This book is the result of two international seminars on the subject of lean combustion. Therefore, the number of lean burn patents did not increase until after the Clean Air Act of 1990.
M OBILE S OURCES
Furthermore, as discussed earlier (and throughout this book), lean burning can represent two strategies: oxidation and dilution, as shown in Table 1.1. For example, Honda Motor Corporation used lean combustion technology to certify the first ultra-low emission vehicle in 1974.
S TATIONARY S OURCES
Because of the theoretical simplicity of achieving complete combustion and low emissions simultaneously, lean combustion is common in both mobile and stationary applications. Lean combustion (premixed or rapid mixing) Catalytic aftertreatment (eg SCR) Particulate aftertreatment (eg baghouse) Flue gas recirculation.
LEAN COMBUSTION APPLICATIONS AND TECHNOLOGIES
BRIEF HIGHLIGHTS OF THE CHAPTERS
This chapter focuses on the issues of acoustic oscillations that dominate near the lean limit. This chapter also discusses control strategies and the particular challenges for their implementation and practical application for the suppression of lean combustion oscillations.
ACKNOWLEDGMENTS
Based on the above points, it is clear that lean combustion has several cross-sectoral characteristics. In any case, it seems likely that the continued tightening of emissions regulations will continue to require the smart implementation of lean combustion technology.
COMBUSTION AND ENGINE PERFORMANCE
In the case of laminar flow, the seminal paper of Burke and Schumann (1928) defined the flame as "the locus of those points where the rate of diffusion of combustible gas from the outside and the rate of diffusion of oxygen from the inside have the ratio required by the stoichiometric equation for Neither the reciprocating speed of the engine nor the combustion speed of the mixture is taken into account.
BURNING IN FLAMES
- L AMINAR F LAMES AND F LAME S TRETCH R ATE
- T URBULENT F LAMES AND F LAME Q UENCHING
At the crest of the wave, the streamlines diverge in the unburned gas ahead of the flame surface. The heat release rate of a propagating flame is the product of the heat of reaction, the flame area, the unburned gas density and the combustion rate.
B URNING V ELOCITY
AUTOIGNITIVE BURNING 1. I GNITION D ELAY T IME
- A UTOIGNITIVE F RONT P ROPAGATING V ELOCITIES
Self-ignition at several such "hot spots" can create a high change in heat release rate. ONs are a guide to whether gasoline will "knock". This can be taken as an indication of spontaneous combustion, but not for the details of it.
RECIRCULATION OF HEAT FROM BURNING AND BURNED GAS
The combustion products then spiraled outward and transferred heat to the reactants in the counterflow heat exchanger. A pressure wave is generated ahead of the combustion zone and the asonic velocity regime is entered, with a sudden jump in the velocity of the reaction wave.
FLAME STABILIZATION
In laminar flow flashback occurs when the burning velocity is greater than the low flow velocity along the tube wall along which the reactants flow. For both sets of contours, the swirl number was 0.72, and the mean axial velocity entering the flame tube was 10 m/s.
CONCLUSIONS
Strain rate measured along the creased flame contour within turbulent non-premixed jet flames. Burning. Turbulent burning rate variations of equivalent ratio methane, methanol and iso-octane air mixtures at elevated pressure. Burning. 1987). Combustion, Flames and Explosions of Gases, 3rd ed.
INTRODUCTION
Ydil Local mass fraction of diluent Yf1 Fuel mass fraction in the fuel stream Yf Local mass fraction of fuel. DTIGN Maximum ignition–inlet temperature difference DTIGN0 Standard auto-ignition temperature difference DTMAX Maximum temperature rise.
MILD COMBUSTION
The processes located in the upper right quadrant of the figure are classified as spontaneous combustion. It is defined by the lower left quadrant of the figure, where the inlet temperature is lower than the ignition temperature.
