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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.
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 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.
Efficient heating methods
A compact regenerator developed by the BFI for recovering exhaust heat is used in combination with a flat flame burner as a heating system (ROREBS).
BINE Projektinfo brochure 15/2009