Environmental Control Devices
NOx Control Technologies

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Participant
New York State Electric & Gas Corporation (NYSEG)

Locations
Lansing, Tompkins County, NY (NYSEG's Milliken Station, Unit No. 1) This station is currently owned by AES Corporation and is designated AES Cayuga.

Rochester, Monroe County, NY (Eastman Kodak Company's Kodak Park Power Plant, Unit No. 15)

Plant Capacity/Production
Milliken Station: 148-MWe tangentially fired boiler

Kodak Park: 60-MWe cyclone boiler

Coal
Pittsburgh seam bituminous, medium- to high-sulfur (3.2% sulfur and 1.5% nitrogen at Milliken, and 2.2% sulfur and 1.6% nitrogen at Kodak Park)

Technology
Micronized coal reburning using DB Riley's MPS mill (at Milliken Station) and Fuller's MicroMill™ (at Eastman Kodak) technologies for producing micronized coal

Additional Team Members
Eastman Kodak Company
host and cofunder
CONSOL (formerly Consolidation Coal Company)
coal sample tester

D.B. Riley
technology supplier

Fuller Company
technology supplier

Energy and Environmental Research Corporation (EER)
reburn system designer

Empire State Electric Energy Research Corporation
cofunder

New York State Energy Research and Development Authority
cofunder

Project Funding

MicroMill is a trademark of the Fuller Company.
LNCFS is a trademark of ABB Combustion Engineering, Inc.

Project Objective
To achieve at least 50% NOx reduction with micronized coal reburning technology on a cyclone boiler; to achieve 25-35% NOx reduction with micronized coal reburning technology in conjunction with low-NOx burners on a tangentially fired boiler; and to determine the effects of coal micronization on electrostatic precipitator (ESP) performance.

Technology/Project Description
The reburn coal, which can constitute up to 30% of the total fuel, is micronized (pulverized to achieve 80% below 325 mesh) and injected into a pulverized coal-fired furnace above the primary combustion zone. At the Milliken tangentially fired boiler site, NOx control is achieved by: (1) close-coupled overfire air (CCOFA) reburning in which the top coal injector of the LNCFS III™ burner is used for injecting the micronized coal, and the separated overfire air system completes combustion; and (2) the remaining burners and air ports are adjusted for deep-staged combustion by re-aiming them to create a fuel-rich inner zone and fuel-lean outer zone providing combustion air. At the Kodak Park cyclone boiler site, the Fuller MicroMill™ is used to produce the micronized coal, reburn fuel is introduced above the cyclone combustor, and overfire air is employed to complete the combustion.

Results Summary

Environmental

• Using a 14.4% reburn fuel heat input on the Milliken Station tangentially fired boiler at full load resulted in a NOx emission rate of 0.25 lb/10⁶ Btu, which represents a 29% NOx reduction from the 0.35 lb/10⁶ Btu achieved with the LNCFS III™ burner alone (baseline).

• Using a 17.3% reburn fuel heat input (reburn stoichiometry of 0.89) on the Kodak Park cyclone boiler resulted in a NOx emission rate of 0.59 lb/10⁶ Btu, which represents a 59% NOx reduction from 1.36 lb/10⁶ Btu (baseline). Higher reburn rates (estimated at 18.4% reburn or stoichiometry of 0.87) would be required for long-term compliance with 0.60 lb/10⁶ Btu NOx emission limits.

Operational

• Reburning was successfully applied at Milliken Station using the top coal injector of the LNCFS III™ burner for the reburn fuel and reducing the top burner level air flows. This eliminated the need for a separate reburn system. Testing on the tangentially fired boiler at Milliken Station showed:

  • Unburned carbon-in-ash, also referred to as loss-on-ignition (LOI), was maintained under 5%;

  • Increasing the economizer O₂ generated the classical response of higher NOx emissions and lower LOI— the sensitivity was estimated at 0.1 lb/10⁶ Btu per 1% change in O₂ and was relatively independent of coal fineness;

  • Increasing coal fineness reduced both NOx emissions and LOI—the effect on NOx was significant only for large variations in coal fineness; and

  • Pulverizing the reburn coal to the micronized level (greater than 80% passing 325 mesh) was not a requirement for the successful application of reburning, but significantly impacted LOI.

