Combustion Science and Technology

Bell, S.R.; Caton, J. A.

doi: 10.1080/00102208608923932pmid: N/A

Abstract An engine cycle simulation was developed to investigate the role of volatiles and liquid carriers on the combustion process and performance of a coal-fueled, reciprocating engine operating at 1000 rpm. Models for coal particle combustion and devolatilization, liquid droplet vaporization, fuel vapor combustion, cylinder heat transfer, piston work, and mass flow rates were combined with a thermodynamic analysis of the engine to yield instantaneous cylinder conditions and overall indicated engine performance. Four representative devolatilization rate expressions for bituminous coal were selecled from the literature for comparison in this study. With rapid devulatilization of the coal, no external energy source was necessary to ignite and completely combust the fuel for an initial (at BDC) gas temperature of 450 K. In contrast. for no volatile evolution, an initial gas temperature of 600 K was needed to achieve maximum engine performance. The initial gas temperature was identified as a significant parameter with respect to evaluating the ignition characteristics of coal fuels in engines. In addition to dry coal and dry coal-char (no volatiles), three coal slurries were considered: coal/oil.coal methanol. and coal/water. For the coal/oil and coal/water slurry fuels, maximum efficiency was obtained for an initial ges temperature of ahout 370 K and 575 K, respectively.

Joulin, Guy

doi: 10.1080/00102208608923933pmid: N/A

Abstract We consider a slab of absorbing-emitting-reacting medium bounded by two parallel black walls held at constant, equal, temperatures. Taking an Arrhenius law with large activation energy as chemical rate, and a differential equation of radiative transfer, we study analytically the conditions under which the radiant losses, with in-depth absorption included, can balance the heat release due to chemistry. For slabs of small optical thickness, the criticality conditions corresponding to ignition degenerate to those given by a Semenov-like problem, whereas optically thick slabs lead to results that tend towards the Frank-Kamenetskii problem.

Pohl, John H.; Tichenor, Bruce A.; Lee, Joannes; Payne, Roy

doi: 10.1080/00102208608923934pmid: N/A

Abstract Emissions of incompletely burned hydrocarbons from industrial flares may contribute to air pollution. Available data on flare emissions are sparse, and methods to sample operating flares are unavailable. The study reported herein was developed to provide additional data on flare emissions. A Flare Test Facility (FTF) was designed and constructed. Tests were conducted on 3-, 6-, and 12-in.† diameter flare heads. Propane was used as the flare fuel, diluted with nitrogen to control the heating value. The following results were obtained: (1) soot (from smoky flares) accounts for less than 0.5 percent of the unburned hydrocarbon emissions; (2) the size of the flare head did not influence hydrocarbon combustion efficiency; (3) the stability of the flare flame influenced combustion efficiency, with unstable flames tending to promote inefficient combustion. A relationship between gas heating value and exit velocity was developed to denote the region of flame instability.

Gandhi, P. D.; Kanury, A. M.

doi: 10.1080/00102208608923935pmid: N/A

Abstract In analytical studies of radiant ignition of organic solids, one of the important tasks is to quantitatively describe what ignition is. Experimentally, ignition is signalled by the inception of a flame or of an abrupt change in such crucial gas property as temperature. Some ignition criteria which evolved from experimental observations and intuition are the attainment of a critical: (i) temperature of the exposed surface; (ii) pyrolyzate efflux rate; (iii) char depth in the pyrolyzing slab; (iv) rate of local gas temperature in the boundary layer; and (vii) gas temperature gradient at the solid-gas interface. In this paper, a one-dimensional mathematical model is presented for the radiantly induced spontaneous ignition of solids to examine the issue of ignition criterion. The model takes into account the transient heat conduction within the heated solid, consequent pyrolysis, and exothermic oxidation reactions in the developing natural convective boundary layer in the gas-phase. The results show that for gas-phase ignition of organic solids, ignition is satisfactorily indicated by the criterion of gradient reversal of the gas temperature profile at the solid-gas interface. The study also shows that this criterion makes it possible to predict a number of hitherto unexplained experimental observations. Examples are: the dependence of surface temperature at ignition on the radiant flux, and the influence of slab thickness on the ignition delay. A comparison of the present predictions with some available experimental ignition data on cellulosic solids leads to confirm the validity of the gradient reversal criterion under a variety of conditions. The criteria based on surface temperature, a pyrolyzate efflux rate, and a total reaction rate in the boundary layer are shown to lead to error in determining the ignition delay of materials of different solid pyrolysis kinetic parameters. The proposed new criterion, being based on more fundamental grounds is shown to be better.

