By J. Pham, P.Eng, PhD, MBA, Babcock & Wilcox Canada; D. Wasyluk, PE, Babcock & Wilcox Co.; W. Small, P.Eng, Nova Scotia Power
Nova Scotia Power is a vertically integrated, regulated subsidiary of Halifax-based Emera Inc. which owns and operates most of the electricity generation for the province of Nova Scotia. The generation totals 2,293 MWe, including four 160 MW tangentially fired units at the Lingan Station.
Since 2005, Nova Scotia Power has developed a strategy for sustainable reductions in emissions including greenhouse gases. The plan focuses on three priorities: increase generation from renewable sources, take a leadership role in advancing and delivering electricity-based conservation and energy efficiency, and decrease emissions from current generating facilities. Considerable progress has been made toward reduction of emissions through installation of a new bag house at the Trenton station, low sulfur fuel switching, low NOX combustion retrofitting and adding new equipment for mercury capture at Lingan, Trenton and Point Tupper stations.
This article provides an overview of the NOX formation and reduction techniques, addresses the separated overfire air (SOFA) design approach, and presents baseline and post-retrofit performance test results of the NOX reduction program at Lingan Generating Station.
NOX Formation Mechanism
NOX in the flue gas is a result of oxidizing either nitrogen in fuel (fuel NOX) or nitrogen in combustion air (thermal NOX). Generally for coal-firing, influenced by the type of fuel, less than 25 percent of the NOX produced is thermal NOX and the balance is fuel NOX. Nitrogen oxides consist of 90 to 95 percent nitric oxide (NO) with the balance occurring as nitrogen dioxide (NO2). Once the flue gas leaves the stack, the bulk of the NO is oxidized in the atmosphere forming NO2.
Thermal NOX refers to the NOX formed through high temperature oxidization of the nitrogen found in combustion air. The mechanism of thermal NOX formation was first described by Zel’dovich (1946) and later modified to what is referred to as the extended Zel’dovich mechanism (Sarofim and Pohl, 1972) as shown in the following equations:
Sarofim and Pohl observed that the rate of thermal NOX formation is strongly dependent on temperature, as well as on the residence time that the nitrogen element is oxidized at high temperature. MacKinnon (1974) later found that significant levels of NOX are usually formed above 1,538 C, under oxidizing conditions with exponential increase, as the temperature is increased. Using data obtained from bench-scale tests that measure NOX in a heated mixture of N2, O2 and argon, he showed that thermal NOX can be predicted using the following equation:
From the above equation, thermal NOX can be reduced by limiting residence time, reducing temperature and concentrations of nitrogen (N2) and oxygen (O2). Furthermore, the exponential temperature expressed in the equation demonstrates its importance in the control of thermal NOX. In practice, the Zel’dovich mechanism is sufficient to predict NOX only in the regions downstream from the flame front. Because of the short residence time, the Zel’dovich reactions proved inadequate. Subsequent investigation by De Soeto (1973) showed that, as in Zel’dovich’s reaction, the temperature reduction in all cases reduced prompt NOX; however, additional O2 increased NO for fuel-rich flame fronts but decreased NO for fuel-lean flame fronts. The understanding of the effect of fuel-rich versus fuel-lean flame fronts on NOX production rate leads to the development of air and fuel staging methods for thermal NOX reduction.
Although the kinetics involved in the conversion of organically bound nitrogen compounds found in fossil fuels are not yet fully understood, numerous investigations (Habelt & Howell 1976, Habelt 1977, Pershing & Wendt 1976, Pohl & Sarofim 1976) have shown that the major source of NOX emissions is nitrogen-bearing fuels such as coal and oil. Although little doubt exists that fuel-bound nitrogen is an important contributor in the formation of NOX, the mechanism involved in the transformation of fuel bound nitrogen to NO is complex.
It appears the majority of the fuel NOX formation occurs by two separate paths. The first involves the oxidization of volatile nitrogen species during the initial phase of combustion. During the release, and prior to the oxidization of the volatile compounds, nitrogen reacts to form several intermediate compounds in the fuel rich flame regions. These intermediate compounds are then oxidized to NO or reduced to N2 in the post-combustion zone. The formation of either NO or N2 is strongly dependent on the local fuel/air stoichiometric ratio. The second reaction path involves the release of nitrogen radicals during combustion of the char fraction of the fuel.
These reactions occur much more slowly than the reactions involving the volatile species. These investigators have made quantitative measurements in laboratory-scale tests conducted by Habelt (1977) burning fuel oil in a mixture of oxygen and carbon dioxide, which has shown a remarkable correlation between the percentage of nitrogen in the fuel and NOX formation. However, similar tests run for various coals have not produced similar results. Clearly, the coal test burn results indicate that the fuel-nitrogen conversion rate is not constant, but varies widely depending more on coal rank than on actual nitrogen content. Pershing and Wendt (1976) further isolated fuel NOX by burning coal in a synthetic oxidant mixture that has a specific heat similar to air, however, containing no nitrogen: the mixture consists of 21 percent oxygen, 19 percent carbon dioxide and 61 percent argon. On the basis of four different coals, the studies showed that the fuel NOX, unlike thermal NOX was relatively insensitive to flame temperature.
