Synthetic Coal Research Articles

The effect of phosphine in syngas on Ni–YSZ anode-supported solid oxide fuel cells

Ni–YSZ cermet is commonly used as the anode of a solid oxide fuel cell (SOFC) because it has excellent electrochemical performance, not only in hydrogen fuel, but also in a clean blended synthetic coal syngas mixture (30% H 2 , 26% H 2 O, 23% CO, and 21% CO 2 ). However, trace impurities, such as phosphine (PH 3 ), in coal-derived syngas can cause degradation in cell performance [J.P. Trembly, R.S. Gemmen, D.J. Bayless, J. Power Sources 163 (2007) 986–996]. A commercial solid oxide fuel cell was exposed to a syngas with 10 ppm PH 3 under a constant current load at 800 °C and its performance was evaluated periodically using electrochemical methods. The central part of the anode was exposed directly to the syngas without an intervening current collector. Post-mortem analyses of the SOFC anode were performed using Raman spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The results show that the impurity PH 3 caused a significant loss of the Ni–YSZ anode electrochemical performance and an irreversible Ni–YSZ structural modification. Ni 5 P 2 was confirmed to be produced on the cell surface as the dominant nickel phosphorus phase.

In situ Van der Pauw measurements of the Ni/YSZ anode during exposure to syngas with phosphine contaminant

Abstract Solid oxide fuel cells (SOFCs) represent an option to provide a bridging technology for energy conversion (coal syngas) as well as a long-term technology (hydrogen from biomass). Whether the fuel is coal syngas or hydrogen from biomass, the effect of impurities on the performance of the anode is a vital question. The anode resistivity during SOFC operation with phosphine-contaminated syngas was studied using the in situ Van der Pauw method. Commercial anode-supported solid oxide fuel cells (Ni/YSZ composite anodes, YSZ electrolytes) were exposed to a synthetic coal syngas mixture (H 2 , H 2 O, CO, and CO 2 ) at a constant current and their performance evaluated periodically with electrochemical methods (cyclic voltammetry, impedance spectroscopy, and polarization curves). In one test, after 170 h of phosphine exposure, a significant degradation of cell performance (loss of cell voltage, increase of series resistance and increase of polarization resistance) was evident. The rate of voltage loss was 1.4 mV h −1 . The resistivity measurements on Ni/YSZ anode by the in situ Van der Pauw method showed that there were no significant changes in anode resistivity both under clean syngas and syngas with 10 ppm PH 3 . XRD analysis suggested that Ni 5 P 2 and P 2 O 5 are two compounds accumulated on the anode. XPS studies provided support for the presence of two phosphorus phases with different oxidation states on the external anode surface. Phosphorus, in a positive oxidation state, was observed in the anode active layer. Based on these observations, the effect of 10 ppm phosphine impurity (or its reaction products with coal syngas) is assigned to the loss of performance of the Ni/YSZ active layer next to the electrolyte, and not to any changes in the thick Ni/YSZ support layer.

Coal fly ash and synthetic coal fly ash aggregates as reactive media to remove zinc from aqueous solutions

Coal fly ash (CF) and synthetic coal fly ash aggregates (SCFAs) were evaluated as low-cost reactive media for the remediation of groundwater contaminated with Zn. The SCFAs were prepared by mixing CF, sodium silicate, and deionized (DI) water. Serial batch kinetic and static tests were conducted on both CF and SCFAs, under various conditions (i.e., pH, initial Zn concentration, reaction time, and solid dosage), using Zn(NO 3) 2·6H 2O solutions. Serial column tests were also conducted on both CF and SCFAs. The final rather than the initial pH of the solution had a greater effect on the removal of Zn. At pH > 7.0, the removal of Zn was due to precipitation, whereas at <7.0, the removal of Zn was due to adsorption onto the reactive media. The removal of Zn increased with increasing dosage of the reactive medium and decreasing initial Zn concentration. The results of the column and batch tests were comparable. Preferential flow paths were observed with CF, but not SCFA. The hydraulic conductivity of CF was more significantly decreased than that of SCFA with increasing dry density of the specimen.

