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03193 Inhibition of premixed methane-air flames by fluoroethanes and fluoropropanes
09 cross-section was detected. These measurements indicate the principal usability of the new sensor and a strategy to prevent NO, generation in industrial combustion systems in an early stage of development.
Influence of bubble and emulsion phase on NzO formation in bubbling fluidized bed coal combustion by conditional gas sampling
90l93199
Naruse, 1. et al. Fluid. VIII, Proc. Eng. Found. Conf. Fluid., Sth, 1995 (Pub. 1996) 113-120. Edited by Large, J.-F. and Laguerie, C., Engineering Foundation, New York. It is possible to sample the gas in a bubble without contamination from the emulsion phase under fluidized bed coal combustion conditions. For finer fluidizing particles, the volatile matter evolved from coal burns mainly in the bubble phase, promoting NzO formation. 98103191
The influence of pressure on char combustion
kinetics Croiset, E. et al. Symp. (tnt.) Cornbust., [Proc.]. 1996, 26, (2) 3095-3102. Westerholt bituminous coal semi-coke was used in fixed-bed combustion experiments carried out at 0.2, 0.6, and 1.0 MPa and 850-1200 K. The rate constants were first determined with the so-called ‘nth-order’ overall expression of the combustion rate. A simplified heterogeneous reaction mechanism including a two-stage adsorption-desorption process was proposed to understand the influence of the total pressure on the adsorption and desorption reactions. The total pressure had no effect on the desorption rate constants and because combustion is controlled by the adsorption of oxygen, the adsorption rate constant appeared very similar to the overall rate constant. Increasing total pressure up to 0.6 MPa decreased the rate constants if the oxygen ‘concentration’ is expressed as the oxygen partial pressure (r = kP&). If the oxygen concentration is expressed as the oxygen mole fraction (r=ky’&), there is only a weak effect of the total pressure on the rate constant. Increased pressure also decreases the transition temperature from a kinetics-controlled regime to a regime cocontrolled by pore diffusion. To clarify the effect of pressure on combustion, a Langmuir adsorption isotherm was chosen and the adsorption constant was determined at 0.2, 0.6, and 1.0 MPa. The Langmuir adsorption isotherm was particularly suitable when working at elevated pressure. 99103192
Influence of pressure on the combustion rate of
carbon Essenhigh, R. H. and Mescher, A. M. Symp. (Int.) Combust., /Proc./, 1996, 26, (2). 3085-3094. When predictions were compared with actual experimental results, the influence of pressure on the combustion rates of carbon (or coal) particles was shown to be zero to minor in the temperature range studied. This contradicts the empirical (nth order) assumption widely adopted in much of the literature that predicts a substantial pressure dependence at all temperatures Two models were used in the comparison, and the results were compared with three independent experimental sets of data. These experiments were measurements of burning times of single coal particles by Tidona at I, 1.5, and 2 atm; reaction rates of char particles by Monson et al. at I, 5, 10, and I5 atm; and (non-critical) ignition temperatures of coal particles in the pressure range 0.4-1.7 atm. The first model was based on the fundamental Langmuir-Nusselt-Thiele suite of theoretical equations in the form of the extended resistance equation (ERE). The second model combined the Nusselt BLD analysis with the empirical nth order assumption that the reaction rate at all temperatures is proportional to the nth power of the partial pressure of the oxygen concentration (p”,). It is concluded that the empirical model has no experimental support for the assumptions made and that fundamentally based equations can be developed or already exist that can be used.to predict carbon combustion reaction rates at elevated or reduced pressure with acceptable confidence.
