Efforts are underway to provide more cost-effective options for coal-fired power plants to meet stringent emissions limits.
By Bruce W. Lani, Thomas J. Feeley, III, Charles E. Miller, Barbara A. Carney and James T. Murphy
Commercially available technologies for reducing nitrogen oxide (NOx) emissions from coal-fired boilers are enabling industry compliance with today’s regulatory requirements. Low-NOx burners (LNB) and selective catalytic reduction (SCR) technology have enabled hundreds of power plants to reach mandated emission levels. For a number of plants, however, these technologies cannot meet the tighter limits at acceptable cost.
This article highlights the Department of Energy, National Energy Technology Laboratory’s (DOE/NETL) advanced NOx emissions control research and development (R&D) efforts to provide more cost-effective options for coal-fired power plants to comply with ever more stringent emission limits. Regulatory and legislative requirements have predominantly driven the need to develop NOx control technologies for existing coal-fired power plants. The most recent regulatory driver is the Clean Air Interstate Rule (CAIR), which impacts 28 eastern states and the District of Columbia and which will be implemented in two phases under a cap-and-trade program. 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 beginning in 2009 and 0.125 lb/MMBtu for Phase II beginning in 2015. Another regulatory driver, the Clean Air Visibility Rule (CAVR), could impose additional NOx controls on plants in the western U.S. under best available retrofit technology (BART) requirements.
Advanced NOx emissions control technology R&D has been an important component of the Innovations for Existing Plants (IEP) program conducted by DOE/NETL. The current short-term goal of the research is 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 and 0.10 lb/MMBtu by 2010, while achieving a levelized cost savings of at least 25 percent compared to state-of-the-art SCR technology. The program’s long-term goal is to further develop a combination of advanced in-furnace and SCR control technologies to achieve a NOx emission rate of 0.01 lb/MMBtu by 2020.
Advanced In-furnace NOx Control
Several recently completed DOE/NETL R&D projects were successful in achieving the short-term goal of controlling NOx emissions at 0.15 lb/MMBtu using in-furnace technologies. In anticipation of CAIR and possible congressional multi-pollutant legislation, DOE/NETL issued a solicitation in 2004 to continue R&D efforts to meet the 2007 goal and to initiate R&D targeting the 2010 goal of achieving 0.10 lb/MMBtu using in-furnace technologies in lieu of SCR. As a result, four new NOx R&D projects are currently underway and will be completed over the next three years. These projects focus on boiler applications that are firing medium-volatile eastern bituminous coal, which historically has been more difficult in controlling NOx emissions using combustion technologies compared to low-volatile bituminous and subbituminous coals. (See sidebar.)
Enhanced Combustion Low-NOx Burner for Tangentially-Fired Boilers – ALSTOM Power, Inc. is developing an enhanced combustion, low-NOx burner for tangentially-fired boilers. The objective is to optimize combustion via control of near-burner time, temperature, turbulence and stoichiometry. Candidate low-NOx burner components being tested include enhanced coal nozzle tips and internal and external air and fuel separators. These components are being integrated into ALSTOM’s latest generation of the TFS 2000 firing system. The enhanced low-NOx burner is designed to achieve an emission rate of less than 0.15 lb/MMBtu and have minimal balance-of-plant impacts while burning high-volatile bituminous coal. The project includes computational fluid dynamics (CFD) modeling and large pilot-scale testing (approximately 50 MMBtu/hr) to provide information for designing a full-scale version of the enhanced low-NOx burner.
ALSTOM conducted pilot-scale testing of improved coal nozzle tips firing an Illinois No.6 high-volatile bituminous coal in November 2005. It is apparent that coal nozzle tip design can have a significant impact – some of the new tip designs resulted in NOx emissions 40 percent lower than those achieved with the baseline P2 tip. ALSTOM will test the new coal nozzle tip designs in a full-scale tangential-fired boiler to evaluate burner-to-burner interactions.
Advanced In-furnace NOx Control for Wall- and Cyclone-Fired Boilers – Babcock & Wilcox Company (B&W) is developing and demonstrating an advanced NOx control technology capable of achieving an emission rate of 0.10 lb/MMBtu while burning high-volatile bituminous coal for both wall- and cyclone-fired boilers. The technology is based on a “layered” strategy that combines deep air staging using overfire air (OFA), continuous corrosion monitoring, advanced combustion control enhancements and a proprietary combustion technique using oxygen injection. The layered process includes three zones: main combustion, reburn and burn-out. The stoichiometric ratio (SR) in the main combustion zone is varied from 0.8 to 1.1. The reburn zone features oxygen-enhanced combustion of the reburn fuel and flue gas recirculation (FGR), at a SR of less than 1. The burnout zone utilizes OFA to achieve complete combustion, at a SR greater than 1.
