Coal Char Samples Research Articles

Theoretical and experimental investigation on the effect of CO on N migration and conversion during air-staged coal combustion

Air-staging combustion is the widely-used low-nitrogen combustion technology. In this process of NOx reduction, the role played by CO resulted from the air-staged combustion cannot be ignored. In this study, quantum chemistry method is used to investigate the effects of the CO on the NOx generation/reduction and on the char surface. Theoretical calculations indicate that CO is involved in nitrogen fixation during the migration and transformation of N. During the oxidation of N-containing char, CO increases the binding energy of N of pyridine with adjacent C atoms, which is not conducive to the N precipitation. During reduction of NOx by char, CO decreases the desorption energy barrier of N2 and promotes NO heterogeneous reduction. On the other hand, CO promotes the re-embedding of N in generated NO into the char to form char-N. XPS experiments are carried out for the coal char samples after the air-staged combustion. The experimental results show that high CO concentration is conducive to retention N. When the excess air coefficient (α) is 0.8, the increase of N-6 content is 7.2% larger than that of α = 0.9. This work discovers the nitrogen fixation of CO, enriches and expands the understanding of the role of CO.

On the modelling of the peripheral fragmentation during coal char gasification with CO2

In this paper, a novel mathematical model for the coal char gasification with CO2 is developed. This new approach considers the non-uniformity of pore size distribution, the longitudinal growth of pores due to the surface reaction between CO2 and char, the pore overlapping effect, and the peripheral fragmentation phenomenon. This phenomenon was simulated by incorporating a fragmentation criterion based on both a critical porosity and a critical porosity gradient. Model predictions were compared with experimental data taken by sieving, gas adsorption porosimetry and surface area analysis, and a remarkable agreement was obtained. It was found that critical porosity and critical porosity gradient do not depend either on the operating conditions or the initial particle size but on the initial microstructure of the coal char. Furthermore, these two parameters could not vary significantly between different coal char samples. Regarding the fragmentation, it is observed that as the diffusional limitation increases, this phenomenon takes place in even earlier stages of reaction due to the appearance of porosity gradients within the particle. It was also observed that a maximum on the specific surface area appears due to the pore overlapping phenomenon, and that under chemical control conditions, the average specific surface area that can be reached is larger than that obtained under conditions with diffusional limitations.

Investigation of coal-biomass interaction during co-pyrolysis by char separation and its effect on coal char structure and gasification reactivity with CO2

The interaction between coal and biomass has been widely investigated. However, the mechanism is always proposed based on physicochemical structure and reactivity of char mixture. In this work, char mixture after co-pyrolysis of anthracite and biomass was separated based on different shape and size, and then structure and reactivity of the coal char were analyzed to reveal mechanism of coal-biomass interaction. Anthracite char samples with different corn straw (CS) blending ratios were prepared by pyrolysis in a fixed bed reactor at 600 and 900°C. The AAEM concentration and microcrystalline structures of coal char were examined by inductively coupled plasma-optical emission spectrometry (ICP-OES) system and X-ray diffraction (XRD). The gasification reactivity of char sample after separation was analyzed by TGA under CO2. The results show that concentration of active K and Mg in coal char samples gradually increased and more disordered carbon structure formed as the CS proportion in the blending increased from 0 to 80%. The coal char in the blending captured more AAEM species by volatile-char interactions instead of escaping with volatile from biomass during co-pyrolysis process. Meanwhile, higher pyrolysis temperature led to volatilization and inactivation of K and Na, and also decrease in graphitization degree. Moreover, both addition of CS and low pyrolysis temperature could promote gasification reactivity of coal char sample. Furthermore, a satisfactory linear correlation (R2=0.9009) between alkali index AI and R0.5 of the char samples was established. This indicated that AAEMs performed the dominate effect to enhance gasification reactivity of coal char during co-gasification of coal and biomass.

