High Temperature Stage Microscope Research Articles

Comparative study on the effects of heating rate on char gasification behaviors by thermogravimetric analyzer and high-temperature stage microscope under non-isothermal condition

Understanding the intrinsic gasification reactivity and kinetics of coal char is vital in the process optimization, reactor design, and scaling of industrial gasifiers. Thermogravimetric analyzer (TGA) and visual high-temperature stage microscope (HTSM) were developed to investigate the mass loss and morphological evolution of coal char under non-isothermal conditions at different heating rates of 2.5, 5, and 10 °C/min, respectively. The gasification reactivity and kinetics obtained from different systems were comprehensively compared with the consideration of heating rate. Results showed that the consumption of carbon matrix and sintering could be responsible for the appearance of two reaction rate peaks in the rapid reaction stage in the HTSM experiments. As the heating rate increased from 2.5 to 10 °C/min, the limitation of heat and mass transfer and shortened residence time were conducive to the increase of observed reactivity and characteristic temperatures. Also, more serious diffusion and limitation could be responsible for the lower reactivity monitored by TGA and the difference in increasing amplitude of characteristic temperatures. Kissinger-Akahira-Sunosen (KAS) iso-conversional method was efficiently proposed to predict the gasification process. The activation energy varied with the carbon conversion, and then the average activation energies determined by TGA and HTSM were 191.51 and 199.01 kJ/mol, respectively. The evidence proved powerfully that the results close to the intrinsic behaviors could be provided by HTSM due to lower diffusion and limitation from heat and mass transfer.

Volatile potassium transfer behavior in carbon matrix and its effect on char gasification

Coal catalytic gasification is one of the effective ways to produce natural gas from coal. However, high cost and low recovery efficiency of catalyst increase the cost of catalytic gasification, which has become a restrictive factor for large-scale promotion of coal catalytic gasification. Therefore, the study on the migration behavior of potassium-based catalysts in the gasification process is of great significance for catalysts recovery. To understand the volatilization behavior between potassium and carbon matrix, the experiments were carried out in high temperature stage microscope (HTSM) and fixed-bed reactor. The migration of volatile potassium between particles was analyzed by SEM-EDS, while the chemical fraction of potassium species was determined by AAS, and the surface elements of the samples was investigated by XPS. The results showed that potassium would volatile at 765-770℃, C/K≈6.2–9.5(wt%), which would be adsorbed by the other char particles, leading to an increasing of the content of soluble potassium and ion-exchanged potassium, promoting carbon conversion in a limited effect and a decreasing of hydroxyl functional groups on the char particles. Meanwhile, the potassium content was proportional to the migration distance, the migration distance was about 720–800 μm at maximum under HTSM. The migration of potassium at high temperature caused the formation of liquid phase around the surface of char particles, which would prevent the contact between the gasifier agent and char particles and increase mass transfer resistance, thus preventing the increase of particle carbon conversion. In addition, no obvious liquid phase formation occurred at 800 °C in SFA-K, indicating that the formation of potassium-containing liquid phase and the volatilization of potassium at 800 °C were directly related to the carbon in the char.

Pyrolysis and CO2 gasification of biomass in high-temperature stage microscope: Morphological evolution and thermal behaviors

The morphological evolution of biomass particles during pyrolysis and char gasification was investigated by a high-temperature stage microscope (HTSM), and realistic particle shrinkage characteristics were obtained in combination with weight loss properties. Also, product properties were obtained on a fixed-bed and the influence of the physicochemical properties of the char on the gasification activity was considered. Several typical biomass samples were selected, including poplar sawdust, rice husk, and steamed rice. As a starchy biomass, steamed rice particles underwent intense swelling at 300–330 °C, followed by shrinkage, and finally formed an internal loose and porous char, which was significantly different from lignocellulosic biomass. For lignocellulosic biomass, poplar sawdust and rice husk particles showed similar shrinkage processes during pyrolysis. By comparing the curves of particle shrinkage and weight loss, the development of internal pore volume of particles was reflected in a general way, which can better guide the construction of biomass shrinkage mechanism model. The gasification activity of the three biomass pyrolysis chars differed considerably and consistent conclusions were obtained in terms of particle shrinkage, weight loss, and gas products. The poplar sawdust char showed acute shrinkage of particles and loss of weight, exhibiting the strongest gasification activity, 6 times that of rice husk char and steamed rice char. The gasification activity of the char depended mainly on the degree of graphitization and was almost independent of the mesoporous structure. In the temperature range of 800–900 °C, higher gasification temperatures corresponded to more rapid particle shrinkage and weight loss, higher gas product yield, and greater gasification activity.

