Richard A. Yetter

Functionalized graphene sheet colloids for enhanced fuel/propellant combustion.

We have compared the combustion of the monopropellant nitromethane with that of nitromethane containing colloidal particles of functionalized graphene sheets or metal hydroxides. The linear steady-state burning rates of the monopropellant and colloidal suspensions were determined at room temperature, under a range of pressures (3.35-14.4 MPa) using argon as a pressurizing fluid. The ignition temperatures were lowered and burning rates increased for the colloidal suspensions compared to those of the liquid monopropellant alone, with the graphene sheet suspension having significantly greater burning rates (i.e., greater than 175%). The relative change in burning rate from neat nitromethane increased with increasing concentrations of fuel additives and decreased with increasing pressure until at high pressures no enhancement was found.

Development of a microreactor as a thermal source for microelectromechanical systems power generation

An alumina ceramic 12.5 12.5 5.0 mm microreactor was constructed using a modified stereolithography process. The design was based on a ‘‘Swiss roll’’ concept of double spiral-shaped channels to facilitate a high level of heat transfer between the reactants and combustion products and wall surface contact of the flow through the microreactor body. Self-sustained combustion of hydrogen and air mixtures was demonstrated over a wide range of fuel/air mixtures and flow rates for equivalence ratios from 0.2 to 1.0 and chemical energy inputs from 2 to 16 W. Depositing platinum on gamma alumina on the internal walls enabled catalytic ignition at or near room temperature and self-sustained operation at temperatures to 300 C. Catalyst degradation was observed at higher operating temperatures and reignition capabilities were lost. However, sustained operation could be obtained at wall temperatures in excess of 300 C, apparently stabilized by a combination of surface and gas-phase reaction phenomena. A global energy balance model was developed to analyze overall reactor performance characteristics. The reactor design and operating temperature range have potential applications as a heat source for thermoelectric and pyroelectric power generation at small scales compatible with microelectromechanical systems applications.

Synthesis gas combustion : fundamentals and applications

Gasification Technology to Produce Synthesis Gas, G.A. Richards and K.H. Casleton Syngas Chemical Kinetics and Reaction Mechanisms, M. Chaos, M.P. Burke, Y. Ju, and F.L. Dryer Laminar Flame Properties of H2/CO Mixtures, J. Natarajan and J.M. Seitzman Fundamental Combustion Characteristics of Syngas, G. Ribert, P. Thakre, Z. Wang, R.A. Yetter, and V. Yang Turbulent Combustion Properties of Premixed Syngas, R.K. Cheng Pollutant Formation and Control, K.J. Whitty, H.R. Zhang, and E.G. Eddings Syngas Utilization, G.A. Richards, K.H. Casleton, and N.T. Weiland Catalytic Combustion of Syngas, J. Mantzaras Operability Issues Associated with Steady Flowing Combustors, T. Lieuwen, V. McDonell, D. Santavicca, and T. Sattelmayer Combustion of Syngas in Internal Combustion Engines, M.K. Fox, G. Lilik, A.L. Boehman, and O. Le Corre Solid Oxide Fuel Cells Using Syngas, R.J. Kee, H. Zhu, and G.S. Jackson Index

Combustion of Nanoscale Al/MoO3 Thermite in Microchannels

Abstract : Microscale combustion is of interest in small-volume energy-demanding systems, such as power supplies, actuation, ignition, and propulsion. Energetic materials can have high burning rates that make these materials advantageous, especially for microscale applications in which the rate of energy release is important or in which air is not available as an oxidizer. In this study we examine the combustion of mixtures of nanoscale aluminum with molybdenum trioxide in microscale channels. Nanoscale composites can have very high burning rates that are much higher than typical materials. Quartz and acrylic tubes are used. Rectangular steel microchannels are also considered. We find that the optimum mixture ratio for the maximum propagation rate is aluminum rich. We use equilibrium calculations to argue that the propagation rate is dominated by a convective process where hot liquids and gases are propelled forward heating the reactants. This is the first study to report the dependence of the propagation rate with a tube diameter for this class of materials.Wefind that the propagation rate decreases linearly with 1=d. The propagation rate remains high in tubes or channels with dimensions down to the scale of 100 m, which makes these materials applicable to microcombustion applications.

