Introduction

Premixed combustors for aero engines have been under development for nearly forty years, yet, at the time of writing, the first airplane with premixed combustion still awaits its entry into service. On the other hand, industrial gas turbines have made the transition to premixed combustion within ten years and the level of emissions of nitrogen oxides has decreased tenfold. The differences are due to the peculiarities of gas turbines in flight and a large part of the chapter will be devoted to the understanding of the consequences of those differences for premixed or partially premixed combustion. An obvious difference between both applications lies in the fuels, which are predominantly gaseous for industrial gas turbines and exclusively liquid for aero engines and will continue to be for the foreseeable future. Therefore, premixing in aero combustors always needs to be discussed together with prevaporization, and the differences imposed on the liquid fuel preparation by full or partial prevaporization and premixing are responsible for a large part of the overall development effort. The other determining differences result from the thermodynamic cycles specific to high bypass ratio engines and the impact of the flight profile on the implementation of part load operation. The latter has already been described in Chapter 1.5 and the concept of staging in lean aero engines has been presented in Section 1.5.3 such that this chapter will concentrate more on the implementation of staging and its consequences on the design of the combustor components.

The chapter consists of three parts that partly also follow a historical order: Some results of research are presented that are relevant for lean premixed, prevaporized (LPP) combustion, which for a large part were concurrently achieved with development efforts on LPP combustors. Understanding the limitations and difficulties in the way of fully prevaporized premixed combustion, the concept with the highest emissions reduction potential, will then supply the base for the discussion of partially premixing combustors and their operability aspects.

Introduction

This chapter discusses emissions from systems with extensive levels of exhaust gas recirculation (EGR) or that use oxygen rather than air as a reactant (referred to here as oxyfuel combustion). Such systems have unique attributes that warrant a dedicated chapter in this treatment. First, the systems in which EGR or oxyfuel would be deployed have different degrees of freedom and requirements. For example, both are prominent candidates for carbon capture and storage (CCS) (Griffin et al., 2008; Budzianowski, 2010), where emissions requirements are driven by pipeline or geologic reservoir constraints rather than by atmospheric pollution considerations. Second, while CO2 and H2O dilution have been discussed in Chapters 5 and 7, their presence at very high levels in systems with EGR can provide a significant perturbation of the nominal reactant kinetics (such as in the radical pool) and requires a focused treatment.

As noted earlier, EGR and oxyfuel combustion for gas turbine applications are promising approaches to implement CCS in gas turbine power plants. EGR has also been proposed as a means of promoting fuel flexibility (enabling use of fuels with low heating value (Danon et al., 2010) and high hydrogen content (Lückerath et al., 2008)), and for increasing static stability (resistance to flashback/blowout) (Kalb and Sattelmayer, 2004) and dynamic stability (ElKady et al., 2009) relative to lean premixed combustors, while enabling low levels of pollutant emissions.