Fireside and steamside corrosion of alloys for USC plants

A program on fireside corrosion is being conducted at Argonne National Laboratory to evaluate the performance of several structural alloys in the presence of mixtures of synthetic coal ash, alkali sulfates, and alkali chlorides. Experiments in the present program, which address the effects of deposit chemistry, temperature, and alloy chemistry on the corrosion response of alloys, were conducted at temperatures in the range of 575 – 800 ∘ C for time periods up to ≈ 1850 h . Alloys selected for the study included both ferritic and austenitic steels and Ni-base alloys. Data were obtained on weight change, scale thickness, internal penetration, microstructural characteristics of corrosion products, mechanical integrity, and cracking of scales. Results showed that the relationship of corrosion rates to temperature followed a bell-shaped curve, with peak rates at ≈ 725 ∘ C , but the rate itself was dependent on the alloy chemistry. Several Fe-base alloys showed acceptable rates in the sulfate-containing coal-ash environment; but NaCl in the deposit led to catastrophic corrosion at 650 and 800 ∘ C . Ni-base alloys generally exhibited less corrosion than the Fe-base alloys under similar exposure conditions; however, they were susceptible to localized corrosion in the form of pits. Experiments were also conducted to evaluate the corrosion performance of several Fe-base alloys in a steam environment. Results, presented on both the fireside and steamside corrosion, can be used to compare the performance of the various Fe-base alloys. Attaining adequate creep strength, fireside corrosion resistance, and steamside corrosion resistance is still a challenge in materials development for advanced steam-cycle applications.

Synthetic coal chars for the elucidation of NO heterogeneous reduction mechanisms

In the present work, the mechanisms involved in NO–char heterogeneous reduction have been studied using a synthetic coal char (SC char) as carbon source. Another synthetic char (SN char) without nitrogen in its composition has also been employed in these studies. Isothermal reduction tests at different temperatures have been carried out. Two temperature regimes were considered: low temperature ( T < 250 °C) where NO chemisorption takes place and high temperature ( T > 250 °C) where NO–C reaction occurs. Step response experiments combining consecutive reaction stages with NO and 15NO were performed in order to determine the role of nitrogen surface complexes, C(N), in the reduction process. The results revealed N 2 and CO 2 to be the main reduction products under the experimental conditions employed in this work. NO chemisorption at lower temperatures results in N 2 emission and surface complexes (mainly oxygenated) formation, while char gasification by NO involves a direct NO attack on the char surface to form surface complexes. As a consequence of desorption of these complexes new sites of reaction are created.

06/01092Heterogeneous reduction of nitric oxide on synthetic coal chars: Pevida, C. et al. Fuel, 2005, 84, (17), 2275–2279

06/01092Heterogeneous reduction of nitric oxide on synthetic coal chars: Pevida, C. et al. Fuel, 2005, 84, (17), 2275–2279

Heterogeneous reduction of nitric oxide on synthetic coal chars

Model compounds, with a controlled heteroatoms content and well-defined functionalities, were used to study the release of nitrogen compounds from char combustion. In the present work, the mechanisms involved in NO-char heterogeneous reduction were studied with a synthetic coal (SC) char as carbon source. Another synthetic char (SN) without any nitrogen in its composition was also employed in these studies. Temperature programmed reduction (TPR) tests with a gas mixture of 400 ppm NO in argon and with isotopically labelled nitric oxide, 15NO (500 ppm 15NO in argon), were carried out. The gases produced were quantitatively determined by means of MS and FTIR analysers. Under the conditions of this work the main products of the NO-C reaction were found to be N 2 and CO 2. The main path of reaction involves the formation of surface nitrogen compounds that afterwards react with nitrogen from the reactive gas to form N 2. It was observed that fuel-N also participates in the overall heterogeneous reduction reaction, although to a lesser extent.

Modeling Long-Term Transport of Contaminants Resulting from Dissolution of a Coal Tar Pool in Saturated Porous Media

In this study, a three-dimensional mathematical model for simulating the transport of dissolved contaminants originating from an elliptic-shaped coal tar pool in homogeneous saturated porous media is developed. The methodology of the solution is based on a semianalytical approach that assumes the contaminant source input can be represented by the superposition of a series of consecutive short dissolution pulses. The model accounts for possible solidification of polycyclic aromatic hydrocarbon compounds as the coal tar pool dissolves. A circular-shaped synthetic coal tar pool consisting of 22 components is used for model simulation. Simulations show that even after 130 years of dissolution, the coal tar pool remains a potential groundwater pollution threat despite a significant reduction in overall pool volume.

