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During coal combustion, NOₓ reduction occurs by two possible routes: homogeneous reduction by hydrocarbons and heterogeneous reduction by char formed during coal devolatilization. This paper investigates the latter route, which also has potential as the basis for post-combustion NOₓ clean-up processes, including reburning. The purpose of this investigation is to develop a kinetic model for the reduction of NOₓ by char during coal combustion and to understand the role of coal rank and char surface area on the resulting char reactivity. This investigation reports original kinetic data for nine char samples including graphite, coconut char, and five coal chars ranging in rank from lignite to low-volatile bituminous (Beulah-Zap, Dietz, Utah Blind Canyon, Pittsburgh #8, and Pocahontas #3). An empirical kinetic model with six universal (not char-specific) parameters reproduces the experimental data for all chars. The investigation also presents an alternative and simpler model with only two parameters that differ for each char type. Correlations for the two model parameters were then developed as a function of two char surface areas: (1) active mineral matter surface area measured using CO₂ titration after high-temperature exposure and (2) total sample surface area measured using CO₂ at room temperature and Dubinin–Polanyi theory. Predictions of the rate constant values over a wide range of temperature using this universal approach with only the surface areas differing among the six chars generally fit the experimental data within ±50%.

A series of silver-promoted, 20 wt% cobalt Fischer-Tropsch synthesis (FTS) catalysts supported on an alumina modified with 5 wt% silica were prepared using two methods: traditional incipient wetness impregnation (IWI) and a new solvent deficient precipitation (SDP) technique. Catalysts containing silver promoter concentrations of 0.3, 0.6, 1.2, and 2.5 wt% were prepared using each of the two methods. Silver improved the reducibility of the cobalt significantly, lowering reduction temperatures by up to 100 °C, and increasing the extent of reduction by up to 35%. Further, in both preparation methods, changing the silver loading altered the cobalt dispersion. The smallest Co crystallite size was achieved with 1.2 wt% Ag loading, which produced average cobalt crystallite sizes of about 7 nm. The Fischer-Tropsch CO consumption rate increased with decreasing crystallite size and thus was highest for 1.2 wt% Ag. Intrinsic CO consumption rates per Co site (CO turnover frequency) were also measured for each catalyst. A clear increase in the intrinsic turnover frequency (TOF) was observed as Ag loading was increased from 0.6% to 1.2 wt% for both preparation methods. Both 1.2 wt% Ag catalysts produced TOF’s of ∼0.050 s −1 (equivalent to a CO consumption rate of ∼65 mmol g cat −1 h −1 ), which are 20–30% higher than the catalysts containing 0.3, 0.6, and 2.5 wt% Ag and are comparable to reported rates for commercial FTS catalysts. Higher loadings of Ag (2.5 wt%) resulted in higher extents of reduction, but led to lower TOFs and larger Co crystallite diameters, which is assumed to be due to blockage of active sites and changes in the Ag-Ag and Ag-Co coordination ratio. Thus, silver promotion appears to improve catalytic performance by enhancing cobalt reduction, dispersion, and electronic properties. The SDP and IWI methods produced catalysts with essentially the same properties, but the SDP method is a simpler, one-pot technique that offers many potential advantages.

Four identically-prepared 20 wt% cobalt/0.15 wt% ruthenium Fischer-Tropsch catalysts, supported on different aluminas, were synthesized by incipient wetness impregnation, characterized, and compared based on their performance under standard Fischer-Tropsch synthesis conditions. Three alumina supports were obtained from commercial sources (Sasol, St. Gobain, and Alfa Aesar), while the fourth was made using a published technique that included 5 wt% silica. The silica-stabilized alumina catalyst produced superior Fischer-Tropsch rates of 49 mmol CO/gcat h, compared to the other catalysts on the commercial supports with rates of 26–39 mmol CO/gcat h. The improved performance of the silica-stabilized alumina support compared to the commercially available aluminas is ascribed to a bimodal pore structure with larger average diameters and enhanced thermal stability. The support can be thermally treated to higher temperatures (e.g., 1100 °C) without transformation into α-alumina, allowing dehydroxylation of the surface prior to impregnation to stabilize a nearly ideal dispersion of cobalt crystallites.

A novel solvent deficient precipitation (SDP) method to produce nanoparticles was studied for its potential in Fischer-Tropsch synthesis (FTS) catalysis. Using Fe(NO3)3·9H2O as the iron-containing precursor, this method produces ferrihydrite particles which are then dried, calcined, reduced, and carbidized to form the active catalytic phase for FTS. Six different drying profiles, including final drying temperatures ranging between 80 and 150°C, were used to investigate the effect of ammonium nitrate (AN), a major by-product of reaction between Fe(NO3)3·9H2O and NH4HCO3 in the SDP method. Since AN has two phase-transitions within this range of drying temperatures, three different AN phases can exist during the drying of the catalyst precursors. These AN phases, along with physical changes occurring during the phase transitions, may affect the pore structure and the agglomeration of ferrihydrite crystallites, suggesting possible reasons for the observed differences in catalytic performance. Catalysts dried at 130°C showed the highest FTS rate and the lowest methane selectivity. In general, better catalytic performance is related to the AN phase present during drying as follows: phase III > phase II > phase I. However, within each AN phase, lower drying temperatures led to better catalytic properties.

