John M. Stencel

Catalytic reduction of nitric oxide over activated carbons

Abstract The reduction of NO to N 2 using NH 3 as a reductant over activated carbons was investigated using an integral reactor system. The effects of varying the gas-phase and surface oxygen concentrations were studied in detail. Surface oxygen was introduced by treating a coconut shell activated carbon with sulphuric acid at 100–300 °C. The type and concentration of surface oxides were investigated by infrared spectroscopy, linear temperature-programmed desorption of surface oxides, and adsorption of reactant gases. Increasing the gas-phase oxygen concentration increased NO conversion significantly. The activity of activated carbon at reaction temperatures > 150 °C was enhanced by sulphuric acid treatment. Sulphuric acid acted as an oxidizing agent and created acidic surface functionalities including carboxyl and carbonyl groups.

Dry triboelectrostatic beneficiation of fly ash

A laboratory-scale triboelectrostatic separation system in conjunction with analytical techniques was used to study fly ash beneficiation. Fly ash samples were characterized by size analysis and carbon content and then subjected to dry triboelectrostatic separation. Due to differences in the surface physical and chemical properties of the carbon and ash, particles of unburnt carbon and fly ash were triboelectrically charged to opposite polarity and then separated by passing them through a static electric field. Ash fractions deposited on the positive and negative electrodes were collected, analysed for carbon content and subjected to SEM and petrographic analyses. The results indicate that the physical and chemical properties of the ash dictate the maximum carbon-ash separation that would be possible. In addition, the potential of dry separation technology for removing unburnt carbon from coal ash was demonstrated.

The effect of low-concentration SO2 on the adsorption of NO from gas over activated carbon

The effect of low-concentration SO2 on the adsorption of NO over activated carbon was studied using adsorption-desorption profiles obtained during thermal analysis-mass spectrometry. Using three different gas mixtures and adsorption temperatures between 293 and 413 K with NO and SO2 as the reactants, it was determined that NO2 and SO2 were the primary adsorbed species. The uptake of NO2 decreased with increasing temperature, whereas the uptake of SO2 was independent of temperature. Except at the highest adsorption temperatures, the amount of NO2 adsorbed was greater than that of SO2 adsorbed. The presence of SO2 inhibited adsorption of NO2, but the co-adsorption of NO2 promoted the adsorption of SO2. These data point to the possibility that the SO2 binding sites on the carbon are associated with the reaction NO + 12 O2 → NO2, and to the different adsorption mechanisms which control NO2 and SO2 uptake.

Particle tribocharging characteristics relating to electrostatic dry coal cleaning

Abstract The velocity, number density and size of representative coal mineral matter particles were measured using laser phase Doppler velocimetry subsequent to their triboelectrification and while flowing through a constant electric field. For 60 μm diameter silica particles, the accumulated negative charge, q, could be represented by a normal distribution having an average value which was linearly dependent on the gas velocity in the tribocharger, V, and represented by qSi = [2.6V + 4] × 10−14C. The width of the distribution increased with increasing gas velocity. Physical mixtures of similar sized silica and glassy carbon, and coal, were also subjected to electrostatic separation while being monitored by the non-intrusive laser optical technique. The purity of the carbon separated from silica was >90%, and dependent on the accumulation of either positive charge or no charge on the silica. For a high volatile A, Elswick seam, Pike County, Kentucky coal, the removal of mineral matter was comparable to that obtainable by wet processes, decreasing mineral matter content from 6.4% to 3.7% at a 72% combustible recovery.

Maceral/microlithotype partitioning through triboelectrostatic dry coal cleaning

Abstract Three eastern Kentucky and two Illinois Basin coals were tested in a bench scale triboelectrostatic separation unit. The three eastern Kentucky samples provided a rank series of petrographically comparable coals. The Illinois Basin bituminous coals were lower rank and had high vitrinite ( ∼ 80%) and sulfur contents in comparison to the other three coals. Triboelectrostatic beneficiation provides efficient maceral and mineral partitioning in the high volatile A and B bituminous coals tested, with vitrinite, as vitrite and vitrinite-enriched microlithotypes, reporting to the clean fractions and the inertinites, liptinites, and minerals reporting to the tails. The high volatile C bituminous Springfield coal had a lower separation efficiency than the petrographically similar, but higher rank, Herrin coal. The decreased separation efficiency in the behavior of the Springfield coal may be a response to its higher moisture content. Compared to bench-scale fuel oil agglomeration of some of the same coals, triboelectrostatic separation provides clearer partitioning of mineral matter, sulfur, and macerals.

CH4 storage on compressed carbons

Abstract Compressibility indices and CH 4 uptake capacities were determined for three commercially produced carbons, using a specially designed cell, developed for a high pressure thermal gravimetric analyzer. The cell was used during CH 4 uptake measurements on non-compacted and compacted carbon materials, while maintaining a constant sample volume. Compressibility, piece density, and CH 4 uptake capacities were measured for carbons having different particle sizes, after physical blending, and before and after coke deposition. Pore size distributions and pore volumes was obtained. Compaction of the activated carbons and a graphite powder increased the CH 4 uptake per unit volume of the material beyond that expected from densification. The data suggested that compaction and physical blending provides additional CH 4 adsorption sites in inter-particle voids.

