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Show COMBINED EFFECTS OF PREHEAT AND OXYGEN ENRICHMENT METHANE, 2% Oz IN FLUE, 2400°F FLUE GAS TEMPERATURE ~r-----------------------------------~ Preheat Temperatura ("F) 80 2000 1500 -- --:::.=-_----------------- 1000~ ............ ~ .. /?,,/- ~ /80 ..J I W I ::;) I ~ I .,. I 20' 20 30 40 50 80 70 80 90 100 %OXYGEN IN ENRICHED AIR Figure 6 that for a 100% oxygen system, the overall economics of the preheated oxygen enriched combustion system, relative to that of preheating alone or 100% oxygen, could not be judged without analyzing both the costs of the recuperator and oxygen. This is best accomplished by evaluating the incremental economics of a recuperator to preheat enriched air. The benefits of preheating enriched air consist of (1) incremental fuel savings, (2) savings in equivalent pure oxygen associated with the fuel savings. Since the size of the recuperator is proportional to the amount of heat transferred from the flue gas to enriched air for given flue gas and preheat temperatures, the above benefits can be evaluated based on a unit amount of heat transferred. Figure 7 shows the combined savings for fuel and equivalent pure oxygen by preheating enriched air for various oxygen concentrations at three different ratios of oxygen cost to fuel cost, Y, expressed in $/ton O 2 t $/MMBTU. The base line curve (Y=O) corresponds to the fuel savings by recuperation without a credit for the associated enriched air savings. The incremental fuel savings per 1 MMBTU of heat transferred from the flue gas to the enriched combustion air is l/a MMBTU, where a is the available heat fraction. Since a increases with the level of oxygen enrichment the incremental fuel savings decreases with enrichment . For example, 3.3 MMBTU of fuel is saved by recovering 1 MMBTU of heat in the recuperator for air, while only 1.4 MMBTU of fuel is saved for 100% oxygen by recovering the same amount of heat. On the other hand a unit amount of fuel savings results in a grea~er amount of savings in equivalent pure oxygen with higher enrichment. The difference between the base line curve and each curve above is the value of enriched air savings . As the ratio of oxygen cost to fuel cost becomes greater, the 158 portion of savings from enriched air increases and hence the overall benefits. In all cases, however, the overall economic benefits of preheating enriched air diminish with the enrichment level. The economics of preheating enriched air is further penalized by the progressively smaller volume of enriched air available for preheating and the more stringent material requirements for the recuperator with the higher concentration of oxygen. Both the smaller volume of enriched air and the more severe material constraints translate into higher capital costs of heat recovery per unit amount of heat transferred. Thus, the rate of the return on investment becomes significantly lower for preheating of oxygen enriched air as compared with preheating of air. 20 40 12 eo OxYgenlfuel coet mIo (I/Ion O:! - I/MMBlu HHV) ao 'III OXYGEN IN ENRICHED AIR INCREMENTAL COST SAVINGS BY PREHEATING ENRICHED AIR Met"" .... 2% O:! In Flu • • 245a"F Flue Gee Tempemure • Equlvll.nl MMBtu ... eeI In luel end 0_" pe' MMBtu 01 heet - Figure 7 100 Figure 8 compares the net operating cost savings achieved by oxygen enrichment alone and by the combination of oxygen enrichment and preheating as a function of the ratio of oxygen cost to fuel cost. The assumed conditions are natural gas as the fuel, 24000 F flue gas temperature, and 2% excess oxygen in the flue gas. The apparent fuel savings are 22%, 41%, and 57% at oxygen enrichment levels of 25%, 35%, and 100% respectively at Y=O, which corresponds to zero oxygen cost. The net operating cost savings after subtracting the cost of equivalent pure oxygen, decrease linearly with Y and becomes zero at Y=15.2 for an~ enrichment levels. Preheating of air to 1000 F results in a fuel savings of 37%, which is independent of the oxygen to fuel cost ratio. If the current oxygen cost to fuel ratio ratio is 10, for example, then the net operating cost savings become about one third of the apparent fuel savings for oxygne enrichment. Although the net operating cost savings of oxygen enrichment are lower than that of preheating of air, the economics of oxygen enrichment without preheating can be better than that of preheating of air due to lower capital investment required. This point will be discussed further in a later section. The apparent fuel savings by preheating enriched air increase to 44%, 52% and 59% for |