145
Hydrothermal Processes in Subcritical Water
found that hydraulically milled coal significantly increased the liberation of minerals
(97% vs. 90% for ball milling) and led to an improved overall mineral separation.
Ultrafine coal milling is an essential part of firing coal into gas turbines or diesel engines. For gas turbines, the particle size of ≤10 μm is desirable. For diesel
engine, the top size of 20 μm has been specified, although this depends on the size
and speed of the engine. Milling energy depends on both the type of mill and the
material especially at small particle size required for CWF in turbines and engines.
A number of advanced mills that are now available include ball mills, centrificial
or planetary mills, nutating mills, opposed flow jet mills, impact jet mills, spiral
jet mills, and high-pressure water jet mill [125–129] (Penn State’s coal-water slurry
fuel program, 2012, pers. comm.). While nutating mill appears to offer the lowest energy consumption, the final choice of mill may depend on the interaction of
milling with the deashing process. Another factor affecting the choice may be the
particle size distribution required for the deashing technology to be employed for
each CWF product.
Two types of processes for chemical cleaning of coal are (1) those that attempt
to dissolve the mineral components of coal (e.g., UCC, CENfuel) and (2) those that
dissolve the coal leaving a mineral-rich insoluble coal byproduct (e.g., Hypercoal).
We briefly examine these three important processes in Sections 5.5.3.1.1 through
5.5.3.1.3.
5.5.3.1.1 Ultra Clean Coal
The UCC production process [116] involves two main steps: a caustic pressure leach
to convert silicates and clays to dissolved sodium silicates and sodalite-type minerals. The sodalite material is then dissolved in acid so that it can be removed with the
filtrate in a simple filtering operation [125–129]. The key features of the technology
are as follows:
1. Since coal pulverization is not required, the solid–liquid separation easy.
2. Digestion removes both extraneous and a large portion of minerals within
the coal particles. The process also removes most of alkalis, all of the inorganic sulfur, and some of the organic sulfur. The UCC product contains
about 30% moisture.
3. The process is capable of treating most bituminous coals.
The process is capable of meeting gas turbine specifications for all bituminous coals
[125–129,133] (Penn State’s coal-water slurry fuel program, 2012, pers. comm.).
5.5.3.1.2 CENfuel
CENfuel produces ultralow-ash coal by an acid regeneration and with the removal
of other deleterious elements from coal. In the process, the main ash components
such as SiO 2 , Al 2 O 3 , and TiO 2 are removed by leaching granular coal (2 mm)
with an aqueous solution of hydrofluoric acid and fluosilicic acids. Sulfoides such
as iron pyrites are not affected by leach. The rich liquor contains soluble fluosilicates and undissolved FeS particles. The liquor is passed to a distillation unit
where metal fluorides are recovered and removed from the system. The residue