CO2 Gasification Rate Research Articles

Increasing CO2 gasification rates of longan-seed char by a technique of char pre-oxidation

A simple and effective technique was presented to increase the longan-seed char gasification rate during CO2 activation by pre-oxidizing the char with air. The kinetics of char gasification was studied in a thermogravimetric analyzer (TGA) for the temperatures of 850–950 °C for char samples being oxidized in a tube furnace at the temperatures of 250, 280, and 300 °C. It was found that the gasification rate of the oxidized char was about twice higher than that of the non-oxidized char. The increase in the char reactivity for CO2 gasification is due to the increasing number of reaction sites caused by the disruption of functional group bonding created during the oxidation step. Additionally, the increase in the oxidation temperature gave rise to the increase in the char reactivity but the maximum temperature was limited by the ignition temperature of the longan-seed char. Four gasification kinetic models were tested against the experimental rate data and it was discovered that the modified volume reaction model (MVRM) provided the best description of the gasification conversion. The pre-exponential factor and the activation energy of the gasification reaction were determined from the Arrhenius equation. The estimated activation energy was found to be in the range of 156.87–218.98 kJ/mol and decreased with the increase of char oxidation temperature.

CO2 Gasification Reactivity of Char from High-Ash Biomass

Biomass char produced from pyrolysis processes is of great interest to be utilized as renewable solid fuels or materials. Forest byproducts and agricultural wastes are low-cost and sustainable biomass feedstocks. These biomasses generally contain high amounts of ash-forming elements, generally leading to high char reactivity. This study elaborates in detail how chemical and physical properties affect CO2 gasification rates of high-ash biomass char, and it also targets the interactions between these properties. Char produced from pine bark, forest residue, and corncobs (particle size 4–30 mm) were included, and all contained different relative compositions of ash-forming elements. Acid leaching was applied to further investigate the influence of inorganic elements in these biomasses. The char properties relevant to the gasification rate were analyzed, that is, elemental composition, specific surface area, and carbon structure. Gasification rates were measured at an isothermal condition of 800 °C with 20% (vol.) of CO2 in N2. The results showed that the inorganic content, particularly K, had a stronger effect on gasification reactivity than specific surface area and aromatic cluster size of the char. At the gasification condition utilized in this study, K could volatilize and mobilize through the char surface, resulting in high gasification reactivity. Meanwhile, the mobilization of Ca did not occur at the low temperature applied, thus resulting in its low catalytic effect. This implies that the dispersion of these inorganic elements through char particles is an important reason behind their catalytic activity. Upon leaching by diluted acetic acid, the K content of these biomasses substantially decreased, while most of the Ca remained in the biomasses. With a low K content in leached biomass char, char reactivity was determined by the active carbon surface area.

The significance of intraparticle and interparticle diffusion during CO2 gasification of biomass char in a packed bed

This study investigates the influences of intraparticle and interparticle diffusions on the reaction rates of char gasification in a packed bed without forced convective flows. The main objective is to elucidate how the dominant scales of mass diffusion resistance change based on particle size distributions (PSD). CO2 gasification rates were measured by thermogravimetric analyses (TGA) of spruce char produced from pilot-scale reactors. Experimental setups using two TGA devices highlighted the effects on different rate-limiting steps. Effects of intraparticle diffusion were investigated with a single layer of monodispersed particles between 75 µm and 6.3 mm using a commercial TGA. Effects of interparticle diffusion were investigated with a packed bed of monodispersed and polydispersed particles using a macro-TG. At the particle scale, gasification rate decreased with the increase of particle size when the reaction was controlled by intraparticle diffusion. This effect can be described by the effectiveness factor with Thiele modulus. At the bed scale, void fraction and tortuosity of the packed bed are influential parameters on diffusivity of CO2 through the bed channels. Due to its non-sphericity of the char particles, the bed of relatively large particles had high void fraction and the presence of smaller particles were essential to lower the bed void size. Consequently, smaller size fraction in the PSD had a major impact on the diffusion resistance at bed scale. It means that the diffusion resistances at particle and bed scales are sensitive to different size fractions in the PSD. It allows one to tweak the overall reaction rates in packed beds by manipulating the PSD if the dominant mass transport mechanism is diffusion.