Inhibition of premixed fluoroethanes and fluoropropanes
99103193
methane-air
flames by
Linteris, G. T. et al. Comhusrion and Flame, 1998, 113, (l/2), 164-180. Results from experimentation and modelling of laminar premixed methane-air flames inhibited by the fluoroethanes CzF6, CzHFs, and CZH2F4 and experimental results for the fluoropropanes C3Fs and C3HF7 are detailed. Good agreement is found between the modelling results and the measurements with respect to reproducing flame speeds. For the fluoroethanes, calculated flame structures were used to determine the reaction pathways for inhibitor decomposition and the mechanisms of inhibition, as well as to explain the enhanced soot formation observed for the inhibitors CzHFs, CzHzFd, and CsHF7. The burning velocity of rich and stoichiometric flames was reduced by the agents, primarily by raising the effective equivalence ratio and lowering the adiabatic flame temperature. For lean flames, the inhibition is primarily kinetic, since inhibitor reactions help to maintain the final temperature. The peak radical concentrations are reduced beyond that due to the temperature effect through reactions of fluorinated species with radicals.
Kinetic behavior of solid particle in chemical-loop ing combustion: suppressing carbon deposition in reduction
98103194
Ishida, M. ef al. Energy Fuels, 1998, 12, (2), 223-229. Carbon deposition on the solid particle is one of the key problems when applying chemical-looping combustion to a practical power plant. Six kinds of solid particles were examined to clarify the kinetic behaviour of carbon
Combustion (burners, combustion systems)
deposition. The effects of the solid composition, feed gas composition and reaction temperature on carbon deposition were investigated by thermogravimetric reactor on the basis of NiOlYSZ particle. The particle of NiO mixed with YSZ was a good candidate for chemical-looping combustion, both from a reactivity and a carbon resistance viewpoint. Carbon deposition could be completely avoided with very low concentration of water vapour. A proposed model was used to identify the conditions for avoiding carbon deposition. A kinetic model of carbon burnout in pulverized coal combustion
98/03195
Hurt, R. et al. Combusrion and Flame, 1998, 113, (l/2), 181-197. In full-scale suspension-fired coal combustion systems, the degree of carbon burnout is an important operating characteristic. Prediction of carbon loss requires special char combustion kinetics valid through the very high conversions targeted in industry and valid for a wide range of particle temperature histories occurring in full-scale furnaces. High-temperature kinetic data are presented for five coal chars in the form of time-resolved burning profiles that include the late stages of combustion. The development and validation of the carbon burnout kinetic model (CBK), a coalgeneral kinetics package that is specifically designed to predict the total extent of carbon burnout and ultimate fly ash carbon content for prescribed temperature/oxygen histories typical of pulverized coal combustion systems, is then detailed. Combined in the model are the single-film treatment of char oxidation with quantitative descriptions of thermal annealing, statistical kinetics, statistical densities and ash inhibition in the late stages of combustion. The predictions of the model are outlined. A laboratory study on the NO, NOz, SOz, CO and CO* emissions from the combustion of pulverized coal, municipal waste plastics and tires
99lO3196
Courtemanche, B. and Levendis, Y. A. Fuel, 1998, 77, (3) 183-196. The combustion emissions from pulverized solid fuels are studied: NO,, SOz, CO and CO*. Coal, waste tyre crumb and waste plastics were burned in an electrically heated drop-tube furnace at high particle heating rates (104-105 K ss’) and elevated gas temperatures (1300-1600 K). The fuel to air bulk equivalence ratio, d, was varied in the range of 0.4-1.8. Air or a nitrogen-free mixture of OZ-CO*-Ar were used as oxidizing gases. Results showed that fuels which contain nitrogen generated the highest NO, emissions. Coal combustion generated four times more NO, than combustion of tyre crumb, in proportion to their nitrogen content, and ten times more NO, than that of the nitrogen-free plastics. The specific NO, emissions decreased dramatically (3-6 times) with increasing bulk equivalence ratio for all fuels. However, the NOziNO ratio increased with the equivalence ratio in the fuel-rich region. Depending on the fuel, an increase in the gas temperature, between 1300 and 1600 K, resulted in lo25% more NO,. Atmospheric nitrogen contributed 20% of the total NO, emissions for coal, 30% for tyres and 100% for the plastics. SO? emissions of the particular coal and tyre crumb tested were comparable. While absolute SOz emissions of coal and tyre increased with the equivalence ratio, specific emissions exhibited a mild downward trend. SOz emissions were higher in the absence of atmospheric nitrogen, especially at fuel-rich conditions. CO emissions were mild in the fuel-lean regions, but they increased exponentially in the fuel-rich region accounting for as much as 10% of the carbon at d = 2. Overall, the CO? emissions were proportional to the carbon content of the fuels. 99103197
Low-emission vortex combustion of biomass and
fossil fuel Finker, F. Z. ef al. Proc. Biomass Conf Am.: Energy, Environ., Znd, 1995,480-482.
