Ramón Álvarez García

Coal structure and reactivity changes induced by chemical demineralisation

The aim of this work was to determine the influence that an advanced demineralisation procedure has on the combustion characteristics of coal. A high-volatile bituminous coal with 6.2% ash content was treated in a mixture of hydrofluoric and fluorosilicic acids (HF/H2SiF6). Nitric acid was used either as a pretreatment, or as a washing stage after HF/H2SiF6 demineralisation, with an ash content as low as 0.3% being attained in the latter case. The structural changes produced by the chemical treatment were evaluated by comparison of the FTIR spectra of the raw and treated coal samples. The devolatilisation and combustibility behaviour of the samples was studied by using a thermobalance coupled to a mass spectrometer (TGA-MS) for evolved gas analysis. The combustibility characteristics of the cleaned samples were clearly improved, there being a decrease in SO2 emissions.

Grindability and combustion behavior of coal and torrefied biomass blends.

Biomass samples (pine, black poplar and chestnut woodchips) were torrefied to improve their grindability before being combusted in blends with coal. Torrefaction temperatures between 240 and 300 °C and residence times between 11 and 43 min were studied. The grindability of the torrefied biomass, evaluated from the particle size distribution of the ground sample, significantly improved compared to raw biomass. Higher temperatures increased the proportion of smaller-sized particles after grinding. Torrefied chestnut woodchips (280 °C, 22 min) showed the best grinding properties. This sample was blended with coal (5-55 wt.% biomass). The addition of torrefied biomass to coal up to 15 wt.% did not significantly increase the proportion of large-sized particles after grinding. No relevant differences in the burnout value were detected between the coal and coal/torrefied biomass blends due to the high reactivity of the coal. NO and SO2 emissions decreased as the percentage of torrefied biomass in the blend with coal increased.

Determination of kinetic parameters for biomass combustion.

The aim of this work is to provide a wide database of kinetic data for the most common biomass by thermogravimetric analysis (TGA) and differential thermogravimetry (DTG). Due to the characteristic parameters of DTG curves, a two-stage reaction model is proposed and the kinetic parameters obtained from model-based methods with energy activation values for first and second stages in the range 1.75·10(4)-1.55·10(5)J/mol and 1.62·10(4)-2.37·10(5)J/mol, respectively. However, it has been found that Flynn-Wall-Ozawa and Kissinger-Akahira-Sunose model-free methods are not suitable to determine the kinetic parameters of biomass combustion since the assumptions of these two methods were not accomplished in the full range of the combustion process.

Hydrogen-transfer ability of extrographic fractions of coal-tar pitch

Coal-tar pitch was fractionated using extrography into classes of compounds of similar functionality and molecular weight. Hydrogen acceptor and donor abilities of the whole pitch and its extrographic fractions were evaluated by reaction at 360°C with tetralin and anthracene, respectively, and related to structural characteristics by elemental analysis, VPO, 1H NMR and HPLC. Hydrogen acceptor ability (HAA) of the fractions increases markedly with the elution depth. There is a good correlation between HAA and the total oxygen content in the fraction. On the other hand, all extrographic fractions show comparable relatively low hydrogen donor abilities (HDA) in the initial period of reaction with anthracene due to a similar content of hydroaromatic rings and methylene bridges. On further treatment, F2 and F4 are differentiated by a stronger HDA than other fractions. Among the fractions, only in F2 the hydrogen donor ability outweighs over hydrogen acceptor ability. The opposite situation is observed for F5, F6 and F7. In the case of F3 and F4, a comparable amount of hydrogen is transferred from tetralin to fraction and from fraction to anthracene. There is no straight correlation between the observed hydrogen transfer behaviour of fraction and its ability to develop an anisotropic texture in carbonization product. Possible contribution of various fractions to the creation of optical texture of pitch coke is discussed.

