Oxidation Of Brown Coal Research Articles

Solid-Phase Reactions of Humic Acids with Sodium Percarbonate as a Way to Study Processes Occurring on the Mechanochemical Oxidation of Brown Coal

Solid-Phase Reactions of Humic Acids with Sodium Percarbonate as a Way to Study Processes Occurring on the Mechanochemical Oxidation of Brown Coal

Solid-Phase Reactions of Humic Acids with Sodium Percarbonate as a Way to Study Processes Occurring on the Mechanochemical Oxidation of Brown Coal

The reactions of humic acids in a pure form and in the composition of coal with sodium percarbonate occurring on solid-phase mechanochemical treatment were compared. Analysis for CHNO, IR spectroscopy, and potentiometric titration were used to study changes in the composition of humic acids mechanochemicallytreated after isolation from brown coal. The sorption properties in relation to heavy metal ions (Zn2+, Cu2+, and Cd2+) and their changes as a result of the treatment were studied. The results were correlated with data similar to those for humic acids in coal that underwent mechanochemical treatment. It was demonstrated that, at present, an increase in the yield of humic acids and the concentrations of phenolic and carboxyl groups in brown coal as a result of treatment with sodium percarbonate can be explained by the oxidation of fractions other than humic acids in the organic matter of brown coal.

Combined ReaxFF and Ab Initio MD Simulations of Brown Coal Oxidation and Coal-Water Interactions.

In this manuscript, we use a combination of Car–Parrinello molecular dynamics (CPMD) and ReaxFF reactive molecular dynamics (ReaxFF-MD) simulations to study the brown coal–water interactions and coal oxidation. Our Car–Parrinello molecular dynamics simulation results reveal that hydrogen bonds dominate the water adsorption process, and oxygen-containing functional groups such as carboxyl play an important role in the interaction between brown coal and water. The discrepancy in hydrogen bonds formation between our simulation results by ab initio molecular dynamics (CPMD) and that by ReaxFF-MD indicates that the ReaxFF force field is not capable of accurately describing the diffusive behaviors of water on lignite at low temperatures. The oxidations of brown coal for both fuel rich and fuel lean conditions at various temperatures were investigated using ReaxFF-MD simulations through which the generation rates of major products were obtained. In addition, it was observed that the density decrease significantly enhances the generation of gaseous products due to the entropy gain by reducing system density. Although the ReaxFF-MD simulation of complete coal combustion process is limited to high temperatures, the combined CPMD and ReaxFF-MD simulations allow us to examine the correlation between water adsorption on brown coal and the initial stage of coal oxidation.

Scaling up the Mechanochemical Technology for Producing Materials Enriched in Humic Acids

Mechanochemical treatment is widely used in industry as it allows one to reduce the number of processing stages, either reduce or completely eliminate the use of solvents, automate the technology and ensure its mobility. Energy input is an important parameter of mechanochemical treatment. In this study, the technology of mechanochemical oxidation of brown coal was scaled up using the planetary mills of AGO-2 and APF types, as well as K-7 and CEM-20 flow-type vibrocentrifugal mills. The technology of mechanochemical oxidation of brown coal makes it possible to increase the availability of humic acids in brown coal and the content of oxygen-bearing groups capable of complexation, as well as to improve sorption characteristics of the resulting product. Materials enriched in humic acids can be used as heavy metal sorbents in water bodies and as agents for soil amelioration. The rotor speed of 1400 rpm and the feed rates of 15 kg/h and 1.8 kg/h for CEM-20 and for K-7 vibrocentrifugal mills, respectively, are the optimal modes from the point of view of chemical and economic efficiency. These modes increase the yields of alkali-soluble and water-soluble humic acids from 23 to 59 % and from <1 to 7%, respectively, which corresponds to the values reached using the laboratory-scale equipment.

Effect of an Initiating Additive of CuSO4 on Changes in the Characteristics of Brown Coal Oxidation and Pyrolysis

The oxidation and pyrolysis of brown coal containing 5 wt % CuSO4 as an initiating additive was studied. The experiment was performed by thermogravimetry at a heating rate of 2.5 K/min in an atmosphere of air and nitrogen. The process characteristics in the pyrolysis and oxidation mode were evaluated, and the activation energy of the process was determined by the Coats–Redfern method. It was established that the addition of the initiating agent CuSO4 led to a significant decrease in the initial temperature of oxidation and pyrolysis processes to shift the reaction toward the low-temperature region. Maximum changes in the reaction initiation temperatures in the oxidation (ΔTi) and (ΔTd) pyrolysis modes were 35 and 50°С, respectively. It was established that the introduction of CuSO4 led to a decrease in the activation energy of oxidation by 7.1 kJ/mol, and ΔEa was 10 kJ/mol for the pyrolysis process. A decrease in the residence times of a sample within the limits of the sublimation of volatile substances was observed: Δti was 12 min for oxidation, and Δtd was 18 min for pyrolysis. According to mass-spectrometric analysis data, the presence of SO2 (peaks at 230 and 320°C) was detected in the oxidation and pyrolysis reaction products of modified samples; this was explained by the chemical interaction of copper sulfate with brown coal components.

