Gasification Mechanism Research Articles

Enhancement of depolymerization slag gasification in supercritical water and its gasification performance in fluidized bed reactor

Supercritical water gasification (SCWG) can achieve efficient utilization of solid wastes, including depolymerization slag, biomass, municipal sludge, and others, and can produce hydrogen-rich gas. But phenolic intermediates produced during the gasification process are difficult to be gasified. It restricted the increase of conversion efficiency. Thus, in this paper, two effective schemes were put forward to raise the gasification efficiency of depolymerization slag. The first one was to add oxidants to promote the decomposition of hydrolysates. Carbon gasification efficiency (CGE) and hydrogen gasification efficiency (HGE) reached 71% and 74% respectively at the time of 1200 s. They raised by 41.6% and 60.5%, separately compared with non-oxidants addition. The second one was to raise the reaction temperature since a higher temperature can accelerate the reaction rate and obtain a high hydrogen yield. The results showed that hydrogen percentage increased by 32.82% and CGE and HGE increased by 35.39% and 219.85%, respectively when the reaction temperature raised by 200 °C. Based on the obtained gasification mechanism, the catalytic gasification.

Chemical looping gasification of maceral from low-rank coal: Products distribution and kinetic analysis on vitrinite

The product distribution and kinetic analysis of low-rank coal vitrinite were investigated during the chemical looping gasification (CLG) process. The acid washing method was used to treat low-rank coal, and the density gradient centrifugation method was adopted to obtain the coal macerals. By combining thermogravimetric analysis and online mass spectrometry, the influence of the heating rate and oxygen carrier (Fe2O3) blending ratio on product distribution was discussed. The macroscopic kinetic parameters were solved by the Kissinger-Akahira-Sunose (KAS) method, and the main gaseous product formation kinetic parameters were solved by the iso-conversion method. The results of vitrinite during slow heating chemical looping gasification showed that the main weight loss interval was 400–600 °C, and the solid yield of sample vitrinite-Fe-10 at different heating rates was 64.30%–69.67%. When β = 20 °C·min−1, the maximum decomposition rate of vitrinite-Fe-10 was −0.312%·min−1. The addition of Fe2O3 reduced the maximum decomposition rate, but by comparing the chemical looping conversion characteristic index, it could be inferred that the chemical looping gasification of vitrinite might produce volatile substances higher than the pyrolysis process of vitrinite alone. The average activation energy of the reaction was significantly reduced during chemical looping gasification of vitrinite, which was lower than the average activation energy of 448.69 kJ·mol−1 during the pyrolysis process of vitrinite alone. The gaseous products were mainly CO and CO2. When the heating rate was 10 °C·min−1, the highest activation energy for CH4 formation was 21.353 kJ·mol−1, and the lowest activation energy for CO formation was 9.7333 kJ·mol−1. This study provides basic data for exploring coal chemical looping gasification mechanism and reactor design by studying the chemical looping gasification process of coal macerals.

Kinetic analysis of CO2 gasification of biochar and anthracite based on integral isoconversional nonlinear method

AbstractThe nonisothermal thermogravimetric analysis was implemented for gasification of sawdust char (SD-char), wheat straw char (WS-char), rice husk char (RH-char), bamboo char (BB-char) and anthracite coal (AC) in the presence of CO2. The dependence of activation energy upon conversion for different biochars and AC was obtained by the integral isoconversional nonlinear (NL-INT) method which is a model-free method. Based on the activation energy values from the NL-INT method, a model-fitting method called random pore model (RPM) was used to estimate the kinetic parameters including the preexponential factor and pore structure parameter from the experimental data. The results are shown that the gasification reactivity of different samples from high to low can be sorted as that of WS-char, SD-char, BB-char, RH-char and AC. In the early stage of gasification, the activation energy values of biochars increase generally with an increase in the conversion degree, whereas the value of AC decreases. Thereafter, the activation energy values remain almost unchanged when the conversion is up to some extent. When the conversion degree varies between about 0.3 and 0.9, these carbon materials can be sorted in the order of average activation energy from low to high as WS-char, SD-char, AC, RH-char and BB-char, respectively, 134.3, 143.8, 168.5, 184.8 and 193.0 kJ/mol. It is shown that a complex multistep mechanism occurs in the initial stage of gasification, while a single-step gasification mechanism exists in the rest of the gasification process. The RPM is suitable for describing the gasification of biomass chars and AC except the initial gasification. Additionally, it is found that the kinetic compensation effect (KCE) still exists in the gasification reactions of biochars and AC. However, the AC deviates markedly from the KCE curve. This may be caused by the similarity of carbonaceous structure of biochars and the difference in reactivity between biochars and AC.

