Page 37 - Energize April 2021

TECHNICAL

mixing and combustion take place simultaneously in the primary
combustion zone. The high temperature of the flame is the cause of
high NOx levels and the mixture is typically diluted by adding water
to reduce the flame temperature.
The problem with using dilutants to reduce high flame
temperature has been overcome by using lean premix combustion
(LPM), where air and fuel are thoroughly mixed to form a lean
mixture before delivery to the combustor (Figure 2).

Figure 4: A typical DLN Combustor (GE)

Figure 2: NOx production rate (GE)

Figure 5: A multicluster combustor (Mitsubishi)

Sequential combustion
Current development of H 2 fuelled turbines is moving in the direction
Figure 3: Lean premix burner (Mitsubishi) of sequential combustion (SC) or two stage/reheat combustion,
where fuel is combusted in two stages at different temperatures.
Figure 3 shows the basic construction of a LPM combustor. Keeping detrimental NOx emissions below the limits requires
In LPM systems, atmospheric nitrogen acts as a diluent. The extremely short combustion chambers and rapid mixing, so that the
fuel/air ratio typically approaches one-half of the ideal stoichiometric post-flame residence times are sufficiently low. Longer residence
level, resulting in excess air. This excess air is a key to limiting time leads to higher NOx formation. Sequential combustion
NOx formation, as very lean conditions cannot produce the high essentially entails two combustion stages: a conventional stage and
temperatures which create thermal NOx. The most common forms an auto-ignited second stage. It comprises two short combustion
of LPM are the dry low emissions (DLE) or dry low NOx (DLN) chambers to enable rapid mixing, so post-flame residence times are
combustors. Dry combustors do not use water for dilution. low enough to keep NOx emissions below limits. Having two stages
Stable, efficient, low-NOx combustion requires rapid, of combustion also allows individual control of each stage. The first
homogeneous mixing of fuel and air, which is a challenge with stage uses lean combustion which results in excess oxygen in the
highly reactive hydrogen. The higher turbulent flame speed of H 2 second stage. Fuel is added to the inlet gas of the second stage to
relative to natural gas increases the propensity for flashback in LPM complete the combustion cycle.
combustors and can result in the flame advancing into the mixing Several versions have been developed by different
chamber. To mix H 2 and air in a short period of time, it must be manufacturers. Figure 6 shows sequential combustor designs from
done in a confined space. This results in a configuration where the various manufacturers.
fuel nozzle jets and flame are in close proximity, making flashback In the Ansaldo GT26, the two stages are separated by a high-
increasingly likely. The solution to this is to disperse the flame pressure turbine, whereas in the GT36 and other designs, no high-
over the combustor interface. The key technology to this method pressure turbine is implemented.
is the fuel delivery nozzle, and current designs use a multi-cluster For a SC turbine, the second combustor can be switched off,
combustor, which incorporates many nozzles. allowing the engine to be parked at extremely low loads. This is an

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