The catalytic effect of Benfield waste salt on CO2 gasification of a typical South African Highveld coal

The possible catalytic effect of a 5% loading by mass of crystallized salt from spent Benfield solution (SBFS) was investigated and compared with the catalytic influence of a 5 mass% loading of K2CO3 on the char–CO2 gasification reactivity of a bituminous medium rank C South African Highveld coal. The effect was investigated using known thermogravimetric investigation techniques. Coal char samples were prepared using the coal and catalyst-loaded samples by heating the samples in nitrogen to 1000 °C. The prepared char samples were further subjected to thermogravimetric analyses by heating them at isothermal temperatures between 900 and 990 °C in a CO2 atmosphere to simulate gasification reactions of the coal char. Results from isothermal char–CO2 gasification experiments showed that the catalyst-loaded coal samples exhibited higher gasification reactivities than the coal sample and the reactivities increased in the order: coal 3.5 in comparison with the coal char sample. The addition of catalysts to the coal was found to reduce the estimated activation energies of the catalyst-loaded samples during the gasification process. The estimated activation energies (calculated from the initial reaction rate of the gasification reaction) of the samples were 215 kJ mol−1 for the coal char, 146 kJ mol−1 for the char from the coal + 5% SBF and 118 kJ mol−1 for the char from the coal + 5% K2CO3. The crystallized spent Benfield salt may be utilized as a CO2 gasification catalyst for coal.

Study on coal char ignition by radiant heat flux.

The study on coal char ignition by CO2-continuous laser was carried out. The coal char samples of T-grade bituminous coal and 2B-grade lignite were studied via CO2-laser ignition setup. Ignition delay times were determined at ambient condition in heat flux density range 90–200 W/cm2. The average ignition delay time value for lignite samples were 2 times lower while this difference is larger in high heat flux region and lower in low heat flux region. The kinetic constants for overall oxidation reaction were determined using analytic solution of simplified one-dimensional heat transfer equation with radiant heat transfer boundary condition. The activation energy for lignite char was found to be less than it is for bituminous coal char by approximately 20 %.

The Phenomena of Secondary Weight Loss in High-Temperature Coal Pyrolysis

Studies on coal pyrolysis via a thermogravimetric analyzer (TGA) are typically performed under a variety of temperature increases beginning at ambient temperature. A main limitation in these procedures is that the reaction temperatures (generally lower than 1000 °C) are typically lower than those met in pulverized coal boilers and ash flow temperatures (AFTs; generally higher than 1100 °C). In this paper, five Chinese coals are clarified to low- or mid-AFT coals as their AFTs increase. They were then prepared in TGA at several temperatures below or above their AFTs under N2 atmosphere for approximately 30 min after a heating procedure at a rate of 80 °C/min. The coal and residual char samples were collected and analyzed via X-ray diffraction and compared to the chemical thermodynamic calculation. The secondary weight loss appeared between 1200 and 1450 °C in the TG curves of four mid-AFT coals but not the low-AFT coal. The results showed that this secondary weight loss was caused by the carbothermal reduc...

56. Characterization of partially gasified coal char samples using XRD and HRTEM techniques

56. Characterization of partially gasified coal char samples using XRD and HRTEM techniques

Kinetics of coal char gasification in a carbon dioxide medium

Solid fuel samples with different carbon contents are gasified by successively subjecting to pyrolysis in argon and oxidation in carbon dioxide at various temperatures to determine the rate of the chemical reactions and the activation energy required for simulating and optimizing the operation of gas generators. The samples were prepared from bituminous coal, lignite, and anthracite of the Kuznetsk and Kansk-Achinsk coal basins. The gasification of coal char samples in a carbon dioxide medium at 900–1200°C is analyzed by thermogravimetry. The temperature dependences of the weight change rate and gasification time of coal char samples are measured and used to calculate the preexponential factor and activation energy of the carbon oxidation reaction. It is found that, with increasing oxidizing medium temperature from 900 to 1200°C, the gasification time of the coal char samples obtained from anthracite and bituminous coal decrease 8- and 22-fold, respectively. A physicomathematical model of coal char gasification in a fixed bed, with the oxidizing gas diffusing through the ash layer formed, is proposed.

