Thermodynamic Equilibrium Predictions Research Articles

In-situ redox conditions in hydrothermal diamond-anvil cell experiments using various metal gaskets

Hydrothermal diamond-anvil cells (HDACs) have been widely used in various research fields since they were introduced in 1993. Unlike temperature (T) and pressure (P), the redox states of the samples in HDAC experiments were poorly controlled or measured; they were usually estimated by observing the reduction/disappearance of redox-sensitive mineral(s) (e.g., Mo metal) and corresponding formation and growth of such mineral(s) (e.g., MoO2) without in-situ redox measurement. In this study, we demonstrated the generation of H2 during experiments using various metal gaskets, including inert (Re) and chemically more reactive metals (Ni, Co, Mo, and W), in the pure H2O system under various P-T conditions. A technique for quantitative Raman spectroscopic analysis of H2 in HDAC is designed. Based on this technique, the oxygen fugacity (fO2) values of Mo-MoO2 buffer at the experimental P-T conditions were then calculated from these measured H2 pressures. We demonstrated that steady-state redox control can be maintained at a fixed T between 400 and 500 °C in HDAC experiments using a Mo gasket for durations of up to two hours and for those using a Re gasket with an additional Mo rod in the sample between 400 and 800 °C for durations of up to 90 min, regardless of the considerable changes in sample pressure. This demonstration was established by the balance between the rates of H2 generation and H2 loss from the sample chamber. The loss of H2 enhanced the consumption of H2O and therefore reduced the sample pressures during the experiments.We believe that hydrogen contents in every HDAC experiment, which attempt to control the redox state, need in situ measurement, then estimate the real redox state and compare it to thermodynamic equilibrium prediction. To enhance in situ redox control and measurements, modifications of HDAC are required for improving the quantitative Raman spectroscopic analysis technique in both sensitivity and accuracy.

Influence of MgO content on bio-ash slagging propensity and flowability under mild reducing environment

In this work, an inclined plate technique was used to investigate the slagging propensity of bio-ashes and the flowability of the resulting bio-slags in a reducing gas environment at 1300 °C. A number of different approaches were trialled to visualise and measure the slagging propensity of bio-ash as a function of MgO content, as well as their comparison with reference coal ashes and several synthetic ashes generated by blending bio-ash with CaO, Al2O3, SiO2 and K2O individually and/or collectively. As has been found, the presence of 6–16 wt% MgO in bio-ash and coal ash exerted little influence on the ash fusion temperature. However, the slag flowability is slow for a relatively high viscosity of the bio-slags. In contrast, the coal slags flew much faster, although the formation of spinel crystals was confirmed in the high MgO content. Among the major basic oxides within bio-ash and coal ash, CaO is most influential in depolymerising ash matrix to promote its flowability. In contrast, the presence of Fe2O3 is subtle, a small increase of whose content can cause the interaction with MgO for the formation of crystallised spinel. The co-presence of K2O and Al2O3 is the most negative, causing a significant decrease in the slag flowability. Finally, regarding the traditional methodologies for ash study, the base/acid (B/A) ratio for an overall balance between basic and acid oxides is unable to differentiate the discrepancy between coal ash and bio-ash. Likewise, the thermodynamic equilibrium prediction on liquidus amount and viscosity should be deemed as the empirical index that overlooks the kinetic control and formation of crystalline for the Mg-rich ash samples.

Study of CMAS infiltration and evaporation behaviour under water vapour/sulphur oxide conditions in EB-PVD 7YSZ

The infiltration behaviour of CaO-MgO-Al2O3-SiO2 (CMAS) in Electron Beam Physical Vapour Deposited (EB-PVD) 7 wt% yttria stabilised zirconia (7YSZ) under controlled atmospheres (dry, wet and sulphurizing) was studied in detail. The microstructure of the CMAS-infiltrated 7YSZ and the CMAS residue on top were analysed to interpret the corrosion mechanisms. One of the major goals was to investigate the stability and the infiltration of CaSO4 into the 7YSZ columns. At the testing temperature of 1250 °C, CaSO4 was stabilised only under the SO2 atmosphere within the TBC coating and only in CaSO4-containing CMAS. Hydroxylation of CMAS elements was found under wet conditions, which changed the local CMAS composition and infiltration behaviour. These chemistry changes have provoked acidity changes in the CMAS which directly affected their aggressivity and viscosity. Thermodynamic equilibrium predictions on the volatility of CMAS components and the anhydrite stability under various atmospheres were used to understand the experimental findings and the underlying mechanisms.

