Showing papers on "Combustion published in 1991"

Circulating fluidized bed technology III

Abstract: These proceedings contain 88 technical papers, grouped under the following headings: hydrodynamics; heat transfer; combustion; emissions; pressurized circulating fluidized bed; new processes and applications; catalytic processes; reactor modelling and mixing in addition; 5 overviews present an account of the 'state-of-the-art' of different facets of this technology. Thirty-eight papers have been abstracted separately.

Fullerenes C60 and C70 in flames.

Abstract: THE fullerenes C60 and C70 were first identified1 in carbon vapour produced by laser irradiation of graphite, and have recently been produced in macroscopic quantities2–5 by vaporization of graphite with resistive heating. It has also been suggested6–9 that fullerenes might be formed in sooting flames, and indeed all-carbon ions with mass/charge ratios suggestive of fullerenes have been detected in flames10–12. These species were assumed to have the cage structures of fullerenes, but the mass spectroscopic evidence could not establish this conclusively. We have now collected samples of condensible compounds and soot from hydrocarbon combustion under a range of conditions, and analysed these using conventional techniques in an effort to detect fullerenes. Spectroscopic studies reveal the presence of C60 and C70 in yields and ratios that depend on temperature, pressure, carbon/oxygen ratio and residence time in the flame. Control of these conditions allows optimal yields of 3 g of fullerenes per kilogram of fuel carbon burned, and variation of the C70/C60 ratio over the range 0.26–5.7.

Quenching processes and premixed turbulent combustion diagrams

Abstract: The structure of premixed turbulent flames is a problem of fundamental interest in combustion theory. Possible flame geometries have been imagined and diagrams indicating the corresponding regimes of combustion have been constructed on the basis of essentially intuitive and dimensional considerations. A new approach to this problem is described in the present paper. An extended definition of flamelet regimes based on the existence of a continuous active (not quenched) flame front separating fresh gases and burnt products is first introduced. Direct numerical simulations of flame/vortex interactions using the full Navier–Stokes equations and a simplified chemistry model are then performed to predict flame quenching by isolated vortices. The formulation includes non-unity Lewis number, non-constant viscosity and heat losses so that the effect of stretch, curvature, transient dynamics and viscous dissipation can be accounted for. As a result, flame quenching by vortices (which is one of the key processes in premixed turbulent combustion) may be computed accurately. The effects of curvature and viscous dissipation on flame/vortex interactions may also be characterized by the same simulations. The influence of non-unity Lewis number and of thermo-diffusive processes in turbulent premixed combustion is discussed by comparing flame responses for two values of the Lewis number (Le = 0.8 and 1.2). An elementary (‘spectral’) diagram giving the response of one flame to a vortex pair is constructed. This spectral diagram is then used, along with certain assumptions, to establish a turbulent combustion diagram similar to those proposed by Borghi (1985) or Williams (1985). Results show that flame fronts are much more resistant to quenching by vortices than expected from the classical theories. A cut-off scale and a quenching scale are also obtained and compared with the characteristic scales proposed by Peters (1986). Results show that strain is not the only important parameters determining flame/vortex interaction. Heat losses, curvature, viscous dissipation and transient dynamics have significant effects, especially for small scales and they strongly influence the boundaries of the combustion regimes. It is found, for example, that the Klimov–Williams criterion which is generally advocated to limit the flamelet region, underestimates the size of this region by more than an order of magnitude.

Gas fired radiant tube heater

Abstract: An in situ thermal system is disclosed for enhanced oil recovery and the like from a subterranean formation. The system pressurizes the formation with water whereupon the entire formation is heated to relatively high temperatures in the absence of gas formation to significantly decrease the viscosity of substantially all the crude in the formation and permit recovery thereof. A down hole, fuel fired radiant tube burner of long length is provided to achieve the desired heat patterns within the formation. The burner includes three concentric, closed end tubes with the innermost tube containing a burner and spaced closely adjacent the intermediate tube. A hole pattern in the innermost and intermediate tube establishes a laminar flow of the burner's products of combustion therebetween which modifies the radiation heat flux pattern so that uniform heat is emitted from the outermost tube along its length.

