White Papers & Presentations Advanced Erosion Protection Technology Provides Sustained Low NOx Burner Performance Download PDF here (155 KB)  Download Presentation here (273 KB) Abstract   Coal-fired electric power generators investing in NOx reduction technologies are challenged with maintaining equipment to ensure that the NOx rates remain within specified tolerances.  Pulverized coal traveling at high velocities through burners and burner tips produces significant erosion, forcing plants to repeatedly replace components, make operation adjustments and replace entire burner assemblies. During periods between repairs, alterations in burner geometry caused by excessive erosion can impact combustion characteristics, causing upward trending NOx emissions.   Advanced burner designs and the utilization of erosion-resistant wear protection ensure homogenous non-turbulent coal mixing and controlled burn rates are maintained over extended periods of operation. Retaining burner component geometry with advanced wear protection prolongs compliance with NOx emission requirements, reduces the risk of unplanned downtime, and increases overall unit productivity, predictability, and reliability.   Innovative low NOx burner technologies utilize erosion-resistant materials to protect components against wear and to maintain sustained low NOx performance.  Standardized laboratory analyses conducted by Riley Power Incorporated and field applications evaluating various erosion-resistant materials have found that infiltration brazed tungsten carbide claddings outperform other types of wear protection, including chrome carbide weld overlays, in low NOx burner environments.  These claddings are shown to aid in the retention of burner component geometry, controlling NOx emissions between scheduled plant outages.   The Conflict Before the days of the Clean Air Act Amendment and deregulation, power producers were relatively unconstrained in the fuels, technologies, and production strategies they employed to meet the market demand for power. Their primary objective was to provide a reliable source of quality power. Whatever equipment was in place per original design could be used without a great deal of concern for the quality or type of fuel being burned. Original equipment design took into account the planned fuel formulation, and auxiliary equipment was selected primarily based upon these original specifications. Equipment deterioration was accepted as a normal cost of operation, and was dealt with through frequent and relatively long maintenance outages. In recent years, however, competitive market conditions combined with tough emissions standards have created entirely new challenges that generate potential conflicts between fuel procurement, asset managers, and maintenance teams. Fuel procurement teams are rewarded for saving a nickel per ton even if the cheaper fuel has higher than originally specified Sulphur or other undesirable constituents that may cause increased rates of equipment deterioration and emissions. Asset managers are confronted with the need to invest in SCRs, scrubbers, and other high-cost assets in order to comply with tougher emissions restrictions. Meanwhile, maintenance teams that have experienced significant reductions in staff are striving to increase operating runs between maintenance windows. Because fuel and emissions management assets represent such a large percentage of operating costs, maintenance teams are often left with the conflicting challenge of keeping equipment online for longer periods when changes in fuel and equipment might otherwise increase equipment wear rates.   Introduction   NOx emissions have challenged boiler designers for the greater part of three decades.  Advanced burner configurations have become a standard approach to NOx abatement, and burner designers are continually developing new technologies to maintain low level NOx output.  Burner designers are faced with several NOx reduction challenges, including varying coal specifications which cover a wide range of fuel properties, high heat release rates of some wall-fired cell configurations, the requirement for burner parts to last four plus years between major outages, and the struggle to decrease NOx emissions while maintaining reasonable levels of UBC in the fly ash.   State-of-the-industry wall-fired low NOx burner designs combine sophisticated mixing and stabilization designs with advanced wear protection technologies. Extended low NOx burner performance is achieved by reducing erosion-driven alterations in burner component geometry, resulting in prolonged compliance with NOx emissions requirements, reduced risk of planned and unplanned downtime, and increased unit productivity and reliability.   This paper will discuss a standardized laboratory analysis evaluating various erosion-resistant materials in extreme burner environments.  