Applications of REACH Technology to Reduce NOx

Applications of REACH Technology to Reduce NOx and Particulate Matter Emissions at Oil-Fired Boilers

Prepared for the EPRI-DOE-EPA Combined Utility Air Pollutant Control Symposium (The “Mega” Symposium)
Washington Hilton & Towers, Washington DC
August 25-29, 1997

Robert C. Carr, Coen Company, Inc.
Dan V. Giovanni, Electric Power Technologies, Inc.

ABSTRACT:

Reduced Emissions and Advanced Combustion Hardware (REACH) Technology has been retrofitted to 115 gas- and oil-fired boilers of different designs and capacities to reduce NOx and particulate matter (PM) emissions. The total installed capacity using REACH is presently 17,000-MWe worldwide. Combustion and emissions performance equivalent to that available for new, low-NOx burners has been achieved at costs between $0.25 and $1/kW depending on the unit size. Two versions of REACH are commercially available: (1) Combustion Performance REACH (CP-REACH) for solving a variety of site-specific boiler performance, maintenance, and operating problems related to poor combustion conditions, and (2) Low-NOx REACH (LN-REACH) which provides simultaneous reductions in NOx and PM emissions, while retaining the performance and operating advantages of CP-REACH. This paper describes recent LN-REACH applications using Segmented V-Jet atomizers (patented) at Collins Unit 4 of ComEd, a 550-MW, opposed-wall fired boiler, and at Kahe Unit 6 of Hawaiian Electric Company (HECO), a 140-MW, single-wall-fired boiler. Also described is a CP-REACH application to reduce PM emissions at a 365 metric tons/hr (approximately 100-MW), tangential-fired boiler at the Montova Plant of Frene in Italy.

Results at Collins Unit 4 showed that LN-REACH reduced NOx emissions from 0.55 to 0.30 lb/MBtu (883 to 481 mg/Nm3) at 500-MW without overfire air (OFA) and flue gas recirculation (FGR). The application of OFA and 7% FGR further reduced NOx emissions to 0.21 LB/MBtu (338 mg/Nm3) with 10% opacity. At Kahe Unit 6, NOx emissions at full load were reduced 13% below levels achieved with a combination of 1st-generation LN-REACH, OFA and FGR, i.e., from 0.22 to 0.19 LB/MBtu (353 to 306 mg/Nm3) with 10% opacity. For the tangential-fired boiler, CP-REACH reduced PM emissions from 0.11 to 0.04 LB/MBtu (200 to 60 mg/Nm3) without an increase in NOx emissions (which were approximately 0.45 LB/MBtu, or 725 mg/Nm3).

INTRODUCTION:

Reduced Emissions and Advanced Combustion Hardware (REACH) Technology was developed jointly by Electric Power Technologies, Inc., (EPT), the Electric Power Research Institute (EPRI), the Empire State Electric Energy Research Corporation in New York (ESEERCO), and Consolidated Edison Company of New York for retrofit to existing burners to solve a variety of site-specific boiler problems related to poor combustion conditions, including high stack opacity, high unburned carbon and NOx emissions, acidic stack fallout, flame impingement, poor boiler turndown, and high excess oxygen (1).

The main elements of REACH are oil atomizers and flame stabilizers. The design philosophy for REACH was that it: (1) can be retrofit to the existing range of burner and boiler designs to maximize applicability, and (2) retain as much as possible of the original burner to realize cost advantages relative to other retrofit options (e.g., complete burner replacement). Consistent with this philosophy, REACH adapts to the major existing components of a burner.

OIL ATOMIZATION:

REACH oil atomizers are custom-designed to adapt to the existing oil supply conditions (i.e., pressure, temperature, and capacity)(2). For steam-atomized systems REACH uses internal-mix (I-Mix) atomizers, which produce superior spray quality compared to other common atomizer designs (The quality of the oil spray is characterized by Sauter Mean Diameter (SMD), which is defined as the diameter of a hypothetical droplet that has the same surface-to-volume ratio as that of the total spray. SMD is most often used to characterize oil sprays because it is relevant to evaporation and combustion of oil droplets).

For reducing NOx emissions in steam-atomized systems a novel atomizer design was developed – the Segmented V-Jet atomizer (patented) – which divides the oil spray into distinct segments at the base of the flame. For mechanically-atomized burners, REACH oil atomizers can be designed to operate at supply pressures from 200 to 1,300 psig. Special low-NOx mechanical atomizers that produce oil spray characteristics conditions similar to the Segmented V-Jet atomizer are also available.

