Showing papers on "Combustion published in 1998"

An Introduction to Fire Dynamics

Abstract: Machine generated contents note: About the AuthorPreface to the Second EditionPreface to the Third EditionList of Symbols and Abbreviations1 Fire science and combustion 1.1 Fuels and the Combustion Process 1.2 The Physical Chemistry of Combustion in Fires Problems2 Heat transfer 2.1 Summary of the heat transfer equations 2.2 Conduction 2.3 Convection 2.4 Radiation Problems3 Limits of flammability and premixed flames 3.1 Limits of flammability 3.2 The structure of a premixed flame 3.3 Heat losses from premixed flames 3.4 Measurement of burning velocities 3.5 Variation of burning velocity with experimental parameters 3.6 The effect of turbulence Problems4 Diffusion flames and fire plumes 4.1 Laminar jet flames 4.2 Turbulent jet flames 4.3 Flames from natural fires 4.4 Some practical applications Problems5 Steady burning of liquids and solids 5.1 Burning of liquids 5.2 Burning of solids Problems6 Ignition: The initiation of flaming combustion 6.1 Ignition of^

Steam reformer with internal hydrogen purification

Abstract: A steam reformer with internal hydrogen purification includes internal bulk hydrogen purification, internal hydrogen polishing to remove trace levels of carbon monoxide and carbon dioxide, an integrated combustion method utilizing waste gas to heat the reformer, efficient integration of heat transfer and a compact design. The steam reformer includes a concentric cylindrical architecture with the outermost shell (50) nesting an annular combustion region (60), and annular reforming region (62), and annular hydrogen transport region (54) and cylindrical polishing region (56).

Sources of Fine Organic Aerosol. 9. Pine, Oak, and Synthetic Log Combustion in Residential Fireplaces

Abstract: Combustion of wood in residential fireplaces contributes approximately 14% on an annual average of the total primary fine particle organic carbon (OC) emissions to the Los Angeles urban atmosphere and up to 30% of the fine particulate OC emissions on winter days. This paper presents comprehensive organic compound source profiles for smoke from burning pine, oak, and synthetic logs in residential fireplaces. Mass emission rates are determined for approximately 200 organic compounds including suites of the n-alkanes, n-alkenes, cyclohexylalkanes, n-alkanals, n-alkanoic acids, alkenoic acids, dicarboxylic acids, resin acids, hydroxylated/methyoxylated phenols, lignans, substituted benzenes/benzaldehydes, phytosterols, polycyclic aromatic hydrocarbons (PAHs), and oxy-PAHs. Wood smoke constituents reflect to a great extent the underlying composition of the wood burned: pine and oak logs produce smoke that is enriched in lignin decomposition products, pine smoke is enriched in resin acids and their thermal alteration products, while smoke from the synthetic log burned here bears the major signature of the petroleum products combined with traces of the sawdust components from which it is made. Resin acids are discussed as potential wood smoke tracers in the environment, and it is shown that the time series of resin acids concentrations in the Los Angeles atmosphere follows the extreme seasonal variation in wood use reported in previous emissions inventories for the Los Angeles urban area.

Direct numerical simulation analysis of flame surface density concept for large eddy simulation of turbulent premixed combustion

