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Show 9 the more stringent regulation of 4 lb/ton that became effective in southern California on December 31, 1992 (and might be adopted by other regions in the future), the glass industry will have to either develop advanced methods for natural gas firing or use post-combustion treatment technologies, the latter currently having high capital, as well as high operating costs. As a result, GRl, SoCalGas, and IGT have initiated a program, together with industrial partners CTI, APCI and Anchor, to develop a cost-effective 10w-NOx second-generation combustion technology for the near- and mid-term needs (2 kg/tonne or 4 lb/ton, respectively) of the U.S. regenerative glass melters. This second-generation technology utilizes a unique method of combustion air staging to reduce the oxygen availability in the flame's high-temperature zone and improve flame temperature uniformity to control NOx formation. As discussed, versions of these techniques were tested on the IGT glass tank simulator during the earlier work and showed potential for excellent NOx reduction. OXYGEN-ENRICHED COMBUSTION AIR STAGING Combustion air staging is accomplished by reducing the combustion air flow (primary air) to the port and injecting secondary air downstream. The bulk of the combustion is relatively oxygen deficient (or even fuel-rich) to inhibit NOx formation. Splitting the combustion air in a regenerative glass tank is difficult because 1) it can require major modifications and 2) properly mixing the secondary air with the primary combustion gases requires higher secondary air pressures that are not desirable. A more attractive method is to operate the furnace with near-stoichiometric air and inject a small amount of high-velocity ambient (or preheated) secondary air near the exit port to burn out any residual CO and THC. This method of air staging was tested by IGT on its glass tank simulator using ambient secondary air and was found to be very effective in reducing NOx emissions. Figure 4 shows that in a furnace operating with a typical stoichiometric ratio of 1.15 (150/0 overall excess air) a NOx reduction of 35% (from the current 4 to 2 lb/ton) could be achieved by operating the port at a stoichiometric ratio of 1.04 to 1.06, which should not be very difficult. In the tests at IGT, there was a significant increase in heat transfer (Figure 5) at this level of primary air, even though the secondary air was ambient and was injected downstream of the exhaust port. The data also show that even greater NOx reductions could be achieved by further decreasing the primary stoichiometric ratio. The heat transfer would, however, decrease somewhat compared to the optimum, but would be comparable to the levels achieved at 15% excess air. The use of ambient secondary air, however, may not be desirable because it could adversely affect the furnace productivity and furnace thermal efficiency. In the proposed technique (illustrated in Figure 6), the secondary air is aspirated from the regenerator top by using a small amount of oxygen that is normally available at high supply pressures. This technique also provides a way for oxygen enrichment to potentially increase furnace production rate. The use of oxygen-enriched secondary air could increase secondary-stage combustion temperatures to increase heat transfer to the load and combustible burnout. The increase, however, is not expected to be high enough to impact the overall NOx formation. Five specific variations of secondary oxidants are currently being investigated: 1) 02-enriched hot air, 2) 02- enriched ambient air, 3) pure 02, 4) hot air, and 5) ambient air. (P-002\ 1194Maui.doc |