Diego Alvarez

Phase-mineral and chemical composition of coal fly ashes as a basis for their multicomponent utilization. 1. Characterization of feed coals and fly ashes

Abstract The phase-mineral and chemical composition of feed coals and their fly ashes (FAs) produced in four large Spanish thermo-electric power stations was characterized as a basis for multicomponent FA utilization. The feed fuels used are bituminous coals, semi-anthracites and anthracites with high detrital mineral abundance and mixed carbonate and sulphide–sulphate authigenic mineral tendency. Their mineral composition includes quartz, kaolinite, illite–muscovite, pyrite, chlorite, plagioclase, K-feldspar, gypsum, siderite, calcite, dolomite, marcasite, montmorillonite, jarosite, and ankerite. The FAs studied have aluminosilicate composition with higher concentrations of alkaline and alkaline-earth oxides than Fe oxide. Elements such as Ag, As, Ba, Cr, Cs, Li, P, Sb, Sc, Sn, Sr, Ti, V, Zn, and Zr are relatively enriched in these FAs in comparison with the respective mean values for bituminous coal ashes worldwide. The FAs consist basically of aluminosilicate glass, to a lesser extent of mineral matter (with high silicate abundance and dominant oxide tendency) and moderate char occurrence. The phase-mineral composition (in decreasing order of significance) of these FAs is normally glass, mullite, quartz, char, kaolinite–metakaolinite, hematite, cristobalite, plagioclase, K-feldspar, melilite, anhydrite, wollastonite, magnetite and corundum plus 42 important accessory minerals or phases. A scheme of conventional separation procedures was applied to recover sequentially six initial and potentially useful and/or hazardous products from FAs, namely: (1) a ceramic cenosphere concentrate; (2) a water-soluble salt concentrate; (3) a magnetic concentrate; (4) a char concentrate; (5) a heavy concentrate; and finally (6) an improved FA residue.

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.

SURFACE AREA AND PORE SIZE CHANGES DURING SINTERING OF CALCIUM OXIDE PARTICLES

Changes in surface area and pore size during the sintering of CaO samples between 0·71 to 1 mm, were studied. Typical operating conditions in fluidized bed combustors (temperature between 973 and 1173  K and CO2 concentrations up to 15%) were chosen. The longest sintering time was 90min. The decrease in the surface area with sintering time was described using an empirical equation deduced from the German-Munir model. Surface area reduction and increase in pore size was strongly affected by the presence of CO2. Shrinkage of the CaO particles was not observed during sintering. The pore volume and porosity remain unaffected by sintering. A qualitative model, based on SEM microphotographs, for describing the sintering process was proposed.

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.

In-Situ Capture of CO2 in a Fluidized Bed Combustor

Increasing atmospheric concentration of CO2 and concern over its effect on climate is a powerful driving force for the development of new advanced energy cycles incorporating CO2 capture. This paper investigates the feasibility of CO2 capture using the carbonation reaction of CaO “in situ” in a fluidised bed combustor, where natural gas or petroleum coke (or any other fuel with low ash content) is being burned. The sorbent can be partially regenerated for CO2 capture by combustion of part of the fuel with O2 /CO2 in a separate FBC. The thermodynamic limits in the proposed cycles, in terms of CO2 capture efficiencies, are examined along with the limits imposed by the rapid decay in the sorbent activity during repeated carbonation/calcination cycles, which will be exacerbated by the presence of S. Despite these limitations, it is shown that operating windows exist where it is possible to integrate fuel combustion, CO2 and SO2 capture in a single dual reactor facility. The decay in activity in the sorbent appears to be the major practical limitation to this concept, but this can be compensated for by using a relatively large supply of fresh sorbent, which appears to be acceptable considering the low cost of limestone. Also, a novel concept to reactivate the spent sorbent using sonic energy is outlined here as an alternative to reduce the use of fresh limestone.Copyright

Microscopic examination of coals and carryover from wet and preheated carbonization

Abstract A blend of two Spanish coals was carbonized on a semi-industrial scale using wet and preheated charging. The wet and preheated coal blends were characterized by optical microscopy, and structural changes due to the preheating process i.e. development of porosity, fissuring, etc., were observed. Preheater fines and material carried over in the charging gases from the wet and preheated coal blend were characterized by proximate analysis and optical microscopy. Carryover material was characterized optically according to the ICCP char classification system. Preheating not only produced an increase in the amount of carryover but also modified its structural features. Preheating also resulted in an increase of anisotropic material in the carryover particles.