Hecker, WC

2000

Modeling High Pressure Char Oxidation Using Langmuir Kinetics with an Effectiveness Factor

Hong, J.; Hecker, W.C. and Fletcher, T.H.
Proceedings of the Combustion Institute, 28 (in press, 2000).
Contact: Fletcher

Improving the Accuracy of Predicting Effectiveness Factors for m-th Order and Langmuir Rate Equations in Spherical Coordinates

Hong, J.; Hecker, W.C. and Fletcher, T.H.
Energy & Fuels, 14, 663-670 (2000).
Contact: Fletcher

Modeling Condensed Phase Kinetics and Physical Properties in Nitramines: Effect on Burning Rate Temperature Sensitivity

Washburn, E.B.; Beckstead, M.W.; Hecker, W.C.; Howe, J. and Waroquet, C.
37th JANNAF Combustion Meeting, CPIA No. 689, Vol I, 2000, pp. 639.
Contact: Beckstead

1999

Kinetics of High-Pressure Char Oxidation

Sawaya, R.J.; Allen, J.W.; Hecker, W.C.; Fletcher, T.H. and Smoot, L.D.
ACS Preprint, Div. Fuel Chem., 44, pp xx (Aug 1999).

The kinetics of char oxidation at atmospheric pressure have been much studied and are fairly well agreed upon. However, the kinetics of char oxidation at elevated pressures have not been studied to any significant extent, and standard kinetic models which work at low pressure do not work at high pressure. This paper reports the results of a study to determine the high-pressure kinetics of char oxidation for Pittsburgh #8 char under Zone I conditions. Rate data were obtained for total pressures from one to 64 atmospheres and oxygen mole fractions between 0.03 and 0.40. Temperature dependencies as well as oxygen partial pressure dependencies were determined and the suitability of using various Langmuir-Hinschellwood expressions to fit the data were explored.

Kinetics of NO Reduction by Char: Effects of Coal Rank

Guo, F. and Hecker, W.C.
Twenty-Seventh Symposium (International) on Combustion/The Combustion Institute, 1999/pp.3085-3092

The heterogeneous reaction of NO with coal char has potential as the basis for both reburning and postcombustion clean-up processes to control NOx emissions from combustion. The reaction is also important in understanding the formation and reduction of NO during coal combustion. In this study, the kinetics of NO reduction by chars made from coals ranging in rank from lignite to low-volatile bituminous (Beulah-Zap [NDL], Dietz, Utah Blind Canyon [UBC], Pittsburgh #8, and Pocahontas #3) were investigated in a packed-bed reactor at temperatures between 723 and 1173 K. Graphite and coconut char were also studied.

The low-rank chars were found to be significantly more reactive than the high-rank chars (NDL> Dietz>> coconut ~ Pittsburgh #8 ~ UBC ~ Pocahontas #3 >> graphite) with the T50 (temperature required for 50% NO conversion) varying from 870 K for NDL to 1100 K for graphite for a given set of conditions. For all chars studied, the reaction was found to be first order with respect to NO partial pressure and to exhibit an activation energy 0(EA) shift from 100-160 kJ/mol at low temperatures to 190-250 kJ/mol at high temperatures. The shift to distinctly different and higher EA's at higher temperature is opposite to what would be expected if a reaction is shifting from chemical rate control to mass transfer control and suggests different mechanisms or rate-determining steps at high and low temperatures. Although all chars exhibited the shift in EA, the shift temperature and the EA within each temperature regime tended to increase with increasing rank. Also, the relative reactivity of the chars depends not only on organic char surface area but also on inorganic content, specifically, CaO surface area.

1998

Kinetics of NO Reduction by Char: Effects of Coal Rank

Guo, F. and Hecker, W.C.
Twenty-Seventh Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, accepted.

The heterogeneous reaction of NO with coal char has potential as the basis for both reburning and post-combustion clean-up processes to control NOx emissions from combustion. The reaction is also important in understanding the formation and reduction of NO during coal combustion. In this study the kinetics of NO reduction by chars made from coals ranging in rank from lignite to low-volatile bituminous (Beulah-Zap (NDL), Dietz, Utah Blind Canyon, Pittsburgh #8, and Pocahontas #3) were investigated in a packed bed reactor at temperatures between 723 and 1173 K. Graphite and coconut char were also studied.

The low rank chars were found to be significantly more reactive than the high rank chars (NDL > Dietz >> Coconut ~ Pitts ~ UBC ~ Pocah >> Graphite) with the T50 (temperature required for 50% NO conversion) varying from 870 K for NDL to 1100 K for graphite for a given set of conditions. For all chars studied the reaction was found to be first order with respect to NO partial pressure and to exhibit an activation energy (EA) shift from 100-160 kJ/mol at low temperatures to 190-250 kJ/mol at high temperatures. The shift to distinctly different and higher EA's at higher temperature is opposite to what would be expected if a reaction is shifting from chemical rate control to mass transfer control, and suggests different mechanisms or rate determining steps at high and low temperatures. While all chars exhibited the shift in EA, the values of the shift temperature and the EA within each temperature regime tended to increase with increasing rank. Also, the reactivity of chars seemingly depends not only on organic char surface area, but also on mineral content, specifically CaO surface area.

The data also indicate that the rate of the heterogeneous reduction of NO by char is significant and comparable with the rate of homogeneous reburning of NO by CHI, and thus, the heterogeneous reaction should be included in models of NO formation during coal combustion.

1997

NO Reduction by Char: Effects of Coal Rank, Burnout Level, and Burnout Conditions

Guo, F. and Hecker, W.C.
Carbon 97:392(1997). Presented at the 23rd Biennial Conference on Carbon, Penn State University, July 16, 1997. Funded in part by ACERC.

The heterogeneous reaction of NO with coal char has potential as the basis for both reburning and post-combustion clean-up processes to control NOx Emission from coal Combustion. The reaction is also very important in understanding the formation and reduction of NO during coal combustion. However, the heterogeneous kinetics and mechanism of the NO-char reaction are still poorly understood. Many questions regarding the mechanism and kinetics of the reaction, regarding the effects of char surface area, mineral matter, coal rank, burnout, and flue gases on the reaction remain. In this work, we explore the effects of coal rank, burnout level, and burnout conditions on the kinetics of the NO-char reaction.

Catalyst Characterization by Quantitative FTIR: Determination of Rh-CO Extinction Coefficients for Rh/ZMS-5

Hickenlooper, J.L. and Hecker, W.C.
Environmental Catalysis II, (in press), 1997. Also presented at the AIChE Annual Meeting, Los Angeles, California, November 17, 1997. Funded in part by ACERC.

