Staying Competitive:Emissions Control
By: Nolan Frederick, Entergy Services, Inc., and Ravindra K. Agrawal and Stephen C. Wood, Entropy Technology and Environmental Consultants, LP
Competitive forces are compelling asset owners to consider low-cost emissions control technologies such as induced flue gas recirculation
National, regional and local regulations to reduce NOx emissions have prompted the development and implementation of a number of NOx control technologies – from selective catalytic reduction and low-NOx burners to overfire air and flue gas recirculation. Induced flue gas recirculation (IFGR), a design derivative of conventional forced flue gas recirculation, has demonstrated NOx reductions as high as 85 percent and NOx levels as low as 0.04 lb/MMBtu on gas-fired boilers, at costs of less than $3/kW.
The NOx Control Paradigm Shift
Historically, NOx control retrofits have involved equipment-intensive technology such as low-NOx burners (LNB), overfire air (OFA), forced flue gas recirculation (FFGR), and selective catalytic reduction (SCR), all of which can be very costly. Studies published by EPA1 and EPRI2 indicate that costs for conventional combustion modification NOx control techniques (e.g., LNB, OFA, FFGR) are in the range of $8-35/kW, while costs for flue gas control techniques (e.g., SCR) are in the range of $50-80/kW. Furthermore, due to the extremely mixed results from LNB installations, and the operational problems encountered with LNB, OFA and FFGR systems, the power generation industry needs an innovative, low-cost NOx control technology that can achieve lower NOx emissions with minimal impact on performance and operation.
IFGR was pioneered in 1997 by the electric utility industry and Entropy Technology and Environmental Consultants LP (ETEC), initially through EPRI tailored collaboration programs at Entergy and Reliant Energy. Since then, ETEC has added several features to enhance its performance. ETEC initially simplified the technology by eliminating the intensive equipment modifications required with FFGR, i.e., the hot gas fans and combustion air duct mixing devices. Instead, the exhaust gas is routed through ducting directly into the forced draft fan inlet, where exhaust gas products are thoroughly mixed with combustion air prior to entering the burners. NOx reduction levels are consistent with those of FFGR technology, and since cost levels have been reduced to $1-3/kW, IFGR technology can be the preferred NOx control technology in certain applications.
FGR technology is based on reducing thermal NOx formation by introducing inert flue gas, which reduces oxygen concentration and absorbs heat, thereby reducing peak flame temperatures. Studies indicate that peak flame temperature governs thermal NOx formation.3-4 Because FGR reduces thermal NOx, its use is most effective for natural gas and low nitrogen-containing fuels, where it can reduce NOx by up to 85 percent. On oil and coal-fired units, reductions are in the range of 20-60 percent.
In conventional applications, the recirculated flue gas is typically extracted from the boiler outlet duct upstream of the air heater. The flue gas is then returned through a separate duct and fan to the combustion air duct that feeds the windbox. The recirculated flue gas is mixed with the combustion air using airfoils or other mixing devices in the duct. Most of the cost associated with traditional FGR technology is due to an additional hot gas fan that is required to transport the flue gas. ETEC’s patented IFGR technology is based on utilizing the existing forced draft fan to induce flue gas into the combustion air at the fan inlet. Since no major equipment costs are involved, the typical cost of IFGR technology is less than $3/kW, depending on the plant layout.
ETEC has installed its IFGR technology on more than 36 utility boilers ranging in size from 42 MW to 815 MW (Figure 1). Typical NOx reductions obtained with IFGR technology range from 50-85 percent, with NOx levels as low as 0.04 lb/MMBtu. IFGR technology has been demonstrated on a wide variety of utility boilers, including tangentially fired, turbo fired, opposed fired and face-fired units. Cyclone furnaces and wet bottom boilers are also good candidates for IFGR, since thermal NOx formation is high for these designs. IFGR technology requires very minor modifications and can be installed
in a few weeks. Physical modification includes tapping the exhaust duct to draw flue gas and recirculate it back to the fan. Minor modification to controls will also be needed for IFGR dampers. The only operational change in most applications is recalibration of the total airflow curve.
