J.M. Beér

Combustion in swirling flows: A review

Swirling flows have been commonly used for a number of years for the stabilization of high-intensity combustion processes. In general these swirling flows are poorly understood because of their compelexity. This paper describes the recent progress in understanding and using these swirling flows. The main effects of swirl are to improve flame stability as a result of the formation of toroidal recirculation zones and to reduce combustion lengths by producing high rates of entrainment of the ambient fluid and fast mixing, particularly near to the boundaries of recirculation zones. Two main types of swirl combustor can be identified as follows: The Swirl Burner. Here swirling flow exhausts into a furnace or cavity combustion occurs in and just outside the burner exit. The Cyclone Combustion Chamber. Here air is injected tangentially into a large, usually, cylindrical chamber and exhausts through a centrally located exit hole in one end. Combustion mostly occurs inside the cyclone chamber. Initially the isothermal performance of swirl combustors is considered, and it is demonstrated that, contrary to many previous assumptions, the flow is often not axisymmetric but three-dimensional time-dependent. Under most normal nonpremixed combustion conditions, the swirling flow returns to axisymmetry, although there is still a residual presence of the three-dimensionality, particularly on the boundary of the reverse flow zone. Swirl increases considerably the stability limits of most flames; in fact with certain swirl burners, the blow-off limits are virtually infinite. Cyclone combustion chambers have large internal reverse flow zones which provide very long residence times for the fuel/air mixture. They are typically used for the combustion of difficult materials such as poor quality coal or vegetable refuse. In contrast to the swirl burner which usually has one central toroidal, recirculation zone, the cyclone combustor often has up to three concentric toroidal recirculation zones. Sufficient information is also available to indicate that stratified or staged fuel or air entry may be used to minimize noise, hydrocarbon, and NOx emissions from swirl combustors.

Combustion technology developments in power generation in response to environmental challenges

Abstract Combustion system development in power generation is discussed ranging from the pre-environmental era in which the objectives were complete combustion with a minimum of excess air and the capability of scale up to increased boiler unit performances, through the environmental era (1970–), in which reduction of combustion generated pollution was gaining increasing importance, to the present and near future in which a combination of clean combustion and high thermodynamic efficiency is considered to be necessary to satisfy demands for CO 2 emissions mitigation. From the 1970s on, attention has increasingly turned towards emission control technologies for the reduction of oxides of nitrogen and sulfur, the so-called acid rain precursors. By a better understanding of the NO x formation and destruction mechanisms in flames, it has become possible to reduce significantly their emissions via combustion process modifications, e.g. by maintaining sequentially fuel-rich and fuel-lean combustion zones in a burner flame or in the combustion chamber, or by injecting a hydrocarbon rich fuel into the NO x bearing combustion products of a primary fuel such as coal. Sulfur capture in the combustion process proved to be more difficult because calcium sulfate, the reaction product of SO 2 and additive lime, is unstable at the high temperature of pulverized coal combustion. It is possible to retain sulfur by the application of fluidized combustion in which coal burns at much reduced combustion temperatures. Fluidized bed combustion is, however, primarily intended for the utilization of low grade, low volatile coals in smaller capacity units, which leaves the task of sulfur capture for the majority of coal fired boilers to flue gas desulfurization. During the last decade, several new factors emerged which influenced the development of combustion for power generation. CO 2 emission control is gaining increasing acceptance as a result of the international greenhouse gas debate. This is adding the task of raising the thermodynamic efficiency of the power generating cycle to the existing demands for reduced pollutant emission. Reassessments of the long-term availability of natural gas, and the development of low NO x and highly efficient gas turbine–steam combined cycles made this mode of power generation greatly attractive also for base load operation. However, the real prize and challenge of power generation R&D remains to be the development of highly efficient and clean coal-fired systems. The most promising of these include pulverized coal combustion in a supercritical steam boiler, pressurized fluid bed combustion without or with topping combustion, air heater gas turbine-steam combined cycle, and integrated gasification combined cycle. In the longer term, catalytic combustion in gas turbines and coal gasification-fuel cell systems hold out promise for even lower emissions and higher thermodynamic cycle efficiency. 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.

