OCR Text |
Show Secondly, the amount of heat loss to combustion products is significantly reduced, because the flue gas volume in oxygenJfuel combustion systems is less than one third of that in airlfuel combustion system. Thirdly, the oxygen/fuel system allows much smaller equipments for gas transfer, due to less volume of oxygen and flue gas than that of the conventional system and due to removal of air handling equi pments. Fourthly, carbon-dioxide in the flue gas can be easily and efficiently removed and may be recovered, because the flue gas is mostly composed of CO2 and H20 in the oxygen/fuel combustion system. Figure 1 shows the reheating furnace system flow using a pure-oxygen/COG gas firing burner and the composition of flue gases. The concentration of carbon-dioxide in the dry flue gas in the pure-oxygen/COG system is 820/0 (9~ rv!) in the case of LNG), and such high concentrated carbon-dioxide can be taken out of the sys em using the one-step PSA(Pressure Swing Adsorption). compsition of flue gas 0 2 /COG air/COG CO 2 27% 10% H2O 67% 17% 4% 72% reheating furnace N2 C2 2% 1% COG j 800° C recuperator (boiler) gas dryer H20 (steam) I gascooler~ cooling tower , greengas ~ container 02/COG air/COG 82% 12% 12% 87% 6% 1% ~--~---- burner gas storage compressor Figure 1 Reheating furnace system ilsing 0 ICOG gas firing burner On the other hand, in slab reheating processes w here temperature uniformity is crucial, particularly in wide continuous reheating furnaces, we must consider some questions as follows: For one thing, short and high temperature flame causes hot spots on slabs and refractories (such as walls, ceiling and beams) in the furnace. What is more, low flame momentum results in poor gas recirculation and causes non-unifonn gas temperature and fluctuation in the furnace pressure. Furthennore, high flame temperature gives rise to high NOx emission, typically in COG firing, which contains nitrogen. Economically oxygen cost is another important factor, but here we limit the discussion to technical problems. |