Temperature-resolved weight loss and tar yield during atmospheric devolatilization of pulverized coal have been obtained on a wire mesh reactor (WMR), which imposes prescribed thermal histories covering a wide range of heating rates on coals from lignite to anthracite. The accuracy of measurements has been improved by diminishing non-isothermality in the sample, ensuring independence of yields on loading density, and the development of a convenient tar collection method that inhibits secondary pyrolysis but also secures capture completeness. We reconfirm the continuous rank effects in terms of reaction dynamics and partitioning between tar and non-condensables but at disparate heating rates of 5 and 1000 K/s. In addition, we depict the constant variation in gas evolution histories among various coals before the cessation of tar release, whereas since the end of tar evolution, variations in gas formation kinetics for different coals gradually shrink with increasing temperatures. A larger fraction of total gases is found expelled after tar evolution by coals of higher rank. The sensitivity of the tar yield to heating rate is maintained the same over the range of 5−1000 K/s but varies with rank, being greatest for lignites and low-volatile bituminous coals but exhibiting a minimum for high-volatile bituminous coals.
A phenomenological extension is developed for coal devolatilization, based on FLASHCHAIN's mechanism for tar and lumped noncondensables. It segregates the reaction center population further into bridges and side chains with dissimilar reactivities that vary with rank. At low and medium temperatures, most bridges dissociate to produce tar precursors and noncondensables, whereas only a minor portion of side chains decomposes into gases; a fraction of both types of reaction centers may also be shuttled away as an element in tar molecules. When devolatilization continues to sufficiently high temperatures, bridges are depleted and side chain decomposition takes over to dominate gas formation. In the evaluations against atmospheric reaction dynamics of 16 coals from lignite to anthracite, the extension predicts with accuracy the disparate time scales for the evolution of tar and noncondensables, as well as the rank-dependent devolatilization rates. Also, except for the earlier evolution history at low heating rates, the extension demonstrates extrapolations within experimental uncertainty over a broad domain of operating conditions (heating rates from 1 to 10 4 K/s and pressures from vacuum to 9 MPa) for coals across the entire rank spectrum.
A gas-fired
coal preheating (GFCP) technology, offering a flexible
method to reduce NO
x
, could be used with
other de-NO
x
combustion technology such
as air staging to seek maximum NO
x
reduction.
The preheating chamber key apparatus of GFCP was investigated with
the help of infrared camera. The results show that devolatilization
and partial oxidation (combustion) of coal occurred in the preheating
chamber, and this may prove the main heat source of preheating chamber
is combustion of coal volatiles and gas is only used to prevent flameout.
A self-sustaining combustion drop furnace was used to investigate
the NO
x
reduction potential of GFCP with
air staging. Gas species concentrations along furnace are plotted
for several runs, offering details to further study and analyze the
GFCP. With GFCP, much HCN, C
i
H
j
, and soot were produced in the preheating chamber,
so under the similar air staging condition, the NO destructed by HCN
and soot was stronger than that without. NO
x
reduction could archive up to 72% with GFCP and air staging,
if the residence time in the combustion zone could be prolonged, the
NO
x
reduction will be even higher.
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