Reaction Rate Of Char Research Articles

Using molecular dynamics simulations to study the non-thermal effects of microwave radiation on the mechanism of char gasification

Studying the chemical effects of alternating electric fields on char gasification is key to exploring the non-thermal effects of microwaves on the reaction mechanism. The gasification mechanism of char in the presence of an electric field was explored through molecular dynamics (MD) simulations and quantum chemical calculations. The results show that the promotion effect of 0.02 V/Å alternating electric field on the gasification reaction rate of char is about 8 times that of non-electric field, and the square wave electric field and static electric field are 6.79 times and 5.39 times respectively, this effect increases exponentially as the field intensity increases. Interestingly, the use of alternating electric fields showed, for the first time, that the changes in the electric field intensity and direction also have a significant effect. In terms of the adsorption process during gasification, the electric field promotes the formation of O and OH free radicals (about 5 times of no electric field) and increases the polarity of the char edge, both of which are conducive to adsorption. The electric field can weaken the curling of char macromolecules and expose more active carbon sites, thus increasing the overall reaction rate. For the desorption process during gasification, the presence of the electric field promotes the deformation and tear of char, which destroys its aromaticity, loosening the carbon-oxygen compounds at the carbon edge, which facilitates the desorption of CO, the desorption rate is about 10 times that of non-electric field system. Further, the change in direction of the electric field also promotes the desorption process. The results from this study suggest that the remarkable effects of alternating electric fields on char molecules can have an impact on the field of solid fuel gasification.

A ReaxFF study of the effect of pressure on the contribution of char-CO2 gasification to char conversion during pressurized oxy-fuel combustion

The effect of pressure on the contribution of char-CO2 gasification to char conversion during pressurized oxy-fuel combustion (POFC) at 10–25 bar was investigated using reactive force field molecular dynamics (ReaxFF MD) simulation. The results showed that char conversion rates under air (O2/N2), CO2/N2, and oxy-fuel (O2/CO2) atmospheres all increased with the increase of total pressure. Both the char-O2 oxidation under air atmosphere and char-CO2 gasification under CO2/N2 atmosphere followed the Langmuir-Hinshelwood (L-H) mechanism. The char reaction rates calculated by the L-H model based on the assumption of separate active sites and common active sites were compared with the ReaxFF MD simulation results under O2/CO2 atmosphere at different pressures. It was found that char-O2 oxidation competed with char-CO2 gasification under pressure of 10–15 bar, while the competition did not occur under pressure of 20–25 bar. The contribution of char-O2 oxidation and char-CO2 gasification to char conversion during char combustion in O2/CO2 atmosphere were quantitatively determined from the consumption of O2 and CO2. The char-O2 oxidation and char-CO2 gasification rates in O2/CO2 atmosphere at different char conversions were also examined. It indicated that the char-CO2 gasification rate was mainly controlled by the competition between char-CO2 gasification and char-O2 oxidation under low pressures, but was controlled by the partial pressure of CO2 under high pressures.

Study on the steam gasification reaction of biomass char under the synergistic effect of Ca‐Fe : Analysis of kinetic characteristics

Biomass char gasification is an important part of biomass conversion technology; the addition of additives can effectively change the gasification reaction rate of char. The kinetic characteristics of steam gasification reaction of biomass char particles were explored by adding CaO, Fe2O3, and Ca-Fe mixed catalyst to biomass, and the apparent activation energy and pre-exponential factors of char particles were obtained. Experimental results show that the kinetic reaction process of char steam gasification follows the shrinking cylinder model. Through linear fitting, it can be obtained that there is a good linear correlation between activation energy and pre-exponential factor, which follows the dynamic compensation effect. Under the same temperature conditions, CaO can accelerate the reaction rate of char, while Fe2O3 can delay the gasification rate of char particles because of sintering phenomenon at high temperature. The synergistic effect of Ca-Fe can reduce the total activation energy of the reaction. The addition of CaO can greatly change the kinetic performance of the Fe2O3 catalyst, inhibit the sintering deactivation reaction of Fe2O3 at high temperature, increase the diffusion path of steam, and promote the char steam gasification reaction.

