Showing papers in "Progress in Energy and Combustion Science in 1992"

Combustion instability related to vortex shedding in dump combustors and their passive control

Abstract: This review summarizes recent research program related to the driving mechanism of dump combustor instability. Its scope includes experimental research in dump combustors using gaseous fuel. The emphasis here is on vortex shedding as a driving mechanism of combustion instabilities. It is shown that the development of coherent flow structures and their breakdown into fine-scale turbulence can lead to periodic heat release, which, when in phase with the pressure oscillation, can drive the oscillations as stated by the Rayleigh criterion. The physical processes associated with the vortex breakdown are described. This understanding is used to passively control and reduce the pressure oscillations, as demonstrated for dump and bluff body stabilized comnbustion flows.

Optimization and analysis of large chemical kinetic mechanisms using the solution mapping method—combustion of methane

Abstract: A method of systematic optimization, solution mapping, as applied to a large-scale dynamic model is presented. The basis of the technique is parameterization of model responses in terms of model parameters by simple algebraic expressions. These expressions are obtained by computer experiments arranged in a factorial design. The developed parametrized responses are then used in a joint multiparameter multi-data-set optimization. A brief review of the mathematical background of the technique is given. The concept of active parameters is discussed. The technique is applied to determine an optimum set of parameters for a methane combustion mechanism. Five independent responses—comprising ignition delay times, pre-ignition methyl radical concentration profiles, and laminar premixed flame velocities— were optimized with respect to thirteen reaction rate parameters. The numerical predictions of the optimized model are compared to those computed with several recent literature mechanisms. The utility of the solution mapping technique in situations where the optimum is not unique is also demonstrated.

Theory of opposed-flow flame spread

Abstract: A critical, historical review of the flame spread literature is given, beginning with the first systematic studies of opposed-flow flame spread. Important modeling effects are described, including qualitative, simplified, μg and comprehensive numerical modeling. A brief discussion of subjects with the potential for further development is also given. Although this review focuses on flame-spread theory the emphasis is on the logical development, not the detailed mathematics.

Interactive processes in gasification and combustion. Part I: Liquid drop arrays and clouds

Abstract: A comprehensive review is presented of the interactive transport processes in the gasification and combustion of a cloud of drops and solid particles. The review is divided into two parts. Part I is concerned with the interactive processes for arrays, streams and clouds of drops while Part II presents a review of interactive processes for solid particles clouds. Isolated drop evaporation and combustion are briefly reviewed, as they relate to the interactive processes followed by approximate criteria for interactive combustion. The literature review reveals three distinctive types of studies: Arrays, streams and clouds in the areas of evaporation, ignition and combustion. An integrated approach starting from arrays to clouds and evaporation to combustion is presented. Array problems, analyzed with bispherical coordinates, method of images (MOI) and point source method (PSM) are reviewed. The results are briefly presented. Comparison of results between the different techniques and between the theory and experiment are given. A few of the array were interpreted with the “partial mass fraction” (analogous to partial pressure in thermodynamics) concept introduced in this review article. Convective effects on the array results are discussed. The point source resulting from the array studies are extended to the cloud problems by using the averaging technique for a statistically distributed cloud. The cloud problems of evaporation in confined and unconfined volumes, ignition and combustion are then reviewed. The relation of the results to spray combustion modeling is briefly discussed.

