Low-rank coals

Easily oxidized coal low-rank coal such as subbituminous coal or lignite coals (ASTM D-3302). 

Fossil fuels Coal (low rank lignite / sub-bituminous 15-19 5-20 1.0-3.0 0.02-0.3 1300... 

Conversion of carbon in the coal to gas is very high. With low rank coal, such as lignite and subbituminous coal, conversion may border on 100%, and for highly volatile A coals, it is on the order of 90—95%. Unconverted carbon appears mainly in the overhead material. Sulfur removal is faciUtated in the process because typically 90% of it appears in the gas as hydrogen sulfide, H2S, and 10% as carbonyl sulfide, COS carbon disulfide, CS2, and/or methyl thiol, CH SH, are not usually formed. 

W. G. Willson md co-workers, " AppHcation of Liquefaction Process to Low-Rank Coals," paper presented at /0th BiennialEignite Symposium, Grmd Forks, N.D., May 1979. 

Sondreal, 1993. Everett A. Sondreal, "Clean Utilization of Low-Rank Coals for Low-Cost Power Generation," from "Clean and Efficient Use of Coal The New Era for Low-Rank Coal," Organization for Economic Co-Operation and Development/International Energy Agency, Paris, France, 1993. 

Modern charcoal retorts are charged with wood, biowaste (bark, sawdust, etc.), peat, and sometimes low-rank coals. Yield and properties (hardness, density, surface area, etc.) can vary widely so the desired end use must be considered. Charcoal from coniferous trees is soft and porous, while that from hardwoods is dense and strong. For barbecuing, charcoal is usually compressed into briquettes, with binders and additives chosen to improve handling and ease of ignition. 

In absolute terms, the quantities of reactor solids found in various processes do vary considerably. The rate of accumulation is related to several factors, such as coal characteristics, recycle solvent quality and reactor design. However, it can be stated in general terms that liquefaction of low rank coals (sub-bituminous C and lignites) does result in higher rates of accumulation of solids than do similar operations with bituminous coals. For example, during normal operations of the SRC-I pilot plant at Wilsonville, Ala., it has been found that the amount of solids retained varies from about 0.2-0.5 wt.% (moisture-free) for bituminous coals to 1.0-1.9 wt.% (moisture free) for a subbituminous C coal (Wyodak) (72). Exxon also reports much larger accumulations for lignites and subbituminous coals than those found for bituminous coals (73). 

Miller, R. N. "A Geochemical Study of the Inorganic Constituents in Some Low-Rank Coals, Ph.D. Thesis, Pennsylvania State University, 1977, 314 pp. 

The most significant Tertiary coals are represented by the vast brown coal deposits in Victoria, particularly in the Latrobe Valley. These brown coals with 68-70% carbon, occur in very thick seams (up to 200 meters) under shallow cover (<30 meters). These coals differ from the Tertiary brown coals of North America in that they have a much lower ash yield and significant amounts of the ash-forming inorganic constituents are present as cations on the carboxylic acid groups which are a characteristic of low rank coals ... 

The Role of Ether-Linkages in Solubilization of Low-Rank Carboniferous Coals by H-Donors... 

Formation of asphaltenes during solubilization of low-rank bituminous coals has been attributed to cleavage of open ether-bridges (6). But while the presence of such configurations in high- and medium-rank bituminous coals is well established (7), their existence in less mature coals remains to be demonstrated. From reactions of low-rank bituminous coals with sodium in liquid ammonia or potassium in tetrahydrofuran, it has, in fact, been concluded that open ether-bonds are absent (8) or only present in negligible concentrations (9). 

Detailed discussion of these findings will be presented elsewhere. Here we only wish to point out that responses to a hydrogen donor tend to be critically affected by minor structural differences between the compounds. Thus, while diphenyl ether remains substantially unaffected by the donor, its hydroxy-derivatives (phenoxy phenols) often display fairly high reactivity. Taken in conjunction with the failure of low-rank coals (7) and phenoxy phenols (10) to suffer reductive cleavage when treated with sodium in liquid ammonia, this lends some support for the existence of phenoxy phenol entities in low rank coals. 

The inertness of phenols and phenoxy phenols toward Na/liq. NH3 can be attributed to the fact that phenols are powerful proton-donors in this system, and resistance of the resultant anions toward reduction is believed to result from stabilization by resonance (10). While alkylation of low-rank coals before treatment with Na/liq. NH3 therefore offers means for establishing the presence of phenoxy phenol ethers in them, an alternative is afforded by the observation that some phenols can be reduced by concentrated solutions of lithium (11). If this latter reaction also reduces phenoxy phenols in coal, a second treatment should then cause ether-cleavage. 

Treatment of low-rank coal (or of a vitrinite fraction from such coal) with variously concentrated solutions of potassium in liquid ammonia did not cause an increased -OH content in the reacted material. Nor was the hydroxyl content affected by such treatment after prior exhaustive methylation of the coal with dimethyl sulphate and CO acetone (13). On the other hand,... 

Non-Destructive Solubilization of Low-Rank Bituminous Coal (by non-reductive alkylation)... 

We therefore draw attention to a novel technique which allows solubilization of coal without rupture of covalent bonds. This utilizes the fact that the acidity of low-rank coals, which is largely due to their high -OH contents, can be enhanced by proper choice of a medium. 

To test this approach, 5 g samples -300 mesh Tyler, of a low-rank vitrinite, were stirred for 6 hrs in liquid ammonia (150 ml -33°C) containing 5 gms of potassium amide and 5 g of sodium amide. (The amides were formed in the medium, before introducing the coal, by action of anhydrous ferric oxide (1 g) or ferric chloride (1.5 g) on alkali metals.) Thereafter, 100 ml of anhydrous ethyl ether was added, the suspended coal material ethylated with C2H5Br (32 ml), and the reaction mixture stirred until all ammonia and ether had evaporated. Following... 

The most interesting outcome of this work is the observation that low-rank vitrinites can be rendered substantially soluble in chloroform and pyridine by alkylating coal salts formed in a non-reducing medium and under conditions that appear to preclude cleavage of covalent bonds. 

Selection of Solvents. The extraction yield of a low rank coal (Annesley) has been determined after digestion using a selection of solvents (Table II). The results show large variations in solvent power and, in particular, the high extraction yields obtained with hydrogen donor solvents. It is important to differentiate between the ability of a solvent to prevent polymerisation of the dissolved coal by hydrogen transfer, and its ability to retain the dissolved coal in solution. For example, Tetralin is frequently quoted as an... 

The petrological composition is important when considering the solvent extraction of prime coking coals but with lower rank British coals the variations in petrology are less pronounced. A more frequent cause of variations in extraction yield with low rank coals (CRC 802 and CRC 902) results from ageing. The reactivity of a coal decreases substantially as the coal becomes oxidised by exposure to the atmosphere (Table III). 

A possible explanation is that the phenolic content of low rank coal products are too high to allow solubility in even pyridine. This point has yet to be proven, however. 

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