Char Gasification Kinetics Research Articles

Gasification Kinetics of Date palm seed using Carbon Dioxide

Gasification is a well-known reaction owing to its relevance to generation of sustainable energyfrom biomass and development of porous carbons. The present paper attempts to experimentally investigate the kinetics of palm shell char gasification using carbon dioxide (CO2) in a controlled environment using Thermo Gravimetric Analyzer (TGA) at temperatures ranging from 800 to1000°C. A relevant kinetic model representing the experimental data was identified by fitting the experimental data with popular semi empirical kinetic models such as Linear Model (LM), Volume Reaction Model (VRM), Shrinking Core Model (SCM), and Random Pore Model (RPM). The model kinetic parameters were evaluated by minimizing the sum of root mean square error (RMSE). Among the models tested the RPM exhibited very close adherence to the experimental data evidenced from the minimum RMSE of 0.0046. The ability of the RPM model to represent the gasification kinetics was attributed to its ability to account for the pore growth during initial stages of gasification and destruction of pores due to coalescence in later stages of gasification. The rate of reaction increased with increase in temperature and activation energy was found to be 64.5 KJ/mol.

High-Pressure Char Gasification Kinetics: CO Inhibition of the C–CO2 Reaction

Results of high-pressure (up to 3.0 MPa) CO-inhibited char–CO2 reaction rate measurements are presented and analyzed using a Langmuir–Hinshelwood (LH) kinetic model commonly used to describe gasification kinetics at atmospheric pressure. Despite previous success by the authors and others in using this reaction scheme to describe high-pressure kinetics of uninhibited gasification reactions, this work shows that it is unable to describe well CO-inhibited reaction kinetics at (relatively) high partial pressures of CO and CO2. Measurements of the char surface area suggest significant impact of CO and CO2 partial pressures on the development of the char surface area during the reaction (and, therefore, most likely on the term describing the number of active sites on the char surface in the LH rate equation). This term is considered to be a constant in the standard LH formulation; therefore, an alternative LH approach to describe the kinetics is presented, whereby rates are described using the LH equation on a ...

Gasification reactivity of char with CO2 at elevated temperatures: the effect of heating rate during pyrolysis

ABSTRACTIntegrated coal gasification combined cycle (IGCC) technology is being promoted to make better use of energy resources, and an entrained‐flow gasifier is a key item of equipment in the process due to its high gasification efficiency, smooth discharge of molten ash, etc. The temperature and heating rate under entrained‐flow gasification conditions are very high, and these two parameters can influence the gasification reactivity of a char with CO2. Therefore, the clarification of characteristics and mechanisms of char gasification under conditions close to entrained‐flow gasification is very important. To obtain a valid kinetics of char gasification, it is necessary to clarify the effect of heating rate during pyrolysis on char reactivity. The effect of heating rate during pyrolysis on the gasification reactivity of three types of chars in CO2 was investigated experimentally at elevated temperatures in a novel fluidized bed. It was shown that even at elevated temperatures, the heating rate has significant influence on the char reactivity during gasification. A higher heating rate during pyrolysis leads to higher char reactivity of gasification, however it has only limited effect on the activation energy of char gasification with CO2. The char reactivity is closely related to its specific surface area and pore volume. The high reactivity of a char derived from pyrolysis at a high heating rate is most likely due to its enhanced porous structure as a result of the rapid release of volatile matter at a high heating rate. The effect of heating rate during pyrolysis on char reactivity is pronounced at low temperatures, and is very different for various coal types, i.e. very pronounced for a low‐rank coal. The gasification reactivity of a char is related to both the char pore structure and temperature, which suggests that even at elevated temperatures the gasification of a char with CO2 is controlled by both chemical reaction and diffusion inside the particles. Our results demonstrate that it is necessary to derive the gasification kinetics of a char at a high heating rate to obtain valid kinetic equations for an entrained‐flow gasifier. Copyright © 2010 Curtin University of Technology and John Wiley & Sons, Ltd.

