Coal Char Oxidation Research Articles

Synergistic reduction on PM and NO source emissions during preheating-combustion of pulverized coal

The present research focuses on the synergistic source control of particulate matter (PM) and NOx formation from pulverized coal combustion. Comparative experiments of preheating-combustion and conventional combustion were conducted in a lab-scale high-temperature preheating-combustion furnace, and PM10 and NO were measured by an electrical low pressure impactor and a flue gas analyzer, respectively. The results of the experiment indicate that preheating-combustion has a significant reduction in PM10 (especially PM0.3 up to 37.51 %) and NO, which can achieve the synergistic control of PM10 and NO source emissions during the combustion process. The fragmentation in preheating-combustion was weaker compared to the conventional combustion. Meanwhile, the relatively weak preheating-combustion coal char oxidation reaction leads to a decrease in ultrafine mode PM yielded due to the inhibition on vaporization of mineral inclusions. The PM0.3/PM1 mass ratio of the preheating-combustion has a decreasing trend, implying an elevated yield of PM0.3-1 and a shift of the average PM1 particle size toward a larger particle size. Higher preheating temperature (Tp) presented the potential to further reduce NO formation, and the NO reduction efficiency increased from 46.59 % to 56.60 % when the Tp was increased from 1200 K to 1600 K. All our preliminary results throw light on the nature of synergistic source control of preheating-combustion PM and NO formation.

Studying the mechanism and impact of H2O-rich atmosphere on N oxidation during ammonia combustion and ammonia-coal co-combustion

Carbon-free fuel ammonia and coal co-combustion, an effective way to reduce thermal power CO2 emissions, has attracted more and more attention in recent years. Due to the hydrogen-rich property of ammonia, more H2O is generated during combustion, and the high H2O atmosphere significantly affects the NOx formation. Therefore, it is necessary to explore the oxidation path of N in the combustion of ammonia and its coupling coal in the H2O atmosphere. In this study, quantum chemistry theory and transition state theory are used to investigate the influence mechanism of H2O atmosphere on the oxidation of ammonia and fuel-N in the ammonia-coal co-combustion. The results show that the presence of OH in the H2O atmosphere promotes the formation of NH2 and NH, which is conducive to their oxidation to HNO and NO. The condition without OH is conducive to the oxidation of ammonia to HNO, and the presence of OH promotes the further oxidation of HNO to NO in the co-combustion of ammonia and coal. The H2O atmosphere reduces the energy barrier value of rate-determing step for ammonia and coal char oxidation during co-combustion, and increases the oxidation rate of ammonia-N and char-N. This work reveals the molecular mechanism by which H2O atmosphere promotes the combination of OH and O2 to generate HO2, and discovers the temperature conditions for HO2 formation. This study provides theoretical support for the development of N oxidation mechanism in the ammonia-coal co-combustion.

Optimized TGA-based experimental method for studying intrinsic kinetics of coal char oxidation under moderate or intense low-oxygen dilution oxy-fuel conditions

Accurate assessment of coal char oxidation kinetics is essential for the design of efficient combustion systems; nonetheless currently, there is little research on this aspect under moderate or intense low-oxygen dilution (MILD) oxy-fuel (MO) conditions. In this study, experimental and modeling studies were conducted for analyzing the intrinsic kinetic properties of coal char oxidation under MILD air-combustion (MA) and MO conditions based on thermogravimetric analysis (TGA). The experimental conditions were improved to minimize various diffusion resistances in the crucible during the TGA experiments. Under the optimal conditions of TGA experiments, the reactivity differences of coal char under the MA and MO atmospheres were small. Both nonlinear model-fitting and linear model-free analysis methods were used to analyze kinetic parameters of coal char oxidation. The results showed that the random pore model correlated the best with the experimental results. The activation energies of coal char under the optimal conditions in MA and MO atmospheres (i.e., 104.4 kJ/mol and 98.2 kJ/mol, respectively) were much higher than those obtained by the conventional method. The reaction orders of coal char oxidation in MA and MO atmospheres were close to 1.0, and the specific values were affected by the diffusion effect and background atmosphere.

