The Babcock & Wilcox CompanyBR-1800
Technical Paper
Presented to:32ndInternationalTechnicalConferenceonCoalUtilization&FuelSystems(akaClearwaterCoalConference)June 10 - 15, 2007Clearwater, Florida, U.S.A.
Minimizing NOx with the Innovative AireJetTM Pulverized Coal Burner
H.Sarv,A.LaRue,D.Rowley,A.SayreandW.KahleThe Babcock & Wilcox CompanyBarberton, Ohio, U.S.A.
Minimizing NOx with the Innovative AireJetTM Pulverized Coal Burner
AbstractA new, staged low-NOx, pulverized coal burner named
AireJet was developed with the aid of computational fluid mechanics modeling and tested at pilot scale. Tests were conducted in a 100 million Btu/hr facility that was set up with separated and close-coupled overfire air (OFA) ports at different elevations above the burner centerline. An eastern high-volatile bituminous Middle Kittanning coal and a west-ern Powder River Basin Black Thunder coal were chosen for testing. Effects of burner hardware, combustion stoichiom-etry, and coal fineness on NOx (NO + NO2), CO, and loss of ignition (LOI) were evaluated. Under the normal operating conditions of 17% excess air and 0.85 burner stoichiometry, and with the OFA ports being separated about 21 ft above the burner centerline, we achieved 0.148 lb NOx/MBtu and 3.84% LOI for the eastern bituminous Middle Kittanning coal and 0.076 lb NOx/MBtu and 0.35% LOI for the western Black Thunder coal, respectively. In the close-coupled OFA arrangement, the OFA ports were 2 and 3 ft above the burner centerline, resulting in 0.240 lb NOx/MBtu and 1.26% LOI for the Middle Kittanning coal, versus 0.122 lb NOx/MBtu and 0.34% LOI for the Black Thunder coal. In the first com-mercial application of twelve AireJet burners in a 95 MWe utility boiler that burns a Powder River Basin coal, the NOx emissions averaged around 0.13 lb/MBtu and the LOI levels were less than 1%.
IntroductionIdeal implementation of deep staging for maximum NOx
reduction entails complete consumption of oxygen in the fuel-rich combustion zone between the burner and the overfire air (OFA) ports. In a utility boiler, the NOx reduction residence time is approximated on the basis of flue gas movement from
the mid-plane of each burner row to the OFA ports. As an example, in an opposed-fired boiler the residence time from the bottom burner row to OFA ports is typically 2 to 3 seconds, but less than 0.5 sec from the top row (based on OFA ports 10 ft above the burner row). Since the required burner-to-OFA residence time for flue gas mixing and reaction is often greater than 0.5 sec, the upper-row burners are less effective in NOx reduction. A properly designed low-NOx burner can minimize the flame length and maximize the available time for NOx reduction reactions ahead of the OFA ports.
Fundamentally, oxygen-deficient and high-temperature coal flames with large internal recirculation zones (IRZ) in the near-burner region have been known to accelerate ignition, improve flame stability, and generate low NOx emissions. Based on these principles, a new prototype, staged low-NOx burner named AireJet [1] was developed for pulverized coal firing with the aid of COMOSM [2], a multi-dimensional com-putational fluid dynamics code. The burner design features a central core air nozzle for introducing a portion of secondary air to enhance combustion. Primary air and pulverized coal (PC) flow through an annulus surrounding the core nozzle. Additional secondary air flows through two concentric inner and outer annuli encircling the coal annulus. Swirl is imparted on those flow streams via fixed and adjustable spin vanes. A diverging air separation vane (ASV) can be mounted at the outer secondary air annulus exit to enhance the IRZ.
COMO simulated the flow patterns, coal particle dispersion and trajectories, coal pyrolysis chemistry, gas-phase and char reactions, flame structure, and heat transfer. No attempt was made to predict NOx emissions. Instead, COMO was applied to identify arrangements that produced oxygen-deficient and high-temperature flames with large internal recirculation zones. Figure 1 shows the computed flow fields and tempera-
H.Sarv,A.LaRue,D.Rowley,A.SayreandW.KahleThe Babcock & Wilcox Company
Barberton, Ohio U.S.A.
