Mei C., Zhou J., Peng X. Simulation and Optimization of Furnaces and Kilns for Nonferrous Metallurgical Engineering
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6 Simulation and Optimization of Electric Smelting Furnace
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Coupling Simulation of Four-field
in Flame Furnace
Ping Zhou, Zhuo Chen and Kai Xie
A tower-type zinc distillation furnace and a copper flash smelting furnace are
taken as the typical examples of single-phase combustion and two-phase
combustion, respectively. The applications of the coupling multi-field numerical
simulation are also described.
7.1
Introduction
In metallurgical engineering, chemical engineering, mechanical manufacturing,
electric power industries, etc., various industrial furnaces, in which fuels are the
heat sources, are generally called the flame furnace. In these furnaces, flame is
generally in direct contact with material. However, in some situations, flame has
to be isolated from the material to prevent oxidation of charges or workpieces. In
such cases, heat is transferred from the flame to the material via a partition wall.
Flame furnaces are used extensively in roasting, drying, melting, smelting,
heating and heat treatment of material or workpieces, since they have the
following features (Lu, 1995):
a) Flame furnaces are quite flexible to fuels. Various furnaces of different
configurations can be constructed according to the types and specifications of the
fuel, so as to meet the needs of the production processes.
b) In general, the fuel supply of flame furnaces is sufficient and the cost is low.
c) Flame furnaces have few limitations on the shape, size and specification of
charges or workpieces. All materials, from big metal ingot in hundreds of tons to
small particle, can be processed in flame furnaces.
Ping Zhou, Zhuo Chen and Kai Xie
d) Flame furnaces can be opevated at a wide range of femperatures. Given the
same fuel, different furnaces can be operated at low, mild or high temperatures
determined by the features and requirements of the production processes.
e) Flame furnaces often use direct heating means, but are also able to adopt
indirect means.
Flame furnaces play an important role in various industrial processes. However,
they consume a large amount of fuel, and are a major source of pollution to the
atmosphere in China.
Today, the major problems of flame furnaces in China are:
a) Low energy-utilization ratio: 15%̚30% in reverberatory smelting furnace,
50%̚65% in rotary kiln and 55%̚70% in boiler.
b) Serious atmospheric pollution. Among all pollutants to the atmosphere, 99%
NO
x
, 99% CO, 91% SO
2
, 78% CO
2
, 60% dust and 43% hydrocarbons are
produced by combustion of fossil fuels (Bao, 1997; Mao et al., 1998).
c) Low automation level.
To improve combustion efficiency, design new types of furnaces, optimize
their operation parameters, prevent accidents, and enhance the equipment
reliability and economy, It is imperative to thoroughly understand the velocity
and temperature distributions of gas-flow in the furnaces, so as to develop
reasonable calculation methods and design schemes, as well as establish optimal
operation modes. In other words, thermal engineering theories in flame furnaces
must be further studied.
Thermal engineering theory in flame furnaces is an interdisciplinary subject. It
correlates many subjects such as chemistry, fluid dynamics, combustion, heat and
mass transfer, metallurgical principles, numerical simulation methods, measuring
technique and the principles of furnace and kiln. What takes place inside a flame
furnace involves complet gas-solid multi-phase flow with chemical reactions,
non-isothermal conditions, and variable masses and diameters of particles.
Historically, research of flame furnaces follows the “empirical analogy
method → model experimental method
→ static simulation of single
parameter
→hologram simulation” method. These days most researchers adopt
the isothermal model experiment or simulation of a single parameter to study the
performance of flame furnaces. The designs of the furnaces, however, are still
limited to using lumped or averaged parameters, and their operation controls are
usually performed with the fixed-point monitoring method.
Flame furnaces are complicated, multi-variable, nonlinear and strong coupling
systems. The distributions of various parameters are highly complex. The
velocity field, heat-releasing field, temperature field and concentration fieldü
known as Ā the four-fieldā are correlated and mutually-restrained. Hence,
methods that integrate the “three transfer principles and two reaction dynamics
7 Coupling Simulation of Four-field in Flame Furnace
(i.e., the momentum, heat and mass transfer, and the dynamics of chemical and
high-temperature reactions) must be employed, to develop mathematical models
of the thermal and production process, and to carry out four-field hologram
simulations in flame furnaces. Based on the simulation results, optimization
experiments are performed to further exploit the equipment production potential,
and to intensify the production process. With these methods, both construction
and thermal processes of the entire equipment can be optimized to achieve high
system productivity, good product quality, long equipment life, low energy
consumption, and reduced pollutant emissions
(Mei et al., 1999; Ou et al., 2007;
Ma et al., 2006).
