Thermoplastic properties of coal at elevated pressures: 2. Low-temperature preoxidation of a Pittsburgh Seam coal

The thermoplastic properties of a preoxidized Pittsburgh Seam HVAb coal (air-treated at 383 K to different levels of weight gain) were examined using a high-pressure microdilatometer over a range (0.1–2.9 MPa) of pressures. The results suggest that the thermoplastic properties of the coal, depending on the extent of preoxidation, can be significantly different at elevated pressures from those at atmospheric pressure. The maximum swelling parameters of this preoxidized coal are always lower than those of untreated coal at pressures < 1.0 MPa. However, > 2.0 MPa, the maximum swelling parameters of the mildly oxidized samples are, in general, greater than those of untreated coal.     

Swelling and plastic properties of coal devolatilized at elevated pressures of H2 and He: Influence of potassium and silicon additives

The influences of SiO₂ and potassium additives on the swelling and plastic properties of a low volatile bituminous coal have been characterized at elevated pressures of H₂ and He using a high-pressure microdilatometer. The results suggest that non-porous SiO₂ serves as a ‘diluent’ as it has only a minor influence on the nature of the thermoplastic properties of coal. In marked contrast, K₂CO₃ or KOH significantly reduced the swelling of coal at low pressures (< 1.0 MPa). The effectiveness of K₂CO₃ and KOH as de-caking additives increased with the increase in loading of the additives. In addition, the effectiveness of potassium additives depended on the type of anions present. While K₂CO₃ or KOH served as strong decaking additives, KCl showed little effect. At elevated pressures of H₂ or He (>2.0 MPa), the swelling behaviour of K₂CO₃ or KOH loaded coal was reduced only slightly compared with the behaviour without additives. The influence of potassium additives is a function of coal particle size, heating rate and mode of addition (dry-mixed or solution impregnation). The presence of K₂CO₃ or KOH resulted in increased coke yield (i.e., reduced weight loss of coal during pyrolysis). This increase in coke yield was accompanied by a slight reduction in total light gases monitored (primarily C₂C₄). It is suggested that char-forming crosslinking reactions that can be catalysed by K₂CO₃/KOH facilitate increased thermosetting solid yield. At elevated pressures of H₂, the maximum swelling parameter in the presence of these potassium compounds was further increased compared with that noted in He. This behaviour is explained by suggesting that a hydrogen atmosphere reduces the extent of cross-linking reactions.     

Swelling and plastic behavior of coal devolatilized at elevated pressures of air in the absence and presence of calcium oxide

Swelling and plastic properties of an lv bituminous coal (PSOC 1197) devolatilized at elevated air pressures (at 60 or 150 K/min) were monitored using a high-pressure microdilatometer. It was observed that the maximum swelling parameter (V₈) of the coal was markedly reduced when devolatilized at elevated air pressures, provided that the heating rate of the coal was sufficiently low (60 K/min to 923 K). In marked contrast, at a relatively higher heating rate (e.g., 150 K/min), the swelling of coal at elevated air pressures was closer to V_s obtained in N₂ pressures. An implication of this finding is that coal swelling can be very high at actual utilization conditions (rapid heating of coal in air). The reduction in coal V_s at a relatively slow heating rate is attributable, at least in part, to the preoxidation (chemisorption of oxygen to form oxygen crosslinks) of coal during initial heat-treatment in air (before there is significant devolatilization). Devolatilization of coal in the presence of CaO, however, markedly reduced the swelling of coal at various air pressures and heating rates. This reduction in coal swelling is attributed to crosslinking reactions of pyrolyzing coal that have been catalyzed by CaO.     

Influence of coal preoxidation and the relation between char structure and gasification potential

A study was carried out to ascertain the effects of coal preoxidation and carbonization conditions on the structure and relative gasification potential of a series of bituminous coal chars. Chars were prepared from two freshly mined bituminous coals and preoxidized samples derived from them. Carbonization conditions included a wide range of heating rate (0.2–10000K s^?1), temperature (1073–1273 K) and time (0.25–3600 s). Char properties were characterized in terms of analysis of char morphology, surface area, elemental composition, and gasification reactivity in air. Over the range of conditions used, preoxidation substantially reduced coal fluid behaviour and influenced macroscopic char properties (char morphology). Following slow heating (0.2 K s^?1), preoxidized coals yielded chars having higher total surface areas and higher reactivities toward gasification in air than did similar chars prepared from fresh coal. Following rapid heating (10000 K s^?1) and short residence times (0.25 s), chars prepared from preoxidized and fresh coals exhibited similar microstructural and chemical properties (surface area, $\text{C}\text{H}$ ratios, gasification rates). Carbonization time and temperature were found to be the critical parameters influencing char structure and gasification potential.     

