The recuperators were tested on the test rig at the burner test facility.
The demonstrator can be seen as a silvery cube.
© BFI
Ceramic high-temperature heat exchangers
03.08.2016
Ceramic cube reduces gas losses
Scientists have developed a new ceramic heat exchanger for industrial
furnaces. This can transmit considerably more exhaust heat to the
combustion media than the metal recuperators currently used in
high-temperature furnaces in the process industry. The new ceramic
recuperator thereby decreases the exhaust gas losses to less than 25 per
cent.
Industrial furnaces are used for heating or heat treating steel, glass,
ceramic and non-ferrous metals. Using recuperators, the energy efficiency
of such industrial process furnaces can be improved: they use the exhaust
gas heat to preheat the combustion media. In collaboration with industry
partners, the VDEh Betriebsforschungsinstitut (BFI) has developed a new,
more fuel-efficient heating technology for such applications. Their goal
was to reduce the energy consumption through better utilisation of the waste heat in the heating system. They
constructed a new, high-temperature ceramic recuperator to supplement an existing refractory burner. This
consists of a ceramic cube with 30 centimetre-long edges. The exhaust gas and the combustion air being heated
flow through it in a cross flow, whereby the hot exhaust gas warms up the combustion air.
The heat exchanger elements considerably
shrink during the course of the thermal
treatment from the wood fibre model to the
silicon carbide cube: initial state (WT3.5),
carbonised (WT3.4) and silicised state
(WT3.3). After treatment, the elements have
30-centimetre-long edges.
© Schunk Kohlenstofftechnik GmbH
Ceramic improves the efficiency of industrial furnaces
The researchers tested the performance of the new heating system using the BFI's burner test facility. In this new
system, the recuperator and burner are connected as separate components by short lines. This enables them to
be adapted and combined very flexibly. The new system is designed for a capacity of 300 kilowatts and
temperatures above 1,000 degrees Celsius.
By using the ceramic recuperator, the exhaust gas losses in high-temperature furnaces can be reduced to below
25 per cent. Previously used metallic recuperators provide a maximum combustion air temperature of 500 to 600
degrees Celsius and attain flue gas losses of almost 40 per cent. When used with heating and heat treatment
furnaces with exhaust temperatures above 1,000 degrees Celsius, the new ceramic systems can reduce the
exhaust gas heat losses by about 15 per cent compared with systems with conventional heat recovery.
Creation of the ceramic heat exchanger
Wood as a renewable resourceraw material provides the developers with the starting material for producing the
Wood as a renewable resourceraw material provides the developers with the starting material for producing the
biogenic recuperators. They cut medium- and high-density fibreboard (MDF and HDF) to the required dimensions
and then bond them with wood glue to form a blank with the desired structure. They are then carbonised in a
controlled atmosphere furnace, whereby the components are pyrolysed at temperatures up to about 1,000
degrees Celsius. This is then followed by graphitisation, which occurs through further heat treatment at
temperatures up to about 2,000 degrees Celsius.
As a result of this heat treatment, the structured heat exchanger cube loses much of its volume and mass. The
element shrinks uniformly in its breadth and depth by about 23 per cent, and by even about 44 per cent vertically.
The total mass reduces by around 70 per cent. As the density reduces the porosity increases sharply. This porous
carbon template is now ready for the next treatment step: the siliconisation. Here the component is infiltrated with
liquid silicon, which reacts with the carbon to form silicon carbide. The cubes take on 3 to 3.5 times their own
weight with silicon during this process. The researchers have adapted the furnace program so that this large
amount of silicon can homogeneously penetrate the complex component and a uniform material is obtained.
The final ceramic heat exchanger elements made of reaction-bonded silicon-infiltrated silicon carbide (SiSiC) are
particularly hard, have good thermal conductivity and low thermal expansion. In addition, the material is highly
resistant to wear, temperature changes as well as oxidation and corrosion at high temperatures. SiSiC can be
used at temperatures of up to 1,380 degrees Celsius.
The researchers also investigated eight different manufacturing processes and then constructed a demonstrator.