Char Density Research Articles

Relations of True Relative Density, Micro-Strength and Micro-Crystal Structure of Chars Produced by Different Rank Coals

From gas coals to anthracites, 16 different rank coals were carbonized at 3°C/min in crucible. Chars and cokes produced at different temperatures were analyzed by x-ray diffraction; their true relative density and micro-strength were also measured. The true relative density of all chars produced by different rank coals increases as temperature increases. As the coals volatile increase, cokes true relative density goes down linearly. Cokes produced by weakly caking coals have the lowest true relative density, but have almost the highest micro-strength with anthracites. X-ray diffraction showed chars produced by weakly-caking coal have the largest d002 and least Lc in all chars. Coke structure approaching graphic carbon has high true relative density and low micro-strength, while coke structure approaching non-graphic carbon has low true relative density and high micro-strength.

Size‐dependent resource limitation and foraging‐predation risk trade‐offs: growth and habitat use in young arctic char

Variation in growth and habitat use is closely connected to individual responses to habitat specific resource levels and predation risk. In three mountain lakes which differed in the density of young‐of‐the‐year (YOY) arctic char (Salvelinus alpinus), we studied the growth, diets and habitat use of YOY char in relation to resource levels. With two model approaches, we further examined the extent to which YOY and 1‐yr old char were resource limited and, if so, whether resource limitation was associated with habitat use and small char densities. Benthic prey dominated the diet of YOY char and YOY char habitat use was restricted to the near‐shore habitat in all lakes. In all lakes were chironomid densities higher in the near‐shore habitat than in the offshore benthic habitat whereas zooplankton densities were higher in the pelagic than in the near‐shore habitat. Growth of YOY char in all lakes was close to predicted maximum growth despite large variation in YOY densities between lakes. Model results suggested that density dependent resource limitation in YOY char is unlikely to occur despite restricted near‐shore habitat use. In contrast, strong density dependent resource limitation was predicted in 1‐yr old char with a restricted habitat use to the near‐shore habitat. Correspondingly, field data suggested that the habitat use of 1‐yr old char was density dependent as the use of the offshore habitat increased earlier in time and to a larger extent at high densities. As small individuals are vulnerable to predation but constrained by their low food processing capacity relative to their encounter capacity, we suggest that resource limitation in small individuals should be less pronounced and habitat use should mainly depend on predation risk. A trade‐off in habitat use between foraging gain and predation risk is therefore expected to be more likely for individuals, large enough to be resource limited but still small enough to be vulnerable to predation.

Variation of the pore structure of coal chars during gasification

The variation of the pore structure of several coal chars during gasification in air and carbon dioxide was studied by argon adsorption at 87 K and CO 2 adsorption at 273 K. It is found that the surface area and volume of the small pores (<10 Å) do not change with carbon conversion when the coal char is gasified in air, while those of the larger pores (10–20 Å, 20–50 Å, 50–2500 Å) increase with increase of carbon conversion. However in CO 2 gasification, all the pores in different size ranges increase in surface area and volume with increase of carbon conversion. Simultaneously, the reaction rate normalized by the surface area of the pores >10 Å for air gasification is constant over a wide range of conversion (>20%), while for CO 2 gasification similar results are obtained using the total surface area. However, in the early stages of gasification (<20%) the normalized reaction rate is much higher than that in the later stage of gasification, due to existence of more inaccessible pores in the beginning of gasification. The inaccessibility of the micropores to adsorption at low and ambient temperatures is confirmed by the measurement of the helium density of the coal chars. The random pore model can fit the experimental data well and the fitted structural parameters match those obtained by physical gas adsorption for coal chars without closed pores.

Density of softwood bark and softwood char: procedural calibration and measurement by water soaking and kerosene immersion method

Particle density plays an important role in the design and operation of thermoconversion reactors fed with bed of particles such as softwood bark (SB). Little information is available on the single particle density of SB and charcoal derived. As SB and softwood char (SC) particles are highly irregular in shape and size, conventional methods of particle density measurement cannot be applied. A method known as the water soaking and kerosene immersion procedure, used for density measurement of sugarcane bagasse, has been tested and adapted to the density measurement of individual particles of softwood (SW), SB and SC. The particle density has been determined to be 360, 482 and 299 kg m −3 for SW, SB and SC, respectively. The average particle density of a typical SB feedstock sample comprised of ca. 30% SW and 70% SB was calculated to be 438 kg m −3.

