Showing papers on "Combustion published in 1996"

Combustion: Physical and Chemical Fundamentals, Modelling and Simulation, Experiments, Pollutant Formation

Abstract: Introduction * Fundamental Definitions and Phenomena * Experimental Investigation of Flames * Mathematical Description of Premixed Laminar Flat Flames * Thermodynamics of Combustion Processes * Transport Phenomena * Chemical Kinetics * Reaction Mechanisms * Laminar Prefixed Flames * Laminar Nonpremixed Flames * Ignition Processes * The Navier-Stokes Equations for Three-Dimensional Reacting Flows * Turbulent Reacting Flows * Turbulent Nonpremixed Flames * Turbulent Premixed Flames * Combustion of Liquid and Solid Fuels * Low-Temperature Oxidation, Engine Knock * Formation of Nitric Oxides * Formation of Hydrocarbons and Soot.

Large-eddy simulation of turbulent confined coannular jets

Abstract: Large-eddy simulation (LES) was used to study mixing of turbulent, coannular jets discharging into a sudden expansion. This geometry resembles that of a coaxial jet-combustor, and the goal of the calculation was to gain some insight into the phenomena leading to lean blow-out (LBO) in such combustion devices. This is a first step in a series of calculations, where the focus is on the fluid dynamical aspects of the mixing process in the combustion chamber. The effects of swirl, chemical reactions and heat release were not taken into account. Mixing of fuel and oxidizer was studied by tracking a passive scalar introduced in the central jet. The dynamic subgrid-scale (DM) model was used to model both the subgrid-scale stresses and the subgrid-scale scalar flux. The Reynolds number was 38000, based on the bulk velocity and diameter of the combustion chamber. Mean velocities and Reynolds stresses are in good agreement with experimental data. Animated results clearly show that intermittent pockets of fuel-rich fluid (from the central jet) are able to cross the annular jet, virtually undiluted, into the recirculation zone. Most of the fuel-rich fluid is, however, entrained into the recirculation zone near the instantaneous reattachment point. Fuel trapped in the recirculation zone is, for the most part, entrained back into the step shear layer close to the base of the burner.

Boiler deposits from firing biomass fuels

Abstract: Alkali in the ash of annual crop biomass fuels creates serious fouling and slagging in conventional boilers. Even with the use of sorbents and other additives, power plants can fire only limited amounts of these fuels in combination with wood. The National Renewable Energy Laboratory (NREL). U.S. Department of Energy (DOE), and the biomass power industry conducted eight full-scale firing tests and several laboratory experiments to study the nature and occurrence of deposits. The goal was to increase the quantities of these biofuels which can be used. This paper describes the results of the laboratory and power plant tests which included: tracking and analyzing fuels and deposits by various methods; recording operating conditions; and extensive laboratory testing. These analyses have advanced the understanding of the role of minerals in the combustion of biomass, and their occurrence in biofuels. Deposits occur as a result of the boiler design, fuel properties and boiler operation. The limited furnace volume and high flue gas exit temperatures of most biomass boilers promote slag or deposits from biofuels which contain significant amounts of alkali, sulfur or chlorine and silica. All annual growth, whether from urban tree trimmings, annual crops and residues or energy crops contains sufficient volatile alkali, 0.34 kg GJ − (0.8 lb MMBtu −1 ) or more, to melt in combustion or vaporize and condense on boiler tubes and refractory. Special boiler designs are required for annual crops, including grasses and straws. Addition of magnesium oxide and other additives may be necessary to inhibit alkali volatilization while burning these biofuels.

Open-path Fourier transform infrared studies of large-scale laboratory biomass fires

Abstract: A series of nine large-scale, open fires was conducted in the Intermountain Fire Sciences Laboratory (IFSL) controlled-environment combustion facility. The fuels were pure pine needles or sagebrush or mixed fuels simulating forest-floor, ground fires; crown fires; broadcast burns; and slash pile burns. Mid-infrared spectra of the smoke were recorded throughout each fire by open path Fourier transform infrared (FTIR) spectroscopy at 0.12 cm−1 resolution over a 3 m cross-stack pathlength and analyzed to provide pseudocontinuous, simultaneous concentrations of up to 16 compounds. Simultaneous measurements were made of fuel mass loss, stack gas temperature, and total mass flow up the stack. The products detected are classified by the type of process that dominates in producing them. Carbon dioxide is the dominant emission of (and primarily produced by) flaming combustion, from which we also measure nitric oxide, nitrogen dioxide, sulfur dioxide, and most of the water vapor from combustion and fuel moisture. Carbon monoxide is the dominant emission formed primarily by smoldering combustion from which we also measure carbon dioxide, methane, ammonia, and ethane. A significant fraction of the total emissions is unoxidized pyrolysis products; examples are methanol, formaldehyde, acetic and formic acid, ethene (ethylene), ethyne (acetylene), and hydrogen cyanide. Relatively few previous data exist for many of these compounds and they are likely to have an important but as yet poorly understood role in plume chemistry. Large differences in emissions occur from different fire and fuel types, and the observed temporal behavior of the emissions is found to depend strongly on the fuel bed and product type.