ReaxFF Reactive Force Field Development and Applications for Molecular Dynamics Simulations of Ammonia Borane Dehydrogenation and Combustion

Ammonia borane (AB) has attracted significant attention due to its high hydrogen content (19.6% by mass). To investigate the reaction mechanism associated with the combustion of AB, a reactive force field (ReaxFF) has been developed for use in molecular dynamics (MD) simulations. The ReaxFF parameters have been derived directly from quantum mechanical data (QM). NVT-MD simulations of single- and polymolecular AB thermolysis were conducted in order to validate the force field. The release of the first equivalent H(2) is a unimolecular reaction, and MD simulations show an activation energy of 26.36 kcal mol(-1), which is in good agreement with experimental results. The release of the second H(2) is also a unimolecular reaction; however, the release of a third H(2) requires the formation of a B-N polymer. Similar simulations were conducted with a boron and oxygen system, since the oxidation of boron will be an integral step in AB combustion, and show good agreement with the established mechanism for this system. At low temperatures, boron atoms agglomerate into a cluster, which is oxidized at higher temperatures, eventually forming condensed and gas phase boron-oxide-species. These MD results provide confidence that ReaxFF can properly model the oxidation of AB and provide mechanistic insight into the AB dehydrogenation and combustion reactions.

Enhanced Thermal Decomposition of Nitromethane on Functionalized Graphene Sheets: Ab Initio Molecular Dynamics Simulations

The burning rate of the monopropellant nitromethane (NM) has been observed to increase by adding and dispersing small amounts of functionalized graphene sheets (FGSs) in liquid NM. Until now, no plausible mechanisms for FGSs acting as combustion catalysts have been presented. Here, we report ab initio molecular dynamics simulations showing that carbon vacancy defects within the plane of the FGSs, functionalized with oxygen-containing groups, greatly accelerate the thermal decomposition of NM and its derivatives. This occurs through reaction pathways involving the exchange of protons or oxygens between the oxygen-containing functional groups and NM and its derivatives. FGS initiates and promotes the decomposition of the monopropellant and its derivatives, ultimately forming H(2)O, CO(2), and N(2). Concomitantly, oxygen-containing functional groups on the FGSs are consumed and regenerated without significantly changing the FGSs in accordance with experiments indicating that the FGSs are not consumed during combustion.

Electrostatically self-assembled nanocomposite reactive microspheres.

Nanocomposite reactive microspheres with diameters of approximately 1-5 mum were created via electrostatic self-assembly of aluminum and cupric oxide nanoparticles. The ability to utilize this novel approach of bottom-up assembly to create these reactive materials allows for the potential for a more intimate mixture between the two nanoreactants and, thus, an overall more energetic combustion process. Experiments with the self-assembled material demonstrate the ability to achieve ignition and sustain a combustion wave in rectangular microchannels, which does not occur with material having similar amounts of organics mixed via the traditional sonication method.