Low-temperature air oxidation of caking coals. 1. Effect on subsequent reactivity of chars produced

The effect of preoxidation of two highly caking coals in the temperature range 120–250 °C on weight loss during pyrolysis in a N₂ atmosphere up to 1000 °C and reactivity of the resultant chars in 0.1 MPa air at 470 °C has been investigated. Preoxidation markedly enhances char reactivity (by a factor of up to 40); the effect on char reactivity is more pronounced for lower levels of preoxidation. For a given level of preoxidation, the oxidation temperature and the presence of water vapour in the air used during preoxidation have essentially no effect on weight loss during pyrolysis and char reactivity. An increase in particle size of the caking coals reduces the rate of preoxidation as well as subsequent char reactivity. Preoxidation of caking coals sharply increases the surface area of the chars produced. Compared to heat treatment in a N₂ atmosphere, pyrolysis in H₂ of either the as-received or preoxidized coal results in a further increase in weight loss and a decrease in subsequent char reactivity.     

Thermoplastic properties of coal at elevated pressures: 1. Evaluation of a high-pressure microdilatometer

Thermoplastic behaviour of a Pittsburgh seam hvA coal (PSOC1099) was characterized by the use of a high-pressure microdilatometer. Phenomena such as softening, swelling, final resolidification, and the temperatures at which they occur were measured as functions of heating rate (25 ° and 65 °C min^?1), particle size (= 75 μm and 250 × 425 μm), gaseous atmosphere (N₂, H₂, $\text{CO}\text{H}{}_2$) and applied gas pressure (atmospheric to 2.8 M Pa). The results obtained illustrate several important aspects of thermoplastic properties of this coal under the conditions utilized. It is observed that pressure alone can play a major role in determining its overall thermoplastic behaviour. Compared to that at atmospheric pressure, swelling is significantly reduced at 2.8 MPa of pressure for any given heating rate or particle size. In these experiments, the chemical composition of the gaseous atmospheres ($\text{CO}\text{H}{}_2$, H₂ and N₂) does not appear to alter significantly the plastic phenomena at any given pressure. Increasing the heating rate or decreasing the particle size results in increased swelling at all applied pressures and atmospheres.     

Effect of pre-oxidation on reactivity of chars in steam

The effects of pre-oxidation of char from Taiheiyo coal, a non-caking bituminous coal, in the 400–550 °C temperature range on its gasification reactivity with N₂-H₂O at 0.1 MPa (steam partial pressure of 13.2 kPa) have been investigated. The pre-oxidation of char markedly enhances gasification rates at temperatures between 800 and 900 °C. Reactivity is found to parallel the burn-off level during preoxidation at low temperatures (400–430 °C), whereas at relatively high temperatures (480–550 °C), the burn-off level only affects the reactivity slightly. The amount of CO and CO₂ evolved from the preoxidized char by heat treatment is proportional to the burn-off level at low temperatures (400–430 °C), being closely related to the enhancement of the gasification reactivity in steam.     

High pressure hydropyrolysis of coals by using a continuous free-fall reactor

Rapid hydropyrolysis of coal was carried out at temperatures ranging from 923 to 1123 K and H₂ pressures up to 7 MPa by using a continuous free-fall pyrolyzer. The effects of the reaction conditions on product yields were investigated. Carbon mass balance was fairly good. It was revealed that a large amount of methane was produced due to the hydrogenolysis of higher hydrocarbons and the hydrogasification of char. The influence of pyrolysis temperature was significant on both reactions while H₂ pressure mainly affected the latter. A considerable amount of reactive carbon was formed during hydropyrolysis of coal. It was converted to methane at high temperatures and high H₂ pressures, while the hydrogasification of reactive carbon takes place relatively slowly at low temperatures and low H₂ pressures, resulting in a low overall carbon conversion. The coal conversions observed in the present study were much higher than those obtained with using reactors where the contact between coal particles and H₂ is insufficient.     