Abstract: Large eddy simulation (LES) is a promising tool for numerical simulations of reacting flows, especially when combustion instabilities are encountered. Nevertheless, a difficulty occurs in developing subgrid scale models for premixed combustion, because the flame front is generally too thin to be resolved on the computational grid. An approach based on the filtering of the balance equation for the progress variable c using a LES filter larger than the mesh size is proposed and investigated. Despite its similarities with the field description based on the G-equation, the advantage of this approach is that c and related quantities such as subgrid-scale flame surface density are physically well defined and may be easily extracted from direct numerical simulations (DNS) or experimental data to analyze and validate models. A three-dimensional DNS database where a laminar premixed flame interacts with a homogeneous and isotropic turbulent flowfield is used to investigate unresolved turbulent scalar transport and filtered reaction rate. The unresolved transport is found to exhibit a gradient or a countergradient feature, depending on the heat release parameter and the turbulence level. This finding is in aggreement with previous observations of the turbulent transport in Reynolds-averaged Navier-Stokes (RANS) equations. Nevertheless, the unresolved convective flux is lower than the resolved one. Accordingly, model uncertainties will have probably less dramatic consequences than in RANS because a part of the countergradient phenomenon will be incorporated into the motion of the resolved flow structures. The filtered reaction rate is closed, introducing a subgrid-scale flame surface density, Σ, modeled, in a first step, with an algebraic expression similar to the Bray-Moss-Libby (BML) formulation widely used in RANS context. This concept is very attractive because it could be refined using, for example, a dynamic algebraic formulation or a balance equation for Σ.

A Simple Method for Estimating f(E) and k0(E) in the Distributed Activation Energy Model

Abstract: A new method that is simpler and more accurate than the author's previous method is presented for estimating f(E) and k0(E) in the distributed activation energy model. It utilizes at least three se...

The science and technology of combustion in highly preheated air

Abstract: Recent advances in heat-recirculating combustion in industrial furnaces, particularly of the alternating flow type, are reviewed. A large amount of waste heat can be recovered by this type of system. Highly preheated combustion air, typically above 1300 K, is easily obtained due to advanced design and metarials employed. Although preheated air combustion generally produces high nitric oxide emissions, it has been used to generate high-temperature flames for some special applications. The energy saving achieved simultaneously by heat recirculation has become more attractive, from an ecological point of view. However, to enjoy the energy saving brought by a high rate of heat recirculation by applying highly preheated air combustion to generic industrial furnances, a reduction of nitric oxide emission is required. The possibility of low nitric oxide emission from highly preheated air combustion is intensively discussed. Dilution of the air with burned gases and combustion occurring in air with low oxygen concentration are shown to be indispensable factors in realizing low nitric oxide emissions. This has led to advanced furnance technology.

The role of equivalence ratio oscillations in driving combustion instabilities in low NOx gas turbines

Abstract: This paper presents a theoretical investigation of combustion instabilities in low NOX gas turbines (LNGT) that burn fuel in a lean premixed mode. It is shown that these instabilities may be caused by interactions of combustor pressure oscillations with the reactants' supply rates, producing equivalence ratio perturbations in the inlet duct. These perturbations are convected by the mean flow to the combustor where they produce large-amplitude heat-release oscillations that drive combustor pressure oscillations. It is shown in this study that in contrast to earlier analyses, which assumed a uniform instantaneous heat release throughout the flame region, the heat release within the flame may exhibit strong spatial dependence that can significantly affect the combustor stability. The proposed instability mechanism is incorporated into a model that is used to predict LNGT stability limits. The model results show that LNGT are highly prone to combustion instabilities, especially under lean operating conditions, and that the regions of instability can be approximately described in terms of a ratio of the reactants' convective time from the fuel injector to the combustor and the period of the oscillations (with some modifications that account for the structure of the combustion region). Significantly, the developed model's predictions are in good agreement with available experimental data, strongly suggesting that the proposed mechanism and the developed model properly account for the essential physics of the problem.

Introduction to Catalytic Combustion

Abstract: Thermodynamics, Kinetics and Transport Phenomena. Modelling of Catalytic Combustion Reactors. Homogeneous Gas Phase Reactions. Experimental Studies. Combustion Applications: Examples of Modelling Studies

