In
five specific areas within the Southern Ute Indian Reservation along the
Basin-rim, coalbeds burning beneath the land surface have recently been
discovered. While there is evidence
that Fruitland coals have burned pre-historically, moribund trees and recent
surface collapse features point to a recent resurgence of fire activity. The heat-of-hydration can facilitate
spontaneous combustion of underground coal when the water table
fluctuates. Coal most susceptible to
self-heating is characterized by high intrinsic moisture and oxygen content, as
found in low-rank coal such as sub-bituminous coal and lignite (Sarnecki,
1991). The heat of wetting can be
greater than the heat of oxidation (Kuchta et al,. 1980). If the coalbed is an aquifer (as it tends to
be in these areas), and the water table normally fluctuates, if only slightly,
with seasonal precipitation recharge, the heat of wetting potential is
increased dramatically by water removal.
When water levels drop in these confined aquifers, ambient air is drawn
into the coalbeds, thus supplying the necessary oxygen to support combustion or
further oxidation of the coals. Once
the lower self-heating temperature (SHT) of the coal (defined by the rank of
the specific coal, with lower-ranked coals having the lowest SHT) is breached,
the self heating tendency of the coal produces a sustained exothermic reaction
(Smith, 1989) increasing oxidation until smoldering and combustion occur. The self-heating temperatures for some coals
can be as low as 30 degrees centigrade in lignite and subbituminous coals, and
that for bituminous coals can be as low as 60 degrees centigrade (Kuchta et al,
1980). In areas where current coal
fires have been recognized in 1998-99, annual precipitation is low. Therefore down-dip extraction of water could
have a substantial effect by dewatering the shallow coals if the seams are
hydraulically connected to the nearby producing gas wells. Several of the coal fire sites are in areas
of recent wildfires. Actual ignition of the coals in these particular areas may
have been perpetrated by smoldering tree roots penetrating shallow coalbeds.
Figure 8: Coal fires near
Cinder Buttes
Figure 9: Coal Fires near
Cinder Buttes
The
newly recognized coal fires (figures 8
and 9) were first detected by the presence of steam condensate plumes
evident in cold weather, smoky vents, and distinct pungent odors. Vents are high in carbon monoxide with
smaller hydrogen sulfide and sulfur dioxide components. Only minor amounts of methane are detected
in coal fire vapors due to the methane being consumed by combustion. Many vents are moist, host mossy growths,
and show black stains of soot, scorched roots and grass. Infrared thermometer readings indicate vent
temperatures at the surface as high as several hundred degrees Fahrenheit.