Determination of organic oxygen content of coals

Fast neutron activation analysis has been used to determine organic oxygen (O_org) content of coals by subtracting the oxygen content of the mineral matter from the total oxygen content of the coals. Mineral matter was isolated by low temperature ashing in an oxygen plasma. Optimum ashing conditions produce minimal changes in chemical composition of mineral matter; these changes were taken into account when calculating the O_org content. The O_org contents of whole coals are substantially different — in some cases by as much as 47% — from the ASTM oxygen contents and those calculated on a dry, mineral matter free basis from the ultimate analysis data. Excellent agreement between the FNAA O_org contents of the whole and demineralized coals lends support to the reliability of this experimental approach.     

Modification of sub-bituminous coal by steam treatment: Caking and coking properties

A Chinese sub-bituminous Shenfu (SF) coal was steam treated under atmospheric pressure and the caking and coking properties of the treated coals were evaluated by caking indexes (G_RI) and crucible coking characterizations. The results show that steam treatment can obviously increase the G_RI of SF coal. When the steam treated coals were used in the coal blends instead of SF raw coal, the micro-strength index (MSI) and particle coke strength after reaction (PSR) of the coke increased, and particle coke reactivity index (PRI) decreased, which are beneficial for metallurgical coke to increase the gas permeability in blast furnace. The quality of the coke obtained from 8% of 200 °C steam treated SF coal in coal blends gets to that of the coke obtained from the standard coal blends, in which there was no SF coal addition in the coal blends. The removal of oxygen groups, especially hydroxyl group thus favoring the breakage of the coal macromolecules and allowing the treated coal formation of much more amount of hydrocarbons, may be responsible for the modified results. The mechanism of the steam treatment was proposed based on the elemental analysis, thermo gravimetric (TG) and FTIR spectrometer characterizations of the steam treated coal.     

Studies of inorganics added to low-rank coals for catalytic gasification

Inorganic complexes were added to low-rank coals by step-wise pH adjustment of a mixture of inorganic solution and brown coal while avoiding the formation of precipitates. The nature and amounts of the added inorganic depend on the pH of the coal/solution mixture and concentration of inorganic salts. The amount of hydroxide added for high loading of iron to coal is consistent with added multinuclear complexes. Computer generated models of brown coal with multi-iron species account for observed OH/Fe ratios. X-ray photoelectron spectroscopy (XPS) data for these samples are consistent with multi-iron species in coal. Computer molecular modelling of two brown coal models with added inorganics shows monodentate carboxyl coordination to metals is sterically favoured. Mononuclear Fe(III) with bidentate carboxyl coordination form distorted structures and are energetically unfavoured. Modelling indicates significant reductions in partial charges on metal centres, consistent with a redistribution of electron density on complexation. Low temperature pyrolysis of brown coals with added inorganics provides increased yields of gases, no detectable tar and lower char, compared with acid washed coals. Gasification of these coals using a nitrogen/oxygen mix at 150–200 °C yields CO, while steam gasification at 250–450 °C yields CO₂, CO and CH₄. Iron oxide/carbonate complexes are postulated during the pyrolysis and gasification.     

A world pattern of the optimum ash levels of cleans from the washability data of typical coal seams

