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Hydrothermal Processes in Subcritical Water
either remove fine residual ash (e.g., ultra clean coal [UCC] and CENfuel), from
coal or coal dissolution processes that produce ash free synthetic coal-like material
(e.g., Hypercoal). Although none of these processes reached immediate commercial
development, these advanced coal beneficiation techniques gave a significant impetus to the use of CWF for the following reasons [125–129] (Penn State’s coal-water
slurry fuel program, 2012, pers. comm.):
1. CWF produced from ultra clean coal can replace fuel oils used in highefficiency gas turbines and low–medium speed diesel engines. The cycle
efficiency for gas turbines or diesel engines is not negatively impacted by
the water content in CWF.
2. CWF facilitates pipeline transportation and storage, and gives additional
reductions in greenhouse gases (GHGs). The convenience of easy transport of CWF is a benefit for many countries with overloaded transport
infrastructure.
3. Large and fuel-efficient diesel engines for stationary power generation are
especially suitable for retrofit to burn CWF. Small gas turbines and diesel
engines can also utilize CWF with high efficiency.
The preparation of CWF involves crushing the coal particles to 10–65 μm particle
size, although the particle size of 10–25 μm is more desirable. This can be achieved
by the standard grinding or crushing processes. This is generally followed by the
wet milling and homogenization process using standard milling processes. Some
additives may be used to facilitate the process. CWF has been prepared with a
number of coals such as lignite, flame and gas flame coals, anthracite, and brown
coals [125–129] (Penn State’s coal-water slurry fuel program, 2012, pers. comm.).
If CWF is to be used for gas/oil boilers, the ash content should be <10%. For coal
boilers, no limit on ash content is necessary. According to the literature [125–129]
(Penn State’s coal-water slurry fuel program, 2012, pers. comm.), CWF for brown
coal (lignite) has been successfully tested. For flame coal with 40–45 vol% slurry
and gas flame coal and gas coal with 28–40 vol% slurry, the systems are well
developed. For anthracite with 7–28 vol% slurry, CWF is possible and has been
successfully tested [125–129] (Penn State’s coal-water slurry fuel program, 2012,
pers. comm.).
CWF can be used in several different applications [125–129] (Penn State’s
coal-water slurry fuel program, 2012, pers. comm.) such as a possible substitute
for heavy-grade fuel oils such as diesel #6, bunker C, and bunker D residual fuel
oils. When a particle size is ≤80 μm, it can be used as co-fuel and substitute fuel
in diesel engines [125–129]. Low-speed marine and modular power plant diesels
can operate on pure CWF. Medium-speed diesels such as locomotives sometimes
need coinjection of CWF and diesel #2 fuel that acts as an ignition source for the
CWF. For the use of CWF in gas turbines, fine particles such as 5–10 μm of coal
are needed to substitute petroleum and natural gas in these usages. The particle
size of coal is an important factor in making homogeneous CWF that can be easily
atomized in various types of engines [125–129] (Penn State’s coal-water slurry fuel
program, 2012, pers. comm.).