Effect of inorganic matter removal from coals and chars on their surface areas

Removal of inorganic matter from coals by acid treatment brings about random and, in some cases, significant changes in surface areas measured by adsorption of N₂ and CO₂. Changes in surface areas of chars are generally more pronounced than those found in coals. However, the surface area changes in chars are markedly dependent upon whether the acid treatment is given to the coal precursor prior to charring or to the char produced from the raw coal. Changes in surface areas of raw coals and chars produced therefrom have been attributed to: (i) ‘physical’ removal of inorganic matter from the aperture-cavity system, (ii) bonding of HCl to basic nitrogen present in pyridine-like structures, and (iii) adsorption of acid. Results suggest that the removal of inorganic matter from coals prior to charring affects the carbonization process and, hence, the surface area of the resultant char.     

Microstructural variations of lignite,subbituminous and bituminous coals and their high temperature chars

Microstructure of a North Dakota lignite, a Washington subbituminous and a New Mexico bituminous coal and their chars produced by devolatilization in nitrogen at 1000 to 1300°C was investigated in this work using the CO₂ adsorption method conducted at 25°C. For each coal and char, specific surface area, micropore volume, micropore surface area, mean equivalent radius of micropores and characteristic energy of adsorption, as well as micropore volume distribution, were determined, and their variations with devolatilization temperature studied and interpreted. It was found that, overall, specific surface areas, micropore volumes and micropore surface areas of chars decreased monotonically as devolatilization temperature was raised from 1000 to 1300°C, although most of these values were much larger than that of their parent coals. The micropore volume distributions of the three coals and their high temperature chars were interpreted and found to provide an interesting insight into the micro structural variations of these coals and chars.     

Reactivity study of combustion for coals and their chars in relation to volatile content

To determine the effect of volatile matter on combustion reactivity, the pyrolysis and combustion behavior of a set of four (R, C, M and K coals) coals and their chars has been investigated in a TGA (SDT Q600). The maximum reaction temperatures and maximum reaction rates of the coals and their chars with different heating rates (5–20 °C/min) were analyzed and compared as well as their weight loss rates. The volatile matter had influence on decreasing the maximum reactivity temperature of low and medium rank coals (R, C and M coals), which have relatively high volatiles (9.5–43.0%), but for high rank coal (K coal) the maximum reactivity temperature was affected by reaction surface area rather than by its volatiles (3.9%). When the maximum reaction rates of a set of four coals were compared with those of their chars, the slopes of the maximum reaction rates for the medium rank coals (C and M coals) changed largely rather than those for the high and low rank coals (R and K coals) with increasing heating rates. This means that the fluidity of C and M coals was larger than that of their chars during combustion reaction. Consequently, for C and M coals, the activation energies are lower (24.5–28.1 kcal/mol) than their chars (29.3–35.9 kcal/mol), while the activation energies of R and K coals are higher (25.0-29.4 kcal/mol) than those of their chars (24.1–28.9 kcal/mol).     

Reactivity of chars from New Zealand coals

Equipment for measuring the reactivity of chars from highly reactive coals, based on recent Norwegian developments, has been assembled. Measuring techniques are described, and the results of measurement with a range of New Zealand coals are presented. The measured reactivities are compared with results obtained by using the German standard method for reactivity, and the comparison confirms the advantages of the Norwegian method for use with coals of high reactivity. By overseas standards, New Zealand's subbituminous coals have phenomenally high reactivities.     

Characterization of chars made of solvent extracted coals

Thermal extraction of a sub-bituminous coal (Roto south) using 1-methylnaphthalene solvent has produced ash-free coals successfully. The extracted (EC) and residual coal (RC) as well as its parent coal (PC) were pyrolyzed at 300–900 °C and then the carbonized products were characterized. The extracted coal (EC) contained lower molecular weight components than PC and RC, showing much higher fuel ratio after the pyrolysis. EC is expected to be advantageous over PC and RC when applied to coal gasification and reforming, because EC is readily decomposed and volatized. The heating value of EC chars (7,610–8,120 kcal/kg) was independent of the pyrolysis temperature and was higher than those of PC and RC chars, especially for the chars carbonized below 600 °C. The oxygen content of PC chars at T≤600 °C was mostly at least twice that of EC/RC chars, pointing out the difference in the chemical composition. 13CNMR and FT-IR spectra revealed the release of aliphatic hydrocarbons and reactive functional groups with increasing temperature, in agreement with ultimate/proximate analysis results.     

