Farhang Shadman

The kinetics and mechanism of alkali removal from flue gases by solid sorbents

Kaolinite, bauxite and emathlite have been found suitable for alkali removal from hot flue gases in coal conversion systems. The effect of temperature on the kinetics and mechanism of alkali adsorption/reaction on these sorbents was studied under a simulated flue gas atmosphere. Kaolinite and emathlite reacted irreversibly with the alkali; however for bauxite, 10% of the total weight gained was due to physisorption. Kaolinite was found to have the highest capacity and the largest activation energy for alkali removal. The overall sorption process is not just physical and non-selective, but rather a combination of physical and chemical processes, which are dependent on the temperature and sorbent chemistry. The reaction product of alkali with emathlite has a melting point of approximately 1270 K, while kaolinite and bauxite form compounds with a melting point of about 1870 K. Consequently, kaolinite and bauxite are more suitable for in situ removal of alkali, while all three can be used for downstream alkali removal.

Metal capture by sorbents in combustion processes

Abstract The use of sorbents to control trace metal emissions from combustion processes was investigated, and the underlying mechanisms governing the interactions between trace metals and sorbents, were explored. Emphasis was on mechanisms in which the metal vapor was reactively scavenged by simple commercial sorbents, to form water unleachable products, which are easy to collect and isolate from the environment. Results are presented from two different scales of experimentation, involving a bench scale thermo-gravimetric reactor and a 17 kW down-fired laboratory combustor, respectively. Results from the bench scale tests showed that lead and cadmium, vaporized from the chloride salt, could be reactively captured at temperatures above the dew point. Both kaolinite and bauxite were effective sorbents for lead, while bauxite but not kaolinite was effective for cadmium. The primary reaction products, as identified by X-ray diffraction analyses, consisted of lead and cadmium aluminosilicates. Laboratory combustor tests, completed in the absence of coal char or coal ash particles, showed that lead could be effectively reactively scavenged in situ, in a combustor, downstream of the primary flame. Here, the high temperatures of the combustion process were being exploited to promote the reactions between the metal vapor and kaolinite sorbent, that were identified in the bench scale tests.

Kinetics of catalyst loss during potassium-catalysed CO2 gasification of carbon

A significant fraction of the potassium catalyst can be lost by vaporization during catalysed carbon gasification. The extent of this loss depends primarily on the reaction start-up procedure. Temperature programmed experiments show that, under inert atmospheres, both KOH and K2CO3 react with carbon to give a reduced form of potassium-carbon complex. The formation of this complex appears to be a prerequisite for the vaporization of potassium. The rate of vaporization at 800 °C follows a first-order expression. Under gasification conditions, only a fraction of the catalyst is in this reduced form; therefore, the rate of catalyst loss during gasification is lower than that under inert atmospheres. The effect of catalyst loss on both the initial gasification rate and the variation in rate with conversion has been determined.

Arrhenius Characterization of ILD and Copper CMP Processes

To date, chemical mechanical planarization (CMP) models have relied heavily on parameters such as pressure, velocity, slurry, and pad properties to describe material removal rates. One key parameter, temperature, which can impact both the mechanical and chemical facets of the CMP process, is often neglected. Using a modified definition of the generalized Prestons equation with the inclusion of an Arrhenius relationship, thermally controlled polishing experiments are shown to quantify the contribution of temperature to the relative magnitude of the thermally dependent and thermally independent aspects of copper and interlayer dielectric (ILD) CMP. The newly defined Prestons equation includes a modified definition of the activation energy parameter contained in the Arrhenius portion, the combined activation energy, which describes all events (chemical or mechanical) that are impacted by temperature during CMP. Studies indicate that for every consumable set combination (i.e., slurry and polishing pad) a characteristic combined Arrhenius activation energy can be calculated for each substrate material being polished.

Utilization of coal-ash minerals for technological ceramics

Glasses synthesized from Utah bituminous coal-ash melts were crystallized to form glass ceramics to determine the feasibility of coal-ash utilization. The use of additives to promote glass formation and catalysts to serve as nucleation sites for crystallization was studied. The microstructure of the crystalline phase was investigated using X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. The bulk glasses and glass-ceramics were evaluated by Knoop microhardness and density measurements. The crystalline phase formed has been identified as anorthite, CaAl2Si2O8. Crystallization of the ash was possible up to a maximum of approximately 40%. The use of TiO2 as a nucleation catalyst did little to improve the degree of crystallinity; however, the crystal phase became better defined when this catalyst was used, even in small amounts.

