EPA non-attainment areas are constantly under pressure to meet the Clean Air Standards of 1990. While a number of non-attainment areas exist throughout the country, California has been dealing with this issue for decades. As cost-effective combustion solutions have been developed, that state’s Air Resource Boards have responded accordingly by lowering the NO_X emissions on industrial and commercial stationary sources.

The most recent round of emission reductions focused on units with heat inputs of 2 million BTU/hr or more. Two California air-pollution agencies — the South Coast Air Quality Management District (SCAQMD) and the San Joaquin Valley Air Pollution Control District (SJVAPCD) — mandated that NO_X emissions on gas-fired stationary sources not exceed 9 ppm corrected to 3 percent excess oxygen.

Formation of NO_X in Industrial Thermal Processing Equipment

NO_X is produced from the nitrogen present in the combustion air and in the fuel being burned. The generation of NO_X is, in large part, governed through chemical kinetics, and it results from high flame temperatures in the presence of oxygen over a period of time (commonly referred to as residence time).

The two main components when burning gaseous fuels are thermal NO_X and prompt NO_X:

• Thermal NO_X results when the nitrogen and oxygen in the air combine at the elevated temperatures of the combustion process. (Remember that the air we breathe consists of approximately 21 percent oxygen and 79 percent nitrogen.)
• Prompt NO_X is formed in the early, low-temperature and fuel-rich stages of combustion. Hydrocarbon fragments can react with nitrogen to form fixed nitrogen species such as CN, NH₃, HCN, H₂CN, etc. These, in turn, can be oxidized in the leaner zones of the flame, forming NO.

While prompt NO_X is usually a relatively small part of the total NO_X produced, it can be a significant portion of the total amount for ultra-low-NO_X combustion systems (9 ppm or lower).

Reducing NO_X Emissions in Industrial Heating Equipment

In general, the reduction of NO_X in the combustion process involves three methods:

• The reduction of the peak flame temperatures, which drive the chemical kinetics.
• The control of stoichiometry, or the amount of oxygen present during these critical reactions.
• The reduction of nitrogen in the fuel. (This generally pertains to fuel oils.)

Effective burner design is how these goals are accomplished. Burners can be classified as diffusion flame (nozzle-mix) type, premix or a hybrid of both types.

Nozzle-Mix Burners. While low excess-air operation generally results in a higher flame temperature, low excess-air operation does reduce the amount of free oxygen and nitrogen available for the formation of NO_X. For every percentage point of reduction of the flue-gas oxygen level, the NO_X level generally is reduced by 2 to 8 ppm of NO_X.

Staged Combustion Burners. Staged air and fuel burners operate by using a first stage that is high in excess air, which results in lower peak flame temperatures in this zone. Then, in the second-stage flame zone, the burner operates at lower oxygen concentrations, which retards NO_X formation. NO_X emissions are further reduced by the quenching effect of inert products of combustion from the first stage, which lowers the second-stage flame temperature.

Induced Flue-Gas Recirculation (IFGR). This well-known method uses cooled flue gases from the boiler stack as a source of dilution. These gases are very low in oxygen content and are composed of inert compounds like nitrogen, water vapor and carbon dioxide, which are excellent heat sinks. These inert compounds reduce the peak flame temperature in the combustion process and can effectively reduce the formation of thermal NO_X by 80 percent.

During operation of a burner equipped with IFGR, the burner’s combustion air blower induces a flow of recirculated flue gas that is mixed with the combustion air within the burner. Flue gas enters the burner through an IFGR adapter, which prevents recirculation during purge periods and controls the volume of flue-gas flow during burner operation. Maintaining stable combustion when inducing flue-gas recirculation is critical to the success of this method of NO_X reduction.

IFGR burners can be used to achieve sub 30 ppm easily in most applications, and sub 20 ppm NO_X levels are achievable in favorable furnace designs. Because the combustion-air fan delivers both the necessary air for combustion and the recirculated flue gases, additional fan capacity may need to be incorporated. In some instances, burner model size may increase for a given application.

In general, IFGR is a cost-effective and proven method for reducing NO_X to sub 30 ppm levels. The addition of flue-gas recirculation piping to deliver the flue gases to the burner needs to be considered in the total installation cost of this low-NO_X burner system.

Full Premix Burners (Non-Surface Type). Premix low-NO_X burners reduce NO_X emissions by controlling the fuel-air mix and temperature in each combustion zone. These burners do not require any external flue-gas piping, and they utilize normal (unfiltered) boiler room combustion air. Burner maximum sizes generally are limited due to increased pressure-drop requirements across the burner head and relatively large diffuser openings (for premix technology) to ensure no flash-back conditions. Similar NO_X reductions can be obtained if the premix strategy is combined with flue-gas recirculation. This approach will reduce the higher excess-air levels normally required with the full premix strategy, allowing for more efficient operation.

Full Premix Surface-Combustion Burners. These are surface-burning, radiant-type burners, usually constructed of metal, ceramic or metal-fiber mesh surfaces that use a premix gas/air arrangement. The flame burns at temperatures just below the threshold where thermal NO_X is formed, and the rapid combustion of the fuel minimizes the formation of prompt NO_X.

The premix strategy alone can achieve as much as 90 percent NO_X reduction. NO_X levels are reduced with increased oxygen (O₂) operating levels. Typically, stack O₂ levels in the 4 to 5.5 percent range will produce sub 30 ppm NO_X (corrected to 3 percent O₂) levels. With O₂ in the 5.5 to 6.5 percent range, sub 20 ppm NO_X levels can be achieved. Sub 9 ppm and lower NO_X levels can be achieved with even higher excess oxygen levels, in the 7.5 to 9 percent range.

Ultra-Low-NO_X Solutions for Industrial Heating

The two most proven combustion approaches for achieving single-digit NO_X emissions are surface-stabilized, 100 percent premix and high IFGR with staged combustion.

Surface-Stabilized, 100 Percent Premix. A typical 100 percent premix, surface-stabilized burner can be fitted with a burner control panel, an air-filter assembly, flame-safeguard controls, venturi-mixing chamber and a woven-fiber mesh element. Additional safety devices — typically differential-pressure switches — are advised to monitor the filter condition to warn when the filter is getting dirty and to shut down the burner when the filter is too dirty to maintain proper air-fuel ratios. The ease with which premix burners can achieve single-digit NO_X emissions offsets the small loss in system efficiency due to operating at high excess oxygen levels.

High IFGR with Staged Combustion. A typical high-IFGR, staged-combustion burner combines two proven technologies to effectively reduce thermal NO_X and prompt NO_X while operating at lower excess-oxygen levels than the fully premixed burners. To closely monitor the amount of flue gas being introduced into the burner and the effective control of the staged air/fuel ratios, these types of burners require more sophisticated safety and combustion controls. Minimally, a parallel-positioning control system is required, and oxygen trim is recommended. Fully metered combustion controls sometimes are required due to the level of staged combustion being utilized and the rapid mixing of the air and fuel in combination with the IFGR.

Both types of ultra-low-NO_X burner systems have been applied effectively in boiler, process heater and steam generator installations within California and other non-attainment areas throughout the country.

 As combustion technology continues to advance, the development of efficient, even-lower emission burners are possible. If your plant needs to achieve ultra-low-NO_X emissions, take a fresh look at how retrofitting a new burner can improve emissions control on your process heating system.