Examination of inorganic gaseous species and condensed phases during coconut husk combustion based on thermodynamic equilibrium predictions

Multi-component thermodynamic equilibrium predictions of coconut husk combustion products were performed in this work. The calculation results showed that the concentration of the chlorine species in the flue gas and ash were clearly influenced by the combustion temperature. At temperatures below 820 °C the highest concentration was HCl(g), and above 820 °C was KCl(g). Chlorine species were also observed in ash, as KCl–NaCl–RbCl solid solution (when the combustion temperature was lower than 700 °C), and KCl–NaCl–K2SO4–Na2SO4 liquid solution (in the range of 600–960 °C). At low combustion temperatures (600 °C), chlorine retention in solid ash was the most effective. Speciation of inorganic gaseous species and condensed phases were investigated also during flue gas cooling from 1000 to 200 °C. Major condensed phase compositions were dominated by alkali metal salts in both solid and liquid phase states. In addition, the Gibbs energy of substance formation, and Gibbs reaction energy, in order to thoroughly interpret the thermodynamic predictions were determined. Finally, thirty seven eutectic points for binary systems calculated in the “Phase Diagram” module of the FactSage package were presented. For the purposes of interpreting the results, the melting point ranking for pure substances and binary systems were created.

The Effect of Micro-Segregation on the Quantification of Microstructural Parameters in Grade 91 Steel

The quantification of key microstructural parameters as a function of aging or creep exposure time is commonplace in the assessment of 9Cr Creep Strength Enhanced Ferritic (CSEF) power-plant steels. In these studies, the sample is either assumed chemically homogenous at the micro-scale or that a material average will be achieved by collecting enough images at random locations. In this paper, the micro-scale chemical homogeneity of two ex-service boiler components, a pipe and a forging, are quantitatively assessed using high sensitivity chemical mapping from µ-XRF. The compositional variation was as expected most pronounced in the larger elements Mo and Nb, where a > 20 pct difference in composition was present between positively and negatively segregated areas. The effect of this micro-segregation on local variations in Laves phase particle characteristics was investigated using SEM images. This showed a factor of two difference in the number of particles and the area coverage between positively and negatively Mo-segregated regions. This result was consistent with the thermodynamic equilibrium predictions of the phase content based on the observed level of segregation.

Gaseous thermochemical synthesis and characterization of mixed interstitial phases in the Ti-C-O system

Titanium grade 2 thin foils were carbo-oxidized at 800 °C in a thermogravimetric analyzer in CO gas for 20 - 100 hours. The synthesized thin foils were characterized by infrared carbon determination for composition analysis, transmission X-ray diffraction (XRD), light optical microscopy (LOM) and nanoindentation. Thermodynamic equilibrium predictions were performed using ThermoCalc. Oxygen and carbon expanded h.c.p. α and NaCl type δ carbo-oxide were identified from the XRD patterns. Rietveld refinements of the XRD patterns revealed an increase in expansion of the h.c.p. α lattice from interstitially dissolved C and O: the c/a ratio increases from 1.588 for interstitially free h.c.p. α to 1.607 after 100 hours of exposure. The overall carbon and oxygen content after 100 hours was 4.2 wt% and 5.5 wt%, respectively; these overall compositions correspond to the two-phase α + δ region in the calculated ternary phase diagram. The microstructure from LOM revealed large grains of expanded α-phase and a surface zone consisting of porous δ carbo-oxide. The hardness, as determined by indentation, of the expanded α increased linearly with the c/a ratio, resulting in an increase to 11748 MPA after 100 hours exposure from 2193 MPa for the untreated Ti grade 2.