In-Furnace NOX Reduction Techniques
The investigations of the nature of NOX formation have enabled scientists and engineers to develop control techniques for reducing NOX emissions in fossil-fuel power plants. Because NOX is formed during the combustion process, initial research has focused primarily on controlling NOX at the source, which is referred to as combustion control or in-furnace NOX reduction techniques. It has been recognized that NOX formation is promoted by rapid fuel-air mixing. This produces high peak flame temperatures and excess available oxygen, which in turn promotes NOX formation. The countermeasures involved in low NOX combustion system developments include generous furnace sizing to reduce the thermal loading to the combustion zone, low NOX burners to reduce the rate of combustion and peak flame temperatures, air staging and fuel staging with over fire air (OFA) ports and reburners to reduce combustion zone stoichiometry, and flue gas recirculation to the combustion air to depress flame temperature. Depending on the desired level of NOX reduction, for wall fired boilers, low NOX burners with OFA are a popular choice to meet moderate (20 to 50 percent) NOX reduction target. For 70 percent or higher NOX reduction, reburning with flue gas recirculation to burners is used with low NOX burners and OFA (Pham & MacLean, 2006, 2007).
In a tangentially fired boiler, staged combustion has been widely used in NOX control. This technique involves removing a portion of the combustion air (staged air) from the main combustion zone to reduce stoichiometry during combustion and thus reduce formation of both fuel and thermal NOX. This staged air is then introduced through ports downstream to complete the combustion process. The reduction of NOX emissions with this technique ranges from 20 percent to 60 percent depending on the unit’s baseline NOX emissions levels and other factors including operating excess air concentration, fuel combustion equipment design and arrangement as well as fuel type. The use of OFA in tangentially fired unit results in significant decreases in NOX emissions and is by far the most cost-effective technique for reducing NOX emissions from units of this design.
However, while reducing (sub-stoichiometric) conditions results in significant NOX reduction, increased levels of unburned carbon (UBC) and CO can occur. The extent of these increases is most dependent on the design of the OFA system and on fuel properties. Waterwall wastage may also occur with the degree dependent on such factors as unit design (eight versus four corners), heat release rates, coal composition (high versus low sulphur), UBC and tube temperature.
Low NOX Combustion Retrofit
Tangentially fired boilers are characterized by the introduction of fuel and combustion air at alternating elevations through the corners of the boiler, tangent to an imaginary circle in the middle of the furnace. The firing system design is characterized by the introduction of fuel with primary air at distinct elevations. Each elevation of coal nozzles receives coal from one pulverizer. A small portion of the secondary combustion air is introduced through the windbox co-annular to the pulverized coal primary stream. The majority of secondary air is introduced through air compartments located above and below each fuel admission assembly. This method of air and fuel introduction results in lower NOX emissions than those produced with other methods of fuel firing. Slower mixing of fuel and air results in the creation of hydrocarbon fragments that contain nitrogen in a reducing atmosphere and thus reduces the production of NOX.
Traditionally, air staging in tangentially fired boilers has involved a close-coupled overfire air system with compartments located integral to and at the top of the existing windbox and/or just above the main windbox. However, providing separation distance between the OFA levels and the main combustion air zone, delivers the most effective and maximum reduction in NOX emissions. Thus, to maximize the reduction of NOX emissions from a tangentially fired unit via modifications to the combustion system, Babcock & Wilcox (B&W) has developed a standard approach for the utilization of a separated overfire air (SOFA) low NOX system that is customized on a unit-by-unit basis. The typical system includes the following additions and modifications to the existing combustion equipment:
- Installation of SOFA ports above the main windboxes in bent-tube waterwall openings
- Increased separation between the auxiliary air and fuel admission nozzles to allow for delayed mixing of fuel and air during the initial stages of combustion
- Reduction of secondary air flow through reduced size air nozzles while maintaining system pressure drop characteristics and injection velocities for efficient mixing of air and fuel under staged conditions
- Reduction of the secondary airflow control damper size for better control of the secondary airflow
- Addition of separate tilt control of under bottom end air admission nozzles for improved CO control.