FLAME STABILIZERS:

For flame stabilization and aerodynamic control of fuel and air mixing, REACH uses a compound-curved-vane swirler (CVS) for applications on both wall- and tangential-fired boilers(2). The CVS provides better performance than conventional diffusers and flat-bladed swirlers that are commonly in use. The CVS flame stabilizers supplied with REACH are custom-designed to produce the proper entrainment and swirl of combustion air at the discharge plane of the burner, and to match the oil spray of the REACH oil atomizer.

REACH TECHNOLOGY:

Two versions of REACH have been applied in a number of commercial applications. Combustion Performance REACH (CP-REACH) is designed to reduce PM emissions and opacity and to provide operational improvements including increased burner turndown, reduced excess air requirements, improved flame stability, and elimination of flame impingement on furnace walls. Low-NOx REACH (LN-REACH) is specifically aimed at retrofit projects where NOx reduction is the major goal. The key difference between CP-REACH and LN-REACH is the design of the oil atomizer. Boilers equipped with CP-REACH can be easily converted to LN-REACH. Detailed descriptions of these technologies and commercial applications have been published elsewhere(3-6).

Adapting REACH to existing boilers typically requires the custom design of retrofittable oil atomizers and flame stabilizers. The major components of the burner are retained, and significant changes to air registers, burner auxiliary equipment, pumping and heating equipment, or combustion controls are not required. In some instances, the burners have been changed from mechanical to steam atomization. EPT has designed and supplied REACH for more than 115 boilers totaling 17,000-MWe of generating capacity. In a typical retrofit project, REACH atomizers and flame stabilizers can typically be supplied within 6-8 weeks, and installed during a 3-5 day boiler outage. Alternatively, REACH technology may be incorporated in the design of new burners.

REACH technology has been licensed from EPRI by EPT, COEN Company, and Ansaldo Energia.

This paper describes results from three recent REACH applications. Two are LN-REACH projects: (1) Collins Unit 4 of ComEd, a 550-MW, opposed-wall fired boiler; and (2) Kahe Unit 6 of Hawaiian Electric Company (HECO), a 140-MW, single-wall-fired boiler. The third is a CP-REACH application at a 365 metric tons/hr (approximately 100-MW), tangential-fired boiler at the Montova Unit B6 of Frene in Italy.

BOILER DESCRIPTIONS:

Descriptions of the three boilers which were retrofitted with REACH are provided below.

Collins Unit 4 of ComEd

Collins Unit 4 is an opposed-wall-fired boiler manufactured by the Babcock & Wilcox Company and operated by ComEd. The unit has a gross generating capacity of 550-MW, and is equipped with windbox flue gas recirculation (FGR) and overfire air (OFA). The boiler was originally designed to burn No. 6 fuel oil (only), and is equipped with 28 dual-register burners. The burner arrangement is two elevations of seven burners each on the front and rear walls. A dividing plate in the windbox separates the two burner elevations. However, there are no dampers to bias combustion air between the burner elevations. There are seven OFA ports on each firing wall, with one port above each burner column. Combustion air for the OFA ports is supplied from the windbox for the top burner elevation. When 100% open, the OFA system is designed to divert approximately 11% of the total combustion air flow to the OFA ports. The unit was converted to gas and oil operation by EPT in 1996(4).

CP-REACH was retrofitted by EPT in 1991 to improve oil atomization and combustion performance. The retrofit included: (1) replacement of Y-jet atomizers with internal-mix atomizers, (2) replacement of diffusers with compound-curve-vane swirlers for flame stabilization, and (3) conversion of the atomization steam system from constant steam pressure at 150 psig to constant steam-to-oil differential pressure of 10 psid over the load range. As part of a gas conversion project performed in 1996, EPT retrofitted LN-REACH flame stabilizers and low-NOx, Segmented V-Jet oil atomizers. Results with the Segmented V-Jet oil atomizers are presented in this paper.

Kahe Unit 6 of Hawaiian Electric Company

Kahe Unit 6 is a single-wall-fired boiler manufactured by the Babcock & Wilcox Company and operated by HECO. The unit has a gross generating capacity of 146-MW, and is equipped with windbox FGR and OFA. The OFA system was designed to divert up to 30% of the total combustion air to six OFA ports located on the front and rear boiler walls (three ports per wall). The boiler burns No. 6 fuel oil (only), and is equipped with nine PG-DRB burners in a 3 x 3 array on the front wall(7).

