The influence of char surface oxidation on thermal annealing and loss of combustion reactivity

Thermal annealing associated with heat treatment of coal chars affects gasification reactivity and levels of unburned carbon in residual ash from coal-fired furnaces. The present study addresses the effect of char surface oxidation, occurring upon exposure to oxygen, on the course of thermal annealing, and related loss of combustion reactivity. This goal is pursued by comparing the extent of thermal annealing suffered by coal char upon heat treatment in a nitrogen atmosphere with that of chars that underwent oxidation prior to or during heat treatment. Oxidation of char was accomplished by supplying single or multiple pulses of air during the heat treatment, which were sufficient to oxidize the char surface but small enough to limit carbon gasification to less than 5%. The extent of thermal annealing was characterized both in terms of the loss of combustion reactivity and of the development of structural anisotropy of char samples, investigated by HRTEM. Results of the present study confirm that heat treatment reduces oxyreactivity of char samples, the effect being more pronounced at temperatures exceeding 1200 °C. Oxidation of samples mitigates the effects of heat treatment, as demonstrated by the smaller loss of gasification reactivity and by the more limited development of structural anisotropy of oxidized samples. Correspondingly, elemental analysis of samples indicates the formation of stable surface oxides upon oxidation, that are subsequently desorbed upon heat treatment. At temperatures exceeding 1200 °C, the effect of oxidation vanishes. Results are analysed and discussed in the light of the possible hindrance of thermal annealing due to the formation of stable surface oxides and of the parallel modifications occurring to the ash constituents.     

A semi-detailed kinetic model of char combustion with consideration of thermal annealing

The present paper presents a semi-detailed kinetic model of coal char combustion which embodies consideration of thermal annealing as a mechanism leading to the loss of char combustion reactivity along burn off. The distinctive feature of this model is that deactivation induced by thermal annealing is followed along with combustion. Thermodeactivation is modelled according to the power-law equation proposed by Senneca and Salatino [1]. A semi-detailed combustion mechanism was taken after Hurt and Calo [2] and includes three steps: formation of carbon–oxygen complexes (chemisorption), switch-over of surface oxides and desorption of oxygen complexes to yield combustion products. Computation results allow to discuss the impact of thermal annealing on char combustion under conditions of practical interest.     

The effect of potassium bromide and sodium carbonate on coal char combustion reactivity

The addition of halogens, particularly iodine, to the gas during coal char oxidation has been used in previous studies to quench gas-phase chemistry, thereby allowing one to separate the effects of homogeneous and heterogeneous reactions. Halogen addition suppresses the gas-phase radicals to near-equilibrium levels. A similar effect can be expected from other compounds with high efficiency as fire suppressants, such as alkali metals. The effectiveness of the use of additives in distinguishing homogeneous and heterogeneous reactions during char oxidation relies on the assumption that radicals are suppressed while heterogeneous reactions occurring on the char surface are not affected. The present work tests this assumption for potassium bromide (KBr) and sodium carbonate (Na₂CO₃) reacting with a pulverized eastern bituminous coal char during oxidation. An increase in CO and a slight reduction in particle temperature were observed with the addition of KBr, consistent with known effects of halogens on gas-phase chemistry. An increase in particle size was also observed with the KBr addition. This observation and the results of model calculations suggest that there is significant incorporation of liquid KBr on the char surface under the conditions examined. With Na₂CO₃ addition, the particle temperature did not change, the particle size showed a slight decrease, and CO production increased. Although the mechanisms for Na interaction with radicals at combustion conditions are not well established, char oxidation modeling suggests that a decrease in OH concentration in the particle boundary layer is the cause for the observed increase in CO production. It is concluded that Na₂CO₃ has clear advantages over KBr for inhibiting gas-phase chemistry without affecting char oxidation for the conditions investigated here.     

The role of porosity in char combustion

A detailed study has been conducted on the nature of porosity developed during the combustion of chars prepared from different coals, and how this relates to the apparent reactivity of the char towards oxygen. An Illinois No. 6 coal char and a Wyodak coal char have been examined in both Zone I and Zone II conditions (intrinsic rate control and diffusional mass transfer zones, respectively). These results strongly suggest that there exists some amount of free volume in a char that has not been burned off. Such an unburned char does not reveal much porosity when standard nitrogen adsorption is applied, but rapidly develops such porosity upon burnoff. The free volume opens, but is not truly accessible to reactant gases. It appears likely, based upon the present results, that the only truly available surface for reaction exists in pores larger than 10–15 Å.     

