Reginald E. Mitchell

Numerical Solution of Two-Dimensional Axisymmetric Laminar Diffusion Flames

Abstract We apply a detailed chemistry, complex transport combustion model to a two-dimensional, axisymmetric laminar diffusion flume in which a cylindrical fuel stream is surrounded by a coflowing oxidizer jet. Unlike some models in which diffusion in the axial direction is neglected, we treat the fully elliptic problem. A discrete solution is obtained by combining a steady-state and a time-dependent solution method. A time-dependent approach is used to help obtain a converged numerical solution on an initial coarse grid using a flame sheet starting estimate. Grid points are then inserted adaptively and Newtons method is used to complete the problem. We investigate both a confined coflowing and an unconfined coflowing methane-air diffusion flame and comparisons with experimental data are made.

Toward a comprehensive chemical kinetic mechanism for the oxidation of acetylene: Comparison of model predictions with results from flame and shock tube experiments*

A chemical kinetic model for the oxidation of acetylene is presented. The model is usedto calculate flame speeds for adiabatic, freely propagating flames, composition profiles for lean and moderately rich burner-stabilized flames, and induction times and exponential growth constants for shock induced ignition. These results are then compared to the corresponding experimental results currently available. Rate coefficient information is derived predominantly from reaction-specific experiments. Since the comparisons made here are generally favorable, the kinetic model is consistent with a very large body of experimental, and in some cases theoretical, information. Important aspects of the reaction mechanism are discussed.

Unified high-temperature char combustion kinetics for a suite of coals of various rank

This article summarizes the results of a series of combustion kinetic investigations on chars from a suite of ten US coals of various rank. An optical technique was used to measure in situ single-particle sizes, temperatures, and velocities, from which combustion rates were computed using a model of gas-particle transport processes. Combustion rates were also determined independently by an inorganic tracer technique based on chemical analysis of extracted samples. The data set was acquired in a single experimental facility, using a standardized experimental procedure, and was analyzed in a uniform way to allow direct comparisons between coals. A pronounced trend is evident in the data set; char reactivity decreases with increasing carbon content of the parent coal. A unified treatment of this data set is presented, culminating in a rank-dependent reactivity correlation, allowing the prediction of char combustion rates for a range of coals under conditions relevant to pulverized coal fired combustors.

Conversion of solid carbonaceous fuels in a fluidized bed fuel cell

A fluidized bed direct carbon fuel cell was employed to achieve direct conversion of solid fuels into electricity. Power was generated from pulverized Lower Kittanning (bituminous) coal, synthetic carbon, and biomass in a single process step. Current-voltage characteristics exhibited typical fuel cell behavior. Fluidization in flowing CO{sub 2} overcomes the difficulty of attaining solid fuel-to-anode contact and generates CO in situ via the Boudouard reaction. A mechanistic reaction pathway is proposed for anodic oxidation of the solid fuel. Conversion was verified by gas analysis of oxidation products in the flue stream and by oxygen mass balance.

Experimentally Determined Overall Burning Rates of Coal Chars

Abstract Abstract-Simultaneous measurements of the size, temperature, and velocity of single pulverized-coal char particles are used to determine the overall burning rates of the chars flowing in hot. oxidizing gaseous environments. The single film model of a burning carbon particle is employed with CO as the sole heterogeneous reaction product. Arrhenius parameters which describe the chemical reaction rate coefficients are derived for the chars of a hvb-biturninous coal from Missouri, a North Dakota lignite and a high-swelling, bituminous coal from Kentucky. Gaseous environments in the temperature range 14S0 to 1550 K, having one, three and six mole-percent oxygen were employed. The measurements indicate that in the three and six mole-percent oxygen environments, the temperatures of particles in the size range 70 to 150 𝛍m exceed the local gas temperatures by as much as 150 K. The results suggest that at high particle temperatures, the chars of the coals burn in a regime in which the overall particle bur...

