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Hydrothermal Processes in Subcritical Water
In China and Japan, CWF has been produced in large plants for more than
30 years. In a typical CWF production plant, the coal is mixed with water and
some additives (to help forming good mixture) and passed through one or more
pulverizers and multiple milling (high-load and low-load) processes. For boiler
application of CWF, high slurry concentration (65–70 wt%) and better stability
requires a wide particle size distribution (often bimodal) with mass mean particle
size between 5 and 10 μm. This is often achieved by using several mills and/or
recycle streams.
The efficient atomization of CWF slurry in a combustor governs the required
particle size and slurry concentration. For gas turbine and diesel engine applications
of CWF, generally lower slurry concentration and mass mean particle size of coal
are required. For diesel engines, the preferable slurry concentration is 50–55 wt%
with a mass mean particle size of 5–15 microns. For turbines, the preferable slurry
concentration is 55–60 wt% with a mass mean particle size of 4–6 microns. Since
the cost of milling and pulverizing rises exponentially with a decrease in particle
size below 30–50 microns, low particle sizes required in diesel engines and turbines
will necessitate the use of special milling and pulverizing processes.
CWF exhibits the rheological properties different from fuel oils. Fuel oils tend to
be more Bingham fluids. CWF is pourable and pumpable, but its viscosity decreases
significantly with the shear caused by agitation and pumping. Also unlike fuel oils,
the viscosity of CWF is unaffected by the temperature. Slurry viscosities are strongly
affected by the coal characteristics, concentration, and flow conditions. Currently,
CWF containing 65%–70% coal has an apparent viscosity around 1000  MPa-s at
room temperature. This is too high for atomization of slurry in the combustion chamber, and it is normally reduced either adding water (10%–20%) or heating before the
injection, which can promote flashing.
The use of CWF in diesel engines and gas turbines besides its use as a substitute
for heavy oil in boilers requires the following considerations and additional research:
1. CWF is more difficult to atomize than diesel fuel due to its much higher viscosity. The effective atomization is more critical to combustion due to the
effect of droplet size on ignition delay (which is caused by the time required
for water evaporation) and burnout. Pressure atomization can be improved
by increasing the liquid velocity through the nozzles; however, this greatly
increases the nozzle wear.
2. CWF causes chronic wear of injection nozzles, with wear being exacerbated by cavitation effects.
More research has been continuing to address these issues. However, CWF combustion has a very promising future.
5.5.3.3 Combustion of CWF
Fu et al. (2012, pers. comm.) carried out the earlier combustion experiments for
CWF in a boiler using oxygen-enriched air. Their study for 700 hp watertube
boiler with bituminous coal indicated that the use of oxygen-enriched air resulted
in the required lower air preheating and the improvement in the carbon burnout.