CO2 Gasification Reactivity Research Articles

Combustion Interactions of Blended Chars under Oxidation and Gasification Competition in Oxy–Fuel Condition

The combustion interactions during blended coal combustion affect their combustion characteristics, and the combustion interactions under air–fuel conditions have been well studied. However, oxy–fuel combustion considerably differs from air–fuel firing. Significantly, the char–CO2 gasification reaction under oxy–fuel and high-temperature conditions substantially affects the combustion behaviors, which may influence the combustion interactions. This paper is to study the effect of the char oxidation and gasification reaction competition on comprehensive interactions in O2/CO2 mixture. The results show that under high-temperature and low-oxygen (O2<4%) conditions, an intense gasification reaction is observed. The competition between the char–CO2 gasification and char–O2 oxidation reactions influences the comprehensive interactions during blended chars combustion. As the char–O2 oxidation reaction is strengthened and the char–CO2 gasification reaction is weakened, the comprehensive interactions during blended chars combustion gradually decrease. Moreover, under the competition between the char–CO2 gasification and char–O2 oxidation reactions, only enhancing the char–CO2 gasification reaction or enhancing the char–O2 oxidation reaction weakens the comprehensive interactions. The weaker comprehensive interactions result in the smaller nonadditive behavior of the comprehensive combustion index. The findings shed light on the interaction effects observed during blended chars combustion, which can provide useful information to improve the combustion behaviors of the blended coals in oxy–fuel conditions.

Co-gasification reactivity and kinetics of municipality and coffee waste residue hydrochar and South African density separated coal blends

Residues obtained from hydrothermal liquefaction of municipality waste (MSWR) and coffee waste (CR) were blended with coal and charred at 1000 °C. Co-gasification experiments were conducted on the coal, MSWR, CR, and their respective blended chars using a thermogravimetric analyser. The reactivities of the char samples decreased in the order: MSWR > CR > MSWR-coal blend > CR-coal blend > coal. RPM was found to best predict the reaction profile of the coal, and the blended chars, except for the MSWR and CR chars, which fitted satisfactorily upon the application of the MRPM. The activation energies obtained using the RPM, MRPM, and the different reactivity models showed that the MSWR, CR, and the blend chars have lower activation energies when compared to the feed coal char. Although the MSWR and CR exhibited catalytic influence on the coal char during co-gasification, the synergistic effect was inhibitory with all determined SE < 1.

Experimental and numerical study on degradation behavior of coke with CO2 or H2O gasification reaction at high temperature

Coke undergoes gasification reactions of CO2 and H2O in the blast furnace, and the gasification reactions degrade the coke and affect the operation of the furnace. In this study, to evaluate the factors that govern the strength of coke in the furnace, the fracture behavior of coke under blast furnace conditions was examined both experimentally and numerically. The three-point bending test was performed on coke subjected to the gasification reaction under conditions similar to those inside a furnace. It was found that coke gasified with CO2 is more likely to fracture than that with H2O since the conversions at fracture of the samples gasified with CO2 and H2O were 0.71 and 0.78, respectively. X-ray computed tomography images of coke taken before and after the gasification reactions showed that increase rate of porosity was almost uniform from the central part to the external surface in the case of CO2 gasification reaction, while it increased from the sample center towards the external surface in the case of the H2O gasification reaction. In addition, the finite element analyses using an isotropic damage model were performed. As a result, the damage spread in the analytical object in which the pore structure after the CO2 gasification reaction was reflected, while the damage was localized in the analytical object in which the pore structure after the H2O gasification reaction was reflected. These results suggested that the difference in the spatial distribution of the porosity increase rate within the coke samples owing to the differences in the rate-limiting steps of the gasification reactions greatly affected the strength of the coke samples.