For every coal of specified size there is an optimal cut point, which more or less demarcates the free and fixed dirt in the coal. Technically speaking, one can expect the maximum advantage in washing a coal by separating it at this optimal cut point. A method is described for determining this point (the Optimum Degree of Washability) for any coal, from normal float-and-sink data. The Washability Number (directly calculated from the Optimum Degree of Washability) and the cleans ash at the optimum separation level are useful parameters for defining the ease or difficulty of washing. This Optimum Degree of Washability generally bears a rectilinear relation to the earlier Washability Index. While the values of Washability Number are generally higher and the optimum cleans ash level usually lower for the coal seams formed under quiescent conditions, the opposite is true for coal seams deposited under turbulent conditions. Thus, in the association of dirt (minerals) with coal (macerals), the higher the Washability Number, the greater is the heterogeneity of a coal seam inclusive of the dirt and vice versa. The Washability Number and the optimum cleans ash also undergo changes according to the size of crushing and grading of the raw coal. Characteristics on the above new basis are recorded for regional variation of some typical coal seams covering deposits of both ‘in situ’ and ‘drift’ origin, and significant observations were made regarding the trend of variation of the same seam at different sectors of the regions. In respect of deposits of drift origin, some regular sequence was also noted in the related values from the upper to the lower horizons. On the basis of typical data collected from more than 25 coal-producing countries, a worldwide projection has been made in this paper. A general pattern emerges with significant limits of variation between the coals of the Northern and Southern Hemispheres. This pattern could be better defined if further studies were conducted with coals of the same geological age and their washability characteristics were evaluated under standard conditions of test by crushing them initially to a particular top size. Further, from a knowledge of the different ranges of Washability Number, it is possible for a coal-preparation engineer to take tentative decisions on a size of crushing and a system of washing specifically suited to the beneficiation of any run-of-mine coal.     

Lower emission plant using processed low-rank coals

Low-rank coals can be processed into non-fouling coal, ultra-low ash coal, and coals containing catalysts. Systematic studies of the action of acid to reduce ash in a number of low-rank coals have shown total ash reduction varied from 96% to 30%; the extent of ash reduction was limited by the nature of minerals, which may be removed using various acids, including HF, to produce ultra-low ash coal. A commercial process must operate at elevated temperatures, but this must not produce toxic wastewater; data are provided to show that wastewater may be treated and water recycled without polluting the environment. The addition of effective catalysts results in enhanced reactivity of the coal to oxygen and steam; experimental data show high yield of H₂ from char and steam, and accompanying post-gasification chemistry. Development of catalytic steam gasification requires an understanding of aqua-chemistry and the thermal transformations of inorganics as the coal is heated. The scientific basis exists for processing low-rank coals, but commercial application requires: (i) high production rates, (ii) treating wastewater produced from coal treatment plant, (iii) catalysts that increase the yield of H₂ from steam gasification, and (iv) plant that achieves high power and thermal efficiencies. Modelling studies for super-critical plant, and for direct coal-fuelled turbine in combined cycle, illustrate the potential for lower-emission technology; catalytic steam gasification offers the cleanest option for future coal-fuelled plant.     

The characterization of surface properties and steam reactivities of two Spanish coals of high ash content

Studies were made on two Spanish coals of high as content (a semi-anthracite and a high volatile bituminous coal) and on the coals after heat treatment and on acid demineralisation in HCl and HF. X-ray diffraction revealed that the mineral matter content of the coals included quartz, siderite and aluminosilicate minerals; siderite and the aluminosilicates were decomposed by heat treatment. Mineral matter content was substantially reduced by acid treatment, but the metallic element content, as revealed by EDAX, remained similar to that of the raw coals. N₂ and CO₂ adsorption isotherms and mercury porosimetry show that the coals contain mainly micropores and macropores, the semi-anthracite having the greater microporosity. For bituminous coal, macro- and micro-porosity increase substantially upon heat treatment; for anthracite there is a smaller increase in macroporosity and a decrease in microporosity. Microporosity in both coals is unaffected by acid demineralisation, but macroporosity is increased. The steam reactivity of the bituminous coal char is greater than that of the anthracite coal char, and demineralisation of the coals increases steam reactivity. N₂ surface areas and steam reactivities for both coal chars follow trends with parent coal rank previously established for US coals.     