The original oil atomizers supplied by the boiler manufacturer were capable of meeting the NOx emissions limit of 0.23 LB/MBtu (370 mg/Nm3) when used with maximum FGR and OFA. However, there was no operating margin with the 20% opacity limit to allow for variability in operation or oil properties. First-generation, LN-REACH atomizers were retrofitted by EPT in 1990 to improve oil atomization and combustion performance. Results showed that the NOx emissions limit could be met with reduced levels of OFA and FGR and with significantly lower opacity (10-13%). In the spring of 1997, EPT retrofitted low-NOx, Segmented V-Jet oil atomizers. The objective was to increase the margin of compliance with the NOx and opacity limits. Results with the Segmented V-Jet oil atomizers are presented in this paper.

Mantova Unit B6 of Frene in Italy

Mantova Unit B6 is a tangential-fired boiler manufactured by Franco Tosi and operated by Frene of the ENI Group in Italy. The unit has a steam generating capacity of 365 metric tons/hr (approximately 100-MW), and is not equipped with windbox FGR. However, the boiler does have a small, close-coupled OFA compartment above each burner in the top elevation. The boiler is capable of burning No. 6 fuel oil and natural gas, and has four burner elevations and 16 burners. Each burner has single fuel-air compartment and auxiliary-air compartments immediately above and below the fuel-air compartment. The auxiliary-air compartments between the 2nd/3rd and 3rd/4th elevations are bricked shut. To avoid excessive opacity (smoking) the unit is normally operated with the fuel-air compartment dampers 100% open, and the functional auxiliary-air compartment dampers virtually closed (i.e., 10% open for cooling).

In 1997 EPT installed CP-REACH flame stabilizers and internal-mix oil atomizers to reduce PM emissions. The CP-REACH retrofit included: (1) replacement of Y-jet atomizers with internal-mix atomizers, (2) replacement of diffusers with compound-curve-vane swirlers and extenders for flame stabilization, and (3) conversion of the atomization steam system from constant steam pressure at 150 psig to constant steam-to-oil differential pressure of 10 psid over the load range, and (4) reactivation of the close-coupled OFA. To increase swirler flow entrainment, extender assemblies (i.e., bluff-body rings) were attached to the exit of the fuel-air nozzles to increase airflow turbulence and promote the formation of a strong internal recirculation zone.

REACH CHARACTERIZATION TESTS & MEASUREMENT METHODS:

The REACH installations at the three boilers were commercial applications. Consequently, tests to optimize combustion and emissions performance emphasized development of operational guidelines over the load range for the plant operators, instead of parametric tests to characterize the sensitivity of NOx and PM emissions to FGR, OFA, excess O2, etc.

At Collins Unit 4, a multi-point extractive system was used to obtain flue gas samples for analysis of NOx, CO, CO2, and O2. Samples were extracted from probes installed in the “A” (north) and the “B” (south) flue gas ducts immediately upstream of the air heaters. Gaseous samples were obtained from probes installed in four ports in each duct. Three sampling probes were installed in each port (long-, intermediate-, and short-length probes). Thus, samples were obtained from 24 points in the flue gas duct, ensuring a representative sample of the boiler exhaust gases. Twenty-four sample lines were strung from the probes to a mobile emissions monitoring laboratory that was located at the ground level adjacent to the unit. The gas samples were conditioned to remove water, and were then directed to emission monitors.

NOx, CO, CO2, and O2 were measured using EPA continuous monitoring procedures, and EPA certified calibration gases. O2 was measured with a Servomex Model 570A instrument, CO2 with a ACS Model 3300 instrument, CO with a TECO Model 48 instrument, and NOx with a TECO Model 10A chemiluminescence instrument. Opacity was measured with the plant continuous emissions monitor system (CEMS).

At Kahe Unit 6 and Montova Unit B6, the plant CEMS were used for measurement of NOx, excess O2, and opacity.

RESULTS:

The objectives of the LN-REACH retrofits at Collins Unit 4 and Kahe Unit 6 were to reduce NOx emissions without increasing PM emissions and opacity. The CP-REACH retrofit at Montova Unit B6 was intended to reduce PM emissions without increasing NOx emissions. Results are presented below.

Collins Unit 4

As described above, CP-REACH was retrofitted at Collins Unit 4 in 1991 to improve oil atomization and combustion performance. The CP-REACH retrofit eliminated high opacity which caused derating of the unit.

As part of the gas conversion project at Collins Unit 4, EPT supplied new LN-REACH flame stabilizers and replaced the CP-REACH internal-mix atomizers with Segmented V-Jet atomizers. Figure 1 compares NOx emissions vs. load for CP-REACH and LN-REACH, and also shows results from parametric tests to characterize the effects of OFA and FGR on NOx emissions. For CP-REACH, NOx emissions without OFA and FGR varied from 0.35 LB/MBtu at 300-MW to approximately 0.6 LB/MBtu at 540-MW. The downward-pointing arrows at 525-MW show the beneficial effects of: (1) opening the OFA ports, which reduced NOx emissions to approximately 0.46 LB/MBtu, and (2) combining OFA with approximately 7% FGR (using one GR fan), which further decreased NOx emissions to 0.31 LB/MBtu. NOx emissions less than 0.30 LB/MBtu could be achieved with CP-REACH at full load by operating two GR fans to deliver 10-12% FGR.