Surface-scale affinity and adsorption selectivity of alkaline earth metal oxides to H2O and CO2: Insight into SOFC anode modification

Periodic density functional theory (DFT) calculations have been performed to investigate the adsorption selectivity of H2O over CO2 on alkaline earth metal oxide (AEMO) surfaces. A simple gridding-based interpolation method is innovatively employed to determine the surface-scale affinity. The results show that AEMO surfaces exhibit higher affinity to H2O than CO2 in perspective of whole surface scale. Both the affinity strengths to H2O and CO2 follow the trend BaO > SrO > CaO > MgO. The adsorption selectivity of H2O over CO2 on AEMO surfaces is sensitively dependent on temperature and partial pressure ratio. Based on thermodynamic equilibrium prediction, the estimated adsorption selectivity of H2O over CO2 on AEMO surface is extremely high under SOFC operating condition. This indicates that the anti-coking behavior of AEMO-modified Ni-based solid oxide fuel cell (SOFC) anodes is for the most part governed by H2O capture, which serves an explanation for the experimental finding and anti-coking mechanism proposed in literatures.

Thermodynamic equilibrium predictions of zinc volatilization, migration, and transformation during sludge co-incineration.

The effects of interactions between and among chlorine (Cl), sulfur (S), phosphorus (P), and minerals on migration, transformation, and volatilization of zinc (Zn) were numerically simulated in sludge co-incineration using the chemical thermodynamic equilibrium method. Our results showed that all the minerals of Fe2 O3 , Al2 O3 , Fe2 O3 , and TiO2 except for CaO in the sludge co-incineration system reacted with Zn which inhibited the Zn volatilization. The presence of S and P was beneficial to the formation of ZnSO4 (s) and Zn3 (PO4 )2 (s). Cl weakened the chemical reactions between the minerals and Zn, thus increasing the Zn volatilization. Changes in Zn transformation and migration induced by the coupling of Cl+S were mainly controlled by Cl, S, and the minerals, while those induced by Cl+P and S+P were mainly controlled by P and S+P. The presence of P+Cl, S+Cl, S+P, S+Cl+P, Cl, and Al2 O3 in the coexisting mineral system controlled the reactions between the minerals and Zn.

Characterization of products derived from the high temperature flash pyrolysis of microalgae and rice hulls

Flash heating and high temperature processing of two biomass feed materials is presented as a sustainable thermochemical technique to leverage the entire carbon content of the starting material for various end-uses. Microalgae and rice hulls were flash pyrolyzed in a drop-tube reactor to form exclusively gas and solid products in the temperature range 1573–1873 K at gas residence times of 3–5 s. Product gas yields and compositions approached thermodynamic equilibrium predictions with increasing temperature, which were used in calculations for a conceptual solarthermal processing framework. The derived carbonaceous residue was further characterized for composition (atomic mass and crystalline structures), morphology (surface area, pore volume, surface roughness), and reactivity (via CO2 thermogravimetry). Solid residues generated under flash conditions tended to become nearly indistinguishable, becoming spherical with high macroporosities and similar apparent reactivities. These results were contrasted with slow-pyrolyzed chars of the same starting materials, whose characteristics were markedly opposite for microalgae and rice hulls, further evidencing the importance of char particle thermal history. Results from solarthermal operation show a high calorific value syngas, especially for microalgae at 1873 K. The applicability of carbonaceous residue in a particular downstream use requires further characterization and study.

Gamma Prime Precipitate Evolution During Aging of a Model Nickel-Based Superalloy

The microstructural stability of nickel-based superalloys is critical for maintaining alloy performance during service in gas turbine engines. In this study, the precipitate evolution in a model polycrystalline Ni-based superalloy during aging to 1000 hours has been studied via transmission electron microscopy, atom probe tomography, and neutron diffraction. Variations in phase composition and precipitate morphology, size, and volume fraction were observed during aging, while the constrained lattice misfit remained constant at approximately zero. The experimental composition of the γ matrix phase was consistent with thermodynamic equilibrium predictions, while significant differences were identified between the experimental and predicted results from the γ′ phase. These results have implications for the evolution of mechanical properties in service and their prediction using modeling methods.