Kinetic data were measured for Fischer-Tropsch Synthesis (FTS) on a cobalt catalyst supported on silica-modified alumina at various partial pressures of CO and H 2 and at four different temperatures (210, 220, 230, and 240 °C). The data were sufficient to determine power law rate expressions at each of the four temperatures which indicate that the dependence of rate on P H2 increases with increasing temperature while the rate coefficient for P CO decreases with temperature going from positive order (+0.3) at 210 °C to negative order (−0.6) at 240 °C. Several mechanisms were explored and Langmuir–Hinshelwood (LH) rate models derived in an attempt to explain the data trends. Traditional FT rate expressions have a denominator term, K CO *P CO which, since the denominator is squared in LH expressions, can allow for an overall negative order P CO dependence. However, since K CO is the equilibrium constant for the adsorption of CO, its value decreases with increasing temperature which causes the overall P CO dependence to become more positive with increasing temperature instead of more negative. In this work we propose a mechanism based on parallel hydrogen-assisted mechanistic pathways that leads to a LH model with the denominator term, k’ CO *P CO where k’ CO is effectively an activated rate constant instead of an equilibrium constant and therefore increases with increasing temperature instead of decreasing. This model, which fits the data extremely well, also explains the presence of atomic carbon on the surface of the catalyst.

The kinetics of a supported iron Fischer–Tropsch (FT) catalysts were investigated and a physically meaningful model that fits the data very well is proposed. Kinetic data (reported herein) were obtained at 250 °C and 20 atm in a fixed bed reactor at a variety of P H 2 and P CO. Measured P H 2 and P CO power law dependencies were found to be in the same range as those for unsupported Fe FT catalysts previously reported. The kinetic models in this study were tested using a statistical lack-of-fit test. Eight, two-parameter Langmuir–Hinshelwood rate expressions based on various mechanistic routes and assumptions were derived and tested, but all gave relatively poor fits to the data. An adjustment of the P H 2 dependency of the derived expressions to the 0.875 power resulted in three reasonable semi-empirical models, one of which fit the data extremely well. This approach also allowed us to determine the best function of P CO dependency. The results suggest that supported Fe FT catalysts follow a direct CO dissociation pathway, that carbon is one of the most abundant species on the surface of the catalyst, and that the hydrogenation of either C* or CH* is the rate-determining step.

Effects of CO and H 2 partial pressures on the deactivation by carbon, in the absence of other deactivation forms, of a CoPt/AlSi Fischer–Tropsch catalyst were investigated during six 800–900 h runs at 230 °C. Sintering was eliminated by using a catalyst with relatively large Co crystallites; cobalt-aluminate formation and oxidation were eliminated by operation at lower conversions and confirmed with TPR and XANES/EXAFS results; and physically blocking of pores by heavy hydrocarbons was avoided using a large pore support. Both (a) decreasing H 2 /CO ratio at constant P CO and (b) increasing H 2 /CO ratio at constant P H2 increase deactivation rate, possibly due to (a) formation of less hydrogenated and (b) more hydrogenated polymeric carbon forms, respectively. Deactivation rate was found to increase with increasing polymeric carbon deposition and decreasing CO-uptakes of aged catalyst samples. Methane selectivity increases as the catalyst deactivates only for runs at high CO pressures.

A previously developed one-dimensional reactor model was employed to understand the effects of pellet size and geometry on the performance of a wall-cooled multitubular fixed-bed Fischer–Tropsch reactor for producing hydrocarbons from synthesis gas. The effects of pellet size/shape on catalyst effectiveness, bed void fraction, and overall heat transfer coefficient were studied through a comprehensive parametric study of a reactor with cobalt catalyst. The relative impact of each of these parameters on the overall required amount of catalyst was also determined. The simulations show that the amount of catalyst required to achieve a specified conversion increases with pellet size and shape in the order: trilobes < hollow cylinders < cylinders < spheres. The pressure drop per unit length can be significantly reduced and the catalyst effectiveness increased by using advanced extrudates, i.e., trilobes or hollow cylinders.

Recent breakthroughs in the preparation of transition metal oxide nanoparticles by solvent-deficient precipitation (SDP) provide exciting new possibilities for simpler and faster methods to prepare heterogeneous catalysts with small uniformly sized crystallites of the active phase. In this study, the authors apply the SDP method to prepare an unsupported iron Fischer–Tropsch (FT) catalyst with a nominal composition of 100 Fe/5 Cu/4 K/16 SiO2 (relative mass) and an average crystallite size of 10–11 nm. Three preparations of this catalyst were made using SDP, and each was tested for over 200 h in a fixed-bed reactor to determine activity, selectivity, and stability. Values of the apparent first-order rate constants for syngas consumption at 260 °C were 215–251 mmol gFe–1 h–1 MPa–1 for our catalyst compared to 155–265 mmol gFe–1 h–1 MPa–1 for literature catalysts not using a proprietary pretreatment (Bukur, D. B.; Lang, X. Ind. Eng. Chem. Res. 1999, 38, 3270−3275). Our catalyst was tested for more than 500 h...

