Environmental Control Devices
NOx Control Technologies

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Participant
The Babcock & Wilcox Company

Location
Aberdeen, Adams County, OH (Dayton Power and Light Company's J.M. Stuart Plant, Unit No. 4)

Plant Capacity/Production
605 MWe

Coal
Bituminous, 1.2% sulfur

Technology
The Babcock & Wilcox Company's low-NOx cell burner (LNCB®) system

Additional Team Members
The Dayton Power and Light Company
cofunder and host
Electric Power Research Institute
cofunder
Ohio Coal Development Office
cofunder
Tennessee Valley Authority
cofunder
New England Power Company
cofunder
Duke Power Company
cofunder
Allegheny Power System
cofunder
Centerior Energy Corporation
cofunder
Cincinnati Gas & Electric Company
cofunder
Columbus and Southern Power Company
cofunder

Project Funding

LNCB is a registered trademark of The Babcock & Wilcox Company.

Project Objective
To demonstrate, through the first commercial-scale full burner retrofit, the cost-effective reduction of NOx from a large,

baseload coal-fired utility boiler with LNCB^® technology and to achieve at least a 50% NOx reduction without degradation of boiler performance at less cost than that of conventional low-NOx burners.

Technology/Project Description
The LNCB^® technology replaces the upper coal nozzle of the standard two-nozzle cell burner with a secondary air port. The lower burner coal nozzle is enlarged to the same fuel input capacity as the two standard coal nozzles. The LNCB^® operates on the principle of staged combustion to reduce NOx emissions. Combustion is staged by providing only about 58% of the air theoretically required for complete combustion through the lower burner and the balance of the air through the secondary air port (NOx port).

The demonstration was conducted on a Babcock & Wilcox-designed, supercritical once-through boiler equipped with an electrostatic precipitator (ESP). This unit, which is typical of cell burner boilers, contained 24 two-nozzle cell burners arranged in an opposed-firing configuration. Twelve burners (arranged in two rows of six burners each) were mounted on each of two opposing walls of the boiler. All 24 standard cell burners were removed and 24 new LNCBs^® were installed. Alternate LNCBs^® on the bottom rows were inverted, with the air port then being on the bottom to ensure complete combustion in the lower furnace.

B&W Low-NOx Cell Burner Retrofit Process Flow Diagram
Larger jpeg or wmf version

Results Summary

Environmental

• Short-term optimization testing (all mills in service) showed NOx reductions in the range of 53.0-55.5%, 52.5-54.7%, and 46.9-47.9% at loads of 605 MWe, 460 MWe, and 350 MWe, respectively.

• Long-term testing at full load (all mills in service) showed an average NOx reduction of 58% (over 8 months).

• Long-term testing at full load (one mill out of service) showed an average NOx reduction of 60% (over 8 months).

• Carbon monoxide (CO) emissions averaged 28-55 ppm at full load with LNCB^® in service.

• Fly ash increased, but ESP performance remained virtually unchanged.

Operational

• Unit efficiency remained essentially unchanged.

• Unburned carbon losses (UBCL) increased by approximately 28% for all tests, but boiler efficiency loss was offset by a decrease in dry gas loss due to a lower boiler economizer outlet gas temperature.

• Boiler corrosion with LNCB^® was roughly equivalent to boiler corrosion rates prior to retrofit.

Economic

• Capital cost for a 600-MWe plant in the Midwest, with a 1.2 lb/10⁶ Btu initial NOx emission rate and 65% capacity factor, was $9/kW (1994$).

• Levelized cost (15-year) for the same 600-MWe plant was estimated at 0.284 mills/kWh and $96.48/ton of NOx removed (constant 1994$).

Project Summary
Utility boilers equipped with cell burners represent 7.4% or approximately 24,000 MWe of pre-NSPS coal-fired generating capacity. Cell burners are designed for rapid mixing of fuel and air. The tight burner spacing and rapid mixing minimize flame size while maximizing the heat release rate and unit efficiency. Combustion efficiency is good, but the rapid heat release produces relatively large quantities of NOx.

To reduce NOx emissions, the LNCB^® has been designed to stage mixing of fuel and combustion air. A key design criterion was accomplishing delayed fuel-air mixing with no modifications to boiler walls. The plug-in LNCB^® design reduces material costs and outage time required to complete the retrofit, compared to installing conventional, internally staged low-NOx burners, thereby providing a lower cost alternative to address NOx reduction requirements for cell burners.

Environmental Performance
The initial LNCB^® configuration resulted in excessive CO and hydrogen sulfide (H₂S) emissions. Through modeling, a revised configuration was developed (inverting alternate burners on the lower rows), which addressed the problem without compromising boiler performance. The modification served to validate model capabilities.

