OCR Text |
Show These models have been partially validated at baseline with furnace design and field operating data. The models were then used to predict two scenarios: gas reburn with and without OEA. Furnace Process Modelling The costs and benefits of applying the proposed technologies is under evaluation. This preliminary assessment is based on the following approach which serves as a model for the first phase of the proposed work: • The development of a reference furnace specification and flow sheet which is typical of m o d e m practice • A parametric study of the effects (heat transfer and thermally efficiency improvement and the resulting N O x increase due to high temperature operation) of oxygen enrichment or oxy-fuel boost burner use applied to the heat zones • The effectiveness of GR as the primary control of NOx and CO. Reference Furnace A 115 TPH reference billet furnace has been defined which is typical of furnaces found in both mini-mill and integrated steel. The model furnace is of modern design and high efficiency (about 1.2 MMBtu/ton continuous or about 1.5 MMBtu/ton average with mill down time of about 2 5 % ) . The recuperated furnace preheats combustion air to 1000°F and exhausts flue gas at 1600°F. T w o applications using gas reburning and/or oxygen as shown in Figure 6, have been studied: • A gas fired furnace operating in a nonattainment area for ozone and requiring 5 0 % N O x reduction • A furnace requiring about a 20% increase in billet heating capacity, and a N O x reduction of 30 to 4 0 % relative to the N O x baseline to achieve offset emissions Computational Fluid Dynamic (CFD) Model The CFD modeling is used to establish steel and flue gas temperature profiles for key operating conditions (oxygen enrichment and gas reburn) and to support the location of reburn and overfire air (OFA) injectors. The furnace is sufficiently wide to be modeled as two dimensional (2D). This implies that the heat loss from the side walls can be neglected. This is reasonable since losses account for about 4 % of the gross fuel energy input, and the side walls are small as compared to the roof and bottom walls. Another implication is the amount of energy distribution by radiation as a result of the side walls. Since the refractory walls have very little heat loss, they are close to perfectly reflective walls of the third coordinate direction as assumed in the 2 D model. Therefore, the 2 D result will be representative of the true situation except, perhaps, in a small steel volume immediately next to the side walls. The bottom zone is assumed to have no radiative communication with the top heat zone except through the steel, and their flue gas flows are assumed not to merge until the flows are vertically directed. In reality, the furnace width increases 6 |