Transfer limitations were successfully modeled and optimized for iron-based catalysts for Fischer–Tropsch synthesis. The simulation model predicts the effect of changing reaction temperatures, reaction pressures, catalyst pellet size, and the feed CO composition on pore diffusion, film heat transfer, internal heat transfer, and pressure drop. The comprehensive contour maps obtained from the model quantitatively display the effects of these various design variables to both optimize catalyst design and provide guidance for kinetic experiments. The optimization results favor higher reaction temperatures and pressures, smaller pellet sizes, and lower feed CO compositions to maintain high activity of kinetically-controlled reaction rates. The optimal temperature (255.8 °C) was constrained by the rate of catalytic deactivation. The model was validated by experimental data acquired from a fixed-bed reactor and shows excellent agreement. The model predicts the observed rate to be 79% of the intrinsic rate at 250 °C, 20 bar, equimolar H2:CO, and 425 μm pellet size, while experimental results showed this percentage was 74 ± 7% for 250–600 μm pellets. The model predicts no pore-diffusion limitations at pellet sizes smaller than 250 μm, indicating that the reaction rate is kinetically-controlled. Furthermore, the resistance due to film temperature gradients is more limiting than that due to intraparticle temperature gradients. Finally, pressure drop was well below 10% of the inlet reactor pressure under laboratory-scale conditions. The model was used to predict the effect of using smaller catalyst pellets on pressure drop for a commercial-scale reactor, which showed that acceptable operation could be expected with a pressure drop of 20% of the inlet reactor pressure.

The effects of support properties including pore size, hydroxyl group concentration, and support stabilizer were investigated for six alumina-supported FeCuK Fischer–Tropsch catalysts containing 20% or 40% iron. Catalysts were supported on one of four aluminas stabilized with La2O3 or SiO2. A large pore support was found to accommodate 40% Fe without pore blockage. Catalyst activity was found to increase with increasing support pore size, lower OH group concentrations, and higher extents of reduction. SiO2 was found to be a more effective hydrothermal stabilizer than La2O3 as it suppresses the high-temperature transformation of γ-Al2O3 to α-Al2O3 up to 1200 °C allowing SiO2-stabilized catalysts to be dehydroxylated at higher temperatures. SiO2 stabilization also inhibits deactivation by sintering and increases activity, possibly due to surface silicate groups. Indeed, the activity of Fe/Si–Al2O3 continues to increase after 700 h on stream, while the Fe/La–Al2O3 catalysts lose activity over the same time period.

This paper documents the performance and kinetics of an iron/manganese oxide catalyst in a fixed-bed reactor by Fischer–Tropsch synthesis (FTS) under conditions favoring the formation of gaseous and liquid hydrocarbons (P: 1–12 bar; T: 513–543 K; H 2 /CO:1, 1.5, 2 mol/mol; gas hourly space velocity: 4200–7000 cm 3 (STP)/h/g cat ). Based on the hypothesis that water inhibits the intrinsic FTS reaction rate, eight kinetic models are considered: six variations of the Langmuir–Hinshelwood–Hougen–Watson representation and two empirical power-law models. The kinetic expression/mechanism that most precisely fits the data assumes the following: (1) CO dissociation is reversible and does not involve hydrogen; (2) all hydrogenation steps are irreversible, or the first hydrogenation step is slow and rate determining. Also, the performance of the catalyst for FTS and the hydrocarbon product distributions were investigated under different reaction conditions.

The effect of noble metal (NM) promoter (Pt, Re, or Ru) deposition order was investigated for commercially-representative Co Fischer–Tropsch catalysts (nominally 25 wt% Co on a La stabilized alumina support). The effects of deposition sequence (co-deposition vs. sequential deposition) were studied in terms of NM retention and distribution, Co crystallite size, Co reducibility, and catalyst activity and selectivity. Ru retention was

Acid site properties of silica-doped aluminas prepared by a simple solvent deficient hydrolysis of the alkoxides was investigated. The total acid concentration (Bronsted and Lewis sites) of silica-doped aluminas (SDAs) calcined in the range of 700–1200 °C with Si/Al ratios of 5, 15, 27 wt% was determined using temperature-programmed desorption of ammonia (ammonia-TPD). 27 Al solid state MAS NMR (Al SS MAS NMR) was used to measure the intrinsic Lewis acid site concentration, and FTIR was also used as a separate measure of the Bronsted and Lewis acid site concentration. Results indicate that removing hydroxyl groups in the form of water molecules through calcination result in a lower concentration of Bronsted acid sites. Calcination at higher temperature also results in the transformation of unsaturated 5-coordinated aluminum (a strong Lewis acid) to higher concentrations of 6 and 4-coordinated aluminum in 5, 15 and 27% silica-doped alumina samples. Therefore, the total acid site concentration (Bronsted and Lewis sites) decreases by increasing the calcination temperature. In addition, the data show that increasing the silica/alumina ratio increases both the Bronsted and Lewis acid site concentrations. Based on these results, the acid site concentrations can be controlled by altering the Si/Al ratio and calcination temperature while maintaining high surface areas, large pore volumes, and large pore diameters.

