ASME FACT-18, Combustion Modeling, Scaling and Air Toxins, 1994. LOW NOx EMISSION FROM AERODYNAMICALLY STAGED OIL-AIR TURBULENT DIFFUSION FLAMES

An experimental investigation on the reduction of nitrogen oxide emission from swirling, turbulent diffusion flames was conducted using a prototype multi-annular burner. The burner utilizes swirl-induced centrifugal body forces to dampen turbulent exchange between the fuel and air streams, allowing an extended residence time for fuel pyrolysis and fuel-N conversion chemistry in a locally fuel-rich environment prior to burnout. This aerodynamic process therefore emulates the conventional staged combustion process, but without the need for physically separate fuel-rich and -lean stages. Parametric studies of swirl intensity and external air staging were carried out to investigate the feasibility of aerodynamic staging for low NOx combustion with No. 6 heavy fuel oil. NOx emission was reduced from an uncontrolled 300 ppm (3% O2) to 91 ppm in the optimal configuration. A further reduction from 91 ppm to 53 ppm was realized by external staging (primary stage fuel equivalence ratio = 1.13). A detailed flame structure investigation was carried out for a parametrically optimized, staged flame. INTRODUCTION Combustion air staging has proven to be a highly effective method for reducing NOx emission in a number of practical systems. Typically, these systems rely on physically separate fuel-rich and fuel-lean combustion zones, between which “overfire” air is injected. Drawbacks to this method include higher operating costs, corrosion of the heat transfer surfaces in the fuel-rich first stage, and difficulty in retrofitting existing systems. As an alternative to relying on physically separate zones for staging, the Radially Stratified Flame Core (RSFC) burner was developed at MIT to implement staging by aerodynamic means, so that all of the combustion air is introduced at the burner. Analogous to the conventional overfire air systems, the RSFC burner aerodynamically creates two combustion zones, one in which fuel-air mixing is suppressed by radial density stratification, and the other characterized by a high degree of mixedness. In the density stratified zone, relatively cool, highly swirling combustion air surrounds a hot, fuel-rich flame core, which is created by injecting a small portion (~15%) of the combustion air near the central fuel jet. Under certain conditions, the centrifugal body forces associated with the swirl damp turbulent mixing between the fuel-rich core and the surrounding air, effectively containing the fuel within a locally fuel-rich “first stage” for an extended residence time, during which the NO reduction chemistry is active. Further downstream, vortex breakdown occurs, and the peripheral combustion air mixes with the products of the fuel-rich core, creating a fuel-lean burnout stage. A schematic of the internal staging process is shown in Figure 1. primary air + fuel + products FIGURE 1 INTERNAL STAGING SCHEMATIC. In previously published studies of natural gas flames (Toqan et al., 1992), the RSFC burner achieved 70 ppm NOx emission at 3% O2 (56 ppm CO) without flue gas recirculation, and 15 ppm NOx (< 10 ppm CO) with 32% of the flue gas recirculated. Using detailed velocity, species concentration, and temperature measurements, those studies demonstrated the role of swirlinduced radial stratification in producing aerodynamically-staged low NOx flames. In the current work, the applicability of the RSFC burner to No. 6 heavy fuel-oil (0.3 wt % N) flames was investigated. Because the nitrogenous species (fuel-N) in No. 6 fuel-oil are readily converted to NOx in the presence of O2, particular attention was given to the residence time available in the fuel-rich core to ensure maximum fuel-N conversion to N2. The principal source of NOx emission when burning fuels containing chemically bound nitrogen is the conversion of these nitrogen species (Pershing and Wendt, 1977). BURNER AERODYNAMICS Figure 2 illustrates the important aerodynamic features of a ‘typical’ low NOx RSFC burner flame. An initial fuel-rich flame core is created by mixing initiated within the burner between the central fuel jet and the primary air. The secondary air, typically constituting 85% of the total burner air, is introduced through a radially displaced annulus. The fuel jet penetrates through an annular internal recirculation zone (IRZ) that extends into the burner quarl. Hot combustion products and some fuel peel off the fuel jet during its passage through the reverse flow zone, and are carried back into the quarl. In the emerging fuel-rich flame, fuel mixes slowly, due to turbulence damping, with the surrounding air. Further downstream of the burner (~ 5 burner diameters), stratification ends, largely due to the decay in tangential velocity, and the remaining secondary air mixes rapidly with the fuel-rich core, completing the combustion. FIGURE 2 TYPICAL LOW NOx RSFC FLOW FIELD. Turbulence damping in the density stratified zone results from a combination of the centrifugal force field and the positive radial density gradient (created by the density difference between the hot burning core and the surrounding air). When an individual fluid eddy displaces a parcel of the relatively dense combustion air toward the flame axis, work is expended in the force field, dissipating turbulent energy. The concept is rooted in the work of Rayleigh (1916), which showed that a rotating fluid is stable with regard to radial interchanges if ρWr, the product of density, tangential velocity, and radial position, increases with radial distance from the axis