NO_x species are faster than those of major species, which confirms the suitability of the flamelet tables. The a priori analyses show that the gas temperature and major/intermediate species can be predicted well by the flamelet model, while the NO_x species show significant discrepancies in certain regions. Finally, the budget analyses explain why the flamelet model performs differently for major/intermediate and NO_x species.     

The [4+2]-Cycloaddition of α-Nitrosoalkenes with Thiochalcones as a Prototype of Periselective Hetero-Diels–Alder Reactions—Experimental and Computational Studies

The [4+2]-cycloadditions of α-nitrosoalkenes with thiochalcones occur with high selectivity at the thioketone moiety of the dienophile providing styryl-substituted 4H-1,5,2-oxathiazines in moderate to good yields. Of the eight conceivable hetero-Diels–Alder adducts only this isomer was observed, thus a prototype of a highly periselective and regioselective cycloaddition has been identified. Analysis of crude product mixtures revealed that the α-nitrosoalkene also adds competitively to the thioketone moiety of the thiochalcone dimer affording bis-heterocyclic [4+2]-cycloadducts. The experiments are supported by high-level DFT calculations that were also extended to related hetero-Diels–Alder reactions of other nitroso compounds and thioketones. These calculations reveal that the title cycloadditions are kinetically controlled processes confirming the role of thioketones as superdienophiles. The computational study was also applied to the experimentally studied thiochalcone dimerization, and showed that the 1,2-dithiin and 2H-thiopyran isomers are in equilibrium with the monomer. Again, the DFT calculations indicate kinetic control of this process.     

New standards for the solution of geometric calculation problems by using computers

The availability of computers and of software for the geometric tasks of construction, measurement and calculation, as well as the availability of software for numerical and symbolic computation induce new methods for the solution of geometric calculation problems: computer-aided graphical, numerical and algebraic methods of solution. These computer-aided solutions are explained in relation to suitably chosen tools (Mathematica and Cabri Géomètre) and traditional examples.     

A new benzoquinone-bridged porphyrin

The bridged porphyrin 6 was prepared via the bis(dipyrrylmethane) compounds 3, 4 and 5, by demethylation of 6 and oxidation the benzoquinone/porphyrin system 2 was obtained. The strong fluorescence quenching of 2, as compared to 6, is explained on the basis of an electron-transfer from porphyrin to quinone in the S₁ state.