Gas Turbine Dry Combustion Technology

Dry low-NOx (DLN) combustion technology utilizing lean pre-mixed combustion continues to be the primary trend in development efforts for combustion gas turbines. Premixing marks a change from traditional diffusion flames, in which mixing and combustion take place simultaneously with high temperature peaks and short residence time. Already, NOx emissions levels of 9

Fig. 17-20 Illustration of Pre-Chamber with Respect to the Main Combustion Chamber on Dual-Fuel Compression-Ignition Reciprocating Engine. Source: MAN B&W

Fig. 17-21 Influence of the Pilot Fuel Quantity on Pollutant Emissions and Thermal Fuel Efficiency with Direct Pilot Fuel Injection. Source: MAN B&W

ppmd (@ 15% O2) in several systems have been reported, though 15 to 25 ppmd is typically the current lowest guaranteed level. There are numerous dry combustion designs. Three challenges associated with very lean combustors are:

• The need to maintain flame stability at the design operating point.

• Precise turndown capability over the required load range.

• Balancing NOx reductions with CO (and VOC) increases that occur with low flame temperature and short residence time.

Figure 17-22 illustrates a basic pre-mixing design. This is accomplished by operating the combustor primary zone at increased airflow and lower average temperature. Dilution zone airflow is reduced to keep total combustor airflow and exit temperature unchanged. Gas turbine output and heat rate are therefore not affected. To provide a uniform flame temperature and avoid hot spots, the fuel and primary zone air are pre-mixed upstream. Figure 17-23 shows a lean pre-mixed gas turbine combustor liner for a 5,000 hp (3,700 kW) capacity unit. The sheet metal liner is similar to conventional combustor liners in geometry and construction. Low metal temperatures are maintained through film-cooling, using a small fraction of the compressor discharge air.

Figure 17-24 shows a 12,400 hp (9,250 kW) industrial gas turbine with dual silo-type DLN combustors. Figure 17-25 is a cutaway illustration of this silo-type combustor. Air at about 660°F (350°C) enters the lower combustion chamber area, feeding the burner and cooling the mixing tube, flame tube shell, ceramic holder, and flame tube

Primary Air Inlet

Natural Gas Injection

Premixing Zone

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