The influence of K2CO3 and KCl on H2 formation during heat treatment of an acid-treated inertinite-rich bituminous coal–char

Thermogravimetric-mass spectrometry (TG-MS) studies were conducted in order to investigate the effect of potassium carbonate (K2CO3) and potassium chloride (KCl) on the conversion and hydrogen evolution of chars derived from acid-leached South African inertinite-rich bituminous coal. K2CO3 and KCl (0.5, 1, 3, 5 potassium ion mass percentages) were loaded to the acid-leached (demineralized) coal samples prior to charring. The resulting ‘doped’ coal–char samples were subjected to heat treatment in a CO2 atmosphere up to 1200 °C, and the thermogravimetric curves, as well as the temperature ranges of evolution of hydrogen, were investigated. The results obtained indicate that the temperatures at maximum rate of mass loss (from the DTG curves), as well as the temperatures at maximum rate of hydrogen (H2) evolution (from the MS curves) are lowered with increasing potassium salt loadings. The relative coal gasification reaction reactivity (1/Tmax) determined from the DTG curves increases with increasing potassium salt loading and increases more for K2CO3 than for KCl. The catalytic influence of K2CO3 on the CO2 gasification of the acid-treated coal sample was found to be greater than that of KCl at a loading of 5 % potassium ion mass percentage.

Correlation between the Combustion Behavior of Brown Coal Char and Its Aromaticity and Pore Structure

In this study, the effects of aromaticity and pore structure, particularly fractal dimension, on the combustion behavior of brown coal char were investigated. For this purpose, several partially steam-gasified brown coal char samples with diverse pore structures and predictable aromaticity were chosen. Thermogravimetry was employed for analyzing the combustion behavior of the chars, and their apparent activation energies were obtained by the Arrhenius equation. Results from combustion experiments showed that the ignition temperature increased with char aromaticity, the changes in the temperature of the maximum combustion rate and burnout primarily depended upon the ratio of small aromatic rings to large rings of chars, and the total pore volume of chars predominantly affected their combustion reactivities. Two activation energies (i.e., chemical and diffusion activation energies) were distinguished. The chemical activation energy was controlled by the total aromatic C═C and ratio of small aromatic rings t...

Effects of Pyrolysis Atmosphere and Temperature on Coal Char Characteristics and Gasification Reactivity

AbstractCoal chars are usually produced under complex atmospheres in industrial production. However, most of the research about char characteristics and gasification reactivity uses the coal char samples prepared under an inert atmosphere. In this study, the influences of the pyrolysis atmosphere and temperature on coal char characteristics and gasification reactivity were investigated. A Shanxi bituminous coal was pyrolyzed in a bubbling fluidized bed to produce char samples. A mixture of hydrogen (H2), carbon dioxide (CO2), carbon monoxide (CO), and methane (CH4) was used as simulated pyrolysis gas (SPG). The coal was pyrolyzed in the SPG and N2 atmospheres, respectively. SEM, Raman spectroscopy, and FTIR spectroscopy were used to study the influences of the pyrolysis atmosphere and temperature on the coal char characteristics. The char–H2O and char–CO2 gasification reactivities were measured by using a modified thermogravimetric analysis (TGA) system. The experimental results indicate that the SPG atmosphere has no obvious influence on the coal char morphology, but it has significant influences on the carbon structures and surface chemical groups. The SPG char samples have lower gasification reactivity. The disproportionation reaction of CO, the decomposition reaction of CH4, and the loss of oxygen‐containing structures induced by H2 lead to a higher‐ordered degree of char, and the graphite generated from the disproportionation reaction and the decomposition reaction can decrease the specific surface area of the coal char. These two reasons result in the lower gasification reactivity of SPG char.