Presented to:32ndInternationalTechnicalConferenceonCoalUtilization&FuelSystemsJune 10 - 15, 2007Clearwater, Florida, U.S.A.
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ture profiles for the AireJet burner equipped with a uniquely-designed ASV. With this burner, the hot sub-stoichiometric IRZ was significantly larger relative to the commercial low-NOx DRB-4Z burner.
Quantitative comparisons of the calculated reverse flows within the flame envelopes of both burners are shown in Figure 2. It is easily observed that the AireJet burner creates a larger recirculation (reverse flow) volume within the flame. Based on the computer modeling results, a detailed mechanical design was prepared and a prototype burner was constructed for testing. Figure 3 shows the burner schematic.
TestfacilitydescriptionPrototype-scale testing of the staged AireJet burner was
conducted in the B&Ws 100 MBtu/hr Clean Environment Development Facility (CEDF). Pulverized coal flow was transported to the burner by 150F primary air and at desired air-to-fuel ratios. Secondary air was preheated to 600F and a part of it was diverted and reintroduced through two OFA ports. Figure 4 shows the construction of the CEDF furnace and the overfire air port arrangements.
In the separated (upper) OFA arrangement, two ports were put in service at 21.3 ft above the burner centerline with a burner-to-OFA bulk flow residence time of 3.5 sec during staged firing (SR= 0.85). Commercial utility boil-ers have significantly less gas flow residence time between most of the burners and OFA ports. To examine the effect of shorter residence times, two close-coupled OFA ports were also installed in the tunnel section of the furnace at 2 ft and 3 ft above the burner centerline, and 26 ft away horizontally from the burner throat. For this arrangement, the calculated residence time above the burner centerline was 0.3 sec, rep-resenting the time from the upper row of burners to the OFA ports in a utility boiler.
Flue gas was sampled continuously through a heated sample line at the convection pass exit. After filtering and drying, the composition of key species (e.g., CO, NOx, and O2) was measured by standard analyzers. Flyash was also sampled across the convection pass exit duct via a multi-hole probe and analyzed for loss on ignition (LOI). LOI values closely approximate the unburned carbon levels in the flyash.
Pilot-scaleresults
Coal composition and fineness An eastern bituminous Middle Kittanning coal and
a Powder River Basin subbituminous Black Thunder coal were used in testing. Table 1 lists the proximate, ultimate, and heating value analyses of the as-received coals. Fixed carbon-to-volatile matter ratio (FC/VM) for the Middle Kit-tanning coal was 1.63 versus 1.10 for the Black Thunder coal. Table 2 lists the 30 to 200 sieve (595 to 74 m) cut sizes for the representative standard grind (~72% through 200 mesh) and fine grind (~87% through 200 mesh) PC samples.
Optimum burner and OFA port settings for minimum NOx and CO were established at full load (100 MBtu/hr), 0.85
Fig.1 Predicted flow fields and temperature contours for the AireJetTM burner. Plots correspond to combustion of standard fineness eastern bituminous Middle Kittanning coal at 0.85 stoichiometry and 100 million Btu/hr.
Fig.2 Predicted reverse flow variations within the flame envelope versus axial distance-to-burner diameter ratio (x/D) for the AireJetTM and DRB-4Z burners. Plots correspond to combustion of standard fineness eastern bituminous Middle Kittanning coal at 0.85 stoichiom-etry and 100 million Btu/hr.
Fig.3 General schematic of the low-NOX AireJetTM burner.
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burner stoichiometry, and 17% overall excess air. Lowest pollutant emission levels were generally achieved when the OFA flow was split equally between the opposed side ports.
Initialburnerhardwareoptimizationswithsepa-ratedOFAarrangement
Figure 5 compares the effects of coal rank and burner stoi-chiometry on NOx and LOI at a fixed overall excess air level of 17% when the separated OFA ports were in service. Raising the burner stoichiometry increased the oxygen availability in the flame zone and elevated the NOx emissions. LOI and CO levels remained relatively low with burner stoichiometry variations, but their expected reductions at higher burner stoichiometries (for a fixed overall excess air value) were in most cases offset by slower burnout due to decreasing OFA flow penetration and mixing in the furnace.
