José Vicente Ibarra

Release of volatile sulfur compounds during low temperature pyrolysis of coal

Abstract The behaviour of sulfur structures in coal during low temperature pyrolysis has been studied. Nine low rank coals with high organic and pyritic sulfur contents were heated in a swept fixed bed reactor (N2) up to 850 °C, and the evolved sulfur compounds were determined by sulfide electrode (H2S) and Fourier transform infrared (FT-i.r.) spectroscopy (SO2, COS). Evolution of H2S as a function of temperature passes through two peaks between 500–560 °C and 630–700 °C, related to the decomposition of organic and pyritic sulfur, respectively. Assignment of organic sulfur structures and pyritic sulfur to the two observed peaks of H2S evolution were tested using float-sink fractions with different mineral matter and pyrite contents (0.3–33 wt%). An important role of coal organic matter in pyrite decomposition to H2S was found. The evolution of COS with pyrolysis temperature, followed a trend similar to that for H2S. The evolved SO2 was related to the decomposition of iron sulfate from weathering of pyrite, but also with the presence of oxidation reactions during pyrolysis. The amount of sulfur removed from coal by pyrolysis varied from 35 to 64% of the total sulfur content for the assayed samples. Pyrolysis temperatures below 700 °C were sufficient to obtain desulfurization levels close to 90% of the total amount of sulfur which can be removed by pyrolysis.

Detection of weathering in stockpiled coals by Fourier transform infrared spectroscopy

Abstract The process of natural oxidation of two low-rank coals exposed to the atmosphere for 11 months has been studied by Fourier transform infrared (FT-IR) spectroscopy. The study was carried out on samples taken at different time intervals and additionally from zones where signs of high oxidation and self-ignition were detected. The aliphatic hydrogen (3000-2800 cm −1 ) and oxygen-containing structures (1800-1500 cm −1 ) regions of the spectra were examined by curve-fitting analysis and a series of structural parameters based on ratios of integrated absorbance areas of curve-fitted bands were established. The aliphatic hydrogen content of samples tended to decrease with increasing time of storage and carboxyl groups only increased slightly under conditions of low pile activity. When oxidation and self-ignition processes took place, the structural changes were more significant. Aliphatic structures decreased drastically and net production of oxygen-containing structures was observed. Aliphatic hydrogen content evaluated from integrated absorbance measurements of normalized spectra and the CO/aliphatic hydrogen ratio seemed to be very sensitive in detecting signs of weathering even at very low levels of activity.

Coal characterization using pyrolysis-FTIR

In this paper the application of combined slow pyrolysis—Fourier transform infrared spectroscopy to structurally characterize coal is studied. On-line FTIR analysis of evolved gases from pyrolysis reveals marked structural differences in a series of Spanish coals varying in rank from peat to bituminous. Using slow heating conditions the pyrolysis reactions were resolved into separate peaks for the evolution of individual species such as CO2, CO, CH4 and sulphur compounds, giving information on oxygen-containing functional groups, coal decomposition temperature and the thermal stability of sulphur structures. Furthermore, kinetic parameters such as activation energies and frequency factors for these reactions can be calculated from the gas evolution profiles. Additional information on structural units present in coal was obtained by studying pyrolysis tars. The structural parameters deduced from FTIR data indicated that tars seem to be structurally related to parent coals, although, they have higher aliphatic hydrogen content and aromaticity.

Characterisation of Spanish coal by means of Mössbauer spectroscopy

Seven Spanish coals from lignite to semi-anthracite have been characterised by Mossbauer spectroscopy (MS) at room temperature (RT) and 80 K. The aromaticities as obtained from Fourier transform infrared spectroscopy (FTIR) varied from 0.44 to 0.96. From the analysis of the Mossbauer spectra the iron bearing minerals were found to be mainly pyrite and sulphates in all the samples. Jarosite was only found to be present in lignite, brown and sub-bituminous coal. However, iron carbonates, mainly as siderite were only detected in the bituminous and semi-anthracite that have higher aromaticities (0.71, 0.95 and 0.96).

Fourier transform infrared spectroscopic study of coke deposits on a Cr2O3Al2O3 catalyst

Abstract The process of coke deposition during the dehydrogenation of butene to butadiene over a chromia-alumina catalyst has been studied by Fourier transform infrared spectroscopy. The study was carried out on samples of coked catalyst dispersed on KBr pellets. The aromatic CC stretching (1630-1500 cm −1 ) and aliphatic CH bending (1450-1300 cm −1 ) regions of the spectra were examined by curve-fitting analysis in order to improve the information provided by Fourier transform infrared spectroscopy. Different steps in the process of coke deposition were observed. At low temperature the coke is formed by polyolefinic chains rather than by aromatic structures. Aromatization of coke takes place with increasing temperature and can be followed by observing the peak at 1590 cm −1 . The aging of the coke in nitrogen does not produce modifications in its aromaticity and the observed dehydrogenation is due to the loss of aliphatic structures as is revealed by the 1385 cm −1 band.