Experimental study of synthesis gas production by coal and natural gas co-conversion process

A moving bed was used as the reactor in experiments to produce synthesis gas by coal and natural gas co-conversion process. The effects of coal types on the temperature in the flame zone, the ingredients and the H₂/CO ratio of synthesis gas, together with the methane and steam conversions were investigated by using coke, anthracite, lean and fat coals as the raw materials. By comparing the results between coals and coke, it can be seen that the temperatures in the flame zone and the content of the active compounds (H₂, CO) of coals are higher than those of coke. In addition, the H₂/CO ratio of synthesis gas closes to the calculated value by thermodynamic equilibrium. For the produced crude synthesis gas with coals by coal and natural gas co-conversion process, in which the H₂/CO ratio varies in 1.0–2.0, the content of the active compounds (H₂, CO) is more than 92%, and the residual methane is less than 2%, the methane and steam conversion rates are more than 90% and 75%, respectively. All these results demonstrated that the concept of coal and natural gas co-conversion process is positive and feasible.     

Carbonization of coals to produce anisotropic cokes: 5. Structural factors of coals influencing carbonization properties

The carbonization properties are studied of two particular coals (Zontag Vlei and Metla coals) which are markedly different despite their similar coalification rank, maceral composition, and oxygen and exinite contents. These coals possess different structural features which influence their carbonization. A demineralizing pretreatment improves the properties of Metla coal. However, this is still inferior to the Zontag Vlei coal. O-alkylation of the Metla coal improves fusibility in single carbonizations and susceptibilities, equalling those of the Zontag Vlei coal. Preheat-treatment differentiates between the coals: Metla coal loses its susceptibility at lower temperatures. The chemical analyses of oxygen functionalities of both the original and preheated coals show that their hydroxyl groups behave differently in carbonizations at lower temperatures, indicating that oxygen functionality may be another influential factor. Hydrogen shuttling within the coal may be a third factor as it may remove the oxygen functionality.     

Characterization of Canadian coals by nuclear magnetic resonance spectroscopy

Apparent aromaticities of a series of Canadian coals of different rank were estimated by solid state nuclear magnetic resonance spectroscopy. The aromaticities varied from 0.57 for a lignite up to 0.86 for a semi-anthracite coal. The aromaticities correlated well with fixed carbon and oxygen content of the coals as well as with the mean reflectance of the coals. Correlations were also established between aromaticities and the H/C and H_aru/C_ar ratios of the coals. Uncertainties in calculation of the hypothetical H_aru/C_ar ratios, from experimental data were pointed out.Structural parameters of the chars derived from the coals by pyrolysis at 535°C were, also, estimated. The H/C and H_aru/C_ar ratios of the chars were markedly lower than those of coals. This was complemented by higher apparent aromaticities of the chars compared with the coals.     

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.     

Gas composition and temperature profiles in a spouted bed coal gasifier

Verification of scale-up models of the spouted bed gasifier requires experimental gas composition and temperature profiles within the reactor. Radial and axial profiles are presented for key gaseous species in a 0.30-m. dia, atmospheric pressure gasifier fed at about 30 kg coal/h. Low calorific value gases were generated at air/coal mass ratios of 2.87-4.08, while medium calorific value gases were produced at an oxygen/coal mass ratio of 0.84 and steam/coal ratios of 2.23-2.84. The combustion zone within the gasifier is delineated and the effects of operating conditions on gas composition and temperature profiles are illustrated for typical sub-bituminous and bituminous coals. Recycle of char to the bed from the primary cyclone and bed depth show little effect on profile shape.     

Effect of hydrothermal treatment on the extraction of coal in the CS2/NMP mixed solvent

The extraction of four Chinese different rank bituminous coals with the carbon disulfide/N-2-pyrrolidinone (CS₂/NMP) mixed solvent (1:1 by volume) was carried out in room temperature. It was found that one of middle bituminous raw coal of the four coals gave more than 74% (daf) extraction yield, suggesting an associative structural model for the coal. The four coals were hydrothermal treated under different conditions, and it was found that the extraction yields of the treated coals obviously increased. This will have great significance for coal liquefaction. FTIR measurements show the removal of minerals after the hydrothermal treatment of coals suggesting the dissociation of the coal aggregation structure due to ionic interactions and/or hydrogen bonds broken because of the removal of oxygen and hydroxyl oxygen proceeded through ionic pathways, resulting in the extraction yields of the treated coals increase. However, breaking of π-cation interactions by hydrothermal treatment may be one of possible mechanisms for the enhancement of extraction yield of higher rank of treated coal. The mechanism of hydrothermal treatment of coal was discussed in the paper.     