The sintering analysis of biomass waste ash based on the in-situ exploration and thermal chemical calculation in the gasification process

The plentiful biomass wastes are promising fossil energy alternatives to generate indispensable energy. Biomass gasification is an effective way of producing high-value-added productions and reducing CO2 emissions. However, some ash-related issues, especially ash sintering, restrict the wide utilization of biomass energies. To minimize the negative effect of the sintering problem, this work studied the ash sintering process of typical cornstalk waste ash by in-situ experimental methods, including high-temperature stage microscope (HTSM) and thermomechanical analysis (TMA). Then the thermochemical calculation was combined to clarify the sintering mechanism. The result indicated that sintering was the first step during the ash fusion process. The initial sintering temperature (624 °C) was lower than the ash deformation temperature (919 °C), which was the first characteristic temperature of ash fusion tests. The chemical reaction of the inherent ash-related components was the main reason for the appearance of the initial sintering temperature. From the element perspective, the chemical reaction of alkali and alkali earth metals (AAEMs) was mainly involved. The alkali metals tended to react with SiO2 while alkali earth metals were prone to combine with P2O5. Except for ash sintering behavior during the heating process, the TMA trace showed another two stages, swelling and melting. Massive bubbles would be generated from the inside melts at the swelling stage and cause the drastic fluctuation of the ash-melting system until the end of the melting range. Moreover, the high heating rate was beneficial for alleviating the initial sintering temperature while having a tiny influence on changing the total range of sintering and swelling zones.

Insights into the effect of particle size on coal char particle gasification by thermogravimetric analyzer and high temperature stage microscope

The comparison of gasification characteristics obtained by different methods can provide an in-depth understanding for intrinsic reactivity which is a crucial basis for predicting gasification behavior and optimizing gasifier. Comparative study on the effects of particle size on the reactivity and kinetics was conducted by thermogravimetric analyzer (TGA) and high temperature stage microscope (HTSM). Then the influence of diffusion on gasification characteristics by different methods was analyzed. Results showed that the shrinkage ratio of coal char increased with increasing gasification temperature, and the shrinkage ratio of small particle was higher than that of large particle. The reason was that the shrinkage ratio was determined by sintering when the ash content in char was less different. Also, the observed reactivity in TGA was far less than that in HTSM and the reactivity difference between small particle and large particle in HTSM was much larger than that in TGA, which was determined by the fact that the significant external and bed diffusion effects could not really be eliminated in TGA. Whereas those diffusions were completely eliminated due to sparse dispersion of particles in HTSM. Meanwhile, the modified volume model (MVM) was the best model to describe the char gasification process. The validity of MVM was verified by using the model-free method. Additionally, the internal diffusion effectiveness factor revealed that the internal diffusion of large particle measured in HTSM was more significant than that in TGA, indicating that the result of smaller particle measured in HTSM was closer to the intrinsic reactivity.

Insights into the Effect of Particle Size on Coal Char Particle Gasification by Thermogravimetric Analyzer and High Temperature Stage Microscope

Insights into the Effect of Particle Size on Coal Char Particle Gasification by Thermogravimetric Analyzer and High Temperature Stage Microscope

Effect of B2O3 on the melting characteristics of model municipal solid waste incineration (MSWI) fly ash

This paper presents the melting characteristics of the multicomponent system of CaO-SiO2-Al2O3-Fe2O3-MgO under the influence of B2O3. Experiments on modelled ash fusibility were conducted in which mixtures of SiO2, Al2O3, CaO, Fe2O3, MgO, and B2O3 were investigated using a high-temperature stage microscope (HTSM) and ash fusion temperature (AFT) test. Thermodynamic calculations combined with X-ray diffraction (XRD) were used to obtain the reaction mechanisms that contributed to the melting process. The results showed that the temperature at which the particles agglomerated and melted decreased as the B2O3 content increased. B2O3 decreased the melting temperatures of the model ash samples, which could be related to the formation of the minerals Ca11B2Si4O22, Ca2B2O5, and CaB2O4 as well as the decrease of refractory minerals such as melilite.