Improving the Cyclic-Oxidation Resistance of Ti3AlC2 at 550°C and 650°C by Preoxidation at 1100°C

Preoxidation of Ti₃AlC₂ at 1100°C for 2 h was conducted to improve its cyclic-oxidation resistance at the testing temperature of 550°C and 650°C in air. The cyclic oxidation of the preoxidized Ti₃AlC₂ was found to follow a parabolic rate law rather than the linear oxidation rate for that without preoxidation. Through the X-ray diffraction and SEM analysis, the remarkable improvement of the cyclic-oxidation resistance of preoxidation Ti₃AlC₂ is suggested due to the existence of protective α-Al₂O₃ layers formed during the preoxidation treatment, which inhibits the formation of amorphous Al₂O₃, which can result in larger thermal stress and stress-induced microcracks.     

Preparation of active carbons from coal: Part III: Activation of char

Active carbons with a burn-off of 52% have been prepared from four coals of different rank and origin after preoxidation to different degrees at 543 and 473 K, and further carbonization at 1123 K. The activation has been carried out with CO₂ at 1123 K at two flow rates viz. 7 cm³ min⁻¹ and 500 cm³ min⁻¹. Active carbons have also been prepared from a preoxidized coal by activation to different degrees of burn-off between 10 and 80%. The effect of the degree of oxidation, the flow rate of the activating gas and the extent of burn-off on the porous structure development of active carbons has been examined. The active carbons prepared from unoxidized coal have poor textural characteristics (porosity, N₂ and CO₂ surface area). Nevertheless, the textural characteristics are enhanced as the degree of preoxidation of the coal is increased. The low flow rate of CO₂ (activating gas) produces active carbons with a better microporous character. The degree of activation (the extent of burn-off) of the carbon determines the porous structure of the active carbon. At low degrees of burn-off (less than 50%) the product is largely microporous. At higher degrees of burn-off between 35–65% the product has a mixed porous structure and contains all types of pores. Active carbons with a given textural character can be obtained by controlling the degree of oxidation of coal and the degree of activation of the carbonized material.     

The thermoplasticity of coal and the effect of K2CO3 addition in relation to the reactivity of the char in gasification

The thermoplastic properties of a medium-volatile and a high-volatile A bituminous coal have been studied by means of high-pressure dilatometry as a function of the heating rate (10 and 65 K min⁻¹), particle size (< 44 μm, < 75 μm, 106–200 μm and 212–400 μm) and gas pressure (1–28 bar). The thermoplastic properties of the coals are significantly different at elevated pressures from those at atmospheric pressure. At atmospheric pressure the volume swelling increases strongly with increasing heating rate and, at 10 K min⁻¹, with increasing particle size. At a pressure of 28 bar however, the swelling is nearly independent of heating rate and particle size. The effect of addition of K₂CO₃ (20% by weight) was investigated at 65 K min⁻¹ and turned out to depend on the gas pressure and particle size. At atmospheric pressure, K₂CO₃ reduces the dilatation of the coals almost completely. This reduction decreases with increasing pressure, especially for the larger particle size fraction (212–400 μm). A detailed mechanism for the interaction of alkali metal carbonates with the coal is suggested. The softening and swelling of coal particles has consequences for the available and accessible surface area of the char formed and thus for the reactivity of the char in gasification. Results of reactivity measurements in a CO₂ atmosphere in a thermobalance that illustrate this effect are presented and related to the morphology of the char.     

Gas evolution kinetics of two coal samples during rapid pyrolysis

Quantitative gas evolution kinetics of coal primary pyrolysis at high heating rates is critical for developing predictive coal pyrolysis models. This study aims to investigate the gaseous species evolution kinetics of a low rank coal and a subbituminous coal during pyrolysis at a heating rate of 1000 °C s^− 1 and pressures up to 50 bar using a wire mesh reactor. The main gaseous species, including H₂, CO, CO₂, and light hydrocarbons CH₄, C₂H₂, C₂H₄, C₂H₆, C₃H₆, C₃H₈, were quantified using high sensitivity gas chromatography. It was found that the yields of gaseous species increased with increasing pyrolysis temperature up to 1100 °C. The low rank coal generated more CO and CO₂ than the subbituminous coal under similar pyrolysis conditions. Pyrolysis of the low rank coal at 50 bar produced more gas than at atmospheric pressure, especially CO₂, indicating that the tar precursor had undergone thermal cracking during pyrolysis at the elevated pressure.     

