Environmental Control Devices
Combined SO₂/NOx Control Technologies

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Participant
The Babcock & Wilcox Company

Location
Lorain, Lorain County, OH (Ohio Edison's Edgewater Station, Unit No. 4)

Plant Capacity/Production
105 MWe

Coal
Ohio bituminous, 1.6, 3.0, and 3.8% sulfur

Technology
The Babcock & Wilcox Company's (B&W) limestone injection multistage burner (LIMB) system;
Babcock & Wilcox DRB-XCL^® low-NOx burners; Consolidation Coal Company's Coolside duct injection of lime sorbents

Additional Team Members
Ohio Coal Development Office
cofunder

Consolidation Coal Company
cofunder and technology supplier

Ohio Edison Company
host

Project Funding

DRB-XCL is a registered trademark of The Babcock & Wilcox Company.
TAG is a trademark of the Electric Power Research Institute.

Project Objective
To demonstrate, with a variety of coals and sorbents, that the LIMB process can achieve up to 50% NOx and SO₂ reductions, and to demonstrate that the Coolside process can achieve SO₂ removal of up to 70%.

Technology/Project Description
The LIMB process reduces SO₂ by injecting dry sorbent into the boiler at a point above the burners. The sorbent then travels through the boiler and is removed along with fly ash in an electrostatic precipitator (ESP) or baghouse. Humidification of the flue gas before it enters an ESP is necessary to maintain normal ESP operation and to enhance SO₂ removal. Combinations of three bituminous coals (1.6, 3.0, and 3.8% sulfur) and four sorbents were tested. Other variables examined were stoichiometry, humidifier outlet temperature, and injection elevation level in the boiler.

In the Coolside process, dry sorbent is injected into the flue gas downstream of the air preheater, followed by flue gas humidification. Humidification enhances ESP performance and SO₂ absorption. SO₂ absorption is improved by dissolving sodium hydroxide (NaOH) or sodium carbonate (Na₂CO₃) in the humidification water. The spent sorbent is collected with the fly ash, as in the LIMB process. Bituminous coal with 3.0% sulfur was used in testing.

Babcock & Wilcox DRB-XCL^® low-NOx burners, which control NOx through staged combustion, were used in demonstrating both LIMB and Coolside technologies.

B&W LIMB/Coolside Process Flow Diagram
Larger jpeg or wmf version

Results Summary

Environmental

• LIMB SO₂ removal efficiencies at a calcium-to-sulfur (Ca/S) molar ratio of 2.0, and minimal humidification across the range of coal sulfur contents were 53-61% for ligno lime, 51-58% for calcitic lime, 45-52% for dolomitic lime, and 22-25% for limestone ground to 80% less than 44 microns (325 mesh).

• LIMB SO₂ removal efficiency increased from 22-25% to 32% using limestone ground to 100% less than 44 microns, and increased an additional 5-7% when ground to 100% less than 10 microns.

• LIMB SO₂ removal efficiencies were enhanced by about 10% with humidification at a 20°F approach-to-saturation temperature.

• LIMB, which incorporated Babcock & Wilcox DRB-XCL^® low-NOx burners, achieved 40-50% NOx reduction.

• Coolside SO₂ removal efficiency was 70% at a Ca/S molar ratio of 2.0, a sodium-to-calcium (Na/Ca) ratio of 0.2, and a 20 °F approach-to-saturation temperature using commercial hydrated lime and 2.8-3.0% sulfur coal.

• Sorbent recycle tests demonstrated the potential to improve sorbent utilization.

Operational

• Humidification enhanced ESP performance, which enabled opacity levels to be kept well within limits.

• LIMB availability was 95%. Coolside did not undergo testing of sufficient length to establish availability.

• Humidifier performance indicated that operation in a vertical rather than horizontal mode would be better.

Economic

• LIMB capital costs were $31-102/kW (1992$) for plants ranging from 100 to 500 MWe and coals with 1.5-3.5% sulfur, with a target SO₂ reduction of 60%. Annual levelized costs (15-year) for this range of conditions were $392-791/ton of SO₂ removed.

• Coolside capital costs were $69-160/kW (1992$) for plants ranging from 100 to 500 MWe and coals with 1.5-3.5% sulfur, with a target SO₂ reduction of 70% . Annual levelized costs (15-year) for this range of conditions were $482-943/ton of SO₂ removed.

Water mist, sprayed into the flue gas, enhanced sulfur capture by the sorbent by approximately 10% in the LIMB process when 20 °F approach-to-saturation was used.

Project Summary
The initial expectation with LIMB technology was that limestone calcined by injection into the furnace would achieve adequate SO₂ capture. Use of limestone in lieu of the significantly more expensive lime would keep operating costs relatively low. However, the demonstration showed that, even with fine grinding of the limestone and deep humidification, performance with limestone was marginal. As a result, a variety of hydrated limes were evaluated in the LIMB configuration, demonstrating enhanced performance. Although LIMB performance was enhanced by applying humidification to the point of approaching adiabatic saturation temperatures, performance did not rely on this deep humidification.

