The combustion of fossil fuels has a significant impact on the environment. Products such as carbon monoxide (CO), carbon dioxide (CO2), oxides of nitrogen (NOX), oxides of sulfur (SOx), and particulates are created in the combustion process and are discharged into the environment. While no entirely benign method of burning fossil fuels has been developed, numerous technological developments in combustion and combustion control permit these systems to be operated in a manner that significantly reduces their environmental impact.
In a Combustion Turbine, there are several specific ways in which NOX reduction can be achieved:
There are five chemical compounds that are defined as NOx. These are:
Nitric oxide (NO) and nitrogen dioxide (NO2) are the most common forms of NOX created in the combustion process. NO is the primary chemical formed in the combustion process. The NO generally oxidizes to form NO2.
The global emissions of NOX into the atmosphere have been increasing steadily since the middle of the last century. A significant amount of the increased emissions is attributed to human activities, in particular increased combustion of biomass such as wood, trash and other organic materials and fossil fuels.
NOX is characterized by the source of nitrogen in the combustion process which contributes to the formation of NOX. When the source of nitrogen is the fuel, it is referred to as “fuel NOX.” When the source of nitrogen is air, it is referred to as “thermal NOX.” Fuels with high fractions of chemically bound nitrogen, such as coal and heavy fuel oil, produce significant quantities of fuel NOX. Fuels with low fractions of chemically bound nitrogen, such as natural gas and light fuel oils, tend to produce very little fuel NOX. They primarily form thermal NOX.
The NOX, regardless of how it is created, is chemically identical. The designation of thermal or fuel NOX is important in terms of understanding the processes which create these nitrogen compounds and the types of control methods which are most effective in reducing the formation of the NOX.
The creation of thermal NOX, as the name implies, is temperature dependent. It is created when oxygen (O2) and nitrogen, from the air supplied for combustion, combine at temperatures of approximately 2,900°F or higher. Control strategies for minimizing the formation of thermal NOX include reducing peak combustion temperatures and controlling the supply of air to the combustion process. Some of the chemical processes involved in the formation of thermal NOX are shown below.
Fuel NOX is created by nitrogen bound in the fuel mixing with oxygen. A significant portion of the fuel NOX is created during the early stages of combustion during the devolatilization of the fuel. Devolatilization of the fuel refers to the release of volatile chemical components from the fuel. This occurs early in the combustion process of coal and heavy fuel oil. Fuel NOX is most commonly associated with coal. It is also created to a lesser degree from heavy fuel oil. The control of fuel NOX production includes limiting the amount of air provided during the early combustion stages of the fuel.
While all the chemical processes that form NOX are not completely understood, certain factors are known to be primary in NOX formation. These are: the amount of nitrogen in the fuel, the amount of excess air provided to the combustion process and the peak combustion temperatures. Technologies and operational procedures to limit the formation of NOX in combustion focus on controlling these factors.
In combustion turbine operation, the fuel source (fuel oil or natural gas) has directly impacted the fuel NOX produced in the combustion process. As stated previously, although fuel oil combustion produces less NOX than coal, the production of NOX is still a concern. Natural gas, however, does not contain nitrogen, and, therefore, does not produce any fuel NOX. Therefore, NOX generation in a Combustion Turbine can be lowered by minimizing the use of fuel oil.
In both fuel oil and natural gas firing modes, low- NOX burners help to lower NOX by providing a more thorough mixing of fuel and air. With better mixing, increased power output can be obtained while minimizing flame temperature, reducing the amount of thermal NOX produced. Therefore, NOX generation in a Combustion Turbine can be lowered by conducting routine maintenance to ensure the proper operation of the burners.
Flame temperature can also be reduced in a Combustion Turbine by adding staged water sprays, commonly referred to as NOX Water. In a typical NOX Water System, high-pressure demineralized water is injected into the combustion chamber, lowering the flame temperature and reducing the amount of thermal NOX produced. However, the amount of water injected must be tightly controlled, as excessive water injection could extinguish the flame. Therefore, NOX generation in a Combustion Turbine can be lowered by conducting routine maintenance to ensure the proper operation of the NOX Water Systems.
NOX produced in the Combustion Turbine can be lowered by using an absorption system – typically referred to as a Selective Catalytic Reduction System (SCR) or Selective Non-Catalytic Reduction System (SNCR). Both systems use a chemical process to remove NOX from the exhaust gas. A diagram showing the processes involved in an SCR is shown below.
However, these systems have a limited range of temperature for operations, typically 575 to 750°F. The exhaust gases exiting a Combustion Turbine generally are well over this temperature; when operating in the Simple Cycle Mode, using an SCR or SNCR system is not possible.
However, in the Combined Cycle Mode, the hot exhaust gases are passed through a Heat Recovery Steam Generator (HRSG) – the heat in the exhaust gas is transferred to the water in the HRSG to produce steam which can be provided to other loads, such as a steam turbine. This results in significant cooling of the exhaust gas, bringing the temperature to within the range for using an SCR or SNCR system. Therefore, NOx generation in a Combustion Turbine can be lowered by minimizing the time operating in Simple Cycle mode and by conducting routine maintenance to ensure proper operation of the SCR or SNCR systems.
In a Combustion Turbine, there are several specific ways in which NOx reduction can be achieved:
It is recognized that some of these factors are beyond the scope of the Combustion Turbine Operator. The decision to use fuel oil or natural gas for operations, for example, is made by plant management. Similarly, the decision to operate in the Simple Cycle or Combined Cycle mode of operation is not a decision made directly by the operator.
However, proper maintenance of NOx reduction systems – specifically the NOx Water System, the burners, and the SCR or SNCR systems – has a DIRECT impact on the production of NOx. Therefore, it is crucial that these systems are adequately maintained to ensure that air quality permit requirements are met at all times.