DDGS chars gasification with CO2: a kinetic study using TG analysis

Dry Distiller’s Grains with Solubles (DDGS) is a by-product during ethanol production from cereals which is currently mainly used as feedstock for cattle. With the growth of the ethanol industry, the increasing supply of DDGS may saturate the livestock feed market; thus, its potential applications need to be explored. DDGS gasification in a 100-kWth circulating fluidized bed (CFB) steam-O2 blown gasifier has been studied. However, the modeling of DDGS gasification process encounters difficulties due to the unavailable knowledge of DDGS char gasification kinetics. Therefore, in this paper, gasification kinetics of DDGS char with CO2 was investigated using thermogravimetric analysis (TGA). Two different types of char samples have been tested. Char type one (PYR-Char) was obtained after DDGS pyrolysis in a TGA at a final temperature of 750°C or 850°C for 20 min. Char type two (CFB-Char) was obtained after DDGS gasification in the 100-kWth CFB gasifier within the temperature range of 790°C to 820°C with a steam/biomass mass ratio of 0.81 and oxygen to biomass stoichiometric ratio of approximately 0.38. The influences of pyrolysis temperature (750°C, 850°C), heating rate (10°C/min, 30°C/min, 50°C/min, 70°C/min), CO2 concentration (10, 20, 30 vol.%), and gasification temperature (900°C, 1,000°C, 1,100°C) on the reaction rate of char-CO2 reaction were determined. Two representative gas–solid reaction models, the volumetric reaction model (VRM) and the shrinking core model (SCM) were applied in order to determine kinetic parameters. It was found that the calculated activation energy (E a) values using SCM were slightly lower than those using VRM. The calculated E a values for PYR-Char using both models were in the range of 100–165 kJ/mol, while the calculated E a values for CFB-Char were in the range of 55–100 kJ/mol. It was observed by scanning electron microscopy (SEM) that CFB-Char was more fragile and PYR-Char obtained at lower heating rate had a less porous structure. Generally, the predicted results using both models showed a fairly good agreement with experimental results, and SCM model suited slightly suitable better for char gasification at high temperature.

Gasification of woodchip particles: Experimental and numerical study of char–H 2O, char–CO 2, and char–O 2 reactions

In wood gasification, oxidation of char particles by H 2O, CO 2 or O 2 plays a major role in the performance and efficiency of air gasifiers. These reactions are generally analyzed under carefully design and controlled laboratory conditions, using either micro-samples to focus on the reaction kinetics or large spherical particles, but rarely using the real shape encountered in industrial processes. The objective of this work was to conduct a complete parametric study on char gasification kinetics at particle scale in operating conditions like those of industrial applications. Experimental results from a macro-Thermo Gravimetric reactor are compared to those from a char particle model, which analyzes reactivity versus conversion through the surface function F( X). We first show that particle thickness is a representative dimension of a char particle with respect to its apparent kinetics. Second, considering the three reactions independently, we compared the influence of temperature (800–1050 °C) and reacting gas partial pressure (0.03–0.4 atm) and determined the intrinsic kinetic parameters and surface function F( X). Simulations provided profiles of temperature and gas concentrations within the particle, mainly revealing internal mass diffusion limitation. The experimental data base proposed and the model results improve our understanding of the gasification reaction and support the elaboration of process models.

Kinetics of woodchips char gasification with steam and carbon dioxide

Kinetics of woodchips char gasification has been examined. Steam and CO 2 were used as the gasifying agents. Differences and similarities between kinetics of steam gasification and CO 2 gasification have been discussed. Comparison was conducted in terms of gasification duration, evolution of reaction rate with time and/or conversion, and effect of partial pressure on reaction rate. Reactor temperature was maintained at 900 °C. Partial pressure of gasifying agents varied from 1.5 bars to 0.6 bars in intervals of 0.3 bars. Steam and CO 2 flow rates were chosen so that both gasifying agents had equal amount of oxygen content. CO 2 gasification lasted for about 60 min while steam gasification lasted for about 22 min. The average reaction rate for steam gasification was almost twice that of CO 2. Both reaction rate curves showed a peak value at certain degree of conversion. For steam gasification, the reaction rate peak was found to be at a degree of conversion of about 0.3. However, for CO 2 gasification the reaction rate peak was found to be at a conversion degree of about 0.1. Reaction rates have been fitted using the random pore model (RPM). Average structural parameter, ψ for steam gasification and CO 2 gasification was determined to be 9 and 2.1, respectively . Average rate constant at 900 °C was 0.065 min −1 for steam gasification and 0.031 min −1 for CO 2 gasification. Change in partial pressure of gasifying agents did not affect the reaction rate for both steam and CO 2 gasification.

