Eric M. Suuberg

A review of the kinetics of the nitric oxide-carbon reaction

The literature on the kinetics of the NO-carbon reaction is reviewed. Data are examined both from studies in which catalysis played no role and from studies in which catalysts were present as a result of addition or as natural inclusions (i.e. in coals). It is concluded that there generally exist three distinct reaction regimes, defined in terms of reaction temperatures. There is a near-ambient-temperature chemisorption regime, in which steady gasification is not possible. In this regime, both reversible and irreversible chemisorption processes are observed. A low-temperature gasification regime is observed at temperatures above ∼ 500 K, but below ∼ 1000 K. A high-temperature gasification regime is observed above this transition temperature. The low-temperature gasification regime is characterized by low activation energies, and the high-temperature regime by high activation energies. These two distinct gasification regimes however are not observed in all cases, and there is as yet no clear understanding of what determines this. In the reaction of pure NO with carbons, the order of reaction with respect to NO appears to be near unity in the gasification regimes and two in the low-temperature chemisorption regime. The apparent order in gasification is strongly influenced by the presence of CO in the reactant gas stream, and in certain reaction configurations the CO may be self-generated. In the presence of CO, the apparent order with respect to NO decreases, and there are several reports of apparent fractional order. Other oxidizing gases (e.g. O2, H2O) can also influence the kinetics and course of the NO-carbon reaction. The thermal history of the carbon can have a significant effect on its reactivity in NO. It appears however that the annealing of carbons towards NO cannot be reliably predicted from the annealing behaviour of the carbons towards other oxidizing gases (i.e. O2). Reaction rates of NO with carbon can also be influenced by the presence of mineral matter. There is no general agreement on the quantitative importance of mineral catalysis in the reaction of carbons with NO.

Mercury capture by native fly ash carbons in coal-fired power plants.

The control of mercury in the air emissions from coal-fired power plants is an on-going challenge. The native unburned carbons in fly ash can capture varying amounts of Hg depending upon the temperature and composition of the flue gas at the air pollution control device, with Hg capture increasing with a decrease in temperature; the amount of carbon in the fly ash, with Hg capture increasing with an increase in carbon; and the form of the carbon and the consequent surface area of the carbon, with Hg capture increasing with an increase in surface area. The latter is influenced by the rank of the feed coal, with carbons derived from the combustion of low-rank coals having a greater surface area than carbons from bituminous- and anthracite-rank coals. The chemistry of the feed coal and the resulting composition of the flue gas enhances Hg capture by fly ash carbons. This is particularly evident in the correlation of feed coal Cl content to Hg oxidation to HgCl2, enhancing Hg capture. Acid gases, including HCl and H2SO4 and the combination of HCl and NO2, in the flue gas can enhance the oxidation of Hg. In this presentation, we discuss the transport of Hg through the boiler and pollution control systems, the mechanisms of Hg oxidation, and the parameters controlling Hg capture by coal-derived fly ash carbons.

Size distribution of unburned carbon in coal fly ash and its implications

Abstract A set of nine coal fly ashes, obtained from various US utilities, were fractionated by standard dry-sieving techniques. The carbon contents of the different size fractions were measured, and the nature of the carbon particles was microscopically examined. Significant differences were found in the distribution of carbon in class F and class C ashes. The ‘foam index’ test is commonly used for quick evaluation of the suitability, with respect to air entrainment, of pozzolanic additives for concrete. This test measures adsorption of air entraining admixtures (AEAs) by the carbon in the fly ash. Application of this test to the different ash fractions confirmed that the smallest particle size fractions of ash make the major contribution to AEA adsorption. The carbon from class F ash has a comparable capacity for AEA adsorption as carbon from class C ash, when compared on a surface area basis. What makes the class C carbons apparently ‘worse’ is the fact that they have much higher surface areas than class F carbons (and it is only by virtue of the low carbon mass in most class C ashes that problems with these ashes are not more common). The importance of accessibility of the surface is also clearly seen from these results.

Interactions of carbon-containing fly ash with commercial air-entraining admixtures for concrete

The most important commercial outlet for coal ash is as a partial replacement for Portland cement in the concrete industry. High levels of unburnt carbon can render ash samples unsuitable for this high-value market by interfering with the action of air-entraining admixtures, which are specialty surfactants used to stabilize air bubbles in concrete mixtures. An initial laboratory investigation was carried out to identify the fundamental interaction mechanisms between fly ash and air-entraining admixtures. The results indicate that the interaction is time-dependent and occurs to a degree that correlates only crudely with the amount of carbon present. Measurements made on a variety of model additives suggest that admixtures are preferentially adsorbed from the aqueous phase on non-microporous carbonaceous surfaces.