The competitive behavior for O2 and CO2 reaction during char oxy-fuel combustion: effects of temperature and inherent minerals

Coal O2/CO2 combustion is a promising carbon capture and storage technology for coal-fired power plant. Char consumption rate prediction is essential for the boiler design, but challenging for O2/CO2 combustion since char reaction behavior within dual reactive gas media is complex. There is also a doubt of whether or not the O2 reaction and CO2 reaction occur independently, whereby the overall rate would be simply additive, or the two reactions compete for the same char actives, in which case the rate prediction could be more complicated. Here, the competitive behavior for O2 and CO2 reactions of char active sites was investigated via thermogravimetric approach, with a focus on the effect of temperature and inherent mineral composition. For char combustion within O2/CO2 environment with molar fractions of 30/70, the contribution of O2 to char consumption is the same as the O2 reaction alone. However, as being out-competed by O2, only 37% CO2 could occupy char active sites and react. Consequently, the overall char reaction rate within O2/CO2 is only 83% of the sum of that for individual gases at 900 °C. With increasing temperature, this ratio becomes even smaller, as the contribution of CO2 that being out-competed (to the sum of individual gases) is larger. With increasing O2 fraction, it occupied more active sites, thus further inhibiting CO2 reaction. The inherent alkali and alkaline earth metal provides the char with more active sites, thus muting the competition and inhibition for CO2 gasification.

The New Equations for “Rate-Determining Chemisorption of Coal”

The purpose of the new formulas, Cml, CmlK, and CmlY, which express the slowest char combustion rate, is to show the controlling mechanism of single coal burning. Oxygen diffusion through the boundary layer (as a result of releasing volatile matter from coal) to the char surface is the slowest step rate and can also represent as the rate determining. This step has not yet been taken into account in the literature and may effect incomparable decisions between numerical and experimental results of coal combustion studies. In the 1920s, Wilhelm Nusselt found the coal combustion equation for a single coal, which is based on initial coal diameter, and its burning time, or Nusselt square law (NSL). Also, the burning constant in NSL expressed oxygen partial pressure and the ambient temperature level. Nevertheless, recent studies according to char combustion have explained the effect of coal density on char combustion. Consequently, to help understand the slowest rate of char combustion, NSL as well as ordinary char combustion equations can be used together to establish the rate-determining factor. For this purpose, in this study, the slowest step of the char reaction rate is given as “Cml” of stable position for single coal particle, “CmlK” and “CmlY” for a coal particle in a motion.

SENSITIVITY ANALYSIS OF COAL GASIFICATION IN TWO-STAGE ENTRAINED-FLOW GASIFIER: SYNGAS AND CARBON CONVERSION PREDICTION

The energy production from coal-fired power plant is increasing day by day, which result in increased CO 2 emission from the existing power plant. However, CO 2 emission from coal gasification can be reduced if an efficient CO 2 /O 2 /N 2 coal gasification is implemented in IGCC system. Numerical simulations of coal gasification under CO 2 /O 2 /N 2 gasification condition are carried out with the aim of describing the effects of model parameters, char reaction rates, operating conditions and heat losses to increase the syngas heating value and carbon conversion in a two stage entrained flow coal gasification process. The Eulerian–Lagrangian approach is applied to solve the Navier–Stokes equation and the particle dynamics. Finite rate/eddy dissipation model is used to calculate the rate of nine homogeneous gas-to-gas phase reactions. While only finite rate is used for the heterogeneous solid-to-gas phase reactions. It is found that the carbon conversions of combustor coal lie in the ranges from 97 wt% to 99 wt% for most of the calculated conditions. On the other hand, the carbon conversion of reductor coals varies from 45 wt% to 57 wt%. A noticeable change is obtained when the gasification occurs under a high-temperature condition. Remarkable outlet results of about 32 wt% CO, 0.58 wt% H 2 and 89 wt% overall carbon conversion are predicted if a high temperature of 1673K is maintained in the reductor. On the other hand, a reduced soot concentration is predicted if the O 2 concentration and/or the reductor gas temperature increase(s) in the gasifier.

1-1-2 酸素キャリア存在条件における石炭チャー反応速度の解析

1-1-2 酸素キャリア存在条件における石炭チャー反応速度の解析

Measurement of reaction rates for pulverized fuel combustion in air and oxyfuel atmosphere using a novel fluidized bed reactor setup