Investigation of N Transfer during Coal Char Oxidation by Reactive Molecular Dynamics

Investigation of N Transfer during Coal Char Oxidation by Reactive Molecular Dynamics

A new thermodeactivation model for soot surface reactivity toward O2 in turbulent hydrocarbon diffusion flames

ABSTRACT Accurate soot modeling in flames is one important tool contributing to an understanding of soot behavior in combustion. Soot models reflect the four principal processes that govern soot dynamics: nucleation, coagulation, surface growth, and oxidation. Existing oxidation models reflect both the temperature-dependent surface oxidation chemical kinetics and also the fact that the surfaces become less reactive as they age. This surface deactivation process has been modeled primarily by using in-flame soot measurements as a basis for developing functional forms and parameters to describe the decay of α, the ratio of active oxidation sites divided by total surface sites. These existing models, in general, overpredict oxidation rates in turbulent diffusion flames. This study presents an alternative soot aging factor model that is based on the analogous process of coal char oxidation deactivation, a well-studied problem. The proposed framework provides a basis for application to a wide range of turbulent diffusion flames. This new model is configured to provide the same parameter α that appears in existing approaches. As such it can be easily applied to the existing published models. The approach is implemented in the fixed-pivot sectional soot model, with the performance compared against literature data for ethylene/air turbulent diffusion flames. With only very minor adjustment from the baseline parameters developed for coal char, the model shows good agreement with the literature flame data.

Particle-resolved simulation and modeling of the conversion rate of coal char in chemical looping with oxygen uncoupling

The high concentration of CO2 and low concentration of O2 at relatively low temperatures of 1123-1223 K are typical reactive atmospheres in the fuel reactor of a chemical looping with oxygen uncoupling (CLOU) unit. Char within this environment can be converted via lean-O2 oxidation and/or rich-CO2 gasification. Conventional models, which either considered the oxidation only or considered the oxidation and gasification contributions independently by simply adding the two individual conversion rates together, seem arbitrary. Therefore, it is necessary to quantitatively explore the contributions of the oxidation and the gasification reactions as well as their interactions in CLOU conditions through more detailed particle-resolved simulations.In this work, a single particle model which described a spherical reacting porous particle with its reacting boundary layer was developed to investigate the conversion characteristics of pulverized coal char in lean-O2 and/or rich-CO2 conditions (analogous CLOU conditions). The heterogeneous reaction kinetics developed by Tilghman and Mitchell [35] and the simplified GRI-Mech 3.0 gas-phase reaction kinetics were adopted. It was found that the char conversion rate in CLOU conditions could be basically modeled as the sum of the full oxidation consumption rate and the partial gasification consumption rate, i.e. rmix=φ1rgasi+roxid. The interactions between the oxidation and gasification reactions were elaborated in two aspects: on the one side, the oxidation of coal char in CLOU conditions is within the weak zone II burning region, where the oxidation reaction only happens peripherally and is nearly unaffected by the high concentration of CO2; on the other side, the gasification reaction within the external layer of the coal char particle is inhibited by the oxidation reaction, consequently the gasification reaction only happens inside the coal char particle where O2 cannot penetrate into. Based on these understandings, a correlation between the coefficient, φ1, and the effectiveness factor of the oxidation reaction, η, was proposed to describe the overall char conversion rate in CLOU conditions. Through fitting the simulation results under typical CLOU conditions, the quantitative relationship between φ1 and η was finally attained.