Emissions of nitrous oxide from combustion sources

Abstract: Nitrous oxide (N 2 0) has recently become the subject of intense research and debate, because of its increasing concentrations in the atmosphere and its known ability to deplete the ozone layer and also to contribute to the greenhouse effect. There are both natural and anthropogenic sources for N 2 O; however, the man-made sources are increasing at a much higher rate than natural ones. Until very recently it was believed that the combustion of fossil fuels, especially coal, was the major contributing factor to these anthropogenic sources. For example, 30% of all N 2 0 released into the atmosphere was once attributed to combustion sources, with 83% of the combustion sources coming from coal combustion. Correction of a recently discovered sampling artifact, whereby SO 2 , H 2 O and NO in combustion gases react in a sampling vessel to produce N 2 O, has revealed that, in fact, less than 5 ppm of N 2 0 are found in most product gases from combustion systems. Fluidized bed coal combustors are the exception, though, yielding N 2 O levels of ca. 50ppm in their off-gases. The gas-phase reactions of N 2 0 in flames are reviewed first. It is clear that in most cases N 2 0 is a very reactive intermediate, which is quickly destroyed before being emitted from a flame. The important homogeneous reactions removing N 2 0 are thermal decomposition to N 2 and O 2 and also radical attack in e.g. N 2 O + H → N 2 + OH. Nitrous oxide is formed from nitrogen-containing species by NO reacting with a radical derived from either HCN or NH 3 ; the reactions are NCO + NO → N 2 0 + CO and NH + NO → N 2 0 + H. The levels of N 2 O observed are a balance betwen its rates of formation and destruction. It turns out that HCN is a more efficient precursor than NH 3 at producing N 2 0. The removal of N 2 O is fastest at high temperatures and in fuel-rich systems, where free hydrogen atoms are present in relatively large amounts. When coal burns in a fluidized bed, most of the N 2 O detected is produced during devolatilization, rather than in the subsequent stage of char combustion. It is clear that HCN and NH 3 are produced from nitrogenous material released during devolatilization; these two compounds give N 2 0 when the volatiles burn. The burning of char, on the other hand, involves the chemi-sorption of O 2 on to sites containing carbon or nitrogen atoms, followed by surface reaction, with one of the products being N 2 0, in addition to CO, CO 2 and NO. Fluidized coal combustors have temperatures around 900°C, which is low enough for the thermal decomposition of N 2 O to be relatively slow. In addition, the presence of the solid phase provides a large area for radical recombination, which in turn reduces the rate of removal of N 2 O by free radicals. Parametric studies of fluidized bed combustors have shown that factors such as: temperature, amount of excess air, carbon content and O/N ratio of the coal, all have a significant effect on N 2 O emissions. It is important to note that heterogeneous reactions with solids, such as CaO and char, can cause large decreases in the amount of N 2 O produced during the combustion of coal in a fluidized bed. In fact, there are several methods available for lowering the yields of N 2 O from fluidized bed combustors generally. Areas of uncertainty in the factors affecting N 2 O emissions from fluidized bed combustors are identified.

Combustion accelerated swirling flows in high confinements

Abstract: The effect of combustion on properties of swirl induced internal recirculation zone (IRZ) formed in the vicinity of swirl stabilized burners is studied. Nine cold flows, fifteen well-mixed flames and eight type II diffusion flames of coke-oven gas are measured. An LDA technique is used for velocity and turbulence measurement while sampling probes are used for gas composition and temperature measurements. The inlet swirling vortices are in solid body rotation. It has been demonstrated that the basic effect of combustion is to reduce both the size and the strength of the IRZ. Combustion reduces importance of the centrifugal forces with respect to flow inertia by increasing the latter substantially. The overall combusting flow pattern shows strong similarities to the corresponding cold flow provided that the ratio of centrifugal to inertial forces is identical. The internal recirculation zone strength is correlated with the effective swirl number which accounts for the combustion induced decrease in the ratio of centrifugal to inertial forces. It is found that the ratio of mass-mean temperature of the forward, expanding flow at the position of maximum reverse flow to ambient air temperature is a good measure of how much the initial vortex has been accelerated. Formulae for calculating the effective swirl number are given in the paper. In the experiments reported, the original swirling cold flows are combustion accelerated. Both the degree of acceleration and the position where the acceleration is applied (combustion front location) are systematically varied. The effects stemming from variation in magnitude of the acceleration have been decoupled from the effects which are due to alteration in the flame front location. The experimental results substantially enhance present understanding of the effect of combustion on the swirling flows. It is demonstrated that when the cold vortex is combustion accelerated in the vicinity of the quarl inlet the IRZ strength and size are drastically reduced. When the same vortex is combustion accelerated downstream of the burner quarl a larger IRZ can be observed. Diffusion type II flames were generated with annular fuel injectors. Criteria for establishing such flames are given. Despite of high inlet swirl level of either 0.7 or 1.4 the effective swirl number of the measured flames is in the range 0.1 to 0.4 and the amount of the reverse flow is in the range 10% to 20%. It is demonstrated that in type II diffusion flames the bluff body effects are also very important.