Study on the mechanism of calcium catalyzed CO2 gasification of lignite

The gasification of solid fuels with CO2 as gasifying agent is one way to utilize the greenhouse gas. As a catalyst, calcium (Ca) can improve the CO2 gasification rate of coal char, while its catalytic mechanism remains unclear. In this study, the chars derived from the pyrolysis of lignite samples loaded with different calcium-based catalysts were subject to CO2 gasification, enabling the comparative evaluation of catalytic performance. X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) analyses were employed to systematically reveal the transformation of Ca species during the CO2 gasification process. The results indicate that the catalytic effect of organically-bonded Ca2+ is more pronounced than that of CaO, which was long considered to be the unique catalytic Ca species. The electron distribution of the aromatic macromolecules in char matrix could be altered by the OCa structure bonded with it, thus promoting the cleavage of aromatic CC bond under the attack of gasifying agent (CO2). Consequently, the gasification reaction would take place at a lower temperature with enhanced reaction rates. In addition, the pyrolysis process activates the organic calcium to gain the capability of electrons transfer.

Thermally induced changes in microstructure and reactivity of biogenic and fossil fuel particles

In the present study, thermally induced changes in microstructure and CO2-gasification rates of fuel particles from Rhenish lignite and torrefied poplar wood are experimentally investigated. A literature review reveals a distinct gap in the field of quantitative analysis of thermal deactivation influencing low-rank and biogenic fuels reacting under gasification conditions. Therefore, the reactivity of Rhenish lignite and torrefied poplar wood towards carbon dioxide is examined after the process of heat treatment. Experiments are conducted in a small scale fluidized bed reactor. Assessment of thermal deactivation is achieved by varying heat treatment temperature between 1023–1173 K and heat treatment time between 0–1800 s. Flue gases are analyzed using a Fourier-transform infrared spectrometer to deduce a reaction rate from the temporal evolution of the product gas. Experimental results are reproduced by applying an nth-order power law and a model with distributed activation energies to propose parameters for implementation in comprehensive gasification/combustion codes and for comparison of these two model types: Both models approximate the experimental results almost equally well. In parallel to that, heat treated particles are analyzed by high resolution transmission electron microscopy. Thereby, a progressive transformation of the carbon from an amorphous microstructure to turbostratic arrangements is revealed for both types of fuel. In case of Rhenish lignite, formations of wrinkled carbon layers were detected, indicating a faster progress of transformation for lignite compared to biomass. This finding is in accordance with the kinetic study conducted, where the determined deactivation rate of lignite exceeds the one of biomass. In this work it is successfully shown that both reactivity and microstructure are affected by heat treatment on equal time scales.

Gasification of carbon/carbon composite prepared from pyrolyzed char of low-grade coke and low-rank coal

Carbon/carbon composites were prepared by pyrolysis of mixtures of coke oven gas tar and low-grade cokes or pyrolyzed chars from lignite and sub-bituminous coals. The indirect tensile strength of each composite prepared reached approximately 7.5 MPa, which is comparable to those of high-grade cokes used in commercial blast furnaces. Although the CO2 gasification rates of the composites prepared from low-grade cokes decreased compared to those of the original samples, those of the composites prepared from pyrolyzed chars were almost the same. From the results of mercury intrusion porosimetry analysis and O2 adsorption temperature-programmed-desorption tests of samples before and after compositing, it was suggested that the decrease in gasification rates of composites prepared from cokes occurred owing to a decrease in the pores and number of active sites on the original cokes.