Fundamental issues in control of carbon gasification reactivity

Abstract: Chemical Kinetics of Carbon and Char Gasification.- 13C/12C Isotope Spectrometry in the Study of Carbon Reactivity.- Mass Transport and Carbon Reactivity at High Temperature.- Oxidation of Single Char Particles in an Electrodynamic Chamber.- The CO/CO2 Ratio in the Products of the Carbon-Oxygen Reaction.- Oxy-Reactivity of Coal at Low Temperature and High Pressure during Great Depth Underground Gasification Tests.- Forty Years Ago, "La Combustion du Carbone".- Carbon Gasification: the Active Site Concept.- Active Sites and Carbon Gasification Kinetics: Theoretical Treatment and Experimental Results.- Active Sites in Relation to Gasification of Coal Chars.- Specific Reactivities of Pure Carbon of Diverse Origins.- Active Sites in Carbon Gasification with CO2 Transient Kinetic Experiments.- Reactive Surface Area: An Old But New Concept in Carbon Gasification.- Coke Microtexture: One Key for Coke Reactivity.- Thermally Induced Changes in Reactivity of Carbons.- Surface Complexes on Carbon During Oxidation and Gasification.- Applications of Energetic Distributions of Oxygen Surface Complexes to Carbon and Char Reactivity and Characterization.- The Nature of Isothermal Desorption of Carbon - Oxygen Surface Complexes Following Gasification.- Radical Sites as Active Sites in Carbon Addition and Oxidation Reactions at High Temperatures.- Catalytic Gasification of Carbon: Fundamentals and Mechanism.- An Approach to the Mechanism of the CO2-Carbon Gasification Reaction Catalyzed by Calcium.- The Determining Role of Mineral Matter on Gasification Reactivities of Brown Coal Chars.- Protection of Carbon Against Oxidation: Role of the Active Sites.- Inhibition of Carbon Gasification.- Passivation of Carbon Active Sites for Oxidation Protection.- Controlled Gasification of Carbon and Pore Structure Development.- Control of Anode Consumption During Aluminium Electrolysis.- Synthesis and Recommendations for Future Work.- List of Participants.

Chemical reactions of mercury in combustion flue gases

Abstract: Atmospheric Hg is present in different physical and chemical forms, which determine its atmospheric transformation and transport capacities The chemistry of Hg in flue gases is thus of importance for the deposition pattern around point source emissions In order to apply Hg cleaning methods in flue gases its speciation is also of importance To investigate this under realistic conditions, a 17 kW propane fired flue gas generator was used, while the kinetics of specific Hg reactions were investigated in a continuous flow reactor Elemental Hg is readily oxidized by Cl2 and HCl both at room and at elevated temperatures (up to 900 °C) but not by NH3, N2O, SO2 or H2S It reacts with O2 if a catalyst, such as activated carbon, is present A slow reaction between Hg and NO2 has also been noted

Oxygen gas sensors based on CeO2 thick and thin films

Abstract: Ceria is a promising material for resistive-type oxygen exhaust gas sensors. This paper describes the preparation of ceria thick and thin films and the development of prototype sensors with an additionally integrated platinum temperature sensor and a sensor heater. The CeO2 films exhibit very good adhesion, high thermal shock resistance and no apparent aging under changing gas compositions and at temperatures up to 1000 °C. The electrical conductivity of CeO2 films as a function of the oxygen partial pressure and the temperature behaves analogously to that of bulk material. In a comparison of model calculations with measured data, it is shown that, depending on the material composition, CeO2 sensors are suitable for closed-loop combustion control of stoichiometric or lean air/fuel mixtures. Response times from 5 to 10 ms can be realized with thin- or thick-film CeO2 sensors.

Diagrams of premixed turbulent combustion based on direct simulation

Abstract: The structure and morphology of premixed turbulent flames is a problem of fundamental interest in combustion theory. Diagrams indicating the typical structure of a flame submitted to a given turbulent flow have been constructed on the basis of essentially intuitive and dimensional considerations. Knowing the turbulence integral scale and the turbulent kinetic energy, these diagrams indicate if the flow will feature flamelets, pockets or distributed reaction zones. A new approach to this problem is described in the present paper. The method is based on direct numerical simulations of flame/vortex interactions. The interaction of a laminar flame front with a vortex pair is computed using the full Navier-Stokes equations. The formulation includes non-unity Lewis number, non-constant viscosity and heat losses so that the effects of stretch, curvature, transient dynamics and viscous dissipation can be accounted for. As a result, flame quenching by vortices (which is one of the key-processes in premixed turbulent combustion) may be computed accurately. An elementary (‘spectral’) diagram giving the response of one flame to a vortex pair is constructed. This spectral diagram is then used along with certain assumptions to establish a turbulent combustion diagram similar to those proposed by Borghi 2 or Williams 5 . Results show that flame fronts are more resistant to quenching by vortices than expected from the classical theories. A cut-off scale and a quenching scale are also obtained and compared to the characteristic scales proposed by Peters. 3 Stretch is not the only important parameter determining flame/vortex interaction. Curvature, viscous dissipation and transient dynamics have large effects, especially for small scales and they strongly influence the boundaries of the combustion regimes. For example, the Klimov-Williams criterion which has been advocated to limit the flamelet region, underestimates the size of this region by more than an order of magnitude.