In situ catalyzed Boudouard reaction of coal char for solid oxide-based carbon fuel cells with improved performance

The use of industrial coal char as a fuel source for an anode-supported solid oxide-based carbon fuel cell (SO-CFC) with a yttrium-stabilized zirconia electrolyte and La0.8Sr0.2MnO3 cathode was investigated. Both the Boudouard reactivity and electrochemical performance of the coal char samples are higher than those of activated carbon samples under the same conditions. The inherent catalytic activity of the metal species (FemOn, CaO, etc.) in the coal char mineral matter leads to good cell performance, even in the absence of an external catalyst. For example, the peak power density of a cell fueled with pure coal char is 100mWcm−2 at 850°C, and that of a cell fueled with coal char impregnated with an FemOn-alkaline metal oxide catalyst is 204mWcm−2. These results suggest that using coal char as the fuel in SO-CFCs might be an attractive way to utilize abundant coal resources cleanly and efficiently, providing an alternative for future power generation.

Kinetic study of coals gasification into carbon dioxide atmosphere

The solid fuel gasification process was investigated to define chemical reactions rate and activation energy for a gas-generator designing and regime optimizing. An experimental procedure includes coal char samples of Kuznetskiy and Kansko-Achinskiy deposits consequent argon pyrolysis into argon and oxidating into carbon dioxide with different temperatures. The thermogravimetric analysis data of coal char gasification into carbon dioxide was obtained in the temperature range 900–1200 oC. The mass loss and gasification time dependencies from temperature were defined to calculate chemical reaction frequency factor and activation energy. Two coal char gasification physico-mathematical models were proposed and recommendations for them were formed.

Effect of Coal Rank on Steam Gasification of Coal/CaO Mixtures

The effect of coal rank on hydrogen generation during the reaction of coal/CaO and char/CaO mixtures with high-pressure steam was investigated using a flow-type reactor, and the results were compared with the results for the gasification of the same coal and char sample in the absence of CaO. For the coal/CaO mixture, reactions of coal, CaO and CO with steam, and CO2 absorption by CaO occurred simultaneously in the reactor and H2 was the primary gas product. CO2 was fixed by CaCO3, and CO was completely converted to H2. The rates of H2 generation per mole of carbon in the coal (char) samples decreased in the order of lignite > sub-bituminous >bituminous. The rate of H2 generation seemed to be affected by both the reactivity and the carbon content of the coal.

Pyrolytic Mercury Removal from Coal and Its Adverse Effect on Coal Swelling

Coal combustion is one of the largest anthropogenic contributors of mercury emissions to the environment. The high level of mercury toxicity and its potential to bioaccumulate have prompted strict regulations for mercury emission control in many countries. While end-of-pipe technologies for mercury control are currently undergoing increased international development, precombustion pyrolytic mercury removal has also been proposed as a possible option. Mercury is highly volatile at low temperatures, and its removal from coal can be controlled by low-temperature pyrolysis. This work investigated the effect of temperature on mercury release while monitoring the adverse effects of the thermal pretreatment process on coal using coal swelling as an indicative parameter for combustion quality and char burnout rate. Three coal samples were pyrolyzed for 20 min to selected temperatures ranging between 200 and 600 °C at a heating rate of 50 °C/min. Mercury content in the coal chars was determined by a cold vapor atomic fluorescence method. The total mercury in coal was found to decrease between 5 and 25% when the coal samples were pyrolyzed at 200 °C, while at 300 °C, the amount of mercury removed from coal ranged between 55 and 90%. Representative coal char samples were also observed by long-distance microscopy in a single-particle reactor in which they were pyrolyzed at a heating rate of 200 °C/s. The results showed a significant decrease in the maximum transient swelling behavior when the char samples were thermally treated at temperatures above 400 °C for one coal and 500 °C for the remaining two coal samples. The results suggested that mercury removal from coal by prior thermal upgrading to around a maximum temperature of 300 °C may provide an option for partial mercury control for some coals. However, a better understanding and quantification of the strongly bonded fraction of mercury in the coal is required to model a feasible mercury retention process using the option of thermal upgrading.

