Isabel Suárez-Ruiz

Thermal behaviour during the pyrolysis of low rank perhydrous coals

Abstract Perhydrous coals are characterised by high H/C atomic ratios and so their chemical structure is substantially modified with respect to that of conventional coals. As a result, perhydrous coals show different physico-chemical properties to common coals (i.e. higher volatile matter content, enhancement of oil/tar potential, relatively lower porosity and higher fluidity during carbonisation). However, there is little information about thermal behaviour during the pyrolysis of this type of coal. In this work, six perhydrous coals (H/C ratio between 0.83 and 1.07) were pyrolysed and analysed by simultaneous thermogravimetry/mass spectrometry. The results of this work have revealed the influence of high H/C values on the thermal behaviour of the coals studied. During pyrolysis the perhydrous coals exhibit very well defined, symmetrical peaks in the mass loss rate profiles, while normal coals usually show a broader peak. The shape of such curves suggests that in perhydrous coals fragmentation processes prevailed over condensation reactions. The high hydrogen content of perhydrous coals may stabilise the free radicals formed during heat treatment, increasing the production of light components.

Control of the chemical structure of perhydrous coals; FTIR and Py-GC/MS investigation

Abstract This work analyses a set of perhydrous coals (mainly composed of huminite/vitrinite maceral group) in order to determine the inter-relations between the hydrogen content and the modifications in the coal structure at a molecular level. The study involves the direct solid state characterisation of the coal combined with the analysis of representative fragments of the coal network obtained through flash-pyrolysis. The perhydrous character of the coals is not reflected either in the aliphatic hydrogen concentration (from FTIR data) or by the presence of straight-chain aliphatic moieties in the pyrolysates. This structural study shows that perhydrous coals contain mainly aromatic structures with 1–2 rings and a very small concentration of aromatic rings of large size. In agreement with this, phenol and alkyl phenols are the most prominent degradation products whereas other aromatic compounds (mainly benzene and naphthalene derivatives) are minor and probably evaporative compounds. The major structural elements in the samples studied are simple phenols with a preponderance of substituted para alkyl. The results obtained show that the processes of hydrogen enrichment affect the reactions of aromatisation and condensation. During the natural evolution of the perhydrous coals the transformations of the oxygenated functionalities in the lignin precursor seem to have taken place without the parallel structural reorganisation of the lignin framework responsible for the formation of polycyclic aromatic systems. As a result, the chemical structure of perhydrous vitrinites in coals is substantially modified with respect to that described in ‘normal’ coals. The results obtained also indicate that the source of hydrogen content and the effect that it has during the subsequent evolution process of the coals, affects the chemical structure of the perhydrous vitrinite and hence its properties and behaviour.

Geochemistry, mineralogy, and technological properties of the main Stephanian (Carboniferous) coal seams from the Puertollano Basin, Spain

This study is focused on the occurrence and distribution of mineral matter and major and trace elements in the high volatile bituminous coal from Puertollano (south-central Spain). The relationship between ash behaviour and inorganic composition, as well as the possible formation of fouling and slagging deposits in boilers during the conversion process, were investigated. The Puertollano coals do not exhibit plastic properties, despite their rank, probably because of their high ash and inertinite contents. The Puertollano coal has medium to low total S content (0.48% to 1.63% db, with a mean of 1.0% db) and is characterised by relatively high contents of Si, Pb, Sb, and Cs. Some elements such as As, Cd, Co, Cr, Cu, Ge, Li, Mn, Ni, W, and Zn are also present in relatively high contents. The enrichment in a number of heavy metals could be attributed to the common sulphide ores occurring near the Puertollano coal deposit. The following trace elements affinities are deduced: (a) sulphide affinity: As, Co, Cd, Cu, Ni, Sb, Tl, and Zn; (b) aluminum–silicate affinity: K, Ti, B, Co, Cr, Cs, Cu Ga, Hf, Li, Nb, Rb, Sn, Ta, Th, V, Zr, and LREE; (c) Carbonate affinity: Ca, Mg, Mn, and B; (d) organic affinity: B. The very high Si levels and the anomalous enrichment in Cs, Ge, Pb, Sb, and Zn shown by the Puertollano coals account for the high contents of these elements in the Puertollano fly ash when compared with the other Spanish coal fly ashes. The chemical composition of the high temperature ash (HTA) is consistent with the trend shown by the ash fusion temperatures (AFT) and also with the predictive indices related to slagging and fouling propensities. Thus, the ash fusion temperatures increase with high values of Al2O3 as well as with the decrease in Fe2O3, CaO, and MgO.