The application of quantitative FTIR to heterogeneous catalysis allows the determination of surface concentrations of active intermediates. Successful application of quantitative FTIR depends upon understanding the factors affecting integrated extinction coefficients. Extinction coefficients relate integrated IR absorbance to surface concentration. Research on catalyst systems with 1% Rh has determined the Rh-CO extinction coefficients are not dependent upon oxide support, using the following supports: SiO2, Al2O3, Ce/SiO2, Ce/Al2O3, TiO2, and MgO. In addition, varying temperatures between 50° and 200°C and varying weight loadings from 0.2% to 9% appeared to have no effect on the extinction coefficients for Rh/SiO2 catalysts. Similar studies are being carried out on Rh/ASM-5. The following band frequencies have been observed on 0.8% Rh/AMS-5 catalyst: 2181 and 2150 cm^-1, 2100 (sh) and 2020 (sh) cm^-1, 2114, and 2048 cm^-1, 2117 and 2083 cm^-1 and 2069 cm^-1. The two weak bands at 2181 and 2150 cm^-1 are assigned to Rh (III)-CO where CO is bonded reversibly to the Rh(III) ion (Shannon et al. 1984). The bands at 2100 (sh) and 2020 (sh) cm^-1, 2114, and 2048 cm^-1, 2117 and 2083 cm^-1 are assigned to three types of RH(I) carbonyls (Shannon et al. 1984). The band at 2069 cm^-1 is proposed to be a linear Rh-CO species. Extinction coefficients for these Rh-Co species are being determined by combining peak areas from IR placed in a high temperature transmission reactor cell. The effects of temperature on the extinction coefficients for CO chemisorbed on Rh/ZMS-5 are also being determined. The results of this study will allow catalysis researched to apply quantitative FTIR to better understand catalytic surface reaction or Rh/ZMS-5 as well as demonstrate the use of an analytical system for determining extinction coefficients.

The Kinetics of NO Reduction by Char: The Effects of Coal Rank, Burnout Level, and Burnout Conditions

Guo, F. and Hecker, W.C.
Western States Section/ The Combustion Institute, Livermore, California, Paper #97S-068 (1997). (Also presented at the Western State Section/The Combustion Institute, Livermore, California, April 15, 1997.)

The heterogeneous reaction of NO with coal char has potential as the basis for both reburning and post-combustion clean-up processes to control NOx emissions from combustion. The reaction is also very important in understanding the formation and reduction of NO during coal combustion. In this study the kinetics of NO reduction by chars made from coals ranging in rank from lignite to low-volatile bituminous (Beulah-Zap, Dietz, Utah Blind Canyon, Pittsburgh #8, and Pocahontas #3) were investigated in a packed bed reactor at temperatures between 723 and 1173 K. Graphite and coconut char were also studied.

The reaction is first order with respect to NO partial pressure and exhibits an activation energy shift from 20-40 kcal/mol at low temperature to 45-60 kcal/mol at high temperatures for all chars studied. The low-temperature activation energy increases as coal rank increases. The shift to a distinctly different and higher activation energy at higher temperature is opposite to what would be expected if a reaction is shifting from chemical rate control to mass transfer control, and suggests different mechanisms or rate determining steps at high and low temperatures. The low rank chars are generally more reactive than the high rank chars. Also, the reactivity of char seemingly depends not only on organic char surface area, but also on mineral content and specifically CaO surface area. The experimental data also indicate that for NO formation in coal combustion the heterogeneous reduction of NO by char is significant, compared with homogenous reburning of NO by Chi.

The trend of variation of rate constant with char burnout significantly depends on char burnout conditions. The rate constant consistently decreases burnout when the chars are burnt out to different levels in a drop tube reactor at 1800 K and 3-5% O2. However, the rate constant increases as char burnout increases (up to 90%) when the char burnout levels result from reacting the char with 3000 ppm NO in a packed bed at 723-1173K. For the latter case, the relationship of rate constant (based on maf char mass) and char burnout is approximately linear with roughly the same slope between 20 and 80% burnout for all coal chars. The activation energy to the reaction is apparently independent of both char burnout level and burnout conditions.

Survey Results of Outcome Assessment at Brigham Young University

Terry, R.E. and Hecker, W.C.
Presented at the AIChE Annual Meeting, Los Angeles, California. November 17, 1997. Funded in part by ACERC.

We have conducted several surveys to identify needs and successes in the area of outcomes assessment. A survey of engineering recruiters was conducted to determine the characteristics of graduates being sought for by these recruiters and to determine the characteristics that make employees successful in their companies. A second survey involving alumni and current students was conducted to determine forms of outcomes assessment that had the most meaning to them as students and to them as working engineers.

The survey conducted with engineering recruiters has supported the hypothesis that differing factors are important in the initial recruiting of college students and in the success of the company employees after joining the work force. These results will be presented in detail.

The survey conducted with alumni and current students has been used to guide the direction of assessment techniques in the chemical engineering curriculum at Brigham Young University. The paper will present the results of the survey and attempts to incorporate meaningful assessment techniques into the curriculum.

NO Reduction by Coal Char: Kinetics and Modeling

Guo, F. and Hecker, W.C.
Presented at the Combustion Reaction Engineering Symposium at the AIChE Annual Meeting, Los Angeles, California, November 17, 1997. Funded in part by ACERC.

The heterogeneous reaction of NO with coal char has potential as the basis for both reburning and pos-combustion lean-up processes to control NOx emissions from coal combustion. The reaction is also very important in understanding the formation and reduction of NO during coal combustion. However, the heterogeneous kinetics and mechanism of the NO-char reaction are still poorly understood. Many questions regarding the mechanism and kinetics of the reaction, and the effects of char surface area, mineral matter, coal rank, burnout, and flue gases on the reaction remain. In this work, we explore the effects of coal rank, burnout level, and burnout conditions on the kinetics of the NO-char reaction. We also present and discuss a model that predicts the rate of the NO-char reaction over a wide range of temperatures, coal types, and burnout levels.

1996

The Effects of CaO and Burnout on the Kinetics of NO Reduction by Beulah Zap Char

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.

Modeling High Temperature Char Oxidation

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.

Nitric Oxide Reduction by CaO-Containing Coal Char: Correlation of Kinetics with CaO Surface Area

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%).

Effects of CaO Catalysis on the Kinetics of NO Reduction by Beulah Zap Char

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.

NO Reduction by CH4 Over ZSM5-Supported Rhodium and Palladium Catalysts

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.

Effects of CaO and Burnout on the Kinetics of NO Reduction by Beulah Zap Char

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.

Effects of Burnout Level and CaO Catalysis on the Kinetics of Nitric Oxide Reduction by Beulah Zap Char

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%).

1994

Catalytics Reactor Design: Keeping the Catalyst in Mind From the Beginning

Bartholomew, C.H. and Hecker, W.C.
Chemical Engineering, 70-75, June 1994. Funded by Brigham Young University.

