ABSTRACT

Combustion process modifications and post combustion flue-gas treatment have produced significant reductions in power plant emissions. Further reductions, especially in green house gas emissions can be achieved by increasing the thermodynamic efficiency of the power cycle. Reassessments of increased long term availability of natural gas, and the development of highly efficient and low NOx emitting gas turbines made gas turbine-steam combined cycles greatly attractive, especially for smaller units operating in the mode of distributed power generation. For central power stations of larger unit capacity the main challenge is to develop clean and highly efficient coal fired systems at competitive costs. The most promising of these include Pulverized Coal Combustion in Supercritical Pressure Boiler; Pressurized Fluid Bed Combustion without or with Topping Combustion, Integrated Gasification Combined Cycle, and Air Heater GT-Steam Combined Cycle. In the longer term Catalytic Combustion in Gas Turbines and Coal Gasification-Fuel Cell combinations hold out promise. The present state of these advanced power generating cycles, together with their potential for application in the near future is discussed, and the key role of combustion science and technology as a guide in their continuing development highlighted.

ABSTRACT

The great improvement of understanding the structure of turbulent flames in the last two decades of this century is due to the development of advanced laser- based techniques for flow field and scalar measurements. The structure of turbulent premixed and partially premixed flames, in particular, is discussed in this article based on established advanced laser-based measurements. Some burners and flames are selected for the present discussion that cover most of turbulent combustion regimes. A brief review of these burners is presented and followed by a brief review of the most successful techniques in this field and possible measurements. Finally the measured data that describe both premixed and partially premixed flames are briefly reviewed in the context of the most recent regimes diagram.

The published data reveals the existence of thin flame structure beyond the Klimov-William criterion in both turbulent premixed and partially premixed flames. This structure is accompanied by some events of broaden preheat zone. The variations of other important parameters in turbulent premixed flames are also discussed. The effects of turbulence and chemistry are clear in the published data. Testing and validation of turbulent combustion theories are now feasible.

The data presented for partially- premixed flame structure show that two variables, namely the reaction progress variable and mixture fraction, are necessary for proper description of the flame structure. Thus for modelling purposes these variables should be included.

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Oral presentation.

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Oral presentation

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Oral presentation

ABSTRACT

A review of the current literature on NO-char kinetics shows that the governing chemical reactions are incompletely understood and that disagreement exists between the results of different researchers. Newly emerging quantum mechanical tools show promise of resolving some of these issues in the future. In the interim a simple model with a simple single NO formation step during char nitrogen oxidation and a first order reaction between NO and char has been used to generate a single particle model that captures the main characteristics of the NO-char kinetic reactions at pulverized combustion conditions, particularly the very significant decrease in the apparent conversion of char nitrogen to NO with increasing ambient NO concentrations. The model was calibrated with experimental data obtained during char combustion at pulverized coal conditions in a controlled atmosphere in a pilot scale combustor. Incorporation of the model in a CFD code provides interesting insights of the role of char nitrogen on NO formation in pulverized coal boilers. Tracing of over a thousand particles as a post-processor showed that individual particles had an apparent char nitrogen conversion to NO which ranged from slightly negative to ninety percent. The average conversion was in the range of ten percent, satisfyingly close to values that had been found to empirically fit data on a series of coals fired in a pilot-scale furnace.

ABSTRACT

The paper surveys the studies on comminution phenomena of non fossil fuels carried out essentially by the research group operating in Naples in the field of fluidized bed combustion and gasification of solid fuels and wastes which embodies staff of the Istituto di Ricerche sulla Combustione of CNR and of the Dipartimento di Ingegneria Chimica of the University of Naples "Federico II".

The paper compares the relevance of comminution phenomena of four nonfossil, fuels: Ebonite, Tyre-Derived fuel (TDF), Refuse-derive fuel (RDF) and a biomass from Mediterranean area (Robinia Pseudoacacia) outlining peculiar aspects and examining differences and similarities with the relevance of comminution in case of fluidized bed combustion of coals.

