OCR Text |
Show began in the early 1980's on some of the larger industrial gas turbines, where availability of enough space in the combustor can allowed staging of the fuel/ air mixture to meet the u. S. EPA requirement of 75 ppmv. Recent developments are aimed at NO reductions to a range near 25-30 ppmv for small to medium-sizeJ units, and as low as 7 ppmv for some very large machines. Many of the new systems are proposed to be retrofittable to existing units. Most Canadian gas turbines have annular or can-annular combustors, and dry IOw-NOx solutions will consist of new lean pre-mix combustors. Fuel will be mixed with compressor discharge air to achieve a uniform mixture, prior to entering the combustion zone. This prevents the mixture from passing through a stoichiometric ratio when mixing and combustion take place simultaneously. The fuel/air ratio is also quite lean to minimize NO formation, but this must be closely controlled during off-design conditions to prevent flameout. There is often a parallel or series fuel-staging process, and normally a diffusion flame in a primary zone for startup, acceleration and turndown. This flame will produce the bulk of the NOx at part load, although reduced mass flow would keep down the actual amount of NO emissions. Some companies also try to regulate airflow int~ the combustor. Difficult challenges are faced in trying to design a reliable fuel control system which can maintain good combustion and low CO emissions over a wide range of ambient temperature and loading conditions. These problems are greater with the compact, high pressure annular combustors on modern aero-derivative engines. Cost-effectiveness should be in the range of $500-2000 per tonne of NOx removal. For the large industrial units to be used in Canadian cogeneration projects, General Electric already has available a two-stage premix combustor can arrangement for the Frame 6 and Frame 7 line. GE has received its first order from Belgium for an LM6000 with its new mUlti-cup annular arrangement, which should serve as the basis for its other smaller aeroderivative units. Westinghouse is presently developing low-NOx combustor cans for its W251 and W501 units for commercial introduction in 1994. For the smaller engines aimed at the pipeline market, Solar Turbines has some demonstration units in the united States, with at least two planned for eastern Canada in 1994. Rolls-Royce will shortly be providing three RB211 units for a u.S. pipeline expansion, where an annular combustor has been converted to a ~ow emission can-annular system. Some very large gas turbines employ external single or dual silo combustors which, in the dry uncontrolled state, produced very high amounts of NOx (300-400 ppmv) because of the long residence time of the large flame. Recent developments by European companies (such as Asea Brown Boveri) have resulted in NOx reductions to the 10-25 ppmv level through the selective use of a large number of pre-mix conical burners. Siemens is using a smaller quantity of "hybrid" burners which burn a diffusion flame at low loads and a lean premix flame in the upper range. |