Char Particles Research Articles

Understanding fly-ash formation during fluidized-bed gasification of high-silicon-aluminum coal based on its characteristics

Investigations on fly-ash formation in fluidized-bed gasification are important in mitigating ash-related problems and exploiting its further usage. In this study, the characteristics of ash fusion, size distribution, and the elemental composition of fly ash from the fluidized-bed gasification of high-silicon–aluminum coals were examined, and its formation process during gasification was explored. The ash fusion temperatures of the fly ashes were lower than those of the corresponding raw coal. Although the mean particle size of fly ashes from Jincheng anthracite is smaller than that from Lu'an bitumite, they both have a two-peak distribution. The carbon content and elemental distribution in the two fly ashes vary obviously because of the differences in maceral distribution and mineral composition of original coal. For high-silicon-aluminum coal, fly-ash formation occurred through the char gasification of a shrinking nucleus, the agglomeration of some fine particles into large particles by sintering and collision, and the entry of char particles into a cyclone separator that is entrained by syngas.

Evolution of pore structure and produced gases of Zhundong coal particle during gasification in supercritical water

Supercritical water gasification technology provides a promising way for the efficient and clean utilization of high-sodium Zhundong coal. Pore structure evolution rules of char in the gasification process can provide important information for the heat and mass transfer in char particles and the optimization of kinetic models. In this work, supercritical water gasification experiments of Zhundong coal were conducted and the increase of temperature was found to promote the hydrogen production and gas yield increasing. Residual chars were characterized using techniques of nitrogen adsorption at 77 K, mercury intrusion porosimetry and micro-computed tomography. The results indicated that pores mainly appeared in two regions of 1.7–2 nm and 600–2000 nm. The specific surface area showed an increasing trend in the gasification process with micropores dominating the total surface area. The increase of temperature promoted the pore volume development, and a good linear relationship between pore volume and gas production was found.

The influence of CO2 gas concentration on the char temperature and conversion during oxy-fuel combustion in a fluidized bed

In spite of the extensive theoretical and experimental work carried out on coal/char oxy-combustion in a fluidized bed (FB), the effect of changing the atmosphere from O2/N2 to O2/CO2 for a high O2 concentrations is not entirely understood. In this work, experiments with single char particles are conducted in a bi-dimensional FB at 800 and 850 °C, varying the O2 concentration from 11 to 50%v/v in N2 or CO2. The FB reactor has a quartz window for visual observation, allowing the measurement of temperature and tracking the char conversion process by pyrometry with a digital camera. The method is shown to overcome the inherent limitations of other methods used in FB, such as thermocouples or pyrometry with an optical probe. Results indicate that the transfer of O2 from the bulk gas of the bed to the surface of a char particle controls the overall rate of char conversion in O2/N2 and in O2/CO2. In the latter gas mixture, the carbon consumption by gasification is significant even at a relatively low char temperature (850 °C). This additional carbon consumption makes the apparent char consumption rate in both atmospheres roughly equal (at the same O2 concentration) for char temperatures below 925 °C, and higher in O2/CO2 than in O2/N2 for char temperatures above 925 °C. Moreover, during the time in which the char stays in the emulsion phase, its temperature is roughly the same in both atmospheres, but when the char is in the bubble or splash zone its temperature is much higher than that in the emulsion phase. As a result, the difference in char conversion rate, observed in both atmospheres, is mainly controlled by the time in which the char particle is out of the emulsion phase. These results underline the importance of paying attention to the movement of a char particle through the different phases of the bed in order to improve the understanding of the oxy-fuel behavior in FB.

Determination of coal gasification kinetics from integral drop tube furnace experiments with steam and CO2

Lab scale drop tube furnace reactors with conditions similar to large-scale gasification reactors are able to generate applicable reaction-kinetic expressions for reactor simulation and dimensioning. In the present work, a method is introduced to determine reaction kinetic parameters from integral drop tube furnace experiments. A one-dimensional model of the gasification process was developed incorporating the influence of reaction gas composition, char particle structure change, pore diffusion and gas phase reaction. Suitable kinetic parameters were determined by fitting the gasification model to experimental data. Gasification experiments with gas mixtures of steam, carbon dioxide, carbon monoxide and hydrogen were carried out in an isothermal pressurized drop tube furnace reactor at 6 bar total pressure and temperatures of 900–1100 °C. It was shown that the determined kinetic expressions fit the experimental data. The best fit to the experiments was achieved by application of a Langmuir-Hinshelwood type rate expression with respect to product gas inhibition. Integrating a pore diffusion model did not improve the fitting quality.

