Margaret S. Wooldridge

Co-firing of coal and biomass fuel blends

This paper reviews literature on co-firing of coal with biomass fuels. Here, the term biomass includes organic matter produced as a result of photosynthesis as well as municipal, industrial and animal waste material. Brief summaries of the basic concepts involved in the combustion of coal and biomass fuels are presented. Different classes of co-firing methods are identified. Experimental results for a large variety of fuel blends and conditions are presented. Numerical studies are also discussed. Biomass and coal blend combustion is a promising combustion technology; however, significant development work is required before large-scale implementation can be realized. Issues related to successful implementation of coal biomass blend combustion are identified.q 2001 Published by Elsevier Science Ltd.

Gas-phase combustion synthesis of particles

Abstract The current work summarizes recent experimental and theoretical investigations of the fundamental processes governing gas-phase combustion synthesis of particles. Various experimental methods and results are reviewed for the production of non-oxide, single-oxide, and mixed-oxide powders. Parameters influencing particle morphology and composition including electric field effects are discussed. Nucleation and growth models are presented for the different growth regimes, including homogeneous nucleation, agglomeration, and coalescence.

On the Chemical Kinetics of n-Butanol: Ignition and Speciation Studies

Direct measurements of intermediates of ignition are challenging experimental objectives, yet such measurements are critical for understanding fuel decomposition and oxidation pathways. This work presents experimental results, obtained using the University of Michigan Rapid Compression Facility, of ignition delay times and intermediates formed during the ignition of n-butanol. Ignition delay times for stoichiometric n-butanol/O(2) mixtures with an inert/O(2) ratio of 5.64 were measured over a temperature range of 920-1040 K and a pressure range of 2.86-3.35 atm and were compared to those predicted by the recent reaction mechanism developed by Black et al. (Combust. Flame 2010, 157, 363-373). There is excellent agreement between the experimental results and model predictions for ignition delay time, within 20% over the entire temperature range tested. Further, high-speed gas sampling and gas chromatography techniques were used to acquire and analyze gas samples of intermediate species during the ignition delay of stoichiometric n-butanol/O(2) (χ(n-but) = 0.025, χ(O(2)) = 0.147, χ(N(2)) = 0.541, χ(Ar) = 0.288) mixtures at P = 3.25 atm and T = 975 K. Quantitative measurements of mole fraction time histories of methane, carbon monoxide, ethene, propene, acetaldehyde, n-butyraldehyde, 1-butene and n-butanol were compared with model predictions using the Black et al. mechanism. In general, the predicted trends for species concentrations are consistent with measurements. Sensitivity analyses and rate of production analyses were used to identify reactions important for predicting ignition delay time and the intermediate species time histories. Modifications to the mechanism by Black et al. were explored based on recent contributions to the literature on the rate constant for the key reaction, n-butanol+OH. The results improve the model agreement with some species; however, the comparison also indicates some reaction pathways, particularly those important to ethene formation and removal, are not well captured.

An experimental investigation of gas-phase combustion synthesis of SiO2 nanoparticles using a multi-element diffusion flame burner

Abstract The current work presents the results of an experimental investigation of gas-phase combustion synthesis of silica (SiO 2 ) particles using a multi-element diffusion flame burner (MEDB, a Hencken burner). Silane (SiH 4 ) was added to hydrogen/oxygen/argon (H 2 /O 2 /Ar) flames to produce SiO 2 nanoparticles at various burner operating conditions (φ = 0.47–2.16). To characterize the burner performance, temperature measurements were made using water absorption spectroscopy and uncoated, fine-wire thermocouples. The results demonstrated the non-premixed flow arrangement of the fuel tubes and oxidizer channels of the MEDB provided uniform, ∼1D conditions above the surface of the burner, with temperature variations of less than ±3% in the transverse direction (parallel to the surface of the burner) for elevations above the mixing region (z = 0–7 mm), extending to heights ≥ 30 mm. At heights above the mixing region, approximately constant axial temperatures are also observed. Silica particle formation and growth were examined for comparison with current understanding of the physical mechanisms important in combustion synthesis of SiO 2 . The particle properties were determined using transmission electron microscope (TEM) imaging. Geometric mean diameters of the primary particles varied from d p = 9 to 18 nm. The current study demonstrates the utility of the MEDB in providing a controlled environment for fundamental studies of gas-phase combustion synthesis phenomena, as well as offering broad flexibility in experimental design with control over process variables such as temperature field, particle residence time, scalable reactant loading, and particle precursor selection.