Agric. Ind.,
The results of development and industrial experience of low-emission vortex combustion technology (LEVC) of biomass and fossil fuel in industrial and utility boilers in the Russian timber and paper industries and Polish power plants are introduced. The LEVC technology is based on the aerodynamics method of multiple circulation of gases and fuel in the furnaces, LEVC technology accumulates the advantages of conventional and fluidized bed combustion technology. Existing boilers could be easily retrofitted for the application of LEVC technology without requiring major investment. The repowering of boiler with LEVC was the result the reduction NO, emission to the level 170 g/GJ without installation additional flue gas cleaning equipment and it gave the opportunity for an injection of sulfur sorbent in the furnace. A Russian-Polish experiment on a utility boiler retrofitted with the application of LEVC is discussed: the boiler efficiency increased by 2% and NO, and SOz emissions reductions were 40 and 17%, respectively.
Mathematical pulverized coal flame
98103198
modelling
of a 2.4 MW swirling
Peters, A. A. F. and Weber, R. Combust. 5%. Technof., 1997, 122, (l-6) 131-182. The paper presents predictions and measurements of a swirling unstaged, high NO,, pulverized coal flame. Relatively simple models for turbulence, turbulent combustion and turbulent nitric oxide chemistry were used for the predictions. Turbulence is modelled using the standard k-epsilon model. The turbulent combustion model incorporates a two-step reaction scheme together with an eddy break-up model. In the NO-chemical model, thermalNO and fuel-NO chemical reaction rates are statistically averaged over the fluctuating temperature using the Beta probability density function.
Fuel and Energy Abstracts
July 1998 295
Influence of bubble and emulsion phase on NzO formation in bubbling fluidized bed coal combustion by conditional gas sampling
90l93199
Naruse, 1. et al. Fluid. VIII, Proc. Eng. Found. Conf. Fluid., Sth, 1995 (Pub. 1996) 113-120. Edited by Large, J.-F. and Laguerie, C., Engineering Foundation, New York. It is possible to sample the gas in a bubble without contamination from the emulsion phase under fluidized bed coal combustion conditions. For finer fluidizing particles, the volatile matter evolved from coal burns mainly in the bubble phase, promoting NzO formation. 98103191
The influence of pressure on char combustion
kinetics Croiset, E. et al. Symp. (tnt.) Cornbust., [Proc.]. 1996, 26, (2) 3095-3102. Westerholt bituminous coal semi-coke was used in fixed-bed combustion experiments carried out at 0.2, 0.6, and 1.0 MPa and 850-1200 K. The rate constants were first determined with the so-called ‘nth-order’ overall expression of the combustion rate. A simplified heterogeneous reaction mechanism including a two-stage adsorption-desorption process was proposed to understand the influence of the total pressure on the adsorption and desorption reactions. The total pressure had no effect on the desorption rate constants and because combustion is controlled by the adsorption of oxygen, the adsorption rate constant appeared very similar to the overall rate constant. Increasing total pressure up to 0.6 MPa decreased the rate constants if the oxygen ‘concentration’ is expressed as the oxygen partial pressure (r = kP&). If the oxygen concentration is expressed as the oxygen mole fraction (r=ky’&), there is only a weak effect of the total pressure on the rate constant. Increased pressure also decreases the transition temperature from a kinetics-controlled regime to a regime cocontrolled by pore diffusion. To clarify the effect of pressure on combustion, a Langmuir adsorption isotherm was chosen and the adsorption constant was determined at 0.2, 0.6, and 1.0 MPa. The Langmuir adsorption isotherm was particularly suitable when working at elevated pressure. 99103192
Influence of pressure on the combustion rate of