• Testing on the cyclone boiler at Kodak Park showed:

  • The reburn stoichiometry had a significant effect on both NOx emissions and LOI— lower reburn stoichiometries reduced NOx emissions and increased LOI to 40–45% compared with a LOI baseline of 10–15%.

  • Short-term testing indicated that LOI could be maintained at levels similar to baseline levels without significantly affecting NOx emissions by maintaining a baseline cyclone heat input.

Economic

• The estimated capital cost for retrofitting a generic 300-MWe tangentially fired boiler with micronized coal reburning is $4.3 million, or approximately $14/ kW (1999$). The corresponding O&M costs are estimated at $0.30 million per year (1999$). The resulting total 15-year levelized cost is $1,329/ton of NOx removed (current 1999$) or $1,023 (constant 1999$).

• The estimated capital cost for retrofitting a generic 300-MWe cyclone boiler with micronized coal reburning is $16.9 million, or approximately $56/kW (1999$). The corresponding O&M costs are estimated at $0.80 million per year (1999$). The total 15-year levelized cost is $741/ton of NOx removed (current 1999$) or $571 (constant 1999$).

Micronized Coal Reburning Demonstration for NOx Control Process Flow Diagram
Larger jpeg or wmf version

Project Summary
NYSEG demonstrated the micronized coal reburning technology in both tangentially fired and cyclone boilers. The tangentially fired boiler was NYSEG's Milliken Station 148-MWe tangentially fired Unit No. 1 (also the host for another CCT Program demonstration). The cyclone boiler was Eastman Kodak Company's Kodak Park Power Plant 60-MWe cyclone Unit No. 15.

The challenge with this coal reburning demonstration was to achieve adequate combustion of the reburn coal in the oxygen deficient, short residence time reburn zone to reduce NOx emissions without detrimentally increasing the unburned carbon in the ash, i.e., loss-on-ignition. The primary objective of this two-site project was to demonstrate improvements in coal reburning for NOx emission control by reducing the particle size of the reburn coal. In this demonstration, the coal was finely ground to 80% or more passing 325 mesh and injected into the boilers above the primary combustion zone. The resulting typical particle size is 20 microns compared to 60 microns for normal pulverized coal particles. This smaller size increases surface area ninefold.

With this increased surface area and coal fineness (micronized coal has the combustion characteristics of atomized oil), carbon combustion occurs in milliseconds and volatiles are released at an even rate.

Operating Performance
At the Milliken Station, the existing ABB Low-NOx Concentric Firing System™ (LNCFS-III), which includes both CCOFA and separated overfire air (SOFA) ports, was used for the reburn demonstration. Four D.B. Riley MPS 150 mills with dynamic classifiers provided the pulverized coal. With LNCFS-III, there are four levels of burners. To simulate and test the coal reburning application, the top-level coal injection nozzles fed micronized coal to the upper part of the furnace for this demonstration. The coal injection nozzles at the three lower elevations were biased to carry approximately 80% of the fuel required for full load. The speed of the dynamic classifier serving the mill feeding the top burners was increased to produce the micronized coal (greater than 80% passing 325 mesh).

During the evaluation, several conclusions were reached on how operating variables affected performance. While maintaining a constant economizer O₂ level, no single operating variable had a dominant effect on reburning performance. A combination of operating settings determined from short-term testing were selected for long-term operation to achieve the lowest NOx emissions and reliable operation. Operating settings for long-term operation were 14–16% reburn coal, 105 rpm top mill classifier speed (corresponds to 70–72% passing 325 mesh), –5 degrees main burner tilt and 2.8% economizer O₂. No additional improvement in LOI was observed at top mill classifier speeds above 105 rpm.

