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Mathematical modeling of fluidized bed combustion. 4: N2O and NOx emissions from the combustion of char
09 Combustion (burners, combustion systems) increase in the moisture increases the unburned carbon fraction and decreases the NO, conversion at the outlet of the furnace. These trends agree well with the previous experimental results. It is found that the contribution of thermal NO, to the total NO, drastically decreases with an increasing moisture content, whereas the emission of fuel NO, increases.
Evaluation of waste pyrolysis characteristics in a pressurized fluidized bed reactor
02/01630
Ono, A. er al. Wasfe Management, 2001, 21, (5). 451-456. To obtain the distribution of fuel components to gas, tar and char in a pressurized fluidized bed waste pyrolyser, experiments were conducted with a laboratory scale fluidized bed reactor. Waste samples were fed batchwise from the top of the reactor into the fluidized bed of silica sand and pyrolysed by nitrogen/nitrogen-Oz gas and the effects of pressure, particle size, heating rate and oxygen addition were investigated. In the case of rubber, the char yield tended to increase a little and the tar yield decrease over the pressure of 304-709 kPa. In comparison with the thermogravimetry data it was clearly demonstrated that the char yield from fluidized bed pyrolysis is much lower. A small amount of oxygen addition decreased both tar and char yields but its further increase did not affect them very much.
02lO1631 Experimental and numerical study of autoignition and pilot ignition of PMMA plates in a cone calorimeter Tsai, T-H. et a/. Combusrion and Flame, 2001, 124, (3), 466-480. This work presents experimental and numerical results for the autoignition and pilot ignition of horizontal black polymethylmethacrylate (PMMA) plates in a cone calorimeter. Experimentally, the histories of plate surface temperature as well as ignition times have been measured for various rediative heating fluxes. Numerically, a twodimensional axisymmetric model is proposed to simulate the transient processes in the gas and the solid plate. The radiative absorption by the gas phase monomer is considered in a simplified way. With a set of parameters for the one-step Arrhenius kinetics for the gas-phase reaction and a set of parameters for PMMA pyrolysis, the numerical results of surface temperatures and ignition times agree well with the experimental data for both autoignition and pilot ignition. Both experimental and numerical results indicate that while the pilot ignition is pyrolysis-controlled, the autoignition process depends not only on PMMA pyrolysis but also on gas-phase reaction, especially for lower heating fluxes. According to the numerical calculations for autoignition, the ignition times of thermally thin plates are significantly shorter than those of thermally thick plates. For pilot ignition, there exists a linear l/Jlia - qrad relation for thermally thick plates, as indicated in the literature.
02iO1632 Features of the high-temperature air combustion technique and its application prospects Liu, M. et a/. Nengyuan Yanjiu Yu Xinxi, 2001, 17, (1). 27-35. (In Chinese) The high temperature air combustion (HTAC) technique was developed in Japan in the early 1990s. This technique can bring about significant fuel savings, low NO, emission, high heat-utilization efficiency and size reduction of the facility. In this paper, the characteristics of the flame temperature distribution and the principles of NO, reduction in the HTAC technique were analysed. The results of the theoretical calculation for the blast furnace gas, coke oven gas and mixed gas (BFG 94.5% by volume and COG 5.5% by volume) applied in the HTAC technique showed that the theoretical combustion temperature increases with the temperatures of preheated air and fuel gas. Low heat-value gas, which cannot be fired in conventional combustion systems, can therefore be used in industrial furnaces by the HTAC technique. In addition, using preheated gas and air can make the exit gas temperature lower and the heat-utilization efficiency higher. In the end, the possibilities of application of the HTAC technique to oil- and gas- fired boilers and coal or combustible solidwaste gasification systems were presented.