Synergistic effects of mixing waste activated carbon and coal in co-slurrying and CO2 co-gasification

As a raw material, applications of waste activated carbon (WAC) in synergy with coal powder to prepare coal water slurry (CWS) and perform high-temperature gasification is a novel strategy for disposing WAC. Combining BET surface analysis, contact angle measurement, and thermogravimetric-Fourier transform infrared (TG-FTIR) methods, the physico-chemical properties and CO2 gasification behaviors of coal-waste activated carbon-slurry (CWACS) were evaluated. Compared to coal, WAC exhibited an excellent adsorption capacity and a stronger surface wettability. When the WAC mixing ratio was increased from 0 to 15%, the CWS solid concentration decreased from 62.28% to 59.44%. CWACS exhibited a shear-thinning flow behavior, and the higher the WAC mixing ratio, the more obvious the pseudoplastic characteristics. Moreover, WAC enhanced the storage stability of the CWS. Under the catalysis of Na, Ca, and Fe among others, the CO2 gasification reaction of CWACS coke moved forward to the low-temperature stage. The high fixed carbon content of WAC, as well as its unique tar adsorption and catalytic cracking effects, increased the CO gas component in the synthesis gas.

Strength and Gasification Reactivity of Coke Prepared by Blending a Ca/C Composite and Coal

In this study, a calcium/carbon composite (Ca/C) was prepared from porous CaO by the water vapor swelling method; its pores were filled with tar-derived carbonaceous material to produce high-strength and highly reactive coke. The properties of coke prepared by blending the Ca/C composite and caking-coal were then investigated. The mesopores in the swelling Ca disappeared and the crushing strength was developed by filling tar-derived carbonaceous material into the porous Ca. Thus, a Ca/C composite could be produced by the abovementioned method, wherein the tar-derived carbonaceous material and Ca species were in close proximity. When the Ca/C composite was blended into the caking-coal, the strength of the coke obtained increased by up to 50% of Ca/C blending, which made it possible to produce high-strength coke. On examining the C structure of the prepared coke by X-ray diffraction, that of the original coke was found to be almost unchanged by adding the Ca/C composite. Conversely, the Ca/C blended-coke showed higher CO2 gasification reactivity than the original coke prepared from caking-coal. Based on the CO2 gasification reactivity test of the demineralized coke, it was clear that an increase in the gasification reactivity of the Ca/C blended-coke depended upon the catalytic effect of Ca. Thus, this method helped produce high-strength and highly reactive coke.

Kinetic Modeling of CO2 and H2O Gasification Reactions for Metallurgical Coke Using a Distributed Activation Energy Model.

A distributed activation energy model (DAEM) was applied to the kinetic analysis of CO2 and H2O gasification reactions for pulverized metallurgical coke. The results of the scanning electron microscopy observations and CO2 gas adsorption suggested that the gasification reaction occurs at the particle surface. Therefore, a grain model was employed as a gasification reaction model. The reaction rates of CO2 and H2O gasification were evaluated based on the DAEM. The activation energy changed as the reaction progressed and hardly depended on the particle size. The activation energies were 200–260 kJ/mol in CO2 gasification and 220–290 kJ/mol in H2O gasification. The frequency factor of H2O gasification was approximately 10 times larger than that of CO2 gasification, regardless of the progress of the reaction. At the same activation energy level, the frequency factor showed a higher value with a decrease in the particle size. This result was consistent with the theory of the grain model and indicated that the gasification reaction of the pulverized coke with a micrometer scale occurs on the surface of the coke particle. Furthermore, the value predicted by the DAEM was in good agreement with the experimental one.

Multiscale CFD-DEM model for the CO2 gasification reaction of carbon anode

The reactivity of carbon anodes with CO2 is one of the main concerns in aluminum smelters using the Hall–Héroult process. Such reactivity is undesirable because it increases the net carbon consumption and thus shortens anode lifetime. Anode overconsumption is affected by anode intrinsic reactivity and mass-transport phenomena. Herein, as a first step toward the simulation of anode gasification with CO2, an anode particle bed was considered. Numerical multiscale computational fluid dynamics (CFD)–discrete element method (DEM) model was developed based on an Eulerian–Lagrangian concept. The model includes an Eulerian finite-element method for the gas and solid particles, and a Lagrangian DEM for the particle phase. The was intended to capture the particle-shrinkage effect (movement of particles during gasification). The physical (e.g., porosity and specific surface area) and thermochemical (e.g., heat of reaction) properties of particles are ultimately tracked. Geometric changes in particles, heat and mass transfer, particle shrinkage, and chemical reactions are considered during anode gasification with CO2. The dynamic concentration and temperature profiles of the reactant and product gases, as well as the solid conversion, were modeled in the voids between the particles and the pores inside each particle. To validate the model, experimental tests were performed using a bed of anode particles.

