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01482 Dynamic modelling for simulation and control of a circulating fluidized-bed combustor
09
Combustion
(burners, combustion
systems)
A comparison of the Ahlstrom Pyroflow pressurized circulating fluldlzed bed and competing pressurized bubbling fluldlzed bed combustion technologies
90lO1475
Provol. S. J. and Matousek. W. F. Proc. Int. Conf. Fluid. Bed Combusl.. 1995, i3, (i), 379-390. The two pressurized fluid bed combustion technologies now under development and demonstration [the pressurized circulating fluidized bed (PCFB) and pressurized bubbling fluidized bed (PBFB) 1, show great promise for clean, efficient, and cost effective generation of electricsy in the next century. The market is becoming increasingly competitive, as these technologies are developed and commercially demonstrated, leaving potential customers with many questions regarding the similarities and differences between the two processes. The PBFB system has now achieved significant operational successes, proving pressurized fluidized bed combustion is a commercially viable power generation technology. The alternative PCFB technology, at an earlier stage of commercial development, may offer significant cost and operational advantages when compared with the PBFB. Ahlstrom Pyropower, Inc. (API) has been involved in the development of the Ahlstrom PYROFLOW PCFB technology since 1986. API operates a 10 MW thermal PCFB test facility at its Hans Ahlstrom Laboratory in Karhula, Finland. A project to demonstrate the PCFB system in the 120 MW electricity scale, is in the engineering phase under the DOE Clean Coal III Technologies Programme. That demonstration is scheduled to start operation in 1998. This will provide a market for competing options for users of pressurized fluidized bed systems in the future. This paper will present a brief description of the PCFB process, followed by a comparison of certain key aspects of the PCFB and PBFB technologies. gal01 476 The complex kinetics of carbon burnout Hurt, R. Chemical Physical Processes Cornbust., 1997, 5-12. Carbon burnout kinetics in coal combustion are discussed. Carbon burnout kinetic studies have focused on fuel property distributions, and on the effects of furnace environment on char reactivity modelling of carbon burnout and carbon surface properties of coal ash and ash from co-firing coal and petroleum coke as applied to concrete studies are reviewed.
98101477 Concentration decrease of nitrogen oxides in a case of multistage combustion application : Kipshidze, M. et al. Soobshch. Akad. Nauk Gruz., 1994, 150, (l), 109-111. Multistage combustion was found to significantly decrease formation of nitrogen oxides.
gal01 470 Conditions for complete combustion of solid fuels and improved ash properties
Voina, N. I. et al. Proc. Annu. Int. Pittsburgh Coal Conf., 1995, 12, 789792. The work recommends that the residence time of the coal particles in the flow region of the combustor be extended and the temperature of the ash particle be increased to 10,000 to achieve complete coal combustion and produce ash with no unburned carbon content. This presumes that only lignites with ash melting temperature >lO,OOo”C are used. In addition, finely ground limestone can be incorporated into the combustion chamber to add a calcium component to the ash while simultaneously maintaining sulfur control for decreased air pollution. The added calcium component further increases the properties of the ash for use in cement manufacture.
98101479 Continuous co-firing of coal water slurry Battista, J. J. and Zawadzki, E. A. Proc. Annu. Int. Pittsburgh Coal Conf., 1995, 12, 469-474. Using coal fines recovered from a coal producer and supplied by an independent supplier, low-solids coal-water slurry was continuously cofired with pulverized coal in a utility-scale boiler. The coal-water slurry contains 50 wt% solids and has a target viscosity
98iO1481 Development of pressurized internally circulating fluldized bed combustion technology Nagato, S. et al. Proc. In?. Conf. Fluid. Bed Cornbust., 1995, 13, (l), 645652. In relation to coal fired thermal power plant with low emissions, the paper discusses the configuration of pressurized internally circulating fluidized bed boiler (PICFB), design features, conceptual design of a 350 MW, class PICFB plant, and present and future research plans.