Supercharged Homogeneous Charge Compression Ignition

Abstract: The Homogeneous Charge Compression Ignition (HCCI) is the third alternative for combustion in the reciprocating engine. Here a homogeneous charge is used as in a spark-ignited engine, but the charge is compressed to autoignition as in a diesel. The main difference compared with the Spark Ignition (SI) engine is the lack of flame propagation and hence the independence from turbulence. Compared with the diesel engine, HCCI has a homogeneous charge and hence no problems associated with soot and NOdx formation. Earlier research on HCCI showed high efficiency and very low amounts of NOdx, but HC and CO were higher than in SI mode. It was not possible to achieve high IMEP values with HCCI, the limit being 5 bar. Supercharging is one way to dramatically increase IMEP. The influence of supercharging on HCCI was therefore experimentally investigated. Three different fuels were used during the experiments: iso-octane, ethanol and natural gas. Two different compression ratios were used, 17:1 and 19:1. The inlet pressure conditions were set to give 0, 1, or 2 bar of boost pressure. The highest attainable IMEP was 14 bar using natural gas as fuel at the lower compression ratio. The limit in achieving even higher IMEP was set by the high rate of combustion and a high peak pressure. Numerical calculations of the HCCI process have been performed for natural gas as fuel. The calculated ignition timings agreed well with the experimental findings. The numerical solution is, however, very sensitive to the composition of the natural gas. (Less)

Emission factors of hydrocarbons, halocarbons, trace gases and particles from biomass burning in Brazil

Abstract: Airborne measurements of the emissions of gases and particles from 19 individual forest, cerrado, and pasture fires in Brazil were obtained during the Smoke, Clouds, and Radiation-Brazil (SCAR-B) study in August-September 1995. Emission factors were determined for a number of major and minor gaseous and particulate species, including carbon dioxide, carbon monoxide, sulfur dioxide, nitrogen oxides, methane, nonmethane hydrocarbons, halocarbons, particulate (black and organic) carbon, and particulate ionic species. The magnitude of the emission factors for gaseous species were determined primarily by the relative amounts of flaming and smoldering combustion, rather than differences in vegetation type. Hydrocarbons and halocarbons were well correlated with CO, which is indicative of emissions primarily associated with smoldering combustion. Although there was large variability between fires, higher emission factors for SO2 and NOχ were associated with an increased ratio of flaming to smoldering combustion; this could be due to variations in the amounts of sulfur and nitrogen in the fuels. Emission factors for particles were not so clearly associated with smoldering combustion as those for hydrocarbons. The emission factors measured in this study are similar to those measured previously in Brazil and Africa. However, particle emission factors from fires in Brazil appear to be roughly 20 to 40% lower than those from North American boreal forest fires.

Physical and optical properties of young smoke from individual biomass fires in Brazil

Abstract: Physical and optical characteristics of particles in smoke from 19 fires were measured in Brazil during the 1995 burning season as part of the Smoke, Clouds, and Radiation-Brazil (SCAR-B) project. The University of Washington C-131A measured particle sizes and absorption and scattering properties in very young smoke (<4 min old). These properties are related to fuel type, fire intensity, combustion efficiency, and particle composition. The count median diameter (CMD) of particles from tropical forest fires were strongly and positively correlated with the combustion efficiency. The particle volume median diameter (VMD) of the particles from forest fires did not correlate well with combustion efficiency, but it was highly correlated with the emission factors of particles and unsaturated hydrocarbons. The median diameter and standard deviation of the particle size spectra for smoke from grass and cerrado fires did not correlate with either the combustion efficiency or any emission factor. The measured particle radiative properties correlated well with the measured particle sizes and compositions, and the relationships between these parameters are described fairly well by Mie theory. The optical properties of smoke from individual biomass fires in Brazil differ significantly from those of smoke from biomass burning in North America. In particular, the total light-scattering coefficient for smoke particles in Brazil is, on average, 15% less than for smoke particles in North America. Also, the average values of the single-scattering albedos of smoke particles in Brazil are 0.05 to 0.1 less than those in North America.

