Adsorptive and Optical Properties of Fly Ash from Coal and Petroleum Coke Co-firing

Abstract

Eight commercial fly ash samples including two from petroleum coke/coal co-firing were characterized to determine the origin, structure, and properties of the unburned carbon. Because of its relevance to ash utilization, the characterization included measurements of adsorptivity toward concrete surfactants, and its dependence on carbon form, particle size, and contact time. The coal/coke co-firing ashes are observed to possess unique optical and adsorptive properties which are related to the size, density, and internal porosity of the coke-derived residual char.

Strategies and Technology for Managing High-Carbon Ash

“…The result is expressed as a "surfactant adsorptivity" in milliliters of standard surfactant solution/gram-sorbent. Alternative approaches using direct surfactant assay by UV adsorption have been attemped [11] but fail to provide an appropriate index of adsorptivity, likely due to differences between the components with strong optical absorbance and those with high surfactant effectiveness [11]. In the experiments with single component surfactants, results will also be expressed as molar uptakes (mol surfactant / g-carbon) to aid in interpretation.…”

“…Here the carbon is a contaminant and its undesirable adsorption of surfactant from the aqueous concrete paste reduces the surfactant's ability to stabilize sub-millimeter air bubbles that help improve freeze-thaw resistance in set concrete [10]. Previous studies [5,7,11,12] have related the extent of adsorption to four factors: (1) the amount of residual carbon in ash, (2) the total carbon surface area, (3) the accessibility of the surface as governed by pore and particle size distribution, and (4) the state of carbon surface oxidation. Oxidation of carbon surfaces by either air or ozone has been shown to reduce the extent of concrete surfactant adsorption [7].…”

Strategies and Technology for Managing High-Carbon Ash

“…In some regions of the world the most serious problem with carbon is its undesirable effect on the air entrainment process. Carbon adsorbs the surfactants or "air entraining admixtures" used to stabilize air bubbles that impart freeze/thaw resistance in the set concrete [1][2][3][4].…”

“…A series of recent studies on commercial ash samples has attempted to relate fly ash quality (surfactant adsorptivity) to residual carbon properties, such as surface area, surface chemistry, and pore size distribution [2][3][4]. This cited work does not provide direct answers to two important practical questions: how are adsorptivity and other residual carbon properties influenced by (1) solid fuel type, and (2) combustion conditions.…”

“…In addition a set of 53 commercial ash samples were acquired from North American utility companies burning a range of bituminous and sub-bituminous coals [12]. One sample originated from a coal/petroleum-coke cofiring test burn described previously [3]. Selected measurements were also made on commercial carbon blacks (Cabot, Billerica, MA) as model materials for studying the effects of carbon surface oxides by X-ray Photoelectron Spectroscopy (XPS) in the absence of interference from the abundant inorganic oxides in ash.…”