In addition, field results comparing similar wear-resistant materials in actual burner applications will also be reviewed.  RPI Standarized Laboratory Testing   Riley Power Incorporated’s (RPI) Controlled Combustion Venturi® (CCV) advanced burner technology employs a venturi coal nozzle to control fuel mixing and a low swirl coal spreader to provide good mixing without excessive turbulence.  Integral air diverters and stabilizer rings improve flame attachment and reduce NOx emissions.  The combination of sophisticated design component geometries and erosion-resistant wear protection ensures that homogenous, non-turbulent coal mixing and controlled burn rate is maintained over extended periods of operation   CCV® burner technology has evolved significantly since its initial inception in the early 1980s.  Complex computational fluid dynamic (CFD) analyses and full-scale test facilities are now employed to design high-performance burners.  Today, on a burner unit firing bituminous fuel with burners only, and no overfire air, NOx levels of 0.36 lb/MMBTU can be obtained.  This is achieved by using a plug-in retrofit that requires no pressure part replacement, over fired air (OFA), or burner respacing.  A burner turndown ratio of 2.5:1 is still maintained.  Similar retrofits on units burning sub-bituminous coal can achieve NOx emissions as low as 0.15 lb/MMBTU or less.   The spreader’s useful life is 1-2 years, depending on the operating conditions and the type of coal being burned.  RPI would like to extend the spreader’s life to 3-4 years with the use of erosion-resistant wear protection.  In an attempt to protect their low swirl coal spreaders against abrasion degradation and maintain long-term performance, RPI burner designers conducted standardized laboratory testing on industry accepted wear protection materials.  Tested materials included: Stoody 101HC Conforma Clad® WC219 Infiltration Brazed Tungsten Carbide Cladding Chrome Carbide Weld Overlay Stellite 31 Stellite 6 Stellite 12 A560 Grade 50Cr-50Ni Silicon Carbide   Figure 1. ASTM G73 Test Fixture with Sample   The experiment followed ASTM standard G73 methods (see test fixture in Figure 1) and analyzed highly abrasive fine grit black beauty coal slag as the erodent materi­al.  Testing was conducted at a 90° impingement angle, with particle velocities of 240 ft/s for 30 minutes.    Figure 2 represents the testing results.  Silicon carbide was the most erosion-resistant material with a loss of 0.06 grams.  Conforma Clad WC219 tungsten carbide cladding also performed well with a material loss of 0.21 grams.  These materials were followed by Stellite 6 (1.15 g material loss), chrome carbide weld overlay (1.24 g material loss), Stellite 12 (1.49 g material loss), Stoody 101 (1.52 g material loss), Stellite 31 (2.00 g material loss) and 50Cr/50 Ni (2.36 material loss).    Figure 2. ASTM G73 Test Results. 90° Impingement Angle, 240 ft/second, 30 minute test                                                                                         In order to verify these lab results, RPI began testing several protective materials in actual boiler applications, including low swirl coal spreaders and coal flow distributor elements exposed to the extremely erosive boiler environments. Real-World Field Applications Power generators have found success in reducing the erosive wear experienced by their burner components with infiltration brazed tungsten carbide cladding, including low swirl coal spreaders, dual air zone burners, and burner tips.  Coal Nozzle Low Swirl Coal Spreaders As part of the development and evaluation of selected erosion resistant materials, RPI burner designers chose to install coal spreaders to confirm field performance.  In order to verify previous ASTM G73 laboratory testing, RPI began field-testing protective materials on low swirl coal spreader burner components. Carbon 61.29 — 69.31% Hydrogen 4.18 — 4.82% Nitrogen 1.36 — 1.51% Oxygen 8.92 — 9.31% Sulfur 0.74 — 0.98% Ash 4.19 — 15.37% Moisture 8.14 — 10.51% Hard Grove 46 HGI Table 1. Pulverized Coal Content at We Energies, Valley Power Plant RPI, working in partnership with We Energies, chose the Valley Power Plant as a test site due to their highly-erosive burner velocities and fuel properties.  Typical coal/primary airflow velocity through burners at full load is approximately 87 ft/sec.  The pulverized coal fired at this station is blended with approximately 9% petroleum coke (Valley Stationstopped receiving pet coke in April 2003), with the ultimate analysis shown in Table 1.    The erosive environment was considered to be moderately high because of the high nozzle velocity, and the high silica and alumina content of the coal.  