Gasification Reactivity Of Coal Char Research Articles

Ca-Si-Al interactions induced char structure and active site evolution during gasification–A comparative study based on different gasifying agent

Catalytic steam gasification, as a promising technology for producing hydrogen-enriched syngas, has been a hotspot in recent studies. This study aims to analyze the role of calcium-silicon-aluminum (Ca-Si-Al) interactions in char gasification and distinguish the difference of the interaction in different gasifying agent. The gasification reactivity of coal char was closely linked with the physiochemical structure and the mineral interactions. It was found that whether in CO2 or H2O, the calcium oxide (CaO) has function to promote the generation of mesopores, which is beneficial to the diffusion of reactant and product gases. The scanning electron microscopy (SEM) images showed that no SiO2 particles agglomerated in coal char containing silicon-aluminum ((Si-Al)-coal), which was a good reason why Si-Al inhibited gasification severely; the well dispersed SiO2 particles have no function to adsorb the gasifying agent molecular to transfer oxygen. New minerals including Ca2Al2SiO7, Ca2SiO4 and CaSiO3 were formed in CO2 and H2O, while CaAl2Si2O8 only was detected in H2O. These new minerals exert a positive effect on active site in CO2 but negative effect in H2O. While in H2O, compared with active sites, the mesopores dominate the reaction more. Overall, the interactions catalyze gasification to a moderate degree whether in CO2 or H2O.

Structural Characteristics of Lignite Char from Different Pyrolysis Reactors and the Influence on Their Gasification Reactivity

The gasification reactivity of coal char is affected by numerous experimental variables, and char structure is one of the dominant factors. In this work, Raman spectroscopy, powder X-ray diffraction (XRD), and N2 adsorption were used to investigate the physical and chemical structure of char prepared under different pyrolysis conditions. Three kinds of pyrolysis reactors, fluidizedbed reactor (FL), entrained-flow-bed reactor (EF), and fixed-bed reactor (PT), were designed and used to prepare the char samples. Lignite was pyrolyzed in a fixed-bed reactor with a heating rate of 10 ºC min-1, and the final temperature was 1000 ºC. The gasification reactivity of char was characterized in a quartz fixed-bed reactor under CO2, H2O, and their mixtures at 750 ºC. FB reactor produces chars with a smaller interlayer spacing of aromatic layers (d002), and FL reactor produces chars with a larger mean crystallite size along the c-axis (Lc) and aromaticity (ƒa) but inhibits the growth of mean crystallite size along the a-axis (La). The content of small aromatic rings, which is higher in the FL reactor, positively affects the initial intrinsic reactivity.

Effect of iron-based catalyst from coal liquefaction on coal char gasification reactivity and kinetics

Gasification technology can help the resource recovery of solid product from coal liquefaction, where the iron-based catalyst widely exists. In the present work, the effects of iron-based catalysts from coal liquefaction on the coal structure and reactivity were studied, using the Hami raw coal and demineralized coal. The surface morphology, element distribution and mesoporous characteristics of coal char were investigated by SEM-EDS and physical adsorption analyzer. The gasification reactivity was performed in a thermogravimetric analyzer. The gasification kinetics was studied through the model-fitting and model-free methods. The results showed that the demineralization and catalyst loading had more obvious effect on the surface attachments than the carbon matrix. The coal char with catalyst loading had significant larger specific surface area (SSA). The reactivity improvement by iron-based catalyst could be attributed to the enrichment of Fe and AAEMS and increase of SSA for coal char. More pronounced relative catalytic activity was observed for the catalytic gasification of demineralized coal char, and its activity was not sensitive to the change of heating rate and carbon conversion. The gasification characteristics difference would be reduced with the increase of heating rate. The iron-based catalyst can increase the pre-exponential factor A for the demineralized coal char gasification, and reduce the activation energy Ea for the raw coal char gasification. Under non-isothermal conditions, the Ea decreased with conversion. According to fitting performance and kinetic compensation effect, the random pore model was the best model to describe gasification, especially for the (catalytic) gasification of the demineralized coal char.

In-situ study of effect of migrating alkali metals on gasification reactivity of coal char

The migrating and diffusion of alkali metals affect catalytic gasification of coal char. The paper investigated in-situ gasification behaviors of coal char by TG and hot stage microscope, and studied catalytic performance of single NaAlO2 particle and Na distribution by SEM-EDX. The results show that in the beginning stage, the curve of carbon conversion obtained by area method is consistent with that by TG method. In the ending stage, the ash in coal char acts as framework of the particles, as a result, area of the particles keeps unchanged. Meanwhile, the ash hinders diffusion of gasifying agent into coal char, and decreases the gasification rate, so carbon conversion rate calculated by area change is lower than that from TG. The single NaAlO2 particle has catalytic performance. The coal char particles, which are closer to NaAlO2, have higher gasification reactivity, because the amount of migrating alkali is higher. The migrating distance of NaAlO2 is higher than 840 μm at 900°C.