Conventional and microwave pyrolysis of a macroalgae waste from the Agar–Agar industry. Prospects for bio-fuel production

A comparative study of the pyrolysis of a macroalgae industrial solid waste (algae meal) in an electrical conventional furnace and in a microwave furnace has been carried out. It was found that the chars obtained from both pyrolyses are similar and show good properties for performing as a solid bio-fuel and as a precursor of activated carbon. Bio-oils from conventional pyrolysis have a greater number of phenolic, pyrrole and alkane compounds whereas benzene and pyridine compounds are more predominant in microwave pyrolysis with a major presence of light compounds. The bio-gas fraction from microwave pyrolysis presents a much higher syngas content (H2+CO), and a lower CO2 and CH4 proportion than that obtained by conventional pyrolysis. Yields are similar for both treatments with a slightly higher gas yield in the case of microwave pyrolysis due to the fact that microwave heating favors heterogeneous reactions between the gases and the char.

Molecular characterisation of vitrinite in relation to natural hydrogen enrichment and depositional environment

The Py-GC/MS results of the study carried out on two groups of vitrinites (perhydrous and non-perhydrous) of diAerent age and properties and on a trimaceralic coal associated with one of the perhydrous group are discussed. Such a study provides information about the eAect of natural hydrogen enrichment on vitrinite structure at the molecular level. Moreover, the influence of the diAerent conditions in the sedimentary environment on the chemical structure of the vitrinite is also discussed. This influence is inferred through diAerences in the distribution and relative amount of phenolic compounds found in the pyrolysates from two samples of two diAerent coal-beds in the same basin but formed under diAerent paleoenvironmental conditions. For vitrinites with a high H/C atomic ratio, despite having a strong perhydrous character, their pyrolysates exhibit the highly phenolic signature typical of lignin-derived material with only minor aliphatic compounds. Thus, the major chemical structural elements in these vitrinites are simple phenols with a high contribution of para alkyl-substituted derivatives. However, there is no parallel relationship between the evolution of the oxygenated functionalities and the reflectance values. From the results obtained a coalification pathway where hydrogenation processes predominate over thermal ones is proposed. The presence of resin-like substances and/or oils (which are two of the causes of natural hydrogen enrichment) in the molecular structure of vitrinites have, therefore, aAected the normal evolution of the lignin and contributed to the special properties of this type of materials. # 2000 Elsevier Science Ltd. All rights reserved.

Paleoenvironmental conditions of Puertollano coals (Spain): petrological and geochemical study

Abstract In the present work, petrographic, geochemical and mineralogical data have been combined with results from prior paleobotanical studies in order to determine the sedimentary and postsedimentary diagenetic factors responsible for the physicochemical properties of coal seams and associated sediments from the Puertollano Stephanian coal basin. Although the coal seams (4) originated under similar lacustrine conditions, the highest coal seams (1 and 1 bis ) show higher production/preservation of structured tissues (telinite and collotelinite) than the lowest coal seams (2 and 3) where destruction of the cell tissues (collodetrinite) is predominant. Differences in degree of preservation as a result of changes in the environmental sedimentary conditions in the water table (pH, redox) may explain the different maceral distribution. This feature may also be due to the increase in degree of humification from the highest seams (1 and 1 bis ) to the lowest seams (2 and 3). However, the gentle thermal maturation undergone by these Stephanian coals only produced a slight modification in the physicochemical properties of the organic matter. In general, alkaline synsedimentary conditions could be deduced from the dominance of organic sulphur with respect to pyritic sulphur. Reducing conditions for seam 1, 1 bis and 2 and more oxidant conditions for seam 3 can be deduced from maceral, sulphur and carbonate distribution. Other diagenetic processes involved organic and inorganic sediments. For example, a kaolinitisation process with a different intensity for the early diagenetic stages suggests the occurrence of leaching processes, which affected both organic and inorganic sediments.

Structure of tars derived from low-temperature pyrolysis of pure vitrinites: influence of rank and composition of vitrinites