The effect of bulk gas diffusivity on apparent pulverized coal char combustion kinetics

Apparent char kinetic rates are commonly used to predict pulverized coal char burning rates. These kinetic rates quantify the char burning rate based on the temperature of the particle and the oxygen concentration at the external particle surface, inherently neglecting the impact of variations in the internal diffusion rate and penetration of oxygen. To investigate the impact of bulk gas diffusivity on these phenomena during Zone II burning conditions, experimental measurements were performed of char particle combustion temperature and burnout for a subbituminous coal burning in an optical entrained flow reactor with helium and nitrogen diluents. The combination of much higher thermal conductivity and mass diffusivity in the helium environments resulted in cooler char combustion temperatures than in equivalent N₂ environments. Measured char burnout was similar in the two environments for a given bulk oxygen concentration but was approximately 60% higher in helium environments for a given char combustion temperature. To augment the experimental measurements, detailed particle simulations of the experimental conditions were conducted with the SKIPPY code. These simulations also showed a 60% higher burning rate in the helium environments for a given char particle combustion temperature. To differentiate the effect of enhanced diffusion through the external boundary layer from the effect of enhanced diffusion through the particle, additional SKIPPY simulations were conducted under selected conditions in N₂ and He environments for which the temperature and concentrations of reactants (oxygen and steam) were identical on the external char surface. Under these conditions, which yield matching apparent char burning rates, the computed char burning rate for He was 50% larger, demonstrating the potential for significant errors with the apparent kinetics approach. However, for specific application to oxy-fuel combustion in CO₂ environments, these results suggest the error to be as low as 3% when applying apparent char burning rates from nitrogen environments.     

The stoichiometry and kinetics of carbon combustion at low temperature: A surface complex approach

The stoichiometry and rate of carbon combustion at low temperature (673 K) were investigated. Oxidation and TPD experimental data provide quantification of gaseous products and stable surface complexes over a broad range of conversion. Our analysis distinguishes between surface complexes forming CO and CO₂ and has assumed a certain fraction of each complex type decomposes instantaneously upon formation, leaving the remainder on the surface as stable complexes, C(O) and C(O₂). This analysis suggests that a maximum of 25% of CO-complexes and 89% of CO₂-complexes are unstable upon formation. At low conversion, unstable complex formation is the dominant pathway for the CO product. As conversion increases, decomposition of stable CO-complexes eventually becomes the main source of CO. Formation of unstable CO₂-complexes is the dominant pathway for the CO₂ product at all times. The combustion rate is initially high due to a high availability of vacant active sites, decreases sharply as these sites are filled with stable complexes, and gradually increases as the stable complexes promote CO₂-complex formation, in turn, driving their decomposition. The dynamics of formation and decomposition of C(O) and C(O₂) dictates their ratio on the carbon surface at any moment, which may be measured by TPD. This work may help in developing new kinetic models of carbon combustion which can predict the stoichiometry as well as the rate.     

Char characteristics and particulate matter formation during Chinese bituminous coal combustion

The characteristics of char particles and their effects on the emission of particulate matter (PM) from the combustion of a Chinese bituminous coal were studied in a laboratory-scale drop tube furnace. The raw coal was pulverized and divided into three sizes, <63, 63–100, and 100–200 μm. These coal samples were subjected to pyrolysis in N₂ and combusted in 20 and 50% O₂ at 1373, 1523, and 1673 K, respectively. Char samples were obtained by glass fiber filters with a pore size of 0.3 μm, and combustion-derived PM was size-segregated by a low pressure impactor (LPI) into different sizes ranging from 10.0 to 0.3 μm. The characteristics of char particles, including particle size distribution, surface area, pore size distribution, swelling behavior and morphology property, were studied. The results show that, coal particle size and pyrolysis temperature have significant influence on the char characteristics. The swelling ratios of char samples increase with temperature increasing from 1373 to 1523 K, then decrease when the temperature further increases to 1623 K. At the same temperature, the swelling ratios of the three size fractions are markedly different. The finer the particle size, the higher the swelling ratio. The decrease of swelling ratio at high temperature is mainly attributed to the high heating rate, but char fragmentation at high temperature may also account for the decrease of swelling ratio. The supermicron particles (1–10 μm) are primarily spherical, and most of them have smooth surfaces. Decreasing coal particle size and increasing the oxygen concentration lead to more supermicron-sized PM formation. The influence of combustion temperature on supermicron-sized PM emission greatly depends on the oxygen concentration.     