Coal char reactivity and structural evolution during combustion—Factors influencing blast furnace pulverized coal injection operation

A fresh char was prepared and reacted with oxygen under conditions similar to those prevailing in the raceway region of the blast furnace (BF) during pulverized coal injection (PCI), using a well-characterized drop-tube furnace (DTF). Char combustion under the present conditions was found to be controlled by the combination of pore diffusion and chemical reaction. Both the char density and size gradually decrease with burnoff, while the char surface area increases up to a burnoff of 40 to 50 pct due to the formation of a large amount of meso- and micropores, which were observed by high-resolution field-emission scanning electron microscopy (FESEM) and gas adsorption measurements. Despite the obvious increase in surface area, the char combustion reactivity decreases with burnoff. This is due to the loss of the intrinsic reactivity of char during combustion, as confirmed by fixed-bed (FB) measurements of fresh char and chars partly burnt in a DTF. The structural characterization by quantitative X-ray diffraction analysis (QXRDA) shows that the amorphous concentration (f am ) of the char decreases during combustion, while the aromaticity (f ar ) and the average crystallite size (L 002) of the char increase. The char becomes more ordered during combustion, which is in accordance with the observations made using high-resolution transmission electron microscopy (HRTEM). The char structural ordering observed was found to be responsible for the loss of char intrinsic reactivity during combustion. Based on the QXRDA, a char structure model has also been suggested to explain the char structural evolution observed during combustion. The implications of char structural evolution for char combustion during a PCI operation are also discussed.

Coupling thermal deactivation with oxidation for predicting the combustion of a solid fuel

A thermal annealing model has been coupled with a detailed single particle model for char oxidation to predict the burning of solid fuels in combustion facilities. The combined model is tested against reported laminar flow reactor burning profiles of several coal chars at the intermediate stages of combustion and for a single coal char also at the later stages of combustion. The model shows for all coals a transition from combustion controlled by oxygen bulk diffusion at low char conversions to kinetically controlled combustion at high conversions. The impact of annealing on char reactivity is strongest at the early stages of combustion, and when the simultaneous annealing of the char is not accounted for, shorter burning times are obtained. In many cases, the experimental burning profiles of the coal chars can only be explained by taking into account the presence of fragments being released at the periphery of the particle. This type of fragmentation appears to be the prime means of particle shrinking and is most accentuated at higher oxygen concentrations in the reactor. The model was able to predict reasonably well the unburned carbon fraction of a single coal char at the latest stages of combustion (up to ∼98%). The model relies on reasonable estimates of the temperature and oxygen profiles in the reactor, as well as the char density, particle size, porosity and initial reactivity (e.g., at room temperature) of the non-annealed fuel. The latter is particularly important for determining burnout at the final stages of combustion. This can be determined by simply combining the char’s reactivity (extracted from a single TGA experiment of the fuel involving pyrolysis at high temperature and subsequent char oxidation), with the proposed annealing model.

00/02316 Empirical relationships for the determination of true density of coal chars

00/02316 Empirical relationships for the determination of true density of coal chars

Empirical relationships for the determination of true density of coal chars

Derived and discussed are empirical relationships that describe dependencies of the true density and yield of coal chars on carbonization temperature in the 550–1000°C range. They include formulae for the determination of mass and true density changes with temperature for both organic matter and mineral matter in the coal chars. The relationships are based on results of investigations of laboratory-scale made coal chars under comparable conditions to industrial-scale carbonization processes and, therefore, should also be valid for coke products. Additionally, technological parameters such as a bulk density of coal blend and specifics of grinding process are found not to influence significantly the true density and yield of the coal chars.

Thermal treatment and foaming of chars obtained from almond shells: kinetic study

The influence of a combined thermal treatment on the density of chars obtained from almond shells has been investigated. The thermal treatment involves two steps at different heating rates: the first one up to temperatures of 225–400°C; and the second one up to 850°C. Thermogravimetric experiments have been carried out with the intermediate chars obtained after treatment revealing the evolution of the different fractions of the almond shells (i.e. hemicellulose, cellulose and lignin), and their influence on the final density of the chars. An intermediate treatment, where the gases evolved from the last remaining fractions of the cellulose coincide with the char in a fluid or soft state, is required to foam the particles. A kinetic model has been applied to quantify, with excellent results, the evolution of the different fractions.

