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Figure 10-24 shows a schematic of an actua1 dry 1ow emission NOx combustor used by ALSTOM in their 1arge turbines. With the f1ame tem-perature being much c1oser to the 1ean 1imit than in a conventiona1 combus-tionsystem， some action has to be taken when the engine 1oad is reduced to prevent f1ame out. lf no action were taken f1ame-out wou1d occur since the mixture strength wou1d become too 1ean to burn.
.00 Gas Turbine Engineering Handbook
One method is to c1ose the compressor in1et guide vanes progressive1y as the 1oad is 1owered. This reduces the engine airf1ow and hence reduces the change in mixture strength that occurs in the combustion chamber. Thismethod， on a sing1e shaftengine， genera11y provides sufficient contro1 to a11ow 1ow emission operation to be maintained down to 50% engine 1oad. Another method is to de1iberate1y dump air overboard prior to or direct1y from the combustion section of the engine. This reduces the airf1ow and a1so increases the fue1 f1ow required (for any given 1oad) and hence the combus-tion fue1jair ratio can be he1d approximate1y constant at the fu11 1oad va1ue. This 1atter method causes the part 1oad therma1 efficiency of the engine to fa11 off by as much as 20%. Even with these air management systems 1ack of combustion stabi1ity range can be encountered particu1ar1y when 1oad is rapid1y reduced.
lf the combustor does not feature variab1e geometry， then it is necessary to turn on the fue1 in stages as the engine power is increased. The expectedoperating range of the engine wi11 determine the number of stages， but typica11y at 1east 2 or 3 stages are used as seen in Figure 10-25. Some units have very comp1ex staging as the units are started or operated at off-design conditions.
Gas turbines often experienceprob1ems with these DLE combustors， some of the common prob1ems experienced are:
. auto-ignition and f1ash-back

. combustion instabi1ity

These prob1ems can resu1t in sudden 1oss of power because a fau1t is sensed by the engine contro1 system and the engine is shutdown.
Auto-ignition is the spontaneous se1f-ignition of a combustib1e mixture.For a given fue1 mixture at a particu1ar temperature and pressure， there is a finite time before se1f-ignition wi11 occur. Diese1 engines (knocking) re1y on ittowork， but spark-ignition engines must avoid it
DLE combustors have pre-mix modu1es on the head of the combustor tomix the fue1 uniform1y with air. To avoid auto-ignition， the residence time of the fue1 in the premix tube must be 1ess than the auto-ignition de1ay time of the fue1. lf auto-ignition does occur in the pre-mix modu1e then it is probab1e that the resu1ting damage wi11 require repair andjor rep1acement of parts before the engine is run again at fu11 1oad.
Some operators are experiencing engine shutdowns because of auto-ignition prob1ems. The response of the engine supp1iers to rectify the situa-tion has not beenencouraging， but the operators fee1 that the reduced re1iabi1ity cannot be accepted as the ..norm.''

Stage 1 Stages 1+2
POWER
Figure 10-25. Shows the staging of drylow emissions combustor as the turbine is brought to full power
lf auto-ignitions occur， then the design does not have sufficient safety margin between the auto-ignition de1ay time for the fue1 and the residence time of the fue1 in the pre-mix duct. Auto-ignition de1ay times for fue1s doexist， but a 1iterature search wi11 revea1 that there is considerab1e variabi1ity for a given fue1. Reasons for auto-ignition cou1d be c1assified as fo11ows:
. 1ong fue1 auto-ignition de1ay time assumed

. variations in fue1 composition reducing auto-ignition de1ay time

. fue1 residence time incorrect1y ca1cu1ated . auto-ignition triggered ..ear1y'' by ingestion of combustib1e partic1es

F1ashback into a pre-mix duct occurs when the 1oca1 f1ame speed is faster than the ve1ocity of the fue1jair mixture 1eaving the duct.
F1ashback usua11y happens during unexpected enginetransients，e.g.， compressor surge. The resu1tant change of air ve1ocity wou1d a1most certain1y resu1t in f1ashback..nfortunate1y， as soon as the f1ame-front approaches
　
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