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destruction during pulverised coal combustion
09 Use of fluidized bed technologies 00100966 small capacity coal-fired boilers
for retrofitting
of
Yu, M. A. Prom. Teplotekh., 1997, 19, (6). 106-109. (In Russian) The modification of coal-fired boilers using a variety of fluidized-bed combustion technologies is described in this paper. Recommendations are provided on the application of the retrofitting methods for firing of coals of different degrees of metamorphism. The modification of small coal-fired boilers is extremely valuable, because it extends their operation lifetime and increases their combustion efficiency of high ash coals without the addition of gas and fuel oil.
00100967 atmosphere
Ways to decrease nitrogen oxide emissions to the during coal combustion in industrial boilers
Volodarskii, 1. Kh. K/t&r. TLerd. Topi. (Moscow), 1998, (2). 75-80. (In Russian) A review of nitrogen oxide emission reduction methods for use in industrial boilers. Flue gas recirculation decreases nitrogen oxide emissions from a coal-fired stoker boiler (DKV 6.5113) by 2540%. Particle emissions are lowered from 19.8 to 4.2 g/m7 by using granulated coal.
09
COMBUSTION Burners,
00100966 pulverised
Combustion
Adiabatic oxidation coals to spontaneous
Systems
study on the propensity combustion
Analysis
of an annular
finned
(burners, combustion
systems)
00/00971 An asymptotic and numerical investigation of homogeneous ignition in catalytically stabilized channel flow combustion Mantzaras, _I. and Benz, P. Combustion & Name, 1999, 119, (4). 455-472. The gas-phase ignition of a fuel-lean premixed combustible gas is investigated in a forced convection two-dimensional laminar channel flow configuration established by two catalytically-active parallel plates placed at a distance 26 apart. The gaseous mixture has uniform inlet properties and both plate temperatures are constant and equal. First-order matched activation energy asymptotics are used to describe the reactive gaseous flow in conjunction with the boundary layer approximation. a one-step large activation energy gaseous reaction and an infinitely fast (mass-transportlimited) catalytic reaction. A closed form ignition criterion is obtained for the gas-phase ignition distance in terms of nondimensional groups that are relevant to confined flows. The characteristic chemical and transverse diffusion time scales are included explicitly in the ignition criterion clearly demonstrating the competition between gaseous and catalytic fuel conversion while the effect of flow confinement (h) is included implicitly. The ignition criterion is valid over the range 0.002 < xi(hRefr) < 0.16, withr the streamwise distance, Re the flow Reynolds number based on the channel halfwidth b and the uniform inlet properties and Pr the Prandtl number. The temperature and transport parameter ranges of applicability are I.5 < TWIT,,+, < 3 (with TWIT/N the ratio of the catalytic wall to the inlet temperature) and 0.9 c Le < 2.0 (with Le the Lewis number) respectively. rendering the ignition criterion of particular interest to hydrocarbon catalytically stabilized combustion (CST) applications. Numerical simulations are performed for channel flow catalytic combustion of a fuel-lean (equivalence ratio 0.32) propane-oxygen-nitrogen mixture using the same underlying chemistry assumptions as in the analytical asymptotic approach. The analytically calculated ignition distances are in good agreement with those numerically predicted. The effect of flow confinement (finite h) on gaseous ignition is examined by comparing ignition distances with the corresponding ones of the unconfined (flat plate) case. Flow confinement (decreasing h) increases the ignition distances due to the resulting increase in the channel surface-to-volume ratio. Moreover. the effect of flow confinement is important already from x/(hRePr) = 0.002.
of
Ren, T. X. et al. Fuel, 1999, 78, (14) 1611-1620. This paper presents the results of adiabatic oxidation studies on the propensity of I8 pulverized coals to spontaneous combustion. All the coal samples were tested at an initial temperature of 40°C and three samples at hO”C to mimic the condition in a coal mill. Their propensities to spontaneous combustion were ranked according to their initial rate of heating (IRH) and total temperature rise (TTR) values. The influence of initial temperature, moisture content, particle size and coal ageing on spontaneous combustion were also studied. The results demonstrated that air humidity is an important factor in determining whether a heating will progress rapidly or not. The particle size distribution of the coal affects the IRR and TTR values, with relatively smaller particles tending to be more reactive. Aged and pre-oxidized coals have higher IRH and lower TTR values, and the coal becomes less reactive. The magnitude of the temperature rises (TTR) increases with increasing initial temperature. No direct correlation was observed between the adiabatic oxidation values and the individual coal properties.