Reaction kinetics of pulverized coal-chars derived from inertinite-rich coal discards: Characterisation and combustion

An investigation was undertaken to determine the chemical, structural, mineralogical and petrographic (maceral groups) properties of coal-chars derived from typical South African discards, in order to evaluate a suitable combustion reaction rate model. Detailed characterisation results of typical coal-chars involving conventional and advanced measurements (such as CCSEM) are presented and the following unique properties were found (1) the chars have very low porosities (<5%), which can be attributed to the presence of a high content of inertinite (>75%) in the parent coals and the minerals with low porosities. (2) The coal-chars consists of large amounts of kaolinite and quartz and to a lesser extent of pyrite and calcite, very different to conventionally used bituminous coals, and (3) a microscopic inspection of the coal-chars (CCSEM) showed that the pulverised form of the coal-char samples, used for the reaction rates studies, were a collection of mainly carbon rich particles (<10% minerals) and mineral rich particles (<10%) carbon) which had been liberated in the milling process to particles with diameters of 20 and 70 μm. Combustion experiments were carried out in a TGA at atmospheric pressure with varying mixtures of oxygen and nitrogen and consisted essentially of generating results for the validation of a suitable overall reaction rate model and associated kinetic constants. Using isothermal and non-isothermal methods it was found that the shrinking core model with a rate controlling surface reaction (no internal effects) was applicable for temperatures between 643 and 823 K. A description of a non-isothermal method using linear heating rates and analytical expressions with application to combustion is given and sets of intrinsic reaction rate constants for the coal-chars examined are reported. It was found that the intrinsic reaction rates were determined essentially by the activity of the carbon-rich particles with a low concentration of minerals (ash).

Effect of Microstructural Changes on Gasification Reactivity of Coal Chars during Low Temperature Gasification

An attempt has been made to quantitatively investigate the effect of microstructural changes on gasification reactivity of coal chars. Pocahontas No. 3, Illinois No. 6, and Beulah-Zap coal char samples were gasified in 1% O2 at 500 °C or 600 °C up to 90% (daf) conversion, and their structure were observed under a high-resolution transmission electron microscope (HRTEM). A quantitative structural analysis was done based on HRTEM images to obtain structural parameters such as number of stacks, layer size diameter, and their distributions using an image analysis algorithm. Effect of mineral matter on structural changes was also studied by carrying out gasification experiments with demineralized chars. The gasification reactivity of these chars was correlated with the structural parameters.

00/00658 Characterization of partially gasified coal char samples using XRD and HRTEM techniques

00/00658 Characterization of partially gasified coal char samples using XRD and HRTEM techniques

00/00659 Crystallinity, true density and intrinsic reactivity of intermediate temperature coal char

The true density and crystallinity of char samples of a kind of bituminous vitrinite were measured and studied These char samples were obtained at various temperature and residence time The true density and crystallinity of the residual carbon in the fly ash from a stoker boiler were also investigated The results showed that the crystallinity of coal char samples generated at intermediate temperature increases with an increase in temperature and residence time It also showed that there is a certain relationship between crystallinity, true density and the intrinsic reactivity of coal chars

Experimental investigation of dissolution rates of carbonaceous materials in liquid iron-carbon melts

Interactions of carbonaceous materials in liquid Fe-C melts have been investigated experimentally by determining the rates of dissolution at temperatures ranging from 1623 to 1935 K. The rates of dissolution of spectroscopic graphite and an industrial coke obeyed the correlation for natural convection under turbulent conditions. The experimental data for the graphite suggested that the rate of dissolution was controlled by mass transfer in liquid boundary layer adjacent to the solid sample. The value of the empirical parameter correlating the dissolution coefficient and the operating variables was found to be 0.19, which was close to that reported in the literature. The comparison of the results obtained for coke and low-volatile coal char samples with those for the graphite revealed that impurities and porosity of the samples can effect the dissolution rates. The values ofk 1, for coke decreased with increasing the dissolution time. The examination of some of the partially dissolved coke samples by electron micro-scopy revealed that a thin, viscous ash layer was forming on the sample surface, which must be the main reason for the behavior. The dissolution rates were controlled by both mass transfer and phase boundary reactions when sulfur was present in the bath. The extent of devolatilization and dissolution of coal particles when they were injected into an Fe-C melt depended on the particle size and location.