Adsorptive and Optical Properties of Fly Ash from Coal and Petroleum Coke Co-firing

Eight commercial fly ash samples including two from petroleum coke/coal co-firing were characterized to determine the origin, structure, and properties of the unburned carbon. Because of its relevance to ash utilization, the characterization included measurements of adsorptivity toward concrete surfactants, and its dependence on carbon form, particle size, and contact time. The coal/coke co-firing ashes are observed to possess unique optical and adsorptive properties which are related to the size, density, and internal porosity of the coke-derived residual char.

00/00659 Crystallinity, true density and intrinsic reactivity of intermediate temperature coal char

The true density and crystallinity of char samples of a kind of bituminous vitrinite were measured and studied These char samples were obtained at various temperature and residence time The true density and crystallinity of the residual carbon in the fly ash from a stoker boiler were also investigated The results showed that the crystallinity of coal char samples generated at intermediate temperature increases with an increase in temperature and residence time It also showed that there is a certain relationship between crystallinity, true density and the intrinsic reactivity of coal chars

A friction model for iron powder pressing: B.Wikman et al. (Lulea University of Technology, Sweden.)

Maceral concentrates, extracted by selective sampling from the coal face, were burnt as pulverised fuel in order to obtain the temperatures of many hundred individual particles of pulverised coal at several combustion times by two-colour pyrometry.The concentrate having a 93.7% vitrinite-rich analysis generates chars of fine walled cenospheres, with openings in the walls, which burn at high temperature, whereas the concentrate with a 89.2% intertinite-rich analysis generates chars of lower porosity and higher density which burn at a lower, and more uniform, temperature. The vitrinite-rich concentrate also burns with an increasing spread in the population of particle temperatures, the inertinite-rich concentrate burns with a decrease in this spread. Due to higher char densities and lower reactivities the burning time of the inertinite-rich concentrate is estimated to be from 2.9 to 3.8 times that of the vitrinite-rich concentrate.Reactivity distributions are estimated from the temperature populations by fixing the activation energy of the chemical rate coefficient and allowing the pre-exponential factor to accommodate the reactivity spread. An increase in the spread of the estimated distributions is evident at high burnoffs.

Influence of initial particle density on the reaction mode of porous carbon particles

In combustion of porous carbon particles, the relation between particle density and diameter: σ σ 0 = ( d d 0 ) α ; was obtained in an earlier analysis [1]. This present work extends that earlier analysis to show with experimental support that (1) the power index, α, varies with the initial char density, σ 0, according to α = α 0(1 − σ 0 σ s ), where σ s is the He density, and (2) that the limit value of α at zero density, α 0, is inversely proportional to the square root of the velocity constant of the reaction. Available experimental data values for α 0 yielded an effective value of activation energy, E, of 23 kcal/mol, and 46 kcal/mol after correction for Zone II behavior in agreement with earlier analysis of the same data by a different method [4]. For values of α, supporting the linear decline of α with σ 0, the experimental data base was a set of 37 values obtained from 15 literature sources. The set showed a surprisingly small range in α values, from 0 to 7, and with most in the range 1–3, in spite of the different methods of experiment, char types, origins, particle sizes, and reaction temperatures. This suggests that, while Zone II reaction may be common, the depth of penetration of the reaction in Zone II combustion may be quite shallow. It also supports the conclusion that intrinsic reactivity from different char sources may be very similar and that differences in combustion rate may be determined less by chemical reactivity differences and more by physical access to the surface and interior of the particle, with access to the interior largely determined by initial char density, σ 0, at the start of char combustion.