Greenberg, J. B.; Alibagli, D.; Tambour, Y.

doi: 10.1080/00102208608923936pmid: N/A

Abstract Abstract— A theoretical study of an opposed jet spray diffusion flame is presented. The fuel is introduced into the system in the form of liquid droplets suspended homogeneously in a nitrogen jet stream opposed to an air jet stream of the same hydrodynamic properties. An initial monodisperse droplet size distribution is considered and it is assumed that the droplets follow the flow streamlines. Tambour's (1984) Eulerian "sectional" approach is employed to account for the downstream droplet size changes due to vaporization. Since the monodisperse distribution does not persist, a single "monosectional" equation for quasi-monodisperse sprays is used. The effects of (i) initial droplet and vapor mass fractions, (ii) initial droplet size, and (iii) droplet rate of vaporization, on flame location, temperature and species concentration distributions, for an n-decane fuel spray, are analyzed.

Radhakrishnan, P.; Zakkay, V.; Agnone, A.

doi: 10.1080/00102208608923937pmid: N/A

Abstract Abstract—The results of pressurized fluidized hed combustion (PFBC) tests of lignite in a 780 mm diameter bed operated at 7 atm, using three types of bed material (dolomite, alumina and sand) are presented. The alkali and gas emissions are correlated with bed operating parameters, i.e., excess air, bed temperature and bed materials Sulfur dioxide and nitric oxide emissions, well below EPA limits, were attained, even when inert bed materials (sand or alumina) were used. The inherent alkali oxides in lignite ash have been identified as responsible for removal of SO2 from flue gas. Negligible SO2 emissions were measured with dolomite as a bkd material. However, the measured vapor phase alkali concentrations (−0.2 to 4 ppmw) are one to two orders of magnitude larger than suggested by gas turbine manufacturers (0.024 ppmw). Alumina, as a bed material, results in lowered alkali vapor emissions, due to fixation of alkali in the bed as stable, high melting temperature alumino-silicates. While some results on alkali emissions for lignite combustion at atmospheric conditions have heen published previously, to the authors knowledge. these are the first results that are made available for the combustion of lignite at pressurized conditions.

CHEN, CHIUN-HSUN; T'IEN, JAMES S.

doi: 10.1080/00102208608923938pmid: N/A

Abstract A theoretical model of a laminar diffusion flame at the leading edge of a fuel plate in a forced convective flow is presented and solved numerically to study the flame stabilization and blowoff phenomena. The system of governing equations consists of the two-dimensional Navier-Stokes momentum, energy and species equations with a one-step overall chemical reaction and second-order, finite rate Arrhenius kinetics. The computation is performed over a wide range of Damköhler numbers. For large Damköhler numbers, envelope flames are found to exist where the computed fuel evaporation rate, the flame stand-off distance and the velocity profiles show certain similitude. As the Damköhler number is lowered, a transition to open-tip flame takes place where the flame becomes stabilized on the sides of the fuel plate. Further decreasing of the Damköhler number pushes the diffusion flame downstream out of the leading edge region. In this paper, the flame structures of the envelope and the open-tip flames are presented together with a description of the transition sequence. The implication of this work to downstream boundary layer combustion is also discussed.

KIMBALL-LINNE, MARK A.; KYCHAKOFF, GEORGE; HANSON, RONALD K.

doi: 10.1080/00102208608923939pmid: N/A

Abstract A new application of fiberoptics to spectroscopic measurement of minor species in combustion products is discussed. Fiberoptics could be applied to other optical combustion diagnostics as well. They provide optical access to enclosed combustion systems and allow sensitive equipment to be located remotely. Optical fibers and their use are dicussed in detail, as is probe design. The two most promising spectroscopic techniques for use with fiberoptics are absorption and fluorescence. Examples of both techniques are discussed in detail. We were able to detect 30 ppm of OH (at 310 nm) in a flow reactor using our fluorescence probe. Suggestions for improvement of the technique are given.