Non-isothermal oxidation of coal with Ce(NO3)3 and Cu(NO3)2 additives

Non-isothermal oxidation of brown coal with 5 wt% of Cu(NO3)2, 5 wt% of Ce(NO3)3 and {2.5 wt% Cu(NO3)2 + 2.5 wt% Ce(NO3)3} additives was studied. The introduction of additives was carried out by an incipient wet impregnation method to ensure uniform distribution of cerium and copper nitrates within the structure of coal powdery samples (according to SEM and EDX mapping). The samples reactivity was studied in an isothermal oxidation regime at 200 °C (1 h) and by DSC/TGA at 2.5 °C/min heating rate. The additives implementation was found to reduce significantly the oxidation onset temperature (ΔTi = 20–55 °C), the samples oxidation delay time (Δti = 2–22 min) and overall duration of the oxidation process (Δtc = 8–16 min). The additives efficiency could be graded in accordance with the activation on the coal oxidation in the following row: Cu(NO3)2 > {Cu(NO3)2 + Ce(NO3)3} > Ce(NO3)3. According to the mass spectroscopy, the obtained row of activation correlates well with the initial temperature of the studied nitrate’s decomposition (from 190 to 223 °C). A presence of nitrates was found to change significantly the trend of heat release taking place during the oxidation of coal samples (according to DSC/TGA data). The influence of coal morphology and volatiles content in initial sample on the parameters of the oxidation process was studied as well. Activation energy (Ea) of the coal oxidation was calculated using Coats–Redfern method. Maximum decrease in Ea from 69 to 58 kJ/mol was observed for the samples with Cu(NO3)2.Graphical abstract

Research of thermal destruction dynamics of coal particles in oxidizing medium with copper nitrate

Oxidation of brown coal containing 5% by weight of (Cu(NO3)2) initiation additive has been studied. The experiment was conducted using thermogravimetric analysis at a heating rate of 2.5°C/min up to a maximum temperature of 600°C in air. Based on DTA results, the oxidation characteristics were analytically evaluated. It has been discovered that addition of (Cu(NO3)2) initiation agent leads to a significant decrease of initial temperature of oxidation, which serves to shift the reaction to the low-temperature region. The effect of the initiation additive also manifested in significant reduction of sample residence time within sublimation of volatile compounds. Mass-spectrometric analysis has revealed the presence of NOx (intense peak at 180°C) in oxidation products of the modified sample, which is explained by decomposition of copper nitrate. SEM results for the coal partially oxidized in the muffle furnace indicate significant change of surface and internal structure of the modified sample particles.

Change of brown coal oxidation kinetic characteristics by promoting additives appending in the form of copper salts

Process of brown coal oxidation with 5%wt content of copper-salt additives of various nature (Cu (NO3)2, CuSO4 and Cu(CH3COO)2) was studied. The experiment has been performed by thermogravimetric analysis at a heating rate of 2.5°C/min to a maximum temperature of 600°C in the air. Analytical evaluation of oxidation process kinetic characteristics has been conducted based on the results of TGA. It has been established that addition of initiating agents leads to significant reduction in the initial ignition temperature of coal (ΔTi = 15÷40°C), shortening of the sample warm-up time to the ignition point (Δte = 6÷12 min) and reduction of the sample burning time (Δtf = 40÷54 min). The following series of additives activity affecting ignition temperature of coals has been established: Cu(CH3COO)2 > Cu(NO3)2 > CuSO4. Additionally, the opposite can be said about the effect of additives on residence time of the sample in its combustion area (CuSO4 > Cu(NO3)2 > Cu(CH3COO)2). According to mass spectrometric analysis, presence of NOx, SO2, CO2 (intense peaks at 190÷290°C) was recorded in oxidation products of modified samples, which is explained by partial or complete decomposition of salts.

Partial and complete oxidation of brown coal in a supercritical water–oxygen fluid under conditions of counterflowing reactant streams

The oxidation of brown coal continuously fed as part of a coal–water slurry into a counterflowing stream of a supercritical water–oxygen fluid at a temperature difference along the reactor axis of 673–873 K and a pressure of 30 MPa has been studied. It has been found that, in the case of a partial combustion of coal (under conditions of O2 deficiency), the yield of hydrogen-enriched products increases owing to heat evolution. Under conditions of excess O2, coal undergoes complete oxidation. In this case, the heat evolved per unit volume of furnace space is about 1.0 MW/m3. It has been shown that the heat consumed for the implementation of the process using external sources can be partially or completely compensated for by heat evolution during homogeneous and heterogeneous combustion coupled with coal thermolysis.