Theoretical understanding of coal char oxidation and gasification using reactive molecular dynamics simulation

Coal char possesses high carbon and low ash features, making it suitable for many industrial uses. Despite the extensive interest in coal utilization, it is the char that has the great influence on the combustion reactivity, where char gasification reaction by CO2 can affect the combustion characteristics during oxy-fuel combustion. In this work, a carefully established surrogate coal char model, coupled with the selected ReaxFF force field, is used to simulate the processes of char oxidation by O2, char gasification by CO2, and char combustion under O2/CO2 and O2/N2 conditions. In comparison with the oxidation process, high CO2 concentration hinders gaseous molecules (C2-C4) and light tar (C5-C13) from reacting with O radicals to form smaller carbonaceous molecules. The C–C bond breaking activation energies are 164 and 217 kJ/mol for char-O2 oxidation and char-CO2 gasification, respectively. Specifically, O radical and CO2 will adsorb and destroy the edge of representative poly-aromatic hydrocarbon molecules in the char model during char oxidation and gasification. The CO/CO2 molar ratio decays almost exponentially with the extent of char consumption due to the subsequent oxidation of CO in char oxy-fuel combustion. The C–C bond breaking activation energies are 196, 190 and 167 kJ/mol for char combustions under O2/CO2 ratios of 25%/75%, 50%/50% and 75%/25%, respectively. At the same O2 concentration, the oxidation ratio under the O2/CO2 environment is always higher than that under the O2/N2 environment, as a result of the additional O radicals provided by CO2. This work is devoted to deepen the understanding of the thermochemical conversion process of char at the atomistic level, and is expected to further optimize char utilization.

CaFe2O4 oxygen carrier characterization during the partial oxidation of coal in the chemical looping gasification application

The calcium ferrite (CaFe2O4) oxygen carrier characterization during chemical looping coal gasification was conducted using X-ray diffraction and Raman spectroscopic analysis. Density functional theory (DFT) analysis and experimental data were used to elucidate the mechanism for the selective oxidation of coal char. The Raman spectroscopic analyses indicated that the initial reduction of CaFe2O4 were at the Fe(8)O6 and Fe(7)O6 octahedral sites which are related to the morphological changes at the [4 0 0] and [2 1 2] crystal planes. After the initial reduction, the Raman spectroscopic data indicated the remaining oxygen from Fe-O sites containing Ca (e.g. FeOCa) participated in the reduction reaction. Deep reduction of the CaFe2O4 led to the removal of oxygen from the FeOCa bonds, indicated by the decrease in Raman intensities of the corresponding species, and the appearance of the CaO and Fe0. The oxygen ion diffusion controlling regime was due to the oxygen transfer occurring at the Fe octahedral sites. These results indicated calcium functions to control both the movement of oxygen ions through the solid and the rate of carbon oxidation in coal char leading to the selective oxidation to produce CO, the desirable product of the chemical looping gasification application.

Mechanism and Thermochemistry of Coal Char Oxidation and Desorption of Surface Oxides

The present study investigates the coal char combustion by a combination of thermochemical and X-ray photoemission spectroscopy (XPS) analyses. Thermoanalytical methods (differential thermogravimetry, differential scanning calorimetry, and temperature-programmed desorption) are used to identify the key reactive steps that occur upon oxidation and heating of coal char (chemisorption, structural rearrangement and switchover of surface oxides, and desorption) and their energetics. XPS is used to reveal the chemical nature of the surface oxides that populate the char surface and to monitor their evolution throughout thermochemical processing. XPS spectra show the presence on the carbon surface of three main components. It is shown that the most abundant oxygen functionality in the raw char is epoxy. It decreases with preoxidation at 300 °C and even more at 500 °C, where carboxyl and ether oxygen functionalities prevail. The rearrangement of epoxy during preoxidation goes together with activation of the more s...

Coal char oxidation kinetics in air medium

Study of oxidation kinetics for three types of coal char with different carbon content in air is presented. The coal char powders of anthracite, bituminous T-grade coal and 2B-grade lignite with particle size less than 80 μm were tested. The coal char oxidation was researched by isothermal method via simultaneous TG-DSC analyzer Netzsch STA 449 Jupiter F3 in the temperature range of 1000–1200 °C. Measurements were carried out at ambient pressure. Volumetric flow rate of oxidizing medium into analyser chamber was 250 ml/min. Flow consisted of air and argon with volumetric ratio 24/1. Carbon average rate of oxidation reaction at each temperature were defined based on experimental results. Kinetic constants (the frequency factor and activation energy) were defined for Arrhenius equation modified with three submodels: volumetric model, shrinking core model and random pore model. The activation energy values for anthracite are 1,6-1,7 times higher than for chars of bituminous coal and lignite.

