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02065 Interactions between coal and biomass when cofiring
70
pulverized coal flames was used for the extrapolation. Constant velocity and constant residence time-scaling criteria were employed. When the prototype experiments are carried out at thermal inputs larger than 4 MW, the NO, emissions are representative of full industrial-scale applications. The scaling can be successfully conducted using either the constant velocity or the constant residence time principal. When the prototype experiments are carried out at thermal inputs lower than 2-3 MW, the NO emissions decrease with thermal input. This decrease was attributed to the solid-phase aerodynamics. To obtain identical NO emissions in prototype experiments carried out at a thermal input lower than 1 MW, it is recommended to use the constant residence time scaling, and mill the pulverized fuel more finely.
Shock-tube and modeling study of ethane pyrolysis and oxidation 00102061
Hidaka, Y. Comhu.sfiort trrrd f/ucrme. 2000. 120, (3). 245-264. Pyrolysis and oxidation of ethane were studied behind reflected shock waves in the temperature range 950-1900 K at pressures of 1.2-4.0 atm. Ethane decay rates in both pyrolysis and oxidation were measured using time-resolved infrared (IR) laser absorption at 3.39 /Lrn, and CO2 production rates in oxidation were measured by time-resolved thermal IR emission at 4.24 /trn. The product yields were also determined using a single-pulse method. The pyrolysis and oxidation of ethane were modelled using a reaction mechanism with 157 reaction steps and 48 species including the most recent submechanisms for formaldehyde, ketene, methane, acetylene, and ethylene oxidation. The present and previously reported shock tube data were reproduced using this mechanism. The rate constants of the reactions CzH, --* CH3 + CHs, C2Hs + H -) CzHJ + Hz and CzHs + Oz -+ C2HI + HOz were evaluated. These reactions were important in predicting the previously reported and the present data, which were for mixture compositions ranging from ethane-rich (including ethane pyrolysis) to ethane-lean. The evaluated rate constants of the reactions CzHs + H -+ CzH4 + Hz and CzHs + 02 + CzH4 + HO2 were found to be significantly different from currently accepted values.
Simplification of the mechanism of NO, formation in a CHJair combustion system
00102062
Xu, M. f,rr. J. 01‘ Encrgr Rc.F.. 1999. 23. ( 14). 1267-1276. The mechanism of detailed chemical reactions in combustion is very complicated, especially when the formation of pollutant such as NO, is considered. Based on Glarborg’s and Bilger’s full model (Glarborg P. er crl.. Comhusrion ttntl Fltmtu, 65. 177 (1986). Bilger R. W.et al. Comhwtio~t ant/ Fhme. 80, 135 (1990)), a simplified model of the mechanism of NO, formation in a CH.,/air system is presented. Sensitivity calculation and eigenvalue-eigenvector analysis (principal component analysis (Vajda. S. ct ul. ht. J. Chentid Kitwtkv, 17:55 (1985)) are used in the model. Compared with the conventional methods of simplification of chemical reaction mechanisms such as quasi-steady-state approximation. the eigenvalueeigenvector analysis can indicate the important reactions efficiently without providing any balance hypothesis. The new model consists of only I5 elementary reactions. which is simpler than the full model composing 39 reactions. The deviations in NO, concentration and other major species between the simplified and the full model are rather small over a wide range of fuel-air ratto. Meanwhile. the calculated NO, concentrations by the simplified model are in quite good agreement with Bartok’s experimental data (Bartok W. et ol.. .4lCHE S,wtp. Ser. 126 (1972)). especially in the fuellean region.
Temperature measurements in steady twodimensional partially premixed flames using laser intetferometric holography
00102063
Xiao, X. Cortthrrstiort cttttlFkmw. 2000. 120. (3). 318.-332. The local flow temperature is a key consequence of combustion and must, therefore, be accurately measured. Holographic interferometry can be employed to accurately determine the refractive index in flames and, thereafter, to infer the temperature distribution. This investigation focuses on the utility of laser interferometric holography as a tool to measure the temperature of two-dimensional partially premixed flames (PPFs). Methane-air PPFs are established on a rectangular Wolfhard-Parker slot burner. These flames contain two reaction zones, one in an outer nonpremixed region and the other in an inner rich-premixed region. We examine flame structure effects (that produce a varying composition in the flame) on the local refractive index and show that a relation that contains relatively minor errors, which depend upon the rich-side equivalence ratio, can model the refractive index in PPFs. This is the first investigation to discuss the effects of a realistically varying composition due to combustion on the refractive index distribution in flames. The maximum error in the temperature is 6-34% when the rich-side equivalence ratio lies in the range 1.5-w, while the corresponding average error is 2.4-12.3%. Relatively large discrepancies arise in the case of non-premixed flames. We discuss the experimental configurations required to reconstruct clear interferometric fringe patterns. Image plane holography is employed because it offers two advantages: (1) the holograms can be read with white light, and (2) the fringe count may be considered to occur in a straight line as though no refraction had occurred. We confirm the inefficacy of using thermocouples as a measurement tool in two-dimensional PPFs. Questions related to the beam path length are resolved. The portion of the flame-beam interference length that lies between the high and low temperatures are accounted for by assuming a uniform composition and by introducing a density weighting
Engines
(power generation
and propulsion)
function. Thereafter, we examine the holographtc fringe patterns, discuss the discrepancies that arise in the interpreted temperatures as the PPF equivalence ratio is varied and present the inferred temperature distributions. The temperature distribution is found to correlate with the heat release jn the inner rich premixed and outer non-premixed reaction zones. Large temperature gradients exist in the inner premixed reaction zone, whereas the outer non-premixed region contains smaller gradients, since it is transport limited. The spatial temperature resolution required to resolve the higher gradients is z 400 k mm-‘.
