Adel F. Sarofim

Gas-phase transformations of mercury in coal-fired power plants

Because mercury enters the food chain primarily through atmospheric deposition, exposure models require accurate information about mercury emission rates and mercury speciation from point sources. Since coal-fired power plants represent a significant fraction of the anthropogenic emissions of mercury into the atmosphere, the speciation of mercury in coal-fired power plant flue gas is currently an active topic of research. We have demonstrated that the assumption of gas-phase equilibrium for mercury-containing species in coal-fired power plant exhaust is not valid at temperatures below approximately 800 K (500°C). Chlorine-containing species have been shown to be the most important for oxidation of elemental mercury in the post-combustion gases. The conversion of HCl to Cl2 in the flue gas of a coal-fired power plant is kinetically limited. Kinetic calculations of the homogeneous oxidation of elemental mercury by chlorine-containing species were carried out using global reactions from the literature. The levels of mercury oxidation, while of comparable magnitude to field observations, are still below the 40% to 80% oxidation typically observed in field measurements.

Kinetics of the NOcarbon reaction at fluidized bed combustor conditions

Abstract The reduction of NO by carbonaceous solid has been studied in a packed bed reactor. After an initial transient, a steady rate of NO reduction is observed with N 2 , CO and CO 2 as the dominant products. Kinetic parameters of the reaction were obtained for various NO and CO concentrations, temperatures, and carbon types. The reaction rate per unit total surface area is found to be first order in NO, to have an apparent activation energy of 44 kcal/mole for temperatures higher than 873K, to be enhanced by the presence of CO, and to vary by approximately an order of magnitude between graphite and coal chars.

The Physical Transformation of the Mineral Matter in Pulverized Coal Under Simulated Combustion Conditions

Abstract The physical transformation of the mineral matter in coal has been studied in a laboratory furnace using size-graded, pulverized samples of a lignite and a bituminous coal. The mineral matter is originally distributed in micron-size inclusions in the coal particles. The paper illustrates how the final particle size distribution of the ash produced at combustion temperatures of 1250 to 1830K is determined by a combination of agglomeration of fused mineral matter, cenosphere formation due to gas evolution and vaporization and recondensation of volatile constituents

Deposition of bituminous coal ash on an isolated heat exchanger tube: Effects of coal properties on deposit growth

Fouling deposits were collected on a steam-cooled tube at the exit of a pilot scale furnace during combustion of bituminous coal. The objective of the work is to identify the coal properties and combustion conditions with which one may anticipate fouling and slagging of superheaters in electric utility boilers. Because of the high fusion temperatures of the ashes investigated, deposition on a time scale practical for the pilot scale experiments was observed only at furnace exit gas temperatures above 1700 K (2600°F), higher than the temperatures expected in the convective section of a boiler. A method is proposed for analyzing such measurements to obtain quantitative descriptions of deposit growth under less severe conditions. The model utilizes ash melting temperatures and slag viscosities to account for the variation of particle sticking probabilities with gas and surface temperatures. These parameters are sufficient to reproduce the strong influence of iron during the early stages of deposit growth. As deposit thickness increases, the correlation of deposit loading with iron content disappears. An empirical shedding frequency was introduced to simulate the average loading at long times. The model provides a quantitative description of deposit growth on tubes, and a means for comparison of fouling propensities of different ashes under various conditions. Comparisons with field data are needed to determine whether the proposed model and parameters derived from pilot scale measurements at high temperatures can provide useful estimates of deposit growth on heat exchangers in boilers.

Factors governing the surface enrichment of fly ash in volatile trace species

Abstract Fly ash produced in high-temperature coal combustion has a bimodal size distribution with most of the mass occurring in the size range 1 to 20 Am diameter. Most of the ash surface area is provided by submicron particles (d - 0.03 μm) even though these constitute only of the order of 1% of the total mass. Both kinds of particles are coated with a surface deposit of volatile trace elements such as As, Sb, and S. The distribution of these species between the different size modes of the ash is influenced by the amount of submicron particulate produced during combustion. The amounts of trace species deposited on the larger particles are greatly in excess of those expected from either direct vapor condensation or a two-step process of condensation on the submicron particles followed by the scavenging of these fine particles by their larger counterparts. The data are explicable in terms of a chemically controlled surface deposition model.