The Effects of the Location of Au Additives on Combustion-generated SnO2 Nanopowders for CO Gas Sensing

The current work presents the results of an experimental study of the effects of the location of gold additives on the performance of combustion-generated tin dioxide (SnO2) nanopowders in solid state gas sensors. The time response and sensor response to 500 ppm carbon monoxide is reported for a range of gold additive/SnO2 film architectures including the use of colloidal, sputtered, and combustion-generated Au additives. The opportunities afforded by combustion synthesis to affect the SnO2/additive morphology are demonstrated. The best sensor performance in terms of sensor response (S) and time response (τ) was observed when the Au additives were restricted to the outermost layer of the gas-sensing film. Further improvement was observed in the sensor response and time response when the Au additives were dispersed throughout the outermost layer of the film, where S = 11.3 and τ = 51 s, as opposed to Au localized at the surface, where S = 6.1 and τ = 60 s.

On the Combustion Chemistry of n-Heptane and n-Butanol Blends

High-speed gas sampling experiments to measure the intermediate products formed during fuel decomposition remain challenging yet important experimental objectives. This article presents new speciation data on two important fuel reference compounds, n-heptane and n-butanol, at practical thermodynamic conditions of 700 K and 9 atm, for stoichiometric fuel-to-oxygen ratios and a dilution of 5.64 (molar ratio of inert gases to O(2)), and at two blend ratios, 80%-20% and 50%-50% by mole of n-heptane and n-butanol, respectively. When compared against 100% n-heptane ignition results, the experimental data show that n-butanol slows the reactivity of n-heptane. In addition, speciation results of n-butanol concentrations show that n-heptane causes n-butanol to react at temperatures where n-butanol in isolation would not be considered reactive. The chemical kinetic mechanism developed for this work accurately predicts the trends observed for species such as carbon monoxide, methane, propane, 1-butene, and others. However, the mechanism predicts a higher amount of n-heptane consumed at the first stage of ignition compared to the experimental data. Consequently, many of the species concentration predictions show a sharp rise at the first stage of ignition, a trend that is not observed experimentally. An important discovery is that the presence of n-butanol reduces the measured concentrations of the large linear alkenes, including heptenes, hexenes, and pentenes, showing that the addition of n-butanol affects the fundamental chemical pathways of n-heptane during ignition.

An experimental investigation of the sensitivity of the ignition and combustion properties of a single-cylinder research engine to spark-assisted HCCI

Spark-assisted homogeneous charge compression ignition (HCCI) combustion may be a method to improve the operation of HCCI engines. In the current study, the impact of spark assist on the fundamental properties of ignition and combustion was investigated in a single cylinder, optically-accessible research engine. Early port fuel injection and air preheating were used with indolene fuel in the study. The effects of a range of air preheat (T in = 256–281 °C), fuel/air equivalence ratio (ϕ = 0.38–0.62) and spark assist timing (10°−90° before top dead centre) conditions on maximum in-cylinder pressure and timing, cycle variability, indicated mean effective pressure (IMEP) and heat release rate were investigated. Additionally, high-speed imaging was used to capture the piston-view ignition and combustion events during spark-assisted and unassisted HCCI operation. Methods were developed and applied to the imaging sequences to quantify the physical characteristics (e.g. location of autoignition sites) and the rate of propagation of the reaction fronts formed during spark-assisted and unassisted HCCI operation. The imaging data show that autoignition sites appear with increasing frequency as air preheat temperature is increased. The addition of spark assist led to the formation of reaction fronts at all conditions that propagated outward from the spark electrode at average speeds between 1.9 and 4.3 m/s. The imaging data indicate the effects of spark assist are due to compression heating of the unburned gases by the propagating reaction fronts which also leads to more consistent location of autoignition. Comparison of the imaging and engine data show the initial formation of the reaction fronts are not significant sources of heat release. While the engine data show that spark assist can affect phasing, heat release rate, IMEP and engine stability at the marginal HCCI operating conditions studied, the results also indicate spark assist has a narrow temperature range where the changes will be significant compared to the effects of the inherent thermal stratification of the HCCI fuel/air charge.