Evolution of surface functional groups of coal char during coal combustion under O2/CO2 atmospheres

AbstractIn this study, quantitative analysis was conducted to study the changes in characteristics of surface functional groups of coal char under O2/CO2 atmospheres using Fourier transform infrared analysis. A drop tube furnace was explored. Besides, the effect of the oxygen‐containing functional group on the generation characteristics of NO emission was also investigated. The results showed that CH2/CH3 is highly influenced by the process and atmosphere of coal combustion. The burning of coal increases the contents of aromatic hydrogen and aromaticity on the surface of coal char particles, and it decreases the content of lipid hydrogen. The oxygen concentration exhibits a significant effect on the oxygen‐containing functional groups in coal char, which promotes the reduction reaction of NO. There is an optimum oxygen concentration of 30% that creates the best NO reduction effect caused by the oxygen‐containing functional groups.

Morphological Evolution of a Single Char Particle with a Low Ash Fusion Temperature during the Whole Gasification Process

Morphological evolution of single char particle with low ash fusion temperature (Tf ) was investigated during the whole gasification process using high temperature stage microscope. The experimental results showed that the shrinkage ratio of char particles above the Tf (1300 °C) was higher than that below the deformation temperature (1000 °C) finally. The TEM and SEM analysis results showed that the ash was a dispersed distribution in coal char. Dispersed molten slag aggregated to molten slag layer during the gasification process at the temperature above Tf, which evidently hindered the diffusion of gasifying agent. Therefore, there was a critical shrinkage ratio during the gasification process when the temperature above the Tf, and the critical shrinkage ratio of XLT char particles was 0.7-0.8. The SEM-EDS analysis results indicated that the critical points in the shrinkage rate curve were related to the molten slag on char surface. And the reaction mechanism was analyzed, the calculating critical thickn...

Filtration Performance of Coal Pyrolysis Flying Char Particles in a Granular Bed Filter

The filtration of flying char particles from coal pyrolysis vapors plays a very important role in enhancing yields and quality of pyrolysis oil. In this work, the performance of coal pyrolysis flying char particles in a granular bed filter (GBF) was studied in cold model experiments. A filtration model was developed using a macroscopic phenomenological method that describes the filtration of the GBF. The polynomial expression of the relative filter coefficient (F) and the nonlinear expression of the relative pressure drop ratio (G) were applied in the new model. The unsteady state of granular filtration was captured, demonstrating that the GBF performance could be predicted by the new model. Effects of superficial gas velocity, thickness of granular layer, and dust mass concentration on collection efficiency and pressure drop were analyzed. An excellent performance of the GBF was obtained and the total collection efficiency could reach a span between 98% and 99.9%. In the case of lower dust mass concentra...

Bone char effects on soil: sequential fractionations and XANES spectroscopy

Abstract. The acceptability of novel bone char fertilizers depends on their P release, but reactions at bone char surfaces and impacts on soil P speciation are insufficiently known. By using sequential fractionation and synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy we investigated whether and how the chemical composition of bone char particles has been altered in soil and has consequently affected the P speciation of amended soils. Therefore, two different kinds of bone char particles (BC produced by the pyrolysis of degreased animal bone chips at 800 °C and BCplus, a BC enriched with reduced sulfur compounds) were manually separated from the soil at the end of two different experiments: incubation leaching and ryegrass cultivation. Sequential P fractionation of amended soils showed P enrichment in all fractions compared to the control. The most P increase between all treatments significantly occurred in the NaOH–P and resin-P fractions in response to BCplus application in both incubation-leaching and ryegrass cultivation experiments. This increase in the readily available P fraction in BCplus-treated soils was confirmed by linear combination fitting (LCF) analysis on P K-edge XANES spectra of BC particles and amended soils. The proportion of Ca hydroxyapatite decreased, whereas the proportion of CaHPO4 increased in BCplus particles after amended soils had been incubated and leached and cropped by ryegrass. Based on P XANES speciation as determined by LCF analysis, the proportion of inorganic Ca(H2PO4)2 increased in amended soils after BCplus application. These results indicate that soil amendment with BCplus particles leads to elevated P concentration and maintains more soluble P species than BC particles even after 230 days of ryegrass cultivation.