7.2
Simulation and Optimization of Combustion Chamber of
Tower-Type Zinc Distillation Furnace
Tower-type zinc distillation furnace is a key equipment in zinc pyrometallurgy,
and one of the most complicated equipment in nonferrous metallurgical industry.
Its equipment efficiency directly influences the refined zinc yield and energy
consumption.
In the tower-type zinc distillation furnace, coal-gas and air are supplied into
the combustion chamber in a fractional mode, to achieve delayed burning and
uniform temperature distribution, thus ensuring normal operation of tower trays.
Recent innovations of the distillation furnace include the adoption of large
tower trays, large furnace space and no column supporting structure. Although
it has been proven that the innovations are helpful for higher productivity, some
problems are encountered in practice. For example, the temperature in the upper
combustion chamber was found higher than that in the lower chamber. The
uneven temperature distribution, to some extent, has resulted in problems in
operations. To rectify the problems, series of attempts has been made, such as
increased flue draft, use of coal gas of higher heating value, and increased mass
flux of the secondary coal gas. Although certain improvements were achieved
in the operation of the furnaces, the problem of uneven temperature distribution
remained ansolved.
In order to achieve a uniform temperature distribution in the combustion
chamber, in-situ measurements and numerical simulations were combined to
study the velocity field, concentration field, heat release field and temperature
field in the combustion chamber of different structures. Based on the
measurements and simulation results, it is possible for the researchers to propose
an optimal structure of the combustion chamber, so that higher productivity and
longer equipment life can be achieved (Xie et al., 2003).
Ping Zhou, Zhuo Chen and Kai Xie
7.2.1
Physical model
As a large system, a tower-type zinc distillation furnace is made up of combustion
chambers, a set of tower trays, a condenser, an air preheater, a coal gas
(combustible gas produced from other industrial furnace) preheater, a flue and a
chimney. Every part influences and imposes restraints on each other. Two
combustion chambers are located on the east and west sides of the tower tray.
Due to symmetry, the computational domain consists of, only the west
combustion chamber, which includes air ducts and coal gas ducts away from
preheaters, as well as a portion of the flue.
For the computational domain, the south end is a wall dissipating heat into the
environment; the north is adjacent to the preheaters and the ascending flue, and
the east is the tower tray set. On the west wall, five levels of inlets are arranged
top to bottom, in the order of primary coal gas, primary air, secondary air,
secondary coal gas and tertiary air, A flue gas outlet is located on the bottom.
There are four ports (nozzles) for each level of inlets and outlets (refer to
Fig.7.1).
Fig.7.1 The sketch of the combustion chamber in a zinc distillation furnace
7 Coupling Simulation of Four-field in Flame Furnace
Fresh air from the global air inlet is redirected into three horizontal channels,
of which two streams flow up and down separately, and enter the combustion
space through the primary and secondary air nozzles; the third flows down along
the southern channel, then enters the combustion space through tertiary air
nozzles.
The coal gas from the global coal gas inlet is divided into two paths. One
flows up and enters the combustion space through the primary coal gas
nozzles. The other flows down into the northern channel and enters the
combustion space through secondary coal gas nozzles.
The coordinate system is shown in Fig.7.1. The origin is located at center of
the combustion chamber.
7.2.2
Mathematical model
In the tower-type zinc distillation furnace, coal gas is preheated to 600 , air is ć
preheated to 700 , and ć the temperature of the gas mixture in the combustion
chamber is over 1,000ćühigher than the ignition temperature. Hence in the
combustion chamber, reactions take place at a rather high rate once the fuel
comesin contact with air. Here, fuel and air are fed separately into the chamber, so
there is no premixing prior to combustion. This means the reaction rates between
the fuel and air are determined by their mixing and diffusion process, i.e., the
gas-phase combustion is i.e. diffusion-controlled process. So, a mixed-is-burnt
rapid chemical reaction model can be adopted.
The definition and equations describing the mixed-is-burnt model can be found
in Section 2.3.
7.2.3
Boundary conditions
In order to determine the boundary conditions of the combustion chamber, some
smelters carry out heat balance measurement for the tower-type zinc distillation
furnace. The major parameters are shown in Table 7.1.