Further development of Raman Microprobe spectroscopy for characterization of char reactivity

The effect of heat treatment on reactivity of cellulose char was investigated, using two methods: (1) Raman Microprobe spectroscopy analysis (RMA) and (2) thermogravimetric analysis (TGA). The heat-treatment was in the temperature range of 600–2600 °C, temperature prevailing in combustion of coal-chars. In the RMA, first- and second-order Raman spectra in the range of 800–2000 and 2000–3600 cm⁻¹, respectively, were measured for all samples. In the first-order Raman spectra, the following bands have been observed: D band and G (at 1350 and 1590 cm⁻¹ respectively), 1150 and 1450 cm⁻¹. In the second-order Raman spectra, four bands have been observed at 2450, 2700, 2940 and 3250 cm⁻¹. Both first- and second-order Raman spectra were fitted by Lorentzian functions. The Lorentzian parameters (bandwidth and intensity ratio) showed significant changes with heat treatment, which is consistent with structural modification. Also, from TGA experiments we observed the expected significant influence of heat treatment on char reactivity. Attempts were made to correlate the Lorentzian parameters with char reactivity. A good correlation was found between the 2940 cm⁻¹ bandwidth in the second-order Raman spectrum and char reactivity, confirming the strong connection between char structure and its reactivity, and illustrating the usefulness of RMA in such studies.     

Growth of β-gallium oxide nanostructures by the thermal annealing of compacted gallium nitride powder

Various β-gallium oxide (β-Ga₂O₃) nanostructures such as nanowire, nanobelt, nanosheet, and nanocolumn were synthesized by the thermal annealing of compacted gallium nitride (GaN) powder in flowing nitrogen. We suggest that Ga₂O₃ vapor might be formed by the reaction of oxygen with the gaseous Ga formed by GaN decomposition. The Ga₂O₃ vapor diffuses into voids derived by compacting GaN powder and is supersaturated there, resulting in the growth of Ga₂O₃ nanostructures via the vapor–solid (VS) mechanism. Ga₂O₃ plate-like hillocks and nanostructures were also grown on the surface of a c-plane sapphire placed on the GaN pellet.     

Crystallization of hydrogenated amorphous silicon carbon films with laser and thermal annealing

The crystallization of silicon rich hydrogenated amorphous silicon carbon films prepared by Plasma Enhanced Chemical Vapor Deposition technique has been induced by excimer laser annealing as well as thermal annealing. The excimer laser energy density (E_d) and the annealing temperature were varied from 123 to 242 mJ/cm² and from 250 to 1200 °C respectively. The effects of the two crystallization processes on the structural properties and bonding configurations of the films have been studied. The main results are that for the laser annealed samples, cubic SiC crystallites are formed for E_d ≥ 188 mJ/cm², while for the thermal annealed samples, micro-crystallites SiC and polycrystalline hexagonal SiC are observed for the annealing temperature of 800 and 1200 °C respectively. The crystallinity degree has been found to improve with the increase in the laser energy density as well as with the increase in the annealing temperature.     

Effects of thermal annealing on the properties of GaN MSM UV photodetectors

Metal-semiconductor-metal-structured GaN ultraviolet photodetectors have been fabricated on sapphire substrates by metalorganic chemical vapor deposition. The properties of GaN photodetectors have been improved through thermal annealing. With a 3 V bias, the very low dark current is about 200 pA, the maximum responsivity of 0.19 A/W is achieved at 362 nm, and the corresponding detectivity is 1.2×10¹¹ cm Hz^1/2/W. The physical mechanism of the effects of thermal annealing also has been studied.     