Deactivation mechanism of coal char gasification reactivity induced by cow manure biomass volatile–coal char interactions

Cow manure is a special biological waste with potential for energy utilization and heat recovery. Various aspects of biomass utilization process is affected by volatile–char interaction. In order to reveal the changes of chemical structure and reactivity of coal char induced by cow manure biomass volatile–coal char interactions, five types of volatile–char contact modes were designed to obtain coal chars through fixed bed reactor. Fourier infrared transform spectroscopy (FT–IR), X-ray photoelectron spectroscopy (XPS), temperature programmed oxidation (TPO), and laser Raman spectroscopy (Raman) were used to analyze the change of char chemical structure. The results indicate that the type and content of O-containing functional groups have important influence on char reactivity, and the aromatic C–O is particularly obvious. Volatile–char interactions lead to the decrease of the gasification reactivity which is attribute to the decreased aromatic C–O, C = O and increased graphite carbon structures in coal char. By the deconvolution of the TPO curves, it found that the volatile–char interaction inhibits the cracking of alkyl aryl ethers structures. This work is helpful to understand the mechanism of volatile–char interaction on the structure evolution of coal char.

A perspective on sodium-induced agglomeration of Zhundong coal gasification: Experiments and calculations

In order to provide basic knowledge for the gasification utilization of low-rank Zhundong (ZD) coal in the process of circulating fluidized bed (CFB), this study focuses on gasification reactivity and ash related agglomeration of ZD coal mainly induced by sodium transformation. Typical ZD coal was pyrolyzed in a horizontal tube reactor by a rapid pyrolysis method at 900 °C. Isothermal and non-isothermal thermogravimetric analyses were used to study the gasification reactivity of ZD char under CO2 atmosphere. The effect of temperature, ranging from 850 °C to 1050 °C, on isothermal gasification was investigated, and the obtained gasification residues were analyzed to observe microscope morphology and evaluate agglomeration degree. The ternary system relationship of Na2O–SiO2–Al2O3 at each gasification temperature was calculated with FactSage to compare with the experimental results. The results show that: the gasification reactivity of ZD coal char is greatly affected by the gasification temperature. Higher temperature can increase the average specific gasification rate, thus shorten the reaction time that the carbon conversion reaches equilibrium. The gasification reactivity of coal char is proved to be better at higher temperature. The agglomeration characteristics were also affected by the gasification temperature. There is almost no melting and agglomeration phenomenon for 850 °C gasification residue, while a strong tendency to agglomeration is showed in other samples. However, when the mass fraction of local CaO on the surface of large particles is more than 40%, the melting phenomenon can be alleviated. According to the Na2O–SiO2–Al2O3 phase diagram, the positions of ZD coal and ZD char are located near the complete Slag-liq region, indicating that using ZD coal and ZD char at the operation temperature of CFB above 950 °C has strong risk of slagging, and a relatively lower gasification temperature, ∼900 °C, is suitable for ZD coal/char gasification.

CO2 gasification of Yangchangwan coal catalyzed by iron-based waste catalyst from indirect coal-liquefaction plant

Coal char-CO2 gasification is a strong endothermic reaction with high energy consumption, and a cheap catalyst use is one of the keys to save process costs. The iron-based waste catalyst (IWC) from indirect coal-liquefaction plant contains active component, but its effect on the gasification reactivity of coal char is seldom reported. Thus, CO2 gasification of Yangchangwan coal (YCW) was conducted in thermogravimetric analyzer (TGA) with IWC as the catalyst. The change of functional group on coal char surface was measured by a Fourier transform infrared (FT-IR) spectroscopy. The morphology of coal char was characterized by focused ion beam scanning electron microscope (FIB-SEM). X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were employed to analyze phases and valences of iron. The results indicate that IWC was effective for YCW gasification. In the range of 1000–1200 °C, the optimal temperature of catalytic gasification was 1000 °C. The FT-IR spectra of YCW gasified char showed the changes of small molecule active groups, such as –OH, C–O, –CH2, –CH3, C=C and Fe–O in the process of catalytic gasification. The surface structure of coal char was rough and there were lots of tiny pores. The metallic Fe formed by IWC at 0 min was finally oxidized by gasification agent CO2 to FeO and Fe3O4. The redox cycle between Fe0, Fe2+ and Fe3+ is the main reason for the catalytic YCW-CO2 gasification reactivity in the presence of IWC.