The effects of dispersants (PSS–NSF) used in coal–water slurries on the grindability of coals of different structures

The effects of PSS (sodium polystyrene sulphonate) and NSF (naphtalene sulfonate formaldehyde condensate) chemicals used to control the pulp rheology, on the grindability of coals were studied. Zonguldak region bituminous coal and İstanbul region brown coal samples, which have different structures, were used in the test work. Wet grinding tests with two coal samples were performed with and without PSS and NSF under constant grinding conditions; the solid ratios ranged from 50 to 60% with 0–60 min grinding periods.The grinding conditions in the preparation of coal–water slurries were improved by the use of the dispersing agents PSS and NSF and viscosity values much less than 1000 mPa were obtained. Under the optimum concentrations (0.3 and 0.7% to mass of coal) attainable in the presence of PSS and NSF, it was observed that the conservation of energy would be 70% with coal from Zonguldak and 60% with that from Istanbul if PSS were present in the grinding medium. On the other hand, the values obtained in the presence of NSF were 45 and 47%, respectively.     

Nickel catalysed steam gasification of chars obtained from coal-alkali reaction at 600 °C

Studies on the steam gasification of washed residual chars (obtained from coal-alkali reaction at 600 °C) were carried out at 500 °C and 100 kPa pressure in a fixed bed glass reactor with or without nickel (as nickel nitrate) as catalyst. The results when compared with the corresponding data on coal, revealed that under similar reaction conditions, the coals yielded more gas with higher H₂ and CO contents than their corresponding chars. It was concluded that presence of functional groups, especially oxygen containing is a requirement for nickel catalysed steam gasification of coals/lignites. The recovery of nickel achieved was about 80%.     

The catalytic steam gasification of one Australian and three Japanese coals using potassium and sodium carbonates

The catalytic steam gasification of four different coals using potassium and sodium carbonates as catalysts was carried out in a semi-flow type fixed-bed reactor. The coal was gasified with or without the catalyst under a steam—argon atmosphere at a heating rate of 50°C/s at 700–800°C. The catalytic activity of carbonates for gasification was remarkable for Japanese high-volatile coals (Miike and Takashima coals), and moderate for Australian medium-volatile coal (New Lithgow coal); however, the carbonates had little effect on gasification of Japanese lignite (Taiheiyo coal). It is assumed that Miike and Takashima coals soften and melt during the heating process to make the contact between char and catalyst better. New Lithgow and Taiheiyo coals do not have this property. Gasification was promoted significantly at lower temperatures when the catalyst was used. In both catalyzed and uncatalyzed runs the main products were hydrogen and carbon dioxide; the reaction temperature did not affect the composition of the gases much. A water—gas shift reaction occurred during gasification resulting in a large amount of carbon dioxide under a large excess of steam flow.     

Comparison of coal gasification in fluidized and spouted beds

Two sub-bituminous coals, a bituminous coal and a reject coal of 55% ash were gasified with steam and air in a 0.31-m dia. reactor operated in the fluidized and spouted bed modes. Inert beds of 1.1 mm dia. Ottawa sand and 2.1 mm dia. gravel provided stable fluidization and spouting respectively, at superficial velocities in the range 1.3–1.6 m/s. Coals of 1.19–3.36 mm diameter were fed to the gasifier at rates up to 45 kg/h and a range of air/coal ratios. For a given coal, results in the two systems were basically similar, although differences in gas heating value, gas yield and thermal efficiency were noted for some coals.     