Simulation Study of the Formation of Corrosive Gases in Coal Combustion in an Entrained Flow Reactor

Gaseous sulfur species play a major role in high temperature corrosion of pulverized coal fired furnaces. The prediction of sulfur species concentrations by 3D-Computational Fluid Dynamics (CFD) simulation allows the identification of furnace wall regions that are exposed to corrosive gases, so that countermeasures against corrosion can be applied. In the present work, a model for the release of sulfur and chlorine species during coal combustion is presented. The model is based on the mineral matter transformation of sulfur and chlorine bearing minerals under coal combustion conditions. The model is appended to a detailed reaction mechanism for gaseous sulfur and chlorine species and hydrocarbon related reactions, as well as a global three-step mechanism for coal devolatilization, char combustion, and char gasification. Experiments in an entrained flow were carried out to validate the developed model. Three-dimensional numerical simulations of an entrained flow reactor were performed by CFD using the developed model. Calculated concentrations of SO2, H2S, COS, and HCl showed good agreement with the measurements. Hence, the developed model can be regarded as a reliable method for the prediction of corrosive sulfur and chlorine species in coal fired furnaces. Further improvement is needed in the prediction of some minor trace species.

Performance improvement of a direct carbon solid oxide fuel cell via strontium-catalyzed carbon gasification

As a new way of power generation, direct carbon solid oxide fuel cells (DC-SOFCs) exhibit great potential in solution of energy crisis and environmental pollution. According to the working principle, the cell operation is a kinetically controlled process, and the reverse Boudouard reaction is the rate-determining step of the whole system. In this study, a Sr-based catalyst is successfully introduced to accelerate carbon gasification and thus enhance cell performance of DC-SOFCs. The electrochemical performance of DC-SOFCs operated on coconut active charcoal with various Sr loading contents (3 wt%–10 wt%), are studied and compared with that of DC-SOFCs with traditional Fe-catalyzed carbon fuel. Experimental results demonstrate that the best output of 316 mW cm−2 is achieved from the single cell powered with 5 wt% Sr-loaded coconut active charcoal at 850 °C, higher than those of DC-SOFCs fueled by pure and 5 wt% Fe-loaded active charcoal. The superiority of the Sr-based catalyst is also demonstrated by the operation stability of the corresponding DC-SOFC, which displays a relatively long operation time of 22.68 h at 0.25 A cm−2 with the fuel utilization of 18.3%. The SEM/EDX results indicate that the Sr-based catalyst exhibits good stability without agglomeration during cell operation at high temperature. In addition, the carbon gasification mechanism catalyzed by Sr-based catalyst is also proposed on the basis of these properties. This study indicates that the designed Sr-loaded coconut active charcoal is expected to be an alternative carbon fuel for DC-SOFCs.

Investigation on the Microstructure, Thermal Strength and Gasification Mechanism of Modified Ferro-Coke with Coal Tar Pitch

Ferro-coke is prepared by the slight coking of coal and iron ore. It is a new type of fuel used for blast furnace ironmaking for the purpose of energy saving and emission reduction. However, with the increase of iron ore content, the metallurgical properties of ferro-coke gradually deteriorate. To fabricate high-strength ferro-coke, coal tar pitch was used as a binder in this study. The microstructure, cold drum strength, thermal crushing strength, gasification reactivity and gasification kinetics of this modified ferro-coke were systematically studied and compared with those of gas-coal coke and ferro-coke without binder. The results showed that the coal tar pitch could reduce the porosity of ferro-coke by 4.57 pct, enhance the adhesion of carbon matrix to minerals and increase the microcrystalline structure of ferro-coke. The cold drum strength of ferro-coke increased from 28.04 to 86.34 pct after adding coal tar pitch, while the thermal strength of ferro-coke during the gasification process rose by only 100 N. The coal tar pitch had little effect on the gasification reactivity of ferro-coke, so the activation energy of modified ferro-coke was still low. The first-principle calculation results revealed that the promotion of Fe on the ferro-coke gasification was attributed to the reducing role of Fe on the reaction energy of ketone group decomposition.