The gasification of wood-char spheres in CO 2N 2 mixtures: analysis and experiments

The gasification of charcoal spheres in an atmosphere of carbon-dioxide—nitrogen mixtures involving diffusion and reactions in the pores is modelled and the results are compared with experiments of Standish and Tanjung and those performed in the laboratory on wood-char spheres to determine the effects of diameter, density, gas composition and flow. The results indicate that the conversion time, t c ∼ d 1.03 for large particles (5 mm), departing substantially from the t c ∼ d 2 law valid for diffusion limited conditions. The computational studies indicate that the kinetic limit for the particle is below 100 μmm. The conversion time varies inversely as the initial char density as expected in the model. Predictions from the model show that there is no significant change in conversion time up to 60% N 2 consistent with the CO 2N 2 experiments. The variation of diameter and density with time are predicted. The peculiar dependence of conversion time on flow velocity in the experiments is sought to be explained by opposing free and forced convection heat transfer and the attempt is only partly successful. The studies also indicate that the dependence on the CO concentration with low CO 2 is significant, indicating the need for multistep reaction mechanism against the generally accepted single-step reaction.

The temperature, burning rates and char character of pulverised coal particles prepared from maceral concentrates

Maceral concentrates, extracted by selective sampling from the coal face, were burnt as pulverised fuel in order to obtain the temperatures of many hundred individual particles of pulverised coal at several combustion times by two-colour pyrometry. The concentrate having a 93.7% vitrinite-rich analysis generates chars of fine walled cenospheres, with openings in the walls, which burn at high temperature, whereas the concentrate with a 89.2% intertinite-rich analysis generates chars of lower porosity and higher density which burn at a lower, and more uniform, temperature. The vitrinite-rich concentrate also burns with an increasing spread in the population of particle temperatures, the inertinite-rich concentrate burns with a decrease in this spread. Due to higher char densities and lower reactivities the burning time of the inertinite-rich concentrate is estimated to be from 2.9 to 3.8 times that of the vitrinite-rich concentrate. Reactivity distributions are estimated from the temperature populations by fixing the activation energy of the chemical rate coefficient and allowing the pre-exponential factor to accommodate the reactivity spread. An increase in the spread of the estimated distributions is evident at high burnoffs.

On the products of the heterogeneous oxidation reaction at the surfaces of burning coal char particles

The combustion of the char particles of two bituminous coals were investigated in a laminar flow reactor having gaseous environments containing 6 and 12 mole-% oxygen in the temperature range 1550 to 1700 K. Measurements of the size, temperature and velocity of individual particles were made at selected heights above the reactor inlet and hence, at different extents of burnoff. Samples of partially reacted chars, extracted from the reactor at these same heights, were analyzed in order to determine the fraction of the initial mass remaining, m/mo, and the char density. The data were used in the governing mass, momentum and energy balance equations in order to determine overall particle burning rates per unit external surface area. The burning rates determined indicate that account must be made of CO2 formation in the vicinity of the particle. Whether the CO2 is formed heterogeneously at the particle surface, or homogeneously in the particle's boundary layer cannot be established definitively. It is shown, however, that the single-film model of a burning carbon sphere with a fraction ψ of the carbon being converted to CO2 at the particle, surface adequately describes the burning behavior of coal char particles in the 75 to 125 μm size-range. Employing such a model, for the char particles of the bituminous coals investigated, as much as 15% of the carbon content of the particle can be converted to CO2 at temperatures in the range 1600 to 1700 K. At temperatures above 1800 K. CO is essentially the sole heterogeneous reaction product.

Retinal Structure of Sympatric and Allopatric Populations of Cutthroat Trout (Salmo clarki clarki) and Dolly Varden Char (Salvelinus malma) in Relation to their Spatial Distribution

Retinal structures of cutthroat trout (Salmo clarki clarki) and Dolly Varden char (Salvelinus malma) were compared to determine if these features were consistent with the different light regimes in which the species live, depending on their sympatry or allopatry in lakes. Cone cell density in sympatric trout was over twice that of sympatric char and rod cell density was slightly less. The cross-sectional size of cone cells in sympatric trout was approximately 30% less than in sympatric char. The ratios of photoreceptor cells to ganglion cells in sympatric trout and char retinas were approximately 40:1 and 80:1, respectively. There were no differences between sympatric and allopatric trout with respect to these structural features. Estimates of cone cell density, cone size, and number of photoreceptor cells per ganglion cell for allopatric char were significantly different and intermediate between those for sympatric trout and char. All four populations had similar cone cell mosaics with four paired cones surrounding each single cone. Results are considered in relation to differences in vertical distribution and feeding of sympatric and allopatric populations in three study lakes.