Most major processes in the chemical process industries are built around heterogeneous chemical reactions. A solid catalyst is an integral part of almost all these operations. In new-construction or retrofit project for such plants, process engineers must design and specify not only the reactors but also the catalysts. Independent design of the two, without concern for how they will mesh, can mean a more costly design, a low production rate and more-frequent shutdowns. It may even cause the catalyst to fail. Consider, for instance, this debacle at a methanol plant. A carbon-steel pipe had been installed at the entrance to the methanol reactor. High-pressure carbon monoxide in the feed stream reacted with the steel to produce iron carbonyls, which poisoned the catalyst. Remedying the situation cost several million dollars.

With the hope of avoiding such situations, we first summarize the principles of catalyst and reactor design, with emphasis on maintaining interdependence between the two activities. Then we apply the principles to industrial reactors. The focus is solely on heterogeneous catalysis, in which the catalyst (virtually always in solid form) is not the same phase as the process stream. Even with this limitation, the technology is far too detailed for full presentation here. Instead, out aim is to enable readers to keep the big picture in mind whenever getting immersed in the specifics of the project.

 

Effects of CaO Surface Area on Intrinsic Char Oxidation Rates for Beulah Zap Chars

Cope, R.F.; Arrington, C.B. and Hecker, W.C.
Energy & Fuels, 8(5):1095, 1994. Funded by ACERC, National Science Foundation and US Department of Energy.

This work examines the effect of char burnout level and calcium content on the intrinsic char oxidation rates and physical properties of three series of chars. Three starting chars were prepared by devolatilizing a 63-74 mm fraction of North Dakota Zap lignite in a flat-flame burner; then, a portion of this char was washed in HCl to remove mineral matter; finally, a portion of the acid-washed char was reloaded with calcium. The three starting chars (ND (untreated), NDW (acid-washed), and NCa (Ca reloaded)) were then oxidized to various levels of conversion (6-92%) in a heated-wall drop-tube reactor (DTR) at high temperature (1523 K) in a 5-7% O2 environment. Low-temperature intrinsic oxidation rates were determined for each resulting sample using isothermal TGA (648-748 K, 10% O2). Other measured properties include burnout, N2 BET and CO2 DP surface areas, and CaO surface area. The latter was determined using a selective CO2 chemisorption technique. Intrinsic oxidation rate decreased as burnout increased for the calcium containing chars (ND and Nca). For the NDW char (69% Ca removed), the intrinsic rate was independent of burnout. Increased burnout produced general decreases in N2 BET and CO2 DP surface areas for all three chars, but they did not correlate well with rate. Increased burnout also produced decreases in C1) surface areas for the ND and Nca chars, but not for the NDW. The decreases in CaO surface area paralleled the decreases in intrinsic rates. This led to good correlation of CaO surface area with rate. Furthermore, the normalized values of rate per CaO surface area were essentially independent of burnout for the Ca-containing char series. This result suggests that catalysis by Ca is very significant during low-temperature oxidation. These results also indicate that the reason for decreasing rate with burnout is due to sintering of CaO with increased time of exposure to the high-temperature environment. Activation energies for the chars in the three series were found to be independent of burnout level and calcium content. Average values were 32.9 ± 1.4, 34.7 ± 3.3, and 33.6 ± 1.3 kcal/mol (uncertainties expressed as 95% confidence intervals) for ND, NDW, and Nca, respectively.

Modeling the Effects of Burnout on High-Temperature Char Oxidation

Reade, W.C. and Hecker, W.C.
Proceedings of the 17th Sympsium of the Rocky Mountain Fuels Society, Golden, CO, March 1994; Western States Section/The Combustion Institute, Davis, CA, March 1994; and AIChE Annual Meeting, San Francisco, CA, November 1994. Funded by ACERC.

In this work a char oxidation model that predicts changes in high-temperature reactivity with particle burnout is described. This model accounts for changes due to pore structure evolution and reactivity distributions, although there are many factors that can affect high-temperature reactivity (e.g., densification, mineral matter effects, etc.). Some factors, such as reactivity changes due to gasification-induced densification (Hurt, 1988), are implicitly included in the reactivity distribution effects. The only input parameters needed for this model are low-temperature kinetics (as measured by isothermal thermogravimetric analysis) and an approximate pore structure. This model is unique in that the pore structure evolution/reactivity distribution effects are lumped together in a preexponential that is a function of char conversion. The model predictions of high-temperature rates for two chars (Dietz and Pitt. #8) are compared with experimental data (Mitchell, et al., 1992) and show excellent agreement.

Catalysts for Cleanup of NH3, NOx and CO from a Nuclear Waste Processing Facility

Gopalakrishnan, R.; Davidson, J.E.; Stafford, P.R.; Hecker, W.C. and Bartholomew, C.H.
ACS Symposium Series, 552:74-88, 1994. Funded by Westinghouse Idaho Nuclear Company, Brigham Young University and ACERC.

Performance of Cu-ZSM-5, Pt/Al2O3 and Cu-ZSM-5 + Pt/Al2)3 for NH3 (425-750 ppm) and CO (~1%) oxidation in the presence of NO (250 ppm), O2 (14-15%) and H2O (~20%) was studied as a function of temperature. Pt/Al2O3 is more active for NH3 and CO oxidation, while Cu-ZSM-5 is more selective for conversion of NO and NH3 to N2. NH3 and CO are completely oxidized above 300°C on Pt/Al2O3, while on Cu-ZSM-5 about 99% of NH3 and NO are converted to N2 at 450-500°C, although only about 50% of CO is converted to CO2. The selectivity of Cu-ZSM-5 for conversion of NH3 and NO to N2 is about 100%, while selectivities of Pt/Al2O3 for N2 and N2O are 35-40% and 20-40% respectively. However, the activity and selectivity of a Cu-ZSM-5 + Pt/Al2O3 dual catalytic systems are very high, converting 99% of NH3, 94% of NO, and 100% of CO simultaneously at 485°C with a 100% selectivity to N2.

Improved Diameter, Velocity, and Temperature Measurements for Char Particles in Drop-Tube Reactors

Cope, R.F.; Monson, C.R.; Germane, G.J. and Hecker, W.C.
Energy & Fuels, 8(4):925-931, 1994. Funded by ACERC, Advanced Fuel Research and US Department of Energy.