Effect of Heating Rate on Chemical Kinetics of Chicken Manure With Different Gas Agents

Abstract This paper presents the study on the effect of different heating rates on the pyrolysis and gasification process of the chicken manure. The obtained results are shown by the extent of reaction, the kinetics of the reaction, and differential thermal analysis. In total, 24 cases were carried out; eight heating rates with three different gas agents each. The results show that when using nitrogen or carbon dioxide as gas agents, the reactions were endothermic. Consequently, the energy must be supplied in terms of heating to sustain the reaction. Furthermore, the air gasification was exothermic, which means that the reaction can be sustained without external heating, where the self-ignition was observed between 450 °C–600 °C. The thermal degradation of the three main components of the chicken manure was obtained. The pyrolysis process was divided into two regions at 360 °C and the order of reaction of five for both regions. For the gasification process, it was observed that carbon dioxide had the most complicated mechanism with four stages. Finally, it is recommended to use the lowest heating rate to allow a quasi-equilibrium state through slow heating. Consequently, the delay in response or any transient error can be avoided as they are the main reason for measurement errors. These chemical kinetic parameters can be used in the future for the chicken manure simulation using the order of reaction mechanism for solid-state gasification.

Study on gasification mechanism of biomass waste in supercritical water based on product distribution

Supercritical water gasification technology is widely applied to convert organic waste into valuable substances as a clean and efficient method. Biomass gasification in SCW is a complex process and complicated chemical reactions like decomposition and poly-condensation take place, thus, reaction mechanism of real biomass needs to be further investigated. In this paper, experimental study on cornstalk gasification in SCW was conducted at the temperature of 500–800 °C, reaction time of 1–15min and feedstock concentration of 1–9%. The effects of various operating parameters on evolution of gas, liquid and solid products were conducted. It was discovered that pore structure and carbon microspheres appeared successively on the surface of solid residue. Mechanism study showed that the biomass was first depolymerized into monomer and its derivatives, then cracked and poly-condensed into a nuclear to generate carbon microspheres as its concentration reached the critical concentration. As the reaction proceeds, reduction reaction, coke combustion and secondary reaction occurred, thus carbon microspheres decreased. The results indicated that higher reaction temperature, longer reaction time and lower feed concentration were conducive to improving reaction performance of biomass. Finally, it was discovered that carbon gasification efficiency reached 99% at the temperature of 700 °C, reaction time of 15 min and biomass concentration of 3%.

Determination of a Complete Conversion Model for Gasification of Lignite Char

The conversion of solid fuels via gasification is a viable method to produce valuable fuels and chemicals or electricity while also offering the option of carbon capture. Fluidized bed gasifiers are most suitable for abundantly available low-rank coal. The design of these gasifiers requires well-developed kinetic models of gasification. Numerous studies deal with single aspects of char gasification, like influence of gas compositions or pre-treatment. Nevertheless, no unified theory for the gasification mechanisms exists that is able to explain the reaction rate over the full range of possible temperatures, gas compositions, carbon conversion, etc. This study aims to demonstrate a rigorous methodology to provide a complete char gasification model for all conditions in a fluidized bed gasifier for one specific fuel. The non-isothermal thermogravimetric method was applied to steam and CO2 gasification from 500 °C to 1100 °C. The inhibiting effect of product gases H2 and CO was taken into account. All measurements were evaluated for their accuracy with the Allan variance. Two reaction models (i.e., Arrhenius and Langmuir–Hinshelwood) and four conversion models (i.e., volumetric model, grain model, random pore model and Johnson model) were fitted to the measurement results and assessed depending on their coefficient of determination. The results for the chosen char show that the Langmuir–Hinshelwood reaction model together with the Johnson conversion model is most suitable to describe the char conversion for both steam and CO2 gasification of the tested lignite. The coefficient of determination is 98% and 95%, respectively.