Kinetic behaviour and reactivity of zinc ferrites for hot gas desulphurization

Different zinc ferrite samples have been prepared with varying Fe:Zn atomic ratios and preparation procedures in order to optimize their behaviour as hot gas desulphurization agents. Kinetic studies on metal oxide sulphidation were performed in a thermobalance and sulphur removal tests were conducted in a fixed-bed reactor. Fresh and exhausted samples were characterized by several physical techniques. The results show that reaction rate is mainly controlled by mass transfer processes of the reactant through the gas film in contact with the solid and through the sulphide layer. Bulk ferrites displayed the best performance for sulphur removal. Ferrites deposited on an alumina substrate by impregnation are highly dispersed, however they exhibited a very poor efficiency for sulphur removal. Solid-state reactions of single oxides at the alumina interface, i.e. zinc and/or iron aluminate formation, instead of pore occlusion by the active ingredient seem to be responsible for a low reactivity.

Vibrational spectroscopy study of zinc-containing mixed oxides as regenerable sulphur sorbents at high temperature

Abstract Four series of zinc-based mixed oxides doped with Fe, Cu or Ti, as potential regenerable desulphurization sorbents of coal gas at high temperature were characterized using vibrational spectroscopies, in conjunction with previous studies in which the same sorbents were studied by XRD, SEM-EDX and XPS. For each series, the influence of the composition, within a broad range of concentration in their component oxides, and the temperature of calcination of the fresh sorbents, as well as the structural modifications undergone during sulphidation/regeneration cycles were investigated. It is shown that by analysing the end-process materials, vibrational spectroscopies can reveal fairly well the formation of spinel-structure compounds, sulphides and probable sulphates during calcination, sulphidation and regeneration, respectively. Additionally, vibrational spectroscopies can detect short-range structural changes, not evidenced by XRD, and can be used with advantage to study the evolution of these compounds under real operating conditions.

Influence of cross-linking on tar formation during pyrolysis of low-rank coals

Abstract In this paper the pyrolysis behaviour of two Spanish low-rank coal in the 275–600° C range is studied. Gases evolved from pyrolysis were analyzed by on-line Fourier transform infrared spectroscopy (EGA-FTIR) and gas chromatography and the changes in the macromolecular structure of coal by a solvent swelling technique. The studied coals cross-link very early and the differences found in their tar yields can be explained in terms of cross-linking reactions. Cross-linking of chars correlates with the evolution of CO 2 and pyrolysis water and the extent of the cross-linking reactions is related to the coals initial content of labile functional groups, namely carboxyl and hydroxyl groups.

Distribution of the activation energies for the thermal decomposition of organic and pyritic sulphur in coal

The activation energies for the thermal cracking of the organic sulphur of several coals ranking from lignite to subbituminous have been calculated. Likewise, the activation energies for the thermal decomposition of the pyrite associated with different density fractions obtained from a subbituminous coal by sink-float analysis have been determined. The model of multiple, parallel, independent (MPI) first order reaction has been assumed for the thermal cracking. The kinetic equations associated with a Gaussian distribution of the activation energies (DAE) have been used for calculating the average activation energy, E 0 , and the standard distribution, σ. The results obtained show small differences between the parameters calculated for the cracking of the organic sulphur contained in the different coals tested

Isolation of the phenolic fraction of coal pyrolysis tars by ion-exchange chromatography

Abstract The isolation of the phenolic fraction from coal pyrolysis tars by ion-exchange chromatography is reported. Tars were obtained from low temperature pyrolysis (600°C) of four coals varying in rank from lignite to bituminous. The phenolic fraction was extracted from tars using an anion-exchange resin (Amberlyst A-26, OH-form) activated by consecutive treatments in a column with 2 M HCl, 2 M NaOH and isopropanol-water (1:1). Tar samples dissolved in dichloromethane-diethyl ether (5:1) were shaken with the resin (10 cm 3 /g sample) for 30 min. The resin was then separated by filtration and the acid fraction was quantitatively eluted from the resin with chlorotrimethylsilane-dichloromethane. The acid fraction extracted in this way represented between 46 and 58% (w/w) of the whole tar for the analysed samples. The efficiency of the separation was tested by enthalpimetric titration, gas chromatography, Fourier transform (FT) IR spectroscopy and 1 H NMR. The residual fraction obtained after removal of the acid fraction has no acid character and does not contain hydroxyl groups. In contrast, phenolic structures are concentrated in the acid fraction. The main compuonds in the acid fraction were identified and quantified by wide-bore gas chromatography using external standards. Several structural parameters of the fractions were deduced from FT-IR an 1 H NMR data. The acid fraction is mainly aromatic in nature whereas the aliphatic structures tend to concentrate in the residue.