Dynamic modelllng for simulation and control of a circulating fluidfzed-bed combustor
90lO1482
Muir, J. R. et al. MChE J., 1997, 43, (5). 1141-1152. In order to predict the transient behaviour of the temperature, the heat removal rate by the in-bed heat exchanger and the flue-gas oxygen concentration for a circulating fluidized-bed (CFB) combustor, a dynamic model was developed. The model was incorporated into a control simulator to reproduce the combustion process within the overall programme. The behaviour of the combustor under manual or automatic control can be predicted by the model, to allow testing of control strategies. It is validated by comparison with step-response tests carried out on a pilot CFB combustor. Un-modelled disturbances account for discrepancies. Further validation, necessary to ensure the applicability of the simulator to control development, is provided by comparing control models identified experimental using the pilot CFB to those obtained by simulation. Favourable comparison suggests that the dynamic model is suitable for use in control simulation.
98lO1483 The effect of staging of reburning fuel to reduce NO, and NzO levels during fluidized bed coal combustion
Gulyurtlu, I. K. et al. Proc. Int. Conf. Fluid. Bed Combust., 1995, 13, (2), 1351-1357. A study into fluidized bed co-combustion of a coal with straw. The staging of the secondary air was also implemented as a further measure for controlling the emissions of NO, and NsO. The NzO amounts were lowered by about 20 to 40% and NO, values were decreased to below 250 ppm. With coal alone, combustion the levels of NO, and NzO were about 400 and 100 ppm, respectively. No adverse effects on SOz capture were observed. 90101484 Effects of mixing schemes on kerosene combustion In a supersonic airstream
Owens, M. et al. J. Propul. Power, 1997, 13, (4), 525-531. Kerosene combustion in a supersonic vitiated airflow at Mach 4.75 flight enthalpy was conducted in direct-connect tests at Mach 1.8 at a stagnation temperature of 1000 K. The authors evaluated the effects of shock-and vortex-enhanced mixing mechanisms on the combustion efficiency and also the effects of fuel heating and jet penetration. The experimental conditions corresponded to the low end of the hypersonic flight regime. The following geometric configurations were employed: (1) a generic, rearward-facing step, (2) a modified rearward-facing step with bevelled edges to facilitate vortex-enhanced mixing, and (3) a rearward-facing wedge placed downstream of the rearward-facing step to induce shock-enhanced mixing. In all experiments, locally rich stratified kerosene combustion took place in a layer close to the injection wall. The wedge flame holder contributed to an increased kerosene combustion efficiency by the generation of shock-jet interactions. The bevelled-edge step improved far-field mixing, thereby reducing the local kerosene equivalence ratio, resulting in the highest kerosene combustion efficiency among all configurations tested. Fuel heating below levels required for flash vaporization (one-third of the flash vaporization energy, in this case) did not contribute to increased combustion efficiency. On the contrary, this level of heating reduced the fuel density with adverse effects on penetration and mixing.
Effects of the Lewis number of water vapor on the combustion and extinction of methanol drops
98lO1485
fma/“,“;oB. L. and Williams, F. A.
Development of a simple model fuel for kerosene droplet Ignition
gal01 480
Eigenbrod, C. et al. Int. Annu. Conf. ICT, 1997, 28, 42.1-42.14. With regard to droplet combustion, a simple fuel mixture for the complex practical fuel kerosene was developed with special considerations on the two-stage ignition process with cool and hot flame. Spontaneous ignition of fuel droplets was investigated in the temperature range from 550 to 1000 K, at pressures from 0.1 to 2.0 MPa. To detect the appearance of cool and hot flames, the time-dependent temperature field around the droplet was determined by an interferometer. The behaviour of one- and twocomponent model fuels was compared with the ignition characteristics of Jet Al kerosene. The one-component fuel n-dodecane showed a good agreement to kerosene for pressures 20.4 MPa and temperatures >700 K, but the duration and temperature range of the cool flame was not described well. The addition of an aromatic component was necessary to reproduce the chemical behaviour of kerosene. Two-component fuels consisting of the alkanes n-decane or n-dodecane, and up to 40% of the aromatic compounds 1,2,4_trimethylbenzene or 1-methyl-naphthalene were investigated, and the experimental results showed that an important requirement for the mixture was a small difference in the boiling point of the two components. A mixture of 60% n-decane and 40% 1,2,4_trimethylbenzene fulfilled this requirement and gave a good reproduction of the cool and hot flame behaviour of kerosene over the investigated temperature and pressure range.
