ACERC
Abstracts - 1996 |
ACERC
Abstracts - 1996 |
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Thrust Area 1: Fuel Structure and Reaction Mechanisms |
Ma, J.
Soot Formation During Coal Pyrolysis, Ph.D./BYU, August 1996. Advisor:
Fletcher
Ueng, S.-K.
Scientific Visualization for Finite Element Analysis Data Sets, Ph.D./U
of U, June 1996. Advisor: Sikorski
Facelli, J.C.; Pugmire,
R.J. and Grant, D.M.
Journal of the American Chemical Society, 118(23):5488-5489, 1996. Funded
by National Institutes of Health and US Department of Energy/Basic Energy Services.
In a recent paper the principal values of the N-15 chemical shift tensor in benzamide have been reported. The tensor principal values have been measured using the dynamic nuclear polarization (DNP) experiment and also by cross polarization (CP) in static sample of N-15-benzamide at room temperature. Both experimental approaches produce very similar principal values. The values obtained by fitting the CP static spectrum with the Powder method are the following: delta11= -176 ppm, d22= -321 ppm, and d22= -326 ppm. These values are referenced to an external sample of nitromethane and yield and isotropic shift value of -274 ppm, which is close to the -271.5 ppm MAS (Magic Angle Spinning) value and the liquid values of -278.4 ppm in DMSO and -282 ppm in CDC13, from the literature. Because the measurements were done in a disordered sample no information was available on the orientation of the principal components and only reliable quantum chemical calculations of the tensor would allow these assignments to be made.
The results presented in this communication highlight the importance of including HB and, in general, intermolecular interactions in the calculation on N-15 chemical shifts tensors. This has been observed before in a previous study of the N-15 chemical shifts tensors of uracil but it appears that the magnitude of the effect is greater for an amide nitrogen than for the aromatic nitrogens in uracil. Further evidence of the importance of including intermolecular interactions in the calculation of N-15 chemical shifts tensors can be found in a recent study of these tensors in a series of heterocycles. For these compounds the RMS between experimental and calculated values, which do not include intermolecular effects, is approximately 30-40 ppm. The large RMS value is comparable with the values obtained here when the HB is neglected and almost one order of magnitude larger than the RMS obtained when the HB is included in the calculations. This may indicate that the findings presented here are not limited to benzamide, but are a general feature in the calculation of N-15 chemical shift tensors. The extreme sensitivity observed in the tensor components from HB indicates that the study of N-15 tensor components may be used to complement the determination of the tertiary structures of molecules of biological significance in solid state samples and/or in frozen solutions. It is well known that the HB is the principal governing factor in the tertiary components to these interactions makes N-15 tensors premier indicators of biomolecular structure and portends high value for these NMR methods. Finally, due to the high sensitivity and specificity of tensors, this work emphasizes some of the advantages of measuring tensor components instead of the traditional isotropic.
Facelli, J.C.; Orendt, A.M.;
Jiang, Y.J.; Pugmire, R.J. and Grant, D.M.
The Journal of Physical Chemistry, 100(20):8268-8272, 1996. Funded by
ACERC and US Department of Energy/Basic Energy Services.
The first direct measurement of the ortho steric effect of the methoxy group on the C-13 chemical shifts in anisole is reported. The ortho steric effect on the isotropic C-13 chemical shifts was obtained from a low-temperature MAS spectrum, and the effect on both the isotropic and the tensor principal components was determined from a low-temperature 2D magic angle turning (MAT) experiment. Form the low-temperature MAS spectrum, the C-13 chemical shift of the ortho carbon cis to the methoxy carbon is found to be 7.0 ppm from the low-temperature MAT experiment, a 6.8 ppm decrease in the chemical shift is observed in the isotropic chemical shift, while the effects on the difference (cis minus trans) between the individual tensor components are measured to be -9 ppm in delta1, 1 ppm delta22 and -14 ppm in delta33, in reasonable agreement with the results of a previous linear regression substituent analysis on several di- and trimethoxybenzenes. Comparison of the experimental results with calculations, including thermal averaging considerations, further demonstrates that at room temperature the methoxy group in anisole undergoes stochastic jumps between the two equivalent planar configurations. This work demonstrates the feasibility of using the low-temperature MAT experiments at low temperature to measure the principal values of the C-13 chemical shift tensors in molecules that are liquids at room temperature.
