383
11.1 General Concept and Components
combustion temperature is high and much NOx is formed. By mixing fuel and
air prior to combustion into a lean mixture, the combustion temperature is much
lower, so that much less NOx is formed. The drawback is then that much CO is
formed due to incomplete combustion. It is therefore essential for completing the
combustion that air is added in a second stage to the combustion gas. The difficulty with premixed combustion is that the combustion may be quite unstable for
a mixture with composition near to the lean flammability limit (there is also a rich
limit, but this one is not relevant here).
The burner system of Fig. 11.9 is started by injecting pure fuel through the central pipe of the pilot burner. The mixture that forms by diffusion is ignited by a spark
and a diffusion flame is formed, which is very stable. From a small load on, the
pure fuel stream is stopped and replaced by a premixed flow, but with a composition sufficiently richer than the lean flammability limit, so that stable combustion
is possible. The premixing is realised by swirling the air flow and injection of the
fuel through holes in the hollow swirl vanes (or holes downstream of the vanes as in
Fig. 11.9). The mixture passes a cone. Due to the swirl and the deceleration of the
flow, a low pressure zone forms in the centre of the flow. By the low pressure, combustion gas is sucked from downstream so that a circulating flow pattern is formed
(Fig. 11.9). The conveyed hot combustion gas ignites the oncoming mixture. The
stabilisation and ignition process is called swirl stabilisation and is used in many
industrial burners. The peripheral burners with premixing of fuel and air into a very
lean mixture, under the lean flammability limit, are started in stages. In case of eight
peripheral burners, the stages may be a first group of four burners, followed by another four, as load increases. This mixture burns as it reaches the stable pilot flame.
Air is added downstream of the primary combustion zone through slots or holes in
the liner surface in order to allow the further combustion of the formed CO (second
stage of the combustion). More air is added further downstream (dilution air). With
can liners, there are transition tubes which bring the combustion gas to the turbine
inlet. Also these transition tubes have to be cooled. The air used for convection
cooling of these tubes is mixed in just before the turbine inlet. The same principles
are used in other combustion systems, but the technical realisation may be different
from described above. In particular, with an annular combustion chamber, all burners are equal. A possible realisation is then a burner in the style of the pilot burner of
Fig. 11.9, with a second ring of swirl vanes around it with injection of fuel forming
a very lean mixture (e.g. Siemens, Alstom). The basic ingredients are always: gas
injection in a swirling air flow, swirl-stabilisation of the flame, and staging of the
combustion air. This technology is commonly denoted by the term dry low NOx
(DLN) or dry low emissions (DLE). With the most advanced systems, nowadays
(2014), the production of NOx and CO can be kept below 9 ppm over a broad range
of the load. Alternative techniques for reducing NOx are injection of water or steam
in the combustion chamber. These wet techniques are sometimes used with smaller
gas turbines, but not with large turbines that run almost continuously.
11.1 General Concept and Components
combustion temperature is high and much NOx is formed. By mixing fuel and
air prior to combustion into a lean mixture, the combustion temperature is much
lower, so that much less NOx is formed. The drawback is then that much CO is
formed due to incomplete combustion. It is therefore essential for completing the
combustion that air is added in a second stage to the combustion gas. The difficulty with premixed combustion is that the combustion may be quite unstable for
a mixture with composition near to the lean flammability limit (there is also a rich
limit, but this one is not relevant here).
The burner system of Fig. 11.9 is started by injecting pure fuel through the central pipe of the pilot burner. The mixture that forms by diffusion is ignited by a spark
and a diffusion flame is formed, which is very stable. From a small load on, the
pure fuel stream is stopped and replaced by a premixed flow, but with a composition sufficiently richer than the lean flammability limit, so that stable combustion
is possible. The premixing is realised by swirling the air flow and injection of the
fuel through holes in the hollow swirl vanes (or holes downstream of the vanes as in
Fig. 11.9). The mixture passes a cone. Due to the swirl and the deceleration of the
flow, a low pressure zone forms in the centre of the flow. By the low pressure, combustion gas is sucked from downstream so that a circulating flow pattern is formed
(Fig. 11.9). The conveyed hot combustion gas ignites the oncoming mixture. The
stabilisation and ignition process is called swirl stabilisation and is used in many
industrial burners. The peripheral burners with premixing of fuel and air into a very
lean mixture, under the lean flammability limit, are started in stages. In case of eight
peripheral burners, the stages may be a first group of four burners, followed by another four, as load increases. This mixture burns as it reaches the stable pilot flame.
Air is added downstream of the primary combustion zone through slots or holes in
the liner surface in order to allow the further combustion of the formed CO (second
stage of the combustion). More air is added further downstream (dilution air). With
can liners, there are transition tubes which bring the combustion gas to the turbine
inlet. Also these transition tubes have to be cooled. The air used for convection
cooling of these tubes is mixed in just before the turbine inlet. The same principles
are used in other combustion systems, but the technical realisation may be different
from described above. In particular, with an annular combustion chamber, all burners are equal. A possible realisation is then a burner in the style of the pilot burner of
Fig. 11.9, with a second ring of swirl vanes around it with injection of fuel forming
a very lean mixture (e.g. Siemens, Alstom). The basic ingredients are always: gas
injection in a swirling air flow, swirl-stabilisation of the flame, and staging of the
combustion air. This technology is commonly denoted by the term dry low NOx
(DLN) or dry low emissions (DLE). With the most advanced systems, nowadays
(2014), the production of NOx and CO can be kept below 9 ppm over a broad range
of the load. Alternative techniques for reducing NOx are injection of water or steam
in the combustion chamber. These wet techniques are sometimes used with smaller
gas turbines, but not with large turbines that run almost continuously.