Bi-reforming of methane on Ni/SBA-15 catalyst for syngas production: Influence of feed composition

Bi-reforming of methane (BRM) was evaluated for Ni catalyst dispersed on SBA-15 support prepared by hydrothermal technique. BRM reactions were conducted under atmospheric condition with varying reactant partial pressure in the range of 10–45 kPa and 1073 K in fixed-bed reactor. The ordered hexagonal mesoporous SBA-15 support possessing large specific surface area of 669.5 m2 g−1 was well preserved with NiO addition during incipient wetness impregnation. Additionally, NiO species with mean crystallite dimension of 14.5 nm were randomly distributed over SBA-15 support surface and inside its mesoporous channels. Thus, these particles were reduced at various temperatures depending on different degrees of metal-support interaction. At stoichiometric condition and 1073 K, CH4 and CO2 conversions were about 61.6% and 58.9%, respectively whilst H2/CO ratio of 2.14 slightly superior to theoretical value for BRM would suggest the predominance of methane steam reforming. H2 and CO yields were significantly enhanced with increasing CO2/(CH4 + H2O) ratio due to growing CO2 gasification rate of partially dehydrogenated species from CH4 decomposition. Additionally, a considerable decline of H2 to CO ratio from 2.14 to 1.83 was detected with reducing H2O/(CH4 + CO2) ratio due to dominant reverse water-gas shift side reaction at H2O-deficient feedstock. Interestingly, 10%Ni/SBA-15 catalyst was resistant to graphitic carbon formation in the co-occurrence of H2O and CO2 oxidizing agents and the mesoporous catalyst structure was still maintained after BRM. A strong correlation between formation of carbonaceous species and catalytic activity was observed.

Modeling Soot Oxidation and Gasification with Bayesian Statistics

This paper presents a statistical method for model calibration using data collected from literature. The method is used to calibrate parameters for global models of soot consumption in combustion systems. This consumption is broken into two different submodels: first for oxidation where soot particles are attacked by certain oxidizing agents; second for gasification where soot particles are attacked by H2O or CO2 molecules. Rate data were collected from 19 studies in the literature and evaluated using Bayesian statistics to calibrate the model parameters. Bayesian statistics are valued in their ability to quantify uncertainty in modeling. The calibrated consumption model with quantified uncertainty is presented here along with a discussion of associated implications. The oxidation results are found to be consistent with previous studies. Significant variation is found in the CO2 gasification rates.

CO2 Reforming of Methane over Ni0/La2O3 Catalyst Without Reduction Step: Effect of Calcination Atmosphere

La-Ni precursor prepared by EDTA-cellulose method was calcined under different atmosphere (Air or Ar), and the catalysts were characterized by various techniques. In this study, the possibility of reduction free catalyst for dry reforming of methane was investigated as well. It was observed that LaNiO3 perovskite structure was formed under the calcination of Air atmosphere, while Ni0/La2O3-C structure was obtained under the calcination of Ar atmosphere due to the reducing and the oxidizing agents generated by the decomposition of organic species under inert atmosphere. It was found that even if LaNiO3-Ar had much larger size of nickel particle than LaNiO3-Air, the remained carbon species derived positive effect: the interfacial area among carbon, La2O3 and Ni0 could lead to synergetic sites such as basic sites, which enhanced resistance to carbon deposition. Furthermore, the higher CH4 activation energy and basicity of LaNiO3-Ar catalyst might ascribe to equilibrium between CH4 decomposition and CO2 gasification rates. Thus, it is suggested that remained carbon species in Ar calcined catalyst did not negatively affect the catalytic activity, but it affected stability positively.

Investigation of the anode reactions in solid oxide electrolyte based carbon fuel cells

The anode reactions of solid oxide electrolyte based carbon fuel cells (SO-CFCs) are explored by comparing the electrochemical behaviors of SO-CFCs under varying anode carrier gas flow rates (FAr) and at different contact modes. The electrochemical performance of four raw carbon fuels, including a graphitic carbon (GC), two coals (lignite CF and anthracite YQ) and an activated carbon (AC), and their chars is tested to investigate the influence of carbon fuel properties on the cell performance. The results show that CO electro-oxidation and C-CO2 gasification were main anode reactions. The direct carbon electro-oxidation is insignificant under high FAr. Polarization performance of the chars under high FAr was similar with that of 5–10% CO. It is also concluded that the cell performance is greatly dependent on the carbon fuel gasification reactivity with CO2. Thermal pretreated AC displays the best durability performance for its stable and moderate CO2 gasification rate. Additionally, the coal ash does not affect the cell performance significantly.