Catalytic effect of potassium on the rate of char-CO2 gasification

The effect of potassium on the rate of char-CO2 gasification at 800 °C was investigated. The instantaneous rate depends on both catalyst concentration (KC) and the internal porous structure of the solid. At low values of KC atomic ratio, the rate increases sharply with the addition of catalyst. As catalyst concentration is increased, the rate first levels off and then decreases. The levelling off is attributed to the saturation of the surface with catalytic sites. The subsequent decrease in rate seems to be due to the plugging of micropores by catalyst deposits. The reaction rate changes significantly during gasification and drops sharply before gasification is completed. The drop in rate before total conversion can be explained by catalyst accumulation and pore plugging.

Adsorption of Moisture and Organic Contaminants on Hafnium Oxide, Zirconium Oxide, and Silicon Oxide Gate Dielectrics

Hafnium oxide (HfO 2 ) and zirconium oxide (ZrO 2 ) are two high-K materials having the potential to replace silicon oxide (SiO 2 ) as the gate dielectric. Atmospheric molecular contamination can affect the quality of the new gate dielectric film in a manner similar to SiO 2 . Characterization of contaminant adsorption behavior of these high-K films should assist in deciding their potential for successful integration in silicon metal oxide semiconductor technology. The interaction of moisture and organics (in particular, isopropanol, IPA) as common interfacial contaminants with a 5 nm HfO 2 film deposited by atomic layer chemical vapor deposition (ALCVD), which is a trademark of ASM International) is investigated using atmospheric pressure ionization mass spectrometry (APIMS); the kinetics and mechanism are compared to that of ZrO 2 and SiO 2 . HfO 2 and ZrO 2 have similar moisture adsorption loading, but are significantly higher than that of SiO 2 . However, almost all the adsorbed moisture can he removed from SiO 2 and HfO 2 after a 300°C bake under nitrogen purge, whereas ZrO 2 surfaces retain 20-30% of the adsorbed moisture. Experiments with IPA show that the adsorption loading on the three surfaces has the following order: ZrO 2 > HfO 2 > SiO 2 . A multilayer model for adsorption of water and IPA is developed to understand the mechanism of interactions of contaminants with the three surfaces. Results from the application of this multilayer model to the experimental data indicate that ZrO 2 forms the strongest surface-hydroxyl (X-OH) bond.

Multi-functional sorbents for the removal of sulfur and metallic contaminants from high-temperature gases

A multi-functional sorbent is developed for the simultaneous removal of alkali vapor, toxic metal vapors, and sulfur oxides from combustion gases. The sorbent is tested in a bench-scale reactor at the 800-1000{degree}C temperature range, using simulated flue gas containing controlled amounts of sodium, potassium, lead, and sulfur vapor compounds. The kinetics of sorption for thse contaminants, both individually and in combination, are measured. In general, the sorption process consists of adsorption followed by the diffusion of the metal in the product layer and finally reaction with the sorbent. The product layer is a porous alkali aluminosilicate in the case of alkali and a molten lead aluminosilicate in the case of lead. SO{sub -2} reacts with the calcium sites distributed over the aluminosilicate matrix. The tailored sorbent is effective in simultaneous removal of the tested contaminants; it even shows synergistic removal in some cases. 15 refs., 10 figs., 1 tab.

Variation of rate during potassium-catalysed CO2 gasification of coal char

Abstract The instantaneous rate of catalysed CO 2 gasification of char at 800 °C was measured at various levels of conversion. One important reason for the change in rate during the gasification is the change in the solid surface area, measured in the present study by CO 2 adsorption at 25 °C. The models which have been successful in representing the char porous structure under noncatalytic conditions were found inadequate for catalytic gasification at low conversions. Other important factors contributing to the variation in rate during conversion are the catalyst loss and the change in the catalyst/carbon ratio. A model is presented which combines the effects of these contributing factors and gives a satisfactory representation of the experimental data.

Significance of the reduction of alkali carbonates in catalytic carbon gasification

Abstract The catalytic effect of alkali carbonates on carbon gasification depends not only on the initial catalyst loading but also on how these carbonates are reduced to surface complexes during the initial heating of the samples. This effect is primarily due to three important processes which take place simultaneously during the initial heat treatment: 1, catalyst re-distribution on the surface; 2, catalyst loss by vaporization; and 3, change in the substrate surface area due to carbon conversion.