John W. Graydon

Sorption and stability of mercury on activated carbon for emission control.

A leading strategy for control of mercury emissions from combustion processes involves removal of elemental mercury from the flue gas by injection of activated carbon sorbent. After particulate capture and disposal in a landfill, it is critical that the captured mercury remains permanently sequestered in the sorbent. The environmental stability of sorbed mercury was determined on two commercial, activated carbons, one impregnated using gaseous sulfur, and on two activated carbons that were impregnated with sulfur by reaction with SO(2). After loading with mercury vapor using a static technique, the stability of the sorbed mercury was characterized by two leaching methods. The standard toxicity characteristic leaching procedure showed leachate concentrations well below the limit of 0.2mg/L for all activated carbons. The nature of the sorbed mercury was further characterized by a sequential extraction scheme that was specifically optimized to distinguish clearly among the highly stable phases of mercury. This analysis revealed that there are two forms in which mercury is sequestered. In the sorbent that was impregnated by gaseous sulfur at a relatively low temperature, the mercury is present predominantly as HgS. In the other three sorbents, including two impregnated using SO(2), the mercury is predominantly present in the elemental form, physisorbed and chemisorbed to thiophene groups on the carbon surface. Both forms of binding are sufficiently stable to provide permanent sequestration of mercury in activated carbon sorbents after disposal.

The behaviour of selected heavy metals in MSW incineration electrostatic precipitator ash during roasting with chlorination agents

Abstract The thermal behaviour from 500–1050°C of selected heavy metals (Pb, Zn, Cu and Cd) in the electrostatic precipitator (ESP) ash from a municipal solid waste (MSW) incinerator was studied on a laboratory scale. The parameters studied included temperature, reaction time, type of chlorination agent, and amount of chlorination agent. The vaporization and removal of these metals are characterized by the formation and volatilization of the metal chlorides, and are dependent on temperature and time. As a result of volatilization, ≈ 90% of Pb, Zn, Cu and Cd can be removed from the ESP ash under the conditions studied. The fraction of metal removed is determined mainly by the relative stabilities of the metal chloride and oxide. The results provide a basis for the development of a thermal treatment process to convert the ESP ash from potentially hazardous into non-hazardous material and to recover the selected metals from the ash.

Determination of the Avrami exponent for solid state transformations from non-isothermal differential scanning calorimetry

Abstract Many solid state transformations follow the Johnson-Mehl-Avrami kinetic equation, particularly crystallization of polymers, glasses and amorphous metals. The value of the Avrami exponent in this relationship, important for understanding the reaction mechanism, is normally determined by isothermal differential scanning calorimetry. A computer model has been developed to generate non-isothermal differential scanning calorimeter curves. This model was used to construct a function that consists of parameters easily derived from non-isothermal experiments and which is strongly dependent on the values of the Avrami exponent and is nearly independent of all other variables. Thus, from the values of peak temperature, peak height, activation energy and enthalpy, the value of the function may be calculated and the Avrami exponent read from a correlation curve. The validity of this procedure has been demonstrated using an experimental example of the crystallization of an amorphous metal alloy. The Avrami exponent determined by this procedure was in good agreement with the value of the exponent determined by the conventional isothermal method and resulted in model differential scanning calorimeter curves which closely agreed with those measured experimentally.

Predicting Carbon Dioxide Corrosion of Bare Steel under an Aqueous Boundary Layer

Abstract The corrosion of bare steel under an aqueous boundary layer with dissolved carbon dioxide (CO2) was modeled to investigate the effect of CO2. The model incorporated the coupled effect of CO2 diffusion, hydration, local ionic equilibria, ferrous carbonate (FeCO3) precipitation, and steel corrosion. The model was verified against published experimental data under both FeCO3-saturated and unsaturated boundary layers. Good agreement was shown under a variety of conditions. For saturated boundary layers, the results show that the corrosion rate in carbonic acid (H2CO3) is greater than in hydrochloric acid (HCl)for a given pH and that H2CO3 reduction is the cause for the increase of corrosion rate in H2CO3. Increasing temperature was found to increase corrosion rate substantially. This work provides further understanding of the CO2 corrosion mechanism and is a reliable, convenient, and practical tool for predicting the rate of CO2 corrosion.