Investigation of coal-biomass interaction during co-pyrolysis by char separation and its effect on coal char structure and gasification reactivity with CO2

The interaction between coal and biomass has been widely investigated. However, the mechanism is always proposed based on physicochemical structure and reactivity of char mixture. In this work, char mixture after co-pyrolysis of anthracite and biomass was separated based on different shape and size, and then structure and reactivity of the coal char were analyzed to reveal mechanism of coal-biomass interaction. Anthracite char samples with different corn straw (CS) blending ratios were prepared by pyrolysis in a fixed bed reactor at 600 and 900°C. The AAEM concentration and microcrystalline structures of coal char were examined by inductively coupled plasma-optical emission spectrometry (ICP-OES) system and X-ray diffraction (XRD). The gasification reactivity of char sample after separation was analyzed by TGA under CO2. The results show that concentration of active K and Mg in coal char samples gradually increased and more disordered carbon structure formed as the CS proportion in the blending increased from 0 to 80%. The coal char in the blending captured more AAEM species by volatile-char interactions instead of escaping with volatile from biomass during co-pyrolysis process. Meanwhile, higher pyrolysis temperature led to volatilization and inactivation of K and Na, and also decrease in graphitization degree. Moreover, both addition of CS and low pyrolysis temperature could promote gasification reactivity of coal char sample. Furthermore, a satisfactory linear correlation (R2=0.9009) between alkali index AI and R0.5 of the char samples was established. This indicated that AAEMs performed the dominate effect to enhance gasification reactivity of coal char during co-gasification of coal and biomass.

Gasification reactivity and characteristics of coal chars and petcokes

Gasification reactivity and characteristics of three coal chars and two petcokes were investigated in detail. The temperature programmed reaction technique were used to test the gasification reactivity. The characteristics of the various chars were analyzed by XRD, SEM, N2 adsorption technique and FT-IR. As expect, the coal chars and petcokes showed significantly different gasification reactivity. The reactivity of the two petcokes was much poorer than that of the three coal chars. The gasification reactivity of the coal char increased with the decreasing of coal rank. The gasification reactivity was closely related to the characteristics of samples. It was found that the lower degree of graphitization, less compact surface and more developed pore structure corresponded to higher gasification reactivity of chars. These characteristics of the carbon structure was more favorable to gas diffusion, and hence resulted in high gasification reactivity. In addition, coal char contained more oxygen-containing functional groups and active components such as Ca/Mg/Fe/K/Na compounds, which all had promoting effect on gasification reaction.

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.

Gasification reactivity of co-pyrolysis char from coal blended with corn stalks

The gasification reactivity of coal and corn stalks co-pyrolyzed char is studied using thermogravimetric analysis, and the influence of co-pyrolysis on co-gasification reactivity is quantitatively characterized by synergy index. The results demonstrate that with increasing the pyrolysis temperature, the gasification reactivity of coal char gradually decreases, however, the gasification reactivity of CS char does not change monotonically. Furthermore, inhibition effect on co-gasification reactivity of 75% CS co-pyrolyzed char and se-pyrolyzed mixed char is shown at early stage of co-gasification and it gradually becomes synergistic effect as the co-gasification process progresses. However, the effect of the interaction of coal and CS in co-pyrolysis process on co-gasification reactivity is more important than that of the interaction of coal char and CS char in the co-gasification process. Finally, the optimal co-pyrolysis condition for obtaining highly reactivity char is that the ratio of CS in blend exceeds 70% at the pyrolysis temperature about 600 °C.

Effect of pressure and H2 on the pyrolysis characteristics of lignite: Thermal behavior and coal char structural properties

The aim of the present work was to investigate the effect of pressure and H2 on the pyrolysis characteristics of lignite. Thermogravimetric behavior during lignite pyrolysis was studied under different Ar or H2 pressures using a pressurized thermogravimetric analyzer (P-TGA). Structural properties of coal chars obtained from P-TGA were investigated by the application of elemental analyzer, Fourier transform infrared spectroscopy, X-ray diffraction analyzer, and gas sorption surface area and pore size analyzer. CO2 gasification reactivity of coal chars was also evaluated by TGA tests. The results demonstrated that the weight loss for lignite pyrolysis increased in the presence of H2 and it became more remarkable with rising pressure. Meanwhile, conversion of C and H during pressurized hydropyrolysis was promoted to a higher level considerably. Aromaticity as well as graphitization degree for coal chars were both intensified with addition of H2, especially under elevated pressures. As to the mesopore structure, BET specific surface area of coal chars from hydropyrolysis increased obviously in comparison with that of coal chars obtained in Ar atmosphere, especially under pressurized conditions. Moreover, coal chars from hydropyrolysis exhibited higher CO2 gasification reactivity.

