Dry Low Nox System

The significant products of combustion for gas turbine emissions are oxides of nitrogen (NO and NO2, collectively called NOx), carbon monoxide (CO), unburned hydrocarbons (UHC; these are usually expressed as equivalent methane - CH4 - particles, and arise from incomplete combustion), oxides of sulfur (SO2 and SO3), and particulates. There are two sources of NOx emissions in the exhaust of a gas turbine. Most of the NOx is generated by the fixation of atmospheric nitrogen in the flame. This is called thermal NOx. Other NOx are also generated by the conversion of a fraction of any nitrogen chemically bound in the fuel (called fuel-bound nitrogen or FBN).

Thermal NOx is generally regarded as being generated by a chemical reaction sequence. This set of well-verified chemical reactions postulates that the rate of generation of thermal NOx is an exponential function of the temperature of the flame. It, therefore, follows that the amount of NOx generated is a function not only of the temperature but also of the time the hot gas mixture is at flame temperature. It turns out to be a linear function of time.

In response, GE ENERGY PRODUCTS - EUROPE recommends staged combustors use. The dry low NOx combustion system operates in four distinct modes (illustrated in fig.1) during natural gas fuel operation:

 

Mode	Operating Range
(A) Primary	Ignition, acceleration and operation to 30% load (fuel to first stage)
(B) Lean-lean	30% load to approximately 50%* load (fuel to both stages, flame in both stages)
(C) Secondary (transfer)	Staging at 50%* load (fuel to second stage only)
(D) Premixed	50%* to 100% load (fuel to both stages, flame in second stage only)

*50 % for the gas turbines equipped with a bleed heating system.
*70 % for the gas turbines not-equipped with a bleed heating system.

Natural gas operation
The combustion chamber operates in the following way (figure 2):

100 % of the fuel is injected (during the start-up), through valve VGS-3, by the primary nozzles and burns as a propagation flame (A mode).

This combustion (with a lean mixture of fuel/air) is the normal operating mode during ignition acceleration of the turbine up to its nominal speed, synchronization and load pick up, up to 30 % of the nominal load (emissions of NOx and CO are then comparable to the emissions of classical combustion chambers, equipped with only one fuel nozzle). The percentage of NOx remains low during that phase (lower than 100 ppmvd).

During the load-up, more and more fuel (up to 30 % of the base nominal flow) is injected, through valve 3 way VGS-3, in the secondary combustion zone and is ignited by the high temperature combustion products coming from the primary combustion zone (B mode). At this stage, valve VGD-1 sends the gas in the "internal" part of the secondary nozzle, i.e. through the peripheral nozzles, standing back from the central nozzle.

A "clean" combustion is maintained in a wide range of loads (up to 70 % of base load for turbines not equipped with a bleed heating system and up to 50 % for equipped turbines), burning a lean fuel mixture in the primary zone in a first time, then in the two zones (primary and secondary) in a second time.

At 50 % or 70 % of the base load, the fuel flow in the primary zone is shut and the fuel is totally injected, for a brief period, in the secondary zone (C mode), this with the help of the second circuit of the secondary zone : the transfer circuit. Through this circuit, the gas is injected at the end of the secondary nozzle.

This "transfer" mode extinguishes the flame in the primary zone.

When no flames are detected by the flame detectors, the fuel is sent again in the primary zone and simultaneously its flow is reduced in the secondary zone. The injected fuel in the primary zone is thoroughly mixed to the combustion air to obtain a low fuel/air ratio and an unvarying mixture.


The mixture goes to the secondary zone where it is ignited. The transfer nozzle maintains the flame during the transfer and the premixed operation, it's being used as a pilot nozzle. The aerodynamics of the combustion chamber has been conceived so that the flame cannot go back to the primary zone.

The NOx emission rate is extremely low owing to the lean and unvarying fuel/air mixture.

This final operating form is called premixed mode (D mode). 

 

 

Combustion Chamber Operating on Natural Gas Fuel