The reaction rate of char from pulverized Columbian coal (Mina Norte) is investigated in synthetic air (N2/O2), oxyfuel atmosphere (CO2/O2) and CO2/N2 using a lab-scale fluidized bed reactor (FBR). Reactor temperatures range from 823 to 1273K for combustion and 1173 to 1373K for gasification (Boudouard). The oxygen volume concentration is varied between 15 and 30vol.%, while gasification is investigated in a mixture of 10–75vol.% CO2 in nitrogen. Using an nth order Arrhenius approach, activation energies as well as apparent order of reaction are calculated for the combustion and gasification reactions.It is found that the combustion reaction with this particular fuel evolves between +17% (873K) and +75% (1223K) faster in N2/O2 than in CO2/O2. The results of Arrhenius fit suggest that activation energy of combustion reaction does not differ significantly between synthetic air (regime I: 120.9kJ/mol, regime II: 62.9kJ/mol) and oxyfuel atmosphere (116.6kJ/mol, 64.3kJ/mol). Comparing results for oxyfuel and air, a difference of approximately 50K in the transition temperature from regime I to regime II is observed but this finding is not statistically firm, yet. The apparent order of reaction has been calculated to n=0.72 in air (combustion), n=0.66 in oxyfuel (combustion) and n=0.49 in CO2/N2 (gasification).A comparison with available literature data confirms that the results achieved with the fluidized bed are comparable to the two most common experimental setups used in combustion research: Entrained flow reactors and thermogravimetric analyzers. The experimental setup also represents a novelty in FBR systems, as it quantitatively captures reactions with an apparent 90% carbon conversion timet90 of 3s, which is a third of the time of comparable setups described in literature.

Model development for biomass gasification in an entrained flow gasifier using intrinsic reaction rate submodel

Intrinsic reaction rate submodel is established in this study to consider the effects of diffusion rate and kinetic rate for simulating the char reactions due to their slow reaction rates and important controlling steps. The biomass gasification model for an entrained flow gasifier is developed with the Euler–Lagrange method using ANSYS FLUENT software. Gas phase is treated as continuous phase in standard k–ε model to close governing equations whereas biomass particles are treated as discrete phase in discrete phase model (DPM) to track the movement of particles. For homogeneous phase reactions, finite rate/eddy dissipation model is applied to calculate the reaction rates. For heterogeneous phase reactions, intrinsic reaction rate model is realized by coding the user-defined functions (UDFs) to calculate char reaction rates. The results obtained from this study show that the relative errors of volumetric concentrations are mainly within the range of 1–18% and the relative errors of lower heating value, gas production, cold gas efficiency and carbon conversion efficiency are within the ranges of 1–13%, 1–8%, 1–12% and 1–11%, respectively. The CFD model developed in this study can be used to simulate biomass gasification processes for entrained flow gasifiers.

MICROWAVE PLASMA GASIFICATION OF OIL PALM BIOCHAR

Conventional pressurized gasification operates at higher pressure than atmospheric pressure and requires heat up time during startup. In this study, microwave plasma gasification was used to compensate this problem. The objectives of this paper is to investigate the CO2 microwave gasification of EFB and OPS biochar, and optimizing the char reaction rate through the addition of activated carbon as the microwave absorber. A microwave plasma gasification test rig was designed to produce syngas from oil palm biochar. From the study, it was found that EFB char performed better than OPS char as gasification fuel due to its high porosity and surface area that increased the char reactivity towards CO2. The temperature increment promoted by the addition of MW absorber using activated carbon (AC) has increased the CO composition. The optimum condition for microwave plasma char gasification of EFB was 3 lpm with 25 wt% AC that produced syngas with 1.23 vol% CH4, 20.88 vol% CO2, 43.83 vol% CO, 34.06 vol% H2 and 9.40 MJ/kg gas CV. For OPS is at 2 lpm with 1.12 vol% CH4, 35.11 vol% CO2, 35.42 vol% CO, 28.35 vol% H2 and 7.32 MJ/kg gas CV. As EFB char has larger BET surface areas and larger pores than OPS char, the ability to react with the gasifying gas is better than the OPS. Thus, resulting in higher carbon conversion. The best gasification efficiency was 72.34% at 3 lpm, 10% AC for EFB biochar plasma gasification with 12% unreacted carbon. For OPS biochar plasma gasification, the best gasification efficiency was 69.09% at 2 lpm, 10% AC with 18% unreacted carbon.

Establishment of kinetic parameters of particle reaction from a well-stirred fluidized bed reactor

A novel method is presented for experimental study of gas-particle reactions, based on realizing a well-stirred reactor as a small scale fluidized bed. This reactor is evaluated against the drop tube reactor and the thermo-gravimetric analyser. It shows to enable high heat up rates (∼104 K/s), long timescale observation (up to several hours), operation with small fuel particles (∼100 μm) and accurate control of reaction conditions. Char reaction rates are established from real-time gas product analysis by FTIR spectroscopy, through a detailed data-analysis procedure. This procedure employs a particle surface-evolution model and accounts for sampling system signal attenuation. The validity of the well-stirred conditions is established, and the method is employed for char combustion and gasification. Highly consistent results for char gasification over a wide range of conditions (T = 800–1100 °C, CCO2 = 19–76%), are used to demonstrate the establishment of kinetic parameters for an n-th order approach. Activation energy and order of reaction are found and compare well with the literature.