OUO1633 ignition and propagation of a reaction front in cross-current bed combustion of wet biofuels Thunman, H. and Leckner, B. Fuel, 2001, 80, (4), 473481. Grate firing is the most common way to burn bio-fuels in small-scale units. Different combustion modes are achieved depending on how fuel and primary air are introduced. In continuous systems fuel and air are usually fed in cross-current and counter-current flow. Here, combustion of wet biofuels is studied in a 31 MW reciprocating grate furnace (a cross-current flow combustor), and additional experiments have been made in batch-fired pot furnaces. The fuel was forest waste with moisture content of approximately 50%. The combustion in a crosscurrent flow furnace is generally assumed to start by ignition on the surface of the bed, followed by a reaction front propagating from the surface down to the grate. Measurements and visual observations presented in this paper show, however, that in the case of wet fuels the
ignition takes place close to the grate, followed by a reaction front propagating from the grate up to the surface of the bed. Hence, the progress combustion in the bed is opposite to the expected one.
02/01634 Low NO. combustion technologies for high temperature applications Flamme, M. Energy Conversion and Managemenr, 2001, 42, (15-17) 1919-1935. Because of the high process temperature and the high temperature to which the combustion air is preheated, NO, emissions from glass melting furnaces are extremely high. Even at these high temperatures, NO, emissions could be reduced drastically by using advanced combustion techniques such as staged combustion or flameless oxidation, as experimental work has shown. In the case of oxy-fuel combustion, the NO, emissions are also very high if conventional burners are used. Staged combustion achieves similar NO, reducti0ns.x reduction techniques
02/01635 Mathematical modeling of fluidized bed combustion. 4: N20 and NOx emissions from the combustion of char Chen, Z. et al. Fuel, 2001, 80, (9), 1259-1272. Batch experiments were conducted to investigate the emissions of carbon oxides and nitrogen oxides from the combustion of char prepared from a commercial coal in a bubbling fluidized bed. Combustion gases escaping from the surface of the bed were sampled and analysed for NzO, NO, NOz, CO and COz as a function of time by means of a Fourier Transform Infrared Spectrometer. The experimental variables include char size and loading, inlet oxygen concentration, inlet CO concentration and bed temperature. A single particle reaction-diffusion model was developed first. The detailed chemistry of NO and NsO formation destruction is complex. Homogeneous and heterogeneous reactions considered most pertinent were included in an extension of the single particle char combustion model reported earlier. This single porous char particle model was integrated into a three-phase hydrodynamic description of the fluidized bed reactor. This system model for fluidized bed combustion was developed in response to the observation that most previous system models, including those that incorporate details of NOX emissions, are based on the two-phase theory of fluidization. However, two-phase models are unable to predict the gas back-mixing and the recycle peak in solids-mixing. The non-isothermality of the bed resulting from the gasphase reactions was taken into account through inclusion of an energy balance for the bubble phase. The effect of the variation in superficial gas velocity on bubble properties and cross-flow was included through an overall mass balance. Calculations from the system model, including details of homogeneous NOx reactions far from the char particles, compared well with data on the emissions of CO, COr, NO and NzO for various experimental conditions. The validated model was then used to investigate the influence of operating conditions on the conversions of char-nitrogen to NO and NzO from the simultaneous combustion of char and propane in a fluidized bed with an air/propane mixture as the fluidizing medium. The most significant factors were found to be bed particle size and char diameter.
02lO1636 Modeling on combustion characteristics of biocoal briquettes Kim, H-J. el of. J. Energy Resour. Tech&., 2001, 123, (l), 27-31. A biocoal briquette, a new artificial solid fuel, is manufactured by a mixture of coal and biomass under a high-compression pressure. The combustion characteristics of biocoal briquettes were investigated in this study experimental and numerically. The combustion process of biocoal briquettes appears in two stages: the volatile combustion stage followed by the char combustion stage. It was found that the volatile combustion happens over the whole pellet of biocoal briquette, whereas the char combustion proceeds in a shrinking-core mode. A volume model and a shrinking-core reaction model were introduced and modified here to simulate the two stages of combustion process. The simulation results are found to be consistent with the experimental results.
