Environmental Engineering Reference
In-Depth Information
However, the combustors are operated at lower equivalence ratios in the LPM
approach. The range of operation of such LPM combustors is much narrower
compared to conventional combustors (Glassman 1996 ). So this operation results in
low NO x production, at the cost of overall stability of the combustion process.
Flame stabilization in lean premixed gas turbine combustor is a challenge due to
weak reaction rate and thereby reduced
fl
flame speed against the high-speed
fl
ow of
combustor. The popular stabilization mechanisms used for sustaining the
fl
ame
inside the combustor are bluff-body
flame holders (Cheng and Kovitz 1958 ;
Hertzberg et al. 1991 ; Chaudhuri and Cetegen 2008 ), dump plane, swirlers
(Ateshkadi et al. 2000 ; Schefer et al. 2002 ; Muruganandam et al. 2005 ), and pilot
fl
fl
flame (Jensen and Shipman 1958 ). If the
fl
flame blowout from the combustor due to
insuf
cient stabilization mechanisms,
the phenomenon is called lean blowout
(LBO). The equivalence ratio (
), at which blowout takes place, is called LBO
limit (characterized by the limiting equivalence ratio
Ф
Φ LBO ). Further small distur-
bances in combustor operating conditions such as velocity
fluctuations or equiva-
lence ratio perturbations caused due to fuel/air unmixedness (Barnes and Mellor
1998 ) elevate the risk of blowout. The major risk develops when the combustor
faces power setting changes (Shashvat et al. 2005 ; Ateshkadi et al. 2000 ; Chao et al.
2000 ), which causes loss of power.
Blowout is a serious problem in both land-based engines and aeroengines. The
blowout in stationary power station gas turbines may cause shutdown of the power
station. This leads to an expensive as well as lengthy process of restart, which
increases the maintenance cost and reduces productivity. Severe problem occurs for
LBO in aircraft engines where combustion is the ultimate source of the vehicle
thrust. During rapid deceleration, sudden changes in throttle setting rapidly reduce
the fuel
fl
flow occurs at a slower rate for the
rotational inertia of the compressor (Rosfjord and Cohen 1995 ). This leads to lean
fuel
fl
flow rate. But the transience of air
fl
air mixture, and the resulting blowout prevents the engine from recovering
from the compressor stall event (DuBell and Cifone 1989 ). This puts a more severe
situation in high-altitude vehicles where the stability limits are narrowed.
To avoid blowout, usually conventional combustors are therefore operated with
a wide margin above the LBO limit as the LBO limit is uncertain and does not
follow well-de
-
ned curve on the operating map of combustor (see Fig. 1 ). For these
reasons, the designers have to design the combustor operability (denoted by solid
line) to assure that the worst possible state (state 2) maintains a proper safety margin
from LBO limit. The designer should also take care while drawing the operating
curve of combustor to include dynamic power setting changes that occur during
transit condition of combustor like throttling of turbine for reducing the power
output (dotted curve adjacent to solid line) and similarly maintain an optimal safety
margin from LBO limit.
These requirements keep all other states (like state 1) in a regime of high equiv-
alence ratio (rich condition) which is not optimal state from emission point of view.
The best operating condition in terms of NO x for state 1 will be the leanest condition
possible for that velocity of the combustor (i.e., state 1). But due to design constraint,
the combustor operated at higher equivalence ratio which produces more NO x.
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