Pyrolysis by thermogravimetric analysis of blends of peat with coals of different characteristics and biomass

In this study, an investigation was carried out into the thermal behaviours of peat, reed, lignite, bituminous coal and blends of these with peat. The blends were prepared in 20:80, 40:60, 60:40, 80:20. The samples were pyrolysed in a TG analyzer in a nitrogen atmosphere (50 mL/min) at temperatures ranging from 25 to 900 °C. Using TG/DTG graphs, variations were investigated, which occurred in reaction intervals, percent of weight loss, peak temperatures and maximum devolatilization rate. The activation energy (E) and pre-exponential constant (A) were calculated using the Arrhenius type kinetic model.     

Effect of air-oxidation on the pore-structure of coals and cokes or chars obtained from oxidized coals

Seven kinds of coals (C, 77.8–89.8 wt% daf) were oxidized with air at 150 °C for 1, 5 and 10 h. The oxidized coals were heat-treated at various temperatures between 300 and 1500 °C with intervals of 50 or 100°C. The pore-structure of the oxidized coals and the cokes or chars obtained from the oxidized coals was compared with those of parent coals and their cokes or chars. True densities were measured in methanol and straight-chain hexane and pore volumes were determined by the Dubinin-Polanyi procedure. For the coals, the methanol-density increased with extent of oxidation; the hexane-density increased at first, but then decreased and again increased in the course of the oxidation. The air-oxidation of coals has a marked and controlling effect on the development of the pore-structure of cokes and chars in the course of the carbonization.     

Characterisation of the properties of size fractions from ten world coals and their chars produced in a drop-tube furnace

A range of coals from different parts of the world was studied to determine if there were any common relationships that could be determined to gain a clearer understanding of the distribution of coal properties within different particle-sizes. The properties examined were proximate analysis, maceral analysis and %Unreactives from image analysis. Each fraction was also pyrolysed in a drop-tube furnace at 1300°C, 1 vol% oxygen and a residence time of 200 ms and the resulting chars analysed for morphology using image analysis. There were substantial variations between the particle-size distributions of the different coal samples even though they were ground to the same specification for trials on a combustion rig. Ash distributions showed in all cases that the smallest particle size (−38 μm) had either the highest ash level or was very close to it. However, the trends in ash level for increasing particle size showed variations between coals with some coals showing increases in ash towards the larger particles. Fusinite content did not necessarily concentrate in the smallest size fraction, however, liptinite content did increase with particle size. %Unreactives generally increases with particle size and is related to char morphology through an empirical parameter, the ACA [5]. In addition the ACA [5] parameter showed the effect of both particle size and %Unreactives on char morphology and clearly showed the significant influence of particle size on burnout. A parameter such as this could, therefore, be used in burn-out models and further correlated with %Unreactives and particle size.     

The characterization of weathered discard coals and their behaviour during combustion

With a finite amount of economically viable coal reserves in South Africa, it is becoming increasingly important to consider coal discards as vital energy resources. The energy content of discard coal is generally low and high in ash, but can be effectively utilised in power stations. However, coals weather during storage, and weathered coals behave differently to their fresh counterparts. During this investigation, discard coals from four stockpiles varying in age from 5 to 40 years were characterised chemically and petrographically, with a specific focus on the petrographic-based abnormal condition analysis to determine the degree of secondary weathering. Drop tube furnace tests were conducted to determine the combustibility of the weathered stockpiled or dumped coals relative to typical Eskom coals, and the resultant char forms were characterised. A direct correlation with the extent of weathering and combustion performance was determined, with the more weathered coals reporting enhanced combustion reactions at lower temperatures.     

Structural changes and product distribution of typical Xinjiang coals and chars during pyrolysis

This study used micro-Raman spectroscopy, gas chromatography–mass spectrometry (GC–MS), and gas chromatography–flame ionization detector/thermal conductivity detector (GC–FID/TCD) to analyze the structure and pyrolysis reactions of nine typical coals and chars from Xinjiang. The study fitted 10 Gaussian bands of typical Xinjiang coal and investigated the changes in coal structure during coalification and pyrolysis. The results indicated that the reduction degree of CO structures in coal during coalification had a rough linear relationship with the V_daf (dry ash-free volatile matter) content. During coalification, the condensation of aromatic rings is accompanied by a continuous decrease of CO structures, while the contents of cross-linking and substitution structures decrease persistently relative to the large aromatic ring structures. The influence of coal type on char yield for typical Xinjiang coal is within 15 wt.%; the influence on tar yield is within 8.5%, with a greater impact on the yield of alkanes and phenols in tar; the influence on CO yield in pyrolysis gas is within 6.3%. The relative content of large aromatic ring structures in coal is relatively stable during pyrolysis, while the relative content of small aromatic ring structures declines as coal transforms into char. The study inferred that small aromatic rings might decompose and transform into tar after pyrolysis reaction, which also resulted in a high selectivity of phenolic products in tar from most coal pyrolysis above 40%. This study revealed the structural changes and pyrolysis product distribution of nine typical coals and chars from Xinjiang, providing useful information for their utilization.     

