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Modelling of circulating fluidized bed combustion of coal
Conference
Abstracts
Al, Si, Fe, K, Na and Ca contents). The coals investigated ranged in rank from lignite to low-volatile bituminous. The data obtained were used to develop and validate an accurate model of the overall char combustion process. Gas and particle heat and mass transport models were used to determine char combustion rates from the optical measurements of single-particle sizes, temperatures and velocities. Char combustion rates were also determined by an independent inorganic tracer technique based on chemical analysis of extracted samples. Combined, the two methods for combustion rates allow the evaluation of kinetic parameters and parameters for the CO to CO, heterogeneous product ratio. A unified treatment of the kinetic parameters has resulted in a generalized reactivity correlation based on coal carbon content. Char reactivity was found to decrease with increasing rank of the parent coal. Burning rates vary by a factor of - 7 between the lignite and the Iv-bituminous coals examined in the project, and characteristic burning times. which reflect both reactivity and char physical properties, vary by a factor of - IO. The information has been used to answer fundamental questions concerning the char oxidation mechanisms, the modes of particle burning, the effects of mineral matter on the combustion characteristics of the chars, and the elemental release rates during the char oxidation process. Various aspects of the results of the char combustion experiments are discussed in the paper. This work was performed at the Combustion Research Facility, Sandia National Laboratories and was supported by the IJS Department of Energy through the Pittsburgh Energy Technology Center’s Direct Utilization Advanced Research & Technology Development Program.
is developed. The hydrodynamics of a CFB is described by empirical correlations capable of predicting axial solids concentration profiles in the riser. Volatile matter is assumed to be released instantaneously from coal on entry into the furnace, and is assumed to burn uniformly throughout the riser. The combustion of residual char particles in a CFB combustor is assumed to be controlled by a combination of chemical reaction and oxygen diffusion. It is assumed that char combustion begins with oxygen diffusion to the burning coal particle where the following reaction occurs: C+tO,-CO, then the carbon monoxide produced is oxidized by oxygen to produce carbon dioxide according to reaction: CO +fO, + CO,. A uniform gas temperature is assumed throughout the riser, whereas the temperature of a burning char particle is computed from heat balances of the char particle. Predictions of the model for given operating conditions show that: (I) in a CFB without secondary air injection, oxygen concentration and carbon weight fraction decrease exponentially from the bottom to the top of the riser; (2) as combustion proceeds, the size distribution of char particles shifts to smaller sizes; the spread in the size distribution becomes narrower at the exit than at the inlet of the riser; (3) combustion efficiency increases with increasing combustion temperature, increasing excess air level, and decreasing superficial gas velocity in the riser; the combustion efficiency also increases with overall height of the riser, whereas it is very insensitive to changes of solids circulation rate. Fair agreement has been found between model prediction and experimental results. Prediction of the present model should be of significance for providing useful information on design of a CFB combustor.
Kinetics study of char combustion in a fluidized bed S.
Ignition of coal particles: the influence of experimental technique and theoretical interpretation _ Dong-ke
Zhang
Department Newcastle,
CERCHAR,
and Terry F. Wall
of Chemical Engineering, NSW 2308, Australia
The
University
of
Ignition measurements of the same fuel samples [a bituminous coal (30% VM), a char made from the coal and a petroleum coke] were performed using five different experimental techniques. The experimental techniques involved a pulse ignition and continuous ignition experiments on a drop-tube furnace, TGA ignition measurements. a thermocouple-single particle ignition technique and a newly developed optical fibre-laser induced ignition technique. Attention is focused on the system variables which influence the ignition temperature and the associated ignition mechanism. The experimental results suggest that, apart from the effects of volatile matter content, oxygen concentration and particle size which have been well studied previously, the system variables related to the experimental conditions have strong effects on the particle ignition temperature and the ignition mechanism. The variables examined are the sample mass involved, the particle heating rates used in the techniques, the way in which the particles are heated, the surrounding gas conditions and the definition of particle ignition. Theoretical models for the ignition of both single particles and clouds were also developed to interpret the experimental observations. Hidden information about the coal particle ignition phase was discovered by comparison of the experimental and theoretical results. The combustion reactivity of the fuels was estimated from the onset of ignition temperature data assuming heterogeneous ignition. Comparison of the combustion rates estimated from the ignition data with those obtained by combustion rate measurements was also discussed, which gave further insight into the ignition mechanisms of pulverized fuel particles. It was concluded that not only the ignition temperatures, but also the ignition mechanisms vary with the experimental conditions.