The products of the high temperature oxidation of a single char particle in an electrodynamic balance

Abstract: The ratio CO2/CO from oxidation of Spherocarb char has been measured over a wide temperature range making use of the electrodynamic balance where single particles are heated by laser irradiation but are immersed in room temperature gas. This has allowed measurement of the CO2/CO ratio formed by the heterogeneous reaction on the char surface for temperatures up to 1670 K. For these conditions, an exponential decrease with a temperature coefficient of 3100/K is found. The CO2/CO ratio is proportional to the oxygen partial pressure raised to a power of 0.21. These results are in substantial agreement with work reported at lower temperatures. At normal combustion temperature the CO2/CO ratio from heterogeneous reaction is less than 0.1. Gas phase oxidation of CO to CO2 near the char surface, however, can become important at high char temperatures, even in a gas maintained at room temperature, and can have an important impact on surface temperature. The temperature at which gas phase reactions begin to contribute to CO2 formation and surface temperature was found to be reduced by the presence of water vapor.

Modelling of flamelet surface-to-volume ratio in turbulent premixed combustion

Abstract: A model is proposed, valid in the laminar flamelet regime, for the surface-to-volume ratio of a turbulent premixed flame. The new model is in a form suitable for incorporation into an existing model of turbulent premixed combustion. Exact equations are derived which describe the dynamics of the constant-property surface representing the flame interface. Unknown terms in the exact equations are modelled for the simplified case of constant-density combustion in a specified turbulence field. Numerical solutions of the modelled equations are carried out for a one-dimensional test case. Preliminary results indicate that the model is capable of predicting effects present in turbulent flame propagation, and a parametric study shows that correct trends are observed.

Large-Eddy Simulations of Combustion Instability in an Axisymmetric Ramjet Combustor

Abstract: The large-eddy simulation (LES) model previously developed for cold flow in an axisymmetric ramjet combustor is extended to include a model for premixed fuel combustion. The combustion model explicitly uses a subgrid-scale local turbulent flame speed in the governing equation. This model not only reduces the computational effort, when compared to a model with detailed finite-rate chemical kinetics, but also avoids the potential error in the amount of heat release causd by numerical diffusion. Both stable and unstable combustion are simulated with evidence of decaying and growing pressure oscillations, respectively. The observed flow features on unstable combustion. such as a propagating hooked flame and its phase relation with the pressure oscillation, agree with experimental observations. These simulations clearly establish that the LES model contains the essential physics of combustion instability in a ramjet. Although the simulations show general qualitative features of combustion instability, further ...

Circulating Fluidized Bed Boilers: Design and Operations

Abstract: Hydrodynamics heat transfer combustion gaseous emissions, design considerations gas-solid separators design of CFB components management of solid residues material issues appendix - tables of design data.

Fundamental kinetics of methane oxidation in supercritical water

Abstract: The oxidation kinetics of methane in supercritical water were determined in an isothermal, plug flow reactor over the temperature range 560-650 o C at 245.8 bar. The oxidation rate was found to be first order in methane concentration and 2/3-order in oxygen concentration. The activation energy over the temperature range 560-650 o was 42.8±4.3 kcal/mol. A pressure-corrected elementary reaction model for gas-phase combustion was applied to the oxidation of methane in supercritical water

The combustion of solid fuels and wastes

Abstract: Overview of Solid Fuels Combustion. Characteristics of Biomass Fuels. Combustion Characteristics of Lignite and Coal: The Dominant Solid Fossil Fuels. Characteristics of Petroleum-Related Solid Fossil Fuels. The Combustion of Solid Wastes. Combustion of Solid Fuels in Conventional Systems. Fluidized Bed Combustion of Solid Fuels.

Pyrolytic conversion system

Abstract: A rotary, continuous pyrolytic conversion system converts solid hydrocarbons into gases, liquid hydrocarbons and char by pyrolyzing feed stocks including: plastic waste, tires and plastic from automobile shredding operations; containers and trays of styrofoam and other plastic materials such as used in "fast food" restaurants; rubber; leather; tires; garbage; sewage sludge; coal; oil shale; broken asphalt and the like. These materials are preferably shredded and kinds thereof having different melting points are mixed to facilitate movement of the feed stock through the converter without clogging. Preferably, the materials are baled and injected into the converter where they are severed as they are injected. The materials are fed by gravity from the input end of a converter drum to the discharge end thereof; the input end being elevated above the discharge end. The converter drum is contained within an outer drum which is in substantially air-tight relationship with the injector for the bales and with a discharge chute for the solid products of pyrolysis. A casing around the outer stationary drum defines an oven chamber which is heated by combustion products, from a burner providing a heat source, which circulate in heat exchange relationship with a matrix of fins extending into the oven from the stationary outer drum. A rod extends into the injection end of the converter drum for supporting scrapers against the inner periphery of the converter drum. A crusher bar is carried in the drum at the discharge end thereof and crushes the solid products which consists of char (mostly carbon black), metals and other non-organic materials. A chute containing water receives the pulverized discharge product and balances the pressure in the converter to maintain an air-tight seal therein. A second pyrolysis reactor may receive the solid pyrolysis products and be operative at higher temperature than the first converter to destroy chlorinated hydrocarbons. To assist in such destruction, lime may be injected into the first converter. A plurality of converters may be operated in parallel and utilize common equipment for cleaning the product gases released upon pyrolysis and to supply them to the burners of each of the reactors. Flue gases from the oven chambers of one of the plurality of converters may be supplied to the burner of the next converter for preheating gases for combustion therein.