To evaluate the oxygen injection process, wall- and cyclone-fired pilot-scale testing is being conducted at B&W’s 5 million Btu/hr small boiler simulator (SBS) facility. Oxygen will be injected at various locations in the combustion zone and data gathered by optical sensors will be used to optimize the combustion process. Corrosion monitoring sensors will assess potential waterwall wastage that could occur under severe operating conditions. Testing will also evaluate the effects of oxidizer composition, extent of deep air staging and overall excess oxygen level on combustion behavior, heat transfer characteristics and emissions levels. Finally, results from the pilot-scale testing will be used to design and prepare a cost estimate for a full-scale version of the technology. Pilot-sale testing began in July 2006, but results are not yet available.
Full-Scale Field Testing of ALTA NOx Control for Cyclone-Fired Boilers – Reaction Engineering International (REI) conducted CFD modeling and full-scale field testing to evaluate a NOx control technology known as advanced layered technology application (ALTA). ALTA combines deep staging with OFA, rich reagent injection (RRI) and selective non-catalytic reduction (SNCR) to achieve NOx emissions near 0.10 lb/MMBtu in cyclone boilers. Developed by REI and the Electric Power Research Institute, RRI uses a nitrogen-containing additive, such as ammonia or urea, to non-catalytically reduce NOx in the lower furnace. REI conducted field testing in May and June 2005 at AmerenUE’s Sioux Station Unit 1, a 500 MW cyclone boiler unit that typically burns an 80/20 blend of Powder River Basin subbituminous coal and Illinois No. 6 bituminous coal. Parametric testing was also conducted with 60/40 and 0/100 blends. The testing also evaluated process impacts on balance-of-plant issues such as the amount of unburned carbon in the ash, slag tapping, waterwall corrosion, ammonia slip and heat distribution.
Prior to field testing, REI used CFD modeling to optimize the number and location of the urea reagent injection locations for both the RRI and SNCR systems. Previous ALTA testing in 2001 and 2002 used 20 RRI injectors and four SNCR injectors. Based on the CFD modeling results, eight additional RRI injectors and 14 additional SNCR injectors were installed for the 2005 field testing. The majority of field testing was conducted using the 80/20 coal blend at 480 MW, under two levels of air staging with OFA, resulting in baseline NOx emission rates of 0.25 lb/MMBtu with normal air staging and 0.20 lb/MMBtu with deep staging.
REI conducted parametric testing for the complete ALTA configuration and with RRI and SNCR separately. Using the 80/20 coal blend, the NOx emission rate achieved with ALTA averaged 0.12 lb/MMBtu with ammonia slip less than 5 ppm. For the 60/40 and 0/100 coal blends, the ALTA configuration achieved NOx emission rates of 0.15 and 0.165 lb/MMBtu, respectively. RRI alone reduced NOx emissions to 0.15 to 0.18 lb/MMBtu with less than 1 ppm ammonia slip when firing the typical 80/20 coal blend. Parametric testing with SNCR alone demonstrated average NOx emissions of 0.156 and 0.165 lb/MMBtu for the 80/20 and 0/100 coal blends, respectively, at less than 5 ppm ammonia slip under most test conditions.
Pilot-Scale Testing of ALTA NOx Control for Wall-Fired Boilers – REI is also developing and verifying performance of the ALTA NOx control technology for wall-fired boiler applications to achieve an emission rate of less than 0.15 lb/MMBtu. The burners are being designed for complete near-burner combustion, rather than traditional staged combustion. Near-burner design provides greater homogeneity of the combustion products in the boiler. Not only does this create ideal conditions for combustion-related NOx control, it also results in a stoichiometry and temperature distribution above the burners that is ideal for the chemistry involved in RRI. REI is conducting CFD modeling and pilot-scale testing to optimize the near-burner combustion system and reagent injection. The pilot-scale testing is being conducted on a 5 million Btu/hr coal combustion furnace operated by the University of Utah. Testing began in the summer of 2006, but results are not yet available. REI will be conducting a second set of CFD modeling studies based on initial pilot-scale combustion results to refine the process design. The final task of the project will involve CFD modeling of a full-scale boiler to evaluate the impact of burner modifications combined with deeper staging and RRI on NOx emissions, unburned carbon, waterwall corrosion and boiler heat balance.
Post-Combustion NOx Control R&D
In addition to the four advanced in-furnace NOx control technology projects discussed above, DOE/NETL has also begun an R&D effort to optimize the performance of SCR controls to support the program’s long-range goal to achieve a NOx emission rate of 0.01 lb/MMBtu by 2020. At this time, only one R&D project focuses on SCR performance.
