Ignition temperature of coal. 1. Influence of the coal’s composition,structure, and properties

The reactions of coal with the materials used in determining the ignition temperature of unoxidized coal according to Ukrainian State Standard DSTU 7611:2014 are analyzed. First, the ignition temperature of various types of coal from Ukraine, Russia, Canada, Australia, the Czech Republic, Poland, and Indonesia is determined. The influence of the composition, structure, and properties of the coal on its ignition temperature is assessed. The ignition temperature of the unoxidized coal is found to be closely related to the content of organic carbon C^daf and aromatic carbon C_ar, the structural parameter δ characterizing the degree of saturation of the coal’s organic mass, and also the vitrinite reflection coefficient R_o and the yield of volatiles V^daf.     

Assessing the technological value of coal in coking

Coking coal of the same rank from different countries and fields may be distinguished in terms of use value by rating on the basis of seven technological and petrographic characteristics that determine the coke yield and properties: the ash content A^d; the total sulfur content S_t^d; the yield of volatiles V^daf; the plastic-layer thickness y; the vitrinite reflection coefficient R_o; the content of vitrinite-group macerals Vt; and the basicity index B_b. A range of values and a rating (on a scale from 1 to 10) are established for each of these parameters. Each rating corresponds to a particular score (from 0.1 to 1.0). Ranges of A^d, S_t^d, Vt, and B_b are established for the whole metamorphic series, while ranges of V^daf, y, and R_o are established for individual ranks and groups of ranks. Altogether, 105 coking coals from Ukraine, Russia, the United States, Australia, and Canada that are used at Ukrainian coke plants are investigated. The range of rating scores and their mean values are determined for individual coal ranks and groups. As an example, three bituminous coals from Ukraine, the United States, and Australia are compared by the proposed method. This method permits objective assessment of the technological value of coal within a single rank and the selection of the best purchase option.     

Metamorphic development, degree of reduction, and other characteristics of Donets Basin coal

Analysis of the elementary composition and parameters V ^daf , y, and Q _s ^daf of coal from more than 100 beds in the Donets Basin indicates that the considerable difference in the properties of coal characterized by reduction (in terms of S _t ^d ) and uniform metamorphic development (according to C ^daf ) is due to variation in the ratio of the number of hydrogen and oxygen atoms (H/O_at) in the molecular component of the coal’s organic mass.     

Relations between the reflection coefficients of the basic groups of macerals: A review

Quantitative relations are found between the structural and chemical characteristics of macerals of the basic coal groups (vitrinite Vt, inertinite I, liptinite L), on the one hand, and their reflection coefficients R _r and the corresponding dispersions σ _R , on the other. For coal of a particular metamorphic stage, the reflection coefficient declines in the series I > Vt > L, on account of the reduction in aromatic chemical structure and in the degree of condensation of the aromatic blocks. In the metamorphic series, the reflection coefficients of the macerals rise; the values for Vt and L at intermediate stages converge. The dispersion of the reflection coefficients (and hence the reflectograms) is due to the spread in characteristics of the chemical structure of the coal’s organic content, as confirmed by calculations for the vitrinite of D, G, Zh, and K coal.     

Coal Classification in Terms of the Vitrinite Reflection Coefficient

By statistical analysis of research data for coal samples, coal blends, and coal batches used in Russia and Kazakhstan, relations between the vitrinite reflection coefficient and the fuel ratio are established, with good agreement between the calculated and actual vitrinite reflection coefficients. Statistical analysis of a representative database for coal samples, coal blends, and coal batches from Ukraine, Poland, the United States, Canada, and Australia, based on research in those countries, permits the formulation of very close correlations between the vitrinite reflection coefficient and the fuel ratio F_r, with excellent agreement between the calculated and actual vitrinite reflection coefficients.     

IR Spectroscopy in Rank and Group Assignment of Coal

The possibility of express determination of the characteristics V _IR ^daf , R_{o, IR}, y_IR, ΣLC_IR, and A _IR ^d used in the ranking of coal on the basis of IR spectroscopy is assessed for a specific example: Kuznetsk coals of different maceral composition and metamorphic development. The IR characteristics are compared with values obtained by standard methods (V^daf, R_{o, r, y}, ΣLC, and A^d).     