The products of the high temperature oxidation of a single char particle in an electrodynamic balance

The ratio CO2/CO from oxidation of Spherocarb char has been measured over a wide temperature range making use of the electrodynamic balance where single particles are heated by laser irradiation but are immersed in room temperature gas. This has allowed measurement of the CO2/CO ratio formed by the heterogeneous reaction on the char surface for temperatures up to 1670 K. For these conditions, an exponential decrease with a temperature coefficient of 3100/K is found. The CO2/CO ratio is proportional to the oxygen partial pressure raised to a power of 0.21. These results are in substantial agreement with work reported at lower temperatures. At normal combustion temperature the CO2/CO ratio from heterogeneous reaction is less than 0.1. Gas phase oxidation of CO to CO2 near the char surface, however, can become important at high char temperatures, even in a gas maintained at room temperature, and can have an important impact on surface temperature. The temperature at which gas phase reactions begin to contribute to CO2 formation and surface temperature was found to be reduced by the presence of water vapor.

Soot morphology: An application of image analysis in high‐resolution transmission electron microscopy

Interest in the fine structure of soots and carbon blacks is motivated by a variety of possible applications. The structure provides information on the origins of the particles and on their adsorptive and reactive properties. This paper describes a method for quantification of the structure of soots and carbon blacks based on direct electron microscopic observation followed by image analysis of these materials. High‐resolution transmission electron microscopy (HRTEM) provides a very detailed observation of particle structure. The differences in soot structure, because of its complexity, may not be easily quantifiable with the human eye; therefore, high‐level computer software has been used to manipulate HRTEM images. This technique involves the application of fast Fourier transforms (FFT) to single particles and the measurement of characteristic parameters such as interplanar spacings and crystallite sizes from these particles. The methodology and application of this characterization technique are presented here. Results are shown for different samples obtained from soot and carbon black particles selected to illustrate the capabilities of the methodology. Quantitative information can be obtained on structural characteristics, e.g., interplanar spacing, circularity, orientation, elongation, and length distribution of lattice fringes, as well as on the fractional coverage of the extracted pattern.

The role of microporous surface area in the gasification of chars from a sub-bituminous coal

Abstract The role of microporous surface area in the CO 2 gasification of chars from a sub-bituminous coal was investigated. There is evidence that the reaction primarily takes place outside the microporous network on the surfaces of larger pores. The rate of gasification is insensitive to large changes in total surface area occurring during heat treatment or reaction. This behaviour is consistent with the occurrence of reaction on active sites lying preferentially outside the micropores. Mechanisms which can lead to an enhancement of the active site concentration on large pore surfaces are discussed.

NO/char reactions at pulverized coal flame conditions

The effective rate of the NO/char reaction measured over the temperature range 1250 to 1750 K has been found to be given by (d NO/dt)=4.18×104 exp (−34.70 K cal/RT) AEPNO moles/sec where AE is the external surface area of the char in m2/gm, and PNO is in atmospheres. The rate of the reaction is found to be retarded by water vapor and enhanced by CO by amounts that decrease with increasing temperature. This is consistent with a hypothesis that the NO/C reaction is retarded by the formation of a chemisorbed layer which can be removed by reaction with CO. Support for this hypothesis is provided by transient experiments which show that, at low temperatures, NO reacts with carbon to form N2 and a chemisorbed oxygen layer, and that the chemisorbed oxygen decomposes at higher temperatures to form CO or reacts with CO to form CO2.

Vaporization and condensation of mineral matter during pulverized coal combustion

The factors governing the amount and properties of the submicron aerosol (fume) produced on combustion of coal have been studied by burning size-graded Montana lignite particles in a laminar drop-tube furnace at 1750 K. The coal particle temperatures achieved vary from 1800 K to 2800 K as the oxygen concentration in which the particles burn is increased from 5% to 100%. Size fractionation of the ash yields a bimodal size distribution. The average size of the fine particles varies from less than 50diameter to greater than 300 , depending on the combustion conditions. These fine particles are produced by vaporization of the mineral matter species during combustion and their subsequent recondensation. The amount of fume produced as metal oxides increases from 0.1% of the total ash at particle temperatures around 1800 K to 20% at 2800 K. The chemical composition of the Montana lignite fume is dominated by the refractory oxides MgO and CaO at all but the lowest temperatures (T The oxidation of the vaporized reduced-state species away from the particle surface results in a supersaturation of the refractory oxides, leading to their nucleation. A simple model in which growth of these particles then occurs by coalescent collisions and by heterogeneous condensation of new material released from the burning particles provides a good representation of the observed particle sizes as a function of residence time and degree of ash vaporization.