04/02222 Characterisation of model compounds and a synthetic coal by TG/MS/FTIR to represent the pyrolysis behaviour of coal: Arenillas, A. et al. Journal of Analytical and Applied Pyrolysis, 2004, 71,(2), 747–763

04/02222 Characterisation of model compounds and a synthetic coal by TG/MS/FTIR to represent the pyrolysis behaviour of coal: Arenillas, A. et al. Journal of Analytical and Applied Pyrolysis, 2004, 71,(2), 747–763

Difference in the Nonlinearity of Absorption in Natural and Synthetic Coals

It is established that on an increase in the SHF radiation power for powder anthracite, a more rapid increase in the amplitude of the EPR line is observed in comparison to the case of linear absorption, whereas for carbon obtained by deposition of the products of pyrolysis of deuterobenzene (C6D6) on the surface of γ-Al2O3 there occurs saturation of absorption. At an air pressure of ∼10−3 mm Hg, saturation of resonance absorption is observed in both samples. The linear effects observed can be understood within the framework of the formalism of a three-level scheme possible in the case of the triplet ground state of the paramagnetic centers of powder anthracite.

Surface characterisation of synthetic coal chars made from model compounds

Knowledge of surface properties is essential for understanding the reaction mechanisms involved in several coal conversion processes. However, due to the complexity and heterogeneity of coal this is rather difficult and the use of known model compounds could be a valuable tool. Single model compounds have been widely used, but they give a quite simplified picture. In this work a mixture of model compounds in a phenol-formaldehyde matrix was cured in order to create cross-linked structures. The obtained synthetic coal was pyrolysed in a fixed bed reactor, under helium atmosphere. The surface composition of the chars was evaluated by XPS, adsorption gravimetry of water vapour, temperature-programmed desorption and potentiometric titration. Texture was characterised by N 2 and CO 2 adsorption isotherms at 77 and 273 K, respectively, and immersion calorimetry in benzene. The results obtained from the different techniques were contrasted in order to give an overview of the surface properties (chemical and physical) of the samples studied. Chars obtained under the same operating conditions from a high volatile bituminous coal were used as a reference.

Surface characterisation of synthetic coal chars made from model compounds

Knowledge of surface properties is essential for understanding the reaction mechanisms involved in several coal conversion processes. However, due to the complexity and heterogeneity of coal this is rather difficult and the use of known model compounds could be a valuable tool. Single model compounds have been widely used, but they give a quite simplified picture. In this work a mixture of model compounds in a phenol-formaldehyde matrix was cured in order to create cross-linked structures. The obtained synthetic coal was pyrolysed in a fixed bed reactor, under helium atmosphere. The surface composition of the chars was evaluated by XPS, adsorption gravimetry of water vapour, temperature-programmed desorption and potentiometric titration. Texture was characterised by N2 and CO2 adsorption isotherms at 77 and 273 K, respectively, and immersion calorimetry in benzene. The results obtained from the different techniques were contrasted in order to give an overview of the surface properties (chemical and physical) of the samples studied. Chars obtained under the same operating conditions from a high volatile bituminous coal were used as a reference.

Characterisation of model compounds and a synthetic coal by TG/MS/FTIR to represent the pyrolysis behaviour of coal

Coal pyrolysis is the initial, accompanying reaction of a number of coal conversion processes such as hydrogenation, combustion and gasification. However, because of the inherent complexity of coal composition, it is difficult to describe coal pyrolysis clearly. Single model compounds have been used before in order to provide additional insight into the complex processes that occur in the pyrolysis of coal. Yet the picture obtained is a simplified one and certain important aspects such as coal structure, interactions between different surface groups and cross-links are omitted. The approach used in this work involves the preparation of a synthetic coal, SC, with a known structure by curing a mixture of single, well-defined model compounds. By means of chemical characterisation, the SC was shown to contain the macroscopic features of a high volatile coal (proximate and ultimate analyses). FTIR characterisation revealed the presence of functional groups similar to those of coal in the structure of the SC. Temperature-programmed pyrolysis tests were performed in a thermobalance linked to a mass spectrometer and a Fourier transform infrared analyser (TG/MS/FTIR ). The thermal behaviour of the synthetic coal (i.e., rate of mass loss and the evolution profiles of gaseous compounds during pyrolysis tests) is very similar to that of the high volatile bituminous coal which was used as a reference material. The great advantage of using SC lies in the fact that its composition and structure can be accurately determined and employed in subsequent applications in basic and mechanistic studies.