Structural changes during the thermal stabilization of modified and original polyacrylonitrile precursors

A polyacrylonitrile (PAN) precursor fiber of a special grade for preparing carbon fibers was modified by the impregnation of an aqueous KMnO₄ solution. The effects of the modification on the lateral and morphology structure, related to the crystalline properties of both the precursors and preoxidized fibers, such as the orientation index, crystal size, and crystallinity index, were measured by wide‐angle X‐ray diffraction. For both modified and original PAN fibers, a comparative study of the changes of the elemental content during the process of preoxidation, the relations between the thermal stress and heat‐treatment temperature, and the effect of the modification on the skin/core structure of a preoxidized fiber were also introduced by the use of elemental analysis, optical microscopy, and so on. The modification of KMnO₄ was demonstrated to increase the density, increase the crystallinity index, increase the preferred orientation index, and decrease the crystal size for a modified precursor fiber and for a preoxidized fiber developed from a modified precursor fiber after a different heat‐treatment temperature. KMnO₄ also showed a catalytic action, accelerating the rate of preoxidation and reducing the time of thermal stabilization; this improved the homogenization of the cross‐section structure and led to an improvement in the tensile strength of 15–20% and an improvement in the elongation of 20–30% in the resulting carbon fibers. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 2047–2053, 2005     

Performance of a novel Ni/Nb cathode material for molten carbonate fuel cells (MCFC)

In this work the performance of NiO and a novel cathode material preoxidized nickel–niobium alloy were investigated. It is found that under a cathode atmosphere of p(CO₂)/p(O₂) = 0.67 atm/0.33 atm, the equilibrium solubility of nickel ions in (Li_0.62, K_0.38)₂CO₃ melt at 650 °C is about 17 ppm for the nickel oxide electrode and 8 ppm for the preoxidized nickel–niobium alloy electrode. The improvement in the stability of material in the melt may be attributed to the formation of a more dense nodular structure for the nickel–niobium alloy electrode when compared with a Ni electrode during preoxidation. The formation of a dense nodular structure for the nickel–niobium alloy electrode depresses the dissolution of NiO from the electrode into the carbonate melt and, accordingly, enhances the stability of the electrode material in the melt. The polarization performance of the NiO cathode was improved by electrodeposition of niobium. As far as the thermal stability and the polarization performance are concerned, the preoxidized nickel–niobium alloy can be considered as a candidate for the cathode material of MCFCs.     

Coal reduction studies. 4. Hydrogenation in the presence of AlCl3 and AlCl3 + MClx (M = Cu,Zn, Fe,Cr, Mo and Ni)

Coal has been reduced to low-molecular-weight hydrocarbons in the presence of various metal halides by H₂ at pressures up to 19.3 MPa and temperatures to 600 °C, in all quartz reactors. The mixed halides MoCl₃ + AlCl₃ and NiCl₂ + AlCl₃ were found to be superior catalyst combinations. With the former, initial H₂ pressures of 4.1–6.9 MPa and temperatures of 300–400 °C resulted in 75% coal carbon conversion to low-molecular-weight hydrocarbons. It is tentatively suggested that gaseous mixed metal halide complexes of the type Mo(AlCl₄)_x, and Ni(AlCl₄)₂ may be of importance in the catalytic process. It is possible that such complexes may be able to incorporate H₂ in their structure which could account for their catalytic activity.     

Evaluation of the Fluidized Preoxidation for Producing High Behavior PAN Based Carbon Fiber

Summary In order to resolve the present shortcomings of preoxidation such as preoxidation technology taking too much time, lower production and higher energy consumption for producing high performance carbon fiber, a new type of fluidized preoxidation technology and its effect on the oxidated fibers have been studied in detail. The fluidized fibers have been compared with preoxidized fibers made under conventional technology. It has been found that the technology succeed in the excellence of high efficiency of mass transfer, heat transfer and energy saving. The oxygen content of preoxidized fibers fluidized attained to 11.1%, and the density of 1.42 g/cm³ of preoxidized fibers fluidized can be attained. It was founded that the fluidization-treated PAN fibers not only provide an increase in the tensile strength by about 17%, in the elongation at break by about 13%, and in the carbon yield by about 3%, but also provide a decrease in the tensile modulus by about 7%, and in the fineness by about 21%, when compared with the untreated PAN carbon fibers.     

