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Show OXYGEN TECHNIQUES IN COMBUSTION PROCESSES: A REVIEW OF PRACTICAL CASE STUDIES WITHIN THE BOC GROUP Ross M. Hall BOC Ltd. Baildon, Shipley York England THE COMBUSTION OF FUEL depends basically on the presence of oxygen. Why then do the majority of industrialists insist on using 4 parts nitrogen with the addition of 1 part oxygen to burn fuel? Initially this was because air was the only available substance and systems were developed which had to cater for handling and disposing of the extraneous gas contained in air. It is now some 100 years since oxygen was separated from air, albeit in small quantities, and in the past 30 years the availability of pure oxygen has been virtually limitless. Why then is there still resistance to replacing air with oxygen for combustion systems? To answer this basic question is not as simple as the question itself. What answer ever is? We need to look at the benefits that using oxygen gives over those achieved with air and also to look at perceived disadvantages of this substitution. ADVANTAGES OF OXYGEN SYSTEMS There are many theoretical and practical advantages gained by using oxygen instead of air and these can be categorised quite readily: a) Inc~eased flame temperature b) Reduction in waste gas volumes and subsequent reduction in gas handling equipment. c) Reduction in excess air requirements in a furnace. d) Exhaust gas temperatures reduced. e) Increase heat transfer capacity. f) Increase flame stability. g) Reduced ignition temperature of fuels. The only criterion that industrialists look for in initial assessment of possible oxygen use is cost effectiveness. Thus in some way, the use of oxygen has to produce 245 a positive financial contribution to a process operation for the change to oxygen to occur. The requirement of the oxygen producing companies is to demonstrate the ways in which oxygen can be utilised to bring about a costeffective result. There are standard targets and end-results which can save money and they are generally classified as: a) energy saving b) productivity c) improved product qualities d) reduced refractory wear Looked at in isolation there is often difficulty in reaching a cost-effective solution on these parameters alone if they are taken at purely face value. There are addition~l benefits which may not be apparent at the outset. These can include higher yield levels, reduced wear on downstream equipment such as rolling mills, reduced emission levels which obviates the need for expensive scrubbing equipment. In general the perceived disadvantages of oxygen are categorised as cost and possible refractory damage. The cost factor is usually the prime consideration when oxygen use is evaluated and in most cases a positive case can be made. Refractory damage and wear is often cited as a by-product of oxygen usage. Provided the system is correctly installed and operated in accordance with agreed parameters refractory damage does not occur. If, however, the original equipment or the oxygen system is not functioning correctly, e.g. mis -directed burners, then the use of oxygen will exacerbate the problem which would have occurred in the first place. Refractory wear must be calculated on the basis of tonnage produced by the unit during the life of the refractory and not solely on a time figure. |
