October 2012 Tube Products International
77
The need for material upgrade
in recuperators in steel mills
by Johan Wallin and Tommy Sand, Sandvik Materials Technology, Sandviken, Sweden
Introduction
Summary
A steel mill in Turkey experienced low waste energy recovery
and short lifetime from its recuperator in one of its rolling
mills. By upgrading to Sandvik 253 MA, the mill lowered its
fuel consumption and extended the tube lifetime more than
three times.
History and background of the application
A recuperator is a heat exchanging gas-to-gas device used
in high temperature environment usually over 500°C. A
recuperator can be used in many different industries where
energy recovery is possible. The recuperators used in steel
mills are usually tube/shell heat exchangers, where the
convective heat exchange takes place between the hot flue
gases and the cold incoming combustion air through metallic
tube walls.
Tubes carry the cold air for combustion to be pre-heated; the
outside contains the waste heat stream. The hot gases are
cooled by the incoming combustion air, which now carries
additional energy into the combustion chamber. This is energy
that does not have to be supplied by the fuel; consequently,
less fuel is burned for a given furnace loading. This energy
saving will in turn reduce fuel consumption and consequently,
fuel costs. An example of fuel savings can be seen in
Figure 1
[3]
.
The saving in fuel also means a decrease in combustion
air and therefore stack losses are decreased not only by
lowering the stack gas temperatures but also by discharging
smaller quantities of exhaust gas. For the recuperator used
in conjunction with the hot rolling mills of steel mills, the tube
steel grades that historically have been used range from low
alloyed carbon steel to high alloyed high temperature stainless
steel grades. Depending on the steel grade and the service
parameters, the lifetime and efficiency of the recuperator
tubes varies to a large extent.
The environment in which the recuperator operates are
relatively extreme, with temperatures of the flue gases coming
from the furnace ranging typically between 925°C and 1,050°C
[1]
. For a typical recuperator in connection to an annealing
furnace at a rolling mill, you will have corrosion problems
originating from the flue gases at these high temperatures.
Apart from CO
2
, H
2
O, N
2
and O
2
, the hot flue gases can
contain several corrosive compounds and impurities such as
SO
x
, Cl
–
and H
2
S, depending on what fuel is used. Common
fuel types used in furnaces in a steel mill differ from mill to mill
and the type of furnace. Typically, different kinds of fossil fuels
like oils, coke, coal, lignite and natural gas are used.
The tubes will be exposed to, for example, oxidation,
sulphidation, nitridation and carburisation environments,
in which different corrosion mechanism will lead to the
destruction of the tubes (see Figure 2).
One of the energy/fuel saving goals with the furnace operation
is to recover as much heat/energy from the flue gases as
possible, by preheating the incoming air as much as possible.
This will then in the end reduce the fuel cost. At the same
time, another task has been to extend the life/service time of
the recuperator tubes. The recuperator efficiency has been
one of the top priorities from an energy saving point of view
in steel mills, and has been under constant development and
improvement for the past few decades.
Table 1
: Service parameters for the recuperator at the rolling mill furnace
Recuperator service conditions
Service parameters
Shell side
Tube side
Media
CO
2
, N
2
, H
2
O
Fresh combustion air
Impurities
Cl
–
, H
2
S
N/A
Temp in (C°)
800-850
0-25
Temp out (C°)
450-500
480
Dimension
OD x WT x L (mm)
N/A
45 x 3 x 3,250
45 x 3 x 3,301
Figure 1
:
Savings in fuel
consumption
at different
waste gas and
air preheat
temperatures
