JoséV. Ibarra

FTIR study of the evolution of coal structure during the coalification process

Abstract A series of coals varying in rank from peat to semi-anthracite was studied by Fourier Transform infrared spectroscopy (FTIR) and curve-fitting analysis in order to gain additional information on coal structure and the main structural changes that take place during the coalification process. Several structural parameters based on FTIR data and curve-resolved bands were calculated. These parameters provide quantitative evidence of the increase of aromaticity and the loss of aliphatic and oxygen-containing structures with increasing maturation. The analysis of the aromatic CH out-of-plane region (900-700 cm−1) reveals the loss of aromatic substituents up to the stage of bituminous coal and the subsequent increase in the degree of aromatic condensation with increasing coalification. The ratio of aromatic carbons to carboxyl groups (1605 cm−1/1605 cm−1 + 1705 cm−1) appears to be a suitable parameter for assessing the natural maturation of coal. Copyright

FT-i.r. investigation on char formation during the early stages of coal pyrolysis

Abstract The transformation of coal into char during low-temperature pyrolysis was studied by Fourier transform infrared (FT-i.r.) spectroscopy and solvent swelling measurements. Pyrolysis was carried out in a nitrogen-swept fixed-bed reactor at temperatures of 300–600 °C. Three Spanish coals varying in rank from lignite to bituminous were studied. The samples show very different cross-linking behaviour, related to CO 2 and H 2 O evolution at low temperatures and to methane release at higher temperatures. The FT-i.r. examination shows that the aliphatic structures and the oxygen-containing functional groups (carboxyl and hydroxyl) are easily removed from chars, while aromatic hydrogen tends to increase with increasing pyrolysis temperature. Curve-fitting analyses for the 3000-2700 and 900-700 cm −1 regions permit the variations in the distribution of methyl and methylene groups and in the degree of aromatic substitution and condensation, respectively, to be evaluated as a function of temperature. Several structural parameters based on FT-i.r. data have been calculated for the chars. These structural parameters provide evidence on the loss of aromatic substituents and the process of hydrogen transfer to aromatic structures during pyrolysis.

Characterization of zinc oxide and zinc ferrite doped with Ti or Cu as sorbents for hot gas desulphurization

Three series of samples based on zinc oxide and zinc ferrite doped with copper or titanium oxides have been prepared in order to improve their performance as regenerable sorbents for hot gas desulphurization. In each series the oxide concentration was varied over a broad range to enhance the formation of different chemical species. The stability against reducing agents and the performance of these sorbents were studied elsewhere. The characterization of fresh, sulphided and regenerated samples was undertaken using XRD, FT-Raman and XPS techniques. The addition of Ti increased the stability of ZnO against reducing agents such as H2, up to an atomic ratio Ti/Zn= 0.5 through the formation of Zn2TiO4. Furthermore, the Ti excess is segregated as TiO2. The addition of Cu to zinc ferrite did not affect the stability but improved the sorbent performance enhancing the ferrite formation and migrating to the sorbent surface during the calcination and regeneration steps. The addition of Ti to zinc ferrite prevented its decomposition into the two component oxides below 600°C stabilizing the structure through the inclusion of Ti in the ferrite lattice. In the sulphiding process Fe, Zn and Cu oxides were converted into the lowest oxidation state sulphides that facilitated the sorbent regeneration during the regeneration process.

Structural changes in humic acids during the coalification process

Abstract Humic acids from four coals, varying in rank from peat to subbituminous coal, have been characterized by elemental analysis, acidic groups, molecular weight, electrophoresis and visible, FT-i.r. and CP/MAS 13C n.m.r. spectroscopy. The humic acids increase in carbon content, molecular weight, condensation degree and aromaticity (fa) with increasing maturation of the parent coals, while the oxygen content decreases with a loss of oxygen functional groups. The presence of lignin-like polymers, poly-saccharides and peptidic materials in humic acids from peat was established using i.r. and 13C n.m.r. spectroscopy. The structural changes observed in humic acids are in agreement with the recognized coalification theory and tend to support the hypothesis of condensation of humic acids into insoluble humin of coal.

Functional group dependence of cross-linking reactions during pyrolysis of coal☆

This paper reports an investigation of the influence of functional groups in coal on cross-linking reactions during pyrolysis. A humic coal, two low rank coals and a bituminous coal were selected for this study. The evolution of species such as CO2, CH4 and H2O related to cross-linking reactions was monitored by gas chromatography and Fourier Transform infrared spectroscopy. Solvent swelling of coal chars in pyridine was used as a measurement of the extent of cross-linking. The results reported here prove that the early stages of CO2 and H2O evolution are directly related to the cross-linking observed at low temperatures (<400 °C), which inhibits tar production. Cross-linking produced at temperatures between 400 and 500 °C seems to be related to methane evolution. As tar is also evolved in this temperature range, the tar yield is less affected by the cross-linking related to CH4 evolution.