In Situ Device for Real-Time Catalyst Deactivation Measurements in Full-Scale SCR Systems – Fossil Energy Research Corporation (FERCo) is developing an in situ catalyst deactivation measurement device to reduce SCR operating costs through optimized catalyst management. The device will collect real-time SCR performance data by continuously measuring catalyst activity. As the data is collected, it is analyzed by an existing catalyst management software program, providing information on boiler operating conditions that negatively impact catalyst activity. This information can then be used to optimize boiler operation with respect to catalyst deactivation rate and the catalyst replacement schedule.
FERCo is conducting tests on the SCR installed at Southern Company’s 700 MW tangentially-fired Gorgas Unit 10, which burns bituminous coal. The SCR started operation in 2002 and is designed with two parallel SCR reactors containing three extruded honeycomb catalyst layers plus a spare. FERCo installed its in situ measurement device on each of the three catalyst layers in one of the reactors prior to the 2005 ozone season and took six sets of catalyst activity measurements throughout the summer at four-week intervals. The in situ measurement device uses a self-contained ammonia feed system to control ammonia concentration and extracts upstream and downstream flue gas samples to analyze the inlet and outlet NOx concentration.
Catalyst activity is assessed using a metric known as reactor potential (RP), which provides a measure of the overall potential of the SCR reactor to reduce NOx by accounting for both catalyst deactivation and catalyst layer blockage. The reduction in reactor potential can be characterized by the ratio RP/RPo, where RP is the current measurement and RPo is the value for the initial fresh, unexposed catalyst layer. The RP/RP0 results for the individual layers illustrate the accelerated deterioration of catalyst performance that occurs over time in the first layer compared to the second layer and similarly the second layer compared to the third. The RP/RP0 ratio for the overall reactor dropped from approximately 0.7 to 0.6. The results of these 2005 in situ measurements were then compared to ex situ laboratory measurements. Although there was general agreement between the two methods, some differences are being investigated. Additional in situ measurements were taken during the 2006 ozone season, but results are not yet available.
A preliminary DOE/NETL assessment indicates that approximately 150 GW of advanced NOx in-furnace control technologies could cost-effectively replace 75 GW of new SCR controls required for compliance with CAIR. This market potential is beginning to be realized, as the following examples illustrate:
- In November 2005, AmerenUE announced that it was reconsidering its plans to install SCR based on the successful test results of REI’s ALTA NOx control technology at its Sioux Station. Ameren is now evaluating full-scale implementation of ALTA for both 500 MW units at the Sioux Station.
- Many of the components developed for ALSTOM’s TFS 2000 low-NOx firing system project have been incorporated into commercial boiler designs, resulting in improved NOx emissions without significantly affecting unburned carbon levels. To date, 19 commercial boilers firing PRB coal that utilize aspects of the technologies demonstrated in this project are achieving NOx emissions at or below 0.15 lb/MMBtu.
- Under a previous DOE/NETL NOx R&D project completed in 2004, Praxair, Inc. developed a novel in-furnace NOx control technology called oxygen-enhanced combustion (OEC). Subsequent testing of OEC at two utility boilers, City Utilities’ James River Unit 3 and Northeast Utilities’ Mt. Tom Generating Station, has demonstrated the benefits of the technology. The Praxair technology has also been installed on two coal-fired boilers at the P.H. Glatfelter pulp and paper mill in Spring Grove, Pa.
While our knowledge of the formation and capture of NOx from coal-fired power plants has greatly advanced over the past two decades, many challenges remain. As the nation imposes ever-tightening NOx emissions regulations on the electric power sector, it is critical that research continues to address these challenges.
Watts, J.; Mann, A.; and Russell, D. An Overview of NOx Control Technologies Demonstrated under the Department of Energy’s Clean Coal Technology Program. Presented at American Institute of Chemical Engineers Conference, New Orleans, LA, March 1998.
Feeley, T.; Mayne, A.; and Plasynski, S. (2002) ‘The U.S. Department of Energy’s NOx control technology R&D programme for existing power plants’, Int. J. of Environment and Pollution, Vol. 17, Nos. 1/2, pp. 00-00.
Lani, Bruce; et al. A Review of DOE/NETL’s Advanced NOx Control Technology R&D Program for Coal-Fired Power Plants. National Energy Technology Laboratory, March 2005. See http://www.netl.doe.gov/technologies/coalpower/ewr/nox/index.html.
