Lean‐Premix Technology or Dry‐Low NOx

Processes that use air as a diluent to reduce combustion flame temperatures achieved reduce NO_x by premixing the fuel and air before they enter the combustor. This type of process is called lean‐premix combustion and goes by a variety of names, including the dry‐low NO_x (DLN) process of General Electric, the dry‐low emissions process of Rolls‐Royce, and the SoLoNO_x process of Solar Turbines.

Lean premixed designs reduce combustion temperatures, thereby reducing thermal NO_x. In a conventional turbine combustor, the air and fuel are introduced at an approximately stoichiometric ratio and air/fuel mixing occurs at the flame front where diffusion of fuel and air reaches the combustible limit. A lean premixed combustor design premixes the fuel and air prior to combustion. Premixing results in a homogeneous air/fuel mixture, which minimizes localized fuel‐rich pockets that produce elevated combustion temperatures and higher NO_x emissions. A lean air‐to‐fuel ratio approaching the lean flammability limit is maintained, and the excess air serves as a heat sink to lower combustion temperatures, which lowers thermal NO_x formation. A pilot flame is used to maintain combustion stability in this fuel‐lean environment. Lean‐premix combustors can achieve emissions of about 9 ppmvd NO_x at 15% oxygen (~94% control).

To achieve low NO_x emission levels, the mixture of fuel and air introduced into the combustor must be maintained near the lean flammability limit of the mixture. Lean‐premix combustors are designed to maintain this air/fuel ratio at rated load. At reduced load conditions, the fuel input requirement decreases. To avoid combustion instability and excessive CO emissions that occur as the air/fuel ratio reaches the lean flammability limit, lean‐premix combustors switch to diffusion combustion mode at reduced load conditions. This switch to diffusion mode means that the NO_x emissions in this mode are essentially uncontrolled.

Lean‐premix technology is the most widely applied precombustion control technology in natural gas turbine applications. It has been demonstrated to achieve emissions of approximately 9 ppmvd NO_x at 15% oxygen (Catalytica Energy Systems Inc. 2004).

Steam/Water Injection

In steam/water injection, the technology commonly chosen to reduce the NO_x emissions in natural gas turbine, higher combustion temperatures are used to achieve greater thermodynamic efficiency. In turn, more work is generated by the gas turbine at a lower cost. However, the higher the gas turbine inlet temperature, the more NO_x that is produced. Diluent injection, or wet controls, can be used to reduce NO_x emissions from gas turbines. Diluent injection involves the injection of a small amount of water or steam via a nozzle into the immediate vicinity of the combustor burner flame. NO_x emissions are reduced by instantaneous cooling of combustion temperatures from the injection of water or steam into the combustion zone. The effect of the water or steam injection is to increase the thermal mass by mass dilution and thereby reduce the peak flame temperature in the NO_x forming regions of the combustor. Water injection typically results in a NO_x reduction efficiency of about 70%, with emissions below 42 ppmvd NO_x at 15% oxygen. Steam injection has generally been more successful in reducing NO_x emissions and can achieve emissions less than 25 ppmvd NO_x at 15% oxygen (~82% control).

Table 6.17 NO_x emissions, control effectiveness, economics, and environmental impacts.

Summary of NO_x BACT for Turbines and Duct Burners

Table 6.17 provides information on the emissions, control effectiveness, economics, and environmental impacts measures for the control of NO_x discussed in Sections 6.8.2.2, 6.8.2.4, and 6.8.2.5_. The analysis was performed on a unit (turbine and duct burner) basis.

SCONO_X provides the highest level of NO_x reduction. However, this very new technology has yet to prove itself for long‐term commercial operation on large‐scale combined‐cycle plants. It is the relatively high cost per emission reduction of this control technology ($13 627/T of NO_x removed) that rules out SCONOx as a control option. The next most effective control technology for NO_x is a combination of DLN combustors and SCR. The adverse environmental impact of SCR is primarily from the emissions of ammonia which is on the EPA’s list of extremely hazardous substances. Although these adverse impacts of the SCR process can be minimized with proper system design and operation, it is ruled out as a control operation because the cost is prohibitive ($10 191/T of NO_x removed).

The next most effective control technology is DLN combustors. At reduced loads, combustion instability requires that a switch to diffusion combustion mode, in which NO_x emissions are essentially uncontrolled. However, this can be minimized with proper system design and operation.