Measurement of thermal rate constants by flash or laser photolysis in shock tubes: Oxidations of H2 and D2

Abstract: The flash or laser photolysis-shock tube (FP- or LP-ST) technique for measuring thermal rate constants at high temperatures in reflected shock waves is discussed. The method of detection is atomic resonance absorption spectroscopy (ARAS). The hydrodynamic perturbation due to boundary layer formation is also discussed. Of particular interest is the effect of this process on the thermodynamic state of the hot gas in the reflected regime. Then the method is illustrated with recent results on reactions of importance in the oxidation mechanisms for H 2 and D 2 . On the basis of these new direct results, recommendations for the most important rate constants are presented.

Morphology and dynamics of agglomerated particulates in combustion systems using light scattering techniques

Abstract: Knowledge of the morphology and dynamics of particulates in combustion systems is important because of its implications in critical research and practical areas as: (a) studies of the anisotropic characteristics of particulates in reacting systems, (b) particle growth and oxidation studies in flame systems, (c) production and control of ceramic aerosols with specific sizes and shapes, (d) studies of the agglomeration mechanisms and in situ determination of the characteristic fractal dimensions in fractal systems (e) mixing rates in multiphase systems and prediction of radiative transport from such systems, and (f) fine particle control in the electrons industry. The in situ light scattering and extinction techniques in the context of particulate sizing and morphology determination are reviewed and compared. The classical light scattering, extinction and scattering intensity ratios techniques may yield accurate particle size when the refractive indices of the particles are accurately known. However, the inferred particle size from the measured scattering and extinctions ratios may change by 75% or more when the real part of the refractive index varies in the range 1.3–2.0. On the other hand, the inferred total particle mass may change by a factor of two or more for the same range of values of the real part of the refractive index. Furthermore, the scattering intensity ratios that have been used for size measurements vary by 30% or more for particle diameters larger than 0.15 μm and the changes in the real part of the index by 15% or higher. The applications of the single particle light scattering theory to the characterization of agglomerates that consists of Rayleigh size units are reviewed and the limitations are discussed. Since apart from the classical scattering and extinction techniques the dynamics isotropic light scattering has been used for sizing of spherical particles based on their translational diffusion in flame systems, its advantages and limitations are also presented. Furthermore, it should be noted that most particulates in reacting systems depart from sphericity because of coagulation and/or surface growth and become optically anisotropic. Since in such cases the depolarized light scattering is suitable for particle shape characterization, comparisons are made between the gaseous molecular anisotropic scattering and the scattering corresponding to the anisotropic particulates. The suitability of the technique for particle morphology characterization in flames is assessed. In addition, the new approach of characterizing flame particulates using fractal analysis is summarized and its limitations are discussed.

Connecting the chemical composition of a material to its combustion characteristics

Abstract: The chemistry in the surface region during combustion of a solid is extraordinarily difficult to determine. Understanding is important because molecules of the parent material break up in this zone to form the initial reactants for the flame. Hence, the surface reaction zone ties the composition of the material to the flame chemistry. The validity of using fast thermolysis of a thin film of the material to simulate the surface reaction zone is assessed. Sub-global chemical details are extracted by the use of rapid-scan Fourier transform infrared spectroscopy. Such microscale laboratory simulations help establish molecular structure details of nitramines that influence the formation of near-surface flame reactants like NO 2 , HONO, CH 2 O and N 2 O. The effects of pressure and temperature on the surface reaction zone can be probed experimentally. Other infrared spectral methods designed to probe the condensed phase and gas phase are discussed.

Nitrous oxide emissions in fluidized-bed combustion: Fundamental chemistry and combustion testing

