Low scale reheating of semi-finished metal products in furnaces with recuperative burners

Highlights
• Low scale reheating of copper and steel products applying fuel rich combustion.
• Post-combustion and heat transfer in recuperative burners.
• Low NOX emissions due to fuel rich combustion and post-combustion.
• Cost reduction for reheating of high-priced metal products.

Currently, direct gas fired reheating furnaces are most popular for heating slabs or billets prior for hot working. Additionally, industrial furnaces for reheating semi-finished metal products involve the same. As a consequence, oxidation of the metals exposed to the furnace atmosphere, by excess oxygen, carbon dioxide and vapor is inescapable. This oxidation has been seen to cause significant material loss and additional work during furnace operation and in further processing. To counteract this, researchers have previously attempted various reheating treatment options such as heating with fuel rich combustion and recuperative burners. Unfortunately, most of this published work does not focus on the post-combustion of the off-gas or technical limits of the employed recuperative burner concept. Furthermore, there is minimal documentation regarding reheating concepts in furnaces.
Researchers at RWTH Aachen University in Germany: Christian Schwotzer, Matthias Schnitzler, and Herbert Pfeifer proposed a study to quantify scale reduction due to fuel rich combustion for different reheating processes. Moreover, they hoped to cross-examine the influence of fuel rich combustion and post-combustion on the thermal load and the off-gas emissions of the concept for recuperative burners and the economic implications compared to conventional reheating concepts with fuel lean combustion. To achieve this, they hoped to carry out a detailed study about the reheating of copper billets and steel in direct fired furnaces with recuperative burners. Their work is currently published in the research journal, Applied Thermal Engineering.
The researchers commenced their experiments by quantifying the mass change due to scale formation depending on the air ratio of the reheating process. Next, they investigated post-combustion in an annular gap to localize the maximum thermal load on the recuperator material and the off-gas emissions depending on the temperature of the off-gas and air. A comparison of the economic potential of the low scale reheating concept to that of a conventional reheating process with fuel lean combustion based on the costs for fuel and metal loss was then done.
The authors observed that at a constant primary and total air ratio, the maximum temperature in the post-combustion chamber depended on the temperature of the off-gas and the secondary combustion air. Moreover, it was noted that the maximum temperature in the annular gap was lower than the off-gas temperature at the inlet. In relation to the energy efficiency comparisons with conventional reheating concepts, it was seen that a lower total efficiency for the low scale reheating concept was recorded since for the conventional concept, the fuel does not get fully exploited in the primary combustion.

The RWTH Aachen University study successfully presented the examination of the concept for the reheating of copper and steel in furnaces with metallic and ceramic recuperative burners. The results here predict a positive impact on the technical applicability and economic significance of the concept for industrial furnaces with recuperative burners for the reheating of copper and steel. Altogether, the great potential for the low scale reheating concept for the reheating of steel if employed in industrial steel applications will guarantee increased productivity and profitability.

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