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01466 Mathematical modeling of fluidized bed combustion — 2: combustion of gases
09
Combustion
(burners, combustion
systems)
effect of pore diffusion on the reactivity was analysed. Isothermal absorption of the gas was used to find the pore distribution and the specific surface area of the coal and char. The inert component has relatively low reactivity. During combustion the activation energy of char varies. The effects of the maceral are different in different combustion stages. The effect of diffusion is indicated by the decrease of the activation energy in the later stage. 99101463
A mathematical
model for coal combustion on
grates
Chejne, F. et al. hf. Tecnol., 1998, 9, (3) 163-169. (In Spanish) The processes involved in coal combustion on fixed and moving grates are simulated with a recently developed mathematical model. In developing the model, new phenomenol concepts were considered to form a set of ordinary differential equations (ODE) and partial differential equations (PDE). Explicit finite’ difference techniques and the Richardson extrapolation method were used to solve the PDE, while the ODE were solved using the Runge-Kutta method. The model predictions and experimental values for small height coal-bed combustion agreed well. 99101464
A mathematical model for fluidized bed combustion
of coal. On a sub-model of CO/COl product ratio
Tomita, M. Hohkaido Kogyo Gijutsu Kenkyusho Hokoku, 1998, 71, 27-34. (In Japanese) model This paper presents a new submodel, based on a homogeneous-film combustion, to predict the CO/COz product ratio in the numerical simulation of the fluidized-bed combustion (FBC) of coal. It was found that the new submodel predicted the CO/CO2 product ratio considerably well without any adjustable parameters when the calculated results were compared with those results from the conventional submodel.
Mathematical modeling of a bubbling fluidized-bed coal gaslfier and the significance of ‘net flow’
99101465
Yan, H.-m. et al. Fuel, 1998, 77, (9/10), 1067-1079. To evaluate the performance of a bubbling fluidized-bed coal gasifier, an isothermal model, incorporating the two-phase theory, has been developed. A distinctive feature of this model is the consideration of a net flow term from the emulsion phase to the bubble phase in the conservation equations. Simulations with consideration of the net flow term indicate that the overall results compare favourably with available experimental data from an industrial fluidized-bed gasifier. The net flow is significant, in the range 7187% relative to the feed gas rate, strongly depending on the coal rank, heteroeeneous reaction rates and volatile matter released in the bed. The higherThe coal rank, the lower the net flow and total excess gas flow. The large volume of net flow generated can significantly change the fluidization conditions in the bed and thus alter the reaction rates and mass transfer properties. Simulations deviate significantly from the experimental results when not using the net flow.
Mathematical modeling of fluidized bed combus99101466 tion - 2: combustion of gases Srinivasan, R. A. et al. Fuel, 1998, 77, (9/10), 1033-1049. Pollutant formation, emission characteristics and the combustion behaviour of carbon are expected to be influenced by the combustion of volatiles within a fluidized bed combustor. System models which include details of the release of volatiles from coal and their subsequent combustion are difficult to develop. This paper considers the behaviour of combustible gas mixtures as a sub-model of the more complex problem. The hydrodynamic description of the bubbling fluidized bed is based on the three-phase disoersion model. which accounts for the variation of the visible bubble flow with height above the distributor. By including an energy balance for the bubble phase, the non-isothermality of the bed resulting from combustion reactions is taken into account. The effect on bubble properties and crossflow of the variation in superficial gas velocity, is included through an overall mass balance. Species (intermediates and products) and energy balances are solved numerically. Transient calculations show that the ignition first occurs close to the top of the bed and then over time its front moves closer to the distributor. This is due to increases in bed temperature and particle size and decreases in the excess gas velocity as shown by steadystate parametric analyses. When the model calculations were compared with experimental data reported in the literature on the axial variation in mole fractions of various reactant and oroduct soecies it was shown that methane and propane burn in a region of parametric sensitivity under typical fluidized bed combustion conditions. Therefore significant variations in calculated mole fractions result from comparatively minor variations in reaction rate parameters. However, good agreement is obtained with experimental data for combustion of gases in fluidized beds with activation energies well within acceptable limits established in independent kinetic studies. L
1
Measurement of mass transfer between the bubble and dense phases in a fluidized bed combustor
99iQ1467
Camoos. J. B. L. M. et al. Cotnbust. Flame. 1999. 116. (l/2). 105-119. A description is given of an experimental study on mass‘ transfer between the bubble and dense phases in a fluidized bed, used as a coke combustor. The experimental technique allowed quantification of the mass transfer rate during bubble formation and during a bubble’s rise through the bed. The combustion experiments were performed at 1 atm and 1223 K, in a fluidized bed of sand with static heights of 0.10-0.21 m. The bubbling flow rate
148
Fuel and Energy Abstracts
March 1999
raneed from 2.5 to 5.0 times that at incioient fluidization. The coke oarticles weri 3.0 or 3.5 mm in diameter. Results indicate that the equivalent bed height, L,, (the height a bubble must rise to transfer to the dense phase the same quantity of oxygen as during its formation) is independent of the bubbling air flow rate. The mean value L, = 50 mm suggests that for shallow beds the mass transferred during bu \ ble formation is a significant part of the total mass transferred. 99101466 Measurement of sulfur fixing efficiency of sulfur capturer in combustion of coal He, Y. and Zhang, Y. Huanjing Baohu (Beijing), 1997, (5), 29-31. (In Chinese) By using the principle of sulfur balance, a method for determining the sulfur fixing efficiency of a sulfur capturer during the combustion of coal was developed. A formula for calculating this sulfur fixing efficiency is presented.