A theoretical model to estimate steam and CO2 gasification rates based on feedstock characterization properties

An improved model for the estimation of gasification rate constants using CO2 and steam as gasifying agents is presented in this work. The theoretically deduced model only considers the micropore surface area and catalyst (alkali) content of the raw feedstock as inputs for the gasification rate estimation. The main advantage compared with other models is its simplicity in determining the rate constant with only one regression parameter, which is a result of introducing, for the first time, theoretical limits to the activation energy of the main gasification reactions (steam gasification and Boudouard reactions).Experiments were carried out in a setup that reduces the most common problems associated with laboratory gasification procedures, such as misinterpretation of a maximum gasification rate, adverse effects on the char properties during the pyrolysis phase, and interparticle diffusion. Statistical analysis showed that the model fitted well with experimental data, since the determination coefficient was higher than 0.96 for 13 coal samples with a broad range of ash contents (6.4% to 74.8%).The new model gives an accurate mixing rule that is useful in determining gasification rates of different feedstock mixtures. The new concepts presented in this research can be used for simulating and determining suitable process conditions for industrial gasifiers operating with different feed mixtures.

Evaluation of CO2 gasification kinetics for low-rank Indian coals and biomass fuels

Gasification of solid fuels such as coals, lignite and biomasses has been studied using isothermal and non-isothermal thermogravimetric analysis (TG) with CO2 as gasifying agent. Non-isothermal TG of three Indian coals (two bituminous and one sub-bituminous coal), one lignite and two biomass fuels (Casuarina and empty fruit bunches) at a constant heating rate of 20 °C min−1 in the temperature range from 25 to 1200 °C showed a clear separation of DTG peaks associated with pyrolysis and CO2 gasification. Based on these studies, isothermal TG experiments were conducted in the temperature range from 900 to 1100 °C for coals and from 800 to 1000 °C for biomass fuels. These results show that the CO2 gasification rate follows coal rank for the three coals and the lignite. The two biomasses have significantly higher reactivities than the three coals. The higher reactivity of one coal is attributed to the presence of calcium-containing minerals in its inorganic matter. The kinetic parameters for each fuel were extracted from the isothermal TG results using the volumetric reaction model for the coals and a zeroth-order model for biomass fuels. Biomass and lignite are found to have a much higher reactivity index and much lower conversion time than the three coals under identical conditions.

Kinetic Study on Low-Rank Coal Char: Characterization and Catalytic CO2 Gasification

AbstractA kinetic characterization of the CO2 gasification of chars from Chinese low-rank lignite coal is performed by isothermal thermogravimetry. The isothermal condition is 1,073 K, and the CO2 concentration range is 10–90%. Alkali and alkali earth catalysts were used to promote the gasification reactivity. The gasification rate of the char was increased with the carbon dioxide (CO2) concentration. The gasification rate increment was reduced at a higher concentration, which inhibited the CO2 gasification rate. The best CO2 ratio for gasification was 70%. The gasification rate of the char with CO2 is fast and proceeded in the following order: sodium carbonate (Na2CO3)>potassium carbonate (K2CO3)>calcium carbonate (CaCO3)>dolomite. In this study, the following gas–solid reaction models are compared: the shrinking core model, the volumetric reaction model, the modified volumetric reaction model (MVRM), and the random pore model. The correlation coefficient values of the MVRM were the highest of all the mo...