Reaction kinetics and mass transfer studies of biomass char gasification with CO 2

Gasification kinetics of wheat straw char with CO 2 was investigated using Thermogravimetric apparatus (TGA). The main objective was to identify the diffusional and surface reaction phenomena that may occur during biomass char gasification experiments with CO 2. The effects of temperature (750–900 °C) and particle size (<60–925 μm) on gasification rate of char-CO 2 reaction were determined. The 50% conversion ( r 50) rate showed that the reactivity increases with temperature, and it decreases as the particle size increases. The important diffusional parameters such as effective diffusivity, effectiveness factor and Thiele modulus were calculated based on the experimental data and the results showed that the impact of physical factors is prominent at high temperatures and large particle sizes. It was found that char gasification within the temperature range studied followed the chemically controlled reaction regime and the influence of pore diffusion was negligible for fine powder particles.

Kinetic study of Argentinean asphaltite gasification using carbon dioxide as gasifying agent

The kinetics of Argentinean asphaltite char gasification using carbon dioxide as gasifying agent was studied between 1048 and 1223 K. The chars were obtained by pyrolysis of an asphaltite in a fixed bed reactor. The relative mass change during the gasification reaction was continuously monitored using a high resolution thermogravimetric system. The starting temperature for the reaction of the char with carbon dioxide was determined at about 800 K. The influence of gaseous flow rate, sample mass, carbon dioxide partial pressure and temperature in the reaction rate was analyzed. The experimental condition under which the progress of gasification reaction is chemically controlled was established. In those conditions, an activation energy of 185 kJ mol −1 was obtained with an isoconversional method. Concerning the influence of carbon dioxide partial pressure, it was determined that pressures greater than 50 kPa do not alter the kinetics regime. Finally a reaction order of 0.5 for carbon dioxide partial pressure was determined and a global rate equation that includes these parameters was developed. Rate = d α dt = ( 1 - α ) 2 / 3 · 1.1 × 10 5 exp - 185 kJ mol - 1 R · T · P CO 2 0.5

Kinetics of Char Gasification with CO2 under Regime II Conditions: Effects of Temperature, Reactant, and Total Pressure

There are relatively few studies reported in the literature characterizing the kinetics of high-pressure, high-temperature char gasification reactions. Of particular interest to the application of kinetic data to gasification models are studies that provide links between reaction rate data relevant to high-temperature conditions, where some extent of pore diffusion limitation may apply, and lower-temperature, intrinsic gasification reactivity data. This work describes the effects of temperature, reactant partial pressure, and total pressure on the kinetics of the char−CO2 reaction under conditions where reactant diffusion through the pores of the reacting char has been shown to impact the overall conversion rate. A global nth-order rate equation was used to describe these kinetics, in particular, the effect of temperature (via the activation energy) and reactant pressure (via the reaction order). As expected, activation energies at high temperatures (≥∼1300 K) were consistently less than those obtained un...

木質バイオマスチャーのガス化速度に及ぼす熱分解条件の影響(チャー物性とガス化反応性の関係)

This paper reports on the relationship between biomass char property and gasification reactivity. The gasification kinetics of biomass char has been revealed by measuring the rate of weight loss during reaction with CO2, as a function of temperature. First-order kinetic rate constants are determined by fitting the weight loss data using a random pore model. The relationship between char structure and CO2 gasification reactivity was investigated in the range of 873 to 1073K (pyrolysis temperature), and 0.1 to 3.0 Mpa (pyrolysis pressure) at 1073K. The experimental results indicate that the reactivity of the biomass char is determined by pyrolysis condition. The gasification rate of biomass char decreases with increasing pyrolysis temperature. This is due to promotions of carbonization and dehydrogenation of biomass char. The CO2 gasification rates in char generated at 0.1 MPa exhibited approximately twice values compared to those at 3.0 MPa. This is because the uniformity of carbonaceous structure rises with increasing pyrolysis pressure. The uniformity of carbonaceous structures would affect the CO2 gasification reactivity, and the decreasing uniformity would lead to the progression of cavities on the char surface during the CO2 gasification process.

