OCR Text |
Show Major variables were oxygen content of the combustion air (21-50% vol.), enrichment of BFG (0-10% vol. NG), preheat of air and/or gas (20-500°C), furnace load (1.5-3 MW) and heat sink furnace temperature (800-1000°C). Continuous trials were made over a six week period in order to gain data of heat transfer efficiency with full information of temperature regimes, aerodynamics gas composition and pollutant emissions for the specified ranges of variables. Using the results of these trials and particularly the comparative heat transfer information, a previously developed well stirred mathematical model £9] of the system was employed to correlate the data. The confidence with which these results, which were obtained on a pilot scale system, may be translated to a full scale reheating furnace, is discussed in detail in the investigation report £8] where it is shown that providing the burner load distribution is maintained, the results should be independent of the furnace. In the presentation of the final results of the investigation [jQ two main techniques were used. The first was the presentation of heat transfer efficiency data normalized in relation to natural gas/air firing with no preheat. For example, let us assume that it is proposed to convert a reheating furnace from NG/air firing to say BFG/2.8% vol. NG firing with oxygen enriched air and to install recuperator facilities for gas and air preheating. It may be seen from fig. 5 that to maintain the overall thermal efficiency, for example in'the heating zone with an average stock temperature of 30 0°C, the rate of oxygen enrichment of the combustion air will depend upon the available preheat. Assuming an overall preheat 0.2 MW/MW total thermal input which would coincide with preheats of 500°C and 300°C for the fuel gas and comburant respectively, then it would be necessary to enrich the combustion air to achieve an oxygen concentration of 29% which is within the bounds of technical feasibility. Thus using these data it is possible to check the technical feasibility and to compare the technical merits, of beneficia-tion techniques for a wide variety of conversion situations. This on itself is not a statement on the economic viability and indeed due to cost variations in different steelworks and in different countries it would be clearly impossible to generalize economically. Nevertheless, it was possible to make assumptions for hypothetical cases in order to demonstrate how the data may be used for economic assessment purposes. For example in fig. 6 it is possible to pursue the above mentioned [9] J.B. MICHEL, R. PAYNE and P.A. ROBERTS: An optimization procedure for the use of fuel gases based on a well stirred mathematical model. Seminar on Heat and Mass Transfer in Metallurgical Systems, 3/7 Sept. 1979 Dubrovnik - 11- |