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Show struggle over the prior fifty years from the soft. billowy. poorly-mixed flames that made yesterday's fireboxes as big as cathedrals, through the eventual discovery of the benefits of turbulence intensity. to today's thoroughly mixed, intensely turbulent diffusion flames. "Yo(J mea.n tlJep''re p(Jtting in gas JYltlJ no air? Thllt's crazy/" And perhaps it 'Was from the well entrenched burner design perspective that emphasized rapid mixing . But the nelt time back that same wizard excitedly confided, "I tried it . .. 811d cO(Jldn 't believe it. Grea.t NOr reduction 8.l1d 8 better OIl111C t1J811 my st8.tJdard b(Jrncr/" At the time, the non-standard low NOx state-of-the-art was the clumsy air-staged burner that offered only about half the NOx reduction of fuel-staging and an absolutely dreadful flame. Thereafter, in due course. there was introduced to American refiners the first-generation fuel-staged low NOx burner, descendents of which today enjoy their day in the sun . But there is more to the burner evolution story, Who today remembers the "self-recirculation gasification II burner? By the end of the 1970s everybody knew that, on gas firing. about 15 percent flue gas recirculation produces about 50 percent reduction in NOx emissions. That old rule of thumb still holds up preuy well today. physics and chemistry not having changed much in the interim. But. in refineries at least. we didn 't like the complication and expense of hot ducts and fans. We still don 't. actually. and that's why some of the most recent ultra low NOx burner developments are so exciting to us. In any event, when around 1980 another Japanese development emerged that induced flue gas recirculation internally by means of burner configuration. we stuck in a bunch and. sure enough, got about 50 percent NOx reduction on refinery fuel gas without hot ducts and fans . But soon 50 percent became "not enough II and we lost interest. particularly when fuel-staged burners emerged to offer substantially larger reductions. Then. of course. in Southern California even fuel-staged reductions became "not enough II and eventually there emerged the concept of combining self-recirculation and fuel-staging in a single burner. Perhaps this idea should have dawned at a much earlier stage in the evolution of combustion modification techniques. But hindsight is 20/20 and it took a lot of hard work to make the dream come true. a natural-draft refinery burner offering significant NOx reductions without resorting to expensive flue gas treatment. Other ways have emerged to produce significant NOx reductions by combustion modification . We have already mentioned premixed surface combustion . On gaseous fuels. nitrogen oxides are produced by thermal fixation of combustion air nitrogen. a process that is reaction rate limited. Thus, the fiber matrix burner's low surface combustion temperature around 1800·F assures ultra low NOx levels. Though more expensive than its natural-draft. fuel-staged, self-recirculated counterpart. the premixed surface combustion burner's well defined combustion surface. noiselessness, uniform radiation and high efficiency are attractive for specialized process applications. Other approaches to ultra low NOx combustion that offer significant NOx reductions 'Without resorting to expensive flue gas treatment include off-stoichiometric combustion to moderate the peak combustion temperatures and reburning to destroy nitrogen oxides produced in an earlier combustion stage. Combining these ideas has led to another whole evolution of complex multi-annular distributed mixing burners. However, our experience 'With distributed mixing suggests that these burners can 'Work very well indeed but their performance is sensitive to fuel gas composition and furnace configuration, both of Which, in refineries, vary considerably. Thus, this approach seems best suited to applications in which the distributed mixing burner's performance can be "tuned" in a specific furnace to a constant fuel gas composition . At Chevron, we have always favored combustion modification techniques for NOx reduction, not the least of the reasons being that selective catalytic reduction flue gas treatment plants are more costly per pound of NOx removed. You don't need a flue gas treatment plant to make a furnace work but you do need a burner. So why not a low NOx burner? But not all furnaces can be retrofitted with low NOx burners and, in any event, in the refinery low NOx technology mix, it is hard to pass up those really big furnaces, or that group of smaller furnaces that can be breached together to produce one really big flue gas maker, 3 |