During the combustion of coal and other carbonaceous materials, the heterogeneous reaction of NO with the evolving char created in the combustion process is important for understanding the formation and reduction of NO. This investigation quantifies the effects of char burnout level and conditions on 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) as well as graphite and coconut char. Kinetic data were measured in a packed-bed reactor at temperatures between 723 and 1173 K. The rate constant for the NO−char reaction was found to depend upon the extent of burnout/conversion and conditions under which the char was burned out. The NO−char rate constant consistently decreases with increasing burnout when the char burnout levels are accomplished in a drop tube reactor at 1800 K and 3−5% O2. However, the NO−char rate constant increases as char burnout increases (up to 90%) when the char burnout levels result from reacting the char with 3050 ppm of NO in a packed bed at 723−1173 K. For the latter case, the relationship of the NO−char rate constant (on the basis of moisture ash-free char mass) and char burnout is approximately linear with roughly the same upward slope between 20 and 80% burnout for all coal chars studied. The activation energy of the NO−char reaction is apparently independent of both char burnout level and burnout conditions. The CO2/CO ratio in the exhaust stream appears to correlate with the NO−char rate constant for the chars burned out at low-temperature conditions.

Fischer–Tropsch synthesis (FTS) is a technically proven and economically viable route for the conversion of coal, biomass, and natural gas to hydrocarbon fuels. Although unsupported Fe catalysts are proven for FTS, they lack the physical strength and durability that would make them more viable for large-scale commercial reactors, and their activity is still significantly less than that of Co FT catalysts. In this work, we report on a very active and stable supported Fe FT catalyst that is more active than any supported Fe FT catalyst previously reported and competitive with the best unsupported catalysts. In addition, its productivity, which takes into account selectivity to desired hydrocarbon products, is also very competitive. More importantly, the catalyst is extremely stable, as evidenced by the fact that after 700 h on stream, its activity and productivity are still increasing. These catalyst properties result from using a novel γ-alumina support material doped with silica and pretreated at 1100 °C....

This paper investigates the effects of various, carefully-chosen preparation methods on the performance of Fischer–Tropsch (FT) alumina-supported iron/copper/potassium (FeCuK/Al2O3) catalysts. Two tested preparation methods (co-impregnation and non-aqueous slurry impregnation) yielded supported Fe catalysts with better catalyst performance than previously thought possible. These two supported iron catalysts have high reaction rates (114–154 mmol (CO + H2) gcat−1 MPa−1 h−1), good productivity (0.26–0.29 gHC gcat−1 h−1), and reasonable stability. In fact, both catalysts are more active than any supported Fe catalyst reported outside our group and compare favorably with unsupported catalysts. Superior activity, coupled with the high strength of a supported catalyst, make these catalysts excellent candidates for use in slurry bubble column reactor (SBCR) applications.

Significance A unique combination of classic packed bed friction factor equations and newly refit correlation constants is proposed which produces a new friction factor correlation which significantly improves predictions in high turbulence regimes, high porosity regimes, and high wall effect regimes.

The distribution, retention, chemical state, and bonding of noble metal (NM) promoters (Pt, Re, or Ru), their influence on Co crystallite size, and the resulting Co reducibility and dispersion were investigated in commercially-representative Co Fischer–Tropsch catalysts (nominally 25 wt% Co and 0 to 0.6 wt% NM on a La stabilized alumina support). NM retention during preparation ranged from complete for Pt and Re to 0 bonds directly to Co 0 , Ru forms separate metal crystallites and Re remains as Re 2 O 7 . Despite these differences, all three NMs substantially improved Co reducibility. Pt is the most effective promoter for increasing cobalt reduction rate and gives the highest extent of reduction (EOR). Preliminary activity tests showed no statistical differences in the CO depletion rates of the four catalysts of this study.

Sulfation occurs when commercial vanadia SCR catalysts are exposed to SO 2 -laden coal combustion flue gases. Effects of sulfation on the surface chemistry of vanadia/titania catalysts and SCR activity have not been adequately addressed in previously published work. In this work, in situ FTIR spectroscopy and post situ XPS investigations were performed during vanadia/titania catalyst sulfation under simulated coal combustion flue gas conditions. In situ FTIR spectroscopy combined with XPS analyses on fresh and sulfated TiO 2 , 2% and 5% V 2 O 5 /TiO 2 indicate that sulfate does not form on vanadia sites but rather on titania sites. FTIR spectroscopy data show that sulfation inhibits NO adsorption in the presence of oxygen, but greatly enhances NH 3 adsorption by generating additional Bronsted acid sites while reducing the concentration of Lewis acid sites. Observed increases in the intrinsic NO reduction activity of sulfated vanadia/titania catalysts (relative to fresh catalysts) are consistent with spectroscopy data showing that the sulfation enhances NO reduction activity by increasing the number of active sites without changing the activation energy or site acid strength.