With LN-REACH, NOx emissions without OFA and FGR varied from 0.18 LB/MBtu at 300-MW to approximately 0.30 LB/MBtu at 500-MW (the maximum load possible during the tests due to boiler feed pump problems). The downward-pointing arrows at 500-MW show that OFA reduced NOx emissions to 0.26 LB/MBtu, and OFA combined with approximately 7% FGR (one GR fan) further decreased NOx emissions to 0.21 LB/MBtu. Opacity was less than 10% over the load range. Emissions compliance tests performed with LN-REACH at a later date showed NOx emissions of 0.25 LB/MBtu at 550-MW with the OFA ports open and 7% FGR. Opacity was 10-13%.


Figure 1, NOx Emissions vs. Load for Collins Unit 4 With CP-REACH (open symbols) and LN_REACH With Segmented V-Jet Atomizers (Closed Symbols)

The solid lines represent baseline NOx emissions without FGR and OFA. The singular data points show NOx reductions achieved at 540-MW for CP-REACH and 500-MW for LN-REACH with OFA (squares) and OFA + 7% FGR (triangles). The Segmented V-Jet atomizers alone (baseline) reduced NOx emissions by 45%. Adding OFA and FGR further reduced NOx emissions by 32%.

The reductions in NOx emissions with LN-REACH were substantial. At 500-MW without OFA and FGR, LN-REACH delivered a 45% reduction in NOx emissions compared to CP-REACH. With OFA and 7% FGR, LN-REACH achieved a 32% reduction in NOx emissions compared to CP-REACH. A major advantage of LN-REACH in this application was that NOx emissions compliance was achieved without OFA and FGR below 490-MW. At higher loads, plant operators had the flexibility to choose either OFA or FGR to meet the NOx emissions limit depending upon steam temperature or other operational considerations. In any case, there was sufficient NOx and opacity compliance margin available to allow for variations in fuel composition (e.g., fuel nitrogen) and boiler operation (e.g., only one GR fan). The experience at Collins Unit 4 showed that for boilers not equipped with OFA and FGR, LN-REACH can achieve significant reductions in NOx emissions at very low cost, i.e., about $0.25/kW.

Kahe Unit 6

LN-REACH Segmented V-Jet atomizers were installed in Kahe Unit 6 in April 1997. Figure 2 compares NOx emissions vs. load for the Segmented V-Jets and the first-generation LN-REACH atomizers installed in 1990. The data were obtained with approximately 15% OFA and 12% FGR. Results showed that the Segmented V-Jet atomizers reduced NOx emissions at maximum load from 0.22 to 0.19 LB/MBtu (approximately 13%). At lower loads, the reduction in NOx emissions by using the Segmented V-Jet atomizers was approximately 20%. As was the case at Collins Unit 4, the lower NOx emissions with LN-REACH allowed plant operators to meet the NOx and opacity limits with less than the maximum OFA and FGR, thus providing a margin to allow for variations in fuel composition and boiler operation.

Montova Unit B6

CP-REACH flame stabilizers, extenders, and internal-mix atomizers were installed in Montova Unit B6 in January 1997, and tests to characterize particulate matter (PM) and NOx emissions were performed in February 1997. As summarized in Figure 3, PM emissions with CP-REACH were reduced by 70% to 56 mg/Nm3 (0.036 LB/MBtu) from 200 mg/Nm3 (0.11 LB/MBtu). NOx emissions remained essentially unchanged at 715-735 mg/Nm3 (The NOx and PM emissions reported for the original operating condition were “typical values” provided by Frene based on historical data, and were not results from specific tests performed as part of the REACH retrofit.).


Figure 2, NOx Emissions vs. Load for Kahe Unit 6 With 1st-Generation LN-REACH (Open Symbols) and LN-REACH With Segmented V-Jet Atomizers (Closed Symbols)

The data were collected with 15% OFA and 12% FGR to the windbox. The Segmented V-Jets reduced NOx emission by 13% compared to the 1st-generation LN-REACH atomizers. Opacity at full load was 11-13% for the 1st-generation LN-REACH compared to 8-10% with the Segmented V-Jets.