Retention of metallic cations by lignites and humic acids

Abstract Adsorption of strontium, lead, uranium and thorium ions from their solutions by several Spanish lignites have been studied. Adsorption isotherms are adequately described by means of the Langmuir equation and can be characterized by two numerical constants: the sorption capacity of lignite, and the geochemical enrichment factor (GEF). Very high values of this GEF have been found for heavy metals. Humic acids were extracted from the lignites and their metal binding ability was determined as a function of pH. Higher pH favours metal retention, also higher atomic weight and valency. A role of humic substances in the retention and accumulation of metals in coals is suggested.

Catalytic effects of zinc chloride in the pyrolysis of Spanish high sulphur coals

Abstract Study of the catalytic effects of zinc chloride (ZnCl2) in the slow pyrolysis of two Spanish high sulphur coals showed that the behaviour of ZnCl2 was modified by the presence of organic sulphur and minerals such as calcite and pyrite in the coals. The desulphurization of chars from impregnated coals was lower due to the formation of ZnS from the H2S evolved during pyrolysis. ZnCl2 produced a specific dehydrogenation of coal structure from aliphatic structures. This reaction took place at temperatures below 400 °C and produced a loss of tar-forming structures and an increase of cross-linking. This implies a reduction of tar yield for impregnated coals.

Analysis of coal and char ashes and their ability for sulphur retention

Abstract Changes in the mineral matter of two high sulphur Spanish lignites and their respective low temperature chars during combustion were investigated by Fourier transform infrared spectroscopy (FT-i.r.), X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX). During the low temperature ashing of coals (LTA), fixation of organic sulphur as sulphate occurred. For this reason, FT-i.r. spectral subtraction seems to be a more suitable technique than LTA for identifying original mineral matter in high sulphur high calcium content coals. Ashes from chars contained less fixed sulphur and had lower S Ca atomic ratios than those from coals. The study shows that sulphur is retained in ashes only as calcium sulphate. If calcite is present in the original coal, it is active in the retention process through its conversion into calcium sulphate. This conversion, however, is partial because only the outer particle layers play a real role, especially if calcite particles are large. Pyrolysis transforms the outer layers of the calcite particles into CaS. In combustion this film prevents further participation of calcite in sulphur retention and even destabilizes the calcium sulphate. Consequently its role in that process is small or negative.

Influence of weathering on the sulphur removal from coal by pyrolysis

Abstract In this paper evidence of the negative effect of weathering on the sulphur removal by pyrolysis is reported. The study of the evolution of sulphur forms during the pyrolysis of a weathered coal has been conducted by chemical analysis, transmission Mossbauer spectroscopy and scanning electron microscopy with energy dispersive X-ray analysis. This study shows that in the pyrolysis of weathered coals some special chemical changes occur when compared with the pyrolysis of unweathered coals. In particular, no accumulation of organic sulphur in chars has been observed, however, inorganic iron sulphates formed by the oxidation of pyrites are decomposed above 500°C and yield complex sulphides which accumulate in chars, decreasing the efficiency of the desulphurization. These sulphides, because of their unreactivity, cannot be determined completely by chemical analysis, which implies an overcalculation of organic sulphur when calculated by difference.

The release of nitrogen during the combustion of coal chars: the role of volatile matter and surface area

Abstract The effect of pyrolysis conditions on the release of nitrogen during char combustion was investigated. A fluidized bed pyrolysis unit was used to produce several chars at different temperatures and pressures from a Spanish coal. In this way, a number of chars differing in volatile matter and specific surface area were obtained. Gas evolution profiles of CO, CO 2 , NO and N 2 from temperature-programmed char combustion were recorded by a thermogravimetric analyser linked to a quadrupole mass spectrometer. The NO evolution profiles show two zones whose intensity depends on the volatile matter and specific surface of the chars. These zones of NO release are attributed to two different nitrogen sources. The fraction of the total nitrogen char released as NO depends on the volatile matter and the specific surface of the chars. The higher the volatile matter and the lower the surface area, the higher the NO/N ratio, i.e. the fraction of the char nitrogen that is released as NO. The nitrogen released as NO + N 2 accounts for nearly all the nitrogen in chars of low surface area and high volatile matter. This is not the case for the chars of high specific surface and low volatile matter, showing that other nitrogen species, probably N 2 O, are formed during oxidation of such chars. The results are discussed on the basis of the mechanisms proposed in the literature for NO release that involve primary NO formation and subsequent reduction to N 2 and N 2 O by CO and char carbon.