TG-FTIR Study of the Influence of potassium Chloride on Wheat Straw Pyrolysis

Abstract: The interest in utilizing biomass as a CO2 neutral fuel by combustion, gasification, or pyrolysis processes is increasing due to concern about the emission of greenhouse gases from fossil fuel combustion. In thermal fuel conversion, pyrolysis is an important step which determines the split of products into char, tar, and gas. In this work, a combination of thermogravimetry and evolved gas analysis by Fourier transform infrared analysis (TG-FTIR) has been applied to study the influence of potassium chloride (KCl) on wheat straw pyrolysis. Raw straw, washed straw, and washed straw impregnated with KCl have been investigated. To facilitate interpretation of the results, pyrolysis of biopolymers (cellulose, xylan, lignin) in the presence and absence of KCl was investigated as well. The raw straw decomposed in a single broad featureless peak. By washing, two peaks appeared in the derivative weight loss curve, corresponding to the decomposition of hemicellulose and cellulose components in the straw. Washing red...

Laseroptical investigation of highly preheated combustion with strong exhaust gas recirculation

Abstract: Laseroptical measurements using OH laser-induced predissociative fluorescence and Rayleigh thermometry were carried out in a combustion chamber with highly preheated air and strong exhaust gas recirculation. Under these conditions, combustion takes place without any luminousity and is referred to as flameless oxidation. It is realized by a configuration with exhaust gas recirculation that is entrained by air nozzles operating as a jet pump. This leads to low temperatures even at significant air preheating so that thermal NO formation is largely suppressed. In the optically accessible combustion chamber, the reaction zones of flameless oxidation were visualized, and burning and mixing regions were identified. The instantaneous flame structure is compared to measurements in turbulent premixed flames. Only disconnected reaction zones due to strong flame stretch resulting from high flow velocities were observed. In the flowfield ahead of the burner, fresh gas was mixed with the hot exhaust gases. The fresh gas was diluted and heated up simultaneously. Flameless oxidation could only be observed if the temperature of the unburnt mixture was above 950 K. The temperature rise between the unburnt and burnt side of the reaction zones varied between 200 and 400 K, depending on the dilution of the unburnt mixture with exhaust gases. This temperature rise was compared with a theoretical analysis of a temperature equation for the well-stirred reactor. It can be shown that flameless oxidation takes place in the well-stirred reactor regime. Maximum local temperatures below 1650 K were measured. The OH concentration in the combustion zones of flameless oxidation is lower than in nonpreheated undiluted turbulent premixed flames.

Application of a flame-wrinkling les combustion model to a turbulent mixing layer

Abstract: The necessity for turbulent combustion modeling in the large-eddy simulation (LES) of premixed turbulent combustion is evident from the computational cost and the complexity of handling flame kinetics reaction mechanisms directly. In this paper, a new flame-wrinkling LES combustion model using conditional filtering is proposed. The model represents an alternative approach to the traditional flame-surface density based models in that the flame distribution is represented by a flame-wrinkle density function and that the effects of flame stretch and curvature are handled through a modeled transport equation for the perturbed laminar flame speed. For the purpose of validating the LES combustion model, LESs of isothermal and reacting shear layers formed at a rearward-facing step are carried out, and the results are compared with experimental data. For the isothermal case, the agreement between LES and the experimental data is excellent. For the reacting case, the evolution and topology of coherent structures is examined, and direct comparisons are made with time-averaged profiles of velocity and its fluctuations. temperature, and reaction products. Good agreement is obtained, to a large extent due to accurate modeling of the flame-wrinkle density but also to the novel treatment of the strain-rate effects on the laminar flame speed of the lean propane-air mixture.

Vaporization of volatile materials

Abstract: Vaporization of volatile materials while avoiding combustion and denaturation of such material provide an alternative to combustion as means of volatilizing bioactive and flavor compounds to make such compounds available for inhalation without generating toxic or carcinogenic substances that are by-products of combustion and pyrolysis.