Devolatilization and Combustion of Coarse-Sized Coal Particles in Oxy–Fuel Conditions: Experimental and Modeling Studies

A generalized unsteady-state kinetic model, coupled with all modes of heat transfer, was developed to describe the combined coal devolatilization and the subsequent combustion of the residual char under oxy–fuel condition in both O2–CO2 and O2–N2 environments. Experiments were conducted to validate the model, which was also found to predict the experimental data published in the literature well. The effect of coal particle diameter, temperature of the reactor, and oxygen concentration on devolatilization time was investigated. Peaks in devolatilization and char combustion rates and particle center temperature were studied, and the effect of different parameters assessed. Higher reaction time was observed in an O2–CO2 environment compared to that in an O2–N2 environment due to lower particle temperatures resulting from endothermic gasification reaction and the difference in thermo-physical properties. Simulation studies were carried out to generate temperature, carbon, O2, CO, and CO2 contours to understan...

Numerical investigation of different effects of carbon dioxide properties and carbon monoxide oxidation on char particle combustion in actual and fictitious O2/CO2 environments

The overall and individual effects of carbon dioxide properties and carbon monoxide oxidation on char combustion were numerically simulated with a continuous-film model in different O2/CO2, O2/N2 and O2/Ar environments, and different CO2 physicochemical properties were artificially changed to distinguish the individual effect on char particle temperature and combustion rate. The results show that the char combustion rate in 21%O2/79%CO2 environment is slightly higher than that in air environment because of two opposite effects of higher char gasification reaction rate with high concentration CO2 and lower char oxidation rate with O2 resulting from lower particle temperature. At the same time, the CO flame front in 21%O2/79%CO2 environment is farther away from char particle surface than that in 21%O2/79%N2 environment because of lower diffusion coefficient of oxygen in CO2 environment although the gas temperature is lower. Furthermore, the net effect of molar heat capacity of CO2 on char combustion rate decreases and the net effect of char gasification reaction with CO2 on char combustion rate distinctly increases with the increase of ambient gas temperature and O2 mole concentration.

Modeling the Vaporization of Inorganic Matter from a Single Coal Char Particle Burning in an O2/CO2 Atmosphere

The present work aims to model ash vaporization from a burning char particle during pulverized coal combustion in an oxygen-enriched O2/CO2 atmosphere. The modeling development was to apply the classic ash vaporization model of Quann and Sarofim to cover oxy-coal combustion over a wide range of O2 concentrations by coupling it with the double-film char burning submodel to properly describe the burning process of a single char particle. The model was extensively validated with literature data on the vaporization of SiO2, MgO, and CaO in both O2/N2 and O2/CO2 atmospheres. The model was proved to be able to predict the vaporization behavior of SiO2 during char conversion stages in both atmospheres, covering the effects of coal type, particle size, and O2 concentration. The model predicts the vaporization of MgO and CaO during char combustion of bituminous coal, but improvement is required to describe the vaporization of MgO and CaO during char combustion of lower-rank coals by considering the occurrence of M...