The effects of temperature, mixing and volatile release on NO reduction mechanisms by coal reburning
00/02064
Yang, Y. B. Srmp. f1ttt.j Cmhtrst.. [Pvoc.]. 199X. 2. 3009-3017. An experimental investigation into the effects of mixing, temperature and devolatilization rates on nitrogen oxide by coal reburning is presented. A 0.2-MW pilot-scale furnace is used in the study. A mathematical model was used to analyse the sensitivity of the reburning process to the devolatilization hehaviour and mixing rates of the reburn coal. The model considers the major NO reduction pathways relevant to reburn using a global chemical approach, namely, hydrocarbon fragments, volatile fuel-nitrogen species, and heterogeneous NO reduction by char particles. It is evident from both the predicted and experimental results that increased volatile yields and faster devolatilization rates can lead to improved NO reduction in the reburn zone provided that the reburn zone is operated fuel rich so as to limit the consumption of volatile gases by oxygen. As reburn zone conditions become increasingly leaner. rapid coal pyrolysis results in the earlier consumption of volatile gases by oxygen as mixing takes place, leading to less NO reduction. Less efficient NO reduction is achieved with increasing temperature, because the rate of coal devolatilization near the reburn coals injectors is increased. The magnitude of this effect was found to vary with the coal type. Consequently, while it is known that rank plays a significant role in determining the effectiveness of a coal as a reburn fuel, the results presented also show that simple rank indicators such as proximate volatile yield can only provide an initial indication of the suitability of a particular coal as a reburn fuel. Of further significance to the reburning process is the specific coal pyrolysis behaviour, which is controlled by the process temperature and heating rate and can vary between coals of similar rank but different morphology. The model predictions for coal reburn indicate that the reduction of NO hy CH fragments is the dominant NO reduction mechanism in the rehurn zone. accounting for virtually 70% of the observed total reduction.
00/02065
lnteractlons between coal and biomass when cofiring
Robinson, A. L. Srmp. (1nt.i Cotnhwt., /Proc.]. 1998 27th. I. 1351~1359. Combustion Institute. The most promising short-term options for reducing the net COZ emissions from coal-fired power plants is to co-fire the coal with biomass. This paper examines the effects of co-firing biomass and coal on ash deposition, corrosion, and NO, emissions under conditions representative of those found in pulverized-coal (PC) boilers. Experiments were conducted with blends of six different fuels, two types of bituminous coal, two types of straw, switch-grass and wood. The deposition rates for the hlends of fuel\ considered by this investigation lay between the measured deposition rates of the unblended fuels. Therefore, co-firing can mitigate some of the fouling difficulties associated with high-fouling coals and biofuels. By accounting for factors other than fuel composition, we can improve our predictions of the effects of co-firing on ash deposition rates, For the fuel blends examined here, available alkali is a relatively good predictor for the ash deposition rate. During the co-firing process sulfation of alkali chlorides is the major interaction that occurs within forming deposits. Sulfation reduces the chlorine content of the deposits, thus, reducing their corrosion potential. Co-firing biomass can decrease NO, emissions relative to firing unblended coal under typical pc-combustion conditions when the biomass fuel contains little nitrogen. There does not appear to he any fundamental synergistic interaction between the coal and biomass that significantly reduces NO, emissions. The test results show that co-firing biomass with coal can result in a decreased in pollutant production, ash deposition and effective C02.