Modelling
of circulating
fluidized bed combustion of coal
X. S. Wang, B. M. Gibbs and M. J. Rhodes*
Department
of Fuel and Energy,
LS2 9JT, UK * Department of Chemical Bradford, Bradford BD7 A model for the combustion
702
Fuel 1993
University
Engineering, lDP, UK
of Leeds, Leeds
University
of coal in a circulating
Volume
72 Number
of
fluidized bed (CFB)
5
Brunello,
Ph. Maiisa BP
and I. Flour
19, 62670
Mazingarbe,
France
The aim of this work is to study the kinetics of char combustion in a Ruidized bed reactor. The effect of temperature and particle size on the combustion rate and also the rate controlling step (chemical or diffusion) are reported. The experiments were carried out using eight different chars and a graphite in a batch fluidized bed reactor. The combustion rate was determined by measuring the CO and CO, concentration in the flue gas. We showed that the progressive conversion model and the shrinking spherical particle model both described the combustion rate well for all the tested chars. With small particle size, the temperature dependency of the reaction rate constant was determined and expressed by Arrhenius’ equation providing the apparent activation energy and the pre-exponential factor. We extended the experiments using larger sized particles and by analysing the temperature dependency of the reaction rate constant. We found that the high volatile bituminous char combustion was under external diffusion control when the temperature was > X50-900°C and the particle size was > 2 mm. This was not clearly shown for the anthracite under the same conditions.
Numerical study of combustion and NO formation in a low NO, burner J. L. 7: .4zevedo Instituto Rovisco
and M. G. Carvalho
Superior TCcnico Pais, 1096 Lisboa
Mechanical Eng. Codex, Portugal
Dept,
Av.
Public concern over NO emissions has focused attention on detailed experiments and numerical analysis of pulverized coal burners. Experimental studies of full scale boilers are restricted due to the difficulties of gaining access into the boiler, so a lot of research is performed using smaller furnaces with a single burner with scales from < 1 MWt up to the burner size found in utility boilers. Numerical modelling provides a tool to predict single burner and utility boiler behaviour. In the present work, a numerical model is applied to a large scale 38 MWt low NO, burner operating in a laboratorv furnace. The numerical model considers transport equations for momentum enthalpy, chemical species concentrations and turbulent quantities. Pulverized coal is described by a Lagrangian model allowing the calculation of the interaction between gas and particle phase. Fuel NO, is calculated based on the model proposed by De’Soete. This model was previously applied to a small furnace burner and to a utility boiler predicting the experimental trends correctly. The tests considered here correspond to a secondary air swirl number of 0.4, with and without tertiary air swirl. Comparison of experimental results is performed with the computed results including NO concentrations.
Abstracts
Al, Si, Fe, K, Na and Ca contents). The coals investigated ranged in rank from lignite to low-volatile bituminous. The data obtained were used to develop and validate an accurate model of the overall char combustion process. Gas and particle heat and mass transport models were used to determine char combustion rates from the optical measurements of single-particle sizes, temperatures and velocities. Char combustion rates were also determined by an independent inorganic tracer technique based on chemical analysis of extracted samples. Combined, the two methods for combustion rates allow the evaluation of kinetic parameters and parameters for the CO to CO, heterogeneous product ratio. A unified treatment of the kinetic parameters has resulted in a generalized reactivity correlation based on coal carbon content. Char reactivity was found to decrease with increasing rank of the parent coal. Burning rates vary by a factor of - 7 between the lignite and the Iv-bituminous coals examined in the project, and characteristic burning times. which reflect both reactivity and char physical properties, vary by a factor of - IO. The information has been used to answer fundamental questions concerning the char oxidation mechanisms, the modes of particle burning, the effects of mineral matter on the combustion characteristics of the chars, and the elemental release rates during the char oxidation process. Various aspects of the results of the char combustion experiments are discussed in the paper. This work was performed at the Combustion Research Facility, Sandia National Laboratories and was supported by the IJS Department of Energy through the Pittsburgh Energy Technology Center’s Direct Utilization Advanced Research & Technology Development Program.
is developed. The hydrodynamics of a CFB is described by empirical correlations capable of predicting axial solids concentration profiles in the riser. Volatile matter is assumed to be released instantaneously from coal on entry into the furnace, and is assumed to burn uniformly throughout the riser. The combustion of residual char particles in a CFB combustor is assumed to be controlled by a combination of chemical reaction and oxygen diffusion. It is assumed that char combustion begins with oxygen diffusion to the burning coal particle where the following reaction occurs: C+tO,-CO, then the carbon monoxide produced is oxidized by oxygen to produce carbon dioxide according to reaction: CO +fO, + CO,. A uniform gas temperature is assumed throughout the riser, whereas the temperature of a burning char particle is computed from heat balances of the char particle. Predictions of the model for given operating conditions show that: (I) in a CFB without secondary air injection, oxygen concentration and carbon weight fraction decrease exponentially from the bottom to the top of the riser; (2) as combustion proceeds, the size distribution of char particles shifts to smaller sizes; the spread in the size distribution becomes narrower at the exit than at the inlet of the riser; (3) combustion efficiency increases with increasing combustion temperature, increasing excess air level, and decreasing superficial gas velocity in the riser; the combustion efficiency also increases with overall height of the riser, whereas it is very insensitive to changes of solids circulation rate. Fair agreement has been found between model prediction and experimental results. Prediction of the present model should be of significance for providing useful information on design of a CFB combustor.