Reaction of nitrous oxide with methane to synthesis gas: A thermodynamic and catalytic study

The aim of the present study is to explore the coherence of thermodynamic equilibrium predictions with the actual catalytic reaction of CH4 with N2O, particularly at higher CH4 conversions. For this purpose, key process variables, such as temperature (300°C–550°C) and a molar feed ratio (N2O/CH4= 1, 3, and 5), were altered to establish the conditions for maximized H2 yield. The experimental study was conducted over the Co-ZSM-5 catalyst in a fixed bed tubular reactor and then compared with the thermodynamic equilibrium compositions, where the equilibrium composition was calculated via total Gibbs free energy minimization method.The results suggest that molar feed ratio plays an important role in the overall reaction products distribution. Generally for N2O conversions, and irrespective of N2O/CH4 feed ratio, the thermodynamic predictions coincide with experimental data obtained at approximately 475°C–550°C, indicating that the reactions are kinetically limited at lower range of temperatures. For example, theoretical calculations show that the H2 yield is zero in presence of excess N2O (N2O/CH4= 5). However over a Co-ZSM-5 catalyst, and with a same molar feed ratio (N2O/CH4) of 5, the H2 yield is initially 10% at 425°C, while above 450°C it drops to zero. Furthermore, H2 yield steadily increases with temperature and with the level of CH4 conversion for reactions limited by N2O concentration in a reactant feed. The maximum attainable (from thermodynamic calculations and at a feed ratio of N2O/CH4= 3) H2 yield at 550°C is 38%, whereas at same temperature and over Co-ZSM-5, the experimentally observed yield is about 19%.Carbon deposition on Co-ZSM-5 at lower temperatures and CH4 conversion (less than 50%) was also observed. At higher temperatures and levels of CH4 conversion (above 90%), the deposited carbon is suggested to react with N2O to form CO2.

Thermodynamic equilibrium prediction of bed agglomeration tendency in dual fluidized-bed gasification of forest residues

Dual fluidized-bed (DFB) gasification is one of the recently developed technologies for production of heat, power, transportation fuels and synthetic chemicals through steam gasification of biomass. Bed agglomeration is a serious ash-related problem that should be taken into account when biomass-based fuels are selected for fluidized-bed gasification and combustion. This study developed a thermodynamic equilibrium model to assess the risk of bed agglomeration in gasification and combustion reactors of a DFB gasifier using biomass (forest residues) as feedstock. The modelling approach combined thermodynamic equilibrium calculations with chemical fractionation technique to predict the composition and melting behaviour of the fuel-derived ash as well as bed particles coating layer in the gasification and combustion reactors. FactSage was employed for the thermodynamic equilibrium calculations. The modelling results were then compared with experimental data obtained from a full-scale DFB gasifier to estimate the reliability and validity of the predictive model. In general, a good agreement was found between the modelling results and experimental observations. For the forest residues as feedstock and olivine as bed material, the modelling results indicate a low risk of bed agglomeration in the DFB gasifier, as long as the dominant temperature in the combustion zone is below 1020°C. In contrast, quartz as bed material in the DFB gasifier was shown to significantly increase the risk of bed agglomeration through coating-induced agglomeration mechanism.

Characterization of an ilmenite ore for pressurized chemical looping combustion

This work presents the characterization results of a Canadian ilmenite ore tested in pressurized chemical looping combustion cycles. The ilmenite ore was isothermally cycled in a pressurized thermogravimetric analyzer using carbon monoxide as a reducing fuel and air as an oxidizing gas. All samples were calcined prior to submission to cycles. X-ray diffraction (XRD) comparing raw and calcined ore samples indicate the disappearance of ilmenite crystals and the formation of rutile and ferric pseudobrookite. Cycled ilmenite ore surface morphology was found to be insensitive to the total pressure (up to 51bar) and CO partial pressure (3.2–8.0bar). These findings are in agreement with thermodynamic equilibrium predictions using FactSage. SEM images reveal the development of cracks in the oxidized particles after 4 redox cycles at 950°C and 16bar, which was correlated to the original lamellar structure of the raw ilmenite ore. Increasing the number of redox cycles from 3.5 to 19.5 resulted in large cracks near iron-rich lamellae. Furthermore, the average grain size was smaller after 19.5 cycles. Increasing the reaction temperature from 850°C to 1050°C resulted in similar surface morphologies, but increased grain size. Despite being below melting temperatures, the sample cycled at 1050°C did agglomerate. Local temperature excursions during sample oxidation may have led to melting. This study provides insights into the phase transformations and morphological changes accompanying the use of this type of ilmenite ore in a pressurized chemical looping combustion process, highlighting the need for proper adjustments to the design and operation of the process.