Figures 3 and 4 demonstrate the typical wear experienced by unprotected burner components and those protected with Stellite 31 weld overlay.  The Stellite-protected coal spreader was missing approximately 1 1/2" of the coal spreader vane after 22 months of service.      Figure 3. Unprotected burner component showing typical wear after 22 months of service Figure 4. Stellite 31 Weld Overlay on the leading edge shows approximately 1 ½” off vane leading edge after 22 months of service   In February 2003, multiple low swirl coal spreaders were installed into the existing CCV® low NOx burners at We Energies Valley Station in order to compare the two ASTM G73 test top performers. They were installed in burners fed by the same mill.    One coal spreader test piece was protected with 0.040" thick Conforma Clad WC219 tungsten carbide cladding, applied directly to the leading edge of the spreader base material using a proprietary infiltration brazing process.  Two other cast silicon carbide coal spreader test pieces were installed. The three coal spreaders were installed in Unit 2, Boiler 3 D1 burner in February 2003.  In October 2003, after approximately 9 months of continuous service, the prototype test pieces were inspected.    Figure 5. Silicon Carbide spreader experienced .080” leading edge erosion and had significant cracking Figure 6. Conforma Clad infiltration brazed tungsten carbide spreader measured .007” material erosion loss The silicon carbide spreader showed 0.070" of wear, experienced severe thermal cracking, and broke during disassembly (see Figure 5). The prototype test spreader protected with tungsten carbide cladding showed no visible signs of erosion (see Figure 6).  It is believed thatthe silicon carbide did not outperform the tungsten carbide cladding in field results because of the environment’s temperature gradient, which was not present in previous lab tests.  It is likely that the temperature gradient also accelerated erosion due to micro-fracture of the grain structure.   LEADING EDGE   BASE MIDDLE TIP VANE 1 .042" .036" .039" VANE 2 .040" .033" .043" VANE 3 .038" .036" .039" VANE 4 .039" .037" .040" BODY LOCATION 1 2 3 4 .038" .039" .037" .036" Table 2. Tungsten Carbide Cladding Thickness Measurements after Nine Months in Operation Due to the non-magnetic nature of the tungsten carbide cladding, it was possible to measure the remaining cladding thickness using an Elcometer eddy current thickness gauge.  Measurements showed that the maximum extent of erosion was 0.007", or less than 20% of the total protective layer thickness (see Table 2).  From the measured results, the predicted life of the coal spreader protected by the tungsten carbide cladding is estimated at approximately 5 years. As a result of the ASTM G73 lab testing and field performances, RPI burner designers selected Conforma Clad® WC 219 infiltration brazed tungsten carbide cladding as the best available control technology for protecting burner components against erosion deterioration to ensure long-term performance.  Due to the success experienced on low swirl coal spreaders, RPI extended their use of infiltration brazed tungsten carbide wear protection to their CCV® dual air zone burners. CCV® Dual Air Zone Burners A second installation of infiltration brazed tungsten carbide cladding was applied to the new coal flow distributor elements installed at We Energies, Presque Isle Power Plant (PIPP).  In 2001, PIPP installed RPI’s low NOx CCV® second generation dual air zone burners.  In conjunction with this installation, modifications were made to the coal mill system to improve the coal pipe-to-pipe balance and overall unit performance. The coal flow distributor installed by RPI was designed for installation in the coal stream exiting the mill.  Boiler components at this location often experience severe erosion due to sliding and impact abrasion.   Conforma Clad agreed to test their tungsten carbide cladding on this application due to the unique location of the device and the wear characteristics of the fuel (see Table 3).   Fuel Composition Ash Composition Carbon 73.8 – 74.2% Silicon Dioxide 56% Hydrogen 5.0 – 5.1% Aluminum Oxide 25% Nitrogen 1.6% Iron Oxide 5% Oxygen 8.8 – 9.1% Sulfur Trioxide 2% Sulfur 0.82 – 0.85% Calcium Oxide 4% Ash 9.1 – 9.9% Other 8% Table 3. Fuel and Ash Composition at We Energies, Presque Isle Power Plant   The coal flow distributors were installed in Unit #6 pulverizers in December 2002.  The coal flow distributors for Unit 5 were installed in February 2003.  Although the two units have slightly different operating times, a good comparison can be made between the unprotected flow elements in Unit 6, which were inspected in September 2003, and elements in Unit #5, protected with tungsten carbide cladding and inspected in October 2003.   Figure 7 shows the unprotected element installed in the Unit 6 D pulverizer.  