Swelling and plastic properties of coal devolatilized at elevated pressures: an examination of the influences of coal type

Attempts have been made to correlate the measured thermoplastic properties of 24 coals (hvCb to Ivb) pyrolysed at elevated pressures (0.1 to 2.9 MPa) with some commonly reported characterization parameters. While the thermoplastic transition temperatures can be correlated somewhat with some coal properties (e.g., vitrinite mean-maximum reflectance, or carbon content), the maximum swelling parameter of these coals, especially at elevated pressures, shows little correlation with the usual coal characteristics which are measured at atmospheric pressure (e.g., ASTM FSI or vitrinite reflectance). In general, the behaviour of the coals at elevated pressures cannot be predicted from their properties obtained at atmospheric pressure. These data stress the importance and profound influence of elevated pressure on swelling and plastic properties of coals. All thermplastic properties were measured by a high-pressure microdilatometer.     

In situ Y2Si2O7 coatings on SiC fibers: Thermodynamic analysis and processing

It is shown using thermodynamic analysis and kinetic modeling that a processing window exists for the formation of Y₂Si₂O₇ coatings on SiC. The proposed method is validated using an experimental procedure in which the in situ formation of Y₂Si₂O₇ on a commercial SiC-based fiber is demonstrated. The method involves the deposition of YPO₄ on preoxidized fine diameter SiC-based fibers, and heat treating the coated fibers within a calculated processing window of oxygen partial pressure, temperature, degree of preoxidation, and coating thickness. The results are promising for the development of environmentally resistant interfacial coatings for SiC-fiber reinforced SiC-based matrix composites. The proposed and validated approach allows a low-cost method to obtain continuous hermetic coatings on SiC fibers with interfacial properties adequate for tough composite behavior that resists degradation under turbine engine conditions.     

Petrography and anisotropy of carbonized preoxidized coals

Vitrinite-rich samples from a weakly caking and a strongly coking coal have been carbonized both fresh and after preoxidation for 14 days at 105 °C to points within the temperature range 300–800 °C. Conventional reflected-light microscopy shows that pronounced reduction in softening capacity and devolatilization of carbonized preoxidized reactive macerals are accompanied by microscopical features which contrast with those of the same reactive macerais when carbonized fresh. Most noticeable is the retention of the original petrographical structure of the coals when carbonized after preoxidation, even at temperatures of 800 °C. No mosaic is formed in the preoxidized reactive macerals, although a general anisotropy gradually develops; some vitrinitic particles attain a very high degree of anisotropy though all of this development takes place in the solid state. The level and trends of the average bulk bireflectances of both forms of carbonized vitrinite are similar, but the rapid bireflectance rise characteristic of vitrinites carbonized above ≈ 600 °C probably begins earlier in preoxidized than in fresh vitrinites. This could happen if preoxidation raises the aromaticity of the original vitrinite, so allowing reorientation of the lamellae to begin at lower temperatures. Comparisons are made with earlier observations.     

Effect of temperature and holding time on preoxidation for aligned electrospun polyacrylonitrile nanofibers

In this article, aligned electrospun polyacrylonitrile nanofiber bundles were prepared as the precursor fibers to prepare preoxidized nanofibers through washing, drying densification, damp‐heat drafting, and preoxidation process. Effects of preoxidation temperature and holding time on appearance and microstructure of the preoxidized fibers were studied. Fiber density is increased from 1.159 to 1.193 g cm^?3 after drying densification. Crystallinity is increased from 22.66 to 45.90% after fourfold drafting. The aligned preoxidized nanofibers were prepared at the optimum preoxidation temperature of 283°C, heating speed of 1°C min^?1, and holding time of 1 h show a sufficient reaction degree of cyclization and crosslinking. Moreover, there is no occurrence of adhesion between fibers. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1158‐1163, 2013