The role of weathering on fly ash charge distribution during triboelectrostatic beneficiation

Triboelectrostatic beneficiation of coal combustion fly ashes with high-unburned carbon contents can produce low-carbon ash products having value as mineral admixtures and meeting technical requirements for replacing cement in concrete. This capability is a result of establishing bipolar charge on mineral ash versus carbon particles where, typically, unburned carbon attains positive surface charge and ash attains negative surface charge under the tribocharging conditions employed in triboelectrostatic technologies. However, long-term exposure of fly ash to weathering conditions, such as moisture or high humidity, before beneficiation is known to dramatically diminish carbon-ash separation efficiencies. Although experimentation has shown that water soluble surface species can be redistributed on fly ash particles after exposure to moisture, which could affect the extent of charging and polarities, measurement of the actual amount of charge and polarity on particles after weathering exposure versus after removal of surface moisture has not been accomplished. Hence, a new experimental methodology was developed and applied to measure charge distributions on tribocharged ash and carbon particles in a fly ash that had been exposed to weathering conditions for 6 months before and after removal of the surface moisture. Weathered ash particles were found to have an average zero charge, whereas carbon particles attained an average negative charge, opposite of the normal polarity for carbon. Although the extent of uncharged particles decreased and ash particles attained an average negative charge after drying, carbon particles attained only an average zero charge. These changes were reflected in very small increases in carbon-ash separation efficiency, in contrast to previous beneficiation tests in which fly ash drying led to significant increases in carbon-ash separation efficiency. It is suggested that removal of surface moisture in the absence of other processes like surface ion redistribution would beneficially impact carbon-ash triboelectrostatic beneficiation.

Structural and surface characterization of ultrafine iron carbide particles generated by laser pyrolysis. I. High temperature He treatment

Three samples of passivated ultrafine iron carbide particles, synthesized by laser induced pyrolysis of gaseous precursors, were examined by x‐ray diffraction, x‐ray photoelectron spectroscopy, and thermogravimetry/mass spectroscopy at temperatures as high as 600 °C under a He atmosphere. The approximate 6–7 nm diam particles began to sinter at temperatures between 300 and 400 °C and formed a complex mixture of carbide, oxide and metallic phases. Preservation of the carbide structure on heating was dependent on the purity of the carbide, and the concentration of oxygen and carbon in the particles. A topotactic transformation from Fe7C3 to Fe0.98O was observed and is discussed relative to the as‐synthesized/passivated particles composition and structure. This topotaxy leads to the selective formation of γ‐Fe2O3 from specific ultrafine particle carbides.

Triboelectrostatic Cleaning of Coal In-Line Between Pulverizers and Burners at Utilities

Pulverized coal combustors are the principal means for converting coal to electricity in the United States. Coals used in these combustors have to be beneficiated because of escalating specifications on fuel quality that help to increase boiler efficiency and decrease acid gas emmissions. Because coal is pulverized before its combustion in PC boilers, during which time substantial mineral liberation occurs, it may be beneficial to remove mineral matter from the combustibles by using dry beneficiation techniques after the pulverizers and within the burner pipes leading to the coal burners. One such technique is based on triboelectrostatics. It relies on establishing a differential charge on coal and mineral matter and then separating this charged mixture in an electrostatic field. This paper presents data obtained at a utility site that quantify the charge imparted on coal during pulverization and transport in a burner pipe, and compares these values to data obtained within a laboratory setting. The electr...

AIR FLOW AND PARTICLE TRANSPORT IN A TRIBOELECTRIC COAL/ASH CLEANING SYSTEM-COUNTER FLOWING STRAIGHT DUCT DESIGN

ABSTRACT The process of triboelectric coal/ash cleaning is outlined and a new design for scale-up to industrial systems is described. A straight rectangular duct forms the mixing chamber and the main body of this triboelectric separator. To improve mixing and to generate a uniform concentration in the main electrostatic separator duct, the mixture enters the mixing box in the form of two opposing jets. The stress transport model of the FLUENT code is used for simulating the mean gas flow and to guide the overall design of the electrostatic coal/ash separator. The three-dimensional computational grid covers the mixing chamber with the opposing inlets and the upper part of the main separator duct. The gas flow field conditions including the directional intensities of turbulence are evaluated. The instantaneous fluctuating velocity field was simulated by a Gaussian filtered white noise model. A recently developed computational model for Lagrangian particle tracking is used to study the transport of particles from the inlets to the main body of the separator duct. The model accounts for the drag and lift forces acting on the particle, in addition to the Brownian motion and gravitational sedimentation effects. The particles are also dispersed by the action of the high level of turbulence that is generated in the mixing chamber. The computational results show that the new design generates roughly uniform flow conditions in the triboelectric separator duct. Furthermore, the larger 100 µm particles will have a more uniform distribution when compared with the smaller (40 µm) particles. The effect of the presence of an electric field perpendicular to the flow direction is also studied. It is shown that the charged particles are significantly deflected by the action of the electrostatic forces. As a result, the system could separate coal and ash particles that carry different charges. The simplicity and effectiveness of the straight duct separator makes it a prime candidate for scale-up for use in industrial applications for online coal beneficiation and separation of minerals from ash in combustion product.