Structural differences in the tars and chars from the pyrolysis of coals of different rank in hydrogen and in nitrogen

Two British coals—Linby high-volatile bituminous and Abernant anthracite — and an Australian brown coal were pyrolysed at 575 °C in hydrogen or nitrogen at 0.1, 5 and 30 MPa in an inclined, single-stage reactor. Hydrogen pressures of ? 5 MPa led to increased yields of tars which were more aromatic than those produced in nitrogen. The high-volatile bituminous coal fused under all conditions, whereas the anthracite and brown coal fused and agglomerated only in hydrogen at 30 MPa.     

Pyrolysis of South Wales coals: 1. Preliminary thermogravimetric studies

Thermogravimetry (TG) has been applied, in a preliminary investigation, to the pyrolysis of seven samples of South Wales coals varying in rank from bituminous to anthracite. Coal samples (4–10 mg) were investigated within the temperature range 150–900 °C at a heating rate of 80 °C min⁻¹, under a nitrogen atmosphere (40 cm³ min⁻¹). The chart recorder method of collecting data, was replaced by a BBC Model B microcomputer system which included a monitor screen, dual disc drive and a printer. The results indicate a significant increase in the sensitivity, compared with the chart recorder method. In addition, the technique shows promise as an aid in the characterisation of coals, due to the distinct pattern of differential thermogravimetric (DTG) peaks for each sample. It is possible these patterns may be of use in predicting qualities of coal important in gasification processes.     

Adsorption of dyes from aqueous solution by coals, chars, and active carbons

Adsorption of two acidic dyes (amaranth red and metanil yellow) and a basic dye (methylene blue) from aqueous solution has been studied using a wide range of American coals and chars. The adsorption capacity of coals and chars has been compared with commercially available active carbons. The amount of dye removed from solution by raw coals is made up of two parts, one due to interaction with some mineral constituents and the other due to adsorption on the porous surface. There are some indications that pyrite and soluble alkali salts are the reactive minerals. When the coals are washed by refluxing in distilled water for 8 h, reproducible adsorption capacity is obtained. With washed coals, good correlation between the nitrogen BET surface area and dye adsorption capacity is observed. Except for lignites, adsorption of dyes is not influenced by the oxygen groups on the surface. Chars show enhanced capacity over coals, where under the most favourable conditions adsorption approached 25% of the active carbon capacity. The initial rate of adsorption of dye from solution by coals, chars and active carbons has been found to be dependent on the square root of time.     

A reactivity study of chars obtained at different temperatures in relation to their petrographic characteristics

This study reports on the reactivity of chars obtained at 1000°C and 1300°C (within the range of temperatures reached by coal particles in the near-burner zone of pulverised fuel boilers) from three different coals. The coals were selected according to petrographic criteria: two of them are high volatile bituminous coals differing in maceral composition and the third one is a vitrinite-rich low volatile bituminous coal.The morphology and optical texture of the chars were studied by optical microscopy. The kinetic parameters for the combustion of the high temperature chars under Regime I (combustion controlled by chemical kinetics) have been obtained and related to the optical texture and reflectance of the chars. The intrinsic reactivity of the high temperature chars was found to be lower than that of the low temperature chars, whereas the enhanced porosity observed in the high temperature chars had a positive effect on their combustion reactivity under Regime II (combustion controlled by oxygen pore diffusion). The intrinsic reactivities of the chars decreased following the sequence: vitrinite-rich low rank char>inertinite-rich char>vitrinite-rich high rank char. As the combustion temperature increases, the reactivity of the inertinite-rich char approaches that of the low rank vitrinite-rich char, which justifies the good performance observed for high volatile bituminous inertinite-rich coals in power plants.     