Gasification reactivity and morphology of coal chars formed in N2 and CO2 atmospheres

In this study, chars derived from Zhundong coal were prepared in a tubular reactor under CO2 and N2 atmospheres in the temperature range 500–800 °C. The structure features of char were proven using scanning electron microscopy, X-Ray diffraction, N2 adsorption analyzer and Raman spectroscopy. Besides, the correlations of structure features and gasification reactivity of coal char were also investigated. The results reveal that char yield during devolatilization under CO2 atmosphere is relatively lower than that under N2 atmosphere, which could be attributed to heterogeneous reactions of CO2 with tar and char under CO2. The char formed in CO2 atmosphere exposes more pores in char matrix accompanied by the rough surface, resulting in a higher specific surface than that obtained under N2. Additionally, the char formed under CO2 atmosphere has lower degree of graphitization and higher ratio of small to large aromatic ring systems, compared to that obtained under N2. As a result, the gasification in a thermo-gravimetric analyzer indicates that the char prepared under CO2 is more reactive, due to more amorphous carbon structures formed under CO2. The homogeneous and heterogeneous reactions of CO2 with gases, tar and char occur when CO2 is introduced in the pyrolysis process. Moreover, the introduction of CO2 changes morphology of char, since CO2 participates in reaction process on the char surface, resulting in promoting the gasification reactivity.

The carbon dioxide gasification characteristics of biomass char samples and their effect on coal gasification reactivity during co-gasification

The carbon dioxide gasification characteristics of three biomass char samples and bituminous coal char were investigated in a thermogravimetric analyser in the temperature range of 850–950 °C. Char SB exhibited higher reactivities (Ri, Rs, Rf) than chars SW and HW. Coal char gasification reactivities were observed to be lower than those of the three biomass chars. Correlations between the char reactivities and char characteristics were highlighted. The addition of 10% biomass had no significant impact on the coal char gasification reactivity. However, 20 and 30% biomass additions resulted in increased coal char gasification rate. During co-gasification, chars HW and SW caused increased coal char gasification reactivity at lower conversions, while char SB resulted in increased gasification rates throughout the entire conversion range. Experimental data from biomass char gasification and biomass-coal char co-gasification were well described by the MRPM, while coal char gasification was better described by the RPM.

Non-isothermal study of gasification process of coal char and biomass char in CO2 condition

AbstractNon-isothermal method was used to study gasification characteristics of three coal chars and one biomass char. Four chars were made from anthracite coal (A), bituminous coal (B), lignite coal (L), and wood refuse (W), respectively. The gasification process was studied by random pore model (RPM), unreacted core model (URCM) and volumetric model (VM). With an increase in metamorphic grade, the gasification reactivity of coal char decreased, and the gasification reactivity of biomass char was close to that of low metamorphic coal char. With an increase in heating rate, the gasification of all samples moved towards high temperature zone, and the whole gasification time decreased. It was concluded from kinetics analysis that the above-mentioned three models could be used to describe the gasification process of coal char, and the RPM fitted the best among the three models. In the RPM, the activation energies of gasification were 193. 9, 225. 3 and 202. 8 kJ/mol for anthracite coal char, bituminous coal char and lignite coal char, respectively. The gasification process of biomass char could be described by the URCM and VM, while the URCM performed better. The activation energy of gasification of wood refuse char calculated by the URCM was 282. 0 kJ/mol.

Investigation into the structural features and gasification reactivity of coal chars formed in CO2 and N2 atmospheres

Coal structure undergoes a series of complex changes during pyrolysis which can affect the gasification reactivity. Lignite was used as raw sample to prepare char samples by pyrolyzing under CO2 and N2 atmospheres in the temperature range 500–800°C in a fixed-bed reactor. The structural features of chars were characterized by N2 adsorption analyzer and FT–Raman spectroscopy. The reactivity of chars was investigated in a thermogravimetric analyzer under CO2 atmosphere. The results indicated that the yields of char from pyrolysis under CO2 at 500 and 600°C were higher than that under N2; however, the specific surface area of the chars prepared under CO2 was nearly the same as that prepared under N2. At 700 and 800°C, the char yields prepared under CO2 were less than that under N2; however, the specific surface areas exhibited reverse trend, which could be attributed to the occurrence of char-CO2 reaction. Notably, the reactivity of chars formed at lower temperatures was higher than that formed at higher temperatures under both CO2 and N2 atmospheres. The reactivity of chars obtained under CO2 atmosphere was lower than that under N2 atmosphere at same pyrolysis temperature, which was mainly due to the higher aromaticity and the low ratio of small to large rings.