Richards, G.; et al. Development of an Enhanced Combustion Low NOx Pulverized Coal Burner. Proceedings of the 31st International Technical Conference on Coal Utilization & Fuel Systems, Clearwater, FL, May 2006.
Cremer, Marc; et al. Sub 0.15 lb/MBtu NOx Emissions Achieved with ALTA on a 500 MW Cyclone-Fired Boiler. Proceedings of the 31st International Technical Conference on Coal Utilization & Fuel Systems, Clearwater, FL, May 2006.
Cyclone Boiler Field Testing of Advanced Layered NOx Control Technology in Sioux Unit 1; Final Report to the U.S. Department of Energy under Contract No. DE-FC26-04NT42297; Reaction Engineering International, September 2006.
Smith, R; et al. In situ Determination of SCR Catalyst Activity. Proceedings of the NETL 2006 Environmental Controls Conference, Pittsburgh, PA, May 2006. See http://www.netl.doe.gov/publications/proceedings/06/ecc/index.html
DOE-Funded Technology Slashes NOx, Costs in Coal-Fired Cyclone Boiler. NETL TechNews, November 7, 2005. See http://www.netl.doe.gov/publications/TechNews/tn_doe_fund.html.
Bool, Lawrence; Bradley, Jurron. NOx Reduction from a 44 MW Wall-Fired Boiler Utilizing Oxygen Enhanced Combustion. Presented at the A&WMA/EPA/DOE/EPRI Combined Power Plant Air Pollutant Control Mega Symposium, Washington, DC, May 2003.
Oxygen Enhanced Combustion for NOx Control; Final Report to the U.S. Department of Energy under Contract No. DE-FC26-00NT40756; Praxair, Inc., March 2004.
Bruce W. Lani, Thomas J. Feeley, III, Charles E. Miller and Barbara A. Carney are with the U.S. Department of Energy, National Energy Technology Laboratory. James T. Murphy is with Science Applications International Corporation.
Recent regulatory drivers for NOx control
Regulatory and legislative requirements have driven NOx control technology development for existing coal-fired power plants. The first driver was the Title IV acid rain program, created through the 1990 Clean Air Act Amendments (CAAA). This included a two-phase strategy to cut NOx emissions from coal-fired power plants – Phase I started January 1, 1996 and Phase II started January 1, 2000. The Title IV NOx program was implemented through unit-specific NOx emission rate limits ranging from 0.40 to 0.86 lb/MMBtu depending on the type of boiler/burner configuration.
The second driver was Title I National Ambient Air Quality Standards (NAAQS) for ozone that led to EPA’s NOx SIP (state implementation plan) Call Rule in 1998, requiring 21 eastern states and the District of Columbia to participate in a regional cap-and-trade program based on an equivalent NOx emission rate of 0.15 lb/MMBtu beginning in 2003-04. Compliance required development and implementation of post-combustion NOx controls such as SCR. Being a cap-and-trade program, power plants could either reduce their NOx emissions to the level of their allowance allocation or acquire allowances.
A third regulatory driver resulted from EPA’s revision to the fine particulate matter (PM) and ozone NAAQS in 1997, which led to the Clean Air Interstate Rule (CAIR) finalized in May 2005. The NOx emission reductions affect 28 eastern states and the District of Columbia and will be implemented as a cap-and-trade program in two phases: January 1, 2009 and January 1, 2015. NOx emission caps were calculated using emission rates of 0.15 lb/MMBtu for 2010 and 0.125 lb/MMBtu for 2015.
New Coal-Fired Power Plants
New coal-fired power plants must meet both NSPS and New Source Review (NSR) NOx emission requirements. Existing sources that see reconstruction or a major modification are also subject to NSR. The NOx NSPS for coal-fired plants built after August 17, 1971 and before July 9, 1997 ranges from 0.5 to 0.8 lb/MMBtu depending on coal rank and construction date. The NOx NSPS was changed to a generation output-based standard of 1.6 lb/MWh (roughly equal to 0.15 lb/MMBtu at a heat rate of 10,500 Btu/kWh) for new coal-fired plants that started construction after July 9, 1997. The most recent NSPS revision applies to units built after Feb. 28, 2005.
The NOx NSR requirements are set on a case-by-case basis by the state environmental agency during a new plant’s permitting process and are likely to be more stringent than NSPS. Under NSR, a new plant must install either Best Available Control Technology (BACT) if located in an ozone NAAQS attainment area, or Lowest Achievable Emission Rate (LAER) technology if located in an ozone NAAQS nonattainment area. Recent state BACT/LAER determinations have set NOx emission rate limits for new coal-fired plants between 0.05 and 0.10 lb/MMBtu and required installation of LNB and SCR.