SIMPLE PROCESSES IN MILD COMBUSTION
- HBBI IN W ELL -S TIRRED R EACTORS
- HCCI IN B ATCH R EACTORS
- HCDI IN C OUNTER -D IFFUSION F LOW R EACTOR
- HDDI IN C OUNTER -D IFFUSION F LOW R EACTOR
- HFFI IN P LUG F LOW R EACTOR
The maximum temperature rise (DT) is the difference between the maximum temperature occurring in the reactor and the temperature of the inlet reactants Tin. In figure 3.8, the vertical black bar in the lower part of the figure (heat release) and the dash-dot. In the same plot, the autoignition temperature is also reported as a function of the mixture fraction for minimum oxidation for a residence time of one second.
PROCESSES AND APPLICATIONS OF MILD COMBUSTION IN GAS TURBINES
- MILD C OMBUSTION WITH E XTERNAL C ONTROL
A schematic of the core of this plant is shown in Figure 3.24, in which the numbers p = 10 bar. Expansion, cooling and re-expansion of the working fluid takes place in the 2-3, 3-4 and 4-5 branches. This stream feeds the last part of the plant depicted on the right side of Figure 3.27.
CONCLUSION
HiTAC (2002). Proceedings of the 5th International Symposium on High Temperature Air Combustion and Gasification, Yokohama. Using a regenerative burner for a forge furnace. Proceedings of the High Temperature Air Combustion Technology Forum, Japan. An overview of the development and commercial use of the LNI technique. Proceedings of the 4th High Temperature Air Combustion and Gasification, Rome, document no.
INTRODUCTION
In this chapter we will first examine the theoretical performance and efficiency of the ideal air standard internal combustion engine. This will provide the fundamental background needed to understand how leaner air-fuel ratios can be used to improve engine performance. Further lean of the mixture can be achieved with homogeneous charge compression ignition (HCCI) strategies and with hydrogen (as described in Chapter 8), but this chapter concentrates on the lean combustion performance possible using conventional gasoline fuel and spark -infection.
PERFORMANCE OF THE IDEAL INTERNAL COMBUSTION ENGINE
The ideal thermal efficiency of the standard air Otto cycle as a function of compression ratio is shown in Figure 4.1, compared with that for the standard air-Diesel cycle at different values of cut-off ratio. The length of the burn time, or burn duration, also affects thermal efficiency. As the duration of combustion increases, the optimal value of ignition time also increases, as shown in the diagram.
ENGINE COMBUSTION AND EMISSIONS
- H OMOGENEOUS C HARGE S PARK -I GNITION E NGINES
- S TRATIFIED -C HARGE E NGINES
- S PARK -I GNITION E NGINE E MISSIONS
The homogeneous fuel-air mixture that always exists in the cylinder leads to another characteristic of the spark-ignition engine knock. The stratified charge engine is therefore capable of operating over a much wider range of air-fuel ratios than a. These engines can then operate at leaner overall air-fuel ratios and higher efficiencies than conventional spark ignition engines.
EXTENDING THE LEAN LIMIT OF OPERATION
Airflow significantly improved the turbulence field near the spark plugs, leading to a significant increase in burn rate. A lambda sensor mounted on the exhaust pipe was used to control the air-fuel ratio of the mixture burning in the engine. It can be seen that operation with the PSC system provides a 10% extension of the lean burn limit.
SUMMARY
The optimal spark advance, specific fuel consumption and specific emission levels are greatly influenced by the burning rate in the combustion chamber, which in turn is controlled by both the intensity and scale of the mixture turbulence just before ignition and during the early combustion process. The "squish-jet" turbulence-generating chamber design was tested in a spark-ignited research engine and was compared to a conventional bowl-in-piston design. Rapid combustion chamber development for natural gas engines. Proceedings of the Windsor Workshop on Alternative Fuels, pp. The Effects of Turbulence and Combustion Chamber Geometry on Combustion in a Spark Ignition Engine, UBC Alternative Fuels Laboratory Report AFL-89-02.
INTRODUCTION
Although the primary motivation for using lean burn in gas turbines is generally associated with NOx. Gas turbines play a prominent role in the stationary power generation market and should remain a critical part of the market mix for at least the next few decades. As a result, the lean burn gas turbine market should increase significantly over the next two decades.