The paper also discusses how comminution phenomena are embodied into a simplified model of bubbling fluidized bed combustor which is useful to highlight differences and similarities between modes of operation of a fluidized bed combustor fuelled with either low- or high-volatile fuels.

ABSTRACT

After a brief introduction presenting background information, methods of treating chemical reaction rates in combustion theory are addressed. Activation- energy asymptotics with one-step chemistry is quite successful in describing flame propagation, instabilities and extinction. Quantitative application of this approach relies on empirical determination of effective overall rate parameters. Detailed elementary reaction-rate information has now improved sufficiently that detailed-chemistry computations often are made, and systematic reduction of full chemistry is becoming an increasingly viable approach in applications of combustion theory. This is especially true for emissions of pollutants such as CO and NO, for which use of relevant elementary rates can extend ranges of validity beyond those provided by purely empirical correlations. The methods employed are to introduce suitable steady-state and partial-equilibrium approximations into short starting mechanisms and then apply rate-ratio asymptotics to the resulting reduced chemistry. Some research along these lines is to be presented by other papers at this meeting. The present lecture will address some applications to laminar flame structure, to NO production and to pollutant emissions in turbulent combustion.

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Oral presentation.

ABSTRACT

This review of transient filtration combustion is intended to examine the fundamentalþ of this type of combustion present an overview of recent activities and potential applications. Filtration combustion in porous media differs substantially from combustion in a homogeneous media. The difference is the heat transfer between the filtrated gas and the porous medium under conditions of active heat transfer over a highly developed internal solid surface, The reactive transfer is characterized by a thermal wave velocity that determines the velocity of heat accumulation in a porous medium due to the filtrated flow and by a reaction wave velocity.

This paper discusses theoretical and experimental results of filtration combustion of methane, hydrogen, and acetylene in porous media. Two general cases were examined: linear propagation of a slow, thermal combustion wave during fast fuel filtration, and a reverse unsteady-state combustion process, when the fuel flow direction is periodically switched from one end to the other.

The intensive heat transfer between the heat releasing filtrating gas and high thermal capacity, porous medium (through the highly developed internal solid surface) results in energy accumulation in the solid body and in the so called excess enthalpy or superadiabatic effect, wherein gas temperatures within the porous combustor can significantly exceed the adiabatic temperature of a feeding gas fuel.

It was found that in such a superadiabatic combustor: 1) a very fuel lean gas mixture can be burned, and 2) conversely even a very small amount of oxygen in such gas fuels as CH₄ or H₂S can support a combustion process associated with the high temperature pyrolysis, hydrogen production and hydrocarbon or hydrogen sulfide partial oxidation.

Possible SAC applications are discussed. These include: applications in chemistry and energy systems, partial oxidation of very fuel rich gas mixtures, hydrogen production from hydrocarbon fuels, and applications for environmental control, in particular for air purification of volatile organic compounds.

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Oral presentation

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The concept of laminar flamelets provides a useful tool to describe the turbulent premixed flames using simple but reasonable assumptions to overcome some of the challenges posed by the problem. This concept assumes that at high Damköhler numbers, a premixed flame front can be taken as consisting of regions of reactants and products separated by thin laminar flamelets. Since the instantaneous behaviour of these thin layers is the same as those of laminar flames, turbulent burning velocity can be approximated by the product of the flamelets' surface area and laminar burning velocity corrected for the effect of stretch and flame curvature. The two approaches that have been recently used for estimating a measure of the wrinkled flame surface area are the flame surface density concept and fractal geometry. In this paper, experimental approaches used to estimate the flame surface density and the fractal characteristics of turbulent premixed flames are briefly introduced. The formulations that link the flame surface density and fractal characteristics to turbulent flame velocity have been reviewed. Available experimental data on fractal characteristics and surface density of turbulent premixed flames have been compared and implications have been discussed. A critical assessment of the experimental fractal parameters obtained so far indicated that these are not capable of correctly predicting the turbulent burning velocity using the available fractal area closure model. A similar conclusion has been reached after examining the surface density data from flames of different geometries. One of the reasons for this is that the current flamelet models based on fractal geometry and flame surface density concepts are unable to account for the non-trivial thermo-diffusive effects in turbulent premixed flame propagation.