Overview of combustion and gasification of rice husk in fluidized bed reactors

Abstract: Rice is cultivated in more than 75 countries in the world. The rice husk is the outer cover of the rice and on average it accounts for 20% of the paddy produced, on weight basis. The worldwide annual husk output is about 80 million tonnes with an annual energy potential of 1.210 9 GJ corre- sponding to a heating value of 15 MJ/kg. India alone generates about 22 million tonnes of rice husk per year. If an eAcient method is available, the husk can be converted to a useful form of energy to meet the thermal and mechanical energy requirements of the rice mills themselves. This paper provides an overview of previous works on combustion and gasification of rice husk in atmospheric bubbling fluidized bed reactors and summarizes the state of the art knowledge. As the high ash content, low bulk density, poor flow characteristics and low ash melting point makes the other types of reactors like grate furnaces and downdraft gasifiers either ineAcient or unsuitable for rice husk conversion to energy, the fluidized bed reactor seems to be the promising choice. The overview shows that the reported results are from only small bench or lab scale units. Although a combustion eAciency of about 80% can nor- mally be attained; the reported values in the literature, which are more than 95%, seem to be in higher order. Combustion intensity of about 530 kg/h/m 2 is reported. It is also technically feasible to gasify rice husk in a fluidized bed reactor to yield combustible producer gas, even with suAcient heating value for application in internal combustion engines. A combustible gas with heating value of 4-6 MJ/Nm 3 at a rate of 2.8-4.6 MWth/m 2 seems to be possible. Only very little information is available on the pol- lutant emissions in combustion and tar emissions from gasification. The major conclusion is that the results reported in the literature are limited and vary widely, emphasizing the need for further research to establish suitable and optimum operating conditions for commercial implementations. # 1998 Pub- lished by Elsevier Science Ltd. All rights reserved

Oxidation of Automotive Primary Reference Fuels at Elevated Pressures

Abstract: Automotive engine knock limits the maximum operating compression ratio and ultimate thermodynamic efficiency of spark-ignition (SI) engines. In compression-ignition (CI) or diesel cycle engines, the premixed burn phase, which occurs shortly after injection, determines the time it takes for autoignition to occur. In order to improve engine efficiency and to recommend more efficient, cleaner-burning alternative fuels, we must understand the chemical kinetic processes that lead to autoignition in both SI and CI engines. These engines burn large molecular-weight blended fuels, a class to which the primary reference fuels (PRF) n -heptane and iso-octane belong. In this study, experiments were performed under enginelike conditions in a high-pressure flow reactor using both the pure PRF fuels and their mixtures in the temperature range 550–880 K and at 12.5 atm pressure. These experiments not only provide information on the reactivity of each fuel but also identify the major intermediate products formed during the oxidation process. A detailed chemical kinetic mechanism is used to simulate these experiments, and comparisons of experimentally measured and model predicted profiles for O 2 , CO, CO 2 , H 2 O and temperature rise are presented. Intermediates identified in the flow reactor are compared with those present in the computations, and the kinetic pathways leading to their formation are discussed. In addition, autoignition delay times measured in a shock tube over the temperature range 690–1220 K and at 40 atm pressure were simulated. Good agreement between experiment and simulation was obtained for both the pure fuels and their mixtures. Finally, quantitative values of major intermediates measured in the exhaust gas of a cooperative fuels research engine operating under motored engine conditions are presented together with those predicted by the detailed model.

Theoretical Energy Release of Thermites, Intermetallics, and Combustible Metals

Abstract: Thermite mixtures, intermetallic reactants, and metal fuels have long been used in pyrotechnic applications. Advantages of these systems typically include high energy density, high combustion temperature, and a wide range of gas production. They generally exhibit high temperature stability and possess insensitive ignition properties. For the specific applications of humanitarian demining and disposal of unexploded ordnance, these pyrotechnic formulations offer additional benefits. The combination of high thermal input with low brisance can be used to neutralize the energetic materials in mines and other ordnance without the "explosive" high-blast-pressure events that can cause extensive collateral damage to personnel, facilities, and the environment. In this paper, we review the applications, benefits, and characteristics of thermite mixtures, intermetallic reactants, and metal fuels. Calculated values for reactant density, heat of reaction (per unit mass and per unit volume), and reaction temperature (without and with consideration of phase changes and the variation of specific heat values) are tabulated. These data are ranked in several ways, according to density, heat of reaction, reaction temperature, and gas production.