Effect of Changes in Mechanical Properties of Coke Matrix Caused by CO&lt;sub&gt;2&lt;/sub&gt; or H&lt;sub&gt;2&lt;/sub&gt;O Gasification Reaction on the Strength of Lump Coke

In this study, we evaluated the effect of the CO2 or H2O gasification reaction on the mechanical property of the coke matrix by measuring the elastic modulus of the coke matrix before and after the gasification reaction. We also investigated the effect of the distribution of the elastic modulus in the coke matrix on the strength of the lump coke by conducting the fracture analysis for the coke model with porosity of 0–0.4 in which the distribution of the elastic modulus obtained by the experiment was reflected. The nanoindentation measurements of the elastic modulus of the coke matrix before and after the gasification reaction implied that the distribution of the elastic modulus in the coke matrix differs depending on the gasification agent. In the case of the CO2 gasification reaction, both the coke matrices with high and low elastic moduli were consumed by the gasification. On the other hand, in the case of the H2O gasification reaction, only the coke with an elastic modulus of over 30 GPa before the reaction was consumed by the gasification reaction. Also, the numerical results showed that the distribution of the elastic modulus in the coke matrix affects the strength of the coke model with low porosity whereas the one did not affect that with high porosity.

Study on Reactivity and Synergy Behavior of Cogasification between Biomass Char and Coal Char

The cogasification reactivity of Shenmu bituminous coal (SM) char and corn straw (CS) char in CO2 was evaluated by a fixed-bed/fluidized-bed reactor and mass spectrometry. The effects of blend rati...

Investigation on the structural feature and gasification reactivity of bio-char derived from energy crop

A kind of energy crops (Arundo donax) was selected to investigate effects of pyrolysis heating rate (PHR) on char structure and CO2 gasification reactivity. The gasification reactivity and microstructure of char obtained at various PHR were explored. Besides, the surface morphology, microcrystalline and aromatic structure of bio-chars were analyzed via scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. The results reveal that the main thermal decomposition of Arundo donax occurs in the range of 180 °C - 570 °C. The surface of chars obtained at rapid pyrolysis appears to be rougher than that under slow pyrolysis. Rapid pyrolysis results in high temperature gradient inside and outside the particles, leading to condensation of free radical fragments in a short time. The bio-char obtained at rapid heating rate has more structural defects and imperfections of the carbon crystallites. Besides, the crystallization size, graphitization and aromatization degree decrease of bio-char were inhibited as the PHR increased. In addition, some H2O-soluble K was volatilized with the increase of heating rate, and a small part was converted to NH4Ac-soluble K during pyrolysis. Therefore, the gasification reactivity of chars obtained from rapid pyrolysis rate was higher than that from the low pyrolysis rate, which was associated with the formation of alkali metal distribution and micro-areas structure evolution in the bio-chars.

Effect of hydrothermal carbonization temperature on reactivity and synergy of co-gasification of biomass hydrochar and coal

In this study, co-gasification reactivity, synergy and kinetics of blended char of bituminous coal (SF) and raw/hydrothermal-carbonization-treated (HTC) rice straw (RS/RSH) at different HTC temperatures (180–240 °C) and gasification temperatures (800–950 °C) were investigated. Moreover, mineral matter transformation and chemical structure evolution during co-gasification were quantitatively investigated to reveal the influence mechanism of binary particle interaction on co-gasification synergy behavior. Reactivity results indicate that co-gasification reactivity of SF-RSH blended char was lower than SF-RS blended char and was decreased with increasing HTC temperature. Synergy factor results show that the existence of synergistic effect during co-gasification contributed to the higher actual blended char reactivity than the corresponding calculated value, which was mainly due to the inhibition effect on active K transformation and the positive effect on carbon structure evolution (i.e., the decrease of ordering degree of carbon structure and the increase of amorphous carbon structure amount) resulted from binary particle interaction during co-gasification. Moreover, synergistic effect was weaker for SF-RSH blended char than SF-RS blended char and was reduced as HTC temperature and gasification temperature increased, which could be well correlated by the weakened inhibition effect on active K transformation, the enhanced promotion effect on active Ca transformation and the weakened inhibition effect on carbon structure graphitization process at higher HTC temperature and gasification temperature. Kinetics analysis demonstrates that activation energies of SF, RS, RSH, SF-RS and SF-RSH char samples were 208.21, 144.74, 160.62–174.01, 161.09, 169.20–178.97 kJ·mol−1, respectively. Moreover, binary particle interaction improved blended char co-gasification kinetics with activation energy reduction of 19.96–31.41 kJ·mol−1 than the calculated values.