Predicting the classification characteristics of coal. Part 3. Gross calorific value of dry,ash-free coal

For 63 samples of Ukrainian, Russian, and imported coal, equations for predicting the gross calorific value Q_s^daf on the basis of the following coal characteristics are developed: W^a, O_d^daf, Q_s^af, and C_ar. The error is within the standard tolerances (σ ≤ 0.3 MJ/kg). With sufficient accuracy, Q_s^daf may be predicted from equations based on petrographic characteristics such as the vitrinite reflectance, the content of liptinitegroup minerals, and the sum of lean macerals (I + 2Sv/3). In these equations, the coefficients correspond to the heat of combustion of the vitrinite components at different metamorphic stages, the liptinite, and the lean macerals.     

Molecular models of coal’s organic matter and the reflectance of the macerals: A review

The mean reflectance R _o,n of vitrinite and other macerals in coal whose structure is represented by molecular chemical models may be calculated on the basis of the contributions of structural-chemical groups to the molecular refraction, if the relation between the reflection, refraction, and absorption is taken into account on the basis of the Fresnel-Beer law. The findings confirm the relation between the reflectance R _o,n and the chemical structure of the coal macerals. Consequently, reliable analysis of the possible chemical transformations in coal may be based on molecular models of the structure.     

Predicting the classification characteristics of coal. Part 2. Maximum moisture content

Analysis of 63 samples of coal concentrates (from Ukraine, Russia, the United States, Canada, Australia, and Poland) currently employed at Ukrainian coke plants indicates that the prediction of the maximum moisture content of coal may expediently be based on R _o and Q _s ^daf , determined, respectively, in plant laboratories and in power-station laboratories. The maximum moisture content of metamorphically distinct coals does not depend on their ash content (in the range 3.7–35.3%) nor on the chemical composition of the ash, expressed by the basicity index B _b (in the range 1.24–27.18) and the base/acid ratio I _b (in the range 0.198–1.832). Although the oxidation of coal also increases the maximum moisture content, this change is less than the error in its determination (0.5%). The oxidation of practically 30% of the coal’s organic mass increases the maximum moisture content by no more than 0.4%     

Rapid Quality Assessment of Coal

No satisfactory methods are available for rapid and reliable prediction of one or more coal characteristics. The ignition temperature t_ig of coal, determined in assessing the oxidation of coal in accordance with Ukrainian State Standard DSTU 7611:2014, may be regarded as a useful predictor of coal quality. Research shows that t_ig depends on the composition and ordering of the coal’s organic mass. Mathematical and graphical means of predicting the V ^daf and R_o values of coal are developed.     

Alkaline activation of the donbass coals of different ranks

The effect of the rank of coal (C ^daf = 80?95.2%) on the yield and characteristics of activated carbons prepared under the conditions of alkaline activation (800°C, 1 h, Ar) at KOH/coal ratios of 1 g/g was studied. Under these conditions, the ability of coals to form porous materials decreased in the metamorphic series. Grade D coal (C ^daf = 80%) exhibited a maximum activation ability to form a material with S _BET = 1560 m²/g, V _Σ = 0.71 cm³/g, and V _mi = 0.51 cm³/g. A minimum activation ability was found in anthracite (C ^daf = 95.2%), which forms activated carbon with poorly developed porosity (S _BET = 306 m²/g, V _Σ = 0.15 cm³/g, and V _mi = 0.11 cm³/g).     

Metamorphism and mineral composition of coal

The influence of ash content on the chemical composition of the ash is considered, along with the dependence of the Fe₂O₃, CaO, MgO, and SiO₂ content in the ash of Kuznetsk and Donetsk coal on the yield of volatiles V ^daf , which characterizes the degree of metamorphism. Correlation analysis shows that this dependence is described by second-order polynomial equations within the ranges considered. Within the interval V ^daf = 20–28% (from the overall range 8 ≤ V ^daf ≤ 45% considered), the content of most oxides in the ash passes through an extremum. The idea that high contents of ore components (Fe₂O₃, CaO, and MgO) in the ash are useful should be revised, since they impair the CSR and CRI values of the coke.     