Wang, W.; Pugmire, R.J.
and Grant, D.M.
J.Phys. Chem (in press), 1996. Funded by US Department of Energy/Pittsburgh
Energy Technology Center and US Department of Energy/Basic Energy Services.
The phase diagram constructed form differential scanning calorimetry data indicated that the binary mixture of dibenzofuran (DBF) and hexamethylbezene (HMB) forms a simple eutectic system. Comparative studies of proton T1 Values for the annealed and quenched samples show the annealed material can be best described as a two-phase mixed crystals, while a rapidly quenched sample is a combination of a metastable one-phase glass and two-phased mixed crystals. It is found that glass formation is the key to T1 reduction of DBF in the HMB doping technique reported previously. The interesting rends in the TA and the relative spin population of DBG is explained with the competition between glass formation and crystalline phase separation.
Jakab, E.; Huai, H.; Nie,
H. and Meuzelaar, H.L.C.
Process Control & Quality, 8:55-67, 1996. Funded by Consortium for Fossil
Fuel Liquefaction Science, ACERC and Rocketdyne.
Gas chromatography-mass spectrometry instruments were interfaced to high-pressure flow-through microreactors to monitor the product formation on-line. Three types of instrumental setup are described illustrating the versatility of this kind of coupling. The first example shows the thermal and catalytic conversion of dibenzyl ether in solution under 155 bar hydrogen atmosphere. In the second application, thermal decomposition of JP-7 jet fuel was carried out under supercritical conditions to study the gaseous product evolution. This system features the application of a microbalance to monitor the weight of the total liquid sample plus products. The third system was designed to perform conversion of solid sample (wood) in liquid/vapor environment. This reactor can be applied to model two-step liquefaction processes with catalytic conversion of the primary product. All three systems provide information on the product distribution and kinetic profiles of the conversion processes.
Liu, K. and Meuzelaar, H.L.C.
Fuels Processing, 49:55-67, 1996. Funded by Consortium for Fossil Fuel
Liquefaction Science.
High pressure thermogravimetry (TG) with rapid on-line gas chromatography/mass spectrometry (GC/MS) has been used to investigate the effects of different catalysts on decomposition reactions of commingled wasted plastics (predominately PE), high density polyethylene (HDPE) and mixtures of DECS-6 with waste plastics in H2 at 900 psig. This permits direct evaluation of relative decomposition and residual char amounts as well as yield and include solid super acids such as Fe2O3/SO42-, Al2O3/SO, Al2O3/SO42- promoted by 0.5% Pt, and ZrO2/SO42- (all added at 10 wt%), as well as a conventional cracking catalyst of SiO2/Al2O3 in a 4:1 ratio, a hydrocracking catalyst of NiMo/Al2O3, HZSM-5>NiMoAl2O3 mixed with SiO2 Al2O3> (both added at 50%), and a HZSM-5 zeolite catalyst (added at 10%). Under these conditions cracking activity for waste plastics reveals the following order: SiO2/Al2O3, HZSM-5>NiMo/Al mixed with Si2O3O2Al2O3 solid super acids. Of the solid super acids studied the ZrO2/SO42- catalyst possesses the highest cracking activity, and the approximate order of cracking activity is ZrO2/SO42- > Al2O3/SO42- > Pt/Al2O3/SO42- > Fe2O3/SO42->no catalyst. The stronger the cracking catalyst, the lighter the aliphatic products and the more abundant the isometric constituents. Similar results are found for HDPD with these catalysts. For co-processing of coal with commingled waste plastic the HZSM-5 zeolite catalyst shows the most promising results by increasing the rate of the decomposition reactions at 420ºC nearly tenfold. Hydrocracking catalysts, such as NiMo/Al2O3 mixed with SiO2/Al2O3, show potential promise for processing of coal with commingled waste plastic due to their combined hydrogenation and cracking ability. By contrast, a superacid such as ZrO2/SO42- or cracking catalyst such as SiO2/Al2O3 appears to have little effect on the decomposition rate of the mixture. To what extent these findings are influenced by transport limitations (e.g., due to incomplete mixing or degree of crystallinity) and/or catalyst pretreatment is being studied further.