3-D numerical simulation for co-firing of torrefied biomass in a pulverized-fired 1 MWth combustion chamber

Torrefaction process is a promising technology for changing the chemical and physical properties of biomass so, that it can be used in existing pulverized fuel firing systems. A numerical 3-D simulation to study the combustion behaviour of torrefied biomass in a pulverized-fired furnace has been carried out. In the model different reaction kinetics for devolatilization and char oxidation of three biomass components, namely hemicellulose, cellulose and lignin, have been applied. The reaction kinetic parameters for pyrolysis and char oxidation were determined by experimental work using thermogravimetric analysis (TGA). In numerical studies three different fuel blends were examined, which include pure coal, ∼9% and ∼17% (thermal basis) torrefied sawdust in coal. Gas concentrations, temperature distribution, pyrolysis and char reactions were analyzed along the 1 MWth combustion chamber. The numerical investigation suggests that the torrefied biomass can be used as a substitute fuel for coal without modifications in the co-firing system. However, the case-study with the highest biomass substitute leads to an incomplete decomposition of the lignin component, due to coarser biomass particles and decreased char reaction rates.

ICOPE-15-1134 Fundamental study on the char reaction rate of pulverized coals

ICOPE-15-1134 Fundamental study on the char reaction rate of pulverized coals

Eulerian–Eulerian multi-fluid methods for pulverized coal flames with nongray radiation

A new Favre averaging method based on gas phase density applied to Eulerian–Eulerian simulations of turbulent multiphase reacting flows is proposed. The new Favre averaging method can reproduce turbulent dispersions of mass, momentum, heat and species of solid phases from their averaged transport equations. A new radiation model that can account for nongray gas–solid mixtures is developed for pulverized coal flames. The new models are validated against a pulverized coal ignition flame. Predictions of velocity, temperature and gas major species are found in good agreement with experiments. Pyrometer measurements of temperature are predicted by a radiation transport calculation. It is found that radiation cooling effects can lead to a 500K temperature decline in solid phases. Radiation also has indirect influence on CO2 predictions through the slow yet strongly temperature-dependent char reaction rates. The consideration of radiation improves predictions of both temperature and CO2 concentrations. Effects of devolatilization model and number of phases are also discussed.

Gasification kinetics during entrained flow gasification – Part II: Intrinsic char reaction rate and surface area development

The reaction behaviour of two different coals is analysed in a pressurised entrained flow reactor (PiTER) and a pressurised thermogravimetric analyser (PRETA). The entrained flow reactor is used to produce char samples under conditions relevant to larger scale entrained flow gasifiers, i.e. at temperatures up to 1600°C and pressures up to 2.5MPa. The conversion rate of these char samples is measured under defined conditions in the PRETA using CO2 and H2O atmospheres.Furthermore, the development of char surface area during devolatilisation and char gasification is analysed in the PiTER. The initial surface area after devolatilisation is dependent on pressure. In the later stages of conversion a significant loss of surface area is detected. The decrease in surface area is attributed to a melting of mineral matter as the PiTER is operated above the ash melting temperature of the coals. The development of the mass specific surface area during gasification cannot be described by the Random Pore Model probably because of the ash melting, but by an empirical correlation.The surface area data are combined with the thermogravimetric analysis to derive the intrinsic reaction rates at the char surface. The intrinsic rate is modelled by nth order and Langmuir–Hinshelwood rate equations. The temperature influence on the char–CO2 reaction can be described by an activation energy of 200kJ/mol, and on the char–H2O reaction by an activation energy of 212kJ/mol. The activation energy is found to be independent of the char preparation conditions, whereas the pre-exponential factor significantly decreases for a higher heat treatment severity of the char sample.The model equations can be used to predict the intrinsic gasification behaviour in the absence of mass transport limitations.

Ash of palm empty fruit bunch as a natural catalyst for promoting the CO2 gasification reactivity of biomass char

Palm empty fruit bunch ash (EFB-ash) was used as a natural catalyst, rich in potassium to enhance the CO2 gasification reactivity of palm shell char (PS-char). Various EFB-ash loadings (ranging from 0 to 12.5wt.%) were implemented to improve the reactivity of PS-char during CO2 gasification studies using thermogravimetric analysis. The achieved results explored that the highest gasification reactivity was devoted to 10% EFB-ash loaded char. The SEM–EDS and XRD analyses further confirmed the successful loading of EFB-ash on PS-char which contributed to promoting the gasification reactivity of char. Random pore model was applied to determine the kinetic parameters in catalytic gasification of char at various temperatures of 800–900°C. The dependence of char reaction rate on gasification temperature resulted in a straight line in Arrhenius-type plot, from which the activation energy of 158.75kJ/mol was obtained for the catalytic char gasification.