A novel, solvent-deficient precipitation (SDP) method for catalyst preparation in general and for preparation of iron FT catalysts in particular is reported. Eight catalysts using a 23 factorial design of experiments to identify the key preparation variables were prepared. The catalysts were characterized by electron microprobe, N2 adsorption, TEM, XRD, and ICP. Results show that the morphology of the catalysts, i.e., surface area, pore volume, pore size distribution, crystallite sizes, and promoter distribution are significantly influenced by (1) whether or not the precursor catalyst is washed, (2) the promoter addition step, and (3) the drying condition (temperature). Consequently, the activity, selectivity, and stability of the catalysts determined from fixed-bed testing are also affected by these three variables. Unwashed catalysts prepared by a one-step method and dried at 100 °C produced the most active catalysts for FT synthesis. The catalysts of this study prepared by SDP compared favorably in activity, productivity, and stability with Fe FT catalysts reported in the literature. It is believed that this facile SDP approach has promise for development of future FT catalysts, and also offers a potential alternate route for the preparation of other catalysts for various other applications.

A high-temperature, high-pressure flat-flame burner reactor was developed to prepare char at different pressures. This system achieves particle heating rates of 105 K/s, which better mimics industrial conditions than conventional drop tube or radiative flow reactors. Previous data at atmospheric pressure demonstrated a significant decrease in particle swelling during devolatilization as heating rates increased from 104 K/s (the typical drop tube heating rate) to 105 K/s. Pyrolysis experiments were performed at pressures from 1 to 15 atm at 1300 °C for two bituminous coals and a lignite. Average swelling was determined from a combination of the mass release and the average density. The results indicate significantly lower swelling ratios at elevated pressures than reported in the literature. Scanning electron micrographs show that the bubbles in the bituminous coal particles popped before significant swelling at these elevated heating rates. Lignite particles exhibited shrinkage rather than swelling, but still showed a small effect of pressure. TGA oxidation reactivities were determined for the Pitt #8 and Knife River lignite char samples at their respective char preparation pressures. The oxidation reactivities of both the bituminous and lignite chars decreased with increasing pressure.

Chars were produced from two coals (North Dakota lignite and Pittsburgh No. 8 hva bituminous coal) in a flat flame burner at atmospheric pressure. Intrinsic char reactivities to oxygen were then me...

The effect of support material pretreatment temperature, prior to adding the active phase and promoters, on Fischer–Tropsch activity and selectivity was explored. Four iron catalysts were prepared on silica-stabilized alumina (AlSi) supports pretreated at 700 °C, 900 °C, 1100 °C or 1200 °C. Addition of 5% silica to alumina made the AlSi material hydrothermally stable, which enabled the unusually high support pretreatment temperatures (>900 °C) to be studied. High-temperature dehydroxylation of the AlSi before impregnation greatly reduces FeO·Al2O3 surface spinel formation by removing most of the support-surface hydroxyl groups leading to more effectively carbided catalyst. The activity increases more than four-fold for the support calcined at elevated temperatures (1100–1200 °C) compared with traditional support calcination temperatures of

Char oxidation is often modeled using an mth order intrinsic reaction rate in conjunction with an effectiveness factor (η) to account for intraparticle diffusion of gas species. This approach invol...

The global nth order rate equation has been criticized for lack of theoretical basis and has been shown to be inadequate for modeling char oxidation rates as a function of total gas pressure. The simple Langmuir rate equation is believed to have more potential for modeling high pressure char oxidation. The intrinsic Langmuir rate equation is applied to graphite flake oxidation data and agrees well with reaction rates at three temperatures over the entire range of oxygen pressure (1–64 atm). It also explains the change of reaction order with temperature. In this work, the intrinsic Langmuir rate equation is combined with (1) an effectiveness factor to account for pore diffusion effects and (2) a random pore structure model to calculate effective diffusivity. The resulting model is able to predict the reaction rates of large (ca. 8 mm) coal char particles as a function of gas velocity, total pressure, oxygen partial pressure, oxygen mole fraction, initial particle size, and gas temperature. This approach is also able to correlate the particle burnouts of pulverized (70 lm) coal char particles in a drop tube reactor as a function of total pressure, oxygen mole fraction, gas and wall temperatures, and residence time. The ability of the model to correlate data over wide range of temperature and pressure is promising.

The heterogeneous reaction of NO with coal char has potential as the basis for both reburning and postcombustion clean-up processes to control NO x 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 T 50 (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 ( E A ) shift from 100–160 kJ/mol at low temperatures to 190–250 kJ/mol at high temperatures. The shift to distinctly different and higher E A '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 E A , the shift temperature and the E A 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.

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 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 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 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 three 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.