02/01637 burner
Numerical simulation of a mild combustion
Coelho, P.J. and Peters, N. Comhusfion and Flame, 2001, 124, (3), 503518. A combustor with high air preheating and strong internal exhaust gas recirculation is numerically simulated. The combustor operates in the mild combustion mode, characterized by relatively low flame temperatures, low NO. emissions, no visible flame and no sound. The Eulerian Parttcle Flamelet model is employed to calculate NO in a postprocessing stage. Predictions of the mean and fluctuating velocity components, as well as local residence times, are compared with experimental data. It is found that the predicted profiles reproduce the data reasonably well, but some discrepancies were found. The NO Fuel and Energy Abstracts
May 2002
207
Evaluation of waste pyrolysis characteristics in a pressurized fluidized bed reactor
02/01630
Ono, A. er al. Wasfe Management, 2001, 21, (5). 451-456. To obtain the distribution of fuel components to gas, tar and char in a pressurized fluidized bed waste pyrolyser, experiments were conducted with a laboratory scale fluidized bed reactor. Waste samples were fed batchwise from the top of the reactor into the fluidized bed of silica sand and pyrolysed by nitrogen/nitrogen-Oz gas and the effects of pressure, particle size, heating rate and oxygen addition were investigated. In the case of rubber, the char yield tended to increase a little and the tar yield decrease over the pressure of 304-709 kPa. In comparison with the thermogravimetry data it was clearly demonstrated that the char yield from fluidized bed pyrolysis is much lower. A small amount of oxygen addition decreased both tar and char yields but its further increase did not affect them very much.
02lO1631 Experimental and numerical study of autoignition and pilot ignition of PMMA plates in a cone calorimeter Tsai, T-H. et a/. Combusrion and Flame, 2001, 124, (3), 466-480. This work presents experimental and numerical results for the autoignition and pilot ignition of horizontal black polymethylmethacrylate (PMMA) plates in a cone calorimeter. Experimentally, the histories of plate surface temperature as well as ignition times have been measured for various rediative heating fluxes. Numerically, a twodimensional axisymmetric model is proposed to simulate the transient processes in the gas and the solid plate. The radiative absorption by the gas phase monomer is considered in a simplified way. With a set of parameters for the one-step Arrhenius kinetics for the gas-phase reaction and a set of parameters for PMMA pyrolysis, the numerical results of surface temperatures and ignition times agree well with the experimental data for both autoignition and pilot ignition. Both experimental and numerical results indicate that while the pilot ignition is pyrolysis-controlled, the autoignition process depends not only on PMMA pyrolysis but also on gas-phase reaction, especially for lower heating fluxes. According to the numerical calculations for autoignition, the ignition times of thermally thin plates are significantly shorter than those of thermally thick plates. For pilot ignition, there exists a linear l/Jlia - qrad relation for thermally thick plates, as indicated in the literature.
02iO1632 Features of the high-temperature air combustion technique and its application prospects Liu, M. et a/. Nengyuan Yanjiu Yu Xinxi, 2001, 17, (1). 27-35. (In Chinese) The high temperature air combustion (HTAC) technique was developed in Japan in the early 1990s. This technique can bring about significant fuel savings, low NO, emission, high heat-utilization efficiency and size reduction of the facility. In this paper, the characteristics of the flame temperature distribution and the principles of NO, reduction in the HTAC technique were analysed. The results of the theoretical calculation for the blast furnace gas, coke oven gas and mixed gas (BFG 94.5% by volume and COG 5.5% by volume) applied in the HTAC technique showed that the theoretical combustion temperature increases with the temperatures of preheated air and fuel gas. Low heat-value gas, which cannot be fired in conventional combustion systems, can therefore be used in industrial furnaces by the HTAC technique. In addition, using preheated gas and air can make the exit gas temperature lower and the heat-utilization efficiency higher. In the end, the possibilities of application of the HTAC technique to oil- and gas- fired boilers and coal or combustible solidwaste gasification systems were presented.
OUO1633 ignition and propagation of a reaction front in cross-current bed combustion of wet biofuels Thunman, H. and Leckner, B. Fuel, 2001, 80, (4), 473481. Grate firing is the most common way to burn bio-fuels in small-scale units. Different combustion modes are achieved depending on how fuel and primary air are introduced. In continuous systems fuel and air are usually fed in cross-current and counter-current flow. Here, combustion of wet biofuels is studied in a 31 MW reciprocating grate furnace (a cross-current flow combustor), and additional experiments have been made in batch-fired pot furnaces. The fuel was forest waste with moisture content of approximately 50%. The combustion in a crosscurrent flow furnace is generally assumed to start by ignition on the surface of the bed, followed by a reaction front propagating from the surface down to the grate. Measurements and visual observations presented in this paper show, however, that in the case of wet fuels the
ignition takes place close to the grate, followed by a reaction front propagating from the grate up to the surface of the bed. Hence, the progress combustion in the bed is opposite to the expected one.