Combustion and Conversion Efficiency of Nanoaluminum-Water Mixtures

An experimental investigation on the combustion behavior and conversion efficiency of nanoaluminum and liquid water mixtures was conducted. Burning rates and chemical efficiency of aluminum-water and aluminum-water-poly(acrylamide-co-acrylic acid) mixtures were quantified as a function of pressure (from 0.12 to 15 MPa), nominal aluminum particle size (for diameters of 38, 50, 80, and 130 nm), and overall equivalence ratios (0.67 < φ < 1.0) under well-controlled conditions. Chemical efficiencies were found to range from 27 to 99% depending upon particle size and sample preparation. Burning rates increased significantly with decreased particle size attaining rates as high as 8 cm/s for the 38 nm diameter particles above approximately 4 MPa. Burning rate pressure exponents of 0.47, 0.27, and 0.31 were determined for the 38, 80, and 130 nm diameter particle mixtures, respectively. Also, mixture packing density varied with particle size due to interstitial spacing, and was determined to affect the burning rates at high pressure due to inert gas dilution. The presence of approximately 3% (by mass) poly(acrylamide-co-acrylic acid) gelling agent to the nAl/H2O mixtures had a small, and for many conditions, negligible effect on the combustion behavior.

The Effect of Added Al2O3 on the Propagation Behavior of an Al/CuO Nanoscale Thermite

Three types of experiments were performed on an Al/CuO nanoscale thermite to understand the effect of adding a diluent (40 nm Al2O3 particles) to the mixture: the constant volume pressure cell, the unconfined burn tray, and the instrumented burn tube. The addition of Al2O3 decreased the pressure output and reaction velocity in all three experiments. Burn tube measurements showed three reaction velocity regimes: constant velocity observed when 0% (633 m/s) and 5% (570 m/s) of the total weight is Al2O3, constant acceleration observed at 10% (146 m/s to 544 m/s over a distance of 6 cm) and 15% (69 m/s to 112 m/s over a distance of 6 cm) Al2O3, and an unstable, spiraling combustion wave at 20% Al2O3. The pressure measurements correlated to these three regimes showing a dropoff in peak pressure as Al2O3 was added to the system, with relatively no pressure increase observed when 20% of the total weight was Al2O3. Equilibrium calculations showed that the addition of Al2O3 to an Al/CuO mixture lowered the flame temperature, reducing the amount of combustion products in the gas phase, thus, hindering the presumed primary mode of forward heat transfer, convection.

Flow reactor studies of methyl radical oxidation reactions in methane‐perturbed moist carbon monoxide oxidation at high pressure with model sensitivity analysis

New rate constant determinations for the reactions CH3 + HO2 CH3O + OH(1) CH3 + HO2 CH4 + O2(2) CH3 + O2 CH2O + OH(3) were made at 1000 K by fitting species profiles from high-pressure flow reactor experiments on moist CO oxidation perturbed with methane. These reactions are important steps in the intermediate-temperature burnout of hydrocarbon pollutants, especially at super-atmospheric pressure. The experiments used in the fit were selected to minimize the uncertainty in the determinations. These uncertainties were estimated using model sensitivity coefficients, derived for time-shifted flow reactor experiments, along with literature uncertainties for the unfitted rate constants. The experimental optimization procedure significantly reduced the uncertainties in each of these rate constants over the current literature values. The new rate constants and their uncertainties were determined to be, at 1000 K: k1 = 1.48(10)13 cm3 mol−1 s−1 (UF = 2.24) k2 = 3.16(10)12 cm3 mol−1 s−1 (UF = 2.89) k3 = 2.36(10)8 cm3 mol−1 s−1 (UF = 4.23) There are no direct and few indirect measurements of reactions (1) and (2) in the literature. There are few measurements of reaction (3) near 1000 K. These results therefore represent an important refinement to radical oxidation chemistry of significance to methane and higher alkane oxidation. The model sensitivity analysis used in the experimental design was also used to characterize the mechanistic dependence of the new rate constant values. Linear sensitivities of the fitted rate constants to the unfitted rate constants were given. The sensitivity analysis was used to show that the determinations above are primarily dependent on the rate constants chosen for the reactions CH3 + CH3 + M C2H6 + M and CH2O + HO2 HCO + H2O2. Uncertainties in the rate constants of these two reactions are the primary contributors to the uncertainty factors given above. Further reductions in the uncertainties of these kinetics would lead to significant reductions in the uncertainties in our determinations of k1, k2, and k3.