Potentiometric Study of Oxidized Coals in Aqueous Solutions of Surface-Inactive Electrolytes

Stationary electrode potentials of oxidized synthetic coals of the SCN type are determined in aqueous solutions. The contributions of redox reactions with participation of the surface oxygen-containing functional groups and the potential of unoxidized areas of the coal surface to the total potential of oxidized coals in aerated solutions are studied. The potentials of oxidized carbons in deoxygenated solutions are also measured.

Dissolution of lime into synthetic coal ash slags

One of the alternate processes presently being investigated to produce electrical power from coal is Integrated Gasification Combined Cycle (IGCC). The ash, which remains when the coal is gasified in this process, is removed by tapping the molten ash at 1400–1500°C. To ensure that the coal ash is molten at 1400–1500°C, the melting temperature of the coal ash may need to be reduced by addition of a flux, usually limestone, which is added with the coal to the gasifier. The rate of dissolution of the flux is uncertain. This paper reports the investigation of the rate of lime dissolution into synthetic coal ashes, consisting of SiO 2, Al 2O 3 and CaO. Results previously reported have shown that the free dissolution of fine particles (50–200 μm) is mass-transfer controlled [S.M. Wang, T.F. Wall, J.A. Lucas, L. Elliott, A.C. Beath, Experimental studies and computer simulation of dissolution of lime particles into coal ash slags. Australian Symposium on Combustion and the Fourth Flame Days, Univ. of Adelaide, South Australia, November 9–10, 1995]. To investigate forced dissolution, a high-temperature viscometer was used to rotate a cylinder of lime in the molten slag for a given period. At temperatures between 1450°C and 1650°C, reaction products of 3CaO·SiO 2/3CaO·Al 2O 3, 2CaO·SiO 2/3CaO·Al 2O 3 form around the lime cylinder. The concentration gradient involved in the mass transfer and the diffusion coefficient are currently being investigated.

96/06426 Dissolution of lime into synthetic coal ash slags

As part of the European Brite Euram Low Emisson Combustor Technology programme, a two-head combustor sector, designed by SNECMA and ONERA, that includes an LPP main zone and an RQL pilot zone within the same volume, was tested at ONERA. A significant 50% reduction in NOx emissions was recorded for a pressure, air inlet temperature and fuel air ratio of 7 bar, 750 K, and 2.4%, respectively. The optimisation of each combustor component allowed to understand the phenomena and to identify solutions to the main difficulties associated with this new concept.Dans le cadre du programme européen Brite Euram sur les technologies de chambre de combustion à basses émissions polluantes, un secteur de chambre à deux têtes qui inclut les concepts LPP (prévaporisation à prémélange pauvre) et RQL (combustion riche, figée puis pauvre) dans un même volume, a été défini conjointement entre SNECMA et ONERA et essayé à l'ONERA. Une réduction significative des émissions de NOx de 50 % a été enregistrée aux conditions de pression, temperature d'air et rapport de mélange respectivement de 7 bar, 750 K, 2.4 %. L'optimisation de chaque composant de cette chambre de combustion a permis de comprendre les phénomènes de combustion liés à chaque concept et d'identifier des solutions aux principales difficultés rencontrées.

Temperature dependence of the cation distribution in CoAl2O4 spinel

The cation distribution in synthetic CoAl 2 O 4 spinel has been determined in samples quenched from 750 to 1200°C, by Rietveld structure refinement of powder X-ray diffraction data. The inversion parameter, x, increases smoothly from 0.11 at 750°C to 0.25 (± 0.005) at 1200°C, and can be described by a conventional thermodynamic model (O'Neill & Navrotsky, 1983, 1984) with α Co-Al = 45.3 ± 0.5 and β = -18.3 ± 1.2 kJ/mol. The β term agrees well with the -20 kJ/mol suggested for all 2-3 spinels by O'Neill & Navrotsky (1984), and CoAl 2 O 4 thus behaves similarly to the majority of other oxide spinels which show temperature-dependent cation distributions

Characterization of Synthetic‐Coal Char Particles using fractal dimension analysis

AbstractThe development and change of surface ruggedness in chars was studied at conditions typical in a pulverized coal furnace. The fractal dimension, a measure of surface ruggedness, of chars was measured using physisorption techniques. By adjusting the temperature encountered (1173 to 1773 K) and residence time (0.1 to 1.5 s) of the synthetic coal (sized to 46–106 μm diameter), chars at different stages of combustion were prepared in a laminar flow (drop‐tube) furnace. The particles were quickly cooled and quenched in an inert atmosphere. The samples were examined using a scanning electron microprobe, and their fractal dimensions were determined using gas physisorption. The adsorption data were used to test if the char surface was fractal on a molecular scale, to determine the fractal dimension, and to quantify changes in the fractal dimension during combustion. The fractal dimension of the unburned synthetic coal was approximately 2. The fractal dimension increased as high as 2.85 as the carbon matrix burned away and exposed mineral moieties. However, as combustion continued the carbon burned completely away leaving a mineral fly ash particle with a fractal dimension as low as 2.47.