In IFGR applications, 10-25 percent of the flue gas is recycled back to the combustion zone. IFGR flow requirements are similar to those of FFGR, but NOx reductions are higher due to better mixing of flue gas and air in the fan. The extent of NOx reduction depends on the amount of flue gas recirculated. Increasing the recirculation rate causes a decrease in peak flame temperature, resulting in lower thermal NOx formation.4 Many LNB vendors are packaging FGR with LNB to reduce NOx. In most applications, however, the cost of LNB with FGR is more than $10/kW, and the typical NOx reduction of the combined system is not much greater than with IFGR. Therefore, in certain situations, the high cost of LNB for incremental reduction in NOx may not be justifiable.
Since IFGR technology uses the existing fan to induce FGR, IFGR operation at full load depends on the forced draft fan capacity. For fan limited units, IFGR capacity may need to be reduced at or near full load. Since most plants have to meet a rolling average limit, over-controlling at lower loads is one option to overcome this drawback. In many projects, IFGR has been combined with combustion modifications, such as Burners-Out-Of-Service (BOOS), to achieve dramatic NOx emissions reductions. In addition, several low-cost debottlenecking options such as re-tipping fan blades and/or rewinding fan motor are available to accommodate IFGR flow.
IFGR flow rates achievable at high loads also may depend on steam temperature conditions. IFGR causes an increase in mass gas flow through the boiler, resulting in increased heat transfer in the convection sections. At high loads, therefore, steam temperatures are increased, but can usually be overcome cost-effectively by utilizing existing steam temperature control systems. In many cases, where units suffer steam temperature sag during lower load operation, the increased steam temperatures have provided substantial improvement in net heat rate, as well as substantially improved turndown capability. In fact, in some cases IFGR has been primarily considered for its turndown enhancement for standby boilers, operating at minimum capacity, reserved as emergency backup for main steam supply interruption.
IFGR at Reliant Energy
Prompted by requirements for NOx RACT compliance in the Houston area, Reliant Energy widely implemented IFGR technology on oil and gas-fired boilers within its system (Table 1). It should be noted that the baseline NOx emissions were relatively low due to the fact that these units had been operating with combustion modifications, i.e., BOOS and/or OFA. For many of the units, the combination of combustion modifications and IFGR has resulted in an effective NOx reduction on the order of 90 percent from the original baseline levels.
To characterize NOx reduction as a function of load, ETEC and Reliant conducted performance testing at P.H. Robinson Unit 2 (Figure 2). The baseline data, collected prior to the IFGR installation, show relatively lower NOx formation at lower loads. At higher loads, the unit fires harder, increasing peak flame temperature and increasing NOx formation. This behavior is typical of thermal NOx formation. Figure 2 indicates that IFGR is very effective at controlling NOx emissions through the load range and that effectiveness increases with the amount of recirculation rate.
Entergy Sabine Unit 4
Entergy hosted the first utility boiler application of IFGR at Willow Glen Unit 3,3 and based on this success, implemented IFGR at nine additional units. During a 2002 outage, Entergy incorporated IFGR at Sabine Unit 4 to reduce NOx emissions. Two IFGR ducts were installed from the exhaust duct to the two forced draft fan inlets. Each IFGR duct has three sets of dampers. The first damper, located closest to the exhaust duct, is mounted horizontally and is manually controlled with a chain. The IFGR flow control damper, also mounted horizontally, comes next and is used for making IFGR flow adjustments to optimize system performance. The last damper, mounted vertically, is referred to as the “fresh air” damper. The IFGR control damper and the fresh air damper are designed to operate together such that as the IFGR control damper is opening, the fresh air damper is closing. The total open position of both dampers is always 100 percent, i.e., if the IFGR control damper is 30 percent open, then the fresh air damper is 70 percent open.
Burner Out Of Service (BOOS) operation is used in conjunction with the IFGR System operation to lower NOx emissions and optimize boiler performance. BOOS operation involves terminating the fuel flow to selected burners while leaving the air registers open. The remaining burners operate fuel-rich, thereby limiting oxygen availability, lowering peak flame temperatures, and reducing NOx formation. The unreacted products combine with the air from the terminated fuel burners to complete burnout before exiting the furnace.