Determination of a Complete Conversion Model for Gasification of Lignite Char

The conversion of solid fuels via gasification is a viable method to produce valuable fuels and chemicals or electricity while also offering the option of carbon capture. Fluidized bed gasifiers are most suitable for abundantly available low-rank coal. The design of these gasifiers requires well-developed kinetic models of gasification. Numerous studies deal with single aspects of char gasification, like influence of gas compositions or pre-treatment. Nevertheless, no unified theory for the gasification mechanisms exists that is able to explain the reaction rate over the full range of possible temperatures, gas compositions, carbon conversion, etc. This study aims to demonstrate a rigorous methodology to provide a complete char gasification model for all conditions in a fluidized bed gasifier for one specific fuel. The non-isothermal thermogravimetric method was applied to steam and CO2 gasification from 500 °C to 1100 °C. The inhibiting effect of product gases H2 and CO was taken into account. All measurements were evaluated for their accuracy with the Allan variance. Two reaction models (i.e., Arrhenius and Langmuir–Hinshelwood) and four conversion models (i.e., volumetric model, grain model, random pore model and Johnson model) were fitted to the measurement results and assessed depending on their coefficient of determination. The results for the chosen char show that the Langmuir–Hinshelwood reaction model together with the Johnson conversion model is most suitable to describe the char conversion for both steam and CO2 gasification of the tested lignite. The coefficient of determination is 98% and 95%, respectively.

Catalytic supercritical water gasification mechanism of coal

Catalytic supercritical water gasification (SCWG) for H2 production is a hopeful way of coal conversion to replace the traditional coal utilization mode. At present, the detailed catalytic mechanism in the process remains unknown. Herein, a comprehensive catalytic SCWG mechanism of coal is proposed by establishing a novel catalytic kinetic model. It shows that catalysts (K2CO3) break up the coal matrix by a cyclic redox reaction to produce plenty of mesopores, accelerating steam reforming of fixed carbon and coal pyrolysis. Water-gas shift reaction is facilitated by K2CO3 via formation of formate, which then promotes steam reforming of CH4 at high temperature (≥700 °C) due to the decreasing CO. The proposed mechanism provides important insights in catalytic SCWG process of coal.

Influencing factors and reaction mechanism for catalytic CO2 gasification of sawdust char using K-modified transition metal composite catalysts: Experimental and DFT studies

Gasification of renewable biomass, sawdust char in this study, was carried out with CO2 to observe the effect of the gasification temperature, CO2 adsorption, and K-modified transition metal composite catalysts; this was known as the Boudouard reaction. The catalysts were characterized using inductively coupled plasma-optical emission spectrometry, X-ray photoelectron spectroscopy, X-ray diffraction analysis, BET surface area analysis, and scanning electron microscopy. The reaction mechanism of the catalytic gasification was investigated using thermogravimetry and density functional theory. The results showed that temperature was one of the most important operating variables for char gasification, and carbon conversion increased 2.55 times within 40 min when the temperature increased from 750 °C to 800 °C. Composite catalysts effectively improved the char conversion at low temperatures, and the order of catalytic performance was KCo > KNi > KFe > KCe. The catalytic CO2 gasification activity depended not only on the CO2 quantity adsorbed, but also on the CO2 decomposition activity on the catalyst surfaces. The CO2 reaction mechanism over Co, Ni, and Fe surfaces can be written as follows: M + CO2 → M-Oads + CO, M-Oads + C* → M + CO. Over CeO2 surface, the reaction mechanism was the following: CeO2 + C* → CeO2vacO + CO, CeO2vacO + CO2 → CeO2 + CO. These results can aid in the CO2 catalytic gasification mechanism and provide further theoretical guidance to improve the industrialization of biomass gasification.