At Kodak Park, EER designed the micronized coal reburn system using a combination of analytical and empirical techniques. The reburn fuel and overfire air (OFA) injection components were designed with a high degree of flexibility to allow for field optimization to accommodate the complex furnace flow patterns in the cyclone boiler. Two Fuller MicroMills™ were installed in parallel on Kodak Park Unit No. 15 to provide the capacity necessary for high reburn rates, with the second mill serving as a spare at lower reburn rates. The mills produced the micronized coal reburn fuel at greater than 90% passing 325 mesh. Eight injectors, six on the rear wall and one on each of the side walls, introduced the micronized coal into the reburn zone. The optimization variables included the number of injectors, swirl, and velocity. Four ports on the front wall provided OFA using EER's second-generation, dual-concentric overfire air design, which has variable injection velocity and swirl. To maximize NOx reduction, the reburn fuel was injected with flue gas rather than air. The flue gas was extracted downstream of the electrostatic precipitator and was boosted by a single fan. A new boiler control system was also installed on Unit No. 15.

Environmental Performance
At the Milliken Station, micronized coal reburning with 14.4% reburn fuel at full load reduced NOx emissions from the 0.35 lb/10⁶ Btu baseline level to 0.25 lb/10⁶ Btu, a 29% reduction. This reduction represents an addition to the 39% reduction achieved with the LNCFS III™ low- NOx burner alone. Boiler efficiency was maintained at 88.4–88.8%. Furthermore, concentrating the overfire air through fewer and higher ports and using finer grind reburn coal maintained LOI below 5%. Based on long-term testing consisting of 23 days of continuous measurements, the achievable annual NOx emissions using 15.1% coal reburn heat input were estimated at 0.245 ± 0.011 lb/ 10⁶ Btu (95% confidence), and the estimated average fly ash LOI was 4.4 ± 0.4%. Based on replicated performance tests and a 95% confidence level, variations in NOx emissions less than 0.006 lb/10⁶ Btu and in fly ash LOI less than 1.5 percentage points were assumed to be of no statistical significance. There were large uncertainties with respect to the effects on LOI, possibly because LOI generally varied within a relatively narrow range (between 3% and 5%) in response to changing operating variables.

With regard to reburn coal fineness and reburn coal quantity, using a finer grind reburn coal (top mill) reduced both NOx emissions and LOI. The effect on NOx was significant (relative to the uncertainty level of 0.006 lb/10⁶ Btu) only for relatively large variations in the top mill classifier speed (and hence coal fineness). Using a finer grind coal (all mills) reduced both NOx emissions and LOI. Decreasing the reburn coal fraction from 25% to 14% decreased NOx emissions from 0.25 to 0.23 lb/10⁶ Btu and had a minor effect on LOI (generally less than 1.5 percentage points). The decrease in NOx from decreasing the coal reburn fraction was attributed to lower excess air levels in the primary combustion zone as more coal was diverted to the lower burners.

Reducing the boiler load reduced NOx emissions, and the effect was greater when the second mill was taken out of service. Thus, reducing the boiler load by taking the second mill out of service is a recommended option. Taking the second mill out of service while maintaining the same boiler load reduced NOx emissions at both high (140 MW) and low (110 MW) boiler loads, possibly due to longer residence times in the primary combustion zone.

Changes in air flow resulted in measurable changes in both NOx reduction and LOI. An increase in the reburn coal transport air (top burner primary air), corresponding to a 20% increase in the air-to-fuel ratio from 2.05 to 2.45, increased NOx emissions from 0.28 lb/10⁶ to 0.31 lb/10⁶ Btu. This increase in NOx was attributed to less reducing reburn zones with the additional introduction of an oxidant with the reburn fuel. Increasing the top level auxiliary airflow increased both NOx emissions and LOI. This increase in NOx was attributed to less reducing reburn zones as more oxidant was introduced through the auxiliary air nozzle situated directly below the reburn coal nozzle. The increase in LOI from increasing the top level auxiliary airflow was attributed to lower excess air levels in the primary combustion zone as more air was diverted away from the lower burners. Increasing the economizer O₂ generated the classical response of higher NOx emissions and lower or stable LOI. The economizer O₂ sensitivity was estimated at 0.1 lb NOx/10⁶ Btu per 1% change in O₂ and was relatively independent of the reburn coal fineness.

