By Douglas J. Smith IEng, Senior Editor
Conducting airflow tests to determine the size of air register orifices used to balance airflow in burners. Photograph courtesy RJM Corporation.
Unlike sulfur oxides, which are formed only when coal contains sulfur, nitrogen oxides (NOx) are formed from nitrogen in the fuel and the combustion air. Currently there is no method for removing the nitrogen from coal before it is burned. Annually, coal-fired power plants account for approximately 25 percent of the total NOx emissions in the U.S.
When coal is burned, two types of NOx are formed: thermal NOx and fuel NOx. Thermal NOx, which accounts for 25 percent of the total emissions of NOx, results from the dissociation and oxidation of nitrogen in the combustion air. Fuel NOx on the other hand results from the oxidation of nitrogen that is organically bound in the fuel.
The total thermal NOx and fuel NOx generated from these reactions depends upon the actual characteristics of the combustion process. Different boiler designs and configurations affect how much NOx is produced. In addition to the boiler’s design, flame temperature, residence time at high temperatures, quantity of excess air (oxygen) and the nitrogen content of the fuel all affect the amount of NOx produced. Table 1 summarizes the amount of NOx emissions allowed under the Clean Air Act of 1990 for different coal-fired boilers.
Controlling NOx Emissions
Low-NOx combustion systems reduce the availability of oxygen in the primary combustion zone of the boiler. There are three ways to reduce oxygen in the combustion area: staged combustion using low-NOx burners (LNB) alone, using LNB with over-fire air (OFA) or by reburning. In reburning, coal and natural gas, from separate burners, are burned together.
With staged combustion, the formation of NOx is lowered by reducing the amount of combustion air going to the burners. Any additional air required for complete combustion is introduced into the burners as secondary air or above the burners as over-fire air. Staged combustion using LNB and OFA is used on wall-fired and tangential coal-fired boilers. These two types of boilers can also use reburning to control NOx.
CP&L Reducing NOx Emissions
New federal regulations and North Carolina’s “State Implementation Plan” require that Carolina Power and Light (CP&L) reduce zone NOx emissions from their coal-fired power plants from 29,000 tons in 2000 to 11,300 tons by 2006. According to a CP&L representative, the company expects to spend $370 million to install NOx reduction technologies between now and 2006.
Besides being the first utility to use SCR in North Carolina, CP&L is reported to be the first in the Southeast to install a combination of lean gas reburn and SNCR. In addition, their Cape Fear plant is the first in the world to install a Swedish combustion control technology, ROFA, on a coal-fired boiler. They are also the first in the U.S. to test a Russian developed technology, WIR, to reduce NOx.
Following the successful installation of “Rotating Opposed Fire Air” (ROFA) on Unit 5 at their Cape Fear Power Station in 2000, Table 2, CP&L installed a ROFA and “Rotating Mixing” (ROTAMIX) on Unit 6 during the spring 2001 outage. Cape Fear’s Unit 5 is a 150 MW tangential coal-fired unit and Unit 6 is a 175 MW twin furnace tangential coal-fired unit. The ROFA and ROTAMIX systems, which have been developed in Sweden, were supplied and installed by Mobotec, USA.
According to Mobotec, ROFA is an improved over-fire air system and ROTAMIX is a second generation SNCR. When ROFA and ROTAMIX are used together they can reduce NOx emissions by up to 75 percent. The remaining NOx can be reduced another 50 percent by adding an SCR. Mobotec reports that combining all three will result in lower NOx, lower CO, improved boiler efficiency and enhanced control of unburned carbon.
The over-fire air system installed on Unit 6 includes a booster fan that supplies high velocity air to the furnace. This high velocity air is injected into the boiler through injection boxes that have been installed asymmetrically at different levels of the boiler. Each box has several nozzles with dampers that control the amount of air entering the boiler.
Ammonia is injected into the furnace through lances that are inserted into the over-fire air nozzles. The ammonia mixing with the high velocity air carries it into the center of the furnace. The amount of ammonia added into the furnace is governed by the furnace temperature, fuel flow and steam production. Limestone can also be injected through the nozzles to reduce SOx emissions.
Once the high-pressure air and ammonia mixture is in the furnace, it mixes with the flue gases. At this point turbulence and rotation of the mixture occurs. The rotation reduces the maximum temperature of the flame while increasing the heat absorption within the furnace. The new over-fire air system also improves the efficiency of the unit because less excess oxygen is required. There is also no increase in CO.
