Angeles G. Borrego

Influence of pyrolysis temperature on char optical texture and reactivity

In this study a set of 10 coals varying in rank and maceral composition has been devolatilised in a flat flame burner at 1000 and 1300°C in order to investigate the changes in char reactivity and structure as a function of pyrolysis temperature. The intrinsic reactivity of the chars, measured isothermally in a thermobalance at 500°C, has been related to the petrographic characteristics of the chars and their CO2 surface areas. The temperature of pyrolysis has shown to have a strong effect on char reactivity for certain coals. Overall, the increase of temperature provoked an enhanced plasticity and a more extensive consolidation during the metaplast stage of pyrolysis and reduced the amount of unfused material in the chars. This effect was more pronounced in low volatile bituminous vitrinites and low rank inertinites. The presence of inertinite derived materials in high rank coal chars enhanced their reactivities, whereas the opposite was observed for low rank coal chars. This was attributed to the fact that inertinite macerals yield both isotropic and anisotropic materials: inertinite in low rank coals increases the amount of anisotropic, less reactive char, but reduces it in high rank coals.

Unbiased methods for the morphological description of char structures

Abstract This paper shows how the most relevant morphological parameters of chars can be accurately measured with the aid of standard image analysis techniques. The descriptions of char particles thus obtained allow a more precise application of most of the classification schemes in use to date, or the production of useful data for the modelling of p.f. combustion. The application of these techniques is illustrated with a simplified scheme for the classification of char structures, with emphasis on the discrimination of the main structural types considered in current classification systems. Data obtained from the application of the proposed techniques to char samples from three coals of different rank and maceral composition clearly reflect the differences that could be expected from the plastic properties of the parent coals. The accuracy and objectivity of the measurements by these techniques make them a valuable tool for improving the repeatability and reproducibility of char petrographic analyses.

Physicochemical transformations of coal particles during pyrolysis and combustion.

Abstract The pyrolysis and combustion behaviour of a set of 11 Canadian coals with different ranks (lignite to low volatile bituminous) and maceral compositions has been investigated by TGA techniques. Temperature programmed heating of the coals was carried out both under nitrogen and under air, and the weight loss rates occurring in the two experimental conditions were compared in the whole temperature range studied (100–1000°C). Results showed that the pyrolysis curves of the coals do not match at all with any specific feature of the corresponding combustion profiles, and that the temperatures of initiation of both processes are very different in the low-rank end (higher initiation temperatures for pyrolysis), to become similar only for coal ranks of ∼0.8% vitrinite reflectance and above. This contradicts existing theories which state that coal combustion under TGA conditions is a three-stage process, namely volatiles release, vitrinite combustion and inertinite combustion. The processes leading to the weight loss rates occurring in the early stages of combustion were also investigated, with special emphasis in the temperature interval where no substantial weight losses had occurred yet in either a combustion or a pyrolysis experiment. This was done by heating the coals to 300°C in the TGA under air, and then switching the gas flow to nitrogen and allowing the sample to further devolatilise until 1000°C. Also, partly burnt and/or pyrolysed samples were obtained from the TGA and characterised by optical microscopy techniques. It was observed that the volatile yields of all the coals were substantially reduced as a consequence of their initial heating under air. Besides, evidences of melting and thermal annealing in the inner core of burning coal particles were noticed to occur at lower temperatures than in pyrolysing particles. This was attributed to a sealing effect of the oxidation rim formed in the early stages of combustion, which might give rise to a higher pressure build-up in the inner part of burning particles, thus enhancing the likelihood for condensation reactions to take place in the newly formed metaplast. As combustion profiles are commonly used to infer about combustion behaviour of coals, much care should be exercised in interpreting them, since even in a pure vitrain, two rather than one single material will be involved in the measured weight losses, and, more, these materials will often display fairly different reactivities.