DTF를 이용한 초청정 석탄 촤 산화 반응률 특성 연구

본 연구는 낮은 회 성분의 함량과 높은 발열량의 특성을 지닌 초청정 석탄의 촤 반응율 특성을 알아보았다. 실험은 DTF(Drop Tube Furnace)를 통해서 다양한 온도조건 하에 산소의 분율을 바꾸어가며 수행하였다. 촤 반응률을 도출하기 위하여 FT-IR을 통해 배기가스(CO, CO2 )를 측정하였으며, 이색온도계를 통하여서 입자 온도를 측정하였다. 또한, Arrhenius 경험식을 토대로 초청정 석탄 촤의 활성화 에너지와 빈도인자를 도출하였다. 결과는 초청정 석탄 촤의 반응특성은 온도와 산소 분율이 높아질수록 뚜렷한 증가를 보였고, 초청정 석탄 촤의 활성화 에너지는 역청탄의 수치와 비슷한 값을 보임을 알 수 있었다.

Influence of carbon structure and mineral association of coals on their combustion characteristics for pulverized coal injection (PCI) application

The influence of carbon structure and mineral matter of three pulverized coals on their char characteristics including reactivity was studied under a range of combustion conditions in a drop tube furnce (DTF) and thermogravimetric (TGA) furnace for PCI application. Physical and chemical properties of coals and their combustion derivatives were characterized by automated reflectogram, X-ray diffraction, scanning electron microscope, and BET N2 adsorption. The QEMSCAN technique was used to characterize the heterogeneous nature of minerals of discrete coal particles. The TGA char reactivity was related to the proportion of coal particles displaying strong association of calcium/sulfur phases with carbon matrix to highlight the catalytic influence of minerals on char reactivity at low temperatures. The study suggested that during DTF combustion tests at 1200 °C, char reaction rates might have been catalyzed by coal minerals, particularly due to illite and its association with carbon. Under the same combustion conditions, most of the coal minerals did not transform significantly to slag phases. Coal burnout was found to improve significantly in a combustion temperature range of 1200 °C to 1500 °C. The improvement of coal burnout with temperature appeared to be influenced by coal properties, particularly as a function of the chemical nature of minerals, as well as the degree of associations with other minerals. The study implies that coals with similar mineral compositions might not necessarily reflect similar combustion behavior due to the differences in their associations with other phases. The study highlighted the significance of the characterization of the heterogeneity of coal particles including mineral associations for a comprehensive and reliable assessment of the combustion performance of PCI in an operating blast furnace.

Effects of gas temperature fluctuation on the instantaneous char reaction of pulverized coal particle

The heterogeneous char reaction processes of pulverized coal particle in a hot gas flow with temperature fluctuation are investigated. The instantaneous mass variations and char reaction rates of the particles with initial diameters of 10–50 μm are calculated under different conditions. The gas temperature fluctuation has evident influences on the instantaneous char reaction processes of the pulverized coal particles. The instantaneous char reaction rates with the gas temperature fluctuation are different from those without the gas temperature fluctuation. The gas temperature fluctuation leads to more rapid char reaction and faster mass loss of the particles. The effects of fluctuation amplitude of the gas temperature and particle Reynolds number on the instantaneous char reaction processes are delineated.

Effect of Heating Rate on Steam Gasification of Biomass. 1. Reactivity of Char

Steam gasification or pyrolysis of biomass was conducted at heating rates of 1, 10, and 100 K s-1 using a thermobalance reactor. Three kinds of biomass samples (cellulose, lignin, and bagasse) were used. Time profiles of weight change of biomass samples during steam gasification and pyrolysis were measured at 973 K. Char obtained in steam gasification of lignin was analyzed with a scanning electron microscope and a CHNS elemental analyzer. The effect of the heating rate on final conversion and reaction rate of char was investigated. It was found that a higher heating rate substantially increased the reaction rate of lignin char in steam gasification because porous char was produced during devolatilization due to rapid evolution of volatiles. A higher heating rate also increased final conversion of biomass. The heating rate showed no pronounced influence on elemental composition of char.