A TGA Investigation to Co‐Pyrolysis Characteristics of Shenhua Direct Coal Liquefaction Residue and Huolinhe Lignite and the CO2 Gasification Behavior of the Derived Char

AbstractCo‐pyrolysis of direct coal liquefaction residue (DCLR) and low rank coals was regarded as reliable and efficient method to process DCLR. The treatment of the derived co‐pyrolysis char is important for the comprehensive utilization of DCLR. In this study, the non‐isothermal co‐pyrolysis of Shenhua DCLR and Huolinhe lignite, and the isothermal CO2 gasification of the co‐pyrolysis char were performed by the thermogravimetric analysis method. The interactive effect of co‐pyrolysis and the CO2 gasification behavior of the derived char were explored by comparing the experimental and calculated thermogravimetric/derivative thermogravimetric (TG/DTG) curves and reactivity – conversion (R ‐ x) curves, respectively. The results show that the devolatilization pattern at 200–600 °C was modified and part of volatiles were released in advance. As the experimental co‐pyrolysis Tmax was lower by 7–13 °C than the calculated value, while it was almost equal to the Tmax of lignite pyrolysis. We tentatively concluded that lignite devolatilization could promote the evaporation of molten components of DCLR. Moreover, DCLR and lignite could polymerize or condensation with each other at 650–900 °C. The lignite char within co‐pyrolysis char could support the char skeleton and hold a high porosity. Therefore, the CO2 gasification reactivity of co‐pyroysis char was promoted and the experimental reactivity exceeded the calculated value over the conversion range of 12–90 %.

Experimental and density functional theory study of the influence mechanism of oxygen on NO heterogeneous reduction in deep air-staged combustion

Deep air-staged combustion is one of the important technologies to further reduce NOx emissions, and the mechanism of oxygen influence on NO heterogeneous reaction under deep air-staged conditions is still unclear. This study explores the influence of minor change in oxygen concentration on NO formation in the primary combustion zone under deep air-staged combustion by experimental and density functional theory (DFT). Experimental results show that there is an optimal oxygen concentration for NOx formation under the deep staged combustion. The theoretical calculation further verifies the experimental results, and reveals that char model with one hydroxyl group is more conducive to the N2 desorption than the char model with two hydroxyl groups at the mechanism level. Oxygen changes the reaction pathway of char-NO, and the reduction of NO to N2 is accompanied by the formation of CO or CO2. Moreover, the char and CO2 gasification reaction promotes CO formation under low oxygen condition under deep air staging. The kinetic analysis also reveals that there is an optimal concentration of oxygen in promoting CNO heterogeneous reduction. The carbonaceous surface with one hydroxyl group enhances char reactivity and increases the desorption rate of N2 compared to char surface without hydroxyl group. The char surface containing two hydroxyl groups promotes the retention of the pyridine nitrogen in the structurally stable aromatic ring, which greatly reduces the N2 desorption rate and inhibits NO reduction.