These elements are made from a heat-treated alloy, with a hardness of 300+ Brinell.  Figure 8 shows the same element design clad with infiltration brazed tungsten carbide supplied by Conforma Clad.   Figure 7. Heat Treated Alloy with no Protective Cladding after Nine Months in Operation Figure 8. Leading Edge Protected with Brazed Tungsten Carbide Cladding after Eight Months in Operation   The infiltration brazed tungsten carbide cladding had an initial base thickness of 0.040". With braze scale (un-melted chrome), the resulting total cladding thickness was approximately 0.045 – 0.050".  From the thickness measurements shown in Table 4, it can be seen that the braze layer, which has a hardness of approximately 57Rc and is relatively erosion-resistant, had not yet been penetrated.     Inboard Outboard Vane Location 1 2 3 4 5 6 7 Left 0.047 0.048 0.049 0.048 0.050 0.048 0.050 Left Center 0.048 0.048 0.047 0.046 0.046 0.046 0.048 Right Center 0.046 0.047 0.046 0.046 0.046 0.047 0.048 Right 0.049 0.051 0.047 0.051 0.049 0.049 0.049 Table 4. Infiltration Brazed Tungsten Carbide Cladding Thickness Measurements   Follow-up inspections were performed on the dual air zone burner elements in January 2005.  Unit 5’s coal distributors showed no visual or measurable wear in 23 months of continuous operation.  Since the tungsten carbide clad coal flow distributors have experienced minimal wear, it is difficult to extrapolate the anticipated life.  The expected life of the tungsten carbide clad elements is projected to be several times that of the unprotected elements.  Infiltration brazed tungsten carbide cladding has performed similarly well in low NOx burner tip applications. Low NOx Burner Tips In May 2004, Dynegy, Wood River generating station installed Foster Wheeler low Nox burner tips protected with infiltration brazed tungsten carbide cladding for evaluation. The plant’s T-fired boiler was converted to balanced draft during PRB conversion in spring 2002. The burner tips had been experiencing severe erosion damage due to burner velocities of 83 feet/second and the erosive properties found in the station’s fuel and ash composition (see Table 5).  A typical burner tip protected with a standard weld overlay required replacement within two years     Fuel Composition Ash Composition Carbon 49.5 – 51.2% Silica 46% Hydrogen 3.97 – 4.1% Aluminum Oxide 18% Nitrogen 0.74% Iron Oxide 4% Oxygen 10.71 – 10.9% Sulfur Trioxide 8% Sulfur 0.33 – 0.35% Calcium Oxide 16% Ash 8.2 – 8.4% Other 8% Table 5. Fuel and Ash Composition at Dynegy, WoodRiverGenerating Station   The station tested the erosion protection provided by infiltration brazed tungsten carbide cladding and standard weld overlay on their burner tips.  The two tips were installed in May 2004 and inspected during an outage in January 2005.  The weld overlay tip had developed holes in the casing and splitter vane leading edge.  The side of the tip also showed substantial material loss (see Figure 9). Inspections of the infiltration brazed tungsten carbide-protected tip revealed no visual or measurable erosion or material loss (see Figure 10).   Figure 9. Standard weld overlay protected Foster Wheeler tip Figure 10. Conforma Clad infiltration protected Foster Wheeler tip   Dynegy successfully increased the life of their burner tips with the use of infiltration brazed tungsten carbide wear protection.  It is anticipated that the tungsten carbide clad burner tips will increase life several times over the weld overlay tips.  The weld overlay tips showed significant erosion after only 9 months, while the tungsten carbide clad tips were virtually unchanged. Conclusion Plant maintenance teams are experiencing increasing pressures to reduce the cost of maintaining critical low NOx burner equipment, and are expected to use the latest technology to maintain emissions compliance while extending the operating period between unit shutdowns. Standardized laboratory analyses and various field applications evaluating various erosion-resistant materials have found that infiltration brazed tungsten carbide claddings outperform other types of wear protection in low NOx burner environments. Burner designers and plant personnel who protect their burner components with infiltration brazed tungsten carbide cladding are capable of extending equipment run times between repairs and reduce component replacements, ensuring their boilers continuously perform at peak levels.  By maintaining the geometry of these critical burner components, NOx levels do not deteriorate due to component degradation. Written By: Chris Harley, Senior Applications Engineer, Conforma Clad Inc.; Douglas Goebel, Engineer, We Energies; Michael Saari, Senior Mechanical Engineer, We Energies; Bonnie Courtemanche, P.E., Senior Engineer, Riley Power Inc.; Corby Valentine, Maintenance Director, Dynegy.