Char Types Research Articles

Characterization of coals for combustion using petrographic analysis: a review

Petrographic analysis can give useful information about the combustion of pulverized coal. However, the relations between maceral types and reactivity are not simple; for example, not all inertinites are inert, and not all vitrinites are reactive. This is a consequence of the formal definition of macerals and the variations between coals from different parts of the world. The reflectance values of the maceral groups and the associations of the different macerals, determined as microlithotypes, also need to be taken into account. Mineral matter also has an effect on the type of char formed, and the situation is further complicated by the different char characterization schemes in the literature. The main conclusion from this review is that any predictive system needs to take all these factors into account, especially reflectance.

Reactive (fusible) inertinite in pulverized fuel combustion: 2. Determination of reactive (fusible) inertinite

A laser microreactor was used to study six coals to determine the proportion of their inertinite macerals that fused or melted under p.f. combustion conditions, i.e. their percentage of reactive (fusible) inertinite. The method pyrolyses individual monomaceral particles, giving one-to-one correlation between the maceral and the char formed. Photomicrographs of macerals and their chars revealed the wide diversity of char types that derive from inertinite. Classifying these into fused and unfused chars showed that the inertinite divided into fusible and infusible macerals at a particular reflectance value for each coal. Calculations using the whole coal reflectogram gave the percentage of the inertinite that was reactive. This averaged 75% for four Australian coals and 51% for two Laurasian coals, both much larger than previous methods had indicated. Predictions from earlier methods indicated that either the heating rate or the reaction temperature, or both, affect the fusible-infusible boundary and hence the proportion of the inertinite that exhibits reactive (fusible) behaviour.

Fluoride sorption characteristics of different grades of bone charcoal, based on batch tests.

Although bone charcoal (char) has been recommended for use in the de-fluoridation of drinking water in developing countries, parameters relating to fluoride (F) sorption characteristics by grade or type of bone char are unclear. Based on batch tests, the rate and capacity of F uptake were highest with black grade, followed by grey grade and lowest with white grade. Twenty-four-hour contact-time F sorption isotherms indicated saturation at capacities of 11.4 mg F per g of black grade, 2.4 mg F per g of grey grade, and less than 0.3 mg F per g of white grade bone char. Additional investigations showed that F-removal efficiency of black grade char decreased steadily with increasing F concentration. Its initial sorption kinetics were dependent on particle size, and the highest sorption rate was observed with particle sizes below 0.42 mm. For maximum sorption rate, the critical mixing time was between five and 15 min. The presence of chloride anions and elevated temperatures increased the rate of F uptake by black bone-char. The pH of distilled water following contact with bone char rose to 11.5 for white bone-char, 8.3 for grey bone-char, and 7.8 for black bone-char. The effects of processing temperature on the availability of intra- and extra-apatitic F-binding sites, levels of oxides of calcium and magnesium, and the carbon content were viewed as possible causes of observed variations. These findings suggest that the black grade of bone char was the most efficacious for use in partial de-fluoridation of drinking water.

Relation between reactivity and structure for coals and chars

Causal relationships were determined for the chemical and physical properties and reaction characteristics of three coals and two types of chars derived from these coals. The reactivities of the virgin coals and extract chars correlated well with the fuel rank. The higher proximate volatiles contents in these prepared fuels resulted in reactivities higher than those measured for the low-volatile pyrolysis chars. The reactivities of the pyrolysis chars were strongly correlated with fuel properties, whose relative order of importance was: catalytic elements > porosity > hydrogen types > cluster size.

A char morphology system with applications to coal combustion

A char morphology system is outlined, based on the physical and optical characteristics known to influence the burning of pulverized coal. Application of the system to high temperature char has established links between coal microlithotype predecessors and char types for subbituminous and bituminous coals. Quantities of high density and thick-walled char produced by pyrolysis at 1500 °C correlate with quantities of high density char in combustion residue, and with unburnt carbon content in fly ash for seven Australian energy coals. The proportions of different pyrolysis char types produced by any coal are shown to depend on coal rank, petrographic composition, maceral fusibility, and possibly ash content and composition, Quantities of high density chars in combustion residue can also be related to petrographic properties, including infusible inertinite content and percentage of microlithotypes of high inertinite content. Vitrinite reflectance is found to be a good parameter to differentiate burnout performance of coals with significant differences in rank. By comparing the proportions of high density, thick-walled pyrolysis char that form at 1500 °C, an estimate of burnout behaviour based on the combustion characteristics of char is obtained. Study of char types remaining after combustion also indicates potential for improvement of burnout performance, based on knowledge of the nature and origin of the unburnt carbon particles.

Entrained flow reactor and image analysis study of maceral effects in coal-char oxidation

Coal char contains particles of varying morphology reflecting their maceral origin. Optical image analysis has been used to measure quantitatively the changes in morphology of three types of char after various extents of burnout. The oxidations were carried out under well-defined conditions of temperature, oxygen concentration and residence time in an entrained flow reactor. Surface oxidation rates were found to be independent of particle diameter (35–100 μm) and proportional to oxygen concentration. After oxidation of a low rank bituminous coal at 1323 K and 10% O 2 all types of char shrank, became more porous and disappeared. The relative oxidation rates corrected to a particle temperature of 1350 K of chars derived from vitrinite, low-reflectance inertinite and high-reflectance inertinite were in the ratio 1:0.8:0.5. For a medium rank coal the differences were less pronounced. This work, taken with other results, shows that maceral composition is an influence on the reactivity of coal but that other factors, including rank and ash composition, are at least as important. Measurements of the volume to surface ratio of the large pores of char particles of various sizes indicate that the particles are geometrically similar. This implies that the pore structure of the char depends on the size of the coal particle pyrolysed to produce it and this needs to be taken into account in modelling char oxidation.