Pyrolysis and gasification of food waste: Syngas characteristics and char gasification kinetics

Characteristics of syngas from the pyrolysis and gasification of food waste has been investigated. Characteristic differences in syngas properties and overall yields from pyrolysis and gasification were determined at two distinct high temperatures of 800 and 900 °C. Pyrolysis and gasification behavior were evaluated in terms of syngas flow rate, hydrogen flow rate, output power, total syngas yield, total hydrogen yield, total energy yield, and apparent thermal efficiency. Gasification was more beneficial than pyrolysis based on investigated criteria, but longer time was needed to finish the gasification process. Longer time of gasification is attributed to slow reactions between the residual char and gasifying agent. Consequently, the char gasification kinetics was investigated. Inorganic constituents of food char were found to have a catalytic effect. Char reactivity increased with increased degree of conversion. In the conversion range from 0.1 to 0.9 the increase in reactivity was accompanied by an increase in pre-exponential factor, which suggested an increase in gasifying agent adsorption rate to char surface. However, in the conversion range from 0.93 to 0.98 the increase in reactivity was accompanied by a decrease in activation energy. A compensation effect was observed in this range of conversion of 0.93–0.98.

Modeling of catalytic gasification kinetics of coal char and carbon

Calcium- and potassium-catalyzed gasification reactions of coal char and carbon by CO 2 are conducted, and the common theoretical kinetic models for gas–carbon (or char) reaction are reviewed. The obtained experimental reactivities as a function of conversion are compared with those calculated based on the random pore model (RPM), and great deviations are found at low or high conversion levels as predicted by theory. Namely, calcium-catalyzed gasification shows enhanced reactivity at low conversion levels of <0.4, whereas potassium-catalyzed gasification indicated a peculiarity that the reactivity increases with conversion. CO 2 chemisorption analysis received satisfactory successes in both interpreting catalytic effects and correlating the gasification reactivity with irreversible CO 2 chemical uptakes (CCU ir) of char and carbon at 300 °C. In details, calcium and potassium additions led to significant increases in CCU ir and correspondent high reactivities of the char and carbon. Furthermore, CCU ir of char and carbon decreased with conversion for calcium-catalyzed reaction but increased for potassium-catalyzed one, corresponded to the tendency of their reactivity. The RPM is extended and applied to these catalytic gasification systems. It is found that the extended RPM predicts the experimental reactivity satisfactorily. The most important finding of this paper is that the empirical constants in the extended RPM correlate well with catalyst loadings on coal.

Pyrolysis and Gasification Experiments of Biomass under Elevated Pressure Condition

This paper reports on the pyrolysis and CO2-gasification of various biomass materials in a pressurized thermobalance. In particular, the primary yields of total volatiles, tar and non-condensable gases, together with the composition of non-condensable gases, are measured as a function of temperature at 1 atm and 10.8 ata. The use of a high-intensity infrared heating source, in conjunction with a non-absorbing carrier gas (viz. argon), is reported to reduce the significance of secondary gas-phase pyrolysis reactions. Linear trends with atomic H/C ratio are observed in the tar yield, CH4 yield and total volatile yield during pyrolysis. The gasification kinetics of biomass chars (pyrolysed to 700°C at a medium heating rate) have also been measured by measuring the rate of weight loss during reaction with CO2, as a function of temperature. First-order kinetic rate constants are then determined by fitting the weight loss data using a random pore model. The results indicate that the gasification rate of biomass char increases as pressure increases.