Method for reducing the flame temperature of a burner and burner intended therefor

Abstract: A method for reducing the flame temperature of an oxy-fuel burner, such a burner comprising a central oxygen nozzle at least one fuel nozzle radially spaced from the central oxygen nozzle, and at least one peripheral oxygen nozzle at greater radial distance from the central oxygen nozzle than the fuel nozzle, all nozzles being at least substantially parallel, a casing surrounding the nozzles and whose forward end lies in a plane at right angles to the longitudinal axis of the burner, and whose rear portion defines an oxygen chamber, and means for the supply of oxygen-containing gas and fuel, respectively, to the nozzles. The peripheral oxygen nozzle is in the form of a Laval nozzle or, if it is an annular nozzle, is of the same cross-sectional variation as the Laval nozzle throughout its entire length. The major fraction of the volume of oxygen requisite for combustion is caused to pass at great speed through at least one oxygen nozzle disposed about the burner nozzle and thereby aspirate ambient atmosphere which is at a lower temperature than the flame, before the oxygen from the oxygen nozzle reacts in the flame.

The Autoignition Chemistry of Paraffinic Fuels and Pro-Knock and Anti-Knock Additives: A Detailed Chemical Kinetic Study

Abstract: A numerical model is used to examine the chemical kinetic processes which lead to knocking in spark-ignition internal combustion engines. The construction and validation of the model is described in detail, including the low temperature reaction paths involving alkylperoxy radical isomerization. The numerical model is then applied to C{sub 1} to C{sub 7} paraffinic hydrocarbon fuels, and a correlation is developed between the Research Octane Number (RON) and the computed time of ignition for each fuel. Octane number is shown to depend on the rates of OH radical production through isomerization reactions, and factors influencing the rate of isomerization such as fuel molecule size and structure are interpreted in terms of the kinetic model. The knock behavior of fuel mixtures is examined, and the manner in which pro-knock and anti-knock additives influence ignition is studied numerically. The kinetics of methyl tert-butyl ether (MTBE) is discussed in particular detail. 28 refs., 5 figs., 5 tabs.

Supercritical fluids as diluents in combustion of liquid fuels and waste materials

Abstract: The present invention is directed to processes and apparatus in which supercritical fluids are used as viscosity reduction diluents for liquid fuels or waste materials which are then spray atomized into a combustion chamber. The addition of supercritical fluid to the liquid fuel and/or waste material allows viscous petroleum fractions and other liquids such as viscous waste materials that are too viscous to be atomized (or to be atomized well) to now be atomized by this invention by achieving viscosity reduction and allowing the fuel to produce a combustible spray and improved combustion efficiency. Moreover, the present invention also allows liquid fuels that have suitable viscosities to be better utilized as a fuel by achieving further viscosity reduction that improves atomization still further by reducing droplet size which enhances evaporation of the fuel from the droplets.

The effect of diameter on the burning of crude oil pool fires

Abstract: In order to understand the combustion characteristics of crude oil pool fires, an experimental study was carried out at the Fire Research Institute (FRI) large scale test facility. The radiative output, burning rate, and the concentrations of CO, CO2, and smoke (above the flame tip) were measured during the burning of Arabian light crude oil, heptane, toluene, and kerosene floating on water. The effect of scale was studied by using steel pans ranging from 0.6 to 3 meters in diameter.

Synthesis of molybdenum silicides by the self-propagating combustion method

Abstract: The synthesis of the silicides of molybdenum (Mo3Si, Mo5Si3, and MoSi2) by the self-propagating combustion method is investigated. Only the reactants corresponding to the last phase can be reacted in a self-sustaining mode without preheating. The product of such a reaction is single-phase MoSi2. Although reactant mixtures corresponding to the other two silicides can react in a self-sustaining mode with prior heating, the products of combustion were multi-phase. The dependence of the combustion temperature and velocity on the stoichiometry of the silicide was determined. The activation energy for the combustion synthesis of MoSi2 determined in this investigation, 139.4 kJ mol−1, is considerably lower than activation energies reported for the diffusion of silicon in molybdenum or in MoSi2.