Method and apparatus for gasification and combustion of solid waste using fluidired bed with oxygen supply
99101469
Fujinami, S. et al. PCT Int. Appl. WO 98 23,898 (Cl. F23G5/30), 4 Jun 1998, JP Appl. 96/329,079, 26 Nov 1996, 24 pp. (In Japanese) Subjected to thermal decomposition and gasification at 450-650°C in a fluidized bed of a fluidized-bed gasification furnace, the solid waste is then subjected to melt combustion at 1200-1500°C in a whirling-flow-type melt combustion furnace, whereby the ash is turned into fused slag. Selected as a gas supplied to the fluidized bed is at least one of oxygen, vapour and air. The quantity of oxygen in the gas supplied to the fluidized bed is set to lo30% of a ouantitv of theoretical combustion oxveen. Generated are the following whirling’movements of a fluidized mediug. The fluidized medium flows down as it is fluidized in a slow fluidized bed in a central portion of a furnace bottom, flows up as it is fluidized in an active fluidized bed in a peripheral portion of the furnace bottom, flows from a central portion to a oerioheral oortion in a lower nortion of the fluidized bed and from a peripheral portion to a central portion in an upper portion of the fluidized bed as the fluidized medium is fluidized and flows from a central portion to a peripheral portion in the lower portion of the fluidized bed as the fluidized medium is fluidized. 99101470
A method of NO. reduction during the combustion
of oil Wilk, R. and Szlek, A. Energy Comers. Manage., 1998, 39, (16-18), 19571965. A new method of reducing the emission of NO, during the combustion of oil, making use of bi-fractional atomization, is discussed. Theoretical investigations have shown the possible reducing role of large droplets. This theory was verified on a test stand with a power rating of 50 kW. The results obtained confirmed the abatement of the emission of NO, by about 20-30% without any significant increase of the CO emission. The authors suggest that this method may be applied together with other methods of decreasing the NO, emission.
99101471 A model of low-speed combustion of methane-aircoal dust suspensions
Tunik, Y. V. Arch. Cornbust., 1996 (Pub 1998), 16, (3-4), 177-187. A consideration of the problems of low-speed combustion propagation in methane-air mixtures takes place. Only one overall chemical reaction (CH., + 2Oz = CO2 + 2HzO) is accounted for to describe CHI burning. Its heat releasing rate is assumed to be defined by delay time. The suggested method obtains relations to calculate normal flame propagation velocity and good correlation between theoretical and experimental data in steady flame propagating problems for different gaseous carbon-hydrogen fuel. The examination of the coal or sand dust effect on the gas dynamics of a methane-air mixture combustion was found to be an unsteady problem. Complete gas dynamics equations of two-temperature and two-velocity medium are solved numericallv. CH, is assumed to burn in an infinite thin flame front propagating across the-dusty medium. Particle ignition and combustion occur behind the flame front. The heterogeneous burning of coke occurs in kinetic and diffusion regimes.
