William C. Hecker

Prediction of propagating methane-air flames☆

Abstract The kinetics and propagation of laminar methane-air flames were studied using a one-dimensional, flame propagation model. The model is based on a numerical, unsteady-state solution of transformed species and energy conservation equations using explicit techniques for diffusion terms and linearized, implicit techniques for kinetic terms. A methaneoxygen kinetic mechanism consisting of 28 elementary reactions was postulated and used in the flame model. Flame velocity, flame thickness, temperature profile and concentration profiles of 13 species were predicted for a series of methane-air flames. The effects of pressure, methane concentration, initial temperature, rate constants, and transport coefficients were investigated. Many of the model predictions were compared with experimental data, and agreement was generally very good. The concentrations of the radicals H, OH, and O were major factors in the propagation of methane-air flames. The relative importance of each of the 28 reactions was examined; five were found to be negligible, while several were shown to be important in determination of propagation velocity and flame characteristics.

Selective catalytic reduction of nitric oxide by propane in oxidizing atmosphere over copper-exchanged zeolites

Abstract 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.

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

Multiple techniques (CO2 and N2 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 (CO2 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) CO2 surface areas (involving mainly micropores, rpore < 1.5 nm) increase two- to three-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 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 CO2 and N2 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.

Low temperature char oxidation kinetics: effect of preparation method

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.

Kinetics of NO reduction by char: Effects of coal rank

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.

MODELING HIGH-PRESSURE CHAR OXIDATION USING LANGMUIR KINETICS WITH AN EFFECTIVENESS FACTOR

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.

Effects of burnout on char oxidation kinetics

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).

Preparation of Fe-Mn/K/Al2O3 Fischer-Tropsch Catalyst and Its Catalytic Kinetics for the Hydrogenation of Carbon Monoxide

Abstract A K promoted iron-manganese catalyst was prepared by sol-gel method, and subsequently was tested for hydrogenation of carbon monoxide to light olefins. The kinetic experiments on a well-characterized Fe-Mn/K/Al 2 O 3 catalyst were performed in a fixed-bed micro-reactor in a temperature range of 280-380 °C, pressure range of 0.1-1.2 MPa, H 2 /CO feed molar ratio range of 1-2.1 and a space velocity range of 2000-7200 h −1 . Considering the mechanism of the process and Langmuir-Hinshelwood-Hogan-Watson (LHHW) approach, unassisted CO dissociation and H-assisted CO dissociation mechanisms were defined. The best models were obtained using non-linear regression analysis and Levenberg-Marquardt algorithm. Consequently, 4 models were considered as the preferred models based on the carbide mechanism. Finally, a model was proposed as a best model that assumed the following kinetically relevant steps in the iron-Fischer-Tropsch (FT) synthesis: (1) CO dissociation occurred without hydrogen interaction and was not a rate-limiting step; (2) the first hydrogen addition to surface carbon was the rate-determining steps. The activation energy and adsorption enthalpy were calculated 40.0 and −30.2 kJ·mol −1 , respectively.

No reduction activity and FTIR characterization of rhodium on niobia-modified SiO2

Abstract 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.

Effects of preparation variables on an alumina-supported FeCuK Fischer–Tropsch catalyst

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.