In situ experimental study of CO2 gasification of petcoke particles on molten slag surface at high temperature

The gasification reaction of petcoke on wall slag layer was an important issue that should be studied during the co-gasification process of coal and petcoke. In present work, the gasification characteristics of petcoke particles on molten slag surface was investigated using a high temperature stage microscope. The catalytic effect of slag on petcoke gasification on slag surface was demonstrated. The experimental results demonstrated that the catalytic effect of slag only worked when the particle was on the surface of slag. The catalyst vapor had no influence on the reactivity of petcoke. What’s more, the SEM analysis result showed that the reaction surface area of petcoke was decreased as a result of the deposition process. The effect of molten slag on particle temperature could be neglected as a result of the weak endothermic effect. Therefore, the change of petcoke reactivity rely on the synergy between catalytic effect of slag and the decreased reaction surface area. Besides, the increased residence time of petcoke also could increase the carbon conversion compared with that in gasifier chamber.

Sub-particle scale study on the melting behavior of Zhundong coal ash based on the heterogeneous distribution of elements

The current research work focused on the effect of the heterogeneous distribution of elements on the ash melting behavior of Zhundong coal at the sub-particle scale by using a high temperature stage microscope (HTSM) and scanning electron microscope (SEM) equipped with energy dispersive spectrometry (EDS). Experimental results showed that both molten and unmolten areas existed in the residual ash sample after combustion below 1200 °C. Below this temperature, the partial ash melting behavior was significantly found during the ash melting process. Comparing the element composition of local molten and unmolten areas, Na, Si and Mg played predominant roles in the partial melting behavior. The enrichment of Na and Si promoted the formation of low temperature eutectic mixture and further molten areas, while the unmolten areas were rich in Mg. Combined with the micro-Raman analysis, minerals like spinel and forsterite that containing Mg were the main refractory components in the unmolten areas. Thermodynamic calculation results from FactSage software showed that the mass fraction of the liquid phase started to form rapidly at about 1100 °C in molten areas mainly with the fast melting of olivine and nepheline. However, the unmolten areas started to melt swiftly at about 1200 °C mainly with the fast melting of merwinite. Spinel was the main refractory component in both molten and unmolten areas.

Effect of element content fluctuation on partial melting behavior of coal ash particles

This study investigates the partial melting behavior difference of ash particles at sub-particle scale, correlating to the element fluctuation, with the experiment in a high temperature stage microscope (HTSM). Scanning electron microscopy (SEM) coupled with energy dispersive spectrometry (EDS) was employed to clarify the different local melting behavior of residual ash particles. It was found that the element enrichment status differed in the local molten and unmelted areas existing in the residual ash particles. Sodium and silicon tended to be rich in the molten areas while the element content of Mg, Al, Ca, Fe and S was comparatively higher in the unmelted areas. As the combustion temperature approached the ash deformation temperature of the three coals, the difference in the major element content was reduced. Results calculated by FactSage software showed that the initial liquid phase formed at the temperatures lower than the ash deformation temperature of the three coal ash samples. Na-rich minerals such as nepheline and albite are closely related to the formation of the liquid phase with low melting temperature. Element fluctuation may lead to the composition of local areas locating in the areas with low liquidus temperature on the phase diagram. In addition, relatively large fluctuations in silicon content have a significant effect on the position in the phase diagram. Thus, the ash melting behavior differs at sub-particle scale.

Studying effects of solid structure evolution on gasification reactivity of coal chars by in-situ Raman spectroscopy

This study focuses on characteristics of structure evolution during pyrolysis and subsequent char gasification of lignite and bituminous coal. Coal pyrolysis was conducted on induction-heating reactor. As pyrolysis temperature increased, the graphitization degree of char increased, and the amount of C-O and -CH in char decreased, resulting in poorer gasification reactivity of char. Char gasification was conducted by thermogravimetric analyzer (TGA), high-temperature stage microscope (HTSM) and in-situ Raman system to investigate gasification reactivity, in-situ evolution of morphology structure and in-situ evolution of carbon structure, respectively. As gasification proceeded, the projected area of char particle decreased rapidly first and then kept consistent, and degree of graphitization of char gradually increased. In-situ Raman parameters indicated difference in increasing rate of graphitization between lignite char and bituminous char decreased with increasing temperature. Higher temperature deactivated the alkali metals, leading to attenuation of their inhibitory effect on graphitization. Gasification reactivity index R0.9 was in positively correlated with in-situ ID1/IG value, and corresponding correlation coefficient (R2) for TGA and HTSM were 0.8108 and 0.9829, respectively. The higher R2 for HTSM was due to the weaker diffusion effect. In-situ Raman parameter was more suitable for predicting gasification reactivity of single particle char.