At Sabine, the upper level burners on Unit 4 were modified to enhance the flexibility of BOOS operation. Originally, each burner gas valve controlled the gas flow to two burner cells, which is still the configuration for the lower level burners. In the modified configuration, ETEC replaced the piping downstream of the double block and bleed valves with piping to each cell, with each pipe run containing a manual gas valve and a flow control orifice.
Figure 3 shows the NOx emissions
profile for baseline operation and for IFGR/BOOS operation at Sabine. It should be noted that the baseline NOx emissions include OFA operation. The NOx emissions level increases above 450 MW with IFGR/BOOS operation because the IFGR control damper begins closing as full load conditions are approached in order to provide forced draft fan capacity. The NOxemissions reduction requirements are calculated on a 24-hour and a 30-day rolling average basis.
Typical power plant IFGR installation.
In some cases, power plants may have to use SCR to control NOx levels below 0.01 lb/MMBtu. In such a case, costs can be reduced by combining IFGR installation with SCR. ETEC has recently patented the use of FGR with post-combustion flue gas clean-up technologies such as SCR. Analysis has shown that SCR costs can be reduced as much as 65 percent with a hybrid system combining IFGR and SCR.4 Studies published by EPA5 and the TCEQ6 also show that hybrid systems can effectively reduce the costs and improve performance of the SCR. Hybrid systems lower the concentration of NOx to the SCR, resulting in reduced requirements for catalyst and ammonia handling systems. Due to lower NOx concentrations to the SCR, ammonia consumption can be reduced by as much as 80 percent. For a 300 MW unit, reduction in ammonia usage alone could result in a payback of the IFGR installation cost in less than 6 months! Thus, hybrid systems can lower not only capital costs but also reduce operating costs significantly.
Acknowledgement: The authors wish to express their appreciation to Mr. Ben Carmine of Reliant Energy for his review and commentary.
Nolan Frederick is a Senior Engineer in Entergy Corp.’s Beaumont Area Plant Support division. He has been with Entergy for 14 years, and currently serves as Project Manager for Entergy’s Texas NOx Projects. Mr. Frederick holds a Bachelor of Science degree from the University of Louisiana at Lafayette and is an EIT in Louisiana
Dr. Ravi K. Agrawal is Vice President of Entropy Technology and Environmental Consultants, Inc., [email protected] He has 20 years of experience in project management, business development, sales, process design, advanced process control, process optimization, combustion engineering, and air pollution control. Dr. Agrawal has previously been associated with Argonne National Laboratory, M.W. Kellogg, Fluor Daniel, and Woodward Clyde Consultants. He holds Ph.D. and M.S. degrees in chemical engineering from Clarkson University and a B.Tech degree from Osmania University. He is also a registered professional engineer in Texas and Pennsylvania.
Stephen C. Wood is President of Entropy Technology & Environmental Consultants, Inc., [email protected] He is a consulting/design engineer with over 30 years of combustion process engineering experience, and recently pioneered the application of Induced Flue Gas Recirculation (IFGR) technology for NOx control at large central station boilers. Mr. Wood has previously been associated with NASA, KVB, Tenerx, Energy Technology Consultants, and Woodward Clyde Consultants. He holds a BS degree from the University of Houston, and is currently a member of ASME.
1. “Nitrogen Oxides, Why and How They are Controlled,” EPA-456/F-99-006R (1999).
2. Lange, H., Himes, R., and Jantzen, T., “Retrofit NOx Control Guidelines for Gas and Oil Fired Boilers,” EPRI TR-108181 (1997).
3. Wood, S., “Reliant Energy Induced Flue Gas Recirculation Program,” EPRI (June 2000).
4. Agrawal, R.K. and Wood, S.C., “Cost-Effective NOx Reduction,” Chemical Engineering, 108(2), 78-82 (2001).
5. Petroleum Refinery Tier 2 BACT Analysis Report, U.S. EPA (Jan. 2001).
6. “Control of Air Pollution from Nitrogen Compounds,” Texas Register, Chapter 117, August 25, 2000 and January 12, 2001.