Dry, ash-free basis

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...

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. 

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...

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. 

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. 

It is therefore impossible to determine accurately the composition of the pure coal substance from the usual ultimate analysis simply by making allowance for the quantity of ash left behind as a residue when the coal is burned. Results obtained in this fashion are, as a consequence, quoted as being on a dry, ash-free basis, and no claim is therefore made that these results do in fact represent the composition of the pure coal substance. If, however, it were possible to calculate accurately the quantity of mineral matter originally present in the coal sample, then by making due allowance for this material, the composition of the pure coal material could be deduced with reasonable precision and certainly with a greater accuracy than could be obtained by adopting the analytical figures calculated to a dry, ash-free basis. 

An ultimate analysis that can claim to represent the composition of the organic substance of a coal is said to be on a dry, mineral-matter-free (dmmf) basis. The dmmf basis is a hypothetical condition corresponding to the concept of a pure coal substance. Since the dry, ash-free basis for coal neglects changes in mineral matter when coal is burned, the dmmf basis is preferred whenever the mineral matter can be determined or calculated. 

Dry, ash-free basis data calculated to a theoretical base of no moisture or ash associated with the sample (ASTM D-3173 ASTM D-3174). 

Analytical results (Table 1) show that SRC have very high volatile matter and nitrogen contents (52-60% and 1.8-1.9%, respectively, on a dry-ash-free basis) and very low moisture and ash contents (0.1-0.3%, as-received basis in each case). The Higher Heating Values for the SRC (15,920-16,115 Btu/lb, dry-ash-free basis) are much higher than those of reference coals (13,290 and 14,110 Btu/lb for the WSB and KHB coals, respectively). 

Note V.M. contents are shown as percent on dry, ash free basis Inerts contents are shown as volume percent. 

About 70-95% of the organic matter in carbonaceous chondrites consists of an ill-defined, insoluble macromolecular material, often referred to as polymer or kerogen . A typical elemental composition for Murchison polymer (Hayatsu et al., 1980a), on a dry, ash-free basis, is C 76.5%, H 4.5%, N 2.4%, S 4.3%, and O 12.4% (by difference). 

Elemental data reported on a dry, ash-free basis. Oxygen calculated as the % mass difference (100-%C-%N-%H) for 74 FAs, 71 HAs acids, and 49 NOM samples. 

Carboxyl and phenolic contents reported on a dry, ash-free basis. 

A in the normal Arrhenius equation. Note that k is the rate constant at T. The algorithm was used to fit kinetic constants to the pyrolysis of wheat straw at 5,10 and 40°C/min (one data set per heating rate). The algorithm use the local temperature and does not rely on a constant heating rate. The data from an experiment were converted to dry ash free basis and the mass loss rate was normalized by the maximum mass loss rate. The data in the range where the normalized mass loss rate was above 0.1 was then used. This excludes the lignin tail from the data. The mass data were then converted to degree of conversion and normalized so the conversion of the final data point was 1.300 points were used per data set. Kinetic parameters were fitted to the individual data sets as well as to all three data sets simultaneously, The kinetic values are listed in Table 1. 

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