High-temperature pyrolysis of petroleum coke and its correlation to in-situ char-CO2 gasification reactivity

The understanding of the pyrolysis behavior of petroleum coke (PC), a by-product of oil refining, is very critical to its energy utilization. Different from most previous work, this investigation particularly focused on PC pyrolysis at high temperatures > 1273 K. The CO2 gasification reactivity of in-situ char, rather than quenched char, was evaluated and correlated to PC pyrolysis behavior at different temperatures. A Chinese PC with sizes below 100 μm was tested on a high-temperature thermogravimetric analyser (TGA). Three sets of tests were carried out for different purposes. The first set was to simultaneously obtain sample mass loss and gas evolution data during pyrolysis through thermogravimetry-mass spectrometry (TG-MS). The second set aimed to collect char samples for subsequent analyses by scanning electron microscopy (SEM) and Raman spectrometry. The third set was to evaluate in-situ char-CO2 gasification reactivity through the TGA. The results showed that, in addition to the commonly-observed primary pyrolysis stage at low temperatures, there was a secondary PC pyrolysis stage at high temperatures > 1300 K. In this process, the gases such as HCN, CO2 and SO2 were significantly released. The observed changes of char morphology suggested a four-staged thermoplastic transformation of the PC during pyrolysis, which has little been discussed previously. At different stages, i.e. softening, plasticizing, resolidification and graphitization, the rate of carbon ordering was different. The in-situ char-CO2 gasification reactivity was found to first increase, then decrease and finally increase again with increasing temperature. Such changes coincided with the thermoplastic state of the pyrolyzed char, but not with the changes of char surface area or carbon ordering. The obtained knowledge is new and highlights the potentially important roles of char thermoplastic state in determining its reactivity towards CO2.

Modeling and numerical simulations of lignite char gasification with CO2: The effect of gasification parameters on internal transport phenomena

The purpose of this study is to develop an experimental data-based char particle gasification model in order to assess the effects of particle size, gasification temperature and char generation heating rate on global gasification parameters. Also, the effect of initial porosity is observed by performing parametrical numerical simulations. A continuum-based model is used to solve the gasification inside a char particle and within the external boundary layer. The intrinsic rate of CO2 gasification reaction is computed according to Langmuir-Hinshelwood (LH) mechanism. External mass transfer is modeled by Stefan-Maxwell relations, and Cylindrical Pore Interpolation Model (CPIM) is used for intra-particle molecular diffusion. In the model, all the effects due to particle internal structure changes are represented by a global conversion function, f(X) which is computed from local reaction rate values. In this study, f(X) is deduced from experimental results instead of phenomenological models almost impossible to validate. The best reproduction of the experimental gasification results is obtained for the function f(X) postulated as a summation of two Gaussian functions which represent the char particle random pore structures and their dynamics during gasification. Comparative simulation results show that the Gaussian for low conversion interval is shifted to even lower conversion values for higher gasification temperature and higher initial porosity. Thereby, the Gaussian function for low conversion rates (large particle sizes) is interpreted as representative of the diffusion-limited gasification regime in conjunction with the network of macropores and molecular diffusion rates. The modification of the pore structure due to char generation heating rates causes a shift of the second Gaussian towards higher conversion rates. It is therefore postulated that the second Gaussian function corresponds to the boundary layer diffusion-controlled regime related to available outer surface area of the particle.

Effect of calcium ferrites on carbon dioxide gasification reactivity and kinetics of pine wood derived char

Biochar gasification is an essential stage of chemical looping biomass gasification, a promising sustainable source of energy that may limit anthropogenic carbon dioxide emissions. Here, the performance of calcium ferrite oxygen carriers (CaFe2O4, and Ca2Fe2O5) on the CO2 gasification reactivity and kinetics of pine wood char is investigated by a thermogravimetric analyzer. The morphology and graphitization degree of the pine wood derived biochar were characterized by using high-resolution transmission electron microscope (HRTEM) and Raman spectroscopy. A random pore model is established and the best matching structural parameter, Ψ, is used to indicate the pore structure of the initial char samples. Results indicate that the activation energy under biochar with no oxygen carrier, biochar loaded with 10 wt% Fe2O3, 10 wt% Ca2Fe2O5, and 10 wt% CaFe2O4 are 169.76 kJ/mol, 176.03 kJ/mol, 173.81 kJ/mol, and 178.83 kJ/mol, respectively. The introduction of oxygen carriers increases the activation energy for CO2 gasification, potentially due to competition with calcium ferrite reduction, resulting in decreased CO2 partial pressure.