A Numerical Study of a Bluff-Body Stabilized Diffusion Flame. Part 2. Influence of Combustion Modeling And Finite-Rate Chemistry

Abstract: This is the second and last part of a paper on numerical prediction of a bluff-body stabilized turbulent diffusion flame of syngas and air. Part 1 investigates the effect of the turbulence model an...

Combustion in a Porous Medium-Advances and Applications

Abstract: In this paper research and development work of the authors is described, which led to the design of a new combustor-heat exchanger system based on the combustion in porous media. Combustion in inert porous media is possible if the Peclet-number is high enough ( > 65), so that quenching of the flame inside the pores is prohibited. The heat transfer from the combustion zone, to the porous medium itself is very effective because of the very large surface between them. The combustion temperature can be controlled through the porous medium temperature. Prompt and thermal NOx formation, which is temperature dependent, can be controlled by appropriate cooling of the combustion zone. The heat transfer mechanisms in the combustor are discussed and new designs of porous materials are proposed, which allow a high power density and a better control of the temperature level in the combustion zone. The possible application field and the expected benefits of this combustion technique are discussed.

Flame-Retardant Treatments of Cellulose and Their Influence on the Mechanism of Cellulose Pyrolysis

Abstract: Cellulose, either as a major component in wood or as the prime textile fiber cotton, is most frequently implicated in fire, causing injuries and fatalities [1] When ignited, cellulose undergoes thermal degradation, form-ing combustible volatile compounds which become involved in the propaga-tion of fire Fortunately cellulose has a chemical composition which makes it easily amenable to interactive flame-retardant treatments Because flam-mability is a relative rather than an absolute concept, there are no truly flame-retardant fabrics, and the best that can be attained is some given level of flame resistance Barker and Drews [2] proposed that with cellulose, the problem of fire can be described as two distinct phenomena, glowing and flaming, which present different potential hazards and should be ap-proached in different ways Glowing is a direct oxidation of solid cellulose or its degradation products It is generally a slow combustion and is of great concern for only specific items, such as c

Mercury Speciation in Coal Combustion and Gasification Flue Gases

Abstract: Coal combustion and gasification are an anthropogenic source of mercury emission to the atmosphere. Effectively minimizing the emission and understanding the atmospheric fate and transport of mercury require knowledge of its speciation in flue gases. Hg0(g) is the thermodynamically stable form in the highest temperature regions of combustors and gasifiers. Hg0(g) remains as the dominant form in the relatively reducing conditions of a gasification flue gas, but with decreasing temperature in a combustion flue gas Hg0(g) will react to form Hg2+ compounds. Current mercury speciation analysis results suggest that generally >50% of the Hg0(g) reacts with oxidants in coal combustion flue gases; results for gasification conditions are lacking. Oxidation is beneficial because Hg2+ compounds are generally water-soluble and are therefore more effectively captured by wet scrubber pollution control systems and are more apt to deposit locally or regionally. Conversely, Hg0(g) is difficult to control and is likely to e...

Simulation of the catalytic partial oxidation of methane to synthesis gas

Abstract: The modeling and simulation of reactors for the catalytic partial oxidation of natural gas to synthesis gas is complex and requires detailed kinetics if it is to be representative and reliable. Adiabatic fixed bed reactors with a catalytic combustion zone fed with methane/oxygen or methane/air mixtures were simulated based upon the kinetics of total combustion, steam reforming and water-gas shift on a Ni catalyst. The steam reforming reactions and water-gas shift reaction are parallel or more or less consecutive to the total combustion, depending upon the degree of reduction of the catalyst, which is determined by the temperature and the gas phase composition. The calculation of the net rates of coke formation was included in the simulation. The influence of carbon dioxide and steam was also investigated.