PRODUCTION OF HYDROGEN FROM THE STEAM AND OXIDATIVE REFORMING OF LPG: THERMODYNAMIC AND EXPERIMENTAL STUDY

The objective of this paper was to use a thermodynamic analysis to find operational conditions that favor the production of hydrogen from steam and oxidative reforming of liquefied petroleum gas (LPG). We also analyzed the performance of a catalyst precursor, LaNiO3, in order to compare the performance of the obtained catalyst with the thermodynamic equilibrium predictions. The results showed that it is possible to produce high concentrations of hydrogen from LPG reforming. The gradual increase of temperature and the use of high water concentrations decrease the production of coke and increase the formation of H2. The reaction of oxidative reforming of LPG was more suitable for the production of hydrogen and lower coke formation. Furthermore the use of an excess of water (H2O/LPG =7.0) and intermediate temperatures (973 K) are the most suitable conditions for the process.

Indirect Carbonation of Victorian Brown Coal Fly Ash for CO2 Sequestration: Multiple-Cycle Leaching-Carbonation and Magnesium Leaching Kinetic Modeling

In this paper, a closed-loop multistep process which allows leaching and precipitation of magnesium and calcium as carbonate from Victorian brown coal fly ash has been examined. Victorian brown coal fly ash has a distinctively high concentration of alkaline earth metals and low amounts of aluminum and silica. The main objective here is to clarify the dissolution kinetics of magnesium and calcium in regenerative ammonium chloride and subsequent carbonation of dissolved cations. Instead of a once-through test with fresh ammonium chloride, multiple locked circuits were adopted to assess the leaching capability of regenerated ammonium salt, as well as the accumulation of impurities upon the recycling and reuse of the leaching agent. As has been revealed, upon increasing cycles of ammonium chloride use, the extraction yields of both target cations decreased significantly. Their extraction by ammonium chloride was favored by the presence of the oxide form in the original ash sample, with the extraction of calcium occurring much faster than that of magnesium. Both phenomena were in agreement with the thermodynamic equilibrium prediction on the lowest Gibbs function for the dissolution of oxides, especially calcium oxide in ammonium chloride solution. Carbonation results dropped gradually upon the increase in the cycle number; meanwhile, the size and morphology of precipitates were changed from the first to last cycle. By fitting the observed results with a shrinking core model, it was shown that the extraction of Mg2+ followed a pseudo-second-order reaction with a nonconstant ammonium chloride concentration in the film layer on the surface of a solid particle. The activation energy of 20.7 kJ mol–1 was obtained for the dissolution of magnesium from both Hazelwood fly ash and pure MgO in ammonium chloride solution.

Comparison of Thermodynamic Equilibrium Predictions on Trace Element Speciation in Oxy-Fuel and Conventional Coal Combustion Power Plants

The importance of coal as the main energy source has posed the challenge to use this fuel in a sustainable way while new clean energy technologies are developed. Carbon capture and storage is one of the main short-term solutions to the problem of CO2 emissions and involves a wide range of technologies. Oxy-fuel combustion is one of them, and even though it has emerged as an attractive alternative, a better understanding of the performance of the process is still required. The current work was aimed at enhancing the understanding of the different thermodynamic equilibrium behaviors of metallic trace elements in coal combustion under air and oxy-fuel environments. The effect of coal composition on the speciation of these elements was analyzed, and a sensitivity analysis of the effect of various concentrations of Cl, S and the mineral content of coal was performed. It was found that the increase in the concentration of CO2 and H2O did not have a significant effect on the thermodynamically stable forms of trace element compounds that were predicted to form. The recycling of a larger amount of trace elements did not affect the speciation either but only increased the concentration of trace elements inside the boiler. The speciation and volatility of the species predicted by thermodynamic modeling in the majority of the trace elements considered in this study was found to be sensitive mainly to changes in the concentration of Cl, Ca, S, and Si. In terms of species that may enhance corrosion at tube wall temperatures, the thermodynamic calculations predicted the condensation of the majority of trace elements as sulfates as well as the formation of V2O5.

