Lean Premixed Burners
Interest in lean premixed burners as an attractive alternative for "state-of-the-art" practical combustion systems began in the early 1980s. In addition to their common application in NOx control strategies (Bowman, 1992), these burners were independently introduced in high-efficiency condensing boilers in domestic central heating appliances as a method for the efficient use of the fan, which is necessary to overcome the pressure drop in the condensing heat exchanger. In this sense, these appliances were "low emission" before NOx emission standards ever existed. In this chapter, we describe a number of developments in the design of lean premixed burners for use in heating applications. As will be discussed, lean premixing presents two challenges to burner design: achieving acceptable flame stability (flashback/blow-off) over the desired range of turndown ratio and maintaining stability with varying fuel composition. Although the effects of fuel variability are well known in gas engines and premixed gas turbines, the analogous principles governing the operation of burner systems are often neglected. Chapter 2 discusses the foundations of some of these principles in general. We shall review them briefly in the particular context germane to lean premixed burners.