Boilers

NOx-reducing Technologies

Issue 4 and Volume 115.

Compliance that Can Lead to Profit

Babcock & Wilcox AireJet burner was developed for use with overfire air systems. The burner accelerates coal ignition and intensifies combustion for rapid achievement of fuel rich conditions in the burner zone. Photo courtesy Babcock & Wilcox.

By Lindsay Morris, Associate Editor

Regulations to reduce nitrous oxide (NOx) emissions have been around for decades, but frequent changes have kept the industry on its toes regarding which types and how many measures to install. But now that most of the largest NOx emitters have been regulated, the U.S. Environmental Protection Agency’s (EPA’s) NOx trading program has become an area of investment and earning for many power providers. That means the installation of NOx reducing technologies can be an investment in more than just a better environment: They can also become a profit source.

NOx emissions regulations started with passage of the Clean Air Act (CAA) in 1990. Compliance with CAA rules resulted in a 29 percent reduction in NOx emissions from 1990 to 2003, according to a government report, “A Review of DOE/NETL’s Advanced NOx Control Technology R&D Program for Coal-Fired Power Plants.” In 2005, the Clean Air Interstate Rule (CAIR) introduced another change for NOx compliance: It required 25 eastern states to reduce NOx emissions by 1.7 million tons in 2009 and 2.0 million tons by 2015. Since that time, a federal court has tossed out CAIR, leading EPA to introduce the Interstate Trading Rule, which attempts to resolve issues the court had with the CAIR program. It does this in part by proposing trading among states. The CAIR NOx emission caps are based on an equivalent emission rate of 0.15 pounds NOx per million Btu heat input (lb/MMBtu) for Phase I, which began in 2009 and 0.125 lb/MMBtu for Phase II beginning in 2015.

A Babcock & Wilcox SCR retrofit at a U.S. utility. Photo courtesy Babcock & Wilcox.

Another up-and-coming potential change for NOx control could come as a result of the Boiler MACT rules, which were released by EPA in February.

While it’s still unclear how much these rules will affect NOx limits, the impression is that there will be incremental decreases in NOx emissions, said Barney Racine, software development manger for the environmental solutions group of Honeywell Process Solutions. “Boiler controls will have to be updated and NOx emissions will come down accordingly. It will drive investment over the next five years after that standard comes into place.”

The Boiler MACT rules are a game-changer for all emissions control and will re-emphasize the need for NOx control, said Bruce Lani, project manager for the existing plantsdivision of the U.S. Department of Energy’s National Energy Technology Laboratory (NETL). However, data on NOx technology generated by NETL for the EPA indicates that “we’re fairly far down the road on meeting these regulations,” Lani said.

Coen’s Dynaswirl-low NOx burners, ready for shipment. Photo courtesy Coen Co.

As an added concern for power producers, NO2 National Ambient Air Quality Standards(NAAQS) rules are also in effect, which set annual averages for NO2 at 0.053 parts per million (ppm). Coal-fired units are affected, as well as the Reasonably Available Control Technology (RACT) for the state of operation. Gas-fired units must abide by NO2 NAAQS rules and state RACT guidelines.

Emissions compliance can prove to be more difficult than just meeting the federal regulations in many areas, said Racine. Specific areas of ozone non-attainment tend to be regulated more closely by local standards than federal standards. This can lead to more stringent emission limits, more investments in NOx reduction technology within those areas, and greater participation in regional emissions programs.

For example, “you see more investments in the Houston area than in the middle of New Mexico,” Racine said.

NOx reduction technologies can also become an investment strategy for some companies, notably major power producers who participate in credit trading programs. “Every pound of NOx for them is highly valuable,” Racine said. “They tend to invest a lot in NOx emissions technology if they think they can turn that into money.”

With NOx compliance technology serving as a bridge to more than just environmental compliance, being aware of all of the modification options available is a must for power providers. The right technology could lead to future profit as well as a boost in power production for the entire system. 

