OCR Text |
Show emissions that cause adverse environmental effects such as global warming, acid rain, and smog (urban ozone). Nitrogen oxides (NOx) represent a particularly important precursor to these environmental hazards. With stringent industrial combustion emissions requirements already in place in some areas, ABB recognizes that a significant, positive contribution can be made by developing and commercializing low N O x gas-fired systems. NOx emissions from a natural gas-fired combustion system are a result of the oxidation of nitrogen by the combustion air (thermal NOx). The most widely-used design strategy for N O x reduction is staged combustion. The creation of a high-temperature, fuel-rich zone, followed by a reduced-temperature, fuel-lean combustion zone, is known to decrease N O x formation. Although the principles of staged combustion are well-understood, the difficulty lies in executing the theory when designing a practical combustor. A practical application is often hampered by a lack of information on the overlapping processes of nitrogen-hydrocarbon chemistry and the mixing temperature history of the fuel in the flame. Another way to control NOx from natural gas flames is to use recirculated flue gas as means to drop the flame temperature to a level at which the N O x formation rates are insignificant. The amount of recirculated flue gas used in practical designs is often limited by its effect on flame stability. Achieving the ideal conditions for low N Ox combustion is made even more difficult when ultra-low N O x emissions are desired. The fixation of N 2 by hydrocarbons (prompt NOx) mechanism also becomes a major source of N O x at ultra-low levels. In practical combustion systems, flue gas recirculation (FGR) is used to suppress thermal N O x formation from both diffusion and premixed burners. ABB recently developed and evaluated at a large laboratory scale (70 x 106 Btu/hr), two low N O x burners : 1) the L N V ™ low N O x venturi diffusion burner, and 2) the Environmental (EV) premixed burner. These burners operate in diffusion and premixed combustion modes, respectively. LNV BURNER DESIGN The LNV burner is a, diffusion, internally staged burner. The fuel is burned in a sub-stoichiometric zone to minimize the N O x formation in the vicinity of the flame. In the post-flame domain, secondary air is added to complete the combustion process and limit the production of thermal N O due to the lower temperatures in the downstream region. If required, recirculating flue gas enters through a separate annuli to reduce the oxygen partial pressure, and reduces the flame temperature to suppress the formation of thermal NOx. The LNV low NOx venturi burner is a dual air register burner with a refractory tile quarl (Figure 1). It can fire natural gas and No. 2 through No. 6 oil fuels. Nominally, natural gas is injected in a ring of gas canes or spuds near the periphery of the burner. Oil is fired through a center oil gun and diffuser plate assembly. 3 IV-24 |