Characteristics of single petcoke particle during the gasification process at high temperatures

Abstract Particle concentration significantly affected the gasification of petcoke particles according to our previous studies. In this work, gasification characteristics and morphological evolution of single petcoke particle were investigated using a high temperature stage microscope experimental setup. The results showed that the reaction temperature significantly affected the reactivity of petcoke in the temperature range of 1200–1300 °C. While the promoting effect on gasification reactivity decreased with further increasing the reaction temperature, the SEM analysis demonstrated the pore development during the gasification process, which attributed to the increase of reaction rate with conversion. The Raman analysis, HRTEM and SEM–EDX analysis showed that the heterogeneous graphitization of petcoke and non-uniform distribution of catalytic elements in petcoke attributed to the development of surface pores with limited depth. The gasification mechanism of petcoke particle can be briefly described as the reaction rate mainly contributed from the fast-reaction area. Besides, the pore development in fast-reaction area also enlarged the surface area of petcoke particle.

Investigation of fluctuation behavior in viscosity of coal slags used in entrained-flow gasifiers

Entrained-flow gasification has attracted extensive attention because of its clean and efficient utilization of coal as well as flexible application in industry for power generation and chemical production. Numerous theoretical and experimental investigations on slag flow in entrained-flow gasifiers have been performed to ensure a smooth operation, but an unscheduled shutdown due to slag tap blockage remains an ongoing challenge. A fluctuation phenomenon of slag viscosity gives a rise in the risk of slag tap blockage. In this work, three Chinese coal slags with low silicon content and high iron content were used to investigate the viscosity fluctuation phenomenon. It was found that the crystalline phases and suspended bubbles can cause the fluctuation phenomenon. The fluctuation degree is related to the volume fraction of crystalline phases and the crystallization rate. In the search for the mechanism of the viscosity fluctuation, an in-situ observation using a high temperature stage microscope system (HTSM) in combination with XRD and SEM analyses was carried out. In addition, the viscosity of investigated slags with the suppression of solid phases as well as the phase distribution at high temperatures was calculated. Based on a comprehensive analysis of the results, a possible mechanism has been proposed that the viscosity fluctuation is attributed to an instant local change in the macrostructure such as the deformation of suspended bubbles, the change of the orientation of acicular crystals and the interaction of the acicular crystals and suspended bubbles.

Study on the Fragmentation Behaviors of Deposited Particles on the Molten Slag Surface and Their Effects on Gasification for Different Coal Ranks and Petroleum Coke

Coal char particles were deposited on the molten slag surface and continued to react with the gas near the gasifier wall slag surface in an entrained flow gasifier, which significantly affected the total carbon conversion and slag flow properties. The current research work is to investigate the fragmentation and gasification characteristics of coal char particles, which were captured on the molten slag surface. A high-temperature stage microscope was used to study and analyze the particle evolution and char gasification process on the molten slag surface with the effects of different coal ranks. Results indicated that char particles first shrank and then broke into several fragments during the gasification on the molten slag surface. Char particles with fragmentation behaviors on the molten slag surface had higher reaction rates and carbon conversions than the particles in the shrinking period. The initial fragmentation time of a char particle showed an increasing linear relationship with the initial part...

Morphology and structure evolution of flaky char particles during CO2 gasification

High temperature stage microscope was applied to observe morphology evolution of flaky char particles during gasification. Raman spectroscopy was used to analyze crystalline structures of gasification semicoke. Effect of gasification temperature (1000–1200°C) and initial equivalent diameter (1.00–1.60 mm) on morphology and structure evolution were examined. The results show that particle shrinkage in later stage of gasification is more intense than that in early stage. Within tested gasification temperature, the particle ASR (area shrinkage ratio) and VSR (volumetric shrinkage ratio) decrease with increasing temperature. The initial particle size of char has a significant effect on particle shrinkage. At 1100°C the shrinkage trend of particle marks a turning point at initial diameter of 1.30 mm. The variation of char apparent density is dominated by carbon consumption. When the carbon conversion reaches 80%, the apparent density ratio linearly decreases below 0.4. At the same gasification temperature, with increasing carbon conversion the graphitization of char reduces first and then increases, while the amorphous carbon is opposite.