Abstract: Growing scientific and public concern over global warming and ozone layer depletion has led researchers to study nitrous oxide (N2O) emissions from natural and anthropogenic sources. N2O is reportedly increasing in the atmosphere at an estimated rate of 0.7 ppb per year and was measured in the atmosphere at 307 ppb in 1988. Since N2O is a stable compound, it is transported to the stratosphere where it is photochemically oxidized to nitric oxide (NO), a contributor to catalytic ozone depletion. Potential anthropogenic sources of N2O that have been investigated include biomass burning, fertilization, groundwater release through irrigation and fossil fuel combustion. Until recently, pulverized-coal combustion was implicated as the main source of N2O emissions due to errors encountered in sampling. Emissions from these units are typically less than 10 ppm N2O. Fluidized-bed combustion (FBC) has emerged as an advanced method of energy production by utilities, but under current designs these units have been shown to emit greater (N2O) emissions than their pulverized-coal counterparts. This fact is related to lower combustion temperatures used to enhance SOx/NOx control, which, as a consequence, increases N2O emissions. Nitrous oxide emissions from FBCs range from 50–200 ppm. At N2O concentrations of 100 ppm, and assuming a 2.5% growth rate in U.S. coal-derived energy production with 50% of this generated by FBCs, by the year 2000 this production would account for only 1% of the total current global N2O inventory. Nitrous oxide emissions from FBCs are strongly dependent upon fuel type, operating temperature, and excess air level. Based on current measurements at standard operating conditions (1550°F and 3.5% O2), fuels such as wood, peat and lignite generally will have N2O emissions ranging from 15–50 ppm. Subbituminous and bituminous coal combustion generate emissions ranging from 40–100 ppm and 70–200 ppm, respectively. Petroleum coke combustion is similar to bituminous coal. Actual emissions from a plant will decrease with increasing operating temperature (0.2-1.1 ppm N2O/°F) or decreasing excess air levels (7–21 ppm N2O/% O2). Possible N2O abatement strategies include afterburning of a gaseous fuel in the freeboard or cyclone, increasing bed temperatures, decreasing excess air, or catalytic reduction of N2O by metal oxides.

Kinetic aspects of oxidation and combustion of silane and related compounds

Abstract: Present day knowledge of oxidation and combustion of SiH4 and related species are summarized from three different view points; (1) phenomenology of oxidation and combustion, in particular ignition, (2) kinetic modeling of combustion, and (3) the present status of relevant elementary reactions.

On global climate change, carbon dioxide, and fossil fuel combustion

Abstract: Global climate change (GCC), greenhouse (GH) gases and the uncertainties swirling about them are of considerable concern to the Congress and to the public. Although the uncertainties leave GCC and GH gas implications open, the only GH gas of significance that results from fossil fuel combustion, namely CO 2 , has drawn much attention. This review discusses the concerns in two parts. The first part covers in some detail the scientific and technical aspects of GCC and the relationships of CO 2 and fossil fuels to it. It includes sections on the GH effect; on the anthropogenic atmospheric emission of CO 2 which has increased some 25% since the preindustrial period; on the analyses of historical temperature data which suggest we currently may be in a warming period; on global climate modeling, presently inadequate but necessary to analyze the past as well as to make future predictions; on climatic effects; and on possible mitigation methods for CO 2 injections resulting from fossil fuel combustion. Intended primarily as a primere on GCC for the fossil fuel technical community, the relationships between fossil fueled systems and atmospheric injections of CO 2 are addressed only in a general fashion. The details of various technologies and their systems (conventional and future) are left for the reader to obtain from other sources. The second part notes in more abbreviated form some economic, demographic and international considerations. The use of an identity points to population increases as one key determinant of future GCC effects. The uncertainties, scientific and other, are noted throughout; they inhibit the reaching of conclusions regarding GH impacts on GCC. A large number of literature citations are provided, leading to further references on the subject.

Promotion of high-temperature self-ignition

Abstract: Available information on kinetics and mechanisms of small accelerating chemical additive interaction in self-ignitable systems in the subject of this review. Experimental studies of promoted high-temperature self-ignition indicate that promoting additives are of special importance for alternative fuels which are usually less reactive than the traditional petroleum-derived energy carriers. The review paper includes three parts: The first contains data on kinetic analysis of various promoter mechanisms. Additives producing radicals upon decomposition can reduce ignition delays by several orders of magnitude in systems with non-branched chain reactions but only weakly affect branched reactions. The analysis shows that there are optimal promoters leading to maximum promoter efficiency. Analytical and numerical solutions for promoter efficiency and rate constant of the optimal promoter are presented for cases using various interaction mechanisms. The second part considers experimental results on promoted self-ignition and oxidation of hydrogen, carbon monoxide, methane, higher hydrocarbons, methanol, ethanol, ammonia, as well as systems including nitrous oxide. The best promoters for hydrocarbon and alcohol oxidation are additives which are able to increase radical concentration without disappearing during the induction period. The experimental results and kinetic model calculations are compared. Possibility of the promoter use is discussed for pyrolysis and partial oxidation acceleration. The third part considers kinetics and mechanisms of high-temperature reactions for efficient promoters (organic nitrates, organic nitrites, and difluoroamino derivatives decomposing with NF 2 -radical formation). In conclusion, promoter efficiency limits and strategy for effective promoter search are discussed.