Heat Of Gasification For Pyrolysis Of Charring Materials

The relationships between the heats of reaction, pyrolysis and gasification are examined for vaporizing and charring materials. During burning of thermally thick, vaporizing materials the heat of gasification hg E qiet/m biW is equal to the heat of pyrolysis hp. However, no simple relationship exists between h and hp during the burning of charring materials. Solutions for pyrolysis of cellufose are obtained using a one dimensional, numerical model with material properties estimated from the literature. For this material it is found that after an initial transient hg is about twice h For a semi-infinite slab the value of h g then remains nearly constant; for a finite slab t%e value of h may decrease with time to values less than hp. Changes in the assumed chemical reaction rate are found to have a minor effect on hg. The heat of gasification is also relatively insensitive to changes in the char density and thermal conductivity. The variation of h with time is nearly independent of the assumed heat of reaction, but the magnitude of kg varies almost directly with the value of the heat of reaction. Changes in the incident heat flux give corresponding changes in the gasification rate, but much smaller changes in the heat of gasification.

Reactivity and combustion of coal chars

Seven industrial process chars were tested to determine their reactivities and/or combustion characteristics. Char densities, total surface areas, and elemental and proximate compositions were measured. Reactivity tests were conducted over three temperature ranges: thermogravimetric analysis (TGA) with air (550–750 K), with CO 2 in a fixed-bed (1070–1270 K) and with air in a drop-tube furnace (1300–2100 K). In TGA tests, reactivities among chars differed by almost two orders of magnitude at the same temperature. Fixed-bed CO 2 tests gave mass reactivities that differed, by about one order of magnitude, while mass reactivities at high temperature in the drop-tube furnace differed by only a factor of two. Parent coal type and char formation conditions are major factors influencing reactivity in low and intermediate temperature ranges. N 2 -BET surface area does not adequately explain observed reactivity in any temperature range while variation in external surface area does correlate reactivity at high temperature. High temperature reactivities compared well with predictions from a pore tree/char oxidation model, after adding char volatiles percentage. Only high temperature reactivity data characterized the char consumption rate in a laboratory combustor, but a significant residual char volatiles content appeared to be essential for flame stability.

Effect of charring temperature on observed L/S ratio

Amniotic fluid lecithin (L) sphingomyelin (S) ratio {L/S) is commonly determined by thin layer chromatography (TLC), acid spray. and high temperature charring, after Cluck who initially employed a quantitative method for L and S using transmission densitometry [l]. Although Cluck et al found that the ‘observed L/S of 2: 1 by reflectance de~sitometry actually represented a ‘ratio of concentration’ or actual L/S of 4: 1 [Z], control of conditions and thorough evaluation resulted in clinically meaningful data. An ‘observed’ ratio of 2 or more was found to represent maturity of the fetal surfactant system. Cluck did not comment on the prior observation that charring be carried out at lower temperatures, or the recommendation that reference standards be used whenever available f 1, op tit 3, 4] but chose to char at 280°C and ‘avoid standard curves, measurement constants, or correction factors’. Sarkozi et al [5] noted that when lipid concentrations were plotted against char density, the slopes of lecithin and sphingomyelin were different and both varied from plate to plate. Curiously, the plot of observed L/S versus the ‘ratio of concentration’ is quite linear. The observation that the observed L/S of 2: 1 reflected a ‘ratio of concentration’ of nearly 6: 1, compared to Gluck’s 4: 1, went without comment. Fainter [6] charred at less than 100°C I?]+ and obtained agreement between observed L/S and the ‘ratio of concentrations’ of standards. This study indicated an expected amniotic fluid L/S of 2 at 35 weeks gestation, and the absence of respiratory distress syndrome (RDS) if the L/S was greater than 1.5 in normal deliveries, or greater than 2 in deliveries by cesearean section. Thus, some report agreement between observed and actual L/S of standards (61, some do not [2,5], while others recommend that standards not be used for quantitation at all [g]. In addition, other phospholipids may interfere [9] because of migration with L and S. Yet, many agree that an L/S of 2 or greater indicates fetal lung maturity. Evaluation of several L/S methods in our laboratory indicated poor ~thin-run reproducibility, and charring conditions were suspected. It was desired to determine the effect of charring temperature on observed L/S.