Coolside design was dependent upon deep humidification to improve sorbent reactivity and the use of hydrated lime. Sorbent injection was downstream of the furnace. In addition, sorbent activity was enhanced by dissolving sodium hydroxide (NaOH) or sodium carbonate (Na₂CO₃) in the humidification water.

Environmental Performance (LIMB)
LIMB tests were conducted over a range of Ca/S molar ratios and humidification conditions while burning Ohio coals with nominal sulfur contents of 1.6, 3.0, and 3.8% by weight. Each of four different sorbents was injected while burning each of the three different coals with one exception. Other variables examined were stoichiometry, humidifier outlet temperature, and injection elevation level in the boiler. Exhibit 27 summarizes SO₂ removal efficiencies for the range of sorbents and coals tested.

While injecting commercial limestone with 80% of the particles less than 44 microns in size (minus 325 mesh), removal efficiencies of about 22% were obtained at a stoichiometry of 2.0 while burning 1.6% sulfur coal. However, removal efficiencies of about 32% were achieved at a stoichiometry of 2.0 when using a limestone with a smaller particle size (i.e., all particles were less than 44 microns). A third limestone with essentially all particles less than 10 microns was used to determine the removal efficiency limit. The removal efficiency for this very fine limestone was approximately 5-7% higher than that obtained under similar conditions for limestone with particles all sized less than 44 microns.

During the design phase, it was expected that injection at the 181-foot plant elevation level inside the boiler would permit the introduction of the limestone at close to the optimum furnace temperature of 2,300 °F. Testing confirmed that injection at this level, just above the nose of the boiler, yielded the highest SO₂ removal. Injection was also performed at the 187-foot level and similar removals were observed. Removal efficiencies while injecting at these levels were about 5% higher than while injecting sorbent at the 191-foot level.

Removal efficiencies were enhanced by approximately 10% over the range of stoichiometries tested when using humidification down to a 20 °F approach-to-saturation temperature. The continued use of the low-NOx burners resulted in an overall average NOx emissions level of 0.43 lb/10⁶ Btu, which is about a 45% reduction.

Operational Performance (LIMB)
Long-term test data showed that the LIMB system was available about 95% of the time it was called upon to operate. Even with minimal humidification, ESP performance was adequately enhanced to keep opacity levels well below the permitted limit. Opacity was generally in the 2-5% range (limit was 20%).

Environmental Performance (Coolside)
The Coolside process was tested while burning compliance (1.2-1.6% sulfur) and noncompliance (2.8-3.2% sulfur) coals. Objectives of the full-scale test program were to verify short-term process operability and to develop a design performance database to establish process economics for Coolside. Key process variables-- Ca/S molar ratio, Na/Ca molar ratio, and approach-to-saturation temperatures-- were evaluated in short-term (6 - 8 hours) parametric tests and longer term (1 - 11 days) process operability tests.

The test program demonstrated that the Coolside process routinely achieved 70% SO₂ removal at design conditions of 2.0 Ca/S molar ratio, 0.2 Na/Ca molar ratio, and 20 °F approach-to-saturation temperature using commercially available hydrated lime. Coolside SO₂ removal depended on Ca/S molar ratio, Na/Ca molar ratio, approach-to-adiabatic-saturation, and the physical properties of the hydrated lime. Sorbent recycle showed significant potential to improve sorbent utilization. The observed SO₂ removal with recycled sorbent alone was 22% at 0.5 available Ca/S molar ratio and 18 °F approach-to-adiabatic-saturation. The observed SO₂ removal with simultaneous recycle and fresh sorbent feed was 40% at 0.8 fresh Ca/S molar ratio, 0.2 fresh Na/Ca molar ratio, 0.5 available recycle, and 18 °F approach-to-adiabatic-saturation.

Operational Performance (Coolside)
Floor deposits experienced in the ductwork with the horizontal humidification led designers to consider a vertical unit in a commercial configuration. Short-term testing did not permit evaluation of Coolside system availability.

Economic Performance (LIMB & Coolside)
Economic comparisons were made between LIMB, Coolside, and a wet scrubber with limestone injection and forced oxidation (LSFO). Assumptions on performance were SO₂ removal efficiencies of 60, 70, and 95% for LIMB, Coolside, and LSFO, respectively. The EPRI TAG™ methods were used for the economics, which are summarized in Exhibits 28 and 29.

Commercial Applications
Both LIMB and Coolside technologies are applicable to most utility and industrial coal-fired units, and provide alternatives to conventional wet flue gas desulfurization processes. LIMB and Coolside can be retrofitted with modest capital investment and downtime, and their space requirements are substantially less than for conventional flue gas desulfurization processes.

Contacts

Greg Bielawski
  The Babcock & Wilcox Company
  20 South Van Buren Avenue
  P.O. Box 351
  Barberton, OH 44203-0351
  (330) 860-1591
  (330) 860-9292 (fax)
  gtbielawski@babcock.com

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

John C. McDowell, NETL, (412) 386-6175
  john.mcdowell@netl.doe.gov