Combustion Phenomena of High Temperature Air Combustion - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 127

temperature. These are examples on Arrhenius plots of the emission process of volatile
matter. This figure shows that the rate constant differs according to the types of coal
and the combustion conditions. The above-mentioned scattering of data is due to the
fact that the rate constants are not normalized by the specific surface area of the particle
and are not corrected by actual particle temperatures because the rate of the emission
process of volatile matter is controlled by the heat transfer.
Further, the multistage parallel model is proposed by Ubhayakar et al., 41 Pitt, 42
Kobayashi et al., 43 etc. The FLASHCHAIN model proposed by Niksa, et al. 44 is mainly
used for modeling of the emission process of volatile matter. What is important in this
case is that the volume of volatile matter is obtained by industrial analysis only at the
temperature of 1173 K. The temperature of actual combustion would often be different
from analysis temperature and therefore it is necessary to confirm temperature depen-
dence of the amount of evaporation in advance. In this case, Q factor (VM*/VM) is
often taken into account for the analysis (VM* is volatile matter at 1173 K).
Currently, there are few reports on the research of clarifying experimental distri-
bution of gas concentration around solid particles in the region of volatile matter
combustion because of the difficulty of its measurement. Therefore, we estimate the
general combustion characteristics in a furnace by sampling combustion gas. But the
results of gas analysis obtained in the particles dispersion field such as combustion of
pulverized coal is generally the average value of all mixed gas around the particles.
Hence investigations 45-47 that deal with mathematical analysis of flame structure using
the modeling of single particles are reported.
Figure 2.93 46 sho ws an example of the calculation of distribution of temperature
and gas concentration around particles by the mathematical analysis of the single
particle model. This figure is the result of analysis assuming that volatile matter is
evolved uniformly from around the particle. This figure shows that the flame tem-
perature becomes higher than that of the particle surface because volatile matter
burns separately from the surface. Under this condition, combustion of volatile matter
is predominant and little fixed carbon burns.
The analytical calculation indicates that after emission of volatile matter, fixed
carbon (char) begins to burn at the particle surface. However, it is reported 48,49 that
the emission of volatile matter occasionally occurs in a jet-like state, in which
combustion of fixed carbon occurs simultaneously because oxygen can diffuse onto
the particle surface.

2.5.2.3 Combustion Phenomena inside a Particle

The cases necessary to consider the combustion phenomena inside particles are the
following:

• Char combustion after the process of emission of volatile matter in case

of pulverized coal

• Combustion of lump coal that is used in stoker boilers and coal stoves

and the case to analyze reaction phenomena inside desulfurization reagent
in furnace desulfurization