The SOFA and main burner tilts had minimal effects on performance. Variations in the SOFA tilt between 0 and 15 degrees (above horizontal) had minor effects on both NOx emissions and LOI in both LNCFS III™ and reburn configurations. Operating the main burner tilt slightly below the horizontal (about -5 degrees) improved the reburning performance (lower LOI without increasing NOx), relative to the horizontal setting, which was attributed to longer residence times in the furnace prior to overfire air introduction. Overall, the effect was difficult to quantify due to the limited number of tests.

At Kodak Park, the application of micronized coal reburning reduced NOx emissions and increased LOI, as expected. Micronized coal reburning with 17.3% reburn fuel at a reburn stoichometry of 0.89 reduced NOx emissions to 0.59 lb/10⁶ Btu from a baseline of 1.36 lb/10⁶ Btu, a 59% reduction, and reduced the boiler efficiency from 87.8% to 87.3%. At greater reburn rates, further NOx reduction was achieved to a degree comparable with gas reburning systems. At full load, LOI was 40–45%, compared with a baseline level of 10–12%.

Based on long-term testing, the achievable annual NOx emissions (at 15.6% reburn or stoichiometry of 0.90) were 0.69 ± 0.03 lb/10⁶ Btu (95% confidence), corresponding to an LOI of 38% ± 2%. Higher reburn feeds (estimated at 18.4% reburn or stoichiometry of 0.87) would be required for long-term compliance with the 0.6 lb/10⁶ Btu NOx emissions limit.

The reburn stoichiometry had a significant effect on NOx emissions and a significant effect on the LOI. Lower reburn stoichiometries reduced NOx emissions and increased the LOI, typically dropping below 0.6 lb/10⁶ Btu at reburn stoichiometries below 0.9 and corresponding to 40–45% LOI. The increase in the LOI relative to baseline was partially due to a lower cyclone heat input, which resulted in lower temperatures in the primary combustion zone. The lower temperatures produced less thermal NOx formation and less efficient char burnout. The LOI increase was also partially due to the staged combustion resulting in shorter residence times under oxidizing conditions. At constant heat input levels, the LOI was not significantly different with or without reburning, suggesting that in reburn applications, the LOI could be maintained at levels similar to baseline by maintaining a high cyclone heat input. The contribution of reburning alone (assuming no change in the cyclone heat input) to the increase in the LOI was estimated at 0–12% (absolute).

Economic Performance
Estimates were prepared for retrofitting micronized coal reburning on generic 300-MWe tangentially fired and cyclone boilers. For the tangentially fired boiler, the capital costs were estimated at $4.3 million, or approximately $14/kW (1999$). The O&M costs were estimated at $0.30 million per year (1999$). Costs were levelized both on a current dollar and constant dollar basis. The 15-year levelized cost for the 300-MWe unit is $1,329/ton of NOx removed on a current dollar basis, and $1,023/ton of NOx removed on a constant dollar basis (1999$).

For the cyclone boiler, the estimated capital cost is $16.9 million, or approximately $56/kW (1999$). The estimated O&M costs are $0.80 million per year (1999$). The total 15-year levelized cost is $741/ton of NOx removed on a current dollar basis or $571 on a constant dollar basis (1999$).

Commercial Applications
Micronized-coal-reburning technology can be applied to existing and greenfield cyclone-fired, wall-fired, and tangentially fired pulverized coal units. The technology reduces NOx emissions by 20-59% with minimal furnace modifications for existing units.

The availability of a coal-reburning fuel, as an additional fuel to the furnace, enables switching to lower heating value coals without boiler derating. Commercial units can achieve a turndown of 8:1 on nights and weekends without consuming expensive auxiliary fuel.

Contacts

Jim Harvilla
  New York State Electric & Gas Corporation
  Corporate Drive--Kirkwood Industrial Park
  P.O. Box 5224
  Binghamton, NY 13902-5224
  (607) 762-8630
  (607) 762-4002 (fax)
  jjharvilla@nyseg.com

Victor K. Der, DOE/HQ, (301) 903-2700
  victor.der@hq.doe.gov

Thomas A. Sarkus, NETL, (412) 386-5981
  sarkus@netl.doe.gov