Li, W.; Lazar, I.M.; Wan,
Y.J.; Butala, S.; Shen, Y.; Malik, A. and Lee, M.L
Energy & Fuels, 1996 (in press). Funded by ACERC and Gas Research
Institute.
Conventional analytical techniques, such as headspace gas chromatography and Soxhlet extraction, can provide compositional information for the gaseous (C1-5) and heavy (C15+) hydrocarbon constituents, respectively. The volatile (C6-14) hydrocarbons, if present, usually go undetected because of volatility fractionation and loss. In this study supercritical CO2 was used to extract the C6-C14 volatile hydrocarbons from pulverized coal samples. Capillary column gas chromatography/mass spectrometry was used to identify the mixture components, and packed capillary column supercritical fluid chromatography was used to separate and quantify the aliphatic and aromatic hydrocarbon class fractions. It was found that the compositions of the light hydrocarbon fractions included several homologous series of normal and branched aliphatic hydrocarbons, cyclic and aromatic hydrocarbons, and alkyl-substituted benzenes and naphthalenes; the concentrations of these volatile hydrocarbons ranged between 0.01 to 0.2% (by weight) of the bulk material for the different coal and shale samples.
Guo, F. and Hecker, W.C.
ASC Division of Fuel Chemistry, (in press), 1996. (Also presented at
the Twenty-Sixth Symposium (International) on Combustion, The Combustion
Institute, Pittsburgh, Pennsylvania, 1996.) Funded by ACERC.
The heterogeneous reaction of NO with char is important in understanding the formation and reduction of NOx from coal combustion processes. The kinetics of NO reduction by North Dakota lignite char (NDL, its acid-washed char (NDW), and its calcium-reloaded char (NCa) were investigated in a packed bed reactor at temperatures from 723 to 1073 K. The results show that the reaction rate of NO with char increases significantly as the CaO content of the char increases. They also indicate clearly that the reaction is first order with respect to NO pressure and that there is a sharp increase in the apparent activation energy with increasing temperature. In the low temperature regime, the activation energies for all three char types are essentially the same (22-26 kcal/mol); in the high temperature regime, they are all higher, but decrease form 60-45 kcal/mol as the CaO content increases. The temperature at which the shift takes place also decreases as the CaO content increases.
Using a series of six NDL chars, the effect of Char burnout level on the reaction of NO with char was also studied. The transition temperatures and apparent activation energies were found to be independent of char burnout, but both the reaction rate constant and CaO surface area (determined by CO2 uptake at 573K) decreased as char burnout level increased from 0 to 80%. When the reaction rates are normalized by CaO surface area, they become essentially independent of burnout level, which suggests the importance that CaO sites play in reduction process. The correlation of rate with CaO surface area is quantitative and also holds for 3 char types in the high temperature regime.
Gale, T.K.; Bartholomew,
C.H. and Fletcher, T.H.
Energy & Fuels, 10(3):766-755, 1996. Funded by ACERC.
The main objective of this work was to determine the effects of pyrolysis heating rate on intrinsic O2 reactivity of coal chars. Relationships of intrinsic reactivity to other pyrolysis conditions and char physical and chemical structure were also investigated, and empirical correlations were obtained. Two different entrained flow reactors (a flat flame methane-air burner and a drop tube reactor) were used to prepare chars under a variety of different pyrolysis conditions at maximum particle temperatures and heating rates of 840-1627 K and 104 to 2 s 105 K/s, respectively. Intrinsic reactivities of a lignite and two bituminous coal chars decrease with increasing preparation heating rate. Maximum particle temperature and heating rate are difficult preparation parameters to separate and were closely coupled in this work, as in most entrained flow coal research. Indeed, much of the work described in the literature defining the effects of pyrolysis heating rate on coal char reactivity; has utilized vast residence time differences, comparing data from fixed bed (residence time of ~ 1 h) and entrained flow reactors (residence time of ~100 ms). It is concluded from this work that observations made on the basis of such experimentation are a function more of residence time and reactor variations (packed or fixed bed, as opposed to entrained flow) than particle heating rate. This work also provides evidence that intrinsic reactions of O2 with coal char (for the three coals observed in this study) are not significantly influenced by large differences in char meso- or micropore surface area obtained by varying pyrolysis conditions.