A trickle fixed-bed reactor model for the Fischer-Tropsch synthesis applicable to both cobalt and iron catalysts which accounts for gas and liquid recycle is described. A selection of kinetic models for both iron and cobalt catalysts (4 each) is included in the reactor model and their effect on model predictions is compared. While the model is 1-D and reaction rates are determined for quasi-average radial bed temperatures, a correlation is used to account for radial thermal conductivity and radial convective heat transfer. Traditional pressure drop calculations for a packed column were modified with a correlation to account for trickle-flow conditions. In addition to describing the model in detail and showing validation results, this paper presents results of varying fundamental, theoretically-based parameters (i.e. effective diffusivity, Prandtl number, friction factor, etc.). For example, the model predicts that decreasing effective diffusivity from 7.1E-09 to 2.8E-09 m2/s results in a lower maximum temperature (518 K vs. 523 K) and a longer required bed length to achieve 60% conversion of CO (8.5 m vs. 5.7 m). Using molar averages of properties to calculate the Prandtl number for the gas phase (recommended by the authors) results in average bed temperatures up to 10 K higher and reactor lengths 17-45% shorter than assuming a Prandtl number of 0.7. Using the Tallmadge equation to estimate friction losses, as recommended by the authors, results in a pressure drop 40% smaller than using the Ergun equation. Validation of the model was accomplished by matching published full-scale plant data from the SASOL Arge reactors.

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

Selective catalytic reduction of nitric oxide with propane and oxygen was investigated on Cu-exchanged ZSM-5, mordenite, X-type and Y-type zeolites at temperatures in the range of 200 to 600°C. Catalytic activities of Cu-X and Cu-Y are negligible, activity of Cu-mordenite moderate, and that of Cu-ZSM-5 very high, converting > 90% of nitric oxide to nitrogen at 400°C and at a space velocity of 102 000 h −1 . Effects of space velocity, nitric oxide concentration, C 3 H 8 /NO ratio, oxygen concentration, and water vapor on the activities of Cu-ZSM-5 and Cu-mordenite were investigated. Nitric oxide conversion decreases with increasing space velocity, decreasing propane and nitric oxide concentrations, and decreasing propane/NO ratio. Water vapor decreases the activity significantly at all temperatures. At temperatures above 400°C, propane oxidation by oxygen is a significant competing reaction in decreasing the selectivity for nitric oxide reduction. The results indicate that Cu-ZSM-5 is a promising catalyst for selective catalytic reduction of nitric oxide by hydrocarbons.

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/SiO 2 . Absorption intensities for both linear and bridged CO surface species ( A 1 and A b ) were determined by combining peak area data from IR spectra with uptake measurements obtained in gravimetric experiments. This resulted in an A 1 value of 13 (±2) and an A b 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 which may broaden the application of quantitative FTIR. Rh dispersions were determined for Rh/SiO 2 samples of five 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/SiO 2 catalysts. Also, it was observed that the ratio of linear to bridged 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.

Selective catalytic reduction of nitric oxide with propane and oxygen was investigated on Cu-exchanged ZSM-5, mordenite, X-type and Y-type zeolites at temperatures in the range of 200 to 600°C. Catalytic activities of Cu-X and Cu-Y are negligible, activity of Cu-mordenite moderate, and that of Cu-ZSM-5 very high, converting > 90% of nitric oxide to nitrogen at 400°C and at a space velocity of 102 000 h −1 . Effects of space velocity, nitric oxide concentration, C 3 H 8 /NO ratio, oxygen concentration, and water vapor on the activities of Cu-ZSM-5 and Cu-mordenite were investigated. Nitric oxide conversion decreases with increasing space velocity, decreasing propane and nitric oxide concentrations, and decreasing propane/NO ratio. Water vapor decreases the activity significantly at all temperatures. At temperatures above 400°C, propane oxidation by oxygen is a significant competing reaction in decreasing the selectivity for nitric oxide reduction. The results indicate that Cu-ZSM-5 is a promising catalyst for selective catalytic reduction of nitric oxide by hydrocarbons.

Chars derived from Beulah-Zap (lignite A) and Dietz (subbituminous 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, 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 at temperatures between 550 K and 950 K. Reactivities at different oxidation burn-out 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 burn-out levels. It was also found that the lower burn-out levels are more highly influenced by diffusional effects. This was manifest by a decrease in the slope of the Arrhenius plot which began at a temperature of ~ 750 K for the char at 10% burn-out compared with a temperature of nearly 900 K for the char at 75% burn-out. 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 2.5-5 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 118 ± 3 kJ mol−1. A comparison of the reactivities for the flat flame burner chars of the lignite and the subbituminous showed that the lignite chars were more reactive by a factor of two. This was consistent over all burn-out levels.

The effects of both extent and type of burnout on char oxidation rates and rate parameters (apparent activation energy and oxygen reaction order) have been investigated for chars prepared from Dietz (subbituminuous B) coal. Intrinsic rates were determined using isothermal thermogravimetric analysis (TGA). N2 BET and CO2 DP surface areas were measured, as was hydrogen to carbon ratio (H/C). CaO surface area was measured for selected samples. Three types of burnouts 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. Low-temperature oxidation burnout was studied by oxidizing a FFB char to a continuum of burnout oxidation rates. High-temperature oxidation burnout was studied by taking the same FFB char and oxidizing it to various conversion levels in a drop-tube reactor (DTR) at 1400 K (particle temperature). The oxidation rates and kinetics of these partially burned out char samples were then determined using TGA. The rate of oxidation was found to decrease with increasing devolatilization residence time, even after mass loss (DML) and H/C had become essentially constant. This decrease in reactivity was shown to correlate with a decrease in CaO surface area, consistent with the importance of CaO catalysis in low-temperature char oxidation. H/C shows an inverse linear correlation with DML. N2 and CO2 surface area data indicate dramatic increases in the mesopore surface are during devolatilization but not in the micropore surface are. Intrinsic rates of chars partially burned out at high temperatures were found to decrease with burnout level, while those of chars burned out at low temperatures were essentially constant with burnout level. Values of apparent activation energy increased with burnout level (or DML) for all three types of burnout by 20 to 30%. Oxygen reation orders ranged from 0.74 to 0.53 and generally showed a decrease with burnout (or DML).