Kinetics of the NOcarbon reaction at fluidized bed combustor conditions

Abstract The reduction of NO by carbonaceous solid has been studied in a packed bed reactor. After an initial transient, a steady rate of NO reduction is observed with N 2 , CO and CO 2 as the dominant products. Kinetic parameters of the reaction were obtained for various NO and CO concentrations, temperatures, and carbon types. The reaction rate per unit total surface area is found to be first order in NO, to have an apparent activation energy of 44 kcal/mole for temperatures higher than 873K, to be enhanced by the presence of CO, and to vary by approximately an order of magnitude between graphite and coal chars.

Deposition of bituminous coal ash on an isolated heat exchanger tube: Effects of coal properties on deposit growth

Fouling deposits were collected on a steam-cooled tube at the exit of a pilot scale furnace during combustion of bituminous coal. The objective of the work is to identify the coal properties and combustion conditions with which one may anticipate fouling and slagging of superheaters in electric utility boilers. Because of the high fusion temperatures of the ashes investigated, deposition on a time scale practical for the pilot scale experiments was observed only at furnace exit gas temperatures above 1700 K (2600°F), higher than the temperatures expected in the convective section of a boiler. A method is proposed for analyzing such measurements to obtain quantitative descriptions of deposit growth under less severe conditions. The model utilizes ash melting temperatures and slag viscosities to account for the variation of particle sticking probabilities with gas and surface temperatures. These parameters are sufficient to reproduce the strong influence of iron during the early stages of deposit growth. As deposit thickness increases, the correlation of deposit loading with iron content disappears. An empirical shedding frequency was introduced to simulate the average loading at long times. The model provides a quantitative description of deposit growth on tubes, and a means for comparison of fouling propensities of different ashes under various conditions. Comparisons with field data are needed to determine whether the proposed model and parameters derived from pilot scale measurements at high temperatures can provide useful estimates of deposit growth on heat exchangers in boilers.

NO/char reactions at pulverized coal flame conditions

The effective rate of the NO/char reaction measured over the temperature range 1250 to 1750 K has been found to be given by (d NO/dt)=4.18×104 exp (−34.70 K cal/RT) AEPNO moles/sec where AE is the external surface area of the char in m2/gm, and PNO is in atmospheres. The rate of the reaction is found to be retarded by water vapor and enhanced by CO by amounts that decrease with increasing temperature. This is consistent with a hypothesis that the NO/C reaction is retarded by the formation of a chemisorbed layer which can be removed by reaction with CO. Support for this hypothesis is provided by transient experiments which show that, at low temperatures, NO reacts with carbon to form N2 and a chemisorbed oxygen layer, and that the chemisorbed oxygen decomposes at higher temperatures to form CO or reacts with CO to form CO2.

Flame stabilization in recirculation zones of jets with swirl

An experimental study has been made of recirculation zones set up in the central region of a large diameter swirl burner with air introduced into the burner through tangential slots. Measurements of temperature, velocity, and flow direction were made in flames in which fuel gas was injected into the burner. Flames and recirculation zones extended to distances of more than two diameters inside the burner mouth. High intensity combustion was completed at distances of 1 1/2 diameters downstream of the burner exit. Recirculation zones occupy 75% of the cross-sectional area at the burner mouth, with reverse flow velocities into the burner of the same order of magnitude as mean exit velocities. The shape and size of recirculation zones is slightly reduced under combustion conditions as compared to isothermal conditions. Temperature measurements show high temperatures (1300°C), with small variation throughout the flame and with rapid temperature decay for x/D between 1.5 and 2. Maximum reverse mass flow rates are 80% of input mass-flow rates under cold conditions and 70% for combustion conditions. Kinetic energy of turbulence, as measured by a hot-wire anemometer in cold isothermal conditions, shows maximum values at 160% at the burner exit. Temperature decay closely follows the decay of kinetic energy of turbulence. Results of the experiments show that, under strong swirl conditions, aerodynamic forces are so dominant that little change occurs to flow fields as a result of chemical reaction in the flame. Quantitative information is provided for calculation of heat transfer in recirculation zones required in the theory of flame stabilization in reversal zones.