Abstract In this paper, a gas chromatography (GC)/mass spectrometry (MS) analysis of the volatile fraction of low temperature tars obtained at 550°C by means of the Gray–King pyrolysis of four Stephanian (Carboniferous) monomaceralic coals (pure vitrinites) and a structural study of the whole material by means of nuclear magnetic resonance (NMR, 1H and 13C) are undertaken. Furthermore, a precise assignment of the different spectral regions is achieved through 13C editing of the protonated carbons, identification of quaternary carbon atoms and 2D proton detected 1H, 13C correlation experiments. The study gives valuable information about chemical variability within the vitrinite maceral group. Apart from variation with rank, the results show the effect of factors such as the nature of the precursors, degree of gelification and preservation of the structure, and depositional environment. This effect is not very well understood and it is not always reflected in the conventional bulk characterisation parameters. However, there is no doubt that it affects the thermal behaviour of the material and, therefore, has an indirect effect on its conversion processes. The low-temperature tars are made up mainly of mono- and di-aromatic structures with a preponderance of phenolic compounds. All of the above mentioned factors (rank, nature of precursors, degree of gelification, preservation of the structure, depositional environment) affect the internal distribution of phenols in a complicated manner. Nevertheless, the higher proportion of telinite and initially structured macerals in the raw vitrinite and its precursors is clearly reflected in the amount of 2,4-dimethyl phenol and 4-ethyl phenol. Vitrinites made up of more gelified and homogeneous material give rise to tars with a higher amount of aromatic hydrogen and a higher quantity of condensed aromatic systems. Variation in the maceral composition within the vitrinite maceral group leads to differences in the amount of aliphatic carbon and in the distribution of the aliphatic moieties. Such variations seem to be closely related to the relative amounts of telinite and collotelinite and the proportion of collodetrinite present in the raw materials.

Combustion behaviour of ultra clean coal obtained by chemical demineralisation

The increasing environmental concern caused by the use of fossil fuels and the concomitant need for improved combustion efficiency is leading to the development of new coal cleaning and utilisation processes. However, the benefits achieved by the removal of most mineral matter from coal either by physical or chemical methods can be annulled if poor coal combustibility characteristics are attained. In this work a high volatile bituminous coal with 6% ash content was subjected to chemical demineralisation via hydrofluoric and nitric acid leaching, the ash content of the clean coal was reduced to 0.3%. The original and treated coals were devolatilised in a drop tube furnace and the structure and morphology of the resultant chars was analysed by optical and scanning electron microscopies. The reactivity characteristics of the chars were studied by isothermal combustion tests in air at different temperatures in a thermogravimetric system. Comparison of the combustion behaviour and pollutant emissions of both coals was conducted in a drop tube furnace operating at 1000 °C. The results of this work indicate that the char obtained from the chemically treated coal presents very different structure, morphology and reactivity behaviour than the char from the original coal. The changes induced by the chemical treatment increased the combustion efficiency determined in the drop tube furnace, in fact higher burnout levels were obtained for the demineralised coal.

The influence of impregnation by hydrocarbons on coal structure during its thermal evolution

The present work analyses the changes in the chemical structure of a perhydrous coal during its thermal evolution at different temperatures in an open-medium pyrolysis system. The results obtained were compared with those described for non-perhydrous coals in order to establish the effect of the substances assimilated by the coal structure (hydrocarbon/oil-like substances) on the thermal evolution of the coal. The transformation ratio at each stage of thermal treatment was determined and the chemical-structural characterisation of the resultant products was performed. Changes in textural and microtextural properties associated with structural modifications during the evolution were also tested. The results obtained show that this perhydrous coal develops a specific evolution pathway different from that followed by non-perhydrous coals with a normal H/C ratio. The substances assimilated by the perhydrous coal cannot be easily and totally released from its structure so that they can be only partially removed after thermal treatment. Thus, the treatment debilitates the interactions between the substances and the coal matrix in addition to weakening and cracking the matrix during the thermal process. The increase in temperature also leads to the conversion of some of the heavy assimilated substances into lighter compounds. However, the assimilated substances are present even at high temperatures of the thermal process, providing hydrogen which has the effect of stabilising the radicals originated during the pyrolysis and improving the fluidity properties in the reacting medium.

Chapter 2 – Basic Factors Controlling Coal Quality and Technological Behavior of Coal

Publisher Summary Coal is used in processes such as combustion, gasification, and liquefaction and in carbonization for the manufacture of metallurgical coke. Coal and its derivative products are also used as precursors of other materials and in the production of chemicals. There are two characteristics that influence the use of coal: its composition and its rank. Coal composition is, in turn, represented by two essentially independent factors: type and grade. Coal is a heterogeneous material, and evaluation of coal type may be approached on two different levels: the macroscopical and microscopical, both of which form a part of coal petrology. The chapter also details the coal metamorphism, which involves the physical and chemical transformation from peat through bituminous coal through anthracite and meta-anthracite to graphite. It is a function of heat and pressure acting over a period of time. It is also denoted as the coal rank, which is marked by a progressive decrease in moisture and volatile functional groups with a consequent increase in the carbon content of the coal.