An investigation of mineral distribution in coking and thermal coal chars as fuels for the direct carbon fuel cell

For the first time, a combination of coal grain analysis (CGA) and mineral liberation analysis (MLA) has been applied to coal chars prepared at different pyrolysis temperatures. Information collected from these powerful techniques is used here to assess electrochemical activity of coal chars in a solid anode of the direct carbon fuel cell. Mineral distribution and consideration of particle size gives very useful information in terms of showing differences between chars of differing origin, although major differences in mineral distribution between pyrolysis HHT are not observed here. As a result of this analysis, it is proposed that electrochemical performance of coal chars at solid anodes in molten carbonate appear to be dependent on several, interlinked factors. The physical properties of the char including its porosity, resistivity, surface area and crystalline structure have previously been demonstrated to affect electrochemical oxidation of carbons, and these factors are also observed to be important here. Further, however, the prevalence, size and type of mineral matter in the coal char and its contact with char particles also is suggested to play an equally important role. The mechanism of this interaction may include alteration of previously observed sensitivities, such as mineral enhanced gasification of the coal char leading to carbon morphology changes. Additional influences of mineral matter are suggested here to include modification of surface polarity and subsequent contact between carbon and molten carbonate electrolyte (effectively enhancing surface area), or the ability of mineral components to act as a shuttle for active species.

Catalytic activity of char produced from brown coal for steam-gasification of bitumen oil

ABSTRACTThere are several kinds of catalyst for tar reduction such as Ni-based catalysts, alkali catalysts, metal catalysts, and char. Char is a nonmetallic material which be generated by the devolatilization of organic materials. In this article, by using an experimental setup we tried to study the effect of char particles as a catalyst on the tar concentration and produced gas from steam gasification of bitumen oil and to introduce effective parameters in the process of syngas generation. With the increase of gasification temperature from 800 to 950°C, the hydrogen increased sharply from 28.4 vol% to 34.9 vol% and 18.5 vol% to 21.3 vol%, while CO decreased from 12.5 vol% to 14.9 vol% and 14.8 vol% to 18.1 vol% for gasification with and without char, respectively. As a result, it was found that the gas produced is not sensitive to the pressure changes.

Benzene Conversion in a Packed Bed Loaded with Biomass Char Particles

This study investigates the conversion of benzene in a packed bed containing fine char particles. Benzene and steam were simultaneously supplied to a tubular ceramic reactor that was heated electrically. Fragmented char particles were suspended and continuously supplied via a separate supply line. A packed bed of crushed alumina balls was positioned in the reactor to retain the char particles. The benzene conversion in the hot char bed was investigated by varying the bed temperature (900–1100 °C), steam concentration (0–27 vol %), and char concentration (5–50 g Nm–3). The highest conversions achieved in the experiments were approximately 75%. At comparable char concentrations, similar benzene conversions occurred at 900 and 1000 °C. Increasing the temperature to 1100 °C or increasing the steam concentration reduced the benzene conversion. The results indicate that the reduced conversion was due to enhanced char gasification reactions at elevated temperatures and steam concentrations and thus to reduced ch...

Experimental study on combustion characteristics of pulverized coal preheated in a circulating fluidized bed

A new technique to preheat pulverized coal in a circulating fluidized bed was adopted. This technique consists of two stages: the pulverized coal is first self-preheated in a circulating fluidized bed, and then the preheated fuel gas and char particles are burned out in a down-fired combustor under air-staging conditions. Experiments were conducted with two types of coals, two air ratios in the circulating fluidized bed and different air distributions in the combustor. The preheating and combustion processes ran stably under all experimental conditions. For both types of coals, the preheating temperatures in the circulating fluidized bed could be adjusted broadly within 800–950°C, and the ratios of primary air could be lowered to 0.15. The combustion efficiency and nitrogen conversion were remarkably influenced by the air distribution, and were controlled above 98% and below 10% respectively, although the two targets could not be simultaneously achieved.

Steam gasification of solid recovered fuel char derived from landfill waste: A kinetic study

A kinetic study of steam gasification of char derived from solid recovered fuel (SRF) was performed. The char was prepared by using pyrolysis and using a nitrogen atmosphere at a temperature of 900 °C for 1 h. Gasification in a steam atmosphere was conducted for the char product using isothermal thermogravimetry analysis (TGA). The gasification was carried out at three different temperatures: 800, 850, and 900 °C for at least 1 h. The calculation of the kinetic parameters were done by considering the volumetric model, shrinking core model, and random pore model. The results show that the increase of the gasification temperature was linearly correlated to the rise of the reaction rate of the carbon conversion. Moreover, the activation energy of the char reaction is known to range between 58-83 kJ/mol. The kinetic parameters of char gasification were affected by their corresponding SRF component and the char particle size. Also, there were no significant different in the kinetic parameters calculated using the three different models as the difference between the results was less than 3.5 %.