130
Fuel and Energy Abstracts
March 1998
Combustion and Flame, 1997, 112, (l/2),
The extension of a simplified model for the time-dependent, spherically symmetrical combustion and extinction of methanol droplets incorporates a Lewis number of water vapour that is different from unity. A parametric study revealed that, with water absorption and good mixing in the liquid, the burning time and extinction diameter achieve maxima for water Lewis numbers slightly less than unity in the gas phase. At high Lewis numbers, the smaller diffusion coefficient reduces the amount of water absorbed, and at lower Lewis numbers, the larger diffusion coefficient increases the rate of water loss to the atmosphere, reducing the water concentration at the flame and the water absorbed in the liquid. The extent of absorption, therefore, maximizes at an intermediate Lewis number.
Evaluation of an improved code for the performance of AFBCs
98lo1486
Sel.quk, N. et al. J. Institute of Energy, March 1997, 70, 31-50. Bed and freeboard models were coupled in a previously developed system model for the continuous combustion of lignite particles of widely varied size distributions, burning in their own ash in a fluidized combustor. This was modified to incorporate a procedure for faster computation of particlesize distributions (PSDs) without compromising accuracy, an energy balance on char particles, for determining temperature variation with particle size and an assumption of plug flow for the interstitial gas. The improved computer code was evaluated by being applied to prediction of
Combustion
(burners, combustion
systems)
A comparison of the Ahlstrom Pyroflow pressurized circulating fluldlzed bed and competing pressurized bubbling fluldlzed bed combustion technologies
90lO1475
Provol. S. J. and Matousek. W. F. Proc. Int. Conf. Fluid. Bed Combusl.. 1995, i3, (i), 379-390. The two pressurized fluid bed combustion technologies now under development and demonstration [the pressurized circulating fluidized bed (PCFB) and pressurized bubbling fluidized bed (PBFB) 1, show great promise for clean, efficient, and cost effective generation of electricsy in the next century. The market is becoming increasingly competitive, as these technologies are developed and commercially demonstrated, leaving potential customers with many questions regarding the similarities and differences between the two processes. The PBFB system has now achieved significant operational successes, proving pressurized fluidized bed combustion is a commercially viable power generation technology. The alternative PCFB technology, at an earlier stage of commercial development, may offer significant cost and operational advantages when compared with the PBFB. Ahlstrom Pyropower, Inc. (API) has been involved in the development of the Ahlstrom PYROFLOW PCFB technology since 1986. API operates a 10 MW thermal PCFB test facility at its Hans Ahlstrom Laboratory in Karhula, Finland. A project to demonstrate the PCFB system in the 120 MW electricity scale, is in the engineering phase under the DOE Clean Coal III Technologies Programme. That demonstration is scheduled to start operation in 1998. This will provide a market for competing options for users of pressurized fluidized bed systems in the future. This paper will present a brief description of the PCFB process, followed by a comparison of certain key aspects of the PCFB and PBFB technologies. gal01 476 The complex kinetics of carbon burnout Hurt, R. Chemical Physical Processes Cornbust., 1997, 5-12. Carbon burnout kinetics in coal combustion are discussed. Carbon burnout kinetic studies have focused on fuel property distributions, and on the effects of furnace environment on char reactivity modelling of carbon burnout and carbon surface properties of coal ash and ash from co-firing coal and petroleum coke as applied to concrete studies are reviewed.