At full load and 0.85 burner stoichiometry, average NOx, CO, LOI, and P values were 110 ppmv (0.153 lb NO2/MBtu), 29 ppmv, 4.62 %, and 4.7 in. wc, respectively for Middle Kittanning coal combustion. Another hardware variation resulted in 105 ppmv NOx (0.148 lb NO2/MBtu), 27 ppmv CO, 3.84% LOI, 4.2 in. wc P under the same operating conditions. Staged combustion of Black Thunder coal at 0.85
burner stoichiometry and 17% excess air had 3.4 in. wc P and generated 56 ppmv NOx (0.076 lb NO2/MBtu), 12 ppmv CO, and 0.35% LOI. Higher reactivity (lower FC/VM), and lower coal-nitrogen content for this coal were responsible for low NOx emissions and high carbon burnout. Overall, good combustion efficiency was exhibited by the AireJet burner with low NOx, LOI, and CO levels. Part load operations generally resulted in lower NOx and higher LOI levels due to cooler furnace environment.
Coal fineness effectIncreasing the Middle Kittanning coal fineness from 72%
to 87% through a 200 mesh screen had the expected effect of faster combustion and oxygen depletion within the staged flame, leading to a longer NOx reduction residence time for the flue gas prior to reaching the overfire air ports. A 6% drop in NOx was attributed to raising the PC fineness in the close-coupled OFA configuration. Corresponding LOI levels were low and relatively unaltered by PC fineness variations.
Finalburnerhardwareoptimizationswithclosed-coupledOFAarrangement
More design modifications were made to further im-prove upon the previous results. Additional tests were then conducted using the close-coupled OFA arrangement to gauge the modified burner performance with regard to high combus-tion efficiency, short flame length, low NOx and CO emissions,
Fig.4 CEDF furnace and OFA arrangements.
Fig. 5 Coal type and stoichiometry effects on NOx and LOI for the AireJet burner using separated OFA ports. Nominal operat-ing conditions: 100 million Btu/hr, 17% overall excess air, and 70-72% through a 200 mesh screen PC fineness.
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low burner P, good flame stability, and low LOI values. NOx and LOI performance results are plotted in Figure 6. At 100 million Btu/hr, 17% excess air and 0.85 burner stoichiometry, the combustion of high fineness eastern bituminous Middle Kittanning coal produced 195 ppmv NOx (0.266 lb/MBtu), 19 ppmv CO, 1.47% LOI, and 3.2 in. wc P. Burning the same coal at 0.80 burner stoichiometry generated 173 ppmv NOx (0.238 lb/MBtu), 14 ppmv CO, and 1.60% LOI while operating at 2.6 in. wc P. Deeper staging from 0.80 to 0.75 stoichiometry showed only slightly more NOx reduction.
Firing the standard fineness western sub-bituminous Black Thunder coal in the AireJetburner resulted in 91 ppmv NOx (0.122 lb/MBtu) and 0.34% LOI in the flyash. Within the plotted burner stoichiometry range, the NOx emissions were nearly unchanged and averaged around 0.124 lb/MBtu. CO and LOI levels were below 25 ppmv and 0.4% LOI, respectively.
NOx emissions remained near their lowest levels when the burner stoichiometry was at 0.80 or less for the Black Thunder coal and at or below 0.85 for the Middle Kittanning coal. In the end, the configuration that was optimum for Black Thunder coal firing also turned out to be optimum for firing the Middle Kittanning coal, achieving 176 ppmv NOx (0.240 lb/MBtu), 17 ppmv CO, and 1.26% LOI at 0.85 burner stoichiometry and 17% overall excess air.
ComparisonwiththeDRB-4ZburnerperformanceComparative tests were also carried out with a 100 million
Btu/hr version of the commercial B&W staged DRB-4Z low-NOx burner[3]. Figure 7 compares the NOx and LOI levels of the staged AireJet and DRB-4Z burners for the separated OFA arrangement. NOx emissions from firing the standard
fineness Black Thunder and Middle Kittanning coals in the AireJet burner at 0.85 burner stoichiometry and 17% excess air were 12% and 21% below their respective measured values for the DRB-4Z burner. Similar plots for the close-coupled OFA arrangement are also shown in Figure 8. Relative to the standard DRB-4Z burner, the AireJet burner generated 18% and 33% lower NOx when firing the Middle Kittanning and Black Thunder coals, respectively.