Coal combustion researchers have typically used the average temperature and residence time of a burning particle cloud to determine the high-temperature reactivity of coals and chars. These average values, however, cannot account for particle-to-particle variations or their possible causes. Researchers at Sandia National Laboratories developed a pyrometry technique to simultaneously measure the temperature, velocity, and diameter of individual char particles burning in a transparent-wall flat-flame facility. This work reports two significant advances relative to the optical pyrometry technique. First, pyrometer modifications together with a new analysis technique now permit the particle properties to be measured for smaller/cooler particles. Second, the modified pyrometer has been implemented in two heated-wall drop-tube reactors, rather than transparent-wall, flat-flame burners. This is significant because drop-tube reactors allow greater flexibility/control of gas environments and operating pressures during char oxidation. Glowing reactor walls, however, present some unique challenges for these optical measurements. Means of overcoming these challenges are discussed, and reliable in situ measurement of particle temperatures, velocities, and diameters is verified. The results of measurements made in these drop-tube reactors, both for calibration tests and actual oxidation tests with Spherocarb and a Utah bituminous coal char, are also presented.

1993

Catalyst Characterization Using Quantitative FTIR: Co on Supported Rh

Rasband, P.B. and Hecker, W.C.
Journal of Catalysis, 139:55l, 1993. Funded by Brigham Young University.

Qualitative FTIR has been and continues to be one of the most utilized tools in the characterization of supported metal catalysts. Quantitative FTIR has the potential to allow catalysis researchers to determine the surface concentrations of active intermediates. However, its successful application depends upon an understanding of the factors affecting integrated absorption intensities (coefficients relating IR absorbance to surface concentration). This work addresses the effect of metal particle size and temperature on the absorption intensities for CO chemisorbed on Rh/SiO2. Absorption intensities for both linear and bridged CO surface species (a1 and Ab) were determined by combining peak area data from IR spectra with uptake measurements obtained in gravimetric experiments. This resulted in an A1 value of 13 (+/-2) and an Ab value of 42 (+/-6) cm/µmol. No statistically significant particle size effect has been observed for average spherical particle diameters ranging from 13 to 58 angstroms (100 to 22% dispersion). Also, integrated absorption intensities for linear and bridged CO were shown to vary little over the temperature range of 323 to 473 K. The discovery that absorption intensities determined for one temperature and metal dispersion may be used for other temperatures and dispersions is a welcome result that may broaden the application of quantitative FTIR. Rh dispersions were determined for Rh/SiO2 samples of 5 different weight loadings using the absorption intensities determined in this study. The variation of Rh dispersion with Rh loading was practically identical to that observed in nitrogen chemisorption experiments conducted on another series of Rh/SiO2 catalysts. Also, it was observed that the ratio of linear to bridges CO surface concentrations increased from 2 to 5 as Rh dispersion increased from 22 to 100%. These observations demonstrate the usefulness of a more fully developed quantitative FTIR technique.

1992

Catalyst Characterization Using Quantitative FTIR: CO on Supported Rh

Rasband, P.B. and Hecker, W.C.
Journal of Catalysis, 1992 (in press). Funded by Brigham Young University.

Qualitative FTIR has been and continues to be one of the most utilized tools in the characterization of supported metal catalysts. Quantitative FTIR has the potential to allow catalysis researchers to determine the surface concentrations of active intermediates. However, its successful application depends upon an understanding of the factors affecting integrated absorption intensities (coefficients relating IR absorbance to surface concentration). This work addresses the effect of metal particle size and temperature on the absorption intensities for CO chemisorbed on Rh/SiO2. Absorption intensities for both linear and bridged CO surface species (a1 and Ab) were determined by combining peak area data fro, IR spectra with uptake measurements obtained in gravimetric experiments. This resulted in an A1 value of 13 (± 2) and an Ab value of 42 (± 6) cm/µmol. No statistically significant particle size effect has been observed for average spherical particle diameters ranging from 13 to 58 angstroms (100 to 22% dispersion). Also, integrated absorption intensities for linear and bridged CO were shown to vary little over the temperature range of 323 to 473 K. The discovery that absorption intensities determined for one temperature and metal dispersion may be used for other temperatures and dispersions is a welcome result that may broaden the application of quantitative FTIR.

Rh dispersions were determined for Rh/SiO2 sample of 5 different weight loadings using the absorption intensities determined in this study. The variation of Rh dispersion with Rh loading was practically identical to that observed in hydrogen chemisorption experiments conducted on another series of Rh/SiO2 catalysts. Also, it was observed that the ratio of linear to bridges CO surface concentrations increased from 2 to 5 as Rh dispersion increased from 22 to 100%. These observations demonstrate the usefulness of a more fully developed quantitative FTIR technique.

Low-Temperature Char Oxidation Kinetics: Effect of Preparation Method

McDonald, K.M.; Hyde, W.D. and Hecker, W.C.
Fuel, 71(3):319-323, 1992. Funded by ACERC.

Chars derived from Beulah-Zap (lignite A) and Dietz (sub-bituminous B) coals were prepared by three different methods utilizing three different reactor systems. These included a high heating rate method achieved in a methane flat flame burner, a moderate heating rate method achieved in a drop tube reactor, and a slow heating rate method achieved in a muffle furnace. The flat flame char was produced in a flame environment with excess oxygen, while the drop tube and muffle furnace chars were produced in inert environments. Low temperature oxidation rates and kinetic parameters were determined using isothermal thermogravimetric analysis (TGA) at temperatures between 550 and 950 K. Reactivities at different oxidation burnout levels (10%-75%) were compared on both an initial mass and an available mass basis. Using the available mass basis, rates in the intrinsic regime were found to be nearly identical for the different burnout levels. It was also found that the lower burnout levels are more highly influenced by diffusion effects. This was manifest by a decrease in the slope of the Arrhenius plot that began at a temperature of about 750 K for the char at 10% burnout compared to a temperature of nearly 900 K for the char at 75% burnout. In comparing the chars produced by the three different methods, reactivities in the reaction control regime showed that, for both coals, the drop tube char was more reactive than either the flat flame or muffle furnace char. Further tests indicated that the drop tube chars had a hydrogen to carbon ratio that was 2.5 to 5 times greater than the char from either of the other reactors and the devolatilization conversion was significantly less. The activation energies for all three Beulah-Zap chars and for the Dietz muffle furnace and flat flame chars were found to be 28.1±0.6 kcal/mol. A comparison of the reactivities for the flat flame burner chars of the lignite and the sub-bituminous showed that the lignite chars were more reactive by a factor of two. This was consistent over all burnout levels. Further work with the Dietz flat flame char showed the dependency on oxygen concentration, yielding an apparent reaction order of 0.67±0.03. This is in excellent agreement with data found in the literature.

Molybdena, Ceria, and Niobia Addition to Supported Rh Catalysts: Effects on NO Reduction by CO

Hecker, W.C.; Wardinsky, M.D.; Clemmer, P.G. and Rasband, P.B.
Proceedings of the Twelfth Canadian Symposium on Catalysis, 211-218, Alberta, Canada, May 1992. Funded by Brigham Young Unversity.