Table 7.1 Inlet boundary conditions of coal gas and air
Mass fraction / %
Item
T
Temperature
/K
Flow rate
/m
3
gh
−1
Area
/m
2
Velocity
/mgs
−1
N
2
O
2
CO CO
2
H
2
CH
4
H
2
O
Coal
gas
873 751 0.1666 4.004 58.28 0 29.34 7.26 0.78 0.6 3.74
Air 973 943 0.1123 8.311 76.8 23.2 0 0 0 0 0
According to the test results, the flue pressure at the outlet of the computational
Ping Zhou, Zhuo Chen and Kai Xie
domain is -83.4Pa. The heat fluxes through the south and north walls are identical
180W/m
2
. Because the north wall is adjacent to preheaters and the ascending flue,
heat flux from the wall may be ignored. Therefore, adiabatic boundary is assumed
on the north wall.
The east wall of the tower tray set is of mixed boundary condition. The
tower trays are always filled with liquid and gaseals zinc. Liquid zinc has
high heat conductivity and latent heat of vaporization, whereby the
temperature of liquid zinc in the tower tray set generally does not vary greatly.
Therefore, the temperature T
0
at the inner wall of the tower tray is assumed to
be a constant and equal to the vaporization temperature of zinc, i. e., 920 . ć
The heat transfer from the combustion chamber to liquid zinc depends on the
temperature in the combustion chamber and can be calculated by means of the
following steps.
The tower tray is made of silicon carbide (SiC) and refractory coating. The
total thermal resistance of tower trays is given by:
2
2
1
1
λ
δ
λ
δ
R +=
(7.1)
where δ
1
and δ
2
represent the thickness of the tower tray and refractory coating
respectively; λ
1
and
λ
2
are the thermal conductivity of the tray and coating
respectively.
Therefore, the relationship between the heat flux
q through the tray wall and
the temperature
T outside the tray wall can be expressed as:
()
TT
R
q
0
−=
1
(7.2)
7.2.4
Simulation of the combustion
chamber prior to structure
optimization
The computational Combustion chamber
simulation is performed prior to structure
optimization. The computational grid is
shown in Fig.7.2.
Using the commercial CFD software
CFX4, multi-field simulation of the
combustion chamber is carried out. The
standard
k-
ε
turbulence model and the
mixed-is-burnt gas-phase combustion model
are used. The primary results are given in
Fig.7.3 and Table 7.2.
Fig. 7.2 Grid on vertical section of
combustion chamber
7 Coupling Simulation of Four-field in Flame Furnace
Fig. 7.3 Temperature distribution on the vertical section of x=−0.68m
Table 7.2 Predicted flow of air and coal gas nozzles prior to structure optimization
The ratio of nozzle flow to total flow /%
Item
No.1(south) No. 2 No. 3 No. 4(north) Σ
Primary coal gas 14.7 15.0 19.9 27.7 77.3
Secondary coal gas 3.4 5.3 7.0 7.0 22.7
Primary air 12.0 8.9 7.8 4.2 32.8
Secondary air 10.3 9.2 8.2 6.0 33.8
Tertiary air 9.3 10.4 8.1 5.6 33.4
Total coal gas 18.1 20.3 26.9 34.7 100
Total air 31.6 28.5 24.1 15.8 100
Fig.7.3 shows that the temperature distribution in the combustion chamber
tends to be triangular, that is, high temperature flames exist near the four primary
air nozzles and the maximum temperature is up to 2058.6K As the flue gas flows
downward, the temperature greatly decreases while the temperature remains
relatively high on top. In addition, the flames appear to extend into the flue. The
triangular temperature distribution indicates that a portion of the trays operates at
high loads while others at low loads, implying negative effects on yield and
longevity of the trays.
Ping Zhou, Zhuo Chen and Kai Xie
Table 7.2 clearly reveals that the ineffective allocation of air and coal gas
passages is the major reason that the triangular temperature distribution in the
combustion chamber is formed. Furthermore, the measurement results of the
tower-type zinc distillation furnace in operation indicate the temperature
distribution outside the wall appears to have the same characteristics as the
computational results. For the temperature outside the wall, the maximum
deviation between simulation results and measured data is less than 3.5%.
7.2.5
Structure simulation and optimization of combustion chamber
In order to reallocate the air and coal gas passages, the sizes of air and coal gas
nozzles are adjusted and dozens of designs are obtained. For every design,
multi-field coupling simulations of the combustion chamber are performed. By
analyzing the simulation results in detail, the optimal design is identified where
temperature is uniformly distributed in the combustion chamber. The simulation
results of the optimal design are shown in Fig. 7.4.
Fig. 7.4 Temperature distribution on the vertical section of x=−0.68m
after structure optimization