Probing the chemical nature of surface oxides during coal char oxidation by high-resolution XPS

Coal chars, like most solid carbons, have a pronounced tendency to chemisorb oxygen at low and moderate temperatures. Characterization by XPS of surface oxides on carbon has been accomplished with the aim of providing a better atomistic insight into: (a) reactions involved in molecular oxygen adsorption on coals and (b) the relations between the nanostructure of solid carbons and the chemistry of oxidation. High-resolution C 1s and O 1s core level and valence band XPS spectra effectively reflected the oxidative functionalization of different types of coals and synthetic carbons upon oxidation in air at moderate temperatures (300 and 500°C). More specifically, analysis of C 1s and valence band spectra could be directed to monitor the structural evolution of the carbons in terms of extension of sp2 versus sp3 conjugation, carbon vacancies and oxidized carbon. Comparison of the O 1s spectra, on the other hand, provided a tool to characterize the nature of oxygen bonding on carbon and to determine the relative abundance of carbon–oxygen species. Results underline the important role of epoxy groups in the early stages of oxidation, providing a mechanistic framework for the identification of the stable and metastable intermediates in the heterogeneous oxidation of coal by molecular oxygen.

A comparative process study of chemical-looping combustion (CLC) and chemical-looping with oxygen uncoupling (CLOU) for solid fuels

A solid-fuel combustion system based on chemical-looping combustion (CLC) and chemical-looping with oxygen uncoupling (CLOU) has the potential to assist in the capture of CO2 from coal-fired power plants. In both processes an air separation unit is not required, and the flue gas streams from CLC and CLOU contain primarily carbon dioxide and water, which facilitates CO2 capture. CLOU offers a potential advantage for solid fuels as it uses combustion reactions. The O2 for the combustion reactions in CLOU is supplied from the reduction of a metal oxide (e.g. CuO). Iron-based materials are being considered for oxygen carriers in CLC, wherein the coal is gasified, and subsequently the product gas is oxidized to CO2 and H2O by reaction with the circulating oxygen carrier. CLOU affords faster coal char oxidation reaction rates, as compared to CLC coal gasification reactions, but CuO-based materials for CLOU will necessarily be more expensive. Furthermore, the stability of CuO-based oxygen carrier materials is also an important concern. In this paper, ASPEN PLUS process engineering models were developed for combustion of a Wyoming Powder River Basin coal using an iron-based oxygen carrier for CLC and a copper-based oxygen carrier for CLOU. The objective of these process models was to evaluate the material and energy requirements for a process development unit by incorporating insights from previously reported kinetic studies on laboratory scale units. A relative economic analysis has also been performed to address key technical challenges which will subsequently help in addressing the development of CLC and CLOU for solid fuels. Due to slower char gasification reaction times, CLC requires a larger reactor, which results in a relatively higher capital cost. It also manifests in a higher pressure drop and consequently higher energy costs for fluidizing the oxygen carrier.

Arsenic emissions and speciation in the oxy-fuel fly ash collected from lab-scale drop-tube furnace

Combustion of three coal samples of different types and origins has been conducted in a lab-scale drop-tube furnace (DTF) to examine the speciation and emission of As during air and oxy-fuel combustion. The synchrotron XANES has been used to quantify the contents of toxic As3+ and less toxic As5+ in char and fly ash samples. Irrespective of coal type and the original mode of occurrence of As, the substitution of N2 by CO2 for oxy-fuel combustion caused little changes to the emission and speciation of As at the initial step of coal combustion, pyrolysis. The reduction of original As5+–O and/or oxidation of the As(g) derived from the decomposition of As–S association favoured the retention of approximately 20% of As as As3+ in char, which remained unchanged with the progress of the bituminous coal char oxidation, but was further vaporised during the combustion of lignites. For the oxy-fuel mode with no less than 27% oxygen to match air in terms of adiabatic flame temperature, the fraction of toxic As3+–O in the final fly ash of the bituminous coal and high-ash lignite is much lower than in air, due to the higher oxygen partial pressure in the system. However, for the As-doped low-ash brown coal, its high CO2-gasification reactivity slightly promoted the fraction of As3+–O in oxy-fuel mode that decreased with increasing oxygen content. In addition, the amount of inherent Ca and/or Fe in coal has been proven insignificant in affecting the stabilisation of As vapours, instead relying more on the availability of Ca and/or Fe active sites for As capture. For the addition of external free lime (CaO), its capture capability is also lower in oxy-fuel mode than in air, probably due to the preferential carbonation caused by vast CO2 in the reactor.

