Page 48 - New Trends In Coal Conversion

16 New Trends in Coal Conversion

coal. The ASTM system uses fixed carbon and gross calorific value on a mineral matterefree
basis to classify coals. The higher rank coals (mainly bituminous coals and anthracite) are
classified according to fixed carbon on a dry basis, and the lower rank coals (wbelow
high-volatile A bituminous coal) are classified according to the gross calorific value on a
moist basis. Moreover, the agglomerating character can be applied to differentiate between
some adjacent coals.
The United Nations Economic Commission for Europe (UNECE, 1988) replaced the first
1956 coal classification system (which had significant flaws) with a new international codi-
fication system for medium- and high-rank coals (>0.6% R or >24 MJ/kg) in international
trade. This new UNECE (1988) system was applicable to all coals of different origin and
geological age from different types of deposits as well as to single seams and multiseam
blends of runoff mine coals and washed coals. Medium- and high-rank coals were character-
ized by means of a 14-digit code number comprising eight coal quality parameters as follow:
(1) mean random vitrinite reflectance, either measured directly or estimated as maximum vit-
rinite reflectance divided by 1.06; (2) the character of the reflectogram; (3) maceral compo-
sition expressed as (a) the percentage of inertinite and as (b) the percentage of liptinite, which
provides a means of distinguishing, in part, between Gondwana and Carboniferous coals; (4)
the free swelling index (FSI); (5) volatile matter (dry, ash free); (6) ash percentage (dry basis);
(7) the total sulfur content (dry basis); and (8) gross calorific value (dry, ash free) in MJ/kg.
Other parameters may be appended to provide a more detailed description of coal quality,
such as ash composition, ash fusion characteristics, and Hardgrove grindability index
(HGI) for steam coals or AudiberteArnu dilation properties, and phosphorous content for
metallurgical coals.
UNECE (1998) proposed an international classification of inseam coals. This system was
intended to serve as a means of classifying coals and ensuring a better characterization of
coal deposits. Unlike the previous UNECE (1988) classification system, which was intended
for commercial purposes, this new system was clearly described as not intended for use in
commerce or trade. The 1998 coal classification is based on three fundamental coal charac-
teristics to be used in combination: coal rank (or degree of coalification), petrographic
composition, and grade or amount of impurities (ash yield).
For the low-rank coals and taking into consideration the many criteria used to establish the
boundary between brown coals and medium-rank coals, once again the UNECE developed a
new classification and codification system for low-rank coals (UNECE, 2000). In this clas-
sification, the boundary between low-rank coals and medium-rank coals is established on the
basis of gross calorific value (lower than 24.0 MJ/kg moist and ash-free basis) and a random
vitrinite reflectance value of less than 0.6%. According to these criteria, the low-rank
coals are broadly classified as low-rank C (the former ortho-lignites), low-rank B (former
meta-lignites), and low-rank A coals (subbituminous).
The ISO-11760 (2005) system “classification of coals” was developed by the International
Organization for Standardization but has been guided by the classification system of the
UNECE (1998). The ISO-11760 (2005) system is based on the three fundamental coal prop-
erties: vitrinite reflectance (mean random reflectance), vitrinite content expressed as percent
per volume on a mineral-free basis, and ash yield. In line with vitrinite reflectance, this sys-
tem classifies coals into three broad categories of rank: low-rank (which covers lignite and
subbituminous coals), medium-rank (bituminous coals), and high-rank coals (anthracites).
Each category in turn contains a series of rank subcategories. In accordance with vitrinite
content (% vol. mineral-free), the system groups the coals into four categories: coals with
low, medium, moderately high, and high vitrinite contents. The inorganic content of coal
(ash yield) requires coals to be classified into five categories: very low ash, low, medium,