Combustion Phenomena of High Temperature Air Combustion - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 124

phenomena, the emission of volatile matter, the combustion of char, and the burnout
phenomena. The fundamentals of each phenomenon will be explained in this order.
2.5.2.1 Properties of Coal
The combustion phenomena of coal are clearly affected by its fuel properties. If the
characteristics of the combustion of coal can be estimated without combustion tests,
the total cost of tests of combustion will be reduced as combustion tests on funda-
mental parts will be unnecessary. The important methods for the purpose are to
estimate quantitatively the properties of coals in detail. The results of the industrial
chemical analysis based on the JIS standards can be an index to indicate the com-
bustion characteristics of coal. But at present more detailed properties analysis is
required, because many kinds of coal are imported to Japan in large quantities from
all over the world.
Figure 2.90 3 8 is an example of a series of property analyses prediction index of
combustion characteristics. In addition to the industrial chemical analysis, various
data such as chemical composition of metals in ashes, maceral compositions (fine
structure), base compositions containing oxygen, calorific value, carbon structure,
mineral compositions, molecular weight and chemical compositions of volatile mat-
ter, specific surface area, densities, porosity, functional base analysis, and extraction
characteristics are listed in this figure. This information is useful for considering
theoretically the results of reaction experiments. The newest analysis instruments
are necessary for obtaining some of these data.
2.5.2.2 Combustion Phenomena around Particles
The most fundamental combustion of solid fuels is combustion of carbon. A model
of its combustion phenomena is shown in Figure 2.91 . 39 In case of a low surface
temperature, the surface reaction (I) C(S) + O 2 → CO 2 occurs predominantly as shown
in Figure 2.91a , because the combustion rate of carbon is low and O 2 diffuses to the
surface of carbon particles. Moreover, reaction (II) 2C(S) + O 2 → 2CO occurs simul-
taneously, through which CO is oxidized to CO 2 immediately near the surface. In the
case of high surface temperatures, the surface reaction (III) C(S) + CO 2 → 2CO
becomes predominant and the generated CO is oxidized to CO 2 in the atmosphere in
the same way. Accordingly the temperature of the gas phase, some distance from the
surface, becomes higher than that of the particle surface. The former reaction (a) is
called the Single Film Model, the latter reaction (b) the Double Film Model.
Ignition of coal is accompanied by the emission process of volatile matter.
Combustion of pulverized coal takes place at comparatively high temperatures, so
that its emission process is accomplished as quickly as under 100 ms, but the reaction
is an important process to obtain stable and low NO x combustion.
In general, the emission process of volatile matter depends strongly on the initial
and succeeding changes in the properties of coal during combustion. Therefore, an
exact modeling of coal combustion has not yet been realized. But the simplest model
is the following Equation 2.25, assuming that the emission process is a first-order
reaction.