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Show 2,000 1,500 1,000 o 50 100 150 200 New Firing Rate (MMBtu/hr) Approxlma'e Aan.ea of To'al Ina'aned Coa'a for Aecupe .. 'orx and Ox,. en Enriched Combua,lon S,a'ema for Ae'ro'" Figure 10 enriched combustion systems are significantly greater than those with the conventional recuperators for high levels of oxygen enrichment, the net savings after subtracting the cost of equivalent pure oxygen are often lower than those with recuperators. The capital cost for an oxygen enriched combustion is, however, much lower. Therefore, the payback periods for oxygen enriched combustion can be significantly shorter than those for recuperators, making capital investment more attractive. The foregoing discussions are solely based on the fuel savings and cost of oxygen. The differences in maintenance costs and the other benefits beyond fuel savings have to be considered for proper evaluation of oxygen enriched combustion. Since fuel savings with oxygen enriched combustion are primarily achieved by a direct reduction in flue heat losses, there is no dependency on the performance of heat recovery equipment. By comparison, the fuel savings with recuperators vary depending on the efficiency of heat transfer from the hot flue has to the heat recovery system. Since the air preheat temperature can significantly fluctuate with the furnace temperature and the firing rate, the performance of the equipment is more difficult to predict and can be expected to deteriorate with time. This is particularly true when the flue gas contains corrosive gases and particulates. The faster heating available with oxygen enriched combustion can be used to advantage for a production rate increase or for additional fuel savings. The productivity increase can often be advantageous, even when an overall production increase in not required, to facilitate operating flexibility and potentially reduce man-hour expenditures per unit amount of product. Faster heating can also significantly reduce the requirement for preheating furnaces or kilns and their associated fuel and man-hour inefficiencies. 162 SUMMARY Oxygen enriched combustion has gained wide acceptance in recent years for fuel savings and productivity improvements in a broad range of industrial furnaces. An analysis of key technical and economic factors has been conducted with the following results: o Oxygen enriched combustion technologies to control the high flame temperature and resulting high NO emission are available today. The use 6f up to 100% oxygen is technically possible in virtually all industrial furnaces for fuel savings and productivity improvements. o Equivalent pure oxygen in oxygen enriched air is the key parameter influencing process economics. The incremental fuel savings per unit amount of equivalent pure oxygen are constant regardless of enrichment level for most applications where flue gas temperature remains relatively constant. For continuous type furnaces with unfired counter-current preheat sections, fuel savings per unit amount of equivalent pure oxygen are enhanced substantially due to the reduction in flue gas temperature, while incremental fuel savings diminish with enrichment level. o Since substantial benefits of oxygen enriched combustion are achieved at relatively low enrichment levels of, say 35% O 2 or above, the choice of optimum level of oxygen enrichment depends primarily on the cost of equivalent pure oxygen from different oxygen supply methods. The lowest cost equivalent pure oxygen available today is from large scale cryogenic air separation plants. For small furnace applications with a steady demand pattern, membrane and PSA systems can offer better economics than delivered liquid oxygen. Advancements in membrane and PSA technologies may offer some potential in providing relatively low volume oxygen enriched air at costs approaching that of a modern large scale cryogenic air separation plant. o There is an inherent advantage of oxygen enriched combustion at normal process loads and an increasing advantage at higher productivity conditions where the furnace gas temperature increases significantly due to the greater heat transfer requirement. A high flame temperature is not necessarily important to achieve a higher heat transfer rate with oxygen enriched combustion. "Low flame temperature" oxygen enriched combustion is preferred in furnaces where both uniform heating and high heat transfer efficiencies are required . o Combined preheating and oxygen enriched combustion increase the overall fuel savings substantially at relatively low levels of oxygen enrichment, while at high levels of |