Mechanisms of surfactant adsorption on non-polar, air-oxidized and ozone-treated carbon surfaces

Ozone treatment of fly ash carbon has recently been reported to inhibit the adsorption of commercial surfactants in concrete paste, thus mitigating the known negative effects of carbon on ash utilization. This paper examines the general mechanism of surfactant adsorption on carbon and its suppression by surface oxidation. Experimental results are presented for two carbon types (carbon black, fly ash carbon), both raw and surface oxidized (by ozone and molecular oxygen) and several commercial anionic and non-ionic surfactants (Darex II, SDS, Tergitol). The treated carbon surfaces were characterized with XPS, FT-IR, thermal desorption in N and H / He, surface acidity, hygroscopic behavior, interfacial 22 energy and its components through contact angle measurement involving standard liquid probes. Surface oxidation is found to decrease surfactant adsorption in each of the carbon / oxidant / surfactant systems examined, and its effect correlates with the amount of surface oxides by XPS. The combined results suggest that surfactant adsorption primarily occurs on non-polar carbon surface patches where it is driven by hydrophobic interactions. The main mechanism of oxidative suppression is the destruction of this non-polar surface, though micropore blockage and increased negative surface charge may also contribute for some systems.  2003 Elsevier Science Ltd. All rights reserved.

Temperature dependence of crosslinking processes in pyrolysing coals

Changes in the macromolecular structure of a lignite and a bituminous coal during rapid pyrolysis in the temperature range 300–1200 K are described. Solvent swelling techniques have clearly demonstrated that crosslinking occurs in lignites at somewhat lower pyrolysis temperatures than it does in bituminous coals. The onset of the crosslinking processes in bituminous coals coincides with the end of the tar formation period. In lignites, crosslinking occurs very early in the pyrolysis process, coinciding with low temperature release of CO2. The presence of natural moisture in the lignite appears to have a significant effect on pyrolysis chemistry, increasing the amount of crosslinking observed at any temperature.

Molecular weight distributions of tars produced by flash pyrolysis of coals

Abstract The molecular weight distributions of tars produced during flash pyrolysis of coal have been determined by a combination of gel permeation chromatography and vapour phase osmometry. Small particles (62–88 μm) of two high-volatile and one low-volatile bituminous coal, and a lignite have been pyrolysed at heating rates of ≈1000 K s −1 at temperatures from 600 to 1300 K in a heated wire mesh apparatus. The molecular weight distributions range from ≈100 to 4000 and peak in the range from 250 to 750 in all cases. The evidence gathered on a softening bituminous coal clearly indicates a selective evaporation of light fractions of the metaplast.

Physical changes accompanying drying of western US lignites

Abstract Experiments have been performed to measure the effect of drying procedures of different degrees of severity on the physical properties of four North Dakota lignites. Moisture desorption-readsorption behaviour was relatively reversible in some cases and irreversible in others. Shrinkage measurements commonly showed volume loss greater than that calculated from moisture loss. The shrinkage was always irreversible, even when drying was relatively incomplete; only ≈ 80% of original volume was regained upon reswelling in water. Severity of drying had a significant effect on measured N 2 surface areas, and on pore size distributions throughout most ranges of pore sizes. All the results are consistent with the view that low-rank coals are essentially colloidal gels, many of whose properties are irreversibly altered by drying.

Development and Application of a Three-Dimensional Finite Element Vapor Intrusion Model

Abstract Details of a three-dimensional finite element model of soil vapor intrusion, including the overall modeling process and the stepwise approach, are provided. The model is a quantitative modeling tool that can help guide vapor intrusion characterization efforts. It solves the soil gas continuity equation coupled with the chemical transport equation, allowing for both advective and diffusive transport. Three-dimensional pressure, velocity, and chemical concentration fields are produced from the model. Results from simulations involving common site features, such as impervious surfaces, porous foundation sub-base material, and adjacent structures are summarized herein. The results suggest that site-specific features are important to consider when characterizing vapor intrusion risks. More importantly, the results suggest that soil gas or subslab gas samples taken without proper regard for particular site features may not be suitable for evaluating vapor intrusion risks; rather, careful attention needs to be given to the many factors that affect chemical transport into and around buildings.

Experimental study on mass transfer from pyrolysing coal particles

Abstract The molecular weight distributions of coal tars and coal char extracts were examined in an effort to learn more about the process of mass transfer during coal pyrolysis. Evidence was obtained which suggests that the majority of the tar evolved during rapid pyrolysis of pulverized coal escapes by a process limited by gas film diffusion. However, there is also some evidence that the tar includes a small amount of heavy material which could have been ejected from the particle in a condensed phase. Data were also obtained which suggest that the tar precursors (within the parent coal) are formed over a wide range of temperature and do not seem to be present as such in the raw coal. The rather large effect of pressure on yields of tar from bituminous coal pyrolysis has previously been attributed to the effect of pressure on evaporation rates of tar precursors from the particle surface. This study shows that the molecular weight distributions of both the tar and extractable tar precursors within the particle are consistent with such a mechanism.