Environmental Engineering Reference
In-Depth Information
8.1 Coals: The Earliest Modern Fuels
Coals are sedimentary rocks whose main constituents are
partially decomposed organic matter, various minerals,
and water. Their genesis begins with anaerobic decom-
position of accumulated plant debris in wetlands. Peats
are the youngest, only partly fossilized products of this
process. The Carboniferous period (360-286 Ma ago)
left behind the largest coal resources, followed by the
Jurassic (213-145 Ma ago), the Cretaceous (145-65
Ma ago), and the Paleocene (65-55.5 Ma ago). Poor-
quality lignites and peats are the products of the Quater-
nary period (starting 1.8 Ma ago). The bulk of coal mass
is composed of macerals, highly heterogeneous organic
compounds derived from woody phytomass (Stopes
1935; Carpenter 1988). This complex mixture is mostly
composed of vitrinite produced by the transformation of
cellulose and lignin; H-rich liptinite from the waxy and
resinous biomass; and C-rich inertinite from charred phy-
tomass. Their specific density is 1.3-1.4 g/cm
3
, and
commonly present minerals except for sulfides are about
twice as heavy. Ultimate elementary analysis on a
moisture-free basis reveals the expected dominance of
carbon: over 90% in anthracites, 80%-85% for good bitu-
minous varieties, less than 75% for lignites.
Oxygen's share is 3%-20%, hydrogen's 4%-6%, and
nitrogen's 0.5%-2% in U.S. bituminous coals. Sulfur in
sulfides (pyrites), sulfates, and organic matter ranges
from fractions of a percent to over 5%, and some coals
have elevated content of trace elements, including As
and various heavy metals (Mo, Cd, Hg, Pb) and trace
amounts of U, Th, and Ra. More customary proximate
and empirical analysis reports the shares of carbon, vola-
tile matter, moisture, ash, sulfur, and energy content of
coals. The World Coal Quality Inventory is the most
comprehensive repository of both proximate and ulti-
mate analyses of samples from all coal-producing coun-
tries (USGS 2001). Coal qualities span a very wide
continuum, with substantial variations common even in
a given seam. While the best anthracites contain less
than 5% moisture and 95% C, the mass of some lignites
may have up to 60% water and 15% C. Volatile matter
may be totally absent or as high as 85%. Ash is a collec-
tive term for incombustible minerals whose presence
varies widely, from little to more than 40% of the fuel's
mass in the poorest lignites.
The heating value of coals ranges about fourfold
on fresh-weight basis, between 8.3 MJ/kg and 35.0
MJ/kg. Poor lignites are greatly inferior to air-dried
wood (@15 MJ/kg), and the heat content of the best
anthracites approaches that of some crude oils. This vari-
ability makes classification of coals difficult and explains
the absence of a uniform global standard (Lett and Rup-
pel 2004). U.S. classification recognizes anthracite, bitu-
minous (at least 26.7 MJ/kg), and subbituminous (at
least 22.1 MJ/kg) coals and lignites (down to 14.7
MJ/kg). European classification has nine classes of hard
coals (at least 23.86 MJ/kg) and six classes of lignites
(brown coals). For decades international energy statistics
defined the standard coal equivalent as the fuel with 29.3
MJ/kg or 7 Mcal/kg. Only the best bituminous coals
rate that high, with most steam coals having 20-25
MJ/kg. Reliable conversions to energy equivalents re-
quire a detailed knowledge of specific coal qualities.
Coal seams outcrop in many parts of the world, and
hence the fuel was known and locally used from antiq-
uity, when its most important application was in iron
making during the Han dynasty (Needham 1964). Euro-
pean extraction is documented in Belgium in 1113, the
first shipments of coal to London date to 1228, and
the first exports from the Tynemouth region to France