A mechanistic study on kinetic compensation effect during low-temperature oxidation of coal chars

This paper provides mechanistic insights into the low-temperature oxidation of a range of carbon materials (graphite, a sub-bituminous coal char, and a brown coal char). Kinetic analysis was carried out on oxidation of the chars, prepared from fast-heating pyrolysis, under chemical-reaction-controlled regime. FT-Raman spectroscopic analysis was adopted to provide direct structural information on the carbon structure of reacting carbon materials throughout oxidation. The results demonstrate the significance of selective oxidation under the conditions, and parallel to this, the kinetic compensation effect of carbon oxidation reaction throughout conversion for all samples. Supported by the results from FT-Raman spectroscopy, the kinetic compensation effect seems to be a result of the selective oxidation of these carbon materials with heterogeneous carbon structures. Oxidation of all samples, with or without catalysts, appears to be similar in terms of the ‘nature’ of carbon structural condensation during low-temperature oxidation, suggesting a similar increase in apparent active sites population with respect to increase of apparent energy barrier. Under the current experimental conditions, a general kinetic compensation effect correlation has been deduced for various materials, requiring only the initial char kinetic parameters. The inherent inorganic species in chars also seem to alter the ‘degree/extent’ of carbon structural condensation as results of selective oxidation. In this case, the use of the compensation effect correlation will require more information on the catalysis during oxidation, apart from the initial char kinetic parameters.

Releases of NO and its precursors from coal combustion in a fixed bed

Experiments have been carried out to investigate the emissions of nitrogen species including NO and its precursors during temperature-programmed coal combustion by TG/EGA method. Experimental results show that the conversion ratio of fuel nitrogen to NO is the highest, followed by that of fuel nitrogen to HCN and the conversion ratio to NH 3 is negligibly small. Nitrogen is retained in the char and released mainly as NO at the later stages of coal combustion. HCN and NO are both primary products from coal char oxidation. Coal rank, heating rate, indigenous minerals and external additives are the major influential factors of the nitrogen species release. Higher rank coals with higher fuel ratio have higher NO releases. HCN release decreases as fuel ratio increases for most coals. The fuel nitrogen conversion to NO increases and the fuel nitrogen conversion to HCN decreases with the increase of heating rate, which may imply that the char nitrogen prefers to react with oxygen to form NO instead of HCN while coal char is combusted at higher temperatures. Different metallic additives show different effects on nitrogen species emission and the effects of indigenous minerals on nitrogen release can be qualitatively estimated by ash analyses.

The effect of potassium bromide and sodium carbonate on coal char combustion reactivity

The addition of halogens, particularly iodine, to the gas during coal char oxidation has been used in previous studies to quench gas-phase chemistry, thereby allowing one to separate the effects of homogeneous and heterogeneous reactions. Halogen addition suppresses the gas-phase radicals to near-equilibrium levels. A similar effect can be expected from other compounds with high efficiency as fire suppressants, such as alkali metals. The effectiveness of the use of additives in distinguishing homogeneous and heterogeneous reactions during char oxidation relies on the assumption that radicals are suppressed while heterogeneous reactions occurring on the char surface are not affected. The present work tests this assumption for potassium bromide (KBr) and sodium carbonate (Na 2CO 3) reacting with a pulverized eastern bituminous coal char during oxidation. An increase in CO and a slight reduction in particle temperature were observed with the addition of KBr, consistent with known effects of halogens on gas-phase chemistry. An increase in particle size was also observed with the KBr addition. This observation and the results of model calculations suggest that there is significant incorporation of liquid KBr on the char surface under the conditions examined. With Na 2CO 3 addition, the particle temperature did not change, the particle size showed a slight decrease, and CO production increased. Although the mechanisms for Na interaction with radicals at combustion conditions are not well established, char oxidation modeling suggests that a decrease in OH concentration in the particle boundary layer is the cause for the observed increase in CO production. It is concluded that Na 2CO 3 has clear advantages over KBr for inhibiting gas-phase chemistry without affecting char oxidation for the conditions investigated here.