CP&L’s Gary Tonnemacher says that after two weeks of operation with the combination of the ROFA and ROTOMIX systems, Unit 6 NOx emissions were down to approximately 0.15 lb/MMBtu, a reduction of 40-50 percent.
Carolina Power and Light (CP&L) installed the first U.S application of WIR, a Russian developed technology for reducing NOx emissions, on Weatherspoon Unit 3 and Lee Unit 1. Both of the units are vintage 1952, Combustion Engineering 78 MW tangentially fired, dry bottom ash, single reheat boilers. At night the units are reduced to 50 to 65 percent of rated output. The WIR system at Weatherspoon was completed in September 1999 and the Lee unit in June 2000, Figure 1.
The WIR technology involved:
- Changing the tilt angles of the boiler’s existing burner and secondary nozzles.
- Re-routing some of the upper furnace air to the bottom of the furnace.
- Varying the secondary air through each level of nozzles.
- Installing air nozzles along the length of the ash hoppers.
Re-routing some of the air from the top of the furnace produces an oxygen rich, turbulent region that in turn reduces NOx formation and improves heat transfer. Installation of WIR does not require any changes to the wind box or water wall tubes.
Secondary air is injected into the boiler through coal burners on levels 2, 4 and 6, oil burners at levels 3 and 5 and through the auxiliary air ports on levels 1 and 7. Levels 1 and 2 are mainly used to regulate reheat and superheat temperatures and to control carbon moNOxide in the furnace.
In operation the system reduces the use of excess air (oxygen) and combustion temperatures in strategic areas of the furnace. The emphasis is placed on altering the aerodynamics to increase particle residence times and solids density in the lower part of the furnace. Without reducing the feed rate of the coal, the secondary air to the burners is also reduced.
Redirecting the fuel and air creates a separate sub-stoichiometric region of coal combustion in the lower part of the furnace, which lowers the formation of NOx. Since the fuel travels along longer trajectories the residence time of the fuel in the furnace is increased. The upper burners can also provide over-fire air. Besides reducing the formation of NOx, the WIR system improves the boiler’s efficiency by reducing excess air levels.
At Weatherspoon the aim was to reduce NOx from 0.65 lb/MMBtu down to 0.39 lb/MMBtu and at Lee to reduce it from 0.85 to 0.49 lb/MMBtu without reducing boiler efficiency. In addition, the flue gas carbon moNOxide was to be held below 200 ppm. After the installation of the WIR system, the Weatherspoon unit reduced NOx to 0.435 lb/MMBtu, slightly higher than anticipated. Lee Unit 1, however, met the project’s goal. Table 3 summarizes the operating conditions of the Weatherspoon and Lee units before and after installation of WIR.
Although CP&L only looked at the two systems, ROFA/ROTOMIX and WIR, Tonnemacher believes the systems are the most cost effective technologies for reducing NOx emissions. “We are very satisfied with the reduction of NOx at Cape Fear, Weatherspoon and Lee,” says Tonnemacher.
Texas Plant Lowers NOx by 52 Percent
Austin Electric has lowered emissions of NOx on Unit 3 at their Holly power plant by 52 percent. The 165 MW unit is a Babcock & Wilcox front and rear fired boiler burning natural gas. After evaluating different systems, including SCR, the utility decided to use RJM Corporation’s layered NOx reduction technology.
Essentially, the RJM technology involved redesigning and modifying the existing burners to balance the secondary air and the fuel flow between all of the unit’s burners. According to John Halloran, Vice President, RJM Combustion and Environmental Group, balancing the fuel flow between each burner ensures that the minimum furnace oxygen level is achieved. Flame stabilizers added to each of the burners help to stabilize the combustion process and lower excess O2, says Halloran.
In addition to modifying the burners, Austin Electric is installing a NOx tempering system. With this system, micronized water droplets are injected into the high NOx zones near the burners. NOx tempering can reduce NOx an additional 30 percent. However, there is no scheduled startup date for the NOx tempering system, says Reggie Horton, Power Plant Manager, Austin Electric.
Since being put back into service with modified burners, Holly Unit 3 has reduced NOx emissions by 52 percent. Once NOx tempering is put into service the plant expects to reduce NOx emissions a further 25 percent, an overall reduction of 65 percent from the initial baseline emission level of 0.2 lb/MMBtu. “The RJM layered approach for reducing NOx has enabled Austin Energy to achieve its NOx emission objectives at the lowest cost/ton ratio in the industry,” says Horton.