Dry, ash-free

Operating parameters of this German plant, on the basis of one cubic meter of raw gas, iaclude 0.139 m O2, 0.9 kg briquettes, 1.15 kg steam, 1.10 kg feed water, 0.016 kWh, and 1.30 kg gas Hquor produced. Gasifier output is 1850 m /h and gas yield is 1465 m /t dry, ash-free coal. The coal briquettes have a 19% moisture content, 7.8% ash content (dry basis), and ash melting poiat of 1270°C. Thermal efficiency of the gas production process is about 60%, limited by the quaHty and ash melting characteristics of the coal. Overall efficiency from raw coal to finished products is less than 50%. 

The effects of rank on both compressive and impact strength have been studied, and usual minima were found at 20—25% dry, ash-free volatile matter (88—90 wt % carbon). Accordingly, the Hardgrove grindabiUty index exhibits maximum values in this area. 

Fig. 9. Composition of volatile matter as a function of rank (bright coals) at (a) 500°C and (b) 900°C. The wt % of C is on a dry ash-free basis of unheated...

Eig. 4. Product yields for EDS process on D on once-through and a bottoms recycle basis for various types of coal. DAF = dry ash free. 

Coal analyses are reported on severalbases, and it is customaiy to select the basis best suited to the application. The as-received basis represents the weight percentage of each constituent in the sample as received in the laboratoiy. The sample itself may be coal as fired, as mined, or as prepared for a particular use. The moisture-free (diy) basis is generally the most useful basis because performance calculations can be easily corrected for the ac tual moisture content at the point of use. The dry, ash-free basis is frequently used to approximate... 

The relationships between specific heat and water content and between specific heat and ash content are hnear. Given the specific heat on a dry, ash-free basis, it can be corrected to an as-received basis. The specific heat and enthalpy of coal to 1366 K (2000°F) are given in Coal Conversion Systems Technical Data Book (part lA, U.S. Dept, of Energy, 1984). 

The Australian Permian coals vary widely in rank (maturity) and type (vitrinite content) from the Oaklands (N.S.W.) coal at 72% (dry ash-free basis) carbon, a hard brown coal (6), containing 17% vitrinite, at one extreme - through high volatile bituminous coals such as Galilee (Queensland) coal at 77% carbon, 16% vitrinite Blair Athol (Queensland) coal at 82% carbon, 28% vitrinite, Liddell (N.S.W.) coal at 82% carbon, and >70% vitrinite - to low volatile bituminous such as Peak Downs (Queensland) at 89% carbon, 71% vitrinite, and Bulli seam (N.S.W.) 89% carbon, 45% vitrinite. 

Studies initiated by the author in CSIRO (13) seek to throw light on the role of the various macerals by studying the conversion, under catalytic hydrogenation conditions, in Tetralin as vehicle, of maceral concentrates from a high volatile bituminous coal. Some preliminary results, given in Fig. 3, show conversions as almost complete for the hand picked vitrain (>90% vitrinite) from a high volatile bituminous coal (Liddell seam N.S.W., 83.6% carbon and 43% volatile matter both expressed on a dry ash-free basis). However, it is evident that the conversion of the whole coal increases rapidly with increase in hydrogen pressure (under otherwise similar conditions - batch autoclave, 4h. 400°C). 

The steep dependence on hydrogen content of the tar yields obtained during the low temperature (500°C) fluidized bed carbonization of 14 Australian coals, ranging in rank from 72% to -89% (dry ash-free basis) carbon content, is clearly demonstrated in Fig. 5 (15,16). 

Further reference to Fig. 6 shows that the latter tar yield now plots with the bituminous coals with reference to the effect of the atomic H/C ratio. Similarly a second brown coal sample (Loy Yang) which, as recovered from the seam, has a very low ash yield (0.4% dry ash-free basis), and most of the carboxyl groups in the acid form, plots with the bituminous coals in Fig. 6 however, when the sodium-salt is produced from this coal before flash pyrolysis the tar yield is almost complete supressed. 

Coal Rank Natural Water (wt%) Volatile Matter (Dry, Ash Free) (wt%) Total Carbon (Dry, Ash Free) (wt%) Heat of Combustion (MJ/kg)... 

Table HI. Results of Experiments in Hydrogen and Nitrogen at 275 , 6.9MPa Gas Pressure (cold) for 30 min. Results are expressed on a dry, ash-free basis...

AD = air dried AR = as received DRY = moisture free DAF = dry, ash free (moisture free and ash free) DMF = dry, mineral free (moisture free and mineral free)... 

Complete analytical details of most of the samples referred to here were given in an earlier article (18). The results are summarized in Table I and are presented on a dry ash-free basis fully corrected for contamination by other macerals. 

Semibituminous Coat. Coal that ranks between bituminous coal and semianthracile. It is harder and more brittle than bituminous coal, has a high fuel ratio and bums without smoke. Semibituminous coal is also known as metabituminous coal which is defined as containing 89-91.2% carbon, analyzed on a dry. ash-free basis. The term smokeless coal also is used. 

Analyses reported on a dry basis are calculated on the basis that there is no moisture associated with the sample. The moisture value (ASTM D-3173 ISO 331 ISO 589 ISO 1015 ISO 1018 ISO 11722) is used for converting as determined data to the dry basis. Analytical data that are reported on a dry, ash-free basis are calculated on the assumption that there is no moisture or mineral matter associated with the sample. The values obtained for moisture determination (ASTM D-3173 ISO 589) and ash determination (ASTM D-3174) are used for the conversion. Finally, data calculated on an equilibrium moisture basis are calculated to the moisture level determined (ASTM D-1412) as the equilibrium (capacity) moisture. 

Lignite (brown coal) has been classified arbitrarily as coal having a moist, ash-free calorific value below 10,260 Btu/lb. A code number that is a combination of a class number and a group number classifies these coals. The class number represents the total moisture of the coal as mined, and the group number represents the percentage tar yield from dry, ash-free coal (Table 1.9). 

Group Number Yield, Dry, Ash-Free (%) Class 10 0-20% Class 11 20-30% Class 12 30-40% Class 13 40-50% Class 14 50-60% Class 15 60-70%... 

The final results of the proximate analysis of coal (ASTM D-3172 ASTM D-3173 ASTM D-3174 ASTM D-3175 ASTM D-5142 ISO 562) are usually reported to the first decimal place any subsequent figures have little or no significance. The final report of the analysis should always contain the results on a basis of air-dried coal (i.e., coal in its most stable condition and in which it was analyzed), but for purposes of classification or comparison it is often necessary to convert to another basis, such as dry coal, dry, ash-free coal, or as-received coal. 

Moisture and ash (Chapter 3) are not determined as a part of the data presented for ultimate analysis but must be determined so that the analytical values obtained can be converted to comparable bases other than that of the analysis sample. In other words, analytical values may need to be converted to an as-received basis, a dry basis, or a dry, ash-free basis. When suitable corrections are made for any carbon, hydrogen, and sulfur derived from the inorganic material, and for conversion of ash to mineral matter, the ultimate analysis represents the elemental composition of the organic material in coal in terms of carbon, hydrogen, nitrogen, sulfur, and oxygen. 

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