Reduction of Nitric Oxide by Coal Char at Temperatures of 1250–1750 K

INTRODUCTION An important factor to be considered in developing NO control strategies for pulverized coal combustors is the potential for the reduction of NO by char. Such reaction has been shown to reduce appreciably the emissions of NO from fluidized bed coal combustors (Pereira et al., 1975). A number of experimental studies were carried out of the NO/char reactions using fixed and fluidized bed reactors (Furusawa et al., 1977; Chan, 1977; Sprouse, 1977; Beer et al., 1977) in which the products of the reaction were identified and kinetic parameters of the reaction were determined. The present paper is concerned with the destruction of NO by char at temperatures of interest in pulverized coal flames.

Laminarization of turbulent flames in rotating environments

The stability limits for transition from laminar to turbulent flow in free boundary layers can be substantially increased by imposing an external centrifugal force field on the boundary layer. For the case of an axial jet introduced into a rotating cylindrical flow, the centrifugal forces can have a stabilizing effect and impede transverse motion of fluid. The centrifugal forces act against the turbulent viscous forces with a consequent reduction in the entrainment of surrounding fluid into the boundary layer. For the case of burning jets, strong density gradients are set up which further influence the stability of the flow. When the density increases radially outward within a centrifugal force field, a stable radial density stratification is set up which again impedes turbulent mixing. The combination of these two stabilizing forces leads to the “laminarization” of flames and results in an increase in the length of the diffusion flames. Experiments were carried out in three systems. In the first, a rotating wire-mesh screen generated a free vortex with a central fuel-gas jet diffusion flame. The second system consisted of a vertical stationary cylindrical tube mounted on a variable swirl generator with a central burning gaseous fuel jet. The third system used was an isothermal model of the second system with a helium jet replacing the fuel jet. Measurements of temperature, gas concentration, velocity, and turbulence characteristics show that the imposition of a rotating flow field on a turbulent diffusion flame results in increase in flame length, reduction of the rate of spread of the flame, and laminarization of the boundaries of the flame. A modified Richardson number is proposed as a criterion for laminar-turbulent transition stability.

Time-resolved burnout of coal particles in a fluidized bed

Abstract The mechanism of combustion of coal particles in a fluidized bed of sand was studied by injecting small batches of a Montana lignite coal into an electrically heated fluidized bed of sand. The partially burned particles were retrieved by means of a movable gauze basket and quenched in a stream of nitrogen. In this manner relationships between weight loss and time and burnout time versus oxygen concentration were obtained. Qualitative observations dealing with salient aspects of coal devolatilization and residue combustion were also recorded. The burnout histories obtained in this investigation were found to be well represented by the mathematical model of Borghi, Sarofim and Beer (presented at the 70th Annual AIChE Meeting, New York, 1977), a model that quantitatively describes the processes of devolatilization and burning of coal particles in a fluidized bed.

Jet flames in rotating flow fields

Experimental studies have been made on methane and propane flames introduced along the axis of vertical free-vortex air flows set up by a cylindrical rotating wire-mesh screen. Flame lengths are shown to be mereased by a factor of 2, and blowoff velocties are increased by a factor of 7. Schlieren photographs, together with velocity and turbulence measurements in an isothermal model, show that mixing and entrainment are considerably reduced and that turbulence is damped over the major portion of the flame length. Near the nozzle exit high rates of mixing are found, which result in increased stability of the flames.