00100969
Combustion
pyrolyser-II
Batra. D. and Rajeswara Rao, T. Energy Convers. & Manage., 2000.41, (6), 573-583. Experiments are conducted in a pyrolyser with eight fins with rice husk and coffee husk. Model equations for the heat transfer and pyrolysis processes, considering wail heat resistance at the pyrolyser and fin walls, are solved for predicting the temperatures and conversions in the pyrolyser. It is found that the agreement between the experimental and predicted values is very good.
Approaches to modelling heterogeneous char NO 00/00970 formation/destruction during pulverised coal combustion Jones. J. M. et al. Carbon, 1999, 37, (lo), 1545-1552. Emissions of nitrogen oxides during combustion pose a major environmental problem and hence there is considerable interest in reducing the NO, levels encountered during pulverized coal combustion. The chemically bound nitrogen in the fuel is known to account for up to 80% of total NO,. Whilst the homogeneous production pathways of NO, from the nitrogen present in the coal volatiles are relatively well known, the heterogeneous conversion of the nitrogen retained in the char to NO is less well understood. As part of an ongoing project to develop an advanced coal model to describe the combustion process in pulverized coal flames this study examines the possible mechanisms involved in both nitric oxide formation and reduction at the char surface for a more accurate prediction of char-NO interactions. The use of a model to predict the partition of coal nitrogen between volatiles and char has also been explored. Various mechanisms of NO, production/destruction at the char surface are discussed and it is concluded that the best approach for computational modelling is to treat the formation and reduction pathways separately.
00/00972 nonuniform
The Burke-Schumann flow field
spray diffusion
flame in a
Khosid, S. and Greenberg, J. B. Comhustim & Flame. 1999. I IX. (l-2). 13-24. The uniform, constant velocity profile assumption of the classical BurkeSchumann diffusion flame model is relaxed to permit more realistic duct entrance velocity profiles to be accounted for. An approximate model is formulated to include the supply of a spray of liquid fuel in the inner duct of the coflow configuration. For appropriate operating conditions (the characteristic flow time less than the characteristic vaporization time) it is shown that a fully developed parabolic velocity profile located at a short distance downstream of the inlet region provides the background flow field for the establishment of a homogeneous spray diffusion flame. The characteristic constriction of the streamlines immediately downstream of the inlet is also taken into consideration. Analytic soluttons are found for the cases of negligible and notable axial diffusion. Due to difficulties that arise in the numerical calculation of the solutions. the large eigenvalues of the problem are derived asymptotically, thus enabling the full range of parametric values to be employed in predicting the spray flame characteristics. Computed results for flame shapes and heights are compared with those of a model from the literature that is based on the assumption of a uniform velocity profile. The sensitivity of the flame profiles to the initial droplet loading, the Peclet number, the vaporization Damkohler number and the use of a parabolic velocity profile are clearly demonstrated. The net effect of the non-uniform flow field is found to be expressible in terms of a resealing of the vaporization Damkohler number.
00100973 to unsteady
Chemical response strain rate
of methane/air
diffusion
flames
Im, H. G. et al. Combustion & Flame, 1999, 118, (l-2), 204-212. Effects of unsteady strain rate on the response of methane/air diffusion flames are studied numerically. The numerical simulations are carried out for the finite-domain opposed flow configuration in which the nozzle exit velocities are prescribed as a function of time. The chemical kinetics is computed with the GRI mechanism ~2.11 including NO, in methane/air combustion. The response of individual species to monochromatic oscillation in strain rate with various frequencies reveals that the fluctuation of slow species, such as CO and NO,, is quickly suppressed as the flow time scale decreases. Furthermore, it is observed that the maximum CO concentration is very insensitive to the variation in the scalar dissipation rate. It is also demonstrated that, for high-frequency oscillations, the scalar dissipation rate is a more appropriate parameter than strain rate to characterize the unsteady flame behaviour. An extinction event due to an abrupt imposition of high strain rates is simulated by an impulsive velocity with various frequencies. For a fast impulse, a substantial overshoot in NO: concentration is observed immediately after extinction. The overall fuel burning rate shows a weak response to the vartation in characteristic unsteady time scale, while the emission indices for NO, show a monotonic decay in response as the impulse frequency increases.