Modification Options 

A number of options exist for combustion and post-combustion modifications that lead to a decrease in NOx emissions. The most common method (and as many manufacturers would agree, the “first step” in NOx emissions control) is to install low-NOx burners. These control fuel and air mixing to achieve staged combustion, which, in turn reduces both flame temperature and oxygen concentration during some phases of combustion. In turn, this reduces both lower thermal NOx and fuel NOx production.

Overfire air is often used in conjunction with low-NOx burners to complete the combustion process at a lower temperature. Air is injected into the furnace above the normal combustion zone. When overfire air is used, the burners function at a lower-than-normal air-to-fuel ratio, which reduces NOx formation.

An additional combustion process known as reburning injects boiler fuel (anywhere from 10 to 25 percent) into a separate reburn zone. The fuel-rich reducing conditions in this zone lead to a reduction of NOx formed in the normal combustion zone.

Flue gas recirculation (FGR), in which part of the flue gas is recirculated to the furnace, can also be used to alter conditions in the combustion zone by lowering the temperature and reducing the oxygen concentration. Additionally, FGR is used as a carrier to inject fuel into a reburn zone to increase penetration and mixing.

Post-combustion processes include selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR) and a hybrid of the two. With SCR, a catalyst vessel is installed downstream of the furnace. Ammonia (NH3) is injected into the flue gas before it flows over the fixed-bed catalyst. The catalyst prompts a reaction between NOx and NH3 to form nitrogen and water vapor. NOx reductions as high as 90 percent are possible.

In SNCR, a reducing agent (usually NH3 or urea) is injected into the furnace above the combustion zone. There it reacts with NOx as in the case of SCR. Critical factors in SNCR applications include sufficient time in the appropriate temperature range and equal distribution and mixing of the reducing agent across the full furnace cross section.

SCR and SNCR can be used together and can also be used in combination with low-NOx burners.

Craig Penterson, manager of fuel equipment design for Riley Power Inc., a division of Babcock Power Inc., said that each unit may require a different level of retrofit, often depending on the type of coal being combusted. Riley Power low-NOx burners can typically reduce NOx emissions by 40 to 50 percent from an uncontrolled level, Penterson said. Combining low NOx burners with an over-fire air system, NOx emissions can be reduced by 60 to 70 percent.

Because the low-NOx burners combined with over-fire air system have such high levels of attainment, power producers can, at times avoid an SCR installation, Penterson said. However, overall NOx reduction can exceed 90 percent for units that install SCR in addition to low-NOx burners and over-fire air. Penterson said units in the eastern part of the U.S. are commonly retrofitted with all three methods due to lower reactive nature of eastern coal as compared to western coals.

DOE NOx Program

To support NOx regulations, NETL has funded several projects advancing NOx control technologies for the existing fleet of coal-fired utility boilers. The short-term goal of one of the projects was to develop advanced in-furnace technologies for coal-fired power plants capable of controlling NOx emissions to a level of 0.15 lb/MMBtu by 2007 in preparation for Clean Air Act compliance. That goal has been achieved and several companies that tested at NETL are now manufacturing NOx technology that controls NOx to levels at or below 0.11 lb/MMBtu.

An Alstom SCR retrofit with optimized catalyst management design, incorporating provisions for additional levels of catalyst at a future date. Photo courtesy Alstom.

NOx emission control costs can be significant, exceeding 20 percent of the total cost for environmental controls on coal-fired power plants. The capital and operating costs of SCR controls are relatively high and may not be cost-effective for older, smaller coal-fired power plants, or even for some larger baseload plants. Therefore, NETL’s NOx R&D program, which was in place from 2000 to 2007, focused on developing advanced in-furnace NOx controls that can approach the performance of SCR, but at lower costs.