Interpretation of activation energies calculated from non-isothermal transformations of amorphous metals

Abstract Non-isothermal differential scanniong calorimetry is often used to determine the activation energy of transformation in amorphous metals by the way in which the transformation peaks shift with changes in heating rate. The two relationships used for this purpose were developed by Kissinger and Ozawa. There have been attemptsto compare the activation energie obtained from each of these methods and therey to infer the order of the underlying kinetic process. In fact, the differences found between the activation arise from different approximations used in developing the two relationships. When a correction is applied, both techniques give identification energies for th transformation and yield no information about the reaction order nor even about the type of kinetics involved.

Steel Corrosion under a Disbonded Coating with a Holiday—Part 1: The Model and Validation

Abstract A model was developed to investigate the corrosion of a coated pipeline surface in a thin, narrow channel crevice formed between the pipe surface and the coating. Such crevices are often found on natural gas pipelines when the coating is high-density polyethylene (HDPE). Imposed cathodic protection current passage through the holiday and oxygen diffusion through both the holiday and the coating were included in the model. The effect of the corrosion product, ferrous hydroxide (Fe[OH]2), on the transport of oxygen and ionic species in the crevice solution was neglected because it is porous. Mass balance for oxygen pressure and Laplaces equation for the steel potential in the crevice solution were used to derive the model equations. For a crevice solution with uniform pH and uniform resistivity, the model equations were solved using a collocation technique programmed in MATLAB. This new model was validated with published experimental data and model results—good agreement was found.

Steel corrosion under a disbonded coating with a holiday-Part 2: Corrosion behavior

Abstract Corrosion of a pipe surface in a channel crevice has been modeled assuming oxygen diffusion through a holiday in a permeable coating. The crevice was thin and narrow with a finite length. ...

Long-term electrolytic hydrogen permeation in nickel and the effect of vanadium species addition

Nickel cathodes have been found to become deactivated under long-term polarization in the H2 evolution region during alkaline water electrolysis. The cause of deactivation was examined using steady state polarization and measurement of hydrogen permeation through nickel foil in 8 mol/l KOH at 70 8C and 100 mA/cm 2 . The long-term (over 50 h) permeation behaviour was explained by formation and growth of a nickel hydride phase. The rise in hydrogen overpotential was ascribed to an increase of the hydrogen surface coverage on the newly formed hydride. The effect of an electrolyte additiv e( avanadium salt) on the hydrogen overpotential and permeation rate was also investigated. Upon addition of dissolved V2O5, the permeation rate was found to increase quickly and then slowly decrease to a steady value close to that measured for hydride-free nickel. Meanwhile, the hydrogen overpotential was observed to recover back to nearly its initial value for fresh nickel. The exhibited behaviour was attributed to decomposition of the hydride phase after deposition of a vanadium-bearing compound. The prolonged contact between Ni and V was proposed as the main reason for hydride decomposition. The addition of more vanadium had no further result on the hydrogen overpotential. # 2002 Elsevier Science Ltd. All rights reserved.

Characterization of vanadium deposit formation at a hydrogen evolving electrode in alkaline media

Although nickel exhibits a high electrocatalytic activity toward the hydrogen evolution reaction, it undergoes extensive deactivation as a cathode during alkaline water electrolysis. The addition of dissolved V 2 O 5 to the electrolyte results in partial reactivation of nickel cathodes in 8 mol/L KOH at 70°C by formation of a vanadium-rich deposit. Various analytical techniques were employed to characterize the deposit in terms of surface morphology, phase analysis, and chemical composition. The deposit on nickel had a smooth and compact surface with a thickness of 1-2 μm. X-ray diffraction results indicated an amorphous structure with a Scherrers length less than 2 nm. Chemical and thermal analyses led to an empirical compound formula of K 2 H 2 V 10 O 26 .4H 2 O. That formula implied a mixed-valence (+4/+5) vanadium compound, later confirmed by X-ray photoelectron spectroscopy. The mechanism of deposit formation was then investigated by cyclic voltammetry on a mercury electrode. A cathodic peak representing the reduction of vanadium (+5) species was apparent near the onset of hydrogen evolution on mercury. The behavior of the cathodic peak with concentration and sweep rate revealed key steps in the mechanisms of deposit formation. Deposit formation was modeled as an irreversible precipitation/polymerization reaction following a charge transfer step, to produce a large, mixed-valence vanadium compound.

Effect of ferrous ion oxidation on corrosion of active iron under an aerated solution layer

Abstract A new model was developed to study the effect of ferrous ion oxidation on the corrosion of active iron. When ferrous hydroxide precipitate is present at the iron surface as the result of c...