Thermogravimetric study on the influence of structural, textural and chemical properties of biomass chars on CO2 gasification reactivity

The present investigation aims to examine the influence of textural, structural and chemical properties of biomass chars on the CO2 gasification rate. Various lignocellulosic biomass chars were prepared under the same conditions. Different analytical techniques were used to determine the char properties such as Scanning Electronic Microscopy, nitrogen adsorption manometry, Raman spectroscopy and X Ray Fluorescence. Gasification tests were carried out in a thermobalance under 20% CO2 in nitrogen at 800 °C.Significant differences of the total average reactivity were observed with a factor of 2 between the prepared chars. Moreover, different behaviors of gasification rate profiles versus conversion were obtained. This difference of behavior appeared to be correlated with the biomass char properties. Hence, up to 70% of conversion, the gasification rate was shown to depend on the char external surface and the potassium content. At higher conversion ratio, a satisfactory correlation between the Catalytic Index and the average gasification rate was identified. The results highlight the importance of knowing both textural and structural properties and mineral contents of biomass chars to predict fuel reactivity during CO2 gasification processes. Such behavior prediction is highly important in the gasifiers design for char conversion.

CO2 gasification rates of char particles from torrefied pine shell, olive stones and straw

Single char particles with a diameter of 8 mm were produced in situ from pellets of raw and torrefied pine shell, olive stones and straw. The single char particles were then gasified at 800, 900 and 1000 °C in two or three CO2 concentrations at each temperature. In the experiments, CO concentrations were measured from the product gases. From the measured CO concentrations, the char conversions versus time were determined. The reactivity of the char from the torrefied olive stones was lower than the reactivity of the char from the raw olive stones, while the reactivity of the char from the torrefied straw was higher than the reactivity of the char from raw straw. For the raw and torrefied pine shell the char reactivities were similar. At 900 °C the influence of torrefaction on char gasification rates was minor, however, since the conversion occurred under Regime II conditions, i.e., the conversion rate is partly limited by mass transfer. A detailed single particle model, taking into consideration mass transfer effects, was used to extract kinetic parameters from the experimentally determined char conversions. For the six chars, activation energies of the adsorption step were in the range 175 and 285 kJ/mol, while activation energies of the desorption step were in the range 145 and 195 kJ/mol. In the study it was also tested whether the char conversion could be computed with unified activation energies for all the chars. The results show that by using unified activation energies – 240 kJ/mol for the adsorption step and 168 kJ/mol for the desorption step – the computed char conversions were in good agreement to the experimental char conversions.

Kinetics of steam and CO 2 gasification of high ash coal–char produced under various heating rates

Abstract Char particles of high ash Indian coal with different sizes are produced using thermogravimetric method at different heating rates. Particle diameter and porosity changes during devolatilization significantly affect char gasification rates. The effect of coal particle size on char production under various heating rates is reviewed. Char gasification is affected by operating conditions such as reaction temperature, char production method, and particle size in addition to chemical composition and physical structure of char. The measurements of the steam and CO2 gasification rate of the produced chars are performed by an isothermal thermo-gravimetric analysis at the temperatures of 900, 950, and 1000 °C under ambient pressure conditions. It is found that the reactivity of char gasification is increased along with heating rate of char. Three kinetic models are applied to describe the varying conversion rate: volumetric model, grain model, and random pore model. The activation energy of the gasification is varied based on the char generation method and particle sizes. The activation energy is varying from 122 to 177 kJ mol−1 for steam gasification and 130 to 214 kJ mol−1 for CO2 gasification to different sized char particles. The effect of char heating rate on gasification is more pronounced in the activation energies of smaller sized particles.