99101472 A model of the combustion of a single small coal particle using kinetic parameters based on thermogravimetric analysis Vamvuka, D. and Woodburn, E. T. Int. J. Energy Res., 1998, 22, (7), 657670. A mathematical model for the combustion in air of a sinzle entrained spherical coal particle, 30 pm in diameter, has been deyeloped. The thermogravimetric analysis data of Whihvick coal was used. Ordinary differential equations, describing the reaction rates and the mass and heat transport processes, solved numerically formed the basis of the model. The combustion mechanism of the particle was described by locating the reaction zone at the solid surface, where gas-phase combustion of volatiles and heterogeneous reaction between gaseous oxygen and the carbon and hydrogen in the solid occurred in parallel. The combustion process was chemical-reaction-rate-controlled. The oxygen partial pressure at the surface was almost the same as the surrounding bulk gas. The simulation results using this model are consistent with previously reported combustion lifetimes of approximately one second, for particles of this size and rank. Furthermore, they are consistent with the anticipation that higher ambient gas temperatures should result in shorter burn-out times. The use of
Combustion
(burners, combustion
systems)
effect of pore diffusion on the reactivity was analysed. Isothermal absorption of the gas was used to find the pore distribution and the specific surface area of the coal and char. The inert component has relatively low reactivity. During combustion the activation energy of char varies. The effects of the maceral are different in different combustion stages. The effect of diffusion is indicated by the decrease of the activation energy in the later stage. 99101463
A mathematical
model for coal combustion on
grates
Chejne, F. et al. hf. Tecnol., 1998, 9, (3) 163-169. (In Spanish) The processes involved in coal combustion on fixed and moving grates are simulated with a recently developed mathematical model. In developing the model, new phenomenol concepts were considered to form a set of ordinary differential equations (ODE) and partial differential equations (PDE). Explicit finite’ difference techniques and the Richardson extrapolation method were used to solve the PDE, while the ODE were solved using the Runge-Kutta method. The model predictions and experimental values for small height coal-bed combustion agreed well. 99101464
A mathematical model for fluidized bed combustion
of coal. On a sub-model of CO/COl product ratio
Tomita, M. Hohkaido Kogyo Gijutsu Kenkyusho Hokoku, 1998, 71, 27-34. (In Japanese) model This paper presents a new submodel, based on a homogeneous-film combustion, to predict the CO/COz product ratio in the numerical simulation of the fluidized-bed combustion (FBC) of coal. It was found that the new submodel predicted the CO/CO2 product ratio considerably well without any adjustable parameters when the calculated results were compared with those results from the conventional submodel.
Mathematical modeling of a bubbling fluidized-bed coal gaslfier and the significance of ‘net flow’
99101465
Yan, H.-m. et al. Fuel, 1998, 77, (9/10), 1067-1079. To evaluate the performance of a bubbling fluidized-bed coal gasifier, an isothermal model, incorporating the two-phase theory, has been developed. A distinctive feature of this model is the consideration of a net flow term from the emulsion phase to the bubble phase in the conservation equations. Simulations with consideration of the net flow term indicate that the overall results compare favourably with available experimental data from an industrial fluidized-bed gasifier. The net flow is significant, in the range 7187% relative to the feed gas rate, strongly depending on the coal rank, heteroeeneous reaction rates and volatile matter released in the bed. The higherThe coal rank, the lower the net flow and total excess gas flow. The large volume of net flow generated can significantly change the fluidization conditions in the bed and thus alter the reaction rates and mass transfer properties. Simulations deviate significantly from the experimental results when not using the net flow.
Mathematical modeling of fluidized bed combus99101466 tion - 2: combustion of gases Srinivasan, R. A. et al. Fuel, 1998, 77, (9/10), 1033-1049. Pollutant formation, emission characteristics and the combustion behaviour of carbon are expected to be influenced by the combustion of volatiles within a fluidized bed combustor. System models which include details of the release of volatiles from coal and their subsequent combustion are difficult to develop. This paper considers the behaviour of combustible gas mixtures as a sub-model of the more complex problem. The hydrodynamic description of the bubbling fluidized bed is based on the three-phase disoersion model. which accounts for the variation of the visible bubble flow with height above the distributor. By including an energy balance for the bubble phase, the non-isothermality of the bed resulting from combustion reactions is taken into account. The effect on bubble properties and crossflow of the variation in superficial gas velocity, is included through an overall mass balance. Species (intermediates and products) and energy balances are solved numerically. Transient calculations show that the ignition first occurs close to the top of the bed and then over time its front moves closer to the distributor. This is due to increases in bed temperature and particle size and decreases in the excess gas velocity as shown by steadystate parametric analyses. When the model calculations were compared with experimental data reported in the literature on the axial variation in mole fractions of various reactant and oroduct soecies it was shown that methane and propane burn in a region of parametric sensitivity under typical fluidized bed combustion conditions. Therefore significant variations in calculated mole fractions result from comparatively minor variations in reaction rate parameters. However, good agreement is obtained with experimental data for combustion of gases in fluidized beds with activation energies well within acceptable limits established in independent kinetic studies. L
1
Measurement of mass transfer between the bubble and dense phases in a fluidized bed combustor
99iQ1467
Camoos. J. B. L. M. et al. Cotnbust. Flame. 1999. 116. (l/2). 105-119. A description is given of an experimental study on mass‘ transfer between the bubble and dense phases in a fluidized bed, used as a coke combustor. The experimental technique allowed quantification of the mass transfer rate during bubble formation and during a bubble’s rise through the bed. The combustion experiments were performed at 1 atm and 1223 K, in a fluidized bed of sand with static heights of 0.10-0.21 m. The bubbling flow rate
148
Fuel and Energy Abstracts
March 1999
raneed from 2.5 to 5.0 times that at incioient fluidization. The coke oarticles weri 3.0 or 3.5 mm in diameter. Results indicate that the equivalent bed height, L,, (the height a bubble must rise to transfer to the dense phase the same quantity of oxygen as during its formation) is independent of the bubbling air flow rate. The mean value L, = 50 mm suggests that for shallow beds the mass transferred during bu \ ble formation is a significant part of the total mass transferred. 99101466 Measurement of sulfur fixing efficiency of sulfur capturer in combustion of coal He, Y. and Zhang, Y. Huanjing Baohu (Beijing), 1997, (5), 29-31. (In Chinese) By using the principle of sulfur balance, a method for determining the sulfur fixing efficiency of a sulfur capturer during the combustion of coal was developed. A formula for calculating this sulfur fixing efficiency is presented.