Effect of biomass blending on coal ignition and burnout during oxy-fuel combustion

Oxy-fuel combustion is a GHG abatement technology in which coal is burned using a mixture of oxygen and recycled flue gas, to obtain a rich stream of CO₂ ready for sequestration. An entrained flow reactor was used in this work to study the ignition and burnout of coals and blends with biomass under oxy-fuel conditions. Mixtures of CO₂/O₂ of different concentrations were used and compared with air as reference. A worsening of the ignition temperature was detected in CO₂/O₂ mixtures when the oxygen concentration was the same as that of the air. However, at an oxygen concentration of 30% or higher, an improvement in ignition was observed. The blending of biomass clearly improves the ignition properties of coal in air. The burnout of coals and blends with a mixture of 79%CO₂-21%O₂ is lower than in air, but an improvement is achieved when the oxygen concentration is 30 or 35%. The results of this work indicate that coal burnout can be improved by blending biomass in CO₂/O₂ mixtures.     

The effect of the activating agent and temperature on the porosity development of physically activated coal chars

An experimental work on the influence of temperature and the activating agent on the development of surface area and porosity for activated carbons was carried out. Three coals from different regions of Colombia were activated with CO₂, steam and a CO₂–steam mixture. Coal from the Antioquia Region (La Capotera) was activated with a CO₂–steam mixture at 1073, 1123 and 1173 K and with CO₂ and steam at 1073 K. Other two coals from Antioquia and Cesar regions (La Grande and Borrego) regions were activated with a CO₂–steam mixture at 1073 K and these were compared with the La Capotera char for the same conditions. The content of ash was confirmed to affect the development of surface area: coals with lower amount of ash developed higher specific surface areas. Activation temperature also affected the development of surface area: the use of high temperature produced low surface areas. Results indicate that CO₂–steam produces larger surface areas than CO₂ and steam alone, and reactions with CO₂–steam and CO₂ develop a more uniform porosity than reaction with steam. The pore sizes are larger when steam is used and smaller when CO₂ is used.     

Material balance in coal. 1. Material balance and oxygen stoichiometry of six coals from Wyoming

Oxygen was determined in six coals from the Wyodak Bed, Campbell County, Wyoming. Data from the U.S. Bureau of Mines' coal analysis reports were used to calculate the material balance of these coals based on accurate oxygen determination by a fast-neutron activation method developed at the University of California, Irvine. A computer program recalculates the data based on moisture determined in our laboratory and tabulates the results, comparing the ‘oxygen by difference’ to oxygen determined on ‘as received’ basis. Oxygen in samples dried at 105 °C was also determined in order to estimate the possible effect of oxidation and loss of volatile components other than water during the drying process. Summations of all data were derived using the accurate oxygen values determined. This approach permits a better interpretation of stoichiometry of coal analysis. The completeness of an analysis can be thus evaluated rapidly. One may be able to indicate and pinpoint probable errors in the determination of sulphur and nitrogen in coal, also the presence of considerable CaCO₃. The gross effect of evolution of CH₄ and gases other than H₂O can be detected. One also is able to estimate the composition of the coal ash and the low-temperature ash as well as the mineral matter (Parr basis) in terms of their varying total oxygen and cation contents. This work shows that five of these six soft, water-rich, coals (20–31% H₂O, 105 °C) do balance well stoichiometrically. One coal does not balance based on USBM moisture data. This is apparently due to an error in the moisture determination because oxygen data on this ‘as received’ versus ‘dried, 105 °C’ coal match very closely. It is shown how adding the oxygen determination to routine coal analysis may permit the coal chemist to interpret the data better in terms of true material balance. Oxygen is the only major constituent not routinely determined during the ultimate coal analysis. Adding this element permits the chemist to ascertain the accuracy of the analysis, completes the analysis in the sense of stoichiometry, and makes the customary summations of coal analysis more meaningful.