Laser measurements of scalars in turbulent diffusion flames

Abstract: Laser diagnostics based on incoherent interaction between fight and matter can provide quantitative scalar data in reacting flows as linear calibration procedures are achievable. However, in turbulent diffusion flames, where mixing and chemistry interact, a closure relation is still required as the signal intensity depends on both the instantaneous composition and the reaction progress in the probed volume. The properties of the main scalars in tubulent non-premixed flames are examined in order to provide coupling functions that can be used to complete the laser diagnostics, and also to underline the quantities and assumptions that are worth experimental investigation. Laser measurements by Mie, Rayleigh, Raman and induced fluorescence techniques are successively reviewed as well as recent combined measurements where two diagnostics are simultaneously performed to provide correlated data which are essential for interpretation.

Entropic efficiency of energy systems

Abstract: Thermodynamic foundations of the thermal entropy production are rested on the concept of lost heat, (Q/T)6T The thermomechanical entropy production is shown to be in terms of the lost heat and the lost work as = 1 Q where the second term in brackets denotes the lost (dissipated) work into heat The dimensionless number 1-I s describing the local entropy production s" in a quenched flame is related to l'I~ ~ (Pe°) -e where 1-1~ = s"~2/k, E = otis ° a characteristic length, k thermal conductivity, ct thermal diffusivity, S o the adiabatic laminar flame speed at the unburned gas temperature, Pe°D = S°uD/C~ the flame Peclet number, D the quench distance The tangency condition dPe°/OOb = 0, where 0 b = Tb/T~b , T b and T~b denoting respectively the burned gas (nonadiabatic) and adiabatic flame temperatures, is related to an extremum in entropy production The distribution of entropy production between the flame and burner is shown in terms of the burned gas temperature and the distance from burner A fundamental relation between the Nusselt number describing heat transfer in any (laminar, transition, turbulent) forced or buoyancy driven flow and the entropy production is shown to be

Heat transport model for fibre burners

Abstract: A model for the heat transport in a fibre burner is presented. The model contains heat transfer by conduction in the fibres as well as in the gas. Radiation inside the fibre material is modelled by the two-flux formulation, i.e. isotropical scattering is not assumed. Constant thermophysical values and a heat release function are used. The combustion inside the fibre layer is modelled with a heat release function. The system of differential equations is solved by the method of finite differences. The radiation components, temperatures of the gas and solid material are shown as functions of the input power, the excess air and the location. The results are in a qualitatively agreement with measurements. The apparent emissivity of the porous layer is also found as a function of the input power and the excess air.

Alternative fuels for mobile transport

Abstract: In an attempt to reduce photochemical ozone from the atmosphere, various regulatory agencies have encouraged the use of alternative fuels, which produce exhaust emissions that are believed to have less ozone-forming potential. Another potential advantage of alternative fuels is that they could reduce dependence on crude oil. Although the potential reduction in crude oil dependence may be significant, a critical review of the available data regarding the potential air quality advantages of alternative fuels indicate that they are less significant. Continuing progress in reducing emissions from gasoline-fueled vehicles has reduced the advantages offered by electricity and gaseous fuels. In addition, data from an extensive study jointly sponsored by 14 automobile and oil companies indicates that methanol-fueled vehicles may lead to greater ozone formation than state-of-the-art gasoline-fueled vehicles. This finding is consistent with the very recent report by the National Research Council on “Rethinking the Ozone Problem in Urban and Rural Air Pollution”, which cautions that it is “premature” to use any alternative fuel, except for electricity, on a large scale. Because they are non-sooting, methanol and natural gas may be more effectively used in heavy-duty applications (such as buses) where the fuel being replaced is Diesel.

Rate constants of reactions with no activation energy

Abstract: Current theoretical methods of predicting rate constant values for fast reactions with no activation energy are described and discussed. For many reactions in this category, the overall rate is controlled by the rate of capture over the centrifugal barrier in a long-range electrostatic potential. For such systems, remarkably good agreement with experiment can be obtained by quasi-classical or quantum trajectory calculations of barrier-crossing rates, and even by statistical methods based on RRKM theory. Complications may arise when secondary barriers or multiple potential energy surfaces are involved: methods of dealing with such cases are discussed. Examples are given of applications of the theory to bimolecular and termolecular radical-radical and ion-molecule reactions, and to bimolecular quenching reactions.