Effect of catalyst on coal char structure and its role in catalytic coal gasification

The structure of the coal chars with and without catalyst was characterized using X-ray diffraction and laser Raman techniques. The catalyst changes the structure of the organic unit in coal char. The mechanism by which the catalyst affects the structure of coal char in pyrolysis was investigated by X-ray photoelectron spectroscopy. For the first time, the direct evidence of electron transfer in catalyst–coal interactions was obtained, and a K-Char intermediate forms. This makes it easier for H2O to attack the K-Char intermediate, thus increasing the gasification reactivity of coal char.

Effect of high-temperature pyrolysis on the structure and properties of coal and petroleum coke

In this study, for the purpose of substituting part of petroleum coke with low ash anthracite to prepare carbon anode used in aluminum electrolysis, the effects of high-temperature pyrolysis on the structure and properties of coal char and petroleum coke were studied at the temperature range of 1000°C–1600°C. Results showed that the carbon crystallite structure of coal char and petroleum coke became more ordered with the increase of the temperature. However, the graphitization degree of coal char was lower than that of petroleum coke at the same pyrolysis temperature. The Brunauer-Emmett-Teller (BET) surface area of coal char decreased with the increase of temperature, whereas the BET surface area of petroleum coke decreased first and then increased. The increase of pyrolysis temperature generally inhibited the gasification reactivity of coal char, whereas the gasification reactivity of petroleum coke was inhibited first and then promoted. Moreover, the increase of pyrolysis temperature led to the rapid decrease of powder resistivity of coal char and petroleum coke. In particular, the powder resistivity of coal char was significantly higher than that of petroleum coke at the same pyrolysis temperature. Additionally, the real density of coal char and petroleum coke showed a different trend with the increase of temperature. Specifically, the real density of the former decreased, whereas that of the latter increased. The causes of the above observations were also discussed. Overall, this study provides further understanding about the differences between coal char and petroleum coke, which will facilitate the preparation of carbon anode from the mixture of coal char and petroleum coke.

Effect of fuel origin on synergy during co-gasification of biomass and coal in CO2

The effect of fuel origin on synergy in coal/biomass blends during co-gasification has been assessed using a congruent-mass thermogravimetry analysis (TGA) method. Results revealed that synergy occurs when ash residuals are formed, followed by an almost complete gasification of biomass. Potassium species in biomass ash play a catalytic role in promoting gasification reactivity of coal char, which is a direct consequence of synergy during co-gasification. The SEM–EDS spectra provided conclusive evidence that the transfer of potassium from biomass to the surface of coal char occurs during co-pyrolysis/gasification. Biomass ash rich in silica eliminated synergy in coal/biomass blends but not to the extent of inhibiting the reaction rate of the blended chars to make it slower than that of separated ones. The best result in terms of synergy was concluded to be the combination of low-ash coal and K-rich biomass.

Structural features and gasification reactivity of coal chars formed in Ar and CO2 atmospheres at elevated pressures

The structural features and gasification reactivity of chars derived from pyrolysis of a bituminous coal under Ar (Ar char) and CO2 atmosphere (CO2 char) have been investigated, respectively. The pyrolysis was performed in a fixed bed reactor at a final temperature of 700 °C and pressures ranging from 0.1 to 1.5 MPa. It was found that CO2 affect the char yield, pore structure and surface area. The N2 surface area of the CO2 char at ambient pressure increased by nearly 42 times compared to the Ar char. The chemical structure features were characterized by using Raman spectroscopy. The recorded spectra between 800 and 1800 cm−1 were curve-fitted with 10 Gaussian bands representing typical structural features of chars to quantitatively compare the char structure difference. The ratio I(Gr+Vl+Vr)/ID between the band intensities of amorphous char structures with small aromatic ring (3–5 rings) systems and condensed aromatic ring systems (>6 rings) is seen to decrease with increasing pyrolysis pressure. The I(Gr+Vl+Vr)/ID of CO2 char is always lower than that of Ar char in the whole pressure range. The non-isothermal CO2 gasification from 700 to 1000 °C in a TGA (thermogravimetric analyzer) indicates that the char prepared under Ar atmosphere was more reactive.