On the role of the boudouard reaction in the isothermal reduction of iron ore by char and graphite

The kinetics of gasification of graphite and two types of char by carbon dioxide, as well as the kinetics of reduction of iron ore by these carbonaceous species were studied by thermogravimetry. Potassium carbonate was found to be more effective than sodium carbonate in catalysing both the Boudouard reaction and the reduction of iron ore. Although the activation energies of the reduction reactions were much lower than those of the corresponding gasification reactions, the reductants revealed the same order of activation energies in both types of reaction. It was shown that the Boudouard reaction played a predominant role in controlling the rate of the reduction reaction.

The influence of rank and maceral composition on ignition and char burnout of pulverized coal

Investigation of combustion characteristics for different ranks of coal was conducted in a one-dimensional vertical tube furnace without external heating. Though fuel ratio (fixed carbon content/volatile matter content) is generally believed a good index to determine combustibility,carbon burnout of various coals and fuel ratio didn't always show good correlation in this study. Various types of char were produced at the devolatilization zone of the tube furnace during combustion. Morphology of those chars depended on coal rank and maceral composition of individual coal particles. Relationship between combustion rate and morphology and properties of chars from different coals was discussed.

Microscopic investigation of unburnt char in fly ash

A microscopic method to characterize unburnt char particles in fly ash is proposed. Polished samples of fly ash in epoxy resin to which a few drops of blue pigment have been added are observed by incident light microscopy. The char particles are classified into four types (vesicular, dense, mixed and mineral-rich) and categorized into five volumetric size classes (>10 6, 10 6−10 5, 10 5−10 4, 10 4−10 3, <10 3μm 3) Percentages of individual types and classes are calculated and can be illustrated diagrammatically. To test the feasibility of the proposed method, eight fly ash samples were analysed. Reasonable relationships between the level of unburnt char, particle size and proportions of char types were recognized. When the total carbon content in a fly ash was high, the proportion of coarse classes was also high, and there was a broad inverse correlation between the total carbon and total percentage of the dense char. The proposed microscopic method may be useful in understanding various technical problems related to coal combustion in pulverized-fuel fired furnaces.

Characterization of rdf properties through high pressure differential scanning calorimetry

High Pressure Differential Scanning Calorimetry was used to study the thermal-analytical properties of refuse derived fuels. Under oxidizing conditions, thermograms exhibit four characteristic peaks ∼ 215°C, ∼ 325°C, ∼ 435°C and ∼ 475°C. By comparison with studies on newsprint and polyethylene the peaks are identified with the smoldering combustion of plastics, cellulosic materials and two types of char, respectively. Similarities and differences between thermograms of refuse derived fuels and coal samples are noted. The variability of thermal-analytical properties as a function of process parameters, sampling, and source was determined. Finally, data from pyrolytic studies using thermogravimetry as well as scanning calorimetry are presented.

Influence of char porosity on strengths of hot-briquettes

The effect of char porosity on the strength-porosity relation of coal/char hot-briquettes has been investigated. Six different char and coke samples are characterized with respect to pore-size and pore size distributions, using mercury porosimetry and SEM. An empirical correlation between the ultimate compressive strength ( U) and total porosity ( p) shows that Ryshkewitch's expression U = U oe − ap is obeyed. The porosity coefficient ‘a’ is sensitive to interfacial bonding and decreasing values are indicative of penetration of the binder phase into the pores of the chars. The ability of a binder phase to penetrate very small size (submicron) pores of certain types of char appears to be a contributing factor in the strength in the hot-briquetted composites.

Ignition characteristics of chars made in the G R & D coal-gasification and coal-to-liquids processes

Chars are obtained as by-products in the coal-gasification and coal-to-liquids processes currently in the pilot-plant stage at Garrett Research & Development Company, Inc. Ignition tests have been carried out on these chars using an ignitability test, i.e. dispersing the chars in oxygen and igniting the mixture in a hot tube. DTA—TGA determinations have also been used to study the static reactions of the chars in air. Both types of char, under roughly simulated pulverized-fuel-firing conditions probably with a significantly lower heating rate, ignited readily at temperatures equal to or lower than those required to ignite the parent coals. Partial oxidation of the char raised the ignition temperature. The heats of combustion of the chars are slightly higher than those of the parent coals and approach that of petroleum coke; the sulphur content per unit of heating value is slightly lower. They are porous materials with a particle-size distribution suitable for use as pulverized fuel. Enough volatile matter remains to give a bright flame after ignition.

Fusibility of ash from coals of the United States

A char morphology system is outlined, based on the physical and optical characteristics known to influence the burning of pulverized coal. Application of the system to high temperature char has established links between coal microlithotype predecessors and char types for subbituminous and bituminous coals. Quantities of high density and thick-walled char produced by pyrolysis at 1500 °C correlate with quantities of high density char in combustion residue, and with unburnt carbon content in fly ash for seven Australian energy coals. The proportions of different pyrolysis char types produced by any coal are shown to depend on coal rank, petrographic composition, maceral fusibility, and possibly ash content and composition, Quantities of high density chars in combustion residue can also be related to petrographic properties, including infusible inertinite content and percentage of microlithotypes of high inertinite content. Vitrinite reflectance is found to be a good parameter to differentiate burnout performance of coals with significant differences in rank. By comparing the proportions of high density, thick-walled pyrolysis char that form at 1500 °C, an estimate of burnout behaviour based on the combustion characteristics of char is obtained. Study of char types remaining after combustion also indicates potential for improvement of burnout performance, based on knowledge of the nature and origin of the unburnt carbon particles.