In situ experimental and modeling study on coal char combustion for coarse particle with effect of gasification in air (O2/N2) and O2/CO2 atmospheres

This study applied the high temperature stage microscope to investigate coal char combustion in air (O2/N2) and O2/CO2 (volume, 21%/79%) atmospheres with the effect of gasification. A reaction front combustion model was also proposed, coupled with effect of char gasification, to predict the key parameters of char combustion in the O2/CO2 atmospheres. The combustion process and particle evolution of coarse chars were observed and measured to calculate the carbon conversion. Experimental results showed that the burnout time of the char particle in the O2/CO2 atmosphere in the was prolonged about 20–25%, compared to the burnout time of chars in air. The overall reaction rates of char particles in the O2/CO2 atmosphere were lower than the rates in the air for different particle sizes. In addition, reaction rates from model prediction showed good agreements with the experimental data for different particle sizes both in the air and O2/CO2 atmospheres, which proved the reaction front combustion model was applicable for the char combustion in this study. The reaction front combustion model also predicted that the reaction front decreased with burning time and mass consumption of char particle. For the char combustion in the O2/CO2 atmosphere, the gasification with CO2 absorbed part of the combustion heat or radiation heat and caused lower particle temperatures and heat fluxes for different particle sizes. The proposed combustion model also predicted that although the effect of the gasification reaction was dominant at the beginning of combustion and then reduced, the conversion of particles was hindered compared to the combustion of char in air.

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...

Modeling study for the effect of particle size on char gasification with CO2

A high temperature stage microscope to investigate the temperature effect caused by particle size on char gasification is applied in this study. Experiments were carried out with different particle sizes for raw chars and chars on molten slag surface, respectively. Heat transfer models were built for the raw char of two temperature distributions and char particle on molten slag, respectively. Results showed that reaction layer temperature of raw char decreased in the reaction dominant while char on molten slag had higher temperature. Temperature difference between two distributions increased with the initial particle size, indicating the temperature effect on large particles was obvious. Shrinking core model was applied and modified herein coupled with the modification of reaction layer temperature and reaction area. Model prediction and experimental data showed good agreements of carbon conversion and reactivity index for raw char and char on molten slag, respectively. © 2016 American Institute of Chemical Engineers AIChE J, 63: 716–724, 2017

In situ experimental study on the combustion characteristics of captured chars on the molten slag surface

Captured char particles on the molten slag continue to burn or transform into residual carbon in a slagging combustor or furnace, which affect the complete carbon conversion. This study applied a high temperature stage microscope to investigate the combustion behavior of captured chars on the molten slag surface. The combustion process of captured chars with air on the slag surface was observed and recorded, compared to the original char combustion. Particle size evolution calculated from the measured cross section area versus time indicated that the burnout time of chars was prolonged on the molten slag surface. Besides the heat transfer analysis also showed that the temperatures of char particles on the molten slag were decreased due to the thermal conduction between char and slag. Molten slag layer reduced the char reactivity from the analysis of combustibility index, indicating the captured char combustion on the molten slag surface was hindered. Coupled with heat transfer analysis, shrinking particle model (SPM) was applied and modified to predict the combustion time at carbon conversion of 0.9, and results showed an agreement with the experimental data.

In situ study on the formation mechanism of bubbles during the reaction of captured chars on molten slag surface

Bubbles in the liquid slag affect the rheology and thermal conductivity, and formed porous structures after cooling or quench in the industrial gasifier. In this study, we applied a high temperature stage microscope (HTSM) to investigate the formation mechanism of bubbles during char gasification on molten slag surface. The initial bubble diameter and the formation time were measured by ImageJ software. Bubbles formed on the char–slag interface and formation time of the initial bubble increased with increasing initial particle size. Bubble accumulative volume for a single char particle increased with the shrinkage of particle. Results also showed that gas product from fine particles tended to diffuse into liquid slag and form the bubbles, with the comparison of bubble accumulative volume and gas product volume from a single particle. In addition, particle diameter at the initial bubble formation time was proportional to the initial particle size. The formation mechanism of bubbles was analyzed and predicted, which showed approximate results for the experiment data and was available for the bubble formation.