of rotation. Beér et al. (1971) adapted Richardson’s dimensionless group for the characterization of turbulence damping under conditions of atmospheric inversion to flames with swirling air flow around a hot central core. More recently, the adaptation of the Richardson number to a radially stratified natural gas flame in a low NOx gas burner was presented by Toqan et al. (1992). EXPERIMENTAL Parametric tests were conducted to study the relationship between exit NOx emission and swirl number, S, defined as the non-dimensional ratio of angular momentum to axial momentum and burner radius: S ≡ Gφ GxR (Beér and Chigier, 1963). In addition, axial profiles of centerline species concentration and temperature were measured for zero and maximum swirl settings to demonstrate the role of swirl in mixing suppression. Parametric studies of external staging (using conventional overfire air) were also conducted to give a relative indication of the efficacy of the RSFC burner internal staging process. Finally, a detailed flame structure study was conducted in which gas composition and temperature measurements were taken at many locations in an optimized flame. The detailed study was used to elucidate the overlapping mixing and chemistry processes, particularly to address the question of fuel-N conversion. The measurements would indicate the extent to which fuel was confined within a fuel rich core, and whether the temperature in the fuel-rich region was high enough to allow fuel-N conversion to N2 in the available fuel-rich residence time. The thermal input was maintained at 0.9 MW and the fuel was No. 6 fuel-oil with a 1.5 C/H ratio and a 0.3 wt% N content. The air preheat temperature was 555 K. A twin fluid Y-jet atomizer with air as the atomizing medium was used, except for the detailed flame study, in which steam atomization was employed. Experimental Burner A schematic of the RSFC burner is given in Figure 3. The combustion air is introduced through three concentric annular nozzles, of which the positions of the primary and secondary (as well as the fuel gun) can be adjusted to produce a particular flow field. To produce the desired internally staged flame for the experiments, the primary air nozzle was used to introduce approximately 15% of the combustion air while the remaining 85% was introduced through the tertiary nozzle (for the purpose of this discussion, the terms “tertiary air” and “secondary air” are interchangeable). Each nozzle is equipped with moveable block-type swirlers capable of infinitely variable swirl control. For the results reported below, the degree of swirl used in any experiment is indicated by "swirl setting" which is a linear scale of swirler adjustment angle, with 0 representing no swirl and 10 representing maximum swirl. For moveable block swirlers, the theoretical swirl number of the flow issuing from any nozzle is strictly a function of the burner geometry, and can be calculated for any particular swirl setting, as shown in Beér and Chigier (1972). For the burner geometry of the experiments reported here, a swirl setting of 10 corresponds to a swirl number of 0.3 and 0.6 for the primary and tertiary nozzles, respectively. Experimental Furnace The MIT Combustion Research Facility (CRF) was used to conduct the experiments. The CRF is an approximately 10 m long tunnel furnace that consists of several interchangeable water-cooled sections, each with either a bare metal or refractory brick lined surface, and a square (1.2 x 1.2 m) or cylindrical (φ 0.5 m) cross-section. The thermal capacity of the CRF is 3 MW, though typically it is operated at 1 MW. By varying the sequence of bare-metal and refractory brick sections, the heat extraction along the flame axis can be varied to simulate the temperature history of large scale practical flames. An overfire air injection port is located 3.4 m downstream of the burner. An access door in each of the furnace sections allows measurement of gas temperature and composition with intrusive traversing probes, including a suction pyrometer for temperature, a water cooled suction probe for major species, and a steam jacketed suction probe for hot cell fourier transform infrared (FTIR) spectrometry for various trace species. The sampling methods and furnace have been described in detail elsewhere (Beér et al., 1985). EXPERIMENTAL RESULTS Tertiary Air Swirl Figure 4 illustrates the effect of tertiary air swirl on NOx emission, demonstrating a reduction from 300 ppm at zero swirl, to 120 ppm at the maximum setting, which corresponds to a swirl number of 0.6. The non-zero slope at the maximum setting suggests that further reductions in NOx might be achieved if the burner were modified by increasing the maximum swirl angle. These results are consistent with previous studies of natural gas RSFC flames which demonstrated the presence of radial stratification as a result of swirl (Toqan et al., 1992). As a simple test that the same process was at work in the oil flames, centerline measurements of gas composition and temperature were taken along the flame axis for maximum and zero tertiary swirl cases. The results, shown in Figures 5 through 9 indicate that in the high swirl case, fuel-air mixing is suppressed, while in the zero swirl case, mixing was rapid. Figure 5 shows that a significant amount of oxygen reaches the flame axis in the no-swirl case, whereas practically none is found at the axis when swirl is applied. Similarly, the CO and CO2 profiles shown in Figures 6 and 7 indicate that with swirl, fuel consumption proceeds more slowly, partly accounting for the lower temperatures shown in Figure 9. B B B B B B B B B