Study on CO2 Gasification Reaction during the Combustion of Pulverized Coal Char Particle

Although much research on the effect of CO2 gasification on char combustion has been reported, few studies give clear information on the proportion of carbon consumed by the gasification and oxidation respectively. In this work, a drop-tube furnace is used to study the combustion of Huangling bituminous coal char in different oxygen and carbon dioxide concentrations at different residence times. It is found that with increasing CO2 content the carbon conversion ratio decreases first and then goes up in both oxygen content of 21 and 36 vol.%. There always exists a minimal point around CO2 content of 8-15 vol.% along with increased residence time during pulverized coal char combustion. Meanwhile, a preliminary formula is proposed to calculate the overall carbon conversion ratio of char and the proportion of carbon consumed by the gasification and oxidation respectively.

Effect of Char Particle Size on NO Release during Coal Char Combustion

In this study, the effect of particle size on the release of NO during the char combustion under particle packed-layer conditions was investigated at combustion temperatures of 900 °C~1400 °C. The results show that after pyrolysis, the small size char favored more pores and specific surface area. No significant variations of the intrinsic reactivity of O2 and NO with char particle size were measured. By increasing char particle size, more NO conversion from nitrogen in char (char-N) was found regardless of the combustion temperature and bulk oxygen concentration. This can be explained as follows. By increasing char particle size, less O2 penetrated into the pores and more NO formed because of a less accessible pore surface area and a decreased NO reduction time. The effects of reaction temperature and bulk oxygen concentration were also discussed. A quantified description using a first-order reaction model shows that the char particle size will finally influence the accessible pore surface area and depth ...

Char particle burning behavior: Experimental investigation of char structure evolution during pulverized fuel conversion

The present work provides experimental results on structural changes of burning char particles derived in an entrained flow reactor. Changes in the char particle diameter, apparent and true density, specific surface area, and porosity are studied at different residence times using a bituminous coal with a moderate ash content of 11.8wt%. Results show the significance of mineral matter on structural properties of samples extracted along the combustion. The apparent density of a sample composed of mineral and organic matter increases progressively with proceeding char conversion, whereas the isolated char density decreases. The mean particle diameter decreases during conversion, indicating regime II conditions, with pore diffusion being the rate-controlling step. The decrease in char density and diameter follow well-known power law expressions, where the exponents are shown to be dependent on the effectiveness factor, and, thus the reaction regime. However, an ideal burning behavior in regime III, where the diffusion of oxidant through the boundary layer is rate-controlling, is not observed. Instead, large openings in the char structure enable the penetration of oxidant into the particle interior, in particular for highly porous cenospherical char particles. Hence, it is recommended to consider large openings and voids in the char structure and their changes, as well as mineral matter effects in advanced char combustion models.

Particle Size Distributions of Fly Ash Arising from Vaporized Components of Coal Combustion: A Comparison of Theory and Experiment

A 100-kW-rated down-fired pilot-scale combustor was used to explore sub-micrometer coal ash aerosol formation for two coals under various air and oxy-combustion atmospheres. Particle size distribution (PSD) data were obtained through isokinetic sampling and then by electron mobility and light-scattering particle sizing. The sub-micrometer portion of the PSD exhibited an “accumulation” mode at ∼0.3 μm and, in some cases, an additional “nucleation” mode between 0.03 and 0.07 μm. Predictions of the temporal evolution of the sub-micrometer aerosol were made using a sectional coagulation model. A comparison to experimental measurements suggested that the “accumulation” mode was formed by coagulation of vaporized silicon-rich species, which occurred and was completed very close to the parent char particle and not in the mixed flue gas. This showed the importance of carefully characterizing microscale mixing phenomena around individual particles. For the sodium-rich species that had heretofore been thought to nu...