Co-processing methane in high temperature steam gasification of biomass

High temperature steam gasification/reforming of biomass-methane mixtures was carried out in an indirectly heated entrained flow reactor to analyze the feasibility of controlling the output composition of the major synthesis gas products: H2, CO, CO2, CH4. A 23 factorial experimental design was carried out and compared to thermodynamic equilibrium predictions. Experiments demonstrated the product gas composition is mostly dependent on temperature and that excess steam contributes to CO2 formation. Results showed that with two carbon-containing reactants it is possible to control the gas composition of the major products. At 1500°C, the equilibrium results accurately predicted the syngas composition and can be used to guide optimization of the syngas for downstream liquid fuel synthesis technologies.

Phase Behavior of Sick House Syndrome(SHS) Chemicals

The vapor/liquid phase equilibrium data of sick house syndrome (SHS) materials were measured experimentally. SHS materials are chemical species to which the human body is exposed, such as the volatile organic chemicals emitted from constructing finishes. In this study, acetone(1) + toluene(2) and benzene(1) + m-xylene(2), as two sets of mixed SHS, were selected in the experimental system. These SHS are used as solvents for various chemical processes and finishing materials for residential dwellings, as well as the components contained in interior construction materials. It is very important to identify their vapor phase concentration and diffusion characteristics under atmospheric pressure. The vapor phase equilibrium concentration of the mixed SHS was measured experimentally, the data were correlated with their phase behavior characteristics, and then their nonideal behaviors were predicted by means of calculating their activity coefficients. The NRTL model and Wilson equation were employed as the thermodynamic equilibrium prediction model, in which the parameters of the experimental system were optimized in the course of calculation. The vapor phase concentration for each pair of mixed SHS was proposed with the given temperature range.

Solar Gasification of Biomass: Kinetics of Pyrolysis and Steam Gasification in Molten Salt

The use of concentrated solar energy as a heat source for pyrolysis and gasification of biomass is an efficient means for production of hydrogen rich synthesis gas. Utilizing molten alkali carbonate salts as a reaction and heat transfer media promises enhanced stability to solar transients and faster reaction rates. The present study establishes and compares the reaction kinetics of pyrolysis and gasification of cellulose from 1124 K to 1235 K in an electric furnace. Data are presented in an inert environment and in a bath of a ternary eutectic blend of lithium, potassium, and sodium carbonate salts. Arrhenius rate expressions are derived from the data supported by a numerical model of heat and mass transfer. The molten salt increases the rate of pyrolysis by 74% and increases gasification rates by more than an order of magnitude while promoting a product gas composition nearer to thermodynamic equilibrium predictions. These results justify using molten carbonate salts as a combined catalyst and heat transfer media for solar gasification.

Biomass steam gasification in fluidized bed of inert or catalytic particles: Comparison between experimental results and thermodynamic equilibrium predictions

In order to improve the understanding of biomass gasification in a bed fluidized by steam, the thermochemical equilibrium of the reactive system was studied. The equilibrium results were compared to LGC experimental results, obtained by the gasification of oak and fir in a laboratory-scale fluidized bed of different catalysts: sand, alumina, and alumina impregnated with nickel. The research was completed by a study of the influence on the equilibrium of additional parameters such as the quantity of steam, the pressure or the kind of biomass. Those results of simulation may be used for evaluating the limits of actual reactors. The following conclusion may be drawn from all the results: The thermodynamic equilibrium state calculated is far away from the experimental results obtained on sand particles. The steam to biomass ratio, between 0.4 and 1 kg steam/kg dry biomass, has a strong influence on the gas mixture composition. The temperature increase and the use of catalyst allow producing a gas mixture with a high content of hydrogen and carbon monoxide. The H 2:CO ratio may reach values greater than 3. The use of catalyst allows the system to get closer from the equilibrium, especially for the nickel based catalyst.