| OCR Text |
A target no greater than 9 ppm is usually established. Selective catalytic reduction (SCR) is generally designated as the technology to meet L A E R standards. C A A A control requirements for ozone apply to over 100 ozone non-attainment areas which do not meet the ozone health standard and to "ozone transport regions" which may meet the standard but into which ozone can migrate. Much of the industrial U S A is non-attainment for ozone: California, the Great Lakes, the Gulf Coast, as well as the North East. N e w sources in some ozone non-attainment areas will be subject to the same L A E R N O x target. Thus emission offsets and 9 p p m or lower N O x emissions will be required. Further, the C A A A will affect smaller sources than previous regulations and consequently will impact industrial-scale furnaces and boilers directly. O f course, industry could opt for S C R systems. But because of the high cost of S C R systems, very low-emitting combustion equipment which does not generate typical pollution levels could be expected to capture the lion's share of the market. If available at a truly competitive cost, very low emission combustion equipment could be used even where it is not required. In response to these market demands, Thermo Power Corporation and John Zink Company, with funding support from D O E , are developing a natural gas burner system capable of meeting these new emission requirements for boilers and process heaters in the size ranges of 3 to 120 MMBtu/hr, without resorting to costly post-processing of exhaust gases. The specific objectives are to achieve N O x emission levels of 9 p p m or less and C O levels of 50 p p m or less, both at 3 % 02 , under all operating conditions, and without the use of external flue gas recirculation. Additional goals include high turndown ratio, low levels of unburned hydrocarbons or air toxics, and field demonstration as an operating industrial burner. VIStA The VIStA burner concept, shown in Figure 1, consists of two combustion stages. In the first stage, natural gas and pan of the combustion air are premixed and tangentially admitted at high velocity into the inertial reactor through multiple ports. By exploiting the radial pressure difference created in the reactor by the vortex, part of the combustion products are taken out through tangential openings and returned into the combustor axially at a center opening via multiple recirculation tubes. The stoichiometry, recirculation rate, and residence time are controlled to reform the natural gas in this first stage thereby eliminating or minimizing the hydrocarbon and nitrogen-bound compounds which contribute to prompt N O x formation. In the second stage, secondary air is introduced axially through tubes located concentrically around the burner periphery. The air stream velocity is designed to aspirate sufficient furnace gases into the annular space between the first stage products and secondary air stream so as to lower both the temperature and the oxygen concentration in the secondary flame zone thereby minimizing thermal N O x formation. In summary, the VIStA burner concept utilizes advanced aerodynamic principles to provide high internal flue gas recirculation in both combustion stages of an air-staged burner to achieve very low (less than 9 ppm) thermal and prompt N O x emissions. This is accomplished without the use of external flue gas recirculation (FGR) or post combustion treatment, both of which result in efficiency losses and increased capital, maintenance, and operating costs. 2 |