10 ENGINES Power generation and propulsion 00102066 A feasibility study of an alternative power generation system based on biomass gasification/gas turblne concept De Souza, S. and Marcia, L. Ftd, 1999. 78. (5). 5299538. A newly proposed power generating system based on atmospheric gasification of sugar cane bagasse combined with fluidized-bed heat exchanging and gas turbine has been experimentally studied. The technology’s thermodynamic viability has been investigated under imposed conditions; several configurations of the power unit have been tried until
Fuel and Energy Abstracts
July
2000
229
pulverized coal flames was used for the extrapolation. Constant velocity and constant residence time-scaling criteria were employed. When the prototype experiments are carried out at thermal inputs larger than 4 MW, the NO, emissions are representative of full industrial-scale applications. The scaling can be successfully conducted using either the constant velocity or the constant residence time principal. When the prototype experiments are carried out at thermal inputs lower than 2-3 MW, the NO emissions decrease with thermal input. This decrease was attributed to the solid-phase aerodynamics. To obtain identical NO emissions in prototype experiments carried out at a thermal input lower than 1 MW, it is recommended to use the constant residence time scaling, and mill the pulverized fuel more finely.
Shock-tube and modeling study of ethane pyrolysis and oxidation 00102061
Hidaka, Y. Comhu.sfiort trrrd f/ucrme. 2000. 120, (3). 245-264. Pyrolysis and oxidation of ethane were studied behind reflected shock waves in the temperature range 950-1900 K at pressures of 1.2-4.0 atm. Ethane decay rates in both pyrolysis and oxidation were measured using time-resolved infrared (IR) laser absorption at 3.39 /Lrn, and CO2 production rates in oxidation were measured by time-resolved thermal IR emission at 4.24 /trn. The product yields were also determined using a single-pulse method. The pyrolysis and oxidation of ethane were modelled using a reaction mechanism with 157 reaction steps and 48 species including the most recent submechanisms for formaldehyde, ketene, methane, acetylene, and ethylene oxidation. The present and previously reported shock tube data were reproduced using this mechanism. The rate constants of the reactions CzH, --* CH3 + CHs, C2Hs + H -) CzHJ + Hz and CzHs + Oz -+ C2HI + HOz were evaluated. These reactions were important in predicting the previously reported and the present data, which were for mixture compositions ranging from ethane-rich (including ethane pyrolysis) to ethane-lean. The evaluated rate constants of the reactions CzHs + H -+ CzH4 + Hz and CzHs + 02 + CzH4 + HO2 were found to be significantly different from currently accepted values.
Simplification of the mechanism of NO, formation in a CHJair combustion system
00102062
Xu, M. f,rr. J. 01‘ Encrgr Rc.F.. 1999. 23. ( 14). 1267-1276. The mechanism of detailed chemical reactions in combustion is very complicated, especially when the formation of pollutant such as NO, is considered. Based on Glarborg’s and Bilger’s full model (Glarborg P. er crl.. Comhusrion ttntl Fltmtu, 65. 177 (1986). Bilger R. W.et al. Comhwtio~t ant/ Fhme. 80, 135 (1990)), a simplified model of the mechanism of NO, formation in a CH.,/air system is presented. Sensitivity calculation and eigenvalue-eigenvector analysis (principal component analysis (Vajda. S. ct ul. ht. J. Chentid Kitwtkv, 17:55 (1985)) are used in the model. Compared with the conventional methods of simplification of chemical reaction mechanisms such as quasi-steady-state approximation. the eigenvalueeigenvector analysis can indicate the important reactions efficiently without providing any balance hypothesis. The new model consists of only I5 elementary reactions. which is simpler than the full model composing 39 reactions. The deviations in NO, concentration and other major species between the simplified and the full model are rather small over a wide range of fuel-air ratto. Meanwhile. the calculated NO, concentrations by the simplified model are in quite good agreement with Bartok’s experimental data (Bartok W. et ol.. .4lCHE S,wtp. Ser. 126 (1972)). especially in the fuellean region.
Temperature measurements in steady twodimensional partially premixed flames using laser intetferometric holography
00102063
Xiao, X. Cortthrrstiort cttttlFkmw. 2000. 120. (3). 318.-332. The local flow temperature is a key consequence of combustion and must, therefore, be accurately measured. Holographic interferometry can be employed to accurately determine the refractive index in flames and, thereafter, to infer the temperature distribution. This investigation focuses on the utility of laser interferometric holography as a tool to measure the temperature of two-dimensional partially premixed flames (PPFs). Methane-air PPFs are established on a rectangular Wolfhard-Parker slot burner. These flames contain two reaction zones, one in an outer nonpremixed region and the other in an inner rich-premixed region. We examine flame structure effects (that produce a varying composition in the flame) on the local refractive index and show that a relation that contains relatively minor errors, which depend upon the rich-side equivalence ratio, can model the refractive index in PPFs. This is the first investigation to discuss the effects of a realistically varying composition due to combustion on the refractive index distribution in flames. The maximum error in the temperature is 6-34% when the rich-side equivalence ratio lies in the range 1.5-w, while the corresponding average error is 2.4-12.3%. Relatively large discrepancies arise in the case of non-premixed flames. We discuss the experimental configurations required to reconstruct clear interferometric fringe patterns. Image plane holography is employed because it offers two advantages: (1) the holograms can be read with white light, and (2) the fringe count may be considered to occur in a straight line as though no refraction had occurred. We confirm the inefficacy of using thermocouples as a measurement tool in two-dimensional PPFs. Questions related to the beam path length are resolved. The portion of the flame-beam interference length that lies between the high and low temperatures are accounted for by assuming a uniform composition and by introducing a density weighting
Engines
(power generation
and propulsion)
function. Thereafter, we examine the holographtc fringe patterns, discuss the discrepancies that arise in the interpreted temperatures as the PPF equivalence ratio is varied and present the inferred temperature distributions. The temperature distribution is found to correlate with the heat release jn the inner rich premixed and outer non-premixed reaction zones. Large temperature gradients exist in the inner premixed reaction zone, whereas the outer non-premixed region contains smaller gradients, since it is transport limited. The spatial temperature resolution required to resolve the higher gradients is z 400 k mm-‘.