The kinetics of the reduction of NO by CO over Rh/molybdena/silica, Rh/ceria/silica, Rh/niobia/silica, and Rh/ceria/alumina catalysts have been studied. Catalysts have been characterized using H2 chemisorption and quantitative FTIR techniques for dispersion determination, and using in-situ and post reaction FTIR spectroscopy for site distribution study. The chief effect of molybdena, ceria, and niobia addition under the conditions of this study appears to be in the alteration of effective Rh particle size (dispersion) although Mo addition does seem to more directly affect catalyst activity. A plot of turnover does seem to more directly affect catalyst activity. A plot of turnover frequency versus Rh particle ize shows that all of the catalysts with the exception of the Rh/molybdena/silica exhibit a common structure sensitivity. This sensitivity has been explained qualitatively through mechanistic arguments.

 

Effects of Burnout on Char Oxidation Kinetics

Hecker, W.C.; McDonald, K.M.; Reade, W.; Jackson, C.D. and Cope R.F.
Twenty-Fourth Symposium (International) on Combustion, Sydney, Australia, July 1992. (Previously presented at the 1991 Annual Meeting of the American Institute of Chemical Engineers, Los Angeles, CA, November 1991). Funded by ACERC.

The development and validation of accurate coal combustion modeling requires separate characterization of the coal devolatilization and char oxidation processes. The characteristic time for heterogeneous oxidation of a char particle in a commercial combustor is 1-2 orders of magnitude greater than the time required for devolatilization and homogeneous oxidation of the volatiles. Thus the rate of combustion and hence the efficiency of the coal combustion process will be governed by the rate of oxidation of the nonvolatile coal char.

The effects of both extent of burnout and type of burnout on char oxidation rates and rate parameters have been investigated for chars prepared from Dietz (subbituminous B) coal. Intrinsic rates and rate parameters (reaction order, activation energy, and pre-exponential factor) were determined using isothermal thermogravimetric analysis (TGA). N2BET and CO2DP surface areas were measured, as was hydrogen to carbon ratio. Three types of burnout were studied and compared. Devolatilization mass loss (DML) was studied by devolatilizing the Dietz coal to various extents in a flat-flame methane burner (FFB) and then comparing the oxidation rates and other properties of the resulting chars. High-temperature oxidation burnout was studied by taking a given FFB Dietz char and oxidizing it to various conversion levels in a drop tube reactor at high temperatures characteristic of industrial combustors. The oxidation rates and kinetics of these partially burned out char samples were then determined using TGA. Low-temperature oxidation burnout was studied by oxidizing the given FFB Dietz char to a continuum of burnout levels in the TGA at low temperature (550 to 750 K) and obtaining the instantaneous oxidation rates, which are inherent in the TGA experiment.

The rate of oxidation was found to decrease with increasing devolatilization residence time, even after devolatilization mass loss has become constant. Increasing N2 and CO2 surface areas with devolatilization residence time are inverse to the decreasing rates, and a constant difference between N2 and CO2 surface areas indicates dramatic changes in the mesopore surface area during devolatilization, but not in the micropore surface area. Intrinsic rates of chars oxidized at high temperatures were found to decrease with burnout level, while those of chars oxidized at low temperatures were essentially constant with burnout level.

1991

Changes in Surface Area, Pore Structure and Density During Formation of High-Temperature Chars from Representative U.S. Coals

White, W.E.; Bartholomew, C.H.; Hecker, W.C. and Smith, D.M.
Adsorption Science & Technology, 4:180-209, 1991. Funded by ACERC.

Multiple techniques (CO2 and N2 adsorptions, NMR spin relaxation of adsorbed water, He pycnometry, and Hg porosimetry) were combined in a comprehensive study to determine changes in surface area (CO2 and nitrogen), density (solid, particle, and bulk), and pore structure (pore size and volume distributions of micro-, meso-, and macropores) in high temperature char formation from rank-representative U.S. coals of the ANL and PETC Banks (i.e. Beulah Zap, Dietz, Utah Blind Canyon, Pittsburgh No. 8, and Pocahontas No. 3). Chars were formed at high heating rates in a flat flame burner (maximum temperature of 1473 K), a process representative of char formation in pulverized coal combustion. It was determined that most of the surface area of coals was found in micropores with radii less than 1.5 nm, while 95% or more of the pore volume in the coals (85% of that in chars) is contained in mesopores (radii > 20 nm). During high temperature formation of char in a flame: (1) CO2 surface areas (involving mainly micropores, rpore < 1.5 nm) increase 2-3 fold, while N2 surface areas, (involving mesopores, 1.5 nm < rpore < 20 nm) increase 20-200 fold, (2) solid densities increase about 25% due to graphitization, while particle densities decrease by about a factor of two due to large increases in particle porosity, (3) pore volumes are increased 5-10 fold, and (4) total porosities are increased 3-4 fold, most of this increase occurring in the macropore range. The larger surface areas and porosities of chars relative to coals may be explained by (i) the removal by pyrolysis of strongly adsorbed molecules or volatile hydrocarbons from micropores and small mesopores that would otherwise hinder access of CO2 and N2, (ii) creation of new pores during the restructuring process involved in charification, and (iii) opening up by gasification with oxygen of new pores previously blocked to gas adsorption. Preparation conditions (e.g. atmosphere, heating rate, and temperature) greatly affect the physical properties including surface area, porosity and density of the resulting chars. The degree of carbon burnout is an important correlating factor affecting these properties.

1990

NO Reduction Activity and FTIR Characterization of Rhodium on Niobia-Modified SiO2

Rasband, P.B. and Hecker, W.C.
Catalysis Today, 8, 99-111, 1990. Funded by Brigham Young University.

Several 2% Rh/silica catalysts containing from 0 to 6% Nb205 were prepared by consecutive impregnations with aqueous solutions of niobium oxalate and rhodium trichloride. These catalysts were studied using Fourier Transform Infrared Spectroscopy (FTIR) to determine Rh oxidation state and dispersion. The addition of Nb205 to Rh/SiO2 resulted in a decrease in Rh(0) for relatively low niobia loadings (0 to 3% Nb2O5) and an increase in Rh(1) for higher niobia loadings (3 to 6% Nb205). These two effects combined to give a minimum Rh dispersion for a catalyst containing approximately 3% Nb2O5. For the reduction of NO by CO the niobia addition decreased the observed rate (per gram catalyst) but had little effect on activation energy or concentration dependencies. A combination of the observed rate and Rh dispersion data suggests that specific rate varies inversely with Rh dispersion for these catalysts and conditions.

Low Temperature Kinetics of Coal Char Oxidation

McDonald, K.M.; Hyde, W.D. and Hecker, W.C.
Western States Section/The Combustion Institute, San Diego, CA, 1990. Funded by ACERC.

