OCR Text |
Show cal~br~te the. accuracy ~f the computations. Positive feedback to verify and adjust for changes in emissivity, non~ tOlchi?metnc CO~~ustIon, ~cale fonnation rate, burner misadjustment and other deviations from theoretical estimates IS reqUIred to optImIze heatmg practices. Several concepts have been researched but the industry claims the need has not been satisfied. Measurement of steel temperature in a reheat furnace is subject to errors caused by the varying thicknesses of the scale laye~ on t.he steel. The measurement can be affected by the furnace atmosphere in the area where the m~asurement IS b~mg t~en . A device is needed to directly measure the temperature of steel in the furnace as it is b~mg heated. ThIs deVIce must be unaffected by the hot combustion gases surrounding the steel and the varying thlc~esses o~ scale on the swface of the steel. Additionally, the device must be robust to withstand the unfriendly envlf~n.ment I~ ~~ch it must operate. While several manufacturers have produced and marketed such instrumentation, d~ablhty, r~habl!lty, accuracy and repeatability have not been acceptable. It is recommended that research be ~ec~ed to eIther Improve existing equipment or develop a measurement system that does not depend on optical vlewmg of the steel. 4.2.1.5 Combustion Completion Measurement. Combustion efficiency is maximized when the fuel is ~urne~ to completion with a minimum of excess air. A fuel cost penalty results if either too much air or too much fuel IS put mto the furnace. Since combustibles or excess air in the flue gas from anyone zone affect the measurement of combustion in succeeding downstream zones in the furnace, each zone must be monitored and controlled separately, thereby requiring a multiplicity of instruments in a multiple zone, multiple burner furnace. Several manufacturers have m~keted i~strumentation to attempt to measure and control the completeness of combustion. None of these has been entIrely satIsfactory because they have failed to meet one or more of the criteria for durability, reliability, accuracy, or repeatability. However, if a reliable instrument is developed to look at the flame to detennine completion of combustion, excess air could be better controlled, with an attendant fuel saving. It is recommended that a research effort be directed to develop an instrument capable of measuring completeness of combustion and determining proper air fuel ratio, taking into account the following: • Flame length and size, which need not be constant as a function of fuel content and turndown ratio. The length of the flame and the point where any test instrument must be positioned changes as the rate of fuel input is varied. • Variable fuel composition. By-product fuels are known to vary appreciably in analysis and Btu content. Similar variations occur to a larger degree in purchased fuels . • Reliability-based design criteria, including attention to water cooling system design, clouding of lenses, and other problems inherent to the hot, dirty, vibration prone reheat furnace environment. 4.2.1.6 Continuous Annealing Furnace Burners. Continuous annealing furnaces are very large, costly, and inefficient. Expensive radiant tube burners must be used throughout because of requirements for various furnace atmospheres. A direct-fired burner that would heat the steel much more rapidly than the present radiant burners, and that would not oxidize, decarburize, or carburize the steel, would effect major fuel savings in the continuous anneal process, even if applicable in only a few furnace sections. This would not be an easy process to achieve, but it would most certainly have significant benefits, both in furnace size and thenl1al efficiency. Areas that promise possible efficiency gains include: • Identifying new methods of rapidly heating strip to temperatures below those where furnace atmospheres would result in oxidation, decarburization, or carburization of the product. Pulse heating may be an application in this area. • New direct firing methods at higher temperatures that would not cause adverse effects on the product. 4.2.1. 7 Computer Modeling of Reheat Furnace Burner Flames. The accuracy of the computer models for predicting the steel temperatures developed in the furnace depends on the model for flame radiation and convection in the furnace. The advent of the many different types of low NOx burners, as well as the low NOx oxygen enriched burners, complicates the problem. of developing accurate .models to predict the heat transfer of these burn.ers. The ~ac~ that varying firing rates and ambIent temperatures occur m the reheat furnace adds to the problem. Any maccuracles m the calculation of the heat transfer affected by the bunlers will a:ITect the accuracy of the reheat furnace computer model. 10 |