Influence of initial iron ore particle size on CO2 gasification behavior and strength of ferro-coke

Highly reactive ferro-coke has been widely studied due to its contribution to the energy saving and emission reduction in blast furnace ironmaking. To optimize the coking process of ferro-coke and improve its metallurgical properties, it is necessary to clarify the influence of initial iron ore on the strength, micro-morphology and CO2 gasification reaction behavior of formed ferro-coke. The effects of initial iron ore particle size (0.50–1.00, 0.25–0.50 and 0.074–0.125 mm) on the CO2 gasification reaction of ferro-coke were analyzed using thermo-analysis technique. In addition, the effects of initial iron ore particle size on the strength and morphology of ferro-coke were investigated by drum test, digital microscopy and scanning electron microscopy. The results show that iron reduced from iron ore has a great promotion effect on the CO2 gasification reaction of ferro-coke. The smaller the particle size of initial iron ore, the more intense the gasification reaction, and the lower the starting temperature for gasification reaction of ferro-coke. The results of kinetic calculation show that the apparent activation energy of ferro-coke decreases with the decreasing particle size of blended iron ore. The particle size of initial iron ore has a great impact on the strength of ferro-coke. The ferro-coke prepared by 0.25–0.50 mm iron ore presents the best strength in this experiment.

Fractionation of coal through organo-separative refining for enhancing its potential for the CO2-gasification

Coal gasification has already been extensively studied earlier under varying conditions of steam, CO2, O2, inert conditions. Belbaid coal and its e, N and NMP-DETA SCC products recovered through organo-refining under milder ambient pressure conditions were subjected to CO2-gasification in a fixed bed reactor under varying conditions. CO2 being an inert gas becomes the most challenging to be utilized during the gasification process. The SCCs showed better CO2-gasification reactivity than the raw Belbaid coal at 900 °C. The use of the catalyst K2CO3 tremendously increased the gasification reactivity for both raw coal and the SCCs. The use of sugarcane bagasse for CO2-gasification along with raw coal as well as with residual coal was also studied. Gasification under CO2 atmosphere conditions was used to structurally understand the coals as the coal structure gets loosened after extraction.

Comparison of CO2 gasification reactivity and kinetics: petcoke, biomass and high ash coal

It is desirable to exploit biomass energy along with coal and petcoke through gasification, and understanding the differences between petroleum coke (petcoke), coal and biomass gasification behaviour becomes very essential. Consequently, present investigation compares gasification components of petcoke, sawdust and high ash coal with their physico-chemical properties under isothermal conditions in CO2 atmosphere in the temperature range of 1173–1623 K. Physico-chemical characterisation includes proximate and ultimate analyses, porous structure analysis by Brunauer-Emmett-Teller (BET) surface area from nitrogen (N2) adsorption isotherm, ash composition analysis and ash fusion temperature. The effects of temperature and nature of different solid fuels on gasification reactivity have been discussed. Gasification kinetics has been investigated using two nth-order kinetic models, such as homogeneous model (HM) and shrinking core model (SCM). Influence of diffusion resistance on gasification behaviour of different solid fuels is also reported. Thus, the present study will be helpful to realise the effects of high ash in gasification behaviour as well as in designing and modelling of the suitable gasifier and to establish optimum gasification conditions for petcoke, biomass and high ash coal.

Linking char reactivity to structural and morphological evolution during high pressure pyrolysis of Morupule coal

Understanding the influence of pressure on char physicochemical properties and reactivity during pyrolysis is principal for the design and optimisation of coal gasification technologies. This work investigated the pyrolysis behaviour of Morupule coal at various pressures and temperatures in a high-pressure wire-mesh reactor (HPWMR). The effect of pressure was pronounced at 600 and 800 °C, resulting in lower total volatile yields at higher pressures due to the repolymerisation of volatiles. Although graphitisation was limited during the heating period, an increase in the concentration of ordered structures was observed during holding at 1000 °C. Marked blowholes on the char surface suggested an explosive bubble transport phenomenon during pyrolysis. Chars produced during the ramping period under high pressure showed higher combustion reactivities than their low-pressure counterparts. However, prolonged holding at peak temperature produced chars of similar reactivity, with their CO2 gasification reactivity found to be independent of pyrolysis pressure.