SOFA ports are most effective when located approximately 15 to 30 feet above the top fuel firing elevation. Locating the ports lower reduces their effectiveness (that is, requiring the use of additional OFA); locating them at a higher elevation, while not affecting their NOX performance, may affect combustible losses and possibly steam temperatures. The SOFA system for tangentially fired boilers is designed to:
- Maximize NOX emissions reductions
- Maximize mixing of OFA with the flue gases to allow for minimal increases in unburned combustibles;
- Minimize modifications to the boiler buckstays and sootblowers and
- Maximize access to affected elevations of the boiler.
As mentioned above, tangential firing involves injecting fuel and air at the corners of the furnace at an angle tangent to an imaginary circle located in the middle of the furnace. It is typical that the fuel and secondary air are injected at an angle of 4 to 6 degrees off the diagonal of the furnace. The angle allows for imparting rotation to the reacting jets of fuel and air. Another significant factor influencing the rotation of reacting streams (fireball) is the injection velocity. Specifically, the velocity, along with the mass of the fuel and air, imparts a swirling action, which is necessary to stabilize the flames in the furnace.
The secondary air velocity is set based on two major criteria: fuel reactivity and furnace size. Fuels with high reactivity, such as natural gas or pulverized western sub-bituminous coal, require higher injection velocities of secondary air. This allows for flame fronts to be located away from the burner corners while maintaining proper flame stability. Proper operation of tangentially fired fuel combustion systems requires maintaining acceptable secondary air velocities whenever changes are made to the combustion system, such as the addition of separated overfire air. This is accomplished by reducing the auxiliary air compartment and nozzle sizes.
Proper sizing of the damper box opening for each compartment is important in achieving smooth unit operation over the load range. During load changes, the unit’s windbox-to-furnace differential pressure is controlled, to assure proper secondary air distribution within the main windbox. The auxiliary air dampers modulate to maintain windbox-to-furnace differential pressure setpoint. The reduction in secondary airflow of the nozzle tips requires a proportionate reduction in the corresponding damper’s flow area. Typically, the damper-free area is approximately twice as large as the nozzle tip’s free flow area. Should the free area of the damper box not be appropriately reduced, minor changes in damper position will create large changes in flow through the compartment, causing changes in windbox-to-furnace differential pressure and resulting in less stable control of air flow. If the existing damper system is in good working order, damper box free area reduction is accomplished by trimming and/or disabling damper blades and installing low pressure drop Venturi plates to reduce flow area.
The main windbox nozzles and dampers are resized using a proprietary program to optimize secondary air mass flow velocity and distribution. This is to assure that all main windbox dampers will remain in control of windbox pressure over the load range when the SOFA ports are in service.
The Lingan Generating Units
The boilers at Lingan Generating Station were commissioned between 1979 and 1984 and are sub-critical, single reheat, controlled circulation, balanced draft, four-corner-tangential-fired tower units. Each unit is rated at 1,080,260 lb/hr superheated steam at 1,005 F and 1,850 psig and 957,500 lb/hr reheat steam at 1005 F. The boilers were originally designed to burn high volatile, bituminous coal with full load capability firing heavy fuel oil. Currently, the units fire a variety of blended solid fuels including petroleum coke (petcoke), low sulfur South American coals that may have either high or low heating value, U.S. mid-sulfur, Powder River Basin, and others. Blending of fuels takes place at the front end and is supplied to all mills as opposed to dedicating mills to individual fuels.
Pulverized coal is supplied by four 663 RS exhauster mills. There are four coal elevations per corner for a total of 16 burners. Each mill supplies coal to one complete coal elevation (four burners, one per corner). Full load can be achieved with three mills in service depending on coal types and blended compositions.
Prior to the NOX reduction retrofits, the original windbox compartment arrangement and nozzle designs had never been modified to reduce NOX. Each corner windbox assembly consisted of 10 compartments: four coal, two auxiliary air/oil, one intermediate auxiliary air, one top end air, one bottom end air and one close-coupled OFA. The intermediate auxiliary air compartment had been decommissioned in that the oil gun has been removed and the dampers locked closed. The remaining two oil levels could achieve approximately 65 percent load or 100 MWe.
Design and performance guarantee fuels used for each unit are as shown in Table 1 on page 48. Regarding petroleum coke, having extremely low volatile content and grindability index, it exhibits similar ignition and combustion characteristics to that of a hard to burn anthracite coal. With its high fixed carbon (FC) content and low volatile matter (VM), the resulting high FC/VM ratio is an indication that petcoke is a high NOX producing fuel. In addition to increasing NOX emissions, it is known that raising the petcoke input above 20 percent will significantly increase unburned carbon (UBC).
Combustion System Modifications
The low NOX modification consisted of adding separated overfire air (SOFA) to provide deeper staging capability, and modifying the main windbox air nozzles and air control dampers. The main windbox modifications involved the replacement of existing air and coal nozzle tips with reduced nozzle sizes and corresponding reductions in damper size. These changes are to compensate for reduced secondary air flow to the main windboxes resulting from the SOFA addition. They are to ensure that proper secondary air velocity damper control be maintained over the normal load range during low NOX operation.