The characteristics of coal-char oxidation under high partial pressure of oxygen

The characteristics of coal-char oxidation under high partial pressure of oxygen

The characteristics of coal-char oxidation under high partial pressure of oxygen

Pulse technique was proposed in this paper to study the characteristics of three kinds coal-char combustion with high partial pressure of oxygen under the temperature of 1273–1573K to simulate the process of coal combustion with pure oxygen. Oxygen or the mixture of nitrogen and oxygen was taken as the pulse gas. An on-line gas chromatography with TCD was employed to detect the concentration of CO and CO2 in the reacted pulse gas. Helium was used as both the carrier gas and reference gas in TCD. The results showed that the apparent reaction rate increased strongly as a function of oxygen concentration up to 100% under the operation pressure 0.18MPa. The reaction orders for the three chars were all close to unity. The activation energies were about 20kcal/mol in the temperature range of 1273–1573K.

Interactions of gaseous no with char during the low-temperature oxidation of coal chars

The role of nitric oxide (NO) during the low-temperature oxidation (873 K, ca. 2% O2/He) of subbituminous and bituminous coal chars was studied experimentally in fixed-bed reactor systems. Analysis of the gas-phase products by gas chromatography (GC) and Fourier transform infrared spectroscopy (FTIR) identified nitrogenous products with typical abundance N2(54%), NO (22%), HNCO (12%), HCN (6%), and N2O (1%). Significant retention of char nitrogen (char N) occurred within the char during the early stages of burnout. This may be explained via the following mechanism: NO + 2C() → C(N) + C(O) (1) C(N) + NO → N2 + C(O) or CO (2) C(N) + O2 → NO + C(O) or CO (3) The observed extent of nitrogen enrichment of the char during burnout requires that up to 90% of the NO formed by oxidation of char N (3) is reincorporated into the char (1) in the burnout. Experiments also were conducted using a dispersed-bed reactor in which isotopically labeled 15NO (0–1000 ppm) was added to the reactant gases (2% O2 in He) during the oxidation of coal char. The concentrations of labeled HC15N and H15NCO that were observed by FTIR indicated a significant interaction between gaseous NO and the char surface. The ratios of isotopes present in the HCN and HNCO product distributions were similar, suggesting that these species may be formed by related processes.

Mechanism of N 2 formation during coal char oxidation

In spite of many studies, the reaction mechanism of the evolution of nitrogen-containing gases during coal oxidation is not fully understood yet. One reason is that the nitrogen mass balance has not been well established in most studies. In the present study, we attempted to clarify the N 2 formation mechanism during the reaction between coal char and O 2 by paying special attention to nitrogen mass balance. The closure of the nitrogen mass balance was 100±6%. Blair Athol coal char was used as a sample, and the formation of N-containing species was determined during temperature-programmed reaction as well as isothermal reactions at 700 and 850°C in a packed-bed reactor. In all the cases, N 2 was the major product, with a little amount of NO as a minor product. Under the isothermal reaction conditions, the ratio of NO/N 2 was low in the initial stage and increased in the later stage. The effect of bed height on the product gas distribution was examined, and it was found that the shallower the bed, the higher the ratio of NO/N 2 . When the O 2 /He gas mixture was switched to He, the formation of N 2 and NO became almost negligible. From these observations, we proposed the following reaction mechanism for the N 2 formation: the formation of NO as a primary oxidation product, and the reaction of NO with surface nitrogen species to form N 2 . The NO capture on carbon surface to form surface nitrogen species is thought to be an important step during the course of this reaction.