Comparison of methods of predicting the higher thermal combustion of coal

Formulas for predicting the higher heat of combustion of coal on the basis of technical analysis (the yield of volatiles), elementary analysis, and petrographic analysis are verified. It is found that the formulas are only valid for the samples used in their derivation. Statistical analysis of data for coal from the Donetsk and Lvov-Volynsk basins shows that the higher heat of combustion of such coal is most accurately described by a mathematical formula in which the predictors are petrographic characteristics of the initial coal components, including the reflection coefficient of vitrinite. The mean higher heat of combustion may be determined for vitrinite at different metamorphic stages (for ranks from D to T) and also for liptinite and the sum of fusinized (lean) components in Ukrainian Donetsk and Lvov-Volynsk coal.     

Properties of Kuznets Basin gas coal

Six samples of G coal characterized by different vitrinite reflection coefficient (R _o,r = 0.64–0.74%) are investigated. With increase in R _o,r , the plastic-layer thickness y increases from 7 to 17 mm, the free-swelling index SI from 0.5 to 3.5, and the Roga index RI from 1 to 21. The temperature range of plasticity expands from 34 to 94°C. The results clarify the difference in clinkering properties of the gas coal.     

Composition and Properties of Brown Coal from the Bodonskoe Deposit in Buryatia

The composition and properties of coal from the Bodonskoe deposit were studied. It was shown that this is low-sulfur (S ^d = 0.3–1.0%) and medium-ash grade 1B brown coal with a high yield of volatile substances (V ^daf = 56.1–60.9%). The humic acid content varies from 32.3 to 50.8%. The heat of combustion of coal is Q _s ^daf = 26.0–27.4 MJ/kg. The concentrations of toxic elements in the coal samples are at a background level.     

Moisture-holding capacity of coals

As a result of a comprehensive study of 63 samples of coal concentrates (from Ukraine, Russia and countries outside the former Soviet Union), it was established that the prediction of the moisture-holding capacity of coals can be appropriately performed according to their values of W ^a, R ₀, O _d ^daf , and O _s ^daf . It was found that the oxidation of coal increased its moisture-holding capacity; however, in this case, the absolute change in this parameter was smaller than the error of its determination (0.5%). Therefore, upon the oxidation of almost 30% of the organic matter of coal, the moisture-holding capacity increased by only 0.4%. There is a close correlation between the maximum moisture capacity of coals and the water pore volume, and this correlation was described by a linear equation in the studies.     

Changes in the petrographic composition of coal batch on crushing

The influence of crushing on the maceral composition of coal is studied. The experimental data indicate not only redistribution of the petrographic microcomponents by size but also change in the maceral composition, depending on the intensity of grinding. With increase in the degree of crushing from 55.5 to 96.2%, the vitrinite content in the batch declines from 70 to 63%, while the total content of fusinized components increases from 29 to 35%. For coal with 50–80% vitrinite, which is less strong than coal with smaller vitrinite content, crushing is probably accompanied by disruption of the molecular interactions and rupture of the chemical bonds in the organic macromolecules. That leads to partial destruction of the brittle vitrinite structure, especially in the intermediate stages of metamorphism (R _o = 0.9–1.39%).     

Petrographic aspects of the hot strength of coke

The strength CSR of metallurgical coke after high-temperature reaction with CO₂ is determined by the depth of reaction and depends on the petrographic composition of the initial coal, the degree of metamorphic development, and the content of catalytically active ash components. Qualitative aspects of coke’s hot strength are reflected in the mathematical model, whose parameters include the vitrinite content Vt in the batch, the inertinite content I, the reflective index R _o,r of vitrinite, and the basicity index of the ash.     

Structural parameters of Donetsk and Lvov-Volynsk coal

The parameters ??, B, f _a , and cA calculated from the elementary composition of Donetsk and Lvov-Volynsk coal of different metamorphic stages characterize not so much the structure of the organic mass as the relative rates of chemical reactions determining the carbon, hydrogen, and oxygen content in two successive coalification periods: specifically, the rate of decarboxylation in the first period; and the rates of dehydrogenation and dehydration in the second.     

Determining the petrographic inhomogeneity of mixed-rank coal

It is shown that the inhomogeneity of coal mixtures (enrichment-plant concentrates) differing in vitrinite reflection coefficient and the content of clinkering components (i.e., in the deviation of these parameters S _R and S _CC ) may only be objectively estimated on the basis of refractogram and petrographic analysis of bed samples of the coal from the mines that supply the enrichment plant. A method and corresponding program for calculating S _R and S _CC is proposed. This program permits correct determination of the petrographic inhomogeneity of coal batch, which is important in evaluating the influence of this factor on coal quality.