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Water for Energy and Fuel Production
spectroscopy (TG-FTIR), solvent extraction, donor solvent liquefaction, and FTIR.
The study resulted in the following conclusions:
1. At short pretreatment times, the process loosened up the coal structure
resulting in the increase of extractables and the yield. The oxygen content
also decreased when coal was subjected to an accelerated aging process.
However, the liquefaction yields appear to decrease relative to the raw coal.
2. At longer pretreatment times, the process partly recombines the structure
resulting in a decline of extractable and tar yields. Oxygen continues to be
removed, but ether groups go through a maximum. The liquefaction yields
were closer to values for the raw coal.
3. The solvent adduction may be the reason for the decline in liquefaction
yields for coals with short pretreatment times.
4. For Illinois coal, the yields were very sensitive to the amount of oxygen
exposure. The participation by the oxidized form of pyrite in the liquefaction pretreatment chemistry appeared possible.
Bienkowski et al. [109,110] evaluated the effect of steam pretreatment on coal liquefaction. For a Wyodak coal stored under water (to avoid weathering), they pretreated
the coal using 750 psig steam for 30 min at 200°C. Pretreatment of suction dried coal
at 200°C increased the production of extractables at 400°C from 30.5% to 38.5%.
While an increase in the pretreatment temperature to 240°C increased the yield to
40.3% an increase in the pretreatment temperature to 320°C reduced the conversion
to 33.8%. Bienkowski et al. [109,110] argued that an increase in the pretreatment
temperature increased coal matrix loosening and stabilization of some reactive components of the coal resulting in higher conversion. A further increase in temperature
set up higher rate of retrogressive reaction, which in turn decreased the conversion.
Bienkowski et al. [110] also found that an addition of ammonia in both the pretreatment and subsequent liquefaction stages gave even higher conversion due to the reactions between hydrogen and oxygen functional groups.
Graff and Brandes [111,112] (Graff and Brandes, 2012, pers. comm.) and
Brandes et al. [113] observed higher yields of liquid products from pyrolysis and solvent extraction of Illinois No. 6 coal that was pretreated by steam at
320°C–360°C and 50 atm pressure. A similar pretreatment with helium had no
effect and the exposure to air of steam-pretreated coal lost the increase in yields.
The study concluded that the pretreatment disrupts the hydrogen bonds, reduces
the number of covalent cross-links, and increases the hydroxyl groups in the coal
[111,112] (Graff and Brandes, 2012, pers. comm.). The exposure to air weathers
the coal with a negative effect on liquefaction yield. Khan et al. [114] showed
that the steam pretreatment at a pressure of 1100–1300 psig and a temperature of
300°C–320°C for five coals of different ranks did not increase the tar yields when
pyrolyzed at a slow heating rate. The steam treatment reduced the concentration
of oxygen functional groups for the low-rank coals and increased the tar yields
when pyrolyzed at a rapid heating rate.
Ross et al. [115–119] evaluated the effects of water pretreatment on Illinois No. 6
and Wyodak subbituminous coals and found no effects on toluene solubles in a