Baxter, L.L.; Reginald,
E.M.; Fletcher, T.H. and Hurt, R.H.
Energy & Fuels, 10:188-196. Funded by US Department of Energy/Sandia
National Laboratories.
Experiments in entrained flow reactors at combustion temperatures are performed to resolve the rank dependence of nitrogen release on an elemental basis for a suite of 15 U.S. coals ranging from lignite to low-volatile bituminous. Data were obtained as a function of particle conversion, with overall mass loss up to 99% on a dry, ash-free basis. Nitrogen release rates are presented relative to both carbon loss and overall mass loss. During devolatilization, fractional nitrogen release from low-rank coals is much slower than fractional mass release and noticeably slower than fractional carbon release. As coal rank increases, fractional nitrogen release rate relative to that of carbon and mass increases, with fractional nitrogen release rates exceeding fractional mass and fractional carbon release rates during devolatilization for high-rank (low-volatile bituminous) coals. At the onset of combustion, nitrogen loss rates increase significantly. For all coals investigated, fractional nitrogen loss rates relative to those of mass and carbon pass through a maximum during the earliest stages of oxidation. The mechanism for generating this maximum is postulated to involve nascent thermal rupture of nitrogen-containing compounds and possible preferential oxidation of nitrogen sites. During later stages of oxidation, the fractional loss rate of nitrogen approaches that of carbon for all coals. Changes in the relative release rates of nitrogen compared to those of both overall mass and carbon during all stages of combustion are attributed to a combination of the chemical structure of coals, temperature histories during combustion, and char chemistry.
Eatough, C.N. and Smoot,
L.D.
Fuel, 75(3):1601-1606, 1996. Funded by ACERC.
Devolatilization times of large (0.1 and 0.2g) Utah hvBb and North Dakota lignite coal particles, in the range 15-30 s, were measured in the air at 101 and 507 kPa, at air temperatures of 900 and 1200 K in a connective flow reactor. Visual observations indicated infrequent heterogeneous ignition of the lignite prior to devolatilization and occasional explosion of bituminous coal particles during devolatilization. Devolatilization times were correlated with the temperature, pressure and particle size. Power-law exponents for tests at 101 kPa and 900 K were determined to be 2.5 for Utah hvBb coal and 2.2 for North Dakota lignite. At 507 kPa and 900 K, exponents decreased to 1.6 for both Utah and North Dakota lignite.
Wan, Y.J.; Butala, S.; Li,
W. and Lee, M.L.
Proceedings of the Pittcon '96 (Pittsburgh Conference), Chicago, Illinois,
March 6, 1996. Funded by ACERC.
To understand the original chemical features of the intact coal structure, a new method has been developed for depolymerization and extraction of coal. It involves successive catalyzed depolymerization reactions under mild and organic solvent free conditions, each immediately followed in in-situ supercritical fluid (carbon dioxide) extraction and enhanced solvent (methylene chloride) extraction of the reaction matrix. The procedure is simple and the system is easy to operate. The timely removal of coal products avoids any retrograde reactions that complicate the products. Compared with coal head distillation, supercritical fluid extraction and enhanced liquid extraction is fast and can be used to analyze a very small amount of coal for analytical purposes. Experimental detail, extraction yields, and GC and GC/MS characterization of the products associated with coal structure will be presented. From the comparison of the composition of products from different depolymerization steps, it appears that coal has a common macromolecular skeletal structure, despite the well-established heterogeneity of coal.
Reade, W.C.; Morris, K.;
Ness, T. and Hecker, W.C.