Fuel and Energy Abstracts
March 2000
107
for retrofitting
of
Yu, M. A. Prom. Teplotekh., 1997, 19, (6). 106-109. (In Russian) The modification of coal-fired boilers using a variety of fluidized-bed combustion technologies is described in this paper. Recommendations are provided on the application of the retrofitting methods for firing of coals of different degrees of metamorphism. The modification of small coal-fired boilers is extremely valuable, because it extends their operation lifetime and increases their combustion efficiency of high ash coals without the addition of gas and fuel oil.
00100967 atmosphere
Ways to decrease nitrogen oxide emissions to the during coal combustion in industrial boilers
Volodarskii, 1. Kh. K/t&r. TLerd. Topi. (Moscow), 1998, (2). 75-80. (In Russian) A review of nitrogen oxide emission reduction methods for use in industrial boilers. Flue gas recirculation decreases nitrogen oxide emissions from a coal-fired stoker boiler (DKV 6.5113) by 2540%. Particle emissions are lowered from 19.8 to 4.2 g/m7 by using granulated coal.
09
COMBUSTION Burners,
00100966 pulverised
Combustion
Adiabatic oxidation coals to spontaneous
Systems
study on the propensity combustion
Analysis
of an annular
finned
(burners, combustion
systems)
00/00971 An asymptotic and numerical investigation of homogeneous ignition in catalytically stabilized channel flow combustion Mantzaras, _I. and Benz, P. Combustion & Name, 1999, 119, (4). 455-472. The gas-phase ignition of a fuel-lean premixed combustible gas is investigated in a forced convection two-dimensional laminar channel flow configuration established by two catalytically-active parallel plates placed at a distance 26 apart. The gaseous mixture has uniform inlet properties and both plate temperatures are constant and equal. First-order matched activation energy asymptotics are used to describe the reactive gaseous flow in conjunction with the boundary layer approximation. a one-step large activation energy gaseous reaction and an infinitely fast (mass-transportlimited) catalytic reaction. A closed form ignition criterion is obtained for the gas-phase ignition distance in terms of nondimensional groups that are relevant to confined flows. The characteristic chemical and transverse diffusion time scales are included explicitly in the ignition criterion clearly demonstrating the competition between gaseous and catalytic fuel conversion while the effect of flow confinement (h) is included implicitly. The ignition criterion is valid over the range 0.002 < xi(hRefr) < 0.16, withr the streamwise distance, Re the flow Reynolds number based on the channel halfwidth b and the uniform inlet properties and Pr the Prandtl number. The temperature and transport parameter ranges of applicability are I.5 < TWIT,,+, < 3 (with TWIT/N the ratio of the catalytic wall to the inlet temperature) and 0.9 c Le < 2.0 (with Le the Lewis number) respectively. rendering the ignition criterion of particular interest to hydrocarbon catalytically stabilized combustion (CST) applications. Numerical simulations are performed for channel flow catalytic combustion of a fuel-lean (equivalence ratio 0.32) propane-oxygen-nitrogen mixture using the same underlying chemistry assumptions as in the analytical asymptotic approach. The analytically calculated ignition distances are in good agreement with those numerically predicted. The effect of flow confinement (finite h) on gaseous ignition is examined by comparing ignition distances with the corresponding ones of the unconfined (flat plate) case. Flow confinement (decreasing h) increases the ignition distances due to the resulting increase in the channel surface-to-volume ratio. Moreover. the effect of flow confinement is important already from x/(hRePr) = 0.002.
of
Ren, T. X. et al. Fuel, 1999, 78, (14) 1611-1620. This paper presents the results of adiabatic oxidation studies on the propensity of I8 pulverized coals to spontaneous combustion. All the coal samples were tested at an initial temperature of 40°C and three samples at hO”C to mimic the condition in a coal mill. Their propensities to spontaneous combustion were ranked according to their initial rate of heating (IRH) and total temperature rise (TTR) values. The influence of initial temperature, moisture content, particle size and coal ageing on spontaneous combustion were also studied. The results demonstrated that air humidity is an important factor in determining whether a heating will progress rapidly or not. The particle size distribution of the coal affects the IRR and TTR values, with relatively smaller particles tending to be more reactive. Aged and pre-oxidized coals have higher IRH and lower TTR values, and the coal becomes less reactive. The magnitude of the temperature rises (TTR) increases with increasing initial temperature. No direct correlation was observed between the adiabatic oxidation values and the individual coal properties.