Pyrolysis behaviour of pulverised coals at different temperatures

This study reports the chemical and petrographic characteristics of chars obtained at 1000 and 1300°C (within the range of temperatures reached by coal particles in the near burner zone of pulverised fuel boilers) from three different coals. The coals were selected according to petrographic criteria: two of them are low and high volatile bituminous vitrinite-rich coals, and the third one has similar rank to the high volatile bituminous coal but a very different maceral composition (97% inertinite). The residual volatiles remaining after pyrolysis at the two temperature tested were determined by heating the chars in a thermobalance at 1100°C under nitrogen. Also, the morphology and optical texture of the chars were studied by optical and scanning electron microscopy. The increase in preparation temperature enhanced the plastic behaviour of the low volatile bituminous coal, resulting in an extensive development of secondary porosity. The drop in reactivity from low to high temperature char might be due to the enhanced fluidity of the metaplast, which increases the likelihood for polymerisation reactions to take place, with a parallel reduction in the number of active sites. Incomplete devolatilisation and poor plastic properties were achieved during low temperature pyrolysis of the inertinite-rich coal, whereas at 1300°C most of the material passed through a fairly plastic stage and was very efficiently pyrolysed. No significant differences were observed between the low and the high temperature chars from the high bituminous vitrinite-rich coal. This behaviour could be due to either extensive crosslinking reactions involving hydroxylic oxygen in an early stage of the pyrolysis process, or, which seems more likely, to the sheltering effect of soot particles formed in the vicinity of pyrolysing coal particles, which could minimise the effect of the increased flame temperature.

Organic matter composition and distribution through the Aptian-Albian lacustrine sequences of the Araripe Basin, northeastern Brazil

Abstract The main characteristics and distribution of organic matter have been studied through several Aptian–Albian lacustrine units (Rio da Batateira, Crato and Ipubi Formations, Araripe Basin, NE Brazil). Six main facies have been distinguished on the basis of their lithological–sedimentological characteristics and the type and amount of organic matter: (1) marginal lacustrine deltaic massive to coarsely laminated mudstones and sandstones found as lenticular sheet and channel-infills; (2) oxic marginal lacustrine massive to poorly laminated mudstones; (3) oxic–suboxic inner lacustrine mudstones and laminated bioclastic mudstones; (4) anoxic, inner lacustrine dark mudstones; (5) anoxic, inner lacustrine dark clay-carbonate rhythmites; and (6) anoxic, inner lacustrine finely laminated dark micritic limestones. These facies make up decimeter- to several meter-thick parasequences, which stack up in several tens of meters thick successions. Water level changes and variations of the detritic sediment contribution in the basin controlled the evolution of the depositional system. Palynomorph associations showed that the lacustrine system evolved in an arid–semiarid tropical area where the most characteristic flora consisted of conifers, Gnetales, Bennettitales and early angiosperms. In facies 1 to 3, organic matter was scarce, poorly preserved and mainly derived from higher land plants. Facies 4 is characterised by containing abundant algal/bacterial-derived well-preserved organic matter. Facies 5 and 6 contained mainly branchlets and pollen debris blown by the wind or fed by lacustrine underflows. The pattern of organic matter accumulation and preservation corresponds well with cyclical palaeoenvironmental changes (from anoxic to oxic bottom conditions and from a fresh to saline upper water layer), which affected the shallow lacustrine system. The deposition of thin, organic matter-rich mudstones (facies 4) probably occurred during higher water levels, which resulted in relatively low salinity waters and low detrital contribution to the inner lacustrine zones. Less significant organic matter accumulation is recorded in the clay-carbonate rhythmites and in the limestones (facies 5 and 6) due to scarce autochthonous aquatic organic matter deposition and pervasive intense bacterial degradation, caused by persistent meromictic conditions. Large differences in the degree and path of evolution of the humic material derived from higher land plants have been detected, depending on the depositional conditions. The humic material in the oxic facies underwent humification and gelification and had the highest huminite reflectances, whereas it showed suppressed reflectance values in the oil shales (facies 4) and in the carbonate-dominated facies. In the latter, extensive bacterial reworking of humic material is also observed. These differences are considered significant and applicable to other case studies in terms of evaluating the maturity of organic matter in ancient lacustrine systems.

Influence of petrographic and mineral matter composition of coal particles on their combustion reactivity

Combustion at programmed temperature in a thermobalance is a rapid technique, which monitors coal burning characteristics and has shown its utility to classify coals according to their combustion performance. However, combustion profiles are affected by different coal properties and characteristics such as particle size, rank, maceral composition and mineral matter content, whose separate effects are difficult to determine. The objective of this work was to ascertain the influence of coaly and mineral matter composition and distribution on burning profiles as determined by thermogravimetric analysis, by using coals of different rank, and fractions of these coals obtained by density separation. Five coals ranging in rank from lignite to anthracite and with variable mineral matter content and composition were used in this study. Density fractions were separated from each coal to obtain samples with different organic/mineral matter proportions. Some of the factors influencing coal combustion profiles are widely recognised as the negative effect of increasing both rank and inertinite content on the reactivity. The favorable effect of mineral matter content on the reactivity has shown to be related to the maceral size within the density fractions and the intimate association organic/mineral matter that favors the diffusion of the reacting gas. Catalytic effects of the mineral matter could not be demonstrated.

