OCR Text |
Show COMBUSTION MEASUREMENTS IN AN INDUSTRIAL GAS FIRED ALUMINUM RECYCLING FURNACE 6_ are reported here. Table 1 presents a summary of the stack measurements in the furnace. It includes the furnace settings for methane, air, and oxygen flow rates in standard cubic feet per hour (SCFH) as obtained from the furnace operators. Based on these flow rates, the overall stoichiometry (the variable "Stoich" included in Table 1) and dry concentrations of oxygen and carbon dioxide were calculated. The last five columns in Table 1 are the average measured results for the dry concentrations of 02, C 0 2 , N O , and C O , and gas temperature. N O concentrations are presented for the actual oxygen concentration (uncorrected for variations in oxygen concentration). Average exhaust gas temperatures varied between a low of 1558°F (1121K) and a high of 2281°F (1523K) for the conditions of low-fire and highest oxygen enrichment (35.4%), respectively. Furthermore, the differences in exhaust gas temperature for the high-fire conditions were not significant. With the exception of the highest enrichment case, average temperatures were about 2132°F (1440K) for the high-fire settings. It is also observed in Table 1 that changes in furnace stoichiometry were important in controlling the overall N O production. The highest N O level (3509 ppmvd) was observed for the normal high-fire condition when the temperatures were high in the furnace and there was enough excess air available (stoichiometry > 2.2). A significant reduction in N O production (from 3509 to 1390 ppmvd) was observed when the stoichiometry was reduced (from 2.22 to 2.01). Only a small increase in N O levels were observed for the case of the highest oxygen enrichment, which had an excess air of about 5 % (Stoich = 2.1) based on the plant settings. It is believed that this N O level could be further reduced by optimizing the stoichiometry of the furnace. C O concentrations were negligible except for cases with minimum oxygen excess. Values of C O concentrations of 10,820 and 7700 ppmvd were measured with oxygen excess values of 0.5 and 5%, respectively. Generally speaking, there was good agreement between the calculated and measured 0 2 and C 0 2 concentrations. The only discrepancies observed were for the highest-enrichment and air/fuel conditions, it being larger for the latter experimental condition where the C 0 2 concentration agreed well, but for which the 0 2 concentration was off by almost 100%. It should be noted here that (i) before and after each test the analyzers were calibrated following the procedure previously described here, and (//) during all tests there was a very good agreement between the concentration values measured with the three analyzers used for all species. Therefore, the explanation for the differences in measured and calculated 0 2 concentrations is related to the possible variation in operating conditions, particularly variations in air flow rates. For example, the high C O concentration value (7700 ppmvd) for a stoichiometry of 2.10 in a gaseous flame (highest enrichment condition) is unexpected. Based on the measured 0 2 concentration for that case (0.3 vol%) and accepting the 0 2 flow rate as being correct, the stoichiometry should be more nearly 2.0136 instead of the 2.10 calculated based on the furnace settings. This change in stoichiometry corresponds to a variation in air flow rate of less than 9%. If both the air and oxygen flow rates are assumed to have an error associated with them, the furnace stoichiometry should be 2.0146 based on the measured exhaust concentrations of 0 2 and CO:. It is important to point out here that, with nearly stoichiometric combustion, there is a strong sensitivity of 0 2 effluent concentration to errors in measurements of both air and oxygen flow rates. In order to quantify the sensitivity of measured effluent oxygen and carbon dioxide concentrations to errors in inlet air and oxygen flow rate measurements, the following analysis was performed for a generalized lean complete combustion of methane for variable volumetric amounts of air and oxygen, a and b, respectively. The combustion equation is C//4 +a(02+3J6N2) + b02 -> C02 + 2H20 + (a+b-2)02 + 3.16aN2 (1) For this equation, the dry concentrations in volume percent of oxygen and carbon dioxide are, respectively: L 2idn U + 4.76a-lJ |