A novel process for the production of aromatic hydrocarbons from brown coal in water medium by hydrothermal oxidation and catalytic hydrothermal decarboxylation

With no use of hydrogen or a hydrogen donor, a novel process is proposed for the production of aromatic hydrocarbons from brown coal in water medium, which consists of hydrothermal oxidation at 240°C and Cu2O-catalysed hydrothermal decarboxylation at 350°C. Using this hydrothermal oxidation and catalytic hydrothermal decarboxylation method, aromatic carboxylic acids were formed from brown coal by an oxidation process, and then decarboxylated to the corresponding aromatic hydrocarbons. The validity of the decarboxylation method was examined using several aromatic carboxylic acids as model compounds for the acid products after hydrothermal oxidation of brown coal, including o-phthalic acid, isophthalic acid, terephthalic acid, hemimellitic acid, trimellitic acid, trimesic acid, 1-naphthoic acid, 2-naphthoic acid, and 2,6-naphthalic acid. Use of Cu2O catalyst increased the decarboxylation yields of all the aromatic carboxylic acids. Especially for benzene carboxylic acids, all decarboxylation yields were above 90% with 100% selectivity. The Cu2O-catalysed hydrothermal method was proven to be effective, not only for the decarboxylation reactions from monocarboxylic acids to hydrocarbons, but also for the decarboxylation reactions from tricarboxylic acids to dicarboxylic acids and from dicarboxylic acids to monocarboxylic acids. In the presence of alkali and oxygen during the oxidation process, various valuable aromatic hydrocarbons and heterocyclic compounds, especially benzene, were obtained from Loy Yang coal using the hydrothermal oxidation and catalytic decarboxylation process. Based on analyses of aromatic carboxylic acids or ions before and after the decarboxylation process, it was found that decarboxylation reactions were the major reactions and the use of Cu2O was effective not only for the decarboxylation from monocarboxylic acids but also from polycarboxylic acids to monocarboxylic acids, consistent with the results for the model aromatic carboxylic acids. Additionally, it was found that a Cu2O catalyst was also effective for the conversion of ‘heavy’ compounds in oxidised coal to ‘light’ compounds.

Insight into the intrinsic reaction of brown coal oxidation at low temperature: Differential scanning calorimetry study

The oxidation of coal at low-temperature involves a series of physical and chemical processes and many parallel reactions. But the intrinsic oxidation reaction between O2 and coal is the main source responsible for the self-heating and spontaneous combustion of brown coal. In this research, differential scanning calorimetry (DSC) was introduced to determine the intrinsic reaction of Ximeng brown coal oxidation at low temperature. The heat evolution of the intrinsic reaction after eliminating the evaporation of water and thermal decomposition of inner oxygen-containing functional groups was obtained by subtracting the DSC curve in N2 from the DSC curve in air. It is considered that the intrinsic reactions between coal and oxygen could be divided into three stages, including the slow oxidation, accelerated oxidation and rapid oxidation stages. Compared with DSC-air curve, the DSC-sub curve based on the subtracting results elucidated the exothermic characteristics of intrinsic oxidation reaction in each stage more clearly. In addition, DSC-sub curve reduced the experimental errors inborn from the heating rate and the sample mass, so it had more practical application value than DSC-air curves. Activation energies obtained from DSC-sub curves can better reflect intrinsic oxidation reaction and be used as important indicators for the evaluation of coal spontaneous combustion tendency.

No.74 湿潤雰囲気下での褐炭の低温酸化挙動(熱分解・コークス素(5))

No.74 湿潤雰囲気下での褐炭の低温酸化挙動(熱分解・コークス素(5))

Reactive molecular dynamics simulations of the initial stage of brown coal oxidation at high temperatures

To investigate the detailed mechanisms for brown coal oxidation at high temperatures, a ReaxFF reactive forcefield was used to perform a series of molecular dynamics simulations from 1000 K to 2500 K. Analyses indicated that the chemical system tend to be more reactive with increasing temperature. It was found that the oxidation process of brown coal primarily initiates from hydrogen abstraction reactions by O2 and related oxygenated radicals from phenolic hydroxyl groups, methyl groups, especially carboxyl groups in lower temperature to form peroxygen species, or by either thermal decomposition of brown coal backbone in higher temperature. These peroxygen species usually could chemically adsorb on the C-centered radicals of brown coal backbone. The weak O–O bond in peroxygen makes them easier to break into oxygenated radical, which could also chemically adsorb on the C-centred radical to form hydroxyl group and other oxygenated compounds. In the oxidation process of brown coal, the decomposition and oxidation of aliphatic chain is easier than aromatic ring. The chemisorption of peroxygen radical induces the breakage of aromatic ring and accelerates the depth oxidation of brown coal. An increasing number of products are observed with increasing temperature.

