Page 97 - New Trends In Coal Conversion

62 New Trends in Coal Conversion

variations of TE concentration in coal seams are indicative of postdepositional and
different coalification processes. Xu et al. (2003) have compiled and tabulated the
mean values of concentrations of 10 TE contents in seven Chinese coals and 38 TE
contents in the US, British, and Australian coals based on different sources and inves-
tigations concerning more than 1500 coal samples. These authors considered that
although the TE content seems to vary strongly with the coal type, four main groups
of content level can be established as follows: (1) concentration >50 ppm, (2) ranging
between 10 and 50 ppm, (3) 1e10 ppm, and (4) content <1 ppm (Table 3.1). Most
TEs are less concentrated than 50 ppm. Focusing on elements of major concern
only for boron concentrations higher than this value have been found. Arsenic and
lead are commonly found in concentrations ranging from 10 to 50 ppm. Concentra-
tions for the other TEs of major concern range between 10 and 50 ppm. On another
hand, mercury contents of coal differ by coal basin, by rank, and within individual
coal beds, reflecting diagenetic, coalification, and postdiagenetic differences at all
scales. For most coals over the world, mercury concentrations range between 0.02
and 1 ppm, concentrations exceeding 1 ppm being rare (Xu et al., 2003; Yudovich
and Ketris, 2005).
The coal samples were analyzed with methods based on modern instrumental tech-
niques including inductively coupled plasma mass spectrometry, inductively coupled
plasma atomic emission spectrometry, hydride generation and cold vapor atomic fluo-
rescence spectrometry, X-ray fluorescence spectrometry, and proton induced gamma
emission (Dale, 2006). The results in Table 3.1 show that the most significant differ-
ence between the Australian and international coals are the levels of As, Se, Hg, and B,
which are of major environmental concern. Australian coals contain lower concentra-
tions of these elements, which are volatile and therefore may be released to the atmo-
sphere during coal conversion processes. Significantly higher concentrations of B and
Hg were noted in Chinese coals. The average value for Se in Chinese coals was similar
to international coals. High levels of As were found in a number of international coals.
Relatively high levels of Hg were present in some US coals (>0.1 ppm). The levels of
other TEs of major concern were generally similar in all coals.
The release of TEs during coal conversion processes is notably affected by their
chemical affinities. The elements associated mostly with the coal organic and sulfide
fractions tend to vaporize first and are then easily adsorbed on fine particles during
flue gas cooling. In contrast, elements combined with the discrete mineral matters
most possibly remain in the ash. However, the numerous studies carried out until
now have not concluded in a general rule. The physical separation technique such
as density separation resulted in higher organic associations as compared with sequen-
tial leaching. Comparison of multiple density separation and the sequential leaching
results led to different levels of confidence regarding the modes of association of
TEs in coal. For some of the TEs such as Mn, Pb, Se, and As, data were consistent,
whereas for other elements such as Co, Ni, Be, and Sb, little agreement was observed
(Davidson and Clarke, 1996). Querol et al. (2001) characterized the affinity of TEs in
selected coals by the density separation methodology coupled with the mathematical
model developed by Klika and Kolomazník (2000). Six to seven density fractions
3
between <1.3 and >2.8 g/cm were obtained from the bulk coal samples by using