Proceedings of the Tenth Annual Technical Review Meeting of the Advanced
Combustion Engineering Research Center, Salt Lake City, Utah, March 6, 1996.
Funded by ACERC.
A char oxidation model that predicts char burnout behavior over a broad range of temperature and oxygen concentration has been developed. The model accounts for variations in intrinsic reactivity and pore structure with char burnout. Intrinsic rates are obtained as a continuous function of char conversion from low-temperature isothermal TGA measurements. A port structure model based on three sizes of pores is included to calculate the effective diffusivity of oxidizer through the particle, which in turn is used to calculate the oxygen concentration profile as a function of particle burnout levels. Since the TGA data are base on reactivity per residual mass of the char sample, the effects of char heterogeneities and surface area evolution are implicit in the intrinsic kinetic, thus eliminating the need to predict surface area evolution which has been a problem in previous models.
Model predictions of high-temperature global rates and mass loss as a function of residence time, oxygen partial pressure, and parent coal rank have been compared with experimental data, both reported in the literature and obtained in our high temperature reactor. Chars tested to date include those from Pittsburgh #8, Pocahontas #3, Illinois #6, Deitz and North Dakota coals. To validate the model's ability to predict the effects of oxygen concentration, Pittsburgh #8 char was oxidized at two different temperatures and five different oxygen concentrations (ranging from 0-15%) in the high temperature reactor. The model predictions fit the experimental data very well for all 10 data pints, with variations ranging between 1 and 7%. Model predictions were also compared to the extensive high-temperature data of Sandi (Hurt and Mitchell, 1992) in order to test the model's ability to predict combustion behavior as a function of parent coal rank. The comparison was made for chars from 5 different coals and the agreement in burning rate and burning time for all five chars was again excellent.
Guo, F. and Hecker, W.C.
Proceedings at the AIChE Annual Meeting, Chicago, Illinois, November
12, 1996. Funded by ACERC.
The heterogeneous reaction of NO with coal char has potential as a flue gas clean-up process. It appears that the inorganic constituents in coal, especially CaO and K2O, increases the rate at which NO is reduced by carbon. The kinetics of NO reduction by North Dakota Beulah Zap lignite char (NDL, its acid washed char (NDW), and its calcium-reloaded char (NcA) were investigated in a packed bed reactor at temperatures form 723 to 1073 K. The results show that the reaction rate of NO with char increases significantly as the CaO content increases. They also indicate that the reaction is first order with respect to NO partial pressure and show an increase in the apparent activation energy with increasing temperature. In the low temperature regime, the activation energies for all three char types are essentially the same (22-26 kcal/mol); in the high temperature regime, they are all higher, but decrease form 60-45 kcal/mol as the CaO content increases. The temperature at which the shift takes place also decreases as the CaO content increases.
The effect of char burnout level on the reaction of No with char was also studied using tow methods. In one method, a series of NDL chars with different burnout levels was made by oxidation with 3% O2 in a drop tube reactor (DTR) at about 1800 K. In the other method, NDL parent char was continuously reacted with NO in a packed bed reactor at 948 K until a 90% burnout level was achieved. Different kinetic behaviors of the NO/char reaction for the two cases were observed. For the DTR chars (high temperature), the reaction rate constant and CaO surface area decreased as char burnout area, they became essentially independent of burnout level, which suggests the important part that CaO sites play in the reduction process. The transition temperatures and apparent activation energies were found to be independent of char burnout. For the char made by continuous reaction with NO (low temperature), however, the reaction rate constant increased with increasing burnout level up to a burnout level of 65%, then, quickly dropped off. Like the high temperature char, a shift in activation energy with temperature was also found for the lower burnout level chars, but the shift disappeared for the high burnout level chars (>35%).
Rigby, J.R.; Webb, B.W.
and Fletcher, T.H.
Proceedings of the Spring Meeting of the Western States Section of the Combustion
Institute, Tempe, Arizona, March 11-12, 1996. Funded by ACERC.