CO2 gasification rate analysis of coal in molten blast furnace slag—For heat recovery from molten slag by using a chemical reaction

The coal gasification rate of Datong coal in molten blast furnace slag using carbon dioxide as gasifying agent was studied at the temperature ranging from 1473 to 1723 K. The kinetic model was developed. The influence of reaction temperature and coal/slag mass ratio in the reaction rate was analyzed. The reaction rate has a strong dependence on reaction temperature and coal/slag ratio. The peak value of reaction rate increases firstly, and then levels off, or even decreases a little as temperature increases. Within the same reaction time, the carbon conversion and peak value of reaction rate increase with increasing coal/slag ratio. Molten BF slag is active catalyst for carbon gasification. The volume reaction model A1 has the best fit on coal gasification in molten BF slag. The kinetic parameters applicable to volume reaction model at high temperature with different coal/slag ratios were obtained. The value of intrinsic activation energy is between 50 kJ/mol and 90 kJ/mol. The global rate equation that includes these parameters was developed.Rate=ⅆxⅆt=k0iexp(−EaiRT)(1−x)

CO2 Char Gasification Rates of Sawdust, Switchgrass, and Corn Stover in a Pressurized Entrained-Flow Reactor

An entrained-flow flat-flame burner reactor was used to measure apparent CO2 gasification rates of near-spherical biomass chars of poplar sawdust, switchgrass, and corn stover using particle residence times <270 ms at total pressures of 10 and 15 atm. The gas temperature and bulk CO2 partial pressure ranges in these gasification experiments were 1258–1891 K and 6.1–13.5 atm, respectively. A new method was developed to produce near-spherical particles from nonspherical biomass chars. The apparent CO2 gasification rates for the three biomass chars with mean diameters near 100 μm were fit to a global first-order model, and the optimal kinetic parameters are reported. The measured gasification rate of poplar sawdust was about 3.9 times faster than that of switchgrass char, but only about 20% faster than that of the corn stover char.

Enhancement of Gasification Reactivity of Low-Rank Coal through High-Temperature Solvent Treatment

We have recently proposed a degradative solvent extraction method to upgrade and fractionate various types of low-rank coals by a nonpolar solvent at temperatures below 350 °C. The main products obtained are solvent-soluble fractions (extracts) and an insoluble fraction, which we call “upgraded coal (UC)”. The UC samples were prepared from eight low-rank coals and one bituminous coal in this work. It was found that the gasification reactivities of the UC chars prepared from seven coals of the eight low-rank coals were larger than those of the raw coal chars. The CO2 gasification rates of the UC chars prepared from an Australian brown coal, Loy Yang (LY/UC), and an Australian bituminous coal, Gregory (GR/UC), were surprisingly larger than the gasification rates of the corresponding raw coal chars by 2.4–1.9 times. The mechanism of gasification reactivity enhancement was examined from the viewpoints of the catalytic effect of coal inherent minerals, pore surface area, and carbon structure of the chars. The ...

Effect of CO2 gasification reaction on char particle combustion in oxy-fuel conditions

CO2 gasification of coal char may play an important role in oxy-combustion environments with flue gas recirculation (FGR), but its effect on the overall reaction rate has not been clearly understood. To give clarity to the likely impact of CO2 gasification on the oxy-combustion of pulverized coal chars, burnout simulations of coal char particles were carried out, adopting apparent char reactivity and a single-film model that includes the Stefan flow effect on mass and energy transfer. Three oxygen concentrations (21%, 30%, and 5% O2), representing air, oxy-fuel, and oxygen-deficient combustion environments were simulated. A new experimental approach was used to directly measure the CO2 gasification rate of a subbituminous coal char at high temperatures and atmospheric pressure. The measured gasification rate is somewhat higher than previous measurements. The simulation results show that the endothermic gasification reaction reduces the char particle temperature and thereby reduces the oxidation rates. However, due to the contribution of the direct gasification reaction on carbon consumption, the char burnout time and the carbon consumption were improved. The gasification reaction has a greater influence on the char burnout time and the relative carbon consumption in an oxygen-deficient environment and on the drop of particle temperature in an oxygen-enriched environment (for a given gas temperature). In addition, the influence of the gasification reaction on char combustion increases as the gas temperature increases and as the particle size increases. Further, it was observed that the impact of the gasification reaction is dependent on the presumed kinetic rate, which highlights the importance of using reliable kinetic parameters in simulations. Based on the present results, it is important to include the gasification reaction by CO2 when simulating char combustion in oxy-fuel combustion environments.