02/01634 Low NO. combustion technologies for high temperature applications Flamme, M. Energy Conversion and Managemenr, 2001, 42, (15-17) 1919-1935. Because of the high process temperature and the high temperature to which the combustion air is preheated, NO, emissions from glass melting furnaces are extremely high. Even at these high temperatures, NO, emissions could be reduced drastically by using advanced combustion techniques such as staged combustion or flameless oxidation, as experimental work has shown. In the case of oxy-fuel combustion, the NO, emissions are also very high if conventional burners are used. Staged combustion achieves similar NO, reducti0ns.x reduction techniques
02/01635 Mathematical modeling of fluidized bed combustion. 4: N20 and NOx emissions from the combustion of char Chen, Z. et al. Fuel, 2001, 80, (9), 1259-1272. Batch experiments were conducted to investigate the emissions of carbon oxides and nitrogen oxides from the combustion of char prepared from a commercial coal in a bubbling fluidized bed. Combustion gases escaping from the surface of the bed were sampled and analysed for NzO, NO, NOz, CO and COz as a function of time by means of a Fourier Transform Infrared Spectrometer. The experimental variables include char size and loading, inlet oxygen concentration, inlet CO concentration and bed temperature. A single particle reaction-diffusion model was developed first. The detailed chemistry of NO and NsO formation destruction is complex. Homogeneous and heterogeneous reactions considered most pertinent were included in an extension of the single particle char combustion model reported earlier. This single porous char particle model was integrated into a three-phase hydrodynamic description of the fluidized bed reactor. This system model for fluidized bed combustion was developed in response to the observation that most previous system models, including those that incorporate details of NOX emissions, are based on the two-phase theory of fluidization. However, two-phase models are unable to predict the gas back-mixing and the recycle peak in solids-mixing. The non-isothermality of the bed resulting from the gasphase reactions was taken into account through inclusion of an energy balance for the bubble phase. The effect of the variation in superficial gas velocity on bubble properties and cross-flow was included through an overall mass balance. Calculations from the system model, including details of homogeneous NOx reactions far from the char particles, compared well with data on the emissions of CO, COr, NO and NzO for various experimental conditions. The validated model was then used to investigate the influence of operating conditions on the conversions of char-nitrogen to NO and NzO from the simultaneous combustion of char and propane in a fluidized bed with an air/propane mixture as the fluidizing medium. The most significant factors were found to be bed particle size and char diameter.
02lO1636 Modeling on combustion characteristics of biocoal briquettes Kim, H-J. el of. J. Energy Resour. Tech&., 2001, 123, (l), 27-31. A biocoal briquette, a new artificial solid fuel, is manufactured by a mixture of coal and biomass under a high-compression pressure. The combustion characteristics of biocoal briquettes were investigated in this study experimental and numerically. The combustion process of biocoal briquettes appears in two stages: the volatile combustion stage followed by the char combustion stage. It was found that the volatile combustion happens over the whole pellet of biocoal briquette, whereas the char combustion proceeds in a shrinking-core mode. A volume model and a shrinking-core reaction model were introduced and modified here to simulate the two stages of combustion process. The simulation results are found to be consistent with the experimental results.
02/01637 burner
Numerical simulation of a mild combustion
Coelho, P.J. and Peters, N. Comhusfion and Flame, 2001, 124, (3), 503518. A combustor with high air preheating and strong internal exhaust gas recirculation is numerically simulated. The combustor operates in the mild combustion mode, characterized by relatively low flame temperatures, low NO. emissions, no visible flame and no sound. The Eulerian Parttcle Flamelet model is employed to calculate NO in a postprocessing stage. Predictions of the mean and fluctuating velocity components, as well as local residence times, are compared with experimental data. It is found that the predicted profiles reproduce the data reasonably well, but some discrepancies were found. The NO Fuel and Energy Abstracts
May 2002
207