Another technology tested by NETL’s NOx R&D program led to the development of the Advanced Layered Technology Approach (ALTA), which uses a targeted combination of overfire air, rich reagent injective and SNCR processes in the boiler to lower NOx formation and ammonia slip. ALTA technology was one of the first to prove that NOx emissions can be achieved below 0.15 lb/MMBtu on a coal-fired cyclone boiler when it was tested at AmarenUE’s Sioux Unit 1 Station near St. Louis in 2005. Since then, companies like Reaction Engineering International (REI) have adopted the technology and made improvements. ALTA has been commercially installed on 11 cyclone-fired units ranging in size from 60 MWe to 500 MWe. NOx reduction at these boilers has ranged from 50 percent to 90 percent depending on baseline levels. ALTA installations are currently in process on five additional cyclone boilers.

An Alstom advanced low NOx tangential firing systems. Photo courtesy Alstom.

REI’s ALTA technology uses a combination of deep staging from overfire air, rich reagent injection (RRI) and SNCR processes to reduce NOx formation and ammonia slip. RRI is an Electric Power Research Institute technology licensed by Reaction Engineering, which injects reagents into fuel-rich zones in a furnace to reduce NOx. The ALTA technology shares equipment such as tanks, pumps and injection components between the SNCR and RRI systems to reduce capital and operational costs. ALTA often results in lower capital and lifecycle costs compared to SCR and requires significantly shorter plant outage time for installation.

Alstom Power also took part in DOE/NETL’s program and now boasts one of the highest emissions-reducing low-NOx burners on the market. Alstom’s goal was to develop system hardware to further reduce the NOx emissions from both Alstom’s new boilers and the tangentially fired boiler fleet. The project developed a globally air staged low NOx firing system, which provides a means to achieve less than 0.11 lb/MMBtu NOx at less than three-quarters the cost of an SCR. It does so with little or no impact on balance of plant when firing a high volatile bituminous coal.

As a result of the knowledge gained with this program as well as internally funded research efforts, Alstom developed advanced burner technologies for new and retrofit applications for both tangentially fired and wall-fired boilers.

Alstom has retrofitted over 340 units in the U.S. with low-NOx burners and has achieved some of the lowest in-furnace NOx emission levels. Based on data reported by the EPA, over 70 percent of the lowest 50 NOx emitting units in the U.S. firing pulverized coal and using in-furnace technology only, have been retrofitted with Alstom low-NOx technology. Several units have reported NOx emission levels at or below 0.10 lb/MBtu firing sub-bituminous coals.

Tangential firing is one of the keys to obtaining the lowest in-furnace NOx emission levels.

“Tangential firing is inherently a lower NOx-producing combustion system compared to wall firing burners, all other parameters being equal,” said Douglas Hart, manager of engineering in Alstom’s boiler retrofits organization. “Advancements to tangential firing with low-NOx burners have produced even lower emissions, while maintaining efficient combustion.” 

Post-combustion NOx Reduction 

As NOx emission regulations have become more stringent for operating units, there is frequently a need for NOx levels lower than those obtainable with in-furnace technology alone. In these cases, SCRs can also be retrofit for post combustion NOx reduction, creating a benefit by integrating low-NOx burners and SCR.

“By reducing the inlet NOx concentration into the SCR, the quantity of catalyst and the amount of ammonia or urea consumption is reduced,” said Donald Borio, principal engineer for Alstom’s SCR technology. Integrating the two technologies is part of a process Alstom uses to seek the lowest cost per unit of NOx reduction at the final stack outlet.

An important challenge to SCR system retrofit is fitting the new equipment into the existing plant footprint and building the system without affecting the unit operation. The only impact to the operating unit occurs when it is shut down to tie in the SCR. 

Spurlock Units 1 and 2 

An example of a retrofit that integrated both low-NOx burners and SCR for NOx control is the East Kentucky Power Cooperative Spurlock Unit 1 and 2. The emissions control system retrofit project for the Spurlock units provided a significant reduction in the emissions of NOx and particulates from the plant. Today these boilers are consistently among some of the lowest NOx emitters of any coal-fired units in the U.S.

