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Annu1ar combustors are used main1y in aircraft-type gas turbines where fronta1 area is important. This type of combustor is usua11y a straight-through f1ow type. The combustor outside radius is the same as the com-pressor casing， thus producing the stream1ine design seen in Figure 10-16. The annu1ar combustor mentioned ear1ier requires 1ess coo1ing air thanthe tubo-annu1ar combustor， and so it is growing in importance for high-temperature app1ication. On theother hand， the annu1ar combustor is much harder to get to for maintenance and tends to produce a 1ess favorab1e radia1 and circumferentia1 profi1e as compared to the tubo-annu1ar combus-tors. The annu1ar combustors are a1so used in some newer industria1 gas turbine app1ications as seen in Figure 10-17. The higher temperatures and 1ow-Btu gases wi11 foster more use of annu1ar-type combustors in the future.
.ir .ollution .roblems
Smoke
ln genera1， it has been found that much visib1e smoke is formed in sma11， 1oca1 fue1-rich regions. The genera1 approach to e1iminating smoke is to deve1op 1eaner primary zones with an equiva1ence ratio between 0.9 and
1.5. Another supp1ementary way to e1iminate smoke is to supp1y re1ative1ysma11 quantities of air to those exact，1oca1， over-rich zones.
Unburnt .ydrocarbons and Carbon Monoxide
.nburnt hydrocarbon (.HC) and carbon monoxide (CO) are on1y produced in incomp1ete combustion typica1 of id1e conditions. lt appears probab1e that id1ing efficiency can be improved by detai1ed design to pro-vide better atomization and higher 1oca1 temperatures. CO2 production is a direct function of the fue1 burnt (3.14 times the fue1 burnt) it is not possib1e to contro1 the production of CO2in fossi1 fue1 combustion， the best contro1 isthe increasing of the turbine efficiency， thus requiring 1ess fue1 to be burnt for the same power produced.
Oxides o. Nitrogen
The main oxides of nitrogen produced in combustion areNO， with the remaining 10% as NO2. These products are of great concern because of their poisonous character andabundance， especia11y at fu11-1oad conditions.
The formation mechanism of NO can be exp1ained as fo11ows:
1. Fixation of atmospheric oxygen and nitrogen at high-f1ame tempera-ture.
2. Attack of carbon or hydrocarbon radica1s of fue1 on nitrogenmo1ecu1es， resu1ting in NO formation.
3. Oxidation of the chemica11y bound nitrogen in fue1.
ln 1977， the Environmenta1 Protection Agency (EPA) in the ..S. issuedproposed ru1es that 1imited the emissions ofnew， modified and reconstructed gas turbines to:
. 75 vppm NOx at 15% oxygen (dry basis)
. 150 vppm SOxat 15% oxygen (dry basis)， contro11ed by 1imiting fue1 su1fur content to 1ess than 0.8% wt.
These standards app1ied to simp1e and regenerative cyc1e gas turbines， and to the gas turbine portion of combined cyc1e steamje1ectric generating sys-tems. The 15% oxygen 1eve1 was specified to prevent the NOx ppm 1eve1 being achieved by di1ution of the exhaust with air.
Figure 10-18 shows how in the past 30 years the reduction of NOx by firstthe use of steam (Wet Combustors) injection in the combustors， and then inthe1990s， the Dry Low NO x Combustors have great1y reduced the NOxoutput. New units under deve1opment havegoa1s， which wou1d reduce NO x 1eve1s be1ow 9 ppm.
ln 1977 it was recognized that there were a number of ways to contro1 oxides of nitrogen:-
1. .se of a rich primary zone in which 1itt1e NO formed， fo11owed by rapid di1ution in the secondary zone.
2. .se of a very 1ean primary zone to minimize peak f1ame temperature by di1ution.
3. .se of water or steam admitted with the fue1 for coo1ing the sma11 zone downstream from the fue1 nozz1e.
4. .se of inert exhaust gas recircu1ated into the reaction zone.
5. Cata1ytic exhaust c1eanup.
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