Figure 3, PM and NOx Emissions at Montova Unit B6 With the Original Combustion Equipment (Left) and CP-REACH

The CP-REACH data shown on the far right were obtained with the dampers 50% open on the close-coupled OFA compartments above the top burner elevation. Prior to installation of CP-REACH it was not feasible to open the OFA compartment dampers due to high opacity.

Figure 3 also shows the effect on NOx emissions of opening the auxiliary-air compartments above the top burner elevation. As discussed previously, to avoid high opacity the boiler was normally operated with the fuel-air compartment dampers 100% open and the functional auxiliary air compartments 10% open (for cooling) on all the burners. As shown in the figure, setting the auxiliary air compartments above the top burner elevation to 50% open reduced NOx emissions by approximately 9% from 0.45 to 0.41 LB/MBtu (725 to 660 mg/Nm3) with a very slight increase in PM emissions from 56 to 69 mg/Nm3 (0.036 to 0.045 LB/MBtu). This result was not surprising, and suggests that further biasing of combustion air to the auxiliary-air compartments in the top elevation or application of LN-REACH at Montova Unit B6 would be effective in lowering NOx emissions.

CONCLUSIONS:

LN-REACH applications using Segmented V-Jet atomizers (patented) at Collins Unit 4, a 550-MW, opposed-wall fired boiler, and Kahe Unit 6, a 140-MW, single-wall-fired boiler, showed significant reductions in NOx emissions without increases in opacity or PM emissions. At Collins Unit 4, LN-REACH reduced NOx emissions at 500-MW from 0.55 to 0.30 LB/MBtu (883 to 481 mg/Nm3) without OFA and FGR. The application of OFA and 7% FGR further reduced NOx emissions to 0.21 LB/MBtu (338 mg/Nm3) with 10% opacity. At Kahe Unit 6, NOx emissions were reduced 13% below levels achieved with a combination of 1st-generation LN-REACH, OFA and FGR, i.e., from 0.22 to 0.19 LB/MBtu (354 to 306 mg/Nm3) with 10% opacity. At both plants the lower NOx emissions with LN-REACH allowed plant operators to meet the NOx and opacity emissions limits over the load range with less than the maximum OFA and FGR, and thus providing a margin to allow for variations in fuel composition and boiler operation.

CP-REACH flame stabilizers, extenders, and internal-mix atomizers were installed in Montova Unit B6 of Frene in Italy in January 1997. Compared to emissions with the original combustion equipment, PM emissions with CP-REACH were reduced by 70% to 56 mg/Nm3 (0.036 LB/MBtu) from 200 mg/Nm3 (0.11 LB/MBtu). NOx emissions remained essentially unchanged at 715-735 mg/Nm3.

REFERENCES:

1. Giovanni, D.V., M.W. McElroy, and S.E. Kerho, “REACH: A Low-cost Approach to Reducing Stack Emissions and Improving the Performance of Oil-fired Boilers,” EPRI/EPA Joint Symposium on Stationary Combustion NOx Control, Kansas City, Missouri (May 1995).

2. Kerho, S. E., and D. V. Giovanni, “Atomizer and Swirler Design for Reduced NOx and Particulate Emissions,” EPRI Workshop on NOx Controls for Utility Boilers, Boston, Massachusetts (July 1992).

3. Giovanni, D.V., R.C. Carr and S.E. Kerho, “Reduction in NOx Emissions by Retrofit of Low-NOx Atomizers on a 550 MW Oil-fired Boiler,” Third International Conference on Combustion Technologies for a Clean Environment, Portugal (July 1995).

4. Carr, R. C., Marco Alberti, and Christopher J. Nagel, “Retrofit of Gas Combustion Equipment and Low-NOx Oil Atomizers at a 550-MW Oil-Fired Utility Boiler,” International Joint Power Generation Conference, Houston, Texas (October 1996).

5. Conti, A.V., S.E. Kerho and J. Lucente, “Low Cost Retrofit Combustion Hardware for Emissions Control on Industrial Boilers,” ASME/EPRI International Joint Power Generation Conference & Exposition, Minneapolis, Minnesota, (October 1995).

6. Conti, A.V. and J. Lucente, “Reduction of NOx Emissions on Oil Firing at Bowline Point Unit No. 2,” ASME/EPRI International Joint Power Generation Conference & Exposition, Minneapolis, Minnesota (October 1995).

7. Kerho, Stephen, E., Dan V. Giovanni, et al, “Reduced NOx , Particulate, and Opacity on the Kahe Unit 6 Low-NOx Burner System,” 1991 EPA/EPRI Joint Symposium on Stationary Combustion for NOx Control, Washington D.C., March 1991.