Utilization of biomass ash for upgrading petroleum coke gasification: Effect of soluble and insoluble components

In this paper, the catalytic effect of different components in biomass ash on petroleum coke (PC) gasification was studied. Cotton straw ash with high content of AAEMs and low content of silica, was prepared and pretreated by washing to obtain the soluble (ash leachate) and insoluble (washed ash) components. These ash-based catalysts were added to PC via physical mixing or impregnation. The catalytic gasification performance and kinetics were investigated using a thermal-gravimetric analyzer (TGA), and the carbon structure evolution was determined by Raman spectroscopy. The results showed that all of these ash-based catalysts, even for the washed ash, had catalytic effect on PC gasification. The soluble component, mainly containing K, could speed up the gasification rate especially at the late stage. The catalytic activity of the soluble component was sensitive to temperature and conversion, while that of the insoluble component was steady. Moreover, the modified random pore model and nucleation and growth method were applied to analyze the gasification kinetics. In addition, the washed ash could inhibit the graphitization during gasification, whereas the ash leachate and original ash could enhance the generation of amorphous carbon. Possible mechanisms of catalytic gasification were proposed to explain these phenomena.

Activating ABO3-type coating by additive for coke inhibition in supercritical thermal cracking of endothermic hydrocarbon fuel

Coking during endothermic hydrocarbon fuel cracking is a key problem that has to be considered. In this study, anti-coking effect was systematically studied by using water, ethanol and HPW as additives on the coatings of CsOH and BaCeO3 during RP-3 cracking. The morphology, type and amount of coke deposits on the coatings were characterized by means of scanning electron microscopy (SEM), temperature programmed oxidation (TPO) and quantitative carbonization method, respectively. The results show that either the additive or the catalytic coating can effectively remove the coke deposits on the reaction tube. Remarkably, BaCeO3 coating is more effective in inhibiting the formation of fiber coke compared with CsOH coating，which is conducive to the cracking reaction. The best anti-coking effect is achieved by using 5 wt% ethanol as additive on the BaCeO3/HPW coating, and the inhibiting percentage is 97%. In addition, a possible anti-coking mechanism of carbon-steam gasification through BaCeO3/HPW coating and ethanol has been proposed.

Gasification kinetics of bulk coke in the CO2/CO/H2/H2O/N2 system simulating the atmosphere in the industrial blast furnace

The gasification characteristics and gasification kinetics of coke in complex CO2/CO/H2/H2O/N2 systems similar to the gas system of industrial blast furnace (BF) were studied by the method of isothermal thermogravimetric analysis. The experimental gas compositions and the corresponding temperature were chosen according to data reported for industrial BFs. The gasification behavior of coke was described by the Random Pore Model (RPM), Volumetric Model (VM), and Grain Model (GM). Results showed that the gas composition of the coke gasification zone in BF changes slightly and that the temperature is the most important factor affecting coke gasification. The lower activation energy of coke samples (Coke Reaction Index (CRI) > 50) is due to the high Fe2O3 in the ash, lower degree of graphitization, and larger pore structure. In addition, the choice of kinetic model does not differ substantially in describing the gasification mechanism of coke in a BF.

Novel Kinetic Model for Petcoke Gasification at High Temperature

Gasification kinetic parameters are valuable for designing a gasifier and optimizing operation conditions. In this work, a novel kinetic model named the apparent kinetic model (AKM) is presented and used for kinetic modeling of petcoke gasification under CO2 or steam atmospheres at high temperatures. The AKM was obtained from the Boltzmann equation distribution, which was consistent with the simplified gasification mechanism of petcoke at high temperature. The AKM was used for modeling petcoke gasification at various conditions to verify the applicability of the model. The kinetic modeling results of AKM and three common models were also compared in detail. The fitting results showed that the AKM prediction agrees very well with the gasification experimental results under two atmospheres at high temperature. It was amazing to find that the fitting values, such as t0.5, were close to the experimental values, with a very limited deviation. The fitting-corresponding R2 values of AKM were above 0.99 for most conditions, which indicated that AKM was suitable for petcoke gasification at high temperature with high accuracy. It was found that the gasification profile can be described well by AKM, whereas the other three common models showed great deviations in both low and high carbon conversion ranges. It can be concluded that the results of AKM are the best when used to simulate petcoke gasification at high temperature. The kinetic parameters obtained by AKM were accurate and conformed with the gasification reactivity of petcoke, which indicated that the AKM was an appropriate kinetic model for modeling the gasification of petcoke at high temperature.

