Steadily tightening federal and state government regulations for allowable pollutants that can be discharged into the atmosphere have created an incentive for the power generation industry to find new cost-effective means to reduce power plant flue gas emissions.
Today's primary concern for operators of new and existing power plants - whether coal-, oil-, or natural gas-fired - is the reduction of nitrogen oxides (NOx) emissions. Under the Clean Air Act Amendments (CAAA) of 1990 and best available control technology (BACT) for new units, U.S. EPA increasingly requires reduced NOx emissions from power plants.
More recently, California adopted BACT standards limiting NOx emissions from new simple-cycle gas turbine plants to 5 ppmvd, and for new combined cycles, 2.5 ppmvd. Other states have since followed with similar restrictive limits for new combined cycle and cogeneration plants.
NOx in fuel gas results from nitrogen oxidizing either in the combustion air (thermal NOx), or in the fuel (fuel NOx) - 90 percent to 95 percent being nitric acid (NO), the remainder being nitrogen dioxide (NO2).
• Fuel NOx formation depends on the fuel/air ratio, thus most low NOx burners are designed to carefully introduce reduced oxygen levels for fuel combustion.
• Boiler thermal NOx emissions can be minimized by reducing combustion zone temperature; although firing techniques that limit fuel NOx formation are far more effective in reducing total NOx.
Combustion NOx emissions reduction methods include: combustion control and post-combustion control.
Combustion control is the primary method for reducing NOx emissions from boilers burning coal, oil, or natural gas. These control systems include low-NOx burners and dry low-NOx combustors, with the technology selected for a particular plant dependent on the required NOx emissions limits and the specific fuel to be fired.
Low NOx Burners -
Used for new and retrofit applications on boilers and combustion turbines burning nearly any fuel. However, low-NOx burners are not used on fluidized-bed boilers, because the low combustion temperature and complete mixing within the combustion chamber generally limit NOx emissions from CFB boilers to as little as 0.2 lb/Mbtu.
Pulverized coal (PC) boilers, however, use low-NOx burner systems installed as wall-fired, or corner-fired units. Wall-mounted burners not only control the fuel/air flow to each burner, but also control the amount and position of secondary air in the burner zone reducing oxygen content in the fuel devolatization and high-temperature zones.
Corner-fired, low NOx burners are configured either as concentric firing systems for retrofit applications, or as pollution minimum systems for new power plants.
• Low NOx Concentric Firing System (LNCFS)-reduces NOx about 20 percent to 50 percent by directing a portion of auxiliary air toward the furnace wall at an angle with the primary air/coal stream, thus reducing air available to the fuel stream devolatization. Adding overfire air to an LNCF system will deliver a vertical air stream throughout the furnace's height.
• Pollution Minimum (PM) System - reduces NOx about 60 percent, using both overfire air (OFA) and local stoichiometry control during the early part of the combustion process. Centrifugal separation splits the fuel/primary air stream into fuel-rich and fuel-lean streams producing air ratios limiting NOx production. PM systems, however, cannot be retrofitted because the windbox is taller than a conventional tangential windbox.
Among a number of systems currently available to provide low NOx emissions are Foster Wheeler's Vortex Series/Split Flame low NOx burner for wall-fired boilers, and its Tangential Low NOx systems for corner-fired boilers that provide not only ultra-low NOx emissions, but also minimizes CO emissions.1