A New Method for Direct Preparation of Tin Dioxide Nanocomposite Materials

In the current work, a novel combustion method is demonstrated for the direct synthesis of nanocomposite materials. Specifically doped tin dioxide (SnO 2 ) powders were selected for the demonstration studies due to the key role SnO 2 plays in semiconductor gas sensors and the strong sensitivity of doped SnO 2 to nanocomposite properties. The synthesis approach combines solid and gas-phase precursors to stage the decomposition and particle nucleation processes. A range of synthesis conditions and four material systems were examined in the study: gold–tin dioxide, palladium–tin dioxide, copper–tin dioxide, and aluminum–tin dioxide. Several additive precursors were considered including four metal acetates and two pure metals. The nanocomposite materials produced were examined for morphology, phase, composition, and lattice spacing using transmission and scanning electron microscopy, x-ray diffractometry, and energy-dispersive spectroscopy. The results using the combustion synthesis approach indicate good control of the nanocomposite properties, such as the average SnO2 crystallite size, which ranged from 5.8 to 17 nm.

A Regime Diagram for Autoignition of Homogeneous Reactant Mixtures with Turbulent Velocity and Temperature Fluctuations

A theoretical scaling analysis is conducted to propose nondimensional criteria to predict weak and strong ignition regimes for a compositionally homogeneous reactant mixture with turbulent velocity and temperature fluctuations. This leads to a regime diagram that provides guidance on expected ignition behavior based on the thermo-chemical properties of the mixture and the flow/scalar field conditions. The analysis extends the original Zeldovich’s theory by combining the turbulent flow and scalar characteristics in terms of the characteristic Damköhler and Reynolds numbers of the system, thereby providing unified and comprehensive understanding of the physical and chemical mechanisms controlling autoignition. Estimated parameters for existing experimental measurements in a rapid compression facility show that the regime diagram predicts the observed ignition characteristics with good fidelity.

A shock tube study of the CO+OH→CO2+H reaction

The rate coefficient for the reactionCO+OH→CO 2 +H has been determined using mixtures of nitric acid (HNO 3 ), carbon monoxide (CO), and argon in incident shock wave experiments. Upon shock heating, the nitric acid rapidly decomposes into OH and NO 2 . The OH subsequently reacts predominantly via reaction (1). Quantitative OH time histories were obtained by continuous-wave (cw) narrow-linewidth UV laser absorption of the R 1 (5) line of the A 2 Σ + «X 2 Π i (0,0) transition at 32,606.56 cm −1 (vacuum). In some experiments, helium was added to the reactant mixture to examine CO vibrational excitation effects on the rate coefficient determination. It was found that the rate of excited CO ( v =1) with OH is less than the rate of ground-state CO ( v =0) with OH, which is in agreement with previous state-dependent work. The experiments were conducted over the temperature range 1090–2370 K and the pressure range 0.19–0.82 atm. The second-order rate coefficient was determined to be k 1 ( T )=2.12×10 12 exp[−2630/ T (K)] (cm 3 mol −1 s −1 ) with overall uncertainties of +16, −12% at high temperatures and +19, −22% at low temperatures. These results are in good agreement with recent studies of reaction (1) and are well fit by a chemically activated intermediate model. The current work also provides a link to previous low-temperature data.