Summaryofpilot-scaleresultsThe Babcock & Wilcox Company (B&W) has developed
a new staged low-NOx burner named AireJet for pulverized coal firing. Close-coupled and separated OFA ports were setup for testing to represent burner-to-OFA residence times of top and bottom row burners in a commercial boiler. At 17% excess air and 0.85 burner stoichiometry, NOx, CO, and LOI levels were 105 ppmv (0.148 lb/MBtu), 27 ppmv, and 3.84% for the eastern bituminous Middle Kittanning coal and 56 ppmv NOx (0.076 lb/MBtu), 12 ppmv CO, and 0.35% LOI for the western subbituminous Black Thunder coal, respec-tively, when using the separated OFA arrangement. In the close-coupled OFA arrangement, the combustion of Middle Kittanning coal produced 176 ppmv NOx (0.240 lb/MBtu), 17 ppmv CO, and 1.26% LOI, versus 91 ppmv NOx (0.122 lb/MBtu), 11 ppmv CO, and 0.34% LOI for the Black Thunder coal. These favorable results also indicate the suitability of the AireJet burner for firing a broad range of coals.
Highlightsofcommercial-scaleexperienceFollowing the successful single burner performance test-
ing at pilot scale, the next logical step was commercial-scale demonstration at a utility boiler. Our first commercial-scale demonstration was done at the PRB coal-fired, Black Hills Generation, 95 MWe, Wygen 1 Unit 3 in Wyoming. All twelve pre-retrofit, opposed-wall, B&W low-NOx DRB-4Z burners
Fig. 7 Burner hardware and coal rank effects on NOx and LOI at 100 million Btu/hr, 0.85 burner stoichiometry, and 17% over-all excess air with separated OFA ports. Nominal PC fineness (percent through a 200 mesh screen): 70-72%.
Fig.6 Coal type and stoichiometry effects on NOx and LOI for the AireJet burner using close-coupled OFA ports. Nominal operating conditions: 100 million Btu/hr and 17% overall excess air. PC fineness (percent through a 200 mesh screen): 87% for Middle Kittanning coal and 72% for Black Thunder coal.
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were replaced with AireJet burners but the existing four dual-zone opposed-wall OFA ports remained unchanged. NOx emissions at full load operation with the AireJet burners were 0.13 lb/MBtu versus 0.19 lb/MBtu with the DRB-4Z burners. Excess O2 level at the boiler exit was also reduced to 2.0% on a dry volumetric basis while holding the CO emissions below 100 ppmv. Post-retrofit and pre-retrofit LOI levels were less than 1%. Further details can be found in another paper[4]. More commercial installations are planned for the AireJet burners in units firing bituminous and subbituminous coals.
References
1. Patent pending.2. Fiveland, W., and Jessee, P., Mathematical Modeling
of Pulverized Coal Combustion in Axisymmetric Ge-ometries, Presented at the Joint EPRI/ASME Power Generation Conference, Phoenix, AZ, October 1994.
3. Sarv, H., Sayre, A., Warchol, J., and LaRue, A., De-velopment and Evaluation of a Full-Scale Plug-In Ultra Low-NOx (0.2 lb NO2/106 Btu Unstaged) Pulverized Coal Burner for Utility Applications, Presented at the 25th International Technical Conference on Coal Utilization and Fuel Systems, Clearwater, FL, March 2000.
4. Larue, A, Rowley, D., Sarv, H., Kahle, W., and Sayre, A., B&Ws AireJet Burner for Low NOx Emissions, Presented at the Power-Gen International Conference, Orlando, FL, November 2006.
Fig.8 Burner hardware and coal rank effects on NOx and LOI at 100 million Btu/hr, 0.85 burner stoichiometry, and 17% overall excess air with close-coupled OFA ports. Nominal PC fineness (percent through a 200 mesh screen): 87% for Middle Kittanning coal and 72% for Black Thunder coal.
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