On the temperature and reaction rate of burning pulverized fuels

A two-color pyrometer technique has been used to measure the temperature of burningpulverized fuel particles flowing in dilute suspension on the centerline of a laminar flow reactor. The transparent reactor is fed by a premixed flat flame fueled with CH4−H2−O2−N2 mixtures, which permit widely variable post-flame temperatures and oxygen concentrations to be obtained. The two-color system employs an optical-electronic system which includes two monochromators to acquire light emission intensity at two wavelengths simultaneously from individual burning particles passing through a 1 mm square area on the centerline of the reactor. Plancks law and the gray body assumption (verified here by measurements) are used to compute particle temperatures, and the average temperature is then determined by averaging the values for some 500 individual particles. Particle temperatures have been determined for various gas temperatures over a range ofoxygen mole fractions from 0 to 0.2 for size-graded (90 μm mean diameter) samples of a petroleum coke and a flash pyrolysis char. Particle burning rates per unit external surface area were determined from the measured temperatures by a heat balance analysis. The functional variation in burning rate with temperature was then used to infer whether the burning rate was controlled by chemical rates (Zone I), the combined effects of chemistry and pore diffusion (Zone II), or bulk diffusion (Zone III). For the petroleum coke, both zone I and Zone II conditions were observed, and chemical rate coefficients for each zone were derived. For the char, Zone II and a transition to Zone III conditions were observed, and a chemical rate coefficient for Zone II was derived. Reaction orders and activation energies were in the range expected from previous oxidation studies, and the chemical rates for both samples were in good agreement with previous measurements by a different method in another laboratory. The results show that careful temperature measurements can provide a sensitive method for the determination of pulverized fuel burning rates.

Optically determined temperatures, sizes, and velocities of individual carbon particles under typical combustion conditions☆

This study utilizes an in situ optical method to determine the temperature, size, and velocity of individual particles of burning pulverized fuels. Temperature-size and velocity-size correlations were determined for both nonreacting and ignited suspensions of a spherical, nonvolatile, molecular sieve carbon flowing in one dimension. The measurements at three axial positions when no oxygen was present illustrate the strong sensitivity to particle size in the transient temperatures. A comparison with predictions from a heat transfer model with no adjustable parameters assesses the experimental errors. Sizes were determined to within 10 μm (120 < dp, μm < 240), temperatures to within 50K (1150 < Tp, K < 1950), and velocities to within 5% (150 < νp, cm/s < 300). Four temperature-size correlations for suspensions burning in 24% excess oxygen show that heterogeneous reaction steepens the correlations beyond their levels for conduction heating alone. For a 75 μm size range, the observed ranges of temperature during heatup and ignition span several hundred degrees, which raises serious doubts about the utility of using average temperatures and sizes to assign kinetic parameters throughout the first 100–150 ms of this process.

Characterization of coal char and biomass char reactivities to oxygen

Studies were undertaken to quantify the extent of carbon deactivation during the combustion of coal and biomass chars at high temperatures. To this end, chars extracted at successive residence times from a flow reactor were subjected to oxidation tests in order to determine char reactivity as a function of conversion under zone II burning conditions. Measured char conversion rates during oxidation tests in a thermogravimetric analyzer were combined with surface areas determined from gas adsorption measurements to yield intrinsic reactivities as functions of char conversion under zone I burning conditions. These reactivities were characterized using a heterogeneous reaction mechanism that not only accurately deseribes variations in char reactivity with conversion at low temperatures but also predicts high enough reaction rates at high temperatures to yield the mass loss rates observed in the laminar flow reactor. Results indicate that char intrinsic reactivity decreases progressively during oxidation at high temperatures. Coal-char reactivity just subsequent to develocatization in an environment containing 12 mol % O 2 . at nominally 1650 K was about 33% higher than the reactivities of chars at later extents of conversion. In 8 mol % oxygen at nominally 1200 K, the reactivity of a biomass-char at 92% conversion was about 45% higher than the reactivity of the biomass char at 98% conversion. Results indicate that mass transport limitations become evident at oxidation temperatures as low as 900 K: Calculations indicate that a quasi-steady burning rate is not established until relatively late in burnoff, suggesting that char oxidation models that use rates based on steady-state adsorbed oxygen concentrations may not accurately predict the times for char burnout under all reaction conditions.

Partial equilibrium in the reaction zone of methane-air diffusion flames

Abstract Analysis of the temperature and composition profiles obtained by probing a confined, axisymmetric-laminar methane-air diffusion flame suggests that a state of partial equilibrium exists in the high-temperature primary reaction zone. Free-radical concentrations were estimated to be several orders of magnitude in excess of the full-equilibrium values.

The release of iron during the combustion of Illinois No. 6 coal

Abstract A series of investigations of iron release during the combustion of an Illinois No. 6 coal are reported. Experimental measurements are described in which 40%–70% of the iron in the coal is released within a 25-ms period immediately following coal devolatilization. This iron release dominates the evolution of all other inorganic elements in this coal (exculding sulfur) on a mass basis. The release is consistently observed in controlled combustion environments over a particle size range from 75 to 125 μm and a gas oxygen concentration range of 6%–12%. Statistical uncertainty of the results is also reported and is small compared with the measured effect. Further experiments and analysis were conducted to determine the mechanism of iron release. Forms of iron with sufficient volatility to explain the results as a vaporization process were investigated. None was found that could reasonably be expected to form under the experimental conditions. Further experimentation suggests that the mechanism is related to the chemical reactivity of the iron species with oxygen and that the release proceeds through the chemical formation of an iron fume.