All hydrocarbon-based fuels have the potential to form soot during combustion of devolatilization reactions. Soot resulting from incomplete combustion is the main contributor to luminosity in flames. Because of its high surface area and spectrally continuous radiation, sot is a very efficient thermal radiator. The optical properties of coal-derived soot have not received as much attention as soot derived from gaseous hydrocarbon fuels. The reason for this neglect may be the difficulty in separating the radiation effects of the coal-derived soot, char and fly ash. The radiative properties of coal-derived soot have not been characterized, nor have the influences of coal type, volume fraction and morphology been examined. The radiative properties of coal-derived soot char can be sued in combustion modeling and burner design. In the near-burner region, the stoichiometry is very fuel rich providing for the high soot volume fraction and for large radiative heat fluxes being transmitted to the burners and walls. In this region, neglecting soot can result in inaccurate radiant flux predictions, as well as inaccurate predictions of gas temperatures, species concentrations, pressure fields, and velocity profiles. Radiative properties of soot can also be used to determine soot volume fraction and soot temperature in-situ.
Based on preliminary results from this study, trends for C-lambda from coal-derived and propane-derived differ as a function of residence time in the post flame environment, even though the ranges of magnitudes of C-lambda overlap. All measurements indicate an increase in C-lambda at increasing wavelengths. Further work is needed to examine these trends for different coal-derived soots. Explanations of why the optical properties of soot change and parameters to characterize these changes are also needed. This work provides the basis for futures in-situ measurements that will measure soot volume fraction in a coal pyrolysis experiment.
Guo, F. and Hecker, W.C.
ACS Division of Fuel Chemistry, (in press), 1996. (Also presented at
the 211th ACS National Meeting, Division of Fuel Chemistry Symposium on Gasification
Mechanisms, New Orleans, Louisiana, March 24-28, 1996.) Funded by ACERC.
The reduction of NO emissions from combustion processes has become increasingly important in protecting the world's environment. It has been shown that Selective Cataytic Reduction (SCR) with ammonia is an effective commercial technique to remove NOx from combustion flue gas. However, the implementation of this technique is limited by high investment and operating costs, "ammonia slip," and SOx poisoning which motivate the search for alternatives. Carbon (activated carbon or char) is a promising reducing agent for NOx reduction with many potential advantages, such as low cost, easy availability, high efficiency, simplicity of process, and no secondary pollution. Moreover, the heterogeneous reaction of NO with char is very important for the understanding of the formation of NOx from coal combustion processes. The reaction may significantly destroy the NOx formed earlier in coal combustion, which partially contributes to low NO emission from fluidized bed combustion. Therefore, the reaction of NO with char is receiving significant attention in the literature. Previous investigations on the reaction of NO with char involve the kinetics and mechanism, the effects of char surface area, the effects of feed gas composition, and the catalytic effects of metals. The reaction of NO with char has generally been reported to be first order with respect to NO partial pressure, bur reaction orders between 0.42 and 0.73 have also been reported. A sharp shift in the activation energy has been observed in the temperature range of 873-973 K, which suggests a complex reaction mechanism. Several mechanisms have been proposed. However, questions concerning N2 formation, the surface complexes, the nature of active surface sites, and the effects of minerals in char ash are still not well understood. In most previous studies, chars were taken to be pure carbon, thus the effects of the ash in chars and its composition on kinetics and mechanism of the reduction reactions are not well known. Although the catalytic effects of certain metals of metal oxides on the reactions have been investigates, little is known about their effects on the kinetics of the reactions. Therefore the objectives of this study are to investigate the kinetics of the reaction of NO with Beulah Zap chars to study the effects of CaO on kinetics.
Walker, A.C. and Hecker,
W.C.
Presented at the Tenth Annual Technical Review Meeting of the Advanced Combustion
Engineering Research Center, Salt Lake City, Utah, March 7, 1996. (Also
at the Tenth Annual Symposium of the Western States Catalysis Club, Albuquerque,
New Mexico, March 1, 1996.) Funded by Brigham Young University.