Alstom integrates enhanced low-NOx tangential firing systems with post combustion SCR technology in new plant designs as well. Typically, best available control technology (BACT) and other regulations for new coal plants in the U.S. require installation of SCR; however, the same cost and performance benefits through optimization of firing systems with SCR exist for new units as for retrofits. One advantage of new installations is that furnace geometry and residence times can be optimized for lower NOx emissions from the furnace to the SCR inlet, Borio said. In addition, the SCR footprint is incorporated into the plant design, allowing for optimal locations, future catalyst levels and other features that may be more difficult in a retrofit situation.

During the last few years, Alstom has commissioned several new coal fired boilers in the U.S. that incorporate the latest evolution of their low NOx firing systems and SCR technology. These include Kansas City Power & Light’s Iatan Unit 2 and Xcel Energy’s Comanche Unit 3. These sliding pressure supercritical pulverized coal plants burn Powder River Basin (PRB) coal and use integrated low NOx firing systems and SCR for emissions control. The SCRs are large single reactors and utilize efficient modular construction. They also feature multi-zone ammonia injection grids to enhance distribution of ammonia. Both units are operating and successfully meeting NOx emissions requirements. 

Fuel Types 

To respond to the needs of utilities that fire PRB coals, the Babcock & Wilcox Co. has been focusing on SCR system design enhancements to enable reliable post-combustion NOx emissions control when these fuels are fired, said Betty Hansen, project manager. While an inherently low NOx producer, PRB coal is infamous for being an ashy fuel, and the ability to expose the SCR catalyst surface to the flue gas becomes limited if ash plugs the SCR catalyst or piles on its surface.

“That means you must carefully manage ash distribution in the flue gas entering the SCR reactor and minimize obstruction to its flow through the system,” Hansen said.

An example of the Separated Overfire Air (SOFA) windbox, manufactured by R-V Industries Inc. The windbox features remote directional control and air flow control. Photo courtesy R-V Industries Inc.

As a response to this problem, Babcock & Wilcox designed a reconfigured reactor and taken steps to minimize obstructions in fluework and the reactor that can serve as ash collectors. Another challenge, Hansen said, is that PRB ash can form a cementatious substance, coating the catalyst surface or forming hard deposits over the top of catalyst modules if it is allowed to build up within the reactor.

In addition, when the combustion system includes overfire air or when combustion is poor, PRB firing can result in the formation of gas-phase phosphorus, which is released in the flue gas and can poison or prematurely deactivate catalyst.

Babcock & Wilcox identified a solution to address the gas-phase phosphorus problem: the injection of an additive onto the fuel as it makes its way into the boiler. The additive acts in the combustion process to tie up the gas-phase phosphorus, making it unavailable to poison the catalyst.

“Full scale testing is underway and results to date are demonstrating the viability of this approach,” Hansen said.

Babcock & Wilcox has also expanded its low-NOx combustion products with the roll out of its AireJet burner. This burner significantly reduces NOx through its unique “inside out/outside in” combustion arrangement. Air supplied to the center of the flame accelerates ignition and combustion under fuel rich conditions to most effectively reduce NOx. Adding air to the center of the flame promotes cleaner combustion, producing lower carbon monoxide (CO) emissions in combination with lower excess air and improved efficiency. The AireJet burner is now in service in five units firing PRB coals.

“It’s a change in the philosophy of how you burn the fuel,” Hansen said.

The AireJet burner enables users to achieve simultaneous low NOx and low CO production. And since the duty on the SCR system is a function of the NOx level exiting the boiler, using these burners ahead of an SCR system can reduce catalyst volumes and ammonia consumption, Hansen said.

R-V Industries Inc. designs and manufactures low-NOx tangential firing systems and components and recommends addressing high CO emissions through special adjustable air nozzles. These help reduce furnace CO emissions by vectoring air toward high CO areas, thereby oxidizing CO within the combustion zone, said John Grusha, R-V’s director of combustion systems and product development.

By introducing remote control to these nozzles, “it makes it easier and much quicker to optimize the direction of the air, reducing CO emissions,” Grusha said. Some new systems coming on line will have smart closed-loop control whereby CO profiles will be measured in real time and nozzles remotely directed to reduce the high CO zones.