Decentralized biomass combustion: state of the art and future development

Abstract: The present amount of biomass used for heat, and to a smaller extent electricity production, is already considerable in several European countries but the potential unused in Europe is even higher. Combustion is the most mature conversion technology utilized for biomass. The systems addressed in this paper are plants with a nominal boiler capacity greater 0.5 MWth. The main combustion technologies used for these systems are underfeed stoker furnaces, moving grate firings (over-bed stoker fired units), bubbling and circulating fluidized beds. The most important biomass fuels are sawdust, wood chips, bark, straw, cereals and grass. The essential differences between them are their different combustion behaviour as well as the different concentrations of combustion relevant elements (such as N, S, Cl, K) they contain, influencing the necessary combustion, process control and flue gas cleaning technology. Important points that are now under development focus on possibilities of NOx reduction by primary measures, as well as on higher plant efficiencies by efficient biomass drying by well adjusting the excess oxygen level in the flue gas to the requirements for a complete combustion and by recovering energy from the flue gas. Furthermore, possibilities of influencing the material fluxes of ash forming elements by primary measures aiming at a sustainable ash utilization and an efficient dust precipitation are in progress. Problems still unsolved that need comprehensive R&D in the near future are reactions taking place in the hot flue gas, causing depositions and corrosion in furnaces and boilers (especially when K-, S- and Cl-rich biomass fuels such as straw, cereals and grass are used). Research on possibilities to prevent or control them (by material selection or appropriate technologies) are of great importance. Furthermore, the ash melting behaviour and its influencing variables have to be treated as urgent. Staged combustion systems, hot fly ash precipitation as well as specially designed boilers could represent solutions for these ash and aerosol related problems. Combined heat and power (CHP) production, already realized in plants with a nominal boiler capacity greater 10 MWth based on steam turbines, is also of growing importance for small-scale applications. Moreover, the lower limit for CHP plants is a nominal boiler capacity of about 5 MWth at the moment, due to the lower electric efficiencies achievable and to the economy of scale. Interesting technologies which are right now under development are sterling engines, a newly developed steam engine (screw-type motor) and organic rankine cycles (ORC) with hydrocarbons as working fluids operating at low temperature and pressure levels (in comparison with conventional steam processes). The basic requirements for the selection of an appropriate CHP process are a high electric efficiency to investment costs ratio and a well tested technology to ensure a continuous and undisturbed operation of the plant.

Combustors for micro-gas turbine engines

Abstract: The development ofa hydrogen-air microcombustor is described. The combustor is intended for use in a 1 mm 2 inlet area, micro-gas turbine engine. While the size of the device poses several difficulties, it also provides new and unique opportunities. The combustion concept investigated is based upon introducing hydrogen and premixing it with air upstream of the combustor. The wide flammability limits of hydrogen-air mixtures and the use of refractory ceramics enable combustion at lean conditions, obviating the need for both a combustor dilution zone and combustor wall cooling. The entire combustion process is carried out at temperatures below the limitations set by material properties, resulting in a significant reduction of complexity when compared to larger-scale gas turbine combustors. A feasibility study with initial design analyses is presented, followed by experimental results from 0.13 cm 3 silicon carbide and steel microcombustors. The combustors were operated for tens of hours, and produced the requisite heat release for a microengine application over a range of fuel-air ratios, inlet temperatures, and pressures up to four atmospheres. Issues of flame stability, heat transfer, ignition and mixing are addressed. A discussion of requirements for catalytic processes for hydrocarbon fuels is also presented.