Oxidation of brown coal of Angren deposit with a mixture of nitric and sulfuric acids

Oxidation of brown coal of Angren deposits by a mixture of nitric and sulfuric acids was studied in respect of dependence on the concentration of nitric acid (5–30%), concentration of sulfuric acid in nitric acid solution (2.5–20%), and temperature (40–80°C) at the process duration 2 h and the ratio of coal: HNO3 = 1: 2. Optimal conditions of the process in terms of maximum yield of humic acids and the minimum loss of nitrogen oxides were found.

Reactivity for low-temperature oxidation of brown coal and upgraded brown coal

褐炭および油中脱水褐炭とその熱処理炭について，自然発火の要因である低温酸化反応性を調べるため，低温での酸素吸収量と空気気流中の酸化過程での重量変化の速度解析を行った．また，一般炭と比較し炭質の影響を明らかにした．速度解析では，酸素の吸収による酸化物の生成とその分解反応および石炭自身の分解反応を仮定し，各々1次反応として速度定数を求めた．酸素吸収反応について，低温での吸収量測定と熱天秤から求めた速度定数のアレニウス・プロットが同一の直線で表されたことから，自然発火に至るまでの広範囲にわたる低温酸化反応が同一の反応スキームで表され，各反応が定量的に評価できた．また，低炭化度炭ほど酸素吸収速度と自身の分解速度が大きく，反応温度が低いほど炭質の影響が顕著となることが明らかとなった．油中脱水褐炭は真空乾燥褐炭に比べて，低温での酸素吸収と分解速度が抑制され，自然発火性が小さくなった．

97/01633 Low-temp oxidation of brown coal and its reactivity in thermal treatment

97/01633 Low-temp oxidation of brown coal and its reactivity in thermal treatment

A comparative thermoanalytical study of low-temperature reactivity of brown coal with dioxygen and radiofrequency-activated oxygen

The kinetic features of oxidation of a Spanish brown coal by dioxygen (studied by means of TG and DTA) and by oxygen plasma (carried out in a low-temperature ashing apparatus) are compared. It is concluded that slowness in brown coal oxidation by radiofrequency-activated oxygen is associated with formation of sulphate and nitrate artifacts which prevent the access of the oxidizing agent to the surface of organic coal matter.

Low-temperature oxidation of Victorian brown coal

A study has been made of low-temperature oxidation of Victorian brown coal at 35 °C and oxygen pressure of 0.1 M Pa and regression analysis of the experimental results shows that the reaction can best be described by the continuous reaction model. The reaction is subject to reaction product inhibition, apparently caused by product adsorption. Progressive conversion is obtained through a series of cycles consisting of a reaction step followed by evacuation of the coal. The rate equation developed from the continuous reaction model describes satisfactorily the progress of the reaction in each of these cycles. A study of earlier findings on the nature of Victorian brown coal oxidation products indicates that the product water may be significant in this reaction inhibition.

Low-temperature oxidation of brown coal. 3. Reaction with molecular oxygen at temperatures close to ambient

Yallourn brown coal was oxidized with pure molecular oxygen at temperatures of 35 and 70 °C until, after about 45 days, no further gain in mass occurred. At this stage, despite the loss of considerable quantities of carbon and hydrogen from the coal as carbon dioxide, carbon monoxide and water, the coal had gained in mass by 15 g per kg of dry coal. Virtually all the gain was accounted for by increases in the content of carboxyl, carbonyl and phenolic groups. These functional groups, which subsequently break down to yield the stable products already mentioned, are formed apparently by attack on the aliphatic structures of the coal. At 35 °C, reactions leading to the formation of functional groups such as phenolic groups, and their breakdown to water, predominated; however, at 70 °C, the formation of carboxyl groups and their breakdown to carbon dioxide became more important, the amount of water formed being less than that at 35 °C. The general course of these reactions was qualitatively similar to that observed by other workers using bituminous coals, but the extent of the reactions was an order of magnitude greater.

Low-temperature oxidation of brown coal. 2. Elovich adsorption kinetics and porous materials

Computer-generated diffusion data are shown to be fitted closely for long periods by the Elovich equation. The graphs obtained also show the discontinuities that frequently occur in Elovich plots of experimental adsorption data. Use of the Elovich equation to interpret sorption on porous materials is therefore a dubious procedure.