This type of combustion air flow vectoring should also help reduce the amount of unburned carbon, particularly relevant for units burning less reactive bituminous coals, Grusha said. 

Low-NOx Systems for Gas Boilers 

While NOx emissions are an obvious concern for coal-fired systems, they also pose a challenge for gas boilers. John Zink/TODD Combustion produces technology for gas-fired utilities that seek to achieve over 90 percent NOx reduction on a boiler retrofit without affecting capacity or requiring replacement of ancillary components.

In natural gas combustion, NOx is produced through two main routes: a thermal route, where high flame temperatures cause nitrogen molecules from the combustion air to disband and combine with oxygen to form nitric oxide, and the prompt mechanism. John Zink used various methods to lower peak flame temperatures and oxygen availability by adding various diluents to the combustion process. Two common methods including using recirculated flue gases from the boiler outlet and steam injection, which lowers flame temperature and helps reduce thermal NOx formation.

Another method of lowering NOx is through burner designs that incorporate staged combustion, slowing the mixing of fuel and air. The staged combustion process creates an initial fuel-rich combustion and air is added downstream to complete combustion. Oxygen availability is limited, peak flame temperature is lowered and thermal NOx formation is reduced. Yet another staging method involves creating a fuel-lean primary combustion zone, with lower flame temperatures and lower thermal NOx formation, followed by the injection of additional fuel downstream to consume excess oxygen and complete the combustion process.

TODD uses three technologies to address NOx concerns for new and existing boilers. The first approach is the Dynaswirl low-NOx burner, in which a stratified flame structure with specific sections of the flame operating fuel rich and other sections operating fuel lean was developed. This design allows for internal staging of the flame to achieve reductions of NOx emissions while maintaining a stable flame.

TODD’s second approach is the Dynaswirl low excess air burner. The burner features a venture air sleeve, which provides for the primary and secondary airflow in the burner. To increase combusting staging and flame shaping capabilities, the burner is equipped with a tertiary air sleeve assembly. Tertiary air is combined with bulk furnace gas to achieve complete fuel blackout. This allows for the complete burnout of the fuel in the post combustion zone where NOx formation is inhibited by lower combustion temperature and reduced O2 concentration.

Dynaswirl burners have been installed at a number of gas-fired and combined cycle plants with results of lowered NOx emissions. The Southern Energy gas-fired plant in Pittsburg, Calif., needed daily average emission rate of its existing utility boiler to be reduced to less than 36 ppm. The plant installed 24 Dynaswirl burners, which resulted in a 40 percent reduction of NOx emissions. In addition, excess O2 and CO levels were lowered to 0.5 percent O2 and 120 ppm CO. The Dynaswirl retrofit also improved the boiler turndown and operating flexibility. 

Ormond Beach Power Plant 

TODD also installed its Dynaswirl low-NOx burners at the gas-fired Ormond Beach power plant in Southern California owned by Reliant Energy. The plant experienced combustion-induced boiler vibrations in two of its units after a series of modifications were made to meet state NOx regulations. The vibrations caused several costly repairs, frequent maintenance, reduced loads due to unstable flame formation and poor turndown that limited the ability to maximize reserves during off-peak periods. The plant tried several modification attempts to correct the vibration, but each attempt proved unacceptable as forced outages occurred, equipment was damaged and flame shapes became highly unstable and potentially hazardous.

The TODD burner engineers recommended a two-part solution to address the plant’s predicaments. First, Coolflow, a gas conditioning technology that reduces thermal NOx by lowering the flame temperature as well as local oxygen concentration, was implemented. Baffles were installed to straighten the airflow and provide uniform air velocity into and around the entrance of each burner. The engineers then retrofitted one of the units with 32 Dynaswirl low-NOx burners. As a result of these two applications, NOx was reduced by 23 to 35 percent over a wide range of loads. In addition, the plant was able to fire all burners in service at full load without vibration. The plant was able to increase peak load from 740 MW to 790 MW.

