An important part of the hot rolling process is the reheating of steel products to target temperature using the combustion of natural gas in a furnace. The reheat furnace is the most energy intensive part of the hot rolling mill; the reheat furnace commonly comes second only to the energy required by the blast furnace in the entire steelmaking process. Methods of reducing fuel and energy usage for industrial furnaces relate back to the combustion process. Increasing the combustion air temperature using recuperative or regenerative processes on the flue gas is one commonly implemented method. Control of the combustion itself is also important; while stoichiometric mixing is the theoretical ideal, turbulent mixing and dilution requires some excess air, commonly assumed as 10% by industrial rules of thumb. Improvement of mixing could reduce the necessary excess air and improve fuel economy. Oxygen enrichment of the combustion air beyond 21 vol% oxygen is also used to improve mixing. In this scenario, the molar fraction of oxygen is increased, replacing nitrogen in the combustion air. Out of the three, the least implemented in steelmaking is the use of oxygen enrichment.Oxygen enrichment can be used with high entrainment of reaction product gases to distribute the combustion reaction area. This entrainment of furnace gases slows the reaction, which lowers the overall temperature and spreads the high-temperature area throughout the zone for a more uniform heating to the product. Higher temperatures may yield higher heating rates, but they will also lead to the generation of larger amounts of nitrous oxides and may cause damage to the internal structures of the furnace. Natural gas is largely used inside reheat furnaces, but it does not contribute anything to the reaction besides being a diluent. Nitrogen reduces the possibility of oxygen meeting methane and requires energy to be brought from its original temperature to the temperature of the combustion products6. As such, any reduction in nitrogen content via oxygen enrichment can increase the flame and gas temperatures within the furnace through basic thermodynamic principles. This is usually accompanied by a reduction in the firing rate of the furnace and an overall fuel savings. There are many studies on reheating furnaces focused on modeling the combustion between air and a fuel, usually natural gas or coke oven gas. However, there are less studies regarding oxygen enrichment, which can be implemented in many ways.The entire reheating furnace process is quite complex. Implementing operational changes can be costly due to the downtime required and the time required to run any necessary experiments. For example, it can be laborious and expensive to add thermocouples into a slab to study the temperature evolution of slab as they traverse the furnace. As such, many researchers have turned to computational methods to study the reheating furnace and the benefits of various operational changes. While many of the computational methods are simplified models based on empirical work, when adequately validated, these methods can still give an accurate view of the reheating furnace process. For the purposes of this paper, the numerical modeling of various burner configurations was undertaken using computational fluid dynamics to use oxygen enrichment or oxy-fuel to maintain a given slab heating rate while reducing fuel usage. Medium oxygen enrichment (46 vol% O2) and oxy-fuel (100 vol% O2) were applied to the preheating zone which has the highest fuel usage of the three zones.

Application of Oxygen-Enriched Combustion in an Industrial Reheating Furnace Using CFD

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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