Several 2% Rh/silica catalysts containing from 0 to 6% Nb 2 O 5 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 Nb 2 O 5 to Rh/SiO 2 resulted in a decrease in Rh(0) for relatively low niobia loadings (0 to 3 % Nb 2 O 5 ) and an increase in Rh(I) for higher niobia loadings (3 to 6% Nb 2 O 5 ). These two effects combined to give a minimum Rh dispersion for a catalyst containing approximately 3% Nb 2 O 5 . 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.

This is the Final Report for DOE Cooperative Agreement No: DE-FC26-00NT40753. The goal of the project was to develop cost-effective analysis tools and techniques for demonstrating and evaluating low-NOx control strategies and their possible impact on boiler performance for boilers firing US coals. The Electric Power Research Institute (EPRI) provided co-funding for this program. This project included research on: (1) In furnace NOx control; (2) Impacts of combustion modifications on boiler operation; (3) Selective Catalytic Reduction (SCR) catalyst testing and (4) Ammonia adsorption/removal on fly ash. Important accomplishments were achieved in all aspects of the project. Rich Reagent Injection (RRI), an in-furnace NOx reduction strategy based on injecting urea or anhydrous ammonia into fuel rich regions in the lower furnace, was evaluated for cyclone-barrel and PC fired utility boilers. Field tests successfully demonstrated the ability of the RRI process to significantly reduce NOx emissions from a staged cyclone-fired furnace operating with overfire air. The field tests also verified the accuracy of the Computational Fluid Dynamic (CFD) modeling used to develop the RRI design and highlighted the importance of using CFD modeling to properly locate and configure the reagent injectors within the furnace. Low NOx firing conditions can adversely impact boiler operation due to increased waterwall wastage (corrosion) and increased soot production. A corrosion monitoring system that uses electrochemical noise (ECN) corrosion probes to monitor, on a real-time basis, high temperature corrosion events within the boiler was evaluated. Field tests were successfully conducted at two plants. The Ohio Coal Development Office provided financial assistance to perform the field tests. To investigate soot behavior, an advanced model to predict soot production and destruction was implemented into an existing reacting CFD modeling tool. Comparisons between experimental data collected in a pilot scale furnace and soot behavior predicted by the CFD model showed good agreement. Field and laboratory tests were performed for SCR catalysts used for coal and biomass co-firing applications. Fundamental laboratory studies were performed to better understand mechanisms involved with catalyst deactivation. Field tests with a slip stream reactor were used to create catalyst exposed to boiler flue gas for firing coal and for co-firing coal and biomass. The field data suggests the mechanisms leading to catalyst deactivation are, in order of importance, channel plugging, surface fouling, pore plugging and poisoning. Investigations were performed to better understand the mechanisms involved with catalyst regeneration through mechanical or chemical methods. A computer model was developed to predict NOx reduction across the catalyst in a SCR. Experiments were performed to investigate the fundamentals of ammonia/fly ash interactions with relevance to the operation of advanced NOx control technologies such as selective catalytic reduction. Measurements were performed for ammonia adsorption isotherms on commercial fly ash samples subjected to a variety of treatments and on the chemistry of dry and semi-dry ammonia removal processes. This work resulted in the first fundamental ammonia isotherms on carbon-containing fly ash samples. This work confirms industrial reports that aqueous solution chemistry takes place upon the introduction of even very small amounts of water, while the ash remains in a semi-dry state.

This is the twelfth Quarterly Technical Report for DOE Cooperative Agreement No: DE-FC26-00NT40753. The goal of the project is to develop cost effective analysis tools and techniques for demonstrating and evaluating low NOx control strategies and their possible impact on boiler performance for boilers firing US coals. The Electric Power Research Institute (EPRI) is providing co-funding for this program. This program contains multiple tasks and good progress is being made on all fronts. During this quarter, a new effort was begun on the development of a corrosion management system for minimizing the impacts of low NOx combustion systems on waterwalls; a kickoff meeting was held at the host site, AEP's Gavin Plant, and work commenced on fabrication of the probes. FTIR experiments for SCR catalyst sulfation were finished at BYU and indicated no vanadium/vanadyl sulfate formation at reactor conditions. Improvements on the mass-spectrometer system at BYU have been made and work on the steady state reactor system shakedown neared completion. The slipstream reactor continued to operate at AEP's Rockport plant; at the end of the quarter, the catalysts had been exposed to flue gas for about 1000 hours. Some operational problems were addressed that enable the reactor to run without excessive downtime by the end of the quarter.