Gasification reaction kinetics on biomass char obtained as a by-product of gasification in an entrained-flow gasifier with steam and oxygen at 900–1000 °C

The purpose of this study was to investigate the gasification kinetics of biomass char, such as the wood portion of Japanese cedar char (JC), Japanese cedar bark char (JB), a mixture of hardwood char (MH) and Japanese lawngrass char (JL), each of which was obtained as a by-product of gasification in an entrained-flow type gasifier with steam and oxygen at 900–1000 °C. Biomass char was gasified in a drop tube furnace (DTF), in which gasification conditions such as temperature ( T g), gasifying agent (CO 2 or H 2O), and its partial pressure ( P g) were controlled over a wide range, with accompanying measurement of gasification properties such as gasification reaction ratio ( X), gasification reaction rate ( R g), change of particle size and change of surface area. Surfaces were also observed with a scanning electric microscope (SEM). By analyzing various relationships, we concluded that the random pore model was the most suitable for the biomass char gasification reaction because of surface porosity, constant particle size and specific surface area profile, as well as the coincidence of R g, as experimentally obtained from Arrhenius expression, and the value is calculated using the random pore model. The order of R g was from 10 −2 to 10 −1 s −1, when T g = 1000 °C and P g = 0.05 MPa, and was proportional to the power of P g in the range of 0.2–0.22 regardless of gasifying agent. Reactivity order was MH > JC > (JB, JL) and was roughly dependent on the concentration of alkali metals in biomass feedstock ash and the O/C (the molar ratio of oxygen to carbon) in biomass char.

High-pressure gasification reactivity of biomass chars produced at different temperatures

The knowledge of biomass char gasification kinetics is of considerable importance in the design of advanced biomass gasifiers, some of which operate at high pressures. In the present work the effects of pyrolysis temperature, total pressure and CO 2 concentration on the gasification of biomass chars have been studied using the thermogravimetric approach. The chars were obtained by pyrolysis in a drop tube furnace reactor at temperatures of 1000 and 1400 °C. The gasification tests were carried out in a pressurized thermogravimetric analyser (PTGA) at different temperatures, pressures and CO 2 concentrations. The reactivity measurements were conducted under the kinetically controlled regime, and three nth-order kinetic models as well as the Langmuir–Hinshelwood model were applied to determine the kinetic parameters.

Effect of pyrolysis conditions on gasification reactivity of woody biomass-derived char

The effect of pyrolysis conditions on char reactivity has been studied using Raman spectroscopy. This paper reports on the relationship between the properties of biomass char and the gasification rate. The gasification kinetics of biomass char have been revealed by measuring the rate of weight loss during its reaction with CO 2 as a function of temperature. First-order kinetic rate constants are determined by fitting the weight loss data using a random pore model. The relationship between the char structure and CO 2 gasification reactivity was investigated in the range of 15–600 °C/min at a constant pyrolysis pressure (0.1 MPa), and 0.1–3.0 MPa at a constant heating rate (15 °C/min). The experimental results reveal that the reactivity of biomass char is determined by the pyrolysis condition. The CO 2 gasification rates in char generated at 0.1 MPa exhibited approximately twice the values as compared to those obtained at 3 MPa. This is because the uniformity of the carbonaceous structure increases with the pyrolysis pressure. The uniformity of carbonaceous structures would affect the CO 2 gasification reactivity, and the decreasing uniformity would lead to the progression of cavities on the char surface during the CO 2 gasification process. The gasification rate of biomass char increases with the heating rate at pyrolysis. This is due to the coarseness (surface morphology) of biomass char and rough texture, which increases with the heating rate.

CO 2 and H 2O Gasification Kinetics of a Coal Char in the Presence of Methane

The kinetics Of CO2 and H2O gasification of a Chinese coal char in the presence of methane has been studied using a thermogravimetric analyzer (TGA) in the temperature range from 1173 to 1273 K. It was observed that the rate of carbon conversion increased with an increasing temperature and decreased with an increasing CH4 concentration. The rate of carbon conversion during the gasification of the char experienced a maximum without the presence of methane but decreased continuously with carbon conversion in the presence of methane. Carbon deposition from CH4 cracking reduced the overall rate of carbon conversion during gasification, but this effect was reduced as the temperature and concentration Of CO2 or H2O were increased. A gas-solid reaction shrinking core model was developed and applied to the char-CH4-CO2/H2O systems. The experimental results were used to determine the kinetic parameters. The model predictions compared favorably to the measured data. The overall carbon conversion rate has been shown to be a combination of carbon gasification, by CO2 or H2O, respectively, and methane cracking.