98101477 Concentration decrease of nitrogen oxides in a case of multistage combustion application : Kipshidze, M. et al. Soobshch. Akad. Nauk Gruz., 1994, 150, (l), 109-111. Multistage combustion was found to significantly decrease formation of nitrogen oxides.
gal01 470 Conditions for complete combustion of solid fuels and improved ash properties
Voina, N. I. et al. Proc. Annu. Int. Pittsburgh Coal Conf., 1995, 12, 789792. The work recommends that the residence time of the coal particles in the flow region of the combustor be extended and the temperature of the ash particle be increased to 10,000 to achieve complete coal combustion and produce ash with no unburned carbon content. This presumes that only lignites with ash melting temperature >lO,OOo”C are used. In addition, finely ground limestone can be incorporated into the combustion chamber to add a calcium component to the ash while simultaneously maintaining sulfur control for decreased air pollution. The added calcium component further increases the properties of the ash for use in cement manufacture.
98101479 Continuous co-firing of coal water slurry Battista, J. J. and Zawadzki, E. A. Proc. Annu. Int. Pittsburgh Coal Conf., 1995, 12, 469-474. Using coal fines recovered from a coal producer and supplied by an independent supplier, low-solids coal-water slurry was continuously cofired with pulverized coal in a utility-scale boiler. The coal-water slurry contains 50 wt% solids and has a target viscosity
98iO1481 Development of pressurized internally circulating fluldized bed combustion technology Nagato, S. et al. Proc. In?. Conf. Fluid. Bed Cornbust., 1995, 13, (l), 645652. In relation to coal fired thermal power plant with low emissions, the paper discusses the configuration of pressurized internally circulating fluidized bed boiler (PICFB), design features, conceptual design of a 350 MW, class PICFB plant, and present and future research plans.
Dynamic modelllng for simulation and control of a circulating fluidfzed-bed combustor
90lO1482
Muir, J. R. et al. MChE J., 1997, 43, (5). 1141-1152. In order to predict the transient behaviour of the temperature, the heat removal rate by the in-bed heat exchanger and the flue-gas oxygen concentration for a circulating fluidized-bed (CFB) combustor, a dynamic model was developed. The model was incorporated into a control simulator to reproduce the combustion process within the overall programme. The behaviour of the combustor under manual or automatic control can be predicted by the model, to allow testing of control strategies. It is validated by comparison with step-response tests carried out on a pilot CFB combustor. Un-modelled disturbances account for discrepancies. Further validation, necessary to ensure the applicability of the simulator to control development, is provided by comparing control models identified experimental using the pilot CFB to those obtained by simulation. Favourable comparison suggests that the dynamic model is suitable for use in control simulation.
98lO1483 The effect of staging of reburning fuel to reduce NO, and NzO levels during fluidized bed coal combustion
Gulyurtlu, I. K. et al. Proc. Int. Conf. Fluid. Bed Combust., 1995, 13, (2), 1351-1357. A study into fluidized bed co-combustion of a coal with straw. The staging of the secondary air was also implemented as a further measure for controlling the emissions of NO, and NsO. The NzO amounts were lowered by about 20 to 40% and NO, values were decreased to below 250 ppm. With coal alone, combustion the levels of NO, and NzO were about 400 and 100 ppm, respectively. No adverse effects on SOz capture were observed. 90101484 Effects of mixing schemes on kerosene combustion In a supersonic airstream
Owens, M. et al. J. Propul. Power, 1997, 13, (4), 525-531. Kerosene combustion in a supersonic vitiated airflow at Mach 4.75 flight enthalpy was conducted in direct-connect tests at Mach 1.8 at a stagnation temperature of 1000 K. The authors evaluated the effects of shock-and vortex-enhanced mixing mechanisms on the combustion efficiency and also the effects of fuel heating and jet penetration. The experimental conditions corresponded to the low end of the hypersonic flight regime. The following geometric configurations were employed: (1) a generic, rearward-facing step, (2) a modified rearward-facing step with bevelled edges to facilitate vortex-enhanced mixing, and (3) a rearward-facing wedge placed downstream of the rearward-facing step to induce shock-enhanced mixing. In all experiments, locally rich stratified kerosene combustion took place in a layer close to the injection wall. The wedge flame holder contributed to an increased kerosene combustion efficiency by the generation of shock-jet interactions. The bevelled-edge step improved far-field mixing, thereby reducing the local kerosene equivalence ratio, resulting in the highest kerosene combustion efficiency among all configurations tested. Fuel heating below levels required for flash vaporization (one-third of the flash vaporization energy, in this case) did not contribute to increased combustion efficiency. On the contrary, this level of heating reduced the fuel density with adverse effects on penetration and mixing.
