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Show 3 engine equipped with lean-premixed combustors. Although the goal is to have all of the ammonia react with the NOx to fonn N2, some ammonia slip occurs. A "rule" used by at least one regulatory agency in the US is that 9 ppm NOx plus the typical ammonia slip ~s "equivalent" to 15 ppm NOx from a gas turbine engine for which the NOx control IS effected by lean-premixed combustion. Until recently, SCR could not be applied to simple cycle gas turbine engines, that is, to engines which were not attached to heat recovery steam generators. This is because the SCR catalysts could not withstand the temperature of the uncooled gas turbine engine exhaust. However, molecular sieve technology, which overcomes this temperature limit, is now available for some SCR catalysts. The water/steam injection and SCR methods of NOx control have been applied mainly to electrical generation gas turbine engines. Generally, these methods have not been regarded as suitable for the control of NOx from pipeline gas turbine engines. The exceptions are a few engines in British Columbia, which have been equipped recently for water injection (e.g., see Robinson, 1994), and a new three-engine pipeline site in Southern California, which is trying SCR as the sole NOx control element. Although these NOx control techniques are well established commercially, costs can be substantial, as much as several tens of thousands of dollars per ton of NOx removed per year. Thus, there has been a substantial interest in controlling the NOx fonnation through advanced combustion technology. Lean-premixed combustion has emerged as the preferred method for this generation of land based gas turbine engines. 3. NOx FORMATION CHEMISTRY IN GAS TURBINE ENGINE COMBUSTORS Generally, NOx can form by three distinct chemical kinetic routes, which are the Zeldovich, nitrous oxide, and prompt mechanisms. These mechanisms fonn NO; the N02 component of the NOx sum fonns by oxidation of some of NO in the combustor or turbine, or in the emissions sampling system. The Zeldovich mechanism involves attack of O-atom on N2. This reaction yields NO and N-atom, which is subsequently oxidized to NO. The nitrous oxide mechanism also involves attack of O-atom on N2. However, in this case, the reaction leads to the fonnation of nitrous oxide (N20), a reaction intermediate. Through attack by O-atom and H-atom, some of the N20 is converted to NO. The balance of the N20 is converted to N2. (Emission levels of N20 are only about 1 to 3 ppm.) For flame temperatures greater than about 1800K, the Zeldovich route is favored, whereas for temperatures below about 1800K, the nitrous oxide route becomes significant. High pressures appear to favor the nitrous oxide route, because the N20-forming step is a three-body reaction. In leanpremixed combustion turbines, the nitrous oxide route appears to be important because of the relatively modest flame temperatures (of about 1650 to 1850K), and the high pressure levels (of 12 to 40atm). The prompt mechanism involves attack of CH-radicals on N2, leading to HCN and N-atom, which subsequently are converted to NO. For additional information on the behavior of these mechanisms in lean-premixed combustion turbines, see for example Nicol et al. (1993 and 1994) and Steele et al. (1994). The term thermal NOx refers to NO formed by the Zeldovich mechanism in a field of free radicals (i.e, O-atom, H-atom, and OH-radical) which are assumed to be at their local thermo-chemical equilibrium concentrations. Conventional diffusion flame combustors provide an opportunity for thermal NOx to form in hot, stoichiometric post-flame zones, |
