Page 54 - New Trends In Coal Conversion

22 New Trends in Coal Conversion

• Koppers test. The Koppers test uses an oven with unidirectional heating applied from one
side. In this test, carbonization is performed with 80 g of coal, which is subjected to a con-
stant pressure of 10 kPa by means of a piston, and the change in volume is measured. Mod-
ifications to the early Koppers test have been introduced by INCAR (Spanish Patent No.
524.258, 1983) based on Mott and Spooner’smodifications. From this test, a criterion
for classifying coals has been established: coals undergoing a final contraction greater
than 10 mm and no expansion can be considered to not be dangerous during carboniza-
tion. Dangerous coals, however, are those showing a final threshold of 10 mm
contraction.
• SHO test. This type of coking oven has unidirectional heating from the sole, while a spec-
ified force is applied uniformly from the top by a piston plate. Basically, high-volatile coals
tend to contract or shrink during the test, and higher rank coals from medium- to low-
volatile content expand. The corresponding ASTM standard specifies the conditions under
which the expansion or contraction of a coal or coal blends during carbonization can be
measured.
• Movable-wall oven. Direct measurement of coking pressure of coals and coal blends can be
obtained in a movable-wall oven, which is widely used. In these ovens, one wall is
mounted on runners such that it can move, or tend to move, away from the other oven
wall. Its movement is nowadays restrained by a load cell that measures the force necessary
to prevent the wall from moving. By monitoring this force during a coking cycle, the pres-
sure exerted by the charge on the wall can be calculated. There is no standard oven and
procedure, although all movable-wall ovens operate on the same basic principles. These
ovens with a 250e400 kgcapacityhaveasimilar widthtocommercialovens,sothat heat-
ing regimes can be accurately reproduced on a pilot scale. During the coking process, not
only can the rank and type of coal be related to the generation of gas and wall pressure but
also moisture content, particle size distribution, and bulk density are key factors to be
considered. In addition to coking pressure measurements, the amount of coke produced al-
lows to run full-scale coke tests in terms of chemical and physical properties and in partic-
ular the mechanical resistance of lump coke before and after the reaction with carbon
dioxide.

1.5.2.3 Coal weathering and coal oxidation

Coal weathering and/or coal oxidation is a major problem in coal utilization in gen-
eral, but in particular in the field of cokemaking because weathering reduces the coal
quality by altering its chemical and physical properties. As explained by Crelling
(2008), weathering is a normal geological process that can eventually break
down all rocks to fine particles. The exposure of coals to the atmosphere or to
oxygenated ground waters promotes the oxidation of the organic and inorganic frac-
tions in coals.
In weathered coals, the oxygen, moisture, and volatile matter content increases,
whereas the sulfur content decreases. The decrease in sulfur is the result of pyrite
changing into forms of soluble sulfate that contribute to mine acid drainage. Weath-
ering also generates humic acids in coal that lower the pH of coal-water slurries.
Weathering also reduces the particle size of coals and can cause dust-related problems.
Oxidation alters the surface properties of coal so that coal recovery by flotation is more
difficult.