With respect to the new SOFA, four SOFA ports, one at each corner, are located approximately 22 feet above the top coal elevation. The selected SOFA elevation was deemed best suited for the modifications. That is, it provides adequate separation distance and residence time above the top coal elevation for effective NOX control, while allowing sufficient time to complete combustion prior to leaving the furnace. Complete combustion minimizes unburned carbon in flyash and avoids upper furnace slagging. Moreover, the selected SOFA location would facilitate easy supply air duct routing while not interfering with the existing buckstays. Furthermore, locating the SOFA port higher may provide lower NOX, but would be too close to the superheater and would affect heat absorption and steam temperature as well as increased unburned carbon due to reduced residence time.
Figure 1 Isometric view of SOFA windbox assembly and wall opening
The SOFA assembly as shown in Figure 1 (on page 46) consists of two separate air compartments with integral turning vanes and independent flow control dampers. This feature provides operational flexibility, in that one or both compartments can be controlled to provide optimal air velocity and furnace penetration over the entire boiler load ranges, to control NOX, CO and carbon burnout. Moreover, the flow control dampers are opposed blade type, providing even air distribution and a linear air flow relationship, compared to the parallel blade type design. Furthermore, each SOFA assembly is equipped with a manual tilt wheel drive to tilt both air compartments in unison online through a range of +/- 30 degree and two manual yaw mechanisms for online horizontal yaw adjustment through a range of +/- 15 degree for additional NOX and CO control.
Pre and Post Modification Results
The low NOX combustion retrofit program at Lingan Generating Station was conducted in three stages, each stage involving a separate competitive bidding and contract award process. The Unit 3 low NOX conversion contract was awarded in 2005 and completed in 2006, Units 4 and 2 in 2006 and completed in 2007 and Unit 1 in 2007 with work completed in 2008.
With respect to the performance fuels, Lingan station utilizes a wide range of imported bituminous coals and petroleum coke as shown in Table 1. The fuels and blended compositions used for performance guarantees were different for each unit. The performance fuel used for Unit 3 is based on 80 percent Columbian coal and 20 percent petcoke; Units 2 and 4 were based on 65 percent low sulfur Venezuelan coal and 35 percent mid sulfur U.S. Appalachian coal; Unit 1 was based on 80 percent Columbian coal and 20 percent mid sulfur U.S. Appalachian coal. Pre and post fuel analyses were similar to enable a true comparison.
Nova Scotia Power conducted pre modification tests to establish base line boiler performance and emissions for each unit, under normal full load operating conditions, burning the performance fuels as specified above. After the modification for each unit was completed, post retrofit tests were conducted for each unit to prove performance guarantee compliance.
Unit 3 was tested with 4 mills in service and burner tilt at +10 degrees, Units 2 and 4 tests were conducted having top 3 mills in service and burner tilt was set at+10 degrees, Unit 1 was tested with top 3 mills in service and burner tilt at level. Figures 2, 3, and 4 compare pre and post modification NOX, UBC, and CO, respectively.
Each unit achieved over 50 percent reduction in NOX emissions with small increases in UBC and CO. The pre-modification NOX emissions as shown in Figure 2 (on page 48) were higher with petcoke, as expected. Approximately 35 percent higher NOX emissions were observed for the 80/20 coal and petcoke blend composition in compared to Units 3 and 1 with the same proportion of Columbian coal. With regard to Unit 4 performance, post modification CO was higher. Although the percentage increase was large, the absolute CO value was 32 ppm. CO less than 100 ppm is considered normal for a coal fired unit with SOFA, burning these blends of coals. It should also be noted that the NOX emissions with all mills or bottom 3 mills in service were substantially lower than shown on Figure 2 and ranged from 0.18 to 0.22 lb/MBtu at full load with similar CO levels.
The addition of the SOFA ports extends the furnace combustion zone further to the furnace exit plane. This would alter the furnace absorption, however the effect was insignificant in changing the main steam and reheat steam temperature profile or the boiler efficiency. The post modification steam temperatures, as well as boiler efficiency were essentially unchanged from baseline.
The SOFA additions to the Lingan units have enabled Nova Scotia Power to achieve over 50 percent NOX reduction with small increases in CO and UBC. Lower furnace corrosion risk has been one of the biggest concerns associated with stage combustion. However, Unit 3 has been the first unit in operation with the SOFA at the reduced atmosphere condition since 2006 with no sign of corrosion. Furthermore, all units have been able to maintain the same emissions performance and there has been no degradation since the acceptance tests. Achieving reduction of NOX emissions from all of its coal fired boilers is helping Nova Scotia Power fulfill one of its initiatives in reaching its goal going toward a cleaner and greener future for Nova Scotia.
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