Ash particle size and composition evolution during combustion of synthetic coal and inorganic mixtures

To develop methods to predict and mitigate ash deposition problems, a firm understanding of the fundamental mechanisms of ash formation and deposition must be obtained. The first step to understanding the mechanisms of ash deposition is to determine how the initial inorganic constituents in the coal are transformed to produce intermediate species. To avoid some of the complexities involved in coal minerals, simplified, precisely formulated model mixtures consisting of a synthetic coal organic matrix and minerals and inorganic species were prepared, t ~ Mineral inclusions in the synthetic coal were limited in order to isolate specific chemical and physical transformations during combustion under controlled conditions in a laminar flow furnace. Char and ash products were analyzed to determine their chemical and physical characteristics. Although these model mixtures were not exhaustive representations of real coal, they provided a system which could be used to determine the effect of gas temperature and coal inorganic composition on the particle size and composition of specific ash types. Two calcium, silica, and sulfur synthetic coal systems were prepared: one system having calcium in the mineral form as 10-/zm calcite (Ca [min.]-Si-S), and the other having calcium associated in a simulated organic association as ironically dispersed calcium acetate (Ca[org.]-Si-S). A third system consisted of sodium, silica, and sulfur, with the sodium being associated as sodium benzoate simulating an organic association (Na[org.]-Si-S). Silica, in all three systems, consisted of 10-/.Lm quartz that was associated within the synthetic coal particles. Sulfur was extraneously added to the model mixtures in an elemental form and was also included in the organic polymer in very small amounts. The calcite particles were added to the organic matrix during polymerization to allow for their incorporation into the synthetic coal particles. The mineral grains associated with coal particles can be described as being included. The organic matrix consisted of a furfuryl alcohol polymer. The purpose of the Ca -S i -S and Na-S i -S systems was to demonstrate the differences in interactions between included quartz and 1) Ca that is mineral bound as included calcite, 2) Ca that is organically bound or dispersed on an ionic level within the organic matrix, and 3) Na that is organically bound. Reactions involving Ca, Si, and S during combustion are of particular interest when burning low-sulfur and calcium-rich western U.S. subbituminous coals, since calcium sulfate and calcium silicate phases play important roles in the formation of convective pass ash deposits. The effect of temperature on the particle size and composition distribution of ash produced from included calcite and quartz and organically associated calcium and included quartz are of specific interest, as are the reactions of sulfur oxides with the available calcium oxide. The Na (org.)-Si-S system adds another dimension to the role of organically associated elements in the formation of ash, especially with regard to the formation of low melting point Na sulfates and Na silicates on the surface of quartz particles. Quartz was chosen instead of clay (aluminosilicate) as the silicate material to test reactions with the calcium and sodium because lower viscosity and lower melting point liquid phases will form alkali// alkaline earth elements reacting with silica. Lower viscosity liquid phases usually lead to more severe fouling deposits.

Fly ash development from sodium, sulphur and silica during coal combustion

A synthetic coal was made from a furfuryl alcohol polymer with appropriate amounts of sodium, sulphur and silica to duplicate the characteristics of a low rank coal. The synthetic coal was burned in a laminar flow (drop-tube) furnace at 900, 1100, 1300 and 1500 °C. The resulting fly ash particles were quickly quenched, collected and analysed using a scanning electron microscope to determine size and composition. Below 1100 °C, fly ash formation was dominated by coalescence of the mineral matter in the burning particle to form close to one fly ash particle per original coal particle. At higher temperatures, fly ash formation was dominated by fragmentation of the coal/char particle followed by coalescence within the resulting fragments. At the lower temperatures, submicrometre sodium sulphate particles were found adhered to the larger sodium-rich silicate particles. At higher temperatures, the abundance and size of the sodium sulphate particles decreased.