00100969
Combustion
pyrolyser-II
Batra. D. and Rajeswara Rao, T. Energy Convers. & Manage., 2000.41, (6), 573-583. Experiments are conducted in a pyrolyser with eight fins with rice husk and coffee husk. Model equations for the heat transfer and pyrolysis processes, considering wail heat resistance at the pyrolyser and fin walls, are solved for predicting the temperatures and conversions in the pyrolyser. It is found that the agreement between the experimental and predicted values is very good.
Approaches to modelling heterogeneous char NO 00/00970 formation/destruction during pulverised coal combustion Jones. J. M. et al. Carbon, 1999, 37, (lo), 1545-1552. Emissions of nitrogen oxides during combustion pose a major environmental problem and hence there is considerable interest in reducing the NO, levels encountered during pulverized coal combustion. The chemically bound nitrogen in the fuel is known to account for up to 80% of total NO,. Whilst the homogeneous production pathways of NO, from the nitrogen present in the coal volatiles are relatively well known, the heterogeneous conversion of the nitrogen retained in the char to NO is less well understood. As part of an ongoing project to develop an advanced coal model to describe the combustion process in pulverized coal flames this study examines the possible mechanisms involved in both nitric oxide formation and reduction at the char surface for a more accurate prediction of char-NO interactions. The use of a model to predict the partition of coal nitrogen between volatiles and char has also been explored. Various mechanisms of NO, production/destruction at the char surface are discussed and it is concluded that the best approach for computational modelling is to treat the formation and reduction pathways separately.
00/00972 nonuniform
The Burke-Schumann flow field
spray diffusion
flame in a
Khosid, S. and Greenberg, J. B. Comhustim & Flame. 1999. I IX. (l-2). 13-24. The uniform, constant velocity profile assumption of the classical BurkeSchumann diffusion flame model is relaxed to permit more realistic duct entrance velocity profiles to be accounted for. An approximate model is formulated to include the supply of a spray of liquid fuel in the inner duct of the coflow configuration. For appropriate operating conditions (the characteristic flow time less than the characteristic vaporization time) it is shown that a fully developed parabolic velocity profile located at a short distance downstream of the inlet region provides the background flow field for the establishment of a homogeneous spray diffusion flame. The characteristic constriction of the streamlines immediately downstream of the inlet is also taken into consideration. Analytic soluttons are found for the cases of negligible and notable axial diffusion. Due to difficulties that arise in the numerical calculation of the solutions. the large eigenvalues of the problem are derived asymptotically, thus enabling the full range of parametric values to be employed in predicting the spray flame characteristics. Computed results for flame shapes and heights are compared with those of a model from the literature that is based on the assumption of a uniform velocity profile. The sensitivity of the flame profiles to the initial droplet loading, the Peclet number, the vaporization Damkohler number and the use of a parabolic velocity profile are clearly demonstrated. The net effect of the non-uniform flow field is found to be expressible in terms of a resealing of the vaporization Damkohler number.
00100973 to unsteady
Chemical response strain rate
of methane/air
diffusion
flames
Im, H. G. et al. Combustion & Flame, 1999, 118, (l-2), 204-212. Effects of unsteady strain rate on the response of methane/air diffusion flames are studied numerically. The numerical simulations are carried out for the finite-domain opposed flow configuration in which the nozzle exit velocities are prescribed as a function of time. The chemical kinetics is computed with the GRI mechanism ~2.11 including NO, in methane/air combustion. The response of individual species to monochromatic oscillation in strain rate with various frequencies reveals that the fluctuation of slow species, such as CO and NO,, is quickly suppressed as the flow time scale decreases. Furthermore, it is observed that the maximum CO concentration is very insensitive to the variation in the scalar dissipation rate. It is also demonstrated that, for high-frequency oscillations, the scalar dissipation rate is a more appropriate parameter than strain rate to characterize the unsteady flame behaviour. An extinction event due to an abrupt imposition of high strain rates is simulated by an impulsive velocity with various frequencies. For a fast impulse, a substantial overshoot in NO: concentration is observed immediately after extinction. The overall fuel burning rate shows a weak response to the vartation in characteristic unsteady time scale, while the emission indices for NO, show a monotonic decay in response as the impulse frequency increases.
Fuel and Energy Abstracts
March 2000
107