1070-4272/04/7709-1547C2004 MAIK [Nauka/Interperiodica]
Russian Journal of Applied Chemistry, Vol. 77, No. 9, 2004, pp. 1547!1555. Translated from Zhurnal Prikladnoi Khimii, Vol. 77, No. 9,
2004, pp. 1558!1567.
Original Russian Text Copyright + 2004 by Boiko, Pachkovskii.
OF FOSSIL FUEL
A Kinetic Model of Thermochemical Transformation
of Solid Organic Fuels
E. A. Boiko and S. V. Pachkovskii
Krasnoyarsk State Technical University, Krasnoyarsk, Russia
Received February 17, 2004
Abstract-A kinetic model of thermochemical transformation of solid organic fuels was constructed. The
mathematical model describes the kinetics of drying, release and combustion of volatiles, and combustion
and gasification of the nonvolatile (coke) residue at linear heating. The kinetic curves of burn-out of Irsha!
Borodino brown coal at various heating rates and model parameters were calculated.
The experience of using solid organic fuels at
thermal power plants shows that there are still no
pretreatment and combustion procedures ensuring
high efficiency, reliability, and environmental safety
of boiler operation .
To formulate recommendations concerning condi-
tions and modes of combustion of solid organic fuels,
it is necessary to elucidate an interrelation, based on
kinetic parameters, between various steps and proc-
esses of thermochemical transformation of coals .
Description of heat and mass exchange and of aero-
dynamics in combustion chambers of boilers involves
elucidation of the mechanism and construction of an
adequate kinetic model of burn-out of a solid organic
fuel . The following questions should be ans-
wered : What steps control the overall process ki-
netics? What steps are excessive in the kinetic scheme
under consideration, and can this scheme be simpli-
fied? What kinetic mechanism (among several alter-
natives) is the most probable?
When a solid fuel is burnt as a dust, the degree of
its thermochemical conversion is determined by a set
of parameters of consecutive-parallel processes, in
particular , by the rate of moisture evaporation,
rate of release and combustion of volatiles, and rate of
combustion of the nonvolatile residue. The scheme of
burn-out of a coal substance is complicated in certain
steps by such processes as low-temperature chemi-
sorption of oxygen, gasification of the nonvolatile
residue with carbon dioxide, steam, and hydrogen,
transformations of chemical components of the miner-
al matter of the fuel, etc. .
As these processes strongly influence each other
and their endo- and exothermic effects are significant,
they should be taken into account in constructing the
pattern of thermochemical transformation. The rates
and parameters of the above processes are largely
determined by the quality of the initial fuel. In this
connection, to correctly describe and calculate the coal
burn-out, it is necessary to elucidate the mechanism
and construct a kinetic model of themochemical trans-
formation of a solid organic fuel under conditions of a
combustion chamber. This would allow proper choice
of the process modes and conditions for pretreatment
and combustion of solid organic fuels.
Analysis of the mechanism of complex physico-
chemical processes that occur in combustion of coals
involves evaluation of the significance of particular
process steps  and construction of a hierarchy of
this steps following preset criteria . To construct
a mathematical model of thermochemical transforma-
tion of a solid organic fuel, we used the following
process scheme. The process is subdivided into sever-
al relatively independent parallel-consecutive multi-
step stages (Fig. 1): drying and heating of a particle
until release or ignition of volatiles; release of vola-
tiles and their combustion near the particle; combus-
tion of the nonvolatile (coke) residue consisting of
organic and mineral matter .
The physicochemical model of thermal transforma-
tion of a solid organic fuel in a wide temperature
range suggests that drying of a moist material is char-
acterized by moisture evaporation from the bulk of a
coal particle, with the evaporation front moving inside
the particle as a phase transition front, under the influ-
ence of an increase in the temperature of the dry sur-
face. Since the moisture present in fuel forms bonds