Mesophase from a coal tar pitch: a Raman spectroscopy study

Raman spectroscopy has been utilised to detect changes in the structural order accompanying the process of mesophase spherules preparation. A coal tar pitch was selected as the precursor and it was carbonised under the appropriate conditions to produce a discrete amount of anisotropic material present almost completely in the form of spherules. These anisotropic spherules were concentrated by partial removal of the isotropic phase by hot filtration. The analysis by Raman spectroscopy confirmed an increase of the graphene-like orientation from the original coal tar pitch to the final anisotropic concentrate, with the carbonisation semicoke presenting an intermediate behaviour. The results indicate that Raman spectroscopy can be employed to monitor the process of production of mesophase spherules from coal tar pitches by studying the degree of structural order of the resultant product.

Desulphurisation of a spanish lignite via low temperature supercritical gas extraction

Abstract Supecritical gas (SCG) extraction of a high sulphur Spanish lignite with methanol has been conducted at temperatures between 250 and 350°C and the resultant extracts have been characterised by GC/MS and other appropriate techniques. Char prepared at 350°C contained 40% less organic sulphur than the initial coal but a similar reduction in organic sulphur content was achieved by pyrolysis at 500°C. Thus the extent of desulphurisation is influenced primarily by thermal processes rather than by the nature of the SCG. Nevertheless, the methanol had a significant impact on product distribution; it was incorporated into the extracts and residues as both methyl ethers and esters and decomposed to generate hydrogen in situ, some of which was transferred to the lignite.

Influence of wet and preheated coal charging on the nature of quinoline insolubles of coal tars and their derived pitches

Differences between primary quinoline insoluble (QI) material of coal tars and their derived pitches caused by preheating carbonization have been studied using microscopic techniques, Coulter Laser particle size analysis and solid-state 13C NMR. The selected tars were produced from the same coal using wet and preheated charging at the INCAR experimental coking test plant, which operates on a semi-industrial scale. Specific operational conditions were successfully applied not only to reduce the ash content and the QI and toluene insoluble (TI) contents in the tars produced by the preheating process, but also to modify the nature of the QI particles. In addition, structural changes in the insoluble materials formed during the pitch production from the tar have been monitored.

PYROLYTIC DESULPHURISATION OF SOME HIGH SULPHUR COALS

Publisher Summary This chapter discusses the pyrolytic desulfurization of some high sulfur coals. For the effective utilization of high sulfur coals, a thorough understanding of the chemical transformations of organic and pyritic sulfur forms during devolatilization is required. Mild thermal processing as a route to achieve liquid products from coal and a low sulfur char for combustion has received considerable attention. The three coals display completely different trends for the removal of organic sulfur. For Cayirhan lignite, nearly all the organic sulfur is eliminated as tar and gas in pyrolysis compared to ∼ 75% for Mequinenza lignite. The implication is that little of the organic sulfur exists in condensed thiophenic structures for Cayirhan lignite. The use of a sulfided molybdenum catalyst in hydropyrolysis gives rise to levels of desulfurization in excess of 90% excluding unreacted sulfatic sulfur.

HYDROGEN TRANSFER AND ALKYLATION REACTIONS DURING SUPERCRITICAL GAS EXTRACTION OF A HIGH SULPHUR LIGNITE WITH METHANOL

Publisher Summary This chapter discusses hydrogen transfer and alkylation reactions during supercritical gas extraction of high-sulfur lignite with methanol. In a study described in the chapter, semicontinuous supercritical gas (SCG) extractions were carried out. Batch extractions with 13C-labeled methanol were performed in a vertical agitated tubing bomb reactor. Phase-transfer-catalyzed methylation reactions were conducted using normal and 13C-labeled iodomethane following Rose and Franciscos procedure. Elemental compositions of the extracts and residues were determined together with the forms of sulfur in the residues. In addition, maceral analysis of Mequinenza lignite (ML) and the SCG residues were carried out. The chapter presents the product yields obtained from supercritical methanol extractions of ML at different temperatures and pressures. Total conversions and yields of extracts and gases increase as temperature and pressure increase, the influence of temperature being greater than pressure. However, the evolution and composition of extracts and gases, and the mass balances (>100%) suggest the existence of different pathways for the extraction process and the reactions between coal, methanol, and their thermal decomposition products.