An accurate char oxidation submodel is an essential element of a realistic model of coal combustion. To aid in the development and validation of such a submodel, which will be used in comprehensive combustion codes being developed at ACERC, oxidation rates and rate parameters were measured for chars prepared from ACERC coals. In this study, chars derived from Beulah-Zap (lignite A) and Dietz (sub-bituminous B) coals were prepared in three different reactor systems: (1) a flat-flame methane burner, (2) an inert-atmosphere drop tube reactor, and (3) a muffle furnace. Low temperature oxidation rates and kinetic parameters were determined using isothermal thermogravimetric analysis (TGA) at temperatures between 550 and 950 K.

Reactivities at different oxidation burnout levels (10%-75%) were compared on both an initial mass and an available mass basis. The initial mass basis provided no clear distinction from which behavioral differences in the different burnout levels could be deduced. Basing rates on the available mass, however, elucidated the fact that in the intrinsic regime, rates are nearly identical for the different burnout levels. The available mass basis also made clear the fact that the lower burnout levels are more highly influenced by diffusion effects. This was manifest by a decrease in the slope of the Arrhenius plot beginning at a temperature of about 750 K for the char at 10% burnout compared to a temperature of nearly 900 K for the char at 75% burnout.

In comparing the chars produced in the three different reactors, reactivities in the reaction control regime showed that, for both coals, the drop tube char was more reactive than either the flat flame or muffle furnace char. Further tests indicated that the drop tube chars had a hydrogen to carbon ratio that was 2.5 to 5 times greater than the char from either of the other reactors and the percent conversion for devolatilization was significantly less. The activation energy for all three Beulah-Zap chars and for the Dietz muffle furnace and flat flame chars was found to be 28.10.6 kcal/mol. Data from the Dietz drop tube char is as yet insufficient to yield an accurate activation energy value.

A comparison of the reactivities for the flat flame burner chars of the lignite and the sub-bituminous showed that the lignite chars were more reactive by a factor of two. This was consistent over all burnout levels. Further work with the Dietz flat flame char showed an apparent reaction order of 0.66. This is in excellent agreement with data found in the literature.

Changes in Surface Area, Pore Structure and Density During Formation of High-Temperature Chars from Representative U.S. Coals

White, W.E.; Bartholomew, C.H.; Hecker, W.C. and Smith, D.M.
Adsorption Science & Technology, 1990 (In press). Funded by ACERC.

Multiple techniques (CO2 and N2 adsorptions, NMR spin relaxation of adsorbed water, He pycnometry, and Hg porosimetry) were combined in a comprehensive study to determine changes in surface area (CO2 and nitrogen), density (solid, particle, and bulk), and pore structure (pore size and volume distributions of micro-, meso-, and macropores) in high temperature char formation from rank-representative U.S. coals of the ANL and PETC Banks (i.e. Beulah Zap, Dietz, Utah Blind Canyon, Pittsburgh No. 8, and Pocahontas No. 3). Chars were formed at high heating rates in a flat flame burner (maximum temperature of 1473 K), a process representative of char formation in pulverized coal combustion. It was determined that most of the surface area of coals was found in micropores with radii less than 1.5 nm, while 95% or more of the pore volume in the coals (85% of that in chars) is contained in mesopores (radii > 20 nm). During high temperature formation of char in a flame: (1) CO2 surface areas (involving mainly micropores, rpore < 1.5 nm) increase 2-3 fold, while N2 surface areas, (involving mesopores, 1.5 nm < rpore < 20 nm) increase 20-200 fold, (2) solid densities increase about 25% due to graphitization, while particle densities decrease by about a factor of two due to large increases in particle porosity, (3) pore volumes are increased 5-10 fold, and (4) total porosities are increased 3-4 fold, most of this increase occurring in the macropore range. The larger surface areas and porosities of chars relative to coals may be explained by (i) the removal by pyrolysis of strongly adsorbed molecules or volatile hydrocarbons from micropores and small mesopores that would otherwise hinder access of CO2 and N2, (ii) creation of new pores during the restructuring process involved in charification, and (iii) opening up by gasification with oxygen of new pores previously blocked to gas adsorption. Preparation conditions (e.g. atmosphere, heating rate, and temperature) greatly affect the physical properties including surface area, porosity and density of the resulting chars. The degree of carbon burnout is an important correlating factor affecting these properties.

1989

The Effects of Rank and Preparation Method on Coal Char Oxidation Rates

Hyde, W.D.; Hecker, W.C.; Cope, R.F.; Painter, M.M.; McDonald, K.M. and Bartholomew, C.H.
Western States Section/The Combustion Institute, Livermore, California, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).

Coal char reactivity has been found to vary greatly depending on the rank and type of the parent coal. Also, the conditions under which a given coal is devolatilized to produce char can significantly effect the reactivity of the resulting char. Preparation conditions such as gas environment, heating rate, peak temperature, residence time, and particle size are very important in determining the resulting char reactivity in that they effect its chemical and physical structure.

The general objectives of this work are to (1) understand the effect of the rank of the parent coal on the oxidation rate (reactivity) of its derived char, (2) understand the effect of devolatilization conditions on the char oxidation rate, and (3) determine any correlations which may exist between char oxidation rates and the chemical and physical properties of the chars. Specifically, oxidation rates for char from 5 coals of various ranks were measured and compared. The differences in char reactivity of chars produced in three different char preparation apparatus: a muffle furnace, a flat-flame methane burner, and a high temperature inert-atmosphere reactor, were studied. The effects of peak temperature, residence time, and particle size were also studied. Finally, correlations of oxidation rate with hydrogen content, cluster size, and surface area were attempted.

Samples of chars from Beulah Zap (Lignite), Dietz (Subbituminous A), Utah Blind Canyon (hvC Bituminous), Pittsburgh #8 (hvA Bituminous), and Pocahontas #3 (lv Bituminous) were prepared at different residence times in the Flat-Flame Char Preparation Apparatus; samples of Pittsburgh #8, Beulah Zap, and Dietz were also prepared in the muffle furnace and the high temperature inert-atmosphere reactor. The low temperature reactivity of all the coal char samples was determined in a TGA using Tcrit as the reactivity indicator. Tcrit is defined at the temperature at which the mass loss of the sample reaches 11 percent per minute.

Effects of Char Preparation Variables on Reactivity and Surface Properties of Chars Derived from ANL Coals

Hecker, W.C.; Hyde, W.D. and McDonald, K.M.
International Chemical Congress of Pacific Basin Societies, Honolulu, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).