RATIONALE FOR LEAN COMBUSTION IN GAS TURBINES
- S TABILITY
- F LASHBACK
- F UEL F LEXIBILITY
- T URN D OWN
As can be seen from Figure 5.8, the ordinate value in this case is expressed in the form of an emission index, namely grams of NOx per kilogram of fuel consumed. The challenges associated with lean combustion are illustrated in Figure 5.10 in the context of a typical burner "stability loop". For a given inlet pressure and temperature, the fuel/air ratio for a given mass flow through the burner can be increased or decreased to a point where the burner can no longer sustain the reaction. This locus of conditions where the reaction is no longer sustained is shown as the static stability limit in Figure 5.10.
LEAN GAS TURBINE COMBUSTION STRATEGIES
- A VIATION
The pilot usually generates a relatively high amount of NOx due to the nature of the diffusion flame of the reaction it generates. This requirement increases drastically in complexity with catalytic combustion due to the operational requirements of the catalytic reaction. The other major limitation of catalytic combustion is the stability of the catalyst and its support structure.
SUMMARY
Field Test Validation of the DLN2.5H Combustion System on the 9H Gas Turbine at Baglan Bay Power Plant, Paper GT2005 68843. ASME Turbo Expo 2005, Reno. Reduction of nitrogen oxides by lean premixed prevaporized combustion. Proceedings on Gas Turbine Engine Combustion, Emissions, and Alternative Fuels, RTO-MP-14, p. Evaluation of Hydrogen Addition to Natural Gas on the Stability and Emission Behavior of a Model Gas Turbine Combustor, Paper GT2005 -68785.ASME Turbo Expo 2005, Reno.
INTRODUCTION
PRINCIPLES OF FUEL VARIABILITY
Thus, the thermal input to a burner is directly proportional to the Wobbe Index of the gas, not its highest heating value. The gas momentum flux is, to a good approximation, determined only by the pressure drop across the orifice. For stability, changes in burn rate caused by changes in equivalence ratio can create serious challenges for system design in terms of knockback and rebound.
STABILIZATION METHODS
- L OW -S WIRL S TABILIZATION C ONCEPT
In the submerged case, there is also significant heat transfer to the burner surface downstream of the primary flame front. Increasing the exit velocity of the combustible mixture reduces heat transfer to the burner surface (and increases NOx emissions). As discussed above, the ST in an LSB is defined by the centerline velocity at the leading edge of the flame brush.
SUMMARY
The profile similarity feature U=Uo means that the normalized rate of axial divergence [ie, (dU=dx)=Uo] has a constant value. The first term of the RHS tends to a very small value for large Uo because the laminar flame velocities for hydrocarbons are from 0.2 to 0.8 m/s. As long as flow uniformity is maintained, the flame position at large Uo tends to an asymptotic value independent of the laminar flame velocity.
INTRODUCTION
Combustion oscillations in channel flames have generally been quantified in terms of the amplitude of pressure oscillations in the combustion chamber cavity. Close to the lean limit, stoves can give rise to large oscillation amplitudes, which would limit the range of operation of the combustion chamber and prevent the realization of the full potential of low emissions (Bradley et al., 1998; De Zilwaet al., 2001). Close to extinction, a broadband low-frequency oscillation dominates, with amplitude increasing with proximity to the lean flammability limit (De Zilwaet al and accompanied by acoustic or bulk-mode frequencies in the combustion chamber cavity.
OSCILLATIONS AND THEIR CHARACTERISTICS
- T HE P ROCESS OF E XTINCTION
- S TRATIFIED F LOWS
Flame studies close to the lean extinction limit in turbulent premixed flames have shown that a series of cycles of extinction and re-ignition precede global flame extinction (Bradley et al., 1998; however, part of the increase in frequency at Sw = 0.25 is ", due to the larger equivalence ratio. In addition, the frequency of the extinction and relight cycle, as discussed by De Zilwaet al. 2001), could reach values as high as 100.