The effects of diffusional resistance on wood char gasification

The steam gasification kinetics of wood char is studied. First, the kinetic expression of wood char gasification is derived (kinetically controlled). The modelling, uses the random pore model, has been experimentally validated. The char gasification study is performed in a tubular kiln, at various temperatures (850 °C, 900 °C, 950 °C and 1000 °C) and vapour pressures (0.2 bar, 0.5 bar and 0.7 bar). The steam gasification kinetics parameters (activation energy…), together with the physical properties of the char (porosity, specific surface…) are experimentally determined. By performing experiments with different particle sizes, steam flow rates and initial sample weights, the experimental conditions which permit the minimization of the steam penetration limitation, are found. Secondly, a 1D isothermal and non-equimolar reaction-diffusion model is developed in order to assess the diffusional effects taking place during steam gasification in a fixed bed. The model takes into account local chemical reaction rate and effective transport properties dependant on time and position within the char bed. Inter particle diffusion and structural changes of the char bed during the gasification are taken into account. The model shows a good agreement with the experimental results, obtained at various bed heights, and point out the relevant role played by diffusional effects within a fixed bed reactor. Finally, the kinetic results are used to estimate, in a down draft gasifier, the total carbon yield conversion as a function of both the mass fraction of stagnant zones within the bed char and of the characteristic length of these stagnant zones.

Kinetic Study of Various Chars Steam Gasification

Gasification is an attractive technology for waste thermal treatment. The successful design and modelling of a gasifier requires reliable kinetic data. The purpose of this work is to study the steam gasification kinetics of chars produced by municipal wastes pyrolysis. The municipal solid wastes (MSW) are modelled as a mixture of four organic constituents: paper, wood, plastics, and vegetables. The various char samples are obtained by pyrolysis of each waste constituent, in a fixed bed reactor at 1000°C, in order to minimize their volatile content and thus, to eliminate any subsequent devolatilization of the carbonaceous residues. These chars are used as raw materials in steam gasification experiments. The gasification studies are performed on each char separately, in a tubular kiln at various temperatures (900°C, 950°C and 1000°C) and various vapour pressures (0.2 bar, 0.5 bar and 0.7 bar). The gases produced are analysed by gas chromatography in order to determine the gasification kinetics. The kinetics parameters, with respect to H2O, together with the influence of the char's physical properties are experimentally determined. A kinetic expression for the gasification reaction, based on the random pore model is deduced. It is shown that the char resulting from the pyrolysis of MSW constituents, essentially paper, wood and vegetables have the same gasification kinetics. On the contrary, the plastic char steam gasification kinetic appears to be significantly slower.

Pyrolysis and Gasification Experiments of Biomass Under Elevated Pressure Condition

This paper reports on the pyrolysis and CO2-gasification of various biomass materials in a pressurized thermobalance. In particular, the primary yields of total volatiles, tar and non-condensable gases, together with the composition of non-condensable gases, are measured as a function of temperature at 1 atm and 10.8 ata. The use of a high-intensity infrared heating source, in conjunction with a non-absorbing carrier gas (viz. argon), is reported to reduce the significance of secondary gas-phase pyrolysis reactions. Linear trends with atomic H/C ratio are observed in the tar yield, total volatile yield and CH4 yield during pyrolysis. The gasification kinetics of biomass chars (pyrolysed to 700°C at a medium heating rate) have also been measured by measuring the rate of weight loss during reaction with CO2 as a function of temperature. First-order kinetic rate constants are then determined by fitting the weight loss data using a random pore model. The results indicate that the reactivity of the biomass materials is intermediate between lignin and cellulose and that they are much more reactive than a black coal char.