The objective of this study is to understand the effect of devolatilization conditions on the oxidation rate (reactivity) and surface properties of the resultant char. Chars were produced from 3 ANL coals (Pittsburgh No. 8, Beulah Zap, and Pocahontas No. 3) and Dietz subbituminous (PETC) by three different methods: (1) a flat flame methane burner, (2) a rapid heating-rate inert atmosphere reactor, and (3) a slow heating-rate muffle furnace. Thermogravimetric analysis (TGA) was used to determine the low-temperature reactivity of these chars as a function of residence time and peak temperature. CO2 surface areas pore size distributions were also determined for some samples. Trends of increased reactivity with decreasing rank and decreasing residence time were observed. Preliminary results indicate that surface area goes through a maximum with increasing devolatilization time for Pittsburgh No. 8 while it is essentially unaffected for the Zap lignite char. Char reactivity appears to be correlated with hydrogen content.

Effects of Preparation Variables on Reactivity and Surface Properties of ACERC Coal Chars

Hyde, W.D.; McDonald, K.M.; Cope, R.F.; Bartholomew, C.H. and Hecker, W.C.
Twelfth Symposium of the Rocky Mountain Fuels Society, Denver, 1989. (Also presented at the Rocky Mountain Regional AIChE Meeting, Salt Lake City, 1989). Funded by ACERC (National Science Foundation and Associates and Affiliates).

The combustion of coal consists of devolatilization and heterogenous char oxidation. The objective of this study is to understand the effect of devolatilization conditions on the oxidation rate (reactivity) and surface properties of the resultant char. Chars were produced by devolatilizing Pittsburgh No. 8 bituminous coal and North Dakota lignite at varying residence times and peak temperatures in a flat flame methane burner. Thermogravimetric analysis was used to determine the low-temperature reactivity of these chars. Trends of increased reactivity with decreased preparation temperature and residence time are observed. N2 and CO2 surface areas and pore size distributions were determined. Preliminary results indicate that surface areas increase with increased devolatilization. Char reactivity is also correlated with hydrogen content and aromatic cluster size.

Oxidation Rate Measurements of Coals and Derived Chars

Wells, W.F.; Hyde, W.D.; Cope, R.F.; Smoot, L.D.; Hecker, W.C. and Bartholomew, C.H.
Twelfth Symposium of the Rocky Mountain Fuels Society, Denver, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).

To aid in the development and provide validation data to the char oxidation submodel, measurements of oxidation rates at high and low temperatures are being collected for chars prepared from five select ACERC coals. Chars are prepared using a flat flame burner and a recently constructed inert atmosphere drop-tube reactor heated using an inductively coupled plasma. The drop-tube reactor is also used to obtain reaction rates at high temperatures; low temperature rates were measured in a TGA. Chemical and physical properties of the fuel were measured. Multivariate statistics are used to correlate fuel properties to reaction rates.

Surface Properties and Pore Structure of ACERC Coals and Chars

White, W.E.; Bartholomew, C.H.; Thornock, D.; Wells, W.F.; Hecker, W.C. and Smoot, L.D.
Twelfth Symposium of the Rocky Mountain Fuels Society, Denver, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).

Results of an ongoing collaborative study of the surface properties and pore structure of a suite of 11 coals selected for comprehensive study by ACERC are reported. The principal objective is to correlate the surface, pore, and chemical properties of coals and chars with their rates of combustion. Surface areas, pore volumes, pore size distributions, and solid densities were measured for Pittsburgh No. 8, Wyodak, Beulah Zap Lignite, Lower Wilcox, Dietz and a Utah Scofield coal and for chars derived from these coals. Surface areas, pore volumes and pore size distributions were measured using nitrogen and carbon dioxide adsorptions, mercury porosimetry and NMR spin-lattice relaxation measurements for samples saturated with water vapor. Solid densities were obtained using helium displacement. The results indicate that chars have larger surface areas and pores volumes relative to coals. New mesopores are created and micropore volume increases during devolatilization. Large fractions of the internal pore volume of coals are not penetrated by nitrogen molecules during adsorption, but are penetrated by carbon dioxide, suggesting that a fraction of the pore volume is microporous, or involves blocked pores. By using several techniques for measuring surface properties (e.g. N2 and CO2 adsorption isotherms, NMR,etc.), the pore structure of coals and chars can be defined more accurately, and char oxidation models can be evaluated with more understanding.

Pore Structure Characterization of Coals & Chars Via NMR

Davis, P.J.; Smith, D.M.; Bartholomew, C.H.; White, W.E. and Hecker, W.C.
Twelfth Symposium of the Rocky Mountain Fuels Society, Denver, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).

Due to the wide pore size range and complexity of coals and chars, it is difficult to study the pore structure. Multiple techniques such as gas adsorption, mercury porosimetry, and density measurements are often used. These techniques suffer from limited pore size range, pore shape assumption, network/percolation effects, and sample changes during analysis. To obtain a more complete description of coals and chars, NMR spin-lattice relaxation measurements of water saturated coals and chars have been performed. The NMR technique does not suffer from network/percolation effects, sample changes, pore shape assumption (rp>5nm). Three coals and their chars were compared.

Surface Areas and Pore Structures of ANL and PETC Coals and Derived Chars

Bartholomew, C.H.; White, W.E. ; Hecker, W.C.; Smith, D.M.
4th Annual Meeting of the Western States Catalysis Club, Denver, Colorado , 1989. (Also presented at the Western States Section,The Combustion Institute, Livermore, California, 1989). Funded by ACERC (National Science Foundation and Associates and Affiliates).

Surface areas, pore volumes, and pore size distributions of five Argonne National Lab (ANL) coals (Pittsburgh No. 8, Illinois No. 6, Pocahontas No. 3, Beulah Zap lignite, and Utah Blind Canyon) and of two PETC coals (Lower Wilcox and Dietz) and chars derived from these coals area being measured in an ongoing study. Data obtained for several of these coals and chars will be reported. Surface areas, pore volumes and pore size distributions were measured by nitrogen and carbon dioxide adsorptions at 77 K and 195-300 K respectively; pore volumes and pore size distributions were also determined by NMR spin-relaxation measurements of samples saturated with water. Comparisons of accuracy and precision for static vacuum and flow desorption methods were made. Surface areas and pore volumes measured by adsorption in a static vacuum system and by desorption in a flow/TC detector system agree to better than 1-5% where adsorption equilibrium has obtained.

The results provide new insights into the surface and pore structure of coals and chars as functions of rank and charification. Surface areas of coals generally increase with decreasing rank. Chars have larger surface areas and pore volumes than the parent coals; indeed surface areas measured by nitrogen adsorption are up to two orders of magnitude larger, while those measured by carbon dioxide adsorption are 2-3 times larger. Pore volumes of chars measured by nitrogen adsorption are 10-20 time those of the parent coals. Large fractions of the internal surfaces of coals and pore diameters are microporous (pore diameters of 1 nm or smaller) and are not easily penetrated by nitrogen molecules at 77 K. In the case of some coals, while the pore volume increases during devolatilization, the shape of the pore size distribution stays the same. For other coals, the pore size distribution changes radically during devolatilization. This systematic study of surface areas and pore structures of coals and chars provides insights into physical changes that occur during coal devolatilization and char burnout. This information can be useful in characterizing the evolution of pore structure and its effect on diffusion of reactant in and products out during combustion of coal chars.

