Page 52 - New Trends In Coal Conversion

20 New Trends in Coal Conversion

cokemaking, this section only includes descriptions of specific tests that are necessary
for coals that are to be used in these processes. Fig. 1.10 summarizes the most impor-
tant characteristics and standardized tests, which serve as a useful guide for selecting
coals for use in these conversion processes.
In addition to the chemical data obtained from proximate and ultimate analyses and
petrographic information, coal quality parameters that need to be taken into account in
coal combustion and gasification include those obtained from the following specific
tests (Fig. 1.10):
1. The total sulfur content. This may be obtained from a combination of the organic constituents
and the mineral matter. It may as well in terms of overall percentage be expressed in some
cases as the amount of sulfur dioxide emissions expected in relation to the heating (or calo-
rific) value, for example, as kg SO 2 /GJ.
2. The heating value, calorific value, or specific energy indicates the amount of heat liberated
per unit of mass of combusted coal and is of fundamental importance in setting the price
of specific coals for combustion applications. Although generally regarded as a rank-
related parameter, the calorific value is also dependent on the macerals in the coal, which
are determined by petrographic analysis and the mineral composition.
3. The ash fusion temperatures. These indicate the behavior of the ash residues from the coal at
high temperatures and are mainly related to the chemical composition of the ash and the na-
ture of the mineral matter contained in the coal. They are used to indicate whether the ash will
remain as a fine powder within the furnace system after the coal has burned or whether it
might partially melt and form a slag on the boiler’s heat exchange surfaces.
4. The Hardgrove Grindability Index (HGI). This indicates the ease with which the coal can be
ground to a fine powder and is important for gauging the coal’s compatibility in blend prep-
aration processes where the precombustion pulverization system of the plant is concerned.
The HGI is most directly related to the maceral and maceral group composition but is also
dependent on rank and mineral content.
The specific tests that provide information about the potential behavior of coals dur-
ing carbonization processes include the following (Fig. 1.10):
1. Tests related to the swelling and caking properties of coals:
• The FSI or crucible swelling number. This is a measure of the increase in volume of the
coal when it is heated in a small crucible in the absence of air. This test is also used to
characterize coals for combustion, especially where the coal is burned in beds of
coarse-crushed material in stoker-based systems.
• The Roga index. This test provides information on the caking properties of the coal, in a
similar way to the FSI. The index itself is derived from the strength or cohesion of the
coke produced in the crucible, as evaluated by a subsequent tumbler test.
• The GrayeKing and Fischer assays. These determine the proportions of coke or char
(carbonaceous solids), tar (organic liquids), liquor (ammonia-rich solutions), and gas
produced when the coal is carbonized (heated in the absence of air) under particular
laboratory conditions and hence may provide a basis for estimating the yields of coke
and by-products obtained from the coal in an industrial coke oven.
• Gieseler plastometer and AudiberteArnu dilatomer tests. These monitor how the coal be-
haves as the different macerals melt, devolatilize, and resolidify at different temperatures
during the carbonization process. The Gieseler plastometer evaluates the coal’s behavior