Geology Reference
In-Depth Information
Table 4.1.2
Analyses of fuel ash compositions.
Weight percent oxides in ash in ancient fuels
Fuel
Charcoal
Coal ash
Peat
Oxide in fuel ash
SiO
2
8.0
25
-
53
3
-
30
Al
2
O
3
0.9
20
-
40
0.3
-
5
Fe
2
O
3
tr
9
-
26
12
-
19
MnO
2
31.6
0
-
0.2
-
MgO
7.6
0
-
10
1
-
7
CaO
25.0
2
-
12
24
-
30
Na
2
O+K
2
O+CO
2
22.0
1.0
-
2.0
0.6
-
4.0
P
2
O
5
4.9
0.2
-
2.0
0.2
-
2.5
Source: From Tylecote (1980, p. 205) tr, trace amount.
Tillandsia
was collected in Peru for analysis, but was not permitted into the United States; therefore, chemical data
on plant ash, also in Table 4.1.4, is from Kabata-Pendias and Pendias (1992).
Oxide and Trace Element Data
D
ata for a number of oxides and trace elements from regional coal samples are presented in Table 4.1.3. However,
only specific oxides (bold-faced data in Table 4.1.3) such as SiO
2
,Al
2
O
3
, CaO, and K
2
O (Tylecote, 1980, p. 205),
and trace element data, specifically zirconium, compiled in this study provide key information for determination of
the original material used as fuel at the Tschudi burn. This is because only a few elements form refractory
compounds: for example, the melting point of SiO
2
is 1713°C, CaO is 2570°C, Al
2
O
3
is 2050°C, and ZrO
2
is
2710°C (Norton, 1968, p. 230). During coal burning, or ashing for geochemical analysis, some elements, such as
arsenic, lead, mercury, or silver, are volatized. Other elements and their oxides, such as aluminum, calcium, silicon,
and zirconium, are not volatized (Fletcher and Skeen, 1995) and therefore, are especially useful for comparison and
determination of the original composition of the unknown fuel material.
The geological environment of coal formation is also an important factor in understanding and interpreting the
analytical data. Coal formation begins in a warm, humid region, in a basin that acts as a catch-all for plant material
as well as a variety of intercalated stream-transported, detrital or airborne, volcanogenic minerals in varying
amounts that are included in the coal. Over tens of thousands of years, coal forms from the accumulation, burial,
and compaction of plant and other intercalated material. Carbon becomes concentrated during this process and
water is expelled until a higher rank, such as bituminous or anthracite coal, is attained. Plants, in contrast, use and
accumulate specific minerals from local soils for their growth and development.
The most distinctive of several oxides and elements used to differentiate coal ash from other ash compositions are
discussed below:
SiO
2
,Al
2
O
3
—
53wt% as the range of SiO
2
content of coal compared with
8 wt% SiO
2
for charcoal. Similarly, the Al
2
O
3
content of coal is much higher (20
Tylecote (1980, p. 205) established 25
-
1.0wt
%). This is directly related to the accumulation of detrital grains of quartz (SiO
2
) and feldspar (Al
2
O
3
is an important
constituent of this mineral series) in the coal-forming basin. In general, these elements are not as abundant in trees
and the SiO
2
content of common plants is 0.3
-
40wt%) than in plants (
<
-
1.2wt% (Kabata-Pendias and Pendias, 1992, p. 151).
CaO, K
2
O
—
Tylecote (1980, p. 205) indicates that the CaO content of charcoal ash is 25.0 wt% and peat ash is
24
-
30 wt%, compared with only 2
-
12 wt% in coal ash. In this study, the CaO content of wood ash is 41.0
-
53.0 wt
% compared with 1.0
-
7.0 wt% in coal ash (Table 4.1.4). Calcium would bioaccumulate in plants as an element