Ignition and devolatilization of pulverized bituminous coal particles during oxygen/carbon dioxide coal combustion

Oxygen/carbon dioxide recycle coal combustion is actively being investigated because of its potential to facilitate CO₂ sequestration and to achieve emission reductions. In the work reported here, the effect of enhanced oxygen levels and CO₂ bath gas is independently analyzed for their influence on single-particle pulverized coal ignition of a U.S. eastern bituminous coal. The experiments show that the presence of CO₂ and a lower O₂ concentration increase the ignition delay time but have no measurable effect on the time required to complete volatile combustion, once initiated. For the ignition process observed in the experiments, the CO₂ results are explained by its higher molar specific heat and the O₂ results are explained by the effect of O₂ concentration on the local mixture reactivity. Particle ignition and devolatilization properties in a mixture of 30% O₂ in CO₂ are very similar to those in air.     

Effect of thermal annealing on nitrogen implanted epitaxial Fe films

Nitridated iron is a promising material for potential applications in permanent magnets. Recent work on stabilization of nitridated iron in a foil form through nitrogen ion implantation and annealing motivates to study effect of thermal annealing on the surface of nitrogen-implanted iron. In this work, we show effect of annealing on chemical state and magnetism of nitrogen implanted epitaxial iron films. It is observed that nitrogen in the lattices only stays at the lower temperatures than 450 °C. In addition, significant reduction and lattice modification are taken placed, when the film is annealed at 450 °C. The increases of saturation magnetization and coercivity, where it is annealed at 450 °C, are likely to be triggered by reduction of oxygen contents at the surface and thinning of Fe₂O₃.     

Behavior of oxygen in zinc oxide films through thermal annealing and its effect on sheet resistance

Behavior of oxygen in sputtering deposited ZnO films through thermal annealing and its effect on sheet resistance of the films were investigated. The crystallinities of the ZnO film were improved by post-deposition annealing in vacuum. However, the sheet resistance of ZnO film was dramatically decreased after post-deposition annealing in vacuum at more than 300 °C, while O₂ desorbed from the film. The oxygen vacancies which acted as donors were formed by the thermal annealing in vacuum. The sheet resistance of the films was recovered by annealing in oxygen ambient. In this paper, ¹⁸O₂ gas as an oxygen isotope was used as the annealing ambient in order to distinguish from ¹⁶O, which was constituent atom of the ZnO films. SIMS analysis revealed that ¹⁸O diffused into the ZnO film from the top surface by ¹⁸O₂ annealing. Therefore oxygen vacancies formed by the post-deposition annealing in vacuum could be compensated by the annealing in oxygen ambient.     

Volatile–char interactions: Roles of in situ volatiles with distinctly-different chemistry in determining char structure and reactivity

This study reports the roles of volatiles with distinctly-different chemistry in determining char reactivity and char structure during in situ volatile–char interactions under non-catalytic conditions. Volatiles were generated in situ from polyethylene (PE), double-acid washed biosolid (DAWB), polyethylene glycol (PEG) or cellulose and interacted with char prepared from DAWB that is free of catalytically-active inorganic species in a two-stage reactor at 1000 °C. The experimental results show that both H- and O-containing reactive species play different roles during in situ volatile–char interactions. It has been found that char reactivity decreases substantially after in situ volatile–char interactions. Results from Raman analysis of the char after in situ interactions with the PE volatiles show H-containing reactive species substantially enhance the condensation of the aromatic ring systems within the char, thus slightly decreasing the H content in char and also making char carbon structure considerably less reactive. It has also been found that the reactivity of char after in situ volatile–char interactions increases with increasing O/H molar ratio of volatiles. The results indicate that O-containing reactive species in volatiles can react with char to form CO complex oxides that mitigate the carbon structure from condensing into large aromatic ring systems, thus increasing O and H contents in char and enhancing char reactivity.     