This is the fourteenth Quarterly Technical Report for DOE Cooperative Agreement No: DEFC26-00NT40753. The goal of the project is to develop cost effective analysis tools and techniques for demonstrating and evaluating low NOx control strategies and their possible impact on boiler performance for boilers firing US coals. The Electric Power Research Institute (EPRI) is providing co-funding for this program. Using the initial CFD baseline modeling of the Gavin Station and the plant corrosion maps, six boiler locations for the corrosion probes were identified and access ports have been installed. Preliminary corrosion data obtained appear consistent and believable. In situ, spectroscopic experiments at BYU reported in part last quarter were completed. New reactor tubes have been made for BYU's CCR that allow for testing smaller amounts of catalyst and thus increasing space velocity; monolith catalysts have been cut and a small reactor that can accommodate these pieces for testing is in its final stages of construction. A poisoning study on Ca-poisoned catalysts was begun this quarter. A possible site for a biomass co-firing test of the slipstream reactor was visited this quarter. The slipstream reactor at Rockport required repair and refurbishment, and will be re-started in the next quarter. This report describes the final results of an experimental project at Brown University on the fundamentals of ammonia / fly ash interactions with relevance to the operation of advanced NOx control technologies such as selective catalytic reduction. The Brown task focused on the measurement of ammonia adsorption isotherms on commercial fly ash samples subjected to a variety of treatments and on the chemistry of dry and semi-dry ammonia removal processes.

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 H 2 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 frequency versus Rh particle size 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.

Publisher Summary This chapter describes the effects of a burnout on char-oxidation rates. The chapter describes a study in which the chars used were prepared from Dietz subbituminous B coal with an average particle size of 70 micrometers. They were prepared in a flat flame burner (FFB) in a methane-flame environment, at a high heating rate, at particle temperatures of approximately 1300K, and at various residence times. Some of the FFB char produced at a residence time of 104 ms was further converted by oxidizing it to various extents in a drop tube reactor in the presence of 5% oxygen and at particle temperatures of approximately 1300 K. Most of the devolatilization occured between 20 and 50 ms. The oxidation rates decrease significantly as the preparation residence time increases and appear to be inversely proportional to the devolatilization mass loss.

Multiple techniques (CO 2 and N 2 adsorptions, NMR spin relaxation of adsorbed water, He pycnometry and Hg porosimetry) have been combined in a comprehensive study to determine changes in surface area (CO 2 and nitrogen), density (solid, particle and bulk), and pore structure (pore size and volume distributions of micro-, meso- and macro-pores) 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-frame burner (maximum temperature of 1473 K), a process representative of char formation in pulverized coal combustion. Most of the surface area of the 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) was contained in mesopores less than 20 nm). During the high-temperature formation of char in a flame: (I) CO 2 surface areas (involving mainly micropores, r pore < 1.5 nm) increase two- to three-fold, while N 2 surface areas (involving mesopores. 1.5 nm < r pore < 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 increase 5–10-fold; and (4) total porosities increase three- to four-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 CO 2 and N 2 molecules; (ii) the 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. The preparation conditions (e.g. atmosphere, heating rate and temperature) greatly affect the physical properties including the surface area, porosity and density of the resulting chars. The degree of carbon burnout is an important correlating factor affecting these properties.

The kinetics of NO reduction by CO have been investigated over a RhSiO2 catalyst. These studies have been complemented by in situ infrared studies. The specific activity is found to be sensitive to the nature of the catalyst pretreatment. Preoxidation in NO increases the specific activity for NO reduction by 50% over that observed when the catalyst is prereduced. Pretreatment has little effect, though, on the selectivity for forming N2 versus N2O. For NO conversions below 50%, the kinetics for NO reduction, and for N2 and N2O formation, are positive order in CO but inverse order in NO. The activation energies for all three processes are equivalent. Infrared spectra taken under reaction conditions show that the surface is nearly saturated by adsorbed NO. Smaller coverages by CO and NCO species are also observed. The results of both the spectroscopic and rate studies can be interpreted on the basis of a relatively simple reaction mechanism. The rate-limiting step in this model is assumed to be the dissociation of chemisorbed NO. The higher specific activity for NO reduction observed upon preoxidation of the catalyst is ascribed to the more facile dissociation of adsorbed NO on a partially oxidized Rh surface.

The construction and performance are described of a gas chromatographic system for the analysis of gas mixtures formed during the catalytic reduction of NO with CO, H/sub 2/, and NH/sub 3/. Reactants and products are separated by using three packed columns contained within two isothermally operated gas chromatographs. The system operation, data acquisition, and data reduction are accomplished by means of a microprocessor. Quantitative analysis of NO, NH/sub 3/, N/sub 2/, N/sub 2/O, CO, and CO/sub 2/ can be obtained down to levels in the range of 10 to 35 ppM, and H/sub 2/ can be analyzed down to 400 ppM. Water present in the sample is separated from the other components but is not quantified.