Effects of the Lewis number of water vapor on the combustion and extinction of methanol drops
98lO1485
fma/“,“;oB. L. and Williams, F. A.
Development of a simple model fuel for kerosene droplet Ignition
gal01 480
Eigenbrod, C. et al. Int. Annu. Conf. ICT, 1997, 28, 42.1-42.14. With regard to droplet combustion, a simple fuel mixture for the complex practical fuel kerosene was developed with special considerations on the two-stage ignition process with cool and hot flame. Spontaneous ignition of fuel droplets was investigated in the temperature range from 550 to 1000 K, at pressures from 0.1 to 2.0 MPa. To detect the appearance of cool and hot flames, the time-dependent temperature field around the droplet was determined by an interferometer. The behaviour of one- and twocomponent model fuels was compared with the ignition characteristics of Jet Al kerosene. The one-component fuel n-dodecane showed a good agreement to kerosene for pressures 20.4 MPa and temperatures >700 K, but the duration and temperature range of the cool flame was not described well. The addition of an aromatic component was necessary to reproduce the chemical behaviour of kerosene. Two-component fuels consisting of the alkanes n-decane or n-dodecane, and up to 40% of the aromatic compounds 1,2,4_trimethylbenzene or 1-methyl-naphthalene were investigated, and the experimental results showed that an important requirement for the mixture was a small difference in the boiling point of the two components. A mixture of 60% n-decane and 40% 1,2,4_trimethylbenzene fulfilled this requirement and gave a good reproduction of the cool and hot flame behaviour of kerosene over the investigated temperature and pressure range.
130
Fuel and Energy Abstracts
March 1998
Combustion and Flame, 1997, 112, (l/2),
The extension of a simplified model for the time-dependent, spherically symmetrical combustion and extinction of methanol droplets incorporates a Lewis number of water vapour that is different from unity. A parametric study revealed that, with water absorption and good mixing in the liquid, the burning time and extinction diameter achieve maxima for water Lewis numbers slightly less than unity in the gas phase. At high Lewis numbers, the smaller diffusion coefficient reduces the amount of water absorbed, and at lower Lewis numbers, the larger diffusion coefficient increases the rate of water loss to the atmosphere, reducing the water concentration at the flame and the water absorbed in the liquid. The extent of absorption, therefore, maximizes at an intermediate Lewis number.
Evaluation of an improved code for the performance of AFBCs
98lo1486
Sel.quk, N. et al. J. Institute of Energy, March 1997, 70, 31-50. Bed and freeboard models were coupled in a previously developed system model for the continuous combustion of lignite particles of widely varied size distributions, burning in their own ash in a fluidized combustor. This was modified to incorporate a procedure for faster computation of particlesize distributions (PSDs) without compromising accuracy, an energy balance on char particles, for determining temperature variation with particle size and an assumption of plug flow for the interstitial gas. The improved computer code was evaluated by being applied to prediction of