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Oral presentation

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Laser-based non-intrusive spectroscopic methods for turbulent reacting flows are presented. Turbulent combustion processes are characterised by the complex time- dependent and three-dimensional phenomena which govern the interaction between a large number of chemical elementary reactions and turbulent transport of mass, momentum, energy and chemical species. The limitation in the complete understanding of these processes is mainly due to the inability to experimentally probe them to the extent of empirical and theoretical modelling. However, no techniques came closer to overcoming these limitations than laser- based diagnostic methods. In this paper the laser spectroscopic techniques used for combustion studies are reviewed in relation to the different experimental applications where they were applied. The use of linear (Raman, Rayleigh, and LIF) optical methods as well as the nonlinear (CRAS, DFWM) ones are reviewed, and emphasis was put on those methods used by the author of this paper. Namely, a method for simultaneous measurements of NO and temperature profiles in a turbulent methane/air turbulent diffusion flame using CARS, Rayleigh, and LIF was discussed in detail. Recently emerging techniques such as diode-laser based methods and infrared laser planar imaging were also presented.

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The practical importance of coal char combustion is such that this topic has never ceased to be popular among combustion researchers. Nevertheless, the fundamental aspects of this topic have had their "waves" of popularity and a new one is on the horizon after a decade-long hiatus. This anticipated new "wave" will be a consequence of the renewed interest in complete char burnout which in turn is due to the increasing popularity of low-NO_x combustion conditions. The main argument in this presentation is that the understanding of char burnout in general, and of l00% char burnout in particular, requires a more detailed knowledge of carbon surface chemistry. The key aspects of coal char surface chemistry will thus be summarized. Their importance in other relevant combustion situations, such as NO and/or N₂O reduction by carbons, will also be emphasized.

The "holy grail" of coal char combustion, at least from a kinetic point of view, can be summarized in the following two questions: (a) Why do different coal chars bum out at different rates even though they have the same temperature-time history? (b) Which main factor is responsible for the drastic variations in char reactivity between 0 and 100% burnout (at constant temperature)? The evolution and the meaning of the concept of (re)active surface area (ASA or RSA) of coal chars, which claimed to contain the answer to both questions -- as formulated during the last fundamental "wave" -- will be reviewed briefly. The presentation will end with a speculation regarding the necessary modifications of this concept, both in order to formulate a more reliable predictive parameter for complete char burnout and to reflect better our improved understanding of carbon surface chemistry.

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A low pressure flat premixed H₂/O₂/Ar flame doped with different precursors was used to produce nanosized silica, alumina, tin-oxide, and iron-oxide particles. The gaseous and liquid precursors silane (SiH₄), trimethylaluminum (Al(CH₃)₃), tetramethyltin (Sn(CH₃)₄), and iron-pentacarbonyl (Fe(CO)₅), respectively, were used as source materials in different concentrations. The particles were characterized due to their composition, specific surface area, morphology, and size utilizing FT-IR spectroscopy, BET analysis by nitrogen absorption, X-ray diffraction, transmission electron microscopy, and particle mass spectrometry. In the case of silica (SiO₂), the experimental results were validated by a theoretical model including the chemical kinetics and transport properties of burner stabilized flames as well as particle dynamics. A detailed reaction mechanism was used to simulate the formation of gaseous SiO₂ which has been incorporated in a sectional model for particle growth. It has been shown that the precursor concentration along with flame temperature are the most important factors for particle size, shape, and structure. In the case of SnO₂ formation, two distinct lattice structures could be identified depending on the exposure time in the hot flame gases during sampling. Al₂O₃ particles of spherical shape were found in the size range 4.7 nm < d_p < 8.4 nm. Their size depend on the precursor concentration and the sampling position, indicated by the flow coordinate x. The Fe₂O₃ particles were shown to consist of both, the g- and the a- phase. Average particle sizes were measured in the range 7.4 to 16 nm depending on precursor concentration and flame condition.