Method and apparatus for gasification and combustion of solid waste using fluidired bed with oxygen supply
99101469
Fujinami, S. et al. PCT Int. Appl. WO 98 23,898 (Cl. F23G5/30), 4 Jun 1998, JP Appl. 96/329,079, 26 Nov 1996, 24 pp. (In Japanese) Subjected to thermal decomposition and gasification at 450-650°C in a fluidized bed of a fluidized-bed gasification furnace, the solid waste is then subjected to melt combustion at 1200-1500°C in a whirling-flow-type melt combustion furnace, whereby the ash is turned into fused slag. Selected as a gas supplied to the fluidized bed is at least one of oxygen, vapour and air. The quantity of oxygen in the gas supplied to the fluidized bed is set to lo30% of a ouantitv of theoretical combustion oxveen. Generated are the following whirling’movements of a fluidized mediug. The fluidized medium flows down as it is fluidized in a slow fluidized bed in a central portion of a furnace bottom, flows up as it is fluidized in an active fluidized bed in a peripheral portion of the furnace bottom, flows from a central portion to a oerioheral oortion in a lower nortion of the fluidized bed and from a peripheral portion to a central portion in an upper portion of the fluidized bed as the fluidized medium is fluidized and flows from a central portion to a peripheral portion in the lower portion of the fluidized bed as the fluidized medium is fluidized. 99101470
A method of NO. reduction during the combustion
of oil Wilk, R. and Szlek, A. Energy Comers. Manage., 1998, 39, (16-18), 19571965. A new method of reducing the emission of NO, during the combustion of oil, making use of bi-fractional atomization, is discussed. Theoretical investigations have shown the possible reducing role of large droplets. This theory was verified on a test stand with a power rating of 50 kW. The results obtained confirmed the abatement of the emission of NO, by about 20-30% without any significant increase of the CO emission. The authors suggest that this method may be applied together with other methods of decreasing the NO, emission.
99101471 A model of low-speed combustion of methane-aircoal dust suspensions
Tunik, Y. V. Arch. Cornbust., 1996 (Pub 1998), 16, (3-4), 177-187. A consideration of the problems of low-speed combustion propagation in methane-air mixtures takes place. Only one overall chemical reaction (CH., + 2Oz = CO2 + 2HzO) is accounted for to describe CHI burning. Its heat releasing rate is assumed to be defined by delay time. The suggested method obtains relations to calculate normal flame propagation velocity and good correlation between theoretical and experimental data in steady flame propagating problems for different gaseous carbon-hydrogen fuel. The examination of the coal or sand dust effect on the gas dynamics of a methane-air mixture combustion was found to be an unsteady problem. Complete gas dynamics equations of two-temperature and two-velocity medium are solved numericallv. CH, is assumed to burn in an infinite thin flame front propagating across the-dusty medium. Particle ignition and combustion occur behind the flame front. The heterogeneous burning of coke occurs in kinetic and diffusion regimes.
99101472 A model of the combustion of a single small coal particle using kinetic parameters based on thermogravimetric analysis Vamvuka, D. and Woodburn, E. T. Int. J. Energy Res., 1998, 22, (7), 657670. A mathematical model for the combustion in air of a sinzle entrained spherical coal particle, 30 pm in diameter, has been deyeloped. The thermogravimetric analysis data of Whihvick coal was used. Ordinary differential equations, describing the reaction rates and the mass and heat transport processes, solved numerically formed the basis of the model. The combustion mechanism of the particle was described by locating the reaction zone at the solid surface, where gas-phase combustion of volatiles and heterogeneous reaction between gaseous oxygen and the carbon and hydrogen in the solid occurred in parallel. The combustion process was chemical-reaction-rate-controlled. The oxygen partial pressure at the surface was almost the same as the surrounding bulk gas. The simulation results using this model are consistent with previously reported combustion lifetimes of approximately one second, for particles of this size and rank. Furthermore, they are consistent with the anticipation that higher ambient gas temperatures should result in shorter burn-out times. The use of