1988-1987

Surface and Pore Properties of ANL and PETC Coals

Bartholomew, C.H.; White, W.E.; Thornock, D.; Wells, W.F.; Hecker, W.C.; Smoot, L.D.; Smith, D.M. and Williams, F.L.
Preprint ACS Fuels Chem. Divl., 1988, Los Angeles. 9 pgs. Funded by ACERC (National Science Foundation and Associates and Affiliates).

Surface areas, pore volumes, pore size distributions, and solid densities were measured for three ANL coals (Pittsburgh No. 8, Wyodak, and Beulah Zap Lignite), two PETC coals (Lower Wilcox, and Dietz) and a Utah Scofield coal and for chars derived from these coals. Surface areas were measured using nitrogen and carbon dioxide adsorptions; pore volumes were determined using nitrogen adsorption, mercury porosimetry, and NMR spin-lattice relaxation measurements of samples saturated with water. Solid densities were obtained using helium displacement. The results indicated that chars have larger surface areas and pores relative to coals; large fractions of the internal surfaces of coals are not penetrated by nitrogen molecules but are penetrated by carbon dioxide suggesting that the pores are mostly smaller than 1 NM

Char Preparation Facility

Merrill, R.; Bartholomew, C.H. and Hecker, W.C.
ACERC Report, 1987. Funded by ACERC (National Science Foundation and Associates and Affiliates).

During summer 1987 a facility for preparation of coal chars was designed and constructed in a cooperative effort of the catalysis and combustion laboratories. The system devolatilizes coal particles fed up through a flat-flame burner after which they are collected on a water-cooled, gas-quenched probe. The system has been successfully tested in the preparation of a char from a Texas lignite coal. The system is capable of producing about 5-10 g/hr of high-temperature char.

Fuel Characteristics and Reaction Mechanisms

Lee, M.L.; Bartholomew, C.H. and Hecker, W.C.
ERC Symposium, Annual ASEE Meeting, 1987, Reno, Nevada. Funded by ACERC (National Science Foundation and Associates and Affiliates).

The main goal of the Advanced Combustion Engineering Research Center is the development and implementation of advanced combustion models. The Center research is organized around six major thrust areas focused on the clean and efficient use of low-grade fuels such as coal. These thrust areas will provide data on kinetics, fuel properties, and process-performance design characteristics that will be integrated into a comprehensive computer model used in the design and optimization of advanced combustion systems. This paper deals with the work in the fuel characterization and reaction mechanisms thrust area.

The research project in the fuel characteristics and reaction mechanisms thrust area are focused on relating the kinetic rates and mechanisms of rapid coal devolatilization and char reactivity with the physical and chemical structure of coal and pyrolysis tars and chars. This paper summarizes the results from four integrated research programs in this thrust area.

Supercritical solvent extraction is employed to determine the amount and nature of hydrocarbons that are physically absorbed or only weakly bound within the coal structure and are expected to be liberated early in the devolatilization process. Both paraffin (n-alkanes, isoprenoids, and pentacyclic triterpanes) and polycyclic aromatic (two to five fused aromatic rings) hydrocarbons have been identified. Pyrolysis mass spectroscopy provides both rate data and pyrolysis tar data on coals during slow devolatilization. The physical properties (surface area and pore size distribution) of the parent coals and pyrolysis chars are studied in order to relate these properties to coal and char reaction rates. Advanced solid-state NMR techniques are used to obtain the carbon skeletal structure of the parent coals and pyrolysis chars. High field, high-resolution NMR spectroscopy experiments provide data on the structural features of the pyrolysis tars.

The experiments in this thrust area are carried out on a common set of standard coals. The devolatilization studies to be initially carried out in collaboration with other laboratories will be summarized. A description will be provided for the analysis and integration of the various experimental data. These data are used in the development of coal devolatilization and char reaction sub-models in comprehensive combustion models. The means for integrating the chemical data into the combustion code will be described.

The Effect of Weight Loading and Reduction Temperature on Rh/Silica Catalysts for NO Reduction by CO

Hecker, W.C and Breneman, R.B.
Catalysis and Automotive Pollution Control, 30, 257-265, 1987. Funded by Brigham Young University.

Rhodium catalysts with very low weight loadings (0.01 to 0.06%) are used to efficiently reduce the nitrogen oxides in automobile exhaust. Many academic studies, however, are done suing catalysts with high weight loadings (1 to 10%). The study reported herein explored differences in activity and surface properties between high and low weight-loaded Rh catalysts before and during NO reduction by CO. Initial and steady state turnover numbers were found to increase significantly (factor of 7) as weight loading was increased from 0.2% to 12%. At the same time the activation energy decreased form 36 to 24 kcal/more. Power rate laws determined by varying NO and CO partial pressures were found to be fairly similar for the high and low weight-loaded catalysts. These results seem to indicate that NO reduction is a structure sensitive reaction. The effect of varying the reduction temperature of the catalysts between 200 and 450ºC was also explored, but no significant effect was seen on hydrogen uptakes, infrared spectra, initial rates, or steady state rates.

The Effects of Nitric Oxide Reduction Activity of Molybdena and Ceria Addition to Rh/Silica Catalysts

Hecker, W.C.; Wardinsky, M.D.; Clemmer, P.C. and Breneman, R.B.
Proc. of 9th Inter. Congress on Catalysts, 1988. Funded by Brigham Young University.

The effects of molybdena and ceria addition on the catalytic activity of silica-support rhodium for the reduction of NO by CO have been measured. FTIR spectroscopy was used as a qualitative and quantitative probe of the catalyst surface to determine the type and number of Rh surface sites present under reaction conditions. The addition of Mo was seen to have only a small effect on the bulk rate, but had a significant effect on transient behavior, turnover frequency, Rh oxidation state, and power rate law. A 1% Rh / 4% MO catalyst which was prepared by consecutive impregnation with intermediate calcination exhibited no transient decay and had a steady-state turnover frequency 2.5 times that observed for 1% Rh/silica. Infrared spectra of this catalyst showed a new dicarbonyl species with bands at 2110 cm-1 that indicate that the Rh is in a more oxidized state in the Rh/MO catalyst than in straight Rh. The NO partial pressure dependency was seen to be much more negative on Rh/MO than on Rh, probably due to MO-assisted NO adsorption. The addition of CE was seen to give opposite behavior to MO addition in terms of Rh dispersion, turnover frequency, and NO partial pressure dependency. While MO addition decreased the Rh dispersion, the addition of CE was seen to increase it; however, it decreased turnover frequency. The NO partial pressure dependency became less negative than that for straight Rh.