The RCL Catalytic Combustor manufactured by Precision Combustion Inc. This solution is intended for use on gas turbines from microturbines to F-Class central station size units. Photo courtesy Precision Combustion.

Zeeco is another manufacturer offering low-NOx burners for gas-fired units. Its GLSF Free-Jet burner provides for ultra low-NOx emissions by optimizing the internal flue gas recirculation and fuel gas mixing, thereby lowering the thermal NOx. The Free-Jet design maximizes the internal flue gas recirculation NOx control methodology, said Scott Reed, vice president.

“We have found a simple method to best utilize 100 percent of the fuel gas to inspirate more internal flue gas back into the flame zone for maximum NOx control.”

Zeeco’s burner design provides one of the smallest physical footprints on the market, Reed said, saving power producers from having to cut into boiler steel, refractory or tubes and risk encountering other retrofit hazards that would result in escalated total installed cost. The Free-Jet burner has a wide turndown and can transition quickly from cold to normal operation, offering an added safety element. It also has the ability to switch from fresh air to turbine exhaust combustion air firing without any burner modifications.

Hamworthy-Peabody Combustion offers ultra-low NOx burners for gaseous and liquid fuels that can achieve 9 ppm NOx on a packaged boiler. Bill Gurski, national sales manager, said Hamworthy-Peabody also offers add-on technologies that can add gas to existing boilers: 30 to 50 percent gas as a supplement to coal. Gurski said with EPA regulations being more stringent on coal-fired units, a gradual transition to gas through staged combustion technology could be of interest to some coal-fired operators.

“I sense trepidation in customers who have been firing a solid fuel for many years, but once they enter the discussion of using a gaseous fuel, they realize it can be achieved,” Gurski said.

He said the conversion to a gaseous fuel, rather than a full gas conversion, can be beneficial.

“When a plant is going to a full gas conversion, the amount of materials or equipment that is used to fire a solid fuel takes a lot of power and maintenance. When you go to a gaseous fuel, all the power and maintenance costs go back to the generator.”

Precision Combustion Inc. offers another way to control NOx emissions through catalytic combustors. This solution is intended for use on gas turbines from microturbines to F-Class central station size units.The compact and retrofittable device resembles a catalytically coated tube and shell heat exchanger with a rich fuel and air mixture passing over a catalyst coated side of the tube and cooling air passing over the other side of the tube with the resulting mixture being combined downstream for overall lean combustion.

Lean premixed combustion, the current dominant gas turbine combustor technology, balances high flame stability against cool maximum firing temperatures, achieving near-zero unburned hydrocarbon and CO levels while producing NOx in the high single digit ppm range. This method usually requires costly and complex post-combustion NOx control systems such as SCR which also increase emissions of importance, like ammonia. PCI’s catalytic combustors effectively demonstrate ultra-low single digit NOx emissions with no need for post-combustion treatment.

Catalytic combustors increase gas phase combustion stability by catalytically reacting a portion of the fuel and increasing overall gas phase reactivity and flame speed. Reacting a portion of the fuel in the catalytic region of the combustor improves flame stability as well as creating the conditions for more rapid, complete combustion. Extending flame stability provides many opportunities for gas turbines including the ability to burn lean enough to avoid NOx formation as well as achieve extended low NOx turndown. In the case of Precision Combustion’s technology, the combustor’s fuel flexibility is broadened, and the technology can burn low Btu fuels that otherwise would require supplementation.

The ongoing drive to reduce NOx and other emissions is a fact the industry must battle daily. To offer the most cost-effective solution and have the smallest impact on existing systems operation, the appropriate combustion system for each particular application needs to be engineered and supplied. When power producers apply the right system, not only can NOx emissions be reduced, but profits are also possible through EPA’s NOx trading program. And while the NOx emissions control regulations front has been relatively quiet in the last couple years, Boiler MACT regulations may result in greater reduction requirements for NOx and other emissions.

Preparation is key, and with many technologies available, retrofits for NOx come with a wide spectrum of possibilities. 

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