Structural analysis of chars generated from South African inertinite coals in a pipe-reactor combustion unit

An inertinite-rich medium rank C bituminous South African coal was utilized to generate chars in a pipe-reactor combustion unit. This unit generates chars at atmospheric pressures and temperature was controlled with N₂ to a maximum of 1250 °C. Chemical structural changes were investigated at different reaction zones identified in the pipe-reactor combustion unit. A combination of FTIR, XRD and Solid State NMR experiments were used to characterize the coal/char/ash fractions produced in the reactor. These techniques revealed that the coal structure becomes disordered in the drying zone as well as in the beginning of the pyrolysis zone in the reactor. As the temperature increases towards the base of the reactor the coal structure becomes more ordered and well aligned until char is formed and converted. Major structural changes were seen to occur in the drying to the pyrolysis zones. Structural changes within the molecular core were observed with FTIR and XRD results obtained from samples taken from the drying zone to the combustion zone. However, ¹³C CP/MAS and dipolar dephasing experiments were not able to corroborate these structural changes of the coal/char/ash fractions produced in the reactor occurring in the reduction and combustion zones.     

Desulphurization of coals and chars by treatment in various atmospheres between 400 and 600 °C

Desulphurization of fifteen US coals of rank ranging from anthracite to sub-bituminous B and five high-temperature chars by carbon monoxide and other gases and gas mixtures between 400 and 600 °C has been studied. The sulphur content of the parent coals ranged between 3.0 and 7.3% and that of the chars between 1.3 and 3.8% by weight. A comparison between air, nitrogen, carbon monoxide, and steam-carbon monoxide mixtures as desulphurizing gases shows the order of desulphurizing ability as

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Low-temperature air oxidation of caking coals. 1. Effect on subsequent reactivity of chars produced

The effect of preoxidation of two highly caking coals in the temperature range 120–250 °C on weight loss during pyrolysis in a N₂ atmosphere up to 1000 °C and reactivity of the resultant chars in 0.1 MPa air at 470 °C has been investigated. Preoxidation markedly enhances char reactivity (by a factor of up to 40); the effect on char reactivity is more pronounced for lower levels of preoxidation. For a given level of preoxidation, the oxidation temperature and the presence of water vapour in the air used during preoxidation have essentially no effect on weight loss during pyrolysis and char reactivity. An increase in particle size of the caking coals reduces the rate of preoxidation as well as subsequent char reactivity. Preoxidation of caking coals sharply increases the surface area of the chars produced. Compared to heat treatment in a N₂ atmosphere, pyrolysis in H₂ of either the as-received or preoxidized coal results in a further increase in weight loss and a decrease in subsequent char reactivity.     

Preparation and characterization of chars and activated carbons from Saskatchewan lignite

Saskatchewan lignite was used as a precursor to prepare carbonaceous adsorbents for use as SO₂ adsorbent from flue gases. The lignite was carbonized producing char in a fixed bed microreactor system at different temperatures from 350 to 550 ^°C in nitrogen atmosphere. The chars obtained at 475 ^°C for 120 min exhibited the highest micropore surface area (136 m²/g) and volume (0.062 cm³/g) and the smallest median pore diameter (∼ 0.7 nm). Carbon dioxide and steam were used as activating agents. Activation of char at optimum conditions of 650–675 ^°C for 15 min with carbon dioxide and steam resulted in a further increase in micropore surface area (220 and 186 m²/g for CO₂ and steam, respectively) and volume (0.090 and 0.085 cm³/g for CO₂ and steam, respectively). The yield of char was 64 wt.%, while the yields of activated carbon were 60 and 57 wt.% for CO₂ and steam activation, respectively; all based on the mass of original lignite.     

Comparison of the associative structure of two different types of rich coals and their coking properties

Solvent extractions of two different types of Chinese rich coals i.e. Aiweiergou coal (AG) and Zaozhuang coal (ZZ) using the mixed solvent of carbon disulfide/N-methyl-2-pyrrolidinone (CS₂/NMP) with different mixing ratios were carried out and the caking indexes of the extracted residues were measured. It was found that the extracted residues from the two types of coals showed different changing tendencies of the caking indexes with the extraction yield. When the extraction yield attained about 50% for ZZ coal, the extracted residue had no caking property. However for AG coal, when the extraction yield reached the maximum of 63.5%, the corresponding extracted residue still had considerable caking property with the caking index of 25. This difference indicated the different associative structure of the two coals although they are of the same coalification. Hydro-thermal treatment of the two rich coals gave different extract fractionation distributions for the treated coals compared to those of raw coals respectively. The coking property evaluations of the two coals and their hydro-thermally treated ones were carried out in a crucible coking determination. The results showed that the hydro-thermal treatment could greatly improve the micro-strengths of the resulting coke from the two coals, and the improvement was more significant for the more aggregated AG coal. The reactivities of hydro-thermally treated AG coal blends were almost the same as those of raw coal blends. The higher coke reactivities of AG raw coal and its hydro-thermally treated ones than those of ZZ coal might be attributed to its special ash composition.