Coal
is one of the major fuels for power generation, and it will
continue in this capacity for the next several decades. Two types
of coal are mainly used: lignite and bituminous coals. When exposed
to air, post-mining, the coal surface undergoes LTO (low-temperature
oxidation) at RT-150 °C according to the atmospheric oxygen level.
The LTO process decreases the calorific value of the coal, and consequently,
different gases are released [mainly carbon oxides (CO, CO2), water vapor, hydrogen (H2), and also some low molecular-weight
organic gases (C1–5)]. Some of these gases are toxic
and flammable. In extreme cases, fires erupt. The mechanism by which
the molecular oxygen oxidizes the coal macromolecule at the temperature
range of 30–150 °C (LTO process) is complex and also involves
a chain of radical reactions that take place; however, the exact underlying
mechanism is not yet clear. The LTO process was studied in detail
by simulating the processes occurring in the coal piles by using two
coal types: an American Bailey coal, used in Israeli coal-fired utilities
and a German Hambach lignite, used in German utilities. The mechanism
underlying the LTO process and the radical reactions that are involved
are discussed in detail.
Coal is intensively
used worldwide as a main fuel source. However,
it may undergo oxidation processes [i.e., low-temperature oxidation
(LTO)] when stored under an air atmosphere in piles post-mining at
low temperatures ranging from 300 to 425 K, specifically, a surface
gas/solid reaction with molecular oxygen. Therefore, it is of major
importance to prevent or appreciably slow down such reactions, which
result in a loss in the energy content (calorific value) of coal.
Previously, we showed that radicals are formed during the LTO process.
In this work, the dependence of radical formation on coal rank as
a function of heating (temperature) and the presence of oxygen gas
were studied using electron paramagnetic resonance spectroscopy. It
was shown that lignite coals are more sensitive than bituminous coals
to the atmospheric environment (i.e., molecular oxygen and nitrogen
content) and to temperature, as reflected by the formation of surface
carbon-centered radicals. Moreover, this is the first publication
showing the effects of LTO on micro- and macro-pores by assessing
how these structures affect O
2
diffusion. The LTO process
blocks the micro-pores, such that radicals form mainly at the surface
of the coal macromolecules, in both bituminous and lignite coals.
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