Thermal stabilization of catalyst supports and their application to high-temperature catalytic combustion

Abstract: Recent studies concerning thermal stabilization of catalyst supports and their application to a catalytic gas turbine combustor were reviewed. Development of heat-resistant catalysts has received considerable attention for the development of high-tempearture combustion catalysts. The use of additives such as La, Si, Ba, etc., is a promising way to improve the thermal stability of alumina-based support materials. Preparation routes also influence the surface area and the pore structure at operation temperatures. The effects were discussed from the solid state sintering mechanism of transition alumina. The authors have developed a noble alumina-based catalyst, hexaaluminate, as a thermally stable combustion catalyst, which can be used above 1200°C. In the second half of this review, we discussed fundamental catalytic properties of hexaaluminates and the results of practical applications to a gas turbine combustor.

A Fundamental Study of a New Kind of Medium Material for Chemical-Looping Combustion

Abstract: A new kind of medium material is developed for chemical-looping combustion, in which fuel is oxidized by metallic oxide medium in a reactor and reduced metal is oxidized by air in another reactor. This scheme may yield great advantage of savings of energy and suppressing the effect on environment. Two kinds of methods, sol−gel method and dissolution method, are examined to prepare the particle of NiO mixed with YSZ (i.e., yttria-stabilized zirconia) as an excellent medium material from the viewpoint of chemical kinetics and mechanical strength. The effects of reaction temperature, particle size, and gas composition are clarified experimentally. The experimental results on kinetics are interpreted by unreacted-core shrinking model. The cyclic use of the medium particle is also tested. The results obtained in this paper disclose the high potentiality that the chemical-looping combustion could be applied in a practical power plant.

Biomass combustion for power generation

Abstract: An overview is given of the state of the art of biomass combustion power generation technologies with a capacity of more than 10 MWe. Biomass combustion technologies have been compared on a qualitative basis and a selection of individual biomass combustion power plants has been compared on a quantitative basis. Collected data were modified for comparison of the various power plants in the quantitative analysis. The qualitative analysis focused on the following technologies: pile, grate, suspension and fluidised-bed combustion. Fluidised-bed systems are found to have relatively high efficiencies and are also flexible with regard to fuel properties. Both fluidised-bed systems and vibrating grates are successful in limiting thermal NOx formation. Some recently built plants and some planned concepts are compared quantitatively on the basis of efficiency, investment costs and emissions. All electric efficiencies are close to or above 30% (at lower heating value). Of the plants fired solely by biomass, vibrating grates and circulating fluidised beds turn out to have the highest efficiency at the moment. Co-firing of 4.5% biomass in a pulverised coal boiler has an efficiency of about 37% (LHV). Expected efficiencies for large-scale (100 to 250 MWe) promising concepts are in the 39–44% (LHV) range. Investment costs range from 1200 to 2900 (1992)US$/kWe. High costs are often caused by additional features such as the firing of difficult fuels or combined heat and power production. None of the existing technologies is found to be superior with respect to all the criteria selected.

The Dilution, Chemical, and Thermal Effects of Exhaust Gas Recirculation on Diesel Engine Emissions - Part 1: Effect of Reducing Inlet Charge Oxygen

Abstract: This is a first of a series of papers describing how the replacement of some of the inlet air with EGR modifies the diesel combustion process and thereby affects the exhaust emissions This paper deals with only the reduction of oxygen in the inlet charge to the engine (dilution effect) The oxygen in the inlet charge to a direct injection diesel engine was progressively replaced by inert gases, whilst the engine speed, fuelling rate, injection timing, total mass and the specific heat capacity of the inlet charge were kept constant The use of inert gases for oxygen replacement, rather than carbon dioxide (CO 2) or water vapour normally found in EGR, ensured that the effects on combustion of dissociation of these species were excluded In addition, the effects of oxygen replacement on ignition delay were isolated and quantified Results from final set of tests are also presented during which the inlet charge temperature was raised progressively to quantify the effect that EGR temperature has on combustion and emissions The reduction in the inlet charge oxygen (dilution effect) resulted in very large reductions in exhaust NO x level at the expense of rises in particulates and unburnt hydrocarbon emissions The engine power output and fuel economy also deteriorated substantially Raising the inlet charge temperature increases NOx but also, substantially, the exhaust smoke and particulate emissions © 1996 Society of Automotive Engineers, Inc