Abstract
Utilising the oxyfuel practice for CH4-fuelled combustion has positive effects on the emissions, efficiency and cost of high temperature furnace practices. However, especially in older installations, oxyfuel usage requires retrofitting and alters the atmosphere in which the oxidation of the steel occurs, when compared to using air as the oxidiser. Stainless steel slab oxide growth during reheating was studied in different atmospheres. The simulated post-burn atmospheres from oxyfuel, lean oxyfuel and air-fuel practices were used to compare oxide-scale layer growth and morphology during simulated typical AISI 304 stainless steel slab reheating prior to hot rolling. Thermogravimetric measurements, glow discharge optical emission spectrometer (GDOES) and field-emission scanning electron microscope energy dispersive X-ray (FESEM-EDS) methodology were applied to discern differences between oxide growth and inner oxide layer morphology between the three practices. Switching from air to oxyfuel practice at a single temperature had the same increasing effect on the scale formation amount as a 25 °C temperature increase in air atmosphere. Inner oxide layer depth profiling revealed C, Si and Ni to be the main elements that differed between temperatures and atmospheres. A morphology study showed Si and Ni behaviour to be linked to breakaway oxidation.
Highlights
Reheat and annealing furnaces during steel manufacturing are usually fuelled by propane or methane (NG) [1,2,3,4]
The thermogravimetric results and glow discharge optical emission spectrometer (GDOES) depth profiles for the inner oxide layer, and the field-emission scanning electron microscope (FESEM) and energy dispersive X-ray (EDS) images with analyses are presented
The x-axis is cropped to start at 4800 s in the TG figures, as no measured oxidation happened before this point in the tests
Summary
Reheat and annealing furnaces during steel manufacturing are usually fuelled by propane or methane (NG) [1,2,3,4]. Reheating furnaces contributes to a large proportion of the steel industry’s energy usage and emissions, yet only around 40% of the energy from burning fuels goes to heating the steel [5]. While recuperating heat from effluent furnace gases is used to preheat the combustion air [6], nitrogen is not part of the combustion process, and heat is still wasted to heat a nonreacting ingredient. By utilising the oxyfuel practice, air is replaced with pure oxygen for the combustion. This leads to inherently lowered CO2 [9] emissions from not using the fuel to heat nitrogen, and if effluent gas recirculation is used, to lower NOx emissions [9]. In using oxyfuel as the oxidiser, the atmosphere resulting from the combustion is altered, and leads to an increase in water vapour content when compared to using air as the oxidiser [10]
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