As states and local jurisdictions impose ever-more stringent emission limits for six criteria pollutants -- particulate matter (PM), nitrogen oxides (NOX), sulfur oxides (SOX), ozone, carbon monoxide (CO) and lead -- subject to the National Ambient Air Quality Standards, operators of boilers (and, in some areas, other fired process equipment) are being forced to adopt new burner technologies. Many or all of these states and local jurisdictions constantly review their emission standards as they relate to the best available control technology (BACT), and BACT can change with each new advance in burner development. Therefore, the operators of combustion equipment must be more knowledgeable than ever of emerging and evolving burner technologies as they become technically feasible, economically practical and commercially available.
To understand current burner technology, one must have a basic understanding of combustion fundamentals and how the pollutants, particularly NOX and CO, are produced. Combustion can be defined simply as the rapid reaction of fuel and oxidant. In this case, the oxidant is the oxygen (O 2) in air and the reaction is burning, resulting from the application of heat. The process produces heat (as well as light and noise) and, in the case of a hydrocarbon fuel such as natural gas, the principal byproducts are carbon dioxide (CO 2) and water vapor (H 2O).
There also are small amounts of NOX and CO produced in this process and, because of their significance as pollutants, are the substances primarily driving emission controls on natural gas burners.
CO is produced in varying amounts by all hydrocarbon-fueled combustion processes and is a measure of combustion efficiency (that is, high levels of CO in the exhaust gases generally are an indication of incomplete combustion). And, because the air and fuel -- as a practical matter -- cannot be perfectly mixed prior to combustion, more air is provided to aid the combustion. The amount of air added, beyond that required for perfect (stoichiometric) combustion, is referred to as excess air and is related to the O 2 in the air not used during combustion (i.e., excess O 2). Because O 2 is relatively easy to measure, conversions have been developed to relate excess air to the excess O 2 found in the flue gases. For example, 3 percent O 2 represents approximately 15 percent excess air. Although operation at high levels of excess air can reduce the emission of pollutants, it has an adverse effect on efficiency (table 1).
NOX is produced in burners through three mechanisms. The first, which accounts for the majority of the NOX going up the stack, is thermal NOX. As its name implies, this is the NOX formed as a high temperature reaction of the nitrogen (N 2) and O 2 in the combustion air. It is produced in the hottest part of the flame, and its formation increases (approximately) exponentially with flame temperature. It is obvious, then, that by lowering the flame temperature, thermal NOX formation can be reduced. This has led to the development of such techniques as flue gas recirculationand steam or water injection. However, the use of diluents reduces efficiency and therefore is an important economic consideration.
The second mechanism for NOX formation is prompt NOX.
Because it is a relatively low temperature phenomenon, it is not readily controlled by simply reducing the peak flame temperatures with flue gas recirculation or steam/water injection. Control of prompt NOX generally employs such techniques as combustion staging (air or fuel), either with or without flue gas recirculation.
The third NOX formation mechanism is fuel NOX. This is the conversion of fuel-bound N 2 to NOX by direct oxidation. This mechanism is not a significant consideration in natural gas combustion because generally natural gas has no organically bound N 2. However, it becomes extremely important when burning fuel oils or coal, but these fuels typically are not encouraged or permitted in low NOX applications -- and certainly not in ultralow NOX (less than 10 PPMv) requirements.
In addition to controlling NOX in the burner, there are a number of post-combustion treatment methods for removing NOX from the exhaust gases. These usually are classified as dry or wet. Common examples of dry post-treatment include selective catalytic reduction (SCR), selective noncatalytic reduction (SNCR) and activated carbon beds. Typically, the wet techniques employ oxidation-absorption-reduction in various combinations. All of these methods tend to be capital intensive, both in first cost and maintenance.
Another ApproachOne approach for ultralow NOX applications is a forced internal recirculation burner. This burner, unlike most burners developed in response to the original requirements of the 1990 Clean Air Act Amendments, uses no efficiency-reducing diluents such as external flue gas recirculation or steam/water injection, nor does it rely on costly post-combustion NOX reduction treatment or on energy wasting levels of excess air.
Firing natural gas, a forced internal recirculation burner utilizes staged combustion with internal recirculation of partial combustion products. This minimizes formation of both thermal and prompt NOX and reduces CO emissions.
During operation of a forced internal recirculation burner, the first stage of combustion incorporates both premixed substoichiometric combustion and the recirculation of partial combustion products around a Hastelloy recirculation insert (sleeve). This results in stable, uniform combustion with lower peak flame temperatures and pockets of high O 2 concentrations. The combustion temperatures in the second stage are reduced by enhanced heat transfer between the stages, and the second stage combustion controls further minimize the peak flame temperature. As a result, both thermal and prompt NOX formation is minimized.