A comparison of the activity/selectivity, kinetic, and infrared properties of Rh/ZSM5 and Pd/ZSM5 for the reduction of nitric oxide (NO) with methane will be reported. Tests at 300-400°C indicate that catalysis made with Pd and Rh ion-exchanged into ZSM5 are significantly more active than corresponding SiO2-supported catalysts. Both ZSM5 catalysts are quite active in the absences of oxygen while Pd/ZSM5 has low activity in the presence of oxygen. Rh/ZSM5 on the other hand shows fairly high activity and some interesting trends in the presence of oxygen. Two Rh/ZSM5 catalysts were used, one which has both ion-exchanged and crystalline sites and one with nearly exclusively ion-exchanged sites that are active in the presence of oxygen while the crystalline sites are not. The crystalline sites are active for methane combustion and the presence of crystalline sites has a dramatic influence on selectivity.
Possible reactive intermediates have been identified. They are considered to by NO2 type species attached to exchanged rhodium sites. Transient studies show that a species characterized by a band at 1570 1/cm is more reactive towards methane than other absorbed species. In addition, an isocyanate species has been observed which is a possible pathway for N-N bond formation. The mechanistic implications of these findings will be discussed.
Ma, J.; Fletcher, T.H. and
Webb, B.W.
Twenty-Sixth Symposium (International) on Combustion, (in press) 1996.
(Also presented at the Twenty-Sixth Symposium (International) on Combustion,
Naples, Italy, July, 1996.) Funded by ACERC.
Coal pyrolysis experiments were performed in the post-flame region of CH4/H2/air flat flame burner running in fuel-rich conditions, where the temperature and gas compositions were similar to those in the near burner region of a large-scale coal-fired furnace. Volatiles released form the coal particles formed a cloud of soot particles at high temperatures in the absence of oxygen. The soot particles in the cloud were collected at different residence times using a water-cooled, nitrogen-quenched suction probe. The test variables included the reaction temperature and coal type. Soot yields in terms of weight percentage of dry ash-free coal were measured based on the bulk soot collection experiments. The measured soot yields were related to coal rank, reaction temperature, and residence time. Size changes of soot particles due to soot agglomeration were also observed. The information obtained bout coal-derived soot is useful in predictions of radiative heat transfer and pollutant formations in the near-burner region of pulverized coal-furnaces.
Guo, F. and Hecker, W.C.
Twenty-Sixth Symposium (international) on Combustion/The Combustion Institute,
2251-257(1996). Funded in part by ACERC.
The heterogeneous reaction of NO with char is important in understanding the formation and reduction of NOX from coal combustion processes. The kinetics of NO reduction by North Dakota lignite char (NDL), its acid-washed char (NDW), and its calcium-reloaded char (NCa) were investigated in a packed-bed reactor at temperatures from 723-1073 K. The results show that the reaction rate of NO with char increases significantly as the CaO content of the char increases. The also indicate clearly that the reaction is first order with respect to NO pressure and that there is a sharp increase in the apparent activation energy with increasing temperature. In the low temperature regime, the activation energies for all three char types are essentially the same (22-26 kcal/mol); in the high temperature regime, they are all higher, but decrease from 60 to 45 kcal/mol as the CaO content increases. The temperature at which the shift takes place also decreases as the CaO content increases.
Using a series of six NDL chars, the effect of char burnout level on the reaction NO with char was also studied. The transition temperatures and apparent activation energies were found to be independent of char burnout, but both the reaction rate constant and the CaO surface area (determined by CO2 uptake at 573 K) decreased as char burnout level increased from 0 to 80%. When the reaction rates are normalized by CaO surface area, they become essentially independent of burnout level, which suggests the importance that CaO sites play in the reduction process. The correlation of rate with CaO surface area is quantitative and also holds for the tree char types (NDL, NDW, and NCa) in the low-temperature regime. It does not hold for the three char types in the high-temperature regime.
Guo, F. and Hecker, W.C.
Presented at the Eighteenth Symposium of the Rocky Mountain Fuels Society,
Albuquerque, New Mexico, February 29, 1996. Funded by ACERC.
The heterogeneous reaction of NO with chars is very important in understanding the formation and reduction of NOx in coal combustion processes. The reaction also has potential as the basis for and inexpensive, flexible approach to reduce NO emissions in post-combustion clean-up processes. Thus, the reaction is receiving increased attention in the literature; nevertheless, the kinetics of the reaction and the factors that influence them are not yet well understood.