The effects of temperature, mixing and volatile release on NO reduction mechanisms by coal reburning
00/02064
Yang, Y. B. Srmp. f1ttt.j Cmhtrst.. [Pvoc.]. 199X. 2. 3009-3017. An experimental investigation into the effects of mixing, temperature and devolatilization rates on nitrogen oxide by coal reburning is presented. A 0.2-MW pilot-scale furnace is used in the study. A mathematical model was used to analyse the sensitivity of the reburning process to the devolatilization hehaviour and mixing rates of the reburn coal. The model considers the major NO reduction pathways relevant to reburn using a global chemical approach, namely, hydrocarbon fragments, volatile fuel-nitrogen species, and heterogeneous NO reduction by char particles. It is evident from both the predicted and experimental results that increased volatile yields and faster devolatilization rates can lead to improved NO reduction in the reburn zone provided that the reburn zone is operated fuel rich so as to limit the consumption of volatile gases by oxygen. As reburn zone conditions become increasingly leaner. rapid coal pyrolysis results in the earlier consumption of volatile gases by oxygen as mixing takes place, leading to less NO reduction. Less efficient NO reduction is achieved with increasing temperature, because the rate of coal devolatilization near the reburn coals injectors is increased. The magnitude of this effect was found to vary with the coal type. Consequently, while it is known that rank plays a significant role in determining the effectiveness of a coal as a reburn fuel, the results presented also show that simple rank indicators such as proximate volatile yield can only provide an initial indication of the suitability of a particular coal as a reburn fuel. Of further significance to the reburning process is the specific coal pyrolysis behaviour, which is controlled by the process temperature and heating rate and can vary between coals of similar rank but different morphology. The model predictions for coal reburn indicate that the reduction of NO hy CH fragments is the dominant NO reduction mechanism in the rehurn zone. accounting for virtually 70% of the observed total reduction.
00/02065
lnteractlons between coal and biomass when cofiring
Robinson, A. L. Srmp. (1nt.i Cotnhwt., /Proc.]. 1998 27th. I. 1351~1359. Combustion Institute. The most promising short-term options for reducing the net COZ emissions from coal-fired power plants is to co-fire the coal with biomass. This paper examines the effects of co-firing biomass and coal on ash deposition, corrosion, and NO, emissions under conditions representative of those found in pulverized-coal (PC) boilers. Experiments were conducted with blends of six different fuels, two types of bituminous coal, two types of straw, switch-grass and wood. The deposition rates for the hlends of fuel\ considered by this investigation lay between the measured deposition rates of the unblended fuels. Therefore, co-firing can mitigate some of the fouling difficulties associated with high-fouling coals and biofuels. By accounting for factors other than fuel composition, we can improve our predictions of the effects of co-firing on ash deposition rates, For the fuel blends examined here, available alkali is a relatively good predictor for the ash deposition rate. During the co-firing process sulfation of alkali chlorides is the major interaction that occurs within forming deposits. Sulfation reduces the chlorine content of the deposits, thus, reducing their corrosion potential. Co-firing biomass can decrease NO, emissions relative to firing unblended coal under typical pc-combustion conditions when the biomass fuel contains little nitrogen. There does not appear to he any fundamental synergistic interaction between the coal and biomass that significantly reduces NO, emissions. The test results show that co-firing biomass with coal can result in a decreased in pollutant production, ash deposition and effective C02.
10 ENGINES Power generation and propulsion 00102066 A feasibility study of an alternative power generation system based on biomass gasification/gas turblne concept De Souza, S. and Marcia, L. Ftd, 1999. 78. (5). 5299538. A newly proposed power generating system based on atmospheric gasification of sugar cane bagasse combined with fluidized-bed heat exchanging and gas turbine has been experimentally studied. The technology’s thermodynamic viability has been investigated under imposed conditions; several configurations of the power unit have been tried until
Fuel and Energy Abstracts
July
2000
229