Kinetics and mechanism of CO2 gasification of coal catalyzed by Na2CO3, FeCO3 and Na2CO3–FeCO3

The purpose is to develop a mild catalytic CO2-gasification technology that can promote CO2 utilization and reduce cost in air separation systems with improving system efficiency and obtaining desirable gaseous products. In this study, the influence of Na, Fe and their composite catalysts on the structure and gasification reactivity of chars derived from pyrolysis of Powder River Basin (PRB) coal was investigated. The results showed that a strong positive synergistic effect between Na and Fe catalyst in the gasification process was observed, the catalytic activity of the added catalysts was in order of: 4% Na > 3% Na–1%Fe > 2% Na-2% Fe > 1% Na-3% Fe > 1% Na-2% Fe > 4% Fe > raw coal. The catalysts inhibited the growth of the aromatic ring structure and enriched the generation of O-containing functional groups. Compared to Fe, the Na-based catalyst could easily diffuse into inner pores of coal char, forming C–O–Na structure and thus increasing the gasification reactivity of chars. In addition, due to the formation of inert material between SiO2 and Na, the catalytic activity of Na catalysts was significantly decrease at the late stage of char conversion. Comparatively, the Fe-based catalysts showed better stability life. Moreover, it was found that the activation energy for CO2-gasification of PRB coal can be decreased by 50% due to the addition of Na catalyst.

Effects of gaseous agents on the evolution of char physical and chemical structures during biomass gasification

Bio-char samples were prepared from gasification of corn straw under N2, CO2 and H2O conditions, and systematically characterized to reveal the effects of gaseous agents on the evolution of char structural features during the gasification process. The results showed that the increase of reacting temperature had positive effects on the gasification of char in both H2O and CO2 atmospheres. The evolution of char pore structures under H2O and CO2 was quite different. The formation of micropores was facilitated by CO2, while mesopores and macropores were developed more in H2O condition. Besides, char structures obtained at 800 °C were more ordered than those obtained at 600 °C. Compared with the longitudinal merging, the aromatic layers preferred to grow laterally. Moreover, the mechanisms of gasification between char and gaseous agents were different. CO2 preferred to react with amorphous carbon, while the cross-linked carbon was more likely to be consumed during char gasification with H2O.

3-3-4 草本バイオチャーのガス化速度とガス化機構

3-3-4 草本バイオチャーのガス化速度とガス化機構

Experimental study on gasification mechanism of unburned pulverized coal catalyzed by alkali metal vapor

The catalytic behavior of alkali vapor on unburned pulverized coal (UPC) prepared from bituminous coal, anthracite, semi coke and coke have been investigated. Experiment results show that the alkali metal vapor has catalytic effect on the gasification reaction of UPC. The catalytic effect of coke UPC is the most obvious, followed by anthracite UPC and bituminous coal UPC. Semi coke UPC is the least catalyzed by alkali metals but its gasification reactivity is the best. As the concentration of alkali metal vapor increases, the alkali metal content adsorbed by UPC increases, and there may be a phenomenon of saturated adsorption. The amount of alkali metal adsorbed is between 2% and 6%. AUPC has the highest ability to adsorb alkali metals, and SUPC is the weakest, which may be related to the minerals (CaO, Al2O3) in the coal ash. Changes in stomatal parameters are not the main factors affecting the catalytic effect. The results of X-ray diffraction experiments show that alkali metal aluminosilicate exists in the ash of coal after adsorption of alkali metal, and the average stack height of carbon atoms decreases significantly. The alkali metal catalytic gasification reaction is mainly achieved by destroying the carbon basic structural unit. The alkali metal nepheline and the interlayer compound may be the main catalytic substances, and the destructive ability of alkali intercalation is obviously stronger than that of alkali nepheline. The catalytic mechanism of alkali metals on unburned coal and the destruction mechanism of alkali metals on carbon crystals were discussed. It can provide reference for reducing alkali metal corrosion on coke and improving pulverized coal utilization rate.