carbon Essenhigh, R. H. and Mescher, A. M. Symp. (Int.) Combust., /Proc./, 1996, 26, (2). 3085-3094. When predictions were compared with actual experimental results, the influence of pressure on the combustion rates of carbon (or coal) particles was shown to be zero to minor in the temperature range studied. This contradicts the empirical (nth order) assumption widely adopted in much of the literature that predicts a substantial pressure dependence at all temperatures Two models were used in the comparison, and the results were compared with three independent experimental sets of data. These experiments were measurements of burning times of single coal particles by Tidona at I, 1.5, and 2 atm; reaction rates of char particles by Monson et al. at I, 5, 10, and I5 atm; and (non-critical) ignition temperatures of coal particles in the pressure range 0.4-1.7 atm. The first model was based on the fundamental Langmuir-Nusselt-Thiele suite of theoretical equations in the form of the extended resistance equation (ERE). The second model combined the Nusselt BLD analysis with the empirical nth order assumption that the reaction rate at all temperatures is proportional to the nth power of the partial pressure of the oxygen concentration (p”,). It is concluded that the empirical model has no experimental support for the assumptions made and that fundamentally based equations can be developed or already exist that can be used.to predict carbon combustion reaction rates at elevated or reduced pressure with acceptable confidence.
Inhibition of premixed fluoroethanes and fluoropropanes
99103193
methane-air
flames by
Linteris, G. T. et al. Comhusrion and Flame, 1998, 113, (l/2), 164-180. Results from experimentation and modelling of laminar premixed methane-air flames inhibited by the fluoroethanes CzF6, CzHFs, and CZH2F4 and experimental results for the fluoropropanes C3Fs and C3HF7 are detailed. Good agreement is found between the modelling results and the measurements with respect to reproducing flame speeds. For the fluoroethanes, calculated flame structures were used to determine the reaction pathways for inhibitor decomposition and the mechanisms of inhibition, as well as to explain the enhanced soot formation observed for the inhibitors CzHFs, CzHzFd, and CsHF7. The burning velocity of rich and stoichiometric flames was reduced by the agents, primarily by raising the effective equivalence ratio and lowering the adiabatic flame temperature. For lean flames, the inhibition is primarily kinetic, since inhibitor reactions help to maintain the final temperature. The peak radical concentrations are reduced beyond that due to the temperature effect through reactions of fluorinated species with radicals.
Kinetic behavior of solid particle in chemical-loop ing combustion: suppressing carbon deposition in reduction
98103194
Ishida, M. ef al. Energy Fuels, 1998, 12, (2), 223-229. Carbon deposition on the solid particle is one of the key problems when applying chemical-looping combustion to a practical power plant. Six kinds of solid particles were examined to clarify the kinetic behaviour of carbon
Combustion (burners, combustion systems)
deposition. The effects of the solid composition, feed gas composition and reaction temperature on carbon deposition were investigated by thermogravimetric reactor on the basis of NiOlYSZ particle. The particle of NiO mixed with YSZ was a good candidate for chemical-looping combustion, both from a reactivity and a carbon resistance viewpoint. Carbon deposition could be completely avoided with very low concentration of water vapour. A proposed model was used to identify the conditions for avoiding carbon deposition. A kinetic model of carbon burnout in pulverized coal combustion
98/03195
Hurt, R. et al. Combusrion and Flame, 1998, 113, (l/2), 181-197. In full-scale suspension-fired coal combustion systems, the degree of carbon burnout is an important operating characteristic. Prediction of carbon loss requires special char combustion kinetics valid through the very high conversions targeted in industry and valid for a wide range of particle temperature histories occurring in full-scale furnaces. High-temperature kinetic data are presented for five coal chars in the form of time-resolved burning profiles that include the late stages of combustion. The development and validation of the carbon burnout kinetic model (CBK), a coalgeneral kinetics package that is specifically designed to predict the total extent of carbon burnout and ultimate fly ash carbon content for prescribed temperature/oxygen histories typical of pulverized coal combustion systems, is then detailed. Combined in the model are the single-film treatment of char oxidation with quantitative descriptions of thermal annealing, statistical kinetics, statistical densities and ash inhibition in the late stages of combustion. The predictions of the model are outlined. A laboratory study on the NO, NOz, SOz, CO and CO* emissions from the combustion of pulverized coal, municipal waste plastics and tires