North Dakota Beulah Zap lignite char (NDL), a portion of the NDL washed with HCI to remove mineral matter (NDW), and a portion of NDW reloaded with calcium oxide (NCa) were prepared previously in our lab and used for this study. Kinetic experiments were carried out in a 10 mm ID packed-bed reactor at temperatures from 723 to 1073 K, at an inlet NO concentration of 3130 ppm, and at flow rates of 100 to 500 ml/min. Compositions of inlet outlet gases were measured by GC and chemiluminescence NOx analyzer. Nitrogen and oxygen mass balances were determined for each run and always within ±5%.
Experimental results show that the NO/char reaction rate varies with char type, increasing significantly as the CaO content increases. They also indicate that the reaction is first order with respect to NO partial pressure and show an increase in the apparent activation energy with increasing temperature. In the low temperature regime, the activation energies for all three char types are essentially the same (22-26 kcal/mol); in the high temperature regime, they are all higher but decrease form 60 to 45 kcal/mol as the CaO content increases. The temperature at which the shift takes place also decreases as the CaO content increases. The shift in activation energy suggests a complicated reaction mechanism.
The effect of char burnout level on the reaction of NO with char was also studied using two methods. In one method a series of NDL chars with different burnout levels was made by oxidation with 3% O2 in a drop tube reactor (DTR) at about 1800 K. In the other method, NDL parent char was continuously reacted with NO in a packed bed reactor at 984 K until a 90% burnout level was achieved. Different kinetic behaviors of the NO/char reaction for two cases were observed. For the DRT chars (high temperature), the reaction rate constant with CaO surface are decreased as char burnout level increased from 0 to 80%. When the reaction rates were normalized by CaO surface area, they became essentially independent of burnout level, which suggests the important part that CaO sites play in the reduction process. The transition temperatures and apparent activation energies were found to be independent of char burnout. For the char made by continuous reaction with NO (low temperature), however, the reaction rate constant increased with increasing burnout level up to a burnout level of 65%, the quickly dropped off. Like the high temperature char, a shift in activation energy with temperature was also found for the lower burnout level chars, but the shift disappeared for the high burnout level chars (>35%).
Fletcher, T.H.; Watt, M.;
Bai, S.; Solum, M.S. and Pugmire, R.J.
Twenty-Sixth Symposium (International) on Combustion, (in press) 1996.
(Also presented at the Twenty-Sixth Symposium (International) on Combustion,
Naples, Italy, July, 1996.) Funded by ACERC and US Department of Energy/University
Coal Research.
Three coals of different rank were pyrolyzed in a drop tube reactor at a maximum temperature of 900 K and residence time of 160 ms. The coal and char were analyzed with solid state C-13 NMR. The tar was dissolved in deuterated methylene chloride. It was found that the tar was only partially soluble in CD2CL2-. The non-soluble tar portion was analyzed using a recently developed high-resolution C-13 NMR technique developed for liquid phases. The tar structure was found to be significantly different from the structure of the char and coal. The number of bridges and loops per cluster in the tar was up to 65% lower than in the char. In addition, the number of aromatic carbons per cluster in the tar was significantly lower than that found in either the coal or the char. Since the molecular weight per cluster in the tar is lower than reported average tar molecular weights, these data imply that tar is made up of a number of multiple clusters (dimers, trimers, etc.) as well as single clusters (i.e., monomers). The mass of nitrogen per cluster in the tar was found to be significantly lower in the tar than in either the coal or the char. These experimental findings suggest that changes may be necessary in current network devolatilization models to accurately describe the changes in chemical structure.
Groberg, E.T.
Validation of the Gas Species and Particle Composition Data Taken at the
Milliken Power Plant Furnace, M.S./BYU, July 1996. Advisor: Cannon
Picket, L.
Velocity Measurements in a Pulverized Coal Flame Using Laser Doppler Anometry,
M.S./BYU, December 1996. Advisor: Tree
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