A reactivity study of chars obtained at different temperatures in relation to their petrographic characteristics

Abstract This study reports on the reactivity of chars obtained at 1000°C and 1300°C (within the range of temperatures reached by coal particles in the near-burner zone of pulverised fuel boilers) from three different coals. The coals were selected according to petrographic criteria: two of them are high volatile bituminous coals differing in maceral composition and the third one is a vitrinite-rich low volatile bituminous coal. The morphology and optical texture of the chars were studied by optical microscopy. The kinetic parameters for the combustion of the high temperature chars under Regime I (combustion controlled by chemical kinetics) have been obtained and related to the optical texture and reflectance of the chars. The intrinsic reactivity of the high temperature chars was found to be lower than that of the low temperature chars, whereas the enhanced porosity observed in the high temperature chars had a positive effect on their combustion reactivity under Regime II (combustion controlled by oxygen pore diffusion). The intrinsic reactivities of the chars decreased following the sequence: vitrinite-rich low rank char>inertinite-rich char>vitrinite-rich high rank char. As the combustion temperature increases, the reactivity of the inertinite-rich char approaches that of the low rank vitrinite-rich char, which justifies the good performance observed for high volatile bituminous inertinite-rich coals in power plants.

Devolatilisation behaviour of petroleum coke under pulverised fuel combustion conditions

The combustion of petroleum coke in large scale facilities has been limited due to its high sulphur content, but the increasing installation of flue-gas desulphurisation units makes possible the firing of petroleum coke either as a primary fuel or blended with coals. This study focuses on the behaviour of three fuel-grade petroleum cokes of different provenance under pulverised fuel combustion conditions. These cokes, ground and sieved 125– 20 mm were fed to a drop tube reactor operating at 1300 8C under different atmospheres to produce chars with different combustion degrees. Char reactivity assessment was performed isothermally in a thermobalance at 550 8C and morphology and optical texture of the chars were studied by optical and scanning electron microscopy. Petroleum coke chars are composed of two main types of particles: (i) porous anisotropic particles that passed through a plastic stage and generated either cenospheric or network-like chars and (ii) angular particles with fine-mosaic optical texture that did not swell and show abundant contraction cleats. The relative proportions of both types of particles were very different in the three petroleum coke chars indicating significant differences in their devolatilisation patterns. The morphology and optical texture of the petroleum coke chars were related to their reactivity (as measured in a thermobalance) and to the characteristics (chemical composition and optical texture) of the parent petroleum cokes, in an attempt to understand the implications of their different devolatilisation behaviours on the combustion efficiency. q 2003 Elsevier Ltd. All rights reserved.

Preliminary results of extraction experiments in an oil shale

Abstract Soxhlet versus ultrasonic extraction efficiency on the immature Puertollano oil shale has been tested. The yields from the two methods are compared. The extracts were compared by gas chromatography and geochemical parameters such as CPI and pristine/phytane ratio calculated. The ultrasonic extraction procedure is an effective, quick and easy method which does not change the biomarker ratios of the bitumen. Minor efficiency differences can be observed among the solvents allowing a choice on the basis of handling properties. Carbon disulfide and chloroform are easiest to eliminate from the extract.

Pyrolytic behaviour of Spanish oil shales and their kerogens

Abstract A comparative study of the thermal behaviour of selected Spanish oil and their kerogen concentrates has been carried out. The oil shales differ in geological age, depositional environment, source location and degree of maturity. The influence of the amount and composition of mineral matter in the thermal behaviour of the oil shales and its influence on the amount of hydrocarbons released has been assessed. In mineral matter-rich samples, the mineral matter is responsible for a delay in the hydrocarbon generation during thermal heating and also retains part of the generated hydrocarbons, which might lead to erroneous kerogen typing when using thermal methods. The most immature samples are characterised by relatively higher proportion of oxygen-bearing functional groups which are more labile bonded to the kerogen and are released at lower temperatures during pyrolysis. Temperatures of initiation of kerogen cracking have shown to be more sensitive to reflect the maturity than temperatures of maximum rate of hydrocarbons release under the low heating rates used in this study. The most aliphatic and oxygen-rich kerogen have shown to yield the highest conversion to shale oil whereas the initially most aromatic kerogens are more prone to condense yielding higher coke amounts.