M.C. Mayoral

DIFFERENT APPROACHES TO PROXIMATE ANALYSIS BY THERMOGRAVIMETRY ANALYSIS

Abstract The experimental optimization by the simplex method of the proximate analysis of coal and biomass by thermogravimetry analysis (TGA) is reported. Heating rate, final temperature, holding time, Ar flow rate and sample size were the control variables. The response function used was chosen to minimize the difference in percentage of volatile matter with the ASTM characterization. The relative accuracy of the method was demonstrated by determination of the volatile matter contents of a number of coals in parallel with the ASTM certified method. The method is successfully used with biomass samples.Authors acknowledge the European Commission for the financial support by contract No. ECSC 7220-PR/048. M.C.M. and M.T.I. are thankful to the Ministry of Science and Technology.

Aluminosilicates transformations in combustion followed by DSC

Abstract The behavior of quartz, kaolinite and calcite, as main components of coal mineral matter, has a direct impact on slags formations and development in pulverized coal combustion. The mineral transformations along temperature can be followed by differential scanning calorimetry (DSC), provided that the formation and crystallization of mullite (3Al2O3·2SiO2) are exothermic phenomena. Peak heights and areas allow to quantify the occurrence and extent of those mineral transformations for pure kaolinite and its mixture with calcite as fluxing agent. On the other hand, sinterization time at temperatures bellow mullite formation has a clear impact on the onset and the shape of the peaks. In this way, it is demonstrated that mineral transformations at solid state have a direct effect on the final structure of mineral species generated by slagging.

Mechanism of interaction of pyrite with hematite as simulation of slagging and fireside tube wastage in coal combustion

Abstract Iron-bearing minerals have been recognised as a mayor source of fire-side wall slagging in pulverised fuel boilers, which not only reduces the thermal efficiency of heat transfer of the exchangers surface, but also affects its integrity as a result of corrosion and erosion. Nevertheless, the root cause of adhesion and growing of deposits has not been clearly addressed. Our approach suggests that differential scanning calorimetry (DSC) combined with simultaneous thermogravimetry can follow the chemical reaction between pyrite and the outer layer of iron oxide on tubewalls. The changes in composition are followed by X-ray diffraction (XRD). The results indicated that the mechanism of wetting and adherence of molten pyrite over iron oxides is chemically induced: both di and mono iron sulphides interact with the oxide layer, changing the oxidation state of iron in oxide scale, from hematite to magnetite. This would imply a change in the protective ability of the scale as well as a great increment in corrosion tendency.

Inhomogeneous oxygen interchange during annealing and cooling of textured bulk Bi2Sr2CaCu2O8+δ superconductors

The optimized annealing of bulk Bi2Sr2CaCu2O8+δ (Bi-2212) materials textured using the laser floating zone technique requires a two-step process, consisting of a first step at 870 °C and a second step at 801 °C. In this paper, we present an analysis of the changes induced within the samples along this thermal treatment and the subsequent cooling to room temperature. During the initial step the cationic diffusion takes place, while the oxygen content is adjusted during the second step, at lower temperature. The evolution of the superconducting properties reveals that the oxygen diffusion into the sample is not homogeneous and that the different oxygen contents along the annealing are also a consequence of a strong interchange between the central and the external sample regions. It has been shown that, at the different stages of the thermal process, the samples present different degrees of homogeneity in their superconducting properties, this being caused by the improvement of one region to the detriment of the other.

Approximation to the laser floating zone preparation of high temperature BSCCO superconductors by DSC

Differential scanning calorimetry (DSC) has been used to simulate laser fusion of ceramic precursors to prepare BSCCO high temperature superconducting materials. At fast heating rates, typical of the laser floating zone, the energy required for complete melting of the precursor increases with the Bi to Sr+Ca ratio, in agreement with the critical current results obtained. At low heating rates, solid-state reactions involving ionic rearrangements take place before incongruent melting. These changes can be divided into three processes and can be quantified from calorimetric measurements. Lower heating rates and isothermal experiments allow detailed visualisation of the ionic arrangements taking place. The results prove that the mechanism of the final phase formation is dependent on the fabrication procedure.

Impact of iron-sulfide deposits on oxidized austenitic steels as simulation of corrosion and fireside-tube wastage in coal combustion

The sulfidation effect of molten iron sulfides was studied on oxidized austenitic steels as a simulation of furnace-wall corrosion in PC combustion environments. The test coupons were oxidized to produce an external oxide scale and pyrite was placed on the oxide and thermally treated in an inert atmosphere to decompose the pyrite into pyrrhotite. DSC-TGA and XRD indicated that FeS interacts with the Fe2O3 oxide layer, even at 700°C if the contact is good, changing the oxidation state of iron and the physical structure. On the other hand, the interaction of FeS with Cr2O3 between 1100 to 800°C, 24 hr in the inert atmosphere, consisted of the formation of a chromium sulfide layer beneath the oxide scale. SEM-EDX showed that the diffusion of sulfur in the steel matrix can be 30 μm deep, indicated by small particles of chromium sulfide. It is demonstrated that iron sulfide deposits could be responsible for sulfidation of the alloys.

Fabrication of Bi-2212 Coatings Using Thermospraying

This work presents the characteristics of Bi-2212 coatings on silver substrates obtained by thermo-spraying. The system allows moving the substrate to obtain coatings over surfaces several centimeters long. The thickness of the superconducting layer can be controlled modifying the thermo-spraying parameters and the substrate speed. The phase distribution and evolution during annealing has been monitored. An important amount of Bi-2212 phase is on the sample after the thermo-spraying process and this phase can be aligned by subsequent laser zone melting. The influence of the different treatments applied on the phase composition, microstructure and superconducting properties is shown.

Iron from two different catalytic precursors in coal hydrogenation

Publisher Summary Iron dispersed onto coal surface as iron disulphide show different state and behavior depending on the type of coal; over the sub-bituminous coal, the detection of added iron by X-ray photoelectronic spectroscopy (XPS) and X-ray diffraction (XRD) appears to be screened by the rapid oxidation by the heteroatoms present in coal composition. Apart from the better characterization, the iron dispersed on the high rank coal presents higher disposition to be reduced to pyrrhotite under hydrogenation conditions. The extent of reduction to pyrrhotite and the number of its vacancies do not present a direct relationship to the conversion degree achieved in dry hydrogenation nor to the selectivity to oil production for catalytic precursors, iron sulphide, and iron oxide from red mud. The results obtained with red mud characterization indicate an acid behavior; the iron oxide does not seem to develop the sulphate ion chimisorption that would confer to the solid superacid characteristics.