Dehydration and recrystallization of radiation-damaged titanite under thermal annealing

Abstract

Dehydration of oriented sections of a radiation-damaged titanite crystal, CaTiSiO₅, at temperatures up to 1500K was analysed using infrared spectroscopy. The IR spectra of the untreated sample show only a very weak orientational dependence. The absence of sharp absorption peaks at wave number near 3486 cm^?1 in metamict titanite shows that the local environmental configurations of OH species in the metamict titanite differ strongly from that of crystalline titanite. The OH spectra of radiation-damaged titanite can be decomposed into two components: the first component shows anisotropic and sharp spectral features while the second component consists of a broad spectral feature like those observed in disordered silica glasses. It is proposed that the first component is related to the crystalline part of the titanite sample while the second is from the defected and disordered part which suffered strong radiation damage. With increasing annealing temperature, a decrease in the broad absorption between 2500 and 3200 cm^?1 is accompanied by a recovery of sharp IR bands near 3486 cm^?1 which display the same orientational dependence as undamaged single crystals. Annealing the sample at 1000K leads to the line profiles and orientational dependence of the main OH stretching bands near 3486cm^?1 that are virtually identical with those of crystalline, undamaged titanites. At temperatures above 1500 K, the crystal starts to melt and the orientational dependence of the IR absorption is destroyed. The recrystallization processes are quantified and discussed in terms of a percolation behaviour of amorphous and crystalline titanite. It is proposed that hydrogen transport is strongly enhanced during recrystallization.     

The effects of thermal annealing in self-assembled Ge/Si quantum dots

The effects of thermal annealing in Si base self-assembled Ge dots have been investigated by Raman spectra and PL spectra. An obvious Raman frequency shift under different annealing temperature can be observed. There are two main effects during the annealing procession: one is the inter-diffusion of the Si and Ge quantum dots; the other is the relaxation of the elastic strain. With the calculated results, PL blue shift can be related to strain relaxation effects, and/or a general decrease of Ge content due to the Ge-Si intermixing.     

Effect of thermal annealing on nanoimprinted Cu-Ni alloys using molecular dynamics simulation

The mechanical behaviors of nanoimprinted Cu-Ni alloys before and after annealing were studied using molecular dynamics simulation with a tight-binding potential. The results showed that when the punch is advancing, the punching force obtained from the simulation with a tight-binding potential is lower than with the Morse potential. During and after withdrawing the punch from the specimen, the adhesive phenomena are observed and the large residual stress in the Cu-Ni alloys is induced. During the annealing process, the internal energy of Cu-Ni alloys decreased with increasing the temperature and the component of Cu. In addition, comparing the maximum residual stress in the Cu-Ni alloys with and without annealing treatment, the stress is significantly released after annealing, especially in the higher component of Ni.     

Thermal annealing of ZnO substrates

Zinc oxide crystals were grown by Chemical Vapor Transport using Contactless Crystal Growth technique. After X-ray examination the 8 off-axis oriented slices were polished using alumina powder followed by a mixture of oxides. The mechanical polishing was followed by chemo-mechanical polishing using colloidal silica in water and supplemented by thermal annealing in an air atmosphere. The range of temperature was between 770 and 1070 C, time of annealing ranged up to 160 h. The quality of surfaces was studied using atomic force microscopy. A wide spectrum of surface morphology was observed. The morphology was dependent on the annealing conditions and additionally on the quality of chemo-mechanical polishing and crystallographic orientation of the surfaces. It was found possible to obtain the lowest RMS surface roughness factor in extremely different annealing conditions. The best annealing procedure for surface improvement was investigated.     

Enhancement of carbon nanotube growth yield on Inconel 600 substrates through the surface pretreatments combining thermal annealing and plasma ion bombardment

We report the drastic growth yield enhancement of carbon nanotubes (CNT) on Inconel 600 metal substrate which contains catalytic components through the surface pretreatments combining thermal annealing and plasma ion bombardment. We comparatively investigated the effect of different substrate pretreatments of air annealing, plasma treatment, and their combination affecting CNT growth yield. The roughness of the substrates was significantly enhanced by the air annealing at 725 °C but reduced after the following plasma treatment, which produce nano-sized particle structures. Finally, a significant enhancement in the CNT growth yield was observed when the Inconel 600 substrate was undergone the pretreatment, which consists of oxidative annealing and subsequent plasma treatment.