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Commercial entrained bed slagging gasifiers use a carbon feedstock of coal, petcoke, or combinations of them to produce CO and H 2 . These carbon sources contain mineral impurities that liquefy during gasification and flow down the gasification sidewall, interacting with the refractory linear and solidifying in the cooler zones of the gasifier. Proper slag flow is critical to good gasifier operation. A hot-stage confocal scanning laser microscope (CSLM) was used to analyze the kinetic behavior of slag crystallization for a range of synthetic coal-petcoke mixtures. On the basis of the observed precipitation during cool down studies in the 1200-1700 °C temperature range, a time-temperature-transformation (TTT) diagram was created. The crystallization studies were conducted with a CO/CO 2 (=1.8) corresponding to a gasification P O 2 of approximately 10 -8 atm at 1500 °C. Ash chemistries were chosen such that they correspond to coal-petcoke feedstock mixtures with coal ash amounts of 0, 10, 30, 50, 70, and 100% (by weight), with the balance being petcoke ash. The TTT diagram exhibited two crystallization areas, one above and one below 1350 °C. At the nose of the higher temperature curves, karelianite (V 2 O 3 ) crystallization occurred and was fastest for a 30% coal-petcoke ash mixture. The second nose was located below 1350 °C and had spineltype phases that formed at 1200 °C, in which preferred atomic occupation at the octahedral and tetrahedral sites varied depending upon the ash composition. At 1200 °C, an Al-rich spinel formed for 100% coal slag and a Fe-rich spinel formed in petcoke-enriched slags. The addition of petcoke ash to coal ash promoted crystallization in the slag, with additional crystalline phases, such as V-rich spinel, forming at the lower temperatures. These phases were not predicted using commercially available databases.

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The objective of this study is to determine the temperature dependence of the nickel/nickel oxide phase transition using additions of dissolved hydrogen in subcritical and supercritical water. The dissolved hydrogen at the Ni/NiO boundary was measured by direct exposure of nickel coupons at 320 to 450 • C and 25 MPa. The stable form of nickel at each exposure condition was determined by optical examination and confirmed by nuclear reaction analysis. Henry's law constant as a function of temperature was determined in-situ by measuring the fugacity of hydrogen through a palladium-silver tube using known dissolved hydrogen concentrations. Below 320 • C, Henry's constant decreases linearly up to the critical temperature, above which the slope becomes temperature independent. The effect of water density on Henry's law constant was determined. A thermodynamic model for hydrogen fugacity at the Ni/NiO phase boundary was developed and was shown to agree with the measured data. The deviation in the fugacity at the critical temperature was accounted for by the inclusion of activity of water in the model, highlighting the importance of thermodynamic properties of solvents in high temperature oxidation. Stress corrosion crack (SCC) initiation and growth experiments on nickel-based alloys has been conducted by many researchers to characterize cracking in pressurized water reactor (PWR) primary water conditions. [1][2][3][4][5] Testing conducted by varying the dissolved hydrogen concentration has shown that nickel-based alloys have a maximum in the cracking susceptibility near the Ni/NiO phase transition.6-8 Both exposure data and calculations show that the dissolved hydrogen concentration required to stabilize nickel increases with temperature. 9Given the importance of the Ni/NiO transition in SCC, accelerated testing at higher temperatures than in service should then account for this shift in the transition with temperature. Increasing the testing temperature beyond PWR conditions (∼320• C) is appealing for accelerated experiments, but the extent to which this can be done is limited, given the transition to the supercritical water phase (SCW) at 374• C. Testing in this temperature range is also appealing for addressing supercritical water reactor conditions which operate at water temperatures up to 625• C at 25 MPa. 10 However, the location of the Ni/NiO boundary in water is not known at temperatures higher than the typical accelerated testing temperature of 360• C. Extensive experiments on the stability of nickel in water as a function of dissolved hydrogen concentration have been conducted by Attansio and Morton 9 by contact electrode resistance measurements of oxide films up to 360• C. Further work was conducted to determine thermodynamics of nickel oxidation by measuring the fugacity of hydrogen using a palladium-silver thimble and estimations of the Henry's law constant from unpublished data of Moshier and Witt. 11However, Attanasio and Morton 8 found that the measured standard state entropy and enthalpy disagree sligh...

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