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Show 2 The goal of many of the gas turbine engine combustor development programs is a guaranteed NOx emission of 9 ppmvd (15% 02). In order to reach this objective, it is important that the NOx formation behavior be well understood in lean-premixed combustion. Much research is currently underway to gain this detailed understanding. This paper examines this effort. Methods of NOx control for gas turbine engines are summarized in Section 2, followed by a description in Section 3 of the chemical routes affecting NOx formation in gas turbine engines. Differences between conventional diffusion flame combustors and lean-premixed combustors are discussed in this regard. In Section 4, experimental results from our laboratory lean-premixed combustion reactors are shown, and in Section 5 these results are compared to results from other leanpremixed burners, including industrial scale burners. In Section 6, results from our modeling are presented, and in Section 7, conclusions are given. Throughout this paper, unless stated otherwise, the NOx emission is expressed in terms of ppmvd (15% 02): that is, as parts per million by volume, adjusted to a 15% 02, dry basis. For the remainder of this paper this is written simply as ppm. 2. SUMMARY OF NOx CONTROL FOR GAS TURBINE ENGINES Gas turbine engines equipped with conventional diffusion flame combustors, and fired on natural gas, have based-loaded NOx emissions in the 70 to 300 ppm range. Generally, across this range, the engines of small compressor ratio (i.e., 7 to 10) have the lowest emissions, whereas the engines of high pressure ratio (i.e., about 30) and large combustor size have the largest emissions. Firing these engines on distillate oil generally increases their NOx emissions by 40 to 50% compared to gas firing. Control of gas turbine engine NOx began in Southern California in the early 1970s. This was accomplished by water injection, and was closely followed (at other sites) by steam injection. Since that time water/steam injection has been widely applied to the control of NOx emissions from gas turbine engines. Many engines are now equipped with this NOx control method. For many gas-fired gas turbine engines equipped with conventional combustors, water/steam injection reduces the NOx emission to about 40 ppm. Typically, this requires a water to fuel ratio (by mass) of 0.8 to 1.0. Steam to fuel ratios (by mass) are about a factor 1.5 higher. The water/steam is injected into the primary zone of the gas turbine engine combustor in order to reduce the flame temperature, and thus, the rate of NOx formation. Some engines are tolerant of water and steam injection ratios greater than the ratios just given above. For example, some engines equipped with silo combustors are tolerant of steam to fuel ratios as great as 3.0, yielding NOx emissions in the 15 ppm range. Further, some aero-derivative engines have been injected with steam to fuel ratios in the 2.0 to 2.5 range, yielding NOx emissions as low as about 15 ppm. Selective catalytic reduction (SCR) is another method of NOx control which has been applied to gas turbine engines fIred on gas. This has been done since the mid 1980s. A typical emission control system begins with water or steam injection to effect an NOx concentration of about 40 ppm in the engine exhaust stream. Next, within the heat recovery steam generator (of the co-generation or combined cycle system), a CO oxidation catalyst is used to reduce the enhanced CO levels, which are caused by the water/steam injection. The concluding components (also located within the heat recovery steam generator) are the ammonia injectors and the SCR catalyst bank. Systems of this type are frequently designed to attain a final NOx emission of 9 ppm. Lower final levels of NOx are possible in some cases; for example, when SCR is added to a gas turbine |