Following parametric testing to establish optimal operating modes, a series of optimization tests were conducted on the LNCB^® to assess environmental and operational performance. Two sets of measurements were taken, one by Babcock & Wilcox and the other by an independent company, to validate data accuracy. Consequently, the data provided is a range reflecting the two measurements.

The average NOx emissions reduction achieved at full load with all mills in service ranged from 53.0-55.5%. With one mill out of service at full load, the average NOx reduction ranged from 53.3-54.5%. Average NOx reduction at intermediate load (about 460 MWe) ranged from 52.5-54.7%. At low loads (about 350 MWe), average NOx reduction ranged from 46.9-47.9%. NOx emissions were monitored over the long-term at full load with all mills in service and one mill out of service. Each test spanned an 8-month period. The NOx emission reductions realized were 58% for all mills in service and about 60% for one mill out of service.

Single LNCB^® Retrofit

Complications arose in assessing CO emissions relative to baseline because baseline calibration was not sufficiently refined. However, accurate measurements were made with LNCB^® in service. Carbon monoxide emissions were corrected to 3.0% O₂ and measured at full, intermediate, and low loads. The range of CO emissions at full load with all mills in service was 28-55 ppm, and 20-38 ppm with one mill out of service. At intermediate loads (about 460 MWe), CO emissions were 28-45 ppm, and at low loads (about 350 MWe), 5-27 ppm.

Particulate emissions were minimally impacted. The LNCB^® had little effect on flyash resistivity, largely due to SO₃ injection, and therefore ESP removal efficiency remained very high. Baseline ESP collection efficiencies for full load with all mills in service, full load with one mill in service, and intermediate load with one mill out of service were 99.50%, 99.49%, and 99.81%, respectively. For the same conditions, in the same sequence with LNCB^® in operation, ESP collection efficiencies were 99.43%, 99.12%, and 99.35%, respectively.

Operational Performance
Furnace exit gas temperature initially decreased by 100 °F, but eventually rose to within 10 °F of baseline conditions. The UBCL increased by approximately 28% for all tests. The most significant increase from baseline data occurred for a test with one mill out of service. A 52% increase in UBCL resulted in an efficiency loss of 0.69%.

Boiler efficiency showed very little change from baseline. The average with all mills in service increased by 0.16%. The higher post-retrofit efficiency was attributed to a decrease in dry gas loss with lower economizer gas outlet temperature (and subsequent lower air heater gas outlet temperature), offsetting UBCL and CO emission losses. Also, increased coal fineness mitigated UBCL.

Because sulfidation is the primary corrosion mechanism in substoichiometric combustion of sulfur-containing coal, H₂S levels were monitored in the boiler. After optimizing LNCB^® operation, levels were largely at the lower detection limit. There were some higher local readings, but corrosion panel tests established that corrosion rates with LNCB^® were roughly equivalent to pre-retrofit rates.

Ash sample analyses indicated that ash deposition would not be a problem . The LNCB^® ash differed little from baseline ash. Furthermore, the small variations observed in furnace exit gas temperature between baseline and LNCB^® indicated little change in furnace slagging. Startup and turndown of the unit were unaffected by conversion to LNCB^®.

Cell burner AOFA connection with air control vanes open (right) laying next to cell burner housing showing primary air directional vanes and coal tube (left).

Economic Performance
The economic analyses were performed for a 600-MWe nominal unit size and typical location in the Midwest United States. A medium-sulfur, medium-volatile bituminous coal was chosen as the typical fuel. For a baseline NOx emission level of 1.2 lb/10⁶ Btu, 65% capacity factor, and a 50% reduction target, the estimated capital cost was $9/kW (1994$). The 15-year levelized cost of electricity was estimated at 0.284 mills/kWh, or $96.48/ton of NOx removed in constant 1994 dollars.

The S-Type burner impellers used in the LNCB^® design.

Commercial Applications
The market for LNCB^® technology is 33, two-nozzle type cell burner boilers in the U.S. (5 cell burners are three-nozzle types) with a total generating capacity of 25,200 MWe. The LNCB^® system installed at the Dayton Power & Light Company's J.M. Stuart Plant unit No. 4 has been retained for commercial service.

Commercial success to date, and likely to come, is owed largely to the establishment of the LNCB^® Advisory Committee composed of most of the cell burner equipped boiler owners. The Committee participated in the demonstration, becoming familiar with the technology, supporting numerical models, providing inputs to the demonstration, and reviewing field data.

The demonstration project received R&D magazine's 1994 R&D Award.

Contacts

Dot K. Johnson
  The Babcock & Wilcox Company
  20 South Van Buren Avenue
  P.O. Box 351
  Barberton, OH 44203-0351
  (330) 860-1757
  (330) 860-2348 (fax)
  dkjohnson1@babcock.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