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Show > % % ~ CD ~ ~ ~ Z W ~ W ~ :; 0 W ~ oJ W ::J ~ 6 5 4 3 %~/ PREHEAT TEMP (oF) .-----------------------------~~no FUEL REQUIREMENT TO PROVIDE 1 MMBTU OF AVAILABLE HEAT FUEL CH4 <nIN FLUE 2% ~/500 ~/1000 -_.- ~~--------------------------_11oo/~ AVAILABLE HEAT 2000 2200 2400 2600 2800 3000 FLUE GAS TEMPERATURE (oF) Figure 1 excess oxygen in the flue gas and, to a lesser degree, on the type of fuel used. When the concentration of excess oxygen in the flue gas is increased for both the air and oxygen enriched combustion systems to the same level, fuel efficiency of the air system deteriorates much faster than that for the oxygen enriched system. Consequently, fuel savings by switching from air to oxygen enriched air become greater when the concentration of excess oxygen in the flue gas is higher. Figure 2 illustrates energy savings at different enrichment levels for combustion conditions leading to 30 to 80% fuel savings with 100% oxygen. Since the energy savings at a given enrichment level is uniquely related to the energy savings at 100% oxygen, each curve represents any combination of combustion conditions leading to the same energy savings at 100% oxygen. For natural gas and No.2 fuel oil, the flue gas temperatures corresponding to different energy savings at 100% oxygen are indicated in the graph. For example, the same 50% energy savings are obtainable at flue gas temperatures of 2200 0 F and 2280 0 F for natural gas and No.6 fuel oil respectively. For these conditions about 33% energy savings, or 66% of the savings achieved with 100% oxygen, are possible at a 35% oxygen enrichment level. Thus, it is not critical to use high purity oxygen to obtain substantial energy savings, even though the greatest savings are always achieved at 100% oxygen. The apparent levelingoff of energy savings at high concentrations of oxygen, however, should not be misinterpreted as diminishing return in using oxygen . The specific energy savings, or the energy savings achieved per unit amount of pure oxygen used, remain the 154 same at any enrichment level as long as the flue gas temperature, flue oxygen concentration and furnace heat load are kept constant. The pure oxygen portion of the oxygen contained in enriched air, assuming that it is a mixture of dry air and pure oxygen, is often termed as equivalent pure oxygen. Since the portion of the oxygen coming from air does not contribute to the reduction of sensible heat loss, it is important to use equivalent pure oxygen as the basis for comparison in evaluating the effects of different enrichment levels. FUEL SAVINOS WITH OXYGEN ENRICHMENT II: C II: ~ °II) "z ~ 40 ...I ~ if. 30 20 10 0 20 80 80 100 % <>2 IN ENRICHED AIR Figure 2 Figure 3 shows the specific fuel savings, R, expressed in MMBTU per ton of equivalent pure oXY8en for a flue gas temperature range between 300 F and 30000F using methane as a fuel with 2% excess oxygen in the flue gas. Each curve represents fuel savings against a base air case with a different combustion air preheat temperature. For example if the flue gas temperature is 24000 F, R = 15.2 and 5.1 MMBTU/ton 02 against cold air and 10000F air respectively. Specific fuel savings is an important parameter to judge the economics of oxygen enrichment. If the costs of fuel and oxygen are $4 per MMBTU and $40 per ton of equivalent pure oxygen, then the break-even fuel savings becomes 10 MMBTU/ton 02. In the above example oxygen enrichment would result in $21 saving per ton of equivalent pure oxygen used against cold air, while $20 net loss would result if the air is preheated to 10000 F. Since specific fuel savings are independent of the level of oxygen enrichment for the fixed combustion conditions described before, Figure 3 can be used as a convenient screening guide in considering oxygen enrichment for energy savings for any level of oxygen enrichment. The foregoing discussions assumed the same flue gas temperature before and after oxygen enrichment. Although it is a reasonable assumption for most batch type furnaces, significant changes take place for continuous type process furnaces with counter-current heat transfer sections. Continuous steel reheating furnaces and unit melters for glass are examples of such furnaces. In these furnaces, hot combustion |