99lO3196
Courtemanche, B. and Levendis, Y. A. Fuel, 1998, 77, (3) 183-196. The combustion emissions from pulverized solid fuels are studied: NO,, SOz, CO and CO*. Coal, waste tyre crumb and waste plastics were burned in an electrically heated drop-tube furnace at high particle heating rates (104-105 K ss’) and elevated gas temperatures (1300-1600 K). The fuel to air bulk equivalence ratio, d, was varied in the range of 0.4-1.8. Air or a nitrogen-free mixture of OZ-CO*-Ar were used as oxidizing gases. Results showed that fuels which contain nitrogen generated the highest NO, emissions. Coal combustion generated four times more NO, than combustion of tyre crumb, in proportion to their nitrogen content, and ten times more NO, than that of the nitrogen-free plastics. The specific NO, emissions decreased dramatically (3-6 times) with increasing bulk equivalence ratio for all fuels. However, the NOziNO ratio increased with the equivalence ratio in the fuel-rich region. Depending on the fuel, an increase in the gas temperature, between 1300 and 1600 K, resulted in lo25% more NO,. Atmospheric nitrogen contributed 20% of the total NO, emissions for coal, 30% for tyres and 100% for the plastics. SO? emissions of the particular coal and tyre crumb tested were comparable. While absolute SOz emissions of coal and tyre increased with the equivalence ratio, specific emissions exhibited a mild downward trend. SOz emissions were higher in the absence of atmospheric nitrogen, especially at fuel-rich conditions. CO emissions were mild in the fuel-lean regions, but they increased exponentially in the fuel-rich region accounting for as much as 10% of the carbon at d = 2. Overall, the CO? emissions were proportional to the carbon content of the fuels. 99103197
Low-emission vortex combustion of biomass and
fossil fuel Finker, F. Z. ef al. Proc. Biomass Conf Am.: Energy, Environ., Znd, 1995,480-482.
Agric. Ind.,
The results of development and industrial experience of low-emission vortex combustion technology (LEVC) of biomass and fossil fuel in industrial and utility boilers in the Russian timber and paper industries and Polish power plants are introduced. The LEVC technology is based on the aerodynamics method of multiple circulation of gases and fuel in the furnaces, LEVC technology accumulates the advantages of conventional and fluidized bed combustion technology. Existing boilers could be easily retrofitted for the application of LEVC technology without requiring major investment. The repowering of boiler with LEVC was the result the reduction NO, emission to the level 170 g/GJ without installation additional flue gas cleaning equipment and it gave the opportunity for an injection of sulfur sorbent in the furnace. A Russian-Polish experiment on a utility boiler retrofitted with the application of LEVC is discussed: the boiler efficiency increased by 2% and NO, and SOz emissions reductions were 40 and 17%, respectively.
Mathematical pulverized coal flame
98103198
modelling
of a 2.4 MW swirling
Peters, A. A. F. and Weber, R. Combust. 5%. Technof., 1997, 122, (l-6) 131-182. The paper presents predictions and measurements of a swirling unstaged, high NO,, pulverized coal flame. Relatively simple models for turbulence, turbulent combustion and turbulent nitric oxide chemistry were used for the predictions. Turbulence is modelled using the standard k-epsilon model. The turbulent combustion model incorporates a two-step reaction scheme together with an eddy break-up model. In the NO-chemical model, thermalNO and fuel-NO chemical reaction rates are statistically averaged over the fluctuating temperature using the Beta probability density function.
Fuel and Energy Abstracts
July 1998 295