Kinetics study of char combustion in a fluidized bed S.
Ignition of coal particles: the influence of experimental technique and theoretical interpretation _ Dong-ke
Zhang
Department Newcastle,
CERCHAR,
and Terry F. Wall
of Chemical Engineering, NSW 2308, Australia
The
University
of
Ignition measurements of the same fuel samples [a bituminous coal (30% VM), a char made from the coal and a petroleum coke] were performed using five different experimental techniques. The experimental techniques involved a pulse ignition and continuous ignition experiments on a drop-tube furnace, TGA ignition measurements. a thermocouple-single particle ignition technique and a newly developed optical fibre-laser induced ignition technique. Attention is focused on the system variables which influence the ignition temperature and the associated ignition mechanism. The experimental results suggest that, apart from the effects of volatile matter content, oxygen concentration and particle size which have been well studied previously, the system variables related to the experimental conditions have strong effects on the particle ignition temperature and the ignition mechanism. The variables examined are the sample mass involved, the particle heating rates used in the techniques, the way in which the particles are heated, the surrounding gas conditions and the definition of particle ignition. Theoretical models for the ignition of both single particles and clouds were also developed to interpret the experimental observations. Hidden information about the coal particle ignition phase was discovered by comparison of the experimental and theoretical results. The combustion reactivity of the fuels was estimated from the onset of ignition temperature data assuming heterogeneous ignition. Comparison of the combustion rates estimated from the ignition data with those obtained by combustion rate measurements was also discussed, which gave further insight into the ignition mechanisms of pulverized fuel particles. It was concluded that not only the ignition temperatures, but also the ignition mechanisms vary with the experimental conditions.
Modelling
of circulating
fluidized bed combustion of coal
X. S. Wang, B. M. Gibbs and M. J. Rhodes*
Department
of Fuel and Energy,
LS2 9JT, UK * Department of Chemical Bradford, Bradford BD7 A model for the combustion
702
Fuel 1993
University
Engineering, lDP, UK
of Leeds, Leeds
University
of coal in a circulating
Volume
72 Number
of
fluidized bed (CFB)
5
Brunello,
Ph. Maiisa BP
and I. Flour
19, 62670
Mazingarbe,
France
The aim of this work is to study the kinetics of char combustion in a Ruidized bed reactor. The effect of temperature and particle size on the combustion rate and also the rate controlling step (chemical or diffusion) are reported. The experiments were carried out using eight different chars and a graphite in a batch fluidized bed reactor. The combustion rate was determined by measuring the CO and CO, concentration in the flue gas. We showed that the progressive conversion model and the shrinking spherical particle model both described the combustion rate well for all the tested chars. With small particle size, the temperature dependency of the reaction rate constant was determined and expressed by Arrhenius’ equation providing the apparent activation energy and the pre-exponential factor. We extended the experiments using larger sized particles and by analysing the temperature dependency of the reaction rate constant. We found that the high volatile bituminous char combustion was under external diffusion control when the temperature was > X50-900°C and the particle size was > 2 mm. This was not clearly shown for the anthracite under the same conditions.
Numerical study of combustion and NO formation in a low NO, burner J. L. 7: .4zevedo Instituto Rovisco
and M. G. Carvalho
Superior TCcnico Pais, 1096 Lisboa
Mechanical Eng. Codex, Portugal
Dept,
Av.
Public concern over NO emissions has focused attention on detailed experiments and numerical analysis of pulverized coal burners. Experimental studies of full scale boilers are restricted due to the difficulties of gaining access into the boiler, so a lot of research is performed using smaller furnaces with a single burner with scales from < 1 MWt up to the burner size found in utility boilers. Numerical modelling provides a tool to predict single burner and utility boiler behaviour. In the present work, a numerical model is applied to a large scale 38 MWt low NO, burner operating in a laboratorv furnace. The numerical model considers transport equations for momentum enthalpy, chemical species concentrations and turbulent quantities. Pulverized coal is described by a Lagrangian model allowing the calculation of the interaction between gas and particle phase. Fuel NO, is calculated based on the model proposed by De’Soete. This model was previously applied to a small furnace burner and to a utility boiler predicting the experimental trends correctly. The tests considered here correspond to a secondary air swirl number of 0.4, with and without tertiary air swirl. Comparison of experimental results is performed with the computed results including NO concentrations.